Hydrophobicization of silicas under oxidizing conditions

The invention relates to a process for the preparation of hydrophobic silicas.

The surface modification of highly disperse silicas with dimethyldichlorosilane in the fluidized bed is known (<patcit id="pcit0001" dnum="DE1163784"><text>DE 1163784</text></patcit>). In this case, hydrophobic silicas are obtained which, in addition to a considerable proportion of unreacted silanol silanol groups, have a highly hydrophobic surface modification. When using these silicas, for example for the rheology control of polar coatings or adhesives and sealants, this can lead to a prolonged period of incorporation into the formulations despite unreacted silanol groups. Furthermore, the process is uneconomical due to the high temperatures required.

It is also known to modify highly disperse silicas with low-volatility of siloxanes (<patcit id="pcit0002" dnum="EP686676A"><text>EP 686676</text></patcit>). In this case, highly hydrophobic silicas are obtained with a very small proportion of unreacted silanol groups. The highly non-polar surface siloxane layer can lead to a worsening of the particles in polar coating, adhesive or sealants, and thus to a prolonged incorporation time of the silicas into the liquid medium.

It was an object of the present invention to improve the prior art, in particular to develop a silica and a process which leads to silica particles which have a relatively polar surface modification with at the same time a constant or even a lower residual silanol content compared to hydrophobic silicas obtained by known methods.

Out <patcit id="pcit0003" dnum="EP924269A"><text>EP 924269</text></patcit> The surface modification of silicas with dimethyldisiloxy groups and monomethyltrisiloxy groups is known. Cyclic siloxanes are reacted with the silica at high temperatures (550 ° -600 ° C.). This results in silicas with an increased carbon content, ie better hydrophobation of the silica. The particles described there have a higher methanol number, ie a low polarity, than particles which have been reacted at low temperature. The reaction at high temperatures is uneconomical due to energy costs. A disadvantage of these particles is that they have a smaller proportion of surface silanol groups, but at the same time also have a higher methanol number, ie a higher hydrophobicity.

The co-silylation of silicic acid with dimethyldichlorosilane and methyltrichlorosilane is also known. The problem here is the distinctly different reactivity of the silanes, which can lead to an inhomogeneous distribution of D and T groups and the formation of large amounts of HCl, which must be purified from the product.

The invention relates to silicas which are mixed with groups of the general formulas (I) and (II)         R₂Si (O-)₂     (I)         RSi (O-)₃     (II) Wherein:<ul><li>R is a monovalent, optionally mono- or polyunsaturated, optionally branched hydrocarbon radical having 1 to 24 carbon atoms, where</li><li>The relative proportion of the T groups from T¹-, T²- and T³Groups on the total silicon-organic content of the silica is greater than 0.1%, the relative proportion of the T¹Groups (F_T1 = I_T1/ (I_T1 + I_T2 + I_T3)) </li><li>Less than 10% and the relative proportion of the T2 groups (F_T2 = I_T2/ (I_T1 + I_T2 + I_T3)) Is greater than 5%, wherein a T group is a monoalkyl trisiloxy group R-Si (O-)₃, The high number being the number of siloxane bonds, such that:</li><li>T¹: R-Si (OR')₂-O-Si</li><li>T² : R-Si (OR ') (- O-Si)₂</li><li>T₃: R-Si (-O-Si)₃, That is,</li><li>Where R can be an Si-C-bonded optionally substituted alkyl radical and R 'can be an alkyl group or a hydrogen atom.</li></ul>

