Patent Application
20120250451
1. Technical Field
The invention relates to a method of homogenizing a liquid, in particular a colorant for paint, in a container. The invention further relates to an assembly for dispensing a liquid comprising a container and a pump and to an apparatus comprising a plurality of such assemblies.
2. Background of the Related Art
Brownian motion of particles suspended in liquids is not sufficient to overcome gravitational forces and such particles tend to settle out of the liquid and agglomerate. The paint industry has struggled with this problem for more than a hundred years. It is still widely believed within this industry that in order for a pigmented colorant to be successfully applied using colorant dispensers the dispersion must be stirred. This requirement places stringent demands on the manufacturers of dispensers, especially when colorants contain a high loading of pigment.
EP 2 198 950 relates to an apparatus (denoted by numeral 1 in the Figures of that publication) for dispensing a plurality of fluids, comprising a support, such as a turntable (2) or a linear table, a plurality of containers (4) for holding a fluid mounted on the support (2), pumps (3) connected to respective containers (4), a common actuator (31) for sequentially operating the pumps (3), stirring elements mounted rotatably inside and extending from the containers (4), and a drive mechanism (21) for rotating the stirring elements, wherein the support (2) on the one hand and the actuator (31) and drive mechanism (21) on the other are movable relative to each other. The drive mechanism (21) comprises a protrusion (26) movable between at least a first, extended position (
WO 2008/110606 relates to a canister assembly comprising an annular outer shell (21) with an outer cylindrical wall connected to a bottom which, in turn, is connected to an inner cylindrical wall that serves as a standpipe. The bottom includes an outlet (29) connected to a pump. A hollow agitator shaft (21) slides over the inner standpipe and includes a plurality of outwardly extending blades (32) for stirring or “agitating” the fluid.
EP 1 908 510 relates to a device for mixing fluids, particularly paints and varnishes. The Figures of this publication shows a storage tank (2) containing a stirring element. GB 1,004,721 also discloses stirring devices.
In practice, stirrers get contaminated, the colorants will dry in, and over time a buildup of thin layers of colorant will form on the stirrers. The dried layers can be a serious cause of contamination when pieces of the layers break away and drop in the colorant. This might even cause blocking of a downstream pump or contamination of the paint.
Another disadvantage is that stirrers inevitably cause a lot of interaction between the colorant and the atmosphere above the colorant, resulting in evaporation of solvent and in oxidation.
It is an object of the present invention to provide a method of homogenizing a liquid, in particular a colorant for paint, in a container, that does not require a stirrer.
The invention relates to a method of homogenizing a liquid, in particular a colorant for paint, in a container, comprising the steps of withdrawing liquid from the container at a first flow speed (S1) and then returning the liquid to the container at a second flow speed (S2), higher than the first flow speed (S1).
These measures were found to result in effective mixing, even without the use a stirrer. In an embodiment, part, preferably most of the liquid is returned in a direction having an axial component, preferably returned along the central axis of the container.
In a further embodiment the second flow speed (S2) is in excess of 0.2 A*(V1/2)/M, where M is the percentage of the volume of the liquid withdrawn relative to the maximum volume of the container, V is the percentage of the height of the column of the liquid in the container (before withdrawing) relative to the height of the container, and A is a correction for viscosity and in range of 0.6 to 1.4, preferably in a range from 0.8 to 1.2. In yet another embodiment, an amount of at least 5% preferably at least 10% of the content of the container is withdrawn from and returned to the container, typically no more than 50% of the content of the container.
Further, the liquid is preferably returned at a pressure in a range from 2 to 10 bar, more specifically 3 to 8 bar, more specifically 4 to 6 bar.
By adjusting the flow speed to the viscosity of the colorant in such a way that a pulse flow is created at the exit of the conduit connecting the container to a pump, the colorant will flow upwards trough the centre of the container and reach the top portion of the liquid in the container. To avoid that the returned colorant will break through the colorant surface in the container, the pump stroke and pump speed can be adjusted to the actual colorant level and colorant specification (density, viscosity, reologic structure, etc.).
To efficiently homogenize the whole contents of the container, it is preferred to withdraw liquid from the lower half, preferably the bottom portion of the liquid in the container and return it to the upper half, preferably the top portion of the liquid in the container.
Another embodiment comprises consecutively repeating the steps of withdrawing liquid from the container and then returning the liquid to the container until a volume corresponding to at least 50%, preferably at least 70% of the liquid in the container has been withdrawn and returned. This procedure can be carried out e.g. between dispensing paint recipes or when the dispenser has been idle for a pre-selected period, e.g. six hours.
To compensate for variations in viscosity resulting from, e.g., variations in temperature, in an embodiment, the method comprises establishing, e.g. measuring, calculating or retrieving from a database, the viscosity of the liquid in the container and adjusting the first and/or second flow speeds depending on the viscosity.
The invention further relates to an assembly for dispensing a liquid comprising a container for holding a liquid, a pump connected to the container via at least one conduit and a controller for operating the pump, wherein the controller is arranged to operate the pump to withdraw liquid from the container at a first flow speed (S1) and then return the liquid to the container at a second flow speed (S2), higher than the first flow speed (S1).
In an embodiment, the conduit is oriented in a direction having a component extending along the central axis of the container, i.e. an axial component, and preferably extends coaxial with the central axis of the container.
In another embodiment no stirrer is present in the container.
The invention further relates to an apparatus for dispensing a plurality of liquids, in particular colorants for paint, comprising a plurality of assemblies in accordance with the invention as described above.
Within the framework of the present invention the term “liquid” is defined as any flowable material that comprises a liquid phase and thus includes suspensions and emulsions. Colorants often contain high density pigments or particles suspended in (instable) liquid binder/solvent solutions.
The disclosed method, assembly, and apparatus will be further explained with reference to the accompanying drawings in which presently preferred embodiments of the invention are shown schematically.
It is noted that the drawings are not necessarily to scale and that details, which are not necessary for understanding the present invention, may have been omitted.
This particular dispensing apparatus 1 is an automated version and includes a horizontal turntable 2, with a plurality of containers 3 mounted along its circumference. Each container 3 is provided with a pump 4 and a valve 5 (
The turntable 2 can be rotated between discrete positions, e.g. twelve or sixteen positions including a dispensing position, i.e. a position where the pumps and valves are operated by means of a central actuator. Liquids are dispensed in a receptacle, in this example a bucket 6 on an adjustable shelf 7. The apparatus 1 includes a base 8 made e.g. by injection moulding a polymer. A computer 9 for entering and storing information, such as customer data and recipes, and generating instructions for driving the turntable 2, pumps and valves, is positioned on a separate stand 10.
Further information regarding suitable turntables and procedures for driving the various components, are disclosed in, for instance, European patent applications EP 800 858, EP 1 492 970, EP 1 688 652, and EP 2 198 950.
As shown in
An example of the method of the present invention is illustrated in
In
Homogenization can be further enhanced in various ways.
The assembly shown in
The area and length of the central opening on the one hand and the area and length of the radial opening, the diameter, D1, of the top surface, and the inner diameter, D2, of the container at the axial position of the structure, on the other hand, have been selected to increase the amount of liquid that is directed in axial direction during the returning of the liquid at the second speed.
In general, good results were obtained with structures that fulfil the following criterion:
(D22−D12)/D1=C/π
where D1 and D2 are the diameters defined above and C is in a range from 3 to 300. In the example shown in
The method and assembly of the present invention provide efficacious homogenization of colorants in containers, obviating the need for a stirrer.
As a matter of course, the invention is not restricted to the above-disclosed embodiment and can be varied in numerous ways within the scope of the claims. E.g., the container can made of a rigid material or of a flexible material. Flexibility of the container and/or a bag inside the container enables the container to compensate for the amount of liquid that is withdrawn temporarily.
