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Most plastics contain various additives to improve certain characteristics of the polymer, such as ductility, hardness, and durability. Biodegradable additives are additives that enhance the biodegradation of polymers by allowing microorganisms to utilize the carbon within the polymer chain itself.

Biodegradable additives attract microorganisms to the polymer through quorum sensing after biofilm creation on the plastic product. Additives are generally in masterbatch formation that use carrier resins such as polyethylene, polypropylene, polystyrene or polyethylene terephthalate.

Chemical and physical properties of plastics play important roles in the process of biodegradation. Biodegradable additives can influence the mechanism of biodegradation by changing these chemical and physical properties.

Mechanism of biodegradation

There are several different mechanisms and conditions in which microorganisms can carry out the process of plastic degradation. In general, the process of plastic biodegradation results in a considerable decrease in molecular weight leading to a loss of structural integrity of the plastic.

Direct Action

Some microorganisms can directly consume plastic fragments and use the carbon as a nutritional source.

Indirect Action

Microbes involved in the breakdown of fossil-based plastics typically used an indirect mechanism in which microbial enzymes break down the plastic. Through indirect action, the metabolic products of the microorganism will affect the properties of the plastic, resulting in degradation.

Enzyme-based microbial degradation can occur under two conditions: aerobic and anaerobic. Plastics are typically made up of hydrophobic polymers, so the first step of biodegradation under both conditions involves the breakdown of the polymer into smaller constituents such as oligomers, dimers, and monomers by the enzyme. The microorganisms can then act on the lower molecular weight products. This process of breaking down the plastic into smaller molecules in known as hydrolysis or oxidation, and it is the most important step since it initiates the process of biodegradation.

Common enzymes involved in microbial plastic biodegradation include lipase, proteinase K, pronase, and hydrogenase, among others. The efficacy of these enzymes will depend on the type of plastic that they are degrading.

Once hydrolysis or oxidation occurs, microorganisms can use the monomers as a source of energy. Depending on the conditions, the products of microbial degradation will differ.

Aerobic

Under aerobic conditions, the microorganisms will use oxygen as an electron acceptor. The resulting products will be carbon dioxide (CO₂) and water (H₂O).

Anaerobic

Under anaerobic conditions, the lack of oxygen requires that the bacteria use a different source for an electron acceptor. Common electron acceptors used by anaerobic bacteria are sulfate, iron, nitrate, manganese and carbon dioxide. The resulting products under anaerobic conditions will be carbon dioxide (CO₂), water (H₂O), and methane (CH₄).

A simple chemical equation of the anaerobic process is: C₆H₁₂O₆ → 3CO₂ + 3CH₄

Types of biodegradable Additives

Starch can converted into plastic pellets that can then be used as a biodegradable additive to other plastics, such as polyethylene.

Starch

Experiments have been done on the feasibility of using starch as a biodegradable additive.

Bioaugmentation

Experiments have been done on bioaugmentation - the addition of certain microbial strains to plastics - and its role in increasing biodegradability.

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. Some important physical properties that can be measured experimentally are tensile strength, molecular weight, elasticity, and crystallinity.

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. 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.

Testing environmental conditions

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. Typically, samples are buried in biologically active soil for six months and are exposed to air to ensure that there is sufficient oxygen. The experimental conditions must reflect natural conditional closely, so the moisture and temperature of the soil are carefully controlled. The type of soil used must also be recorded, as it can affect the degradation process.

Specific Testing Methods

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

1. ASTM D5511-12 testing is for the "Anerobic Biodegradation of Plastic Materials in a High Solids Environment Under High-Solids Anaerobic-Digestion Conditions"
2. ASTM D5526-12 testing is for the "Standard Test Method for Determining Anaerobic Biodegradation of Plastic Materials Under Accelerated Landfill Conditions"
3. ASTM D5210-07 testing is for the "Standard Test Method for Determining the Anaerobic Biodegradation of Plastic Materials in the Presence of Municipal Sewage Sludge"

Laboratories performing ASTM testing methods

• Eden Research Labs
• Respirtek
• NE Laboratories
• National Science Foundation (NSF)

Environmental Impact

Large areas of land are currently covered in plastic waste. Biodegradable additives will help speed up the biodegradation process of plastics so that plastic pile ups will be less frequent.

Plastics are ubiquitous in everyday life and are produced in huge quantities each year. Many common plastics, such as polyethylene, polypropylene, polystyrene, poly(vinyl chloride), and poly(ethylene terephthalate), that can be found in most consumer products are not biodegradable. These, and other, non-biodegradable plastics accumulate in the environment, which is threat to both human and animal health and leads to harmful changes in our planet.  

Current solutions to dealing with the amount of plastic being thrown away include burning the plastics and dumping them into large fields or landfills. Burning plastics leads to significant amounts of air pollution, which is harmful to human health. When dumped into fields or landfills, plastics can cause changes in the pH of the soil, leading to infertile land once the plastics are degraded.

Because of the substantial growth in plastic consumption, biodegradable additives are becomingly increasingly necessary to increase the rate of degradability of common plastics. Current research is focused on finding new biodegradable additives that will shorten the degradation process from taking decades to centuries to taking only a few months to a few years.