Reactions of lithographic papers to variations in humidity and temperature

Abstract

In studies previously reported, information was obtained relative to causes of serious economic waste in offset lithography resulting from misregister in color printing. At the request of the Lithographic Technical Foundation, which is cooperating with the Bureau of Standards, further studies were made to obtain scientific data on the reactions of lithographic papers to variations in atmospheric humidity and temperature. Paper is a hygroscopic substance. Its moisture content is determined by the hygroscopic condition of the surrounding air, and its properties, particularly dimensional, are governed to considerable extent by moisture content. Since precise register requires constant dimensions, the influence of atmospheric changes was closely studied. The dimensions of the papers varied directly with the miosture content regardless of the cause of the moisture changes. Moisture content was influenced by relative humidity, temperature, and history of conditioning Humidity changes were most important. A 10-point change affected the moisture content sufficiently to alter the dimensions enough to cause serious misregister. Temperature effects were less important, but of sufficient magnitude to call for close control in printing. History of conditioning had an important influence on equilibrium moisture content, hence on dimensions. The paper that gave best register in printing had the lowest coefficient of expansion in the machine direction, and the paper that gave the largest misregister had the highest. The papers with low expansion in the machine direction have correspondingly high expansion in the cross direction, which indicates the desirability of large directional difference in fiber formation of offset papers. The low coefficient of expansion in machine direction for the papers giving best register, indicated that distortion in printing which affects register is caused largely by moisture content variations. The usual sizing materials had little influence on the moisture content response of paper to changing relative humidity except with respect to the rate of change. Both surface sizing and mineral coatings increased the total dimensional changes per unit of moisture content change. The effects of ordinary atmospheric changes on strength properties were not of sufficient magnitude to be of significance in lithography. CONTENTS Page