Polyurethane Foams Obtained from Castor Oil-based Polyol and Filled with Wood Flour M. A. MOSIEWICKI, G. A. DELL’ARCIPRETE, M. I. ARANGUREN N. E. MARCOVICH*
AND
Institute of Materials Science and Technology (INTEMA) University of Mar del Plata - National Research Council (CONICET) Av. Juan B. Justo 4302, (7600) Mar del Plata, Argentina
ABSTRACT: A natural polyol was prepared from castor oil by alcoholysis with triethanolamine. The oil and the oil-based polyol were characterized by infrared spectroscopy and through the analytical determination of their functional groups, both techniques indicating that the hydroxyl content increased significantly after the alcoholysis reaction. The modified oil was subsequently used as the polyol component in the formulation of rigid polyurethane foams. Wood flour was chosen to be incorporated as filler in these materials. Physical, thermal, and mechanical properties of the neat and reinforced foams were measured, analyzed, and compared to a reference commercial system. The chemical reaction between wood flour and isocyanate strongly affected the composites’ response to thermo-gravimetric tests. Compression modulus and yield strength decreased as wood flour content increased. The effect of the foam density on the compression properties was also investigated. KEY WORDS: castor oil, natural polyol, filled polyurethane foams, wood flour.
INTRODUCTION produced to display a wide range of properties from those of flexible elastomers to rigid crosslinked polymers, which can be used to fit in different applications. Among those, polyurethane foams are materials with very interesting properties such as a high capacity of energy absorption, particularly useful for shock damping, and low thermal conductivity due to the presence of a skeleton made of cells (open or closed) more or less regular [1]. The mechanical response of these cellular materials depends on the architecture of the cell walls (wall width, the size distribution, and the shape of the cells) and the intrinsic properties of the polymer constituent [1].
P
OLYURETHANES CAN BE
*Author to whom correspondence should be addressed. E-mail:
[email protected]
Journal of COMPOSITE MATERIALS, Vol. 43, No. 25/2009 0021-9983/09/25 305716 $10.00/0 DOI: 10.1177/0021998309345342 ß The Author(s), 2009. Reprints and permissions: http://www.sagepub.co.uk/journalsPermissions.nav
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OH O OH
O
O
O O
O
OH Figure 1. Chemical structure of castor oil.
Presently, most of the polyols used in the manufacture of polyurethanes are derived from the petroleum industry [2], thus alternative sources that present economical and environmental advantages must be investigated. In this context, natural oils are widely available throughout the world. Their chemical structure offers reactive sites, which can be used to obtain products useful in the polymer industry [3,4]. One of the possible chemical modifications is to introduce hydroxyl groups in an unsaturated triglyceride by hydroxylation of C¼C double bonds and/or by alcoholysis of the triglyceride to obtain a monoglyceride [3,5]. Castor oil is particularly interesting because it contains a high percentage of ricinoleic acid (Figure 1), which already carries one hydroxyl group in its structure [6]. Although the hydroxyl amount in the unmodified oil is not high enough to formulate rigid polyurethanes, castor oil can be reacted with triethanolamine (alcoholysis) in order to increase the groups capable of reacting with isocyanates [5]. On the other hand, the use of natural fibers as reinforcing material is an attractive alternative to the use of synthetic fibers/fillers, since they are of low cost and are also available from renewable resources, reducing environmental concerns [7,8]. The densities of natural fibers are of the same order as those of plastics and only 4050% of that of glass fibers. Because of this, plastics can be reinforced or filled without having significant effects on the density of the composites [9]. Besides, lignocellulosic fibers have surface hydroxyl groups that can interact with the isocyanate groups, leading to excellent adhesion fibermatrix. For some years, the study of cellular materials has been extended to composite foams reinforced by the introduction of fibers [10] and metallic or mineral particles [1114]. This addition to the polymer matrix can lead to a better absorption of the heat released during the polymerization reaction, an increase of the composite rigidity still maintaining a large capacity of energy absorption, and an acceptable density. The foam architecture obviously depends on the filler dispersion, and the cell-wall width compared to the particle size. Among others, natural-fiber-reinforced foamed materials have considerable importance because of the possibility of reducing the density of automobile parts by virtue of the cellular structure of the polymer and the use of low density fillers [9,1518]. Some authors have reported the use of oil-based polyols in polyurethane foam formulations [5,19,20] with similar properties to those from petrochemical origin. There have also been results on the incorporation of particulate reinforcements into synthetic formulations. However, there are not many published studies on the formulation and characterization of bio-based foamed materials reinforced or filled with vegetable particles/fibers [21]. The aim of the present work was to synthesize and characterize a polyol from castor oil to be used in the production of wood flour filled rigid polyurethane foams and to analyze the effect of different filler contents and foam densities on the final thermal and mechanical properties of these composites.
