The present invention relates to a removable protective coating deposited over at least a portion of the surface of a vitreous substrate.

The protection of vitreous surface such as glass surface against damages during transport is a very important challenge for glass manufacturers. This includes ordinary glass substrates without any functional coating as well as glass substrates bearing a functional coating such as a low emissivity coating or a solar control coating, for example silver based coatings. This includes also mirrors or spandrel glass. During the transport of glass sheets piles the friction between the surfaces generates deep scratches that causes the rejection of these damaged pieces. To avoid that, interleaving powder such as Lucite ^® (acrylic) beads or polyethylene particles is dispersed on the glass surface to reduce the friction between the two surfaces. The main problem with these organic powders is that they are not fixed on the surfaces and that the vibrations during the transport results in the sedimentation of the particles to the bottom of the pile thereby cancelling the protection.

Another way to avoid mechanical damage during transport or handling of the glass surfaces is to place a "plastic" film (polymer like polyethylene or polypropylene) on top of it. This is a very efficient way to solve the transport scratches and all other mechanical damages. However this method is only applicable on cold surfaces and it generates a lot of wastes at the customer site. Furthermore most of the low cost polymers used for this application are not biodegradable so the environmental cost of this option is high and make it unattractive. Moreover, such polymer films are not available for substrate of great dimensions such as glass sheets having the width of float ribbon as it exits the float production apparatus.

Film forming polymers, such as wax or coating based on copolyamide as disclosed by French patent application published under the number FR 2.295.100 Al, which can be peeled off, can be used for temporary protection of the glass. But they present the disadvantage to need large amount of organic solvents like alkanes or alcohol to be applied and removed. The environmental and safety costs of these solutions are so high that they can hardly be envisaged on an industrial point of view. Further, considerable time is required to peel the coating completely off the substrate surface.

Another way to temporary protect the surface of a glass substrate is the use of a polymer film which is adapted to be removed by industrial washing machines.

Recently a photocurable film forming polymer based on a mixture of polyfunctional (meth)acrylate, monofunctional (meth)acrylate, a resin having a cyclopentadien skeleton and an initiator has been described by the International patent application WO 2007080936 Al. The main problem with this kind of protection is that the film is not soluble in water and that the protection stays "in a film form" in water. This means that very rapidly the film will contaminate the washing machine and will accumulate with the consequence that the glass will no longer be cleaned properly because the washing machine is full off polymer or the water jets are blocked with film residue.

US patent N°5.026.597 and WO 01/022496 A2 disclosed a temporary protective coating formed from a water soluble film forming polymer. The water soluble polymer described in those patents is either polyvinylpyrrolidone or a polyvinyl alcohol (PVOH). The film can be filled with organic spacer particles such as polyethylene or acrylic beads that are water non soluble and non biodegradable compounds. Some filler are also hydrophobic reducing the wetting of the substrate surface and so making the product not washable or less easily washable. In WO

01/022496 A2, the polymer contains dyes to identify the product and a variety of polymeric molecules and additives. Colorants are often environmental unfriendly materials, furthermore use dyes or pigments in the washing machine make the film useless industrially because they will pollute the water of the washing machine since on industrial equipment most of the water is recycled. So the proposed film will soot up the washing machine. In WO 01/022496 A2, the water soluble film is also described to give a chemical protection to the glass bearing a functional coating based on silver layer, but additives like mineral oils, lubricants, biocides and some resins that the film may contain can react with silver based coating and degrade the functional coating in hot and humid storage conditions.

There is a need to mechanically and chemically protect vitreous surfaces having light-transmitting characteristics, especially but not exclusively glass surface bearing silver based functional coating, by a temporary coating that is water soluble, and can easily be removed in an industrial washing machine.

The present invention relates to a vitreous substrate bearing a removable protective coating deposited over at least a portion of its surface, wherein the removable protective coating is a polymeric film based on polyvinyl alcohol which contains natural inorganic filler having hardness in the Mohs hardness scale lower than that of said surface of the vitreous substrate.

The invention can easily provide a good protection for surfaces of vitreous substrates by an easily washable film which is respectful to the environment.

