JP7613054B2 - Silane-modified ethylene-based polymer composition for heat-shrinkable film, silane-crosslinked ethylene-based polymer composition, heat-shrinkable film, and method for producing heat-shrinkable film - Google Patents

Description

The present invention relates to a silane-modified ethylene polymer composition for heat-shrinkable films, a silane-crosslinked ethylene polymer composition, a heat-shrinkable film, and a method for producing a heat-shrinkable film.

Conventionally, multilayer films made of ethylene-based polymers cross-linked by electron beam irradiation have been known as heat-shrinkable packaging films. However, cross-linking by electron beam irradiation requires expensive dedicated equipment and complicated processes. Therefore, silane-crosslinked films have been developed to simplify the process, and for example, a film containing an olefin-based resin with a silane-crosslinked structure is known (see Patent Document 1).

Boxed lunches and prepared foods sold in the food sections of convenience stores and supermarkets are wrapped in stretch film. Recently, the types, sizes, and shapes of the packed lunches and prepared foods have become more diverse, and in order to accommodate this variety of packaged items, the bag-making process has been improved by increasing the margin (the ratio of the size of the film to the size of the packaged item) when making bags using automatic packaging machines, using a tunnel-shaped heating device to heat and shrink the film so that it adheres closely to the shape of the product, and providing small holes to allow the air contained in the bag to escape after it has been sealed and bagged. Under these circumstances, there is an increasing demand for films that have a high heat shrinkage rate that can accommodate the large margin in the bag-making process, and are strong enough not to tear from small holes or protrusions (such as the corners of folding boxes) during packaging or transportation.

However, the film described in Patent Document 1 was not sufficient in terms of heat shrinkage rate and strength to meet the above requirements.

JP 2017-203145 A

The present invention aims to solve the problems of the conventional techniques described above and to provide a silane-modified ethylene polymer composition for heat-shrinkable films, a silane-crosslinked ethylene polymer composition, a heat-shrinkable film, and a method for producing a heat-shrinkable film, which enable the production of heat-shrinkable films with excellent heat shrinkage and strength.

As a result of intensive research conducted by the inventors to solve the above problems, they discovered that a heat-shrinkable film with excellent heat shrinkage and strength can be obtained by using a silane-modified ethylene polymer composition with a specific density, and thus completed the present invention.

In other words, the gist of the present invention is as follows:

[1] A silane-modified ethylene polymer composition for a heat-shrinkable film, the silane-modified ethylene polymer composition for a heat-shrinkable film having a density, as measured by an underwater displacement method, of 880 kg/m3 ^or more and 905 kg/m3 or ^less .

[2] The silane-modified ethylene-based polymer composition for heat-shrinkable films described in [1], wherein the ethylene-based polymer composition before modification of the silane-modified ethylene-based polymer composition contains two or more types of linear low-density polyethylene.

[3] The silane-modified ethylene polymer composition for heat-shrinkable films according to [1] or [2], which has a melt flow rate of 0.05 g/10 min or more and 10 g/10 min or less at 190°C and 2.16 kg.

[4] A silane-modified ethylene polymer composition for heat-shrinkable films according to any one of [1] to [3], which has a melting peak temperature of 110°C or higher as measured by a differential scanning calorimeter (DSC).

[5] A silane-crosslinked ethylene-based polymer composition comprising a crosslinked product obtained by crosslinking the silane-modified ethylene-based polymer composition for heat-shrinkable films described in any one of [1] to [4].

[6] The silane-crosslinked ethylene polymer composition described in [5], in which a No. 3 test piece is prepared with reference to JIS K7113 (1995), and the tensile breaking strength in the molding flow direction measured at 23°C and a test speed of 50 mm/min is 15 MPa or more.

[7] A heat-shrinkable film using the silane-crosslinked ethylene polymer composition described in [5] or [6].

[8] A method for producing a heat shrinkable film, comprising the steps of obtaining a silane-modified ethylene-based polymer composition for a heat shrinkable film according to any one of [1] to [4], stretching an extrusion composition obtained by melt-kneading the silane-modified ethylene-based polymer composition and a crosslinking catalyst to obtain a stretched film, and crosslinking the stretched film to obtain a heat shrinkable film.

[9] The method for producing a heat-shrinkable film according to [8], wherein the crosslinking catalyst is a silanol condensation catalyst.

According to the present invention, it is possible to provide a silane-modified ethylene polymer composition for a heat-shrinkable film, which enables production of a heat-shrinkable film having excellent heat shrinkage and strength, a silane-crosslinked ethylene polymer composition, a heat-shrinkable film, and a method for producing the heat-shrinkable film.
The silane-modified ethylene polymer composition for heat shrinkable films of the present invention is expected to be applied to applications in which heat shrinkable films have conventionally been used.

The present invention will be described in detail below, but the present invention is not limited to the following description, and can be modified in any manner without departing from the gist of the present invention.
In this specification, when an expression is used using "~" followed by a numerical value or physical property value, the values before and after the expression are included.

In addition, the term "film" generally refers to a thin, flat product that is extremely thin compared to its length and width, with an arbitrarily limited maximum thickness, and is usually supplied in roll form (Japanese Industrial Standard JIS K6900). In addition, the term "sheet" generally refers to a thin, flat product, as defined in JIS, whose thickness is generally small compared to its length and width. However, the boundary between film and sheet is unclear. In this specification, both terms are used as having the same meaning, and both are referred to unified as "film."

