MXPA01007206A - Heat-shrinkable, irradiated, polyethylene mono-layer film - Google Patents

Abstract

A process of manufacturing a heat-shrinkable polyethylene mono-layer film with a thickness variation of less than 20%, preferably less than 18%and even more preferably less than 15%, which process comprises extrusion of the film resin through a flat die, quenching of the cast extruded sheet, irradiation thereof, re-heating to the suitably selected orientation temperture and stretching of the irradiated sheet. Preferably the polyethylene comprises an ethylene-&agr;-olefin copolymer. The heat-shrinkable polyethylene mono-layer irradiated film with a thickness variation of less than 20%, preferably less than 18%and even more preferably less than 15%is also claimed.

Description

POLYETHYLENE MONOCAPA FILM, THERMOENCOGIBLE, IRRADIATED The present invention relates to a monolayer, irradiated, heat-shrinkable polyethylene film, characterized by a very low variation in thickness, a process for its manufacture and the use of the film as a packaging material. Biaxially oriented thermo-shrink films are films that have been oriented by stretching in two perpendicular directions, typically the longitudinal direction of the machine (MD = Machine Direction) and the transverse direction (TD = Transverse or Crosswise Direction) at a temperature comprised between the Tg and the melting point of the resin used, ie a temperature where the resin is not in the molten state. Biaxially oriented thermo-shrink films are made by extruding the polymer from a melt in a coarse sheet which is rapidly cooled to prevent or retard crystallization of the polymer, and then oriented to stretch under temperature conditions, as indicated above , where the molecular orientation of the film occurs and the film does not tear. In the event of subsequent reheating at a temperature close to the orientation temperature, the oriented film, shrinkable, will tend to shrink in order to recover its original dimensional state. The bi-axially oriented thermo-shrinkable polyethylene mono-layer films are known in the literature and widely used in the market. Typically, they are obtained by extruding the polymer through a round matrix to obtain a thick tubular film called "tape", which is cooled immediately and quickly by means of a water bath or cascade, re-heated with or without previous irradiation to the temperature of conveniently selected orientation and stretch bi-axially, while at this temperature, by the so-called "trapped bubble" technique, which uses internal gas pressure to expand the diameter of the belt to form a large "bubble" and advance the expanded tube at a speed greater than the extrusion rate in order to obtain directions of transverse orientation and of the machine, respectively. The film is then cooled and rolled up in the cooled state to retain the thermo-shrink property. In general, in the above process, the tape is interlaced before stretching in order to impart greater mechanical strength and in this way allow better control of the process by increased stability of the blown bubble.

Films obtained by the trapped bubble technique always show poor thickness control and variations of at least about 25-30% are typically obtained. EP-A-319,401 describes an alternating process for the manufacture of mono-layer, thermo-shrinkable, bi-axially oriented films of ethylene-α-olefin copolymers. The process allows the extrusion of the copolymer through a flat matrix in the form of a sheet, and after a cooling step, for heating the sheet to a first orientation temperature and its stretching in a longitudinal direction, followed by heating the longitudinally stretched film to a second orientation temperature higher than the first, and for its transverse stretching. Irradiation stage is not expected in the method, which unlike the blown bubble process there are no problems of process stability ("bubble") in the process of flat orientation. The films obtained by the method described in the prior art document show an extremely high thickness variation. Considering the working examples contained in EP-A-319, 401, the film made from the copolymer of ethylene-octene-1 with a density of 0.919 g / cm3 and MI of 6 g / 103 (resin A), shows a variation in thickness of > 25%, while that made from a copolymer of ethylene-octene-1 with a density of 0.917 g / cm3 and MI of 2.3 g / 103 (resin B) shows a variation in thickness of >; 33%. These values can be acceptable when the film is obtained by the trapped bubble method, since the use of a rotating platform will allow the distribution of the thicknesses over the film width, they are unacceptable in the case of a flat orientation process in where thickness distribution can not be achieved. In this latter case a variation in thickness as indicated, will not allow the winding of the final film in acceptable rolls. It has been found that it is possible to control the variation of thickness in a thermo-shrinkable monolayer polyethylene film which is obtained by the flat orientation process, by irradiating the sheet before stretching. It has been found that depending on the level of irradiation, it is possible to achieve a variation in thickness of less than 20%, preferably less than 18% and even more preferably less than 15%. A first objective of the present invention is therefore a thermo-shrinkable, irradiated single-layer polyethylene film characterized by a thickness variation of less than 20%, preferably less than 18% and even more preferably less than 15% .

