WO2001029314A1

WO2001029314A1 - A method to achieve a release material coating, release material and base material for release material

Info

Publication number: WO2001029314A1
Application number: PCT/FI2000/000830
Authority: WO
Inventors: Mikko Rissanen; Kaj Koskinen; Jarmo Merisalo
Original assignee: UPM Kymmene Oy
Current assignee: UPM Kymmene Oy
Priority date: 1999-10-18
Filing date: 2000-09-28
Publication date: 2001-04-26
Anticipated expiration: 2002-04-18
Also published as: FI19992251L; FI19992251A7

Abstract

In the method for producing a release coating in the manufacture of a release material (R), such as release paper, a release agent based on silicone polymers is spread on top of a backing material (1), which release agent is catalytically cross-linked. The backing material (1), onto which the release coating (3) is spread, incorporates the coating's cross-linking catalyst. The cross-linking catalyst may be platinum or another catalytically active noble metal or tin, and it is spread onto the surface of the backing material acting as a base paper or added to the pulp stock for the base paper before formation of the paper web. There is no need for heating as used in previous methods to activate the catalyst.

Description

A method to achieve a release material coating, release material and base material for release material

The present invention relates to a method for producing a release coating, in which a release agent based on silicone polymers is spread on top of a backing material, which release agent is catalytically cross-linked. The present invention also relates to a release material manufactured by the method. In addition, the invention relates to a backing material for the release material.

Release materials are used as backing materials for various self-adhesive labels, said release materials having on the surface that is against the adhesive layer of the self- adhesive label, a release agent that is easily detachable from glue, which release agent is usually a layer formed of silicone polymer based material. The release material is usually so-called release paper, whose body layer, the base paper, is formed of cellulose-based fibres by a paper manufacturing technique. The backing material of the release material may also be synthetic, for example film.

In the following description, release paper manufactured on a paper machine is used as an example of a release material.

The finished self-adhesive laminate consists of a self-adhesive label and a release paper, which comes against the adhesive layer of the self-adhesive label, the layer being comprised of a face paper and an adhesive. The release paper should be detachable from the self-adhesive label in such a way that the adhesive layer remains on the back of the face paper. For this purpose, the release paper is provided with a suitable release agent on the surface coming against the adhesive layer. Usually a coating comprising silicone polymers is used as the release agent for this kind of release paper. A silicone polymer based coating is suitable in terms of its surface energy and chemical properties for use as a release agent with which the self-adhesive label glue does not react or mix. Low surface energy gives this kind of coating low adhesion, even for the stickiest self- adhesive label glues, whereby the laminate functions reliably even after a long period of time. The Si-Si bond typical of silicone is durable and thermally stable. The chemical nature of the silicone coating derives from the properties of the polydimethylsiloxane polymer that characterises the coating. On the surface of the release coating, the silicone molecules are oriented in such a way that the methyl groups point outwards from the coating. The release values of the self-adhesive laminate may be regulated by adding a regulating agent to the silicone coating recipe. The regulating agent functions on the basis of the disturbance of surface chemistry caused by the methyl groups.

The current functioning of a solvent-free silicone coating is based on a coating recipe where, among the silicone polymer, which contains vinyl groups, there is a catalyst that makes the cross-linking possible on the basis of an addition reaction (usually platinum in metal-complex form). The other main component of the coating recipe is a so-called cross-linker, whose functioning is based on the presence of Si-H groups. When the catalyst is present, the reaction enthalpy required for the double bond of the vinyl groups and the addition reaction taking place between the Si-H groups is so low that the reaction may take place even at room temperature.

The coating composition comprising a silicone polymer, a catalyst and a cross-linker is difficult to handle, because the silicone tends to cross-link prematurely in the coater rolls and mixing tanks. For this reason, an inhibitor has been added to the coating composition, which inhibitor prevents the catalyst from reacting prematurely. Usually the added agent is an organic compound that forms with platinum a complex that degrades again in certain conditions. The so-called 'thermal cure' silicone curing method is based on this phenomenon. The synthetic or fibre-based base paper is heated with the silicone coating in an oven to a temperature at which the catalyst-inhibitor compound and the catalyst are freed to catalyse the cross-linking reaction. The chemistry relating to the cross-linking of the silicone is based on the same principles in solvent and emulsion siliconisation. In the addition reaction, other noble metals beside platinum (Pt) may be used as the catalyst. In the cross-linking reaction based on condensation, the catalyst used is usually tin (Sn).

The cross-linking reaction based on polyaddition in the presence of a platinum catalyst has been described in the international publication WO 99/35181, among others. This publication describes the use of a polyaddition inhibitor so that a water-based silicone emulsion which already contains a platinum catalyst could be stored at normal temperature. The use of an inhibitor in a release agent composition dissolved in an organic solvent and containing a platinum catalyst, is described, e.g. in the publication EP-860 487, on increasing the storage stability of the composition at room temperature.

