Polyolefins are the most-used non-biodegradable polymers in the US and the world, and are responsible for the largest amount of plastics in municipal solid waste (MSW), followed by PET.
Pakistan and Argentina
A number of countries have adopted legislation promoting the use of biodegradation-promoting additives in polyolefins and PET. For example, in 2012, the UAE banned all disposable plastic bags with the exception of those made from oxo-biodegradable and compostable plastic. Similar legislation is being considered in Pakistan and Argentina.
The underlying goal behind using biodegradable plastics is the assimilation of these materials back into the environment. Biodegradation of plastics takes place when microorganisms use the plastics as a source of energy for metabolic processes as well as a source of carbon for growth or reproduction.
Six researchers from the School of Packaging and the Biosystems and Agricultural Engineering department, both at Michigan State University, Michigan, US, carried out tests on LDPE/LLDPE (PE) films (commonly used for bread, supermarket, and trash bags) and poly(ethylene terephthalate)—PET sheets.
They investigated the effect of three different types of biodegradation-promoting additives in anaerobic digestion, aerobic degradation (compost), and soil burial environments.
According to the public information at the time of additive selection, one of these was of the oxo-biodegradable type, one was a nonoxo additive, and the third was a combination.
Three environments for PET & PE
The base experiments evaluated all three types in all three environments for PE, and both available additives in all three environments for PET, to understand the effect of disposing these polymers in the environment.
“None of the five different additives tested significantly increased biodegradation in any of these environments,” said Susan Selke, researcher, Michigan State University.
“Thus, no evidence was found that these additives promote and/or enhance biodegradation of PE or PET polymers.
“As a result, anaerobic and aerobic biodegradation are not recommended as feasible disposal routes for non-biodegradable plastics containing any of the five tested biodegradation-promoting additives.
“Complete assimilation of polymers into the environment requires eventual total breakdown of the molecular structure of the polymer.
“A key factor in such assimilation is the time period that is required. For there to be substantive environmental benefit, such assimilation must occur within a reasonable time frame.
“While there is disagreement on the exact length of time that is “reasonable,” there is general consensus that time frames must be on the order of months to a few years, rather than decades to centuries.
“Making improper or unsubstantiated claims can produce consumer backlash, fill the environment with unwanted polymer debris, and expose companies to legal penalties.”
Short degradation times
As part of the study, films were produced from a blend of LDPE/LLDPE at a ratio of 70:30 wt % (PE) containing 1% and 5% by weight of biodegradation-promoting additive masterbatches obtained from Wells Plastics (Reverte for PE)—W1 & W5—a combined oxo-degradable and nonoxo-degradable additive, Ecologic (Eco-one EL 10)—E1 & E5—a nonoxo-degradable additive, and Symphony (d2w)—S1 & S5—an oxo-degradable additive.
PET sheets were produced containing 1% and 5% by weight of biodegradation promoting additive masterbatches obtained from Wells Plastics—W1 & W5—and Ecologic—E1 & E5. Controls without additives were also produced. Samples were exposed to anaerobic and aerobic biodegradation.
“We recognize that three years testing for PE films and PET sheets are very short degradation times for these polymers,” added Selke.
“However, considering littering in the environment, this time frame provides a good indication of the short-term consequence of disposing these polymers in the environment.
“Medium term studies were conducted by Albertsson and Karlsson studing the aerobic degradation of LDPE films labeled with C14 (Mn = 18.2 and Mw = 84 kDa) with thicknesses of 0.02 and 0.16mm with and without a photochemical degradation additive for 10 years in dark soil cultivation at 25°C.
“The evolved CO2 was trapped in KOH solutions and measured with a liquid scintillation counter. PE films without oxidation additives only evolved 0.2% wt. in 10 years of CO2, and PE films with UV sensitizer irradiated for 42 d lost around 5% wt. in 10 years.
“In this work, the particular degraded compound was not discussed, but it was also observed that even samples exposed to high irradiation for 42 d did not significantly biodegrade.
“Therefore, it is safe to conclude PE films and PET sheets disposed in sandy soil will litter the environment, since they do not fragment or biodegrade for at least 1095 d, and there is no reason to suggest that they will undergo considerable biodegradation when exposed to this environment for longer periods of time.”
The project was funded by the Center for Packaging Innovation and Sustainability at the School of Packaging, Michigan State University and the USDA National Institute of Food and Agriculture and Michigan AgBioResearch.
Source: School of Packaging, Michigan State University, East Lansing, Michigan, US and Biosystems and Agricultural Engineering, Michigan State University, East Lansing, Michigan, US.
Author (s): Susan Selke, Rafael Auras, Tuan Anh Nguyen, Edgar Castro Aguirre, Rijosh Cheruvathr and Yan Liu.
Publication: Environ. Sci. Technol., 2015, 49 (6), pp 3769–3777, American Chemical Society
Date: February 27, 2015.
DOI: 10.1021/es504258u