EP0396789A1 - Process for making release paper with a special surface coating - Google Patents

Abstract

A process for modifying the surface of raw release papers is described, in which a suspension of film-forming substances is applied within or outside the paper machine by means of conventional applicators to the paper web, if appropriate with the addition of a white pigment, which suspension already contains a fraction of up to 25% of a catalyst in the form of an aqueous emulsion.

Description

The invention relates to a process for the production of release paper with special surface coats (primer coats) inside or outside the paper machine.

It is known to coat release papers within the paper machine. Common impregnation or surface solutions made from polyvinyl alcohol (PVA), starch derivatives, carboxymethyl cellulose (CMC), alginate, protein etc. are used. as well as polymer dispersions (latices) applied to the paper surface alone or in combination with known application devices such as roller, blade or air brush application devices or immersion impregnation devices. Depending on the concentration, temperature and viscosity of these coating compositions, a pure surface covering of the paper, a partial or complete penetration of the coating compositions into the base paper is achieved (see "Coating" (1986), No. 7, pp. 218-221 and (1986) , No. 8, pp. 262-268).

Release papers can also be coated with pigment-binder combinations using the application devices already mentioned. For this purpose, mainly latices are used as binders and white pigments such as kaolin and / or calcium carbonate (see "Coating" (1987), No. 10, pp. 366-372 and (1987), No. 11, pp. 396-398).

It is also known to coat release papers within the paper machine with aqueous silicone systems (emulsions) to achieve a certain abhesive effect, whereby Various aqueous film formers and thickeners, mostly PVA or CMC, can be added to these aqueous silicone systems. However, the silicone resin used always forms the main component, since it primarily influences the adhesive effect of the papers coated in this way with a special purpose (for example baking release papers, sack papers with hydrophobic properties, etc.).

However, these papers coated in this way cannot be used for the technical sector with the strictly defined and reproducible release force level required there. For this, siliconization must be carried out on separate coating systems, e.g. with silicone emulsions. The usually 40..50% aqueous, thermally crosslinkable emulsions are then applied using known application devices, such as e.g. the anilox roller, applied to the base paper.

The aqueous silicone systems contain organopolysiloxanes, e.g. Polydimethylsiloxanes, to which catalysts, crosslinking agents (e.g. based on methyl hydrogen siloxanes), optionally also adhesives (e.g. water-soluble reactive silane esters) and "controlled release" additives are added (see "Adhesion" (1973), No. 7). The manufacturers of these aqueous silicone systems predominantly already offer the base polymer with crosslinker or catalyst additive (crosslinker or catalyst emulsions). The further components are then added by the processor shortly before use (raw paper coating) in order to achieve the desired abhesive properties of the papers coated in this way.

However, the object of the present invention is within the paper machine release paper with special surface coatings, but without producing any additional abhesive effects, which enable better adhesion and faster crosslinking of subsequent customary separate coatings with a wide variety of silicone resins.

The object is achieved in that a suspension of film-forming substances, optionally with the addition of white pigments, which contains up to 25% (calculated) catalyst in the form of an aqueous emulsion is applied to the paper web inside or outside the paper machine by means of conventional application units.

Surprisingly, it was found that a relatively small addition of catalysts in the form of aqueous catalyst emulsions to customarily used surface preparations for release base papers leads to improvements in the adhesion and crosslinking of subsequent separate silicone applications of different chemical compositions, without the release agent paper thus surface-treated (primer-coated) in this way having additional adhesive properties would have been awarded.

The following substances, for example, can be used as the catalyst component for the aqueous emulsion:
Finely divided platinum, ruthenium, rhodium, palladium, iridium and compounds or complexes of these elements, in particular platinum halides such as PtCl₄ platinum (IV) chloric acid and Na₂PtCl _n · 4H₂O, platinum-olefin complexes, platinum-alcohol or platinum Alcoholate complexes, platinum ether complexes, platinum aldehyde complexes, platinum ketone complexes and platinum vinyl siloxane complexes; also iron, nickel and cobalt carbonyls.

