Note: Descriptions are shown in the official language in which they were submitted.
2150856
A sawin~ suspension
The present invention relates to a sawing suspension, consisting essentially of an abrasive
substance in a liquid aqueous phase.
It is already known that inorganic solids can be sawn with a multiple wire saw by using
an oil slurry. In this case, the oil slurry which contains an abrasive substance is drawn
along by a sawing wire and thus saws up the solid being divided into sections. An
application example is, inter alia, the division of silicon blocks into individual silicon
10 discs for use in the photovoltaic field. Alternative methods according to the prior art
include slurry baths in which the solid being sawn is surrounded by the suspension during
the sawing process.
The normally used sawing slurries consist of a mixture of oil and a powdered hard
15 substance. Mixing ratios of oil to hard substance (l/kg) of 0.7 to l.l, for example, are
used. The oil has a kinematic viscosity of ca. 32 mm2/s (40C) and the hard substance
has a grain size of about S-20 ~lm. During the cutting procedure, the slurry is circulated
in a cycle, i.e. from storage vessel through a cooler, through nozzles and back to the
storage vessel. The slurry can be used for about 2-5 saw-cuts.
There are many disadvantages when using oil-bonded slurries. The sawn discs are
difficult to separate from each other due to the intense wetting and smearing of the discs
by oil and can only be cleansed by using so-called cold cleansers and large amounts of
surfactants.
- The organic compounds in oils and cold cleansers can only be disposed of at great
expense and in an ecologically unacceptable manner. Furthermore processing the oil
slurry in order to re-use the oil and hard substance is difficult and technically
complicated. The high costs of using oils and cleansing agents and of disposal of and
3 0 recycling the substances need to be reduced.
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It is also known, from EP-A 84 966 that inorganic solids can be sawn using "overbased
materials" or metal colloids in an aqueous dispersion using organic solvents and/or oils.
In order to prepare these slurries, the colloidal particles used must first be prepared using
a complicated reaction. The disadvantages described for oil slurries are still not
5 sufficiently elimin~ted by means of this combination.
The object of this invention is, therefore, the provision of a sawing slurry which does not
have the disadvantages described for the prior art.
10 Surprisingly, it was found that an aqueous suspension which contains no or only
negligible proportions of organic solvents and oils, in which an abrasive substance is
suspended in an aqueous phase which contains a thickener, e.g. water-soluble polymers,
can be used for sawing inorganic solids.
15 Thus, the invention provides a sawing suspension consisting essentially of an abrasive
substance in a liquid aqueous phase, wherein the aqueous phase contains one or more
water-soluble polymers. The sawing suspension according to the invention preferably has
an oil content of less than O.Ol wt.%.
2 0 The water-soluble polymer is preferably one, several and/or one or more copolymers from
the group of polyoxides, polyethers, polyethylenimines, polycarboxylates, modified
celluloses, polyvinylalcohols, polyvinylacetates, polyvinylpyrrolidones,
polyvinyloxazolidones, polyethylenesulphonic acids, polystyrenesulphonic acids,
polyphosphoric acids, polysilicy acids, polyvinylamines, polyvinylpyridines,
25 polyvinylsulphuric acids, polydiallyldimethylammonium chlorides, polybrenes
[= poly-(dimethylimino)-trimethylene-(dimethylimino)-hexamethylene dibromides],
polyvinylphosphonic acids, polyethylenephosphonic acids, polymaleic acids, poly(2-
methacryloyloxyethane-l-sulphonic acids).
30 It should be present in the aqueous phase in amounts such that the viscosity of the
aqueous phase is between lO and lOOO mPas, measured at a shear rate of lO s~'. By
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2150856
adding acids or bases, e.g. hydrochloric acid or caustic soda solution, and using specific
polymers, e.g. those with weakly acidic groups, the sedimentation behaviour of the
abrasive substance in the slurry can be adjusted by varying the pH. The polymersaccording to the invention preferably contain weakly acidic groups. Polyacrylic acids
5 and/or polyacrylates with average molecular weights of lOO 000 to 2 000 000 are
particularly preferred water-soluble polymers.
The water-soluble polymers act as thickeners in the context of this invention. This effect
can also be achieved by the presence of polyhydric alcohols and/or inorganic solids such
10 as, for instance, silica sol.
