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Feature - Biodegradability of INNOBITE products

24/11/2015

The biodegradability of a material can heavily influence its end-of life treatment. Whilst there is currently a strong drive towards the re-use and recycling of materials, for most non-biodegradable materials the current most economical end-of-life option is still landfill

– an option which takes up land space and may cause air, water and soil pollution. On the other hand, a biodegradable material, and in particular a compostable material, has several more environmentally favourable end-of-life treatment options. For instance, because these materials easily decompose they can potentially be composted or used in bio-gas production. Thus, the INNOBITE project has aimed for and evaluated the biodegradability of its products.

What is a biodegradable material and why is biodegradability important?

There is no single consensus across the various scientific sources over the accurate definition of the different terms used to describe some environmental benefits of biobased materials, yet a common approach can be important when trying to compare different materials and products. For instance, there is often a lack of distinction between the terms degradable, biodegradable and compostable, and then again between terms biodegradable, biobased and biobased product.

The term “degradable” is used to describe materials that can be degraded by different processes, which include physical degradation, chemical degradation and biodegradation through biological mechanisms. However, a biodegradable material, as described by CEN/TR 15932 is capable of undergoing biological anaerobic or aerobic degradation leading to CO2, H2O, methane, biomass and mineral salts depending on the environmental conditions of the process. Thus, a degradable material is not always biodegradable - since some materials will only degrade though physical or chemical degradation and not biological degradation. Differentiation can also be made between compostable and biodegradable materials. According toEN 13193:2000, a compostable material is one which can be disintegrated and biodegraded without hindrance in a composting process. Thus a biodegradable material is not always compostable – this can only be true if the material can be biodegraded or disintegrated in a composting process.

Whilst the term biodegradable is used to describe a function of a material, the term “Biobased” describes the origin of the raw materials – and whilst “biobased” is defined by CEN BT/WG 209 as being derived from biomass, a “biobased product” is described as a product wholly or partly bio-based.

Both the biodegradability (or more often the compostability) and the biobased nature of the product can be important when considering the benefits of a material or product. However, whilst the biodegradability of a product can be considered important when evaluating the products end-of-life treatment, the biobased nature of a product can be considered more important for evaluating the sustainability of the consumption of raw materials since a biobased product can be considered a (at least partly) renewable.

Biodegradable plastics represent a promising alternative to petroleum-based plastics. Petroleum-based products use oil in their synthesis and usually, uses a lot of space in landfills. However, biodegradable plastics have the potential to be disposed of in a way that is less damaging to the environment. There are several obstacles for the replacing of petroleum-based polymers into bio-based polymers, one of them being the proper disposal of biodegradable plastics. In order for biodegradable plastics to be effectively disposed of, the current waste management infrastructure must change, or methods with less economic and environmental costs must be developed. 

How are biodegadable or more specifically, compostable claims assessed?

Due to concern founded in the composting industry about claims for different compounds biodegradable or compostable credentials, a standard which laid down the criteria to establish what could be considered compostable or biodegradable, was established - the European Standard EN 13432, based on the ISO Standard. Other standards have since been implemented – many of which are closely related to EN 12431, including the American Standard ASTM D6400-99 or the Australian Standard AS4736-2006 where the main difference lies in the inclusion of a worm eco-toxicity and other chemical tests.

Different criteria established in these standards needs to be adhered to in order to meet the definition of compostability, but each point alone is not sufficient. For example, a biodegradable material is not necessarily compostable because it must also break up during one composting cycle. On the other hand, a material that breaks up, over one composting cycle, into microscopic pieces that are not totally biodegradable, is not compostable. So compostable material must possess the following characteristics:

  • Biodegradability: measured by metabolic conversion of the material to carbon dioxide to at least 90% in less than six months. (90% is used to account for sampling error, not to allow for non-biodegradable material).
  • Disintegrability: there should be fragmentation below a certain size with no visible contamination. This is tested by composting the materials for three months then screening through a 2 mm sieve. The mass of residues above 2mm must be less than 10% of the original mass.
  • Absence of negative effects on the final compost: This is tested by a plant growth test and physical/chemical analyses. There must be no difference from the control compost.
  • Other chemical/physical parameters that must not be different from those of the control compost after the degradation are: the pH, salinity, volatile solids, nitrogen, phosphorous, magnesium and potassium.

Biodegradable polymers are certified according to any of the following international standards:

  • ISO 17088:2012
  • EN 13432:2000, EN 14995:2006
  • ASTM D6400-12

MFC is utilised in the INNOBITE project. Is it biodegradable?

