Novel Biobased Plastics, Rubbers, Composites, Coatings and

Downloaded by UNIV OF MISSOURI COLUMBIA on August 20, 2013 | http://pubs.acs.org Publication Date (Web): August 11, 2010 | doi: 10.1021/bk-2010-1043.ch007

Chapter 7

Novel Biobased Plastics, Rubbers, Composites, Coatings and Adhesives from Agricultural Oils and By-Products Yongshang Lu and Richard C. Larock* Department of Chemistry, Iowa State University, Ames, IA 50011 *[email protected]

A remarkable range of exciting new plastics and rubbers have been made by the cationic, thermal, free radical and ring-opening metathesis polymerization of regular and modified vegetable oils with a number of readily available, commercial comonomers, including styrene, divinylbenzene, acrylonitrile and dicyclopentadiene. The bioplastics and rubbers possess excellent thermal and mechanical properties, plus outstanding damping and shape memory properties. Fillers, such as glass fibers, organic clays, and agricultural co-products, such as soybean hulls and spent germ, have been used to reinforce these vegetable oil polymer resins, resulting in biocomposites with significant improvement in their mechanical properties and thermal stability. A number of novel new vegetable oil-based waterborne polyurethane dispersions and polyurethane/ acrylics hybrid latexes have also been prepared and show promising applications as decorative/protective coatings and pressure sensitive adhesives.

Introduction The utilization of fossil fuels for the manufacture of plastics accounts for about 7% of the worldwide use of oil and gas, which will arguably be depleted within the next one hundred years (1). Therefore, a change from fossil feedstocks to renewable resources is important for sustainable development into the future (2). The utilization of renewable resources can consistently provide raw materials for © 2010 American Chemical Society In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


every day products, effectively avoiding further contribution to greenhouse gas effects, because of the minimalization of CO2 emissions (3). The renewable raw materials most widely used to replace petroleum are polysaccharides (mainly cellulose and starch), proteins, sugars, natural rubbers and plant oils (4, 5). Among these, vegetable oils are considered to be the most promising materials for the chemical and polymer industries, due to their superb environmental credentials, including their inherent biodegradability, low toxicity, avoidance of volatile organic chemicals, easy availability, and relatively low price (6). Vegetable oils have been used as an ingredient or component in many manufactured products, such as soaps, drying agents, paints, coatings, insulators, hydraulic fluids and lubricants. In recent years, the amounts of vegetable oils and fats produced have increased by approximately 4% per year, one third of which results from the growing industrial use of vegetable oils. Vegetable oils possess a triglyceride structure linked with different fatty acids as shown in Scheme 1. There are several positions that are amenable to chemical reactions: ester groups, C=C double bonds, allylic positions and the α-position of the ester groups. These functional groups can be used to directly polymerize triglycerides or to modify the triglyceride structure with more readily polymerizable groups to obtain thermosets (7). Recently, a variety of new polymeric materials have been prepared from vegetable oils and derivatives, which possess industrially viable thermophysical and mechanical properties and thus may find many applications. One of the major efforts in this field has taken advantage of the carbon-carbon double bonds of the vegetable oils themselves for cationic polymerization (8) or the carbon-carbon double bonds of vegetable oil derivatives for free radical polymerization (9) or olefin metathesis polymerization (10). Other major processess have involved the conversion of vegetable oils or fats into epoxidized vegetable oils (EVO) (11) or polyols (12). The EVO can be cationically polymerized by latent thermal catalysts (11) or cured by amines (13) or anhydrides (14) to produce thermosetting epoxy resins, whereas the polyols can react with diisocyanates to produce vegetable oil-based polyurethane foams, thermosets (15, 16) or waterborne polyurethane dispersions (17, 18). In this review, we highlight the most recent advances made in novel polymers, biocomposites and nanocomposites based on vegetable oils, which have been subjected to cationic, free radical, olefin metathesis and step growth polymerizations.

