Device for continuously mixing liquid and powder

Abstract: A device for continuously mixing liquid and powder is disclosed which is able to provide liquid-powder mixtures of lower apparent viscosities and which is particularly suitable for the preparation of low-viscosity products comprising blends of powder fillers in liquid polymers such as liquid silicones. The invention further provides a method of continuously mixing a liquid and a powder to provide liquid-powder mixtures of lower apparent viscosities.

Description: This invention relates to a device for continuously mixing liquid and powder (hereinafter a continuous liquid-powder mixer). More specifically, it is a continuous liquid-powder mixer that is able to generate lower apparent viscosities for liquid-powder mixtures and is highly adapted for the preparation of low-viscosity products comprising blends of powder fillers in liquid polymers such as liquid silicones. Liquid silicone rubber compounds are employed in molding operations such as injection and compression molding. They are also used in various other operations as materials, such as moldmaking materials, and architectural or building sealants. Liquid silicone rubber compounds are viscous mixtures of liquid silicone with a powder filler such as reinforcing silica. As is well known, lower apparent viscosities for these compounds provide a better processability in the aforementioned operations, while higher apparent viscosities impair processability. Liquid silicone rubber compounds with low apparent viscosities are prepared by mixing the highest possible dispersion of powder filler within the liquid silicone rubber. Compact devices that efficiently mix liquid and powder are disclosed in US-A-3 998 433 (JP-A-53038828) and JP-A-2 002 610. These are continuous mixers that contain a scraper-equipped rotating disk which is installed within a casing to divide the interior of the casing into upper and lower mixing compartments. However, at high compounding ratios for powder fillers, such as fumed silica at ratios up to 10%, it is almost impossible using these prior-art devices to rapidly and inexpensively achieve an apparent viscosity for the compound (mixture) low enough to avoid negative consequences for the processability during molding. The present invention introduces a continuous liquid-powder mixer that is able to provide lower apparent viscosity values for liquid-powder mixtures. An additional object is to blend larger amounts of powder for a given liquid-powder mixture viscosity. According to the invention there is provided an apparatus for continuously mixing liquid and powder comprising a continuous mixing device, consisting of a feed opening for the introduction of liquid and powder positioned on top of a casing and a discharge outlet located on the bottom of said casing, a rotating disk installed within said casing and thereby separating the casing interior into an upper and a lower mixing compartment, and scrapers fixed on both upper and lower surfaces of said rotating disk, characterized in that said device has a liquid feed line which is connected to said lower mixing compartment, a ring plate installed on the inside wall of the lower mixing compartment, notches furnished in said scrapers on the lower surface of said rotating disk, and the inner edge of said ring plate being inserted into said notches in such a manner that the ring plate does not contact the scrapers and that the scrapers, while in this interpenetrated condition, are able to move relative to the ring plate. According to the invention there is furthermore provided a method of continuously mixing a liquid and a powder as indicated in claim 1. In our continuous mixing device, the liquid and powder are introduced into the upper mixing compartment and are subjected to a first-stage mixing process by the scrapers installed on the upper surface of the rotating disk. The resulting mixture is then transferred into the lower mixing compartment where it is subjected to a second-stage mixing process by the scrapers installed on the lower surface of the rotating disk. In this second-stage mixing process, the liquid/powder mixture is subjected to strong shear between the ring plate and the notches in the scrapers as the mixture flows down onto the ring plate. This strong shear improves the quality of our powder dispersion. The apparent viscosity is substantially reduced as a result of this improved dispersion and as a result of the fresh liquid supplied into this zone from the liquid feed line. Liquids which may be subjected to the present invention are exemplified by water, liquid candy with a starch base, edible oils, liquid chemical compounds and liquid polymers. The liquid polymers are exemplified by silicones, polybutadienes, and epoxy resins. The powders are exemplified by wheat