Good sawing powers are achieved when the sawing suspension has a pH higher than 6.
Furthermore, the sawing power of the sawing suspension according to the invention can
be improved by the presence of surfactants. In this case, the surface tension of the
15 suspension should advantageously be less than 40 mN/m.
Finally, corrosion of the sawing wire and of metal-containing components in the saw can
be reduced by the
2 0 presence of an anti-corrosion agent, which in addition also has a positive effect on the
sawing power.
The inorganic hard substance in the context of this invention is preferably a powder from
the group of metal or metalloid carbides, nitrides, carbonitrides, oxinitrides, oxides,
25 borides, hard metals, diamond or mixtures thereof, wherein the metal or metalloid
elements are from groups 3A, 4A, SA, 3B, 4B, 5B, 6B, 7B or 8B in the periodic system.
Aluminium oxide, boron carbide, iron oxide, titanium carbide, zirconium oxide, titanium
nitride, silicon nitride and/or silicon dioxide are particularly preferred.
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2150856
,
Diamond and/or silicon carbide are particularly suitable as a hard substance for sawing
silicon. In this case, the hard substance preferably has an average particle size of 0.2 to
100 llm.
5 The sawing slurry can be produced by simply dispersing the abrasive solid in the aqueous
phase with the desired composition.
By using, for example, polymers with weakly acidic groups, the aqueous sawing
suspension can be recycled without using a complicated procedure since, after altering
10 the pH, the viscosity is lowered, the sedimentation time is reduced and the abrasive solid
can rapidly be separated. The viscosity and possibly the surface tension or the
concentration of anti-corrosion agent are then readjusted to their initial values.
The solid can be cleansed by simply rinsing with water. The amount of waste consisting
15 of abrasive material, rubbings and a small proportion of the aqueous phase, including
added components, is reduced. Pollution of the environment is thus reduced in
comparison to known processes. The use of this type of aqueous suspension leads to a
reduction in the costs of the sawing process because both the cleansing costs and the
waste disposal costs incurred are higher in the two processes described above. In
2 0 addition, the normally used oils plus abrasive at current prices are more expensive than
the aqueous sawing slurry.
The sawing suspension is produced, for example, as follows. Water is initially introduced
and then the desired amount of polymer is added. The pH is adjusted with acid or base.
25 The viscosity of this solution is between lO and lO00 mPas at a shear rate of lO s-l. A
specific amount of anti-corrosion agent is added and the pH is readjusted with acid or
base. In addition surfactant is added until the surface tension is at most 40 mN/m,
preferably at most 25 mN/m, and the pH is once again checked and readjusted.
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Cationic, anionic or non-ionic surfactants may be used. Which charge sign preferably has
to be carried by the surfactant depends on the hard material, polymer and inorganic
thickener used.
5 Polymers and surfactants are preferably used which have the same charge sign as the
abrasive in the aqueous slurry. The charge sign on the abrasive material can be obtained
by determining the æta potential. This æta potential can be determined by using known
techniques such as micro-electrophoresis, streaming potential measurements or
acoustophoresis.
To do this, the abrasive substance is dispersed in water and the æta potential is
determined at the pH to which the slurry has been adjusted, using one of the methods
mentioned. For abrasives which have a negative æta potential, anionic polymers and
surfactants are preferably used and for abrasives which have a positive æta potential,
15 cationic polymers and surfactants are preferably used.
When using inorganic thickeners, it is also advantageous if these have the same charge
sign as the æta potential of the abrasive substance.
20 Solids which have been cut up using the sawing suspension according to the invention
are cleansed by rinsing with water. The amount of waste consisting of abrasive material,
rubbings and a small proportion of the aqueous phase, including added components, is
reduced. Pollution of the environment is thus reduced in comparison to known processes.
2 5 The sawing suspension according to the invention can completely replace the oil slurries
described. It has to be called surprising that an aqueous suspension can achieve the same
sawing power as a suspension prepared using a mineral oil base.
