GB2028716A - Laminar thermoplastic film constructions - Google Patents

Abstract

A thermoplastic film construction comprises a layer 17 of low density polyethylene bonded to a layer 16 of a polymer blend comprising high density polyethylene and an ethylene-alpha olefine copolymer. The copolymer may have a density below 0.94 g./cc. and the alpha olefine may contain 3 to 15 carbon atoms. The film may be made by co-extrusion, the blend being fed via extruder 11 to die 13 and the other layer to die 13 via extruder 12. <IMAGE>

Description

SPECIFICATION Laminar thermoplastic film constructions Description of the PriorArt Thermoplastic bags, and in particular polythene 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 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 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 most 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 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 laminar 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 laminate thickness, of a resin or blend or resins which comprises a blend of polymeric resins. For example, the second layer may be constituted by a relatively thin layer of a resinous blend which comprises a high density polyethylene 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.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 low density polyethylene layer preferably constituting the interior bag surface, such bag structures offer improved strength characteristics as contrasted to the aforedescribed prior art non-laminar polyethylene bag structures. Additionally, such strength characteristics are achieved by not sacrificing material economics as hereinabove discussed since the laminar bag structure of the present invention contains a predominant amount, i.e., up to about 80% of the overall laminar thickness, of low cost general purpose polyethylene resin.

Brief Description of Drawings Figure l 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 semi-molten layers of different polymer melts are brought into contact and subsequently cooled. Examples of such coextrusion techniques are described in U.S. Patent Numbers 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. An example of such concentric extrusion of dissimilar thermoplastic melts is described, for example, in U.S. Patent No. 3,926,706, the disclosure of which is incorporated in its entirety herein by reference.

In producing the laminates 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 of that layer.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 layers machine direction (direction of layer extrusion).

The degree orientation in each of the respective laminar layers is an important factor with respect to the overall physical properties of the laminate 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 in the direction of flow (MD). With a material completely amorphous in nature, this flow orientation has little or no effect, while there is an increasing amount of orientation in materials as the crystallinity increases. In a linear polymer with long, straight chains, the crystallites are oriented in the machine direction.With more branching of the chain, the crystallites tend to be in a somewhat more random orientation and these materials also contain more amorphous regions which do not orient. The orientation of high density polyethylene, since it is linear and more crystalline, thus is quite strong compared to 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 progression from 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 susceptabilityto 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 pulls the bubble to larger diameters, the pull 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 (TD). 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 with the additional stiffness and strength of the high density polyethylene-ethylene and ceolefn blend in the outer layer.

