JP2002534297A - Heat shrinkable, irradiated polyethylene monolayer film - Google Patents

Abstract

  (57) [Summary] A method for producing a heat shrinkable polyethylene monolayer film having a thickness variation of less than 20%, preferably less than 18%, more preferably less than 15%, the method comprising passing the film through a flat die. A method comprising: extruding a resin, quenching the extruded cast sheet, irradiating the sheet, reheating the irradiated sheet to a properly selected orientation temperature, and stretching. The polyethylene preferably contains an ethylene-α-olefin copolymer. Heat-shrinkable, polyethylene monolayer, irradiated films having a thickness variation of less than 20%, preferably less than 18%, more preferably less than 15% are also claimed.

Description

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to heat-shrinkable, irradiated, single-layer polyethylene films characterized by very low thickness variations, to a process for the production of these films, and to the use of said films as packaging material.

[0002] Biaxially oriented, heat-shrinkable films are used in two perpendicular directions, typically in the machine or machine direction (MD), and in the transverse or transverse direction (TD). and T _g of the resin, at a temperature, i.e. the resin is not molten temperature comprised between the melting point, a film which is oriented by stretching.

[0003] Biaxially oriented, heat-shrinkable films extrude the polymer from the melt into a thick sheet, which rapidly quench quenches to prevent or slow down the crystallization of the polymer, It is produced by stretching under a temperature condition in which molecular orientation of the film occurs and the film does not tear. Thereafter, upon reheating at a temperature close to the orientation temperature, the oriented heat-shrinkable film tends to shrink in an effort to restore its original dimensional state.

[0004] Biaxially oriented, heat shrinkable polyethylene monolayer films are known in the literature and are widely commercially available.

[0005] The films are typically extruded through a circular die to give a thick, tubular film called a "tape", which is immediately and quickly quenched by a water bath, or water cascade, With or without prior irradiation treatment, it is reheated and biaxially stretched at a properly selected orientation temperature, at which point the internal gas pressure is used to expand the tape diameter and increase the size of the large "
A so-called “trapped bubble” that forms a “bubble”
ble) "technique, and by advancing the expanded tube at a speed higher than the extrusion speed so as to obtain the transverse and machine direction orientation, respectively. The film is then cooled and wound up in a cooled state so as to retain heat shrinkable properties. In general, in the above method, the tape is crosslinked before stretching to give the tape greater mechanical strength and thus better control of the process by increasing the stability of the blown and expanded foam. Is done.

[0006] Films obtained by trapped bubble technology always show poor thickness controllability,
Thickness variations of at least about 25-30% are typically obtained.

[0007] EP-A-319401 describes an alternative method for the production of biaxially oriented, heat-shrinkable monolayer films of ethylene-α-olefin copolymers. The method comprises extruding the copolymer in the form of a sheet through a flat die and, after a quenching step, heating the sheet to a first orientation temperature and stretching it longitudinally, Heating the vertically stretched film to a second orientation temperature higher than the first temperature and stretching it transversely. Unlike the blown molding method, the flat alignment method requires a step (
The irradiation step is not foreseen in the method, since there is no "foam") stability problem.

[0008] The films obtained by the methods described in the above-mentioned prior art documents show very high thickness variations. Looking at the examples contained in EP-A-319401, a film made from an ethylene-octene-1 copolymer (resin A) having a density of 0.919 g / cm ³ and a melt index (MI) of 6 g / 10 min. Shows a thickness variation> 25%, and a film made from an ethylene-octene-1 copolymer (resin B) having a density of 0.917 g / cm ³ and an MI of 2.3 g / 10 min has a thickness variation of > 33%. If the film is obtained by the capture bubble method,
These values may be acceptable, as the thickness can be distributed along the width of the film by the use of a rotating platform, but in the case of a flat orientation method where no thickness distribution can be achieved, these values are Not acceptable. In this latter case, in fact, the thickness variation as shown would not allow the final film to be wound up as an acceptable roll.

[0009] It has now been found that irradiation of the sheet before stretching can control thickness variations in the heat shrinkable single-layer polyethylene film obtained by the flat orientation method. It has been found that irradiation levels can achieve a thickness variation of less than 20%, preferably less than 18%, more preferably less than 15%.

Accordingly, a first object of the present invention is to provide a heat-shrinkable, irradiated monolayer characterized by a thickness variation of less than 20%, preferably less than 18%, more preferably less than 15%. It is a polyethylene film.

A second object of the invention is to produce a heat-shrinkable, polyethylene monolayer film characterized by a thickness variation of less than 20%, preferably less than 18%, more preferably less than 15%. The method involves extruding a film resin through a flat die, quenching the extruded cast sheets, irradiating them, and orienting the irradiated sheets to a properly selected orientation. Reheating to temperature and stretching.

A third object of the present invention is to provide a heat-shrinkable, irradiated, monolayer of polyethylene characterized by a thickness variation of less than 20%, preferably less than 18%, more preferably less than 15%. The use of the film in packaging food or non-food products.

