GB2057454A - Polymeric compositions of matter for use in agriculture - Google Patents

Abstract

A polymeric composition of matter substantially transparent to solar radiation and constituting an effective barrier to infra-red radiation, especially in the region of about 7 mu to 15 mu , comprises polyethylene or a copolymer of mainly polyethylene, which contains an effective quantity e.g. from 1 to 20% by weight, of an insoluble metaphosphate, e.g. sodium metaphosphate, potassium metaphosphate, aluminum metaphosphate or calcium metaphosphate. The composition is mainly used in sheet or film form as a cover for agricultural crops.

Description

SPECIFICATION Polymeric compositions of matter for use in agriculture The invention relates to novel polymeric compositions of matter, for use in agriculture.

More specifically it relates to certain novel polymers or copolymers of ethylene adapted to pass substantially unhindered light in the visible range, yet be substantially impervious to a substantial fraction of light in the infra-red range.

Such polyethylenes or copolymers of polyethylene in sheet and film form are especially suitable for use as plastic sheets or films covering structures under which winter or spring crops are grown.

Preferred thicknesses are about 0.15 to 0.5 mm.

Other and further aspects of the invention will become apparent hereinafter.

The use of polyethylene sheets or films for mulching is an efficient method for heat sterilization of soil. Polyethylene films containing metaphosphate filler enhance the sterilization process by reducing the heat losses from the soil due to serving as infra-red barrier to infra-red radiation, given by the soil.

Quite large areas of land devoted to the cultivation of winter crops are covered with tunnel-like structures, low tunnels, walk-in tunnels and greenhouses; using polyethylene sheeting for protection of the crops from the cold. The polymeric substances are chosen in such manner as to permit light in the visible range to penetrate said sheeting. When the visible light penetrates the sheeting and reaches the ground or the growing plant, part of it is converted into light of longer wavelength, in the infra-red range, and generally the temperature inside such tunnels is higher than the outside temperature. At night, when no illumination by sun-light takes place, heat will be lost through the polyethylene sheeting through infra-red radiation passing through said sheeting, as this is generally quite pervious to such radiation.The resulting drop of temperature is harmful to the growth of the plants and hitherto various attempts have been made to provide compositions of matter which will substantially prevent the heat loss due to infra-red radiation.

In French Patent Specification No. 1,574,088 there are described plastic films, which are pervious to solar radiation and constitute a barrier to infra-red radiation, which comprise a polymer of ethylene or a copolymer of ethylene and of vinyl acetate containing at least 10 per cent by weight of finely dispersed silica or aluminum hydroxide.

Water formed in the process by exudation from the inorganic filter spoils the mechanical properties of such films. There exists a further French Patent, No. 2,213,954 which provides plastic sheeting of polyethylene or copolymers or polyethylene and vinyl acetate containing 1 to 20 per cent by weight of anhydrous alumo silicate.

The work-up of the compositions of matter according to the latter patent requires the use of esters of polyalcohols or of polyamino alcohols.

This process is rather complicated and the resultant product seems to be rather expensive.

The main advantage of the metaphosphate filler as compared to the aluminum silicate is that the sodium metaphosphate filler is substantially transparent to the visible light while the aluminum silicate filler absorbs as much as 50% of the visible light. The visible light is both essential for crops grown under the plastic tunnel and is essential for heating the soil.

The present invention relates to polymeric compositions of matter, in sheet or film form, which sheets are substantially pervious to solar radiation, yet constitute an efficient barrier to infra-red radiation, said sheets being characterized in that the matrix is polyethylene or a copolymer in which polyethylene predominates, containing a sufficient quantity of a metaphosphate of a metal chosen from among group 1 to 3 of the periodic table. The film must be pervious to solar radiation, and it is well known that polyethylene polymers or copolymers have this property. The addition of certain phosphates, and especially metaphosphates does not perceptibly impair the transmission of solar radiation, but changes in a very pronounced manner the transmission of infrared radiation.

The critical region is that of about 7 y to 1 5 u, and it is well known that there exist certain ions which absorb in this region. Thus it is known that PO4-3 absorbs strongly in this region. The spectra of polyanions of the MPO3- type is described in: a. D.E.C. Corbridge et al., J. Chem. Soc. (1954) 493-504: b. Infrared Spectra of Inorganic Compounds, R. A. Hyquist, Academic Press, New York, 1971.

c. Bertoluzza et al., Atti della Academia Nationale, Lincel Rend. Sc. fis. Mat. c. Nat. Vol.

LVI, 1974.

The region of 7 to 1 5 u corresponds to about 1300 cm-' to 800 cm-1. The polyanions (NaPO3)x,(KPO3)xl (CaPO3) and (AlPO3)x all have strong absorption spectra in the 800 cm-1 to 1400 cm-1 region and they are substantially transparent in the visible region.