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EXPERIMENTAL Materials A bio-based polyol was obtained from castor oil (ParafarmÕ , Argentina, hydroxyl value ¼ 169.3 mg KOH/g) by alcoholysis with triethanolamine (>99%) from Laboratorios Cicarelli. Lithium hydroxide (>99%) from Fluka was used as catalyst in this reaction. Additionally, a commercial 95% pure toluene-di-amine based polyether polyol, having a functionality of 3.9 (Rubinol R124, Hunstman Polyurethanes, hydroxyl value ¼ 392 mg KOH/g) was used to obtain the reference foams. For the preparation of rigid polyurethane foams, the isocyanate used was a 4,40 diphenylmethane diisocyanate prepolymer, p-MDI (Rubinate 5005, Huntsman Polyurethanes, USA) with an equivalent weight of 131 g/eq. Besides, different additives were incorporated to the formulation in the percentages indicated (with respect to the total weight): 1.5 wt% of surfactant agent (Tergostab B8404 Hunstman Polyurethanes), 0.1 wt% catalyst (tertiary amine, Tergoamin DMCHA, Hunstman Polyurethanes) and 10 wt% of foaming agent (HCFC, 141b, Hunstman Polyurethanes). Pine wood flour, WF (Jorge Do Santos Freire, Buenos Aires, Argentine) was selected as the reinforcement/reactive filler with particle sizes 64 mm. The hydroxyl value of the wood flour was determined using a back titration technique. For this determination, a measured weight of wood flour was mixed with excess of p-MDI. After the reaction was completed, the free NCO groups were determined according to the technique described elsewhere [22]. The resulting value was 233.3 mg KOH/g. Synthesis of Castor oil-based Polyol The castor oil, dry triethanolamine, and lithium hydroxide (catalyst) were added together in a reactor with mechanical stirrer. The temperature was raised to 150 C in 0.5 h and it was maintained at this value for 2.5 h. The castor oil/triethanolamine molar ratio was 1/3 and the catalyst was used in a 0.2% by weight of the total reactants. The molar excess of triethanolamine is used to favor the equilibrium of this reversible reaction towards the formation of the modified oil. A scheme of the chemical reaction is presented in Figure 2. It is worth noticing that the final product is a complex mixture of different molar masses components, since glycerol and amine based polyols containing 0, 1, 2, or 3 fatty acids chains in their structure result from this reaction. Physical Characterization of the Castor Oil-based Polyol Hydroxyl value: It was determined by acetylation with acetic anhydride in pyridine solution, according to the method suggested by Urbanski [23]. Size exclusion chromatography (SEC): Small aliquots of castor oil or its derived polyol (15 mg) were dissolved in 5 ml tetrahydrofuran (THF). The resulting solutions (25 mL) were injected in the size exclusion chromatograph (SEC, Waters 510 with HR 0.5, 1 and 3 ultrastyragel columns, UV detector at 240 nm, and a THF flow rate of 1 ml/min). Fourier transform infrared spectroscopy (FTIR): The samples were also characterized by transmission FTIR spectroscopy, using NaCl windows. All spectra were recorded at
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CH
CH
OOC
CH
CH
CH
CH
OOC
+
N
CH2
CH2
OH
CH2
CH2
OH
CH2
CH2
OH
ET AL.
