GREENHOUSE SCREEN

Abstract

A greenhouse screen is described with strips of film material that are interconnected by a yarn system of transverse threads and longitudinal threads by means of knitting, warp-knitting or weaving process to form a continuous product is disclosed. At least some of the strips include a film material in the form of a single- or multilayer polyethylene film having a thickness of 10-70 micrometers. Said film has at least 2-4 wt.-% SiO.sub.2 particles having a D.sub.50 of 2-10 micrometers. The film is advantageously used as a screen providing light scattering properties particularly suited for greenhouse applications.

Claims

1. A greenhouse screen comprising strips (11) of film material that are interconnected by a yarn system of transverse threads (12, 14, 18) and longitudinal threads (13a, 13b; 15; 19) by means of knitting, warp-knitting or weaving process to form a continuous product, characterized in that at least some of the strips (11) comprise a film in the form of a single- or multilayer polyethylene film with a thickness of 10-70 micrometers, said film comprising at least 1.7 wt.-% and a maximum of 4.5 wt.-% SiO.sub.2 particles having a D.sub.50 of 2-10 micrometers.

2. The greenhouse screen according to claim 1, characterized in that said polyethylene is a high density polyethylene (HDPE) resin.

3. The greenhouse screen according to claim 2, characterized in that said high density polyethylene (HDPE) resin has a melt flow rate of 1.6-2.0 g/10 min at 190° C./5.0 kg, and 19-26 g/10 min at 190° C./21.6 kg (ISO 1133-1), and a density of 0.940-0.955 g/cm.sup.3 (ISO 1183-1).

4. The greenhouse screen according to any one of the preceding claims, characterized in that said film comprises 2.0-4.2 wt.-%, such as 2.5-4.0 wt.-% of SiO.sub.2 by weight, based on the total weight of the film.

5. The greenhouse screen according to any one of the preceding claims, characterized in that said SiO.sub.2 particles have a D.sub.50 value of 3-9 micrometers, such as a D.sub.50 value of 4-8 micrometers, such as a D.sub.50 value of 5-7 micrometers.

6. The greenhouse screen according to any one of the preceding claims, characterized in that said film has a transparency of at least 70%, such as at least 75%, 80%, 85%, 86%, 87%, 88%, 90%, 91%, 92%, 93% or higher.

7. The greenhouse screen according to any one of the preceding claims, characterized in that said film has a haze should be 50-75%, such as 54-70%, such as 57-67%.

8. The greenhouse screen according to any one of the preceding claims, characterized in that said film has a spreading factor (SF) is between 1.5 and 7, such as between 1.8 and 6, such as between 1.9 and 5, such as between 2 and 4.

9. The greenhouse screen according to any one of the preceding claims, characterized in that said film has a total thickness of 15-50 micrometers, such as 20-40 micrometers, such as 20-35 micrometers.

10. The greenhouse screen according to any one of the preceding claims, characterized in that said film is selected from the group consisting of UV absorbers, excited-state quenchers, or Hindered-Amine Light Stabilizers (HALS).

11. The greenhouse screen according to claim 10, characterized in that the UV-stabilizer is selected from the group consisting of Hindered-Amine Light Stabilizers.

12. The greenhouse screen according to claim 11, characterized in that the UV-stabilizer is Flamestab™ NOR 116 available from BASF Schweiz AG, or Tinuvin™ NOR 371 available from BASF Schweiz AG.

13. The greenhouse screen according to claim 12, characterized in that Flamestab™ NOR 116 is complemented by a phosphorus based Flame Retardant (FR).

14. The greenhouse screen according to claim 13, characterized in that the Flame Retardant (FR) is Aflammit® PCO 700, Aflammit® PCO 800 or Aflammit® PCO 900 available from Thor Group Limited, England.

15. The greenhouse screen according to any one of claims 10-14, characterized in that UV-stabilizers are added to the film in quantities of between 0.2 and 4 wt.-%, based on the weight of the film.

16. The greenhouse screen according to any one of the preceding claims, characterized in that the film is stretched before it is cut into strips.

17. The greenhouse screen according to any one of the preceding claims, characterized in that the film is stretched after it has been cut into strips.

