Patent Application
20100233060
The invention relates to a method of producing a nanoscale silicon dioxide.
The addition to polymeric materials such as, for example, polyurethanes, polyureas or what are called reactive resins, of fillers, for the purpose of modifying certain properties of the polymeric material, is known. For example, it is possible in this way to improve impact strength, flexural strength, hardness or electrical insulation capacity.
The use of silica or silicon dioxide (SiO2) as a filler in polymers is already known. Various methods of producing SiO2 fillers are known from public prior use.
Natural (mineral) SiO2 can be brought to a desired particle size by grinding, for example, and can be mixed with the polymer or with a polymer precursor. Ground SiO2 generally has a very broad particle size distribution and irregular particle structure. Particle sizes of below 1 μm are difficult, if not impossible, to obtain by mechanical comminution of the SiO2.
Also known is the precipitation of SiO2 from aqueous alkali metal silicate solutions by acidification, and its subsequent drying. This precipitated SiO2 is mixed with the polymer or with a precursor. Here again, irregular particle structures with very broad particle size distributions are obtained.
A further possibility is the production of fumed silica by flame hydrolysis of silicon halogen compounds. This produces particles with a very complex morphology and an extremely broad particle size distribution, since some of the primary particles formed in the flame hydrolysis undergo agglomeration and form other associated superstructures. Fumed silica, moreover, is expensive to produce.
The hydrolysis and condensation of organofunctional silanes (especially alkoxy silanes) to produce aqueous or aqueous-alcoholic silica sols, and the mixing of these sols with a polymer precursor, are also known. Subsequently it is possible to remove water and/or alcohol from the mixture. This method is expensive and on an industrial scale is difficult to control.
The methods described have the disadvantage, furthermore, that it is not possible to produce, specifically, SiO2 fillers having a monomodal, narrow particle size distribution; this disadvantage is particularly pronounced for the three first-mentioned methods. As a result of this, dispersions of the filler in polymer precursors, even at relatively low filler concentrations, exhibit unwanted rheological properties, more particularly a high viscosity, which make processing more difficult.
EP A 0 982 268 discloses a method of producing colloidal silica that involves silanizing an aqueous suspension of a colloidal SiO2.
The invention is based on the object of providing a method of the type specified at the outset that provides a hydrophobic, monodisperse, nanoscale silicon dioxide that can be put to diverse use.
The method of the invention comprises the following steps:
The method of the invention starts from a nanoscale, colloidal silica sol. The pH of this sol is set preferably at 5 or less, more preferably at 4 or less. In the case of a basic sol, this can be done by adding acid or by using an acidic cation exchanger.
In the next step, an organosilane or organosiloxane in an aprotic cyclic ether (e.g., dioxane, more preferably THF) is added, and the system is mixed with stirring. A silanization takes place, in the course of which stirring is carried out, preferably intensively. After about an hour, the reaction is over, and phase separation has taken place. The organic phase comprises the solvent (THF), the silanized colloidal SiO2, and small amounts of water. The aqueous phase is separated off and discarded. The term “aqueous phase”, in the context of the invention, identifies the phase with the more polar solvent. It preferably comprises substantially water, but may also comprise water-miscible or water-soluble (preferably polar) organic solvents. The term “organic phase” identifies the less polar phase.
In a subsequent step, silanization is carried out a second time by further addition of an organosilane or organosiloxane. The reaction is again carried out until two phases are formed. The upper phase contains the greatest fraction of the residual water, the bottom phase the silanized colloidal SiO2.
The colloidal SiO2 used in step a) preferably has an average particle size of 2 to 300 nm, more preferably 3 to 200 nm, more preferably 4 to 150 nm, more preferably 4 to 80 nm, more preferably 10 to 40 nm.
The nanoscale silicon dioxide produced in accordance with the invention is preferably hydrophobic or hydrophobicized as a result of the silanization of the surface. It can therefore be incorporated particularly effectively into an apolar and hence hydrophobic matrix such as, for example, a polymer matrix.
The nanoscale silicon dioxide of the invention is composed preferably to an extent of at least 50% of separate, unaggregated and unagglomerated primary particles. This separation is preferably retained when the particles, either from the solvent or else after removal of the solvent, in the form of a redispersible powder, are incorporated into a polymer matrix. Other preferred lower limits are 70%, 80%, 90%, 95%, and 98%. These percentages are % by weight. According to this aspect of the invention, then, it is possible to provide a dispersion or a redispersible powder that is substantially free from aggregates and/or agglomerates of the silicon dioxide particles. This improves the processing properties (lower viscosity) and the mechanical properties of intermediates and end products that are produced using the silicon dioxide particles produced in accordance with the invention.
