Coating processes – Direct application of electrical – magnetic – wave – or... – Electrostatic charge – field – or force utilized
Reexamination Certificate
2000-03-15
2002-02-26
Parker, Fred J. (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Electrostatic charge, field, or force utilized
C427S486000, C106S491000, C501S016000
Reexamination Certificate
active
06350495
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a process for the production of ceramic and vitreous coatings, in particular glazes and engobes, comprising electrostatic application of a coating powder onto a ceramic substrate and firing of the coated substrate. The invention furthermore relates to a coating powder particularly suitable for the performance of the process and to the use thereof.
Ceramic and vitreous coatings, such as engobes and glazes, on ceramic, in particular unfired or partially fired, substrates are predominantly produced using aqueous slips. After application of the slip, the substrate coated therewith is fired, wherein the stovable material contained in the slip melts or sinters together to yield a ceramic or vitreous layer. Due to the disadvantages associated with the use of aqueous slips, such as effluent problems and elevated energy consumption, electrostatic powder coating of ceramic products is becoming increasingly significant.
Providing ceramic substrates, such as porcelain, earthenware and stoneware, but in particular unfired or only partially fired substrates, with an electrostatic coating still occasions various problems with regard to electrostatic application of the coating powder, inadequate adhesion of the powder to the substrate and, often, inadequate handling resistance. In addition to these problems, there are glazing defects which only occur on firing.
Various approaches have been used to solving the electrostatic and adhesion problems: thus, according to DE-PS 29 41 026, glaze powder may be adjusted to a specific surface resistance value suitable for electrostatic application of greater than 1·10
10
Ohm·m by coating with a polysiloxane. Engobes may also be applied electrostatically as powders after such hydrophobing treatment (EP-A 0 442 109). While, according to DE-A 42 39 541, the adhesion of an electrostatically applied glaze powder may indeed be improved by initially applying an aqueous coupling layer containing a polymer and a glass frit onto the substrate, the use of an aqueous system is regarded as disadvantageous.
WO 94/26679 discloses an improvement to the adhesion and handling resistance of a stovable coating powder, such as a glaze powder, which has been electrostatically applied to a substrate: in this case, the coating powder contains, apart from a glaze powder, a chemically or physically activatable coupling agent which combusts without leaving a residue on firing, such as polyolefins and dextrins, by means of which, after activation, the particles of the layer are fixed to each other and to the substrate. Preferred coating powders contain polysiloxane-coated glass frits mixed with to 15 wt. % of thermoplastic or 5 to 10 wt. % of dextrin. Usable glazes may be obtained on porcelain biscuit bodies only under specially optimised conditions, which, however, entail increased costs. If the conditions are only slightly modified, depending upon the substrate, sometimes considerable glaze defects and deficiencies occur before the required layer thickness is obtained.
WO 97/08115 discloses a remedy to the above-stated problems: the production process may be simplified without degrading glaze quality by using a glaze or engobe composition having a particular grain size distribution, namely a d
50
value of 5 to 25 &mgr;m, a d
90
value of less than 35 &mgr;m and a d
10
value of greater than or equal to 2 &mgr;m, in a coating powder additionally containing a coupling agent.
While the two above-mentioned processes do allow the production of defect-free glazes on fired or biscuit-fired porcelain bodies, when the same methods are used to produce glazes and engobes on unfired bodies, such as wall and floor tiles, the stoved coating exhibits considerable deficiencies in quality. In the case of biscuit-fired bodies too, the frequency of glaze defects is often excessively high. The defects often take the form of large, “frozen-in” blisters and extensive areas without glaze (the occurrence of such defects is also known as “crawling”). These defects often occupy 10 to 30% of the stated area. The cause of this defect is suspected to involve the following interactions: the electrostatically applied layer is much looser and moreover exhibits much lower adhesion to the substrate than the layer obtained when a conventional slip is used. During firing of a green body provided with a glaze layer, gas-forming reactions occur in the green body, in the boundary layer between the body and the glaze layer and in the glaze layer, such reactions including, for example, the conversion of kaolin into metakaolin, which in particular proceeds at a temperature in the range from approx. 700 to 800° C., and the elimination of CO
2
from carbonates, such as dolomite, present in the body, which occurs at around 900° C. If the layer is too thin and adheres inadequately, it is labile from when an initially present organic binder has combusted during the firing until the temperature at which the glaze melts, such that it may be destroyed by slight mechanical action, such as vibration and air currents in the kiln and by degassing processes in the body. One of these disruptions is manifested by lifting of an area of the electrostatically applied layer, so resulting in the formation of blisters and glaze-free areas once the blisters have burst. Clearly, the capillary forces which occur when a slip is applied using conventional methods firmly anchor the layer to the green body, such that the stated disruptions do not occur, whereas such anchoring is absent when the layer is applied electrostatically.
SUMMARY OF THE INVENTION
The object of the present invention is accordingly to overcome the stated problems in the production of ceramic and vitreous coatings, in particular glazes, wherein the coating powders are applied electrostatically. Once fired, the coatings may contain crystalline and/or amorphous fractions.
A process has been found for the production of a ceramic and/or vitreous coating, in particular a glaze, on a ceramic substrate, comprising electrostatic application of a coating powder and firing of the coated substrate, which process is characterized in that a coating powder CP
AB
is used which contains 1 to 50 wt. % of a layer-forming composition A having a softening onset in the range from 400 to 750° C. and 99 to 50 wt. % of a layer-forming composition B having a softening onset in the range from above 750 to 1100° C., or in that an underlayer is applied onto the ceramic substrate using a coating powder CP
U
containing at least 5 wt. % of the layer-forming composition A and an upper layer is applied thereon using a coating powder CP
O
containing at least 50 wt. % of the layer-forming composition B, wherein at least one of the two layers is applied electrostatically and the underlayer constitutes at least 1% of the total coating. The term “layer-forming” is taken to mean that, on firing, the composition is capable of forming a layer of a crystalline and/or amorphous, i.e. ceramic and/or vitreous, material.
The essence of the invention is the use of a layer-forming composition A, which has a softening onset T(EB)
A
below the softening onset T(EB)
B
of a layer-forming composition B, wherein composition B constitutes the majority of the ceramic coating to be produced. The layer-forming composition A preferably comprises a low-melting glass composition. The majority of the coating comprises compositions as are used for glazing, engobing and otherwise decorating ceramic substrates. According to the invention, the ceramic substrates to be coated in particular comprise those in which or at the boundary layer of which with the applied coating degassing processes occur during firing; such substrates include unfired or biscuit-fired earthenware and stoneware as well as unfired and biscuit-fired porcelain. Unfired wall and floor tiles and roofing tiles are particularly suitable substrates. Composition A is either a constituent in an effective quantity of the coating powder CP
AB
directly applied as a single layer or a constituent in an effective quantity o
Jackson Philip Robert
Schriener Andreas
Shingler David
Triptrap Herbert
Withington Steven Charles
dmc
Parker Fred J.
Wenderoth , Lind & Ponack, L.L.P.
LandOfFree
Electrostatically applicable coating powder and processes... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Electrostatically applicable coating powder and processes..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Electrostatically applicable coating powder and processes... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2938847