Liquid scouring cleaning compositions containing cristobalite

Liquid scouring cleaning compositions containing 30% to 60% by weight of cristobalite having a grain size of less than 150 .mu., a surfactant, a thickening agent, and a liquid carrier, which compositions are white, stable suspensions.

THE RELATED ART 
Cleaning agents which have a scouring action, and which are used for the 
cleaning of very dirty, hard surfaces, contain as an essential constituent 
a water-insoluble mechanically-cleansing abrasive component in admixture 
with a component which has a cleaning action in aqueous solution. The 
scouring effect of this abrasive component is chiefly determined by the 
hardness of the material and the size of grain. Thus, pulverized minerals, 
such as mineral meal of marble, dolomite, feldspars and calcspars, as well 
as quartz sand are used for scouring agents having a particularly 
effective cleaning action. These minerals have a more or less pronounced 
inherent color, so that the products produced with these minerals have a 
usually undesirable grey or brown tint. The low degree of whiteness of 
such products generally deteriorates when these products are suspended in 
water. In the case of liquid products, this phenomenon causes an 
unsightly, dirty appearance which gives the user the impression of reduced 
cleaning power. This disadvantage cannot even be avoided by using dyestuff 
additives, since this usually results in undesirable mixed colors and not 
in clear tints. 
It is also known to improve the low degree of whiteness of these scouring 
agents by the use of additives, such as optical brighteners or so-called 
white pigments such as titanium dioxide, zinc oxide or zinc carbonate. 
However, this measure has proved to be unsatisfactory and also leads to a 
considerable increase in the cost of the final product. 
A further disadvantage of the natural mineral powders or meal, which 
combine great mineral hardness with a good scouring effect, is their high 
specific gravity which impairs the suspension stability and thus the 
storage stability of liquid products. 
OBJECTS OF THE INVENTION 
An object of the present invention is the development of liquid scouring 
cleaning compositions which are white, stable suspensions, thus overcoming 
the disadvantages of the previously known compounds. 
Another object of the present invention is the development of a liquid 
scouring cleaning composition consisting essentially of: 
(a) from 30% to 60% by weight of cristobalite having a grain size of less 
than 150 .mu., 
(b) from 1% to 10% by weight of at least one surfactant selected from the 
group consisting of anionic surface-active compounds, zwitterionic 
surface-active compounds and non-ionic surface-active compounds, 
(c) from 0.1% to 2% by weight of an organic thickening agent, and 
(d) from 27% to 68.9% by weight of a predominantly aqueous liquid carrier. 
These and other objects of the invention will become more apparent as the 
description thereof proceeds. 
DESCRIPTION OF THE INVENTION 
We have now found that cristobalite, a modification of silicon dioxide, is, 
in a finely distributed form, eminently suitable as an abrasive component 
for liquid cleaning compositions having a scouring action. In addition to 
having the high degree of hardness of 6.5 in accordance with Mohs' scale 
of hardness, which is required for a satisfactory cleaning effect, 
cristobalite also has a very high degree of whiteness which is largely 
maintained even when it is suspended in water. Furthermore, cristobalite 
has a lower specific gravity than the conventional mineral powders used as 
abrasive substance. By way of example, compared with quartz, whose 
specific gravity is 2.65, or compared with fluorite, whose specific 
gravity is 3.18, the use of finely distributed cristobalite having the 
specific gravity of 2.32 renders it possible to produce pourable liquid 
products which have a distinctly greater suspension stability. 
The cristobalite used in the cleaning compositions in accordance with the 
invention is produced in a high temperature process by roasting quartz 
sand at temperatures in excess of 1200.degree. C and by the iron-free 
grinding of the cristobalite sand, thus obtained, into a powder having the 
desired granular size. The cristobalite used in accordance with the 
invention is, crystallographically, the tetragonally trapezohedral crystal 
modification of silicic acid which is designated low-cristobalite or 
.beta.-cristobalite. 
The liquid cleaning compositions in accordance with the invention are 
products based on mixtures of water, a surfactant component and a 
thickening agent together with the abrasive substance and, if required, 
other additives. The compositions of the invention comprise: 
30% to 60% by weight of cristobalite having a grain size less than 150 
.mu., 
1% to 10% by weight of a surfactant component comprising anionic and/or 
zwitterionic and/or non-ionic surfactants, 
0.1% to 2% by weight of a thickening agent, and 
27% to 68.9% by weight of a liquid carrier, preferably water. 
