Abstract:
Black polyethylene storage tanks are an inexpensive and effective way to store water and simultaneously inhibit algae growth. However, based on other considerations, it may be desirable to use a storage tank with a non-black color. By producing a polyethylene water storage tank with a black inner layer and a non-black outer layer, the desire to inhibit algae growth and the desire to have a non-black exterior can both be met.

Description:
BACKGROUND OF THE INVENTION 
     As the availability of potable drinking water diminishes, it is becoming more and more necessary to utilize alternate sources of water to supply the needs of a household. One readily available and easy to collect alternate source of water is rainwater. Using appropriately configured gutters, the water shed off of a roof can be directed into water storage tanks to collect many gallons of water over a short period of time. For example, 1 inch of rain over a 1000 ft 2  of roof surface corresponds to over 600 gallons. On a rainy day, a light rain of 0.1 inches per hour might fill a 1000-gallon tank. 
     There are various means for storing water, but a preferred method for residential water storage is the use of a polyethylene tank. All polyethylene tanks are made from virgin polyethylene has been approved by the United States Food &amp; Drug Administration for use with drinking water. They are also durable, relatively inexpensive, and available in a wide variety of colors. These properties make them an ideal choice for many residential users. 
     Proper use of a water storage tank includes addressing the problem of algae growth, which is a common problem associated with standing water. If left unchecked, algae growth may make the stored water unsuitable for its intended purpose. Thus, there is a need to prevent algae growth within water storage tanks. The growth rate of algae can be inhibited by limiting the water&#39;s exposure to ultraviolet radiation. Because most water storage tanks are placed outdoors, this generally means being exposed to sunlight. 
     The ability of a polyethylene tank to block ultraviolet radiation is dependent on the color of the tank. The darker the color of the tank, the more ultraviolet radiation that is blocked. Therefore, a black polyethylene tank would provide the best ultraviolet radiation protection and best inhibit the growth of algae. However, for a variety of reasons, a user might find that a black water storage tank is unacceptable. In some cases, a home owner&#39;s association limits the color of storage tanks. In some situations, the user just insists on having a non-black storage tank. Regardless of the reason, using a single-layer non-black polyethylene storage tank will result in accelerated algae growth. Thus, there is a need to provide water tanks of a variety of colors that still inhibit the growth of algae. By providing a polyethylene tank with a black inner layer and a non-black outer layer, a user can select a water storage tank color and still obtain improved algae inhibiting properties. 
     SUMMARY OF INVENTION 
     Using a modified molding process, a multi-layered storage tank can be produced with different colored external layers with an internal layer of black polyethylene. By combining layers of different materials, the properties of these materials can be combined into a single storage tank with advantages corresponding to the chosen materials. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an example of multi-layered water storage tank 
         FIG. 2  is an illustration of the wall of a two layer water storage tank. 
         FIG. 3  is an illustration of the wall of a three-layer water storage tank. 
         FIG. 4  is a diagram illustrating the steps necessary to manufacture a two-layer storage tank. 
         FIG. 5  is a diagram illustrating the steps necessary to manufacture a three-layer storage tank. 
         FIG. 6A  is a diagram of a single-port dropbox. 
         FIG. 6B  is a diagram illustrating a single-port dropbox attached to a mold. 
         FIGS. 7A and 7B  are diagrams of a dual-port dropbox. 
     
    
    
     DETAILED DESCRIPTION 
     This disclosure can be understood in the context of the following examples. However, what follows is merely exemplar and not intended to define the scope of this patent, which is defined by the claims. 
     At a very high level, producing a rotation-molded plastic structure involves heating and rotating polyethylene resin within a mold until the resin heats up and coats the interior of the mold. The mold is then cooled until the plastic structure is ready for extraction. If the process works correctly, the resulting structure should have the shape of the mold with a thickness and color determined by the mold design and material weight. 
     One of the important steps in creating a rotation-molded structure is calculating the proper length of time to leave the mold in the oven (i.e., “curing time”). The proper curing time depends on a number of factors including, but not limited to, the shape of the mold, the quantity of resin, the type of resin, and the color of the resin. 
