Re: Resin kits and Barge Cement


Dennis Storzek <destorzek@...>
 

--- In STMFC@..., "radius158" <gard158@...> wrote:

What is "contact mode" ? Doug gardner
The way any contact cement is designed to be used:

Apply an even coat to each side of the joint;
Allow to dry THROUGHLY;
Press the two parts together.

The intention is to allow the fast evaporating solvents that keep the
cement liquid to flash off, otherwise they will be trapped in the
joint and have to migrate through one or both pieces to escape. Since
the solvents used to dissolve the synthetic rubbers used to make the
cement also soften styrene, this is bad.

Denny makes the point because many people were in the habit of using
Walthers GOO like it was Ambroid cement; just plunk the part in a
puddle of liquid glue and expect it to be fastened in place when the
glue finally dries. Doing this in an enclosed plastic carbody is a
recipe for disaster as the solvents evaporate, then permeate through
the plastic parts enclosing the space.

Keep in mind, however, that contact cements are formulated with a
mixture of solvents that have different rates of evaporation. The fast
solvents are expected to flash off quickly, before the parts are
brought together, while the slower solvents keep the glue soft and
pliable so it will bond to itself. This is why contact cements
normally specify a maximum "open time"; exceed it and likely the
cement will be too hard to form a bond. These slower solvents will
also soften styrene.

Silicone RTV caulk isn't immune from this; that vinegar like odor is
the acetic acid used to inhibit the cure while it is in the tube. It
also has to gas off before the bond will develop, but at least acetic
acid doesn't soften styrene.

Dennis

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