Crosslinking
All polyvinyl alcohol grades are crosslinkable through their secondary hydroxyl functionality. Even lower hydrolysis grades—which are so exceptional on paper surfaces for oil, grease and organic solvent resistance and Gurley porosity—can be made water resistant. Degrees of water resistance vary from grade to grade. The table below shows the effect of glyoxal, a commonly used and favored crosslinker for polyvinyl alcohol. When glyoxal was added to Celvol grades 540, 350 and 165 polyvinyl alcohol at 20% dry-on-dry, significant water resistance improvements resulted. Note that the wet tensile of Celvol 540 polyvinyl alcohol increased from no measurable wet strength uncrosslinked to 6.1 pli when crosslinked. Also, the wet tensile of crosslinked Celvol 350 polyvinyl alcohol was more than double that of uncrosslinked Celvol 350, and crosslinked Celvol 165 polyvinyl alcohol was 28% higher than uncrosslinked Celvol 165.

A vast array of crosslinkers or insolubilizers are available. They include several classes: (1) aldehydes, of which glyoxal, a simple dialdehyde, is the most common, along with higher aldehydes, such as gluteraldehyde and hydroxyadipaldehyde; (2) thermosetting resins, such as urea-formaldehyde and melamineformaldehyde; and (3) salts of multivalent anions, such as zirconium ammonium carbonates.

More recently, there has emerged a growing interest in zero-formaldehyde, or low-formaldehyde-type crosslinkers. Two such products are Polycup 172, a water soluble, polyamide-epichlorohydrin-type resin, and Bacote-20, a zirconium ammonium carbonate salt. The results in the table below indicate that the addition of Polycup 172 to Celvol 165 polyvinyl alcohol at 5% dry/dry parts was as effective as glyoxal added at 20%, both resulting in a 26% wet tensile improvement. The addition of 5% Bacote-20 resulted in an 11% wet tensile improvement.

Effect of Glyoxal on Polyvinyl Alcohol Wet Tensile Strength Chromatography Base Paper*