Relating Extender Size to Gloss

As discussed, the particle size of the extender is important in controlling both gloss and gloss retention during drying of the coating film. This is illustrated , where the 20° gloss measured for a decorative alkyd gloss paint is shown as a function of drying time for different barium sulphate grades. The total PVC for this paint system was 21.5% (including the extender), with a primary pigment volume concentration of 2.5%.

The gloss value increases significantly as the extender Dv97 decreases. This relates to improved packing of the extender particles within the paint film, leading to a more uniform surface finish. Furthermore, the rate of degradation of the gloss value over time (caused by film shrinkage during drying) is also reduced . This shows how important it is to consider the extender particle size as well as the pigment particle size during product development and manufacture.