(730g) Polymer Applications in Pulp and Papermaking- A Review

For many practical uses the water and moisture sensitivity of ordinary paper proves to be an insurmountable obstacle. Water leaf paper show excellent dry strength properties, but almost all of this strength is lost when the paper is wetted by water because dry strength is due primarily to hydrogen bonding (and probably some ionic bonding) and these bonds are destroyed by water, which is very effective at forming hydrogen bonds itself. Therefore, development of wet strength requires the formation of different types of bonds which are not adversely affected by exposure to water .The earliest method for developing wet strength was to fuse the cellulose fibers to one another, either by heating the paper to very high temperature or by partially solubilizing the paper with 75% sulfuric acid. The later is the method by which parchment paper is produced. The first chemical method for improving wet strength involved the use of formaldehyde, which can form methylene bridges between hydroxyl groups on the surface of cellulose fibers.

Wet strength is usually reported in terms of the ratio of wet tensile to dry tensile strength, expressed as a percentage. Paper possessing a wet tensile strength greater than 15% of dry tensile is considered to be wet strength papers.

 

The most important wet-strength agents are applied by beater or wet end addition; i.e., they are added to the pulp before the sheet is formed at the “wet-end” of the machine. Aluminium sulphate is often added to cure resin or to retain anionic Urea-formaldehyde resins. Additives that are not substantive to (absorbed by) paper fibers must be added to the paper after sheet formation. This procedure is usually less cumbersome than addition to the paper stock.

Saturation of paper with water reduces its strength to about 3-10% of its dry strength. Animal glue decreases the rate of absorption of water: if the paper is treated with both formaldehyde and animal glue it will have true wet strength. Treatment with 75% sulfuric acid produces “vegetable parchment,” which has high wet strength, but the treatment seriously changes other sheet properties. By means of modern commercial resins, wet-strength of 20- 50% of dry strength can be obtained. Addition of 1% resin can cause a three to six fold increase in wet strength.