GB2384488A

GB2384488A - Continuous mixing of silicone rubber base

Info

Publication number: GB2384488A
Application number: GB0201562A
Authority: GB
Inventors: Koichi Ozaki; Toyohiko Yamadera; Mitsuo Hamada
Original assignee: Dow Corning Toray Silicone Co Ltd
Current assignee: DuPont Toray Specialty Materials KK
Priority date: 2001-02-16
Filing date: 2002-01-24
Publication date: 2003-07-30
Also published as: GB0201562D0; JP4155718B2; JP2002241508A

Abstract

A method for continuously producing a liquid silicone rubber base by mixing a composition comprising 100 parts by weight of a liquid diorganopolysiloxane containing alkenyl groups 1 from 5 to 100 parts by weight of a reinforcing silica filler 2, and a hydrophobing agent adapted to hydrophobe the silica filler 3 in an amount equivalent to from 1 to 30 % by weight of the reinforcing silica filler into and through a first co-rotating twin-screw continuous kneader/extruder 10 at a temperature of from 0 to 100{C to form a mixture and then feeding the mixture continuously into and through a second co-rotating twin-screw continuous kneader/extruder 15 at a temperature of from 150 to 300{C. Reagent 3 is preferably hexamethyldisilazane.

Description

-1- 2384488

METHOD FOR CONTINUOUSLY PRODUCING LIQUID SILICONE RUBBER BASE

1] The present invention relates to a method for continuously producing a liquid silicone rubber base with the aid of co-rotating twinscrew continuous kneader/extruders.

5 [0002] Liquid silicone rubber bases, the primary ingredients of silicone rubber compositions which are curable by addition reactions, typically comprise liquid diorganopolysiloxanes containing alkenyl groups, processing aids and reinforcing silica fillers. Organohydrogen polysiloxanes and platinum-based catalyst are added to the base in order to cure the composition. Liquid silicone rubber compositions which are curable by 10 addition reactions have excellent moldability and curability, and are hence used in the manufacture of silicone rubber moldings.

t0003] Although liquid silicone rubber bases are commonly produced by batch methods, continuous production systems have also been proposed because of the need to rapidly increase production in order to satisfy increased demand. JP Patent Publication 15 (Kokoku) Hei 4-28008, describes the mixing section of a twin-screw continuous kneader/extruder which is heated to a temperature of between 200 and 300 C and into which an inorganic filler and 30-100 wt% of a polyorganosiloxane containing alkenyl groups are fed, the remaining 70 wt% of the polyorganosiloxane containing alkenyl groups is fed to a mid-portion of the mixing section and mixed in order to obtain a liquid silicone rubber base 20 by means of the twin-screw continuous kneader/extruder. A drawback to this approach, however, is that the liquid silicone rubber base is produced without the addition of a silica filler treatment agent and as such the resulting base has an inadequate storage stability.

4] JP patent application publication (Kokai) Hei 9-3332 discloses a method for producing a liquid silicone rubber base in which ammonia, a mixing aid, and part of a liquid 25 polyorganosiloxane containing alkenyl groups are premixed in a prescribed ratio, a powdery silica filler is introduced into the mixture to produce a mixed powder. The resulting mixed powder is then mixed to uniformity in a prescribed ratio in a continuous kneader with the remainder of the liquid polyorganosiloxane containing alkenyl groups. However, ammonia is toxic and has an acrid odour, and hence presents operating problems. Another drawback is 30 that it is difficult to continuously produce a uniformly mixed powder or to feed the resulting nixed powder to a continuous kneader at a constant rate.

[BOOS] According to JP (Kokai) 9-155950 a mixture of liquid polyorganosiloxane and fine powdery silica can be continuously produced by mixing liquid polyorganosiloxane, fine

-2- powdery silica, liquid organosilazane, and water after continuously introducing them into the starting material inlet port of a twin-screw continuous extruder/kneader. In the process, the twin-screw continuous extruder/kneader is cooled and kept at a temperature of 50 C or less when the length/diameter (LID) ratio of the screw ranges from 1 to 13; and at a temperature 5 of from 80 to 280 C, 50 to 280 C, or from 50 to 200 C when the LID ratio ranges from 13 to 47. A drawback to this approach, however, is that the cooling region and the heating region lie close to each other, impairing temperature control. Another drawback is that the mixing conditions are difficult to set for each region because it is impossible to set different rotational speeds for the screws in the cooling and heating regions.

