By Nigel Larsen
Grain components — starches, lipids, proteins and non-starch polysaccharides — are proving to have considerable scope in films, coatings, chemical pre-cursors for plastics and other materials in packaging composites and polystyrene-like foams.
The biodegradable polymer market may still be in its infancy but it has enormous potential for expansion in the 21st century.
An example of the current expansion is the Cargill-Dow polylactic acid (PLA) facility in Savage, Minnesota, U.S. The plant is designed to turn maize starch into 300 million pounds (136.2 million kg) of PLA per year. This will be used to make polymers for a range of agricultural, apparel, furnishing and other markets.
Early generation biodegradable polymers were usually comprised of starch granules added to petroleum-based polymers. Current research now aims for polymers almost entirely derived from grains and other renewable, biological resources.
All this is being driven by strategic, political, scientific and market factors such as:
• Impending scarcity of oil
• Environmental and economic impacts of oil-based polymers (disposal costs of oil-based polymers used as agricultural mulches are estimated to be U.S.$125 per acre)
• Renewable energy input into synthesizing the polymer in the plant ("plants as factories")
• Advanced microbial and plant gene technologies
• Consumer demands
• Legal requirements (such as Germany’s packaging laws)
• International Standards — ISO14000 supports the use of eco-friendly plastics
• Government research and development inducements for industry.
Raw materials for biodegradable polymers
Biological polymers and monomers, such as proteins, starches, cellulose, pectins, non-starch polysaccharides, chitin, waxes and resins, can be extracted from a range of sources — grains, seeds, tubers, legumes, fruit, nuts, seaweeds, eggs, milk, meat, wool, feathers and fish.
A lot of research is now focused on using cereal grains as a starting point for new biomaterials. Most of this research is based on the starch and protein fractions of grains that are readily available on the world commodities markets or are produced as co-products of other industrial processes, such as ethanol manufacture. For example, maize, wheat and soy starches and proteins have been the main raw materials, and major advances in the science and use of these for biodegradable materials have been made. However, this approach to large-scale industrial manufacture of commodity biodegradable polymers is, in general, both industrially and economically inefficient. Currently the products cannot easily compete with cheaper oil-based equivalents.
Crop & Food Research is aiming for biodegradable polymer formulations utilizing as much of the grain as possible. The first step is to understand the composition and functionality of each grain component that could be used in biomaterials.
Grains generally comprise about 65% starch, 12% protein, 5% non-starch polysaccharides (NSP), 3% lipid and 15% minor biochemicals, minerals and water. Of most interest and relevance for biomaterials are starches, proteins, NSPs and lipids. Any process that can be developed to use all four components would therefore use over 90% of the dry matter of the grain.
Starch is the major grain component — around 65% — and a commodity raw material. In most grains it is usually, apart from very small quantities of starch lipids and surface proteins, a binary mixture of linear amylose and branched amylopectin, each of which affect biomaterial properties in different ways.
Because the genetic control of amylose and amylopectin ratios is increasingly achievable in the major grains (waxy maize, wheat, rice and barley), the composition of starch extracted from the grain can be somewhat pre-determined by breeding.
Chemical derivation of starch is becoming increasingly important in the design of new polymers. Starch is being used to produce films, coatings, co-polymers, packaging, and encapsulants and, increasingly, starch foams are extruded or molded to produce polystyrene analogues. Because starch is a polymer of glucose monomers, it can also readily be used as a chemical feedstock to produce monomers (e.g., polylactic acid and 1,3-propanediol) for the plastics industry. It has been estimated that to produce 1 billion pounds (450 million kilograms) of PLA would utilize only 0.5% of the annual U.S. corn crop.
Although protein is a minor component at approximately12% in most grains, maize, soy and wheat proteins are readily available as commodity raw materials.
However, unlike starch, the proteins are usually complex mixtures, and each component has different, sometimes strikingly different, physical and chemical properties. For example, from what we know about wheat gluten proteins, it might be expected that wheat with high molecular weight (HMW) glutenins will form strong, extensible films. Although for casting films, HMW glutenins will only dissolve in extreme conditions such as high urea concentrations or acid or alkali and the resulting films are poor quality. Unlike starch, themoplasticity is often only achievable within narrow ranges of conditions and, therefore, extrusion of protein-based biomaterials is difficult.
Further fractionation of proteins is possible to achieve specific solubility or functional end-points, but this is usually uneconomical. Fractions from gluten and proteins are therefore best used in high-value, low-volume products such as pharmaceutical encapsulants. Grain proteins, such as zein, gluten, sorghum kafirin, rice bran protein and soy have already been used to make films and edible coatings for foods and pharmaceuticals. Zein has also been used to make fibers for clothing — an example of this was "Vicara." This fiber was manufactured in the U.S. after WWII until oil-based fibers such as acrylic became cheaper alternatives. Protein-based manufactured fibers would have to offer exceptional and unique properties to compete with wool and fibers made from PLA.
The complex amino acid composition of proteins means that currently they have much less scope than starch for manufacturing chemical precursors for polymers. However, it is possible that genetic modification and microbial fermentation technologies might enable this possibility in the future.
NSPs are a very minor component (around 5%) of grains and usually comprise mixtures of ß-glucans and pentosans. As such, they are not generally available as an industrial commodity. However, this situation may change with the development in New Zealand of a process to extract ß-glucan fractions from barley. This process has now been proven viable on a semi-industrial scale.
Unlike starches and proteins, the genetic control and biochemical pathways for NSPs in the major grains are not well understood. Despite this, work done in New Zealand and elsewhere shows substantial increases in ß-glucan yields can be achieved through breeding. If a viable extraction process is established, this may make NSP use in biomaterials more economical in the medium to longer-term future.
NSPs can be used to make films and encapsulants and there is a patent to cover their use as bioremediation membranes for the removal of heavy metals from water.
Grain lipids have been used as functional additives in films and coatings to control moisture migration. For example, sorghum wax has been studied in bi-layer films where it was used to form a physical barrier to moisture movement between the two layers of film. Mostly, lipids are co-cast with proteins but control of moisture migration is usually at the expense of tensile strength.
Despite a long history of research and utilization, the potential for renewable grain raw materials to replace oil-based sources of polymers is only just starting to gather momentum. There is still much research that can be done to explore combinations of different ingredients for biomaterials to efficiently utilize as much of the grain as possible and to identify or breed grains with the unique combinations for specific biomaterials. To achieve this presents many exciting challenges for cereal and materials scientists, engineers, biotechnologists and plant breeders.