A SOLAR-REACTIVE MULCH FILM

Abstract

A solar-reactive agricultural mulch film including a sheet of heat-shrink material having one or more layers and uniform thickness, formed by a mechanical process. The process includes elongating the sheet, in a longitudinal direction, evenly across a width of the sheet, while the sheet is at a stretching temperature to pre-strain the sheet evenly across the width, and yields the solar-reactive agricultural mulch film having the uniform thickness throughout and being configured to, in operation, shrink when an exposure temperature of the material is elevated, by exposure to solar energy, to a shrinking temperature. The process may include elevating the sheet to an above-ambient stretching temperature for the elongating, or elongating the sheet at an ambient temperature.

Claims

1-51. (canceled)

52. A solar-reactive agricultural mulch film comprising a sheet of heat-shrink material having one or more layers and uniform thickness, formed by a mechanical process comprising: elongating the sheet, being at a stretching temperature, in a longitudinal direction, evenly across a width of the sheet, to pre-strain the sheet evenly across the width, yielding the solar-reactive agricultural mulch film having the uniform thickness throughout and being configured to, in operation, shrink when an exposure temperature of the material is elevated, by exposure to solar energy, to a shrinking temperature.

53. The solar-reactive agricultural mulch film of claim 1 wherein the elongating is performed to evenly pre-strain the sheet in the longitudinal direction, across an entirety of the sheet, beyond a yield point of the material, by a machine direction orientation sub-process.

54. The solar-reactive agricultural mulch film of claim 2 wherein: the mechanical process comprises elevating a temperature of the sheet to the stretching temperature being above ambient; the pre-strained one or more layers is caused or allowed to cool after reaching a predetermined level of yield whereat resultant molecular deformation stresses are retained in the material to, in operation of the solar-reactive agricultural mulch film, cause shrinkage, and not expansion, of the solar-reactive agricultural mulch film when heated by the solar energy to the shrinking temperature.

55. The solar-reactive agricultural mulch film of claim 2 wherein: the stretching temperature is an ambient temperature; and the elongating is performed to stretch the sheet to beyond the yield point of the material, and retaining the pre-strained one or more layers at the ambient temperature.

56. The solar-reactive agricultural mulch film of claim 1 wherein: the mechanical process comprises elevating a temperature of the sheet to the stretching temperature being above an ambient temperature; the elongating is performed with a predetermined tension to stretch the sheet beyond a yield point of the material; and the mechanical process comprises, after the elongating, immediately cooling the pre-strained one or more layers to below the ambient temperature.

57. The solar-reactive agricultural mulch film of claim 1 wherein resultant molecular deformation stresses retained in the material, through the pre-straining of the mechanical process, facilitate, in operation of the solar-reactive agricultural mulch film, the shrinking, with no expanding, of the material in one or more directions when heated by the solar energy to the shrinking temperature.

58. The solar-reactive agricultural mulch film of claim 1 wherein longitudinal pre-straining of the one or more layers of heat-shrink material increases tensile strength of the solar-reactive agricultural mulch film, allowing the solar-reactive agricultural mulch film to, in operation, be machine-handled, by mechanical apparatus, to be: laid without tendency to tear; and removed without tendency to lose material.

59. The solar-reactive agricultural mulch film of claim 1 wherein the sheet comprises: a single-wound plastic sheet; or a bonded and blocked double-wound plastic sheet forming a single resulting sheet.

60. The solar-reactive agricultural mulch film of claim 1 wherein by the mechanical process: the sheet is not subjected to any post orientation to anneal, de-stress, or pre-relax shrinking forces entrapped in the plastic sheet prior to winding the sheet onto a spool; and shrinking stresses trapped in the film are such that the film will, in operation, if unrestrained, shrink by approximately 10% to 30% in surface area when the film reaches a temperature of only about 55 degrees Celsius.

