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3.6.4.4 Requirements and Measurements of Sized Papers
The purpose of sizing is to modify the surface of the fibers to control penetration of aqueous liquids into the paper. The penetration usually correlates with the ab¬sorbency, repellency (hydrophobicity), and spreading of the aqueous liquid con¬cerned. Control of these important properties may be required for three pur¬poses:
1. 1. Control of the penetration rate of the aqueous phase in a converting operation such as size press treatment, coating, glueing (influences also the machine runnability).
2. 2. Control of liquid absorption or wetting in a printing process.
3. 3. Control of the serviceability of many grades of paper and board, e. g. milk/juice carton, packaging papers, wallpaper, printing writing papers, etc.
Besides a certain degree of water and ink resistance the papermakers and their customers often look for improvements in dimensional stability, surface strength (pick and rub), internal bond, linting, dusting, stiffness, smoothness, porosity, or friction coefficient, depending on the converting process and use of the paper.
Several test methods are used to measure the degree of water and ink resis¬tance:
. • Water drop absorption: the time required for defined droplets of water to be ab¬sorbed by paper.
. • Ink flotation test: the time required for writing ink to penetrate through a floating piece of paper and to change the color of the upper surface.
. • Hercules sizing test: the decrease in reflectance of the opposite side of a paper sheet which has been covered with a given amount of ink.
. • Cobb test: the amount of water absorbed by a given area of paper in a specified period of time.
. • Pen and ink feathering test: the extent of spreading of lines drawn on a paper sample with a steel pen and ink.
. • Contact angle: the tangential angle from the horizontal which the base of a drop of liquid develops when carefully placed on a paper sheet.
. • Edge penetration/edge wicking test: the lineal penetration of liquid in the in-plane dimension of the paper.
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