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Issue 05/2017

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Basics Biodegradation

Basics Biodegradation Bioplastics and their behaviour in different biodegradation environments The efficient management of plastic waste plays a key role within the circular economy. Good waste management requires the implementation of the waste hierarchy, as set out by EU legislation in the form of Directive 2008/98/CE [1], which aims to encourage solutions providing a better environmental result. The waste hierarchy sets out the following hierarchy of steps for prioritising waste management practices: (1) prevention; (2) preparation for reutilisation; (3) recycling; (4) other kind of recovery, such as energy recovery; and (5) disposal, such as in the case of landfilling. Moreover, the package of circular economy measures adopted by the European Union requires that waste be transformed into resources again, so they can be returned cyclically to the productive system, until reaching the very ambitious target of “zero waste” to landfill (European Commission, 2014). Thus, the end of life of plastics continues to be a controversial point, since landfilling is still a common practice. In the year 2014, 31 % of the post-consumer plastic waste generated in Europe went to landfill. The situation in Spain is even more unfavourable: here, just over 50% of all post-consumer plastic waste ends up in a landfill [2] These decisions, combined with the push towards creating a sustainable environment - in addition to a desire to get away from landfilling and to reduce the amount of litter in the environment – have led to heightened interest in the production of bioplastics. Biodegradable plastics are considered to be eco-friendly materials. In recent years, they have been promoted in the market as substitutes for conventional plastics in specific applications in which biodegradability, as an end-of-life solution, provides environmental benefits. However, we must not forget their limitations regarding manufacturing costs, mechanical properties or variable biodegradability behaviour depending on the aggressiveness of the different media, and focus the efforts on the research and development of solutions and improvements. There are several important factors affecting the process or mechanism of biodegradation of biodegradable plastics. On the one hand, the chemical structure, the polymeric chain, crystallinity or complexity of the polymeric formulation are key points to be studied. In this way, the specific functional groups of the polymeric chain that forms the bioplastic are selected by certain enzymes and processed by them to trigger what is known as “material biodegradation”. We can say that, normally, polymers with short chains and more abundant amorphous area are more susceptible to being biologically degraded. On the other hand, the different environments in which biodegradable plastics initiate the biodegradation processes, must be studied. In this case, pH, temperature and the presence of oxygen and microbial content are the most significant factors in determining the aggressiveness level, depending on the conditions under which the material undergoes biodegradation. In such a reaction, the carbon that is a part of the material’s polymeric chains will, in the presence of biomass and hence of microorganisms, temperature, light and water, turn into CO 2 and new biomass. The different possibilities that can occur regarding environment are, among others: compost, natural, soil, soil with normalized characteristics according to the test standards, fresh water, seawater or sewage sludge. This window of environments represents the existence of a wide range of very different conditions when studying the biodegradability of materials. More specifically and bearing in mind the possibility of recovering the plastic materials at the end of their shelf life, the medium offering this possibility is compost. The composting process is defined as the complete biological recovering process, aerobic (in presence of oxygen) and exothermal (with an increase of the temperature) of waste fermentation in controlled conditions whose result is the obtaining of CO 2 , water and fertilizer or compost where wastes are not visually distinguishable and do not produce eco-toxicological effects in the environment. With regard to composting, it is important to highlight the difference between industrial composting and home composting due to the temperature difference in both processes (58 ºC in the case of industrial composting and below 30 ºC in the case of home composting) that makes a material to biodegrade faster in an industrial facility than home composting. If we analyse another key factor, such as the presence of microorganisms, the most important role is played by fungi. The presence of fungi is needed for a good biodegradation process. However, they can only be found in compost and soil, although they are more abundant in compost and less in soil. Both compost and soil have high microbial and populations that biodegrade bioplastic materials and a high diversity that is not found in other environments such as fresh water and seawater (aquatic ecosystems), so this environment is less aggressive. Therefore, an estimation of the aggressiveness of different environments can be given and we can affirm that the most active environment is compost, followed by soil, fresh water, seawater and finally ambient and landfill conditions (this last option must be ruled out if we talk about efficient waste management) [3]. In order to calculate the percentage of biodegradability that a specific material has reached in an environment, there are lab-scale tests that evaluate parameters such as the amount of carbon dioxide generated when subjecting plastic materials to certain conditions. This parameter is an indirect measurement of the amount of carbon from the polymeric chain that is transformed into carbon dioxide by the mechanism of biodegradability. Based on this measurement, different standards have been developed with which the different biodegradation environments must 48 bioplastics MAGAZINE [05/17] Vol. 12