The organosilanes or organosiloxanes are preferably selected from the group consisting of organosilanes of the formula R1aHbSiX4-a-b and organosiloxanes of the formula R1nSiO(4-n)/2, in which each R1 independently is selected from hydrocarbon radicals having 1 to 18 carbon atoms or organofunctional hydrocarbon radicals having 1 to 18 carbon atoms, each X is selected independently from a halogen atom or alkoxy radicals having 1 to 18 carbon atoms, a=0, 1, 2 or 3, b=0 or 1, a+b=1, 2 or 3, with the proviso that if b=1, then a+b=2 or 3 and n is an integer from 2 up to and including 3.
Particular preference is given to using a halosilane, more preferably a chlorosilane. The silanes may be functionalized, as for example with polymerizable groups, more particularly vinyl groups. In the context of the invention it is possible to carry out the two silanization steps with different silanes. For example, a functionalized silane, preferably a vinyl silane, can be used only in one of the two silanization steps. It is likewise possible to use mixtures of functionalized and nonfunctionalized silanes in one silanization step.
In the context of the invention it is preferred, when using functionalized silanes, for them to be used entirely or predominantly in the second silanization step. It has been found that in that case the functionalization of the particle surface that is achieved is greater.
The silanization in steps b) and d) of claim 1 is carried out preferably at 0 to 65° C., more preferably 10 to 65° C. The first silanization step, in one variant of the invention, can be carried out at lower temperatures (preferably 0 to 20° C., more preferably 0 to 10° C.) and the second step, which can be carried out, for example, at 20 to 65° C.
In the context of the invention it is possible to carry out a silanization additionally before the first silanization step (step b) of claim 1) is carried out, by adding an alkoxy silane to the aqueous suspension itself.
After the second silanization has been carried out, it is preferred to replace the cyclic ether by another aprotic organic solvent, preferably toluene. For this purpose, the cyclic ether may be removed by distillation. This is done preferably with addition of the second solvent (e.g., xylene, butyl acetate, methyl isobutyl ketone, or toluene) as an azeotrope former. It is preferred, following the removal of the cyclic ether, to carry out further heating under reflux, in which case, preferably, the refluxing solvent is neutralized with a base. For the neutralization it is possible to use a basic salt such as, for example, an alkali metal or alkaline earth metal carbonate or hydrogen carbonate. The solvent may be passed, for example, through a column filled with the basic salt.
In accordance with the invention it is possible to prepare solvent-free powders from the suspension. For this purpose, the solvent is removed at elevated temperature under reduced pressure. The resulting powder, by means of simple stirring, can be redispersed monodispersely in a multiplicity of solvents, monomers, and polymers. The particle size remains constant; agglomeration or aggregation takes place not at all or at most to an insubstantial extent.
The dispersion produced in accordance with the invention, or the redispersible powder obtained from the dispersion by removal of the solvent, can be incorporated into a very wide variety of base polymers and can improve or modify their physical, and more particularly mechanical, properties. Base polymers which can be used in the context of the invention include a multiplicity of known polymers. For example, thermoplastic of thermoset plastics can be modified by means of silicon dioxide particles produced in accordance with the invention. Examples include polyolefins, polycarbonates, polyamides, polyimides, polyacrylates, polymethacrylates, polyetherketones, polysulfones, polyurethanes, polyureas, epoxy resins, polyester resins, and polysiloxanes (silicones). Examples of elastomers which can be modified include natural rubber, butyl rubbers, acrylate rubbers, styrene-butadiene rubber (SBR), unhydrogenated or hydrogenated nitrile-butadiene rubbers, etc. For many of these groups of materials it is particularly advantageous to incorporate the nanoparticles produced in accordance with the invention in the form of a redispersible powder, since their introduction via solvent is disadvantageous and is associated with high expense and complexity.
With particular advantage the nanoscale silicon dioxide produced in accordance with the invention can also be incorporated into polymers or resins having a low boiling point, such as, for example, methyl methacrylate (MMA).
Nanoscale particles produced in accordance with the invention may likewise be used for modifying plasticizers such as, for example, adipates and phthalates. With these plasticizers they form stable dispersions of low viscosity.
Polymeric or polymerizable mixtures modified with the particles produced in accordance with the invention are stable and storable dispersions and have good flow properties (low viscosity, low structural viscosity). They are therefore suitable, for example, for producing dental formulations which are applied, for example, from a static mixer and hence must not have too high a processing viscosity. With particular preference they can be used with dental formulations based on silicones.
Another possible field of application is the modification of LSRs (Liquid Silicone Rubbers), which are processed generally by injection molding and in which, therefore, a low processing viscosity is of great advantage. In accordance with the invention, in LSRs, a high filler content and hence good mechanical properties of the cured end product can be achieved, without the processing properties suffering from too high a viscosity.
In principle the invention makes it possible to provide polymerizable mixtures which on account of their low viscosity have good processing properties and, as a cured polymer, have improved properties brought about by means of a high filler content, more particularly mechanical properties, improved thermal conductivity, and the like.