The need is to have a better protection for the surface of the vitreous substrate. However, this protection must be temporary and "removable" that is to say that the protective coating, which may comprises one or more films, must be removed before final use of this vitreous substrate. These requirements are somewhat in conflict because the better is the protection the more difficult may be the removal of said protection. The level of protection may be evaluated with a normalised test called the brush (standard number is ASTM D2486) as detailed here below so as to check if no scratches or damage are made to the protected surface. The easiness of removal of the protective coating is evaluated with a wash ability test of samples in a dish washer as disclosed here below.

The contact between the surfaces of two vitreous substrates, for example glass substrates, facing each other in a pile during transportation is impeded by the use of the polymeric film based on polyvinyl alcohol (also known as PVOH or PVA) that covers the surface. We have found that the addition of natural inorganic filler into the polymeric matrix, according to the invention, affords a number of advantages. Damages, such as scratches, at the facing surfaces are avoided or at least reduced, probably due to the increase of the friction coefficient between the two surfaces so blocking or reducing the displacement between them. As the hardness of the natural inorganic filler is lower than that of the surface of the vitreous substrate, micro-cracks due to the contact between the surface and the filler are avoided. As a result, we have found that the invention can afford a good protection for the vitreous substrate. Also, each substrate can be removed more easily from a pile.

It is a bit surprising because one could have thought that the rough patches created by the inorganic filler fixed by the PVOH film would have some harmful consequences to the surface of the vitreous substrate or to the functional coating deposited on the vitreous substrate.

Since the polymeric film forming material is polyvinyl alcohol which is a water soluble biodegradable compound and that the fillers are natural minerals, the invention can provide a safe, environmental friendly and industrially compatible solution for the protection of vitreous substrate.

By the expression "a polymeric film based on polyvinyl alcohol", we mean that the matrix is principally formed by polyvinyl alcohol. So, polyvinyl alcohol is the main component which remains on the surface after drying, and preferably, except for additives and fillers, it is the only component of the removable protective coating. Preferably, at least 70%, and advantageously at least 75% or 80% weight of the protective coating is PVOH.

By the expression "vitreous substrate" in the present specification, we mean a substrate of vitreous material like ordinary soda-lime silicate glass or borosilicate or a substrate of vitreous ceramic. It can be a clear or extra-clear glass or a tinted or otherwise coloured one. It can be tempered, heat strengthened glass or not, or it can be curved glass. This expression also means a vitreous substrate, such a glass sheet, bearing a functional coating like a low emissivity coating based on F and/or Sb doped SnO ₂ layer or a low emissivity coating and/or solar control coating comprising at least one Ag based layer, or other solar control layer like coatings based on nitride layer such as TiN or oxide layers, or other functional layers such as reflective or anti-reflective layers or conductive layers, for example electrodes for photovoltaic cells.

By the expression "natural inorganic filler", we mean an inorganic material which can be found in the nature and can serve as filler for the polymeric film.

The Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material. It was created in 1812 by the German mineralogist Friedrich Mohs and is one of several definitions of hardness in materials science. In the Mohs hardness scale the talc is the softest mineral with a hardness of 1 and diamond is the hardest with a value of 10, glass has a hardness of 5.5; calcium carbonate has a hardness of 2.5 and gypsum has a hardness of 2.

The polymeric film forming material is a water soluble polymer composed of an aqueous solution of polyvinyl alcohol (C ₂ H ₄ O) _x (PVOH) or mixture of PVOH with variable degree of hydrolysis and variable molecular weight and natural inorganic particles. The film forming polymer containing PVOH and mineral particles can be applied on the glass surface by classical techniques used for painting or surface treatment like dipping, inkjet, spray, roller, etc.

When the vitreous substrate is an ordinary glass sheet without any functional coating, natural inorganic filler having hardness in the Mohs hardness scale of about 5 may be appropriate. Preferably, the hardness in the Mohs hardness scale of the natural inorganic filler is lower than 4.5, and advantageously lower than or equal to 3.5. This is more appropriate for vitreous substrate bearing a hard coating such as coating deposited by chemical vapour depositing (CVD) process directly on hot glass ribbon in the float forming process, inside or outside the float chamber, such coatings having in general a hardness slightly higher than or close to that of the glass.

