Richard Mitchell wrote: > I don't know about all plastics but my AquaTerra Chinook age 7 > years was definitely showing cracks in rounded areas and had > always been stored indoors. Dave Kruger can give you the > chemistry but one dramatic demo of the fragility of poly > containers came years ago on a mountaineering expedition I'd > organized on the Monarch Icecap in BC. We arranged for a bush > plane to resupply us with an air drop after reaching the icecap > plateau. [snip] We figured the tin [steel, actually] > would not make it but counted on the plastic. Wrong, wrong, > wrong. *Every* poly container shattered upon impact. [snip] > The tin [steel]? Everything was fine! The Coleman fuel can > distorted and stretched in shape, as did many of the other tin > [steel] containers, but all remained intact. I was going to stay out of this, but Rich invoked my mantle. (Rich can be very persistently devious -- he extracted *all* my intellectual worth over the course of a week's queries on materials science -- and I thought I was on a don't-need-to-use-my-brain (relaxing) sea kayaking trip! Shoulda got a consulting fee.) Per previous posts, Rich's poly bottles probably were not cold enough for that to contribute significantly to their fragility. However, it is mistaken to think that poly bottles have to be below 32 F (what we regard as "cold") for the temperature to have that effect. Somebody with an honest materials science background would be better versed in this, but I'll give it a shot: 1. Every plastic has a "glass transition temperature," below which it functions like a crystalline material, and above which it is more flexible -- hence resistant to fracture. This temperature might be higher than 32 F for polyethylene, depending ... 2. The tendency toward crystallinity for a polyethylene sample is a function of its initial polymerization conditions, its plasticizer content, and its age. a. Plasticizers are small molecules incorporated into the polymer during manufacture -- they are thought to "lubricate" the meshing of polymer molecules. They diffuse out of the plastic over time. Maybe topical application of "replacements" can forestall this process, the basis for application of Armorall and similar products to rubber tires and vinyl dashboards. b. Initial polymerization conditions determine whether the polyethylene is "linear," "cross-linked," "high-density," or "low-density." Each type has its property profile, with cross-linked high density polyethylene usually the stiffest, and least flexible. (Inexpensive poly bottles are usually linear, low-density polyethylene.) Reasons for the differences? We don't want to go there ... this is a family-oriented list! <G> If anybody wants to know more about polymers, check out some of the links at this URL: http://pw2.netcom.com/~uncleal0/net2.htm (ignore Uncle Al's politics -- his knowledge of chemistry is, however, impeccable). c. Age. Yeah. This is probably the biggy. Just like our own collagenous tissues, practically every plastic stiffens with age. [This applies equally to epoxy/polyester/vinylester/you name it - based polymers, though epoxy seems to be more resistant than the others, *if UV damage is minimized.*] The main factor is probably gradual loss of the random arrangement of the polymer chains. As they pack more efficiently, they more closely resemble an ordered crystal. Thus, they become more "crystalline," and are much less resistant to breakage from mechanical shock. Loss of plasticizers and oxidation of the polyethylene contribute. Bottom line: all our boats are aging, with polyethylene boats perhaps having the shortest lifetime. Hank Hays, and others, however, can no doubt attest to the fragility of well-aged polyester layups, which use styrene as a copolymer adduct / plasticizing ingredient. Sorry for running on ... I'm an academic, folks, I can't help it! -- Dave Kruger Astoria, OR *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************Received on Sat Dec 05 1998 - 09:39:39 PST
This archive was generated by hypermail 2.4.0 : Thu Aug 21 2025 - 16:30:01 PDT