10 [OOOG] The inventors perfected the present invention as a result of research aimed at overcoming the aforementioned shortcomings. Specifically, an object of the present invention is to provide a highly productive method for continuously producing a liquid silicone rubber base that offers improved storage stability and makes it easier to establish the mixing conditions.

15 [0007] In accordance with the present invention there is provided a method for continuously producing a liquid silicone rubber base comprising the following steps: i) mixing a composition comprising 100 parts by weight of a liquid diorganopolysiloxane containing alkenyl groups, (component A), from 5 to 100 parts by weight of a reinforcing silica filler, (component B), and an agent 20 adapted to hydrophobe the silica filler (component C) in an amount equivalent to from 1 to 30 % by weight of component B. by passing said components into and through a first co-rotating twin-screw continuous kneader/extruder at a temperature of from O to 100 C to form a mixture and ii) feeding the mixture resulting from step (i) continuously into and through a 25 second co- rotating twin-screw continuous kneader/extruder at a temperature of from 150 to 300 C.

8] Preferably the mixture leaving the first co-rotating twin-screw continuous kneader/extruder is transferred directly into the second corotating twin-screw continuous kneader/extruder without the need for any intermediary steps.

30 [0009] It is to be understood that the term mixing as used herein means the function of mixing effected by the kneader/extruder, which might also be referred to as milling or any other appropriate term used in the industry.

-3- [0010] Selecting a low mixing temperature of from 0 to 100 C for the first co-

rotating twin-screw continuous kneader/extruder in accordance with the method of the present invention reduces the volatilization of the agent adapted to hydrophobe the silica filler (henceforth referred to as the hydrophobing agent) and allows the reinforcing silica filler to 5 be rendered sufficiently hydrophobic even when the hydrophobing agent is a comparatively easily volatilized compound such as hexamethyldisilazane or 1,3-divinyltetramethyldisilazane. [0011] The mixing temperature in the second co-rotating twin-screw continuous kneader/extruder is from 150 to 300 C, making it possible to adequately heat the uniform 10 mixture of components obtained in step (i). Consequently, a liquid silicone rubber base having high flowability, excellent storage stability, and low viscosity for the desired content of the reinforcing silica filler can be continuously obtained with high productivity.

2] Whilst cleanliness represents a problem during component replacement in cases in which liquid silicone rubber bases with a variety of compositions or properties are 15 produced using the same kneader/extruder, the use of self-cleaning kneader/extrudersisuch as corotating twin-screw continuous kneader/extruders during steps (i) and (ii) of the present invention enables the time taken to replace a component to be reduced and the formation of waste minimized. Operability and productivity can therefore be improved.

3] The co-rotating twin-screw continuous kneader/extruders used in the present 20 invention are preferably commercially available. Suitable examples for the first co-rotating twin-screw continuous kneader/extruder are made sold as KRC (manufactured by Kurimoto) and Mixtron (manufactured by Kobe Steel) and for the second co-rotating twin-screw continuous kneader/extruder are sold as TEM (manufactured by Toshiba Machine Co.) and ZSK (manufactured by Werner Pfleiderer).

25 [0014] Typically in a co-rotating twin-screw continuous kneader/extruder, there is provided one or more inlet ports and a discharge port situated at or adjacent opposite ends of an extruder barrel. Twin screws are disposed in parallel in the barrel, with typically the end of each screw nearest the inlet port connected to a drive unit. The drive unit is adapted to rotate both the screws at the same speed and in the same direction (i.e. they are synchronized).

30 The screws commonly have double or triple threads and are adapted to mix and knead material travailing along the barrel from the inlet port to the discharge port.

5] In accordance with the present invention the preferred method for introducing the components into the first co-rotating twin-screw continuous kneader/extruder is as

follows, the starting material components A to C are transferred from their storage tanks to their respective inlet port(s) in the first kneader/extruder. The liquid starting materials (e.g. components A and C are fed through lines to the starting material inlet ports at constant rates.

