GB1572519A - Process for the manufacture of mechanical pulp - Google Patents

Description

(54) PROCESS FOR THE MANUFACTURE OF MECHANICAL PULP (71) We, PAPIRINDUSTRIENS FORSKNINGSINSTITUTT, a Co-operative Research Institute of Forskningsveien 3, Oslo 3, Norway, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a process for the manufacture of dry mechanical wood pulp.

Mechanical pulp is a general term used for pulp obtained by mechanical defibration of different wood species. It can be produced by the use of grinding stones or disk refiners, optionally after mild chemical treatment.

As mechanical pulp is a relatively inexpensive product compared with cellulose pulps obtained by chemical digestion of wood, such pulp has been widely used as a raw material for the manufacture of newsprint and magazine paper. However, mechanical pulp possesses the undesirable property that it undergoes "self sizing" on storage, causing it to exhibit hydrophobic properties to a greater or lesser degree, dependent on storage time, rosin content, and storage conditions. This makes re-wetting difficult when the pulp is to be further processed, and greatly restricts the use of mechanical pulp for absorption purposes, for instance in napkins, sanitary pads, tampons, towels and similar products. For such purposes high quality bleached celluloses have previously been necessary.

It has been shown that if the rosin can be removed from the mechanical pulp, such pulp can advantageously be utilized for the above mentioned purposes. Norwegian Patent No.

124,193 discloses that the rosin can be removed from the pulp by extraction with acetone, whereby a product with good absorption properties is obtained. Acetone extraction of mechanical pulp has also been suggested to improve the strength of mechanical pulp and to facilitate wet defibration in a subsequent process (Brandal & Lindheim, Pulp & Paper Mag.

Canada 67 T 431, 1966). However, an industrial extraction process is a relatively complicated and expensive process requiring large capital investment in extraction apparatus, and solvent recovery apparatus.

In Norwegian Patent Specification No. 136583, there is disclosed a process for treatment of a mechanical pulp in order that the hydrophilic properties of the pulp be maintained after ageing. A mechanical pulp treated in accordance with Norwegian Patent Specification No.

136583 will during storage remain hydrophilic and can thus easily be defibrized during subsequent processing. It has further been shown that such pulp is eminently suitable for absorption purposes, in spite of the fact that the new treatment affects the rosin content of the mechanical pulp to only a minor degree.

In accordance with Norwegian Patent Specification No. 136583 the mechanical pulp is treated or impregnated with a buffer buffering in the pH range 6 - 10, preferably in the range 7 - 9. Suitable buffers are alkali metal salts and ammonium salts of, for instance, phosphoric acid, carbonic acid, citric acid and tartaric acid. The buffer, preferably in the form of an aqueous solution, is added to the mechanical pulp in such amount that the pulp will contain at least 0.2 % by weight of the buffer, based on the dry weight of the pulp, preferably 0.5 % by weight or more.

Such treatment of the mechanical pulp with an aqueous solution of the buffer can be effected at a low price and in a simple manner. The addition can be effected at different stages in the production of the mechanical pulp, depending on the field of use of the product and available equipment. The addition can for instance take place during the manufacturing step in the refiner or grinder, in the white water system in the paper machine, in the size press, by spraying during or after the sheet formation, injection in the pulp before bale formation. The treatment can thus be effected with existing equipment and thus utilised in existing processing plants.By means of that process, absorbing materials can be manufactured from mechanical pulps, which means a substantial saving compared with highly refined cellulose products, and from a resource and environment point of view, also, that process is advantageous.

It has now surprisingly been found that it is possible to manufacture a mechanical pulp capable of maintaining its hydrophilic properties without effecting the separate buffer addition described in Norwegian Patent Specification No. 136583.

The present invention accordingly provides a process for the manufacture of dry mechanical wood pulp wherein wood raw material, in the presence of an alkali metal hydroxide or a precursor thereof, is mechanically defibrated at a temperature in the range from 50 to 1500C, the alkali metal hydroxide or precursor being introduced before or during mechanical defibration, so as to react alkali metal hydroxide with organic acids, formed from the wood lignin and carbohydrates, at 50 to 1500C, thereby forming in situ a buffer buffering in the range of pH 7 - 10, whereafter the resulting buffered pulp is dried with the pH remaining in said range during the drying.

We have found that alkali metal hydroxide, conveniently the sodium hydroxide resulting from mild chemical pre-treatment of the wood chips with sulphite solutions, or from injection of sulphite in the refiner, can react with organic acids, formed from the wood lignin and carbohydrates during treatments at 50 to 1500C, to form appropriate amounts of buffer systems buffering in the range pH 7 - 10. These buffer systems are thus obtained in situ as by-products in the pulp.

All mechanical pulps submitted to such mild chemical treatment are normally subjected to post-treatment by washing with water and subsequent pH adjustment to pH 5.0 - 5.5 using sulphuric acid or sulphur dioxide. That pH adjustment shifts the pH range of the pulp out of the desired range, so that the pulp will no longer maintain its hydrophilic properties on storage.

The buffer systems formed in the pulp as a consequence of the mild chemical treatment can be directly retained by omitting the subsequent washing treatment, or the washing treatment can be effected with weak aqueous alkali, or, if it is desired to lower the pH e.g.

to the preferred range of 7 - 9, the washing of the pulp can be effected utilizing suitable weak acids. More preferably, the pH of the washed pulp will be reduced to a level in the range 7.0 - 8.5. Phosphoric acid, carbonic acid, citric acid or tartaric acid, or mixtures thereof, can be used to increase the amount of buffer material in the pulp.

A mechanical pulp can thus be obtained containing a suitable amount of buffer buffering in the desired pH range of 7 - 10, preferably in the range 7 - 9, whereby the obtained pulp will remain hydrophilic and not undergo "self sizing" during storage.

