Oriented Film Produced In-Process For Use in the Stretch Film Market

Abstract

Pre-stretched films may be used to increase the rate at which loads can be wrapped and to minimize the exertion required when using conventional stretch films. However, pre-stretched films must generally be stretched in a separate step and stored for several days in order for cling to fully develop. The present disclosure describes compositions, devices, systems, and methods for producing film that eliminate the stretching and storage steps. In particular, the present disclosure relates to the use of selected resins and an angled die to increase the level of orientation in the film as it is formed, thus eliminating the need to stretch the film in a separate step. The present disclosure also relates to the use of a cling agent which eliminates the storage time traditionally required to develop the film's cling properties.

Description

FIELD OF THE DISCLOSURE

The present disclosure generally relates to compositions, devices, systems, and methods for producing oriented film in-process for use in the stretch film market. In particular, the present disclosure relates to the use of selected resins and an angled die to increase the level of orientation in the film as it is formed, thus eliminating the need to stretch the film in a separate step. The present disclosure also relates to the use of a cling agent which eliminates the storage time traditionally required to develop the film's cling properties.

BACKGROUND OF THE DISCLOSURE

Stretch films are widely used in a variety of bundling and packaging applications. For example, manually applied stretch films (i.e., handheld stretch films) have become a common method of securing bulky loads such as boxes, merchandise, produce, equipment, parts, and other similar items on pallets. Stretch films may be stretched at the time of use, which requires the application of force in order to stretch the film as much as 200 percent to properly contain the load. In contrast, stretch films may be “pre-stretched” by a film converter prior to delivery to the end-user. Pre-stretched films are described as films that are taken from master rolls of film that have already been produced, stretched in a separate step, and re-wound onto film rolls for later use. Many end-users have chosen to use pre-stretched films to increase the rate at which loads can be wrapped and to minimize the force required to wrap loads.

Pre-stretched films are typically made from various polyethylene resins and may be single or multilayer products. An additive known as a cling agent is frequently used to ensure that adjacent layers of film will cling to each other. A cling agent typically used in pre-stretched films is polybutene with a Saybolt Universal Viscosity of 3,000 SUS at 99.degree. C. with a number average molecular weight of 1,290. This cling agent requires time to migrate or “bloom” to the film's surface after the film is produced and typically starts to reach equilibrium in 12 to 24 hours under optimum storage conditions. If the film is stretched before the cling agent has fully migrated, the resulting film will have little or no appreciable cling. Films that are produced with excessive winding tension or stored at low temperatures will also have little or no cling due to the lack of migration of the cling agent.

As a result, conventional pre-stretched films require that master rolls of film be stored for several days before stretching in order for the cling agent to migrate and the cling to fully develop. This necessary delay between the time the film is produced and the time the film is stretched increases the cost and decreases the efficiency of making pre-stretched films.

After the cling has fully developed, pre-stretched films are stretched in a separate operation. This process orients the molecules in the film in a longitudinal direction, parallel to the direction of the film's travel through the stretching machine. This orientation in the machine direction removes most of the stretch in the film. The resulting film is relatively stiff for its thickness and has very little residual orientation or stretch remaining before the film fails in the machine direction. These characteristics are desirable because much less effort is required to secure a load using pre-stretched film as compared to conventional handheld stretch films. However, this separate operation requires additional material handling, dedicated converting equipment, increased warehouse space, and the manpower needed to manage the operation. This process also results in increased film scrap and higher raw material usage, further increasing the cost and decreasing the efficiency of making pre-stretched film.

As can be seen, there is a need for compositions, methods, systems, and devices which can produce oriented film in a single, continuous process. In addition, there is a need for compositions, methods, systems, and devices which can simplify the production process by eliminating the need to stretch the film. Eliminating the stretching and storage steps makes oriented film simpler, quicker, and less expensive to produce. In addition, the production line may be configured so that the film, as initially produced, meets the length and width specifications for the final product.

