Question

Types of biodegradable plastics

Answer 1

The two main types are oxo-biodegradable and hydro-biodegradable.

Answer 2

Biodegradable plastics can be biobased or fossil fuel based. New types of plastics have been produced in recent years to address the plastic pollution problem, by trying to shorten the time needed to degrade them, especially in natural conditions. However, not all the current biodegradable plastics have achieved this aim.

Definition of Biodegradable Plastics

Biodegradable plastics are those that can be degraded by microbial action to produce natural end products, like water and carbon dioxide, in a reasonable period of time. The time needed to decompose completely depends on the material, environmental conditions such as temperature and moisture, and location of decomposition according to Biodegradable Products Institute 

Compostable plastics are those that biodegrade rapidly and turn into humus that is not contaminated by metals.

Plastics that are derived from plants are called biobased plastics. Not all of these are biodegradable; for example there are biobased pet bottles made to be durable. The biobased plastics that biodegrade are made of two materials: biomass and polysters derived from plants. There are two kinds of biobased polysters: polylactide acid (PLA) and polyhydroxyalkanoate (PHA).

Polyhydroxyalkanoate (PHA)

PHA is produced naturally by bacteria and Genetically Modified Organisms (GMO) plants, but there are plans to tryproduction from food waste. Polyhydroxybutyrate or PHB is also a kind of PHA that is widely used. PHAs are expensive to make as only limited quantities can be produced from bacteria.

Uses: PHAs are used as food wraps, cups, plates, coating for paper and cardboard, and 'many medical uses, including sutures, gauzes, and coatings for medicines' according to a report by the Centre for Industry and Education Collaboration (CIEC report). It can replace most of the major fossil fuel based plastic types currently used, such as PE, PS, PVC, and PET points out Bio Based Press.

PHA-blended starch/cellulose plastics: Some plastic items are made entirely of PHA, as in case of water bottles notes Bio Based Press. However, since production of PHA is expensive, it is also blended with starch and cellulose to make it more economical. This has the added advantage of improving the rate of decomposition according to the Dartmouth Undergraduate Journal of Science (DUJS).

Biodegradation: It can be completely compostable in environments that are rich in microbes and fungi, especially soil. These microbes breakdown the PHA with the help of enzymes. The time necessary to degrade depends on the concentration of microbes in the environment.

PHA takes two months to decompose in backyards, according to Bio Based Press.

The rate of decomposition is much slower in marine waters where less than 50% is broken down after six months adds CalRecycle (pg. 6). PHA passed the ASTM D7081 test by showing 30% decomposition in six months (pg. 7).

Polylactide Acid (PLA)

The DUJS explains that PLA is a thermoplastic made through fermentation by bacteria. PLA are actually a long chain of many lactic acid molecules. Since there are many inexpensive means of producing lactic acid, these have only to be polymerized or joined. Therefore, PLA is less expensive than PHA. However, PLA is brittle and its application is more restricted than PHA. Manufacturers get around this problem by including additives or polymers.

Uses: It is made into grocery bags, food packaging, bottles, cups, and plates. Since it decomposes well in the presence of acids, it is used in some medical applications like medical sutures and plates, where it dissolves after 90 days notes the CIEC report. It is also used in 3-D printing of objects.

PLA and polymer blends: PHA can also be blended with polymers from renewable sources to improve its qualities according to DUJS.

Biodegradation: PLA cannot be composted easily in the backyard because temperature and water levels needed are not available in this environment.