Surprisingly and in no way by the person skilled in the art, it has now been found that a surface modification layer produced by the process according to the invention, comprising dimethyldisiloxy groups and monomethyltrisiloxy groups, leads to silicas with a low proportion of unreacted surface silanol groups, while at the same time not relatively polar surface modification layer. Silicas are reacted with dialkyldihalogeno or dialkyldialkoxysilanes or polydimethylsiloxanes (PDMS) under oxidizing conditions, ie, for example, in air at temperatures below 400.degree. Under these reaction conditions, relatively polar SiOH-containing siloxane chains are formed. The SiOH groups can be prepared by means of²⁹Si-CMPAS NMR spectroscopy as T² Groups. A T-group is a monoalkyl trisiloxy group R-Si (O-)₃, The number being raised represents the number of siloxane bonds. That is,<ul><li>T¹: R-Si (OR')₂-O-Si</li><li>T²: R-Si (OR ') (- O-Si)₂</li><li>T³ : R-Si (-O-Si)₃,</li></ul>Where R can be an Si-C-bonded optionally substituted alkyl radical and R 'is an alkyl group or hydrogen atom.

Analogously, a D-group is understood as meaning a dialkyldisiloxy group (R-)₂Si (O-)₂.

The silicas obtained according to the invention are mixed with groups of the general formulas (I) and (II)         R₂Si (O-)₂     (I)         RSi (O-)₃     (II), wherein<ul><li>R is a monovalent, optionally mono- or polyunsaturated, optionally branched hydrocarbon radical having 1 to 24 carbon atoms.</li></ul>

Preferred radicals R are alkyl radicals such as methyl, ethyl, propyl, hexyl or n-hexyl or i-hexyl, octyl or n-octyl or i-octyl, n-dodecyl, N-hexadecyl or n-octadecyl radicals. The methyl radical is particularly preferred.

Preferred groups on the silica surface are the dimethylsilyldioxy group (CH₃) "₂Si (O-)₂ And the methylsilyltrioxy group CH₃Si (O-)₃.

The silicic acid surface of the silicas obtained according to the invention is simultaneously modified with both groups of the general formulas (I) (= D groups) and (II) (= T groups). Preferably, the relative proportion of the T groups in the total silicic acid of the silica is preferably greater than 0.1%, more preferably 0.1% to 30%, most preferably 0.5% to 20% and in a particular embodiment 1% to 10% %. The individual proportions of the total silicon organic can be obtained, for example, by means of the integration of the corresponding signal intensities of a²⁹Si CPMAS NMR spectrum, ie, F_T = I_T/ (I_T + I_D), Where F_T The proportion T groups and I_T Respectively_D The NMR signal intensities of the T and D groups, respectively.

The T groups of the silicas obtained according to the invention are composed of T¹-, T²- and T³-Groups. The proportion of T¹Groups (F_T1 = I_T1/ (I_T1 + I_T2 + I_T3)) Is preferably less than 10%, preferably less than 5%, particularly preferably less than 1%, and in a special embodiment no T¹Groups.

The relative share of T²Groups (F_T2 = I_T2/ (I_T1 + I_T2 + I_T3)) Is preferably greater than 5%, preferably 5% to 75 ° C., particularly preferably 10% to 50%.

The silicas obtained according to the invention have a smaller proportion of unreacted surface silanol groups than silicas, which have been reacted with identical amounts of silylating agent but under inert gas. The proportion of unreacted surface silanol groups is preferably up to 90% lower, preferably up to 75%, and more preferably up to 50%, than in the case of silicas which have been reacted with identical amounts of silylating agent but under inert gas.

The silicas obtained according to the invention have an equal or smaller methanol number than silicas which have been reacted with identical amounts of silylating agent but under inert gas. The methanol number is preferably up to 50% lower, preferably up to 25%, and more preferably up to 20%, than in silicas which have been reacted with identical amounts of silylating agent but under inert gas.

The silicas obtained according to the invention have a carbon content of greater than 0.1% by weight, preferably a carbon content of from 0.1% by weight to 10% by weight and particularly preferably a carbon content of from 0.5% by weight to 7.5% by weight, on.

The silicas obtained according to the invention have a small proportion of extractable organosilicon constituents. This proportion is preferably less than 10% by weight, more preferably less than 5% by weight.

