From: "Barry L. Ornitz" <ornitz@tricon.net>
Subject: Silicones - Chemistry and Corrosion
Date: May 13 1997
Newsgroups: rec.radio.amateur.antenna

Ian White, G3SEK <G3SEK@ifwtech.demon.co.uk> wrote:

> One of the best things about this newsgroup is its 
> enormous depth of experience. But one of the side-
> effects is that it's sometimes hard to tell exactly 
> WHEN that experience relates to. 
>
> Jim's posting above is unusual in admitting that the 
> experience happened 25 years ago. The formulations of 
> silicone sealants have changed since then. It isn't 
> clear whether most of the other postings refer to 
> recent experience, or to the old days when the chemists 
> were still trying to get the formulation right.
>
> Modern acetoxy formulations contain free acetic acid 
> before curing, but the intention is for all that acid 
> to be absorbed - chew a lump of cured silicone and 
> taste for yourself! 
>
> I'd speculate that the earlier formulations 
> (sometimes?) contained too much free acid, which was 
> left behind to cause corrosion. The main mechanism for 
> corrosion is probably electrolysis of dissimilar 
> metals, which would be accelerated by the presence of 
> acetate ions.
>
> Please can we re-state the question: Does anyone have 
> experience of corrosion caused by MODERN silicone 
> sealants?

Certainly.  The corrosion is entirely dependent on the particular type of
silicone sealant used, and most common ones generally available to the
public still release acetic acid when curing.

To begin this discussion, however, it should be stated that silicone
polymer is a term much like "plastic" - it covers a rather wide range of
materials and properties.  The chemistry of a particular polymer
determines it characteristics, from hard and brittle varnishes, to soft
and flexible rubbers.  The initial work on polysiloxane chemistry dates
back almost 60 years and it continues today, so "getting the formulation
right" is hardly a meaningful expression.

Modern silicone elastomers are generally linear polymers of methyl
silicone "oils" of extremely high molecular weight which will remain
largely independent of one another but which may deliberately be
cross-linked at a number of selected points.  Two methods are commonly
used to obtain such structures:

1) Benzoyl peroxide or other strong oxidizers can be used to oxidize side
groups and so establish cross links.

2) Some vinyl or other organic groups groups can be incorporated into the
siloxane structure, and then a catalyzed condensation polymerization of
these is accomplished to establish cross links.

The first of these two methods is common in two-part silicone
formulations such as potting compounds.  In particular this is used in
situation where a deep-section cure must be obtained.

The latter of the above two methods is generally found in the room
temperature vulcanized (RTV) sealers generally available to the public.
When the organic groups polymerize, a volatile species is generally
released - the most common being acetic acid.  Other species may be
released if appropriate organic groups have been incorporated into the
silicone structure; these include alcohols and even amines.  These
compounds are released as part of the cross-linking (curing) process.
Most eventually evaporate from the cured silicone.  They are NOT absorbed
by the polymerization.

Single part silicone sealers generally contain a cross-linking catalyst
(such as a tin compound) that is activated upon exposure to moisture in
the air.  These single-part sealers should not be expected to cure deeper
than about a centimeter as the diffusion of moisture through the cured
silicone, and the diffusion of the acetic acid or other compound out of
the curing silicone is limited by the thickness.  To demonstrate this,
leave the cap off a tube of silicone sealer for a few days.  The silicone
around the neck of the tube will be hard, but the rest of the tube will
be uncured.

Since the acetic acid is released during curing, it can attack the
underlying substrate material.  This can cause corrosion of certain
metals and prevent the proper adhesion of the silicone.  However, on
other materials, the acid can etch the surface slightly and increase the
adhesion.  Aluminum is one such material.  Copper and zinc, however, are
corroded by the acid.  Thus brass and galvanized steel should not be used
with silicones which release acid.  Dissimilar metals can form
electrolytic couples and corrode severely underneath a covering of acetic
acid releasing silicone.  Silicones do not adhere well to all other
plastics either.

General Electric's Silicone Division can provide information about the 
compatibility of their products with different surfaces.  GE manufactures 
silicones for industrial as well as domestic use.  The following condensed 
table is from a GE publication on silicone sealants for domestic use.  N-R 
means not recommended.


            Adhesion Characteristics of General Electric Silicones 
                    Commonly Available to Homeowners*

			Silicone II		Silicone        Paintable
					        Rubber          Sealant 5091
 
 Metals:
     			
Alclad Aluminum		Excellent		Excellent	Excellent
Anodized Aluminum    	Excellent     		Excellent	Excellent
Brass			Good			N-R		Good
Chrome			Excellent		Excellent	Excellent
Copper			Good			Good		Good
Galvanized Steel	Fair			N-R		Fair	
Carbon Steel		Excellent		Excellent	Excellent
Stainless Steel	 	Excellent		Excellent	Excellent
			
Plastics: 			
			
ABS			Excellent		Excellent	Fair
Fiberglass		Excellent		Good		Fair
Formica			Excellent		Excellent	Good
PVC	(rigid)		Excellent		Fair		Good
LEXAN polycarbonate  	Good			Fair		Good
Vinyl (soft)	  	Good			Fair		Good
Acrylic/Plexiglas	N-R			N-R		N-R
Glass			Excellent		Excellent	Excellent
Rubber (any type) 	N-R			N-R		N-R

* Note: The contractor grade silicone is similar to the Silicone II
adherence properties, however, contractor grade silicone differs from
Silicone II by having a longer tooling time, faster curing time and
different formulations and ingredients.

GE Contractor Bath/Plumbing products, GE6040 (clear) and GE6070 (white)
are specifically formulated to be non-corrosive to metals.  In
conclusion, using common silicone sealants around antenna connections is
asking for trouble.  Much like the Plasticizers in the vinyl jackets of
coaxial cable, if you do not know they are corrosive, assume they are as
the special non-corrosive grades are normally labeled as such.

            73, Dr. Barry L. Ornitz   WA4VZQ   ornitz@tricon.net
	Eastman Chemical Company Research  ornitz@eastman.com