A PROCEDURE FOR ELECTROCHEMICAL POLISHING OF AN ALUMINIUM SUBSTRATE TO OBTAIN CERTAIN TOPOGRAPHICAL PROPERTIES THEREOF

Abstract: The present invention concerns the removal/reduction of the directionality or anisotropy of a surface and the formation of certain topographical properties of the same on a substrate material consisting of aluminium or an aluminium alloy. The procedure comprises the following process stages: chemical etching, preferably using phosphoric acid, and electrochemical polishing, preferably using phosphoric acid. The present invention has made it possible to produce surfaces with good optical qualities on substrate materials of inexpensive Al alloys which may contain recycled aluminium.

Description: A PROCEDURE FOR ELECTROCHEMICAL POLISHING OF AN ALUMINIUM SUBSTRATE TO OBTAIN CERTAIN TOPOGRAPHICAL PROPERTIES THEREOF The present invention concerns a procedure for removing/reducing the directionality or anisotropy of a surface and forming certain topographical properties of the same on a substrate material consisting of aluminium or an aluminium alloy. Moreover, the invention concerns a surface produced by implementing the procedure. The substrate material can be produced by casting, extrusion, rolling or other shaping techniques and may have an anisotropic or directional surface before it is treated in accordance with the present procedure. Moreover, it will be possible, in accordance with the present invention, to form an isotropic surface with a grainy appearance. A surface formed in accordance with the present invention has visual and optical properties equivalent to materials which are often designated as being "reflector matt" in the lighting trade. The surface will, therefore, be described in the following as reflector matt. Furthermore, the surface produced in accordance with the present invention will have different functional or aesthetic properties depending on the intended application. For example, a substrate material which is to be used for reflector purposes, for example in connection with a lighting product, must possess a number of properties. Lighting products are regarded as being extremely surface-critical and the material which is used must have high reflectivity and it should preferably have certain light dispersion properties. Moreover, the material must have sufficient scratch resistance to allow cleaning. Any layers on the surface of the material must adhere well to the substrate. The material should also be as light as possible and may typically be based on rolled aluminium. In recent years there has been a trend in the European lighting market away from polished materials to materials with lower brightness. The need for a reduction in price has been the most important driving force behind this development but the trend is also a consequence of better understanding of the connection between lighting and health/well-being. Since the light dispersion properties of the material are created today mainly in rolling mills, most of the work involved in producing the new types of materials has also been carried out there. Generally, it is easy to produce materials with a lower brightness in a rolling mill; this is what was done before it was possible to roll bright workpieces. However, the problem exists that the reduction in brightness is directional so that there is a difference between the rolling direction and the transverse direction. The materials become anisotropic regarding the optical properties and the rolling direction can clearly be observed on the surface of the material. For lighting purposes, this has negative functional and aesthetic consequences. In recent times, rolling mills have been able to produce materials with a lower brightness without directionality. This is done by first rolling a bright isotropic material using a relatively pure alloy and then introducing an isotropic roughness in the surface by using a structured, for example sandblasted, roll in the last roll pass. Such additional treatment is, however, expensive and the price of such materials is often as high as or higher than expensive high-gloss qualities. After the rolling has been completed, a chemical or electrochemical treatment/polish is performed which, in most cases, does not significantly alter the rolled topography. The topography is usually characterised by small shingle-like structures which give the surface isotropic optical properties with a grainy appearance. With both chemical and electrochemical polishing, so-called gassing defects may occur as a consequence of the formation of gas bubbles on the surface of the substrate. In today's production processes, it is necessary to avoid these defects. The present invention concerns a procedure which makes it possible to produce a material with qualitatively good surface properties based on a starting material which may have an undesired directionality in its surface, for example as the result of the shaping operation. This means that the requirements for mechanical processing/shaping can be reduced. If the present procedure is to be used for the formation of a lighting material, the starting material of the process will be characterised by two properties which distinguish it from the norm in today's production of isotropic lighting materials: - The rolling quality is considerably rougher and - The alloy is less pure than normal. While lighting products are produced today in rolling mills without the subsequent chemical or electrochemical process altering the starting material's surface topography significantly, an essential feature of the present invention is that it allows the use of a less expensive raw material by improving the chemical or electrochemical process. The same applies to aluminium-based starting materials shaped in another way. When used, the present invention will allow less stringent requirements to be made for the processing of the substrate by using an improved chemical or electrochemical process. As mentioned earlier, such processes are usually associated with the emergence of gassing defects, which are normally not wanted when treating aluminium surfaces. However, by controlling the emergence of