61. The solar-reactive agricultural mulch film of claim 1 wherein by the mechanical process: the sheet is not subjected to any post orientation to anneal, de-stress, or pre-relax shrinking forces entrapped in the plastic sheet prior to winding the sheet onto a spool; and the shrinking stresses trapped in the film are such that the film will, in operation, if unrestrained, commence shrinking once the film, in response to the solar energy, reaches the shrinking temperature.

62. The solar-reactive agricultural mulch film of claim 1 wherein the mechanical process comprises subjecting the sheet to a post orientation relaxing process to fully de-stress or pre-relax the sheet from residual elastic forces, comprising stretching the sheet below a yield point of the sheet prior to winding the sheet onto a spool.

63. The solar-reactive agricultural mulch film of claim 1 wherein the one or more layers comprises at least one layer of a polyolefin polymer combined with an additive promoting a longer lifespan for the mulch film than the film would have without the additive.

64. The solar-reactive agricultural mulch film of claim 12 wherein the additive comprises at least one item selected from a group consisting of: a process aid; an anti-oxidization additive; a hindered amine light type ultra violet light stabilizer; an ultraviolet light absorber; a plasticizer; and a lubricant.

65. The solar-reactive agricultural mulch film of claim 1 wherein the heat-shrink material comprises two or more co-extruded layers.

66. The solar-reactive agricultural mulch film of claim 1 wherein at least one of the layers of heat-shrink material is substantially impermeable to gas.

67. The solar-reactive agricultural mulch film of claim 1 wherein at least one of the layers of heat-shrink material contains between about 2% and about 8% of carbon black particles of between about 17 and about 100 nanometers in size.

68. The solar-reactive agricultural mulch film of claim 1 wherein the at least one layer of heat-shrink material is black in color or opaque to ensure the sheet blocks all light transmission and absorbs sufficient solar energy to heat the sheet sufficiently to initiate shrinkage of the heat-shrink material.

69. The solar-reactive agricultural mulch film of claim 1 wherein longitudinal pre-straining of the one or more layers of heat-shrink material increases tensile strength of the solar-reactive agricultural mulch film, allowing the solar-reactive agricultural mulch film to be produced at a relatively thinner gauge, yet have strength properties of a thicker film produced in another manner.

70. A solar-reactive agricultural mulch film comprising a sheet of heat-shrink material of uniform thickness, formed by a mechanical process comprising: elevating a temperature of the sheet to a stretching temperature, greater than ambient temperature; elongating the sheet longitudinally while the sheet is at the stretching temperature; and cooling the sheet, or allowing the sheet to cool, to at least the ambient temperature, yielding the solar-reactive agricultural mulch film being configured to, in operation, shrink when an exposure temperature of the material is elevated, by exposure to solar energy, beyond a shrinking temperature.

71. A process of forming a solar-reactive agricultural mulch film comprising a sheet of heat-shrink material of uniform thickness, the process comprising: elevating a temperature of the sheet to a stretching temperature, greater than ambient temperature; elongating the sheet longitudinally while the sheet is at the stretching temperature; and cooling the sheet, or allowing the sheet to cool, to at least the ambient temperature, yielding the solar-reactive agricultural mulch film being configured to, in operation, shrink when an exposure temperature of the material is elevated, by exposure to solar energy, beyond a shrinking temperature.

Description

DESCRIPTION OF THE DRAWINGS

[0137] In order that the technology be better understood and put into practical effect, reference will now be made to the accompanying drawings wherein:

[0138] FIG. 1 shows film sheet formed by the blown film process according to the present technology.

[0139] FIG. 2 shows film sheet formed by the cast film extrusion process according to the present technology.

[0140] FIGS. 3, 4 and 5B show a preferred embodiment and its use according to the present technology.

[0141] FIG. 5A shows the contrasting operation of prior art mulch film hen compared to the present technology.

[0142] The invention is not limited to the example embodiments illustrated.

