CA1174423A

CA1174423A - Shrink films of ethylene/alpha-olefin copolymers

Info

Publication number: CA1174423A
Application number: CA000399852A
Authority: CA
Inventors: Ralph C. Golike
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1981-04-23
Filing date: 1982-03-30
Publication date: 1984-09-18
Also published as: BE892927A; GB2097324B; DE3215120C2; GB2097324A; NL189515C; IT1151736B; FR2504537B1; FR2504537A1; DE3215120A1; JPH0354048B2; NL8201675A; IT8220883A0; JPS57181828A

Abstract

ABSTRACT
A shrink film having high optical clarity, good shrink properties, and good mechanical properties is obtained by stretching biaxially a film made of a copolymer of ethylene with at least one C8-C18 .alpha.-olefin, which copolymer has two distinct crystallite melting points below 128°C, the difference between these melting points being at least 15°C, and stretching being carried within the temperature range defined by these melting points.
The above copolymer may be blended with a homopolymer of ethylene or copolymer of ethylene with an ethylenically unsaturated comonomer, which may constitute up to about 95 weight % of the blend. The shrink film of this invention is particularly suitable for wrapping consumer goods which have to maintain good sales appeal.

Description

7~3 _tl Back~round of the Invention This invention rela~es to shrink i~ms based 5- on selected linear, low d~nsity copolymers of e~hylene with cer~ain ~-olefins, which films have ou~standing optieal properties and a good balance of other physical prc~perties and shrink properties.
Shrink films of oriented polyethylene and lC v~rious copolymers of ethylene are well known; see, ~or e~ample~ U.S. l?a~ents 3f299,194 ~:o Go~.ike and 3,663,662 to Golike et al.
A polyolefin shrink film, used mainly for wrapping ~ood products and a var iety of ~onsumer 15 goods, should hav~ good optical clarity; otherwise, the consumer appeal of the packaged article wi~hin the wrapping s~ould be diminished or lost. For practical applications, the film should shrink within a temperature range of approxima~ely 100 tc> 120C to 20 a degree of at least 15% in the direction of orientation and with sufficient ~orce to provide a tight-fittLng ~kin around the arti.cle enclosed within ~he wrapping. The film also shou3.d have good mechanical propertie~, such as tensile s~rength and 25 modulus, so that i'c will stretch and then shrirlk without 'cearin~ 9 will maintain good physical corltac~
wi'ch the packaged article at all times, and will no~
get easily damaged in handling.
One prior art technlque iEor making ethylene 30 polymer hrink films required polymer crosslinking prior 'co stretching in order to impart to tbe film greater me~hanical strengthO This crosslinking usually was accomplished by irradiation wi~h high energy particles or with gamma rays.

'7'~Z3 In order to obtain a res.in composition yielding films with satisfactory properties for shrink film applications without crosslinking prior to stretching, it has been generally necessary in the S past to blend low density ~nd high density ethylene polymers. Naturally, it would be desirable to be able to make shrink films from a single low density ethylene polymer resin. In this context, the term ~low density~ means 0.940 g/cm3 or less, and ~high density" means more than 0.940 g/cm3.
A recent commercial offering of the Dow Chemical Company, DOWLEX* low density "polyethylene~
resins, are described in a Dow bulletin as giving ~lown film having excellent optics and superior strength properties. Yet, the same bulletin indicates that these resins are not suitable for making shrink films because they will shrink less than conventional low density polyethylene film and will shrink within a narrower temperature range.
DOWIEX* resins are in fact copolymers of ethylene with 1-octene.
- 5ummary of the Inv~ntion According to this invention, there i5 now provided a shrink film having high op ical clarity, 25 ~od shrank properties, and good mechanical properties, said film being obtained by stretching at least three times its original linear dimension in at leas~ one direction a film made of the following homogeneous polymeric composition:
~1) 5-100 weight % of at least one linear copolymer of ethylene with at least one C8-C18 ~-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/cm3;
*denotes trade mark .~ - 2 ~:~74~

