Rolf-Joachim Müller

Biodegradation of polyesters containing aromatic constituents.

Polymers, which undergo a controlled biological degradation by micro-organisms came to remarkable interest during the last years. Composting for instance could so be established as an alternative waste management system for parts of the plastic waste. Within this group of innovative polymer, polyesters play a predominant role, due to their potentially hydrolyzable ester bonds. While aromatic polyesters such as poly(ethylene terephthalate) exhibit excellent material properties but proved to be almost resistant to microbial attack, many aliphatic polyesters turned out to be biodegradable but lack in properties, which are important for application. To combine good material properties with biodegradability, aliphatic-aromatic copolyesters have been developed as biodegradable polymers for many years. This article reviews the attempts to combine aromatic and aliphatic structures in biodegradable plastics and work, which has been done to evaluate the degradation behaviour and environmental safety of biodegradable polyesters, containing aromatic constituents.

New biodegradable polyester-copolymers from commodity chemicals with favorable use properties

Copolyesters composed of aliphatic and aromatic compounds were synthesized by the polycondensation of 1,2-ethanediol, 1,3-propanediol, 1,4-butanediol, sebacic acid, adipic acid, and terephthalic acid. By applying an appropriate ratio of aliphatic to aromatic acids, the synthesized materials proved to be biodegradable, as was verified by several degradation test methods such as aqueous polymer suspension inoculated by a soil eluate (Sturm test), a soil burial test (at ambient temperature), and a composting simulation test at 60°C. The degradability of the polyester-copolymers (measured as weight loss) was investigated with respect to the aliphatic monomer components and the fraction of terephthalic acid. Excellent biodegradability was observed even for copolymers with a content of terephthalic acid up to 56 mol% (of the acid fraction) and melting points in the range up to 140°C. Degradation by chemical hydrolysis of the polyesters was determined independently and was found to facilitate microbial attack significantly only at higher temperatures. The findings demonstrate that biodegradable polymers with advantageous usage properties can easily be manufactured by conventional techniques from commodity chemicals (adipic acid, terephthalic acid, and ethylene glycol or 1,4-butanediol).

Degradation of natural and synthetic polyesters under anaerobic conditions

Often, degradability under anaerobic conditions is desirable for plastics claimed to be biodegradable, e.g. in anaerobic biowaste treatment plants, landfills and in natural anaerobic sediments. The biodegradation of the natural polyesters poly(beta-hydroxybutyrate) (PHB), poly(beta-hydroxybutyrate-co-11.6%-beta-hydroxyvalerate) (PHBV) and the synthetic polyester poly(epsilon-caprolactone) (PCL) was studied in two anaerobic sludges and individual polyester degrading anaerobic strains were isolated, characterized and used for degradation experiments under controlled laboratory conditions. Incubation of PHB and PHBV films in two anaerobic sludges exhibited significant degradation in a time scale of 6-10 weeks monitored by weight loss and biogas formation. In contrast to aerobic conditions, PHB was degraded anaerobically more rapidly than the copolyester PHBV, when tested with either mixed cultures or a single strained isolate. PCL tends to degrade slower than the natural polyesters PHB and PHBV. Four PHB and PCL degrading isolates were taxonomically identified and are obviously new species belonging to the genus Clostridium group I. The depolymerizing enzyme systems of PHB and PCL degrading isolates are supposed to be different. Using one isolated strain in an optimized laboratory degradation test with PHB powder, the degradation time was drastically reduced compared to the degradation in sludges (2 days vs. 6-10 weeks).

Biodegradable Polymeric Materials—Not the Origin but the Chemical Structure Determines Biodegradability

It is completely plausible that unmodified materials of natural origin, such as the native macromolecules cellulose or starch, are biodegradable. If these materials are modified then degradation may, depending on the degree of modification, be more difficult or even impossible. In the same manner synthesized macromolecules, whether from renewable or petrochemical sources, could be inert or completey biodegradable, depending on their chemical structure.

