BRENT TISSERAT, LOUIS REIFSCHNEIDER, DAVID GREWELL, GOWRISHANKER SRINIVASAN, and ROGERS HARRY O’KURU

There is a need to identify usable lignocellulosic materials that can be blended with thermoplastic resins to produced commercial lignocellulosic plastic compos­ites (LPC) at lower costs with improved performance. The core objectives of this study are to: i) evaluate the use of dried distillers grain with solubles (DDGS) and Paulownia wood (PW) flour in high density polyethylene-composites (LPC); ii) as­sess the benefit of chemically modifying DDGS and PW flour through chemical extraction and modification (acetylation/malation); and iii) to evaluate the benefit of mixing DDGS with Pine wood (PINEW) in a hybrid LPC. Injection molded test specimens were evaluated for their tensile, flexural, impact, environmental durabil­ity (soaking responses), and thermal properties. All mechanical results from com­posites are compared to neat high-density polyethylene (HDPE) to determine their relative merits and drawbacks. HDPE composites composed of various percentage weights of fillers and either 0% or 5% by weight of maleate polyethylene (MAPE) were produced by twin screw compounding and injection molding. Chemical modi­fication by acetylation and malation of DDGS and PW fillers prior to compounding was done to evaluate their potential in making an improved lignocellulosic mate­rial. Composite-DDGS/PINEW mixture blends composed of a majority of PINEW were superior to composites containing DDGS only. Composites containing MAPE

had significantly improved tensile and flexural moduli compared to neat HDPE. Impact strength of all composites were significantly lower than neat HDPE. Chemi­cal modification substantially improved the tensile, flexural, water absorbance, and thermal properties of the resultant composites compared to untreated composites. Differential scanning calorimeter and thermogravimetric analysis were conducted on the HDPE composites to evaluate their thermal properties as this may indicate processing limitations with conventional plastics processing equipment due to the exposure of the bio-material to elevated temperatures. Finally, because exposure to the moisture in the environment can affect the physical and color properties of wood, changes in the size and color of test specimens after prolonged soaking were evaluated.

Contact information: a) Functional Foods Research Unit, National Center for Agricultural Utilization Research, Agricultural Research Service, United States Department of Agriculture, 1815 N. University St., Peoria IL 61604, USA; b) De­partment of Technology, College of Applied Science and Technology, Illinois State University, Normal IL 61790, USA; c) Polymer Composites Research Group: Ag­ricultural and Bio systems Engineering, College of Agriculture and Life Science, Iowa State University, Ames, IA, 50011, USA; d) Bio-oils Research Unit, National Center for Agricultural Utilization Research, Agricultural Research Service, United States Department of Agriculture, Peoria, IL 61604, USA; Corresponding author: Brent. Tisserat@ars. usda. gov

13.1 INTRODUCTION

The U. S. wood plastic composite (WPC) industry is projected to increase 13% a year to amount to $5.3 billion by 2015 and is likely thereafter to continue to increase at a similar rate in the foreseeable future.1-3 There is an ever increasing need to improve the quality of lignocellulosic plastic composite and WPC in order to obtain more useful, reliable and inexpensive commercial products.410 The most common type of LPC is WPC, which uses wood flour fillers derived from wood waste materials such as shavings and sawdust generated from lumber processing.61012 WPC thermoplas­tics typically include polyethylene (PE), polypropylene (PP), and polystyrene and are mixed with up to 50% wood flour (w/w) depending on the desired mechanical and physical properties and industrial acceptance.61112 Cost is the most important consideration in the commercialization of any LPC/WPC products. Generally, wood flour fillers are used without elaborate chemical preparations; however, the fillers are sized (sieved) and dried to enhance their processing. The price of LPC is dictated by the price of petroleum and the cost of wood/lignocellulosic fillers. Currently, PE and PP sell for ~$1.85 to $2.27/kg ($0.91 to $1.12/lb.) and ~$2.23 to $2.47/kg ($1.10 to $1.22/lb.), respectively.213 Commercial hardwood flour blends are derived from lumber milling byproducts is composed of various tree species (e. g., maple, birch, ash) and sells for ~$0.18 to $0.48/kg ($0.08 to $0.22/lb.).u Wood waste mate­rial prices fluctuate on the basis of availability (housing demand) and the demand for their utilization.14 For example, in 2006-2008, when the US housing market con­tracted, sawdust prices quadrupled due to a lack of supply.14 Biomass energy us­age competes with LPC/WPC filler availability and price. Currently, 85% of wood waste is consumed for energy production (fuel pellets and direct combustion).15 The Energy Independence and Security Act of 2007 mandates that 36 billion gallons of biofuels be produced by 2022 and woody biomass materials will be increasingly used to achieve this goal.16 A number of government subsidy programs are diverting the woody biomass into bio-energy facilities from their traditional markets. Changes in the cost, availability, and utilization of the biomass and wood waste markets are in flux.16 As previously noted, since the demand for wood flour needed by the WPC in­dustry will also increase and its cost will undoubtedly increase due to the bio-energy mandates, new sources of woody biomass are clearly needed.

