Controlled release fertilizers and methods of manufacture

A method of manufacturing a controlled release fertilizer wherein the fertilizer is deposited onto a first polymer film and then covered by a second polymer film to encapsulate the fertilizer between the first polymer film and the second polymer film.

BACKGROUND

Most commercial fertilizers, such as urea, dissolve quickly in water. When applied to lawns or agricultural crops, most of the fertilizer dissolves rapidly in the moisture in the soil. Often, such fertilizers dissolve at a rate that exceeds the rate at which it can be used by plants. This can cause a number of significant problems. First, the excess fertilizer can leach into the groundwater, where it can cause potentially serious environmental damage. In addition, excessive concentration of fertilizer in the vicinity of a plant may result in “burning” of the plant roots.

Because of these problems, it is often necessary to apply these fertilizers in several light doses throughout the growing season, rather than in a single heavy application. However, the need for repeat applications increases labor costs. Worse, repeat applications may cause physical damage to the growing plants because the fertilizer spreading equipment must pass over the plants several times.

In order to avoid the need for repeat applications, various slow or controlled release fertilizers have been developed. For examples, see U.S. Pat. Nos. 4,042,366, 5,147,443, 5,435,821, and 6,231,633. These fertilizers can be applied at the beginning of the growing season and provide an initial high release of fertilizer followed by a slower, gradual release throughout the growing season. Slow or controlled release fertilizers are widely used on home lawns, public lawns, golf courses, home gardens, plant nurseries, and on horticultural crops.

Polymer coated fertilizers are the most technically advanced controlled release fertilizers. Different methods of manufacturing are utilized to produce polymer coated fertilizers. For examples, see U.S. Pat. Nos. 4,019,890, 4,369,055, 5,176,734, 5,211,985, 5,374,292, 5,858,094, and 6,338,746. Polymer coated controlled release fertilizers are typically produced by coating water insoluble semi-permeable polymer layer onto the fertilizer granules. Over time the fertilizer is released into the soil through diffusion from this semi-permeable coating. One of the disadvantages in existing methods is that uniform polymer coating thickness is difficult to achieve. Another disadvantage is that it is hard to control the thickness of the polymer that is applied the fertilizer. Yet another disadvantage is that most of these manufacturing processes are batch type processes which are expensive. Yet another disadvantage with existing processes is that they require multiple applications of polymer to increase the thickness of the polymer coating around the fertilizer.

SUMMARY

A method of manufacturing a controlled release fertilizer wherein the fertilizer is deposited onto a first polymer film and then covered by a second polymer film to encapsulate the fertilizer between the first polymer film and the second polymer film.

DETAILED DESCRIPTION

The invention provides a method of manufacturing a controlled release fertilizer14wherein the fertilizer14is deposited onto a first polymer film and then covered by a second polymer film to encapsulate the fertilizer14between the first polymer film and the second polymer film. The preferred polymer film is any suitable polymer. The fertilizer14may be any suitable fertilizer, including urea.

FIG. 1shows an embodiment wherein the first polymer film is a roll11of polymer film10that is directed under a fertilizer deposition station16which deposits fertilizer14onto the moving polymer film10.FIGS. 1-3show that the roll11of polymer film10allows the controlled release fertilizer production process to be continuous in that the steps of the process can be performed sequentially without interruption. In some embodiments, a conveyor belt20or a similar conveyance device may be used to assist with moving the polymer film10through the production process. As will be discussed below, in some embodiments the conveyance device is made of a material having a relatively high thermal conductivity, such as steel.

The fertilizer14is deposited onto the film10at the fertilizer deposition station16, which may be any suitable device capable of depositing fertilizer14onto the film10. Examples of fertilizer deposition stations16are described in U.S. Pat. Nos. 6,398,989 and 5,395,560. The preferred embodiment deposits molten (liquid) fertilizer14onto the film10, however, in other embodiments, solid granules of fertilizer may be deposited onto the polymer film10. The preferred deposition station16is a Rotoformer (developed by Sandvik Corporation). This fertilizer deposition station16deposits the drops of fertilizer14at approximately equal distances from each other onto the film10.FIG. 5shows several molten fertilizer14droplets that have been deposited onto the polymer film10.

