Biodegradable additives

Biodegradable additives are additives that enhance the biodegradation of polymers.^{[citation needed]} Some microorganisms, which could be used as additives, have been identified to enhance the biodegradation of some plastics.^[1] The topic has remained niche.

Starch

Starch is a biodegradable additive that is blended with polyvinyl alcohol (PVA).^[3] Starch and polyester blends have also been found to be completely biodegradable.^[4] The presence of a continuous starch phase allows direct consumption of the plastic by microorganisms because the material becomes more hydrophilic. Microorganisms can directly attack and remove the starch from the plastic, leading to its degradation. Starch is most commonly used as a biodegradable additive for both low-density polyethylene (LDPE) and high-density polyethylene (HDPE).^[5] Since polyethylene is used for a wide range of uses, from plastic bags to plastic water bottles to outdoor furniture, large amounts of PE plastic is thrown away each year, and determining ways to increase its biodegradability has become an important area of research.

Cornplast, produced by the National Corn Grower Association (USA), is a specific starch additive that can be used to increase the biodegradability of synthetic polyethylene. Cornplast is a material whose composition is 20% polyethylene and 80% starch. 50%-50% by weight blends of Cornplast with both LDPE and HDPE have been studied to determine the effectiveness of starch as a biodegradable additive.^[5]

Bioaugmentation

The addition of certain microbial strains to plastics is known as bioaugmentation. The method is intended to increase the biodegradability of plastics.^[6] The bacterium Geobacillus thermoleovorans has been proposed for bioaugmentation of poly(lactic acid).^[6]

Pro-oxidant additives

Pro-oxidant additives are proposed to accelerate thermo-oxidation and photo-oxidation of polymers.^[7] Pro-oxidant additives have been shown to accelerate biodegradation of polyethylene, a very common polymer.^[8] Metal stearates are one family of such agents.^[8] The use of such OXO-biodegradation additives was banned in the EU in 2019^[9].^[10]

Testing of biodegradable additives

Testing methods

Several tests can be performed on a certain plastic in order to determine whether a potential additive increases its biodegradability.

Comparison of the changes in physical properties of the plastic both with and without potential biodegradable additives throughout the degradation process can provide insight into the efficacy of the additive. If the degradation is significantly affected with the addition of the additive, it could indicate that biodegradation is improved.^[11] Some important physical properties that can be measured experimentally are tensile strength, molecular weight, elasticity, and crystallinity. Measuring the physical appearance of the plastic before and after potential microbial biodegradation can also provide insight into the efficacy of the degradation.^[12]

Thermal analysis is a useful method for characterizing the effects of degradation on the physical properties of polymers. Information about the thermal stability and the kinetic parameters of thermal decomposition can be obtained through thermogravimetric analysis. These kinetic parameters provide information about the breakdown of molecular chains, an indicator of degradation. From measurements of enthalpies in the melt state and the crystalline state, the evolution of the crystallinity content of plastics can be recorded. Changes to crystallinity can indicate that degradation was either successful or unsuccessful. Lamellar thickness distribution of the plastic can also be measured using thermal analyses.^[5]

Another way to determine the efficacy of biodegradation is by measuring the amount of carbon dioxide and/or methane produced by the microorganisms that are degrading the plastic. Since carbon dioxide and methane are products of the microbial degradation process, large amounts of these products in the air indicate that the synthetic plastic has been consumed and converted into energy.^[6]

Testing environmental conditions

Thermo-oxidative treatments

Thermo-oxidative treatments of synthetic plastics can replicate the conditions under which a plastic will be used (ex. storing water for a water bottle). These tests can be used to observe changes in the plastic during its service life in a much shorter period of time that would be necessary to naturally observe the plastic. Typical air atmosphere conditions are controlled using specific instrumentation (ex. Heraeus UT 6060 oven).^[5]

Soil burial

Accelerated soil burial tests are used to record the degradation process of the plastic in the ground by replicating the conditions of a landfill, a typical disposal site for plastics. These tests are used after the service life of the material has been depleted, and the next step for the material is disposal. Typically, samples are buried in biologically active soil for six months and are exposed to air to ensure that there is sufficient oxygen so that the aerobic mechanism of degradation can occur. The experimental conditions must reflect natural conditions closely, so the moisture and temperature of the soil are carefully controlled.^[11] The type of soil used must also be recorded, as it can affect the degradation process.^[5]

Specific testing methods

The following testing methods have been approved by the American Society for Testing and Materials:^[13]

ASTM D5511-12 testing is for the "Anerobic Biodegradation of Plastic Materials in a High Solids Environment Under High-Solids Anaerobic-Digestion Conditions"^[14]
ASTM D5526-12 testing is for the "Standard Test Method for Determining Anaerobic Biodegradation of Plastic Materials Under Accelerated Landfill Conditions"^[15]
ASTM D5210-07 (withdrawn by ASTM in 2016)^[16] testing is for the "Standard Test Method for Determining the Anaerobic Biodegradation of Plastic Materials in the Presence of Municipal Sewage Sludge"^[17]