SILICA MOLDED BODIES HAVING LOW THERMAL CONDUCTIVITY

Abstract

Hydrophobic shaped silica bodies having low density and low thermal conductivity are produced by forming a dispersion of silica in a solution of binder and organic solvent, and removing the solvent and shaping to form a shaped body. The shaped bodies retain their hydrophobicity, are stable with regards to shape, and are useful in acoustic and thermal insulation.

Claims

1.-7. (canceled)

8. A process for producing shaped silica bodies having a C content of less than 8% by weight, a density, determined by Hg porosimetry, of less than 0.30 g/cm.sup.3, a pore volume for pores smaller than 4 m, determined by Hg porosimetry, of more than 2.0 cm.sup.3/g, a proportion of the pores smaller than 4 m, based on the total pore volume, of at least 60% and a thermal conductivity, determined by a non-steady-state method, of less than 30 mW/K*m, comprising: i) producing a dispersion containing silica, at least one binder and an organic solvent, and ii) evaporating the solvent from the dispersion, and shaping to form the shaped silica bodies.

9. The process of claim 8, wherein hydrophilic silica or a mixture of hydrophilic silica and partially hydrophobic silica is as the silica.

10. The process of claim 8, wherein silanes containing a C.sub.1-C.sub.3-alkyl group, C.sub.2-3 alkenyl group, methoxy group, ethoxy group, or a mixture thereof are used as a binder.

11. The process of claim 8, wherein at least one solvent is selected from the group consisting of alkanes, ethers, alcohols, and mixtures thereof.

12. A shaped silica body produced by the process of claim 8.

13. Acoustic or thermal insulation comprising shaped silica bodies of claim 12.

Description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] The advantage of the high-viscosity silicone rubber compositions of the invention is that, without exception, they afford a high-quality printed image, whereas noninventive high-viscosity silicone rubber compositions lead either to blocking and sticking of the print valve or to a crust-like appearance.

[0025] The high-viscosity silicone rubber compositions of the invention for the 3D printing of silicone moldings by the ballistic additive DOD method have a viscosity n.sub.1 (in each case at 25 C. and 0.5 s.sup.1) in the range from 300 Pa.Math.s to 10 kPa.Math.s, and comprise

[0026] (A) 50% to 95% by weight of at least one organosilicon compound having an average of at least two aliphatically unsaturated groups per molecule,

[0027] (B) 1% to 10% by weight of at least one organosilicon compound having an average of at least two SiH groups per molecule,

[0028] or, in place of (A)+(B) or in addition to (A) and (B), (G) 0%-95% by weight of at least one organosilicon compound having an average of at least two aliphatically unsaturated groups and at least two SiH groups per molecule,

[0029] (C) 0.1 to 500 ppm by weight of at least one hydrosilylation catalyst, based on the content of the metal relative to the overall silicone composition,

[0030] (D) 1% to 50% by weight of at least one actively reinforcing material,

[0031] (E) 0% to 30% by weight of auxiliaries other than (D),

[0032] characterized in that the nominal melt flow index n of the silicone rubber composition is within the following range:

1.00<n<0.40,

where n is calculated from formula (X):

log =log K+n*log v(X)

[0033] where

[0034] represents the viscosity at shear rate v,

[0035] K represents the nominal consistency index,

[0036] v represents the shear rate and

[0037] log represents the decadic logarithm,

[0038] and the viscosity .sub.1, the nominal melt flow index n and the nominal consistency index K in formula (X) are determined by the rheological test method disclosed in the description.

[0039] The nature and the extent of the shear-thinning characteristics and hence the degree of the shear-thinning characteristics are characterized by the nominal melt flow index n, where n<0 describes shear-thinning flow characteristics, n>0 describes shear-thickening flow characteristics and n=0 describes newtonian flow characteristics.

[0040] The high-viscosity silicone rubber compositions of the invention preferably have a viscosity .sub.1 (in each case at 25 C. and 0.5 s.sup.1) in the range from 400 Pa.Math.s to 5 kPa.Math.s, and more preferably from 500 Pa.Math.s to 3 kPa.Math.s,

[0041] The nominal melt flow index n of the silicone rubber compositions of the invention is preferably in the range of 0.80<n<0.45 and more preferably in the range of 0.70<n<0.50.

[0042] Constituent (A) of the silicone rubber compositions of the invention is an organosilicon compound having at least two aliphatic carbon-carbon multiple bonds, preferably linear or branched polyorganosiloxanes composed of units of the formula (I):

R.sub.aR.sup.1.sub.bSiO.sub.(4-a-b)/2(I)

[0043] where

[0044] R may be the same or different and is a C.sub.1-C.sub.20 radical which is free of aliphatic carbon-carbon multiple bonds, is optionally halogen-substituted and optionally contains oxygen, nitrogen, sulfur or phosphorus atoms,

[0045] R.sup.1 may be the same or different and is a monovalent, optionally substituted organic radical having an aliphatic carbon-carbon multiple bond,

[0046] a is 0, 1, 2 or 3 and

[0047] b is 0, 1 or 2,

[0048] with the proviso that a+b<4 and there is an average of at least 2 R.sup.1 radicals per molecule.

[0049] The R radical may comprise mono- or polyvalent radicals, in which case the polyvalent radicals, such as bivalent, trivalent and tetravalent radicals, connect a plurality of, for example two, three or four, siloxy units of the formula (I) to one another.

[0050] Preferably, the R radicals are bonded to the silicon via a carbon or oxygen atom. Examples of SiC-bonded R radicals are alkyl radicals (e.g. methyl, ethyl, octyl and octadecyl radicals), cycloalkyl radicals (e.g. cyclopentyl, cyclohexyl and methylcyclohexyl radicals), aryl radicals (e.g. phenyl and naphthyl radicals), alkaryl radicals (e.g. tolyl and xylyl radicals) and aralkyl radicals (e.g. benzyl and beta-phenylethyl radicals). Examples of substituted R radicals are 3,3,3-trifluoro-n-propyl, p-chloropbenyl, chloromethyl, glycidozypropyl and (CH.sub.2).sub.n(OCH.sub.2CH.sub.2).sub.mOCH.sub.3, where n and m are identical or different integers from 0 to 10. Examples of SiO-bonded R radicals are alkoxy groups (e.g. methoxy, ethoxy, iso-propoxy and tert-butoxy radicals) and the p-nitrophenoxy radical.