     In the absence of pigment, the natural color of polyethylene resin is an off-white color. In order to change the color of the resulting plastic structure, pigmented resin can be used. It may be possible to obtain resin that already contains a particular pigment color. Alternatively, pigment can be added to natural resin using a high-speed mixer. In either case, pigmented resin will change the color of a resulting plastic structure in accordance with its pigmentation. In addition to affecting the color of the resulting structure, resin color also changes the curing time. Darker pigmented resin cures faster than lighter pigmented resin. 
     The wall thickness of a plastic structure depends on the quantity of resin used, which is usually measured in pounds. Generally, adding more resin results in thicker walls and removing resin results in thinner walls. In addition to affecting wall thickness, increasing the quantity of resin also increases the curing time and vice versa 
     Resins are provided in what are commonly called “powder” for use in molding processes. Powder sizes are measured in terms of mesh. A lower mesh number means larger particles. For example, 35 mesh powder has larger particles than 50 mesh powder. Using a higher mesh results in a shorter curing time as compared to using lower mesh. 
     Different materials have different curing times. For example, nylon generally takes longer to cure than a similar amount of polyethylene. The specific properties of the material used will affect the curing time as well as the dependency of the other factors discussed above. 
     The process of forming a rotation-molded plastic structure requires the use of a mold with an interior surface corresponding to the outer surface of the desired plastic structure. The mold should be constructed of a material that will maintain a constant shape with minimal expansion during the curing process. The mold should also be sufficiently durable for repeated use in the production of identical plastic structures. In order to shape and cure the contents of the mold, the mold should also be comprised of a heat conductive material. 
     During the rotational-molding process, the mold should be rotated from the beginning of the curing time until the completed plastic structure is ready to be removed. During the curing process, the resin powder softens and then spreads over the inner surface of the mold. The rotation of the mold keeps the resin from settling into one spot. The precise curing time is determined by a number of factors including, but not limited to, the quantity of resin, the pigmentation of the resin, the shape of the mold, and the temperature of the oven. 
     After the curing process has completed, the mold is put in a cooling phase to lock the melted resin into the shape of the mold. Because the resin is still soft upon exiting the oven, it must continue to be rotated during the cooling process. Using air circulation and possibly a water mist, the mold can be cooled to a workable temperature in a relatively short amount of time. 
     Once the mold has sufficiently cooled, the plastic structure can be removed from the mold. If performed properly, the resulting structure should have the same shape as the interior of the mold and possess a thickness and color consistent with the pigmentation and quantity of resin. The process described above is effective for producing a rotation-molded structure with a single layer 
     The above process can be modified by adding additional resin to the mold during the curing process using a dropbox, which is an insulated resin container with a release valve. The release valve may be spring loaded for ease of opening. The dropbox can be attached to the mold via an access port in the mold structure. The dropbox and the mold are connected in such a manner that when the release valve on the drop box is opened, resin within the dropbox can flow into the mold. The dropbox is sufficiently insulated that its resin contents will not cure within the dropbox even when the dropbox is within an oven and attached to the mold. 
     As discussed above, curing time is inversely proportional to the darkness of the pigmented resin. Accordingly, by beginning the curing process with lightly-pigmented resin and adding darkly-pigmented resin via a drop box, the curing times of the colors can be matched such that the entire structure cures by the end of the cycle. Moreover, because the lightly-colored resin has been partially cured within the mold prior to introduction of the darkly-pigmented resin, it has had the opportunity to coat the inner surface of the mold. When the darkly-pigmented resin is added to the mold, the lightly-pigmented resin acts as a mold for this new layer. 
       FIG. 1  illustrates a polyethylene storage tank comprising multiple layers. Storage tank  100  is one possible shape of a multilayer polyethylene water tank. Tank  100  contains a black polyethylene inner layer and one or more outer layers. The inner layer contacts any stored water or other consumable fluid. The outer layers are isolated from the stored water and can serve any number of other purposes including, but not limited to aesthetic coloring and insulation. 
