Heat-shrinkable co-extruded multilayer polyolefin film having an improved heat seal resistance

Heat shrinkable co-extruded multilayer polyolefin film having an improved heat seal resistance, comprising: PA1 A) A central layer consisting of a blend obtained by an homogeneous dispersion of an ethylene-propylene-butene terpolymer (EPB) produced by catalysis, in a linear ethylene-alpha olefin C.sub.4, C.sub.5, C.sub.6 or C.sub.8 copolymer, said blend being optionally added with a linear ethylene-alpha olefin C.sub.4, C.sub.5, C.sub.6 or C.sub.8 copolymer, PA1 B) At least two layers externally placed with respect to said central layer and constituted by an ethylene-propylene-butene terpolymer (EPB) produced by catalysis, mixed with an ethylene-propylene copolymer (EPC).

FIELD OF THE INVENTION 
The invention regards the sector of the heat shrinkable films produced by 
extrusion of polyolefin polymers, used for packaging of various commercial 
products and also known as display films. Such films may be single or 
multilayer, in particular 3 or 5. 
PRIOR ART 
In the heat shrinkable films field known as display films an important 
problem is constituted by the heat seal resistance. Generally the heat 
seal represents a weak point for any kind of packaging films, but this 
fact is particularly accentuated in the case of the heat shrinkable films 
owing to the following reasons. 
First of all in the heat seal area the copolymer deteriorates and then its 
resistance decreases. 
The concomitant action of the force and of the retraction percentage in the 
application step causes a stress in the whole film structure and in 
particular it finds a weak point in the heat seal itself. 
Finally it is clear that the heat seal resistance depends either on the 
resistance of the polymer and on the thickness of the film heat sealing 
layer itself. 
Considering the co-extruded multilayer films, they have total thicknesses 
ranging from 12.5 to 25 micron while the heat sealing layers thicknesses, 
in the normal technique, are ranging from 1.2 to 5 micron. 
This implies that the heat seal resistance, being linked directly to the 
thicknesses of the two heat sealed layers, is not proportional to the 
total thickness of the film. 
In particular we notice that in any kind of heat seal known in the 
technique (filament, hot plate, radio frequency and ultrasonic) it never 
happens that layers differing from the adjacent ones join together. This 
is widely proved by the microscopic analysis of the heat seals. In order 
to improve the heat seal resistance several techniques have been proposed. 
For example according to U.S. Pat. No. 4,352,844 (Grace) an improved 
resistance of the heat seals is obtained in a heat shrinkable film having 
3 or 5 layers all based on various kinds of EVA, if a copolymer having 
melting point higher than those relative to the EVA used for the other 
inner layers is used for the heat sealing external EVA layers. The 
European Patent EP 92897 (American Can) refers of a heat shrinkable 
multilayer structure with barrier characteristics, easy heat sealing and 
high heat seal resistance, constituted by a 3 or 5 layers film wherein the 
external heat sealed layers are constituted by EVA having 6-12% of VA and 
m.i. (melt index) equal to 0.3/0.9, if necessary mixed with LLDPE having 
m.i. equal to 0.5/1.5. 
The EVA amount in the mixture may range from 90 to 10% while the LLDPE may 
range from 10 to 90%. The barrier layer is formed by PVDC or EVOH or by 
mixtures of the same ones with EVA. 
The possible layer intermediate between the barrier one and the heat 
sealing layer is composed by a mixture of 90-10% LLDPE and 10-90% EVA. The 
European Patent EP 409615 (Du Pont) refers of the following composition as 
an excellent example of heat sealing layer: 
1. from 10 to 80% by weight (preferably from 45 to 60%) of at least a 
linear ethylene-alpha olefin copolymer having a density ranging from 0.890 
to 0.915, 
2. from 15 to 70% by weight (preferably from 30 to 45%) of at least a kind 
of polyethylene selected from: linear polyethylene-alpha olefin copolymer 
having a density about equal to 0.915-0.940 and LDPE having a density 
about equal to 0.920-0.925, 
3. from 0 to 25% by weight (preferably from 5 to 15%) of at least an 
ethylene copolymer having at least one of the following comonomers: 
vinylalkanoate, alkyl acrilate or alkyl metacrilate. 