A second objective of the present invention is a process for the manufacture of a mono-layer film of thermo-shrinkable polyethylene, characterized by a variation in thickness of less than 20%, preferably less than 18% and even more preferably less than 15%. %, this process comprises extruding the film resin through a flat die, cooling the cast extruded sheet, irradiating it, reheating to the conveniently selected orientation temperature and stretching the irradiated sheet. A third objective of the present invention is to use an irradiated mono-layer film of thermo-shrinkable polyethylene, characterized by a thickness variation of less than 20%, preferably less than 18% and even more preferably less than 15%, the packing of food or non-food products. DEFINITIONS As used herein, the term "film" is used in a generic sense to include a plastic web, regardless of whether it is a film or sheet. Typically, films of and used in the present invention have thicknesses of 100 μm or less, preferably have a thickness of 80 μm or less, more preferably a thickness of 50 μm or less, in particular TO. more preferably at a thickness of 35 μm or less, and even more preferably a thickness of 25 μm or less. As used herein, the phrase "thickness variation" refers to the percentage value that is obtained by measuring the maximum and minimum thickness of the film, calculating the average thickness value and applying these numbers to the following formula: film thickness variation (max) thickness (%) = film thickness (min) X 100. average film thickness) The maximum and minimum thicknesses are determined by taking a total of 10 thickness measurements at regular distance intervals over the entire cross direction of a film sample, recording the highest and lowest thickness values as the maximum thickness values and minimum, respectively, while the average value is determined by adding the same 10 thickness measurements and dividing the result by ten. The thickness variation is then calculated (as a percentage value) using the formula above. A variation in thickness of 0% represents a film without measurable differences in thickness. A variation in thickness over 25% is industrially unacceptable while a variation in thickness less than 20% is acceptable. As used herein, the phrase "machine direction", abbreviated "MD", refers to a direction "on the length or length" of the film, ie in the direction of the film as the film is formed during extrusion and / or coating. As used herein, the phrase "transverse direction", abbreviated here "TD", refers to an address through the film, perpendicular to the machine direction or longitudinal. As used herein, the phrases "orientation ratio" and "stretch ratio" refer to the product of multiplication of the extent to which the plastic film material expands in the two directions perpendicular to each other, ie the direction of machine and the transverse direction. As used here, the phrase "thermo-shrinkable", "thermo-shrink" and the like refer to the tendency of the film to shrink before the application of heat, ie to contract when heated, such that the size of the film decreases while the film is in an unrestricted state. As used herein, the term refers to films with a total of free shrinkage (ie, free shrinkage in the machine direction plus free shrinkage in the transverse direction), ^ as measured by ASTM D 2732, of at least 30 percent a 120 ° C, more preferably at least 40 percent, even more preferably at least 50 percent and still more preferably at least 60 percent. As used herein, the term "mono-layer" refers to a relatively simple compound, which usually contains carbon and of low molecular weight, which can react to form a polymer when combined with ^ Same or with other molecules or similar compounds. As used herein, the term "comonomer" refers to a monomer that is copolymerized with at least one different monomer in a copolymerization reaction, the result is a copolymer. As used herein, the term "polymer" refers to the product of a polymerization reaction and includes homo-polymers and co-polymers. ? As used herein, the term "homo-polymer" as used with reference to a polymer resulting from the polymerization of a single monomer, ie a polymer that essentially consists of a single type of grouper, i.e. repeating unit. As used herein, the term "copolymer" refers to polymers formed by the reaction of polymerization of at least two different monomers. For example, the term "copolymer" includes the reaction product of copolymerization of ethylene and an α-olefin, such as 1-hexene. However, the term "copolymer" also includes for example, the copolymerization of a mixture of ethylene, propylene, 1-hexene and 1-octene. The term "copolymer" also includes random copolymers, block copolymers and graft copolymers. As used herein, the term "polyethylene" refers to homo- and copolymers of ethylene. As used herein, the term "ethylene homopolymer" identifies polymers that essentially consist of a repeating unit of ethylene. Depending on the polymerization process used, polymers with a different degree of branching and a different density can be obtained. Those characterized by a low degree of branching and showing a density greater than 0.940 g / cm3 are called HDPE, while those with a higher branching level and density of up to 0.940 g / cm3 are called LDPE. As used herein, the term "ethylene copolymer" refers to copolymers of ethylene with one or more other olefins and / or with a non-olefinic comonomer copolymerizable with ethylene, such as vinyl monomers, modified polymers thereof and the like . Specific examples include ethylene-α-olefin copolymers, ethylene-vinyl acetate copolymers, ethylene-ethyl acrylate copolymers, ethylene-butyl acrylate copolymers, ethylene-methyl acrylate copolymers, ethylene-acrylic acid copolymers. ethylene-methacrylic acid copolymers, ionomer resins, ethylene-alkyl acrylate-maleic anhydride terpolymers, etc. As used herein, terminology employing a "-" with respect to the chemical identity of a copolymer (for example "an ethylene-α-olefin copolymer"), identifies the comonomers that are copolymerized to produce the copolymer. As used herein, the phrase "heterogeneous polymer" refers to the polymerization reaction products of a relatively broad variation in molecular weight and a relatively wide variation in composition distribution, ie typical polymers prepared for example using Ziegler-Natta catalysts. conventional Heterogeneous polymers are useful in various layers of the film used in the present invention. Although there are a few exceptions (such as linear homogeneous TAFMER ™ ethylene-α-olefin copolymers produced by Mitsui Petrochemical Corporation, using Ziegler-Natta catalysts), heterogeneous polymers typically contain a relatively wide variety of chain lengths and comonomer percentages.

