Abstract:
The method of producing a colored powder of a polymeric material includes the steps of selecting a feedstock of said polymeric material, pulverizing said polymeric material in a pulverizer to produce a powder, moving the powder directly from the pulverizer to a mixer; spraying a liquid formulation including a colorant into the powder within the mixer, and mixing the liquid formulation and powder.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This claims the benefit of U.S. Provisional Application No. 61/432,777, titled IN-LINE COLOR MIXER, filed Jan. 14, 2011, and is a divisional of U.S. patent application Ser. No. 13/350,078, filed Jan. 13, 2012, each of which is herein incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     A. Field of Invention 
     This invention relates to a polymeric material. Preferred embodiments relate to coloring of a powderous polymeric material for use in making components by melt-processing, for example by rotational molding. 
     Rotational molding is used for the production of hollow containers and other products of large size and complicated structure which cannot be readily molded by other conventional molding techniques such as injection molding and sheet thermoforming. Such structures include, for example, gasoline tanks, casks, storage tanks and toys. 
     Rotational molding includes the steps of feeding a powder comprising polymeric material to a mold, heating the mold from the exterior while rotating the mold and melting the powder, thereby forming a molten synthetic polymeric material layer within the mold, then cooling the mold to solidify the material and finally separating the mold from the molded product. 
     B. Description of the Related Art 
     To color the polymeric material powder prior to molding, it may be mixed with one or more pigments in dry powder and/or solids form. However, such solid materials can be difficult to handle and can mix unevenly with the polymeric material powder leading to inconsistent color distribution in the final molded product. Also, such a process is a time-consuming discontinuous process. Attempts have been made to use liquid color formulations. However, there are also problems associated with use of such formulations. For example, the formulations may be added during pulverization of polymeric material to produce the polymeric material powder. However, disadvantageously, this necessitates extensive and prolonged cleaning of the pulverization apparatus should it be desired to change the color of the polymeric material powder to be produced. This is generally uneconomic. Furthermore, it is challenging to homogenously color the polymeric material. It is an object of the present invention is to address the above-described problems. 
     SUMMARY 
     It is an object of the present invention to address problems associated with coloring powderous polymeric material for use in rotational molding. 
     According to a first aspect of the invention, there is provided a method of producing a colored powder of a polymeric material, the method comprising: 
     (a) selecting a feedstock of said polymeric material; 
     (b) pulverizing said polymeric material in a pulverizer to produce a powder; 
     (c) passing the powder directly from the pulverizer to a mixer; 
     (d) spraying a liquid formulation comprising a colorant into the powder in the mixer; and 
     (e) mixing the liquid formulation and powder. 
     Said feedstock may comprise pellets or granules of polymeric material. Said feedstock suitably does not include a colorant; it may be substantially uncolored. It may comprise virgin polymeric material. 
     Said polymeric material may comprise a thermoplastic polymer. It may comprise any polymeric material usable in rotational molding, for example a polyolefin such as polyethylene or polypropylene. 
     Said pulverizer may be a crusher, grinder, chopper masher, hammer, mill, pounder or other means to grind the polymeric material into a dust or powder. Said pulverizer may be of a standard design. It may be used to comminute the polymeric material to produce a powder. It may include means (for example a sieve device) for selecting pulverized polymeric material of predetermined particle size characteristics (e.g. particle sizes less than a specified value and/or within a specified particle size range). 
     Said pulverizer may be arranged to increase the temperature of the polymeric material during pulverization. For example, the difference between the temperature of said polymeric material entering the pulverizer and the temperature of said powder leaving the pulverizer may be at least 10° C., suitably at least 20° C. The difference may be at least 30° C. or at least 40° C. 
     An outlet of the pulverizer is preferably coupled to an inlet of the mixer. Thus, the pulverizer is suitably arranged to deliver powder directly into the mixer. The linear distance between a mixing chamber of the mixer and the outlet of the pulverizer may be less than 50 cm, less than 40 cm, less than 30 cm, less than 20 cm or less than 10 cm. 
     Step (c) preferably includes continuously delivering powder from the pulverizer to the mixer. 
     Preferably, the temperature of powder delivered to the mixer is above ambient temperature. It may be at least 10° C., at least 20° C. or at least 30° C. above ambient temperature. Suitably, the temperature of the powder is above ambient temperature by virtue of the polymeric material becoming heated in the pulverizer and at least some of the heat being retained after the powder has passed from the pulverizer to the mixer. 
     The difference between the temperature of powder at an outlet of the pulverizer and the temperature of powder at an inlet of the mixer (and/or immediately downstream of an inlet of the mixer, for example in a mixer chamber of the mixer) is suitably less than 40° C., preferably less than 30° C., more preferably less than 20° C., especially less than 10° C. 
