The properties of the materials combined in corrugated
board are set to give the board its strength, flexibility
and protection towards impact and pressure. The raw material is
of course the single fibre but additives that enhance e.g. the wet
and dry paper strength are also added. As the strength
enhancing additives used today are most efficient between pH 4
and pH 7 there is a need for new types of additives that can be
used under alkaline conditions. In the present report polyallylamine
hydrochloride (PAH) and polyelectrolyte complexes
(PEC) of PAH and polyacrylic acid (PAA) were investigated as
strength enhancing additives. The components can be used
under alkaline conditions and results showed that PAH alone or
incorporated into a PEC gave different effects regarding
strength properties of sheets from kraft pulps of different yields.
The results showed that by treating the fibres with PEC´s of
PAH and PAA it was possible to considerably increase the
tensile properties, the Z-strength and the compression strength
of papers made from the treated fibres. The results showed an
increase of as much as 54 % to 180 % in dry Z-strength and 14
% to 53 % in compression strength, when using the PEC as an
additive, indicating an increase in inter-fibre joint strength. It
was also found that a heat treatment of the non-treated papers
and of the papers from the PAH treated fibres gave a large
improvement in tensile properties as well as Z-strength properties
for the different pulps. For the PEC treated fibres it was not
necessary to heat-treat the paper to achieve a higher dry
strength. The molecular mechanism behind the large improvements
is not identified in the present work but the problem is
currently being investigated in the lab of the authors.