JPS6351744B2 - - Google Patents

Description

[Detailed Description of the Invention] Summary of the Invention When applying a coating to both sides of a paper web moving at high speed, the paper web is pressed against a roll by an elastic and flexible doctor, and at the same time the coating is formed by the roll and the doctor. When both surfaces are wetted with the coating material through the coating material existing in the space, and immediately after coming out of the doctor-roll gap, it is bent in the direction away from the applicator roll at an angle to the tangent to the roll gap. If it is greater than 14 g/m ² , serious operational disruptions occur due to significant fog formation of the coating material.

In order to eliminate this confusion and to be able to controllably apply a coating weight of 14 g/ ^m2 or more, the paper web to be coated must be passed through a coating material reservoir at a speed of 400 m/min or more. The viscosity of the coating material is maintained at 400 to 200 cps, and the paper web is then separated from the applicator roll beyond the doctor edge at an angle of 8° to 12° to the tangent at the roll nip immediately after exiting the doctor-roll nip. bend in the direction.

The present invention relates to a method of coating both sides of a paper web moving at high speed, where the coating amount is calculated as the total amount on both sides of the paper web.
g/m ² , the paper web is pressed against the roller by an elastic and flexible doctor and at the same time passes through the coating material present in the space formed by the roller and the doctor to absorb this coating material. Immediately after coming out of the doctor-roll gap, it is bent away from the applicator roll at an angle to the tangent to the roll gap.

It is known that coating paper improves its properties in various ways, for example by allowing it to print better. In this case, dispersions of coating materials, preferably containing binders and other additives, are often used. The degree of improvement achieved depends on various factors. Generally, a constant coating weight of coating material is targeted, and for example, to produce paper suitable for drawings, it is often 10 to 12 coats of dry coating material per side.
It appears that a coating weight of g/m ² is required.

However, other factors must also be taken into account during coating, such as, for example, the energy consumption during the subsequent drying of the paper web. A high dry matter content of the dispersion used is also generally targeted, keeping a relatively low absorption of water into the paper substrate for a given coating weight and using a correspondingly small amount of energy to cover this water content in the coating process. must be removed later. However, the dry matter content of the coating material also influences its rheological properties; in general, for a given pigment and binder, the higher the dry matter content, the higher the viscosity.

From the above point of view, when producing high quality coated paper,
Those skilled in the art will be able to optimize coating performance by paying attention to the amounts of various factors.

The known so-called Billblade method (German Announcement No. 1696153) teaches that both sides of a paper web are coated simultaneously with a relatively simple device, in which case a relatively large application amount is applied to both sides of the paper. be able to. Furthermore, the Billblade method teaches that the resulting coating is uniform and that the so-called citron skin appearance (prevention of this citron skin appearance is an important prerequisite for achieving good printability of paper) ) is avoided.

According to this known method, double-sided application of the coating is achieved as follows. That is, the moving paper web is pressed against the roll by an elastic and flexible doctor, simultaneously supplying coating material to both sides, and bending away from the roll. In this case, the paper web is guided in a known manner into the coating material present in the space formed by the roll and the doctor and then removed therefrom, which ensures that the paper web is sufficiently wetted with the coating material and the doctor ―
Immediately after emerging from the roll nip, it is bent away from the coating roll at an angle greater than 5° to the tangent at the roller nip, and subjected to a drying process.

With this known method, for example, double-sided coated printing paper can be produced with very good coating weights of 20-24 g/m ² and a uniform distribution on both sides of the paper at low speeds and speeds up to 400 m/min. Obtained quality. The applicator roll is provided with a relatively flexible rubber layer in this known method and rotates at a speed that approximately matches the speed of the paper web or exceeds this speed by about 5%. Thickness 0.25-0.5 for doctor
mm steel doctor is used. The coating material is applied in sufficient excess to both sides of the paper web in a so-called space (reservoir), and in this reservoir there is a continuous flow of coating material in an amount that overflows over the forked edges of the space; This overflowing coating material is then screened and returned to the reservoir together with fresh coating material.
In this case, in practice a withdrawal angle of about 20° to the tangent to the applicator roll is used.

