Hydrophilic silica as filler for silicone rubber formulations

Abstract

Provided is a hydrophilic precipitated silica which is well suited to use in silicone rubber formulations (RTV-1, RTV-2, HTV and LSR), particularly well suited to use in HTV silicone rubber formulations. It has a BET surface area of 185˜260 m.sup.2/g, a CTAB surface area of 100˜160 m.sup.2/g, a BET/CTAB ratio of 1.2˜2.6, and a conductivity of <250 μS/cm. Also provided are a process for producing the precipitated silica and the use of the precipitated silica for thickening and reinforcing silicone rubber formulations.

Claims

1. A precipitated silica, having as physical properties: a BET surface area of 185 to 260 m.sup.2/g; a CTAB surface area of 100 to 160 m.sup.2/g; a BET/CTAB ratio of 1.2 to 2.6; a conductivity of from 1 to 250 (μS)/cm, a median particle size d.sub.50 of 8 to 25 μm, and a tamped density of 50 to 140 g/l.

2. The precipitated silica of claim 1, having a median particle size d.sub.50 of 10 to 20 μm.

3. The precipitated silica of claim 1, wherein a median particle size d.sub.5 value is from 4 to 10 μm.

4. The precipitated silica of claim 1, wherein the conductivity is 1 to 200 (μS)/cm.

5. The precipitated silica of claim 1, having a DBP absorption of 250 to 300 g/100 g.

6. The precipitated silica of claim 1, having a moisture content of less than 7% by weight.

7. The precipitated silica of claim 1, having a pH of 5 to 8.

8. A process for preparing the precipitated silica according to claim 1, the process comprising: a) preparing an initially introduced mixture having a Y value of 10 to 30; b) simultaneously metering in at least one selected from the group consisting of an alkali metal silicate and an alkaline earth metal silicate, and an acidifying agent into the initially introduced mixture with stirring at 80 to 95° C. for 60 to 90 min, to obtain a precipitation suspension; c) reacidifying the precipitation suspension, to obtain a reacidified suspension; d) ageing the reacidified suspension for 5 to 50 min; e) filtering, washing and drying, wherein the at least one selected from the group consisting of the alkali metal silicate and the alkaline earth metal silicate employed in at least one selected from the group consisting of a) and b) has an alkali metal oxide or an alkaline earth metal oxide content in a range of 4 to 7% by weight and a silicon dioxide content in a range of 12 to 28% by weight, and wherein the acidifying agent employed in at least one selected from the group consisting of b) and c) is at least one acidifying agent selected from the group consisting of a concentrated mineral acid, carbonic acid, CO.sub.2 gas, sodium hydrogensulphite, and SO.sub.2 gas, in any correspondingly possible concentration, and wherein the Y value of the precipitation suspension is kept constant at a value between 10 and 30 during the precipitation, wherein kept constant means that the Y value varies by not more than 3% from a starting Y value, i.e. the value directly before beginning the simultaneously metering of the acidifying agent and the at least one selected from the group consisting of the alkali metal silicate and the alkaline earth metal silicate, during b).

9. The process of claim 8, wherein a demineralized water is employed in at least one of a) through e).

10. The process of claim 8, wherein a high shear force acts on the precipitated silica in one of a) through e).

11. The process of claim 8, wherein the drying is effected by spray drying.

12. A method of making a silicone rubber formulation, the method comprising: combining a filler with a silicone rubber formulation wherein the filler comprises the precipitated silica of claim 1.

13. A silicone rubber formulation, comprising the precipitated silica of claim 1.

14. The precipitated silica of claim 1, having a moisture content of 5 to 7% by weight.

15. The precipitated silica of claim 1, having a moisture content of 5.5 to 7% by weight.

Description

EXAMPLE 1

(1) 45.94 m.sup.3 of water are initially introduced into a precipitation container having a capacity of 85 m.sup.3 and 5.08 m.sup.3 of waterglass (Be=29.0; weight ratio of SiO.sub.2 to Na.sub.2O=3.23) are added. The initially introduced mixture is then heated to 64.1° C. The Y value of the initially introduced mixture at the beginning of the precipitation, i.e. the addition of waterglass and sulphuric acid together (about 98.0±0.5% by weight) to the initially introduced mixture, is 19.65. Within 75 min, waterglass (as specified above) and sulphuric acid (as specified above) are then added while maintaining a constant precipitation temperature of 87° C. (maximum variation±0.2° C.) and with introduction of shear energy by means of a turbine, in such a way that the Y value remains constant, i.e. is subject to a maximum variation of ±1.9% about the starting value. After 75 min, the addition of waterglass is stopped and the sulphuric acid is further added until a pH of 3.68 is reached. Thereafter, the suspension is stirred for 20 min at a pH between 3.68 and 3.74.

