The basic constituent of paper is cellulose fiber, but fillers are added to alter one or more of a paper's properties, depending on the desired end-use characteristics. Loading modifies such paper properties as texture, opacity, brightness, basis weight, dimensional stability, ink absorbency, and overall printability. It is important for fillers to scatter light well to increase opacity, impart high brightness, be nonabrasive, and not be chemically reactive with either the printing system to be used or with any proximate end-use materials.
The most commonly used fillers are clay and titanium dioxide, the latter of which is ideal for increasing opacity. Calcium carbonate is used in producing alkaline papers. Other fillers that are used on occasion are talc, calcium sulfate, barium sulfate, hydrated alumina, silicas or silicate pigments, and zinc pigments. Fillers added for internal sizing include acid or alkaline rosin and alum. Gums, starches, and synthetic polymers are added to aid in sheet formation. Dyes and pigments are added to alter the color of a paper. In writing and printing papers, filler content can range from 5 to 30% of a paper's weight.
Although fillers primarily improve the optical properties of paper, it is not without compromising other properties. Increasing the amount of fillers in general increases brightness, dimensional stability, ink absorbency, opacity, and smoothness, but it can decrease a paper's strength and stress endurance. Fillers reduce paper bulk and stiffness, which may or may not be desirable.
Adding fillers is a function of the intended printing method and end-use requirements of the paper. (See also Paper and Papermaking: Paper Properties.)
In typography, the term filler refers to any extraneous or less important material added to a page to fill out a short column.