Rubber Contact Glue

From: jblank (jblank_at_wanadoo.nl)
Date: 07/06/04 

Date: Tue, 6 Jul 2004 22:02:16 +0200

Hi all,

I've been been trying to bond a certain type of rubber using a contact glue
for a while now, with mixed results (in terms of contact/bond strength -
some areas are super strong, other places I can peel off as if no glue was
applied) and am now trying to obtain a better understanding of how the
application and bonding processes actually work (from a chemical
perspective). I've been searching around for a while now, but have only
managed to find some nice sales brochures. Friends and sales reps have
helped out quite a lot but I haven't managed to get any chemical process
specific information on the materials and application steps I'm using. Can
anyone help me out here or point me to a good source of information?

I'm currently using a neoprene/polychloroprene based glue. I'm uncertain
what family the rubber falls under, though I'm led to believe it's of the
neoprene variety, hence the choice for a neoprene based glue.

The application processes I'm using involves a few steps: grinding/sanding
the rubber surfaces to be bonded, cleaning with a neoprene based thinner,
applying a layer of thinned neoprene glue as a primer, allowing to dry,
applying a layer of neoprene glue, allowing to dry for a few hours,
reactivating with IR lamps for a minute, joining surfaces and applying
pressure for a few hours.

The rubber I'm using has quite an oily/greasy sweaty feel to it after
grinding/sanding. One bloke I spoke to said that this may be a result of
compounds in the rubber that contribute to it's suppleness being released
during sanding, or even the carbon black compound used to colour it.
Whatever it is, these compounds seem to be interfering with the bonding
process of the glue (my theory) hence I tried to thoroughly scrub and clean
the surface with a neoprene based thinner (after which the cleaned surface
feels nice and sticky) and then apply a super thinned layer of glue as a
primer to lock these compounds away. I let the primer layer dry for about
5 - 10 mins before applying a thin layer of glue, that I allow to dry for a
few hours.

First question - does the idea of locking away the evil greasy-feel forming
compounds with a primer have any justification, or is it just one of those
ideas that sounds good at the time but serves no practical purpose. I had
the idea that the primer forms a barrier between the evil compounds and the
glue, preventing them from interacting and affecting the bonding process.
However, I was also kind of curious that if the primer forms a barrier, how
does the glue layer achieve contact with cure points on the rubber polymer
chain - is it possible that a primer forms a selective barrier - only
blocking contact with the evil compounds in the chain whilst leaving cure
points open? or does the increased concentration of solvent in the primer
hold the evil compounds back and permit the glue elements in the primer to
lock on to the cure points and create a larger cure point area for the
subsequent glue layer? (does this question make sense - sorry, maybe I've
even answered my own question; I've been working around glues all day and so
maybe I'm not thinking too straight at the moment...)

In relation to allowing the glue to dry. My idea here is that the glue
basically consists of the material one is attempting to bond, only held in
solution by means of solvents (?). Hence, allowing the glue to dry permits
the solvents to evaporate from the glue and concentrate the effective
elements of the glue that will form the crosslinking bonds between the
rubber chain polymers. If this is correct then one might assume it best to
allow the glue to dry for as long as (is practically) possible. Is this the
idea behind allowing the glued surfaces to dry before joining? I've also
been putting the glued surfaces into a vacuumn to speed up the
evaporation/drying time and visual inspection seems to indicate it works.
The last batch of glued test pieces that I vacuumed are still in the press,
so I'm not sure if this will enhance the bond strength - any ideas?

Next question relates to reactivation of the glue with the IR lamp - what
the hell is this actually doing - I'm just following blind instructions from
the manual and lots of other people that keep saying "You've got to use the
IR reactivation step!". These people seemed to have obtained this
information from other people that told them the same thing. Does anybody
know the person who first said "You've got to use the IR reactivation step!"
and why he said this?

Last question - cure time after the surfaces. I read about this just today
and what I read seemed to indicate that months are required to attain a
super strong bond. This might be glue specific though as the properties of
the glue I'm using indicate max strength is attained within 24 hours of
joining the surfaces. What chemical processes are actually taking place
when joining the glued surfaces, pressing them and letting them "cure". I
had the idea that joining the surfaces was effectively sticking the
crosslink chains together and then pressing them is physically pushing the
bonds between the crosslink chains and rubber polymers closer together -
increasing the "density" or strength of the bond?

Whoa, I now realise this is quite a long post. I would be grateful if
someone could even answer just a few of my questions, or perhaps point me to
reference on glues etc that seems to elude me every time I search for it.

Thanks a million for anything you've got on this topic
ciao for now
jonny b

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