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DESCRIPTION OF THE PRIOR ART
Thermoplastic bags, and in particular polyethylene bags, have in recent
years gained prominence in the packaging of a wide variety of goods such
as dry goods, comestibles and the like. Most recently, polyethylene bags
have emerged as the preferred packaging material for refuse materials and,
in fact, many communities across the country have mandated that refuse be
packaged and contained in such a manner. The advantages offered are
obvious and include a hygenic means for the containment of garbage and
waste materials; the bag provides some protection of the contents from
insects, ruminants and other animals which would normally be attracted by
the bag contents. Such bags are conventionly employed as disposable liners
for trash cans whereby when the trash containers have been filled to
capacity, the bag mouth is gathered and twisted closed and raised out of
the container, leaving the interior of the container free from
contamination and ready to receive another bag liner. The twisted bag
mouth may be secured in a conventional manner employing wire-twistems or
similar fasteners and subsequently the closed, loaded bag is disposed of.
Alternatively, such bags may be employed in an unsupported condition as
receptables. Prior art polyethylene branched low density homopolymer bags
however lack stiffness and when articles are loaded into such bags
difficulties are encountered in keeping the bag mouth open, requiring
excessive digital manipulation.
Another of the most common drawbacks in the employment of polyethylene bags
in waste disposal is their tendency to rupture under load stresses and,
also, their fairly low puncture resistance. When a loaded bag is
punctured, by an internal or external element, it is characteristic of the
polyethylene film to zipper, i.e., the puncture tear rapidly propagates
across or down the bag wall.
Numerous attempts have been made in the past to remedy the aforenoted
deficiencies, the most obvious being to increase the film gauge, i.e.,
make the bag walls thicker and therefore stronger. However, substantial
gauge increases are necessary to achieve substantial bag strengthening, on
the order of 50% to 150%, and the product costs are increased in direct
proportion to the increased amount of resin employed in each bag. Attempts
to replace the relatively low cost polyethylene with other resins which
exhibit improved strength characteristics have been largely unsuccessful
for reasons including the unfavorable economics associated with the more
costly resin substitutes.
SUMMARY OF THE INVENTION
In accordance with the present invention it has been found that
thermoplastic film structures which contain a predominant amount of
relatively low cost resinous materials commonly used in the prior art
fabrication of bags such as, for example, general purpose, low density
polyethylene branched homopolymer resin may be fabricated into articles
such as bags which have improved stiffness, i.e., modulus, and strength
characteristics over prior art polyethylene bags. In general it has been
found that a multi-layer structure comprising at least one layer of low
density, general purpose polyethylene resin having a thickness on the
order of from about 50% to 90% and preferably from about 65% up to about
85% of the overall laminate thickness may be bonded to a second layer, the
second layer contributing the balance of the overall thickness of a blend
of resins. For example, the second layer may be constituted by a
relatively thin layer of a resinous blend which comprises a high density
polyethylene homopolymer resin and a linear low density polyethylene
copolymer, which may be a copolymer of ethylene and another alpha olefin
having from about three up to fifteen carbon atoms and a density of below
about 0.94 grams per c.c. The preferred alpha-olefin comonomers comprise
at least one C.sub.4 to C.sub.8 olefin. Minor amounts of a colorant
masterbatch material, on the order of less than about 5% by weight, such
as a blend of low density polyethylene and an inorganic pigment may also
be used. It has been found that when structures such as bags are
fabricated from such laminar film materials, the branched low density
polyethylene layer preferably constituting the interior bag surface, such
bag structures offer improved strength characteristics as contrasted to
the aforedescribed prior art polyethylene bag structures. Additionally,
such strength characteristics are not achieved by sacrificing material
economics as hereinabove discussed since the laminar bag structure of the
present invention contains a predominant amount, i.e., up to about 85% of
the overall laminar thickness, of low cost general purpose branched low
density polyethylene resin.
BRIEF DESCRIPTION OF THE DRAWINGS
The FIG. 1 is a schematic side elevation, in cross section, of one form of
extrusion apparatus which may be employed for the production of the
laminar films of the present invention, with certain segments enlarged for
clarity.
DESCRIPTION OF SPECIFIC EMBODIMENTS
Numerous techniques have been described in the prior art for the formation
of multilayer laminar thermoplastic film constructions including
preforming a first film and subsequently melt extruding another film onto
its surface whereby a two layer laminate is formed. Other techniques which
have been developed in more recent years include a technique which is
referred to as coextrusion, a process whereby molten or semimolten layers
of different polymer melts are brought into contact and subsequently
cooled. Examples of such coextrusion techniques are described in U.S. Pat.