More particularly, the present invention relates to a liquid scouring 
cleaning composition consisting essentially of: 
(a) from 30% to 60% by weight of cristobalite having a grain size of less 
than 150 .mu., 
(b) from 1% to 10% by weight of at least one surfactant selected from the 
group consisting of anionic surface-active compounds, zwitterionic 
surface-active compounds and non-ionic surface-active compounds, 
(c) from 0.1% to 2% by weight of an organic thickening agent, and 
(d) from 27% to 68.9% by weight of a predominantly aqueous liquid carrier. 
Preferably, the surfactant component predominantly comprises non-ionic, 
surface-active compounds or tensides. Embodiments which are particularly 
preferred are those having a surfactant component which is composed 
entirely of non-ionic tensides. The content of thickening agent can be 
kept low in such compositions. 
The term "liquid scouring cleaning compositions" refers to those liquid 
compositions which range from easily pourable to highly viscose and pasty 
cleaning agents. Those which are particularly preferred are the pourable 
liquid agents having a viscosity of 1000 to 8000, particularly 2000 to 
5000, cP (measured with a Brookfield rotary viscosimeter at 20 r.p.m. and 
room temperature). 
In addition to water, suitable liquid carrier substances are water-soluble 
organic solvents such as alkanols containing 1 to 4 carbon atoms, or 
alkanepolyols containing 2 to 4 carbon atoms, in quantities less than 
those of the water. By adding these organic solvents, one frequently 
obtains a substantial increase in the solubility of a portion of the 
water-soluble constituents, particularly the solubility of the 
surfactants. It is usually sufficient for the said solvents to replace 
from 1% to 20% by weight of the water used. 
The predominantly aqueous liquid carrier present in an amount of from 27% 
to 68.9% by weight therefore can consist of 80% to 100% by weight of water 
and from 0 to 20% by weight of a water-soluble organic solvent, both based 
on the total aqueous liquid carrier. 
Finely distributed cristobalite, suitable for the use in accordance with 
the invention, is in the form of cristobalite powder whose grain size is 
substantially less than 150 .mu.. This is obtained by iron-free grinding 
and multi-stage sifting to obtain such a product, so that at least 95% of 
the cristobalite particles pass through a sieve having an inside mesh size 
of 150 .mu.. Cristobalite powders which are particularly preferred are 
those which are substantially finer than 100 .mu., in grain size, i.e., 
those of which at least 99.5% pass through a sieve having an inside mesh 
size of 105 .mu.. There is no lower limit to the grain size. Of course, a 
substantial proportion of particles having a grain size of less than 5 
.mu. is not particularly desirable, since such extremely fine particles 
are too small to have an adequate scouring action. Preferably the grain 
size of the cristobalite is from 5 .mu. to 150 .mu.. 
The cleaning agents in accordance with the invention can also optionally 
contain up to 5% by weight of conventional additives from the group of 
water-soluble inorganic and organic bases, particularly alkali metal 
hydroxides, foam stabilizers, preservatives, antimicrobials, perfumes and 
dyes, as well as soluble inorganic alkaline reacting alkali metal salts, 
such as sodium carbonate, sodium bicarbonate, sodium sulfate, sodium 
silicate, borax and/or organic complex-forming acids, such as citric acid, 
gluconic acid, nitrilotriacetic acid, ethylenediaminetetraacetic acid, 
etc. and alkali metal salts thereof, which assist the cleaning action by 
the dissolving of metal oxide coatings. 
The surfactant component to be used in accordance with the invention can 
comprise anionic, zwitterionic or non-ionic surface-active compounds or 
tensides and mixtures thereof. In general, the surfactant component 
contains at least one non-ionic tenside in amounts of from 50% to 100% of 
the surfactant component. In the present context, the term "anionic 
tensides" primarily refers to the synthetic sulfonate and sulfate 
surface-active compounds in addition to the alkali metal soaps. The 
zwitterionic surface-active compounds primarily involve betaines, i.e., 
derivatives of aliphatic quaternary ammonium compounds. The non-ionic 
surface-active compounds are primarily the polyglycol ethers and preferred 
are the ethoxylated C.sub.8 -C.sub.20 alkanols of all types, ethoxylated 
C.sub.4 -C.sub.12 alkylphenols and the fatty acid alkylolamides. In a 
wider sense, the anionic, zwitterionic and non-ionic tensides are suitable 
to the extent to which they will be further discussed in the description 
of the invention. 