The layer obtained by this composition may form by itself a single layer 
film or the heat sealing layer in a multilayer film. 
SUMMARY 
Now it has been found that a multilayer heat shrinkable film based on 
ethylene-propylene-butene terpolymer (EPB) and on linear low density 
polyethylene (LLDPE) suitably added and combined one another, allows to 
overcome the problems relating to heat seal resistance creating a product 
having the following characteristics: 
low temperature heat sealing with heat sealing resistance higher than 
analogous known films one, good optical and mechanical characteristics and 
high tear resistance. 
The film according to the present invention comprises: 
A) a central layer consisting of an intimate mixture or blend obtained by a 
homogeneous dispersion of an ethylene-propylene-butene terpolymer (EPB) 
produced by catalysis (I), in a linear ethylene-alpha olefin C.sub.4, 
C.sub.5, C.sub.6 or C.sub.8 copolymer (LLDPE) (II). 
The percentage of said terpolymer ranges from 20 to 70% by weight, 
preferably from 30 to 50% by weight and the percentage of said copolymer 
ranges from 30 to 80% by weight and preferably from 50 to 70% by weight. 
Said intimate mixture may be in turn added with a linear ethylene-alpha 
olefin C.sub.4, C.sub.5, C.sub.6 or C.sub.8 copolymer (III) in an amount 
such that (I+II) ranges from 20 to 80% by weight and (III) ranges from 80 
to 20% by weight. 
B) At least two layers placed externally with respect to said central layer 
and composed by an ethylene-propylene-butene terpolymer (EPB) obtained by 
catalysis (I) mixed with an ethylene-propylene copolymer (EPC) (IV). The 
mixture comprises from 90 to 40% by weight of (I) and from 10 to 60% by 
weight of (IV). The so obtained film gives heat seals having an average 
resistance greater or equal to about 70 MPa against values equal about to 
40 MPa found on analogous products obtained by the known techniques.

DETAILED DESCRIPTION OF THE INVENTION 
The characteristics and the advantages of the heat shrinkable co-extruded 
multilayer polyolefin film according to the present invention, will be 
mostly pointed out during the following detailed description. 
The main advantages of the film object of the present invention are 
constituted by the easy heat sealing in a wide temperature range with 
different heat sealing systems and by the high resistance of the obtained 
heat seal. 
Said film comprises: 
A) a central layer consisting of an intimate mixture or blend suitably 
prepared according to the method later described and constituted by a 
homogeneous dispersion of an ethylene-propylene-butene terpolymer (EPB) 
produced by catalysis (I), uniformly distributed in a linear 
ethylene-alpha olefin copolymer (LLDPE) (II). 
Such copolymer has comonomers of butene, methyl-pentene, hexene or octene. 
In turn LLDPE having as comonomer an alpha olefin C.sub.4, C.sub.5, 
C.sub.6 or C.sub.8 (LLDPE III); many be added to this homogeneous blend; 
LLDPE II may be equal to LLDPE III. 
B) At least two layers placed externally with respect to said central layer 
and composed by an ethylene-propylene-butene terpolymer obtained by 
catalysis process (EPB) (I) with addition of ethylene-propylene copolymer 
(EPC) (IV). 
In order to improve the characteristics of formability and the behavior of 
the film in the application phase, little amounts of substances commonly 
used in the production technique of the plastic films as for example 
erucic amide, micronized silica and if necessary hydrogenated hydrocarbon 
resins may be added in the various layers. The basic polymers of the film 
according to the present invention and the polymers added to them have the 
following characteristics. 
The terpolymer EPB (I) is a catalysis product comprising a matrix 
consisting of a polypropylene homopolymer and of a disperse phase 
consisting of synthesis elastomers ethylene-propylene and propylene-butene 
in amounts which may range according to the final desired characteristics. 
The density of this terpolymer ranges from 0.885 to 0.895 g/cm.sup.3 and 
its melt index from 3.5 to 7 g/10 min at 230.degree. C. 
An used product is for example the Adsyl 5C 37F (Montell) having a density 
equal to 0.890 g/cm.sup.3, melt index .sup..about. 5 and starting heat 
seal temperature near to 105.degree. C. 
