CA2359323A1 - 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-.alpha.-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

HEAT-SHRINKABLE, IRRADIATED, POLYETHYLENE MONO-LAYER FILM
The present invention refers to a heat-shrinkable, irradiated, mono-layer polyethylene film characterized by a very low thickness variation, to a process for the manufacture thereof and to the use of said film as a packaging material.
Bi-axially oriented, heat-shrinkable, films are films that have been oriented by stretching in two perpendicular directions, typically the longitudinal or machine direction (MD) and the transverse or crosswise direction (TD), at a temperature comprised between the Tg and the melting point of the resin employed, i.e. at a temperature where the resin is not in the molten state.
1o Bi-axially oriented, heat-shrinkable, films are made by extruding the polymer from a melt into a thick sheet that is quickly quenched to prevent or delay polymer crystallization, and then oriented by stretching under temperature conditions, as indicated above, where molecular orientation of the film occurs and the film does not tear. Upon subsequent re-heating at a temperature close to the orientation temperature, the oriented, heat-shrinkable.
film will tend to shrink in seeking to recover its original dimensional state.
Bi-axially oriented, heat-shrinkable polyethylene mono-layer films are known in the literature and widely used in the market.
They are typically obtained by extruding the polymer through a rov.md die to get a tubular thick film called "tape", that is immediately and quickly quenched by means of a 2o water bath or cascade, re-heated, with or without prior irradiation, at the suitably selected orientation temperature and stretched bi-axially, while at this temperature, by the so-called "trapped bubble" technique that uses internal gas pressure to expand the diameter of the tape to form a large "bubble" and advancing the expanded tube at a faster rate than the extrusion rate so as to obtain transverse and machine directions of orientation respectively. The film is then cooled and rolled up in the cooled state so as to retain the property of heat-shrinkability.
Generally in the above process the tape is cross-linked prior to stretching in order to impart greater mechanical strength thereto and thus allow a better control of the process via an increased stability of the blown bubble.
The films obtained via the trapped bubble technique always show a poor thickness control and thickness variations of at least about 25-30 % are typically obtained.
EP-A-319,401 describes an alternative process for the manufacture of bi-axially oriented, heat-shrinkable, mono-layer films of ethylene-a-olefin copolymers.
Said process provides for the extrusion of the co-polymer through a flat die in the form of a sheet, and after a quenching step, for the heating of the sheet to a first orientation temperature and the stretching thereof in the longitudinal direction, followed by heating of the longitudinally stretched film to a second orientation temperature higher than the first one, and by the transversal stretching thereof. No irradiation step is foreseen in said method as, unlike the blown bubble process, there are no problems of process ("bubble") stability in the flat orientation process.
The films obtained by the method described in said prior art document show an extremely high thickness variation. Considering the working examples contained in EP-A-319,401, the film made from an ethylene-octene-1 co-polymer with density of 0.919 g/cm' and MI of 6 g/10' (resin A) shows a thickness variation > 25 %, while that made from an ~5 ethylene-octene-1 co-polymer with density of 0.917 g/em' and MI of 2.3 g/10' (resin B) shows a thickness variation > 33 %. These values, that might be acceptable when the film is obtained via the trapped bubble method, as the use of a rotating platform would allow the distribution of the thicknesses along the film width, are unacceptable in case of a flat orientation process where no thickness distribution can be achieved. In this latter case in fact a thickness variation as indicated would not allow the winding up of the end film into acceptable rolls.
It has now been found that it is possible to control the thickness variation in a heat-shrinkable, mono-layer polyethylene film obtained by the flat orientation process, by irradiating the sheet prior to stretching. It has been found that depending on the irradiation level it is possible to achieve a thickness variation less than 20 %, preferably less than 18 and even more preferably less than 15 %.
A first object of the present invention is therefore a heat-shrinkable, irradiated, mono-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 object of the present invention is a process for manufacturing a heat-shrinkable polyethylene mono-layer film characterised by a thickness variation of less than 20 %, preferably less than 18 % and even more preferably less than 15 %, which process s 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 temperature and stretching of the irradiated sheet.
A third object of the present invention is the use of a heat-shrinkable, polyethylene, irradiated. mono-layer film characterized by a thickness variation of less than 20 %, to preferably less than 18 % and even more preferably less than 15 %, in the packaging of food or non-food products.
DEFINITIONS
As used herein, the term "film" is used in a generic sense to include plastic web, regardless of whether it is film or sheet. Typically, films of and used in the present invention 15 have a thickness of 100 um or less, preferably they have a thickness of 80 p.m or less, more preferably a thickness of 50 p.m or less, still more preferably a thickness of 35 ~m or less, and yet, still more preferably, a thickness of 25 p.m or less.
As used herein, the phrase "thickness variation" refers to the percent value obtained by measuring the maximum and minimum thickness of the film, calculating the average 2o thickness value and applying these numbers to the following formula:
film thickness~max~ - film thickness~m;", Thickness variation (%) = x 100.
film thickness,,~e~

The maximum and minimum thicknesses are determined by taking a total of 10 thickness measurements at regular distance intervals along the entirety of the transverse direction of a film sample, recording the highest and lowest thickness values as the maximum and minimum thickness values, respectively, while the average value is determined by summing up the same lU thickness measurements and dividing the result by ten.
The thickness variation is then computed (as a percent value) using the formula above. A
thickness variation of 0 % represents a film with no measurable differences in thickness. A
s thickness variation over 25 % is unacceptable industrially while a thickness variation below 20 % is acceptable.
As used herein, the phrase "machine direction", herein abbreviated "MD", refers to a direction "along the length" of the film, i:e., in the direction of the film as the film is formed during extrusion and/or coating.
As used herein, the phrase "transverse direction", herein abbreviated "TD", refers to a direction across the film, perpendicular to the machine or longitudinal direction.
As used herein, the phrases "orientation ratio" and "stretching ratio" refer to the multiplication product of the extent to which the plastic film material is expanded in the two directions perpendicular to one another, i.e. the machine direction and the transverse direction.
15 As used herein, the phrases "heat-shrinkable," "heat-shrink," and the like, refer to the tendency of the film to shrink upon the application of heat, i.e., to contract upon being heated, such that the size of the film decreases while the film is in an unrestrained state. As used herein said term refer to films with a total free shrink (i.e., free shrink in the machine direction plus free shrink in the transverse direction), as measured by ASTM D 2732, of at least 30 percent at zo 120 °C, more preferably at least 40 percent, still more preferably, at least 50 percent, and, yet still more preferably, at least 60 percent.
As used herein, the term "monomer" refers to a relatively simple compound, usually containing carbon and of low molecular weight, which can react to form a polymer by combining with itself or with other similar molecules or compounds.
2s As used herein, the term "co-monomer" refers to a monomer that is co-polymerized with at least one different monomer in a co-polymerization reaction, the result of which is a copolymer.
As used herein, the term "polymer" refers to the product of a polymerization reaction, and is inclusive of homo-polymers, and eo-polymers.
_ As used herein, the term "homo-polymer" is used with relerence to a polymer resulting from the polymerization of a single monomer, i.e., a polymer consisting essentially of a single type of mer, i.e., repeating unit.
As used herein, the term "co-polymer" refers to polymers formed by the polymerization reaction of at least two different monomers. For example, the terns "co-polymer" includes the co-polymerization reaction product of ethylene and an a-olefin, such as 1-hexene. However.
the lern~ "co-polymer" is also inclusive of, for example, the co-polymerization of a mixture of ethylene, propylene, 1-hexene, and 1-octene. The term "co-polymer" is also inclusive of I o random co-polymers, block co-polymers, and graft co-polymers.
As used herein, the term "polyethylene" refers to ethylene homo- and co-polymers.
As used herein the term "ethylene homopolymer" identify polymers consisting essentially of an ethylene repeating unit. Depending on the polymerization process employed, polymers with a different degree of branching and a different density can be obtained. Those ~5 characterized by a low degree of branching and showing a density higher than 0.940 glcm' are called HDPF while those with a higher level of branching and a density up to 0.940 g/cm' are called LDPE.
As used herein the term "ethylene copolymer" refers to the copolymers of ethylene with one or more other olefins and/or with a non-olefinic comonomer copolymerizable with 2o ethylene, such as vinyl monomers, modified polymers thereof, and the like.
Specific examples include ethylene-a-olefin copolymers, ethylene-vinyl acetate copolymers, ethylene-ethyl acrylate copolymers, ethylene-butyl acrylate copolymers, ethylene-methyl acrylate copolymers, ethylene-acrylic acid copolymers, ethylene-methacryIic acid . copolymers, ionomer resins, ethylene-alkyl acrylate-malefic anhydride ter-polymers, ctc..
z5 As used herein, terminology employing a "-" with respect to the chemical identity of a copolymer (e.g., "an ethylene-a-olefin copolymer"), identifies the co-monomers which are co-polymerized to produce the copolymer As used herein, the phrase "heterogeneous polymer" refers to polymerization reaction wo ooiaisn rcr~Paoiooia~
products of relatively wide variation in molecular weight and relatively wide variation in composition distribution, i.e., typical polymers prepared, for example, using conventional Ziegler-Natta catalysts. Heterogeneous polymers are useful in various layers of the film used in the present invention. Although there are a few exceptions (such as TAFMERTM
linear s homogeneous ethylene-a-olefin copolymers produced by Mitsui Petrochemical Corporation, using Ziegler-Natta catalysts), heterogeneous polymers typically contain a relatively wide variety of chain lengths and co-monomer percentages.
As used herein, the phrase "homogeneous polymer" refers to polymerization reaction products of relatively narrow molecular weight distribution and relatively narrow composition 1 o distribution. Homogeneous polymers are structurally different from heterogeneous polymers, in that homogeneous polymers exhibit a relatively even sequencing of co-monomers within a chain. a mirroring of sequence distribution in all chains, and a similarity of length of all chains, i.e., a narrower molecular weight distribution. Furthermore, homogeneous polymers are typically prepared using metallocene, or other single-site type catalysts, rather than using 15 Ziegler Natta catalysts.
More particularly, homogeneous ethylene-a-olefin copolymers may be characterized by one or more methods lmown to those of skill in the art, such as molecular weight distribution (M"/M"), composition distribution breadth index (CDBI), and narrow melting point range and single melt point behavior. The molecular weight distribution (M"/M"), also known as 2o polydispersity, may be determined by gel permeation chromatography. 1'he homogeneous ethylene-a-olefin copolymers useful in this invention generally have (M"/M") of less than 2.7;
preferably from about 1.9 to about 2.5; more preferably, from about 1.9 to about 2.3. The composition distribution breadth index (CDBI) of such homogeneous ethylene-a-olefin copolymers will generally be greater than about 70 percent. The CDBI is defined as the weight 25 percent of the copolymer molecules having a co-monomer content within 50 percent (i.e., plus or minus 50%) of the median total molar co-monomer content. The CDBI of linear polyethylene, which does not contain a co-monomer, is defined to be 100%. The Composition Distribution Breadth Index (CDBI) is determined via the technique of Temperature Rising Elution Fractionation (TREF). CDBI determination clearly distinguishes the homogeneous copolymers used in the present invention (narrow composition distribution as assessed by CDBI values generally above 70%) from VLDPEs available commercially which generally have a broad composition distribution as assessed by CDBI values generally less than 55%.
s The CDBI of a copolymer is readily calculated from data obtained from techniques know,i in the ari, such as, for example, temperature rising elution fractionation as described, for example, in Wild et. al., J. Poly. Sci. Poly. Phvs Ed , Vol. 20, p.441 (1982).
Preferably, the homogeneous ethylene-a-olefin co-polymers have a CDBI greater than about 70%, i.e., a CDBI of from about 70% to about 99%. In general, the homogeneous ethylene-a--olefin co-polymers in the multi-layer films of the present invention also exhibit a relatively narrow melting point range, in comparison with "heterogeneous copolymers", i.e., polymers having a CDBI of less than 55%. Preferably, the homogeneous ethylene-a-olefin copolymers exhibit an essentially singular melting point characteristic, with a peak melting point (Tm), as determined by Differential Scanning Calorimetry (DSC), of from about 60°C to about 110°C. Preferably t5 the homogeneous copolymer has a DSC peak T", of from about 80°C to about 105°C. As used herein, the phrase "essentially single melting point" means that at least about 80%, by weight, of the material corresponds to a single T", peak at a temperature within the range of from about 60°C to about 110°C, and essentially no substantial fraction of the material has a peak melting point in excess of about 11S°C, as determined by DSC analysis. Melting information reported 2o are second melting data, i.e., the sample is heated at a programmed rate of 10°Clmin. to a temperature below its critical range. The sample is then repeated (2nd melting) at a programmed rate of 10°C/min. The presence of higher melting peaks is detrimental to film properties such as haze, and compromises the chances for meaningful reduction in the seal initiation temperature of the final film.
25 A homogeneous ethylene-a-olefin copolymer can, in general, be prepared by the co-polymerization of ethylene and any one or more a-olefins. Preferably, the a-olefin is a Ca-C,~, a-mono-olefin, more preferably, a C4-C,, a-mono-olefin, still more preferably, a C4-Cg a-mono-olefin. Still more preferably, the a-olefin comprises at least one member selected from the group consisting of butene-1, hexenc-1, and octene-1, i.e., 1-butene, 1-hexene, and 1-octene, respectively. Most preferably, the a-olefin comprises octene-1, and/or a blend of hexene-1 and butene-I.
Processes for preparing and using homogeneous polymers are disclosed in U.S.
Patent s No. 5,206,075, U.S. Patent No. 5,241,031, and PCT International Application WO 93/03093, each of which is hereby incorporated by reference thereto, in its entirety.
Further details regarding the production and use of homogeneous ethylene-a-olefin copolymers are disclosed in WO-A-90/03414, and WO-A-93/03093.
Still another genus of homogeneous ethylene-a-olefin copolymers is disclosed in U.S.
Patent No. 5,272,236, to Lai, et. al., and U.S. Patent No. 5,278,272, to Lai, et. al.
As used herein, the phrase " ethylene-a-olefin copolymer" refer to such heterogeneous materials as linear low density polyethylene (LLDPE), linear medium density polyethylene (LMDPE) and very low and ultra low density polyethylene (VLDPE and ULDPE); and homogeneous polymers such as metallocene-catalyzed FXACTT"' linear homogeneous 15 ethylene-a-olefin copolymer resins obtainable from the Exxon Chemical Company, single-site AFFINITYTM linear homogeneous ethylene-a,-olefin copolymer resins obtainable from the Dow Chemical Company, and TAFMERTM linear homogeneous ethylene-a-olefn copolymer resins obtainable from the Mitsui Petrochemical Corporation. All these materials generally include co-polymers of ethylene with one or more co-monomers selected from C4 to C,o a-20 olefin such as butene-l, hexene-l, octene-1, etc. in which the molecules of the copolymers comprise long chains with relatively few side chain branches or cross-linked structures. The heterogeneous ethylene-a-olefin co-polymer commonly known as LLDPE has a density usually in the range of from about 0.915 g/cm' to about 0.930 g/cm', that commonly known as LMDPE has a density usually in the range of from about 0.930 g/cm' to about 0.945 g/cm', 25 while those commonly identified as VLDPE or ULDPE have a density lower than about 0.915 g/cm'.
As used herein, the phrase "free shrink" refers to the percent dimensional change in a 10 cm x 1 p cm specimen of film, when subjected to selected heat (i.e., at a certain temperature), with the quantitative determination being carried out according to ASTM D
2732, as set forth in the 1990 Annual Book of ASTM Standards, Vol. 08.02, pp.368-371. "Total free shrink" is determined by summing the percent free shrink in the machine direction with the percentage of free shrink in the transverse direction.
For the purpose of the present invention, the film "haze", i.e. the percentage of transmitted light which 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", i.e. the surface reflectance of a film specimen, is measured according to ASTM D 2457 - 90 at a 60°
angle.
DETAI1;ED DESCRIPTION OF THE INVENTION
The film according to the present invention preferably comprises an ethylene-a-olefin co-polymer.
Ethylene-a-olefin copolymers that can suitably be employed for the film according to the present invention are heterogeneous and homogeneous ethylene-a-olefin copolymers having a density of from about 0.880 g/cm' to about 0.940 g/cm;, preferably from about 0.890 g/cm' to about 0.935 g/cm', more preferably from about 0.900 g/cm' to about 0.930 g/cm;, and still more preferably from about 0.905 g/cm' to about 0.925 g/cm'.
Preferably said film will contain at least about SO % by weight of one or more ethylene-a-olefin copolymers, more preferably at least 60 % by weight of one or more 2o ethylene-a-olefin copolymers, still more preferably at least 70 % by weight of one or more ethylene-a-olefin copolymers, and yet still more preferably at least 80 % by weight of one or more ethylene-a-olefin copolymers.
Preferably, the ethylene-a-olefin copolymer of the film according to the present invention 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, still 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).
Other polymers that can be blended with the ethylene-a-olefin copolymers in said preferred film of the present invention arc polyolefins compatible therewith, e.g. ethylene, propylene, and butene homo- or co-polymers.
_ In a most preferred embodiment said 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 co-polymers, ethylene-(meth)acrylic acid copolymers, ethylene-alkyl (meth}acrylate copolymers, ionomers, and the like polymers.
Thus in a most preferred embodiment of the present invention the film consists essentially of one or more ethylene-a-olefin co-polymers of different densities or of a blend thereof with one or more ethylene homo-polymers and/or ethylene co-polymers with a non-olefinic comonomer copolymerizable with ethylene.
The polymers) used for the film of the present invention may contain appropriate amounts of additives as known in the art. These include slip and anti-block agents such as talc, waxes, silica, and the like, antioxidants, fillers, pigments and dyes, cross-linking enhancers, UV
absorbers, antistatic agents, anti-fog agents or compositions, and the like additives known to ~ 5 those skilled in the art of packaging films.
The film according to the present invention is obtained by extrusion of the resin or blend of resins through a flat die, cooling quickly the mono-layer sheet exiting from the extrusion die by means of a chill roll, irradiating the cast sheet thus obtained to a suitably selected irradiation dosage, repeating this tape to the orientation temperature, and stretching the ~0 heated tape in both directions, MD and TD, by any sequential or simultaneous tenter apparatus.
The obtained bi-axially oriented heat-shrinkable film may then optionally be stabilized by an annealing step, and finally cooled and wound up.
In a preferred embodiment the film will comprise up to 30 % by weight, preferably up to about 20 % by weight and even more preferably up to about 1 U % by weight of recycle 25 material from the manufacture of polyolefin films. When orientation is carried out by means of a tenter, the film edges that have been clipped during orientation need to be trimmed off at the end of the process before the end bi-axially oriented heat-shrinkable film is wound up.
These trimmed edges are collected, pelletized and recycled, preferably in-line. In a most preferred embodiment, therefore, said recycle material will come from the manufacture of the same polyethylene film and the scrap recycle will be carried out in-line.
The bi-axially oriented, heat-shrinkable, mono-layer film prepared in accordance with the present invention can have any total thickness desired, so long as the film provides the desired properties for the particular packaging operation in which the film is used, e.g. optics, modulus, seal strength, ete. . In a most preferred embodiment however the thickness of the film is lower than 25 p.m; typically it is comprised between about 6 and about 20 p,m; and even more preferably between about 7 and about 19 Vim.
The process according to the present invention involves feeding the solid polymer beads to an extruder, where the polymer beads are melted and then forwarded into a flat extrusion die.
The obtained cast sheet, that is preferably from about 0.2 mm to about 3 mm thick, is then chilled on a chill roll, typically with the aid of an air knife that keeps the sheet in contact with the chill roll. Preferably the chill roll is partially immersed in a water bath at a low temperature (e.g. 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 where a continuous and substantially unifornl layer of water or of any other cooling liquid flows onto the surface of the sheet that does not contact the chill roll. Any other known means for cooling the cast web can however be employed.
The cooled sheet is then fed through an irradiation unit, typically comprising an ~o irradiation vault surrounded by a shielding, the flat sheet is irradiated with high energy electrons (i.e., ionizing radiation) from an iron core transformer accelerator.
Irradiation is carried out to induce cross-linking. The flat sheet is preferably guided through the irradiation vault on rolls. It is thus possible by suitably combining the number of rolls and the path of the traveling web within the irradiation unit to get more than one exposure of the sheet to the ionizing radiation.
Preferably, the sheet is irradiated to a level of from about 15 to about I50 kGy, more preferably of from about 20 to about 130 kGy, still more preferably of from about 25 to about I 10 kGy, and yet still more preferably of from about 30 to about 100 kGy, wherein the most preferred amount of radiation is dependent upon the polymers employed and the film end use.

C

As an example polymers with a high MFI requires a higher dosage of irradiation to obtain the desired thickness control while with polymers with a iow MFI, low irradiation dosages are sufficient. Furthermore highly branched polymers are more susceptible to irradiation than the more linear ones and thickness control with an highly branched polymer can be achieved using a low irradiation dosage. The person skilled in the art can set the minimum irradiation dosage required to get the desired thickness control by one or just few simple tests.
While irradiation needs to be earned out on the extruded cast sheet just before orientation, in order to get a suitable control of the end film thickness, an additional irradiation step could be carried out also after orientation in order to further broaden the film heat-sealing i o window.
fhe irradiated cast sheet is then fed to the pre-heating gone of a sequential or simultaneous flat stretching apparatus.
In a sequential flat stretching apparatus, the film is generally first stretched in the MD
and then in the TD. The MD stretching is accomplished by drawing the heated sheet between ~ s sets of heated rolls with the downstream set moving at a higher speed. The TD stretching is on the other hand obtained by means of a tenter frame, a machine that consists of two continuous chains on which are mounted clamps gripping the two edges of the film and carrying it along as the chain is driven forward. The two chains gradually move part and as they do they draw the film in the TD between them.
2o Conventional stretching ratios for the flat, sequential orientation process are up to about 8:1, preferably from 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 oven in the pre-heating cone, the length thereof and the time spent by the traveling web in said zone (i.e. the web speed) can suitably be varied in order to 25 bring the sheet up to the desired temperature for MD orientation. In a preferred embodiment the MD orientation temperature is comprised between about SO °C and about 100 °C
depending on the composition of the polyethylene film, and the temperature of the pre-heating zone is kept between about 60 °C and about 120 °C. The longitudinally oriented film is then heated to the suitably selected TD orientation temperature that is generally higher than the MD one. In particular, suitable TD orientation temperatures in the sequential flat stretching process according to the present invention are comprised between about l00 °C
and about 140 °C depending on the composition of the polyethylene film, and the temperature of said second pre-heating zone is therefore kept between about 105 °C and about 1 SO °C. In said second pre-heating zone the sheet is clipped but it is not yet stretched.
Thereafter, the resulting hot, irradiated, and clipped sheet is transversally stretched by means of a tenter apparatus.
Alternatively the extruded mono-layer polyethylene sheet is bi-axially stretched by means of a simultaneous tenter apparatus providing for a simultaneous stretching of the sheet in both the machine and the transverse directions.
Simultaneous stretching of a continuous traveling flat sheet in the longitudinal and transversal directions is a technique known in the literature since many years. tJS-A-2,923,966, issued in 1960, described an apparatus for carrying out such a simultaneous flat ~5 stretching. The apparatus there claimed comprised two endless conveyors, positioned on the two sides of the web and disposed along divergent paths, said conveyors being formed of a plurality of links pivotally interconnected at their ends to provide a lazy-tongue structure and carrying a series of spaced clips to grip the web edges.
The use of endless loop linear motor systems for the simultaneous stretching of a 2o continuous traveling flat sheet has later been described, e.g. in US-A-3,890,421, and improvements thereto, with particular reference to the problem of controlling synchronism, have been described in e.g. US-A-4,825,111, US-A-4,853,602, and US-A-5,051,225.
Actually there are various commercial simultaneous bi-axial film tenters.
A suitable line for simultaneous stretching with linear motor technology has been 25 designed by Brueckner GmbH and advertised as LISIM~ line. The configuration of the tenter can be varied depending on the stretching ratios desired. Using a synchronous linear motor tenter, the stretching ratios that may be applied are generally comprised between about 3:1 and about 10:1 for MD stretching and between about 3:1 and about 10:1 for TD

stretching. Preferably however stretching ratios of at least about 4:1 in both directions are applied, wherein stretching ratios of at least about 5:1 are more preferred, and stretching ratios of at least about 6: I are even more preferred.
In case of a simultaneous flat stretching process, there is only one pre-heating zone and the temperature in said pre-heating zone is kept between about 105 °C and about 145 °C, i.e. close to the orientation temperature that is generally comprised between about 100 °C and about 140 °C.
Thereafter, the resulting hot, irradiated, and clipped sheet is directed to the stretching zone of the simultaneous tenter, and stretched simultaneously in both directions.
In both flat stretching processes, the bi-axially stretched film is then transferred in a relaxation or annealing zone, heated to a temperature of about 70-90 °C.
Following the annealing step the film is transferred to a cooling zone where generally air, either cooled or kept at the ambient temperature, is employed to cool down the film. The temperature of said cooling zone is therefore typically comprised between about 20 and about ~5 40 °C. At the end of the line, the edges of the film, that were grasped by the clips and have not been oriented, are trimmed off and the obtained bi-axially oriented, heat-shrinkable polyethylene Flm is then wound up, with or without prior slitting of the film web to the suitable width.
The heat-shrinkable, mono-layer, irradiated polyethylene film thus obtained has a 2o thickness variation of less than 20 percent, preferably less than 18 percent, and more preferably less than 15 percent.
The bi-axially oriented, heat-shrinkable, mono-layer polyethylene film, obtained by the above process, has a total free shrink, at 120 °C, of from about 60 to about 170 percent, generally, from about 70 to about 170 percent, typically, from about 80 to about 160 percent.
25 When the film is obtained by a simultaneous flat stretching process, the free shrink properties thereof are more balanced in the two directions and differences of less than 15, preferably less than 10, and even more preferably less than 5, between the %
free shrink in MD and the % free shrink in TD are obtained.

The film thus obtained, when heated under restraint, exhibits a shrink tension in either directions of at least 40 psi, and preferably of at least 50 psi. Shrink tension is measured in accordance with ASTM D 2838.
The obtained film may then be subjected to a corona discharge treatment to improve the print receptivity 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 close proximity to a film surface, the ionization initiated by a high voltage passed through a nearby electrode, and causing oxidation and other changes to the film surface, ~ 0 such as surface roughness. Corona treatment of polymeric materials is disclosed in e.g. US-A-4,120,716.
The invention is further illustrated by the following examples, which are provided for the purpose of representation, and are not to be construed as limiting the scope of the invention.
Unless stated otherwise, all percentages, parts, etc. arc by weight.
t 5 Examples 1 to 3 Three mono-layer, heat-shrinkable films of a heterogeneous ethylene-octene-1 copolymer with a density of 0.920 g/cm' and a melt index of 1.0 g/10 min (DowlexTM 2045 by The Dow Chemical Company) have been manufactured on a sequential tenter frame line, with an MD stretching ratio of 5:1 and a TD stretching ratio of 8:1.
20 The thickness of the extruded sheet and the stretching ratios were set to obtain end heat-shrinkable films 15 ~m-thick.
In Examples 1 to 3 all the parameters of the manufacturing process but the level of irradiation were exactly the same. In particular, the MD pre-heating zone was kept at about 95-105 °C, the MD stretching was carried out at about 78-88 °C, pre-heating for the TD
zs stretching was kept at about 125-135 °C, TD stretching was carried out at about 120-125 °C
and the relaxation was kept at about 80-85 °C. The irradiation level was 0 kGy (Example 1 ), 27 kGy (Example 2) and 54 kGy (Example 3).
The film thickness was continuously monitored with a beta-gauge instrument and the l5 obtained values have then been statistically evaluated.
Following Table I reports the average thickness, the maximum thickness, the minimum thickness, the 2a value, and the thickness variation (%) of the films of Examples I
to 3.
The 26 value is the double of the standard deviation a, is expressed in pm and is the parameter employed industrially to evaluate thickness control. It is calculated by the following equation 26 = 2. n.E" ~ x;' - (E"~ x;)'-/(n-1 ) wherein "n" is the number of thickness measurements, which is at least ten, and "x;"
~ o are the actual readings.
TABLE I

Film of Ex. Av. thick.Max. thick. Min. 2a 'thick. variation thick.

(!gym) (lrm) (gym) ~ s 1 15.5 30.9 11.9 7.0 123 2 14.3 16.8 I1.5 2.4 37 3 15.0 16.9 14.5 0.9 16 The optical properties of the film of Example 3 were excellent : haze was 2.5 % and 20 gloss was 134 gu.
The shrink properties also were excellent as the film of Example 3 showed a total free shrink of 126 % at 120 °C.
Examples 4 to 6 The procedure of the Examples I to 3 has been repeated using however a 25 heterogeneous ethylene-butene-1 co-polymer with density of 0.919 g/cm' and a melt index of 1.0 g/10 min (FlexireneT"' FG 20 by Yolimeri Europa). While the non irradiated film (Example 4) could not be obtained due to a breakage of the film in the middle during transversal orientation, the results obtained with a level of irradiation of 27 kGy (Example 5) WO 00/41872 PCTlEP00/00127 and 54 kGy (Example 6) are reported in following Table II.
TABLE II
Film of Ex. Av. thick. Max. thick. Min. thick. 2a Thick. variation (gym) (p,m) (p.m) 4 _ _ _ _ -5 15.0 19.8 13.3 2.7 43 6 15.0 15.9 14.1 0.7 12 l0 Example 7 A mono-layer, heat-shrinkable film of a heterogeneous ethylene-octene-1 copolymer with a density of 0.902 g/cm' and a melt index of 3.0 g/10 min (TeamexT"' 1000F by DSM) has been manufactured on a sequential tenter frame line, with an MD stretching ratio of 5:1 and a TD stretching ratio of 8:1. The thickness of the extruded sheet was set to obtain an end heat-shrinkable film 15 ~m-thick.
In the orientation process the MD pre-heating zone was kept at about 60-65°C, the MD stretching was carried out at about 50-55 °C, pre-heating for the TD
stretching was kept at about 105-110 °C, TD stretching was carried out at about 100-105 °C and the relaxation temperature was kept at about 70-75 °C. The irradiation level was 72 kGv.
2o The results thus obtained are reported in following Table I1I.

Fihn of Ex. Av. thick. Max. thick. Min. thick. 2a Thick. variation %
(gym) (pm) {p,m) 7 17.4 18.2 16.6 0.8 9 Examples 8 to 10 Three mono-layer, heat-shrinkable films of a heterogeneous ethylene-octene-1 WO 00/41872 PCTlEP00/00127 copolymer with a density of 0.911 g/cm' and a melt index of 6.6 g/10 min (StamylcxT"' 08-076 by DSM) have been manufactured on a sequential teeter frame line, with an MD
stretching ratio of 5:1 and a TD stretching ratio of 8:1. The thickness of the extruded sheet was set to obtain an end heat-shrinkable films 15 p.m-thick.
s The orientation temperatures were the same in both cases : the MD pre-heating zone was kept at about 70-7s °C, the MD stretching was carried out at about 60-65 °C, pre-heating for the TD stretching was kept at about 105-110 °C, TD stretching was carried out at about 100-105 °C and the relaxation temperature was kept at about 70-75 °C. The irradiation level was 0 kGy (Example 8), 54 kGy (Example 9) and 72 kGy (Example 10).
io The results thus obtained are reported in following Table IV.

Pilm of Ex. Av. thick.Max. thick. Min, 2a Thick. variation thick.

(>anl) (Eim) (Irm) ~ s 8 18.2 30.1 12.8 6.2 95 9 15.3 19.8 12.7 3.1 46 IO 18.8 20.7 18.1 1.3 14 The optical properties of the film of Example 10 were excellent : haze was 1.0 % and 2o gloss was 145 gu.
The shrink properties also were excellent as the film of Example 10 showed a total free shrink of 160 % at 120 °C.
Exam,~les I 1 to 13 Three mono-layer, heat-shrinkable films of a homogeneous ethylene-octene- I
25 copolymer with a density of 0.920 glem~' and a melt index of 0.9 gll0 min (EliteT"' S 100 by The Dow Chemical Company) have been manufactured on a sequential teeter frame line, with an MD stretching ratio of S:1 and a TD stretching ratio of 8:1. The thickness of the extruded sheet was set to obtain an end heat-shrinkable films 15 p.m-thick.

The orientation temperatures were the same in all the three cases : the MD pre-- heating zone was kept at about 105-1 15 °C, the MD stretching was carried out at about 88-98 °C, pre-heating for the TD stretching was kept at about 135-145 °C, TD stretching was carried out at about 130-135 °C and the relaxation temperature was kept at about 90-95 °C.
The irradiation level was 0 kGy (Example 11 ), 27 kGy (Example 12), and 54 kGy (Example 13).
The results thus obtained are reported in following Table V.
TABLE V
Film of Ex. Av. Max. thick.Min. thick.2a Thick. variation thick.

(!gym) (wm) (N~m) 11 16.0 27.8 12.1 5.8 98 12 15.G 17.1 11.9 2.2 33 13 15.4 16.8 14.5 0.8 l5 The optical properties of the film of Example 13 were excellent : haze was 2.
~ % and gloss was 135 gu.
The shrink properties also were excellent as the film of Example 13 showed a total free shrink of 132 % at 120 °C.
2o Examples 14 to 16 Three mono-layer, heat-shrinkable films of a heterogeneous ethylene-octene-1 copolymer with a density of 0.910 g/em' and a melt index of 2.2 g/10 min (StamylexT"' 08-026F by DSM) have been manufactured on a sequential tenter frame line, with an MD
stretching ratio of 5:1 and a TD stretching ratio of 8:1. The thickness of the extruded sheet was set to obtain an end heat-shrinkable films 15 pm-thick.
'fhe orientation temperatures were the same in both cases : the MD pre-heating zone was kept at about 70-75 °C, the MD stretching was carried out at about 60-65 °C, pre-heating for the TD stretching was kept at about 105-110 °C, TD stretching was carried out at about CA

100-105 °C and the relaxation temperature was kept at about 70-75 °C. The irradiation level was 0 kGy (Example 14), 54 kGy (Example 15), and 72 kGy (Example 16).
The results thus obtained are reported in following Table VI.
TABLE VI

Film of Ex. Av. thick.Max. thick. Min. 2a Thick. variation thick.

(lam) (N~m) (!gym) 14 17.7 30.4 13.0 6.5 98 18.U 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 : haze was 0.9 % and doss was 145 gu.
The shrink properties also were excellent as the film of Example 16 showed a total 15 free shrink of 158 % at 120 °C.
The films obtained according to the present invention can be used in the packaging of food and not food products as known in the art. To this purpose they can be used in the flat form to be wrapped up around the product to be packaged or they may be first converted into bags or pouches by conventional techniques well known to the person skilled in the art. They 2o can also be coupled or laminated to other films or sheets to obtain a packaging material of improved performance.
Although the present invention has been described in connection with the preferred embodiments, it is to be understood that modifications and variations may be utilized without departing from the principles and scope of the invention, as those skilled in the art will readily understand. Accordingly, such modifications may be practiced within the scope of the following claims.

Claims (9)

1 . A heat-shrinkable, irradiated, mono-layer, polyethylene film characterized by a thickness variation of less than 20 %, preferably less than 18 % and even more preferably less than 15 %.

2. The film of claim 1 which comprises at least about 50 % by weight of one or more ethylene-.alpha.-olefin copolymers, more preferably at least 60 % by weight of one or more ethylene-.alpha.-olefin copolymers, still more preferably at least 70 % by weight of one or more ethylene-.alpha.-olefin copolymers, and yet still more preferably at least 80 % by weight of one or more ethylene-.alpha.-olefin copolymers.

3. The film of claim 2 wherein the ethylene-.alpha.-olefin co-polymer is selected from the group consisting of heterogeneous and homogeneous ethylene-.alpha.-olefin copolymers having a density of from about 0.880 g/cm3 to about 0.940 g/cm3, preferably from 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 still more preferably from about 0.905 g/cm3 to about 0.925 g/cm3.

4. The film of claim 3 wherein the ethylene-.alpha.-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, still 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 of manufacturing a heat-shrinkable, mono-layer, polyethylene film 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 mono-layer polyethylene sheet is irradiated.

6. The process of claim 5 wherein the cast mono-layer polyethylene sheet is irradiated to an irradiation level of from about 15 to about 150 kGy, more preferably of from about 20 to about 130 kGy~ still more preferably of from about 25 to about 110 kGy, and yet still more preferably of from about 30 to about 100 kGy..

7. The process of claim 6 wherein the cast mono-layer polyethylene sheet is bi-axially stretched with an orientation ratio in the longitudinal direction higher than 4:1, preferably higher than 4.5:1, more preferably of at least 5:1 and an orientation ratio in the transverse direction higher than 4:1, preferably higher than 4.5:1, more preferably of at least 5:1.

8. The process of claim 7 wherein the stretching is carried out sequentially at a temperature for the MD stretching of from about 50 °C to about 100 °C and at a temperature for the TD stretching of from about 100 °C to about 140 °C.

9. The process of claim 7 wherein the stretching is carried out simultaneously at a stretching temperature of from about 100 °C to about 140 °C.