What is important in this known technique is that the paper web is pulled out at a constant angle over the blade edge when emerging from the doctor-roll nip. This prevents the paper web from being entrained by the applicator roll after exiting the gap, so that the feared effect of forming a citron skin appearance due to membrane splitting will not be directly apparent.
When using this known device at speeds up to 400 m/min, a withdrawal angle of 15-20° is used in order to avoid the above-mentioned effects.

Developments in the paper industry have led to ever-increasing production speeds, and this of course also applies to coating methods. Regardless of whether a coating device is included in the papermaking machine or a separate coating device is used to effect the coating during papermaking, there is a need for a device that can function even at very high speeds.

It has now been found that the known billblading process is not able to simultaneously apply the desired high coating weights without difficulty at the high speeds currently required.

Thus, the implementation of the billblading method surprisingly showed that the use of known types of coating materials at speeds ranging from 400 to 500 m/min leads to a significantly stronger fog formation, which is caused by the formation of a fog in the coating equipment. Deposits on machine parts can cause serious operational disruption, forcing the continuous coating process to be interrupted. However, this drawback can be eliminated or in each case reduced in various ways, for example by increasing the contact pressure of the doctor or by lowering the dry matter content of the coating material, thereby reducing the amount of application. A certain degree of improvement can be obtained if it is kept at . That is, it has been found that the above-mentioned fog formation can be eliminated at a coating weight of about 14 g/m ² or less.

It is confirmed from the above that due to the above-mentioned drawbacks, the known billblading method cannot be used when a coating weight of 14 g/m ² or more is required and a web speed of 400 m/min or more is required. is that its usability is severely limited.

Surprisingly, it has now been found that the above drawbacks can be completely eliminated by combining the following measures in the method described at the beginning. a) moving the paper web through the space containing the coating at a speed greater than 400 m/min; b) maintaining a viscosity of 400 to 2000 cps in the coating in the space; and c) moving the paper web through the doctor-roll nip. Immediately thereafter, it is bent away from the applicator roll beyond the doctor edge at an angle of 8° to 12° with respect to the tangent in the roll nip.

According to the solution proposed by the present invention, the above-mentioned significant fog formation is surprisingly
It was possible to completely avoid this problem even at a coating weight of more than 14 g/m ² .

The invention will now be explained in more detail with reference to the accompanying drawings, which show a known billblading device and partially explain the solution to the problem proposed by the invention.

FIG. 1 is a diagram illustrating the principle of the known bill blade method, in which a rotary coating roll having a rubber layer is designated by the reference numeral 1, a blade or doctor support is designated by the reference numeral 2, and the doctor support 2 is shown in FIG.
A flexible covering blade is designated by reference numeral 3. The blade 3 brings the paper web (running in the direction of arrow A) into contact 5 with the surface of the applicator roll.
Press at. The applicator roll rotates in the direction of arrow B. The tangent to the applicator roll 1 at the contact point 5 is represented by a broken line and designated by the reference numeral 6-6. The paper web 4 is drawn out over the blade edge 5 at an angle α, which angle is approximately 20° to the tangent 6--6.

It has now been shown that with the method shown in FIG. 1, neither the desired high coverage nor at the same time the desired uniform distribution on both sides of the paper web can be obtained at high web speeds. The problems that arise with the coating apparatus of FIG. 1 are illustrated in FIG. At high web speeds and under certain conditions (described in detail below), the doctor
In the area defined by the roll surface after the roll nip and the paper web side facing this roll surface, a more or less intense fog formation of fine particles of the coating material occurs. The mist thus formed is indicated by reference numeral 7 in FIG.

FIG. 3 mainly shows the method according to the present invention in the first step.
It is shown with the apparatus corresponding to FIG. Again, the rotating applicator roll is referenced 1, the doctor support is referenced 2, and the flexible blade is referenced 3.
The paper web is designated by reference numeral 4, the contact point between the blade and the applicator roll is designated by reference numeral 5, and the tangent line passing through contact point 5 is designated by reference numeral 6-6.

