Cellulosic market paper pulp is manufactured by cooking a raw material of wood chips in suitable diges¬ tive chemicals with a view to removing the lignin of the wood and exposing its cellulosic fibres. The woody raw material may be both softwood and hardwood, however in most cases being softwood, e.g. from pine and spruce. After cooking, the delignified fibres are washed to re¬ move dissolved organic substance and residual digestive chemicals by being abundantly diluted in water so as to form a suspension which is passed on to a suitable dewa- tering device, generally a fourdrinier wire on which the fibres are dewatered, whereupon they are finally dried, e.g. by being subjected to a combination of pressing and heating operations. In many cases, the pulp is also bleached in a special bleaching step between cooking and drying, primarily in order to increase the bright¬ ness of the pulp. Such drying is conducted to dry so¬ lids contents considerably exceeding 80%. In practice, the dry solids content of paper pulp in sheet or web form is about 90% and that of fluff pulp about 92-93%. Dried paper pulp of the above generally described type is supplied to special paper mills where paper is manufactured (in some cases, there are also integrated manufacturing plants in which the pulp mill is integrat- ed with or directly connected to the paper mill. In these cases, the pulp is however not supplied in the dried state, but in a still water-suspended state from the pulp mill to the paper mill or paper machine). Tradi¬ tionally, papermaking is carried out in such a manner that the pulp, optionally together with one or more additional paper pulps, is slushed in water in a so- called machine vat so as to yield a stock which is

processed in different ways, primarily by beating in a mill or a refiner, before it is distributed via a head box or the like onto a forming wire (alterna¬ tively, a cylinder or the like) on which a coherent web of fibres bonded together is formed as the water in the stock drains through the wire. The web is there¬ after caused to pass a number of press rolls and a drying station in which it is dried so as to yield finished paper. For many applications, the paper requires colour¬ ing. This is done either by so-called stock or vat colouring or, alternatively, by surface colouring. Stock colouring, which is the commonest colouring method, is done by adding a colouring agent to the stock, always containing uncoloured pulp fibres, in a step somewhere between the slushing of the pulp in the machine vat and the forming on the wire, gene¬ rally as early as the machine vat, and fixing it to the fibres in a suitable manner. Surface colouring is carried out by adding a dye solution to the formed paper web when it has passed the drying station, for instance in a size press located after the drying station. Common agents for stock colouring are so¬ dium salts of organic sulphonic acids (C H _? , , S0 ₂ OH) and substantive dyes made up of straight-chain molecules which can form hydrogen bonds to the fibre surfaces. In practice, such stock colouring yields good results as to the task of providing a homogeneously vat-coloured paper at minimal costs. However, this technique is restricted to a single colour or shade throughout the paper. As compared with stock colouring, surface colouring is more expensive and involves many more problems. A serious drawback of the surface-colouring technique also is that it often yields a paper with different colour strength on the different sides.

Besides the conventional wet-forming technique related above, methods for dry-forming of paper have

also been developed in recent years. Dry-formed paper is manufactured basically in that the pulp supplied in sheet or web form (alternatively in fluff form) is shredded and finely divided, and incorporated in a flow of air which, like the stock, is caused to pass a form¬ ing wire on which a paper web of substantially uniform thickness is formed. As opposed to the case of wet- formed paper, this technique requires the addition of special binders serving to bond the separate fibres together so as to form a coherent web. In the dry-form¬ ing method, which is especially suitable for manufactur¬ ing a fluffy tissue paper having a structure which highly resembles textile, the colouring of the paper in particular is a problem which is difficult to solve. Thus, colouring is effected by spraying a suitable dye solution onto the finished paper web. In practice, this technique provides poor colouring throughout the paper material, generally yielding a more or less patchy paper with different colour strength on the different sides. Such tissue paper is unsuited for making e.g. napkins, cloths and the like, i.e. products on which the demands for uniform and homogeneous colouring are great.