     The temperature of the powder at the outlet of the pulverizer may be at least 30° C., at least 40° C. or at least 50° C. It is preferably at least 20° C. (or at least 40° C.) less than the glass transition temperature (Tg) of the polymeric material. 
     The temperature of the powder at the inlet of the mixer may be at least 30° C., at least 40° C. or at least 50° C. It is preferably at least 20° C. (or at least 40° C.) less than the glass transition temperature (Tg) of the polymeric material. 
     Said mixer may be a churn, blender, beater, agitator or other means to mix a liquid with a powder. Said mixer suitably comprises a mechanical mixer which is suitably arranged to produce a fluidized bed in which the powder and liquid formulation can be intimately mixed. The mixer may comprise a paddle mixer. The mixer suitably has an inlet for receiving powder from the pulverizer and an outlet for directing colored powder away from a mixing chamber of the mixer, for example to a storage location. Said mixer is suitably a continuous mixing device (e.g. it is not a batch mixer) which is suitably arranged to continuously convey material between its inlet and outlet. Thus, in the method, polymeric material may pass continuously between an inlet of the pulverizer and an outlet of the mixer, during which time it is colored. 
     In step (d), liquid formulation is suitably sprayed directly at the powder, suitably avoiding as far as possible, contacting a wall which defines a mixing chamber of the mixer. Preferably, droplets of liquid formulation are sprayed. Suitably, a spray device is used in step (d). It may comprise an atomizing spray device arranged to atomize the liquid formulation, an atomizer, vaporizer, mister, sprinkler, nebulizer, or any other suitable liquid dispenser. In order to facilitate substantially homogenous mixing of liquid formulation and powder (while avoiding clumping of colorant and/or overly concentrated regions of colorant) the spraying in step (d) is suitably carried out at relatively low pressure. For example, the spray device may be arranged to atomize the liquid formulation using less than 50 psi, preferably less than 35 psi, more preferably less than 25 psi, especially less than 15 psi of pressure. This may produce a softer application of the liquid formulation to the powder and may avoid the formulation being jetted through a mass of powder thereby contacting the walls of the mixing chamber. If liquid formulation is allowed to build up on walls of the mixing chamber, there is a tendency for occasional highly concentrated regions of color to develop within the powder which is undesirable. 
     In step (e), the liquid formulation and powder are suitably mixed intimately and conveyed to an outlet of the mixer. 
     Said liquid formulation preferably comprises a vehicle and a colorant. The vehicle is suitably arranged to carry the colorant and largely be removed after delivery into the powder, during mixing and/or in downstream processing (e.g. melt-processing) of the colored powder. 
     The liquid formulation may have a viscosity measured as described herein in the range 100-50,000 cP. 
     Said colorant may be dissolved or dispersed in the vehicles. It may comprise a pigment or dye. 
     Said vehicle is suitably selected so that some may evaporate on and/or after contact with the powder in step (d) due to residual heat present in the powder as a result of pulverization in step (b). 
     Said vehicle suitably has a boiling point of less than 120° C. or less than 110° C. at STP. Preferably, said vehicle comprises water. It may consist essentially of water. Advantageously, some of the water will evaporate on contact of the liquid formulation and powder which may facilitate coating of the powder with the liquid formulation and/or colorant. Said liquid formulation may include at least 30 wt % of water. 
     According to a second aspect of the invention, there is provided apparatus for use in the method of the first aspect, the apparatus comprising: 
     (i) a pulverizer for producing a powder; 
     (ii) a mixer for mixing powder with a liquid formulation; 
     (iii) a spray device for spraying a liquid formulation into powder within the mixer; 
     wherein an outlet of the pulverizer is coupled to an inlet of the mixer and the pulverizer is arranged to deliver powder directly into the mixer. 
     The apparatus may have any feature described in accordance with the first aspect. 
     The apparatus may include a feedstock supply containing feedstock and being arranged to deliver said feedstock to said pulverizer. 
     The apparatus may include a liquid formulation supply, for example a receptacle, containing liquid formulation, said supply being operatively connected to the spray device. 
     According to a third aspect of the invention, there is provided a method of producing a component which comprises: 
     (A) selecting a colored powder made as described according to the first aspect and/or using an apparatus according to the second aspect; 
     (B) melt-processing the colored powder to produce said component. 
     Step (B) preferably comprises rotational molding of said colored powder. 
     The invention extends to a component produced in the method of the third aspect per se. 
     According to a fourth aspect of the invention, there is provided apparatus for differently coloring respective powders, the apparatus comprising: 
     (1) a pulverizer for producing a powder; 
     (2) a first combination comprising a mixer for mixing powder with liquid formulation and a spray device for spraying a liquid formulation into powder within the mixer, said first combination being associated with, for example dedicated to, a first colorant; 
     (3) a second combination comprising a mixer for mixing powder with liquid formulation and a spray device for spraying a liquid formulation into powder within the mixer, said second combination being associated with, for example dedicated to, a second colorant. 