18. The greenhouse screen according to any one of the preceding claims, characterized in that the film is uniaxially stretched to a ratio of 1:3 to 1:10, such as from 1:4 to 1:8, such as from 1:5 to 1:7.

19. The greenhouse screen according to any one of claims 1-18, characterized in that one or more of said strips of film material (11) has a width that is smaller than the distance between the longitudinal threads (13a, 13b; 15; 19).

20. The greenhouse screen according to claim 19, characterized in that a gap is formed between said one or more strips of film material (11) and the adjacent strip(s) of film 11, said gap permitting ventilation through said screen.

21. The greenhouse screen according to any one of claims 1-20, characterized in that at least 10%, preferably at least 20%, more preferably at least 30%, more preferably at least 40%, more preferably at least 50%, more preferably at least 60%, more preferably at least 70%, more preferably at least 80%, more preferably at least 90% of the strips of film material (11) in the greenhouse screen comprise said single- or multilayer polyethylene film.

22. The greenhouse screen according to any one of claims 1-21, characterized in that all strips of film material (11) in the greenhouse screen are of said polyethylene film.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0067] The invention will below be described with reference to some embodiments shown in the drawings.

[0068] FIG. 1 shows on an enlarged scale part of warp-knitted screen according to a first embodiment.

[0069] FIG. 2 shows a part of a warp-knitted screen according to a second embodiment.

[0070] FIG. 3 shows on an enlarged scale a part of a woven screen.

[0071] FIG. 4 shows a part of a woven screen according to a further embodiment.

DETAILED DESCRIPTION

[0072] Films of the invention are outstandingly suitable as light scattering film, in particular for the production of screens in greenhouses.

[0073] The greenhouse screen 10 according to the invention comprises a plurality of narrow strips of film material 11 held together by a yarn framework 12, 13a, 13b; 14, 15; 18, 19.

[0074] The strips of film material 11 are preferably arranged closely edge to edge, so that they form a substantially continuous surface. In all embodiments the distance between the strips 11 has been exaggerated for the sake of clarity to make the yarn framework visible. The screen has a longitudinal direction, y, and a transverse direction, x, wherein the strips of film material 11 extend in the longitudinal direction. In some embodiments strips of film material 11′ may extend also in the transverse direction. A typical width of the strips is between 2 mm and 10 mm.

[0075] In FIG. 1 strips of film material 11 are interconnected by a warp knitting procedure as described in EP 0 109 951. The yarn framework comprises warp threads 12 forming loops or stitches and primarily extending in the longitudinal direction, y. The warp threads 12 are connected to one another by weft threads 13a and 13b extending across the film strips.

[0076] FIG. 1 shows an example of a mesh pattern for a fabric manufactured through a warp knitting process in which four guide bars are used, one for the strips of film material 11, two for the connecting weft threads 13a and 13b extending transversely to the film strips and one for the longitudinal warp threads 12.

[0077] The spaces between the strips of film material 11 have been strongly exaggerated in order to make the mesh pattern clear. Usually the strips of film material 11 are located closely edge to edge. The longitudinal warp threads 12 are arranged on one side of the screen, the underside, while the transverse connecting weft threads 13a and 13b are located on both sides of the fabric, the upper and the underside. The term “transverse” in this respect is not restricted to a direction perpendicular to the longitudinal direction, but means that the connecting weft threads 13a and 13b extends across the strips of film material 11 as illustrated in the drawings. The connection between the longitudinal warp threads 12 and the transverse weft threads 13a and 13b are preferably made on the underside of the fabric. The strips of film material 11 can in this way be arranged closely edge to edge without being restricted by the longitudinal warp threads 12.

[0078] The longitudinal warp threads 12 in FIG. 1 extend continuously in unbroken fashion along opposite edges of adjacent strips of film material 11, in a series of knitted stitches, in a so called open pillar stitch formation.

