Optical: systems and elements – Mirror – With support
Reexamination Certificate
1999-12-22
2002-12-03
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Mirror
With support
C427S488000, C427S489000, C427S491000
Reexamination Certificate
active
06488384
ABSTRACT:
INTRODUCTION AND BACKGROUND
The present invention concerns a method for the coating of substrates made of plastic with a light-reflecting layer, preferably an aluminum layer and with another layer placed between the substrate and the light-reflecting layer.
The reflecting capacity of reflectors, which are produced on plastic substrates by applying thin metal layers in a vacuum, is frequently affected unsatisfactorily by production and usage conditions. This can have various origins, which frequently are to be found in the characteristics of the surface of the plastic:
On the one hand, the roughness of the plastic surface can be too high. The light is diffusely scattered by this, partially at the expense of the desired reflection. This effect can be reinforced even more unfavorably by the method for the application of the metal layer. Thus, it is known that in the thermal vaporization of metal layers, particle layers are formed, as a rule, whose particle size, and thus roughness, increases with the thickness of the layer. For this reason, there is an optimal layer thickness for the reflecting capacity of metal layers that are applied by vaporization; this thickness ensures a coating which is still sufficient (optically dense) with a minimum particle size. This effect manifests itself, in a particularly undesirable manner, on molded articles. Often very different layer thicknesses are found on them, so that on certain surfaces, a sufficiently thick, and therefore optically dense, and thus completely reflecting layer has not yet been formed. In comparison on other layers, the reflecting capacity is diminished by a particle size which has progressed to an excessively large magnitude. This effect is particularly reinforced with an oblique incidence of the vapor particles on the surface e coated, since protruding peaks are preferably coated and areas of the surface which lie behind them are blocked, and therefore a greatly diminished layer growth takes place there. This problem can be reduced, but not solved, by moving, such as, the molded articles during vapor deposition.
On the other hand, the reflecting capacity of the metal layer depends on its purity. The more the layer is contaminated, for example, by oxide fractions, the more its reflecting capacity declines. In this respect, aluminum layers, which are by far most frequently used in technology, are particularly sensitive, since aluminum vapor is especially reactive. This is particularly troublesome because the contaminated aluminum layers also manifest themselves by a more or less pronounced yellow or brown shade. The important thing therefore is to keep reactive foreign gases away from the metal vapor to the greatest extent possible during the vacuum coating. One common measure for this is to maintain the surfaces as clean as possible in a vacuum. However, this is not sufficient if gases or vapors exit from the plastic material during the coating. These are particularly disadvantageous because they appear precisely at the point where the metal layer is being formed so that with a particularly high probability, they contribute to the contamination of the layer. The problem appears, above all, with plastics which have a strong gas evolution, such as polyamide or BMC (bulk molded compound), and particularly if the plastic was not prelacquered before the vacuum coating.
Finally, an optically satisfactory metal layer can clearly lose reflecting capacity in the course of use, in particular, at elevated temperatures, as may occur in a headlight, if substances from the plastic material migrate and spread in the form of vapors in the headlight. The vapors can condense on colder surfaces of the headlight in the form of a dull, unattractive coating. These coating have a particularly disturbing effect on the reflector, a contemplated border region, and the transparent headlight lens or covering pane. In these cases, the optical appearance of the headlight and frequently also the light efficiency is disturbed in a sensitive manner. This phenomenon is very disturbing, particularly with plastics which have a strong evolution of gas, such as B.C. materials.
To increase the reflecting capacity, therefore, headlight reflectors are frequently provided with a lacquer layer prior to the metal coating. This lacquer layer is supposed to compensate for the roughness of the surface and in the case of plastic parts, suppress the migration and desorption of substances from the plastic material during the metal coating and later operations.
Thus, DE 37 31 686 proposes the application of a polymer layer, with the aid of a plasma, on a previously applied lacquer layer, so as to improve the adhesion, the corrosion resistance, and the reflecting capacity of the subsequently produced metal layer.
A method is also known, in accordance with EP 0 136 450, for the production of a mirror layer, in particular, for headlight reflectors, wherein a layer of aluminum is sputtered on a preferably thermally curable lacquer layer, in particular, synthetic resin-powder layer foundation, in an evacuatable recipient. The sputtering of the aluminum layer takes place after an evacuation to pressures of 8×10
−3
Pa to 3×10
−2
Pa with inert gas sputter pressures between 6×10
−2
Pa and 1 Pa with coating rates of ca. 5 nm/sec, using a magnetron, preferably a planar magnetron.
It is therefore an object of the present invention to improve the process for forming reflecting surfaces on plastic substrates.
SUMMARY OF THE INVENTION
The above and other objects of the present invention can be achieved by applying an intermediate layer with as high as possible a barrier effect toward substances which can migrate and evolve gases from plastic material, onto unlacquered plastics, as reflector or border blanks, before the metal coating by means of a vacuum method, preferably a plasma-aided coating method. Among these substances, that can migrate and evolve gases from plastic material is water. As a rule, however, organic substances, such as the residues of unreacted monomers, for example, acrylonitrile from ABS, or styrene (from B.C.), may also emerge, depending on the plastic material.
Highly crosslinked hydrocarbon layers, silicon dioxide-, silicon nitride-, or silicon oxynitride layers are suitable as the intermediate layer according to the invention. For the barrier effect of the latter layers, it is essential that the carbon content not be selected too high if the layers (as preferred) are produced using silicon-organic compounds. The carbon content should be <15%, preferably <8%. It became evident that such barrier layers not only clearly reduce the migration of turbidity-causing substances from the plastic during the use of the headlight, but rather also enhance the reflecting capacity of the metal layers, in particular, the aluminum layers, produced therefrom. In this respect, the application of the intermediate layer in a large layer thickness, so as to compensate for the roughness of the plastic surface, is, surprisingly, not required. A thickness of 15 nm can be sufficient for an intermediate layer, depending on the plastic material, produced with plasma-CVD or remote plasma-CVD methods.
For layers produced according to this method, a thickness of 30 nm to 60 nm has proved to be particularly good.
DETAILED DESCRIPTION OF INVENTION
The present invention will now be described in further detail.
One variant of the method of the invention resides in another production method of the layer, namely, condensing a radically polymerizable monomer on the plastic parts, under a vacuum, from the gas phase, and subsequently thoroughly polymerizing with the aid of a plasma. This method step (plasma-induced polymerization) includes of the following steps:
First, the part to be coated is tempered to room temperature or preferably a temperature below that and then it is immediately introduced into a vacuum chamber with heated walls. The vacuum chamber is thereupon typically evacuated to 1 Pa. Then, a short plasma pretreatment follows, according to the
Dicken Wilfried
Grünwald Heinrich
Nauenburg Klaus
Schulte Thomas
Boutsikaris Leo
Leybold Systems GmbH
Smith , Gambrell & Russell, LLP
Spyrou Cassandra
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