Definitions As used herein, the term “film” is used in a generic sense to include a plastic web, whether it is a film or a sheet. Typically, the films of and used in the present invention have a thickness of 100 μm or less, and they preferably have a thickness of 80 μm or less, more preferably 50 μm or less, even more preferably 35 μm or less. Preferably 25
It has a thickness of less than μm.

As used herein, the phrase “thickness variation” refers to measuring the maximum and minimum thickness of a film, calculating an average thickness value, and converting these numbers to the following formula:

[0015]

(Equation 1) Means the percentage value obtained by applying

The maximum and minimum thicknesses are taken over ten thickness measurements at regular distance intervals along the entire lateral direction of the film sample, and the highest and lowest thicknesses are taken as the thickness, respectively. And the average is determined by summing the same ten thickness measurements and dividing the result by ten. The thickness variation is then calculated (as a percentage) using the above formula.
A thickness variation of 0% represents a film with no measurable difference in thickness. 25
Thickness variations greater than 20% are unacceptably industrially acceptable, but thickness variations less than 20% are acceptable.

As used herein, the phrase “machine direction” is used herein to refer to “MD
"Refers to the direction" along the length "of the film, i.e., in the direction of the film as it is formed during extrusion and / or coating.

As used herein, the term “lateral” refers to “TD” herein.
Abbreviated and refers to the direction across the film, perpendicular to the machine or machine direction.

As used herein, the terms “orientation ratio” and “stretch ratio” refer to the product of multiples of the extent to which a plastic film material extends in two mutually perpendicular directions, the machine direction and the transverse direction. Say.

As used herein, the terms “heat-shrinkable,” “heat-shrinkable,” and the like, refer to a film that shrinks when heat is applied, ie, when heated, when the film is in an unconstrained state. The tendency of the film to shrink so that the size of the film decreases. The term as used herein refers to at least 30%, more preferably at least 40% at 120 ° C. as measured by ASTM D2732.
More preferably at least 50%, more preferably at least 60%, of a film having a total free shrinkage (i.e. free shrinkage in the machine direction plus free shrinkage in the transverse direction).

[0021] As used herein, the term "monomer" usually contains carbon and is of low molecular weight and reacts to bind itself or with other similar molecules or compounds to form a polymer. Refers to relatively simple compounds that can be formed.

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 of which is:
It is a copolymer.

As used herein, the term “polymer” refers to the product of a polymerization reaction and includes homopolymers and copolymers.

As used herein, the term “homopolymer” is used in reference to a polymer resulting from the polymerization of a single monomer, ie, a polymer consisting essentially of a single type of mer, ie, a repeating unit. Is done.

As used herein, the term “copolymer” refers to a polymer formed by a polymerization reaction of at least two different monomers. For example, the term “copolymer” includes the copolymerization reaction product of ethylene and an α-olefin such as 1-hexene. However, the term "copolymer" also encompasses, for example, copolymerization of a mixture of ethylene, propylene, 1-hexene, and 1-octene. The term `` copolymer ''
Also encompasses random copolymers, block copolymers, and graft copolymers.

As used herein, the term “polyethylene” refers to ethylene homopolymers and copolymers.

As used herein, the term “ethylene homopolymer” specifies a polymer consisting essentially of ethylene repeating units. Depending on the polymerization method used, polymers with different degrees of branching and different densities can be obtained. Those characterized by a low degree of branching and exhibiting densities in excess of 0.940 g / cm ³ are called high density polyethylene (HDPE) and also have a higher level of branching and 0. Those having a density of up to 940 g / cm ³ are called low density polyethylene (LDPE).

As used herein, the term “ethylene copolymer” refers to one or more other olefins and / or a non-olefinic comonomer copolymerizable with ethylene, such as a vinyl monomer. , Ethylene copolymers and their modified polymers. Specific examples include ethylene-α-olefin copolymer, ethylene-vinyl acetate copolymer, ethylene-ethyl acrylate copolymer, ethylene-butyl acrylate copolymer, ethylene-methyl acrylate copolymer, Ethylene-acrylic acid copolymer, ethylene-methacrylic acid copolymer, ionomer resin, ethylene-alkyl acrylate-maleic anhydride terpolymer and the like are included.

As used herein, the nomenclature using “-” for the chemical identification of a copolymer (eg, “ethylene-α-olefin copolymer”) is used to refer to a copolymer being a copolymer. The comonomer produced is specified.

As used herein, the phrase “heterogeneous polymer” refers to a polymerization reaction product that varies relatively widely in molecular weight and also varies widely in composition distribution.
That is, for example, it refers to a typical polymer prepared using a conventional Ziegler-Natta catalyst. Heterogeneous polymers are useful in the various film layers used in the present invention. With a few exceptions (TAFMER ™ linear homogeneous ethylene-α produced by Mitsui Petrochemical Company using Ziegler-Natta catalysts)
-Olefin copolymers), heterogeneous polymers typically contain a relatively wide variety of chain lengths, and comonomer percentages.