       FIG. 2  illustrates a cross-section of a two-layer plastic wall, such as the wall of storage tank  100 . Layer  201  is the outer layer and  202  is the inner layer. In this embodiment, layer  201  is comprised of a white-pigmented polyethylene and layer  202  is comprised of black-pigmented polyethylene. However, layer  201  can also be comprised of different colors and materials including, but not limited to, nylon, polypropylene, or crosslink materials in addition to white-pigmented polyethylene. The resistance to ultraviolet penetration is determined by the combination of layers  201  and  202 . However, the visual appearance of a corresponding tank, such as tank  101  is largely controlled by the color of exterior layer  201 . Accordingly, a two-layer plastic storage tank can provide ultraviolet protection greater than that of a wall comprised solely of layer  201  while providing the outward appearance of layer  201   
       FIG. 3  illustrates a cross-section of a three-layer plastic wall, such as the wall of storage tank  100 . Layer  301  is the outer layer, layer  302  is the inner layer, and layer  303  is the middle layer. In this embodiment, layer  301  is comprised of a white-pigmented polyethylene and layer  302  is comprised of black-pigmented polyethylene. Layer  301  can also be comprised of different colors and materials including, but not limited to, nylon, polypropylene, or crosslink materials in addition to white-pigmented polyethylene. In this embodiment, layer  303  is comprised of closed cell white-pigmented polyethylene foam. In addition to the properties of the wall illustrated in  FIG. 2 , the addition of layer  303  provides additional ultraviolet protection as well as insulation. 
       FIG. 4  illustrates a process that can be used to construct a multilayer storage tank such as  100 . Specifically,  FIG. 4  describes steps in the process of producing a two-layer water tank. By way of example and not limitation, the steps shown in  FIG. 4  will be discussed in the context of a specific embodiment for producing a 100-gallon tank of the type of shown in  FIG. 1  with a white-pigmented polyethylene outer layer and a black polyethylene inner layer. The values, equipment, configurations and materials discussed below are examples and do not define the scope of this patent, which is defined by the claims. Based on this disclosure, a skilled artisan could apply this technique to different colors of resin, different resin powder size, different tank shapes, different tank sizes, and different layer compositions without undue experimentation. 
     Step  401  includes preparing the mold and dropbox. In this embodiment, the mold corresponds to the shape of the storage tank shown in  FIG. 1 . This may involve using a water hose, air hose, or other mechanism to remove any residual plastic or other containments that might the adversely affect the rotational molding process. The dropbox is also prepared in a similar manner for use in the disclosed rotational molding process. 
     Step  402  involves adding a first resin to the mold. This first resin will form the outer layer of the resulting structure and will correspond to layer  201  in  FIG. 2 . If the outer layer is polyethylene, then step  402  involves adding pigmented polyethylene resin powder. Step  402  also includes closing the mold with the first resin inside. The quantity of resin comprising the first resin is based on the characteristics of the mold, and the desired thickness of the resulting plastic structure. In this embodiment, the first resin comprises 44 pounds of white-pigmented 35 mesh resin, which would be added to the mold in step  402 . 
     Step  403  involves adding a second pigmented resin to the dropbox. The second pigmented resin corresponds to layer  202  in  FIG. 2 . When the second resin is added to the dropbox, the port on the dropbox should be closed so that the second resin will stay within the dropbox until deployed. Step  403  also includes closing any other openings in the dropbox. The overall thickness of a resulting plastic structure depends on the quantity of the second resin as well as the quantity of the first resin. In this embodiment, the second resin comprises 16 pounds of black-pigmented 35 mesh resin, which would be added to the dropbox in step  403 . 
     Step  404  involves attaching the dropbox to the mold. The drop box port is attached to an access port on the mold. The access port on the mold may have a cover that needs to be removed or slid away to expose the access port. However, the port on the dropbox remains closed during this step. 
     Step  405  is the first curing phase. This step involves setting the rotational parameters and placing the mold in an oven. In this specific embodiment, the mold is rotated continuously along two axes using a 4:1 ratio or other as needed, meaning that for every 4 times that the mold is rotated around its central axis, it has flipped once end-over-end. This rotational configuration is maintained while the mold is the oven. In this specific embodiment, the mold is placed within a gas oven set to 600° F. The duration of curing phase one is based on the properties of the mold, and the resin properties including, but not limited to, pigmentation and weight. In a specific embodiment, curing phase one has a duration of approximately 16 minutes. During curing phase one, the first resin should soften and coat the inner surface of the mold. The second resin inside the dropbox should remain relatively cool because of the insulation of the dropbox. 
