Optical coating having a low refractive index

Present invention relates to a novel composition for the preparation of porous coatings of low refractive index, e.g. for optical purposes such as antireflex coatings, optical de- vices or porous substrates, a method for preparing such coatings, and the use of such coatings e.g. in photovoltaic devices, security elements or features, waveguiding applications (e.g. as cladding layer), lighting applications, light concentration devices, optical adhesives. Coatings of low refractive index are widely used, especially in optical devices, for reducing undesired reflections on surfaces between ambient air or vacuum and optical materials such as glass or suitable plastics. As described in US-7821691 and documents cited therein, a convenient method for obtaining a material whose refractive index is lowered in comparison to the one of these typical bulk materials is the inclusion of voids to obtain porous phases, e.g. by incorporating optically neutral particles into such materials. Optically neutral particles are to be understood as particles which do not cause noticeable absorption or scattering of the light transmitted.

Porous layers, and layers comprising particles, have also been used as ink receiving layer in printable materials, or for improving abrasion resistance of a surface equipped with such a layer. WO 2006/040304 describes certain coatings comprising modified silica particles, polyvinyl alcohol and boric acid, for use in ink jet recording materials. WO 2014/193573 describes a reinforced coating prepared by dispersing silica nano- particles in polyvinyl alcohol and crosslinking.

WO 2013/019770 proposes an antireflex coating comprising Si02-nanoparticles, a matrix polymer and a silane binder; a similar coating is described in WO 2013/055951. WO 2008/01 1919 and WO 2013/1 17334 disclose certain nanoporous layers for optical applications, whose polymer matrix is typically based on polyvinyl alcohol hardened with boric acid; layers of WO 2013/1 17334 contain silica particles with positively charged surface (PCS), and show a refractive index below 1.2.

It has now been found that nanoporous layers of low refractive index, which are based on silica particles in a crosslinked matrix of hydrophilic polymer, show surprisingly good mechanical and optical properties such as low haze, good stability against UV exposure and/or exposure to humidity, adhesion to polymer substrate such as PET or a further layer of hydrophilic polymer, and that such layers may conveniently be laminated to such further layers, if an alkoxysilane such as tetraethoxysilane (TEOS) is used as the crosslinking agent. Using a silane crosslinker instead of boron compounds may further improve the environmental compatibility of such materials. Besides the effect of low reflectance, porous coatings of the invention are further useful for modifying inter- facial tension, reducing hydrophobicity and improving interlayer adhesion. A convenient way to obtain materials showing especially low refractive properties is to increase the particle content, typically expressed as the weight ratio of silica particles to hydrophilic polymer in such materials. The dependency of refractive index from particle loading is shown in the below table (compositions consisting of modified Si02 nanoparticles, pol- yvinylalcohol and water as shown in the control compositions of the below example 3): ratio

20 : 1 12 : 1 4 : 1 2 : 1 1 .5 : 1 1 .2 : 1 1 : 1 Si02/PVA

refraxtive

1 .153 1 .158 1 .201 1 .294 1 .338 1 .497 1 .500 index

It is a further advantage of present invention, that such highly particle-loaded materials according to the invention show especially low haze. The invention thus primarily pertains to a composition comprising

a) a polar polymer,

b) a tetraalkoxysilane, and

c) modified silica nanoparticles having a positively charged surface. The polar polymer component (a) generally is water soluble or well dispersable in water, and/or contains hydroxy groups. Examples for such polymers are polyvinyl alcohol, hydroxyacrylates and copolymers of vinylalcohol and/or hydroxyacrylates; typically, such copolymers may be statistical or block copolymers, consisting of at least 30 %, preferably at least 50 %, more preferably at least 70 %, and most preferably at least 90 % repeating units containing a hydroxy group such as vinylalcohol, hydroxyalkylacry- late, hydroxyalkylmethacrylate. Especially preferred as component (a) is polyvinyl alcohol. Preferably, the polar polymer (a) is added to the present composition in form of a homogenous aqueous solution. The tetraalkoxysilane (b) typically is of the formula Si(OR) ₄ , wherein R is Ci-C ₄ alkyl; especially preferred is tetraethoxysilane (TEOS).

Silica nanoparticles having a positively charged surface useful as present component (c), as well as methods for their preparation, are well known in the art; see, for exam- pie, WO 2013/1 17334. These particles are characterized by a Zeta potential larger than 0 mV, typically of more than +20 mV; for example, the Zeta potential is from the range +20 to +50 mV. Due to the preparation, these particles typically contain a certain amount of aluminium and/or zirconium. Thus, suspensions containing the present particles typically are of pH 7 or lower. Useful particles of these classes are commercially available, e.g. CAB-O-SPERSE ^® PG022 from Cabot Corp. (US).

