Hydrophobic precipitated silica with enhanced properties.

0001The present invention relates to a precipitation silica, which can be used in particular for the preparation of compositions for hot-vulcanizable silicone elastomers, as well as to a process for the preparation of such a precipitation silica.

0002It is known to prepare compositions for hot-vulcanizable silicone elastomers from combustion silica. In general, combustion silicas are satisfactory from the technical point of view for this application and are treated (to make them hydrophobic) before their introduction into the compositions, with a polysiloxane, preferably with cyclopolysiloxanes, in particular with a cyclotetrasiloxane, and more especially with octamethylcyclotetrasiloxane (D₄), at around 400 ° C. Patents mentioning this kind of processing are known, in particular by: - French patent FR-A-1 234 245 from the company RHONE-POULENC, in which, in Example 2, the combustion silica is treated with D₄ at 375 ° C, as well as with - French patent FR-A-1,520,663 from the same Company, in which (see example 1), the operation is carried out at a temperature of 500 ° C.

0003It is also known from Examples 2, 3 and 5 of patent application FR-A-2 411 157 that silica obtained by pyrogenation (combustion) can be treated to make it hydrophobic with a mixture of hexamethylcyclotrisiloxane (D₃) and (D₄), in the presence of a tin compound. This patent application FR-A-2 411 157 further specifies on page 6, in the comparative tests C₁ to C₅ that the pyrogenic silica treated at 200 ° C with the mixture D₃ + D₄ but without the tin compound ( see C₁ to C₄) or without acidic compound (see C₅), does not have a hydrophobic character. It is also mentioned in the description on page 3, lines 31 to 35, that this treatment can take place at temperatures between 50 and 350 ° C.

0004At present, it is sought to use as much precipitation silica as possible, instead of (in whole or in part), combustion silica, since precipitation silica is less expensive. However, it turns out that, unlike combustion silicas, the precipitation silicas currently available on the market, and subjected to a treatment in particular with octamethylcyclotetrasiloxane (D₄) at 400 ° C lose their initial whiteness; they get gray. This is inconvenient because the elastomers obtained from compositions containing such precipitating silicas are themselves intoxicating and they cannot be colorless, or have clear colors when pigments have been introduced into such compositions. It also turns out that by treating a precipitated silica with a mixture D₃ + D₄ at a temperature of 290 ° C or more that this silica loses its initial whiteness in the same way as when it is treated with D₄ to 400 ° C.

0005An object of the present invention is therefore a hydrophobic precipitation silica which can be used as a reinforcing filler in compositions for hot vulcanizable silicone elastomers (EVC), the elastomers obtained having no graying colors, when the latter are not not desired. More particularly, an object of the present invention is a hydrophobic precipitation silica allowing hot vulcanizable elastomers having clear colors to be obtained.

0006Another object of the present invention is a hydrophobic silica which makes it possible to obtain hot vulcanizable elastomers (EVC) having mechanical properties similar to those of EVC obtained from combustion silica.

0007Another object of the present invention is a process which makes it possible to obtain this hydrophobic and non "graying" precipitation silica.

0008It has now been found and this is what is the subject of the present invention, a precipitation silica characterized in that it has a methanol index greater than 20, preferably greater than 25, a luminance greater than 84 (i.e. between 84 and 100), a BET specific surface area greater than 50 m² / g, an average particle size between 0.1 and 20 microns and in that it is treated with a mixture of hexamethylcyclotrisiloxane (D₃) and octamethylcyclotetrasiloxane (D₄), in the absence of a tin compound.

0009The object of the present invention also relates to a process for obtaining silicas according to the invention, as well as compositions for EVC containing such silicas, and the elastomers (vulcanisates) obtained with these compositions.

