Polymer compositions may be rendered molten for manufacture into a wide variety of articles. Such articles may include films, fibers, and tubes. Various polymer processing techniques are known, including extrusion, blowing, molding, compression, and injection, in which the molten polymer is cooled and shaped into a solid mass. Each process has its own particular physical and chemical effects upon the polymer. Further, each process is customized to achieve exactly the performance required from the polymer, using the least amount of energy, and at the maximum rate of production. In general, the use of one compound or formula in one type of polymer processing technique does not predict success using the same formula in another type of processing technique. Extensive trial and experimentation is needed to determine that a particular formulation is or is not suitable for a particular type of polymer process.
Thermoplastic compositions must exhibit certain physical characteristics to facilitate widespread use. Specifically within polyolefins, for example, uniformity in arrangement of crystals upon crystallization is sometimes necessary to provide an effective, durable, and versatile polyolefin article. To achieve desirable physical properties, certain compounds and compositions can be employed to provide nucleation sites for polyolefin crystal growth during molding or fabrication. Nucleating agents are known to modify the crystalline structure of thermoplastic polymers.
The use of nucleating agents may increase the temperature and the rate of crystallization. Compositions containing such nucleating compounds crystallize at a much faster rate than non-nucleated polyolefins. Crystallization at higher temperatures results in reduced fabrication cycle times and a variety of improvements in physical properties such as stiffness.
Nucleating agents provide nucleation sites for crystal growth during cooling of a thermoplastic molten formulation. The presence of such nucleation sites results in a larger number of smaller crystals. As a result of the smaller crystals formed therein, clarification of the target thermoplastic may be achieved. However, excellent clarity is not always a result. The more uniform (and smaller) the crystal size, the less light is scattered. Thus, the clarity of the thermoplastic article itself may be improved. Thus, thermoplastic nucleator compounds are important to the industry, as they may provide enhanced clarity, improved physical properties and faster processing.
Dibenzylidene sorbitol derivatives are nucleator compounds, commonly used in polypropylene end-products. Compounds such as 1,3-O-2,4-bis(3,4-dimethylbenzylidene)sorbitol (hereinafter DMDBS), available from Milliken Chemical under the trade name Millad® 3988, provide excellent nucleation and clarification characteristics for polypropylene.
Other well known nucleator compounds include sodium benzoate, sodium 2,2′-methylene-bis-(4,6-di-tert-butylphenyl)phosphate (from Asahi Denka Kogyo K.K., known as “NA-11™”), aluminum bis[2,2′-methylene-bis-(4,6-di-tert-butylphenyl)phosphate] (also from Asahi Denka Kogyo K.K., known as “NA-21™”), and talc.
U.S. Pat. Nos. 6,599,971 and 6,562,890 each disclose using metal salts of hexahydrophthalic acid (HHPA) in polypropylene (PP) to provide desirable properties in polypropylene. U.S. Pat. No. 6,562,890 teaches, for example, the extrusion of disodium HHPA salts with calcium stearate in polypropylene homopolymer in an extrusion process. Extrusion of polypropylene is followed by injection molding, to form polypropylene 50 mil PP plaques. A Killion single screw extruder is used in the process. The polypropylene is passed through an extruder die, according to the examples of the reference. Lithium stearate was used as an acid scavenger in some polypropylene samples which were passed through an extruder die in the disclosed extrusion process.
U.S. Pat. No. 6,599,971 discloses various HHPA compounds used in polypropylene (PP) homopolymer and molded into plaques by melt compounding on a Killion single screw extruder through an extruder die. The performance of various HHPA compounds were measured in molded polypropylene plaques as stated in the reference, using acid scavengers such as calcium stearate and lithium stearate. This patent also discloses the nucleation of polyester polymer.
Extrusion of polymer is a common manner of making extruded plastic articles. Other processes, however, are known for processing polymers. Processing techniques, temperatures, and the like vary greatly among various types of polymer processing techniques. In general, it is not predictable or certain that any particular formulation used in one type of processing (such as extrusion) could apply or work in a different type of polymer processing technique, using different temperatures, mechanical processing methods, cure times and the like. Further, each type of polymer itself provides unique properties, and it is not predictable that an additive or procedure used with one type of polymer would perform satisfactorily with another polymer.
A full and enabling disclosure of this invention, including the best mode shown to one of ordinary skill in the art, is set forth in this specification.
