biodegradability of polymers-regulations and methods for testing

12

Biodegradability ofPolymers: Regulations andMethods for Testing

Dr. Rolf-Joachim M¸llerGesellschaft f¸r Biotechnologische Forschung mbH, Braunschweig, Germany;Tel.: �49 (0)531 6181 610; Fax: �49 (0)531 6181 175; E-mail: [email protected]

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 366

2 General Mechanism of Biodegradation, and Definitions . . . . . . . . . . . . . 367

3 Testing Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3703.1 General Principles in Testing Biodegradable Plastics . . . . . . . . . . . . . . 3733.2 Analytical Procedures for Monitoring Biodegradation . . . . . . . . . . . . . . 3753.2.1 Visual Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3753.2.2 Changes in Mechanical Properties and Molar Mass . . . . . . . . . . . . . . . 3753.2.3 Weight Loss Measurements: Determination of Residual Polymer . . . . . . . 3763.2.4 CO2 evolution/O2 Consumption . . . . . . . . . . . . . . . . . . . . . . . . . . 3773.2.5 Determination of Biogas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3783.2.6 Radiolabeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3783.2.7 Clear-zone Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3783.2.8 Others . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3793.3 Development of Standardized Biodegradation Tests . . . . . . . . . . . . . . . 3793.3.1 Testing Compostability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3813.3.2 Testing Anaerobic Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . 3833.3.3 Testing Biodegradation in Soil . . . . . . . . . . . . . . . . . . . . . . . . . . . 384

4 Regulations Concerning Biodegradable Plastics . . . . . . . . . . . . . . . . . . 385

5 Certification and Labeling of Biodegradable Plastics . . . . . . . . . . . . . . . 387

6 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 388

ASTM American Society for Testing and MaterialsAFM atomic force microscopyCEN European Committee for Standardization (Comitÿ Europÿen de Normalisation)

365

DIN German Institute for Standardization (Deutsches Institut f¸r Normung)DOC dissolved organic carbonISO International Standardization OrganizationJIS Japanese Institute for StandardizationMITI Ministry of International Trade and Industry (Japan)OECD Organization for Economic Co-operation and Development (Europe)PHB poly(�-hydroxybutyrate)SEM scanning electron microscopyUNI Italian National Standards Body (Ente Nationale Italiano di Unificazione)

1

Introduction

Since the first developments of polymericmaterials, scientists and engineers havemade intensive efforts to increase thestability of these materials with regard totheir diverse environmental influences. As aresult, polymeric materials (plastics) arenow used in all sectors of life as verydurable products with tailor-made proper-ties. During the past decade the intense useof modern plastics, combined with theirenormous stability, has created seriousproblems with plastic waste, with the mainproblems being caused by plastic packaging.As possible alternative waste managementstrategies to landfilling, incineration orplastics recycling are not optimal andremain the subject of much controversyand discussion among both scientists andthe public. On the basis of these problems,an intensive activity has been undertakensince the early 1990s to develop novelplastics which have performance compara-ble with that of conventional polymers, butare also susceptible to microbial degrada-tion. The intention was that these materialswould reduce waste deposit volume whileundergoing degradation in a landfill, oralternatively they could be treated in com-posting plants. These technologies offered anew approach to themanagement of plasticswaste. Moreover, when this waste manage-

ment system is combined with the use ofrenewable resources to produce the poly-mers initially, it is likely that biodegradableplastics may simply become part of a naturalcycle.The biodegradability of plastics provides

these materials with novel and additionalproperties which may also be beneficialduring their use. For example, in agriculturebiodegradablemulch films would not need tobe collected after use (a procedure which ishighly labor-intensive) and then landfilled orincinerated, but would decompose with timeand could simply be ploughed into the soil,where they would biodegrade.Hence, it is notsurprising that this concept of using bio-degradable plastics has become of majorinterest during recent years.Biodegradable plastics, as novel materials,

make claims to be environmentally friendly.Consequently, it must be proved by usingscientifically based and generally acceptedmethods that this is indeed the case. A firstgeneration of biodegradable plastics consist-ed simply of polyethylenes blended withstarch. Initially, these were sold as bio-degradable plastics, but in practice they didnot fulfill the expectations of the users.Arguments for claiming these blends asbiodegradable included the growth of micro-organisms on the material's surface, or acertain loss in mechanical properties (e.g.,tensile strength) when they were exposed tothe environment. However, the evaluation