(2) The suspension obtained is filtered with a chamber filter press and the filter cake is washed with water. The filter cake is then fluidized in a dissolver. The silica feed having a solids content of about 18.9% by weight and a pH of about 5.7 is then spray-dried so that a pH of 6.3, measured as 5% strength suspension, is established in the end product.

(3) The spray-dried product is then milled by means of a mechanical mill having a classifier (Vortex Pulverizing Mill QWJ-60).

(4) The physicochemical data of a representative sample of the spray-dried and unmilled product obtained (Example 1a) and of the milled product (Example 1b) are listed in Table 2.

EXAMPLE 2

(5) 20.52 m.sup.3 of water are initially introduced into a precipitation container having a capacity of 45 m.sup.3 and 2.42 m.sup.3 of waterglass (Be=23; weight ratio of SiO.sub.2 to Na.sub.2O=3.28; SiO.sub.2 content=14.7±0.5% by weight) are added. The initially introduced mixture is then heated to 86° C. The Y value of the initially introduced mixture at the beginning of the precipitation, i.e. the addition of waterglass and sulphuric acid together (about 98.0±0.5% by weight) to the initially introduced mixture, is 20.35. Within 75 min, waterglass (as specified above) and sulphuric acid (as specified above) are then added while maintaining a constant precipitation temperature of 86° C. (maximum variation±0.2° C.) in such a way that the Y value remains constant, i.e. is subject to a maximum variation of ±3.1% about the starting value. After 75 min, the addition of waterglass is stopped and the sulphuric acid is further added until a pH of 3.74 is reached. Thereafter, the suspension is stirred for 20 min at a pH between 3.74 and 3.78.

(6) The suspension obtained is filtered with a chamber filter press and the filter cake is washed with water which was fully demineralized beforehand via reverse osmosis. The filter cake is then fluidized in a dissolver. The silica feed having a solids content of about 18.2% by weight and a pH of about 5.7 is then spray-dried so that a pH of 6.5, measured as 5% strength suspension, is established in the end product.

(7) The spray-dried product is then milled by means of a mechanical mill having a classifier (Vortex Pulverizing Mill QWJ-60).

(8) The physicochemical data of a representative sample of the spray-dried and unmilled product obtained (Example 2a) and of the milled product (Example 2b) are listed in Table 2.

COMPARATIVE EXAMPLES 1 to 4

(9) A conventionally prepared standard silica Ultrasil VN3 from Evonik Degussa GmbH was chosen as Comparative Example 1. In Comparative Examples 2 to 4, precipitated silicas which are developed and sold specially for the reinforcement of silicone rubber elastomers were chosen. Comparative Example 2 is Nipsil LP from Nippon Silica. In Comparative Examples 3 and 4, two different samples of Zeosil 132 from Rhodia Chimie were tested. These two samples show very different properties. The reason is not known to the applicant. It might be, for example, that Rhodia Chimie sells different product qualities under the same name.

(10) TABLE-US-00002 TABLE 2 Physicochemical properties of different silicas Example Example Example Example Comparative Comparative Comparative Comparative 1a 1b 2a 2b Example 1 Example 2 Example 3 Example 4 Moisture 6.0 6.6 6.0 6.7 5.5 7.9 6.4 6.4 content (% by wt.) BET (m.sup.2/g) 249 241 231 223 181 208 183 166 CTAB (m.sup.2/g) 138 136 143 143 146 164 130 124 DBP, based on 280 267 280 271 252 250 262 260 dried substance (g/100 g) Median 55.2 15.6 88.7 15.9 137.5 15.8 11.9 17.5 particle size d.sub.50 (μm) Particle size 10.0 4.5 11.3 5.3 11.8 5.6 5.1 5.6 d.sub.5 (μm) Tamped 245 105 236 112 274 153 173 119 density (g/l) pH 6.3 6.4 6.5 6.5 6.1 5.6 6.2 6.3 Conductivity 146 136 26 30 656 66 166 392 (μS/cm)