As used herein, the phrase "homogeneous polymer" refers to polymerization reaction products with relatively narrow molecular weight distribution and relatively narrow composition distribution. Homogeneous polymers are structurally different from heterogeneous polymers, since homogeneous polymers exhibit a relatively uniform sequence of comonomers within a chain, a mirror image of sequence distribution in all chains and similarity in length of all chains, that is, a narrower molecular weight distribution. In addition, homogeneous polymers are typically prepared using metallocene, or other single site type catalysts instead of using Ziegler Natta catalysts. More particularly, the homogeneous ethylene-α-olefin copolymers can be characterized by one or more methods known to those skilled in the art, such as molecular weight distribution (Mw / Mn) composition distribution amplitude index (CDBI = Composition). Distribution Breadth Index) and single melting point behavior and narrow melting point range. The molecular weight distribution (Mw / Mn) also known as polydispersity, can be determined by gel permeation chromatography. The homogeneous ethylene-α-olefin copolymers using this invention generally have (Mw / Mn) less than 2.7; preferably from about 1.9 to about 2.5; more preferable from about 1.9 to about 2.3. The composition distribution amplitude index (CDBI) of these homogeneous ethylene-α-olefin copolymers will generally be greater than about 70 percent. The CDBI is defined as the weight percent of the copolymer molecules having a comonomer content in 50 percent (ie, plus or minus 50%) of the average total molar comonomer content. CDBI of linear polyethylene that does not contain comonomer is defined as 100%. The Composition Distribution Amplitude Index (CDBI) is determined by the Elution Fractionation with Temperature Increase (TREF = Temperature Rising Elution Fractionation) technique. The CDBI determination clearly distinguishes the homogeneous copolymers employed in the present invention (narrow composition distribution as estimated by CDBI values generally over 70%) of commercially available VLDPEs that generally have a broad composition distribution as estimated by CDBI values, generally less than 55%. The CDBI of a copolymer is easily calculated from data obtained by techniques known in the art such as, for example, elution fractionation with temperature increase as described for example by Wild et al., J. Poly. , Sci. Phvs. Ed., Vol. 20, p. 441 (1982). Preferably, the homogeneous ethylene-α-olefin copolymers have a CDBI greater than about 70%, ie a CDBI from about 70% to about 99%. In general, the homogeneous ethylene-α-olefin copolymers in the multilayer films of the present invention also exhibit a range of relatively narrow melting points compared to "heterogeneous copolymers", ie polymers having a CDBI of less than 55%. %. Preferably, the homogeneous ethylene-α-olefin copolymers exhibit an essentially unique melting point characteristic, with a peak melting point (Tm) as determined by Differential Scanning Calorimetry (DSC) of approximately 60 °. C at approximately 110 ° C. Preferably, the homogeneous copolymer has a DSC peak Tm from about 80 ° C to about 105 ° C. As used herein, the phrase "essentially simple melting point" means that at least about 80% by weight of the material corresponds to a single peak Tm at a temperature in the range of about 60 ° C to about 110 ° C, and essentially without substantial fraction of the material reaching a peak melting point that exceeds approximately 115 ° C, as determined by DSC analysis. Reported fusion information is second melting data, ie the sample is heated at a programmed rate of 10 ° C / minute, at a temperature lower than its critical range. The sample is then reheated (2nd melting) at a programmed speed of 10 ° C / minute. The presence of higher melting peaks is detrimental to film properties such as turbidity and compromises the possibility of a significant reduction in the seal initiation temperature of the final film. A homogeneous ethylene-α-olefin copolymer can generally be prepared by the copolymerization of ethylene and any one or more α-olefins. Preferably, the α-olefin is an α-mono-olefin of 4 to 30 carbon atoms, more preferably an α-mono-olefin of 4 to 13 carbon atoms, even more preferably an α-mono-olefin of 4 to 8 carbon atoms. Still more preferably, the α-olefin comprises at least one member selected from the group consisting of butene-1, hexene-1 and octene-1, ie 1-butene, 1-hexene and 1-octene, respectively. More preferably, the α-olefin comprises octene-1 and / or a mixture of hexene-1 and butene-1. Processes for preparing and using homogeneous polymers are described in U.S. Pat. No. 5,206,075, the US patent. No. 5, 241.031 and PCT International Application WO 93/03093, each of which is hereby incorporated by reference in its entirety. Additional details regarding the production and use of homogeneous ethylene-α-olefin copolymers are described in WO-A-90/03414 and WO-A-93/03093. Still another homogeneous ethylene-α-olefin copolymer type is described in U.S. Pat. No. 5,272,236 issued to Lai, et al., And the US patent. No. 5,278,272, granted to Lai, and collaborators. As used herein, the phrase "ethylene-α-olefin copolymer" refers to heterogeneous materials such as linear low density polyethylene (LLDPE = Linear Low Density Polyethylene), linear medium density polyethylene (LMDPE = Linear Medium Density Polyethylene) and very low and ultra low density polyethylenes (VLDPE = and Very Low and ULDPE = Ultra Low Density Polyethylene); and homogenous polymers such as the homogeneous linear metallocene catalyzed ethylene-α-olefin copolymer copolymer EXACTMR resins which are obtained from Exxon Chemical Company, homogeneous linear single-site AFFINITY ™ single ethylene-α-olefin copolymer copolymer resins obtained from the Dow Chemical Company, and linear homogeneous ethylene-α-olefin copolymer copolymer resins TAFMERMR which are obtained from Mitsui Petrochemical Corporation. All of these materials generally include copolymers of ethylene with one or more comonomers selected from α-olefin of 4 to 10 carbon atoms such as butene-1, hexene-1, octene-1, etc., wherein the molecules of the copolymers comprise long chains with relatively few side chain branches or interlaced structures. The heterogeneous ethylene-α-olefin copolymer commonly known as LLDPE has a density usually in the range of about 0.915 g / cm3 to about 0.930 g / cm3, which is commonly referred to as LMDPE, has a density usually in the range of about 0.930 g / cm3 at approximately 0.945 g / cm3, while those commonly identified as VLDPE or ULDPE have a density less than about 0.915 g / cm3. As used herein, the phrase "free shrink" refers to the dimensional change in percent in a specimen of 10 cm x 10 cm of film, when subjected to select heat (ie at a certain temperature), with the quantitative determination which is carried out in accordance with ASTM D 2732, as established in the 1990 Annual Book of ASTM Standards, Vol. 08.02, pgs. 368-371, "Total Free Shrinkage" is determined by adding the percentage of free shrinkage in the machine direction with the percentage of free shrinkage in the transverse direction.For the purpose of the present invention, the "Turbidity" of film ie the percent of transmitted light that is scattered forward while passing through the sample, is measured by ASTM D 1003 (Method A). For the purpose of the present invention, the "film gloss", ie the surface reflectance of a film specimen, is measured according to ASTM D 2457-90 at an angle of 60 °. DETAILED DESCRIPTION OF THE INVENTION The film according to the present invention preferably comprises an ethylene-α-olefin copolymer. The ethylene-α-olefin copolymers which can be conveniently employed for the film according to the present invention are heterogeneous and homogeneous ethylene-α-olefin copolymers having a density of about 0.880 g / cm3 to about 0.940 g / cm3, preferably about 0.890 g / cm3 to about 0.935 g / cm3, more preferably about 0.900 g / cm3 to about 0.930 g / cm3, and even more preferably about 0.905 g / cm3 to about 0.925 g / cm3. Preferably the film will contain at least about 50% by weight of one or more ethylene-α-olefin copolymers, more preferably at least 60% by weight of one or more ethylene-α-olefin copolymers, still more preferably at least 70% by weight. % by weight of one or more ethylene-α-olefin copolymers, and even more preferably at least 80% by weight of one or more ethylene-α-olefin copolymers. Preferably, the ethylene-α-olefin copolymer of the film according to the present invention has a melt index of from about 0.3 to about 8 g / 10 minutes, more preferably from about 0.5 to about 7 g / 10 minutes, even more preferably from about 0.6 to about 6 g / 10 minutes, further more preferably from about 0.8 to about 5 g / 10 minutes (as measured by ASTM D 1238). Other polymers that can be mixed with the ethylene-α-olefin copolymers in the preferred film of the present invention are compatible polyolefins, for example homo- and co-polymers of ethylene, propylene and butene. In a more preferred embodiment, the other polymers are selected from the group consisting of ethylene homo-polymers and ethylene co-polymers, with a non-olefinic comonomer copolymerizable with ethylene such as ethylene vinyl acetate copolymers, ethylene copolymers ( met) acrylic and ethylene-alkyl (meth) acrylate copolymers, ionomers and similar polymers.

