Definitions and Norms of Biodegradable and Compostable Polymer

The ability of plastics to biodegrade is determined by the material’s chemical structure and final composition, not just the raw materials used in its production. Biodegradable plastics can be made from natural or synthetic resins.

Bioplastics differ from conventional plastics in two ways: they are produced using renewable resources and they are biodegradable or compostable. However, there are no standardized measures for evaluating the proportion of renewable resources needed to classify a product as a bioplastic. While there are scientific methods for measuring the “renewable carbon” in a product, no standard has been established for this purpose.

On the other hand, scientific methods for accurately determining the biodegradability and compostability of plastics have been developed. The European Norm EN 13432, a harmonized standard, was introduced in Europe in 2000 to define requirements for packaging to be considered recoverable. The harmonized standard EN 13432 sets laboratory test procedures for determining biodegradability, compostability, and potential harmful material constituents in packaging and packaging materials.

EN 13432 must be used to assess compliance with the requirements of the EU Directive on Packaging and Packaging Waste (94/62/EC) whenever a packaging product is marketed as “degradable.”

European Bioplastics is advocating for proper certification of plastic products that are marketed as “compostable” or “biodegradable” in accordance with EN 13432. To address the misuse of these terms, the association has also provided information on “degradable” or “ox-degradable” plastic products. Producers have committed to a voluntary self-certification process that has been recognized by the European DG Enterprise.

To ensure the highest quality of bioplastic products, the industry has established that materials validated under EN 13432 are usually certified. Certification guarantees that not only the plastic component of the product is fully biodegradable and suitable for composting, but also that other components such as inks, glues, labels, and even the content of the product if it is packaging, meet the composting requirements. Certification requires disclosing the complete set of components to the certifying institute, which also enables determination of the proportion of renewable resources used in the product.

The certification process includes assigning a protected label, which is a valuable tool for communication. However, the terms “compostable” and “biodegradable” cannot be protected. The compostability logo, known as the “seedling,” helps consumers identify compostable products and is also necessary for the proper sorting and disposal of compostable plastic waste. The European Bioplastics association is currently working on a unified certification and labeling scheme for bioplastic products in Europe.

There are various types of polymers that are often labeled as “biodegradable” but are actually classified as “bioerodable”, “hydrobiodegradable”, or “photo-biodegradable”. All of these polymer categories fall under the larger umbrella of “environmentally degradable polymers”. These biodegradable plastics are categorized based on their specific mechanisms of degradation, which include:

  • Biodegradable
  • Compostable
  • Hydro – biodegradable
  • Photo – biodegradable
  • Bioerodable

European standards for biodegradability and compostability are outlined in EN 13432:2000 (Packaging – Requirements for packaging recoverable through composting and biodegradation – Test scheme and evaluation criteria for the acceptance of packaging). In addition, the Australian Standard AS 4736 – 2006 (Biodegradable plastics – Biodegradable plastics suitable for composting and other microbial treatment) specifies the requirements and procedures to determine the compostability or anaerobic biodegradation of plastics. The standard addresses biodegradability, disintegration during biological treatment, the effect on the biological treatment process, and the quality of the resulting compost. By meeting these standards, materials or products made from plastics can be labeled as “compostable” for use in facilities like municipal or industrial composters. The definitions provided by these standards are as follows:

  • Biopolymer: Material that contains natural starch additives and has the properties of a plastic product.
  • Degradable plastic: Plastic designed to undergo significant chemical changes under specific environmental conditions, leading to a loss of plastic properties. Changes and degradations are measured by standard test methods.
  • Biodegradable plastic: Degradable plastic that breaks down due to the activity of natural micro-organisms such as bacteria, fungi, and algae. It’s a type of biopolymer.
  • Photodegradable plastic: Degradable plastic that breaks down due to natural daylight.
  • Oxidatively degradable plastic: Degradable plastic that breaks down due to oxidation.
  • Hydrolytically degradable plastic: Degradable plastic that breaks down due to hydrolysis.
  • Compostable plastic: Plastic that undergoes biological degradation during composting and results in carbon dioxide, water, inorganic compounds, and biomass at a rate comparable to other compostable materials without leaving any distinguishable or toxic residue.

The failure of early “biodegradable” plastics to degrade properly led to the American Society for Testing and Materials (ASTM) creating definitions on what constitutes “biodegradability”. The ASTM created new standards of testing methods and labeling terminology for biodegradable and compostable packaging materials.

The ASTM defines “biodegradable” as: “capable of undergoing decomposition into carbon dioxide, methane, water, inorganic compounds, or biomass in which the predominant mechanism is the enzymatic action of microorganisms, that can be measured by standardized tests, in a specified period of time, reflecting available disposal conditions.” It is important to note that the definition of biodegradation should specify a time limit. If the biodegradation process is sufficient to mineralize organic matter into carbon dioxide or methane, water, and biomass, the material can be termed “biodegradable”.

