SIZING 117
the size is so much diluted as to be resolved into its component parts, free
resin and sodium hydroxide. In the same way, hydrolysis of the sulphate of
alumina produces, in its place, sulphuric acid and aluminium hydroxide. The
sulphuric acid and sodium combine to some extent to form neutral salts, thus
leaving as effective agents for sizing the free resin particles and particles of
aluminium hydroxide.
Now, research into electro-kinetics has shown that some substances are
charged with positive and some with negative electricity. Substances of opposite
charges attract each other. It will at once be seen that this has a vital bearing
on the subject.
Fibres suspended in water are charged negatively, so also are the minute
particles of resin. Therefore the addition of resin particles to the beater will
not result in the fibres being covered by the resin. They will be repelled
instead, and anyone who has watched the behaviour of stuff and resin size in
a beater before the alumina has been added will at once perceive that some
disturbing action is taking place which seems to be too great to be accounted
for by the formation of gas, causing an increase in bulk.
The particles of aluminium hydroxide, on the other hand, are charged with
a positive charge. On the addition of the alum solution the disturbance in
the beater subsides and the stuff contracts in bulk. The cellulose has attracted
to itself the positively charged aluminium, and as this is always added in excess,
it may be concluded that the negatively charged fibres have now become
positively charged, or at least coated with the positively charged aluminium
particles. Thus the fibres are now in a condition to attract the negatively
charged particles of resin, when we may assume that the action has ceased.
The excess of alumina assures that no resinous particles will remain
unattracted by the fibres or by the alumina particles.
On passing over the machine, some of these wandering particles of resin
and alumina will pass through the wire with the water, but the bulk remains
on or between the fibres, forming an ink-resisting coating or filler for air
spaces.
It has generally been assumed that the resin size must be first added to the
stock in the beater, so that it may be 'beaten into the fibres', but this theory
seems to indicate that the opposite would be more effective. Indeed, many
mills do hold this to be the case, and put the sulphate of alumina solution into
the beater first, to neutralise the hardness of their water, and find their sizing
improved thereby.
Bewoid Size.—For years paper-makers have been looking for a method by
which resin may be incorporated in the finished sheet without the use of alkali
and alum. Until the advent of the Bewoid sizing process the nearest approach