Heat-shrinkable laminate tubular film

Disclosed herein is a heat-shrinkable laminate tubular film comprising a gas-barrier layer of a copolymer of vinylidene chloride, outer layers of a polyolefin, at least one intermediate layer of a polyamide or a thermoplastic polyester both of which show a crystal melting point of not more than 240.degree. C. and have a specified thickness, and adhesive layers disposed between any of the above layers.

BACKGROUND OF THE INVENTION 
The present invention relates to a heat-shrinkable laminate tubular film 
showing an adequate rigidity and a favorable stretchability and comprising 
a gas-barrier layer of a copolymer of vinylidene chloride and at least one 
layer of a polyamide or a thermoplastic polyester having a specified 
thickness. 
More in detail, the present invention relates to a heat-shrinkable laminate 
tubular film comprising a gas-barrier layer of a copolymer of vinylidene 
chloride, outer layers of a polyolefin, at least one intermediate layer of 
a polyamide or a thermoplastic polyester both of which show a crystal 
melting point of not more than 240.degree. C. and have a specified 
thickness, and adhesive layers disposed between any of the above layers. 
For packaging the foodstuffs non-uniform and irregular shape, a method of 
heat-shrink packaging is generally applied, and since the thus packaged 
foodstuff should be preserved safely for a long time period, it is 
necessary that the packaging used in such a packaging is impermeable to 
gases, particularly to gaseous oxygen, in other words, the packaging 
material must have a gas-barrier property. 
As the heat shrinkable gas-barrier film for packaging the foodstuffs, a 
single film of a copolymer of vinylidene chloride (hereinafter referred to 
as VDC) has been utilized. However, because of the poor mechanical 
properties of the single film of VDC at low temperatures, a composite 
laminate film comprising a gas-barrier layer of VDC and an outer layer of 
polyolefin has been developed. For instance, Japanese Patent Application 
Laying-Open No. 58-128821 (1983) discloses a laminate film comprising a 
gas-barrier layer of VDC and outer layers of a copolymer of ethylene and 
vinyl acetate (hereinafter referred to as EVA), and the disclosed film has 
solved the problem of the poor mechanical properties at low temperatures. 
However, because of the lack of rigidity in such a film, such a laminate 
film cannot exhibit a satisfactory operational processability in packaging 
foodstuffs. 
As a result of the present inventors' studies, the problem of the lack of 
rigidity has been solved by incorporating a layer of a polyamide or a 
thermoplastic polyester between the gas-barrier layer and the outer layer 
of polyolefin of the laminate film. 
SUMMARY OF THE INVENTION 
In a first aspect of the present invention, there is provided a 
heat-shrinkable laminate tubular film comprising a gas-barrier layer of a 
copolymer of vinylidene chloride having a thickness of not less than 6 
.mu.m and not more than 30% of the total thickness of the tubular film, 
outer layers of a polyolefin, at least one intermediate layer of a 
polyamide or a thermoplastic polyester both of which show a crystal 
melting point of not more than 240.degree. C. and have a thickness of 5 to 
40% of the total thickness of the tubular film, and adhesive layers 
disposed between any of the above layers. 
In a second aspect of the present invention, there is provided a process 
for producing a heat-shrinkable laminate film, comprising 
feeding a copolymer of vinylidene chloride as a gas-barrier layer, a 
polyolefin as outer layers, a polyamide or a thermoplastic polyester as at 
least one intermediate layer and an adhesive material as adhesive layers 
to an annular die provided with passages for the gas-barrier layer, the 
outer layers, the intermediate layer(s) and the adhesive layers and with 
adiabatic spaces disposed on the both sides of the passage(s) of the 
intermediate layer(s), in a melt state, each of the end portions of the 
passages of the gas-barrier layer, the outer layers, the intermediate 
layer(s) and the adhesive layers being assembled, 
laminating and extruding the copolymer of vinylidene chloride, the 
polyolefin, the polyamide or the thermoplastic polyester and the adhesive 
material, thereby forming laminate tubular film at the outlet of the 
annular die, 
cooling the resultant laminate tubular film by quenching, and 
after heating the laminate tubular film, biaxially stretching the resultant 
laminate tubular film, thereby forming a heat-shrinkable laminate tubular 
film.

