Evaluation of Foaming and Nucleation and Growth

Materials Science Forum ISSN: 1662-9760, Vol. 913, pp 738-745 doi:10.4028/www.scientific.net/MSF.913.738 © 2018 Trans Tech Publications, Switzerland

Submitted: 2017-08-15 Revised: 2017-09-29 Accepted: 2017-12-04

Evaluation of Foaming and Nucleation and Growth Mechanism of Soy-Based Polyurethane Foams Xiao Zhang1,2,a, XinXing Zhou2,b 1

National Engineering Laboratory For Surface Transportation Weather Impacts Prevention, Broadvision Engineering Consultants, Kunming 650041, PR China

2

Key Laboratory of Highway Construction and Maintenance in Loess Region, Shanxi Transportation Research Institute, Taiyuan 030006, PR China a

[email protected], [email protected]

Keywords: Composites; Curing of polyurethane; Foaming; Nucleation and growth mechanism; Scanning electron microscopy

Abstract. The foaming and nucleation and growth mechanism of soybean oil-based polyurethane (SPU) were determined by the degree of hydrogen bonding, and isocyanate groups. New types of SPU were prepared by the different NCO/OH molar ratio (isocyanate index) from 1.0 to 2.0 in a soy polyol/polyether polyol (MDI) system. Foaming and nucleation and growth mechanisms of SPU were studied by fluorescence microscope (FM), scanning electron microscope (SEM), energy disperse spectroscopy (EDS) and Fourier transform infrared spectroscopy (FT-IR). It indicated that the isocyanate index affected remarkably the velocity of foaming and the critical nucleation radius of SPU and the ester functional group increased with the increase of isocyanate index. The nucleation and growth phase transition were dominated by the diffusion controlled nucleation and isocyanate content was the key factor of foam formation. Introduction Polyurethane (PU) foams are widely used in many engineering applications, such as slip casting materials, coatings, sealants, and structural glue [1]. As natural resources become exhausted, many researches and industries are beginning to investigate the various renewable resources. Previous studies [2] demonstrated the use of vegetable oil polyols to increase the renewable content of PU, such as soybean oil, seed oil, and castor oil [3-5]. Isocyanate and soy based polyols were used to prepare soybean oil-based polyurethane (SPU) that could compete in many aspects with PU derived from petrochemical polyols. Thermal stability and mechanical stability of the SPU are better than those of the conventional PU. Soybean oil is highly hydroxy that controls the cross linking rate of SPU. Soybean oil contains triglycerides, which are three-aim structures in which the arms can be either saturated or unsaturated fatty acids [6]. Foaming and nucleation and growth mechanism of SPU are determined by the degree of hydrogen bonding, isocyanate groups, and critical nucleation radius. Moreover, foaming and nucleation and growth mechanisms is closely related to the thermal and mechanical properties of SPU. Owing to this characteristic, SPU produced from soy-based polyols normally existed the improved physical and chemical properties, and soybean oil had been used to synthesize SPU in the foams. Several research efforts have been centered on fabricating elastomeric, adhesive and SPU thermosets. Liu et al [7] investigated that carbon nano-fibers reinforced SPU, and it showed that the isocyanate index changed the cross-linking density of SPU as well as the amount of dangling ends within the networks. Researches has been found that the cross-linking density of PU networks from such polyols and methane diisocyanate is not very high, rendering PU with moderate glass transition temperatures (Tg=50-70°C) [8]. Petrović [9] studied the effect of NCO/OH molar ratio on properties of SPU networks and concluded that tensile strength decreased and elongation increased with decreasing molar ratio of isocyanate and hydroxyl groups. The synthetic routes of vegetable oils synthesizing PU were investigated and it revealed that olefinic functionalities of triglycerides could

