Sizing

Purpose
Papers are sized in order to hydrophobize the material in a targeted manner, i.e. to achieve controlled liquid absorption. Originally, sizing reduces the absorbency of paper materials, while simultaneously influencing their strength-related properties like bending stiffness, braking length and tear growth resistance, as well as “rattle”-the sound made when shaking sheet of material.

Principle
In order to observe the interactions between paper and liquid, test inks are frequently applied.
Measuring techniques :
Ink stroke test : assesses the ink bleed when writing on a paper with drawing pen in a definer manner
Ink flotation sizing test : a paper specimen is floated on a writing ink and the time for ink to penetrate the paper is determined visually
Sizing test, e.g. with a PLG sizing tester : similar to the ink flotation sizing test, this tester method measures the time of ink penetration by means of a photocell

Sizing / Cobb
Because paper is composed of a randomly felted layer of fiber, it's structure has a varying degree of porosity. Thus, the ability of fluids, both liquid and gaseous, to penetrate the structure of paper becomes a property that is both highly significant to the use of paper. The need to limit the spreading of ink resulted in "sizing" the paper with gelatinous vegetable materials which had the effect of sealing or filling the surface pores. Later, the term "sizing" was applied to the treatment of paper stock prior to the formation of the sheet, with water-repellent materials such as rosin or wax. Resistance towards the penetration of aqueous solution / water is measured by Sizing or Cobb values.
The surface water absorption over 60 seconds, expressed in g/m2, measured by Cobb Test. The procedural Standards are explained in TAPPI T 441.