Triethanolamine
Tryglyceride 150°C LiOH
N
CH2
CH2
R
CH2
CH2
R
CH2
CH2
R CH or R = OH
R = OOC
H2 C
R
HC
R
H2 C
R
CH
Figure 2. Scheme of the synthesis of the natural polyol from castor oil (alcoholysis reaction).
2 cm1 resolution using a Genesis II Fourier transform infrared spectrometer. Reported results are the averages of 16 scans. Viscosity determination: Viscosity measurements were carried out at 26 C using a Brookfield Digital Viscometer, model DV-II (USA). Foam Production The index (moles of NCO groups/moles of OH groups) was adjusted for each system in order to obtain stable non-collapsing foams. The contributions of both, polyol and wood flour, were taken into consideration in the index calculation, since isocyanate groups are also consumed in a heterogeneous covalent reaction with the surface OH groups of the vegetable particles added [21,24]. In the reference system the index was maintained between 1.0 and 1.1, while the foams obtained with the natural polyol were formulated with an index kept between 1.15 and 1.25. In all cases, polyols were dehydrated under vacuum and strong stirring during 2 h at 80 C, before its use, in order to eliminate absorbed moisture. Similarly, wood flour was dried at 70 C until constant weight in a vacuum oven, previously to be used. The foams were obtained by free-rising in a mold, at room temperature. The polyol, surfactant agent, catalyst, and foaming agent were manually mixed together in a container and then, the p-MDI (or p-MDI plus wood flour, to obtain filled foams) was added. The system was mechanically mixed for 20 s and then the foam was allowed to freely rise. Foam Characterization Density Measurements: the density of the foams was obtained as the ratio between the weight and the volume of a cubic specimen. The weight was measured with a precision of
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0.001 g and the sample linear dimensions (50 mm side) with 0.01 mm. At least five replicated specimens of each sample were measured. Thermal Conductivity: was determined using the ‘hot wire method’ (HWM), a transient test, using an ad-hoc equipment [2527]. Scanning Electron Microscopy (SEM): was used to obtain photographs of unfilled and wood flour filled foams. Small specimens were cut from the foam samples using a thin blade and coated with gold in preparation for the SEM study (scanning electron microscope Philips model SEM 505). Thermogravimetric Analysis (TGA): Thermogravimetric tests were performed using a TGA-50 SHIMADZU Thermogravimetric Analyzer at a heating speed of 10 C/min under nitrogen atmosphere. Compression Tests: Specimens of 50 50 30 mm3 were cut from the center of the foams, and tested at room temperature and 2.5 mm/min in an INSTRON 8501 Universal testing machine, according to the ASTM D1621. The compression force was applied in the foam rise direction. At least three specimens of each sample were used, and the average values are reported. The initial slope was used to calculate the compressive modulus. The yield stress was directly determined from the maximum in the stressstrain curve when possible. In the rest of the cases, the yield stress was measured at the intersection of the initial slope and the level of the final plateau.
RESULTS Castor Oil-based Polyol Castor oil is a clear, pale yellow colored liquid. This compound is a fatty-acid triglyceride, with approximately 90% of the fatty-acid chains corresponding to ricinoleic acid. Hence, by considering castor oil as containing just ricinoleic acid chains, a molar mass of 932 g/mol is estimated. After the alcoholysis step, the synthesized polyol is a mixture of different components (Figure 2) with molar masses ranging from 92 to 149 (pure glycerol and unreacted amine, respectively) to that of the unreacted castor oil. Table 1 shows the hydroxyl and viscosity values of castor oil-based polyol and the corresponding values of the commercial polyol. It is clear that the hydroxyl value increased significantly after the alcoholysis reaction (2.6 times the original value for the unmodified oil) reaching a value higher than (but still comparable to) that of the commercial polyol. The viscosity of the natural polyol was lower than that of the commercial one, which is very desirable in the production of the rigid foams, in order to reduce the mixing times and the filler agglomeration in reinforced foams containing high filler contents. Although, the higher viscosity of the commercial polyol may be related to its higher molar mass (approximately 1500 according to manufacturer data) one must consider that the chemical structures of the polyols are also different. Figure 3 shows the FTIR spectra of the unmodified and modified castor oils. The spectrum of the original oil shows the band corresponding to the absorption due to hydroxyls (32503550 cm1). There is a small peak at 3008 cm1 corresponding to CH stretching of aliphatic CH¼CH. The peaks at 2925, 2855, and 1463 cm1 were assigned to
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Table 1. Hydroxyl values and viscosities of polyols.