The linear polyethylene (LLDPE) (II) preferably used is a linear 
ethylene-alpha olefin copolymer wherein the comonomer is of the C.sub.8 
kind (octene), however the comonomer may be of the C.sub.4, C.sub.5 or 
C.sub.6 kind (butene, pentene or hexene). The density ranges from 0.911 to 
0.925 g/cm.sup.3 and the melt index ranges from 0.9 to 2.2 g/10 min at 
190.degree. C. In particular the polyethylene ClearFlex FG 308 by the 
Polimeri Europa having density equal to 0.920 and melt index equal to 1 is 
used. 
The blend composing the central layer is prepared as follows: 
an amount ranging from 20 to 70% by weight (preferably from 30 to 50%) of 
terpolymer EPB (I) and an amount of LLDPE (II) ranging from 30 to 80% by 
weight (preferably from 50 to 70%) are mixed very homogeneously to the 
obtainment of an uniform dispersion of the two phases one into the other; 
some linear polyethylene of the kind III above described may be added to a 
such blend in such an amount that (I+II) ranges from 20 to 80% by weight 
and (III) ranges from 80 to 20% by weight. In particular in a preferred 
form of the invention the use of said blend is provided in a mixture with 
said linear polyethylene in 1:1 ratio by weight. 
The amount and the quality of the various polymers used in the preparation 
of the film depends on the characteristics desired for the film and on the 
characteristics of the polymers themselves. We found that preliminarily 
dispersing in an homogeneous way the terpolymer EPB (I) in the LLDPE (II) 
without altering by thermal and/or mechanical effect the characteristics 
of the above mentioned polymers (for example by dispersion in a low 
revolutions number double screw counter rotating mixer, or in a double 
screw co-rotating mixer having a special screws profile and operating at 
low temperature) an intimate mixture or blend which once extruded in the 
central layer of the film gives to the film itself some special 
characteristics, is obtained. For example the three-layer film having the 
central layer consisting of a blend having a 1:1 ratio by weight of EPB 
(I) and LLDPE (II) and the external layers consisting of a mixture 
containing 90% by weight of EPB terpolymer (I) and 10% by weight of EPC 
copolymer (IV) has good optical and mechanical resistance characteristics 
but in particular leads to heat seals having a resistance to 70% greater 
with respect to the analogous films on the market one. 
In particular distribution of the external layers thicknesses with respect 
to the products available on the market being equal, the film object of 
this patent shows a heat seal resistance about 70% greater. This is the 
case of a three-layer film having a thicknesses distribution equal to 
22%/56%/22%. On the contrary in the case of reduced thicknesses of the 
external layers (for example 14%/72%/14%) the heat seal resistance turns 
out to be always greater but to a lower extent (from 10% to 20%). This 
alternative version of the product also shows some advantages in that 
reducing the percentage of polypropylene on the outside, present as 
copolymer and terpolymer structure of co-extruded film having a higher 
tearing propagation resistance is obtained. 
The result surprising in heat seal resistance terms combined with the good 
mechanical and optical characteristics of the film is obtained when the 
above mentioned central layer in a three-layer structure as described 
above is constituted by an homogeneous blend EPB/LLDPE obtained using 
particular compounding conditions. In particular a suitable preparation of 
the above mentioned blend provides for the following steps: 
a) Homogeneous mixing of the granules of the two substances by a vertical 
or horizontal conveyor mixer or by a low velocity blade mixer. 
b) Extrusion of the mixture by a single screw extruder having a special 
profile screw for LLDPE and with a dynamical mixer of the so-called 
Maddock kind or by a counter rotating double screw extruder or by 
co-rotating double screw extruder. Preferably a double screw extruder of 
counter rotating kind having diameter equal to 60 mm and screws length 
equal to 22 diameters is used. 
The operative conditions are such to limit as much as possible a thermal or 
mechanical stress to the mixture of polymers and therefore we operate at 
low revolutions number (about 25/min) and at a relatively low temperature. 