flour, metal powders, and powder fillers. The powder fillers are themselves specifically exemplified by fumed silica, wet-process silica, calcium carbonate, and carbon black. The continuous mixer of the present invention is effectively applied to viscous liquids whose viscosity is further raised by the admixture of powder. It is particularly effective when applied to the production of silicone rubber compounds in which microparticulate fillers are blended in large quantities into a liquid polymer such as liquid silicone. The instant invention will be explained in greater detail hereinafter with reference to the example and the drawings. Figure 1 is a vertical cross section of a continuous mixer of the instant invention. Figure 2 is a profile view of the continuous mixer of Figure 1 . Figure 3 is the cross section at the A-A level in Figure 1 . Figure 4 is the cross section at the B-B level in Figure 1 . Figure 5 is the cross section at the C-C level in Figure 1 . Reference Numbers 1 casing 1a center of upper plate 1b inclined surface 2 feed opening 3 discharge outlet 4 cylindrical casing 5 liquid feed lines 6 liquid reservoir 7 overflow tube 8 powder feed conduit 9 rotating disk 10 upper mixing compartment 11 lower mixing compartment 12 scrapers 13 scrapers 14 scrapers 14a notches 15 rotating shaft 15a shaft bearing 16 pulley 17 ring plate 18 liquid feed lines 19 conical element 20 mixer body 30 starting material feed section Figure 1 contains the vertical cross section and Figure 2 contains the profile of a continuous mixer according to our invention. Figures 3 through 5 contain cross sections at the A-A , B-B , and C-C lines, respectively, in Figure 1 . In the figures, 20 refers to the mixer body and 30 refers to the starting material feed section for the mixer. A cylindrical casing 1 forms the outer shell of mixer body 20 , and a feed opening 2 that receives liquid/powder mixture is installed at the center of the upper plate 1a of this casing. The lower part of the casing forms an inclined surface 1b having the shape of an inverted cone, and a discharge outlet 3 is installed in said inclined surface 1b . A conical element 19 is installed at the center of the bottom of the casing to form an annular V-shaped bottom with the inclined surface 1b . A cylindrical casing 4 forms the outer shell of the starting material feed section 30 . A liquid feed line 5 is connected tangentially at the side of this casing, and a liquid reservoir 6 is formed within the casing. An overflow tube 7 having the shape of an inverted cone is connected on the top of the feed opening 2 on the mixer body 20 . This overflow tube 7 ascends vertically into the liquid reservoir 6 . The lower end of a powder feed conduit 8 faces the inlet to the overflow tube 7 . The starting viscous liquid is fed into the starting material feed section 30 through the liquid feed line 5 , while the starting powder is fed from the powder feed conduit 8 . The liquid supplied from the liquid feed line 5 is first stored in the liquid reservoir 6 in the starting material feed section 30 and then flows down along the inner wall of the overflow tube 7 from its top edge. At this point the liquid is mixed with the powder supplied through the powder feed conduit 8 and descends into the feed opening 2 . A rotating disk 9 is horizontally installed within the casing 1 of the mixer body 20 to face the feed opening 2 . This rotating disk 9 divides the interior of the casing into an upper mixing compartment 10 , where the first-stage mixing operation is implemented, and a lower mixing compartment 11 , where the second-stage mixing operation is implemented. The center of rotation of this rotating disk 9 is fixed on the upper end of a rotating axle 15 . Said rotating axle 15 is supported by an axle bearing 15a and extends to the exterior of the casing 1 . A pulley 16 is fixed at the bottom end of the rotating axle 15 , and the power for rotation is input from a motor (not shown) across this pulley 16 . The preferred range for the rotation rate is 400 to 1,500 rpm. The upper surface, outside edge, and lower surface of the rotating disk 9 each carry three scrapers separated by equal angles (the scrapers in each set are respectively designated 12 , 13 , and 14 ), and the mixture is mixed through the stirring and scraping actions of these scrapers. Mixing occurs as follows: the scrapers 12 in the upper mixing compartment 10 scrape off the mixture adhering to the top plate 1a ; the scrapers 13 scrape off the mixture adhering on the inner wall of the casing at the boundary between the upper mixing compartment 10 and the lower mixing compartment 11 ; and the scrapers 14 in the lower mixing compartment 11 scrape off the mixture adhering on the inclined surface 1b of the casing bottom. The mixer need not have 3 scrapers in each set 12 , 