Polyolefin-based wrapping film

Abstract: A polyolefin-based wrapping film having one or more layers which satisfies the following conditions (1) to (4): (1) the film has a thickness ranging from 6 to 100 µm; (2) the film has a birefringence Δn A of from 0.6×10⁻³ to 8×10⁻³; (3) the film has a ratio of percent heat shrinkage in MD to percent heat shrinkage in TD of from 0.65 to 1.50 in a silicon oil bath at 140°C for 3 minutes; and (4) the film has a strain recovery of at least 18 mm, which provides excellent performance properties in stretch wrapping, especially in the area of high deformation recovery.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a resin film for use in wrapping, such as stretch wrapping of foods or pallet stretch wrapping, or in bag formation and a process for producing the same. Discussion of the Background In the field of stretch wrapping or prepackaging of fresh foods, such as vegetables, fruits, fishes, and meats, and cooked foods, either with or without a plastic tray, films made of polyvinyl chloride-based resins are conventionally used. Recently, however, the use of ethylene-based resins, such as low-density polyethylene and ethylene-vinyl acetate copolymer, as a substitute for the polyvinyl chloride films has drawn much interest for safety and hygiene reasons. However, films of known low-density polyethylene-based resins, such as low-density polyethylene or ethylene-1-butene copolymer, tend to be difficult to use as stretch wrapping because they are hard and less stretchable. Forced stretching of these films results in non-uniform stretching or breaking of the film, or causes deformation or breakage of the food tray to which the film is applied causing the film to wrinkle. This fails to provide sufficient tension for achieving commercially valuable wrapping which must protect the food from loss of freshness and avoid contamination. While a film of an ethylene-vinyl acetate copolymer, having appropriate properties, such as vinyl acetate content and melt flow rate, would solve above-described problems associated with low-density polyethylene films, these ethylene-vinyl acetate copolymer films, when used in stretch wrapping of sharp-edged foods or trays, tear easily at the sharp edge. Additionally, any small initial tear of the film which occurs during delivery of the wrapped articles tends to run or spread into a big tear, eventually leading to unwrapping. Use of a laminate film composed of the above-mentioned low-density polyethylene film or a linear low-density polyethylene film and an ethylene-vinyl acetate copolymer film as disclosed in JP-B-2-12187 (the term "JP-B" as used herein means an "examined Japanese patent publication") would also solve the problems of hardness and non-stretchability, but still leads to a film which tears easily during wrapping or delivery. This is especially true when the film is of a reduced thickness, as is now preferred for reasons of conservation of raw materials and reduction of packaging costs. In order to overcome the disadvantages associated with conventional stretch-wrapping films, a laminate film has been proposed. The laminate film, composed of an intermediate layer comprising a specific 1-butene polymer having laminated on both sides thereof a layer mainly comprising an ethylene-vinyl acetate copolymer having a vinyl acetate content of from 5 to 25% by weight, is obtained by blown-film extrusion at a blow-up ratio of from 3 to 7. This film seeks to provide a thin stretch-wrapping film which is excellent in stretchability, softness and particularly tear strength as well as moderate slip, self-adhesion, sufficient heat-sealability, and transparency, as disclosed in JP-A-61-89040 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), U.S. Patent 4,713,282, and EP-B-198091. Nevertheless, even the above-described stretch-wrapping film does not always satisfy market demands for wrapping, such as maintaining a tight fit to the shape of an article without wrinkling and recovering from deformation caused during delivery or display without wrinkling. A wrapping film which exhibits excellent deformation recovery has been proposed as disclosed in JP-B-2-14898, U.S. Patent 4,619,859 or EP-B-137454 and put to practical use. That film is composed of at least four layers: 1) at least one base layer comprising a mixed composition selected from (A)+(B)+(C) , (A)+(B), and (B)+(C); where (A) is at least one ethylene polymer selected from low-density polyethylene, a copolymer of ethylene with a monomer selected from a vinyl ester monomer, an aliphatic unsaturated monocarboxylic acid, and an alkyl ester of an aliphatic unsaturated monocarboxylic acid, and derivatives of the copolymer; (B) is a soft elastomer having a Vicat softening point of 60°C or lower; (C) is at least one polymer selected from crystalline polypropylene and crystalline poly-1-butene; 2) an H layer comprising a polymer selected from (C); and 3) a surface layer (S layer) comprising at least one polymer selected from (A), (B), and crystalline 1,2-polybutadiene, such that at least one base layer is adjacent to the H layer. The above-described multi-layer film is produced by co-extrusion of the layers through a layered annular die, followed by quenching of the extruded tubular film by a water cooling ring. The tubular film is then passed through two pairs of delivery nip rolls and draw nip rolls while heating to 37°C with hot air. During this heating process, air is blown into the tube to inflate the tube continuously by means of a rectifying contact guide, thereby stretching the film about 3.3 to 3.6 times in the longitudinal direction and about 3.2 to 3.4 times in the transverse direction. The end region of stretching is cooled by directing cold air of 18°C from an air ring onto the stretched tube, and the stretched film is folded flat with a deflator and drawn up with nip rolls. The stretched film is then subjected to heat setting with hot air of 40°C for a few seconds. The heat-set film is cut into two sheets of film by slitting its edges in the longitudinal direction, and each film sheet is respectively wound up under constant tension to obtain the desired film having excellent shrinkability and strength. Because the above-mentioned wrapping film is an oriented film obtained by cold stretching, when it is applied to a tray, such as a food tray, the tray tends to deform when the wrapping film is heat sealed. Additionally, JP-B-2-12187 discloses a three-layered stretch-wrapping film composed of a linear low-density polyethylene layer having laminated on both sides thereof an ethylene-vinyl acetate copolymer film which is obtained by blown-film extrusion at a blow-up ratio of 3 to 6. However, the film still has deformation (strain) recovery which is inferior to polyvinyl chloride films. EP-A-348 794, DE-A-35 02 136, EP-A-351 744, EP-A-92 897 and EP-A-436 196 refer to shrinkable films made of a polyolefin such as polypropylene or polyethylene and optionally also other polymers such as EVA. However, these references do not pertain to stretch wrapping films and are not based on the concept of attaining an appropriate balance between the heat shrinkage in the machine direction (MD) and the shrinkage in the transverse direction (TD). EP-A-198091 relates to a film for use in stretch-packing. The examples given display a tensile elastic modulus in machine direction between 850 and 1850 kg/cm 2 and in transversal direction of 930 to 1700 kg/cm 2 . The concept of adapting certain parameters to the use in automatic wrapping machines to ensure that the film does not tear, is not deformed or wrinkled is not anticipated by this document. SUMMARY OF THE INVENTION Accordingly, one object of the present invention is to provide a stretch wrapping film which has excellent stretch-ability, softness, tear strength and, in particular, deformation recovery. A further object of the present invention is to provide a wrapping resin which has, in addition to the above mentioned properties, sufficient slip properties for use in automatic stretch wrapping machines, self-tack, and transparency. A further object of the present invention is to provide a method for preparing the stretch wrapping films discussed above. Another object of the present invention is to provide an apparatus for performing the above method. The above object of the present invention is accomplished by a polyolefin-based stretch wrapping film having properties which satisfy the following conditions (1) to (6): (1) the film has a thickness ranging from 6 to 100 µm; (2) the film has a birefringence Δn A ranging from 0.6×10 -3 to 8×10 -3 , the birefringence being defined by equation: Δn A = |n MD - n TD | wherein n MD is a refractive index in the machine direction; and n TD is a refractive index in the transverse direction; (3) when the film is allowed to stand in a silicone oil bath at 140°C for 3 minutes, the ratio of percent heat shrinkage in the machine direction S MD to percent heat shrinkage in the transverse direction S TD (S MD /S TD ; hereinafter referred to as heat shrinkage ratio) ranges from 0.65 to 1.50; (4) the film has a strain recovery of at least 18 mm as measured by a test method comprising clamping a circular film specimen having a diameter of 100 mm, indenting the center of the specimen with a hemispherical indenter of 20 mm in diameter to cause deformation, and measuring the maximum strain (indentation) which disappears completely upon removal of the indenter; (5) the film has a tensile modulus of from 58.8 to 137.2 MPa (600 to 1400 kg/cm 2 ) in both machine and transverse directions. (6) the film has a ratio of stress at 50% elongation in the machine direction (T MD ) to stress at 50% elongation in the transverse direction (T TD ) of from 0.50 to 2.50. Since the stretch wrapping film of the present invention has a good stress balance between the machine direction (MD) and the transverse direction (TD), one can wrap an article therein without causing tearing of the film, deformation of a tray, or wrinkling of the film. The wrapping film of the present invention additionally has excellent deformation recovery. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross-sectional view of a cooling apparatus used in blown-film extrusion for the production of the stretch wrapping film according to the present invention. Figure 2 is a side view of the cooling apparatus of Fig. 1. Figure 3 is a cross-sectional view of another cooling apparatus used in blown-film extrusion for the production of the stretch wrapping film according to the present invention. Figure 4 is a plan view of a blown-film extrusion machine used in the production of the stretch wrapping film according to the present invention. Figure 5 is a plan view of a tester used for measurement of deformation recovery of a stretch wrapping film. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention relates to a stretch wrapping film comprising one or more layers of a polyolefin-based resin, wherein the layers may be the same or different, wherein the film has properties which satisfy the following conditions (1) to (6): (1) the film has a thickness ranging from 6 to 100 µm; (2) the film has a birefringence Δn A ranging from 0.6×10 -3 to 8×10 -3 , the birefringence being defined by equation: Δn A = |n MD - n TD | wherein n MD is a refractive index in the machine direction; and n TD is a refractive index in the transverse direction; (3) when the film is allowed to stand in a silicone oil bath at 140°C for 3 minutes, a ratio of heat shrinkage in the machine direction, S MD , to heat shrinkage in the transverse direction, S TD , (S MD /S TD ) ranges from 0.65 to 1.50; (4) the film has a strain recovery of at least 18 mm as measured by a test method comprising clamping a circular film specimen having a diameter of 100 mm, indenting the center of the specimen with a hemispherical indenter of 20 mm in diameter to cause deformation, and measuring - the maximum strain (indentation) which disappears completely upon removal of the indenter; (5) the film has a tensile modulus of from 58.8 to 137.2 MPa (600 to 1400 kg/cm 2 ) in both machine and transverse directions; (6) the film has a ratio of stress at 50% elongation in the machine direction (T MD ) to stress at 50% elongation in the transverse direction (T TD ) of from 0.50 to 2.50. The stretch wrapping film of the present invention is preferably prepared from polyolefin-based resins. Suitable resins include ethylene-vinyl acetate copolymer having a content of ethylene-based structural units from 5 to 25% by weight and a melt flow rate (ASTM D-1238-86; 190°C, 2.16 kg; hereinafter abbreviated as MFR) of from 0.3 to 5 g/10 min, linear low-density polyethylene comprising from 80 to 99% by weight of ethylene-based structural units and from 1 to 20% by weight of structural units based on an α-olefin having from 3 to 8 carbon atoms, high-density polyethylene, low-density polyethylene, ethylene-acrylic acid copolymer, ethylene-methyl acrylate copolymer, ethylene-methyl methacrylate copolymer, polypropylene, a copolymer comprising 88 to 99.5% by weight of propylene-based structural units and from 0.5 to 12% by weight of ethylene-based structural units or structural units based on an α-olefin having from 4 to 8 carbon atoms, poly(4-methyl-1-pentene), and polybutene. Examples of the above-mentioned α-olefin include 1-butene, 1-heptene, 1-hexene, 4-methyl-1-pentene, and 1-octene. These polyolefin-based resins may be used either individually or in combinations of two or more. If desired, the polyolefin-based resin can contain an impact modifier in an amount which does not impair the transparency of the film, preferably 0.5 to 20% by weight, more preferably 1 to 10% by weight. Suitable impact modifiers include hydrogenated petroleum resin, hydrogenated styrene-butadiene-styrene copolymer, ethylene-propylene copolymer rubber, 1,2-polybutadiene and ethylene-propylene-ethylidenenorbornene copolymer rubber. The polyolefin-based resin may further contain a lubricant for improving slip properties, a tackifier, a nucleating agent for improving transparency or any combination of these agents in a total amount of (lubricant + tackifier + nucleating agent) of from 0.05 to 5% by weight, preferably from 0.1 to 3% by weight. Suitable lubricants include fatty acid amides, such as oleamide, stearamide, and erucamide; fatty acid glycerin esters, such as glycerol monostearate, glycerol distearate, glycerol mono-oleate, and glycerol dioleate; and polyethylene glycol adducts of these glycerides. Suitable nucleating agents include inorganic substances, such as talc and silica. Suitable tackifiers include castor oil derivatives, low-molecular weight viscous polybutylene, and sorbitan higher fatty acid esters having from 12 to 22 carbon atoms in the fatty acid group, preferably from 15 to 22 carbon atoms. The stretch wrapping film of the present invention has a single layer structure or a laminate structure. A multi-layer laminate structure is preferred from the standpoint of moldability and balance of physical properties of the film. The layer structure of the multi-layer film includes the following embodiments (1) to (8): (1) A laminate film composed of an intermediate layer comprising a linear low-density polyethylene-based resin having an MFR (ASTM D1238-86; 190°C, 2.16 kg) of from 0.1 to 5 g/10 min and a Q-value (ratio of weight average molecular weight to number average molecular weight) of from 1 to 6 having laminated on both sides thereof a surface layer comprising a copolymer resin comprising from 60 to 95% by weight of ethylene-based structural units and from 5 to 40% by weight of structural units derived from a monomer selected from vinyl acetate, an aliphatic unsaturated carboxylic acid, and an aliphatic unsaturated carboxylic acid alkyl ester. (2) A laminate film composed of an intermediate layer comprising from 10 to 90% by weight of a crystalline polyolefin-based resin selected from an ethylene-based resin and a propylene-based resin and from 10 to 90% by weight of an olefinic thermoplastic elastomer having laminated on both sides thereof a surface layer comprising a copolymer resin comprising from 60 to 95% by weight of ethylene-based structural units and from 5 to 40% by weight of structural units derived from a monomer selected from vinyl acetate, an aliphatic unsaturated carboxylic acid, and an aliphatic unsaturated carboxylic acid alkyl ester. (3) A laminate film composed of an intermediate layer comprising from 80 to 95% by weight of linear low-density polyethylene-based resin and from 5 to 20% by weight of a copolymer resin comprising from 60 to 95% by weight of ethylene-based structural units and from 5 to 40% by weight of structural units derived from a monomer selected from vinyl acetate, an aliphatic unsaturated carboxylic acid, and an aliphatic unsaturated carboxylic acid alkyl ester, having laminated on both sides thereof a surface layer comprising a copolymer resin comprising from 60 to 95% by weight of ethylene-based structural units and from 5 to 40% by weight of structural units derived from a monomer selected from vinyl acetate, an aliphatic unsaturated carboxylic acid, and an aliphatic unsaturated carboxylic acid alkyl ester. (4) A laminate film composed of an intermediate layer comprising from 10 to 90% by weight of a 1-butene-based resin and from 10 to 90% by weight of (a) a polyolefin-based resin (exclusive of a 1-butene-based resin), (b) an olefinic thermoplastic elastomer or both (a) and (b), having laminated on both sides thereof a surface layer comprising a copolymer resin comprising from 60 to 95% by weight of ethylene-based structural units and from 5 