HK1259845B - A method of manufacturing a coated polymer substrate having low emissivity - Google Patents

Description

Method for producing a coated polymer substrate having low emissivity

Technical Field

The present invention relates to a method for producing a coated polymeric substrate having low emissivity properties, high hardness and good scratch resistance. The present invention further relates to a coated polymeric substrate and the use of such a coated polymeric substrate as a low emissivity substrate.

Background Art

Thermal insulation of glass surfaces, such as those in buildings and vehicles, has become increasingly important over the past few decades. This is driven not only by the need for greater living comfort, such as temperature control, but also by the growing concern to limit energy consumption.

Low-e (low emissivity) coatings have been developed to minimize the amount of infrared (IR) light that can pass through the glass without compromising the amount of visible light transmitted.

A first type of low-e coating known in the art comprises coatings having at least one metal layer. Typically, such low-e coatings comprise at least one sputter-deposited silver layer deposited between dielectric layers, such as titanium oxide (TiO ₂ ).

However, this low-e coating has several drawbacks. The silver layer has low stability, limited hardness, low durability, and poor resistance to moisture and weathering. Consequently, this coating remains quite fragile and must be protected, for example, by a hard polymer top layer. However, this hard polymer top layer negatively impacts the structure's emissivity due to the high absorption of IR heat.

The second type of low-e coating consists of ceramic coatings deposited on glass substrates. These coatings are bonded to the glass substrate in a semi-molten state (pyrolytic coatings). Typical ceramic coatings contain oxides such as indium oxide, tin oxide, indium tin oxide, or zinc oxide. The oxides become part of the glass, making the low-e coating more durable. Obviously, these coatings cannot be deposited on polymer substrates because ceramic synthesis requires high temperatures, and polymer substrates cannot withstand temperatures above 150°C.

Summary of the Invention

It is an object of the present invention to provide a method for producing a coated polymer substrate and to provide a coated polymer substrate having low emissivity properties, avoiding the disadvantages of the prior art.

Another object of the present invention is to provide a coated polymeric substrate that combines the characteristics of high hardness, good scratch resistance and low emissivity.

Another object of the present invention is to provide a coated polymeric substrate whereby the coating has excellent adhesion to the polymeric substrate.

Another object of the present invention is to provide a coated polymer substrate such that a silica layer or a silica-based layer can be deposited on the polymer substrate at a sufficiently low temperature that the polymer substrate is not damaged.

It is a further object of the present invention to provide use of the coated polymeric substrate as a low emissivity film.

According to a first aspect of the present invention, there is provided a method for producing a coated polymer substrate having low emissivity properties and high hardness. The method comprises the following steps:

- providing a polymer substrate;

- coating at least one adhesion-promoting layer on one side of the polymer substrate;

Starting from a mixture comprising at least one partially condensed alkoxide precursor, at least one silicon dioxide layer or a silicon dioxide-based layer is applied to the at least one adhesion-promoting layer by a sol-gel process.

As polymeric substrate, any polymeric substrate may be considered, such as a polymer sheet or foil. Preferably, the polymeric substrate is flexible and transparent. Any material conventionally used for polymeric substrates may be considered, in particular any material conventionally used for window films or daylight control films. Preferred substrates include polymer films comprising polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyurethane (PU), polycarbonate (PC), polyimide (PI) and polyetherimide (PEI). Typical substrates include PET substrates with a thickness ranging between 12 μm and 125 μm, for example 75 μm.

The silicon dioxide layer or the layer based on silicon dioxide is deposited by sol-gel method.Sol-gel is a known technology for producing oxides of silicon, for example. The method comprises converting a monomer into a colloidal solution (sol) and reacting subsequently to form a network (or gel). In the sol-gel method, at least one precursor is dissolved in a suitable liquid, which is typically water or an organic solvent (for example, alcohol). For catalytic reaction, a catalyst, for example an acid or a base, is preferably added. In the sol-gel method according to the present invention, the sol-gel method starts from a mixture comprising at least one partially condensed alkoxide precursor.

The sol can be deposited on a polymer substrate, and more particularly on an adhesion-promoting layer deposited on a polymer substrate, for example by wet chemical deposition. The deposited layer can be further crosslinked, for example thermally or by using radiation (IR and/or UV radiation). Through these processing steps, the solvent and/or water is evaporated.

To obtain the inorganic glass material, the gel is annealed in an oxygen atmosphere to a temperature above 450° C. Through this calcination step, the organic compounds of the material are calcined and a pure inorganic glass material is obtained.

