PROCESS FOR MAKING SILICONE RUBBER BASE COMPOSITIONS

Abstract

A process and apparatus for the continuous preparation of silicone rubber base compositions as well as the resulting compositions produced therefrom. This disclosure aims to cover a new continuous manufacturing process for making silicone rubber base compositions using an in situ silica treatment. The new continuous manufacturing process uses twin-screw extruder (TSE) technology.

Claims

1. A continuous method for the preparation of a silicone rubber base composition, the composition comprising: (A) one or more polyorganosiloxanes containing at least two unsaturated groups per molecule selected from alkenyl groups and alkynyl groups, and (B′) a hydrophobically treated reinforcing silica filler; said method comprising the steps of: (a) introducing (C) at least one hydrophobing treating agent, and component (A), and optionally (D) water, into a first static mixer (25) to form a step (a) mixture and then introducing the step (a) mixture on to a first twin-screw extruder (4); (b) introducing (B) reinforcing silica filler into the step (a) mixture via a reinforcing silica filler (B) entry port (27) in the first twin-screw extruder (4) while maintaining the temperature in a range of between 20 to 80° C. to form a viscous paste and providing at least one vent (5, 8) to the atmosphere up and/or downstream of the reinforcing silica filler (B) entry port (27) to allow gases present to escape; (c) mixing the viscous paste resulting from step (b) in a dispersive mixing and kneading zone in the first twin-screw extruder (4) to form a silica dispersion; (d) further mixing the silica dispersion produced in step (c) in a residence zone (17) downstream of the first twin-screw extruder (4) to provide an unstripped silicone rubber base composition; (e) stripping the unstripped silicone rubber base composition with vacuum stripping to provide a silicone rubber base composition at a temperature of at least 100° C.; and (f) introducing component (C) and optionally one or both of components (A) and (D) either into the first twin-screw extruder (4) between steps (c) and (d) and/or during step (d) to dilute and further hydrophobically treat silica from the silica dispersion of step (c) and subsequently form a diluted silica dispersion.

2. The continuous method for the preparation of a silicone rubber base composition in accordance with claim 1, wherein components and component (D) when present are introduced into the first static mixer (25) at pre-defined controlled rates which may be varied relative to each other within a pre-determined range as and when required.

3. The continuous method for the preparation of a silicone rubber base composition in accordance with claim 1, wherein components (C) and (A), and optionally component (D), are pumped into the first static mixer (25) by one or more weight loss meters, mass flow meters, gear pumps, syringe pumps, or piston pumps, or a combination thereof.

4. The continuous method for the preparation of a silicone rubber base composition in accordance with claim 1, wherein: (i) component (A) comprises one or more polydiorganosiloxanes containing at least two alkenyl groups per molecule and optionally a polyorganosiloxane resin containing at least two alkenyl groups per molecule; and/or (ii) additives are introduced into the mixture during the preparation.

5. The continuous method for the preparation of a silicone rubber base composition in accordance with claim 1, wherein the first twin-screw extruder (4) is a co-rotating twin-screw extruder, an intermeshing twin-screw extruder or a co-rotating and intermeshing twin-screw extruder.

6. The continuous method for the preparation of a silicone rubber base composition in accordance with claim 1, wherein the first twin-screw extruder (4) has an axial length LL to screw diameter (D) ratio of from 25:1 to 65:1.

7. The continuous method for the preparation of a silicone rubber base composition in accordance with claim 1, wherein component (B) is fed to the reinforcing filler (B) entry port (27) at a constant rate by a continuous feeder for powder selected from one or more tables, belts, loss in weight feeders, side feeders, feed enhancement technology systems, or screws.

8. The continuous method for the preparation of a silicone rubber base composition in accordance with claim 1, wherein in step (f) there is provided first and second additional entry ports (28), and wherein the first additional entry port is utilized for the introduction of additional amounts of component (C) and optionally component (D) and the second additional entry port is utilized for the introduction of additional amounts of component (A).

