WO1992014595A1

WO1992014595A1 - Mixing process for reactive liquids

Info

Publication number: WO1992014595A1
Application number: PCT/GB1992/000266
Authority: WO
Inventors: Gary W. Boyce
Original assignee: Exlan Limited
Priority date: 1991-02-15
Filing date: 1992-02-14
Publication date: 1992-09-03
Also published as: AU1243692A; GB9103286D0; CA2104109A1

Abstract

Rapid, homogeneous mixing of different liquids having substantially different viscosities from one another and inter-reactable rapidly upon contact to generate a gaseous reaction product, is accomplished by introducing the different liquids into a preliminary mixing chamber and effecting mutual contact and intermixing thereof under conditions of mild shear, then continuously moving the viscous liquid mixture into a closed main mixing chamber where it is subjected to conditions of severe shearing agitation, whilst maintaining the mixture under pressure conditions suitable to prevent substantial escape of the gaseous reaction product, followed by expulsion of the mixed liquid composition from the mixing chamber accompanied by release of pressure to permit release of gaseous reaction product, in a total time from mutual contact to expulsion from the chamber of not greater than 1 minute.

Description

MIXING PROCESS FOR REACTIVE LIQUIDS

This invention relates to mixing processes, and more particularly to processes for intimately mixing liquids of widely different viscosities and which are chemically reactive with one another upon contact.

The mixing of liquids in which one of them is of high viscosity, in order to provide a homogeneous mixture of them, is energy intensive, and normally demands mixing conditions of high shear. When the two liquids are chemically reactive with one another upon contact, and the desired product is the reaction product of them, additional factors need to be addressed. The speed and nature of the reaction which takes place between them dictates that the intimate mixing to a homogeneous condition must be achieved rapidly, before the chemical reaction has proceeded to any great extent, or an unsatisfactory, non-uniform reaction product will be obtained. The alternative is to complete the mixing at conditions under which the reaction will not start or will proceed extremely slowly, and then change the conditions to those promoting the reaction, but in many cases this is not practical.

These and other problems are exemplified by processes for the manufacture of polysilicone foams, to which the present invention is primarily directed. In these processes, a first viscous liquid comprising a siloxane polymer, inorganic filler, water, diluent and catalyst, is mixed with a second liquid siloxane polymer (the curative), which has a much lower viscosity. The first siloxane may contain vinyl groups and the second siloxane hydride groups, so that on contact, in the presence of complex platinum based catalyst, they inter-react to form a high molecular weight polysilicone. Hydrogen gas is generated as a reaction product within the reaction mixture. Then the product rapidly gels and eventually cures. By proper control of the chemical reaction, the evolved hydrogen gas can be arranged to blow and expand the reacting mixture into a cellular foam so that the end product is a cured, stable, flexible foam material, plastic or elastomeric in nature. Examples of formulations for making such polysilicone flexible foams can be found in United States patent 4,189,545 Modic.

The quality of such foam materials is dependent to a large extent upon the uniformity and size of the cells, which in turn derives largely from the homogeneity of the mixture during the foaming process. There are three process steps taking place rapidly, namely chemical reaction with evolution of gas, foam formation and expansion, and gelling of the high molecular weight polysilicone product so formed. When the gelling process is well advanced, no further foam expansion takes place. Eventually, but much more slowly, the foamed polysilicone cures, to its final stable condition - a separate oven-curing step is normally adopted to accomplish this. The three rapidly occurring processes, however, are substantially complete within one minute of the initial contact of the silicone reactants and catalyst, a room temperature.

The process is further complicated by the fact that best uniformity in the final foam is achieved by ensuring uniformity of temperature throughout the viscous liquid reaction mixture as the reactions take place. Temperature primarily affects the generation of gas in the mixture. Hot-spots in the reaction mixture should be minimized. This is difficult to accomplish, especially on the commercial production scale, particularly since the reaction itself is mildly exothermic.

The present invention provides a simple and economic process for the production of high quality flexible polysilicone foams of good uniformity, capable of operation at high speeds on a commercial scale. The process of the present invention involves bringing together two mutually reactive liquids which have widely different viscosities, in a first, preliminary mixin chamber in which they encounter conditions of mild shear to intermix them, and then passing the mixture to a main mixing chamber where it encounters conditions of severe shearing agitation, with repeated subdivision and recombination of portions of the mixture, whilst being maintained under pressure conditions effectively preventing gas release and foaming. Then the mixture is discharged from the main mixing chamber and the pressure released, so that foam expansion can occur end be completed before the gelling process has advanced to any great extent. The time elapse between initial material contact of the reactants and discharge of the homogeneously mixed reaction mixture is not greater than one minute.

