Other than achieving its maximum mechanical properties, at molecular level does it mean that all epoxide groups have underwent ring opening reaction with hardener?
If yes, a fully cured epoxy will less likely have epoxide groups on its surface?
if epoxy is made of a stoechiometric mixture of epxoide + amine group, a fully cured epoxy does not contain epoxide ring anymore.
if epoxy is made of a non stoechiometric mixture, the excess functions will remain after the total cure
regards
A little correction to Emmanuel's answer:
if epoxy is made of a stoechiometric mixture of epoxide + amine group, a fully cured epoxy STILL contains a small amount of uncured epoxy-groups - due to topological features of the stoechiometric network (about a few %) because at the very end of reaction all the amino-groups "would-be-available-to react with remaining epoxy-groups" are fully exhausted. In otter words - the remaining epoxy-groups - have no amino-group-neighbours .
See for example: M. Sharifi,a C. W. Jang,a C. F. Abramsa and G. R. Palmese*a
J. Mater. Chem. A, 2014,2, 16071-16082
DOI: 10.1039/C4TA03051F
Regards, Leonid
Epoxy resins are characterized by the presence of a three-membered ring containing two carbons and an oxygen (epoxy group or epoxide or oxirane ring). The curing of epoxy resins is associated with a change in state from a low molecular weight resin to a highly cross-linked network. Many commercial hardeners suitable as curing agents have been used for epoxy resins. The most common types of curing agents are (1) primary, secondary polyamines and their adducts, (2) anhydrides, (3) polyamides, and (4) catalytic types etc.
Curing may be achieved by reacting an epoxy with itself (homo-polymerisation) or by forming a copolymer with polyfunctional curatives or hardeners. In principle, any molecule containing reactive group may react with the epoxide groups of the epoxy resin. In theory, if epoxy is made of a stoechiometric mixture of epoxide + hardener, curing agent group, a fully cured epoxy should not remain any uncured groups. Practically, small amount of uncured epoxy and unreacted groups are presence in the final product. Therefore, post curing need to be taken sometime. When the epoxy reached the high level of cure (network), at the very end of reaction, the movement of the molecules become more difficult. In addition due to topological features of the stoechiometric ratio, and curing conditions the curing level of epoxy may vary. However, the cured stage should typically attain the glass transition temperature (Tg) of the fully cured network in order to achieve maximum properties.
Epoxy when cured is Maximum 70% crosslinked due to the difficulties in movement of chains in the crosslinked structure. With time(aging) and temperature effect the percent of crosslinking increases until it reaches 90% or more. Degradation with time contests with crosslinking causing the chains to break and deterurate
As already indicated in previous answers, complete cure of epoxies is something that never happens. Vitrification is a temperature dependent, steric hindrance and diffusion limited phenomenon. The final few degrees of cure that can be achieved at any given cure temperature can make a significant positive difference to the Tg, to the elastic modulus of the resin, as well as to its durability, but not necessarily its strength.
Thank you very much for all the answers.
Basically base from all the answers:
Complete curing is difficult to attain due to chain movement and orientations when reactive groups react with one another.
Due to this, there will be very small amount remaining unreacted groups
Hence, epoxide groups may be found on its surface. But the amount of epoxide groups will also depend on the mix ratio.
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