Any suitable pumping system may be utilised if required, for example, a gear pump. The 5 powdered starting materials may be fed to their respective starting material inlet ports at constant rates with the aid of continuous feeders in the form of tables, belts, or screws. The starting material components A to C may also be introduced through the starting material inlet ports directly into the barrel, but because component A is liquid and component B is a powder, it is preferred to introduce these materials through a double pipe as described in JP 10 Patent Application Publication (Kokai) Hei 9-155950. Specifically, component A should be introduced through an outer pipe of a double pipe, and component B should be introduced through the inner pipe. Because it is commonly a liquid material used to treat component B. component C should preferably be introduced through a line that opens into the inner pipe.

6] The following elements may also be disposed between the starting material 15 inlet ports and the discharge port: inlet ports for introducing optional components (for example, additional component A or C) , vents for the release of volatile components of the mixture, temperature sensors for measuring the temperature of the mixture, other instrumentation sensors, and the like.

7] The first co-rotating twin-screw continuous kneader/extruder preferably has a 20 greater axial length L versus screw diameter D (henceforth referred to as the IdD ratio) which increases the residence time during mixing. The IdD ratio is preferably from 6 to 13, and more preferably from 7 to 10. The peripheral speed of the screws of the first co-rotating twin-

screw continuous kneader/extruder are preferably from 0.2 to 1.7 m/s, and most preferably from 0.5 to 1.5 m/s.

25 [0018] As frictional heat is generated when components A, B and C are moving through the first kneader/extruder, the resulting mixture is heated during the mixing process.

Hence it is preferred for the external surface of the barrel of the first co-rotating twin-screw continuous kneader/extruder to be cooled and most preferably the first kneader/extruder is enclosed in a coolant-circulated jacket in order to keep the said material at a temperature of 30 no more than 100 C.

9] The second co-rotating twin-screw continuous kneader/extruder has the saline basic structure as the first co-rotating twin-screw continuous kneader/extruder. No particular restrictions are imposed on the port for charging the mixture as long as the mixture

-5 discharged from the first co-rotating twin-screw continuous kneader/extruder may be introduced into the second co-rotating twin-screw continuous kneader/extruder barrel.

0] The port for charging the mixture into the second co-rotating twinscrew continuous kneader/extruder may be disposed directly below the discharge port of the first 5 co-rotating twin-screw continuous kneader/extruder. Alternatively, a line, in the form of a pipe or tube can be extended from the discharge port of the first co-rotating twin- screw continuous kneader/extruder to the port for charging the mixture into the second co-rotating twin-screw continuous kneader/extruder.

1] The following elements may be disposed between the port for charging the 10 mixture and the discharge port of the second co-rotating twin-screw continuous kneader/extruder: vents to enable release of the volatile components of the mixture, temperature sensors for measuring the temperature of the mixture, other instrumentation sensors, and the like.

2] The second co-rotating twin-screw continuous kneader/extruder preferably 15 has a greater axial length L versus the screw diameter D because it increases the residence time during mixing and improves the heat treatment of the material. Preferably the ratio should be from 20 to 50, and most preferably from 45 to 50.

3] The peripheral speed of the screws of the second co-rotating twinscrew continuous kneader/extruder should be from 0.2 to 1.7 ants, and most preferably from 0.5 to 20 1.5 m/s. 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.

4] In accordance with the method of the present invention a liquid diorganopolysiloxane A containing alkenyl groups, a reinforcing silica filler B. and an agent 25 adapted to hydrophobe the silica filler (the hydrophobing agent) C are introduced into the co-

rotating twin-screw continuous kneader/extruder of the first step in accordance with the production method of the present invention.

