Using Optical Brightening Agents (OBA) for Improving the Optical Properties of HYP-Containing Paper Sheets

EXPERIMENTAL A softwood (mainly spruce) bleached kraft pulp (SWBKP), and a hardwood (eucalyptus) bleached kraft pulp (HWBKP) were refined in a PFI mill to 470 and 490 ml CSF freeness, respectively. Three grades of commercial aspen high-yield pulp (325/85, 325/83, 250/80) were obtained from Tembec Inc., without further refining treatment. The properties of the pulp samples are listed in Table I. igh-yield pulps (HYP), also known as bleached chemi-thermomechanical pulps (BCTMP), can be used to replace bleached hardwood chemical pulp in the manufacturing of highquality paper products, including printing and writing paper, due to their unique properties and lower cost [1-5]. Mechanical pulps have superior bulk and opacity compared with bleached chemical pulp, properties which are desirable for the production of printing and writing paper. Brightness is one of the most important quality criteria for mechanical pulps. Aspen HYP pulp can be bleached to 85% ISO or even higher brightness, but it is relatively expensive to do so. The production cost of HYP increases exponentially with brightness levels. In the pulp and paper industry, optical brightening agents (OBAs) or fluorescent whitening agents (FWA) are commonly used to improve the brightness of paper products [6-11]. Essentially all of this literature refers to bleached chemical pulps. Very few references are available which refer to application of OBAs on mechanical pulps. Since the substitution rate of HYP for hardwood chemical pulp in fine paper grades is usually at about 20% or lower, it was proposed that HYP with a brightness of a few units lower than 85% ISO might not affect the optical properties of final products if fillers, such as PCC, and optical brightening agents (OBA) are used. The production cost of HYP would decrease significantly if the brightness of HYP pulp can be lowered from 85% ISO to about 83% ISO without any sacrifice in the quality of final paper products. The objective of this research project included: i) to investigate the influence of adding OBA on the brightness of HYP-containing paper sheets, and ii) to determine the feasibility of using a Y. ZHOU Tembec Inc. H Abstract: High yield pulps (HYP) with different brightness levels were used to replace hardwood bleached kraft pulp to determine the influence of HYP substitution on the optical properties of paper products. The effects of OBA, PCC filler, and dyestuff on paper brightness, whiteness, and colour shade were investigated under various HYP substitution rates. The HYP substitution decreases the brightness and CIE whiteness of paper sheets, particularly at a high HYP substitution rate (e.g. 30%). The brightness loss due to the HYP substitution can be recovered by the addition of OBA and/or PCC.