CA2124576A1

CA2124576A1 - Cardboard with coating on one side

Info

Publication number: CA2124576A1
Application number: CA002124576A
Authority: CA
Inventors: Werner Bergmann; Paul-Heinz Dahling
Original assignee: Individual
Current assignee: Stora Feldmuehle AG
Priority date: 1991-11-29
Filing date: 1992-10-30
Publication date: 1993-06-10
Also published as: EP0614502A1; DE4139386C1; DK0614502T3; ATE124741T1; FI942441A0; WO1993011299A1; FI942441A; ES2074368T3; JPH07501362A; EP0614502B1; DE59202829D1

Abstract

ABSTRACT OF THE DISCLOSURE

The cardboard described has a coating applied by cast-coating, the side of the cardboard on which the coating is applied having, before application of the coating, a water-absorption capacity, measured by the Cobb sizing test, of 25 to 40 g/m2 per minute, and the coating consisting of a film-forming synthetic-resin binder and 10 to 60% by wt., relative to the total binder content, of a natural binder, the remainder being thickening and/or peeling and/or cross-linking agents present in an amount not exceeding 10% by wt., relative to the total binder content.

Description

212~ 7~:3 CARDBOARD WITH COATING ON ONE SIDE

The present invention relates to a cardboard with a coating applied on one side by the cast-coating process, a method for its production, and its use.

Cast-coated paper and cardboard of good surface smoothness and high gloss have long been known in the state of the art. DE-B-1233248 describes a cast-coating process in which a coating composition containing a metallic pigment and oxganic binding agent is applied to a paper web, for example, and dried in contact with a heated high gloss cylinder.

For the production of papers with a high metallic gloss, coatings have already been proposed which are produced by the deposition of metal vapors produced in a high vacuum onto papers provided with special coatings.
According to US-A-3113888, a paper or cardboard with a coating produced by the cast-coating method is used for this purpose. The coating consists of a synthetic resin binding agent which does not form a film at normal temperature and whose particles fuse between 71C and 99C during the cast-coating process to form a hard and glossy continuous film. If desired, the coating can also be applied to a base coating which contains one of the common synthetic resin binding agents plus filler.
Part of the synthetic resin binding agent to be used in the base coating can also be replaced by natural binding agents, so that 10 to 30 wt-% of the total binding agent content of the base coating can consist of, for example, starch, modified starch, methylcellulose, carboxymethylcellulose, hydroxyethyl-cellulose and similar substances.

?J ~ 2 b~

For the production of the coating that is to be metalized the so-called "direct method" is used, as described in DE-B-1233248 referred to above, and also the "indirect method" in which the coating composition is applied to the glazing cylinder, and before it is dry, the dry paper is brought in contact with this still wet film, the latter being absorbed by the paper and drawn from the drum.
In the process disclosed in DE-B-2310891, it is not the cast-coating process disclosed in DE-A-1233248 (also called "direct method") that is involved. Instead, first a part of the water is evaporated with the simultaneous formation of a continuous film from the polymer contained in the coating composition, and the film that has formed is finished against a high gloss cylinder at a temperature above 100C and a minimum pressure of 5 kp/cm2. To prevent the film from sticking to the glazing cylinder during the finishing taking place under pressure, up to less than 30~ of a substance reducing its tackiness can be added to the coating composition. Among other such substances, sodium carboxymethylcellulose, methylcellulose, polyvinyl alcohol and polyvinyl pyrrolidone are proposed, for example, in amounts up to 10% of the weight of the main polymer. As the base coating, the common prime coatings known in paper manufacture are provided, with and without pigment additives.
EP-B-98368 discloses a metalized paper in which the coating provided for metalizing is formed under process conditions similar to those stated in DE-B-2310891. The coating composition contains 5 to 25 weight-parts of film-forming binding agent per 100 weight-parts of pigment, and thus it has the pigment-to-coating ratio .- 212A.J'7~' of conventional paper coatings. What differs it from the other paper coatings, however, is the addition of a synthetic polymer pigment, which is added in an amount of 5 to 100 weight-percent of the total pigment content. This proposal is said to eliminate the former disadvantage which was that the coating surface provided for metalizing first had to be provided with a varnish pri~er on the basis of organic solvents.

However, the proposal of EP-B-98368 is not free of disadvantages, because the coating to be metalized has to be prepared in a minimum amount of 10 g/m2, preferably with an applied weight of 18 to 26 g/m2.
Another disadvantage results from the high costs of the synthetic polymer pigment.

