Plastic substrate having a porous layer and method for producing the porous layer

Abstract

A plastic substrate having a porous layer is disclosed. In an embodiment, the porous layer is formed at least partially from a material of the plastic substrate and has pores. The proportion by volume of pores is greater in a first region of the porous layer than in a second region of the porous layer. The second region follows the first region, as seen proceeding from the plastic substrate. The porous layer can be produced by a plasma process that simultaneously effects structuring of the plastic substrate by ion bombardment and coating of the plastic substrate.

Claims

1. A plastic substrate comprising: a porous layer on a surface, wherein the porous layer is formed at least partially from a material of a plastic substrate and has pores, wherein a proportion by volume of the pores is greater in a first region of the porous layer than in a second region of the porous layer, wherein the first region is located between the second region and the plastic substrate, wherein the porous layer has a thickness of between 50 nm and 500 nm, wherein the pores, on average, have a lateral extent of between 20 nm and 200 nm, and wherein a solid phase of the porous layer is formed to an extent of at least 10% from the material of the plastic substrate.

2. The plastic substrate according to claim 1, wherein the first region is formed to an extent of at most 70% by volume from the material of the plastic substrate.

3. The plastic substrate according to claim 1, wherein the first region is formed to an extent of at most 50% by volume from the material of the plastic substrate.

4. The plastic substrate according to claim 1, wherein the first region of the porous layer has a lower effective refractive index than the second region of the porous layer.

5. The plastic substrate according to claim 1, wherein the plastic substrate is a flexible plastic substrate.

6. The plastic substrate according to claim 1, further comprising a further layer overlying the porous layer.

7. The plastic substrate according to claim 6, wherein the porous layer and the further layer form an optical interference layer system.

8. The plastic substrate according to claim 1, wherein the porous layer comprises at least one material selected from the group consisting of Al, Mg, Zn, Sn, Si, Ti, C, V, Cr, Fe, Cu, In, Ag, Zr, Hf, Ta, W, Ce, a chemical compound comprising at least one of these materials or an alloy comprising at least one of these materials.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be explained in more detail herein below with reference to exemplary embodiments in conjunction with FIGS. 1 to 4, in which:

(2) FIG. 1 shows a schematic illustration of a cross section through a plastic substrate having a porous layer according to one exemplary embodiment;

(3) FIG. 2 shows an enlarged illustration of a section of the surface of the plastic substrate with the adjoining porous layer according to one exemplary embodiment;

(4) FIG. 3 shows a schematic illustration of a cross section through an apparatus for carrying out the method for producing the plastic substrate having the porous layer according to one exemplary embodiment; and

(5) FIG. 4 shows a graphical illustration of the transmission and the reflection of an IR filter, which comprises a plastic substrate having a porous layer according to one exemplary embodiment.

(6) Identical component parts or component parts having an identical action are provided in each case with the same reference signs in the figures. The component parts shown and also the size relationships of the component parts among one another should not be regarded as true to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

(7) In the exemplary embodiment shown schematically in cross section in FIG. 1, a porous layer 20 is applied to a plastic substrate 10. The porous layer 20 has a first region 21 and a second region 22, the second region 22 following the first region 21, as seen proceeding from the plastic substrate 10. The second region 22 is thus at a greater distance from the plastic substrate 10 than the first region 21. The regions 21, 22 of the porous layer 20 each contain pores, the proportion by volume of the pores being greater in the first region 21 than in the second region 22. The porous layer 20 preferably has a thickness of between 50 nm and 500 nm.

(8) At least one further layer 30 is preferably arranged on the porous layer 20. The further layer 30 may be in particular an optically active layer. The further layer 30 is, for example, an optically transparent dielectric layer, e.g., an oxide, nitride or fluoride layer. By way of example, the at least one further layer may comprise TiO.sub.2 or SiO.sub.2.

(9) Together with the porous layer, the at least one further layer 30 may form, for example, an antireflection coating for the plastic substrate or an optical filter.

(10) Instead of a single layer 30, it is possible for a plurality of further layers to be arranged on the porous layer 20, these preferably forming an optical interference layer system. The optical interference layer system can comprise in particular a plurality of oxide, nitride or fluoride layers.

(11) The porous layer 20 arranged between the plastic substrate 10 and at least the further layer 30 advantageously reduces mechanical stresses between the plastic substrate 10 and the at least one further layer 30. This is advantageous particularly when the plastic substrate 10 is a flexible substrate, for example a film. Without the porous layer 20 between the plastic substrate and the further layer 30, there would otherwise be the risk that the further layer 30 would become damaged in the case of buckling of the plastic substrate 10, or would even become completely or partially detached from the plastic substrate 10.

(12) In addition to this mechanical function, the porous layer 20 advantageously also has an optical function. The porous layer 20 has an effective refractive index dependent on the proportion by volume of the pores in the porous layer 20. Owing to the greater proportion by volume of the pores, the first region 21 has a lower effective refractive index than the second region 22. For the calculation of the optical action of the porous layer 20, this can be regarded approximately as a first layer 21 having a first effective refractive index n.sub.eff,1 and a second layer 22 having a second effective refractive index n.sub.eff,2>n.sub.eff,1. In actual fact, the porous layer has a gradient of the refractive index, which is caused by the three-dimensional structure of the pores.

