Proceeding of the SAMPE Conference, Long Beach, CA, pp. 1-10, May 1-4, 2006 DEVELOPMENT AND CHARACTERIZATION OF SOY-BASED EPOXY FOAMS

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1 Proceeding of the SAMPE Conference, Long Beach, CA, pp. 1-10, May 1-4, 2006 DEVELOPMENT AND CHARACTERIZATION OF SOY-BASED EPOXY FOAMS R.Yadav, A. Shabeer, S. Sundararaman, K. Chandrashekhara*, V. Flanigan and S. Kapila Center for Environmental Science and Technology University of Missouri - Rolla, Rolla, MO ABSTRACT Epoxy foams have been developed using soy-based resin system synthesized at University of Missouri Rolla. The soy-based resin system was developed by the process of transesterfication and epoxidation of soybean oil. A physical blowing agent was used to foam the soy-based resin. The performance of foams was evaluated by testing for compression in both parallel and perpendicular to rise direction. The tests were carried out at room temperature and at the elevated temperature. The compression properties showed a decreasing trend for increasing amounts of soy resin. The density and the thermal properties of the epoxy foams were also evaluated. The relation between the composition, density and properties of the foam was analyzed. Soy-based epoxy foams are potential low cost and environmentally benign materials for structural applications. KEY WORDS: Epoxidized allyl soyate, Soy resin, Soy foam. *Author for Correspondence: K. Chandrashekhara, Professor, Department of Mechanical and Aerospace Engineering, University of Missouri-Rolla, Rolla, MO E mail: chandra@umr.edu, Phone: ; Fax:

2 1. INTRODUCTION The cellular structure of foamed plastics imparts them a lower density, lower heat conductivity, superior sound absorption, and, in some cases better resilience than ordinary plastics [1]. In the past 20 years foams have been used increasingly where regular plastics cannot satisfactorily meet requirements. Rigid polyurethane foam has many applications as a light weight structural material. Unfortunately, polyurethanes are based on isocyanates, and continuous exposure even to very low concentrations of these materials poses a serious health hazard [2]. In addition to health hazard problems during foam manufacture, the highly toxic nature of foam pyrolysis products must be considered if the foam is likely to be exposed to a fire risk situation during service. Among the newer materials, structural thermoset foams have caught attention, because they combine good mechanical resistance with a low weight [3]. In particular, epoxy foams have been recently developed due to their high thermal and chemical stability. Fiber-reinforced epoxy resin foams represent a new group of materials that are now industrially used. Epoxy foams are used in the production surfboards, automobiles and railway carriages [4]. Epoxy foams has been established to be well suited for lightweight electronic encapsulation because of its long pot life, excellent strength, low thermal conductivity, and its ability to be blown to a relatively low density [5]. Impact and dynamic mechanical properties of natural fiberreinforced plastics have also been reported [6, 7]. The processing and final physical properties of combinations of epoxy and curing agent depend primarily on their chemical composition and degree of cure. Gas blown foams are formed when a gas is released in the system during cure. The gas can either be formed by the curing reaction itself, or from a compound present in the formulation. The technology of gas-blown epoxy foams is very similar to that of polyurethane foams. The main difference between the two is in the chemistry of the cure. Epoxies normally react with curing agent without the evolution of volatiles, and thus require addition of a blowing agent. The isocyanates can, by the addition of water to the system, react to evolve carbon dioxide, which as blowing agent for the foams. In recent years polymeric materials prepared from natural and renewable resources such as triglyceride vegetable oils are finding numerous applications. Among the triglyceride oils, soybean attracts great interest because of plentiful supply in the United States, low cost and the biodegradability [8-11]. Soybean oil contains 85% unsaturated oleic, linoleic and linolenic fatty acids. This high degree of unsaturation makes it possible to polymerize it into useful materials. Several researchers have attempted to use epoxidized soybean oil (ESO) for polymeric and composite applications [12-14]. However, due to its low reactivity and its tendency to intramolecular bonding ESO normally has a low cross-linking density and therefore limited thermal and mechanical properties. Most ESO industrial applications are still limited to nonstructural applications as coating and PVC additives with low strength requirements. In our previous study, a soy based epoxy resin system, Epoxidized Allyl Soyate (EAS), was synthesized by the addition of Epoxidized Soybean Oil to the base Shell Epon resin [15-17]. Soy based epoxy foam was developed by foaming the epoxidized allyl soyate using an appropriate blowing agent and simultaneously curing it with a hardener. The resulting foam will be cheaper than commercial epoxy foam and will have better thermal properties. In this present