In the embodiment where the deposited fertilizer14is in its molten state, the fertilizer14is shaped like a hemisphere after being deposited on the film10. This hemispherical shape is seen best inFIG. 4. The hemispheric shape allows the fertilizer14to comprise a curved top portion and a generally flat bottom portion. The flat portion can be used as a carrier for other chemicals by laminating paper onto it. To accomplish this, a polymer film is first laminated to a paper web. Then the film is fed under the fertilizer deposition station16for fertilizer14deposition onto the polymer side.

As shown inFIGS. 1-3, in some embodiments of the invention the fertilizer14droplets encounter a cooling zone70after being deposited onto polymer10. The cooling zone70subjects the fertilizer14to cooler temperatures to help solidify and cool the molten fertilizer14droplets after they have been deposited onto the film10and before the second polymer film is applied. In one embodiment shown inFIGS. 1-3, the cooling effect is achieved by spraying cool water onto the underside of the conveyor belt20through nozzles19. After contacting and cooling the conveyor belt20, the water drains back into a holding tank for reuse. The heat from the fertilizer14is transferred to the cooled conveyor belt20. In this embodiment, it is beneficial for the conveyor belt20to be made of metal to allow the heat from the fertilizer14to be easily transferred to the belt20. A chilling tower18or some similar means may be used to cool the water before it is sprayed onto the conveyor belt20. In an alternate embodiment, the cooling zone70may be achieved by blowing cool air over the top of the fertilizer14.

After the fertilizer14droplets have hardened, the second polymer film is deposited onto the top of the fertilizer14. In the embodiment shown inFIG. 1, the second polymer film is polymer12which is placed over the fertilizer14in a thermal lamination process to encapsulate the fertilizer14between the second (top) polymer film12and the first (bottom) layer of polymer film10. This polymer film12film may be stored on a roll15that is similar to the roll11used to store the bottom polymer layer10. A heating unit26such as a hot air blower is used to apply heat after the top layer of laminate polymer12has been applied. The heat from the heating unit26helps to soften the polymer layers10,12and fuse them together to encapsulate the fertilizer14between the layers of polymer10,12. Other heating devices such as radiant heaters may also be used in place of hot air blower26.

In an alternate embodiment show inFIG. 2, the second polymer film is polymer40. In this embodiment, the top polymer film40is extruded onto the fertilizer14after the fertilizer14has been deposited onto the polymer film10. In this embodiment, the lower polymer film10containing deposited fertilizer14is directed toward the extruder43where an extrusion die42deposits a layer of molten polymer40onto the deposited fertilizer14. This results in the fertilizer14being encapsulated between the lower polymer film10and the upper polymer film40. In some embodiments, a compressed air blower22is used to help spread the extruded polymer40onto the fertilizer14.

In an alternate embodiment shown inFIG. 3, the second polymer film is polymer30. In this embodiment, after the fertilizer14is deposited onto the film10, the lower polymer film10is directed into polymer spraying station32where a polymeric solution30is sprayed onto deposited fertilizer14through nozzle21. The sprayed polymer30results into polymer film30. The fertilizer14is encapsulated between the lower polymer layer10and the upper polymer layer30.

As shown inFIGS. 1-3, in some embodiments of the invention the fertilizer14droplets encounter a heating zone72before and/or during the time when the second polymer film is applied. The heating zone72helps to remove any moisture present on the polymer film10to help the second polymer film stick to the fertilizer14and fuse with the bottom layer of polymer10. In one embodiment, the heating zone72is created by spraying warm water onto the underside of the conveyor belt20through nozzles19. The heat is transferred through the conveyor belt20to the fertilizer14droplets while the water drains into a holding take for reuse. A heat exchanger28or some similar means may be used to heat the water. Other heating devices such as radiant heaters can be used.

As shown inFIGS. 1-3, some embodiments use a static charge generator50to help fuse the second polymer film with the fertilizer14surface and the first polymer film. In one embodiment, the static charge generator50consists of a DC high voltage generator and one or several charging electrodes. The generator generates the high voltage for the high voltage points in the electrodes. The high voltage points produce ions to charge the materials, which then adhere to each other or to other surfaces electrostatically.

