Evaluation of the biodegradability of copolyesters containing aromatic compounds by investigations of model oligomers

Model oligo esters of terephthalic acid with 1,2-ethanediol, 1,3-propanediol, and 1,4-butanediol have been investigated with regard to their biodegradability in different biological environments. Well-characterized oligomers with weight-average molar masses of from 600 to 2600 g/mol exhibit biodegradation in aqueous systems, soil, and compost at 60°C. SEC investigations showed a fast biological degradation of the oligomer fraction consisting of 1 or 2 repeating units, independent of the diol component used for polycondensation, while polyester oligomers with degrees of polymerization higher than 2 were stable against microbial attack at room temperature in a time frame of 2 months. At 60°C in a compost environment chemical hydrolysis also degrades chains longer than two repeating units, resulting in enhanced degradability of the oligomers. Metabolization of the monomers and the dimers as well by the microorganisms could be confirmed by comparing SEC measurements and carbon balances in a Sturm test experiment. Based on these results degradation characteristics of potential oligomer intermediates resulting from a primary chain scission from copolyesters consisting of aromatic and aliphatic dicarbonic acids can be predicted depending on their composition. These results will have an evident influence on the evaluation of the biodegradability of commercially interesting copolyesters and lead to new ways of tailor-made designing of new biodegradable materials as well.     

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).Dedicated to Prof. J. Klein's 60th birthday.     

The veterinary antibiotic oxytetracycline and Cu influence functional diversity of the soil microbial community

There are increasing concerns over the effects of veterinary antibiotics and heavy metals in agricultural soils. The widely used veterinary antibiotic oxytetracycline (OTC), Cu and their combination on soil microbial community function were assessed with the Biolog method. The microbial community was extracted from the soil and exposed to a 0.85% sodium chloride solution containing OTC (0, 1, 5, 11, 43, 109 and 217 microM), or Cu (0, 10, 20, 100 and 300 microM), or combination of the two pollutants (OTC 0, 5, 11 microM and Cu 0, 20 microM). Functional diversity, evenness, average well color development (AWCD) and substrate utilization decreased significantly with increasing concentrations of OTC or Cu (p < 0.005). The critical concentrations were 11 microM for OTC and 20 microM for Cu. The combination of OTC and Cu significantly decreased Shannon's diversity, evenness and utilization of carbohydrates and carboxylic acids compared to individual one of the contaminants. The antibiotic OTC and Cu had significant negative effects on soil microbial community function, particularly when both pollutants were present.     

Biodegradation behavior and material properties of aliphatic/aromatic polyesters of commercial importance

Copolyesters of aliphatic monomers with a defined amount of terephthalic acid recently have been shown to be biodegradable. This group of plastic materials exhibits very interesting material properties with regard to their technical application potential. A tensile strength of 25 N/mm² combined with elongations at break up to 1500% was achieved for BTA materials. Melting points varied from 80 to 140‡C. Biodegradation rate under compositing conditions were determined, showing typical erosion rates of films, in the range of 5 to 10 Μm/week. The material properties and the degradation rate as well can be adjusted by the copolymer composition. Stretching of the polymer in the cold state leads to 10-fold higher mechanical strength of the material. The polyester chain can be extended to high molar masses, resulting in melt viscosities suitable, e.g., for melt below extrusion.     

Calculation of carbon balances for evaluation of the biodegradability of polymers

Establishing carbon balances has been proven to be an applicable and powerful tool in testing biodegradability of polymers. In controlled degradation tests at a 4-L scale with the model polymer poly(-hydroxybutyrate) (PHB), it was shown that the degree of degradation could not be determined with satisfactory accuracy from CO₂ release alone. Instead, the course of degradation was characterized by means of establishing carbon balances for the degradation of PHB withAcidovorax facilis and a mixed culture derived from compost. Different analytical methods for determining the different carbon fractions were adapted to the particular test conditions and compared. Quantitative determination of biomass and residual polymer were the main problems in establishing carbon balances. Amounts of biomass derived from protein measurements depend strongly on assumptions of the protein content of the biomass. Selective oxidation of biomass with hypochlorite was used as alternative, but here problems arose from insoluble metabolic products. Determination of soluble components with the method of chemical oxygen demand (COD) also includes empirical assumptions but seems acceptable if the dissolved carbon fraction is in the range of some 10% total carbon. Results confirm both analytical assays and theoretical approaches, in ending up at values very close to 100%, within an acceptable standard deviation range under test conditions comparable to standard test practice.Paper presented at the Bio/Environmentally Degradable Polymer Society—Third National Meeting, June 6–8, 1994, Boston, Massachusetts.     

Biodegradation of aliphatic-aromatic copolyesters: evaluation of the final biodegradability and ecotoxicological impact of degradation intermediates.

The biological degradation behaviour of the aliphatic-aromatic copolyester Ecoflex was investigated with regard to the degree of degradation and the intermediates formed during the degradation process. The individual thermophilic strain Thermomonospora fusca, isolated from compost material, was used for the degradation experiments in a defined synthetic medium at 55 degrees C. After 22 days of degradation more than 99.9% of the polymer had depolymerized and with regard to the degradation of the diacid and diol components of Ecoflex only the monomers of the copolyesters (1,4-butanediol, terephthalate and adipate) could be detected by gas chromatography/mass spectroscopy (GC-MS) measurements in the medium. In interrupted degradation experiments predominantly the monoesters of adipic acid and terephthalic acid with 1,4-butanediol were observed in addition to the monomers. In toxicological tests with Daphnia magna and Photobacterium phosphoreum no significant toxicological effect was observed, neither for the monomeric intermediates nor for the oligomeric intermediates. From a risk assessment it can be concluded that there is no indication for an environmental risk when aliphatic-aromatic copolyesters of the Ecoflex-type are introduced into composting processes.