Figure 2.9: Molecular structures of Ebecryl 860 and Esperox 28. ..... groups and more hydroxyl groups per monomer than Ebecryl 860 ...... 78: p. 115-122. 77.
BIO-BASED POLYMERIC FOAM FROM SOYBEAN OIL AND CARBON DIOXIDE
by Laetitia M. Bonnaillie
A dissertation submitted to the Faculty of the University of Delaware in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemical Engineering
Fall 2007
Copyright 2007 Laetitia M. Bonnaillie All Rights Reserved
BIO-BASED POLYMERIC FOAM FROM SOYBEAN OIL AND CARBON DIOXIDE
by Laetitia M. Bonnaillie
Approved:
__________________________________________________________ Norman J. Wagner, Ph.D. Chair of the Department of Chemical Engineering
Approved:
__________________________________________________________ Michael J. Chajes, Ph.D. Interim Dean of the College of Engineering
Approved:
__________________________________________________________ Carolyn A. Thoroughgood, Ph.D. Vice Provost for Research and Graduate Studies
I certify that I have read this dissertation and that in my opinion it meets the academic and professional standard required by the University as a dissertation for the degree of Doctor of Philosophy.
Signed:
__________________________________________________________ Richard P.Wool, Ph.D. Professor in charge of dissertation
I certify that I have read this dissertation and that in my opinion it meets the academic and professional standard required by the University as a dissertation for the degree of Doctor of Philosophy.
Signed:
__________________________________________________________ Annette D. Shine, Ph.D. Member of dissertation committee
I certify that I have read this dissertation and that in my opinion it meets the academic and professional standard required by the University as a dissertation for the degree of Doctor of Philosophy.
Signed:
__________________________________________________________ Antony N. Beris, Ph.D. Member of dissertation committee
I certify that I have read this dissertation and that in my opinion it meets the academic and professional standard required by the University as a dissertation for the degree of Doctor of Philosophy.
Signed:
__________________________________________________________ Anette M. Karlsson, Ph.D. Member of dissertation committee
iv
TABLE OF CONTENTS
LIST OF TABLES ............................................................................................................. xi LIST OF FIGURES.......................................................................................................... xiii ABSTRACT ................................................................................................................... xx 1.
INTRODUCTION ........................................................................................................1 1.1. Objective and motivation.....................................................................................1 1.2. Introduction to polymeric foams..........................................................................4 1.2.1. General foam architecture ..........................................................................4 1.2.2. Apparent foam density................................................................................6 1.2.3. Cell shape ...................................................................................................8 1.2.4. Cell size ......................................................................................................9 1.2.5. Effects of structure on foam properties ......................................................9 1.3. Foam production ................................................................................................11 1.3.1. Principles of foam formation ....................................................................11 1.3.1.1. Bubble nucleation ..........................................................................12 1.3.1.2. Bubble growth ...............................................................................13 1.3.2. Commercial foaming processes ................................................................15 1.3.2.1. Polystyrene and other thermoplastic foams ...................................15 1.3.2.2. Polyurethane foams and other thermosets .....................................16 1.4. Background on bio-based polymeric foams ......................................................18 1.4.1. Starch-based foams ...................................................................................18 1.4.2. Bio-based polyurethanes...........................................................................20 1.5. ACRES monomers and polymers ......................................................................24 1.5.1. Plant oils ...................................................................................................24 1.5.2. Functionalization of triglycerides .............................................................25 1.5.3. Acrylated Epoxidized Soybean Oil Monomer..........................................27 1.5.4. AESO polymer .........................................................................................29 1.6. Choice of CO2 as a blowing agent .....................................................................32
v
1.6.1. Properties of CO2......................................................................................32 1.6.2. Common high-pressure solubility measurement methods........................33 1.6.3. Solubility of CO2 in a liquid .....................................................................34 1.6.3.1. Definitions .....................................................................................34 1.6.3.2. Raoult’s Law .................................................................................35 1.6.3.3. Henry’s law ...................................................................................35 1.6.3.4. Solubility parameters.....................................................................36 1.6.3.5. Rapid estimate of the solubility of CO2 in organic solvents .........37 1.6.4. Solubility of Supercritical CO2 in polymers .............................................37 1.6.5. Solubility of CO2 in plant oils ..................................................................38 1.7. Research goals and strategy ...............................................................................39 2.
EXPERIMENTAL METHODS .................................................................................42 2.1. Equipment ..........................................................................................................42 2.1.1. High-pressure reactor ...............................................................................42 2.1.2. Reactor add-ons ........................................................................................44 2.1.2.1. Reactor stand and shield ................................................................44 2.1.2.2. Reactor liners.................................................................................44 2.1.2.3. Rotameter ......................................................................................44 2.1.2.4. Heating mantle ..............................................................................45 2.1.2.5. Temperature Controller .................................................................45 2.1.2.6. Dip tube .........................................................................................46 2.1.2.7. Stirrer assembly .............................................................................46 2.1.2.8. External heater...............................................................................47 2.1.2.9. Micrometric valve .........................................................................49 2.1.3. Other equipment used for foam processing ..............................................49 2.1.3.1. Reactor loading station ..................................................................49 2.1.3.2. Molds.............................................................................................50 2.1.3.3. Vacuum oven.................................................................................50 2.1.3.4. Programmable oven.......................................................................51 2.2. Materials ............................................................................................................51 2.2.1. Foam components.....................................................................................51 2.2.2. Reactives ..................................................................................................51 2.3. Foam preparation ...............................................................................................52 2.3.1. Air purge...................................................................................................52 2.3.2. Foam fabrication .......................................................................................53
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2.4. Foam Characterization methods ........................................................................53 2.4.1. Viscosity measurement .............................................................................53 2.4.2. Digital imaging .........................................................................................54 2.4.2.1. Liquid foams: photographs............................................................54 2.4.2.2. Solid foams: scans .........................................................................55 2.4.3. Foam density ............................................................................................56 2.4.4. Extent of cure with 13C-NMR ..................................................................56 2.4.4.1. Introduction and sample preparation .............................................56 2.4.4.2. Magnetic shifts ..............................................................................58 2.4.5. Compression tests .....................................................................................60 2.4.5.1. Definitions .....................................................................................61 2.4.5.2. Sample preparation ........................................................................61 2.4.5.3. Size limitations ..............................................................................63 2.4.6. Flexural tests.............................................................................................63 2.4.7. Cytotoxicity ..............................................................................................64 2.4.8. Biodegradability .......................................................................................64 3.
SOLUBILITY OF CO2 IN AESO ..............................................................................66 3.1. Experimental Methods .......................................................................................66 3.1.1. CO2 dissolution at T and P .......................................................................67 3.1.2. Sample extraction .....................................................................................69 3.1.3. Solubility measurements ..........................................................................70 3.2. Results and discussion .......................................................................................73 3.2.1. Solubility data for CO2 in AESO..............................................................73 3.2.2. Experimental error due to gas leak under vacuum ...................................77 3.2.3. Data modeling ..........................................................................................78 3.2.3.1. Thermodynamic Model .................................................................78 3.2.3.2. Henry’s law constant and LF equation of state models .................82 3.2.4. Comparison with unmodified triglycerides ..............................................85 3.2.5. Foam density versus CO2 solubility .........................................................86 3.2.5.1. In-reactor foam density as a function of TR and P ......................86 3.2.5.2. Extracted foams: theoretical density vs. actual density .................92 3.3. Conclusions .......................................................................................................96
4.
AGING OF LIQUID AESO/CO2 FOAMS ................................................................98 4.1. Introduction........................................................................................................98
vii
4.1.1. Purpose .....................................................................................................98 4.1.2. Foam fabrication .......................................................................................99 4.1.2.1. Method ..........................................................................................99 4.1.2.2. Definitions ...................................................................................101 4.1.2.3. Thermodynamic bubble growth ..................................................102 4.1.3. Foam aging .............................................................................................104 4.1.3.1. Elementary bubble dynamics ......................................................104 4.1.3.2. Bubble instability ........................................................................106 4.1.3.3. General appearance of liquid AESO/CO2 foam ..........................107 4.1.3.4. Parameters affecting foam behavior ............................................108 4.2. Bubble growth during foam aging ...................................................................109 4.2.1. Growth of individual foam cells .............................................................109 4.2.2. Average cell size vs. time .......................................................................110 4.2.2.1. Volumetric bubble density ..........................................................110 4.2.2.2. Average bubble volume ..............................................................111 4.2.2.3. Second order model for coalescence ...........................................114 4.2.3. Effect of temperature ..............................................................................116 4.2.4. Effects of other parameters .....................................................................120 4.3. Conclusions on bubble growth during foam aging ..........................................121 5.
LOW-TEMPERATURE AESO/CO2 EXTRACTED FOAMS................................122 5.1. Introduction......................................................................................................122 5.2. Experimental Method ......................................................................................123 5.2.1. Equipment Setup ....................................................................................123 5.2.2. Sample preparation .................................................................................124 5.2.2.1. Preparation of the monomer mixture and air purge ....................124 5.2.2.1.1. Procedure #1 ........................................................................ 125 5.2.2.1.2. Procedure #2 ........................................................................ 126 5.2.2.2. Dissolution of CO2 in the monomer mixture ..............................126 5.2.2.3. Foam extraction ...........................................................................127 5.2.2.4. Foam temperature versus heater temperature ..............................128 5.2.3. Foam density customization ...................................................................128 5.2.3.1. Foam density after extraction ......................................................128 5.2.3.2. Foam density reduction under vacuum .......................................130 5.2.4. Detailed step-by-step experimental procedure .......................................132 5.3. Foam Cure .......................................................................................................134
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5.3.1. Reactives ................................................................................................134 5.3.2. Isothermal foam cure ..............................................................................135 5.3.2.1. Method for viscosity measurement during foam cure .................136 5.3.2.2. Viscosity curve during foam cure at 45ºC ...................................137 5.3.2.3. Timing for the application of vacuum .........................................139 5.3.3. Effects of Temperature ...........................................................................143 5.3.3.1. Cure at room-temperature ...........................................................143 5.3.3.2. Viscosity profiles during cure at 35-62ºC ...................................144 5.3.3.3. Foam cure rate as a function of temperature ...............................146 5.3.3.4. Definition of the ‘vacuum-timing’ window ................................148 5.3.4. Determination of the optimal cure temperature......................................150 5.3.5. Modelling of viscosity (t,T) for ACRES foams .....................................152 5.3.6. Cure temperature-cycle...........................................................................155 5.4. Structure of cured samples...............................................................................157 5.4.1. Foams cured without vacuum .................................................................157 5.4.1.1. General structure .........................................................................157 5.4.1.2. Effect of cure temperature ...........................................................159 5.4.2. Vacuum-expanded foams .......................................................................163 5.4.2.1. Typical result ...............................................................................163 5.4.2.2. Color/cure gradient in molded foams ..........................................164 5.4.2.3. Foam structure versus process parameters ..................................168 5.4.2.3.1. Foam structure versus tvac, at constant Tcure ......................... 171 5.4.2.3.2. Foam structure versus Tcure, at constant tvac ......................... 171 5.5. Process limitations ...........................................................................................174 5.5.1. Minimum cell-size..................................................................................174 5.5.2. Maximum sample size ............................................................................175 5.5.3. Repeatability of experiments ..................................................................177 5.5.3.1. Variations in initial density .........................................................177 5.5.3.2. Variations in initial foam temperature ........................................178 5.6. Summary..........................................................................................................181 6.
FOAM PROPERTIES ..............................................................................................185 6.1. Cell architecture ...............................................................................................185 6.1.1. Cell shape ...............................................................................................185 6.1.2. Open-cell and closed-cell foam ..............................................................187 6.2. Monomer conversion .......................................................................................187
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6.3. Compressive properties ...................................................................................190 6.3.1. Compressive properties vs. foam structure ............................................192 6.3.1.1. Effect of cell architecture ............................................................193 6.3.1.1.1. Anisotropy ........................................................................... 193 6.3.1.1.2. Cell size ............................................................................... 193 6.3.1.2. Effect of foam density .................................................................194 6.3.1.2.1. Elastic Modulus ................................................................... 194 6.3.1.2.2. Compressive Strength.......................................................... 196 6.3.1.2.3. Modulus versus strength ...................................................... 197 6.3.2. Comparison with industrial foams .........................................................198 6.4. Flexural properties ...........................................................................................201 6.5. Flammability ....................................................................................................201 6.6. Cytotoxicity .....................................................................................................204 6.6.1. Growth of human fibroblast cells ...........................................................204 6.6.2. Potential improvements ..........................................................................206 6.7. Bio-degradability .............................................................................................207 6.8. Conclusions .....................................................................................................209 7.
CONCLUSIONS & RECOMMENDATIONS FOR FUTURE WORK ..................210 7.1. Conclusions .....................................................................................................210 7.2. Recommendations for future work ..................................................................213 7.2.1. Increasing monomer viscosity to slow foam aging ................................214 7.2.1.1. Unmodified oils ...........................................................................214 7.2.1.2. Acrylated oils ..............................................................................215 7.2.1.3. Hydroxylated and maleinized oils ...............................................215 7.2.1.4. Mixture with a co-monomer........................................................216 7.2.2. Modification of the low-temperature foaming process ..........................216 7.2.3. Monolithic foam-recycled paper sandwich composite ...........................217 7.2.4. Additives: nucleating agents and surfactants..........................................219 7.2.4.1. Nucleating agents ........................................................................219 7.2.4.2. Surfactants used commercially....................................................219 7.2.4.3. Potential “green” surfactants .......................................................220
REFERENCES ................................................................................................................222 8.
