Abstract:
In a first step, basic components including (A) 100 parts by weight of an organopolysiloxane gum, (B) 5-100 parts by weight of a reinforcing silica filler, and (C) 0.1-30 parts by weight of a processing aid are fed batchwise to a closed mixer for mixing them until uniform. In a second step, the mixture is continuously fed into a kneader/extruder for heat treating. The first step achieves quick dispersion and cooperates with the second step for the efficient production of a heat-curable silicone rubber compound.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an efficient method for preparing heat-curable silicone rubber compounds. 
     2. Prior Art 
     Heat-cured silicone rubber is improved in weather resistance, durability, heat resistance, physiological activity and colorability. It is used in a variety of applications including building materials, electronic materials, electric materials, business machine parts, automotive parts, and medical parts. 
     A variety of methods are known for the preparation of heat-curable silicone rubber compounds which heat cure into silicone rubbers. Most customarily, they are prepared using large size, open kneader/mixers having a pair of mixing blades in a chamber. 
     The kneader/mixers have the advantage that a large amount of mass can be produced in a single pass on account of its maximum capacity reaching several thousands of liters, but the drawback that a long time is needed until the mixture becomes homogeneous, and specifically, the mixing step takes several hours to several tens of hours. The throughput rate at which heat-curable silicone rubber compounds are prepared by means of kneader/mixers can be increased by increasing the size of the apparatus or using a multi-stage apparatus. These modifications, however, are difficult to implement partially because of limits on the installation space. 
     With respect to the preparation of heat-curable silicone rubber compounds, a number of methods have been investigated for the purpose of improving productivity. For example, JP-A 56736/1989 discloses a continuous production process comprising the step of feeding a diorganopolysiloxane gum to a twin-screw kneader/extruder along with an inorganic filler which has been surface treated with an organic silicon compound under pressure. The drawback of this method is that the pretreatment of the filler takes a long time. JP-A 102007/1990 discloses a continuous production process comprising the steps of continuously pulverizing raw materials in a high-speed shearing machine and feeding them to a continuous twin-screw kneader/extruder. In order to obtain uniform particles, the pulverizing conditions must be strictly controlled. This makes the process complex. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide a novel and improved method for the efficient preparation of a heat-curable silicone rubber compound through simple steps within a short time. 
     We have found that when basic components including an organopolysiloxane gum, a reinforcing silica filler, and a processing aid are first fed to a batchwise closed mixer for mixing them into a uniform mixture, and the mixture is continuously fed into a kneader/extruder for continuously heat treating it, the components can be mixed, homogenized and heat treated within a short time and by simple steps and a heat-curable silicone rubber compound having improved curing properties is prepared. 
     In well-known methods for preparing silicone rubber compounds, a series of steps from mixing and homogenization of raw ingredients to heat treatment are carried out in a kneader/mixer. Since the kneader/mixer is rather incompetent in mixing and homogenization operation, the present invention carries out mixing and homogenization in a batchwise closed mixer capable of achieving homogenization within a short time, as the first step. In the kneader/extruder of the second step, only heat treatment is continuously carried out. The two steps are assigned to the respective apparatus. This enables brief production of silicone rubber compounds, achieving an increased throughput rate. 
     The invention provides a method for preparing a heat-curable silicone rubber compound, comprising: 
     a first step of feeding basic components to a batchwise closed mixer for mixing them into a uniform mixture, said basic components including (A) 100 parts by weight of an organopolysiloxane gum, (B) 5 to 100 parts by weight of a reinforcing silica filler, and (C) 0.1 to 30 parts by weight of a processing aid, and 
     a second step of continuously feeding the mixture of the first step into a continuous kneader/extruder for continuously heat treating the mixture. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The heat-curable silicone rubber compound to be prepared by the method of the invention is a millable compound containing basic components: (A) an organopolysiloxane gum, (B) a reinforcing silica filler, and (C) a processing aid. 
     The organopolysiloxane gum as component (A) is preferably a linear high viscosity organopolysiloxane of the general formula (1): 
     