EXAMPLE 3

(11) The comparative experiments shown in Table 3 are based on compounding by means of a laboratory kneader. The comparative experiments shown in Table 4 are once again based on compounding by means of a two-roll mill. The experiments were carried out as follows:

(12) Apparatuses:

(13) 1. Laboratory kneader with thermostat, model No.; HI-KD-5 (from Hongyi, Taiwan). 2. Two-roll mill, model No.: LRM-S-110/3E (from Labtech, Taiwan). Roll diameter: 100 mm Roll length: 200 mm Roll nip: 1.0+/−0.05 mm Speed: 15 rpm/20 rpm Friction: 1:1.3 3. Heating press, model No.: GT-7014-A50 (from Gotech, China). Pressure: 0-50 t Temperature range: RT-300° C. Press area: 315.Math.288 mm 4. Two stainless steel plates having the dimensions 330 mm 330 mm.Math.2 mm and a spacer plate of the same thickness with 4 recesses having the dimensions 130 mm.Math.130 mm and two stainless steel plates having the dimensions 300 mm 300 mm.Math.6 mm and a spacer plate of the same thickness with 2 recesses having the dimensions 60 mm.Math.60 mm. 5. High-temperature oven (from ESPEC, China). 6. Tensile tester (from Gotech, China). 7. Datacolor (Datacolor 400™). 8. Further testers for measuring the hardness (Shore A), Williams plasticity and sample thickness (from Gotech, China). 9. Laboratory balance, accuracy at least ±0.01 g.
Raw Materials Used:

(14) HTV Polymer:

(15) Dongjue 110-2 molecular weight 600 000 g/mol, vinyl content: 0.15%, from Nanjing Dongjue, China

(16) Distributor oil/processing auxiliaries:

(17) α,ω-Dihydroxysiloxane, content OH functional groups approx.: 8%, from Shanghai HuaRun Chemical, China.

(18) Crosslinking agent/organic peroxide:

(19) DHBP-C8BS paste (45%) from Qiangsheng Chemical. Co., Ltd., China.

(20) Silicas according to the invention and comparative products (cf. Table 2)

(21) Experimental Section:

(22) Part 1 Compounding by Means of Laboratory Kneader

(23) Formulation:

(24) 100 parts of HTV polymer (1500 g)

(25) 40 parts of silica (600 g)

(26) 3.2 parts of distributor oil/processing auxiliaries (48 g)

(27) 1.0% of crosslinking agent/organic peroxide

(28) Compounding/Preparation:

(29) After adjustment of the rotational speed of tae kneader tool to 20 rpm, 1500 g of HTV polymer are introduced into the kneader chamber. The silica and the processing auxiliary were then added at room temperature in four portions of (40%, 25%, 20% and 15%). 1. 40% by weight=240 g of silica+19.2 g of processing auxiliary 2. 25% by weight=150 g of silica+12.0 g of processing auxiliary 3. 20% by weight=120 g of silica+9.6 g of processing auxiliary 4. 15% by weight=90 g of silica+7.2 g of processing auxiliary

(30) After complete dispersing of the silica, the incorporation time required for the silica was determined (noted). After incorporation of the complete amount of silica, the heating power of the kneader thermostat is set at 170° C. and compounding is then effected for two hours at a temperature of 170° C. (without vacuum). Thereafter, further dispersing in the kneader is effected for 0.5 hour in vacuo but without heating (cooling process). After cooling of the compound, the Williams plasticity (according to ASTM D 926-67) was measured after 24 h. Before the measurement of the optical properties of the compound, special compound test specimens must be prepared. For the preparation of the 6 mm thick compound test specimens, 2 stainless steel plates having the dimensions 300 mm.Math.300 mm.Math.6 mm and a spacer plate of the same thickness with two recesses having the dimensions 60 mm.Math.60 mm are used.