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easily be epoxidized, leading to reactive epoxide groups [10]. Zou et al [11] investigated the effect of hard segments on the thermal and mechanical properties of polyurethane foams. Hamilton et al [12] thought that the huge impact of foam shape on the module of the glass fiber and nano-composite foams. Micro-morphology or micro-structure of PU has been investigated and the influence of soft segment composition on segmented PU co-polymers. Raftopoulos et al [13] used molecular dynamics to investigate the polyurethane-poss hybrids and it concluded that a fraction of polymer was immobilized, making no contribution to the relaxation. Micro-structure of PU membranes were studied by Monte Carlo and molecular dynamics. The highly permeable PU membranes were investigated and it revealed that the highest diffusivity and permeability coefficients were observed for H2; while the highest values of solubility coefficient were found for H2O and CO2 gases [14]. There are several research about nucleation and foaming mechanism of PU. Ji et al [15] investigated the nucleation mechanism of thermosets PU foam and considered that the nucleation mechanism of thermosetting PU was air dispersion. Adding solid nucleating agents, dissolved gas and increasing stirring rate could promote nucleation in a large extent. Pielichowski [16] used the isoconversional methods of Ozawa-Flynn-Wall and activation energy to analyze the thermal degradation of PU. However, few literatures on micro-morphology of SPU foams during the curing process had been reported from multi-perspective. Furthermore, the exact foaming and nucleation and growth mechanism of SPU was still largely unclear. The main objective of this study is to investigate the foaming and nucleation and growth mechanism of SPU. In addition, the micro-morphology of SPU at different curing time and isocyanates index were studied by fluorescence microscope (FM); the foaming mechanism was investigated by scanning electron microscopy (SEM), energy disperse spectroscopy (EDS), and Fourier transform infrared spectroscopy (FT-IR). The velocity of foam formation and the critical nucleation radius were calculated in terms of micro-morphology and part thermodynamic parameters. Materials and Methods Materials. Soybean oil (SO), with OH#=305mg KOH/g and functionality 4.2, was bought from Lianyuangang City Jiaxiang Grain and Oil Trade Co., Ltd. Polyether polyol N303 and N204, with OH#=450±20 and 270±20, were bought from Wanhua Chemical Co., Ltd and the mass ratio was 1:1. All isocyanates (MDI), with 30.5%NCO, were supplied by Guangzhou Hongna Chemical and Technology Co., Ltd. Catalysts were triethanolamine and stannous octoate and they were supplied by Sinopharm Chemical Reagent Co., Ltd. 1,4-butanediol and silicone AK 8805 were selected as chain extender and foam stabilizer, respectively. Their compositions were shown in Table 1. MDI

Table 1 The compositions of SPU(g). SO NCO index Catalysts Butanediol

100 120 140 150

33.3 33.3 33.3 33.3

1.0 1.2 1.4 1.5

180 33.3 1.8 200 33.3 2.0 *The SO is represent the mass of soybean oil.

Silicone

0.3 0.3 0.3 0.3

2 2 2 2

0.2 0.2 0.2 0.2

0.3 0.3

2 2

0.2 0.2

Methods. SO was treated with hydrogen peroxide to form peroxyacetic acid and water. The samples were prepared by mixing a proper amount of polyol, isocyanates, catalyst, chain extender, and foam stabilizer, and then pouring in the cast iron mold. The mixture was cured at room temperature overnight. Fourier transform infrared spectroscopy (FT-IR) (Nicolet IS5) was performed to evaluate the functional group. A fluorescence microscope (FM) (XSP-63XV) was used to study the SPU

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morphology and the magnification ranging from 40× to 1000×. A scanning electron microscopy (SEM) (ZEISS EVO 10) with the accelerating voltages 15kV and the magnification ranging from 10× to 20,000× was also used to examine the internal morphology of SPU foams; and the SPU foams SEM samples of the different curing time were acquired using polymerization inhibitor to prevent the curing reaction. The foam areas and non-foam areas of SPU were measured by energy disperse spectroscopy (EDS) (Bruker) attached by SEM. Results and Discussions Foaming Mechanism. FM Analysis. As shown in Fig. 1, the fractured surface of pure PU and SPU foams were observed. In general, the average foam size of SPU were smaller than that of the pure PU foam. The decreased in the foam size of SPU as the isocyanate index increased when isocyanate index was less than 1.5. Furthermore, the increased in the foam size of SPU as the isocyanate index increased when the isocyanate index ranged from 1.5 to 2.0. It indicated that the isocyanate index could improve the foam formation capacity, uniform, and stability when the isocyanate index was less than 1.5, because the isocyanate index could affect the cross-linking density, mixture viscosity, and chemistry reaction ratio. Some larger foam size in SPU might be due to the excessive NCO groups when the isocyanate index ranged from 1.5 to 2.0. Furthermore, fractured surface of SPU were smoother than pure PU foam. (b)

