NL7906246A - THERMOPLASTIC FOIL LAMINATE STRUCTURES. - Google Patents

Description

S 2909-523 Ned.

P&C

»

Thermoplastic film aminate structures.

Thermoplastic bags, and especially polyethylene bags, have become important in recent years for packaging a wide variety of materials, such as textiles, food and the like. Recently, polyethylene bags have emerged as the preferred packaging material for waste, and in many places there is an ordinance that waste should be packed in these bags. The obvious advantages of this include: a hygienic way of packaging household waste and other waste materials; and any protection of the contents 10 provided by the bag against insects, ruminants and other animals which would normally be attracted to the contents of the bag. These bags are suitably used as a disposable "liner" for trash cans; when the bin is filled, the opening of the bag is closed and lifted out of the bin, leaving the interior of the bin free of contamination and ready to place another bag.

The closed opening of the bag can be closed in the usual manner with a wire or a similar fastening device, after which the closed, filled bag is discharged. These bags can also be used in an unsupported state. However, the known polyethylene bags are not rigid and when filling these bags difficulties arise in keeping the opening of the bag open, whereby too much manual work has to be done.

Another very common drawback of using polyethylene bags for waste is their tendency to tear under stresses caused by the contents, as well as their fairly low puncture resistance. When a filled bag is punctured by a material contained herein or outside, it is characteristic of the polyethylene film that it further tears, ie the tear caused by the puncture rapidly spreads over the wall of the bag.

Numerous attempts have been made so far to correct the above shortcomings. The most obvious method is to make the bag thicker and therefore stronger. However, a significant increase in thickness is required to achieve a significant reinforcement of the bag of the order of 50-150%, and the cost of the product increases in direct proportion to the increase in the amount used for each bag resin. Attempts to replace the relatively inexpensive polyethylene with other resins exhibiting improved strength properties have been largely unsuccessful, including

790 ê 2 4S

* «· * - 2 - because of the higher cost of the resins eligible as substitution materials.

In accordance with the invention, it has been found that thermoplastic film structures with a predominant amount of relatively inexpensive resin materials common to the prior art bag manufacturing process, for example, general purpose low density polyethylene resin, can be formed into articles such as bags with improved stiffness (i.e., improved modulus) and improved strength properties over known polyethylene bags. In general, it has been found that a laminate structure comprising at least one general purpose layer of low density polyethylene resin having a thickness of the order of about 50-90% and preferably about 65 to about 85% of the total thickness of the laminate can be adhered to a second layer, the second layer providing the remainder of the total thickness of the laminate and constituted by a mixture of polymeric resins. For example, the second layer may be formed by a relatively thin layer of a mixture of a high-density polyethylene resin and a linear low-density ethylene copolymer, for example, a copolymer of ethylene and another α-olefin of about 3-15 carbon. 20 material atoms, which copolymer may have a density of less than about 0.94 g per 3 cm. A small amount of a colorant, on the order of less than about 5% by weight, may also be used, for example, a mixture of low density polyethylene and an inorganic pigment. It has been found that when structures such as bags are made from these laminate film materials, where the layer of low density polyethylene preferably forms the interior surface of the bag, these bag structures have improved strength properties over the known, not described above -laminar polyethylene bag structures. Furthermore, these strength properties are obtained without compromising the economy of the material, since the laminate bag structure of the invention contains a predominant amount (up to about 80% of the total thickness of the laminate) of inexpensive general purpose polyethylene resin. .

The accompanying drawing is a schematic cross-sectional view of an extruder that can be used to manufacture the film laminate of the invention, with certain parts shown enlarged for clarity.

