SE537924C2 - Low surface roughness based glazes - Google Patents

Abstract

14 ABSTRACT The present invention relates to treatment of ceraniic bodies. More specifically the inVention relates to glossy glazes providing a high gloss surface of the sanitary Ware 5 With low surface roughness, by providing a ceraniic glaze, Which, after f1ring coniprisesfrom 3.0 to 10.0 % (Weight) opacifier and from 0.5 to 5 % (Weight) SrO. To be published With: Fig. 2

Description

LOW ROUGHNESS GLAZES WITH OPACIFIERS Technical fieldThe present invention relates to treatment of ceramic bodies. More specificallythe invention relates to glossy glazes providing a high gloss surface of the sanitary Ware With loW surface roughness.

Background Ceramic Ware such as china, sanitary Ware, faience, tiles or similar are usuallycoated With glaze to create an aesthetical, hygienic, chemical and mechanical resistantsurface. A glazed ceramic surface such as sanitary Ware is more resistant to dirt, soils,urine and faeces, stains, food and drug rests, lime and/or silica deposits, bacterialgroWth, etc.

Today, most White sanitary Ware glossy glazes are based on zirconium silicate,zirconium dioxide, tin oxide or similar, Which makes the color White. Generally, theWhiteness of the glaze creates a psychological association to clean surfaces. Whenglazing a ceramic body, or biscuit, it is thus desirable to cover the dark color of thebiscuit. The amount of opacifier may vary depending on the darkness of the biscuit andon its opacifying strength. The normal amount of opacif1er in a glaze is betWeen l0 andl5 % (Weight) opacif1er. The most efficient opacif1er is fine grinded zirconium silicate.

HoWever, use of opacifiers increases the roughness of the glaze after the f1ringprocess, due to its stiff, coarse and non-meltable grains.

Dirt, lime, silica from flushing Water, bacteria, etc. accumulate around thesegrains Which give deposits over time. The rougher the glaze surface, the highertendency to build deposits, Which requires more cleaning and shorten the life length ofthe sanitary Ware.

A solution to this problem is to cover the rough glaze, comprising opacifiers,With a second layer of transparent glaze.

HoWever, this makes the manufacturing process complicated and may evenrequire tWo f1ring steps Which is expensive and not environmentally friendly.

Hence, there is a need for a glaze Which comprises opacif1ers to make it sufficiently opaque, but With increased surface smoothness.

Summary of the InventionAccordingly, the present invention preferably seeks to solve at least the above mentioned problems by providing a glaze according to the appended patent claims.

The general solution according to the invention is to provide a glaze With aspecific composition, Which is sufficiently opaque and yet has a smooth surface.

According to a first aspect, a ceramic glaze is provided, Which, after f1ringcomprises from 3.0 to 10.0 % (Weight) opacif1er and from 0.5 to 5 % (Weight) SrO.

This is advantageous, since it is sufficiently opaque to use as a sanitary Ware,and still has a high surface smoothness or gloss.

In an embodiment, the ceramic glaze, after f1ring, comprises from 0.5 to 2.5 %(Weight) SrO such as 1.2 % (Weight) SrO.

This is advantageous, since it provides a sufficiently high surface smoothnessor gloss, With low amounts of Sr, Which is economical.

In an embodiment, the ceramic glaze, after f1ring, consisting essentially of from3.0 to 10.0 % (Weight) opacifier; from 8.0 to 12.0 % (Weight) AlgOg; from 1.0 to 2.0 %(Weight) MgO; from 8.0 to 14.0 % (Weight) CaO; from 0.5 to 2.5 % (Weight) SrO; from0.5 to 2.5 % (Weight) NagO; from 0.5 to 2.5 % (Weight) KQO; and up to 100 % (Weight)SiOg.

The opacif1er may be ZrOg, ZrSiO4 or SnO.

In an embodiment, the ceramic glaze, after f1ring, consisting essentially of 6.7% (Weight) ZrOg; 12.2 % (Weight) AlgOg; 1.4 % (Weight) MgO; 11.6 % (Weight) CaO;1.2 % (Weight) SrO; 2.5 % (Weight) NagO; 2.1 % (Weight) KQO; and 62.3 % (Weight)SiOg.

In an embodiment the ceramic glaze does not comprise Zn, Li, La, Ba or W.

This is advantageous, since the f1red glaze is an inert material With retainedhigh surface smoothness or gloss, but no toxic or environmental harrnful materials areincluded in the glaze recipe.

In a second aspect, use of a ceramic glaze according to the first aspect, forproducing a glazed ceramic body, is provided.