to 40% by weight of structural units derived from a monomer selected from vinyl acetate, an aliphatic unsaturated carboxylic acid, and an aliphatic unsaturated carboxylic acid alkyl ester. (5) A laminate film composed of a low-density polyethylene resin layer having laminated on one or both sides thereof a linear low-density polyethylene resin layer. (6) A laminate film composed of a high-density polyethylene resin layer having laminated on one or both sides thereof a low-density polyethylene or linear low-density polyethylene resin layer. (7) A laminate film composed of a linear low-density polyethylene-based resin layer having laminated on one or both sides thereof a low-density polyethylene or propylene-based resin layer. (8) A laminate film composed of a propylene-based resin layer having laminated on one or both sides thereof a linear low-density polyethylene-based resin layer. Of the above-mentioned layer structures, (1) and (3) are particularly preferred from the viewpoint of strain recovery. Suitable linear low-density polyethylene-based resins used in (1), (3), and (5) to (8) include ethylene-based copolymers having an MFR of from 0.1 to 5 g/10 min, a Q-value of from 1 to 6, and a density of from 0.89 to 0.92 g/cm 3 e.g., ethylene-1-butene copolymers, ethylene-1-hexene copolymers, ethylene-1-butene-1-hexene copolymers, ethylene-4-methyl-1-pentene copolymers, ethylene-1-hexene copolymers, ethylene-1-octene copolymers, and mixtures thereof. The crystalline polyolefin-based resin to be used in the intermediate layer of (2) is a stereoregular polyolefin-based resin or mixture of stereoregular polyolefin-based resins. Examples of these include crystalline ethylene-based resins and crystalline propylene-based resins. The crystalline ethylene-based resins, noted above, include ethylene homopolymer and copolymers comprising at least 65% by weight, preferably 75 to 95% by weight, of ethylene-based structural units and structural units derived from at least one C 3 -C 10 α-olefin, such as propylene, 1-butene, 1-hexene, 4-methyl-1-pentene, and 1-octene. The crystalline ethylene-based resins have a crystallinity of from 20 to 80%, preferably 40 to 80% and an MFR (ASTM D1238-86) of from 0.1 to 10 g/10 min, preferably 0.1 to 5 g/10 min. Preferred crystalline ethylene-based resins include low-density polyethylene and linear low-density polyethylene, each having a density of from 0.89 to 0.93 g/cm 3 . The crystalline propylene-based resins noted above, include propylene homopolymer and random or block copolymers comprising at least 60% by weight, preferably 70 to 95% by weight, of propylene-based structural units and structural units derived from a C 2 -C 10 α-olefin (except C 3 ), such as ethylene, 1-butene, 1-octene, 1-hexene and 4-methyl-1-pentene. The crystalline propylene-based resins have a crystallinity of from 20 to 80%, preferably from 40 to 80%, an isotactic index of from 40 to 99%, preferably from 55 to 90%, and an MFR (230°C, 2.16 kg) of from 0.1 to 10 g/10 min, preferably from 0.3 to 10 g/10 min. Suitable 1-butene-based resins for use in (4) include 1-butene homopolymer and random or block copolymers comprising at least 60% by weight, preferably 70 to 95% by weight of 1-butene-based structural units and structural units derived from a C 2 -C 10 α-olefin (other than 1-butene), such as ethylene, propylene, 1-octene, 1-hexene and 4-methyl-1-pentene. Suitable propylene-based resins used in (7) and (8) include a homopolymer of propylene, a copolymer of propylene and ethylene, a copolymer of propylene and 1-butene, and a copolymer of propylene and 4-methyl-1-pentene. The olefinic thermoplastic elastomer to be mixed with the crystalline polyolefin-based resin in (2) includes copolymers comprising from 65 to 90%, preferably 75 to 90%, by weight of ethylene-based structural units and from 10 to 35%, preferably 10 to 25%, by weight of propylene-based structural units, 1-butene-based structural units or a mixture thereof. The elastomers have a density of from 0.86 to 0.89 g/cm 3 . The olefinic thermoplastic elastomer serves to improve stretchability and elasticity of the wrapping film. If desired, the intermediate layer of (1) to (8) may contain from 3 to 25%, preferably 5 to 15%, by weight of the same ethylene-based copolymer resin as used in the surface layer. This can be accomplished by returning the flash during the molding process so that it becomes incorporated into the intermediate layer. The copolymer resin used in the surface layer of (1) to (4) is a copolymer comprising from 60 to 95%, preferably 70 to 95%, by weight of ethylene-based structural units and from 5 to 40%, preferably 7 to 30%, by weight of structural units derived from a monomer selected from vinyl acetate, an aliphatic unsaturated carboxylic acid, and an aliphatic unsaturated carboxylic acid alkyl ester. Examples of such an ethylene-based copolymer include ethylene-vinyl acetate copolymers, ethylene-acrylic acid copolymers, ethylene-methacrylic acid copolymers, ethylene-ethyl acrylate copolymers, and ethylene-methyl methacrylate copolymers. Ethylene-vinyl acetate copolymers having a vinyl acetate-derived content of from 5 to 25% by weight and an MFR of from 0.3 to 5 g/10 min are preferred. The copolymer resin used in the intermediate layer of (3) is a copolymer comprising from 60 to 95% by weight of ethylene-based structural units and from 5 to 40% by weight of structural units derived from a monomer selected from vinyl acetate, an aliphatic unsaturated carboxylic acid, and an aliphatic unsaturated carboxylic acid alkyl ester. Examples of such an ethylene-based copolymer include ethylene-vinyl acetate copolymers, ethylene-acrylic acid copolymers, ethylene-ethyl methacrylate copolymers, and ethylene-methyl methacrylate copolymers. For the purpose of endowing the film with moderate slip properties, self-tack, anti-fogging properties, and antistatic properties, the intermediate layer, surface layer or both, of the above-mentioned laminate structures may contain from 0.5 to 5% by weight, and preferably from 1 to 3% by weight, of at least one compound selected from aliphatic alcohol fatty acid esters and polyalkylene ether polyols. The aliphatic alcohol fatty acid esters include those obtained from an aliphatic alcohol having from 1 to 12 carbon atoms, and preferably from 1 to 6 carbon atoms, and a fatty acid having from 12 to 22 carbon atoms, and preferably from 12 to 18 carbon atoms, such as glycerol mono-oleate, glycerol polyoleate, glycerol triricinoleate, glycerol acetylricinoleate, methyl acetylricinoleate, ethyl acetylricinoleate, butyl acetylricinoleate, propylene glycol oleate, propylene glycol laurate, pentaerythritol oleate, polyethylene glycol oleate, polypropylene glycol oleate, polyoxyethylene glyceride, polyoxypropylene glyceride, sorbitan oleate, sorbitan laurate, polyethylene glycol sorbitan oleate, and polyethylene glycol sorbitan laurate. The polyalkylene ether polyols include polyethylene glycol and polypropylene glycol. The stretch wrapping film of the present invention can be produced by any suitable film production method, such as blown-film extrusion using blown-film extrusion machine 1 shown in Fig. 4. In blown-film extrusion machine 1, starting thermoplastic resin a is stored in storage hopper 2. Under instructions from a computer, load cell 3 works in accordance with the blown film take-off speed to feed thermoplastic resin a automatically to feed hopper 4. Thermoplastic resin a is fed from feed hopper 4 to extruder 5 having screw 7 driven by screw motor 6, where it is melt-and extruded upward from the head of the extruder. Blowing head containing ring die 9 is provided at the head of extruder 5 in the vertical direction via direct coupling 8. To blowing head 9 is connected inflator (air pump) 12 via pipe 11 having electromagnetic valve 10 so that air is blown into the inside of the extruded resin tube to form tubular bubble b at a blow-up ratio of from 8 to 20. Tubular bubble b is then cooled with air fed from cooling blower 14 through air cooling equipment 13 having an air cooling ring set above blowing head. Tubular bubble b is guided by a pair of guide plates 15 and passed through a pair of nip rolls 17 driven by take-off motor 16 to obtain flattened tube c . Flattened tube c is introduced into width measuring means 18 in which the flattened width is measured by width sensor 19 and then, while being guided by guide rolls 20, 21, 21, wound up around paper reel d held by reel holder 23 in wind-up 22. In Fig. 4, e is a film wind-up roll. When the blown film is to be used as a bag-forming film, flattened tube c is wound up around paper reel d as a flat tube. When the blown film is to be used as a wrapping film or a stretch film, the width of flattened tube c is slit into a desired number of sheets and wound up around the respective