Appendix A ..............................................................................................................237
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LIST OF TABLES Table 1.1:
Industrial classification of foams by their bulk density [3]. .........................6
Table 1.2:
Major oil crops world production [100].....................................................24
Table 3.1:
Experimental data for the measurement of the solubility of CO2 in AESO at 21 and 60ºC, between 1 and 80 bar CO2 pressure. .....................74
Table 3.2:
Fractionation of CO2 between the liquid and bubble phases, and experimental error due to leaking under vacuum.......................................77
Table 3.3:
Reduced LF-EoS parameters for CO2 and soybean oil (SO) [147, 151]. ...........................................................................................................83
Table 5.1:
Extracted foam temperature and density versus CO2-dissolution time, reactor pressure and heater temperature..........................................129
Table 5.2:
Liquid foam volume before and after isothermal expansion in a vacuum oven, where the pressure was decreased from P0~1 bar to Pvac............................................................................................................131
Table 5.3:
Experimental values of rvisc and tind as a function of temperature, during isothermal foam cure. ...................................................................146
Table 5.4:
Foam structure as a function of cure conditions. No vacuum. ................161
Table 5.5:
Structural characteristics of foams cured with vacuum application.........169
Table 5.6:
Seven steps for the production of cured foams with the lowtemperature foam-extraction process. ......................................................182
Table 6.1:
13
Table 6.2:
Compressive properties of AESO/CO2 foams, calculated according to the methodology detailed in Chapter 2. ...............................................191
Table 6.3:
Flexural properties of three AESO foams with different cell sizes. ........201
C-NMR magnetic shifts, ppm-range, areas, and corresponding number of carbon atoms in cured AESO/CO2 foams. .............................188
xi
xii
LIST OF FIGURES Figure 1.1:
Internal architecture of: (A) reticulated foam with 100% open cells [2]. (B) Liquid AESO foam with closed cells. ............................................5
Figure 1.2:
Lamination process for rigid PU foams [65]. ............................................17
Figure 1.3:
Reaction-injection-molding (RIM) process for rigid polyurethane foams [64]. .................................................................................................17
Figure 1.4:
Chemical reactions during the formation of polyurethane foam [64]. ............................................................................................................20
Figure 1.5:
Structure of a triglyceride from plant oil. A glycerol center connects three fatty acids of variable lengths and numbers of unsaturations. .............................................................................................21
Figure 1.6:
Structure of a polyol made from soybean oil triglyceride. The triglyceride is first epoxidized with hydrogen peroxide, then the epoxy rings are opened with methanol [89, 90]. ........................................22
Figure 1.7:
Monomers from soybean oil triglycerides [105]. Pink: glycerolysis. Blue: functionalization of the double bonds. Green: glycerolysis + functionalization of the double bonds. ...............................26
Figure 1.8:
Acrylated epoxidized soybean oil monomer (AESO). ...............................27
Figure 1.9:
Organic peroxides decompose into two radicals with heat [111]. .............29
Figure 1.10:
Curing of triglyceride-based AESO resins. The application of heat depends on the choice of initiator [103]. ...................................................30
Figure 1.11:
Overall foaming process for thermoset polymers. .....................................40
Figure 2.1:
Experimental setup: high-pressure foaming process (as used in Chapter 5)...................................................................................................43
Figure 2.2:
Flexible fabric heating mantle used to heat the reactor [ColeParmer]. ......................................................................................................45
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Figure 2.3:
Schematic of the stirring system added to the Parr high-pressure reactor. .......................................................................................................47
Figure 2.4:
Half of the custom-design aluminum heater (interior view), displaying the path taken by liquid foam flowing through the intricate channels (red arrows). The 6 holes on the side are bolt holes. ..........................................................................................................48
Figure 2.5:
Molecular structure of the free-radical initiator Esperox 28. .....................51
Figure 2.6:
Molecular structure of CoNap accelerator. ................................................52
Figure 2.7:
Digital picture of the central section of foam inside a graduated cylinder, 61 minutes after pouring; the number of cells per unit area is counted for cell-size calculation (Chapter 4). Two large cells #1 and #2 are highlighted. .................................................................55
Figure 2.8:
13
Figure 2.9:
Molecular structures of Ebecryl 860 and Esperox 28. The different carbon atoms that correspond to each 13C-NMR shift are highlighted and numbered according to Fig. 2.13......................................59
Figure 2.10:
Example of stress/strain curve for an AESO foam (foam = 0.23 g/cm3). Compressive strength: c = 1.13 MPa; Elastic modulus: E = 22.2 MPa; Proportional limit = 0.75 MPa. .............................................62
Figure 3.1:
Experimental setup to measure the solubility of CO2 in AESO as a function of pressure and temperature. ........................................................67
Figure 3.2:
Pressure decay PR(t) during CO2 dissolution into AESO via mixing at 21ºC, as a function of mixing time, for two experiments: PR(0)=59 bar and PR(0)=20 bar. .................................................................68
Figure 3.3:
Different balloon stages during the AESO/CO2 solubility measurement experiments: A. just after filling with the equilibrated AESO/CO2 mixture from the reactor. B. After emptying the free CO2 in the burette under water. C. Expansion under vacuum in a vacuum oven to induce foam collapse. ......................................................71
Figure 3.4:
Experimental setup for the volumetric measurement of the amount of CO2 dissolved in AESO in the reactor at PR, TR. ..................................72
C-NMR spectrum of foam with 96 wt.% Ebecryl 860 + 3 wt.% Esperox 28 + 1 wt.% CoNap, after cure (CPMAS). ..................................58
xiv
Figure 3.5:
Diagram of the fractionation of an element of pressurized AESO+CO2 between 3 phases (gas, bubbles, and liquid) during sampling, and list of nomenclature from Table 3.2. ..................................75
Figure 3.6:
Mole fraction solubilities of CO2 in AESO at 21ºC and 60ºC, as a function of CO2 pressure. The reference lines at both temperatures are also indicated........................................................................................76
Figure 3.7:
Normal log of xCO2 as a function of pressure at 21 and 60ºC, calculated from experimental data (lines = manual smoothing). ...............79
Figure 3.8:
Negative plot of the parameter a (from Eq. 3.2) versus pressure. .............80
Figure 3.9:
Experimental mole fraction solubility of CO2 in AESO vs. pressure at 21ºC and 60ºC, and thermodynamic model fit (Eq. 3.6) at both temperatures. ..............................................................................................81
Figure 3.10:
Mass fraction solubility of CO2 in AESO vs. pressure, at 21ºC and 60ºC. Fits with Henry’s law and the LF model (Courtesy of A. Shine) at both temperatures are plotted. At 21ºC, the data are best fitted with a polynomial expression. ..........................................................84
Figure 3.11:
Mass fraction solubility of CO2 in AESO [this work], soybean oil (SO) and castor oil (CO) [Ndiaye et al.] as a function of pressure and temperature. .........................................................................................86
Figure 3.12:
Foam density in the reactor as a function of initial pressure P1, final pressure P2, and P=P1-P2, at 21ºC, 60ºC, and 90ºC (log scale). Calculations with the thermodynamic solubility model for CO2 (extrapolated to 90ºC). ...............................................................................89
Figure 3.13:
Parameter =(x1-x2)/((1-x1)(1-x2)) as a function of P and P2. .................90
Figure 3.14:
Foam density inside the reactor as a function of the pressure drop applied (P) and the final pressure (P2) at 60ºC, calculated from CO2 solubility data using the thermodynamic model. ...............................91
Figure 3.15:
Theoretical foam densities calculated from solubility data, ρcalc, or from the total volume of gas measured, ρmin; and actual measured foam density, ρexp, as a function of CO2 pressure at 21ºC and 60ºC. .........94
Figure 3.16:
Percentage of gas escaping the liquid matrix during extraction. ...............95
xv
Figure 3.17:
Actual density of various foam samples extracted from the reactor after saturation with CO2 at TR and PR [131]. ............................................96
Figure 4.1:
Growth of a volume of AESO/CO2 mixture during extraction from the pressurized reactor (~60 bar) into atmospheric pressure (~1 bar). ..........................................................................................................100
Figure 4.2:
Monomer drainage along cell ribs and walls due to gravity, resulting in wall thinning. ........................................................................104
Figure 4.3:
Schematic of the coalescence of two bubbles via rupture of a thin cell wall. ...................................................................................................105
Figure 4.4:
Coarsening of the bubble population via continuous diffusion of gas from small bubbles into large ones, which accents the size difference. ................................................................................................105
Figure 4.5:
View of whole AESO/CO2 foam, at room temperature, 80 min. after extraction. ........................................................................................107
Figure 4.6:
Evolution of volume vs. time for two individual large bubbles at 23ºC, and illustration of the phenomenon of gas diffusion from small bubbles into the largest bubble of a neighborhood.........................109
Figure 4.7:
Evolution of the volumetric bubble density, Nc, due to foam dynamics, in the center area of AESO/CO2 foam as a function of time at 23ºC..............................................................................................111
Figure 4.8:
Volume-equivalent bubble diameter, d, as a function of time at 23ºC..........................................................................................................112
Figure 4.9:
Evolution of the average bubble volume vs. time, at 23ºC. The red data point is the extrapolation of the linear fit to t=0. ..............................113
Figure 4.10:
Coalescence of two bubbles, and the associated bubble parameters. ......114
Figure 4.11:
Inverse of bubble volumetric density as a function of time at 23ºC. .......116
Figure 4.12:
Evolution of volumetric bubble density as a function of time at 23ºC and 48ºC (2 samples). .....................................................................117
Figure 4.13:
Evolution of the average bubble diameter versus time in foams kept isothermally, at either 23ºC (1 sample) or 48ºC (2 samples). ..........118
xvi
Figure 5.1:
Schematic of the apparatus for the low-temperature foamextraction process.....................................................................................124
Figure 5.2:
Typical industrial measurement of cure performance [166]. ...................137
Figure 5.3:
Viscosity profile of bulk foam samples during isothermal cure at Tcure=Tfoam= 45ºC, PCO2= 58 bar, and foam density ~ 0.4 g/mL. Two different spindle rotation speeds were used: 10 rpm (pink data), or 4 rpm (dark blue data). The linear portion is highlighted. ........138
Figure 5.4:
Profile of foam viscosity during cure at 45ºC. Three foam samples with a composition of 96% AESO +3% Esperox 28 +1% CoNap were vacuumed at different times: A. tvac=3 min. B. tvac=7 min. C. tvac=10 min. ..............................................................................................140
Figure 5.5:
Scans of vertical cuts of three samples foamed at PCO2 = 58 bar and Tfoam = 45C, then cured at Pvac = -17.5 Torr and Tcure = 45C after a time tvac. A) tvac = 3 min. B) tvac = 7 min. C) tvac = 10 min. ................142
Figure 5.6:
Viscosity versus time during cure at 20ºC, for an un-foamed AESO sample containing 3 wt.% Esperox 28 and 1 wt.% CoNap......................144
Figure 5.7:
Bulk foam viscosity profiles during isothermal cure, versus cure time and cure temperature. Tcure=35, 36, 38, 43, 45 and 62ºC. ...............145
Figure 5.8:
Experimental values of the induction time during foam cure versus cure temperature, tind(T). ..........................................................................147
Figure 5.9:
Plot of the experimental rate of viscosity increase during foam cure versus cure temperature, rvisc(T). ..............................................................147
Figure 5.10:
During foam cure, the grey-shaded area represents the ideal timewindow in which to apply vacuum for the production of lowdensity AESO foams. ...............................................................................149
Figure 5.11:
Fit of the experimental viscosity data during foam cure as a function of temperature with the autocatalytic model..............................154
Figure 5.12:
Large cracks created during improper cure of two foam samples. Cure cycles: A: 54ºC, 48h +100ºC, 2h +140ºC, 12h. B: 45ºC, 12h +80ºC, ½ h +100ºC, 1h +120ºC, 12h +140ºC, 12h. .................................156
Figure 5.13:
Foam structure after isothermal cure in the absence of vacuum. .............158
xvii
Figure 5.14:
Cores of foam samples cured between 38 and 62ºC, with no vacuum. Identical scale. Individual cell measurements are shown as an example. (White cells were filled with dust during cutting) ..........160
Figure 5.15:
Center cut of foams cured without vacuum. Sample numbers refer to Table 5.4 (the picture for sample #4 was lost). ....................................162
Figure 5.16:
Structure of successful foam with medium cells and low density. Cell size range: 0.1-1.2 mm; core density ρ=0.16 g/cm3 (sample #12). .........................................................................................................164
Figure 5.17:
Color gradient throughout cured foam samples, after vacuum application (identical scale). #7, 6, 10 were poured in cold molds; #6 was uncovered; #12 was poured in a pre-heated mold. ......................167
Figure 5.18:
Foam samples characterized by their cure temperature, Tcure, time of vacuum-application, tvac, and the resulting foam structure (colored squares). Blue = vacuumed too early; aqua = too early, partial success; green = success; yellow = vacuumed too late, partial success; orange = vacuumed too late. ...........................................170
Figure 5.19:
Four foam samples expanded at tvac=3 min, with cure temperatures of 40C, 45C or 46C. Identical scale, original colors. .........................172
Figure 5.20:
Foam samples expanded at tvac=7 min, with cure temperatures of 40C, 45C or 47.5C. Identical scale, original colors. ..........................174
Figure 5.21:
Range of initial foam temperature obtained at constant heater temperature, due to variations in mass flow rate during foam extraction (accidental: blue; intentional: red). The line corresponds to temperatures calculated with a flow rate of 1 g/s.................................179
