Development of a fast brightness testing method for mechanical pulp based on microwave oven drying

Brightness is an important quality parameter for pulp products, and it is important to have reliable measurement of pulp brightness in a timely manner for process control and/or quality control purposes. In these circumstances, a quick testing method for pulp brightness is highly desirable. A rapid handsheet brightness testing method for lignin-rich mechanical pulp has been developed, which is based on the use of tap water to make handsheets and microwave ovens to rapidly dry the handsheet. Microwave oven fast drying decreased the handsheet brightness of mechanical pulp by 5–6 points due to the lignin-originated discoloration reactions. The spray of ascorbic acid and ethylenediaminetetraacetic acid (EDTA) solutions to the handsheet can effectively inhibit these lignin discoloration reactions. With 0.2% ascorbic acid and 0.2% EDTA spraying on the wet pulp handsheet, the brightness of the handsheet from a peroxide-bleached stone groundwood pulp after the microwave oven fast drying method was similar to that obtained from the same pulp but following TAPPI Standard Test Method T 272 sp-12 “Forming handsheets for reflectance testing of pulp (sheet machine procedure)”. The effect of handsheet dryness on the handsheet brightness was also studied, and the results showed that the brightness reading was almost constant in the dry-ness range of 70% to 90%. The method developed is a reliable, fast brightness testing method for lignin-rich pulp that is of practical interest in industrial operations.