Analysis method for epoxy equivalent

The epoxy equivalent and epoxy value (epoxy equivalent = 100/epoxy value) are primarily determined through addition reactions that involve halogen hydrides, thiosulfates (esters), and thiols in order to quantify the epoxy groups, or through ionization reactions for the analysis of epoxides. Since 1984, many imported epoxy materials have required the use of the “hydrogen bromide-ice acetic acid non-aqueous titration method” for quality control. This method is faster, more accurate, and more sensitive than the salt-acid triphenylmethane back-titration method that has been widely used in China. It can be used to determine common epoxy resins as well as those containing fillers or being dark in color; compared to the salt-acid triphenylmethane method, it requires less testing time, makes it easier to determine the endpoint, provides more accurate data, and has a wider range of applicability. Its results are comparable to those obtained using the perchloric acid method specified in standard GB/T 4612, but it is safer to use, cheaper, and simpler. In domestic research and production facilities, the salt-acid triphenylmethane method, the hydrogen bromide-ice acetic acid method, and the perchloric acid method have been compared in practical applications. The supplementary notes in national standard GB/T 13657-1992 (bisphenol A epoxy resins) state that this standard was formulated with reference to ASTM 1763-1981 (standard specifications for epoxy resins), and ASTM specifies the use of ASTM D1652 (hydrogen bromide-ice acetic acid method) for determining the epoxy equivalent. The main reasons for this are: first, the salt-acid triphenylmethane method relies on back-titration, requiring the sample to be left standing for 1 hour before titration, and the reagents need to be prepared just before use; second, the hydrogen bromide-ice acetic acid method and the perchloric acid method utilize non-aqueous titrations, allowing the sample and reagents to dissolve easily, enabling rapid reactions and making it easier to determine the endpoint. In contrast, some samples treated with the salt-acid triphenylmethane method may become cloudy around the endpoint due to poor dissolution, which affects the determination of the endpoint. Additionally, in the analysis of imported and domestically produced epoxy adhesives and various potting compounds, most samples are not transparent resins like E-51 (EP01441-310 or CYD-128); instead, they contain large amounts of fillers, colored additives, or dark-colored by-products

The epoxy equivalent is an important parameter for epoxy resins, but it is difficult to measure. Is there a fast, accurate, simple, and practical method for determining the epoxy equivalent? The “hydrogen bromide-ice acetic acid non-aqueous titration method” introduced by experts recently is a scientific approach that can be utilized by the industry. This method is used in an aprotic medium of glacial acetic acid, with crystal violet as an indicator; hydrogen bromide reacts rapidly and equimolarly with epoxy groups. The key reagent is 0.1 mol of HBr-HOAe. There are also specific techniques for testing colored samples, samples with high filler content, or dark samples that are difficult to decolorize, and the potentiometric titration method using hydrogen bromide/glacial acetic acid involves various secrets. Compared with other commonly used epoxy analysis methods, this method offers the advantages of simplicity, reliability, and higher accuracy. The epoxy equivalent (epoxide value) is a very important parameter for the use and quality control of epoxy resin adhesives, potting compounds, composites, etc. (Epoxy equivalent = 100/epoxide value); as such, it has received extensive attention over the years, leading to numerous studies and the development of various chemical analysis methods. These methods are primarily based on the stoichiometric addition reactions or ionization reactions of hydrogen halides, thiosulfates (esters), and thiols with epoxy groups, and are used for the analysis of epoxides. Budvak et al. provided detailed reports and discussions on analytical methods for epoxides as early as 1969, while Sorokina et al. did so in 1978. In the past, the salt-acid-propanol back-titration method (HG2-741) was widely used in China and remains in use to this day; however, it is often difficult to conduct measurements when the sample is not pure resin but contains a certain amount of fillers, or when dark substances interfere with the determination of the endpoint (such as adhesives or potting materials). Since 1984, many imported epoxy materials have required a \"hydrobromic acid-ice acetic acid non-aqueous titration method\" for quality control. This method is faster, more accurate, and more sensitive than the salt-acid triphenylmethane back-titration method that has been widely used in China for a long time. It can not only determine common epoxy resins but also be applied to the rapid analysis of epoxy materials containing fillers or those with dark colors. Compared with the method using salicylic acid, it features shorter detection time, easier determination of the endpoint, more accurate and consistent data, and a wider range of applicability. It is comparable in effectiveness to the high-chloride* method specified in **standard GB/T 4612. However, the hydrogen bromide-ice acetic