Current agriculture growth is limited to relatively few consumable crops though expanded use via industrial applications promises to broaden their market potential. Maize, soybean, and rapeseed are examples of consumable crops that are currently being considered as industrially significant fuels, solvents, coating resins, and/or lubricants. In addition, a wide variety of new, "alternative" crops are being examined as substitutes or adjuvants to existing sources of food and non-food raw materials. The use of agricultural materials as industrial raw materials is accomplished via accurate analysis of raw material composition(s) and structural identification of chemical components. Given this information, the value of the raw material source can be determined either as an unmodified source and/or as derivatized products. Finally the competitive use or application of the material and/or its derivatives must be determined by its incorporation into and testing of new industrial products.
The field of polymer application is immense and offers much potential for growth. Markets for polymeric industrial materials include plastics, coatings, adhesives, and elastomers. More than one half of all polymer personnel are employed in the coatings industry. In 1994, the coatings industry generated 1200 million gallons of coating shipments or an equivalent to 14 billion dollars in revenues. Moreover, the coatings industry is ever mindful of existing and impending environmental regulations and disposal issues that must be solved.
The question of new crops versus old crops is mute. The decision of which to use is entirely dependent upon raw material composition, processing, future risk (i.e., pollution cleanup), and product performance properties. An "old," over-produced crop can be instantly revitalized by the advent of innovative technologies. Future needs/demands will expand for renewable raw materials, for disposal control, and for pollution. Clearly, the science of biodegradation will receive much impetus in the years ahead. Indeed, a well-planned, consistent product design, synthesis, and market and investment protocol for future product development exist and will be the subject of this paper.
Agriculture began in four independent centers: southwest Asia, southeast Asia, Mexico, and Peru (Solbrig and Solbrig 1994; Fig. 1), which influenced other geographical areas. Several experts believe these centers to be the ancestors of modern horticulture and animal husbandry (Vavilov 1926; Wilkinson 1972).
Southwest Asia, comprised of India, south Afghanistan, near Cashmere, Iran, Asia Minor, and the Caucasus, was responsible for soft wheats, rye, small-seeded flax, lentils, broadbeans, chickpeas, assorted vegetables, and Old World cotton. Southeast Asia, China, Tibet, Nepal, and surrounding areas domesticated barley, naked oats, millets, and soybeans. Southeast Asians also founded crucifers and many fruit trees. The Americas were responsible for maize, potatoes, beans, tobacco, Jerusalem artichokes, and American cotton. Vavilov ascribes the development of hard wheats, cultivated oats, large-seeded flax, peas, large-seeded lentils, beets, and other vegetables to the lands around the Mediterranean Sea. He credits an Ethiopian region as the nesting ground for barley, wheat, sorghum, and coffee. As people migrated from region to region, they carried crops with them, spreading agriculture from centers of origin into "noncenters" (Solbrig and Solbrig 1994). These Old World "noncenters" expanded to physical barriers, such as the oceans.
Once the physical barriers to the Old World were crossed, crop species were further exchanged. Sugar, a storage-stable confection of little nutritional value was the first world crop unrelated to food/feed survival. It was grown strictly for cash profits beginning around 600 AD. Tobacco brought the habit of smoking to Europe in the early 17th century, while coffee was smuggled to Martinique by a young French naval officer spawning a coffee industry in Brazil and Columbia (Solbrig and Solbrig 1994). Cotton fabric, the cloth of luxury, was popular when Eli Whitney invented the cotton gin in 1793 and revolutionized cotton fiber. Likewise, seedlings from the rubber tree, Hevea, were smuggled to England from South America by Henry Wickham and later transported to the British Malaysian colony where hevea rubber has become a thriving industry (Allen 1949). Coffee, sugar, wheat, and grapes are examples of crops introduced into the New World, while potatoes, tomatoes, maize, and tobacco were returned from the New World to the Old World.
