SOLAR-REACTIVE MULCH FILM

Abstract

A solar reactive mulch film comprising one or more layers of heat shrink material pre-strained at a predetermined temperature and tension. Immediately cooling the pre-strained layers retains molecular deformation stresses in the material which facilitate shrinkage but not expansion on exposure to solar radiation. The mulch film adapted to be laid over longitudinal growing beds and held tautly by burying its side edges in soil. Exposure of uncovered portions of film to solar radiation causes shrinkage in one or more directions whereby in combination with the weight of the soil holding it down, maintains tautness of the mulch over a growing period. Methods of manufacture and use of the same.

Claims

1-20. (canceled)

21. A method of creating a solar-reactive agricultural mulch film, comprising: heating one or more polymers beyond a melting temperature of each polymer to produce one or more melted polymers, wherein one or more of the polymers is a linear low density polyethylene (LLDPE) and/or a metallocene linear low density polyethylene (mLLDEP) having dispersed therein carbon black particles in an amount that ranges between 2% and 8% by weight; extruding one or more of the melted polymers to produce a film that includes one or more layers of LLDPE and/or mLLDPE having dispersed therein the carbon black particles; heating the film to a stretching temperature; stretching, at the stretching temperature, the film in a longitudinal direction beyond a yield point of each of one or more of the layers of LLDPE and/or mLLDPE such that main chain molecules of LLDPE and/or mLLDPE and/or side branched molecules of LLDPE and/or mLLDPE in each of the one more layers of LLDPE and/or mLLDPE are elongated in the longitudinal direction; cooling the stretched film; maintaining a stretching tension throughout the stretched film while cooling the stretched film such that resultant plastic deformation stresses, resulting from stretching beyond the yield point, are retained and each of the one or more layers of LLDPE and/or mLLDPE does not shrink until one or more of the layers of LLDPE and/or mLLDPE having dispersed therein the carbon black particles to absorb radiant energy from sunlight and convert the radiant energy to heat, such that a surface temperature of one or more of the layers of LLDPE and/or mLLDPE is raised to at least a shrinking temperature of at least 40 degrees Celsius.

22. The method of creating the solar-reactive agricultural mulch film of claim 1, wherein the extruding further comprises: extruding one or more melted polymers through a circular die; drawing one or more of the extruded polymers upwards as a bubble; cooling and solidifying the collapsed tube by a cooling air ring; and collapsing the bubble into a tubular configuration through a set of collapsing boards.

23. The method of creating a solar-reactive agricultural mulch film of claim 1, wherein the extruding further comprises: extruding one or more melted polymers through a flat die; and passing the one or more of the extruded polymers onto chilled rollers wherein it cools and solidifies.

24. The method of creating the solar-reactive agricultural mulch film of claim 1, further comprising adding, to one or more of the polymers, at least one material selected from a group comprising an anti-oxidation additive, an ultraviolet light stabilizer, an ultraviolet light absorber, a plasticizer, and a lubricant.

25. The method of creating the solar-reactive agricultural mulch film of claim 1, wherein the film is heated to a stretching temperature that ranges from between 20 degrees Celsius and 120 degrees Celsius.

26. The method of creating the solar-reactive agricultural mulch film of claim 1, wherein the film is heated to a stretching temperature that ranges from between 40 degrees Celsius and 90 degrees Celsius.

27. The method of creating the solar-reactive agricultural mulch film of claim 1, wherein the film is heated to a stretching temperature that ranges from between 60 degrees Celsius and 70 degrees Celsius.

28. The method of creating the solar-reactive agricultural mulch film of claim 1, wherein the stretching includes stretching using a machine direction orientation machine having a heated infeed nip roller and a chilled outfeed nip roller rotating at a faster velocity than the infeed nip roller such that the outfeed nip roller has a surface linear velocity that is greater than the infeed nip roller, thereby stretching the film.

29. The method of creating the solar-reactive agricultural mulch film of claim 28, wherein the outfeed nip roller surface linear velocity is between 1.25 and 10 times faster than the infeed nip roller linear speed velocity.

30. The method of creating the solar-reactive agricultural mulch film of claim 28, wherein the infeed nip roller is rotated at the same velocity as a haul off nip roller employed during the extruding.

31. The method of creating the solar-reactive agricultural mulch film of claim 1, wherein the film is cooled to a temperature that ranges between 10 degrees Celsius and 40 degrees Celsius after the film has been stretched and while the resultant plastic deformation stresses are retained in the film.

32. The method of creating the solar-reactive agricultural mulch film of claim 1, wherein the film is cooled to a temperature that ranges between 15 degrees Celsius and 20 degrees Celsius after the film has been stretched and while the resultant plastic deformation stresses are retained in the film.

33. The method of creating the solar-reactive agricultural mulch film of claim 1, further including printing planting lines longitudinally along the film.

34. The method of creating the solar-reactive agricultural mulch film of claim 1, further including trimming away edges of the film to ensure the film is of substantially uniform thickness across its width.

35. The method of creating the solar-reactive agricultural mulch film of claim 1, further including rolling the film onto a spool.

36. The method of creating the solar-reactive agricultural mulch film of claim 1, further comprising subjecting the cooled film to a relaxing process to remove residual elastic forces entrapped in the film by some polymer molecules being stretched to below their yield point.

37. The method of creating the solar-reactive agricultural mulch film of claim 1, wherein the sheet is not subjected to any post orientation process designed to anneal, de-stress, or pre-relax shrinking forces entrapped in the plastic sheet.

38. The method of creating the solar-reactive agricultural mulch film of claim 1, wherein the film is a single wound sheet.