In the present invention, the "silane-modified ethylene polymer composition" contains one or more silane-modified ethylene polymers, and may contain not only silane-modified ethylene polymers but also non-silane-modified ethylene polymers, and may further contain unreacted residues of the unsaturated silane compound used in the silane modification of the ethylene polymer composition and a radical generator, and therefore, although it is referred to as a "composition," it is also generally referred to as a "silane-modified ethylene polymer."
In addition, with regard to the "ethylene-based polymer composition" described later, the term "composition" is a general term for a composition consisting of one type of ethylene-based polymer and a composition consisting of two or more types of ethylene-based polymers, but is also generally referred to as an "ethylene-based polymer."

[Silane-modified ethylene polymer composition for heat-shrinkable films]
The silane-modified ethylene polymer composition for heat shrinkable films of the present invention (hereinafter, may be referred to as "the silane-modified ethylene polymer composition of the present invention") is characterized in that it has a density, as measured by an underwater displacement method, of 880 kg/m3 ^or more and 905 kg/m3 or ^less .
If the density of the silane-modified ethylene polymer composition exceeds the upper limit, the rigidity of the silane-crosslinked ethylene polymer composition obtained by crosslinking or the heat-shrinkable film obtained by crosslinking becomes high, and for example, the degree of freedom of shape during use is impaired. If the density of the silane-modified ethylene polymer composition is less than the lower limit, the heat shrinkage rate when applied to a heat-shrinkable film is reduced, and the film strength is reduced, which may cause holes or breakage. From the above viewpoint, the lower limit of the density of the silane-modified ethylene polymer composition is preferably 885 kg/ ^m3 or more, and the upper limit is preferably 903 kg/m3 or ^less .
The density of the silane-modified ethylene polymer composition of the present invention is specifically measured by the method described in the Examples section below.

<Mechanism>
The mechanism by which a heat-shrinkable film excellent in heat shrinkage rate and strength can be obtained from the silane-modified ethylene polymer composition of the present invention is presumed to be as follows.
In the silane-modified ethylene polymer composition, the silyl group is mainly present in the amorphous part of the ethylene polymer constituting the silane-modified ethylene polymer composition by graft polymerization. The silyl groups present in the amorphous part are crosslinked with each other to obtain a three-dimensional network structure, which shows a high heat shrinkage rate as a film. Therefore, the more amorphous parts there are, the more likely this characteristic is to be expressed. The density of the silane-modified ethylene polymer composition is correlated with the amount of amorphous parts present in the silane-modified ethylene polymer composition, and the lower the density of the silane-modified ethylene polymer composition, the more amorphous parts there are, and the higher the heat shrinkage rate as a film tends to be. On the other hand, as the density decreases, the effect of stress relaxation appears as the flexibility increases, and the heat shrinkage rate decreases. Therefore, by setting the density of the silane-modified ethylene polymer composition to 880 kg/m ³ or more and 905 kg/m ³ or less, a film with excellent heat shrinkage rate when molded into a film can be obtained. Furthermore, the unmodified ethylene polymer composition which enables the preparation of a silane-modified ethylene polymer composition in this density region also has excellent strength, and therefore provides a heat shrinkable film with high strength.

<Silane-modified ethylene polymer composition>
The silane-modified ethylene polymer composition of the present invention may be a blend of two or more kinds of silane-modified ethylene polymers, or a blend of a silane-modified ethylene polymer and a non-silane-modified ethylene polymer.
Preferably, it is an ethylene-based polymer composition graft-modified with an unsaturated silane compound, and can be obtained by copolymerizing an olefinically unsaturated silane compound having a hydrolyzable organic group with an ethylene-based polymer composition described below in the presence of a radical generator. In this reaction, the unsaturated silane compound is grafted to each polymer so as to become a crosslinking point at which the ethylene homopolymer and ethylene-α-olefin copolymer contained in the ethylene-based polymer composition crosslink with each other.

<Ethylene-Based Polymer Composition>
Examples of ethylene-based polymers that can be used as raw materials for the silane-modified ethylene-based polymer include ethylene homopolymers and copolymers of ethylene units and α-olefin units other than ethylene units or monomer units other than α-olefins. The ethylene-α-olefin copolymer may be a copolymer of ethylene and one type of α-olefin described below, or a copolymer of ethylene and two or more types of α-olefins in combination.

The α-olefins typically include α-olefins having 3 to 20 carbon atoms, such as propylene, 1-butene, 1-pentene, 1-hexene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-octadecene, 1-eicosene, 3-methyl-1-butene, 4-methyl-1-pentene, and 6-methyl-1-heptene, with propylene, 1-butene, 1-hexene, and 1-octene being preferred.

The content of ethylene units in an ethylene-based copolymer is usually 51 to 99.9% by mass relative to the entire ethylene-based copolymer. For the same reason, the content of α-olefin units other than ethylene units and monomer units other than α-olefins in an ethylene-based copolymer is 0.1 to 49% by mass in total relative to the entire ethylene-based copolymer.

When the ethylene-based copolymer is an ethylene-α-olefin copolymer, the structural units constituting the copolymer are preferably one in which 2 to 45% by mass of one or more α-olefins are copolymerized with 55 to 98% by mass of ethylene. If there is more ethylene and less α-olefin than this range, it tends to be difficult to obtain sufficient flexibility, and if there is less ethylene and more α-olefin than this range, the melting point tends to be lower and heat resistance tends to be reduced.

