Indian Patents. 254722:FUMED SILICA

Title of Invention	FUMED SILICA
Abstract	Fumed silica having a BET surface area of 200 + 15 m2/g, a grindometer value of less than 30 µm and a median value by PCS of 65 to 85 nm is prepared by grinding a fumed silica having a surface area of 200 + 25 m2/g, and silicone rubber containing his silica.

Full Text	Fumed silica The invention relates to fumed silica, to a process for preparing it and to its use. Fumed silica (pyrogenically prepared silicon dioxide) is known from Ullmanns Enzyklopadie der technischen Chemie, Volume 21, page 4 64 (1982). It is prepared by burning a vaporizable silicon compound, such as silicon tetrachloride, for example, in a mixture with hydrogen and oxygen. The comminution of materials to form coarse powders (50-500 µm) , fine powders (5-50 µm) and even greater finenesses (less than 5 µm) is common and widespread practice. For all . comminution tasks there is a multiplicity of technical and industrial equipment offered and operated, all adapted to the particular circumstances of the specific tasks. A good overview of the comminution problems and of the diverse machines is given in Ullmanns Enzyklopadie der technischen Chemie, 3rd Edition, Volume 1, pages 616 to 638. For fumed silica the average primary particle diameters are notably lower (5-50 nm) than can be obtained by mechanical comminution. The primary particles and aggregates of the fumed silica with a surface area of 200 m2/g can be visualized in the electron microscope. The primary particles and aggregates of a fumed silica agglomerate to form larger assemblies whose size is generally in inverse proportion to the primary particle size or in proportion to the specific surface area. The agglomerate size also increases in line with the extent to which the fumed silica is compacted. The binding forces holding these agglomerates together are relatively weak. Nevertheless, when these agglome- rates are incorporated into and broken down in a liquid system for the purpose of homogeneous distribution of the primary particles and aggregates, or particles with a low degree of agglomeration, a certain shearing energy is required. Depending on the particular field of application, dispersing is carried out using any of a very wide variety of mixing devices, with determining factors for the selection being not only the viscosity and polarity of the system but also the agglomerate strength and the desired homogeneity. With simple agitator mechanisms, such as paddle stirrers, it is usually not possible to carry out to satisfaction the direct incorporation of small amounts of silicas, especially not when the systems in question are of low viscosity. However, paint and varnish makers, and also those who carry out processing, have an interest in obtaining performance-optimum distribu- tion of the silicas, which are used predominantly as thickeners and thixotropic agents, by means of very simple devices, with a very low energy input and in a very short time. In the case of paddle-stirrer dispersing, the coarse silica agglomerates are not sufficiently comminuted and hence are able to make only a small contribution to raising viscosities and thixotropy. The data relate to a UP resin (unsaturated polyester resin) as dispersion medium. Reducing the agglomerate size by dispersing outside a liquid system, in other words, in practice, in the air, or by grinding in the conventional sense, is possible only to a limited extent, since in the case of material with a given propensity to agglomerate the comminution is followed immediately by the re-establishment of the old agglomeration state. This effect comes about no later than after recompaction of the material which as a result of the mechanical intervention has undergone a high degree of loosening and which in this form is not amenable to dispatch and storage. The storage time as well would have the effect of renewed agglomerate enlargement. A value taken as a dimensional number and evaluation variable for the stage of distribution of a dispersible silica and maximum agglomerate size of the dispersion (granularity) is the so-called grindometer value to DIN 53203. A known procedure is to render fumed silica hydro- phobic, to grind it in a pinned-disc mill and then to classify it (US 2004/0110077 A1) . This known silica is used as an external additive in toner mixtures. Hydrophilic fumed silica with a BET surface area possesses a grindometer value as determined in UP resin (unsaturated polyester resin Ludepal P6 from BASF, 2% dispersion) in accordance with the DIN specification, of 50 to 60. If this fumed silica is also relatively highly com- pacted (100 to 120 g/l), the grindometer value as well is also significantly higher, specifically more than 100, whereby necessitating an additional, not inconsiderable energy as a thickener and thixotropic agent. A known procedure is to grind a highly dispersed silica having a surface area of approximately 300 m2/g in a pinned-disc mill. The grindometer value achieved is initially, for the uncompacted silica, 25. If this silica is compacted to 50 g/l, the grindometer value rises to 30, and in the case of further compac- tion to 75 g/l it rises to as far as about 40. In the case of storage over a period of three months, the ground silica, not modified but compacted to 50 g/l, has a grindometer value of 50 to 60. Reagglomeration can only be prevented in accordance with the prior art if the hydrophilic silica is mixed with 3% by weight of a hydrophobic silica and if this mixture is ground by means of an air-jet mill or a pinned-disc mill (EP 0 076 377 B1). In this case, for a fumed silica having a BET surface area of 200 m2/g, even after compaction to 73 or 107 g/l, a grindometer value of 35 is achieved. For a fumed silica having a BET surface area of 300 m2/g, the addition of hydrophobic silica prior to grinding produces a grindometer value of 10 for a tamped density of 28.1 g/l and of 15 to 20 for a tamped density of 50 g/l. The known fumed silicas have the disadvantage that they contain an unwanted fraction of hydrophobic silica. The invention provides a fumed silica which is characterized in that it has the following physico- chemical data: BET surface area 200 ± 15 m2/g Grindometer value less than 30 µm Median value PCS 65 to 85 nm The invention further provides a process for preparing the silica of the invention which is characterized in that a fumed silica having a BET surface area of 200 ± 25 m2/g is ground. In one preferred embodiment of the invention the grinding can take place by means of a pinned-disc mill or an air-jet mill. The fumed silica can have an average primary particle size of 12 nm ± 5 nm, preferably 12 nm. In a further embodiment of the invention a fumed silica can be used which has a tamped density of approximately 50 ± 10 g/l, preferably 50 m2/g. In one particular embodiment of the invention a fumed silica can be used which has the following physico- chemical data: 1) To DIN ISO 9277 2) To DIN EN ISO 787-11, JIS K 5101/20 (unsieved) 3) To DIN EN ISO 787-2, ASTM D 280, JIS K 5101/23 4) To DIN EN 3262-20, ASTM D 1208, JIS K 5101/24 5) Based on the substance dried at 105°C for 2 hours 6) To DIN EN ISO 787-9, ASTM D 1208, JIS K 5101/26 7) Based on the substance calcined at 1000°C for 2 hours 8) HC1 content in constituent from loss on ignition The Aerosil® 200 silica used in accordance with the invention, with a BET surface area, differs from the known Aerosil® 300, which has a surface area of 300 m2/g, not only in the surface size but also in the average primary particle size. The Aerosil® 300 has an average primary particle size of 7 nm. On the basis of these differences these silicas have different performance properties. The fumed silica of the invention contains no hydro- phobic fractions. In spite of this the silica of the invention shows no propensity to reagglomerate. The grindometer value is below 30. The fumed silica of the invention can be used as a filler in silicone rubber compounds. Silicone rubber compounds and the use of fumed silica (AEROSIL®) in silicone rubber compounds are known (Ullmann's Encyclopaedia of Industrial Chemistry, Volume A 23, Rubber, 1, 221 ff.; Rubber 3, 3,6 ff.; Volume A 24, Silicones 57 ff. 1993). Fumed silica is used on account of its excellent thickening effect (thixotroping) in silicone sealants, this thickening effect being desirable in the context of use as a jointing compound. Where, however, the silicone rubber compounds are to be used as coating materials, a low level of thickening is desired (US 6,268,300). Of decisive importance in both cases is the optical quality of the surface of the silicone vulcanizates. It is an object of the present invention, therefore, to provide silicone rubber compounds which through the use of fumed silica as filler exhibit an optically high- grade surface after vulcanization. The invention provides silicone rubber compounds containing 0.5% to 60% by weight, based on the total mass, of the fumed silica of the invention having the following characteristic physicochemical data: BET surface area 200 ± 15 m2/g Grindometer value less than 30 Median value by PCS 65 to 85 nm and 40% to 99.5% by weight, based on the total mass, of an organopolysiloxane of the formula where R = alkenyl, alkoxy, aryl, oxime, acetoxy, alkyl radicals, having 1 to 50 carbon atoms, unsubstituted or substituted by 0, S, F, Cl, Br, I, in each case identical or different, and/or polystyrene, polyvinyl acetate, polyacrylate, polymethacrylate and polyacrylo- nitrile radicals having 40-10 000 repeater units. Z = OH, Cl, Br, acetoxy, amino, amineoxy, oxime, alkoxyamido, alkenyloxy, acryloxy or phosphate radicals, it being possible for the organic radicals to carry up to 20 carbon atoms, and in each case identical or different. Z' = oxime, alkoxy, acetoxy, amino, amido, n = 1-3 x = 100-15 000. As organopolysiloxane it is possible to use all poly- siloxanes which have or have been able to be used to date as a basis for room-temperature-crosslinking (room-temperature-vulcanizing) (RTV) compositions. They may be described for example by the general formula where x, R, Z' and Z have the following definitions: where R = alkenyl, alkoxy, aryl, oxime, acetoxy, alkyl radicals, having 1 to 50 carbon atoms, unsubstituted or substituted by 0, S, F, Cl, Br, I, in each case identical or different, and/or polystyrene, polyvinyl acetate, polyacrylate, polymethacrylate and polyacrylo- nitrile radicals having 40-10 000 repeater units. Z = OH, Cl, Br, acetoxy, amino, amineoxy, oxime, alkoxyamido, alkenyloxy, acryloxy or phosphate radicals, it being possible for the organic radicals to carry up to 20 carbon atoms, and in each case identical or different. Z' = oxime, alkoxy, acetoxy, amino, amido, n = 1-3 x = 100-15 000. Within and/or along the siloxane chain in the formula indicated above it is also possible for there to be other siloxane units present, usually only as impuri- ties, in the form of diorganosiloxane units, for example those of the formula RSiO3/2, R3O1/2 and SiO4/2, R in each case having the definition indicated for it above. The amount of these other siloxane units ought not to exceed 10 mol per cent. Examples of R with the definition alkyl radical are, for example, methyl, ethyl, propyl, hexyl and octyl radicals; possible alkenyl radicals are vinyl, allyl, ethylallyl and butadienyl radicals; and as aryl radicals it is possible to use phenyl and tolyl radical. Examples of substituted hydrocarbon radicals R are in particular halogenated hydrocarbon radicals such as 3,3,3-trifluoropropyl radical, chlorophenyl and bromotolyl radical; and cyanoalkyl radicals, such as the (3-cyanoethyl radical. Examples of polymers as radical R are polystyrene, polyvinyl acetate, polyacrylate, polymethacrylate and polyacrylonitrile radicals which are attached to silicon via carbon. On account of the greater ease of access the predominant fraction of the radicals R is composed of methyl groups. The other radicals R are, in particular, vinyl and/or phenyl groups. Particularly in the case of the presence of formula- tions which are storable in the absence of water and which cure to elastomers at room temperature on ingress of water, Z and Z1 are hydrolysable groups. Examples of such groups are acetoxy, amino, amineoxy, alkenyloxy (for example H2C = (CH3CO-) ) , acyloxy and phosphate groups. Primarily on account of the greater ease of access, preferred groups Z are acyloxy groups, especially acetoxy groups. Excellent results, however, are also achieved using, for example, oxime groups, such as those of the formula -ON=C (CH3) (C2H5) , as Z. Examples of hydrolysable atoms Z are halogen and hydrogen atoms; examples of alkenyl groups Z are, in particular, vinyl groups. The viscosity of the organopolysiloxanes used in the context of the invention ought not to exceed 500 000 cP at 25°C, preferably 150 000 cP at 25°C. Accordingly the value x ought preferably not to exceed 40 000. Examples of organopolysiloxanes which can be used are, for example, the silicone polymers E50 (α, ω-hydroxydi- methylsiloxypolydimethylsiloxane) or M50 (α, ω-hydroxy- dimethylsiloxypolydimethylsiloxane) from GE Bayer Silicones. Mixtures of different organopolysiloxanes can also be used. The mixing of these organopolysiloxanes with the fumed silica and, where appropriate, with the further constituents of the formulation of the invention can take place in any desired known way, for example in mechanical mixing devices. It is accomplished very rapidly and easily, irrespective of the sequence in which the mixing constituents are added. Preferably the fumed silicas of the invention are used in amounts of 0.5% to 60% by weight, preferably 3% to 30% by weight, based on the total weight of the compounds which can be cured to elastomers. If the only reactive terminal units present in the diorganopolysiloxanes which contain reactive terminal units are those having Si-bonded hydroxyl groups, then