(a)

(c)

2μm

2μm

(e)

(d)

2μm

2μm

(f)

2μm

2μm

Fig. 1 Fractured surface of pure PU and SPU at the magnification of 40×: (a)pure PU, (b)1.2 isocyanate index SPU, (c)1.4 isocyanate index SPU, (d)1.5 isocyanate index SPU, (e)1.8 isocyanate index SPU, and (f)2.0 isocyanate index SPU. In Fig. 2, it was clear that SPU foam size increased with increased curing time. The numbers of open foams decreased with increased curing time. Size and shape of the foams was uniform as the curing time increased at a certain level. SPU foams showed incomplete opening pore, while the closed pore was uniform. The SPU foams curing 8h and 16h had fewer open foam compared to the foams curing 1h, 2h, and 4h, which conformed to their higher modulus and compression strength. The morphology results matched well with mechanical properties results. The timing of events, i.e., phase separation and foam porosity, was significantly different, whereas the difference in the morphological changes was insignificant in terms of the number of closed foams. Moreover, the higher the curing time is, the more uniform the soybean oil will disperse.


(b)

(a)

(c)

2μm

2μm

2μm

(f)

(e)

(d)

741

2μm

2μm

2μm

Fig. 2 Fractured surface of SPU at the magnification of 40×: (a)curing 5min, (b)curing 1h, (c)curing 2h, (d)curing 4h, (e)curing 8h, and (f)curing 16h. SEM Analyses. Low-magnification (200×) SEM micro-graphs of fractured surfaces were taken to analyze the development of surface texture and foaming mechanism. As shown in Fig. 3, the surface texture and foam size increased with increased curing time. The thin closed pore (Fig. 3(a)) became the opening pore (Fig. 3(b)) and the opening pore became bigger and bigger (Fig. 3(c-d)) with the increased curing time and foam size. The foam size (Fig. 3(a)) ranged from 150μm-200μm, the foam size (Fig. 3(b)) ranged from 200μm-250μm, the foam size (Fig. 3(c)) ranged from 250μm-300μm, while the foam size (Fig. 3(d)) ranged from 300μm-350μm. The uniformity (size and shape) of SPU foams improved with increased curing time. This showed that the curing time had influenced on the morphology of foams. (a)

(b)

(c)

(d)

Fig. 3 SEM micro-graphs for SPU with different curing time: (a)curing 1h, (b)curing 2h, (c)curing 4h, and (d)curing 8h. Fractal Geometry Analyses. According to fractal geometry, the roughness of SPU fractured surface could be evaluated by the length of irregular curve and fractal dimension (D). In fractal

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geometry, the relationship between the length of irregular curve and radius of measured circle could be used to evaluate the roughness. The relationship between the length of irregular curve (L(r)) and fractal dimension can be described as follows, L(r ) = Ar − ( D −1)

