The present invention relates to a soft, bulky absorbent paper. Soft paper or tissue paper, which is used in household paper products, toilet paper, serviettes, paper handkerchiefs, face tissues, etc., can be produced from a number of different types of pulp fibres. When disregarding recycled fibres, which at present are used to a large extent primarily in the manufacture of toilet paper and household paper, and considering solely the use of primary fibres, chemical pulp is the most usual type of pulp used in the manufacture of soft paper. Chemical pulp is produced by impregnating wood chips with chemicals and thereafter cooking the chips so that the lignin, resins and hernicellulose present pass into the liquor used. Upon completion of the cooking process, the pulp is screened and washed prior to being bleached, Such pulp contains practically no lignin and the fibres, which consist substantially of pure cellulose, are relatively slender and flexible. Chemical pulp may be comprised of either long fibres or short fibres, depending on the nature of the wood used, and may be either a sulphate pulp or a sulphite pulp, depending on the composition of the cooking liquor. Chemical long-fibre pulps, particularly sulphate pulps, have a favourable influence on the strength properties of soft paper with regard to both dry strength and wet strength.
Chemical pulp is a low yield pulp, since it gives a yield of only about 50% calculated on the wood starting material used. Chemical pulp is therefore a relatively expensive pulp. Consequently, less expensive so-called high-yield pulps have been used in soft paper, for instance mechanical or thermomechanical pulps; see GB Patent Specification 1,533,045 in this regard. Mechanical pulps are produced by grinding or refining the wood starting material. The principle applied in the manufacture of mechanical pulp is to break down the wood mechanically. All of the wood material is used and the lignin thus remains in the fibres, which are relatively short and stiff. Thermomechanical pulp (TMP) is produced by refining the wood in disc refiners at an elevated steam pressure. The lignin also remains in the fibres of thermomechanically produced pulp.
Chemithermomechanical pulp (CTMP) is the designation given to a thermomechanical pulp that has been modified by adding small quantities of chemicals, usually sulphite, which are added prior to the refining process. The admixture of a given amount of chemithermomechanical pulp in soft paper has positive effects on such properties as bulk and absorbency.
All of the aforementioned pulp types are at present used in the manufacture of soft paper.