This is advantageous, since the ceramic body produced has a high surfacesmoothness, or gloss.

In a third aspect, a glazed ceramic body is provided, comprising a ceramicglaze according to the first aspect.

This is advantageous, since the ceramic body has a high surface smoothness, orgloss.

The present invention has the advantage over the prior art that it provides asmooth surface of an opaque glaze, in one fire. This is cost-effective and environmentally friendly, and the finished product is easy to clean.

Brief Description of the Drawings These and other aspects, features and advantages of which the invention iscapable of will be apparent and elucidated from the following description ofembodiments of the present invention, reference being made to the accompanyingdrawings, in which Fig. 1 is a an illustration of a cross-section of a sanitary Ware according to priorart; Fig. 2 an illustration of a cross-section of a sanitary Ware according to anembodiment of the invention; and Fig. 3 is a graph showing the temperature curve of the firing process according to an embodiment.

Description of embodiments Several embodiments of the present invention will be described in more detailbelow with reference to the accompanying drawings in order for those skilled in the artto be able to carry out the invention. The invention may, however, be embodied in manydifferent forms and should not be construed as limited to the embodiments set forthherein.

Usually, sanitary ware glazes based on opacifiers, such as zirconium silicate(ZrSiO4), zirconium dioxide (ZrOg) or tin oxide (SnO), are known for their roughnessdue to the unmeltable coarse and sharp grains in the glaze surface. It is also known thatglazes with low viscosity in mo lten state spread and flow out properly over the ceramicbody, and during the cooling phase of the firing process it get gradually stiffer until itsolidify when the cooling process get closer and reaches ambient temperature. Duringthis process the sharp grains of opacifiers set in the glaze matrix, in a way orientingtheir sharp peaks outwards. These peaks, surface roughness, are later the start point ofthe inorganic and/or organic deposits of lime, silica, dirt, or similar which may triggerthe bacterial growth on the glaze surface and affect the aesthetical and hygienicproperties.

It has been found that by choosing a specific chemical composition of mineraloxides with special behavior during the firing, it is possible to decrease the roughness ofthe glaze surface yet retaining a sufficient opacity giving the desired white color.

Specifically, by using specific raw materials, minerals or oxides comprising strontium (Sr) ions, such as strontium carbonate (SrCOg), strontium sulfate (SrSO4), or strontium oxide (SrO) it Was surprisingly discovered that the nanotopography of thesurface of the resulting glaze is smoother, even though the glace comprises opacif1ers.

Strontium is a commonly used fluxing agent, norrnally added to control thefluxing behavior of the mo lten glaze.

HoWever, the present inventor discovered that addition of strontium alsoeffects the cooling process so that, even though the percentage of opacifier is the sameas in a common glaze, the resulting surface of the glaze is much smoother than WhatWould be expected after the f1ring process. This is advantageous, since the resulting surface is easier to keep clean.

The glaze An unf1red ceramic glaze comprises a suspension of fine particles of rawmaterial minerals dispersed into Water, i.e. raW glaze. The raW material minerals areWell known to a person skilled in the art, and may be china clays, kaolins, mo lochite,nepheline, Wollastonite, pegmatites feldspars, quartz sand, silica, talk, dolomite, chalk,strontium carbonates, silicates and/or similar.

As knoWn to a person skilled in the art, the glaze may altematively compriseminerals that have been pre-treated by a fritting process, i.e. fritted glaze. Furthermore,it may also comprise some binders Which prevent the sedimentation of the particles andtherefore improve the stability of the glaze slurry. The rheo lo gy (viscosity andthixotropy) and the density of the raW glaze have to be optimal for that purposedepending on the glazing technique, Which is appreciated by a person skilled in the art.It is extremely important to get an even and flat surface, When applying the glaze ontothe object to be glazed. Thus, during glazing, the suspension is applied to the ceramicbody and a Water transport process starts. The Water from the suspension is migratingthrough the pores of the ceramic body and the particles from the suspension are layingon the surface.

In the case of sanitary Ware, it is most common to use raW glaze, since this is cost-effective. HoWever, for the purpose of the invention, also fritted glaze may be used.

Figure 1 is an illustration of a cross-section of a sanitary Ware 10, Where the ceramicbody 11 is coated With a glaze 12 according to the state of the art. The surface of theglaze 12, either raW glaze or fritted glaze, typically has some roughness due to theunmeltable coarse grains of the opacifiers 13 such as zirconium silicate (ZrSiO4), zirconium dioxide (ZrOg) or tin oxide (SnO) or similar, Well knoWn to a person skilled in the art. However, opacif1ers are needed in the glaze to give the glazed sanitary Warean appealing look, usually white.

