GB1559503A

GB1559503A - Method of producing a protective coating on the surface ofoptical reflectors and reflectors manufactured in accordance with this method

Info

Publication number: GB1559503A
Application number: GB30483/78A
Authority: GB
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1976-06-05
Filing date: 1976-08-20
Publication date: 1980-01-23
Also published as: DE2625448B2; DE2625448A1; DE2625448C3

Description

(54)METHOD OF PRODUCING A PROTECTIVE COATING ON THE SURFACE OF OPTICAL REFLECTORS, AND REFLECTORS MANUFACTURED IN ACCORDANCE WITH THIS METHOD (71) We, ROBERT BOSCH GmbH, a German Company, of Postfach 50, 7 Stuttgart 1, Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a method of producing a protective coating on the surface of optical reflectors, preferably aluminized reflectors, in a vacuum container, and to a reflector manufactured in accordance with this method.

Known coatings for protection against corrosive effects, hitherto used in, for example, reflectors for motor vehicle lamps, were produced by vaporization of inorganic substances. The method used was such that a vaporizer for applying the protective coating was located in the vapour deposition plant in addition to the vaporizer for applying the highly reflective metal coating. The protective coating can comprise, for example, magnesium fluo ridc (MgF2), or it can be produced by reactive vaporization of SiO in an oxygen atmosphere, a silicon oxid having a high degree of oxidation (SiO1) being formed on the substrates. If protective coatings of this type have to be mass-produced economically, they are unsuitable for reflectors of motor vehicle lamps having high demands with respect to quality.

An object of the invention is to provide a protective coating having satisfactory anti-corrosive and optical properties, particularly to improve the anti-corrosive effect on headlamp reflectors to which aluminium has been applied by vapour deposition. The novel protective coatings are to ensure great durability of the protected part, an additional requirement to be met being that of optimum optical neutrality when the protective coatings are used in optically effective devices. A method of producing the coating is to be specified which, with only a few working steps and with low cost, ensures that a resistant and high-grade coating can be mass-produced. The method is to be operationally reliable and simple, so that sourccs of error in the factory are substantially excluded and manufacturing waste can be reduced to a minimum.

Accordingly the present invention provides a method for producing a protective coating on the surface of a metallized optical reflector in which the reflector surface is exposed in a vacuum container to a monomeric gas and a hydrophobic protective coating is formed on the reflector surface by polymerization of the monomeric gas and in which the surface of the protective coating is then rendered hydrophilic, preferablfy by after-treatment with oxygen.

It is advantageous if the polymerization is effected by means of radiation and if the radiation is continued during the aftertreatment for rendering the surface of the protective coating hydrophilic.

The course of the manufacturing process is rendered particularly simple if the metallizing operation for producing the reflective coating, the polymerization of the hydrophobic protective layer on the reflective coating, and the subsequent rendering hydrophilic of the surface of the protective coating are carried out in successive working operations in the same container.

The present invention also provides a metallized optical reflector having a protective coating produced on its optical surface by polymerization of an organic compound, preferably a monomeric gas, the surface of the protective coating having been rendered hydrophilic by after-treatment with oxygen.

Organo-silicon compounds have proved to be particularly advantageous as starting substances both with respect to the manufacturing conditions and with respect to the protective action, although, with polymerization out of the gas phase, purely organic compounds, preferably unsaturated low-molecular weight hydrocarbons result in satisfactory anticorrosive protective coatings. Compounds of this type are, for example, Olefines: ethylene, propylene and higher homologues thereof, Aromatics: benzene, toluene and xylene, Vinyl compounds: styrene, acrylic acid ester, vinyl halide, vinyl alcohol ester, and fuorosubstituted hydrocarbons such as tetrafluoroethylene.

The organo-silicon coatings have the advantage that they are particularly resistant to temperature and are less sensitive to influences such as ageing and discolouration. Furthermore, the nuisance caused by odour is less than that of most of the purely organic substances, while some of the latter in turn have higher rates of polymerization.

Advantageously, the coating is formed by polymerization of low-molecular weight siloxanes containing methyl-, vinyl-, or phenyl groups, preferably hexamethyldisiloxane (HMDS), or by polymerization of methyl-, vinyl-, chloro- or alkoxy silane preferably by the polymerization of vinyl trimethyl silane (VTMS).