Thus, in a more preferred embodiment of the present invention, the film consists essentially of one or more ethylene-α-olefin copolymers of different densities or of a mixture thereof with one or more homo-polymers of ethylene and / or copolymers of ethylene with a non-olefinic comonomer copolymerizable with ethylene. The polymer (s) used for the film of the present invention may contain appropriate amounts of additives as is known in the art. These include slip and anti-blocking agents such as talc, waxes, silica and the like, antioxidants, fillers or fillers, pigments and dyes, crosslinking improvers, UV absorbers, antistatic agents, antifog agents or compositions, and similar additives. known by those skilled in the specialty of packaging films. The film according to the present invention is obtained by extrusion of the resin or mixture of resins through a flat matrix, rapidly cooling the mono-layer sheet that leaves the extrusion matrix by means of a cooling roller, irradiating the sheet Thus cast at a suitably selected irradiation dose, reheat this tape to the orientation temperature, and stretch the heated tape in both directions, MD and TD, by any sequential or simultaneous voltage apparatus. The bi-axially oriented thermo-shrinkable film obtained can then be optionally stabilized by an annealing step and finally cooled and wound. In a preferred embodiment, the film comprises up to 30% by weight, preferably up to about 20% by weight and even more preferably up to about 10% by weight of recycled material from the manufacture of polyolefin films. When the orientation is carried out by means of a tensioner, the edges of the film that have been clamped during the orientation required to be trimmed at the end of the process before the final bi-axially oriented thermo-shrinkable film are wound. These trimmed edges are harvested, pelletized and recycled, preferably in line. In a more preferred embodiment, therefore, the recycling material will come out of the manufacture of the same polyethylene film and the waste recycling will be carried out online. The bi-axially oriented thermo-shrinkable mono-layer film, which is prepared in accordance with the present invention can have any desired total thickness, provided that the film provides the desired properties for the particular packaging operation wherein the film is employed. , for example optical components, modules, seal resistance, etc.

