FIELD OF THE INVENTION: This invention relates to agricultural products, in particular but not exclusively to a Biodegradable mulch film a crop cover with uniform degradability and which disintegrates substantially without leaving harmful residue by the time of the next cycle of cultivation.

BACKGROUND OF THE INVENTION:

Prior art agricultural mulch film is invariably a sheet of plastic the width of a planting row, plus an additional width on each side of the row onto which soil is piled to hold the sheet in place. In use, the sheet is rolled out for the entire length of the plant row after which holes are punched into it along the length at intervals suitable for the expected crop.

It is also common practice for a drip irrigation hose to be placed along the length of the row under the mulch sheet to supply water and fertilizer to the crop plant.

Mulch film is usually made from either linear low density or high density polyethylene but may be made from any polyolefin or styrene. These plastics are extremely strong and flexible having very long chain polymerized molecules, but under the effect of the energy supplied by UV light or elevated temperatures, these long chains can be degraded. Mulch film is supplied in a number of colours and forms each having a different effect on the plant, for example, clear film allows large amounts of light through the plastic onto the soil where it is converted to heat which is trapped in the soil by the plastic. Black film is used to control weeds by not allowing any light through to the soil. The black film absorbs the light striking it which heats the film which in turn heats the soil. The carbon black used in this film also absorbs ultra violet (UV) light which is a major cause of degradation in plastic, thus black film has a longer life than standard unprotected plastic. Coextruded film uses two (2) or more extruders to extrude the film through a die in two (2) or more layers. One layer is generally black to prevent weed growth and as such is placed against the soil. The outer layer is commonly white and reflects light away from the soil, thereby keeping the soil cool and increasing the light striking the plant. Other colours may be used in the topmost layer which may have other advantageous properties, such as reflective silver to prevent insect attack. Red colour may encourage additional growth and fruit protection in tomato plants. Blue colour is believed to assist with melon production. When using colours it is common to have a three layered film consisting of a black under layer, intermediate white layer, and a coloured or white top layer. It is also possible to produce black mulch in which all layers incorporate black colour additive plus additional additives. Agricultural mulch film is designed to last the entire growing time of the crop without breaking up or disintegrating. It needs to remain strong enough to be pulled up at the end of the season for safe disposal.

A major problem with the removal of mulch film is that the plastic partially degrades and the film breaks up into large pieces making it difficult to pull up. Certain weeds can also grow through the film and anchor the film to the ground making lifting difficult and causing the film to split into pieces. The weight of soil piled onto the edges of the film can also cause the plastic to tear and break.

Once removed from the soil there remains the problem of disposing the film. It is not easy to recycle the mulch as it is usually heavily contaminated with soil. In many areas it is not permitted to burn the film. Many landfills will not accept the waste and often burying it on the farm can cause problems with water flow and soil contamination.

The best solution to these problems is for the mulch film to be of a type that does not require removal but can merely be ploughed into the soil at the end of the season or before the next planting season where it will degrade and disintegrate, thereby becoming a useful component of the soil and removing the chance of any contamination. OXO-BIODEGRADABLE PLASTIC

Oxo-biodegradable plastic is made from prior art petrochemical polymers such as polyethylene or polystyrene. In this type of plastic, a small percentage of a chemical prodegradant is added to the petrochemical polymer blend.

Under the influence of UV light, heat or mechanical action, the prodegradant becomes an active free radical undergoing catalytic reactions with the long chain polymer molecule whereby the long chain is oxidized at any point by the prodegradant. Wherever it has been oxidized the polymer will undergo a schism and will break apart at that point.

Once the polymer is oxidized at a particular point, the free radical prodegradant compound attaches to another portion of the plastic and repeats the process causing the polymer to break again. As this process continues, the long chain molecules are reduced in length, lose their elasticity, and cause the plastic to become brittle and easily disintegrated. This schism of the polymer will continue until the plastic molecules become small enough to either be assimilated by microbes or become part of the soil's humus. Although all plastic sheets will ultimately degrade, the addition of a prodegradant allows the rate at which the sheet degrades to be accelerated and controlled by type and dose level of prodegradant. A number of prodegradants are commercially available in masterbatch form and are normally an ion of a transition metal. Common metals used include cobalt, manganese and iron.

Although all of the prodegradant chemicals are made more active by both UV light and heat, some elements are more initiated by heat than others.

The rate at which an oxo-biodegradable plastic will disintegrate is determined by the amount of UV light and the amount of heat the plastic is exposed to, or the amount of physical action applied. It is also affected by the proportional amount of prodegradant contained in the blend making up the plastic sheet.