[0079] The transverse weft threads 13a and 13b pass above and below the strips of film material 11 at the same location, i.e. opposed to each other, to fixedly trap the strips of film material. Each knitted stitch in the longitudinal warp threads 12 has two such transverse weft threads 13a and 13b engaging with it. FIG. 2 shows another example of a mesh pattern for a fabric similar to the one shown in FIG. 1. The difference is that the transverse weft threads 13a and 13b pass over one and two strips of film material 11 in an alternating way.

[0080] FIG. 3 shows a woven screen in which the strips of film material 11 are interconnected by warp threads 14 extending in longitudinal direction, y, and interwoven with weft threads 15 extending across the strips of film material 11 primarily in the transverse direction, x.

[0081] FIG. 4 shows another embodiment of a woven screen as described in U.S. Pat. No. 5,288,545 comprising strips of film material 11 (warp strips) extending in longitudinal direction, y, and strips of film material 11′ (weft strips) extending in transverse direction, x. The weft strips 11′ in the transverse direction may as shown in FIG. 4 always be on the same side of the warp strips 11 in longitudinal direction or may alternate on the upper and underside of the warp longitudinal strips 11. The warp and weft strips 11 and 11′ are held together by a yarn framework comprising longitudinal and transverse threads 18 and 19. The screen may comprise open areas that are free from strips to reduce heat build-up under the screen.

[0082] The strips (11) of inventive film can be combined with strips of other films. Such strips may be of materials providing desired heat transporting and shading properties and be of plastic, metal foil or laminates of plastic and metal. It is also possible to make a screen having “open” areas free from strips permitting ventilation through said screen.

[0083] In order to provide the desired light scattering properties at least 10%, preferably at least 20%, more preferably at least 30%, more preferably at least 40%, more preferably at least 50%, more preferably at least 60%, more preferably at least 70%, more preferably at least 80%, more preferably at least 90% of the surface area of the screen should be strips (11) of the single or multilayer film according to the invention. According to one embodiment all strips (11) in the screen are of the single or multilayer polyethylene film described herein and the strips (11) are arranged closely edge to edge, so that they form a substantially continuous surface. Alternatively, the film itself is installed in the greenhouse.

[0084] In some embodiments the film strips may be interconnected by a yarn framework having liquid-transporting capacity by capillary action. Advantageously the yarn framework is thermally bonded to at least one side of the strips of film material, and wherein also those parts of the yarn framework that are thermally bonded to the strips have liquid-transporting capacity by capillary action. The installations described above result in a reduction of the amount of light hitting the plants and thus lead to cooling during the day. At the same time they distribute the remaining amount of light homogenously due to the high light scattering in space, thereby ensuring a good illumination of all plants and plant parts. During the night these installations lead to a lower heat loss from the greenhouse to the outside.

Polyethylenes

[0085] Suitable high density polyethylenes (HDPE) to be used in the film material as described herein include ethylene homopolymer and copolymers of ethylene and alpha-olefins (about 0.1 to about 10 wt. %). Suitable alpha-olefins include 1-butene, 1-hexene, and 1-octene, the like, and mixtures thereof. Advantageously the HDPE has a melt flow rate of 1.6-2.0 g/10 min at 190° C./5.0 kg, and 19-26 g/10 min at 190° C./21.6 kg, preferably the melt flow rate is 1.7-1.9 g/10 min at 190° C./5.0 kg, and 20-25 g/10 min at 190° C./21.6 kg, more preferably the melt flow rate is 1.8 g/10 min at 190° C./5.0 kg, and 21-23 g/10 min at 190° C./21.6 kg (as determined according to ISO 1133-1).

[0086] Advantageously the HDPE has a density of 0.940-0.955 g/cm.sup.3, preferably a density of 0.943-0.950 g/cm.sup.3, and more preferably a density of 0.946-0.948 g/cm.sup.3 (ISO 1183-1). Examples of HDPEs to be used are Hostalen ACP 7740 F2 from Lyondellbasell Industries Holdings, B.V. (Melt flow rate of 1.8 g/10 min at 190° C./5.0 kg, and 23 g/10 min at 190° C./21.6 kg (as determined according to ISO 1133-1); Density of 0.948 g/cm.sup.3 (ISO 1183-1)) or HDPE Hostalen GF 9045 F from LyondellBasell Industries Holdings, B.V. (Melt flow rate of 1.8 g/10 min at 190° C./5.0 kg, and 21 g/10 min at 190° C./21.6 kg (as determined according to ISO 1133-1); Density of 0.946 g/cm.sup.3 (ISO 1183-1)).