In the pourable liquid cleaning agents, particularly preferred within the 
scope of the present invention, the amount of the water-insoluble 
component having a mechanical cleaning action, preferably entirely 
comprising cristobalite having a grain size of less than 100 .mu., is 
preferably 35% to 50% by weight of the total preparation. These agents 
comprise a stable suspension of the cristobalite in an aqueous solution 
which contains 1% to 6% by weight of a surfactant component which 
preferably comprises only non-ionic surface-active compounds, and 0.1% to 
1.5% by weight of a thickening agent, wherein the amount of water in the 
suspension is at least 40% by weight of the total agent. Compositions 
within this range exhibit optimum cleaning power and suspension stability. 
The preferred composition of the invention, therefore, relates to a liquid 
scouring agent composition consisting essentially of: 
(a) from 35% to 50% by weight of cristobalite having a grain size of less 
than 100 .mu. and over 5 .mu., 
(b) from 1% to 6% by weight of at least one non-ionic surface-active 
compound, 
(c) from 0.1% to 1.5% by weight of a water-soluble polycarboxylate 
polymeric thickening agent, 
(d) from 40% to 63.9% by weight of water, and 
(e) from 0 to 20% by weight of a water-soluble organic solvent. 
Particularly suspension-stable liquid preparations contain a water-soluble 
polycarboxylate polymeric compound as the thickening agent. 
This polymeric polycarboxylate acting as a thickening agent is present as 
an alkali salt and/or as ammonium or substituted ammonium salt in the 
liquid agents. In the present context, the term "substituted ammonium" 
refers to the salt form of primary, secondary or tertiary alkylamines 
having a maximum of 2 carbon atoms per alkyl radical and a maximum of 3 
carbon atoms per alkylol radical, respectively. These alkylamines and 
alkylolamines are for example, methylamine, dimethylamine, ethylamine, 
diethylamine, trimethylamine, triethylamine, ethanolamine, diethanolamine, 
triethanolamine, 2-hydroxypropylamine, bis-2-hydroxypropylamine, 
2,3-dihydroxypropylamine or bis-2,3-dihydroxypropylamine, etc. Preferred 
substituted ammonium salts are di- and triethanolammonium salts. 
The water-soluble polycarboxylates used, in accordance with the invention, 
as thickening agents are polymers based on acrylic and/or methacrylic 
acid. In addition to the acryl or methacryl monomer units, the copolymers 
contain preferably differing proportions of monomer units selected from 
the group consisting of acrylic acid esters or methacrylic acid esters of 
the lower, particularly C.sub.1 -C.sub.3 alkanols, acrylamide or 
methacrylamides, maleic acid or maleic acid anhydride, styrene, 
vinyl-lower-alkyl ethers, such as vinylmethyl ether, and the polyallyl 
ethers of sugar. These mixed polymerizates can be present in the form of 
two-component or three-component copolymers. Preferably the water-soluble 
polycarboxylate polymers employed are those which contain, in addition to 
the acryl and/or methacryl monomer units up to 75 mol percent of monomer 
units of a methyl, ethyl or propyl ester of acrylic acid and/or 
methacrylic acid, as well as copolymers from acrylic acid and polyallyl 
polysugar ether monomer units, the latter amount to 0.2% to 2.5% by weight 
of the polymer. 
Adequate solubility in aqueous alkalis is obtained in the copolymers of 
acrylic acid or methacrylic acid with the lower alkyl esters of these 
acids when the content of the acryl or methacryl monomer units in the 
copolymer amounts to at least 25 mol percent. Preferably, this content is 
at least 30 mol percent, whereby these preferred copolymers can also 
contain styrene monomer units. Suitable copolymers of this type comprise, 
for example, methacrylic acid and methyl methacrylate in the molar ratio 
2:1, or methacrylic acid and ethyl acrylate in the ratio of 1:2. An 
example of a product of the thickening agents, based on acrylic acid and 
polyallyl sugar ethers, is that copolymer which has approximately 1% of 
polyallylsucrose having an average of 5.8 allyl groups with acrylic acid. 