Enzymatic degradation of a model polyester by lipase from Rhizopus delemar

Abstract Enzymatic degradation of a model polyester was studied under varying conditions. Poly(trimethylene succinate) was hydrolyzed by the use of lipase from Rhizopus delemar . An enzyme assay was adjusted for the use of insoluble substrates and gave well-reproducible data. Ester bond cleavage was measured with respect to time. Comparison of ester cleavage and weight loss—a commonly used technique in the evaluation of polymer biodegradation—indicated that oligomers with an average length of five to six monomers are released from polymer bulk. Thus, weight loss as well as dissolved organic carbon measurements will not give information on real enzymatic degradation activity, because solubility properties of oligomers are superimposed. Time-dependent degradation profiles are strongly influenced by the materials surface (film or polydispers powder) as well as the addition of surfactants. The use of Triton X-45 did not assist the degradation of the insoluble substrate, as it did with commonly applied emulsions of liquid substrates. On the contrary, at concentrations above 0.5% (vol/vol), the addition of Triton X-45 inhibited enzymatic degradation to a great extent.

Production of a Polyester Degrading Extracellular Hydrolase from Thermomonospora fusca

The production of a polyester‐degrading hydrolase from the thermophilic actinomycete Thermomonospora fusca was investigated with regard to its potential technical application. Only in the presence of a polyester (random aliphatic‐aromatic copolyester from 1,4‐butanediol, terephthalic acid, and adipic acid with around 40–50 mol % terephthalic acid in the acid component), the excretion of the extracellular enzyme could be achieved with an optimized synthetic medium using pectin and NH4Cl as nitrogen source. Compared to complex media, a significantly higher specific activity at comparable volumetric yields could be obtained, thus reducing the expenditure for purification. The activity profile in the medium is controlled by a complex process involving (1) induction of enzyme excretion, (2) enzyme adsorption on the hydrophobic polyester surface, (3) inhibition of enzyme generation by monomers produced by polyester cleavage, and (4) enzyme denaturation. Diafiltration with cellulose acetate membranes as the sole downstream processing step led to a product of high purity and with sufficient yield (60% of total activity). Scaling‐up from shaking flasks to a fermentor scale of 100 L revealed no specific problems. However, the excretion of the hydrolase by the actinomycete turned out to be inhibited by the degradation products (monomers) of the aliphatic‐aromatic copolyester used as inductor for the enzyme production. The crude enzyme exhibited generally similar properties (temperature and pH optimum) as the highly purified hydrolase described previously; however, the storage capability and thermal stability is improved when the crude enzyme solution is diafiltrated.

Biologisch abbaubare Polymerwerkstoffe – nicht die Rohstoffquelle, sondern die chemische Struktur entscheidet über die Abbaubarkeit

Vollig plausibel ist es, das aus der Natur stammende und unveranderte Stoffe wie die nativen Makromolekule Cellulose und Starke biologisch abbaubar sind. Werden diese Stoffe modifiziert, so kann, abhangig vom Grad dieser Veranderung, der Abbau erschwert bis verhindert werden. Auch synthetisierte Makromolekule, ob auf Basis nachwachsender oder fossiler Rohstoffe, konnen – je nach ihrer chemischen Struktur – inert bis vollstandig biologisch abbaubar sein.

Structure-Biodegradability Relationship of Polyesters

The biodegradability of polymers and also polyesters is solely determined by the structure and the morphology of the plastics. To ensure environmental safety of products and to be able to design new tailor made biodegradable plastics, it is important to know the correlation of structure and biodegradability. Based on especially synthesised aliphatic polyesters, aromatic polyesters, aliphatic-aromatic copolyesters and low molecular weight oligo-esters we studied the degradation behaviour with a lipase from Pseudomonas sp. For aliphatic polyesters the difference between melting temperature of the polymer and the degradation temperature turned out to be predominantly determining the degradation. The missing degradability of aromatic polyesters is obviously not caused by a steric hindrance of the polymer — enzyme complex but must be correlated with the high melting point and also the low flexibility of the polymer chains. The degradability of aliphatic-aromatic copolyesters is not determined by the number of aliphatic ester bonds and the length of aliphatic sequences, but the length of aromatic domains. The length of these domains correlates with the melting temperature of the materials.