There is illustrated in Figure 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 orifices14 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 now 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 Figure 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 Figure 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) only one resinous component, i.e., 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 4 1/2 inch screw diameter and an L/D ratio of 24:1. The extruder barrel for extruder 12, was likewise equipped with hollow 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 Figure 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 14 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 14 by approximately 1/32 inch. As a result of the separation, the film layers are joined above the die as illustrated in Figure 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 diameters. 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 Figure 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 convenionally collapsed by a frame of horozontally wooden slats located in an inverted V shape with the angle between the legs of the V approximately 30 to 350. 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. Such copolymers include polyethylene copolymerized with another alpha olefin including 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%. In the following specific embodiments the linear low density copolymer of polyethylene with about 4.8% by weight of ocente 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 1331 D-638-68 (20"/min).psi Tensile at Break 1688 D-638-68 (20"/min).psi Elongation at Break,% 603 D-638-68 Elastic Modulus, psi 24635 D-638-68 Stiffness in Flexure, 800 D-747-63 psi Hardness, Shore D D44 D-2240-68 Vicat Softening 217 D-1525-65T Point, OF Brittleness Tempera- below D-746-64T ture, 0F -105 Physical Properties - Linear Low Density Polyethylene - Octene-1 Copolymer Resin ASTM Property Value Test Method Melt Index 2.0 D-1238 Density 926 D-1505 Molecular Weight 89,000 % by Weight Octene-1 4.8 TABLE I (continued) 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 Ibf/in2 4100 kgf/cm2 288 Elongation, % 800 D-638 Flexural Modulus D-790 Ibf/in2 205,000 kgf/cm2 14,400 Hardness, Shore D 70 D-1706 Izod Impact, ft Ibf/in of notch 6.9 D-256 Tensile Impact D-1822 ft Ibf/in2 60 cm kgf/cm2 128 Brittleness Tempera- < -70 D-746 ture Vicat Softening D-1525 Point 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 Figure 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 A dual wall tubular thermoplastic film laminate averaging 1.5 mils in thickness, the inner wall being formed from the low density polyethylene hereinbefore defined and the outer wall being formed from a blend of high density polyethylene, ethylene vinyl acetate copolymer containing 18 percent vinyl acetate by weight, and low density fractional melt index polyethylene hereinbefore defined was prepared by melt extruding 98 parts by weight of low density polyethylene resin and 2 parts of black masterbatch colorant through extruder 12 and concurrently melt extruding from extruder 11 a resinous blend mixture comprising 35 percent by weight high density polyethylene, 35 percent by weight of ethylene vinyl acetate copolymer (18 percent VA), and 25 percent by weight of the fractional melt index, low density polyethylene and 5% redwood masterbatch colorant.The respective molten layers assumed a concentric 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 Figure 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 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 laminar tube construction constituted 22% of the overall laminarthickness. The structure was further modified in the Example 3 The tubular laminate 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% by volume 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 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% by volume. In this case, the resin blend comprising the outer layer of the tubular laminate comprised 65% by weight of ethylene-octene1 copolymer; 30% by weight of high density polyethylene and 5% by weight of the inorganic-low density polyethylene pigmented 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 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/in3 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 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 pigment comprising 50% by weight of inorganic pigment and about 50% of weight of low density polyethylene as a carrier.

TABLE 3 Extruder 12: (inner layer) Barrel Dia. (in.) 6" Screw RPM 49 Plastic Melt Temp., F. 396 Plastic Melt Press. (psi) 4600 Extruder 11: (outer) 4.5 Barrel Dia. (in.) Screw RPM 41 Plastic Melt Temp., F. 500 Plastic Melt Press. (psi) 5400 Die 13: Orifice Width (in.) outer .040 inner .040 Tubular Film: LayflatWith (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 ethVlene-alpha-olefin such as octene-1 when blended together with a major amount of high density polyethylene resin provides excellent resistance to tear and high modulus properties. Moreover, such properties are either equivalent or superior than three component blend mixtures such as those containing high density polyethylene or low density polyethylene and vinyl ethylene acetate copolymer which have been employed in prior art constructins.

The advantages in processing and in material handling properties of the improved two component blend system over a three component system in such blended layers is obvious to one skilled in the art.

Although the present invention has been described with 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.

Claims (5)

1. A laminar film structure characterized in that it contains at least one layer of low density polyethylene resin and a second layer comprising a resinous blend of a high density polyethylene and an ethylene-alpha-olefin copolymer, wherein said blend contains a major amount of said copolymer.

2. A laminar film structure in accordance with claim 1 wherein said second layer comprises from about 10 percent up to about 50 percent of the total laminarfilm thickness.

3. A laminar film structure in accordance with claim 1 wherein said alpha olefin comprises about 3 up to about 15 carbon atoms.

4. A laminar film structure in accordance with claim 1 wherein the concentration by weight of said alpha-olefin in said copolymer is from about 1.5 percentto about 10 percent.

5. A laminar thermoplastic bag structure comprising at least two layers, an inner layer and an outer layer, characterized in that the inner layer contains low density polyethylene and the outer layer contains a resinous blend of a high density polyethylene and an ethylene-alpha-olefin copolymer, wherein said blend contains a major amount of said copolymer.