WO2006000626A1

WO2006000626A1 - A method for coating a substrate

Info

Publication number: WO2006000626A1
Application number: PCT/FI2005/000298
Authority: WO
Inventors: Juha Kokko; Kari Lehtinen; Martin Taxacher
Original assignee: Kemira Oyj
Priority date: 2004-06-28
Filing date: 2005-06-28
Publication date: 2006-01-05
Also published as: FI20040886A; FI20040886A0

Abstract

The invention is directed to a method for coating a substrate with a coating suspension using application means of a coating apparatus, which is susceptible to wear due to the effect of the coating suspension. In the present invention a less abrasive coating suspension of the invention is used.

Description

A method for coating a substrate

Field of the invention

The invention relates to a new method for reducing the abrasive effect of the coating suspension on application means of a coating apparatus for paper and paperboard industry, wherein a coating suspension is applied as a spray via application means of a coating apparatus. Furthermore, the invention relates to the new use of calcium sulfate.

Brief description of the prior art

Coating is a well-known surface treatment method i.a. in paper industry. Paper and paperboard can be coated with one or more coating layers for modifying the mechanical and/or optical properties thereof depending on their application area. For producing e.g. a printing paper, the paper formed from a wood pulp needs to be coated for improving the printing properties of the surface.

Coating mixtures are often suspensions. They typically comprise a binder, one or more solid inorganic and/or organic coating components including so-called pigments, and optionally other additives. For instance calcium carbonates, kaolins, calcium sulfate and titanium dioxide can be mentioned as examples of the inorganic coating components in the paper and paperboard industry, of which calcium carbonates and kaolins are commonly used.

The coating may be carried out during the paper making process in a coating unit arranged in the wire section of the paper machine, whereby, after an initial drying step and, optionally, a sizing step of the formed paper or paperboard web, one or more coatings are then applied on the surface of the moving web in the following coating unit. The coating can also be effected simultaneously with the sizing step.

Alternatively, the coating step may be carried out off-machine in a separate coating line.

The paper or paperboard web can be coated on one or both sides with one or several layers using the same or different coating(s). In order to achieve a coating with an even and uniform quality, the coating suspension should have i.a. a uniform composition during the application from the coating apparatus and application rate should be constant to avoid e.g. variations in the thickness of the coating along the coated web.

Various coating methods can be used for forming coatings for the different applications of the paper and paperboard. The coating can be applied on the surface of the web by using a mechanical contact, such as rolls or blades. The mechanical contact, however, may bring limitations to the web speed, and cause defects on the relatively thin and wet paper or paperboard web.

One of the developments were so called jet applicators, wherein the coating is premetered as a thin film via a pressurized application chamber on the web surface. The jet of the coating coming out of the applicator is of a laminar shape. After the application of the coating on the web a further, final metering step of the formed coating layer on the surface of the web is normally required. For said further metering a blade and/or film transfer nip system has typically been used. Accordingly, in jet application methods a mechanical contact during the coating process is normally required for forming the coating on the web (see e.g. Papermaking Science and Technology, no. 11 , Pigment Coating and Surface Sizing of Paper, editors Johan Gullichsen, Hannu Paulapuro and Esa Lehtinen, Helsinki University of Technology, published by Fapet Oy, Helsinki Finland, 2000, pp. 433-439).

More recently, there have been developed non-contacting coating methods for the paper industry, so called spray-coating method, which i.a. enable to overcome the problems encountered with the mechanical contact. The coating is formed directly from the small droplets of the spray. No subsequent metering of the sprayed coating with a mechanical blade or film transfer nip system is needed.

In said spray-coating method the coating suspension is atomized under pressure to small droplets by means of a nozzle array arranged in one or more rows in the cross-wire direction and the formed spray is applied on the surface of the paper or paperboard web. Such spray-coating method typically involves a high-pressure spraying method. The spray devices and methods are known in the prior art. The coating suspensions containing solid inorganic coating components in the form of particles, e.g. said conventional pigments, can be very abrasive for a coating equipment. Particularly the application means including the openings thereof of a coating apparatus, such as nozzles, may wear considerably, especially when the coating suspension is applied under pressurized conditions through said application means. Therefore, in order to obtain a coating with an even and uniform quality, the worn parts may need to be changed at certain intervals to ensure the constant application rate of the coating via the application means, such as nozzles.