[0051] The R.sup.1 radical may be any desired groups amenable to an addition reaction (hydrosilylation) with an SiH-functional compound. The R.sup.1 radical preferably comprises alkenyl and alkynyl groups having 2 to 16 carbon atoms, such as vinyl, ally, methallyl, 1-propenyl, 5-nexenyl, ethynyl, butadienyl, hexadienyl, undecenyl, cyclopentenyl, cyclopentadienyl, norbornenyl and styryl radical, particular preference being given to vinyl, allyl and hexenyl radicals.

[0052] If the R.sup.1 radical comprises substituted aliphatically unsaturated groups, preferred substituents are halogen atoms, cyano groups and alkoxy groups. Examples of substituted R.sup.1 radicals are allyloxypropyl and isopropenyloxy radicals.

[0053] Preference is given, as constituent (A), to the use of vinyl-functional, essentially linear polydiorganosiloxanes having a viscosity of 100 to 500,000 mPa.Math.s, more preferably between 1000and 50,000 mPa.Math.s (at 25 C. and 0.8 sec.sup.1). Constituent (A) may be a mixture of different organosilicon compounds of the type described above.

[0054] The content of constituent (A) in the silicone rubber composition of the invention is 50% to 95% by weight, preferably 70% to 90% by weight, more preferably 65% to 80% by weight.

[0055] Constituent (B) is any SiH-functional organosilicon compound having an average of at least two SiH groups. Constituent (B) functions as cross-linker of the silicone rubber composition. Constituent (B) may also be a mixture of various SiH-functional organosilicon compounds. Preferably, constituent (B) is linear, cyclic, branched or resinous polyorganosiloxanes having Si-bonded hydrogen atoms, composed of units of the formula (II)

R.sub.cH.sub.dSiO.sub.(4-c-d)/2(II)

[0056] where

[0057] R may be the same or different and has the definition given above,

[0058] c is 0, 1, 2 or 3 and

[0059] d is 0, 1 or 2,

[0060] with the proviso that the sum total of (c+d) is not more than 3and there is an average of at least two Si-bonded hydrogen atoms per molecule.

[0061] Preferably, constituent (B) contains Si-bonded hydrogen in the range from 0.04 to 1.7 percent by weight (% by weight) based on the total weight of the organopolysiloxane (B). The molecular weight of constituent (B) may vary within wide limits, for instance between 10.sup.2 and 10.sup.6 g/mol. For example, constituent (B) may be an SiH-functional oligosiloxane of relatively low molecular weight, such as tetramethyldisiloxane, but may also be high-polymeric polydimethylsiloxane having SiH groups in chain or terminal positions or a silicone resin having SiH groups. Preference is given to the use of SiH-functional compounds of low molecular weight, such as tetrakis(dimethyl-siloxy)silane and tetramethylcyclotetrasiloxane, and SiH-containing siloxanes, such as poly(hydrogenmethyl)siloxane and poly(dimethylhydrogenmethyl) siloxane having a viscosity of 10 to 1000 mPa.Math.s (at 25 C. and 0.8 sec.sup.1). Preference is given to constituents (B) that are compatible with constituent (A) (homogeneously miscible or at least emulsifiable). According to the type of constituent (A), it may therefore be necessary to suitably substitute constituent (B), for example by replacing some of the methyl groups with 3,3,3-trifluoropropyl or phenyl groups.

[0062] Constituent (B) may be used individually or in the form of a mixture of at least two different (B) and is preferably present in the silicone rubber composition of the invention in such an amount that the molar ratio of SiH groups to aliphatically unsaturated groups is 0.1 to 20, preferably between 0.5 and 5, more preferably between 1 and 2. The content of constituent (B) in the silicone rubber composition of the invention is 0.1 to 15% by weight, preferably 0.5%-10% by weight, more preferably 2%-5% by weight.

[0063] Constituent (G) can be used in place of (A)+(B) or in addition to (A) and (B). In the addition-crosslinking compositions of the invention, the following combinations are thus possible: (A)+(B) or (A)+(G) or (B)+(G) or (A)+(B)+(G) or (G) alone. (G) is an organosilicon compound having at least two aliphatically unsaturated groups and at least two SiH groups per molecule and can thus crosslink with itself. Compounds (G) are known to those skilled in the art from the prior art. If compounds (G) are used, they are preferably those composed of units of the general formulae

R.sup.7.sub.kSiO.sub.(4-k)/2(VI),

R.sup.7.sub.mR.sup.6SiO.sub.3-m)/2(VII)

and

R.sup.7.sub.oHSiO.sub.3-o/2(VIII),

[0064] where

[0065] R.sup.7 is a monovalent, optionally substituted hydrocarbyl radical which is free of aliphatic carbon-carbon multiple bonds and has 1 to 18 carbon atoms per radical and

[0066] R.sup.6 is a monovalent hydrocarbyl radical having a terminal aliphatic carbon-carbon multiple bond having 2 to 8 carbon atoms per radical,

[0067] k is 0, 1, 2 or 3,

[0068] m is 0, 1 or 2,

[0069] o is 0, 1 or 2,

[0070] with the proviso that, in (G), there is an average of at least 2 R.sup.6 radicals and an average of at least 2 Si-bonded hydrogen atoms.

[0071] It is possible to use a single compound (G) or a mixture of at least two compounds (G).

[0072] The content of constituent (G) in the silicone rubber composition of the invention is 0%-95% by weight, preferably 0%-50% by weight, more preferably 0%-10% by weight.

