MXPA06008535A - Preparation of polyethylene films - Google Patents

Abstract

An essentially transparent, high density polyethylene film is disclosed. The film has a haze of 20%or less, a gloss of 40%or greater, and density within the range of 0.935 to 0.948 g/cc. The film is made by a high-stalk blown extrusion process and then uniaxially oriented in the machine direction.

Description

PREPARATION OF POLYETHYLENE FILMS FIELD OF THE INVENTION The invention relates to polyethylene films. More particularly, the invention relates to polyethylene films having high density and high transparency.

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 from 0.910 to 0.925 g. / cc), and low linear density (LLDPE, density 0.91 0 to 0.925 g / cc). (See ASTM D4976-98: Standard Specification for Polyethylene Plastic Holding and Extrusion Materials). One of the main uses of polyethylene (HDPE, LLDPE, and LDPE) is in film applications, such as grocery bags, consumer and institutional can liners, market bags, shipping bags, food packaging films, linings multi-walled bags, production bags, wraps for quality food, stretch wrappers and shrink wrappers. The key physical properties of polyethylene film include tear strength, impact resistance, tensile strength, inflexibility and transparency. The total film strength is desirable so that the films can be used with confidence in various applications. Film transparency is desirable because a transparent film allows users to see inside the bag without opening it. Although HDPE films have superior strength, roughness and puncture resistance, they have low transparency and low gloss. The direction of machine direction (MDO) is known for the polyolefin industry. When a polymer is tensioned under uniaxial tension, the orientation is aligned in the pull direction. Most commercial MDO films are produced by orienting cast extrusion films. When an HDPE film undergoes MDO, the resulting film usually shows improved transparency and gloss. However, the improvement is very limited and the MDO films remain opaque. Other ideas to improve the transparency and brightness of the film are also known. For example, Pat. from USA UU No. 5,989,725 teaches a multilayer film of which at least one layer is an HDPE. The multilayer film shows brightness and increased transparency. However, multilayer film is usually more expensive than a single layer film. In addition, the claimed multilayer film still has a high nebulosity and low gloss compared to LLDPE films. Similarly, European Patent Application EP 0246328 teaches a transparent film made of a mixture of HDPE and LLDPE. It would be desirable to prepare a polyethylene film having a physical strength similar to HDPE film but the transparency and gloss similar to the LLDPE film. It is more desirable Prepare a transparent polyethylene film without using multiple layers or mix two or more different resins.

BRIEF DESCRIPTION OF THE INVENTION The invention is a polyethylene film. The film has a nebulosity of 20% or less, a brightness of 40% or greater, and density within the range of 0.935 to 0.948 g / cc. The film is made by a high-speed blown extrusion process and then oriented uniaxially in the machine direction. Unlike the known HDPE films, the polyethylene film of the invention has a high density and high physical strength, and is essentially transparent. The invention also includes a method for making the film. The method comprises converting a polyethylene, having a density within the range of 0.935 to 0.948 g / cc and Ml2 within the range of 0.03 to 0.1 5 dg / min, into a film by a high-tracking blow extrusion and then orienting the film uniaxially in the direction of the machine. The oriented film is essentially transparent with a nebulosity of 20% or less and a brightness of 40% or greater. Unlike the methods unknown in the art, the method of the invention does not require the use of multiple layers or a mixture of two or more resins.

