Polymers of all types, whether from renewable resources or not, whether recyclable or not, have a role to play in the circular economy and the quest toward net zero. Moveable items made from lightweight plastics save energy during every motion. Recycling is obviously desirable, but even though some advanced composites are difficult to recycle (though not impossible), they can still make a massive contribution to fuel saving. The idea of motorists pumping their tanks full of fuel made from forestry and agricultural scrap is fanciful – better to make sustainable polymers from renewable resources. It is far better to make plastics from oil than to burn oil as a fuel.
Sadly, microplastics and the infusion of our oceans with microscopic polymer particles is a scourge that is giving a bad name to one of the most valuable materials in the race to save our planet. And it needs to be addressed without delay.
Currently, most industrial polymers and plastics are produced from non-renewable, oil or gas-based resources. However, due to recent concerns about fossil resource depletion, efforts have been made to replace conventional oil- and gas-based plastics with others based on hydrocarbons derived from renewable resources, such as biomass.
Polymer production is a major source of greenhouse gas (GHG) emissions. To reduce this, the polymer industry needs to shift towards renewable carbon feedstocks such as biomass and CO2. Both feedstocks have been shown to reduce GHG emissions in polymer production, however, often at the expense of increased utilization of the limited resources of biomass and renewable electricity.
Generating polymeric materials from renewable resources is nothing new. Naturally occurring polymers were amongst the first materials used by man. In the 19th century, natural materials, such as casein, natural rubber and cellulose, were modified to obtain useful polymeric materials. Over the past few decades, the production and application of synthetic polymers have seen an almost exponential increase. However, concerns regarding depletion of fossil resources, disposal and related issues, as well as government policies, have led to a continuously growing interest in the development of sustainable, safe and environmentally friendly plastics from renewable resources.
The importance of bio-based polymers lies in the great variety of renewable feedstock and the environmentally friendly perspectives of materials. Current achievements in polymer chemistry and the involvement of biotechnology have impressively accelerated the progress of multifunctional bio-based polymers. This is often through chemical modification of natural polymers, such as starch, cellulose or chitin. Bio-based polymers can also be synthesized through a two-step process from biomass (lignin, cellulose, starch, plant oils).
In addition, traditional monomers such as ethylene, 1,2-ethanediol, terephthalic acid, or novel monomers like lactide, 2,5-furandicarboxylic acid, 1,4-cyclohexane dicarboxylic acid, furfuryl alcohol, or isosorbide can be obtained through chemical or biochemical conversion. They can then be polymerized to produce bio-based versions of more familiar plastics. Even thermosetting polymers can be synthesized from renewable monomers.
Finally, natural bio-based polymers are synthesized by living organisms, essentially in the form in which they are finally used. Examples of naturally produced bio-based polymers include:
- bacterial polyhydroxyalkanoates
- After extraction and purification, direct industrial exploitation is possible.
By using carbon dioxide (CO2) or by micro-organisms, polymer synthesis can be achieved in plants through photosynthesis.
Biomass is also already used in various industrial-scale applications, in particular to provide bio-energy or as a feedstock for biofuels. It is also used industrially to produce surfactants or polymers such as polyethylene, polyethylene terephthalate or polyols for polyurethanes. Sugar cane bagasse, vegetable oils, and corn stover are mostly used as feedstocks for bio-based production. However, there are concerns about the large-scale implementation of bio-based production, which often increases other environmental impacts, such as acidification; biomass is seen as an increasingly important renewable energy source, for example, in the revised Renewable Energy Directive of the European Union (2018/2001/EU). However, the total amount of available biomass is limited, making efficient use of this resource essential.
Polymers may be tailored by any of four main strategies:
- selection of the most appropriate monomers for homopolymer production
- use of fillers, fibers and additives, including lignocellulose fibers or other bio-based fillers.
Uses of polymeric materials from biomass typically include packaging, antimicrobial films, fibers, foams or coatings. Applications in medicine and pharmaceutics include drug delivery systems. Polyethylene biocomposites for 3D printing are commercially available. Biomedical polymer hybrid composites hold a unique position in the fast-growing and active multidisciplinary world of biomaterials, which are crucial to solving complex medical and biomedical problems involving replacement, repair and regeneration of tissues.
The ability of a polymer to biodegrade is independent of the origin of its raw material. Instead, it strongly depends upon the structure of the polymer. For example, while some bio-based plastics may be biodegradable (e.g., polyhydroxyalkanoates), others are not (e.g., polyethylene derived from sugar cane). Some polymers degrade in only a few weeks, while others take several months.
Biodegradable polymers are finding applications in a diverse range of niche market sectors, including:
- Medical Devices: orthopedic, dental, drug release and tissue engineering
- Agriculture: mulch films, flowerpots and encapsulation of fertilizers for controlled release
- Packaging: carrier bags, waste bags and food wrapping and containers
For a plastic to be considered compostable, it must meet the following criteria:
- Biodegrade – Break down into carbon dioxide, water and biomass. 90% of the organic material is converted into CO2 within six months.
- Disintegrate – After three months’ composting and subsequent sifting through a 2 mm sieve, no more than 10% residue may remain
- Eco-toxicity – The biodegradation does not produce any toxic material, and the compost can support plant growth.
A plastic, therefore, may be degradable but not biodegradable, or it may be biodegradable but not compostable (i.e., it breaks down too slowly or leaves toxic residues).
The worldwide biodegradation standards will be reviewed for biodegradable plastics, including PHA, PLA, starch-based plastics and other biodegradable plastics, in common disposal environments, including compost, marine, anaerobic digestion, soil and landfill.
- Thinking Green: Sustainable Polymers from Renewable Resources
- Synthetic polymers from renewable feedstocks: an alternative to fossil-based materials in biomedical applications
- Renewable carbon feedstock for polymers: environmental benefits from synergistic use of biomass and CO2
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