Method for modifying silicic acid in the liquid phase

Abstract

The invention relates to a method for the surface modification of hydrophilic silicic acid with a specific surface area of 10 to 1000 m2 / g (measured according to the BET method, according to DIN EN ISO 9277 / DIN 66132), wherein a suspension of the silicic acid in an organic solvent is reacted with liquid polyorganosiloxane of the following general formula (I): [R1R2R3SiO1 / 2][R4R5SiO2 / 2]n[R6R7R8SiO1 / 2], where R1, R2, R3 are each independently a hydroxy group or a monovalent hydrocarbon group with 1 to 24 C atoms, wherein at least one group R1, R2, R3 is a hydroxy group, R4, R5 are each independently a hydroxy group or a monovalent hydrocarbon group with 1 to 24 C atoms, in particular a monovalent hydrocarbon group with 1 to 24 C atoms, R6, R7, R8 are each independently a monovalent hydrocarbon group with 1 to 24 C atoms, n is an integer from 0 to 20, and wherein no more than 20 mol.% are hydroxy groups, based on all groups R1 to R8.

Description

[0001] Method for modifying silica in the liquid phase

[0002] The invention relates to a method for surface modification of hydrophilic silica with liquid, asymmetrically substituted polyorganosiloxane in a suspension of silica in an organic solvent.

[0003] Hydrophobic, surface-modified silicas modified with polydimethylsiloxanes (PDMS) or chloromethylsilanes are used as thickening and thixotropic agents in composites, coatings and adhesives, especially in vinyl ester, epoxy and polyurethane systems.

[0004] To obtain surface-modified silica, a hydrophilic silica is formed, for example according to W02008077814 or EP2824148.

[0005] Silica is fluidized in the gas phase and functionalized with a PDMS-containing plasticizer. Subsequent annealing at 150 to 350 °C is particularly important to ensure excellent bonding of the PDMS chains to the silica surface. This results in very low volatility levels of less than 0.6% (2 h at 105 °C). However, the material has the disadvantage of very long incorporation times. Since the incorporation of silica into polymer matrices is the cycle time-determining step in numerous commercial applications, the maximum throughput is strongly linked to the incorporation time.

[0006] Pyrogenic silicas, which have been hydrophobized in the organic liquid phase, exhibit very good matrix compatibility, resulting in very short incorporation times (<20 s) in matrices such as epoxy resins. This represents a significant improvement compared to conventional gas-phase modified silicas, such as the commercial type HDK® H18 (polydimethylsiloxane-modified pyrogenic silica with a specific surface area of ​​the parent silica of 200 m²). 2 / g and a carbon content of 4.0% to 5.2%), which has incorporation times of 300 to 360 s.

[0007] However, these liquid-phase hydrophobized silicas exhibit unfavorable aging behavior: an exemplary silica modified in the liquid phase with a symmetrical reactant, α,α-di-hydroxy-polydimethylsiloxane (PDMS), can be incorporated into an epoxy resin. This silica-epoxy resin mixture exhibits excellent shear thinning behavior (thixotropy) with a high shear viscosity (at 10 s⁻¹). -1 ) of 167 Pas and a low shear viscosity (at 0.1 s -1 ) of 4810 Pas. After storage (e.g. 6 months) the high-shear viscosity remained approximately constant, but the mixtures reached significantly lower low-shear viscosities of 3922 and 2625 Pas, corresponding to a reduction of 18% and 46% respectively.

[0008] It would therefore be desirable to provide liquid-phase modified silicas which, for example, do not exhibit adverse aging behavior in a silica-epoxy resin mixture, thus representing storage-stable additives for resin mixtures.

[0009] Surprisingly, it has been shown that liquid-phase modification of silica with asymmetric hydroxy polyalkylsiloxanes enables excellent storage stability.

