To the best of my knowledge, "ziplock bags" are made of polyethylene.
Never saw one advertised that wasn't.  If in doubt, ask the manufacturer.

Regards,
Charlie
******************************************************************************
Charlie Sturm, Jr
Research Associate - Section of Invertebrate Zoology
                     Carnegie Museum of Natural History, Pittsburgh, PA
Assistant Professor - Family Medicine

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On Wed, 25 Jul 2001, Ellen Bulger wrote:

> Are all brands of ziplock-style bags okay for packing shells that may be in
> storage for a year or two? Are any of them?
>