Having sold Tyvek for so many years, I knew it was a combination of plastics. Here is a good web page that is quite interesting http://www.fibersource.com/f-tutor/olefin.htm. I learned that olefin is a combination of polyethylene and polypropylene and that olefin has the LOWEST SPECIFIC GRAVITY (it floats) of ALL fibers (this site appears to be only about man-made fibers). Seems like we both had a piece of the puzzle and now I'm having fun tying with this yarn. Will let you know how well the flys float from the yarn Harmen supplied me.
Next I did an AOL search for Phentex yarn and #3 result was the pike fly page, #4 was Hans' page on Wally Lutz's egg cluster. Cool how FFing and tying is so prominent on the web. :-)))
Murf
Tyvek is not poly propylene but polyethylene.
Polyethylene floats in water, but just barely.