Rural communities, initially located near the food or agricultural source, expanded rapidly into highly populated areas of commerce. The fastest growing cities were historically located on the most fertile lands (Brown 1981; Solbrig and Solbrig 1994) reducing available land for cultivation. In 1980, for example, 2,000 acres (800 ha) of U.S. prime farmland were gobbled up by urban sprawl each day (Jackson 1980). W.H. Davis (1972) stated that a million acres (400,000 ha) of farmland were destroyed annually from urban expansion alone, and projected a decrease in agricultural land from 2.6 acres (1 ha) in 1972, to 1.2 acres (0.5 ha) per person by the year 2000. As population increases, a greater demand on agricultural production continues regardless of the reduced land area.
Probably the largest influence on modern society is fossil fuels, primarily petroleum. Fossil fuels are now a necessary evil to our present day society although chemical refineries, ocean tankers, and automobiles pollute land, water, and air. In addition to modern conveniences, energy is required to subsidize our fertilizers, pesticides, and harvesters permitting maximum crop yields on minimal acreage in order to meet today's demands for food, shelter, clothing, transportation, and heat. Petroleum products have not only allowed enhanced agricultural production but permeated raw material markets due to ebbing natural productivity compared to demand and scarcity in new cropland (Brown 1981). For example, world consumption of wood products for housing and paper production was at 0.62 m3 per capita in 1976 (Brown 1981), converting to 2.5 billion m3 of wood harvested. Wood capacity has since dropped off per capita (0.60 m3 per person in 1980) from a high of 0.67 m3 per person in 1964, as it was unable to keep pace with world population growth. Petroleum filled this production gap in the form of polymeric wood composites. Similarly, petroleum in the form of fertilizer is essential for producing more sustenance from less available land.
Plants extract nutrients from the soil during their growth cycle. If land is over planted, the crop yields are reduced over time. Crop rotation, allowing the ground to lie fallow or planting crops that promote nitrogen fixation, restores soil fertility as well as the application of natural or man-made fertilizers. Nutrients are being replenished by synthetic fertilization in ever intensifying amounts. In the U.S. during the period of 1945 to 1975, there was an increase from 10,000 to 400,000 t, producing only a twofold yield (Jackson 1980). World use of fertilizer increased from 15,000,000 to 114,000,000 t/year from 1950 to 1980, respectively. Herbicide and pesticide use has also escalated (Brown 1981; Solbrig and Solbrig 1994). Pesticides were applied at an accelerating dosage of 168% per unit production increase in the years 1950 to 1967 (Jackson 1980). It is important to remember that most fertilizers and pesticides are petroleum in origin.
Yet petroleum is ultimately limited in supply, therefore coal, the probable substitute for petroleum, will eventually compete for land and water use. Fig. 2 shows a disturbing overlap of coal reserves and agricultural regions in the maize and wheat production belts. Technologies beyond petroleum must include not only energy and chemical resources for agriculture, i.e., fertilizers, agricultural chemicals, and fuel for harvesters, but also industrial materials for civilian and critical/strategic military needs (Wheaton 1990).
Of greatest importance to society's future is that only agriculture can provide sustenance for our growing population on an otherwise shrinking planet. Agricultural production is limited only by the number and type of plants grown. Diverse planting avoids widespread dependence on a single plant, for example, the Irish potato famine of 1845 (Solbrig and Solbrig 1994). Recent trends in agriculture thus seek alternatives that diversify rather than concentrate on a few crops (New Farm and Forest Products Task Force 1987; Conrad 1993; Williams and Haq 1993; Babb 1990). Alternative crops can be either food or non-food related and are usually produced in smaller volumes, thus not competing with large, existing markets. However, alternative crops have the potential for spawning new industries such as soybean. Soybean was a North American alternative crop until just a few decades ago. Examples of alternative crops for food include canola, sunflower, rapeseed, high-bush blueberries, and broccoli. Non-food alternative crops include vernonia, lesquerella, tung, guayule, and kenaf as well as industrial strains of sunflower and rapeseed.
Well established crops are also enjoying innovative successes. Maize is attempting to expand in the area of alternative fuels, i.e., ethanol. Newly constructed factories produce poly(lactide) from maize carbohydrates for biodegradable and medical/surgical polymers. Tung oil derivatives such as methyl eleostearate are useful as a reactive diluent in high solids, low volatile organic content (VOC) coatings. Soy protein is again showing promise in adhesives and high performance plastics, e.g., Environ of Phenix Composites. Soy oils are finding use in diesel fuel, letter-print inks, and industrial solvents.
