WO1999013013A1

WO1999013013A1 - Coated substrate for lighting appliances, and a method for the production of such a material

Info

Publication number: WO1999013013A1
Application number: PCT/NO1998/000260
Authority: WO
Inventors: Cornelis Spooren; Steve Taylor; Merete Hallenstvet
Original assignee: Norsk Hydro Asa
Priority date: 1997-09-09
Filing date: 1998-08-28
Publication date: 1999-03-18
Also published as: NO974171L; AU9098098A; NO974171D0

Abstract

The present invention relates to a material for lighting appliances and a method for the production of such a material. The material is produced by coating a substrate material, preferably of aluminium or an al-alloy, by a transparent lacquer. The lacquer comprises elements, such as transparent particles embedded therein. By different combinations of transparent particles of different types and sizes added to the transparent lacquer, the layered material will have different optical scattering properties. According to the invention a material with high total reflection can be produced at relatively low cost.

Description

Coated substrate for lighting appliances, and a method for the production of such a material.

The coated substrate may be used in appliances provided with a separate light source, such as lamps, fixtures etc., but in general the coated substrate may be used in any application suitable. For instance, ceiling elements or partition walls formed of the coated substrate may be used in systems for indirect lighting or in daylight systems.

A material suitable for use as a reflecting material for lighting purposes has to fulfil certain demands. The material should have the highest possible reflectivity. Depending on the application, the material should in addition have certain light scattering properties. It should withstand mechanical stresses e.g. in forming operations and it should withstand cleaning operations without the formation of scratches. Protective layers on the surface of the material must adhere properly to the substrate. Finally, the material should have the lowest weight possible.

As early as in the thirties aluminium material was found to comply with many of the above mentioned demands. Compared with other materials, silver is the only material that is found to have better reflection properties than aluminium. The ductility properties of aluminium are good while its weight is low.

Until recently, bright lighting reflector surfaces were often preferred due to functional and aesthetic standards at the time. Chemical and electrochemical brightening processes were developed, in order to transform the relatively rough surface of a rolled material into a surface having a bright finish. In addition, these processes induce an improvement of the reflectivity of the rolled substrate.

Aluminium is a soft material, and in physical contact with other materials it is easily damaged. In addition, an unprotected surface of aluminium is disposed to corrosion. In most applications of aluminium, lighting appliances included, it is therefore necessary to protect the surface. The most common protection methods involve anodising or lacquering.

The production of reflectors was originally a typical batch process. First, reflector parts are formed from the rolled material. Following this, the parts are assembled to form complete reflectors or louvres. Then the louvres are brightened by a chemical or an electrochemical process and are given a protective layer in an anodising step or by coating with a lacquer.

In order to produce bright, high reflective materials with the anodising process a minimum alloy purity is required. This is necessary to avoid substantial reductions in the reflection- and brightness properties. In other words, the quality achieved before the final anodising step is otherwise being destroyed by the anodising process. On the other hand, lacquering of the alloy surface does not put any restriction on the alloy purity.

The traditional batch processes are time consuming partly due to the relatively rough input material and partly due to time for handling. As time went by, the rolling mills improved their processes, and are able to offer brighter rolled materials. As a result, immersion times in the brightening operations could be reduced and opened up for the introduction of more cost-efficient continuous brightening processes for the treatment of aluminium strips.

The rolling mills continued to improve their processes, and as a result, the need for brightening the material chemically or electro-chemically was reduced. Today, the material brightness is mainly created by the rolling mill. The role of the following coil process is merely to increase the reflectivity of the rolled substrate (without reducing the gloss), and to anodise the material for chemical and mechanical protection.

In the past decade the lighting market showed an increased interest in less bright materials implying more light scattering. The need for cost reduction has been the dominating driving force, but this development is also partly explained by a better understanding of the relation between lighting conditions and health and comfort. Since the light scattering properties of lighting materials are mainly created by the rolling mills, most of the efforts towards the development of new types of material have been carried out there.

By conventional rolling it is not straight forward to produce non bright material surfaces with isotropic light scattering properties, and both the functional and aesthetic optical properties of the surfaces show a distinct variation with respect to the rolling direction. This is generally regarded as being a negative consequence of the rolling operation. In order to circumvent this problem, the rolling mills have developed a technique in which the first step is to produce an isotropic, bright material, followed by the introduction of isotropic roughness on the surface by the use of a textured working roll in the final rolling step. As a consequence, the price of such materials will usually be as high as or even higher than the expensive bright qualities.

There are very few possibilities available to influence the light scattering properties of the material in the anodising process. It is possible to achieve aluminium-based materials with reduced and isotropic gloss by the application of alloys containing substantial amounts of intermetallic particles. The substrate material prior to anodising should have a relatively high, isotropic brightness. Intermetallic particles that precipitate in the anodic layer in the anodising process are effective light scatters. The particles also increase the interface topography between substrate and the anodic film as the anodising proceeds. As a result the brightness is substantially reduced. This is a well-known process that results in a material type that in commercial terms is often named as "semi-specular". One typical feature of this material is that it has a hazy appearance that often is unwanted, mainly of aesthetic reasons. In addition the reflectivity of these materials is reduced.