The coating reservoirs on each side of the paper web 4 are designated by reference numerals 8 and 9. If the angle denoted β between the tangent 6-6 at contact point 5 and the running direction of the paper web after the roll-doctor gap in FIG. 3 is kept below 12°, high web speed and high coating It has now surprisingly been found that fog formation is completely eliminated even at low concentrations.

As an example, when coating a paper material for printing purposes, this paper web is
It can be run at a speed of 22 min, and at this time, the total amount of 22
A coating amount of g/m ² could be applied. The viscosity of the coating is 800 cps in this case and the dry matter content is 56%
It was hot. Further, the pull-out angle β was 10°.

In other cases, the coating material had a dry matter content of 60% and a viscosity of 1000 cps. The speed of the paper web is 700 m/min, and the total amount of coating is 20 g/m ²
It was hot. Further, the pull-out angle β was 8° in this case. Very good coating results were obtained in this example.

With regard to the viscosity of the coating material, within the scope of the present invention this viscosity should reasonably be between 400 and 2000, preferably between 1200 and 2000.
Should be kept at 1500 (according to Burdskfield).

The advantage of the solution according to the invention is that the withdrawal angle
At 12°, the danger of yuzu skin appearance was completely eliminated.

It is difficult to give a basic explanation for the effect itself leading to fog formation and to explain why this effect can be eliminated according to the invention. There are hypotheses for this, but only some of them can be proven.

In this connection, FIG. 4 is shown. The applicator roll surface corresponds to the roll surface in FIG. 1 and is designated here by reference numeral 10 in the area of the roll-doctor gap. The lower part of the reservoir 8 in FIG. 3 is designated by the reference numeral 11 in FIG. 4, and the corresponding lower part of the reservoir 9 is designated by the reference numeral 12. blade 3
The lower part of the blade is designated by reference numeral 13, the blade surface facing the paper web is designated by reference numeral 14, and the blade tip is designated by reference numeral 15. Paper web 4 is
Reference number 1 in the area of the roll-doctor gap
6. When performing this method, the reservoir 11
and 12 coating materials form a coating in the region of the roll-doctor nip. The contact pressure of the blade 13 in this range exerts a directed force on the roll, which force is designated by the reference F in FIG. A force in the opposite direction develops along the roll surface 10 and is designated F ₁ . Drawing the paper web 16 from the roll over the blade edge 15 creates a force F ₂ in the same direction as force F ₁ . Moderate web speed or 400
When using the known billblading method at speeds up to m/min, it is possible in practice to obtain a uniform distribution of the coating amount on both sides of the paper web, so that in this case such an equilibrium relationship exists in this doctor-roll nip. However, it can be assumed that the film thickness between the roll surface 10, the paper web 16 and the blade surface 14 will be approximately the same. However, as web speeds increase,
Due to the high speed of this paper web, both reservoirs 8 and 9
turbulence increases within the It has been found that the turbulence in reservoir 8 at high speeds is significantly increased compared to the turbulence in reservoir 9. The cause of this is
This is because two surfaces move in the reservoir 8, resulting in a lowering of the coating material relative to the contact 5, partly by the movement of the roll surface 10 and partly by the movement of the paper web 16. Therefore, at high speeds, the hydraulic pressure due to the movement of the coating material will increase to a greater extent in the lower part of the reservoir 8 than in the lower part of the reservoir 9. Then, perhaps the doctor-roll nip relationship changes, the equilibrium is broken, and the paper web 16 is forced toward the blade surface by the high hydraulic pressure between the roll surface 10 and the paper web 16. This hydraulic pressure increases with increasing viscosity and correspondingly increasing dry matter content of the coating material.

This results in non-uniform distribution of coating material on both sides of the paper web when coating at high web speeds. For example, with a total coating weight of 20 g/m ² a distribution of approximately 10 g/m ² per side of the paper web is achieved at low speeds, although the application rate is selected by appropriate selection of the blade pressure. At high speeds and a suitably selected blade pressure of 20 g/ ^m2 , the roll side of the paper web is approximately 13 g/m2.
m ² , and a distribution of 7 g/m ² occurs on the blade side.