SUMMARY OF THE DISCLOSURE

The present disclosure provides an oriented film that is produced in-process and mimics the properties of pre-stretched handheld film. The oriented film may be comprised of a linear low density polyethylene (LLDPE) copolymer resin. The LLDPE copolymer resin may be blended with resins chosen from the group consisting of polyethylene, polyethylene copolymers, and polypropylene copolymers. The oriented film may have a majority layer and a plurality of minority layers. The majority layer may be comprised of a LLDPE copolymer resin and the minority layers may be resins chosen from the group consisting of polyethylene, polyethylene copolymers, and polypropylene copolymers.

The present disclosure further provides an oriented film that is produced in-process and contains a cling agent comprised of a polybutene polymer. The selected cling agent may be a polybutene polymer with a Saybolt Universal Viscosity of 14,900 SUS at 99.degree. C. with a number average molecular weight of 2,060.

The present disclosure further provides an oriented film that is produced in-process and contains a non-migratory cling agent. The non-migratory cling agent may be incorporated into one or more layers of the oriented film.

The present disclosure further provides an apparatus for producing oriented film. The apparatus comprises one or more extruders that receive and melt the resins. The apparatus also comprises an angled die that delivers a layer of melted resin from the extruder onto a casting roll to produce a film.

These and other features, aspects, and advantages of the present disclosure will become better understood with reference to the following drawings, description, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure will be better understood from the following description and the accompanying drawings given as non-limiting examples, and in which:

FIG. 1 illustrates the steps for producing oriented cast film in-process, according to an embodiment disclosed herein;

FIG. 2 illustrates the means for producing a film from molten resins, according to an embodiment disclosed herein;

FIG. 3 illustrates the standard placement of a cast film die according to the prior art; and

FIG. 4 illustrates the placement of a cast film die at an angle, according to an embodiment disclosed herein.

DETAILED DESCRIPTION

The following detailed description is of the best currently contemplated modes of carrying out the disclosure. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the disclosure, since the scope of the present disclosure is best defined by the appended claims.

Broadly, the current disclosure includes compositions, systems, devices, and methods for producing oriented film in-process for use in the stretch film market. More specifically, according to one aspect of the disclosure, the film may be comprised of higher molecular weight resins than are conventionally used for stretch films. The inventive resins may increase the level of orientation in the film as it is formed. In addition, the resins may be extruded onto the casting roll through an angled die, which may further increase the level of orientation in the film. As a result of the increased level of orientation, the film does not have to be stretched in a separate operation. Eliminating the stretching step may make the film simpler, quicker, and less expensive to produce.

In another aspect of the disclosure, a cling agent may be provided to enable an oriented film to be produced without an extensive storage time between steps in the manufacturing process. The inventive cling agent does not require an extended period of time to migrate to the surface of the film. As a result, the cling properties of the film will be immediately apparent. Eliminating the storage time may further reduce the time and cost associated with producing stretch film.

The film of the present disclosure may be comprised of one layer or multiple layers, and the composition of each layer may vary. Materials that may be used to produce the film layers include, but are not limited to, Ziegler Natta (ZN) catalyzed linear low density polyethylene (LLDPE), metallocene catalyzed linear low density polyethylene (mLLDPE), polyethylenes, polyethylene copolymers, polyethylene terpolymers, polyethylene blends, polypropylenes, polypropylene copolymers, and blends thereof The majority of the ZN catalyzed LLDPE and mLLDPE molecules inherently lack long chain branching. In addition, ZN catalyzed LLDPE has a composition depth breadth index (CDBI), which is defined as the weight percent of the copolymer molecules having a comonomer content within 50 percent of the medium total molar comonomer content, of less than 70 percent. As one example, the CDBI of ZN catalyzed LLDPE may range from 30 to 60 percent. Cling agents may be used as discrete layers or as blends to produce one-sided, differential, or two-sided cling structures. Such cling agents may be migratory or non-migratory.

One embodiment of the present disclosure may be a three-layer film with a majority layer sandwiched between two minority layers. The thickness of the minority layers may range from 0 to 49 percent of the total film thickness. The preferred thickness for the minority layers may be approximately 16 percent of the total film thickness.