According to an embodiment, the glaze comprises strontium (Sr) ions, such asstrontium carbonate (SrCOg), strontium sulfate (SrSO4), or strontium oxide (SrO).

This is advantageous, since it results in a smoother surface on the finishedglaze.

According to a non-limiting theory of the inventor, illustrated in a cross-sectionof a sanitary Ware 20 as shown in Figure 2, the coarse and sharp grains of opacif1er 23in the glaze 22, applied on the ceramic body 21, are forced to a favorable angularorientation during the cooling process, because of the specifically chemical compositionof the glaze, and by that substantially decreasing the roughness of the glaze surfacecompared to what would be expected. According to the roughness measurement results(see below), the roughness average, the density of the peaks and the range of the angularorientation of the peaks in the low roughness glaze is lower than in an ordinary sanitaryware glaze with identical type and amount of opacif1er.

The raw glaze may also comprise oxides such as SiOg, AlgOg, NagO, KgO,CaO, and/or MgO.

Certain metal oxides are classif1ed as a very toxic agent for aquaticmicroorganisms. For example, use of oxides of zinc (Zn), lithium (Li), lanthanum (La),barium (Ba) or tungsten (W) is thus disadvantageous because it may cause long-terrnadverse effects in the aquatic environment. Thus, in an embodiment, a glaze is provided,which is free from Zn, Li, La, Ba or W, such as oxides of Zn, Li, La, Ba or W.

In an embodiment, the unfired glaze essentially consists of: 3.0-10.0 % (weight) opacifier, such as ZrOg; 8.0-12.0 % (weight) AlgOg; 1.0-2.0 % (weight) MgO; 8.0-14.0 % (weight) CaO; 0.5-2.5 % (weight) SrO; 0.5-2.5 % (weight) NagO; 0.5-2.5 % (weight) KZO; 3.0-10.0 % (weight) loss of ignition; and up to 100 % (weight) SiOg.

As known to a person skilled in the art, the loss of ignition is a collective term for substances, which are a part of the unfired glaze but subsequently bumt out during the f1ring process, so that they are not part of the f1red glaze. Such substances are wellknown to a person skilled in the art, and may be additives such as binders, stabilizers,antimicrobial agents or similar.

In an embodiment, the different components are milled to a median particulardiameter of 2.5-3.5 um and, after weighing and mixing the specific amounts, water isadded forrning slurry. The mill may be a traditional ball mill or a flow-through millsuch as Drais mill.

The Viscosity of the slurry is adjusted to 0.30 - 0.90 Pas by adding water andthe density of the slurry is adjusted to 1750 - 1850 g/1. As will be appreciated by aperson skilled in the art, the typical weight distribution between particular matter andwater is about 2/3 particular water and 1/ 3 water.

As known to a person skilled in the art, the slurry making up the unf1red glazemay then be applied to the object to be glazed by any suitable means and subjected to af1ring process to produce a finished product coVered with fired glaze. Applying theglaze may be done by spraying, dipping, painting or a combination of these. Any kindof glazing tool known to a person skilled in the art may be used, such as a glazing robot.Altematively, manual or other similar transfer and/or deposition technical or manualdevices may be applied. The application is preferably done with the ceramic bodyplaced on a carrier inside a glazing cell.

The thickness of the glaze layer when applied is between 500-800 um. As willbe appreciated by a person skilled in the art, the exact thickness may be adjusted according to the desirable thickness after the f1ring with respect to the loss of ignition.

T he firíng process After application of the glaze, the sanitary ware, such as the toilet bowl, issubject to a f1ring process. The f1ring takes place in a tunnel or interrnittent kiln, wellknown to a person skilled in the art, during a total f1ring time of 12-24 hours. It isknown that a glaze has a melting interVal and not a melting point. When the glaze entersthe melting interVal its Viscosity increases gradually. It reaches the highest Value whenthe temperature reaches the top temperature. It is supposed that all raw materials andoxides in the glaze melt during the up going f1ring curVe and complete the melting at thetop temperature and during the holding time at the sealing temperature. The mo ltenglaze which also has a low Viscosity flows out over the ceramic body covering theobject to be glazed, i.e. the biscuit.

The starting point of the melting temperature interval of the respective glaze isdepending on the specific glaze, but occurs typically around 1120 °C.

The sealing of the glaze occurs at the top temperature which depends on therespective glaze, but it occurs typically in the interval between 1190 °C and 1300 °C.