Clearly, a sol-gel layer deposited on a polymer substrate may not be subjected to such high temperatures because the polymer substrate itself cannot withstand such high temperatures.

Therefore, the sol-gel process according to the invention is modified to obtain a uniform or substantially uniform layer which is free or substantially free of organic material, and this by using a process which involves only processing steps at relatively low temperatures, i.e. temperatures which are low enough not to damage the polymer substrate.

“Free of organic materials” means that the silicon dioxide layer or the silicon dioxide-based layer does not contain organic materials.

"Substantially free of organic material" means that the amount of organic material present in the silica layer or silica-based layer is less than a few weight percent.

In contrast to the silica coatings or silica-based low-e coatings known in the art which have a (poly)crystalline structure, the silica coating or silica-based coating of the coated polymer substrate according to the invention preferably has a quasi-amorphous structure.

As already mentioned above, the sol-gel process according to the invention preferably starts from a mixture of at least one precursor, a solvent and preferably also a catalyst.

The at least one precursor preferably comprises an alkoxide precursor. The alkoxide precursor is preferably at least partially condensed.

Preferred alkoxide precursors include alkoxysilane precursors, such as silanes selected from the group consisting of tetraethoxysilane (TEOS), tetramethoxysilane (TMOS), methyltriethoxysilane (MTES), vinyltrimethoxysilane (VTMS), 3-aminopropyltrimethoxysilane (APS), methacryloxypropyltrimethoxysilane (MAPTS), bis(triethoxysilyl)hexane, 1,6-bis(trimethoxysilyl)hexane, or any combination thereof.

The degree of pre-condensation of the silane precursor in the sol is preferably high, i.e., greater than 60%, greater than 70%, greater than 80%, or greater than 90%. In a preferred embodiment, the mono-, di-, tri-, and tetra-substituted siloxane bonds designated Q1, Q2, Q3, and Q4, respectively, are fully condensed. The degree of pre-condensation can be determined by NMR.

The solvent preferably comprises an organic solvent, for example an alcohol such as methanol or ethanol.

The catalyst preferably comprises at least one base or at least one acid. Preferred catalysts comprise hydrochloric acid, acetic acid or formic acid or any mixture thereof.

The sol-gel coating is applied by any technique known in the art, such as by dipping, spin coating, spray coating, printing or roller coating.

Preferred techniques include roller coating. In roller coating methods, a liquid film is formed on a continuously moving substrate by using one or more rotating rollers. Preferred roller coating methods include gravure coating methods. In gravure coating methods, a coating is applied using a patterned roller (which is a roller with holes or grooves).

After application of the coating, the coated substrate is preferably dried, for example in an oven at a temperature of 100°C.

Possibly, in addition to or instead of drying, the coated substrate is subjected to irradiation, for example infrared or UV radiation.

Drying and/or irradiation further stimulates polymerization of the network and causes evaporation of the solvent.

In order to ensure sufficient adhesion of the silica layer or the silica-based layer to the polymer substrate, an adhesion-promoting layer is applied to the polymer substrate before the silica layer or the silica-based layer is applied.

The adhesion promoting coating may comprise any material. Preferably, the adhesion promoting layer comprises a metal oxide, preferably a metal oxide selected from the group consisting of doped or undoped titanium oxide, indium oxide, tin oxide, zinc oxide, indium tin oxide, niobium oxide, zirconium oxide, and any mixtures thereof.

The adhesion promoting layer may be deposited by any technique known in the art. Preferred techniques include chemical vapor deposition, sputter deposition and evaporation.

According to a second aspect of the present invention, there is provided a coated polymer substrate having low emissivity properties and high hardness. The coated polymer substrate comprises

- polymer substrate;

- at least one adhesion-promoting layer deposited on one side of said polymeric substrate;

- at least one silicon dioxide layer or silicon dioxide-based layer deposited on the adhesion-promoting layer.The silicon dioxide layer or silicon dioxide-based layer deposited on the adhesion-promoting layer is deposited by a sol-gel process.

The coated polymeric substrates according to the invention are characterized by a low emissivity and a high hardness.

The emissivity of the coated polymeric substrate is preferably below 0.2. More preferably, the emissivity of the coated polymeric substrate is below 0.1, such as below 0.06 or below 0.04.

The hardness of the coated polymer substrate is preferably higher than a 2H pencil hardness. More preferably, the hardness of the coated polymer substrate is higher than a 3H pencil hardness.