The process of the present invention thus uses only a single mixer, having a pre-chamber and a main chamber. It operates continuously and at high speeds, to produce a high quality, uniform foam product economically on a commercial scale.

Thus according to the present invention, there is provided a process for the rapid, homogeneous mixing of at least two liquids which have substantially different viscosities from one another, and under conditions such that they will, upon contact, inter-react rapidly to generate a gaseous reaction product, the process comprising:

continuously introducing said at least two liquids through separate inlet ports into a preliminary mixing chamber and effecting mutual contact and intermixing thereof under conditions of mild shear;

continuously moving the viscous liquid mixture so formed into a closed main mixing chamber and subjecting the mixture therein to conditions of severe shearing agitation, with subdivision and recombination of mixture portions, whilst maintaining the mixture under pressure conditions suitable to prevent substantial escape of gaseous reaction product from the liquid mixture;

continuously expelling the intimately mixed liquid composition from the main mixing chamber accompanied by release of pressure restrictions therefrom to permit release of gaseous reaction product from the mixture;

the time elapse from initial mutual contact of the viscous liquids to expulsion of the mixture from the main mixing chamber being not greater than one minute.

Preferably, the process of the present invention is conducted at room or slightly elevated temperatures, eg. from 15^βC to 32^βC.

The process of the present invention has its primary application in the production of foam polysilicone products, in which the first highly viscous liquid comprises a polydiorganosiloxane composition and the second lower viscosity liquid comprises a polydiorganosiloxane curative, at least one of the two liquids also including a catalyst so as to create — o —

conditions under which the polydiorganosiloxanes are inter- reactable on contact, with generation of hydrogen to form a cured, high molecular weight polysilicone foam. In such a process, one of the polydiorganosiloxanes has vinyl groups therein, and the other has hydride groups. In the presence of a suitable complex platinum catalyst, these materials interact on contact very rapidly, so that in the present invention the inter- reactive mixtures are kept separate until they are carefully contacted in the preliminary mixing chamber, to which they are fed through separate inlet ports. The reaction between them to generate hydrogen gas, used as the in situ blowing agent, is slightly exothermic. Nevertheless, in accordance with the present invention, the temperature throughout the reacting mixture can be kept substantially uniform and within the range of 15^β- 32^βC, preferably 26^β- 28^βC, by suitable adjustment of throughput rates and mixing speeds.

In the preferred embodiment of the process of the present invention, the main mixing chamber in which the mixture is subjected to severe shearing agitating conditions comprises a dynamic rotary mixer with a multiplicity of sub-chambers, each of the sub-chambers having a perforated rotor mounted to rotate about a substantially horizontal axis, and a perforated stator disposed in close tolerances to the rotor to define tortuous paths of travel of the mixture through the chamber, along with continuous subdivision and recombination of the portions of the mixture. Preferably also, the direction of travel of the mixture through the main mixing chamber is predominantly axial with respect to the rotor axis, and the direction of expulsion from the main mixing chamber is radially upwards, most preferably substantially vertically upwards. This effectively prevents suck back of air into the mixing chamber to mix with the reaction mixture as the process proceeds.

Upon expulsion from the main mixing chamber, the polysilicone mixture is suitably received in continuously moving, shallow forms, so that it foams and cures therein in the form of slabs. The slabs are suitably conveyed through an oven, to complete the curing thereof at elevated temperatures.

A specific preferred embodiment of the invention will now be described, with reference to the accompanying drawings, in which:

FIGURE 1 is a diagrammatic process flow sheet of the overall process;

FIGURE 2 is a perspective view of the mixer and associated parts used in this specific embodiment of the process; FIGURE 3 is a vertical cross sectional view, with parts cut away, through the center of the mixer shown in Figure 2.

In the drawings, like reference numerals indicate like parts.

