Abstract:
A method of coloring thermoplastic olefin resins and pulverizing resin pellets in a single step involving the steps of metering liquid coloring onto resin pellets and then passing the combination of liquid color and resin pellets into a mill blender. The liquid color is metered onto the plastic pellets at a rate of from less than 0.2% to less than 1.0% by weight of the plastic pellets. The mill blender receives the combination of plastic pellets and liquid color and pulverizes the mixture to produce a colored thermoplastic resin powder that is suitable for subsequent processing, such as roto-molding or slush-molding.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates generally to the field of polymeric resin powder coloring. More specifically, the present invention relates to an apparatus and system for coloring powdered resins, including polymer resin powders of all types.  
           [0003]    2. Description of Related Art  
           [0004]    Pulverized resins, particularly resins intended for use in thermoplastic molding and shaping operations, are typically sold as fine powders, with particle sizes generally being between 200 to 350 microns. These polymer powders are generally colored by the addition of dry pigment after it has been pulverized.  
           [0005]    One traditional method of coloring polymer resins include admixing the polymer powder with a finely ground pigment which electrostatically clings to the outer surface of the polymer powder particles. Other methods of coloring exist that involve the use of high speed machinery to agitate a combination of polymer and pigment to produce a colored polymer. One example involves an apparatus with agitating blades and legs mounted on a frame that are lowered into a vessel for mixing the coloring agent with the polymer.  
           [0006]    Another process involves incorporating a coloring agent with the polymer powder in a mixing device, such as a twin-cone blender. The mixed composition is then further mixed in any of a number of apparatuses that generates or adds heat to the composition, where the heat is controlled to aid in the blending of the coloring agent to the pigment. The mixing apparatuses listed include a twin-roll mill, a Banbury mixer, ribbon blender, a tumble blender, a conventional screw extruder, or any other device that allows heat to be added to the composition, or produced by the mixing. Examples of patents that include these devices are Scheibelhoffer et al., U.S. Pat. No. 5,670,561, Hahn, U.S. Pat. No. 3,632,369, Lerman et al., U.S. Pat. No. 3,674,734, Lerman et al, U.S. Pat. No. 3,449,291, and Tanaka, U.S. Pat. No. 5,779,360.  
           [0007]    However some disadvantages exist regarding the previously known pigmenting processes of polymer powders. Pigment applied electrostatically can be easily removed by contact with solvents or friction caused the handling of the pigmented powder. It has been found that admixing pigment with the polymer powders results in a reduced structural strength of objects formed from those polymers. Additionally, the known pigmenting processes are applicable to polymer powder and do not work with polymer pellets. Current methods of pigmenting polymer involve pulverizing the polymer pellets to produce a polymer powder, then pigmenting the polymer powder.  
           [0008]    Therefore, there exists a need for a process to add pigment to polymer, where the resulting polymer does not easily shed its color and whose structural integrity is not compromised by coloring. It is further desired to have a process where polymer pellets can be mixed with pigment and a colored polymer powder is produced, without the added step of pulverizing the pellets prior to the addition of the pigment.  
         BRIEF SUMMARY OF THE INVENTION  
         [0009]    Disclosed herein is a method of pulverizing and coloring thermoplastic resin pellets coloring in a single step, particularly olefin based polymer resins. According to this invention, the process for coloring thermoplastic resins comprises adding a suitable amount of liquid coloring compound to the thermoplastic olefin based resin. The rate of added liquid should be sufficient to thoroughly color the thermoplastic olefin based resin. After the liquid color is added to the resin, the mixture of liquid color and thermoplastic olefin based resin is passed into a mill blender where the mixture is pulverized for a time sufficient to fuse the liquid coloring compound onto each particle of the thermoplastic olefin based resin.  
           [0010]    The process also includes maintaining the temperature of the mixture inside of the mill at between 85° C. and 125° C. during the milling process. The amount of coloring compound added to the thermoplastic olefin based resin can range less than 0.2% to in excess of 1.0% by weight, and preferably less than 0.2% by weight. It is important that the flow of liquid coloring compound onto the thermoplastic olefin based resin be at a constant rate. The particle size of the final product should be less than 600 microns. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0011]    [0011]FIG. 1 provides a schematic view of Process for Coloring and Pulverizing Thermoplastic Resin Pellets.  
         [0012]    [0012]FIG. 2 illustrates a combination of resin pellets and liquid color being pulverized with a rotor and stator to produce a colored polymer powder.  
