PROCESS

This invention relates to a film for agricultural use and the relative manufacturing process.

More in detail, the invention relates to a plastic film for agricultural use containing high-density polyethylene (FIDPE).

As is well known, the polymeric films currently used in agriculture are usually based on polyolefinic polymers having optical and mechanical properties which comply with the requirements of EU Regulation EN 13206:2017, which classifies the plastic covering films used in agriculture and horticulture.

The plastic films in agriculture are mainly used for covering greenhouses in order to protect and favour the growth of the plants. In particular, they are used in greenhouses for forming a barrier which provides a protection for the cultivated plants against the weather conditions, for filtering solar radiation, for determining a balanced thermal effect between day and night.

The films for agriculture are mainly produced with the following variants of polyethylene:

- low-density polyethylene (LDPE);

- medium-density polyethylene (MDPE);

- linear low-density polyethylene (LLDPE);

- metallocene-based linear low-density polyethylene a base (mLLDPE); - ethylene copolymers (EVA, EBA, etc.).

Said variants can be produces in mono-extrusion mode or in co extrusion with several layers.

However, systems of this type can only be used for a limited time since they are exposed to solar radiation and to the use of chemical agents inside cultivations and the are often not able to filter the UV rays in an optimum manner.

In order to resolve these problems, more resistant films have been proposed, such a, for example, that described in patent EP0888404, which provides for cross-linking of the polyolefinic layers with silanes.

However, this system obtained with cross-linking additives (chemical reaction) is not only very expensive but also requires modifications to the extrusion system.

HDPE is, on the other hand, widely used in the production of films in the food packaging sectors (small bags, sheets for wrapping food, etc.), in the industrial sector, for example for making counter covers, large bags, etc. In agriculture, HDPE is only transformed for the production of mono filaments and raffia which are then woven and/or interlaced with frames for packaging covering netting or meshes but not into films for covering greenhouses or tunnels, either the mono-extruded or co-extruded variants.

In light of the above, there is an evident need for a device which overcomes the drawbacks of the prior art, which does not need any chemical reaction during the production step and does not require any modification of the extrusion systems.

The solution according to the invention falls within this context, providing a film for agricultural use containing high-density polyethylene (HDPE) with a physical process of combination of the HDPE with layers of polymers and copolymers, and thus without the need for any chemical reaction during the production step and no need for the modification of the standard extrusion systems.

These and other results are obtained according to the invention with a polyolefinic polymer having a density greater than those mentioned above, that is to say, the high-density polyethylene (HDPE) in co-extrusion with traditional polyolefinic polymers and copolymers.

The main advantages of the film according to the invention are as follows:

- better mechanical characteristics compared with all the traditional films;

- possibility of reducing thicknesses with consequent reduction of the quantity of plastic materials which at the end of the life cycle must be disposed of in authorised collection centres;

- greater impermeability, which is advantageous since the HDPE finds it more difficult to absorb the pesticides used in agriculture which can attack, and thus chemically deactivate, the UV stabilisers (HALS, NOR HALS)

- providing diffused light, thus avoiding the burning of the leaves of the crops, caused by the high energy of the direct solar light;

- regularising the migration of any anti-drop and/or anti-fog additives;

- better resistance to the degradation caused by UV rays of a film made of HDPE compared with those made of LDPE, LLDPE, MDPE, mLLDPE and the ethylene copolymers.

Table 1 shows the permeability values of the polyolefinic polymers and copolymers to carbon dioxide (CO2), oxygen (O2) and transmission of water vapour (WVTR), in 25 pm mono-extruded films.

The data shows a better barrier effect of the HDPE and consequently, since the HDPE is more impermeable, a greater resistance to the absorption of the treatments with pesticides inside the greenhouse and consequently a controlled migration, because it is slower, of any anti-drop and/or anti-fog additives inserted in the structure of the co-extruded film.

Table 1

Table 2 shows comparative values of diffused transmittance (haze) of the light, measured according to ISO 14782, between a mono-extruded film made of LDPE and one made of HDPE, both with thicknesses of 70 pm, which can be further modified by adding mineral additives of spheres of various kinds.

Table 2

Table 3 shows the values of the resistance to UV radiation of the film, expressed in kLangley (kly), for various mono-extruded films with a thickness of 50 pm, measured in Florida, hie is an exposure zone characterised by a solar radiation of approximately 140 kly/year. The measurements have been carried out on polymers without the addition of any UV stabilising additive, upon reaching 50% of their initial mechanical characteristics

Table 3

The film according to the invention contains in co-extrusion:

- in the outer layer(s) the HDPE leaving the possibility of using the traditional polyolefinic polymers or/with copolymers inside in order to have all the advantages described above (high mechanical strength, better resistance to UV degradation, better impermeability to any passage of treatments with pesticides);

- in one or more of the inner and/or outer layers, the HDPE in combination in the remaining layers with traditional polyolefinic polymers and/or copolymers (LDPE, LLDPE, mLLDPE, EVA, EBA, etc.) to mainly enhance just the mechanical characteristics of the latter, with the consequent possibility of reducing the thicknesses (if the HDPE is present in the outer layer(s) the performance levels are those described above). Any addition of HDPE in the inner layer(s) would drastically improve the mechanical characteristics of the film as indicated in Table 4).