As used herein, the phrase “homogeneous polymer” refers to a polymerization reaction product having a relatively narrow molecular weight distribution and a relatively narrow composition distribution. A homogeneous polymer,
Homogeneous polymers are unsuitable in that they have a relatively uniform alignment of the comonomers within the chains, a mirror of the alignment distribution in all chains, and a similarity in all chain lengths, i.e., a narrow molecular weight distribution. It is structurally different from a homogeneous polymer. further,
Homogeneous polymers are typically prepared using metallocene or other single-site catalysts instead of using Ziegler-Natta catalysts.

More specifically, a homogeneous ethylene-α-olefin copolymer has a molecular weight distribution (
M _w / M _n ), composition distribution breadth index (CDBI), and narrow melting point range and single melting point behavior can be characterized by one or more methods known to those skilled in the art. The molecular weight distribution ( _Mw / _Mn ) is also known as the polydispersity,
It can be measured by gel permeation chromatography. Homogeneous ethylene-α-olefin copolymers useful in the present invention are typically less than 2.7, preferably from about 1.9 to about 2.5, more preferably from about 1.9 to about 2.3 (M _w / _Mn ). The composition distribution breadth index (CDBI) of such a homogeneous ethylene-α-olefin copolymer will typically be greater than about 70 percent. CDBI is defined as the weight percent of copolymer molecules having a comonomer content within 50 percent (ie, plus or minus 50%) with respect to the median total comonomer molar content. The CDBI of linear polyethylene without comonomer is defined as being 100%. The composition distribution width index (CDBI) is calculated by temperature-elution fractionation (TRE).
It is measured by the technique of F). By CDBI measurement, a homogeneous copolymer (narrow composition distribution, typically greater than 70%, as assessed by CDBI value) used in the present invention generally has a broad composition distribution, typically less than 55%, as assessed by CDBI value. Is clearly distinguished from commercial very low density polyethylene (VLDPE) having The CDBI of the copolymer is described, for example, in Wild et al. , J. et al. Poly. S
ci. Poly. Phys. Ed. , Vol. 20, p. 441 (1982), and are readily calculated from data obtained from techniques known in the art, such as, for example, elevated temperature elution fractionation. A homogeneous ethylene-α-olefin copolymer is
It is preferred to have more than about 70% CDBI, ie, about 70% to about 99% CDBI. In general, the uniform ethylene-α-
Olefin copolymers may also be referred to as "heterogeneous copolymers", that is, less than 55% CD.
It shows a relatively narrow melting range compared to polymers with BI. Homogeneous ethylene
The α-olefin copolymer can exhibit essentially a single melting point characteristic with a peak melting point (T _m ) of about 60 ° C. to about 110 ° C. as measured by differential scanning calorimetry (DSC). preferable. Homogeneous copolymer is preferably from about 80 ° C. has a DSC peak _{T m} of a about 105 ° C.. As used herein, the phrase “essentially a single melting point” means that at least about 80% by weight of a material has a single point at a temperature in the range of about 60 ° C. to about 110 ° C., as determined by DSC analysis. This corresponds to one _Tm peak, meaning that a substantial portion of the material has essentially no peak melting point above about 115 ° C. The reported melting information is secondary melting data, ie, the sample is heated at a set rate of 10 ° C./min to a temperature below its critical range. Next, the sample is reheated (secondary melting) at a set rate of 10 ° C./min. The presence of higher melting peaks is detrimental to film properties, such as haze, and impairs the opportunity to significantly lower the onset temperature of sealing of the final film.

A homogeneous ethylene-α-olefin copolymer generally comprises ethylene and any one
It can be prepared by copolymerization with one or more α-olefins. α- olefin _is preferably a α- monoolefins _C 4 ~C _20, C 4 ~
More preferably α- monoolefin C _12, more preferably a α- monoolefins C 4 -C _8. The α-olefins are butene-1, hexene-1, and octene-1, ie, 1-butene, 1-hexene, respectively.
It is even more preferred to include at least one member selected from the group consisting of: and 1-octene. Most preferably, the α-olefin comprises octene-1 and / or a blend of hexene-1 and butene-1.

Methods for preparing and using homogeneous polymers are described in US Pat. No. 5,206,075,
U.S. Patent No. 5,241,031, and Patent Application Treaty (PCT) International Application WO93
No./03093, each patent is hereby incorporated by reference in its entirety. Further details regarding the production and use of homogeneous ethylene-α-olefin copolymers can be found in WO-A-90 / 03414, and WO-A-93 / 0309.
3 is disclosed.

Still other types of homogeneous ethylene-α-olefin copolymers are disclosed in US Pat. No. 5,272,236 to Lai et al. And US Pat. No. 5,278,272 to Lai et al.