As mentioned above, the silicon dioxide layer or silicon dioxide-based layer is free of organic material or substantially free of organic material.

The silicon dioxide layer or silicon dioxide-based layer preferably has a thickness of 0.1 μm to 1 μm. More preferably, the silicon dioxide layer or silicon dioxide-based layer has a thickness between 0.2 μm and 0.6 μm, such as 0.25 μm or 0.40 μm.

The adhesion promoting layer preferably has a thickness in the range of 0.01 μm to 0.1 μm. More preferably, the adhesion promoting layer has a thickness between 0.02 μm and 0.06 μm, such as 0.04 μm or 0.05 μm.

As mentioned above, the adhesion promoting coating may comprise any material. Preferably, the adhesion promoting layer comprises a metal oxide, preferably a metal oxide selected from the group consisting of doped or undoped titanium oxide, indium oxide, tin oxide, zinc oxide, indium tin oxide, niobium oxide, zirconium oxide, and any mixtures thereof.

According to a third aspect of the present invention, there is provided a glass substrate having a coated polymer substrate as described above. The coated polymer substrate is adhered to the glass substrate, for example, by an adhesive.

According to a fourth aspect of the present invention, there is provided use of the coated polymer substrate as described above as a low emissivity substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further explained with reference to the accompanying drawings. Figure 1 shows a cross section of a coated polymer substrate according to the invention.

DETAILED DESCRIPTION

The present invention will be described with reference to specific embodiments and with reference to certain drawings, but the invention is not limited thereto but only by the claims. The drawings are illustrative only and non-restrictive. In the drawings, the dimensions of some elements may be exaggerated and not drawn to scale for illustrative purposes. Dimensions and relative sizes do not correspond to actual reductions in practice of the invention.

The figure shows a cross section of a coated polymer substrate 100 according to the present invention. The coated polymer substrate 100 comprises a polymer substrate 102, an adhesion promoting layer 104 and a silica layer or silica-based layer 106.

The polymer substrate 102 may comprise any polymer substrate. A preferred polymer substrate 102 comprises a polyester foil having a thickness of 75 μm.

The adhesion promoting layer 104 comprises, for example, an oxide layer, such as a titanium oxide ( _TiO2 ) layer preferably having a thickness between 0.02 μm and 0.04 μm. The TiO2 layer may be deposited by any technique known in the art. A preferred technique for depositing the _{TiO2 layer} is by sputtering deposition.

In a preferred preparation of the sol, ethanol is used as the solvent and tetraethoxysilane (TEOS) as the precursor. An organic linker such as bis(triethoxysilyl)hexane or 1,6-bis(trimethoxysilyl)hexane may be added. Hydrochloric acid, acetic acid, and formic acid are added as catalysts. The precursors are mixed and added to the ethanol under rapid stirring. Water is first acidified by mixing in the catalyst, and then the water is added to the precursor mixture under stirring. The mixture is stirred until a clear sol is obtained. The mixture is maintained at a predetermined temperature for a certain period of time for precondensation. For example, the mixture is maintained at 60°C for 1 hour under stirring and reflux.

The degree of pre-condensation of the silane precursor in the sol is preferably high, i.e., greater than 60%, greater than 70%, greater than 80%, or greater than 90%. In a preferred embodiment, the mono-, di-, tri-, and tetra-substituted siloxane bonds, designated Q1, Q2, Q3, and Q4, respectively, are fully condensed. The degree of pre-condensation can be determined by NMR.

A silica layer or silica-based layer 106 is applied on top of the adhesion promoting layer preferably by roller coating and more preferably by gravure coating. The thickness of the applied layer is influenced, for example, by the speed of the substrate and the speed of the roller.

The thickness of the silicon dioxide or silicon dioxide coating is preferably between 0.25 μm and 0.4 μm.

The coated polymeric substrates according to the present invention were subjected to various tests: hardness test, adhesion test and low-e measurement. The tests are described in more detail below.

The hardness of the samples was evaluated using the Wolff-Wilborn method (ASTM D3363). The test was performed using an Elcometer 501 pencil hardness tester. The coated substrate was placed on a firm, horizontal surface and a pencil was held firmly against the coating at a 45° angle away from the operator. The pencil was then pushed away from the operator. The hardness of the pencil was increased until one or both of the following defects appeared in the coating:

a. Plastic deformation: permanent indentation in the coating surface without cohesive fracture.

b. Cohesive Failure: A visible scratch or crack exists in the coating surface where material has been removed from the coating.

