Epoxy Man Strikes Again

[Isaac OLEG]

Del,

I talked about the fact that the soundbord may remain hot (dry) during
the glue curing time because before the glue have totally stiffened it
may be advisable to avoid any wood displacement due to the moisture
coming back, that's all.

Talking about cooking epoxy, I have a quality, that may be cooked to
attain full strength, the temperature needed is far much as 120° .
While it can be used as such it is said that the maximum compression
resistance is obtained after a 24 to 48 h cooking in a hot box.

Even when working with modern glue as UreaFormalehyde, I understand
the panel is heated usually, or may be it may not be heated, only dry
(in a controlled gluing room) , as I suspect that warm wood will
absorb air moisture really faster.

Warming do helps the glue surface tension a little maybe is not it ?

Difficult to me to talk about these in regard of my large experience
:)

Talking about the bridge glue joint, I like to believe that we are not
only in mechanical phenomena there. Things are probably less simple as
they seem (if they look simple )

A glue joint with a "real" glue that is moisten enough the wood may be
more homogenous from a material point of view, than the mechanical
bond of epoxy (poor drip).

But I have yet some time to change my mind !

Best Regards.



Isaac OLEG

Entretien et réparation de pianos.

PianoTech
17 rue de Choisy
94400 VITRY sur SEINE
FRANCE
tel : 033 01 47 18 06 98
fax : 033 01 47 18 06 90
cell: 06 60 42 58 77

> -----Message d'origine-----
> De : Delwin D Fandrich [mailto:pianobuilders@olynet.com]
> Envoyé : samedi 8 mars 2003 17:00
> À : oleg-i@wanadoo.fr; Pianotech
> Objet : Re: Epoxy Man Strikes Again
>
>
>
> ----- Original Message -----
> From: "Isaac OLEG" <oleg-i@wanadoo.fr>
> To: "Pianotech" <pianotech@ptg.org>
> Sent: March 08, 2003 1:00 AM
> Subject: RE: Epoxy Man Strikes Again
>
>
> > Hello,
> >
> > I can't see why the glue joint or glue zone between bridge and
> > soundboard can't act as a sound modifier, or filter. A
> too thick  glue
> > joint certainly should.
>
> I suppose if one were to really use a pva adhesive
> (commonly referred to as
> "white glue") and if one were to make the glue-line thick
> enough (a mm or
> so) it might be possible to introduce enough compliance to
> affect the
> mechanical response of the soundboard. There would also be
> some structural
> problems since there are no adhesives except epoxy that
> have any mechanical
> strength at these thicknesses.
>
> Typical glue lines are in the neighborhood of 0.075 to 0.150 mm in
> thickness. Much less and there is the risk of starved glue
> joints, much
> thicker and the structural strength of most woodworking
> adhesives begins to
> break down. Modern adhesives manufacturers have improved
> the gap-filling
> qualities of their formulations with various additives so
> that adhesives
> such as Titebond and Titebond II work reasonable well even when the
> wood-to-wood joint fitting is not perfect.
>
> While technically thermoplastic, both of these adhesives
> dry hard. As Ron O
> has pointed out, they chip when the squeeze-out is cleaned away.
>
> The idea in fixing the bridge to the soundboard is to get a
> reasonably good
> wood-to-wood fit and a reasonably good glue bond so that
> the motion of the
> bridge is closely coupled to the soundboard. This is more easily
> accomplished that commonly thought. Remember that the
> bridge base has a
> relatively large surface area. The motion of the bridge is
> not isolated to a
> single point. It is, after all, a bridge. When a specific
> string sets the
> bridge in motion, that motion is felt, mechanically, over a
> broad expanse of
> the bridge. It would take an unbelievably viscous adhesive
> to audibly or
> measurable affect the energy transfer from the bridge to
> the soundboard.
>
>
> >
> > And the choice of the glue may change the resonnance
> frequencies of
> > the assembly too.
>
> Keeping in mind that we're really trying to keep the
> soundboard/rib/bridge
> system relatively free of specific resonances. Many things
> will affect the
> resonances found in a soundboard. Among them are changes in
> humidity and
> temperature. Even tuning the piano will cause changes in
> the resonances of a
> soundboard assembly.
>
>
> >
> > Beside I understand that the bridge and the board  have to be warm
> > (and dry) to be glued. Do you use the hot box before gluing ?
>
> This is only true if hot animal hide glue is being used to
> bond the ribs
> and/or bridges to the soundboard. Historically, soundboards
> and ribs were
> heated to temperatures up to 120º F (approx. 50º C) to keep
> the hot glue
> from gelling prematurely.
>
>
> >
> > Epoxyes seems to be cook sometime, to attain their final hardness
> > temperatures as high as boiling water . I suspect we don't want to
> > cook our  soundboards as that !
>
> So, it's OK to heat the soundboard to 120º F (approx. 50º
> C) when using
> animal hide glue but it's a problem to raise the
> temperature of the wood by
> some fraction of a degree (by either scale) when using
> epoxy? Let's think
> this through.
>
> Epoxy adhesives are thermosetting. They cure by chemical
> reaction and they
> do generate heat as they cure. This becomes most obvious
> when you mix up too
> much and it begins to kick over (chemically cure) in the
> pot or can in which
> it was mixed before you can use all of it. If you have
> really mixed up too
> much it can begin to bubble and boil, turning itself into a
> real witch's
> brew. The heat generated by its own chemical reaction feeds
> on itself,
> accelerating the chemical reaction and the mix goes into
> thermal runaway.
> The more epoxy mass you have the faster this process takes
> place. Once it
> starts, it's too late to do anything about it. Set the container on
> something nonflammable, wait for it to settle down, throw
> the batch away and
> start over.
>
> This is one of the most obvious differences between slow-setting and
> fast-setting epoxy. Mix up equal batches of each and the
> fast-setting batch
> will start bubbling much faster than the slow-setting
> batch. And it's one
> reason why, in our shop, we use slow-setting hardeners
> almost exclusively.
>
> One way to control this--if you really must mix up a large
> batch--is to mix
> the resin and hardener in a wide, flat pan. This spreads
> the mix out, giving
> it more surface area to dissipate the generated heat and
> slow the process of
> thermal runaway. It also illustrates why it is not a
> problem in real life.
> The epoxy goes on so thin that, as heat is generated by the
> curing process,
> it is immediately dissipated into the wood (or whatever).
> Yes, the curing
> process will raise the temperature of the wood. But, the
> mass of the wood is
> so large compared to the mass of the epoxy that the actual
> increase in
> temperature is nominal. In other words, the wood will get
> warmer but the
> temperature increase will be so small it would be difficult
> to measure just
> how much warmer.
>
> Confused yet? I can elaborate....
>
> Regards,
>
> Del
>
>