Fiber reinforced composite materials are often made by wet-laminating individual plies or layers of reinforcement made from such fibers as carbon or glass using a two part amine-epoxy resin. After the wet-impregnation of the structural reinforcement the assembly is usually either cured open to the environment, under a vacuum bag, or in a press. Depending on the matrix formulation it may be able to be cured at ambient temperature or require heat to increase the cure speed and complete the reaction. Whatever process is utilized for the manufacture of the part, it is recommended to minimize the exposure of the uncured amine-epoxy matrix to the ambient environment; especially low temperature, high humidity, and excess carbon dioxide. The greater the surface area of the resin exposed to the atmosphere to volume of resin in the part, the greater the possibility of changes in mechanical properties of the composite. The effect is therefore increased as more surfaces are exposed, such as multiple wet-out plies, coupled with longer open time to the environment. If only the uncured matrix/composite surface is exposed, the effects on mechanical properties should not be a factor but surface cosmetics and bonding may be affected. The processing complications described are due to the formation of amine blush (i.e. water spotting), that may be caused by a surface phenomenon or leaching (i.e. blooming or exudate) process.
Amine blush is due to the curing agent (Part B) component, and more specifically, usually the primary amines reacting with atmospheric carbon dioxide and moisture forming ammonium carbamates and/or ammonium carbonates/bicarbonates. The water that is present may be from humidity in the atmosphere but can also come from the substrate, such as glass fabric that has not been stored properly. Depending on the amine type and other components in the formulation, along with the concentration of carbon dioxide to water, the formed blush may appear on the surface as greasy or waxy, or be more white and crusty. Some amines lead to much more water soluble products than other amines. Although ammonium carbamates are highly hygroscopic, they dissolve slowly in water. In many cases the blushing effect will not be observed until the part has actually been subjected to water or high humidity after complete cure. It is commonly observed that amine blush reduces surface gloss, surface hardness, paint adhesion, and solvent resistance, but it can also lead to greater discoloration of the surface with exposure to heat and ultraviolet light (UV). Furthermore, amine blush can reduce the adhesion of an over-coating of the same or different resin or another layer of wet-out reinforcement. Although these are surface effects, amine blush can also affect the mechanical properties of composites arising from subsequent layup of wet-out fiberous reinforcements.
In wet-lamination of individual fabric layers, usually each surface is exposed to the environment. Since each wet out fabric layer has a high surface area to volume of resin, time open to the atmosphere should be minimized if possible. If the surface of the wet out fabric is contaminated by amine blush, it is then transferred to the adjacent interlayer of which it is placed. This can create a loss of mechanical properties from a lower modulus/strength interlayer region or can also result in voids in the cured composite structure if the assembly is heated. The formation of voids is due to the reversible reaction of the ammonium carbamate at elevated temperature, above 60 ⁰C, liberating carbon dioxide gas and ammonia products. This pressure can be enough to delaminate plies or form blisters in outer layers.
While all of Applied Poleramic, Inc.’s (API’s) resin formulations are designed to eliminate or reduce as much as possible the formation of amine blush, there are some resin systems that will have a greater propensity to blush. In general, the faster reacting resins (i.e. faster gelation and shorter pot-life) will have less amine blush than resin systems that are very slow to gel and cure at room temperature. This is because the atmospheric moisture and carbon dioxide is competing with the epoxy groups to react with the active amine hydrogens.