Figure 5.22:
Foam flow rate as a function of micrometric valve-opening. Green squares represent ‘first-try’ measurements with a fresh load; the blue data and the orange data show the evolution of foam flow-rate as a function of trial-number, for one sample load (each). ......................180
Figure 6.1:
Detail of the core and general cell shape of the lightest AESO/CO2 foam produced in this work (foam = 0.16 g/cm3).....................................186
Figure 6.2:
Schematic of a unit foam cell during compression (adapted from [52]). ........................................................................................................192
xviii
Figure 6.3:
Elastic modulus, E, versus foam density, foam. Red data points are experimental values; the blue data point was calculated with the percolation theory [24]. The line is the expected trend. ..........................195
Figure 6.4:
Compressive strength vs. density for AESO/CO2 foams, σc(ρfoam). ........197
Figure 6.5:
Elastic modulus, E, versus compressive strength, σc, of AESO/CO2 foams. The red data point corresponds to an industrial PU foam. ..........198
Figure 6.6:
Structure of an industrial PU foam. Cell size ~0.5 mm, mostly closed-cell, density = 0.08 g/cm3. ............................................................199
Figure 6.7:
General classification of foams as a function of their modulus and density, and comparison with AESO/CO2 foams (in red). ......................200
Figure 6.8:
Flammability test for an industrial polyurethane foam (right) and a 96% AESO +3% E-28 +1% CoNap foam cured at 45ºC (left): samples aspects after a burn time of 10 seconds. .....................................202
Figure 6.9:
Human fibroblast cells on AESO/CO2 foam after incubation for 1 day and 3 days (20× magnification), and 8, 12, 19 and 22 days (10× magnification). The arrows indicate fibroblasts growing inside open foam pores. ......................................................................................205
Figure 6.10:
Fully acrylated, partially-epoxidized AESO, with less acrylate groups and more hydroxyl groups per monomer than Ebecryl 860 (A = 3.4). ..................................................................................................207
Figure 6.11:
Two foam samples after burial 2” deep for 60 days in wet garden soil, with T~60-80C (September-October). Composition (for both): 96 wt.% AESO + 3 wt.% E28 + 1 wt.% CoNap. ..........................208
Figure 7.1:
Cross-section of a structural beam for roofing application: poor adhesion between the AESO resin and the industrial foam core. ............218
Figure 7.2:
Molecular structure of the amphiphilic Triton X-100 surfactant. ............220
Figure 7.3:
Molecular structure of the lecithin lipoprotein from egg yolk. ................221
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ABSTRACT Polymeric foams are complex composite materials, consisting of a multitude of gas cells trapped inside a polymer matrix. The multi-phase architecture provides numerous physical and mechanical advantages over plain polymers. The density of solid polymeric foam varies according to the needs of a wide range of applications. The principal raw materials currently used for the production of foam are petroleum and its derivatives. Over the last decade, researchers have actively seeked new bio-based polymers and polymeric foams in order to replace some petroleum-based foams on the market. Bio-based polyurethane (PU) foams were produced using polyols from modified plant-oil triglycerides, with properties comparable to that of petroleum-based insulating foams. Also, many types of biodegradable starch foams were successfully developed and are currently used in the packaging industry, mostly. The goal of this dissertation is the development of a new type of bio-based foam from plant oil that is both stronger and more resilient than starch foam and with a higher bio-content than bio-based PU foam. The University of Delaware has patented many different monomers synthesized from soybean oil and other plant oils. The best studied and most versatile of these is acrylated epoxidized soybean oil (AESO). AESO possesses a wide range of properties according to its level of functionality, and was chosen here to design resilient, rigid polymeric foams with a very high bio-content. In the presence of a free-radical initiator and catalyst, AESO forms a cross-linked, thermosetting polymer network. Carbon dioxide has many useful attributes, including its high solubility in plant oils and
xx
modified triglycerides, and was chosen as the blowing agent that expands AESO polymers into foam. We designed a new foaming process inspired by the commercial injectionmolding and free-rise foaming processes and adapted it to fit the AESO/CO2 foam system. Foam was generated by saturating the monomer with CO2 inside a stirred, pressurized chamber, then extracting the mixture through a heater to trigger simultaneous foam expansion and polymerization. The architecture of cured foams showed a strong dependency on the foaming procedure and the process parameters, namely timing and the pressure and temperature cycles. The design target was the production of solid foams with a homogeneous, small-celled structure, which should confer better physical and mechanical properties to the foam. The aging behavior of liquid AESO/CO2 foams was studied to understand and quantify the mechanisms of cell growth and foam degradation before cure. During foam cure, conflicts of interests between the polymerization kinetics and the structural integrity of the foam presented the major design difficulties. The process parameters and foaming procedure were progressively optimized to produce foams with increased homogeneity, smaller cells, and a lower density. As a result, strong, semi-rigid thermoset foams with a bio content superior to 80% were successfully obtained. Our new foaming process is flexible and may be applied to other plant oilbased monomers and be customized according to monomer functionality and the desired foam architecture. Bio-based foams with a wide array of densities, cellular architectures, and mechanical properties may be produced. The new AESO/CO2 foams are good candidates for applications ranging from foam-core composite sandwich panels for hurricane-resistant housing to human tissue-scaffolds.
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Chapter 1 1. INTRODUCTION “We live in a universe inundated with foam” Sydney perkowitz.
1.1. Objective and motivation Polymeric foams are complex gas-solid structured materials, formed of a multitude of gas cells inside a solidified polymer matrix. This two-phase architecture presents numerous physical and mechanical advantages over bulk polymers. The density of solid polymeric foams typically range from 0.016 g/cm3 to 0.960 g/cm3, according to the needs of a wide range of applications. High-density foams usually have a high mechanical strength and are used as lightweight structural components for furniture, construction and transportation; that is, everywhere relatively strong, lightweight material is preferred. Flexible foams serve comfort and cushioning purposes in seating, shoe soles, bedding, packaging applications, etc. Most polymeric foams have low thermal and electrical conductivity, and possess good sound-dampening property, which makes them useful for all insulation purposes.
Open-cell foams have a large fluid-absorption
potential; a special type of open-cell foam, reticulated foams, are used as highperformance air filters, such as in vehicle engines. Finally, polymeric foams generally possess good shock absorption properties, which is highly appreciated in the fabrication of impact-resistant parts such as car bumpers [1-3].
1
Polymer foaming is a mature industry; the total US production of foamed plastics, widespread among the many applications, was estimated at more than 7.4 billion pounds in 2001, which accounts for about 10% of the total commodity resin consumption [4]. This production was projected to grow nearly 3% annually to 8.5 billion pounds in 2006, which is valued at more than $18 billion [5, 6]. The numbers for 2006 and 2007 were not published at the time of this writing. The principal raw materials used nowadays for the production of foam are petroleum and its derivatives. Several issues consequently appear. Petroleum in the world is being depleted and the best estimate of global oil reserves indicates that we've already used about half of the world's commercially available oil [7, 8]. Secondly, the United States does not produce enough petroleum for its domestic consumption, and the pricing and supplying of petroleum imported to America depends strongly on the economic and political situations in the oil-producing countries. Thirdly, petroleumbased foams are, like most of the plastics and resins from which they are issued, nonbiodegradable, even over several decades time [9]. This is an important concern in our environmentally-conscious modern world. Research is today actively seeking to replace petroleum with renewable resources for polymer production. An ideal replacement material would offer comparable performance and be price-competitive with petroleum, as well as allow some degree of biodegradability. It is, therefore, economically and environmentally interesting to replace the non-renewable raw material with renewable and affordable plant-derived materials. An attractive raw material candidate is the soybean, the largest worldwide source of vegetable oil [10]. The United States’ own production is plentiful with 600,000 soybean farmers [11] producing more than 3 billion bushels of soybeans in 2004 [12, 13], a
2
potential of roughly 30 billion pounds of soybean oil sold at the inexpensive price of about $0.20 per pound [14, 15]. The triglycerides constituting any plant oil possess many active sites such as double bonds, allylic carbons, and ester groups. These sites can be chemically modified to introduce polymerizable groups in order to form monomers and co-monomers useful for several polymerization reactions [14, 16-35]. Then, in theory, any polymer can be foamed. The chemical, mechanical, physical, and economical properties of bio-based foams should at least equal that of popular petroleum-based foams in order to replace them on the global market. To date, some polymeric foams have successfully been produced out of soybean oil polyurethanes and out of starch (see Section 1.4). The Affordable Composites from Renewable Resources program at the University of Delaware (ACRES), has designed several new thermosetting polymers from plant oils [10, 14, 1828, 33-35]. The present work uses one of the monomers studied by ACRES and already commercialized [36], acrylated epoxidized soybean oil (AESO), to design processes that transform AESO into exciting new structural, thermoset foams using carbon dioxide as a “green” blowing agent. The resulting AESO foams possess a higher bio-based content (~96 wt.% AESO) than any previously produced soy-based polyurethane foam, and are stronger and less biodegradable than starch foams.
Projected applications include
AESO/CO2 foam cores for composite sandwich panels as designed by the ACRES Group for hurricane-resistant housing structures[14, 37-39], as well as windmill blades, sporting goods, tissue scaffolds [40], and all foam core composite applications. For example, we believe AESO foams would be particularly welcome in the building industry, where building “green” houses has become very popular. In 2006, an estimated 7% of houses were built using foam-insulated concrete [41]. The concept of “Green Building” could be brought one step further by utilizing insulating and reinforcing materials that both save
3
energy and are themselves “green”, such as our newly designed soy-based foams with a very high bio-content.
1.2. Introduction to polymeric foams “To grasp the world around us, we need to understand how the atomic building blocks relate to the materials around us. The connections are easiest to trace for pure crystalline solids. […] Liquids are harder to understand than solids. And most complicated of all, there are the combinations of solids, liquids and gases. Solids, liquids and gases are combined in foams. […] Unlike the orderly geometry of a crystal, the inclusions are arranged randomly in the surrounding medium.” Sydney Perkowitz [42] Polymeric foams are complex three-phase materials. An initial mixture of liquid monomer or polymer phase and gas bubbles from a blowing agent is progressively grown into a solid-gas structured material, where the gas phase forms a multitude of randomly packed cells inside the solidified polymer matrix. 1.2.1. General foam architecture Each foam cell is a volume of gas enclosed in polymer walls interconnected by polymer ribs (or plateau borders or struts) and vertices (see Figure 1.1) [43]. In liquid foams, cells are also called bubbles. In solid foams, a cell with all its surrounding walls intact is called a closed-cell; when at least two walls are broken during the solidification phase of the foam, the cell is called an open-cell. Polymeric foams consist of a mixture of open- and closed-cells in varied proportions.
The structure of foams can vary
significantly. The fraction of closed and open cells differs depending on the composition of the polymer phase, the foaming conditions, and the surfactant used. Elastic polymers will often yield open-cell structures, while rigid polymers generally form closed-cell materials. If all cell walls have been removed throughout a foam (either by a physical or
4
chemical method), the foam is called reticulated (Fig. 1.1-A). Reticulated foams are the lightest type of polymeric foams, with an extremely low density (0.016 to 0.1 g/cm3), and are commonly used as filters because gases or liquids can circulate through the interconnected open-cell network.
(A) Polymer rib Vertex
Cell wall (open)
Gas cell
(B)
Figure 1.1:
Internal architecture of: (A) reticulated foam with 100% open cells [2]. (B) Liquid AESO foam with closed cells. 5
Polymeric foams are characterized by their basic macro-structural parameters: the relative number of open and closed cells, the apparent density, the cell size and shape, the wall thickness, the distribution of cells according to size and shape in a given volume, and the specific surface area of the foamed plastic material [2, 3, 44]. 1.2.2. Apparent foam density Polymeric foams are typically classified according to their density (Table 1.1), which ranges from 1.6 kg/m3 (0.016 g/cm3, very light foam) to 960 kg/m3 (0.96 g/cm3, super-heavy foam) according to the needs of a wide range of applications [2, 3].
Table 1.1:
Industrial classification of foams by their bulk density [3]. Foam type
Density range
Very light Light Medium
0.003 P2 ≥ 1 bar, and T2 is inferior to T1 due to the endothermic nature of the vaporization phenomenon. The pressure reduction from P1 to P2 is P = P1 - P2. The amount of gas that goes in the bubble phase, ng,b, is equal to the total gas dissolved at P1 and T1 minus the gas still soluble at P2 and T2. Thus, if there is gas leak during foam expansion: n g ,b n g1 n g 2 n L
87
x1 x nL 2 1 x1 1 x2
(3.13)
Vg ,b
n g ,b R T2 P2
and
n L R T2 ( x1 x2 ) P2 (1 x1 )(1 x2 )
(3.14)
nL M L
(3.15)
The volume of liquid in the foam is: VL
L
Finally, from Eq. 3.10, 3.14 and 3.15, we obtain the foam density as a function of pressure and temperature in the reactor before and after depressurization:
foam
1 R T2 ( x1 x 2 ) 1 L M L P2 (1 x1 )(1 x 2 )
(3.16)
where x1 and x2 are calculated according to Eq. 3.11 (replacing 1 subscripts with 2 subscripts for x2), and all the units are SI units (kg/m3, Pa, kg/mol, and K). This equation was used for all density calculations in this section. In Figure 3.12, the foam density as a function of P was plotted at three temperatures (21, 60 and 90ºC), using two different scenarios for the pressure cycle: Scenario 1 (solid curves): the overhead gas in the reactor is emptied completely during depressurization, leading to P2 = Patm ~ 1 bar, and the dissolution pressure P1 is adjusted to obtain the desired P=P1-P2. Scenario 2 (dotted curves): the dissolution pressure P1 is kept constant (~60 bar at 21ºC, 80 bar at 60ºC, 100 bar at 90ºC), and it is P2 that is adjusted to produce the desired P=P1-P2.