Very matt materials are usually produced by traditional, well known processes that combine chemical etching of an aluminium alloy surface together with anodising. The chemical etching process requires the use of relatively non-pure alloys which again has negative consequences for the material reflectivity after the anodising process. An other drawback of this method for the production of matt lighting materials is the relatively large amount of aluminium that needs to be removed from the surface in the etching process. The drawbacks of the present processes for the production of materials with varying light scattering properties, can be summarised as follows:

The rolling process has limited possibilities for the production of optically isotropic materials with various light scattering properties. The processes which have been developed are cost increasing.

The anodising process demands pure and expensive alloys for the production of bright and/or highly reflective materials.

The anodising process has limited possibilities for the production of various light scattering properties. The exeption is the "semi specular" quality but which has limitations with respect to aesthetic and functional optical properties.

The use of chemical etching for the production of matt materials implies a need for removal of large quantities of aluminium together with relatively non pure alloys that reduces the reflectivity that can be achieved.

- The forming properties of coil-anodised materials are limited. The anodic film may crack by forming operations and the optical properties and corrosion protection may be destroyed.

The cleaning of an anodised material is difficult, in particular the removal of finger prints.

The application of transparent lacquers is a well known alternative for the anodising process in order to protect the surface of a lighting material.

However, according to the knowledge of the Applicant, it has not been suggested to apply modified lacquer systems that may comply with both the desired light scattering properties and the reflectivity properties of a lighting material. The present invention thus relates to a novel lacquer system that is intended to create the possibility of producing high quality materials for lighting purposes without the use of very pure alloy qualities and expensive surface treatment processes

The invention will be further described in the following

The Applicant has observed that the addition of different elements to the transparent lacquer, may give the lacquer very interesting properties for use in relation with aluminium based substrates for lighting purposes

It is observed that it is possible to influence the topography of the surface of the lacquer, and thereby obtaining various textures, by adding chemicals such as amines to the lacquer These can produce a range of textures eg wrinkle/orange peel which may be used in lighting applications Such chemicals can be monomers or polymers and can give specific effects dependant on the binder type / quantity etc

Other elements that can influence the topography of the surface of the lacquer is found to be transparent particles of particular types and sizes added to the transparent lacquer

According to the above mentioned knowledge, it is possible to alter the light scattering properties of a material over a wide range by changing the composition of the transparent lacquer, without significant changes in the reflectivity The aluminium appearance of the surface is maintained

Different mechanisms may apply for the increase of light scattering, when transparent particles are added to a transparent, clear lacquer (1) A difference between the refractive indexes of the particle and the binder system (2) Poor wetting of the particles yielding a discontinuity of refractive index between particle and binder, even when particle and binder have the same refractive index (3) The particles create an optically rough surface

It has been found that one has to avoid light scattering by mechanisms (1) and (2), in order to minimise the reduction in reflectivity, and in order to maintain the aluminium appearance of the surface This means in other words that the binder system and the lacquer should form a medium that is optically homogenous By altering the size and number per volume unit of the particles or clusters of particles, it is possible to create a lacquer surface topography with varying scattering properties, and which can give the coated aluminium substrate a variety of appearances. Provided that the lacquer with the particles included can be regarded as optically homogenous, the reduction of reflectivity is minimised, and the aluminium appearance of the surface is maintained. This is achieved by choosing a binder system and particle type with a small difference in refractive index, and particles with good wetting properties with respect to the binder.

According to the invention, particle materials which may be used in conjunction with standard transparent binder systems (e.g. polyester, acrylate) are transparent materials such as SiO₂ (Silica) or Polyamide (Nylon).

In order to achieve a highest possible reflectivity of the coated material, irrespective of the gloss level of the material, the aluminium substrate should have a highest possible reflectivity prior to coating. A relatively high reflectivity, even on standard aluminium alloys (e.g. AA 1200 or AA 1050), may be obtained by the removal of 1-2 micrometers of the alloy surface, preferably in a chemical or electrochemical process.

Applying various amounts of particles or clusters of particles having a diameter of typically 10 micrometers, the light scattering may be varied over a wide range maintaining the reflectivity of the material. The gloss levels that are achieved depend upon the gloss level of the substrate material. By choosing a bright and sufficient isotropic substrate, it is possible to vary the light scattering properties over the widest possible range, in order to produce relatively matt materials, it is advantageous to use a standard mill finish substrate (in particular with respect to costs). By the addition of a sufficient amount of particles, it is possible to achieve a surface that both with respect to functionality and aesthetic properties is comparable to materials produced by chemical etching and anodising, but with a higher reflectivity and at a substantial lower cost.