DETAILED DESCRIPTION OF THE INVENTION 
The heat-shrinkable laminate tubular film according to the present 
invention comprises a gas-barrier layer of VDC having a thickness of not 
less than 6 .mu.m and not more than 30% of the total thickness of the 
laminate tubular film, outer layers of a polyolefin, at least one 
intermediate layer of a polyamide or a thermoplastic polyester having a 
thickness in a range of from 5 to 40% of the total thickness of the 
laminate tubular film and adhesive layers disposed respectively between 
any of the above layers, the crystal melting point of the polyamide or the 
thermoplastic polyester being not more than 240.degree. C. 
Since the gas-barrier layer of a VDC of the heat-shrinkable laminate 
tubular film according to the present invention is relatively thick, the 
retention time of the VDC in a passage of the die is relatively short and 
accordingly, there is a merit of scarcely causing the thermal 
decomposition of the VDC in the production of the laminate tubular film. 
Further, since the heat-shrinkable laminate tubular film according to the 
present invention has at least one intermediate layer of the polyamide or 
the thermoplastic polyester having a specified thickness, the laminate 
tubular film according to the present invention shows an adequate rigidity 
thereby overcoming the difficulty due to elongation of the laminate 
tubular film caused by the weight of the article to be packaged in the 
packaging operation. In addition, the laminate tubular film according to 
the present invention exhibits more uniform stretchability than that of 
the laminate film disclosed in Japanese Patent Application Laying-Open No. 
58-128821 (1983). The gas-barrier property of the laminate tubular film 
according to the present invention is of course excellent. 
The VDC used as the gas-barrier layer of the heat-shrinkable laminate 
tubular film according to the present invention comprises a copolymer of 
from 65 to 95% by weight of vinylidene chloride and from 5 to 35% by 
weight of at least one comonomer copolymerizable with vinylidene chloride. 
As the comonomer copolymerizable with vinylidene chloride, for instance, 
vinyl chloride, acrylonitrile and, C.sub.1 to C.sub.18 -alkyl acrylates 
may be mentioned. Of these comonomers, vinyl chloride is generally used. 
The VDC may contain a small amount of plasticizer(s) and/or stabilizer(s) 
according to necessity. These additives, i.e., the plasticizers and 
stabilizers, used in the present invention have been known by the persons 
skilled in the art and represented by, for example, dioctyl adipate and 
epoxidized soy-bean oil. 
As has been stated, it is necessary that the thickness of the gas-barrier 
layer of the VDC is not less than 6 .mu.m and is not more than 30%, 
preferably not more than 25% of the total thickness of the laminate 
tubular film according to the present invention. In the case of below 6 
.mu.m, the retention time thereof in the passage of the die becomes so 
large in extruding the VDC with the polyamide or the thermoplastic 
polyester in lamination that the VDC decomposes, thereby having difficulty 
in the co-extrusion. On the other hand, in the case of over 30% of the 
total thickness of the laminate tubular film, the impact strength of the 
laminate tubular film at low temperatures is reduced. 
As the polyolefin of the outer layers, high density polyethylene, middle 
density polyethylene, low density polyethylene, ionomers, a copolymer of 
ethylene and vinyl acetate, a copolymer of ethylene and an ester of 
acrylic acid, a copolymer of ethylene and propylene, polypropylene, a 
copolymer of ethylene and .alpha.-olefin (the so-called linear low density 
polyethylene, hereinafter referred to as LLDPE), a polybutene and mixture 
thereof may be mentioned. 
The preferable embodiments of the outer layers from the viewpoint of 
stretchability of the layer(s) are as follows. 
(i) At least one of outer layers is EVA of a crystal melting point of from 
80.degree. to 103.degree. C. 
(ii) One of the outer layers is EVA of a crystal melting point of 
80.degree. to 103.degree. C., and the other of the outer layers is LLDPE 
of a crystal melting point of from 110.degree. to 130.degree. C. 
(iii) One of the outer layers is EVA of a crystal melting point of 80 
.degree. to 103.degree. C., and the other of the outer layers is a mixture 
of not more than 40% by weight of the LLDPE of a crystal melting point of 
110.degree. to 130.degree. C. and not less than 60% by weight of the EVA 
of a crystal melting point of 80.degree. to 103.degree. C. 
(iv) Both the outer layers are the mixture of not more than 40% by weight 
of LLDPE of a crystal melting point of 110.degree. to 130.degree. C. and 
not less than 60% by weight of the EVA of a crystal melting point of 
80.degree. to 103.degree. C. 
Of the four embodiments, (i), (ii) and (iii) are preferable, particularly 
(i) is more preferable, from the viewpoint of transparency of the thus 
prepared layer(s). 
In addition, the total thickness of the layer(s) of the polyolefin is 
preferably in a range of from 40 to 92% of the total thickness of the 
heat-shrinkable laminate tubular film according to the present invention. 
The use of at least one polyamide or thermoplastic polyester layer as the 
intermediate layer(s) is the characteristic feature of the heat-shrinkable 
laminate film according to the present invention, and by disposing the 
layer(s) of polyamide or thermoplastic polyester of an adequate rigidity 
and a favorable stretchability as intermediate layer, the stretchability 
and the rigidity of the laminate film comprising a VDC layer and the 
polyolefin layer(s) have been improved. 
The laminate film disposed with each of two polyamide or thermoplastic 
polyester layers as intermediate layer between each of two outer layers of 
polyolefin and a core layer of VDC through the adhesive layers is 
preferable. 
As the polyamide, a polymer having a crystal melting point of not more than 
240.degree. C., preferably of 220.degree. C. is used and for instance, 
Nylon 6-66 (copolymer consisting of Nylon 6 and Nylon 66), Nylon 610 
(polyhexamethylene sebacamide), Nylon 612 (a condensate of 
hexamethylenediamine and 1,10-decanedicarboxylic acid), Nylon 11(a 
condensate of 11-aminoundecanoic acid), Nylon 12 (a ring-open polymerizate 
of laurolactam) and Nylon 6 (polycapramide) may be exemplified. 
As the thermoplastic polyester, a polymer having a crystal melting point of 
not more than 240.degree. C., preferably not more than 220.degree. C. or 
showing no crystal melting point is used and the thermoplastic polyester 
comprises the acid moiety selected from an aromatic dibasic acid and an 
aliphatic dibasic acid and the glycol moiety selected from aliphatic 
glycol, cycloaliphatic glycol and aromatic glycol. As such an esters, for 
instance, Vylon (made by TOYOBO Co., Ltd.), Hytel (made by Goodyear Co., 
Ltd.) and PET-G (made by Eastman Kodak Co., Ltd.) may be exemplified. 
For reference, the "crystal melting point" used in the present invention is 
the maximum temperature on a melting curve obtained by the determination 
on a specimen in an amount of 8 mg while using a differential scanning 
calorimeter (made by Perkin-Elmer Co., Model IB) at a rate of raising the 
temperature of 8.degree. C./min. 
In the case where the crystal melting point of the polyamide or the 
thermoplastic polyester is over 240.degree. C., it is necessary to raise 
the extruding temperature, thereby the coextrusion of the polyamide or the 
thermoplastic polyester with VDC becomes difficult. 
In the case where the heat-shrinkable laminate tubular film according to 
the present invention contains not less than two intermediate layers of 
the polyamide or the thermoplastic polyester, the two layers may be the 
same to each other or different from each other. It is necessary that the 
thickness of such layer is from 5 to 40%, more preferably from 5 to 20% of 
the total thickness of the laminate tubular film according to the present 
invention. In the case of the number of such layers of not less than 2, 
the total thickness of such layers is from 5 to 40% of the total thickness 
of the laminate tubular film according to the present invention. On the 
other hand, in the case where the thickness of such layer(s) is below 5% 
of the total thickness of the laminate tubular film according to the 
present invention, such layer(s) cannot contribute to the rigidity of the 
laminate tubular film, and in the case of over 40%, the coextrusion with 
the VDC is difficult. From the viewpoint of co-extrudability with the VDC, 
the thickness of such layer(s) is more preferably not more than 20% of the 
total thickness of the laminate tubular film according to the present 
invention. 
