Abstract:
A continuous process provides a devolatilized liquid injection moldable silicone composition. In the process, a filler, treating agent and silicone polymer are introduced into an extruder having a length to diameter ratio of at least greater than 50. The filler, treating agent and silicone elastomer are continuously compounded in the extruder into a devolatilized liquid injection moldable silicone composition. A system for preparing a liquid silicone rubber composition comprises a mixer to prepare a concentrate of filler and silicone polymer, a long extruder having an L/D ratio of greater than 50, connected to the mixer to receive the concentrate from the mixer and to compound and devolatilize the concentrate, a treating agent and silicone polymer into a liquid silicone rubber composition containing volatiles and a cooler to receive the liquid silicone rubber composition to cool, homogenize and further devolatilized the composition.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The invention relates to a process of preparing a liquid silicon rubber (LSR) composition.  
           [0002]    An LSR composition also referred to as a LIM composition, is a multi-component combination of a vinyl-containing polydiorgansiloxane fluid, a hydrogen-containing polydiorganosiloxane fluid, an effective amount of a platinum catalyst and a reinforcing filler, such as a fumed silica and additional additives. Typically, two component mixtures are mixed and charged into a preheated mold where they are rapidly cured to produce a rubber part. A first component mixture includes a vinyl-containing polydiorgansilozane fluid, a silica filler and an effective amount of a platinum catalyst and a second component mixture includes a hydrogen containing polydiorganosiloxane fluid in combination with other ingredients, including a vinyl containing polyorganosiloxane fluid and a silica filler.  
           [0003]    Typically, the LSR composition is produced by kneading a polydiorganosiloxane, inorganic filler and additives by means of a kneading machine such as a Banbury mixer, a turbulizer, a change can mixer or a low intensity double arm dough mixer. In this process, polydiorganosiloxane, inorganic filler, treating agents and additives are batch mixed until desired properties are obtained. The batch mixing process can take 12 to 30 hours per batch. After mixing, the LSR composition is stripped of volatiles and cooled. This final step requires additional time to complete preparation of the LSR composition.  
           [0004]    A batch process requires long residence times and large amounts of energy. Non-homogeneous shear and extensional stresses across a commercial sized batch can result in non-uniform size distribution of filler that results in variations in properties. Different physical properties may result form batch to batch. Batches processed at different times may be characterized by different physical properties. The batch process is labor, energy and capital intensive and produces materials of only marginal consistency.  
           [0005]    There is a need for a continuous process that consistently produces a devolatilized LSR composition from polymer, filler and additives.  
         BRIEF SUMMARY OF THE INVENTION  
         [0006]    The invention provides a continuous process that prepares a devolatilized LSR composition. The process comprises introducing a filler, treating agent and silicone polymer into an extruder having a length to diameter ratio of at least greater than 50. The filler, treating agent and silicone polymer are continuously compounded and devolatilized in the extruder into a devolatilized LSR composition.  
           [0007]    In an embodiment, the invention provides a system for preparing a liquid silicone rubber composition. The system comprises a mixer to prepare a densified concentrate of filler and silicone polymer, a long extruder having an L/D ratio of greater than 50, connected to the mixer to receive the densified concentrate from the mixer and to compound and devolatilize the densified concentrate, a treating agent and silicone polymer into a liquid silicone rubber composition containing volatiles and a cooler to receive the liquid silicone rubber composition to cool, homogenize and further devolatilized the composition.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is a schematic representation of a continuous LSR composition process;  
         [0009]    [0009]FIG. 2 is a schematic representation of an overall process for preparing an LSR composition;  
         [0010]    [0010]FIG. 3 is a table of processing conditions; and  
         [0011]    [0011]FIG. 4 is a table of testing results including product properties. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]    The invention reduces filler treatment reaction time and provides an economical system for producing silicone compositions including LSR compositions. Two parts of an LSR composition can be made simultaneously. Inventories can be reduced and equipment clean-out required between runs of a batch process can be eliminated.  