0010To measure the luminance Y of the precipitation silica according to the present invention, it was carried out as follows. We use the TRISTIMULUS ELREPHO photocolorimeter manufactured by the company CARL ZEISS and allowing the measurement of diffuse reflection (diffuse lighting / 0 ° observation). This device, well known to those concerned with colorimetry, comprises an integration sphere and two light sources (tungsten filament lamps) which illuminate the sample to be analyzed, placed in a tank which has been placed in the lower part of this sphere of integration, which includes at this location a measurement window having a diameter of 35 mm (millimeters). This sample reflects this light through the Y filter positioned in the upper part of the integration sphere and this light intensity (green component of the sample) is measured after it has passed through this Y filter by zeroing l hand of the device's galvanometer, thanks to a graduated rotating drum. It only remains to read on the graduated drum the value obtained, corresponding to the luminance Y of the sample that is analyzed, this drum having been calibrated before measuring the sample, for example using a standard. secondary (VITROLITHE) for which the value of the luminance Y is known with respect to barium sulphate whose luminance Y is equal to 100. Thus with this apparatus, a sample as white as barium sulfate will have a luminance Y of 100, while a rigorously black sample will have a luminance Y of 0, the intermediate values ​​at these two limits corresponding to all the shades of gray. The cells in which the samples for which the luminance Y is to be measured are placed are delivered by CARL ZEISS with the ELREPHO device. These curves make it possible to obtain samples (of powder) whose diameter is 40 mm and whose height is 3 to 5 mm. The samples to be measured thus all have the same dimensions.

0011To measure the hydrophobicity of precipitated silicas (silicon dioxide) according to the present invention, the methanol index was chosen as test, as described in FR-A-2 411 156 (pages 4-5) already cited.

0012This value is determined as follows:

00130.5 g of the silicon dioxide to be tested is dispersed over 50 ml of water. Using a burette, the tip of which dips into water, methanol is added to the water, while stirring, until approximately 95% by weight of the silicon dioxide is immersed. The methanol index is the consumption of methanol in ml.

0014The precipitation silicas according to the present invention are obtained (after having subjected them to the D₃ + D₄ treatment) from precipitation silicas currently available on the "market" or from precipitation silica manufactured by the company RHONE-POULENC according to Example 1 of French patent FR-A-2 471 947. The particle size of the silica used is 1.5 microns and its BET surface area of ​​175 m². As precipitation silica currently available on the market, there may be mentioned: - precipitation silica, reference FK 160, sold by the company DEGUSSA, with an average particle size equal to 6 microns, with a BET specific surface area equal to 160 m² / g, - precipitation silica, NIPSIL LP reference, co-marketed by the company NIPPON SILICA, with an average particle size equal to 10 microns and BET specific surface area of ​​150 m² / g.

0015The precipitation silicas according to the present invention have a BET surface area greater than 50 m² / g, preferably between 50 and 350 m² / g. They have an average particle size between 0.1 and 20 microns, advantageously between 0.5 and 10 microns.

0016The precipitation silicas according to the present invention have a hydrophobicity (water repellency) expressed in methanol index greater than 20, preferably 25 and have a whiteness, expressed in luminance Y greater than 84.

0017The precipitation silicas according to the present invention have a number of groups<chemistry id="chem0001" num="0001"><img file="EP0340129A1_D0001.tif" /></chemistry> fixed by square nanometer (nm²) of silica greater than 1.5, preferably between 2 and 5.

0018To obtain precipitation silicas according to the present invention, the procedure is carried out, for example, in the following manner in the laboratory. A cylindrical device made of glass or stainless steel is used, the internal diameter of which is for example 66 mm, this device comprising in its lower part a sintered glass (n ° 2) and below the latter at least two tubes. , one for the passage of nitrogen and another for the passage of the mixture D₃ + D₄. The precipitation silica to be treated is placed on the sintered glass. This device (whose longitudinal axis is arranged vertically) includes a metal stirrer to stir the silica. This device comprises in its upper part a tube allowing the passage of nitrogen and the mixture D₃ + D₄ (not fixed on the silica) in a glass cyclone in which the gases arrive tangentially and thus have a circular circulation allowing in the part lower cyclone the deposit of the small part of silica possibly entrained by the gas stream. In the upper part of the cyclone (and in its center) is a tube allowing the passage of gases (nitrogen + D₃ + D₄) through a condenser (refrigerant) in which water circulates at 20 ° C. At the outlet of the condenser, the parts of condensed products are collected.

