JPS61273930A - Preparation of biaxially oriented low density polyethylene film - Google Patents

Abstract

PURPOSE:To improve the operatability of orientation and to prepare biaxially oriented film whose transparency has been remarkably improved, by biaxially orientating a resin compsn. which consists of a specified quantity of linear low density polyethylene and a specified quantity of branched low density polyethylene. CONSTITUTION:An un-drawn film or sheet is prepd. by using a polyethylene resin compsn. which consists of 97 to 55pts.wt. of linear low density polyethylene whose melt index is 0.3 to 4g/10min, pref. 0.5 to 3g/10min and 3 to 45pts.wt. of branched low density polyethylene whose melt index is 0.3 to 4g/10min and the film or sheet is then biaxially oriented by a draw ratio of less than 3 times to the longitudinal direction and a draw ratio of 3 to 9 times to the lateral direction. The biaxially oriented film can be thereby obtd. stably and in high efficiency and the films prepd. has excellent transparency, uniform thickness and excellent strength.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for producing a biaxially stretched film of low density polyethylene.

Specifically, the present invention relates to a method for producing a biaxially stretched film from a composition comprising specific linear low density polyethylene and branched low density polyethylene.

[Prior art]

Linear low-density polyethylene with less branching, produced by copolymerization of ethylene and α-olefin, has higher tensile strength, impact strength, rigidity, and other strengths than high-pressure low-density polyethylene produced by radical polymerization at high temperature and high pressure. It has excellent properties such as environmental stress cracking resistance (WSAX), heat resistance, and heat sealability, and has been used in various fields in recent years. Particularly in the film field, linear low-density polyethylene is rapidly replacing high-pressure low-density polyethylene due to its superior physical properties.

However, linear low-density polyethylene with the above-mentioned excellent properties is a high-pressure low-density polyethylene/
Compared to other products, it has poor transparency, poor commercial value in terms of appearance, and low melt tension, resulting in unstable pull when forming blown films, and neck-in when forming T-die films. There are problems such as the phenomenon of increased shrinkage occurring and the film forming conditions having to be adjusted to narrow conditions. In addition, in order to improve the above-mentioned transparency, an attempt to stretch the film has been proposed, but -axial stretching processing leaves problems with the tear resistance of the film in different directions, especially in the longitudinal direction (stretching direction). . Therefore, it may be possible to biaxially stretch the original film, but since linear low-density polyethylene resin is essentially a crystalline polymer, conventional high-density polyethylene resin cannot be stretched easily.゛
Like polyethylene resin, biaxial stretching is difficult;
□ ,,,・ Even if the film is stretched biaxially, stretching irregularities may occur on the film. However, a stretched film that can be put to practical use has not yet been obtained. Furthermore, a method has been proposed to improve the drawbacks of the above-mentioned nine-biaxial stretching process, ie, a method of biaxially stretching an unstretched film of linear low-density polyethylene under specific conditions. (Unexamined Japanese Patent Publication No. Sho J-r-90924) [Problems to be Solved by the Invention] However, the above proposal also suffers from poor stretching operability when biaxially stretching an unstretched film of linear low density polyethylene. The thickness of the stretched film produced by this method is largely uneven, and the improvement in transparency is not sufficient, so it cannot be said that a biaxially stretched film with sufficient commercial value can be provided.

[Means for solving problems]

In view of such prior art, the present inventors improved the formability of unstretched films and the stretching operability of biaxially stretched films without impairing the above-mentioned properties of linear low density polyethylene,
A method for obtaining a biaxially stretched film with significantly improved transparency.
As a result of extensive research, we formed an unstretched film using linear low-density polyethylene mixed with a specific amount of branched low-density polyethylene having specific physical properties, and then biaxially stretched the film under specific conditions. They have discovered that the objective can be achieved by processing, and have completed the present invention.

That is, the gist of the present invention is that the melt index is 0.3.
~4tt/70 minutes linear low density polyethylene 97~
jj parts by weight and melt index are 0.3 to 41 t
/ /θ minute branched low density polyethylene, 73
An unstretched film or sheet is formed using a polyethylene resin composition consisting of ~%jli parts, and then the film or sheet is stretched up to 3 times in the machine direction and 3 times in the transverse direction.
The present invention relates to a method for producing a biaxially stretched low-density polyethylene film, characterized in that the film is biaxially stretched at a stretching ratio of ~9 times.

Detailed description of the present invention Trap 1 The linear low density polyethylene used in the present invention is ethylene and □ Other α-
Conventionally, it is a copolymer with olefin. □
Engineering □□ Wow yo □9. care.