Biodegradation is degradation caused by biological activity, particularly by enzyme action leading to significant changes in the material’s chemical structure. In essence, biodegradable plastics should break down cleanly, in a defined time period, to simple molecules found in the environment, such as carbon dioxide and water.

Biodegradation rates are highly dependent on the thicknesses and geometries of the fabricated articles. Although rapid breakdown rates are often quoted, these generally apply to thin films. Thick-walled articles such as plates, food trays, and cutlery can take up to a year to degrade biologically.

As per EN 13432, a material or product is considered biodegradable if it undergoes a particular degradation process caused by biological activity within a specific time under certain environmental conditions, as determined by a standardized test method. To be classified as biodegradable and compostable, the material/product must meet the following criteria:

  • biodegradation level: > 90% in comparison with cellulose (positive standard) in 180 days under conditions of controlled composting measured through respirometric methods (EN 14046 ),
  • disintegration level: > 90% in 3 months (EN 14045 ),
  • ecotoxicity: toxicity tests (germination and plant tests), and
  • chemical testing according to product safety.

Polymers that are biodegradable and compostable can be made from renewable agricultural raw materials, either alone or in combination with fossil-derived materials. To evaluate their aerobic biodegradability under controlled composting conditions, one can refer to the ISO 14855-1999 or EN 14046-2003 standards.

Compostable biodegradable plastics must be able to biodegrade and disintegrate in a compost system within a specified time frame (typically around 12 weeks at temperatures above 50°C). The compost must meet certain quality standards, including limits on heavy metal content and ecotoxicity, and should not contain any visible residues from the breakdown of the polymers. It’s worth noting that compostable plastics are a type of biodegradable plastic.

According to the ASTM, compostable materials are defined as those that can undergo biological decomposition in a compost site as part of an available program, such that they break down into carbon dioxide, water, inorganic compounds, and biomass. They should not be visually distinguishable and should break down at a rate consistent with other known compostable materials like cellulose.

A polymer is considered “compostable” if it can biodegrade in composting conditions, meeting the following criteria:

  • it undergoes degradation due to the action of microorganisms like bacteria, fungi, and algae
  • complete mineralization occurs, resulting in the conversion of the polymer into CO2, H2O, inorganic compounds, and biomass under aerobic conditions
  • the rate of mineralization is high and compatible with the composting process.

To determine the degree of degradability of a material, it must be tested under controlled composting conditions according to ASTM D 5338-92. The material is considered compostable if its degree of biodegradation is equivalent to that of cellulose (with a maximum tolerance of 5%). To meet the criteria for compostable plastic products, the following requirements must be met:

  • All criteria of the EN 13432 standard must be complied with.
  • The product must be certified according to EN 13432.
  • The product must be labeled with a protected compostability mark.

Users/vendors of compostable bioplastic products are strongly recommended to ask their supplier about product certification and to demand the certificate with the certification number. Even if composting of the product is not intended, certification guarantees high product safety. Certification, moreover, distinguishes between bioplastic and conventional plastic and offers marketing and communication possibilities.

The norm EN 13432 incorporates the definitions of biodegradability and compostability by reference and is recommended for the applications of compostable packaging according to the Directive on Packaging and Packaging Waste 94/62/EC. EN 13432 can be also the basis for other compostable plastic products or semi-finished goods.

For a material to be called compostable it has to be biodegradable and to disintegrate in a composting system in such a way that this disintegration occurs during a period of end-use of the compost. Product certification guarantees that it is not only the plastic that is compostable, but also all other components of the product, such as colors, labels, glues, and – in the case of packaging products – residuals of the content.

The protected “compostability label” (or “seedling”) may only be used for certified products. Both the logo and the number of the certificate printed on the product allow identification of the producer and confirmation of conformity: the product that has been tested and the marketed one must coincide.

Certification of compostable polymer products based on EN 13432 is performed by DIN CERTECO and other EU certification bodies according to quality management systems such as EN 45011/EN 45012. DIN CERTECO, the certification organization of DIN, established the scheme in 1997 and has gained internationally renowned expertise ever since. They participate in the International Network for Compostability Certification together with institutions from the USA, Japan, and Taiwan, or with other certification bodies in Europe to implement harmonized supranational certification schemes.

As of June 2004, participants include The Composting Association from the UK, Keurmerk from the Netherlands, and COBRO from Poland. Hydro-biodegradable and photo-biodegradable polymers are broken down in a two-step process: an initial hydrolysis or photo-degradation stage, followed by further biodegradation. Single-degradation-phase “water-soluble” and “photodegradable” polymers also exist.

Many polymers that are claimed to be “biodegradable” are, in fact, “bioerodable” and degrade – at least initially – without the action of micro-organisms. This is also known as abiotic disintegration and may include processes such as dissolution in water, “oxidative embrittlement” (heat aging), or “photolytic embrittlement” (UV aging).

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