Preferably, the hardness in the Mohs hardness scale of the natural inorganic filler is lower than or equal to 3, and advantageously lower than or equal to 2.5. This is more appropriate for vitreous substrate bearing a coating produced by sputtering under reduced pressure, in particular in a magnetron, such coatings having a hardness about 3.5 to 4 depending of their composition. But the use of particles of calcium carbonate as natural inorganic filler has also been found especially suitable for vitreous substrate, such as glass sheet, without any functional coating as well as glass substrate bearing a functional coating deposited by spray pyrolysis or CVD apparatus on hot glass ribbon.

Preferably, the hardness in the Mohs hardness scale of the natural inorganic filler is lower than or equal to 2. This is the more convenient for the protection of the surface of a vitreous substrate bearing a soft coating and this was found the more convenient for glass substrate bearing a sputtered silver based stack.

Preferably, the natural inorganic filler is hydrophilic. This was found greatly advantageous because the presence of said filler improves the removal of the protective coating by water in the washing machine. Natural inorganic filler may comprise micas like muscovite, calcium carbonate, gypsum, or similar materials. Among these fillers, calcium carbonate is particularly suitable. Preferably, the natural inorganic filler is based on talc. Talc has been found as especially appropriate filler for the purpose of the invention. Talc is very soft so that damages to the surface of the protected surface are greatly avoided. Talc also improves the water wetting properties of the film, that is to say that the film is then more hydrophilic, generating better performances to remove the protection by washing.

Said natural inorganic filler suitably is in the form of particles. The particles must have a size greater than 200 nm to have a significant effect on the roughness of the protective coating. The particles may have a size from 1 to 50 μm.

Preferably, said natural inorganic filler is in the form of particles having a size between 1 and 25 μm, and advantageously 5 to 20 μm, determined as an average of size distribution curve. This is found to be convenient as filler in a protective coating having a thickness which forms a good protection and which is easily removed from the substrate. A particularly preferred size of particles is around 10-15 μm.

The thickness of the protective coating after drying suitably is in the range from greater than 0 to 20 μm. This is the thickness of the dried solution but without taking account of the inorganic filler. Preferably, the thickness of the removable protective coating after drying is in the range from 0.5 to 10 μm, advantageously in the range from 1 to 5 μm. It is found to be a good compromise between a good protection, an easy removal in the washing machine and production costs.

As already said, the hydrolysis degree of the polyvinyl alcohol may vary within a great range. Preferably, the hydrolysis degree of the polyvinyl alcohol is in the range from 78 to 99%. This range of hydrolysis degree was found to provide a good compromise between the protection of the surface and the ability to be machine-washable . The average molecular weight of the polyvinyl alcohol can vary from

13,000 to 130,000. Preferably, the average molecular weight of the polyvinyl alcohol is in the range from 15,000 to 80,000. Below 15,000, the protective coating is too easy to remove so that the protection is not stable. Advantageously, the average molecular weight of the polyvinyl alcohol is around or above 20,000.

The used PVOH may result from a mixture of at least two different PVOH having different molecular weights and different hydrolysis degrees in order to obtain the required properties.

The natural inorganic filler weight percentage in the protective coating on the substrate surface can vary from greater than 0 to 40%, preferably from 5 to 35% or 5 to 30%, depending on the application, the kind of protection needed, the average molecular weight of the polyvinyl alcohol and the dilution of the PVOH in the film-forming solution used to form the coating. Preferably, the natural inorganic filler weight percentage, and in particular the talc weight percentage when used, in the protective coating is in the range from 10% to 20%. This was found to be suitable to obtain a good protection combined to an easy removal of the protective coating. The ability of the protective coating to be better washable is improved when the natural inorganic filler weight percentage, and in particular the talc weight percentage when used, is equal to or less than 15%, preferably in the range from 10 to 15%.