Thus it would be highly desirable to find coating suspensions which are less wearing, i.e. are less abrasive, so that the time interval between the required changes of the worn parts of a coating apparatus could be extended.

The "wear" and "wearing" of the coating apparatus due to the effect of the coating suspension are used herein equally and mean the wear/wearing of any parts of the apparatus, and particularly of the application means, such as nozzles, including the shape and/or size of the openings of said application means. Typically, the wear/wearing of the coating apparatus gradually deteriorates or prevents the proper functioning of the apparatus. Especially, in the course of a coating procedure the abrasive coating suspension may gradually enlarge the size of the openings and/or deform the shape thereof, whereby increase in the application rate through the openings of the application means, such as nozzles, and/or alterations in the distribution pattern of the coating suspension may occur, which have an detrimental effect on the uniformity of the coating.

Summary of the invention

The object of the invention is to reduce the undesirable wear effect of the coating suspensions.

Particularly, the object of the present invention is to provide a method for reducing the abrasive effect of the coating suspension on application means of coating apparatus especially for the paper and papermaking industry, wherein a coating suspension is applied as a spray by using an application means of a coating apparatus, e.g. nozzles, which method enables i.a. the prolongation of the time interval between the changes of the worn parts of a coating apparatus in cases, where said parts, particularly the application means, such as the nozzles, wear due to the wear effect of the coating suspensions, e.g. due to mechanical stress, such as an abrasive effect, caused by the coating suspension.

Moreover, as a further object of the invention, also the use of calcium sulfate as a solid coating component in a coating suspension for a coating method is provided, which enables to minimize/reduce the wear of a coating apparatus used in the coating method, and particularly the application means, such as nozzles, of the coating apparatus that are susceptible to wear due to the effect of a coating suspension.

The method and use of the invention thus enables to extend the use-life of the application means of a coating apparatus.

Brief description of the drawings

Figure 1 shows the opening of the unused nozzle before the spraying experiment (baseline). Figure 2 shows the opening of the nozzle after the experiment with the coating suspension 5 of the invention. Figure 3 shows the opening of the nozzle after the experiment with the comparative coating suspension 4.

Detailed description of the invention

The inventors have surprisingly found that a coating suspension which contains calcium sulfate as a solid coating component was less abrasive for the coating apparatus and, particularly, for the application means of the apparatus, compared e.g. to solid coating components, such as carbonates and kaolins, which are commonly used in the coating suspensions of the paper and paperboard industry.

The made finding is unexpected, because these solid coating components are known in the field as interchangeable alternatives and the literature in this field have merely been focused on their impact on the mechanical and/or optical properties and on the material differences as coatings. Thus calcium sulfate can have an advantageous effect on a coating method, preferably it enables to lengthen the use-life of the coating apparatus, and more preferably of the application means of said apparatus.

Moreover, the abrasive effect of the present coating suspension is markedly reduced compared e.g. to said conventional pigments. Therefore the present coating suspension containing calcium sulfate is a valuable alternative for minimizing or reducing the above discussed wear/abrasive effect of a coating suspension in coating methods, wherein the suspensions used in the prior art would wear considerably the coating apparatus, e.g. the application means of the apparatus, such as the openings of the nozzles of a spray-coating apparatus.

Accordingly, the invention provides a method for reducing the abrasive effect of the coating suspension on application means of a coating apparatus for paper and paperboard industry, in which a coating suspension is applied as a spray, whereby the characteristic feature of the present method is to use calcium sulfate as a solid coating component in said coating suspension.

Herein the "coating suspension" used in the method of the invention may contain one or more solid coating components suspended in the continuous phase of the coating suspension, provided that one of the solid coating components is calcium sulfate. Thus, in addition to calcium sulfate, the coating suspension may also comprise one or more further solid coating components.