[0010] The invention therefore relates to a method for the surface modification of hydrophilic silica with a specific surface area of ​​10 to 1000 m². 2 / g (measured according to the BET method according to DIN EN ISO 9277 / DIN 66132) , in which a suspension of silica in an organic solvent is reacted with liquid polyorganosiloxane of the following general formula (I):

[0011] [R x R 2 R 3 SiOi / 2] [R 4 R 5SiO2 / 2]n[R 6 R 7 R 8 SiOi / 2] Formula (I) wherein

[0012] R 1 , R 2 , R 3 independently of each other, a hydroxyl group or a monovalent hydrocarbon group with 1 to 24 carbon atoms, where at least one group is R 1 , R 2 , R 3 a hydroxyl group means

[0013] R 4 , R 5 independently of each other means a hydroxyl group or a monovalent hydrocarbon group with 1 to 24 carbon atoms, in particular a monovalent hydrocarbon group with 1 to 24 carbon atoms,

[0014] R 6 , R 7 , R 8 independently of each other, a monovalent hydrocarbon residue with 1 to 24 carbon atoms, n represents an integer from 0 to 20, and where at most 20 mol% based on all residues R 1 to R 8 Hydroxy residues are.

[0015] The siloxanes of formula (I) used according to the invention are thus asymmetric hydroxy-polyalkylsiloxanes, in particular asymmetric mono-hydroxy-polyalkylsiloxanes having only one terminal hydroxy group, and most preferably asymmetric mono-hydroxy-polydimethylsiloxanes. The siloxanes of formula (I) used according to the invention are to be understood in particular as siloxanes of the following polymeric structure: R 1 R 2 R 3 SiO- [R 4 R 5 SiO] n -OSiR 6 R 7 R 8

[0016] The siloxane used according to the invention is used in particular in a mixture of short-chain siloxanes.

[0017] The asymmetrically modified silica according to the invention thus solves the problem of premature aging of a silica-epoxy resin mixture and the associated loss of shear-thinning behavior - in contrast to the silica which is modified in the standard way with symmetrical α,γ-di-hydroxy polydimethylsiloxanes.

[0018] Liquid-phase silicas modified with this asymmetric reactant exhibit surprisingly good aging behavior, with low-shear viscosities that remain well above 4000 Pa·s even after 18 months of storage of the silica, for example.

[0019] The silica modified according to the invention leads, among other things, to the advantages of consistently good shear thinning in polymer matrices and drastically reduced incorporation times compared to conventionally modified silica.

[0020] In a preferred embodiment, R 1 a hydroxy group, while R 2 and R 3 independently of each other, they represent a monovalent hydrocarbon residue with 1 to 24 carbon atoms.

[0021] In a preferred embodiment, the residues R 4 to R 8 identical. In a preferred embodiment, the remainder R represents 1 a hydroxyl group and the R groups 2 to R 8 are identical.

[0022] The silica modification is carried out at moderate temperatures in the liquid phase.

[0023] The silica modified by the process exhibits chain-like siloxane structures on its surface, which have a chain length distribution that is as homogeneous as possible. Preferably, these siloxane chains are permanently fixed to the surface of the silica as completely as possible. Furthermore, the chemical bonding of the siloxane chain to the surface of the silica preferably occurs via a single connection point.

[0024] For example, precipitated silica or pyrogenic silica can be used as silicic acid, preferably pyrogenic silica.

[0025] Particularly preferred is pyrogenic silica produced in a flame reaction from organosilicon compounds, e.g., from silicon tetrachloride or methyltrichlorosilane, or hydrogentrichlorosilane or hydrogen methyldichlorosilane, or other methylchlorosilanes or alkylchlorosilanes, also in mixtures with hydrocarbons, or any volatile or sprayable mixtures of organosilicon compounds, as mentioned, and hydrocarbons, e.g., in a hydrogen-oxygen flame or a carbon monoxide-oxygen flame. The silica can be produced optionally with or without the addition of water, for example, in the purification step; preferably, no addition of water is required. The silica used preferably has a specific surface area of ​​40 to 400 m². 2 / g and especially preferably 150 to 270 m 2 / g (measured according to the BET method according to DIN EN ISO 9277 / DIN 66132).