According to one embodiment of the invention, the silane or siloxane used in the second silanization step has free SiH groups, and so, after this second silanization step, there are free SiH groups on the surface of the silicon dioxide particles. SiH groups are very sensitive to hydrolysis. In the context of the invention, the first silanization step already makes the surface of the SiO2 particles largely water-free, and so in the second silanization step it is possible to apply SiH groups to the surface that are sufficiently stable and are not immediately hydrolyzed by residual moisture.
In accordance with another variant of the invention, this SiH group is then available for hydrosilylation. By means of this hydrosilylation, the surface of the silicon dioxide particles can be provided with specific organic modification, as for example by hydrosilylation with an alkene or an alkyl compound. The invention accordingly also provides nanoscale silicon dioxide particles which have free SiH groups on the surface (process product of claim 20). They permit, so to speak, a building-block chemistry for specific attachment of desired molecules by means of hydrosilylation.
Working examples of the invention are described below. In the drawings,
In the examples below, plastics composites are produced using the nanoparticles of the invention, and their properties are ascertained. This is done using the measurement techniques that are described below.
Evaluation:
Starting Materials Used
The resulting toluene sol, which was still acidic, was heated under reflux, and the distillate flowing back was passed through a column filled with sodium carbonate. After 6 hours of reflux, the sol no longer gave an acidic reaction.
A total of nine experiments are carried out with different proportions of the two silanes in the first and second silanization; details are given in table 1.
On account of the large surface area, the particles tend to form agglomerates when the solvent is removed. On redispersion, these agglomerates must be broken up again. The greater the match between the particle size distribution and the particle size distribution in the original sol, the more redispersible the particles.
Table 5 shows that the particles are readily redispersible in the solvents toluene, butyl acetate, and isopropyl acetate. In toluene, an insubstantially broadened particle size distribution is observed that is shifted to larger particle sizes. These changes, however, are within a range of the kind also observed on fluctuations from batch to batch. In isopropyl acetate the distribution measured is identical; in butyl acetate, it is shifted to somewhat smaller particle sizes, owing to solvent effects.
1050 g of silica sol (Levasil 200/40%, BET=200 m2/g, 40% SiO2, Na+ removed with ion exchanger) were stirred with 62.58 g of gamma-methacryloxypropyltrimethoxysilane for 1 hour. The material was then diluted with 1250 g of THF, and, with stirring, 63 g of chlorotrimethylsilane were added. After an hour, two phases have formed. The top phase contained no solid and was discarded. The bottom phase was diluted with 150 g of THF, 63 g of chlorotrimethylsilane were added with stirring, and, after an hour, a further phase separation was carried out. The top phase was again discarded. The bottom phase was diluted with 400 g of toluene, and THF/water were distilled off with addition of further toluene.
The resulting toluene sol, which was still acidic, was heated under reflux, and the distillate flowing back was passed via a column filled with sodium carbonate. After six hours of reflux, the sol no longer gave an acidic reaction.
The toluene sol obtained was freed from volatile fractions under reduced pressure at 60° C. This gave a white powder.
By stirring with a magnetic stirrer, 50% dispersions of this powder in methyl methacrylate (MMA) can be produced which have a viscosity of only 18 mPas, are highly flowable, and are optically clear.
The pretreatment of the silica with gamma-methacryloxypropyltrimethoxysilane results in high compatibility with methacrylates such as MMA in relatively low dispersion times.
60 g of chlorotrimethylsilane were introduced in 1100 g of THF, and 926 g of silica sol (Levasil 200/40%, BET=200 m2/g, 40% SiO2, Na+ removed with ion exchanger) were metered in over the course of 45 minutes. After an hour, two phases had formed, and were separated in a separating funnel. The 895.1 g bottom phase was reacted with 55 g of chloromethylsilane, with stirring. During the reaction, evolution of gas (hydrogen) was observed. Again, two phases were formed, which were separated in a separating funnel. The 729 g bottom phase was distilled out with a total of 400 g of toluene. The reaction mixture was heated under reflux over sodium carbonate until the condensate gave a neutral reaction.
The product contains 38.8% of solids. The SiH content is 0.4 mmol/g (determined by gas volumetry). After drying under reduced pressure, the resulting solid can be easily redispersed in toluene.
50.4 g of 1-octene were introduced with 0.01 g of hexachloroplatinic acid and the mixture was heated to 90° C. Then, over the course of 10 minutes, 257 g of silica sol from example 8 were metered in and hydrosilylation took place at 100° C. for 1 hour. The SiH content went down to <0.007 mmol/g; i.e., in the reaction, the SiH groups on the surface of the particles were consumed completely by reaction with the 1-octene. This gave a clear, brown dispersion having a solids content of 33.6% and a particle size of 26 nm with a span of 0.6.
In the same way, the hydrosilylation was carried out successfully with styrene, undecylenic acid, allyl alcohol, allyl glycidyl ether, and allyl methacrylate.
| Number | Date | Country | Kind |
|---|---|---|---|
| 07014926.5 | Jul 2007 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP08/04634 | 6/10/2008 | WO | 00 | 1/28/2010 |