paper reel d . The terminology "blow-up ratio" as used in blown-film extrusion means a ratio of the final inner diameter of a blown tube to an outer diameter of the ring die. Olefin resins are generally inflated at a blow-up ratio of from 1.2 to 7. Blown films having a blow-up ratio of from 3 from 7 are called high blown films in the art. In order to obtain stretch-wrapping films having satisfactory strain recovery according to the present invention by blown-film extrusion, the blow-up ratio ranges from 8 to 20 (from 9 to 20 for the production of the above-described laminated films (1) and (3)), and preferably from 10 to 16. Such a high blow-up ratio can be reached by using a specific cooling apparatus as shown in Fig. 1 or Fig. 3 as the air cooling equipment 13 in Fig. 4. Air cooling equipment 13' shown in Fig. 1 or Fig. 3 comprises air ring 26, a plurality of rectifying cylinders 27 and a rectifying cylinder 25a. Air ring 26, which is provided around orifice D of ring die C, has at least three air nozzles 24a (upstream nozzle), 24b (midstream nozzle), and 25 (downstream nozzle) preferably circular air nozzles. Upstream nozzle 24a and midstream nozzle 24b serve as auxiliary air nozzles, while downstream nozzle 25 serves as a main air nozzle, with the air flow rate of these air nozzles increasing in the order 24a<24b<25. A plurality of rectifying cylinders 27 having different diameters are arrayed at certain intervals in the radial direction concentrically with the air nozzles. Between each two adjacent cylinders, there is formed ring air chamber 28, 28' with its downstream end open. The outermost rectifying cylinder has a plurality of intake vents 29 for intake of open air at the positions on the same plane perpendicular to the axis thereof, and closer to air ring 26 than to the downstream end as shown in Fig. 2. All the rest of the rectifying cylinders each have vent 30 at the bottom end (upstream end) thereof so that air chambers 28 are ventilated. The height of rectifying cylinders 27 gradually increases toward the outermost one so that the downstream ends of these cylinders may form a tapered guide for the bubble, making an angle of from 45 to 70° with axis O of orifice D. When the above-mentioned cooling apparatus is employed for the production of a blown film, as blow-up ratio increases with the film thickness and the take-off speed being fixed, the bubble can be supported in a stable manner irrespective of the blow-up ratio by increasing the number of the auxiliary air nozzles and, if desired, also increasing the number of the air chambers supporting the bubble. The air amount from midstream nozzle 24b and from upstream nozzle 24a ranges from 5 to 20 parts by volume and from 0.1 to 3 parts by volume, respectively, with that from downstream nozzle 25 being taken as 100 parts by volume. In air ring 26, the air in first chamber 31 leading to upstream nozzle 24a is usually the atmosphere. If necessary, air may be supplied to first chamber 31 by means of a blower. Air supply to second chamber 32 leading to midstream nozzle 24b and third chamber 33 leading to downstream nozzle 25 is effected by means of blower 14. In Fig. 1, second chamber 32 and third chamber 33 are connected via passageways 34 and 34'. Blower 14 is connected to air ring 26 via six pipes 35 provided on the side wall of air ring 26 and their respective hoses 36. In cooling apparatus 13' shown in Fig. 3, air ring 26 has three auxiliary air nozzles 24a, 24b, and 24c. 32' is another second chamber of air supplying. The chambers leading to the respective air nozzle may be independent of each other as in Fig. 3. In that case, each chamber connects with the respective blower. The speed of air flow from each air nozzle varies depending on the blow-up ratio, the diameter of the bubble, or the take-off speed, and ranges from 0.1 to 10 m/sec at upstream nozzle 24a, from 5 to 40 m/sec at midstream nozzle 24b or 24c (increasing upstream), and from 10 to 50 m/sec at downstream nozzle 25. Since the extruded tube in the molten state is preliminarily cooled with air from auxiliary air nozzles 24a, 24b, and 24c, the amount of blowing air from the main nozzle can be reduced compared to conventional processes. That is, the speed of air flow from the main nozzle does not need to be higher than 50 m/sec. The air from the main nozzle blowing against the bubble and thereby having its temperature slightly raised is then made to flow downstream between the bubble and the tapered conical section formed by the downstream ends of the rectifying cylinders. By the action of the thus produced Venturi effect, the inner pressure of each ring air chamber 28 is diminished thereby letting open air in through intake vents 29 and vents 30. Part of the air in the air chambers joins the air flow from the air nozzles and is made to flow downstream along the bubble to rapidly cool the bubble. At the same time, the bubble can be stably supported from the outer peripheral side by the ring air chambers having diminished inner pressure. Since the height of the rectifying cylinders increases downstream to form a tapered conical section widening downstream, the extruded bubble can be rapidly inflated even to a high blow-up ratio without contacting with the downstream end of the rectifying cylinder immediately after extrusion. Further, open air taken in through intake vents 29 is made to flow through each air chamber via the respective vent 30 in proportion to the degree of pressure decrease. Therefore, the pressure in each air chamber is constantly maintained at a pressure suited for bubble inflation. The wrapping film of the present invention satisfies the following conditions (1) to (6): (1) the film has a thickness ranging from 6 to 100 µm; (2) the film has a birefringence Δn A ranging from 0.6×10 -3 to 8×10 -3 , the birefringence being defined by equation: Δn A = |n MD - n TD | wherein n MD is a refractive index in the machine direction; and n TD is a refractive index in the transverse direction; (3) upon standing in a silicone oil bath at 140°C for 3 minutes, the film has a ratio of percent heat shrinkage in the machine direction S MD to percent heat shrinkage in the transverse direction S TD (heat shrinkage ratio, S MD /S TD ) of from 0.65 to 1.50; and (4) the film has a strain recovery of at least 18 mm as measured by a test method comprising clamping a circular film specimen having a diameter of 100 mm, indenting the center of the specimen with a hemispherical indenter of 20 mm in diameter to cause deformation, and measuring the maximum strain (indentation) which will disappear completely on removal of the indenter; and (5) the film has a tensile modulus of from 58.8 to 137.2 MPa (600 to 1400 kg/cm 2 ) in both machine and transverse directions. (6) the film has a ratio of stress at 50% elongation in the machine direction (T MD ) to stress at 50% elongation in the transverse direction (T TD ) of from 0.50 to 2.50. When the wrapping film of the present invention is to be used in stretch wrapping or pillow type packaging, it preferably satisfies the following additional condition (7): (7) the film has a haze (JIS Z-6714) of not more than 3%. With respect to condition (1), for use as a stretch-wrapping film for foods or a bag-forming film, the preferred thickness is from 8 to 30 µm, from the standpoint of economy and transparency. In the case of the multilayer laminate structures, the intermediate layer has a preferred thickness of from 2 to 10 µm and each surface layer has a preferred thickness of from 3 to 10 µm. For use as a pallet stretch wrapping film, a preferred film thickness is from 20 to 50 µm. With respect to condition (2), the birefringence Δn A of the film is measured with a polarizing microscope "OPTIPHOT-POL" manufactured by Nicon Co., Ltd. according to the following instructions. (1) One drop of "Oil Type B" produced by Nicon Co., Ltd. is placed on a slide glass, followed by a 5 mm-square film specimen on the oil droplet. One more drop of the same oil is then placed on the film specimen. A cover glass is placed thereon to prepare the sample for measurement. (2) The sample is fixed on the stage. (3) A compensator having three through-holes is inserted into the slot and fixed in such a position that the middle hole of the compensator is on the axis of the film specimen. (4) Polarized light is obtained using the button by the side of the upper dial plate. Then, the zero of the upper dial plate is adjusted to the zero of the lower dial plate. (5) The stage is then turned until the field of view gets dark, and the scale value on the stage read. (6) The compensator is then replaced by a quartz wedge. (7) The stage is turned to either +45° or -45° from the scale value read in (5). Then, the quartz wedge is slowly moved forward and backward. When the view begins to turn from white