The majority layer may consist of a LLPDE copolymer resin, such as a higher alpha-olefin LLDPE resin. The melt index of the LLDPE copolymer resin selected for the majority layer may range from 0.5 g/10 min. @ 190° C./2.16 kg to 4 g/10 min. @ 190° C./2.16 kg, with a preferred melt index ranging from 0.8 g/10 min. @ 190° C./2.16 kg to 1.2 g/10 min. @ 190° C./2.16 kg. The density of the LLDPE copolymer resin selected for the majority layer may range from 0.900 g/cm³ to 0.960 g/cm³, with a preferred density of approximately 0.920 g/cm³. Using a LLDPE copolymer resin with a higher molecular weight than is conventionally used in stretch films may increase the level of orientation when the film is extruded through a die. The LLPDE copolymer resin may be also combined with other resins, including, but not limited to, other polyethylenes, polyethylene copolymers, and polypropylene copolymers.

The minority layers may be resins comprised of polyethylene, polyethylene copolymers, polypropylene copolymers, or blends thereof. Depending upon the desired properties of the film, the minority layers may or may not have the same composition. The melt index of the resins selected for the minority layers may range from 0.5 g/10 min. @ 190° C./2.16 kg to 12 g/10 min. @ 190° C./2.16 kg, with a preferred melt index ranging from 3 g/10 min. @ 190° C./2.16 kg to 5 g/10 min. @ 190° C./2.16 kg. The density of the resins selected for the minority layers may range from 0.850 g/cm³ to 0.969 g/cm³, with a preferred density of approximately 0.917 g/cm³.

To impart cling to the film, a cling agent may be incorporated into the film. The cling agent may be a migratory or non-migratory additive. For example, a migratory cling agent may be metered into a three-layer film through one or both extruders for the minority layers. As another example, if the film is composed of a single layer, a migratory cling agent may be metered into the film through the extruder for that layer. The rate at which the migratory cling agent is metered into the film may range from 0 to 25 percent of the total film structure on a weight-to-weight basis, with a preferred rate of approximately 0.6 percent of the total film structure on a weight-by-weight basis. As an alternative, a non-migratory cling agent may be added to the minority layers at a rate of 0 to 25 percent of the total film structure on a weight-to-weight basis, with a preferred rate of approximately 1 percent of the total film structure on a weight-to-weight basis.

As an example, a polybutene polymer with a Saybolt Universal Viscosity of 14,900 SUS at 99.degree. C. with a number average molecular weight of 2,060 may be used as a cling agent. The molecular weight of this cling agent is higher than the molecular weight of a cling agent typically used in stretch films (which is polybutene with a Saybolt Universal Viscosity of 3,000 SUS at 99.degree. C. with a number average molecular weight of 1,290). Unlike the typical cling agent, the higher molecular weight polybutene polymer may not require time to migrate to the film's surface. As a result, the film's cling properties may develop almost immediately. In addition, the higher molecular weight polybutene polymer may be minimally affected over time or winding tension. As a result, the oriented film may be produced in-process, which is more cost-effective and efficient than the standard practice of producing master rolls of film, storing the master rolls for several days while the cling develops, and then converting the master rolls into pre-stretched film.

Referring generally to FIG. 1, the steps 100 for producing oriented cast film in-process, according to an embodiment of the present disclosure, are illustrated. Specifically, the steps may comprise producing a film from molten resins 110, gauging the film 120, longitudinally slitting the film into multiple sections 130, folding the edges of the film 140, oscillating the film 150, and winding the film onto a film roll 160 in a manner that prevents stacking of the edge folds and entraps air between the layers of film. All of the steps may be performed along a single production line. The steps may be performed in a different order, and one or more steps may be eliminated without departing from the scope of the present disclosure.

As shown in FIG. 2, a means for producing a film from molten resins 200 may comprise one or more extruders 210 connected by transfer pipes 220 to a die 230. The number of extruders 210 used in the apparatus may depend upon the desired composition of the film. For example, if the film is desired to have a three-layer composition, then three extruders 210 may be used. As another example, if the film has only a single layer, then one extruder 210 may be used.