5] The liquid diorganopolysiloxane A containing alkenyl groups used herein is the principal ingredient of a liquid silicone rubber base. Such diorganopolysiloxanes are 30 typically expressed by the general formula R3SiO(R2SiO)oSiR3 Wherein each R may be the same or different and is a monovalent hydrocarbon group or a halogenated monovalent hydrocarbon group, of which at least two R groups per molecule

-6- should be alkenyl groups; and is a positive number selected such that the viscosity at 25 C ranges between 0.5 and 1000 Pa s. R is preferably an alkyl group such as methyl, ethyl, propyl or butyl, a cycloalkyl group such as cyclohexyl; an alkenyl group such as vinyl, allyl and hexenyl; an aryl group such as phenyl and tolyl, an aralkyls group such as a benzyl, or 5 phenylethyl; and hydrocarbon groups obtained by substituting some of the hydrogen atoms in these groups by halogen atoms (such as fluorine atoms). More preferably apart from the alkenyl groups each R group is preferably a methyl group or a phenyl group, but most preferably a methyl group. The alkenyl groups act as cross-linking sites, such that at least two of alkenyl groups must be present in each molecule. The alkenyl groups may be disposed at 10 both ends of the molecular chain, on side chains only, or both on the ends and on the side chains. Any suitable terminal blocking groups for liquid diorganopolysiloxane containing alkenyl groups may be used, for example, trimethylsiloxy and dimethylvinylsiloxy groups.

Any suitable main siloxane polymer chains may be utilised, for example dimethylpolysiloxane, dimethylsiloxane/methylphenylsiloxane copolymers, 15 dimethylsiloxane/methylvinylsiloxane/methylphenylsiloxane copolymers, and dimethylsiloxane/(3,3,3-trifluoropropylsiloxane) copolymers.

tO026] The liquid diorganopolysiloxane containing alkenyl groups should preferably have a viscosity of from 0.5 to 1000 Pa s at 25 C. This is because a liquid silicone rubber base whose viscosity is less than 0.5 Pa s has excessively high hardness, whereas a viscosity 20 greater than 1000 Pa s renders the system excessively viscous when the reinforcing silica filler is added. The liquid diorganopolysiloxane containing alkenyl groups may also be moderately branched.

7] The reinforcing silica filler B may be of any suitable form providing that it is a fine powder capable of reinforcing silicone rubber. Typical examples of such fillers include 25 dry process silicas, such as fumed silica and wet process silicas, such as precipitated silica.

The specific surface area of the reinforcing silica filler is preferably 50 m2/g or greater.

Component B should be admixed in an amount of from 5 to 100 parts by weight, and preferably from 5 to 60 parts by weight, per 100 parts by weight of component A because admixing too little of the component has an inadequate reinforcing effect, whereas admixing 30 too much of the component results in excessive viscosity and impairs moulding.

8] The agent adapted to hydrophobe the silica filler or hydrophobing agent (component C) renders component B hydrophobic and makes the reinforcing silica filler

easier to mix with component A. The hydrophobing agent should preferably be an organosilicon compound containing silanol groups or hydrolyzable groups attached to silicon atoms. The hydrophobing agent participates in a condensation reaction with the silanol groups on the surface of component B. making it easier for this component to mix with 5 component A. Specific examples of such hydrophobing agents include hexamethyldisilazane, divinyltetramethyldisilazane, and other hexaorganodisilazanes; trimethylsilanol, a:,o dihydroxydimethylsiloxane oligomers, or,}dihydroxymethylphenylsiloxane oligomers, x, dihydroxymethylvinylsiloxane oligomers, and other organosilane or organosiloxane oligomers having silanol groups; and organosilane or organosiloxane oligomers in which 10 hydrolyzable groups are attached to silicon atoms. Hexamethyldisilazane, divinyltetramethyldisilazane, and other hexaorganodisilazanes are preferred because they exhibit high reactivity in relation to the reinforcing silica filler and have powerful hydrophobing properties.

9] Component C is added in an amount constituting from 1 to 30 wt% of 15 component B. This amount varies with the moisture content, specific surface area, and silanol group content of component B. as well as with the content of silanol groups or hydrolyzable groups attached to silicon atoms in component C. A small amount of water should also preferably be present in order to promote hydrolysis and to enhance the treatment effect when component B is a hexaorganodisilazane. Mixing components A to C alone is 20 sufficient for producing a liquid silicone rubber base, although chemically inert diorganopolysiloxanes, pigments, heat-resistant agents, organopolysiloxane resins containing alkenyl groups, and the like may also be added.

