The present invention relates to a multilayer polymeric film with interfer¬ ence effect. More particularly, the present invention relates to an agricul- tural film for the screening of solar radiation with the aim of influencing plant growth and the morphogenesis.

It is necessary for a controllable growth of plants to set up special light conditions to affect plant growth and morphogenesis. Special light condi- tions for plant growth, for example, within a greenhouse or under a mulch film include some which are of particular interest. The temperature within the enclosed area as well as the light intensity and the spectral distribution of the incoming light are important factors for the plant growth.

Plants in a greenhouse, for example, will not grow so well if the tempera¬ ture variation between day and night is large due to a big difference in visible light and NIR radiation impact.

From this it follows that the plants stay back in growth. Thus, for the pho- tosynthesis in greenhouses the photosynthetically active radiation (PAR) is only desired while other parts of the solar light should be screened be¬ cause they have mainly a negative influence on the microclimate. There¬ fore the following objectives have to be achieved: Firstly, the intensive ir¬ radiation by sunlight in particular by NIR radiation needs to be screened, in order to avoid too high temperatures within the enclosed area. Sec¬ ondly, the incoming light beams are to be scattered in order to prevent burnings on plants. Thirdly, heat within the area is to be stored at night.

However, in case of mulch films the opposite effect is desired. The PAR should be screened and all other parts of the solar light should be trans¬ mitted. Mulch films are used in agriculture to improve the growing condi¬ tions of the useful plants and to minimize the use of chemical herbicides.

For certain crop growers want to intervene in the plant morphogenesis. Morphogenesis is the influence of environmental factors on the shape and appearance of the plant. One of these factors is the spectral energy distri¬ bution.

Raviv and Allingham describe in "Plastic Culture", No. 59, September 1989, pp. 3 to 12, that diffuse light is favourable for a steady growth of plants. Furthermore, it turned out that diffuse light has the advantage of a more uniform illumination and that it does not damage the plants as much as direct radiation. Hancock describes in "Plastic Culture", No. 79, 1988, pp. 4 to 14, that special additives in LDPE have a favourable effect on the microclimate in greenhouses because of the generation of diffuse light and the creation of a favourably thermic effect in the greenhouses. Suitable additives are aluminosilicate in the form of kaolin, calcium carbonate, talc and kaolin clay.

Greenhouses made of glass already fulfil some of the above-mentioned conditions by virtue of the inherent physical properties of inorganic glasses. Furthermore, constructive measures, for example knurled glass or shadow cloth are known.

If the enclosing material is plastic, such as, for example, polyethylene, polyethylene/vinyl acetate copolymers, poiyvinyl chloride, polycarbonate or polyacrylate, it is much more difficult to meet these requirements. The high transparency of many plastics for IR radiation results in good heat transmission which, in the case of films, is additionally assisted by the low film thickness.

A polymethyl methacrylate glazing material for buildings and vehicles containing an interference pigment for the screening of NIR radiation of a wave length between 800 and 1500 nm is disclosed in DE 2,544, 245.

The pigment used has a blue-red colour and the light transmitted through the glazing material has a yellow-green colour. When used in green¬ houses, this glazing material has the disadvantage that portions of visible light which cannot be utilized by the plant are transmitted and, on the other hand, the red portion of visible light which can be utilized by the plant is screened off by said glazing material. Furthermore, the transmitted green portion has the additional disadvantage that it contributes to heating of the greenhouse by virtue of its conversion into long wave light.

EP-A 0,428,937 describes a grey-white coating composition for green¬ houses consisting of a polymer substrate and reflecting particles sus¬ pended therein. These particles are aluminium platelets or mica platelets coated with titanium dioxide. The coating composition is used for tempo¬ rary coatings in extreme weather conditions (summer). In winter, it can again be removed by washing it off with a water jet. More details regarding the pigment used are not given.

The coating composition has the disadvantage that not only the IR beams but also a substantial portion of the visible light which can be utilized by the plant are reflected by the metal particles. The green portion of the transmitted light cannot be utilized by the plant and contributes to heating of the greenhouse.

EP-A-03 98243 describes polymeric mulch sheets and mulch films for use in agriculture which have a green absorption colour and contain a UV stabilizer. The green sheets and films absorb a large part of the solar ra¬ diation which promotes photosynthesis and plant development (PAR), and transmits enough solar radiation so as to heat the soil beneath such films and sheets. Such mulch films and sheets reduce weed growth but they have the disadvantage that the photosynthetically active radiation is ab¬ sorbed and is lost for the useful plant.

L.M. Mortensen and E. Stromme describe in Scientia Horticulturae 33 (1987), 27-36 the effects of light quality on some greenhouse crops. Dif¬ ferent light qualities were established in growth chambers placed outdoors by selective screening of the natural light spectrum in the chambers. The PAR level was the same at all light qualities. Blue light containing a high red/far-red ratio reduced the dry weight compared to natural (N), green, yellow and red light in chrysanthemum, tomato and lettuce. Plant height in chrysanthemum and tomato was strongly reduced by blue compared to N light, and strongly increased by green and yellow light.

