MXPA06008762A - Preparation of polyethylene films - Google Patents

Abstract

A method for making high modulus and high density polyethylene films is disclosed. The method comprises orienting in machine direction (MD) a polyethylene blown film to a draw-down ratio greater than 10:1 to produce an MD oriented film having a 1%secant MD modulus of 1,000,000 psi or greater.

Description

PREPARATION OF POLYETHYLENE FILMS FIELD OF THE I NVENTION The invention relates to polyethylene films. More particularly the invention relates to polyethylene films having high density and high modulus. BACKGROUND OF THE INVENTION Polyethylene is divided into high density polyethylene (HDPE, density 0.941 g / cc or greater), medium density (MDPE, density from 0.926 to 0.940 g / cc), low density (LDPE, density 0.910). at 0.925 g / cc), and linear low density polyethylene (LLDPE, density from 0.910 to 0.925 g / cc). See ASTM D4976-98: standard specification for plastic polyethylene materials for molding and extrusion. The polyethylene can also be divided by "molecular weight," for example, ultra-high molecular weight denotes those having an average molecular weight (Mw) greater than 3,000,000, see US Pat. No. 6,265,504. High molecular weight polyethylene usually denotes that which It has an Mw of 130,000 to 1, 000,000.One of the main uses of polyethylene (HDPE, LLDPE and LDPE) is in the application of films, such as for shopping bags, linings for institutional and consumer cans, bags for merchandise, packaging bags, food packaging films, multi-walled bag liners, fruit and vegetable bags, sausage wraps, stretch wrappers and shrink wrappers The key physical properties of polyethylene film include tear resistance, impact resistance, tensile strength, rigidity and transparency The stiffness of the film can be measured by means of the module. ncia of the film to deformation under effort. Although there are few polyethylene films with a module greater than 1,000,000 psi, there is a growing demand for these films. For example, the upright vertical bag has been the fastest growing segment in the flexible packaging industry in recent years. These bags are used to pack a wide variety of items, including food, industrial and agricultural products. One of the key benefits of the erect bag is its physical shape that gives the packaging a unique "board" effect. That design presents the packaging with additional exposed areas of high quality graphics that can be used to promote the purchase of the item by the consumer. Another benefit of the erect bag is its unique shape allowing the user to differentiate their products from those of the competition. Polymeric films with high stiffness values are necessary to achieve those two unique characteristics of the erect pouch. Another improvement to stiffness by means of the film would allow the user to produce erect bags of larger sizes, thinner packages and / or more unique and creative shapes. These innovations are desirable throughout the erect bag industry to create new products that are visually appealing to the consumer. The orientation of the machine direction (MDO) is known in the polyolefin industry. When a polymer is stretched under uniaxial force, the orientation is alienated in the direction of traction. For example, the North American patent no. 6,391, 41 1 shows the MDO of HPE high molecular weight films (both Mn and Mw greater than 1, 000,000). However, HPE films of high molecular weight are common in the processes of forming films by molding, which are more expensive than the processes of film formation by blowing. In addition, the MDO of high molecular weight HDPE films is limited because those films are difficult to stretch in a high proportion of stretch. It would be desirable to prepare a polyethylene film having a modulus greater than 1,000,000 psi. Ideally, films with a high modulus would be produced by MD orientation of high molecular weight HDPE blown films. SUMMARY OF THE INVENTION The invention is a method for preparing a high modulus, high density polyethylene (HDPE) film. The method consists of orienting a blown HDPE film at a stretch ratio greater than 10: 1 in the machine direction (MD). The MD oriented film has an MD drying modulus of 1% is 1, 100,000 psi or greater.

