JP2000262961A - Slide bead coating method of pva coating liquid - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the generation of coating defects (streak-like ununiformity or the entanglement of air) even at a increased coating speed. SOLUTION: As the condition in the case of applying simultaneously plural PVA coating liquids 14 in multilayer on a web 12 using a slide bead coating device 10, polyvinyl alcohol contained in the PVA coating liquid 14A forming the lowermost layer in the multiple layer coating films has polymerization degree of 1000-4000 and a thickener is added to becomes 0.05-0.5 wt.% per the content of polyvinyl alcohol.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for applying a PVA coating solution to a slide bead, and more particularly, to the production of heat-sensitive and pressure-sensitive recording materials, printing plate materials and the like, in which a continuously running belt-shaped support (hereinafter referred to as "web"). The present invention relates to a slide bead coating method of a PVA coating liquid, which is a multi-layer coating of a plurality of PVA coating liquids containing polyvinyl alcohol (hereinafter referred to as “PVA”) on a coating substrate precisely.

[0002]

2. Description of the Related Art A slide bead type coating apparatus is one of apparatuses for applying a coating solution on a continuously running web surface. When a plurality of PVA-containing coating liquids containing PVA are applied in multiple layers with uniform thickness, the use of this slide bead type coating apparatus is advantageous in the quality of the multilayer coating film and the like.

[0003] In a slide bead coating method using a slide bead type coating apparatus, a plurality of coating liquids flowing down a slide surface form a bead portion in a gap where a continuous running web meets a web at the tip of the slide surface. Then, a coating liquid is applied to the web surface through the bead portion. However, when the coating speed (web running speed) is increased, the coating liquid is rapidly stretched at the bead portion, so that the PVA coating liquid has a remarkably unstable flow due to its characteristics. As a result, streaky unevenness occurs in the multilayer coating film applied to the web surface, and serious coating defects such as air entrapment occur. The reason for the occurrence of such coating defects is that in the case of a coating solution containing a large amount of a high-molecular polymer in a coating solution such as a PVA coating solution, the flow is more unstable due to the forced stretching action than in the case of a normal coating solution. By promoting it. However, if the concentration of PVA contained in the PVA coating solution is too low in the slide bead coating of the PVA coating solution, the viscosity becomes low and the flow is rapidly accelerated, so that stripes parallel to the running direction of the web are generated. The disadvantage of doing so occurs.

Therefore, in practice, the application speed of the PVA coating solution in the slide bead application must be about 100 m / min or less, and there is a problem in application efficiency. From such a background, there is a demand for a slide bead coating method in which a coating defect does not occur even if the coating speed of the PVA coating liquid is increased, and various countermeasures have been conventionally taken.

For example, Japanese Patent Application Laid-Open No. 5-323116 discloses that in a multilayer coating of a deflecting film containing an organic coloring agent, the viscosity of the lower PVA coating liquid is lowered and the viscosity of the upper PVA coating liquid is raised. ing. JP 7
Japanese Patent No. 18599 discloses that a PVA coating solution having a concentration of 12 to 60% is 200 m at 60 to 95 ° C. and 100 to 3000 cP.
It is disclosed to apply to paper at an application speed of at least / min. Japanese Patent Application Laid-Open No. 6-294099 discloses that in the production of a coated printing paper containing a pigment as a main component, a PVA coating solution having a concentration of 0.5 to 2.5% is curtain-coated.

As a conventional technique relating to multilayer coating, Japanese Patent Publication No. 56-20534 discloses that the viscosity of the lowermost coating solution is reduced and the coating amount is reduced to form a thin film. It is disclosed that the viscosity is increased and the coating amount is increased to form a thick film. JP 57
No. 99367 discloses forming a low viscosity layer under a high viscosity layer. JP-A-2-83066 describes that the viscosity of the lowermost layer is adjusted according to the concentration of a solid content insoluble in water. JP 56-1
Japanese Patent Application Laid-Open No. 08566 discloses that the viscosity of the lowermost layer is lower in the high-shear region and higher in the low-shear region as compared with the next layer. Japanese Patent Application Laid-Open No. 9-108615 discloses that the ranges of the shear viscosity and the extensional viscosity are defined for at least one layer of the coating solution.

