CA1104781A - Scrim reinforced plastic film - Google Patents

Abstract

ABSTRACT

Process for forming a scrim-reinforced film wherein a molten, film-forming plastic, such as polyethylene, in a high state of fluidity is fed onto an unsupported scrim and solidified by cooling to produce a scrim at least substantially completely enclosed and surrounded by the plastic. The film, thus produced, can be formed into a high strength bag container by folding and cutting operation utilizing hot knife means capable of fusing and thus sealing the plastic. In a preferred embodiment, the plastic is extruded onto the scrim.

Description

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This invention relates to scrim-reinforced plastic film and a process for the preparation thereof, and particularly to such film having improved strength properties, the film being advantageously adapted for use in preparing various types of containers, such as envelopes, bags etc. The reinforced film material exhibits the looks and feel of a strong material as well as being stronger than the un-reinforced material.
Methods for the preparation of scrim-reinforced plastic sheet are well known in the art. Since plastic materials currently available commercially lend themselves advantageously to a wide variety of uses and particularly in the packaging of consumer goods, there has been an ever-increasing emphasis on developing means to further enhance the strength properties of such materials. Thus, tensile strength, tear and impact resistance, moisture-proofing and the like are especially important properties as regards container utility. The use of scrims, these usually comprising web material formed of spaced-apart strands of high strength thermo-plastic res-inous material~ to reinforce plastic sheets has proved particularly effective in augmenting tensile strength, tear resistance and the like. Additionally, it has been found that a reinforced material, whether in the form of sheet or bags is more appealing to the con-sumer and more saleable if the reinforcement is readily discernible both visually and tactilely. One method commonly used for the prep-aration of scrim-reinforced plastic sheet involves sandwiching a scrim member between two sheets of thermoplastic material the assem-bly thereafter being heatbonded by various means, e.g., contacting opposed sides of the sandwich assembly with heated platens~ radiant heating etc.

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Thus, one such method involves the radiant heating of the edge portions of plastic material folded back on a scrim member to form an envelope portion. Other methods involve, for example, the application of adhesive to the scrim or web and/or the inner surfaces of the plastic sandwiching elements and pressure-sealing the elements together; extruding one or more layers of thermo-plastic material onto a supported scrim or web i.e., provided with a thermoplastic backing layer.
Methods of the aforedescribed type entail significant dis-advantages. Thus, the use of heated rollers invariably results in an undesirably thin enclosing plastic film; moreover, the combined heat-pressure effects often causes structural weakening of the film and particularly in the critical areas situated outside of the strands. Extrusion of plastic onto a supported web or scrim requires the use of a plastic backing or supporting sheet.
Thus, there may be significant differences in the strength char-acteristics as between the opposed plastic members sandwiching the web. Bonding can occur between the outer plastic elements, or between each of these and the intermediate web which can pro-duce sub-optimum strength characteristics in the product.
The foregoing methods require formation of a sandwich type element wherein the outer plastic layers are separately provided.
This requires rather burdensome techniques for carrying out contin-uous operations. This aspect combined with the sub-optimum strength characteristics often obtained in the final product underscore the need for more effective methods for preparing reinforced plas-tic sheets. The prior art methods, typically do not provide the reinforced material with the appearance and feel of reinforce-ment and therefore might not be appealing to -the consumer.

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Thus, a primary object of the invention is to provide a method for preparing reinforced plastic sheets wherein the foregoing and related disadvantages are eliminated or at least mitigated to a substantial extent.
Another object of the present invention is to provide such a method particularly advantageously adapted to continuous processing.
Yet another object of the invention is to provide such a method wherein any necessity for the use of heated pressure mem-bers such as platens to effect the bonding operation is eliminated.
Still another object of the invention is to provide such a method advantageously adapted to be carried out in conjunction with an extruder.
A further object of the invention is to provide such a method capable of producing a reinforced plastic sheet having im-proved strength properties including, tensile strength, tear re-sistance, impact resistance and the like.
A still further object of the invention is to provide such a method which can readily be integrated with a process for producing plastic containers such as envelopes, containers, etc.
Yet a still further object of the invention is to provide a reinforced plastic sheet material having significantly improved strength characteristics.
A still further object of the invention is to provide a scrim reinforced plastic sheet wherein the scrim reinforcement is apparent both visually and tactilely.
Other objects and advantages of the invention will be-come more apparent hereinafter as the description proceeds.