Preferably, the HDPE has a density within the range of 0.950 to 0. 970 g / cc, an average molecular weight (Mw) within the range of 130,000 to 1, 000,000, and a numerical average molecular weight in the range of 10,000 to 500,000. DETAILED DESCRIPTION OF THE NONDION The invention is a method for preparing a high modulus and high density polyethylene (HDPE) film. Polyethylene resins suitable for producing films of the invention have a density in the range of about 0.950 to 0.970 g / cc. Preferably the density is in the range of about 0.955 to 0.965 g / cc. More preferably, the density is in the range of 0.958 to 0.962 g / cc. Preferably, the polyethylene resin has a number average molecular weight (Mn) in the range of from about 10,000 to about 500,000, more preferably from about 1,100 to about 50,000, and more preferably from about 1 1,000 to 20,000. Preferably, the polyethylene resin has a weight average molecular weight (Mw) in the range of about 130,000 to about 1,000,000, more preferably from about 150,000 to about 500,000, and more preferably from about 155,000 to about 250,000. Preferably the polyethylene resin has a molecular weight distribution (Mw / Mn) within the range of about 5 to 20, more preferably about 7 to 1 9, and most preferably about 9 to 17. The Mw, Mn and Mw / Mn are obtained by means of gel permeation chromatography (GPC) in a Waters GPC2000CV high temperature instrument equipped with a mixed bed column GPC (Polymer Labs mixed B-LS) and 1, 2,4-trichlorobenzene (TCB) as a mobile phase. The mobile phase is used at a nominal flow rate of 1.0 m L / min and a temperature of 145 ° C. No antioxidant is added to the mobile phase, but 800 ppm of BHT is added to the solvent used for the dissolution of the sample. The polymer samples are heated at 175 ° C for two hours with gentle agitation every 30 minutes. The injection volume is 100 microliters. The Mw and Mn are calculated using the% cumulative match calibration procedure used by the Waters Millenium software 4. 0. This includes generating a calibration curve using narrow polystyrene standards (PSS, Waters Corporation products), developing a polyethylene calibration by means of the Universal Calibration procedure. Preferably the polyethylene resin has a melt index Ml2 of from about 0.03 to 0.15 dg / min, more preferably from about 0.04 to 0.15 dg / min, and more preferably from 0.05 to 0.10. The Ml2 is measured at 1 90 ° C under a pressure of 2. 16 kg in accordance with ASTM D-1238. In general, the higher the molecular weights, the lower the Ml2 values. Preferably, the polyethylene resin is a copolymer containing from about 90% by weight to about 98% by weight of recurring units of ethylene and from about 2% by weight to about 10% by weight of recurring units of an α-olefin with from 3 to 10 carbon atoms. Suitable α-olefins with from 3 to 10 carbon atoms include propylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1 pentene, and 1-ketene, and the like and mixtures thereof. Suitable polyethylene resins can be produced by means of Ziegl.er catalysts or the newly developed single-site catalysts. Ziegler catalysts are well known. Examples of suitable Ziegler catalysts include titanium halides include titanium hairs, titanium alkoxides, vanadium halides and mixtures thereof. Ziegler catalysts are used with cocatalysts such as alkylammonium compounds. The single-site catalysts can be divided into metallocene and not metallocene. The single-site metallocene catalysts are transition metal compounds containing binders containing cyclopentadienyl (Cp) or Cp derivatives. For example, the North American patent no. 4, 542, 199 shows metallocene catalysts. The catalysts of a single non-metallocene site contain binders other than Cp but have the same catalytic characteristics as the metallocenes. Non-metallocene single-site catalysts may contain heteroatomic binders, for example boroaryl, pyrrolyl, azaborolinyl or quinolinyl. For example, US patents. 6,034,027, 5,539, 124, 5,756.61 1 and 5,637,660. they show catalysts that are not metallocene. The polyethylene is converted to a thick film by means of a blown extrusion process by the high-stalk extrusion process or in pocket (in-pocket). These processes are commonly used to produce polyethylene films. The difference between the high stem and pocket process in which the high stem process, the extruded tube inflates, a distance (this is the length of the stem), from the extrusion die, while the tube extruded in the process in the pocket it is inflated as the tube leaves the extrusion die.

