Modeling Drying of Aqueous Pressure Sensitive Adhesive Coatings

Water-based pressure sensitive adhesives are typically latex suspensions and are often coated onto silicone-coated paper prior to being contacted with the face stock. It is necessary to control drying conditions to achieve low residual moisture in the PSA without formation of defects, and mathematical models can be effective tools to determine appropriate drying conditions. We will present two mathematical models of drying PSA coatings. The first model is a three-layer model of moisture transport in a PSA coating on silicone-coated paper. Moisture transport in each of the three layers is predicted by diffusive transport models. Moisture transport through the silicone layer into the paper leads to faster initial drying. The paper acts as a reservoir for moisture, and at long times moisture can transport back from the paper into the coating. The second model is a particle dynamics model of latex aggregation during the initial water evaporation stage. The model predicts particle arrangement as a function of rate of evaporation and predicts conditions under which long range ordering occurs. Introduction Drying of a coating applied on a silicone-coated paper is a critical step in the manufacture of pressure sensitive adhesive materials (PSA). In this process, a thin release layer of silicone (di-methyl siloxane) is first coated onto a paper substrate (or “liner”), dried and cured. The siliconized coating is then overcoated with an aqueous pressure sensitive adhesive coating and dried. The drying rate of the PSA layer often controls maximum line speed given the size of the adhesive oven. A model for drying of a coating applied on silicone-coated paper is obtained here by combining distinct models for moisture transport in the coating, silicone layer and paper substrate. From experiments it was observed that the drying of the system of interest (acrylate-based PSA) is accurately modeled by Fickian diffusion. Therefore, the Fickian diffusion equation was used to model the moisture transport in the coating. The solubility of water in silicone is very small, but its mobility is high ((Banerjee, Asrey et al. 1997)). The moisture transport through the silicone layer is modeled by a pseudo steady state model. Paper is a hygroscopic porous substrate exhibiting a dual scale pore structure: large pores between the fibers and smaller pores in the lumen ((Lampinen and Ojala 1993)). Water transport takes place on both length scales by several mechanisms. Hartley and Richards (1974) modeled the liquid and vapor transport in paper using the diffusion equation with an effective diffusion coefficient(Hartley and Richards 1974). Their model neglects the effect of the porous structure of the paper on moisture transport. Bandyopadhyay et. al. (2002) accounted for moisture transport in the both the paper fibers and the pore space(Bandyopadhyay, Ramarao et al. 2002). This model with some modifications is used in this paper to model the moisture transport. The pressure sensitive adhesive system of interest is an emulsion of latex particles suspended in water. During drying latex coatings pass through three stages of film Drying of Emulsion Coating + + Emulsion Coating Water evaporation Impermeable substrate Emulsion Impermeable substrate Water evaporation Paper Silicone Layer Water flux from emulsion coating Water flux to paper Pseudo-steady moisture transport in Silicone layer Drying of Paper 3-layer Coated paper drying Emulsion Coating Water evaporation Water evaporation Paper Silicone Layer x Xc