The silicas obtained according to the invention have a DBP number of preferably less than 300, preferably 250-100 and particularly preferably 225-150.

The silicas obtained according to the invention have a high specific surface area (according to DIN EN ISO 9227 / DIN 66132).

Preferably, the specific surface area is 10 m²/ G up to 450 m²/ G, preferably 20 m²/ G up to 400 m²/ G and whole, more preferably 30 m²/ G up to 350 m²/G.

The invention further provides a process for preparing a silica modified with the groups of the general formulas (I) and (II), wherein the hydrophilic silicic acid is reacted with silanes of the general formula III,         R₂SiX₂     (III) Where X is halogen, OH or OR, and R is as defined above Or with siloxanes of the general formula (IV),         (R._ASiX_B(O-)_1/2) (Si (R)₂(O-)_2/2) "_N(-O)_1/2X_BSiR_A) " <ul><li>Where X and R are as defined above and</li><li>A is 2 or 3,</li><li>B is 0 or 1, with the proviso that a + b = 3,</li><li>N is 1 to 10⁴, Preferably 1 to 1000, more preferably 3 to 100, the viscosity of the polysiloxanes used being greater than 0.5 mPas, preferably 1 mPas-10⁶ MPas and more preferably 1 mPas-1000 mPas, at 25 ° C,</li></ul>Or with any mixtures of the silanes of the general formula (III) and / or siloxanes of the general formula (IV), with the proviso that at temperatures below 400 ° C. and under oxidizing conditions, for example air, oxygen or nitrous oxide, Is modified.

The surface-modified silica may be prepared in continuous or discontinuous processes, the process for modification may be constructed from one or more steps. The surface-modified silica is preferably prepared by a process in which the production process is carried out in separate steps: (A) initially preparing the hydrophilic silica, (B) modifying the silica with (1) loading the hydrophilic silica with silanes of the general formula (III) ), Or with siloxanes of the general formula (IV) or with any mixtures of the silanes of the general formula (III) and / or siloxanes of the general formula (IV),

(2) reacting the silica with the compounds provided; and (C) purifying the silica from excess compounds and byproducts introduced.

The surface treatment is preferably carried out in an atmosphere which leads to the partial oxidation of the modified silica, ie preferably nitrous oxide, oxygen or air, preferably more than 5% by volume of oxygen, more preferably more than 10% by volume of oxygen.

Allocation, reaction and purification can be carried out as a discontinuous or continuous process.

For technical reasons, continuous reaction is preferred.

The coating (step B1) is preferably carried out at temperatures below 400 ° C., preferably from -30 to 250 ° C., more preferably 20 to 150 ° C., particularly preferably 20 to 80 ° C. In a very particularly preferred embodiment, the coating step is carried out at from 30 ° to 50 ° C.

The residence time is preferably 1 min-24 h, preferably 15 min-300 min, for reasons of the space-time yield, particularly preferably 15 min to 240 min.

The pressure in the occupancy ranges from preferably low vacuum to 0.2 bar up to an overpressure of 100 bar. For technical reasons, normal pressure, ie, pressure-free work, is preferred to external / atmospheric pressure.

The silanes of the general formula III or siloxanes of the general formula IV are preferably added in liquid form, and in particular are admixed with the pulverulent silicic acid. The compounds can be admixed in pure form or as solutions in known, technically used solvents such as, for example, alcohols such as, for example, methanol, ethanol or i-propanol, ethers such as diethyl ether, THF or dioxane or hydrocarbons, eg hexanes or toluene. The concentration in the solution is preferably from 5 to 95% by weight, preferably from 30 to 95% by weight, particularly preferably from 50 to 95% by weight.

The admixing is preferably effected by means of nozzle techniques, or comparable techniques, such as effective atomization techniques, such as atomizing in 1-material nozzles under pressure (preferably 5 to 20 bar), spraying in 2-substance nozzles under pressure (preferably gas and liquid 2-20 bar) With atomizers or gas-solid exchange units with movable, rotating or static internals which permit a homogeneous distribution of the silanes of the general formula III or siloxanes of the general formula IV with the pulverulent silica.