these diorganopolysiloxanes must be crosslinked. This can be done in a conventional way by means of the water present in the air, with the addition where appropriate of further water, with a crosslinking agent. Here it is possible for example to use the Silopren crosslinker 3034 from GE Bayer Silicones, the ethyltriacetoxysilane optionally in the presence of a condensation catalyst in a known way. Suitable catalysts for all formulations of the invention are, for example, the Silopren catalysts DBTA or type 162 dibutyltin diacetate or dilaurate from the same manufacturer. In one particular variant of the silicone rubber compounds of the invention it is possible additionally for there to be 0.5%-20%, preferably 2%-10% by weight, based on the total weight of the compound, of a crosslinker having the formula with R = alkyl, alkoxy, acetoxy, oxime, aryl, alkene radicals, having 1 to 50 carbon atoms, unsubstituted or substituted by 0, S, F, Cl, Br, I, in each case identical or different, and/or polystyrene, polyvinyl acetate, polyacrylate, polymethacrylate and polyacrylo- nitrile radicals having 5-5000 repeater units, Z1 = OH, Cl, Br, acetoxy, oxime, amino, amineoxy, alkenyloxy or phosphate radicals, it being possible for the organic radicals to carry up to 20 carbon atoms, in each case identical or different, and All weight data relate to the total amount of silicone rubber compounds. Examples of silanes of the formula indicated above are ethyltriacetoxysilane, methyltriacetoxysilane, isopro- pyltriacetoxysilane, isopropoxytriacetoxysilane, vinyl- triacetoxysilane, methyltrisdiethylaminooxysilane, methyltris(cyclohexylamino)silane, methyltris(diethyl- phosphato)silane and methyltris(methylethylketoximo)- silane. Of course it is possible for formulations of the inven- tion to contain, besides organopolysiloxanes, hydro- phobicized silica, crosslinking agents and crosslinking catalysts, if desired, fillers which are conventionally used mostly or frequently in compounds which can be cured to elastomers. Examples of such substances are fillers having a surface area below 50 m2/g, such as coarse quartz powder, kaolin, phyllosilicates, clay minerals, diatomaceous earth, additionally zirconium silicate and calcium carbonate, and also untreated pyrogenically produced silicon dioxide, organic resins, such as polyvinyl chloride powders, organopolysiloxane resins, fibrous fillers, such as asbestos, glass fibres and organic pigments, soluble dyes, fragrances, corrosion inhibitors, curing retardants, such as benzo- triazole, and plasticizers, such as dimethylpoly- siloxanes end-blocked by trimethylsiloxy groups. Optionally the RTV 1K [one-component] silicone rubber compounds of the invention can contain 0.1%-20%, preferably 0.1%-15%, with particular preference 0.1%- 10% by weight (based on the total amount of the formulation (of water-binding substances). Suitable substances for this purpose are, for example, carboxylic anhydrides, for example acetic anhydride or maleic anhydride, and/or carbonic esters, such as for example diethyl carbonate, ethyl carbonate and/or alkenyloxy compounds and/or ketals, such as dimethyldi- oxolane, for example. It is possible to use one or more of these substances. Additionally the silicone rubber compounds may contain 0.01% to 99.5% by weight of an unfunctionalized polysiloxane. Here it is possible to use the poly- siloxanes already specified, provided that they are not functionalized. One suitable, non-functional poly- siloxane is, for example, Baysilone oil M1000 (polydi- methylsiloxane) from GE Bayer Silicones. Additionally the silicone rubber compounds may contain 0.01% to 6% by weight of organic or inorganic compounds of the metals Pt, Sn, Ti and/or Zn as catalyst and/or 0.01% to 6% by weight of inhibitors and/or 0.01%-6% by weight of fungicides and/or bactericides and/or 0.01% to 6% by weight of adhesion promoters (such as, for example, Silopren adhesion promoter 3001 from GE Bayer Silicones, with the composition: di-tert-butoxydiace- toxysilane). As fungicides/bactericides it is possible for example to use isothiazolinone, Vinycin or benziso- thiazolinone. The silicone rubber compounds of the invention can be used as silicone rubber systems from the group of the room-temperature-vulcanizing one-component (1K RTV) silicone rubber sealants and also self-levelling room- temperature-crosslinking silicone rubber compounds (1K RTV) . The silicone rubber compounds can be used as jointing compounds, window sealants, seals in motor vehicles, sports equipment and household appliances, heat- resistant seals, oil-exuding and chemical-resistant seals, and water-vapour-resistant seals, and seals in electrical and electronic appliances. The silicone rubber compounds can be used as coating materials for textiles, e.g. lace tape (antislip) , and textile materials, e.g. woven glass fabric or woven nylon fabric. The vulcanizates of the silicone rubber compounds of the invention advantageously have a high-grade surface. Examples: I. Preparation of the inventive fumed silica The inventive examples were produced by metering commercial AEROSIL® 200 (bagged product) into the mill employed, using a metering balance, and subjecting it to grinding. The Aerosil® 200 has the physicochemical properties listed in Table 1. 1] To DIN ISO 9277 2) To DIN EN ISO 787-11, JIS K 5101/20 (unsieved) 3) To DIN EN ISO 787-2, ASTM D 280, JIS K 5101/23 4) To DIN EN 3262-20, ASTM D 1208, JIS K 5101/24 5) Based on the substance dried at 105°C for 2 hours 61 To DIN EN ISO 787-9, ASTM D 1208, JIS K 5101/26 7) Based on the substance calcined at 1000°C for 2 hours 8) HC1 content in constituent from loss on ignition The experiments were carried out using a pinned-disc mill (Alpine 160Z, rotor diameter 160 mm) or an air-jet mill (grinding chamber diameter: 240 mm, grinding chamber height: 35 mm). The individual parameters of preparation are listed in Table 2. The ground product was isolated with a hose filter (filter area: 3.6 m2, filter material: woven nylon fabric). In further experiments the ground product obtained was packaged into commercially customary bags using a commercially customary bagging machine. In further experiments the bags packed with ground products were levelled prior to palletization, using a method routine in the industry and suitable for their purpose. With virtually the same specific surface areas in the range from 195 to 202 m2/g, preferably from 198 to 200 m2/g, and unchanged pH values, the silicas of the invention exhibit lower grindometer values and smaller median values than the starting silica. Surprisingly the lower grindometer values and lower median values are retained in spite of the compaction, evident through the tamped density, as a result of bagging or bagging/levelling. In some cases the tamped densities are in fact above that of the oxide used. This means that the oxides of the invention, despite the same or even higher compaction, exhibit lower grindometer values and lower median values. The inventive fumed silica can have a D50 (g) (i.e. median value, weight distribution) of 12 to 14 nm, preferably of 12.473 to 13.278 nm. The total span of the particles can be from 5.260 to 34.700 nm, preferably 5.260 to 22.740 nm. Figures 1 to 4 show that the inventive silica has no particles in the range greater than 40 nm when the weight distribution is considered (cf. Figure 4). The arithmetic mean of the particle diameter, DN, can be from 11.5 to 13.5 nm for the inventive silica. The particle diameter averaged over the surface area, DA, can be 12.5 to 17.2 nm. The particle diameter averaged over the volume, DV, can be 13.0 to 21.7 nm. The median value D50 (A) (number distribution) can be 11.0 to 11.7 nm. Determination of characteristic physicochemical data BET surface area The BET surface area is determined in accordance with DIN ISO 9277. Tamped density The tamped density is determined in accordance with DIN EN ISO 787-11. Principles of tamped density determination: The tamped density (formerly tamped volume) is equal to the ratio of the mass to the volume of the powder after tamping in a tamping volumeter under defined condi- tions. According to DIN EN ISO 787-11 the tamped density is reported in g/cm3. Owing to the very low tamped density of the oxides, however, we state the value in g/l. Furthermore, the drying and sieving, and the repetition of the tamping process, are omitted. Apparatus for tamped density determination: Tamping volumeter Measuring cylinder Laboratory balance (reading accuracy 0.01 g) Tamped density determination procedure: 200 ± 10 ml of oxide are introduced into the measuring cylinder of the tamping volumeter so that there are no cavities remaining and so that the surface is horizon- tal. The mass of the sample introduced is determined to an accuracy of 0.01 g. The measuring cylinder containing the sample is inserted into the cylinder holder of the tamping volumeter and tamped 1250 times. The volume of the tamped oxide is read off to an accuracy of 1 ml. Evaluation of tamped density determination: pH The pH is determined in 4% aqueous dispersion. Reagents for pH determination: Distilled or deionized water, pH > 5.5 Buffer solutions pH 7.00 pH 4.66 Apparatus for pH determination: Laboratory balance (reading accuracy 0.1 g) Glass beaker, 250 ml Magnetic stirrer Magnetic rod, length 4 cm Combined pH electrode pH meter Dispensette, 100 ml Procedure for determining pH: The determination takes place in a modification of DIN/ISO 787/IX. Calibration: Prior to pH measurement the meter is calibrated using the buffer solutions. Where two or more measurements are carried out one after another, a single calibration is sufficient. 4 g of oxide are mixed in a 250 ml glass beaker with 96 g (96 ml) of water, with the aid of a dispensette, and the mixture is stirred using a magnetic stirrer (speed about 1000 min-1) for five minutes with the pH electrode immersed. After the stirrer has been switched off the pH is read off after a standing time of one minute. The result is reported to one decimal place. Grindometer value Principles: The degree of dispersion determines the performance properties of the liquid thickened with Aerosil. The measurement of the grindometer value serves to assess the degree of dispersion. By the grindometer value is meant the boundary layer thickness below which the particles or aggregates present become visible on the surface of the sample which has been coated out. The sample is coated out in a groove with a scraper, the depth of the groove at one end being twice the size of the diameter of the largest Aerosil particles, and decreasing steadily down to 0 at the other end. On a scale indicating the depth of the groove, the depth value is read off, in micrometers, the value in ques- tion being that below which a relatively large number of Aerosil particles becomes visible as a result of bits or scratches on the surface of the binder system. The value read off is the grindometer value of the system present. Apparatus and reagents: Hegmann grindometer with a depth range of 100-0 micro- meter. Polyester resin dispersion with 2% Aerosil, prepared according to Testing Instructions 0380. Procedure: The grindometer block is placed on a flat, slip-proof surface and is wiped clean immediately prior to testing. The Aerosil dispersion, which must be free from air bubbles, is then applied to the deepest point of the groove in such a way that it flows off somewhat over the edge of the groove. The scraper is then held by both hands and placed, perpendicularly to the grindometer block and at right angles to its longitudinal edges, with gentle pressure, onto the end of the groove in which the dispersion is located. The dispersion is then coated out in the groove by slow, uniform drawing of the scraper over the block. The grindometer value is read off no later than 3 seconds after the dispersion has been coated out. The surface of the spread dispersion (transverse to the groove) is viewed obliquely from above at an angle of 20-30° (to the surface). The block is held to the light in such a way that the surface structure of the spread dispersion is readily apparent. The grindometer value read off on the scale is the value in micrometers below which a relatively large number of Aerosil particles become visible as bits or scratches on the surface. Individual bits or scratches occurring randomly are not taken into account in this context. The granularity is assessed at least twice, in each case on a newly spread dispersion. Evaluation: From the measured values the arithmetic mean is formed. The relationship between the grindometer value in micrometers and the FSPT units and Hegmann units, which are based on the inch system, is as follows: B = 8-0.079 A C = 10-0.098 A = 1.25 B In this relationship: A = Grindometer value in micrometers B = Grindometer value in Hegmann units C = Grindometer value in FSPT units Particle size determination by means of PCS Purpose: The method of analysis described below is used to determine the particle size distribution by means of photon correlation spectroscopy (PCS, dynamic light scattering) of the dispersed phase (mostly solid particles) in dispersions. This method is particularly suitable for determining particles and their aggregates whose size lies within the submicrometer range (10 nm to 3 µm) . General remarks, theory: The basis for particle size determination by means of PCS is the Stokes-Einstein equation, which demonstrates the relationship between particle size (expressed through the so-called hydrodynamic diameter d(H)) and the diffusion coefficient D of the particles: d(H) = kT/3 η.