       
         R 2 (R 1   2 SiO) n SiR 1   2 R 2   (1) 
       
     
     wherein R 1  is a monovalent hydrocarbon group having 1 to 10 carbon atoms, R 2  is selected from methyl, vinyl and hydroxyl groups, and letter n is a number of at least 1,000. 
     In formula (1), R 1  is selected from substituted or unsubstituted monovalent hydrocarbon groups having 1 to 10 carbon atoms, preferably 1 to 6 carbon atoms. Included are unsubstituted hydrocarbon groups, for example, alkyl groups such as methyl, ethyl, propyl, butyl, pentyl, hexyl, octyl, decyl and dodecyl, alkenyl groups such as vinyl, allyl and butenyl, aryl groups such as phenyl and tolyl, and aralkyl groups such as benzyl and β-phenylethyl, and substituted hydrocarbon groups in which some or all of the hydrogen atoms in the foregoing groups are replaced by halogen atoms or cyano groups, such as 3,3,3-trifluoropropyl and cyanoethyl. Of these, methyl, vinyl, phenyl and 3,3,3-trifluoropropyl are preferred. R 2  is selected from methyl, vinyl and hydroxyl groups. Letter n, representative of an average degree of polymerization, is an integer of at least 1,000, preferably 3,000 to 20,000. 
     With the properties of silicone rubber taken into account, it is preferred that at least 50 mol %, especially at least 80 mol % of the entire R 1  and R 2  groups in formula (1) be methyl and 0.025 to 0.5 mol %, especially 0.05 to 0.3 mol % of the entire R 1  and R 2  groups be vinyl. 
     Component (B) is a reinforcing silica filler which is selected, for example, from fumed silica, fired silica, precipitated silica (or wet silica) and mixtures thereof. Fillers having a specific surface area of at least 50 m 2 /g are desirable. Such fillers may be used alone or in admixture of two or more. 
     The silica fillers may have been surface treated with suitable agents, for example, linear organopolysiloxanes, cyclic organopolysiloxanes, hexamethyldisilazane and reactive silanes. Since the reinforcing silica filler becomes expensive on account of the surface treatment, such surface treatment is unnecessary unless it is desired to impart special properties. From the standpoints of transparency and reinforcement of the reinforced silicone rubber, fumed silica having a specific surface area of 100 to 400 m 2 /g is desirable. From the standpoints of cost, elasticity and physical properties of the reinforced silicone rubber, reinforcing precipitated silica having a specific surface area of 50 to 800 m 2 /g is especially desirable. 
     An appropriate amount of the reinforcing silica filler blended is 5 to 100 parts by weight, preferably 10 to 50 parts by weight, per 100 parts by weight of organopolysiloxane (A). Outside this range, less or larger amounts of the filler result in silicone rubber compositions which have poor properties and cure into rubber parts having insufficient mechanical strength (such as tensile strength and tear strength). 
     Component (C) is a processing aid for improving the dispersibility of the reinforcing silica filler (B) in the silicone rubber and imparting various desirable properties to the silicone rubber. Exemplary are low-molecular weight organic silicon compounds blocked with a hydroxyl group at each end, low-molecular weight organic silicon compounds blocked with an alkoxy group at each end, and silazanes. The processing aid used herein is typically an organic silicon compound represented by the general formula (2): 
     
       
         R 4 O—(R 3   2 SiO) m —R 4   (2) 
       
