Chemical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 176

epoxy-based ones appear to be the most versatile and common. An ubiquitous epoxy
compound used in thermoset materials is diglycidyl ether of bisphenol A (DGEBA)
or 2,2-bis(4-glycidyloxyphenyl)propane:
The value of n determines viscosity and the glass transition temperature of the
DGEBA precursor so that at ambient temperature it can be either low-viscosity liq-
uid or solid [ 24 ]. The compound is usually copolymerized with organic anhydrides
or amines. The epoxy-amine polymerization includes several steps. The first is a
primary amine reaction:
(4.18)
It is followed by a secondary amine reaction:
(4.19)
If DGEBA is reacted with a monoamine, reactions 4.18 and 4.19 would produce a
linear polymer. A diamine should be used to form a network.
The rate of the secondary amine addition is generally slower than that of the
primary one. The ratio of the rate constants for the secondary and primary amine
reaction is reported [ 26 - 29 ] to be around 0.1-0.6. It means that the early stages of
epoxy-amine curing are typically dominated by the primary amine addition. The
primary amine groups become largely consumed close to 50 % cure (i.e., ʱ = 0.5)
and the secondary amine reaction (4.19) becomes dominant [ 30 ]. However, it pro-
ceeds in competition with etherification:
(4.20)