Again according to the invention, the layer or layers of HDPE can have a minimum thickness of 1 .5 pm and a maximum thickness which can correspond to 90%, preferably 60%, of all the structure (for example, a co extruded film consisting of 5 layers and with a thickness of 150 pm, preferably not more than 90 pm which can be made of HDPE, distributed in one or more layers, inside and/or outside).

Moreover, the maximum thickness of total HDPE in the film can be approximately 200 pm, above which the film would have a rigidity and optical characteristics, such as opalescence, which would not be suitable for agriculture.

The film according to the invention is made by means of co-extrusion processes known for making film for agricultural covers of greenhouses or tunnels, both with blow technology and by casting.

Preferably, the film according to the invention is obtained by means of the coex blow process, feeding the various hoppers of the extruder with the polymeric matrices described above, in which the HDPE is placed in one or more inner and/or outer layers of the film.

The mixture of polymers is heated by the extruder to a temperature which can vary between 180 °C and 230 °C in order to obtain a homogeneous mixture.

Again according to the invention, the following can also be added to the polymeric mixture:

- UV stabilisers (for example HALS, NOR HALS, UV absorber, Nickel Quencher), in the standard versions or more resistant to pesticides (for example, methylated HALS, NOR HALS), to protect the polymers from the degradation by the UV rays;

- any mineral additives (for example, kaolin, silicon, nepheline, etc.) for example, spheres of various chemical nature (such as glass, PMMA, ceramic, etc.) to vary the optical/thermal characteristics, for example, in order to increase the diffused transmittance (haze) of the film in specific crops or geographical areas with high solar irradiation;

- any refreshing additives (for example, modified mica) for controlling the passage of the near infrared (NIR), which is the main cause of the temperature increases during the day;

- any anti-drop additives (for example, fatty acid esters) and/or anti fog additives (for example, modified silicones) to limit the problems of dropping of the films and/or preventing the formation of fog inside the greenhouse which represents one of the main causes of the proliferation of plant diseases;

- any anti-blocking additives in the co-extrusion process (for example, synthetic and natural silica) and/or gliding agents (erucamides, polyammidic waxes, etc.) to facilitate the production, collection of the film and laying of the film on the greenhouse structures.

The aim of the invention is therefore to make a plastic film for agricultural use containing high-density polyethylene (HDPE), with thicknesses which can vary according to the desired performance levels for the requested application, in order to increase the resistance to UV radiation, diffuse the light and substantially improve the mechanical characteristics, as well as the resistance to any aggression of plant protection products/pesticides.

A further aim of the invention is that said device can be manufactured with substantially reduced costs, with regard to both the productions costs and the management costs.

Yet another aim of the invention is to provide a process and device which is substantially simple, safe and reliable.

A first specific object of the invention is therefore a plastic film for agricultural use comprising at least one layer of high-density polyethylene (HDPE) and at least one layer of a second polyolefinic material selected between low-density polyethylene (LDPE), medium-density polyethylene (MDPE), linear polyethylene, linear low-density polyethylene (LLDPE) based on metallocenes (mLLDPE), ethylene copolymers (such as, for example, EVA and EBA), which can be obtained by means of a physical process for combination of said high-density polyethylene with said polyolefinic material.

According to the invention said layer of HDPE can have a minimum thickness of 1 .5 pm and a maximum thickness of 200 pm, and the sum of the thicknesses of all the layers of HDPE cannot correspond to more than 90% of the total thickness of the film.

Again according to the invention, said film can comprise at least three layer in which, preferably, at least one of the outer layers is made of HDPE.

Moreover, according to a preferred embodiment of the invention, said plastic film can comprise at least one UV stabiliser, in particular HALS o NOR-HALS, UV absorber, Nickel Quencher and/or at least any ultraviolet rays absorber, and can also comprises at leat one anti-drop and/or anti-fog refreshing additive (for example, modified mica).

Another object of the invention is a process for manufacturing a plastic film as defined above, comprising a co-extrusion step, which can preferably be performed by blowing, of said at least one layer of high-density polyethylene (HDPE) and said at least one layer of at least a second polyolefinic material.

According to the invention, during said co-extrusion step at least one anti-blocking additive can be added, for example synthetic or natural silica and/or at least one gliding agent, for example erucamides or polyammidic waxes.

The invention will be described below by way of example and without limiting the scope of the invention with particular reference to some examples.

Example 1 .

A film was made having five layers and a total thickness of 150 pm, with each layer of equal thickness, co-extruded with HDPE in just the two outer layers and LDPE in the remaining three inner layers and it was compared with a traditional film, having five layers and a total thickness of 150 pm, with each layer of equal thickness, co-extruded in all the five layers with LDPE.

Table 4 indicates the comparative values (according to ASTM D882) of ultimate load (%) and ultimate elongation(%) in a longitudinal (MD) and transversal (TD) direction between the two co-extruded films (respectively, coex film with HDPE and traditional coex film) of equal thickness.

Table 4

The present invention is described by way of example only, without limiting the scope of application, according to its preferred embodiments, but it shall be understood that the invention may be modified and/or adapted by experts in the field without thereby departing from the scope of the inventive concept, as defined in the claims herein.