12 Biodegradability of Polymers: Regulations and Methods for Testing366

methods used ± which had originated fromthe field of plastics biocorrosion ± provedunsuitable to characterize biodegradablematerials. At the time, the failure of thesepolyethylenes led to a generally negativeimage of biodegradable plastics; however,the subsequent development of suitabletesting methods and evaluation criteria forbiodegradable plastics has resulted in thedefinition of standards by various nationaland international standardization bodiesduring the past 10 years. Indeed, thisprocess is ongoing, as the number ofdifferent environments in which plasticsmay be degraded has made necessary theestablishment of a complex and extensivebattery of test methods and evaluationcriteria for these materials.

2

General Mechanism of Biodegradation, andDefinitions

The term ™biodegradable plastics∫ normallyrefers to an attack by microorganisms onnonwater-soluble polymer-based materials(plastics). This implies that the biodegrada-tion of plastics is usually a heterogeneousprocess. Because of a lack of water-solubilityand the size of the polymer molecules,microorganisms are unable to transportthe polymeric material directly into the cellswhere most biochemical processes takeplace; rather, they must first excrete extra-cellular enzymes which depolymerize thepolymers outside the cells (Figure 1). As aconsequence, if the molar mass of thepolymers can be sufficiently reduced togenerate water-soluble intermediates, thesecan be transported into the microorganismsand fed into the appropriate metabolicpathway(s). As a result, the end-products ofthese metabolic processes include water,carbon dioxide and methane (in the case of

anaerobic degradation), together with a newbiomass. The extracellular enzymes are toolarge to penetrate deeply into the polymermaterial, and so act only on the polymersurface; consequently, the biodegradation ofplastics is usually a surface erosion process.Although the enzyme-catalyzed chain

length reduction of polymers is in manycases the primary process of biodegradation,nonbiotic chemical and physical processescan also act on the polymer, either in parallelor as a first stage solely on the polymer.These nonbiotic effects include chemicalhydrolysis, thermal polymer degradation,and oxidation or scission of the polymerchains by irradiation (photodegradation).For some materials, these effects are useddirectly to induce the biodegradation process[e.g., poly(lactic acid); pro-oxidant modifiedpolyethylene], but they must also to be takeninto account when biodegradation is causedpredominantly by extracellular enzymes.Because of the coexistence of biotic andnonbiotic processes, the entire mechanismof polymer degradation could ± in manycases ± also be referred to as environmentaldegradation.Environmental factors not only influence

the polymer to be degraded, they also have acrucial influence on the microbial popula-tion and on the activity of the differentmicroorganisms themselves. Parameterssuch as humidity, temperature, pH, salini-ty, the presence or absence of oxygen and thesupply of different nutrients have importanteffects on the microbial degradation ofpolymers, and so these conditions must beconsidered when the biodegradability ofplastics is tested.Another complicating factor in plastics

biodegradation is the complexity of theplastic materials with regard to their possi-ble structures and compositions. In manycases plastics do not consist simply of onlyone chemical homogeneous component, but

2 General Mechanism of Biodegradation, and Definitions 367

contain different polymers (blends) or low-molecular weight additives (e.g., plasticiz-ers). Moreover, within one polymer itselfdifferent structural elements can be present(copolymers), and these may either bedistributed statistically along the polymerchains (random copolymers) or distributedalternately (alternating copolyesters); theymay also be used to build longer blocks ofeach structure (block-copolymers). Anotherstructural characteristic of a polymer is thepossible branching of chains or the forma-tion of networks (cross-linked polymers).These different structures of a polymer,despite having the same overall composi-tion, can directly influence accessibility ofthe material to the enzyme-catalyzed poly-mer chain cleavage, and also have a crucialimpact on higher-ordered structures of thepolymers (crystals, crystallinity, glass tran-sition) which have been shown predomi-nantly to control the degradation behavior ofmany polymers (Marten, 2000). Addition-ally, the crystallinity and crystal morphologyis dependent upon the processing condi-tions, and can change with time.