[0073] Constituent (C) serves as catalyst for the addition reaction (hydrosilylation) between the aliphatically unsaturated groups of constituent (A) and the silicon-bonded hydrogen atoms of constituent (B) or (G). In principle, it is possible to use any hydrosilylation catalysts typically used in addition-crosslinking silicone rubber compositions. As catalysts (C) that promote addition of Si-bonded hydrogen onto aliphatic multiple bonds, for example, platinum, rhodium, ruthenium, palladium, osmium or iridium, an organometallic compound or a combination thereof are suitable. Examples of such catalysts (C) are metallic and finely divided platinum which may be present on supports, such as silicon dioxide, aluminum oxide or activated carbon, compounds or complexes of platinum, such as platinum halides, e.g. PtCl.sub.4, H.sub.2PtCl.sub.6.6H.sub.2O, Na.sub.2PtCl.sub.4.4H.sub.2O, platinum acetylacetonate and complexes of these compounds, encapsulated in a matrix or a core/shell-like structure, platinum-olefin complexes, platinum-phosphite complexes, platinum-alcohol complexes, platinum-alkoxide complexes, platinum-ether complexes, platinum-aldehyde complexes, platinum-ketone complexes, including reaction products of H.sub.2PtCl.sub.6.6H.sub.2O and cyclohexanone, platinum-vinyisiloxane complexes, such as platinum-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complexes with or without a content of detectable inorganically bound halogen, bis(gamma-picoline)-platinum dichloride, trimethylenedipyridineplatinum platinum dichloride, trimethylenedipyridineplatinum dichloride, dicylopentadieneplatinum dichloride, dimethylsulfoxide-ethyleneplatinum (II) dichloride, and reaction products of platinum tetrachloride with olefin and primary amine or secondary amine or primary and secondary amine, such as the reaction product of platinum tetrachloride dissolved in 1-octene with sec-butylamine or ammonium-platinum complexes, trimethylcyclopentadienylplatinum(IV), trimethyl [(3-trimethoxysilyl)propylcyclopentadienyl]platinum(IV).

[0074] The hydrosilylation catalysts listed generally enable rapid crosslinking of the silicone rubber composition even at room temperature. Since the hydrosilylation reaction sets in immediately after mixing of all constituents, addition-crosslinking compositions are usually formulated in the form of at least two components, where a component A comprises the platinum catalyst (C) and another component B comprises the crosslinker (B) or (G). In order to have a sufficient processing time even after mixing of the two components, inhibitors which delay the onset of the orosslinking reaction are usually added. Rapid crosslinking can then be brought about by supply of neat. For the use of addition-crosslinking compositions in the 3D printing method, however, preference is given to those hydrosilylation catalysts which can barely be activated by thermal means but can be very readily activated by high-energy radiation (UV, UV-VIS), meaning that the deposited silicone rubber composition of the invention is crosslinked not with thermal initiation but preferably with initiation by UV or UV-VIS radiation. This is effected, for example, either via an activatable hydrosilylation catalyst or via a deactivatable inhibitor (F) which is additionally present. Compared to thermal crosslinking, UV- or UV-VIS-induced crosslinking has numerous advantages. Firstly, the intensity, period of action and locus of action of the UV radiation can be judged accurately, whereas the heating of the silicone rubber composition deposited dropwise (and the subsequent cooling thereof) is always retarded by virtue of the relatively low thermal conductivity. Because of the intrinsically very high coefficient of thermal expansion of the silicones, the temperature gradients that are inevitably present in the course of thermal crosslinking lead to mechanical stresses which have an adverse effect on the dimensional accuracy of the molding formed, which in the extreme case can lead to unacceptable distortions of shape. A further advantage of the UV/VIS-induced addition crosslinking is found in the production of multicomponent moldings, for example hard-soft composites which, as well as the silicone elastomer, comprise a thermoplastic, the thermal warpage of which is avoided.

[0075] UV/VIS-induced addition-crosslinking silicone rubber compositions are described, for example in DE 10 2008 000 156 A1, DE 10 2008 043 316 A1, DE 10 2009 002 231 A1, DE 10 2009 027 486 A1, DE 10 2010 043 149 A1 and WO 2009/027133 A2. The crosslinking takes place through UV/VIS-induced activation of a light-sensitive hydrosilylation catalyst (C), preference being given to platinum catalysts activatable by UV or UV-VIS radiation. The technical literature describes numerous light-activatable platinum catalysts which are largely inactive with exclusion of light and can be converted to platinum, catalysts that are active at room temperature by irradiation with light having a wavelength of 250-500 nm. Examples of these are (-diolefin) (-aryl) platinum complexes (EP 0 122 008 A1; EP 0 561 919 B1), Pt (II)--diketonate complexes (EP 0 398 701 B1) and (.sup.5-cyclopentadienyl)tri(-alkyl)platinum(IV) complexes (EP 0 146 307 B1, EP 0 358 452 B1, EP 0 561 893 B1). Particular preference is given to MeCpPtMe.sub.3 and the complexes that derive therefrom through substitution of the groups present on the platinum, as described, for example, in EP 1 050 538 B1 and EP 1 803 728 B1.

[0076] The silicone rubber compositions which crosslink in a UV- or UV-VIS-induced manner can be formulated in single- or multicomponent form.

[0077] The rate of UV/VIS-induced addition crosslinking depends on numerous factors, especially on the nature and concentration of the platinum catalyst, on the intensity, wavelength and action time of the UV/VIS radiation, the transparency, reflectivity, layer thickness and composition of the silicone rubber composition, and the temperature. For the activation of the UV/VIS-induced addition-crosslinking silicone rubber compositions, light of wavelength 240-500 nm, preferably 300-450 nm, more preferably 350-400 nm, is used. In order to achieve rapid crosslinking, which is understood to mean a crosslinking time at room temperature of less than 20 min, preferably less than 10 min/more preferably less than 1 min, it is advisable to use a UV/VIS radiation source with a power between 10 mW/cm.sup.2 and 15,000 mW/cm.sup.8, and a radiation dose between 150 mJ/cm.sup.2 and 20,000 mJ/cm.sup.2, preferably between 500 mJ/cm.sup.2 and 10,000 mJ/cm.sup.2. Within the scope of these power and dose values, it is possible to achieve area-specific irradiation times between a maximum of 2000 s/cm.sup.2 and a minimum of 8 ms/cm.sup.2. It is also possible to use two or more radiation sources, including different radiation sources.

[0078] The hydrosilylation catalyst should preferably be used in a catalytically sufficient amount, such that sufficiently rapid crosslinking is enabled at room temperature. Typically, 0.1 to 500 ppm by weight of the catalyst is used, based on the content of the metal relative to the overall silicone rubber composition, preferably 0.5-200 ppm by weight, more preferably 1-50 ppm by weight. It is also possible to use mixtures of different hydrosilyation catalysts.