The silica nanoparticles, which are present in the composition of the invention, typically have an average particle diameter as determined by dynamic light scattering from the range 10 to 500 nm, preferably 20 to 200 nm, more preferably from the range 30 to 150 nm, for example 70 to 120 nm. Silicon oxide particles like those mentioned above typically are aggregates, whose primary particles often show diameters from the range 1 to 50 nm, especially 5 to 20 nm (as determined by transmission electron microscopy). These particles of aforementioned size ranges generally are referred to as "nanoparticles"; aforementioned size ranges (also referred to as average particle diameters) refer to the diameter, where 50 mass-% (of the aggregates) of the sample have a larger diameter, and the other 50 mass-% have a smaller diameter. The diameter of the aggre- gates can also be measured by further techniques, e.g. using a centrifugal sedimentation particle size analyzer.

The weight ratio of the modified silica nanoparticles to the polar polymer typically is 1 :1 or higher, for example, from the range 1 : 1 to 35:1 . In order to obtain a material of low reflective index, the weight ratio of modified silica nanoparticles : the polar polymer should be higher than 1 :1 , for example from the range 1.5 : 1 to 35 : 1 . In preferred embodiments, the weight ratio of modified silica nanoparticles : the polar polymer is 2:1 or higher, especially 4:1 or higher; for example, from the range 4:1 to 35:1 , especially 5:1 to 30:1 , more especially 6:1 to 30:1 , and most especially 7:1 to 30:1 . In an embodiment of special technical importance, the weight ratio of modified silica nanoparticles : the polar polymer is from the range 8:1 to 25:1. The modified silica nanoparticles preferably are added to the composition of the invention in form of an aqueous dispersion, typically containing from about 5 to about 50 % by weight of the modified nanoparticles, the remainders being mainly water.

The composition of the invention is typically applied as an aqueous composition onto a suitable substrate, typically a transparent or optical substrate like glass or polymer film of further optical layer(s) and dried; drying often supports curing, and leads to removal of (some of the) volatiles such as water.

The invention thus primarily provides an aqueous composition useful inter alia as a coating composition. This composition typically consists, based on the total weight of the composition, of the following components:

from 0.1 to 10 % b.w. of the polar polymer, especially water soluble polymer containing hydroxy groups,

from 0.5 to 20 %, especially 0.5 to 16%, b.w. of the tetraalkoxysilane,

from 4 to 25 % b.w. of the modified silica nanoparticles having a positively charged surface, and optionally

0 to 5 % by weight of further components different from water,

and water ad 100%,

where the weight ratio of the modified silica nanoparticles to the polar polymer is from the range indicated above. Further components different from water typically are selected from polar organic solvents, surfactants, further polymers, further crosslinking agents, light stabilizers, antioxidants, rheologic or thixotropic agents, and/or levelling agents. Antioxidants may be, for example, selected from known phenolic antioxidants. Light stabilizers typically are selected from known radical scavengers such as sterically hindered amines, and/or UV absorbers, especially polar agents of these classes showing a certain miscibility with water such as widely used for water borne coatings. Examples of some useful components are described, inter alia, in Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, Vol. A18, pp. 429-471 , VCH, Weinheim 1991. The wet composition is typically applied onto a substrate by a coating technique such as spin coating, bar coating, printing, curtain coating. Preferably, 100% b.w. of non-volatile components of present composition (also referred to as "solids" hereinbelow) consist of:

3 to 60 % b.w. (especially 3 to 30%) of the polar polymer, especially water soluble polymer containing hydroxy groups,

1 to 30 % b.w. of the tetraalkoxysilane (especially 3 to 30%),

39 to 95 % b.w., especially 80 to 95%, of the modified silica nanoparticles having a positively charged surface, and optionally

0 to 10 % by weight (especially 0 to 6 %) of further components such as noted above. The composition may be dried after application, e.g. under air or a protective gas such as nitrogen, under reduced pressure, and/or application of heat (e.g. 40 to 100°, especially 50 to 80°C). Drying also effects crosslinking (i.e. curing) of the present composition. Surfactants typically are cationic or especially non-ionic. In an embodiment of specific technical interest, the composition of the invention contains little or none of the surfactant; for example, the amount of surfactant in such composition of the invention being from the range 0 - 1 % b.w. of solids, especially less than 0.2 % such as from 0.001 % up to 0.2 % b.w. of solids.

The amount of light stabilizers and antioxidants each typically is from the range 0 to 5 % b.w. of the polar polymer.