EXISTING TECHNOLOGY

Mulch films made from oxo-biodegradable plastic have been supplied by numerous manufacturers but have not given consistently good results because of the different degradation rates of the exposed and buried portions of the sheet.

The major problem experienced with mulch film is its removal, and the thinner the film the more it breaks off during the winding up process of removal. The cost of removal in terms of labour can be as much as 50% of the overall cost of purchasing, and laying the film. The main problem with removing film is that it breaks off when being pulled from the ground due to a number of reasons, including:

1. The film has partially degraded while laying on the ground under the influence of UV light but those portions buried on the edges to retain the film have not degraded and the exposed portion tears away from the edges;

2. The soil placed on the film compacts and locks in with the pathway soil;

3. Weeds such as nut grass grow though the mulch and anchor the film to the ground;

4. Inexperienced tractor drivers can pull the mulch up faster than the tractor is moving putting additional strain on the film, and

5. Too much soil may have been placed on top of the edges of the film when it was laid down.

OBJECT OF THE INVENTION:

It is an object of the invention to ameliorate some or all of the above disadvantages of the prior art by providing a novel agricultural biodegradable mulch film. The principle object of the present invention is to co-ordinate a particular thickness of mulch sheet with the amount of prodegradant included in the plastic which may be varied to achieve a desired time for disintegration when the plastic is placed in an environment receiving a known amount of UV light or heat. STATEMENT OF INVENTION:

In one aspect, the invention resides in a biodegradable agricultural mulch film comprising, a plastic polymer sheet material having sections across its width of varying thickness and having added chemical composition or treatment designed to enhance degradation of the plastic,

wherein in use some sections of the sheet are buried in the soil and other sections of the sheet are exposed to sunlight such that the,

interrelationship between a predetermined thickness profile and chemical composition of sections buried in the soil and sections exposed to sunlight causes the sheet to degrade at a substantially uniform rate and the entire sheet completely disintegrates leaving no harmful residual material by the time of the next cycle of cultivation.

In another aspect, the invention resides in a biodegradable agricultural mulch film comprising in combination a plastic sheet comprised of,

two or more layers of coextruded or laminated plastic material,

such sheet having sections of varying thickness across its width and having added chemical composition or treatment designed to enhance degradation of the plastic,

such added chemical composition or treatment may vary in type or composition in each layer of the sheet,

wherein in use some sections of the sheet are buried in the soil and other sections of the sheet are exposed to sunlight such that, even though the sections buried in the soil and the sections exposed to sunlight degrade at different or varying rates, the difference in thickness and the varied chemicals added per layer will allow the added chemicals to disintegrate the complete sheet at a substantially uniform rate and the entire sheet completely disintegrates leaving no residual material which will adversely affect the next cycle of cultivation.

Suitably, the mulch sheet has holes or apertures cut into the exposed sections for planting seeds or seedlings in the soil.

Preferably, the mulch sheet or film is an oxo-biodegradable plastic, to which prodegradant chemicals are added during production, to result in catalytic oxidation and consequent molecular schism of the polymer materials of the sheet, wherein various prodegradants react under the influence of either UV light, ambient heat, or mechanical action to accelerate the disintegration of the sheet. Preferred prodegradant chemicals include salts of iron, cobalt, zinc, nickel, manganese, copper and other transition metal ions alone or in combination with a calcium compound ,which catalytically oxidises the polymer molecules causing them to break into shorter chains which results in the plastic losing its flexibility and becoming brittle wherein it snaps easily under physical action.

Preferably, the edge sections of the sheet are of different thickness than the middle, wherein the thickness of the edge sections vary according to the amount of sunlight and heat generated in the soil such that the entire mulch sheet degrades and disintegrates adequately by the next cycle of cultivation so as to have no detrimental effect to the crop.

Preferably, the edge sections of the sheet are also micro-perforated for greater exposure of surface area to be degraded.

Preferably, the entire surface of the mulch film is embossed typically with a diamond pattern, wherein each diamond is approximately 4 mm by 2 mm with a depth of between 75 microns and 500 microns and in a repetitive pattern wherein the diamonds are adjacent to each other. However any other suitable embossed pattern may be used.

Preferably, the edge sections of the mulch film which is buried are micro-perforated with puncture holes made by needles having an approximate diameter of 1 mm and perforated in a pattern wherein each hole is between 5 mm and 25 mm apart from the other holes.