[0079] Constituent (D)

[0080] The term actively reinforcing material or, synonymously, reinforcing material is understood in the context of this invention to mean an (actively) reinforcing filler. Compared to (inactive) non-reinforcing fillers, actively reinforcing fillers improve the mechanical properties of the elastomers in which they are used. Inactive fillers, by contrast, act as extenders and dilute the elastomer. The terms actively reinforcing material, actively reinforcing filler, reinforcing material and reinforcing filler are used synonymously in the context of the present invention.

[0081] Constituent (D) is necessary in order to achieve adequate mechanical strength of the silicone elastomer. Mechanical strength is understood to mean the entirety of the properties typical of elastomers, especially hardness, elongation at break, tear resistance and tear propagation resistance. In order to achieve appealing properties in this regard, the addition of actively reinforcing materials is indispensable. These include, in particular,

[0082] (D1) finely divided particulate materials such as fumed silicas, titanium dioxides, aluminum, oxides, aerogels and carbon blacks having a high specific surface area between 50 and 1000 m.sup.2/g (measured by the BET method to DIN 66131 and DIN 66132), and

[0083] (D2) nanoparticies (SiO.sub.2, TiO.sub.2, exfoliated sheet silicates, carbon nanotubes etc.).

[0084] Preferably, component (D) is selected from the group consisting exclusively of (D1) fumed silicas, titanium dioxides, aluminum oxides, aerogels and carbon blacks having a high specific surface area between 50 and 1000 m.sup.2/g, measured by the BET method to DIN 66131 and DIN 66132, and (D2) nanoparticies consisting of SiO.sub.2, TiO.sub.2, exfoliated, sheet silicates or carbon nanotubes.

[0085] Preference is given to using (D1) as a reinforcing filler. A particularly active and preferred reinforcing agent (D1) is fumed silica (produced, for example, by reaction of silicon-halogen compounds in a hydrogen-oxygen flame).

[0086] Fumed silica is hydrophilic because of the silanol groups (SiOH) present on the surface thereof. However, it is customary and preferable to use hydrophobic silicas in silicone rubber compositions in order to achieve higher filler contents (and hence better mechanical properties) without an excessive rise in viscosity and without phase inversion. Moreover, the mixing of the silicone constituents and the silica is significantly facilitated by the hydrophobic character. The hydrophobization of the silica, which is effected mainly by silylation, is known to those skilled in the art and is described, for example, in published specifications EP 686676 B1, EP 1433749 A1 and DE 102013226494 A1. As a result of the hydrophobization (silylation) of the silica surface, there is a reduction in the silanol group density typically from 1.8 to 2.5 SiOH/nm.sup.2 down to less than 1.8 to less than 0.9 silanol groups per nm.sup.2 (determined by means of acid-base titration as stated in G. W. Sears, Anal. Chem. 1956, 28, 1981). At the same time, there is an increase in the carbon content of the silica to 0.4% to 15% by weight of carbon (determined by means of elemental analysis), the weight being based on the hydrophobic silica.

[0087] The use of reinforcing agents (D2) is possible but not preferred, because it is impracticable on the industrial scale. Because of the very minor intermolecular interactions between silicones, the production of true nanoparticulate silicone rubber compositions is found to be very difficult. There is usually rapid re-agglomeration of the nanoparticles, or there is no exfoliation or intercalation of sheet silicates in the silicone.

[0088] It is also possible to use a plurality of different reinforcing agents (D).

[0089] The content of reinforcing agents (D) based on the overall crosslinkable silicone rubber composition is between 1% and 50% by weight, preferably between 5% and 30% by weight, more preferably between 10% and 25% by weight.

[0090] Constituent (E) is known to those skilled in the art and includes ail optional additives that may be present in the silicone rubber composition of the invention in order to achieve specific profiles of properties. These include inhibitors, heat stabilizers, solvents, plasticizers, color pigments, sensitizers, photoinitiators, adhesion promoters, inactive fillers, thixotropic agents, conductivity additives, silicone resins etc. that are different than the other constituents.

[0091] What is to be shown hereinafter is how the nominal melt flow index n and the viscosity .sub.1, which are crucial for the processing of high-viscosity silicone compositions by the DOD-3D printing method, can be adjusted in a controlled manner.

[0092] The particulate materials (D1) and (D2) detailed above are among the reinforcing fillers. By contrast with the non-reinforcing fillers, for example chalk, quartz flour, polymer powders etc., reinforcing fillers have a high specific surface area, which results in a very nigh number of filler-filler and filler-silicone interactions. These interactions bring about the desired high mechanical strength of the resulting silicone elastomer.

[0093] A further means of achieving a high level of mechanical strength and elasticity is the use of long-chain (and hence higher-viscosity) silicone polymers. Silicone compositions containing short-chain silicone polymers can be crosslinked to give very hard materials, but give less tear-resistant and less elastic silicone elastomers.

[0094] The use of reinforcing fillers in combination with long-chain silicone polymers, by virtue of the abovementioned interactions, leads to relatively high-viscosity silicone compositions. However, the viscosity of the silicone composition is often limited at the upper end (and often also additionally at the lower end) by the desired processing method. For instance, the extremely high-viscosity (firm/pasty) solid silicone rubbers are typically processed by the press-molding method or by calendering etc.

[0095] Processing by the DOD-3D printing method is increasingly meeting its limits with silicone compositions of ever higher viscosity in spite of all technological advances. In the individual case, it is therefore necessary to make a compromise between the desired high mechanical strength of the silicone elastomer and the processibility of the uncrosslinked silicone composition.

[0096] In principle, one is confronted with the problem of keeping the viscosity of the silicone composition .sub.1 as low as possible for reasons of processibility, but at the same time of achieving maximum mechanical strength values. The viscosity of the silicone composition .sub.1 (since it is measured at very low shear, which can also be regarded as viscosity at rest or starting viscosity) is fixed in particular by the nature and content of reinforcing filler (D) and the chain length (viscosity) of the silicone polymers used (components (A, (B) and/or (G)).

[0097] One skilled in the art knows of numerous ways of keeping the viscosity low without significantly reducing the mechanical strength values. These include, for example, the hydrophobization of the filler (for example by silylation of finely divided silicas), which can reduce the increase in viscosity caused by the filler to an enormous degree. In addition, it is possible to keep the viscosity of the silicone composition low through the use of relatively short-chain vinyl-terminated silicone polymers (component (A)) in combination with relatively short-chain SiH-terminal silicone polymers (component (B)) or through the use of relatively short-chain silicone polymers of component (G); in this case, it is only in the course of the crosslinking reaction that the long-chain silicone polymers are constructed through chain extension (i.e. only after the actual processing step such as the jetting).