In a preferred embodiment, however, the thickness of the film is less than 25 μm; typically comprises between about 6 and about 20 μm; and still more preferable between about 7 and about 19 μm. The process according to the present invention involves feeding the solid polymer beads to an extruder, wherein the polymer beads are melted and then sent in a flat extrusion die. The cast sheet obtained is preferably from about 0.2 mm to about 3 mm thick, then cooled in a cooling roll, typically with the aid of an air knife which keeps the sheet in contact with the cooling roll. Preferably, the cooling roller is partially submerged in a low temperature water bath (for example from about 5 to about 60 ° C). Alternatively, the cooling step can be carried out by using a liquid knife as described in WO-A-95/26867, wherein a continuous and substantially uniform water layer or any other cooling liquid circulates on the surface of the blade that does not contact the cooling roller. Any other known means for cooling the cast screen however can be employed. The cooled sheet is then fed through an irradiation unit, which typically comprises an irradiation tank, surrounded by shielding, the flat sheet is irradiated with high energy electrons (i.e. ionizing radiation) from an iron core transformer accelerator . The irradiation is carried out to induce entanglement. The flat sheet is preferably guided through the roller radiation tank. In this way it is possible by conveniently combining the number of rollers and the route of the path of travel within the irradiation unit, to obtain more than one exposure of the sheet to the ionizing radiation. Preferably, the sheet is irradiated at a level of from about 15 to about 150 kGy, more preferably from about 20 to about 130 kGy, even more preferably from about 25 to about 110 kGy, and still more preferably from about 30 to about 100 kGy, wherein the most preferred amount of radiation depends on the polymers employed and the final use of the film. As an example, polymers with high MFI require higher irradiation doses to obtain the desired thickness control while with low MFI polymers, low irradiation doses are sufficient. In addition, highly branched polymers are more susceptible to irradiation than linear ones and thickness control with a highly branched polymer can be achieved using a low irradiation dose. The person skilled in the art can adjust the minimum irradiation dose required to obtain the desired thickness control by one or just a few simple tests. While the irradiation requires to be carried out on the extruded cast sheet just before orientation, in order to obtain a convenient control of the final film thickness, an additional irradiation step can also be carried out after orienting in order to further extend the Thermo-sealed film window. The irradiated cast sheet is then fed to the pre-heating zone of a simultaneous or sequential flat drawing apparatus. In a sequential flat stretching apparatus, the film generally narrows first in the MD and then in the TD. MD stretching is achieved by pulling the heated sheet between sets of heated rolls with the downstream play moving at a higher speed. The TD stretch on the other hand is obtained by a tensioner frame, a machine consisting of two continuous chains in which clamps are mounted that hold the two edges of the film and transport it as the chain moves forward. The two chains gradually move part as they pull the film in the TD between them.

Conventional drawing ratios for the plane sequential orientation process are up to about 8: 1, preferably about 5: 1 to about 7: 1 in MD and up to about 12: 1, preferably from about 6: 1 to about 10 : 1, in TD. The temperature of the furnace in the preheating zone, its length and the time dedicated by the path of travel in the area (ie the frame speed) can be conveniently varied in order to bring the sheet to the desired temperature for MD orientation. In a preferred embodiment, the orientation temperature MD is constituted between about 50 ° C and about 100 ° C depending on the composition of the polyethylene film, and the temperature of the pre-heating zone is maintained between about 60 ° C and approximately 120 ° C. The longitudinally oriented film is then heated to the conveniently selected TD orientation temperature which is generally higher than the MD. In particular, suitable TD orientation temperatures in the process are drawn sequentially according to the present invention, are constituted between about 100 ° C and about 140 ° C, depending on the composition of the polyethylene film, and the temperature of the second preheating zone is therefore maintained between about 105 ° C and about 150 ° C. In the second preheating zone, the sheet is held but not yet stretched. Therefore, the resulting hot, irradiated and clamped sheet is stretched transversely by a tensioning apparatus. Alternately, the extruded mono-layer polyethylene sheet is stretched bi-axially by a simultaneous tensioning apparatus by providing both simultaneous sheet stretching in both machine directions and transverse. A simultaneous stretching of a flat sheet of continuous travel in the longitudinal and transverse directions is a technique known in the literature for many years. The patent of the U.S.A. No. US-A-2,923,966, issued in 1960, describes an apparatus for carrying out this stretched simultaneous plane. The apparatus claimed therein comprises two endless conveyors, placed on the two sides of the frame and arranged on divergent paths, the conveyors are formed of a plurality of links pivotally interconnected at their ends, to provide a loose tongue structure and transport a series. of fasteners spaced to hold the weft edges.

