Troubleshooting Tip: Cobb Test

Paper is composed of a randomly felted layer of fiber, its structure has a varying degree of porosity.  Therefore, the ability of fluids, both liquid and gaseous, to penetrate the structure of paper becomes a property that is very significant in 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.  The term "sizing" was later 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 Cobb Test measures surface water absorption over 60 seconds, expressed in g/m2.  The procedural Standards are explained in TAPPI T 441.

There are several actions you can take to improve Cobb Test numbers.  An increase in refining can help close the sheet, which would improve Cobb results.  A change in furnish can also help.  You would increase broke content if possible and/or increase hardwood content.  These fibers are smaller and can fill the holes or pores in the sheet.  If you have alum at your disposal, increasing the addition rate could help.   Also increasing pH can help close up the sheet.  Be careful when carrying out any of these actions.  You increase the risk of sealing the sheet by closing up the sheet.