The Rheological Properties of Paper and Pulp Sheets

Abstract

The explanation of rheological properties of paper and pulp sheets in terms of their structure has hitherto been considered to be very difficult, seeing that they are composed of a lot of macroscopic elements or fibers, though it seems rather easier than those of other materials composed of microscopic elements or molecules. Therefore, rheological studies in this field have not been very actively made in spite of practical importance of these materials. In this article, the rheological studies by the author, especially those on non-destructive elastic and viscoelastic properties, have been reviewed along with the related studies by other investigators.There are two distinct lines in the theories relating the rheological and other physical properties of paper to its structure. Nissan's works are typical of the first of the two. He assumes that the strain of paper under stress is derived from the extension of hydrogen bonds. The author and several others, assuming that paper consists of randomly arranged three-dimensional array of fibers, have put forward theories for elastic and other properties, assuming also that the deformation of paper arises from the bending, stretching and shearing of the fiber segments between the bond sites. The latter theories can presuppose many important facts, some of which have been known experimentally or empirically, though imperfectly yet.Paper is a typically orthotropic material, and its Young's modulus and tensile strength in an arbitrary direction are well represented by the Horio-Onogi equations (8) and (9), which describe a peculiar elastic property of paper, as was pointed out by Campbell and Craver and Tayler. The correlation between the density and modulus of elasticity, the effects of humidity and several manufacturing variables such as beating, wet press, screening and drying on rheological properties of papers and pulp sheets are also discussed. The viscoelastic data reported by the author reveal that paper has essentially the same viscoelastic properties as cellulosic fibers such as viscose and Bemberg rayons if, and only if, they are corrected for the density. This indicates that the junctions between the constituent fibers are very strong and similar to those between the cellulose molecules as was assumed in the above theories, while the macroscopic structural factor should also be taken into account.