FI79732B - FOERFARANDE OCH PRESSFILT FOER STYRNING AV AVVATTNINGEN I ETT PRESSPARTI AV EN PAPPERSMASKIN. - Google Patents

Description

1 79732

The present invention relates to a method for controlling dewatering in a press section of a paper machine, in which a paper web to be dried is passed between two press felts through a press nip formed by rolls to remove water from the web to the felts and direct water drainage from the web.

A problem in papermaking is the difference in the surface quality of the finished paper web in different parts of the web. As a result, the surface of the web is smoother on one side than on the other, which impairs further processing of the web, e.g. the suitability of the web for printing. Such one-sidedness of the paper web is caused by the uneven distribution of different types of fibers in the thickness direction of the web. Generally, more short fibers, fiber fragments, and zero pulp accumulate above the web because most of the water is removed by gravity from the wire side of the web, leaving the fines from the web through the wire.

Attempts have been made to remedy the one-sidedness of the paper web by developing various improved press sections with the aim of applying a stronger suction or other mechanical dewatering effect to the top of the paper web to remove water as evenly as possible from both sides of the web. The article "The influence of pressing on sheet two-sideduess", published in TAPPI Journal September 1982, pages 73-77, by Lau-30 rie Wicks, USA, presents several structural proposals for press-parts. However, such specially designed press parts are complicated and expensive and do not achieve completely satisfactory results either.

To increase production, it is known in papermaking to increase the speed of the paper web, and this higher web speed has been utilized to remove water from the top of the web by centrifugal force by passing the web 2,79732 over a roll forming a reversal point. With such a structure, water can be removed from the dough more evenly on both sides, but the result is not quite good and requires corrective measures in subsequent press nips by means of suction and groove rollers.

The object of the present invention is to provide a method which avoids the above-mentioned drawbacks and makes it possible to achieve a uniform surface quality of the paper web with current conventional press parts without the need for expensive special presses. This object is achieved by the method according to the invention, characterized in that in order to direct water upwards from the web, the felt on the wire side of the paper web is continuously made water-repellent and the felt on the wire side of the paper web is continuously absorbent.

The invention is based on the idea that the uneven dewatering of the paper web is corrected in the press section by controlling the direction of the water leaving the paper web by means of the press felts themselves. This is achieved by adjusting the water repellency and water absorption properties of the press-20 tin felts. In this way, dewatering can be directed from the opposite side of the paper web to the wire, i.e. from the top side, in the desired amount, whereby the fine material from the top side of the web is removed with the water pressed into the top felt.

The paper web to be dried is passed to a press nip between two press felts, from which the wire-side felt of the paper web is made hydrophobic, i.e. water-repellent, and an ordinary press felt or felt which is formed hydrophilic, i.e. water-absorbent, is used as the opposite felt. In this way, uneven dewatering in the nip can be resisted in each press nip until the paper web has reached such a dryness that no more fiber shifts occur in the web.

The invention will be explained in more detail below with reference to the accompanying drawing, in which Fig. 1 schematically shows a press section of a paper machine in which the method according to the invention is applied, and Fig. 2 shows a press nip in a larger size.

Figure 1 of the drawing shows the wire and press part of the paper machine 5. Extending from the pulp headbox 1 is a wire 2, the end of which has a first nip formed by suction rollers 3. A press felt 4 passes through this nip, which further passes through a second nip formed by the suction roll 5 and the groove roll 6 and a third nip formed by the same suction roll 10 and the receiving roll 7. A second press felt 8 also passes through this second press nip. A fourth press nip is formed between the roll 7 and the second groove roll 9, through the nip of which the third felt 10 passes.

15 The paper web to be dried formed on the wire is marked with the reference number 11. The web passes through a second nip between the press felts 4 and 8.

Fig. 2 of the drawing shows in larger size the operating principle of the press nip. The lower press felt ... "20 8 is treated to be water-repellent, while the upper press felt 4 is treated to absorb water. As a result ·: · the water contained in the paper web tends to leave the web Y: more strongly upwards to the upper felt than downwards to the lower felt Arrow A shows the main direction of dewatering achieved in this way in Fig. 2. Such forced dewatering control can counteract the unilaterality in the paper web caused by the predominantly downward dewatering in the first nip. thanks to this, the dried paper web will be of very similar quality on both sides.

'Alternatively, an ordinary press felt can be used as the upper felt, which has therefore not been specially treated to absorb more water. Again, directional dewatering according to the invention, 4 79732, is provided so that more water is discharged from the web to the upper felt than to the lower felt.

