epoxy shelf life

Discussion in 'Materials' started by brendan gardam, Feb 19, 2020.

  1. ondarvr
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    ondarvr Senior Member

    The physical properties of epoxies vary a great deal, and most are tested after being post cured.

    Variables like temperature and humidity before and during curing needs to be taken into account when there are differences in the results in batches of the same product.

    The difference in removing peel ply can be greatly influenced by green strength development, which can be different between an infusion and hand lam epoxy.
     
  2. antonkov
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    antonkov Junior Member

    The point is the structural engineer comes up with a laminate schedule for the project sorely based on the properties of the materials. The properties are specified by the material manufacturer based on certain conditions (humidity, post cure, etc).
    In a more scientific way, the engineer uses mechanical properties declared in the datasheet and those come with a specification of the tests (and conditions) used to measure them. As long as the material corresponds to the datasheet, it should be considered as good. By reproducing the test conditions one can figure if the material still corresponds to the datasheet.

    In a less scientific way, the engineer says I know this laminate works based on my experience (i.e. product has been on a market long enough) and hence anything equal or stronger will also do the job. This way, there are no structural calculations just need to ensure the material or laminate is as strong as the reference one. In this scenario, one would need to test the fresh and old resin under the same conditions without calculating the exact modulus numbers.
     
  3. ondarvr
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    ondarvr Senior Member

    The TDSs for resins frequently have errors, other times they tend to stretch the truth.

    The large epoxy and polyester manufacturers tend to have accurate TDSs, but there are only a few companies that make the resins themselves.

    The major distributors of resins deal directly with these manufacturers so they have the original TDSs and can get the COA (certificate of analysis) for that exact batch. This typically doesn't include cured physical properties, just the gel time, viscosity and a few other things.

    Then you have the smaller resin blenders, they buy it, repackage it with their name and sell it in smaller quantities. They may sell it directly to the fabricators, DIYrs, or to another distributor.

    Some of these distributors buy resin from different manufacturers based on price. So it may not be exactly he same as the last shipment, the manufacturer may be different, the base and other aspects may be similar, but that's as close as it gets.

    They don't change the data sheets because the current base is considered "close enough" to the last one. Although its frequently someone with little to no knowledge of resin making the decision.

    For non critical stuff it works most of the time, but repeatability can be poor in production situations. These are the resins frequently used by the type of people on this site.


    You can't typically get a COA from the smaller distributors, I've seen them fake one when they were forced to produce it though.

    I've also found TDSs to be wrong even from the manufacturer, it can be from a typo, or again, stretching the truth.

    It's sort of like the physical properties listed, many times the numbers are for post cured samples, and without post curing you won't get even close to those numbers. The post cured statement and technique may not be easily found unless you hunt for it.

    This really comes into play when changing hardeners, fast, medium and slow hardeners can yield very different physical properties without the correct post cure procedure.

    The same resin base may be used to make hand laminating resin or infusion resin, and may list the same physical properties for both. But depending on how they designed the infusion version it could have very different physical properties.
     
  4. antonkov
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    antonkov Junior Member

    Now we have several scenarios:
    1) resin from a trusted supplier with accurate datasheet but that is beyond its shelf life
    2) some fresh resin from a questionable source.
    In both cases, one needs to make a decision whether it is still good or not. And it is not only about melting crystallized epoxy, as it was pointed earlier there is more to that.

    The trivial solution is to always plan the job and use unexpired material and source it from the very best (i.e. expensive) possible supplier. Does it mean the next day after the expiry date the material needs to be disposed or where is the line? Does it make the brand and price more reliable decision factors than technical QA?

    I wonder what methods they use in the industry with demanding applications (e.g. advanced aerospace) for quality and consistency assurance?

    Also, when looking for the boat plans, I checked designs from several marine designers (both industrial and DIY) and didn't see them providing any specifics on the material properties other than general resin chemistry and fibre density. If it wasn't a budget-conscious backyard builder but an industrial boatyard, how would they decide on the acceptable properties of resin and fibre when they can differ from brand, type, price, age, humidity, postcure conditions, all within the same chemistry?
     
  5. ondarvr
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    ondarvr Senior Member

    The shelf life is based on how long the manufacturer can ensure the properties listed on the TDS are still in the spec range, all the specs have a range.

    In truth it could be out of spec prior to the end of the stated shelf life, if it is the product is typically replaced at no charge.

    Opened or improperly stored products have a shorter shelf life.

    Customers frequently ask us to re-certify resin because its just past the official shelf life. We won't re-certify, but if its a good customer it's common to check the retain and report the results.

    There are markets where the exact specs are required for each batch. The cost can go up significantly when all the physical properties need to be tested each time.

    In reality just about any marine epoxy will be strong enough to do the job easily on a normal build. The specs may be more specific when a high performance design using carbon is being built.
     
    Last edited: Apr 28, 2020
  6. ondarvr
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    ondarvr Senior Member

    There aren't that many production builders using epoxy, that's mostly the one-off or high end custom shops.

    They pick an epoxy that may have better specs, or meets their production requirements. Once they find a product they like they stick with it.

    New projects may have a spec called out by the engineer. Or as you mentioned earlier, stating it needs to be equivalent to XXX product or better.

    Most epoxies exceed the physical requirements of the average build easly, so there's no spec called out.

    Its different with polyesters, there are some very low cost products that barely meet any spec, so certain specs or resin chemistry may be required.

    Better polyesters and VEs are also good enough for just about any build. The bad rap is from the cheap junk used at times by unknowing or unscrupulous builders.
     
  7. antonkov
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    antonkov Junior Member

    Does it mean that as long as the hardener remained sealed and resin crystals are melted, the epoxy is most likely to be good enough? ;)
    I have an old amine hardener that is discoloured and having flakes on the bottom, looking at it I feel more comfortable having some quantitative way to judge the resin, the methods may vary. Although in reality, there is always some odd, non-structural application that could consume the old resin.
     
  8. ondarvr
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    ondarvr Senior Member

    I never use old stuff without checking the cure, and even then it never gets used where it may cause a problem.

    Most of the time I mix it, let it get hard, and throw it away.

    One other thing that comes into play. Resin manufacturers sometimes get stuck with far to much of a particular resin on hand. Or they make a batch that doesn't meet the spec.

    This product may get dumped on the market at a very low cost. Some small to medium distributors buy up this product for cheap and then resell it to unsuspecting customers.

    These distributors may hold on to the resin for far longer than than its shelf life indicates while selling it as fresh.
     

  9. missinginaction
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    missinginaction Senior Member

    Ondarvr, maybe what you wrote explains why Dave Gerr's scantling formulas are said to be on the high (heavy) side. I've always used 2:1 resins (System 3 and MAS) and never had issues even when I had to melt down some crystals. Thanks for the input.
     
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