Applied Poleramic, Inc.

Do I need to post-cure a room temperature cure resin?

Room temperature cure resin is the biggest misnomer in epoxies. A thermoset resin is only capable of achieving a Tg of about 30oF above the cure temperature. Therefore, unless the ultimate Tg is within the 100oF range, a post-cure will always be required to complete the cure and increase resin strength. The higher the service temperature, the greater the need for a post-cure, assuming the molecular structure is capable of a Tg greater than the use temperature.

What is Tg and is it the same as HDT?

Glass transition is the temperature a polymer turns from a glassy like plastic to a non-structural rubber. On a molecular level, it is the temperature where the main polymer backbone initiates molecular motion. The Tg is controlled by cross-link density and modulus properties of the structure between cross-links (aromatic, aliphatic, heterocyclic, etc.). All polymers have a Tg and can best be determined using Dynamic Mechanical Analysis (DMA) curves. Tg is an intrinsic bulk polymer property totally determined by molecular structure. HDT is an arbitrary value where a set deflection is achieved under a constant load. As the temperature is raised, the stiffness slowly drops and the specimen deforms until it reaches a pre-set deflection where the temperature is reported. The HDT is always lower (approx. 10oF) that Tg due to the applied load.

What does the Part B do and can I change the mix ratio to adjust cure speed?

Amines and amine like (Lewis base) molecules are used to chemically react the epoxy functionality (ethylene oxide). There are two general classes of mechanisms. The most dominate is a direct conversion where the active hydrogens of the amine react one for one on the epoxy group. These are referred to as converters, where a stoicheometric amount, one amine hydrogen for one epoxy, is used. The other mechanism is catalytic, where small amounts of catalyst make the epoxy react with itself. (homopolymerization). DETA and IPDA are examples of amine hardeners, where imidozoles and tertiary amines are common catalyst. In two part epoxies, the hardener should not be changed to adjust cure speed or potlife. The stoicheometry becomes un-balanced with either too little or excess hardener. Too much hardener is just as detrimental or worse than not enough. Excess hardener leaves un-reacted amine that plasticizes the cured resin and can result in poor hot/wet properties and a large loss in stiffness. To change potlife or cure speed it is best to use a different speed hardener or alter the ambient temperature.