DETAILED DESCRIPTION

[0143] Reference is made to FIG. 1 and FIG. 2 indicated by numerical reference in parenthesis in the text. Throughout the detailed description, numbers referring to the same components in different drawings will be kept likewise identical where possible and only varied where the context and avoidance of ambiguity call for a change in the enumeration.

[0144] The process of manufacturing heat shrink mulch according to the blown film process is shown in FIG. 1. This involves polymer being heated until melted in an extruder (4), or multiple extruders, and extruded through a circular die (5). The film is drawn upwards as a bubble while being cooled to a solid state by the cooling air ring (6) and is passed through a set of collapsing boards (7) by a first set of top nip rollers (8). The film travels further along the line as a double sheet or collapsed tube (9).

[0145] Alternatively, the film may be extruded by the process known as a cast film extrusion shown in FIG. 2 wherein the polymer is heated and melted in an extruder or multiple extruders and formed as a co-extruded sheet. The polymer is extruded via a flat die (18) onto chilled rollers (16) (or, chilled casing rollers) to produce a single sheet of mulch film (17).

[0146] The polymer making up the film may consist of either low density polyethylene or high density polyethylene or linear low density polythene or metalocene linear low density polyethylene or blends thereof. The polymer is extruded either as a mono layer or a multilayer co-extrusion. In various embodiments, at least one layer contains high levels of carbon black particlesfor example, 14% to 16% of carbon black master batch containing 40% of 17 to 25 nanometer size particles of carbon black.

[0147] In various implementations, the polymer blend making up the mulch film suitably contains additives to ensure a long lifespan of the mulch film, such as process aids, anti-oxidization additives, HALS type ultra violet light stabilizers, UV light absorbers, plasticizers and lubricants.

[0148] Subsequent to exiting the top nip rollers (8) of the blown film line of FIG. 1, or exiting the chilled rollers (16) of the cast film line of FIG. 2, the mulch film is passed through an in-line, machine direction orientation (MDO) machine wherein there exists a second nip roll set (10) which feeds the film into the MDO machine and which is rotating at the same lineal surface speed as the previous exit nip rollers (8, 16).

[0149] This nip roller set (10) is in various embodiments heated to a temperature that may be up to 150 degrees Celsius and, in some implementations, preferably is heated so as to raise the temperature of the sheet to approximately 55 to 75 degrees Celsius, given consideration to the line speed, film thickness, film input temperature and the distance the film travels in contact with the hot roller.

[0150] The sheet is immediately passed through a third set of nip rollers (12) which are, in some implementations, preferably cooled to 40 degrees Celsius, or ambient temperature, or in some implementations approximately 15 degrees Celsius, to chill the sheet prior to the sheet returning to ambient temperature.

[0151] The third set of nip rollers (12) in various embodiments rotate at between 150% and 300% faster than the second heated nip rollers (10) set. On occasion it may be preferable to rotate the third set of nip rollers (12) at faster rates so as to stretch the sheet by a greater amount.

[0152] The sheet is stretched between the second set of nip rollers (10) and third sets of nip rollers (12) and is maintained under the full stretching stress. In various embodiments the sheet is therein not allowed to relax prior to the film temperature being reduced in temperature, in various implementations, to approximately 15 degrees Celsius or at least ambient temperature.

[0153] Between the second set of nip rollers (10) and third set of nip rollers (12) may be placed smaller idler rollers (11) which reduce the amount of film that is unsupported during the stretching process. The shorter the amount of sheet unsupported the less will be neck-down (width narrowing) due to the stretching.

[0154] Upon exiting the MDO machine the film may pass through a printing station where plant lines are printed onto the film.

[0155] Subsequently the sheet has two edge trims cut away (13) so as to ensure the thicker edge section of the film does not impact on the even thickness of the film across the web. The film may then be rolled onto a cardboard tube inner core as a single-wound sheet from a cast line or blocked blown film line, or if the film is in the form of two flat sheets, the sheets may be split into two separate sheets (14) and then rolled onto two core tubes (15), containing the desired number of meters of mulch film, as single-wound sheet.