         [0013]    [0013]FIG. 3 depicts an overview of the rotor and stator of a mill blender.  
         [0014]    [0014]FIG. 4 is an enlarged view of the teeth formed on the outer circumference of the rotor and the inner circumference of the stator.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    One embodiment of the present invention is disclosed in schematic form in FIG. 1. FIG. 1 illustrates a polymer feeder  10 , a metering pump  15 , a mill blender  20 , and a separating sieve  26 . The process involves combining liquid color  16  with plastic resin pellets  11  and processing the combination in the mill blender  20 . This thoroughly coats the plastic resin pellets  11  with the liquid color  16  to produce a colored polymer powder  25  with excellent color properties. The colored polymer powder  25  produced by the present invention has superior color pigment stability which leads to plastic products where the coloring or pigment is not easily removed from the plastic but instead is vibrant and long lasting.  
         [0016]    The plastic resin pellets  11  can be chosen from a wide variety of plastics, including thermoplastic olefin and polyolefin based resins such as polyethylene, polypropylene, and linear low density polyethylene. However, when used in conjunction with the present invention, the plastic resin pellets  11  should be in a pelletized form. The form of the plastic resin pellets  11  is important because the plastic resin pellets  11  must be in a semi-fluidized state to be handled by the polymer feeder  10 .  
         [0017]    It has been found that coloring plastics with a liquid instead of a dry powder form of pigment better distributes the coloring agent throughout the plastic that is being colored. One of the many advantages of liquid coloring is longer lasting color that is less likely to rub off or be otherwise removed from the plastic. As is known in the art, liquid color is produced by mixing a dry pigment with a carrier. The efficiency of liquid color is better than that of dry color pigmenting. This means that less actual pigment is required when a liquid color is used instead of a dry blend. Further, dry pigment can have the detrimental effect of acting as a foreign body in the finished polymer product. The presence of foreign bodies in the finished plastic can reduce tensile strength of the plastic. This problem of reduced tensile strength can be avoided by the implementation of liquid color. Additionally, liquid color produces more reliable coloring results than dry pigment. Thus the repeatability of achieving the desired color in the finished product is enhanced with using liquid color. A yet additional advantage to liquid color over dry pigment is that it is much easier to control the flow rate of liquid color into the polymer than dry pigment.  
         [0018]    The mill  20 , the preferred type of which is a WEDCO Model UR 28, is used as a one step method to pulverize the polymer pellets  11  into a polymer powder while at the same time bonding the liquid color  16  onto the polymer powder  25 . A functional illustration of this process is illustrated in FIG. 2. The mill  20  differs from the traditional blend mixers in that it does not employ blades or paddles used in prior art devices. Instead, as shown in FIG. 3, the mill  20  includes a rotor  30  radially disposed within a stator  32 . As seen in FIG. 4, formed on the outer radius of the rotor  30  are a series of teeth  33 . Another set of teeth  33  are similarly formed on the inner radius of the stator  32 . The gap between these two sets of teeth  33  is precisely machined to a predetermined distance which produces a desired polymer particle size as will be described in more detail below.  
         [0019]    Before the coloring phase begins the process goes through a “start up” procedure involving activating the mill  20  and feeding plastic resin pellets  11  into the mill  20  via the polymer feeder  10 . When the mill  20  is activated the rotor  30  begins to rotate as depicted in FIG. 2. Once fed into the mill  20  and inside of the mill  20 , the plastic resin pellets  11  pass between the rotating rotor  30  and the stator  32 . The pellets  11  are caught between the respective teeth  33  of the rotor  30  and the stator  32  and pulverized into powder form. The polymer powder particles produced by the mill  20  are very close in size to the gap between the teeth  33  of the rotor  30  and the stator  32 . Accordingly, the gap between the rotor  30  and stator  32  teeth  33  is adjusted based on what size polymer powder particle size is desired. For the purposes of the invention disclosed herein that size is approximately 600 microns. Because the polymer particle size is important, during the life of the mill  20  the gap is closely monitored to ensure that the same size particle is produced by the mill  20 .  
         [0020]    The temperature of the polymer powder exiting the mill  20  during start up is checked until it reaches a steady state of about 85° C. to 125° C. At this time the liquid color or pigment  16  is dosed onto the plastic resin pellets  11  through a metering pump  15 . The metering pump  15  can be any fluid handling device capable of producing a steady flow of liquid at a wide range of flow rates. More importantly, the metering pump  15  must be capable of supplying fluid at low flow rates on the order of 5 kg/hr or less. A standard peristaltic pump is the preferred type of pump for metering the liquid color  16  onto the plastic resin pellets  11 .  