88
Foam density foam (g/cm 3)
1
P2=1 bar, T=21C
0.1 P1=60 bar, T=21C P2=1 bar, T=60C P1=80 bar, T=60C P2=1 bar, T=90C
0.01 P1=100 bar, T=90C
0.001 0
20
40
60
80
100
P (bar)
Figure 3.12: Foam density in the reactor as a function of initial pressure P1, final pressure P2, and P=P1-P2, at 21ºC, 60ºC, and 90ºC (log scale). Calculations with the thermodynamic solubility model for CO2 (extrapolated to 90ºC).
The temperature was considered constant during depressurization in both scenarios, as the temperature drop was experimentally measured to be generally less than 15 K, which represents less than a 5% error. The first observation from Fig. 3.12 is that, with both scenarios and at any temperature, the foam density decreases greatly as the pressure drop applied increases. The second notable observation is that the foam growth behavior appears to not depend on P only, but to strongly depend on the initial and final pressures, as shown by the large difference between the foam densities calculated with scenario 1 (solid curves) and
89
scenario 2 (dotted curves) at the same P. The effects of the molar solubilities x1(P1) and x2(P2) on the foam density are contained within the term , defined as: x1 x 2 (1 x1 )(1 x 2 )
(3.17)
Figure 3.13 presents the value of as a function of P2 and P.
6
(x1 -x 2 ) / ((1-x 1 )(1-x 2 )) at 60C
5 4 3 2 1
0
0
20
10
25
30
13
P2 (bar)
50
1
40
P (bar)
Figure 3.13: Parameter =(x1-x2)/((1-x1)(1-x2)) as a function of P and P2.
At constant P, increases slightly when P2 increases, which should result in a small reduction of foam density when P2 increases, at constant P. However, because of the presence of the term P2 itself in Eq. 3.16 and because of its position in the equation, ρfoam should increase considerably when P2 increases. This later influence of P2
90
on the foam density is much stronger than that of the term , as shown by the following example: at P=30 bar, when P2 decreases from 30 to 1 bar, is reduced from ~2.2 to ~1.4, which is a factor of ~1.6, while P2 itself is reduced by a factor of 30; therefore ρfoam is greatly reduced (from ~0.37 to 0.029 g/cm3) by decreasing P2. Figure 3.14 presents the calculated values of the foam density as a function of pressure drop and final pressure on the entire range of P2 and P of interest in this work, at the fixed temperature of 60ºC. The initial pressure, P1, may be deduced from
1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
1-1.1 0.9-1 0.8-0.9 0.7-0.8 0.6-0.7 0.5-0.6 0.4-0.5 0.3-0.4 0.2-0.3 0.1-0.2
80
70
50
60
13 1
P (bar)
25
40
30
0-0.1
20
10
0
(g/cm3 )
foam density at 60ºC
P1=P2+P.
P2 (bar)
Figure 3.14: Foam density inside the reactor as a function of the pressure drop applied (P) and the final pressure (P2) at 60ºC, calculated from CO2 solubility data using the thermodynamic model.
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We can draw two conclusions of interest for future in-reactor foaming processes: 1. Energy efficiency: when low foam density is desired, it is more energyefficient to set P2 = 1 bar, then calculate the P needed and set P1 accordingly, than to use the maximum allowable initial pressure. For example, if a foam density of 0.2 g/cm3 is desired at 60ºC, set P2 = 1 bar, then from Fig. 3.14 we have P = 3 bar, thus the needed P1 is only 4 bar. If we set P1=80 bar (the max. pressure obtained at 60ºC), to produce foam of density 0.2 g/cm3 we need P=48 bar and P2=32 bar. Pressurizing the reactor to 80 bar instead of 4 bar is a considerable waste of energy. However, it allows more precision in reaching the desired foam density, because the initial slope, /P, becomes smaller when P2 increases (Fig. 3.14). 2. Expansion limit: The initial reflex with in-reactor foaming processes is to apply the maximum possible pressure, then depressurize completely down to atmospheric pressure in order to obtain the lowest possible foam density. This scenario corresponds to P ~ 58 to 100 bar, P2 ~ 1 bar, and calculated foam densities of 0.013 to 0.0022 g/cm3. However, during air-purge treatments we observed that AESO foams may only be expanded up to ~10 times before collapsing spontaneously, which corresponds to a foam density of ~0.1 g/cm3. Any AESO foam with a lower density will not be stable and collapse quickly. 3.2.5.2. Extracted foams: theoretical density vs. actual density In the case of foams extracted from the reactor at PR and TR, as was the case in the solubility measurement experiments in this chapter, the assumption that all CO2 initially dissolved in AESO stays within the liquid matrix during foam expansion becomes untrue and not applicable. Indeed, ‘free’ CO2 constituted the majority of the
92
CO2 collected in the balloon samplers among all three phases, and the actual foam density measured was much higher than the theoretical density. The theoretical density is the density that the foam would have if all the CO2 dissolved at TR, PR stayed within the polymer matrix after depressurization and participated in foam expansion instead of escaping in the free gas phase. The theoretical density was calculated two ways: from the thermodynamic model, calc, using Eq. 3.16; and from the volume of gas measured experimentally, min, according to:
min ~ mAESO / (VAESO + VCO2, gas+VCO2, bubbles)
(3.18)
The experimental foam density, exp, is the actual density of the foam at the bottom of the balloon sampler, measured just after removal of the free CO2:
exp = mAESO / Vfoam. Table 3.1 lists min and exp for each experimental data point.
(3.19) The
difference between theoretical foam density and actual foam density at 21ºC and 60ºC is shown on Figure 3.15. The extreme differences between the theoretical foam density and the actual experimental foam density are due to large amounts of CO2 that escaped the liquid matrix while exiting the extraction tube. Figure 3.16 shows the percentage of gas that escaped the foam versus the total amount of gas originally dissolved in the liquid phase, VCO2, gas / VCO2, total (the ‘free CO2’ column in Table 3.1). With increasing pressure, up to 98% of the gas dissolved joined the ‘free’ gas phase floating above the foam in the balloon sampler (Fig. 4.6 and 4.17). This was the gas located in the outer layer of the foam snake (the coil of foam being pushed out of the tube), that breaks free from the thin liquid matrix due to intense shear stress while traveling the extraction tube. Since pressure is the driving force behind the foam’s extraction, this leaking-gas phenomenon was amplified when the reactor’s pressure increased, because higher pressure meant a higher
93
flow rate and hence a higher shear stress. In addition, when the surface-to-volume ratio of the foam snake is high, a larger proportion of the flowing foam is affected by the shearing effect, and a higher percentage of CO2 will break free from the foam’s outer layer, escaping into the free-gas phase. This was the case in our setup: the ¼” tubing that was used for extraction generated a very narrow foam snake with a high surface-tovolume ratio, which enabled a very large fraction of the CO2 dissolved to escape during extraction.
0.45 0.4
3
Foam density (g/cm )
0.35 Min. density, 21C
0.3
Calc. density, 21C
0.25
Actual density, 21C
0.2
Min. density, 60C Calc. density, 60C
0.15
Actual density, 60C
0.1 0.05 0 0
20
40
60
80
100
Initial CO 2 Pressure P1 (bar)
Figure 3.15: Theoretical foam densities calculated from solubility data, ρcalc, or from the total volume of gas measured, ρmin; and actual measured foam density, ρexp, as a function of CO2 pressure at 21ºC and 60ºC.
94
100%
CO2 lost (vol.%)
95% 90% 85% 80% 75% 70%
21ºC
65%
60ºC
60% 0
20
40
60
80
100
PR (bar)
Figure 3.16: Percentage of gas escaping the liquid matrix during extraction.
The same size tubing (1/4”) was used throughout this work, therefore, the expected density of extracted foams could never be calculated from the process parameters, TR and PR, using the solubility models described here. Figure 3.17 shows an array of actual foam densities obtained in this work as a function of reactor pressure and temperature at saturation. The actual densities of extracted foams did decrease slightly with increasing CO2 pressure at room temperature (‘+’ signs on Fig. 3.17), and decreased considerably with decreasing reactor temperature at constant pressure (e.g., ~50 bar on Fig. 3.17), but in an erratic manner, and not according to any of the models previously studied. Thus, during foam extraction with the present setup, it was possible to lower the foam density to some extent by operating at room temperature and at maximum pressure (~60 bar, see Chapter 5), but the corresponding foam density could never be predicted accurately.
95
Figure 3.17: Actual density of various foam samples extracted from the reactor after saturation with CO2 at TR and PR [131].
3.3. Conclusions We have designed a new, efficient method to measure the solubility of a gas in a foaming liquid at custom high pressure and temperature using an inflatable rubber balloon as a sample container. The solubility of CO2 in AESO is high: as much as 206 mL of CO2 was dissolved in 1 g of AESO at 294 K and 57 bar. Several models were found to adequately describe the experimental data in different conditions: at 60ºC, the mass fraction solubility was best modeled with Henry’s law at pressures up to 80 bar, and Henry’s law constant is defined by Eq. 3.8. At 21ºC, the mass fraction solubility was adequately described by Henry’s law for pressures below 20 bar, and by a polynomial expression
96
(with no physical meaning; Eq. 3.9) for pressures between 20 and 60 bar. When using the mole fraction solubility, the thermodynamic model of Eq. 3.6 proved an excellent fit on the entire range of pressure at both temperatures. The solubility of CO2 in AESO was found very similar to that of CO2 in soybean oil; thus, extrapolation to higher pressures, and with other oil-based monomers is possible using the solubility data for CO2 in SO. With AESO/CO2 in-reactor (free-rise) foaming processes, foam densities as low as 0.005 g/cm3 may potentially be obtained with the help of an adequate surfactant, after saturation at room temperature and at maximum pressure. Foam density as a function of reactor temperature and initial and final pressures was adequately modeled with Eq. 3.16 derived from the thermodynamic model. However, with extracted foams (molding processes), the majority of the CO2 dissolved in AESO escaped the monomer matrix during sample extraction, thereby not contributing to foam expansion, and actual foam densities were in the range 0.18-0.22 g/cm3 at 21ºC, and 0.29-0.43 g/cm3 at 60ºC. To obtain the lowest possible foam density, the process parameters must be set to the lowest temperature (room temperature) and the highest pressure (i.e., vapor pressure of CO2 at room temperature, ~58-60 bar depending on T). A better extraction setup (e.g., wider tubing) could greatly decrease the amount of CO2 that escapes during foam extraction.
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Chapter 4 4. AGING OF LIQUID AESO/CO2 FOAMS “[Foam] is neither fully liquid nor completely gaseous; it flows differently from the first and does not dissipate like the second. Its components are stable, yet it lives only a short while. It is made from clear air and [clear liquid], yet it is opaque. […] In most ways, a foam is totally unlike the substances that make it up.” Sydney Perkowitz [42]
4.1. Introduction 4.1.1. Purpose In order to produce cured, rigid polymeric foams, it is important to analyze the aging behavior of liquid monomer foams as a function of process parameters and foam formulation. During thermosetting foam synthesis, bubbles must first be generated within the liquid monomer phase, then polymerization permits solidification of the foamed material. The foam will generally be set with the precise architecture it possessed at the time of gelation, plus or minus rupture of the cell walls depending on their elasticity. This means that the average cell size, cell shape, cell size distribution, and apparent density of the cured foam are decided by the two steps of foam formation and foam aging. In Chapter 4, we studied the solubility of CO2 in AESO as a function of process parameters, and its theoretical effect on the foam density. The initial bubble size and bubble density of the liquid foam are determined both by the CO2-vaporization stage (bubble nucleation and thermodynamic growth) and by the pattern of flow through the process during foam extraction (bubble growth, breakage and gas loss due to shearing). 98
In this chapter, we will aim to understand the macroscopic behaviors that contribute to the degradation of a liquid foam immediately after its extraction, which is called ‘foam aging’. The target of this study is the design of light, homogeneous liquid foams with small bubbles, that can later be converted to light, homogenous, small-celled solid polymeric foams. 4.1.2. Foam fabrication 4.1.2.1. Method Liquid AESO/CO2 foams were produced using our low-temperature extraction process (Chapter 5). Pure AESO monomer was saturated with pressurized carbon dioxide at room-temperature and at variable CO2 pressure inside the reactor’s chamber. The AESO/CO2 mixture was then extracted through a dip-tube, passed through a heater with variable temperature, and was poured into transparent graduated cylinders at atmospheric pressure (~1 bar). This procedure permitted to customize the amount of CO2 dissolved at saturation by changing the pressure in the chamber, PCO2, and control the temperature of the extracted AESO/CO2 foam, Tfoam, by varying the heater’s temperature. The process was pictured in Fig. 2.1. The maximum pressure used was PCO2 ~60 bar, and the heater’s temperature, TH, ranged from room-temperature (no heat) to 120ºC. The resulting temperature of the extracted AESO/CO2 foam ranged from Tfoam ~ 8ºC (with PCO2 = 60 bar; no heat), to Tfoam ~ 60ºC (with PCO2 = 60 bar; TH = 120ºC). As seen in Chapter 3, the solubility of CO2 increases with increasing pressure, and greatly decreases with increasing temperature. To maximize solubility in this chapter, the lowest convenient temperature and highest convenient pressure were used during CO2-dissolution: PCO2 ~ 60 bar, and TH = room-temperature (Troom ~ 20ºC).