9. The continuous method for the preparation of a silicone rubber base composition in accordance with claim wherein component (C), and component (D) when present, is/are pre-mixed in a second static mixer (31) prior to being introduced through the first additional entry port (28).

10. The continuous method for the preparation of a silicone rubber base composition in accordance with claim 1, wherein the further mixing in step (e) is carried out in a residence zone (17) optionally comprising a third static mixer.

11. The continuous method for the preparation of a silicone rubber base composition in accordance with claim Mc wherein residence zone (17) is controlled at a pressure of between 300 and 2,000_kPa and at a temperature of between 90 and 170° C. and wherein the average residence time in the residence zone (17) is from 5 to 30 minutes.

12. The continuous method for the preparation of a silicone rubber base composition in accordance with claim 1, wherein step (e) is undertaken in a continuous stripping device (19), designed to vacuum strip volatiles and gases from the unstripped silicone rubber base composition resulting from step (d).

13. The continuous method for the preparation of a silicone rubber base composition in accordance with claim 1, wherein one or more temperature sensors, flow rate sensors, pressure sensors and other instrumentation and/or sensors, are utilized during the method.

14. A silicone rubber base composition manufacturing assembly adapted to make a silicone rubber base composition by the continuous method in accordance with claim 1.

15. A silicone rubber base composition obtainable or obtained by the continuous method in accordance with claim 1.

16. A hydrosilylation cure silicone rubber composition comprising the silicone rubber base composition in accordance with claim 15.

Description

FIGURE

[0111] An embodiment of the disclosure herein will now be described by way of example with reference to the accompanying FIG. 1 in which:

[0112] FIG. 1 is a schematic view of a process assembly for an embodiment of the disclosure as described herein.

[0113] Referring to FIG. 1 there is provided a first static mixer (25), first co-rotating twin-screw extruder (4), a pump (16), a residence zone (17) and a means for vacuum stripping in the form of a second co-rotating twin-screw continuous extruder (19).

First co-rotating twin-screw extruder (4) comprises a starting material entry port (26), a reinforcing silica filler entry port (27), vents to the atmosphere (5) and (8), a pair of screws (not shown) a barrel (4a), an additional entry port (28) and a discharge port (29). The residence zone (17) is a combination of pipes and a further static mixer. The second co-rotating twin-screw continuous extruder (19) has an entry port (not shown) a pair of screws (not shown) a barrel (19a) and a discharge port (30). The second co-rotating twin-screw continuous extruder (19) also has several vents for vacuum stripping material being transported therethrough of which three are shown (20, 21 and 22).

[0114] There is also provided a first feed (3) for the one or more polyorganosiloxanes containing at least two unsaturated groups per molecule, typically alkenyl groups (component (A)), a first feed (1) for hydrophobing treating agent (component (C)), a first feed (2) for water (component (D)) if required, each of which, in use, is designed to feed their respective components into the first static mixer (25).

[0115] A reinforcing silica store (6) is provided to, in use, feed silica into first co-rotating twin-screw extruder (4), by way of entry port (27).

[0116] There is also provided a second feed (12) for the one or more polyorganosiloxanes containing at least two unsaturated groups per molecule, typically alkenyl groups (component (A), a second feed (14) for one or more hydrophobing treating agents (component (C)), a second feed (13) for water (component (D)) if required, each of which, in use, is designed to feed their respective components into second static mixer (31) which, in use, is provided to introduce the mixture resulting from second static mixer (31) on to first co-rotating twin-screw extruder (4), by way of entry port (28).

[0117] Feeds (3) and (14) utilised gear pumps (not shown) to introduce component (A) into the first and second static mixers (25, 31). Feeds (1) and (2) utilised syringe pumps or piston pumps (in the case of extruders with larger barrels to introduce components (C) and (D) respectively into the first static mixer (25). Feeds (14) and (13) utilised syringe pumps to introduce components (C) and (D) respectively into the second static mixer (31). Component (B) was introduced onto the first co-rotating twin-screw extruder (4), by way of a loss in weight screw feeder paired with a continuous side-feeder from store (6).