[0007] These conventional techniques relating to coating using a PVA coating liquid and techniques relating to multi-layer coating are basically based on the technique of lowering the viscosity of the lowermost layer and increasing the viscosity of the next layer to prevent coating defects. It is intended. Also, P
The reason why coating defects easily occur in the VA coating liquid is that the PVA coating liquid shows Newtonian flow, and non-Newtonian properties are obtained by adding a thickening agent to the PVA coating liquid (low viscosity property at high shear). To enhance the suitability of high-speed coating. For example, Japanese Patent Laid-Open No.
It is disclosed that a VA coating solution to which carboxymethylated starch, water-soluble cellulose and seaweed polysaccharide are added is coated on paper.

[0008]

However, even if the above-described technique for determining the viscosity of the lowermost layer and the next layer is combined with the technique for adding a thickener to the PVA coating solution, the coating speed is about 200 m / min. If the speed is increased to a higher speed, there is a problem that coating defects are not sufficiently solved. That is, when a thickener is added to a PVA coating solution, high-speed coating is not possible simply by adding the thickener, and if the conditions for adding the thickener are incorrect, the high-speed coating may be adversely affected. There are even things to do.

The present invention has been made in view of such circumstances, and a slide bead coating method of a PVA coating liquid which can prevent coating defects (streak-like unevenness and air entrapment) from occurring even when the coating speed is increased. The purpose is to provide.

[0010]

According to the present invention, in order to achieve the above object, a plurality of PVA coating solutions containing polyvinyl alcohol are prepared by using a slide hopper type coating device.
In the slide bead coating method of a PVA coating liquid for forming a multilayer coating film on the surface of the belt-like support by simultaneously applying multiple layers to the running belt-like support, the PVA coating liquid for forming the lowermost layer of the multilayer coating film The polymerization degree of the polyvinyl alcohol to be contained is set to 1000 to 4000, and the thickener is added so as to be 0.05 to 0.5% by weight based on the content of the polyvinyl alcohol.

According to the present invention, when a plurality of PVA coating liquids are simultaneously multi-layer coated on a belt-like support using a slide hopper type coating apparatus, PVA coating for forming the lowermost layer of the multilayer coating film is performed. Regarding the liquid, the polymerization degree of the contained polyvinyl alcohol was set to 1000 to 4000, and the thickener was added so as to be 0.05 to 0.5% by weight based on the content of the polyvinyl alcohol. Even if the coating speed is increased, coating defects (streak-like unevenness or air entrapment) can be prevented.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of a slide bead coating method for a PVA coating liquid according to the present invention will be described below in detail with reference to the accompanying drawings. FIG. 1 shows a slide bead type coating apparatus 1 to which the slide bead coating method of the present invention is applied.
0, which is an example of an apparatus that performs three-layer simultaneous coating.

As shown in FIG. 1, a plurality (for example, three types) of coating liquids 14 applied to the surface of the web 12 are supplied from supply tanks (not shown) to supply lines 16, 18, 2 respectively.
0 each manifold 2 in the slide hopper 22
4, 26 and 28. Manifold 24, 2
After the PVA coating liquid 14 supplied to the coating liquids 6 and 28 is spread in the coating width direction, each of the slots 3 formed in a slit shape is formed.
The slide hopper 22 is extruded through the slide surfaces 36 inclined downwardly on the upper surface of the slide hopper 22 through 0, 32 and 34. The PVA coating solution 14 extruded onto the slide surface 36 becomes a multilayer coating film-like PVA coating solution 14 without being mixed with each other, flows down the slide surface 36, and reaches a lip tip 38 at the lower end of the slide surface 36. The PVA coating solution 14 that has reached the lip tip 38 forms a bead portion 42 in a gap between the lip tip 38 and the surface of the web 12 that runs around the backing roller 40. In this bead portion 42, the web 12
Runs upward from below along the peripheral surface of the backing roller 40. As a result, the PVA coating solution 14 in the bead portion 42 is stretched by the action of being pulled up on the surface of the web 12 to be thinned. As a result, a thin multi-layer coating film can be formed on the running web 12 surface. The degree of thinning of the multilayer coating film depends on the running speed of the web. In FIG. 1, 14A is a PVA coating liquid for forming the lowermost layer of the multilayer coating film, 14B is a PVA coating liquid for forming the next layer, and 14C is a PA coating liquid for forming the uppermost layer.
3 shows a VA coating solution.