[0098] If the viscosity .sub.1 is found to be too high, i.e. more than 10 kPa.Math.s (at 25 C. and 0.5 s.sup.1), it is thus possible to counteract this by (i) lowering the filler concentration, (ii) increasing the hydrophobicity of the filler (for example by using hydrophobic fillers or by hydrophobizing hydrophilic fillers) and/or (iii) lowering the polymer chain length of the silicone constituents. The exact adjustment of the viscosity .sub.1 can thus be achieved by simple routine experiments.

[0099] However, the viscosity at rest .sub.1 is not the only crucial factor for the processibility of a silicone composition by the DOS method. The viscosity at the extremely high shear rates that occur in the jetting nozzle is also of great significance. Processibility of high-viscosity compounds by the DOD method becomes possible at all only by virtue of the fact that the viscosity decreases enormously with increasing shear rate. These characteristics are referred to as shear-thinning characteristics. The decrease in the viscosity as a result of shear may be several orders of magnitude in the DOB method. For the processibility of high-viscosity silicone compositions by the DOD method, marked shear-thinning characteristics are thus indispensable.

[0100] The shear-thinning characteristics can be well-characterised by the nominal melt flow index n. The nominal melt flow index n describes the deviation from what are called newtonian flow characteristics, which feature a shear rate-independent viscosity and are characterized by the flow index n=0. n values of greater than 0 describe an increase in viscosity with increasing shear rate (shear-thickening characteristics). n values of less than zero describe shear-thinning characteristics, i.e. a decrease in viscosity with increasing shear rate. This relationship can be described by equation (IX) (the logarithmic form of equation (IX) is identical to equation (X)):

(v)=K*v.sup.n(IX)

[0101] where (v) denotes the viscosity at the shear rate v and K is the nominal consistency index (when n=0, i.e. in the case of newtonian characteristics, K=).

[0102] In order to determine the two parameters K and n present in the equation (IX), it is sufficient to know two pairs of values (.sub.1 at v.sub.1) and (.sub.2 at v.sub.2). For the first pair of values, it is possible, for example, to use the viscosity at rest .sub.1 at v.sub.1=0.5 s.sup.1. Since the shear rate range that occurs on jetting is difficult to attain for measurement purposes (requiring measurements in a high-pressure capillary viscometer), for example, the viscosity .sub.2 at v.sub.2=25 s.sup.1 is taken for the second pair of values, but this does not constitute a restriction owing to the experimentally confirmed validity of equation (IX).

[0103] Silicone compositions generally nave shear-thinning characteristics, meaning that the nominal melt flow index n is negative. However, it has been found that, surprisingly, irrespective of the processibility of the silicone compound, which may quite possibly be processible under high shear given a sufficiently low viscosity, a satisfactory printed image is obtained by the DOD-3D printing method only when the nominal melt flow index n is within a particular range. It is thus quite possible that a compound having lower viscosity at high shear will give a poorer printed image than a compound having higher viscosity at the same shear. In fact, a crucial factor is found to be the degree of the shear-thinning characteristics, i.e. the significance of the decrease in viscosity that occurs when the shear rate is increased. In other words: a sufficiently low viscosity at high shear rate is a necessary condition for processibility by the DOB method, but is not a sufficient condition for a good printed image. A necessary and simultaneously sufficient condition for this is the abovementioned relation

1<n<0.40.

[0104] There are in principle the following options for bringing the nominal melt flow index n into the range of the invention:

[0105] Silicone polymers feature virtually newtonian flow characteristics over a wide shear rate range; only at very high shear rates is there any orientation of the polymer chains in the flow direction, which causes a decrease in viscosity. For this reason, it is less effective to adjust the melt flow index n via an altered composition of the silicone constituents (components (A), (B) and/or (G)).

[0106] A much more effective method is found to be the adjustment of the shear-thinning characteristics through suitable choice of the reinforcing filler. More particularly, it is possible to adjust the surface energy of the filler such that the filler particles form a filler network based on physical interactions in the hydrophobic, nonpolar silicone matrix. At rest, this filler network hinders the flowability of the silicone constituents, but breaks down as soon as there is relatively strong shear on the silicone composition, meaning that the flowability of the silicone constituents increases significantly under strong shear, which is equivalent to marked shear-thinning characteristics.

[0107] The breakdown of the filler network brought about by the strong shear is reversible, meaning that the compound returns to its original equilibrium state after the shear has stopped (relaxation).

[0108] More exact analysis of these shear-thinning characteristics caused by the filler shows that an increase in the surface energy of the filler aggregates is associated with stronger and quicker formation of the filler network, which in turn results in a rise in the starting viscosity (viscosity at rest) .sub.1.

[0109] This enhanced structure formation can be brought about, inter alia, by structure-forming additives which increase the surface energy of the filler. The enhanced structure formation can alternatively be brought about through the use of less strongly hydrophobic or hydrophobized fillers. In addition, structure formation can be intensified by an increase in the filler content. Given the same filler content, an increase in the specific surface area of the filler also leads to enhanced structure formation of the silicone composition.

[0110] If the nominal melt flow index n is thus found to be too high, i.e. more than 0.40, it is possible to counteract this by (i) lowering the hydrophobicty of the filler, (ii) by increasing the filler concentration, (iii) by using auxiliaries which can increase the surface energy of the filler (for example thixotropic agents) and/or (iv) by increasing the specific surface area of the filler. The person skilled in the art will thus be able to select the most suitable method taking account of the other boundary conditions to be placed on the silicone composition. The exact adjustment of the nominal melt flow index n can thus be achieved as described by simple routine experiments.

[0111] Silicone rubber compositions of the invention can be produced in one-, two- or multicomponent form. In the simplest case, production is effected in the form of a one-component silicone rubber composition of the invention by homogeneous mixing of all components.

[0112] The silicone rubber compositions of the invention are used for production of elastomeric shaped bodies by means of ballistic additive DOD methods (3D printing).

[0113] The present invention therefore further provides a process for producing elastomeric shaped bodies, characterized in that the shaped bodies are formed from the silicone rubber compositions of the invention by means of ballistic additive DOB methods (3D printing).

