IMPROVING THE LONG-TERM DIFFUSION RATE OF CONTROLLED
RELEASE HERBICIDES IN THIN- WALLED DRIP IRRIGATION LINES
BACKGROUND OF THE INVENTION The slow release of trifluralin and other dinitro-anilines from polymers to protect subsurface drip irrigation systems from root intrusion is taught by Ruskin in South Africa patent 86/1 133 dated September 24. 1986, which is incorporated herein by reference. This patent describes incorporating the trifluralin (or other herbicide or bioactive material) into the drip emitter itself or into the body of a tape or tube. In one embodiment, the trifluralin is present in the emitter or tape or tube wall in a concentration of 2.4% by weight. The trifluralin is slowly released from the buried drip irrigation device in order to inhibit root intrusion into the irrigation device. This technology works well with heavy walled tube and molded emitters. However, there are products which are thin- walled tapes (Gilead U.S. Patent 4.195,784, Chapin U.S. Patent 4,534,515, and Allport U.S. Patent 4.247.051 , etc.) which are also used for drip irrigation. These tapes are formed by various techniques with embossed or printed flowpaths. The essential common factor is that there is no thick molded part or extruded section into which one can conveniently incorporate the trifluralin.
Slow release technology generally is described in Burton et al. U.S. Patent 5,1 16,414. Trifluralin (also referred to herein by the trade name Treflan) in pellet form is slowly released from a polymer into the soil to inhibit root intrusion into that area without killing the plants bearing those roots. Preparation of slow release polymeric materials comprising carbon-filled polymers and an herbicide is described in Burton et al. '414, particularly at column 10, lines 18- 26.
Experience has shown success in protection of buried drip irrigation devices from root penetration lasting for periods of many years. The bioactive chemical is incorporated into a polymer matrix which protects the bioactive chemical from chemical or biological degradation while providing a controlled but sustained release of the bioactive material to the soil adjacent the device for a substantial period of time, typically lasting many years.
The performance and life expectancy of such root growth inhibiting drip irrigation devices can be measured in terms of release rate or diffusion rate of the bioactive chemical from the host polymeric binder. One of the drawbacks of present polymeric carrier delivery systems using trifluralin as a root growth inhibiting chemical is that diffusion rate or release rate of the chemical is difficult to sustain over long periods of time in a thin-walled product. For thin-walled products containing trifluralin. for example, essentially all of the trifluralin diffuses or migrates to the surface. This results in a short useful life of the product.
Because the release rate of the trifluralin is directly proportional to the surface area and inversely proportional to the thickness of the bioactive material, it is not practical to use the methods of Ruskin 86/1133 to achieve a life of many years with tapes. Thin-walled tape
driplines are essentially low in cost, and the market will not pay a substantially higher price for the root intrusion protection. Trifluralin is an expensive product.
SUMMARY OF THE INVENTION According to one embodiment of the present invention, a relatively thick bead of a polymeric carrier containing a slow release herbicide, such as a polyethylene compound containing trifluralin or other dinitro-anilines. is extruded along the edge of the flowpath of the dripper. This bead will either be continuous in cross section, or preferably will be at maximum cross section immediately in line with the water exit hole of the dripper flowpath and of much thinner cross section along the rest of the length of the tube. The bead can be interrupted completely between the thick cross sections adjacent to the water exit hole and the rest of the dripper tube, thereby forming separate beads of bioactive material adjacent the exit holes.
The process of root intrusion protection operates by means of the trifluralin moving from the bead through the polymer of the tape wall and directly into the soil, where it is fixed in the zone around the tape. Hence, if the source of herbicide material is located close to the exit hole it will efficiently protect the exit hole against root intrusion. Because the trifluralin-impregnated bead does not perform any mechanical function in the dripper delivery system, it can be heavily filled with trifluralin and carbon to the maximum as taught by the slow release materials in Burton U.S. Patent 5,1 16,414, for example; this would not be practical if it were part of the tape wall itself as taught by Ruskin 86/1133. The result is a drip irrigation tape or tube product in which the higher amount of bioactive material is available to significantly extend the useful life of the product.
DRAWINGS FIG. 1 is a fragmentary cross-sectional perspective view illustrating a drip irrigation tape or tube having an internal flowpath with a continuous bead of slow release herbicide-impregnated polymeric material extruded as a continuous bead alongside the flowpath.
FIG. 2 is a fragmentary cross-sectional perspective view showing a drip irrigation tape or tube similar to FIG. 1 in which separate beads are located adjacent the exit holes of the dripper flowpath.
DETAILED DESCRIPTION
Figs. 1 and 2 illustrate cross sections of a drip irrigation tape or tube 10 made in accordance with this invention. The tape or tube is a thin-walled product. Such thin- walled tape or tube products are defined by a wall thickness generally in the range of about 4 to about 50 mils. It is believed that the present invention is most useful for a drip irrigation tape product having a wall thickness in the range of about 4 to about 20 mils. A typical wall thickness of such tape products is about 8 mils, although tape products can have a wall thickness up to about 40
mils. The invention also can be useful with thin-walled tubes generally having a thickness of about 30 to about 50 mils. The tape is preferably made of low density polyethylene. The tape or tube includes an internal flowpath 12 of generally inverted U-shaped cross section (when the flowpath is facing downwardly and the tube is under fluid pressure as shown in the drawings). The flowpath extends continuously along the inside of the tape or tube. The flowpath internally contains a labyrinth or other means of reducing the flow rate of water which enters the flowpath from the interior of the tape or tube through an inlet 14 in the flowplath. The water passing through the flowpath exits the tape or tube through external holes 16 spaced apart along the drip irrigation line. The external holes extend from the interior of the flow path through the exterior wall of the tape or tube to supply water at a low drip rate and at predetermined spaced-apart intervals along the irrigation line.