The problem to which the present invention is addressed is to make available, by resorting to lower-cost raw coating materials, a cardboard covered on one side with a single coating, which due to its high gloss, its smoothness and its closedness, is to be used wherever decorative impression is desired. The invention sees a special problem in the formation of a coating which will permit direct metalization, i.e., without previous ~ -priming with a varnish on the basis of organic solvents, so that the result will be a product of maximum smoothness with a high gloss and a metallic appearance. The invention also intends to make a cardboard available which can be imprinted by gravure in the non-metaliæed state, and, at least with special printing inks, by offset printing. The printed product is to have good ink hold-out and a high print gloss. -For the solution of this problem, the present invention provides, in the case of a cardboard having a one-side coating applied by the cast-coating process, that % ~ 3~3 . , before the coating is applied to the side that is to bear the coating, it has a Cobb water absorption rating of 25 to 40 g/m2 in 60 seconds, and the coating consists of a film~forming synthetic resin binding agenthaving a film forming temperature of <40~C, a natural binding agent in the amount of 10 to 60 weight-percent of the total binder content, remainder:
thickening adjuvants and/or release agents and/or crosslinking adjuvants in the amount of not more than 10 wt.-~ of the total binding agent content.
..
Within the scope of the present invention, the term cardboard signifies a flat material, i.e., a cardboard web or a cardboard sheet.
`~
In an especially preferred embodiment, the cardboard is single ply cardboard; especially preferred is a cardboard in which 100% of its fiber mass consists of cellulose fibers.
Another preferred embodiment provides for the admixture of up to 5 weight-percent of ground wood fibers with respect to the total fiber mass of a single-ply cardboard.
According to another cost-attractive embodiment of the present invention, a single-ply cardboard can be used, although it is necessary for the cardboard to have a cover 100~ of whose fiber content consists of cellulose, or ground wood fibers can be present in the cover coating up to a maximum of 5% by weight, remainder cellulose fibers. In this case, of course, the cover coating serves for the application of the finish by the cast-coating process.

2~ 37i3 A cardboard is preferably used whose weight per unit area is between 150 and 350 g/m2, reckoned as the fiber mass.

According to another preferred embodiment, a cardboard is used in which the necessary moisture absorbing capacity of 25 to 40 g/m2 in 60 sec. by the Cobb test has been created by imbibing the cardboard, prior to cast-coating, with a synthetic resin binding agent and treating it after drying on a soft or supercalender.
Suitable synthetic resin binding agents are especially plastic dispersions on a basis of butadiene-styrene, acrylic resin, methacrylic resin, on a basis of polyvinyl acetate or polyvinyl alcohol solutions, as well as mixtures thereof. Polyacrylamide has also proven to be especially well suited. The amount of the synthetic resin binding agent to be applied is at least 1 g/m2 and at most 10 g/m2. Preferably, the amount applied is 1 to 3 g/m2, especially 1 to 2 g/m2. ~-Instead of imbibing with a synthetic resin binding agent, a coating of one or more superimposed layers can be applied to the side that is to receive the cast coating; this coating consists of a composition containing a film-forming synthetic resin binding agent and/or natural binding agent, and the coating that forms is calendered to develop the desired surface qualities. This coating operation can be performed with the customary coating equipment of the paper and cardboard industry; in addition, the coating can also contain mineral pigment or dyes.

After imbibing with synthetic resin binding agent and after the calendering that follows, the thickness of the cardboard should range from 145 to 400 ~/m, 2 ~ 2 ~

preferably the thickness is to be no greater than 370 ~m.

According to an advantageous embodiment, mineral pigment can also be contained in the cardboard, preferably in the range of 9 to 13 wt.-~, expressed as ash content, of the total area mass of the cardboard.

According to an additional, quite preferred embodiment, the proportion of the natural binding agent in the coating applied on one side by the cast-coating process amounts to 20 to 35 weight-percent of the total binding agent content.

The smoothness of the cardboard side that is to receive the cast coating should be about 50 Bekk-sec.;
preferably the smoothness by the Bekk standard is greater than 150 sec.

The amount of the coating applied by cast coating is at least 5 g/m2, but preferably more than 10, up to a maximum of 15 g/m2. Applying a lesser coating weight, reckoned as dry weight percentage, can bring the result that the surface produced on the cardboard does not have the closedness and the desired high gloss, which according to a preferred embodiment is about 88 to 98~, measured at an angle of 75 degrees (Lehmann method).