The aqueous solution is prepared to reach a final solid concentration that varies from about 20% to 1% wt. The limit depends on the viscosity of the aqueous solution. An aqueous solution having a viscosity greater than 10 centipoises cannot easily be applied by industrial techniques uniformly on the substrate surface. The solution must be permanently shacked to avoid sedimentation of the natural inorganic filler.

The aqueous solution may also include other additives, such as surfactants, rheology modifiers, stabilizers, hydrophilic agents, UV filters or the like, to improve application onto the surface, to improve wet ability, or to give another specific property such as the UV resistance.

The vitreous substrate may be constituted by borosilicate glass, vitro- ceramic or the like. Preferably, the vitreous substrate is a glass substrate based on silica-soda-lime glass.

As already discussed here above, in one embodiment, the protective coating may be applied directly onto the vitreous substrate to protect its own surface. In another embodiment, a functional coating may be applied onto the vitreous substrate and the protective coating is applied onto this functional coating. The functional coating may be a single layer or a multilayer stack. It may be a low emissivity coating and/or a solar control coating or it may be a reflective layer such as a mirror layer or an anti-reflective coating. This functional coating may be deposited online, for example onto a hot glass ribbon at the float process plant, such as a coating based on F or Sb doped SnO ₂ with or without a sub-layer such as SiO _x . Preferably, the surface to be protected is the external surface of a multilayer stack deposited by sputtering on said vitreous substrate, and said multilayer stack preferably comprises at least one silver based infrared reflective layer. The invention is particularly well adapted to protect such functional coatings. For example, in a preferred embodiment, the vitreous substrate according to the invention may be a glass substrate bearing a heat treatable functional coating stack comprising one, two or three silver layers, each of which being enclosed between dielectric antireflective layers such as metal nitride or metal oxide such as zinc oxide and/or zinc stannate oxide or the like, onto which a removable protective coating has been deposited. This protective coating may be applied by classical techniques used for painting or surface treatment, such like dipping, inkjet, spray, roller, and the like. This vitreous substrate is stored and then transported to a site where it will be heat treated, for example tempered and/or bent. Before heat treatment, the vitreous substrate is washed in a washing machine where the protective coating is removed. The removable protective coating may be applied to form a non- continuous or "islanded" protective coating. Preferably, the removable protective coating is applied as a substantially uniform layer onto the whole surface. This gives the better protection for the surface.

The invention will now be described in relation to preferred non- limiting embodiments.

Example 1.

A 2 m by 1 m 6 mm thick sheet of standard clear soda-lime float glass was placed in a magnetron-type sputtering device operated with the aid of a magnetic field at reduced pressure (about 0.3 Pa). A functional coating in the form of a multilayer sunshield stack was deposited on this glass sheet comprising in sequence from the glass surface: zinc-tin mixed oxide, silver, titanium barrier, zinc-tin mixed oxide, silver, titanium barrier, zinc-tin mixed oxide and titanium nitride.

After that, according to the invention, a removal protective coating was applied to the coated glass sheet to temporary protect the surface of the vitreous substrate bearing the multilayer sunshield during storage and transportation.

To prepare the film forming solution a PVOH having an average molecular weight of 80,000 and a hydrolysis degree of 99 % was used. Talc was chosen as natural inorganic filler and 15 weight %, in relation to the PVOH, of talc particles, having a size of 10 μm determined as an average on a size distribution curve, was added to the solution. The PVOH and the talc were diluted in water to obtain a solution with, in weight, 90% of water and 10% of PVOH plus talc, from which 15% was talc and 85% was PVOH. The solution had a density close to 1 and its viscosity was about 1 centipoise at room temperature. The polymeric aqueous solution was continuously stirred and was applied on the coated glass by a calibrated metallic roll (ROD 6) leading to a film thickness after drying of 1.5 μm. The film was dried in a furnace at 100 ⁰ C during 2 to 5 minutes under atmospheric pressure. The dry residue represents 10 weight % of the starting solution of which 15 weight % is talc. It was noted that the water contact angle measured on the polymeric film with PVOH and talc was 20° instead of 38° for the polymeric film with PVOH without talc.