to yellow, the scale value on the quartz wedge is read, and the scale value (ψ) on the quartz wedge when the field is whiter than at ±45° is noted. (8) The quartz wedge is removed, and insert a Sénarmont compensator having one through-hole at the center then inserted in its place. The lower lens is covered with a yellow filter (MXA 20168, MF 45 IF 546). (9) The upper dial plate is turned until the field of view gets dark, and that scale value () is read. (10) The birefringence Δn A is calculated according to the following equation: Δn A = [{(/180) x λ} + (λ x ψ)]/thickness of the film wherein λ = 546 nm. The thus obtained Δn A is defined as follows based on the refractive indices of the film in the machine direction (n MD ) and in the transverse direction (n TD ). Δn A = |n MD - n TD | Δn A is a measure indicative of the balance of molecular chain orientation between the machine direction and the transverse direction. As this value increases, it is assumed that the molecular chain orientation in either direction is more predominant. In the present invention, Δn A is in the range of from 0.6×10 -3 to 8×10 -3 , and preferably from 0.8×10 -3 to 6×10 -3 . If Δn A exceeds 8×10 -3 , the molecular chain orientation in one direction is too strong, resulting in difficulty of stretching in that direction. Additionally, the film easily undergoes necking under tension in the direction perpendicular to the direction of stronger orientation. Such a film is liable to wrinkle during stretch wrapping and will show a small strain recovery. The smaller the Δn A value, the smaller the in-plane anisotropy of molecular orientation (it is difficult to obtain a film having Δn A of less than 0.6×10 -3 with the current commercially practical techniques of blown-film extrusion). Accordingly, in order to minimize the anisotropy of orientation, blown-film extrusion which is less causative of residual molten orientation is preferable to a tentering method, in which the film is stretched at a temperature lower than its melting point, or a stretching method, in which the degree of orientation is increased by making use of a difference in peripheral speed between rolls. The planar orientation anisotropy associated with the stretching between rolls may be minimized by making the stretch ratio and the temperature of stretching in one direction the same as those in the other direction. However, this results in production of a film which has a modulus of elasticity which is too high and is therefore liable to cause breakage of a tray during stretch wrapping. Where blown-film extrusion involving molten orientation is adopted, the balance between the blow-up ratio and the take-off speed must be carefully considered. As the blow-up ratio increases, the take-off speed must be increased so that the orientation anisotropy between the machine direction and the transverse direction may be minimized. Accordingly, with the take-off speed ranging from 20 to 150 m/min, the blow-up ratio falls within a range of from 8 to 20, and preferably from 10 to 16. With respect to condition (3), the heat shrinkage ratio (S MD /S TD ) is also attributed to the balance of orientation stress between the machine direction and the transverse direction and is indicative of liability to form wrinkles upon stretch wrapping. The shrinkage ratio should be in the range of from 0.65 to 1.5, preferably from 0.7 to 1.35, and is more preferably as close to 1 as possible. If it is less than 0.65 or more than 1.5, the film wrinkles upon being stretch wrapped. Besides wrinkling, the film will also have a small strain recovery so that the film covering a food, when pressed by a customer with a fingertip, does not recover from the strain and leaves wrinkles. If that particular item of wrapped food is not bought by that customer, another customer would likely care about the wrinkles caused by the residual strain of the film, and it is very likely that the wrapped food would remain unsold. The heat shrinkage ratio is determined by soaking a 100 mm-square film specimen, which is marked with two lines intersecting at right angles and passing the center thereof, in a silicone oil bath at 140°C for 3 minutes, preserving the specimen taken from the bath in a controlled environment kept at a temperature of 22°C and a relative humidity of 57% for 2 hours, measuring the length of the crossing lines (L MD , L TD ), and applying the measured values to the following equation: S MD /S TD = (100 - L MD )/(100 - L TD ) With respect to condition (4), the terminology "strain recovery" as used herein is a value obtained as follows. As shown in Fig. 5, circular film specimen c having a diameter of 100 mm is clamped, and piston rod 37 having a hemispherical indenter foot having a diameter of 20 mm is pressed down onto film c to cause deformation. The maximum strain H of film c which will disappear completely upon removal of the indenter is measured. The wrapping film of the present invention exhibits a strain recovery of at least 18 mm, preferably 20 mm or more, and more preferably from 23 to 35 mm. In Fig. 5, numeral 38 indicates a load cell. With respect to condition (7), the wrapping film of the present invention is preferably as transparent as possible (haze: less than 3%) so that one may see the wrapped article through the film. However, for applications requiring no transparency, for example bag formation, transparency is of no importance. That is, the film may contain from 3 to 40% by weight of inorganic fine particles, such as carbon black, calcined clay, calcium carbonate, and talc, and may have a haze of 3% or more, and preferably from 40 to 100% (semitransparent to opaque). With respect to condition (5), the film has a tensile modulus (ISO R-1184) of from 600 to 1400 kg/cm 2 , preferably 600 to 1200 kg/cm 2 , in both machine and transverse directions so as not to cause tray breakage upon stretch wrapping. If the tensile modulus is less than 600 kg/cm 2 , the film is too soft to provide a sufficiently tight fit to an article or tray. If it exceeds 1400 kg/cm 2 , the film is too hard and tends to cause deformation or, in some cases, breakage of a tray on wrapping. Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified. All the parts and percents are by weight unless otherwise indicated. EXAMPLE 1 Blown-film extrusion was conducted using blown-film extrusion machine 1 of Fig. 4 with its air ring 13 being displaced with cooling apparatus 13' of Fig. 1 according to the following specifications. Diameter of ring dye C: 100 mm Lip width: 1.0 mm Air cooling conditions: Air Flow Rate (m 3 /min) Air Flow Speed (m/sec) Upstream Nozzle (24a) 0.016 1 Midstream nozzle (24b) 3.2 25 Downstream Nozzle (25) 46.7 (open air) 40 Rectifying cylinders (angle of slope: 60°): Diameter (mm) Height (mm) Innermost 495 160 Second 580 185 Third 700 350 Forth 840 410 Fifth 1000 510 Outermost 1300 670 Linear low-density polyethylene "Ultzex 1020L" (a product of Mitsui Petrochemical Industries, Ltd.; MFR: 2.0 g/10 min; ethylene content; 84.9%; 4-methyl-1-pentene content: 15.1%) was kneaded in an extruder (diameter: 65 mm; L/D: 25) at 185°C (resin A). Separately, a resin composition consisting of 98.5% of an ethylene-vinyl acetate copolymer resin (vinyl acetate content: 15%; MFR (190°C): 2.0 g/10 min) and 1.5% of glycerol mono-oleate "Rikemal OL100" (a product of Riken Vitamin Oil Co., Ltd.) was kneaded in another extruder (diameter: 50 mm; L/D: 25) at 160°C (resin B). Two molten resins A and B were fed to a three-layered ring die with resin A as an intermediate layer and resin B as a surface layer on each side of the intermediate layer, co-extruded at a die temperature of 185°C, and inflated at a blow-up ratio of 12.0 and a take-off speed of 50 m/min to obtain a stretch-wrapping film having a total thickness of 11 µm (B/A/B = 4 µm/3 µm/4 µm). Physical properties of the resulting film were measured according to the aforementioned test methods and the following ones. The results obtained are shown in Table 1 below. Strain Recovery (mm): The tester of Fig. 5 was used. A piston rod of 20 mm in diameter was pressed onto the center of a film specimen at a speed of 500 mm/min and then withdrawn. The maximum strain (indentation height: mm) of the film caused by the piston rod which disappeared within 3 minutes from the withdrawal of the piston rod was taken as a strain recovery. Tensile Strength at Break: Measured according to JIS Z-1702. Tensile Elongation at Break: Measured according to JIS Z-1702. Elmendorf Tensile Strength: Measured according to JIS Z-1702. The resulting film was used for wrapping a tray of 210 mm long, 140 mm wide, 20 mm deep, and 2 mm thick made of foamed polystyrene, stretched polystyrene, or high-impact polystyrene with meat, fish, or vegetables on it by means of an automatic stretch wrapping machine "A-18X" (a product