All of the above-described factors must beconsidered when measuring the biodegra-dation of plastics and interpreting theresults, and this makes the testing ofplastics biodegradability a highly interdisci-plinary process.The standardized evaluation of bio-

degradable plastics should always be basedon definitions, and what biodegradationwith regard to plastics actually means.Several different definitions have beenpublished by national and internationalstandardization bodies and organizations(Table 1).Whilst in the ISO definition of bio-

degradable plastics only a chemical changeof the material (e.g., oxidation) by micro-organisms is requested, the CEN and DIN,in contrast, demand in their definitions theconversion of plastics into microbial meta-bolic products. Other definitions such asinherent biodegradability or ultimate biode-gradability are adapted from the area ofdegradation of low molecular- weight chem-icals, but these can also be applied topolymers. Generally, the definitions do not


Fig. 1 General mechanism of plastics biodegradation.

specify any environment or time frames;this must be carried out according tocorresponding standards.Based on these definitions, biodegradable

plastics (or packaging materials) are notnecessarily suitable for composting. In the

definition of compostability, biodegradationof the material is only one requirement, andfurther demands such as good compostquality after composting plastics are alsoincluded.

2 General Mechanism of Biodegradation, and Definitions 369

Tab. 1 Definitions used in correlation with biodegradable plastics

DINFNK 103.2

Biodegradable plastics1)

A plastic material is called biodegradable if all its organic compounds undergo acomplete biodegradation process. Environmental conditions and rates of biodegrada-tion are to be determined by standardized test methods.Biodegradation3)

Biodegradation is a process, caused by biological activity, which leads under change ofthe chemical structure to naturally occurring metabolic products.

ASTMsub-committeeD20-96


A degradable plastic in which the degradation results from the action of naturallyoccurring microorganisms such as bacteria, fungi and algae.

Japanese Bio-degradable Plas-tics Society


Polymeric materials which are changed into lower molecular weight compounds whereat least one step in the degradation process is though metabolism in the presence ofnaturally occurring organisms.

ISO 472 Biodegradable plastics1)

A plastic designed to undergo a significant change in its chemical structure underspecific environmental conditions resulting in a loss of some properties thatmay vary asmeasured by standard test methods appropriate to the plastic and the application in aperiod of time that determines its classification. The change in the chemical structureresults from the action of naturally occurring microorganisms.

CEN Biodegradable plastics1)

A degradable material in which the degradation results from the action of micro-organisms and ultimately the material is converted to water, carbon dioxide and/ormethane and a new cell biomass.Biodegradation2)

Biodegradation is a degradation caused by biological activity, especially by enzymaticaction, leading to a significant change in the chemical structure of a materialInherent biodegradability2)

The potential of a material to be biodegraded, established under laboratory conditions.Ultimate biodegradability2)

The breakdown of an organic chemical compound bymicroorganisms in the presence ofoxygen to biodegradability carbon dioxide, water andmineral salts of any other elementspresent (mineralization) and new biomass or in the absence of oxygen to carbon dioxide,methane, mineral salts and new biomass.Compostability2)

Compostability is a property of a packaging to be biodegraded in a composting process.To claim compostability it must have been demonstrated that a packaging can bebiodegraded in a composting system as can be shown by standard methods. The end-product must meet the relevant compost quality criteria.

1) Pagga (1998);2) Calmon-Decriaud et al. (1998);3) DIN V 94900 (1998)

However, despite these apparently inho-mogeneous definitions, the different stand-ards and evaluation schemes are surpris-ingly congruent.

