New and (already) improve! A report on the first IUPAC International Conference on Bio-based Polymers (ICBP 2003).
While the objective has not changed and the same players are involved--biodegradable products are greatly expanded and now include many synthetic biodegradables. Yoshiharu Doi, the conference chair, opened the first IUPAC International Conference on Bio-based Polymers with this statement:
"Bio-based polymers include various synthetic polymers derived from renewable resources and C[O.sub.2], biopolymers (nucleic acids, polyamides, polysaccharides, polyesters, polyisoprenoids and polyphenols), their derivatives, and their blends and composites. Fossil resources are limited, while renewable resources are sustainable. In the last few years, science and technology on bio-based polymers have experienced a tremendous rise in significance. The bio-based polymers have become important at both the academic and industrial research centres."
The biodegradability target has expanded from microbial polyesters to include all types of plastics as long as they are friendly to the environment. The recent book by E. S. Stevens, Green Plastics, published in 2002 by Princeton University Press is a good layperson's primer on this subject, although most of the 240 registrants (2/3 from Asia) were already "green plastics" enthusiasts.
Life cycle of PHAs
Microbial polyesters are part of the natural biosynthesis/biodegradation cycle, hence they respond to present requirements for biodegradable materials as shown below.
Bacteria can accumulate PHAs, such as poly(3-hydroxybutyrate-co-3-hydroxyvalerate), as carbon reserve. The PHAs are extracted from the cell and are utilized for commodities, such as shampoo bottles, golf tees, fibres, plastic bags, and so on. These items are quickly and easily degraded by soil enzymes when they are thrown away to nature. The enzymes can break them down into small molecules which are the very food for bacteria to produce the PHAs, again.
These biopolyesters have been a model system for learning about biodegradable thermoplastics but have failed to satisfy all possible needs and especially large scale production's requirements. The January 2004 issue of Canadian Chemical News / L'Actualite chimique canadienne (ACCN) had a lead article on the Cargill-Dow Nature Works[TM] poly(lactic acid), PLA, a synthetic biodegradable derived from fermentation of starch to lactic acid, its production by ring-opening polymerization of the lactide, recycling, etc. Large scale production of PLA leaves the bacterial polyesters on the starting blocks, at least for now. However, the storehouse of microbial knowledge concerning this manner of bioplastics production will probably see its future in production using transgenic plants.
Biodegradable synthetic plastics
In Japan, companies such as Toyota Automotive, Mitsui Chemicals, and others are committed to the PLA technology and not PHA. The reason being is the natural fibre or clay/polylactide biocomposite that is a major development in Europe, Asia, and the U.S. to replace the non-sustainable polyolefins in automobile parts. Along with this effort goes the "biorefinery" that implies cracking of natural raw materials to make valuable chemicals, many of which are not possible with present day petroleum refining. Both fundamental and focused research on polylactides is replacing the former effort on PHA. For the automotives, the specific objective is to fabricate some of the 200 or more compression molded components inside the cabin of an automobile with sustainable biodegradable plastics.
Biodegradability is not the exclusive feature of natural polymers. An increasing number of synthetic plastics mimic this "green chemistry" characteristic. It is not only the source of the polymer that confers biodegradability, equally important are texture, conformation, and the aliphatic ester or amide comonomers. The latter seems to be the trigger for initiating biodegradation in synthetic biodegradables. Thus, BASF's Ecoflex[TM], a synthetic biodegradable half-aromatic polyester, is a random copolymer based on 1,4-butanediol and a mix of terephthalic acid and adipic acid. The aliphatic components, reminiscent of PHA biopolyesters, are prominent in the successful synthetic biodegradables. For example, Bionolle[TM], a product of the Showa High Polymer Company is poly (tetramethylene succinate) with biodegradability characteristics comparable to the microbial polyesters. Other successful synthetic biodegradable thermoplastics are listed in the table.
Thus, the meaning of "bio-based" in the title of this article refers to a chemical class of polymers that mimic nature's biopolymers. The mature chemical processing of the synthetic polymer industry will often use biomass fermentation to ensure a sustainable plastics industry. The Biosynthesis-Biodegradation Cycle of PHA is a model where fermentation is the source of the biodegradable plastics. PLA products are based on a combined agro-fermentation-chemistry paradigm. Bio-based polymers for value-added applications such as drug delivery or compatible bone cement can be synthesized/biosynthesized with comb-like or block-like textures. New blends and copolymer compositions of PHAs such as Procter and Gamble's Nodax[TM], a co-polymer of butyrate/hexanoate repeat units [poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)], were described at the conference.
Of the six classes of biopolymers mentioned by Doi in his opening statement only polyesters were prominent in the program. This is in keeping with the ease of chemical synthesis of commercial polyesters. Polyolefins were only present as blends with starch; in spite of the natural abundance of polyisoprenoids, natural rubber was not mentioned. Polyphenols (which can be called lignins) are equally abundant, but their variable structure is too much of a challenge for controlled polymer synthesis. The oral presentations will be published in a special issue of Macromolecular Bioscience later in 2004.
The final lectures of the meeting were dedicated to presentations by organizations such as the Biodegradable Plastics Society of Japan (BPS). They have a trademark "GreenPla" that has guidelines for product approval and wide industrial membership. In the U.S., the Biodegradable Products Institute (BPI) is the equivalent. Products such as "starch loose fill, raincoats from unwoven poly lactic acid, PEA microwaveable trays, etc." are blazing the publicity trail. Life cycle analyses were prominent in these lectures.
This was an outstanding meeting, a rallying cry for much broader biopolymer perspective than was provided by ISBP meetings alone. The research activities in polylactide seem to have a distinct edge for commercial development. The most prominent PHA "push" was for Procter and Gamble's P (3-hydroxybutyrate-co-6mol%-3-hydroxyhexanoate) from vegetable oil fermentation.
Many academic researchers at the conference believe PHAs are the best candidate for the thermoplastic due to the sustainability--in other words--the ideal biosynthesis-biodegradation cycle. However cost and productivity problems are still formidable compared with PEA or other synthetic biodegradables. In contrast, industrial researchers favour PEA in terms of availability and price. At the 1994 ISBP meeting, the PLA success was not anticipated. Technology advances have allowed large scale production. Will the next decade bring the same success for PHAs and other bio-based materials?
Polymer trade name Composition Ecoflex Biodegradable aliphatic- aromatic (BASF) copolyester: Terephthalic acid (22%), 1,4-butanediol (50%) and adipic acid (28%) Biomax Hydro/biodegradable aliphatic-aromatic (DuPont) copolyester: ethylene glycol, diethylene glycol 85% terephthalic acid ~ 15 % adipic acid sulfo isophthalic acid CelGreen PH Homopolyester: Poly ([epsilon]- (Daicel Chemical caprolactone) Industry Ltd.) LACEA/Nature WorksPLA Homopolyester: Poly(L-lactic acid) (Mitsui Chem. Corp/Cargill Dow Polymer) Bionolle Biodegradable aliphatic polyester: (Showa Highpolymer Co.) Poly (tetramethylene succinate)
Robert H. Marchessault, FCIC, is the E.B. Eddy professor, and Jumpei Kawada, MCIC, is a postdoctoral fellow. They both hail from the department of chemistry at McGill University.
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|Author:||Marchessault, Robert H.; Kawada, Jumpei|
|Publication:||Canadian Chemical News|
|Date:||Apr 1, 2004|
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