The invention thus further provides a cured composition obtainable by drying of a wet composition described above.

Curing is generally effected by drying, e.g. under air, dry gas stream (e.g. air or nitrogen), reduced pressure, elevated temperatures (e.g. as noted below) or a combination of such measures. Curing times and temperatures are not critical, curing time generally may range from a few seconds up to minutes or hours, while higher temperatures (e.g. 60-120°C) may be used for shorter curing times (typically below 10 minutes), and lower temperatures (less than 60°C) are used for curing times well above 5 minutes, e.g. 10 minutes or more).

All refractive indices (also mentioned as "index") are as determined for a radiation of 513.7 nm, if not indicated otherwise. In typical embodiments, the term "low refractive index" denotes a refractive index from the range 1 .01 to less than 1.4, e.g. 1.05 to 1.3, especially 1.05 to less than 1 .2.

The following examples illustrate the invention. Unless indicated otherwise, reactions take place at standard conditions, i.e. atmospheric pressure and room temperature

(r.t), which depicts a temperature from the range 22-25°C; "over night" denotes a period of 12 to 15 hours; percentages are given by weight, if not indicated otherwise.

Abbreviations:

aq. aqueous

b.w. by weight

PVA polyvinyl alcohol (in the examples used: Mowiol ^® 90-88, Kuraray)

TEOS tetraethoxysilane (Si(OEt) ₄ (99% grade from Wacker used in the examples) polymethylmethacrylate (used in the examples: EVONIK, Folie farblos 99524 GT, thickness 0.5 mm)

polyethyleneterephthalate (used in the examples: (DuPont Teijin Films, Melinex ST504)

Example 1 : Preparation of a coating composition

Positively charged silica particles are prepared in accordance with example 1 of WO 2013/1 17334 using aluminium chlorohydrate, boric acid, aq. formic acid, n- butylaminopropyltrimethoxysilane, fumed silica and aq. ammonium hydrogen carbonate in amounts and qualities as described. The dispersion containing 23 % b.w. of particles is sonicated for 15 min while stirring the solution from time to time, and subsequently filtered through a 1 μηη glass fiber syringe filter for use in the below composition.

The following components are combined in a closable glass bottle:

Surfactant: ABEX EP1 10 (anionic, Solvay)

The mixture is heated to 60°C under vigorous stirring on a hot-plate (100°C) for 1 h, and then allowed to cool to room temperature under stirring. The majority of air bubbles settles over night. Example 2: Preparation of a coating composition

Positively charged silica particles are prepared in accordance with example 2 of WO 2013/1 17334 using aluminium chlorohydrate, n-butylaminopropyltrimethoxysilane, fumed silica and aq. ammonium hydrogen carbonate in amounts and qualities as de- scribed. The dispersion containing 25 % b.w. of particles is sonicated for 15 min while stirring the solution from time to time, and subsequently filtered through a 1 μηη glass fiber syringe filter for use in the below composition. The following components are combined in a closable glass bottle:

^* Surfactant: ABEX EP1 10 (anionic, Solvay) The mixture is heated to 60°C under vigorous stirring on a hot-plate (100°C) for 1 h, and then allowed to cool to room temperature under stirring. The majority of air bubbles settles over night.

Example 3: Preparation of a coating composition for bar coating

Positively charged silica particles are prepared in accordance with example 2 of WO 2013/1 17334 using aluminium chlorohydrate, n-butylaminopropyltrimethoxysilane, fumed silica and aq. ammonium hydrogen carbonate in amounts and qualities as described. The dispersion containing 25 % b.w. of particles is heated to approximately 40°C and a solution of 6.6% b.w. polyvinyl alcohol (Kuraray, Mowiol 18-88), deionized water and, where applicable, a 5.3% b.w. solution of Olin ^® 10G (non-ionic surfactant, Fitzgerald) in water are added. The mixture is stirred for several minutes (5-60 min) and TEOS (99%, Wacker) is added, where applicable. Before coating, the solutions were allowed to cool to r.t.

Compositions of the coating solutions are listed in the following tables.