3

Testing Methods

Test methods to determine biological actionon man-made materials have been availablefor many years, and for different classes ofmaterials. Nowadays, the evaluation of thedegradability of chemicals in the environ-ment (and especially in waste water) as oneimportant aspect of the ecological impact of acompound has become very important whenattempting to bring a new chemical productto the marketplace. For this reason, a largenumber of standardized tests have beendeveloped for different environments, andwith the use of different analytical methods(Pagga, 1997). An overview of existinginternational standards in this area isprovided in Table 2.Also for the evaluation of the influence of

microorganisms on polymer, test methodshave been established long before bio-degradable plastics were first developed.Although conventional plastics are relative-ly resistant against environmental influen-

ces, in some cases microorganisms canpartially attack the plastics and cause un-desired changes in the material properties,e.g. in the color or in mechanical propertiessuch as flexibility or mechanical strength(Pantke, 1977; Seal and Eggins, 1981)(Table 3).Although this wide range of degradation

tests already existed, it was necessary todevelop special test methods when dealingwith biodegradable plastics. The standardslisted in Table 3 do not consider the specialaspects (see above) of plastics materials, andfor biocorrosion phenomena the question isnot whether the plastic is degraded or not,but rather whether changes in the materialproperties may be caused byminor chemicalchanges in the polymers (e.g., extraction ofplasticizer, oxidation, etc.).The test methods which have been devel-

oped especially for biodegradable plasticsduring the past decade (It‰vaara and Vik-man, 1996) are predominantly based onprinciples used for the evaluation of lowmolecular-weight substances, but have beenmodified with respect to the particular envi-ronments in which biodegradable plasticsmight be degraded. The methods also consid-er the fact that plastics often have a complexcomposition and are degraded mainly by aheterogeneous surface mechanism.


Tab. 2 Standard test methods for biocorrosion phenomena on plastics

ASTM G21-96 Standard practice for determining resistance of synthetic polymer materials to fungi

ASTM G29-96 Standard practice for determining algal resistance of plastic films

DIN IEC 60068±2-10-1991

Elektrotechnik; Grundlegende Umweltpr¸fverfahren; Pr¸fung J und Leitfaden:Schimmelwachstum; (Identisch mit IEC 60068±2-10: 1988)

EN ISO 846±1997

Plastics ± Evaluation of the action of microorganisms

IEC 60068±2-10-1988

Elektrotechnik; Grundlegende Umweltpr¸fverfahren; Pr¸fung J: Schimmelwachstum

ISO 846±1997 Plastics: Determination of behaviour under the action of fungi and bacteria. Evaluationby visual examination or measurement of changes in mass or physical properties

3 Testing Methods 371

Tab. 3 Standard test methods for biodegradability of chemicals

OECD Guidelines (OECD, 1993)301 Ready Biodegradability301 A±1992 DOC Die-Away Test301 B±1992 CO2 Evolution Test301 C±1992 Modified MITI Test301 D±1992 Closed Bottle Test301 E±1992 Modified OECD Screening Test301 F±1992 Manometric Respirometry Test302 Inherent Biodegradability302 A-1981 Modified SCAS Test302 B-1992 Zahn-Wellens Test302 C-1981 Modified MITI Test302 D B draft (2002) Inherent biodegradability-concave test303 Simulation Test303 A-2001 Aerobic Sewage Treatment: Activated Sludge Units304 Biodegradation in Soil304 A-1981 Inherent Biodegradability in Soil306-1992 Biodegradability in Seawater307 B draft (2000) Aerobic and anaerobic transformation in soil308 B draft (2000) Aerobic and anaerobic transformation in aquatic sediment systems309 B draft (2001) Aerobic mineralisation in surface water-simulation biodegradation test301 B draft (2002) Ready biodegradability B CO2 in sealed vessels (Headspace test)311 B draft (2002) Ready anaerobic biodegradability:Gasproduction fromdiluted anaerobic sewage sludgeISO 7827±1994 Water quality ± Evaluation in an aqueous medium of the ™ultimate∫ aerobic

biodegradability of organic compounds ± Method by analysis of dissolved organiccarbon (DOC)

ISO 9439±1999 Water quality ± Evaluation in an aqueous medium of the ™ultimate∫ aerobicbiodegradability of organic compounds ± Method by analysis of released carbondioxide

ISO 9408±1999 Water quality ± Evaluation in an aqueous medium of the ™ultimate∫ aerobicbiodegradability of organic compounds ± Method by determining the oxygendemand in a closed respirometer

ISO 9887±1992 Water quality ± Evaluation of the aerobic biodegradability of organic compounds inan aqueous medium ± Semi-continuous activated sludge method (SCAS)

ISO 9888 B 1999 Water quality ± Evaluation of the aerobic biodegradability of organic compounds inan aqueous medium ± Static test (Zahn-Wellens method)