(31) The preparation of the 6 mm HTV compound test specimens (weight taken: 2 times 25 g of compound) is effected in the heating press at room temperature (without addition of the crosslinking agent/peroxide) for 1 min and at a pressure of about 6 bar (6 mPa).

(32) After preparation of uniform test specimens, the measurement of the optical properties of the compound by means of Datacolor 400 can be effected.

(33) The following were determined: 1. l, a, b values, yellow discolouration according to the standard ASTM E 313/DIN 6167. 2. Turbidity according to the standard DIN 53146. 3. Transparency according to the standard DIN 53147.

(34) After cooling of the compound and after storage for 24 hours, the vulcanization of the compound can be effected—as described in Part 3—for determining further performance characteristics.

(35) Part 2 Compounding by Means of a Two-Roll Mil:

(36) Formulation:

(37) 100 parts of HTV polymer (200 g)

(38) 40 parts of silica (80 g)

(39) 3.2 parts of distributor oil/processing auxiliary (6.4 g)

(40) 1.0% of crosslinking agent/organic peroxide

(41) Compounding/Preparation:

(42) After the two-roll mill has been switched on, important working parameters, such as roll nip (1.0+/−0.05 mm) and dispersing speed (20 rpm/15 rpm, i.e. a friction of 1:1.3), are set.

(43) At room temperature, 200 g of HTV polymer are applied to the two-roll mill and dispersed until the compound is present in homogenized form, i.e. the faster roll is completely coated with said compound and has a smooth surface. For this purpose, the two-roll mill is stopped and about 10% of the amount of silica (total amount of silica is 80 g) are introduced in the middle between the two rolls. Thereafter, the complete amount of processing auxiliary (6.4 g of distributor oil) is metered to the silica already applied. By starting the two-roll mill again, the incorporation of the silica into the polymer is now achieved. The addition of further amounts of silica is effected very slowly and stepwise. After about 50% of the total amount of silica were added and incorporated, the compound is removed from the roll, folded and applied again to the roll. The remaining 50% of silica are now incorporated stepwise. Small amounts of silica may fall under the two-roll mill through the roll nip. In this context, it should be ensured that the amounts of this silica are collected on a clean underlay and applied to the two-roll mill again for complete incorporation.

(44) After complete incorporation of the silica and as soon as the compound is present in homogenized form, i.e. the faster roll is completely coated with said compound and said compound has a smooth surface, the incorporation time required for the silica is determined (noted).

(45) The homogenization of the compound is now continued in order to ensure the complete dispersing of the silica. During this procedure, the compound is removed a further five times from the two-roll mill, folded and applied again. The total subsequent dispersing time should be not more than 5 minutes. After the compounding, the compound is removed from the roll in the form of a single long compound hide. The compound hide is now folded into 4 uniform layers and stored on a stainless steel plate at 170° C. for one hour in a hot-air oven.

(46) After this heating, the compound is removed from the oven and is stored for one hour at room temperature for cooling. Thereafter, the compound is again plasticized by rolling with 5-10 minutes (depending on silica properties), i.e. applied again to the two-roll mill and dispersed until the compound is present in homogenized form, i.e. the faster roll is completely coated with said compound and said compound has a smooth surface.

(47) Thereafter, the compound is removed from the roll and the determination of the Williams plasticity (according to ASTM D 926-67) and the preparation of special compound test specimens for the determination of the optical, properties of the compound, as also described in Part 1, follow. Thereafter, the vulcanization of the compound—as described in Part 3—can also be effected for determining further performance characteristics.

(48) Part 3 Vulcanization:

(49) The vulcanization is effected after compounding by means of a laboratory kneader or after compounding by means of a two-troll mill.

(50) For determining the performance characteristics, vulcanizates or HTV test specimens having a thickness of 2 mm (for determining the tensile strength, elongation at break and tear propagation resistance) and having a thickness of 6 mm (for determining the hardness and the optical properties of the HTV vulcanizates) are required.

(51) After the two-roll mill has been switched on, important working parameters, such as roll nip (1.0+/−0.05 mm) and dispersing speed (20 rpm/15 rpm, i.e. a friction of 1:1.3), are set.

(52) At room temperature, the required amount of HTV compound is applied to the two-roll mill and dispersed until a completely replasticized compound (compound plasticized by rolling) has formed.