Π D Clearly this equation can be derived via an equilibrium of forces between the Brownian motion of the particles and the opposing flow resistance. From this equation it is also comfortably apparent which parameters must be known or constant: the viscosity T\| of the dispersing medium, and the temperature T. If this is ensured, the hydrodynamic diameter can be calculated via the deter- mination of the diffusion coefficient. Expressed in other terms: large particles move slowly, small ones quickly. The movement of the particles in a dispersion can be monitored via the fluctuation in the scattered-light intensity of a laser beam. Where rapid movements are present, the fluctuation in intensity is small; slower particles generate more considerable fluctuations. By way of mathematical operations such as a Fourier transformation or an autocorrelation function, it is possible, from the fluctuation pattern of the scattered-light signal, which is constant on average, to infer a particle distribution. In the simplest case, a monodisperse distribution is assumed. If the logarithm of the autocorrelation func- tion is then plotted against the correlation time, the average diffusion constant is given by the slope of the best-fit line. This form of evaluation is known as "cumulant analysis". In reality this relationship is usually non-linear, since the distributions present are not actually monodisperse. As a measure of the breadth of distribution the polydispersity index "PI" is defined, by adapting a third-degree polynomial rather than a straight line. The "PI" is then PI = 2c/b2 where b and c are the coefficients of second and third degree. In order to determine volume-weighted or mass-weighted averages, however, it is necessary to consider the Mie theory (comprehensive scattered-light theory), which presupposes a knowledge of the refractive indices of particle and dispersion medium. In order to gain even more information from the intensity fluctuations, it is possible mathematically to adapt distributions other than, for example, one (or more) log-normal distribu- tion (s). An established method of achieving this is the so-called "Contin" analysis, which encompasses lopsided and also multimodal distributions. Since the definition of the hydrodynamic diameter derives from the assumption of spherical form of the particles, the measurement result obtained must always be understood, in the case of non-spherical particles, as an equivalent spherical diameter. This means that the diameter assumed is that of a sphere which causes the same intensity fluctuations as the non-spherical particles under determination. Only indirectly, there- fore, does the hydrodynamic diameter have anything to do with an "actual size". Measurement prerequisites, restrictions: The structure of the scattered-light geometry of different instruments gives rise to technical restric- tions on the measurement. The conventional PCS instru- ment, such as the Malvern Zetasizer 3000 instrument used here, uses a scattered-light angle of 90°. In order to rule out multiple scattering, which would falsify the result, the sample under measurement must with this geometry be highly diluted. Indicative values are a concentration of 0.001%-0.01% by weight. Obviously the dispersions under measurement ought to be translucent and to have only a slight turbidity. Other geometries also permit the use of higher concentra- tions. Hence depending on particle type (particle density, morphology) it is possible to carry out measurement even of concentrations from 0.5% to 30% by weight. For this purposes, for example, the Horiba LB 500 instrument uses a back-scattering optical system for which the ratio between single and multiple scattering is virtually constant and can therefore be disregarded. In the case of the alternative 3D cross- correlation technique it is possible to eliminate the fraction of multiply scattered light mathematically through the use of two independent laser beams. Where high concentrations are used, however, there is a further restriction to be observed: the determination of the diffusion constant presupposes the free mobility of the particles, which at high concentrations is no longer the case. For this reason, with instruments which allow the use of higher concentrations, dilution series ought to be routinely measured. Determining the particle size distribution by means of PCS requires knowledge of the following physical parameters: Temperature of the dispersion In the case of the Zetasizer 3000 the sample holder is thermally conditioned by way of a Peltier element (25°C), and the temperature is held constant during the measurement. This is important in order to rule out convection movement within the cell, which would super- impose itself on the free movement of the particles. The Horiba LB500 measures the temperature in the cell and takes account of the temperature it measures in the context of the evaluation. Viscosity of the dispersion medium In the case of the highly diluted systems this is not critical, since in that case the viscosities of the pure solvents at 25°C, for example, are well known. It is necessary, however, to ensure that the right viscosity is used for the calculation. Refractive index of particles and dispersion medium These figures are needed only for the volume-weighted evaluation. For determining a cumulant analysis (zave, PI) these details are unimportant. For standard systems there are tabular values (see instrument handbook); in the case for example of surface-modified substances, however, one ought to be aware of this possible source of error if the refractive index is not known with precision, and in that case only the cumulant analysis ought to be used. Furthermore, the following preconditions must be met: The dispersion must be stable to sedimentation. Sedimentation in the cell not only generates additional movement in the particles (see above) but also alters the scattered-light intensity during measurement. Moreover, it depletes the dispersion of larger parti- cles, which collect on the bottom of the cell. For this reason the scattered-light intensity should be moni- tored during the measurement ("count rate"). If this count rate is not stable, the result ought not to be interpreted. This restriction is the reason why the upper measurement limit for PCS is dependent on the difference in density between particle and dispersion medium. To dilute the dispersion to measurement concentration it is necessary to use extremely clean, dust-free solvent (generally water). This is particularly impor- tant for the measurement of small particles and also in highly dilute dispersions. Instruments: Malvern Zetasizer 3000 HSa instrument Disposable cells 10 x 10 x 48 mm (e.g. Sarstedt Ref. 67.754) for aqueous dispersions or glass cells 10 x 10 x 48 mm for solvent-containing dispersions. Glass beaker Syringe (20 ml) Disposable filter 200 µm (for example Macherey-Nagel Chromafil A-20/25, cellulose mixed ester) Disposable pipette Software, settings The Malvern Zetasizer 3000 HSa instrument is controlled via a computer program which also carries out the evaluation of the measurement signal and allows the results to be archived and printed out. As regards the operation of the program, refer to the handbook. Prior to each measurement it is necessary to perform the following settings within the software: Input of the refractive indices of particle and medium (see above) Input of the viscosity of the dispersion medium (see above) Designation and notes relating to the sample Path and file for archiving of the result In addition the following settings are present as standard and are generally not changed: 10 measurements are collated to form one series and their average is expressed as the result. 3 such measurements are carried out per sample. There is a waiting time of 1 minute before each measurement (for constant temperature). Attenuator "Auto" (instrument automatically attenuates the signal intensity down to a factor of 128, so that it is not necessary to prepare the sample with such precision). Data analysis "Contin" Other settings: dilation 1.20; weighting "quadratic"; size range "Auto"; point select "Auto", by cut off 0.01; optical properties 0.00, core real 1.6, core imag. 0; Mark-Houwink parameters A 0.5 cm2/s, K 0.000138. Procedure: Sample preparation: 20 ml of pure solvent (generally fully demineralized water) are filtered, using a syringe, through a 200 µm filter and then introduced into a clean, dust-free glass beaker. If the dispersion under measurement has been made acidic or basic, demineralized water adjusted accordingly is used, in order to prevent a pH shock when the dispersion is diluted. Then one drop of the dispersion is added, using a disposable pipette, to the solvent, and the glass beaker is swirled gently for some time until the drop has dispersed homogeneously, and a slight turbid solution has formed. This solution is introduced using a second disposable pipette into the cell, which is sealed and fixed in the instrument's cell holder. Instrument preparation The instrument and also the attached computer are switched on and the associated measurement program is started. After the instrument has been switched on it is necessary to wait a while, since laser and Peltier element must first come up to operating temperature. Generally, therefore, the instrument is switched off only in cases of prolonged standstill (several days). Measurement and evaluation: At the beginning of the measurement it is necessary first of all to input the sample parameters and sample designation and also to select the file in which the measurements are to be stored. The actual measurements are started by clicking on the "Go" button. The results of the three measurements are subsequently compared. When there is a good match one of the measurements is selected and represented as a volume-weighted distribu- tion by selection of the menu item "View -> New plot -> Volume". Selecting "Copy -> Table as Text" transfers the result via the Clipboard to Excel, where it is converted, using a macro, into a report file (XLS). The Excel file generated in this way is stored on the server. If the values of the individual measurements do not match, a repeat measurement is carried out. If it is apparent that during the measurements there are sharp fluctuations in the count rate occurring, the result is not evaluated and the particle size measurement is carried out, where appropriate, by a different method. II. Silicone rubber compounds Examples 13 to 16 The experiments for determining the surface properties of silicone vulcanizates were carried out in a speed mixer, with the following formulation: A polyethylene beaker which can be screwed closed with a lid is charged with 31 g (62%) of α,ω-hydroxydi- methylsiloxypolydimethylsiloxane (E 50, GE Bayer Silicones). Subsequently 12.5 g (25%) of α,ω-trimethyl- siloxypolydimethylsiloxane (M1000, GE Bayer Silicones) are added. After addition of 0.5 g (1%) of carbon black (Printex 3, Degussa AG) the mixture is mixed in the speed mixer at 2500 rpm for 15 seconds. Then 2.0 g (4%) of ethyltriacetoxysilane (AC 3034, GE Bayer Silicones) are added to this mixture and the components are mixed in the speed mixer at 2500 rpm for 15 seconds. Subsequently 4.0 g (8%) of fumed silica are mixed in at 2500 rpm within 30 seconds or in two steps (2 X 15 seconds) in the speed mixer. Material clinging to the inner wall is stripped off and the batch is again mixed at 2500 rpm for 30 seconds. Addition of 1 drop (0.01%) of dibutyltin dilaurate is followed by speed mixer mixing at 2500 rpm for 120 seconds. The silicone formulation obtained in this way is coated with a coating bar and vulcanized at room temperature in ambient air over a period of 24 hours. The surface of the vulcanizates is inspected and evaluated on a school-grade system: Grades: 1 - very good, 2 = good, 3 = satisfactory, 4 = unsatisfactory, 5 = deficient Grade 1 is awarded when the vulcanizate has no defects or undispersed particles. For grade 2 there are only a very few defects visible. In the case of grade 3 there are somewhat more. In the case of grade 4 the number of visible defects is such that the surface appearance would be unacceptable for technical use. The same applies to a grading of 5, only more so. When the inventive silicas are used in accordance with Examples 3, 6 and 9, surprisingly, the silicone vulcanizates have good surface properties in comparison to the standard material. The silicone vulcanizate with the standard material shows only an unsatisfactory surface, which is also hardly surprising, since silicas of high surface area cannot generally be used in silicone sealants, for precisely that reason. WE CLAIM: 1. Hydrophilic fumed silica characterized in that it contains no hydrophobic fractions and has the following physicochemical data: BET surface area 200 ± 15 m2/g Grindometer value less than 30 µm Median value by PCS 65 to 85 nm 2. Process for preparing the hydrophilic fumed silica as claimed in claim 1, wherein a hydrophilic fumed silica containing no hydrophobic fractions and having a BET surface area of 200 ± 25 m2/g is ground. 3. Silicone rubber compounds containing 0.5% to 60% by weight of the hydrophilic fumed silica as claimed in claim 1 and 40% - 99.5% by weight of an organopolysiloxane of the formula Zn Si R3-n -O- [Si R2 O]x -Si R3-n -Z'n where R = alkenyl, alkoxy, aryl, oxime, acetoxy, alkyl radicals, having 1 to 50 carbon atoms, unsubstituted or substituted by O, S, F, Cl, Br, I, in each case identical or different, and/or polystyrene, polyvinyl acetate, polyacrylate, polymethacrylate and polyacrylonitrile radicals having 40-10 000 repeat units, Z = OH, Cl, Br, acetoxy, amino, amineoxy, oxime, alkoxyamido, alkenyloxy, acryloxy or phosphate radicals, it being possible for the organic radicals to carry up to 20 carbon atoms, and in each case identical or different, Z' - oxime, alkoxy, acetoxy, amino, amido, n= 1-3 x= 100-15 000. ABSTRACT Title: FUMED SILICA Fumed silica having a BET surface area of 200 + 15 m2/g, a grindometer value of less than 30 µm and a median value by PCS of 65 to 85 nm is prepared by grinding a fumed silica having a surface area of 200 + 25 m2/g, and silicone rubber containing his silica.