Although a 12,000 year history in food/feed production exists, only recently has agriculture focused on production of industrial materials. Biomass currently enjoys use in textiles, paper and paperboard as fibers, sizing, and adhesives. However, use in plastics, coatings, resins, and composites is negligible due to the adoption of synthetics (Narayan 1994). Synthetics displaced agricultural materials, flooding the market because of their low costs. This implies low-to-no profit margin, i.e., strictly large volume farming or a high petroleum price, is required for agricultural materials to replace petroleum. Current strategies target high-priced, low-volume specialty markets to bridge the gap with low-price, high-volume commodity chemicals (USDA 1992). Examples of these markets are additives for foodstuffs, paints and coatings, lubricants, and fuels such as rheological aids, dispersants, adhesion promoters, and anti-knock or smog-reducing products.
The polymer structure of protein lends well to chemical reaction and application in industrial materials (Fig. 3). Although the amino acid composition of proteins can vary widely, the polymer backbone structure remains the same. The polyamide feature of protein contributes both chemical and physical properties that can be altered by processing. Hydrolysis of protein yields lower molecular weight polyamides that can provide better mixing, reactivity, and uniformity.
Plastics and composites have been synthesized from soy proteins in combination with phenol and formaldehyde, filler, and plasticizer (Johnson and Meyers 1994). However, these plastics were susceptible to water absorption and longer cure times though somewhat improved by the addition of wood flour. Water sensitivity is partially due to the presence of carbohydrate impurities within the protein raw material (Lambuth 1989), which can be fixed to reduce water sensitivity via reactions with silicates (Johnson and Meyers 1994), formaldehyde (Brother and McKinney 1940), and perchlorate and periodate (Wurtzburg 1986). Environ, a protein product of Phenix Composites, is made from recycled newspaper, soy flour, and polymer binder. The material is innovative, utilizing agricultural raw materials and paper recycle material, thus reducing the quantity of solid waste. Paper, e.g., newspaper and paperboard, comprises 35%, or 58 million tons, of the landfilled solid wastes produced in the U.S. annually (Wolfe 1991). Other new uses for soybean proteins are expected in the areas of water resistant wood adhesives, biodegradable plastics and composites, textile fibers, and emulsifiers (United Soybean Board 1995).
Soybean oils are being applied as hydraulic fluids, form release for concrete, improved inks, and as a raw material to produce polyester and nylon monomers (United Soybean Board 1995). Saponification of the oil yields a glycerol and fatty acid mixture (Fig. 4). The profits obtained by recovery of glycerol are usually sufficient to cover process costs, thus free fatty acids, by volume, cost nearly the same as the whole oil. Esterification of the fatty acids with methanol affords fatty methyl esters, a class of materials useful for diesel fuels or environmentally friendly, industrial solvents. Methyl esters are less expensive than ethyl esters since ethanol not only costs more than methanol but the same esterification requires nearly twice the volume. Maize oils are also being examined for similar applications.
Cargill and Ecochem have recently installed polymer plants for production of poly(lactide) synthesized from maize-derived lactic acid, a novel biocompatible and biodegradable material for medical sutures and implants, drug delivery, and commercial packaging (Narayan 1994; Fig. 6). Poly(hydroxybutyrate) and poly(hydroxyvalerate) (Fig. 7), biodegradable thermoplastic polymers for personal care containers, medical uses, films, paper coatings, and even fishing nets (Office of Technology Assessment 1993), are also produced directly from fermentation processes (Narayan 1994; Shimamura et al. 1994; Eggink et al. 1993).
Fermentation processing is unique in that many processes are relatively independent of carbon nutrient structure. For example, fermentation of glucose, sucrose, starch, oleic acid, or 3-hydroxybutyrate (Nakamura et al. 1992) can each be used to produce poly(3-hydroxybutyrate). The variable for control of polymer structure is entirely dependent on proper selection of the microbe (bacteria) and its particular genome, or genetic code. Thus, bacteria can be genetically engineered to custom produce a raw material, either a monomer for polymerization like lactic acid or the complete polymer, such as poly(hydroxyalkyrate) from a carbon source.