The aerosol can be impregnated from above onto the fluidized solid or into the fluidized solid.

The silanes of the general formula III or siloxanes of the general formula IV are preferably added as finely divided aerosol, characterized in that the aerosol has a settling rate of 0.1-20 cm / s.

If the silanes of the general formula III or siloxanes of the general formula IV are decomposed by evaporative compounds, ie, compounds whose boiling point at normal pressure is less than 200 ° C., these are preferably admixed as a vapor of the pulverulent silica.

If desired, protic solvents can be added to the silanes of the general formula III or siloxanes of the general formula IV, such as liquid or vaporizable alcohols or water; Typical alcohols are isopropanol, ethanol and methanol. Mixtures of the above-mentioned protic solvents may also be added. Preferably, from 1 to 50% by weight of protic solvent is added based on the silica, more preferably from 5 to 25% by weight. Water is particularly preferred.

Optionally, acidic catalysts of acidic character in the sense of a Lewis acid or a Bronsted acid, such as hydrogen chloride or basic catalysts, of basic character in the sense of a Lewis base or a Brönsted base, such as ammonia or amines such as triethylamine, can also be added. These are preferably added in traces, ie less than 1000 ppm. Catalysts are not particularly preferred.

The loading of the silica and the reaction with the silanes of the general formula III or with siloxanes of the general formula IV are preferably carried out under mechanical or gas-supported fluidization.

A gas-borne fluidization can be carried out by means of inert or oxygen-containing process gases, preferably fluidization by air. The supply of the gases for fluidization preferably takes place in the range of open-tube gas velocities of 0.05 to 5 cm / s, particularly preferably of 0.5 to 2.5 cm / s.

Particular preference is given to the mechanical fluidization which takes place without additional gaseous addition, by blade stirrers, anchor stirrers, and other suitable stirring elements.

The reaction (step B2) is preferably carried out at temperatures of less than 400 ° C., preferably at temperatures of 20 ° -380 ° C., particularly preferably 100 ° -350 ° C. and very particularly preferably 150 ° -350 ° C.

The cleaning (step C) is preferably carried out at a cleaning temperature of 20 to 400 ° C., preferably 50 ° C. to 350 ° C., particularly preferably 100 to 300 ° C.

The cleaning step is preferably characterized by movement, slow movement and low mixing being particularly preferred. The agitating elements are advantageously adjusted and moved in such a way that preferably mixing and fluidizing, but not complete swirling, occur.

The cleaning step can also be characterized by an increased gas input, corresponding to an empty tube gas velocity of preferably 0.001 to 10 cm / s, preferably 0.01 to 1 cm / s. This can be effected by all inert gases which do not react with the silanes of the general formula II, the silicic acid, and the modified silicic acid, ie do not lead to secondary reactions, decomposition reactions, oxidation processes and flame and explosion phenomena, such as, preferably, N₂, Ar, other rare gases, CO₂, Etc., or also by oxygen-containing gases, preferably by air.

In a particularly preferred embodiment, unreacted silanes of the general formula III or siloxanes of the general formula IV and exhaust gases from the purification step are recycled back into the step of the loading and charging of the silica; This can be carried out partially or completely, preferably 10 to 90% of the total volumetric flow of the gas volumes emerging from the purification.

This takes place in appropriately tempered devices. This recycling is preferably carried out in the non-condensed phase, ie as a gas or as a vapor. This return can be carried out as a material transport along a pressure equalization or as a controlled transport of substances with the technically customary systems of gas transport, such as fans, pumps, compressed air diaphragm pumps. Since the return of the noncondensed phase is preferred, the heating of the return lines is recommended if necessary.