         [0013]    In one step of the process, filler surface treatment is effected by mixing and reacting filler, polydimethylsiloxane polymer and treating agents. The mixture can then be stripped of volatiles and cooled. According to the invention, filler treatment is reduced from hours to a few minutes, e.g., 2-6. The is continuously in-situ treated to produce a base, which can be filtered and then catalyzed.  
         [0014]    In one embodiment, the process comprises four steps; 1) filler densification, 2) In-situ treatment, volatiles stripping and filtering, 3) cooling, homogenization and de-airing and 4) catalization. Prior to densification, the filler can be pre-mixed with silicone polymer to facilitate feeding into a reactor. A co-rotating twin screw extruder can be used as the reactor to mix the densified filler with treating agents (HMDZ, water, vinyl silazane) and additional polymer.  
         [0015]    The subsequent reaction is effected at higher temperatures and pressures than in a batch process. The in-situ treatment can require a minimum residence time in the extruder, a temperature in the range of 160-210° C. and pressure between about 70 and about 300 psi.  
         [0016]    Stripping of volatiles can be effected toward the end of the extruder while the material is hot thereby eliminating secondary operations. Residual amounts of nitrogen containing by-products measured as parts per million HMDZ can be at about the 100 ppm level. Filtration can be done at the discharge of the extruder while the material is hot, eliminating secondary operations. Cooling and de-airing can be effected in a separate counter-rotating mixer, which can provide back-mixing and a high residence time to homogenize the material into a base.  
         [0017]    Finally, the base can be divided in two components and separately mixed in static mixers with catalyst and other additives to produce two separate “A” and “B” components. The two components can be made simultaneously to reduce inventories.  
         [0018]    These and other features will become apparent from the drawings and following detailed discussion, which by way of example without limitation describe preferred embodiments of the present invention.  
         [0019]    In FIG. 1, a filler such as fumed silica is continuously in situ treated and compounded with a silicone polymer such as a vinyl-terminated polydimethylsiloxane. In a first step, fumed silica  12 , an initial amount of silicone polymer  14  and a treating agent  16  are charged into a continuous mixer  18  such as a Bepex Turbolizer to form a densified polymer/filler mass.  
         [0020]    The filler that can be used in this invention is any inorganic filler with silanol surface groups that can be compounded with silicone polymers. Examples of inorganic fillers include a reinforcing silica such as fumed silica or precipitated silica. A single filler or a combination of fillers can be used to reinforce the silicone polymer.  
         [0021]    The silicone polymer used in the compositions of the present invention is typically a vinyl-terminated polydimethylsiloxane having a viscosity varying from 100 to 2,000,000 cps at 25° C. The polymer can be represented by recurring units of Formula I:  
                         
 
         [0022]    wherein, R 1  independently at each occurrence represents C 1-4  alkyl, or C 2-4  alkylene; R 2  independently at each occurrence represents C 1-4  alkyl, C 1 -C 4  haloalkyl or C 2-4  alkylene; R 3  independently at each occurrence represents H, C 2-10  alkyl, C 2-4  alkylene, C 4-6  cycloalkyl, OH or C 1 -C 4  haloalkyl; and n represents an integer so that the viscosity of the silicone polymer varies from 100 to 2,000,000 cps at 25° C. Broadly, n can be about 80 to about 3500, desirably, n is a value of about 100 to less than 1000.  
         [0023]    A further preferred composition comprises a silicone polymer wherein, R 1  independently at each occurrence represents, CH 3  or CH═CH 2 ; R 2  independently at each occurrence represents, CH 3 , CH═CH 2  or CH 2 CH 2 CF 3 ; R 3  independently at each occurrence represents CH 3 , CH═CH 2 , OH or CH 2 CH 2 CF 3 ; and n represents an integer from about 200 to about 900.  
         [0024]    Another embodiment provides a composition wherein the vinyl content of the silicone polymer ranges from about 0.05% to about 0.5% by weight of the silicone polymer.  
         [0025]    The amount of filler in the densification step can be from about 20 to about 150 parts by weight, desirably from about 30 to about 100 parts by weight and preferably from about 40 to about 70 parts by weight, per 100 parts by weight of the high viscosity silicone polymer. During the compounding step, addition of further silicone polymer reduces the proportion of silica in the final silicone composition to about 10 to about 100 parts by weight, desirably from about 15 to about 90 parts by weight and preferably from about 25 to about 70 parts by weight, per 100 parts by weight of the total high viscosity silicone polymer.  