0019The temperature of the device is maintained between 230 and 280 ° C, preferably between 240 and 260 ° C, thanks to an electric oven.

002030 g of precipitation silica are loaded into the cylindrical device for example and a stream of nitrogen of between 5 and 20 l / h (liters per hour) and a mixture of D₃ + D₄ are circulated at a flow rate of between 5 and 40 g / h (grams per hour). The mixture of D₃ + D₄ vaporizes instantly (since the device is heated) as soon as it leaves the tubing opening under the sintered glass. The mixture D₃ + D₄ comprises by weight 5 to 35 parts of D₃ per 100 parts of mixture D₃ + D4) (from 5 to 35% by weight of D₃). The gas mixture circulates in contact with the precipitation silica between 30 minutes and 10 hours, preferably between 1 hour and five hours.

0021Industrially, this treatment of precipitated silica can be carried out, for example, in a fluidized bed or under a pulsed gas stream using conventional apparatuses already used for the treatment of charge. Mention may in particular be made of the devices described in French patents FR-A-1,234,245 and FR-A-1,520,663 from the company RHONE-POULENC.

0022The hot-vulcanizable silicone elastomers obtained with the precipitating silicas according to the invention have clear colors, that is to say not "graying" when pigments have been added to their composition, or are translucent or transparent ( without being gray) when the compositions do not contain pigments. It has been found that the best mechanical qualities of these hot vulcanizable elastomers, also called vulcanizates, have been obtained with a precipitation silica according to the invention having an average particle size of 1 to 7 microns.

0023The hot vulcanizable elastomers (vulcanizates) obtained with the precipitation silica according to the present invention have a luminance Y of at least 47. These luminance measurements Y are carried out with the ELREPHO photometer described above and following the same mode of calibration, on vulcanizates having a thickness of 2 mm which are each placed on a calibrated support (VITROLITHE Y = 77.85), the measurements having been carried out as soon as the vulcanizates have been demolded and cooled.

0024The siloxane compositions which make it possible to obtain hot vulcanized elastomers (vulcanisates) having the above properties include by weight:<ul id="ul0001" list-style="none"><li>a) - 100 parts of a diorganopolysiloxane gum having a viscosity greater than 1,000,000 mPa.s at 25 ° C,</li><li>b) - 0.1 to 7 parts of an organic peroxide,</li><li>c) - 5 to 150 parts of a precipitation silica according to the present invention.</li></ul>

0025This diorganopolysiloxane gum, cited in a), of viscosity greater than 1,000,000 mPa.s at 25 ° C, is formed of a chain of units of formula R₂SiO, blocked at each end of its chain by a unit of formula R₃SiO_0,5 and / or a radical of formula OR ′; in these forms the symbols R, which are identical or different, represent methyl, ethyl, n-propyl, phenyl, vinyl and 3,3,3-trifluoropropyl radicals at least 60% of these radicals being methyl and at most 3% being vinyl , the symbol R ′ represents a hydrogen atom, an alkyl radical having from 1 to 4 carbon atoms, the betamethoxy-ethyl radical.

0026These compositions preferably further comprise d) from 0.5 to 15 parts of a diorganopolysiloxane oil of viscosity at most 5,000 mPa.s at 25 ° C. formed from a chain of units of formula R˝₂SiO blocked at each end of its chain with a radical of formula OR ′; in these formulas the symbols R˝, identical or different, represent methyl, phenyl, vinyl radicals, at least 40% of these radicals being methyl and the symbol R ′ has the meaning given above in a).

0027The various constituents of these compositions are explained in more detail below.