     
     wherein R 3  is a monovalent hydrocarbon group having 1 to 10 carbon atoms, R 4  is methyl, ethyl or hydrogen, and letter m is an integer of 1 to 100. 
     In formula (2), R 3  is as defined for R 1 . Preferably R 3  is selected from the group consisting of methyl, vinyl, phenyl, and 3,3,3-trifluoropropyl. R 4  is selected from the group consisting of methyl, ethyl and hydrogen, with hydrogen being preferred. Letter m is an integer of 1 to 100, preferably 1 to 20, more preferably 2 to 5. 
     An appropriate amount of the processing aid (C) blended is 0.1 to 30 parts, preferably 0.2 to 10 parts by weight per 100 parts by weight of organopolysiloxane (A). The processing aids may be used alone or in admixture of two or more. 
     In the silicone rubber compound according to the invention, there may be added additives if necessary. Such additives include extending fillers such as ground quartz and diatomaceous earth, and other fillers (excluding the aforementioned reinforcing ones) such as calcium carbonate, iron oxide, zinc oxide, titanium oxide, carbon black, barium oxide, magnesium oxide, cerium hydroxide, magnesium carbonate, zinc carbonate, asbestos, glass wool, finely divided mica, and fused silica powder; pigments; heat resistance modifiers such as fatty acid salts of metal oxides; flame retardants such as platinum compounds; and agents for preventing cured products from coloring such as organohydrogenpolysiloxanes. These optional components may be added in conventional amounts insofar as the benefits of the invention are not impaired. 
     Now the method of the invention is described. In the first step, the above-described components are fed to a batchwise closed mixer where they are mixed until a homogeneous mixture is obtained. As compared with non-closed atmospheric pressure kneader/mixers, the batchwise closed mixer has the advantage that a fully dispersed state can be established within a short time. 
     The batchwise closed mixer is a mixer having a closed chamber where the contents are mixed under pressure. Exemplary are pressure kneader/mixers, internal mixers, and Banbury mixers. 
     If desired, the batchwise closed mixer has a cooling or heating function so that the internal temperature during mixing may be maintained in the range from room temperature to about 150° C., more preferably from about 50° C. to about 120° C. The mixing time may be about 1 to about 30 minutes. Above 150° C., the process aid would not perform well. 
     The mixture resulting from the first step is fed to a kneader/extruder of the second step directly or through a transfer apparatus such as an extruder while optionally heating. Examples of the kneader/extruder include a two-shaft co-rotational kneader available under the trade name of KRC (by Kurimoto Iron Works K.K.), a single-shaft kneader with functions of screw rotation and an axial back and forth motion available under the trade name of Buss Co-Kneader (by Buss K.K.), and a kneader exerting powerful shear action by means of a rotating blade in combination with a stationary blade available under the trade name of KCK (by KCK K.K.). 
     Preferably, the kneader is provided with a vent which can be evacuated to vacuum for removing volatiles from the mixture that passes through the kneader. 
     The mixture fed to the kneader is preferably heat treated at a temperature of 150 to 250° C., preferably 150 to 200° C. for about 1 to about 30 minutes (residence time). Below 150° C., a silicone rubber compound having desired physical properties would not be obtained. Temperatures above 250° C. can cause degradation of the silicone rubber compound. 
     According to the invention, the first step of mixing and homogenization operation, in which conventionally employed kneader/mixers are rather incompetent is carried out by a batchwise closed mixer capable of achieving homogenization within a short time. In the kneader/extruder of the second step, the mixture is continuously heat treated. The two steps are assigned to the respective apparatus and can be accomplished within short duration. A heat-curable silicone rubber compound can be produced through simple steps at an increased throughput rate. 
    