Full Text

Fumed silica
The invention relates to fumed silica, to a process for
preparing it and to its use.
Fumed silica (pyrogenically prepared silicon dioxide)
is known from Ullmanns Enzyklopadie der technischen
Chemie, Volume 21, page 4 64 (1982).
It is prepared by burning a vaporizable silicon
compound, such as silicon tetrachloride, for example,
in a mixture with hydrogen and oxygen.
The comminution of materials to form coarse powders
(50-500 µm) , fine powders (5-50 µm) and even greater
finenesses (less than 5 µm) is common and widespread
practice. For all . comminution tasks there is a
multiplicity of technical and industrial equipment
offered and operated, all adapted to the particular
circumstances of the specific tasks. A good overview of
the comminution problems and of the diverse machines is
given in Ullmanns Enzyklopadie der technischen Chemie,
3rd Edition, Volume 1, pages 616 to 638. For fumed
silica the average primary particle diameters are
notably lower (5-50 nm) than can be obtained by
mechanical comminution.
The primary particles and aggregates of the fumed
silica with a surface area of 200 m2/g can be
visualized in the electron microscope.
The primary particles and aggregates of a fumed silica
agglomerate to form larger assemblies whose size is
generally in inverse proportion to the primary particle
size or in proportion to the specific surface area. The
agglomerate size also increases in line with the extent
to which the fumed silica is compacted.