The recycling of the unreacted silanes of the general formula III or siloxanes of the general formula IV and the exhaust gases can be between preferably 5 and 100% by weight, based on their total mass, preferably between 30 and 80% by weight. The reflux can be between 1 and 200 parts, preferably 10 to 30 parts, based on 100 parts of freshly used silane.

The removal of the purification products of the modification reaction into the occupancy preferably takes place continuously.

In addition, during the modification or subsequent to the purification processes, the mechanical compaction of the silica may be used, for example, press rolls, mucking units such as auger length and ball milling, continuous or discontinuous, compression by screws or screw mixers, screw compressors, briquetting or compacting Aspiration of the air or gas content by suitable vacuum methods.

Particular preference is given to the mechanical compaction during the modification, in the step (II) of the reaction by means of press rolls, the abovementioned grinding units, such as ball mills or compression by screws, screw mixers, screw compressors, briquettes.

In a further particularly preferred procedure, subsequent to the purification processes, the mechanical compaction of the silica is used, such as compression by sucking off the air or gas content by suitable vacuum methods or press rolls or a combination of both methods.

In addition, in a particularly preferred procedure, subsequent to the purification processes, the deagglomeration of the silica can be used, such as pin mills, hammer mills, countercurrent mills, impact mills or apparatus for grinding screening.

The invention also relates to a process for using the silicas obtained according to the invention in systems of low to high polarity as a viscosity-imparting component. This applies to all solvent-free, solvent-containing film-forming paints, rubber-like to hard coatings, adhesives, sealing and potting compounds, as well as other comparable systems.

The silicas according to the invention can be used, for example, in systems such as:<ul><li>Epoxide systems</li><li>Polyurethane systems (PUR)</li><li>Vinyl ester resins</li><li>Unsaturated polyester resins</li><li>Low-solvent resin systems, so-called "high solids".</li><li>Solvent-free resins which are applied in powder form, for example, as coating materials.</li></ul>

The silicas according to the invention as rheological additive in these systems provide the required necessary viscosity, structural viscosity, thixotropy and a flow limit which is sufficient for the strength at vertical surfaces.

The invention also relates to the use of the silicas obtained according to the invention in catalytically crosslinking coatings, adhesives or sealants such as, for example, 1-component systems, such as, for example, moisture-curing one-component polyurethane adhesives or coating materials for improving the storage stability of the uncrosslinked formulation. The silicas according to the invention are characterized by a retardation of the catalysts conventionally used. Typically employed catalysts are, for example, basic compounds, such as amines, for example triethylamine, ethylene diamines such as triethylenediamine, morpholines such as N-alkylmorpholines, piperazine and its derivatives or pyridine and its derivatives,

The proportion of the silicas according to the invention is preferably from 0.1 to 20% by weight, preferably from 0.5 to 15% by weight and particularly preferably from 1 to 10% by weight, based on the total weight of the formulation.

This means that, in a temperature load test at 80 ° C., moisture-crosslinking, catalyzed 1-component polyurethane adhesives or coating compositions comprising the silicas according to the invention, compared to 1-component polyurethane adhesives or coating compositions comprising hydrophobic silicas without the T groups according to the invention Fraction, exhibit a markedly decelerated increase in viscosity. Preferably, the time until the two-fold higher viscosity is reached is increased by a factor of 10, preferably by a factor of 5 and more preferably by a factor of 2, compared to the starting viscosity.

The silicas obtained according to the invention can also be used, in particular, as a rheological additive and reinforcing filler in non-crosslinked and crosslinked silicone systems, such as silicone elastomers, composed of silicone polymers, such as polydimethylsiloxanes, fillers, and further additives. These can, for example, be crosslinked with peroxides, or crosslinked via addition reactions, the so-called hydrosilylation reaction, between olefinic groups and Si-H groups, or via condensation reactions between silanol groups, for example those which are formed under the action of water.

The silicas obtained according to the invention can be used for improving and controlling the powder flow behavior, and / or for regulating and controlling the triboelectric charge properties of the toner or developer. Such toners and developers can preferably be used in electrophotographic printing and printing processes, and can also be used in direct image transfer processes. The same also applies to the use in powder paints.