99
When the AESO/CO2 mixture is extracted from the pressurized reactor into the atmosphere, the solubility of CO2 greatly decreases and bubbles form within the liquid AESO phase generating foam. Figure 4.1 schematically illustrates the growth of gas bubbles within a constant volume of liquid monomer during pressure reduction.
Figure 4.1:
Growth of a volume of AESO/CO2 mixture during extraction from the pressurized reactor (~60 bar) into atmospheric pressure (~1 bar).
The two steps of bubble creation and growth are the same as described in Chapter 1. When the critical pressure difference needed for nucleation, Pcr, is reached, a multitude of gas nuclei form (but most of the time, the presence of macrovoids replaces nuclei and this step is skipped). Next, each microscopic bubble (or nucleus) grows into a larger and larger bubble due to the progressive diffusion of gas from the liquid phase into the gas phase as the solubility of CO2 in AESO decreases. When atmospheric pressure is reached, the final size of each bubble is defined by the combination of bubble breakage 100
during foam flow, volume increase as a function of pressure according to the ideal gas law, and Laplace’s law (Eq. 1.9). 4.1.2.2. Definitions The foam’s porosity, apparent density, and mean wall thickness were defined in Chapter 1. Additional definitions useful for the study of liquid foam aging are given below. Bubble size and bubble volume: Vb is the volume of one bubble. r is the radius of a spherical bubble, and d its diameter. For non spherical bubbles, r and d are taken as the volume-equivalent bubble dimensions, such as: 4 1 Vb r 3 d 3 3 6
(4.1)
With a foam density of ~0.5 g/cm3, cells are spherical. However, the bubbles’ shape changes when foam density decreases below a certain limit (see Chapter 1).
For
calculation purposes, bubbles will always be considered spherical in this chapter. The foam’s apparent density was defined by Eq. 1.1. If we neglect the mass of the gas in the foam, the foam’s apparent density becomes:
L (V foam Vg ) m L / VL L (1 g ) V foam / VL V foam
(4.2)
where L is the density of the liquid monomer, L = 1.05 g/cm3 for AESO, VL is the volume of liquid phase in the foam, and mL ~ mfoam is the mass of liquid in the foam. Total gas volume: it is useful to express the total volume of gas, which is not easily measurable, as a function of known parameters and measurable quantities:
Vg V foam VL V foam
101
mL
L
(4.3)
and,
Vg
m foam
1 1 mL L L
mL
(4.4)
Bubble density: The volumetric bubble density, Nb, is the average number of bubbles per unit volume of foam, and is calculated with:
Nb
Vg Vb
(4.5)
where Vb is the average bubble volume. 4.1.2.3. Thermodynamic bubble growth Many simple or extensive models for the growth of a single bubble – or a bubble within a polymeric foam – owing to the diffusion of gas at supersaturation can be found elsewhere [152-163] and will not be presented here. In this chapter, we were interested in the size of a single bubble in its thermodynamic equilibrium, and in the fascinating dynamics of bubble growth that are particular to liquid foams. The size of a single bubble in its thermodynamic equilibrium is a function of the number of gas moles, nb, that have transferred from the liquid phase to the bubble via vaporization, or to the bubble from other bubbles via diffusion and breakage. The volume occupied by nb gas moles is a function of the local pressure and temperature, according to the ideal gas law for one bubble (Eq. 1.8). The ideal gas law applies at low pressure, and foams here were all studied at the ambient pressure of 1 atm or under vacuum. We assumed that the temperature is uniform throughout the liquid phase and all the bubbles, and T was defined as the whole foam’s temperature. Next, Laplace’s law (Eq. 1.9) correlates a bubble’s radius to the system’s pressure and surface tension, γ. The surface tension of pure AESO was measured with the method of the Wilhelmy plate at room temperature: γ = 33.5 mN/m.
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Throughout this work, bubbles were found to have sizes of the order of 20 to 1000 m. If we plug the values measured for the surface tension of AESO and these cell sizes into Laplace’s law (Eq. 1.9), we obtain: 0.0007 < P = Pb-PL < 0.035 bar. Therefore, for most calculations involving the ideal gas law, PL and Pb are interchangeable with only a very small error: 0.07 % < (Pb-PL)/Pb < 3.5 %; and this error decreases when the bubble size increases. Next, if we neglect gravity and assume that the hydrostatic pressure in the liquid, PL, is uniform throughout the whole liquid phase and equal to the ambient pressure, P, we can write:
P Vb ~ nb R T
(4.6)
valid for one bubble. Summing on all the bubbles in the entire foam, we have:
ng nbi i
(4.7)
where ng is the total number of moles of gas in the foam, then:
Vg Vbi i
(4.8)
Thus, another way to write the ideal gas law for foam, combining Eq. 4.6, 4.7 and 4.8, is:
P Vg ~ ng R T
(4.9)
where ng and Vg are the total number of moles and the total volume of the entire gas phase, respectively.
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4.1.3. Foam aging 4.1.3.1. Elementary bubble dynamics The high energy associated with the high liquid surface area of foam is a strong drive for bubble-growth dynamics. In liquid foam, bubbles like to re-arrange and create larger and larger bubbles, for which the ratio of surface area over volume is smaller, resulting in a reduction of the total foam energy per unit volume, as dictated by the basic laws of thermodynamics. Every time a thin film between two cells ruptures, the energy associated with the difference in surface area for the two cells is released and the total energy of the system is lowered. The dynamics of bubble growth are very complex. For simplification and further analysis, we may isolate four different phenomena: liquid drainage, bubble coalescence, bubble coarsening, and bubble breakage [43, 164]. Liquid drainage: due to gravity, the liquid contained in all the interstices between bubbles, i.e. in the cell walls and cell ribs, flows downward continuously. This results in wall thinning where the liquid has departed (Figure 4.2). The liquid eventually finishes its course at the bottom of the foam container, where a pool of plain liquid forms progressively.
Figure 4.2:
Monomer drainage along cell ribs and walls due to gravity, resulting in wall thinning.
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Coalescence: When walls between two bubbles are thin enough, they can be easily broken by local motions or thermal fluctuations [43] (Figure 4.3). Then, the two bubbles merge together into one larger one, providing a reduction of the total surface-area and energy of the system.
Figure 4.3:
Schematic of the coalescence of two bubbles via rupture of a thin cell wall.
Coarsening: when two neighboring bubbles have different sizes, the continuous diffusion of gas from the smaller one into the bigger one through the thin cell wall results in the growth of the larger bubble and the progressive shrinkage of the smaller one until total disappearance, as illustrated in Figure 4.4. The driving force is the pressure difference between bubbles of different sizes: Laplace’s law (Eq. 1.9) implies that the gas pressure decreases when bubbles increase.
P1
Figure 4.4:
P2 0.24 g/cm3), and the cell shape became less spherical at lower densities. For example, consider Figure 6.1, which shows a magnification of the foam core of the lightest AESO/CO2 foam produced in this work (foam 0.16 g/cm3). Many large and small spherical cells are found, but there also exist many oval cells, and polyhedral cells with various face-numbers. The oval cells are stretched mostly in the vertical direction, which was the direction of foam rise during vacuum-expansion. During the free-rise of low-density foams, cells frequently expand in the direction of minimum local stress,
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resulting in local cell elongation in the direction of foam rise, and general foam anisotropy. The anisotropy can be characterized by the ratio of the width, w, and length, L, of elongated cells, as pictured on Fig. 6.1. The anisotropy ratio of AESO/CO2 foam with density 0.16 g/cm3 is superior to 0.85. When foam density increases, foam cells become more spherical and the anisotropy ratio becomes closer to 1. Therefore all the AESO/CO2 foams produced in this work were considered isotropic when measuring mechanical properties.
Figure 6.1:
Detail of the core and general cell shape of the lightest AESO/CO2 foam produced in this work (foam = 0.16 g/cm3).
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6.1.2. Open-cell and closed-cell foam The AESO/CO2 foams produced in this work were mostly closed-cells, close to 100% as noted from visual analysis. The standard test method to measure the ratio of open-cells versus closed-cells is ASTM D6226-05, which uses an air pycnometer, but this method is not suitable for closed-cell samples. Thus, ASTM D6226-05 was not used in this work. The open-cell/closed cell ratio of foam is important to determine possible foam applications. Open-cell foams are most useful when low-density, flexibility, and a high absorption potential are desired, while closed-cell foams are stronger and provide better insulation. Insulation panels is a possible application for AESO/CO2 foams.
6.2. Monomer conversion 13
C-NMR was used to estimate the degree of conversion of the AESO
monomer, or the extent of cure, in the AESO/CO2 foams at optimal operating conditions (96 wt.% Ebecryl 860, 3 wt.% Esperox-28, 1 wt.% CoNap; cure at 45ºC, then 70ºC; postcure at 100ºC, then 140ºC). The corresponding 13C-NMR spectrum was presented in Chapter 2 (Fig. 2.8). The magnetic shifts of the different groups of carbon atoms, as highlighted in Fig. 2.9, and the respective peak areas (from Fig. 2.8) are summarized in Table 6.1. Magnetic shifts #1 and 1’ were grouped because their areas were measured together. Next, we used the data of peak areas vs. number of atoms to calculate the monomer conversion.
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Table 6.1:
13
C-NMR magnetic shifts, ppm-range, areas, and corresponding number of carbon atoms in cured AESO/CO2 foams.
Peak #
Carbon type
Shift (ppm)
Range (ppm)
Area Ai
Ni, AESO
Ni, E28
Total (a) Ni
Ratio Ni/Ai
1 1’
R-C-C C-C-C
15.05 30.65
10 - 55
100
44.8 +2x
10
46.5 +2x
?
2
C-C-O
74.02
55 - 98
27.34
12.2
1
12.37
0.45
3
O-C=O
174.12
163 - 190
10.94
6.4
1
6.57
0.60
4
C-C=C
132.60
125 - 142
4.69
6.8 –2x
0
6.8 –2x
?
(a) Ni = Ni,AESO + 0.17×Ni,E28.
The molecular weights of Ebecryl 860 (AESO) and Esperox 28 (E28) were calculated to be 1186 g/mol and 216 g/mol, respectively. The formulation comprised 96% AESO and 3% E28, in mass, which corresponds to a molar ratio of 100:17 (AESO:E28). 1 wt.% CoNap was neglected in the following calculations. Before functionalization, soybean oil had an average of 4.6 double bonds per triglyceride, and 18 carbons per fatty acid. After epoxidation and acrylation, AESO had an average of 3.4 acrylate groups per monomer, the remaining ex-double-bonds being occupied by hydroxyl groups. The number of carbon atoms corresponding to each one of the NMR shifts for AESO or E28 were Ni,AESO and Ni,E28, with i (i = 1…4). The total number of carbon atoms for that shift was Ni = Ni,AESO + 0.17×Ni,E28. The numbers for each magnetic shift were calculated as follows:
Shifts #1 and 1’: N1,E28 = 10 and N1,AESO = 3×18-2×4.6 = 44.8, thus N1 = 46.5 /mol AESO.
Shift #2: N2,E28 = 1 and N2,AESO = 3+2×4.6 = 12.2, thus N2 = 12.37 /mol AESO.
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Shift #3: N3,E28 = 1 and N3,AESO = 3+3.4 = 6.4, thus N3 = 6.57 /mol AESO.
Shift #4: N4,E28 = 0. Before polymerization, N4,AESO = 2×3.4 = 6.8. After polymerization, N4,AESO = 6.8 – 2x, where x is the number of double-bonds in the acrylate groups that reacted to form a crosslink. Thus, N4 = 6.8 – 2x /mol AESO. Also, after polymerization, N1 becomes: N1 = 46.5 – 2x /mol AESO. The extent of reaction, , is the ratio of the number of functional groups that
reacted, over the original number of acrylate groups:
x 3.4
(6.1)
For standard, we can use either one of the two groups of carbon atoms that are not changed by the reaction, that is, the groups that shift to 74 ppm and 174 ppm. The ratio of the number of carbon atoms over the peak’s area, Ni/Ai, was 0.45 at 74 ppm, and 0.60 at 174 ppm. In theory, these two values should be equal; as stated earlier, this 13C-NMR spectrum will only permit an estimate of . x was calculated using the number of atoms at shift #1 or #4 and their corresponding areas, so that the ratio would be included between the two standards: 0.45
Nj Aj
0.60 with
j 1, 4
(6.2)
For j = 1, we obtained: 0 ≤ x ≤ 6.75, which is useless information since, by definition, 0 ≤ x ≤ 3.4. For j = 4, we obtained:
0.45
6.8 2 x 0.60 2 x 2.34 0.59 0.69 4.69
(6.3)
Thus, the estimated average monomer conversion for the tested samples was (0.59+0.69)/2 = 64%. This is much lower than the maximum conversion of 95% for the acrylate groups on AESO that was measured by LaScala et al. [24]. The difference can
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be attributed to two factors: low-temperature cure (the higher the cure temperature, the better the monomer conversion [103]), and an excess of initiator. Indeed, an excess of free-radical initiator was introduced to ensure fast polymerization at low temperatures. The downside is that, hypothetically, a higher concentration of free-radical initiator generates a higher concentration of active free-radicals on the monomers (Mi*, Section 1.5.4) and accelerates the termination reaction, thus limiting the extent of conversion of the monomers that occurrs during propagation. In future work, one could progressively reduce the initiator contents in order to increase monomer conversion, and define the minimum amount of initiator necessary to obtain both a satisfactory foam structure and a satisfactory degree of monomer conversion.