Basics By: Elena Domínguez Solera Sustainability and Industrial Recovery Department, AIMPLAS Valencia, Spain comply. Thus, for example, the standard EN ISO 14855 [4], sets out a method to determine the final biodegradability percentage of a plastic material. The material is subjected to controlled composting conditions (58 ºC and 50 % of humidity) using generated and automatic carbon dioxide detection methods, such as infrared detection or gravimetric methods of carbon dioxide absorption in certain substances. Likewise, there are standards that simulate a medium at an environmental temperature of 25 ºC, where plastic materials are subject to the presence of a natural or normalized soil, as in the standard EN ISO 17556 [5] or to 30 ºC in a natural aqueous medium, normalized or in other environments rich in microorganisms, such as sewage sludge, as in the standard EN ISO 14852, which sets out the testing procedure to determine the final aerobic biodegradability in aqueous medium [6]. It is essential to establish in each case what we want to analyse, in which environments and under what conditions, in order to determine the behaviour of plastics and their utility in different applications. AIMPLAS, has been committed to this vision for more than 20 years, and besides having developed and taken part in different projects in the field of bioplastics, continues to opt for them regarding biodegradability tests in different environments. The institute has taken a further step by becoming the first Spanish laboratory to earn ENAC accreditation (National Accreditation Body) for tests determining the final aerobic biodegradability in composting conditions (EN ISO 14855-1), soil (EN ISO 17556) and tests determining the degree of disintegration of plastic materials under simulated composting conditions in a laboratory scale (EN ISO 20200 [7]). Thanks to this extension of the accreditation scope, AIMPLAS is at the forefront of accredited tests in the field of plastic materials in Europe. The fact that ENAC is a signatory of all the EA (European Accreditation), ILAC (International Laboratory Accreditation Cooperation) and IAF (International Accreditation Forum) international agreements, is very important Therefore, a report or certificate issued under ENAC accreditation is recognized by the other signatories in the entire world and these agreements act like an international passport for trade. References: [1] DIRECTIVE 2008/98/CE OF THE EUROPEAN PARLIAMENT AND THE COUNCIL of 19 November 2008 on waste and repealing certain directives. [2] PlasticsEurope (PEMRG) / Consultic. Plastics - the Facts 2015. An analysis of European plastics production, demand and waste data. [3] Challenges and opportunities of biodegradable plastics: A mini review (Maja Rujnić-Sokele and Ana Pilipović). Waste Management & Research 2017, Vol. 35(2) 132–140. [4] EN ISO 14855-1:2013. Determination of the ultimate aerobic biodegradability of plastic materials under controlled composting conditions - Method by analysis of evolved carbon dioxide - Part 1: General method. Part 2: Gravimetric method. [5] EN ISO 17556:2013. Plastics. Determination of the ultimate aerobic biodegradability of plastic materials in soil by measuring the oxygen demand in a respirometer or the amount of carbon dioxide evolved. [6] EN ISO 14852:2005. Determination of the ultimate aerobic biodegradability of plastic materials in an aqueous medium - Method by analysis of evolved carbon dioxide. [7] EN ISO 20200:2016. Plastics - Determination of the degree of disintegration of plastic materials under simulated composting conditions in a laboratory-scale test. AIMPLAS’ equipment (Plastics Technology Centre) for biodegradability tests in different environments. bioplastics MAGAZINE [05/17] Vol. 12 49

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