(53) As soon as the compound is present in homogenized form, i.e. the faster roll is completely coated with said compound, the peroxide addition can be effected.

(54) 1.0% by weight of DHBP-45-PSI (2,5-dimethyl-2,5-di-tert-butylperoxy)hexane based on the mass of compound used, are now added and dispersed.

(55) In order to ensure good dispersing of the peroxide, dispersing is effected for a further 8 minutes. During this rolling time, the compound is removed from roll 8 times altogether, folded and applied again to the roll.

(56) After the peroxide has been dispersed, the compound is now stored for 24 hours away from dust. Before the actual vulcanization, the compound is completely replasticized again by rolling.

(57) After the heating press has been preheated to 170° C., the actual vulcanization is effected. For this purpose the following polished stainless steel plates are used:

(58) For the 2 mm thick test specimens, 2 stainless steel plates having the dimensions 330 mm.Math.330 mm.Math.2 mm and a spacer plate of the same thickness with 4 recesses having the dimensions 130 mm.Math.130 mm are used.

(59) For the 6 mm thick test specimens, 2 stainless steel plates having the dimensions 300 mm.Math.300 mm.Math.6 mm and a spacer plate of the same thickness with 2 recesses having the dimensions 60 mm.Math.60 mm are used.

(60) The vulcanization of the 2 mm HTV test specimens (weight taken: 4 times 35 g of compound) is effected in the heating press at 170° C. for 7 min and at a pressure of about 15 bar (15 mPa).

(61) The vulcanization of the 6 mm HTV test specimens (weight taken: 2 times 25 g of compound) is effected in the heating press at 170° C. for 10 min and at a pressure of about 15 bar (15 mPa).

(62) What is important is that the stainless steel plates used for the vulcanization must be cooled to room temperature before reuse, since otherwise the optical properties of the vulcanizates may be adversely effected.

(63) In order to ensure complete vulcanization and, to remove cleavage products from the vulcanizates, all test specimens are subsequently postvulcanized in a high-temperature circulation oven (at least 120 l of fresh air per minute and per kg of HTV vulcanizate are required) at 200° C. for 4 hours.

(64) The performance characteristics shown in Table 3 and Table 4 can be tested after storage of the HTV vulcanizates in a conditioned room or conditioned chamber according to the requirements of the respective method of measurement. The following standard test methods were chosen for determining the comparative data:

(65) Hardness (Shore A): DIN 53 505

(66) Tensile strength and elongation at break: ISO 37

(67) Tear propagation resistance: ISO 34-1

(68) Optical Properties

(69) Yellow discolouration, l, a, b values: ASTM E 313/DIN 6167

(70) Turbidity: DIN 53146.

(71) Translucence: DIN 53147.

(72) TABLE-US-00003 TABLE 3 Performance characteristics of different silicas in HTV silicone rubber - compounding by means of laboratory kneader Example Example Example Example Comparative Comparative Comparative Comparative 1a 1b 2a 2b Example 1 Example 2 Example 3 Example 4 Compound properties Incorporation 98 80 104 98 150 97 113 83 time of the silica (min) Williams 200 211 228 198 220 190 213 177 plasticity Compound 86.2 85.4 86.3 81.1 76.7 73.8 n.d. 84.2 translucence (%) Compound 82.6 83.1 83.7 79.2 77.3 79.4 84.0 84.4 lightness, l value (%) Compound a 0.6 0.9 1.1 1.3 1.2 1.9 0.8 0.7 value Compound b 7.7 7.9 6.6 7.3 7.4 6.5 7.4 6.9 value Cured and postvulcanized HTV silicone rubber elastomer Hardness (Shore 52 51 55 53 50 58 56 46 A) Tensile 4.7 7.0 6.2 7.7 6.0 7.2 7.4 7.3 strength (N/mm.sup.2) Elongation at 286 340 346 389 300 330 338 409 break (%) Tear 14.3 13.8 15.9 14.7 17.1 15.6 17.6 13.5 propagation resistance (N/mm) Translucence 87.9 89.5 88.1 84.8 80.7 79.9 77.9 87.9 Lightness 80.6 79.1 81.1 78.2 74.4 76.6 n.d. 81.5 (l value) a value −0.6 −0.3 −0.7 −0.5 2 −0.6 n.d. 0.2 b value 20.1 20.5 16.8 17.2 23.7 20.1 n.d. 15.5 Yellow 30.7 32.0 24.3 26.1 41.4 31.6 28.7 22.4 discolouration
Interpretation of the Test Results from Table 3

(73) The precipitated silicas according to the invention (Examples 1a, 1b, 2a and 2b) have substantially reduced incorporation time in comparison with conventionally prepared precipitated silicas (Comparative Example 1), i.e. they can be more rapidly incorporated and more rapidly dispersed/homogenized. If only the precipitated silicas according to the invention (Examples 1b and 2b) are considered, they too have substantially more advantageous incorporation times than precipitated silicas which are already available on the market and are offered especially for silicone rubber applications (Comparative Example 3). The Williams plasticity, a measure of the thickening and the rheological properties of the compounds, is comparable for all examples shown in Table 3. A further advantage of the precipitated silicas according to the invention is found on consideration of the compound translucency.

(74) In this respect, the comparison of the Examples 1a, 1b, 2a and 2b with Comparative Examples 1 and 2 shows that the compound translucency as a measure of the compound transparency is at an extremely high level for all precipitated silicas according to the invention. The same applies to the compound lightness, since the precipitated silicas according to the invention are clearly distinguished from Comparative Example 1, and Examples 1a, 1b, 2a from Comparative Example 2. Even the very good values of Comparative Example 4 are surpassed here.

(75) The lower Shore A hardness of the precipitated silicas according to the invention (Examples 1a, 1b, 2a and 2b) in direct comparison with Comparative Examples 2 and 3 is likewise advantageous since a lower vulcanizane hardness permits increased silica addition (based on the formulation). This means that better mechanical properties could be achieved by addition of more silica at theoretically the same hardness. The mechanical properties, such as, for example, tensile strength and tear propagation resistance, of Examples 1b and 2b are at a similarly high level in comparison with Comparative Examples 2 and 3. However, the results obtained for Example 1a are to be singled out in this respect. Although this is an unmilled silica, an acceptable tensile strength and an acceptable tear propagation resistance can already be achieved in comparison with a milled silica (Comparative Example 2). According to the prior art at present, this is not achievable with conventional unmilled precipitated silicas. However, the following advantages of the precipitated silicas according to the invention are to be singled cut in particular:

(76) Example 1a, 1b, 2a and 2b show an extremely high translucency of the vulcanizates as a measure of the transparency and clarity of the crosslinked silicone systems. This permits use in novel silicon products which require extremely high transparencies. All Comparative Examples (1, 2 and 3) show substantially lower translucency values and the lightness of the vulcanizates, too, was determined with substantially lower values for Comparative Examples 1 and 2.

(77) In the case of highly transparent vulcanizates, it is usually found that an increased b value is also a measure of the yellow component and an increased yellow discolouration. However, in spite of the highly transparent properties, all vulcanizates based on the precipitated silicas according to the invention show only moderate (Examples 1a, 1b) or very low (Examples 2a, 2b) b values and a very slight yellow discolouration (Examples 2a and 2 b).

(78) In this respect, Examples 2a and 2b not only have a substantial improvement in comparison with conventional precipitated silicas (Comparative Example 1) but could also be substantially improved in direct comparison with the precipitated silicas already established in the market, which are offered for silicone rubber applications: Comparative Examples 2 and 3).