゛、・ It is different from resin. The linear low-density polyethylene may contain, for example, ethylene and other α-olefins such as butene, hexene, octene, decene, dermethylpentene, etc. in an amount of about 17% by weight, preferably 5% to 17% by weight.
It is produced by weight-degree copolymerization using a Ziegler-type catalyst or Phillips-type catalyst used in the production of medium- to low-normally washed high-density polyethylene, and is a product of short branched structures of conventional high-density polyethylene by copolymerization components. Then, use the original short chain branching to lower the density appropriately to 0,9/-0,wo! It should be about t/CrA,
It is a polyethylene with a structure that is more linear than conventional low-density polyethylene and more branched than high-density polyethylene.

Linear low density polyethylene used in the present invention, and
The density is Oori/! ~0. riJJ't/C1d.

Preferably, those in the range of 0.9/l" to 0.2!t/CtA are suitably used. Melt index is 0.
．． 39/If less than 10 minutes', tenter method is used.
If the film tension is excessive at the start of the axial stretching operation,
□ It is necessary to raise the temperature of the tenter to near the melting point. In this case, a part of the film is melted in the flutter, reducing the stretching operability. Also, at/
...j 1 □ ', If it is longer than 70 minutes, the film is likely to break during stretching, and even if it is stretched, the elongation from the center of the film will be excessive, and it will not be uniform in the lateral direction. Stretchability (thickness, good).

□'・: This is not preferable because it results in poor unevenness. Sara. 1.1. has density o, p ′s t
If it is less than /al1, the same phenomenon as when the measurement is greater than %?/10 minutes will occur; .

This is undesirable because the same phenomenon occurs as when the melt index is o and 3t7iθ is not dropped.

! l The amount blended into the above linear low density polyethylene, ),.

, :.

Branched low-density polyethylene is prepared using the usual high-pressure method 1. In other words, using a radical generating agent such as an organic peroxide or a radical generator such as 5-polymers or 5-polymers, a pressure of 1000 - 3θ00 is used. 9/cd, temperature 1jo-4to
Conditions of o℃: ・.

, ゛ Under homopolymerization of ethylene or ethylene and other 0,. . Those obtained by copolymerizing with copolymerization components are used. Examples of copolymerization components include α-olefins such as propylene, butene, hexene, octene, and %-)f-lupentene, and vinyl compounds such as vinyl acetate, ethyl acrylate, and methyl acrylate. The copolymerization amount of the copolymerized component is O6S~/♂% by weight, preferably about 0~10% by weight.

The branched low density polyethylene has a melt index of 0.3 to 'It/10 min, preferably 0.5 to 3.
The range is 2770 minutes, and the density is 75 to 0.
．． 93017M1, preferably 0, ri//~O0ri3o
The range is t/ad. Melt index is 0. !
If the film is not submerged for f//0 minutes, high-temperature stretching is required, and in this case, a portion of the film is melted in the tenter, leading to a decrease in stretching operability. In addition, if the feed f// is larger than 0 minutes, the film is likely to break during stretching, and even if stretched, the thickness unevenness in the transverse direction (direction) will be excessive, and the film strength will decrease, which is undesirable. , if the above density is less than 0.911971, melt-in 1
If the melt index is less than %t/10 minutes, the same phenomenon will occur, and if the melt index is greater than 3ot/aA, the same phenomenon will occur if the melt index is less than 0.39/10 minutes. Therefore, it is desirable that it not occur.

Furthermore, as the above-mentioned branched low-density polyethylene, (melt index value) × (melting □′
The molecular weight distribution index (hereinafter abbreviated as 1ζ α) expressed by the tension value is 3~/! The range of
Desirable in terms of pushability. The above is less than 3.
The molecular weight distribution becomes sharp, and the lateral ■
Uniform stretchability (thickness unevenness) in the direction is poor, resulting in poor fF-/J'"!'.0""(')G jE"0
(1) It is undesirable because the original film is easily cut during stretching in the transverse direction.

8. In the present invention, the melt index is 5
1 day, (,1□゛ The value measured at 790°C based on K67 O / ゛I$ 70 minutes), and the melt tension is TIE K cost・4...", < 740 Used for melt index measurement: lX□, Ru, 8 Yaka, et al.7.
0°C, o, s, t /1nipr tD speed KM J extrusion, tension (P) measured at a position 26 cm from the nozzle when taken at a speed of /, j 2 rx /writt, Furthermore, the density is the value Ct/C1/l) measured in accordance with J.K. 740.

