International conference discusses the future of biodegradable thermoplastics.

The fourth International Symposium on Bacterial Polyhydroxyalkanoates (ISBP'94) examined the environmental applications of this growing area of research

ISBP'94, held at McGill University last August, was the fourth in a series of biennial meetings which started in Toronto in 1988 as a

symposium in the program of the 3rd Chemical Congress of North America, sponsored by The Chemical Institute of Canada, American Chemical Society, Sociedad Quimiica de Mexico, Instituto Mexicano del Ingenieros Quimicos and Associacion Farmaceutica Mexicana. The Toronto meeting was followed by a NATO Advanced Science Institute Conference in Sitges, Spain, in 1990 and by the third symposium in Gottingen, Germany, in 1992. Stimulated by environmental implications of this fast moving field, biologists and polymer scientists have forged a collaboration to understand and exploit these biodegradable thermoplastics.

What are Polyhydroxyalkanoates?

Polyhydroxyalkanoates (PHAs) are thermoplastic polyesters naturally produced by various microorganism (especially bacteria) and are considered leading candidates for the 'clean plastic' material eagerly sought by many environmentalists. These polyester substances are part of the natural biosynthesis/biodegradation cycle, hence they respond to present environmental requirements in various countries for biodegradable materials. The first of these polyesters was discovered by Lemoigne of the Pasteur Institute in 1925. The biopolyesters are presently produced on a relatively small scale by fermentation technology. The most available commercial version is a copolymer, as shown below, of 3-hydroxybutyrate and 3-hydroxyvalerate: P(3HB-co-3HV), produced by the bacterium Alcaligenes eutrophus fed with glucose and proprionic acid. This copolymer is produced by Zeneca Inc., U.K., with an annual production of over 700,000 lbs and nearly 50% of this production is presently sold to Japan, showing a strong commitment to biodegradable plastic development in this country.

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Attendance

A total of 170 people from 24 different countries attended the symposium, making it a truly international event. The largest block of attendance was from Western countries (Canada, 45; U.S., 45; Europe, 40). Japan and Korea had a total of 16 attendees.

Attendees not only enjoyed an excellent scientific program but also a very interesting social program which included a dazzling magic show by Ed Dawes, Hull, U.K., one of the pioneers in the PHA field. The symposium banquet was a bateau-mouche evening cruise around the starlit Montreal islands, including a dash through the Lemoine Channel opposite Isle Ste. Helene.

Scientific Program

The scientific program was comprised of 30 invited oral presentations and of over 100 poster presentations. The oral presentations dealt with various aspects of PHA research including molecular biology, enzymology of PHA and biodegradation, structure and function of inclusions, bio/chemo synthesis, novel PHA production systems, models for PHA systems, fermentation and product recovery, and finally polymer properties and applications.

As for the poster presentations, they also covered several of the aspects discussed during the oral presentations. A significant number of the poster presentations dealt with PHA production and product recovery reflecting increased focus on new substrates such as molasses, hemicelluloses and plant oils. Interestingly, six of the posters dealt with production of PHA in plants, the most promising approach for a cost breakthrough. Symposium Abstracts are available from the McGill Conference Office. The Proceedings will be published in a special issue of Canadian Journal of Microbiology.

A closed Workshop complemented the above scientific program. This Workshop was sponsored by the Centre Quebecois de Valorisation de la Biomasse (CQVB) with the goal of bringing together industry, university, and government in order to explore the 'Obstacles and Opportunities' for PHA businesses. Bruce Ramsay, MCIC (Ecole Polytechnique) acted as the kick-off speaker with a 'Present Status and Future Possibilities' report. Julie Saad (Zeneca Inc., Wilmington, DE) spoke about 'Marketing Biodegradable Polymers' describing the stringent Federal Trade Commission guidelines which have a major impact on how degradables can be marketed in the U.S. In the ensuing discussion, personal products manufacturers were adamant about the present cost for anything but value-added uses for PHAs. John Sheppard, MCIC (McGill University) presented a summary of the Workshop on the last day of the symposium.

Areas of Progress

Successive ISBP meetings have seen remarkable progress in the understanding of genetics, applications and the unique property descriptions of various PHAs. ISBP'94 was no different and several areas of obvious progress are discussed below.

Variety of PHAs

The initial polyester reported by Lemoigne in 1925 was a 'head to tail' macromolecule based on (R)-3-hydroxybutyric acid, P(3HB). This appears to be the most commonly occurring material, but a keynote speaker, Alexander Steinbuchel of Gottingen, listed 62 different repeating units now known to occur in PHAs. Depending on microbial strain and substrate, bacteria can produce polyalkanoates in as wide a variety of polymeric substrates as there are fatty acids. Protein-coated inclusions of PHA can be isolated as a concentrated suspension with all of the characteristics of an organic polymer latex, which can then be used for film formation and coating. However, it is the thermoplastic character of PHAs which has attracted most attention from plastic converters.