As used herein, the phrase “ethylene-α-olefin copolymer” is
, Linear low density polyethylene (LLDPE), linear medium density polyethylene (LMDP)
E) and very low and very low density polyethylene (VLDPE and ULDPE
) And Exxon Chemical Company
Metallocene catalyzed EXACT ™ linear homogeneous ethylene-α-
Olefin copolymer resin, commercially available from Dow Chemical Company
AFFINITY ™ Linear Uniform Ethylene-α-O
Refin copolymer resin and TAFMER ™ available from Mitsui Petrochemical Company
It refers to a uniform polymer such as a linear homogeneous ethylene-α-olefin copolymer resin.
All these materials generally have butene-1, hexene-1, and octene-1
Such as C₄~ C₁₀One or more comonomers selected from α-olefins
And ethylene copolymers, of which the copolymer molecules are relatively small
Includes long molecular chains with branched or cross-linked side chains. Generally as LLDPE
Known heterogeneous ethylene-α-olefin copolymers typically contain about 0.915 g /
cm³From about 0.930g / cm³Having a density in the range of LMDPE
Is usually about 0.930 g / cm³From about 0.945g / cm ³ , But generally identified as VLDPE or ULDPE
What is done is about 0.915 g / cm³Has a density of less than

As used herein, the phrase “free shrink” refers to the percent dimensional change in a 10 cm × 10 cm specimen of a film when subjected to a selected heating (ie, at a certain temperature), 1990 Annual Book o
f ASTM Standards, Vol. 08.02, pp. 368-37
Quantitative measurements are made according to ASTM D2732 described in 1. "Total free shrinkage" is determined by summing the percent free shrink in the machine direction with the percent free shrink in the transverse direction.

In the present invention, the “haze” of the film, ie, the percentage of transmitted light that is forward scattered while passing through the sample, is measured according to ASTM D1003 (Method A).
Is measured by

In the present invention, the “gloss” of the film, ie, the surface reflectance of the film specimen, is measured according to ASTM D2457-90 at a 60 ° angle.

DETAILED DESCRIPTION OF THE INVENTION The film according to the present invention preferably comprises an ethylene-α-olefin copolymer.

The ethylene-α-olefin copolymer that can be suitably used in the film according to the present invention comprises from about 0.880 g / cm ³ to about 0.940 g / cm ³ , preferably about 0.890 g / cm 3 ³ to about 0.935 g / cm ³ , more preferably about 0
. 900 g / cm ³ to about 0.930 g / cm ^3, more preferably from about 0.90
It is a heterogeneous and uniform ethylene-α-olefin copolymer having a density from 5 g / cm ³ to about 0.925 g / cm ³ .

The film preferably 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-olefin copolymers And more preferably at least 7
It contains 0% by weight of one or more ethylene-α-olefin copolymers, more preferably at least 80% by weight of one or more ethylene-α-olefin copolymers.

The ethylene-α-olefin copolymer of the film according to the present invention preferably has a content of about 0.3 to about 8 g / 10 min, more preferably about 0.5 to about 7 g / 10 min, even more preferably about 0 to about 7 g / 10 min. 0.6 to about 6 g / 10 min, more preferably about 0.8 to about 5 g
It has a melt index of / 10 minutes (as measured by ASTM D1238).

Other polymers that can be blended with the ethylene-α-olefin copolymer in the preferred films of the present invention are polyolefins that are compatible therewith, for example,
Ethylene, propylene, and butene homopolymers or copolymers.

In a most preferred embodiment, the other polymer is an ethylene homopolymer, an ethylene-vinyl acetate copolymer, an ethylene- (meth) acrylic acid copolymer,
It is selected from the group consisting of ethylene copolymers with non-olefinic comonomers copolymerizable with ethylene, such as ethylene-alkyl (meth) acrylate copolymers, ionomers, and other similar polymers.

Thus, in a most preferred embodiment of the invention, the film is essentially
One or more ethylene-α-olefin copolymers having different densities, or ethylene copolymers having one or more ethylene homopolymers and / or non-olefinic comonomers copolymerizable with ethylene Consisting of a blend with coalescing.

The polymers used in the films of the present invention can contain any suitable amount of additives known in the art. These additives include talc, wax, slip agents and antiblocking agents such as silica, antioxidants, fillers, pigments and dyes,
Includes crosslinking accelerators, UV absorbers, antistatic agents, anti-fogging agents or compositions, and similar additives known to those skilled in the packaging film art.

The film according to the present invention was obtained by extruding a resin, or a blend of resins, through a flat die, and rapidly cooling the single-layer sheet exiting the extrusion die by means of chill rolls. Irradiating the cast sheet with a properly selected radiation dose, reheating the tape to the orientation temperature, and heating the heated tape in any direction, MD and T by any sequential or simultaneous tenter device.
It is obtained by stretching to D. The resulting biaxially oriented, heat-shrinkable film is then optionally stabilized by an annealing step, and finally cooled and wound up.

In a preferred embodiment, the film comprises up to 30%, preferably up to about 20%, more preferably up to about 10% by weight of recycled material from polyolefin film production. If the orientation is carried out by a tenter, the film ends that have been clipped during the orientation must be trimmed off at the end of the process, before winding up the final biaxially oriented, heat-shrinkable film. . These trimmed ends are collected, granulated and preferably recycled in the line. Thus, in a most preferred embodiment, the recycled material is obtained from the same polyethylene film production. Moreover, the recirculation of the scrap is performed in the line.

The biaxially oriented, heat-shrinkable monolayer film prepared according to the present invention may have the properties desired for the particular packaging operation in which the film is to be used, such as optical properties, modulus, sealing. It can have any desired overall thickness as long as the film can provide strength and the like. However, in the most preferred embodiments, the thickness of the film is less than 25 μm, and the thickness of the film is typically between about 6 and about 20 μm.
μm, more preferably between about 7 and about 19 μm.

The method according to the invention comprises feeding solid polymer beads to an extruder, where the polymer beads are melted and then advanced into a flat extrusion die. The resulting cast sheet preferably has a thickness of about 0.2 mm to about 3 mm, and is then typically cooled on a chill roll with the aid of an air knife holding the sheet in contact with the chill roll. The chill roll is preferably partially immersed in a cold (eg, about 5 to about 60 ° C.) water bath. Alternatively, WO-A-95 / 26
The cooling step can be performed by using a liquid knife described in 867, in which a continuous and substantially uniform layer of water or any other layer of cooling liquid flows over the sheet surface, The layer does not contact the chill roll. However, any other known means can be used to cool the cast web.

The cooled sheet is then fed through an illuminator, typically with an illuminated arch chamber surrounded by a shield, and the flat sheet is fed with high energy electrons (I) from an iron core transformer accelerator. That is, it is irradiated with ionizing radiation. Irradiation is performed to induce crosslinking. Preferably, the flat sheet is guided by being passed through an irradiation arch chamber on a roll. Thus, by properly combining the number of rolls in the irradiation device and the path of the moving web, it is possible to expose the sheet to ionizing radiation more than once.

The sheet is preferably irradiated to a level of about 15 to about 150 kGy (kilo grey), more preferably about 20 to about 130 kGy, even more preferably about 25 to about 110 kGy, more preferably about 30 to about 100 kGy. And
The most preferred radiation dose then depends on the polymer used and the end use of the film. As an example, a polymer with a high melt flow index (MFI) requires a higher dose to achieve the desired thickness control, whereas for a polymer with a low MFI, a lower dose is sufficient. In addition, highly branched polymers are more susceptible to irradiation than more linear polymers,
Using low doses, highly branched polymer thickness control can be achieved. A person skilled in the art can set the minimum dose required to obtain the desired thickness control with one or only a few simple tests.

For proper control of the final film thickness, the irradiation needs to be performed on the extruded cast sheet just before orientation, but in order to further widen the window for heat sealing the film, An additional irradiation step could be performed after orientation.

The irradiated cast sheet is then fed to a preheating zone of a sequential or simultaneous flat stretching apparatus.

In a sequential flat stretching apparatus, the film is usually stretched first in the MD and then in the TD. MD stretching is achieved by moving the heated sheet between several sets of heated rolls, moving the downstream set rolls at a higher speed and pulling. On the other hand, TD stretching is obtained by a tenter frame, and when a machine consisting of two continuous chains with clamps grasps both ends of the film and the chain is driven forward,
Convey both ends along it. The two chains move progressively apart, pulling the film to the TD between them as they separate.

Conventional draw ratios for flat sequential orientation methods are up to about 8: 1 in the MD, preferably about 5: 1 to about 7: 1, and up to about 12: 1 in the TD, preferably about 6: 1 to about 10: 1.

In order to bring the sheet to the desired temperature for MD orientation, the oven temperature in the preheating zone, the length of the preheating zone, and the time spent by the web moving in said zone (ie web speed) , Can be changed appropriately. In a preferred embodiment, the MD orientation temperature depends on the composition of the polyethylene film,
Included between about 50 ° C and about 100 ° C, and the temperature of the preheat zone is maintained between about 60 ° C and about 120 ° C. The vertically oriented film is then heated to a properly selected TD orientation temperature, which is generally higher than the MD orientation temperature. In particular,
The proper TD orientation temperature in the sequential flat stretching method according to the present invention is included between about 100 ° C. and about 140 ° C. depending on the composition of the polyethylene film, and thus the temperature of the second preheating zone is about 105 ° C. and about 150 ° C. Kept between ° C. In the second preheat zone, the sheet is clipped but not stretched yet. Thereafter, the resulting hot, irradiated, and clipped sheet is stretched laterally by a tenter device.

[0059] Alternatively, the extruded single-layer polyethylene sheet is biaxially stretched by a simultaneous tentering apparatus to effect simultaneous stretching of the sheet in both the machine and transverse directions.

Stretching a continuously moving flat sheet simultaneously in the machine and transverse directions is a technique that has been known in the literature for many years. US Patent 2 issued in 1960
No. 923966 described an apparatus for performing such simultaneous flat stretching. The claimed device comprises two endless conveyors located on both sides of the web and arranged along a diverging path, said conveyors being pivotally interconnected at the ends to provide a telescopic arm structure. And a series of clips attached to a conveyor at intervals to grip the end of the web.

Thereafter, the use of an endless loop linear motor system to simultaneously stretch continuously moving flat sheets has been described, for example, in US Pat. No. 3,890,421.
Improvements thereto, with particular reference to the issue of control synchronization, are described in, for example, U.S. Pat. No. 4,825,111, U.S. Pat. No. 4,853,602, and U.S. Pat. No. 5,051,225.

At the present time, there are a variety of commercially available simultaneous biaxial film tenters.

The appropriate line for simultaneous stretching with linear motor technology is Brueckner
Designed by GmbH and advertised as the LISIM® line. The configuration of the tenter can be varied depending on the desired draw ratio. With a synchronous linear motor tenter, applicable draw ratios are generally about 3: 1 for MD draw.
And between about 10: 1 and between about 3: 1 and about 10: 1 for TD stretching. However, it is preferred that a draw ratio of at least about 4: 1 be applied in both directions, with a draw ratio of at least about 5: 1 being more preferred, and a draw ratio of at least about 6: 1 being more preferred.

In the case of the simultaneous flat stretching method, there is only one preheating zone, and the temperature in said preheating zone is between about 105 ° C. and about 145 ° C., ie usually about 100 ° C.
It is kept close to the orientation temperature comprised between C and about 140C.

The resulting hot, irradiated and clipped sheet is then
It is sent to the stretching area of the simultaneous tenter and stretched simultaneously in both directions.

In both flat stretching methods, the biaxially stretched film is transferred to a relaxation or annealing zone and heated to a temperature of about 70-90 ° C.

Following the annealing step, the film is moved to a cooling zone, where
The film is cooled using either air, either cooled or kept at ambient temperature. Thus, the temperature of the cooling zone is typically about 20 ° C. and about 4 ° C.
Included between 0 ° C. At the end of the line, the film ends gripped and unoriented by clips are trimmed away, and the resulting biaxially oriented, heat-shrinkable polyethylene film is then pre-cut into appropriate widths of the film web. Rolled without cutting or cutting.

The heat-shrinkable, single-layer, illuminated polyethylene film thus obtained has a thickness variation of less than 20 percent, preferably less than 18 percent, more preferably less than 15 percent.

[0069] The biaxially oriented, heat-shrinkable, single-layer polyethylene film obtained by the above-described method has, at 120 ° C, about 60 to about 170 percent, usually about 70 to about 170 percent, typically Has a total free shrink of about 80 to about 160 percent.

When a film is obtained by the simultaneous flat stretching method, the free shrink properties of the film are more balanced in two directions, between 15% free shrinkage in MD and% free shrinkage in TD, Preferably a difference of less than 10, more preferably less than 5, is achieved.

[0071] The film thus obtained, the when heated under restraint, at least 40psi in either direction (about 2.8 kg / ^cm 3), preferably at least 5
It shows a contraction tension of 0 psi (about 3.5 kg / cm ³ ). Shrink tension is ASTM
Measured according to D2838.

The resulting film can then be subjected to a corona discharge treatment to improve the print receptive properties of the film surface.

As used herein, the terms “corona treatment” and “corona discharge treatment” refer to corona discharge treatment of the outer surface of a film, ie, the ionization of a gas such as air near the film surface. In other words, ionization is started by a high voltage applied to a nearby electrode, and oxidation of the film surface and other changes such as surface roughness are caused. Corona treatment of polymeric materials is described, for example, in US Pat.
No. 16.

The invention is further illustrated by the following examples, which are provided for illustrative purposes and should not be construed as limiting the scope of the invention. All percentages, parts, etc., are by weight unless otherwise indicated.

Examples 1 to 3 A density of 0.920 g / cm ³ and a melt index of 1.0 g / 10 from the three
Heterogeneous ethylene-octene-1 copolymer (Dow Chemical)
A single-layer, heat-shrinkable film consisting of Dowlex ™ 2045) by the Company was sequentially stretched on a tenter frame line with an MD stretch ratio of 5:
1 and TD stretch ratio 8: 1.

The thickness of the extruded sheet and the draw ratio were set so as to obtain a final heat-shrinkable film having a thickness of 15 μm.

In Examples 1 to 3, all parameters of the manufacturing process except the irradiation level were:
Exactly identical. In particular, the MD preheating zone is maintained at about 95-105 ° C, the MD stretching is performed at about 78-88 ° C, the preheating for TD stretching is maintained at about 125-135 ° C,
TD stretching was performed at about 120-125 ° C and relaxation was maintained at about 80-85 ° C. The illumination levels were 0 kilogray (Example 1), 27 kilogray (Example 2), and 5 kilogray (Example 2).
It was 4 kilogray (Example 3).

The thickness of the film was continuously monitored with a β-ray thickness gage instrument, and the obtained value was evaluated statistically.

Table I below reports the average thickness, maximum thickness, minimum thickness, 2σ value, and thickness variation (%) for the films of Examples 1-3.

The 2σ value is twice the standard deviation σ, expressed in μm, and is a parameter that is used industrially to evaluate the controllability of thickness. The 2σ value is calculated by the following equation.

[0081]

(Equation 2) However, "n" is a thickness of the number of measurement, is at least 10, "x _i" is the actual reading.

[0082]

[Table 1]

The film of Example 3 has excellent optical properties, a haze of 2.5% and a gloss of 1
It was 34 gu.

The film of Example 3 was also excellent in shrinkage properties, showing a total free shrinkage of 126% at 120 ° C.

Examples 4 to 6 The procedure of Examples 1 to 3 was repeated to obtain a heterogeneous ethylene-butene-1 copolymer having a density of 0.919 g / cm ³ and a melt index of 1.0 g / 10 min (
Flexirene ™ FG20 by Pomeri Europa)
It was used. Unirradiated film (Example 4) could not be obtained due to breakage in the center of the film during lateral orientation, but was obtained at illumination levels of 27 kGray (Example 5) and 54 kGray (Example 6). The results are reported in Table II below.

[0086]

[Table 2]

Example 7 A monolayer of a heterogeneous ethylene-octene-1 copolymer (Teamex ™ 1000F from DSM) having a density of 0.902 g / cm ³ and a melt index of 3.0 g / 10 min. Heat shrinkable films were produced on a sequential tenter frame line with an MD stretch ratio of 5: 1 and a TD stretch ratio of 8: 1. The thickness of the extruded sheet was set so as to obtain a final heat-shrinkable film having a thickness of 15 μm.

In the orientation step, the MD preheating zone is maintained at about 60 to 65 ° C., and the MD stretching is performed at about 50 to 65 ° C.
Performed at 55 ° C, preheating for TD stretching was maintained at about 105-110 ° C, TD stretching was performed at about 100-105 ° C, and relaxation temperature was maintained at about 70-75 ° C. The irradiation level was 72 kilogray.

The results thus obtained are reported in Table III below.

[0090]

[Table 3]

Examples 8 to 10 Three with a density of 0.911 g / cm ³ and a melt index of 6.6 g / 10
Ethylene-octene-1 copolymer having a polymer content (Stamy by DSM)
lex ™ 08-076) was produced on a sequential tenter frame line at a MD draw ratio of 5: 1 and a TD draw ratio of 8: 1. The thickness of the extruded sheet was set so as to obtain a final heat-shrinkable film having a thickness of 15 μm.

In both cases, the orientation temperature is the same, the MD preheating zone is maintained at about 70-75 ° C., the MD stretching is performed at about 60-65 ° C., and the preheating for TD stretching is about 105-11
0 ° C., TD stretching is performed at about 100-105 ° C., and relaxation temperature is about 70-75.
C. The illumination levels were 0 kilogray (Example 8), 54 kilogray (Example 9), and 72 kilogray (Example 10).

The results obtained in this way are reported in Table IV below.

[0094]

[Table 4]

The film of Example 10 had excellent optical properties, a haze of 1.0% and a gloss of 145 gu.

The film of Example 10 was also excellent in shrinkage properties, and showed a total free shrinkage of 160% at 120 ° C.

Examples 11 to 13 Three, density 0.920 g / cm ³ and melt index 0.9 g / 10
Ethylene-octene-1 copolymer (Dow Chemical)
A single-layer, heat-shrinkable film consisting of Elite ™ 5100) from the Company was sequentially machined on a tenter frame line with an MD draw ratio of 5: 1 and T
It was manufactured at a D stretching ratio of 8: 1. The thickness of the extruded sheet was set so as to obtain a final heat-shrinkable film having a thickness of 15 μm.

In all three cases, the orientation temperature was the same, and the MD preheating region was about 105 to 1
Maintain at 15 ° C, MD stretching is performed at about 88-98 ° C, preheating for TD stretching is maintained at about 135-145 ° C, TD stretching is performed at about 130-135 ° C, and relaxation temperature is about 90- Maintained at 95 ° C. The illumination levels were 0 kilogray (Example 11), 27 kilogray (Example 12), and 54 kilogray (Example 13).

The results thus obtained are reported in Table V below.

[0100]

[Table 5]

The film of Example 13 had excellent optical properties, a haze of 2.3% and a gloss of 135 gu.

The film of Example 13 was also excellent in shrinkage properties, showing a total free shrinkage of 132% at 120 ° C.

Examples 14 to 16 The density of 0.910 g / cm ³ and the melt index of 2.2 g / 10 of the ^three
Ethylene-octene-1 copolymer having a polymer content (Stamy by DSM)
lex ™ 08-026F) was produced on a sequential tenter frame line at a MD draw ratio of 5: 1 and a TD draw ratio of 8: 1. The thickness of the extruded sheet was set so as to obtain a final heat-shrinkable film having a thickness of 15 μm.

In both cases, the orientation temperature is the same, the MD preheating zone is maintained at about 70-75 ° C., the MD stretching is performed at about 60-65 ° C., and the preheating for TD stretching is about 105-11
0 ° C., TD stretching is performed at about 100-105 ° C., and relaxation temperature is about 70-75.
C. The illumination levels were 0 kilogray (Example 14), 54 kilogray (Example 15), and 72 kilogray (Example 16).

The results thus obtained are reported in Table VI below.

[0106]

[Table 6]

The film of Example 16 had excellent optical properties, a haze of 0.9% and a gloss of 145 gu.

The film of Example 16 was also excellent in shrinkage properties, showing a total free shrinkage of 158% at 120 ° C.

The films obtained according to the present invention can be used for packaging food and non-food products as known in the art. For this purpose, the film can be used in a flat form and wrapped around the product to be packaged, or the film can be first wrapped in a bag or pouch by conventional techniques well known to those skilled in the art. Can be converted to The films can also be combined or laminated to other films or sheets to provide improved performance packaging materials.

Although the present invention has been described with reference to preferred embodiments, modifications and changes may be made without departing from the principles and scope of the invention, as will be readily understood by those skilled in the art. It is to be understood that gains. Accordingly, such modifications can be made within the scope of the appended claims.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme court ゛ (Reference) C08L 23:00 C08L 23:00 (81) Designated country EP (AT, BE, CH, CY, DE, DK, ES, FI, FR, GB, GR, IE, IT, LU, MC, NL, PT, SE), OA (BF, BJ, CF, CG, CI, CM, GA, GN, GW, ML, MR, NE, SN, TD, TG), AP (GH, GM, KE, LS, MW, SD, SL, SZ, TZ, UG, ZW), EA (AM, AZ, BY, KG, KZ, MD, RU) , TJ, TM), AE, AL, AM, AT, AU, AZ, BA, BB, BG, BR, BY, CA, CH, CN, CR, CU, CZ, DE, DK, D M, EE, ES, FI, GB, GD, GE, GH, GM, HR, HU, ID, IL, IN, IS, JP, KE, KG, KP, KR, KZ, LC, LK, LR, LS , LT, LU, LV, MA, MD, MG, MK, MN, MW, MX, NO, NZ, PL, PT, RO, RU, SD, SE, SG, SI, SK, SL, TJ, TM, TR, TT, TZ, UA, UG, US, UZ, VN, YU, ZA, ZW

Claims (9)

[Claims] 1. A heat-shrinkable, irradiated, monolayer polyethylene film characterized by a thickness variation of less than 20%, preferably less than 18%, more preferably less than 15%. 2. At least about 50% by weight of one or more ethylene-α
Olefin copolymer, more preferably at least 60% by weight of one or more ethylene-α-olefin copolymers, even more preferably at least 70% by weight of one or more ethylene-α-olefin copolymers A film according to claim 1, comprising coalesced, more preferably at least 80% by weight, of one or more ethylene-α-olefin copolymers. 3. An ethylene-α-olefin copolymer comprising about 0.880 g / c
m ³ to about 0.940 g / cm ³ , preferably about 0.890 g / cm ³ to about 0
. 935 g / cm ³ , more preferably from about 0.900 g / cm ³ to about 0.930
g / cm ³ , more preferably from about 0.905 g / cm ³ to about 0.925 g / c
^3. The film of claim 2, wherein the film is selected from the group consisting of heterogeneous and uniform ethylene- [alpha] -olefin copolymers having a density of m3. 4. An ethylene-α-olefin copolymer comprising about 0.3 to about 8 g /
10 minutes, more preferably about 0.5 to about 7 g / 10 minutes, and even more preferably about 0.1 g / g.
The film of claim 3 having a melt index (as measured by ASTM D1238) of from 6 to about 6 g / 10 minutes, more preferably from about 0.8 to about 5 g / 10 minutes. 5. A process for producing a heat-shrinkable, single-layer polyethylene film having a thickness variation of less than 20%, preferably less than 18%, more preferably less than 15% by flat extrusion and flat orientation. The single-layered polyethylene cast sheet is irradiated before being oriented. 6. The single-layer polyethylene cast sheet has a thickness of from about 15 to about 150 kilogray, more preferably from about 20 to about 130 kilogray, even more preferably about 2 to about 130 kilogray.
The method according to claim 5, wherein the irradiation is performed to an irradiation level of 5 to about 110 kilogray, more preferably about 30 to about 100 kilogray. 7. The single-layer polyethylene cast sheet has a length of 4: 1 in the longitudinal direction.
With an orientation ratio of more than 4.5, preferably more than 4.5: 1, more preferably at least 5: 1, and more than 4: 1 in the transverse direction, preferably more than 4.5: 1, more preferably at least 5: 1. The method according to claim 6, wherein the film is biaxially stretched at an orientation ratio of 1: 1. 8. An MD stretching temperature of from about 50.degree. C. to about 100.degree.
The method of claim 7, wherein the stretching is performed sequentially at a TD stretching temperature of from about 140 ° C to about 140 ° C. 9. The method of claim 7, wherein the stretching is performed simultaneously at a stretching temperature of about 100 ° C. to about 140 ° C.