Mini-Encyclopedia of Papermaking Wet-End Chemistry
Additives and Ingredients, their Composition, Functions, Strategies for Use


Composition: "ONP" pulp consists mainly of old newspapers. But also it includes coated inserts and a moderate level of other types of paper and contaminants. The newsprint portion is mostly mechanical fiber, containing most of the dry content of wood. In the original production of the pulp the wood was converted either by grinding (the traditional approach) or by thermomechanical refining of chips (TMP). These mechanical pulping processes produce a wide distribution of fibers and fragments of fibers. A minority of the newsprint paper may have been upgraded by hydrogen peroxide bleaching, hydrosulfite bleaching, or by the addition of some clay to improve the brightness and opacity. Because ONP may have been exposed to sunlight, some of it may be considerably more yellow than fresh newsprint pulp. The most common form of ink applied to newspapers consists mainly of carbon black particles dispersed in a heavy oil. Only in the case of premium newspapers does the ink contain soybean oil or additives to promote drying or setting. Rather, the oil mainly absorbs into the fibers. The surface charge of ONP, after it has been dispersed in water, tends to be highly negative due to the presence of wood extractives, dispersants from the coatings on various glossy inserts, and byproducts of peroxide bleaching. Dirt and stickies are partly removed, in most cases, by screening and cleaning.

Function: The main component of recycled newspapers. Also used in other recycled paper and paperboard products where low cost is a main concern.

Strategies for Use: The papermaker may need to modify the process in order to run ONP in place of virgin mechanical pulp. The extent of these modifications depends greatly on how well the pulp has been washed, screening, and cleaned after repulping. Talc and other detackifying agents may be increased in order to handle the variable load and tackiness of the mixture of latex, polyvinylacetate (from labels), and wood pitch. It is likely that the furnish will have different drainage characteristics; again this depends on the extent to which fine materials may have been washed from the furnish during the removal of contaminants. De-inked newsprint pulp may contain surfactants, including fatty acids that are not unlike those originally in the wood. Papermakers have a range of options, including treatment with highly cationic polymers (for charge neutralization and to improve drainage), cationic starch (for strength), and retention aids (to keep the system clean). Also there is likely to be an increased requirement for biocides.

Cautions: A program of safe operating procedures in the mill will include the safe handling of ONP.

Sun exposure yellows mechanical fiber   Cartoon illustrating the rapid yellowing of mechanical fiber exposed to sunlight. Clipping of a coupon has revealed a non-yellowed sheet below the top sheet.

PLEASE NOTE: Users of the information contained on these pages assume complete responsibility to make sure that their practices are safe and do not infringe upon an existing patent. There has been no attempt here to give full safety instructions or to make note of all relevant patents governing the use of additives. Please send corrections if you find errors or points that need better clarification.


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This page is maintained by Martin Hubbe, Associate Professor of Wood and Paper Science, NC State University, .