Drying of Emulsion Adhesives

Maximizing lines speeds on coating/drying equipment is an obvious approach to improving manufacturing economics. Insufficient dryer capacity is often an impediment to taking advantage of the mechanical speed limit for an existing line, but adding additional dryer capacity can be prohibitive due to return on investment and/or space limitations. Optimizing oven settings becomes a critical path to improved economics for existing equipment. The relative contributions of humidity, air flow and volatile content to drying rate are somewhat counterintuitive and often overlooked when optimizing existing drying equipment. Minor differences in polymer composition and emulsion additives can and do impact drying rates as well. These principles will be explained in the presentation, with examples from different oven types and different emulsions, as a foundation to developing a clearer understanding of how to maximize the drying rate for a given water based pressure sensitive adhesive. Introduction The application of pressure sensitive adhesives to generate a useful finished product is a complex process. Whether hot-melt, solventborne, or waterborne, a material must be coated in a manner such that a PSA film of uniform thickness and composition is created with the necessary cohesive strength and PSA profile. Hot-melt adhesives must be applied using very precise heating profiles in order to obtain the appropriate rheology response for high quality PSA films to be formed. Solventborne adhesives are typically applied at room temperature and must be cured in order to generate the appropriate level of cohesive strength for a high quality product to result; in addition a solvent capture / oxidation system is needed to prevent large quantities of volatile organic compounds (VOCs) from being released to the atmosphere. Waterborne latex polymers, on the other hand, are applied at room temperature, typically require no curing or containment, and represent a high performance, high speed option for all segments of PSA applications. One of the requirements for successful coating of a waterborne PSA is the formation of a coalesced, strong polymer film during the removal of water from the PSA coating. The factors that influence the formation of this film and the speed with which it can be formed are the focus of this paper. Emulsion Polymerization Aqueous emulsion polymers are a form of polymer dispersed as discrete particles in water. The process of emulsion polymerization requires some basic ingredients: water, monomer, surfactant, and initiator. There are almost limitless combinations of these ingredients that can be used to tailor properties for specific applications. Emulsion polymers are primarily sold into Adhesives, Coatings, Floor Care, and Leather application areas. Monomer choices depend entirely on the end-use application. For adhesives, butyl acrylate, 2-ethylhexyl acrylate, and isooctyl acrylate are the primary monomers of commerce. Initiators can fall in one of two main categories: 1) Thermal and 2) Redox. The advantages of thermal initiators, such as ammonium and sodium persulfate, are the high degree of water solubility and ease of use; polymerization temperatures normally fall in a range of 75 to 90°C. Thermal initiators are more common than the redox alternatives. The main benefits of a redox initiation system are the ability to run polymerizations at lower temperatures and to minimize the number of charged groups built into the polymer backbone. The most common redox systems rely on iron as a catalyst, an oxidant (e.g. persulfate, hydrogen peroxide, and t-butyl hydroperoxide), and a reductant (e.g. Sodium formaldehyde sulfoxylate, sodium bisulfite). The progression of an emulsion polymerization is very well known. In the beginning stages, most of the monomer for the polymerization is present in monomer droplets. As the polymerization proceeds, the monomer diffuses from the droplets, through the water phase, and into the growing polymer particles. Toward the end of the polymerization, the majority of the monomer is present inside the polymer particles, and the monomer droplets have disappeared entirely. The final product of the polymerization is a discrete polymer particle of well-defined particle size. For adhesive applications, of course, additional formulation ingredients can be added in order to improve the wetting and foam-prevention properties of the latex product. The choices of polymer composition, process type, and formulation package all strongly impact the rate of drying as well as the ultimate film quality and PSA performance. Monomer Structure Tg Butyl Acrylate