Likewise, pouring cold resin out of an enclosed container into a warm environment may also cause water to condense on the surface of the resin/part. Another contributing factor to amine blush on composite surfaces may occur if the reinforcement is not kept dry and exposed to ambient conditions. Glass fiber reinforcement is especially prone to absorbing significant water from the atmosphere which can then be incorporated into the layup. If this does not increase the surface blush, the additional water in the assembly may result in voids or plasticization of the network and lower properties.
In relation to the reaction rate for most amine cured epoxy resin systems, they follow the “rule of thumb” where the rate of reaction doubles for every 10 ⁰C rise in temperature. Accordingly, the critical parameter affecting blush, the gel time, of the resin is affected significantly by temperature. In general, higher temperature reduces amine blush all things being equal since the cure rate is increased. If the temperature is below or drops below 50 ⁰F, cure of the resin system will significantly slow down and more amine blush may occur.
Limiting and controlling the level of carbon dioxide in the wet-laminating or application environment is also necessary to reduce amine blush. It has been mentioned previously that carbon dioxide readily reacts with both primary and secondary amines and therefore should be minimized. In a typical outdoor environment, carbon dioxide is present at 350 to 360 ppm. This can be elevated many times in indoor environments just due to people but there are other influences that cause extreme elevations in carbon dioxide. The use of gas or kerosene fired heaters (e.g. “salamanders”) should be eliminated from the lamination and cure area. Other types of direct fired gas heaters should also be eliminated since they also raise the level of carbon dioxide and moisture in the air significantly. A big problem can result if these types of heaters are used for heating an enclosed laminating space or worse yet a curing tent heated using these types of heaters. Other gas burning motors should also not be present in the laminating or curing environment. These may be used to power forklifts or generators. If used for curing ovens, they should be vented outside away from the laminating area. Other sources of carbon dioxide should also be identified and removed if possible.
4.) Addition of solvents or fillers can lead to problems of amine blush and reduced mechanical properties. If custom modification of an API resin system (Part A/B) is undertaken by a laminator or contractor, the outcome may be hard to predict. If a solvent is added to the mixed resin system for whatever reason, usually to lower the viscosity, this can cause significant changes to the properties as well as slowing the cure rate and corresponding gel time of the resin. This can lead to greater amine blush as one of the problems. Also, the addition of fillers (calcium carbonate, wollastonite) microballoons, thixotropes (fumed silica, clay), or pigments, can also add water to the mixed resin if not properly dried and/or change the reactivity, causing greater amine blush. If different customizations are necessary for the resin, please contact Applied Poleramic, Inc. and we would be glad to make recommendations or modify the resin system accordingly.
What to do if Blush is Problem
If the conditions are constantly changing and/or the environment is such that amine blush is always a problem, there are a few things that can possibly be done from a process standpoint to change the outcome. This assumes that the laminating/curing environment cannot be changed as described above.
1.) If the blush is a result of the part curing under adverse conditions, a plastic release film could be draped on the surface of the part during cure if the cosmetics can allow it. To alleviate the problem completely, the part could be placed in a vacuum bag.