manufactured by Fuji Pack System Co., Ltd.). The appearance of the wrapped article was observed with the naked eye and evaluated in terms of breaking, deformation of the tray, and wrinkling. The results obtained are shown in Table 1. EXAMPLES 2 AND 3 AND COMPARATIVE EXAMPLE 1 A blown film was produced in the same manner as in Example 1, except for changing the blow-up ratio to 8 (Example 2), 10 (Example 3), or 5 (Comparative Example 1) and adjusting the extrusion rates of resins A and B so as to obtain a total film thickness of 11 µm. Physical properties and wrapping suitability of the resulting film were evaluated in the same manner as in Example 1, and the results obtained are shown in Table 1. EXAMPLES 4 AND 5 A blown film was produced in the same manner as in Example 1, except for using cooling apparatus 13' of Fig. 3 having the following specifications and increasing the blow-up ratio to 16 (Example 4) or 20 (Example 5). Diameter of ring dye C: 100 mm Lip width: 1.0 mm Air cooling conditions: Air Flow Rate (m 3 /min) Air Flow Speed (m/sec) Upstream Nozzle (24a) 0.016 1 Midstream nozzle (24b) 3.1 25 Midstream nozzle (24c) 13.4 31 Downstream Nozzle (25) 50.2 (open air) 42 Rectifying cylinders (angle of slope: 60°): Diameter (mm) Height (mm) Innermost 495 175 Second 580 215 Third 700 365 Forth 840 440 Fifth 1000 540 Outermost 1300 715 Physical properties and wrapping suitability of the resulting film were evaluated in the same manner as in Example 1, and the results obtained are shown in Table 1. Compara. Example 1 Example 2 Example 3 Example 1 Example 4 Example 5 Blow-up ratio 5 8 10 12 16 20 Birefringence (x10 -3 ) 2.5 0.9 2.4 4.2 6.3 8.0 Haze (%) 3.3 2.8 2.7 2.7 2.6 2.6 Heat shrinkage ratio (MD/TD) 1.55 1.20 1.29 1.15 1.10 1.05 Percent heat shrinkage (MD/TD) (%) 82/53 73/61 76/59 74/64 75/68 74/70 Tensile modulus (MD/TD) (kg/cm 2 ) 800/800 850/750 900/750 850/720 800/700 800/680 Stress ratio at 50% elongation 1.60 1.19 0.96 0.78 0.63 0.50 Tear strength (MD/TD) (kg/cm) 34/200 72/170 130/130 94/130 120/130 130/120 Tensile strength at break (MD/TD) (kg/cm 2 ) 330/330 340/310 330/360 310/360 330/370 290/370 Tensile elongation at break (MD/TD) (%) 330/360 440/370 430/370 470/330 490/310 500/300 Breaking of film not observed not observed not observed not observed not observed not observed Deformation of tray not observed not observed not observed not observed not observed not observed Wrinkling of film slightly observed not observed not observed not observed not observed not observed Strain recovery (mm) 20 21 23 23 24 25 REFERENCE EXAMPLES 1 TO 4 Four commercially available olefin resin films for stretch wrapping were evaluated in the same manner as in Example 1. The results obtained are shown in Table 2 below. COMPARATIVE EXAMPLE 2 Resin composition A for 1st and 5th layers comprising 98% of an ethylene-vinyl acetate copolymer (vinyl acetate content: 15%; MFR: 1 g/10 min) and 2.0% of glycerol mono-oleate was kneaded in an extruder (diameter: 50 mm; L/D=25) at 160°C. Resin composition B for 2nd and 4th layers comprising 65% of the above ethylene-vinyl acetate copolymer, 20% of an ethylene-propylene-ethylidene-norbornene copolymer elastomer (propylene content: 15 mol%; ethylidenenorbornene content: 2.8 mol%; MFR: 0.45 g/10 min; density: 0.88 g/cm 3 ), 10% of polypropylene (C 2 content: 4%; MFR: 7 g/10 min; density: 0.90 g/cm 3 ), and 5% of poly-1-butene (C 2 content: 4 mol%; MFR: 1.0 g/cm 3 ; density: 0.905 g/cm 3 ) was kneaded in a separate extruder (diameter: 65 mm; L/D=25) at 200°C. Resin composition C for a 3rd layer comprising 80% of the same polypropylene as for the 2nd layer and 20% of the same poly-1-butene as for the 2nd layer was kneaded in another separate extruder (diameter: 50 mm; L/D=25) at 200°C. The molten resin compositions A, B, and C were co-extruded through a five-layered ring die, and the extruded tube was quenched by means of a water cooling ring 6 cm distant from the lip end of the die from which water uniformly flowed out to obtain a tubular film having a diameter of 100 mm and a total thickness of 90 µm (A/B/C/B/A = 10 µm/20 µm/30 µm/20 µm/10 µm). The tubular film was passed through two pairs of delivery nip rolls and draw nip rolls and, meanwhile, heated to 40°C with hot air. The film at that temperature was continuously inflated with air and stretched about 3.5 times in the machine direction and about 3.0 times in the transverse direction. The end portion of the film in the stretching zone was cooled with an air ring from which cold air of about 20°C was blasted, and the stretched film was taken off with nip rolls and subjected to heat setting with hot air of 40°C for several seconds. The flattened tube was separated into two by slitting both edges thereof to finally obtain a 10 µm thick wrapping film. Physical properties and wrapping suitability of the resulting film were evaluated in the same manner as in Example 1, and the results obtained are shown in Table 2. COMPARATIVE EXAMPLE 3 An ethylene-1-butene copolymer resin (C 4 content: 11.0%; MFR: 0.85 g/10 min) was kneaded in an extruder (diameter: 65 mm; L/D=25) at 220°C (resin A). A resin composition comprising 98% of an ethylene-vinyl acetate copolymer resin (vinyl acetate content: 17%; MFR: 3.5 g/10 min) and 2.0% of glycerol mono-oleate was kneaded in a separate extruder (diameter: 50 mm; L/D=25) at 170°C (resin B). The molten resins A and B were fed to a three-layered T-die with resin A as an intermediate layer and resin B as a surface layer, co-extruded at a die temperature of 220°C, and taken off at a speed of 60 m/min to obtain a three-layered laminate film having a total thickness of 16 µm (B/A/B = 4 µm/8 µm/4 µm). Physical properties and wrapping suitability of the resulting film were evaluated in the same manner as in Example 1, and the results obtained are shown in Table 2. Ref. Example 1 Ref. Example 2 Ref. Example 3 Ref. Example 4 Compara. Example 2 Compara. Example 3 Birefringence (x10 -3 ) 10 10 1.6 14 10 1.7 Heat shrinkage ratio (MD/TD) 1.76 1.78 3.5 1.32 1.84 3.75 Percent heat shrinkage (MD/TD) (%) 81/46 72/41 74/21 89/67 73/42 75/20 Tensile modulus (MD/TD) (kg/cm 2 ) 900/1000 1680/1520 790/750 1630/1790 1670/1530 810/760 Stress ratio at 50% elongation 4.16 3.81 1.04 1.61 3.80 1.06 Tear strength (MD/TD) (kg/cm) 95/65 50/70 42/135 17/22 52/72 40/130 Tensile strength at break (MD/TD) (kg/cm 2 ) 330/310 590/590 270/240 860/700 595/585 260/250 Tensile elongation at break (MD/TD) (%) 200/430 150/350 430/540 200/350 150/360 420/550 Breaking of film not observed not observed observed observed not observed observed Deformation of tray not observed observed observed observed observed observed Wrinkling of film not observed not observed observed observed not observed observed Strain recovery (mm) 15 20 10-13 unmeasured 19 11 EXAMPLES 6 TO 9 AND COMPARATIVE EXAMPLES 4 AND 5 A blown film was produced by using the same blown-film extrusion machine as used in Example 1. Resin composition A comprising 70% of poly-1-butene "M 0200" (product of Shell Chemical Co., Ltd.; density: 0.915 g/cm 3 ; MFR (190°C): 1.8 g/10 min), 15% of a propylene-ethylene-1-butene random copolymer resin (ethylene content: 2.0%; 1-butene content: 13.0%; density: 0.896 g/cm 3 ; MFR (230°C): 5.0 g/10 min), and 15% of an ethylene-vinyl acetate copolymer resin (vinyl acetate content: 15%; MFR (190°C): 2.0 g/10 min) was kneaded in an extruder (diameter: 65 mm; L/D=25) at 185°C. Separately, resin composition B comprising 98.5% of an ethylene-vinyl acetate copolymer resin (vinyl acetate content: 15%; MFR (190°C): 2.0 g/10 min) and 1.5% of glycerol mono-oleate "Rikemal OL100" (product of Riken Vitamin Oil Co., Ltd.) was kneaded in a different extruder (diameter: 50 mm; L/D=25) at 160°C. The molten resin compositions A and B were fed to a three-layered ring die with resin composition A as an intermediate layer and resin composition B as a surface layer, co-extruded at a die temperature of 185°C, and inflated at a varied blow-up ratio as shown in Table 3 below to obtain a three-layered laminate film having a total thickness of 11 µm (B/A/B = 4 µm/3 µm/4 µm). Physical properties and wrapping suitability of the resulting film were evaluated in the same manner as in Example 1, and the results obtained are shown in Table 3 below. Compara. Example 4 Compara. Example 5 Example 6 Example 7 Example 8 Example 9 Blow-up ratio 5 8 10 12 16 20 Birefringence (x10 -3 ) 10 7.6 5.1 6.0 5.0 4.8 Haze (%) 2.5 2.4 2.4 1.3 1.3 1.2 Heat shrinkage ratio (MD/TD) 2.26 1.59 1.46 1.33 1.29 1.19 Percent heat shrinkage (MD/TD) (%) 86/38 72/45 83/57 81/61 84/65 84/71 Tensile modulus (MD/TD) (kg/cm 2 ) 1220/1100 1220/1110 1320/1210 1220/1120 1230/1130 1240/1120 Stress ratio at 50% elongation 4.39 3.65 2.32 1.98 1.86 1.70 Tear strength (MD/TD) (kg/cm) 160/26 82/23 56/17 75/20 73/23 72/24 Tensile strength at