GB2240745A

GB2240745A - Improved mulch film

Info

Publication number: GB2240745A
Application number: GB9018655A
Authority: GB
Inventors: Gordon William Gilby
Original assignee: British Visqueen Ltd
Current assignee: British Visqueen Ltd
Priority date: 1989-09-14
Filing date: 1990-08-24
Publication date: 1991-08-14
Anticipated expiration: 2010-08-24
Also published as: GB2240745B; GB9018655D0; GB8920872D0

Abstract

An improved plastics film suitable for use in mulching comprises superimposed layers of two coloured polyolefin films, each of which absorbs light over only a part of the range of wavelengths from 400 nm to 700 nm but which together absorb a major proportion of light in that range. Preferably both films transmit solar energy over at least a major proportion of the range of wavelengths from 700 nm to 2300 nm. Examples are given of such mulch films comprising a red or orange layer and a blue layer, with an optional white layer between them.

Description

Improved Mulch Film The present invention is concerned with plastics films suitable for use in mulching.

Mulching by means of plastics films is now a widespread practice on both the industrial and domestic scales.

The primary purpose of mulching is to provide and maintain conditions favourable for the development of healthy plants or cultivation while at the same time discouraging or suppressing the growth of weeds. Such conditions include maintaining a good soil structure by keeFing the soil warm and reasonably moist, while minimising exposure of the soil to light. Plastics films with various different characteristics have been proposed for this purpose, with varying degrees of success.

Thus plain black film is effective at excluding light from the area of the plants, thereby inhibiting photosynthesis and the growth of weeds. However black film also excludes a major proportion of incident solar energy and therefore fails to provide the benefits of soil heating on plant growth. By contrast a clear film allows ready transmission of solar energy but fails to exclude light and thus does not prevent or inhibit weed growth.

TWD further effects of mulch film may also be taken into account. Firstly, it is desirable that the enhanced soil temperature should remain reasonably steady throughout the daily cycle. To this end, loss of radiated heat from the soil, especially overnight, should be minimised. Heat retention of this type is good with black film but is very limited with clear or white film.

Secondly, mulch film is usually left in place until after the crop is harvested and thus plant growth continues above the film. It has been observed that beneficial effects can arise from light reflected from the upper side of the film on the plants. White film displays this advantage to a significant degree.

In an attempt to combine some of the foregoing effects, it has been suggested to use for mulching a film comprising superimposed black and white layers, the film being laid with the white layer uppermost. However such a combined film, while effectively inhibiting weed growth and giving good light reflection, excludes solar heat corrletely.

It is an object of the present invention to provide an improved mulch film which displays to a greater extent the benefits available fram prior unitary films, while overcoming or reducing at least sane of the disadvantages of such prior films.

The improved mulch film according to the present invention comprises superimposed layers of two coloured polyolefin films, each of which absorbs light over only a part of the range of wavelengths from 400 nm to 700 nm but which together absorb a major proportion of light in that range. Preferably the films both transmit solar energy over at least a major proportion of the range of wavelengths from 700 nm to about 2300 nm.

By way of example, one of the films may be pigmented with a red or orange pigment and the other may contain a blue pigment. The mulch film containing two such layers will then appear to be red or orange when viewed from one face and blue from the other, although sane of the intensity of the colour will be muted by the presence of the second layer behind the layer being viewed. As described below, the clarity of each colour as viewed may be enhanced by interposing a white layer between the two coloured layers.

One preferred form of the mulch film according to the invention comprises superimposed layers of a red polyolefin film and a blue polyolefin film. The red polyolefin film will usually be one which reflects or transmits light within the range of wavelengths from approximately 600into 700 nm but almost totally absorbs light in the wavelength range frcm 400 nm to 600 nm. The blue polyolefin film will usually be one which reflects or transmits light within the wavelength range from approximately 400 nm to 600 nm and almost totally absorbs light in the wavelength range from 600 nm to 700 (see Fig. 2).The ccmbination of these two films in appropriate concentrations therefore absorbs almost totally the light in the wavelength range fran 400 nm to 700 nm.

In a particularly preferred form of the improved mulch film according to the invention, a white polyolefin film is interposed between the two coloured films. While the inclusion of a white film in this way may reduce slightly the transmission of solar heat into the soil, it enhances the reflection of coloured light from the upper surface of the mulch film. An incidental benefit is that the laminated mulch film has a more positive, and therefore more attractive, colour on each of its faces.

The improved mulch film may be used with either of its layers uppermost and gives a beneficial soil-heating effect and greatly reduces or eliminates the formation and growth of weeds. However the choice of which layer is placed uppermost may be made bearing in mind the crop to be produced. For example red light reflected from the upper surface of a mulch film has been reported to favour the growth of tomato crops, while blue reflected light is said to enhance the growth of strawberries.

In the above embodiments of the mulch film according to the invention, the two coloured films which combine to exclude light in the specified range are the two outermost films and therefore determine the colour of the mulch film when viewed fram its two faces. However other effects, both visual and also benefitting plant growth above the film, may be achieved by superimposing one or more additional layers on the film. For example a layer of another colour, for example green, may be superimposed upon superimposed red and blue layers, or upon red, white and blue layers, optionally with a further white layer below the topmost coloured layer.