[0118] The external surface of the first co-rotating twin-screw extruder (4) is enclosed in a jacket (not shown) for heating and/or cooling to maintain the temperatures in the zones within the required ranges. In the case of cooling this may be achieved by e.g. circulating cooling water to reduce e.g. the friction-induced heating of the material in said first co-rotating twin-screw extruder (4).

[0119] In use, hydrophobing treating agent (C) in the form of hexamethyldisilazane, water (D) and a dimethylvinyl terminated polydimethylsiloxane polymer having a viscosity of 53,000 mPa.Math.s at 25° C. are each separately introduced from feeds (1, 2 and 3) respectively, into a first static mixer (25) to form a step (a) mixture.

[0120] Upon leaving the first static mixer (25) the step (a) mixture is introduced into first twin-screw extruder 4 through entry port (26). The first twin-screw extruder (4) is a co-rotating and intermeshing twin-screw extruder having an elongate barrel (4a) which holds the twin screws (not shown) attached to a drive unit (not shown) designed to rotate both the screws at the same speed. The first twin-screw extruder (4) has an L/D ratio of 48. The speed of the screws of the first co-rotating twin-screw extruder are predetermined dependent on the system requirements but, solely for the sake of example, may be from between 200 and 1200 rpm.

[0121] The first zone (7) on the twin-screw extruder (4) is provided for step (b) of the process, the introduction of reinforcing filler (B) into the step (a) mixture, thereby wetting the surface of the reinforcing filler (B) with the step (a) mixture to form a viscous paste. Entry port (26) is provided for the introduction of the step (a) mixture described above. Entry port (27) is used to introduce the silica filler from supply store (6) into the step (a) mixture and any air trapped in the filler prior to or subsequent to the addition of the filler through entry port (27) is allowed to escape via atmospheric pressure vents (5) and (8). At the end of the first zone (9) of first twin screw extruder (4), a viscous paste resulting from the mixing of components (A), (B), (C) and (D) has been made.

[0122] The viscous paste is then transferred downstream to the second zone (10) of twin screw extruder (4) in which dispersive mixing and kneading is carried out to break down the reinforcing silica filler from agglomerates (particle size about 50-100 μm) to aggregates (particle size about 1-10 μm) and ultimately considerable fractions of the silica may comprise silica in the 50-500 nanometer range and provide a silica dispersion of the reinforcing filler (B) in the other components at the end of the second zone (11) along twin screw extruder (4) as a product of step (c).

[0123] Subsequently the resulting silica dispersion of step (c), is transferred to a third zone (15) for in this instance a step (f) dilution step between steps (c) and (d). In the third zone (15) in FIG. 1 components (C), (D) and (A) are supplied from feeds (14, 13 and 12) respectively into second static mixer (31). In an alternative route (not shown) there may be two additional entry ports a first for the introduction of a components (C) optionally in a mixture with component (D) and the second for the introduction of component (A).

[0124] The resulting diluted silica dispersion obtained in step (f) was then transported to the discharge port (29) and further through pump (16) which pumps the diluted silica dispersion into residence zone (17) which comprises pipes and a further static mixer for further mixing and filler treatment. The diluted silica dispersion resulting from step (f) is then mixed in the residence zone (17) for a period of from 5 to 30 minutes, alternatively 5 to 20 minutes, alternatively 10 to 20 minutes at a temperature of between 90 and 170° C. and a pressure of between 300 and 2,000 kPa.

[0125] At the end (18) of residence zone (17) the resulting product is an unstripped silicone rubber base composition which is introduced on to the means for vacuum stripping (19) which in these examples is a second co-rotating twin-screw continuous extruder (19). The second co-rotating twin-screw continuous extruder (19) is utilised to strip out residual treating agent, water, and ammonia, if present under heat and vacuum, through ports (20, 21 and 22) in order to produce a final silicone rubber base composition (23) which exits the second co-rotating twin-screw continuous extruder (19), via discharge port (30).

[0126] The second co-rotating twin-screw continuous kneader/extruder may have any suitable L/D ratio, e.g. for the sake of example 15 to 50, and alternatively from 20 to 40.