3 study, the effect of various concentrations of EAS on the properties of soy-based epoxy foams. The results obtained were compared with that of base Epon resin foams. 2. EXPERIMENT 2.1 Materials Shell Epon 9500 was the base epoxy resin used in this study. Soy-based resin was synthesized using a two-step laboratory scale process from regular food grade soybean oil. At first the triglycerides molecules in the soybean oil were transesterified with allyl alcohol to yield fatty acid allyl esters. Next the fatty acid esters were epoxidized to yield soyate epoxy resin. The blends were prepared by mixing the soy epoxy resin with the base Epon resin. The liquid anhydride, Lindride 56V, used as the curing agent was obtained from Lindau Chemicals (Columbia, SC). Surfactant Dabco 193 was obtained from Air Products (Allentown, PA). Physical blowing agent, decafluoropentane was obtained from Dow Chemical Company (Midland, Michigan). 2.2 Foam Preparation Foams with different formulations were fabricated. EAS content was varied from 0% to 50% with Epon resin. Lindride 56V was used as a hardener with the resin:hardener ratio of 1: % surfactant was added to mix. Surfactant lowers the surface tension of the reactive mix, thus, reducing the free energy associated with dispoersion of the blowing agent vapor. Finally, the blowing agent, decafluoropentane was added in the proportion of 20% by total weight of the resin and hardener. The soy foam folrmuation for the present work is shown in Table 1.The resin and hardener were weighed and mixed together by a high shear mixer for 4-5 minutes, operating at approximately 5000 rpm. Then the surafactant was added and mixture was again stirred for 2-3 minutes. Finally, the blowing agent was added, half in the mix and other half was poured into the mold for effective blowing. The mix, after mixing for seconds, was poured in the mold. Finally the mold sealed and placed into a closed air-tight container. This container had very low headspace to prevent escaping of the blowing agent. Finally, this container was placed into the oven. Curing temperature was based on cure characteristics of EAS resin. Hence, an optimized temperature of 150 C and time of 2 hours was used. Figure 1 shows the developed epoxy foams. 2.3 Density The density of the foams was determined according to the ASTM method D1622. Density measurements were done for five rectangular samples for each formulation. The test specimens were weighed on a balance. The dimensions of the specimens were determined by using a caliper. Finally, the density was calculated by dividing the weight of the sample by volume of the sample. 2.4 Compression Test Compression test was performed in accordance with ASTM D1621 using an Instron Universal Testing Machine (Model 1120). Specimens, 30 mm square by 25.4 mm thick were compressed between two stainless steel plates, and load was applied with a crosshead speed of 1.5 mm/min. Compressive modulus was calculated as the slope of the linear portion of the compression stress-strain curve. The values of the compression yield stress reported refer to the intrinsic property of the polymer measured as the maximum of the stress-strain curve, which is related to the start of yielding of the material. The results reported here are the averages of five specimens. Compression tests were carried out in both the parallel and perpendicular to rise

4 directions. Also, high temperature testing of epoxy foams was done at 150 F to determine the performance of foams at high temperature on the properties. 2.5 Thermal Property Thermal conductivity studies were carried out in accordance with ASTM D5930 by using a Quickline -30 Thermal Properties Analyzer. A needle probe was used to measure the thermal conductivity, thermal diffusivity and volume heat capacity of the specimens. 3. RESULTS AND DISCUSSIONS The mechanical, thermal, electrical and chemical properties of foam are dictated by both its chemical composition and its physical structure. The functionality and chain length of the epoxy resin and curing agents used determine the crosslink density of the foam, and hence influence its rigidity, strength and temperature resistance. The aromaticity of the resins also affects foam mechanical properties. However, perhaps the most important factors influencing foam compressive strength and temperature resistance are the density and cell structure. 3.1 Density All the mechanical properties depend on density. The density of the foam can be adjusted within broad limits. It is determined by the amount of blowing agent and the volume fraction of the unexpanded resin. In this case, some part of the EAS resin remains unexpanded, thus leading to a heavier foam, hence, higher density with increasing EAS content. Figure 2 shows the variation of density with EAS content. 3.2 Compression Test The compressive strength and modulus for the soy foams, measured in a direction parallel to the foam rise, are summarized in Table 2.The strength and modulus decreased with the soy resin concentration at room temperature and at elevated temperature. The compression modulus and strength are higher than pure epoxy foams. This is attributed to increased crosslink density. The pure epoxy foam has highest crosslink density, which produces higher strength and improves modulus. Crosslink density is frequently represented in terms of molecular weight between crosslinks (M c ), where higher crosslink densities correspond to lower M c values. The pure epoxy foam has the lowest M c and thus the highest crosslink density. This explains why the compressive strength and modulus of pure epoxy foam are higher than soy foams. The compressive strength and modulus for the soy foams, measured in a direction perpendicular to the foam rise, are summarized in Table Thermal Property Figure 3 shows the variations of thermal conductivity for the soy based foam. The thermal conductivity is directly related to the exotherm of the reaction taking place. The curing reactions are exothermic, and the rate of reaction increases with temperature. The heat formed by the exothermic reaction can lead to a considerable rise in temperature of the system. The heat evolved in the center of the foam cannot readily escape, owing to the low thermal conductivity of the resin and the gaseous interior of the cell structure. Figure 4 shows the the variations of specific heat capacity for the soy based foam. Figure 5 shows the the variations of thermal diffusivity for the soy based foam.