DETAILED DESCRIPTION OF THE INVENTION The polyethylene resin suitable for making the film of the invention has a density in the range of about 0.935 to about 0.948 g / cc. Preferably, the density is within the range of about 0.939 to about 0.945 g / cc. More preferably, the density is within the range of 0.939 to 0.941 g / cc. Preferably, the polyethylene resin has an Ml2 melt index of from about 0.03 to about 0.1 5 dg / min, more preferably from about 0.04 to about 0.15 dg / min. min, and more preferably from 0.05 to 0.10. Ml2 is measured at 190 ° C under 2.1 6 kg pressure according to ASTM D-1238. Preferably, the polyethylene resin has a numerical average molecular weight (Mn) within the range of about 1,000 to about 25,000, more preferably from about 12,000 to about 20,000, and more preferably from about 14,000 to about 18,000. Preferably, the polyethylene resin has a weight-average molecular weight (Mw) in the range of from about 100,000 to about 250,000, more preferably from about 150,000 to about 250,000, and more preferably from about 150,000 to about 200,000. Preferably, the resin polyethylene has a molecular weight distribution (Mw / Mn) within the range of about 5 to about 20, more preferably about 5 to about 15, and more preferably about 8 to about 15. Mw, Mn and Mw / Mn are obtained by gel permeation chromatography (GPC) on a Waters GPC2000CV high temperature instrument equipped with a bed GPC column mixed (Polymer Labs mixed B-LS) and 1, 2,4-trichlorobenzene (TCB) as the mobile phase. The mobile phase is used at a nominal flow rate of 1.0 mL / min and a temperature of 145 ° C. No oxidant is added to the mobile phase, but 800ppm BHT is added to the solvent used for sample dissolution. The polymer samples are heated at 175 ° C for two hours with gentle agitation every 30 minutes. The injection volume is 100 microliters. Mw and Mn are calculated using the cumulative coupling calibration procedure% used by the Water Millenium 4.0 software. This includes first generating a calibration curve using narrow polystyrene standards (PSS, Waters Corporation products), then developing a polyethylene calibration by the Universal Calibration procedure. Preferably, the polyethylene resin is a copolymer comprising from about 85% by weight to about 98% by weight of recurring units of ethylene and from about 2% by weight to about 15% by weight of recurring units of a C3 α-olefin. a C10- The a-olefins of C3 to C1 Q include propylene, 1 -butene, 1 -pentene, 1 -hexene, 4-methyl-1- pentene, and 1-octene, and the like, and mixtures thereof. Suitable polyethylene resins can be produced by newly developed Ziegler catalysts or single site catalysts. Ziegler catalysts are well known. Examples of suitable Ziegler catalysts include titanium halides, titanium alkoxides, vanadium halides, and mixtures thereof. Ziegler catalysts are used with cocatalysts such as alkyl aluminum compounds. Single site catalysts can be divided into metallocene and not metallocene. The single-site metallocene catalysts are transition metal compounds containing cyclopentadienyl (Cp) or ligands derived from Cp. For example, Pat. from USA U U. No. 4,542, 1 99 teaches metallocene catalysts. The non-metallocene single site catalysts contain ligands other than Cp but have the same catalytic characteristics as the metallocenes. Non-metallocene single site catalysts may contain heteroatomic ligands, for example, boroaryl, pyrrolyl, azaborolinyl or quinolinyl. For example, Pat. from USA UU Nos. 6,034,027, 5,539, 124, 5,756.61 1 and 5,637,660 teach non-metallocene catalysts. The polyethylene is converted into a thick film by a high-speed blown extrusion process. The high-tracking blown film process is known. For example, Pat. from USA UU No. 4,606,879 teaches the high-tracking blown film extrusion apparatus and method. The temperature of the process it is preferably within the range of about 1 50 ° C to about 21 0 ° C. The thickness of the film is preferably in the range of about 3 to about 12 mils, more preferably in the range of about 6 to about 8 mils. The film is then stretched uniaxially in the direction of the machine (or processing) to a thinner film. The ratio of the film thickness before and after orientation is called "throw-down ratio". For example, when a 6-mil film is stretched to 1-mil, the throw-down ratio is 6: 1. Preferably, the roll-down ratio is such that the film is at or near the maximum extension. The maximum extension is the thickness of film pulled downwards in which the film can not be thrown further without breaking. The film, ie, is at a maximum extent when the tensile strength of the machine direction (MD) has less than 100% elongation at breaking under ASTM D-882. During MDO, the film of the blown film line is heated to an orientation temperature. Preferably, the orientation temperature is between 60% of the difference between the glass transition temperature (Tg) and the melting point (Tm) and the melting temperature (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. The heating is preferably carried out using multiple heating rollers. Then, the heated film is fed on a slow feed roller with a tip roller, which has the same winding speed as the heating rollers. The film then enters a fast drag roller. The fast drag roller has a speed that is 2 to 10 times faster than the slow drag roller, which effectively stretches the film on a continuous basis. The stretched film then enters thermal quenching rolls, which allow stress relaxation by keeping the film at an elevated temperature for a period of time. The tempering temperature is preferably within the range of about 1 00 ° C to about 125 ° C and the tempering time is within the range of about 1 to about 2 seconds. Finally, the film is cooled by cooling rollers at room temperature. The film of the invention is essentially transparent. By "essentially transparent", it is understood that the film has a haze of 20% or less. Nebulosity is tested according to ASTM D1 003-92: Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics, Oct. 1992. Preferably, haze is 15% or less. More preferably, the cloudiness is 1 3% or less. The film of the invention has a high gloss. By "high brightness", it is understood that the brightness is approximately 40% or greater. He Gloss is tested according to ASTM D 2457-90: Standard Test Method for Specular Gloss of Plastic Films and Solid Plastics, June 1 990. Preferably, the gloss is approximately 50% or greater. The film of the invention remains at a high physical strength. Preferably, the oriented film has MD modulus greater than or equal to 240,000 psi, MD tensile strength in production greater than or equal to 7,000 psi, MD elongation at production greater than or equal to 3%, MD tensile strength at greater breakthrough ao equal to 30,000 psi, and MD elongation at break greater than or equal to 40%. Preferably, the oriented film has TD modulus (transverse direction) greater than or equal to 210,000 psi, TD tensile strength in production greater than or equal to 4,000 psi, TD elongation in production greater than or equal to 4%, TD tensile strength in break greater than or equal to 4,000 psi, a TD stretch at break greater than or equal to 700%. Tensile strength is tested according to ASTM D-882. Module is tested according to ASTM E-1 1 1-97. Surprisingly, it has been found that the directional orientation of the polyethylene film machine which is made by a cavity film process gives much lower brightness and much higher nebulosity. For example, MDO, high-tracking polyethylene films may have a haze of 20% or less and a brightness of 40% or higher (see Examples 1-6 and 9-16), while MDO, in cavity films made of the same polyethylene resins have much higher nebulosity and shine much lower (see Comparative Examples 7, 8 and 17-23). Both high-tracking and cavity processes are commonly used to make polyethylene films. The difference between the high-tracking process and the cavity process is that in the high-tracking process, the extruded tube is inflated at a distance (ie, the length of the tracking) from the extrusion nozzle, while the tube Extruded in the cavity process is inflated as the tube exits the extrusion nozzle. In addition, it has been found that the densities of polyethylene resins are also determined for nebulosity and brightness of MDO films. For example, Comparative Examples 24-26 show that when the polyethylene resin has a density of 0. 949 g / cc or higher, MDO films have a haze that is greater than 20% and a brightness that is less than 20% at their maximum downcasting rates (see Table 5). The following examples merely illustrate the invention. Those skilled in the art will recognize many variations that are within the spirit of the invention and scope of the claims. EXAMPLES 1-6 MDO, high density and high density polyethylene films High density polyethylene (XL3810, product of Equistar Chemicals, LP, Ml2; 0. 12 dg / min, density: 0.940 g / cc, Mn: 16,000, Mw: 175,000 and Mw / Mn; 1 1) becomes films with a thickness of 6.0 thousand in 200 mm nozzle with 2 mm of space nozzle. The films are produced at a tracking height of 8 nozzle diameters and at blowout ratios (BUR) of 4: 1. The films are then stretched into thinner films in the machine direction with roll-down ratios 1, 2, 3, 4, 5 and 6.5 in Examples 1-6, respectively. When the roll-down ratio is 1: 1, the film is not oriented. The pull down ratio of 6.5: 1 is the maximum pull down ratio. The film properties are listed in Table 1. TABLE 1 Properties vs. MDO Down Roll Ratio, High Tracking Movies COMPARATIVE EXAMPLES 7-8 MDO, Cavity and High Density Polyethylene Films Examples 1-6 are repeated, but films are made in cavity film line. The film properties are listed in Table 2, which shows that the direction of the machine oriented, cavity films have much lower brightness and haze higher than the high-tracking films in their respective maximum drag ratios. The roll-down ratio of 2: 1 is the maximum roll-down ratio of the films in cavity.