Examples

Example 1:

At a temperature of 25 ° C under inert gas N₂ Are added to 100 g of hydrophilic silica with a moisture content of less than 1% and an HCl content of less than 100 ppm and with a specific surface area of ​​150 m²/ G (measured according to the BET method according to DIN EN ISO 9227 / DIN 66132) (available under the name HDK^® V15 at Wacker-Chemie AG, Munich, Germany), 16 g of a trimethylsiloxy-terminated silicone oil having a viscosity of 20 mPas at 25 ° C. were added by atomizing via a two-fluid nozzle (pressure 5 bar). The silica thus charged is reacted for 2 h at 300 ° C. in a 100 l drying cabinet in an air stream of 900 l / h.

The analytical data are shown in Table 1.

Example 2 (comparative example):

At a temperature of 25 ° C under inert gas N₂ Are added to 100 g of hydrophilic silica with a moisture content of less than 1% and an HCl content of less than 100 ppm and with a specific surface area of ​​150 m²/ G (measured according to the BET method according to DIN EN ISO 9227 / DIN 66132) (available under the name HDK^® V15 at Wacker-Chemie AG, Munich, D), by spraying over a two-fluid nozzle (pressure 5 bar), 16 g of a trimethylsiloxy-terminated silicone oil (F._T /% = 0) with a viscosity of 20 mPas at 25 ° C. The silica thus charged is heated for 2 h at 300 ° C. in a 100 l drying cabinet in a N₂Stream of 900 l / h.

The analytical data are listed in Table 1.

Example 3:

At a temperature of 25 ° C under inert gas N₂ Are added to 100 g of hydrophilic silica with a moisture content of less than 1% and an HCl content of less than 100 ppm and with a specific surface area of ​​300 m²/ G (measured according to the BET method according to DIN EN ISO 9227 / DIN 66132) (available under the name HDX^⊗ T30 at Wacker-Chemie AG, Munich, D), 27 g of an OH-terminated silicone oil with a viscosity of approx. 35 mPas at 25 ° C. were added by means of atomization via a two-fluid nozzle (pressure 5 bar). The silica thus charged is reacted for 2 h at 300 ° C. in a 100 l drying cabinet in an air stream of 900 l / h.

The analytical data are shown in Table 1.

Example 4:

At a temperature of 25 ° C under inert gas N₂ Are added to 100 g of hydrophilic silica with a moisture content of less than 1% and an HCl content of less than 100 ppm and with a specific surface area of ​​150 m²/ G (measured according to the BET method according to DIN EN ISO 9227 / DIN 66132) (available under the name HDK^® V15 at Wacker-Chemie AG, Munich, D), 1.89 g of deionized water and then 6.4 g of dimethyldichlorosilane were added by means of atomizing via a two-fluid nozzle (pressure 5 bar). The silicic acid thus charged is reacted for 1 h at 80 ° C. and for 2 h at 300 ° C. in a 100 l drying cabinet in an air stream of 900 l / h.

The analytical data are shown in Table 1.

Example 5 (comparative example):

At a temperature of 25 ° C under inert gas N₂ Are added to 100 g of hydrophilic silica with a moisture content of less than 1% and an HCl content of less than 100 ppm and with a specific surface area of ​​150 m²/ G (measured according to the BET method according to DIN EN ISO 9227 / DIN 66132) (available under the name HDK^® V15 at Wacker-Chemie AG, Munich, Germany), by means of atomization via a two-fluid nozzle (pressure 5 bar), 1.89 g of deionized water and then 6.4 g of dimethyldichlorosilane (F._T /% = 0). The silicic acid thus charged is heated for 1 h at 80 ° C. and for 2 h at 300 ° C. in a 100 l drying cabinet in an N₂Stream of 900 l / h.