[0114] Rheological test method for determination of the nominal consistency index K, the viscosities .sub.1 and .sub.2, and the nominal melt flow index n of the silicone rubber composition

[0115] All measurements were conducted in an Anton Paar MCR 302rheometer with air bearings at 25 C., unless stated otherwise, according to DIN EN ISO 3219. Measurement was effected with plate-plate geometry (diameter 25 mm) with a gap width of 300 m. Excess sample material was removed by means of a wooden spatula after the plates had formed the measurement gap (called trimming).

[0116] Before the start of the actual measurement profile, the sample was subjected to a defined preliminary shear in order to eliminate the rheological history composed of sample application and formation of the measurement position. This preliminary shear (measurement phase 1) comprises a shear phase of 60 s at a shear rate of v.sub.1=0.5 s.sup.1, wherein a viscosity value is established very rapidly and remains constant. This viscosity value which is established at the end of measurement phase 1 is referred to as .sub.1. Immediately thereafter, there is strong shear at a shear rate of v.sub.1=25 s.sup.1that lasts for 60 s (measurement phase 2), which results in an abrupt drop in the viscosity, as a result of the shear-thinning characteristics, to a considerably lower value that remains constant.

[0117] The viscosity value which is established in this case at the end of measurement phase 2 is referred to as .sub.2. By inserting these two pairs of values (v.sub.1; .sub.1) and (v.sub.2; .sub.2) into formula (X), the two unknowns K (consistency index) and n (melt flow index) are calculated (two equations with two unknowns):

log =log K+n*log v(X)

[0118] Determination of Viscosity

[0119] The viscosities were measured in an Anton Paar MCR 302rheometer according to DIN EN ISO 3219:1994 and DIN 53019, using a cone-plate system (CP50-2 cone) with an opening angle of 2. The instrument was calibrated with 10000 standard oil from the National Metrology Institute of Germany. The management temperature is 25.00 C.+/0.05 C., the measurement time 3 min. The viscosity reported is the arithmetic mean of three independently conducted individual measurements. The measurement uncertainty for the dynamic viscosity is 1.5%. The shear rate was chosen depending on the viscosity and is stated separately for each viscosity reported.

Examples

[0120] The examples which follow serve to illustrate the invention without restricting it.

[0121] Rheological Test Method

[0122] Testing in the examples was effected analogously to the manner described above.

[0123] Conditioning of the Silicone Rubber Compositions

[0124] All the silicone rubber compositions used for DOB 3D printing were devolatilized prior to processing, by storing 100 g of the composition in an open PE can in a desiccator under a vacuum of 10 mbar at room, temperature for 3 h. Subsequently, the composition was dispensed into a 30 ml cartridge having a bayonet seal with exclusion of air and sealed with an appropriate expulsion plunger (plastic piston).

[0125] The Luer lock cartridge was then screwed into the vertical cartridge holder of the Vermes dosage valve in a liquid-tight manner with the Luer lock screw connection downward and 3-8 bar compressed air was applied to the pressure plunger at the top end of the cartridge; the expulsion plunger present in the cartridge prevents the compressed air from getting into the previously evacuated silicone rubber composition.

[0126] All UV-sensitive silicone compositions were produced under yellow light (with exclusion of light below 700 nm), devolatilized analogously and dispensed into opaque 30 ml cartridges with a Luer lock bayonet seal.

[0127] In order to prevent the silicone compositions from absorbing air during storage, the cartridge containers were packed under vacuum with aluminum foil-covered PE inliners using a vacuum welding system from Landig+Lava GmbH & Co. KG, ValentinstraBe 35-1, D-88348 Bad Saulgau.

[0128] Raw Materials and, Silicone Rubber Compositions Used

[0129] Vinyl-Functional Polyorganosiloxanes as Per Constituent (A):

[0130] A1: vinyldimethylsiloxy-terminal polydimethylsiloxane having a viscosity of 20,000 cSt., available from ABCR GmbH, Karlsruhe, Germany under the Poly(dimethylsiloxane), vinyldimethylsiloxy terminated; viscosity 20000 cSt. product name, cat. no. AB128873, CAS [68083-19-2] (ABCR catalog).

[0131] A2: vinyldimethylsiloxy-terminal polydimethylsiloxane having a viscosity of 500 000 cSt.; GAS No, [68083-19-2],

[0132] A3: vinyldimethylsiloxyl-terminal trifluoropropylmethyldimethylsiloxane copolymer having a viscosity of 14 Pa.Math.s and a trifluoropropylmethylsiloxy content of 42 mol %, available from ABCR GmbH, Karlsruhe, Germany, under the FMV-4031 name.

[0133] A4: vinyldimethylsiloxy-terminal polydimethylsiloxane having a viscosity of 200 cSt., available from ABCR GmbH, Karlsruhe, Germany under the product name DMS-V22, CAS [68083-19-2] (ABCR catalog).

[0134] A5: vinyldimethylsiloxy-terminal polydimethylsiloxane having a viscosity of 1000 cSt., available from ABCR GmbH, Karlsruhe, Germany under the product name DMS-V31, CAS [68083-19-2] (ABCR catalog).

[0135] SiH-Functional Crosslinkers as Per Constituent (B):

[0136] B1: methylhydrosiloxane-dimethylsiloxane copolymer having a molecular weight of Mn=1900-2000 g/mol and a methylhydrogensiloxy content of 25-30 mol %, available from Gelest, Inc. (65933 Frankfurt am Main, Germany) under the product name HMS-301.

[0137] B2: SiH-terminated polydimethylsiloxane, CAS: 70900-21-9, available from ABCR GmbH, 7 6187 Karlsruhe, Germany, under the DMS-H31 name, viscosity 1000 cSt.

[0138] B3: SiH-terminated polydimethylsiloxane, CAS: 70900-21-9, available from ABCR GmbH, 76187 Karlsruhe, Germany, under the DMS-H21 name, viscosity 100 cSt.

[0139] B4: trimethylsiloxy-terminal methylhydrodimethyltrifluoropropylmethylsiloxane copolymer having a viscosity of 170 mPa.Math.s, an Si-bonded H content of 0.59% by weight and a trifluoropropylmethylsiloxy content of 15 mol %.

[0140] Hydrosilylation Catalyst as Per Constituent (C):

[0141] C1: UV-activatable platinum catalyst; trimethyl-(methylcyclopentadienyl)platinum(IV), available from Sigma-Aldrich, Taufkirchen, Germany.