break (MD/TD) (kg/cm 2 ) 420/380 440/400 440/400 440/470 430/480 430/490 Tensile elongation at break (MD/TD) (%) 140/470 160/330 170/320 190/330 210/320 220/330 Breaking of film not observed not observed not observed not observed not observed not observed Deformation of tray not observed not observed not observed not observed not observed not observed Wrinkling of film seriously observed slightly observed not observed not observed not observed not observed Strain recovery (mm) 15 16 18 20 21 23 EXAMPLES 10 TO 14 AND COMPARATIVE EXAMPLE 6 A blown film was produced by using the same blown-film extrusion machine as used in Example 1. Resin composition A comprising 55% of poly-1-butene "M 0200" (product of Shell Chemical Co., Ltd.; density: 0.915 g/cm 3 ; MFR (190°C): 1.8 g/10 min), 15% of a propylene-ethylene-1-butene random copolymer resin (ethylene content: 2.0%; 1-butene content: 13.0%; density: 0.896 g/cm 3 ; MFR (230°C): 5.0 g/10 min), and 30% of an ethylene-1-butene copolymer elastomer (1-butene content: 20%; MFR (190°C): 4.0 g/10 min; density: 0.88 g/cm 3 ) was kneaded in an extruder (diameter: 65 mm; L/D=25) at 185°C. Separately, resin composition B comprising 98.5% of an ethylene-vinyl acetate copolymer resin (vinyl acetate content: 15%; MFR (190°C): 2.0 g/10 min) and 1.5% of glycerol mono-oleate "Rikemal OL100" was kneaded in a different extruder (diameter: 50 mm; L/D=25) at 160°C. Both of the molten resin compositions A and B were fed to a three-layered ring die with resin composition A as an intermediate layer and resin composition B as a surface layer, co-extruded at a die temperature of 185°C, and inflated at a varied blow-up ratio as shown in Table 4 below to obtain a three-layered laminate film having a total thickness of 11 µm (B/A/B = 4 µm/3 µm/4 µm). Physical properties and wrapping suitability of the resulting film were evaluated in the same manner as in Example 1, and the results obtained are shown in Table 4. Compara. Example 6 Example 10 Example 11 Example 12 Example 13 Example 14 Blow-up ratio 5 8 10 12 16 20 Birefringence (x10 -3 ) 3.9 2.0 2.7 1.9 1.6 1.5 Haze (%) 3.1 2.8 2.6 2.5 2.5 2.4 Heat shrinkage ratio (MD/TD) 1.56 1.14 1.36 1.38 1.36 1.31 Percent heat shrinkage (MD/TD) (%) 88/56 78/68 83/61 85/61 84/62 85/65 Tensile modulus (MD/TD) (kg/cm 2 ) 1050/1100 1050/1000 1000/1000 1000/900 950/900 900/850 Stress ratio at 50% elongation 2.29 1.67 1.88 1.75 1.68 1.65 Tear strength (MD/TD) (kg/cm) 55/26 32/17 48/48 52/42 46/43 48/45 Tensile strength at break (MD/TD) (kg/cm 2 ) 410/380 440/390 430/370 430/390 420/390 440/400 Tensile elongation at break (MD/TD) (%) 190/390 250/320 240/380 250/390 265/390 270/380 Breaking of film not observed not observed not observed not observed not observed not observed Deformation of tray not observed not observed not observed not observed not observed not observed Wrinkling of film slightly observed not observed not observed not observed not observed not observed Strain recovery (mm) 16 18 19 20 22 23 EXAMPLE 15 Resin composition A comprising 45 parts of an ethylene-1-octene copolymer (ethylene content: 89%; 1-octene content: 11%; density: 0.92 g/cm 3 ; MFR: 1.2 g/10 min), 40 parts of an ethylene-1-octene copolymer elastomer (ethylene content: 78%; 1-octene content: 22%; density: 0.906 g/cm 3 ; MFR (190°C): 0.8 g/10 min), and 15 parts of an ethylene-vinyl acetate copolymer (ethylene content: 85%; vinyl acetate content: 15%; MFR: 2.0 g/10 min) was kneaded in an extruder (diameter: 65 mm; L/D=25) at 185°C. Separately, resin composition B comprising 98.5% of an ethylene-vinyl acetate copolymer resin (vinyl acetate content: 15%; MFR (190°C): 2.0 g/10 min) and 1.5% of glycerol mono-oleate "Rikemal OL100" was kneaded in a different extruder (diameter: 50 mm; L/D=25) at 160°C. Both of the molten resin compositions A and B were fed to a three-layered ring die with resin composition A as an intermediate layer and resin composition B as a surface layer, co-extruded at a die temperature of 185°C, and inflated at a blow-up ratio of 12.0 to obtain a three-layered laminate film having a total thickness of 11 µm (B/A/B = 4 µm/3 µm/4 µm). Physical properties and wrapping suitability of the resulting film were evaluated in the same manner as in Example 1. Wrapping suitability was additionally evaluated in terms of stretchability as observed with the naked eye on stretch wrapping by use of "A-18X". The results obtained are shown in Table 5 below. EXAMPLE 16 A three-layered laminate film was produced in the same manner as in Example 15, except for using resin composition A comprising 80 parts of an ethylene-1-hexene copolymer (1-hexene content: 12%; density: 0.91 g/cm 3 ; MFR (190°C): 2.0 g/10 min) and 20 parts of an ethylene-1-butene copolymer elastomer (1-butene content: 31%; density: 0.89 g/cm 3 ; MFR (190°C): 3.6 g/10 min) as an intermediate layer. Physical properties and wrapping suitability of the resulting film were evaluated in the same manner as in Example 15, and the results obtained are shown in Table 5. EXAMPLE 17 A three-layered laminate film was produced in the same manner as in Example 15, except for using, as an intermediate layer, resin composition A comprising 80 parts of a propylene-ethylene-1-butene terpolymer (79:1:20 by weight; MFR (230°C): 5 g/10 min) and 20 parts of the same ethylene-1-butene copolymer elastomer as used in Example 16. Physical properties and wrapping suitability of the resulting film were evaluated in the same manner as in Example 15, and the results obtained are shown in Table 5. EXAMPLE 18 A three-layered laminate film was produced in the same manner as in Example 15, except for using resin composition A comprising 45 parts of the same ethylene-1-octene copolymer as used in Example 15 and 55 parts of an ethylene-1-butene copolymer elastomer (ethylene content: 70%; MFR (190°C): 3.7 g/10 min; density: 0.88 g/cm 3 ) as an intermediate layer. Physical properties and wrapping suitability of the resulting film were evaluated in the same manner as in Example 15, and the results obtained are shown in Table 5. EXAMPLE 19 A three-layered laminate film was produced in the same manner as in Example 15, except for using an ethylene-4-methyl-1-pentene copolymer (ethylene content: 88%; 4-methyl-1-pentene content: 12%; MFR: 2.0 g/10 min; density: 0.910 g/cm 3 ) as an intermediate layer. Physical properties and wrapping suitability of the resulting film were evaluated in the same manner as in Example 15, and the results obtained are shown in Table 5. EXAMPLE 20 A three-layered laminate film was produced in the same manner as in Example 15, except for changing the blow-up ratio to 16.0. Physical properties and wrapping suitability of the resulting film were evaluated in the same manner as in Example 15, and the results obtained are shown in Table 5. COMPARATIVE EXAMPLE 7 A three-layered laminate film was produced in the same manner as in Example 15, except for changing the blow-up ratio to 5.0. Physical properties and wrapping suitability of the resulting film were evaluated in the same manner as in Example 15, and the results obtained are shown in Table 5. Example 15 Example 16 Example 17 Example 18 Example 19 Example 20 Compara. Example 7 Blow-up ratio 12 12 12 12 12 16 5 Birefringence (x10 -3 ) 4.0 4.5 4.6 5.2 3.1 5.9 3.0 Heat shrinkage ratio (MD/TD) 1.14 1.19 1.36 1.12 1.20 1.11 1.60 Percent heat shrinkage (MD/TD) (%) 79/69 76/64 83/61 73/65 79/66 78/70 80/50 Tensile modulus (MD/TD (kg/cm 2 ) 790/700 750/680 1350/1300 810/690 930/810 780/690 780/750 Tensile strength at break (MD/TD) (kg/cm 2 ) 320/340 320/360 370/380 310/290 360/380 310/350 300/270 Tensile elongation at break (MD/TD) (%) 350/420 370/390 380/390 360/460 380/400 340/380 300/490 Tear Strength (MD/TD) (kg/cm) 85/120 70/140 105/250 55/65 90/170 105/95 45/170 Stretchability good good good good good good good Breaking of film not observed not observed not observed not observed not observed not observed not observed Wrinkling of film not observed not observed not observed not observed not observed not observed not observed Strain recovery (mm) 18 18 15 25 18 20 10 It is seen from the foregoing Examples that olefin resin films obtained by blown-film extrusion at a high blow-up ratio of from 8 to 20 and having a birefringence Δn A of from 0.6×10 -3 to 8×10 -3 and a heat shrinkage ratio at 140°C of from 0.65 to 1.50 are excellent in deformation strain recovery and suitable as stretch wrapping film. Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