With reference first to Fig. 1, a rotary dynamic forced shear mixer 10 is utilized for the continuous mixing of liquid components inter-reactable to form polysilicone foam. The reactants include a polydiorganosiloxane with vinyl terminal groups combined with appropriate amounts of inorganic filler (appropriately silica), complex platinum catalyst, water and reactive diluent. This reactant mixture, hereinafter "resin part A^M, is mixed to a substantially homogeneous liquid composition in a pre-mix resin tank 12 equipped with agitators 14, 16. This resin part A is highly viscous, e.g. of the order of 45,000 - 100,000 centipoise (cps) at ambient temperatures. The pre-mixed resin part A can be pumped by means of first resin transfer pump 18 via line 20 and solenoid operated valve 22 to resin holding tank 24. From there, it can be pumped as required, by means of second resin transfer pump 26 and part A inlet line 28 to mixer 10. This flow can be controlled by solenoid operated valve 30. A by-pass line 32 leading directly from pre-mix resin tank 12 to part A inlet line 28 under control of another solenoid operated valve 34 is also provided, so that if desired resin part A can be directly fed to mixer 10 without passing through resin holding tank 24. A further solenoid operated control valve 36 is provided downstream of the junction of by-pass line 32 and part A inlet line 28, for flow control of resin part A immediately upstream of the mixer 10.

The second liquid for mixing, resin part B or curative, comprises a polydiorganosiloxane with hydride groups. It is of much lower viscosity than resin part A, e.g. of the order of 1000 - 1400 cps. Since this is inter-reactable with resin part A on contact, it is kept in a separate curative tank 38 and fed through a separate curative line 40 to the mixer 10. Line 40 is provided with a curative pump 42 and solenoid operated control valve 44. A pressure relief, safety valve 46 is provided in curative line 40. A valve controlled drain outlet 48 is also provided, which can be arranged to drain curative tank 38 and curative line 40 as and when required.

A solvent tank 50 is also provided, connected to mixer 10 by solvent line 52. This is similarly provided with a solvent pump 54, safety valve 56, drain 58 and solenoid operated control valve 60. The solvent is not used during the mixing process itself, but only for purposes of washing and flushing the mixer 10 after operation. The general arrangement also includes a purge line 62 by means of which air can be supplied to the mixer 10 to dry the internal parts after solvent washing. The purge line 62 is provided with an appropriate shut-off valve 64, filter 66 to prevent entry of air-borne particles into the mixer, and solenoid operated control valve 68.

The outlet 70 from the mixer 10 is disposed vertically upwardly, but terminates in a downwardly extending flexible conduit 72 which in operation oscillates slowly from side to side, to deposit foaming product evenly into forms 74 passing therebeneath on a continuously moving conveyer 76.

With reference to Fig. 2, the mixer 10 is generally cylindrical, and mounted in horizontal disposition. At its upstream end, it has a circular face plate 78 in which are provided three separate inlet ports. The first inlet port 80 is of relatively large diameter (approximately 1% inches) and is connected to part A inlet line 28. The second inlet port 82 is of smaller diameter (eg. % inch) and is connected to curative line 40. The third inlet port 84 is connected to the purge line 62 and is approximately h inch d:".ameter. As shown and described in Fig. 3, the mixer 10 has a central rotary shaft disposed along a substantially horizontal axis and an appropriate drive train including a reduction gear box 86 and motor (not shown) are provided beyond the downstream end of the mixer.

With reference to Fig. 3 of the accompanying drawings, this shows a vertical cross section through the mixer 10, with various parts cut away for clarity of illustration. The separate inlet ports 80, 82 and 84 extending through the face place 78 communicate with a preliminary mixing chamber 88 at the upstream part of the mixer 10. Perforated paddle blades 90, three in number, are disposed in the chamber 88, to rotate therethrough upon rotary drive of the central rotation shaft 92 to which they are secured.

Downstream of the chamber 88, the mixer 10 has a main mixing chamber generally designated 94, effectively divided into three sub-chambers by rotor and stator arrangements. The first sub-chamber 96, extending circumferentially around shaft 92, contains a stator 98 integral with the outer cylindrical wall 100 and connected thereto by a perforated ring portion 102 of relatively thin dimension. The upstream boundary of sub-chamber 96 is formed by upstream ring-like rotor 104 which has a series of apertures 106 therethrough providing communication between preliminary mixing chamber 88 and sub-chamber 96. Rotor 104 is provided, at its radially outward edge, with a series of vanes 108, protruding downstream into close proximity with the apertured ring portion 102 of the stator. The downstream boundary of sub-chamber 96 is formed by the upstream side of first central rotor 110. This rotor 110 is essentially similar to rotor 104, being apertured near its radially inner portion but having an upstream presented series of vanes 112 and a downstream presented series of vanes 114 on its downstream side. The next sub-chamber 116 is similarly provided with a stator 118 essentially the same in all respects as stator 98. Sub-chamber 116 is bounded by rotor 110 and second central rotor 120, essentially the same in all respects to first central rotor 110. The next sub-chamber 122 is similarly provided with a stator 124 of the same construction, and is bounded at its downstream end by a rotor 126 essentially identical to rotor 104 but with upstream- extending vanes 128. Thus, main mixing chamber 112 comprised of the three sub-chambers 96, 116 and 122 is essentially closed so that pressure can be exerted on the contents therein.

Sub-chamber 122 communicates with an exit chamber 130 in which are rotated perforated paddles 132 similar to the paddles 90 in the premix chamber 88. The vertically upwardly extending discharge outlet 70 communicates with exit chamber 130. The premix chamber 88 and the exit chamber 130 are provided with respective drain outlets 134 and 136, for washing, flushing and purging purposes. In operation, the mixer 10 is initially flushed by pumping thereto solvent, suitably trichloroethylene, from solven tank 50 via solvent line 52 through third inlet port 84 into the mixer, to clean it thoroughly of residues of previous operations and other contaminants. Residual solvent is drained through drain outlets 134 and 136, and then the mixer is purged by blowing air through it, via purge line 62 and inlet port 84.

Then the apparatus is ready for use in the manufacture of foamed polysilicone. Resin part A premixed in premix resin tank 12 and stored in resin holding tank 24 is pumped to the mixer, and at the same time the resin part B from the curative tank 38 is pumped to the mixer. The overall rates of flow of th resins and the relative rates of flow of the resin part A and th resin part B are carefully controlled to predetermined values using solenoid operated control valves 30, 36 and 44. The shaft 92 of the mixer 10 is rotated at relatively high speeds (for example 200 r.p.m.). The resin part A and the resin part B are thus initially mutually contacted in premix chamber 88 where the are intimately mixed together by the action of rotating paddles 90, but under conditions of relatively mild shear.

Then, under the influence of infeed pumps 26, 42, the mixture is forced through apertures 106 in upstream rotor 104, and thence into the first sub-chamber 96 of the main mixing chamber 94. Here, the mixture encounters the action of the rotors 104 and 110, and the stator 98, so that it is subjected to high shear mixing conditions, whilst at the same time being maintained under pressure. Hydrogen gas produced as a reaction product is consequently held within the body of the liquid resin mixture, and cannot escape therefrom. The resin mixture follows a tortuous path through chamber 96, through the perforations in rotor 106, around the edges of the stator 98, through the perforations in the ring portion of the stator 98 and thence around the edge of the stator and into the downstream portion of chamber 96. The passage of the resin through the apertures in the rotor and the stator ensures sub-division of the mixture into small quantities and recombination thereof, repeatedly, as it moves through the chamber 96. Similar conditions of severe shearing agitation, with sub-division and recombination of mixture portions, following similarly tortuous paths, are encountered by the mixture as it proceeds through rotor 110, into chamber 116, through stator 118 end rotor 120 into chamber 122, and out through rotor 126 into exit chamber 130.

There the mixture is subjected to only mild agitation conditions by perforated paddles 138, prior to being expelled vertically upwardly through outlet 70. This period of mild agitation immediately prior to discharge has an advantageous effect on the foam expansion process. As it enters outlet 70, the pressure on the mixture starts to be relieved, so that foam expansion of the mixture starts, due to the evolution and escape of the hydrogen gas. The mixture proceeds down through flexible conduit 72 to pour out in a foaming condition, into moulds 74 on conveyor 76. The outlet 72 is arranged to oscillate from side to side across the forms 74, so as to fill them evenly with foaming mixture. Foam expansion of the product continues in the form 74, whilst the gelling process also occurs, eventually limiting the foam expansion. Then the forms containing slabs of foam polysilicone are fed into an oven, to complete the curing process.

