CA2104109A1 - Mixing process for reactive liquids - Google Patents

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 ~ROOESS FOR REACTIVE LIOUIDS

Thls lnvention relates to mixlng proces~es, and more partlcularly to processes for intlmstely mlxing llqulds of widely dlfferent viscositles and which nre chemlcally reactlve wlth one ~nother upon contact.

The mlxlng of llgulds ln whlch one of them ls of high vlscosity, ln order to provlde a homogeneous mixture of them, ls energy lntenslve, and normally demands mlxing conditlons of high shear. When the two llquids are chemlcally reactive with one another upon contact, and the deslred product ls the reaction product of them, additlonal factors need to be addrsssed. The speed and nature of the reactlon whlch ta~es place between them dlc~ates that the intlmate mlxing to a homogeneous condltlon must be achieved rapldly, before the chemlcal reactlon has proceeded to any great extent, or an unsatlsfactory, non-unlform reactlon product wlll be obtalned. ~he alternatlve ls to complete the ~lxing at conditions under whlch the reactlon wlll not start or wlll proceed extremely 610wly, and then change the condltlons to those promotln~ the reactlon, but ln many cases this 18 not practical.

These and other problems are exempllfled by processes for the manufacture of polyslllcone foams, to whlch the present lnvention ls prlmar~ly dlrected. In these processes, a first vlscous ll~uid comprlslng a siloxane polymer, inorganic filler, water, dlluent and catalyst, is mlxed wlth a second llquld slloxane pclymer (the curatlve), wh~ch has a much lower WO92/1459s PCT/GB92/00266 ; ; - 2 -2l0~las ` ~"
viscosity. The flrst siloxane may contain vinyl groups ~nd the second siloxane hydride groups, so that on contact, ln the presence of complex platinum based catalyst, they lnter-react to form a high molecular weight polysilicone. Hydrogen gas is generated as a reaction product within the reactlon mixture.
Then the product rapidly gels and eventually cures. By proper control of the chemlcal reaction, the evolved hydrogen gas can be arranged to blow and expand the reacting mixture lnto a cellular foam so that the end product is a cured, stable, flexible foam material, plastlc or elastomerlc ln nature. Examples of formulations for making such polysilicone flexible foams can be found in United States patent 4,189,~45 Modic.

The quality of such foam materials 1~ 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 evolutlon of gas, foam formation and expansion, and gelllng of the high molecular weight polysilicone product ~o formed. When the gelling process is well advanced, no further foam expansion takes place. Eventually, but much more slowly, the foamed polysilicone cures, to lts 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 W~92/1459~ _ 3 ~ O9 PCT/GB92100266 of the initial contact of the silicone reactants and catalyst, at room temperature.

The process is further complicated by the fact that best unlformity in the final foam ls 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. ~his ls d~fficult to accomplish, especially on the commercial production scale, particularly since the reaction itself is mildly exothermic.

The present lnvention provides a simple and economic process for the production of high quallty flexible polysilicone oams 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, ln a first, preliminary mixing 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 condltions effectlvely prevent~ng gas release and foaming. Then the mixture is discharged from the main mixing chamber and the WO92/14S9~ PCT/GB92/00266 9 `-pressure released, so that foam expans$on can occur and be completed before the gelling process has advanced to any great extent. The tlme elapse between inltial material contact of the reactants and dlscharge of the homogeneously mlxed reactlon mixture ls not greater than one mlnute.

~ he 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 mixlng 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 prellminary mlxlng chamber and effecting mutual contact and lntermlxing thereof under conditlons of mild shear;

continuously moving the v~scous liquid mixture so formed into a closed main mixing chamber and subJecting the WO92/14~9~ ~ lU~l~ 9 PCT/GB92/00266 ~: - 5 -mixture thereln to conditions of severe shearing agitation, with subdivision and recombinatlon of mixture portions, whllst maintalnlng the mixture under pressure conditions sultable to prevent substantial escape of gaseous reactlon product from the liquid mixture, continuously expelling the lntimately mixed llquld composltlon from the maln mlxing chamber accompanled by release of pressure restrlctlons therefrom to permit release of gaseous reactlon product from the mixture;

the tlme elapse from lnltlal mutual contact of the vlscous llqulds to expulslon of the mlxture from the maln mixing chamber belng not greater than one mlnute.

Preferably, the process of the present lnvention is conducted at room or ~lightly elevated temperatures, eg. from 15C to 32C.

