Re: [ATM] RE: RTV Silicone question

["Jule DuBois" <jjdubois@insightbb.com>]

Just got back from vacation so I am very late with this thread.  I am 
retired now, but spent 15 years as a chemical engineer in the silicone 
business, 10 of those with GE silicones.  I actually was responsible for a 
time for making the cross linking agents for silicone rubber.

Some random facts about silicones:

1. Adhesion to glass and other silicon containing materials is usually very 
good.

2. Adhesion to organics requires primers to produce a strong bond.

3. Adhesion to metals is usually ok.  Some of the curing systems actually 
use metals for curing.  Mostly tin and zinc.

4. Clear silicone rubber sold commercially contains a large amount of 
filler.  All the common silicone rubber contain lots of fumed silica, for 
strength and dimensional stability - even the "clear" grades.

5. Cheaper silicone RTV rubber contains other fillers to reduce the cost of 
the rubber, and the fillers reduce the quality of the rubber.

6.  Silicone rubber has one great flaw.  It has very poor shear strength.

7, The acetic acid smell is indeed from the cure mechanism of RTV silicone 
(Room Temperature Vulcanization).  The cure requires water from the air for 
the cure, releasing acetic acid in the cross linking process.  Therefore, 
thick sections of curing RTV take a long time to cure, and in the absence of 
air will not completely cure.  For thick sections the water molecules have 
to penetrate the already cured outer sections of the rubber, and the acetic 
acid has to migrate back out.

8.  Silicones are specialty materials, best only for special conditions 
where their good properties are required and their disadvantages tolerated. 
RTV silicone is not a universal adhesive.  In fact generally a very poor 
adhesive that because of its chemical makeup adheres very well to glass.

9. I use silicone for glass (attach mirrors to mirror cells), but use other 
adhesives for other aplications.  Picking compatible adhesives for the 
materials being bonded is the key to trouble free bonding.  Silicone os best 
for only  very limited and special appications.

Hope that this helps.

Jule J. DuBois at jdubois@alum.mit.edu


----- Original Message ----- 
From: "Mark Holm" <mdholm@telerama.com>
To: "ATM Mailing List" <atm@atmlist.net>
Sent: Saturday, October 02, 2004 4:39 AM
Subject: Re: [ATM] RE: RTV Silicone question


> >
>>
>>PVC is slippery stuff. I'd drill a hole in it wherever a RTV pad was 
>>intended and make sure the RTV squirts through the hole for a mechanical 
>>hold as well as adhering to the surface of the PVC.
>>
>
> PVC is not a good surface to try to get silicone to stick to.  For best 
> results, scuff up the PVC with coarse sandpaper, drill the holes as 
> suggested and clean it well with alcohol.  I'm not sure even that will 
> provide a really strong adhesion.
>
> Mark Holm
>
> _______________________________________________
> ATM mailing list http://www.atmlist.net/ 

_______________________________________________
ATM mailing list http://www.atmlist.net/