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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