The coating applied by the cast coating method consists to at least 90 wt.-% of binding agents only, and otherwise contains only the conventional adjuvants such as are used in cast coating, that i5, for example, release agents, such as calcium stearate or one of the common thickening agents on a natural or synthetic basis.

2 f 2~

The invention also comprises a method for the production of a cardboard coated on one side with a high gloss and great smoothness. For the practice of this method, a raw cardboard is imbibed with an impregnating fluid contai~ing a synthetic resin binding agent or consisting of it, dried, and calendered with a supercalender or soft calender. Then, onto the calendered surface, a composition is applied which consists of a film-forming synthetic resin binding agent, a natural binding agent in the amount of 20 to percent of the weight of total binding agent, balance: thickening adjuvant and/or release agent and/or crosslinking adjuvant in a total amount not exceeding 10 percent of the total binding agent content, and the coating applied is evened out and contacted with a heated high gloss cylinder with simultaneous drying to form a high-gloss surface.

According to an alternative embodiment of the process of the invention, instead of imbibing the raw cardboard with a synthetic resin binding agent, the raw cardboard is covered with one or more coats of a composition containing a film-forming binding agent and/or natural binding agent or consisting thereof, and the coating formed is dried and calendered. Then, a cast coating is applied in the manner described above. By each of the two methods there is formed a cast coating which can be considered a closed film of little porosity.

It follows from the above description of the method that the term, cast coating process, used in the patent claims and description in connection with the present invention, means exclusively a method in which the coating composition is evened out immediately after application to the cardboard web and brought in contact ~ 2~2~7~3 with a heated high-gloss cylinder, cf. DE-B-1233248, as weIl as the "direct method" described in US-A-3113888.

The film-forming synthetic resin binding agents to be used in forming the cast coating are used in the form of aqueous dispersions, preferably on a basis of the polymers and copolymers of acrylic acid esters, methacrylic acid esters, butadiene-styrene, vinyl acetate and vinylidene chloride. For example, the synthetic resin binding agents which have film forming temperatures of <30C.
. , I~, according to the first embodiment of the method of the invention, the raw cardboard is impregnated with synthetic resin binding agent, this is preferably performed back in the paper machine, e.g., in a size press. If, instead of the cardboard web being impregnated with a synthetic resin bindiny agent on the side that is to take the cast coating, the web is provided with a coating, it is preferable to use for this purpose the same film-forming synthetic binding agent as is used for the cast coating, but in this case, synthetic resin binding agents with a higher film-forming temperatu~^e can be used, if desired. The natural binding agents for producing the coating and the cast coating include cellulose derivatives, such as carboxymethyl cellulose, hydroxyethyl cellulose, preferably caseine and starch, modified starch, as well as mixtures of the above-named natural binding agents.
In the coating, the content of the natural binding agent is preferably not more than 70 weight-percent, and very especially preferred is a content ranging between 10 and 50 weight-percent, reckoned in each case as dry weight percentages of the total binding agent content.

.~ , -:~ 2~2~7~

Chalk, kaolin and titanium dioxide can be used in the coating as mineral pigment, the total binding agent content, reckoned as dry weight percentages, is around 5 10 to 20 weight-percent with respect to the pigment ;
content.

Due to the content of natural binding agent always present in the cast coating, the cardboard according to the invention can be manufactured at an especially low cost. If, however, more than 60 wt.-% of natural binding agent is added, a disadvantage results insofar as only those coating compositions which have a low solid content can be prepared and the saving is offset by the greater expense of drying. There is also the danger that the sensitivity of the cast coating to moisture increases.

According to a preferred embodiment, the coating has a Bekk smoothness in the range from 200 to 2,000 sec. For the attainment of a smoothness rating of this kind, it has proven necessary to produce a coating with a weight per unit area of at least 5 g/m2. A coating with a weight of 7 to 22 g/m2 has proven especially appxopriate, but preferably, the weight should not exceed 15 g/m2. If such a coa~ing is calendered, with a supercalender for example, it also offers an outstanding basis for the cast coating that follows.
The smoothness of the cast coating is so good that it is not measurable by the conventional Bekk method.
-.:
Data expressed in the description and claims concerning the binding agent content refers to dry-weiqht.

Owing to its great smoothness and high gloss, the cardboard according to the invention is appropriate for ~-~ 2~ 7~j direct metalization without the need to prime the cast coat with a varnish based on organic solvents.

Another especially preferred use of the cardboard according to the invention consists in using it as medium for gravure printing. As a result of the pigment-free cast coating, the gravure ink stands very well on the surface and the print has a high gloss.