(79) TABLE-US-00004 TABLE 4 Performance characteristics of different silicas in HTV silicone rubber - compounding by means of two-roll mill Example Example Example Example Comparative Comparative Comparative Comparative 1a 1b 2a 2b Example 1 Example 2 Example 3 Example 4 Compound properties Incorporation time 20 22 25 25 28 25 20 25 of the silica (min) Williams plasticity 180 191 204 207 213 198 199 180 Compound 70.8 68.6 68.5 70.7 55.5 73.6 76.2 76.8 translucency (%) Compound lightness, 77.4 76.4 76.9 76.7 75.5 77.2 78.0 78 l value (%) Compound a value 1.4 1.0 1.8 1.7 0.1 1.6 1.4 7.3 Compound b value 6.6 6.3 4.7 4.7 2.2 7.0 7.5 7.0 Cured and postvulcanized HTV silicone rubber elastomer Hardness (Shore A) 51 51 53 54 52 60 53 49 Tensile strength 5.3 5.7 5.8 7.0 5.1 7.4 7.3 5.8 (N/mm.sup.2) Elongation at break 288 303 300 340 256 304 349 356 (%) Tear propagation 14.9 14.3 15.1 14.8 14.8 15.7 14.6 15.7 resistance (N/mm) Translucency 85.5 85.6 80.9 83.8 75.3 84.1 87.3 86.5 Lightness (l value) 78.9 78.3 77.1 78.1 69.6 76.9 78.9 83.0 a value 0.4 0.5 0.1 0.2 2.0 0.2 −0.2 −0.4 b value 21.8 23.2 19.3 19.6 23.5 27.3 23.9 17.7 Yellow 43.4 46.5 30.6 31.0 43.5 44.3 37.6 25.4 discolouration
Interpretation of the Test Results from Table 4

(80) The precipitated silicas according to the invention (Examples 1a, 1b, 2a and 2b) have a reduced incorporation time in comparison with conventionally prepared precipitated silicas (Comparative Example 1). However, with the compounding method by means of the two-roll mill which is in any case substantially faster (in comparison with the kneader experiments—Table 3), in some cases substantial differentiation from precipitated silicas which are already available in the market and are offered especially for silicone rubber applications (Comparative Examples 2 to 4) is no longer possible. However, also in comparison with the precipitated silicas which are already available in the market and are offered especially for silicone rubber applications, from Comparative Examples 2 and 4, Examples 1a, 1b according to the invention show up to 20% faster incorporation times.

(81) The Williams plasticity, a measure of the thickening and rheological properties of the compounds, is comparable for all examples shown in Table 4. A further advantage of the precipitated silicas according to the invention is found on consideration of the compound translucency.

(82) In this respect, the comparison of Examples 1a, 1b, 2a and 2b with a conventional precipitated silica (Comparative Example 1) shows that the compound translucency as a measure of the compound transparency is at a high level. However, the very low b values, determined for all precipitated silicas according to the invention, are to be singled out in particular. A low b value is equivalent to very little yellow discolouration of the compound. If the b values of Examples 1a, 1b, 2a and 2b are now compared with Comparative Examples 2 to 4, it becomes clear that the compounds based on the precipitated silicas according to the invention have substantially less yellow discolouration. Unfortunately, the relatively small difference between the b values does not make this as clear as the simple visual assessment of the compounds (even by means of photographs, this difference readily visible to the eye in daylight unfortunately cannot be shown in a reproducible manner).

(83) The lower Shore A hardness of the precipitated silicas according to the invention (Examples 1a, 1b, 2a and 2b) in direct comparison with Comparative Example 2 is likewise advantageous since a lower vulcanizate hardness permits an increased silica addition (based on the formulation).

(84) The mechanical properties, such as, for example, tensile strength and tear propagation resistance, of Example 2b are at a similarly high level in comparison with Comparative Examples 2 and 3. In this respect, however, the results obtained for Examples 1a and 2a are to be singled out in particular. Although these are unmilled silicas, tear propagation resistances which are just as high can be achieved in comparison with the milled precipitated silicas (Comparative Examples 2 and 3).

(85) However, the following advantages of the precipitated silicas according to the invention are to be singled out in particular:

(86) Examples 1a, 1b, 2a and 2b show a high translucency of the vulcanizates as a measure of the transparency and clarity of the crosslinked silicone systems in comparison with a conventional precipitated silica (Comparative Example 1). This permits use in highly transparent silicone products. In the case of highly transparent vulcanizates, an increased b value as a measure of the yellow component and an increased yellow discolouration are also usually found.

(87) However, in spite of the highly transparent properties, all vulcanizates based on the precipitated silicas according to the invention show only moderate (Examples 1a, 1b) or very low (Examples 2a, 2b) b values and very little yellow discolouration (Examples 2a and 2 b).

(88) In this respect, Examples 2a and 2b not only have a substantial improvement in comparison with conventional precipitated silicas (Comparative Example 1) but could also be substantially improved in direct comparison with the precipitated silicas already established in the market, which are offered for silicone rubber applications (Comparative Examples 2 and 3).