The blending ratio of the linear low-density polyethylene and branched low-density polyethylene is linear low-density polyethylene 97~j! Parts by weight, preferably from 95 to 60 parts by weight of branched low density polyethylene, preferably from 3 to 60 parts by weight! ~Z0-ju is used within the metropolitan area. If the amount of branched low-density polyethylene is less than the above range, it will not only be difficult to stretch uniformly but also it will be extremely difficult to stretch to a predetermined draw ratio. ), which is not preferable.

The polyethylene resin composition used in the present invention is prepared by tri-blending the above-mentioned linear low-density polyethylene and branched low-density polyethylene in a blender or the like, or after tri-blending, using a conventional melt-mixing method such as a Banbury mixer. , continuous mixer, mixing roll,
It can be obtained by melt-mixing using a kneading extruder or the like and pelletizing.

The polyethylene resin composition obtained as described above has a molecular weight distribution index (hereinafter abbreviated as α) expressed by (melt index value) x (melt tension value):
A range of .about./.theta. is desirable from the viewpoint of stretching operability. If the above α is less than 7, the molecular weight distribution will be sharp, and the uniform stretchability (thickness unevenness) in the lateral direction will be poor during stretching, and if it is greater than 10, the stress during stretching will be excessive, and the lateral stretching This is undesirable because the film may sometimes be easily cut.

Note that, if necessary, known additives commonly used in polyethylene, such as antioxidants, ultraviolet absorbers, antistatic agents, and lubricants, may be added to the above polyethylene resin composition.

In the present invention, an unstretched film or sheet is formed using the polyethylene resin composition obtained above, and then the unstretched film or sheet is biaxially stretched under specific conditions to produce a biaxially stretched film. do.

The unstretched film or sheet is formed using a conventional film or sheet forming apparatus and forming method, such as an inflation molding method using a circular die or a T-die molding method using a T-die, and the above-mentioned composition is heated to a resin temperature of /! 0-2jO
The molding is carried out under the following conditions: °C and draft rate in the range of 2 to 100.

When performing inflation molding, the blow-up ratio should be 7.
．． It is preferable to carry out the process under the conditions of j-da, θ loquat f-dete, and draft rate in the range of 2 to 100.

Note that in the present invention, the draft rate is obtained as follows.

In the formula, the symbols are as follows, and in the case of T-die molding, they are expressed as BUR=/.

If the draft ratio is less than 2, the optical properties of the formed film will be poor, and if it is more than 100, it will tend to tear longitudinally during stretching, which is not preferred.

The unstretched film obtained as described above is then biaxially stretched. The biaxial stretching treatment is carried out by sequentially or simultaneously biaxially stretching an unstretched film obtained by the T-die method or the inflation method, either as it is or after slitting it to a predetermined width. Successive biaxial stretching is carried out in the longitudinal direction (direction in which the film is taken) and then in the transverse direction (
Stretching is carried out either in the direction (perpendicular to the direction of film draw) or in the reverse order. In addition, in simultaneous biaxial stretching, the time distribution of stretching in the longitudinal direction and the transverse direction can be arbitrarily determined. For example, stretching may be gradually continued in the longitudinal direction until the stretching in the transverse direction is completed, or This is done in such a way that the horizontal direction is completed at the same time, but the vertical direction is completed first.

In the present invention, it is possible to adopt a tenter method, sequential biaxial stretching method, tubular method, simultaneous biaxial stretching method, etc., but in particular, when a tenter method, a sequential biaxial stretching method, is adopted, transparency may be reduced. This is preferable since it is significantly improved.

The biaxial stretching process in the present invention is carried out under the following conditions for stretching temperature, stretching speed, and stretching ratio.

The stretching temperature is -20°C, the melting point of the polyethylene resin composition.
- melting point range, preferably melting point -20°C to melting point -70°C
is within the range of If the stretching temperature is below the melting point -20°C, the molecular chains will have poor mobility and will be easily broken during stretching, and even if stretching is possible, the stretching ratio will not increase, making it impossible to obtain a stretched film with excellent physical properties. 1. At temperatures higher than the melting point, the resin composition will partially melt and no stretching orientation will occur, and even if it appears to be stretched, the stretching will be poor and the transparency will be impaired, reducing the commercial value. It's not a certain film.

The stretching speed is in the range of 1 to 0%/sec, preferably 'i:'
It is in the range of Fi/θ to Coθ%/sec. If the stretching speed is slower than 1.J%/sec, noise will occur due to oriented crystallization during stretching. ÷−・:・ ,゛・′ h,,N(,:'l Stretchability is easily inhibited, and if it is faster than 7 seconds, the deformation of the polymer will follow the stretching speed, ゛・:・′・The film becomes unable to be stretched, causing stretching breakage.