Very special importance is to be placed on the use of the cardboard according to the invention as the substrate for the offset prin~ing process with the simultaneous use of ultraviolet-drying printing inks.
Although the cast coat, since it contains no pigments, forms a virtually closed film, it can be printed outstandingly by the offset process with printing inks that can be dried by ultraviolet radiation. The advantageous effect achieved when printing the cardboard according to the invention with ultraviolet-drying offset printing inks is explained as follows:

- The offset printing inks applied remain standing on the surface since it is so closed that the result is a print of great brilliance and intensity of color.

- Since the ultraviolet-drying inks are dry immediately after leaving the offset press, no smearing and sticking of the freshly applied ink ~0 occurs when the printed sheets are stacked, when the bottom sheets are subjected to a high pressure loadinq.

On the other hand, there has been heretofore the disadvantage in the printing of cast-coated types of cardboard, but even of other coated types of cardboard, r~. 21?~ri7~

that in the case of ordinary printing inks (not ultraviolet-drying), especially in the case of heavy applications of printing ink to achieve an intense print, the printed sheets became smeared. The reason for this, is that, in the case of ordinary printing inks, an oxidative drying process occurs, which is not `
complete until after about 48 hours. Even with the use of ultraviolet-drying pxinting inks, these problems have not been overcome when using the previously known types of cardboard, because even ultraviolet-drying printing inks strike in greatly, and seen that way they are no longer available on the surface and prints of lower intensity and lower color brightness result.

Example 1:

A single-ply cardboard, whose fiber mass consists to 100 wt.-% of cellulose fibers, and having a weight per unit area of 180 g/m2, is impregnated with polyacrylamide as a synthetic resin binding agent, of which 2 g/m2, reckoned as dry weight, is absorbed.
After the cardboard web is dried the web is calendered.
Measurement of water absorption capacity according to Cobb shows that the capacity is reduced to 26 g/m2 for 60 sec. A Bekk gloss of 120 sec. is measured.

The following composition is applied to the cardboard thus prepared:

Cold-water soluble starch 29 weight-parts ~ `
Synthetic binding agent on ~-a polyacrylate resin base 68weight-parts Calcium stearate 3 weight-parts Of this composition 12 g/m2, reckoned as dry weight, is applied to the cardboard, the applied composition is evened outJ and the cardboard web is brought in contact : :`
.~

2 L 2 ~ ~3 7 ~

with a heated hiyh gloss cylinder, so that while the applied composition is drying, a high-gloss coating is formed on the surface of the cardboard.

Measurement of the gloss of the cast coating by the Lehmann method at 75 showed a gloss of 95%. Estimation of the closedness of the sur~ace by a test method of our own, which is called the "microcontour test,"
showed a pore-free and completely closed surface. An outstandingly metalizable cardboard is obtained.

Example 2:

This example describes the use of a multi-ply cardboard with a recycled paper inlay and a cover layer consisting to 100% of cellulose. The cover has a weight of 25 g/m2, while the total weight of the raw cardboard amounts to 210 g/m2.

A doctor is used to apply to the top surface of the cardboard a coating of the composition specified as follows:

Kaolin 71.8 weight-parts Ground chalk 17.4 weight-parts Butadiene styrene latex 9.0 weight-parts Cold-water soluble starch 2.0 weightparts All the compone.nts are stated as dry weight parts.
10 g/m2, reckoned as dry weight, was applied. After calendering, the coating has a Bekk smoothness of 510 seconds.

Using the formula given in Example 1 for the preparation of the cast coat, a cast coat is formed on ~ " 2 ~ 7 ~

the calendered coating by the method specified in Example 1, achieving an applied dry weight of 7 gm/2.
The gloss and closedness of the surface is the same as the result obtained in Example 1. The cardboard is outstandingly metalizable.
Example 3:
The procedure of Example 2 is followed, but the formula for the application of the coating differs as follows in regard to pigment composition:
ICaolin 67.2 wt.-parts Ground Chalk 12.9 wt.-parts Titanium dooxide 8.9 wt.-parts The binding agent composition for the coating is the same as given in Example 2.
The results obtained show good coverage of the recycled paper inlay of the cardboard, so that an outstandingly printable cardboard is the result.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. Cardboard with a coating applied to one side by cast-coating methods, characterized in that, before application of the coating, the side of the cardboard that is to receive the coating has a Cobb water absorption of 25 to 40 g/m2 in 60 seconds, and the coat consists of:
- a film-forming synthetic resin binding agent having a film forming temperature of <40°C;
- a natural binding agent in an amount of 10 to 60 wt.-% with respect to the total binding agent content, - balance: thickening adjuvant and/or release agent and/or crosslinking agent in an amount totaling not more than 10 wt.-%, with respect to the total binding agent content.