The binding forces holding these agglomerates together
are relatively weak. Nevertheless, when these agglome-
rates are incorporated into and broken down in a liquid
system for the purpose of homogeneous distribution of
the primary particles and aggregates, or particles with
a low degree of agglomeration, a certain shearing
energy is required. Depending on the particular field
of application, dispersing is carried out using any of
a very wide variety of mixing devices, with determining
factors for the selection being not only the viscosity
and polarity of the system but also the agglomerate
strength and the desired homogeneity.
With simple agitator mechanisms, such as paddle
stirrers, it is usually not possible to carry out to
satisfaction the direct incorporation of small amounts
of silicas, especially not when the systems in question
are of low viscosity. However, paint and varnish
makers, and also those who carry out processing, have
an interest in obtaining performance-optimum distribu-
tion of the silicas, which are used predominantly as
thickeners and thixotropic agents, by means of very
simple devices, with a very low energy input and in a
very short time.
In the case of paddle-stirrer dispersing, the coarse
silica agglomerates are not sufficiently comminuted and
hence are able to make only a small contribution to
raising viscosities and thixotropy. The data relate to
a UP resin (unsaturated polyester resin) as dispersion
medium.
Reducing the agglomerate size by dispersing outside a
liquid system, in other words, in practice, in the air,
or by grinding in the conventional sense, is possible
only to a limited extent, since in the case of material
with a given propensity to agglomerate the comminution
is followed immediately by the re-establishment of the

old agglomeration state. This effect comes about no
later than after recompaction of the material which as
a result of the mechanical intervention has undergone a
high degree of loosening and which in this form is not
amenable to dispatch and storage. The storage time as
well would have the effect of renewed agglomerate
enlargement.
A value taken as a dimensional number and evaluation
variable for the stage of distribution of a dispersible
silica and maximum agglomerate size of the dispersion
(granularity) is the so-called grindometer value to DIN
53203.
A known procedure is to render fumed silica hydro-
phobic, to grind it in a pinned-disc mill and then to
classify it (US 2004/0110077 A1) . This known silica is
used as an external additive in toner mixtures.
Hydrophilic fumed silica with a BET surface area
possesses a grindometer value as determined in UP resin
(unsaturated polyester resin Ludepal P6 from BASF, 2%
dispersion) in accordance with the DIN specification,
of 50 to 60.
If this fumed silica is also relatively highly com-
pacted (100 to 120 g/l), the grindometer value as well
is also significantly higher, specifically more than
100, whereby necessitating an additional, not
inconsiderable energy as a thickener and thixotropic
agent.
A known procedure is to grind a highly dispersed silica
having a surface area of approximately 300 m2/g in a
pinned-disc mill.
The grindometer value achieved is initially, for the
uncompacted silica, 25.

If this silica is compacted to 50 g/l, the grindometer
value rises to 30, and in the case of further compac-
tion to 75 g/l it rises to as far as about 40.
In the case of storage over a period of three months,
the ground silica, not modified but compacted to
50 g/l, has a grindometer value of 50 to 60.
Reagglomeration can only be prevented in accordance
with the prior art if the hydrophilic silica is mixed
with 3% by weight of a hydrophobic silica and if this
mixture is ground by means of an air-jet mill or a
pinned-disc mill (EP 0 076 377 B1).
In this case, for a fumed silica having a BET surface
area of 200 m2/g, even after compaction to 73 or
107 g/l, a grindometer value of 35 is achieved.
For a fumed silica having a BET surface area of
300 m2/g, the addition of hydrophobic silica prior to
grinding produces a grindometer value of 10 for a
tamped density of 28.1 g/l and of 15 to 20 for a tamped
density of 50 g/l.
The known fumed silicas have the disadvantage that they
contain an unwanted fraction of hydrophobic silica.
The invention provides a fumed silica which is
characterized in that it has the following physico-
chemical data:
BET surface area 200 ± 15 m2/g
Grindometer value less than 30 µm
Median value PCS 65 to 85 nm
The invention further provides a process for preparing
the silica of the invention which is characterized in
that a fumed silica having a BET surface area of 200 ±
25 m2/g is ground.

In one preferred embodiment of the invention the
grinding can take place by means of a pinned-disc mill
or an air-jet mill.
The fumed silica can have an average primary particle
size of 12 nm ± 5 nm, preferably 12 nm.
In a further embodiment of the invention a fumed silica
can be used which has a tamped density of approximately
50 ± 10 g/l, preferably 50 m2/g.
In one particular embodiment of the invention a fumed
silica can be used which has the following physico-
chemical data:

1) To DIN ISO 9277
2) To DIN EN ISO 787-11, JIS K 5101/20 (unsieved)
3) To DIN EN ISO 787-2, ASTM D 280, JIS K 5101/23
4) To DIN EN 3262-20, ASTM D 1208, JIS K 5101/24
5) Based on the substance dried at 105°C for 2 hours

6) To DIN EN ISO 787-9, ASTM D 1208, JIS K 5101/26
7) Based on the substance calcined at 1000°C for 2
hours
8) HC1 content in constituent from loss on ignition
The Aerosil® 200 silica used in accordance with the
invention, with a BET surface area, differs from the
known Aerosil® 300, which has a surface area of
300 m2/g, not only in the surface size but also in the
average primary particle size. The Aerosil® 300 has an
average primary particle size of 7 nm. On the basis of
these differences these silicas have different
performance properties.
The fumed silica of the invention contains no hydro-
phobic fractions. In spite of this the silica of the
invention shows no propensity to reagglomerate. The
grindometer value is below 30.
The fumed silica of the invention can be used as a
filler in silicone rubber compounds.
Silicone rubber compounds and the use of fumed silica
(AEROSIL®) in silicone rubber compounds are known
(Ullmann's Encyclopaedia of Industrial Chemistry,
Volume A 23, Rubber, 1, 221 ff.; Rubber 3, 3,6 ff.;
Volume A 24, Silicones 57 ff. 1993).
Fumed silica is used on account of its excellent
thickening effect (thixotroping) in silicone sealants,
this thickening effect being desirable in the context
of use as a jointing compound.
Where, however, the silicone rubber compounds are to be
used as coating materials, a low level of thickening is
desired (US 6,268,300).
Of decisive importance in both cases is the optical

quality of the surface of the silicone vulcanizates.
It is an object of the present invention, therefore, to
provide silicone rubber compounds which through the use
of fumed silica as filler exhibit an optically high-
grade surface after vulcanization.
The invention provides silicone rubber compounds
containing 0.5% to 60% by weight, based on the total
mass, of the fumed silica of the invention having the
following characteristic physicochemical data:
BET surface area 200 ± 15 m2/g
Grindometer value less than 30
Median value by PCS 65 to 85 nm
and
40% to 99.5% by weight, based on the total mass, of an
organopolysiloxane of the formula

where R = alkenyl, alkoxy, aryl, oxime, acetoxy, alkyl
radicals, having 1 to 50 carbon atoms, unsubstituted or
substituted by 0, S, F, Cl, Br, I, in each case
identical or different, and/or polystyrene, polyvinyl
acetate, polyacrylate, polymethacrylate and polyacrylo-
nitrile radicals having 40-10 000 repeater units.
Z = OH, Cl, Br, acetoxy, amino, amineoxy, oxime,
alkoxyamido, alkenyloxy, acryloxy or phosphate
radicals, it being possible for the organic radicals to
carry up to 20 carbon atoms, and in each case identical
or different.
Z' = oxime, alkoxy, acetoxy, amino, amido,
n = 1-3
x = 100-15 000.

As organopolysiloxane it is possible to use all poly-
siloxanes which have or have been able to be used to
date as a basis for room-temperature-crosslinking
(room-temperature-vulcanizing) (RTV) compositions. They
may be described for example by the general formula

where x, R, Z' and Z have the following definitions:
where R = alkenyl, alkoxy, aryl, oxime, acetoxy, alkyl
radicals, having 1 to 50 carbon atoms, unsubstituted or
substituted by 0, S, F, Cl, Br, I, in each case
identical or different, and/or polystyrene, polyvinyl
acetate, polyacrylate, polymethacrylate and polyacrylo-
nitrile radicals having 40-10 000 repeater units.
Z = OH, Cl, Br, acetoxy, amino, amineoxy, oxime,
alkoxyamido, alkenyloxy, acryloxy or phosphate
radicals, it being possible for the organic radicals to
carry up to 20 carbon atoms, and in each case identical
or different.
Z' = oxime, alkoxy, acetoxy, amino, amido,
n = 1-3
x = 100-15 000.
Within and/or along the siloxane chain in the formula
indicated above it is also possible for there to be
other siloxane units present, usually only as impuri-
ties, in the form of diorganosiloxane units, for
example those of the formula RSiO3/2, R3O1/2 and SiO4/2, R
in each case having the definition indicated for it
above. The amount of these other siloxane units ought
not to exceed 10 mol per cent.
Examples of R with the definition alkyl radical are,