The hardness of the pencil that mars the surface is taken as a measure of scratch hardness, for example, '2H' hardness.

The adhesion of the coating to the substrate was determined by a cross-hatch test. The cross-hatch test is a method of determining the resistance of a coating to separation from a substrate by cutting a right-angled grid pattern through the coating to the substrate using a tool.

In the crosshatch test, a crosshatch pattern is made through the coating onto the substrate. In the next step, the coating's release flap is removed by rubbing with a soft brush. Pressure-sensitive tape is then applied over the crosshatch cuts. The tape is smoothed into place by running a pencil eraser over the cut areas. It is then removed by quickly pulling it back to an angle close to 180°. Adhesion is rated on a scale of 0 to 5. Table 1 further explains the 0 to 5 scale.

Table 1

The emissivity of a sample is measured by briefly exposing the surface to thermal radiation from a blackbody at 100°C. To achieve completely uniform illumination of the measurement surface, the radiator is designed in the form of a spherical half-space. A portion of the reflected radiation strikes the radiation sensor through an opening in the radiator.

The radiation level is determined by comparing the sample's reflectance to stored reference values of two calibration standards. A TIR 100-2 instrument was used in the test, with calibration standards of 0.010 and 0.962.

For the samples described above and shown in the accompanying figures, the following results were obtained:

Hardness: 3H pencil hardness

Adhesion test (cross hatch test): Grade 0

Radiation coefficient test: E=0.04.

Claims (13)

1. A method for manufacturing a coated polymer substrate with an emissivity of less than 0.1 and a hardness of at least 2H pencil hardness, the method comprising the following steps: -Provide polymer substrates; - Coat at least one adhesion-promoting layer on one side of the polymer substrate; - Starting from a mixture containing at least one partially condensed alkoxide precursor, at least one silica layer or silica-based layer is coated onto the at least one adhesion promoting layer by a sol-gel method, wherein the thickness of the adhesion promoting layer is between 0.01 μm and 0.1 μm, and wherein the at least one adhesion promoting layer contains a metal oxide selected from doped or undoped titanium oxide, indium oxide, tin oxide, zinc oxide, indium tin oxide, niobium oxide, zirconium oxide, and any mixture thereof. 2. The method of claim 1, wherein the silica layer or silica-based layer is obtained from a mixture comprising at least one partially condensed alkoxide precursor, a solvent, and a catalyst. 3. The method of claim 1 or claim 2, wherein the degree of condensation of the at least one partially condensed alkoxide precursor is at least 60%. 4. The method of claim 3, wherein the alkoxide precursor is fully condensed in the sol. 5. The method according to claim 1, wherein the alkoxide precursor is an alkoxysilane. 6. The method according to claim 5, wherein the alkoxysilane is selected from: tetraethoxysilane, tetramethoxysilane, methyltriethoxysilane, vinyltrimethoxysilane, 3-aminopropyltrimethoxysilane, methacryloxypropyltrimethoxysilane, bis(triethoxysilyl)hexane, 1,6-bis(trimethoxysilyl)hexane, or any combination thereof. 7. The method according to claim 2, wherein the catalyst comprises at least one base or at least one acid. 8. The method of claim 1, wherein the silicon dioxide layer or silicon dioxide-based layer is deposited by roll coating. 9. The method of claim 1, wherein the at least one adhesion-promoting layer is deposited by sputter deposition. 10. A coated polymer substrate having a low emissivity, said coated polymer substrate comprising: -Polymer substrate; - At least one adhesion-promoting layer deposited on one side of the polymer substrate; - Deposit at least one silica layer or silica-based layer on the adhesion-promoting layer, said at least one silica layer or silica-based layer being deposited by a sol-gel method; starting from a mixture containing at least one partially condensed alkoxide precursor, The coated polymer substrate has an emissivity of less than 0.1, and the hardness of the at least one silica layer or silica-based layer is at least 2H pencil hardness, wherein the thickness of the adhesion promoting layer is between 0.01 μm and 0.1 μm, and wherein the at least one adhesion promoting layer comprises a metal oxide selected from doped or undoped titanium oxide, indium oxide, tin oxide, zinc oxide, indium tin oxide, niobium oxide, zirconium oxide, and any mixture thereof. 11. The coated polymer substrate according to claim 10, wherein the thickness of the silica layer or silica-based layer is between 0.1 μm and 1 μm. 12. A glass substrate having a polymer substrate coated as defined in claim 10 or 11. 13. Use of the coated substrate as defined in claim 10 or 11 as a substrate having a low emissivity.