[0156] During the MDO process, the film fed into the MDO machine is extruded initially thicker than is required, and drawn narrower and thinner; however the thinner film retains the tensile properties of the thicker in-feeding film requiring as much force to ultimately break the thin film as required to break the thicker film.

[0157] On occasion it may be that the extruded film has the thickness profile of the bubble or cast sheet alteredi.e. made thin on the edges, so as to accommodate the thicker edges arising after the film is subjected to MDO. This alteration provides a film being fed into the MDO machine which has thinner edges than the remaining body of the film when measured across the width of the web, so that as the film necks in during MDO the thickening of the edges is compensated for by the infeed film being made thinner at the edges. This effect may be so efficient at removing thick edges that no trim, or a reduced trim, need be taken from the edges.

[0158] By conducting the MDO process at temperatures of between 55 and 75 degrees Celsius and quickly cooling the film while under full stretching stress, the film retains a measure of plastic elongation which allows it to achieves a shrinkage of approximately 25% when the surface of the film is subsequently raised above about 55 degrees Celsius as occurs, for example, when completely opaque mulch film is exposed to solar radiation. This shrinking effect commences at temperatures as low as about 40 degrees C. and the rate of shrinkage increases as the temperature is increased.

[0159] It is possible to stretch linear low density polyethylene at ambient temperatures of around 20 degrees Celsius without having the film snap and break, however the speed at which the film may be stretched is substantially reduced, the extent to which the film may be stretched is reduced and the amount of neck down, and consequent trim wastage experienced may be increased, when compared to heated MDO.

[0160] By raising the temperature of the film during the stretching process, it is possible to achieve a faster, smoother and more uniform stretch than could be achieved by cold stretching the film at ambient temperature and allows the film to be stretched inline. This is especially applicable where lower percentages of stretch are employed. The increase in temperature of the film also allows stretch percentages above 100% to take place, at normal production speeds, without having the film web break off during the stretching process.

[0161] However, mulch film which has been elongated beyond its yield point at ambient temperature and which shrinks when exposed to solar radiation is also included as a method of manufacturing the heat shrinking mulch film.

[0162] In various embodiments, the temperature that the film achieves during stretching will depend on the temperature of the in-feeding nip roller, the speed at which the film passes through the nip and the length of time the film is in contact with the heated nip.

[0163] If the farmer using the mulch film is planting his seedlings close together, say 150 mm or less, high levels of stretch, above about 300%, can cause the film to become very linear and to split between the holes, especially when the film is being lifted from the ground, making it difficult to remove by mechanical means.

[0164] If a high stretch ratio above about 300% is employed the film may become easy to split and may split when placed under the tuck wheels of a laying machine.

[0165] It has been noted that stretch ratios of between about 1:2 and 1:3 will achieve sufficient shrinkage in the film to ensure it shrinks and does not flap in the wind.

[0166] Should the mulch sheet be comprised of two sheets blocked and bonded to form a single sheet the two separate sheets support each other so that any propensity to split in one film is countered by the second bonded sheet, this allows for higher stretch ratios as much as 1:6 to be employed.

[0167] The third set of nip rollers (12) feeds the film out of the MDO machine and is chilled to approximately 15 degrees Celsius and freezes in the stresses imposed on the film during the stretching process by chilling the film while it is still undergoing the stress of being stretched.

[0168] The shrinking stresses in the film arise as to the long chain polymer molecules, which exist as coiled crystals and randomly laid chains are extended in a linear direction while at an elevated temperature, which softens the polymer, and then cooled below that temperature so that they cannot easily return to their original form without being raised to a similar temperature as they were stretched under.

[0169] Subsequent to passing through the MDO section a trim is taken from both sides of the film allowing the two flattened sheets of plastic to be separated and wound up on cardboard tubes as single-wound sheeting.