As the ethylene copolymer, linear low density polyethylene is preferred from the viewpoint of an excellent balance between heat resistance and strength, etc. Among linear low density polyethylenes, ethylene/α-olefin copolymers having 3 to 20 carbon atoms are preferred. Although one type of these linear low density polyethylenes may be used alone, it is preferable to select two or more types.
Among these, an ethylene-based polymer composition containing an ethylene homopolymer and an ethylene/C3-8 α-olefin copolymer, an ethylene-based polymer composition containing an ethylene/1-octene copolymer and an ethylene/1-hexene copolymer, an ethylene-based polymer composition containing an ethylene/1-octene copolymer and an ethylene/1-butene copolymer, or an ethylene-based polymer composition containing an ethylene/1-octene copolymer and an ethylene/propylene copolymer is more preferred.

In an ethylene-based polymer composition containing an ethylene homopolymer and an ethylene-α-olefin copolymer having 3 to 8 carbon atoms, the ratio of the ethylene homopolymer and the ethylene-α-olefin copolymer is not particularly limited as long as it satisfies the density of the silane-modified ethylene-based polymer composition described above. However, in film applications where transparency is required, it is preferable that the main material of the ethylene-based polymer composition is composed of any ethylene-α-olefin copolymer.

The method for producing the ethylene polymer is not particularly limited and may be any of various known methods. For example, the type of catalyst used may be a Ziegler-Natta catalyst or a metallocene catalyst. As a catalyst used for polymerizing ethylene and an α-olefin, a Ziegler-Natta catalyst is preferred because it is easy to produce an ethylene copolymer in a suitable density range. Ethylene-α-olefin copolymers obtained using a metallocene catalyst are usually mainly low-density polymers, so it is recommended to mix them with an ethylene polymer with a high density in order to obtain a density in the desired range.

From the viewpoint of controlling the density of the silane-modified ethylene polymer composition of the present invention to 880 kg/ ^m3 or more and 905 kg/m3 ^{or less} , it is preferable to use one or more types of ethylene polymers having a density of 880 kg/ ^m3 or more and 965 kg/m3 or ^less , and it is preferable to use one or more types of ethylene polymers having a density of 890 kg/ ^m3 or more and 960 kg/m3 or ^less .
However, when two or more kinds of ethylene-based polymers are used in combination, even if the density of each ethylene-based polymer is outside the above-mentioned preferred range, they can be preferably used as well as the mixture having a density within the above-mentioned preferred range.

The density of the ethylene polymer can be adjusted mainly by the amount of other α-olefin introduced to be copolymerized with ethylene.
By setting the density of the ethylene-based polymer within the above range, the strength and heat resistance of the heat-shrinkable film obtained can be well maintained. In addition, appropriate rigidity can be imparted, and the degree of freedom in shape during use of the film can be ensured.
The density of the ethylene polymer is measured in the same manner as the density of the silane-modified ethylene polymer composition of the present invention, and if it is a commercially available product, the catalog value can be used.

The melt flow rate (MFR) of the ethylene polymer at 190° C. and 2.16 kg is preferably 0.1 g/10 min or more and 10 g/10 min or less, and more preferably 0.5 g/10 min or more and 7 g/10 min or less.
As with the density, when two or more kinds of ethylene-based polymers are used in combination, even if the MFR of each ethylene-based polymer is outside the above-mentioned preferred range, they can be preferably used as long as the MFR of the mixture is within the above-mentioned preferred range.

By setting the MFR of the ethylene-based polymer within the above range, it is possible to prevent the viscosity from becoming too high during silane modification, and to maintain good extrusion characteristics during silane modification. In addition, since it is possible to prevent a significant decrease in the MFR of the resulting silane-modified ethylene-based polymer composition, it is possible to prevent pressure increases and drawdown during film molding, and to prevent deterioration of extrusion moldability, such as torque abnormalities and melt fracture that deteriorates surface properties.

In the present invention, commercially available ethylene polymers can be used. Examples of ethylene homopolymers include the "Novatec (registered trademark) LD" series manufactured by Japan Polyethylene Corporation. Examples of ethylene-α-olefin copolymers include the "Novatec (registered trademark) LL" series and "Harmolex (registered trademark)" series manufactured by Japan Polyethylene Corporation, the "Tafmer (registered trademark)" series manufactured by Mitsui Chemicals, Inc., the "Engage (registered trademark)" series and "INFUSE (registered trademark)" series manufactured by The Dow Chemical Company, and the "QAMAR" series manufactured by SABIC.

<Silane modification>
The silane-modified ethylene polymer composition of the present invention can be produced by graft-introducing an alkoxysilane into the above-mentioned raw material ethylene polymer composition to modify it with silane.
Silane modification can be carried out according to a known method, and is not particularly limited. For example, solution modification, melt modification, solid-phase modification by irradiation with electron beams or ionizing radiation, modification in supercritical fluid, etc. are preferably used. Among these, melt modification, which is excellent in equipment and cost competitiveness, is preferred, and melt kneading modification using an extruder, which is excellent in continuous productivity, is more preferred. Examples of devices used for melt kneading modification include single-screw extruders, twin-screw extruders, Banbury mixers, and roll mixers. Among these, single-screw extruders and twin-screw extruders, which are excellent in continuous productivity, are preferred.

In general, grafting of alkoxysilanes onto ethylene-based polymers can be carried out by a graft polymerization reaction in which the carbon-hydrogen bonds of polyolefins are cleaved to generate carbon radicals, to which unsaturated functional groups are added. In addition to the above-mentioned electron beams and ionizing radiation, carbon radicals can also be generated by using high temperatures or radical generators such as organic or inorganic peroxides. From the standpoint of cost and ease of operation, it is preferable to use organic peroxides.

