Shelf life of two-part methacrylate 3

[Paitch]

We use a two-part methacrylate in a manufacturing process. The adhesive has a 6 month shelf life and the activator 12 months after date of shipment from the manufacturer.
Sometimes when we get it from the distributor, it has only two months shelf life left.
We have some adhesive that expired in August. I feel that I can safely use this product until our next shipment arrives in two weeks.
I would like to know what happens to the adhesive when it is past 6 months. I need some justification for an internal deviation. My gut feel is that the reaction may take longer but the product once cured will be fine.
Your thoughts please.

 

[Compositepro]

Shelf life limits are often arbitrary and conservative. They are in place to limit the liability of the manufacturer to a finite time limit.

Adhesives go bad for various reasons. One part adhesives will cure in storage. Usually if you can still apply them they will still work. Two part adhesives have chemically reactive groups that can react with air or moisture or just degrade with time. You may mix the parts together but they don't cure. Or one or both parts may have solidified by itself.

When you say a two part methacrylate, I assume you mean a resin with a peroxide catalyst. Methacrylate will react with air if the container is not tightly sealed. Containers that have been opened have much shorter shelf life than unopened ones. The sign of it going bad is that it will set-up in the container. Peroxide catalysts will degrade with time and temperature and lose the ability to cure the resin.

The quality of the adhesive will be immeasurably different between the day before and the day after the shelf life expires. How do you know the adhesive was good the day before it expired? Do a test bond to see if the adhesive meets your needs. What are the consequences of a bond failure? It is a cost benefit analysis. Often the effort required to convince your Quality group makes the cost too great.

 

[Paitch]

Compositepro, Thanks for the information. In our application, the adhesive is in a capped double barreled syringe and is dispensed through a mixing tip once the cap is removed.

The constituents in this activator are:
3,5-Diethyl-1,2-dihydro-1-phenyl-2-propylpyridine 1-10 %
Methyl Methacrylate Monomer 60-100 %

The assembly is a "clam shell" wherein adhesive is applied to the mating surfaces of the two halves then they are mechanically fastened and clamped. Once the adhesive has cured, a two part insulating foam in injected into the cavity. This operation is also conducted in a clamp fixture. If the adhesive bond were ever to fail, it would do so at this step. Once cured, the foam will further bond the two halves together.

 

[Compositepro]

Well, it sounds as though there is very little risk for a couple of scrap or rework parts. So why not use the adhesive?

 

[Paitch]

Have an ISO audit starting today and wanted to get an internal deviation in place.

I've expedited the next shipment of adhesive and will use the expired product under this deviation until the new shipment arrives. Thanks again for your able assistance.

 

[MintJulep]

It seems that in this particular application there is little risk.

However, this time the adhesive is two weeks past its expiration date. The next time it might be a month, and the time after that three months.

You will get into the "well it's always been ok before" trap, and eventually your production and stores departments won't bother to ask you if it's ok or not, they'll just use it.

Are you ever going to use this adhesive in some other application? Will the risk of bond failure be similarly benign in that application?

You need to work with your purchasing folks and the distributor to make sure that you get "fresh stuff" when you place an order so that the shelf life isn't used up on the distributor's shelves.

 

[Paitch]

MintJulep, you make an excellent point. This adhesive is unique to this application and has been put in MRB and can not be issued without a formal deviation. The deviation is time specific not to exceed 30 days.
I share your concern about the "slippery slope".
Thank you.