gas-generated topography on the surface of the material during the chemical or electrochemical process, it has been found that it is possible to produce a reflector matt product without having to make stringent requirements for the quality of the starting material. Variants of reflector matt products are produced by means of minor differences in exposure times, temperatures and bath composition in the individual process stages. Using the proposed combination of the various process stages, it is possible to control the gas-generated topography formed on the aluminium surface. It has been shown that, using the procedure in accordance with the present invention, it is possible to achieve an even distribution of irregularities, in particular elevations produced in connection with the generation of gas on the surface of the substrate, which is utilised to produce a reflector matt product. The procedure allows the use of material qualities with a low purity (for example, AA 1200) and considerable roughness, for example "mill finish", on account of the improved control of the formation of elevations. The main stages in the process consist of chemical etching followed immediately by electrochemical polishing, preferably without the material being exposed to air. It is expedient to use chemicals which can be regenerated. The present invention will be described in further detail in the following in connection with the formation of reflector materials for lighting purposes as these materials are considered to be particularly surface-critical. It should naturally be understood that it is possible, in connection with the present invention, to produce aluminium-based materials for other purposes in which aesthetic and functional surface properties such as optical properties may be of significance. For example, visible building/structural elements of extruded or cast aluminium/aluminium alloy. Moreover, it can be mentioned that the principles of the present invention can be applied in one-off production as well as in continuous treatment of a strip. The alloy used in the subsequent description may comprise a semi-bright-rolled quality or a mill finish quality, i.e. qualities which are below bright-rolled quality in terms of both price and quality. Before the actual treatment in connection with the present invention is carried out, the material is degreased. This is done mainly in order to avoid rolling oil and dirt entering and contaminating the process bath. Degreasing may be done by using organic solvents, hot water (possibly with tenside), an acid or a basic solution. After degreasing, chemical etching takes place, for example in a bath. The bath for chemical etching may consist of phosphoric acid, water and a certain quantity of dissolved aluminium. The bath may also contain accessory agents of various types, for example to give the bath the desired viscous properties. Such accessory agents are known to experts and will not, therefore, be described in further detail here. The acid bath may have different concentrations and temperatures and the dwell times may also be varied. Varying the parameters provides the opportunity to control the density and size of the gas-generated elevations which occur on the surface of the material. It is assumed that gas-generated elevations are associated with defects and inhomogeneities in the surface itself. Chemical etching before electropolishing will condition the surface in such a way that an optimal distribution of elevations can be achieved. If the material is exposed to air, this conditioning will be altered, which will affect the appearance of the finished product. Control of the emergence of elevations can be achieved in particular by means of the use of prior etching. Electropolishing can be done in the same bath as the chemical etching. In a one-off process, this will be done by switching on the power for the electropolishing after a certain time. For a continuous strip process, this must be done differently. For example, the strip can pass through different zones in the same vessel so that it is not exposed to a significant electric field before the etching process has been carried out. Another solution, perhaps the easiest solution in technical terms for a strip process, is to carry out the prior chemical etching by spraying the strip with phosphoric acid just before it goes down into the electropolishing bath. The electropolishing itself can be carried out both in constant current and in constant voltage mode. It can also be carried out in different acid concentrations and with different additives in the bath. Moreover, the voltage and/or current can be varied to achieve different surface qualities. In order for a strip process to be practically feasible and financially profitable, it is important for the process time to be kept as short as possible. Process times for the electrochemical process of approximately 60 seconds are usually enough but there is potential for improvement of the products if longer process times are allowed. Longer times can often be used for a one-off process. Very good results are achieved with electrochemical treatment of approximately 10 minutes. Products produced in accordance with the present invention may preferably be finished by lacquering. However, anodisation may also be used. The choice of type of finishing will depend on the quality of the raw material, such as its purity. The present invention will be illustrated in the following using two detailed examples and figures showing the production of surfaces for lighting purposes, where: Figure 1 is a picture taken with a microscope of a surface produced in accordance with example 1 with a stated longitudinal scale of 500 micrometres, Figure 2 is a picture taken with a microscope of the surface shown in figure 1 with a stated longitudinal scale of 1 millimetre. Example 1 This example concerns the production of a reflector matt material for lighting purposes in an industrial one-off process. The objective is to achieve a reflector matt-like material with certain optical properties and a visually attractive appearance. The process was optimised for a one-off process and was carried out