Polymer film for use as a cover in agriculture

DESCRIPTION Technical field The present invention relates to a polymer film for use in agriculture as a cover for greenhouses and tunnels in accordance with the features set out in the preamble of the main claim. Technological background

In the technical sector of reference of the present invention, it is known to use polymer films as covers for agricultural greenhouses. These films have to meet numerous technical requirements both in terms of optical properties, in order to ensure maximum transparency to visible light and at the same time an effective barrier to far infrared radiation (greenhouse effect), and in terms of mechanical properties, such as, for example, tensile strength, yield point, impact strength and tear propagation strength. One of the more important technical characteristics sought in a polymer film for applications in the agricultural sector is that of resistance to the degradation caused by the action of sunlight and thermal oxidation. These latter damage the polymer chains over time, draining the film of both its mechanical characteristics and its optical characteristics. In fact, it is known that a polymer film used as a cover for greenhouses has an average service life of a few dozen months (by way of reference, the example given by Appendix A of standard EN 13206:2001: a film of Class D in a climate zone with irradiation of from 100 to 130 kLy/year has a life of approximately 24 months), after which it becomes necessary to replace it because the film loses its inherent elasticity, becomes brittle and tends to become opaque.

Typically, the films used in this specific technical field are obtained from a polyolefin base formed, for example, from low density or linear low density polyethylene, commonly known by the acronyms LDPE and LLDPE, or also from ethylene-vinyl acetate or ethylene-butyl acrylate copolymers, commonly known by the respective acronyms EVA/C and EBA/C. Compounds capable of conferring on the polymer mixture the desired properties in terms of photo-oxidation resistance, surface activity, absorption of infrared radiation and modification of transmitted photosynthetically active radiation (P.A.R.), etc., are therefore advantageously added to those polymers.

As regards resistance to the polymer degradation caused by light (photodegradation), it is known to use light stabilisers which slow down, more or less efficiently, the processes which result in the progressive breakage of the polymer chains, thus prolonging the service life of the film. Many families of compounds suitable for use in polymer mixtures as light stabilisers are available on the market. There may be mentioned, for example, UV absorbers, Isuch as benzophenones, benzotriazoles, hydroxybenzophenones, benzoic acid esters, triazines, as well as some acrylates, salts and/or organometal complexes based on nickel or other metals, some oxamides and others still.

Together with the compounds mentioned above, it is also known to use as stabilisers sterically hindered amines, which are usually referred to in the sector by the acronym HALS (hindered amine light stabiliser) and which are identified thus hereinafter for the sake of brevity. HALS are normally dosed together with UV-absorbing additives in

percentages ranging from 2 to 4 times that of the UV absorber. They act by catalytically blocking the radicals which form owing to the effect of the absorption by the polymer chains of the energy provided by solar radiation, thus slowing down the rate of the photo-oxidation and depolymerisation reactions. All types of HALS are potentially effective but, given their behaviour as Lewis bases, they suffer from interaction with acid substances which may block the functional groups thereof. In particular, in the case of films used as covers for greenhouses and tunnels, contact with numerous chemical products which are based on sulphur and halogens and which are normally used in agriculture may rapidly reduce their efficiency.

Despite the presence on the market of all of the families of compounds indicated generically above, there is still a need in the technical sector of reference to prolong the service life of polymer films used as covers in agriculture by increasing their resistance to photodegradation. Description of the invention

The object of the present invention is to provide a polymer film which is devised structurally and functionally to\ maintain for a longer period the desired functional, mechanical and spectrophotometric characteristics by a greater resistance to the degradation caused by the action of solar radiation.

That object is achieved by the present invention by means of a polymer film produced in accordance with the claims which follow.

Preferred embodiment of the invention

In accordance with a first aspect of the present invention, the film comprises a layer of polyolefin-based polymer material in which a light

stabiliser based on hindered amines (HALS), and an effective amount of thermoplastic polyurethane, which is referred to hereinafter as TPU for the sake of brevity, are dispersed.

The film is normally of the single-layer type but may likewise be of the multi-layer type, which is coextruded or coupled, as long as the presence of a polyolefin material, of TPU and HALS, is provided for in at least one of the layers forming the film.

The polymer base which forms the film according to the invention is a polyolefin selected from those normally used in this type of application, such as, for example, a low density polyethylene (LDPE) or a linear low density polyethylene (LLDPE). Preferably, however, the film is formed from an ethylene-vinyl acetate copolymer (EVA/C) or from an ethylene-butyl acrylate copolymer (EBA/C). Likewise preferred is the use of a polymer mixture based on EVA/C or EBA/C with LDPE or LLDPE, in any proportion. The polyolefin base is advantageously mixed with the normal additives used in the sector in order to confer the desired antioxidant, slip, antistatic and antidrip properties and the desired properties of selective photo-absorption in the visible light range and/or the ultraviolet range and/or the infrared range, etc., at the dosages suggested by the suppliers of the additives and at any rate known to the experts in the art.

On the other hand, as regards resistance to photo-degradation, the light stabiliser added to the polyolefin-based mixture comprises HALS, which are preferably oligomeric or polymeric and of the secondary and tertiary type and also methylated or at any rate modified and substituted, and is constituted in a preferred composition variant by those substances only.

