FR2504537A1 - CONTRACTABLE FILMS OF COPOLYMERS ETHYLENE / ALPHA-OLEFIN - Google Patents

Abstract

THE INVENTION RELATES TO PLASTICS. IT CONCERNS A THERMO-CONTRACTABLE FILM SHAPED BY STRETCHING A FILM CONSTITUTED BY 5-100 BY WEIGHT OF A LINEAR COPOLYMER OF ETHYLENE AND OF AN ALPHA-OLEFIN IN CC AND 0-95 BY WEIGHT OF A HOMOPOLYMER D ' ETHYLENE OR AN ETHYLENE COPOLYMER AND AN ETHYLENICALLY UNSATURATED COMONOMER. USE IN THE PACKAGING FIELD.

Description

250453;

  The present invention relates to contractable films based on linear low density copolymers selected from ethylene with certain alpha-olefins, films which have outstanding optical properties and a good set of other physical properties and

contraction properties.

  Contractable polyethylene film and various ethylene copolymers are well known; see, for example, U.S. Patent Nos. 3,299,194 to Golike and No. 3,663,662 to

Golike's names and others.

  A contractable film of polyethylene, used primarily for packaging food products and various consumer items, must have good optical clarity; otherwise, the consumer appeal of the article thus packaged would be diminished or lost. For practical purposes, the film should shrink in the temperature range of about 100 to 12000 by at least 15% in the direction of orientation and with sufficient force to form a tight skin

  adjusted around the item enclosed in the package.

  The film must also have good mechanical properties, such as tensile strength and modulus, so that it elongates and then contracts without tearing, that it maintains a good physical contact with the packaged article. any time, and that it is not easily damaged at

course of handling.

  Prior art for forming contractable ethylene polymer films required polymerization prior to drawing to

  to give the film greater mechanical strength

  This crosslinking was carried out

  by irradiation with particles of high

or by gamma rays.

  In order to obtain a resin composition giving films having satisfactory properties for contractable film applications without crosslinking prior to drawing, it has been generally necessary in the past to mix low density ethylene polymers. high and mass

  high volume Naturally, it would be desirable

  It is possible to form contractable films from a single ethylene polymer resin of low density. In the present context, the term "low density" means 0,940 g / cm 3 or less and "high density" means more than 0.940 g / cm A recent commercial offer from the Dow Chemical Company, namely low density "polyethylene" resins DOWLEX @, is described in a bulletin of this company as giving a blown film having excellent optical properties and superior mechanical strength properties Yet, the same bulletin indicates that these

  resins can not be used to form film

  Contractable because they will shrink less than conventional low density polyethylene film and contract in a narrower temperature range DOWLE Xe resins are actually ethylene copolymers

with 1-octene.

  According to the present invention, there is now

  a contractable film having high optical clarity, good contraction properties and good mechanical properties, which film is obtained by stretching at least three times its initial linear dimension in at least one direction a film formed of the polymer composition. homogeneous composition: (1) 5-100% by weight of at least one linear copolymer of ethylene with at least one C 8 -C 18 alpha-olefin, said copolymer having the following characteristics: (a) melt index; the melt state of 0.1-4.0 g / 10 min; (b) density of 0.900 to 0.940 g / cm 3 (c) stress exponent of greater than 1.3; and (d) two distinct regions of crystallite melting below 128 C as determined by Differential Scanning Calorimetry (DSC), the temperature difference between these regions being at least 15 C; and (2) 0-95% by weight of at least one selected polymer

  in the group consisting of homopolymers of ethyl-

  ethylene and copolymers of ethylene with a comonomer

  ethylenically unsaturated, this polymer having only one

  a crystallite melting point below 12800 C; with the proviso that the drawing is carried out in the temperature range defined by the two melting points of the oristallites of the copolymers

  ethylene with the C 8-18 alpha-olefins in para-

graph (1) above.

  The drawings show DSC curves for three different resins. Figure 1 is the curve for polyethylene, Figure 2 for a commercial linear ethylene / 1-octene copolymer, and Figure 3 for a mixture of ethylene polymers of

  high and low density.