As an olefinically unsaturated silane compound having a hydrolyzable organic group that is suitably used for the silane modification of an ethylene-based polymer composition, there can be mentioned a silane compound represented by the following general formula (1).
RSiR' _n Y _3-n (1)
(In formula (1), R represents a monovalent olefinically unsaturated hydrocarbon group, Y represents a hydrolyzable organic group, R' represents a monovalent hydrocarbon group other than an aliphatic unsaturated hydrocarbon group or is the same as Y, and n represents 0, 1, or 2.)

In general formula (1), R is preferably a vinyl group, an allyl group, an isopropenyl group, a butenyl group, etc., R' is preferably a methyl group, an ethyl group, a propyl group, a decyl group, a phenyl group, etc., and Y is preferably a methoxy group, an ethoxy group, a formyloxy group, an acetoxy group, a propionoxy group, an alkyl or arylamino group.

More preferred examples of the unsaturated silane compound include compounds represented by the following general formula (2).
_CH2 =CHSi(OA) ₃ (2)
(In formula (2), A represents a monovalent hydrocarbon group having 1 to 8 carbon atoms.)

Specific examples of the unsaturated silane compound represented by the above general formula (2) include vinyltrimethoxysilane and vinyltriethoxysilane.

As the unsaturated silane compound, a compound represented by the following general formula (3) can also be preferably used.
_CH2 =C( _CH3 ) _COOC3H6Si (OA) _{3 (} 3)
(In formula (3), A has the same meaning as in formula (2).)

Examples of unsaturated silane compounds represented by the above general formula (3) include γ-methacryloyloxypropyltrimethoxysilane, γ-methacryloyloxypropyltriethoxysilane, etc.

Among these, vinyltrimethoxysilane, vinyltriethoxysilane, and gamma-methacryloyloxypropyltriethoxysilane are preferred as unsaturated silane compounds.

These unsaturated silane compounds may be used alone or in any combination of two or more.

The addition rate of the unsaturated silane compound used for graft modification is usually 0.01% by mass or more, preferably 0.1% by mass or more, more preferably 0.7% by mass or more, based on the total mass of the ethylene-based polymer composition to be subjected to silane modification, and usually 15% by mass or less, preferably 10% by mass or less, more preferably 7% by mass or less, and even more preferably 4% by mass or less. By setting the addition rate of the unsaturated silane compound within the above numerical range, it tends to be easier to obtain the desired silane introduction rate required to obtain a high heat shrinkage rate and material strength in the silane-modified ethylene-based polymer composition. Here, the addition rate of the unsaturated silane compound has the same meaning as the unit derived from the unsaturated silane compound in the silane-modified ethylene-based polymer composition.

Radical generators used during graft modification include various organic peroxides and peresters with strong polymerization initiation action, such as dicumyl peroxide, α,α'-bis(t-butylperoxydiisopropyl)benzene, di-t-butyl peroxide, t-butylcumyl peroxide, di-benzoyl peroxide, 2,5-dimethyl-2,5-bis(t-butylperoxy)hexane, t-butylperoxypivalate, di-t-butyl peroxide, and t-butylperoxy-2-ethylhexanoate. Of these, dicumyl peroxide, benzoyl peroxide, and di-t-butyl peroxide are preferred. These radical generators may be used alone or in any combination of two or more.

The addition rate of the radical generator is desirably adjusted so that the MFR of the resulting silane-modified ethylene polymer composition is ultimately within the following MFR range, and is usually 0.005% by mass or more, preferably 0.01% by mass or more, more preferably 0.02% by mass or more, and usually 0.5% by mass or less, preferably 0.4% by mass or less, more preferably 0.2% by mass or less, based on the total mass of the resulting silane-modified ethylene polymer composition. By setting the addition rate of the radical generator within the above numerical range, it tends to be easier to obtain the desired silane introduction rate in the silane-modified ethylene polymer composition, while at the same time suppressing a decrease in the MFR of the silane-modified ethylene polymer composition, not impairing extrusion processability, and tending to easily maintain the condition of the molded surface in good condition.

<MFR>
The melt flow rate (MFR) of the silane-modified ethylene polymer composition of the present invention at 190°C and 2.16 kg is preferably 0.05 g/10 min or more and 10 g/10 min or less, and more preferably 0.1 g/10 min or more and 8 g/10 min or less.
When the MFR of the silane-modified ethylene polymer composition is within the above-mentioned numerical range, the occurrence of pressure increase and drawdown during film molding can be suppressed, and deterioration of extrusion moldability, such as the occurrence of torque abnormality and melt fracture that deteriorates surface properties, can be suppressed.

<Melting peak temperature>
The silane-modified ethylene polymer composition of the present invention preferably has a peak melting temperature (hereinafter sometimes referred to as "peak melting temperature") of 110°C or higher as measured by a differential scanning calorimeter (DSC). When the silane-modified ethylene polymer composition has a peak melting temperature of 110°C or higher, the composition can maintain its shape due to crystallization even at high temperatures. From this viewpoint, the silane-modified ethylene polymer composition more preferably has a peak melting temperature of 115°C or higher. However, if the silane-modified ethylene polymer composition has an excessively high peak melting temperature, there is a risk of poor appearance due to unmelted particles during heating during molding or early crystallization during cooling during molding, and therefore the silane-modified ethylene polymer composition usually has a peak melting temperature of 135°C or lower.
The melting peak temperature of the silane-modified ethylene polymer composition is specifically measured by the method described in the Examples section below.