The use of endless loop motor systems for the simultaneous stretching of a flat sheet of continuous path, has been described later for example in US-A-3, 890, 421, and improvements thereto with particular reference to the problem of controlling synchrony, have been described in US-A-4, 825, 111, UA-A-4,853,602, and US-A-5, 051, 225. Currently, there are several commercial simultaneous biaxial film tensioners. A convenient line for simultaneous stretching with linear motor technology has been designed by Brueckner GmbH and announced as the LISIMMR line. The configuration of the tensioner can be varied depending on the desired stretching ratios. Using a tensioner with a synchronous linear motor, the stretching ratios that can be applied in general are constituted between approximately 3: 1 and approximately 10: 1 for MD stretching and between approximately 3: 1 and approximately 10: 1 for TD stretching. Preferably, however, drawing ratios of at least about 4: 1 in both directions are applied, wherein drawing ratios of at least about 5: 1 are more preferred, and drawing ratios of at least about 6: 1 are even more preferred.

In the case of a simultaneous flat drawing process, there is only one preheating zone and the temperature in the preheating zone is maintained between approximately 105 ° C and approximately 1 5 ° C, that is to say close to the orientation temperature which is generally constituted between approximately 100 ° C and approximately 140 ° C. Therefore, the resulting hot, irradiated and clamped sheet is directed to the stretching zone of the simultaneous tensioner and stretches simultaneously in both directions. In both flat drawing processes, the bi-axial drawing film is then transferred into a loosening or annealing zone, heated to a temperature of about 70 to 90 ° C. Following the annealing step, the film is transferred to a cooling zone where, in general, air, either cooled or maintained at room temperature, is used to cool the film. The temperature of the cooling zone is therefore typically constituted between about 20 and 40 ° C. At the end of the line, the edges of the film which were held by the fasteners and have not been oriented, are cut out and the bi-axially oriented thermo-shrinkable polyethylene film obtained is then wound with or without previous grooving of the weft. film at the convenient width. The thermo-shrinkable monolayer, irradiated polyethylene film thus obtained has a thickness variation of less than 20%, preferably less than 18% and more preferably less than 15%. The monolayer, biaxially oriented, thermo-shrinkable, monolayer polyethylene film obtained by the above process has a total free shrink at 120 ° C, from about 60 to about 170%, in general from about 70 to about 170% , and typically from about 80 to about 160%. When the film is obtained by a simultaneous flat drawing process, the properties of free shrinkage are more balanced in the two directions and differences of less than 15, preferably less than 10 and even more preferably less than 5 in percent shrinkage free in MD and free shrink in percent in TD are obtained. The film thus obtained when heated under restriction exhibits a shrinkage stress in both directions of at least 2.812 kg / cm2 (40 psi) and preferably at least 3.515 kg / cm2 (50 psi). The shrinkage tension is measured according to ASTM D 2838.

The film obtained can then be subjected to a corona discharge treatment to improve the print receptive characteristics of the film surface. As used herein, the phrases "corona treatment" and "corona discharge treatment" refer to subjecting the outer surfaces of the film to a corona discharge treatment, i.e. the ionization of a gas such as air in immediate proximity to a film surface, ionization initiated by high voltage that is passed through a nearby electrode, and cause oxidation and other changes to the film surface, such as surface roughness. Corona treatment of polymeric materials is described, for example, in US-A-4, 120,716. The invention is further illustrated by the following examples, which are provided for purposes of representation and are not to be construed as limiting the scope of the invention. Unless otherwise stated, all percentages, parts, etc., are given by weight. Examples 1 to 3 Three mono-layer, heat-shrinkable films of a heterogeneous ethylene-octene-1 copolymer with a density of 0.920 g / cm3 and a melt index of 1.0 g / 10 min (DowlexMR 2045 by Dow Chemical Company ) have been manufactured in a frame line with sequential tensioner, with an MD stretch ratio of 5: 1 and a TD stretching ratio of 8: 1. The thickness of the extruded sheet and the stretching ratios are adjusted to obtain thermo-shrink films with a thickness of 15 μm. In Examples 1 to 3 all the parameters of the manufacturing process except the irradiation level are exactly the same. In particular, the MD preheat zone is maintained at about 95-105 ° C, the MD stretch is carried out at about 78-88 ° C, the pre-heating for the TD stretch is maintained at about 125-135 ° C. , the TD stretch is carried out at approximately 120-125 ° C and the relaxation is maintained at approximately 80-85 ° C. The irradiation level was 0 kGy (Example 1), 27kGy (Example 2) and 54 kGy (Example 3). The film thickness is checked continuously with a beta-calibrator instrument and the values obtained have been statistically evaluated. The following Table 1 reports the average thickness, the maximum thickness, the minimum thickness, the value 2s, and the variation in thickness (%) of the films of Examples 1 to 3.