0] The mixture discharged from the first co-rotating twin-screw continuous kneader/extruder is introduced into the second co-rotating twin-screw continuous 25 kneader/extruder, then mixed whilst being heated and finally is discharged through the discharge port of the second corotating twin-screw continuous kneader/extruder as a liquid silicone rubber base. The material is then cooled naturally or forcedly and introduced into an appropriate container for storage. Uniformly admixing an organohydrogen polysiloxane and a platinum-based catalyst into the liquid silicone rubber base starts a hydrosilylation reaction, 30 initiates cross-linking, and converts the material to silicone rubber. Simultaneously admixing a hydrosilylation inhibitor delays the crosslinking reaction at room temperature and allows the material to be stored or cured upon heating.

-8 [Working Examples] [0031] A specific embodiment of the present invention will now be described by way of the following working example and Figure 1 Fig. 1 is a schematic cross-sectional view of the equipment utilised in the method of the S present invention.

2] Referring to figure 1 there is provided a first co-rotating twinscrew continuous kneader/extruder 8 and a second co-rotating twin-screw continuous kneader/extruder 13. Kneader/extruder 8 comprises a starting material inlet port 7, a pair of screws 9, a barrel 10 and a discharge port 11. Components A, B and C of the composition to 10 be mixed in kneader/extruder 8 are stored in storage tanks 1, 3 and 2 respectively.

Components A and C are transported to inlet port 7 by way of gear pumps 4 and 5 respectively and Component B is transported to inlet port 7 by way of a table-type continuous feeder 6.

3] The components A, B and C are introduced onto the kneader/extruder 8 IS through inlet port 7 and are kneaded, mixed and transported along barrel 10 of kneader/extruder 8 by way of screws 9. The external surface of the barrel 10 of kneader/extruder 8 is enclosed in a jacket (not shown) for circulating cooling water to reduce the friction-induced heating of the material in kneader/extruder 8.The mixture exiting kneader/extruder 8 through exit 11 is immediately transferred to kneader/extruder 13.

20 [0034] Co-rotating twin-screw continuous kneader/extruder 13 comprises a mixture inlet port 12, a pair of screws 14, barrel 15 and discharge port 16. The mixture discharged from discharge port 11 of kneader/extruder 8 is continuously fed through a line to mixture inlet port 12 of kneader/extruder 13. The mixture from kneader/extruder 8 enters kneader/extruder 13 through inlet port 12 and the mixture is kneaded, mixed and transported 25 along barrel 15 of kneader/extruder 13 by way of screws 14. The external surface of barrel 15 of kneader/extruder 13 is enclosed in a jacket (not shown) adapted for circulating a heat medium to keep the temperature of the mixture contained in kneader/extruder 13 between 150 and 300 C. The resulting liquid silicone rubber base obtained is collected from discharge port 11 and stored.

30 [0035] For the following example in accordance with the invention, the following components were continuously fed through the starting material inlet port of the co-rotating twin-screw continuous kneader/extruder 8 in the weight ratios indicated below:

- 9 - A 100 parts by weight of a dimethylpolysiloxane blocked at both ends by dimethylvinylsiloxy groups and provided with a viscosity of 40 Pa s at 25 C B 43.3 parts by weight of fumed silica with a specific surface area of 250 m2/g C 7.88 parts by weight of hexamethyldisilazane.

5 [0036] Twin screws 9, each had a diameter of 100 mm and were rotated at 350 rpm.

The L/D ratio of kneader/extruder 8 was 7.2. components A, B and C being mixed in barrel 10 had a temperature of 50 C, and the mixture in discharge port 11 had a temperature of 70 C. The time between the introduction of the starting material and the discharge of the

mixture (i.e. the mean residence time)for materials moving through kneader/extruder 8 was 10 60 seconds.

7] Kneader/extruder 13 had twin screws 14, each with a diameter of 58 mm which rotated at 500 rpm. The L/D ratio of kneader/extruder 13 was 45. The mixture being further mixed in kneader/extruder 13 had a temperature of about 280 C in the barrel, and the mixture in discharge port 16 had a temperature of 200 C. The mean residence time of the 15 mixture in kneader/extruder 13 was 90 seconds.