39. The method of creating the solar-reactive agricultural mulch film of claim 1, wherein the film is a double wound sheet bonded together to form a single sheet.

40. The method of creating the solar-reactive agricultural mulch film of claim 1, wherein the stretching includes aligning the main chain molecules of LLDPE and/or mLLDPE and/or the side branched molecules of LLDPE and/or mLLDPE in each of one more of the layers of LLDPE and/or mLLDPE.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0166] In order that the invention be better understood and put into practical effect, reference will now be made to the accompanying drawings wherein:

[0167] FIG. 1 shows film sheet formed by the blown film process according to the invention.

[0168] FIG. 2 shows film sheet formed by the cast film extrusion process according to the invention.

[0169] FIGS. 3, 4 and 5B show a preferred embodiment and its use according to the invention.

[0170] FIG. 5A shows the contrasting operation of prior art mulch film when compared to the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0171] 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.

[0172] 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).

[0173] 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) to produce a single sheet of mulch film (17).

[0174] 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 and will have at least one layer contain high levels of carbon black particles—for example 14% to 16% of carbon black master batch containing 40% of 17 to 25 nanometre size particles of carbon black.

[0175] 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.

[0176] Subsequent to exiting the top nip rollers of the blown film line or exiting the chilled casting rollers of the cast film line 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. This nip roller set is preferably heated to a temperature that may be up to 150 degrees Celsius but more 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.

[0177] The sheet is immediately passed through a third set of nip rollers (12) which are cooled to preferably 40 degrees Celsius or more preferably ambient temperature but more preferably to approximately 15 degrees Celsius to chill the sheet prior to the sheet returning to ambient temperature. This third set of nip rollers preferably rotate at between 150% and 300% faster than the second heated nip rollers set. On occasion it may be preferable to rotate the third set of nip rollers at faster rates so as to stretch the sheet by a greater amount.

[0178] The sheet is stretched between the second and third sets of nip rollers and is maintained under the full stretching stress and is not allowed to relax prior to the film temperature being reduced to preferably approximately 15 degrees Celsius or at least ambient temperature.

[0179] Between the second and third set of nip rollers are 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 the neck-down (width narrowing) due to the stretching.

[0180] Upon exiting the MDO machine the film passes through a printing station where plant lines are printed onto the film.

[0181] 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 2 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 metres of mulch film, as single wound sheet.

[0182] During the MDO process the film which is fed into the MDO machine is extruded initially thicker than is required and is 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 is required to break the thicker film.

[0183] On occasion it may be that the extruded film has the thickness profile of the bubble or cast sheet altered, i.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.

[0184] 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 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 40 degrees C. and the rate of shrinkage increases as the temperature is increased.

[0185] 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 is increased, when compared to heated MDO.

[0186] 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.

[0187] 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.

[0188] Obviously 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.

[0189] If the farmer using the mulch film is planting his seedlings close together, say 150 mm or less, high levels of stretch, above 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.

[0190] If a high stretch ratio above 300% is employed the film may become easy to split and may split when placed under the tuck wheels of a laying machine.

[0191] It has been noted that stretch ratios of between 1:2 and 1:3 will achieve sufficient shrinkage in the film to ensure it shrinks and does not flap in the wind.

[0192] 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.

[0193] The third set of nip rollers 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.

[0194] 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.

[0195] 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.

[0196] 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.

[0197] 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.

[0198] 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.

[0199] Secondly when placed in direct sunlight the film will shrink in surface area by a minimum 25%. 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 40 to 55 degrees Celsius. This shrinkage occurs even though the solar energy only needs to raise the surface temperatures to between 40 to 55 degrees Celsius, although much higher temperatures may be achieved in hot climates.

[0200] It will be noticed that the film does not immediately expand when heated by solar radiation.

[0201] 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.

[0202] The speed of this shrinkage is determined by the level of temperature rise which is determined by the strength of the solar energy striking the film and the outer colour 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.

[0203] Shrinking mulch film made according to this application can be made thinner than normally required as the film does not need to have the 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.

[0204] 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.

[0205] This shrinkage is surprizing 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.

[0206] 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.

[0207] Also surprizing 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.

[0208] 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.

[0209] 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 colour additives in the film.

[0210] The polymer normally has to be raised to close to the melting temperature of the polymer prior to shrinkage beginning.

[0211] To have the mulch film shrink by up to 25% when heated to only 55 degrees Celsius is very unusual, but this ability proves to be very useful in preventing flapping and billowing in the mulch film.

[0212] 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 55 degrees Celsius at which temperature the contained stresses in the film are released causing the film to permanently shrink in surface area.

[0213] 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.

[0214] Should the film be made clear it is possible for mulch film made according to this invention 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.

[0215] The shrinking ability of mulch film made according to this invention 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.

[0216] 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 colour additives contained in the film which raises the surface temperature of the mulch sheet to 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.

[0217] 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 invention, 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 but not shrink as the temperature is increased.

[0218] Should sections of the mulch film be buried (30, 32), 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) 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.

[0219] FIGS. 5A and 5B are instructive in the operation of the invention. 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.

[0220] FIG. 5B shows the operation of the present mulch film (22) which 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 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 and its methods of manufacture are hitherto unknown in the field of agricultural sheet mulches and comprises the inventive step of the invention.

EXAMPLES

[0221] 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.

[0222] 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.

[0223] 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.

[0224] The ambient temperature was 25° C. and it was a cloudless day at 1:00 pm in autumn in Brisbane Australia.

[0225] 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.

[0226] 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.

[0227] 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.

[0228] 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.

[0229] It will of course be realized that while the foregoing has been given by way of illustrative example of this invention, 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 this invention as is herein set forth.

[0230] In the specification 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”.