TABLE 2 Properties vs. Proportions of Roll Down MDO, Cavity Films EXAMPLES 9-16 MDO, high density and high density polyethylene films A high density polyethylene (XL3805, product of Equistar Chemicals, LP, Ml2: 0.057 dg / min, density: 0.940 g / cc, Mn: 1 8,000, Mw : 209,000, and Mw / Mn: 1 1) becomes films with a thickness of 6.0 thousand in 200 mm nozzle with 2 mm nozzle space. The films are produced at a tracking height of 8 nozzle diameters and in blowing proportions (BUR) of 4: 1. The films are stretched in thinner films in the direction of the machine with roll-down proportions 1, 2, 3, 4, 5, 7 and 8 in Examples 9-16, respectively. When the roll-down ratio is 1: 1, the film is not oriented. The Ratio of roll down to 8: 1 is the maximum pull down ratio. The film properties are listed in Table 3. TABLE 3 Properties vs. Pull-down ratios of MDO, high-tracking films COMPARATIVE EXAMPLES 1 7-23 MDO, Cavity and High Density Polyethylene Films Examples 9-16 are repeated, but films are made under cavity film conditions. The film properties are listed in Table 4, which shows that the direction of the oriented machine, cavity films have much lower brightness and higher nebulosity than the high tracking films at their respective maximum drag ratios. The roll-down ratio of 7: 1 is the maximum roll-down ratio of the films in cavity.