     Step  406  involves opening the release mechanism on the dropbox, which opens the port on the dropbox. Once the dropbox is opened, the second resin will begin to flow from the dropbox through the access port on the mold into the interior of the developing plastic structure. Step  406  may involve briefly slowing down the rotation and/or removing the mold from the oven just long enough to open the dropbox port. In some cases, the dropbox release is spring-loaded or air opened to minimize any interruption of the normal curing process. Once Step  406  is complete, the second curing phase can begin. 
     Step  407  is the second curing phase and involves curing the plastic structure to completion. In the case of the first resin, curing phase two is a continuation of the first curing phase. In the case of the second resin, the second curing phase is the continuation. In this embodiment, the same parameters as the first curing phase are maintained, which means rotating with a 4:1 ratio at a temperature of roughly 600° F. The duration of curing phase two also is based on the properties of the first and second resins. In this embodiment, the second resin comprises of 16 pounds of black resin which translates to a second curing phase lasting approximately 15 minutes. Once the second curing phase has completed, the first resin and second resin should have formed a two-layer wall like the one illustrated in  FIG. 2 . In this specific embodiment, the durations of curing phase one and curing phase two are roughly the same. However, this may not be the case when different resin or mold configurations are used. 
     Step  408  involves removing the mold from the oven and cooling it to a temperature where the polyethylene resin will cease to flow. Until the mold has sufficiently cooled, its rotational parameters will be maintained. Step  408  may involve the use of fans and water mist to accelerate the cooling of the mold. 
     Step  409  involves removing the plastic structure from the mold. The mold is opened and a two-layer tank is extracted from the mold. In this embodiment, the extracted tank should roughly resemble  FIG. 1 . The actual thickness of the wall layers is based on the quantity of resin used to form each layer. In this embodiment, 44 pounds of white-pigmented resin was used for the outer layer and 16 pounds of black-pigmented resin was used for the inner layer. Accordingly, the outer layer should be thicker than the inner layer. 
       FIG. 5  illustrates a process that can be used to construct a multilayer storage tank such as  100 . Specifically,  FIG. 5  describes steps in the process of producing a three-layer water tank. The process used in the three-layer process is similar to the process used in the two-layer process, so it will discussed in terms of differences from the two-layer process discussed above and illustrated in  FIG. 4 . By way of example and not limitation, the steps shown in  FIG. 5  will be discussed in the context of producing a 100-gallon tank of the type of shown in  FIG. 1  with a white-pigmented polyethylene outer layer, a pigmented polyethylene foam middle layer, and a black-pigmented polyethylene inner layer. The values, equipment, configurations and materials discussed below are examples and do not define the scope of this patent, which is defined by the claims. Based on this disclosure, a skilled artisan could apply this technique to different colors of resin, different resin powder size, different tank shapes, different tank sizes, and different layer compositions without undue experimentation. 
     Step  501  is very similar to step  401 . However, the dropbox used in the process of three-layer process contains two compartments as opposed to the dropbox used in the two-layer process which may contain only one compartment. Therefore, step  501  includes preparing both compartments of the dropbox. 
     Step  502  is similar to step  402  in that the resin that will form the outer layer is added directly to the mold. In this specific embodiment, the first resin comprises of 44 pounds of white-pigmented 35 mesh polyethylene resin. 
     Steps  503  and  504  are similar to step  403  in that resin will be added to a dropbox. However, it is important to note that the second resin referenced in step  503  is the resin that will form the middle layer of the resulting water tank and not the inner layer as in step  403 . In this specific embodiment, the second resin comprises 10 pounds of white-pigmented polyethylene foam resin. By “foam resin”, this means that the resin contains a foaming (blowing) agent that will cause this resin to produce polyethylene foam during the curing process. The resin added in step  504  corresponds to the resin added in step  403  of the two-layer process in that they both involve the resin that will form the inner layer. Of course, step  504  involves a second compartment that may not be present in the two-layer process. In this specific embodiment, the third resin comprises 10 pounds of black-pigmented polyethylene resin. 
