Biodegradability of Polymers: Regulations and Methods for Testing 376 3.2.4 CO 2 evolution/O 2 Consumption

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Biodegradability of Polymers: Regulations and Methods for Testing 376 3.2.4 CO 2 evolution/O 2 Consumption
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  12 Biodegradability of Polymers: Regulations andMethods for Testing Dr. Rolf-Joachim M¸ller  Gesellschaft f¸r Biotechnologische Forschung mbH, Braunschweig, Germany;Tel.:   49 (0)531 6181 610; Fax:   49 (0)531 6181 175; E-mail: rmu@gbf.de 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 CO 2  evolution/O 2  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  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 388ASTM 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 anewapproach tothemanagementofplasticswaste. Moreover, when this waste manage-ment system is combined with the use of renewable resources to produce the poly-mers initially, it is likely that biodegradableplastics may simply become part of a naturalcycle.The biodegradability of plastics providesthese materials with novel and additionalproperties which may also be beneficialduring their use. For example, in agriculturebiodegradablemulchfilmswouldnotneedtobe 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,wheretheywouldbiodegrade.Hence,itisnotsurprising 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 Testing  366  methods used ± which had srcinated 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 of different 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 of these 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 of plastics is usually a surface erosion process.Although the enzyme-catalyzed chainlength 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 useddirectlytoinducethebiodegradationprocess[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 influencethe 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, thepresence or absence of oxygen andthesupply of different nutrients have importanteffects on the microbial degradation of polymers, and so these conditions must beconsidered when the biodegradability of plastics is tested.Another complicating factor in plasticsbiodegradation is the complexity of theplastic materials with regard to their possi-ble structures and compositions. In manycases plastics do not consist simply of onlyonechemicalhomogeneouscomponent,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 itself different 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 of each 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 of the 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 of many 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 of plastics 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 of degradation of low molecular- weight chem-icals, but these can also be applied topolymers. Generally, the definitions do not 12 Biodegradability of Polymers: Regulations and Methods for Testing  368Fig. 1  General mechanism of plastics biodegradation.  specify any environment or time frames;this must be carried out according tocorresponding standards.Based on these definitions, biodegradableplastics (or packaging materials) are notnecessarily suitable for composting. In thedefinition 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  369Tab. 1  Definitions used in correlation with biodegradable plastics DINFNK 103.2Biodegradable plastics 1) 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.Biodegradation 3) Biodegradation is a process, caused by biological activity, which leads under change of the chemical structure to naturally occurring metabolic products.ASTMsub-committeeD20-96Biodegradable plastics 1) 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 SocietyBiodegradable plastics 1) Polymeric materials which are changed into lower molecular weight compounds whereat least one step in the degradation process is though metabolism in the presence of naturally occurring organisms.ISO 472 Biodegradable plastics 1) A plastic designed to undergo a significant change in its chemical structure underspecificenvironmentalconditionsresultinginalossofsomepropertiesthatmayvaryasmeasured 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 plastics 1) 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.Biodegradation 2) Biodegradation is a degradation caused by biological activity, especially by enzymaticaction, leading to a significant change in the chemical structure of a materialInherent biodegradability 2) The potential of a material to be biodegraded, established under laboratory conditions.Ultimate biodegradability 2) Thebreakdownofanorganicchemicalcompoundbymicroorganismsinthepresenceof oxygentobiodegradabilitycarbondioxide,waterandmineralsaltsofanyotherelementspresent(mineralization)andnewbiomassorintheabsenceofoxygentocarbondioxide,methane, mineral salts and new biomass.Compostability 2) 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)
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