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

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A.� Room temperature cure resin is the biggest misnomer in epoxies.� A thermoset resin is only capable of achieving a Tg of

�� �� about 30^oF above the cure temperature.� Therefore, unless the ultimate Tg is within the 100^oF 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.

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Q.� What is Tg and is it the same as HDT?

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A. 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.

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����� 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. 10^oF)

���� �that Tg due to the applied load.�

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

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A.� 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.�

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����� 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.

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Q.� What is a two phase resin?

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A.� With proper formulation, a toughening modifier can precipitate into a separate elastomeric phase that enhances the epoxies

���� �toughness by creating multiple energy dissipation mechanisms.� A cross-section� of a cured two phase epoxy would look

���� �encapsulated and bonded to a continuous epoxy phase.� The second phase enhances fracture toughness, impact

���� �resistance, and improves the composites resistance to premature delamination.

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Two phase technology is beneficial in bonding dissimilar materials. �To maximize peel strengths, an adhesive must adhere to the substrates as well as have exceptional resistance to cracking within the adhesive layer.� If an adhesive has good adhesion but is brittle (as is the case of single phase resins), the bond will split through the adhesive leaving� residual resin on both surfaces and would be considered a cohesive failure.

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Q.� Can we have a tough single phase resin?

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A.� Continuous single phase epoxies can be toughened by putting in reactive and/or un-reactive plasticizers.� Unfortunately, there is a large knockdown on properties such as Tg and stiffness (modulus).� Two phase resins also suffer, but not as bad.� Assuming total phase separation, the modulus goes down with the volume fraction of the second phase, usually 5-10%, but the Tg is not sacrificed since the continuous epoxy phase has not changed.

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����� A two phase resin achieves a better balance of properties than a single phase, although there are trade-offs for both.� For

��� � a given designed stiffness and/or �temperature,� the toughness of a two phase resin exceeds that of a single phase.� The

���� �two phase systems do however decline to single phase properties as the Tg increases beyond 250^oF.�

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Q.� Why can�t I have everything in an epoxy matrix?

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A.� All two component epoxies have to be balanced for cost, Tg, stiffness, toughness, and workability. Unfortunately, many of these requirements work against each other.� For instance, as toughness increases, stiffness and Tg decrease.� Therefore, it is not possible to have a ultra-tough resin with high stiffness.� Only a few properties, like stiffness (modulus) and Tg, actually go hand- in -hand.�� The best formulation technologies, usually involving modifiers and some kind of� heterogeneous morphology,� achieve the best property balances, but are still compromised by trade-offs.� It is the total balance between workability and cured properties that make one product most suitable for a specific application.

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