The process of the present invention has its primary application in the production of foam polysilicone products, in which the first highly vlscous liquld comprises a polydiorganosiloxane composltion and the ~econd lower vlscosity liquid comprlses a polydiorganosiloxane curative, at least one of the two liqulds slso lncludlng a catalyst so as to create WO92/1459~ PCT/GB92/00266 - 6 - ~?
a s conditlons under which the polydiorganosiloxanes are lnter-reactable on contact, with generation of hydrogen to form a cured, hlgh molecular weight polysilicone foam. In ~uch a process, one of the polydiorganosiloxanes has vlnyl groups therein, and the other has hydrlde groups. In the presence of a suitable complex platlnum catalyst, these materials lnteract on contact very rapidly, 60 that in the present lnvention the lnter-reactlve mlxtures are kept separate untll they are carefully contacted ln the preliminary mixlng chamber, to which they are fed through separate lnlet ports. ~he reactlon between them to generate hydrogen gas, used as the in sltu blowing agent, ls ~llghtly exothermic. Nevertheless, in accordance with the present inventlon, the temperature throughout the reacting mixture can be kept substantlally uniform and within the range of 15- 32C, preferably 26- 28C, by sultable adJustment of throughput rates and mixing speeds.

In the preferred embodiment of the process of the present lnventlon, the maln mixlng chamber ln which the mixture ls sub~ected to severe shearlng agltating condltlons comprises a dynamic rotary mlxer wlth a multlpllclty of ~ub-chambers, each of the sub-chambers havlng a perforated rotor mounted to rotate about a substantlally horlzontal axls, and a perforated stator dlsposed ln close tolerances to the rotor to deflne tortuous paths of travel of the mixture through the chamber, along with W092/14595 ~ Y PcT/GB92/oo266 continuous subdivision and recombination of the portions of the mixture. Preferably also, the direction of tra~el of the mixture through the main mixing chamber is predominantly axlal with respect to the rotor axis, and the direction of expulslon from the maln mlxing chamber ls radially upwards, most preferably ~ubstantlally vertlcally upwards. This effectively prevents suck back of alr into the mlxing chamber to mix with the reaction mixture as the process proceeds.

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

A ~pecific preferred embodlment of the invention will now be described, with reference to the accompanying drawings, in which:

FIGURE l ls a diagrammatic process flow sheet of the overall process;

FIGURE 2 ls a perspective vlew of the mlxer and associated parts used in this specific embodiment of the process;

WO92/14595 ~ PCT/GB92~00266 ~0~139 - 8 - ~

FIGURE 3 ls a vertical cross ~ectional view, with parts cut away, through the center of the mixer shown in Flgure 2.

In the drawings, llke reference numerals indicate like parts.

With reference flrst to Flg. l, a rotary dynamic forced shear mixer lO ls utilizad for the continuous mixing of liquid components inter-reactable to form polysillcone foam. The reactants include a polydiorganosiloxane with vinyl terminal groups combined wlth appropriate amounts of inorganic filler (appropriately ~ilica), complex platinum catalyst, water and reactlve diluent. This reactant mixture, hereinafter "resin part A", is mixed to a substantially homogeneous liquid composition ln a pre-mix resin tank 12 equipped with agitators 14, 16. This resin part A ls highly viscous, e.~. of the order of 45,000 -lO0,000 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, lt can be pumped AS required, by means of second resin transfer pump 26 and part A inlet line 28 to m~xer lO. This flow can be controlled by ~olenoid operated valve 30.
A by-pass line 32 leading dlrectly from pre-mix resin tank 12 to part A inlet line 28 under control of another solenold operated valve 34 is also provlded, so that if desired resin part A can be W092/14595 2 1 ~ ~1 0 9 PCT/GB92/00266 . _ g _ .

directly fed to mixer 10 wlthout passing through resin holding tank 24. A further solenold operated control valv~ 36 ls provlded downstream of the Junctlon of by-pass llne 32 and part A
lnlet line 28, for flow control of resln part A immediately upstream of the mixer 10.

The ~econd liquid for mlxlng, resln part B or curative, comprlses a polydlorganoslloxane with hydride groups. It ls of much lower vlscosity than resln part A, e.g. of the order of 1000 - 1400 cps. Slnce thls is lnter-reactable wlth resln part A on contact, it is kept ln a separate curatlve tank 38 and fed through a separate curative llne 40 to the mlxer 10. Llne 40 is provlded with a curative pump 42 and solenold operated control valve 44. A pressure relief, safety valve 46 ls provlded ln curative llne 40. A valve controlled draln outlet 48 ls also provlded, whlch can be arranged to draln curatlve tank 38 and curatlve line 40 as and when requlred.