Viscosity (cp) Hydroxyl value (mg KOH/g)
Absorbance
Castor oil-based polyol
Commercial polyol
670 449
12500 392
1737 cm– 1 1077 cm– 1 1037 cm– 1
castor oilbased polyol
1745 cm– 1
castor oil 4000
3500
3000
2500
2000 1800 1600 1400 1200 1000 800 600 cm–1
Figure 3. FTIR spectra of unmodified and modified castor oils.
CH stretching and bending vibrations of CH3 and CH2 groups, respectively. The bands at 1746 and 1240 cm1 correspond, respectively, to the C¼O and CO stretching vibration of ester groups, while the peak appearing at 1652 is assigned to the C¼C stretching vibration of cis CH¼CH; the bands at 11641101 cm1 are due to CO stretching and the one at 722 cm1 is assigned to the cis CH out of plane deformation, respectively. When comparing the spectra of the unmodified oil with that of the derived polyol, the most important difference is the increase in the band intensity at 3450 cm1 after the alcoholysis reaction, corresponding to hydroxyl absorption, in agreement with the measured increase in hydroxyl number. Moreover, the peak at 3008 cm1 remains unaltered after the chemical modification, indicating the preservation of the C¼C during the alcoholysis reaction. The ester carbonyl peak appears wider and shifts to 1737 cm1. The band with maxima at 1077 and 1037 cm1 correspond to absorption peaks present in the triethanolamine. Figure 4 shows the results obtained by size exclusion chromatography of the unmodified and modified castor oils. The original oil curve shows essentially a single peak at 21.5 min attributed to the castor oil triglyceride molecules. As expected, size exclusion chromatogram of the modified castor oil presents several peaks corresponding to different products: the peak appearing at 21.5 min corresponds to the species containing three fatty acid chains in their molecules (triglyceride and amine-derived molecules), and the peaks that appear at longer times (22.3 and 24.0 min) correspond to lower molar mass products containing two or one fatty acid chains in their structure (Figure 2). Lower molar mass species (unreacted triethanolamine or any glycerol that could have been formed during the alcoholysis) are not detected by this technique.
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Castor oilbased polyol
Castor oil 10
15
20 Time (minutes)
25
30
Figure 4. Size exclusion chromatograms of the unmodified and modified castor oils.
Table 2. Foaming characteristic times of unfilled and filled foams. Commercial polyol
Natural polyol
Times (s)
0% WF
15% WF
0% WF
5% WF
10% WF
15% WF
Cream Half cup Full cup End of rise
25 2 48 4 61 6 135 7
32 3 75 4 106 5 160 9
67 2 94 4 111 4 166 13
64 2 91 5 107 4 156 6
72 3 94 6 112 9 157 8
77 2 111 5 134 5 155 7
Foam Rising Times Table 2 shows the foaming behavior of the samples prepared with different percentages of filler for the natural and the synthetic systems. The characteristics times measured for the oil-based system are higher than those measured for the commercial system selected [19], but still in the range of operation conditions for preparing rigid polyurethanes foams. The lower reactivity of the castor oil-based polyol can be attributed to the secondary hydroxyl groups, while the synthetic polyol contains mainly primary hydroxyl groups, which are more reactive [5]. The foaming reaction rate is slightly lower as wood flour concentration increases, probably because of steric hindrance effects that control the chemical reaction between isocyanate and filler OH groups. The values for the end of rise time are in the range of 155-166 s for all the samples with the exception of the commercial unfilled formulation. Apparently, the addition of WF did not affect noticeably this characteristic time for the castor oil-based system. Microscopy Analysis Figure 5 show the scanning electron micrographs of the rigid foams formulated with the natural polyol as a function of the WF content. There are not preferential orientations in
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Figure 5. Scanning electron micrographs of the natural foams as a function of WF concentration.
