Review: On the Nature of Joint Strength in Paper – A Review of Dry and Wet Strength Resins Used in Paper Manufacturing

The properties of paper are largely dependent on the bonds between the fibres. This is, of course, primarily true of those strength properties that are directly related to the number of bonds in the paper. Other properties are also dependent on such bonds, properties such as the opacity of the paper, its smoothness, porosity, dimensional stability, pore size distribution, linting propensity, density, stiffness, formation, and compressibility to mention a few. The normal way of affecting the number of bonds in a paper is through the choice of fibre material and through a correct beating of the pulp. It is true that properties of paper may be manipulated through the choice of beater type, its specific edge load etc to expand the property or process space in paper manufacture. There are still many limitations as to what can be achieved by beating and other process tools, so the practical paper-maker is continuously looking for ways to expand property and process space to be able to manufacture new products or boost paper machine productivity. In this review the terms “bonding” and “joint strength” will be used interchangeably. “Joint strength” includes both the adhesion zone (2D zone of bonding) and the cohesion zone (3D zone of bonding). Despite massive efforts over the years, our understanding of the molecular mechanisms of bonding is still in its infancy. There is still the fundamental argument as to the relative contribution of hydrogen bonds, ionic bonds, dipolar interactions, induced polar interactions, long-range van der Waals forces, and covalent forces (for wet strength resins) in various situations. Taken to the extreme, it was once believed that lignin contributed little to bonding in lignin-rich pulps, because they were assumed to be poor hydrogen bonding agents. Not anymore, as it has been realised that strong bonding can be created between mechanically liberated pulp fibres. Though critical experiments still need to be formulated to examine such matters, this review will not focus on them. It is acknowledged, that hydrogen bond theories have been formulated by Corte and Shashek (1955), Nissan and Sternstein (1964) and others, but it has not been possible to further expand our knowledge from the initial formulations. This review will instead focus on the use of various dry and wet strength additives to improve bond strength. The authors have made efforts to relate the discussion to the historical and current context of dry and wet strength resins, and to discuss more recent developments in understanding adhesive and cohesive failure. Hence, after some general considerations and introduction to the concepts of process and property space in paper manufacture, a brief discussion of current paper strength theories will be made. A more detailed account of adhesive and cohesive failure mechanisms will follow, after which dry and wet strength resins will be reviewed. As far as wet strength agents are concerned, traditional wet strengthening will be given less emphasis; the focus of this later part will instead be on potential chemistries to alleviate tensile creep and compression creep under moist conditions.