Genetic engineering can also modify plant genomes, analogous to microbial genomes, to improve disease or drought resistance, or alter the product's chemical structure. In 1970, maize crops were devastated by a blight. Although the susceptibility was added through genetic engineering during an attempt to increase maize yields, genetic recombination with other cultivars yielded a disease resistant, high yield strain. Insect resistance, disease resistance, and plant yields, including the chemical structure of the product, are all affected by genome (Solbrig and Solbrig 1994). Transgenic crops are now a current trend in agricultural business (Rotman and Fairley 1995).
Products derived from lesquerella oil can be analogous to those obtained from castor. Dehydration of lesquerella oil produces a drying oil that is superior to dehydrated castor oil (Thames 1995). Hydrogenated waxes may be useful in lubricating greases, cosmetics such as lipstick, polishes, and inks (U.S.D.A. 1991). Pyrolysis and alkali fusion of lesquerolic acid yield tridecenoic acid and heptanal, and dodecanedioic acid and 2-octanol, respectively. The engineering nylons 13 and 6,12 can then be synthesized from the tridecenoic acid and dodecanedioic acid, respectively, similar to the nylons 11 and 6,10 obtained from ricinoleic acid. Stable, low isocyanate index polyurethane foams are prepared from alkoxylated lesquerella oils, and lesquerella oil has performed notably in cationically cured coatings (Thames, S.F. and H. Yu unpubl. results). The industrial future of lesquerella looks bright indeed.
Nylons are a particularly useful class of high value polymers due to their moldability, toughness, and durability (Modern Plastics 1994). Nylons are a good example of the extent that physical properties are affected by structural variations. Nylons typically vary in structure in two ways (Fig. 12): by the carbon length of each unit segment, and by the segment form, -AB- or -AA-BB-, of the repeat unit where the "A" would stand for an amine and "B" for a carboxyl. Changes in physical behavior occur due to the ordering of the polymer chains as a crystal matrix, affecting the extent that nitrogen protons can align with carbonyl oxygens (Miyake 1960). Two forms of crystal dominate, the alpha and gamma structures. For -AA-BB- polymers, even-even carbon unit lengths have the alpha form whereas odd-odd have the gamma form. Even-odd and odd-even both have a gamma form, except that solution processing may produce a third arrangement, the delta form. The -AB- polymers are more complex where even units tend to the alpha form and odd to the gamma form, except nylons 8 and 10 which take on the gamma pattern.
Nitrogen proton to oxygen interactions, indicated by crystal structure, influence the extent of hydrogen bonding which is a primary factor in nylon toughness and other physical properties. In general, hydrogen bonding increases the melting temperature of the polymer as the concentration of amide linkages increases. Nylon 12, an -AB- polymer formed from an omega-aminododecanoic acid, has a melt temperature of about 178deg.C compared to nylon 6, poly-omega-aminohexanoic acid, of 229°C. However, although nylon 11 has a more concentrated amide linkage than nylon 12 and therefore should have an increased melt temperature, the melt temperature actually decreases to 165°C. The odd carbon spaced nylon 11 segments, gamma in crystal structure, are less able to align, decreasing interactions between chains and causing a corresponding decrease in melt temperature. This particular feature makes nylon 11, with nylon 12, useful to the powder coatings industry because powder coatings rely on good melting at a minimum temperature for film formation (Misev 1991).
Lengths of the carbon chain between the amide links also affect the physical properties of the bulk nylon. As observed above, more carbon atoms dilute the amide linkages, and decrease melt temperature and tensile modulus. Other related effects are increased polymer flexibility and elongation, while a constant tensile strength, nearly 83 MPa at break, is common for all carbon spacings. The exceptional performance of nylons commands premium prices (Modern Plastics 1994).