         [0021]    The mill  20  operates in the same manner when the mixture of liquid color  16  and plastic resin pellets  11  passes through it as it does during start up. Except that with the addition of the liquid color  11 , the pulverizing action of the mill  20  acts not only to pulverize the pellets  15  into powder, but also impinges the pigment inherent in the liquid color  16  onto the polymer particles. This impinging action fuses the color onto the polymer powder particles to produce a colored polymer powder  25 . One of the many advantages of utilizing the mill  20  to color the plastic resin pellets  11  is that the pellets  11  can be colored and pulverized in a single step. Further, the polymer powder colored in this manner requires less pigment to be colored than processes using traditional mixing techniques. Also, because of the improved fusion of the pigment onto the polymer powder colored by the process disclosed herein, the subject powder will retain its color better than polymers colored by known mixing processes.  
         [0022]    As is known in the art, the magnitude of the liquid color  16  flow rate will depend upon what color intensity or hue is desired in the final product. Coloring the polymer so that the final product has the desired color is what is known as a color match. The metering pump  15  is calibrated so the flow of liquid color  16  provided by the metering pump  15  mixed with the plastic resin pellets  11  produces the specified color match. Calibrating the metering pump  15  to produce the desired color match is well known to those skilled in the art. After the metering pump  15  is calibrated the colored product will reflect the desired hue as long as the flow rate of plastic resin pellets  11  into the mill  20  does not change. Varying the metering pump  15  flow with changes in voltage to the polymer feeder  10  allows for a continuous coloring process instead of the batch mixing processes that are currently used. Repeatability and consistency in producing a colored polymer powder are some of the advantages of a continuous process over a batch process.  
         [0023]    To compensate for changes in the pellet flow rate, a control feedback system (not shown) is included that operatively couples the mass flow of the metering pump  15  to the mass flow of the polymer feeder  10 . This will serve to assure that the proper mass flow of liquid color  16  is dosed onto the plastic resin pellets  11 . The control feedback system proportionally varies the liquid color  16  feed rate with variations in the plastic resin pellets  11  flow rate. The proportional changes have a linear relationship. This results in colored polymers having a consistent and repeatable amount of coloring. Thus, once the system is calibrated and stabilized, it can operate without excessive supervision thereby ensuring consistent coloring results at a reduced manpower cost. It is appreciated that such a control system would be obvious to one skilled in the art.  
         [0024]    As is well known in the art the polymer feeder  10  can be comprised of a vibratory feeder or a screw feeder. While both embodiments will adequately perform the required function, the screw feeder is preferred due to its more consistent and uniform mass flow characteristics. Further, one embodiment of the flow control system involves the use of corresponding load cells under the metering pump  15  and the polymer feeder  10  to produce a gravimetric flow relationship. The load cells under the metering pump  15  and the polymer feeder  10  can be calibrated to provide an accurate indication which results in a very precise control of the mass flow relationship of the liquid color and the polymer.  
         [0025]    As the colored polymer powder  25  exits the mill blender  20  it is passed through a separating sieve  26 . The separating sieve  26  is designed to pass only particles that are 600 micron and smaller. Particles larger than 600 microns disaffect the aesthetics of the final plastic product by giving it a speckled appearance. To eliminate this problem, these larger particles are separated from the final colored product and are channeled through the return line  27  back onto the mill blender  20  for additional processing. Colored polymer particles emanating from the mill blender  20  that are less than 600 microns in size are transferred through the processing line  28  to a final product bin  29 .  
         [0026]    The powder deposited into the final product bin  29  can then be further processed and formed into a final polymeric object. Numerous processes can be employed using the colored polymer powder  25  formed by the present invention, these include roto-molding and slush molding. Additionally, use of the present invention in conjunction with roto-molding produces final form plastics whose physical properties of tensile or impact strength can be equal to traditional manners of coloring polymeric compounds, which are generally more costly. An example of such a traditional coloring method includes extrusion compounding.  
         [0027]    This description is made with reference to the preferred embodiment of the invention. However, it is possible to make other embodiments that employ the principles of the invention and that fall within its spirit and scope as defined by the following claims.