The effect of additives of industrial importance in modern high speed papermaking is reviewed. These sizes, adhesives and fillers will be detrimental to formation, consolidation, physical and optical properties of paper or board, if they exist in a tightly aggregated condition and are poorly distributed in the fibrous product. Data are presented to illustrate suitable dimensions of the solid particles to supply the desired property with the least amount of addition. The retention of the additives without sacrifice of their efficiency is a difficult problem.
Alumina precipitates can decrease paper strength to very low values, but can increase it again with additional aluminium sulphate. The absence of sulphate ions permits gain in strength above the starting value. Anionic polyacrylamide, carboxymethyl starch, urea-formaldehyde and cationic melamine-formaldehyde resins become considerably more effective with a closely controlled alumina compound. Recent studies on the degree of neutralisation of alum and the advantage of a moderate cationic charge in rosin sizing are reviewed, as well as an explanation for the improved efficiency of fortified rosin and synthetic sizes. Bridging by polyacrylamide molecules, the sorption and movement to fibre contact areas by molecules and macromolecular fragments from cationic starch are discussed.
Evidence is presented for the agglomerated state of pigments in paper and electron micrographs establish that titanium dioxide can be retained at about 0.25 u. Modern criteria for pigment evaluation are given. Debonding reduces pick resistance in offset printings,but helps in the ability to run letterpress paper.