Table 3a: Compositions of the invention (TEOS, no surfactant)

Composition: 3.1 3.2 3.3 3.4 ratio S1O2-NP : PVA 20 : 1 12 : 1 4 : 1 1 .2 : 1 silica dispersion 43.0 g 42.2 g 31 .65 g 18.5 g

6.6% b.w. aq. PVA 8.1 g 13.2 g 29.7 g 58 g

TEOS 0.48 g 0.48 g 0.48 g 0.48 g

H ₂ 0 48.4 g 44.1 g 38.2 g 23.0 g Table 3b: Control compositions (no crosslinker, no surfactant)

Composition: Cntl.1 Cntl.2 Cntl.3 Cntl.4 ratio Si02/PVA 20 : 1 12 : 1 4 : 1 1 .2 : 1 silica dispersion 43.0 g 42.2 g 31.65 g 18.5 g

7% aq. PVA 8.1 g 13.2 g 29.7 g 58 g

H ₂ 0 48.9 g 44.6 g 38.6 g 23.5 g

Table 3c: Comparative compositions (boric acid as crosslinker)

Composition: C1 C2 C3 C4 ratio Si02/PVA 20 : 1 12 : 1 4 : 1 1.2 : 1 silica dispersion 43.0 g 42.2 g 31 .65 g 18.5 g

7% aq. PVA 8.1 g 13.2 g 29.7 g 58 g

5.3 wt% aq. surfactant 1.44 g 1.44 g 1.44 g 1 .44 g

Boric acid 0.29 g 0.29 g 0.29 g 0.29 g

H ₂ 0 47.2 g 42.9 g 36.9 g 21 .8 g

Example 4: Bar-coating of the coating solutions

The coating solutions of example 3 are applied via bar-coater using a meyer-bar (10 or 25 μηη wet film thickness) onto glass or PET substrates. Wet films of thickness 10 micrometer or 25 micrometer are dried at air for 1 -10 min and then heated to 80°C for 5 min. The resulting films are characterized by their transmission (BYK, HazeGard ^® plus), haze (ASTM D1003 - 13; BYK, HazeGard ^® plus), dry film thickness (micrometer; Metricon ^® prism coupler Model 2010/M) and refractive index (Metricon ^® prism coupler Model 2010/M; whereever mentioned in this example, refractive indices (mentioned below as "index") are as determined for a radiation of 513.7 nm). These characteristics are summarized in the following Tables: Table 4.1 : Characterization of dry films on glass (25 micrometer wet thickness; asterisk denotes composition of the invention)

Composition Haze % Transmission

3.1 ^* 0.59 93.4

Cntl.1 0.46 93.6

C1 8.67 92.0

3.2 ^* 0.32 93.4

Cntl.2 0.40 93.7

C2 1.92 92.3

3.3 ^* 0.90 93.5

Cntl.3 0.77 93.6

C3 0.48 93.7

3.4 ^* 1 .09 93.6

Cntl.4 1.71 93.1

C4 1.27 94.2

Table 4.2: Characterization of drv films on PET (10 micrometer wet thickness, drv thickness given in micrometer; asterisk denotes composition of the invention)

Composition Haze % Transmission dry thickness refractive index

3.1 ^* 0.55 92.0 0.87 1 .152

Cntl.1 0.48 92.0 0.82 1 .155

C1 0.71 92.4 0.77 1.147

3.2 ^* 0.58 92.3

Cntl.2 0.60 91.8 1 .162

C2 0.93 92.4 0.91 1 .154

3.3 ^* 0.60 92.7

Cntl.3 0.53 92.2

C3 0.57 92.7

3.4 ^* 1 .09 93.2

Cntl.4 1.42 92.4

C4 1.38 93.3 Table 4.3: Characterization of dry films on PET (25 micrometer wet thickness, dry thickness given in micrometer; asterisk denotes composition of the invention)

Composition Haze % Transmission dry thickness refractive index

3.1 ^* 0.84 92.1 2.42 1 .150

Cntl.1 0.81 91.8 2.59 1 .153

C1 4.40 91.8 1.96 1.150

3.2 ^* 0.69 92.2 2.06 1 .156

Cntl.2 0.63 91.9 1.97 1 .158

C2 1.85 91.8 1.71 1 .155

3.3 ^* 1 .14 92.4 2.02 1 .194

Cntl.3 0.98 92.3 1.51 1 .201

C3 0.82 92.4 1.84 1 .187

3.4 ^* 1 .42 93.0 0.87 1 .478

Cntl.4 2.04 92.3 0.68 1 .497

C4 1.31 92.3 0.59 1 .477

Dry LRI layers of the present invention show good light transmission and low haze.

