ADDITIVES FOR CERAMIC GLAZES
FIELD OFTHE INVENTION
The present invention relates to the use for the preparation of ceramic
glaze slips of extruded pellets comprising a carboxymethyl cellulose and
at least another ceramic glaze additive.
The glaze slips so obtained may be applied on green or fired ceramic
bodies such as artware, tableware, tiles, roofing tiles, bricks, heavy clay
products and sanitary ware.
BACKGROUND OFTHE ART
Most traditional ceramic manufactured products, such as tiles and
sanitary ware, are made of a ceramic body that confers shape and
mechanical properties t o the object; the ceramic body generally has
some porosity and poor aesthetic qualities.
Said ceramic body, which is defined "green" or, alternatively, "fired", is
usually coated with a ceramic layer, called ceramic glaze; the ceramic
glaze is sintered by firing, in such a way t o gain suitable superficial
aesthetic qualities and, in the meantime, t o become a fluid-proof barrier;
as a matter of fact, after firing, the ceramic glaze has usually no porosity
and is generally resistant to abrasion and t o the attack of chemical
agents.
Glaze is mainly applied on the surface of the ceramic body
dispersed/suspended in a opportune vehicle, usually water, or, in some
special application, by dry dusting a dry mixture over the surface of the
ceramic body. Traditional liquid ceramic glazes are suspensions of
l
various powdered minerais and metal oxides that can be applied by
directly dipping pieces into the glaze, pouring the giaze over the piece,
spraying it onto the piece with an airbrush or similar tool, with a brush,
or with any tool that wil! achieve the desired effect.
Liquid ceramic g!azes, also called ceramic giaze slips, generally contain,
finely dispersed in water, silica to form glass, also in form of frit (prefired
vitreous component); mixtures of metal oxides, usually in the form
of pre-treated natural occurring minerals, such as alkaline earth metal
oxides which act as a flux and allow the glaze to melt at a particular
temperature; alumina to stiffen the glaze and prevent it from running off
the piece; ceramic pigments, such as oxides or carbonates of transition
metals.
Since most of the ingredients cited above are heavy ingredients and in
order to obtain a proper coating before and after firing, it is necessary to
add some particular additive into the liquid ceramic glazes. These
additives, often organic in nature, are added, singularly or as
compositions, to glazes to give them specific properties that are required
during application. They do not participate directly to the vitrification
process, but are able to give special characteristics to the glaze slip
useful for the subsequent application on the ceramic body or to the
formed glass for successive treatments.
These ceramic glaze additives are well known in the art and further
information can be found in literature, for example in Fortuna D.,
"Sanitaryware", Gruppo Editoriale Faenza Editrice, p. 61-64 (2000) and
Stefanov S. and Batscharow S., "Ceramic Glazes", Bauverlag GmbH
(1989).
The most common additives for ceramic glazes are: suspending agents,
such as water-swellabie clays; thickening agents, such as carboxymethyl
cellulose, alginates, natural gums and acrylic (co)polymers;
preservatives, biocides, antifoams, dispersants (fluidizers), such as
medium/low molecular weight acrylic acid (co)polymer; binders;
deflocculants; levelling agents and plasticizers.
Many of these additives are added into the glaze slips as powders.
Powders by their nature have very large surface areas susceptible to
humidity and/or bacterial growth.
Handling of such powders and dust generation during processing create
environmental and health problems that must be dealt with by the
manufacturer and the customer.
In addition suspending agents and thickening agents in form of powders
are difficult to dissolve in the thick glaze slip and, if not stirred for
enough time and/or with a high shear mixer, they can create lumps or
aggregates in the g!aze slip. After preparation, the glaze slips are sieved
in order to eliminate residual impurities and aggregates. If not
completely dissolved, the lumps or aggregates of the additives can
increase considerably the time required for the sieving. Moreover, a
partial dissolution of the rheology modifier can require a time-consuming
correction of the viscosity of the glaze slip or, if not corrected, can cause
serious glazing defects on the final products, such as leveling problems,
running or craw!ing, which are well known to those expert in the art.
Furthermore the exact dosing and in-loading of the powdery additives,
which have usually different densities and different particle sizes, are
source of further difficulties.