[0026] The bulk densities of the silica used (determined according to DIN EN ISO 787-11) can be in the range of 10 to 200 g / l, preferably 100 to 190 g / l, particularly preferably 150 to 180 g / l.

[0027] The degree of modification achieved by the process can be analyzed by determining the residual silanol content. The modified silica preferably has a residual silanol content in the range of 30 to 90 mol%, particularly preferably 45 to 85 mol%, and especially preferably 55 to 75 mol%. A suitable method for determining the residual silanol content after modification by acid-base titration is described, for example, in GW Sears et al., Analytical Chemistry 1956, 28, 1981ff.

[0028] The carbon content achieved by the process is preferably 1 wt.% to 15 wt.%, particularly preferably 2 wt.% to 10 wt.%, and especially preferably 3 wt.% to 8 wt.%.

[0029] The groups introduced by the modification are firmly bound to the surface of the silica. A strong bond indicates good chemical bonding and, according to the invention, is quantified by the solvent-extractable fraction of the modified silica, which is preferably at most 10 wt.%. Particularly preferably, the extractable fraction is at most 6 wt.%, more preferably at most 4 wt.%, and most preferably at most 2 wt.%. A suitable method for evaluating the bond strength of a modification is the quantitative determination of the extractable fraction, i.e., the fraction not chemically bound to the surface of the modified silica.

[0030] Po lyorgano siloxane .

[0031] Methyl isobutyl ketone (MIBK) is preferably used for the determination of extractable polyorganosiloxane.

[0032] The monovalent hydrocarbon residues R 1 to R 8can be the same or different and are selected from the group of saturated, monounsaturated or polyunsaturated, unbranched or branched hydrocarbon residues, which may contain heteroatoms and / or functional groups.

[0033] Preferably, the hydrocarbon residues are alkyl, alkenyl and aryl groups such as methyl, ethyl, propyl, such as n-propyl or i-propyl, butyl, such as n-butyl, i-butyl or t-butyl, hexyl, such as n-hexyl or i-hexyl, octyl, such as n-octyl or i-octyl, dodecyl, tetradecyl, hexadecyl, octadecyl, vinyl, allyl, phenyl, o-tolyl, m-tolyl, p-tolyl, xylyl, mesityl or naphtyl groups.

[0034] The alkyl or aryl groups may also contain other heteroatoms or functional groups. Preferably, monovalent organic groups of the general formula R=(CH2)mY with m = 1 to 24 and Y = vinyl, acrylate, methacrylate, glycidoxy, -SH, -OH, primary amine residue (-NH2), secondary amine residue (-NHR), such as N-monomethyl, N-monoethyl, N-monopropyl, N-monobutyl, N-cyclohexyl or anilino residue, tertiary amine residue (-NR2) such as N,N-dimethyl, N,N-diethyl, N,N-dipropyl, N,N-dibutyl, N,N-methylethyl, N,N-methylpropyl, N,N-ethylpropyl, N,N-methylphenyl, morpholino, pyrrolyl, indolyl, pyrazoyl, imidazoyl or piperidyl residue , quaternary amine residue such as N, N, N-trimethylammonium, N, N, N-triethylammonium or N, N, N-tripropylammonium residue, phosphonato-, -P(0) (OR 9 )2(R 9selected from a methyl, ethyl or phenyl group), isocyanato and protected isocyanato group (-N(H)C(O)G, wherein the protecting group G is cleaved off under thermal stress as HG, with HG = 2-methyl hydroxybenzoate, 2-hydroxypyridine, 1-hydroxymethyl-1, 2,4-triazole, N,N-diethylhydroxylamine, 2-butanone oxime, dimethyl malonate, ethyl acetoacetate, diisopropylamine, benzyl-tert-butylamine, tert-butylmethylamine, tert-butylisopropylamine, 2-isopropylimidazole, 3,5-dimethylpyrazole or s-caprolactam) or dihydro-3-yl-2,5-furandione.

