Bioplastic, biodegradable, and compostable are terms that often end up in close proximity; however, I find them confusing. Because these terms are often used in conjuction with words like “green,” and “eco-friendly,” and because most of the companies that produce bioplastic, biodegradable- and compostable-products tend to market them with leaves, flowers, and other nature symbols, it’s difficult not conflate the meanings of these terms.
Bioplastic is a plastic “derived from renewable biomass sources,” according to Wikipeida. The emphasis in the term “bioplastic” is on the source of the material used to produce it. Bioplastics are marketed as advantageous because they are derived from a renewable source, rather than a finite source like petroleum oil. Indeed there are bioplastics that are identical to petroleum-based plastics, with the only difference being the source material.
Bioplastic does not equal biodegradable plastic. Bioplastic can take the chemical structure of a material that will biodegrade or it can be more similar to traditional plastic which does not biodegrade very well at all.
Biodegradable means a material is “capable of being decomposed by biological activity, especially by microorganisms,” according to Wiktionary. When it is used in common English, “biodegradable” usually means that the material will degrade to a set of products that do not harm the biological processes around them. If someone tells us that a piece of packaging is biodegradable, we expect the packaging to disintegrate and not create problems for the environment. We expect it to return to compounds found in nature.
Like most materials, plastic degrades. The question is how far must it degrade to meet the definition of biodegradable, i.e. returning to compounds found in nature. Most plastics photodegrade, which means they are broken into little pieces by sunlight. A plastic spoon shattered into a thousand little fragments in a pile of dirt did not biodegrade, if the fragments still retain the chemical structure of plastic (most of the time they do).
Compostable is a term that shouldn’t be complicated at all. It means that a material will biodegrade in a compost pile at a rate similar to the other materials in the pile. The American Society for Testing and Materials (ASTM) standard for compostability was ASTM D6002, and stated, “That which is capable of undergoing biological decomposition in a compost site such that the material is not visually distinguishable and breaks down into carbon dioxide, water, inorganic compounds and biomass at a rate consistent with known compostable materials.” There is a new standard (ASTM D6400) although it sounds like it suffers the same issues as D6002.
This standard was withdrawn in 2011, because they could not agree on renewing it. Since inorganic compounds include pretty much everything that doesn’t contain carbon, and the standard also includes carbon, it seems to me that the ASTM was wise not to renew it – their standard said something was compostable if it degraded into anything.
You can find an interesting discussion of all these terms at Green Plastics. In addition to all the confusion, there are some manufacturers who simply misstate the truth. There is almost no regulation controlling what a company says about the biodegradability or compostability of their product. Check out this post by Beth Terry, describing her experiences attempting to compost (both in her backyard and at a commercial facility) various utensils that claim to be compostable. And finally, I can’t tell you how many times I’ve seen a “socially responsible” restaurant offer compostable utensils but no compost bin – compostables in the landfill are about as bad as plastics in the landfill.