[Silane-crosslinked ethylene polymer composition]
By subjecting the silane-modified ethylene polymer composition of the present invention to a crosslinking treatment, it is possible to obtain the silane-crosslinked ethylene polymer composition of the present invention, which contains a crosslinked product obtained by crosslinking the silane-modified ethylene polymer composition.
The method for crosslinking the silane-modified ethylene polymer composition of the present invention to obtain the silane-crosslinked ethylene polymer composition of the present invention will be described later in the section [Method for producing heat shrinkable film].

From the viewpoint of the film strength of the heat-shrinkable film obtained, it is preferable that the silane-crosslinked ethylene polymer composition of the present invention has a tensile breaking strength of 15 MPa or more in the molding flow direction measured at 23°C and a test speed of 50 mm/min in accordance with JIS K7113 (1995). By making the tensile breaking strength of the silane-crosslinked ethylene polymer composition equal to or more than the above lower limit, it is possible to obtain a film strength that can withstand various external loads (tears, breakage, etc.) during packaging and during transportation and storage after packaging.

As a method for making the tensile breaking strength of the silane-crosslinked ethylene polymer composition equal to or higher than the above lower limit, the following method can be mentioned in the production of the silane-modified ethylene polymer composition to be subjected to the crosslinking treatment.
(1) From the viewpoint of materials, it is preferable to select a linear low-density polyethylene having excellent material strength as the ethylene polymer to be subjected to silane modification, and to appropriately adjust the blending ratio of the ethylene polymer composition and the types and amounts of the unsaturated silane compound and the radical generator used in the silane modification.
(2) From the viewpoint of the production method, it is preferable to feed the ethylene polymer composition to be subjected to silane modification into an extruder whose temperature is set so as to appropriately cause a graft reaction of the unsaturated silane compound, melt-knead the composition, extrude the molten kneaded product into a string-like shape, cool it, and then cut it to obtain the silane-modified ethylene polymer composition.

The tensile breaking strength of the silane-crosslinked ethylene polymer composition is specifically measured by the method described in the Examples section below.

[Heat shrinkable film]
The heat-shrinkable film of the present invention is obtained by using the silane-crosslinked ethylene polymer composition of the present invention.

<Gel Fraction>
The gel fraction of the heat-shrinkable film of the present invention, i.e., the gel fraction of the silane-crosslinked ethylene-based polymer composition of the present invention constituting the heat-shrinkable film of the present invention, is preferably 55% by mass or more and 85% by mass or less. The gel fraction contained in the heat-shrinkable film is derived from the crosslinked body contained in the silane-crosslinked ethylene-based polymer composition.
The gel fraction can be adjusted by changing the graft ratio (modification amount) of the unsaturated silane compound in the silane-modified ethylene polymer composition, the type and amount of a silanol condensation catalyst used as a crosslinking catalyst in the crosslinking reaction described below, the conditions (temperature, time) for crosslinking, etc.

The gel fraction is generally specified in JIS K6769, the standard for cross-linked polyethylene pipes, and JXPA401, the standard for heating polyethylene pipes. Here, the gel fraction is an index of the degree of cross-linking of the resin; a high gel fraction indicates a high degree of cross-linking, and a low gel fraction indicates a low degree of cross-linking.

By setting the lower limit of the gel fraction of the heat-shrinkable film to 55% by mass or more, it is possible to suppress a decrease in heat resistance and mechanical strength. From this viewpoint, the gel fraction of the heat-shrinkable film of the present invention is more preferably 60% by mass or more, and even more preferably 65% by mass or more. On the other hand, by setting the upper limit of the gel fraction to 85% by mass or less, it is possible to suppress a decrease in the heat shrinkage rate of the heat-shrinkable film. From this viewpoint, the upper limit of the gel fraction of the heat-shrinkable film of the present invention is more preferably 83% by mass or less, and even more preferably 81% by mass or less.

The gel fraction measurement method specified in JIS K6769 and JXPA401 involves carrying out actual molding, crosslinking the molded body, and then cutting the molded body into a sample for measurement. However, for the sake of simplicity, in this invention, the gel fraction of the silane-crosslinked ethylene polymer composition is measured using the following method, and this is regarded as the gel fraction of the heat-shrinkable film.

<Method for measuring gel fraction of silane-crosslinked ethylene polymer composition>
A silane-crosslinked ethylene polymer composition sample was obtained by adding 0.05 mass% of dioctyltin dilaurate to a silane-modified ethylene polymer composition (actually, a master batch in which 1 mass% of dioctyltin dilaurate was added to a linear low-density polyethylene having an MFR of 3.5 g/10 min and a density of 898 kg/ ^m3 was added), melt-kneading the mixture, extruding the mixture at 210°C to a thickness of 0.5 mm, and subjecting the extruded film to a crosslinking treatment in hot water at 80°C for 24 hours. About 0.5 g of the sample (sample weight is designated as G1 (g)) cut into 1 mm squares was refluxed in xylene at 120°C for 8 hours in a 200-mesh wire net, and the boiling xylene insoluble matter remaining on the wire net was dried in a vacuum of 10 Torr at 80°C for 8 hours, and its weight was precisely measured (the weight of the precisely measured boiling xylene insoluble matter is designated as G2 (g)), and the weight was calculated according to the following formula (4).
Gel fraction (%) = G2 (g) ÷ G1 (g) × 100 (4)

<Thickness>
The thickness of the heat-shrinkable film of the present invention is not particularly limited and can be set arbitrarily depending on the required performance, for example, the application, the shape of the final product, the required physical properties, etc. For example, when used for food packaging, industrial product packaging, or as a film for electronic components, the total thickness of the heat-shrinkable film is preferably 1 to 500 μm, more preferably 2 to 800 μm, and even more preferably 3 to 700 μm.
The heat-shrinkable film of the present invention can also be used as a sheet depending on the thickness.