For example, the North American patent no. 4,606,879 shows an apparatus and method of extrusion blown by high stem. The process temperature is preferably within the range of about 50 ° C to about 21 0 ° C. The thickness of the film is preferably in the range of about 3 to 14 mils, more preferably in the range of about 6 to 8. mils The blown film is then stretched uniaxially in the machine direction (or processing) to form a thinner film. The ratio of thin film thickness before and after orientation is called "stretch ratio". For example, when a 6 mil film is stretched to 0.6 mil the stretch ratio is 10: 1. The stretch ratio of the method of the invention is greater than 10: 1. Preferably the stretch ratio is 1 1: 1 or greater. Preferably, the stretch ratio is such that the film is at or near the maximum extension. The maximum extension is the thickness of the stretched film to which the film can no longer stretch without breaking. The film is said to be at its maximum extent when the tensile strength in the machine direction (MD) has a rupture elongation less than 100% under ASTM D-882. During the MDO, the film from the film blowing line is heated to an orientation temperature. Preferably the orientation temperature is between 60% of the difference between the crystalline transition temperature (Tg) and the melting point (Tm). For example, if the mixture has a Tg of 25 ° C and a Tm of 125 ° C, the orientation temperature is preferably within the range of about 60 ° C to about 125 ° C. Heating is preferably done using multiple rollers of heating. Next, the hot film is fed into a slow drawing roller with a cutting roller having the same speed as the heating rollers. The film then goes into a fast stretching roller has a speed that is 2 to 10 times faster 'that the slow stretch roller, which effectively stretches the film on a continuous basis. The stretched film then enters thermal fixing rolls, which allow stress relaxation by retaining the film at an elevated temperature for a period of time. The thermal fixation temperature is within the range of approximately 1 00 ° C to 125 ° C and the fixing time is within the range of approximately 1 to 2 seconds. Finally the film is cooled through cooling rollers at room temperature. The invention includes the MD oriented film produced by the method. The MD oriented film has a 1% MD drying module greater than 1,000,000 psi. The module is tested in accordance with ASTM E-1 1 1-97. Preferably the MD module is greater than 1, 00,000 psi. In addition to the high modulus MD, the oriented film remains high in other physical properties. Preferably, the oriented film has a MD tensile strength with a yield greater than or equal to 7,000 psi, the MD elongation at a yield greater than or equal to 3%, the MD tensile strength at break is greater than or equal to 30,000 psi, and a Elongation greater than or equal to 40%. Preferably the oriented film has a 1% secant modulus TD (in cross section) greater than or equal to 300,000 psi and more preferably 350,000 psi, TD tensile strength with a yield greater than or equal to 4,000 psi, elongation TD with a yield greater than or equal to 4%, the TD tensile strength of rupture greater than or equal to 4,000 psi, and a TD elongation of rupture greater than or equal to 700%. Tensile strength is tested in accordance with ASTM D-882. The module is tested in accordance with ASTM E-1 1 1-97. Preferably, the MD oriented film has an opacity of less than 50%. Opacity is tested according to ASTM D1003-92: Standard test for opacity and light transmittance of transparent plastics, October 1 992. Preferably, the MD oriented film has a gloss greater than 20. The gloss is tested in accordance with ASTM D2457-90: Standard test methods for the specular gloss of plastic films and solid plastics. The following examples only illustrate the invention. Those skilled in the art will recognize that many variations are within the spirit of the invention and the scope of the claims. EXAMPLES 1 -1 1 Orientation in the direction of the machine of blown films by high high density stem (0.959 g / cc) A high density polyethylene. (L5906, product of Equistar Chemicals, LP, Ml2; 0.057 dg / min, density: 0.959 g / cc, Mn: 13,000, Mw: 207,000, and Mw / Mn: 16) is converted into films with a thickness of 6.0 mol in a 200 mm matrix with a 2 mm air gap between the matrices.

The films are produced at a stem height of 8 matrix diameters and blowing ratios (BUR) of 4: 1. The films are stretched in the form of thinner films in the machine direction with a stretch ratio of 1, 2,3,4, 5,6, 7,8,9, 1 0 and 1 1 .6 in the examples 1 -1 1, respectively. When the stretch ratio is the maximum stretch ratio limited by the orientation equipment and not the polymer film. The properties of the film are listed in Table 1. TABLE 1 Properties vs. stretch ratio of blown films per tall stem oriented in the machine direction.