By variations of particle size and number of particles per volume unit it is possible to produce materials with functional and optical properties that may substitute other existing anodised lighting materials (for instance materials with a characteristic grainy appearance often designated "reflectormat"). These materials may be produced with a large number of gloss levels with a maintained high reflectivity. In particular, when using particles of relatively large size (50 micrometers or more) it is important that the surface of the substrate material prior to lacquering is sufficiently bright and isotropic, depending on the desired properties of the end product.

For instance, the bright material as produced in accordance with the Applicant's co-pending Norwegian patent application, filed at the same date as the present application, may be used as a substrate material in the present invention.

The advantages of the proposed lacquer systems for lighting applications may be summarised as follows:

The use of lacquer relaxes the requirements with regard to the alloy purity compared to anodising. The total reflection is as good as or even better than that of an anodised substrate produced from the same material quality. The system allows the use of recycled materials.

A lacquered material has better forming and cleaning properties than an anodised material.

Further, according to the invention, the use of a lacquer system creates the possibility of manufacturing materials with a variety of optical properties, less dependent on the properties of the substrate material.

- Modifying light scattering properties of a material by the use of the lacquer system thus avoiding the introduction of optical anisotropy, is a less expensive solution than applying rolling processes involving the use of textured rolls.

- According to the present invention it is possible to produce matt materials having light scattering properties and an appearance equivalent to that of materials that have been treated by chemical etching and anodising. The reflection properties may even be improved compared to that of the state-of-the-art materials. Further, the invention do not require a good "etching response" of the substrate material. A good response to etching in the material will normally indicate that the material has reduced reflection properties after anodising.

The invention will in the following be further described by two examples and a figure, where example 1 relates to the production of a matt lighting material and example 2 relates to the production of a reflectormat lighting material.

Figure 1 relates to the development of gloss and total reflectivity as a function of the amount of particles in weight % on solid binder for the combination of binder system and particle type and size as described in example 1.

Example 1 : Lighting materials with various gloss levels

Substrate: AA 1200, Semi bright rolled

Gloss at 20° 800 GU according to DIN 67530

Total reflection 87% according to DIN 5036-part 3

Lacquer composition: Binder: acrylate

Solvent: esters, hydrocarbons Particles: Silica (SiO₂), commercial quality (for instance

Degussa ACEMATT 400), nominal size 3 micrometers, amount 0-12 weight% on solid binder

Viscosity: 60 seconds (DIN 4 cup, 21 °C)

Optical properties: Ref. figure 1

The figure shows that it is possible to reduce the gloss, and thereby improve the light scattering properties, in a substantial amount of a substrate material without a significant loss in total reflection. Example 2: reflectormat lighting material

Substrate: AA 1200, Semi Bright rolled

Gloss at 20° 1100GU according to DIN 67530

Total reflection 87% according to DIN 5036-part 3

Lacquer composition:

Binder: polyester, melamine

Solvent: esters, hydrocarbons

Particles: Polyamide, commercial quality

(for instance VESTASINT 2157), nominal size

20-60 micrometers, amount 1 ,5 weight % on solid binder

Viscosity: 60 seconds (DIN 4 cup, 21 °C)

Optical properties coated product:

Gloss at 20°: 500 GU according to DIN 67530 Total reflection: 83% according to DIN 5036-part3

Claims

1. Method for producing a material for lighting appliances, involving the step of coating a substrate material, preferably of aluminium or an Al-alloy, by a

5 transparent lacquer, characterised in that the substrate material is coated with a transparent lacquer having a surface with a topography that changes the optical scattering properties of the material and thereby the gloss, with a minimum reduction of the 10 reflectivity.

2. Method in accordance with claim 1 , characterised in that the lacquer have transparent particles embedded therein. 15

3. Method in accordance with claim 1, characterised in that the lacquer have chemical elements embedded therein.

204. Method in accordance with claim 1 , characterised in that the substrate material is coated in a continuous coil process.

5. Material for lighting appliances comprising a substrate coated with a

25 transparent lacquer, characterised in that the cured coating comprises a binder having elements embedded therein that changes the optical scattering properties and thereby the gloss of the material with a minimum reduction of the reflectivity.

30

6. Material in accordance with claim 5, characteriser in that the elements are constituted by transparent particles or clusters of transparent particles that influence the topography of the surface of the coating.

7. Material in accordance with claim 6, characteriser in that the particles and the binder have approximately equal refractive indexes, which secure the achievement of an optically homogenous material.

8. Material in accordance with claim 7, characteriser in that the binder is of an acrylate type and that the particles are constituted by silica and have a nominal size of 3 micrometers.

9. Material in accordance with claim 7, characteriser in that the binder is of a polyester-melamine type and that the particles are constituted by polyamide and have a size of 20-60 micrometers.

10. Material in accordance with claim 5, characterised in that the binder comprises chemical elements that influence the topography of the surface of the coating.

11. Material in accordance with any preceding claim 5-10, characterised in that the substrate material is made out of aluminium or an Al-alloy.