Heat shrinkable, coextruded polyethylene film laminate

Details Heat shrinkable, coextruded polyethylene film laminate Heat shrinkable, coextruded polyethylene film laminate

2001-05-02

2003-01-21

Tarazano, D. Lawrence (Department: 1773)

Stock material or miscellaneous articles

Structurally defined web or sheet

Including components having same physical characteristic in...

C428S516000, C428S910000

Reexamination Certificate

active

06509087

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to a heat shrinkable, coextruded polyethylene film laminate suitable for the heat-shrink packaging of an article.
A heat-sealable, biaxially oriented polyethylene film laminate is known which includes a core layer of a relatively higher melting point polyethylene between two surface layer of a relatively low melting point polyethylene. A heat-resisting, biaxially oriented polyethylene film laminate is also known which includes a core layer of a relatively low melting point polyethylene between two surface layer of a relatively high melting point polyethylene. Because of poor stretchability of polyethylene in comparison with polypropylene resin, however, it is difficult to prepare the above polyethylene film laminates. In particular, the temperature range in which the stretching of polyethylene films can be carried out smoothly in a satisfactory manner is so narrow that it is difficult to continue stretching of the polyethylene films in a stable manner for a long time. As a consequence, known oriented polyethylene film laminates have defects that the thickness thereof is not uniform and that stretching cannot be continued in a stable manner for a long process run.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a biaxially oriented, coextruded polyethylene film laminate which has uniform thickness.
Another object of the present invention is to provide a biaxially oriented, coextruded polyethylene film laminate which has good stretchability in both high and low temperatures and which can be continuously stretched in a stable manner even when the stretching temperature fluctuates.
It is another object of the present invention to provide a biaxially oriented, coextruded polyethylene film laminate of the above-mentioned type which shows good heat resistance and good surface slippage (good wrapping or packaging property).
In accomplishing the foregoing objects, there is provided in accordance with the present invention a polyethylene laminate film comprising two surface layers F1 and F2, and an intermediate layer M interposed therebetween,
wherein the surface layer F1 comprises
a linear low density polyethylene having a density D
AF1
g/cm
3
of 0.910-0.930 g/cm
3
and a linear high density polyethylene having a density D
CF1
g/cm
3
of 0.925-0.945 g/cm
3
and being present in an amount of W
CF1
% based on a total weight of the linear low density polyethylene having the density D
AF1
and the linear high density polyethylene having the density D
CF1
,
wherein the surface layer F2 comprises
a linear low density polyethylene having a density D
AF2
g/cm
3
of 0.910-0.930 g/cm
3
and
a linear high density polyethylene having a density D
CF2
g/cm
3
of 0.925-0.945 g/cm
3
and being present in an amount of W
CF2
% based on a total weight of the linear low density polyethylene having the density D
AF2
and the linear high density polyethylene having the density D
CF2
,
wherein the intermediate layer M comprises
a linear low density polyethylene having a density D
AM
g/cm
3
of 0.910-0.930 g/cm
3
and
a linear very low density polyethylene having a density D
BM
g/cm
3
of 0.880-0.915 g/cm
3
and being present in an amount of W
BM
% based on a total weight of the linear low density polyethylene having the density D
AM
and the linear very low density polyethylene having the density D
BM
, and
wherein the laminate film satisfies the following conditions (a) through (e) at the same time:
(a) D
CF1
−D
AF1
≧0.010,
(b) D
CF2
−D
AF2
≧0.010,
(c) D
AM
−D
BM
≧0.010,
(d) 0.01≦{(D
CF1
−D
AF1
)×L
F1
×W
CF1
}+{(D
CF2
−D
AF2
)×L
F2
×W
CF2
}≦0.20,
(e) 0.05≦(D
AM
−D
BM
)×L
M
×W
BM
}≦1.40,
 where D
CF1
, D
AF1
, D
CF2
, D
AF2
, D
AM
, D
BM
, W
CF1
, W
CF2
and W
BM
are as defined above and L
F1
, L
F2
and L
M
represent as follows:
L
F1
=t
F1
/(t
F1
+t
F2
+t
M
),
L
F2
=t
F2
/(t
F1
+t
F2
+t
M
),
L
M
=t
M
/(t
F1
+t
F2
+t
M
),
where t
F1
, t
F2
and t
M
represent the thicknesses of the surface layer F1, surface layer F2 and intermediate layer M, respectively.
Other objects, features and advantages of the present invention will become apparent from the detailed description of the preferred embodiments of the invention to follow.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
In the polyethylene film laminate according to the present invention, a linear low density polyethylene (hereinafter referred to as LLDPE) having a density of 0.910-0.930 g/cm
3
is used in each of surface layers F1 and F2 and in an intermediate layer M interposed therebetween. It is preferred that LLDPE have a melt index of 0.5-5.9 g/10 min for reasons of good stretchability. LLDPE is preferably a copolymer of ethylene with an &agr;-olefin having 4-8 carbon atoms. Copolymers produced using a Ziegler Natta catalyst or a single site catalyst may be used for the purpose of the present invention.
In addition to LLDPE, each of the surface layers F1 and F2 contains linear high density polyethylene (hereinafter referred to as LHDPE) having a density of 0.925-0.945 g/cm
3
. The intermediate layer M must contain linear very low density polyethylene (hereinafter referred to as VLDPE) having a density of 0.880-0.910 g/cm
3
in addition to LLDPE.
It is preferred that LVLDPE have a melt index similar to LLDPE and in the range of 0.5-5.0 g/10 min for reasons of good streatchability and of compatibility with LLDPE. LVLDPE is preferably a copolymer of ethylene with an &agr;-olefin having 4-8 carbon atoms. Copolymers produced using a Ziegler Natta catalyst or a single site catalyst may be used for the purpose of the present invention.
It is preferred that LHDPE have a melt index similar to LLDPE and in the range of 0.5-5.0 g/10 min for reasons of good streatchability and of compatibility with LLDPE. LHDPE is preferably a copolymer of ethylene with an &agr;-olefin having 4-8 carbon atoms. Copolymers produced using a Ziegler Natta catalyst or a single site catalyst may be used for the purpose of the present invention.
In the present invention, it is not necessary that the same polyethylene be used in the surface and intermediate layers of the same laminate. Thus, different LLDPEs can used in respective layers of the same laminate. Similarly, respective layers of the same laminate can contain different LVLDPEs and different LHDPES.
The surface layers F1 and F2 and the intermediate layer M of the laminate according to the present invention must meet with specific conditions. These conditions will be next described.
As described above, the surface layers F1 and F2 contain LHDPE in addition to LLDPE, while the intermediate layer M contains LVLDPE in addition to LLDPE. In this case, it is important that the following conditions (a) through (c) are met at the same time:
(a) D
CF1
−D
AF1
≧0.010,
(b) D
CF2
−D
AF2
≧0.010,
(c) D
AM
−D
BM
≧0.010,
 wherein
D
CF1
: density (g/cm
3
) of LHDPE of surface layer F1
D
AF1
: density (g/cm
3
) of LLDPE of surface layer F1
D
CF2
: density (g/cm
3
) of LHDPE of surface layer F2
D
AF2
: density (g/cm
3
) of LLDPE of surface layer F2
D
BM
: density (g/cm
3
) of LVLDPE of intermediate layer M
D
AM
: density (g/cm
3
) of LLDPE of intermediate layer M.
Namely, in each of the surface layers F1 and F2, the density of LHDPE must be greater by at least 0.010 g/cm
3
than that of LLDPE, in order to obtain good stretchability and heat resistance. At the same time, in the intermediate layer M, the density of LLDPE must be greater by at least 0.010 g/cm
3
than that of LVLDPE in order to obtain good stretchability.
Further, it is important that the following conditions (d) and (e) should be met simultaneously:
(d)
0.01≦Z
S1
+Z
S2
≦0.20,
Z
S1
=(D
CF1
−D
AF1
)×L
F1
×W
CF1
Z
S2
=(D
CF2
−D
AF2
)×L
F2
×W
CF2
(e)
0.05≦Z
M
≦1.40
Z
M
=(D
AM
−

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