However, PVA coating solution 1 containing PVA
In the case of the slide bead coating of No. 4, when the coating speed was increased, the PVA coating liquid 14 was rapidly stretched in the bead portion 42, so that the characteristics of the PVA coating liquid 14 caused a markedly unstable flow, and the coating was applied to the surface of the web 12. Streak-like unevenness occurs in the multilayer coating film, and serious coating defects such as air entrapment occur.

As a countermeasure against such coating defects, there is a method of adding a thickener to a PVA coating solution, but simply adding a thickener cannot make high-speed coating possible. Then, the present inventor studied the conditions for adding a thickener to prevent the occurrence of coating defects even when the coating speed was increased to about 200 m / min or more, and obtained the following knowledge. That is, the amount of the thickener added to the lowermost PVA coating solution 14A is
It is important to perform stable high-speed coating without specifying the PVA coating liquid as a reference but specifying the PVA content contained in the PVA coating liquid 14 as a reference. Specifically, it is in the range of 0.05 to 0.5% by weight, preferably 0.06 to 0.4% by weight, and more preferably 0.1 to 0.35% by weight based on the PVA content. It is. This is because when the amount of the thickener added to the PVA content is less than 0.05% by weight, a sufficient thickening effect cannot be obtained, whereas when the amount of the thickener exceeds 0.5% by weight, the viscosity becomes high. This is because coating defects are likely to occur, and conversely, coating defects easily occur.

In order to make the addition of the thickener effective, the degree of polymerization of PVA is important. Specifically, the degree of polymerization is in the range of 1,000 to 4,000, more preferably one to four.
It is in the range of 500-3000. If the degree of polymerization of PVA is less than 1000, the increase in viscosity is small even if a thickener is added, and the effect of the addition is diminished.
If it exceeds 00, PVA becomes difficult to dissolve, which is not practical.

The relationship for preventing coating defects between the PVA coating solution 14A of the lowermost layer and the PVA coating solution 14B of the next layer may be defined by the viscosity.
By making the viscosity of the PVA coating solution 14B of the next layer higher than the viscosity of 4A, the occurrence of streak-like unevenness can be suppressed. However, even if the viscosity of the PVA coating liquid 14A of the lowermost layer is smaller than the viscosity of the PVA coating liquids 14B and 14C of the other layers, coating defects will occur if the PVA concentration contained in the PVA coating liquid of the lowermost layer is too high. appear. As a countermeasure, when a thickener is added to the lowermost PVA coating solution 14A,
The concentration of PVA contained in the VA coating solution 14A is 0.5 to 5
It is good to set to%.

The slide bead coating method of the PVA coating liquid of the present invention regulates the amount of the thickener added to the PVA coating liquid 14A forming the lowermost layer in the PVA coating liquid based on the above findings. The main configuration is that a slide hopper type coating apparatus 10 is used to apply a plurality of PVA coating liquids 14 containing PVA to a running web 12.
In a slide bead coating method of a PVA coating liquid for forming a multilayer coating film on the surface of the web 12 by simultaneously applying a multilayer coating to the web 12,
Regarding the PVA coating solution for forming the lowermost layer of the multilayer coating film, the polymerization degree of the contained polyvinyl alcohol is set to 100.
In addition to 0 to 4000, the thickener was added so as to be 0.05 to 0.5% by weight based on the polyvinyl alcohol. In this case, the lowermost PVA
The PVA content contained in the coating solution is in the range of 0.5 to 5% by weight.

Thus, the multilayer coating film can be stably maintained by increasing the viscosity at the time of low shear when flowing down the slide surface while maintaining the viscosity at the time of high shear in the bead portion. Is applied to the web at high speed,
It can be applied to a constant thickness without unevenness. here,
Polyvinyl alcohol (PVA) used in the present invention
Is a polyvinyl alcohol-based polymer. Polyvinyl alcohol is used as a binder or the like when a chemical is applied, and is used for imparting barrier properties and surface strength. Polyvinyl alcohol is a vinyl ester such as vinyl acetate or a vinyl ester obtained by polymerizing or copolymerizing a vinyl ester and another copolymerizable monomer by a polymerization method such as bulk polymerization, emulsion polymerization, solution polymerization, or suspension polymerization. Obtained by saponification. The degree of saponification of polyvinyl alcohol is 60 to 100%, preferably 7
0 to 99.5%, more preferably 80 to 99.5%.