[0170] Having produced a sheet of mulch film according to the methods described above, several changes to the properties of the film will be noticed when compared to prior art mulch film.

[0171] Firstly it will be noticed that the film has improved tensile properties in the machine direction even though it has been drawn thinner. This is to be expected as the film starts out as a thick film and retains the tensile properties of that thick film even when being drawn thin.

[0172] Also during the MDO process both main chain molecules and side branched molecules are aligned in the machine direction improving the breaking strength in that direction. Less crystallization will also take place as the molecules are forced into a straightened form rather than being allowed to coil into a crystal.

[0173] Secondly when placed in direct sunlight, the film will shrink in surface area by a minimum 25%, in various embodiments. Prior to being placed in the sunlight, the film does not shrink and it will commence shrinking as soon as the solar radiation trapped by the carbon black in the film heats the film to above about 40 to 55 degrees Celsius. This shrinkage occurs even though the solar energy only needs to raise the surface temperatures to between about 40 to 55 degrees Celsius, although much higher temperatures may be achieved in hot climates.

[0174] It will be noticed that the film in some implementations does not immediately expand when heated by solar radiation, but can expand immediately or very quickly. When the stretched heat shrink mulch film reaches sufficient temperature to achieve the freedom of molecular movement to allow expansion, the in-built shrink stresses contained in the film causes it to shrink by a greater proportion than it will expand.

[0175] The speed of this shrinkage is in various embodiments determined or affected by the level of temperature rise, which is determined by the strength of the solar energy striking the film and the outer color of the film. In hot conditions with bright summer sunlight, the film shrinks quickly and, if unrestricted, can be visibly seen to be shrinking as soon as it is exposed to sunlight. In colder conditions with weaker sunlight such as in autumn, the film shrinks slower and may take several minutes to reach its maximum shrinkage.

[0176] Shrinking mulch film made according to the present technology can be made thinner than normally required as the film does not need to have strength to sustain its integrity by being stretched tightly by the tuck wheels over what can often be rough soil and stones. The film may be laid loosely and then employ solar energy to shrink and tighten over the bed. This allows for saving in plastic materials, cost of mulch film, cost of laying time as the laying machine may travel faster if there is no need to pull the film tightly over the bed.

[0177] This slightly delayed shrinkage is advantageous because the mulch film may be laid onto the bed using existing mulch laying equipment without the risk of the film shrinking away from edge tuck wheels and once laid and the edges are buried, the film will then shrink to the limits imposed by the buried edges and the shape of the growing bed.

[0178] This shrinkage is surprising as normally sheets of film made from linear low density polyethylene or metalocene linear low density manufactured by the blown film process does not shrink substantially when subject to heat. Most commercial heat shrink films are made from low density polyethylene which does have a propensity to shrink. Sheets made from pure low density polyethylene do not have the tensile properties to be successfully used as mulch film, especially when compared to the much improved tensile properties of LLDPE or mLLDPE films.

[0179] Commercial heat shrink film, employing heat shrink tunnels or hot air blowers, initially expands as it approaches its melting point temperature, above which it will shrink. This innovative heat shrink mulch film does not initially expand but rather immediately shrinks once its temperature is sufficiently raised.

[0180] Also surprising is the low temperature at which the film shrinks, normally polyethylene has to be raised in temperature to near its melting point (approximately 110 to 120 degrees Celsius) in order to achieve shrinkage in the film. This is due to the shrink memory of the polymer caused by the blown film process in which the polymer is stressed as it is stretched in the transverse direction as it exits the extrusion die over the air trapped in the bubble and is further stretched in the longitudinal direction by being drawn upwards by the top nip rollers. This stress is locked into the sheet at the time it solidifies during the extrusion process at approximately 110 to 120 degrees Celsius.