Alternative woody biomass sources to provide wood flour are being devel — oped.71719 Small-diameter trees obtained from forest under-stories or brush condi­tions offers a source of woody materials to satisfy both the bio-energy as well as wood flour for WPC.1718 Short-rotational woody crops using “fast-growing trees” grown in coppicing plantations are another option to obtain woody materials.20 Mar­ginal land utilization has been suggested as the potential site for planting large acre­ages of bio-energy woody tree crops.52021

Paulownia elongate S. Y. Hu, family Paulowniaceae, a native to China, is an ex­tremely fast-growing coppicing hardwood that is cultivated in plantations in China and Japan. Paulownia wood (PW) is highly valued in the construction and furniture industries.2223 There are several attributes of Paulownia wood that favor using it as a feedstock for WPC: a Paulownia plot containing 2000 trees per hectare can yield up to 150 to 300 tons of wood within 5 to 7 years, growth rate of heights up to 3.7-4.6 m and diameters of 3 to 5 cm a year are common, Paulownia trees are amenable to being established on marginal lands and have deep taproots, which make them drought resistant, PW is light weight, insect resistance, highly durable, and heat re­sistance. Paulownia species such as P elongate, P kawakamii, and P tomentosa, are currently being grown and evaluated in the United States for their commercial wood properties.2324 For example, recent studies conducted at Fort Valley State University, Fort Valley, GA show that two to four-year-old trees can grow to a diameter of 16.5 cm and achieved a height of 10 m.24 In addition, Paulownia could serve as a short — rotational woody crop that could be harvested frequently over a 10 year period. Therefore, in this study the utilization of juvenile wood materials harvested from 3 year old trees were used as a reinforcement materials with thermoplastic resins.

In many cases, lignocellulosic flour cost is less than wood flour; sometimes costing only a few cents a pound, thereby making it a very economically attrac­tive material to be developed as a filler for LPC.2526 Ag-waste materials generated from processing seeds have not been vigorously exploited as possible fillers in bio­composites. In the U. S. Midwest, dried distillers grains and soluble (DDGS) offers an abundant, available and inexpensive lignocellulosic flour for biocomposites.27’31 DDGS are processed corn seeds left over after the distillation of alcohol to generate the bio-based ethanol fuel.31,32 Approximately, 25 million metric tons of DDGSs are produced annually in North America with this figure expected to increase further in the next few years.29,31,33 Currently, DDGS is used almost entirely as an animal feed although other uses have been sought.26,27,29 DDGS sells for about $0.06 to $0.10/kg ($0.03 to $0.05/lb.) which makes it an attractive bio-filler to blend with thermoplas­tic resins. However, to date, studies employing DDGS as a filler with thermoplastic resins have produced composites that have poor mechanical properties compared to the neat thermoplastic resin.30,33,35 Further research is required to produce a DDGS material that has improved mechanical properties in order to become an acceptable filler material.

In addition, there are also numerous other seed residues (seed meals or press cakes) generated from seeds after their oil processing (soybean, cottonseed, penny — cress). Roughly half of the oil seed’s harvest mass remains as a press cake after oil extraction by pressing.36 In 2012, 472 million tons of oil seeds were harvested glob­ally to provide for culinary (e. g., edible oils and food additives) and industrial (e. g., soaps, cosmetics and biodiesel) products.37 Many press cakes are used as an animal feed or fertilizer, when appropriate.38 However, there is much interest in finding higher value uses for press cakes. Also, “new” oil energy-crops, such as jatropha (Jatropha curcas L.) and pennycress (Thlaspi arvense L.) containing even higher oil compositions than current oil seeds crops and are being developed to address the world’s fuel needs.36,39,40 In the U. S. Midwest, pennycress has a promising future as a bio-diesel crop. It contains more oil than soybeans and is unique in that it is a winter annual that can be grown on the same land used for soybeans without competition since their planting and harvesting dates do not coincide.36,39,40 However, pennycress press cake cannot be used as animal feed since it contains high levels of toxic gluco — sinolates.40 Therefore, alternative uses for pennycress press cakes are sought in or­der to maximize the utilization of this oil seed crop.41 In the tropical and subtropical regions, jatropha is becoming a prominent bio-diesel crop. Likewise, its seed meal is also toxic due to the presence of phorbol esters and is not available to be used as an animal feed or fertilizer. Employment of these press cakes as a filler in LPC could be an ideal utilization. Press cakes price between a range of $0.09 to $0.55/kg ($0.04 to $0.25/lb.) depending on the species and extent of their preparation, which makes press cakes an attractive bio-filler to be blended with thermoplastic resins.42 How­ever, press cakes composition differs substantially from other lignocellulosic flour fillers because they contain high concentrations of extractives which includes re­sidual vegetable oil (» 8-15%) and protein (» 20-35%) while having a low cellulose (» 11-25%) and lignin concentration (» 3-15%).36 DDGS is composed of 25-33% protein, 39-60% carbohydrates, 5-12% oils and 2-9% ash.33 In contrast, PW flour contains: water and solvent extractives (» 3-12%), protein content (» 1-2%), cel­lulose (45-50%), hemicellulose (22-25%) and lignin (20-25%).4,43,44 Few published reports have dealt with using press cakes as a lignocellulosic flour filler.45 Attempts to employ press cakes has resulted in composites with relatively poor mechanical properties when compared to neat thermoplastic resins.45