ISO 10634±1995 Water quality ± Guidance for the preparation and treatment of poorly water-solubleorganic compounds for the subsequent evaluation of their biodegradability in anaqueous medium

ISO 10707±1994 Water quality ± Evaluation in an aqueous medium of the ultimate aerobicbiodegradability of organic compounds ±Method by analysis of biochemical oxygendemand (closed bottle test)

ISO 10708±1997 Water quality ± Evaluation in an aqueous medium of the ultimate aerobicbiodegradability of organic compounds ± Method by determining the biochemicaloxygen demand in a two-phase closed ± bottle test

ISO 11733±1995 Water quality ± Evaluation of the elimination and the biodegradability of organiccompounds in an aqueous medium. Activated sludge simulation test

ISO 11734±1995 Water quality ± Evaluation of the ultimate anaerobic biodegradability of organiccompounds in digested sludge. Method by measurement of the biogas production


Tab. 3 (cont.)

ISO/FDIS 14592±1 Water quality ± Evaluation of the aerobic biodegradability of organic compounds atlow concentrations ± Part 1: Shake-flask batch test with surface water or surfacewater/sediment suspension

ISO/DIS 14592±2 Water quality ± Evaluation of the aerobic biodegradability of organic compounds atlow concentrations ± Part 2: Continuous flow river model with attached biomass

ISO 14593±1999 Water quality ± Evaluation of ultimate aerobic biodegradability of organiccompounds in aqueousmedium ±Method by analysis of inorganic carbon in sealedvessels (CO2 headspace test)

ISO/TR 15462±1997 Water quality ± Selection of tests for biodegradabilityISO 16221 B 2001 Water quality ± Guidance for determination of biodegradability in the marine

environmentEN ISO 7827±1995 Water quality ± Evaluation in an aqueous medium of the ™ultimate∫ aerobic

biodegradability of organic compounds ± Method by analysis of dissolved organiccarbon (DOC)

EN ISO 9439±2000 Water quality ± Evaluation of ultimate aerobic biodegradability of organiccompounds in aqueous medium ± Carbon dioxide evolution test

EN ISO 9408±1999 Water quality ± Evaluation of ultimate aerobic biodegradability of organiccompounds in aqueous medium by determination of oxygen demand in a closedrespirometer

EN ISO 9887±1994 Water quality ± Evaluation of the aerobic biodegradability of organic compounds inan aqueous medium ± Semi-continuous activated sluge method (SCAS)

EN ISO 9888±1999 Water quality ± Evaluation of ultimate aerobic biodegradability of organiccompounds in aqueous medium ± Static test (Zahn-Wellens method)

EN ISO 10634±1995 Water quality ± Guidance for the preparation and treatment of poorly water-solubleorganic compounds for the subsequent evaluation of their biodegradability in anaqueous medium

EN ISO 10707±1997 Water quality ± Evaluation in an aqueous medium of the ™ultimate∫ aerobicbiodegradability of organic compounds ±Method by analysis of biochemical oxygendemand (closed bottle test)

EN ISO 11733±1998 Water quality ± Evaluation of the elimination and biodegradability of organiccompounds in an aqueous medium ± Activated sludge simulation test

EN ISO 11734±1998 Water quality ± Evaluation of the ™ultimate∫; anaerobic biodegradability of organiccompounds in digested sludge ± Method by measurement of the biogas production

Inhibition testsISO 8192 B 1986 Water quality ± Test for inhibition of oxygen consumption by activated sludgeISO 9509 B 1989 Water quality ± Method for assessing the inhibition of nitrification of activated

sludge microorganisms by chemicals and waste watersISO 10712 B 1995 Water quality ± Pseudomonas putida growth inhibition test (Pseudomonas cell

multiplication inhibition test)ISO 11348 Part 1, 2, 3± 1998

Water quality ± Determination of the inhibitory effect of water samples and the lightemission of Vibrio fischeri (Luminescent bacteria test)

