The present invention relates to a method and to an apparatus for producing smooth two-sided coated paper, so-called LWC-paper, where me coating and smoothing processes are carried out immediately after each other. The paper may either be coated on both sides at one and the same time, followed by a drying process, or first on one side and after drying, then on the other side, followed by further drying. The method can be carried out in accordance with International Patent Application WO,Al, 95/30049, wherein paper is produced in a conventional manner, dried and thereafter coated with a coating mixture on-line, first on one side and then on the other side, whereafter the paper is dried and smoothed. It is also possible to produce the paper and coat the same off-line.

Hitherto, it has only been possible to produce high-gloss LWC-paper, i.e. a gloss level of 50-70%, with the aid of an off-line calender, a so-called supercalender. The supercalender comprises a high stand of rolls through which the paper is passed forwards and backwards between mutually sequential rolls. A roll temperature of 90-120°C and a roll pressure of 300 kN/m are used. However, such a discontinuous method has considerable economic disadvantages.

Consequently, as described in International Patent Application WO,Al, 95/30049, endeavours have been made to calender paper on-line with paper manufacture and paper coating. There is used to this end a so-called compact calender that has two or more sequential rolls pairs, wherein one roll in each pair is hard and heated and the other roll is made of a somewhat softer material and is unheated. This method has not hitherto been successful in achieving the high gloss required for newsprint. However, such a method is used in the manufacture of MFC-paper, with which a gloss of 20-45% is obtained.

In the development of one such method for the continuous manufacture, coating and smoothing of paper, there was established an optimal temperature of 150-210°C,

suitably about 200°C with respect to the hot roll of each pair of rolls, and a line pressure of 200-470 kN/m, suitably 300-400 kN/m and then preferably 350-380 kN/m. Machine speed is suitably 900-1,800 m/min., e.g. 1,000 m/min. In laboratory tests with calendering of paper that had already been coated with a standard coating under the aforesaid conditions, a gloss according to Hunter of 50% or more at a 75° angle was obtained. However, large scale on-line production has not given the same high gloss values as those achieved with pilot tests carried out on a smaller scale.

The object of the invention is to provide a high-gloss LWC-paper with the aid of a compact calender, where coating and calendering are carried out immediately after each other. The coating quantity is suitably 7-12 g/m ² /side.

According to the invention, this is achieved with a method in which the paper is maintained at a temperature beneath the lignin softening point during the calendering process. This is conveniently achieved by cooling the paper or by the cold roll in one or more roll pairs.

When coating-smoothing in-line, the paper will have a temperature of 75-90°C at the calender inlet, due to the preceding drying of the coating. A hot roll in each roll pair is utilized to heat the coating to a temperature that is sufficiently high to soften the coating. A roll surface temperature in the order of 200°C has been found beneficial in this regard. This means that the paper will be further heated between the roll pairs, wherewith paper and coating may reach a temperature in the order of 105°C prior to the inlet between a following roll pair. The high temperature also means mat the non-heated roll will obtain a high temperature. This roll therewith becomes softer and the pressure in the roll nip decreases. The high paper temperature also causes the paper fibres to soften and be rolled-out, in addition to softening of the coating.

The soft roll will suitably maintain a hardness of Shore D 90-95 or higher, measured at room temperature. The effect of cooling, when cooling is used, is to maintain a greater hardness during the calendering process. This results in an increase in nip pressure.

When the paper web maintains a temperature beneath the lignin softening temperature, the paper web itself will not be compressed by the force in the roll nip and the whole of said force will therefore act on the softened coating on the hot side.

Cooling can be achieved in several ways. For instance, the paper web can be cooled upstream of the calender inlet and also between the calender roll pairs. Cooling can be achieved, for instance, with the aid of cool air or by increasing the distance between coating and drying station and calender respectively and by increasing the spacing between the roll pairs in the calender. It is also possible to cool the soft roll in each roll pair. This can be achieved, for instance, by peimitting the mantle surface of said roll to pass through a cooling medium, which may be liquid or gaseous. CO ₂ , air, water are examples of expedient cooling media. It is also possible, for instance, to permit the paper to pass over or between a pair of cold pipes or rolls. These measures may either be applied separately or in combination.

The invention also relates to apparatus for producing high-gloss LWC-paper, comprising an LWC-paper web coating station in which the paper web is coated on both sides and the coating dried. The apparatus includes a compact calender which has at least two roll pairs and which is arranged in line downstream of the coating station, wherein the surface of the hard roll in each roll pair is maintained at a

temperature of 150-210°C, suitably about 200°C, and the line pressure is 200-470 kN/m, suitably 300-400 kN/m and then preferably 350-380 kN/m, wherein the machine speed is 900-1,800 m/min., e.g. 1,000 m/min., and wherein means are provided for maintaining the temperature of the paper beneath the lignin softening point as the paper passes through the calender, suitably within a temperature range of 40-100°C, particularly 60-90°C.

The cooling means may be comprised of one or more cold pipes or rolls that lie close to the paper or against the paper prior to its entry into the calender or during its passage between the roll pairs, or of cooling air means arranged upstream of the calender inlet and/or between the roll pairs of the calender, or of means operable in cooling the surface of the non-heated roll in one or more roll pairs. Cooling can also be achieved by increasing the distance between coating station and calender and/or increasing the distance between the calender pairs. A combination of these means may also be used.

The invention also relates to plant for the manufacture of high-gloss LWC-paper, comprising a paper machine in line with an inventive apparatus.

Example:

In on-line coating and smoothing in a compact calender, the temperature of the paper is measured upstream of the inlet in the first roll pair, stack 1, between the stacks, and downstream of stack 2, with both stacks closed, with only one stack closed or with both stacks open. The density, gloss (top side and bottom side) and surface smoothness (PPS) of the paper coating are also measured under the same conditions. The results are set forth in the following Tables.

The test conditions were as follows:

Machine speed: 1,000 m/min.

Applied coating: 10+10 g/m ²

Line pressure calendering: 370 kN/m

Temperature at different positions, °C.

Table 1

Table 2

As evident from the results, when both stacks are closed and the paper has a temperature of about 105°C at the inlet to the second stack, the gloss obtained on the bottom side, which is calendered last, is much poorer than the gloss obtained on the top side. The difference in gloss value is 9 units. This difference is only 5 units between the top layer calendered at an input temperature of 77°C (52%) and the bottom layer calendered at an input temperature of 78°C (47). Similarly, the difference in surface smoothness in the first-mentioned case is 0.1 unit, whereas no difference was measured in the latter case.

The elasticity modulus of a soft calender coating having a hardness of 91 ShD at room temperature was also measured at increasing temperatures. The enclosed Figure illustrates the elasticity modulus in dependence on temperatures. The elasticity modulus decreases by about 1 MPa between 50 and 105°C and the elasticity thus greatly increases. Consequently, cooling of the roll surface can result in a considerable increase in nip pressure.

The inventive method and apparatus have been described with a starting point from LWC-paper and with respect to certain embodiments and examples. However, variations are possible within the scope of the following Claims. For instance, the invention can also be applied in the manufacture of fine paper.