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MMD > Archives > June 1999 > 1999.06.20 > 13Prev  Next

Glues and Adhesives
By Craig Brougher

Ern Grover, in his article about glues and adhesives said a few things
that may be a bit misleading, while disagreeing with my article on hot
hide glue.  He said:

> In my opinion there are a number of misleading and perhaps inaccurate
> statements in the forwarded article.  In a short nutshell (or at least
> as short as I can make it):

His entire premise however, is based on hot hide glue being a thermo-
plastic, and able to "creep" just like other glues do, as stated below:

> Hide glues are thermoplastic adhesives and will creep like many other
> adhesives.  Perhaps epoxies are less likely to creep as they are a
> cross-linked polymer system

I would like to correct this central point of his letter and perhaps
dispel other misgivings created as a result.  Hot hide glue isn't a
thermoplastic, and it will definitely not creep like other plastics.
Ern is wrong.  Just because something has a melting temperature doesn't
make it a thermoplastic.  Also, hide glue has phases, like solder, and
in its final phase, it does not fulfill any properties of thermoplastic,
namely, "a thermoplastic may be heated and reheated any number of times
(within its heat range) without losing any of its properties."

Also, a plastic, to begin with, requires a plasticizer and a synthetic
resin system, or must by nature, be a plastic like gutta percha, before
a word like "thermoplastic" would even apply to it.  Cured and fully
dry hot hide glue is no plastic at all.  It is neither pliable,
ductile, malleable, or able to conform to any 3-dimensional shape any
number of times.

Contrary also to Ern claims, is that  the definition of epoxies has
always been that they are made from thermosetting resins and swap
oxygen atoms with the hardener during the exothermic phase.  I also
doubt that they are a cross-linked polymer as a result of this basic
swap, but the reason epoxy works so well in loose joints is because it
doesn't shrink when it hardens, creating a non-plastic, filled joint.
This is how modern furniture succeeds.  Less by its precision joinery,
and more by its microwaved, hard-as-a-rock filled resin cement.

Regarding flexible joints and loose joints, I have for years subscribed
to Fine Woodworking magazine, and keep up with their articles.  I have
seen, as I am sure Ern has, also, ideas and theories propounded as far
as why joints get loose.  But the prevailing one that seems to be
commonly ascribed to is that if you have a loose chair joint, as I
alluded to, the problem isn't the uncured wood or shrinking joints.  It
is a defect in the chair joint itself, and that joint defect will cause
the other joints eventually to loosen up.  Furniture with poorly cured
core wood, despite otherwise good joinery seem more often to loosen up
all over.  Ern wondered:

> Regarding creep of glued joints, this should not be a problem for
> almost any joint.  Plywood will have minimal creep under normal
> loading conditions, for example.  For cabinets or clock cases ...
> well, you figure out the amount of load and determine if this ought
> to be significant.  Does the author of the article know that wood
> will creep?

That is surely begging the question.  I guess he hasn't been reading
this page too long, if he wondered if we know that wood creeps or not.
But proper joinery is designed to swell and contract together.  That's
the whole point of wooden joints, versus butt joints and simple glued
surfaces.  So if joints, by nature, eventually fell apart from weather
and temperature changes alone, how would anybody still be able to sit
in original 450 year old Chinese chairs, for example (as I have done)?
Chairs twice as old as this country, brought over on sailing ships,
jointed with mortices and glued with hot hide glue? Or how would veneer
ever stay on old furniture, which is ten times more vulnerable to
seasonal changes than the most sensitive joint?

One method of jointing furniture didn't even use glue.  It actually
relied on dry wood dowels inserted into green wood holes, and things
like that.  So high humidity changes could actually crack out those
kinds of joints if it happened like Ern said it did.  There is no way
that two pieces of maple, for example, doweled into each other can ruin
its own joint because they swell and contract together.  Only if the
wood used in both parts is not properly cured for furniture, or is a
completely wrong choice of joinery, or if the woods used are too
dissimilar, could this happen.

The day that a chair joint can internally crush its own structure
simply by swelling and then contracting repeatedly, is also the day
that a solid plank of anything you name could also do the same thing!
Forget it.  Good joinery shouldn't even require glue to fit up a tight
chair, for example.  The joinery makes it tight, but the glue makes it
permanent.  A good joint doesn't move around.  That's the purpose of a
good joint-- not to move, at all.  And of all the furniture you've ever
used, when joints start to move and creak, they are no longer good.
They have lost their integrity.

As far as being able to "figure out the amount of load and determine if
this ought to be significant," I don't know how anyone would do that to
determine anything about "creep."

As far as gap-filling glues, versus thin glue or non-gap filling glues,
I will say that hot hide glue can be either, depending on how you want
to mix it and use it.  anybody will tell Ern that hot hide glue is
definitely gap-filling, and that PVA glues definitely are not, as they
are their own worst enemies, and unfill their own space, losing about
40% of their own volume when dry.  But the only truly gap-filling glue
that doesn't shrink some when drying will be epoxy.

Everything else that Ern said about glues, I agree with in general, and
did not contradict myself as he seems to intimate that I did.  And if
he wants to know what glue in particular I describe as a lousy "space
age adhesive, I will be happy to say it is Weldwood White Space age
adhesive glue with the red pull-up cap.  I personally will tell anyone
I think it is lousy, terrible glue.  It dries almost clear and hard, as
opposed to translucent and soft, but has given me very weak bonds, and
refuses to glue simple things for me like a felt punching.  They look
tight and then peel right off with almost no force at all.  That may be
"space age," but that's mediocrity taken to an art form, in my opinion.

Craig Brougher

(Message sent Fri 18 Jun 1999, 17:37:19 GMT, from time zone GMT-0500.)

Key Words in Subject:  Adhesives, Glues

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