Powder Coating

Aug2016

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22 POWDER COATING, August 2016 Time-temperature-particle size cure window in powder coatings It is well known that the reactions of epichlorohydrin (ECH) with hy- droxyl, carboxyl, and amino func- tional groups will form epoxide func- tionalities, which, in comparison to other epoxy derivatives, are capable of withstanding higher shear stresses while exhibiting weaker peel strength. One approach to overcoming the issue of weak peel strength is to modify the epoxide phenolic novolac resins in order to reduce shrinkage during cure as well as to improve chemical resis- tance, film strength, and adhesion. Phenolic novolac resins, which under acidic conditions are the reaction products of formaldehyde and excess phenol, form repeating units. These are shown in Figure 1. Due to the high reactivity of hydroxyl functionalities, phenolic novolacs are easily modified with ECH in the presence of an alkali catalyst. To cure epoxy resins as thermoset net- works, it is necessary to use crosslinkers such as amines, hydrazines, carboxylic acids, or anhydrides. The choice of a crosslinker will strongly influence the mechanical properties and cure time of the final polymer network. The use of a phenolic novolac crosslinker will pro- duce a hard, tough thermoset that ex- hibits excellent adhesion. The mechanism of cure involves two etherfication reactions, as shown in Figure 2. The first is the reaction of a phenolic hydroxyl and epoxide to form an aromatic ether and 2? hy- droxyl via a carbonium ion complex (A). The second involves the reaction of 2? hydroxyl groups and epoxide to form an aliphatic ether (B). The sec- ond reaction depends on the concen- tration of oxirane and the nature and concentration of catalyst. When 2- methylimidazole (2-MI) is used as a catalyst, the extent of reaction for sec- ondary hydroxyl groups has been shown to vary from 50 to 90 percent. One of the advantages of using epoxy phenol novolac (EPN) resins is their ability to be formulated into powder coatings. After application to the sur- face, these powders have to flow in order to form a uniform surface coat- ing. Therefore, the time required to ac- complish this process and the tempera- ture at which a dry powder is converted to a liquid and solid film are key. This concept, often referred to as time- temperature-transformation (TTT), is highly important for liquid coatings, but not quite adequate for powders. The reason is because powders are par- ticles that also have physical dimen- sions. In other words, the larger the particles, the more energy and/or time it takes to melt them. This concept goes beyond just the melting of particles that would be suitable for thermoplastic sys- tems. For thermosetting coatings dur- ing the same process of particle melt- ing, there are also chemical reactions, such as those shown in Figures 1 and 2. If the reactions occur too fast, surface properties will suffer because once the system is crosslinked, it cannot flow. Common defects such as orange peel may then be observed. For that reason, it is more useful to uti- lize the concept of time-temperature- particle size (TTPS) transformation with powder coatings. We then have to focus on the structure-property rela- tionships between the particle size, chemistry of crosslinking, and other properties such as adhesion. PC Editor's note For further reading on raw materials for powder coatings, see Powder Coating magazine's website at www.pcoating .com. Click on Article Index and search by subject category or click on Book- store. For troubleshooting or to submit a question, click on Problem Solving. Powder Coatings Clinic Marek W. Urban, Ph.D. Clemson University Figure 1 OH CH 2 CH 2 OH OH n

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