(13) An example of the three-dimensional structure of the pores 23 is illustrated in FIG. 2, which shows an enlarged illustration of a partial region of the surface of the plastic substrate 10 and of the porous layer 20 adjoining the latter. In the first region 21, in the vicinity of the surface of the plastic substrate 10, the pores 23 have a relatively large lateral extent, whereas in a region 22 lying thereabove the proportion of the solid phase 24 is larger. The effective refractive index in the direction perpendicular to the surface of the plastic substrate 10 therefore has a gradient which increases from the first region 21 toward the second region 22. The pores 23 on average preferably have a lateral extent of between 20 nm and 200 nm. The pores 23 are thus smaller than the wavelength of visible light and therefore are advantageously not visible.

(14) The solid phase 24 of the porous layer 20 is advantageously formed to an extent of at least 10% from the material of the plastic substrate 10. Furthermore, the solid phase 24 of the porous layer 20 advantageously contains at least one of the materials Al, Mg, Zn, Sn, Si, Ti, C, V, Cr, Fe, Cu, In, Ag, Zr, Hf, Ta, W, Ce, a chemical compound comprising at least one of these materials or an alloy comprising at least one of these materials. The fact that the porous layer 20 is formed partially from the material of the plastic substrate 10 and partially from a further material is based on the method for producing the porous layer 20.

(15) The method for producing the plastic layer 10 will be explained herein below in conjunction with FIG. 3, which shows an apparatus for producing the plastic substrate having the porous layer.

(16) The apparatus for producing the porous layer on the plastic substrate 10 has, for example, an arrangement made up of two planar magnetrons 8. The planar magnetrons 8 each comprise an electrode 1, which are connected to a medium-frequency voltage generator 2. Furthermore, the planar magnetrons 8 each contain magnets 3, which generate a magnetic field 4.

(17) The method is carried out in a vacuum installation, into which a process gas 7 is admitted. The method is advantageously suitable for treating substrates 10 with a large surface area. By way of example, the plastic substrate 10 may be a film transported on a roller 9. As the method is being carried out, the plastic substrate 10 is preferably moved continuously past the magnetrons 8 at a distance therefrom.

(18) In the method, a magnetic field assisted glow discharge is produced in the process gas 7 by means of the magnetron 8. In this case, a plasma is produced in the process gas 7, where the magnetic fields 4 generated by the magnets 3 should reduce diffusion of electrons from the plasma.

(19) During the magnetic field assisted glow discharge, positive ions 5, which are accelerated in the direction of the electrodes 1, are firstly produced in the process gas 7. The positive ions 5 are, for example, ions of a noble gas present in the process gas 7. The process gas 7 may contain argon, for example.

(20) In the method described herein, the process gas 7 contains at least one electronegative constituent. The electronegative constituent is preferably oxygen. The process gas 7 preferably comprises at least 15% oxygen. Alternatively, the process gas 7 may contain, for example, fluorine or chlorine as the electronegative constituent. Oxygen, fluorine and chlorine are distinguished by particularly high values of electronegativity. Since the process gas 7 contains at least one electronegative constituent, negative ions 6 form during the magnetic field assisted glow discharge. Some of the negative ions 6 are formed in the plasma. These negative ions generally have relatively low energies and therefore generally cannot leave the plasma. Further negative ions 6 are produced at the surface of the electrodes 1. These negative ions 6 are accelerated in the direction of the plastic substrate 10 by an electrical voltage of preferably more than one hundred volts and, in the method, advantageously serve to modify the surface of the plastic substrate 10. The negative ions 6 impinging on the plastic substrate 10 partially remove the material of the plastic substrate 10 by virtue of the fact that particles 12 are driven out from the plastic substrate. This material removal is locally inhomogeneous and produces structures in the plastic substrate 10 which form the pores.

(21) At the same time, the positive ions 5 produced in the plasma are accelerated in the direction of the electrode 1, where they drive out particles 11 of the electrode material, which are deposited on the plastic substrate 10. This process is known per se as sputtering.

(22) The porous layer is thus produced by means of a plasma process which simultaneously effects structuring of the plastic substrate 10 by bombardment with ions 6 and coating of the plastic substrate 10 by sputtering of the material of the electrode 1.

(23) After the porous layer 20 has been produced, in the method it is advantageous for at least one further layer to be applied to the plastic substrate 10. This may be effected by means of known coating methods, such as for example magnetron sputtering, thermal evaporation or electron beam evaporation.

(24) FIG. 4 shows the calculated transmission and reflection of an IR filter formed from the plastic substrate 10, a porous layer 20 applied thereto and a further layer 30, which is a layer of TiO.sub.2 having a thickness of 145 nm. The IR filter advantageously has a high transmission and a low reflection in the visible range, and is therefore suitable in particular for a transparent optical element. In the infrared spectral range, by contrast, the filter has a relatively high reflection and only a low transmission. The IR filter can therefore have a heat-shield function, for example. The plastic substrate 10 may be, for example, a lamination film, which can be applied for example to panes for applications in architecture or in automotive construction. Such an IR filter can be produced by the method described herein in a relatively simple manner and with relatively few layers. Comparative calculations show that a comparable optical action would be achievable with a layer system of five layers produced by means of magnetron sputtering with an overall thickness of approximately 600 nm. The IR filter shown in FIG. 4, by contrast, advantageously has only the porous layer and the one further layer of TiO.sub.2 on the plastic substrate.

(25) The invention is not restricted by the description on the basis of the exemplary embodiments. Instead, the invention encompasses any novel feature and also any combination of features, which includes, in particular, any combination of features in the patent claims, even if this feature or this combination itself is not explicitly specified in the patent claims or exemplary embodiments.