WO 94/05727 discloses a composite material for the selective screening of radiation comprising a polymer and an interference pigment. The com¬ posite material can be employed in form of films, sheets and webs as cov¬ ering for greenhouses, for the influencing of plant morphogenesis and for mulching purposes.

Surprisingly, it has been found that a multilayer reflective polymeric body of at least first and second diverse generally transparent polymeric mate¬ rials, arranged in a sufficient member of alternating layers of said first and second polymeric materials such that a definite part of the light incident on said body is reflected wherein said first and second polymeric materials differ from each other in refractive index by at least about 0.03 can be employed for the screening of solar radiation with the aim of influencing plant growth and the morphogenesis.

The polymeric body can be used in the form of a film, sheet or web.

Multilayer reflective polymeric bodies and their manufacture are well known and described in US patents Nos. 2408398, 2790202, 2 820249, 2 902 754, 3 024494, 3 147 515, 3 308 508, 5 217794 and 5234 729 and in Polymer Engineering and Science 1969, Vol. 9, No. 6, pages 400-404 under the title "Physical Optics of Iridescent Multilayered Plastic Films" by T. Alfrey, E.F. Gurnee and W.J. Schrenk, in a brochure of The Dow Chemical Company titled "Coextrusion of Blown Multilayer Plastic Films" by "J. Schrenk and T. Alfrey, and in Journal of Plastic Film & Sheeting 1994, Vol. 10, pages 78-79 by J.A. Wheatley and W.J. Schrenk.

Normally, said polymeric body has an iridescent or silvery appearance but by keeping definite tolerances of the layer thickness of about 5 nm it is possible to reflect a definite part of the light incident on the body. In this case the polymeric body appears in a definite color in the reflected light and in the complementary color in the transmitted light.

By variation of layer thickness, number of the alternating layers and the polymeric material the desired color can be adjusted. That means the re¬ flecting band can intensionally be shifted in the spectrum of the sunlight.

The polymeric material used for the preparation of the composite materials are transparent materials like polyolefins, cc- or terpolymers, for example low-density polyethylene (LDPE), vinylester copolymers like ethylene- vinylacetate copolymer (EVA), fluoropolymers, co- or terpolymers and polyvinylidene chloride (PVDC), polyvinyl chloride (PVC), polycarbonate (PC), polymethylacrylate (PMMA), polystyrene (PS), polyesters (PET, PBTE) or mixtures of transparent polymers. The term "polymer" may in¬ clude mixtures of polymers.

The polymeric material has to be choosen so that the difference in refrac¬ tive index is bigger than .03.

The term multilayer reflective polymeric body includes noncontinuous lay¬ ers of films, such as lamellar or paniculate structures incorporated into a polymer matrix.

The protective skin layer improves the mechanical, optical and application properties such as weather resistance, UV stability, and non-drop behav¬ ior.

The number of alternating layers of the polymeric body can be varied be¬ tween 30 and 3000, preferably between 50 and 1000. The total number of layers for the desired effect depends on the difference of refractive indi¬ ces. The smaller the difference the higher the total number of layers.

The entire disclosure of all applications, patents and publications, cited above and below are hereby incorporated by reference.

In order that the invention may be more readily understood, reference is made to the following example, which is intended to be illustrative of the invention, but is not intended to be limiting in scope.

Example 1

Fig. 1 shows the reflectance spectrum of a polymeric multilayer film which consists of alternating layers of polycarbonate (PC refractive index n _D = 1,587) and polymethacrylate (PMMA, n ₀ = 1,491). The total numbers of layers is 100 and the layer thickness is 100 nm for a PC layer resp. 140 nm for PMMA layer.

The spectrum shows that the film is transparent below 700 nm and reflects with a maximum around 740 nm.

This film therefore is suitable for influencing of the growth of plants, such as vegetables and tomatoes, so that this plants become more compact. The background for this photomorphogenetic effect is the change of the red to far red ratio for the phytochrome.

Example 2

Fig. 2 shows the refractive spectrum of a polymeric multilayer film which consists of alternating layers of PC and PMMA. The total number of layers is 100 and the layer thickness is 100 nm for a PC layer resp. 115 nm for PMMA layer.

This film is transparent below 620 nm and above 700 nm and shows re¬ flection inbetween. This film is suitable for influencing the growth of plants, when slender plants are desired, such as for roses.

The background here is the opposite effect of Example 1.

Example 3

Fig. 3 shows the reflective spectrum of a polymeric multilayer film which consists of alternating layers of PC and PMMA. The total numbers of lay- ers is 50 and the layer thickness is 70 nm for PC resp. 105 nm for PMMA.

This film reflects the green light around 540 nm that cannot be utilized for the growth of plants (minimum of PAR). It is therefore suitable for green¬ house films.

Example 4

Fig. 4 shows the reflective spectrum of a polymeric multilayer film which consists of alternating layers of PC and PMMA. The total number of layers is 500 and the layer thickness is (70 + 0.05 n) nm for PC and (75 + 0.0533 n) nm for PMMA, n indicating the number of layer.

This spectrum is similar to the reflection spectrum in T. Alfrey, jr. et al. Polym. Eng. Sci. 9 (1969), 400-404. This film reflects nearly the total PAR spectrum and therefore suitable for mulch foils.