Degradable Biopolymers

The polymer age – which arguably began in 1907 with Leo Baekeland’s Bakelite – has vastly improved our standard of living, but not without undesirable consequences. Worldwide production of synthetic plastics currently accounts for over 60m jobs and yields over 200bn kg of material each year – nearly 30kg per person. Commercial polymers consume around 5% of the earth’s finite reserves of fossil fuels such as natural gas and petroleum from which they are made. Most are designed for durability and will easily persist 1000 years under normal conditions, greatly contributing to the world’s growing trash crisis.

Plastic bag litter is just one example of the problem. More than one trillion grocery/retail polyethylene bags are consumed each year. This amounts to over 4bn kg and over 2% of all plastics. Attempting to counter this blight, many municipalities around the world have banned plastic bags, or prohibited their free distribution.

Where does all the plastic go? Recycling efforts vary considerably from country to county. Recycling rates for polyethylene terephthalate (PET), for example, are high in France (>80%), but low in the US (<30%). While some polymers are incinerated for energy production, most are discarded either to landfill or worse, directly into the open environment. Those plastics that float, principally polyethylene and polypropylene, have an easy route to the ocean, which is the destination for an estimated 5bn kg of plastic/year – equivalent to nearly 10,000kg/minute!

These startling numbers, combined with the deleterious effects of floating plastics on ocean ecosystems, have prompted the X Prize Foundation to develop the Ocean Plastics X Prize, to be awarded to a team that invents an environmentally safe, ocean-degradable alternative to petroleum plastics at scalable volume.

My own research group at the University of Florida, for example, is busy attempting to find a solution by creating ‘sustainable polymers’ that address the major drawbacks of traditional polymers. First, sustainable polymers are designed to have a ‘green birth’ and derive from annually renewable chemical building blocks, such as plant-based polysaccharides (cellulose), lignin, or triglycerides (bio-oils). Second, they are designed to undergo programmed or facile degradation, returning to small molecular units that can be easily metabolised back to carbon dioxide via normal microbial action.

Today, only about 5% of US chemical production employs biorenewable feedstocks. Several US governmental resolutions and (non-binding) targets plan for an increase to 25% by the year 2030, while companies such as Dow Chemical and BASF have disclosed more ambitious goals. The amount of biomass generated annually, mainly by photosynthesis, has been estimated at 170 trillion kg, or about 1000 times more than the quantity of synthetic plastics produced. While this amount of biomass is not enough to satisfy our energy needs, it is more than sufficient to satisfy our need for polymeric building blocks.

Perhaps the most successful synthetic, sustainable plastic is the polyester polylactic acid (PLA). The world’s largest PLA plant is in Blair, Nebraska, US, where cornstarch is converted to glucose, which is then partially fermented to lactic acid. The capacity of this NatureWorks plant is 250m kg/year, which is about 1000 times smaller than the output of the whole polymer industry. Nonetheless, PLA is readily found in disposable packaging goods, such as restaurant cups and fruit trays, and in durable goods, such as Ingeo fibres for textiles.

Many industry observers thought that PLA would never become economically viable because of the high cost: initially 15 to 20 times the price of polyolefins. However, economies of scale mean the price is now only 15% to 25% more than fossil fuel-based PET (polyethylene terephthalate) polyester, widely used in water bottles, films and the textile industry.

While PLA has garnered many environmental accolades; however, it is not perfect. For example, it does not break down or biodegrade in a conventional landfill, so rivalling classical polymers with respect to slow degradation. Sending these materials to industrial composting sites for proper biodegradation is not often feasible. Moreover, the useful temperature range of PLA is limited by its low glass transition temperature of 55°C, above which the polymer loses its rigidity and succumbs to plastic deformation. As such, it is not appropriate for hot beverages or hot food and usually is marked with a warning such as ‘cold drinks only’.

Polymer development in my research group has been inspired by nature. Half of all the organic carbon in our environment is contained in the world’s most abundant polymer: cellulose. Usefully, the glucose monomer units making up cellulose are also connected by hydrolysable acetal (–OCO–) – a functional group that is surprisingly rare in synthetic and commercial polymer chemistry. By installing this readily hydrolysed acetal group into the backbone of polylactic acid (PLA), for example, we have been able to produce a water-degradable PLA variant. Based on the rate at which the polymer loses mass in distilled water or Atlantic seawater, which regular PLA and polyolefins do not, our new PLA polymer is expected to take five to 10 years to degrade.

The group has also developed a variety of other polyacetals that are structurally akin to polyethylene but derived from bio-oils. Future work is aimed at matching the thermal and mechanical properties of polyethylene, but retaining the water-degradability of polyacetals – which could significantly reduce the amount of persistent plastics added to the oceans each year.

Yet another polymer of interest in my group, meanwhile, is polyethylene terephthalate (PET), which accounts for about 18% of the global plastics market. Americans discard over 6m PET bottles/hour. Since PET is an aromatic polyester, a reasonable approach to matching the thermal properties of PET is to commandeer aromatics from nature and employ them as monomeric feedstocks. Lignin, which makes up about 30% of a typical tree, is the second most abundant organic polymer on earth and is the best source of renewable aromatics. Lignin is the major byproduct of the paper pulping industry, but nearly all of it is burned for onsite energy production.

