The influence on paper strength properties when building multilayers of weak polyelectrolytes onto wood fibres

Polyallylamine hydrochloride (PAH) and polyacrylic acid (PAA) have been used to build up polyelectrolyte multilayers (PEM) on wood fibres and on silicon oxide surfaces, under various pH conditions. Consecutive adsorption onto silicon oxide surfaces of PAH and PAA were studied using stagnation point adsorption reflectometry, and the results showed a steady build-up of multilayers. Furthermore, by altering pH, the build-up of the multilayer could be made either linear or exponential in terms of adsorbed amount. Nitrogen analysis of sheets prepared from modified fibres showed that the adsorbed amount of PAH increased throughout PEM build-up, the amount of increase depending on pH during adsorption. Strength measurements of the sheets, i.e., stress at break and strain at break, showed significant improvements ranging from 60 to 200%, depending on both pH during adsorption and type of polyelectrolyte in the outer layer. A good correlation between the adsorbed amount of PAH and the improved strength properties of the paper was also found.