Mechanical pulping

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A cross-sectional view of kenaf is shown in Figure 3d. The ultimate cells are nearly cylindrical with thick cell walls. Kenaf fibers are shorter and coarser than those of jute. Both chemical (kraft) and mechanical pulps have been produced from kenaf, and successful demonstration mns of newsprint have been made for the Dallas Morning Nem, the St. Petersburg Times, and the Bakersfield Californian with a furnish of 82% kenaf chemithermomechanical pulp and 18% softwood kraft pulp. Kenaf fiber is also considered a substitute for jute and used in sacking, rope, twine, bags, and as papermaking pulp in India, Thailand, and the former Yugoslavia. RoseUe bleached pulp is marketed in Thailand. 

Most of the hydrogen peroxide use in Canada is at more recendy constmcted large chemithermal mechanical pulp (CTMP) mills. These modem mills can consume 5000 10,000 t/yr of hydrogen peroxide. 

Certain chemical treatments can be employed during the TMP process to achieve improved strength. Sodium sulfite and hydrogen peroxide have been used either for chip pre- or post-treatment of the TMP pulp such pulp is called chemithermomechanical pulp (CTMP). The strength improvements, which may be 50%, are obtained at some sacrifice to yield and opacity. The yields of mechanical pulps are 90—95% the lower yields are associated with chemical treatment. No principal commercial pulps are produced in the next lower yield range, ie, 80—90%. 

The retention of fillers in the sheet during the forming process is important. Both hydrodynamic mechanisms and colloidal or coflocculation phenomena are significant in determining filler retention (7). Polymeric retention aids are used to bridge between filler particles and fibers. Talc is sometimes used with mechanical pulp furnishes in order to reduce the deposition of pitch-like materials onto paper machinery. 

A high yield chemical pulp, eg, 52—53% bleached yield from softwoods, can be obtained, but strength properties ate inferior to those obtained from the kraft process. If a protector, eg, potassium iodide, is added, an additional 2—3% yield is obtained, as is an improvement in all strength properties. The gas penetration problem can be minimized if ftbetization is accompHshed before treatment with oxygen. Oxygen treatment of virtually all types of semichemical and mechanical pulps has been explored (55). Caustic, sodium bicarbonate, and sodium carbonate have been used as the source of base (56,57). In all cases, the replacement of the kraft by these other processes has not been justified over the alternative of pollution abatement procedures. 

Peroxide Stage. Peroxides in the form of both hydrogen and sodium peroxide have been used to bleach mechanical pulps since about 1940. 

R. A. Leask and M. J. Kocurek, Pulp and Paper Manufacture, Vol. 2, Mechanical Pulping, 3rd ed.. The Joiat Committee of the Paper Industry, TAPPI/CPPA, Adanta, Ga., 1987. 

A smaller but important use for sulfur dioxide is for stabilization of pulp (qv) brightness after hydrogen peroxide bleaching of mechanical pulps. Sulfur dioxide neutralizes the alkalinity and destroys any excess hydrogen peroxide, which if left in the pulp would cause it to lose brightness. 

Basic dyestuffs are usually used for dyeing of unbleached pulp in mechanical pulp such as wrapping paper, kraft paper, box board, news, and other inexpensive packaging papers. Their strong and brilliant shades also make them suitable for calendar staining and surface coloring where lightfastness is not critical. 

Weiss-schliff, m. white ground wood, (Paper) white mechanical pulp, -siedekessel, m. blanching copper. weis(s)sieden, v.t. blanch. 

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