Paper Manufacture

PAPER MANUFACTURE. The problem of the paper 'maker is to make a felted web, fabric or tissue of cellulosic fibres and of appropriate uniform thickness, strength, colour and surface. For this purpose the fibres are "beaten" in water, i.e., cut and bruised, so that the cellulose composing them may be come more or less "hydrated," by imbibition of water, to form a wet "pulp" (see PAPER MATERIALS) of adequate cohesive quality: this wet pulp, as a thin even layer, is then put on to a wire fabric or screen to allow the removal of the superfluous water, and thereafter the wet web of pulp, thus formed, is finished and dried in appropriate manner to produce the paper, in the form in which it comes to the consumer. These operations may be done by hand or by machine : in olden days the beating was actually done by hand process, with a pestle and mortar, but it is nowadays invariably done by machine : the making of the felted web, on the wire, is still done by hand, but only for making the very highest class of so-called "hand made" papers, and it involves a very skilful craftsmanship which attains and maintains its highest development in the "hand made" paper industry of Great Britain.

These are the essentials of the paper making process : but in practice there are also additional auxiliary operations such as tinting, sizing, loading, calendering, etc. The surfacing of paper by the process of "sizing" is sometimes done by hand, in the so called "tub sizing" process, but otherwise it is done by machine— "engine sizing." The processes of tinting, loading with mineral matter, and calendering (glazing) are also generally machine processes. Paper making, as a whole, is therefore now almost entirely a machine process : the vast output and low prices of the industry would be otherwise, of course, quite impossible.

Half Stuff.

The paper maker has a variety of cellulosic ma terials available for his "half stuff," i.e., the mixture of say 3-5% of loose fibres suspended in water, from which to make the paper by the paper making process proper. If he starts with a cellulose material fully purified by removal of non-cellulosic substances, and felts it together, with a minimum of further treatment, he obtains a product which is, in effect, of filter paper or blotting paper type : this sort of paper has little strength, it is absorbent, it is uniform in quality from surface to surface, i.e., it has no added surface quality. In order to make paper of this character the half stuff is given a minimum of treatment in the breaker or beater, i.e., the knives are sharp and cut up the fibres quickly

into suitable short lengths with only a little compression, bruising and comminution : then, when the wet pulp thus obtained is thrown on to the wire to be made into a sheet it parts with much water very easily, and is said to work very "free." In practice a certain amount of compression, bruising and com minution of the fibres is almost always necessary, or the result ing paper will be uneven in texture and hardly capable of holding together at all. The pulp is therefore, even in this instance, "beaten" to a suitable small extent which makes it slightly co hesive, retain more water, and work just sufficiently wet : the cellulose itself is only to some small extent "hydrated" (see CELLULOSE : Hydrated cellulose). For a very "free" paper such as filter paper or blotting paper this small amount of light beat ing and hydrating of the cellulose is as much as is required to give the product its distinctive character. The beating and hy dration may, on the other hand, and if required, be continued un til finally a very slimy mass is produced in which the fibrous character of the pulp has almost completely disappeared : from it a paper of parchment-like character and considerable strength can then be made. These are the two extreme types obtainable by beating and hydration : intermediate types are also, of course, likewise obtainable proportionately. But a number of other auxiliary treatments are also necessary in order to provide the great varieties of qualities and properties required by modern conditions. Thus, special kinds of surface are required: for writing, the paper must not be too penetrable to moisture and the ink must not spread : the capillaries of the cellulose fibres, for this purpose, are therefore protected against excessive penetra tion, by "sizing" with more or less water-resistant substances such as rosin, gelatine, with or without auxiliaries such as casein, starch and mineral loadings and thus the felt or web is closed, covered and compacted. The paper can be also tinted, if re quired, and pressed between "calender" rollers, that can be heated, to give a smooth, glazed surface; or it can be "coated," by the addition to its surface of appropriate mineral matter, and an ad hesive, and by treatment with "super calender" rollers.