Many plants, animals, microorganisms, and fungi are biobased feedstock sources for obtaining and/or deriving monomers, oligomers, polymers, and/or biofibers.
While biobased feedstocks still comprise only a very small percentage of the feedstocks used by the polymer industry, their usage is growing much faster than the usage of petrochemical feedstocks so that they are certain to become increasingly important over the years. For example, in 2014, it was estimated that the revenues of the global plastics industry had ~2.8% annual growth rate, while its biobased portion comprised less than 0.5% of the total revenues of the plastics industry but had ~15% annual growth rate.
The substitution of biobased feedstocks for petrochemical (fossil fuel based) feedstocks in manufacturing polymeric products is often limited to partial replacement. This limitation is a consequence of the need to avoid performance declines and/or price increases that are unacceptable to consumers who usually demand greater sustainability without having to make any sacrifices, as will be discussed below in greater detail. Attaining price parity of biobased polymeric products with their petrochemical counterparts is, therefore, essential for continued gains in market share by such products.
The achievement of price parity by additional biobased products will depend not only on advances in biobased materials but also on the future abundance and prices of fossil fuels. For example, in one scenario, fossil fuel extraction from unconventional resources may become sufficiently widespread to lead to a petrochemical industry renaissance, hampering the commercialization of biobased products. In an opposite scenario, environmental concerns and/or faster renewable energy technology growth may lead to a decline in fossil fuel extraction, accelerating the commercialization of biobased products.
The biobased content of a product is defined according to the following equation, as discussed by Understanding Biobased Carbon Content (Society of the Plastics Industry, Bioplastics Council, February 2012).
ASTM D6866-18, “Standard Test Methods for Determining the Biobased Content of Solid, Liquid, and Gaseous Samples Using Radiocarbon Analysis”, is used to measure the biobased content of products containing carbon-based components combustible in the presence of O2 to produce CO2 gas.
The following flow chart, from R. P. Babu, K. O’Connor, and R. Seeram, Current Progress on Bio-based Polymers and Their Future Trends (December 2013), summarizes some of the most common categories of biobased polymers produced by various processes.
Biobased feedstocks can be obtained from all forms of life; including plants, animals, microorganisms, and fungi. Some biobased feedstocks are used in the polymer industry. Some others are used mostly or solely in applications outside of the polymer industry. For the sake of thoroughness, the lists provided below include many biobased feedstocks that are not currently used in the polymer industry.
The following are agricultural (plant-based) sources of biobased feedstocks:
- Glucose: Sugar cane, sugar beets, corn.
- Starch: Sugar cane, sugar beets, corn.
- Cellulose: Switch grass, bamboo, eucalyptus, many other shrubs and trees.
- Cellulosic biofibers: Cotton, jute, kenaf, hemp, flax, sisal, ramie, corn, wheat, rice, sorghum, barley, sugarcane, pineapple, banana, coconut.
- Alginic acid: Brown algae (brown seaweeds). Alginates are derived from alginic acid.
- Agar: Red algae.
- The thermoplastic polyester poly(ethylene furanoate), which was discussed in a post titled A PLANT-BASED RENEWABLE ALTERNATIVE TO PET (created 09 November 2018), is derived by processing plant-based carbohydrates to obtain furanic monomers and then polymerizing these monomers.
- Lipids (oils): Corn, soy, castor, rapeseed (canola), cottonseed, palm, olive, almond, coconut, eucalyptus, cashew nut, algae.
- Vegetable oils are often used to derive biobased formulation ingredients for manufacturing polymeric products.
- Biobased polyols (used in polyurethane chemistry) and epoxidized soybean oil (used as a plasticizer and/or stabilizer in plastics such as PVC) are familiar examples.
- Proteins: Soy protein, wheat gluten.
- Lignin: A constituent of the cell walls of most dry land plants. The second most abundant natural polymer in the world, surpassed only by cellulose. The only one among the polymers found in plant cell walls that is not composed of carbohydrate (sugar) monomers. The only large-scale biomass source of an aromatic functionality.
The following are the main challenges to the continued growth of the use of agricultural feedstocks:
- The use of agricultural feedstocks raises many issues whose severity can vary greatly between different crops as well as different geographical regions. These potential issues include competition of land allocation for plant-based feedstock generation versus other purposes (i.e., food production), use of water for irrigation, environmental impact of pesticide use, environmental impact of fertilizer use, level of penetration of genetically modified seeds, and natural habitat (such as Amazon rainforest) conversion.
- The substitution of biobased feedstocks for petrochemical feedstocks is often either limited to partial replacement (as in most of their uses in polyurethanes), or not (yet?) possible at all, due to one or a combination of three main reasons:
- Many useful polymer formulation ingredients derivable from fossil fuel based feedstocks are not (yet?) available from biobased feedstocks.
- Ingredients available from biobased feedstocks often require expensive processes to derive and purify.
- Even after derivation and purification, the molecular structures of the resulting biobased monomers are sometimes unable to provide products possessing properties that would enable them to compete with products obtained through the use of monomers of different molecular structure derived from petrochemical feedstocks.
- Many consumers demand greater sustainability but are unwilling to sacrifice performance and/or price. Hence the frequent technical difficulty of providing a product with a similar performance at no higher a cost is a limitation.
The following are animal-based sources of biobased feedstocks:
- Chitin: A primary component of the exoskeletons of arthropods (crustaceans, such as crabs, lobsters, and shrimp) and insects, the scales of fish, and various other animal sources. Chitosan is derived by the deacetylization of chitin.
- Hyaluronic acid: Distributed widely throughout connective, epithelial, and neural tissues. It used to be produced mainly via extraction from cockscomb.
- Lipids (fats): Many animal fats and their derivatives are used in various industries.
- Collagen: Main structural protein in extracellular space in connective tissues, and mostly found in fibrous tissues, such as tendons, ligaments, hide, and skin. Gelatin is derived from collagen.
- Elastin: A fibrous protein that functions in connective tissue in partnership with collagen. The structural protein that gives elasticity to the tissues, organs, and skin of animals.
- Keratin: A fibrous protein forming the main structural constituent of hair, wool, feathers, hoofs, claws, horns, etc.
- Silk: Obtained commercially from the silkworm Bombyx mori. Work is in progress to scale up the production of spider silk.
The following are microbial sources of biobased feedstocks:
- Xanthan gum is produced industrially from carbon sources by fermentation using the gram-negative bacterium Xanthomonas campestris.
- Curdlan is produced by non-pathogenic bacteria such as Agrobacterium biobar.
- Hyaluronic acid is manufactured via fermentation by using the Streptococcus equi zooepidemicus
- Microbial polyesters:
- Polyhydroxyalkanoates are the most important examples of polyesters produced entirely by microorganisms. Bacteria accumulate them as storage materials when cultured by using suitable nutrients in suitable environmental conditions.
- Poly(lactic acid) is the most important example of polyesters produced by the polymerization of monomers prepared by a fermentation process.
While fungal sources of biobased feedstocks hold promise, the exploration and development of such sources is still in its infancy. Most of the work thus far has focused on the use of fungi to produce carbohydrates such as chitin and pullulan.
It is reasonable to conclude that, when all of the factors favoring and limiting the greater use of biobased feedstocks are balanced against each other, the utilization of biobased feedstocks in the polymer industry can be expected to continue to grow faster than the utilization of petrochemical feedstocks, resulting in continued gains in the market share of biobased feedstocks used in manufacturing polymeric products.
The recent review articles by Ferreira-Filipe et al. and Rosenboom et al. may be of interest to readers interested in learning more about this topic.