A typical solution to these problems commonly used in many fields is to
granulate the powdery compounds or compositions. Unfortunately the
granules obtained during the granulation process are different in their
forms and dimensions, thus making it necessary to sieve the granulated
material, for the purpose of selecting the granules presenting dimensions
above a minimum value. Moreover granulation does not eliminate dust.
In fact, a percentage of this dust, even if small, remains imprisoned in
between the granules and tends to spread itself.
It has now been found that the use of compositions of these additives in
form of extruded pellets can solve all the above mentioned difficulties.
The composition and dimensions of the pellets can be easily controlled in
order t o avoid hazard and to optimize processing, handling/shipping, inloading
dosing, etc. At the same time the extruded pellets are really
compact and do not produce dust when handled and have a low
dissolution rate, compared to the powders, which reduce significantly the
formation of tumps in the glaze s!ip.
As far as the Applicant knows, the use of extruded pellets obtained by
extrusion of a mixture of two or more ceramic glaze additives for the
preparation of glaze slips have not been described in the literature.
By "pellet", we mean any solid shaped composition, including but not
limited to, tablets, pearls, flakes, briquettes, bars, or blocks.
DESCRI PTION O FTHE INVENTION
It is therefore a fundamental object of the present invention the use of
extruded pellets comprising : a) from 5 to 85 % by weight as dry matter
of a carboxymethyl cellulose (CMC), b) from 5 to 85 % by weight as dry
matter of at least another ceramic glaze additive, in which the sum of a)
+ b) represents at least 40% by weight of their dry matter, for the
preparation of ceramic glaze slips, the pellets being used in an amount
comprised between 0.05% and 5% by weight based on the weight of the
ceramic glaze slip.
DETAILED DESCRI PTION O FTHE INVENTIO N
Preferably the extruded pellets of the invention comprise: a) from 20 to
75 % by weight as dry matter of a CMC and b) from 20 to 75 % by
weight as dry matter of at least another ceramic glaze additive.
The carboxymethyl cellulose a) suitable for the realization of the present
invention can be chosen among those commonly used in the ceramic
field and known to those expert in the art. The carboxymethyl cellulose
preferred for the realization of the present invention has a degree of
substitution comprised between 0.5 and 1.5, more preferably between
0.6 and 1.2, most preferably from 0.7 to 1.1. Its Brookfield LVT®
viscosity, at 2 % wt (weight) in water, 60 rpm and 20 °C, is comprised
between 5 and 30,000 mPa*s, preferably between 10 and 15000 mPa*s.
The carboxymethyl cellulose useful for the realization of the present
invention can be technical or purified carboxymethyl cellulose, having a
percentage of active substance comprised between 55 and 99.5 % by
weight on dry matter, preferably from 70 t o 98.5, and a content of water
of about 2-12 % by weight.
The ceramic glaze additive b) is preferably chosen in the group
consisting of suspending agents, thickening agents different from CMC,
preservatives, biocides, antifoams, dispersants, binders, deflocculants,
levelling agents, plasticizers, de-airing agents and mixture thereof.
Suspending agents improve the stability and the flowability of the
dispersion and also permit a higher percentage of suspended solids to be
incorporated into the dispersion. Suitable suspending agents are water
swellable clays and sodium or magnesium chloride. Water-swellable clays
are the preferred suspending agents. With the expression "waterswellable
clays" we mean clays which are capable of adsorbing water.
Examples of these clays are bentonite, montmorillonite, kaolinite,
hectorite, attapulgite, smectite and others. The most popular clay is
standard Bentonite, which may contain small amount of iron. Another
useful clay is Hectorite, which is very plastic and iron-free and belongs t o
the family of the smectite minerals. It is sold under various commercial
names, including Bentone®, Hectabright®, Maca!oid® and VeeGum®.
Also synthetic smectites can be used for the same purpose-
Suitable thickening agents different from CMC, which have binding, filmforming,
suspending and water retention properties, are naturally
occurring water-soluble polymer, as such or synthetically derivatized,
such as starch and starch derivatives, guar and guar derivatives,
tamarind and its derivatives, xanthan gum, alginates, diutan gum, gum
Arabic, and gum tragacanth. Cellulose and cellulose derivatives different
from CMC, such as hydroxyethyl cellulose, hydroxypropyl cellulose,
methyl hydroxypropyl celiu!ose, can be also used. Further suitable
thickening agents are synthetic polymers such as a high molecular
weight acrylic acid based polymer or polyvinyl pyrrolidone and its
copolymers. Also mixtures of thickening agents can be advantageously
used.
As its name suggests, the dispersant ensures that the solid is uniformly
dispersed throughout the aqueous medium. Any of the dispersants which
are normally used for ceramic slurries are useful in the preparation of the
pellets of the present invention. Examples of these dispersants are
water-soluble salts of low/medium molecular weight acrylic
(co)polymers, such as (meth)acrylic acids homopolymers;
polyphosphates, for example tripolyphosphate and hexametaphosphate;
humic acids; lignin sulfonates; sodium silicates; sodium carbonate and
mixture thereof. Preferred dispersants are water-soluble salts of
low/medium molecular weight acrylic (co)polymers and polyphosphates.
Suitable biocides and preservatives are, for example, p-chloro-m-cresol,
o-phenyl phenol, 2-bromo-2-nitropropane-l,3-dioi (Bronopol) or
compounds from the class of the derivatized isothiazolin-3-ones such as
benzisothiazolinone (BIT), 5-chloro-2-methyl-4-isothiazolin-3-one (CIT
or CMIT) and 2-methyl-4-isothiazolin-3-one (MIT). Other examples are
sodium or zinc pyrithione, parabens, sodium benzoate, formaldehyde
releasers etc. They are used both in the form of powders and liq uid also
as synerg istic mixtures .
Example of antifoa ms and de-a iring agents suitable for the rea lizatio n of
the present invention are aluminum stea rate, ethylen e/pro pylene oxide
copolymers, poiydi methyl siloxa ne, colloida l silica , minera l oils and
mixtu re thereof.
Exa mple of binders which can be used in the present invention are
polyvi nyl alcoho l, polyvi nyl acetate or partially hydrolyzed polyvi nyl
acetate, anionic polyacri lates or poiyacryl amides, polyu reth anes,
styrene/butadiene resi ns and mixture thereof.
Plasticizer are usua lly added to red uce the elastic modulus of the glaze
slips and its interna l stress during the firi ng, t hus decreasi ng the
proba bility of craking formation . Examples of plasticizer are glyce rol,
sorbitol, glycols, such as triethylene glycol or propylene glycol,
(co) polymers ethylene oxide/propylene oxide ; fatty acids or fatty
amides ; alkanol amin es such as triethanol amine ; fatty acid monoesters
of glycerol or glyco ls; esters such as monobutyl or dibutyl phthalate ;
and mixtu re t hereof. Preferred plasticizers are polyethylene and/or
polypropylene glycols .
Other ingred ients that can be advantageously added to the extruded
pellets of the invention are fillers ; disi nteg rating agents such as
polyvi nylpyrrolido ne, dextra n and sta rch or mixtu re of carboxylic acids,
for example citric or tarta ric acid, and water sol uble carbonate or
bica rbonate, i.e . sodi um carbonate; extrusion plasticizers such as ethy l
cellulose and polyethylene glycol.
The method of preparation of the extruded pellets of comprises the
following phases :
1) preparing a hydrated mixture of a) and b);
2) extruding the hydrated mixture to form an extruded material;
3) comminuting the resulting extruded material to form a shaped
product (pellets).
By conventional means, the CMC, water and the ceramic additives are
mixed to form a hydrated mixture (phase 1). This can be done in an
external mixing device and/or inside the extruder .
The water content of the hydrated mixture is oniy important in that it
should be high enough to allow the intimate and uniform mixing of the
different components and should permit the extrusion of the mixture.
Conversely, the water content of the hydrated mixture should not be so
high that the it does not maintain its shape after it is extruded.
Generally, the water content of the hydrated mixture is from 5.0 to 50 %
by weight.
The material in the extruder is heated to or maintained at a temperature
in the range from about 20 to about 100 °C. The optimum temperature
for extrusion will vary somewhat dependent upon the components of the
mixture, but the optimum temperature can readiiy be determined
empirically. The temperature of the mixture may vary depending upon
where it is in the extruder, but generally a uniform temperature profile is
preferred. The temperature referred to herein is the mixture temperature
in the extruder just before it passes through the die. High temperatures
which can cause decomposition should be avoided.
The hydrated mixture is extruded through a die, preferably a multihole
die. I n general, the shape and size of the orifices fix the cross-sectional
shape and size of the extrudate. Although any shape of orifice may be
used, i.e. circle, triangle, square, or rectangle, it is preferred that the
extrusion of the hydrated mixture be through equiaxial orifices. Equiaxial
orifices are orifices that have approximately equal dimensions in all
directions. The cross-sectional area of the orifices should be small
enough so that the extruded hydrated mixture fibers line up parallel to
each other in a tightly formed filaments (strands). On the other hand,
the cross-sectional area of the orifice should not be so small that an
excessive amount of energy must be exerted to press the hydrated
mixture through the orifices. Generally, the orifices are of dimensions
ranging from 1.0 to 6.0 mm, preferably from 2.0 to 3.5 mm.
The extrusion can be done with any device that applies sufficient
pressure to push the hydrated mixture through the extrusion orifices at a
temperature which keeps the mixture hydrated. For example, a pumptype
extruder, such as a positive displacement piston or a gear pump,
can be used. Another example of suitable extrusion equipment is a
screw-type extruder which advances the hydrated mixture by means of a
screw rotating inside a cylinder. A twin screw extruder in co-rotating or
counter-rotating mode, intermeshing or non-intermeshing may be
utilized in the processes of the invention, but equally a single screw
extruder or a multi screw extruder may also be suitable providing always
that mixing can be achieved. Screw-type extruders are not as energy
efficient as pump-type extruders and convert much of the energy to
heat. This causes the temperature of the mixture to increase and
dehydration occurs. Thus, when a screw-type extruder is used, it is
generally necessary to use a cooling device to keep the hydrated mixture
at a temperature below 100 °C.
Usually the extrusion process is carried out at pressures well above
atmospheric pressure, preferably the extrusion is carried out at
pressures of from about 20 to about 160 bar.
The extruded material is a firm material appearing uniform in texture
and color. Generally, the extruded material is in the form of long, narrow
filaments. The filaments have a uniform cross-sectional area that is
approximately the same as the extrusion orifices described above. The
extruded materia! has a residual moisture content ranging from 5.0 to 50
% by weight, preferably from 15 to 30 % by weight.
In order to reduce the filaments into pellets, it is necessary to
comminute the extrudate (phase 3).
The comminuting can be accomplished by using standard equipment
known in the art. Typical comminuting devices are air-swept impact
mills, ball mills, hammer mills, and disk mills. This is preferably done in
an air-swept impact mill because the other mills, i.e. ball mills, have a
tendency to overmiil the product into fine particles that are dusty. I n
addition, an air-swept impact mill will dry the extruded material, if
necessary, by blowing hot air across the mill.
Another method for comminuting the extruded material is to cut it with a
die-faced cutter. A die-face cutter operates by moving a blade across the
stationary die or by moving a die across the stationary blade. Thus, the
ceramic additive is cut as it is extruded through the plurality of orifices in
the die.
The size of the extrusion orifice fix two of the dimensions of the product.
Therefore, it is only necessary to cut the filaments to shorten the length.
Typically, the extruded material is cut to a fength/diameter ratio of from
0.5 to 3, preferably to a length/diameter ratio of from 1 to 2.
It may be advantageous to dry the extruded pellets . The drying of the
extruded material can be accomplished with standard drying equipment
and methods known in the art. Typical driers include belt driers and fluid
bed driers. The dried extruded pellets have a residual moisture content
generally ranging from 5.0 to 15 % by weight.
I n a preferred embodiment the ceramic glaze slips contains from 0.2 to
3% by weight of the extruded pellets of the invention.
All glazes normally used in the ceramic industries and well known t o
those expert in the art can be prepared using the extruded pellets of the
invention. Various examples of ceramic glaze formulations can be found
in literature, such as in : Fortuna D., "Sanitaryware", Gruppo Editoriale
Faenza Editrice, (2000) and Stefanov S. and Batscharow S., "Ceramic
Glazes", Bauverlag GmbH (1989).
As already said, typical components of ceramic glazes are silica, fluxes,
alumina and ceramic pigments.
Silica and alumina can be added to glazes by the addition of minerals,
such as: quartz, flint, ball clay, kaolin, feldspars or mixtures thereof.
Silica can be also added to the glaze in the form of frits, the term frit
referring to that granulated or particulate material obtained when molten
glass is poured into cold water. Frits are normally mixtures of various
mineral materials containing among the others silica, alumina, metal
oxides, boron oxide.
Fluxes lower the melting point of the glass formers. Non exhaustive
examples of fluxes are alkali and alkali-earth oxides and carbonates.
The ceramic pigments useful in the ceramic glaze of the invention are
solid sinterable materials. Examples of suitable ceramic pigments include
iron, titanium, copper, chromium, zinc, magnesium, aluminum, cobalt,
and cadmium oxides or salts, and zirconium and praseodymium
silicates.
The extruded pellets of the invention can be added to the glaze in the
mill during the grinding or during the preparation of the glaze slip, both
as such or as a water dispersion at a concentration ranging from 1 to
30% wt; preferably they are added to the ceramic glaze slip components
in dry form.
For liquid applications, glazes are normally ground and sieved, then are
suspended in a vehicle such as water obtaining the glaze slip. The ratio
between the solid materials and the vehicle is between 85/15 and 40/60
wt. Often grinding of the glazes is performed directly in the presence of
the vehicle t o provide the glaze slip in a single operation.
The extruded pellets of the invention can also be used for the
preparation of engobes, which are a particular kind of glazes. An engobe
is an opaque coating that is often applied t o the ceramic body before
glazing. Its function is to mask the ceramic body, for example, when it is
produced using red clay. Engobes conventionally contain frits and raw
materials but the frit content is typically much lower than in a normal
glaze.
The glaze slip of the present invention may be applied on green or fired
ceramic bodies such as artware, tableware, tiles, roofing tiles, bricks,
heavy clay products and sanitaryware using anyone of the conventional
application techniques known to those expert in the art. Application
techniques such as disk and bell applications, dipping, spraying, screen
printing, brushing and electrostatic applications can be employed.
The glaze slip of the invention is storage stable for several days without
change of its rheological profile and can be used as if it was freshly
prepared.
EXAMPLES
Examples 1-3
The solid ingredients of Table 1 were homogenized in a mixer, using a
"K" shaped stirrer. During the homogenization, demineralized water was
added slowly (in about 10 minutes) to the mixture avoiding t o produce
agglomerations of the material.
Table 1
* active 74% on dry matter; DS 0.72; moisture 24 % wt; Brookfield®
viscosity LVT (6% sol. as dry matter, 60 rpm, 20 °C) 300 mPa sec.
The mixtures of Example 1 - 3 were transferred into a Bausano extruder
equipped with 2 counter rotating screws, a multihole die with holes of
2.5 mm and a die-faced cutter.
The speed of the screws and the cutter was adjusted to produce about
50-80 g/min of pellets about 2.5 mm large and 2.6 mm long. The
internal temperature and pressure during extrusion were around 60-70
°C and 130 bar respectively.
The extruded pellets were dried on fluid bed at 80 °C to obtain a residual
moisture of about 7 % wt.
Table 2 shows the results of dissolution test of 30 g of each mixture in
300 ml demineralized water under constant stirring with a magnetic bar
stirrer.
The results are reported as the time required to reach the maximum
Brookfield LVT viscosity measured at 25 °C and 60 rpm.
Table 2
The stability of the extruded pellets of Example 1 - 3 to mechanical
stress was evaluated by shaking the pellets for 30 min in a tared
stainless steel sieve (80 mesh) in the presence of hard sphere (2 cm
o.d.) of silica.
At the end of the test the amount of powder which pass through the 80
mesh sieve was determined. For each sample the amount of powder was
below 0.1%.
Applicative test
The extruded pellets of Example 2 were compared with the same
composition of Example 4 (see Table 3), prepared by simply physically
mixing the different ingredients.
Table 3
* active 74% on dry matter; DS 0.72; moisture 6 % wt; Brookfieid® viscosity
LVT (6% sol. as dry matter, 60 rpm, 20 °C) 300 mPa*sec.
The dissolution velocity in a glaze slip of the two ceramic additives was
evaluated.
Two glaze slips for tile-ware were prepared transferring in two 1000 ml
jars the amount in g of the ingredients reported in Table 4.
Table 4
* Comparative
Both glaze slips were ground in a jar mill for 15 minute and then poured
in a 1000 ml glass beaker.
The main parameters (Brookfield RVT viscosity, Ford Cup Viscosity,
Density and % Residue) of the so obtained glaze slips were measured at
25 °C.
The % residue was determined screening the two glaze slips with a tared
150 microns ASTM sieve (100 mesh) and determining the weight
difference after drying in oven at 105 °C for 2 hours.
The result are reported in Table 5.
The results of the dissolution behavior tests show that the extruded
peilets of the invention has a dissolution behavior comparable with that
of a rheology modifier of the known art.
Table 5
* Comparative
Because of their physical characteristic the extruded pellets of the
invention avoid the production of volatile dust when handled and allow
simpler procedures for the preparation of the glaze slip, together with
simpler and more precise dosage of the additives.
CLAIMS
1) Use of extruded pellets comprising
a) from 5 to 85 % by weight as dry matter of a carboxymethyl
cellulose (CMC),
b) from 5 to 85 % by weight as dry matter of at least another
ceramic glaze additive,
in which the sum of a) + b) represents at least 40% by weight of
their dry matter, for the preparation of ceramic glaze slips, the
pellets being used in an amount comprised between 0.05% and 5%
by weight based on the weight of the ceramic glaze slip.
2) The use of Claim 1) wherein the extruded pellets comprise:
a) from 20 to 75 % by weight as dry matter of a carboxymethyl
cellulose;
b) from 20 to 75 % by weight as dry matter of at least another
ceramic glaze additive.
3) The use of Claim 1) wherein the carboxymethyl cellulose a) has a
degree of substitution comprised between 0.5 and 1.5 and a
Brookfield LVT® viscosity, at 2 wt% in water, 60 rpm and 20 °C,
from 5 to 30,000 mPa*s.
4) The use of Claim 3) wherein the carboxymethyl cellulose a) has a
degree of substitution comprised between 0.6 and 1.2 and a
Brookfield LVT® viscosity, at 2 wt% in water, 60 rpm and 20 °C,
from 10 to 15,000 mPa*s.
5) The use of Claim 1) wherein said ceramic additives b) are chosen in
the group consisting of suspending agents, thickening agents
different from CMC, preservatives, biocides, antifoams, dispersants,
binders, deflocculants, levelling agents, plasticizers, de-airing agents
and mixture thereof.
6) The use of Claim 5) wherein said suspending agents are waterswellabte
clays chosen among bentonite, montmorillonite, kaolinite,
hectorite, attapulgite and smectite and mixture thereof.
7) The use of Claim 5) wherein said thickening agents different from
CMC are chosen among natural water-soluble polymers as such or
derivatized, cellulose derivatives different from CMC, synthetic
polymers and mixture thereof.
8) The use of Claim 5) wherein said dispersants are chosen among
water-soluble salts of low/medium molecular weight acrylic
(copolymers, polyphosphates, humic acids, lignin sulfonates,
sodium silicates, sodium carbonate and mixture thereof.
9) The use of Claim 5) wherein said biocides are chosen among
isothiazolin-3-one derivatives, o-phenyl phenol derivatives, p-chiorom-
cresol, pirithione salts, parabens, formaldehyde releasers, and
mixture thereof.
10) The use of Claim 5) wherein said antifoams and/or a de-airing
agents are chosen among aluminum stearate, ethylene/propylene
oxide copolymers, polydimethyl siloxane, colloidal silica, mineral oils
and mixture thereof.
11) The use of Claim 5) wherein said binders are chosen among
polyvinyl alcohols, polyvinyl acetate or partially hydrolyzed polyvinyl
acetate, anionic polyacrilates or polyacrylamides, polyurethanes,
styrene/butadiene resins and mixture thereof .
12) The use of Claim 5) wherein said plasticizers are chosen among
glycerol; sorbitol; glycols, (co)polymer ethylene oxide/propylene
oxide; fatty acids or fatty amides; alkanol amines, fatty acid
monoesters of glycerol or glycols, mono- or di-esters of phthalic acid
and mixture thereof.