Numerous methods are known for forming multilayer thermoplastic film structures, for example, a method in which a first film is pre-formed and then another film using melt extrusion on the surface of 790 62 46 t-3-de first film is applied to form a two-layer laminate. According to another, more recently developed method, known as co-extraction, melted or semi-melted layers of melts of different polymers are contacted and then cooled. Examples of these co-extrusion methods are described in U.S. Pat. Nos. 3,508,944 and 3,423,010. While any of the methods described above may be suitable for forming the laminate tricks of the invention, the present laminates are manufactured by a particularly preferred method by extrusion of individual polymer melts from concentric tubular die openings, thereby individual molten or semi-molten streams are extruded coaxially; these streams are then joined outside the die openings, and a subsequent tubular laminate is obtained by subsequent cooling. An example of this concentric extrusion of thermoplastic melts that differ from one another is described, for example, in US Patent 3,926,706.

In the manufacture of the present laminates which, according to a special application, are intended for bag structures, it has been found that certain particularly desirable physical properties must be exhibited by the individual layers. For example, in bag structures, the outer layer, the thickness of which can be about 10-50% of the total thickness of the laminate, is preferably rigid, i.e., has a relatively high tensile modulus, and is tough, i.e., resistant to impact forces; furthermore, this layer should exhibit a good elongation under tension and finally have a high degree of tear resistance, in particular in the transverse direction of the layer, i.e. the direction transverse to the extrusion direction of this layer. The physical properties particularly desired in the thick inner layer 30 of the bag formed by the laminate include easy heat welding over a wide temperature range and a wide pressure range and a high degree of tear resistance, in particular especially in the running direction of the layers (the direction of the extrusion of the layer).

The degree of orientation in each of the layers of the laminate is an important factor in the overall physical properties of the laminate structure. It has been found that two types of orientation of the polymeric crystallites occur in blown film extrusion by the trapped air method. The first type occurs through flow 40 at the edges of the die opening and tends to 790 62 46 4.

- 4 - orientation of the crystallites in the direction of the current (direction of travel, LR). With a material that is completely amorphous in nature, this orientation by the flow has little or no effect, while the degree of orientation in materials increases with an increase in crystallinity. In a linear polymer with long, straight chains, the crystallites are oriented in the running direction. With increasing branching of the chain, the crystallites tend to be somewhat more random in orientation, and these materials also contain more amorphous regions that are not oriented. The orientation of high-density polyethylene is therefore very strong compared to low-density polyethylene, since high-density polyethylene is linear and more crystalline. This effect of the mold alone has as a net result a strongly oriented film in the running direction (LR) and little orientation in the transverse direction (DR). With increasing density, decreasing branching of the polymer from low density polyethylene to high density polyethylene, the material is more subject to orientation. High density polyethylene is highly oriented and therefore very subject to cracking in the running direction (LR).

It has been found that the second type of orientation in the method of manufacturing films by blown extrusion is the effect of the inflation ratio (OBV). Since this stretching of the film gives the bubble a larger diameter, the tensile force exerted on the polymer crystallites takes place in a number of directions, thus contributing to counteracting the orientation in the running direction caused by the effect of the die. As the inflation ratio increases, the DR orientation effects increase with some deterioration in the LR properties. One can therefore find improved cracking resistance in the normal. obtain a weak transverse direction.

Low density polyethylene is normally extruded at an inflation ratio of 1.5: 1-3.0: 1 (ratio between the bubble circumference and the annular die circumference) to attempt to balance the properties in the running direction and the properties in the transverse direction. In contrast, high density polyethylene is highly oriented in the running direction due to the mold effect, thereby obtaining very poor cross-direction properties in low density polyethylene inflation ratios. The economy and ease with which the molten polymer can be handled greatly hinder the application of these large inflation ratios, but the tear strength is a key property in the bag type product 40.

790 62 46 - 5 -

The invention makes it possible to extrude films at speeds and inflation ratios for low density polyethylene to obtain the stiffness and strength of the blend of high density polyethylene and a copolymer of ethylene and an α-olefin in the outer layer.

