This article is the winner of the PSTC's Carl A. Dahlquist Award, presented at Tech XXVIII in Baltimore.
Electron beam radiation has been used successfully to crosslink polymers, and it has also been applied to curing applications where ultraviolet (UV) light does not work as well (opaque, etc.).1 It has not been generally applied to polymerization, however, which mostly converts monomers (as opposed to oligomers) and, therefore, requires more time to complete. Most curing applications are used to harden a coating, like a chemical equivalent of drying. If the material under consideration is a pressure-sensitive adhesive (PSA), then meeting the more demanding physical property requirements that depend on polymer structure presents a considerable challenge. To make a high-performance PSA, a high-molecular-weight crosslinked network is essential. Irradiation with an electron beam would be a desirable process for making a PSA because of the advantages of direct initiation: 100%-solids, solventless conversion to a polymer on-web, a product-by-process in one step. However, the difficulty to overcome is the very high dose rate and short residence time at productive speeds, which tend to give low conversion and short chains that may be too tightly networked or that remain as low-molecular-weight residue unincorporated into the gel.2 The main problem with free-radical polymerization under a continuous beam (and other methods) is that neutral reactive species are continuously produced and easily terminate one another. To better understand how this problem may be overcome, and to address all of the issues, the three stages of polymerization must be considered (see Figure 1).