[Method of producing heat shrinkable film]
The heat shrinkable film of the present invention can be produced by various conventionally known methods, and the method is not particularly limited. However, the heat shrinkable film is preferably produced through a process including a step of obtaining the silane-modified ethylene polymer composition of the present invention by silane-modifying an ethylene polymer composition, a step of stretching an extruded composition obtained by melt-kneading the silane-modified ethylene polymer composition of the present invention and a crosslinking catalyst to obtain a stretched film (hereinafter, may be referred to as the "stretched film of the present invention"), and a step of crosslinking the obtained stretched film to obtain a heat shrinkable film.

That is, the heat shrinkable film of the present invention contains a silane crosslinked product obtained by subjecting the silane-modified ethylene copolymer composition of the present invention to a crosslinking reaction in the presence of a crosslinking catalyst, and is usually produced by melt-kneading a raw material containing the silane-modified ethylene copolymer composition of the present invention and a crosslinking catalyst in a molding machine equipped with an extruder and a cylindrical die or T-die, stretching the extruded composition to form a stretched film, and then performing a crosslinking treatment.

<Step of obtaining stretched film>
An example of the process for obtaining the stretched film of the present invention will be described.
Examples of this step include inflation film molding in which a molten resin composition obtained by melt-kneading the silane-modified ethylene polymer composition of the present invention and a crosslinking catalyst in an extruder is fed to a die to be molded, and a method in which an unstretched film obtained by T-die film molding or the like is cooled and solidified, then reheated in-line or out-line to a stretching temperature of 60 to 160°C, and stretched at least 1.5 times in area ratio in a uniaxial or biaxial direction using a tenter and compressed air, etc., to obtain a uniaxially or biaxially stretched stretched film. These may be simultaneous stretching or sequential stretching. In the case of an inflation film, a simultaneous inflation biaxial stretching method, and in the case of using a roll and a tenter, a sequential biaxial stretching method, etc. are generally used.

The heat-shrinkable film of the present invention is preferably used as a single layer because of its excellent heat shrinkability and strength, but it may also be a laminated multilayer film. In this case, for example, a method of laminating three molded films, a method of extrusion lamination, or a method of coating may be used, but in particular, a co-extrusion method such as an inflation molding method using multiple extruders and a multilayer die or a T-die molding method is optimal.

<Crosslinking Treatment Step>
An example of a process for obtaining a heat-shrinkable film by crosslinking a stretched film will be described.
The stretched film of the present invention formed in the above-mentioned step of obtaining a stretched film is subjected to a crosslinking treatment. This crosslinking treatment may be carried out, for example, under high temperature and high humidity conditions, for example, in the presence of hot water or water vapor, while the film is wound into a roll.

In the process of crosslinking the stretched film of the present invention to obtain a heat shrinkable film, the extrusion molded product of the silane-modified ethylene-based polymer composition of the present invention is stretched to orient and crystallize the silane-modified ethylene-based polymer, and then the amorphous portion is crosslinked to form a three-dimensional network structure. Such crosslinking after stretching is preferable from the viewpoint of obtaining a film with a larger heat shrinkage rate.

<Crosslinking catalyst>
As a crosslinking catalyst used in the crosslinking reaction of the silane-modified ethylene polymer composition, a silanol condensation catalyst is preferably used. The silanol catalyst will be described in detail below.

Examples of silanol condensation catalysts include tin catalysts such as dibutyltin dilaurate, stannous acetate, dibutyltin diacetate, dibutyltin dioctoate, and dioctyltin dilaurate; lead catalysts such as lead naphthenate and lead stearate; zinc catalysts such as zinc caprylate and zinc stearate; cobalt catalysts such as cobalt naphthenate, titanium catalysts such as tetrabutyl titanate; cadmium catalysts such as cadmium stearate; and organometallic catalysts such as alkaline earth metal catalysts such as barium stearate and calcium stearate. Among these, tin catalysts are preferred. These silanol condensation catalysts may be used alone or in any combination of two or more.

The addition rate of the silanol condensation catalyst is usually 0.01% by mass or more, preferably 0.02% by mass or more, more preferably 0.05% by mass or more, based on the total mass of the silane-modified ethylene polymer composition to which the silanol condensation catalyst is added, and is usually 5% by mass or less, preferably 3% by mass or less, more preferably 2% by mass or less. By setting the addition rate of the silanol condensation catalyst within the above numerical range, a sufficient crosslinking reaction can be promoted.

The silanol condensation catalyst is generally conveniently added in the form of a masterbatch. A masterbatch of the silanol condensation catalyst can be produced, for example, by adding the silanol condensation catalyst to one or more types of polyolefins, such as ethylene homopolymer (polyethylene) or ethylene-α-olefin copolymer, and kneading them.

When the silanol condensation catalyst is used as a masterbatch in which the silanol condensation catalyst is blended with a polyolefin, there is no particular limit to the content of the silanol condensation catalyst in the masterbatch, but it is usually preferable to set it to about 0.1 to 5.0 mass%.

