Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Physical dimension specified
Reexamination Certificate
1995-02-15
2001-03-27
Le, H. Thi (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Physical dimension specified
C428S423100, C428S515000, C428S543000
Reexamination Certificate
active
06207270
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a transparent cover plate of a synthetic plastic material for a headlight of a motor vehicle and other light sources, which is provided at least at its outer side with a coating of a transparent synthetic plastic, as well as to a method of producing the transparent cover plate.
Cover plates for motor vehicle headlights are usually composed of glass. For the reason of accurate shaping of dispersing and lens elements as well as for the reason of the weight saving, cover plates are increasingly produced of synthetic plastic materials. Synthetic plastic materials suitable for this purpose are usually polycarbonate, aromatic modifiers polycarbonate (APEC) and polymethylacrylmethylimide (PMMI), which however require a surface improvement to satisfy corresponding requirements with respect to scratch resistance, stone impact resistance, weather resistance and resistance to solvents, fuels and oil.
German document DE-A-27 48 358 discloses a method of improving the surface layer of the cover plate of synthetic plastic material. The treatment is performed directly on the synthetic plastic of the cover plate, for example in form of a subsequent hardening.
In older time the coating of the synthetic plastic cover plates with a hardenable lacquer was used. The lacquer is applied in several layers until the desired layer thickness is formed. Lacquer resins are based of cross-linkable acrylate, polyester, polysiloxane. Systems are known in which the pre-polymerized cross-linkable synthetic plastic is dissolved in organic solvents. Furthermore, aqueous systems on acrylate bases are known. Also, already proposed, solvent-free systems are utilized, in which a pre-polymer is dissolved in aqueous monomers. The monomers used as solvents are then bonded in the lacquer layer during hardening.
The known coating processes are complicated and pose environmental problems and health risks. Partially, also the desired or required mechanical properties, weathering properties and resistance against chemicals and solvents as well as optical properties are not satisfactory.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide transparent cover plates of synthetic plastic material which can be produced in simpler and cost-favorable manner and possess good mechanical properties, good optical properties, a good weather resistance and resistance against aggressive substances.
In keeping with these objects and with others which will become apparent hereinafter, one feature of the present invention resides, briefly stated, in a transparent cover plate of a synthetic plastic material for a motor vehicle headlight or another motor vehicle light source, which at least at its outer side is provided with a coating of a transparent synthetic material, wherein in accordance with the present invention the coating is burnt powder lacquer.
In accordance with another feature of the present invention a method of producing the cover plate is proposed, wherein a preformed uncoated cover plate of a synthetic plastic material is heated simultaneously on a surface to be coated, the heated plate is treated during a sufficiently long time required for producing the desired layer thickness, with powder lacquer of a cross-linkable synthetic plastic material, with the temperature on the surface of the cover plate maintained so high that the powder is melted and flows to form a closed layer, and then the formed lacquer layer is burnt.
The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In accordance with the present invention the cover plate for a headlight or another light source of a power vehicle is composed of a burnt powder lacquer.
Transparent cover plates must be composed of a transparent synthetic plastic material at least in their optical region, and their manufacture from economical reasons is performed by injection molding of thermoplastic synthetic plastics. Thermoplastic synthetic plastics are however heat sensitive in correspondence with their softening region, so that during thermal treatments or processing at high temperatures deformations of the cover plates must be taken into consideration. In a surprising manner it has been shown that such deformation can be avoided when a method is performed with highly reactive powder lacquers which melt at relatively low temperatures. Such lacquer powder melts at temperatures which are significantly below the melting region of the thermoplastic materials of the cover plates, so as to form a pore-free film with smooth outer surface and to provide the cover plate after its hardening with the desired properties. The inventive cover plates correspond to standards and regulations prescribed for the headlights. The rock impact strength lies for example within the prescriptions given in TRANS/SC1/WP29/GRE (sand jet test). Also a pronounced adherence is obtained. This is provided by the grid section value corresponding to the DIN 53151 Gt 0. The transmission value of the plate is not undesirably affected by the coating. The continuity characteristic of the coating is good. The surface of the lacquer satisfies the following continuity values (measured with wave scan device of the Firm Byk-Gardner):
Long wave characteristic value≦3 especially≦2
Short wave characteristic value≦15 especially≦11.
The coating is composed of a cross-linked synthetic resin, in particular a polyacrylate. Also, other powder lacquer systems can be used, such as based on polyester or polyurethane. The layer thickness of the coating lies in the optical region of the cover plate as a rule between 30 and 200 &mgr;m, preferably between 60 and 150 &mgr;m, normally between 80 and 120 &mgr;m. High layer thicknesses provide a good protection for the cover plates while to the contrary low layer thicknesses are preferably on cost reasons and can be used where the stress of the cover plates is low. As a rule, the layer thickness over 40 &mgr;m.
Because of the formation of the coating from burnt powder lacquer, in a surprising manner very uniform layer thicknesses are obtained. For example, in optical region of the cover plate, deviations in the layer thickness smaller than ±10%, in particular smaller than ±5% can be maintained. In the edge region the deviations are less critical. In this region the layer thickness of the coating can be maintained smaller, in particular in the region between 40 and 60 &mgr;m.
It is especially advantageous when the cover layer of the inventive cover plate is composed of a 1-layer coating. Therefore, the layer is very uniform. Also, the production of the coated cover plate is less expensive.
The cover plate in accordance with the present invention is not only a cover plate, in particular a cover plate, for a headlight. It is also possible to form the cover plate as a combined chassis part which performs both the function of a cover plate and also the function of an orifice or a chassis part. For this purpose it is possible to make the optical regions impermeable and/or colored by painting or pigmenting of the powder lacquer. It is also possible for combined functions of light sources of motor vehicles, for example for rear lights, braking lights, blinking lights and rear headlights, to produce a cover plate of colorless transparent synthetic plastic by injection molding and then coat the same with powder lacquer which is differently color transparent in correspondence with the function of the light source.
The cover plate is as a rule three-dimensional object. Therefore it is necessary to avoid the deformations under the action of increased temperatures. The rear side or inner side of the cover plate is free of a coating, which provides an imp
Fritz Hans-Georg
Gugl Albin
Keck Thomas
Koch Ekkehard
Le H. Thi
Robert Bosch Stewart
Striker Michael J.
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