for example, methyl, ethyl, propyl, hexyl and octyl
radicals; possible alkenyl radicals are vinyl, allyl,
ethylallyl and butadienyl radicals; and as aryl
radicals it is possible to use phenyl and tolyl
radical.
Examples of substituted hydrocarbon radicals R are in
particular halogenated hydrocarbon radicals such as
3,3,3-trifluoropropyl radical, chlorophenyl and
bromotolyl radical; and cyanoalkyl radicals, such as
the (3-cyanoethyl radical.
Examples of polymers as radical R are polystyrene,
polyvinyl acetate, polyacrylate, polymethacrylate and
polyacrylonitrile radicals which are attached to
silicon via carbon.
On account of the greater ease of access the
predominant fraction of the radicals R is composed of
methyl groups. The other radicals R are, in particular,
vinyl and/or phenyl groups.
Particularly in the case of the presence of formula-
tions which are storable in the absence of water and
which cure to elastomers at room temperature on ingress
of water, Z and Z1 are hydrolysable groups. Examples of
such groups are acetoxy, amino, amineoxy, alkenyloxy
(for example H2C = (CH3CO-) ) , acyloxy and phosphate
groups. Primarily on account of the greater ease of
access, preferred groups Z are acyloxy groups,
especially acetoxy groups. Excellent results, however,
are also achieved using, for example, oxime groups,
such as those of the formula -ON=C (CH3) (C2H5) , as Z.
Examples of hydrolysable atoms Z are halogen and
hydrogen atoms; examples of alkenyl groups Z are, in
particular, vinyl groups.
The viscosity of the organopolysiloxanes used in the
context of the invention ought not to exceed 500 000 cP

at 25°C, preferably 150 000 cP at 25°C. Accordingly the
value x ought preferably not to exceed 40 000.
Examples of organopolysiloxanes which can be used are,
for example, the silicone polymers E50 (α, ω-hydroxydi-
methylsiloxypolydimethylsiloxane) or M50 (α, ω-hydroxy-
dimethylsiloxypolydimethylsiloxane) from GE Bayer
Silicones.
Mixtures of different organopolysiloxanes can also be
used.
The mixing of these organopolysiloxanes with the fumed
silica and, where appropriate, with the further
constituents of the formulation of the invention can
take place in any desired known way, for example in
mechanical mixing devices. It is accomplished very
rapidly and easily, irrespective of the sequence in
which the mixing constituents are added.
Preferably the fumed silicas of the invention are used
in amounts of 0.5% to 60% by weight, preferably 3% to
30% by weight, based on the total weight of the
compounds which can be cured to elastomers.
If the only reactive terminal units present in the
diorganopolysiloxanes which contain reactive terminal
units are those having Si-bonded hydroxyl groups, then
these diorganopolysiloxanes must be crosslinked. This
can be done in a conventional way by means of the water
present in the air, with the addition where appropriate
of further water, with a crosslinking agent. Here it is
possible for example to use the Silopren crosslinker
3034 from GE Bayer Silicones, the ethyltriacetoxysilane
optionally in the presence of a condensation catalyst
in a known way. Suitable catalysts for all formulations
of the invention are, for example, the Silopren
catalysts DBTA or type 162 dibutyltin diacetate or

dilaurate from the same manufacturer.
In one particular variant of the silicone rubber
compounds of the invention it is possible additionally
for there to be 0.5%-20%, preferably 2%-10% by weight,
based on the total weight of the compound, of a
crosslinker having the formula

with R = alkyl, alkoxy, acetoxy, oxime, aryl, alkene
radicals, having 1 to 50 carbon atoms, unsubstituted or
substituted by 0, S, F, Cl, Br, I, in each case
identical or different, and/or polystyrene, polyvinyl
acetate, polyacrylate, polymethacrylate and polyacrylo-
nitrile radicals having 5-5000 repeater units,
Z1 = OH, Cl, Br, acetoxy, oxime, amino, amineoxy,
alkenyloxy or phosphate radicals, it being possible for
the organic radicals to carry up to 20 carbon atoms, in
each case identical or different, and

All weight data relate to the total amount of silicone
rubber compounds.
Examples of silanes of the formula indicated above are
ethyltriacetoxysilane, methyltriacetoxysilane, isopro-
pyltriacetoxysilane, isopropoxytriacetoxysilane, vinyl-
triacetoxysilane, methyltrisdiethylaminooxysilane,
methyltris(cyclohexylamino)silane, methyltris(diethyl-
phosphato)silane and methyltris(methylethylketoximo)-
silane.
Of course it is possible for formulations of the inven-
tion to contain, besides organopolysiloxanes, hydro-
phobicized silica, crosslinking agents and crosslinking

catalysts, if desired, fillers which are conventionally
used mostly or frequently in compounds which can be
cured to elastomers. Examples of such substances are
fillers having a surface area below 50 m2/g, such as
coarse quartz powder, kaolin, phyllosilicates, clay
minerals, diatomaceous earth, additionally zirconium
silicate and calcium carbonate, and also untreated
pyrogenically produced silicon dioxide, organic resins,
such as polyvinyl chloride powders, organopolysiloxane
resins, fibrous fillers, such as asbestos, glass fibres
and organic pigments, soluble dyes, fragrances,
corrosion inhibitors, curing retardants, such as benzo-
triazole, and plasticizers, such as dimethylpoly-
siloxanes end-blocked by trimethylsiloxy groups.
Optionally the RTV 1K [one-component] silicone rubber
compounds of the invention can contain 0.1%-20%,
preferably 0.1%-15%, with particular preference 0.1%-
10% by weight (based on the total amount of the
formulation (of water-binding substances). Suitable
substances for this purpose are, for example,
carboxylic anhydrides, for example acetic anhydride or
maleic anhydride, and/or carbonic esters, such as for
example diethyl carbonate, ethyl carbonate and/or
alkenyloxy compounds and/or ketals, such as dimethyldi-
oxolane, for example. It is possible to use one or more
of these substances.
Additionally the silicone rubber compounds may contain
0.01% to 99.5% by weight of an unfunctionalized
polysiloxane. Here it is possible to use the poly-
siloxanes already specified, provided that they are not
functionalized. One suitable, non-functional poly-
siloxane is, for example, Baysilone oil M1000 (polydi-
methylsiloxane) from GE Bayer Silicones.
Additionally the silicone rubber compounds may contain
0.01% to 6% by weight of organic or inorganic compounds

of the metals Pt, Sn, Ti and/or Zn as catalyst and/or
0.01% to 6% by weight of inhibitors and/or 0.01%-6% by
weight of fungicides and/or bactericides and/or 0.01%
to 6% by weight of adhesion promoters (such as, for
example, Silopren adhesion promoter 3001 from GE Bayer
Silicones, with the composition: di-tert-butoxydiace-
toxysilane). As fungicides/bactericides it is possible
for example to use isothiazolinone, Vinycin or benziso-
thiazolinone.
The silicone rubber compounds of the invention can be
used as silicone rubber systems from the group of the
room-temperature-vulcanizing one-component (1K RTV)
silicone rubber sealants and also self-levelling room-
temperature-crosslinking silicone rubber compounds (1K
RTV) .
The silicone rubber compounds can be used as jointing
compounds, window sealants, seals in motor vehicles,
sports equipment and household appliances, heat-
resistant seals, oil-exuding and chemical-resistant
seals, and water-vapour-resistant seals, and seals in
electrical and electronic appliances.
The silicone rubber compounds can be used as coating
materials for textiles, e.g. lace tape (antislip) , and
textile materials, e.g. woven glass fabric or woven
nylon fabric.
The vulcanizates of the silicone rubber compounds of
the invention advantageously have a high-grade surface.
Examples:
I. Preparation of the inventive fumed silica
The inventive examples were produced by metering
commercial AEROSIL® 200 (bagged product) into the mill

employed, using a metering balance, and subjecting it
to grinding. The Aerosil® 200 has the physicochemical
properties listed in Table 1.

1] To DIN ISO 9277
2) To DIN EN ISO 787-11, JIS K 5101/20 (unsieved)
3) To DIN EN ISO 787-2, ASTM D 280, JIS K 5101/23
4) To DIN EN 3262-20, ASTM D 1208, JIS K 5101/24
5) Based on the substance dried at 105°C for 2 hours
61 To DIN EN ISO 787-9, ASTM D 1208, JIS K 5101/26
7) Based on the substance calcined at 1000°C for 2
hours
8) HC1 content in constituent from loss on ignition
The experiments were carried out using a pinned-disc
mill (Alpine 160Z, rotor diameter 160 mm) or an air-jet
mill (grinding chamber diameter: 240 mm, grinding

chamber height: 35 mm). The individual parameters of
preparation are listed in Table 2.
The ground product was isolated with a hose filter
(filter area: 3.6 m2, filter material: woven nylon
fabric). In further experiments the ground product
obtained was packaged into commercially customary bags
using a commercially customary bagging machine.
In further experiments the bags packed with ground
products were levelled prior to palletization, using a
method routine in the industry and suitable for their
purpose.

With virtually the same specific surface areas in the
range from 195 to 202 m2/g, preferably from 198 to
200 m2/g, and unchanged pH values, the silicas of the
invention exhibit lower grindometer values and smaller
median values than the starting silica.
Surprisingly the lower grindometer values and lower
median values are retained in spite of the compaction,
evident through the tamped density, as a result of
bagging or bagging/levelling.
In some cases the tamped densities are in fact above
that of the oxide used. This means that the oxides of
the invention, despite the same or even higher
compaction, exhibit lower grindometer values and lower

median values.