Batches of samples was washed in a dish washer (Miele professional

G7835CD) in demineralised water for 10 minutes at 40 ⁰ C and dried at 60 ⁰ C by air for 15 minutes, no detergent was used. The wash ability is considered OK if by visual inspection under a dark sky (very severe observation) the film is completely removed. The result for the samples was OK.

The mechanical resistance was evaluated with a normalised test called the brush (standard number is ASTM D2486) . The efficiency of the protective coating was considered as good (OK) if after 500 cycles of the brush on a double silver functional coating bearing the removable protective coating no scratches are observed after removal of the protective coating. The mechanical resistance of the samples was evaluated as OK according to said normalised test. In fact, the talc limits or reduces the number of movements between the surfaces so avoiding scratches. It means also that the thickness of PVOH needed will be reduced. The direct consequence of a thinner film is a better wash ability.

In a first alternative embodiment, the polymeric aqueous solution was applied on the coated glass by a calibrated metallic roll (ROD 12) leading to a film thickness after drying of 3 μm for the protective coating. The wash ability and the mechanical resistance also were evaluated as OK.

In a second alternative embodiment, the polymeric aqueous solution was applied on a glass sheet bearing a functional coating based on a single IR reflective layer enclosed within two dielectric layers instead of a functional coating based on double silver reflective layers. The wash ability and the mechanical resistance also were evaluated as OK. In a third and fourth alternative embodiments, the used PVOH had a hydrolysis degree of respectively 78% and 89%. Again, both wash ability and mechanical resistance were evaluated as OK.

Comparative example.

In a comparative example out of the scope of the invention, a film forming solution containing a PVOH having an average molecular weight of 80,000 and a hydrolysis degree of 99 % was applied on a glass sheet bearing a single silver based functional coating. In this case, no natural inorganic filler such as talc was used. The PVOH was diluted in the same way as in example 1 so that the solution had a density close to 1 and its viscosity was about 1 centipoise at room temperature.

The polymeric aqueous solution was applied on the coated glass by a calibrated metallic roll (ROD 12) leading to a film thickness after drying of 3 μm and was dried like in example 1.

The mechanical resistance was evaluated as good, however the wash ability evaluated as in example 1 was not good.

As first and second alternatives of this comparative example, colloidal silica was used as filler in the PVOH, instead of talc, with the same ratio as talc in example 1. The film forming solution was applied onto a coated glass sheet to form a removable protective coating of 1.5 μm and 3 μm respectively.

The wash ability of both protective coatings was found as not good.

Example 2.

On the coated glass sheet of example 1 , a removal protective coating was applied according to the present invention. In example 2, the film forming solution contained 3 weight % of PVOH, 0.5 % of talc particles (having a size of 10 μm determined as an average of a distribution curve), and 0.4 % of wetting agent, the remainder being water. The PVOH had an average molecular weight of 20,000 and a hydrolysis degree of 87-89 %.

The polymeric aqueous solution was continuously stirred and was applied on the coated glass by a calibrated metallic roll (ROD 6) leading to a film thickness after drying of 1.5 μm. The film was dried in a furnace at 100 ⁰ C during 2 to 5 minutes under atmospheric pressure, like as in example 1.

Both wash ability test and mechanical resistance, evaluated as in example 1 , were OK.

Example 3.

In a float line plant, producing standard clear soda-lime float glass sheets, a low-E functional coating comprising SnO ₂ :F was deposited on line by CVD apparatus on the hot ribbon of glass. The ribbon was 3 m 21 wide and 6 mm thick.

A removal protective coating was applied, according to the invention, to the ribbon on the line when the glass was at about 70 ⁰ C, before cutting into sheets.

