Paper machine clothings are long, wide belts that serve to shape and transport the paper web in a paper machine. They are endless, and circulate in the paper machine over rollers. The outer flat side constitutes the paper side, since the paper web is transported on it. The inner flat side is referred to as the machine side, since it runs over the rollers of the paper machine.
A paper machine has substantially three parts, also called sections. In the first section—the sheet-forming section—forming wires are used as the paper machine clothing. These are generally made of fabrics that are produced from monofilaments. The purpose of the forming wires is to form the paper web and to dewater the paper web through the forming wire. In the next section—the press section—the paper web is subjected to high pressure for purposes of additional dewatering. The press section uses, in particular, press felts that comprise a woven or knitted support and a nonwoven fabric needle-felted thereonto (cf. U.S. Pat. No. 4,943,476). In the subsequent dryer section, the paper web is thermally dewatered, being guided over heated rollers by means of dryer felts. Dryer felts exist in numerous embodiments, principally as woven fabrics (cf. U.S. Pat. No. 4,621,663) although knitted fabrics have also been proposed, or as so-called wire element belts (cf. DE 24 19 751 C3, U.S. Pat. No. 4,796,749, U.S. Pat. No. 5,534,333). These wire element belts are made up of coils, extending transversely to the machine direction, that are coupled to one another via coupling wires. In addition, there are also proposals to assemble a paper machine clothing from a plurality of planar element rods, extending transversely to the machine direction, that are connected to one another via coupling wires (cf. DE 37 35 709 C2; EP 1 035 251 A1). Also known are paper machine clothings which are made of plastic layers that are porous or provided with openings (cf. EP 0 817 886 B1; U.S. Pat. No. 4,541,895; WO 92/17643).
In addition to the aforesaid paper machine clothings, there also exist auxiliary belts, such as transfer belts for transferring the paper web from one section to another, and shoe press belts. These, like the aforesaid paper machine clothings, are adapted to the respective purpose in terms of their surface conformation and structure.
Leaving aside felted materials as paper machine clothings, the surfaces of such paper machine clothings are constituted by plastic elements. In the case of woven and knitted fabrics, these are yarns, in particular monofilaments. In the case of wire element belts (cf. U.S. Pat. No. 4,796,749) the surfaces are constituted by the coils. The same applies to paper machine clothings made up of planar element rods or extruded segments (cf. DE 37 35 709 C2; EP 1 036 251 A1). Paper machine clothings that have a plastic layer or are made up of such a layer on at least one side (usually the paper side) have very smooth surfaces (cf. EP 0 817 886 B1).
The problem exists in this context that contaminants in the form of small particles settle on the paper sides of the paper machine clothings, and build up to form larger dirt deposits. They derive from the raw material of paper production. The source of the particles is, in particular, waste paper. At certain times of year, however, cellulose also contributes sticky contaminants. The range of contaminants extends from tree resins, oils, and adhesives to printing inks, etc. The contaminants interfere with production of the paper web and necessitate cleaning or even premature replacement of the paper machine clothing in the paper machine.
Complex and expensive efforts have been made to remove these contaminants from the paper raw material, or mask them, before production of the paper web. Deposits of dirt on the paper machine clothing nevertheless continue to occur. Attempts have therefore been made to equip the plastic elements constituting the flat side of the paper machine clothings with an anti-adhesive coating, made for example of fluorocarbons or silicone compounds. The dirt particles adhere much less effectively to such coatings than to the material from which the plastic elements are produced. The effect of anti-adhesive coatings is only brief, however, since they are rubbed off the surface of the paper machine clothing by abrasion.
U.S. Pat. No. 4,541,895 discloses a paper machine clothing that is constructed from a plurality of layers provided with openings. It is proposed in this context to shape depressions into the paper-side layer and to incorporate detergents into these depressions. During operation of the paper machine clothing, the detergent is slowly delivered out of the depressions to a washing bath, and is intended thereby to achieve a self-cleaning effect. Contaminants of the kind described above cannot, however, be removed with such detergents. They are moreover effective only in an aqueous environment, and then tend to foam. The washing effect ends as soon as the detergent is consumed by continuous delivery.
WO 01/21884 A1 proposes to impart to those plastic elements that constitute the surface facing the paper web (paper side) a roughness having a depth of between 5 μm and 100 μm, and thereby to prevent the deposition of contaminants. This is based on the idea of not offering to the dirt particles any large continuous contact regions that might serve as adhesion surfaces. This is intended to achieve considerably improved detachment of the dirt particle agglomerations, and to prevent the growth of such agglomerations to sizes that cause quality losses in the paper web. The effect of this action is not satisfactory, however, in particular with deposits in the form of dust and powder.