Vegetable Oil-Based Bioplastics and Biocomposites from Cationic Polymerization The carbon-carbon double bonds in vegetable oils are slightly more nucleophilic than those of ethylene and propylene and are susceptible to cationic polymerization (19). However, compared with ethylene, propylene and isobutylene, vegetable oils are a multifunctional monomer because of the multiple carbon-carbon double bonds in the triglycerides and the branching of the monomer, which results in crosslinked polymers with high molecular weights. 88 In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Scheme 1. The typical structure of vegetable oils Protic acids and Lewis acids, such as TiCl4, ZnCl2 and BF3·OEt2 (BFE), are capable of initiating the cationic polymerization of vegetable oils under mild conditions (20). Among the Lewis acids, however, BFE has proved to be the most efficient initiator. Except for tung oil (21) and conjugated linseed oil, cationic homopolymerization of regular vegetable oils or the corresponding conjugated oils initiated by BFE affords only low molecular weight viscous oils or soft rubbery materials consisting of solid polymers and liquid oligomers in most cases. These materials generally possess limited utility. Therefore, alkene comonomers, such as styrene (ST), divinylbenzene (DVB), norbornadiene (NBD) and dicyclopentadiene (DCP), have usually been copolymerized with the vegetable oils (22). The cationic polymerization of a vegetable oil with ST and DVB is illustrated in Scheme 2. The cationic copolymerization of 50-60 wt % of various soybean oils [regular soybean oil (SOY), LowSat soybean oil (LLS) and conjugated LLS (CLS)] with DVB (25-35 wt %) initiated by BFE (1-5 wt %) provides thermosetting polymers ranging from soft rubbers to hard plastics, depending on the reagents, stoichiometry and initiators used (21). The room temperature moduli of these thermosets are approximately 400-1000 MPa, which are comparable to those of conventional plastics. However, due to the poor miscibility between the soybean oils and the initiator and the big difference in the reactivity of the soybean oils and DVB, heterogeneous reactions with a lot of solid white particles occur at the early stages of this copolymerization, leading to phase separated copolymers consisting of oil-rich phases and DVB-rich phases (22). It has been found that homogeneous copolymerization of the various soybean oils with DVB can be achieved by using ST as the main comonomer, instead of DVB, or using an initiator of BFE modified with Norway fish oil ethyl ester (NFO), since NFO is completely miscible with vegetable oils and DVB. The resulting soybean oil (SOY, LLS and CLS)-ST-DVB bulk polymers, consisting of ~ 45-50 wt % of the vegetable oil as a starting material are typically opaque materials with a glossy dark brown color, and have Tgs ranging from approximately 0 to 105 °C. The soybean oil-ST-DVB thermosets exhibit tensile stress-strain behavior ranging from soft rubbers through ductile to relatively brittle plastics (23). The Young’s moduli of these polymers vary from 3 to 615 MPa, the ultimate tensile strengths vary from 0.3 to 21 MPa, and the elongation at break vary from 1.6 to 300%, depending on the stoichiometry and the soybean oil employed. Material damping is one of the most effective solutions to the problem of vibration and noise. Viscoelastic polymers useful as damping materials have attracted considerate interest in recent years because of their high damping values around the glass transition temperature (Tg) (24). Soybean oil-ST-DVB polymers with appropriate compositions exhibit good damping properties over a broad temperature and frequency range with a loss factor maximum (tan δ)max of 0.8-4.3, an overall damping capacity value of 50-124 K after correcting the background, 89 In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Scheme 2. Cationic copolymerization of soybean oils with ST and DVB. and an 80-110 degree temperature range for high damping (tan δ > 0.3) (25). The high damping intensities observed in these materials can be attributed to the large number of ester groups present in the triglycerides directly attached to the polymer chains, whereas the broad damping region is due to the segmental inhomogeneities upon crosslinking. In addition to good damping properties, these soybean oil-ST-DVB polymers can also be tailored to show good shape memory properties (26). It is found that a Tg well above ambient temperature and a stable crosslinked network are two prerequisites for these polymers to exhibit good shape memory effects. Through structural design of the polymer chain rigidity, the resuting soybean oil-based polymers possess excellent processibility in the elastomeric state, being able to fix over 97% of their deformation at room temperature, and completely recover their original shape upon being reheated, making these materials particularly promising in applications where shape memory properties are desirable. In addition to ST and DVB, dicyclopentadiene (DCP) has also been used as a comonomer for cationic copolymerization with SOY or conjugated soybean oil (CSOY) initiated by BFE, resulting in a variety of novel thermosetting polymers ranging from tough and ductile to very soft rubbers (27). The SOY-DCP and CSOY-DCP bulk copolymers have Tgs ranging from -22.6 to 56.6 °C and are thermally stable below 200 °C. New silicon-containing soybean-oil-based copolymers have been prepared from SOY, ST, DVB, and p-(trimethylsilyl)styrene by cationic polymerization using BFE as the initiator. The resulting thermosets exhibit glass transition temperatures ranging from 50 to 62 °C and limiting oxygen index (LOI) values from 22.6 to 29.7, suggesting that these materials may prove to be useful alternatives for current non-renewable-based flame-retardant materials (28). Using soybean/corn oil-based resins as the matrix, a series of novel high performance biocomposites have been prepared by reinforcing the resins with 90 In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