The extruders 210 may be connected to a source 240 of stock resins. The extruders 210 may heat the stock resins to a molten condition and deliver the molten resins through the transfer pipes 220 to the die 230. The film may be extruded through the die 230 onto a casting roll 250. The casting roll 250 may be a 30-inch diameter matt casting roll with a set temperature. As an example, the set temperature of the casting roll may range from 75.degree. F. to 100.degree. F., with a preferred value of approximately 90.degree. F. The film may move from the casting roll to a secondary chill roll 260. The secondary chill roll may be a 20-inch diameter mirror finish secondary chill roll with a set temperature. As an example, the set temperature of the secondary chill roll may range from 65.degree. F. to 90.degree. F., with a preferred value of approximately 85.degree. F.

Oriented film may be produced by a plurality of suitable methods. While the present disclosure specifically relates to chill roll casting techniques, it is to be understood that the present disclosure is not to be limited to that type of film production method. The disclosed compositions, systems, methods, and devices can be successfully employed with other film production methods, including, but not limited to, blown film techniques and tubular bath extrusion.

As shown in FIG. 3, dies 310 in the cast stretch film industry are generally positioned vertically. The placement of the die 310 may affect the melt curtain 320, which is defined as the distance between the end 330 of the die 310 through which the film is extruded and the surface 340 of the casting roll 250. The placement of the die 310 may also affect the intercept angle 360, which is the angle at which the extruded film initially contacts the surface 340 of the casting roll 250. For example, the intercept angle 360 for a vertical die 310 may be approximately 90.degree.

Possible die configurations in the present disclosure may include, but are not limited to, angled, vertical, and horizontal. As shown in FIG. 4, the present disclosure may use an angled die 410. When compared to a vertical die 310, an angled die 410 may reduce the length of the melt curtain 320 and the intercept angle 360 to the casting roll 250. As a result, the molten resins contact the casting roll 250 more quickly, giving the molecules in the resins less time to lose their orientation before they are quenched and frozen in place by the temperature of the casting roll 250 and the secondary chill roll 260. As a result, an angled die 410 may produce thin layers of film with increased machine direction orientation more efficiently than a vertical die 310. Due to the increased machine direction orientation, films produced by the present disclosure do not require stretching in a separate step.

As shown in FIG. 1, the film may undergo additional processing steps once formed. These steps may include, but are not limited to, gauging the film, longitudinally slitting the film into multiple sections, folding the edges of the film, oscillating the film, and winding the film onto a film roll.

As discussed above, oriented film may be produced by a plurality of suitable methods, including cast or blown film processes. Films produced via the cast film process may be made and processed in the manner previously described. The blown film process may use low blow-up ratios and narrow die gaps to achieve the required orientation. Blown film products may be comprised of single or multiple layers. However, multiple layers may be necessary if high melt index resins are to be used to prevent or minimize melt fracture and interfacial instability. The use of high molecular weight cling agents may also be required to achieve a commercially viable product.

As can be seen, the present disclosure provides compositions, methods, systems, and devices for producing oriented film in-process for use in the stretch film market. In particular, the present disclosure relates to the use of particular resins and an angled die to increase the level of orientation in the film as it is formed, thus eliminating the need to stretch the film in a separate step. The present disclosure also relates to the use of a cling agent which eliminates the storage time traditionally required to develop the film's cling properties.

From the foregoing, it will be understood by persons skilled in the art that compositions, devices, systems, and methods for producing oriented film in-process for use in the stretch film market have been provided. While the description contains many specifics, these should not be construed as limitations on the scope of the present disclosure, but rather as an exemplification of the preferred embodiments thereof. The foregoing is considered as illustrative only of the principles of the present disclosure. Further, because numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the present disclosure to the exact methodology shown and described, and accordingly all suitable modifications and equivalents may be resorted to, falling within the scope of the present disclosure. Although this disclosure has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and numerous changes in the details of the method may be resorted to without departing from the spirit and scope of the present disclosure.

