Synthetic dry-strength additives

The addition of a cross-linking agent, glyoxal, to polyacrylamide molecule increases both dry-strength and wet-strength. Such chemically modified additive forms covalent bonds with the hydroxyl groups of cellulose. This additive is used e.g., in the production of paper towels or labels, where temporary wet-strength is required. 

If no or insufficient cationic functional chemical is present, then a separate cationic prompter/fixing agent is needed to ensure 

---

Today, the most common types of synthetic dry strength additive are based on polyacrylamide although polyvinylamine strength additives are growing in popularity. This is because of the relatively low cost, and the ability to readily put an ionic charge onto these polymers. Other resins such as polyvinyl alcohol or lattices are used, but generally these are seen in surface applications to the paper, rather than as wet-end additives. Similarly styrene acrylic resins have shown superior performance when applied through surface application rather than as stock additives [8],... 

Relatively low molecular weight polymers such as polyacrylamides or polyvinylamines are widely used as synthetic dry strength additives. These offer both economic and quality benefits to the papermaker that could not be obtained through increases in refining or wet pressing. Additions of active resin as low as 0.2% can achieve increases in dry strength of up to 50%. Collateral benefits can include increases in retention, drainage, machine runnability, productivity and reduced raw material costs. 

Dry strength additives are usually water soluble, hydrophilic natural or synthetic polymers, the commercially most important of which are starch, natural vegetable gums and polyacrylamides. These polymers are often made in cationic form by the introduction of tertiary or quaternary amino groups into the polymer, and are therefore polyelectrolytes. They are thus also able to function to some extent as drainage and retention aids. 

Figure 5 shows how the addition of 0.5% of oxidized neutral guar to bleached kraft pulp (50% hardwood, 50% softwood) results in a 27% increase in dry strength. The oxidized guar actually performs better than Kymene 557H synthetic resin of Hercules Incorporated. This clearly shows that also neutral oxidized guar has a lot of potential in the production of high performance papers. 

More than 2 million tons of this polymer is produced annually in the United States. Dacron, the fiber produced from this polyester, accounts for approximately 50% of all synthetic fibers. It absorbs very little moisture, and its properties are nearly the same whether it is wet or dry. Additionally, it possesses exceptional elastic recovery properties so it is used to make permanent-press fabrics. This polyester can also be made into films of great strength (e.g.. Mylar), which can be rolled into sheets 1/30 the thickness of a human hair. Such films can be magnetically coated to make audio- and videotapes. 

---