Chemistry Reference
In-Depth Information
case, curing involves the opening of the epoxide groups. Hardening occurs primarily
through the hydroxyl groups when an anhydride is used as the crosslinking agent;
in this situation, the epoxy prepolymer is considered as structopendant. The curing
mechanisms of epoxy binders and the structures of epoxy cured with amines, amides
and anhydride have been studied thoroughly and epoxy resin systems with different
types of hardeners are used extensively in many industrial applications. Curing agents
are either catalytic or corrective. The catalytic curing agent functions as an initiator
for epoxy resin homopolymerisation, whereas the corrective curing agent acts as a
comonomer in the polymerisation reaction.
The curing agent can react with the epoxy and pendant hydroxyl groups on the resin
backbone by either an anionic or a cationic mechanism. Catalytic curing agents can
be used for homopolymerisation, as supplementry curing agents with polyamines
or polyamides, or as accelerators for an anhydride cured system. Catalytic cures are
initiated by Lewis acids, e.g., boron trihalides, and Lewis bases, e.g., tertiary amines.
Several methods based on dynamic mechanical analysis (DMA) [63], rheological
measurement, NIR analysis [64] and thermal differential scanning calorimetry (DSC)
were used to study the mechanism and kinetics of the curing reaction. The reactivity
of the epoxy binders to be cured, and the properties of the end products, affect the
choice of curing system. The type and concentration of curing agents may be able to
control the length of crosslinks (crosslink density) and, consequently, the mechanical
properties of the cured epoxy resins. Mijovic and co-workers [64] highlight the
importance of the crosslinking agent (hardener) concentration and consider that
higher hardener concentrations lead to more excessive intermolecular reactions at
the expense of the intramolecular matrix.
The stoichiometric quantity of polyamine and polyamide used to cure an epoxy resin
(parts by weight of polyamide or polyamine per 100 parts by weight of resin) is a
function of the molecular weight of the hardener and the number of active hydrogens
of the polyamine or polyamide and the EEW of the epoxy binder. The calculation of
the number of active hydrogens in the curing agent follows the method for polyamine
and polyamide [65]. Often, formulae are provided to simplify the calculations for
epoxy resin:hardener ratios. It is therefore important to select the best curing systems
by choosing the appropriate types of hardener and epoxy, concentration of hardener,
type of catalyst and curing temperature. Atta and co-workers [36, 37] used 1 wt%
triethylamine as a catalyst (based on the weight of epoxy resins) for several curing
systems for preparing epoxy resins and using hardeners based on rosin acid derivatives.
The curing exotherms of the prepared formulae were obtained by plotting the curing
temperature as a function of time at different temperatures between 40 and 60 °C.
Since the amount of heat evolved upon curing depends on the sample size, it was