2. Cardboard according to claim 1, characterized in that the cardboard is a single-ply cardboard.

3. Cardboard according to claim 2, characterized in that it consists to 100 wt.-% of cellulose fibers with respect to the total fiber mass.

4. Cardboard according to claim 2, characterized in that it contains up to 5 wt-% of ground wood fibers, with respect to the total fiber mass.

5. Cardboard according to claim 1, characterized in that it is multi-ply and has a cover coating whose fiber mass consists to 100 wt.-% of cellulose or up to a maximum of 5 wt.-% of ground wood fibers, balance:
cellulose fibers.

6. Cardboard according to any one of claims 2 to 5, characterized in that it has a mass per unit area of 150 to 350 g/m2, reckoned as fiber mass.

7. Cardboard according to any one of claims 1 to 6, characterized in that it is imbibed with an impregnating liquid containing a synthetic resin binding agent or consisting thereof.

8. Cardboard according to any one of claims 1 to 7, characterized in that the amount of the synthetic resin binding agent applied by imbibing amounts to 1 to 3 g/m2, reckoned as dry weight parts.

9. Cardboard according to any one of claims 1 to 6, characterized in that it has a calendered coating of one or two superimposed layers provided for receiving the coating to be applied by cast-coating methods, the layer(s) consisting of a composition containing film-forming synthetic resin binding agent, and/or natural binding agent.

10. Cardboard according to any one of claims 1 to 6, characterized in that it has a calendered coating of one or two superimposed layers provided for receiving the coating to be applied by cast-coating methods, the layer(s) consisting of a composition containing film-forming synthetic resin binding agent, and/or natural binding agent and natural pigment.

11. Cardboard according to any one of claims 1 to 10, characterized in that the side of the cardboard that is to receive the coating has a Bekk smoothness of at least 50 sec. before application of the coating.

12. Cardboard according to any one of claims 1 to 11, characterized in that the content of the natural binding agent in the coat applied to one side by cast coating processes amounts to 20 to 35 wt.-% with respect to the total binding agent content.

13. Cardboard according to any one of claims 1 to 12, characterized in that the coating applied by cast-coating processes has a mass per unit area of at least 5 g/m2.

14. Cardboard according to any one of claims 1 to 13, characterized in that the coating applied by cast-coating processes has a mass per unit area of greater than 10 to 15 g/m2.

15. Cardboard according to any one of claims 1 to 14, characterized in that the coating applied by cast-coating processes has a gloss (by the Lehmann method) of 88 to 98% measured at an angle of 75 degrees.

16. Method for the production of a cardboard according to any one of claims 1 to 8 and 11 to 15, characterized in that the cardboard is imbibed with an impregnating liquid containing a synthetic resin binding agent or consisting of same, dried, and glazed with a supercalender or soft calender, and onto the glazed surface a composition is applied which consists of a film-forming synthetic resin binding agent, a natural binding agent in an amount of 10 to 60 wt.-%, with respect to the total binding agent content, balance:
thickening adjuvants and/or release agents and/or crosslinking agents, in an amount totaling not more than 10 wt.-% with respect to the total binding agent content, that the applied composition is evened out and brought in contact with a heated high-gloss cylinder with simultaneous drying for the formation of a high-gloss coating.

17. Method for producing a cardboard according to any one of claims 1 to 6 and 9 to 15, characterized in that a coating is formed on the carton by one or more applications of a composition containing a film-forming binding agent and/or containing natural binding agent or consisting thereof, the coating is dried and glazed, and to the glazed surface a composition is applied which consists of a film-forming synthetic resin binding agent, a natural binding agent in an amount of 10 to 60 wt.-%, with respect to the total binding agent content, balance: thickening adjuvants and/or release agents and/or crosslinking adjuvants in an amount totaling not more than 10 wt.-% with respect to the total binding agent content, and that the applied coating is evened out and contacted with a heated high-glazing cylinder with simultaneous drying to form a high-gloss surface.

18. Use of a cardboard according to any one of claims 1 to 15 as substrate for a metal coating to be applied by the vacuum vapor process to the coating produced by cast-coating processes.

19. Use of a cardboard according to any one of claims 1 to 15 as a print medium for packaging to be imprinted by the gravure process.

20. Use of a cardboard according to any one of claims 1 to 15 as a print medium for ultraviolet-drying offset printing inks.