The main contribution to opacity has to come from the pigments. Synthetic prod¬ucts are usually less opaque than natural products and this can be a problem, especially when coating ULWC (ultra light weight) grades of paper. Products with a very pronounced thickening effect are only added in small quantities, so they hardly have any effect on opacity.
Influencing Smoothness and Gloss
The gloss of paper largely depends on its smoothness. High gloss depends on the evenness of the topology of the paper surface. Like all hydrocolloids, co-binders and thickeners have an effect on the smoothness and the gloss of the paper. They are able to migrate in the wet coating, and migration can lead to them becoming unevenly distributed on the surface. They can also absorb water and swell, which also impairs the smoothness of the paper.
Synthetic co-binders are usually less detrimental to gloss than natural products. One of the reasons for this is that natural products are able to absorb moisture and swell after they are dried, whereas acrylic polymers are much less sensitive to moisture once they have dried to form a film. Another reason is that acrylic polymers are highly thermoplastic, and respond very well to calendering. The films formed by natural products are thermosetting, and so they are not deformed as easily under heat and pressure in the calender nip.