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The foregoing objects are attained in accordance with the :invention which in its broader aspects provide a process for preparing a scrim-reinforced film comprising contacting molten plastic in a state of high fluidity with an unsupported scrim, said plast~c flowing into and around the scrim network, cooling said plastic to solidify same to produce a scrim at least substantially completely enclosed and surrounded by said plastic such that the thickness of the plastic material at intersections of strands in the scrim is at least 50% greater than the th~ckness of the scrim at such intersections and the thickness of the plastic material on the strands is from 1/2 mil greater than the strand thickness, and subsequently shrinking said plastic material to a greater degree th.an said scrim, the plastic material being heat shrinkable to a greater degree than the scrim mater~al.
In accordance with a preferred em~odiment, the molten plastic comprises the extrudate from the discharge orifice of an extruder, and is fed to a contact zone to contact the unsupported scri.m the di.stance between the discharge orifice of the extruder and the contact zone being such that the tempera-ture of the ~olten extrudate as it contacts the scrim is at least about 400F and preferably at least about 600F. Accord-ing to a preferred embodiment of the invention, the scrim method comprises two sets of syntheti.c plafitic fibers or strands disposed at an angle to each other, preferably transversely.
Each. set of strands i.ncludes a plurality of uniformly spaced, parallel fibers. The strands can be from about 1 to about 20 mils thick in their largest cross sectional di.mension, prefer-ably from about 3 to about 8 mils. The fibers can be present in an amount of 1 to 5 fibers per inch per direction.

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In accordance with a further specific aspect of the invention, there is provided a reinforced plastic sheet material comprising a scrim including two sets of angularly disposed intersecting strands, each set comprising a plurality of uni-form:Ly spaced strands, and a continuous film of plastic material which is heat shrinkable to a greater degree than the scrim material covering both sides of said strands and extending between said strands such that the thickness of the plastic material at the intexsections of the strands in the scrim is 10 at lea5t 50% greater than the thickness of the scrim at such intersecti`ons and the thickness of the plastic material on the strands i5 from 1~2 mil greater th.an the strand thickness, a plurali.ty of said strands being buckIed between the points of intersection o~ sald inter5ecting strands and lying out of the plane of said film.
Preferably, the thickness of the scrim network at points of inters.ection of the strands is at least about 50 percent greater than the thickness of the strands. To provide the desixed tactile sensation of reinforcement, according to this aspect of the invention, the thickness of the film measured between -4a-~,:

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strands i.e. in the interstices formed by the strands is less than the scrim thickness at the intersection of the strands and at least half as great as that of the fibers.
In accordance with a further specific aspect of the invention, a method is provided to import a nubby or rough character to the scrim rein-forced product that is both visible and tactilely discernible. In this re-gard, the new method includes the step of heat shrinking the film component of the scrim reinforced product by heating the product to a temperature be-tween about 180 F and 225F and cooling to room temperature. The heat shrinking step functions to bend or distort the scrim fibers, which are of a non-heat shrinkable material, out of the plane of the film to create a rough surface that is both tactilely and visually discernible by the user of the product. The scrim fibers are bent out of the plane of the film because the shrinking film draws the scrim intersection closer together, thereby forcing the fibers to bend. According to this aspect of the invention, the scrim fibers when plastic are made by a process including the step of heat setting which releases stresses, and result in plastic fibers that exhibit virtually no shrinkage when exposed to heat and cooled. The film material which should be a heat shrinkable substance can be supplied as a preformed film that is heat sealed or adhesively sealed to both sides of the scrim or it can be extruded onto the scrim according to the invention and subsequently heat shrunk to bend the scrim fibers and create the desired rough or nubby surface pattern.
The foregoing characteristics of the new reinforced material pro-vides not only an actual and substantial increase in the strength of the ma-terial but also imparts the appearance and "feel" of strength to the product.
The invention is illustrated but not limited by reference to the accompanying drawing herein:

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47~31 Figure 1 - is a schematic view of an embodiment of the invention for producing a scrim-reinforced plastic sheet from a molten extrudate, Figure 2 - is a sectional view taken along the line 2-2 in "Figure 1", before the scrim-reinforced plastic sheet is heat shrunk;
Figure 3 - is a plan view, shGwn partly broken away illustrating the scrim-reinforced sheet folded longitudinally prior to the cutting operation for forming the container;
Figure 4 - is a plan view illustrating schematically the cutting operation for forming the product bag container;