[0181] Prior art film manufactured using the cast extrusion method is a stable product that does not normally shrink at all when re-heated, although it may expand. Mulch films made using this innovative heat shrink mulch film method of manufacture will shrink in the sun even though the initial film has been made by the cast film process.

[0182] By subjecting the agricultural mulch film to heated MDO without post relaxation, stresses are built into the film which, surprisingly, allows it to shrink at the relatively low temperatures achieved by the film when heated under the effects of solar radiation being absorbed by the color additives in the film.

[0183] The polymer normally has to be raised to close to the melting temperature of the polymer prior to shrinkage beginning.

[0184] To have the mulch film shrink by up to about 25% when heated to only about 55 degrees Celsius is very unusual, but this ability proves to be very useful in preventing flapping and billowing in the mulch film.

[0185] The high level of carbon black contained in the film absorbs the light striking the mulch film placed on a crop growing bed, this light is converted into heat which provides the energy to increase the temperature of the film by in excess of about 55 degrees Celsius at which temperature the contained stresses in the film are released causing the film to permanently shrink in surface area.

[0186] Should the mulch film be restrained as it shrinks by, for example, having the edges of the sheet buried, the long chain polymer molecules will shrink to a point where the molecules shrink plastically by more than they will be physically allowed, this draws the film very tight and builds into the film a measure of elasticity. When the film is cooled such as at night elastic tension is formed into the film so that as it re-heats the following day it will remain tight over the bed.

[0187] Should the film be made clear it is possible for mulch film made according to the present technology to shrink as the film will heat the soil which will heat the air trapped between the film and the soil and the air will heat the film causing it to shrink.

[0188] The shrinking ability of mulch film made according to the present technology considerably improves the quality and effectiveness of the mulch film when compared to existing prior art plastic mulch films, particularly when considering that the shrinking effect is present in both black mulch film and black/white mulch film and clear mulch film.

[0189] Referring to FIGS. 3, 4 and 5B, when unwound and applied to the surface of a raised growing bed (20) the mulch film (22) is exposed to solar radiation (24). This radiation is absorbed by the color additives contained in the film which raises the surface temperature of the mulch sheet to a target temperature, in some embodiments as much as 70 degrees Celsius or more. This raising of the surface temperature makes it easier for the stretched molecules to de-stress and shrink back, mainly lengthwise (26) and partially crosswise (28) to their pre MDO stretched length or at least to partially contract wherein as a consequence the entire surface area of the sheet is reduced.

[0190] As the temperature of the mulch sheet rises, should the sheet be unrestrained, the first physical signs of the sheet being affected by the rise in temperature is for the sheet to shrink. In contrast to the present technology, prior art mulch film (see 50 in FIG. 5A), which has not been pre-treated during manufacture according to the methods described above, will instead expand, not shrink, as the temperature is increased.

[0191] Should sections of the mulch film be buried (30, 32), such as due to the weight of the soil (34), the exposed area will not be able to shrink in overall surface area but will shrink as tightly as possible against the profile of the growing bed (20). As a consequence, seedlings (36, 38, 40) shown in FIG. 4, growing through planting holes (42, 44, 46), will not be damaged either by the sheet flapping in the wind or rubbing against the stems of the seedlings.

[0192] FIGS. 5A and 5B are instructive in the operation of the present technology. In FIG. 5A, prior art mulch film (50) even if buried under soil (34) expands rather than contracts in the heat of the sun. The loose film can allow air to lift it and flap during strong wind and to damage seedlings (not shown) as previously described.

[0193] FIG. 5B shows the operation of the present mulch film (22), also buried under soil (34), wherein the more radiation applied to the film (22), the more it shrinks. This is in contrast to prior art mulch film (50) which increasingly stretches when exposed to solar radiation. This result for the present mulch film is due to the longitudinal pre-straining of the present mulch film at a predetermined tension and temperature and immediately cooling it. The resultant molecular stresses in the material facilitate shrinkage but not expansion on exposure to solar radiation. This property of the subject solar-reactive mulch, accomplished by the film structure described, and its methods of manufacture, are hitherto unknown, and not disclosed or suggested in publications, in the field of agricultural sheet mulches.