         [0026]    The treating agent is a silanol or methoxy stopped silicone polymer that reacts with the filler surface hydroxyl or silanol groups to prevent a condensation reaction among filler particles or between filler and gum molecules that otherwise leads to stiffening and loss of elastomeric properties. The treating agent reduces filler silanol groups and reduces the time required for aging of the silicone, to prevent crepe hardening and/or to regulate plasticity. The treating agent can be an organosilane, an organosilazane, a low-viscosity polyorganosiloxance or a silicone resin, which has a silanol group and/or an alkoxy group having 1 to 6 carbon atoms. Examples include diphenylsilanediol, dimethylsilanediol, methyltriethoxysilane and phenyltrimethoxysilane. The low-viscosity polysiloxane may contain one or more kinds of organic groups selected from a methyl group, a phenyl group, a vinyl group and a 3,3,3-trifluoropropyl group. Preferred silanol-reactant treating agents include silanol-stopped polydimethylsiloxane, octamethylcyclotetrasiloxane (D4) and hexamethyldisilazane (HMDZ). The viscosity of the polysiloxane measured at 25° C. is in the range of from about 1 to about 300 cP, preferably from about 5 to about 100 cP.  
         [0027]    The treating agent can be added in the compounding step in a weight proportion of about 0.1 to about 100 parts fluid to 100 parts of filler, desirably about 0.5 to about 75 parts fluid to 100 parts of filler and preferably about 1.0 to about 50 parts fluid to 100 parts of filler. The treating agent can react to reduce available groups of the filler to a concentration of between about 8 to about 2 hydroxyl groups/(nanometer) 2  of filler, preferably between about 7 to about 3 hydroxyl groups/(nanometer) 2  of filler. In an embodiment, the treating agent can be a combination of HMDZ and water. This combination can comprise a weight ratio of HMDZ/water of between about 0.1 to 10, desirably between 0.2 to 5 and preferably between 0.3 to 3.  
         [0028]    In the extruder  22 , a densified filler and polymer concentrate  20  and additional silicone polymer  24 , treating agent and water are fed at barrels  1  to  2 , further additional fluid  26  is added downstream at about barrel  16  and volatiles are stripped in two vacuum vents  28 ,  30  at barrels  18  to  19  and at  21  to  22 . Water  32  is injected between vent sections to facilitate stripping. According to the invention, the L/D ratio of the entire extruder is at least greater than 50 to achieve proper filler treatment and stripping of volatiles. Desirably, the L/D ratio is greater than 60 and preferably greater than 70. The exemplary extruder  22  of FIG. 1 has an L/D ratio of 78.  
         [0029]    During the extruder step, addition of further silicone polymer reduces the proportion of silica in the final silicone composition to about 5 to about 100 parts by weight, desirably from about 10 to about 40 parts by weight and preferably from about 15 to about 30 parts by weight, per 100 parts by weight of the total high viscosity silicone polymer.  
         [0030]    Reaction between silanol groups on the filler and the treating agent takes place under pressure and at elevated temperature in the extruder compounding step. The screws of the treatment section (barrels  3  to  18 ), can be configured to provided a pressure of about 16 and about 400 psi. Desirably, the pressure is about 40 to about 300 psi and preferably about 100 and about 200 psi. Temperature in a feed section of the extruder (L/D&lt;9) is maintained at less than about 150° C., preferably less than about 120° C. Temperature is controlled in the treatment section (L/D=9 to discharge) between about 100 and 250° C., desirably between 120 and 240° C. and preferably between 130 and about 230° C.  
         [0031]    Compounding in extruder  22  produces a base silicone composition  32  that can be used to produce LSR silicone composition components.  