The inventive fumed silica can have a D50 (g) (i.e.
median value, weight distribution) of 12 to 14 nm,
preferably of 12.473 to 13.278 nm. The total span of
the particles can be from 5.260 to 34.700 nm,
preferably 5.260 to 22.740 nm.
Figures 1 to 4 show that the inventive silica has no
particles in the range greater than 40 nm when the
weight distribution is considered (cf. Figure 4).
The arithmetic mean of the particle diameter, DN, can
be from 11.5 to 13.5 nm for the inventive silica.
The particle diameter averaged over the surface area,
DA, can be 12.5 to 17.2 nm.
The particle diameter averaged over the volume, DV, can
be 13.0 to 21.7 nm.
The median value D50 (A) (number distribution) can be
11.0 to 11.7 nm.
Determination of characteristic physicochemical data
BET surface area
The BET surface area is determined in accordance with
DIN ISO 9277.
Tamped density
The tamped density is determined in accordance with DIN
EN ISO 787-11.
Principles of tamped density determination:
The tamped density (formerly tamped volume) is equal to
the ratio of the mass to the volume of the powder after
tamping in a tamping volumeter under defined condi-

tions. According to DIN EN ISO 787-11 the tamped
density is reported in g/cm3. Owing to the very low
tamped density of the oxides, however, we state the
value in g/l. Furthermore, the drying and sieving, and
the repetition of the tamping process, are omitted.
Apparatus for tamped density determination:
Tamping volumeter
Measuring cylinder
Laboratory balance (reading accuracy 0.01 g)
Tamped density determination procedure:
200 ± 10 ml of oxide are introduced into the measuring
cylinder of the tamping volumeter so that there are no
cavities remaining and so that the surface is horizon-
tal.
The mass of the sample introduced is determined to an
accuracy of 0.01 g. The measuring cylinder containing
the sample is inserted into the cylinder holder of the
tamping volumeter and tamped 1250 times.
The volume of the tamped oxide is read off to an
accuracy of 1 ml.
Evaluation of tamped density determination:

pH
The pH is determined in 4% aqueous dispersion.
Reagents for pH determination:
Distilled or deionized water, pH > 5.5
Buffer solutions pH 7.00 pH 4.66
Apparatus for pH determination:

Laboratory balance (reading accuracy 0.1 g)
Glass beaker, 250 ml
Magnetic stirrer
Magnetic rod, length 4 cm
Combined pH electrode
pH meter
Dispensette, 100 ml
Procedure for determining pH:
The determination takes place in a modification of
DIN/ISO 787/IX.
Calibration: Prior to pH measurement the meter is
calibrated using the buffer solutions. Where two or
more measurements are carried out one after another, a
single calibration is sufficient.
4 g of oxide are mixed in a 250 ml glass beaker with
96 g (96 ml) of water, with the aid of a dispensette,
and the mixture is stirred using a magnetic stirrer
(speed about 1000 min-1) for five minutes with the pH
electrode immersed.
After the stirrer has been switched off the pH is read
off after a standing time of one minute. The result is
reported to one decimal place.
Grindometer value
Principles:
The degree of dispersion determines the performance
properties of the liquid thickened with Aerosil. The measurement of the grindometer value serves to assess
the degree of dispersion. By the grindometer value is
meant the boundary layer thickness below which the
particles or aggregates present become visible on the

surface of the sample which has been coated out.
The sample is coated out in a groove with a scraper,
the depth of the groove at one end being twice the size
of the diameter of the largest Aerosil particles, and
decreasing steadily down to 0 at the other end. On a
scale indicating the depth of the groove, the depth
value is read off, in micrometers, the value in ques-
tion being that below which a relatively large number
of Aerosil particles becomes visible as a result of
bits or scratches on the surface of the binder system.
The value read off is the grindometer value of the
system present.
Apparatus and reagents:
Hegmann grindometer with a depth range of 100-0 micro-
meter.
Polyester resin dispersion with 2% Aerosil, prepared
according to Testing Instructions 0380.
Procedure:
The grindometer block is placed on a flat, slip-proof
surface and is wiped clean immediately prior to
testing. The Aerosil dispersion, which must be free
from air bubbles, is then applied to the deepest point
of the groove in such a way that it flows off somewhat
over the edge of the groove. The scraper is then held
by both hands and placed, perpendicularly to the
grindometer block and at right angles to its
longitudinal edges, with gentle pressure, onto the end
of the groove in which the dispersion is located. The
dispersion is then coated out in the groove by slow,
uniform drawing of the scraper over the block. The
grindometer value is read off no later than 3 seconds
after the dispersion has been coated out.
The surface of the spread dispersion (transverse to the
groove) is viewed obliquely from above at an angle of
20-30° (to the surface). The block is held to the light

in such a way that the surface structure of the spread
dispersion is readily apparent.
The grindometer value read off on the scale is the
value in micrometers below which a relatively large
number of Aerosil particles become visible as bits or
scratches on the surface. Individual bits or scratches
occurring randomly are not taken into account in this
context.
The granularity is assessed at least twice, in each
case on a newly spread dispersion.
Evaluation:
From the measured values the arithmetic mean is formed.
The relationship between the grindometer value in
micrometers and the FSPT units and Hegmann units, which
are based on the inch system, is as follows:
B = 8-0.079 A
C = 10-0.098 A = 1.25 B
In this relationship:
A = Grindometer value in micrometers
B = Grindometer value in Hegmann units
C = Grindometer value in FSPT units
Particle size determination by means of PCS
Purpose:
The method of analysis described below is used to
determine the particle size distribution by means of
photon correlation spectroscopy (PCS, dynamic light
scattering) of the dispersed phase (mostly solid
particles) in dispersions. This method is particularly
suitable for determining particles and their aggregates
whose size lies within the submicrometer range (10 nm
to 3 µm) .

General remarks, theory:
The basis for particle size determination by means of
PCS is the Stokes-Einstein equation, which demonstrates
the relationship between particle size (expressed
through the so-called hydrodynamic diameter d(H)) and
the diffusion coefficient D of the particles:
d(H) = kT/3 η.Π D
Clearly this equation can be derived via an equilibrium
of forces between the Brownian motion of the particles
and the opposing flow resistance. From this equation it
is also comfortably apparent which parameters must be
known or constant: the viscosity T| of the dispersing
medium, and the temperature T. If this is ensured, the
hydrodynamic diameter can be calculated via the deter-
mination of the diffusion coefficient. Expressed in
other terms: large particles move slowly, small ones
quickly.
The movement of the particles in a dispersion can be
monitored via the fluctuation in the scattered-light
intensity of a laser beam. Where rapid movements are
present, the fluctuation in intensity is small; slower
particles generate more considerable fluctuations. By
way of mathematical operations such as a Fourier
transformation or an autocorrelation function, it is
possible, from the fluctuation pattern of the
scattered-light signal, which is constant on average,
to infer a particle distribution.
In the simplest case, a monodisperse distribution is
assumed. If the logarithm of the autocorrelation func-
tion is then plotted against the correlation time, the
average diffusion constant is given by the slope of the
best-fit line. This form of evaluation is known as

"cumulant analysis". In reality this relationship is
usually non-linear, since the distributions present are
not actually monodisperse. As a measure of the breadth
of distribution the polydispersity index "PI" is
defined, by adapting a third-degree polynomial rather
than a straight line. The "PI" is then PI = 2c/b2 where
b and c are the coefficients of second and third
degree.
In order to determine volume-weighted or mass-weighted
averages, however, it is necessary to consider the Mie
theory (comprehensive scattered-light theory), which
presupposes a knowledge of the refractive indices of
particle and dispersion medium. In order to gain even
more information from the intensity fluctuations, it is
possible mathematically to adapt distributions other
than, for example, one (or more) log-normal distribu-
tion (s). An established method of achieving this is the
so-called "Contin" analysis, which encompasses lopsided
and also multimodal distributions.
Since the definition of the hydrodynamic diameter
derives from the assumption of spherical form of the
particles, the measurement result obtained must always
be understood, in the case of non-spherical particles,
as an equivalent spherical diameter. This means that
the diameter assumed is that of a sphere which causes
the same intensity fluctuations as the non-spherical
particles under determination. Only indirectly, there-
fore, does the hydrodynamic diameter have anything to
do with an "actual size".
Measurement prerequisites, restrictions:
The structure of the scattered-light geometry of
different instruments gives rise to technical restric-
tions on the measurement. The conventional PCS instru-
ment, such as the Malvern Zetasizer 3000 instrument

used here, uses a scattered-light angle of 90°. In
order to rule out multiple scattering, which would
falsify the result, the sample under measurement must
with this geometry be highly diluted. Indicative values
are a concentration of 0.001%-0.01% by weight.
Obviously the dispersions under measurement ought to be
translucent and to have only a slight turbidity. Other
geometries also permit the use of higher concentra-
tions. Hence depending on particle type (particle
density, morphology) it is possible to carry out
measurement even of concentrations from 0.5% to 30% by
weight. For this purposes, for example, the Horiba LB
500 instrument uses a back-scattering optical system
for which the ratio between single and multiple
scattering is virtually constant and can therefore be
disregarded. In the case of the alternative 3D cross-
correlation technique it is possible to eliminate the
fraction of multiply scattered light mathematically
through the use of two independent laser beams. Where
high concentrations are used, however, there is a
further restriction to be observed: the determination
of the diffusion constant presupposes the free mobility
of the particles, which at high concentrations is no
longer the case. For this reason, with instruments
which allow the use of higher concentrations, dilution
series ought to be routinely measured.
Determining the particle size distribution by means of
PCS requires knowledge of the following physical
parameters:
Temperature of the dispersion
In the case of the Zetasizer 3000 the sample holder is
thermally conditioned by way of a Peltier element
(25°C), and the temperature is held constant during the
measurement. This is important in order to rule out
convection movement within the cell, which would super-