EN ISO 8192 ± 1995 Water quality ± Test for inhibition of oxygen consumption by activated sludgeEN ISO 9509 ± 1995 Water quality ± Method for assessing the inhibition of nitrification of activated

sludge microorganisms by chemicals and waste waterEN ISO 10712 ± 1996 Water quality ± Pseudomonas putida growth inhibition test (Pseudomonas cell

multiplication inhibition test)EN ISO 11348 Teil 1,2, 3 ± 1998

Water quality ± Determination of the inhibitory effect of water samples on the lightemission of Vibrio fischeri (Luminescent bacteria test)

3.1

General Principles in Testing BiodegradablePlastics

When testing the degradation phenomena ofplastics in the environment, there is ageneral problem concerning the type oftests to be applied, and the conclusionswhich can be drawn. In principle, tests canbe subdivided into three categories: fieldtests; simulation tests; and laboratory tests(Figure 2).Although field tests, such as burying

plastics samples in soil, placing it in a lakeor river, or performing a full-scale compost-ing process with the biodegradable plastic,represent the ideal practical environmentalconditions, there are several serious disad-vantages associated with these types of test.One problem is that environmental condi-tions such as temperature, pH, or humiditycannot be well controlled; secondly, theanalytical opportunities to monitor thedegradation process are limited. In mostcases it is only possible to evaluate visiblechanges on the polymer specimen, or

perhaps to determine disintegration bymeasuring weight loss. The latter approachis problematic however if thematerial breaksinto small fragments that must be quantita-tively recovered from the soil, compost orwater. The analysis of residues and inter-mediates is complicated by the complex andundefined environment. Since the purephysical disintegration of a plastic is notregarded as biodegradation in the sense ofmost definitions (see above), these testsalone can never prove whether a material isbiodegradable, or not.As an alternative to field tests, various

simulation tests in the laboratory have beenused to measure the biodegradation ofplastics. Here, the degradation might takeplace in compost, soil or sea-water placed ina controlled reactor in a laboratory. Althoughthe environment is still very close to the field-test situation, the external parameters(temperature, pH, humidity, etc.) can becontrolled and adjusted, and the analyticaltools available are better than would be usedfor field tests (e.g., for analysis of residuesand intermediates, determination of CO2

3 Testing Methods 373

Fig. 2 Schematic overview on tests for biodegradable plastics.

evolution or O2 consumption). Examples ofsuch tests include the soil burial test (Pantkeand Seal, 1990), the so-called ™controlledcomposting test∫ (Pagga et al., 1995; Tosinet al., 1996; Degli-Innocenti et al., 1998;Ohtaki et al., 1998; Tuominen et al., 2002),test simulating landfills (McCartin et al.,1990; Smith et al., 1990; McCarthy et al.,1992) or aqueous aquarium tests (P¸chneret al., 1995). On occasion, in order to reducethe time taken to conduct the tests, nutrientsare added to increase the microbial activityand accelerate degradation.The most reproducible biodegradation

tests are the laboratory tests, where definedmedia are used (in most cases syntheticmedia) and inoculated with either a mixedmicrobial population (e.g., from wastewater) or individual microbial strains whichmay have been especially screened for aparticular polymer. In such tests, which may

be optimized for the activity of the particularmicroorganisms used, polymers often ex-hibit a much higher degradation rate thanwould be observed under natural conditions.This can be regarded as an advantage whenstudying the basic mechanisms of polymerbiodegradation, but in laboratory tests it isonly possible to derive limited conclusionson the absolute degradation rate of plasticsin a natural environment. However, formany systematic investigations these testsare widely used.A move towards more reproducible and

controlled degradation tests involves the useof systems where only those extracellularenzymes known to depolymerize a particulargroup of polymers are used. Comparable toweight loss measurements, this methodcannot be used to prove biodegradation interms of metabolism by a microorganism,but the system is valuable when carrying out


Fig. 3 Comparison of the enzymatic degradation of the polyester poly(tetramethylene adipate) with a lipasefrom Pseudomonas sp. for a polymer film and polymer nanoparticles at pH 7 and 40�C. Degradation expressedas percentage of cleaved ester bonds (maximum ester cleavage of �40% results from the dissolution of lowmolecular-weight esters which are not accessible to attack by the lipase). (From Welzel et al., 2002.)

biodegradability of polymers-regulations and methods for testing

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