Reference now will be made to the embodiments of the invention, one or more examples of which are set forth below. Each example is provided by way of explanation of the invention, not as a limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in this invention without departing from the scope or spirit of the invention.
Polyethylene film is one type of film that finds particular application in the industry. In the past, polyethylene film has provided relatively poor optical properties due to haze in the polyethylene films. It is desirable in the polyethylene film industry to reduce the haze and improve the optical clarity of such films, while maintaining or improving the physical properties of the film. This invention is directed at improved polyethylene film, and methods of making improved polyethylene-based films.
Surprisingly, it has been discovered that employment of certain additives in polyethylene with particular cycloaliphatic salt nucleating agents may improve the properties of a film made with such polyethylene. Use of fatty acid salts as co-additives with such nucleating agents provides benefits in the manufacture of film. Such fatty acid salts may include stearates of zinc, calcium, lithium, magnesium or sodium. Zinc stearates may be particularly advantageous in the practice of the invention. In the invention, an additive package comprising at least one cycloaliphatic salt nucleating agent with a co-additive of a fatty acid salt (with a C12-C22 anion and a cation) is employed. The cation may be zinc, calcium, sodium, lithium, magnesium and others employed in the fatty acid salt. The invention may be applied in polyethylene of various densities, as further described herein.
When using a nucleator of a cycloaliphatic salt, a hexahydrophthalic acid (HHPA) salt compound may be employed in one particular embodiment of the invention. This compound employs a counter-ion, including, for example, a calcium counter-ion. Calcium has been found to be particularly effective in providing a low degree of haze, as compared to other counter-ions, when employed with a co-additive fatty acid salt.
A combination of a fatty acid salt of a C12-C22 anion and a cation of certain specific metals may provide enhanced clarity and reduced % haze. Metals of zinc, calcium, sodium, lithium, magnesium and others may be used in such a fatty acid salt. Results with zinc have been found to be particularly good. A calcium-containing nucleating agent compound and zinc stearate co-additive has been found to provide very favorable properties in blown film. Such films provide reduced % haze, while maintaining and in some instances even enhancing physical properties of the film.
In the practice of the invention, it has been found that polyethylene density ranges of between about 0.910 and about 0.965 grams/cc are quite useful. Further, in other applications, linear low density ranges of about 0.910-0.940 grams/cc are employed. Still other applications of the invention may employ higher density polyethylene in the range of about 0.940 to about 0.965 grams/cc.
Definitions
Improvements in optics and physical properties made possible by the invention may lead to enhancements in packaging operations and packaging performance. For example, improved modulus and stiffness is a desired property in packaging operations, as it enhances the speed and quality of the operation. Improved optics of the package is desired to improve the shelf appeal of the film or package. Improved optics is desired without the loss of other physical properties. Packaging operations that may benefit from the improved physical properties practiced in the invention include, but are not limited to Horizontal Form Fill and Seal, Vertical Form Fill and Seal, Bag Making, Film Wrapping Operations, Forming Films, lidstocks, and pouches. Multi-layer constructions may also benefit from the use of this invention.
The invention in one application employs the addition of cycloaliphatic metal salts with a polyethylene polymer or copolymer to form films having improved properties. In one particular embodiment of the invention, the fatty acid salt comprises an anion and a cation, the anion of the fatty acid salt comprising at least one C18 (stearic) hydrocarbon chain.
In other more specific embodiments of the invention, it may be possible to use various hexahydrophthalate (HPPA) salt compounds compounds similar to that shown in such film articles:
A blown film article further may comprise or include a C12-C22 fatty acid compound, such as for example, a stearate-type compound. Furthermore, the cycloaliphatic metal salts may comprise dicarboxylate salts, as above, including a carbocyclic ring structure, and a cation or metal.
A blown film may be made which is less than about 250 microns in thickness. In other applications, a film may be made which is less than about 75 microns in thickness, or in some instances, less than about 25 microns. A blown film article is particularly useful in the practice of the invention, but other types of film manufacturing processes also can be employed.
In one application of the invention, a film is made comprising a polyethylene polymer or copolymer and a cycloaliphatic metal salt, wherein said cycloaliphatic salt further comprises a compound conforming to Formula (I)
wherein M1 and M2 are independently selected from calcium, strontium, lithium, zinc, magnesium, and monobasic aluminum;
One method of practicing the invention may comprise the steps of: (a) providing a polyethylene polymer or copolymer; (b) blending said polyethylene polymer or copolymer with a cycloaliphatic metal salt to form blended polyethylene material; (c) plowing said blended polyethylene material; and(d) forming a film.