[0035] Furthermore, other organosilicon groups of the general formula R can also be used. 10 Si (01 / 2)3 is present, wherein the substituent R 10 from the above for R 1 -R 8 The aforementioned hydrocarbon residues are selected.

[0036] Preferably, the monovalent hydrocarbon fractions are in R 1 to R 8selected from methyl, ethyl, propyl, butyl, and phenyl residues.

[0037] R is particularly favored 1 a hydroxy group and R 2 to R 8 are selected from methyl, ethyl, propyl, butyl, and phenyl residues, especially from methyl residues.

[0038] Preferably, n represents an integer from 0 to 15, particularly preferably 1 to 10, especially 2 to 10.

[0039] The polyorganosiloxane used in the process is preferably liquid in the range of 0 to 60°C, particularly preferably 10 to 50°C, and especially preferably 15 to 30°C, at a viscosity of 0.10 MPa (abs.). At 20°C, the polyorganosiloxane used in the process preferably has a mean viscosity of 5 to 200, particularly preferably 10 to 100, and especially 20 to 60 mPa s.

[0040] The polyorganosiloxane can be used in any quantity. Preferably, the quantity used is 5 to 50 wt.%, particularly preferably 20 to 40 wt.%, and especially 15 to 25 wt.%, in each case based on the unmodified hydrophilic silica.

[0041] In a special embodiment of the invention, the

[0042] Polyorganosiloxane is used with the addition of an excipient.

[0043] The organic solvent used to prepare the silica suspension is preferably an aprotic solvent, preferably with a boiling point of no more than 120 °C, particularly no more than 100 °C, in each case at 0.10 MPa (abs.), for example a ketone such as acetone, methyl ethyl ketone, ether such as diethyl ether, dioxane, hydrocarbon such as pentane, hexane, aromatic such as toluene, or another solvent such as hexamethyldisiloxane. Mixtures can also be used.

[0044] Optionally, protic solvents can be added to the process. A molecule is considered protic if it possesses a functional group from which hydrogen atoms can be released as protons (dissociation). Due to the high polarity of the OH bond, it can be cleaved relatively easily by releasing a positively charged hydrogen atom, the proton. The most important protic solvent is water, which (in simplified terms) dissociates into a proton and a hydroxide ion. Other protic solvents include, for example, alcohols and carboxylic acids. According to the invention, liquid or vaporizable alcohols such as isopropanol, ethanol, or methanol, or water, can be added as protic solvents. Mixtures of the above-mentioned protic solvents can also be added. Preferably, 1 to 50 wt.% of protic solvent based on the metal oxide is added, particularly preferably 5 to 25 wt.%. The addition of water as a protic solvent is particularly preferred.

[0045] In the surface modification of hydrophilic silica, substances can also be used that reduce the necessary reaction times and / or make it possible to lower the process temperatures. These catalytically or stoichiometrically active substances are referred to below as auxiliary substances. They preferably include acidic or basic substances. They can, for example, be selected from the group of Lewis acids, which include trivalent aluminum and boron compounds. Brønsted acids, such as hydrogen halides or organic acids, are also preferred. Hydrogen chloride or acetic acid are particularly preferred. In another embodiment, basic compounds are used as auxiliary substances, for example, hydroxides of alkali and alkaline earth metals as well as their salts derived from the corresponding alcohols or carboxylic acids.Furthermore, they can be selected from nitrogen-containing compounds such as ammonia or organically substituted primary, secondary, or tertiary amines. The monovalent organic substituents of the aforementioned alcohols, carboxylic acids, and amines include saturated and unsaturated, branched and unbranched hydrocarbon residues, which may also contain further heteroatoms or functional groups. The excipients can be added in their pure form or as a solution in inert or reactive solvents. Aqueous sodium or potassium hydroxide solutions, aqueous ammoniacal solutions, isopropylamine, n-butylamine, isobutylamine, tert-butylamine, cyclohexylamine, triethylamine, morpholine, piperidine, or pyridine are preferably used.