Example 5: Spin-Coating of LRI-Laver

A composition of example 1 or 2 or 3 is filtered via a syringe filter (glas-fiber, 1 μηη) before application. About 3 ml of each of the compositions are spin-coated to cover the whole surface of a 10 cm x 10 cm sheet of PMMA or glass under the following spin- coating conditions:

Speed 700 - 1000 rpm

acceleration typically 100-500 rpm/s

time 1 :00 min

Subsequently, the wet coating is dried using cold air or nitrogen. During drying, the film first gets turbid before getting transparent again. Subsequently, the air-dried film is placed on a hot-plate for the time period indicated below to completely dry and crosslink the film: On PMMA substrate, 10 minutes, plate temperature 55°C; on glass substrate, 5 minutes, plate temperature 120°C. On both substrates, a dry film thickness of 2 μηη is obtained. Example 6: Applying LRI layer by print roller

A composition of example 3 (3.2 or 3.3) is continously applied to the surface of a print roller and transferred to a PET tape of width 27 cm. The wet layer is air dried and crosslinked in an oven directly after printing at a oven temperature of 60°C.

Example 7: Printing a low refractive index ink on security features

Mixing 60.97g positively charged silica particles (23% dispersion in water, H5-042LT from Wifag Polytype) with 2.77% polyvinyl alcohol (Mowiol 90-88, Kuraray) and 32.96g water, 3.3 g tetraethoxysilane (98% from Sigma Aldrich). The mixture is heated to 60°C for one hour and cooled to room temperature over night.The resulting ink viscosity of 330 seconds (Din 5321 1 of 06/1987; cup 4mm) is adjusted to 120 seconds by adding 10% of water.

Printing conditions

Low refractive index ink is printed by gravure on UV casted security holograms (substrate Melinex 506 Dupont Teijin Films) at 20 m/min, and heated at 90°C.

Using a gravure cylinder of 120 l/cm, good transparency and hologram visibility is ob- tained.

Overcoating

The low refractive printed ink is overcoated with a nitrocellulose clear varnish (10% wt DHM10/25, Nobel Enterprise, in n-propylacetate) or with a polyvinyl alcohol based clear varnish (7 wt% in water, Poval 95-88 or Poval 235, Kuraray).

The same process is used with Fresnel lens or tower structures on transparent film. The same process is used with Fresnel lens or tower structures on paper and board.

Example 8: Preparation of coating compositions for bar coating

Positively charged silica particles are prepared in accordance with example 2 of WO 2013/1 17334 using aluminium chlorohydrate, n-butylaminopropyltrimethoxysilane, fumed silica and aq. ammonium hydrogen carbonate in amounts and qualities as described. This results in a 25% b.w. dispersion. The solution containing negatively charged silica particles is used as received from Cabot (Cab-O-Sperse 4012K, 13.1 % b.w.). The dispersion of particles is heated to approximately 40°C and a solution of 6.9% b.w. polyvinyl alcohol (Kuraray, Mowiol 18-88), deionized water and a 5.3% b.w. solution of Abex EP1 10 (anionic surfactant, Solvay) in water are added. The mixture is stirred for several minutes (5-60 min) and TEOS (99%, Wacker) is added, where applicable. Before coating, the solutions were allowed to cool to r.t.

Compositions of the coating solutions are listed in the following tables. Table 8: Comparative compositions (8.1 containing cationic, 8.2 anionic particles)

Composition: 8.1 8.2 ratio Si0 ₂ -NP : PVA 17 : 1 12 : 1 silica dispersion 53.0 g 68.9 g

6.9% b.w. aq. PVA 1 1 .0 g 10.9 g

5.3 wt% aq. surfactant 1.07 g 1.07 g

TEOS 0.48 g 0.48 g

H ₂ 0 34,4 g 18.6 g

Example 9: Bar-coating of the coating solutions

The coating solutions of example 8 are applied via bar-coater using a meyer-bar (40 μηη wet film thickness) onto glass or PET substrates. Wet films of thickness 40 micrometer are dried at air for 1 -10 min and then heated to 80°C for 5 min. The resulting films are characterized by their transmission (BYK, HazeGard ^® plus), haze (ASTM D1003 - 13; BYK, HazeGard ^® plus), dry film thickness (micrometer; Metricon ^® prism coupler Model 2010/M) and refractive index (Metricon ^® prism coupler Model 2010/M; whereever mentioned in this example, refractive indices (mentioned below as "index") are as determined for a radiation of 513.7 nm). These characteristics are summarized in the following Tables:

Table 9.1 Characterization of dry films on glass (40 micrometer wet thickness; asterisk denotes composition of the invention)

Composition Haze % Transmission

8.1 ^* 2.7 93.7

8.2 50.2 91.4 Table 9.2 Characterization of dry films on PET (40 micrometer wet thickness; asterisk denotes composition of the invention)

Composition Haze % Transmission dry thickness refractive index

~ ^* 2~5 9Z9 1^38 1.147

8.2 28.0 92.5 1.17 1.139

Exchanging present cationic particles with anionic particles results in an unacceptable increase of haze.