Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2003-03-11
2004-07-27
Thexton, Matthew A. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S586000, C524S604000, C524S605000, C366S077000
Reexamination Certificate
active
06767956
ABSTRACT:
This invention relates to processes for producing aqueous dispersions of finely divided polymer, and for making powders from these dispersions, to aqueous dispersions made by the process and to powders made from the dispersions. It also relates to coating compositions comprising the finely divided polymer, to a coating process using the compositions and to a coated substrate resulting from the process.
Finely divided solid polymers, either in powder form or as dispersions in a carrier liquid are useful for a number of applications including coatings. For use in coatings and adhesives, it is important that the polymer be very finely divided so that when the powder or dispersion is applied to a substrate surface, it can be heated and melted into a thin film with sufficient flow-out to give a smooth coating.
Grinding solid polymers into fine powders is inconvenient and costly. Only polymers that are brittle and amenable to dry milling when cold can be ground up efficiently. In particular, the grinding process itself generates heat which must be removed by cooling to avoid the polymer melting and fusing into a solid mass again. Also, grinding results in irregularly shaped particles and in a broad size distribution, neither of which are ideal for coatings.
As an alternative to grinding, it is known that dispersions of polymers which are solid at normal temperatures can be made in certain carrier liquids by using an extruder, for example as shown in U.S. Pat. No. 5,124,073. Extruders are conveying single or multi-screw mixers. The use of an extruder enables polymers of high viscosity to be readily dispersed in liquids in a continuous process. In general the polymer is fed into the extruder in pellet form and is heated as it passes down the extruder barrel at least until it becomes sufficiently liquid to be dispersed. The carrier liquid and a dispersant are added through an inlet port along the extruder barrel at a point at which the polymer is liquid enough to disperse. The combined effect of heating the polymer so that it is liquid, and the shear applied by the extruder in the presence of the carrier liquid and dispersant results in the formation of fine polymer dispersions. These can be cooled to solidify the polymer once more so as to produce a dispersion of solid polymer particles in the liquid carrier. When stable dispersions are required, then a stabiliser is usually included in the mixture, usually in the carrier liquid. Alternatively, where powders are desired, the polymer can be allowed to settle out and centrifuged and filtered off and dried. The result of such a known process is spherical particles of polymer of a reasonably uniform size.
U.S. Pat. No. 4,996,259 describes aqueous synthetic wax dispersions which can be used as coating films, and which comprise a partially or completely neutralised copolymer of a C
2
-C
4
olefin, and an unsaturated mono or dicarboxylic acid or dicarboxylic anhydride. The preparation is carried out by neutralising and dispersing the solid material in water in an extruder.
There are, however, limitations to this extrusion process. Clearly, the mixture can not be heated above the boiling point of the carrier liquid, or else the liquid boils and it becomes impossible to disperse the polymer. This means that the boiling point of the carrier liquid must be a safe margin above the temperature at which the polymer is sufficiently liquid to disperse. The process by which the polymer becomes liquid enough to disperse can be generally referred to as melting and the temperature at which the polymer is liquid enough to disperse can be generally referred to as its melting point (Tm). Many polymers that are useful for coatings have melting points above about 90° C. and some above 150° C., for example crystalline polyesters melt at around 120 to 250° C. For these polymers the use of the known extruder process to produce dispersions in water is not possible.
EP-A-0 246 729 describes the use of an extruder to produce what are called aqueous dispersions at extruder temperatures above 100° C. The types of compositions made in this process comprise a polymer and only 3 to 25% by weight of water. These compositions are apparently solid, and are not free-flowing aqueous dispersions suitable for coating compositions. The apparatus used comprises a heated extruder which has a cooling device at its outlet. The combination of the high level of viscous resin and the cooling device appears to create a viscous plug at the extruder outlet, and this plug causes the pressure inside the heated portion of the extruder to rise so as to prevent the water from boiling. Clearly, this process cannot be used when the dispersions produced by the process are of lower viscosity, because no plug would be formed. Such lower viscosity dispersions are generally those with a higher level of water.
The present process represents a modification of the known extrusion processes to allow the production of novel aqueous fine liquid dispersions of polymers having a melting point close to or above 100° C., for example above 90° C.
According to the present invention there is provided a process for producing an aqueous dispersion of a polymer in an aqueous medium, the dispersion having a Brookfield viscosity below 10 Pa.s, in which the polymer is dispersed in an aqueous medium in an extruder at a temperature above 100° C., the pressure inside the extruder being maintained above atmospheric pressure so that the aqueous medium does not boil inside the extruder.
Preferably the dispersion has a viscosity of less than 5 Pa.s and more preferably below 1 Pa.s.
Describing a typical process, the polymer is metered in the form of pellets into the intake of the extruder, where it is melted in the initial melt zone of the extruder at a temperature above the melting point of the polymer, preferably from 5 to 150° C., typically 10 to 130° C., above the melting point. Any other non-volatile components required in the composition, such as pigments or other additives can be fed in along with the polymer.
As the material passes down the extruder barrel, the melt zone is followed by one or more inlet ports through which aqueous medium is injected under pressure, optionally along with a dispersant. In the portion of the extruder following the inlet ports the mixture of aqueous medium and polymer are subjected to shear in the dispersion zone so as to form a dispersion. The aqueous medium can be injected in one portion to produce a dispersion of the polymer in the aqueous medium. Alternatively and preferably, initially a smaller proportion of aqueous medium can be injected, typically along with dispersant, which is believed to form a dispersion of the aqueous medium in the polymer, and this can be followed by more aqueous medium so as to make the required dispersion of polymer in aqueous medium.
At this stage, the polymer is molten and the dispersion comprises droplets of dispersed polymer. Finally, a cooling zone is used to lower the temperature of the dispersion below the melting point, Tm, of the polymer so as to produce a dispersion of solid polymer particles in the aqueous medium.
The pressure in the extruder can be maintained by putting a pressure control device such as a pressure relief valve at the extruder outlet. Alternatively, and preferably, the pressure can be maintained by connecting the outlet of the extruder to a pressurised collection vessel. The use of a pressurised collection vessel is preferred because pressure relief valves are prone to blocking up because of the small exit aperture.
The pressure inside the extruder is maintained so as to prevent the water in the aqueous medium from boiling and so the exact pressure will depend on the temperature at which the dispersion is carried out. The pressure required to prevent water from boiling at various temperatures above 100° C. is well known or can easily be calculated. For example a pressure of at least 3 bar at 130° C., of at least 5 bar at 150° C., and at least 10 bar at 180° C. will prevent the water from boiling. The exact pressure can be chosen
Choudhery Riaz Ahmad
Downing Stephen Barry
Murray Kenneth Andrew
Southgate David John
Imperial Chemical Industries plc
Stachel Kenneth J.
Thexton Matthew A.
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