[0046] In a preferred embodiment, the amount of excipient used is 0.1 to 10 wt.% based on the unmodified silica. Preferably, 0.2 to 5 wt.% is used. Particularly preferred is the use of 0.5 to 1.5 wt.% excipient based on the unmodified silica.

[0047] Silica .

[0048] The temperature during the surface modification of the hydrophilic silica is preferably 20 to 140°C, particularly preferably 30 to 120°C, and especially preferably 40 to 100°C at 0.10 MPa (abs.).

[0049] Removing solvents, excess

[0050] Polyorganosiloxane and by-products can preferably be dried using dryers or by spray drying.

[0051] If necessary, a post-reaction step can be added to the drying step to complete the reaction.

[0052] The post-reaction preferably takes place at temperatures of 20–300 °C, more preferably 20–200 °C, and particularly preferably at 40–180 °C. Additionally, following the drying step, methods for deagglomerating the modified silica can be used, such as pin mills, hammer mills, countercurrent mills, impact mills, or grinding sifting devices.

[0053] Analysis methods:

[0054] Determination of carbon content (%C)

[0055] The elemental analysis for carbon was carried out according to DIN ISO 10694 using a CS-530 elemental analyzer from Eitra GmbH (D-41469 Neuss).

[0056] Determination of the residual content of unmodified silica silanol groups

[0057] The residual silanol content was determined analogously to GW Sears et al. Analytical Chemistry 1956, 28, 1981ff by acid-base titration of silica suspended in a 1:1 mixture of water and methanol. The titration was carried out in the range above the isoelectric point and below the pH range of dissolution of the silica.

[0058] The residual silanol content in % can therefore be calculated using the following formula:

[0059] SiOH = SiOH (silyl) / SiOH (phil) *100% with

[0060] SiOH (phil) : Titration volume from the titration of untreated silica

[0061] SiOH (silyl) : Titration volume from the titration of silylated silica

[0062] Determination of the extractable fraction, i.e., the proportion of extractable polyorganosiloxane

[0063] 2.5 g of the silica to be analyzed are stirred into 47.5 g of MIBK in a PE screw-top container using a spatula, and the container is then sealed. After a 30-minute resting period in an ice bath, the mixture is treated for 30 minutes in an ultrasonic bath under ice cooling (Sonorex Digitec DT 156, BANDELIN electronic GmbH & Co. KG, D-12207 Berlin) and subsequently filtered by pressure filtration (5 bar nitrogen) through a PTFE membrane filter (pore size: 0.2 pm, diameter: 47 mm, Sartorius AG, Göttingen) to obtain the clear filtrate. Exactly 10.00 ml of this filtrate is taken and weighed for analysis to determine the silicon content using atomic absorption spectroscopy (Atom Absorption Spectrometer 2100, Perkin Elmer Waltham, MA, USA).

[0064] The extractable components in wt.% can be calculated as a first approximation as follows:

[0065] Extractable components m(MIBK) XV(Analysate) c(Analysate) XM(R 4 R 5 SiO2 / 2) m(metal oxide) XM(Si) m(analysate) with m(MIBK) : Weight of MIBK (= 47.50 g)

[0066] V(Analysate): Volume of the analyzer (= 10.00 ml) m(Metal oxide): Weight of the surface-modified metal oxide (= 2.50 g)

[0067] M(Si) : Molar mass of silicon (=28.09 g / mol) c(Analysate) : Silicon content of the analyzer in mg / l m(Analysate) : Weight of the analyzer in g

[0068] M (R4R5S1O2 / 2 ): molecular mass of the D groups R4R5S1O2 / 2 in g / mol

[0069] Determination of the viscosity of the polyorganosiloxane

[0070] Determination according to DIN 53019 at 20°C and 0.10 MPa (abs.) Examples

[0071] In the following examples, unless otherwise stated, all quantities and percentages are based on weight, all pressures are 0.10 MPa (abs.) and all temperatures are 20°C.