Preferably, production of the mulch film is by means of a blown film extrusion through a circular die with a cool air blowing ring situated above the die. By manipulation of the air flow through the air ring or the temperature of air blowing onto the film, or by manipulating the amount of polymer extruded through the die aperture, two diametrically opposed areas of the film are made thicker than the remainder of the film, or two diametrically opposed areas of the film can be made thinner than the remainder of the film. Suitably, when oxo-biodegradable mulch is produced, the position of the thinner sections are such that when the extruded film bubble is collapsed, the thinner sections are folded in half at the edges of the flat extrusion. The film is then slit longitudinally along the folded edges thereby forming two sheets which are thicker in the centre than at the edges.

The width of the two thin sections of each sheet is preferably approximately equal to the width of the film that is buried. If oxo-biodegradable additives are not included in manufacture, the thicker sections can be placed along the edges of the sheet so that when the mulch is pulled up it will not break off under the additional strain of being buried.

If oxo-biodegradable additives are not included in the manufacture, but the film is made from a hydro-biodegradable plastic polymer, such as containing polymerized starch and a hydro-biodegradable petrochemical such as polyester, thicker section may be placed along the edges of the sheet in order for these edges to be disintegrated by hydrolysis and biodegradation at the same rate as the centre section. In the alternative, the mulch sheet may be produced by means of a cast extrusion process wherein edges of the film are made thinner by adjusting the dimensions of the gap in the die through which the plastic is extruded. Plastic enhanced with metal ion or other suitable prodegradant will also accelerate degradation when the plastic is subject to mechanical action such as continual flexing. When embossed with a diamond pattern approximately 2 mm by 4 mm with a depth of between 75 and 500 micron, or other suitable pattern the plastic will flex at the embossed areas as the plastic expands or contracts according to changes in the ambient temperature.

This continual flexing assists with degradation and weakens the plastic along the embossed pattern which is degraded more than the flat portions between the embossing. This process helps to disintegrate the sheet into small pieces.

The edge sections of the mulch film which is buried may be micro-perforated with puncture holes made by needles having an approximate diameter of 1 mm and perforated in a pattern where in each hole is between 5 mm and 25 mm apart from the other holes.

These perforations allow more physical action to occur around the edges of the holes when the plastic is buried with additional water movement and providing an area for microbial attack.

The perforations assist with the degradation of the plastic by allowing more areas where the edges of the plastic molecules are exposed to oxygen, thereby increasing the rate of degradation around the holes. The holes give the plastic an existing pathway of accelerated degraded plastic to tear along when being ploughed into the ground, thereby significantly reducing the size of the residual plastic pieces.

BRIEF DESCRIPTION OF THE DRAWINGS:

In order for the invention to be better understood, reference will now be made to the accompanying drawings, wherein;

Figure 1 refers to a preferred embodiment of the invention according to Example 1.

Figure 2 shows a preferred embodiment according to Example 2.

Figure 3 shows a preferred embodiment according to Example 3.

Figure 4 shows a section of the invention showing the edges portions.

Figure 5 shows the invention in use.

DETAILED DESCRIPTION OF THE DRAWINGS:

Referring now to the drawings and initially to Example 1, there is shown a preferred biodegradable agricultural mulch film 10 according to the first aspect of the invention. T e film is shown as a single layer of plastic polymer sheet material having a wider thickness in the central region 10a of the sheet. This is a cross-sectional view of the sheet with thinner side portions 10b, 10c. The thinner portions of the sheet are buried in soil (not shown), wherein the thicker section is exposed to sunlight. The chemical composition or treatment of the sheet with prodegradants is designed such that the thick and thin portions of the sheet degrade at a substantially uniform rate wherein the entire sheet is disintegrated without leaving harmful residual pieces of material at the end of a cycle of cultivation.

Referring now to Figure 2, there is shown a preferred embodiment of the mulch film 20 according to the second aspect of the invention. There is shown a plastic mulch sheet comprising three layers 20a, 20b, 20c of co-extruded plastic material. The top and bottom layers enclose a middle layer 20b which results in the sheet having a thicker portion 22 in the central portion of the mulch film tapering on either side to thinner sections 24, 26. As with the first example, the thinner sections are buried under soil (not shown) wherein the central section is exposed to sunlight and wherein the different chemicals added to the layers result in the complete sheet degrading at a substantially uniform rate, which disintegrates at the end of a cultivation cycle, with little or no residual material left in the soil.