Microfibrillated cellulose (MFC) is manufactured from cellulose which is considered the most abundant organic compound from biomass. The origin of cellulose is wide: wood, plant fibers (cottom, hemp, flax..), marine animals such as tunicate, algae, fungi, invertebrates and bacteria.

MFC can be obtained using different paths: mechanical treatment, primarily shearing, of pulp cellulose fibers to small diameter fibrils through refining and high-pressure homogenization processes. Since the widths of the fibrils in MFC are in submicron range, they are considered as nanofibers. Yet, due to its cellulose origin, MFC is biodegradable.

So far, R&D efforts have been focused on the synthetisation of MFC based composites using non degradable phenolic and other resins. These components are not biodegradable and are still deposed on landfills. Their substitution by MFC components could be a solution for the problems derived at the end of their life.



Figure 1. MFC produced from recycled newspapers

Are Lignin-thermosetting plastics biodegradable?

Thermosetting resins are polymers which play an important role in the current industry inasmuch as these materials have a high flexibility for tailoring desired ultimate properties, leading to their high modulus, strength, durability, and thermal and chemical resistance due to the high cross-linking density. However, they have a low impact resistance and are not able to be reshaped after curing/polymerization.

Lignin, among other, it is a raw material of interest in the design of bio-based thermosetting polymers since it has a vast abundance and high functionality. This compound is the second most abundant in nature after cellulose in and due to its phenolic structure, it has been considered as a promising substitution for phenol in phenol-formaldehyde based resins. Lignin can also be used as curing agent in the processing of epoxy resins. Lignin is (usually) biodegradable and there isn’t record of any ecological build-up for this compound.

The main thermosetting products that can be obtained from lignin are crosslinked elastomeric polyurethanes, lignin-urea-formaldehyde, lignin-phenol-formaldehyde and lignin-polyisocyanate, all of they could be a source of adhesives. Using lignin derivate products as a part of a thermoset network contributes to an increase in vitreous transition temperature and modulus.

The biodegradability of thermoset polymers obtained from lignin is still not clear, however, evidences that fungi are able to degrade lignin-thermoset plastics such as phenolic resins has been found. 

How biodegradable are lignin-thermoplastics?

As part of the INNOBITE project, a new thermoplastic resin, based on cellulose fibres and CIMV biolignin derived from wheat straw, was developed by TECNARO. This resin has many advantages over similar resins which utilise lignin derived from the Kraft process. These include: less sensitivity to shear and heat; less tendency to produce incrustations, a better impact on mechanical properties and available at acceptable prices.

This thermoplastic material is 100 % renewable – and it is being adopted into TECNARO’s commercial ARBOFORM® range. In terms of environmental impact, these thermoplastics based on renewable sources have many advantages compared to petrochemical thermoplastics, including fixation of CO2. Theoretically, these kind of products could be incinerated at the end of their life and the carbon emitted would be the same as that taken from the atmosphere by the plant when growing. However, these materials are also biodegradable so due to their formulation they could be composted.

Figure 2. TECNARO resin production

Biodegradability tests on the final products in INNOBITE project were run, according to standard ISO 14593. The materials were expected to have bio-based origin and/or biodegradability of >95% (thermoplastic materials), or bio-based origin of >50% (thermosetting products). On the basis of sample compositions and relevant literature, it could be concluded that the bio-based and biodegrading target of thermoplastic materials was reached. However, the selected test method was not able to prove the biodegradability. The temperature of the biodegradability test used in the study was not high enough to initiate the biodegradation process of composites which contain natural polymers and additives over short the test period. These somewhat counterintuitive findings were attributed to the presence of lignin which can reduce biodegradability of composites, even if it is regarded as bio-based and biodegradable material.

What does the future hold for biocomposites?

A continued decrease in the use of synthetic fibers in polymer composites is expected - mainly due to their cost instability, non-biodegradability and the concern about the pollution that components generate in the environment. There is a movement created by different scientists and engineers that promotes the use of natural fibers with polymers based on renewable resources and it is expected that it will solve many environmental problems. Green-biocomposites, biofibers with bio-based polymers such as plastic with cellulose or lignin origin, polylactides starch plastics, bacterial polyesters or soy-based plastics, are expected to be adopted more widely

Currently, these biocomposites based on renewable sources are object of a sustained research with targets in construction and automotive industry since these components have very good properties such as lower weight and costs. Home owners are also interested in these compounds for decking, fencing and so on.

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