The two monomers HMDS and VTMS which are preferably used are colourless, combustible liquids of low viscosity having molecular weights of 162 (HMDS) and 100 (VTMS). HMDS has only a slight odour and has a viscosity of 0.6 cST and has a vapour pressure of 40 mbar at 200C. On the other hand, VTMS has a stronger odour and has a lower viscosity, a higher vapour pressure and is more chemically active owing to the fact that it is relatively easy to break open he double bond of the vinyl group. It can be polymerized more readily than HMDS, thus leading to higher rates of growth. The polymer coatings formed from the two substances have comparable properties, the vTMS coating being subject to secondary reactions, while the HMDS coating contains scarcely any radicals which are still free. Finally, the silicone oil HMDS is more readily available on the market and is less expensive than VTMS.

In practice, incandescent lamps used together with reflectors cannot be prevented from producing vapours caused by lamp cement, solder residues etc. These vapours condense in the form of droplets on a hydrophobic surface constituting the polymeric protective coating, particularly in a sealed system, and the droplets produce stray light and thus become visible as deposits.

Therefore, the surface of the hydrophobic protective coating is rendered hydrophilic for the purpose of ensuring satisfactory optical properties of the reflector provided with the protective coating. Advantageously, with the continued action of the radiataion preferably used for polymerization of the monomeric gas, oxygen is admitted into the container preferably directly after the feeding of the monomeric gas has been terminated.

satisfactory results have been obtained when the oxygen pressure is approximately 1/3 of the monomeric gas pressure and when the after-treatment period with oxygen is approximately 30% of the time re quired for producing the polymer coating.

Rendering the surface of the protective coating hydrophilic eliminates the optical efficiency of the deposits on the reflective surface, since the vapours condense in the form of an homogeneous film on the hydrophilic surface of the protective coating. Thus, the advantages of this invention reside, in the first instance, in the uniform and thus non-visible condensation of detrimental vapours on the hydrophilic surfaces produced by the method in accordance with the invention, without appreciably reducing the excellent protection of the hydrophobic polymer coatings against corrosion. Furthermore, with respect to the desired adhesion, the surface enriched by hydroxyl- and carboxyl groups behaves in a more favourable manner when glueing diffusing screens to the coated reflectors.

A particularly economical method of producing the protective coating, in accordance with the invention, on a metallised substrate resides in effecting the polymerization in the same container as the metallization, monomeric gas being admitted by way of a metering valve, after metallization has been effected, from a reservoir in which the polymerizable substance is located in a liquid form. The parts to be protected can remain at their locations in the container, and expensive and time-consuming measures for changing over to, or equipping, a separate plant can be dispensed with. Advantageously, the container is pumped out to a pressure below 10-, bar before the monomeric gas is admitted, and is subsequently flooded with monomeric gas. The process is accelerated, and the protective coating is rendered particularly uniform, when a continuous flow of monomeric gas through the container is obtained by balancing the pressure drop, produced by connecting the container to a vacuum pump, by open ing a metering valve for a monomeric gas.

Advantageously, the polymerization of the coating can be effected by a gas-ampli fied, non-self-maintained discharge which is triggered by thermionic emission electrons and which is hereinafter referred to as "electron thermionic emission". The plant can be of very simple construction when producing the hydrophobic protective coating by electron thermionic emission. Compared with self-sustained glow discharge for example, the occurrence of spark-overs between the high-tension electrode and the substrates is additionally avoided with electron thermionic emission, whereby the number of faulty parts can be greatly reduced during production.

With polymerization by thermionic emission of electrons, the pressure required in the plant is approximately two powers of ten lower than that required with glow polymerization, so that the throughput of monomeric gas is also reduced and thus there is a considerable saving. In addition to the advantages already mentioned, thermionic emission has the further advantage that a uniform coating is produced around the thermionic cathode, i.e., over an angle of almost 360 degrees, while the coating would be limited substantially to the region of the spatial dimension of the electrode when using a glow electrode.

When using the electron thermionic emission for producing the coating, an embodiment of the invention which is particularly advantageous with respect to the cost of production resides in effecting the polymerization by means of an evaporator wire which acts as a thermionic cathode and, after the metallizing operation been completed, is heated to a higher temperature sufficient for the thermionic emission of electrons and is connected to a voltage which is negative relative to the substrates.

