Stronger, lighter, cheaper . . . naturally

Design engineers have an expanding suite of natural fibers available for compounding into base resins. These not only “green up” products due to their renewable nature, but also can lower weight and boost physical 
properties, often at a lower cost than inorganic alternatives. In a series of papers given at the Society of Plastics Engineers (SPE) Global Plastics Environmental Conference (GPEC) held earlier this year in Orlando, presenters promoted everything from corn cobs and wheat straw to sunflower hulls as functional fillers that not only displace petroleum- or natural-gas-based plastics, but can also increase a compound’s performance.

A paper by Michael Fuqua, Venkata Chevali, and Chad Ulven of North Dakota State University (Fargo, ND) detailed how corn cobs mixed into recycled high-density polyethylene (HDPE) yielded improvements in thermal stability and mold tolerances. The paper also showed how the addition of chemical compatibilizers to aid bonding between the resin and the natural filler resulted in “significant” improvements in mechanical properties. The research led the team to conclude that corn-cob-filled recycled HDPE has some strong advantages over neat polymers or traditionally filled compounds.

Key to the compounds is the fact that biomass derived from natural fibers features what the researchers call a “backbone” of crystalline straight-chain polymer cellulose. In contrast, biomass’s other components, including hemicellulose, starch, lignin, and protein represent amorphous polymers that have less inherent strength than cellulose.

The researchers concluded that because of the chemical structure, the development of functional lignocellulosic fillers would require high-cellulose-content biomass.

Hydrophobic polymer meets hydrophilic biomass
To create a robust compound, the NDSU researchers also had to overcome the fact that HDPE is inherently hydrophobic while biomass is hydrophilic. They found that maleic anhydride grafted polyethylene (MAPE) could act as a surface modifier of biomass fillers to help join the fillers and the HDPE in a robust matrix. MAPE essentially acted as a compatibilizer, interacting with the cellulose polymer chains that make up the backbone of lignocellulosic biomass.

As part of their research, the NDSU team created samples loaded 20% by weight with the corn-cob filler, in addition to the MAPE. That was melt-blended with HDPE in a Leistritz co-rotating twin-screw extruder. Team members concluded that because of the inherent polarity differences between lignocellulosic fibers and HDPE, interfacial bonding is required or there would be limited chemical interaction and surface voids could form.

Sunflower power
Jeremy Dworshak, a material engineer at custom molder Steinwall Inc. (Coon Rapids, MN), delivered a paper on his company’s work with NDSU and John Deere to create a biocomposite mixing sunflower hull fibers with polypropylene (PP). In developing the compound, plaques were molded for use in standardized mechanical and physical testing, along with the production of components for full-scale testing.

John Deere wanted lower overall cost and improved properties for the part, a handle with two insert-molded screw threads, which had been molded of an unreinforced copolymer PP supplied by Matrixx Group Inc. Sunflower hull fibers were chosen on the basis of cellulose content and cost (see table), with around 45% cellulose: better than corn cobs (40%) but below flax fibers (70%).

The researchers concluded that these biocomposites could be processed at approximately the same parameters as the neat polymer. Moisture absorption did increase with the addition of fibers, and for the highest peak load in tensile pullout, a grade compatibilized with maleic anhydride did best.

Ford harvests wheat straw 
as functional filler
Also at GPEC, custom compounder A. Schulman (Akron, OH), Ford, and the University of Waterloo (Waterloo, ON) discussed how their collaboration had led to the creation of several products that not only meet the current material specifications, but also offer significant weight savings over conventional mineral- and glass-reinforced composites, while being price competitive.

The group discussed their development of an injection moldable wheat-straw-fiber-reinforced polypropylene (WSPP). At the heart of the compound is wheat straw, an annually renewable agricultural waste byproduct that is widely available at a low cost, and does not compete with food sources.

The wheat straw was supplied by Omtec Inc. (Mississauga, ON) and sourced from farms in southern Ontario. Omtec utilizes a proprietary system to prepare the wheat straw, including cleaning, separating, chopping, and sizing. After that, the wheat-straw fiber was compounded on conventional extrusion equipment by A. Schulman. The study focused on PP homopolymer and some impact-modified grades.

The researchers concluded that the wheat straw could act as a feasible alternative to talc reinforcement, offering a significant density reduction, thanks to the fiber’s lower specific gravity.

Taking the heat, still smelling sweet
Natural fiber composites consistently pose one challenge: Thermal degradation in processing causes changes in odor and/or color. To overcome this, A. Schulman and the University of Waterloo collaborated on an additive system and processing method that eventually eliminated the majority of the odor. A. Schulman developed an additional process improvement that further reduced any aromas, resulting in what the paper described as “an odor that was not offensive, minimal, and dissipates over a short time.”

Wheat-straw-fiber-reinforced PP is currently on the road in the 2010 Ford Flex’s third-row quarter-trim storage bin and cover liner. The compound contains approximately 20% wheat-straw fiber, and several additional wheat-straw PP grades have been developed for interior, exterior, and underhood applications.

The researchers said benefits for the compound include: standard process equipment and tooling can be used; shorter cycle times (8%-12% faster) and reduced processing temperatures (20-30 deg C) lowered energy usage for a smaller carbon footprint and reduced costs; the compound exhibited equivalent or improved properties over inorganic fillers; the dimensional stability enhanced fit/finish; and there was a weight reduction of 5%-10% over equivalent glass- and/or mineral-reinforced composites.

Judging from these and other recent advances, and consumers’ increasing tendency to favor products made with sustainable materials, processors can expect to be offered a wider choice of naturally reinforced compounds in the months and years to come.