0028Diorganopolysiloxane gums a) of viscosity greater than 1,000,000 mPa.s at 25 ° C, preferably greater than 2,000,000 mPa.s at 25 ° C, are linear polymers, of high molecular weight, the diorganopolysiloxane chain of which is consisting essentially of the units of the above-mentioned formula R₂SiO; this chain is blocked at each end by patterns of formulas R₃SiO_0,5 and / or the radical of formula OR ′. The presence, along the diorganopolysiloxane chain, of small amounts of units other than R₂SiO, for example of RSiO formulas_1,5 and / or SiO₂ is however not excluded in the proportion of at most 2% relative to the number of R₂SiO units. Although the meaning of the radicals R and R ′ is explained above, it should be specified that by alkyl radical is meant C₁-C₄ alkyl radicals and more particularly the methyl, ethyl, n-propyl and n-butyl radicals.

0029As concrete examples of patterns of formulas R₂SiO and R₃SiO_0,5 and radicals of formula OR ′, can be cited those of formulas: (CH₃) ₂SiO, CH₃ (CH₂ = CH) SiO, CH₃ (C₆H₅) SiO, (C₆H₅) ₂SiO, CH₃ (C₂H₅) SiO, (CH₃CH₂CH₂) CH₃SiO, CH₃ (n.C₃H₇) SiO, (CH₃) (C₆H₅) (CH₂ = CH) SiO_0,5, -OH, -OCH₃, -OC₂H₅, -On.C₃H₇, -O-iso.C₃H₇, -On.C₄H₉, -OCH₂CH₂OCH₃.

0030These gums a), which can be used as a mixture to represent the 100 parts of gum of the compositions according to the present invention, generally comprise less than 3% of vinyl groups and most often these vinyl groups represent from 0.005 to 1.5% of the number of radicals linked to silicon atoms.

0031The gums a) are marketed by manufacturers of silicones; on the other hand, they can be manufactured by operating according to already known techniques.

0032The diorganopolysiloxane oil mentioned in d) is used in an amount of 0 to 15 parts, preferably from 0.3 to 12 parts per 100 parts of gum a). This oil or these oils are linear polymers of relatively low viscosity , at most 5,000 mPa.s at 25 ° C, preferably at most 4,000 mPa.s at 25 ° C, of ​​which the diorganopolysiloxane chain is formed essentially of units of the above formula R˝ SiO; this chain is blocked at each end by a radical of the above formula OR ′. At least 40% of the radicals R˝ are methyl radicals, preferably at least 45%.

0033The meaning of the symbols R˝ and R ′ has been explained previously.

0034As concrete examples of units of formula R˝₂SiO and radicals of formula OR ′, those of formulas can be cited: (CH₃) ₂SiO, CH₃ (CH₂ = CH) SiO, CH₃ (C₆H₅) SiO, (C₆H₅) ₂SiO, C₆H₅ _(CH2 = CH) SiO -OH, -OCH₃, -OC₂H₅, -On.C₃H₇, -OCH₂CH₂OCH₃.

0035Preferably, are used: - dimethylpolysiloxane oils blocked at each end of their chain by hydroxyl, methoxyl, betamethoxyethoxyl radicals, with a viscosity of 10 to 200 mPa.s at 25 ° C; - methylphenylpolysiloxane oils, consisting of CH₃ (C₆H₅) SiO units, blocked at each end of their chain by hydroxyl and / or methoxyl radicals, with a viscosity of 40 to 2,000 mPa.s at 25 ° C.

0036The use of these oils d) has the object of preventing the compositions of the invention from undergoing an evolution during storage, and more precisely from being structured, from hardening; they are therefore "antistructure" agents. Their use is especially recommended when the quantities of reinforcing silicas c) are high, for example above 30-40 parts per 100 parts of gums a).

0037Other "antistructure" agents can replace all or part of the oils d), for example diphenylsilanediol and the silanes of formulas:<chemistry id="chem0002" num="0002"><img file="EP0340129A1_D0002.tif" /></chemistry>

0038However, they are often more expensive and / or require more work for their dispersion in the compositions of the invention than oils d).

