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Name: Sherry
Status: other
Grade: other
Location: KS
Country: N/A
Date: August 2007

Question:
Fish oil containing high DHA and EPA can melt polystyrene, is it chemical reaction? what reaction happens? or fish oil act as solvent? if fish oil can act as solvent for polystyrene, then what else (oil) can dissolve polystyrene?



Replies:
Yes, Sherry,

It's just a solvent-type action, not a "chemical reaction". "Styrofoam" (tm of Dupont) is a gas-foamed casting of polystyrene plastic. Polystyrene is a -(CH2)- chain with a -(C6H5) benzene-ring group on every second C, replacing one of the H's. Having only C and H atoms, it's a very non-polar molecule. The benzene group with it's de-localized electron cloud several atoms across reaches out farther to do physically attractive interactions with nearby molecules than does the simple waxy or oily -CH2- chain. So non-polar or moderately polar solvents with double-bonded groups like ketones -(C=O)- and benzene-rings (or other "aromatic" groups) will tend to dissolve it. Look up "solubility parameters". Solubility parameters (s.p.numbers) represent the "like dissolves like" rule in numerically-ranked form.

Any groups attached to the carbon backbone, that raise the s.p. number without adding too much electric polarizedness, will help a liquid dissolve polystyrene. Hydroxyl groups are too polar, so glycerin and alcohols don't do it. (Methanol might slightly.) Gasoline does attack styrene foam because it has modest percentages of benzene, toluene, and xylene. Paint-store denatured alcohol has 10% acetone CH3-(C=O)-CH3 in it as the denaturant, so it will attack ps-foam a little. All cousins of acetone, like MEK and MIBK, will attack polystyrene. Old-fashioned plastic model glue is toluene (CH3)-(C6H5) with a little ps already dissolved in it to make it thick. Paraffin wax, drug-store mineral oil, and candle oil are very pure linear (CH2)x molecules and won't quite dissolve it except maybe when heated. Polyester resins typically used in fiberglass really melt ps foam. To get around this I once built water-floats of ps-foam wrapped with fiberglass cloth and painted on epoxy resin instead. Epoxies do vary in molecular formula, but most won't dissolve polystyrene before they harden. DMSO (dimethyl sulfoxide) and NMP (n-methyl pyrolidone) are solvents with high s.p. values; they dissolve ps very well.

Kitchen vegetable oil (unsaturated fat oil) doesn't attack ps that I have noticed. Saturated fats should be safer than that. Silicone oils should not either. I can't think of all the food oils with multiple components you might want to consider in the future. You can probably answer your own question for specific edible oils by looking up their molecular structure on the web. Wikipedia is getting pretty good for that. Any molecule having some rings with double-bonds, or more than one ketone group per eight carbons, is likely to be a problem in the short-to medium term. Being a problem in the long term is easier, more common, and a little time-challenging to test for. Glycerin and ethylene glycol, for example, might be like that. Soluble compatiblity can go both ways: liquid into solid or solid into liquid.

Some food oils will make the ps rubbery by soaking into the ps (look up "plasticizers"), other substances slowly might dissolve away the outer layer of bubble-walls if there is a large quantity and some circulation.

Foam is a structure of very thin skins. Solvents don't have to dissolve very deep or fast to disrupt it. And bubble-walls can pop when made weaker. Some ps-foam is an packed body of foam-beads which are not well attached to each other. Look for that. The cracks between beads are penetrated by liquids and come apart easily. So polystyrene is in it's most vulnerable form when it's ps-foam.

Lining a ps-foam container with something should help. Polyethylene bagging to contain most liquid, surrounded by freezer-paper or aluminum foil to separate the oil-soaked baggy from the ps-foam, should be a rather complete solution. Another reason to use liners or baggies: it's not entirely smart to assume that typical ps-foam coolers are pure enough to be allowed long-term direct contact with your food.

Jim Swenson


Many nonpolar liquids can dissolve or soften polystyrene. Pretty much any fat will do it; you will know this if you ever try to pour pan drippings into a foam cup! (Yes, I have found this the hard way myself.) The warmer the liquid is, the faster it will work, but you will find the pan dripings can eat through the cup when they are well below the melting temperature of polystyrene (which is a bit but not a lot over 100 C).

One of the most dramatic examples of a liquid dissolving polystyrene foam is acetone. It's very dramatic. Just a small drop of acetone will make an impressive hole in the foam. Not to be missed!

Richard Barrans
Department of Physics and Astronomy
University of Wyoming


Polystyrene is soluble in most oils to a greater or lesser degree, depending upon the particular oil, the temperature and contact time. For example, if you drop a piece of lemon skin to a hot cup of tea you will see the polystyrene begin to disintegrate at the level of the tea due to lemon oil in the skin that spreads out across the surface of the hot tea.

Vince Calder



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