If necessary, other compatible thermoplastic resins, stabilizers, lubricants, fillers, colorants, foaming agents, and other auxiliary materials can be added to the silanol condensation catalyst-containing masterbatch. These additives may be any commonly used additives known per se. In addition, these additives can be added to the silane-modified ethylene polymer composition of the present invention together with the silanol condensation catalyst as a third component.

<Crosslinking reaction conditions>
The crosslinking reaction using a silanol condensation catalyst is usually carried out by exposing a composition obtained by blending a silane-modified ethylene polymer composition with a silanol condensation catalyst to a water atmosphere, thereby promoting the crosslinking reaction between silanol groups, without requiring any special crosslinking equipment. Various conditions can be used for the method of exposing to a water atmosphere, and examples of the method include a method of leaving the composition in air containing moisture, a method of blowing air containing water vapor, a method of immersing the composition in a water bath, and a method of spraying warm water in a mist form.

In this crosslinking reaction, the hydrolyzable alkoxy groups derived from the unsaturated silane compound contained in the silane-modified ethylene polymer composition of the present invention react with water in the presence of a silanol condensation catalyst and are hydrolyzed to generate silanol groups, and the reaction proceeds by further dehydration condensation between the silanol groups, and the silane-modified ethylene polymers in the silane-modified ethylene polymer composition bond to each other to generate a silane crosslinked product.

The rate at which the crosslinking reaction proceeds depends on the conditions of exposure to the water atmosphere, but typically a temperature range of 20 to 130°C and exposure time of 10 minutes to 1 week is sufficient. Preferred conditions are a temperature range of 20 to 130°C and a time range of 1 to 160 hours. When using air containing moisture, the relative humidity is selected from the range of 1 to 100%.

[Application]
The heat shrinkable film of the present invention is preferably used as a heat shrinkable film for packaging food, medical products, electronic materials, building materials, etc. In addition, the heat shrinkable film of the present invention is also very useful in various heat shrink packaging fields that utilize the characteristics and functions of the heat shrinkable film.

The present invention will be explained in more detail below using examples, but the present invention is not limited to the following examples as long as it does not exceed the gist of the invention. The values of various manufacturing conditions and evaluation results in the following examples are meant as preferred upper or lower limit values in the embodiments of the present invention, and the preferred range may be a range defined by a combination of the above-mentioned upper or lower limit values and the values of the following examples or values between the examples.

[Ingredients used]
In the following production examples, the following ethylene polymers were used.

<Linear low-density polyethylene>
PE1: Engage (registered trademark) 8200 manufactured by The Dow Chemical Company
Ethylene/1-octene copolymer MFR (190°C, 2.16 kg): 5.0 g/10 min Density: 870 kg/ ^m3
PE2: Engage (registered trademark) 8180 manufactured by The Dow Chemical Company
Ethylene/1-octene copolymer MFR (190°C, 2.16 kg): 0.5 g/10 min Density: 863 kg/ ^m3
PE3: Japan Polyethylene Corporation, Harmolex (registered trademark) NF464N
Ethylene/1-hexene copolymer MFR (190°C, 2.16 kg): 2.0 g/10 min Density: 918 kg/ ^m3
PE4: SABIC QAMAR FD18N
Ethylene/1-butene copolymer MFR (190°C, 2.16 kg): 2.0 g/10 min Density: 920 kg/ ^m3

[Production Example 1]
PE1: Engage (registered trademark) 8200 (ethylene/1-octene copolymer) manufactured by The Dow Chemical Company, and PE3: Harmolex (registered trademark) NF464N (ethylene/1-hexene copolymer) manufactured by Japan Polyethylene Corporation were mixed in a mass ratio of 40:60 to prepare pre-modification composition 1.

To 100 parts by mass of this pre-modification composition 1, 0.04 parts by mass of di-t-butyl peroxide (shown as "Pox" in Table 1) and 2 parts by mass of vinyltrimethoxysilane (shown as "Si" in Table 1) were added and mixed, and then melt-kneaded at a temperature of 220°C, a screw rotation speed of 60 rpm, and an extrusion rate of 20 kg/h using a single-screw extruder PMS50-32 (1V) (D = 50 mmφ, L/D = 32, manufactured by IKG Co., Ltd.). Thereafter, the melt-kneaded product was extruded into a string shape, cooled, and then cut to obtain a silane-modified ethylene polymer composition 1.
The MFR, density and melting peak temperature of the obtained silane-modified ethylene polymer composition 1 were measured by the following methods. The results are shown in Table 1.

<MFR>
The measurement was performed at 190° C. and a load of 2.16 kg in accordance with JIS K7210 (1999).

<Density>
In accordance with JIS K7112 (1999) Method A, a test piece having a thickness of 2 mm, a length of 40 mm and a width of 15 mm, which had been molded by slow cooling press, was used for the measurement by the underwater displacement method.

<Melting peak temperature>
Using a differential scanning calorimeter manufactured by Hitachi High-Tech Science Corporation, product name "DSC6220", in accordance with JIS K7121 (2010), about 5 mg of a sample was heated from 20°C to 200°C at a heating rate of 10°C/min, held at 200°C for 3 minutes, cooled to -10°C at a cooling rate of 10°C/min, and then heated to 200°C at a heating rate of 10°C/min. From the thermogram measured at this time, the melting peak temperature was calculated.

In addition, the gel fraction of silane-crosslinked ethylene polymer composition 1, which was obtained by crosslinking silane-modified ethylene polymer composition 1, was measured by the method described above, and the tensile breaking strength was measured by the following method, and the results are shown in Table 1.