In particular a melt temperature ranging from 195.degree. to 198.degree. C. 
is recorded. 
c) Granulation by the known technique of crosscut by water cooling. 
d) Drying by centrifugation of the granules in a suitable centrifuge which 
may be crossed or not by a hot air jet at 60.degree./70.degree. C. 
according to the productive needs and mixing of the granules obtained in a 
vertical conveyor mixer in order to guarantee the homogeneity of the 
product to use for the intermediate layer. 
A very homogeneous blend wherein the terpolymer EPB (I) is uniformly 
dispersed in the linear polyethylene (II) is so obtained. Such a blend is 
then used as it is or in mixture with further LLDPE of the III kind in 
order to produce the central layer in the co-extruded structure having at 
least two external layers. The heat shrinkable film according to the 
present invention is produced by the process of the twin-bubble technique 
providing for: 
A) Preparation of the polymers mixtures necessary to the various layers 
comprising the already mentioned process additives, in particular 
antiblocking (SiO.sub.2) and sliding (erucic amide). 
B) Co-extrusion of the various layers in suitable annular head. 
C) Fast cooling of the tubular at the outlet from the extrusion head from a 
temperature about equal to 200.degree. C. to a temperature equal to 
15.degree.-20.degree. C. in 5-8s. 
D) Warming of the tubular to a temperature close to softening ranging from 
120.degree. to 150.degree. C. 
E) Film biaxial orienting by the twin-bubble stretching technique. 
F) Fast cooling of the oriented film to a temperature ranging from 
20.degree. to 30.degree. C. 
The so obtained heat shrinkable bioriented films show the following 
characteristics: 
high mechanical resistance with breaking loads always greater than 100 MPa 
and extensibility greater than 110%; 
good tearing resistance, however always greater than 7 g in both the 
directions (longitudinal/transversal) for a film having a thickness equal 
to 15 micron; 
good optical characteristics with haze (opacity) about equal to 1.5% and 
gloss (brightness) about equal to 130 photoelectric cell units; 
heat seal starting temperature lower than the normal polypropylene homo-and 
copolymers existing on the market and close to 100.degree. C. (precisely 
about 105.degree. C.); 
heat seal mechanical resistance particularly high and decidedly higher than 
the other analogous co-extruded films available on the market, in 
particular values greater on the average than 70 MPa against 40-50 MPa for 
the commercial films; 
very high shrinking percentage: from 60 to 70% at 120.degree. C. It turns 
out clear from what has been described that by the technique of the 
present invention it is possible to obtain a heat shrinkable polyolefin 
film which, while keeping good general characteristics, turns out to have 
a weldability and a heat seal resistance markedly greater with respect to 
the analogous products available on the market. 
The following examples are reported by way of illustration, wherein the 
heat seal resistance has been determined by the following method. 
METHOD FOR THE DETERMINATION OF THE HEAT SEAL RESISTANCE ON HEAT SHRINKABLE 
FILMS 
1) Aim 
The aim of the method is the determination of the heat seal resistance 
performed in controlled conditions on a heat shrinkable film by 
dynanometric traction of standard samples, previously united by hot wire 
impulse welding. 
2) Description of the method 
Film samples are heat sealed by the action of the pressure and of the 
temperature exerted by: a warmed wire bar. 
From such samples specimens having known sizes undergoing the action of a 
dynamometer recording the stress needed to open the heat seal are 
obtained. 
3) Equipment 
a) Hot bar flat welder "Sentinel" mod. 12AS, or alternative equipment 
allowing to operate impulse weldings with thread-like bar -in reproducible 
conditions. 
b) Digital dynamometer Instron series 4400 or analogous equipments. 
c) Thickness tester having reading capability equal to 0.001 mm, pressure 
exerted on the sample: 3.18 kg/cm.sup.2. 
d) Hand punch press and hand punches having sizes equal to 100 by 20 mm, or 
plastic or metallic material templates having the suitable sizes for the 
specimens cut. 
4) Process 
From the reel to test pick up at regular distances two couples of samples 
having sizes equal to 30 by 20 cm parallelly to the extrusion direction. 
For any couple execute to heat seals along the minor borders in such a way 
that the heat seals result in a transversal direction with respect to the 
extrusion direction and executing the heat seals on both the surfaces of 
the film (external-external and internal-internal). 
