Method of making mechanical pulp

Details Method of making mechanical pulp Method of making mechanical pulp

1987-05-28

1988-12-06

Lacey, David L.

Paper making and fiber liberation

Apparatus

Digester

162 26, 162 28, D21B 116, D21D 130, D21K 326

Patent

active

047894296

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

The present invention relates to methods of manufacturing mechanical pulps. More particularly, the present invention relates to methods of manufacturing mechanical pulps from lignocellulose-containing materials, and particularly in which the pulps are intended for the production of coated paper having a low grammage, such as so-called LWC paper (light weight coated), magazine paper or similar such paper.


BACKGROUND OF THE INVENTION

In connection with the production of such coated papers or LWC papers, the properties of the pulps so utilized must meet certain very strict requirements. The reason for this is primarily because the paper must have high density, low roughness, low porosity, as well as high strength. Furthermore, a uniform surface structure of these papers is of particular importance.
Paper of this type normally includes both chemical and mechanical pulps. The traditional mechanical pulp component used in this paper has been a groundwood pulp. However, as an alternative, thermomechanical pulp (TMP) has been used in recent years, but with limited success. Experience has thus proven that the employment of such TMP as a mechanical pulp component, even at low freeness values, results in a certain unevenness in the surface structure of the paper. This, in turn, gives rise to poor coating and, thus, to unacceptable printability. It has only been possible until now to avoid these problems by the paper manufacturer taking special steps to either modify or eliminate entirely the negative effects of that fraction in the thermomechanical pulp which constitutes long fibers. This long fiber fraction thus includes long, rigid and unworked fibers, which have a negative effect on the surface structure of the papers produced therefrom.
Most recently, chemically modified TMP (CTMP) has arisen as an attractive alternative to TMP as a component of the mechanical pulp in such LWC-papers and papers of similar quality. It has thus been found that CTMP improves the strength and binding properties thereof, and permits a reduction in the aforenoted problems with long fiber fractions. This is due to the fact that the mild chemical pretreatment used therein substantially improves both the defibering capability of the wood material and the tendency of the fibers in the long fiber fraction of the pulp to swell. These fractions, in turn, provide for a lower shives content and improved flexibility and ductility for the long fibers in CTMP, as compared to TMP.
Both the TMP and CTMP intended for use in LWC-papers and the like are generally manufactured by refining in one or more steps, and then screening and bleaching the refined material. The energy consumption in such a process is relatively high, and considerable equipment for dewatering and washing is required. Therefore, the search has continued for improved methods of producing such papers.


SUMMARY OF THE INVENTION

In accordance with the present invention, these and other objects, including simplification of the equipment and reduction in the energy consumption while maintaining, or even improving, pulp quality and the production of such coated papers and the like have now been accomplished. More particularly, in accordance with this invention, this has now been accomplished by applicants' discovery of a method for producing a mechanical pulp product useful for the production of coated paper and the like from lignocellulose-containing material which includes preheating the lignocellulose-containing material in the presence of impregnation chemicals so as to produce a preheated lignocellulose-containing material, refining the preheated lignocellulose-containing material in a first refining step employing a pair of counter-rotating refining discs under pressure so as to produce a partially refined lignocellulose-containing material, bleaching the partially refined lignocellulose-containing material in the presence of bleaching chemicals of high concentrations so as to produce a bleached lignocellulose-containing material, and refining the bleach

REFERENCES:
patent: 3016324 (1962-01-01), Textor
patent: 3388037 (1968-06-01), Asplund et al.
patent: 3446699 (1969-05-01), Asplund et al.
patent: 3467574 (1969-09-01), West
patent: 4279694 (1981-07-01), Fritzvold et al.
patent: 4294653 (1981-10-01), Lindhahl et al.

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