glass fibers (29, 30). With increasing glass fiber content from 0 to 52 wt%, the composites exhibit a significant increase in Young’s modulus from 150 to 2730 MPa and ultimate tensile strength from 7.9 to 76 MPa. Increasing the crosslinking (e.g. DVB) in the vegetable oil-based matrix results in composites with improved thermal and mechanical properties. When compared with biocomposites from a corn oil-based resin reinforced with glass fiber, the biocomposites based on the soybean oil resin possess higher thermal and mechanical properties (30), which is attributed to the higher crosslinking present in the soybean oil resin, due to the higher unsaturation of the soybean oil. A montmorillonite clay modified with triethyl(4-vinylbenzyl)ammonium chloride (VTAC), abbreviated VMMT, has been used to reinforce corn (31) and soybean oil-based cationically polymerized resins (32), where the polymerizable vinyl groups of the VTAC can be incorporated into the polymer resin through chemical bonding. It has been found that a heterogeneous structure consisting of intercalation and partial exfoliation or an intercalation structure occurs in the resulting materials, depending on the amount of VMMT. The resulting nanocomposites with 1-2 wt % VMMT exhibit a significant improvement in their thermal stability, mechanical properties, and vapor barrier performance. For example, the modulus, strength and strain at failure for the CLS-based nanocomposites increase by 100-128%, 86-92% and 5-7%, respectively, when the VMMT loading is in the range of 1-2 wt % (32).

Vegetable Oil-Based Bioplastics and Biocomposites by Free Radical Polymerization The carbon-carbon double bonds in the vegetable oils are capable of being polymerized through a free radical mechanism. However, due to the presence of chain transfer processes occurring at the allylic positions of the fatty acid chains, the free radical polymerization of triglyceride double bonds has received relatively little attention. The drying oils, such as tung oil, can react with atmospheric oxygen to form polymeric materials with a network structure. The oxidation of drying oils by air involves hydrogen abstraction from a methylene group between two double bonds in a polyunsaturated fatty acid chain (33–35), which leads to peroxidation, perepoxidation, hydroperoxidation, epoxidation, and crosslinking via radical recombination. Using this method, a variety of grafted copolymers with higher biodegradability and biocompatibility have been synthesized by the free radical polymerization of methyl methacrylate (MMA) or n-butyl methacrylate (nBMA) initiated by using polymeric peroxides prepared by the autooxidation of linseed oil (LIN) (33), soybean oil (SOY) (34), and linoleic acid (LIA) (35). When heated above 100 °C, ST undergoes thermal polymerization, which involves the formation of a Diels-Alder dimer from two ST molecules and subsequent hydrogen transfer to styrene to yield two radicals that can initiate polymerization of a drying oil (36). Tung oil-based (30-70 wt %) thermosetting polymers have been synthesized by the thermal copolymerization of tung oil, ST and DVB (37). These fully cured thermosets, ranging from elastomers to tough and rigid plastics, possess Tgs of -2 to +116 °C, crosslink densities of 1.0 × 103 91 In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


to 2.5 × 104 mol/m3, coefficients of linear thermal expansion of 2.3 × 10-4 to 4.4 × 10-4/ °C, compressive moduli of 0.02-1.12 GPa, and compressive strengths of 8-144 MPa. A series of samples with similar properties have also been prepared from conjugated linseed oil (CLIN), ST and DVB (38). Conjugated carbon-carbon double bonds in vegetable oils are relatively easily attacked by free radicals (8). Much more viscous polymerized conjugated oils can be obtained by free radical polymerization of conjugated linseed oil (CLIN) or CLS initiated by either benzoyl peroxide (BPO) or tert-butyl hydroperoxide (TBHP) or combinations of these reagents. Some comonomers, such as DVB, acrylonitrile (AN) and DCP, have been successfully copolymerized with CLIN and CLS to obtain thermosetting polymers with good thermal and mechanical properties ranging from hard and brittle plastics to soft and rubbery materials (39, 40). For example, for the CLIN-AN-DVB thermosets, approximately 61-96 wt % of the CLIN has been incorporated into the final products. The Tgs determined from the tan δ peaks determined by DMA analysis range from 60 to 101 °C and the room temperature storage moduli vary from 160 MPa to 1.6 GPa, depending on the oil content in the formulation. These thermosets are thermally stable up to 300 °C. The wide range of properties attained with these materials makes them suitable for potential applications in which petroleum-based polymers are currently used (39). Novel biocomposites have been prepared by the free radical polymerization of a tung oil-based resin using spent germ (SG), the co-product of wet mill ethanol production, as a filler (41). The composites produced are quite thermally stable with Tmax values in the vicinity of 430 °C. In general, the thermal and mechanical properties of the composites are improved by decreasing the size of the filler. This is most likely the result of enhanced interfacial adhesion and filler-matrix interaction. As more SG is added to the composite, the mechanical properties tend to decrease, due to filler-filler agglomerations and an increase in voids expected when the amount of filler is increased. DVB is used as an effective crosslinker and, as expected, the thermal and mechanical properties of the composites increase as the concentration of DVB in the matrix increases. In addition to tung oil-based biocomposites, biocomposites have been prepared by the free radical polymerization of a CSOY-based resin reinforced with soybean hulls (42). The resin consists initially of 50 wt % CSOY and varying amounts of DVB (5-15 wt %), DCP (0-10 wt %), and nBMA (25-35 wt %). Two soybean hull particle sizes have been tested (

Novel Biobased Plastics, Rubbers, Composites, Coatings and

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