The sawing power was checked using a filament saw Type H7 from Lipoldt KG,
3 0 Freiburg. The solid being sawn was sprayed with slurry via two nozzles. The measure for
sawing power was defined as the time to saw a solid of a specific siæ into two pieces
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with constant settings for the equipment. In addition, the surface profile was determined
by means of a roughness measurement. The surface roughnesses of the sawn solids are
comparable with the aqueous and oil slurries.
5 The extremely small degree of corrosion of the sawing wire and of cast iron at pHs above
8 and using an anti-corrosion agent in the aqueous suspension, in accordance with the
present invention, was surprising.
Slurry according to the invention has, in general, the following effect. It wets the
10 interface being sawn and the sawing wire or sawing blade. The slurry acts as abrasive
and cooling agent between wire or blade and the inorganic solid. Obviously, the entire
sawing process can also be performed completely in the suspension.
The suspension according to the invention enables the sawing of inorganic materials and
15 has the following particular advantages in comparison with the (;ullenlly generally utilised
oil slurry processes:
Removal of suspension residues from solids is performed in a less complicated manner
(solvents and/or cleansing solutions are not used). Cleansing with water is sufficient.
Less waste which has to be disposed of is produced because the aqueous phase can be
re-used.
The suspension according to the invention is explained by way of example in the
25 following, without this being regarded as a limitation.
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Example 1
10 grams of polyacrylic acid (molecular weight = 450,000) was initially introduced and
dissolved in 990 grams of demineralised water with constant stirring. This solution was
5 adjusted to a pH of 10 using 45 wt.% strength LiOH. Then 20 g of Recor 190 (anorganic polycarboxylic acid neutralised with triethanolamine, commercial product from
Ciba-Geigy, Basle, Switzerland) were added and afterwards the pH was again adjusted
to 10. 10 g of dodecylbenzenesulphonate were added to this solution. The pH was again
held constant at 10. 1600 g of silicon carbide were added for each 1000 g of solution and
10 stirred in for 30 minutes using a laboratory stirrer at 2000 rpm. The sawing power was
62 % with respect to oil slurries.
The composition of further sawing slurries according to the invention are tabulated in the
following and the sawing powers produced are listed.
All the slurries contained 61.3 wt.% of silicon carbide with respect to the total slurry. The
pH was adjusted with lithium hydroxide. The oil slurry is cited as a comparison slurry.
The aqueous phase contained the following components in addition to water (the
20 concentrations are with respect to the aqueous phase):
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Polymer Surfactant Anti- pH Sawing
corrosion power
agent
Oil slurry Oil/viscosity= 55 100%
(Standard) mPas
Ex.2 1.1 wt% polyacrylic 0.5 wt.% 10 97%
acid mol wt. 450 000 tetraethylammonium
perfluorooctane-
sulphonate
Ex.3 20 wt.% polyacrylic 0.5% wt.% 10 104%
acid mol. wt. tetraethylammonium
250 000 perfluorooctane-
sulphonate
Ex.4 3 wt.% Viscalex HV 0.1% tetraethyl- 0.5 wt.% 10 102%
30 ammonium cinnamic
perfluoro- acid
octane-
sulphonate
Ex.5 1.63 wt.% Rheovis 0.1 wt.% tetraethyl- 2 wt.% 10 105%
CR ammonium Reocor
perfluoro- 190
octane-
sulphonate
Ex.6 4 wt.% Viscalex HV 2 wt.% 10 105%
Reocor
190
Ex.7 2 wt.% Rheovis CR 2 wt.% 10 106%
Reocor
190
Ex.8 3 wt.% Aerosil 0.5 wt.% tetraethyl- 10 102%
Mean particle ammonium
diameter is 7 nm perfluoro-
octane-
sulphonate
Ex.9 1 wt.% polyacrylic 10 54%
acid mol. wt.
450 000
Ex.10 5 wt.% polyvinyl- 10 50%
pyrrilidone mol wt.
1 250 000
23189-7804
21508S6
Reocor 190: An inorganic polycarboxylic acid neutralised with
triethanolamine from Ciba-Geigy, Basle, Switzerland.
Viscalex HV 30: An acrylic copolymer which contains carboxyl groups,
commercial product from Allied Colloids GmbH, Hamburg.
Rheovis CR: An acrylate associative thickener, commercial product from
Allied Colloids GmbH, Hamburg.
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