0039The organic peroxides b) are used in an amount of 0.1 to 7 parts, preferably 0.2 to 5 parts, per 100 parts of the gums a). They are well known to technicians and more particularly include benzoyl peroxide, 2,4-dichloro-benzoyl peroxide, dicumyl peroxide, bis (t-butylperoxy) 2,5 hexane, t-butyl perbenzoate, t-butyl and isopropyl peroxy carbonate, di-t-butyl peroxide, bis (t-butylperoxy) -1,1-3,3,5-trimethyl cyclohexane, 2,5-dimethyl di- (tert-butyl peroxy -2.5) hexane.

0040These various peroxides decompose at sometimes different temperatures and speeds. They are chosen according to the hardening conditions required.

0041These compositions allowing non-graying vulcanizates to be obtained can also comprise 0.02 to 4 parts (per 100 parts of gum a)), preferably 0.03 to 3 parts of an organosilicon compound comprising, connected to the silicon atom, at least one acryloyloxyalkylene to metharcryloyloxyalkylene group, said compound having the formula:<chemistry id="chem0003" num="0003"><img file="EP0340129A1_D0003.tif" /></chemistry> in which the symbol R˝ represents methyl, phenyl, vinyl radicals, the symbol R₃ represents a hydrogen atom, the methyl radical, the symbol R₄ represents a methyl, ethyl, n-propyl, betamethoxyethyl radical, the symbol w represents a number from 1 to 5 and the symbol u represents a number from 0 to 2.

0042Products of this type are described, for example in American patent US Pat. No. 3,567,497. As concrete examples, the products having the following formulas may be cited: CH₂ = CH-COOCH₂Si (OCH₃) ₃ CH₂ = CH-COOCH₂Si (OCH₂CH₂OCH₃) ₂C₆H₅ CH₂ = CH-COO (CH₂) ₃Si ((OC₂H₅) ₃ CH₂ = C (CH₃) COO (CH₂) ₃Si (OCH₃) ₃ CH₂ = C (CH₃) COO (CH₂) ₃SiCH₃ (OCH₃) ₂ CH₂ = C (CH₃) COO (CH₂) ₅Si (OCH₃) ₃

0043The preparation of these compositions is carried out using known mechanical means, for example kneaders, cylinder mixers, screw mixers. The various components are incorporated into these devices in any order that may be arbitrary. It is however recommended to first load the gums a) and the precipitated silica c) according to the present invention, then to load the peroxide b) last.

- EXAMPLES:

0044Precipitation silica reference RP 175 MS, manufactured by the company RHONE-POULENC according to Example 1 of French patent FR-A-2 471 947, with an average particle size equal to 1.5 microns and a BET surface area equal to 175 m² / g has been processed respectively: - with D₄ at 400 ° C for 2 hours, - with D₄ at 250 ° C, for 2 hours, - by a mixture D₃ + D₄, at 250 ° C, for 4 hours.

0045In each of these treatments, the amount of silica used was 30 g. The apparatus used is that made of glass mentioned in the description above. The agitator was rotating at 90 revolutions per minute. The nitrogen flow was 15 l / h (liters per hour) while the flow of D₄ or D₃ + D₄ was 20 g / h. For the treatment with D₃ + D₄ this mixture contained by weight 33 parts of D₃ and 67 parts of D₄.

0046Elastomers (vulcanisates) were prepared from each of these silicas having undergone a different treatment. These vulcanizates were produced by mixing (by weight) on a three-cylinder mixer, at room temperature: - 100 parts of a diorganopolysiloxane gum, viscosity 5 million mPa.s (millipascals.second) at 25 ° C, blocked at each end of its chain by a trimethylsiloxy unit, comprising 0.2% by mole of methylvinylsiloxy group in his chain, - 40 parts of RP 175 MS silica having undergone one of the 3 treatments mentioned above, 1.4 parts of a dimethylpolysiloxane oil with a viscosity of 40 mPa.s at 25 ° C, blocked at each end of its chain by a hydroxyl radical linked to the terminal silicon atom, - 0.5 parts of 2,5-dimethyl-2,5-di (tert-butylperoxy) hexane.