on laboratory scale. Process conditions Starting material: AA 1200 recycled mill finish (MF), optical specifications below Chemical etching: Bath: industrial phosphoric acid (77-80 % weight) High Al concentration: 24-54 g/l Conductivity of the bath, typical value: = 220 mS/cm Temperature: 70C Exposure time: 60 seconds Electrochemical polishing: Same bath as for chemical etching Exposure to air to be avoided Exposure time 10 min. Voltage 18 V Finished product A product is produced with a reflector matt-like appearance and an even, isotropic distribution of elevations. The material has acquired much reduced visual directionality. The total reflection in the finished product is affected by the oxide layer formed during the electrochemical process. Comparative tests were performed at full scale on full rasters in a test plant. The surfaces were virtually the same. The optical properties for the laboratory test are summarised below. Total reflection Brightness R20 (RD) Brightness R20 (TD) (DIN 5036 part 3) (Longitudinal direction) (Transverse direction) (DIN 67530) (DIN 67530) Starting material: 81.5% 225-230GU 120-123GU Finished: 85% 621 -651 GU 404-434GU Where: Total reflection was measured in accordance with DIN 5036 part 3. Brightness was measured at 20 in accordance with DIN 67530. Figures 1 and 2 show two pictures of a surface produced by the above treatment. Figure 1 is an enlargement of the surface in which the stated longitudinal scale is 500 micrometres. The longitudinal scale in figure 2 is 1 millimetre. The pictures clearly show the topographical phenomena mentioned in the application; they appear as dots in the pictures. The density of dots is in the range of 3.2-3.9 dots per mm2. The typical diameter of a dot is 150 micrometres while the height is estimated at 5 micrometres (3-6 micrometres). Some of the original rolling scores are shown as (horizontal) stripes on the pictures. Example 2 This example concerns the production of reflector matt material for lighting purposes in a coil process. It is difficult to make good reflector matt qualities in a coil process on account of the short process times available. However, this quality can be produced in accordance with the two processes proposed, a and b. Process conditions a) Starting material: AA 1050 semi-bright-rolled, optical properties below Chemical etching: Bath: concentrated phosphoric acid (85 % weight) Low Al concentration: 0-8 g/l Al Temperature: 55C Exposure time: 1 -3 seconds Electrochemical polishing: Same bath as for chemical etching Exposure to air to be avoided Exposure time 120-180 seconds Voltage 18 V Finished product: A product is produced with a reflector matt appearance and a relatively even distribution of elevations. The optical properties are summarised below: Total reflection Brightness R20 (RD) Brightness R20 (TD) (DIN 5036 part 3) (Longitudinal direction) (Transverse direction) (DIN 67530) (DIN 67530) Starting material: 81 % 1050-1200GU 1000-1200GU Finished: 87% 600GU 600GU Process conditions b) Starting material: AA 1050 semi-bright-rolled, optical specifications below Chemical etching: Bath: 60% weight phosphoric acid Low Al concentration: 0-8 g/l Al Temperature: 55C Time: 1 -3 seconds Electrochemical polishing: Same bath as for chemical etching Exposure to air to be avoided Exposure time 60 seconds Voltage 14.5 V Finished product: A product is produced with a reflector matt-like appearance and a relatively even distribution of elevations. The optical properties are summarised below: Total reflection Brightness R20 (RD) Brightness R20 (TD) (DIN 5036 part 3) (Longitudinal direction) (Transverse direction) (DIN 67530) (DIN 67530) Starting material: 81 % 1050-1200GU 1000-1200GU Finished: 87.5% 970GU 900GU

Claims: Claims 1. A procedure for removing/reducing the directionality or anisotropy of a surface and for forming certain topographical properties of the same on a substrate material consisting of aluminium or an aluminium alloy, characterised in that the substrate material is first subjected to chemical etching, preferably by using a solution comprising phosphoric acid, after which the substrate material is subjected to electrochemical polishing by using a solution which may preferably consist of phosphoric acid. 2. A procedure in accordance with claim 1 , characterised in that the surface of the substrate material is not exposed to air between the etching and the electrochemical polishing. 3. A procedure in accordance with claim 1 , characterised in that the treatment of the substrate material is carried out as a continuous process. 4. A procedure in accordance with claim 1 , characterised in that the treatment of the substrate material is carried out as a one-off process. 5. A substrate material of aluminium or an aluminium alloy in which the surface is treated in accordance with the procedure stated in claims 1-4, characterised in that the surface of the substrate material after the treatment comprises an isotropic haystack-like elevations. 6. A substrate material in accordance with claim 5, characterised in that the elevations, seen from above, are virtually circular with an average diameter in the order of 100 micrometres. 7. A substrate material in accordance with claim 5, characterised in that the elevations have an average height in the order of 2 micrometres. 8. A substrate material in accordance with claim 5, characterised in that the substrate material comprises a continuous strip of semi-bright-rolled quality or brighter. 9. A substrate material in accordance with claim 5, characterised in that the substrate material used in the one-off processing is of mill finish quality.

Inventors: HALLENSTVET MERETE, LEIN JOHN ERIK, MAARDALEN JOSTEIN, SOERAKER JAN MORTEN, SPOOREN CORNELIS, HALLENSTVET, MERETE, LEIN, JOHN, ERIK, MAARDALEN, JOSTEIN, SOERAKER, JAN, MORTEN, SPOOREN, CORNELIS
Applicants: GLAMOX FABRIKKER AS, HALLENSTVET MERETE, LEIN JOHN ERIK, MAARDALEN JOSTEIN, NORSK HYDRO AS, SOERAKER JAN MORTEN, SPOOREN CORNELIS, HALLENSTVET, MERETE, LEIN, JOHN, ERIK, MAARDALEN, JOSTEIN, NORSK HYDRO ASA, SOERAKER, JAN, MORTEN, SPOOREN, CORNELIS
Application year: 1998
Main classification: C25F3
IPC codes: C23F3, C25F3