The accompanying drawing shows an extrusion system which can be used to obtain the film laminates of the invention. As shown in the drawing, two molten thermoplastic resins, which are different from each other, are fed to a common mold 13 by two extruders 11 and 12. The tubular extrusion die 13 has two concentric annular passages that receive and form the individual resin streams, after which the resins exit the concentric die openings 14 and 14 '. Shortly after leaving openings 14 and 14 ', the concentric, coaxial, molten, or semi-molten tubes are joined and adhere to each other to form a two-layered tubular laminate 15. (not shown conventional means) to provide air to inflate and support the tube 15 until the tube 15 is squeezed downstream of the die 13 by conventional (counter-rotating) nip rollers, i.e., a conventional extrusion process with enclosed air bubble for manufacturing a tubular structure. The squeezed laminate tube is then fed to a winding device (not shown) or to a further treatment, for example for the manufacture of bags.

In practice, granular resin materials from a supply source to be fed to the extrusion system shown in the drawing are air-conveyed through a vacuum-operated unloader and conveyed to separate feed vessels located above the extruders 11 and 12 shown in the drawing. The volume of each of the resin components in the mixture fed to the extruder 11 (i.e., the extruder which provides a molten resin mixture for the die 13 to form the outer layer 16) is measured and these resin components are poured in a blender located above the extruder 11; the order of addition is not critical. The mixer is left for about 15 seconds. operate at a speed of 120 revolutions per minute; then the resulting "premix" is fed to the extruder feed zone (not shown). For the primary extruder (ie, the extruder 12 used to form the inner layer 17), only one resin material, namely low density polyethylene 790 62 46-6, is used as the raw material.

The primary extruder 12 used in the example below contained a screw about 15.2 cm in diameter, which was driven by a motor of about 186 kW. The screw.

5 had a length to diameter ratio of 28: 1. The extruder cylinder was of a standard design and provided with external jackets for circulating temperature control liquids and / or electrical resistance heating elements in the form of tires around the cylinder.

The secondary extruder 11, ie the extruder containing a molten mixture of. resins to the mold 13 to form the outer layer 16 of the laminate structure, having a screw with a diameter of about 11.4 cm and a length to diameter ratio of 24: 1. The cylinder of the extruder 12 was also provided with hollow shells for circulating temperature controlling liquids and / or electric resistance heating elements in the form of bands along the length of the cylinder to control the temperature. of the molten polymer within the cylinder.

The die 13 shown in the drawing is a coextrusion die, wherein the primary extruder 12 supplies material that ultimately forms the layer 17 and the secondary extruder. 11 supplies material that ultimately forms the outer layer 16. Within the annular edges of the die openings there is an annular space of about 0.102 cm, i.e. the openings 14 and 14 '; the inclined edge portion has a length of about 1.3-5.1 cm and the individual concentric tubes are separated by a distance of about 0.08 cm when leaving the mold. This separation unites the films above the mold, as shown in the drawing, to form the laminate tube 15.

When exiting the die 13, the extruded concentric tubes 16 and 17 are oriented by internal pressure of air trapped in the tube, between the die 13 and the foil pinch rollers (not shown), inflating the tube to a circumference that is 2-2.5 times larger than the circumference of the die openings. The method described above is essentially a conventional trapped air bubble extrusion method.

While the internally trapped air stretches the film, a high velocity airflow provided by the air ring 18 shown in the drawing strikes the extruded tube in a generally vertical direction, thereby displacing the molten polymer. 790 62 46 - 7 - cooled. The combination of internal air expansion and high velocity impact of the tube from air ring 18 causes the layers to contract while still in a molten state; this creates a strong bond between the layers 5 as the layers contact with each other cool and solidify.

Before the tube 15 is fed to the roller pair, the formed foil tube is conventionally squeezed by a frame of horizontal wooden slats with an inverted V-shape, the angle 10 between the legs of the V being about 30-35 °. This V-shaped frame gradually flattens the foil tube until the tube at the top of the V-shape is fully squeezed by the pinch rollers that can be formed by a rubber roller and a powered steel roller. The nip rolls pull the tube out of the extrusion die 13 and also effect a seal 15 for the entrapped air bubble in the tube. After the flattened tube has passed through the rollers, the film is rolled up or passed through a bag-making device or the like to form a finished product.