6.3. Compressive properties Table 6.2 lists all the foam samples tested for their compressive properties, according to the methodology detailed in Chapter 2. Data include the foam’s core density, the compressive strength, the elastic modulus and the proportional limit. Each letter in the column titled ‘Sample #’ represents one particular AESO/CO2 foam sample, from which 2 to 4 parallellograms were cut for testing. Each row in Table 6.2 provides the compressive properties of one parallelogram. The results are presented individually because samples from the same original foam often had a slightly different density and a highly different average cell size, depending on their location within the nonhomogeneous original sample (see Chapter 5). Table 6.2 also lists the properties obtained for a sample of industrial PU foam with similar testing conditions, and the expected modulus of unfoamed AESO polymer as calculated with the percolation theory.
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Table 6.2:
Compressive properties of AESO/CO2 foams, calculated according to the methodology detailed in Chapter 2.
Sample #
Density foam 3 (g/cm )
Compressive strength c (MPa)
Elastic Modulus E (MPa)
Proportional Limit (MPa)
A1
0.24
1.14
25.1
0.8 ± 0.1
A2
0.25
0.85
23.8
0.8 ± 0.1
A3
0.24
0.79
16.8
0.6 ± 0.2
B1
0.22
0.86
12.1
0.6 ± 0.1
B2
0.21
0.82
15.4
0.5 ± 0.1
B3
0.21
0.53
8.1
0.4 ± 0.1
B4
0.23
0.99
18.4
0.6 ± 0.1
C1
0.25
1.33
19.3
1.0 ± 0.1
C2
0.23
1.01
20.7
0.7 ± 0.1
C3
0.26
1.33
24.6
0.7
C4
0.24
0.96
19.4
0.5
D1
0.46
2.6
42.2
1.1
D2
0.48
2.5
37.4
1.1
D3
0.48
2.55
36.1
1.1
E1
0.47
2.08
31.6
1.1
E2
0.47
1.93
27.3
0.95
E3
0.47
1.87
28.3
1.2 ± 0.1
F1
0.23
1.13
22.2
0.75
F2
0.23
1.06
24
0.6 ± 0.1
G1
0.26
1.11
19.2
0.45
G2
0.27
1.08
22.4
0.5
H1
0.16
0.35
5.87
0.23
H2
0.17
0.34
6.16
0.25
H3
0.19
0.31
5.97
0.25
Percolation Theory
1.05
N/A
1.0E+02
N/A
0.08
1.1
42
0.65
0.08
1.1
39.4
0.42
Industrial PU
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6.3.1. Compressive properties vs. foam structure During foam compression, most of the resistance is provided by the stiff cell ribs. Figure 6.2 shows a schematic representation of a foam cell during compression [52]. Consequently, the compressive strength should increase when using a more rigid polymer, or when the foam’s density increases at constant polymer composition, because the cell ribs become thicker. Also, closed-cell foams offer more resistance to deformation during compression, since a large extent of deformation involves breakage of the cell walls via excessive stretching [52]. When the compressive load increases, cell ribs also begin breaking, and the foam deformation becomes irreversible. The point where cell walls and cell ribs begin breaking corresponds to the ‘proportional limit’ defined above. Below the proportional limit, the foam should be able to regain its original size after the load is removed.
Figure 6.2:
Schematic of a unit foam cell during compression (adapted from [52]).
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6.3.1.1. Effect of cell architecture 6.3.1.1.1. Anisotropy The compressive strength depends highly on the anisotropy of the foam, since the cells’ elongation results in a higher concentration of cell ribs per cross-sectional area in the direction of cell elongation than in any other direction [2]. Foam anisotropy can be characterized by the ratio of the average length of foam cells in the direction of elongation, L, over their average width, w. If L/w = in one direction and 1/ in the perpendicular direction, as a result the compressive strength, the tensile strength, and the shear, bending and Young’s moduli can be up to 2 times higher in the direction of elongation. In Figure 6.1, we showed that a fraction of foam cells were indeed elongated in the direction of foam rise (vertical), with an anisotropic ratio as high as = 1/0.85 = 1.18. However, many cells were perfectly spherical, and others had random orientations. The anisotropy of all AESO/CO2 foams was considered negligible, therefore the direction of compression vs. the direction of foaming for each sample was not recorded. 6.3.1.1.2. Cell size All samples tested here were taken from the approximately homogeneous core of larger foam samples. Foams with obvious defects, such as cracks and very large cells, were not tested. Some testing samples extracted from the same cured foam had similar densities, but a different cell-size due to the gradient of extent of polymerization before vacuum-expansion, as described in Chapter 5, which resulted in a cell-size gradient after cure. Foams were cut in a manner as to keep a homogeneous cell size within one test sample. No obvious correlations between cell size and the foam’s compressive strength and modulus were found (thus, the details of cell sizes were not included in
193
Table 6.2). This was expected because, at equal density, when foam cells became larger, the concentration of cell-ribs per unit area decreased, but the ribs themselves became thicker. Thus, at constant foam density, the fraction of polymer material per unit volume or per unit cross-sectional area remained the same independently of cell-size, and should consequently produce the same resistance to compressive loads. 6.3.1.2. Effect of foam density The bulk density of foams was reduced by expansion under vacuum (Chapter 5). During vacuum-expansion, the polymer volume fraction, p, decreased as the foam’s density decreased, resulting in thinning of both cell walls and cell ribs. Since all of the resistance to compression is located in cell ribs and walls, both the compressive strength and elastic modulus decreased with decreasing cured foam density. The values of the compressive strength and elastic modulus should in theory be 0 at zero-density, corresponding to foam with a null polymer fraction. In the next sections, we analyze relationships between foam density and compressive properties. 6.3.1.2.1. Elastic Modulus The values obtained for the elastic modulus, E, as a function of foam density are plotted in Figure 6.3. The blue data point corresponds to the prediction from the percolation theory using Eq. 1.22. Unfoamed AESO polymer can be represented as foam with zeroporosity and a density of 1.05 g/cm3. The value of the elastic modulus for unfoamed AESO polymer should be E ~ 99 MPa according to Eq. 1.22.
194
100 80
E (MPa)
y = 90x1.6 60 40 20 0 0.0
0.2
0.4
0.6
0.8
1.0
foam (g/cm ) 3
Figure 6.3:
Elastic modulus, E, versus foam density, foam. Red data points are experimental values; the blue data point was calculated with the percolation theory [24]. The line is the expected trend.
The considerable data scattering in Fig. 6.3 was most likely caused by three factors: -
The presence of defects in the foam, such as small cracks
-
Potential foam anisotropy (which was neglected)
-
The fact that all samples tested were much smaller than recommended by ASTM D1621-04a. In a small sample, all defects and local deviations from homogeneity appear proportionally larger.
The expected trend typically reported for the modulus of rigid foams as a function of foam density is expressed here as: E ~ 90 foam1.6 (E in MPa, foam in g/cm3) and is plotted on Fig. 6.3. This fit seems to slightly underpredict our data.
195
The elastic modulus expresses the ease for a material to deform under a load, in compression or in tension (i.e., flexibility). Light AESO/CO2 foams had a low modulus (as low as ~6 MPa for 0.16 g/cm3), and appeared much more flexible than medium-density foams, which possessed a higher modulus (as high as ~42 MPa for 0.46 g/cm3). In practice, this means that foam density can be customized in order to change the foam’s elastic modulus to fit the flexibility requirements for a specific application. For example, for cushioning purposes, a high flexibility (or low modulus) is desired, and low-density foams will be preferred. On the other hand, for structural reinforcement applications, a strong resistance to deformation is desired, and medium-to-high density foams are preferred. 6.3.1.2.2. Compressive Strength The compressive strength expresses the maximum load per unit surface area that a material is able to sustain before becoming irreversibly damaged by crushing. The compressive strength of polymeric foams is typically proportional to foam~1.6.
For
example, Butler et al. measured the compressive strength of polyethylene foam as c = 0.08 1.637 [174]. For rigid polyurethane foam, c = 12.77 1.416 was obtained [175]. The values of compressive strength measured with AESO/CO2 foams (Table 6.2) are plotted against foam density in Figure 6.4. The foam’s compressive strength decreased when the foam’s density decreased due to thinning of the cell-ribs and cell walls that support the load [52]. In theory, the strength should be 0 at zero-density. As with the elastic modulus, the foam’s density can be customized to provide a wide array of compressive strength to fit various industrial applications, such as load-bearing (e.g., structural reinforcement) and shock absorption (e.g., car bumpers).
196
3 2.5
c (MPa)
2 1.5 1.6
y = 8.1 x 1 0.5 0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
foam (g/cm ) 3
Figure 6.4:
Compressive strength vs. density for AESO/CO2 foams, σc(ρfoam).
The expected trend for the compressive strength of rigid foams as a function of foam density is plotted on Fig. 6.4. The corresponding equation is:
c 8.1 foam1.6
(6.4)
which would fit our data pretty well if it were not for the extensive experimental scattering (as described above). 6.3.1.2.3. Modulus versus strength The experimental value of foam modulus versus compressive strength for each sample was plotted on Figure 6.5. On the range of foam densities considered (foam = 0.16 to 0.5 g/cm3), the compressive strength and the elastic modulus of AESO/CO2 foams were proportional (except for the scattering):
197
E ~ 17 c
(6.5)
This expression obtained for foamed AESO differs only slightly from the expression derived from Eq. 1.19 and 1.20 for bulk AESO polymer: E 1.2 .
50
y = 16.975x 40
E (MPa)
Industrial PU 30
20
10
0 0
0.5
1
1.5
2
2.5
3
c (MPa)
Figure 6.5:
Elastic modulus, E, versus compressive strength, σc, of AESO/CO2 foams. The red data point corresponds to an industrial PU foam.
6.3.2. Comparison with industrial foams An industrially-produced polyurethane foam (DIAB Inc., DeSoto, TX) was tested alongside the AESO/CO2 foam samples in similar test conditions. Figure 6.6 shows the cell structure of the industrial PU foam. It had a cell size of ~0.5 mm diameter, which was of the same order as the cell sizes of AESO foams, and a density of 0.08 g/cm3, which was twice lower than our lightest foam.
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The values obtained for the modulus and compressive strength of this industrial PU foam were plotted on Fig. 6.3 and 6.4. The modulus and strength of the industrial PU foam were found to match that of much heavier AESO foams, that is with a density of ~ 0.48 g/cm3.
Figure 6.6:
Structure of an industrial PU foam. Cell size ~0.5 mm, mostly closedcell, density = 0.08 g/cm3.
The polyurethane foam industry is a mature one and the formulation and processing conditions of PU foams have been perfected for decades in order to obtain a very high modulus and high strength with very low foam densities. On the other hand, the production of hard foams with a high bio-based content is only in its infancy, and we can expect great improvements in the future that will allow the production of strong foams with a lower density from plant-derived monomers. For now, the AESO foams do not perform badly when compared with the whole array of foams in the industrial market, as seen in Figure 6.7. Fig. 6.7 was drawn using the Cambridge Engineering Selector 4.0
199
software with its extensive materials database, and the range of modulus and corresponding density obtained with AESO foams was added onto it (in red). The AESO/CO2 foams belong to the semi-rigid category, with a modulus similar to that of cork. These foams could hence be useful in many applications, including applications where cork is presently used and applications where medium-to-low density, semirigidity, and closed cells are preferred, such as building insulation.
Figure 6.7:
General classification of foams as a function of their modulus and density, and comparison with AESO/CO2 foams (in red).
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6.4. Flexural properties 3-point-bending flexural tests were performed on three representative foam samples that were tall enough to cut into the required shape. These three samples were samples A, B and F from Table 6.2, which have approximately the same density (~0.23 g/cm3) but with very different average cell sizes (‘small’, ‘medium’, and ‘large’). Table 6.3 present the flexural data for these three samples. As shown in Chapter 5, for the same apparent bulk density, both cell ribs and walls become thicker when foam cells are larger. Thicker cell ribs are harder to bend, therefore the rigidity of the foam increased with increasing average cell size.
Table 6.3:
Flexural properties of three AESO foams with different cell sizes.
Sample #
Specimen dimensions (mm)
Cell Size (mm)
Flexural Stress at 5% strain σf (MPa)
Flexural Stress Strain at at Yield Point Yield Point (MPa)
Flexural Modulus Ef (MPa)
F
5.7×21.2×48.5
0.3-1.5
1.1
1.33
7.00 %
23.5
A
5.3×18×48.5
0.2-0.8
0.72
0.92
8.20 %
16.5
B
5.4×16.3×48.5
0.1-0.6
0.56
> 0.6
>6%
12.5
6.5. Flammability Flammability is an essential property to control in the use of foams for most applications, including seat cushions, structural reinforcements, and insulation panels. The combustion of polymeric foams and various test methods have been well described by Aseeva [176], and we will not go into details here. The flammability of our new AESO/CO2 foam was tested according to ASTM WK2269, a standard test method for
201
combustion of rigid thermoset cellular plastics. During the course of combustion, gases, vapors, or both, are evolved which may be hazardous, thus, the tests were performed in a well-vented fume hood. AESO foams were compared with an industrial polyurethane foam sample (DIAB, Inc.). A sample of each, of similar size, was held vertically with long metallic tweezers, and placed above the same radiant heat source (tealight candles) at the same time. After 10 seconds, the samples were extinguished (if applicable), and the extent of burning compared. Figure 6.8 shows a sample of AESO foam (left) after 10 seconds of burning, compared with a sample of the industrial PU foam after the same burning time.
Figure 6.8:
Flammability test for an industrial polyurethane foam (right) and a 96% AESO +3% E-28 +1% CoNap foam cured at 45ºC (left): samples aspects after a burn time of 10 seconds.
