All About Epoxy - Answers to F.A.Q.

 

What You Wanted to Know About Epoxy

Epoxy resin literature has been less than accurate over the years.  Since conception of Applied Poleramic Inc., it has been one of our goals to educate the end-user with the truth about epoxies.  The following is a list of commonly asked questions that will hopefully correct some misnomers we hear and read about in industry.  You can also visit "Ask Captain Sticky" where you can email our president, Richard Moulton, for answers to questions not detailed below.

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

 

A.  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.

 

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

 

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.

 

      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. 

 

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

 

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. 

 

      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.

 

Q.  What is a two phase resin?

 

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.

 

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.

 

Q.  Can we have a tough single phase resin?

 

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.

     

      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 250oF. 

 

Q.  Why canít I have everything in an epoxy matrix?

 

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.

 

 

 

Did we answer your questions?  If not, click on the link below to visit our President at "Ask Captain Sticky".  Submit your own questions for the Captain to answer!

 

Ask Captain Sticky

        

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