Experience has shown that an amount of 13 g/m ² on the paper web side on the applicator roll side can, under certain conditions, cause a film splitting effect with a so-called orange-skinned appearance. This is because, in the case of such relatively high application rates, part of the coating material is entrained with the roll. In this way, the distribution of the coating between the paper surface and the roll surface occurs as droplet formation. At high velocities, such membrane splitting action would likely result in intense droplet formation as a mist.

If the withdrawal angle of the paper web is reduced, the force F ₂ will also be reduced, otherwise similar conditions.
This increases the force F ₁ by a corresponding amount and thus reestablishes equilibrium in the system over a wide range. If the angle reduction is carried out, other things being the same, ie without changing the blade pressure, the total coverage will be less than the desired amount of 20 g/m ² as results from the previous example. The foregoing aspect, as previously stated, is a hypothesis regarding the surprising technical effects achieved when the withdrawal angle is reduced to less than 12 degrees at high web speeds.

In all of the above cases, the expression total coverage means the total dry coverage (g) applied to both sides of the paper (per 1 m ² ).

As a result of actual experiments using a conventional extraction angle (20°) and an angle within the range specified by the present invention (10°), the coating process was carried out at a relatively high speed of, for example, 600 m/min. However, an unexpectedly significant improvement in the fog generation problem was clearly demonstrated. These comparative experiments were conducted at the same speed and the coating composition used had a dry solids content of 53% by weight and a viscosity of
It was 700 centipoise. Coverage amount is 20 each
g/m ² and 18.5 g/m ² . This slight difference in coverage appears to be caused by the difference in the extraction angle. Using the prior art extraction angle (20°), significant deposits occurred as a result of fogging of the coating composition on the plate located below the coating roller, as seen in FIG. On the other hand, in the case of the extraction angle (10°) according to the present invention (FIG. 6), adhesion of the coating composition to the same plate was almost completely prevented. This indicates that the technique of the present invention, including the take-off angle, represents a significant improvement in the art of coating paper webs.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram of the principle of a coating apparatus using the known bill blade method, Fig. 2 is an explanatory diagram of the apparatus shown in Fig. 1 when using extremely high paper web speeds, and Fig. 3 is an explanatory diagram of the apparatus shown in Fig.
The figure is an explanatory diagram of the apparatus shown in Figure 1 when the method of the present invention is used, Figure 4 is a partially enlarged view of Figure 3, and Figure 5 is a coating showing the drawbacks due to the extraction angle of the prior art. FIG. 6 is an explanatory diagram of a coating roll according to the present invention when operated under the same conditions as in FIG. 5 except that the sampling angle has been changed. 1... Application roll, 2... Doctor support, 3... Coating blade, 4... Paper web, 5... Contact, 6-6
...Tangential line, 7...Mist, 8, 9...Reservoir, 10...Applying roll surface, 11...Lower part of reservoir 8, 12...Reservoir 9
13... Lower part of the blade, 14... Blade surface, 15... Blade tip, 16... Paper web.

Claims (1)

Claims: 1. (a) A speed of at least 400 m/min in the gap formed between a rotating applicator roll and an elastic flexible blade with an edge that presses the paper web in the direction of this roll. (b) supplying a paper web to form a first reservoir between the paper web and the roll and a second reservoir between the paper web and the blade; The coating composition is applied to each of the reservoirs so that the amount of coating material applied to the web is 14 ^/ m2, calculated for both sides of the web.
(c) maintaining a viscosity of the coating composition in the first and second reservoirs between 400 and 2000 centipoise;
and (d) the paper web is bent in a direction away from the applicator roll over the edge of the blade at an angle of 8° to 12° with respect to the tangent to the roll in the gap, thereby extracting the paper web from the gap. A method for continuously applying a coating to both sides of a paper web.