[0142] Reinforcing Agent as Per Constituent (D):

[0143] D1: a hydrophobized fumed silica having a BET surface area of 300 m.sup.2/g and a carbon content of 4.3% by weight was produced analogously to patent specification DE 38 39 900 A1 by hydrophobization using hexamethyldisilazane from a hydrophilic fumed silica, Wacker HDK T-30 (available from WACKER CHEMIE AG, Munich, Germany).

[0144] D2: a hydrophobized fumed silica having a BET surface area of 300 m.sup.2/g and a carbon content of 4.7% by weight and a vinyl content of 0.2% by weight was produced analogously to patent specification BE 38 3 9 900 A1 by hydrophobization using a mixture of hexamethyidisilazane and 1,3-divinyltetramethyldisilazane from a hydrophilic fumed silica, Wacker HDK T-30 (available from WACKER CHEMIE AG, Munich, Germany).

[0145] Optional Constituent (E)

[0146] E1: stabilizer (inhibitor) 1-ethynylcyclohexanol; 99%, CAS No. 78-27-3, 99%, available from ABCR GmbH, 76187 Karlsruhe, Germany

[0147] E2: Plasticizer, trimethylsiloxy-terminated polydimethylsiloxane, CAS No. 9016-00-6, available from ABCR GmbH, 76187 Karlsruhe, Germany, under the DMS-T43 name, viscosity 30,000 cSt.

[0148] E3: thixotropic agent: epoxidized linseed oil, CAS No. 67746-08-1, Edenol B 316 Special; from Emery Oleochemicals GmbH, Henkelstr. 67, 40589 Dsseldorf.

Inventive and Noninventive Examples 1-15

[0149] The silicone rubber compositions specified in tables 1, 2 and 3were produced by, in a first step, intimately mixing constituent (A) and constituent (D) as described hereinafter in the weight ratios specified in tables 1, 2 and 3. For this purpose, 60% by weight of the total mass of constituent (A) in the form of 255 g was initially charged in a double-Z kneader at a temperature of 25 C. and the kneader was heated to 70 C. On attainment of 70 C., the total amount of constituent (D), i.e. the hydrophobic fumed silica described as reactant D1 or D2, corresponding to the weight ratios given in tables 1, 2 and 3, was metered in and kneaded in in portions with continuously kneading over the course of 1 hour, and the material was homogenized. Subsequently, the resultant material of relatively high viscosity was kneaded and devolatilized under an oil-pump vacuum (<100 hPa) at 150 C. over the course of 1 hour. After this baking phase, the vacuum was broken and the temperature was adjusted to room temperature. Then the remaining 40% by weight of the total mass of constituent (A), i.e. 170 g, were mixed in and the material was homogenized at room temperature over the course of one hour.

[0150] The further production of the silicone rubber compounds was effected (under yellow light or with exclusion of light) by intimate mixing of the mixture of (A) and (D) produced by the method as described above with the other constituents (B), (E) and (C) in Speedmixer screw top mixing beakers from Hauschild & Co. KG, Waterkamp 1, 5907 5 Hamm. For this purpose, the components were successively weighed into the appropriate mixing beaker and mixed manually. Subsequently, the beaker that had been closed with an appropriate screwtop was mixed and degassed at 1500 rpm under a vacuum of 100 mbar in a vacuum Speedmixer BAG 400.2 VAC-P from. Hauschild & Co. KG, Waterkamp 1, 59075 Hamm for at least 5 minutes.

[0151] Prior to the vacuum mixing operation in the vacuum Speedmixer, a small hole was drilled into the screwtop in order to allow the air to escape from the mixing beaker.

[0152] Subsequently, the material was dispensed from the mixing beaker into an opaque 30 ml Luer lock cartridge in an air-free manner (with the aid of a Hauschild dispensing system, consisting of an appropriate speed disc and a lever press). Subsequently, the cartridge was sealed with an appropriate expulsion plunger (plastic piston).

[0153] The compositions of the inventive and noninventive silicone rubber compositions are given in tables 1 to 3.

TABLE-US-00001 TABLE 1 (all figures in % by weight except for C1 in ppm by weight based on Pt metal): Constituent Ex. 1*) Ex. 2 Ex. 3*) Ex. 4 Ex. 5*) Ex. 6 A1 52.2 52.2 66.6 66.6 44.7 44.7 A2 21.7 21.7 B1 0.9 0.9 1.0 1.0 1.9 1.9 B2 7.7 7.7 B3 2.9 2.9 C1 25 25 25 25 25 25 D1 22.4 22.4 24.8 24.8 19.1 19.1 E1 0.0025 0.0025 0.0025 0.0025 0.0025 0.0025 E2 34.2 34.2 E3 0.05 0.05 0.05 Starting viscosity .sub.1 (at 25 C.; v.sub.1 = 0.5 s.sup.1) .sub.1 [Pa .Math. s] 448 1060 510 877 351 428 Final viscosity .sub.2 (at 25 C.; v.sub.2 = 25 s.sup.1) .sub.2 [Pa .Math. s] 110 129 152 151 167 67 Nominal melt flow index n n [] 0.36 0.54 0.31 0.45 0.19 0.47 Nominal consistency index K K [Pa .Math. s.sup.n] 349 729 412 643 308 304 *)noninventive

[0154] It can be inferred from table 1 that the shear-thinning characteristics are insufficient, in the non-inventive examples 1, 3 and 5, resulting in a nominal melt flow index of more than 0.4. Through use of the thixotropic agent E3, which increases the surface energy of the filler D1, it is possible to remedy this problem and establish a flow index within the range claimed (cf. inventive examples 2, 4 and 6).