Nos. 3,508,944 and 3,423,010. Although any of the aforedescribed
techniques may be suitable in formation of the laminar structures of the
present invention a particularly preferred technique is to produce the
present laminates by extrusion of separate polymer melts from tubular die
orifices which are concentric causing the separate molten or semi-molten
streams to be extruded coaxially and then merged together outside of the
die orifices whereby upon subsequent cooling a tubular laminate is
produced.
In producing the multi-layer film of the present invention, intended for
bag structures in one particular application, it has been found that
certain particularly desirable physical characteristics should be
exhibited by the individual lamina. For example in bag constructions the
outer layer, which may comprise from about 10% up to 50% of the overall
laminate thickness, must be preferably stiff, i.e., have a relatively high
tensile modulus; it must be tough, i.e., resistant to impact forces; it
should exhibit good elongation under stress; and, finally, have a high
degree of tear resistance particularly in the transverse direction of the
layer, i.e, the direction which is transverse to the extrusion direction.
The physical characteristics which are particularly desirable in the
thicker interior laminar bag layer include ease of heat sealing over wide
ranges of temperature and pressure; and a high degree of tear resistance
particularly in the layer's machine direction.
The degree of orientation in each of the respective laminar layers is an
important factor with respect to the overall physical properties of the
multilayer structure. It has been found that two types of orientation of
the polymer crystallites occur in blown film extrusion by the trapped air
method. The first type occurs by flow through the die lips and this
orientation tends to align the crystallites formed upon cooling in the
direction of flow (MD). In a linear polymer with long, straight chains,
the crystallites are oriented in the machine direction. With more
branching of the chain as in ordinary low density branched polyethylene,
the crystallites tend to be in a somewhat more random orientation. The
orientation of high density polyethylene, since it is linear and more
crystalline, thus is quite strong compared to branched low density
polyethylene. From this die effect alone, the net result is a highly
oriented film in the machine direction (MD) with little transverse
direction (TD) orientation. In the homopolymer progression from ordinary
low density polyethylene to high density polyethylene, as the density
increases and polymer branching decreases, the material is more subject to
orientation. High density polyethylene is highly oriented and thus
susceptability to tearing in the machine direction (MD) is very high.
It has been found that the second type of orientation in the blown film
process is the blow-up ratio (BUR) effect. Since this stretching of the
film expands the bubble to larger diameters, the stress on polymer
crystallites is multi-directional in nature and thus helps counteract the
MD orientation associated with the die effects. As BUR increases, TD
orientation effects increase at some drop in MD properties. Improved tear
resistance thus can be achieved in the normally weak TD direction.
Low density polyethylene normally is run in the range of 1.5-3.0:1 blow-up
ratio (circumference of the bubble: circumference of the annula die) in an
attempt to balance the properties between machine direction (MD) and
transverse direction. In contrast high density polyethylene orients
strongly in the machine direction due to the die effect, giving very poor
TD properties at low density polyethylene type blow-up ratios. Economics
and ease of handling the molten polymer strongly discourage such large
blow-up ratios but tear is a key property in the bag type product. The
present invention permits film to run at low density polyethylene rates
and BUR conditions (i.e. 1.5 to 3:1 ratio) with the additional stiffness
and strength of the high density polyethylene-ethylene and .alpha.-olefin
blend in the outer layer.
There is illustrated in FIG. 1 one form of extrusion apparatus which may be
employed to produce the laminar films of the present invention. As shown
two thermoplastic extruders 11 and 12 feed dissimilar molten thermoplastic
resins to common die member 13. Tubular extrusion die 13 has two
concentric annular passages to separately accommodate and shape the
individual resinous streams until they exist from concentric die orifices
14 and 14'. Shortly after emerging from orifices 14 and 14' the
concentric, coaxial, molten or semi-molten tubes merge and become bonded
together to form a two layered laminar tube 15. Air is provided (by
conventional means not shown) to inflate and support tube 15 until tube 15
is collapsed downstream from die 13 by conventional counter-rotating
collapsing rollers (not shown), i.e., a conventional entrapped air-bubble
tubular extrusion process. The collapsed laminar tubing is subsequently
passed to a wind-up station (not shown) or on to further processing, e.g.,
a bag making operation.