Use the following operative conditions for the welder: 
welding bar temperature: 130.degree. C. 
bar contact time: 1 second 
pressure at the manometer: 40 psi 
From each so obtained sample pick up two specimens having width equal to 20 
mm and length greater than the distance separating the dynamometer clamps 
(100 mm), practically specimens having length about equal to 150 mm are 
used. The heat seal (transversal with respect to the major axis) will 
result at half specimen (see FIG. 1). 
Arrange the dynamometer in the following way: 
insert the traction-compression measure cell having a capacity equal to 
100N 
arrange the crossbar velocity at 100 mm/min 
arrange the distance between the clamps at 100 mm. 
Then submit each specimen to the estimation of the tearing tension by the 
dynamometer. In placing the specimens in the dynamometer clamps it is 
necessary to perfectly align the major axis of the sample with the clamps 
axis in order to apply an uniform traction on the whole length of the heat 
seal (see FIG. 1). 
5) Calculations 
The result of the measure is given in grams/cm and it is obtained by the 
expression: 
EQU R.S.=A/B 
where 
A=load read at the breaking point of the heat seal given in grams; 
B=specimen width, given in cm (equal to 2 cm). 
Sometimes the result is used to be given in terms of maximum stress bear 
sustained by the heat seal, the value being obtained dividing the maximum 
heat seal breaking load recorded by the instrument for the cross section 
of the specimen, according to the formula: 
EQU C.R.=C.M./S 
where: 
C.R.=stress corresponding to the heat seal breaking, given in N/mm.sup.2 
=MPa; 
C.M.=load corresponding to the heat seal breaking, given in N; 
S=cross section of the specimen, given in mm.sup.2. 
Note: the cross section of the specimen is obtained multiplying the minimum 
thickness of the sample, measured by the thickness gage and given in mm by 
the width of the specimen itself. 
EXAMPLE 1 
A co-extruded three-layer film is prepared, with the two external layers 
having the same composition. The compositions are the following: 
Central Layer 
A mixture composed as follows is prepared: 
50% by weight of terpolymer EPB (I) of Adsyl 5C37F (Montell) kind having a 
density equal to 0.89 and melt index equal to 5; 
50% by weight of LLDPE (II) of Clearflex FG 308 (Polimeri Europa) kind 
having a density equal to 0.92 and melt index equal to 1. 
Such a mixture is then drawn in a counter rotating double screw extruder 
having 60 mm diameter screws at a rotation velocity.apprxeq.25 Rpm. The 
temperatures outline does not exceed 200.degree. C. and the melt maximum 
temperature ranges from 195.degree. to 198.degree. C. The so obtained 
granules are mixed with 2% by weight of a sliding masterbatch comprising 
6% of erucic amide and 94% of LDPE (for example the AD Wax 90111 kind 
produced by Frilvam). 
External Layers 
The same terpolymer EPB (I) used in the central layer (Adsyl 5C37F) is used 
as basic polymer. 5% by weight of a copolymer ethylene-propylene (IV) 
having a density equal to 0.897 and melt index equal to 1.8 (Eltex KL 415 
produced by Solvay) is added to said polymer. 0.6% by weight of a 
masterbatch containing 20% of micronized silica and 80% of LDPE (AB 6002 
LD produced by Constab) are added as external antiblocking agents. Then an 
homogeneous mixing in a vertical conveyor mixer is carried out. 
The thickness distribution of the layers is the following: 
external layers: about 22% for each single layer 
intermediate layer: about 56%. 
Film Preparation 
The mixtures for each layer obtained as described above have been 
co-extruded in the respective drawplates pertaining to each layer of the 
twin-bubble stretching device. 
The maximum temperature of the melted material has been about equal to 
210.degree. C. for the central layer and about 220.degree. C. for the 
external layers. The temperature of the tubular at the outlet from the 
annular head (first bubble) has been about equal to 210.degree. C. The 
tubular has been cooled at the outlet from the head at a temperature about 
equal to 18.degree. C. in about 6 seconds by crossing through an external 
annular element from which continuously flowed a liquid cooling at 
12.degree. C. and an inner annular element cooled at 6.degree. C. The same 
tubular has been subsequently warmed at a temperature equal to 130.degree. 
