JPS6184224A - Manufacture of polyethylene film having high tensile strength and high modulus - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a process for making polyethylene films having high tensile strength and high modulus.

BACKGROUND OF THE INVENTION It is known (see Dutch Patent Application No. 256,781) to form fibers from concentrated solutions of low molecular weight polyethylene by coagulation spinning and subsequent drawing; Only strength and modulus filaments are obtained. It is also known to spin a dilute solution of high molecular weight polyethylene into filaments (see Dutch Patent Application No. 6501248), in which layer separation occurs between the polyethylene and the solvent (naphthalene). Even in this case, only filaments with low strength and modulus are obtained.

From European Patent Application No. 64167, a method is known for producing fibers, for example by spinning a solution of polyethylene or the like in a non-volatile solvent (paraffins), then cooling, extraction, drying and finally drawing. It is. The resulting fibers have been found to exhibit fairly high creep properties. The patent application also discloses that the method is applied to produce a film.
It only states that it can be done, but does not provide any further details on this point.

According to European patent application no. Methods for producing films are known. The disadvantage of this method is that a non-smooth product is obtained with very high creep properties and also a high degree of opacity and porosity.

The present invention provides a method for obtaining films with high length strength and modulus from moderately dilute solutions of high molecular weight polyethylene (smooth, glossy) that exhibit little or no of the above-mentioned disadvantages.

In accordance with the present invention, a solution of linear polyethylene having a weight average molecular weight of at least 4 x 105 at a concentration of up to 40% (by weight) in a relatively volatile solvent that is liquid at room temperature is prepared by gelation of the solution. forming a film-like solvent-containing product at a room temperature of 100° C. and rapidly cooling the product below the Kelization temperature to form a film having a homogeneous polymer gel structure having substantially the same composition as the starting solution. This is achieved by subjecting the gel film to uniaxial stretching at an altitude of 75° C. or higher at a stretching ratio of at least IO with or without removal of all or a portion of the solvent.

The basic feature of the invention is that a dilute solution of high molecular weight polyethylene in a volatile solvent is formed into a film-like material, which is then thermoreversibly gelled into a gel film, which is then subjected to ultra-high stretching. From this relationship, thermoreversible gelation is
This means that a solvent-containing substance is transformed into a homogeneous gel solely by a decrease in temperature, regardless of changes in the composition (concentration) of the system.

The term film as used herein refers to any film having a thickness of 0.5 am or less and a width:thickness ratio of at least 1.
00:1, preferably 1000:1, a wide, thin film of variable length.

In the process of the invention, one starts from a linear polyethylene solution having a weight average molecular weight of at least 4.times.10@5, preferably at least 8.times.10@5. In this specification,
The polymeric linear polyethylene contains small amounts, preferably up to 5 mol %, of one or more copolymerized other alkenes such as propylene, butylene, pentene, hexene, 4-methylpentene, octene, carbon atoms 1 per 100 pieces
Also meant is polyethylene having the following side chains, preferably no more than 1 side chain per 300 carbon atoms.

The polyethylene may also contain a small amount, preferably at most 2
It may also contain 5% (by weight) of one or more other polymers, especially alkene-1-polymers such as polypropylene, polybutylene, or copolymers of propylene and small amounts of ethylene.

The polyethylene may contain significant amounts of fillers. It is also advantageous to use polyethylene having a (weight average molecular weight)/(number average molecular weight) ratio of 5 or less.

As the molecular weight of polyethylene increases, the viscosity of the solution also increases, making it more difficult to handle, so the method of the present invention is also applicable to polyethylene of high molecular weight, but in general, 15X1
Polyethylene of molecular weight above 06J2i is not used.

Weight average molecular weight is determined by gel permeation chromatography and light scattering according to known methods.

The concentration of polyethylene in solution depends in part on the nature of the solvent and the molecular weight of the polyalkene.

Solutions with a concentration of 40% (by weight) or higher are rather difficult to handle, especially when using polyethylene of very high molecular weight, for example 1 x 106 or higher, due to their high viscosity. On the other hand, if a solution with a polyethylene concentration of, for example, 0.5% (by weight) or less is used, there is a drawback that the yield decreases and the cost of separating and recovering the solvent increases. Therefore, generally the polyethylene solution is 2-20% (by weight), especially 3-15% (by weight).
Start with a concentration of (by weight).

