From: ewanders@garnet.berkeley.edu ()
Newsgroups: sci.chem
Subject: Re: Dissolving rubber?
Date: 21 Aug 1992 04:42:10 GMT
Organization: University of California, Berkeley
NNTP-Posting-Host: garnet.berkeley.edu
Summary: maybe

Can rubber be disolved?  The short answer is no, the long answer
is yes, but.  It all depends on what you mean by rubber and what
you mean by dissolved.
 
   Rubber as we usually think of it consists of very long unbranched 
molecules, generally made of hydrocarbons.  These molecules are not 
stiff, but rather tend to coil up and assume lengths much shorter
than they would if they were fully extended.  Rubber is stretchy
because these molecules can unwind somewhat when a piece of rubber
is stretched.

   To give rubber strength, these long chains are crosslinked chemically
at random distances.  Without crosslinks, the rubber molecules would
slide against each other and a piece would easily pull apart.  The more
crosslinks, the more cohesive a chunk of rubber will be -- the same
long chains can form a very soft gum, a soft rubber, a hard rubber or
something as stiff as a "hard rubber" comb.  Because of these crosslinks,
a chunk of rubber can in some sense be thought of as one large molecule.

   Like most hydrocarbons, rubber resists dissolving in polar solvents
(such as water).  A non-polar solvent such as hexane, toluene, or even
a mixed solvent such as gasoline (petrol) would normally dissolve 
hydrocarbon chains such as those that form rubber.  In fact, many 
processes for making rubber use these solvents to dissolve the chains
before the crosslinking takes place.  But after the crosslinking has
happened, the chains cannot separate without breaking chemical bonds.
Rubber exposed to organic solvents will often swell to a great extent
(many will double in volume), but they don't really dissolve.  Hence, the
short answer of no.   The extent of crosslinking controls the tendency to
swell just as it controls the stiffness. 
 
   "Rubbers" with few crosslinks can often be broken apart by immersion
in enough solvent.  There just aren't enough bonds to hold the mass
together.  Whether this counts as dissolving rubber rather than just
dissolving polymer (gum, in the jargon of the trade) is up to you. 

   You could also attack the chemical structure with strong oxidizers and/or 
acids or bases, breaking the crosslinks (or the chains) chemically.  What 
will work with a specific formulation depends on the chemical nature of
the chains and the crosslinks).  This can "dissolve" rubber, but 
destroys it.  If your goal is just to remove rubber by getting it to
go into some liquid, yes you can.  But you can't get the rubber back
by evaporating the solvent.
 
   I have left out many details.  The third factor in most rubbers is
the presence of "filler."  Very finely divided particles are mixed into
the crosslinked polymer matrix to strengthen the whole mass -- carbon
black (basically soot) gives rubber the classic black; iron oxide
makes the standard red rubber.  Silicone rubbers usually use silicon
dioxide particles.  Traces of many other ingredients are added to
give specific physical properties.               

   One way to have fun with rubber making is to get some room-temperature
vulcanizing (RTV) silicone rubber, commonly sold as caulking material.
In the tube you have uncrosslinked polymer.  It comes out as a viscous
liquid which you are supposed to use to fill cracks around windows etc.
It then crosslinks, changing from a liquid to a flexible solid.  You can
do lots of other things with it, such as make formed-in-place gaskets
by putting it where you want the seal and letting it harden into shape.

   I hope I haven't run off too much, but this is about as short as I 
could tell it.  I hope it helps.
 
-- 
Eric W. Anderson      |  ewanders@garnet.berkeley.edu  | (510)-486-4986
Chem. Eng. Dept.      |-------------------------------------------------------
Univ. of California   |  I don't speak for the University, the city or the
Berkeley, CA 94720    |  people of Berkeley, and they don't speak for me.