the cells, being the cellular structure of the unfilled foams, predominantly spherical and evenly distributed, with few broken cells. In general, the cells appear closed for the unfilled foam, with diameters ranging from 0.12 to 0.5 mm. The addition of WF to the formulation is responsible for an increase in the viscosity of the mixture. This causes the mixture to be less expandable, giving rise to a more distorted structure of cells, with a larger cell size distribution (less uniform sizes). This effect increases with filler content, since WF particles are large enough to interfere with cell development (average particle size was about 64 microns). Moreover, the mechanism of cell growth is governed by the stiffness of the gas/polymer matrix, the rate of gas diffusion, and the amount of gas loss [28,29] and it was reported that the addition of wood fiber increased both matrix stiffness and the rate of gas loss during foaming [29]. The width of the cell walls for the unfilled foams is, in average, around 10 mm, as it is indicated in Figure 6. Regarding the composite width of the cell wall, it was unfeasible to estimate from SEM observations, although they seem to be thinner contrary to the comments of some researchers on that a thicker wall should be expected [7,30]. Figure 7 reveals that interfacial interaction between wood particles and polyurethane matrix was very good, most likely a result of both components having similar chemical functional groups. Good interfacial interaction suggests that stress can be transferred from the polymeric matrix to the fibers very effectively during deformation [7,31,32]. Density and Thermal Conductivity These two physical properties of the unfilled and filled samples are presented in Table 3. All the materials included in the Table show comparable densities, which are in the range
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Figure 6. SEM image of a cell wall in the unfilled natural foamed polyurethane.
Figure 7. Scanning electron micrographs of cell walls (with interacting wood flour) in the natural foams filled with 10 and 15 wt% WF.
Table 3. Densities and thermal conductivities of unfilled and filled foams.
Commercial polyol Natural polyol
Wood flour (wt%)
Density (kg/m3)
Thermal conductivity (mW/m8C)
0 15 0 5 10 15
37.1 0.7 36.4 1.5 37.6 0.5 38.5 0.9 37.7 0.1 38.8 1.8
47.5 2.2 47.9 1.8 39.4 4.3 44.1 3.2 45.3 1.0
expected for these types of foams [19]. All the foams made from the oil-based PU show slightly lower thermal conductivity than those made from the synthetic polyol, especially the unfilled foam. Unfortunately, the thermal conductivity of the oil-based foam increases with WF content, reaching a 15% increase with respect to the unfilled material, in the case of the 15 wt% WF filled-foam.
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In foamed systems, the dominant heat transfer modes are thermal radiation and gasgas and solidsolid conduction. The total conductivity of typical PU closed-cell foams is about two-thirds of the conductivity of stagnant air because there is low conductivity gas (foaming agent) inside the foam. Although foams aged by the in-diffusion of air and the out-diffusion of the foaming agent, the conductivities are comparatively low. The smaller cells have a larger contribution of heat transfer by radiation due to the more walls found in the way of transfer, thus they have a higher conductivity [33,34]. Summarizing, the thermal conductivity of these materials depends on their cellular structure, which is distorted by the addition of wood flour (see microscopy analysis). The effect of having small and/or clustered-small cells resulted in an increase in the thermal conductivity of the foam. Thermal Stability The thermal stability of wood flour, unfilled and 15 wt% WF filled foams formulated with the castor oil-based polyol and the commercial polyol were examined by TGA and the curves obtained (weight loss versus temperature) are shown in Figure 8. At temperatures below 120 C the absorbed moisture is lost [35] in all cases; however, this loss is significant only in the case of the wood flour because of its hydrophilic nature and it represents about 6.5 wt. % of the initial WF mass. In the case of WF, noncombustible products, such as carbon dioxide, traces of inorganic compounds and water vapor are produced between 100 C and 200 C. At about 175 C, some components begin to break down chemically: low temperature degradation at low rate occurs in lignin and hemicelluloses. The mass lost between 300500 C corresponds to the degradation of cellulose and to the pyrolitic degradation of lignins [36,37]. On the other hand, the thermal degradation mechanism of polyurethanes is usually described as a very complex process that involves dissociation of the initial polyol and isocyanate components, followed by the thermal decomposition leading to the formation of amines, small transition components and carbon dioxide. [4,38,39]. Comparison of the TG signal for the commercial and the castor oil-based formulations shows that the rate of thermal degradation is slower for the natural foams (a)
(b)
100
80 TG (weight %)
TG (weight %)
80 60 40 Commerical foam 15 wt% WF WF
20 0
100
0
100
200 300 400 Temperature (°C)
60 40 Natural polyol foam 15 wt% WF WF
20
500
0
0
100
200
300
400
500
Temperature (°C)
Figure 8. Thermogravimetric behavior of wood flour, unfilled and 15 wt% filled foams. (a) made from the commercial polyol, (b) made from the natural polyol.
