Chapter 12
Soy-Based Fillers for Thermoset Composites
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Paula Watt,*,1 Coleen Pugh,2 and Dwight Rust3 1The
Composites Group, PO Box 281, North Kingsville, Ohio 44068 of Polymer Science, University of Akron, Goodyear Polymer Center, Akron, Ohio 44325 3Omni Tech International, Ltd., 2715 Ashman Street Midland, Michigan 48640 *E-mail:
[email protected] 2Department
Weight reduction of composites is desirable for a number of applications. Thermoset molding compounds have historically utilized mineral fillers to reduce cost through displacement of the more expensive resin matrix. These fillers comprise a significant portion of the compound and have specific gravities of roughly 2.5 g/cc, increasing the density of the composites. Renewable biomass fillers, with a density of 1 g/cc or less, yield compounds at equivalent volume reinforcement with a 20-25% weight reduction. Thermal treatments of the biomass have been shown to improve the hydrophobicity of the fillers, but can exacerbate cure inhibition problems. Recent advances in the processing of soy biomass has yielded fillers that do not inhibit the thermoset cure reaction, allowing complete replacement of mineral fillers for compound densities as low as 1.4 g/cc and biobased carbon (BBC) levels of >40%. These compounds are projected to be near cost neutral to conventional compounds on a per volume basis.
Introduction The Opportunity In the Midwest United States there is a large concentration of polymer industry and agriculture that could work together for local sourcing of raw materials derived from biomass. Thermoset composites such as bulk molding © 2014 American Chemical Society In Soy-Based Chemicals and Materials; Brentin; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.
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compound (BMC) and sheet molding compound (SMC) utilize fillers to reduce cost by replacing the higher cost resins. Mined minerals such as calcium carbonate (CaCO3), clay, and talc have been used historically as fillers. Alternatively, biomass-based flours could be used advantagously because the green content is consistent with the USDA bio-preferred and bio-label programs, as well as other green initiatives such as LEED in construction. The lower specific gravity of these fillers (~1 vs 2.7 for CaCO3) also reduces the density of the molded components, which provides weight savings. Wood flour and other lignocellulosic flours are used commercially for thermoplastic compounds and have also been evaluated in thermosets. It follows that soy, which is abundantly available in the Midwest United States, could serve this function. The CaCO3 market is expected to reach 110 million metric tons by 2015. Roughly 30% of that, ~33 million metric tons, is used in polymer filler applications. The transportation industry uses over 136 thousand metric tons of SMC annually and values weight savings in their designs. In addition, the filler can be marketed to the much larger thermoplastics, coatings, and adhesives markets. An opportunity exists for a significant value added market for what would otherwise be a lower value by-product, or waste, from soy oil and meal processing. Background The main concern with using biomass fillers is their high water absorption (1, 2). Untreated biomass absorbs over an order of magnitude more water than CaCO3 in a composite. According to findings by Marcovich et al. this results in loss of modulus and strength (3). The source of the biomass hydrophilic nature is no mystery. Carbohydrates, and proteins or lignin to a lesser extent, have many hydroxyls available for hydrogen bonding with water as seen in Scheme 1.
Scheme 1. Various Carbohydrate Species in Biomass a) saccharides, b) oligosaccharides, c) cellulose, d) hemicelluloses. It follows that reduction of hydroxyl functionality should result in reduced water affinity. A number of surface treatments to improve water resistance of soy flours were explored under a United Soy Board (USB) grant by the National 266 In Soy-Based Chemicals and Materials; Brentin; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.
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Composites Center. In this work Schultz and Mehta evaluated alkali treatement, wax emulsion coating and protein refinement as means to reduce soy meal water affinity (4). Other treatments for imparting hydrophobicity to natural fillers have been reported in the literature. Chemical modifications reported include alkali treatments, organosilane grafting and transesterifications among others (5–10). Heat treatments, ranging from the denaturing of proteins to carbonization, have also been studied (11–13). Lignotech®’s steam explosion process is used to convert dried distillery grains (DDGs) to a less hydrophilic filler (14). Laurel BioComposite, LLC utilizes a proprietary process to convert DDGs to a filler for thermoplastics (15). Likewise, Biobent Polymers offer a line of thermoplastics with soy meal filler (16). New Polymer Systems (NPS) Neroplast® products are made by torrefaction (French word for roasting), which is basically an anaerobic heat treatment at temperatures that drive off volatiles and selectively decompose less stable constituents (17). This process is used to increase the energy density of wood for use as fuel. It is generally reported in treatment of wood that the major effects are dehydration and elimination of the more hydrophilic components (18, 19). In previously reported work, Watt prepared soy flours treated with soy oil, acetylated and torrefacted at 225 °C (20).