In practice, pelletized resinous materials to be fed to the extrusion
system illustrated in FIG. 1 is air-veyed by a vacuum unloader from a
supply source and fed to separate feeder tanks which are mounted above the
individual extruders 11 and 12 illustrated in FIG. 1. Each of the resinous
components in the blend compositions which are fed to extruder 11 (i.e.,
the extruder which supplies a molten resinous blend to die 13 to form
outer layer 16) are volumentrically measured and dropped into a mixer
located above extruder 11, the order of addition is not critical. The
mixer is actuated at 120 RPM for approximately 15 seconds and then the
premixed blend is fed to the extruder feed zone (not shown). For the
primary extruder (i.e., extruder 12 which is employed to form the inner
layer 17) a resin consisting essentially of branched low density
polyethylene is used as a feed material.
The primary extruder 12 which was employed in the following example
comprised a 6 inch diameter screw which was driven by a 250 HP motor. The
screw had an L/D ratio of 28:1 The extruder barrel was a standard design
and equipped with external jackets employed for the circulation of
temperature control fluids therein and/or conventional electric resistance
band heating elements positioned around the barrel.
The secondary extruder 11, i.e., that extruder which feeds molten resinous
blend mixtures to die 13 to form outer layer 16 of the laminar structure,
had a 41/2 inch screw diameter and an L/D ratio of 24:1. The extruder
barrel for extruder 12, was likewise equipped with external jackets for
circulation therein of temperature control fluids and/or electrical
resistance band heaters spaced along the length of the barrel to control
the temperatures of the molten polymer inside the barrel.
Die 13, as shown in FIG. 1, is a coextrusion die with the primary extruder
12 feeding material which will eventually constitute layer 17 and
secondary extruder 11 feeding material to die 13 which will eventually
constitute outer layer 16. The annular die lips have approximately a 0.040
inch annular gap which form orifices 14 and 14' with a 1/2 to 2 inch
length angled lip section in the die so that the individual concentric
tubes are separated as they exit from die 13 by approximately 1/32 inch.
As a result of the separation, the film layers are joined above the die as
illustrated in FIG. 1 to form laminar tube 15.
Upon exit from die 13 the extruded concentric tubes 16 and 17 are oriented
by internal air pressure trapped within the tube between the die 13 and
the film collapsing nips (not shown) which inflates the tube to between 2
and 2.5 times the circumference of the die orifice. This is essentially a
conventional entrapped air bubble extrusion technique.
While the internally trapped air is stretching the film, a high velocity
air stream supplied by air ring 18 as shown in FIG. 1, impinges in a
generally vertical direction on the extruded tube to cool the molten
polymer. The combination of internal air expansion and high velocity
impingement of air from air ring 18 causes the layers to contract while
still in the molten state and thereby forming a strong interfacial bond as
the contacting layers cool and solidify.
Prior to passage of tube 12 to the nip rollers the formed film tube is
conventionally collapsed by a frame of horizontally wooden slats located
in an inverted V shape with the angle between the legs of the V
approximately 30.degree. to 35.degree.. This V frame gradually flattens
the film tube until, at the apex of the V, the tube is completely
collapsed by the nip rollers which may consist of a rubber roll and a
steel driven roller. The nip rollers function to draw the tube from the
extrusion die 13 and also effect an air seal for the entrapped air bubble
in the tube. Subsequent to passing the flattened tube through the nip
rollers, the film is either wound into rolls or passed through bag making
machinery or the like to form a finished product.
As hereinabove discussed, the outer layer of the laminar film structures of
the present invention preferably comprise a blend of thermoplastic resins
and in particular blends of high density polyethylene together with a
linear low density polyethylene-alpha olefin copolymer. While these
copolymers may contain alpha-olefins having 3 to 15 carbon atoms, the
preferred copolymers include polyethylene copolymerized with another alpha
olefin including C.sub.4 to C.sub.8 alpha-olefins such as octene-1,
butene-1, hexene-1 and 4-methylpentene-1. The preferred concentration by
weight of the alpha olefin which is copolymerized with polyethylene is
from about 2.0% up to about 10% by weight. In the following specific
embodiments the linear low density copolymer of polyethylene with about
4.8% by weight of octene copolymerized therewith. It has been found that
when such a blend comprises the exterior laminar tube layer, the resultant
laminates exhibit greatly improved modulus and tear resistance.
In the following Table I there is presented a listing of pertinent resin
physical properties of the various polyolefin materials which were
employed in the succeeding examples.
TABLE I
______________________________________
Low Density Polyethylene Resin (For Inner Layer
Polyethylene Component
ASTM
Property Value Test Method
______________________________________
Melt Index, g/10 min
2.25 D-1238-65T
Density, g/cc .921 D-1505-68
Tensile at Yield
(20"/min) psi 1331 D-638-68
Tensile at Break
(20"/min) psi 1688 D-638-68
Elongation at Break, %
603 D-638-68
Elastic Modulus, psi
24635 D-638-68
Stiffness in Flexure,
psi 800 D-747-63
Hardness, Shore D D44 D-2240-68
Vicat Softening
Point, .degree.F. 217 D-1525-65T
Brittleness Tempera-
ture, .degree.F. below D-746-64T
-105
______________________________________
Physical Properties - Linear Low Density
Polyethylene - Octene-1 Copolymer Resin
ASTM
Property Value Test Method
______________________________________
Melt Index 2.0 D-1238
Density 0.926 D-1505
Molecular Weight 89,000 --
% by Weight Octene-1
4.8 --
______________________________________
High Density Polyethylene Resin
ASTM
Property Value Test Method
______________________________________
Melt Index, g/10 min.
0.35 D-1238
Density, g/cc 0.963 D-1505
Tensile Yield D-638
lbf/in.sup.2 4100
kgf/cm.sup.2 288
Elongation, % 800 D-638
Flexural Modulus D-790
lbf/in.sup.2 205,000
kgf/cm.sup.2 14,400
Hardness, Shore D 70 D-1706
Izod Impact, ft
lbf/in of notch 6.9 D-256
Tensile Impact D-1822
ft lbf/in.sup.2 60
cm kgf/cm.sup.2 128
Brittleness Tempera-
ture <-70 D-746
Vicat Softening
Point D-1525
______________________________________
The details and manner of producing the laminar tubular structures of the
present invention will be apparent from the following specific examples,
it being understood, however, that they are merely illustrative
embodiments of the invention and that the scope of the invention is not
restricted thereto.
In the subsequent examples the apparatus which was actually used to form
the multi-wall thermoplastic tubing corresponded essentially to that shown
in FIG. 1 of the drawing. Also, the resinous material employed in the
following examples had the physical properties as outlined in preceding
Table I.
EXAMPLE 1
(Comparative Prior Art Film)
A dual wall tubular thermoplastic film was coextruded with an inner layer
of branched low density polyethylene homopolymer and an outer layer of a
blend of crystalline high density polyethylene homopolymer with
ethylene-vinylacetate (EVA) copolymer (18% vinyl acetate by weight), and
ordinary fractional melt branched low density polyethylene. The
homopolymeric inner layer consists of about 96 parts by weight of branched
low density polyethylene and 4 parts of black master batch colorant. The
outer layer consists of a melt blend mixture of about 35 wt% crystalline
high density polyethylene homopolymer, 35 wt% EVA copolymer, 25 wt%
branched low density polyethylene and 5 wt% of redwood master batch
colorant. The master batch colorants are prepared from about 50 wt%
inorganic pigment and 50 wt% ordinary low density polyethylene.
The inner and outer layers are melt extruded concurrently from extruders 12
and 11, respectively, forming a multilayer film having an average
thickness of about 1.5 mils. The respective molten layers assumed a
tubular configuration as they flowed through die 13. The molten tubes exit
from die 13 as concentric tubes through orifices 14 and 14' whereupon they
subsequently merged together to form the laminar tube 15 as shown in FIG.
1. The extruder processing conditions including pressures, temperatures
and die orifice dimensions employed for this, and the following example,
are set forth in subsequent Table II which also includes data on the
physical properties of the multi-wall extruded film produced. No
separation of the two layers occurred when the resultant laminar film was
repeatedly flexed. The branched low density polyethylene layer of the
laminar film constituted approximately 78% of the overall thickness of the
laminate.
EXAMPLE 2
The procedure of Example 1 was followed, however, in this case the outer
layer of the tubular film construction comprised a major amount of linear
low density copolymer. The structure was further modified in the present
example in that the outer laminar layer comprised about 75% by weight of a
linear, low density ethylene-octene-1 copolymer containing about 4.8% by
weight of octene-1; 20% by weight of high density polyethylene and about
5% by weight of a master batch comprising 50% by weight of inorganic
pigment and about 50% of weight of low density polyethylene as a carrier.