In the present invention, calcium sulfate is preferably calcium sulfate dihydrate, which is also known as gypsum.

The terms "pigment" and "solid coating component" both refer herein broadly to any solid inorganic or organic coating component which is in a particle form and is suitable for coating and, preferably, can be used in the paper and paperboard industry for modifying the one or more of the properties of the paper or paperboard including the mechanical/physical properties such as porosity and printing properties, the optical properties such as whiteness, opacity and glossiness. Examples of solid coating components/pigments usable for paper or paperboard coating comprise kaolin including the mineral kaolin product (china-clay, aluminum silicate) obtainable from the mining industry and any derivatives or modifications of kaolin, calcium carbonate in precipitated or ground form including carbonates of natural and synthetic origin and any derivatives thereof, titanium dioxide and any derivatives thereof, talc (magnesium silicate) and any derivatives thereof, aluminum hydroxide, barium sulfate, zinc oxide and sodium silico-aluminate. The list is non-exhaustive. The term "solid coating component" thus includes also components which are known e.g. in the paper and paperboard industry as "pigments".

Calcium sulfate can be used partly or fully in place of other solid coating components, which are usable in the coating suspensions, preferably in the coating suspensions of the paper and paperboard industry, and can be interchangeable with calcium sulfate. Particularly, calcium sulfate can be used at least partly in place of coating components which have an abrasive effect on a coating apparatus.

Accordingly, the coating suspension can comprise calcium sulfate alone as the solid coating component/pigment or, alternatively, the coating suspension can comprise calcium sulfate together with one or more further solid coating components/pigments. In case of said mixture of calcium sulfate with one or more further solid coating components/pigments, the further one or more solid coating components/pigments are preferably selected from a group comprising titanium dioxide, talc, calcium carbonate, kaolin and any mixtures thereof, more preferably from calcium carbonate and/or kaolin.

In one preferable embodiment calcium sulfate is used as a mixture together with one or more further solid coating components, preferably with calcium carbonate, such as precipitated calcium carbonate, kaolin or a mixture thereof.

In the present method the amounts of calcium sulfate and optional further components of the suspension, the total solids content of the suspension, as well as the viscosity of the suspension to be applied via the coating apparatus depend i.a. on the used embodiment, the components of the suspension, the coating apparatus and the application rate, and can be chosen by a person skilled in the art. As a preferable example, the coating suspension may contain calcium sulfate e.g. 20-100 % by weight, preferably 20-80 % by weight, more preferably 40-70 % by weight, such as 40-65 % by weight, of the total solids content of the suspension.

Furthermore, the total solids content of the coating suspension may be up to 75 % by weight, such as 2-70% by weight, preferably 10 to below 65 % by weight, more preferably 40-60 % by weight, of the weight of the suspension.

The viscosity of the coating suspension of the invention can vary in a known manner and may preferably be below 100 mPas when determined as Viscosity Brookfield at 100 rpm.

The coating suspension may also contain further components. Preferably, the coating suspension is a coating suspension for the paper or paperboard industry, and may contain further components usable in the field, e.g. one or more binders, and optionally other additives that are e.g. dissolved in the continuous suspension medium. Such additives may be those conventionally used in the paper and paperboard industry, such as dispersing agents, biocides etc. Additives include herein also any pigment-like agents or colorants dissolved in said suspension medium.

The suspension medium, i.e. the continuous phase of the coating suspension, can be a gas phase, a liquid phase or a mixture of one or more gas and/or liquid phases. The liquid phase may comprise water, one or more organic solvents and a mixture of water with one or more organic solvents. Preferably, the suspension is an aqueous suspension.

The coating suspension can be applied on the surface of a substrate in amounts typically used in the art depending naturally on the used embodiment, i.a. on the specific paper or paperboard application, and can be chosen by a skilled person.

