>> ... RTV, ...

Note that RTV MUST be neutral cure, as Joe noted.

"Standard" RTVs release Acetic Acid during curing and corrosion is probable.

There are two commomn "neutral cure" RTV families, with variations in
characteristics within each family.

Dow Corning offerings here

     http://www.dowcorning.com/contentcomponents/multimedia/Sealants_Flash_Final.swf

Dearest is Alkoxy cure with variants releasing pure Ethanol in some
cases and Ethanol/Methanol mix in others.
(Dow 738, 739, 748, 832) (Se re HM25xx below)

Cheaper and more common in domestic use are the Oxime cure versions
which also release oximes (probably mainly ketoxime) which looks a
little scary on first look at the MSDS but is not too bad on
consideration compared to other typical materials in common use.
 http://en.wikipedia.org/wiki/Oxime
(DOW 735, 737, 236)

Any neutral; cure in retail sale which does not specify cure method is
99%+ liable to be oxime cure.

_____________

The Dow Corning HM-2500 and HN2110 are very special and worth noting.
They cost about twice as much as other Alkoxy versions BUT they are
applies as a hot melt glue and then cure to an RTV. This gives you the
very valuable fast set times of hotmelt and equally valuable long
lifetime characteristsics of silicones.

I haven't tried these yet but intend to as they offer superb
advantages in some manufacturing situations. Not available in New
Zealand but can be imported from next-door plus an ocean (in the more
enlightened) Australia.

___________

An important point to note. The long life outdoor performance of RTVs
aka "silicone rubber" is due to the high energy Si-Si bonds which are
essentially UV impervious. A properly formulated TTV wioll last 20+
years in extremely adverse outdoor conditions. The shorter lifetime of
epoxy products is due to the lower energy C-C bonds which are UV
attacked. I do not goes into Joe's recommended co-polymer sealants,
but if long term performance similar to that of silicones is desired
it would be worth seeing what their chemical makeup is and whether
there are liable to be any lifetime issues.

______

Joe mentioned loss of adhesion of RTVs to some materials with time.
Manufacturers (DC and others) supply comprehensive tables of
compatible materials, and surface primers are often available to
improve adhesion.



     Russell McMahon
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