The sealing of the glaze has to be secured by holding constantly the toptemperature which may also vary depending on the respective glaze, body and firingconditions, but generally it has to be kept in the interval between at least 40 and 100minutes, such as 80 minutes, depending of the firing conditions, kiln construction andthe temperature gradients inside the kiln.

In an embodiment, the fired glaze essentially consists of: 3.0-10.0 % (weight) opacifier, such as ZrOg; 8.0-l2.0 % (weight) AlzOg; 1.0-2.0 % (weight) MgO; 8.0-l4.0 % (weight) CaO; 0.5-2.5 % (weight) SrO; 0.5-2.5 % (weight) NagO; 0.5-2.5 % (weight) KQO; and up to 100 % (weight) SiOg.

This composition is advantageous, since it has a smoother surface, while beingsufficiently opaque to be used as a sanitary Ware without any fiarther treatment, such as addition of a second glaze layer etc.

Specific embodiment The following specific description focuses on an embodiment of the presentinvention applicable to a sanitary product and in particular to a toilet bowl. However, itwill be appreciated that the invention is not limited to this application but may beapplied to many other ceramic bodies suitable for glazing, including for examplesanitary products such as wash basins or tiles, made from materials such as vitreouschina, fine clay, or fine fire clay.

In an embodiment, a glaze for a toilet bowl is obtained by weighing and mixingparticular raw materials with water, forrning aqueous slurry, which is milled in a Draismill to a median particular diameter of 3 um and adjusting the viscosity of the slurry to0.50 Pas and the density ofthe slurry to 1785 g/l.

The particular raw materials essentially consist of: 6.3 % (weight) ZrOg; 11.3 % (Weight) AlgOg; 1.3 % (Weight) MgO; 10.8 % (Weight) CaO; 1.2 % (Weight) SrO; 2.3 % (Weight) NagO; 2.9 % (Weight) KZO; 5.8 % (Weight) loss of ignition; and58.1 % (Weight) SiOg.

The glaze slurry is subsequently applied, or coated, onto a ceramic body, suchas a toilet boWl, by spraying. Said body may be an unf1red dry ceramic body, Withmoisture content of 0.1-0.8 % (Weight), or on the unglazed f1red ceramic body.

The thickness of the glaze layer When applied is around 600 um.

After application of the glaze, the toilet boWl is subject to a f1ring process. Thef1ring takes place in a tunnel or interrnittent kiln, Well knoWn to a person skilled in theart, during a total f1ring time of 16 hours.

The firing process comprises a first step of heating of the ceramic body fromambient temperature to a top temperature, Which secures that the body is Vitrified orsintered and the glaze starts to melt properly. Next, the f1ring process comprises a stepof keeping the ceramic body at the top temperature to alloW the glaze to melt and floWout completely, and also a step of cooling doWn the ceramic body from the sealingtemperature to ambient temperature.

A temperature curve of the f1ring process according to an embodiment isprovided in Figure 3. The glaze melts sloWly at loWer temperatures, With a starting pointTm, Which in this embodiment is 1120°C.

During the time betWeen t0 and t1, i.e. the heating, the glazed body is heatedfrom ambient temperature, to the top temperature Tmax, also called sealing temperature,Which in this embodiment is 1200°C.

During the time betWeen t1 and t2, i.e. the sealing or fusion, the glaze is sealedby keeping the temperature substantially constant, as is knoWn to a person skilled in theart. The time period t1 to t2, the hold time, has to be optimal to alloW complete meltingand proper sealing of the glaze, but also cost effective from technological point of VieW.In this embodiment, the hold time is 80 minutes, Which alloW the glaze to melt and floW out completely.

During the time between t2 and t3, i.e. the Cooling, the glazed body is cooledfrom the top temperature Tmax to ambient temperature. The viscosity of the mo lten glazeincreases when the temperature decreases below the respective end point of the meltinginterval and the glaze hardens, creating a finished, f1red glaze.

The fired glaze essentially consists of: 6.7 % (weight) ZrOg; 12.2 % (weight) AlgOg; 1.4 % (weight) MgO; 11.6 % (weight) CaO; 1.2 % (weight) SrO; 2.5 % (weight) NagO; 2.1 % (weight) KQO; and 62.3 % (weight) SiOg.

This composition is particularly advantageous, since it has a smoother surface,while being sufficiently opaque to be used as a sanitary Ware without any fiarthertreatment.

Roughness values of this glaze were measured and compared to a state of theart glaze, not comprising any strontium oxide.

