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
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
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
What does the Part B do and can I change the mix ratio to adjust
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
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,
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.
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
What is a two phase resin?
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,
resistance, and improves the
composites resistance to premature delamination.
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.
Can we have a tough single phase resin?
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.
A two phase resin achieves a
better balance of properties than a single phase,
although there are trade-offs 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 250oF.
Why canít I have everything in an epoxy matrix?
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.