Tailoring PSA-Dispersion Rheology for High-Speed Coating
Successful aqueous PSA dispersions combine outstanding adhesive properties such as tack, peel strength and shear resistance with excellent defect-free coating properties. Cost-effective production of adhesive tapes, labels or foils demands ever-increasing machine speeds. A careful control of dispersion rheology in a wide shear rate range is mandatory for good runnability and coating quality. Typical shear rates relevant for different aspects of the coating process1 are shown in Figure 1. A well-balanced viscosity level at shear rates below 1 s-1 is necessary to provide excellent leveling and minimize dewetting simultaneously. The viscosity at intermediate shear rates between 10 and 103 s-1 determines not only the stirability and pumpability, but also the transfer to the roll or cylinder. The highest shear rates occur at the coating head. On a curtain coater, typical shear rates are between 102 and 104 s-1. In gravure coating systems, shear rates of more than 105 s-1 are reached. The fastest coating line at BASF's coating center in Ludwigshafen, Germany, runs at speeds of up to 1,500 m/min, corresponding to shear rates up to 106 s-1.
The challenge is always to minimize the high shear viscosity while maintaining the highest possible solids content to reduce drying time and energy consumption, i.e., cost.