The following examples illustrate these unexpected reactions better.

example 1

a) A mixture of 22 g of polyvinyl alcohol and 3 g of carboxymethyl cellulose in 475 g of water is prepared in a glass container and heated to 90 ° C. in a water bath. The cooking time of the PVA-CMC suspension is 20 minutes, with the CMC and PVA completely dissolving in the water. After the cooking process, 3 g of at least 3 Si-bonded hydrogen atoms per molecule having organopolysiloxane are added as the base polymer with the addition of Pt catalyst in the form of a 40% emulsion. The pH value of this mixture was adjusted to 4.0 using sulfuric acid.

The dispersion produced in this way is applied with a laboratory size press to unsatinated silicone base paper with a basis weight of 66 g / m². The application weight is about 1.5 g / m² (calculated). The uncoated base paper had a porosity according to Schopper of approx. 60 ccm³ / min and a degree of sizing according to Cobb-Unger of 50 g / m².
After drying and rewetting, the paper treated with it was satinized in a two-roll laboratory calender. The line pressure was 4000 dN / cm. The surface temperature of the steel roller was 100 ° C.

b) The paper thus obtained was siliconized in a laboratory process. The siliconization was carried out with a KCC 302 squeegee applicator, which uses various wire-wrapped metal rods to apply the respective silicone to the paper sheets at a constant speed.
A conventional solvent-free polysiloxane system with the following composition was used for the silicone coating:
100 parts of base polymer silicone from Wacker
80 parts white spirit (boiling range 60-80 ° C)
2.5 parts of crosslinker
1.0 part catalyst
The amount of silicone applied was about 1 g / m² (calculated).

To crosslink the applied silicone layer, the coated paper was placed on a metal sieve in a circulating air oven operated at 150 ° C.

The crosslinking time was set differently in order to follow the influence of the catalyst emulsions on the crosslinking and anchoring process. In Table 1 below, only the shortest crosslinking times are recorded, during which complete hardening and anchoring (adhesion) of the silicone layer is still guaranteed. The paper samples hardened at different times were immediately subjected to a scratch test, with the finger being rubbed 8-10 times over the silicone film. The pressure is selected so that the fingertip heats up significantly when rubbed. A disturbance in the silicone coating shows itself in the form of rubbed off beads ("rub off") and as a matt spot ("smear") if you look at the sheet of paper under the bevel.

As a reference paper (blank), i.e. a paper without the addition of catalysts, a paper was used which had also gone through the treatment steps described above, but contained no further additives in the PVA / CMC solution.

Example 2

The procedure of Example 1 was repeated. After adding the catalyst emulsion, however, the pH was raised to 9.0 using ammonia. The results of the crosslinking tests are shown in Table 1 below.

Example 3

The procedure of Example 1 was repeated, but instead of coating the test paper with the solvent-free silicone system described in part b) of Example 1, a solvent-containing system with the following composition was chosen:
74 parts white spirit
20 parts of base polymer silicone
0.2 parts crosslinker
0.8 parts catalyst
The siliconization was carried out analogously to Example 1, part b) using a laboratory doctor device, the silicon application again amounting to approximately 1 g / m 2 (calculated). The shortest networking times determined are shown in Table 1.

Example 4

The procedure of Example 2 was repeated. However, the laboratory siliconization was carried out using the solvent-containing silicone systems described in Example 3. The minimum required networking times are shown in Table 1.

Example 5

The procedure of Example 1 was repeated. However, instead of 3 g, this time 6 g of catalyst emulsion was added to the solution of 22 g of polyvinyl alcohol and 3 g of carboxymethyl cellulose in 475 g of water. The pH of this mixture was again adjusted to 4.0 with sulfuric acid. The further processing corresponded to the procedure described in Example 1. The results of the crosslinking test are summarized in Table 1.