(c) ~tr~ss exponent above 1.3; and (d~ two distinct crystallite melting regions below 1~8 JC as de~ermined by differential scanning calorimetry 5- (DSC)~ the ~emperature difference between those regions being at le~s~
15C; and (2~ 095 weight ~ of at l~ast one polym~r selected from the group consisting o ethylene 10 homopolymers and ~opolymers of ethylene with an ethylenically ~n~aturated comonomer, said polymer having only one crystallite mel ing point below 128C; with the proviso that ~tretching is done within the temperature range defined by the two 15 crystallit@ mel~ing points of the linear copolymers of ethylene with C8-Cl~ a-olefin of the aboYe paragraph (1).
Brief Descri~ion of the Drawings The drawings represent DSC plots for three 20 different resins. FIG. 1 is the plot for polyethylene, FIG. 2 for a commerc:ial linear ~thylene/l-octene copolymer, and FIG. 3 for a blend of high and low densi~y ethylene polymers.
Detailed Des~riPtion of_t:he Invention The principal resin used in the composi~iong of the present invention is a linear copolymer o~
ethylene with an a-olefin. Typical a-olefins which can be copolymerized with ethylene are l-octene, l-decene, l-undecene, l~dodecene, and l-hexadecene.
30 The copolymers are prepared at a low to moderate pressure (about 29.4 MPal in the presen~e of a coordination catalyst according to the ~enerally known technique of the so-called Ziegler and Natta processes. Typical catalysts are various 35 organoaluminum, organotitanium, and oryanovanadium compounds, and especially titanium-modified organoaluminum compounds. The preparation o~
ethylene copolymers with ~-olefins is taught, for example, in U.S. Patents 4,076,698 to Anderson e~ al.
5_ and 4,~05,021 to ~orita et al.
Suitable co~mercially available copolymers of ethylene with higher a-olefins include the above-mentioned DOWLEX~ resins, and the preferred copolymer is that with l-octene. As the proportion 10 of ~-olefin in ~he copolymer or the molecular weight of ~olefin increases, the density-of the copolymer decreases. For l-octene~ the amount of this ~-olefin in the copolymer normally will be between about 3 and 16 weight percent. However, the amount of each ~uch 15 c~monomer will be so chosen that proper values of melt indexr density, and stress exponent of the copolymer are obtained. These proportions are easily established from known relationships and ca~ be verified experimentally by means of standard 20 techniques. Thus, the melt index is determined according to ASTM method D123B (condition E) and the . density according to ASTM D1505. The stress exponent is the slope of the plot of log f:Low rate versus log extrusion force. Since the plot .is not linear, the 25 slope is determined according to ;~STM D1238 using 2160 g and 640 g weights, both at 190C.
~ he copolymers ~hould give two distinct crystallite melting peaks, which means that they have two different groups of crystallites, each having it~
30 own distinct melting region. ~or ethylene/l~octene copolymers, such regions will be at about 107C and 125C. FIG. 1 is a typical ~SC plot of ~ in milliwatts vs. temperature in C for conventional polyethylene having a density of 0.917. This polymer 35 has only one pe~k, which lies at about 1075C. A DSC
*denotes trade mark -.~ ~