Technology exists, however, to extract two discrete molecules, vanillin and ferulic acid, from lignin and lignocellulose from wheat or rice bran. These aromatic molecules have the potential to be abundantly available and are key starting materials investigated by my group. For example, the aromatic polyester polydihydroferulic acid (PHFA) exhibits a glass transition temperature that slightly exceeds that of PET (Tg = 73°C vs. 67°C). Importantly, the hydrolytic degradation product of PHFA is dihydroferulic acid, a dietary antioxidant found in tea, coffee, whole grains, and other antioxidant rich foods. Its relative, ferulic acid, is even available in 250mg tablets as a dietary supplement. More recent efforts have developed copolymerisation strategies that afford PHFA copolymers with glass transition temperatures over 150°C. The wider temperature range of such materials makes them excellent candidates for replacing polystyrene (Tg = 95°C) in cups, plates, utensils, disposable packaging etc, and for hot-fill bottling and canning processes.

Finally, an ongoing collaboration with fashion designer Natalia Allen seeks to spin these novel aromatic polyesters into fibres for use in eco-friendly textiles.

Ethylene is the most abundantly produced commercial organic molecule and much of it goes into the 80bn kg of polyethylene produced each year — about a 40% market share of all synthetic polymers. Because of increasing and tangible efforts to replace fossil fuels with sustainable biomass, it has been proclaimed that sugar (glucose, the monomer of cellulose) is the ‘new oil’. If that is the case, then it is logical to ask: ‘What is the new ethylene?’ After some consideration my group has heralded formaldehyde as the new ethylene.

Formaldehyde and ethylene have nearly identical mass and both contain carbon participating in a reactive double bond, but formaldehyde also shares the oxidation state and empirical formula of biomass, which is approximately that of cellulose and sugar, CH₂O. One targeted application of formaldehyde is its perfectly alternating copolymerisation with carbon monoxide, which has been optimised to yield polyglycolic acid (PGA) that is functionally equivalent to commercial PGA, but is made in fewer steps with significantly less generation of wasteful byproducts.

Previously, the cost and diminutive production scale of PGA – a relative of PLA – has precluded its use in commercial packaging applications. Importantly, the inexpensive C1 monomers formaldehyde and carbon monoxide are potentially made from methanol, which 100 years ago was sustainably produced via the destructive distillation of wood (hence its common name of ‘wood alcohol’).

A methanol economy has received serious consideration as a sustainable, biobased successor to the fossil fuel economy. To the extent that the methanol economy enjoys future success, polymers derived from this C1 feedstock will become economically privileged.

An impediment to the growth of green polymers is the enormous momentum of the existing fossil fuel-based polymer industry. The chief technology officer of a Fortune 100 Company recently wrote to me to say that they are ‘highly interested in finding bio-based routes to [incumbent] commodity plastics which fit with current processing and disposal infrastructures,’ but are ‘less concerned with developing new-to-the-world biodegradable plastics for our packaging and products.’ This mindset partly explains the success of bio-polyethylene and the PET PlantBottle. Using bioethanol from fermented sugarcane sugar, Dow Chemical and Braskem already make bio-polyethylene, which is technically the volume leader in biorenewable synthetic polymers. Similarly Coca-Cola has exploited bioethanol to synthesise the ethylene glycol comonomer of the PET PlantBottle, thus achieving 30% plant-based material. These approaches have nominally addressed the ‘green birth’ challenge, but accomplish little with regards to the ‘green death’ of a polymer. Indeed, bio-polyethylene and the PET PlantBottle possess the same woeful degradation behaviours as their fossil-fuel cognates.

The global plastics market is a $1200bn/year industry. Short-term packaging applications constitute about one-third of this amount, or $400bn/year. In 2007, the green polymer industry in the US was worth a meagre $1bn, but is rapidly headed toward $10bn in 2020. Sustainable polymers will continue to displace traditional materials.

Akin to turning an oil tanker, the industry momentum is changing fractionally and slowly. A quantum leap will likely occur only in response to considerable consumer demand coupled with a willingness to absorb the extra cost, at least during the introduction and growth phases of the next green polymer.

The technical substitution potential of biobased plastics, including synthetic fibres, is 90%. That is, only about 10% of the polymers we use today must derive from fossil fuels because of specific performance characteristics that have not been achieved with green polymers. Hence, it is possible that the bioplastics packaging industry will grow to several hundred billion dollars/year.

This growth will be accelerated by decreasing fossil fuel resources, increasing fossil fuel prices and an evolving consumer attitude abetted by improved marketing campaigns. And, of course, technical innovations such as those being explored by my own group in Florida will hopefully drive this growth as well.

Further reading

1 Feedstocks for the future; J. J. Bozell; M. K Patel, eds., ACS Symposium Ser. 921, Oxford University Press, 2006.
2 J. J. Bozell, Soil, Air, Water, 2008, 36(8), 641.
3 L. Shen, E. Worrell and M. Patel, Biofuels, Bioproducts and Biorefining, 2010, 4 (1), 25.
4 T. Kijchavengkul and R. Auras, Polym. Int., 2008, 57, 793.
5 G. A. Olah, Angew. Chem. Int. Ed., 2005, 44, 2636.
6 G. A. Olah, A. Goeppert and G. K. S. Prakash, Beyond oil and gas: the methanol economy; Wiley-VCH: Weinheim, 2006.
7 A. G. Pemba, J. A. Flores and S. A. Miller, Green Chemistry, 2013, 15, 325.
8 L. Mialon, R. Vanderhenst, A. G. Pemba and S. A. Miller, Macromol. Rapid Commun., 2011, 32, 1386.

Stephen A. Miller is professor of chemistry in the department of chemistry at the University of Florida, Gainesville, FL, US.