By Vinay Mishra, Kevin Biller, Jeff Dimmit, and Nikola Bilic
Introduction
Epoxy-based products are used in many applications that face aggressive operating environments. Some examples (Figure 1) are electrically insulative encapsulants or powder coatings (e.g., inside electrical motors), chemical-resistant pipe linings, adhesives, and composite parts. Under extreme service conditions (e.g., regarding heat, chemical exposure, and mechanical stresses, often in combination), many traditional epoxy formulations fail because they suffer from a loss of integrity over time. Traditional solutions require switching from epoxy to alternate chemistries such as cyanate ester, bismaleimide, and polyimides, which, while suitable, can add complexity to the process and increase cost.
Aromatic dianhydrides such as BTDA® (3,3’,4,4’-benzophenone tetracarboxylic dianhydride) have been known to impart high crosslinking densities to epoxy formulations.1,2 See details on this molecule in Table 1 and its chemical structure in Figure 2. The resulting dense crosslinking, in combination with the structure of the BTDA linkages, leads to epoxy powder coatings with high glass-transition temperatures (Tg) and heat resistance. These formulations also offer superior dielectric properties, mechanical properties, and chemical resistance. As a result, BTDA-based powder coatings find uses in aggressive environments such as those that are high temperature, involve chemical exposure, or are for long-term electrical applications. It is noteworthy that such successes are achieved using simple, bisphenol-A based solid epoxy resins (Figure 2). Specialized resins such as epoxy novolacs and other multifunctional resins can certainly raise the performance but are not necessary when using a dianhydride curing agent.
A Note on Proper Stoichiometric Treatment of Dianhydride-Epoxy Formulations
To properly design dianhydride-epoxy formulations, a few points require consideration. Dianhydrides cure epoxy formulations to extremely high levels of crosslinking through esterification reactions. A proper review of the complex reaction mechanism3-6 is outside the scope of this paper. Figure 3 describes a simplified two-step esterification reaction model that has been adopted by the epoxy industry. In the overall esterification reaction, one epoxide group reacts with one anhydride group. At first glance, this reaction implies a stoichiometric ratio of anhydride to epoxide groups (A/E ratio) to be 1.0. However, optimum A/E ratios for most dianhydride-epoxy formulations are far less than 1.0, typically between 0.65 and 0.80 for powder coatings. There are two main reasons for this.
First, an A/E ratio less than 1.0 helps address the side-reaction of epoxy etherification (also known as homopolymerization), a process which can consume epoxides without participation from anhydride groups7 (Figure 4). This is a well-established fact, and epoxy formulations using mono-anhydrides such as methyl tetrahydrophthalic anhydride (MTHPA) routinely use A/E ratios in the range of 0.90–0.96 for this reason. This approach minimizes residual anhydride groups after cure while optimizing the Tg and other properties.
However, dianhydrides require a second, crucial consideration due to the extremely high levels of crosslinking that they produce. At near-stoichiometry, formulations will readily vitrify before full cure, thus locking in unreacted functional groups, which are undesirable for long-term performance. Although it is true that post-curing at elevated temperature can lead to full cure, in many cases, the final crosslink density may be too high for the application, thus leading to suboptimal performance.
Therefore, in most epoxy powder coatings, the dianhydride usage must be well below stoichiometric—typically at A/E ratios in the range of 0.65–0.80. Any excess epoxide groups in the formulation will be consumed via etherification side reactions (epoxy homopolymerization). This approach helps avoid over-crosslinking while optimizing performance. The optimum A/E ratio for a specific application is best determined experimentally, but the recommendations in Table 2 are a good starting point.
Continue reading in the July-August digital issue of CoatingsTech.