From: wjc@ll.mit.edu (Bill Chiarchiaro)
Newsgroups: sci.chem
Subject: Answers to silicone question.
Date: 3 Sep 92 14:50:30 GMT
Organization: MIT Lincoln Laboratory


  Now I see why I didn't find an entry for silicone in the CRC tables!

  I received several responses to my question about the formula and structure
of silicone.  They were all in agreement, and here they are below:


From: tony2@prefect.cc.bellcore.com
Return-Path: <tony2@prefect.cc.bellcore.com>
Date: Fri, 28 Aug 1992 08:58:15 -0400
To: wjc@ll.mit.edu
Subject: Re: Silicone
Newsgroups: sci.chem
Organization: Bellcore, Livingston, NJ

There are many varieties of silicone elastomers and
oils, but the basic one is poly(dimethylsiloxane):

         CH3
         |
       -(Si-O-)x-
         |
         CH3

x=several to many thousends

CH3 may be replaced completely or in part by H
(poly(hydrosilanes), -CH=CH2 (curable silicones),
pbenyl (high-temp polymers, high Tg), C2H5, etc.

Tony

-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*
Antoni S. Gozdz                tel  (908) 758-2931  |  Bellcore  NVC 3X-261
tony2@prefect.cc.bellcore.com  FAX  (908) 741-2891  |  Red Bank, NJ 07701, USA
-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*


From: ewanders@garnet.berkeley.edu
Return-Path: <ewanders@garnet.berkeley.edu>
Date: Fri, 28 Aug 92 10:56:20 -0700
To: wjc@ll.mit.edu
Subject: Re: Silicone
Newsgroups: sci.chem
Organization: University of California, Berkeley

Silicones are a family of polymers with -Si-O-Si backbones.
Of course, the silicon atoms need to form 2 more bonds 
each, usually to carbon-containing groups.  The simplest
silicone is polydimethylsiloxane:

          CH3        CH3
          |          |
       -- Si -- O -- Si -- O --  ... etc.
          |          |
          CH3        CH3

Many other side groups are possible.  Common ones include
benzene rings, ethyl groups, or vinyl groups.  Using -CF3
groups instead of -CH3 groups adds to chemical resistance.

The chains are often ended with trimethyl silicons, but there
are several products that are mainly made up of rings (no
ends).  

Depending on the side-groups and the chain length you can get
a volatile oil, a low-vapor-pressure oil, adhesives, anti-foam,
release agents, rubbers or hard resins.  All they have in
common is the Si-O-Si backbone.
           
Compared to hydrocarbon polymers, silicones usually offer greater
resistance to oxidation, better high and low temperature properties,
and lower toxicity.  They are more subject to attack by acids and 
bases, however and generally cost quite a bit more.  They are 
generally much less flammable than a similar hydrocarbon polymer or
resin would be.

I hope this helps.  Since you are posting from .mit.edu, you have
easy access to a variety of sources that can give you much more
detail.  For questions of this sort, the Kirk-Othmer Encyclopedia
of Chemical Technology is a great place to start.
-- 
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.


From: dblackbu@alleg.edu
Return-Path: <dblackbu@alleg.edu>
Date: Sat, 29 Aug 92 17:36:40 EDT
To: wjc@ll.mit.edu (Bill Chiarchiaro)
Subject: Re: Silicone

Silicone is a generic name for polymers -(OSiR2)-n where R = methyl,  
phenyl, etc. It's generally made by reaction of R2SiCl2 + H2O. 

Various applications of silicones (rubber, greases, etc.) use  
different R groups, production methods affect polymer chain length,  
and fillers/plasticizers/crosslinkers are used as in any other  
polymer commercial product.

Dave Blackburn (dblackbu@alleg.edu)


From: R_Tim_Coslet@cup.portal.com
Return-Path: <R_Tim_Coslet@cup.portal.com>
To: wjc@ll.mit.edu (Bill Chiarchiaro)
Subject: Re: Silicone
Date: Sun, 30 Aug 92 13:04:43 PDT
X-Origin: The Portal System (TM)

>
>
>  Could someone tell me the chemical or structural formula of
>Silicone?

Which Silicone?

Many carbon based compounds, when the carbons are replaced with silicons
are called Silicone. They would have identical chemical formulas as their
carbon equivalents if the "C"'s are simply replaced by "Si"'s. Structural
formulas would also be very similar, although some details would change
because of the larger atomic size of silicon atoms.

                                        R. Tim Coslet

Usenet: R_Tim_Coslet@cup.portal.com
California could REALLY use 40 days and 40 nights of rain right now!

Return-Path: <brueck@inm.uni-sb.de>
Date: Wed, 2 Sep 92 11:32:35 +0200
From: brueck@inm.uni-sb.de (Stefan Brueck)
To: wjc@ll.mit.edu (Bill Chiarchiaro)
Subject: Silicone

hi

your question is rather difficult to answer because it's very general, but
i will try...

silicones are polymeric materials. the network is formed by silicon-oxygen
bonds (siloxane bonds): ...-Si-O-Si-O-Si-... silicon as member of the 4.
main group of the PSE has usually four bonds to other atoms, so in the chain
mentioned above each silicon atom has two free bonds. normally these are
saturated with methyl-groups:

     CH3  CH3  CH3
     l    l    l
 ...-Si-O-Si-O-Si-O-...
     l    l    l  
     CH3  CH3  CH3

with other organic groups (ethyl, phenyl,...) it's possible to create silicons
with specific material properties. so now to the obscure '...' in the formula
above: termination of the chain is achieved by silicon species which carry 3
organic groups and can form only one, the terminating, siloxane bond. right,
there exists an silicon species with only one organic group too. this species
can crosslink two two dimensional chains in a three-dimensional network:

       CH3  CH3  CH3 
       l    l    l    
   CH3-Si-O-Si-O-Si-O-...
       l    l    l
       CH3  CH3  O
                 l
                 ...
       A    B    C

A: termination
B: chain
C: crosslinking

the various ratios of A,B,C in a given silicone determines it's properties:
small chains (high amount of A, little B, no C) gives a low viscosity oil,
high crosslinked network (few A, large amount of C) gives resins up to solid
materials 

i hope my description isn't to strange
stefan
Newsgroups: sci.chem,rec.autos,rec.autos.tech,misc.consumers,sci.environment,talk.environment