Hi Walter

 

You have an excellent point about the shelf life.  If you don't want to retest, then you shouldn't exceed the shelf life of the adhesive.  But remember the whole point of a shelf life is so that the properties are kept within the physical specifications of the product.  The point of testing is to prove they are within specifications.  If tested and it is within specifications, then it by definition, is OK.  A shelf life is stated so that if a material is kept within the specified conditions, its properties will stay within specifications for the stated amount of time.  Testing is used to confirm this and coupons are kept to verify and for future proof of building within specifications.

 

So I don't argue that all things being equal, you would be better off keeping it within the shelf life stated by the manufacturer, but if a material is tested and found to be unchanged and within specs, why throw it out? 

 

This is quite apart from using a material well past its shelf life without testing and verification to what you have.  What I have in my maintenance manual is that if a material has gone past it's shelf life, I have to test it to verify it is still within the stated physical properties.  Coupons are to be kept to verify that I have not used bad material.  These tests are normally reaction time (gel time) and hardness.  Hardness is done typically by Shore D but sometimes may be by Barcol hardness test.  Contaminated resin or hardner will not react as quickly as it should and also will not be as hard as it should so either one outside of spec means you should toss it. 

 

Now we are also talking about unreacted resins here, not prepregs which are a different situation that you probably have some experience with.  I have some prepregs in a freezer that I'll probably have to toss in the next few months because it's too hard to retest for the few pounds in storage, even though it is high performance, higher modulus (T-1000) fiber and (RS-1) resin.

 

I checked with my buddy Rich Molton of Applied Poleramics when I was writing my maintenance manual who is also of the opinion that shelf life must be conservative for the manufacturer but that doesn't mean a resin is bad when it is past it.  Rich is former head chemist for Hexcell and now owner and founder of AP, designer of several Boeing RTM (Resin Transfer Molding) resins for aircraft and some spacecraft.  He has patents on two phase epoxies for high temperature usage with very low viscosity for pumping into molds.  Rich will be testing my resin for specs since I plan on using AP resins in my ES.  The ordinary resins manufactures are toughened which are more fatigue, crack and inpact resistant with higher peel and sheer strength than Jeffco.  Note that some post cure at elevated temperature is needed to meet the Jeffco HDT/GTT.  And Jeffco does make some toughened resins also with the same improved performance over its standard room temperature resins.

 

Where I work, we have a number of materials that we put a 1 year shelf life on (tapes and hot melt adhesives) and have a given set of storage conditions that insures we meet the stated shelf life.  But in fact, our products last almost indefinitely.  We will for a price recertify our products. That's why I said that if the original manufacturer won't retest, I can probably find someone that will.

 

Dan Newland