In order to produce single-component release agent (i.e. a release agent which is a mixture of several substances but may be spread on top of the base paper to be coated as it is, without mixing other substances into it), a release agent that at the same time may be stored without losing its stability, the composition should contain a special inhibitor, which it is important to portion carefully into the composition during manufacture. In addition, the inhibitor requires special treatment to release the catalyst, such as the above-mentioned 'thermal cure' silicone curing method established in the prior art.

The purpose of the invention is to eliminate the above-mentioned drawbacks and present a method for producing a release coating, in which method there is no need for a special inhibitor. In addition, the purpose of the invention is to present a release material manufactured by this method. A further purpose of the invention is to present a new backing material for release material, such as a base paper, with which the new method can be implemented.

The method according to the invention is mainly characterised in that the backing material, such as base paper, onto which the release coating is spread, contains the cross-linking catalyst for the coating. Since the catalyst, for example platinum, needed for the cross-linking reaction in the coating, is already in the base paper in a chemical structure that allows it to participate in the catalysis of the cross-linking reaction, the catalyst may be left out altogether from the ready-to-spread coating, whereby there is also no need for an inhibitor in the coating agent during storage. Since cross-linking only commences when the coating composition comes into contact with the base paper, silicone contamination of the silicone spreading rolls is reduced. In addition, the handling of the coating mixture becomes easier, as there is no need to beware of factors that would trigger the catalytic reaction, and for example, a temporary rise in temperature during storage is not harmful. In addition, the coating process proper becomes easier, as there is no need to introduce thermal energy for degrading the catalyst-inhibitor complex, and thus the need for heating is reduced, or heating even becomes altogether unnecessary. At the same time, changes caused in the base paper by heating, such as deformation and strength loss, are also reduced. The catalyst may also be chosen more freely, since it no longer needs to be chosen specifically to bond with the inhibitors, but it requires certain properties to enable its optimal use, for example, as a coating on the paper, or mixed in with the fibrous stock.

A catalytically active amount of the catalyst is incorporated in the base paper by a suitable addition method. The catalyst should be in such a form that it remains inert in relation to the paper chemicals. It is advantageous for the manufacture of paper provided with a catalyst, that the catalyst should be aqueous-phase soluble.

The silicone-based release agent layer later to be spread on the paper is thin, in the finished release material the grammage being less than 2 g/m² , preferably 0.5-1.5 g/m ² , normally about 1 g/m ² , whereby the catalyst can, from the base paper, affect the cross-linking of the whole layer.

It is characteristic of the release material relating to the invention that the release coating comprises a cross-linked silicone polymer whose cross-linking has been effected by the catalyst incorporated in the backing material.

A backing material incorporating a catalyst that cross-links the silicone polymer may be employed in the invention. This kind of material may be stored and unwound from the storage roll onto the coating unit in the manufacture of release paper.

The invention is described in more detail in the following with reference to the appended figures, in which

Fig 1 shows the finished self-adhesive laminate, in which release material as claimed in the invention is used, and

Fig 2 shows the method for producing the release material. According to Fig 1 , the self-adhesive laminate is formed of a self-adhesive label, which comprises an adhesive layer 4 on the back of the face paper 2, and of release material R, which comprises a layer of release agent 3 on top of the backing material 1 , the release agent layer 3 being against the adhesive layer 4. On the other side of the face paper 2, text and/or images may be printed by different printing methods.

The release material R for the self-adhesive laminate according to Fig 1 is formed of backing material 1, on top of which a release layer 3, comprising silicone polymers is spread, which release layer is, after spreading, cross-linked with the help of the cross- linking catalyst in the backing material 1. If the cross-linking of the release coating to be spread is based on an addition reaction, the catalyst may be platinum (Pt) or another noble metal catalytically active in this reaction. If the cross-linking is based on condensation, the catalyst used may be tin (Sn). The catalyst may be in any suitable form in which it creates a catalytic reaction in the coating when it comes into contact with the coating spread.

If the backing material is fibre-based (release material), the catalyst addition may be done by adding the catalyst to the surface of the base paper, for example, in such a way that it is mixed into the surface sizing recipe of the paper or the coating recipe, into the damping fluids at different stages, or into the steam heating or steam damping. These stages may be carried out with a size press or a coating machine as on-line or off-line stages, after the forming of a different paper web on the same production line or in a separate finishing unit. The catalyst may be included, for example, in thin water-based coating compositions comprising pigment and adhesive that are spread as a pre-coating on top of the base paper and that remain under the release layer in the manufacture of release paper. The devices for adding the catalyst, mixed with a suitable carrier material, to the surface of the face paper, may easily be connected to any stage of the process, for example, to the paper machine, supercalendering or slitting-winding, or they may be used in a separate surfacing or coating process. Addition to the wet end may in some cases also be used, in which case the substance is added to the fibre suspension forming the raw material of the paper. If the base paper is two-layer or multi-layer paper, the catalyst only needs to be added to the pulp fraction that forms the surface layer of the paper. Coating or addition methods such as the above are also suitable to be used in modified form for synthetic backing material such as plastic film.