TABLE 4 Properties vs. Proportions of Roll Down MDO, Cavity Films COMPARATIVE EXAMPLES 24-26 MDO, High Density Polyethylene Films of Various Densities and Ml2 Three Equistar high density polyethylene resins, XL5906 (density: 0.959 g / cc, Ml2: 0.057 dg / min, Mn: 12,900, Mw: 207,000), L4907 (density: 0.949 g / cc, Ml2: 0.075 dg / min, Mn: 14,300, Mw: 1 94, 700), and L5005 (density: 0.949 g / cc, Ml2: 0.057 dg / min, Mn: 12,600, Mw: 212,000) are converted into 6.0-mil thick films by the high-tracking process described in Examples 1-6.

The high-tracking films are then stretched in the machine direction to their maximum downcast proportions. In Table 5 the brightness and nebulosity of each oriented film are listed in their maximum downcasting ratios. The table shows that these films have much higher nebulosity and lower brightness than the films of Examples 6 and 1 6. TABLE 5 Brightness and Film Nebulosity vs. Density and Ml2 a Proportions of Chuck Down Maximum

Claims (18)

1. CLAIMS 1. A polyethylene film having a nebulosity of 20% or less and gloss of 40% or more, and consisting essentially of a Ziegler polyethylene having a density in the range of 0.935 to 0.948 g / cc, wherein the film is made by a process of high-speed blown extrusion and then oriented uniaxially in the machine direction.
2. 2. The film according to claim 1 having a thickness of 1 mil or less.
3. 3. The film according to claim 1, characterized in that the nebulosity is 15% or less.
4. 4. The film according to claim 1, characterized in that the nebulosity is 13% or less.
5. 5. The film according to claim 1, characterized in that the density is within the range of 0.939 to 0.945 g / cc.
6. 6. The film according to claim 1, characterized in that the density is within the range of 0.939 to 0.941 g / cc.
7. The film according to claim 1, characterized in that the brightness is 50% or greater.
8. The film according to claim 1, which is oriented uniaxially in the machine direction with a pull ratio down to 5: 1 or greater.
9. The film according to claim 8, characterized in that the downstream ratio is 6: 1 or greater.
10. 10. A method comprising: (a) converting a polyethylene resin into a film by a high-tracking blown extrusion process, wherein the polyethylene resin has a density within the range of 0.935 to 0.948 g / cc and Ml2 within the range of 0.08 to 0.15 dg / min; and (b) orienting the film of step (a) uniaxially in the machine direction; where the oriented film has a nebulosity of 20% or less. eleven .
11. The method according to claim 10, characterized in that the polyethylene resin has a density within the range of 0.939 to 0.945 g / cc.
12. The method according to claim 10, characterized in that the polyethylene resin has a density within the range of 0.939 to 0.941 g / cc.
13. The method according to claim 1, characterized in that the polyethylene resin has Ml2 within the range of 0.1 0 to 0. 15 dg / min.
14. The method according to claim 10, characterized in that the oriented film has a nebulosity of 15% or less.
15. 15. The method according to claim 10, characterized in that the film is oriented uniaxially in the machine direction with a pull ratio down to 5: 1 or greater.
16. 16. The method according to claim 1 5, characterized in that the downstream ratio is 6: 1 or greater.
17. 17. The method according to claim 1, characterized in that the oriented film has a brightness of 40% or greater.
18. 18. The method according to claim 17, characterized in that the oriented film has a brightness of 50% or greater.