  The main resin used in composi-

  according to the present invention is a copolymer

  ethylene with an alpha-olefin

  Typical olefins which can be copolymerized with ethylene are 1-octene, 1-decene, 1-undecene, 1-dodecene and 1-hexadecene. The copolymers are prepared at low to moderate pressure (about 29.4 ° C.). M Pa) in the presence of a coordination catalyst according to the generally known technique of so-called Ziegler and Natta processes Typical catalysts are various organoaluminum, organotitanium and

  organovanadium, and especially organic compounds

  aluminum modified with titanium The preparation of

  ethylene polymers with alpha-olefins is taught, for example, in US Pat. No. 4,076,698 to Anderson et al.

  NI 4,205,021 to Morita et al.

  Suitable commercially available ethylene copolymers with higher alpha-olefins include the aforementioned DO O WLEX resins,

  and the preferred copolymer is that with 1-octene.

  When the proportion of alpha-olefin in the copolymer or the molecular weight of the alpha-olefin increases, the density of the copolymer decreases. For 1-octene, the amount of this alpha-olefin in the copolymer will normally be between about However, the amount of each of these comonomers will be chosen so that the appropriate values of

  melt flow, density and expo-

  These proportions are easily established from known relationships and can be verified experimentally by means of standard techniques. Thus, the melt flow index is determined according to the method of the invention.

  ASTM D 1238 (condition E) and the mass

  according to ASTM D 1505 The exponent of

  is the slope of the graphical representation of the logarithm of the flow velocity versus the logarithm of extrusion force Since the plot is not linear, the slope according to ASTM D 1238 is determined using 2160 g and 640 g, both at 1900 C. The copolymers must give two distinct crystallite melting peaks, which means that they have two different groups of crystallites, each having its own distinct melting region. For ethylene copolymers These 1-octene regions will be at about 1070 ° C. and 125 ° C. FIG. 1 is a typical DSC curve of AH in milliwatts as a function of C temperature for a conventional polyethylene having a density of 0.917. This polymer has only one peak. at about 107 ° C A DSC curve for the ethylene / 1-octene copolymer DOWLEX b 2045 (d 0.920) is shown in FIG. 2. The higher temperature peak is actually a doublet, and the The highest melting temperature of the doublet is taken as a characteristic of this peak. FIG. 3 is a DSC curve for a mixture of high density linear ethylene / 1-octene copolymer with conventional polyethylene. The density of the mixture is 0.926. It can be seen that the mixing peaks correspond to those of the DOWLEX resins shown in Figure 2. DSC is a well-known technique for measuring polymer crystallite melting temperatures. Linear copolymers of ethylene with 1-octene or another alpha-olefin, in which the alpha-olefin comonomer is present in such small amounts that a second peak of DSC is not observed, can not be used in the present invention.

  crystallites in the ethylene / alpha-

  olefin is their most notable feature because dies formed from these copolymers

  can be oriented between these two temperatures.

  Contractable films formed of these copolymers have excellent properties, very comparable to those of contractable films formed from mixtures of ethylene polymers of low density and high density, for example those described.

  in US Pat. No. 3,299,194.

  However, it has been found that the presence of as little as 5 percent by weight of an ethylene / alpha-olefin copolymer of this class

  in a mixture with a homopolymer or co-polymer

  Ethylene oxide having a single crystallite melting region below 128 ° C. can sometimes improve the properties of the latter polymer to such a large extent that excellent contractable films having advantageous physical properties, including high optical clarity, can be used. These conventional homopolymers or copolymers can be of a high or low density, linear or branched, formed at high pressure or at low pressure. Copolyners can be prepared from the mixture.

  those formed with any comonomer, com-

  for example, alpha-olefins, vinyl esters, alkyl acrylates and methacrylates, and

  acrylonitrile Many of these polymers are available

  commercially available from several

  These mixtures can be prepared by a

  that any classical able to produce a material

homogeneous uniform.