impose itself on the free movement of the particles.
The Horiba LB500 measures the temperature in the cell
and takes account of the temperature it measures in the
context of the evaluation.
Viscosity of the dispersion medium
In the case of the highly diluted systems this is not
critical, since in that case the viscosities of the
pure solvents at 25°C, for example, are well known. It
is necessary, however, to ensure that the right
viscosity is used for the calculation.
Refractive index of particles and dispersion medium
These figures are needed only for the volume-weighted
evaluation. For determining a cumulant analysis (zave,
PI) these details are unimportant. For standard systems
there are tabular values (see instrument handbook); in
the case for example of surface-modified substances,
however, one ought to be aware of this possible source
of error if the refractive index is not known with
precision, and in that case only the cumulant analysis
ought to be used.
Furthermore, the following preconditions must be met:
The dispersion must be stable to sedimentation.
Sedimentation in the cell not only generates additional
movement in the particles (see above) but also alters
the scattered-light intensity during measurement.
Moreover, it depletes the dispersion of larger parti-
cles, which collect on the bottom of the cell. For this
reason the scattered-light intensity should be moni-
tored during the measurement ("count rate"). If this
count rate is not stable, the result ought not to be
interpreted. This restriction is the reason why the
upper measurement limit for PCS is dependent on the

difference in density between particle and dispersion
medium.
To dilute the dispersion to measurement concentration
it is necessary to use extremely clean, dust-free
solvent (generally water). This is particularly impor-
tant for the measurement of small particles and also in
highly dilute dispersions.
Instruments:
Malvern Zetasizer 3000 HSa instrument
Disposable cells 10 x 10 x 48 mm (e.g. Sarstedt Ref.
67.754) for aqueous dispersions or glass cells 10 x 10
x 48 mm for solvent-containing dispersions.
Glass beaker
Syringe (20 ml)
Disposable filter 200 µm (for example Macherey-Nagel
Chromafil A-20/25, cellulose mixed ester)
Disposable pipette
Software, settings
The Malvern Zetasizer 3000 HSa instrument is controlled
via a computer program which also carries out the
evaluation of the measurement signal and allows the
results to be archived and printed out. As regards the
operation of the program, refer to the handbook.
Prior to each measurement it is necessary to perform
the following settings within the software:
Input of the refractive indices of particle and medium
(see above)
Input of the viscosity of the dispersion medium (see
above)
Designation and notes relating to the sample
Path and file for archiving of the result

In addition the following settings are present as
standard and are generally not changed:
10 measurements are collated to form one series and
their average is expressed as the result.
3 such measurements are carried out per sample.
There is a waiting time of 1 minute before each
measurement (for constant temperature).
Attenuator "Auto" (instrument automatically attenuates
the signal intensity down to a factor of 128, so that
it is not necessary to prepare the sample with such
precision).
Data analysis "Contin"
Other settings: dilation 1.20; weighting "quadratic";
size range "Auto"; point select "Auto", by cut off
0.01; optical properties 0.00, core real 1.6, core
imag. 0; Mark-Houwink parameters A 0.5 cm2/s, K
0.000138.
Procedure:
Sample preparation:
20 ml of pure solvent (generally fully demineralized
water) are filtered, using a syringe, through a 200 µm
filter and then introduced into a clean, dust-free
glass beaker. If the dispersion under measurement has
been made acidic or basic, demineralized water adjusted
accordingly is used, in order to prevent a pH shock
when the dispersion is diluted. Then one drop of the
dispersion is added, using a disposable pipette, to the
solvent, and the glass beaker is swirled gently for
some time until the drop has dispersed homogeneously,
and a slight turbid solution has formed. This solution

is introduced using a second disposable pipette into
the cell, which is sealed and fixed in the instrument's
cell holder.
Instrument preparation
The instrument and also the attached computer are
switched on and the associated measurement program is
started. After the instrument has been switched on it
is necessary to wait a while, since laser and Peltier
element must first come up to operating temperature.
Generally, therefore, the instrument is switched off
only in cases of prolonged standstill (several days).
Measurement and evaluation:
At the beginning of the measurement it is necessary
first of all to input the sample parameters and sample
designation and also to select the file in which the
measurements are to be stored. The actual measurements
are started by clicking on the "Go" button. The results
of the three measurements are subsequently compared.
When there is a good match one of the measurements is
selected and represented as a volume-weighted distribu-
tion by selection of the menu item "View -> New plot ->
Volume". Selecting "Copy -> Table as Text" transfers
the result via the Clipboard to Excel, where it is
converted, using a macro, into a report file (XLS). The
Excel file generated in this way is stored on the
server.
If the values of the individual measurements do not
match, a repeat measurement is carried out. If it is
apparent that during the measurements there are sharp
fluctuations in the count rate occurring, the result is
not evaluated and the particle size measurement is
carried out, where appropriate, by a different method.

II. Silicone rubber compounds
Examples 13 to 16
The experiments for determining the surface properties
of silicone vulcanizates were carried out in a speed
mixer, with the following formulation:

A polyethylene beaker which can be screwed closed with
a lid is charged with 31 g (62%) of α,ω-hydroxydi-
methylsiloxypolydimethylsiloxane (E 50, GE Bayer
Silicones). Subsequently 12.5 g (25%) of α,ω-trimethyl-
siloxypolydimethylsiloxane (M1000, GE Bayer Silicones)
are added. After addition of 0.5 g (1%) of carbon black
(Printex 3, Degussa AG) the mixture is mixed in the
speed mixer at 2500 rpm for 15 seconds.
Then 2.0 g (4%) of ethyltriacetoxysilane (AC 3034, GE
Bayer Silicones) are added to this mixture and the
components are mixed in the speed mixer at 2500 rpm for
15 seconds. Subsequently 4.0 g (8%) of fumed silica are
mixed in at 2500 rpm within 30 seconds or in two steps
(2 X 15 seconds) in the speed mixer. Material clinging
to the inner wall is stripped off and the batch is

again mixed at 2500 rpm for 30 seconds. Addition of 1
drop (0.01%) of dibutyltin dilaurate is followed by
speed mixer mixing at 2500 rpm for 120 seconds. The
silicone formulation obtained in this way is coated
with a coating bar and vulcanized at room temperature
in ambient air over a period of 24 hours. The surface
of the vulcanizates is inspected and evaluated on a
school-grade system:
Grades: 1 - very good, 2 = good, 3 = satisfactory, 4 =
unsatisfactory, 5 = deficient
Grade 1 is awarded when the vulcanizate has no defects
or undispersed particles.
For grade 2 there are only a very few defects visible.
In the case of grade 3 there are somewhat more.
In the case of grade 4 the number of visible defects is
such that the surface appearance would be unacceptable
for technical use.
The same applies to a grading of 5, only more so.
When the inventive silicas are used in accordance with
Examples 3, 6 and 9, surprisingly, the silicone
vulcanizates have good surface properties in comparison
to the standard material.
The silicone vulcanizate with the standard material
shows only an unsatisfactory surface, which is also
hardly surprising, since silicas of high surface area
cannot generally be used in silicone sealants, for
precisely that reason.

WE CLAIM:
1. Hydrophilic fumed silica characterized in that it contains no
hydrophobic fractions and has the following physicochemical data:
BET surface area 200 ± 15 m2/g
Grindometer value less than 30 µm
Median value by PCS 65 to 85 nm
2. Process for preparing the hydrophilic fumed silica as claimed in
claim 1, wherein a hydrophilic fumed silica containing no
hydrophobic fractions and having a BET surface area of 200 ± 25
m2/g is ground.
3. Silicone rubber compounds containing 0.5% to 60% by weight of
the hydrophilic fumed silica as claimed in claim 1 and 40% - 99.5%
by weight of an organopolysiloxane of the formula Zn Si R3-n -O-
[Si R2 O]x -Si R3-n -Z'n where R = alkenyl, alkoxy, aryl, oxime,
acetoxy, alkyl radicals, having 1 to 50 carbon atoms, unsubstituted
or substituted by O, S, F, Cl, Br, I, in each case identical or
different, and/or polystyrene, polyvinyl acetate, polyacrylate,
polymethacrylate and polyacrylonitrile radicals having 40-10 000
repeat units, Z = OH, Cl, Br, acetoxy, amino, amineoxy, oxime,
alkoxyamido, alkenyloxy, acryloxy or phosphate radicals, it being
possible for the organic radicals to carry up to 20 carbon atoms,
and in each case identical or different, Z' - oxime, alkoxy, acetoxy,
amino, amido,
n= 1-3
x= 100-15 000.

ABSTRACT

Title: FUMED SILICA
Fumed silica having a BET surface area of 200 + 15
m2/g, a grindometer value of less than 30 µm and a
median value by PCS of 65 to 85 nm is prepared by
grinding a fumed silica having a surface area of
200 + 25 m2/g, and silicone rubber containing his
silica.

FUMED SILICA

Documents:

Inventors:

PCT Conventions: