WO2005021464A1

WO2005021464A1 - Improvements to coated articles

Info

Publication number: WO2005021464A1
Application number: PCT/GB2004/003673
Authority: WO
Inventors: Robert John Schaffer; Howard David Winbow
Original assignee: Johnson Matthey Public Limited Company
Priority date: 2003-08-27
Filing date: 2004-08-27
Publication date: 2005-03-10
Also published as: GB0319995D0

Abstract

A coated article, which is an article of tableware or a kitchen appliance, comprises a body with a glazed or enamelled surface which is provided with at least one primary coating in contact with the surface and optionally a secondary coating in contact with the primary coating. The primary coating comprises at least one inorganic oxide and the optional secondary coating comprises an organic material. The coatings are such that the resistance of the article to metal-marking damage is improved. Methods and apparatus for applying the coatings to improve metal-marking damage are also described.

Description

IMPROVEMENTS TO COATED ARTICLES This invention relates to glazed ceramic and enamelled articles with improved resistance to metal-marking damage, and to a process and apparatus for the production of such articles. In particular, the invention relates to glazed articles such as items of tableware and to metal articles provided with enamel coatings. Tableware, that is ceramic plates, cups and saucers and the like usually comprise a clay body which is provided with a glaze overlayer. The glaze provides a hard outer surface which lessens damage to the ware in use as well as rendering the ware waterproof. The glaze is also used to give an attractive appearance to the ware. Enamelled articles include kitchen appliances such as refrigerators, cookers and other 'white-goods' as well as sanitary wares such as sinks and baths. The enamel layer performs many of the same functions as a glaze overlayer on a ceramic article.

Tableware is subjected to many forms of potential damage in use. These include contact with other items of tableware, chemical and abrasive attack from cleaning products, damage caused by extremes of heat and cold and damage caused by contact with metals i.e. cutlery. This final type of damage is known as metal-marking. Metal- marking is exacerbated by chemical and mechanical attack to the glaze as these tend to roughen the surface of the glaze. Metal-marking results from a film of metal being left on the surface of an article. In the same way that a pencil marks a page during writing whilst not damaging the surface of the paper, so can a knife leave a deposit of metal on the surface of an article of tableware without damaging its surface.

In the domestic environment, glazes are generally, although not always, effective in preventing damage to tableware. This is because the number of use cycles is low. However, in a commercial environment, such as a hotel or canteen, individual tableware items may be subjected to very many repeat use cycles. This can cause damage to the glaze overlayer. Although damage to the glaze surface does not generally affect the physical integrity of the tableware item it increases its susceptibility to metal-marking and reduces the attractiveness of the item to the user. In-service lifetimes of commercial tableware may therefore be relatively short, with items being rejected as unsuitable for use whilst still remaining physically intact.

Additionally, kitchen appliances are often subject to metal-marking when metal dishes and utensils come into contact with the surface of the article in use. This can readily occur during the course of cooking when dishes and utensils are placed on the surface of a kitchen appliance, such as a cooker top.

It has long been common practice in the glass bottle manufacturing industry to apply a 'hot-end' treatment to the bottles in order to reduce their surface co-efficient of friction. Typically, a tin-containing precursor is applied to the bottles immediately following their manufacture whilst they are still hot. The precursor decomposes to produce a thin layer of tin oxide on the bottle surface. Often an additional 'cold-end' treatment is used to provide a lubricious polymer coating over the tin oxide. Such treatments are detailed in for example, GB 2 139 997 A and GB 2 147 288 A. The purpose of the treatments is to prevent damage to the bottles as they contact each other during further stages of manufacture, during filling or in transit. Damage is thus by way of glass-to-glass contact. WO 02/066401 Al, describes a similar procedure to provide the surface of a glazed tile with an increased co-efficient of friction. Only a 'hot-end' treatment is used to provide a metal oxide coating. The tiles may be used as floor tiles where an increase in the co-efficient of friction is desirable to prevent slippage for example, to protect users in areas such as swimming pools.

Surprisingly, the present applicants have found that the incidence of metal- marking damage in articles of glazed or enamelled tableware or kitchen appliances, can be significantly reduced by the application of a coating. Thus in accordance with a first aspect of the present invention, a coated article, which is an article of tableware or a kitchen appliance, comprises a body provided with a glazed or enamelled surface, wherein the surface is provided with at least one primary coating, and wherein the first primary coating is in contact with the surface, the primary coating comprising at least one inorganic oxide.

In accordance with an embodiment of the invention the article is provided with a secondary coating in contact with the primary coating, the secondary coating comprising an organic material.

In one embodiment, the secondary coating is applied during manufacture; however, in an alternative embodiment the secondary coating results from the build up of environmental organic materials, for example, from foodstuffs, dishwasher rinse-aids or detergents.

In one embodiment the article surface further comprises on-glaze decoration, such as a decal, banding or decorative precious metals. Once applied to a glazed or enamelled surface, decals comprise of inorganic oxides contained within a glassy matrix. The matrix allows the decal to adhere to the surface and thus fixes the decoration to the glazed or enamelled article of tableware or kitchen appliance. The use of a primary coating to cover the on-glaze decoration allows for metal-marking damage to be reduced on patterned articles, as well as more simply decorated articles of tableware or kitchen appliances. In the context of the present invention, where the coated article comprises on-glaze decoration, the on-glaze decoration comprises the surface onto which the primary coating is applied.

Metal-marking damage is not a problem encountered in the glass bottle manufacturing industry. Damage caused by glass-to-glass contact proceeds by a different wear mechanism to metal-marking damage and is primarily due to scratching and scuffing. Furthermore, the problems caused by glass-to-glass contact in the bottle manufacturing industry are relevant only to the manufacturers, not to the end consumer. An increase in the surface co-efficient of friction, an object of the invention described in WO 02/066401 Al, would exacerbate metal-marking damage in tableware. Optionally, the at least one inorganic oxide comprises a metal oxide such as tin oxide, titanium oxide, hafnium oxide, silicon oxide or zirconium oxide. Tin oxide is particularly favoured. Non-metallic oxides such as silicon oxide may also be suitable.

It is envisaged that the primary coating may consist of more than one layer. For example, several layers of the same or different inorganic oxides may be provided.

The organic material used in the optional secondary coating should be such that the surface co-efficient of friction of the coated article is low. Waxy materials such as polyolefins and their oxides are preferred. Some non-limiting examples include polyethylene and polypropylene and their oxides. Other suitable materials include stearic acid.

Although not limited thereto, the present invention is particularly useful when the coated article is a ceramic article provided with a glazed surface. Examples include, items of tableware such as plates and cups etc. As described hereinabove, these articles are particularly prone to metal-marking damage.

In accordance with another aspect of the present invention, a process for improving the resistance of a glazed ceramic or enamelled article to metal-marking damage comprises the steps of:

(i) heating the article to a first temperature; and, (ii) applying a primary coating composition to the heated article, wherein the primary coating composition comprises a precursor to at least one inorganic oxide.

Additionally the process may include a third step of: (iii) applying a secondary coating composition to the article, wherein the secondary coating composition comprises an organic material. These processes may include the repetition of step (ii) one or more times. This allows for the formation of multiple layers of an inorganic oxide. Different primary coating compositions may be used to produce layers of different oxides, for example a titanium oxide layer could be applied over a tin oxide layer.

These processes may include a further step of washing the article prior to application of the optional secondary coating composition.

Any material which can be thermally decomposed to produce an inorganic oxide may be used as a precursor to the at least one inorganic oxide. For ease of application, the precursor is conveniently dissolved or suspended in a solvent. Alternatively, the precursor may be in liquid or gaseous form. Usually, the primary coating composition comprises a metal salt in an organic solvent. In the case where the inorganic oxide is tin oxide, a particularly favoured substance is that known as Titasol, a product of Grosvenor Chemicals Ltd, UK. This is a mixture of 20%w/w tin(IV)chloride in 80%w/w n-butyl acetate. Alternative solvents and mixing ratios will be known to the skilled man. Tin oxide may also be formed by application of a tin-organometallic species such as mono-butyl tin trichloride or di-butyl tin dichloride, other tin organometallic species will be known to those skilled in the art. These liquid compositions are conveniently applied to the heated article in vapour form and can be thermally decomposed to provide the inorganic oxide coatings. Other metal oxide coating compositions can be used in place of those based on tin oxide, for example Vertec (VEXP0489), a mixture of 20%w/w tetra- iso-propyltitanate in 80%w/w iso-propanol. The above examples are not intended to be limiting in any way.

The first temperature must be sufficient to decompose the precursor to an inorganic oxide but must not be too high such that the glazed or enamelled surface of the article begins to soften. Suitably, the first elevated temperature is between 350 and 600 °C, more suitably, between 400 and 450 °C, for example, 420 °C. The optimum temperature will depend on the nature of both the precursor material and the surface. In general, it is preferred that the first temperature is as high as the surface can withstand without softening. This allows more rapid decomposition of the precursor. Other considerations, such as the amount of energy used to heat the article may also be important in the choice of the first temperature. The inorganic oxide primary coating is intimately and securely bonded to the glazed or enamelled surface. The organic solvent evaporates during heating and does not form part of the coating. In one embodiment, the primary coating as a whole is very thin, less than 1 μm in thickness, such that the precise chemical and physical characteristics of the coating are difficult to determine. The primary coating can comprise one or more separate primary coating layers. The or each coating can be less than 100 nm thick, possibly less than 50 nm thick, however it is estimated that the or each oxide coating can be as thin as approx. 10 nm in thickness. The inorganic oxide coating, which is not removed during normal use or during cleaning, acts as a key for a secondary coating. Although not wishing to be bound by any theory, it is presently thought that the electronic properties of the primary coating should be such that the secondary coating binds strongly to it. Materials with semi-conducting properties, tin oxide, titanium oxide, hafnium oxide, zirconium oxide, silicon oxide and the like are thought to provide these properties. The primary coating composition may further comprise at least one additional oxide precursor, such as a precursor to aluminium oxide, zirconium oxide, titanium oxide or silicon oxide. Such additional oxides are optionally used in minor amounts and may provide the coating with enhanced properties. For example, they may improve the chemical and/or physical durability of the coating or act as improved keys for a secondary coating. Although the coatings of the present invention are intended to be imperceptible to the eye, some oxides, such as transition metal oxides, may be used to alter the appearance of the article. In some cases a thicker primary coating may be desirable. This can be to provide increased protection against mechanical or chemical attack or to alter the visual appearance of the article. As described above, thicker coatings can be formed by repeated application of a primary coating composition, or by applying a greater amount of coating composition in a single application. Usually, the optional secondary coating composition comprises an aqueous dispersion of a polyolefin or polyolefin oxide. The composition may also contain additives such as emulsifiers. Particularly preferred is a substance known as Polarfin, a product of Neptune Chemicals Ltd, UK, which is an emulsion of polyethylene oxide in demineralised water and includes ammonium oleate and potassium oleate as emulsifiers. Polarfin has a solids content of ca. 23%. Conveniently, the emulsion may be applied further diluted with water, for example at a ratio of 1:100, although other mixing ratios may of course be used.

If an optional secondary coating composition is applied, the process usually comprises the step of cooling the article to a second temperature between application of the primary and secondary coating compositions. The majority of materials suitable for use as secondary coating compositions will be unstable and may decompose at the temperatures where the inorganic oxide precursors decompose. This is undesirable; so some cooling will normally be required. The article is optionally cooled to a temperature between room temperature and 350 °C, possibly to a temperature between 80 and 150 °C, for example, 120 °C. Although the three step process according to the invention is described hereinabove as a sequential process, it need not be. For example, application of the primary and secondary coating compositions may be performed at different times and in different locations. In this instance, the article will be cooled to ambient temperature after the application of the primary coating composition and perhaps stored. Subsequent application of the secondary coating composition may therefore require re-heating of the article. In this context, cooling the article to a second temperature includes the step of re- heating the article, provided that the second temperature is equal to or below that used as a first temperature. It is also conceivable, although not preferred, that the secondary coating composition be applied whilst the article is at a higher temperature than that used during application of the primary coating composition. Unlike the primary coating, any secondary coating is expected to be gradually removed during normal usage. However, it is believed that a secondary coating is able to regenerate by picking up organic molecules from the environment. Regeneration of a secondary coating by re-applying the secondary coating composition is also possible.

The present invention is principally concerned with reducing metal-marking damage in tableware and thus usually, the article is an article of ceramic tableware such as a plate. However, other suitable articles include kitchen appliances such as 'white-goods'.

The coatings of the invention, in addition to providing protection against metal-marking, also provide treated articles with improved resistance to scratching and scuffing damage. Furthermore, resistance to chemical attack is also improved.

In a third aspect, the present invention provides an apparatus for improving the resistance of a glazed ceramic or enamelled article to metal-marking damage comprising means to transfer the article in turn through a heater, a primary coating applicator, optionally a cooler, and optionally a secondary coating applicator; wherein the primary coating applicator applies a primary coating composition comprising at least one inorganic oxide precursor to the article; wherein the optional secondary coating applicator applies an optional secondary coating composition comprising an organic material to the article; and wherein the heater heats the article to a temperature between the decomposition temperature of the at least one inorganic oxide precursor and the softening point of the glaze or enamel.

The invention will now be described by way of example only with reference to the following drawings in which:

Figure 1 is a schematic diagram of a pilot primary coating system for use in a process according to the present invention; Figure 2 is a schematic diagram showing a detailed view of the coating hood used in the system of Fig. 1; Figure 3 is a schematic diagram of a production coating system for use in a process according to the present invention; Figure 4 is a schematic diagram of an alternative production coating system for use in a process according to the present invention;

Figure 5 is a bar chart comparing the variation in co-efficient of friction for plates provided with a tin oxide primary coating according to the present invention with uncoated plates;

Figure 6 is a bar chart comparing the visual appearance of plates provided with a tin oxide primary coating and Polarfin secondary coating, according to the present invention, with uncoated plates after a 3 month trial; and, Figure 7 is a bar chart comparing the variation in co-efficient of friction for plates provided with a titanium oxide primary coating according to the present invention with uncoated plates.

Coating Application Examples and Explanation of Coating Systems

A primary coating composition of 20% w/w tin(IV)chloride in 80% w/w n-butyl acetate was applied to the surface of pre-glazed plates using a modified belt tunnel kiln, Fig. 1. The kiln 1 allows plates 2 to be passed through three hot zones 3a, 3b,3c in a kiln of total length approximately 15m.The hot zones were set at 550 °C, 570 °C and 600 °C. The total cycle time for a plate to pass through the kiln and coater is about 15mins. The coating hood 4 is attached at the exit from the kiln. Feeding the plates into the kiln and collecting the coated plates at the end of the process was carried out by hand.

The coating hood 4 was manufactured from 316 stainless and was approximately 1200 mm long by 460 mm wide by 400 mm high. A needle-less spray nozzle 5 delivering a flat fan directly down onto the plate was used. The height of the nozzle can be adjusted to vary the width of coverage of the spray pattern. All pipe work is manufactured from PTFE and all seals are manufactured from EPDM. The spray nozzle 5 is a B1/4QMJ+SUQF130 supplied by Spraying Systems. The nozzle body is manufactured from PVDF / Kynar and the three internal Viton o- rings were replaced with EPDM equivalents supplied by FTL seals. The spray nozzle uses air assistance to achieve good atomisation. The compressed air supply 6 is also fed to the spray nozzle with PTFE tubing. The primary coating fluid 7 is delivered via suction feed from a 50 kg polymer-lined steel drum by an Almatec A10TTZ PTFE diaphragm pump (not shown) with PTFE / EPDM cylinder valves and PTFE pulsation damper. Flow rate is restricted with a PTFE tap positioned between the pump and the spray nozzle (not shown).

A 15 m³/hr scrubber system 8 was used consisting of a 600 mm diameter pack column, a 1000 mm diameter liquid reservoir tank with Pan World NH401PW-CV re- circulation pump rated at 2.0 kgf/cm², a Bl 160 fan unit, a 4 mm discharge stack and 110 mm gas inlet ducting. With the exception of the gas inlet ducting, all parts that come in contact with the exhaust gases are manufactured exclusively from polymer.

Control of the application process is achieved through adjustments to the spray nozzle height, the fluid pressure supplied by the pump, the fluid tap opening and the pressure of the compressed air supplied to the spray nozzle.

The spray nozzle height controls the width of the spray fan hitting the plate. The compressed air pressure controls primarily the degree of atomisation (droplet size) of the primary coating fluid. It will also influence the width of the fan coming from the spray nozzle and the fluid flow rate through the nozzle (as back pressure is created in the fluid supply). Excessive amounts of atomising air will also have a significant cooling effect. Control of the fluid flow rate through the spray nozzle used in this application would normally be governed, primarily, by the pressure in the fluid supply line. However, this was found to give excess fluid flow (too much coating thickness applied) and a tap was installed between the pump and the spray nozzle. Fluid flow rate through the spray nozzle was, therefore, controlled by a balance between the compressed air pressure (atomising air at the spray nozzle), the pressure behind the tap in the fluid line (controlled by the fluid pump) and the degree of opening in the fluid tap.

A secondary coating of Polarfin was applied using a standard glaze spray line (not shown). An electric pre-heater was used to raise the temperature of the plates. The secondary coating was applied using a single DeVillbiss GTI gun with a 1.4 mm fluid tip and needle with a 100 air cap. Fluid pressure to the spray gun was achieved using a DeVillbiss diaphragm pump. The fluid flow was restricted using a tap set between the spray gun and the diaphragm pump. Over spray was extracted to atmosphere. The secondary coating fluid is far less corrosive than the primary coating fluid so materials compatibility was not considered.

A schematic of a proposed production system is shown in Fig. 3. It is envisaged that a production coating system would apply both a primary and a secondary coating in a single pass. Plates to be coated enter the system 9 at a heater 10 and then pass to a primary coating booth 11 where the primary coating is applied. The plates then pass to a cooler 12 before entering a secondary coating booth 13 where a secondary coating is applied. Extraction units 14 and 15 are used to exhaust and scrub excess coating compositions.

Although the pilot plant system used a stainless steel coating hood, corrosion from the primary coating spray means this would not be suitable in a full production coater. It may be possible to protect the steel by painting, or alternatively the hood could be lined with, or manufactured exclusively from, PTFE. All extraction ducting should to be manufactured from PTFE. Due to the temperatures reached in the coating hood, the polymers used in the pipe work, nozzle and o-rings are at the limit of their operating temperature range. Some form of cooling to protect these components whilst still maintaining the temperatures of the plates during coating may be required

To avoid thermal shock in the plates, the plates must be heated gently (slowly) to the coating temperature. This can be achieved either by using a very long kiln, or by slowing the rate at which the plates pass through a short kiln to increase the residence time. However, the longer the kiln, the more expensive it is to produce. Therefore, a slower speed through the kiln is favoured. However, this limits the rate of production of coated pieces and also increases the residence time of the plates in the coating hood. In order to achieve the required coating thickness, the fluid flow rate through the spray nozzle must be very low. This causes several engineering problems and increases the cost of the apparatus.

In order to address these problems, a system 16 is envisaged that uses a dual speed belt, Fig. 4. Ware is passed through the kiln 1 on a slow belt 17. It then transfers to a faster belt 18 to pass through the coating hood for applying a primary coating 4 and the hood for applying a secondary coating 19. Plates may be stacked or placed side by side through the kiln to allow the throughput to match the rate of plates passing through the coater. Chemical Durability and Wear Testing To investigate the possible loss of coating during product lifetime, mechanical wear testing was carried out before and after a chemical durability test. The chemical durability test involved immersing plates in a 2 % solution of a standard commercial dishwasher detergent for 16 hours at 77 °C. This test aimed to replicate many cycles through a commercial dishwasher. Plates were tested by measuring the increase in the co-efficient of friction between a sliding metal indenter (to replicate a metal-marking situation) and the surface of the plates over a number of passes. A large increase indicates more damage to the surface of the plate. An increase in the amount of damage after chemical durability testing suggests attack of the surface of the plate, which may arise from loss of coating and/or glaze. Two applied loads were used, 400 g and 1000 g. Results are presented in Fig. 5 and Fig. 7, where the dark shaded bars represent plates without a coating and the unshaded / lightly shaded bars represent plates coated, with tin oxide or titanium oxide primary coatings and Polarfin secondary coatings, according to the present invention. It is clear from Fig. 5 and Fig. 7 that coated plates displayed significantly less increase in the coefficient of friction between the first and final passes. This result confirms that the coating provided improved mechanical wear resistance to a sliding metal contact. An increase in load produced more damage for both coated and uncoated plates as would be expected. A key observation is that chemical durability testing has no influence on the wear test performance of the coated plates. This indicates that the coating is not removed during the chemical durability test, thus preserving the mechanical benefits of the coating.