    
     EXAMPLE 
     Examples are given below together with comparative examples to illustrate the invention, and are not intended to limit the scope thereof. All parts are by weight. 
     Comparative Example 1 
     To a pressurizing kneader/mixer were fed basic components: 100 parts of an organopolysiloxane gum end-blocked with a dimethylvinylsiloxy group, consisting of 0.15 mol % of methylvinylsiloxy units and 99.85 mol % of dimethylsiloxy units, and having an average degree of polymerization of about 8,000, 41 parts of wet silica having a specific surface area of about 200 m 2 /g (Nipsil LP by Nippon Silica K.K.), 3 parts of a linear dimethylsiloxane blocked with a silanol group at each end and having an average degree of polymerization of 3 to 4. The components were kneaded for about 10 minutes under pressure. In this way, the components were mixed into a homogeneous mixture which had a temperature of 60° C. 
     Example 1 
     The mixture obtained in Comparative Example 1 was continuously fed at a flow rate of 120 g/min to a kneader/extruder KCK 80x22-35EX(7) (by KCK K.K.) where heat treatment was carried out, yielding a silicone rubber compound. The kneading section was set at a temperature of 200° C., a revolution of 20 rpm, and a residence time of about 10 minutes. 
     Example 2 
     The silicone rubber compound obtained in Example 1 was continuously fed to the kneader/extruder KCK which was operated under the same conditions as in Example 1 whereby the compound was heat treated again. There was obtained a silicone rubber compound. 
     Example 3 
     The mixture obtained in Comparative Example 1 was continuously fed at a flow rate of 60 g/min to the same kneader/extruder KCK as used in Example 1 where heat treatment was carried out, yielding a silicone rubber compound. The kneading section was set at a temperature of 200° C., a revolution of 20 rpm, and a residence time of about 15 minutes. A vacuum pump was connected to the kneader/extruder immediately upstream of the discharge outlet for effecting deaeration in vacuum. 
     Comparative Example 2 
     The mixture obtained in Comparative Example 1 was fed to a 3-liter kneader/mixer where the mixture was heated to a temperature of 180° C. and then heat treated at a temperature of 180 to 190° C. for about 1 hour, yielding a silicone rubber compound. The series of steps took about 1.5 hours. 
     Comparative Example 3 
     The same basic components as used in Comparative Example 1 were fed to a 3-liter kneader/mixer where the components were kneaded and mixed for homogenization at a temperature of about 50 to 100° C. for about 1 hour. Thereafter, the temperature was raised and the mixture was heat treated at a temperature of 180 to 190° C. for about 1 hour, yielding a silicone rubber compound. The series of steps took about 3 hours. 
     Using a roll mill, 100 parts of each of the base compounds obtained in Examples 1-3 and Comparative Examples 1-3 was uniformly compounded with 0.5 part of an organic peroxide, 2,5-dimethyl-2,5-di(t-butylperoxy)-hexane. The compound was press cured at 165° C. for 10 minutes and post cured at 200° C. for 4 hours, obtaining a silicone rubber sheet of 2 mm thick. The sheet was examined by physical tests according to JIS K6301. The results are shown in Table 1. 
     
       
         
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Example 
                 Comparative Example 
               
             
          
           
               
                   
                 1 
                 2 
                 3 
                 1 
                 2 
                 3 
               
               
                   
                   
               
             
          
           
               
                 Components 
                   
                   
                   
                   
                   
                   
               
               
                 Organopolysiloxane 
                 100  
                 100  
                 100  
                 100  
                 100  
                 100  
               
               
                 gum (pbw) 
               
               
                 Wet silica (pbw) 
                 41 
                 41 
                 41 
                 41 
                 41 
                 41 
               
               
                 Silanol-terminated 
                  3 
                  3 
                  3 
                  3 
                  3 
                  3 
               
               
                 dimethylsiloxane 
               
               
                 (pbw) 
               
               
                 Tests 
               
               
                 Hardness 
                 48 
                 47 
                 47 
                 53 
                 48 
                 48 
               
               
                 as press-cured 
               
               
                 Hardness 
                 51 
                 49 
                 49 
                 59 
                 50 
                 51 
               
               
                 as post-cured 
               
               
                 Difference between 
                  3 
                  2 
                  2 
                  6 
                  2 
                  3 
               
               
                 press-cured and 
               
               
                 post-cured states 
               
               
                 Tensile strength 
                 80 
                 78 
                 77 
                 70 
                 72 
                 70 
               
               
                 (kgf/cm 2 ) 
               
               
                 Elongation (%) 
                 360  
                 360  
                 330  
                 365  
                 280  
                 300  
               
               
                 Apparatus 
                 pressurizing 
                 pressurizing 
                 pressurizing 
                 pressurizing 
                 pressurizing 
                 non- 
               
               
                   
                 kneader/ 
                 kneader/ 
                 kneader/ 
                 kneader/ 
                 kneader/ 
                 pressurizing 
               
               
                   
                 mixer 
                 mixer 
                 mixer 
                 mixer 
                 mixer 
                 kneader/ 
               
               
                   
                 ↓ 
                 ↓ 
                 ↓ 
                   
                 ↓ 
                 mixer 
               
               
                   
                 kneader/ 
                 kneader/ 
                 kneader/ 
                   
                 non- 
               
               
                   
                 extruder 
                 extruder 
                 extruder 
                   
                 pressurizing 
               
               
                   
                 KCK 
                 KCK 
                 KCK 
                   
                 kneader/ 
               
               
                   
                   
                   
                   
                   
                 mixer 
               
               
                 Process time 
                 20 min. 
                 30 min. 
                 25 min. 
                 10 min. 
                 1.5 hr 
                 3 hr. 
               