The choice of solvent is not critical. Any volatile solvent can be used, such as halogenated or non-halogenated hydrocarbons. For most solvents, polyethylene
It melts only at temperatures above 90°C. When a solution is formed into a film by spinning, it is generally carried out under atmospheric pressure. Therefore, low boiling point solvents are undesirable because they quickly evaporate from the film and often act as blowing agents, adversely affecting the structure of the film.

To make the solution into a film-like product, it can be extruded through a spinning head with a very wide slit die, etc.
This can be done in various ways.

Of course, instead of spinning, the solution can also be poured, for example onto belts or rolls, extruded, rolled or calendered.

Upon rapid cooling, a solution of polyethylene material in the above concentration range turns into a gel below the critical temperature (gel point).

For example, when spinning, a solution must be used and the temperature must be below the gelling point.

For example, during spinning, the temperature of the solution is preferably less than 100°C, preferably at least 120°C, and the boiling point of the solvent is preferably at least at least 100°C, more preferably at least equal to the processing or spinning temperature. The solvent must have a low boiling point to be easily evaporated from the resulting film.Suitable solvents include cycloaliphatic hydrocarbons or aromatic hydrocarbons with a boiling point of at least 100°C. Hydrogen, such as toluene, xylene, tetralin, decalin, and) lambda halogenated hydrocarbons, such as monochlorobenzene, and other known solvents. Due to their lower cost, unsubstituted hydrocarbons are often preferred;
Included are aromatic hydrocarbons, especially halogenated derivatives of decalin.

In order to substantially prevent thermal decomposition of the polyethylene, the processing and melting temperatures should not be too high.

Therefore, this temperature is generally selected not to exceed 240°C.

The film-like product thus obtained is cooled to below the gelling temperature of the solvent. This is done by any suitable method, such as passing the product through a liquid bath or shaft. When the polyethylene solution is cooled below its gel point, the polyethylene forms a gel. This polyethylene gel film has sufficient mechanical strength to be further processed by known methods, such as guides, rolls, etc.

The gel film thus obtained is then stretched.

During this drawing operation, the gel still contains a significant amount of solvent, ie, to about the same extent as is present in the polymer solution spin. Also, some or essentially all of the solvent may be removed from the film prior to stretching, for example by evaporation or washing with an extractant. Although it may be advantageous to recover most of the solvent before stretching,
A significant amount of J, i.e. 25% (by weight) or more, preferably 50%, as ultimately a higher degree of stretching is obtained, resulting in a higher tensile strength and modulus of the final product film.
(weight) or more of the solvent is preferably stretched.

It is preferable to stretch the film at an elevated temperature, particularly at 75°C or higher. In this operation, stretching is preferably carried out at or below the melting point of the polyethylene. This is due to the mobility of polymers (mobility) at temperatures higher than this degree of blackness.
This is because the desired stretching cannot be achieved or cannot be achieved to a satisfactory degree, since the .lity) immediately becomes very high. Also, intermolecular heat generation that occurs due to film stretching must be taken into account. That is, at high draw ratios, the height of the film increases significantly and care must be taken to avoid reaching near or above the melting point.

It is also possible to bring the film to the stretching temperature by passing it through a layer containing 17 a gaseous or liquid medium and maintained at the desired temperature. As gaseous medium, an air-filled tubular oven is very suitable, but liquid baths or other suitable devices can also be used.

During stretching, the solvent (if present) is removed from the film. This is preferably done by passing hot gas or hot air along the film in a stretching zone, for example, or by stretching in a liquid bath containing an eluent for the solvent, which may optionally be the same as the solvent. This can be done by any suitable method, such as by removing solvent vapors. The final film product must be solvent-free; it is advantageous to choose the conditions such that this condition is already achieved, or at least substantially achieved, in the stretching zone.

Modulus (E) and tensile strength (S) were calculated from (strength)/(elongation) curves obtained by measurements at room temperature on an Instron tensile tester at a test rate of 10%/min and Convert to the cross section of

In the method of the invention, high draw ratios can be applied.

The film has at least (], ]2X106/MW-1-
1 times (Mw is the weight average molecular weight of polyethylene), especially,
It is preferable to stretch at least (14×10 6 /MW+1) times.

The films obtained according to the invention are suitable for various applications. The film can be cut into durable bands, ribbons, and tapes. The film can be made of various known materials reinforced by film or tape.
It can be used as a reinforcing material, and can be used in all applications where light weight and high strength are desired, such as audio recording tape or magnetic tape, medical tape, packaging film, protective sheets, adhesive substrates, etc. can.

If desired, small amounts of conventional additives, stabilizers, fiber treatment agents, etc. may be incorporated in or on the film, particularly from 0.1 to 10% (by weight) for polyethylene.

The present invention will be further explained with reference to the following examples, but the present invention is not limited thereto.

Comparative Example ■ Weight average molecular weight of approximately 2X in paraffin at a temperature of approximately 180°C
A 5% (by weight) solution of high molecular weight polyethylene of the Hifax-19O0 (Vercules) clade of IO6,
Pour onto a cooled conveyor belt to form a gel product approximately 2 mg thick and approximately 100 m wide. The gel film thus obtained was passed through a bed of trichlorethylene for solvent removal and then subjected to a temperature gradient (from 120 to
145° C.) in an oven.

At a draw ratio of 15 times, a film with an E-modulus (measured at room temperature) of 22 GPa was obtained. 25x and 30
At double draw ratio, the E-modulus is 40 and 52, respectively.
It was GPa.

Particularly under load and at elevated temperatures, the product exhibits considerable creep.

EXAMPLE A 2.5% (by weight) solution of high molecular weight polyethylene (Hostaren GUR 412, Luerhiemi/Bechst) with a weight average molecular weight of about 1.5×106 in edecalin was extruded at 175° C. from a sleeve (I×40 mm) and then The gel film obtained by quenching in water was added to an extraction bath (
The solvent was removed by a roller system in dichloromethane). The gel film was stretched at stretching ratios of 20x and 33x.
It was stretched in an oven at 00°C.

The resulting smooth and glossy film had an E-modulus of 50.85 GPa, an opacity of 10% or less, and a moisture permeability of 0.5 or less (Kunststoffe/Plastic 7,
(Kunststoffe/Plastics)
7/73. Determined according to the standard method described on page 25).

Even under long-term loading, the film shows little creep.

Example ■ A 10% (by weight) solution of DSM high molecular weight polyethylene having a weight average molecular weight of approximately 6 x 1.05 in xylene was
Extruded through a slit (ix40mm) at a temperature of 0-180"C, then cooled by passing air through the shaft,
Wet stretching was carried out in an oven with a temperature gradient of ~140°C, during which the solvent was evaporated. At stretch ratios of 10x, 25x and 35x, the E-modulus was 12.20 and 27, respectively.
A smooth film with GPa was obtained.

Claims (6)

[Claims] (1) A weight average molecular weight of at least 4 x 10^ at a concentration of 40% by weight or less in a relatively volatile solvent that is liquid at room temperature.
The linear polyethylene solution of No. 5 is made into a film-like solvent-containing product at a temperature above the gelling temperature of the solution, and the product is rapidly cooled to below the gelling temperature to produce a product having substantially the same composition as the starting solution. forming a film having a homogeneous polymer gel structure, and uniaxially stretching the gel film at a temperature of 75° C. or higher with a stretching ratio of at least 10, with or without removing part or all of the solvent. A method for producing a smooth, glossy polyethylene film with high tensile strength and modulus and low creep, opacity and porosity from a semi-dilute solution of high molecular weight polyethylene. (2) The manufacturing method according to item (1) above, using a polymer solution having a concentration of 2 to 20% by weight. (3) The manufacturing method according to item (1) or item (2) above, which uses a solution of linear polyethylene having a weight average molecular weight of at least 8 x 10^5. (4) A method for producing polyethylene substantially the same as described below and/or as further described in the Examples. (5) A polyethylene film obtained by applying one or more of the manufacturing methods described in items (1) to (4) above. (6) A product manufactured wholly or partially from the polyethylene film of paragraph (5) above.