[0072] Example 1 (according to the invention): Production of a hydrophobic

[0073] Silica in a 2L glass reactor under reflux

[0074] In a round 2L glass reactor equipped with a KPG stirrer, 1140 g of solvent hexamethyldisiloxane were placed under argon blanketing. The reactor was covered with a glass lid with a total of four screw necks and equipped with a reflux condenser with a bubble counter and a thermometer immersed in the reactor.

[0075] A total of 150 g of HDK® N20 silica (silica with an initial surface area of ​​200 m² / g, commercially available from Wacker Chemie AG) was added via a grounded metal funnel using a metal scoop under argon overlay and a suspension was created by vigorous stirring with a KPG stirrer. Finally, 4 g of aqueous ammonia solution (concentration: 5 mol per liter) and 21 g of a mixture of short-chain, monofunctional α-hydroxy polydimethylsiloxanes (PDMS) with a mean viscosity of 16.7 mm were added to the suspension. 2 / s (according to Ubbelohde DIN 51562 ) .

[0076] While stirring vigorously, the 2 L glass reactor is immersed as far as possible in an oil bath, which is heated to 130 °C using a magnetic stirrer (Heidolph Instruments MR Hei-Tec magnetic stirrer). The suspension is heated to a temperature of 98 °C and refluxed for two hours. After the mixture has cooled to room temperature, it is evaporated in a rotary evaporator (Heidolph Instruments) with a vacuum pump and cold trap, maintaining an oil bath temperature of 130 °C, to remove the solvent and any unreacted reactants.

[0077] The inventive silica had a carbon content of 4.7%. The rheological properties were tested as follows: the modified silica was dispersed in epoxy resin, and after one day of storage, the viscosity of the mixture was determined at a shear rate of 0.1 s⁻¹. -1 and 10 seconds -1The thixotropy index is determined by dividing the viscosity at low shear rate by the viscosity at high shear rate. The thixotropy index was determined for two epoxy resin systems:

[0078] Epoxy resin system 1 :

[0079] Mixture of 8 wt% modified silica and 92 wt% epoxy resin (Epikote™ Resin 828 from Hexion, a commercially available epoxy resin based on bisphenol A and epichlorohydrin).

[0080] The modified silica according to the invention had a thixotropy index of 33 (HDK® H18: 34).

[0081] Epoxy resin system 2:

[0082] Mixture of 8 wt% modified silica in epoxy resin (Epikote™ Resin 828 from Hexion, a commercially available epoxy resin based on bisphenol A and epichlorohydrin) and 4 wt% HDK® N20 (pyrogenic silica with a specific surface area of ​​200 m² / g, commercially available from Wacker Chemie AG) in amine curing agent (Epikure™ Curing Agent MGS® RIMH-137, commercially available from Hexion), in a mixing ratio of 79 wt% epoxy resin and 21 wt% modified silica.

[0083] % Amine hardener.

[0084] The silica according to the invention had a thixotrophy index of 48 (HDK® H18: 60) .

[0085] The modified silica according to the invention has a shear thickening comparable to the commercial product HDK® H18.

[0086] The incorporation time of this silica according to the invention was 26 s, which is approximately 91% faster than the incorporation time of a conventional HDK® H18 (300 to 360 s).