A solvent tank 50 is also provlded, connected to mixer 10 by solvent llne 52. Thls ls slmilarly provlded wlth a solvent pump 54, ~afety valve 56, drain 58 and solenold operated control valve 60. The solvent ~s not used during the mixlng process ltself, but only for purposes of washlng and flushlng the mixer 10 after operation.

a 9 - 10 - ~

The general arrangement also includes a purge llne 62 by means of which air can be supplied to the mixer 10 to dry the lnternal parts after solvent washing. The purge line 62 is provided with sn 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 termlnates in a downwardly extending flexible conduit 72 which ln operation oscillates ~lowly from ~ide to side, to deposit foaming product evenly into forms 74 passin~
therebeneath on a continuously movlng conveyer 76.

With reference to Fig. 2, the mixer 10 ls generally cyllndrical, and mounted in horizontal disposition. At its upstream end, lt has a clrcular face plate 78 ln which are provlded three separate lnlet portsD The flrst lnlet port 80 is of relatively large diameter (approximately 1~ lnches) and ls connected to part A lnlet line 28. The second lnlet port 82 ls of ~maller dlameter (eg. ~ inch) and ls connected to curative line 40. The third inlet port 84 i8 connected to the purge line 62 and is approxlmately ~ lnch d..ameter. As shown and descrlbed in Fig. 3, the mlxer 10 has a central rotary shaft dlsposed along a substantially horizontal axis and an sppropriate drive train W092/14595 ~ 9 PCT/~B92/002~6 .. I

lncluding a reductlon gear ~ox 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 vertlcal cross section through the mixer 10, with various parts cut away for clarity of illustration. The separate lnlet 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 rotatlon shaft g2 to whlch they are secured.

Downstream of the chamber 8B, the mixer 10 has a main mix~ng chamber generally designated 94, effectlvely divided into three sub-chambers by rotor and stator arrangements. The first sub-chamber 96, extendlng clrcumferentially around shaft 92, contalns a ~tator 98 lntegral with the outer cylindrical wall 100 and connected thereto by a perforated ring portion 102 of relatively thln dlmension. The upstream boundary of sub-chamber 96 is formed by upstream rlng-like rotor 104 which has a series of apertures 106 therethrough providing communlcation between preliminary mixing chamber 88 and sub-chamber 96. Rotor 104 is provided, at lts radially outward edge, with a ~eries of vanes 108, protruding downstream into close proximity wlth the ~lo~las apertured ring portion 102 of the stator. The downstream boundary of sub-chamber 96 ls formed by the upstream side of flrst central rotor 110. This rotor 110 ls essentlally s$milar to rotor 104, being apertured near its radially inner portlon but having an upstream presented series of vanes 112 and 8 downstream presented serles of vanes 114 on lts downstream side. The next sub-chamber 116 is slmllarly provided wlth a stator 118 essentlally the same ln ~11 respects as stator 98. Sub-chamber 116 ls bounded by rotor 110 and second central rotor 120, essentlally the same ln all respects to flrst central rotor 110.
The next sub-chamber 122 is similarly provided with a stator 124 o ~he same construction, and ls bounded at lts downstream end by a rotor 126 essentially identical to rotor 104 but with upstream-extending vanes 128. Thus, maln m~xlng chamber 112 comprised of the three sub-chamber~ 96, 116 and 122 ls essentlally closed so that pressure can be exerted on the contents therein.

Sub chamber 122 communlcates wlth an exlt chamber 130 ln whlch are rotated perforated paddles 132 slmilar to the paddles 90 ln the premix chamber 88. The vertlcally upwardly extendlng dlscharge outlet 70 communlcates wlth exlt chamber 130.
The premix chamber 38 and the exit chamber 130 ~re provlded with respective drain outlets 134 and 136, for washing, flushing and purging purposes.

2 1 ~ 9 W092tl459~ 13 PCT/GW2/00266 -- ,t In operation, the mixer lO ls lnltially flushed by pumpirg thereto solvent, suitably trlchloroethylene, from solvent tan~ 50 vla solvent llne 52 through third inlet port 84 lnto the mixer, to clean lt thoroughly of resldues of prevlous operatlons and other contaminants. Residual ~olvent ls drained through draln outlets 134 and 136, and then the mlxer ls purged by blowing air through lt, vla purge llne 62 and lnlet port 84.

Then the apparatus ls ready for use in the manufacture of foamed polysillcone. Resin part A premlxed ln premlx resin tank 12 and stored ln resin holdlng tank 24 ls pumped to the mlxer, and at the same tlme the resin part B from the curative tan~ 38 is pumped to the mlxer. The overall rates of flow of the resins and the relative rates of flow of the resin part A and the resin part ~ are carefully controlled to predetermined values using solenold operated con-trol valves 30, 36 and 44. The shaft 92 of the mixer 10 ls rotated at relatlvely hlgh speeds (for example 200 r.p.m.). The resln part A and the resln part B are thus lnltlally mutually contacted ln premlx chamber 88 where they are lntlmately mixed together by the actlon of rotatlng paddles 90, but under condltions of relatlvely mlld shear.

