RTV Shelf Life

[Dan320]

We are reviewing the shelf life of chemicals in general and RTV Silicone sealants in particular. According to the aircraft manufacturers, the shelf life of most RTV's can be extended under certain circumstances. I would like to know more about why some RTV's have short SL and some longer, and what happens to them during storage to make them unfit for use.
Any input is welcome.

Dan

 

[rhodie]

I believe that most if not all 1-part RTV silicones are moisture cure.

Packaging, then, become a significant contributor to shelf life. Container gas permability, humidity infiltration, and the ability to reseal and evacuate moisture after a token use all impact subsequent reuse and shelflife.

 

[Dan320]

Rhodie,
You are absolutely correct. However, some RTV's packaged in un-opened aluminum tubes seem to go bad all by themselves for reasons I would like to know some more about. As an example, RTV630 shelf life can be extended quite a bit, but RTV159 can not.

Dan

 

[rhodie]

By what method are you extending shelf life?

Acetoxy silicones are continually engaged in crosslinking and curing, even in a sealed tube. Perhaps the RTV159 requires less catalyst to cure.

 

[Dan320]

We extend iaw the airframer's data.
I thought Acetoxy Silicones cured in contact with atmosphere moisture. In an unopened aluminum tube there will be no or very little moisture even after long time. They cure slowly even without moisture?
Excuse me if this is basic to you, but chemistry is not a thing I know too well.

Dan

 

[rhodie]

That is a nasty secret about '1-part' compounds. They are in effect, always reacting (and curing).

In the example of the RTV, that reaction is significantly accelerated by the presence of moisture. However, even in a 'sealed' contained that reaction is occuring. The container plays a significant role in the preservation of shelf life.

This explains why you might have expeienced 'old' product or aging product to be somewhat more viscous and stiff compared to new material...

 

[unclesyd]

All RTV's that I've been around have a warranty or shelf life based on not exceeding a specific temperature, normally 40C. Based on working with the early RTV products we always kept the bulk of these products under refrigeration. It was very seldom that any product was used past its expiration or warranty date.
The MSDS normally will tell the end date for use and if not the technical bulletin for a particular product will.

 

[Dan320]

Thanks for your thoughts.
The airframer routinely extends shelf life of some products in his inventory IF they pass retesting. Retesting can be as simple as checking if the sealant flows normally from the tube. That is in line with what you are saying here. I just thought the process was stopped without moisture.
In practise we have never seen a sealant go bad from age. Cool storage helps, probably.

Dan