~L~'7~ 3-- 5 --plot for DOWL~X 2045 ethylene/l-octene ~opolymer (d =
a. s~o) is presented in FIG. 2O The hi~her temperature peak is in reality a doublet, and the higher melting temperature of the dou~let is taken as 5 _characteristic of this peak. FIG. 3 is a DSC plot for a blend of linear high density ethylene/l-octene copolymer with the conventional polyethylene. The blend density is 0.926. It can be seen that the peaks of the blend correspond to those of the DOWLEX*
10 resins shown in FIG. 2. DSC is a well-known technigue ~or measurin~ polymer crystallite melting temperatures. .Linear copolymers of ethylene with l-octene ~r another ~-olefin, wherein the n-Qlefin comon~mes is present is such small amounts that a 15 second ~SC peak is ~ot observed are not suitable in the present invention. The existe~ce of two crystallite melting resions in the ethylene -olefin copol~mers is their most outstanding characteristic because films made from these copolymers can be 20 oriented between those two temperatures. Shrink films made from these copolymers have excellent properties, quite comparable with those of shrink films made from blends of low density and high density ethylene polymers, ~or example, those 25 described in U.S~ Patent 3,299,194.
~ owever, it has been found that the presence of as little as 5 weight percent of an ethylene/~-olefin copolymer of this class in a blend with a conventional ethylene homopolymer or copolymer 30 having only one crystallite melting region below 128C can sometimes improve the properties of the latter copolymer so significantly that excellen~
shrink films having desirable physical properties, including high opt~'cal clarity, can be made 35 therefrom. Such conventional homopolymers or *denotes trade mark : - 5 -. ~3 c~polymers can be both high density and low dens;ty, linear and branched, made at high pressure or at low pressure. The copolymers may be those with any comonomer, including for example, u-olefins, vinyl 5-ester~, alkyl acrylates and methacrylates, 2nd acrylonitrile. Many such polymers are commercial~y available from several sources. The biends can be prepared by any conventional technique capable of producing a uniform, homogeneous material.
. Film is made from the above copolymers or blends by a suitable melt extrusion process. The film i5 either tubuïar. or fla~.. It is stretched, preferably biaxially, in the plane of the filrn to the extent of at least 3 times in each direction, 15 preferably at least 5 times. A convenient process, which combines extrusion and orientation of polymeric films is described in U.S. Patent 3,141,912 to Goldman et al.
When subjected to a temperature of about 100 20 to 120C, an oriented, unconstrained film will shrink at least about 15%~ and this shrinking will be a~companied by a considerable forc:e, usually at least 1400 kPa. The preferred shrink films will shrink at least 30~ at a temperature ju~t below the higher 25 ~rystallite melting peak, at least 15% at 100C. The hrink force at 100C should be greater than about 350 kPa. ~aze should be less than 4%, especially less than 2~. Gloss should ~e greater than 90, preferably greater than 110.
A limited amount of crosslinking can be in roduced after stretching but prior to shrinking, if desired. This can be accomplished with a minimum amount of high ener~y radiation, normally less than 8 ~rad, as described, for example r in U.S. Patent 35 3,663,662 to Golike et al. Irradiated oriented films .

have improved melt s~reng~h and are less sensitive to temperature diferences in the ~hrink tunnelO
This invention is now illus~ra~ed by ~he following representative examples~ where all parts S _and proportions are by weight. In all cases ~he thickness of shrink Eilm was about 0.025 mm~
All data obtained in units other than SX
have been converted to SI units.
The shrinkage of oriented films was 10 de~ermined by scribing a fixed length, usually 100 mm, on a strip of unconstrained film in a 100C
~emperature bath or 10 seconds and calculating the ~hrinkage as the percent change of length.
The shrink force was determined according to 15 ASTM 2838. Modulus, tensile strength, and elongation at break were determined according ~o ASTM ~412~
The ethylene resins used in the examples are listed in Table I, below:

2~

I

4~23 Table I
Melt ~e~sp.,~C ~nsi~y, S~ress ~elt l~ctene Resin (by D6C) ~ 3ex ~ ~2 A 124, 107 O.g20 1.4 1.0 14 Linear, low densi~ copolymer B 126 0 . 9501. 8 0 . 451. 7 Linear, high density copolymer C 103 0.917 - 4.0 - Brarlched, l~w densit~ hwK~lylTer D 126 0.940 1.9 0.45 3.6 Lineart l~w density ~opolymer g Exam~le_l Oriented tubular film ~as prepared by the process of U.S. 3,141,912 to Goldman. A S cm extruder operated at 230C and at a feed rate of 0.9 S- kg of ethylene polymer resin per hour produced film at the rate o 2.7 m/min. The hot tubular film was ~uenched, rehea~ed to 115-120C, and blown at an internal pressure of 2 kPa. The ~lowing was controlled with a q~ench ring to give a fivefold stretch in the transverse direction. The take-up rolls were operated to give a fivefold stretçh in the longitudinal direction.
Shrink film made from resin A according to the present invention was compared with a prior art shrink film made from a blend of resins B and C ~in a respective ratio of ~6:74) according to the teachings of U.S. 3,29g,194 to Golike. The films were placed about objects, hot wire sealed, and shrunk in a tunnel maintained at 167C. The appearance of 20 packages in both cases was idential. The properties of both shrink films are compared in Table II, below. All properties other than haze and gloss are given as a ratio: machine direction/transverse direction~