Another example illustrating the importance of structure is given by associative thickeners. Associative thickeners are used in coatings to control viscosity, or more appropriately, thicken on standing and thin on shear. Shear thinning allows the coating to be easily applied by brush or spray while thickening allows good film build and one-coat hiding of the substrate.
Associative polymers have long, soluble segments with short, insoluble end groups (Fig. 13). Hydrophobic end groups are typical of modified cellulosics and hydrophobically-modified ethoxylated urethanes (HEURs). Performance of the thickener is dependent on the length of the hydrophobic end group as a ratio to the overall polymer length. As the terminal hydrophobe of a HEUR polymer varies from 0 to 16 carbons in length, the low shear viscosity increases from 100 to 100,000 times, corresponding to a terminal hydrophobe increase from 0 to 2.6 weight percent, over the molecular weight range of 100,000 to 16,000 (5% W/W solutions, Fig. 14). For example, extending the terminal hydrophobe length to increase the solution concentration of hydrophobe from 0 to 0.1% W/W increases the solution viscosity 100,000 times, from 3 x 10-2 poise to 2 x 103 poise (Jenkins 1990). Thus, a very small change in polymer structure can create a very large change in physical properties.
New products from agriculture will be needed, not only from transgenic crops that treat resistance and production problems, but also industrial materials. Petroleum replaced the original ag materials on a cost and supply basis. Agricultural materials need to reciprocate, alleviating the demand on petroleum including fertilizer, pesticide, and fuel in support of a sustainable agriculture of maximum yield.
Pollution has historically been associated with population, threatening the very resources upon which life itself is contingent. However, pollution can be reduced through recycling and biodegradation while novel "green" technologies improve the global environment through reduction in VOCs, hazardous pollutants, and toxins (Kirschner 1994). These are cost driven markets that are ideal for new, alternative crops.
Today we consider maize, soybean, wheat, and sugarcane "old" crops, yet soybean, sugar, and wheat are actually new to North America having recently been introduced from southern Asia. It was through agriculture and domestication that plant diversity was given up by early man. Of more than 3,000 plant species used as food by different peoples throughout history, only about 200 are now domesticated while fewer than 20 are heavily cultivated within any region. Promotion of all crops, new or old, are probably best served through the new uses of their produce and service to society.
The question of new crops versus old crops is mute. The decision of which to use is entirely dependent upon raw material composition, processing, future risk (i.e., pollution cleanup), and product performance. An "old," overproduced crop can be instantly revitalized by the advent of innovative technologies. New raw materials can fill future needs; demands will expand for renewable raw materials, for disposal and pollution control. Clearly, the science of biodegradation will receive more consideration in the years ahead. The polymer industry plays a vital role in the development of cost effective, high performance polymers from agricultural materials, where small changes in structural design can greatly affect properties, and will further their industrial acceptance.
Fig. 1. Abbreviated map displaying the centers of origin for plant agriculture.
|Fig. 2. Map of coal reserves (United States) and agricultural production zones (North America). Mixed = grains and livestock; maize = grains and livestock; wheat = grain farming; dairy = dairy livestock; special = primarily rice, cotton, and sugarcane.|
Fig. 3. Materials obtained from soybean.
Fig. 4. Saponification and methylation of triglycerides.
Fig. 5. Molecules obtained through fermentation.
Fig. 6. Polymerization of lactic acid.
Fig. 7. Structures of poly(4-hydroxybutyrate) and poly (3-hydroxyvalerate).
Fig. 8. Products from heat pyrolysis of erucic, lesquerolic, ricinoleic, and oleic acids.
Fig. 9. Products from caustic fusion of erucic, lesquerolic, ricinoleic, and oleic acids.
Fig. 10. Formation of 1,6-hexamethylenediamine and 1,6-hexanediol from adipic acid.
Fig. 11. Polymerization of adipic acid and 1,6-hexanediamine to form nylon 6,6.
Fig. 12. Structure-derived nomencalture of the nylon polymer families.
Fig. 13. Structure of the HEUR associative thickener.
Fig. 14. Change in low shear viscosity as a function of the molecular weight and hydrophobe chain length of HUER associative thickeners.