The analytical data are shown in Table 1.<tables id="tabl0001" num="0001"><table frame="all"><title><b>Table 1</b></title><tgroup cols="6"><colspec colnum="1" colname="col1" colwidth="17mm" /><colspec colnum="2" colname="col2" colwidth="11mm" /><colspec colnum="3" colname="col3" colwidth="24mm" /><colspec colnum="4" colname="col4" colwidth="10mm" /><colspec colnum="5" colname="col5" colwidth="13mm" /><colspec colnum="6" colname="col6" colwidth="14mm" /><thead><row><entry align="center" valign="top">example</entry><entry align="center" valign="top">% C</entry><entry align="center" valign="top">% Residual SiOH</entry><entry align="center" valign="top">MZ</entry><entry align="center" valign="top">F_T /%</entry><entry align="center" valign="top">T₂ / Min</entry></row></thead><tbody><row><entry align="center">1</entry><entry align="center">3.2</entry><entry align="center">19</entry><entry align="center">73</entry><entry align="center">4</entry><entry align="center">4, 5</entry></row><row><entry align="center">2</entry><entry align="center">3,7</entry><entry align="center">29</entry><entry align="center">77</entry><entry align="center">0</entry><entry align="center">2.25</entry></row><row><entry align="center">3</entry><entry align="center">5.4</entry><entry align="center">10</entry><entry align="center">74</entry><entry align="center">7</entry><entry align="center">4,8</entry></row><row><entry align="center">4</entry><entry align="center">1.1</entry><entry align="center">53</entry><entry align="center">35</entry><entry align="center">5</entry><entry align="center">Nb</entry></row><row><entry align="center">5</entry><entry align="center">1.0</entry><entry align="center">55</entry><entry align="center">42</entry><entry align="center">0</entry><entry align="center">Nb</entry></row></tbody></tgroup></table></tables>

Description of analytical methods

<ul><li>1. Carbon content (% C)<ul><li>Elementary analysis on carbon; Burn the sample at over 1000 ° C in the O₂-current, detection and quantification of the resulting CO₂ With IR; Device LECO 244</li></ul></li><li>2. Residual content of unmodified silica silanol groups<ul><li>Method: acid-base titration of the silica suspended in water / methanol = 50:50; Titration in the range above the pH range of the isoelectric point and below the pH range of the dissolution of the silica</li><li>Untreated silicic acid with 100% SiOH (silicas - surface silanol groups): SiOH-phil = 2 SiOH / nm²</li><li>Silylated silicic acid: SiOH-silyl</li><li>Silica residual silanol content:% Residual SiOH = SiOH-silyl / SiOH-phil * 100% (analog <nplcit id="ncit0001" npl-type="s"><text>GW Sears Anal. Chem, 28 (12), (1950), 1981</text></nplcit>) "</li></ul></li><li>3. Methanol number: Shaking an equal volume of the silica with the same volume of water-methanol mixture<ul><li>Start with 0% methanol, in the case of non-wetting, silica swells: Use a mixture with 5% higher MeOH content</li><li>When wetting occurs, silica precipitates: fraction MeOH (%) in water gives MZ (MZ = methanol number)</li></ul></li><li>4. Relative proportion of T groups (F_T): ²⁹Si solid-state NMR spectrum measured in CPMAS mode</li><li>5. Period until the initial viscosity increases by a factor of 2 (t₂): 2.3% by weight of the silica to be tested were dispersed by means of a dissolver into an MDI prepolymer (Desmodur E 210 from Bayer MaterialScience). 0.33 g of the mixture was applied to the measuring plate of a Brookfield rheometer CAP 2000 with cone 2 and mixed with 0.01 g tin catalyst COTIN 200. The measurement was carried out at a constant rotation speed of 50 min^-1 At a temperature of 80 ° C. The measurement was terminated as soon as irregular behavior was observed by wall-sliding effects of the cross-linked sample. Data points were recorded every 5 s.</li></ul>