Preparation of Paper Coatings

Paper coatings impart a variety of properties, both decorative (whiteness, opacity, etc.) and functional (smoothness, absorption rate, etc.) A typical coating has two principle constituents: a dispersion of pigments in water and an adhesive or binder. 

Traditionally coatings were based on clays such as china clay (kaolin) and bentonite, however, in modern paper coatings calcium carbonate is more common. Other materials are also used to obtain certain properties. These include: 

• Talc - for smoothness.
• Silica - for applications requiring good water absorption rate - e.g. ink jet paper.
• Titanium Dioxide - for whiteness and opacity.
• Resins - control of ink absorption rate.

The Process

The clay or other pigment is normally supplied in bulk tankers as slurry containing 60% solids. Some paper manufacturers prepare the slurry themselves, dispersing the powder into water with conventional agitation. This is a relatively simple duty, however long mixing times may be required to produce each batch. Basic coating preparation can be summarized as follows:

• The slurry is diluted to the required working strength and held in a storage tank.
• The binder is prepared separately before being blended with the slurry.
• Other additives such as waxes, coloring, etc. may be added at this stage.
• The finished coating may be passed through a filter to remove agglomerates or impurities.
• The coating is then applied to the paper or board by a number of methods.