Wet-strength development of paper: modification of cellulose fibres by adsorption of a natural biopolymer

Wet strength is one of paper’s most important properties. It can be developed by addition of wet strength additives during the fabrication process. Hereby paper products that require high wet strength, such as kitchen towels and bank note paper, are achieved.

This study aims to examine new polymers for future use as wet-strength additive in paper. One of them, a natural biopolymer, has in earlier studies proved to work well as wet strength additive. The objective was to find out in what extent this biopolymer could improve wet strength. A final evaluation was based on mechanical testing of sheets made from bleached Kraft fibres modified by the biopolymer.

Wet-strength polymers are used to develop or conserve the mechanical strength of paper when wetted. Wet-strength agents are added in various paper products such as; hand towels, hygiene paper and packaging grades. According to the chemical composition of these agents they can act either as protective agents by preventing fibre swelling and protecting already existing bonds, or they form new and water resistant bonds through reinforcement mechanisms.

The natural biopolymer has a structure that is very similar to the cellulose structure. This fact together with the reactive functions of the biopolymer probably makes it possible to adsorb it to cellulose fibres.

Pre-treatments of fibres were done by adding 0.5 and 2% biopolymer to the paper pulp. The adsorption was evaluated by measuring the surface charge of pre-treated fibres.

This most popular wet-strength today is PAE (poly(aminoamide)- epichlorohydrin)) but it has some drawbacks. PAE makes paper stiffer and decreases the absorption capacity which is useful in packaging products but not in tissue paper. Other drawbacks of PAE are its bad repulpability, degradability and toxic monomers.

Author: Nordell, Patricia

Source: Lulea University of Technology

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