[0127] The speed of the screws of the second co-rotating twin-screw continuous kneader/extruder should be from 50-800 rpm. The external surface of the barrel of the second co-rotating twin-screw continuous kneader/extruder should preferably be enclosed in a heater-equipped jacket in order to assist in maintaining keep the mixture in the barrel at a temperature of from 150 to 300° C., alternatively 150 to 250° C. The pressure used is between 10 to 500 mbar (1 to 50 kPa), alternatively to 500 mbar (2 to 50 kPa), alternatively between 50 to 200 mbar (5 and 20 kPa).

[0128] The resulting silicone rubber base composition (23) may then be transported to packaging or for compounding/finishing (24) whichever is required. In the latter case, the silicone rubber base composition as described herein may be used in the preparation of a hydrosilylation cure silicone rubber composition. This may be in two parts one comprising a hydrosilylation cure catalyst and the other comprising a cross-linker.

[0129] As previously discussed the likes of temperature sensors for measuring the temperature of the mixture, flow rate sensors, pressure sensors and other instrumentation and/or sensors, and the like may be utilised during the process as and where required, although not shown. Likewise, as previously discussed, if desired, additives may be introduced into the mixture en route to the means for vacuum stripping.

[0130] The above explanation of the process described will now be exemplified using the following two examples which describe embodiments of the disclosure herein and are conducted in accordance with the depiction of FIG. 1 herein unless otherwise indicated.

EXAMPLES

[0131] Unless otherwise indicated the viscosity measurements described are provided from a TA-Instruments AR2000Ex cone plate rheometer @ 10 s.sup.−1 for viscosities of 1000 mPa.Math.s and above or a Brookfield® rotational viscometer with spindle LV-1 (designed for viscosities in the range between −20,000 mPa.Math.s) for viscosities less than 1000 mPa.Math.s and adapting the shear rate according to the polymer viscosity e.g. 10 s.sup.−1 or 100 s.sup.−1. Unless otherwise indicated, all viscosity measurements were taken at 25° C.

Example 1 (Ex. 1) and Comparative Example 1 (C. 1)

[0132] The compositions used in Ex. 1 and C. 1 are provided in Table 1 a.

TABLE-US-00001 TABLE 1a C. 1 (wt. %) Ex. 1 (wt. %) Polymer 1 73.03 70.85 Water 0.96 2.24 HMDZ 2.82 4.41 Silica 1 23.19 22.50

[0133] The polymer used, polymer 1, was a vinyldimethyl terminated polydimethylsiloxane having a viscosity of about 55,000 mPa.Math.s; and Silica 1 was a fumed silica having a specific surface area of 258 g/m.sup.2 (BET method in accordance with ISO 9277: 2010).

[0134] In Ex. 1 polymer, water and hexamethyldisilazane (HMDZ) were introduced in two parts. In each case they were introduced at a constant rate. The wt. % added for each ingredient together with the step during which it was introduced on to the first twin screw extruder are depicted in Table 1b.

TABLE-US-00002 TABLE 1b amounts of each ingredient added at constant rate per step Step when added C. 1 (wt. %) Ex. 1(wt. %) Polymer 1 (a) 33.16 32.17 Water (a) 0.96 0.93 HMDZ (a) 2.82 2.73 Silica (b) 23.19 22.50 Water (f) 0.00 1.31 HMDZ (f) 0.00 1.68 Polymer (f) 39.87 38.68 Total 100.00% 100.00%

[0135] In these examples the first twin-screw extruder (4) was a Werner-Pfleiderer co-rotating twin-screw extruder with a 25 mm diameter and an L/D ratio of 48. The screws were run at 700 rpm. The temperature of entry port (26) was approximately 20° C. and the temperature at the discharge port (29) of said first twin-screw extruder (4) was approximately 110° C. Feeds (1-3, and 6) were all operated at room temperature.