The molecular engineering of the inclusions has led to various categories of biopolyesters usually identified by the length of the pendant side chain. When the latter is methyl or ethyl, the commercial semi-crystalline product of Zeneca Inc. is obtained: P(3HB-co-3HV), up to 30 mole % HV. However, longer side chains give a product which is a thermoplastic elastomer. Two researchers (Eggink; de Koning) reported that the presence of double bonds, in the side chain allowed the formation of crosslinks, either by chemical or radiation methods, leading to biodegradable rubbers. The so-called LSC (long side chain) PHAs are invariably copolymers which are prepared by a judicious choice of substrates. Certain strains of Pseudomonas are most frequently used for obtaining LSC PHAs. Figure 2 shows a typical composition.

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One researcher (Braunegg) suggested that variations in the backbone chemistry is another way to classify these ubiquitous polyesters. The occurrence of copolymers based on both 3-hydroxy- and 4-hydroxybutyric acid repeating units, P(3HB-co-4HB), has recently been discovered (Doi). Increasing amounts of the 4-hydroxy- repeating unit decreases the well known brittleness of P(3HB) and leads to clear, tough films with good susceptibility to both PHA depolymerases and lipases for biodegradation. Several contributions describing this new type of copolyester, as shown in Figure 1, were reported.

Chemical Synthesis of PHAs

ISBP'94 devoted several lectures to the question of chemical synthesis which was reviewed by R. Lenz. This synthesis approach, complementary to biosynthesis, provides samples of partial tacticity for studies of biodegradation and blending (Doi). However, recent success in catalyst development (Hori) gives reason to anticipate that chemo-synthetic P(3HB) may be a way of the future which will compete with fermentation to supply biodegradable plastics. The distannoxane polymerization catalysts described by Hori are readily available and provide synthetic isotactic polymer in 99% yield with almost the same biodegradability as produced by microbial synthesis (Fuller, Lenz, Marchessault, Saito).

Improving PHB Properties

PHB (poly-3-hydroxybutyric acid) is commonly found in bacteria. Industrial interest in PHB has flagged because of two major drawbacks: poor melt stability and extreme brittleness. A group (de Koning et al., Switzerland and the Netherlands) reported that changing the lamellar morphology of PHB, by using a simple annealing treatment (heat treatment), leads to a toughening of PHB. The morphological change increases the relaxation properties of the amorphous regions in PHB, thus improving the fracture behaviour. This discovery may significantly widen the scope of possible industrial applications of PHB. PHB was said to age because of secondary crystallization.

Another notable trend in PHA technology was appreciation of the colloidal potential of the natural inclusions of PHA. Suspensions of these nascent particles have all the characteristics of an natural or synthetic latex. Their film-forming, adhesive, and electrostatic properties are being explored, the more so since aqueous suspensions of PHA allow one to avoid organic solvents in fabrication processes.

The Natural Plastic: Is It Feasible?

The high cost of PHA, so far, has cooled the interest of corporations which market personal products where biodegradability is desirable and must be relatively affordable. ISBP'94 reports suggest that a cost breakthrough is in progress. The production of PHB in plants is an attractive alternative to bacterial fermentation.

Recombinant DNA technology directed at the bioplastic has produced about 20% 'dry weight' growth of P(3HB) in Arabidopsis thaliana. This is a hundredfold increase in yield over the reports at the Gottingen ISBP meeting (Poirier, Nawrath, Sommerville). Similar work is underway at Zeneca Seeds (Fentem) using rapeseed plants whose seed oil may have a very similar biosynthetic pathway to that of PHAs. There is still a long period before field trails and large-scale processing, but a natural plastic at a cost comparable to starch now seems feasible.

If we extrapolate over the next 50-60 years, during which petroleum feedstocks will be in decline, how much land will agrotechnology require for replacing commodity plastics? Bernard Witholt of ETH, Zurich, estimates that 3% of the earth's arable surface could allow for complete substitution. This is a considerable undertaking. However, biomass transducers yielding versatile macromolecules are now part of the biotechnology tool kit.

For example, genetic engineering has lead to rapid progress in the isolation of pure synthase enzymes for in vitro synthesis of PHAs (Sinskey). A presentation by Merchant (Merchant, Dennis, Battrel) indicated that a fermentation process using a genetically-engineered E. coli strain was successfully scaled up at the Alberta Research Council pilot plant to high purity PHB. A plant design analysis concluded that this material could be produced at a price of $1.50/lb.

Chemical, fermentation, and agrotechnologies are conjugating to make natural plastics and elastomers an economic reality. Polyhydroxyalkanoates, only recently appreciated as ranking with polysaccharides, proteins, polynucleotides and polyisoprenoids as a class of physiologically important biopolymers, have brought a solution to the search for biodegradable plastics. Even more, they have provided new insights to membrane ionic transport (Reusch) and applications ranging from chiral synthons (Seebach) to fat substitutes in low calorie foods (Yalpani).

Next ISBP Meeting

Bernard Witholt (Zurich, Switzerland) has offered to organize the next ISBP meeting in 1996.

R.H. Marchessault, FCIC, is the E.B. Eddy professor in the Department of Chemistry at McGill University (Montreal). D. Groleau works in the Bioprocess Sector at the NRC's Biotechnology Research Institute (Montreal). B. Ramsay, MCIC, is a professor in the Department of Chemical Engineering at Ecole Polytechnique de Montreal.