~ _ g .~

7~

Table II
~ + C
esin TyPe*_ _ A (26~74) MODULUS, MPa 295/260 360/330 TENSILE, MPa 115/108 69/56 ELONGATION, % 240/195 152/128 TEAR, ~/mm 1480/1280 267/462 SHRINRAGE
(1~ûC) % 19/2~ 27/30 SHRINK FORCE
(100C) kPa i810/3590 2960/3450 ElAZE, ~ 3.5 3.6 *See Table I for resin d~scription Example 2 ~esin blends were prepared as shown in Table III, below, melt blended in a standard single-~crew mixing extruder, and melt pressed into Sx5-cm films.
These were stret~hed fivefold at 120C in each direction in a la~oratory stretcher (T. M. Long Co., Inc., Somerville r N.J. ) .
The physical properti~s of the films of this invention (A/B and A/D blendc1 are compared in Table III with those of prior art films made of ethylene pol~ner blends (B/C and C/D blends). The improvement of the physical properties, especially of optical 30 properties, in the films of the present invention is apparent.

. Table III
Resin Blend*
~igher Density Component ~- ~ype** B n ~ D
~ 26 37 2~ 30 Lower Density Component Type ** C C A
% . 74 63 80 70 FILM PROPERTIES
MGDULUS~ MPa 367 458 583 508 ~ENSILE, MPa 82 64 106 ll9 ELONG~TION, % 80 106 131 114 T~AR, g/mm 295 336 380 380 S~RINKAGE 8 8 6 10 ~100~C) %
S~RINK FORCE
(100C) kPa 1170 965 1420 1?40 ~A~E, % 6.5 4O3 3.8 2.4 * Proportions were chosen to give blend density of 0~926 g/cm3 *~ See Table I for resin description Example 3 Oriented films ~ere prepared from blends of resins A and C (see Table I). Stretching was carried 30 out at 110-112C using the same technique and e~uipment as in ~xample 20 The physical properties of ~he stretched films are sh~wn in Table IV, below.
It can be seen that all the properties change as the proportion of the conventional low density 35 polyethylene (Resin C~ increases. The most striking ;~7'~3 e:hange is the lar~e decrease of the shrink force with retention of the high level of shrinkage.
Table IV
Propor tion of 5_ Resin C
in _/C resin blend ~
0 ~5 50 75 FILM_PRO_ERTIES
MODULUS, ~Pa 364 273 240 240 TENSIL~, MPa 144 69 42 30 ELON~TION, % 129 162 144 131 TEARp g/mm 104 510 580 2S0 (1009C) 3 SHRINK FORCE
(100C) kPa 225û 2100 1670 1210 HAZE, ~ 1.0 1.7 2.4 1D6 '.
i !
I

Claims

I CLAIM:

1. A shrink film made by stretching at least three times its original linear dimension in at least one direction a film made of the following homogeneous polymeric composition:
(1) 5-100 weight % of at least one linear copolymer of ethylene with at least one C8-C18 .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/cm3;
(c) stress exponent above 1.3; and (d) two distinct crystallite melting regions below 128°C as determined by differential scanning calorimetry (DSC), the temperature difference between those regions being at least 15°C; and (2) 0-95 weight % of at least one polymer selected from the group consisting of ethylene homopolymers and copolymers of ethylene with an ethylenically unsaturated comonomer, said polymer having only one crystallite melting point below 128°C;
with the proviso that stretching is carried out within the temperature range defined by the two crystallite melting points of the ethylene copolymer with C8-C18 .alpha.-olefin of the above paragraph (1).

2. A film of Claim 1, which is made of a copolymer of ethylene with 1-octene.

3. A film of Claim 2 wherein the proportion of 1-octene is about 3-16 weight percent.

4. A film of Claim l, which is made of a blend of a copolymer of ethylene with 1-octene having two crystallite melting points with a copolymer of ethylene with 1-octene having only one crystallite melting point by differential scanning calorimetry.

5. A film of Claim 1 which is stretched biaxially to the extent of at least five times in each direction.

6. A film of Claim 5 which is subjected after stretching but prior to shrinking to high energy radiation in an amount of less than about 8 Mrad.

7. In a process for wrapping an article in an oriented polyolefin film and heat-shrinking the film to provide a tightly fitting overwrap about the article, the improvement of using a film of Claim 1.

8. The improvement of Claim 7 wherein the film is a copolymer of ethylene with 1-octene.