Preferably, the light stabiliser is constituted by methylated HALS or by sterically hindered aminoxyamines, also known as NOR-HALS, in order to make use of the lower basicity thereof.

According to the invention, an effective amount of TPU is also added to the basic polymer mixture. The TPU may be either of the aromatic type or of the aliphatic type, with a polyester or polyether structure. However, the use of aliphatic, polyester or polyether TPU is preferred because aromatic TPU may give rise, as a result of photo-oxidative reactions, to chromophore groups which tend to confer a brownish yellow colour on the film. Films to which aromatic TPU, which is per se advantageously less expensive, is added, could also be used in particular applications where a possible colouring does not have negative effects. TPUs having a polyester structure are generally less expensive, while TPUs based on polyether generally exhibit greater resistance to hydrolysis. In the polymer mixture used to produce the film according to the invention, the TPU and the HALS are present in a ratio by weight of from 2:1 to 25: 1. In particular, the TPU is added to the polymer mixture in such a manner as to attain a fraction by weight of from 5% to 30%, preferably from 7% to 15%, while, for their part, the HALS are added in such a manner as to attain a fraction by weight of from 0.2% to 2.5%, preferably from 0.8% to 1.6%, with respect to the film. It is known that the polyolefin family and the TPUs have physico-chemical characteristics which are relatively diverse with respect to each other, so that, in liquid phase, the two compounds are poorly miscible in each other. This means that they are mixed homogeneously with each other by physical mixing inside an extruder, into

which they are both fed in the predefined proportion, making use of the mechanical action of the screw in which a transport and compression region, a mixing region and a final compression region are combined. Preferably, the TPU and the polyolefin base are selected in such a manner as to have comparable melting points, differing by less than 40 ⁰ C, and similar apparent viscosities in the range of the speed gradients usual for extrusion processes. Examples

A film based on aliphatic polyester TPU and four films based on EVA/C, having, respectively, the formulations indicated below, were prepared: Specimen 1 : aliphatic polyester TPU: 98.0% antiblocking agent: 2.0%

Specimen 2:

EVA/C with 9% of VA 90.5% aromatic polyester TPU 8.0%

HALS 1.0% antiblocking agent 0.5%

Specimen 3:

EVA/C with 9% of VA 90.5% aliphatic polyester TPU 8.0%

HALS 1.0% antiblocking agent 0.5%

Specimen 4:

EVA/C with 9% of VA 80.5% aliphatic polyester TPU 18.0%

HALS 1.0% antiblocking agent 0.5%

Specimen 5:

EVA/C with 9% of VA 98.5%

HALS 1.0% antiblocking agent 0.5%

The formulations were mixed in a drive extruder of the Bandera single- screw type having a diameter of 45 mm, L/D=25, driven at an extrusion rate of 40 rev/min, with an internal temperature profile increasing from 135 ⁰ C to 180 ⁰ C, and were then extruded in such a manner as to form respective films having a thickness of 0.150 mm, numbered 1 to 5, from which were obtained the samples for the accelerated ageing tests in a SAIREM-SEPAP chamber, at a constant temperature of 6O ⁰ C. In order to determine the conclusion of each test, the criterion of reduction to 50% of the initial value for elongation at break in tensile stress tests according to EN ISO 527-1 and EN ISO 527-3 was used.

Specimen 1: did not reach the 750 hours of exposure, corresponding to the first sampling.

Specimen 2: test interrupted at 750 hours of exposure owing to the intense brownish yellow colour exhibited by the specimen.

Specimen 3: from 3,000 to 3,500 hours

Specimen 4: more than 3,500 hours

Specimen 5: from 2,500 to 3,000 hours.

A practical use check was carried out only for the formulation corresponding

to Specimen 4, which was the best result of the test, using as a comparison (control) the formulation of Specimen 5 and producing in an industrial-scale plant coextruded films having a thickness of 0.150 mm, which were installed from the month of October 2003 on a seVies of individual tunnels, three per type, in a test field belonging to the Applicant located in a climate zone of the northern hemisphere with irradiation intensity corresponding to approximately 130 kLy/year.

All of the films of type 5 were found to be still sound after two years, but with a longitudinal elongation at break of approximately 50% of the initial value.

The films of type 4 were found to be sound even at the end of the third winter of use, but with residual mechanical characteristics which unexpectedly suggest that they might last at least another year, with an increase of from 30 to 50% in resistance to photodegradation with respect to the films of the conventional type used as controls.

The present invention therefore achieves the object proposed of providing a polymer film having greater resistance to polymer degradation caused by exposure to sunlight. At the same time, the invention offers numerous other advantages for use, including the fact that the polymer film so obtained has, compared with a conventional polyolefin-based film, improved characteristics of elasticity and tear resistance and greater IR efficiency (greenhouse effect), evaluated as absorption of radiation in the far infrared range, wavelengths of from 7,000 to 13,000 nm, in accordance with the standard EN 13206:2001. In the example given, in fact, the IR efficiency measured on specimens of type 5

film was 75%, against values higher than 85% for the type 4 film obtained in accordance with the present invention.