(1)

where, A is the content, and 1-D is the ratio of logL(r)/logr. On the basis of Fig. 3, the length of contour line are 510μm(r=20μm), 560μm(r=20μm), 620μm(r=20μm), and 700μm(r=20μm), respectively; while the contour line are 540μm(r=10μm), 580μm(r=10μm), 660μm(r=10μm), and 750μm(r=10μm). The D calculated by Eq. (1) of SPU cured 1h, 2h, 4h, and 8h are 1.2, 1.5, 1.8, and 1.9, respectively. As we know, the bigger the D is, the more roughness the fractured surface is. It dedicated that the roughness of SPU increased as the curing time increased and the effects of the pre-curing of SPU on roughness was bigger than that of the curing late. EDS Analyses. In order to understand the foaming mechanism, the chemical element and material composition of thin foam and thick foam were investigated by EDS. As shown in Fig. 4, the N and O element content of thin foam was less than that of thick foam, while the C element content (66.65wt%) of thin foam was bigger than that of thick foam (69.56wt%). As we all known, the O element comes from isocyanates, polyether polyol, soybean oil and SPU, the N element was from isocyanates and SPU. The results demonstrated that the SPU and isocyanates content of thick foam were higher than that of thin foam, the thin foam edge was the foam formation center and isocyanates tended easily to thin foam edge. Moreover, isocyanates was the key of foam formation. (a)

cps/eV

spectrum 365

20 18 16 14 12 10

Au C

N Au

O

8 6 4 2 0

0

2

1

3 keV

4

5

cps/eV

(b)

spectrum 364

20 18 16 14 12 Au

10

C

N

Au

O

8 6 4 2 0

0

1

2

3 keV

4

5

Fig. 4 EDS of SPU: (a)EDS of thin foam, and (b)EDS of thick foam. FT-IR Analyses. As most of the inherent properties and foaming mechanisms of SPU are strongly influenced by the degree of hydrogen bonding and isocyanate groups[26]. As described elsewhere, the SPU with the hydroxyl groups enriched on their surface and improved their mechanical properties. So in general, the carbonyl region of FT-IR spectroscopy was sensitive the specificity of hydrogen-bonded urea and hydrogen-bonded urethane. The urea groups had stronger specific interaction and high stiffness, making them tend to more comparable micro-phase separation as compared to the urethane groups[28]. As shown in Fig. 5, the 3302cm-1 represents stretching


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vibration of hydrogen bonded N-H, which reveals that urea of urethane participates in the formation of SPU foam. The bands at 1732cm-1, 1719cm-1 and 1207cm-1 can be attributed to C=O(urethane), C=O(urea), and C-O-C of SPU linkages. In the curing 8h of SPU spectrum, no peaks was observed at 2270cm-1, which indicated that all -NCO functional groups had reacted completely and the peaks of 2270cm-1 decreased with increased curing time during the SPU forming process. The C=O(urethane) formation was faster than that of C=O(urea) during the foaming process. It indicated that the strength increased faster than during curing early period that of curing late and the driving force for the reduction in micro-phase separation might be related to changed in reaction kinetics. 200 C=O Urethane

Transmittance(%)

N-H 150

C=O Urea

curing 8h curing 2h curing 1h C-O-C

-NCO 100

50

4000

3500

3000

2500

2000

1500

1000

500

Wavenumber(cm-1)

Fig. 5 FT-IR of SPU during foaming process. Nucleation and Growth Mechanism. Nucleation and Foaming. According to thermodynamic phase transition theory, the foam formation of SPU belong to nucleation and growth phase transition. Nucleation are divided by homogeneous nucleation and heterogeneous nucleation. At the heterogeneous nucleation point of view, the velocity of foam formation (IS) are calculated by the follows, ∆Gh* ) I S = K S exp(− kT (2) * where, KS is the nucleation content, k is the Boltzmann content and ΔGh is the Gibbs free enthalpy. According to Fig. 3, the critical radius of foam is 2μm. ΔGh* is simplified as, 3 16πγ LX (2 + cos θ )(1 − cos θ ) 2 * ∆Gh = 3∆GV2 4 (3) where, ΔGV is the Gibbs free enthalpy at content pressure, γ is the interface energy and θ is the contact angle between liquid phase and solid phase. ΔGV are calculated by follows, ∆GV =