[0194] Example Implementations

[0195] Using a non-damaging felt tipped pen, two lines were drawn 100 mm apart along the machine direction of a sample of black mulch film and a sample of black/white mulch film. A further two lines were drawn on each sample 100 mm apart along the transverse direction of the samples.

[0196] The mulch film samples had both been manufactured according to this application and had been stretched at a temperature of 75 C. having the outfeed chilled nip rollers rotating 3 times faster than the infeed heated rollers. The samples had a uniform thickness of 18 to 21 microns and had been drawn down from an infeed blown film having a thickness of 55 microns.

[0197] The samples were taken from rolls of film which had been stored indoors and the sample pieces were then taken outdoors and placed flat on soil.

[0198] The ambient temperature was 25 C. and it was a cloudless day at 1:00 pm in autumn in Brisbane Australia.

[0199] The temperature of the exposed film was assessed using a contactless thermometer and rose to between 60 and 70 degrees for the black sample and between 50 and 60 degrees for the white sample.

[0200] The film could be seen to commence shrinking within seconds of being placed on the ground and appeared to have ceased shrinking within one minute. The distances between the lines was re-measured and it was found that the black mulch film had shrunk by 28% in the machine direction and by 4% in the transverse direction, the black/white film, which had the white side facing upwards, had shrunk by 24% in the machine direction and 3% in the transverse direction.

[0201] Further practical trials of full length mulch rolls have been performed on various farms having different soil types and climatic conditions and all have shown that the film can be laid by machine at higher speeds and lower tension than prior art mulch and that the mulch film manufactured according to this application, proceeds to shrink and tighten onto the growing bed sufficiently to overcome wind caused flapping.

[0202] The trials have indicated that the film does last, intact, for a full growing season of 9 months and is able to be lifted from the ground by mechanical means at the end of the season.

[0203] Select Characteristics and Benefits

[0204] The present section describes some of the features and benefits of the technology, many of which are referenced above.

[0205] The present technology includes a proven and successfully tested solar heated, shrinking agricultural mulch film, and differentiate it from the unsuccessful conventional materials described above.

[0206] The sheet is strained in the longitudinal direction only.

[0207] The sheet has a uniform thickness over the entire surface area with no thin or weak strips.

[0208] The sheet may have high and consistent levels of carbon black, short of rendering the film opaque, to in a balanced manner generate solar heat, prevent weed growth and protect the polymer from UV light degradation. When mulch film is made to be completely opaque by having a dense black layer, it restricts the growth of weeds. Weed seedling do not receive any light for photosynthesis and die shortly after germinating.

[0209] The sheet has improved tensile properties in the longitudinal direction which assist in maintaining a solid sheet when being lifted from the ground.

[0210] The material is strained while heated and immediately chilled while having the strain maintained so as to lock in the shrinking forces in the material, allowing the mulch to shrink at very low temperatures normally experienced in a field 45 to 70 C.

[0211] The sheet shrinks majorly in the lengthwise direction with slight but sufficient shrinkage in the transverse direction, evenly over the entire surface area, without creating weak lines or placing tearing stress on plant cuts and holes.

[0212] The rolls are of an exact and even width and dispense as a flat sheet on application.

[0213] The method of manufacture and consequent physical properties is completely different than prior methods. Prior agricultural plastic mulch film comprises two main types. Plastic sheet mulch is a unique product that is designed to perform under some of the most difficult weather conditions encountered by any agricultural product.

[0214] The first type is generally relatively transparent and designed for short term use to warm the soil and promote early germination of seeds. The second type is opaque and designed to enhance growing conditions, as is described below, for the entire growing season.