As the thickener, water-soluble cellulose compounds, seaweed polysaccharides, carboxymethylated starch and the like can be used, and water-soluble cellulose compounds are particularly preferable. Examples of the web used in the present invention include paper, a resin film such as a polyethylene film, a resin laminated paper, and a metal foil, and a resin film is particularly preferable.

Further, in the present invention, the web 12 is subjected to a corona treatment or a surface treatment such as a charging treatment immediately before the application of the PVA coating liquid 14 on the surface of the web 12. FIG. 2 is an example of a configuration diagram when the web 12 is subjected to corona discharge treatment. As shown in FIG. 2, the running web 12 is applied to a web charging device 44 provided in the preceding stage of the slide bead type coating device 10 by a high voltage from a high voltage power supply 46 and is connected to a grounded support roller 48. Web 1 traveling together
2 is subjected to corona discharge. Thereby, a unipolar charge is given to the surface of the web 12. Next, web 1
2 passes through a surface voltmeter 50 where the potential on the web surface is measured and recorded on a recorder. The web 12 that has passed through the surface voltmeter 50 reaches the backing roller 40 of the slide bead type coating apparatus 10 via the pass roller 52, and the PVA coating solution 14 is applied to the web 12 from the slide hopper 22.
Is applied.

As described above, by applying a unipolar charge to the surface of the web 12 by the web charging device 44, the affinity and adhesion between the web 12 and the PVA coating solution 14 are improved, and the coating performance is improved. it can.

[0023]

Next, examples and comparative examples in which two layers were simultaneously coated under the coating conditions shown in Table 1 using the slide hopper apparatus described in the embodiment will be described. The surface treatment of the web was not performed. Examples are performed in three test sections of Examples (1) to (3) satisfying the conditions of the slide bead coating method of the present invention, and Comparative Examples are Comparative Examples (1) to (3) deviating from the conditions of the present invention. The test was performed in 6) of 6). Table 2 shows the types, concentrations and viscosities of the lower and upper PVA coating liquids of the two-layer simultaneous coating in Examples and Comparative Examples, and the amount of the thickener added to the lower layer. The concentration in Table 2 is the concentration by weight of PVA in the PVA coating solution, and the viscosity is 50%.
(1 / sec). In addition, the amount of application was shown as a value in a liquid state. For water-soluble cellulose thickener,
The amount of addition was expressed in% by weight based on the PVA content.

When the coating temperature, operating conditions, and web conditions in Table 1 were all the same and only the coating speed was increased in each of the test sections of the Examples and Comparative Examples, coating defects occurred up to several coating speeds. The limit speed was checked to see if it could be.
In addition, the coating defect generated at the limit speed is "streak-like unevenness".
And “air entrainment”.

[0025]

[Table 1] (Remarks) The bead interval refers to the gap between the tip of the slide hopper and the web. The slide angle refers to an inclination angle of the slide surface with respect to a horizontal plane. PVA used was Kuraray Co., Ltd. The concentrations of (a) and (c) of the PVA coating solution were adjusted by adding water. P in the PVA coating liquid indicates the degree of polymerization of PVA contained in the PVA coating liquid.

The results are shown in Table 2.

[0027]

[Table 2]

As can be seen from Table 2, Comparative Examples (1) to (6) deviating from the conditions of the slide bead coating method of the present invention.
In the six test plots, the critical speed for preventing the occurrence of coating defects was 100 to 180 m / min. On the other hand, in the three test sections of Examples (1) to (3) satisfying the conditions of the slide bead coating method of the present invention, the critical speed for preventing the occurrence of coating defects is 220 to 300 m / min (min). Even when the coating speed was 200 m / min or more, no coating defects occurred.

As described above, according to the embodiment of the present invention, the occurrence of "streak-like unevenness" and "air entrapment" is suppressed as compared with the comparative example, and the limit coating speed until these coating defects are generated. Was higher than that of the comparative example. Considering this result in detail, Comparative Example (1)
Is the case where the thickener was not added to the PVA coating liquid forming the lower layer, and reached the limit speed at 120 m / min. On the other hand, in the case of Example (1) in which the thickener was added to the lower layer at 0.3% based on the PVA content and the other conditions were almost the same as Comparative Example (1), the limit speed was 300 m / m. Min, which is more than twice the limit speed of Comparative Example (1).