The Roughness Average (Sa), Density of Summits (Sds) and Root MeanSquare Gradient (Sdq) were measured by an optical pro filometer of type WYKONPFlex 3D. The analyzed surfaces were 0.466 x 0.621 mm with 10x magnificationand/or 1.7 x 2.3 mm with 2.75 x magnif1cation. Wave lengths over 80 um were filteredwith a RGOSWP filter. Table 1 shows the result for the smooth, glossy glaze accordingto an embodiment, for the reference glaze and in the case of roughness average also in the case of a transparent glaze (i.e. no rough opacifiers present).

Table I. Comparative measurements regarding surface Characteristicsl Sa (um) Sds (peaks/um2) Sdq (degrees)Reference glaze 25 1669 1.660Smooth glaze 19 794 0.248Transparent glaze 16 - - From the table, it can be seen that the roughness average (Sa) of the glazeaccording to the embodiment is much lower than the reference glaze. Its value is muchcloser to a transparent glaze, which is completely free of opacifiers. Furthermore, thedensity of summits (Sds) on the smooth glaze is much lower compared to the referenceglaze, and the root mean square gradient (Sdq), which actually means the angularorientation of the peaks, is lower compared to the reference glaze. All of these factorsindicate a glossy surface with increased smoothness compared to the reference glaze.

Since the glaze described above has a high surface smoothness, or gloss, andyet is opaque, it is very suitable for glazing ceramic objects.

Thus, in an aspect of the invention use of the ceramic glaze according toembodiments of the invention, for producing a glazed ceramic body, is provided. Saidceramic body may be any kind of ceramic body where a glossy, yet opaque surface isdesired, such as a sanitary ware, such as a toilet, a wash basin etc.

This is advantageous, since the ceramic body produced has a high surfacesmoothness, or gloss.

In an aspect, a glazed ceramic body is provided, comprising a ceramic glazeaccording to the embodiments of the invention. Said ceramic body may be any kind ofceramic body where a glossy, yet opaque surface is desired, such as a sanitary ware,such as a toilet, a wash basin etc.

This is advantageous, since the ceramic body has a high surface smoothness, orgloss.

Although the present invention has been described above with reference tospecific embodiments, it is not intended to be limited to the specific form set forthherein. Rather, the invention is limited only by the accompanying claims and, otherembodiments than the specific above are equally possible within the scope of theseappended claims.

Additionally, although individual features may be included in different claims, these may possibly advantageously be combined, and the inclusion in different claims ll does not iniply that a combination of features is not feasible and/or advantageous. Inaddition, singular references do not exclude a plurality. The terms "a", "an", “first”,“second” etc do not preclude a plurality. Reference signs in the clainis are providednierely as a clarifying example and shall not be construed as liniiting the scope of the clainis in any Way.

Claims (9)

1. A ceramic glaze, Which, after firing comprises from 3.0 to 10.0 % (Weight)opacifier and from 0.5 to 5 % (Weight) SrO.

2. The ceramic glaze according to claim 1, Which, after firing comprises from0.5 to 2.5 % (Weight) SrO.

3. The ceramic glaze according to claim 1, Which, after firing comprises 1.2 %(Weight) SrO.

4. The ceramic glaze according to claim 1 or 2, Which, after firing consistingessentially of: from 3.0 to 10.0 % (Weight) opacifier; from 8.0 to 12.0 % (Weight) AlzOg; from 1.0 to 2.0 % (Weight) MgO; from 8.0 to 14.0 % (Weight) CaO; from 0.5 to 2.5 % (Weight) SrO; from 0.5 to 2.5 % (Weight) NagO; from 0.5 to 2.5 % (Weight) KzO; and up to 100 % (Weight) SiOg.

5. The ceramic glaze according to any of the preceding claims, Wherein theopacifier is ZrOg, ZrSiO4 or SnO.

6. The ceramic glaze according to claim 4, consisting essentially of:6.7 % (Weight) ZrOg; 12.2 % (Weight) AlgOg; 1.4 % (Weight) MgO; 11.6 % (Weight) CaO; 1.2 % (Weight) SrO; 2.5 % (Weight) NagO; 2.1 % (Weight) KQO; and 62.3 % (Weight) SiOg. 5 13

7. Thc ccraniic glazc according to any of the prcccding clainis, Which docs not coniprisc Zn, Li, La, Ba or W.

8. Usc of a ccraniic glazc according to any of the prcccding clainis, for producing a glazcd ceraniic body.

9. A glazcd ccraniic body coniprising a ccraniic glazc according to any of clainis 1 to 7.