Figure 3 shows another embodiment of the invention 30, which is comprised of two layers 32, 34, such sheet having thicker central portions 32a, 34a, which, when combined, result in a combined thicker central portion 39 with tapering side edges 36, 38. The side edges or portions are, similarly as with Examples 1 and 2, buried under soil, and the prodegradant chemicals added to the sheets together with the combination of the varying thickness, enable the sheet to substantially disintegrate completely before the next cultivation cycle.

Figure 4 shows a portion of the sheet 40 showing the dimensional configuration of the side edge portions 42, 44 with respect to the central portion of the sheet wherein the side edges form borders to the sheet that are buried under the soil (not shown). These side edges may in addition be micro-perforated. The central portion 46, as previously discussed, is exposed to sunlight and is of a thicker cross sectional configuration. Referring now to Figure 5, there is shown a diagrammatic view of the invention in use. The cross-sectional depiction of the sheet 50 is shown having its edge portions 52, 54 buried underneath the soil 56 with the thicker central portion 55 exposed to sunlight, and wherein the central portion has apertures or holes 58 cut into the sheet to allow for planting of seeds or seedlings 60 during a cultivation cycle.

The time for degradation of exposed oxo-biodegradable plastic is relatively easy to control, as for any one geographical area, the amount of UV light striking the plastic is almost constant from season to season, as are the ambient temperatures in that area.

Given the climatic conditions in a particular area, the time in which a plastic sheet will disintegrate can be controlled by varying the amount of prodegradant included in the blend. The rate of degradation will also be varied by the thickness of the sheet; with a thin sheet being embrittled and disintegrating quicker than a thicker sheet.

There is however, no prior art study of, or use of, the working interrelationships of thickness in combination with chemical composition or treatment of the sheet mulch to achieve uniform degradation or disintegration according to regional climatic conditions.

In the alternative, or conversely, the amount of prodegradant can be held constant and the thickness of the sheet may be varied to achieve a given time to disintegration.

The rate of disintegration can also be controlled by including antioxidant additives and UV absorbing additives into the plastic blend. These additives protect the polymer from degradation by preventing oxidization of the polymer by free radical prodegradants, or prevent the UV light from energizing the prodegradant, or by absorbing the UV light. However these additives have a given maximum lifespan. Thus, by varying the amount of the antioxidant additives contained in the blend, the time of commencement of degradation can be delayed and controlled. Once the protective additives are no longer effective degradation takes place in a very short time.

Because oxo-biodegradable plastics use mainly commodity petrochemicals with a small percentage of prodegradant, mulch film made from this type of polymer is not substantially more expensive than the standard product, whilst maintaining equal strength and tensile properties.

The rate of oxidation and consequent brittleness and disintegration under the influence of ambient UV light is much quicker than that achieved under the influence of ambient heat. Should the heat levels be raised to around 70 to 80 degrees Celsius, the rate of embrittlement will be much quicker, almost as quick as under ambient UV light.

Thus the plastic that is buried and consequently only receiving ambient heat, will degrade at a slow pace even with the inclusion of a heat activated prodegradant. Therefore, the buried portions should contain less polymer so that these sections commence useful life with lower tensile properties than the centre section so that after oxidation even though these portions receive less energy they will easily disintegrate under the physical action of being ploughed into the ground, because the prodegradant has had the whole growing season to commence the oxidative degradation and thereafter continue to degrade and disintegrate into even smaller pieces ultimately removing any threat to the growing of future crops.

The centre section receives the full strength of the available UV light and so will degrade quickly if not protected by antioxidants or UV light stabilizers. Because this section has to last for the growing season it is better if it commences as a thicker film with consequent greater tensile properties. DEGRADATION OF MULCH FILM

In order to be ploughed into the soil, the mulch film has to degrade to a point where it will quickly disintegrate at the end of the season. As mentioned above, the rate of degradation of oxo-biodegradable mulch films can be controlled to suit a particular crops needs.

However, as that portion of the mulch sheet that is buried under soil will not receive any UV light, it will only degrade under the action of heat from the sun, which, at the ambient temperatures received in most agricultural areas, leads to a slower degradation.

This varying rate of degradation causes much of the buried mulch film to retain its strength with other parts disintegrating. This leads to difficulties in pulling up the mulch or in ploughing it into the soil. After being ploughed into the ground, the level of prodegradant needs to be sufficient to ensure that disintegration continues until no trace of the plastic sheet remains. This may take more than one growing season but because the plastic is broken up into such small pieces it will not affect future crops.

It will of course be realised that while the foregoing has been given by way of illustrative example of this invention, all such and other modifications and variations thereto 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. 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".