Thus, special devices for producing the protective coating are no longer required within the container, thus greatly simplifying the plant. The temperature, to which a, for example, tungsten or tantalum evaporator wire is heated before the admission of the monomeric gas, is advantageously approximately 1800 Celsius, and the gas pressure is below 10-7 bar. The container is subsequently flooded with monomeric gas to a pressure of approximately 5.10-' bar, this being possible without additional technical means in view of the high vapour pressure above the monomeric liquid.

The invention is further described in the following description with reference to the drawings, in which: Fig. 1 is a section through an apparatus for producing the protective coating by the method of the invention, and Fig. 2 is a simplified circuit diagram for an apparatus having thermionic emission of electrons for producing the polymer coating.

A container 10 (Fig. 1) is connected on the one hand to a high-vacuum pump by way of a socket 11 and, on the other hand, to a backing pump by way of a socket 12. Monomeric gas from a reservoir (not illustrated) can be admitted through a metering valve 13 provided opposite the socket 11. The monomeric gas is located in the reservoir as a polymerizable substance in liquid form, sufficient gas for flooding the system to the desired pressure being permanently available owing to its high vapour pressure. Substrate carriers 14 of cylindrical configuration are arranged within the container 10 and rotate about the axis of the container on the one hand and, on the other hand, rotate about their own axes. Substrates 15, which are to be covered with a protective coating effective against corrosive influences, are mounted on the carriers 14. In the embodiment illustrated in Fig. 1, the substrates are reflectors of motor vehicle lamps onto which a reflective coating of aluminium is applied by vapour deposition in the first instance and which are subsequently provided with a protective coating for the purpose of protecting the aluminium coating.

An evaporator wire 16 made from tungsten is provided for the vapour deposition of aluminium and is first heated to a temperature sufficient for the vaporization of the aluminium.

The protective coating is produced by polymerization and is applied in the same vacuum container 10 in which the metallization is effected. Monomeric gas flows continuously through the container 10 by virtue of the fact that the pressure drop, produced by connecting the container to a vacuum pump, is balanced by way of the metering valve 13 for the monomeric gas.

A particularly simple coating method is achieved when a thermionic cathode is used for the polymerization of the protective coating. The tungsten evaporator wire 16 is connected as such a thermionic cathode in the container during the applying of the hydrophobic protective coating and, in accordance with the basic circuit diagram illustrated in Fig. 2, this is effected by connecting one end of the evaporator wire 16 to the insulated secondary winding of a regulable high-current transformer 21 and, during the applying of the protective coating, connecting its other end by way of a limiting resistor Rv to the negative pole of a source Ug of direct voltage whose positive pole is connected to earth. The voltage on the primary side of the transformer 21 is designated Up, and the voltage on its secondary side is designated Us. By increasing the output of the transformer after the aluminium has been applied by vapour deposition, the tungsten wire 16, fitted in an insulated manner, is heated to a temperature of approximately 1800or at which the thermionic emission of electrons is effected. The monomeric gas is then admitted into the plant and a pressure of approximately 5.106 bar is established. In order to discharge the space charge surrounding the hot wire, and to accelerate the electrons towards the substrates 15 connected to earth, the evaporator wire 16 is additionally connected to the direct voltage source Ug by way of a switch S and is thus connected to a negative potential of approximately 300 volts relative to earth. A series resistor Rv of suitable value is additionally connected for the purpose of stabilizing the discharge.

The values of the series resistor and the direct voltage have to be adapted to the particular plant.

The electrons which have been produced and accelerated are multiplied in the gas space by ionizing impacts, so that an amplified charge current acts upon the substrates 15 and its energy renders possible the cross-linking of the adsorbed gas molecules. The growth rates achieved in a factory plant were 2 to 8 nm/min under the described conditions. It will be appreciated that the method in accordance with the invention can be modified by using a separate wire for producing the thermionic emission of electrons, or by producing free electrons in another way, for example by means of an electron beam gun.

In order to render the surface of the protective coating hydrophilic, after treatment with oxygen is effected immediately after the polymerization of the hydrophobic protective coating by closing the valve 13 for feeding the monomeric gas and opening an 0, valve 17, while the therminonic emission of electrons by the evaporator wire 16, used for polymerization, remains effective. The after-treatment time for rendering the surface hydrophilic is approximately 30% of the time required for producing the polymer coating, with an Os pressure of approximately 1/3 of the monomeric gas pressure. During this operation, oxygen is chemically bonded to the surface of the polymer coating, thus leading to the formation of hydroxyl- and carboxyl groups and rendering the surface hydrophilic.