13 , and 14 as shown in the drawings, and any number above one may be employed. Moreover, the scraper sets may all contain the same number of scrapers or may contain different numbers of scrapers, and the scrapers 13 on the outer edge of the rotating disk 9 may even be omitted as desired. The upper surface of the rotating disk 9 may, as necessary, also bear a large number of vertical pins, which through their stirring activity will further promote stirring and mixing. The following structures are installed in the lower mixing compartment 11 to obtain an even greater mixing effect. First, a liquid feed line 18 is attached tangentially to the side wall of the lower mixing compartment 11 . This tangential attachment to the casing side wall functions to promote the mixing effect exercised by the liquid on the mixture within the casing. The installation position of this liquid feed line 18 preferably defines an open angle theta, measured from the discharge outlet 3 along the direction of rotation of the rotating disk 9 , in the range from 180° to 270°. This facilitates the improvement in mixing effect that is due to the incoming liquid. The scrapers 14 installed in the lower mixing compartment 11 comprise plates or mesh plates that extend both radially and vertically, and notches 14a of the scrapers 14 are installed therein that run radially inward from the outside edge. A ring plate 17 is fixed on the inner wall of the casing 1 facing the position of the notches 14a , and the inner edge of this ring plate 17 is interpenetratingly inserted into the notches 14a . The notches 14a stretch horizontally over a surface of the ring plate 17 which is set in narrow spaces of notches 14a . The operation of our instant continuous mixer will now be described. The liquid/powder mixture entering the upper mixing compartment 10 from the feed opening 2 is subjected, while being radially transported to the outside of the rotating disk 9 , to the first-stage mixing process based on stirring and scraping by the scrapers 12 . Due to this structure for the lower mixing compartment 11 , the mixture from the first-stage mixing process descends across the outer edge of the rotating disk 9 onto the ring plate 17 , where it is strongly processed and sheared between the ring plate 17 and the narrow notches 14a in the scrapers 14 . This shearing is all the more forceful because it occurs between narrow notches 14a and the ring plate 17 . Thus, the powder becomes even more uniformly dispersed in the liquid as a result. After shearing on the ring plate 17 , the mixture then descends onto the inclined surface 1b and is sheared while being scraped by the ends of the scrapers 14 . The resulting additional dispersion of the powder induces a further lowering of the apparent viscosity of the mixture. Prior to mixture discharge through discharge outlet 3 , the fresh supply of starting liquid from the liquid feed line 18 and its shear by the scrapers 14 furnishes an additional lowering of the viscosity. The above-described continuous mixer is therefore able to provide a substantial reduction in the apparent viscosity of the mixture, even when large quantities of powder are to be compounded into the liquid. EXAMPLES An invention device, comparison device 1, and comparison device 2 (characteristics described below) were each used to prepare a low-viscosity silicone rubber compound by blending 10 weight parts hydrophobic fumed silica (Aerosil R-972 from Nippon Aerosil Kabushiki Kaisha) into 100 weight parts hydroxyl-endblocked polydimethylsiloxane (viscosity at room temperature = 15 Pa.s). The apparent viscosity at a shear rate of 50s -1 was measured on each of the 3 silicone rubber compounds thus obtained using a flow tester (nozzle diameter = 1 mm, tube length = 10 mm, load = 2 kg). These results are reported in Table 1. The results confirmed that, relative to the comparison devices, the continuous mixer of the instant invention was able to produce the lowest viscosity at the same starting material mixing ratio. Invention Device Structure: according to Figures 1 through 5 diameter of the rotating disk: 300 mm rotation rate of the rotating disk: 900 rpm width of ring plate: 30 mm open angle between the discharge outlet 3 and the liquid feed line 18 : 180° Feed Method: The 10 weight parts of hydrophobic fumed silica were charged through the powder feed conduit 8 , while the feed of 100 weight parts of hydroxyl-endblocked polydimethylsiloxane were subdivided into 60 weight parts through the liquid feed line 5 and 40 weight parts through the liquid feed line 18 to the lower mixing compartment 11 . Comparison Device 1 Structure: device according to Figures 1 to 5 , but contained neither the ring plate 17 nor the liquid feed line 18 (corresponds to prior-art device) diameter of rotating disk: 300 mm rotation