The preferred thickening agents are those copolymers of the specified 
composition which, owing to their structure and their degree of 
polymerization, have a viscosity of at least 500 cP, particularly of from 
1000 to 30,000 cP, when measured in a 3% aqueous solution at a pH of 7 to 
9 and at 20.degree. C. Such products are commercially available. 
The liquid, pourable cleaning agents contain water-soluble inorganic and 
organic bases in accordance with the above definition for the purpose of 
salt formation and thus for rendering the thickening agents, used in 
accordance with the invention, water-soluble. These bases, such as the 
alkali metal hydroxides, particularly sodium and potassium hydroxide, 
and/or alkylamines or alkylolamines, such as mono, di and triethanolamine, 
may be present in the liquid preparations in super-stoichiometric amounts 
relative to the amount necessary to form the corresponding salts with the 
copolymers, whereby they contribute to the increase in the alkalinity and 
thus to assisting the cleaning action of the preparation. An optimum 
cleaning action, suspension stability and satisfactory skin compatibility 
is obtained when the amounts of the bases are proportioned in sufficient 
excess, such that the liquid preparations have a pH value of from 9 to 11. 
The surface-active compounds or tensides incorporated in the cleaning 
agents in accordance with the invention contain, in the molecule, at least 
one hydrophobic organic moiety and one anionic, zwitterionic or non-ionic 
group rendering the cleaning agents soluble in water. The hydrophobic 
moiety is usually an aliphatic hydrocarbon moiety having 8 to 26, 
preferably 10 to 22, and particularly 12 to 18 carbon atoms, or an 
alkylphenyl radical having 6 to 18, preferably 8 to 16, aliphatic carbon 
atoms. 
By way of example, soaps from natural or synthetic, preferably saturated 
fatty acids and, optionally, from resinic or naphthenic acids are suitable 
as anionic tensides. Suitable synthetic anionic tensides are those of the 
type of sulfonates, sulfates and synthetic carboxylates. 
Suitable tensides of the sulfonate type are alkylbenzene sulfonates 
(C.sub.9-15 -alkyl), mixtures of alkene sulfonates and hydroxyalkane 
sulfonates, as well as alkane disulfonates which are obtained, for 
example, from monoolefins having 10 to 22 carbon atoms with either a 
terminal or non-terminal double bond by sulfonation with gaseous sulfur 
trioxide and subsequent alkaline or acid hydrolysis of the products of 
sulfonation. Alkane sulfonates having 10 to 22 carbon atoms are also 
suitable which are obtainable from alkanes by sulfochlorination or 
sulfoxidation and subsequent hydrolysis or neutralization or by bisulfite 
addition to corresponding olefins. Further usable tensides of the 
sulfonate type are the lower alkyl esters of .alpha.-sulfo fatty acids, 
such as .alpha.-sulfonic acids of methyl or ethyl esters of hydrogenated 
coconut fatty acids, hydrogenated palm kernel fatty acids or hydrogenated 
tallow fatty acids. 
Suitable tensides of the sulfate type are the sulfuric acid monoesters of 
primary alkanols (for example, from coconut fatty alcohols, tallow fatty 
alcohols) or primary alkenols (for example, from oleyl alcohol) and those 
of secondary alkanols having 10 to 22 carbon atoms. Also suitable are 
sulfated higher fatty acid alkanolamides, sulfated higher fatty acid 
monoglycerides or sulfated products of the reaction of 1 to 4 mols of 
ethylene oxide with primary or secondary fatty higher alcohols or C.sub.8 
-C.sub.16 alkylphenols. 
Further suitable anionic tensides are the fatty acid esters and amides of 
hydroxycarboxylic acids or aminocarboxylic acids or hydroxysulfonic acids 
or aminosulfonic acids, such as fatty acid sarcosides, fatty acid 
glycolates, fatty acid lactates, fatty acid taurides or fatty acid 
isethionates. 
The anionic tensides may be present in the form of the alkali metal salts, 
such as the sodium or potassium salts, as well as the ammonium salts 
thereof, as well as water-soluble salts of organic bases, for example, 
alkylamines and alkylolamines discussed above, such as mono, di or 
triethanolamine. 