Claims

1. An apparatus for producing oriented film in-process, said apparatus comprising:

a heated enclosure for heating a resin composition to a predetermined temperature that is equal to or greater than an associated melting temperature for said resin composition;

a die disposed in fluid communication with said heated enclosure for extruding a curtain of melted resin; and

a casting roll for receiving the curtain of melted resin.

2. The apparatus of claim 1, further comprising a secondary chill roll for receiving and quenching said curtain of melted resin.

3. The apparatus of claim 1, wherein said die is disposed at an approximately perpendicular angle measured relative to the plane of the curtain of melted resin as said curtain travels from the die to the casting roll.

4. The apparatus of claim 1, wherein said die is disposed at an approximately horizontal angle measured relative to the plane of the curtain of melted resin as said curtain travels from the die to the casting roll.

5. The apparatus of claim 1, wherein said die is disposed at an angle between approximately perpendicular and approximately horizontal measured relative to the plane of the curtain of melted resin as said curtain travels from the die to the casting roll.

6. The apparatus according to claim 5, wherein said die is disposed at an angle of less than about 90° measured relative to the plane of the curtain of melted resin as said curtain travels from the die to the casting roll.

7. The apparatus of claim 1, wherein said curtain of melted resin is measured with respect to a distance established between the die and a receiving surface of said casting roll.

8. The apparatus of claim 7, wherein said die is disposed at an angle between approximately perpendicular and approximately horizontal measured relative to the plane of the curtain of melted resin as said curtain travels from the die to the casting roll, and a reduced distance between the die and a receiving surface of said casting roll is achieved.

9. The apparatus of claim 8, wherein said die is disposed at an angle between approximately perpendicular and approximately horizontal measured relative to the plane of the curtain of melted resin as said curtain travels from the die to the casting roll, and a plurality of resulting film layers having an increased machine direction orientation is achieved.

10. The apparatus of claim 8, wherein said die is disposed at an angle of less than about 90° measured relative to the plane of the curtain of melted resin as said curtain travels from the die to the casting roll.

11. A method for producing oriented film in-process, said method comprising:

heating a resin composition to a predetermined temperature that is greater than an associated melting temperature for said resin composition;

transferring a quantity of melted resin to an associated die; and

extruding a curtain of melted resin from said die onto a casting roll.

12. The method of claim 11, further comprising transferring said curtain of melted resin from said casting roll toward a secondary chill roll for quenching.

13. The method of claim 11, further comprising disposing said die at an approximately perpendicular angle measured relative to the plane of the curtain of melted resin as said curtain travels from the die onto the casting roll.

14. The method of claim 11, further comprising disposing said die at an approximately horizontal angle measured relative to the plane of the curtain of melted resin as said curtain travels from the die onto the casting roll.

15. The method of claim 11, further comprising disposing said die at an angle between approximately perpendicular and approximately horizontal measured relative to the plane of the curtain of melted resin as said curtain travels from the die onto the casting roll.

16. The method according to claim 15, further comprising disposing said die at an angle of less than about 90° measured relative to the plane of the curtain of melted resin as said curtain travels from the die onto the casting roll.

17. The method of claim 11, further comprising measuring said curtain of melted resin with respect to a distance established between a die surface from which the resin composition is extruded and a receiving surface of said casting roll.

18. The method of claim 17, further comprising disposing said die at an angle between approximately perpendicular and approximately horizontal measured relative to the plane of the curtain of melted resin as said curtain travels from the die onto the casting roll, and establishing a reduced distance between the die and a receiving surface of said casting roll.

19. The method of claim 18, further comprising disposing said die at an angle between approximately perpendicular and approximately horizontal measured relative to the plane of the curtain of melted resin as said curtain travels from the die onto the casting roll, and extruding a plurality of resulting film layers having an increased machine direction orientation.

20. The method of claim 18, further comprising disposing said die at an angle of less than about 90° measured relative to the plane of the curtain of melted resin as said curtain travels from the die onto the casting roll.