Table 1 (continued) EXAMPLES 12-22 Orientation in the machine direction of blown films in a density pocket (0.959 g / cc) Examples 1 -1 1 are repeated, but the films are produced in a film line in pocket. The properties of the film are listed in Table 2, which shows that pocket films oriented in the machine direction have MD and TD modules similar to high stem films in their respective maximum stretch ratios. The stretch ratio of 1 1 .3: 1 is the maximum stretch ratio, which is limited by the orientation equipment and not the polymer film TABLE 2 Properties vs. proportion of stretch of blown films in pocket oriented in the direction of the machine.

COMPARATIVE EXAMPLE 23-30 Polyethylene films blown in the direction of the machine of various densities Three resins of high density polyethylene Equistar, XL3805 (density: 0.940g / cc, Ml2: 0.057 dg / min, Mn: 18,000, Mw: 209,000), XL3810 (density: 0.940g / cc, Ml2: 0.12 dg / min, Mn: 16,000, Mw: 175,000), L4907 (density: 0.949g / cc, Ml2: 0.075 dg / min, Mn: 14,000, Mw: 195,000) and L5005 (density: 0.949g / cc, Ml2: 0.057 dg / min, Mn: 1 3,000, Mw: 212,000) are converted into films with a thickness of 6.0 thousand by means of the high stem process described in Examples 1 -1 1 and the pocket process described in Examples 12-22. The films are then stretched in the machine direction to their maximum stretch ratios.

Table 3 shows the MD and TD modules of each film oriented to its maximum stretch ratio. The table shows that these films have low MD and TD modules. TABLE 3 Modules MD and TD vs. density and molecular weight at maximum stretch ratios.

Table 3 (continued)

Claims (19)

1. CLAIMS 1. A method of orienting a polyethylene film blown in the machine direction (MD) to a stretch ratio greater than 10: 1 to produce an MD oriented film having a secant MD modulus of 1% of 1, 000,000 psi or greater.
2. 2. The method of claim 1 wherein the MD oriented film has a modulus decaying at 1% in the transverse direction (TD) of 300,000 psi or greater.
3. The method of claim 1 wherein the blown film is produced from a polyethylene resin having a density in the range of 0.950 to 0.970 g / cc.
4. The method of claim 1 wherein the blown film is produced from a polyethylene resin having a density in the range of 0.955 to 0.965 g / cc.
5. The method of claim 1 wherein the blown film is produced from a polyethylene resin having a density in the range of 0.958 to 0.962 g / cc.
6. 6. The method of claim 1 wherein the blown film is made from a polyethylene resin having a weight-average molecular weight (Mw) in the range of 130,000 to 1,000,000.
7. The method of claim 6 wherein the Mw is within the range of 150.00 to 500,000.
8. 8. The method of claim 6 wherein the Mw is within the range of 155.00 to 300, 000.
9. 9. The method of claim 6 wherein the Mw is within the range of 155.00 to 250,000.
10. The method of claim 1 wherein the blown film is produced with a polyethylene resin having a numerical average molecular weight (Mn) within the range of 1,000,000 to 500,000. eleven .
11. The method of claim 10 wherein the Mn is within the range of 1 1, 000 to 1 00,000.
12. 12. The method of claim 10 wherein the Mn is within the range of 1 1,000 to 50,000.
13. The method of claim 10 wherein the Mn is within the range of 1 1,000 to 20,000.
14. 14. The method of claim 1 wherein the stretch ratio is 1 1: 1 or greater.
15. 15. The method of claim 1 wherein the oriented film has a 1% MD secant modulus of 1, 100.00 or greater.
16. 16. An MD oriented polyethylene film produced by the method of claim 1.
17. 17. An MD oriented polyethylene film produced by the method of claim 5.
18. 18. An MD oriented polyethylene film produced by the method of claim 9.
19. 19. An MD oriented polyethylene film produced by the method of Claim 13