Experimental Procedures Scope This Chapter documents the work done under the United Soy Board (USB) contract #1340-512-5275. Work concluded at the end of USB contract #2456 is first presented as the impetus for the commercialization direction of the torrefaction treatment for soy fillers as a means to reduce water absorption. Next a variety of processes and equipment for torrefaction of soy meal and hulls were evaluated and the resulting fillers were characterized. Experimental procedures and results pertaining to soy hull samples are reproduced or adapted from a previous publication by Watt and Pugh (21). In this Chapter, additional content on meal precursor samples is also reviewed. Finally, a strategy for processing fillers capable of 100% replacement in thermosets is tested in molding compounds. Materials The untreated soy flour (UTSF) was Honeysoy® 90 PDI defatted soya flour provided by CHS Inc. This grade is a high solubility, enzyme active 100% soy flour with minimal heat treatment. The minimum protein specification for this grade is 48%, and the total carbohydrates is 44% with 19% dietary fiber. The flour is granulated to pass a minimum of 95% through a 200 mesh alpine sieve. This translates to less than 75 micron diameter particles. The untreated soy meal (UTSM) was Bunge solvent extracted soy meal purchased from Rome Feed Inc. The minimum protein specification is 47%, crude fiber is not more than 3.5%, and crude fat is not less than 0.5%. The untreated soy hulls (UTSH) were Bunge soybean hulls with a minimum protein level of 9%, ≥ 0.5% crude fat, and ≤ 38% crude fiber. This granular material has a bulk density of 0.37 g/cc. Kraft lignin 267 In Soy-Based Chemicals and Materials; Brentin; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.
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powder was procured from Sigma-Aldrich. The weight average molecular weight for the sample was Mw = 1.0 x 104 g/mol. Chemically treated soy fillers were supplied by the Pugh Research Group in the Polymer Science Department at The University of Akron as described by Watt (20). For the resin casting water absorption experiments, thermoset resin AOC S903, a dicyclopentadiene propylene glycol maleic anhydride-based polyester dissolved in 30% styrene, was initiated with Noury F-85, 40% methyl ethyl ketone peroxide and Shepherd cobalt octoate, a 12% cobalt in mineral spirits accelerator. For BMC screening experiments, maleated acrylated epoxidized soy oil in 30% styrene (MAESO® resin) from Dixie Chemical was used with Premix R-158, a proprietary low profile additive (LPA) comprised of a thermoplastic dissolved in 30% styrene. The curative was Trigonox C, tert-butylperoxy benzoate. Norac Coad 27P zinc stearate (ZnSt) was the mold release; Omya 5, 5 micron calcium carbonate (CaCO3), was the filler; and 1/8" chopped PPG 3075 was used for reinforcement. For the remaining BMC and SMC compounds, AOC S903 and Premix R-158 were again employed. Additional styrene from Total Petrochemical was included for viscosity reduction. The cure package included Trigonox BPIC-C75 (tert-butyl peroxy isopropyl carbonate peroxide) from Akzo-Nobel, Chromoflo’s IN-91029 inhibitor (a solution of 2,6-di-tertbutyl-p-cresol in vinyl toluene) and Chempak’s POWER BLOC 12.5PC (a 12.5% solution of parabenzoquinone). Chromoflo’s black CF-20737 pigment concentrate was used as well as their AM 9033 magnesium oxide thickener slurry. Norac Coad 27P zinc stearate (ZnSt) and Norac Coad 10C calcium stearate (CaSt) were used for mold release. For the control samples, the mineral filler used was BASF ASP200 clay with a particle size where 85% passes through a 325 mesh (
