Calendering of coated paper at high speeds and temperatures

Increasing calendering speeds are challenging paper mills to further develop their coating recipes and calendering conditions to maintain adequate surface properties. In particular, the trend towards higher calendering speeds is putting pressure on mills to raise roll temperatures. At high temperatures the viscoelastic behavior of latex becomes increasingly significant and has an important impact on surface properties. The significance of the gel content and glass transition temperature (T_g) of latex in coating and calendering was studied. The results indicated that at constant T_g the high gel content latex generated a denser, more closed coating structure than medium or low gel content latex. Low gel content latex produced the glossiest but most porous coating layer. In terms of the gloss increase achieved by calendering, it is best to use low T_g latex at low calendering temperature and high T_g latex at high temperature. Latex particle size also had an effect on paper properties. Smaller particles produced a rougher and less glossy surface. The results clearly showed that many coating properties after calendering were related to the coating properties before calendering.