EP0317949B1 - Process for coating firmly closed surfaces - Google Patents

Abstract

In order to coat a solid, closed surface which bears an adhesive protective foil which can be pulled off, the protective foil is prised off the surface and a coating medium of low viscosity is filled into the resulting gap. When pulling off the protective foil, the gap is moved over the surface, coating the surface. The coating medium in the gap is replenished according to use.

Description

The invention relates to a method for coating a solid, closed surface which carries a peelable protective film with a low-viscosity coating agent. For the purposes of the invention, a surface is considered to be solid if it can be stuck to the entire surface with a removable protective film and the protective film can be removed again without destroying or damaging the surface. It is considered closed if it is free of cavities or openings where the protective film is not in contact with the surface. In this sense, fabrics and fleeces do not have a closed surface.

The invention preferably relates to the coating of surfaces of flat substrates, such as sheets, sheets and foils. The coating of such surfaces with decorative or functional cover layers has a significant scope.

State of the art

Sensitive surfaces of flat substrates, such as high-gloss plastic sheets, are often provided with a removable protective film to protect them from damage during storage, transport or processing, which can be easily removed before use. When the protective film is removed, electrostatic charges can arise Tighten dust particles and hold on to the surface. If such a surface is to be coated with a low-viscosity coating agent, careful cleaning is required first, followed by coating under clean room conditions.

Various methods are used for coating solid surfaces with liquid coating agents, such as dipping, brushing, spraying, pouring, knife coating, rolling, etc. When selecting the coating method, the thickness of the coating provided, the viscosity of the liquid coating agent, the process and apparatus costs play a role , the speed of work, the requirements for the quality of the surface etc. a role. Very thin, high uniformity coatings are best made with a low viscosity coating agent by the dip coating process. For this purpose, the substrate to be coated is immersed in a bath of the coating agent and pulled out vertically at a uniform speed. The thickness of the coating depends primarily on the viscosity of the coating agent and to a lesser extent on the rate of ascent. The process requires an immersion bath that is larger than the substrate. In addition, it is only suitable for substrates that can be immersed in a bath at all and should be coated on the entire immersed surface.

Numerous variants are known for coating the surfaces of flat substrates by roller application, which M. Maggi in "Plastic Engineering", March 1984, P.61-65 has put together. In this method, the substrate is passed through a gap formed by two rollers and the coating agent is applied to one of the two rollers and transferred from there to the substrate. The application roller can be moved in the same or opposite direction to the substrate surface. High working speeds and precise adjustment of the layer thickness are achieved. However, the surface quality when using low-viscosity coating agents is not comparable with that of the dipping process.

Task and solution

The aim of the invention was to provide a method for coating a solid, closed surface, which carries a peelable protective film, with a low-viscosity coating agent, in which a surface equivalent to the dipping process is produced without necessarily coating the entire surface and such a large one Amount of bath as required by the immersion process and correspondingly extensive clean room measures would be required. Furthermore, it should be possible to work continuously to coat the surfaces of sheets, strips and foils.

To solve the problem, the protective film is spread from the surface and the thin coating agent is filled into the gap between the exposed surface and the spread protective film and the protective film is pulled off the surface so that the gap moves over the surface to be coated, with the thin coating agent being added according to consumption.

The protective film has either been on the surface since manufacture or is only applied to the surface to be coated for the purposes of the invention; in this case one can speak of an auxiliary film, however the term "protective film" is retained in the following for the purpose of uniform designation.

When the protective film is peeled off, the surface to be coated is gradually exposed, there being always a release gap at the boundary between the surface that has already been exposed and the film-protected area, which gap moves over the surface at the peeling speed. According to the invention, the gap is filled with the low-viscosity coating agent. It is held in the gap by the capillary force, so that the excess coating agent in the gap moves with the migrating gap and leaves the surface coated evenly. If the amount of coating agent in the gap has been consumed and has not been replenished, the coating tears off with a sharp boundary.

With a small filling of the gap with liquid coating agent, the capillary force has the effect of distributing the coating agent evenly over the length of the gap and the coating as the coating moves on Extend the gap to an even small thickness. With a larger filling level, the influence of gravity on the distribution of the coating agent in the gap and on the excess of the coating film flowing back into the gap increases. In this case, it is advantageous if the gap runs horizontally and is open at the top and if the surface to be coated is as steep as possible, preferably vertical, in the region of the gap. This also makes it much easier to add used coating agent in the gap.

The fact that the gap always remains filled with liquid coating agent prevents the formation of electrostatic charges and the attraction of dust particles. Therefore, no foreign particles are included in the coating. In order to produce trouble-free coatings, it is sufficient to prevent dust particles from penetrating into the liquid coating on the way from the gap to the hardening zone. This requires less technical effort than dip coating under clean room conditions.