In addition to the initially mentioned advantageous optical effect of the hydrophilic surface, the after-treatment with 0, prevents the tungsten wires 16, used for the thermionic emission of electrons, from being coated with a deposit of tungsten carbide or some other tungsten compound which considerably reduces the emission and which renders it necessary to exchange the wires after the container has been loaded a certain number of times. In the case of after-treatment with oxygen, the durability of the wires 16 is only limited by the mechanical stability, thus doubling or tripling the service life of the wires 16. The tungsten carbide is oxidized by the after-treatment with 02, wherein the carbon is displaced from the deposit owing to its bonding to oxygen and is drawn off in the form of a gas.

Low-molecular weight substances are particularly suitable for producing the protective coating owing to their high vapour pressure which is adequate for flooding the plant by way of the metering valve 13 without additional aids. The substances mentioned previously viz, hexamethyldisiloxane and vinyltrimethylsilane are particularly low-molecular weight and thus are particularly suitable. Hexamethyldisiloxane produces a coating which is somewhat more chemically stable than vinyltrimethylsilane, while the latter has the advantage that polymerization takes place substantially more rapidly, so that a higher growth rate and thus higher production figures can be obtained.

The organo-silicon or other polymeric coating produced is chemically inactive, temperature-resistant, and soluble only with difficulty. In particular, it is resistant to the corrosive effects caused by, for example, salt scattered on the roads, and for this reason it is particularly well suited as a protective coating for aluminized reflectors of motor vehicles. Furthermore, the protective coating is clear, colour-fast and mechanically durable, thus further increasing its suitability for motor vehicle lamps, since, on the one hand, it is thus optically neutral and, on the other hand, is not damaged by cleaning.

In our co-pending application No.

34703/76 (Serial No. 1 559 502) we have claimed a method for producing a protective coating on the surface of an optical reflector in which the reflector surface is exposed in a vacuum chamber to a monomeric gas and a hydrophobic protective coating is formed on the reflector surface by polymerization of the monomeric gas, the polymerization being effected by a gas-amplified discharge triggered by thermionic emission electrons.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

of direct voltage whose positive pole is connected to earth. The voltage on the primary side of the transformer 21 is designated Up, and the voltage on its secondary side is designated Us. By increasing the output of the transformer after the aluminium has been applied by vapour deposition, the tungsten wire 16, fitted in an insulated manner, is heated to a temperature of approximately 1800or at which the thermionic emission of electrons is effected. The monomeric gas is then admitted into the plant and a pressure of approximately 5.106 bar is established. In order to discharge the space charge surrounding the hot wire, and to accelerate the electrons towards the substrates 15 connected to earth, the evaporator wire 16 is additionally connected to the direct voltage source Ug by way of a switch S and is thus connected to a negative potential of approximately 300 volts relative to earth. A series resistor Rv of suitable value is additionally connected for the purpose of stabilizing the discharge.

The values of the series resistor and the direct voltage have to be adapted to the particular plant.

The electrons which have been produced and accelerated are multiplied in the gas space by ionizing impacts, so that an amplified charge current acts upon the substrates 15 and its energy renders possible the cross-linking of the adsorbed gas molecules. The growth rates achieved in a factory plant were 2 to 8 nm/min under the described conditions. It will be appreciated that the method in accordance with the invention can be modified by using a separate wire for producing the thermionic emission of electrons, or by producing free electrons in another way, for example by means of an electron beam gun.

In order to render the surface of the protective coating hydrophilic, after treatment with oxygen is effected immediately after the polymerization of the hydrophobic protective coating by closing the valve 13 for feeding the monomeric gas and opening an 0, valve 17, while the therminonic emission of electrons by the evaporator wire 16, used for polymerization, remains effective. The after-treatment time for rendering the surface hydrophilic is approximately 30% of the time required for producing the polymer coating, with an Os pressure of approximately 1/3 of the monomeric gas pressure. During this operation, oxygen is chemically bonded to the surface of the polymer coating, thus leading to the formation of hydroxyl- and carboxyl groups and rendering the surface hydrophilic.

