Capillary Absorption Kinetics in Lightweight Coated Virgin Graphic Papers
Capillary absorption kinetics in lightweight coated virgin papers depend on pigment packing geometry, binder distribution, and sub-micron pore radii.
Groundwood Pulp
Fiber Separation
Mechanical disintegration produces groundwood pulp by pressing debarked logs against a rotating abrasive stone to tear fibers apart rather than dissolving chemical bonds. High yield characterizes this manufacturing method because almost the entire dry weight of the wood enters the finished stock without losing lignin fractions to cooking liquors. Mechanical energy breaks the structural integrity of the wood matrix, yielding short and rigid microfibrils that create a bulky network with high opacity.
Subsequent sorting screens separate acceptable fractions from coarse rejects, returning oversized fiber bundles to refiners for additional size reduction.
Opacity Retention
Printing papers utilize groundwood pulp primarily for high light scattering coefficients rather than mechanical strength. Lignin retention preserves stiffness in the wet web during high speed paper machine runs, but light exposure causes photochemical reactions that turn the finished sheet yellow over time. Low cost remains the primary economic driver for publishers selecting this substrate for high volume runs of newsprint and telephone directories.
Residual Staining
Bleaching sequences employ hydrogen peroxide or sodium hydrosulfite to increase brightness values without removing the color-causing lignin fractions that absorb blue wavelengths. High brightness targets require chemical additions that elevate production costs and alter the pH balance of the wet end during paper formation. Residual extractives interfere with sizing agents, demanding careful monitoring of wet end chemistry to prevent deposit buildup on dryer cylinders.