Why Are Epoxy Bi-packs 2.5 Grams?

If we had a nickel for every time we are asked “Why are epoxy bi-packs 2.5 grams?”

Fiber Optic Center provides epoxy in two-part bi-packs that are typically 2.5 grams.  While 2.5 grams might seem small, some modern-day applications are very small, and relatively speaking 2.5 grams is a lot of material, much of which ends up as waste.

Room temperature/heat cured epoxies are typically two-part materials – resin and hardener.  The two parts are mixed in ratios unique for any given epoxy.  Maintaining the mix ratio is essential for maintaining the cured properties of the epoxy.

One of the more common mix ratios is 10:1; 10 parts resin to 1 part hardener.  This means a 2.5-gram bi-pack has 2.27 grams of resin and 0.23 grams of hardener.  Most manufacturers agree that the actual weight of the resin or hardener should be within 5% of the calculated value.  For a small weight like 0.23 grams, 5% is 0.01 grams (a hundredth of a gram).  So the weight of the hardener is 0.23 grams +/- 0.01 grams.  An accurate measurement of 0.01 grams requires a scale that measures 0.001 grams (thousandths of a gram).  Measuring such small amounts becomes a practical problem, and as stated above, maintaining the mix ratio is essential for maintaining the cured properties of the epoxy.

This is why a 1 gram (or smaller) bi-pack is not practical.  Even packaging houses with very accurate and routinely calibrated scales have difficulty maintaining an accurate enough mix ratio to ensure the cured properties of the epoxy.  Can it be done?  Yes, and the cost of achieving this roughly half quantity of epoxy would be expensive and subject always subject to reliability issues.

Is there a way to avoid wasted epoxy?

Given that 2.5 grams is the smallest size that is practicably achievable, FOC recommends saving any extra epoxy in a secondary dispensing container (like a syringe), frozen -40°C.  -40°C essentially stops the curing reaction and will work for a period of ~ 6 months (please verify for the epoxy you are working with).

The small quantity is easily cooled and reheated for later dispensing.  This approach can be tailored for very small applications by mixing a 2.5-gram bi-pack, using immediately what is needed for the application and using the extra epoxy to fill the right amount of syringes for freezing and later dispensing.  Any secondary packaging operation needs to be done expeditiously since any time the epoxy spends at room temperature is time spent curing, and diminishing any future available work time.

Homemade pre-mixed and frozen epoxy

This is the equivalent to making your own pre-mixed and frozen epoxy, which may take some trial and error, but can ultimately save time and solve the epoxy waste problem.  If you can’t chill to -40°C, the slowing of the curing reaction corresponds to how cold you can get the mixed epoxy:  -20°C can work for days to weeks, if that is all the time that is needed to use up the epoxy.  Disclaimer:  Epoxies vary, so please experiment first with the exact epoxy you are using before launching a production plan.

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