     Step  505  is similar to Step  404  in that they both involve attaching a dropbox to a mold. However, the dropbox used in step  505  contains two ports, so the attachment process may involve the use of multiple access ports on the mold. 
     Steps  506 ,  507 , and  508  roughly correspond to steps  405 ,  406 , and  407 . Because the dropbox used step  505  has two ports, only a single port is opened in step  507 . Steps  509  and  510  are additional steps that relate to adding the third resin and utilizing a third curing phase. Step  509 , where the second dropbox compartment is opened, should be substantially similar to step  507 , where the first dropbox compartment is opened. Steps  506 ,  508 ,  510  are curing periods separated by the addition of resin to the mold. In this specific embodiment, all curing phases occur at 600° F. or other needed temperature, wherein the first curing phase  506  has a duration of 16 minutes, the second curing phase  508  has a duration of 10 minutes, and the third curing phase  510  also has a duration of 10 minutes. As with the two-layer variation, the three layers should cure together. 
     Steps  511  and  512  are essentially the same as steps  408  and  409  in that the mold should be cooled before removing the completed plastic structure. In this specific embodiment, the result should be a water tank which resembles water tank  100  in  FIG. 1 , with a three-layer wall structure similar to what is shown in  FIG. 3 . 
     The various curing times and rotational parameters can be altered by a skilled artisan to account for the use of different materials that may have different curing times. The principles discussed above apply when using other plastic materials. For example, a nylon outer layer takes longer time to cure than an equivalent polyethylene outer layer. Accordingly, the duration of curing phase one would need to be extended to account for this property of nylon. A skilled artisan could make the adjustment necessary to account for different materials, different pigments, different powder sizes, and different molds. 
       FIG. 6A  illustrates a single port dropbox  600  that might be used in producing a two-layer tank. Dropbox  600  has an outer surface  601  and an inner surface  602 . Between the outer surface  601  and inner surface  602  is layer of insulation, which may include, but is not limited to Teflon. The surfaces themselves may also have insulating properties. The overall insulation needs to be sufficient to keep the contents of dropbox  600  from curing within the dropbox. Dropbox  600  contains a port  603  that will connect to a corresponding access port on a mold  101  (shown in  FIG. 6B ). Port  603  contains a plug which is connected to release handle  604 . The plug in port  603  will remain closed until such time as release handle  604  is activated. In some embodiments, release handle  604  is spring loaded, such that it need only be pulled to activate. 
       FIG. 6B  illustrates a dropbox  600  that has been attached to mold  101 . The dropbox is attached securely to mold  101  because it will stay attached to the mold throughout the entire curing process. Release mechanism  604  is readily accessible so that it can be activated with as little disruption to the curing process as possible or by other means like pneumatic release. 
       FIGS. 7A and 7B  illustrate a dual port dropbox  700  that might be used in producing a three-layer tank. Dropbox  700  has an outer surface  701  and two inner surfaces  702  and  704 , two ports  703  and  705 , and two release mechanism  706  and  707 . Dropbox  700  contains two compartments  708  and  709 . Inner surface  704 , port  705  and release mechanism  706  correspond to compartment  708 . Inner surface  702 , port  703  and release mechanism  707  corresponding to compartment  709 . Similar to dropbox  600 , dropbox  700  should have insulation between its outer surface  701  and its two inner surfaces  702  and  704 . In addition, surfaces  702  and  704  should be insulated from each other because one compartment may be opened for a substantial period of time before the other compartment is opened. 
     Dropbox  700  can be attached to a mold  101  in a similar manner as dropbox  600 . However, mold  101  will need to have dual access ports to take full advantage of the dual port capability of dropbox  700 . The presence of multiple access ports on mold  101  does not preclude the use of dropbox  600  as additional access ports can remain unused. Although dropbox  700  has two compartments, if can be still be used to produce a two-layer structure if only one compartment is used or if both compartments are used in tandem. 
     Although the above example is described in the context of a water storage tank which inhibits algae growth, a resulting two-layer tank may be used for other purposes or in other applications. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made to the embodiments described herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.