The following describes by way of an example the method for manufacturing self- adhesive laminate, not however restricting the scope of the invention. The release material R is in this case release paper whose backing material 1 is a base paper manufactured in a paper machine.

In manufacturing self-adhesive label stock, the face paper 2 of the self-adhesive label stock and the base paper 1 of the release paper are brought to the production line as rolls of a pre-determined width and length. A catalytically active amount per unit area of the cross-linking catalyst is already in the base paper 1. The base paper is first coated in the roll coating unit 5 with a silicone layer 3, which polymerises after coming into contact with the base paper 1. The amount of silicone to be spread varies between 0.5-2.0 g/m² , usually 0.5-1.5 g/m² , for example, about 1 g/m². The silicone-based coating composition is spread as a single mass, in such a way that it includes all the components for the cross-linking reaction without the catalyst. Thus release paper R is produced, on the surface of which there is a layer of release agent 3. The broken line in the figure describes the previously used activation tunnel functioning at over 100° C, which may now be dispensed with. The catalytic cross-linking reaction takes place at room temperature with sufficient speed (especially cross-linking based on addition in the presence of platinum or other catalytically active noble metal), but in place of the activation tunnel, slight heating may be used if desired, for example, at less than 50 °C.

Next, the back of the release paper R can be printed on in the printing unit 11 and after this the release paper is coated on the siliconised side with an adhesive layer 4 in the roll coating unit 6. Usually the adhesive is a water-based dispersion, from which the excess water is evaporated by drying. After drying in the tunnel 7, the web is conveyed through a damping unit 8, in order to obtain a suitable humidity level on the adhesive side. The release paper R and the face paper 2, on which a primer layer may have been spread in unit 9 and which has been dried in the drier 12, are laminated together by conveying them through the rolls 10 that press the layers together, whereby the adhesive layer 4 remains between the siliconised surface of the release paper R and the face paper 2. The finished self-adhesive laminate according to Figure 1 is wound into a roll. The laminate may be further converted in many ways, for example, by printing on a printing machine. After printing, self-adhesive labels of the desired size may also be die cut from the laminate. The release paper with a layer of release agent may be brought to the production line as a finished product and only there joined to the face paper of the self- adhesive label stock.

The invention is not restricted to the method for manufacturing self-adhesive laminate. The release coating may be spread in catalytic contact onto the face paper, either in the production line of the self-adhesive laminate, whereby the release paper is fed continuously to laminating after cross-linking, or the release paper may be manufactured in the above mentioned way in a separate production line, so that the last processing stage is the possible printing on the back of the paper as shown in Figure 2, after which the paper is wound into a roll and delivered to the laminating line. The release paper or other silicone coated release material is not restricted either to the above-mentioned applications, but it may be used in all purposes where it is supposed to detach easily from a surface, when functioning as a surface cover, especially of an adhesive surface.

Further, the invention is not restricted to a particular backing material, but it may be applied both to synthetic and to paper-based backing materials, onto which the release agent is spread after the backing material has been furnished with the catalyst.

Claims

1. A method for producing release coating, in which a release agent based on silicone polymers is spread on top of a backing material (1), said release agent being catalytically cross-linked, and characterised in that the backing material (1) onto which the release coating (3) is spread, incorporates the cross-linking catalyst of the coating.

2. A method as claimed in Claim 1, characterised in that platinum or another catalytically active noble metal is used as a catalyst.

3. A method as claimed in Claim 1, characterised in that tin is used as a catalyst.

4. A release material comprising a backing material (1), and a release coating (3) based on silicone polymers which is spread on top of it, characterised in that the release coating (3) comprises a cross-linked silicone polymer, in which the cross-linking has been effected by a catalyst incorporated in the backing material (1).

5. A release material as claimed in Claim 4, characterised in that the catalyst is platinum or another noble metal catalytically active in the cross-linking of silicone polymers.

6. A release material as claimed in Claim 4, characterised in that the catalyst is tin.

7. A backing material for a release material, which backing material is intended to receive a silicone polymer-based, catalytically cross-linkable release coating, characterised in that the backing material (1) incorporates the cross-linking catalyst of the silicone polymer.

8. A backing material as claimed in Claim 7, characterised in that the catalyst has been spread on the surface of the backing material (1), in the form, for example, of surface size or coating.

9. A backing material as claimed in Claim 7, characterised in that the catalyst has been incorporated into the body layer of the backing material (1), for example, into the paper in the manufacture of the body layer.

10. A backing material as claimed in any of the Claims 7 - 9, characterised in that it is a base paper formed of fibres.

11. A backing material as claimed in any of the Claims 7 - 10, characterised in that it contains as a catalyst, platinum or another noble metal that is catalytically active in the cross-linking of silicone polymers.

12. A backing material as claimed in any of the Claims 7 - 10, characterised in that it contains tin as the catalyst.