Chapter 7 Dry Strength: Strategies for Stronger Paper Chen Lu, Scott Rosencrance, Darren Swales, Rosy Covarrubias, and Martin A. Hubbe 7.1. INTRODUCTION The dry strength of paper can be defined as the ability of a dry paper specimen to resist a variety of external forces. Common paper dry strength properties can be grouped into in-plane strength properties, out-of-plane strength properties, and structural mechanical properties, based on the type of external force. Paper is a web of cellulose fibers. Many of the strength properties of paper are directly related to fiber packing density, bonding strength among fibers, and the strength of individual fibers. Paper strength is also a function of paper’s uniformity of formation. Failure modes for dry paper are often initiated from a defect-based heterogeneity in sheet structure. This chapter focuses on the various chemical approaches to intensify inter-fiber bonds. By strengthening these bonds, the overall paper product is imparted with more desirable characteristics. To begin the discussion, the next section describes some of the key challenges that modern papermakers face with regards to paper strength. The third section concerns the ways in which paper fails, along with methods for the evaluation of paper strength. The final two sections deal with conventional and non-conventional ways to increase paper’s strength, again with the emphasis being placed on the use of chemical additives at the paper machine wet end. 7.2. CHALLENGING CIRCUMSTANCES FOR PAPER STRENGTH The paper industry has developed several different strategies to optimize paper dry strength properties. Stronger paper can be produced by increasing paper basis weight, which is the weight per unit area of paper. However, because the cost of wood is a large portion of the total cost of making paper, simply increasing basis weight is rarely cost- effective. Desirable dry-strength properties are often achieved by balancing the use of dry-strength chemical additives and optimization of papermaking process mechanics. Lightweighting is an ongoing trend in the industry. This term is used when paper companies prefer to maintain the existing strength properties while lowering paper basis weight. A variety of trends in the industry can lead to reduced paper strength. These approaches include increasing contents of fillers [Li et al. 2002] and increased usage of higher-yield fibers. Both of these trends are motivated by the desire to decrease fiber costs. Specifically, kraft fibers produced at a relatively high yield (~ 61%) exhibit distinctly lower paper strength properties compared with the corresponding fibers pulped to a more typical yield (57%) [Gärdlund et al. 2005]. Furthermore, high-yield pulps, such as groundwood and thermomechanical pulp (TMP), often must be complemented by kraft fibers to meet the final paper strength requirements [Korpinen and Fardim 2009]. In addition to trends of high-yield pulp and filler content there is a growing number of paper products being produced utilizing recycled fiber sources. A significant portion of container board product is produced using old corrugated container (OCC) pulp, which is a recycled pulp from used brown container board. In many cases, the OCC pulp has gone