Figure 5 - is a perspective view illustrating the product container;
Figure 6 - is a sectional view taken along the line 2-2 of Figure 1 after the scrim-reinforced plastic sheet of Figure 2 is heat shrunkO
The invention will now be described by reference to the accom-panying drawing wherein like reference numerals designate similar parts throughout the views. "Figure l" illustrates an arrangement of apparatus for carrying out a preferred embodiment of the invention wherein a molten plastic extrudate lO from the discharge orifice 12 of an extruder (illus-trated partially) and generally designated 14 is directed onto a scrim or netting 16 carried by nip roll 18. Molten extrudate 10 contacts scrim 16 shortly prior to the latter entering the bite provided by chill roll 20 and nip roll 18, these members being positioned so as to tangentially engage opposed sides of the scrim and applied extrudate while the latter is in molten condition. It is essential in the practice of the present invention that the molten plastic extrudate, preferably a film-forming, thermoplastic heat shrunkable polymeric material to be described in greater detail hereinafter, be in a molten, highly fluid state ~hen it contacts the scrim material. In general, the "contact" temperature of the molten plastic extrudate should be at least about 400F and preferably at least about ~478~

600F for optimum results and particularly for assuring a scrim-reinforced product wherein the scrim is completely enclosed and surrounded by the solid-ified plastic extrudate. The upper limiting contact -temperature of the mol-ten extrudate is not particularly critical apart from the requirement that it not exceed the decomposition point o~ the particular plastic material em-ployed. It is generally recommended that the speed of the nip roll be regu-lated to provide a linear scrim speed of from about 500 to 1,000 ft/min. as the scrim 16 passes between chill roll 20 and nip roll 18. Within the lower portion of the linear speed range stated, it is recommended that the temper-ature of the molten extrudate be maintained within the higher range and/orthe distance between the discharge orifice 12 of extruder 10 and the point of contacting the molten extrudate with the scrim be decreased. Conversely, within the higher range of linear scrim speed, it is recommended that the molten extrudate temperature be maintained within the lower range and/or the distance between discharge orifice 12 and the scrim-molten extrudate contact point be increased. These factors can readily be determined in a particular instance having reference to the type of plastic material employed. Thus, operation within the aforestated parameters assures that the molten plastic extrudate in a relatively high state of fluidity flows into and around the scrim network, i.e., the interstices defined by the strands comprising the scrim. This results in the scrim being completely enclosed and surrounded by the extrudate plastic subsequent to cooling and solidification, thereof, this being illustrated, for example, in Figures "2" and "3". Scrim member 16, after contacting molten extrudate 10, is passed between the bite provided by chill roll 20 and nip roll 18 each tangentially engaging opposed sides of the scrim. The width of the bi-te can be adjusted to provide the desired thick-ness of the reinforced product 22.
The temperature of chill roll 20 should be such as to cause sub-stantial solidification of the molten extrudate 10 whereby to render same substantially non-flowable as the scrim assembly exits from the bite of chill X

~3 ~4L781 roll 20 and nip roll 18. The temperature of chill roll 20 is related to the temperature of molten extrudate 10, the linear or coating speed of scrim 16 as we]l as the distance between the latter and the discharge orifice 12 of extruc1er 14. As the coating speed and/or molten extrudate temperature is in-creased, the chill roll temperature is preferably decreased. The scrim-re-inforced assembly 22 is thereafter removed from chill roll 20 by any suitable pickup means (not shown) for further processing (optional) such as the con-tainer-formine operation to be described in more detail hereinafter.
Referring to Figure "2", the scrim-reinforced plastic sheet 22 is shown prior to heat shrinking of the film component according to the inven-tion. The product comprises a scrim member 16 that includes two angularly disposed sets of strands 16a and 16b. Each set includes a plurality of strands uniformly spaced to provide about 1 to 5 strands per lineal inch.
Each strand 16a, 16b is from about 1 to about 20 mils thick, preferably from 3 to 8 mils. As shown in Figure "2", the thickness of the scrim at the inter-section of the strands, 16c is at least 50 percent greater than the thickness of the strands, to provide "~ubs" that are both visible and feelable. In combination with the increased "~ub" thickness, the thickness of the extruded film in the interstices between the strands and the thickness of the film covering the strands is at least 1/2 mil and is preferably from 1/2 the strand thickness to the strand thickness. The thickness of the film can be the same in the interstices 16d between strands and on the strands, or differ-ent within the foregoing limits. The thickness of the extruded film in the interstices between the strands and the thickness of the film covering the strands is at least 1/2 mil. The thickness of the film can be the same in the interstices 16d between strands and on the strands, or different. Refer-ring to Figure 6, the scrim reinforced plastic sheet 22 of Figure 2 is shown after the film component thereof has been heat shrunk according to the inven-tion. Since the scrim fibers according to this aspect of the invention are substantially less heat shrinkable than the plastic film material, the fibers - ~ : .

