Re: [Paddlewise] Life of plastic boats

Dave,

Thanks for your explanation.

I wonder if some other factors should be added.

Thickness of the material.  The walls of a polyethylene boat are of x
thickness.  That would likely mean that they would age a lot faster than
say the half inch thick high density polyethylene cross ribs used in
Feathecrafts.

Length of the material in conjunction with how much it is subjected to
flex.  I sense that polyethylene white water boats (except for the hard
knocks they get against rocks etc.) tend to last a bit longer than their
sea kayak counterparts because of the differences in length and amount
of flex.

ralph diaz


Dave Kruger wrote:

> 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.
-- 
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Ralph Diaz . . . Folding Kayaker newsletter
PO Box 0754, New York, NY 10024
Tel: 212-724-5069; E-mail: rdiaz_at_ix.netcom.com
"Where's your sea kayak?"----"It's in the bag."
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