Chemistry Question #139

Steve Appleby, a 34 year old male from the Internet asks on February 24, 2000,

What catalysts are needed to break down plastic containers, such as yogurt cups?

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The answer

Vance Williams answered on November 24, 2003

In response to a followup question from visitor Joey Hays, "Is there a chemical which I can add to Low Density Polyethylene LDPE and cause it to loose its structural integrity?" Dr. Williams suggest toluene, based on the following paper:

The dissolution/reprecipitation technique applied on high-density polyethylene: I. Model recycling experiments.    Poulakis, J. G.; Papaspyrides, C. D.    Dep. Chemical Engineering,  National Technical Univ. Athens,  Athens,  Greece.    Advances in Polymer Technology  (1995),  14(3),  237-42. 
A model dissolution/re-precipitation process was developed for recycling of high-d. polyethylene (HDPE), using virgin HDPE pellets [Finathene HF 513, Fina Co. Greece].  The process comprises dissoln. of the plastic in an appropriate solvent, re-pptn. by a nonsolvent, thorough washing of the material obtained, and drying.  The solvent mixts. involved are sepd. by distn. for further reuse.  Toluene/acetone was evaluated as solvent/nonsolvent pair and proved to be satisfactory, similarly to the case of recycling low-d. polyethylene.  The effect of sample history through successive recycling cycles was also studied.  The recycled grades were evaluated in terms of: melt flow index (MFI), mol. wt., crystallinity, mech. performance in tensile mode, and grain size anal.  In all cases, the recycled polymer, as pellets, exhibited excellent retention of properties. 

Note that they use toluene to dissolve the HDPE and then precipitate (i.e. make it insoluble) by adding this to the acetone (a nonsolvent).  I would also refer to person to another paper, that talks about HDPE in solution:

Epitaxial crystallization of high-density polyethylene on polypropylene in solution-cast films.    Shen, Yu; Yang, Decai; Feng, Zhiliu.    Changchun Inst. Appl. Chem.,  Acad. Sin.,  Changchun,  Peop. Rep. China.    Journal of Materials Science  (1991),  26(7),  1941-6.

Martin Hocking answered on February 24, 2000

The answer depends significantly on whether this question relates to yoghurt cups which are thrown around as litter, or ones which go through a recycling process. It also depends on the plastics code on this type of cup.

Suppose the cups are made from either high impact polystyrene, or polypropylene. If the first material, then litter decomposition is very slow, of the order of tens of years. If the second material, then a year or so of exposure to sunshine should cause the material to decompose.

In a recycling context, both materials can be thoroughly cleaned, dried, and remelted to make new packaging or other articles, although the law does not allow reuse of recycled plastics for food packaging where the food directly contacts the plastic surface.

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