The temperature of the mixture and the final resin is monitored and controlled as the process proceeds, by standard means, to a range of 26° - 28^βC. Care is taken to ensure that the temperature does not rise above 32^βC, where severe problems of premature curing of the foam, resulting in plugging of the mixer may occur. The time which elapses from the entry of the resin part A and resin part B into the preliminary mixing chamber 88 until the reacting mixture enters outlet 70 is less than 30 seconds.

The provision of the by-pass line 64 from the resin mixing tank 12 directly to the mixer 10 provides a valuable option, in allowing^" direct feed of resin part A to the mixer 10. Whilst in normal operation it is convenient to hold pre-mixed resin part A in resin holding tank 24 so that the preparation of the resin part A mixture is kept essentially separate from the mixing and inter-reaction of resin part A and resin part B, to allow for further testing of the resin part A mixture prior to use, there are occasions when this is not necessary or is indeed disadvantageous. For example, throughput demands of the mixer, instability of the resin part A mixture leading to possible settling out of the filler therefrom, failure of the second resin transfer pump 26 or solenoid operated valve 30, blockage of part A inlet line 28 etc, could all lead to problems or shut down of mixer 10, in the absence of by-pass line 32 and associated parts. Accordingly, the provision of this by-pass facility helps to ensure continous production of product in the event of difficulties.

The process as described and illustrated can be run continuously over extended periods of time. Once mixing and production of foam ceases valves from part A inlet line 28 and curative line 40 are shut-off and solvent from solvent tank 50 and line 52 is provided, thoroughly to wash the mixer and remove silicone residues therefrom, before they have chance to gel and cure to provide obstructions in the mixer. As noted, a suitable solvent for this purpose is trichloroethylene. After this solvent wash and fiush, the solvent is drained from the mixer and then the mixer is purged with air, through line 62,. to dry the mixture and remove solvent residues, so that it is ready for a restart with fresh quantities of resin.

Claims

1. A process for the rapid, homogeneous mixing of at least two liquids which have substantially different viscosities from one another, and under conditions such that they will, upon contact, inter-react rapidly to generate a gaseous reaction product, the process comprising:

continuously introducing said at least two viscous liquids through separate inlet ports into a preliminary mixing chamber and effecting mutual contact and inter-mixing thereof under conditions of mild shear;

continuously moving the viscous liquid mixture so formed into a closed main mixing chamber and subjecting the mixture therein to conditions of severe shearing agitation, with subdivision and recombination of mixture portions, whilst maintaining the mixture under pressure conditions suitable to prevent substantial escape of gaseous reaction product from the liquid mixture:

and continuously expelling the intimately mixed liquid composiτion from the main mixing chamber accompanied by release of pressure restrictions therefrom to permit release of gaseous reaction product from the mixture; the time elapse from initial mutual contact of the liquids to expulsion of the mixture from the main mixing chamber being not greater than one minute.

I

2. The process as claimed in claim 1 wherein said time elapse is less than thirty seconds.

3. The process as claimed in claim 1 or claim 2 wherein the first liquid comprises a highly viscous polydiorganosiloxane composition and the second liquid comprises a polydiorganosiloxane curative of lower viscosity, at least one of the two liquids also including a catalyst so as to create conditions under which the polydiorganosiloxanes are inter- reactable on contact, with generation of gas to form a cured, high molecular weight polysilicone foam.

4. The process as claimed in claim 3 wherein the mixing temperature and reaction temperature is from 15*C to 32'C.

5. The process as claimed in claim 4 wherein the mixing temperature and reaction temperature is from 26*C to 28"C.

6. The process as claimed in claim 4 or claim 5, wherein the main mixing chamber comprises a dynamic rotary mixer with a multiplicity of sub-chambers, each of said sub-chambers having a perforated rotor mounted to rotate about a substantially horizontal axis, and a perforated stator disposed at close tolerances to the rotor, to define tortuous paths of travel of the mixture through the chamber.

7. The process as claimed in claim 6 wherein the direction of travel of the mixture through the main mixing chamber is predominantly axial with respect to the rotor axis, and the direction of expulsion of the mixture from the main mixing chamber is radially upwards.

8. The process as claimed in claim 7, wherein the said direction of expulsion is substantially vertically upwards.

9. The process as claimed in any of claims 5-8 wherein the polysilicone mixture is discharged to continuously moving forms, to foam and cure therein in the form of slabs.

10. The process for mixing at least two viscous liquids as defined in claim 1, substantially as herein described with reference to the accompanying drawings.