0047The mixture obtained is then vulcanized for 10 minutes at 170 ° C. under 150 bars and samples are obtained whose thickness is 2 mm.

0048The table below specifies the properties of RP 175 MS silicas obtained after treatment with D₄ (at 400 ° C and 250 ° C) or with D₃ + D₄ (at 250 ° C), as well as the properties of vulcanizates obtained with silicas. <tables id="tabl0001" num="0001"><table frame="all"><title>BOARD</title><tgroup cols="4" colsep="1" rowsep="1"><colspec colnum="1" colname="col1" colwidth="39.37mm" /><colspec colnum="2" colname="col2" colwidth="39.37mm" /><colspec colnum="3" colname="col3" colwidth="39.37mm" /><colspec colnum="4" colname="col4" colwidth="39.37mm" /><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">Product (s) used for treatment</entry><entry namest="col2" nameend="col2" align="right">D4</entry><entry namest="col3" nameend="col3" align="right">D4</entry><entry namest="col4" nameend="col4" align="right">D3 + D4</entry></row><row><entry namest="col1" nameend="col1" align="left">Processing temperature</entry><entry namest="col2" nameend="col2" align="right">400 ° C</entry><entry namest="col3" nameend="col3" align="right">250 ° C</entry><entry namest="col4" nameend="col4" align="right">250 ° C</entry></row><row><entry namest="col1" nameend="col1" align="left">% of carbon fixed on the silica after treatment</entry><entry namest="col2" nameend="col2" align="right">1,9</entry><entry namest="col3" nameend="col3" align="right">1,3</entry><entry namest="col4" nameend="col4" align="right">2,0</entry></row><row><entry namest="col1" nameend="col1" align="left">Number of groups D attached to silica per nm</entry><entry namest="col2" nameend="col2" align="right">2,9</entry><entry namest="col3" nameend="col3" align="right">1,9</entry><entry namest="col4" nameend="col4" align="right">3,1</entry></row><row><entry namest="col1" nameend="col1" align="left">Methanol index of treated silica</entry><entry namest="col2" nameend="col2" align="right">37</entry><entry namest="col3" nameend="col3" align="right">15</entry><entry namest="col4" nameend="col4" align="right">38</entry></row><row><entry namest="col1" nameend="col1" align="left">Luminance Y of the initial silica</entry><entry namest="col2" nameend="col2" align="right">96</entry><entry namest="col3" nameend="col3" align="right">96</entry><entry namest="col4" nameend="col4" align="right">96</entry></row><row><entry namest="col1" nameend="col1" align="left">Luminance Y of the treated silica</entry><entry namest="col2" nameend="col2" align="right">75</entry><entry namest="col3" nameend="col3" align="right">90</entry><entry namest="col4" nameend="col4" align="right">92</entry></row></tbody></tgroup><tgroup cols="4" colsep="1" rowsep="1"><colspec colnum="1" colname="col1" colwidth="39.37mm" /><colspec colnum="2" colname="col2" colwidth="39.37mm" /><colspec colnum="3" colname="col3" colwidth="39.37mm" /><colspec colnum="4" colname="col4" colwidth="39.37mm" /><thead valign="top"><row><entry namest="col1" nameend="col4" align="center">VULCANISATS (elastomers)</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">DS (A)</entry><entry namest="col2" nameend="col2" align="right">47</entry><entry namest="col3" nameend="col3" align="right">51</entry><entry namest="col4" nameend="col4" align="right">46</entry></row><row><entry namest="col1" nameend="col1" align="left">AR</entry><entry namest="col2" nameend="col2" align="right">420</entry><entry namest="col3" nameend="col3" align="right">260</entry><entry namest="col4" nameend="col4" align="right">430</entry></row><row><entry namest="col1" nameend="col1" align="left">RR (MPa)</entry><entry namest="col2" nameend="col2" align="right">5,2</entry><entry namest="col3" nameend="col3" align="right">3,4</entry><entry namest="col4" nameend="col4" align="right">4,9</entry></row><row><entry namest="col1" nameend="col1" align="left">RD (kN / m)</entry><entry namest="col2" nameend="col2" align="right">15,5</entry><entry namest="col3" nameend="col3" align="right">11,5</entry><entry namest="col4" nameend="col4" align="right">15,0</entry></row><row><entry namest="col1" nameend="col1" align="left">DRC (%)</entry><entry namest="col2" nameend="col2" align="right">15</entry><entry namest="col3" nameend="col3" align="right">32</entry><entry namest="col4" nameend="col4" align="right">20</entry></row><row><entry namest="col1" nameend="col1" align="left">Resistivity (in ohm.cm)</entry><entry namest="col2" nameend="col2" align="right">2.0x10</entry><entry namest="col3" nameend="col3" align="right">1.1x10</entry><entry namest="col4" nameend="col4" align="right">2.3x10</entry></row><row><entry namest="col1" nameend="col1" align="left">Luminance Y of the vulcanizate with untreated silica</entry><entry namest="col2" nameend="col2" align="right">52</entry><entry namest="col3" nameend="col3" align="right">52</entry><entry namest="col4" nameend="col4" align="right">52</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">Luminance Y of the vulcanizate with treated silica</entry><entry namest="col2" nameend="col2" align="right">38</entry><entry namest="col3" nameend="col3" align="right">48</entry><entry namest="col4" nameend="col4" align="right">50</entry></row></tbody></tgroup></table></tables>