The value 2s is twice the standard deviation s, expressed in μm and the parameter used industrially to evaluate thickness control. It is calculated by the following equation 2s = 2 n? "I-l Xi2- (? N i = l Xi) 7n-1) where "n" is the number of thickness measurements that is at least ten, and "xJ 'are the current readings TABLE I Film Thickness Thickness: Thickness 2s Variation of Average Maximum Minimum Thickness Example (μm) (μm) (μm)% 1 15.5 30.9 11.9 7.0 123 22 1144..33 1166..88 11.5 2.4 37 3 15.0 16.9 14.5 0.9 16 The optical properties of the film of Example 3 were excellent: turbidity was 2.5% and brightness was 134 gu. The shrinkage properties were also excellent since the film of Example 3 showed a total free shrink of 126% at 120 ° C. Examples 4 to 6 The procedure of Examples 1 to 3 had to be repeated using however a heterogeneous ethylene-butene-1 copolymer with a density of 0.919 g / cm3 and a melt index of 1.0 g / 10 min (FlexireneMR FG 20 Polimeri Europe). While the non-irradiated film (Example 4) was not obtained due to film rupture in the middle part during transverse orientation, the results obtained with an irradiation level of 27 kGy (Example 5) and 54kGy (Example 6) are reported in the following Table II. TABLE II Film Thickness Thickness Thickness 2s Variation? of the Minimum Maximum Average In espe. Example (μm) (μm) (μm)% 4 5 15.0 19.8 13.3 2.7 43 6 15.0 15.9 14.1 0.7 12 Example 7 A mono-layer heat-shrinkable film of a heterogeneous ethylene-octene-1 copolymer with a density of 0.902 g / cm3 and a melt index of 3.0 g / 10 min (TeamexMR 1000F by DSM) has been manufactured in a frame line with sequential tensioner, with a stretched ratio MD of 5: 1 and a draw ratio TD of 8: 1, the thickness of the extruded sheet is adjusted to obtain a final thermo-shrink film with a thickness of 15 μm .

In the orientation process, the MD preheat zone is maintained at approximately 60-65 ° C, the MD stretch is carried out at approximately 50-55 ° C, preheat for the TD stretch is maintained at approximately 105-110 ° C , the TD stretch is carried out at approximately 100-105 ° C and the relax temperature is maintained at approximately 70-75 ° C. The irradiation level was 72 kGy. The results thus obtained are reported in the following Table III TABLE III Film Thickness Thickness Thickness 2s Variation of Average Maximum Minimum Thickness Example (μm) (μm) (μm)% 7 17.4 18.2 16.6 0.8 9 Examples 8 to 10 Three single-layer thermo-shrink films of a heterogeneous ethylene-octene-1 copolymer with a density of 0.911 g / cm3 and a melt index of 6.6 g / 10 min (Stamylex ™ 08-076 by DSM) are manufactured in a frame line with sequential tensioner, with an MD stretch ratio of 5: 1 and a TD stretch ratio of 8: 1 . The thickness of the extruded sheet is adjusted to obtain a final thermo-shrinkable film with a thickness of 15 μm.

The orientation temperatures were the same in both cases: the MD heating zone is maintained at approximately 70-75 ° C, the MD stretching is carried out at approximately 60-65 ° C, pre-heating for stretching TD is maintained at about 105-110 ° C, the TD stretch is carried out at about 100-105 ° C and the relax temperature is maintained at about 70-75 ° C. The irradiation level was 0 kGy (Example 8), 54 kGy (Example 9) and 72 kGy (Example 10). The results thus obtained are reported in the following Table IV. TABLE IV Film Thickness Thickness: Thickness 2s Variation of Average Maximum Minimum In thickness E Ejjeemmpplloo ((μμmm)) ((μμmm)) (μm)% 8 18.2 30.1 12.8 6.2 95 9 15.3 19.8 12.7 3.1 46 18.8 20.7 18.1 1.3 14 The optical properties of the film of Example 10 were excellent: turbidity was 1.0% and brightness was 145 gu. The shrinkage properties were also excellent since the film of Example 10 showed a total free shrinkage of 160% at 120 ° C. Examples 11 to 13 Three mono-layer, heat-shrinkable films of a homogeneous ethylene-octene-1 copolymer with a density of 0.920 g / cm3 and a melt index of 0.9 g / 10 min (EliteMR 5100 by The Dow Chemical Company) were manufactured in a frame line with sequential sensor, with an MD stretch ratio of 5: 1 and a TD stretching ratio of 8: 1. The thickness of the extruded sheet is adjusted to obtain final thermo-shrink films with a thickness of 15 μm. The cooling temperatures were the same in all three cases: the MD pre-heating zone is maintained at approximately 105-115 ° C, the MD stretching is carried out at approximately 88-98 ° C, pre-heating for the Stretched TD is maintained at approximately 135-145 ° C, stretched TD was carried out at approximately 130-135 ° C and the relax temperature is maintained at approximately 90-95 ° C. The irradiation level was 0 kGy (Example 12), and 54 kGy (Example 13). The results thus obtained are reported in the following Table V.