The present coating method is advantageously carried out using any coating apparatus with an application means which is susceptible to wear due to the effect of a coating suspension and which application means provides a discontinuous flow of the suspension, e.g. droplets, and particularly, a spray. The invention is particularly suitable for embodiments, wherein the coating suspension is applied from the application means using pressure, since under pressurized conditions considerable wear of the application means may occur, whereby in such embodiments the less-wearing effect of the coating suspension of the invention is highly desirable. Thus the invention is particularly preferable for a coating method, wherein the coating suspension is applied under pressure through the opening(s) of the application means. The used pressure depends i.a. on the used method/apparatus and the discontinuous flow desired to be formed. The used pressure has an effect i.a. on the size of the droplets and/or the application rate and may be chosen accordingly in a manner known in the art.

Since the invention is based on the unexpected, i.a. less-wearing/less-abrasive, effect of calcium sulfate in a coating method as disclosed above, preferably in a coating method of the paper and paperboard industry, therefore the present invention is not limited with respect to the coating apparatus or the spray-coating method steps, and these can be chosen in a manner known in the field.

In one preferable embodiment of the invention the application means comprises a nozzle having an opening through which the coating suspension is applied typically under pressure, on the surface of a substrate. Again, the size and shape of the opening of the nozzles may, of course, vary depending i.a. on the components used in the coating suspension, the coating layer to be formed, the form of the discontinuous suspension flow (e.g. droplets, such as spray), the feeding rate and the pressure used in the coating apparatus for applying the coating suspension. The shape may preferably be e.g. circular or circular-like, such as oval. The application means may comprise a plurality of nozzles arranged in one or more rows as know in the art.

"Substrate" as used herein can be any material to be coated with the coating method of the invention. In the method of the invention the substrate is particularly a fiber base which includes any web-like material containing fibers, and more preferably a paper or paperboard of any type, such as a paper or paperboard web which can be formed e.g. in a known manner in the wire section of a paper machine line.

Particularly, the invention provides a a method for reducing the abrasive effect of the coating suspension on application means of a coating apparatus for paper and paperboard industry, preferably of a paper or paperboard web, more preferably of a moving paper or paperboard web, whereby the coating suspension is applied in the form of a spray. The spraying is preferably carried out by means of a nozzle, preferably of a plurality of spraying nozzles which may be arranged in one or more rows in the coating apparatus, e.g. in the cross-wire direction. The spray-coating method of the invention can be a non-contacting spray-coating method, wherein the coating is formed directly from the droplets of the spray of the coating suspension, without further metering of the sprayed coating, e.g. without mechanical metering of the coating suspension applied by spraying on the surface of the web. In spray-coating methods the coating suspension is thus atomized under pressure to fine droplets, i.e. to spray, which is deposited on the web to form said final coating. Such spray-coating methods are typically high-pressure spray- coating methods. The coating methods such as jet applicator methods discussed above under the brief description of the prior art, differ from said non-contacting spray-coating method in that a further metering of the film coating on the surface of the web is normally effected in the jet applicator methods.

In the spray-coating embodiment, the spray may be formed in the nozzles in a manner known in the art, e.g. by using pressure. Thus the spray may be produced by atomizing the suspension into small droplets. The atomization nozzles can be of different type, e.g. atomizers based on rotary centrifugal atomization, air-blast nozzles and preferably high pressure spraying nozzles. The present method is very usable e.g. in low- and high-pressure spraying embodiments, preferably in high-pressure spraying embodiments.

The usable coating apparatuses and methods include e.g. coating apparatuses known in the paper and paperboard industry. Spray-coating apparatuses and methods usable in the present invention are explained and described i.a. in the above referred publication of OptiSpray Coating and Sizing Conference, Finland, 15.3.2001 , pp. 1-24., and articles: Paperi ja Puu - Paper and Timber, 83(7), 2001 , pp. 526 and 527, Wochenblatt fur Papierfabrikation, 10, 2001 , pp. 658-660, and Paperloop.com, May 2001 , pp.12-15.

The invention can be used for coating a substrate, e.g. a fiber base, preferably a paper or paperboard web, from one or both sides as known in the art. Furthermore, each side can be coated with one or more coating layers using the same or different coating mixtures, provided that at least one layer is formed with the method of the invention. The coating can be formed in a known manner, preferably in a manner known from the paper and paperboard industry, e.g. during the paper or paperboard making process in a coating unit placed in the wire section of the paper or paperboard machine. Alternatively, the paper or paperboard obtained from a paper or paperboard machine line may be coated off-machine in a separate coating unit, which are also well known in the art.

Calcium sulfate used in the present coating suspension is suitable for both acidic and neutral paper/paperboard making process.

Due to the use of calcium sulfate as a solid coating component in a coating suspension, the abrasive/wearing effect of a coating suspension on nozzles, particularly on the openings of the nozzles can be markedly reduced. Therefore the present invention further provides more generally the use of calcium sulfate as a solid coating component in a coating suspension, which coating suspension is applied as a discontinuous flow, preferably as droplets, on a surface of a substrate via application means of a coating apparatus that is susceptible to wear due to an abrasive effect of the coating suspension, for reducing the abrasive effect of the coating suspension on said coating apparatus, preferably on a spray-coating device, more preferably on a high-pressure spray-coating device.

The invention also provides the use of calcium sulfate as a solid coating component in a coating suspension for extending the use-life of the application means of a coating apparatus, which application means is susceptible to wear due to the effect of the coating suspension. More preferably, calcium sulfate is used in a spray-coating method, preferably in a high-pressure spray-coating method for extending the time interval between the changes of the worn nozzles of a spray- coating apparatus, preferably of a high-pressure spray-coating apparatus.

Moreover, the invention provides the use of calcium sulfate partly or fully in place of one or more other solid coating components suitable in a coating suspension for the paper or paperboard industry, preferably partly or fully in place of titanium dioxide, talc, calcium carbonate and/or kaolin, preferably calcium carbonate and/or kaolin, for reducing the abrasive effect of the coating suspension on a coating apparatus. The invention further provides the use of calcium sulfate as a solid coating component in a coating suspension for coating a substrate as defined above, preferably a paper or paperboard web, more preferably a moving paper or paperboard web by using the method as defined above, preferably a high- pressure spray-coating method as defined above.

Time- and cost-effective coating method can thus be provided.

The invention is illustrated with the non-limiting examples below. The used materials are commercially available unless otherwise stated.

Examples

In this experiment the abrasive effect of the coating solutions of the invention, which comprised calcium sulfate alone as the solid coating component or a mixture of calcium sulfate and a further solid coating component, on the nozzles of a spray-coating apparatus (OptiSpray, Metso paper Inc, Finland) was compared over the conventional solid coating components, kaolin and calcium carbonate.

Eight different test coating suspensions were prepared in water using commercially available solid coating components/pigments and additives, which were known in the paper industry. The test suspensions 1-8 were prepared by mixing in the water first the calcium sulfate component, if present, in amounts given in table 1 below, followed by 12 weight parts of dispersing agent (commercially available under trade name Fennodisp A41 , Kemira Chemicals Oy), 0.1 weight parts of binder (commercially available under trade name Latex Basonal)and calcium carbonate or kaolin, if present, in amounts given in table 1 below. The weight parts of each component are calculated from the solids content. The amounts of the solid component(s) of the test suspensions are listed in table 1 as follows: Table 1

The solids content of each test suspension 1-8 was 55 weight-% of the weight of the suspension. The viscosity determined as Viscosity Brookfield (100 rpm) was <100 mPas. The hardness of the particles was determined as Mohs¹ Hardness.

In the above test the tested suspensions caused deformation of the openings of the nozzles and increased the size of the opening. The order of the tested components from the most abrasive to the less abrasive was found to be: the used kaolin components were the most abrasive, then the used calcium carbonates and the used calcium sulfate was the less abrasive.

The less abrasive effect of the coating suspension of the invention in the above experimental test is also illustrated in figures 1-3. Figure 1 shows the opening of the unused nozzle before the spraying experiment (baseline), figure 2 the opening of the nozzle after the experiment with the coating suspension 5 of the invention (calcium sulfate having the Mohs's Hardness of 1.5) and figure 3 the opening after the experiment with the comparative coating suspension 4 (kaolin, having the Mohs' Hardness of 1.5-2). It can be seen from figure 3 that the shape of the opening after the spraying with the comparative coating suspension 4 is more deformed and markedly unsymmetrical, and the size of the opening is more increased compared to the shape and size of the opening after the spraying with the coating 5 of the invention (figure 2).