0049In this table the abbreviations have the following meaning: <tables id="tabl0002" num="0002"><table frame="all"><tgroup cols="2" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="78.75mm" /><colspec colnum="2" colname="col2" colwidth="78.75mm" /><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">- DSA:</entry><entry namest="col2" nameend="col2" align="left">Shore A hardness according to ASTMD 2240,</entry></row><row><entry namest="col1" nameend="col1" align="left">- RR:</entry><entry namest="col2" nameend="col2" align="left">breaking strength in MPa (megapascals) according to standard AFNOR T 46002 corresponding to standard ASTMD 412,</entry></row><row><entry namest="col1" nameend="col1" align="left">- AR:</entry><entry namest="col2" nameend="col2" align="left">elongation at break in% according to AFNOR T 46002,</entry></row><row><entry namest="col1" nameend="col1" align="left">- RD:</entry><entry namest="col2" nameend="col2" align="left">tear resistance in KN / m (kilo newton-meter) according to standard ASTM D 624,</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">- DRC:</entry><entry namest="col2" nameend="col2" align="left">Remanent compression deformation measured in%.</entry></row></tbody></tgroup></table></tables>

0050This table shows that the precipitation silica RP 175 MS having undergone the treatment D₃ + D₄ at 250 ° C (that is to say in the temperature range claimed) has a luminance Y greater than 84 and a methanol index greater than 20, while the same silica treated with D₄ at 400 ° C has a substantially identical methanol index but a much lower luminance, and that the silica treated with D₄ at 250 ° C has a slightly higher Y luminance low and a much lower methanol index.

0051This table also shows that the vulcanizate obtained with the silica treated with D₃ + D₄ at 250 ° C has the best luminance Y and mechanical properties corresponding to the properties of the vulcanizate obtained with the silica treated with D₄ at 400 ° C.

0052It should be noted that similar results are obtained with the precipitation silica reference FK 160 sold by the company DEGUSSA, with an average particle size equal to 6 microns.

0053On the other hand, if the precipitated silica FK 160 or RP 175 MS is treated with D₃ + D₄ at a temperature above 280 ° C., the luminance Y of such a silica is less than 84 and the vulcanizate obtained has a luminance less than 47. If, on the other hand, these silicas are treated with D₃ + D₄, at a temperature below 230 ° C., the mechanical properties of the vulcanizates are less good and the methanol index of this silica is less than 20.