TABLE V Film Thickness Thickness 2s Variation of Average Maximum Minimum Thickness Example (μm) (μm) (Atm)% 11 16.0 27.8 12.] 5.8 98 12 15.6 17.1 11. S 2.2 33 13 15.4 16.8 14. E 0.8 15 The optical properties of the film of Example 13 were excellent: turbidity was 2.3% and brightness was 135 gu. The shrinkage properties were also excellent since the film of Example 13 showed a total free shrinkage of 132% at 120 ° C. Examples 14 to 16 Three mono-layer, heat-shrinkable films of a heterogeneous ethylene-octene-1 copolymer with a density of 0.910 g / cm 3 and a melt index of 2.2 g / 10 min (Stamylex ™ 08-026F by DSM ) have been manufactured in a frame line with sequential tensioner, with an MD stretch ratio of 5: 1 and a TD stretching ratio of 8: 1. The thickness of the extruded sheet is adjusted to obtain final thermo-shrink films with a thickness of 15 μm. The orientation temperatures were the same in both cases: the MD pre-heating zone is maintained at approximately 70-75 ° C, the MD stretch is carried out at approximately 60-65 ° C, pre-heating for the TD stretching is maintained at approximately 105-110 ° C, stretched TD was carried out at approximately 100-105 ° C and the temperature of Relaxation is maintained at approximately 70-75 ° C. The irradiation level was 0 kGy (Example 14), and 54 kGy (Example 15), and 72 kGy (Example 16). The results thus obtained are reported in the following Table VI. TABLE VI Film Thickness Thickness: Thickness 2s Variation of Average Maximum Minimum Thickness Example (μm) (μm) (μm)% 14 17.7 30.4 13.0 6.5 98 18.0 24.1 15.3 2.7 49 16 16.8 18.9 15.9 1.3 18 The optical properties of the film of Example 16 were excellent: turbidity was 0.9% and brightness was 145 gu. The shrinkage properties were also excellent since the film of Example 16 showed a total free shrinkage of 158% at 120 ° C. Films obtained in accordance with the present invention can be used in the packaging of food and non-food products, as is known in the art. For this purpose, they can be used in a flat form to wrap around the product to be packaged or they can be first converted into bags by conventional techniques well known to the person skilled in the art. They can also be attached or laminated to other films or sheets to obtain an improved performance packaging material. Although the present invention has been described in connection with the preferred embodiments, it will be understood that modifications and variations may be used without departing from the principles and scope of the invention, as those skilled in the art will readily understand. Accordingly, these modifications can be practiced within the scope of the following claims.

Claims (9)

1. CLAIMS 1. A polyethylene film, monolayer, irradiated, heat-shrinkable, characterized by a thickness variation of less than 20%, preferably less than 18% and even more preferably less than 15%. The film according to claim 1, characterized in that it comprises at least about 50% by weight of one or more ethylene-α-olefin copolymers, more preferably at least 60% by weight of one or more ethylene-α-copolymers olefin, even more preferably at least 70% by weight of one or more ethylene-α-olefin copolymers, and even more preferably at least 80% by weight of one or more ethylene-α-olefin copolymers. The film according to claim 2, characterized in that the ethylene-α-olefin copolymer is selected from the group consisting of heterogeneous and homogeneous ethylene-α-olefin copolymers having a density of about 0.880 g / cm3, preference of about 0.890 g / cm3 to about 0. 935 g / cm3, more preferably from about 0.900 g / cm3 to about 0.930 g / cm3, and even more preferably from about 0.905 g / cm3 to about 0.925 g / cm3. The film according to claim 3, characterized in that the ethylene-α-olefin copolymer has a melt index of from about 0.3 to about 8 g / 10 min, more preferably from about 0.5 to about 7 g / 10 min, even more preferably from about 0.6 to about 6 g / 10 min, even more preferably from about 0.8 to about 5 g / 10 min (as measured by ASTM D1238). 5. A process for manufacturing a polyethylene film, mono-layer, heat-shrinkable, having a thickness variation of less than 20%, preferably less than 18% and even more preferably less than 15%, by flat extrusion and flat orientation characterized in that before orientation, the cast monolayer polyethylene sheet is irradiated. The method according to claim 5, characterized in that the cast monolayer polyethylene sheet is irradiated at an irradiation level of about 15 to about 150 kGy, more preferably about 20 to about 130 kGy, even more preferably from about 25 to about 110 kGy, and still more preferably from about 30 to about 100 kGy. The method according to claim 6, characterized in that the cast monolayer polyethylene sheet is stretched bi-axially with an orientation ratio in the longitudinal direction greater than 4: 1, preferably greater than 4.5: 1, more preferably at least 5: 1 and an orientation ratio in the transverse direction greater than 4: 1, preferably greater than 4.5: 1, more preferably at least 5: 1. The method according to claim 7, characterized in that the stretching is carried out sequentially at a temperature for stretching MD from about 50 ° C to about 100 ° C and at a drawing temperature TD of about 100 ° C to about 140 ° C. 9. The process according to claim 7, characterized in that the stretching is carried out simultaneously at a stretching temperature from about 100 ° C to about 140 ° C.