Improvements in the application properties of water-based low VOC coatings

Abstract

This paper discusses a new class of acrylic latex (NeoCryl TX200) in which the colloid stabiliser is designed to interact with titanium chelates such as Tilcom AT23, thereby building up a weak gel structure which can be broken down by shear. This class of latex can be formulated with associative thickener (and their blends) in conjunction with titanium chelates, thus providing considerable formulation flexibility which cannot be achieved with conventional surfactant stabilised acrylic latex. A key feature of this new class of latex is that the colloid stabilisation system does not introduce water sensitivity, and indeed wet adhesion characteristics are fully comparable to those of state of the art adhesion promoted latexes. A further benefit of this class of latex is that of increased open-time, arising in part from its intrinsic rheological characteristics, and in part from the high film build achievable with thixotropic formulations. Rheological measurements show that introduction of titanium chelates greatly increases the low shear-rate viscosity of the system, and vane rotor measurements show that there is a near linear dependence of gel strength with titanate concentration. This gel, which is responsible for preventing in-can settlement of pigments and good brush pick-up, is partially broken down by the action of brush dripping, and is completely broken down during application (eg by brush or spray). The structure subsequently builds up with time, with gelation occurring in as little as 5–10 minutes, but with full recovery of gel strength taking in the region of 10–12 hours. The titanate chelating agent, even at high concentrations, does not contribute to the high shear-rate viscosity of the system, this being controlled by the addition of associative tickeners, either alone or in combination. The associative thickener(s) also control the low shear-rate viscosity during the period in which the gel structure has been sheared out, and thus good flow and levelling properties can be obtained through the optimisation of associative thickener type(s) and concentration(s). Although, as might be expected, cosolvents affect the rheological properties of formulations containing both associative thickener and titanium chelate, this effect is not a large one, and can be readily taken into account during formulation design.