               
                   
               
             
          
         
       
     
     As is evident from Table 1, a comparison between Examples and Comparative Examples reveals that the invention is successful in producing a silicone rubber compound having satisfactory cured properties within a significantly reduced period of time. When the silicone rubber compound obtained in Comparative Example 1 without the second step according to the present invention is cured, cured properties are unsatisfactory as demonstrated by greater hardnesses and a greater hardness difference between press-cured and post-cured states. 
     Example 4 
     To a Banbury mixer were fed basic components: 100 parts of an organopolysiloxane gum end-blocked with a dimethylvinylsiloxy group, consisting of 0.15 mol % of methylvinylsiloxy units and 99.85 mol % of dimethylsiloxy units, and having an average degree of polymerization of about 8,000, 43 parts of fumed silica having a specific surface area of about 200 m 2 /g (Aerosil 200 by Nippon Aerosil K.K.), 4 parts of a linear dimethylsiloxane blocked with a silanol group at each end and having an average chain length of 15 to 20, and 4 parts of diphenylsilane diol. The components were kneaded for about 13 minutes under pressure. In this way, the components were mixed into a homogeneous mixture which had a temperature of 90° C. The mixture was continuously fed at a flow rate of 50 g/min. to a kneader/extruder S2-KRC (by Kurimoto Iron Works K.K.) where the mixture was heat treated, yielding a silicone rubber compound. The kneading section was set at a temperature of 180° C., a revolution of 80 rpm, and a residence time of about 2 minutes. 
     Comparative Example 4 
     The same basic components as used in Example 4 were fed to a 3-liter kneader/mixer where the components were kneaded and mixed for homogenization at a temperature of about 50 to 100° C. for about 2 hours. Thereafter, the temperature was raised and the mixture was heat treated at a temperature of 180 to 190° C. for about 1 hour, yielding a silicone rubber compound. The series of steps took about 4 hours. 
     Using a roll mill, 100 parts of each of the base compounds obtained in Example 4 and Comparative Example 4 was uniformly compounded with 0.6 part of an organic peroxide, 2,5-dichlorobenzoyl peroxide. The compound was press cured at 120° C. for 10 minutes and post cured at 200° C. for 4 hours, obtaining a silicone rubber sheet of 2 mm thick. The sheet was similarly examined by the physical tests. The results are shown in Table 2. 
     
       
         
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Example 
                 Comparative Example 
               
               
                 Component 
                 4 
                 4 
               
               
                   
               
             
             
               
                 Organopolysiloxane gum 
                 100 
                 100 
               
               
                 (pbw) 
               
               
                 Fumed silica (pbw) 
                 43 
                 43 
               
               
                 Silanol-terminated 
                 4 
                 4 
               
               
                 dimethyisiloxane (pbw) 
               
               
                 Diphenyisilane diol 
                 4 
                 4 
               
               
                 (pbw) 
               
               
                 Hardness 
                 56 
                 57 
               
               
                 Tensile strength 
                 115 
                 110 
               
               
                 (kgf/cm 2 ) 
               
               
                 Elongation (%) 
                 400 
                 420 
               
               
                 Apparatus 
                 Banbury mixer 
                 Non-pressurizing 
               
               
                   
                 ↓ 
                 kneader/mixer 
               
               
                   
                 Kneader/extruder KRC 
               
               
                 Process time 
                 15 min. 
                 4 hr. 
               
               
                   
               
             
          
         
       
     
     Japanese Patent Application No. 257722/1997 is incorporated herein by reference. 
     Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims.