A lot of talk about plastics has been floating around these days- Biodegradable Plastic, Bioplastic, Compostable Plastic- what does it all mean? Can we actually put plastic in the compost heap?
What is Compostable Plastic?
Let’s start with Compostable Plastic. According to American Society for Testing & Materials (ASTM), This is a plastic that is capable of undergoing biological degradation. When it is composting it breaks down into carbon dioxide, water, and “biomass” (biological material from living, or recently living, organisms), that will break down no slower than regular paper. Also, it must not leave any toxic residues or pieces that can be seen to be different than regular plant material. The composting process that does this breaking down can be either industrial or municipal aerobic composting facilities or even a properly managed home compost heap.
One brand of compostable plastic in this category is Natureworks PLA ( Corn Based ) grade 2000D. It has been demonstrated to decompose 97.5% after 60 day in a commercial compost facility. For comparison, standard Cellulose (newpaper) decomposed only 77.8% in the same time period.
What are Bioplastics?
Bioplastics are plastics made from natural materials such as corn starch, a material that can be easily biologically renewed. Actually, about 50 percent of the bioplastics market is made from starch. Bioplastics are NOT made from petroleum products which can not be renewed. They are therefore considered ‘environmentally friendly’. As stated before, the majority of bioplastics are made from corn starch, and some can break down in a few weeks. The cornstarch molecules absorb water and swell up thus causing them to break apart into small fragments that baceria can more readily digest.
An example of bioplastics is Plastarch Material, PSM for short. It is made of starch with several other biodegradable ingredients. PSM is a heat-resistant plastic, one of a few bioplastics capable of withstanding high temperatures. Also PSM can be used in the same manufacturing machines as”regular”plastics, making it very desirable in that respect. According to Wikipedia, “PSM is stable in the atmosphere, but biodegradable in compost, wet soil, fresh water, seawater, and activated sludge where microorganisms exist.” Also, it can be incinerated without producing toxic fumes.
What are Biodegradable Plastics?
Biodegradable plastics are made from traditional petrochemical materials, which are not renewable. Their advantage is that they are made to break down much more quickly that regular plastic. Biodegradable plastics contain additives that cause them to decay more rapidly in the presence of light and oxygen and are known to the industry as “oxos”.
Their disadvantage is that they are the same conventional petroleum-based products with very long breakdown schedules, but with the special additives. Therefore, while biodegradable plastics do break down, they don’t always break down into harmless substances. They can leave behind toxic substances that can make them unsuitable for composting. In addition, it has not been clearly demonstrated that natural composting methods can continue to break down the smaller pieces these plastics chemically break down into. It is difficult to say if these forms of plastic are actually good for the environment or are just one more problem.
So, can we now compost plastic? The answer is… maybe. Check very carefully before you put it in to make sure you won’t have to pick it back out in a few weeks with no noticeable break down.
What is BIOMASS?
Biodegradable Plastic, Bioplastic, Compostable Plastic- What ?
Wikipedia: Plastarch Material