Feb 112007

The other day Tim brought home some food in a container that claimed to be made of corn and therefore compostable. I was intrigued by this, since I remember going to street parties and Christmas markets in Germany where the disposable plates were grain-based and even edible (not that people usually did eat them) but haven’t seen them here. So I looked into it a bit more.

The container was made out of something called PLA, which is made from starch. The label said not to microwave it, which had me puzzled – isn’t corn safe to microwave? What would happen? Would it melt? I tried microwaving some water in the container and nothing untoward happened, so I poked around on the website for the company (NatureWorks LLC, a subsidiary of Cargill) to find out more.

The website didn’t give any details that I could find as to why not to microwave the container, so I emailed them. And got back a rather mixed reply. Some breathless marketing prose (new, revolutionary, bridges the gap between natural and synthetic products), the actual answer in the middle, a somewhat odd paragraph about how all the technical info they’re prepared to disclose is on the website (if the answer to the microwave question had been there, I wouldn’t have emailed them). Anyway, the answer is supposedly that PLA has a low melting point, so the containers might leak in the microwave.

Next Step

So if this container is compostable, where can one compost it? The store has a big bin to take them back, but what about the Vancouver compost facility? Or indeed my compost bin? The cashier said “oh, just throw it in your compost bin.” “Are you sure?” “Yes, absolutely”. According to the PLA brochure, you need 140ºF and humidity between 80% and 90% for extended periods of time. So that rules out my compost bin. And the Vancouver City compost facility as well, unfortunately, since they use an open windrow process. I emailed the relevant people in the Vancouver City engineering department, and they pointed out not only would it not compost, if I put the plastic in the yard waste bin, it might encourage other people to put unsuitable plastic containers in there. So it’s back to the store with the containers; fortunately it’s not a big deal since it’s one of the local stores we shop at anyway.

  3 Responses to “Compostable Plastic”

  1. hi, i first was searching for info on german nuclear fission work during ww2 and wound up here – – in reference to the container dilema, my suggestion is to take the container to your local flea market and sell it as fancy taco shells. have a good one, tom

  2. The trick with biodegradable materials is that we don’t want them to degrade until we are done with them. How does the material know when its time is up? A food container generally needs to be able to withstand water and some microbes. What makes it waterproof will tend to interfere with microbial action. One solution is a material that breaks down when exposed to heat which is produced in compost piles or heat plus water. But the temperature in a microwave can easily be higher than the temperature of a compost pile and can exceed not only the glass transition temperature (50 to 80C for PLA) but also the melting point (173 to 178C). Near or above the glass transition temperature, stiffness of the material may be reduced dramatically. Seams (such as on cups) could fail due to softening of the base material or the adhesive used. Even non-biodegradable hot cops can spring a leak if you let the leftover drink sit for a while. And plastic coated paper food trays can fail after microwaving a very wet food. Chemical reactions occur faster at higher temperatures. Some chemical reactions double in speed for every 10 celsius degree rise in temperature, for example. At higher temperatures, the starch or plastic may start to disolve in water. And these materials probably have a higher melting point and glass transition temperature.

    When microwaved, the container could rupture immediately or soon after, fail while eating, fail while sitting with unconsumed portion, breakdown later while storing the leftover portion, or become to0 soft to handle safely. I once had a non-biodegradable microwavable plastic container containing macaroni and cheese lose its structural integrity and buckle dumping scalding cheese on my arms (the microwave was on top of the fridge and I like my mac & cheese well done). All kinds of accidents can occur when you expect the material to be rigid and it is in a mood to act like a plastic bag instead. It is also possible that at higher temperatures than would normally be encountered in a compost pile, such as while microwaving fatty foods, that you could have toxic breakdown products that don’t form in the temperature range encountered in a compost pile. It is even possible that dry portions of the material (not cooled by evaporation) but still absorbing energy will catch fire. Did you know that a potato will catch fire in a microwave if you nuke it too long? Found that out the hard way too (I hit 6:00 but keybounce apparently changed that to 60:00 or 66:00). It is also possible that the materials are FDA approved for food contact but not when significant amounts leach into the food at higher temperatures. Also, the degradation products might be considered soil safe but not people safe. Fertilizer, for example, is allowed in soil food grows in but probably not added directly to food. Given the large number of natural and unnatural chemicals present in food, there could be the possibility that above some minimum temperature that some of them might react with the plastic to produce a toxic byproduct. Even natural foods can produce carcinogens all by themselves when overcooked. And soil bacteria can safely consume a lot of things we can’t.

    It is possible that the container may work okay in some microwave applications, particularly when backed up by a microwave safe plate, bowl, or other dish, but the company doesn’t want the complaints, customer dissatisfaction, bad publicity, and product liability that results from the other cases when a customer is scalded with hot coffee, soup, butter, grease, or cheese or when the container leaks. Remember the infamous McDonald’s coffee lawsuit?

    Overall, the company not being forthcoming with information is a problem itself and might be indicative of other problems. It does not inspire trust. Many approved “safe” products have turned out not to be as safe as claimed. Are we to believe that the company, whose products we are paying a responsibility premium for, is behaving responsibly in all of its other activities, like safety and environmental testing, when it is being demonstrably irresponsible when it comes to informing consumers? Even if the company performed due diligence, other people with different experience and expertise might recognize potential hazards the company did not anticipate if the information was available.

    Your corn is microwavable, by the way, due to its water content which on the one hand abosorbs microwaves but on the other limits the temperature though I have had corn scorch and if you nuke it long enough to drive off the water you probably have an incident similar to my potato fire. Dried corn is a fuel. The process of manufacturing this stuff resembles traditional plastic manufacturing far more than it does the making of a tortilla plate or bowl. It isn’t like they dip a tortilla in beeswax. This product is made in a highly industrialized chemical process. That may include petrochemicals as well as natural inputs. It may involve significant amounts of CO2 emissions or pollution. stannous octoate is commonly used as a catalyst in making PLA, according to wikipedia. The MSDS for this material does not inspire confidence: “Hazardous in case of eye contact (irritant), of ingestion. Slightly hazardous in case of skin contact (irritant), of
    inhalation (lung irritant).”. LD50 unknown, carinogenic/mutagenic/teratogenic effects and developmnental toxicity: not available. However, this compound is also used in making silicone rubber which is safe for surgical implantation and has been implanted in millions of people. A GE MSDS for 90% stannous octoate mixed with 2-ethylhexanoic acid (the latter can cause liver damage) gives an LD50 of 3400mg/kg, which is safer than table salt and worse than alcohol and vitamin C. Table salt is also listed as an eye and skin irritant.

    Is it green or is it greenwash? Lack of documentation makes it hard to tell.

  3. I see you guys have come across the problems of compostable plastics. Compostable plastics are not a consumer friendly method to dispose of plastics. This makes them not really a viable option as a way to dispose of plastics. The compostable people tout it as the great saviour but as you found out, it is more trouble and may not yield the results we are all lead to believe.
    Consider instead the newer landfill biodegradable additives, that when added to a conventional plastic at time of extrusion, allow the plastic to degrade when put in a landfill. No special conditions required, just throw the plastic in the trash as you do now. (Unless it can be recycled of course). Once in the landfill, naturally occuring bacteria will start to digest it away. The plastic retains all of its original properties of strength and temp properties too. Look up Ecopure or Eco-One for more details on these. THis is where the world needs to go to control this very large plastic trash problem we have.

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