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Using expired prepreg

Using expired prepreg

(OP)
Hello.
I'm wondering if anyone has experience with using expired prepreg (i.e. expired freezer-life and not out-life).
The material in question is an epoxy/glassfiber woven prepreg which will be used locally as a corrosion barrier on a larger epoxy/carbon fiber structure.
I'm thinking that we might get a lower Tg and degree of cure in the glassfiber plies but that might be acceptable since it's not providing any structural support. However what about possible moisture ingress etc.? Is there a risk that the expired prepreg might affect the carbon fiber prepreg in the product?

I should mention that we will be recertifying the prepreg before using it and the reason that we're considering using the expired material is the long delivery times of fresh material.

Any input on the matter is much appreciated.

RE: Using expired prepreg

I have experienced a situation where shelf-lifed product (film adhesive) with a freezer life of 180 days and a bench life of 30 days has been claimed to be in life one week before the freezer life expired despite the fact that the material had been thawed out numerous times so that a small amount could be cut off for use. This is common for repair adhesives.

It must be realised that there are two separate but inter-related measures of material life; Shelf life in freezer, and bench life. It is unrealistic to expect that a material which has been in the freezer for 179 days can then be exposed for 30 days and still be considered "in life". Similarly, if the material has been out for the first 29 days since acceptance, it would be stupid to expect that you can put it back in the freezer and expect it to be useable after 150 days in the freezer.

We introduced a policy where the used bench life and the used freezer life are recorded, and then expressed as a fraction of the nominal bench and freezer life, and the if the total fraction exceeds 1.0 then the material must be requalified.

(Time on bench)/(bench life) + (Time in freezer/freezer life) must be less than 1.0.

Further, if the requalified material meets specs, then the life is extended by a full lifetime for both bench and freezer life for the second re-lifing. In the case of a third re-lifing, if the material meets qualification, then the permitted life is reduced to 50%. Once that life is exceeded, the material is scrapped even if it meets spec.

Regards

Blakmax

RE: Using expired prepreg

Generally epoxy prepregs will become unusable before they become uncurable. As the life is used up the B-staged matrix (B-stage is usually about 30% cured) becomes a bit more cured, but a normal cure will convert the resin to 100% cured (almost; a cured (C-staged) resin is usually more than 90% cured) even if it has become (say) 50% cured sitting around. However, old material may well not be usable because of loss of tack, and it becomes boardy, hard to form to tight curves and won't adhere properly to the tool or material already put down; it will also become impossible to use with automated lay down such as ATL or AFP. It is possible for severely aged resin to reach a point where a normal cure will not quite work, so final resin properties will fall short, but it's rare and it won't happen without becoming noticeably "wrong" when laying up. If it's fit for laying up it'll probably cure ok. Material that's in life will never be unusable. Having said that there's no sure fire way to tell if out of life material will cure properly without curing it and testing.

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