The present invention primarily aims at creating conditions for allowing a simple and high-quality co- louring of dry-formed tissue paper and, to this end, relates to a dried market paper pulp manufactured from a raw material of wood chips, which pulp is character¬ ised in that the fibres therein are coloured by means of a colouring agent added to the fibres while they still are in a water suspension, but prior to final drying.

The invention further relates to a process for mak¬ ing market paper pulp, comprising the steps of cooking a raw material of wood chips for delignifying said raw material and exposing the cellulosic fibres therein, washing the thus exposed fibres by dilution in a water suspension and, after dewatering the fibres, drying them prior to sale. Characteristic of the inventive

process is that the fibres are coloured prior to the final drying, more particularly by adding a colouring agent in a manner known per se or in any optional man¬ ner to said fibre-water suspension, so as to produce a dried market pulp the individual fibres of which are coloured.

Colouring the pulp in accordance with the invention in connection with the very manufacture thereof gives the advantage not only that coloured paper can be manufactured by dry-forming without requiring any co¬ louring operation whatsoever in the paper mill or paper machine, but also that colouring can be carried out by conventional stock colouring technique, yielding a homo¬ geneous and durable colouring of the fibres. Another major advantage of the invention is that it allows manu¬ facturing not only a dry-formed but also a wet-formed paper of veined structure. Thus, according to a pre¬ ferred embodiment of the invention, the paper can be manufactured from two or more paper pulps at least one of which is coloured while the other is uncoloured or differently coloured.

The inventive pulp-making process can be carried out, for example in the manner appearing from the en¬ closed flow diagram. From the digester and different washing steps (and after optional bleaching) in a con¬ ventional fibre line, a suspension of cellulosic fibres and water (pulp concentration of about 1%) is pumped to a first mixer 1 in which a hardening agent is added to adjust the hardness of the water to 5° dH. This addition is made to achieve improved addition of a colouring agent supplied in a following mixer 2. The colouring agent may be an azo dye, for instance of the type commercially available under the trade designation CARTASOL ROT, which is dosed in an amount of 5% by weight based on the amount of dry pulp. After the addition of the colouring agent, this should have a residence time of about 5 minutes in a reaction vessel 3. A

fixing agent, for instance a quarternary polyhydroxy- alkylene polyamine sold under the designation CARTAFIX FF, is dosed at 4 in order that the last dye residues should be added to the cellulose and also in order to obtain sufficient colour fastness. Finally, the fibre suspension passes through a reaction vessel 5 in which the fibres supplied with fixing agent are allowed to remain for about one minute. The cellulosic fibres are now coloured. With present-day techniques, the defibering mills used in dry-forming plants require that the pulp be supplied with a debonding agent which reduces the fibre bonds when the cellulose is dried. The addition of such a debonding agent designated BEROCELL B-584 in an amount of 0.3% by weight is carried out in the mixer 6. The cellulose coloured and treated with debonding agent can now, after a residence time of about one minute in a reaction vessel 7, pass into a pulp drier 8 for conven¬ tional dewatering, final drying and reeling. The process described above can be used whether the pulp is bleached or unbleached. In the case of unbleached pulp, use is advantageously made in a con¬ ventional manner of a basic colouring agent containing an amino group ( - NH- ) while, in the case of bleached pulp, use is made of acid colouring agents, such as sodium salts of organic sulphonic acids, or of substan¬ tive dyes made up of straight-chain molecules which can form hydrogen bonds to the fibre surfaces. In the latter case, no fixing agent is required. It is also conceivable to use different types of pigment dyes which are insoluble in water and precipitate onto the fibre upon sizing with rosin and alum.

It should also be pointed out that a cellulose as above can also be manufactured without any debonding agent but in that case, the defibering mill of the dry- forming plant must have a considerably increased capacity. Although the colouring agent here discussed is added to

the fibres most advantageously while they remain sus¬ pended in water at low concentrations, e.g. about 1%, it is conceivable to add the colouring agent also at higher pulp concentrations, e.g. even up to about 50%, which may prevail in press sections after the wire. The main point is that the addition of the colouring agent is carried out prior to the final drying.