Paper Wet-Strength Improvement With Cellulose Reactive Size And Amine Functional Poly(Vinyl Alcohol)

What is this patent about?

'436 is related to the field of wet-strength additives for cellulosic paper. Paper, being naturally hydrophilic, requires additives to improve its resistance to water, especially for applications like liquid containers and printing. Traditional approaches involve adding substances to the pulp slurry (wet-end) or applying them to the dried sheet (dry-end) to enhance water resistance and overall strength. The invention addresses the ongoing need for improved wet-strength development in paper manufacturing.

The underlying idea behind '436 is the synergistic combination of an amine-functional poly(vinyl alcohol) (PVOH) and a cellulose-reactive size (like alkyl ketene dimer or alkenyl succinic anhydride) to significantly improve the wet strength of paper. The inventor discovered that the combination of these two additives results in a wet strength that is greater than the sum of their individual contributions, indicating a cooperative effect.

The claims of '436 focus on a cellulosic paper product containing both an amine-functional poly(vinyl alcohol) resin and a cellulose reactive sizing agent. The sizing agent is specifically defined as an alkyl ketene dimer or alkenyl succinic anhydride with substituents containing a combined total of at least 8 carbon atoms. The claims cover the composition of the paper product itself, defined by the presence of these two additives.

In practice, the amine-functional PVOH is preferably a copolymer of vinyl acetate and N-vinylformamide, hydrolyzed to introduce amine groups. This copolymer, along with the cellulose-reactive size, is added to the paper pulp slurry or applied to the paper sheet. The amine groups on the PVOH likely enhance its retention within the paper matrix, while the cellulose-reactive size forms covalent bonds with the cellulose fibers, imparting hydrophobicity. The synergistic effect suggests that the PVOH may also facilitate the sizing agent's interaction with the cellulose.

The invention differentiates itself from prior approaches by demonstrating a synergistic effect not observed with other combinations of wet-strength additives and sizing agents. For example, combinations of alkyl ketene dimer with cationic starch or unmodified poly(vinyl alcohol) do not exhibit the same level of wet-strength improvement. The use of a low level of amine functionality (1-25 mole %) in the PVOH is also key, as higher levels can be uneconomical or difficult to synthesize. This specific combination provides a more effective and efficient way to enhance the wet strength of paper compared to traditional methods.