Results: while the industrial PU foam never produced a flame and charred slowly at the point of application of the flame, the AESO foams burst into flames after only 3 s exposition to the heat source. The AESO foam emitted a lot of crackling noises,
202
similar to that of wood fires, and projected small exploding particles. Due to the extreme flammability of AESO foams, it was considered dangerous to pursue these tests, and quantitative analyses such as loss-of-mass and flame height versus time were not done. Polymeric foams are naturally highly flammable (see [176]). The extreme flammability, crackling, and explosivity of AESO foam are fueled by the large percentage of flammable, unreacted monomer that remained in the foam after postcure (~36%, Section 6.2). On the other hand, the industrial polyurethane is most probably fully-cured, with fire-retardant additives in its formulation (trade secret). The flammability of our AESO foams needs to be greatly improved before any further investigation, because of the hazardous condition. Two main ways of improvement include: 1. Extend the monomer’s conversion; this may be done with higher cure temperatures, combined with lower initiator percentages, or with the use of a new initiator more effective than E-28. 2. All industrial foams are usually enhanced with the addition of flame retardant chemicals. A flame retardant is a substance that notably suppresses, reduces or delays the combustion of a material exposed to high-energy sources.
It is usually
incorporated as additives during sample preparation, or applied to external surfaces. The most widely used flame retardant formulations include: halogenated compounds (e.g., decabromodisphenol), phosphorus-based agents (diphenylphosphates), metal hydrates and salts.
More environmentally friendly flame retardants are in the process of being
developed, including nanoclays (e.g., montmorillonite) and silicon powders (e.g., hydrated alkali metal silicates and polysiloxanes) [176].
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6.6. Cytotoxicity Tissue scaffolds are materials whose microstructure and intrinsic properties render useful in tissue-replacement applications.
Scaffolds are crucial to tissue
engineering strategies for a number of reasons: as a three-dimensional structure they provide volume fill, mechanical integrity and a surface that can provide chemical and architectural guidance for live cells to help regenerate tissues [177]. Research in this area has been dominated by developing new scaffold materials, including various biodegradable polyurethanes and polyurethane foams, and new scaffold fabrication techniques [80, 178-186]. Tissue scaffolds foams have previously successfully been synthesized from methacrylate polymers blown with supercritical CO2 [187]. Our new AESO/CO2 foams were tested for cytotoxicity, for potential tissue-scaffolding applications. A sample of cured AESO/CO2 foam was tested for bio-compatibility with human fibroblast cells. The bio-compatibility work was performed by C.L. Noack of C.M. Klapperich’s at Boston University, Department of Manufacturing and Biomedical Engineering.
The test
preparation methods are detailed in Chapter 2. 6.6.1. Growth of human fibroblast cells Incubated foam slices were analyzed after different time intervals, t. After two hours of incubation, human fibroblast cells had begun to attach. After the first 24 hours, a few cells had began attaching and spreading on the foam matrix. These first live cells were provided via initial seeding. Their attachment shows a certain level of biocompatibility with the foam. After 8 days, the number of cells attached on the foam was abundant, and it became clear that the compatibility between the live-cells and the foam was high enough
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to permit and encourage cell multiplication. Eventually, fibroblast cells proceeded to fill up the open pores of the foam. As soon as 19 days into the experiment, completely filled foam pores were visible. Finally, after 22 days of incubation, the foam pores remained filled with fibroblast cells. Since fibroblasts are attachment-dependent cells, this proved that they were still alive, and that the AESO foam was not toxic to them. Figure 6.9 shows the progression of the growth of fibroblast cells within the foam matrix as a function of time.
Figure 6.9:
Human fibroblast cells on AESO/CO2 foam after incubation for 1 day and 3 days (20× magnification), and 8, 12, 19 and 22 days (10× magnification). The arrows indicate fibroblasts growing inside open foam pores.
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The AESO/CO2 foams showed that they are friendly to human fibroblast cells and allow their adhesion and proliferation. The foams also showed no toxicity to the cells after a prolonged contact.
AESO/CO2 foams are therefore considered bio-
compatible and are interesting candidates for tissue engineering scaffold design [40]. 6.6.2. Potential improvements The bio-compatibility of AESO foams could be additionally enhanced, if: 1. The size and shape of foam pores was customized: opened, smaller pores would provide a more favorable foam matrix for the cells to attach upon. 2. The foam was more hydrophilic. 3. The foam was more flexible, to fit current tissue-scaffolds’ lower mechanical properties [40].
Item #1 could be resolved by the introduction of additives into the foam’s formulation, such as certain types of surfactants, which would help create foams with smaller, opened pores. Care should be taken to choose additives that are non-toxic to live cells.
Items #2 and #3 could be improved upon simultaneously, with a simple
modification of the AESO monomer. The two-step reaction used to produce acrylated epoxidized soyoil (Chapter 1) can be customized in order to attach less acrylate groups to the double-bonds of the triglyceride, and more hydroxyl groups in their place [19, 27, 36, 103]. The new theoretical monomer, showed in Figure 6.10, could have a number of acrylate groups per triglyceride as low as 2. When the level of acrylation decreases, the polymer’s crosslink density would decrease and foams would become more flexible. In addition, a higher concentration of hydroxyl groups would increase the polymer’s hydrophillicity and its compatibility with water-rich systems such as human tissues.
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O
O
O
OH
OH
O OH
OH
O O
O
OH
O
O
OH
O
Figure 6.10: Fully acrylated, partially-epoxidized AESO, with less acrylate groups and more hydroxyl groups per monomer than Ebecryl 860 (A = 3.4).
6.7. Bio-degradability AESO foams have the potential to bio-degrade via hydrolysis, due to the presence of ketone and ester bonds in the acrylate groups, the glycerol center, and the fatty acids [188, 189]. Two samples of AESO/CO2 foams from Chapter 7 were buried in garden soil for 60 days. After 60 days, the samples were dug out and compared with the original pictures. A first remark is that it was impossible to thoroughly remove all the soil from inside the small, intricate cells of the foams. Cleaning with water, compressed air, and brushes were all attempted. Therefore, no precise weight-difference measurement was performed. An approximate weight-difference measurement showed no notable change. In future tests, sonication might help with soil-removal. The samples’ surface appearance including color, cell-size and shape, and surface smoothness were compared with pictures of the original samples before burying. No visible micro-organism growth, dark spot, or any other change was noticed. Figure 6.11 shows the two samples after 60-day residency in garden soil.
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In Section 6.6, we concluded that AESO foams were bio-compatible, but apparently they do not readily bio-degrade. Cobalt naphthenate, a toxic component present in 1 wt% in the foams, may be responsible for the resistance to biodegradation, although it did not harm fibroblast cells in the study on cytotoxicity. Additional tests need to be done to measure the bio-degradability of AESO/CO2 foams. Foams may be buried for as long as 6 months, with periodic sample-removal and cleaning (an efficient cleaning method is still to be defined), drying, and precise weight-difference measurements as a function of burial time. One suggestion, to cancel the problem related to soil-cleaning, is to study the bio-degradation of AESO/CO2 foams in ‘living waters’ (such as a reactor as described by ASTM) instead of soil.
Figure 6.11: Two foam samples after burial 2” deep for 60 days in wet garden soil, with T~60-80C (September-October). Composition (for both): 96 wt.% AESO + 3 wt.% E28 + 1 wt.% CoNap.
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6.8. Conclusions The AESO/CO2 foams offered very interesting properties. Their closed-cell architecture renders them useful for insulation and strength-related applications, and compressive tests revealed that they are semi-rigid foams, with a modulus similar to that of cork. In addition, they seemed to show resistance to bio-degradation, which is required in many applications, such as thermal insulation in the building industry. The strength of these foams would be further improved by increasing the monomer’s conversion, currently only ~64% as measured with
13
C-NMR, and by varying the monomer’s
functionalization, for example with the addition of more acrylate groups as predicted with the percolation theory (see Chapter 1). The foam’s strength related to the foam’s density with a power-law exponent of ~1.6 as is commonly found in the literature for rigid foams. The dependence of strength on density is a general design limitation for the foam industry, since one has to choose between high strength and low foam density. However, AESO/CO2 foams are not ready for the industrial world yet because of their extremely high flammability, which needs to be reduced before further study. On another hand, AESO foams showed biocompatibility with human tissue cells and are promising new biomaterials, and considered good candidates for tissue-scaffolding.
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Chapter 7 7. CONCLUSIONS & RECOMMENDATIONS FOR FUTURE WORK
7.1. Conclusions The purpose of this dissertation was the design of new bio-based thermoset foams from soybean oil and carbon dioxide to replace some petroleum-based foams on the market. We conclude that we have successfully synthesized semi-rigid, medium- to low-density foams from AESO and pressurized CO2 that contain more than 80% carbon from soybean origin. To realize this goal, we aimed to gain an understanding of the basics of foam expansion, foam aging, and foam cure. Foam expansion was achieved by the generation of CO2 bubbles within the monomer matrix before gelation, to form a multitude of gas bubbles separated by thin liquid films.
This was performed by
dissolving CO2 into AESO at high pressure, then extracting the saturated AESO/CO2 mixture at atmospheric pressure to vaporize the gas into small bubbles. In Chapter 3, we invented a new balloon-sampler method to efficiently measure the solubility of a gas in a liquid with strong foaming properties. Liquid foaming usually complicates measurements performed with traditional methods. Our experimental data permitted the quantification of the solubility of CO2 in AESO as a function of pressure and temperature in the ranges of T = 21 - 60ºC, and P = 1 - 80 bar. Several models were found to fit our experimental data adequately. These models permit the calculation of the expected foam density after
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depressurization at any temperature, pressure, and pressure-drop within the range considered. In addition, the solubility data of CO2 in AESO was proven to match established literature data for the solubility of CO2 in unmodified soybean oil closely. This means that the addition of functional groups such as acrylate groups, to triglyceride molecules does not affect the solubility of CO2 much. Thus, the solubility models developed in this work may be applied to other triglyceride-based ACRES monomers, in order to calculate the approximate resulting foam density as a function of CO2 pressure and temperature. Before cure, liquid foam aging is responsible for cell growth and foam disappearance. In Chapter 4, the average cell size was found to increase mostly due to bubble coalescence, while the coarsening phenomenon caused a broadening of the cellsize distribution with time. The coalescence phenomenon was adequately represented with a second-order kinetic model. In agreement with this model and our experimental bubble-size measurements, the average cell diameter was defined as increasing proportionally to the cubic root of time, and exponentially with temperature. The loss of total foam volume is caused by the breakage of bubbles on top of the foam (foam collapse) and shrinkage of small bubbles, until total disappearance, at the bottom of the foam combined with liquid pooling (liquid drainage). The rates of foam collapse and liquid drainage were also found to increase exponentially with temperature, which is mostly due to the exponential reduction of the monomer’s viscosity with increasing temperature. In addition, all foam aging dynamics (cell growth, collapse and drainage) greatly accelerate when the foam’s density is decreased, due largely to thinning of the cell walls. Because of all these phenomena, the stability of AESO/CO2 liquid foam is highest at low temperature and when the foam density is high. Thus, to obtain homogeneous, small-celled cured foams, the liquid foam should be generated at the lowest temperature
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possible and with the highest foam density possible. The double paradox in this work is that low-density foams are actually desired, and also that the highest temperature is necessary in order to minimize the polymer’s curing time and thereby minimize the duration of foam aging. In Chapter 5, we developed a foaming process with a detailed experimental procedure that enables control of the cured foam’s density as low as 0.16 g/cm3, while at the same time conserving a small-celled, homogeneous foam architecture. Foam density reduction was achieved with the application of partial vacuum shortly after foam extraction. The key point consisted in finding the best strategic time for the application of vacuum to the foam, tvac. Application of vacuum at the time tvac should cause free foam rise, without accelerating the foam aging dynamics enough to completely degrade the foam’s structure before the polymer gels.
Our working experimental procedure
allowed high flexibility in controlling the process’ temperature and pressure cycles. The viscosity of the polymer during cure was recorded and modeled as a function of time and temperature. Architectural variations in cured foams were analyzed as a function of several process parameters: cure temperature (T), vacuuming pressure (P), or tvac at constant T and P. This systematic study showed that the highest cure temperature should result in the best foam architecture. For the production of low-density foams, tvac had to belong to a narrow expansion window located between the beginning of the propagation reaction and the beginning of polymer gelation. This expansion window became narrower at higher temperature; thus, a ‘medium’ cure temperature was chosen to produce foams with a reasonable processability. Additional parameters such as mold insulation and post-cure temperature cycles were also examined. With the optimized process (Tcure=45ºC; tvac=7 min; pre-heated, insulated molds), we successfully obtained quasi-homogeneous cured foams, whose density is customized with P.
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The new AESO/CO2 foams are semi-rigid structural foams with a closedcell architecture. As required, these foams do not readily biodegrade like starch foams, and are strong and resilient like rigid polyurethane foams, and with a bio content superior to 80%. The foam’s strength is approximately proportional to the foam’s density to the power 1.6. At constant apparent density, the foam’s strength can be greatly increased by utilizing other ACRES monomers with different functional groups and a higher level of functionalization. Our new AESO/CO2 foams are promising structural biomaterials that could serve in the insulation and building industries, as well as with tissue-scaffolding applications. However, these foams still need optimization of their formulation in order to achieve both a higher extent of cure and a lower flammability, as well as a smaller cellsize and lower foam density if required for a specific application (see our recommendations for future work below). Our flexible foaming process could easily accommodate the parametric changes necessary to foam the new formulations.
7.2. Recommendations for future work The present process and formulation limit the quality of the foam that can be produced (limited minimum foam density and minimum cell size).