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(filled and unfilled). This behavior is attributed to the heterogeneous chemical structure of the natural polyol, as was previously discussed. Figure 8b illustrates that the thermal decomposition of the castor oil-based unfilled foam occurs in at least three steps (in addition to the initial water desorption), which reduces the overall rate of thermal degradation of the foam. In both systems, significant degradation takes place above 240 C, approximately. Moreover, the residue left at 500 C is approximately the same for both types of foams (39 and 40% of the initial mass, respectively). At temperatures higher than 350 C, in both systems, the foams with 15% of filler present higher residual mass than the unfilled foams. The lignin content of the filler, added to the reaction between the p-MDI and the hydroxyl groups of the wood flour that originates new covalent bonds, contribute to the higher thermal stability of the filled foams. Additionally, the castor oil-based composite foam produced higher char at 500 C (51% of the initial mass as compared to 43.5% in the commercial filled foam). Both the features, reduced degradation rate and higher char, are characteristics of a better behavior under fire. Figure 9 shows the derivative thermogravimetric curves for the castor oil-based foams, including a curve calculated from the experimental curves of the neat PU foam and the WF, which were combined according to their relative weights in the composite. It can be noticed that the experimental and calculated curves for the 15 wt% filled foam differ significantly. Experimentally, the samples start to degrade at a slightly higher temperature than predicted, but after that, most of the mass is lost in a single wide step, whose maxima is between the first degradation peak for the natural polyurethane and the main degradation peak of WF. On the other hand, the calculated curve predicts a degradation pattern similar to that corresponding to the natural PU, with increased intensity due to the WF presence. Following a similar reasoning, the predicted char yield at 500 C is 35%, while the experimental curve shows a 51% yield. Thus, the shifts in the calculated peaks (dTG), as well as the char yields measured for the composite indicate that very strong interactions exist between the polyurethane and the wood flour, which affects the whole thermal degradation process. This behavior is a consequence of the chemical reaction between filler and matrix, which has been already discussed by the authors previously for a cellulosepolyurethane system [21,24].
DTG
WF Natural polyol foam 15 wt%-calculated 15 wt%-experimental
0
100
200 300 Temperature (°C)
400
500
Figure 9. Derivative thermogravimetric curves of wood flour, natural polyol unfilled and 15 wt% filled foams.