  A film is formed from the copolymers

  mixtures or mixtures above by a suitable extrusion process

  The film is tubular or flat. It is stretched, preferably biaxially, in the plane of the film at least 3 times its initial dimension in each direction, preferably

  at least 5 times A convenient process, which combines the ex-

  trusion and orientation of polymeric films, is described in US Pat. No. 3,141,912.

from Goldman and others.

  When it is subjected to a temperature of

  viron 100 to 1200 C, an oriented film that is not

  stressed will shrink by at least

  15%, and this contraction will be accompanied by a

  considerable force, usually at least 1400 k Pa.

  The preferred contractable films will shrink by at least 30% at a temperature just below the peak crystallite melting peak of at least 1% to 100%. The contraction force at 10,000 must be greater than about 350 kPa. The haze should be less than 4%, preferably less than 21%.

  8 to be greater than 90, preferably greater than 110.

  A limited amount of crosslinking can be introduced after drawing, but before contraction, if desired. This can be done with a minimum amount of high energy radiation, normally less than 8 Mrads, as described, for example, in US Patent No. 5,663,662 to Golike et al. Irradiated oriented films have improved melt strength and are less sensitive to temperature differences in the process.

contraction tunnel.

  The present invention is now illustrated by

  the following representative examples, in which all

  all parts and all proportions are by weight.

  In all cases, the thickness of the film

ble is about 0.025 mm.

  all results obtained in units other than

  SI units have been converted to SI units.

  The contraction of films was determined

  Marked markers of a fixed length, usually 100 mm, of a strip of film placed unconstrained in a bath at a temperature of 1000 ° C and calculating the contraction in the form of

  percentage of change in length.

  Contractive force was determined according to

  in accordance with ASTM 2838 The module was determined,

  tensile strength and elongation at rup-

  according to ASTM D 412.

  The ethylene resins used in the examples

  are shown in Table I below.

r-4 Ln

C'J TABI & LAU I

  Fusion Teemp Resin, 10 (by DSC)

At 124, 10-7

B 126

a 103

D 3 126

Volumetric mass

0, O 950

  Expos 8 Jxt Melt Flow Factor ____ ____ Fade 1.4 1,, 8 0.917 0.940 1.9 0.45 4.0

% from 1-

octene

Description

  Linear low density linear copolymer 0.7 Linear copolymer of high density Low density branched copolymer 3.6 Low density linear copolymer Co c 4537

EXAMPLE 1

  An oriented tubular film was prepared by the method of Eo UA No. 3,141,912 in the name of Goldman. A 5 cm extruder operating at 2300 C and a feed rate of 0.9 kg of ethylene polymer resin per hour produces the film at a speed of 2.7 m / min The hot tubular film is cooled rapidly, heated to 115-1201 ° C and blown at an internal pressure of 2 kPa. The blowing is limited by a cooling ring so to give an elongation to five times the initial dimension in the transverse direction The rollers are operated so as to obtain an elongation

  five times the initial dimension in the long-term direction

gitudinale.

  A contractable film formed from resin A according to the present invention is compared to a prior art contractable film formed from a mixture of resins B and C (in a ratio of 26:74 respectively) according to the teachings of US Pat. US Pat. No. 3,299,194 in the name of Gilike Films are placed around objects, sealed with a hot wire and contracted in a tunnel maintained at 167 C. The appearance of the packages in both cases is identical The properties of the two contractable films are compared in Table II below. All the properties other than the haze and the luster are indicated in the form of a

  report: machine direction / transverse direction.

TABLE II

  Resin Type * Module, M Pa Tensile Strength, Elongation,% Tear Off, g / mm Contraction

(100 C)%

Contraction force

295/260

/ 108

240/195

1480/1280

19/25

360/330

69/56

152/128

267/462

  27/30 (1000 C) k Pa 1810/3590 2960/3450 Sail,% 5,5 5.6 Eclat 85 93

  * See Table I for description of the resins.

ESPLE 2:

  Mixtures of resins are prepared as shown in Table III below, the melt is mixed in a single-screw, single-screw, and melt-pressed extruder in cm x 5 cm films. These films are stretched five times the initial dimension at 120 ° C in each direction in a laboratory stretching machine (TM Long Co,

Inc, Somerville, NJ).

  The physical properties of the films according to the present invention (A / B and A / D mixtures) are compared in Table III with those of prior art films of ethylene polymer blends (B / O and C / D blends). ) The improvement of

  physical properties, especially optimum properties

  in films according to the present invention

is obvious.

TABLE III

  Mixture of resins * Substantially high density component Type **% Density component quite low Type **

B D B D

26 37 20 30

C C A

74 63 80

A 7 o A B + C

(26:74)

  QO t 537 TABLE III (cont'd) Properties of the film Module, M Pa 367 458 583 508 lacquered on the ground 82 64 106 119 n úla î o Agement,% 80 106 131 114 Tearing, g / mm 295 336 380 380 Contraction 8 8 6 10

(10000)%

  Contraction force (10000 C) k Pa 1170 965 1420 1240 Sail,% 6.5 4.3 3.8 2.4 Shatter 65 66 73 121 * The proportions are chosen so as to give a density of the mixture of 0.926 g / cm 3

  ** See Table I for the description of

resins.

EXAMPLE 3

  Films directed from

  mixtures of resins A and C (see Table I).

  Stretching is carried out at 110-112 C using the same

  technique and the same equipment as in Example 2.

  The physical properties of stretched films are shown in Table IV below. It can be seen that all properties change when the proportion of conventional low density polyethylene is increased (Resin C). The most notable change is the significant decrease in the contraction force with retention of the high level of

contraction.

TABLE IV

  Proportion of Resin C in Resin Blend A / C Film Properties Module, M Pa Tensile Strength, M Pa Elongation,% Tear Off, g / mm Contraction (10000 C),% Contraction Strength (100 C), k Pa Sailing,% Eclat

0 25 50 75

1.0 O 30

2100 1670 1210

1.7 2.4 1.6

139,119,100

Có 537

Claims (7)

  A contractable film formed by stretching at least three times its initial linear dimension in at least one direction a film formed of the following homogeneous polymer composition: (1) 5-100% by weight of at least one linear ethylene copolymer with at least one C 8 -C 18 alpha-olefin, said copolymer having the following characteristics: (a) melt index of 0.1-4.0 g / 10 min; (b) density of 0.900 to 0.940 g / cm 3; (c) stress exponent of more than 1.3; and (d) two separate crystallite melting regions below 128 C as determined by Differential Scanning Calorimetry (DSC), the temperature difference between these regions being at least 15 C; and (2) 0-95% by weight of at least one polymer selected from ethylene homopolymers and   copolymers of ethylene with an ethylenic comonomer   unsaturated polymer, this polymer having only a crystallite melting point below 1280 C; with the proviso that the stretching is carried out in the temperature range defined by the two above-defined melting points of the crystallites of the ethylene copolymer with an alpha-olefin in   C 8-C 18 of subsection (1) above.   The film of claim 1, wherein   characterized in that it is formed of a copolymer of ethylene with 1-octene.   The film of claim 2, wherein   in that the proportion of 1-octene is Viron 3-16% by weight.   The film of claim 1, wherein   characterized in that it is formed from a mixture of a copolymer of ethylene with 1-octene having two to 504537 melting points of the crystallites with a copolymer of ethylene with 1-octene having only one melting point crystallites by calorimetric analysis   Differential with power compensation.   Film according to claim 1, characterized in that it is stretched biaxially at least five times its initial dimension in each direction. 6. Film according to claim 5, characterized in that it is subjected after stretching, but before contraction, to a radiation with a high   energy at less than about 8 Nrads.   Method for wrapping an article in oriented polyolefin film and contracting   thermally the film so as to adjust it closely   around the article, characterized in that a film as defined in one of the   any of claims I & 6.