[0087] Example 1: Preserving aging

[0088] A non-inventive silica compound, prepared with a symmetrical α,α-di-hydroxy-polydimethylsiloxane (PDMS), initially exhibited very good thixotropic behavior in epoxy resin systems 1 and 2, with thixotropy indices of 34 and 55, respectively. The silica was stored dry and in the dark for 6 months and then retested for shear thinning in epoxy resin systems 1 and 2: in system 1, the low-shear viscosity decreased from an initial 4790 Pa·s to 2606 Pa·s, while the thixotropy index decreased from an initial 34 to 26. In system 2, the low-shear viscosity decreased from an initial 2258 Pa·s to 1430 Pa·s, while the thixotropy index decreased from 55 to

[0089] 54 sank.

[0090] The silica according to the invention from Example 1 was stored dry and dark for 18 months and also tested again for shear thinning in the epoxy resin systems 1 and 2: in system 1, the low-shear viscosity was initially 4346 Pas, and 18 months later was 4364 Pas, while the thixotropy index decreased from an initial 33 to 32. In system 2, the initial low-shear viscosity was 2126 Pas, while 18 months later 2300 Pas was reached. The thixotropy index increased from an initial 48 to 50.

[0091] This demonstrated that the silica according to the invention continues to exhibit excellent rheological behavior even after 18 months of storage, whereas the silica which was hydrophobized with a symmetrical α,γ-di-hydroxy polydimethylsiloxane already shows clear signs of aging after 6 months, and in particular an enormous loss of performance can be observed in shear thinning.

[0092] Example 2 (according to the invention): Production of hydrophobic silica in a 2L glass reactor at 50°C

[0093] In a round 2L glass reactor equipped with a KPG stirrer, 1140 g of solvent hexamethyldisiloxane were placed under argon blanketing. The reactor was covered with a glass lid with a total of four screw necks and equipped with a reflux condenser with a bubble counter and a thermometer immersed in the reactor.

[0094] A total of 112.5 g of HDK® N20 silica (silica with an initial surface area of ​​200 m² / g, commercially available from Wacker Chemie AG) was added via a grounded metal funnel using a metal scoop under argon overlay and a suspension was created by vigorous stirring with a KPG stirrer. Finally, 4 g of aqueous ammonia solution (concentration: 5 mol / L) and 21 g of a mixture of short-chain, monofunctional α-hydroxy PDMS with a mean viscosity of 16.7 mm² were added to the suspension. 2 / s (according to Ubbelohde DIN 51562) .

[0095] While stirring vigorously, the 2 L glass reactor is immersed as far as possible in an oil bath heated to 60 °C using a magnetic stirrer (Heidolph Instruments MR Hei-Tec magnetic stirrer). The suspension is heated to a temperature of 49.0–51.5 °C and refluxed for two hours. After the mixture has cooled to room temperature, it is evaporated in a rotary evaporator (Heidolph Instruments) with a vacuum pump and cold trap at an oil bath temperature of 130 °C to remove the solvent and any unreacted reactants.

[0096] The inventive silica had a carbon content of 3.9%. The rheological properties were tested as follows: the modified silica was dispersed in epoxy resin, and after one day of storage, the viscosity of the mixture was determined at a shear rate of 0.1 s⁻¹. -1 and 10 seconds -1The thixotropy index is determined by dividing the viscosity at low shear rate by the viscosity at high shear rate. The thixotropy index was determined for two epoxy resin systems:

[0097] Epoxy resin system 1 :

[0098] Mixture of 8 wt% modified silica and 92 wt% epoxy resin (Epikote™ Resin 828 from Hexion, a commercially available epoxy resin based on bisphenol A and epichlorohydrin).

[0099] The modified silica according to the invention exhibited a thixotropy index of 33 (HDK® H18: 34). Epoxy resin system 2:

[0100] Mixture of 8 wt% modified silica in epoxy resin (Epikote™ Resin 828 from Hexion, a commercially available epoxy resin based on bisphenol A and epichlorohydrin) and 4 wt% HDK® N20 (pyrogenic silica with a specific surface area of ​​200 m2 / g, commercially available from Wacker Chemie AG) in amine curing agent (Epikure™ Curing Agent MGS® RIMH-137, commercially available from Hexion), in a mixing ratio of 79 wt% epoxy resin and 21 wt% amine curing agent.