As mentioned above, the outer layer of the laminate structure of the invention is preferably formed by a mixture of thermoplastic resins and in the biponder a mixture of high-density polyethylene and a linear low-density ethylene-α-olefin copolymer. Examples of α-olefins copolymerized with ethylene in these copolymers are octene-1, butene-1, hexene-1 and 4-methylpentene-1.

Preferably, the concentration of the ethylene copolymerized α-olefin is about 2.0-10% by weight. In the following embodiments, a linear low-density polyethylene copolymer containing about 4.8% by weight of octene copolymerized therewith was used. It has been found that when the outer layer of the laminate is formed by such a mixture, the resulting laminate exhibited greatly improved modulus and tear resistance.

Table A below provides relevant physical properties of the various polyolefin materials used in the examples.

- Table A - 790 62 46 - 8 -! Table a

Low density polyethylene resin (for indoor hunting)

Properties Value ASTM test _ _ method._

Melt index, g / 10 min. 2.25 D-1238-65T

5 Density, g / cm3 0.921 D-1505-68

Yield (50.8 cm / min), kPa 9177 D-638-68

Tensile strength (at break) (50.8 cm / min), kPa 11638 D-638-68

Elongation at break,% 603 D-638-68

Modulus of elasticity, kPa 169.852 D-638-68 10 Bending stiffness, - kPa 5516 D-747-63

Hardness, Shore D D44 D-2240-68

Vicat Softening Point, ° C 102 D-1525-65T

Temperature at which material becomes brittle, ° C below D-746-64T

75 °

Physical Properties - Low density linear copolymer resin of polyethylene and octene-1.

Properties Value ASTM test _ method__

Melt index 2.0 D-1238

Density 926 D-1505

Molecular weight 89,000 - 20% by weight of octene-1 4.8

High Density Polyethylene Resin ASTM Test

Properties Value method_

Melt index, g / 10 min 0.35 D-1238 3

Density, g / cm 0.963 D-l505 25 Yield, kPa 28 270 D-638

Elongation,% 800 D-638

Flexural modulus, kPa 1,413,425 D-790

Hardness, Shore D 70 D-1706

Stroke value acc. Izod, Joule / cm notch 3.68 D-256 30 Impact resistance on tension D-1822 2

Joule / cm 12.55

Temperature at which material becomes brittle <1-57 D-746

Vicat softening point D-1525

The details and method of manufacture of the tubular laminate structures of the invention are shown in the following non-limiting examples.

790 62 46 - 9 -

In these examples, the device shown in FIG. 1 is used to form the two-walled thermoplastic tube. The resin material used in the examples below had the physical properties listed in Table A.

EXAMPLE I

A two-layered tubular thermoplastic laminate having an average thickness of about 38 microns was prepared, the inner wall of which was formed from the above defined low density polyethylene and the outer wall from a mixture of high density polyethylene, ethylene vinyl acetate copolymer with 18 "wt.% vinyl acetate and the above defined low density polyethylene with a low melt index, by extruding 98 parts by weight low density polyethylene resin and 2 parts by weight black colorant in the molten state through extruder 12 and simultaneously extruder 11 a resin blend of 35 wt% high density polyethylene, 35 wt% ethylene-ethyl acetate copolymer (18 wt% vinyl acetate), and 25 wt% low density polyethylene with low melt index and 5% colorant of the color of extruded redwood in the molten state The respective molten layers formed a concentric tubular structure after passing through the mold 13 had flowed. The molten tubes exited the die 13 through the openings 14 and 14 'as concentric tubes, after which they were joined together to form the laminate tube 15 shown in the drawing. The operating conditions in the extruder, including the pressures used in this example and the following examples. mold opening temperatures and dimensions are given in the tables below; these tables also include data on the physical properties of the extruded multilayer film. No separation of the two layers occurred upon repeated bending of the resulting laminate. The low density polyethylene layer in the laminate constituted about 78% of the total thickness of the laminate.