[0087] The film also includes a certain amount of low-density polyethylene (LDPE) or linear low density polyethylene (LLDPE). LDPE is defined by a density range of 0.910-0.940 g/cm.sup.3. LDPE has a high degree of short- and long-chain branching, which means that the chains do not pack into the crystal structure as well. It has, therefore, weaker intermolecular forces than HDPE as the instantaneous-dipole induced-dipole attraction is less. By including a certain amount of LDPE in the single- or multilayer film the ductility of the film is improved.

[0088] LLDPE is a substantially linear polymer (polyethylene), with significant numbers of short branches, commonly made by copolymerization of ethylene with longer-chain olefins. Linear low-density polyethylene differs structurally from conventional low-density polyethylene (LDPE) because of the absence of long chain branching. LLDPE is produced at lower temperatures and pressures by copolymerization of ethylene and such higher alpha-olefins as butene, hexene, or octene (about 5 to about 15 wt. %). The copolymerization process produces an LLDPE polymer that has a narrower molecular weight distribution than conventional LDPE and in combination with the linear structure, significantly different rheological properties. The density of LLDPE is within the range of about 0.865 to about 0.925 g/cm.sup.3

EXAMPLES 1-6

[0089] In the examples 1-6 the following polymers and masterbatches are used:

[0090] HDPE1:

[0091] HDPE Hostalen ACP 7740 F2 (LyondellBasell Industries Holdings, B.V.) having a Melt flow rate of 1.8 g/10 min at 190° C./5.0 kg, and 23 g/10 min at 190° C./21.6 kg (as determined according to ISO 1133-1); Density of 0.948 g/cm3 (ISO 1183-1).

[0092] MB3.2:

[0093] 90 wt.-% LDPE+10 wt.-% SiO.sub.2 (D.sub.50=3.2 μm). Plastron ANT PO 10B, Plastron SAS, 15 Rue des Marguerites, 68920 Wintzenheim, France. The SiO.sub.2 was incorporated in the LDPE in a twin-screw extruder.

[0094] MB5.8:

[0095] 80 wt.-% LDPE+20 wt.-% SiO.sub.2 (D.sub.50=5.8 μm). ARGUBLOCK AB 212 LD, ARGUS Additive Plastics GmbH, Oberer Westring 3-7, 33142 Büren, Germany. The SiO.sub.2 was incorporated in the LDPE in a twin-screw extruder.

[0096] MB16:

[0097] 90 wt.-% LDPE+10 wt.-% SiO.sub.2 (D.sub.50=16 μm). Plastron ANT PO 10E, Plastron SAS, 15 Rue des Marguerites, 68920 Wintzenheim, France. The SiO.sub.2 was incorporated in the LDPE in a twin-screw extruder.

TABLE-US-00001 TABLE 1 summarizes the formulations, manufacturing conditions and the resultant film properties. Ex. Ex. Ex. Ex. Ex. Ex. Film Film Film Film Film Film 1 2 3 4 5 6 Thickness of 30 30 30 30 30 30 film (μm) HDPE1 (%) 90 86 90 86 90 86 MB3.2 (%) 10 14 MB5.8 (%) 10 14 MB16 (%) 10 14 Stretch ratio 1:5.8 1:5.8 1:5.8 1:5.8 1:5.8 1:5.8 Concentration 2.0 2.8 1.0 1.4 1.0 1.4 of SiO.sub.2 (wt.-%) SiO.sub.2 particle 5.8 5.8 16 16 3.2 3.2 size D.sub.50 (μm) Transparency 93.4 86.0 93.6 93.6 95.0 95.0 (%) Haze (%) 52 58 29 33 22 25 Spreading factor 1.8 3.0 1.4 1.4 1.2 1.3

[0098] Results:

[0099] The films containing SiO.sub.2 particles with a D.sub.50 of 5.8 μm gave even and smooth films with good light diffusion properties. No particles were visible in the film and the polymer mix was easily extruded.