According to the present invention there are provided polymeric blends of polyethylene or of copolymers containing a preponderant quantity of polyethylene, comprising a quantity of finely dispersed phosphate having a strong absorption in the l.R. region, while being substantially transparent in the visible range. Further according to the present invention, there is provided a structure for the cultivation of agricultural crops comprising a cover or sheeting of film made of a polymeric blend as aforesaid. The thickness of the film is preferably from 0.1 to 0.5 mm.

According to a preferred embodiment, the phosphates used are insoluble forms of sodiumpotassium- calcium- or aluminummetaphosphate. The quantity of these (or other metaphosphates) incorporated in the polymeric matrix may be varied between 1 and 20 per cent by weight, the preferred range being 3 to 10 per cent by weight. The preferred polyethylene is low density polyethylene or copolymers of ethylene with vinyl acetate as e.g. a material known on the market as EVA. The insoluble forms of the above metaphosphates can be blended with a granulate of polyethylene by standard industrial processes, such as milling, double screw extrusion or by the use of a mixer of the Banbury type.There is obtained a homogeneous composition of matter which can be blow-extruded in the form of sheets of conventional thickness used for the coverage of crops by means of structures used in agriculture, and such films have the mechanical strength required for such purpose. Experiments have shown that films of low density polyethylene containing from 5 to 10 per cent by weight of sodium metaphosphate, or of potassium metaphosphate have good mechanical properties; they are substantially transparent to solar radiation, yet absorb at least 80 per cent of infra-red radiation in the 7 to 1 5 y region. Films according to the present invention were immersed in water and during 24 hours essentially no leaching took place and physical properties did not deteriorate.

In a similar manner there can be used other phosphates, and especially metaphosphates which have the required optical properties. While the insoluble polymethaphosphates of sodium and potassium have been found particularly suitable for the purpose of this invention, the invention is not limited to these phosphorus containing salts only. Thus metaphosphates, or polymethaphosphates of aluminum, calcium, barium, etc. may also be suitable.

The following example is given in order to illustrate the invention. Reference is made therein to the accompanying drawing.

In the drawing the single Figure shows infra-red absorption spectra.

EXAMPLE 40 Kg of sodium metaphosphate (Maddrell salt) were mixed with 400 Kg of low density polyethylene in a Banbury internal mixer at 170-1 800C. Subsequently the mix from the Banbury mixer was extruded and pelletized in the conventional manner.

The pellets were then read into an extruder provided with a blowing attachment, whereby a flat tube was formed and rolled up on an uptake mandrel.

From the flat tube, sheets and films of the desired thickness were used for crop covering structures. Good results were obtained with thicknesses from about 0.1 mm to 0.5 mm.

The absorption spectra of a film 0.1 mm thick was compared with those of low density polyethylene 1 PE 111 grade (an SR. PET. IND.

product) and of a commercially available film INFRAME X30, both of 0.1 mm thickness.

The infra-red absorption spectrum is shown by curve 1 in the accompanying drawing.

The spectra of the commercial films known as IPE 111 and Intame X30 are shown by curves 2 and 3. The wavelength is given in microns at the top of the drawing and the Wellenzall wavenumer (cm-1) is given at the bottom.

The spectra show that the film of the invention compares favourably with the commercial film.

However, the commercially available film reduces the transmission of light in the visible region by about 50% as compared to low density polyethylene while the film of the present invention reduces transmission by only 10% of that of polyethylene.

Tensile strength measurement on the film with 10% sodium metaphosphate filler is practically unaltered from that of polyethylene IPE 111.

The invention includes within its scope a tunnel-shaped or similar structure, for use in the cultivation of agricultural or horticultural crops, having a cover of sheeting or film formed from a composition as hereinbefore described. Various arrangements are possible. For example the structure may be in the form of a series of cloches each having a framework on which the sheet or film is mounted or it may be in the form of a metallic or other frame over which the sheet or film is draped, after positioning the frame on the soil, and is secured over the frame by appropriate mechanical fittings or by weighting with stones or soil.

Claims (7)

1. A polymeric composition of matter substantially transparent to solar radiation and constituting an effective barrier to infra-red radiation, especially in the region of about 7 y to 15 , comprising polyethylene or a copolymer of mainly polyethylene, containing an effective quantity of an insoluble metaphosphate.

2. A polymeric composition according to claim 1 in which the metaphosphate is sodium metaphosphate, potassium metaphosphate, aluminum metaphosphate or calcium metaphosphate.

3. A composition according to either of claims 1 or 2 in which the content of the phosphate is from 1 to 20 per cent by weight.

4. A polymer according to any one of claims 1 to 3, in sheet or film form.

5. A structure for the cultivation of agricultural crops comprising a cover of sheeting or film made of a polymer according to any of claims 1 to 4.

6. A structure according to claim 5 in which the sheeting or film is of 0.1 to 0.5 mm thickness.

7. A polymeric composition of matter substantially transparent to solar radiation, substantially as hereinbefore described and illustrated by reference to the foregoing Example.