Areas of
improvements include: increasing the viscosity of the monomer to slow the foam aging dynamics; improve the process’ hardware and setup (flow pattern, heater); introduce additives into the foam formulation, such as nucleating agents to produce smaller initial bubbles, and surfactants to hinder foam aging. The following paragraphs offer potential solutions to treat these different points, and suggest an innovative new application for the AESO/CO2 foam.
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7.2.1. Increasing monomer viscosity to slow foam aging A large proportion of styrene (~40 wt.%) is usually added to AESO to ease flow during the formation of plain polymers, or composites with the VARTM process. On the opposite, the addition of styrene to our foaming process is not recommended because styrene is soluble in CO2 and partially escapes into the gas phase; and also, the viscosity of the AESO monomer mixture is greatly lowered since the zero-shear viscosity of styrene at 25ºC is only 7.0×10-4 Pa.s [190], while the viscosity of AESO is ~10 Pa.s [131]. The foam aging phenomena highly depend on the liquid’s viscosity, and better foam stability will be obtained with higher viscosity of the monomer mixture. However, if the monomer mixture is too thick to flow through the foam-extraction channels of the process, then styrene or another solvent may be added to lower the mixture’s viscosity to a process-able value. But let us not forget that CO2 is an efficient plasticizer itself, thus the addition of styrene will not be necessary until the monomer’s viscosity at room temperature reaches very high values. Several examples of the zeroshear viscosity of ACRES monomers as a function of the type and concentration of functional groups are given below. 7.2.1.1. Unmodified oils The zero-shear viscosity of unmodified oils as a function of their level of unsaturation, U, at 25ºC is well represented by the following equation [103]: η0=0.115 U-0.60
214
in Pa.s
(7.1)
7.2.1.2. Acrylated oils Acrylated triglycerides gain a very polar hydroxyl group and a polar ester linkage for each acrylate group (A) added.
These groups increase intermolecular
interactions via hydrogen bonding and dipole-dipole interactions, as shown by the linear increase of the triglyceride’s dipole moment in ref. [103]. Also, the molecular weight of the monomer, M0, increases by 8 wt.% for each A added. Therefore, according to the Rouse theory, the zero-shear viscosity should also increase proportionally. The Rouse and reptation theories actually severely under-predict the viscosity of acrylated oils versus A, which was found to increase exponentially at 25ºC [103]: η0=0.104 exp(1.34 A) in Pa.s
(7.2)
The type of oil as a small effect on the viscosity after acrylation: at equal A, 0(linseed) > 0(soybean) > 0(high oleic soybean oil (HOSO)), because of the position of the A groups: the further from the glycerol center (i.e., linseed), the stronger the intermolecular polar interactions (e.g., there is steric hindrance in the oleic fatty acids of HOSO). Soybean’s acrylate groups are placed at intermediate positions between those of linseed (C15,16) and HOSO (C9,10). In consequence, the viscosity of acrylated oils can be customized by choosing the level of acrylation, and this can be done by choosing the right oil. About 86-95% of the unsaturations on triglycerides can be acrylated [14]. 7.2.1.3. Hydroxylated and maleinized oils The viscosity of hydroxylated and maleinized oils (Fig. 1.7) increases exponentially with the level of functionality due to polarity effects.
At the same
temperature and level of functionality, the viscosity of hydroxylated oils is slightly higher than that of acrylated oils. On the other hand, maleinized oils have a much higher
215
viscosity than both acrylated and hydroxylated oils due to additional polarity effects and to a higher extent of oligomerization [36, 191]. The viscosity of maleinized oils can be calculated as a function of the original number of hydroxyl groups before maleinization:
0.024 exp 0.61 N hydroxyl 0
in Pa.s
(7.3)
One limitation is that when Nhydroxyl is high, the viscosity becomes too high for processing, even with the addition of styrene. 7.2.1.4. Mixture with a co-monomer The viscosity of any mixture of functionalized triglyceride with any comonomer was established as [192-194]:
comonomer oil 1.2 103 exp 14 comonomer
in Pa.s
(7.4)
where comonomer is the volume percent of co-monomer in the mixture, and comonomer and oil are the zero-shear viscosities of the pure co-monomer and the pure functionalized oil, respectively. 7.2.2. Modification of the low-temperature foaming process Improvements needed for our low-temperature foaming process include:
Modify the flow pattern of extracted foam to minimize the shearing effects that cause CO2 to escape and the original cell size to increase. For example, shorter, wider tubing could be used, or a whole new heating system (e.g., microwaves).
Improve the control of Tfoam with a new heater or better control of the flow rate.
If the loss of CO2 during foam extraction is successfully minimized, the foam density can then be controlled through PCO2 and subsequent vacuum is not necessary anymore. In this case, scale-up of the process for commercial application should be easier.
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Our low-temperature foaming process may be applied to other ACRES monomers.
With each new monomer, the steps necessary to adjust the process
parameters should be:
Study of the cure kinetics of the foamed monomer, using viscosity measurement as a function of time at two temperatures, and the measurement of the final extent of cure with 13C-NMR to find the new parameters for the autocatalytic model.
Choice of the highest practical cure temperature, and definition of tvac accordingly using the autocatalytic model and the vacuum-timing window.
Adjustment of the original cell size through the foam’s formulation (see additives below), and adjustment of the foam density with the degree of vacuum applied, to satisfy specifications for a desired application.
7.2.3. Monolithic foam-recycled paper sandwich composite We present here an idea for an innovative, new application for our AESO/CO2 low-temperature extracted foams.
Previously, the ACRES group had
developed a structural roofing beam consisting of 10-20 layers of recycled paper wrapped around an industrial foam core. The recycled paper is then infused with a AESO/styrene /initiator/CoNap resin mixture utilizing the vacuum-assisted resin transfer molding process (VARTM). This AESO/paper/foam beam was then cured at room-temperature for a day [38, 39]. The main issue with this beam structure is the poor adhesion between the AESO resin skin and the industrial foam at the paper-foam interface (Figure 7.1), which results in a low mechanical integrity.
217
Our idea is to fabricate a one-step,
monolithic foam-recycled paper sandwich composite that would solve this interfacial adhesion problem.
Figure 7.1:
Cross-section of a structural beam for roofing application: poor adhesion between the AESO resin and the industrial foam core.
The intended procedure consists in, first, building an airtight recycled-paper cube. This can be done easily with precut shapes that are taped or glued together. This structure should be held snuggly inside a rigid, porous holder, such as made with chickenwire fencing. The monolithic foam-paper sandwich composite would then be formed by
218
injecting warm AESO/CO2 foam directly inside the paper mold through a little hole at its top, then closing the hole hermetically. When the structure is placed under vacuum at tvac, depressurization will cause air to diffuse out of the porous paper box, and the AESO/CO2 foam will expand to fill the entire paper mold. If sufficient vacuum is applied, the AESO resin will then progressively infuse through the entire paper structure after the mold is filled with foam.
Cure is then performed in-situ at the ambient
temperature and pressure inside the vacuum oven. The one-piece composite thus formed should have excellent integrity at the foam/paper interface, and its mechanical properties will depend on the thickness of the paper skin and the density/cell size of the foam core. 7.2.4. Additives: nucleating agents and surfactants All the additives found to be compatible with AESO should in theory also be compatible with other ACRES monomers. 7.2.4.1. Nucleating agents Nucleating agents are fine powders and other solids that locally lower the surface tension at the liquid-solid interface during foam formation, and help with the initial generation or more numerous, smaller bubbles. Several efficient nucleating agents compatible with the AESO foam system were studied in previous work [131]. These include starch powder, fine metallic powders (aluminum, cobalt, copper), and ground keratin fibers. 7.2.4.2. Surfactants used commercially Polysiloxanes are commonly used as surfactants for the stabilization of nitrogen bubbles within epoxy resins matrices (e.g., epoxy novolacs and bisphenol-A resins mix). Polysiloxanes are silica-based and work even at high temperature [2]. The
219
most widely used are co-polymers based on dimethyl-siloxanes and polysiloxanes (structure: -(-Si(CH3)2-O-)n) with a high viscosity.
The co-polymers can be linear,
branched, or pendant types. These surfactants provide foams with finer cell structures. The proportion of surfactant added should be at least 0.5 % to 1% to prevent large cells. The silicon (Si) content of the surfactant significantly affects its function. Some of the best polysiloxane surfactants are co-polymers of poly-alkylsiloxane and poly-oxyalkylene (structure: -(EO)m-(-Si(CH3)2-O-)n), which give the best mechanical properties and a high content of closed cells. Silicon-based surfactants possess no known health hazard [3]. Oxyalkylenes (oxyethylene, CiEj, or oxypropylene, CiPj) were described in previous work, and C12E5 was shown to be a good surfactant for liquid AESO foam [131].
Another example is polyoxyethylene-(10)-octylphenyl-ether (Triton X-100),
whose structure is presented in Figure 7.2, where n=10.
Figure 7.2:
Molecular structure of the amphiphilic Triton X-100 surfactant.
7.2.4.3. Potential “green” surfactants Natural surfactants that can be used to stabilize modified-oil/CO2 polymeric foams include some sugars, modified oils, and proteins.
Sugars such as acetylated
cyclodextrins have a high solubility in dense CO2, owing to the readily available per-acyl groups that create favorable Lewis acid-Lewis base interactions with CO2 [195]. Fully
220
epoxidized plant oils (soybean and rapeseed oils) have recently been used to synthesize surfactants for foams. The epoxidized oils were made to react with diethanolamine and isopropanolamine, then the resulting products were oxyethylenated to yield non-ionic surfactants that increased foams stability [196]. Finally, one protein commonly used as a surfactant for edible foams and emulsions is the lecithin lipoprotein from egg yolk, shown in Figure 7.3. Egg yolk is a natural emulsifier for mayonnaise, cakes, etc, where it stabilizes the 2-phase architecture. For example, the phospholipids and lipoproteins of egg yolk stabilize oil-in-water emulsions by coating the individual oil droplets3,4. The typical composition of liquid egg yolk is: 49% water, 30.7% Lipids (21.3% triglycerides, 8.0% Lecithin, 1.4% Cholesterol), 16.7% protein, 1.8% sugars, 1.8% ash [197].
O O +
N
O
P O
O O
O O
Figure 7.3:
Molecular structure of the lecithin lipoprotein from egg yolk.
3 http://www.bakingbusiness.com/refbook_results.asp?ArticleID=37152 4 http://www.aeb.org/EggProducts/reference/chapter4.html
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8. APPENDIX A
Properties of CO2
Figure A.1:
Pressure-Enthalpy phase diagram for carbon dioxide (CO2) [198].
237
Table A.1: Properties of carbon dioxide [199, 200]. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 33 34 35 36 37 38 39 40 43 44 45 46 47
Name Formula CAS Number Molecular Weight Melting point Boiling point @ 101.325 kPa (1 atm) Critical Temperature Critical Pressure Critical Volume Critical Density Critical Compressibility Factor Acentric Factor Density of Liquid @ 25 C Coefficient of thermal Expansion of Liquid @ 25 C Surface Tension @ 25 C Density of Gas @ 1 atm and 70 F (21.1 C) Relative Density of Gas @ 1 atm and 70 F (Air =1) Enthalpy of Vaporization @ Boiling Point Enthalpy of Fusion @ Melting Point Heat Capacity of Gas @ 25 C , Constant Pressure (CP) Heat Capacity of Gas @ 25 C , Constant Volume (CV) Ratio of Heat Capacities for Gas , CP/CV Heat capacity of Liquid @ 7 C Heat capacity of Solid @ -103 C Entropy of Gas @ 25 C Entropy of Formation of Gas @ 25 C Enthalpy of Formation of Gas @ 25 C Gibbs Energy of Formation of Gas @ 25 C Solubility Parameter Liquid Volume Solubility in Water @ 25 C Henry's Law Constant for Compound in Water @ 25 C Viscosity of Gas @ 25 C Viscosity of Liquid @ -270 C Thermal Conductivity of Gas @ 25 C Thermal Conductivity of Liquid @ -270 C Lower Explosion Limit in Air Upper Explosion Limit in Air Flash Point Temperature Threshold Limit Value (ACGIH) Permissible Exposure Limit (OSHA) Recommended Exposure Limit (NIOSH) Vapor pressure @ 20 C (68 F) Solubility in water @ 21 C:
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CARBON DIOXIDE CO2 124-38-9 44.01 216.58 K , -56.57 C , -69.83 F 194.7 K , -78.45 C , -109.21 F 304.19 K , 31.04 C , 87.87 F 73.82 bar , 72.85 atm , 1070.67 PSI 3 94 cm /mol 3 0.4682 g/cm 0.274 0.228 3 0.713 g/cm 0.02066 /C 0.57 dynes/cm 3 3 1.823 kg/m , 0.1138 lb/ft 1.52 Hvap.= 571.08 kJ/kg 204.93 kJ/kg , 88.12 BTU/lb 0.873 kJ/(kg K) , 0.209 BTU/(lb R) 0.684 kJ/(kg K) , 0.163 BTU/(lb R) 1.276 3.048 kJ/(kg K) , 0.728 BTU/(lb R) 1.1 kJ/(kg K ) , 0.263 BTU/(lb R) 213.69 J/(mol K) 3.02 J/(mol K) -393.5 kJ/mol -394.4 kJ/mol 3 14.564 (J/cm )^0.5 3 37.278 cm /mol 1950 parts per million (wt) 1635.2 atm/mol fraction 150.5 micropoise 0.064 centipoise 0.01653 watts/(m K) 0.0784 watts/(m K) 5000 parts per million (vol) 5000 parts per million (vol) 5000 parts per million (vol) 58.5 bar -4 3.73×10 mol/L, pH ~ 4