Execution of the invention

At least one of the two surfaces that together form the gap, namely either the surface to be coated or the spread protective film, is curved about an axis parallel to the gap, and possibly both. If the surface to be coated is rigid and is flat, only the protective film is curved. If a film is coated, it can itself be curved. The resulting coating is particularly uniform if the radius of curvature of the curved surface always remains the same as the gap migrates over the surface to be coated.

When the protective film is pulled off freely, a radius of curvature is formed which depends on the pulling force, the pulling angle and the elasticity of the film. If these sizes remain constant, the gap profile also remains constant. Constant radii of curvature are, however, maintained more reliably if the protective film is pulled off on a roller running parallel to the gap. The take-off roller preferably has a soft elastic surface and lies against the still adhering protective film at the take-off limit. The removed film follows the take-off roller so that it has its radius of curvature in the gap. The radius of curvature is preferably between 10 and 200 mm.

With continuous operation, the gap always remains in the same place in the coating system, while the surface provided with the adhesive protective film is guided along the pull-off point. For coating flat substrates, such as plates, strips or foils, a coating system is preferably used which contains two parallel and horizontally arranged rollers at the take-off point, which abut the two sides of the substrate and let it pass upwards. Rigid Substrates emerge tangentially to the rollers, elastic substrates can follow the rollers over a limited distance. Rigid substrates preferably emerge vertically upward from the nip, but angles deviating from the vertical, for example up to 60 degrees C, can usually also be used. If necessary, a protective film can be removed from both sides of the substrate and a coating can be carried out.

The method of the invention can be applied discontinuously in cases in which a surface could otherwise only be coated with difficulty with a uniform, thin layer of the low-viscosity coating agent. Such a case exists, for example, if the surface of an injection molding tool is to be provided with a coating which is transferred to the injection-molded molding in the subsequent injection molding process. An auxiliary film is adhesively adhered to the closed surface of the mold and spread at the edge to form a gap. A sufficient amount of the thin coating agent is poured into the gap and evenly distributed. When the auxiliary film is gradually removed, the coated tool surface remains. In the case of small areas, it is usually possible to meter the amount of coating agent so precisely that it is just sufficient to coat the surface. Otherwise, the coating agent is replenished as required and the excess is sucked in after the auxiliary film has been completely removed.

The coating

The resulting coating is more uniform the thinner the coating agent is. Under the application conditions, a viscosity of 500 mPa s should not be exceeded if possible; 50 to 500 mPas are preferred.

The uniformity of the coating film is further promoted in that the filling height is as uniform as possible and constant over time over the entire width of the nip. This can be achieved by allowing the coating agent to flow continuously from one or more outlet nozzles into the gap in accordance with the current consumption. The outlet nozzles should be so close together that the coating agent is distributed uniformly over the gap length. Distances between 2 and 20 cm are advisable. However, a split nozzle can also be used. The feed of the coating agent into the gap is metered in such a way that it does not run out at the ends of the gap. This can also be prevented by wedge-shaped sealing pieces fitted into the gap or by elastic sealing tabs placed on the side. With a small filling height of the gap, the capillary force alone has the effect of holding the coating agent in the gap in such a way that it is evenly filled to the end, but no coating agent flows off at its open ends.

The gap is preferably guided along the surface at a speed of about 0.1 to 10 m / min, particularly preferably from 0.5 to 2 m / min. Higher Speeds can lead to turbulent flow of the coating agent in the gap and consequently to a non-uniform coating. Lower speeds are generally uneconomical due to low productivity and can lead to malfunctions due to drying in the area of the roll gap.

Disruption of the coating by dust particles should of course be avoided in the process of the invention as well as in the known immersion process. However, it is sufficient to maintain clean room conditions in the limited area from the gap to the drying or curing zone. This area can be easily encapsulated with a relatively small housing and kept dust-free by blowing in cleaned air.

The protective film

As a rule, the protective film covers the entire surface of the substrate and is adhesively bonded to it, for example by means of a pressure-sensitive adhesive. If the protective film is made of corona-treated polyolefin, it can also be adhered by hot lamination without additional adhesive. Soft-elastic protective films made of polyethylene, polypropylene, polyester or soft PVC with a thickness of, for example, 20 to 100 micrometers are preferred. These films have the effect that they bind dust particles that were on the substrate surface before lamination and take them with them when they are removed.

