FR2570982A1 - PROCESS FOR THE PREPARATION OF POLYETHYLENE FILMS WITH HIGH TENSILE RESISTANCE AND HIGH MODULE, POLYETHYLENE FILMS OBTAINED BY THIS PROCESS AND ARTICLES PREPARED THEREFROM - Google Patents

Abstract

THE OBJECT OF THE INVENTION IS SMOOTH AND GLOSSY POLYETHYLENE FILMS HAVING HIGH STRENGTH AND MODULE, AND LOW CREEP, OPACITY AND POROSITY. A SOLUTION AT 1 TO 40 BY WEIGHT OF A POLYETHYLENE HAVING A M 4 10 IN A VOLATILE SOLVENT INTO A FILM IS CONVERTED AT HIGH TEMPERATURE, THE FILM IS CONVERTED BY THERMOREVERSIBLE GELIFICATION INTO A GEL FILM AND IT IS SUBJECTED TO A GEL. HIGH TEMPERATURE AT ULTRA-HIGH UNIAXIAL STRETCH. USING THIS PROCESS, IT IS POSSIBLE TO PREPARE FILMS WITH MODULES GREATER THAN 50GPA, OPACITIES LESS THAN 10 AND WATER VAPOR TRANSMISSIONS OF LESS THAN 0.5, PRACTICALLY WITHOUT CREEP AND HAVING A SMOOTH AND SHINY SURFACE.

Description

The present invention relates to a process for the preparation of films

polyethylene having a

  high tensile strength and high modulus.

  NL-A-256,781 discloses how to convert highly concentrated solutions of poly-

  low molecular weight ethylene by coagulation spinning

  fiber and subject them to further stretching, a process in which only filaments of moderate strength and modulus can be obtained. It is also known from NL-A-6501248 how to spin dilute high molecular weight polyethylene solutions into filaments, in which process the separation between the polyethylene and the solvent (naphthalene) takes place. In this case again, only filaments with resistance and

weak module.

  EP-A-64,167 discloses the manner of

  spinning fibers and then cooling, extracting, drying and finally drawing a solution, for example, polyethylene in a non-volatile solvent

  (paraffin). It can be seen that the resulting fibers

  feel a relatively high creep. This request merely stipulates, without giving any other details on this subject, that by the application of this process, one can

  also prepare dandruff.

  From EP-A-115.192, it is known how to prepare films, in particular by dissolving a high molecular weight polyethylene at a high temperature in a paraffinic wax which is solid at room temperature, by extruding the solution and cooling and successively stretching the film. extrudate. The disadvantage of this method is that a non-uniform product with a very high creep and a high degree of

opacity and porosity.

  The present invention provides a method for obtaining films having very high tensile strength and modulus and practically not exhibiting or demonstrating any

  mentioned drawbacks, based on a semi-permanent solution

  diluted with a high molecular weight polyethylene (smooth, glossy). This result is achieved according to the invention in that a solution of less than 40% (wt) of a linear polyethylene having a molecular weight is converted.

  mass ratio of at least 4 x 105 in a solvent relative to

  volatile, which is liquid at room temperature,

  at a temperature above the gel temperature

  cation of the solution into a film-shaped article containing solvent; we quickly cool the article to

  a temperature below the gelation temperature

  while forming a film having a structure of

  homogeneous polymer gel having virtually the same composition

  sition as the starting solution; and this gel film, at a temperature above 75 ° C, is subjected to uniaxial stretching with a draw ratio of at least one, possibly after the removal of all or

part of the solvent.

  The basic idea of the present application is to convert a dilute solution of high molecular weight polyethylene in a volatile solvent into a film-shaped article and convert it into a film of heat-reversible gelation.

  gel which is then subjected to ultra-high stretching.

  The expression "thermo-reversible gelling" means in this context the conversion of an article containing solvent into a homogeneous gel exclusively by temperature drop and not by a change in the composition.

(concentration) of the mixture.

  Whenever the term "film" appears in this specification, it is used to indicate a product of indefinite length in the form of a thin, broad film less than 0.5 mm thick and having a ratio of width / thickness is at least

  100: 1, preferably at least 1000: 1.

  In the present process, one starts from a poly-

  linear solution ethylene having a weight average molecular weight of at least 4 x 105 and preferably at least 8 x 105. Linear high molecular weight polyethylene means a polyethylene which may contain small amounts of preferably not exceeding mole%, of one or more other alkenes copolymerized with it, such as propylene, butylene, pentene, hexene, 4-methylpentene, octene, etc., with less than 1 chain per 100 carbon atoms and preferably less than 1 side chain per 300 carbon atoms. The polyethylene may contain small amounts, preferably up to 25% by weight, of one or more other polymers, in particular an alkene-1 polymer such as polypropylene, polybutylene or a propylene copolymer with a

small amount of ethylene.

  Here, the polyethylene used may contain significant amounts of filler. It can also

  be advantageous to use a polyethylene whose ratio

  weight average molecular weight to molecular weight

  number average is less than 5.

  Since the viscosity of the solu-

  This increases when the molecular weight of the polyethylene increases, so that it becomes more difficult to treat, in general no polyethylene with a molecular weight greater than 15 x 10 6 is used, although in fact the present process can be achieved with higher molecular weights. Mass average molecular weights can be determined by known methods using permeation chromatography

  of gel and the diffusion of light.

  The concentration of polyethylene in the solution may vary partially depending on the

  nature of the solvent and the molecular weight of the poly-

  alkene. Solutions having a concentration of more than 40% by weight are rather difficult to treat, in particular when a very high molecular weight polyethylene, for example more than 1 × 10 6, is used,

  because of the high viscosity that is then established.

  On the other hand, the use of solutions having a concentration of polyethylene for example less than 0.5% by weight has the disadvantage of a decrease in yield and an increase in the costs of separation and recovery of the solvent. . In general, we start

  therefore with a solution of polyethylene having a concentration

  from 2 to 20%, especially from 3 to 15% by weight.

  The choice of the solvent is not critical.

  Any volatile solvent such as hydrochloric acid

  halogenated or non-halogenated carbide. In most solvents, polyethylene can only be dissolved at temperatures of at least 90 C. If the solution needs to be

  converted into films by spinning, it is generally done

  ral in a space under atmospheric pressure. Low-boiling solvents are then less advantageous because they can be evaporated so fast that they will play the role of more or less foaming agents and interfere with the structure of the films.

  The conversion of the solu-

  in a film-like article in various ways, for example by spinning through a spinning head having a very wide slot die. Of course, instead of spinning, one can simply pour the solution on, for example, a belt or a roller,

  extrude, roll or calender.

  Under rapid cooling, the solutions

  of polyethylene materials in said concentration range

  in a gel below the critical temperature (gel point). For example, during spinning, we must use a solution and we must

  keep the temperature above this gel point.

cation.

  For example, during spinning, the temperature

  solution is advantageously at least 100 C and especially at least 120 C and the boiling point of

  solvent is preferably at least 100 C and in particular

  at least equal to the conversion or spinning temperature. The solvent must have a boiling point sufficiently low to facilitate its evaporation of the films obtained. Suitable solvents are cycloaliphatic and aromatic hydrocarbons having boiling points of at least 100 C, such as toluene,

  xylenes, tetralin, decalin and also hydro-

  halogenated carbons such as monochlorobenzene and other known solvents. As a result of the low price, unsubstituted hydrocarbons which also include hydrogenated derivatives of aromatic hydrocarbons are particularly preferred, in particular

the decalin.

  The conversion temperature and the temperature

  Dissolution should not be so high as to cause significant thermal decomposition of the polyethylene. So we do not usually choose a

temperature above 240 C.

  The product obtained is cooled in the form of

  film below the gel point of the solution.

  tion. This can be done in any convenient manner, for example by passing the product through a liquid bath or into a "well". During cooling below the gel point of the solution of

  polyethylene, polyethylene forms a gel. A film

  This polyethylene gel has a sufficient mechanical strength to permit subsequent processing, for example by guiding members, rollers, and other usual organs in this process.

industry.

  The gel film thus obtained is subsequently stretched. In this stretching operation, the gel may still contain significant quantities of solvent, up to the amounts only slightly lower than in the polymer solution ready for spinning. Similarly, prior to drawing, some or all of the solvent may be removed from the gel film, for example by evaporation or by

  washing with an extractant. We give the preference

  stretching gel films still containing significant amounts, of more than 25% by weight and, preferably, more than 50% by weight of solvent, since if proceeding in this way it is possible to obtain a greater final degree of stretching and, therefore, higher tensile strength and modulus of the final film; in some embodiments, however, it may be more advantageous to recover a large portion of the solvent prior to drawing. The films are stretched at a temperature

  in excess of 75 C. In such an operation,

  tion, the drawing is preferably carried out below the

  melting point or the point of dissolution of the poly-

  ethylene, because above this temperature the mobility

  the macromolecules soon become so high that it is impossible to obtain the desired orientation or

  only to an insufficient degree. Intra-community development

  Molecular heat from the stretching of the films should also be taken into consideration. At high stretching rates, the temperature of dandruff can therefore be significantly increased and care should be taken not to allow it to move closer to

  melting point and especially to exceed it.

  The film can be worn at room temperature

  by stretching them through an area containing

  a gaseous or liquid medium which is maintained at the desired temperature. A tubular furnace with air as a gaseous medium is perfectly suitable, but

  can also use a liquid bath or any other

appropriate.

  During drawing, the possible solvent

  present is separated from the film. This operation is advantageously produced by appropriate measures such as the evacuation of the solvent vapor by passing a stream of gas or hot air along the film in the drawing zone, or by stretching in a liquid bath comprising an extractant for the solvent, which agent may be optionally the same as

  the solvent. The final film must be free of soil

  and it is advantageous that the operating conditions chosen are such that this state is already reached in the drawing zone, at least virtually. -

  The modules (E) are calculated and resisted

  tensile strengths by means of elongation curves.

  Based on the resistance determined at room temperature using an Instron apparatus at a test speed of 10% per minute, they are reduced to the cross section of the film sample. In the present process, high draw ratios can be applied. Stretching of films of at least (12 x 10 6 / M + 1) times is preferred, wherein Mp is the weight average molecular weight of polyethylene and in particular

less (14 x 106 / M + 1) times.

P

  The films according to the invention are

  for many applications. We can discover them

  per to form resistant bands, cords or ribbons. They can be used as reinforcement in many materials that it is usual to reinforce with

  films or ribbons and in all

  in which the combination of low weight and high strength is sought, for example, audio-visual or magnetic tapes,

  tapes for medical purposes, packaging films

  protective foils, adhesive substrates, etc. If desired, small amounts of additives may be incorporated in or on the films

  such as stabilizers,

  fibers and the like, in particular in proportion

  from 0.1 to 10% by weight relative to polyethylene.

  The following examples, in which all

  the proportions are by weight, serve to illustrate the in-

  without limiting its scope.

COMPARATIVE EXAMPLE 1

  On a cooled conveyor belt was poured a 5% high molecular weight, Hifax 1900 (Hercules) high molecular weight polyethylene solution having a weight average molecular weight of about 2 x 10 6 in paraffin at a temperature of about C. to form a gel having about 2 mm thick and about 100 mm wide. The resulting gel film is passed through a bed of trichlorethylene to remove the solvent and is then drawn in an oven at a temperature gradient of 120 ° C to 145 ° C.

variable stretch ratios.

  With a draw ratio of 15x, films having a modulus E (measured at room temperature) of 22 GPa are obtained: At a draw ratio of 25 × and

  30x, the module E is respectively 40 and 52 GPa.

  In particular, under a temperature load

  ture, creep is essential.

EXAMPLE 1

  A 2.5% solution in the high molecular weight polyethylene decalin ("Hostalen GUR 412" from Ruhrchemie / Hoechst) having a mass average molecular weight of about 1 is passed through a 1 × 40 mm slot. , 5x106 at 175 ° C, and then cooled in water, after which the resulting gel film of the solvent is freed by means of a set of rollers in an extraction bath (dichloromethane). The gel film is stretched in an oven at 120 ° C with a draw ratio of 20x and 33x. The resulting smooth and shiny film has a modulus E of 50 and especially 85 GPa, an opacity of less than 10% and a vapor transmission

  less than 0.5 (measured by the normal

  cited by Kunstsoffe / plastics 7/73, page 25).

  Under a prolonged charge, the film does not

  also has virtually no creep.

EXAMPLE 2

  It is passed through a 1 x 40 mm slot at a temperature of 170-180 ° C., a 10% solution in xylene of high molecular weight polyethylene supplied by the company DSM and having a weight average molecular weight of 6x105, then cooled in air in a "well" and stretched in a humid state in an oven with a temperature gradient of 90 to 140 C,

  process in which the solvent evaporates.

  With draw ratios of 10x, 20x and x, smooth films with modules are obtained

Ede 12, 20 and 27 GPa.

2570982.

Claims (5)

R E V E N D I C A T I 0 N S   1. A process for preparing smooth and glossy polyethylene films having high tensile strength and modulus, low creep, and low opacity and porosity, calculated on a semi-dilute solution of high molecular weight polyethylene characterized by converting a solution of less than 40% (eds) of linear polyethylene having a weight average molecular weight of at least 4 x 105 in a relatively volatile solvent, which is liquid at room temperature, at a temperature of   above the gelling temperature of the solu-   in a film-shaped article containing   solvent; the article is rapidly cooled to a temperature   below the gelation temperature while forming a film having a gel structure of   homogeneous polymer having virtually the same composition   tion than the starting solution; and this gel film, at a temperature above 75 ° C, is subjected to uniaxial stretching with a draw ratio of   minus 10, possibly after removal of the total or part of the solvent.   2. A process according to claim 1, characterized in that a solution of   polymer having a concentration of 2 to 20% by weight.   3. Process according to claim 1 or   2, characterized in that a solution of poly-   linear ethylene having a high molecular weight mass of at least 8 x 105.   4. Polyethylene film obtained by the process according to any one of preceding claims.   5.- As a new industrial product, articles prepared in whole or in part from   of a polyethylene film according to claim 4.