The desired effect can also be attained in conventional grinding in a grinder if the pH of the shower is kept in the alkaline range. This can for instance be effected by utilizing the washing water from a peroxide bleaching stage as shower.

The mechanical pulp containing the buffer system formed in situ can be dried directly in a conventional manner or it can be transformed to the desired product such as a tissue product by utilizing a sheet forming machine having an alkaline reacting white water system.

The invention is illustrated by means of the following Example. Different mechanical pulps were prepared and- examined with respect to their water absorbing properties. The prepared pulps were dried in a cyclone and fluffed (in a Wennebeg-mill). The fluffed pulp was then artifically aged at 1500C for 1 hour. whereafter the pulps were allowed to remain at room temperature for 30 - 60 min. before the water absorption rate of the aged pulps was determined. The water absorption rate for non-aged pulp and pulp not prepared in accordance with the invention was also determined.

The water absorption rate was determined by evenly distributing 10 g of air dried, fluffed pulp in a small basket of metal gauze (dimensions 7 cm x 20 cm x 1 cm) which was placed in a container containing 200 ml water. The time taken from the moment the basket was placed in the container until the pulp was completely moistened by the water is a measure of the water absorption rate of the pulp.

Example I A refiner pulp was prepared by soaking 25 kg spruce chips overnight in 220 1 of an aqeuous solution containing 10% by weight NaOSO3 (pH 8.6) for 12 hours at 20"C, and refining the impregnated chips in a disk refiner with a power consumption of 800 kWh/ton.

A part of the resulting pulp (in the following designated as la) was dried and the absorption rate determined before and after ageing.

A part of the pulp was thoroughly washed with water (pulp lb) and subsequently dried and the water absorption rate determined.

A further part of the pulp was washed with SO2 containing water (pulp 1c) in such way that the pH of the pulp was lowered to 5.5, whereafter the pulp was dried and the water absorption rate determined.

The resulting water absorption rates are shown in the subsequent table.

Water absorption rate, seconds/l0 g pulp Pulp After 1 hour number Pulp type: Before ageing at 1050C Refiner pulp from spruce la Dried directly at pH 8.6 2 2.5 1b Dried after a mild 9 52 washing with water lc Dried at pH 5.5 20 130 It can be mentioned that the absorption rate for a conventional refiner pulp of spruce before and after ageing is 8 - 14 seconds and 90 - 600 seconds, respectively.

From the above results it can clearly be seen that mechanical pulp prepared in accordance with the present invention has a high water absorption rate, even after artifical ageing at 105"C for 1 hour.

WHAT WE CLAIM IS: 1. A process for the manufacture of dry mechanical wood pulp, wherein wood raw material, in the presence of an alkali metal hydroxide or a precursor thereof, is mechanically defibrated at a temperature in the range from 50 to 1500C, the alkali metal hydroxide or precursor being introduced before or during mechanical defibration, so as to react alkali metal hydroxide with organic acids, formed from the wood lignin and carbohydrates, at 50 to 1500C, thereby forming in situ a buffer buffering in the range of pH 7 - 10, whereafter the resulting buffered pulp is dried with the pH remaining in said range during the drying.

2. A process according to claim 1 wherein sodium sulphite is used as the precursor for the alkali metal hydroxide.

3. A process according to claim 1 or 2, wherein the pH of the pulp, if above 9 is adjusted to 7 - 9 by the addition of phosphoric acid, carbonic acid, citric acid or tartaric acid, or mixtures thereof.

4. A process according to claim 1 substantially as hereinbefore described with reference to the Example.

5. Mechanical pulp obtained by a process according to any one of the preceding claims.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   and the water absorption rate determined.

   A further part of the pulp was washed with SO2 containing water (pulp 1c) in such way that the pH of the pulp was lowered to 5.5, whereafter the pulp was dried and the water absorption rate determined.

   The resulting water absorption rates are shown in the subsequent table.

   Water absorption rate, seconds/l0 g pulp Pulp After 1 hour number Pulp type: Before ageing at 1050C Refiner pulp from spruce la Dried directly at pH 8.6 2 2.5 1b Dried after a mild 9 52 washing with water lc Dried at pH 5.5 20 130 It can be mentioned that the absorption rate for a conventional refiner pulp of spruce before and after ageing is 8 - 14 seconds and 90 - 600 seconds, respectively.

   From the above results it can clearly be seen that mechanical pulp prepared in accordance with the present invention has a high water absorption rate, even after artifical ageing at 105"C for 1 hour.

   WHAT WE CLAIM IS: 1. A process for the manufacture of dry mechanical wood pulp, wherein wood raw material, in the presence of an alkali metal hydroxide or a precursor thereof, is mechanically defibrated at a temperature in the range from 50 to 1500C, the alkali metal hydroxide or precursor being introduced before or during mechanical defibration, so as to react alkali metal hydroxide with organic acids, formed from the wood lignin and carbohydrates, at 50 to 1500C, thereby forming in situ a buffer buffering in the range of pH 7 - 10, whereafter the resulting buffered pulp is dried with the pH remaining in said range during the drying.
2. 2. A process according to claim 1 wherein sodium sulphite is used as the precursor for the alkali metal hydroxide.
3. 3. A process according to claim 1 or 2, wherein the pH of the pulp, if above 9 is adjusted to 7 - 9 by the addition of phosphoric acid, carbonic acid, citric acid or tartaric acid, or mixtures thereof.
4. 4. A process according to claim 1 substantially as hereinbefore described with reference to the Example.
5. 5. Mechanical pulp obtained by a process according to any one of the preceding claims.