WOOD PULP 65
alpha-cellulose, the further removal of pentosans, oxycellulose, and hydro-
cellulose tends to raise the whiteness, stability of colour, cleanliness, chemical
purity, permanence, tear, softness of fibre, and resistance to hydration. Soft-
wood sulphite fibre is very suitable for high-grade bond, parchment, and other
strong papers, and the beating characteristics become closer to those of rag
stock. Hardwood fibre of exceptional softness, bulk, opacity, and brightness
is used in mixture for fine-textured papers and boards. Sulphate fibre at
moderate whiteness gives the toughest high-grade products. In this range also
are purified pulps designed for photographic paper, absorbent tissue, moulded
products, and high-grade viscose.
The scope of sulphate pulps has been greatly extended by including bleached
grades in addition to unbleached and semi-bleached. As is well known, the
general characteristics are high strength, resistance to beating, freedom from
pitch, relatively high freeness, and colour varying from dark brown to good
white. It should be remembered that not only are breaking length and burst
of sulphate pulps superior to the best that can be reached with sulphite parallels,
but also the tear and fold are outstanding. The properties of sulphate are
more standard for each grade than in the case of sulphite, because the alkali
cooking has a more specific action on the wood fibre with less hydration of
cellulose and pentosans.
Unbleached grades include kraft, light and strong (*L. & S/) kraft,
easy bleaching, and aspen sulphates, according to the usual European
classifications.
Kraft pulp is the well-known, extra-strong brown fibre used in the toughest
grades of paper and board. The best brands for fine wrapping papers and
tissue must be cooked for a fairly long period from fine-fibred woods such as
northern pine or spruce to reach the stage of well-separated fibres. Fairly
long beating time must be expected with kraft pulps.
Light and strong kraft represents somewhat softer cooking to reach brighter
colour. The tear is high, but the fibre has lost some of its bursting strength.
Speciality grades made from kraft or *L. & S/ kraft include pulp which
has received chemical treatment and thorough washing to reduce the electric
conductivity "to a very low figure for the manufacture of condenser tissue,
and pulp which has been treated to swell the fibres for use in absorbent products
such as roofing felt.
Easy-bleaching sulphate is reached by more thorough cooking to yield
pulp which will bleach by single-stage treatment with 12 to 18 per cent bleach.
Bleached in this way, the find colour cannot be expected to equal the white-
ness of bleached sulphite, but the high tear, good bulk, and good opacity are
very useful for soft printing papers.