As shown inFIGS. 1-3, in some embodiments the fertilizer14droplets encounter a second cooling zone74after the second polymer film has been deposited. The second cooling zone74helps to cool the encapsulated fertilizer14and the polymer layers to make it easier to cut the encapsulated fertilizer14apart. The second cooling zone74can operate in a manner similar to the first cooling zone70discussed above. As shown inFIGS. 1-3, the second cooling zone74may use the same water tank and chilling tower18as the first cooling zone70discussed above. In addition to the second cooling zone74, some embodiments also use a cold air fan29to cool the encapsulated fertilizer14after the second polymer film has been applied. The cool air fan29helps to cool the fertilizer14and polymer layers and bring them toward room temperature. The cooling of the fertilizer14and first and second polymer films makes it easier to cut apart the encapsulated fertilizer14droplets.

The final step of the process is to separate the individually encapsulated fertilizer14droplets so that the individual pellets may be spread onto the ground. This cutting step may be performed by any suitable cutting device, including a laser or a punching machine. In one embodiment, the cutting device is a standard cutting machine like those used in processes to cut labels, paper, or packaging cartons.

The release rate of the fertilizer14in the controlled release fertilizer pellets can be controlled by varying the composition and thickness of the polymer films. The present invention allows a high degree of control related to the type and quantity of the polymer used and thus allows the release rate of the final product to be specifically tailored to fit many different growing situations. The ability to use a wide range of polymers with tight control over coating thickness provides a very precise and predictable fertilizer release rate.

One reason the invention allows a high degree of release rate control is because many different types of polymer compositions may be used in the present invention. This is beneficial because different polymers may have different moisture barrier properties which can result in varying fertilizer14release rates. For example, the following polymer films may all be used: virgin/recycled polyethylene, biopolymers, polypropylene, polystyrene, high impact polystyrene, acrylonitrile butadiene styrene, polyethylene terephthalate, polyester, polyamides, polyvinyl chloride, polyurethanes, polycarbonate, polyvinylidene chloride, polylactic acid, starch based polymers, and aminoplastic resin. One reason a wide variety of polymers may be used with this invention is because polymer film formation onto the fertilizer14surface is not required to bind the polymers to the fertilizer. Further, for all embodiments described above, the first polymer film (bottom layer of polymer10) may be made of a different composition than the second polymer film (top layer of polymer12,30,40).

Another reason the invention allows a high degree of release rate control is because the lamination (FIG. 1), extrusion (FIG. 2), and spray methods (FIG. 3) for depositing the top layer of polymer may be intermixed. For example, one embodiment may employ all three methods of depositing the top layer of polymer in the same run cycle. Other embodiments may employ two of the three methods for depositing the top layer of polymer. When intermixing the polymer deposition methods, different polymer compositions may be used. For example, it is possible to laminate one type of polymer film composition onto the top of the fertilizer and then spray another polymer film composition onto the top of that.

Yet another reason why invention allows a high degree of release rate control is because top polymer film12,30,40and bottom polymer film10may be made of multiple layers of different polymers and these layers may have different thickness. For example the top layer12,30,40and bottom layer10may have two or more layers of polymers each and these layers may have different thicknesses.

Yet another reason why the invention allows a high degree of release rate control is because the first and second polymer films may be made of different thicknesses. The first and second polymer films may be pre-manufactured using cast/blown film extrusion or co-extrusion process. These processes are well known commercially and are commonly used to manufacture polymeric films of very precise thickness. Another advantage of this process is the ability to make films of multiple layers. For example a film containing seven or more layers of different polymers of different thicknesses can be made at high speed using cast film co-extrusion process.

Yet another reason the invention provides a high degree of fertilizer release rate control is that various additives may be added into the polymer films during manufacturing for customizing the film properties. For example hydrophilic additives may be added into the polymer during the manufacturing to increase the fertilizer release or biodegradation additive may be added for customizing the polymer rate of degradation in soil.

Having thus described the invention in connection with the preferred embodiments thereof, it will be evident to those skilled in the art that various revisions can be made to the preferred embodiments described herein with out departing from the spirit and scope of the invention. It is my intention, however, that all such revisions and modifications that are evident to those skilled in the art will be included with in the scope of the following claims.