Claims: A stretch wrapping film comprising one or more layers of a polyolefin-based resin, wherein the layers may be the same or different, wherein the film has properties which satisfy the following conditions (1) to (6): (1) the film has a thickness ranging from 6 to 100 µm (2) the film has a birefringence Δn A ranging from 0.6 x 10 -3 to 8 x 10 -3 , said birefringence being defined by equation: Δn A = | n MD - n TD | wherein n MD is a refractive index in the machine direction; and n TD is a refractive index in the transverse direction; (3) when the film is allowed to stand in a silicone oil bath at 140°C for 3 minutes, a ratio of heat shrinkage in the machine direction, S MD , to heat shrinkage in the transverse direction, S TD , (S MD /S TD ) ranges from 0.65 to 1.50; (4) the film has a strain recovery of at least 18 mm as measured by a test method comprising clamping a circular film specimen having a diameter of 100 mm, indenting the center of the specimen with a hemispherical indenter of 20 mm in diameter to cause deformation, and measurinq the maximum strain (indentation) which disappears completely upon removal of the indenter; (5) the film has a tensile modulus of from 600 to 1400 kg/cm 2 in both machine and transverse directions; and (6) the film has a ratio of stress at 50% elongation in the machine direction (T MD ) to stress at 50% elongation in the transverse direction (T TD ) of from 0.50 to 2.50. A stretch wrapping film as claimed in Claim 1, wherein said film has a ratio of percent heat shrinkage in the machine direction (S MD ) to percent heat shrinkage in the transverse direction (S TD ) of from 0.7 to 1.35. A stretch wrapping film as claimed in Claim 1, wherein said film is a blown film obtained by inflating, at a blow-up ratio of from 9 to 20, a laminate tubular film having an intermediate layer comprising a linear low-density polyethylene-based resin having an MFR of from 0.1 to 5 g/10 min and a Q-value of from 1 to 6, wherein said intermediate layer is laminated on both sides thereof with a surface layer comprising a copolymer resin comprising from 60 to 95% by weight of ethylene-based structural units and from 5 to 40% by weight of structural units derived from a monomer selected from the group consisting of vinyl acetate, aliphatic unsaturated carboxylic acids, and aliphatic unsaturated carboxylic acid alkyl esters. A stretch wrapping film as claimed in Claim 3, wherein said blown film has a total thickness of from 8 to 30 µm. A stretch wrapping film as claimed in claim 4, wherein said intermediate layer has a thickness of from 2 to 10 µm and each surface layer has a thickness of from 3 to 10 µm. A stretch wrapping resin film as claimed in Claim 3, wherein said linear low-density polyethylene-based resin is a resin selected from the group consisting of ethylene-1-butene copolymers, ethylene-1-hexene copolymers, ethylene-1-butene-1-hexene copolymers, ethylene-4-methyl-1-pentene copolymers, ethylene-1-hexene copolymers, and ethylene-1-octene copolymers, wherein said resin has a density of from 0.89 to 0.93 g/cm 3 . A stretch wrapping film as claimed in Claim 1, wherein said film is a blown film obtained by inflating, at a blow-up ratio of from 9 to 20, a laminate tubular film having an intermediate layer comprising from 80 to 95% by weight of a linear low-density polyethylene having an MFR of from 0.1 to 5 g/10 min and a Q-value of from 1 to 6 and from 5 to 20% by weight of a copolymer resin comprising from 60 to 95% by weight of ethylene-based structural units and from 5 to 40% by weight of structural units derived from a monomer selected from the group consisting of vinyl acetate, aliphatic unsaturated carboxylic acids, and aliphatic unsaturated carboxylic acid alkyl esters, wherein said intermediate layer has laminated on both sides thereof a surface layer comprising a copolymer resin comprising from 60 to 95% by weight of ethylene-based structural units and from 5 to 40% by weight of structural units derived from a monomer selected from vinyl acetate, aliphatic unsaturated carboxylic acids, and aliphatic unsaturated carboxylic acid alkyl esters. A stretch wrapping film as claimed in Claim 7, wherein said blown film has a total thickness of from 8 to 30 µm. A stretch wrapping film as claimed in Claim 8, wherein said intermediate layer has a thickness of from 2 to 10 µm and each surface layer has a thickness of from 3 to 10 µm. A stretch wrapping film as claimed in Claim 7, wherein said linear low-density polyethylene-based resin is a resin selected from ethylene-1-butene copolymers, ethylene-1-hexene copolymers, ethylene-1-butene-1-hexene copolymers, ethylene-4-methyl-1-pentene copolymers, ethylene-1-hexene copolymers, and ethylene-1-octene copolymers, wherein said resin has a density of from 0.89 to 0.92 g/cm 3 . A stretch wrapping film as claimed in Claim 1, wherein said film is a blown film obtained by inflating, at a blow-up ratio of from 8 to 20, a laminate tubular film having an intermediate layer comprising from 10 to 90% by weight of a crystalline polyolefin-based resin selected from the group consisting of ethylene-based resins and propylene-based resins and from 10 to 90% by weight of an olefinic thermoplastic elastomer, wherein said intermediate layer is laminated on both sides thereof with a surface layer comprising a copolymer resin comprising from 60 to 95% by weight of ethylene-based structural units and from 5 to 40% by weight of structural units derived from a monomer selected from vinyl acetate, aliphatic unsaturated carboxylic acids, and aliphatic unsaturated carboxylic acid alkyl esters. A stretch wrapping film as claimed in Claim 11, wherein said blown film has a total thickness of from 8 to 30 µm. A stretch wrapping film as claimed in Claim 12, wherein said intermediate layer has a thickness of from 2 to 10 µm and each surface layer has a thickness of from 3 to 10 µm. A stretch wrapping film as claimed in Claim 3, wherein said surface layer or intermediate layer or both further contains at least one additive selected from lubricants, tackifiers, and nucleating agents. A stretch wrapping film as claimed in Claim 7, wherein said surface layer or intermediate layer or both contains at least one additive selected from lubricants, tackifiers, and nucleating agents. A stretch wrapping film as claimed in Claim 11, wherein said surface layer or intermediate layer or both further contains at least one additive selected from lubricants, tackifiers, and nucleating agents. A stretch wrapping film as claimed in Claim 14, wherein said at least one additive is present in a total amount of from 0.5 to 5% by weight. A stretch wrapping film as claimed in Claim 15, wherein said at least one additive is present in a total amount of from 0.05 to 5% by weight. A stretch wrapping film as claimed in Claim 16, wherein said at least one additive is present in a total amount of from 0.05 to 5% by weight. A stretch wrapping film as claimed in Claim 14, wherein said lubricant is a compound selected from oleamide, stearamide, erucamide, glycerol monostearate, glycerol distearate, glycerol monooleate, glycerol dioleate, and a polyethylene glycol adduct of the glyceride. A stretch wrapping film as claimed in Claim 15, wherein said lubricant is a compound selected from oleamide, stearamide, erucamide, glycerol monostearate, glycerol distearate, glycerol monooleate, glycerol dioleate, and a polyethylene glycol adduct of the glyceride. A stretch wrapping film as claimed in Claim 16, wherein said lubricant is a compound selected from oleamide, stearamide, erucamide, glycerol monostearate, glycerol distearate, glycerol monooleate, glycerol dioleate, and a polyethylene glycol adduct of the glyceride. A stretch wrapping film as claimed in Claim 14, wherein said tackifier is a compound selected from castor oil derivatives, low-molecular viscous polybutylenes, and sorbitan higher fatty acid esters containing from 12 to 22 carbon atoms in the higher fatty acid moiety. A stretch wrapping film as claimed in Claim 15, wherein said tackifier is a compound selected from castor oil derivatives, low-molecular viscous polybutylenes, and sorbitan higher fatty acid esters containing from 12 to 22 carbon atoms in the higher fatty acid moiety. A stretch wrapping film as claimed in Claim 16, wherein said tackifier is a compound selected from castor oil derivatives, low-molecular viscous polybutylenes, and sorbitan higher fatty acid esters containing from 12 to 22 carbon atoms in the higher fatty acid moiety. A stretch wrapping film as claimed in Claim 14, wherein said nucleating agent is talc or silica powder. A stretch wrapping film as claimed in Claim 15, wherein said nucleating agent is talc or silica powder. A stretch wrapping film as claimed in Claim 16, wherein said nucleating agent is talc or silica powder.

Inventors: KOBAYASHI NORIYUKI, MATSUI TAKASHI, NORO MASATAKA, SANO HIRONARI, YAMAMOTO KOJI, YAZAKI TAKAO, KOBAYASHI, NORIYUKI, MATSUI, TAKASHI, NORO, MASATAKA, SANO, HIRONARI, YAMAMOTO, KOJI, YAZAKI, TAKAO
Applicants: MITSUBISHI CHEM CORP, MITSUBISHI CHEMICAL CORPORATION
Application year: 1992
Main classification: B32B27
IPC codes: B29C47, B65D65, B32B27, B29D7, C08J5