Example 1

An ordinary knitted press felt with a basis weight of 1200 g / m 2 was prepared. Polyamide with a fineness of 6 denier was used as the fibrous and yarn material of the fluff layer and the support fabric. Fluorochemical was injected into the press felt at the last stage of the felt manufacturing process, i.e. at the end of the heat treatment step.

Water-dispersed tetrafluoroethylene with a dry matter content of 50% was used as the fluorochemical. This was injected into the felt as a 4% solution so that the felt became wet and absorbed its own weight of solution. In this case, the so-called wet pick-up was 100%.

15 The felt to be treated was 7 m wide and 20 m long, 2 giving an area of 140 m and a weight of 168 kg. When the felt was dried after spraying the chemical at 130 ° C, about 2 kg of fluorochemical remained, so the weight gain was about 1.2%.

20 The fluorochemical treatment of the press felt made it very water-repellent, which was found in the laboratory in the so-called the drip method.

: According to the drip method, a drop of colored water was carefully dropped on the surface of the felt and it was observed from the stopwatch how many seconds it took for the drop to soak into the felt. With a water-absorbent felt, the droplet absorption time is less than 1 second, while with a water-repellent felt, the time is 1 to 30 seconds, and with a very strongly water-repellent surface, the droplet dries before it has time to be absorbed.

30 The fluorochemical-treated felt was so water-repellent that the droplet was not absorbed into the felt at all even though the surface of the felt was porous.

Alternatively, the hydrophobic treatment can be performed with a silicone-based agent.

35 Instead of spraying, it is possible to use foulardi impregnation or lifting irrigation to apply 5,79732 chemicals. When spraying, the chemical is injected into the felt at a fairly high pressure up to 40 kp / cm so that the felt gets wet properly. In Foulardi impregnation, a part of the felt 5 is immersed in the solution at a time and then passed through a press which smooths the chemical inside the felt. In nos-wetting, the roll rotates in a tray with a chemical solution, and the felt passes in contact with the surface of the roll, thereby transferring the chemical to the felt.

10 Example 2

The untreated knitted press felt of Example 1 was sprayed with ethylene oxide condensate as a 3% solution, which was 100% dry matter before dilution, so that the felt was wetted. The wet pick-up 15 was adjusted with the amount of broth to about 100%. After drying, about 5 kg of material remained in the felt, so the weight gain due to the treatment was about 3%.

When the effect of the treatment on the water absorption capacity of the felt was tested in the laboratory by the drip method, the absorption time of 20 drops was less than 0.5 seconds, i.e. the felt was water absorbent.

In hydrophilic treatment, one is selected as a chemical in which there are polar sites in the chemical molecule to which water as a polar chemical is able to be absorbed. 25 When such a substance is in the form of a film on the surface of the fibers, the nature of the surface becomes more water-absorbent.

Such hydrophilic chemicals include, for example, chemicals containing amines, ethers, esters or oxides.

The drawing and the related explanation are only intended to illustrate the idea of the invention. The details of the method according to the invention and the press felt may vary within the scope of the claims. Thus, instead of or in addition to the chemical treatment described above, it is possible to achieve felt hydrophobicity or hydrophilicity by selecting fibers and other raw materials used in the manufacture of the felt so as to have the desired water absorption capacity (hydrophobicity or hydrophilicity) due to their surface properties. 5 femininity, porosity and / or fineness of diameter.

Claims (5)

A method for controlling the dewatering in a press section of a paper machine, wherein a paper web (11) / 5 to be dried is passed between two press blankets (4, 8) through a press nip formed by water rolls (5, 6) from the web to the blankets and the removal of the water from the web is directed by means of blankets having different water-receiving ability, characterized in that, to direct the water uptake (A) from the web, the continuous-lying felt (8) of the paper web (11) is continuous. and the blanket (4) located on the opposite side of the sheet (4) on the fabric side of the paper web (11) continuously absorbs water. 15 2. A method according to claim 1, characterized in that as a conventional knitted press blanket, a conventional knitted press blanket is used as on the opposite side of the blanket (4) of the relative paper web (11). A lower press felt (8) for applying the method of claim 1 for a paper machine press portion for transporting a paper web (11) / to be dried, with an upper press felt (4) through one of rollers (5, 6) ) formed a press nip, characterized in that the lower press felt (8) is continuously water repellent. 4. A felt blanket according to claim 3, characterized in that it is structurally a conventional knitted blanket (8) which has been treated with a water repellent. 5. Press felt according to claim 3, characterized in that it is structurally a conventional knitted press felt whose fibers and / or other raw materials used in the manufacture are water repellent.