A further test was performed by visually monitoring the apparent damage on coated and uncoated plates during a 3 month trial in the canteen of the applicant. The results are shown in Fig. 6 where a larger value indicates more apparent damage. It is clear from Fig. 6 that the coated plates significantly outperformed the uncoated plates indicating that the coating was not being degraded in normal use.

Claims

1. A coated article, which is an article of tableware or a kitchen appliance, the article comprising a body provided with a glazed or enamelled surface, wherein the surface is provided with at least one primary coating, and wherein the first primary coating is in contact with the surface, the primary coating comprising at least one inorganic oxide.

2. An article according to claim 1, wherein the article is provided with a secondary coating in contact with the primary coating, the secondary coating comprising an organic material.

3. An article according to claim 1 or 2, wherein the article surface further comprises on-glaze decoration, such as a decal, banding or decorative precious metals.

4. An article according to claim 1, 2 or 3, wherein the at least one inorganic oxide comprises at least one metal oxide.

5. An article according to any of claims 1 to 4, wherein the at least one metal oxide comprises tin oxide, titanium oxide, hafnium oxide, silicon oxide or zirconium oxide.

6. An article according to any of claims 1 to 5, wherein the at least one inorganic oxide coating is less than 1 μm think, optionally less than 100 nm thick or possibly less than 50 nm thick.

7. An article according to any of claims 2 to 6, wherein the organic material comprises a polyolefin or polyolefin oxide such as polyethylene, polypropylene or their oxides.

8. An article according to any preceding claim, which is a ceramic article provided with a glazed surface.

9. A process for improving the resistance of a glazed ceramic or enamelled article to metal-marking damage, the process comprising the steps of:

(i) heating the article to a first temperature; and, (ii) applying a first coating composition to the heated article, wherein the first coating composition comprises a precursor to at least one inorganic oxide.

10. A process according to claim 9, wherein a third step is included: (iii) applying a secondary coating composition to the article, wherein the secondary coating composition comprises an organic material.

11. A process according to claim 9 or 10, wherein step (ii) is repeated one or more times applying the same or a different first coating composition.

12. A process according to claim 9, 10 or 11, wherein the at least one inorganic oxide comprises a metal oxide chosen from tin oxide, titanium oxide, hafnium oxide, silicon oxide and zirconium oxide.

13. A process according to any of claims 10, 11 or 12 further comprising the step of cooling the article to a second temperature between application of the first and secondary coating compositions.

14. A process according to any of claims 9 to 13, wherein the precursor to the first coating composition comprises a metal salt in an organic solvent or a liquid or gaseous organometallic material.

15. A process according to any of claims 10 to 14, wherein the secondary coating composition comprises an aqueous dispersion of a polyolefin or polyolefin oxide and optionally, an emulsifier.

16. A process according to any of claim 9 to 15, wherein the first temperature is between 350 and 600 °C.

17. A process according to any of claims 13 to 16, wherein the second temperature is between room temperature and 350 °C.

18. A process according to any of claims 10 to 17 further comprising the step of washing the article prior to application of the secondary coating composition.

19. A process according to any of claims 9 to 18, wherein the first coating composition further comprises at least one additional oxide precursor, such as a precursor to aluminium oxide, zirconium oxide or silicon oxide.

20. A process according to any of claim 9 to 19, wherein the article is an article of ceramic tableware such as a plate.

21. An apparatus for improving the resistance of a glazed ceramic or enamelled article to metal-marking damage, the apparatus comprising means to transfer the article in turn through a heater, a primary coating applicator, optionally a cooler, and optionally a secondary coating applicator; wherein the primary coating applicator applies a first coating composition comprising at least one inorganic oxide precursor to the article; wherein the optional secondary coating applicator applies an optional secondary coating composition comprising an organic material to the article; and wherein the heater heats the article to a temperature between the decomposition temperature of the at least one inorganic oxide precursor and the softening point of the glaze or enamel.