Example 6

The procedure of Example 5 was repeated, but the pH of the coating mixture was raised to 9.0 with ammonia. The results of the crosslinking test are shown in Table 1.

Example 7

The procedure of Example 5 was repeated. However, the test paper was coated with a solvent-containing silicone system from ICI. The composition of this coating composition has already been described in Example 3. The results of the crosslinking test are shown in Table 1.

Example 8

The procedure of Example 6 was repeated. However, the laboratory siliconization was carried out using the solvent-containing silicone system described in Example 3. The shortest networking times required can be found in Table 1.

Examples 9-12

As a comparison (zero sample), paper samples were used which had been surface-finished with a mixture consisting of 22 g of polyvinyl alcohol and 3 g of carboxymethyl cellulose in 475 g of water, without any addition of catalyst emulsion. The pH values of these surface preparations were set to both 4.0 and 9.0. The laboratory siliconization was carried out using those in part b) of the example and in example 3 described catalyst emulsions. The minimum crosslinking force required can be found in Table 1. Table 1 example pH value of the surface preparation shortest crosslinking time in s, at which a complete hardening is given (150 ° C) 4.0 9.0 LF * LH * 1 x 5 2nd x 5 3rd x 5 4th x 10th 5 x 3rd 6 x 3rd 7 5 8th 10th 9.10 (blank) x 10th 20th 11.12 (blank) x 15 20th * LF = solvent-free silicone system) each approx. 1g / m² (solid) LH = solvent-containing silicone system) silicone application

Claims (6)

1. A process for the surface modification of release papers, characterized in that a suspension of film-forming substances, optionally with the addition of white pigments, is applied to the paper web inside or outside the paper machine by means of customary application units, which proportionately up to 25% (calculated) catalyst in the form contains an aqueous emulsion. 2. The method according to claim 1, characterized in that as the catalyst component, for example platinum, ruthenium, rhodium, palladium, iridium and compounds of the complexes of these elements, in particular platinum halides such as PtCl₄ platinum (IV) chloric acid and Na₂PtCL _n · 4H₂O, platinum olefin Complexes, platinum-alcohol or alcoholate complexes, platinum-ether complexes, platinum-aldehyde complexes, platinum-ketone complexes and platinum-vinylsiloxane complexes; iron, nickel and cobalt carbonyls can also be used. 3. The method according to claim 1 and 2, characterized in that these catalysts in the form of aqueous catalyst emulsions in proportions of 1 to 25% (calculated), preferably in proportions of 3 to 10% of conventional surface preparations from film-forming substances for release paper such as polyvinyl alcohol , Carboxymethyl cellulose, starch derivatives, alginate or polymer dispersions (latices) alone or in combination in a pH range from 2 to 11, preferably 4 to 9.5, can be added. 4. The method according to claim 1 and 2, characterized in that these catalysts in the form of aqueous catalyst emulsions in proportions of 1 to 25% (calculated), preferably in proportions of 3 to 10% customary pigmented coating compositions of binders, such as. Copolymer dispersions (latices) based on acrylic acid, acrylic acid ester, acrylonitrile, vinyl acetate and / or styrene, polyvinyl alcohol, carboxymethyl cellulose or polyvinyl alcohol alone or in combination with white pigments, e.g. Kaolin, calcium carbonate, titanium dioxide alone or in combination, in a pH range of 5 to 11, preferably 7 to 9.5, can be added. 5. The method according to claim 1 to 4, characterized gekennzeichent that the surface application by means of conventional application units, based for example on the roller (eg size press) and / or doctor blade (eg blade) principle, on an already formed paper web with a residual moisture content between 1 and 14%, preferably 2 to 8%, inside or outside the paper machine. 6. Use of a release paper produced by the method according to claim 1 to 5 as packaging material for sticky goods, for baked goods, food packaging, for plastic laminates such as so-called laminated boards, clamping plates or the like and for the self-adhesive sector.