Control of degree of sizing through measurement of contact angle and surface energy

Paper during lithographic printing, printing with water based inks, gluing operations and paperboard converting, and water based coating requires controlled hydrophobicity. Contact time of aqueous solutions with paper during all these applications is of milliseconds. Industries use various methods e.g. Cobb & Hercules size test to measure surface wettability of paper. The most widely used test in Indian paper industries is Cobb60wherein a water column is rested on the paper surface in 60 seconds. Interaction of water molecules with paper in the time scale can not be determined through the water absorption test i.e. Cobb test. On the contrary, the rate of absorption of water into the paper surface with respect to time is of paramount importance. The measurement of contact angle and surface energy are the advanced methods in this respect to determine the hydrophocity of paper. Understanding the behavior or interaction of water molecules in contact with paper surface is highly essential to control the sizing of paper. Attempts have been made to observe the wettability of paper through the measurement of both contact angle and Cobb60tests, and find the relationship between the two. In this regard, paper handsheets prepared in laboratory, sized with different sizing chemicals viz. rosin, AKD and ASA, as well as various commercial writing & printing paper samples were analyzed. The effect of filler viz. talc, GCC and PCC has also been shown on the sizing behavior of paper. Generally, the contact angle value decreases with increase in Cobb value. In case of some commercial paper samples, it has been found that the contact angle may not be the same at same Cobb values. In some cases, the contact angle values were higher even at higher Cobb values. The values of contact angle were different with different sizing chemicals even at same Cobb values. This study opens-up an arena to select the right contact angle for a particular grade of paper and fix-up the degree of sizing.