∆H V ∆T Tm

(4)

where, ΔGV is the enthalpy change of each volume, Tm is the melting point and ΔT is the degree of super-cooling. Combining Equation (2) to Equation (4), the velocity of foam formation (IS) of SPU with 1.0, 1.2, 1.5, 1.8, and 2.0 isocyanate index are 1.13×103s-1, 1.25×103s-1, 1.32×103s-1, 1.46×103s-1, and 1.48×103s-1, respectively. It indicated that SPU with 1.5 isocyanate index was the optimum velocity of foam formation. Conclusion The foaming and nucleation and growth mechanism of SPU were studied by FM, SEM, EDS, and FT-IR. Micro-morphology results show that the isocyanate index can improve the foam formation capacity, uniform, and stability when the isocyanate index is less than 1.5. Moreover, the influence of

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foaming time is great on foams morphology. FT-IR indicates that the strength increases fast and the driving force for the reduction in micro-phase separation may be related to changed in reaction kinetics. According to thermodynamic phase transition theory, SPU with 1.5 isocyanate index is the optimum velocity of foam formation. Acknowledgments The authors are supported by Science and Technology Department of Shanxi Province International Cooperation (NO. 201603D421027) and Opening Research fund of National Engineering Laboratory for Surface Transportation Weather Impacts Prevention (No.NELBP201604). Reference [1] A. Guo, I. Javni, Z. Petrović, Rigid polyurethane foams based on soybean oil, Journal of Applied Polymer Science. 77 (2000) 467-473.DOI: 10.1002/(SICI)1097-4628 (20000711) 77. [2] P. Tran, D. Graiver, R. Narayan, Ozone-mediated polyol synthesis from soybean oil, Journal of Oil & Fat Industries. 82 (2005) 653-659.DOI: 10.1007/s11746-005-1124-z. [3] J. John, M. Bhattacharya, R.B.Turner, Characterization of polyurethane foams from soybean oil, Journal of Applied Polymer Science. 86 (2002) 3097-3107.DOI: 10.1002/app.11322. [4] S.S. Narine, X. Kong, L. Bouzidi, et al., Physical properties of polyurethanes produced from polyols from seed oils: II. Foams, Journal of the American Oil Chemists Society. 84 (2006) 65-72.DOI: 10.1007/s11746-006-1006-4. [5] M. Malik, R. Kaur, Mechanical and thermal properties of castor oil-based polyurethane adhesive: effect of TiO2, filler, Advances in Polymer Technology. 35 (2016) 1082-1089.DOI: 10.1002/adv.21637. [6] T.W. Pechar, G.L. Wilkes, B. Zhou, et al., Characterization of soy-based polyurethane networks prepared with different diisocyanates and their blends with petroleum-based polyols, Journal of Applied Polymer Science. 106 (2007) 2350-2362.DOI: 10.1002/app.26569. [7] W. Liu, K. Xu, C. Wang , et al., Carbon nanofibers reinforced soy polyol-based polyurethane nanocomposites, Journal of Thermal Analysis & Calorimetry. 3 (2015) 1-10.DOI: 10.1007/s10973-015-4690-1. [8] Z.S. Petrović, A. Guo, W. Zhang, Structure and properties of polyurethanes based on halogenated and nonhalogenated soy-polyols, Journal of Polymer Science: Part A Polymer Chemistry. 38 (2000) 4062-4069.DOI: 10.1002/1099-0518(20001115)38. [9] Z.S. Petrović, W. Zhang, A. Zlatanić, et al., Effect of OH/NCO molar ratio on properties of soy-based polyurethane networks, Journal of Polymers & the Environment. 10 (2002) 5-12.DOI: 10.1023/A:1021009821007. [10] D. Myriam, E. Maxime, A. Remi, et al., From vegetable oils to polyurethanes: synthetic routes to polyols and main industrial products, Polymer Reviews. 52 (2012) 38-79.DOI: org/10.1080/15583724.2011.640443 . [11] J. Zou, Y. Chen, M. Liang, et al., Effect of hard segments on the thermal and mechanical properties of water blown semi-rigid polyurethane foams, Journal of Polymer Research. 22 (2015) 1-10.DOI: 10.1007/s10965-015-0770-y.


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