[0100] The films containing SiO.sub.2 particles with a D.sub.50 of 16 μm produced films with high transparency but particles were visible in the film and the films were not able to diffuse the light well.

[0101] The films containing SiO.sub.2 particles with a D.sub.50 of 3.2 μm gave films with high transparency but with bad light diffusion properties. No particles were visible.

EXAMPLES 7-18

[0102] In the Tests presented below the following polymers and master batches are used:

[0103] HDPE2:

[0104] HDPE Hostalen GF 9045 F (LyondellBasell Industries Holdings, B.V.) having a Melt flow rate of 1.8 g/10 min at 190° C./5.0 kg, and 21 g/10 min at 190° C./21.6 kg (as determined according to ISO 1133-1); Density of 0.946 g/cm.sup.3 (ISO 1183-1).

[0105] MB ARX F85 LD:

[0106] Master Batch comprising 70 wt.-%+LLDPE+30 wt.-% Flamestab™ NOR 116 (Argus Additive Plastics GmbH)

[0107] MB WPT 1181 N:

[0108] Master Batch comprising 80 wt.-% LLDPE+20 wt.-% CaCO.sub.3 (Walter Kunstoffe GmbH)

[0109] ARX 601 AB02LD:

[0110] 80 wt.-% carrier polymer+20 wt.-% SiO.sub.2, the silica particles have a D.sub.50 of approximately 10 μm (Argus Additive Plastics GmbH). The SiO.sub.2 was incorporated in the LDPE in a twin-screw extruder.

[0111] ARX V17/848:

[0112] 80 wt.-% carrier polymer+20 wt.-% SiO.sub.2, the silica particles have a D.sub.50 of approximately 5.8 μm (Argus Additive Plastics GmbH). The SiO.sub.2 was incorporated in the LDPE in a twin-screw extruder.

[0113] ARX V17/885:

[0114] 90 wt.-% carrier polymer+10 wt.-% SiO.sub.2, the silica particles have a D.sub.50 of approximately 2.8 μm (Argus Additive Plastics GmbH). The SiO.sub.2 was incorporated in the LDPE in a twin-screw extruder.

[0115] For Test films 7-18 the extruded melts were formed into flat films the by blown film processes and the stretched in the machine direction using the settings in Table 2.

TABLE-US-00002 TABLE 2 Machine settings during the manufacture of films Temperature of extruder: T1 190 ° C. T2 200 ° C. T3 210 ° C. T4 210 ° C. T5 210 ° C. Line speed 3.3 m/min MDO Temperature during stretching in the Machine Direction: T1 120 ° C. T2 122 ° C. T3 124 ° C. T4 124 ° C. T5 123 ° C. T6 70 ° C. Line speed of winder 17.9 m/min

[0116] Table 3 summarizes the formulations and resultant film properties.

TABLE-US-00003 TABLE 3 Film formulations and resulting film properties Test Test Test Test Test Test Test Test Test Test Test Test film 7 film 8 film 9 film 10 film 11 film 12 film 13 film 14 film 15 film 16 film 17 film 18 Thickness of film (μm) 30 30 30 30 30 30 30 30 30 30 30 30 HDPE2 (wt.-%) 92 87 82 77 92 87 82 77 92 87 82 77 ARX 601 AB02LD (wt.-%) 5 10 15 20 ARX V17/848 (wt.-%) 5 10 15 20 ARX V17/885 (wt.-%) 10 20 30 40 MB ARX FR85 LD (wt.-%) 2 2 2 2 2 2 2 2 2 2 2 2 MB WPT 1181 N (wt.-%) 1 1 1 1 1 1 1 1 1 1 1 1 Concentration of SiO.sub.2 (wt.-%) 1 2 3 4 1 2 3 4 1 2 3 4 SiO.sub.2 particle size D.sub.50 (μm) 10 10 10 10 5.8 5.8 5.8 5.8 2.8 2.8 2.8 2.8 Stretching ratio 1:5.8 1:5.8 1:5.8 1:5.8 1:5.8 1:5.8 1:5.8 1:5.8 1:5.8 1:5.8 1:5.8 1:5.8 Transparency (%) 94.3 93.2 92.7 93.0 94.5 94.2 93.8 94.0 94.8 94.7 94.7 94.7 Transparency at light 61.5 51.2 43.6 21.7 65.4 55.0 46.3 40.9 72.0 65.0 60.4 56.3 exit opening (%) Spreading Factor 1.5 1.8 2.1 4.3 1.4 1.7 2.0 2.3 1.3 1.5 1.6 1.7 Clarity (%) 66.5 49.4 39.5 18.7 69.5 49.4 35.2 26.6 89.3 83.2 76.0 68.7 Haze (%) 39.2 50.6 60.5 84.3 36.0 48.6 60.8 65.2 26.9 35.4 42.2 47.7 The results presented in Table 3 represent an average of five measurements.