         [0032]    [0032]FIG. 2 illustrates an overall process and system for preparing an LSR composition. In FIG. 2, the system  40  includes long extruder  42  having an L/D ratio of at least greater than 50. In the process, filler is fed from feeder  44  to densifier  46 , e.g., a Bepex turbolizer or a Drais mixer. where it is mixed with initial silicone polymer  48  to form a concentrate of a filler and polymer concentrate  50 . The densified filler and polymer concentrate  50  is crammed by means of screw feed  52 , e.g., a Werner and Pfleiderer ESBV crammer feeder, into the long extruder  42   
         [0033]    Additional silicone polymer  54  is combined in mixer  56  with additives such as HMDZ treating agent  58 , water  60  and silazane  62  and is then charged  64  into the long extruder  42  for compounding with densified filler and polymer concentrate  50 . Further additional silicone polymer  66  can be mixed with vinyl polymer  68  and charged  70  to the long extruder  42  downstream from the charge of concentrate  50 .  
         [0034]    The compounded and reacted components are devolatilized at vents  72  and  74  with water  76  charged between the vents. Devolitilization is important for at least two reasons. First, devolitilization removes unreacted treating agents to “quench” further reaction with filler silanol groups. Otherwise, further reaction can diminish filler reinforcement properties in the heat-vulcanizable silicone composition. Second, devolitilization removes surplus liquids that adversely affect rheological properties of the silicone composition.  
         [0035]    The compounded, reacted and devolatilized LSR composition is filtered  78  and pumped  80  to a cooler  82 . The LSR composition is cooled and back mixed in cooler  82  to de-air and to further homogenize the composition. The cooler  82  can be under vacuum to produce further devolatilization. Cooler  82  can be a counter-rotating twin screw mixer such as a LIST ORP. The volatiles  84  from the cooler are combined with volatiles  86  from the extruder  42  and are exhausted by vacuum  88  to volatiles recovery  90 .  
         [0036]    Cooled LSR composition  92  is divided into two streams, A and B. Stream A is pumped  94  with platinum catalyst and other additives A  96  to static mixer  98  to produce component mixture A  100  and stream B is pumped  102  with hydrides and other additives B  104  to produce component mixture B  106 . Component mixture A  100  and component mixture B  106  can be charged to a mold (not shown) where they can be rapidly cured to produce a rubber part.  
         [0037]    These and other features will become apparent from the following detailed discussion, which by way of example without limitation describes preferred embodiments of the present invention.  
       EXAMPLES  
       [0038]    Filler was mixed with vinyl-terminated polydimethylsiloxane (viscosity about 40,000 cps at 25° C., n about 800) in a Bepex Turbolizer to provide a densified filler concentrate. The densified concentrate was fed into a 53 mm, Krupp Werner &amp; Pfleiderer co-rotating twin screw extruder with a 78 L/D ratio. In the extruder, the filler was mixed with HMDZ, water, vinyl silazane and additional vinyl-terminated polydimethylsiloxane at barrels  1  and  2 . Downstream (barrel  6 ), the filler was mixed with more vinyl-terminated polydimethylsiloxane and the mixture was compounded under elevated temperature in a section of the extruder that was sealed to maintain pressure. FIG. 3 shows conditions for two runs of the extruding step.  
         [0039]    Hot compounded base material was discharged from the extruder to a LIST ORP jacketed mixer for cooling. The base was subjected to back mixing in the mixer to even-out inconsistencies to provide a homogenous product. The mixer was under a vacuum, which served to further strip and de-air the base.  
         [0040]    The base is divided into two parts (A and B), which are fed via metering pumps into static mixers. Platinum catalyst and other additives are introduced into part A base just prior to feed to the mixer. Hydrides and other additives are introduced into part B base before mixing.  
         [0041]    Properties of two final formulations each containing catalyst and hydride are shown in FIG. 4. The two rows of results shown in FIG. 4 correspond to the two runs of FIG. 3.  
         [0042]    These Examples show that a devolatilized LSR composition with excellent properties can be continuously produced from polymer, filler and additives according to the invention.  
         [0043]    While preferred embodiments of the invention have been described, the present invention is capable of variation and modification and therefore should not be limited to the precise details of the Examples. The invention includes changes and alterations that fall within the purview of the following claims.