As the adhesive material forming the adhesive layer, EVA containing from 13 
to 28% by weight of vinyl acetate units, a copolymer of ethylene and an 
ester of acrylic acid containing from 13 to 28% by weight of the ester of 
acrylic acid, a modified copolymer of ethylene and an ester of acrylic 
acid with an unsaturated carboxylic acid and a material obtained by 
further modifying the thus modified copolymer with a metal may be 
exemplified. Of the above adhesive, the modified copolymer of ethylene and 
an ester of acrylic acid with the unsaturated carboxylic acid, and the 
material obtained by further modifying the thus modified copolymer of 
ethylene and an acrylic ester, with a metal are more preferable. 
Any one of the above-mentioned adhesive layer is disposed between the 
polyolefin layer and the VDC layer, between the VDC layer and the 
polyamide or the thermoplastic polyester layer, and between the polyolefin 
and the polyamide or thermoplastic polyester layer. 
The thickness of an adhesive layer is preferably from 0.5 to 3 .mu.m, and 
the total thickness of the heat-shrinkable laminate film is preferably 
from 20 to 120 82 m. 
In the preparation of the heat-shrinkable laminate tubular film, a 
plurality of resins each of which constructs each of layers of the object 
laminate tubular film are respectively co-extruded from a common annular 
die connected to a plurality of extruders corresponding to a plurality of 
resins, thereby being extruded from the die as a laminate tubular film. 
An assembling part of an example of such an annular die is schematically 
shown in FIG. 1 in which 7 is a passage of the assembled flow of the 
molten resins and the respective passages of the molten polyolefin, the 
molten VDC, the molten polyamide or the molten thermoplastic polyester, 
the molten polyolefin and the molten adhesive agent are shown by 1, 2, 4, 
6 and 8. As shown in FIG. 1, it is preferable to provide the respective 
adiabatic spaces 3 and 5 on the both sides of the passage of the molten 
polyamide or the molten thermoplastic polyester. By the thus provided 
adiabatic spaces, even in the case where the thermoplastic polyester is 
extruded at a high temperature, since the direct heat transfer to the VDC 
layer therefrom can be prevented, the VDC which is relatively apt to be 
decomposed can be coextruded with the thermoplastic polyester or the 
polyamide. In the case of preparing a laminate flat film by using a T-die, 
the both end parts of the layer of the VDC coextruded with the polyamide 
layer or the thermoplastic polyester layer are apt to be decomposed and 
accordingly, such an operation is not favorable. 
Although the order of lamination of the layers is optional as far as both 
of the outer layers are a polyolefin, the arrangement of the layers in the 
laminate tubular film is preferably in the following order from the outer 
layer to the inner layer: 
polyolefin layer/adhesive agent layer/VDC layer/adhesive resin 
layer/polyamide layer or the thermoplastic resin layer/adhesive resin 
layer/polyolefin layer. 
The thus extruded laminate tubular film is stretched biaxially following 
the conventional biaxial inflation method to form the heat-shrinkable 
laminate tubular film according to the present invention. 
Since the thus prepared, heat-shrinkable laminate tubular film of the 
present invention shows gas-barrier property and an adequate rigidity, it 
is suitably used in packaging particularly foodstuffs. 
The present invention will be explained more in detail while referring to 
the following non-limitative examples. 
EXAMPLES 1 TO 5 AND COMATIVE EXAMPLES 1 TO 3 
The VDC, the adhesive agent, the polyamide, the thermoplastic polyester and 
the polyolefin respectively shown in Table 1 were extruded from a 
plurality of extruders, respectively, and the thus extruded, molten resins 
were supplied, particularly in Examples 1 to 5, to the annular 
co-extruding die provided with the adiabatic spaces (3 and 5) shown in 
FIG. 1, thereby extruding the thus supplied molten resins as a laminate 
tubular film. The thus prepared laminate tubular film was cooled in a 
quenching bath at 15.degree. to 25.degree. C. to form a laminate tubular 
film of 120 mm in folded width and 540 .mu.m in thickness. A small amount 
of soy-bean oil had been introduced into the inside space of the tubular 
film for the prevention of self-adherence of the inner layer when folded. 
Then, the laminate tubular film was transferred through a hot water bath at 
a temperature shown in Table 3 at a transferring speed of 20 m/min and 
heated for about 12 sec while being pinched by the first pair of nip-rolls 
rotating at a speed of 20 m/min. Thereafter, the thus heated tubular film 
was taken out from the hot water bath and was stretched between the first 
pair of nip-folls rotating at a speed of 20 m/min and the second pair of 
nip-rolls rotating at a speed of 60 m/min, into the longitudinal direction 
at a stretched ratio of 3 (i.e. 60/20=3) while simultaneously being 
stretched to the diametrical direction 3 times by air supplied into the 
inside space of the tubular film. The thus obtained, biaxially stretched 
laminate film was 350 mm in folded width and about 60 micrometer in 
thickness. 
The same procedures were carried out in Comparative Examples 1 to 3 except 
for using a conventional circular co-extruding die not provided with the 
adiabatic space as shown in FIG. 1 instead of the die shown in FIG. 1. 
The physical properties of the resins used in Examples 1 to 5 and 
Comparative Examples 1 to 3 are shown in Table 1; the methods for 
determination of the physical properties of the films produced thereby are 
shown in Table 2 and the layer construction and the test data of the thus 
produced films are shown in Table 3. 
TABLE 1 
__________________________________________________________________________ 
Properties of Resins used 
Melt index 
Crystal melting 
Resin Density 
(g/10 min) 
point (.degree.C.) 
Remarks 
__________________________________________________________________________ 
VDC.sup.1 -- -- -- 
Polyamide 1.14 -- 215 Nylon 6-66 made by TORAY Co., Ltd. 
Thermoplastic polyester 
1.267 
-- not shown 
PET-G, made by Eastman Kodak Co., Ltd. 
LLDPE 0.922 
2.5 122 Made by MITSUITOATSU Co., Ltd. 
EVA-1 0.933 
1.1 96 Content of vinyl acetate units of 
5.5% by weight 
EVA-2 0.93 1.5 91 Content of vinyl acetate units of 10% 
by weight 
EVA-3 0.94 0.6 82 Content of vinyl acetate units of 15% 
by weight 
N--polymer (as adhesive).sup.2 
0.95 6 -- 
__________________________________________________________________________ 
Notes 
.sup.1 A resinuous mixture of 100 parts by weight of a copolymer of 
vinylidene chloride (70% by weight) and vinyl chloride (30% by weight) an 
1 part by weight of an epoxidized soybean oil. 
.sup.2 A modified copolymer of ethylene and an ester of acrylic acid, 
obtained by modifying the copolymer of ethylene and an ester of acrylic 
acid with an unsaturated carboxylic acid, made by Nippon Petrochemical 
Co., Ltd. 
TABLE 2 
______________________________________ 
Item Method 
______________________________________ 
Shrinkage in 
After immersing a specimen (10 cm 
hot water 
square) of each of the laminate films at a 
relaxed state thereof in hot water at 90.degree. C. 
for one min, the percentage of the length 
to the original length and the percentage 
of the width to the original width are 
shown, respectively as the averaged values 
of the test data on 20 specimens. 
Gas-barrier 
The volume of gaseous oxygen (ml/m.sup.2 
property of the film .multidot. 24 hours .multidot. atm) permeated 
through 
a specimen of each of the laminate films at 
30.degree. C. and relative humidity of 100%, re- 
spectively as the averaged values of the 
test data on 3 specimen. 
______________________________________ 
TABLE 3 
__________________________________________________________________________ 
Layer Construction of Tubular Film and Temperature of Hot Water Bath 
Examples and 
Layer Construction Temperature 
Comparative 
1st layer 
2nd layer 
3rd layer 
4th layer 
5th layer 
6th layer 
7th layer 
of Hot Water 
Example 
(.mu.m) 
(.mu.m) 
(.mu.m) 
(.mu.m) 
(.mu.m) 
(.mu.m) 
(.mu.m) 
Bath (.degree.C.) 
__________________________________________________________________________ 
Ex. 1 EVA-3 
N--polymer 
VDC N--polymer 
Polyamide 
N--polymer 
EVA-3 
85 
(12) (1) (8) (1) (6) (1) (31) 
Ex. 2 EVA-3 
N--polymer 
VDC N--polymer 
PET-G N--polymer 
EVA-3 
87 
(12) (1) (8) (1) (6) (1) (31) 
Ex. 3 LLDPE/ 
N--polymer 
VDC N--polymer 
Polyamide 
N--polymer 
LLDPE/ 
94 
EVA-1*.sup.1 
(1) (10) (1) (6) (1) (29) 
(12) 
Ex. 4 LLDPE 
N--polymer 
VDC N--polymer 
PET-G N--polymer 
EVA-2 
95 
(9) (1) (8) (1) (6) (1) (34) 
Ex. 5 LLDPE/ 
N--polymer 
VDC N--polymer 
Polyamide 
N--polymer 
EVA-1 
93 
EVA-1*.sup.1 
(1) (10) (1) (4) (1) (31) 
(12) 
Comparat. 
EVA-3 
N--polymer 
VDC N--polymer 
EVA-3 N--polymer 
EVA-3 
89 
Ex. 1 (15) (1) (8) (1) (6) (1) (28) 
Comparat. 
EVA-3 
N--polymer 
VDC N--polymer 
Polyamide 
N--polymer 
EVA-3 
-- 
Ex. 2 (12) (1) (4) (1) (15) (1) (26) 
Comparat. 
LLDPE/ 
N--polymer 
VDC N--polymer 
PET-G N--polymer 
LLDPE/ 
98 
Ex. 3 EVA-1*.sup.1 
(1) (8) (1) (27) (1) (15) 
(7) 
__________________________________________________________________________ 
Notes: 
*.sup.1 A mixture of 30 parts by weight of LLDPE and 70 parts by weight o 
EVA1 
*.sup.2 A mixture of 30 parts by weight of LLDPE and 70 parts by weight 
EVA2 
TABLE 3-2 
__________________________________________________________________________ 
Physical Properties of Tubular Film 
Example or 
Comparative Rate of heat-shrinkage 
Gas-barrier 
Coextrudability 
Example 
Stretchability 
Rigidity 
(longitudinal/transversal) 
property 
of tubular film 
__________________________________________________________________________ 
Example 
1 A A 51/53 58 A 
2 A A 55/56 59 A 
3 A A 43/45 50 A 
4 A A 44/46 59 A 
5 A A 45/47 50 A 
Comparative 
Example 
1 B C 50/53 60 A 
2 -- -- -- -- C 
3 C -- -- -- B 
__________________________________________________________________________ 
(Notes) 
(1) Stretchability: 
A: Excellent, particularly excellently stretchable with unevenness of the 
thickness of the film of below 30%, 
B: Inferior, although stretchable, with unevenness of the thickness of th 
fim of from 30 to 50%, 
C: Poor, although stretchable, without steadiness and with unevenness of 
the thickness of the film over 50%. 
(2) Rigidity: 
A: Excellent, defects in the packaging operation such as the elongation o 
the film due to the content scarcely occur, 
B: Inferior, defects in the packaging operation occur some times, 
C: Poor, defects in the packaging operation are apt to occur. 
(3) Coextrudability of the layers into a composite laminate tube: 
A: Coextrusion was possible continuously for more than 10 hours, 
B: The time of sufficient, continuous extrusion was below 10 hours, 
C: Coextrusion could be continued only less than one hour. 
As are clearly seen in the results of Examples 1 to 5 shown in Table 3, the 
heat-shrinkable laminate tubular film according to the present invention 
is excellent in stretchablity and adequate rigidity, thereby excellent in 
processability in the packaging operation. In addition, the combination of 
the resins for producing the film was excellent in coextrudability. 
On the other hand, although the resin combination of Comparative Example 1 
was excellent for coextruding, the rigidity of the film was poor resulting 
in the poor processability in the packaging operation due to the absence 
of the layer of the polyamide or the thermoplastic polyester. 
Although the resin combination of Comparative Example 2 includes the 
polyamide for the inner layer, because of the too thin layer of the VDC, 
decomposition of the VDC occurred to inhibit the continuous operation of 
coextrusion. In Comparative Example 3, because of the too large thickness 
of the layer of the thermoplastic polyester, the coextrudability was poor 
to cause the decomposition of the VDC. In addition, the co-extruded film 
was poor in stretchability. By the above-mentioned demerits, any favorable 
laminate film could not be obtained.