8] The liquid silicone rubber base thus produced was allowed to cool to room temperature; 0.83 parts by weight of a methylhydrogenpolysiloxane blocked at both ends by trimethylsiloxy groups having a viscosity of 0.50 Pa s, 0.000275 parts by weight of a complex salt of chloroplatinic acid and 1,3-divinyltetramethyldisiloxane, and 0.014 parts by 20 weight of 3,5dimethyl-1-hexyn-3-ol were added per 100 parts by weight of the base; the components were mixed to uniformity; and the mixture was kept for 10 minutes at a temperature of 150 C and an increased pressure, yielding a cured sheet. Table 1 shows the physical properties of the silicone rubber sheet. Hardness was measured at room temperature in accordance with JIS K 62249 (using Type A durometer), and tensile strength, elongation, 25 and tear strength were measured at room temperature in accordance with JIS K 6249.

9] The liquid silicone rubber base (naturally cooled to room temperature) was kneaded and degassed in a tabletop kneader and pumped into a viscosity-measuring cylinder.

A plunger was pushed in, the weight of the liquid silicone rubber base squeezed out from the nozzle within a given period was measured, and apparent viscosity was calculated using the 30 Hagen-Poiseuille equation. Apparent viscosity was calculated in the same manner after the liquid silicone rubber base had been pumped into a cylinder and kept there for 1 month. The results are shown in Table 1.

-10 [0040] For the sake of comparison, 100 parts by weight of a dimethylpolysiloxane blocked at both ends by dimethylvinylsiloxy groups having a viscosity of 40 Pa s at 25 C, 43.3 parts by weight of fumed silica with a specific surface area of 250 m2/g, and 7.88 parts by weight of hexamethyldisilazane were introduced into a bladed mixer and mixed for 5 60 minutes at 25 C, yielding a liquid silicone rubber base. The silicone rubber was prepared as described above and the physical properties thereof and the apparent viscosity of the liquid silicone rubber base were measured in the same manner as above, yielding the results shown in Table 1.

Table 1

Working Comparative example example Initial apparent viscosity (Pa s) 740 1000 Apparent viscosity after standing for 1 850 1240 month (Pa s) Hardness 33 37 Elongation (%) 790 675 Tensile strength (MPa) 9.4 9.8 Tear strength (N/mm) 34 34 tO041] The production method of the present invention entails mixing an diorganopolysiloxane containing alkenyl groups, a reinforcing silica filler, and a hydrophobing agent at a temperature of from O to 100 C in a specific ratio by way of a first 15 co-rotating twin-screw continuous kneader/extruder, introducing the resulting mixture into a second co-rotating twin-screw continuous kneader/extruder, and mixing the material there at 150-300 C, and is a highly productive method for the continuous production of a liquid silicone rubber bases that have low viscosity and excellent fluidity and storage stability.

Claims

-11 CLAIMS

1. A method for continuously producing a liquid silicone rubber base comprising the following steps: i) mixing a composition comprising 100 parts by weight of a liquid diorganopolysiloxane containing alkenyl groups, (component A), from 5 to 100 parts by weight of a reinforcing silica filler, (component B), and an agent adapted to hydrophobe the silica filler (Component C) in an amount equivalent to 1 to 30 % by weight of component B. by passing said components into and through a first co-rotating twin-screw continuous kneader/extruder at a temperature of from O to 100 C to form a mixture and ii) feeding the mixture resulting from step (i) continuously into and through a second co- rotating twin-screw continuous kneader/extruder at a temperature of from 150 to 300 C.

2. A method for continuously producing a liquid silicone rubber base as defined in Claim 1, characterized in that component A has a viscosity of from 0.5 to

1000 Pa À s at 25 C.

3. A method in accordance with claim 1 or 2 wherein component B is fumed silica.

4. A method in accordance with any preceding claim wherein component C is hexaorganodisilazane.

5. A method for continuously producing a liquid silicone rubber base as defined in any preceding claim wherein the first co-rotating twin-screw continuous kneader/extruder has an IdD ratio of from 6 to 13.

6. A method for continuously producing a liquid silicone rubber base in accordance with any preceding claim as defined in any preceding claim wherein the second co-rotating twin-screw continuous kneader/extruder has an I]D ratio of from 20 to 50.

7. A liquid silicone rubber base prepared in accordance with the method of any one of claims 1 to 6.

8. A method as hereinbefore described with reference to figure 1.