By definition, once a resin and hardener are mixed, the system becomes a B-stage epoxy system. This definition however, is too vague for our purposes. What we need is a better explanation of this effect, and what it means in terms of handling a B-Stage epoxy system such as epoxy mold compounds.
Simply put, a B-stage epoxy system is one wherein the cross-linking reaction (ie. the curing, or hardening phase) is initiated, but is not yet complete. The rate of this reaction is dependent on two factors: time and temperature. At higher temperatures (ie epoxy molding conditions), this hardening reaction occurs very quickly. At lower conditions, this reaction occurs more slowly. The general rule of thumb for epoxy mold compounds is that every 10°C either doubles or halves the rate of reaction.
The problem with this general rule is that it is oversimplified. Yes, the rate of reaction is affected significantly by exposure to higher temperatures. However, at lower temperatures — especially at typical epoxy storage temperatures of 5–8°C, this reaction occurs at such a slow rate that it is barely measurable.
Furthermore, for epoxy mold compounds formulated with catalysts – these lower storage temperatures are far below the minimum activation temperature (sometimes called the “kick-off” temperature) is never reached. This activation temperature is essential for achieving full cure, and without it, the complete reaction or cross-linking never fully takes place.
It is important to understand therefore, which properties are affected during this B-stage staging process, to which extent these properties are affected, and what effect these changes have on final material properties.
As it concerns one-part epoxies and epoxy mold compounds, exposure to higher than recommended storage temperatures over time affects two specific material properties: minimum achievable viscosity and spiral flow.
Please stay tuned for the next part in this series to explore the behaviour of epoxy molding compounds.
Please visit us at www.caplinq.com to learn more about our whole range of epoxy mold compounds including our Transfer Mold Epoxy Compounds and our Compression Mold Epoxy Compounds. If you have any other questions about how to process and cure epoxy molding compounds please feel free to leave a comment below or don’t hesitate to contact us.