22.214.171.124.5 Additives Influencing the Quality and Printability of the Paper Surface
126.96.36.199.5.1 Co-binders and Thickeners
The structure formed by the pigments plays an important part in determining the physical properties of the coating and, because co-binders and thickeners have a high affinity for pigments, they play an important part in influencing the structure of the coating by controlling dewatering. The physical properties of the coating that are affected most by the use of synthetic co-binders and thickeners are the smooth¬ness and porosity and the brightness of coatings that contain fluorescent whiten¬ing agents. Careful attention needs to be paid to all these factors when the paper or board is to satisfy all the market demands made on its gloss and print gloss, ink uptake and holdout, glueability, brightness and opacity, water resistance, and ab¬sence of mottling. Synthetic co-binders also help to bind pigments to each other.
They largely contribute to the pick strength of the coating. The amount of binder employed in the coating can be reduced if large proportions of synthetic co-binder are employed. Generally speaking, two parts of co-binder can be used to replace one part of binder. They also increase the water resistance of coatings. Once they have formed a film, they become firmly integrated within the structure of the coating and are no longer soluble in water.