C. and submitted to bi-orienting by the bubble stretching technique. The 
stretching ratio turned out to be equal to 1:5.2 in the longitudinal 
direction and 1:5.0 in the transversal direction. 
Then the film has been cooled at a temperature about equal to 25.degree. C. 
by air conditioning blowing at about 8.degree. C. The weight ratio between 
the central layer and the external ones was 75:25 and the total thickness 
was equal to 15 micron. 
The characteristics of the obtained film are reported in the table 1 which 
shows that the film has good optical and mechanical characteristics, in 
addition to an exceptional heat seal resistance. 
EXAMPLE 2 
The Example 1 is repeated with the difference consisting in the following 
thickness distribution of the layers: 
external layers about 14% each 
intermediate layer about 72%. 
In this case the heat seal resistance is reduced to the products on the 
market one, while the tearing resistance sensibly increases. 
EXAMPLE 3 
The Example 1 is repeated with the difference that the central layer is not 
obtained from the blend produced by the double screw mixer, but by a 
mixture not previously drawn and directly extruded on the double bubble 
co-extrusion device. 
The results reported in the table 1 show that, even if the heat seal 
characteristics and the other film properties continue to be good, the 
product has characteristics lower than the Example 1 ones. 
EXAMPLE 4 
The Example 1 is repeated with the difference that the blend of the central 
layer prepared on a double screw extruder turns out to be so composed: 
EPB (I)/LLDPE (II) 30/70% by weight. The external layers remain the same. 
The characteristics of the obtained film are reported in table 1. 
EXAMPLE 5 
The Example 1 is repeated with the difference that in the central layer 50% 
by weight of LLDPE (II) of the same kind used for the preparation of the 
same blend (Clearflex FG 308) is added to the blend 50/50 EPB (I) LLDPE 
(II). The external layers remain the same. 
The characteristics of the obtained film are reported in table 1. 
EXAMPLE 6 
The Example 4 is repeated with the difference that in the central layer 50% 
by weight of LLDPE (II) of a kind different from that used for the 
preparation of the same blend is added to the blend 50/50 EPB (I)/LLDPE 
(II). In particular LLDPE Innovex LL 7909 AA by BP Chemicals kind having a 
density equal to 0.936, melt index equal to 0.9 and comonomer 4 
methyl-pentene is used. 
The external layers remain the same. 
The characteristics of the obtained film are reported in table 1. 
EXAMPLE 7 (comparison) 
A film having the central layer composed by the LLDPE (II) only is prepared 
as a comparison example. The external layers remain the same. The 
characteristics of the obtained film are reported in table 1. 
EXAMPLE 8 (comparison) 
The Example 6 is repeated as further comparison example with the difference 
that the external layers are composed by the terpolymer EPB (I) only. The 
characteristics of the obtained film are reported in table 1. 
NOTE: In order to point out the improvements obtained by the film object of 
the invention, the characteristics of a heat-shrinkable polyolefin film 
diffused on the market are reported in table 1 too. 
TABLE 1 
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ASTM Film on 
N. Ex. 1 
Ex. 2 
Ex. 3 
Ex. 4 
Ex. 5 
Ex. 6 
Ex. 7 
Ex. 8 
the market 
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Thickness D 374 
15 15 15 15 15 15 15 15 15 
(.mu.) 
Ultimate tensile stress 
D 882 
127 
124 
120 
116 
114 
109 
117 
112 
110 
TD (MPa) 
Elongation at break 
D 882 
105 
110 
103 
115 
123 
120 
123 
128 
100 
Tearing strength 
D 1938 
6.2 
7.8 
6.0 
7.0 
7.5 
6.6 
7.1 
8.0 
5.0 
TD (g) 
Shrinking at 120.degree. C. 
D 2732 
58 61 58 60 63 59 64 65 60 
TD % 
Heat seal resistance 
70 56 58 63 60 45 42 37 50 
(inner method) 
Haze (%) D 1003 
1.6 
1.7 
1.7 
1.7 
1.8 
2.0 
1.5 
1.4 
1.6 
Gloss D 2457 
128 
126 
126 
127 
126 
128 
127 
120 
122 
(photometric unit) 
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