I just did some reading. The resin has infinite shelf life, but the hardener (MAS Hardeners, in this case) have about 1 yr shelf life in a closed container. I am not sure what you are using. MAS hardeners (I am using those products now) are phenol free, as is West Systems)
here's some good cure info:
Customizing Cure Times- the blending of Slow and Fast Hardeners. The time it takes for an epoxy mixture to change from liquid to solid is the cure time. There are four phases:
* Open (working) time or wet lay-up time.
* The mixture is still liquefied.
* Initial cure is reached when the mixture has turned to gel.
* Final cure is when the epoxy is now solid.
The speed and the length of these phases and total cure time varies relative to temperature and which hardener was used - slow or fast or combination of both - and if additives have been added to the mixture. Cure times can be speeded or slowed depending on your needed application by the combining of the Fast and Slow hardeners. Cure times for the Fast hardener, combined with the low viscosity Epoxy resin can be lengthened by the addition of 35-40% by volume of the Slow hardener. This happens because the Slow generally has a lower peak exotherm temperature and the Slow molecules are bulkier, which slows down the whole chemical reaction between the resin and hardener. The Slow hardener can be speeded up by the addition of Fast hardener, however, it is important that the amount of Fast does not exceed 30% by volume. Adding over 30% of the Fast compromises the "no-blush" characteristics on the Slow. The mix ratio between the resin and hardener still continues to be 2:1 resin : hardener. NOTE - - The cure times are not directly proportional to the amount of hardener used to customize the blend. For example, if 25% Fast is added to Slow, the cure time is speeded by 12-15 %. Conversely, if 50% of Slow is added to Fast, the cure time is slowed down by 25%.
The above ratios should get you in the curing ball park of a perfect shop/lab which warms and cools by increments of 18° F (see above). However, using both temperature variation and blending ratios which favor the "hot" side to manipulate a pot life, will probably get you close to your desired cure speed.
Even if the pot life and thin film set of your first hybrid mix does not fall right on the money, the mix will cure as long as the resin to catalyst ratio is 2:1 and temperatures are not severely cold. We have been working with Hybrid blends of Slow and Fast and the following ratios are the most popular for the listed common conditions:
Coating and Encapsulate: If temperatures are slightly cool (50-60°), we recommend speeding up Slow with approximately 20% of Fast. This still provides a blush free coating (but do check), and is normally sandable in the morning (remember, always mix resin and catalyst in a 2:1 ratio, mix thoroughly).
Filleting and Bonding: Normally, since users are looking for maximum strength and minimum clamp time, we recommend 100% Fast. However, if the weather gets hot (over 85°), this mix can be controlled by adding approximately 25% Slow. (Remember always mix resin and catalyst in a 2:1 ratio, mix thoroughly).
19. Controlling Cure Time through mixed Quantity, Container Shape and Temperature
Mixed Quantity - A larger quantity of mixed epoxy will create more heat and have a shorter open (working) time and overall cure time. Smaller batches of epoxy create less heat than larger batches and also have longer working and cure times. In other words, a thicker layer of epoxy will cure sooner than a thin layer.
Container Shape - The mixture's heat can be distributed by pouring the mixture into a larger, flatter container (like a roller pan, for example.) This also extends the open time.
Temperature - Heat can be applied or removed from the epoxy to shorten or extend open and cure times. After the epoxy is applied, a fan can be used to draw heat from the lay-up or application and extend the epoxy's open time. Moderate heat (an industrial hot air gun, hair dryer or heat lamp) applied to the lay-up or to the resin and hardener before mixing, will shorten the epoxy's cure time.