The non-ionic surface-active compounds or tensides are preferably 
polyglycol ethers of hydrophobic aliphatic hydrocarbon moieties as 
discussed above. The products of addition of 4 to 40, preferably 4 to 20, 
mols of ethylene oxide to 1 mol of a fatty alcohol, an alkylphenol, a 
fatty acid, a fatty amine, a fatty acid amide, or an alkanesulfonamide may 
be used as non-ionic tensides. Particular importance is attached to the 
products of addition of 5 to 16 mols of ethylene oxide to coconut or 
tallow fatty alcohols, to oleyl alcohol or to secondary alkanols having 8 
to 18, preferably 12 to 18 carbon atoms, and to mono or dialkylphenols 
having 6 to 14 carbon atoms in the alkyl radicals. 
However, in addition to these water-soluble non-ionics, polyglycol ethers 
which are not water-soluble, or which are not fully water-soluble, and 
which have a 1 to 4 ethylene glycol ether radicals in the molecule, are of 
interest, particularly when they are used together with water-soluble 
non-ionic or anionic tensides. Also usable as non-ionic tensides are the 
water-soluble block polymers of ethylene oxide and propylene oxide, such 
as the products of addition, containing 20 to 250 ethylene glycol ether 
groups and 10 to 100 propylene glycol ether groups, of ethylene oxide to 
polypropyleneglycol, to alkylenediaminepolypropyleneglycol and to 
alkylpolypropyleneglycols having 1 to 10 carbon atoms in the alkyl chain, 
in which the polypropyleneglycol chain acts as a hydrophobic radical. 
The non-ionic tensides which may be used also include the fatty acid 
alkylolamides having one or two hydroxyethyl or hydroxypropyl groups, such 
as coconut and tallow fatty acid ethanolamide and diethanolamide, oleic 
acid diethanolamide, etc., and compounds which are derived from other 
polyhydroxyalkylamines, such as glycamines. 
Suitable non-ionic tensides are also the surface-active amine oxides which 
are usually derived from tertiary amines having a hydrophobic C.sub.10 
-C.sub.20 -alkyl group and two shorter alkyl and/or alkylol groups each 
containing up to 4 carbon atoms. Typical representatives are, for example, 
the compounds: 
N-dodecyl-N,N-dimethylamine oxide 
N-tetradecyl-N,N-dihydroxyethylamine oxide 
N-hexadecyl-N,N-bis-(2,3-dihydroxypropyl)-amine oxide. 
Suitable zwitterionic surface-active compounds or tensides contain in the 
molecule, in addition to a hydrophobic, usually aliphatic group, both 
hydrophilic acid groups, such as carboxyl, sulfo, sulfuric acid 
semi-ester, phosphono, or phosphoric acid partial ester groups, and basic 
hydrophilic groups, such as amino, imino or ammonium groupings. 
Zwitterionic compounds having a fourfold substitution, i.e., quaternary 
ammonium groups, include the betaine type when they also have in the 
molecule a covalent bound acid group and the positive and negative charge 
is balanced intramolecularly. In a wider sense, the betaine class also 
includes the correspondingly substituted quaternary phosphonium and 
tertiary sulfonium compounds. 
Owing to their good cleaning properties and their good compatibility with 
other tensides, particular importance is attached to the carboxy betaines, 
sulfonate betaines and sulfate betaines with nitrogen. Typical 
representatives of zwitterionic tensides are, for example, the compounds: 
3-(N-hexadecyl-N,N-dimethyl-ammonio)-propane sulfonate 
3-(N-hexadecyl-N,N-bis-[2-hydroxyethyl]-ammonio)-2-hydroxypropyl sulfate 
3-(N-coconut alkyl-N,N-bis-[2,3-dihydroxypropyl]-ammonio)-propane sulfonate 
N-tetradecyl-N,N-dimethyl-ammonio-acetate. 
In less preferred embodiments, the abrasive component, used in accordance 
with the invention and comprising fine-particulate cristobalite, can be 
partially replaced, i.e., approximately up to half the quantity, by other 
conventional water-insoluble finely-ground minerals which have a 
mechanical cleaning action, such as quartz, feldspar, marble, fluorite, 
kaoline or pumice. Alternatively, these inorganic abrasive substances can 
be replaced by subordinate quantities of finely-ground water-insoluble 
organic synthetic polymers, such as polyethylene and polypropylene powder 
or the mineral abrasive substances coated with a film of synthetic resin. 
The advantages of cristobalite are still clearly shown even when using 
such mixtures of abrasive substances where the cristobalite amounts to 
60%, preferably 75% to 100%, by weight of the abrasive component.

The present invention will now be further described by means of the 
following examples. These examples are not limitative in any manner. 
EXAMPLES 
The compositions and the methods of producing some liquid cleaning agents 
will be specified in the following examples for the purpose of explaining 
the invention. The viscosities were measured in a Brookfield viscosimeter, 
Model RVT, spindle number 4, at 20 r.p.m. and 20.degree. C. 
EXAMPLE 1 
This example describes a storage-stable, white, homogeneous suspension 
produced by mixing the individual components, wherein, advantageously, a 
specific mixing sequence was observed. This composition was: 
______________________________________ 
Percent 
by Weight 
______________________________________ 
3.0 Coconut fatty alcohol (C.sub.16 -C.sub.18) + 10 EO 
(EO = ethylene oxide) 
0.45 Copolymer of 1 mol of methacrylic acid 
and 2 mols of ethyl acrylate (viscosity 
of the 1% solution in water at 20.degree. C and 
pH 9: 5,500 to 14,000 cP) 
1.5 Diethanolamine 
50.0 Cristobalite powder, grain size up to 
a maximum of 100 .mu. 
Remainder - demineralized water. 
______________________________________ 
In order to produce the agent, the water was introduced, less the amount 
required to produce a 30% dispersion of the copolymer, to the mixing 
vessel. The non-ionic tenside and the organic base were then introduced 
under agitation. The cristobalite powder was added to this mixture. The 
thickening agent was then added slowly under agitation. The mixture became 
increasingly thick. The viscosity of the product was 5,000 cP; pH 10 to 
10.5. 
EXAMPLE 2 
A white, homogeneous, storage-stable suspension was obtained by mixing the 
individual components by the method described in Example 1: 
______________________________________ 
Percent 
by Weight 
______________________________________ 
4.0 Nonylphenol + 9.5 EO 
0.27 Copolymer of Example 1 
0.9 Diethanolamine 
0.2 Formalin (as 30% solution) 
0.2 Perfume 
30.0 Cristobalite powder, grain size up to 
a maximum of 100 .mu. 
20.0 Cristobalite powder, grain size up to a 
maximum of 150 .mu. 
Remainder - demineralized water. 
______________________________________ 
Viscosity of the product 3,300 cP; pH 10 to 10.5. 
EXAMPLE 3 
A white, homogeneous, storage-stable suspension of the following 
composition was obtained by mixing the constituents in the manner 
specified below: 
______________________________________ 
Percent 
by Weight 
______________________________________ 
2.0 Nonylphenol + 9.5 EO 
0.05 Sodium hydroxide 
1.0 A polymer from 99% by weight of acrylic 
acid and -% by weight of allylsucrose 
(average 5.8 allyl groups per molecule), 
molecular weight approximately 1,000,000 
45.0 Cristobalite powder, grain size up to 
a maximum of 100 .mu. 
0.2 Perfume 
Remainder - demineralized water. 
______________________________________ 
For the purpose of production, the tenside was first dissolved in the water 
after holding back the quantity of water required to produce a 10% caustic 
soda solution. The thickening agent was introduced under agitation into 
the tenside solution, and the caustic soda solution was subsequently 
added. A clear solution was then formed having an increased viscosity. 
After introducing the perfume, the cristobalite powder was added and 
agitation was effected until an homogeneous distribution was obtained. 
If the cristobalite powder, used in accordance with the invention, is 
replaced in the formulations of the specified examples by a marble powder 
having a grain size distribution up to a maximum of 140 .mu., in order to 
ensure a comparable cleaning action of the product owing to the inferior 
hardness of the marble (hardness 3 of the Mohs' scale), these preparations 
exhibit, even after a short period of storage at room temperature, 
sedimentation of the coarser portions of the abrasive substance of the 
marble with the formation of a sediment which, finally, cannot be shaken 
up. The preparations in accordance with the invention meanwhile remain 
fully suspension-stable for this period of time. Replacing the 
cristobalite powder by quartz powder results in products which have a 
dirty, greyish-brown appearance irrespective of the composition of 
surfactant component, while the products in accordance with the invention 
have a pure white appearance. 
The preceding specific embodiments are illustrative of the practice of the 
invention. It is to be understood, however, that other expedients known to 
those skilled in the art or disclosed herein, may be employed without 
departing from the spirit of the invention or the scope of the appended 
claims.