In the practice of the invention, films can be made by several different means: blown, cast, oriented, and be either monolayer or co-extruded films, having polyethylene as the only component or as one of many components in the monolayer or co-extruded film.
An acid scavenger compound may be applied in the method prior to the blowing step (c). An acid scavenger compound employed in such a method may comprise essentially any fatty acid salt, including for example a stearate, such as for example zinc stearate. Zinc stearate has been shown to provide surprisingly beneficial results, as shown in examples herein.
In the method, one may employ a dicarboxylate salt comprising one or two cations, at least one of said cations being calcium.
Compounds and compositions comprising specific metal salts of hexahydrophthalic acid (HHPA) in order to provide highly desirable properties within thermoplastic articles are provided. The inventive HHPA derivatives are useful as nucleating and/or clarifying agents for such thermoplastics, are practical and easy to handle. Such compounds, when added to the thermoplastic provide good (and sometimes excellent) crystallization temperatures, stiffness, and acid scavenger compatibility.
The term polyolefin or polyolefin resin as used herein is intended to encompass any materials comprised of at least one semicrystalline polyolefin. Examples include polyethylene, isotactic and syndiotactic polypropylene, poly(4-methyl)pentene, polybutylene, and any blends or copolymers thereof, whether high or low density in composition. The polyolefin polymers of the present invention may include aliphatic polyolefins and copolymers made from at least one aliphatic olefin and one or more ethylenically unsaturated co-monomers.
In the practice of the invention, it is possible to make the cycloaliphatic salts that may be applied in the invention, according to the synthesis procedure set forth in U.S. Pat. No. 6,562,890 (column 7, Examples 1 and 2). Calcium HHPA (or other HHPA's) can be made in a manner similar to that shown in U.S. Pat. No. 6,562,890 for Cis-disodium HHPA, as recognized by a person of skill in the art.
Referring to
The bubble 36 is tube shaped, and is cooled to below Tc, crystallization temperature. Then, the polymer is rolled into a flattened tube or wound. The blown polymer bubble 36 passes by guide rolls 38a-b, and through nip rolls 40. The bubble 36 is sealed by nip 40, and thus air cannot easily escape. The bubble 36 acts like a permanent shaping mandrel once it has been injected. The bubble 36 becomes a film 42 that may be passed over a treater bar 44 and rolled among various guide rolls 46a-e to wind-up roll 48. Orientation in the machine direction (i.e. the direction of travel) can be induced by tension from the nip rolls 40.
In the practice of the invention, it also may be possible to make cast film using the novel compositions disclosed herein. Cast film may be made using techniques known in the cast film manufacturing industry, and the invention may apply equally as well to cast film forming techniques. A cast film may be manufactured in which the film comprises a polyethylene polymer or copolymer; and a cycloaliphatic metal salt; and a fatty acid salt, said fatty acid salt having an anion of C12-C22 and a cation, in which the cation is selected from the group consisting of: zinc, calcium, lithium, magnesium and sodium.
Referring to
To a common Linear Low Density Polyethylene (LLDPE) in the film industry and having a density of 0.917 grams per cubic centimeter, a mixture of 1000 ppm of the following Calcium HHPA compound was applied:
The above compound and a standard stabilization package (500 ppm Irganox® 1010, 1000 ppm Irgafos 168, and 800 ppm zinc stearate) were also added to the formulation. The resulting mixture was physically blended, twin screw compounded, and pelletized. The resultant compounded resin was then made into film of 25 micron in thickness using a standard blown film process with a blow up ratio of 2.5. The resultant film had the following properties:
Values are Given as Machine Direction/Transverse Direction
To, a common MDPE in the film industry, having a density of 0.934 grams per cubic centimeter, a mixture of 1000 ppm of Calcium HHPA and a standard stabilization package (500 ppm Irganox® 1010, 1000 ppm Irgafos 168, and 800 ppm zinc stearate) were added. The resulting mixture was physically blended, twin screw compounded, and pelletized. The compounded resin was then formed into film of 25 micron in thickness using a standard blown film process with a blow up ratio about 2.5.