Chemical modification (acetylation and malation) of lignocellulosic and wood flour fillers is a common method to improve their physical and mechanical proper- ties.6,8,11,12,33,46,53 Chemical modification of a lignocellulosic material is defined as a chemical reaction between a reactive portion of the lignocellulosic material (hy­droxyl group) and a chemical reagent, with or without a catalyst, to create an ester group.6,8,11,12,33,46,52 Acetylation is the most common method to chemically modify lignocellulosic materials.8,51,54,55 Acetylation offers a number of benefits to WPC/ LPC compared to nonacetylated WPC/LPC including superior weathering resis­tance,8,53 greater thermal stability,51 and enhanced mechanical properties.56 Because there is no accepted method to administer acetylation and/or chemical modification treatments to lignocellulosic and wood flours there are a myriad of acetylation/mala — tion techniques presented in the literature.8,47,48,50,51,53,55,68 Generally, however, chemi­cal medication by acetylation involves the treatment of lignocellulosic and wood flour particles by soaking or coating with a coupling agents (e. g., acetic anhydride) in order to reduce the presence of hydroxyl groups in exchange for esterification linkages (i. e., covalent bonds between the wood and the reagent).8,47,53 In this study, chemical modifications were made on both Paulonia wood flour and DDGS prior to their blending with HDPE to determine if a chemical modification techniques could improve the mechanical properties of these lignocellulosic materials in the resulting composites.

There are three core objectives of this study. The first is to perform an assess­ment of the mechanical properties of thermoplastic composites made with DDGS. The methods developed to produce a usable DDGS composite that exhibits high mechanical properties can be transferred to the development of composites contain­ing seed press cake residues from various species. This is a reasonable assumption due to the chemical compositional similarity between the DDGS and press cakes. Lignocellulosic materials are polar (hydrophilic) due to the occurrence of hydroxyl groups and are not compatible with thermoplastic resin polymers, which are non­polar (hydrophobic). In order to obtain a LPC/WPC with superior physical and me­chanical properties a coupling agent is often employed to aid in the binding of the lignocellulosic materials to thermoplastic resins.50 A variety of different types of couplings agents are blended with LPC/WPC but maleate polyolefins are the most common due to their cost, performance and acceptability.6,11,12,33,46,49,50,52 Since inclu­sion of a coupling agent is typical in WPC,6,11,12 the effects of employing a commer­cial maleate polyethylene (MAPE) on the mechanical properties of HDPE-DDGS composites is included in this study. Residual oils in DDGS may adversely affect the performance of DDGS composites due to their lubricating effect. Therefore, a solvent extracted DDGS material was tested to assess the benefit of oil extraction.

In addition, DDGS was subjected to chemical modification treatments in order to obtain an improved DDGS composite.

The second core objective was to evaluate the mechanical, physical, and thermal properties of WPC obtained from blending Paulownia wood flour with high density polyethylene because there have been relatively few studies of the use of Paulownia wood (PW) as a fiber reinforcement for thermoplastics.10,43,44’69 There is interest in using Paulownia wood flour derived from juvenile trees since small diameter short — rotation woody crop trees are likely to be a source of woody biomass needed by the US in the future. This study used PW flour derived from juvenile tree biomass (i. e., 36-month-old). The use of a maleate PE was employed as part of the scope of the project. Further, because chemical modification through acetylation or malation of filler materials may affect the performance of reinforcement, the mechanical and flexural properties of WPC derived from PW that had been acetylated and acety — lated/maleate was examined.

The third core objective was an evaluation of the physical and mechanical prop­erties of “mixed” composites composed of DDGS mixed with pine wood (PINEW) was conducted due to their relatively unique chemical make-up. The mechanical and flexural properties of these composites were benchmarked to formulations con­taining just PINEW or DDGS as well as to neat HDPE. The mechanical property outcomes were normalized to the control HDPE for ease of assessing the benefit of various filler treatments.

Because bio-composites are subject to degradation by water, water immersion tests were administered on tensile bars composites to evaluate their environmental durability. Weights, thickness, and mechanical properties were measured before and after the immersion tests. Finally, because bio-fiber materials are sensitive to heat exposure during processing, differential scanning calorimetry and thermogravimet­ric analysis were conducted on DDGS, PW, and PINEW composites to evaluate their thermal properties to assess any implications of processing on these materials.