TABLE-US-00002 TABLE 2 (all figures in % by weight except for C1 in ppm by weight based on Pt metal): Constituent Ex. 7 Ex. 8 Ex. 9 Ex. 10 Ex. 11 A1 65.5 62.4 64.9 A3 72.0 70.0 B1 3.5 4.6 4.1 B4 3.0 3.0 C1 25 25 25 25 25 D1 25 27 20 D2 10 33 31 E1 0.0025 0.0025 0.0025 0.0025 0.0025 E3 Starting viscosity .sub.1 (at 25 C.; v.sub.1 = 0.5 s.sup.1) .sub.1 [Pa .Math. s] 3410 4095 1780 3930 2570 Final viscosity .sub.2 (at 25 C.; v.sub.2 = 25 s.sup.1) .sub.2 [Pa .Math. s] 265 307 230 372 254 Nominal melt flow index n n [] 0.65 0.66 0.52 0.60 0.59 Nominal consistency index K K [Pa .Math. s.sup.n] 2173 2592 1239 2589 1706 *) noninventive

TABLE-US-00003 TABLE 3 (all figures in % by weight except for C1 in ppm by weight based on Pt metal): Constituent Ex. 12*) Ex. 13*) Ex. 14* Ex. 15*) A1 16.2 16.2 31.7 31.7 A2 48.6 48.6 48.6 48.6 A4 18.3 18.3 A5 4.9 4.9 E1 3.9 3.9 3.9 3.9 Cl 25 25 25 25 D1 8.1 8.1 15.8 15.8 E1 0.0025 0.0025 0.0025 0.0025 E3 0.05 0.05 Starting viscosity .sub.1 (at 25 C.; v.sub.1 = 0.5 s.sup.1) .sub.1 [Pa .Math. s] 99 126 311 354 Final viscosity .sub.2 (at 25 C.; v.sub.2 = 25 s.sup.1) .sub.2 [Pa .Math. s] 78 79 131 136 Nominal melt flow index n n [] 0.06 0.12 0.22 0.24 Nominal consistency index K K [Pa .Math. s.sup.n] 95 117 267 299 *)noninventive

[0155] It can be inferred from tables 2 and 3 that, in the noninventive examples 12 to 15, the shear-thinning characteristics are insufficient, resulting in a nominal melt flow index of more than 0.4. This problem can be remedied by increasing the filler concentration, and a melt flow index within the range claimed can be established (cf. inventive examples 7 to 11 in table 2). The increase in the filler concentration simultaneously leads to a rise in the viscosity .sub.1 into the range claimed.

[0156] DOD-3D Printer:

[0157] The silicone rubber compositions produced were processed by the DOD method in a NEO-3D printer manufacturing system from German RepRap GmbH to give silicone elastomer parts. For this purpose, the abovementioned 3D printer was modified and adapted. The thermoplastic filament dosage unit that was originally installed in the NEO-3D printer was replaced by a jetting nozzle from Vermes Microdispensing GmbH, Otterfing, in order to be able to deposit higher-viscosity to firm pasty materials such as the silicone rubber compositions of the invention in the DOD method.

[0158] Since the NEO printer was not equipped as standard for the installation of jetting nozzles, it was modified. The Vermes jetting nozzle was incorporated into the printer control system such that the start-stop signal (trigger signal) of the Vermes jetting nozzle was actuated by the G code controller of the printer. For this purpose, a special signal was recorded in the G code controller. The G code controller of the computer used this merely to switch the jetting nozzle on and off (starting and stopping of metering).

[0159] For the signal transmission of the start-stop signal, the heating cable of the originally installed filament heating nozzle of the NEO printer was severed and connected via a relay to the Vermes nozzle.

[0160] The other dosage parameters (metering frequency, rising, failing etc.) of the Vermes jetting nozzle were adjusted by means of the MDC 3200+ Microdispensing Control Unit.

[0161] The 3D printer was controlled by means of a computer. The software control and control signal connection of the 3D printer (software: Repetier-Host) was modified to the effect that both the movement of the dosage nozzle in the three spatial directions and the signal for droplet deposition were thus controllable. The maximum, speed, of movement of the NEO 3D printer was 0.3 m/s.

[0162] Dosage System:

[0163] The dosage system used for the silicone rubber compositions used was the MDV 3200A microdispensing dosage system from Vermes Microdispensing GmbH, consisting of a complete system having the following components: a) MDV 3200 Adosage unit with an attachment for Luer lock cartridges, with which 3-8 bar compressed air (hose with adapter) was applied to the top end of the cartridge, b) Vermes MDH-230tfl left nozzle trace-heating system, c) MDC 3200+ MicroDispensing Control Unit, which was in turn connected to the PC controller and via movable cables to the nozzle, enabled the setting of the jetting dosage parameters (rising, falling, opentime, needlelift, delay, no pulse, heater, nozzle, distance, voxel diameter, air supply pressure at the cartridge). Nozzles having diameters of 50, 100, 150 and 200 m are available. It was thus possible to precisely position ultrafine droplets of the silicone rubber composition in the nanoliter range at any desired xyz position on the workbench or the already crosslinked silicone elastomer layer. Unless stated otherwise, the standard nozzle set installed in the Vermes valve was a 200 m nozzle. The reservoir vessels used for the silicone rubber composition were upright 30 ml Luer lock cartridges that were screwed onto the dispensing nozzle in a liquid-tight manner and pressurized with compressed air.

[0164] The modified NEO 3D printing and the Vermes dosage system were controlled with a PC and an open source software package Simplify 3D.

[0165] Jetting:

[0166] The silicone rubber compositions were repeatedly deposited dropwise in layers of the desired geometry with the jetting nozzle parameters specified hereinafter on a glass microscope slide of area 2575 mm, with continuous irradiation and resultant crosslinking of the deposited material over the entire printing operation (about 50 sec) with a BLUEPOINT irradiation system having an output of 13,200 mW/cm.sup.2. Nozzle diameter: 200 m, rising: 0.3 ms, failing: 0.1 ms, open time: 15 ms, needle lift: 100%, delay (surface pressure): 25 ms, delay (individual points for the voxel size measurement): 100 ms, heating: 45 C., cartridge supply pressure: 3 bar.

[0167] In this way, it was possible to use the silicone rubber compositions of the invention to obtain transparent silicone elastomer molded parts of different geometry.

[0168] While the inventive high-viscosity silicone rubber compositions listed in tables 1-3, without exception, gave a high-quality printed image, the noninventive high-viscosity silicone rubber compositions led either to blocking and sticking of the print valve or gave a crust-like appearance. The inventive silicone rubber compositions have a melt flow index n of less than 0.40 and could be jetted without any problem by the DOD method (cf. inventive dot matrix in FIG. 1 and inventive spiral silicone elastomer molding in FIG. 2). The appearance in the case of use of the noninventive compounds is shown by way of example by FIG. 3 for the printed dot matrix and by FIG. 4 for the spiral silicone elastomer molding.