rate of rotating disk: 900 rpm Feed Method: 10 weight parts of hydrophobic fumed silica were fed through the powder feed conduit 8 , and the 100 weight parts of hydroxyl-endblocked dimethylpolysiloxane were fed through the liquid feed line 5 . Comparison Device 2 Structure: device according to Figures 1 to 5 , but lacked ring plate 17 diameter of the rotating disk: 300 mm rotation rate of the rotating disk: 900 rpm open angle between the discharge outlet 3 and the liquid feed line 18 : 180° Feed Method: 10 weight parts of hydrophobic fumed silica were charged through the powder feed conduit 8 , while the feed of 100 weight parts of hydroxyl-endblocked dimethylpolysiloxane were subdivided into 60 weight parts through the liquid feed line 5 and 40 weight parts through the liquid feed line 18 to the lower mixing compartment 11 . Apparent Viscosity, Pa.s Device of the Present Invention 70 Comparison Device 1 140 Comparison Device 2 130 One distinctive feature of the continuous mixer of this invention is the fresh supply of liquid through the installation of a liquid feed line into the lower mixing compartment created by the subdividing effect of the rotating disk. Another distinctive feature is the provision of notches in the scrapers in the lower mixing compartment, and the configuration of these notches in such a manner that the inner edge of the ring plate installed on the inner casing wall is interpenetratingly inserted into the notches. As a result, our continuous mixer is able to generate a substantial reduction in the apparent viscosity of mixtures (i) due to an improved powder dispersion generated by the strong shear exercised on the mixture between the notches and ring plate and (ii) due to the fresh liquid feed into this zone. The continuous mixer of our invention is therefore able to produce lower viscosity products for a given powder addition and is also able to blend larger amounts of powder for a given viscosity value.

Claims: An apparatus for continuously mixing liquid and powder comprising a continuous mixing device, consisting of a feed opening (2) for the introduction of liquid and powder positioned on top of a casing (4) and a discharge outlet (3) located on the bottom of said casing, a rotating disk (9) installed within said casing and thereby separating the casing interior into an upper and a lower mixing compartment (10, 11), and scrapers (12, 13, 14) fixed on both upper and lower surfaces of said rotating disk, characterized in that said device has a liquid feed line (18) which is connected to said lower mixing compartment (11), a ring plate (17) installed on the inside wall of the lower mixing compartment, notches (14a) furnished in scrapers (14) on the lower surface of said rotating disk (9), and the inner edge of said ring plate (17) being inserted into said notches in such a manner that the ring plate does not contact the scrapers and that the scrapers, while in this interpenetrated condition, are able to move relative to the ring plate. Apparatus according to claim 1 for continuously mixing liquid and powder, in which a vertically ascending overflow tube (7) for the liquid feed is connected to the feed opening (2) on the top of the casing (4) and a powder feed conduit (8) faces the entrance to said overflow tube. A method of continuously mixing a liquid and a powder comprising continuously feeding a viscous liquid and a powder into a first upper mixing compartment (10) where the liquid and powder are continuously mixed to form a first mixture in a first-stage mixing operation by a rotating disk (9) which divides the first mixing compartment from a second lower mixing compartment (11), the first mixture continuously passes into the second mixing compartment by first scraper means (12) fixed on the rotating disk which provides stirring and scraping action and radially transporting the first mixture to the outside of the rotating disk into the second mixing compartment where the first mixture is strongly sheared between a ring plate (17) and notches (14a) in second scraper means (14) fixed on the rotating disk and mixed with a fresh supply of the viscous liquid from a liquid feed line (18) which is connected to the second compartment to continuously produce a second mixture, in which the ring plate is installed on the inside wall of the second compartment and in which the inner edge of the ring plate is inserted into the notches in such a manner that the ring plate does not contact the second scraper means.

Inventors: HAMADA MITSUO, MORI HIDEYUKI, HAMADA, MITSUO, MORI, HIDEYUKI, HAMADA, MITSUO, DOW CORNING TORAY SILICONE CO.,LTD, MORI, HIDEYUKI, DOW CORNING TORAY SILICONE CO.LTD.
Applicants: DOW CORNING TORAY SILICONE, DOW CORNING TORAY SILICONE CO., LTD.
Application year: 1995
Main classification: B01F5
IPC codes: B01F3, B01F5, B01F7, B01F13