are bent by the shrinking of the film in which they are securely embedded.
In producing the fibers, stresses should be relieved to render the fibers, virtually non-heat shrinkable or at least substantially less heat shrinkable than the film material. The film material extruded according to the method shown in Figure 1 and described herein, is 1aid down in one direction and would be heat shrinkable in that direction due to the stresses set up during the extrusion process.
A product similar to that shown in Figure 6 can be alternatively made from a laminate of preformed outer layers of a heat shrinkable film and an inner layer of scrim material that is substantially less heat shrinkable than the film. The film and s^rim layers can be heat or adhesively sealed to each other and subsequently heat shrunk.
According to this aspect of the invention, the film material is typically stressed in one direction when produced and accordingly heat shrinks in that same one direction to relieve these stresses. When preformed film is utilized the film components on either side of the scrim should be oriented so that they heat shrink in the same direction. Film extruded onto the scrim is inherently aligned in the same direction. In this regard, when the film layers are heat shrunk the fibers of the scrim parallel to the heat shrunk direction are buckled, while the transversely dispersed fibers are unaffected, i.e., unbuckled.
The heat shrinking step includes heating the scrim reinforced ma-terial of Figure 2 to a temperature between about 180 F and 225 F and cooling to room temperature. The shrinking of the film without a corresponding degree of shrinking of the fibers results in a bending of the fibers out of the plane of the film and provides the desired nubby or rough appearance and feel as shown in Figure 6.
The plastic material used in forming the molten extrudate 10 can be selected from a wide variety of materials well known in the art generally including preferably thermoplastic polymeric materials commonly used in the _ 9 _ 7~3~

manufacture of plastic bag containers. Preferred for use herein are poly-ethylene and polypropylene materials capable of forming heat sealable films.
Other suitable materials include without necessary limitation polyvinyl chloride, copolymers of vinylidene chloride with vinyl chloride, polyacryl-ates, copolymers of styrene and acrylonitrile~ cellulose triacetate, cellu-lose acetate butyrate, polyurethane elastomers, polystyrene, polyolefins in-cluding homo and interpolymers; generally, thermoplastic polymeric material capable of forming a continuous film on drying from a solvent solution thereof are usable herein, exemplary of synthetic fibers useful herein; useful fi-bers of the natural type include, without limitation, cotton, wool, hogshair, horse hair, vicuna and the like. Suitable materials available commer-cially include the plastic netting available under the trademark "Vexar" man-ufactured by E.I. du Pont de Nemours & Company which may be prepared from high density polyethylene, low density polyethylene, blends of high and low density polyethylene, polypropylene, polybutylene copolymers of ethylene and propylene and blends of polyethylene. The fiber material may comprise twist-ed and/or untwisted multifilamentary material, the range of twist ranging from about one-half to 20 turns or more per inch. The strands, as stated, may be of any desired mesh design such as the rectangular configuration illus-trated in Figure "3" and may be of the woven or non-woven type. Usually, there are provided from 1 to about 10 strands per inch with a range of from 3 to 4 strands per inch being particularly preferred. Although scrims of varying thickness and patterns may be used, it is nevertheless, preferred that the scrim weight be from about 1 to 2 lbs./Mft. . In accordance with a particularly preferred embodiment in terms of cost and performance, the thermoplastic material comprising the molten extrudate is polyethylene while the scrim material comprise polyæropylene, preferably of the oriented type as is well known in the art.
As explained, the process is preferably carried out utilizing an extruder for producing the molten form of the film forming plastic material.

