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MMD > Archives > June 1999 > 1999.06.24 > 14Prev  Next

Hot Hide Glue a Thermoplastic?
By Craig Brougher

Robbie left this definition as the one to go by for the word

>[ These definitions given at
>[ Thermoplastic:  A glue or resin having the property of softening
>[ or fusing when heated and of hardening again when cooled.

Since the Webster's Unabridged is closer and authoritative, I will
go to it.  If we want "just the facts", Robbie, then why pick just
anybody's facts?  I too have found articles about hot hide glue on the
web that call it a thermoplastic, but it is just technically incorrect
idiom, used improperly, as so many words today are.

Webster's Third New International Dictionary:

  "ther-mo-plas-tic (therm + plastic) having the property of softening
  or fusing when heated and of hardening and becoming rigid again when
  cooled  (~ synthetic resins / materials can be remelted and cooled
  time after time without undergoing any appreciable chemical change --
  J.S.  Campbell) -- distinguished from thermoset and thermosetting."

The reason hot hide is not a thermoplastic is because in its final
phase (fully cured and stable) it requires both moisture first, and
then heat, to remelt.  You cannot remelt fully dry hot hide glue at
135-150 degrees F.!  Fully dry and cured hot hide glue remelts at 425
degrees F., unless water or steam can be re-introduced into the joint,
chemically dissolving it again.

This water comes from the surrounding wood, which contains on average,
6-10% water, by weight.  That is more than enough for several reposi-
tioning attempts.  However, were you to glue a material with no water
at all, then unless the joint still has water in it, the glue gets only
partially soft before it starts to burn.  It is not a thermoplastic
therefore, because it needs a solvent plus heat, and each time its
characteristics change, or degrade.  I have also written Ern Grover's
expert, Terry Connor, who has agreed that hot hide glue cannot be
classified as a true thermoplastic.

If we were to take the dried chips and stuff out of the hot glue pot
and allow them to fully cure and completely dry in a warm oven so they
will get rid of even air moisture, and then put them in a little pot,
mostly airless, with a cooking thermometer inside, they melt at about
425 degrees F. (according to Hudson Industry's article some time ago).
However, left at 425 F. it changes form, flows slightly, gets black and

The reason it is important to know that hot hide glue is not thermo-
plastic is because they tend to cold-flow over time.  Hot hide glue
when fully hard will not cold flow -- ever.  There is no creep, period.

As far as glass goes: glass flows slowly, even when cold.  It would
have a better chance to be classified as a high-viscosity liquid,
rather than a thermoplastic.  It is definitely not classified as a
thermoplastic by anybody I knew.  Why?  Because it still doesn't
qualify.  You cannot reheat glass any number of times and still have
the same properties.  I used to live close to Pittsburg Corning Glass
plant, and discovered that they used ground glass to make glass blocks.
I asked a plant manager if they could turn it into glass plate, and he
explained that they could not.  It would be too brittle.  That's why
glass cannot be a thermoplastic.

Here, I think, are the facts you wanted.  I doubt that you will find
anything different from a chemical dictionary, either.  Lots of words
are tossed around carelessly, not realizing that those words convey,
often enough, a wrong concept.

Craig Brougher

(Message sent Thu 24 Jun 1999, 17:21:49 GMT, from time zone GMT-0500.)

Key Words in Subject:  Glue, Hide, Hot, Thermoplastic

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