Cure Terms Glossary
A harder is the term used for a curing agent when the formulation will set hard - i.e. turn glassy. Thus, the term is used for the curatives for epoxy, phenolic and unsaturated polyester resins.
Commonly used hardeners for epoxy resins include active-hydrogen compounds (especially amines) and anhydrides.
For the epoxide-amine reaction, each active hydrogen on the amine is considered potentially active - i.e. both hydrogens on a primary amino (NH2) group. As a result, a functionality of four or more is not unusual for amine hardeners of epoxy resins. Examples of aliphatic polyamine hardeners (which provide for room-temperature curing), include:
The high cure rates achievable with aliphatic amine hardeners are useful for many applications including adhesives and surface coatings. However a rapid cure is just one consideration; state of cure is another. A fast cure itself is not necessarily a guarantee of a high level of cure, as the onset of vitrification effectively seals the fate of the cure. There are advantages in delaying the onset of vitrification by opting for a high temperature cure, where less active hardeners can prove easier to handle. Examples of less active NH types include: dicyandiamide (DICY), aminopolyamides or aromatic amines, e.g.,
(H2N)2C=NC≡N dicyandiamide, cyanoguanidine functionality = 4
Aminopolyamides are typically liquid condensation products of aliphatic polyamines with unsaturated fatty acids.
The epoxy ring opening reaction by active hydrogen generates further active hydrogen in the form of a secondary alcohol.
The rate of ring opening by amino N-H is very much faster than that by alcoholic OH, such that the latter has little consequence in the curing by amine hardeners. The stoichiometry is effectively 1:1 (one epoxy group per NH). Commercial epoxy resins are at least difunctional in epoxide.
Other active-H compounds include polymercaptans, polyphenols and polybasic acids. In the case of polyphenols and polybasic acids, the alcohol produced by epoxy ring opening is also competitive, and the simple stoichiometry of the NH/epoxy reaction is lost. Tertiary amines are catalysts for all these hardeners.
The alcohol from ring opening is also thought to be involved in the cure by anhydride - in this case to initiate sequential ring opening of anhydride and epoxide, i.e.,
The sequence appears to give an overall stoichiometry of 1:1 anhydride:epoxy, although the alcohol/epoxy reaction is also competitive and the optimum stoichiometry will depend on the type and level of any catalysts used. With glycidyl ether resins, anhydrides can offer better high-temperature stability than amine-cured systems.
The hardeners for phenolic (novolak) resins are usually sources of formaldehyde. Hexamethylenetetramine (usually) or paraformaldehyde are two such examples.
A novolak resin is a phenol-formaldehyde condensation prepolymer obtained by using a stoichometric deficiency of formaldehyde. The molar excess of phenol (MW 94.1) over formaldehyde (MW 30.0) is typically in the region of 25% (e.g. 0.8:1 formaldehyde to phenol). A molar excess of formaldehyde over phenol (ideally 1.5:1) is required for a full cure, which suggests something approaching 20 parts (by weight) formaldehyde would be required to cure 100 parts of resin. In practice, 10-15 pph of hexamethylenetetramine are used in typical moulding compounds.
Unsaturated polyester resins undergo a different type of cure from those of epoxies and phenolics. The cure of unsaturated polyester is a free-radical copolymerisation of a polyfunctional polyester with a monomer such a styrene. The hardener is the radical initiator. Typical loading levels are low (e.g. around 1%) which may explain why the initiator is sometimes (incorrectly) called a catalyst. Peroxide initiators are used, the choice depending on the activation required. Diacyl peroxides or peroxyesters usually provide for elevated temperature curing, e.g.,
Room temperature curing is usually required for hand lay-up procedures and is attainable with a suitable peroxide decomposition catalyst. For example with benzoyl peroxide, N,N-dimethylanilne can be used.
An alternative and
commonly-used system is a combination of a hydroperoxide and a redox metal.
Hydroperoxides (ROOH) are amongst the mixture of products from ketone
oxidation. Typical examples are methyl ethyl ketone peroxide and cyclohexanone
peroxide. In unsaturated polyester technology, the commonly-usual term
for a peroxide decomposition catalyst is accelerator.