In addition to the initially mentioned advantageous optical effect of the hydrophilic surface, the after-treatment with 0, prevents the tungsten wires 16, used for the thermionic emission of electrons, from being coated with a deposit of tungsten carbide or some other tungsten compound which considerably reduces the emission and which renders it necessary to exchange the wires after the container has been loaded a certain number of times. In the case of after-treatment with oxygen, the durability of the wires 16 is only limited by the mechanical stability, thus doubling or tripling the service life of the wires 16. The tungsten carbide is oxidized by the after-treatment with 02, wherein the carbon is displaced from the deposit owing to its bonding to oxygen and is drawn off in the form of a gas.

Low-molecular weight substances are particularly suitable for producing the protective coating owing to their high vapour pressure which is adequate for flooding the plant by way of the metering valve

13 without additional aids. The substances mentioned previously viz, hexamethyldisiloxane and vinyltrimethylsilane are particularly low-molecular weight and thus are particularly suitable. Hexamethyldisiloxane produces a coating which is somewhat more chemically stable than vinyltrimethylsilane, while the latter has the advantage that polymerization takes place substantially more rapidly, so that a higher growth rate and thus higher production figures can be obtained.

The organo-silicon or other polymeric coating produced is chemically inactive, temperature-resistant, and soluble only with difficulty. In particular, it is resistant to the corrosive effects caused by, for example, salt scattered on the roads, and for this reason it is particularly well suited as a protective coating for aluminized reflectors of motor vehicles. Furthermore, the protective coating is clear, colour-fast and mechanically durable, thus further increasing its suitability for motor vehicle lamps, since, on the one hand, it is thus optically neutral and, on the other hand, is not damaged by cleaning.

In our co-pending application No.

34703/76 (Serial No. 1 559 502) we have claimed a method for producing a protective coating on the surface of an optical reflector in which the reflector surface is exposed in a vacuum chamber to a monomeric gas and a hydrophobic protective coating is formed on the reflector surface by polymerization of the monomeric gas, the polymerization being effected by a gas-amplified discharge triggered by thermionic emission electrons.

ive coating on the surface of a metallized

optical reflector in which the reflector surface is exposed in a vacuum container to a monomeric gas and a hydrophobic protective coating is formed on the reflector surface by polymerization of the monomeric gas and in which the surface of the protective coating is then rendered hydrophilic.
2. A method as claimed in claim 1 in which the surface of the protective coating is rendered hydrophilic by after-treatment with oxygen.
3. A method as claimed in claim 2 in which the metallization for producing the reflective coating, the polmyerization of the hydrophobic protective coating on the reflective coating and the subsequent rendering of the surface of the protective coating hydrophilic are carried out in successive working operations in the same container.
4. A method as claimed in claim 2 or 3 in which the oxygen pressure is approximately one third of the pressure of the monomeric gas, and the after-treatment with oxygen is approximately 30% of the time required for producing the polymeric coating.
5. A method as claimed in any of claims l to 4 in which the polymerization is effected under the influence of radiation.
6. A method as claimed in claim 5 in which the radiation is continued during the after-treatment for rendering the surface of the protective coating hydro ph ilic.
7. A method as claimed in any one of the preceding claims in which the reflector is an aluminized reflector.
8. A method as claimed in any of claims 1 to 7 in which the protective coating is formed by polymerization of an organo-silicon compound.
9. A method as claimed in claim 8 in which the organo-silicon compound is a low molecular weight silicon compound containing methyl-, vinyl- or phenyl groups.
10. A method as claimed in claim 9 in which the silicon compound is hexamethyldisiloxane.
11. A method as claimed in claim 8 in which the organosilicon compound is methyl-, vinyl-, chloro- or alkoxysilane.
12. A method as claimed in claim 11 in which the organo-silicon compound is vinyltrimethylsilane.
13. A method as claimed in any one of the preceding claims in which a continuous flow of monomeric gas through the container is maintained during polymerization of the protective coating.
14. A method as claimed in any preceding claim in which, prior to the introduction of the monomeric gas, the container is drained by pumping at a pressure of below 10-7 bars and is subsequently flooded with monomeric gas at a pressure of 10-" to 10-, bars.
15. A metallized optical reflector having a protective coating produced on its optical surface by polymerization of an organic compound, the surface of the protective coating having been rendered hy hydrophilic by after-treatment with oxygen.
16. A reflector as claimed in claim 15 in which the organic compound from which the coating is polymerized is a monomeric gas.
17. A reflector as claimed in claim 15 or 16 which is an aluminized reflector.