[0136] In step (a) components (C), (D) and (A) were introduced via feeds (1, 2 and 3) respectively into first static mixer (25) and after mixing therein produced a step (a) mixture which was transported into a Werner-Pfleiderer (now Coperion) co-rotating twin-screw extruder (4), through entry port (26).

[0137] In step (b) silica was introduced onto the Werner-Pfleiderer co-rotating twin-screw extruder 4 via entry port (27) from store (6) and mixed with the step (a) mixture to form the viscous paste. The viscous paste was broken down in step (c) as previously discussed and then in this instance instead of the arrangement described with respect to FIG. 1, there were two entry ports in the third zone (15) for introducing components onto the Werner-Pfleiderer co-rotating twin-screw extruder (4). A first entry port for a mixture of components (C) and (D) and a second entry port for introducing a second amount of component (A), in each case diluting the material in the extruder (4) such that the diluted silica dispersion obtained in step (f) was then transported to the discharge port (29).

[0138] The mixture exiting through discharge port (29) was then transported and pumped through residence zone (17), which comprised pipes and a static mixer. The temperature and pressure of the residence zone were 140° C. and 50 psi (344.74 kPa) and the material had an average residence time in residence zone (17) of about 13 minutes.

[0139] The means for vacuum stripping (19) utilised in example 1 was a Welding Engineers Model 0.8″ (2.032 cm) twin-screw devolatilizing extruder. It was non-intermeshing and counter rotating. Material travelling through Welding Engineers Model 0.8″ twin-screw devolatilizing extruder (19) was stripped (devolatilized) at 190° C.

[0140] The viscosity of the silicone rubber base compositions for Ex. 1 and C. 1 were determined using a TA Instruments AR 2000 parallel plate rheometer at 25° C. at a frequency of 0.1 s.sup.−1. It was found that the viscosity for Ex. 1 having HMDZ and water introduced during step (f) had significantly better (lower) viscosity result (787 Pa.Math.s) than did C. 1 which had no additional HMDZ or water added during step (f) resulting in a viscosity of 3198 Pa.Math.s).

Examples 2 and 3

[0141] In Ex. 2 and 3 identical compositions were prepared into base compositions using exactly the same equipment and process as described in Ex.1 and C. 1 above with the following exceptions [0142] (i) silica (ii) was used which was a fumed silica having a specific surface area of 248 g/m.sup.2 (BET method in accordance with ISO 9277: 2010); [0143] (ii) Ex. 2 the temperature used in the residence zone was 90° C.; and [0144] (iii) Ex. 3 the temperature used in the residence zone was 170° C.

[0145] The composition used for both Ex. 2 and Ex. 3 is depicted in Table 2a.

TABLE-US-00003 TABLE 2a Ex. 2 & Ex. 3 (wt. %) Polymer 1 67.88 Water 2.32 HMDZ 5.10 Silica (ii) 24.70

TABLE-US-00004 TABLE 2b Step when added Ex. 2 & Ex. 3 (wt. %) Polymer 1 (a) 33.94 Water (a) 0.93 HMDZ (a) 4.63 Silica (ii) (b) 24.70 Water (f) 1.39 HMDZ (f) 0.46 Polymer (f) 33.94 Total 100.00%

[0146] The viscosity of the silicone rubber base compositions for Ex. 2 and Ex. 3 were determined using the method described above. The viscosity of Ex. 2 was 1909 Pa.Math.s and for Ex. 3 was 1222 Pa.Math.s. It is considered that both provided base compositions as herein before described but that the base composition of Ex. 3 gave better results as the viscosity was lower indicating an improved filler treatment.

[0147] The base composition of Ex. 3 was then used to prepare a curable two-part silicone rubber composition wherein the base of Ex. 3 was mixed with a platinum-based catalyst to form a part A composition, wherein the platinum-based catalyst was present in the part A composition in an amount of 0.33 wt. %. A part B composition was prepared by mixing the base of Ex. 3 with a cross-linker in the form of trimethyl terminated dimethylmethylhydrogen polysiloxane having a viscosity of 45 mPa.Math.s at 25° C. in an amount of 1.6 wt. % of the part B composition and inhibitor in an amount of 0.095 wt. % of the part B composition. The part A and part B compositions were then mixed together, and the final composition was cured at a temperature of 150° C. for a period of 5 minutes, followed by a 4 hour, 200° C. post cure.

[0148] Once cured the physical properties of the resulting elastomer was determined and was compared with the physical properties of a reference elastomer made with an identical list of ingredients using a standard batch technique. The comparison of the physical properties between Ex. 3 and the ref elastomer are depicted in Table 2c below.

TABLE-US-00005 TABLE 2c Ex. 3 Ref. Shore A Durometer (ASTM D2240-97) 33.4 30.6 Tensile Strength (MPa)_ASTM D412-98A 7.53 8.03 Elongation ASTM D412-98A 651 737 Tear Strength (N/mm) ASTM D624 (Die B) 24.69 15.24

[0149] It can be seen that the results are similar indicating that the process used as described herein results in similarly treated filler when make silicone bases from a continuous process herein and a standard batch process.

Example 4

[0150] In this example the base composition was prepared using a 40 mm screw and piston pumps were utilised as appropriate. The composition used is depicted in Table 3a.

TABLE-US-00006 TABLE 3a Ex. 4 (wt. %) Polymer 1 69.57 Water 2.41 HMDZ 5.93 Silica 1 22.09

[0151] Polymer 1 and silica 1 were the same as used in Ex. 1 and C. 1 above. In Ex. 4 polymer, water and hexamethyldisilazane (HMDZ) were introduced in two parts. In each case they were introduced at a constant rate. The wt. % added for each ingredient together with the step during which it was introduced on to the first twin screw extruder are depicted in Table 3b.

TABLE-US-00007 TABLE 3b Step when added Ex. 4(wt. %) Polymer 1 (a) 28.05 Water 1 (a) 0.80 HMDZ 1 (a) 3.58 Silica (b) 22.09 Water 2 (f) 1.61 HMDZ 2 (f) 2.35 Polymer 2 (f) 41.52 Total 100.00%

[0152] Again referring to FIG. 1, in Ex. 4 the first twin-screw extruder (4) was a ZSK 40 McPlus co-rotating twin-screw extruder from Coperion with a 40 mm diameter and an L/D ratio of 62. The screws were run at 700 rpm and the temperature at the discharge port (29) of said first twin-screw extruder (4) was 110° C.

[0153] Feeds (1-3, and 6) were all operated at room temperature. In step (a) components (C), (D) and (A) were introduced via feeds (1, 2 and 3) respectively into first static mixer (25) and after mixing therein produced a step (a) mixture which was transported into the ZSK 40 McPlus co-rotating twin-screw extruder (4) through entry port (26).

[0154] In step (b) of example 4 silica was introduced onto the ZSK 40 McPlus co-rotating twin-screw extruder (4) via entry port (27) from store (6) and mixed with the step (a) mixture to form the viscous paste. The viscous paste was broken down in step (c) as previously discussed and then in this instance like in Example 1 instead of the arrangement described with respect to FIG. 1, there were three entry ports in the third zone (15) for introducing ingredients onto the ZSK 40 McPlus co-rotating twin-screw extruder (4). A first entry port for (C) a second entry port for component and a third additional entry port was for introducing a second amount of component (A). Components (C) and (D) were introduced at room temperature.

[0155] The mixture exiting through discharge port (29) was then transported and pumped through residence zone (17), which comprised pipes and a static mixer. The temperature and pressure of the residence zone were 140° C. and 145 psi (999.74 kPa) and the material had an average residence time in residence zone (17) of about 18 minutes.

The second extruder (19) was a ZSK 32 McPlus co-rotating twin-screw devolatilizing extruder from Coperion. Material travelling through said ZSK 32 McPlus co-rotating twin-screw devolatilizing extruder (19) was stripped (devolatilized) at 160° C.

[0156] The viscosity of the silicone rubber base composition for Ex. 4 were determined using the same test methodology as described for Ex. 1 and C. 1 and the viscosity was determined to be 4094 Pa.Math.s.