Paper360 - March/April 2012 - (Page 38)

OBA/COLOR CONTROL MILL WISE Sappi Biberist Tests a NEW INLINE SENSOR to Control OBAs and COLORS Over-dosage is avoided and results correlate with the pope scanner and laboratory KLAUS KUNSCHERT, HEINZ ZIEGLER and HELMUT LEYERER T he appearance of graphic paper is determined by its brightness and color which is adjusted by optical brightening agents (OBAs) and dyes. OBAs absorb non-visible UV light (with a wavelength of 300-400 nm) and reflect this as visible blue light with a wavelength between 400 and 450 nm. OBAs are generally anionic, water-soluble and colorless. These brighteners are all based on Diaminostilbene-disulfonic acid and can be categorized according to the number of sulphonic acid groups (Di-, tetra- or hexa-sulphonated). Brighteners can be dosed batch-wise or continuously into the pulp suspension as well as used in surface treatments. In grades with a high brightness demand, OBAs are added both internally and to the surface. The efficiency of optical brightening agents depends on the dosing point and even mixing as well as other process parameters such as the furnish used, fillers, the pH-value, sizing agents and other cationic additives. OBAs and dyes are normally added according to scanner values of the pope roller. This common practice has many flaws including the delay between dosage of additives and the actual measurement, risk of over-dosage upon grade changes, as well as missing scanner data during sheet breaks. To help measure and control OBAs and dye, Sappi Biberist installed an inline BT-5400 UV brightness transmitter on their PM6, in combination with a control loop. They also wanted to see if it was possible to avoid overdosage and correlate with brightness and color results of the pope scanner and the laboratory. Paper360º MARCH/APRIL 2012 HOW DOES THE BT-5400 UV WORK? There are approximately 14 laboratory methods for brightness/whiteness and color measurements. Among the most common ones are base brightness, CIE brightness (W10), R457 brightness (ISO brightness), fluorescence measurement and the CIE L*a*b* color space. All are optical methods where the finished paper sheet is examined. Laboratory measurements are regularly performed to verify the produced paper quality. They are also necessary for calibration purposes for other equipment (pope scanner, inline sensors). The BT-5400 UV brightness transmitter is designed for monitoring and control Figure 1. BT-5400 brightness transmitter. Figure 2. Installation point in a simplified flow sheet of PM6. 38

Table of Contents for the Digital Edition of Paper360 - March/April 2012

Over the Wire . . . News Summary
Asian Innovation on the Rise
SPECIAL FEATURE: Tissue Industry
North America Shifts to Specialization
A Measured Success
Aligning Rolls in a Paper Machine Winder
Understanding Lightweighting
Sappi Biberist Tests a New Inline Sensor to Control OBAs and Colors
Process Control for Stickies
An Innovative Yankee Coating Program
Biopolymers in Papermaking
Best Practices in Product Development
Employee Work Restrictions Challenge Human Resources
Paper360° Online Exclusives
Association News

Paper360 - March/April 2012