The test showed clearly the less-wearing effect of the calcium sulfate and this property is also surprising in view of the physical properties of the tested components. E.g. all the tested components had very similar hardness values and the most abrasive kaolins had i.a. the highest aspect ratio which reflects to the particle size/form, whereby it would have been expectable that the calcium sulfate with a higher aspect ratio than the tested calcium carbonates would have been more abrasive compared to carbonates. Unexpectedly, however, the carbonates were found to be less abrasive than calcium carbonate components.

In general, i.a. the deformation of the openings and the increase in size of the openings of the nozzles of e.g. a spray-coating device is markedly reduced when the coating suspension of the present invention is used compared e.g. to the conventional coating suspensions which do not contain calcium sulfate as a solid pigment.

Claims

1. A method for reducing the abrasive effect of the coating suspension on application means of a coating apparatus for paper and paperboard industry, in which a coating suspension is applied as a spray, characterized in that calcium sulfate is used as a solid coating component in said coating suspension.

2. The method according to claim 1 , characterized in that the method is a non- contacting coating method.

3. The method according to claim 1 or 2, characterized in that a mixture of calcium sulfate with one or more further solid coating components is used in said coating suspension.

4. The method according to claim 3, characterized in that calcium sulfate is used together with one or more solid coating components selected from a group including kaolin, precipitated and ground calcium carbonate, titanium dioxide, talc, aluminum hydroxide, barium sulfate, zinc oxide and sodium silico-aluminate, preferably selected from a group including calcium carbonate and kaolin.

5. The method according to any of the preceding claims, characterized in that calcium sulfate is used in an amount of 20-100 % by weight, preferably 20-80 % by weight, more preferably 40-70 % by weight, such as 40-65 % by weight, of the total solids content of the suspension.

6. The method according to any of the preceding claims, characterized in that calcium sulfate is calcium sulfate dihydrate.

7. The method according to any of the preceding claims, characterized in that the coating suspension is an aqueous coating suspension.

8. The method according to any of the preceding claims, characterized in that the coating suspension further comprises a binder and optionally other additives.

9. The method according to any of the preceding claims, characterized in that the solids content is up to 75 % by weight, such as from 2 to 70% by weight, preferably from 10 to below 65 % by weight, more preferably from 40 to 60 % by weight, of the weight of the suspension.

10. The method according to any of the preceding claims, characterized in that the coating suspension is applied by spraying on the surface of the substrate using a coating apparatus provided with a plurality of spraying nozzles arranged in one or more rows in the coating apparatus.

11. The method according to any of the preceding claims for spray-coating a paper or paperboard web, more preferably a moving paper or paperboard web.

12. The use of calcium sulfate as a solid coating component in a coating suspension for extending the use-life of the application means of a coating apparatus, which application means is susceptible to wear due to the effect of the coating suspension.

13. The use of claim 12 in a spray-coating method, preferably in a high-pressure spray-coating method for extending the time interval between the changes of the worn nozzles of a spray-coating apparatus, preferably of a high-pressure spray- coating apparatus.

14. The use of calcium sulfate as a solid coating component in a coating suspension, which coating suspension is applied as a discontinuous flow, preferably as droplets, on a surface of a substrate via application means of a coating apparatus that is susceptible to wear due to an abrasive effect of the coating suspension, for reducing the abrasive effect of the coating suspension on said coating apparatus, preferably on a spray-coating device, more preferably on a high-pressure spray-coating device.

15. The use of calcium sulfate partly or fully in place of one or more other solid coating components suitable in a coating suspension for the paper or paperboard industry, preferably partly or fully in place of titanium dioxide, talc, calcium carbonate and/or kaolin, preferably calcium carbonate and/or kaolin for reducing the abrasive effect of the coating suspension on a coating apparatus.

16. The use of any of claims 12-15 in the method according to any of claims 1-11.