5 4. CONCLUSIONS Soy foams were synthesized using decafluoropentane as the blowing agent. The density of soy foam increased with increasing soy resin content but it had a negative effect on the compression properties. The higher density can be explained by the non-expanding part of the resin which gives the foam additional weight but does not add to the strength because of the long-chain flexible nature of EAS. Even though EAS has lower properties than the Epon resin, its properties are much higher as compared to polyurethane foams. EAS thus provides low cost, hazard-free foam which has better mechanical and thermal properties than the conventional polyurethane foam. 5. ACKNOWLEDGEMENTS This project is sponsored by National Science Foundation (Grant # NSF CMS ).Partial support from the Missouri Soybean Merchandising Council and the University Transportation Center is gratefully acknowledged. 6. REFERENCES 1. D. Klempner, and K. Frisch Handbook of Polymeric Foams and Foam Technology, Hanser Verlag, München, M. Hajimichael, A. Lewis, D. Scholey, and C. Simmonds, Investigation and development of epoxy foams, British Polymer Journal, Vol. 18, pp , P. Stefani, A. Barchi, J. Sabugal, and A. Vazquez, Characterization of epoxy resin foams, Journal of Applied Polymer Science, Vol. 90, , A. Bledzki, K. Kurek, and J. Gassan, The influence of micropores on the dynamicmechanical properties on reinforced epoxy foams, Journal of Materials Science, Vol. 33, pp , H. Mclroy, and C. Smith, Characterization of an Epoxy foam, Society of Plastics Engineering - Annual Technical Conference, Vol. 28, pp , A. Bledzki, J. Gassan, and W. Zhang, Impact Properties of Natural Fiber-Reinforced Epoxy Foams, Journal of Cellular Plastics, Vol. 35, pp , A. Bledzki, and W.Zhang, Dynamic Mechanical Properties of Natural Fiber-Reinforced Epoxy Foams, Journal of Reinforced Plastics and Composites, Vol. 20, pp , G.S. Kumar, Biodegradable Polymers: Prospects and Progress, Marcel Dekker, New York, D.L.Kaplan, Biopolymers from Renewable Resources, Springer, New York, J. Rosch, and R. Mulhaupt, Polymers from Renewable Resources: Polyester Resins and Blends Based Upon Anhydride-cured Epoxidized Soybean Oil, Polymer Bulletin, Vol 31, pp , J.Ivan, and Z.Petrovic, Annual Technical Conference of Society of Plastic Engineering, Vol 55, pp , 1997.

6 12. J.V. Crivello, R. Narayan, and S.S. Sternstein, Fabrication and Mechanical Characterization of Glass Fiber Reinforced UV-cured Composites from Epoxidized Vegetable Oils, Journal of Applied Polymer Science, Vol. 64, pp , G.I. Williams, and R.P. Wool, Composite from Natural Fibers and Soy Oil Resins, Applied Composite Materials, Vol. 7, pp , R.B. Lu, and O.J. Stoffer, Composite Prepared from Epoxidized Soybean Oil, Polymer Preprints, Vol. 41, pp , J. Zhu, K. Chandrashekhara, V. Flanigan and S. Kapila, Manufacturing and Mechanical Properties of Soy-based Composites Using Pultrusion, Composites Part A: Applied Science and Manufacturing, Vol. 35, pp , G. Liang, A.Garg, J. Zhu, K. Chandrashekhara, V. Flanigan and S. Kapila, Cure Characterization of Pultruded Soy-based Composites, Proceedings of the SAMPE Conference, pp , Long Beach, CA, May 11-15, A. Shabeer, A. Garg, S. Sundararaman, K. Chandrashekhara, V. Flanigan, and S. Kapila, Dynamic Mechanical Characterization of a Soy-based Epoxy Resin System, Journal of Applied Polymer Science, Vol. 98, pp , 2005.

7 Table 1 Soy foam formulations Sl. No. Epon 9500 (gm) EAS (gm) LS 56 (gm) Dabco 193 (gm) Blowing agent (gm) Table 2 Compressive modulus and strength parallel to rise Parallel to Rise Modulus (kpa) Strength (kpa) Sample Room Temp Elevated Temp Room Temp Elevated Temp 0% EAS % EAS % EAS

8 Table 3 Compressive modulus and strength perpendicular to rise Perpendicular to Rise Modulus (kpa) Strength (kpa) Sample Room Temp Elevated Temp Room Temp Elevated Temp 0% EAS % EAS % EAS Figure 1 Soy foam developed at UMR

9 Density (Kg/m 3 ) Soy resin content (%wt) Figure 2 Variation of density with soy resin concentration Thermal conductivity (W/mK) Soy resin content (%wt) Figure 3 Variation of thermal conductivity of soy foam with soy resin concentration

10 Specific heat (J/m 3 K) Soy resin content (%wt) Figure 4 Variation of specific heat capacity of soy foam with soy resin concentration Thermal diffusivity (m 2 /s) Soy resin content (%wt) Figure 5 Variation of thermal diffusivity of soy foam with soy resin concentration