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Table 4. Compression mechanical properties of castor oil-based unfilled foams with different densities. Density (kg/m3) 39.8 0.7 41.8 0.7 46.4 1.1
Compressive modulus (Mpa)
Yield stress (KPa)
Yield deformation (%)
4.1 1.2 4.7 0.1 5.8 0.9
163.6 32.4 217.6 32.0 264.0 14.4
5.4 0.5 6.2 0.5 6.7 0.3
Mechanical Properties The effect of the foam density on the mechanical properties was investigated. To do this, specimens for compression tests were cut from different heights of the free-rise foam prepared with the unfilled oil-based PU. All the selected specimens showed homogeneous cell sizes and shape. The compression modulus, yield strength, and deformation at yield of these foams, as a function of their density, are presented in Table 4. The compression modulus and strength increase with density (overall trend), since in compression the stiffness arises from buckling of cell walls [40]. The higher density is related to more compact cellular structures, thus there is more material per unit area and the modulus and strength increase [41]. The yield strain also shows a slight increase with density and falls in a narrow range of 5.4 to 6.7% strain. The compression mechanical properties of the filled foams as a function of the wood flour content are presented in Table 5. Typical stressstrain curves for unfilled and WF filled foams are also included in Figure 10. Both, the compression modulus and strength decrease as wood flour concentration increases. Since in the filled foams, the isocyanate groups are consumed not only to form polyolpMDI bonds, but also wood flour pMDI bonds, the reduction in mechanical properties could be attributed to more fragile cell walls of the filled foams, with respect to the cells of the neat PU foams. Moreover, upon the application of loading, bending and shrinkage of cell walls occur, which results in the development and propagation of microcracks. Hence, the material strength is highly dependent on the initiation of microcracks and the constraints on their growth. A decrease in strength of the filled foams indicates that crack initiation and growth are accelerated in the presence of WF particles. Besides, as WF can be considered as a non-deformable filler embedded into the foam architecture, it would act as a defect, leading to the embrittlement of the cell walls. Finally, a non-uniform distribution of filler contributes to the embrittlement effects, since areas of high stress concentration can be induced, which lead to the failure of samples in an unexpected manner at random locations in the samples [42]. This behavior, although unexpected, has been reported in the literature. Saint-Michel and coworkers [1] indicated that eventually, filler addition could lead to an embrittlement of the cell walls and therefore favor their rupture instead of their deformation. Shen et al. [43] indicated that the moduli of phenolic foams filled with up to 10 wt% aramid fibers are lower than that of the unfilled counterpart. Alonso et al. [44] found that the modulus of an epoxy foam with 2.5 wt% aramid fibers is slightly lower than the unfilled foam. Additionally, Guan et al. [41] reported that foams had higher mechanical properties when cells were uniform in size, evenly distributed, hexagonal/pentagonal in shape, and unbroken. Unfortunately, as it was previously discussed (microscopy analysis), wood flour incorporation causes a severe disruption of the foam morphology, since the average wood flour particle size (64 mm) is larger than the cell wall width (10 mm for unfilled samples).
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Polyurethane Foams Obtained from Castor Oil-based Polyol Table 5. Compression mechanical properties of filled foams.
Commercial polyol Castor oil-based polyol
Wood flour (wt.%)
Compressive modulus (MPa)
Yield stress (KPa)
Yield deformation (%)
Density (Kg/m3)
0 15 0 10 15
5.9 0.5 4.2 0.7 3.4 0.8 2.7 0.3 2.1 0.2
261.0 22.0 146.0 30.0 178.8 59.6 135.6 17.2 116.8 4.8
5.0 0.4 4.1 0.7 7.1 0.7 6.3 1.2 7.1 0.5
37.1 0.7 36.4 1.5 37.6 0.5 37.7 0.1 38.8 1.8
300 Stress (kPa) 250
200
150
100 CP - 0% WF CP - 15% WF NP - 0% WF NP - 10% WF NP - 15% WF
50
0 0
5
10 Strain (%)
15
20
Figure 10. Compression stress-strain curves of unfilled and WF filled foams. CP: commercial polyol derived foams; NP: natural polyol derived foams.
This change in the cell structure reduces the resistance to fracture under load during the compression test and thus, decreases the composite compression strength.
CONCLUSIONS A low viscosity, highly hydrophilic bio-based polyol was obtained by castor oil alcoholysis. Rigid PU foams and wood flour filled rigid PU foams were successfully prepared. The characteristic foam rising times are slightly longer as compared with a commercial foam system, for both, unfilled and filled samples. On the other hand, a longer cream time is an advantage, because it allows achieving more uniform mixtures. The wood flour reacts with the pMDI and thus, acceptable foams can be prepared using up to 15% wt of filler, even though the compression properties decrease and the thermal conductivity increases slightly as wood flour concentration increases. However, the thermal stability of the foams is increased by addition of WF and the replacement of the
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