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Precautions For Spraying Water-based Alicyclic Epoxy Resin Coatings

Jun 16, 2020

       Water-based coatings that do not contain VOCs and HAPS (high-altitude polluting solvents) can be prepared from liquid alicyclic epoxy resins and caprolactone polyols with a narrow degree of polymerization. These coatings can be applied with traditional equipment and may be substituted Powder coating. What are the precautions for the preparation of waterborne alicyclic epoxy liquid coatings? Experts from China Epoxy Resin Industry Association made a special introduction a few days ago.

   The use of traditional liquid paint spraying equipment can avoid the basic equipment investment necessary for a new powder coating line. After modification, these paints can be diluted with water or thickened with cellulose thickeners for water-based paints. Compared with the traditional bisphenol A diglycidyl ether resin, alicyclic epoxy resin has the following differences: it is an aliphatic main chain, reacts quickly with acid, but is basically the same as amine/amide epoxy commonly used curing agent It does not react, does not contain chlorine, has a viscosity of 350 centipoise, and the bisphenol A diglycidyl ether resin is 11,000 centipoise. It also has excellent weather resistance. The application of low-viscosity alicyclic epoxy resins in cationic UV coatings has grown significantly. There have been many reports on the formulation and performance of such “va”-free 100% solid coatings, and it has been used in thermal curing coatings. There are few exhaustive literatures on the application. In principle, the formulation know-how of cationic UV coatings can also be used in thermal curing systems, but the two are also different. For example, UV coatings generally do not need to be sprayed, so the construction viscosity can be higher than sprayed thermally cured coatings. Water can be used in baked cationic coatings. As a thinner for VOC.

       Caprolactone polyol is an effective reactive diluent that has been used in solvent-based coatings and can accelerate the curing of cationic UV coatings. In the cationic UV coatings, the hydroxyl groups of such polyols can accelerate the curing and lead to a certain degree of humidity Curing sensitivity. In heat-cured coatings, the humidity in the oven is low and the moisture does not inhibit curing. The presence of polyol increases the tolerance to water, so water can be used as a VOC-free diluent in coating formulations composed of alicyclic epoxy and polyol. This type of coating is actually a high-solid organic coating diluted with water . Dilute with water has two advantages: low viscosity when no VOC, common and well-known water-based paint can be used to achieve anti-sagging and rheological control. Because the components in the formula are only partially miscible with water, the amount of water that can be absorbed is limited. Care should be taken to avoid adding too much water to cause phase separation.

   Cellulose thickener can effectively control sag in water-based coatings, but it is unclear whether it is effective in 100% solids, water-diluted alicyclic epoxy coatings. Earlier work showed that water-diluted 100% solid epoxy had thixotropy when measured with a Brookfield viscometer. Investigations on various thickeners show that cellulose thickeners give the best overall performance. When heated to 40 ~ 60 ℃, the best paint film appearance can be obtained by spray gun construction. The low construction temperature makes the cured paint film produce more orange peel. Experts from the China Epoxy Resin Industry Association said that the main advantage of powder coatings is high coating efficiency. High coating efficiency is mainly achieved through the recycling of powders. To be equivalent to powder coatings, such liquid coatings need some kind of Ways to recycle flying paint. Since the coating formulations mentioned here do not contain organic solvents, it is much easier to recycle. By measuring the conductivity, water can be used when necessary to make it easy to adjust the water loss during the recovery process, compared with the powder coating. Compared with the high baking temperature, the lower baking temperature required for this type of coating can save energy. Another program that is being evaluated is to use the "Unicarb" method to spray the water-based epoxy, polyol and pigment formula products. The "Unicarb" method is more efficient than powder coating. For VOC-free coatings, simply add a solvent to the above formula.

       Regarding the formulation considerations, experts of the China Epoxy Resin Industry Association said that for cationic curing, the alkali will stop curing, so all additives and rheology control agents must be free of alkaline substances. Basic pigments and amine-treated pigments will Inhibit curing. Basic rhodamine pigments will also inhibit cationic curing. Basic pigments and additives cannot be used in cationic curing coatings. Curing depends on the volatilization of a small amount of amine blocking agent in catalyst FC-520. If the paint film is too thick or the oven does not Ventilation prevents the amine from being completely removed from the paint film and inhibits curing. Viscosity increases with the degree of branching of the polyol: diols have a lower viscosity than ternary or quaternary alcohols. When used in combination with diols, the amount of epoxy needs to be increased (the R value is large) to obtain hard For the paint film, a high amount of epoxy can improve the corrosion resistance. The increase in the amount of catalyst will accelerate the curing speed but also reduce the corrosion resistance. The molar ratio R of epoxy to hydroxyl has a significant effect on the curing speed.

       In short, cycloaliphatic epoxy and caprolactone polyol can be used to formulate low-viscosity or even VOC-free coatings. The coating can be diluted with water to spray viscosity. Cellulose thickener can give water-diluted 100% solid epoxy and polyhydric Alcohol coatings have effective anti-sagging properties. The curing speed of the system is determined by the following variables: the molar ratio of epoxy to hydroxyl, the functionality of polyol and epoxy, the amount of catalyst and curing temperature, and the formulation can be easily used Preparation of color paints. Since these paints are cation cured, basic pigments and substrates will inhibit curing.