Fig. 1 illustrates a continuous bead 18 of a bioactive material extending continuously alongside the edge of the flowpath on the inside of the dripper. Fig. 2 illustrates the same bead of material only in discontinuous form comprising separate self-contained beads 20 of the bioactive material located adjacent the exit holes of the irrigation line. As a further alternative
(not shown), the bead of bioactive material can be formed with a larger cross section (as in Fig. 2) and of thinner or reduced cross section between the exit holes so that the largest surface area of the bead is at least present in the vicinities of the exit holes.
The bead of bioactive material is preferably formed by incorporating the bioactive material into a suitable polymeric binder that preferably can be extruded along the inside of the tape or tube and bonded to the inside wall of the tape or tube. The figures illustrate the bead extruded as an elongated cylinder 18 or as separate cylindrical shapes 20 essentially circular in cross section. This configuration maximizes the surface area to volume ratio of the bead which has a beneficial effect on long-term diffusion rate during use, inasmuch as the release rate of the bioactive material is directly proportional to surface area. The presently preferred bioactive material is trifluralin (Treflan) or other dinitro-aniline material, although other herbicides may be used. These would include the bioactive materials referred to in Ruskin 86/1 133. The trifluralin is combined with a suitable carrier, such as carbon black, and impregnated uniformly throughout a polymeric binder material such as low-density polyethylene resin or other polyolefin resins.
Slow release products such as Treflan which are incorporated into polymers such as polyethylene used as the encapsulation matrix are compounded by initially absorbing the Treflan into particles of carbon black. The carbon black is used because it is inert, in particulate form, and capable of absorbing and retaining within it the Treflan molecules similar to the absorption characteristics of a sponge. The carbon black and Treflan preferably are absorbed at a one-to-one ratio and then blended with a small amount of polyethylene resin in particulate form, and this blended material is later mixed into a standard polyethylene carrier material which can then be formed into the bead of bioactive material. A presently preferred process for making the
extrudable bioactive polymeric material is described at page 8 of Ruskin 86/1133, although the proportions of bioactive material as used in the present invention are much greater.
In a preferred form of the invention the ranges of components for the bioactive bead material are: about 10% to about 25% carbon black, about 10%) to about 25% trifluralin, and about 80%) to about 50% polyethylene, by weight. A useful level of trifluralin (or other dinitro- aniline) can be essentially the same as or up to about 35% greater than the level of carbon black carrier contained in the polymeric resin-based bioactive material. For example, a bioactive extruded bead containing about 50% polymeric binder can contain about 25% to about 28%) trifluralin with the remaining 25%> to 22% comprising the carbon black carrier. Useful results also can be produced in which the proportions of bioactive material and its active carrier (e.g., trifluralin and carbon black) exceed 50%.
In one embodiment a polyethylene tape having a wall thickness of about 8 mils, the tape can be made with a 20 mil diameter bead coextruded inside the tube adjacent the flowpath. The embodiment of Fig. 2 can be made by coextruding the beads 20 at say 12-inch intervals, in which the separate beads are approximately one inch long and extruded in the vicinity of the dripper exit holes. This can be done by a timer control on the extrusion die output to stop and start the bead extrusion process. The beads are bonded by hot melt adhesion to the polymeric sheet or tape material.
As mentioned, the cylindrical bead with its circular cross section is preferred because it maximizes the surface to volume ratio which can maximize the life of the product. However, during manufacturing operations, the bead can be pressed flat somewhat to produce a minute amount of adhesion in the flattened area between the bead and the inside wall of the tape or tube.
During use, the trifluralin material diffuses from the bead and passes essentially transparently through the polyethylene wall of the tape and to the outside of the irrigation line to the vicinity of the exit hole where it provides continuous protection against root intrusion over an extended period of time. By providing the bioactive material in the separate bead of polymeric binder material, the loading of bioactive material in its binder can be at a very high percentage level as compared with incorporating such a material into the wall of the tape or tube or into a drip emitter itself. The separate extruded bead of bioactive material, because of its high-loading level of trifluralin or other herbicide, need not have the structural strength or tensile strength necessary for any mechanical function for the dripper system, which would otherwise be necessary if such a material is incorporated into the wall of the tape or tube or into the drip emitter itself. The high loading of herbicide material and its carrier in the polymeric binder reduces structural properties, but since the bead is not necessary for any mechanical or structural function, the resultant loading of bioactive material can be at a high level. This increases the useful life of the product when compared with incorporating such bioactive materials into the wall of the tape or tube or into the drip emitter itself. The invention is particularly useful in
extending the useful life of thin- walled tape products because such a high level of loading is not otherwise functionally practical for the thin tape wall.