The resultant film had the following properties:
Values are Given as Machine Direction/Transverse Direction
To several types of polyethylene (PEs), a mixture of 1000 ppm of Calcium HHPA and a standard stabilization package (500 ppm Irganox® 1010, 1000 ppm Irgafos® 168, and 800 ppm zinc stearate) were added. The resulting mixtures were physically blended, single screw compounded, and pelletized. The resultant compounded resin was then made into film of approximately 50 micron in thickness using a standard blown film process with a blow up ratio of approximately 2.0. The resultant films had the following optical properties:
To a common LLDPE in the film industry a mixture of approximately 1000 ppm of Calcium HHPA and a standard stabilization package (500 ppm Irganox® 1010, 1000 ppm Irgafos 168, and 800 ppm zinc stearate) were added. The resulting mixture was physically blended, single screw compounded, and pelletized. The resultant compounded resin was then made into film of varying thicknesses using a standard blown film process. The resultant film had the following properties, for both clarified and unclarified film percent haze:
To a common LLDPE in the film industry, a mixture of various potential clarifiers and a stabilization package (500 ppm Irganox® 1010, 1000 ppm Irgafos 168, and 800 ppm zinc stearate) were added. The resulting mixtures were physically blended, single screw compounded, and pelletized. The resultant compounded resins were then made into film of approximately 2 mil thickness using a standard blown film process. The resultant film had the following properties:
To a common LLDPE in the film industry a mixture of 1000 ppm of Calcium HHPA and a standard stabilization package (500 ppm Irganox® 1010, 1000 ppm Irgafos 168, and 800 ppm zinc stearate) were added. To a common MDPE in the film industry (density=0.934 g/cc), a mixture of 1000 ppm of Calcium HHPA and a standard stabilization package (500 ppm Irganox®) 1010, 1000 ppm Irgafos 168, and 800 ppm zinc stearate) were added. The resultant compounded resin was then made into film of 25 micron in thickness using a standard blown film process with a blow up ratio of 2.5.
Water Vapor Transmission Rate (WVTR) was then measured using ASTM F 372-94. The results are shown in the table below.
To a common LLDPE in the film industry a mixture of HHPA salt and a stabilization package (500 ppm Irganox® 1010, 1000 ppm Irgafos 168, and 800 ppm zinc stearate) were added.
The type of HHPA salt was varied as a function of its counter ion. The counter ions included Zinc, Sodium, Lithium, Calcium, and Magnesium. The resulting mixtures were physically blended, single screw compounded, and pelletized. The resultant compounded resins were then made into film of approximately 50 micron thickness using a standard blown film process.
Haze was measured according to ASTM D 1003 (“Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics”), Procedure A. This testing procedure employs a hazemeter as described in Section 5 of ASTM D 1003, and is considered an industry standard for such measurements.
Surprisingly, the calcium counter ion showed substantially improved % Haze levels as compared to other counter ions. This is an unexpected result, and very beneficial.
The resultant films had the following properties:
To a common LLDPE in the film industry a mixture of (1000 ppm) Calcium HHPA and a stabilization package (500 ppm Irganox® 1010, 1000 ppm Irgafos 168, and 800 ppm of an acid scavenger) were added. The type of acid scavenger was varied to include Zinc Stearate (ZnSt), Calcium Stearate (CaSt), and Sodium Stearate (NaSt). The resulting mixtures were physically blended, single screw compounded, and pelletized. The resultant compounded resins were then made into film of approximately 2 mil thickness using a standard blown film process. It was surprisingly discovered that the use of ZnSt provided unexpected and significant benefits in % Haze as compared to the other stearate compounds tested. The resultant films had the following properties:
To a common LLDPE in the film industry (density=0.917 g/cc) a mixture of 1000 ppm of Calcium HHPA and a stabilization package (500 ppm Irganox® 1010, 1000 ppm Irgafos 168, and 800 ppm of an acid scavenger) were added. The resulting mixture was physically blended, single screw compounded, and pelletized. The resultant compounded resin was then made into film of approximately 25 micron thickness using a standard cast film process. The resultant films had the following properties.
To a common HDPE in the film industry and having a density of 0.958 grams Ic per cubic centimeter, a mixture of Calcium HHPA (1000 ppm) and a stabilization package (500 ppm Irganox® 1010, 1000 ppm Irgafos 168, and 800 ppm of an acid scavenger) were added. The resulting mixtures were physically blended, single screw compounded, and pelletized. The resultant compounded resins were then made into film of approximately 2 mil thickness using a standard blown film process. The resultant films had the following properties.
It is understood by one of ordinary skill in the art that the present discussion is a description of exemplary embodiments only, and is not intended as limiting the broader aspects of the present invention, which broader aspects are embodied in the exemplary constructions. The invention is shown by example in the appended claims.