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01 | 2010

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Materials Misleading

Materials Misleading Claims and Misuse Proliferate in the Nascent Article contributed by Ramani Narayan University Distinguished Professor Michigan State University Department of Chemical Engineering & Materials Science Chairman of ASTM Committee D20.96 on Environmentally Degradable Plastics & Biobased Products Chairman of ISO/TC 61(Plastics) SC1 (Terminology) and US expert to TC 61/SC5/WG22 on biodegradable plastics Biodegradation takes place when microorganisms utilize carbon substrates to extract chemical energy that drives their life processes. The carbon substrates become ‘food’ which microorganisms use to sustain themselves. For this to occur, the carbon substrate needs to be transported inside the cell. Molecular weight is an important but not only criterion for transport across cell membrane. Factors like hydrophobic-hydrophilic balance, molecular and structural features also govern transport across the cell membrane. Under aerobic conditions, the carbon is biologically oxidized to CO 2 inside the cell releasing energy that is harnessed by the microorganisms for its life processes. Under anaerobic conditions, CO 2 +CH 4 are produced. Thus, a measure of the rate and amount of CO 2 or CO 2 +CH 4 evolved as a function of total carbon input to the process is a direct measure of the amount of carbon substrate being utilized by the microorganism (percent biodegradation). This is fundamental, basic biology and biochemistry taught in freshman classes and can be found in any biochemistry textbook. This forms the basis for various National (ASTM, EN, OECD) and international (ISO) standards for measuring biodegradability or microbial utilization of chemicals, and biodegradable plastics [1,2]. It would seem obvious and logical from the above basic biology lesson that to make a claim of biodegradability, all that one needs to do is the following: Expose the test plastic substrate as the sole carbon source to microorganisms present in the target disposal environment (like composting, or soil or anaerobic digestion or marine), and measure the CO 2 (aerobic) or CO 2 +CH 4 (anaerobic) evolved. A measure of the evolved gas provides a direct measure of the plastics substrate carbon being utilized by the microorganisms present in the target disposal environment (% biodegradation). ASTM and ISO test methods teach how to measure the percent biodegradability in different disposal environments based, again, on the fundamental biochemistry described above. It has been claimed by a few companies for quite some time that the addition of a low percent (about 1-5%) of proprietary additives in the form of a masterbatch to polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polyethylene terephthalate (PET), and other carbon chain polymers renders the carbon chain polymer completely (the claim has been 100%) biodegradable in both aerobic (composting, soil) and anaerobic (landfills) environments – that would mean that 100% of the polymeric carbon is completely utilized by microorganisms as measured by the evolved CO 2 (aerobic) or CO 2 +CH 4 (anaerobic) – if this is true, then such data should be provided to substantiate the claim. There are two classes of additives being marketed – ‘oxo’ and ‘organic’ which are sold as masterbatch concentrates. The ‘oxo’ 38 bioplastics MAGAZINE [01/10] Vol. 5

Materials of Standards Continues to BioPlastics Industry Space additive is supposed to promote chain scission, thereby making the polymer small enough to be utilized by the microorganisms present in the disposal environment. The ‘organic’ additive initiates or promotes microbial attack, and that in some way triggers the microorganism to begin breaking down the carbon-carbon backbone chain polymer. Unfortunately, the scientific data and the literature do not support the actual claims being made in the market place. Many reports in the peer-reviewed literature include ‘biodegradation’ in the title; however, the meaning and context of the term is very broadly and loosely applied. Let’s look at several examples: Evidence of microbial growth on the surface of the polymer is reported as ‘biodegradable’ This is then extrapolated by manufacturers to claim that their product is 100% biodegradable, and some go onto claim that this can occur anywhere from 9 months to 5 years. Some studies use the ‘biodegradable’ term to indicate that the PE samples were subjected to a biotic environment (soil, compost) as part of their experimental procedure. They go on to measure weight loss, molecular weight reductions, carbonyl index, mechanical property loss (films becoming brittle). Additive manufacturers reference these studies and extrapolate to stating that their product is ‘completely (100%) biodegradable’ in the environment based on weight loss and physical, chemical, or mechanical property loss. However the fundamental biology/biochemistry data showing carbon utilization by the microorganisms as measured by the evolved CO 2 (aerobic) or CO 2 +CH 4 (anaerobic) is missing. A peer reviewed Chem Communication journal (an established, well respected journal) paper [3] reported increasing the rates of biodegradation of polyolefins, by anchoring minute quantities of glucose, sucrose or lactose, onto functionalized polystyrene. A mere 2-12% weight loss and formation of carbonyl groups was evidence for biodegradation. In another peer reviewed scientific journal paper, polyethylene and polypropylene were put in a composting environment after solvent extraction to remove the antioxidants present, and it was reported that PP lost 60% mass over six months, whereas low density polyethylene lost only 10%. It is well known that unstabilized PP will degrade in the environment. Professor Scott summarizes this in his book chapter as follows: PP biodegrades much more rapidly than LDPE by mass loss in compost, and ethylene-propylene copolymers biodegrade at rates intermediate between polypropylene and ethylene. This implies that 60% of the PP carbon has been utilized by microorganisms present in compost What does Biodegradable Mean? Can the microorganisms in the target disposal system (composting, soil, anaerobic digestor) assimilate/utilize the carbon substrate as food source completely and in a short defined time period? Environment - soil, compost, waste water plant, marine Hydrolytic Oxidative STEP 1 Enzymatic Polymer chains with susceptible linkages Biodegradation (Step 2): Only if all fragmented residues consumed by microorganisms as a food & energy source as measured by evolved CO 2 in defined time and disposal environment Oligomers & polymer fragments Complete microbial assimilation defined time frame, no residues STEP 2 CO 2 + H 2 O + Cell biomass bioplastics MAGAZINE [01/10] Vol. 5 39

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