acid non-aqueous titration method is safer, cheaper, and simpler to use. The main reason why the hydrogen bromide-ice acetic acid method is not widely used in China is the lack of supply of the standard titrant required for it in the country. Researchers from the Lanzhou Institute of Chemical Physics of the Chinese Academy of Sciences and the School of Materials Science at Northwest Polytechnical University developed specialized reagents through research on testing methods and experiments, which enabled the improvement of this method and led to the establishment of enterprise standards. It has received positive feedback from several relevant organizations after trial use, addressing the need for long-term quality control of various imported epoxy materials. In production practice, experts conducted comparative tests on the propionic acid salt method, hydrogen bromide–glacial acetic acid method, and perchlorate method. The additional notes to the national standard GB/T 13657-1992 (Bisphenol A epoxy resins) state that this standard was formulated with reference to ASTM 1763-1981 (Standard Specification for Epoxy Resins), and that ASTM standard specifies the use of ASTM D1652 (hydrogen bromide–glacial acetic acid method) for determining the epoxy equivalent. The main reason is that the salicylic acid method employs back-titration; the sample must be left to stand for 1 hour alone before titration, and the reagents need to be prepared just before use ； Secondly, since the hydrogen bromide-ice acetic acid method and the perchloric acid method employ non-aqueous titration, the samples and reagents dissolve well, the reactions occur rapidly, and it is easy to determine the endpoint. In contrast, with the salicylic acid method, some samples may become cloudy around the reaction endpoint due to poorer sample dissolution, which affects the determination of the endpoint. Furthermore, in the analysis and testing of imported and domestically produced epoxy adhesives as well as various potting compounds, most samples are not transparent resins like E-51 (EP0144l-310 or CYD-128); rather, they are samples containing large amounts of fillers, colored additives, or dark-colored by-products. For this reason, when applying the hydrogen bromide–glacial acetic acid method in practical applications, different epoxy samples are treated in 3 distinct categories. Compared to the perchloro* method, its principle is also based on an addition reaction between hydrogen bromide and epoxy functional groups. The difference is that in the perchlorate method, perchlorate reacts with tetraethylammonium bromide, and the resulting hydrogen bromide then reacts with the epoxy functional groups. The titrants used in both methods require calibration before use, and their calculation methods and procedures are quite similar (the hydrogen bromide-ice acetic acid method generally does not take temperature/volume corrections into account), with relatively small errors. The hydrogen bromide and glacial acetic acid method is more practical due to the simpler and more readily available reagents used. Regarding the potentiometric titration of hydrogen bromide–glacial acetic acid, the epoxy equivalent of various epoxy materials can be determined by potentiometric titration. Under normal circumstances, direct titration with reagents can be used directly. However, due to factors such as the volatilization of hydrogen bromide and oxidation, standard ASTMD1052 specifies the use of perchloric acid in combination with tetraethylammonium bromide for potentiometric titration; for epoxy materials with low activity, tetrabutylammonium iodide is used (which is exactly the same as the method using perchloric acid specified in national standard GB/T 4610). In fact, this high-chloride* method is another application form of the hydrogen bromide-ice acetic acid method. This is because the testing principle relies on the reaction of perchlorate with tetraethylammonium bromide to synthesize elemental hydrogen bromide in situ, and the resulting hydrogen bromide immediately reacts equimolarly with the epoxy groups; the calculation and testing methods are the same. During testing, due to volatile oxidation, a sealed, automatic liquid-fed burette can be used. A comparison of the test results showed that the home-made reagent is comparable to the imported reagent, with a deviation of ≤±1.5%, which meets the usage requirements. After analyzing different testers in laboratories of 3 units, it was concluded that this method and reagents can meet the requirements for product quality control. In summary, compared with the results obtained using the hydrogen bromide–glacial acetic acid method and the salt-acid propyl method, the former offers the advantages of consistent data, high accuracy, cost-effectiveness and safety, as well as simplicity in operation, and its performance is comparable to that of the perchlorate method. However, the hydrogen bromide-ice acetic acid non-aqueous titration method is safer, cheaper, and simpler to use. The HBr-HOAc 0.1 mol/titrant can be prepared simply and effectively by reacting hydrobromic acid with acetic anhydride. Experts emphasize that since the hydrobromic acid–glacial acetic acid method uses standard titrants that can be prepared domestically, it can be widely applied in China, especially for the testing and analysis of imported epoxy materials.