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However, it will be understood that other methods may be effectively employed for provid;ng the requisite form of the molten plastic e.g., in the form of a molten extrudate. Moreover, other methods for forming the solidified, coated scrim assembly can be used such as an air knife coating technique utili~ing cold air to effect cooling and solidification of the mol-ten plastic. In any event, the essential cri-teria is the fluidity of the molten plastic at the time it contacts the scrim network. As explained previously, the extruding parameters should be chosen to effect at least substantial solidification of the molten plastic by the time it emerges from the bite of chilling roll 20 and nip roll 18. Thus, as the scrim speed and/or molten plastic temperature is increased, the temperature of the chilling roll should correspondingly be decreased consistent with the foregoing requirements.
The scrim-reinforced plastic sheet produced as described is par-ticularly advantageously adapted to be formed into plastic bags of signifi-cantly improved strength. Thus, by virtue of contacting the molten plastic as described with an "unsupported" scrim, the quoted term connoting the ab-sence any backing or support sheet whatsoevér, there is obtained a sheet product having improved impact strength, tear and -tensile strength. For ex-ample, when processed as described herein, it is found that the scrim imparts strength improvements of 33% in impact, doubled tear and quadrupled breaking strength. Plastic bag containers fabricated with the thus-reinforced plastic sheet material are accordingly capable of accommodating larger payloads, under higher stress conditions for longer periods of use. Moreover, since any requirement for a backing or support sheet is eliminated, the reinforced sheets are more economical to manufacture than the scrim-reinforced materials heretofore provided. One of the particularly surprising and unexpected ad-vantages of the present invention is the fact that the product sheet material exhibits vastly improved strength properties despite the fac-t that a backing or support sheet is not used. In addition to reducing cost, this aspect en-ables the sheet forming processing to be simplified significantly leading to ~34781 even greater savings.
The scrim-reinforced fllm 22 is collected by any suitable means, not shown, from chill roll 20 and folded longitudinally upon itself as illus-trated in Figure "3" into equal half sections. This can be accomplished, for example, by any conventional folding means as is well known in the art. In -this manner, the film 22 of original width W (Figure 3) is divided into sec-tions 28 and 30 (Figure 5) of width W/2 by means of longitudinal fold 32.
The film 22 is transported in the direction indicated by arrow in Figure "3"
to a hot knife member generally designated 34 comprising a blade portion 36 rotatable about an axle portion 38 in the direction indicated. The hot knife is maintained at a temperature which is sufficient to effect fusing i.e., heat bonding of Sections 28 and 30 along the cutting locii of the hot knife as illustrated at 40 and 42, Figure "5". The rotary speed of Blade 36 and the linear speed of film 22 in the direction indicated by arrow 34 are con-trolled so that cutting is effected at predetermined portions of film 22 to produce plastic bag containers such as illustrated in Figure "5". Thus, heat sealed side portions 40 and 42 in combination with the bottom portion formed by longitudinal fold 32 provide a restraining portion 44 for accommodating various types of articles e.g., as part of a packaging operation, articles 20 normally accumulated during shopping, or as a trash container, etc. ~he di-mensions of the plastic bag container can be varied as desired by for ex-ample, appropriately controlling the rotary speed of hot knife 34 and/or the linear speed of film 22.

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Claims (11)

THE :EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Process for preparing a scrim-reinforced film compris-ing contacting molten plastic in a state of high fluidity with an unsupported scrim, said plastic flowing into and around the scrim network, cooling said plastic to solidify same to produce a scrim at least substantially completely enclosed and surrounded by said plastic such that the thickness of the plastic material at intersections of strands in the scrim is at least 50% greater than the thickness of the scrim at such intersections and the thickness of the plastic material on the strands is from 1/2 mil greater than the strand thickness, and subsequently shrinking said plastic material to a greater degree than said scrim, the plastic material being heat shrinkable to a greater degree than the scrim material.

2. Process according to claim 1 wherein said shrinking is performed by the application of heat to said scrim reinforced film.

3. Process according to claim 2 wherein said plastic is heat shrunk by heating to a temperature from about 180°F to about 225°F.

4. Process according to claim 1 wherein said scrim-reinforced film is folded along a first direction and cut with hot knife means along a second direction substantially per-pendicular to said first direction, the temperature of said hot knife means being sufficient to fuse said plastic whereby to produce a container.

5. A reinforced plastic sheet material comprising a scrim including two sets of angularly disposed intersecting strands, each set comprising a plurality of uniformly spaced strands, and a continuous film of plastic material which is heat shrinkable to a greater degree than the scrim material covering both sides of said strands and extending between said strands such that the thickness of the plastic material at the inter-sections of the strands in the scrim is at least 50% greater than the thickness of the scrim at such intersections and the thickness of the plastic material on the strands is from 1/2 mil greater than the strand thickness, a plurality of said strands being buckled between the points of intersection of said intersecting strands and lying out of the plane of said film.

6. Plastic sheet material according to claim 5 wherein said strands are from about 1 to about 20 mils thick and completely surrounded by said plastic material.

7. Plastic sheet material according to claim 5 wherein said sets of strands are perpendicularly disposed.

8. Plastic sheet material according to claim 5 wherein said plastic comprises thermoplastic, film-forming polymer.

9. Plastic sheet material according to claim 8 wherein said polymer comprises polyethylene.

10. Plastic sheet material according to claim 8 wherein said scrim comprises a network of spaced apart strands of thermoplastic, film-forming polymer.

11. Plastic sheet material according to claim 5 wherein said film thickness is from 1/2 to 1 times the strand thickness.