In faster cure resin systems, the primary amines are consumed by epoxy groups at a greater rate and therefore greatly reduce the carbamate formation. Therefore, if a composite manufacturer that has been commonly using a faster curing agent/hardener (i.e. a Part B that lead to faster gel and cure) but would like more working time, and therefore requests a slower curing agent/hardener, it is possible that more amine blush could occur. While again the Part B will be formulated to resist amine blush, more open time to the environment may cause more blush. Likewise, if a composite manufacturer desires a lower viscosity resin system version than previously used along with the same reactivity, time to gel time and cure rate, this also may have a greater tendency to blush. While there are some low viscosity amine type curing agents that have extremely low tendency to blush, oftentimes these amines cannot be used entirely as the curing agent. This is due to the balance of processing and properties that the curing agent (Part B) imparts to the overall resin system (Part A&B) which most likely cannot be obtained from a single amine. As a result, it is possible that multiple amines are combined to create the curing agent/hardener part in addition to catalysts to modify the reactivity. Components in the Part A (epoxy side) are also selected to reduce amine blush as the reactivity and compatibility of the epoxy resin(s) play a factor in the blush formation.
Formation of Amine Blush
Low molecular weight primary amines are hygroscopic and can react with atmospheric carbon dioxide. While it is known that carbon dioxide (CO2) can react with water (H2O) forming carbonic acid (H2CO3), this is actually quite limited at ambient conditions, and therefore most carbon dioxide is present as dissolved, not reacted. In either case, this is a reversible reaction and can lead to carbon dioxide being present in water on the surface of an uncured composite part. Primary amines, and to a lesser extent secondary amines, react rapidly with carbon dioxide to ultimately form ammonium carbamate through an intermediate carbamic acid zwitterion. A simplified equation is shown below for this reaction where 2 moles of a primary amine are shown to react with one mole of carbon dioxide.
Low molecular weight primary aliphatic amines tend to form ammonium carbamates more than cycloaliphatic primary amines. In general, the lower the molecular weight of the amine and greater the functionality (i.e. number of active amine hydrogens), the greater the chance for amine blush. Even aromatic primary amines, which are significantly less basic, can form carbamates but to a much lesser degree. Usually, this does not present itself as amine blush which may be due to the low surface concentration or/and the compatibility of the carbamate formed with the more aromatic epoxy resins. While ammonium carbamates are identified to be the major component of amine blush, other reactions can lead to the formation of amine blush due to the diffusion of carbon dioxide and water into the matrix resin followed by reaction with the amine curing agents. This results in salts of amine carbonates and bicarbonates. An often cited blush product is ammonium bicarbonate. This may result from the spontaneous decomposition of ammonium carbonate liberating ammonia or from a reaction of a tertiary amine with carbon dioxide in the presence of water. With any of the aforementioned reactions, the stoichiometry of the system is changed. This is due to the consumption of active amine hydrogens. As a result, the epoxy matrix may be under cured and most likely excess epoxy groups will be present.
Factors Affecting Amine Blush
With the use of the following guidelines, blush can be eliminated or reduced so that it does not affect cosmetics or performance of the composite product.