In Comparative Examples (2) and (3), the thickener was added to the PVA coating solution for forming the lower layer so as to be 0.3% or 0.5% with respect to the PVA content. However, since the liquid type used for the lower layer had a low degree of PVA polymerization, the limit speed was reached at 100 m / min or 120 m / min. On the other hand, as in Examples (1) to (3), when the degree of PVA polymerization of the liquid type used for the lower layer is large, the critical speed is increased to 220 to 300 m / min.

In Comparative Examples (4) and (5), although the used liquid type had a high degree of PVA polymerization, the amount of the thickener added was 0.04% and 0.05% with respect to the PVA content. The limit speed was reached at 140 m / min or 180 m / min due to too small an amount. On the contrary, Comparative Example (6) showed that the added amount of the thickener was 0.6%.
And the limit speed was reached at 160 m / min because it was more than 0.5%. However, it is improved as compared with the limit speeds of 100 to 120 m / min of Comparative Examples (1) to (3). This result indicates that the amount of the thickener added to the PVA content was 0.0
Even if the value slightly deviates from the range of 5 to 0.5% by weight, it means that improvement can be achieved by increasing the degree of polymerization of the PVA coating solution used for the lower layer. This is shown in Comparative Examples (4) to
Among (6), the limit speed of Comparative Example (5) in which the degree of polymerization of the PVA coating solution used for the lower layer is the largest is 180 m / min, which is close to the limit speed of Examples (1) to (3). You can see from

From this, in order to increase the critical speed, it is important to appropriately define the PVA polymerization degree of the PVA coating solution forming the lowermost layer and the amount of the thickener added to the PVA content. You can see that there is.

[0033]

As described above, according to the slide bead coating method for the PVA coating liquid of the present invention, even if the coating speed is increased, coating defects (streak-like unevenness or air entrapment) do not occur. it can. That is, if the slide bead coating method of the present invention is performed, coating defects can be prevented from occurring even if the coating speed is increased from about 100 m / min or less to 200 m / min or more.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a slide bead applying apparatus to which a slide bead applying method of the present invention is applied.

FIG. 2 is a configuration diagram in a case where corona discharge treatment is performed on a web in the slide bead coating method of the present invention.

[Explanation of symbols]

10: slide bead type coating device, 12: web, 14
... PVA coating liquid, 14A ... PVA coating liquid of the lowermost layer, 14
B: PVA coating solution of the next layer, 14C: PVA coating solution of the uppermost layer, 22: slide hopper, 24, 26, 28: manifold, 30, 32, 34: slot, 36: slide surface, 40: backing roller, 42 ... Bead part, 4
4: Web charging device, 50: Surface potentiometer

Claims (7)

[Claims] (1) using a slide hopper type coating device,
A slide bead coating method of a PVA coating liquid, wherein a plurality of PVA coating liquids containing polyvinyl alcohol are simultaneously coated on a running belt-like support in multiple layers to form a multilayer coating film on the surface of the belt-like support. Of the PVA coating liquid forming the lowermost layer, the degree of polymerization of the contained polyvinyl alcohol is 1
000-4000, and the thickener is 0.05-0.5% by weight based on the content of the polyvinyl alcohol.
A method for applying a slide bead of a PVA coating solution, wherein 2. The method according to claim 1, wherein the concentration of polyvinyl alcohol contained in the PVA coating liquid for forming the lowermost layer is 0.5 to 5% by weight. 3. The viscosity of the PVA coating liquid other than the lowermost layer is as follows:
3. The method according to claim 1, wherein the viscosity of the lowermost PVA coating liquid is larger than the viscosity of the lowermost PVA coating liquid. 4. The method according to claim 1, wherein the type of said thickener is water-soluble cellulose. 5. The method according to claim 1, wherein said belt-shaped support is a resin film. 6. The slide of the PVA coating liquid according to claim 1, wherein the coating speed, which is the running speed of the belt-like support, is a high-speed coating of 100 to 300 m / min. Bead coating method. 7. The belt-shaped support is subjected to a corona treatment or a surface treatment such as a charging treatment immediately before the PVA coating liquid is applied to the belt-shaped support.
4. A method for applying a slide bead of a PVA coating liquid according to 4, 5, or 6.