Additives to Enhance Biodegradability

Biodegradability implies that a material is capable of degrading efficiently through natural processes. Biodegradability is determined by measuring the amount of carbon dioxide produced over a certain period of time from this natural degradation. EN13432 requires 90% biodegradation within 45 days for a product to be considered biodegradable.

Biodegradable polymer materials have been developed in a variety of forms, and thus have potential uses in a range of industries. Many of these polymers are well suited for adhesive applications such as environmentally friendly packaging, recyclable envelope adhesives, carpet backing, and many other products that are eventually destined for the municipal waste disposal facility.

However, most synthetic polymers are not biodegradable. Polymers such as polyethylene and polypropylene can persist in the environment for many years after their disposal. Biodegradable polymers are generally obtained via polymerization of bio-based raw materials. These raw materials are either isolated from plants and animals or synthesized through modern industrial processes. Examples of biodegradable polymers are provided in Table 1.

Additives that impart controlled degradation behavior to conventional thermoplastics, as well as to improve biodegradability in inherently agriculturally-based polymers are a potential strategy to achieve biodegradability due to the currently high price of biopolymers. These prodegradative additives are being examined primarily for packaging applications, but the technology is worth watching by adhesive formulators as well. Today, these additives fall into two broad classes: "oxo-biodegradables" and "biodegradable organic additives".

Oxo-biodegradables consist of transition metals that theoretically foster oxidation and chain scission in plastics when exposed to heat, air and/or light. Such additives are also known as prodegradant concentrates, and are generally based on catalytic transition metal compounds such as cobalt stearate or manganese stearate. The additive is typically used at levels of 1-3% and, therefore, may be a practical solution for many biodegradable adhesive formulators. The theory behind chain scission is that it is supposed to shorten the polymer chains to the point where they can be consumed by microorganisms found in the disposal environment and used as a food source.

The principal company that has developed these prodegradant additives is EPI Environmental Technologies, Conroe, TX, USA. Their products are trade named TDPA - an acronym for Totally Degradable Plastic Additives. Polymers modified with TDPA progressively degrade to lower and lower molecular weights. They become brittle, disintegrate, and are ultimately digested by micro-organisms. Another company developing these additives is High-Green Bioenergy in Ahmedabad. For a more detailed review of oxo-biodegradable plastic additives and additional suppliers, the reader is directed to a previous SpecialChem article.¹

In the organic material class of biodegradable additives, some portion of the additive itself (which is organic) will biodegrade and generate carbon dioxide or methane. The additives consist of cellulose, starch, or other petrochemical resins, which are known to biodegrade, such as EVA or PVOH. The problem with these types of additive is that even though the additives will biodegrade within a reasonable time, this does not assure that the remaining polymer will degrade sufficiently to meet the EN13432 specifications. Manufacturers with this type of technology include ECM BioFilms, Bio-Tec Environmentals EcoPure, BioBatch, Green Films, and Good Earth. These materials are also being developed primarily as film packaging. Since most of the individual materials are now being used as adhesives, transfer of the technology into the adhesives sector should be relatively straight-forward providing that there is sufficiently perceived value.