Connecting the Dots: A New Method to Determine Radiation Yield during Electron Beam (EB) Polymerization Reactions via Raman Spectroscopy

Electron-beam (EB) polymerization is a fast, solvent-free, low-energy means of polymerizing inks, thin films, and coatings such as those used in food packaging. During EB polymerization, accelerated electrons interact with liquid monomer molecules to form radicals, which continue to react with other monomer molecules and form long chain, solid polymers. The final polymer properties are dependent on the monomer chemistry, as well as processing conditions such as dose (i.e., the amount of energy absorbed by the sample), belt speed (i.e., the rate at which the sample travels through the electron-beam unit), and dose rate (i.e., the rate at which energy is delivered to the system). Unfortunately, the relationships among formulation chemistry, processing conditions, and final polymer properties are not well understood, thereby limiting the growth of EB polymerization in industry. One reason for this is the unpredictable nature of radical formation during EB polymerization. Determination of apparent radiation yield (i.e., number of measurable radicals created per 100 eV of energy absorbed by the system) can facilitate understanding of how radicals are formed during EB reactions, which will allow for better prediction of final polymer properties. Limited research has been conducted to determine the apparent radiation yield of EB-cured polymers, and the work that has been done relies on multiple assumptions. The goal of this research project was to develop a method for determining the apparent radiation yield. Apparent radiation yield is proportional to the apparent rate of initiation (i.e., the change in radical concentration with respect to time). The radical concentration could not be measured directly because the concentration of radicals in the system is so small and because radicals are not easily detected with spectroscopy or other analytical techniques. Instead, a highly reactive inhibitor molecule was added to the formulation, which reacted with the radicals formed by the EB. Each inhibitor molecule reacts with one radical, thus the concentration of radicals is equal to the concentration of inhibitor. Raman Spectroscopy was used to find the delay in conversion caused by the added inhibitor. The inhibitor concentration was plotted versus that conversion delay, and the slope of the resulting best-fit line was the apparent rate of radical formation, which was used to calculate the apparent radiation yield. Using this method, the apparent radiation yield of benzyl acrylate was determined to be 60±40, which is consistent with the theoretical number of radicals that could be produced by 100 eV of energy.