     Advantageously, the apparatus may allow color changes to be undertaken rapidly with minimal cleaning, since the pulverizer contains uncolored powder in use and the first and second combinations may be selected according to the desired color for the powder. 
     Said apparatus may include a third combination which is substantially the same as the first combination except it is associated with, for example dedicated to, a third colorant, different to that of the first and second colorants. 
     Fourth and/or fifth and/or sixth combinations may be provided for different colorants mutatis mutandis. 
     Any invention described herein may be combined with any feature of any other invention or embodiment described herein mutatis mutandis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may take physical form in certain parts and arrangement of parts, embodiments of which will be described in detail in this specification and illustrated in the accompanying drawings which form a part hereof and wherein: 
         FIG. 1  which is a schematic representation of an apparatus for producing a colored powder, according to one embodiment; 
         FIG. 2  is a perspective view of an apparatus for producing a colored powder, according to one embodiment; 
         FIG. 3  is a perspective view of a paddle and paddle rod located within a mixer; according to one embodiment 
         FIG. 4  is a cross-sectional view of mixer, according to one embodiment; 
         FIG. 5  is a top view of a paddle and paddle rod, according to one embodiment; 
         FIG. 6  is a side view of a paddle and paddle rod, according to one embodiment; 
         FIG. 7  is a perspective view of a shaft located within a mixer, according to one embodiment; 
         FIG. 8  is a perspective view of an interior of a mixer, according to one embodiment; 
         FIG. 9  is a side view of a mixer, according to one embodiment; 
         FIG. 10  is a top view of a mixer, according to one embodiment; 
         FIG. 11  is a perspective view of an apparatus for producing a colored powder, according to one embodiment; and 
         FIG. 12  is a perspective view of an apparatus for producing a colored powder, according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings wherein the showings are for purposes of illustrating embodiments of the invention only and not for purposes of limiting the same, and wherein like reference numerals are understood to refer to like components,  FIGS. 1-10  show an apparatus  20  including a pulverizer  30  which is arranged to deliver uncolored pulverized polymer powder via its outlet  40  directly into a mixer  60  via its inlet  80 . A spray device  100  is operatively connected to the mixer  60  and is arranged to produce a spray  120  of a colored liquid formulation which is directed at the pulverized polymer powder thereby to color the powder. The powder and liquid formulation are mixed resulting in an evenly colored powder passing from an outlet  140  of the mixer, after which the colored powder may be stored for subsequent use in rotational molding. Further details on the apparatus are provided below. 
     The pulverizer  30  may be of conventional design. It is arranged to receive pelletized polymer and pulverize it to produce a powder of predetermined particle sizes. During pulverization the polymer becomes heated, for example to a temperature of the order of 50-60° C. due to the mechanical work done on it. 
     The mixer  60  is closely coupled to the pulverizer and is arranged to receive the heated powder from the pulverizer. Suitably, the outlet  40  of the pulverizer and inlet  80  of the mixer are directly coupled to one another so that powder may be continuously fed from the pulverizer to the mixer without a significant drop in temperature of the powder on passing between pulverizer and mixer. Thus, powder entering the mixer is suitably at an elevated temperature which is approximately the same as the temperature of the powder within the pulverizer. 
     In this way, the powder is still hot due to the mechanical work done on it by the pulverizer  30 . The additional heat produces greater color adhesion than would the cooled powder. This process happens without the additional step of re-heating the powder when it enters into the mixing apparatus. 
     The mixer  60  is a mechanical mixer, for example a paddle mixer, and is arranged to develop a fluidized bed in which the powder and liquid formulation can be intimately mixed. The mixer has a paddle  71  and a paddle rod  61 . The paddle rod  61  is connected to the shaft  62 . The top of the paddle rod  61  contains a weld cap  65 . Additionally, the mixer  60  has cleaning ports  64 , which are holes or apertures in a cylindrical paddle rod  61 . The cleaning ports  64  facilitate easy cleaning of the device. In one embodiment, a water or cleaning fluid source is operatively connected to the shaft  62 . The cleaning fluid then travels through the shaft  62  into each of the paddle rods  61 . The cleaning fluid travels through the paddle rods  61  and exits the cleaning ports  64  on each paddle rod  61 . In some embodiments, the shaft  62  may also include cleaning ports  64 . 
     The spray device  100  comprises a low pressure (e.g. 5-15 psi) air atomizing nozzle which is arranged to spray a continuous stream of liquid formulation into the powder at relatively low velocity. It is preferred to avoid the liquid formulation being sprayed so forcefully that it penetrates the mass of powder and contacts walls of the mixer. It is also preferred to direct substantially the entire spray of formulation at the powder itself and avoid coating walls of the mixer. Any coating of walls of the mixer may result in regions of relatively concentrated color being produced in the powder which is undesirable. 