[0101] The silica according to the invention had a thixotrophy index of 55 (HDK® H18: 57) .

[0102] The modified silica according to the invention has a shear thickening comparable to the commercial product HDK® H18.

[0103] The incorporation time of this silica according to the invention was 15 s, which is approximately 95% faster than the incorporation time of a conventional HDK® H18 (300 to 360 s).

[0104] Example 2: Preserving aging

[0105] A non-inventive silica compound, prepared with a symmetrical α,γ-di-hydroxy-polydimethylsiloxane (PDMS), initially exhibited very good thixotropic behavior in epoxy resin systems 1 and 2, with thixotropy indices of 34 and 55, respectively. The silica was stored dry and in the dark for 6 months and then retested for shear thinning in epoxy resin systems 1 and 2: in system 1, the low-shear viscosity decreased from an initial 4790 Pa·s to 2606 Pa·s, while the thixotropy index decreased from an initial 34 to 26. In system 2, the low-shear viscosity decreased from an initial 2258 Pa·s to 1430 Pa·s, while the thixotropy index decreased from 55 to 54.

[0106] The silica according to the invention in Example 2 was stored dry and dark for 18 months and also tested again for shear thinning in epoxy resin systems 1 and 2: in system 1, the low-shear viscosity was initially 5166 Pas and was 4900 Pas 18 months later, while the thixotropy index decreased from an initial 33 to 32. In system 2, the initial low-shear viscosity was 2570 Pas, while 2074 Pas was reached 18 months later. The thixotropy index decreased from an initial 55 to 45.

[0107] The silica according to the invention in Example 2 also continues to show excellent rheological behavior after 18 months of storage, whereas the non-inventive silica, which was hydrophobized with a symmetrical α,γ-di-hydroxy polydimethylsiloxane, already shows clear signs of aging after 6 months, and in particular an enormous loss of performance can be observed in shear thinning.

Claims

Claims 1. Method for surface modification of hydrophilic silica with a specific surface area of ​​10 to 1000 m² 2 / g (measured according to the BET method according to DIN EN ISO 9277 / DIN 66132) , in which a suspension of silica in an organic solvent is reacted with liquid polyorganosiloxane of the following general formula (I): [R 1 R 2 R 3 SiOi / 2] [R 4 R 5 SiO2 / 2 ] n [R 6 R 7 R 8 SiOi / 2] Formula (I) wherein R 1 , R 2 , R 3 independently of each other, a hydroxyl group or a monovalent hydrocarbon group with 1 to 24 carbon atoms, where at least one group is R 1 , R 2 , R 3 a hydroxyl group means R 4 , R 5 independently of each other means a hydroxyl group or a monovalent hydrocarbon group with 1 to 24 carbon atoms, R 6 , R 7 , R 8 independently of each other, a monovalent hydrocarbon residue with 1 to 24 carbon atoms, n represents an integer from 0 to 20, and where at most 20 mol% based on all residues R 1 to R 8 Hydroxy residues are.

2. The method of claim 1, wherein pyrogenic silica is used.

3. A method according to one or more of the preceding claims, wherein the carbon content achieved by the method is 1 wt.% to 15 wt.%.

4. Method according to one or more of the preceding claims, wherein the monovalent hydrocarbon residues are in R 1 to R 8 They are selected from methyl, ethyl, propyl, butyl, and phenyl residues.

5. Method according to one or more of the preceding claims, wherein the polyorganosiloxane is liquid in the range of 0 to 60 °C at 0.10 MPa (abs.).

6. A method according to one or more of the preceding claims, wherein the organic solvent used to produce the suspension of silica is an aprotic solvent.

7. The method of claim 6, wherein a protic solvent is additionally added.

8. Surface-modified hydrophilic silica obtainable by a process according to any of the preceding claims.

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