<Tensile breaking strength>
100 parts by mass of the silane-modified ethylene polymer composition 1 was dry-blended with 5 parts by mass of a silanol condensation catalyst masterbatch (a masterbatch of linear low-density polyethylene (MFR: 2 g/10 min, density: 920 kg/ ^m3 ) containing 1% by mass of tin catalyst (dioctyltin dilaurate)) and fed to a sheet molding machine equipped with a 20 mm single-screw extruder to obtain a film having a thickness of 1 mm at a die temperature of 220°C and a line speed of 2 m/min.
The obtained film was immersed in warm water at 80° C. for 24 hours for crosslinking treatment, and then a No. 3 test piece was prepared in accordance with JIS K7113 (1995), and the tensile breaking strength was measured in the molding flow direction at 23° C. and a test speed of 50 mm/min.

[Production Examples 2 to 3]
A silane-modified ethylene polymer composition 2 and a silane-modified ethylene polymer composition 3 were obtained in the same manner as in Production Example 1, except that the formulation of the pre-modification composition was as shown in Table 1. The obtained silane-modified ethylene polymer compositions were measured in the same manner as in Production Example 1. The measurement results are shown in Table 1.
In addition, silane-crosslinked ethylene polymer compositions 2 and 3 were obtained from the obtained silane-modified ethylene polymer compositions 2 and 3 in the same manner as in Production Example 1. The obtained silane-crosslinked ethylene polymer compositions were measured in the same manner as in Production Example 1. The measurement results are shown in Table 1.

[Example 1]
100 parts by mass of the silane-modified ethylene polymer composition 1 was dry-blended with 5 parts by mass of a silanol condensation catalyst master batch (a master batch of linear low-density polyethylene (MFR: 3.5 g/10 min, density: 898 kg/ ^m3 ) containing 1% by mass of tin catalyst (dioctyltin dilaurate)) and fed to a T-die film molding machine equipped with a 20 mm single screw extruder to produce an uncrosslinked film having a thickness of 660 μm at a die temperature of 220°C and a line speed of 2 m/min. The uncrosslinked film obtained was uniaxially stretched twice in the molding flow direction in an Island Industrial Co., Ltd. biaxial stretching device at an internal temperature setting of 80°C and a stretching speed of 1.8 m/min to produce a stretched film having a thickness of 600 μm. The stretched film was left to stand in a thermo-hygrostat under an environment of 85°C and 85% RH for 16 hours to perform a crosslinking treatment, and a heat-shrinkable film 1 was obtained.

[Example 2]
An uncrosslinked film was prepared in the same manner as in Example 1, except that silane-modified ethylene polymer composition 2 was used instead of silane-modified ethylene polymer composition 1 in Example 1, and a stretched film was prepared by stretching the film uniaxially by 2 times in the same manner, and a heat-shrinkable film 2 was obtained by crosslinking the film in the same manner.

[Comparative Example 1]
An uncrosslinked film was prepared in the same manner as in Example 1, except that silane-modified ethylene polymer composition 3 was used instead of silane-modified ethylene polymer composition 1 in Example 1, and a stretched film was prepared by stretching the film uniaxially by 2 times in the same manner, and a heat-shrinkable film 3 was obtained by crosslinking the film in the same manner.

The heat shrinkage rates of heat shrinkable films 1 to 3 were evaluated using the following method. The results are shown in Table 1.

<Heat shrinkage rate>
The obtained heat-shrinkable film was cut into a piece of 3 cm in length and 3 cm in width, and left to stand in a Geer oven at 120° C. for 1 hour. The length of the film in the molding flow direction was then measured, and the heat shrinkage rate was calculated using the following formula (5).
Heat shrinkage rate (%) =
(Length before contraction (L0) - Length after contraction (L1)) / Length before contraction (L0) x 100 (5)

The above results show that the present invention can provide a heat-shrinkable film with excellent heat shrinkability and strength. The heat-shrinkable film of the present invention can be produced in a single layer and does not require special crosslinking equipment, making it highly advantageous in practical use.

Claims (5)

A heat-shrinkable film using a silane-modified ethylene-based polymer composition obtained by blending a silane-modified ethylene-based polymer having a density of 880 kg/ ^m3 or more and 905 kg/m3 ^or less, as measured by an underwater displacement method, with a silanol condensation catalyst, wherein the blending amount of the silanol condensation catalyst is 0.05 mass% or more and 5 mass% or less, based on the total mass of the silane-modified ethylene-based polymer. The heat-shrinkable film according to claim 1, wherein the melt flow rate of the silane-modified ethylene polymer at 190°C and 2.16 kg is 0.05 g/10 min or more and 10 g/10 min or less. The heat-shrinkable film according to claim 1 or 2, wherein the silane-modified ethylene polymer has a melting peak temperature of 110°C or higher and 135°C or lower as measured by a differential scanning calorimeter (DSC). The heat-shrinkable film according to any one of claims 1 to 3, which contains a crosslinked product obtained by crosslinking the silane-modified ethylene polymer. A method for producing the heat shrinkable film according to any one of claims 1 to 4 , comprising the steps of:
A step of obtaining the silane-modified ethylene polymer;
a step of stretching an extrusion composition obtained by melt-kneading the silane-modified ethylene polymer and the silanol condensation catalyst to obtain a stretched film;
And,
The method for producing a heat-shrinkable film further comprises a step of crosslinking the stretched film to obtain a heat-shrinkable film.