     The liquid formulation comprises water as a vehicle and a colorant, for example a dye or pigment. It suitably has a viscosity measured on a Brookfield viscometer at 20 rpm using a #6 spindle of 100-50,000 centipoises immediately prior to entering the spray device  100 . 
     The use of water as a vehicle is believed to facilitate coating of the liquid formulation, in particular the colorant, on the powder particles. In this regard, the fact the powder is hot at the time it is contacted with the liquid formulation leads to some water being driven off from droplets of formulation which contact the relatively hot powder. This makes the droplets less mobile and more likely to remain adhered to powder particles. 
     It should be appreciated that, since no colorant is introduced into the pulverizer  30 , it does not need to be cleaned between color changes. Furthermore, mixers  60  and/or spray devices  100  may be sufficiently inexpensive that it is cost effective to dedicate respective mixers/spray devices to particular colors. When it is desired to produce powder of a different color, one mixer/spray device used to produce one color may be replaced with a spray device dedicated to production of a different color. Thus, it may not be necessary to undertake extensive cleaning between color changes. 
     Furthermore, it should be appreciated that the process described can be used to continuously produce colored powder from polymer pellets with great efficiency. Additionally, by contacting liquid formulation with heated powder from the pulverizer, not only is the coloring process improved but also less energy is needed to dry the colored powder after is preparation. 
     The powder may be used as a feedstock to a rotational molding apparatus. Alternatively, it could be used in other coating processes or in melt-processing to produce components. 
     With reference to  FIGS. 11 and 12 , an apparatus  200  for producing a colored powder of a polymeric material can include a pulverizer P arranged to deliver uncolored pulverized polymer powder from the outlet of the pulverizer P directly into a mixer  220  via the inlet  290  of the mixer  220 . The apparatus  200  can function and operate in the same manner as the previously described apparatus  20 . The pulverizer P can include a control panel  240 , which houses a controller for the pulverizer. The pulverizer P can include a milling chamber  260 , mill motors  250  and  252 , a vibratory feeder  270 , and an inlet tube  280  for the milling chamber  260 . The mixing chamber  220  can include a mixer inlet  290 , a mixer outlet  292 , a spray nozzle  296 , and a drive motor  300 . The spray nozzle  296  sprays a colorant into the mixing chamber  220  to color the pulverized polymer powder. The mixing chamber  220  can have a generally semi-circular cross-section with a substantially curved or rounded lower or bottom portion and a substantially flat upper or top portion. Many other shapes and configurations for the mixing chamber  220  have been contemplated and are within the scope of this invention. A cabinet or housing  210  can include a container of colorant and a pump for transferring the colorant from the container to the spray nozzle  296 . The housing  210  can enclose the container of colorant and the pump within the interior of the housing. In some embodiments, the pump can be either a progressive cavity pump or a peristaltic pump. The housing  210  can include a controller  230  for the pump. The controller  230  controls the amount of colorant the pump provides to the spray nozzle  296 . The colorant can be a liquid or a powder. A hose or tube can fluidly connect the container to the pump and the pump to the nozzle. The housing  210  can include wheels, rollers, or casters for readily moving and positioning the housing. The mixing chamber  220  can be operatively connected or attached to the housing  210 . In some embodiments, the mixing chamber  220  can be fixedly attached to the housing  210  to form a single unit, which can be moved or transported together. The housing  210  can be easily moved for cleaning the mixing chamber  220  offline. 
     The controller  230  can be integrated with the controls  240  of the pulverizer P to deliver the proper amount of colorant to the mixing chamber  220  for the corresponding amount of pulverized powder. The controller  230  determines the proper amount of colorant to match the production rate of the pulverizer P. In some embodiments, the controller  230  controls the amount of colorant provided to the mixing chamber  220  based upon the feed rate of the pulverizer P. In other embodiments, the controller  230  controls the amount of colorant provided to the mixing chamber  220  based upon the speed of an extrusion screw. In other embodiments, the controller  230  controls the amount of colorant provided to the mixing chamber  220  based upon the flow rate of the pulverized powder. In other embodiments, the controller  230  controls the amount of colorant provided to the mixing chamber  220  based upon the weight basis of the material delivered to the mixing chamber  220 . 
     The invention is not restricted to the details of the foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. 
     Numerous embodiments have been described herein. It will be apparent to those skilled in the art that the above methods and apparatuses may incorporate changes and modifications without departing from the general scope of this invention. It is intended to include all such modifications and alterations in so far as they come within the scope of the appended claims or the equivalents thereof.