[0117] Results

TABLE-US-00004 Test film Results Test film 7 Film is diffuse and grainy with visible particles. Film has a “paper-like” feel. Test film 8 Film is slightly diffuse and grainy with visible particles. Film has a “paper-like” feel. Test film 9 Film is not homogenous with short visible streaks due to improper mixing and stretching of the film. Film has a “paper-like” feel. Test film 10 Lumps are formed and the film cannot be stretched properly. Test film 11 Film is diffuse but homogenous. Test film 12 Film is slightly diffuse but homogenous. Test film 13 Film is transparent and homogenous. Test film 14 Film is transparent with a few visible streaks. Test film 15 Film is transparent, but diffuse. Film is not completely homogenous. Test film 16 Film is transparent, but diffuse. Film is not completely homogenous. Test film 17 Film is transparent, milky but diffuse. Film is not completely homogenous. Test film 18 Film is transparent, milky but diffuse. Film is not completely homogenous.

[0118] Analytical Methods

[0119] The following analytical methods were used to determine parameters used:

[0120] Measurement of the Mean Particle Diameter D.sub.50

[0121] Determination of the average particle size D.sub.50 was carried out using a Malvern Mastersizer 2000. For this, the particles were dispersed in water and transferred to a cuvette that was analyzed in the meter wherein the size of the particles was determined by laser diffraction. In general, while the detector captures an image intensity of the diffracted laser light from the angle-dependent light intensity using a mathematical correlation function, the particle size distribution is calculated. The particle size distribution is characterized by two parameters, the median value D.sub.50 (=measure of location for the average value) and the degree of scatter SPAN98(=measure of the scatter of the particle diameter). The test procedure was carried out automatically and included the mathematical determination of the D.sub.50 value.

[0122] Transparency

[0123] Transparency was measured in accordance with ASTM-D 1003-61 (Method A) by haze-Gard plus from BYK-Gardner GmbH Germany.

[0124] Clarity

[0125] 5 Determination of the clarity is carried out according to ASTM-D-1003 and by haze-gard plus from BYK-Gardner GmbH (BYK-Gardner GmbH, Lausitzer Strasse 8, 82538 Geretsried, Germany). The light is deflected within a small solid angle, so that the amount of scattered light is concentrated in a narrow lobe. Clarity is measured in an angular range of less than 2.5°. To measure the clarity, the film is applied close to the light exit-opening when taking the measurement. (Image sharpness)

[0126] Assessment of Light Scattering Property (Measurement of Scattering Factor SF)

[0127] The light scattering properties are of particular importance for the inventive film. The measurement was carried out by means of a “haze-gard plus” transparency/opacity meter from BYK Gardner (BYK-Gardner GmbH, Lausitzer Strasse 8, 82538 Geretsried, Germany). To measure SF, the transparency of the film is measured when it is held by tension in a clamping ring by holding the film flush to the measurement opening used for haze and transparency measurements as described in ASTM D-1003-61 (Method A). Then, the clamped film is held flush against the light exit-opening (as in the Clarity measurement) while transparency is measured again. The light scattering factor SF corresponds to the ratio of these two readings:

[0128] The Scattering Factor (SF)=transparency (measured according to ASTM D-1003-61 Method A)/transparency measured in front of the light exit opening (i.e. the clarity measurement).