The surface to be coated

The method of the invention is suitable for coating solid, closed surfaces of any kind, provided that their geometry allows a protective film to be applied. This is the case for all flat and uniaxially curved surfaces, such as cylindrical or conical surfaces. Spherically curved surfaces can sometimes be covered with elastic protective films. Flat surfaces of flat substrates with a uniform thickness, such as sheets, sheets or foils, are preferably coated. The coating of flat, rigid substrates in the form of flat sheets or webs is particularly preferred.

The surface can consist of any coatable material and may differ from the material of the layer lying below the surface. The surface is preferably made of plastic. Thermoplastic extrudable plastics are preferred; eg polycarbonate, acrylic glass (polymethyl methacrylate or copolymers of methyl methacrylate), polyethylene, polypropylene, ABS plastics, polystyrene or polyester. They can be crystal clear or clouded or colored by pigments or fillers and can contain impact-resistant or other customary additives. The thickness ranges from about 10 microns to 1 mm for foils and from 1 to 20 mm for sheets. The width of the surface is at most limited by the machine width and can be, for example, 0.2 to 3 m. The length is arbitrary. For example, it is possible to process individual cut sheets or sections of film, preferably with a butt joint are successively passed through the nip. In this case, it is advisable to place the protective films on successive panels, which are coated in a continuously operating system, at the joints - e.g. B. with an adhesive tape - to connect. Endless webs of substrates can also be processed. In this case, the coating system can be connected directly to the production system. The substrate can be coated on one or both sides in the same operation, but foils are mostly coated on one side only and panels are mostly coated on both sides.

The coating agent

With the method of the invention, any type of decorative or functional cover layers can be applied to the surfaces. All low-viscosity curable lacquers can be used as the coating agent, provided that they sufficiently wet the surface and are continuously curable at a sufficient speed. Curing can occur physically through evaporation of a solvent or chemically through crosslinking or polymerization. Preferred coating compositions are cured by both methods by first evaporating a solvent under the action of heat and then polymerizing or crosslinking, for example under the action of UV radiation. Corresponding coating compositions are known in large numbers. They are used, for example, for generation Power-resistant, UV-protective, reflection-reducing, adhesion-promoting or matting top layers and may contain the undissolved ingredients required for this purpose. The production of glossy top layers is of outstanding importance in the context of the invention.

Typical thicknesses of the hardened cover layers are between 1 to 20 micrometers; they are made from e.g. 3 to 60 micrometer thick layers of the low viscosity coating agent produced. The protective film is inevitably co-coated in the process of the invention. Since the coating produced is thin, the amount of coating agent remaining on the auxiliary film is economically insignificant. In order to avoid losses of the coating agent, the coating agent can be scraped off the surface of the protective film just behind the gap filling so that it flows back into the gap. The peeled off auxiliary film can optionally be laminated with the uncoated side as a removable protective film back onto the panel or film coated according to the invention.

Although the aim of the invention is achieved with the generation of a uniform coating on the treated surface, the coating always follows in practice. The curing conditions depend in a manner known per se on the nature of the coating agent. Physically drying coatings harden by evaporation of the solvent. Evaporation can be done by heating with Radiant heaters, rear-facing heating surfaces or warm air are conveyed. Chemically curing coating compositions can be cured in the same way by heating or by activating radiation, such as UV, gamma or electron radiation. In the case of continuous operation, this is expediently carried out in a stationary, dust-free hardening tunnel, the length of which is such that the coating is cured through the tunnel at the same speed at which it is applied. Usual curing times are between 1 and 60 ec.

Claims (9)

1. A process for coating a firm, closed surface, carrying a peelably adhering protective film, with a runny coating compound, characterised in that the protective film is separated from the surface and the runny coating compound is filled into the gap between the exposed surface and the separated protective film, and that the protective film is pulled away from the surface such that the gap moves across the surface to be coated, the runny coating compound used up being replenished accordingly.
2. A process according to claim 1, characterised in that the protective film is separated and pulled off such that the gap extends horizontally.
3. A process according to claim 2, characterised in that the protective film is separated and pulled off such that the gap is opened in the upward direction.
4. A process according to claim 3, characterised in that the surface is held vertically in the region of the gap.
5. A process according to claims 1 to 4, characterised in that the coating compound is allowed to flow into the gap from one or more outlet nozzles as it is used up.
6. A process according to claims 1 to 5, characterised in that a runny coating compound with a viscosity below 500 mPa s is used.
7. A process according to claims 1 to 6, characterised in that a surface made of plastics is coated.
8. A process according to claim 7, characterised in that a surface of a panel or a tape made of plastics is coated.
9. A process according to claim 8, characterised in that the surfaces of a panel or a tape are coated, each surface of which carries a peelably adhering protective film on either side of the panel or the tape, and that the protective films are peeled off and the coatings are applied simultaneously on both sides.