[0215] Distinctions between plastic mulch film and any other plastic product, such as bundle heat shrink film or pallet stretch wrap or other plastic film, include the following properties:

[0216] Plastic mulch film has to perform the specific functions of warming soil, conserving water, preventing soil erosion, protecting fruit, preventing weed growth, surviving for a complete growing season outdoors and having adequate strength to be laid and lifted to and from the growing soil bed by mechanical means.

[0217] The plastic mulch film according to the present technology in various embodiments preferably has any or all of the following qualities: [0218] is generally thin to conserve polymer usage so as to reduce cost and is normally between 15 and 40 microns thick the most commonly 25 microns thick; [0219] blocks virtually all light transmission to prevent weed growth; [0220] is able to have a silver or light colored surface to reflect light to minimize soil temperatures; [0221] resists degradation by ultra violet light so as to be lifted from the soil at the end of the growing season by mechanical means; [0222] remains intact for up to 12 months or more, if the farmer decides to grow a second crop on the field without lifting the mulch, while exposed to all outdoor aspects of an agricultural environment including wind, rain, direct sunlight, hot conditions, frozen conditions, animal movement, machinery movement, and soil settling; [0223] resists degradation from agricultural chemicals sprayed on the crop. [0224] is strong enough to be laid onto a growing bed and have a portion of the film buried under soil by mechanical means, even though it is a thin sheet; [0225] is strong enough to be applied to a bed and buried while under substantial tension so as to be laid tautly against the bed in to prevent flapping and lifting in the wind; [0226] has a uniform roll profile because any raised regions across the roll can result in the mulch being damaged as the film unwinds on the machine, due to the style of machines used to lay the mulch film; [0227] resists splitting and tearing when holes or slits are cut into the film to plant seeds or seedlings in the underlying soil; [0228] retains sufficient strength so as to be lifted from the ground at the end of the growing season by mechanical means without disintegratingany plastic film remaining in the soil after removal can be detrimental to future crops grown on the same ground.

[0229] Growing crops using plastic mulch film according to the present technology also has any or all of the following several advantages.

[0230] The mulch can be colored in order to control the temperature of the soil. Black mulch film will warm the soil around a seedling allowing for early growth, whereas white mulch film will not heat the soil as much and is effective particularly in hot climates.

[0231] White mulch reflects a greater amount of light up into the plant which accelerates growth and development.

[0232] When combined with a drip irrigation system under the mulch, the amount of water received by the plant can be accurately controlled and reduced.

[0233] Water evaporation around the plant is reduced thereby also reducing overall water consumption or user cost for water.

[0234] When using a dripper system, the plant may also be fed fertilizer and other supplements to assist with growth. The amount of fertilizer applied to the plant can be reduced as the mulch film also contains the spread of the fertilizer.

[0235] For plants with ground-resting fruit such as strawberries, melons, cucumbers, pumpkins, etc., the plastic mulch film protects the fruit from damage by contact with the soil and animals living in the soil. These fruits are also kept clean by being grown on plastic rather than in contact with the soil.

[0236] Mulch film also prevents the loss of tilled soil through wind and water erosion.

CONCLUSION

[0237] While the foregoing has been given by way of illustrative example of the present technology, all such and other modifications and variations there to as would be apparent to persons skilled in the art are deemed to fall within the broad scope and ambit of the present technology as is herein set forth.

[0238] In the specification and claims, the terms comprising and containing shall be understood to have a broad meaning similar to the term including and will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. This definition also applies to variations on the terms comprising and containing, such as comprise, comprises, contain, and contains.

[0239] Terms such as approximately, about, and around are used in the specification and the claims on occasion to indicate that while the technology may use or express exactly the relevant value, bound, or range, the technology may use or express a value, bound, or range that is within a reasonable extension. As just one example, by disclosing that the film may shrink by approximately 10% to 30% in surface area, the film may shrink by anywhere between and including 10% and 30%, as well as reasonable extensions, such as 9%, 32%, etc., in various embodiments.