To prepare the film forming solution a PVOH having an average molecular weight of 30,000 and a hydrolysis degree of 85 % was used. In this example, calcium carbonate was chosen as natural inorganic filler. The film forming solution contained 6.5 weight % of PVOH, 0.5 weight % of CaCO ₃ particles, having a size of 15 μm determined as an average on a size distribution curve and screened by a sieve of 30 μm to exclude particles above 30 μm, and 93% of water. The polymeric aqueous solution was continuously stirred and was applied on the coated glass by a bank of 9 sprayers adjusted so as to lead to a uniform film thickness after drying of about 2 μm. The film was dried during the cooling of the glass along the end of the float line until the ribbon was cut into sheets. Both wash ability and mechanical resistance, evaluated as in example 1, were OK.

Example 4

A silver-based low-E multilayer stack functional coating, comprising in sequence from the glass surface: zinc oxide, silver, titanium barrier, zinc oxide, zinc- tin mixed oxide, zinc oxide, silver, titanium barrier, zinc oxide, zinc-tin mixed oxide, tin oxide, was deposited by magnetron coater at industrial scale on 3.21 m wide by 6 m long soda lime float glass.

After that, according to the invention, a removal protective coating was applied to the coated glass sheets to protect the surface of the vitreous substrate bearing the multilayer low-E stack during transportation. The film forming solution was the same as the one described in example 2. The protective coating solution is spayed on the coated side of the glass using spray technology. The nozzle and the spraying parameters are selected in a way to obtain a homogeneous cloud of very fine droplets. The protective coating is applied on pre-heated glass substrates between 60 to 65 ⁰ C at atmospheric pressure. The preheating is optimized to obtain an evaporation of the water and thus a dried film within 1 minute. In these conditions of pressure, if the temperature is lower, the drying process takes more time and this can become a logistic problem on industrial lines. If the temperature is higher, the sprayed droplets will dry immediately on the glass and will not form a good uniform layer. In some cases partial destruction of the coating could occur if temperature is too high. The quantities applied on the low-E coated glass were between 10 and 100 ml/m ² of solution.

A transport test of 8 piles of 9 sheets (full truck) was organized through the Alps. The distance covered by the truck was about 3000 km. The glass sheets are loaded automatically on a standard A-frame rack (pile receiver). Cardboard spacers are placed between two piles on every pile of 9 sheets. The first glass of the pile (when unloading the line) is oriented so that the coating is directly in contact with the cardboard spacers. No uncoated clear glass coversheet was used for this transport so that the low-E coated glass itself, having the removable protective coating, was used as cover sheet.

At the end of the journey, the A-frame rack was unloaded and 4 piles were stored and controlled for 12 months. The second half of the rack was processed on an industrial double glazing line. This mean that the sheet of 3,21 x 6 m are unloaded on the cutting table, cut, edge deleted, washed and assembled in insulating glass unit. At the end of this process about 700m ² were assembled and inspected carefully. The result of this industrial test is that the removable protective coating allows normal handling of the large sheets at an industrial scale without adaptation of the said process: same sucker, same grinding stones, same cutting wheel (small increase of the pressure) and same washing conditions (10 m/min, water temperature 35°C) may be used. The low-E multilayer stack was carefully inspected after washing, looking for residues of the protective coating. But the washing quality was excellent and no residue was found. Furthermore some pieces were selected (after the washing machine) for a visual inspection under an artificial sky that is the most appropriate way to detect scratches. No scratch was seen in the low-E silver based multilayer stack showing that a good protection against scratches during transport and handling can be reached with such a removable protective coating. At the end of the IGU line, the yield was 100% (no glass rejected).

The protective influence of the coating during storage was evaluated during this test for a period of 12 months. To evaluate this protection, every 2 months the wash ability (as in example 1, using a dish washer), the scratch resistance (as in example 1, using the normalized test called the brush) and the quality of the low-E functional coating after washing was controlled on one sheet. The results shown that after 12 months the protective coating is still completely washable in an industrial line and that no residue were detected on the low-E coated glass. The scratch protection measured by a brush test on the protective coating is still efficient after 12 months and no scratch is visible after washing the tested sample. The visual inspection under artificial sky of the low-E multilayer stack protected by the removable coating shows no particular defect or corrosion that could be attributed to the removable protective coating. On the contrary the quality of that Ag based stack is better (less and smaller Ag corrosion spots) than a reference batch without removable protective coating that was stored in the same conditions.