In further preferred forms of the invention, the heatretaining properties of the mulch film may be improved by using a modified polyolefin and/or by the inclusion of appropriate additives in the film. For example, at least one layer of the mulch film may be a copolymer of ethylene with 4 to 28 per cent by weight of vinyl acetate. Alternatively or in addition, from 1 to 20 per cent of an infra-red absorbing inorganic compound may be included in at least one of the layers of the mulch film; suitable such compounds include silica, an aluminosilicate, clay, calcinated clay and a metaphosphate.

Thus the polyolefin films may each be formed from an olefinic homopolymer or from a copolymer of an olefin with another mm=aner, for example an ethylene/vinyl acetate copoliner. The two films may be of the same or different cotttpositions, apart from the pigments imparting colour to the respective films. The preferred polyolefin films are of polyethylene, especially low density or linear low-density polyethylene.

The mulch film may be produced by extruding the films forming the various layers as separate films and then bonding the extruded films together, for example by means of adhesive and/or heat. However it is much preferred to produce the improved film by co-extrusion of the twn or more compositions. to form a laminated film at the point of extrusion. Co-extrusion methods of this sort give films in which the bonding together of the layers is such as to lead to an almost unitary film, in which the respective layers are nonetheless readily distinguished by their colours.

The mulch film according to the present invention may conveniently be approximately 15 to 150 micrometers thick, preferably 30 to 80 micrometres. Thus a typical preferred film may be, say, about 50 or about 65 micrometres in thickness. The individual films which make up this total thickness will usually be each of the same thickness but if desired the mulching properties of the laminated film may be modified by varying these individual film thicknesses.

The invention is further described and illustrated by reference to the accompanying drawings and by means of the following Examples. The two figures of the drawings illustrate the light transmission properties of the red and blue pigments respectively referred to in the Examples. Percentages in the Examples are by weight.

Example 1 A three-layer mulch film was prepared by co-extrusion of three linear low-density polyethylene film compositions, which differed from each other only in respect of the colour-imparting ooopounds present. The three layers consisted of red, white and blue individual layers, superimposed in that order and coloured as follows: The red layer contained 12.5% of Shulman "Polybatch" Red 5170, a colour concentrate containing a lead molybbate orange together with a calcium-based azo red toner.

The white layer contained 5.5% of Shulman "Polybatch" White T3440, a colour concentrate containing titanium dioxide as white pigment.

The blue layer contained 14.0% of Shulman "Polybatch" Blue 4025, a colour concentrate containing a phythalocyanine blue pigment The resulting mulch film, which was approximately 60 micrometres thick, was used under strawberry plants with the blue layer uppermost. In comparison with a brown film used in a similar manner, an early version of the film according to the present invention advanced the developnent of the strawberry crop by about two days. The soil temperature was monitored regularly and was found to be approximately 3 degrees Celsius higher than that of a black film used similarly. Weed growth was totally inhibited.

Example 2 A film was prepared in accordance with Example 1 but having orange, white and blue layers.

The orange layer contained 12.5% of Shulman "Polybatch" Orange 1096, a colour concentrate containing a lead molybdate orange pigment.

The white and blue layers were as described in Example 1.

Example 3 A film was prepared in accordance with Example 2 except that The orange layer contained 12.5% of Shulman "Polybatch" Orange 1046, a colour concentrate containing a mixture of yellow diarylide dimethoxy choranilide, orange diarylide pyrazilone and a strontium based azo red pigment.

The white and blue layers were as in Example 1.

The beneficial results obtained by the use of the films of Examples 2 and 3 were comparable with those obtained in Example 1.

Claims

1. An improved mulch film comprising superinposed layers of two coloured polyolefin films, each of which absorbs light over only a part of the range of wavelengths from 400 nm to 700 nm but which together absorb a major proportidn of light in that range.

2. An improved mulch film as claimed in claim 1, wherein said films both transmit solar energy over at least a major proportion of the range of wavelengths from 700 nm to about 2300 nm.

3. An improved mulch film as claimed in either of the preceding claims, wherein one of said two films is pigmented with a red or orange pigment and the other with a blue pigment.

4. An improved mulch film as claimed in claim 3, wherein said red film absorbs a major proportion of light in the wavelength range from 400 nm to 600 nm.

5. An improved mulch film as claimed in either of claims 3 and 4, wherein said blue film absorbs a major proportion of light in the wavelength range from 600 nm to 700 nm.

6. An improved mulch film as claimed in any of the preceding claims, including a white polyolefin film between said coloured films.

7. An improved mulch film as claimed in any of the preceding claims, including a polyolefin film of a third colour superimposed on said layers.

8. An improved mulch film as claimed in any of the preceding claims, wherein each of the films is a homDpolymer of ethylene.

9. An improved mulch film as claimed in any of claims 1 to 7, wherein at least one of the films is a copolymer of ethylene with vinyl acetate.

10. An improved mulch film as claimed in any of the preceding claims, wherein at least one of the films contains an infra-red absorbing inorganic compound.

11. An improved mulch film as claimed in any of the preceding claims, which has been produced by co-extrusion of two or more polyolefin compositions to form a laminated film at the point of extrusion.

12. An improved mulch film as claimed in any of the preceding claims, having a total thickness lying within the range from 15 to 150 micrometres.

13. An improved mulch film substantially as hereinbefore described in any of the foregoing Examples.