New Horizons in R&D for Bio-Based Plastics
Versatile Materials - Going beyond natural polymers, the use of bio-based monomers from biorefineries and "renewable oil," gained from plastics' and bio wastes, renders synthetic polymers "green" and renewable without impairing their efficient processing and recycling. Independent of their new greenish marketing touch, highly versatile durable polymeric materials enable sustainable development by materials innovations and by simultaneously improving efficiency of cost, resources and energy.
Toward Sustainable Development
The quest for a green economy and the growing public awareness of climate change stimulate the surging demand for green and renewable products with a low carbon footprint. Moreover, the rapidly growing world population, projected to increase from 7 billion to 9 billion by 2050, aspires to higher quality of life. Ultimately, this will lead to drastically increased demand for energy and resources, accompanied by massive greenhouse gas emission. Sustainable development implies "meeting the needs of the present without compromising the ability of future generations to meet their own needs," according to the U.N. Brundtland Commission in 1987.
In principle, there are two strategies for achieving sustainability:
Producing sustainable materials by green chemistry.
Developing new materials and systems for sustainable development, thus enhancing energy and resource efficiency.
In both strategies, polymers play a prominent role, owing to their high versatility in terms of facile tailoring of their property profiles, processing, attractive eco-balance, flexible base of feedstocks, broad range of applications and recycling capability.
Bio-Based Plastics and Green Economy
In the euphoric bioeconomy vision, the move away from petrochemistry toward renewable biofuels, biopower and biomaterials, exploiting biomass conversion, genetic engineering and biotechnology, is seen as the holy grail in a shiny "green" future. A closed carbon dioxide cycle, powered by solar energy, is intended to produce carbon-dioxide-neutral fuels and materials (see figure 1).
At the beginning of the 21st century, we experienced a renaissance of biomaterials and a major thrust toward the development of bio-based plastics. In spite of their increasingly rapid growth, renewable natural polymers such as natural rubber, cellulose, starch, chitosan, proteins, natural-fiber-reinforced composites, and wood plastic compounds account for less than 5% of annual plastics consumption. Since the early days, most of them fail to match the high performance of synthetic polymers. In fact, many natural polymers such as carbohydrates are infusible, lack thermal stability, and exhibit rather narrow molar mass distribution, thus impairing polymer melt-processing. Bio-based polymers such as polylactide have a rather narrow processing window. Hence, instead of redesigning biopolymers, synthetic melt-processable polymers are rendered green and renewable by exploiting bio-based monomers, supplied by modern biorefineries. Unlike the biosynthesis of polymers in water, polymerization of bio-based monomers in bulk and gas phase affords far superior control of key polymer properties such as shear-thinning, melt-strengthening, and crystallization.
Prominent examples of bio-based plastics include polylactide from lactic acid (NatureWorks), green polyethylene from bioethanol (Braskem), bio-based PET ("PlantBottle" technology, Coca-Cola), and even bio-based epoxy resins, derived from bio-polyols and bio-based epichlorohydrin, which is produced from glycerol (Epicerol, Solvay). Frequently, on close inspection, several bio-based products do not meet the requirements of green chemistry. Yet the new greenish touch of polymer technology represents a powerful marketing tool.
Sustainable development requires avoiding competition with food production by using bio-wastes and lignocellulose in biomass conversion. Clearly, the envisioned closed carbon dioxide cycle is an illusion. Especially the large-scale production of biofuels requires extensive farming. This threatens food production and endangers biodiversity. Extensive use of fertilizers and pesticides causes pollution and emissions. The conversion of swamps and rainforest into farmland drastically intensifies carbon dioxide emission.
The development of biopolymers and biodegradation does not solve the plastics littering problems. In landfill, similar to durable plastics, most biopolymers do not degrade in the absence of water and air. Opposite to public expectations, most biopolymers do not instantaneously degrade to form water and carbon dioxide. Frequently, chain scissions account for embrittlement and subsequent disintegration. The resulting micro- and nanoparticles are carried away by wind and rain and may serve as food and breeding grounds for microorganisms such as fungal spores and bacteria.
'Renewable Oil' by Plastics' Waste Recycling
By definition green chemistry is not synonymous with biomaterials and biotechnology.
Polyolefins such as polyethylene and polypropylene, accounting for close to half the annual plastics production, meet the demands of green chemistry. In view of their life-cycle analysis and low carbon footprint they outperform many biopolymers. As hydrocarbon resins, equivalent to a high-molecular weight modification of oil, they represent a valuable source of energy and "renewable oil and gas."
In addition to effective recycling by remolding, oil and gas are recovered in essentially quantitative yields by thermal chain scission above 400° C. Furthermore, "renewable oil and gas" are obtained by catalytic liquefaction of biomass and even carbon dioxide conversion combined with water splitting. In sharp contrast to pulping and many biotechnological polymerization processes, polyolefins are produced in highly energy-, resource- and atom-efficient catalytic gas phase and liquid pool processes. Today, polyolefin granules are formed in the reactor without requiring solvents, purification and pelletizing extrusion.
Advanced polyolefins serve the needs of diversified markets, ranging from automotive parts to pipes, packaging, rubbers, appliances and textiles. Advanced catalyst generations and processing produce ultrastrong ultrahigh molecular weight polyethylene fibers and novel self-reinforcing "all polyolefin" composite materials for lightweight engineering. The exothermic olefin polymerization can be used as thermal power generator.
Fighting the battle of the bag - plastics against paper - and banning of polyolefin packaging is counterproductive. Both packaging materials represent a severe source of pollution when recycling fails. As compared to paper, polyethylene films, produced by blow molding, have much lower weight, higher strength, reduce both energy demand and carbon dioxide emission, cause less pollution during production, and afford "renewable oil" in recycling. In an ideal way, hydrocarbon resins preserve resources and energy for future generations.
Plastics For Sustainable Development
The current thrust toward substitution of bio-resources for fossil resources affords marginal improvements of resource and energy efficiency. For sustainable development, performance, durability, cost and recycling have much higher priority than biodegradation. Independent of their new greenish marketing touch, durable polymers have a profound influence on sustainable development and safeguard natural resources such as oil, gas, potable water, rare metals, clean air and fertile soil. In transportation and construction, lightweight engineering plastics and insulators save energy and reduce carbon dioxide emission. Without polymer composites, electro mobility would not be feasible.
Today, plastics consume around 7% of the oil but save more than twice the amount needed to produce them. Modern polymer packaging, agricultural films, and water pipes for irrigation are essential for securing the food supply. Moreover, polymer innovations preserve natural resources by desalination of water, air purification and by enabling recovery of valuable resources from wastes. As cost-efficient materials, polymers render sustainable high technologies affordable in both industrialized and developing countries.