Then, under the lnfluence of lnfeed pumps 26, 42, the mixture is forced through apertures 106 in upstream rotor 104, and thence lnto the flrst sub-chamber 96 of the main mlxlng WO92/1459~ - 14 - PCT/GB92/00266 chamber 94. Here, the mlxture encounters the actlon of the rotors 104 and llO, and the stator 98, 80 that lt i~ subJected to hlgh ~h~ar mixlng condltlons, whilst at the ~ame time being maintalned under pressure. Hydrogen gas produced as a reactlon product ls consequently held wlthin the body of the liqu~d resin mlxture, and cannot escape therefrom. The resln mlxture follows a tortuous path through chamber 96, through the perforations ln 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 portlon of chamber 96. The passage of the resln through the apertures in the rotor and the stator ensures sub-divlslon of the mixture into small quantities and recomblnatlon thereof, repeatedly, as lt moves through the chamber 96. Slmilar condltlons of severe shearlng agltatlon, wlth ~ub-dlvlslon and recom~ination of mixture portlons, followlng similarly tortuous paths, are encountered by the mixture 8S lt proceeds through rotor llO, into chamber 116, through ~tator 118 and rotor 120 into chamber 122, and out through rotor 126 lnto exlt chamber 130.

There the mlxture ls sub~ected to only mlld agltatlon condltions by perforated paddles 138, prior to belng expelled vertically upwardly through outlet 70. Thls perlod of mlld agltatlon lmmedlately prlor to discharge has an advantageous effect on the foam expanslon process. As lt enter~ outlet 70, WO92/1459~ ~lV'i~ ~ 9 PCT/GB92/00266 ~'f; - 15 -. ., .:

the pressure on the mlxture ~tarts to be relieved, so that foam expanslon of the mixture ~tart~, due to the evolution and escape of the hydrogen gas. The mixture proceeds down through flexible condult: 72 to pour out in a foamlng condltion, into moulds 74 on conveyor 76. The outlet 72 is arranged to osclllate from side to slde across the forms 74, so as to flll them evenly with foaming mixture. Foam expanslon of the product continues ln the form 74, whilst the gelllng process also occurs, eventually limiting the foam expansion. Then the forms contalnlng slabs of foam polysilicone are fed into an oven, to complete the curlng process.

The temperature of the mlxture and the flnal resln is monltored and controlled as the process proceeds, by standard means, to a range of 26 - 28~C. Care i8 taken to ensure that the temperature does not rlse above 32-C, where severe problems of premature curing of the foam, resultlng ln plugglng of the mixer may occur. ~he tlme whlch elapses from the entry of the resin part A and resln part B lnto the prelimlnary mlxing chamber 88 until the reactlng mlxture enters outlet 70 ls less than 30 seconds.

The provision of the by-pass llne 64 from the resln mixlng tank 12 directly to the mlxer 10 provldes a valuable option, in allowlng dlrect feed of resln part A to the mlxer lO.

WO92/14~9; ; PCT/GB92/00266 l 9 `"`
Whilst in normal operation it s convenient to hold pre-mixed resin part A ln resin holding tank 24 ~o that the preparation of the resin part A mixture is kept essentially separate from the mixlng snd lnter-reaction of resin part A and resin part 8, to allow for further testing of the resin part A mixture prior to use, there are occasions when this ls not necessary or ls indeed dlsadvantageous. For example, throughput demands of the mixer, lnstabillty 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 lO, in the absence of by-pass llne 32 and associated parts.
Accordingly, the provlsion of thls by-pass facility helps to ensure continous productlon of product ln the event of d~fflculties.

The process as descrlbed and lllustrated can be run cont~nuously over extended periods of time. Once mixing and production of foam ceases valves from part A lnlet llne 28 and curative line 40 are shut-off and solvent from ~olvent tank 50 and line 52 is provided, thoroughly to wash the mlxer and remove silicone residues therefrom, before they have chance to gel and cure to provide obstructlons ln the mixer. As noted, a sultable solvent for this purpose ls trlchloroethylene. After thls Eolvent wash and flush, the solvent ls dralned from the mixer and WO92/14595 21~ 9 PCT/GB92/00266 then the mixer is purged with air, through line 62, to dry the mixture snd remove solvent resldues, so that it ls ready for a restart wlth fresh quantities of resln.

Claims (10)

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 compris-ing:

continuously introducing said at least two vis-cous 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 main-taining 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 composition from the main mixing chamber in a radially upward direction 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.

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 polysilicon 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.

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 continu-ously 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.