Toughening agent for epoxy thermoset

1. An epoxy thermoset comprising the reaction product of:

a toughening agent prepared from a reaction of a first epoxy resin and a polyether polyamine, where the toughening agent is a liquid and the polyether polyamine has a number average molecular weight in a range from 4,500 to 20,000 and includes at least three primary amines each having at least one reactive hydrogen atom reactive with an epoxide group of the first epoxy resin;

a second epoxy resin, where the second epoxy resin is a liquid; and

a liquid amine hardener that reacts with both the toughening agent and the second epoxy resin,

wherein at least a portion of the toughening agent is phase separated into particles formed in the second epoxy resin and the liquid amine hardener at an onset temperature of exotherm for a first time interval and the second epoxy resin and the liquid amine hardener are cured after the first time interval at an curing temperature that is greater than the onset temperature of exotherm,

wherein the particles have a volume average diameter in a range of from 20 nanometers (nm) to 200 nm.

2. The epoxy thermoset of claim 1 , where the first epoxy resin and the second epoxy resin are chemically identical.

3. The epoxy thermoset of claim 1 , where during the reaction to prepare the toughening agent the first epoxy resin is in a molar excess relative the polyether polyamine.

4. The epoxy thermoset of claim 1 , where the toughening agent is 4 to 30 weight percent of the total weight of the epoxy composition.

5. The epoxy thermoset of claim 1 , where the first epoxy resin and the second epoxy resin are each at least one or more of the formula:

wherein R 4 is a C6 to C18 substituted or unsubstituted aromatic, a C1 to C8 aliphatic or cycloaliphatic; or heterocyclic polyvalent group and a has an average value of from 2 to 8.

6. The epoxy thermoset of claim 5 wherein the first epoxy resin and the second epoxy resin are each one or more of a diglycidyl ether of resorcinol, catechol, hydroquinone, bisphenol, bisphenol A, bisphenol AP (1,1-bis(4-hydroxylphenyl)-1-phenyl ethane), bisphenol F, bisphenol K, bisphenol S, tetrabromobisphenol A, phenol-formaldehyde novolac resins, alkyl substituted phenol-formaldehyde resins, phenol-hydroxybenzaldehyde resins, cresol-hydroxybenzaldehyde resins, dicyclopentadiene-phenol resins, dicyclopentadiene-substituted phenol resins tetramethylbiphenol, tetramethyl-tetrabromobiphenol, tetramethyltribromobiphenol, tetrachlorobisphenol A, or a combination thereof.

7. The epoxy thermoset of claim 1 , where the first epoxy resin and the second epoxy resin are each at least one or more of the formula:

wherein R 2 is C6 to C18 substituted or unsubstituted aromatic, a C1 to C8 aliphatic, or cycloaliphatic; or heterocyclic polyvalent group and b has an average value of from 2 to 8.

8. The epoxy thermoset of claim 1 , where the liquid amine hardener is isophorone diamine and the first epoxy resin and the second epoxy resin are each a bisphenol A diglycidyl ether.

9. A structure, comprising:

a surface; and

an epoxy thermoset of claim 1 on the surface of the structure.

10. A method of curing an epoxy thermoset, comprising:

supplying a toughening agent prepared from a reaction of a first epoxy resin and a polyether polyamine, where the toughening agent is a liquid and the polyether polyamine has a number average molecular weight in a range from 4,500 to 20,000 and includes at least three primary amines each having at least one reactive hydrogen atom reactive with an epoxide group of the first epoxy resin;

mixing the toughening agent with a second epoxy resin and a liquid amine hardener, where the second epoxy resin is a liquid;

phase separating at least a portion of the toughening agent into particles having a volume average diameter in a range from 20 nm to 200 nm in the second epoxy resin and the liquid amine hardener, where phase separating at least a portion of the toughening agent into particles includes allowing the particles to form in the second epoxy resin and the liquid amine hardener at an onset temperature of exotherm for a first time interval; and

curing the second epoxy resin and the liquid amine hardener, where the particles form a soft dispersed phase and the second epoxy resin and the liquid amine hardener form a hard continuous phase, relative the soft dispersed phase, of the epoxy thermoset, where curing the second epoxy resin and the liquid amine hardener includes increasing the onset temperature of exotherm second to a curing temperature after the first time interval.