EXAMPLE 3
The tubular blend comprising the outer laminar layer was identical with
that defined in preceding Example 2, however, the total thickness of the
outer laminar layer comprised about 26% of the overall laminate thickness.
EXAMPLE 4
The tubular laminar construction was prepared in accordance with the
procedure defined in Example 1, however, in this case the external tubular
layer comprised 22% of the overall laminate thickness. Additionally, the
outer laminar layer blend in this example comprised a blend of about 60%
by weight of the ethylene-octene-1 copolymer; 20% by weight of high
density polyethylene; 5% by weight of the low density-inorganic pigment
colorant; and about 15% by weight of ordinary branched low density
polyethylene as hereinbefore defined.
EXAMPLE 5
A tubular laminar construction was prepared in accordance with the
procedure set forth in Example 1, wherein the overall thickness of the
outer laminar layer was approximately 22%. In this case, the resin blend
comprising the outer layer of the tubular laminate comprised 65% by weight
of ethylene-octene-1 copolymer; 30% by weight of high density polyethylene
and 5% by weight of the pigmented master batch material.
The physical properties of the tubular laminates prepared in accordance
with the preceding Examples are set forth in following Table 2. Table 3
sets forth the process conditions which were employed to produce the
laminar structures as described in preceding Examples 1 through 5
inclusive.
TABLE 2
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Example 1 2 3 4 5
______________________________________
Outer Layer
Percentage of Total bags
22% 22% 26% 22% 22%
Ethylene - olefin (%)
-- 75 75 60 65
HDPE (%) 35 20 20 20 30
Redwood Masterbatch (%)
5 5 5 5 5
LDPE (%) 25 -- -- 15 --
EVA (%) 35 -- -- -- --
Inner Layer 78 78 74 78 78
LDPE 96 96 96 96 96
Black Masterbatch (%)
4 4 4 4 4
Elmendorf Tear
MD MD 447 549 547 550 582
(6 MS) TD TD 222 176 202 199 210
1% Secant Moduls
MD 24.3 24.9 25.8 26.6 28.6
(K PSI) TD 31.7 30.7 29.9 33.7 35.0
Tensile Yield MD 1316 1418 1296 1406 1463
(PSI) TD 1433 1470 1414 1546 1604
Tensile Ultimate
MD 3588 3104 3089 3328 3185
(PSI) TD 2146 2095 2089 2151 2232
Tensile Toughness
MD 482 473 508 464 527
Ft.-lb/in.sup.3
TD 736 727 702 744 783
Tensile Elongation
MD 201 227 244 208 242
(%) TD 576 525 555 560 574
Directional Spencer
70 75 78 61 71
Opacity (Light Transmission %)
11.9 8.2 7.0 7.2 6.3
______________________________________
TABLE 3
______________________________________
Extruder 12: (inner layer)
Barrel Dia. (in.) 6"
Screw RPM 49
Plastic Melt Temp., .degree.F.
396
Plastic Melt Press. (psi)
4600
Extruder 11: (outer)
Barrel Dia. (in.) 4.5
Screw RPM 41
Plastic Melt Temp., .degree.F.
500
Plastic Melt Press. (psi)
5400
Die 13:
Orifice Width (in.) outer
.040
Orifice Width (in.) inner
.040
Tubular Film:
Layflat Width (in.) 72
Wall Thicknesses (mils)
Inner Wall 1.2 mil
Outer Wall 0.3 mil
______________________________________
As will be apparent from the foregoing Examples and Tables, it has been
found that blend compositions comprising a linear low density copolymer of
an ethylene-alpha-olefin such as octene-1 when blended together with an
appropriate amount of a high density polyethylene resin provides excellent
resistance to tear and high modulus properties. Moreover, such properties
are either equivalent or superior to prior art blend mixtures such as
those containing high density polyethylene and large amounts of ordinary
low density polyethylene and/or ethylene-vinyl acetate (EVA) copolymer
which have been employed in prior art constructions.
Although the present invention has been described which preferred
embodiments, it is to be understood that modifications and variations may
be resorted to, without departing from the spirit and scope of this
invention, as those skilled in the art will readily understand. Such
modifications and variations are considered to be within the purview and
scope of the appended claims.
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