EXAMPLE II

The procedure of Example 1 was repeated, the outer layer of the tubular laminate structure constituting 22% of the total thickness of the laminate. A further modification of the structure in this example was that the outer layer of the laminate was formed by about 75% by weight of a linear low density ethylene-octene-1 copolymer containing about 4.8% by weight of octene-1. 20 wt.% High density polyethylene and about 5 wt.% Of a pigment formed by about 50 wt.% Inorganic pigment and about 50 wt.% Low density polyethylene as a carrier.

790 62 46 <> - 10 -

EXAMPLE III

The tubular laminate of this example was identical to that of the previous Example II, except that the total thickness of the outer layer was about 26% of the total thickness of the laminate 5.

EXAMPLE IV

A tubular laminate structure was prepared by the method of Example I, the thickness of the outer layer being 22% of the total thickness of the laminate. Furthermore, the blend 10 for the external layer of the laminate consisted of about 60% by weight of the ethylene-octene-1 copolymer, about 20% by weight of high density polyethylene, about 5% by weight of the low density polyethylene colorant and inorganic pigment formed and about 15% by weight of the above defined low density polyethylene.

EXAMPLE V

A tubular laminate structure was prepared by the method of Preparation I, wherein the thickness of the outer layer of the laminate was approximately 22% of the total thickness. In this case, the resin mixture for the outer layer of the laminate consisted of 65% by weight ethylene-20 octene-1 copolymer, 30% by weight high density polyethylene and 5% by weight of the mixture of inorganic pigment and low density polyethylene .

The physical properties of the tubular laminates prepared in the above examples are set forth in Table B above. Table C lists the operating conditions used to manufacture the laminate structures of Examples I-V.

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TABLE C

Extruder 12: (inner layer)

Diameter cylinder 15.2 cm

Screw, revolutions / minute 49 5 'Plastic melt temperature, ° C 202

Pressure on plastic melt, kPa 31720

Extruder 11: (outer layer)

Diameter cylinder 11.4 cm

Screw, revolutions / minute 41 10 Plastic melt temperature, ° C 260

Press plastic melt, kPa 37230

Marijs 13:

Diameter opening outside 0.102 cm inside 0.102 cm

Tubular film: 15 width in flat condition 183 cm

Wall thickness

Inner wall 30.5 microns

Outer wall 7.6 microns

As shown in the previous examples and tables, it has been found that blends of a linear copolymer of ethylene and an α-olefin, for example octene-1, have a low density and a predominant amount of a high density polyethylene resin provide tear resistance and good modulus properties. In addition, these properties are equivalent to or superior to those obtained with blends of three components, such as those containing high-density polyethylene or low-density polyethylene and a vinyl acetate-ethylene copolymer which blends in known structures have been used.

The advantages in terms of processing and material handling properties of the improved two-component system over a three-component system in the blend layers will be apparent to those skilled in the art.

The invention is not limited to the above-described preferred embodiments, since numerous modifications are of course possible within the scope of the invention.

- Conclusions - 790 62 46

Claims (5)

1. Laminate structure, characterized by at least one layer of low-density polyethylene resin and a second layer, comprising a mixture of the high-density polyethylene and an ethylene-cc-olefin copolymer, which mixture contains a predominant amount of the copolymer. Laminate structure according to claim 1, characterized in that the second layer constitutes about 10-50% of the total thickness of the laminate. Laminate structure according to claim 1 or 2, characterized in that the cc olefin contains about 3-15 carbon atoms. Laminate structure according to claims 1-3, characterized in that the concentration of the α-olefin in the copolymer is about 1.5-10% by weight. 5. Thermoplastic laminate bag structure with at least two layers, namely an inner layer and an outer layer, characterized in that the inner layer is a low-density polyethylene and the outer layer is a mixture of high-density polyethylene and an ethylene-a-olefin copolymer containing a major amount of the copolymer in the mixture. 790 62 46