Theory, 1. The stretching ratio is important in terms of stretching operability (ease of stretching), 1:, -) Comparison view゛□ Preferably 3 times or less in the vertical direction of the ilm /, 7
~2·'', 1,2 times the range, and 3 to 9 times in the lateral direction, preferably in the range of 9 to 7 times.

Vertical (, blunder) Qi (.

; Stretching operability is improved when the stretching ratio in the direction is greater than 3 times.

However, if the stretching ratio in the transverse direction is less than 3 times, the thickness of the stretched film becomes more uneven, and a satisfactory stretched film cannot be obtained. If the stretched film was not obtained, and if it was twice as large, the stretching operability deteriorated, but the stretched film was not satisfactory.

, yl ”° Linear low-density polyethylene (melt index souse (hereinafter abbreviated as M): 0.J
-f// 0 minutes, density f20, tacho of /lxt, flow ratio: 20. Copolymerization component Nibutene-/, copolymerization amount: 10% by weight)? ! Branched low density polyethylene (high pressure polyethylene, M work: /, 39710 minutes, density: 0,
After tri-blending 9.2 parts by weight of J flad
The mixture was melt-kneaded at a temperature of 90°C and extruded into pellets.

The obtained polyethylene resin composition (melting point/2θ℃) was
Resin temperature (die temperature) 200℃ using die film device
An unstretched film having a thickness of /0μ was formed under the following conditions: cooling roll temperature /j°C, winding speed 30 ml/min, and draft rate j. This original film was stretched using a tenter method sequential biaxial stretching device at a temperature of //θ°C, a stretching speed of Qchi/sec,
Stretching ratio (vertical direction/0.26 times and horizontal direction!, O゛ times)
A 0-axis stretched film with a thickness of μ was manufactured under the conditions of ″″.

The obtained biaxially stretched film was subjected to a film physical property test using the following evaluation method, and the results shown in Table 1 were obtained.

■ Molding stability ○ Can be stably stretched continuously for 7 hours or more.

Δ 20 minutes or more × Cutting (vertical tearing) at the start point of biaxial stretching
arise.

,・, ■ Film thickness unevenness Film on dial gauge with minimum scale/μ.

Width direction 3 (measured at 1m intervals 1.5., thickness unevenness with respect to average thickness 10 and -
100 on the side (,'1, it fraction display, Pa ■ Transparency (Haze)?-,.

,−・,]°,・1,),÷: AE! TM D 10θ3
Based on "3ko・51.・:・,, ■ Rigidity A, I, fit, i,, AS'rM Dr?,
Compliant with 24, :,) $j ■ Impact strength □' Compliant with A8TM D/7θ9-67, ■ Tear strength 4. Compliant with J Engineering EI Pryl-73 □: Vertical tear Notches were made in the machine direction. Strength of tearing. Horizontal tearing. Strength of tearing with notches in the film width direction. Same as Example/, except that the physical properties and blending amount of linear low-density polyethylene and branched low-density polyethylene, unstretched film forming conditions, and biaxial stretching conditions were changed as shown in Table 1. went. The results are shown in Tables 1 and 2.

In Examples /9t~/ and Comparative Examples ♂~/θ Example/, the physical properties and blending amounts of linear low density polyethylene and branched low density polyethylene were changed as shown in Table 3, and unstretched films were prepared. Example/Except that the molding was performed by the inflation molding method instead of the T-die method (the molding conditions are shown in Table 3), and the resulting unstretched film was biaxially stretched as it was (one film at a time). I plan to do the same.

The results are shown in Table 3.

〔Effect of the invention〕

According to the production method of the present invention, a biaxially stretched film using linear low-density polyethylene, which was previously difficult to produce, can be obtained stably and with high efficiency, and the obtained film has excellent transparency and has a uniform thickness. It has excellent strength.

Sender: Mitsubishi Chemical Industries, Ltd. Representative: Patent attorney (if required) - (7 others)

Claims (2)

[Claims] (1) 97 to 55 parts by weight of linear low density polyethylene with a melt index of 0.3 to 4 g/10 minutes and 3 to 45 parts by weight of branched low density polyethylene with a melt index of 0.3 to 4 g/10 minutes. It is characterized by forming an unstretched film or sheet using a polyethylene resin composition, and then biaxially stretching the film or sheet at a stretching ratio of 3 times or less in the machine direction and 3 to 9 times in the transverse direction. A method for producing a biaxially stretched low-density polyethylene film. (2) Branched low-density polyethylene has a melt index of 0.3 to 4 g/10 minutes and a molecular weight distribution index expressed as (melt index value) x (melt tension value) in the range of 3 to 15. The method according to claim 1, characterized in that: