FI81109C - Polyethylene blend suitable for film production and methods for improving film production properties by irradiation - Google Patents

Description

1 81109

Polyethylene blend suitable for film production and a method for improving the fabrication properties of a well by irradiation Polyethylene blend with heat for the production of film on the same film as for the production of film on the film production genome bestraining

The invention relates to a polyethylene blend suitable for the production of a film, comprising a substantially linear crosslinked copolymer of ethylene and alpha-olefin (s) having a low molecular weight distribution and a low density. The invention also relates to a method for improving the film-forming properties of a polyethylene blend and to a plastic film produced by this method.

Linear, low density ethylene / alpha-olefin copolymers having molecules that do not contain substantially long side branches and have a narrow molecular base distribution (such polymers are hereinafter referred to as "LLDPE" in this application) show an excellent balance of mechanical properties over others. Both LLDPE and conventional low branched (hereinafter referred to as LCB) low density polyethylenes have an excellent balance of high density polyethylenes in terms of properties such as total tensile strength, impact strength, low puncture resistance, tear strength, tear strength, tear strength, tear strength chemical resistance and dielectric strength. In addition, LLDPE is excellent compared to low density LCB polyethylene with increased tensile cross-section reduction resistance, which can lead to unbalanced film properties and thus higher production costs when using low density LCB polyethylene. The special quality properties of LLDPE, which form an excellent balanced combination of mechanical properties, together with the improved film-making rheology make it a versatile raw material with a wide range of uses.

However, the special properties of LLDPE also present difficulties in certain applications such as film production. For example, when using LLDPE, commercially desired film quality at high manufacturing speeds can be achieved only to a limited extent without difficulties such as film bubble instability in blown film expansion or draw resonance in surface-patterned flat film extrusion by slit extrusion or extrusion.

Solutions to the aforementioned problems in forming the blown film have included modifying the methods and apparatus as disclosed in U.S. Pat. 4,330,501 to Jones et al., Which discusses refrigerant flow modification. An improved extrusion coating process has been achieved using blends of LLDPE and low density LCB polyethylene, as disclosed in U.S. Pat. 4,339,507 (Kurtz et al.).

Irradiation treatment has been widely used to modify the Theology of Polyolefin. Much of the prior art involves irradiating a preformed plastic film to improve certain mechanical properties. For example, GB patent publication no. 2,019,412 (Clarke et al.) Relates to the irradiation of an LLDPE film with a radiation dose of 2 to 80 billion, which results in a higher elongation at break values. Irradiation at low levels, between 0.05 and 0.25 billion, is described in U.S. Pat. 3,563,870 (Tung et al.) Reported to improve melt strength and extensibility with high density polyethylenes. Low-level irradiation is also reported in the CA patent

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3 81109 788 440 have been reported to lower the melt index and increase the intrinsic viscosity of low crystallized, low density LCB polyethylenes. In the article "Crosslinking Effects on Flow of Irradiated Polyolefins" on the low level irradiation of polyolefins (including polypropylene, high density polyethylene and various low density LCB polyethylenes), Markovic et al., Modern Plastics, October 1979, page 53, such irradiation has a greater effect on absolute viscosity at low shear rates than at high shear rates. However, irradiation of polyethylene can often be undesirable because it degrades surface properties such as smoothness, which is required in various coating applications.

More specific objects of the present invention appear from the appended claims, which are polyethylene comprising a low level irradiated copolymer of ethylene and one or more alpha-olefins with a substantially linear, narrow molecular weight distribution and low density. Such a copolymer has crosslinks to such an extent that the extensional viscosity is increased and the high shear shear viscosity is substantially the same as that of the corresponding non-crosslinked polyethylene. Such a copolymer has been subjected to and has absorbed a low level, i.e. usually less than about 2 billion, ionizing radiation to an extent sufficient to provide crosslinks to the copolymer, but insufficient to provide significant gelation. After irradiation, such a copolymer can be made into a film by extruding a melt containing the irradiated copolymer into a film and then stretching the film. According to the invention, mixtures of crosslinked and non-crosslinked copolymers are also provided.

Figure 1 is a graph illustrating the effect of irradiation on the tensile viscosity of LLDPE.

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Figure 2 is a graph illustrating the effect of irradiation on the shear viscosity of granular LLDPE.

Figure 3 is a graph illustrating the effect of irradiation on the shear viscosity of pelletized LLDPEs.

Figure 4 is a graph illustrating a comparison of the effects of radiation doses on the melt and flow indices of granular and pelletized LLDPE.

Figure 5 is a graph illustrating film production rates for irradiated and non-irradiated LLDPE, and mixtures of such materials.

According to the present invention, we have found that increased film production rates and better efficiency in LLDPE film production can be achieved by modifying the chemical structure of LLDPE to achieve improved rheology during film extrusion and expansion. Conversion of LLDPE is accomplished using low levels of ionizing radiation to the extent that crosslinking occurs to the copolymer but insufficient to produce a significant gel strain, with a radiation dose typically between 0.05 and 2 billion. Such transformation provides LLDPEs with a higher tensile viscosity during film expansion without any sacrifice in other Theological properties of the resin being treated.

LLDPE copolymer i t

Suitable linear, narrow molecular weight distribution and low density ethylene / alpha-olefin copolymers useful in the present invention do not substantially have long side branches prior to irradiation. The term "linear" is used to define a polymer chain which, for the most part, lacks long side branches. "Formation of long side branches" describes branching by means of polymeric structural units such that the sizes of the side branches exceed the chain lengths of the pendant groups resulting from the incorporation of the individual alpha-olefin comonomers. The long polyethylene side branch should have at least a sufficient number of carbon atoms to bring about significant changes in Theological behavior caused by the entanglement of chains ("Chain entanglement"). The number of carbon atoms required for this is usually greater than about 100. The chain length of the short side branches resulting from the polymerization of the comonomer is usually less than about 10 carbon atoms per side branch. LLDPE without crosslinks has very few or no long side branches so that the only branching that can be spoken is short chains and the chain length of the side branches is determined by the dependent chains formed from the alpha-olefins used as comonomers.

The term "narrow molecular weight distribution" as used in this application means the ratio of the weight average molecular weight to the number average molecular weight. This ratio may be between 1 and 10, preferably between about 2 and about 6.5, and most preferably between about 3 and about 5. The lower limit in this respect is determined by the theoretical limit value, since the number average molecular weight is not by definition may exceed the weight average molecular weight. The upper limit of this ratio is reached when the width of the molecular weight distribution expands to such an extent that the rheology of the polymer is not significantly altered by low-level irradiation so as to improve the film-making properties. Preferred regions comprise LLDPEs with a narrower molecular weight distribution that are commercially available.

6,8109 LLDPE resins are those copolymers of ethylene and one or more alpha-olefins having at least 3, preferably 4 to about 18, and most preferably 4 to 8 carbon atoms in the alpha-olefin comonomers. The term copolymer in this application also includes terpolymers which, in addition to ethylene, contain two or more comonomers. The LLDPE of the present invention preferably has a higher molar percentage (more than about 80%) of ethylene and a lower molar percentage (less than about 20%) of one or more other alpha-olefins. Typical olefins include unsubstituted or alkyl-substituted mono- or diolefins. Preferred alpha-olefins include 1-butene, 4-methyl-1-pentene, 1-hexene, 1-heptene, 1-octene, and 1,4-butadiene. A particularly preferred alpha-olefin is 1-butene.

The density of LLDPEs is normally no greater than about 0.94 g / cm 2, and typically between about 0.86 and about 0.93 g / cm 2. LLDPE can be prepared by the solution, slurry or gas phase method using standard procedures in the art.

The constant melt index of LLDPE useful in the production of the blown film prior to irradiation may be in the range of about 0.1 to about 5.0 g / 10 min (dg / min), preferably in the range of about 0.5 to about 2.5 g / min. 10 min and most preferably between about 1 ... about 2 g / 10 min. Resins with a higher melt index, up to 10 g / 10 min and more, can be used in other film making applications such as extrusion coating, where the properties typical of resins with a low melt index are less important. Irradiation causes a decrease in the melt index of LLDPE. The amount of such reduction depends on the LLDPE in question and the irradiation conditions as described below.

7 81109 Irradiation

The conditions under which irradiation of the LLDPE of the present invention occurs are not critical except for the effect on the radiation dose absorbed by the copolymer. The radiation dose varies depending on how sensitive the irradiated LLDPE is in the following ratios: (1) the minimum amount of irradiation that results in an improvement in the film-making properties without (2) undesirable side effects such as gelling. Generally, the minimum radiation dose for the practice of the present invention is greater than about 0.05, preferably greater than 0.1, and most preferably greater than 0.2 billion. The maximum radiation dose is usually less than 2, preferably less than about 1, and most preferably less than about 0.5 billion. usually falls between about 0.2 and about 0.5 billion.

In the broadest sense, the radiation dose should occur in the range from at least the required minimum dose to provide a degree of crosslinking to improve the film-making rheology in the copolymer, and ends at a maximum just below that that would cause significant gelation in the copolymer. The provision of crosslinks to LLDPE can be determined by measuring the number of long side branches after irradiation. The presence and number of long side branches in the polymer can be indicated, for example, if the number of short side branches, which can be determined by, for example, nuclear magnetic resonance spectroscopy, is less than the total number of side branches determined by, for example, infrared spectroscopy. In this application, the term "significant gelation" is used to indicate the maximum amount of radiation dose and refers to the amount of gel concentration that causes (a) a deterioration in theological properties such as a significant increase in high shear viscosity in copolymers during film production, or (b) a significant deterioration the grade is denoted by the abbreviation FAR in this application. FAR is determined by examining the membrane sample with the naked eye to detect and compare gel areas or other foreign particles in it to standard membrane samples with different levels of gel concentrations. FAR is rated on a scale ranging from -100 (very poor) to +100 (excellent). Substantial deterioration in FAR rating occurs when low-level radiation produces large gel particles. Gel particles smaller than 250 microns in diameter do not usually cause a decrease in FAR rating. Gel particles with a size between about 250 and 500 microns will cause a decrease in FAR rating if the concentration of the particles becomes larger. If gel particles larger than 500 microns are present in measurable amounts, there is usually a significant deterioration in their FAR score.

Within the scope of the present invention, the substantially low gel content of irradiated LLDPE alloys is reflected in the fact that they are substantially soluble in boiling xylene. Significant gelation usually occurs at gel concentrations of 1% or greater based on the total weight of the polymer. The gel content is preferably less than 0.1% by weight. The maximum amount of absorbed radiation varies depending on the gel formation sensitivity of the LLDPE in question, as well as environmental factors affecting gelation such as chemical reactivity to free radicals formed during irradiation from components of the atmosphere in contact with LLDPE, such as oxygen.

The irradiation can be carried out in an air atmosphere, but more preferably in an oxygen-free atmosphere such as a vacuum or in the presence of an inert gas. The inert atmosphere is preferably chemically non-reactive with the LLDPE to be irradiated and relatively well permeated by the radiation flux used. The preferred inert atmosphere contains nitrogen or argon. Irradiation can be performed separately or in conjunction with other copolymer treatment steps such as in a reactor simultaneously with polymerization or in an extruder prior to extrusion.

The radiation dose is sometimes reported as the targeted dose, but the term is misleading because it does not determine the amount of radiation actually absorbed by the sample. In the present invention, the radiation dose is defined to mean the amount of radiation absorbed, thus separated from the amount of radiation emitted by the radiation source (measured as X-rays), and is expressed in megarads (abbreviated Mrd), which means the amount of radiation absorbed (1 Billion = 106rd = 10 ).

Irradiation conditions and methods can be selected from those well established in the art. The quality of the source of ionizing radiation is not critical. For example, electron accelerators can be used to produce high energy electrons or cobalt 60 to produce gamma rays for irradiation. Other sources of ionizing radiation such as X-rays, and other radioisotopes that emit gamma rays and other high energy rays or particles can also be used. Commercial electronic accelerators are particularly useful for the purposes of this invention. Such devices typically operate in the voltage range of 0.5 MEV (= MeV, 1,602,219 x 10 "13j) to 3 MEVs and beam power from 500 to 100,000 W. The radioactivity of cobalt 60 sources used in industrial irradiation varies from a few thousand Curies over one million Curiee (lCi = 3.7 x1010 1 / s).

The term ionizing radiation includes accelerated electrons and other particles as well as electromagnetic radiation having a wavelength of less than about 250 Angstroms (= 25 nm). When ionizing radiation is absorbed into a substance, sufficient energy is transferred to it to collect the electrons in the orbitals from the atoms. This separation of charges, i.e. ionization, causes chemical changes such as chain breakage and crosslinking in the polymers.

Exposure of LLDPE to ionizing radiation can be accomplished by passing the copolymer through the radiation field at such a rate that the residence time in the field is sufficient to absorb the desired dose into the copolymer.

LLDPE can be irradiated as a solid or melt, i.e. in molten form, at any time from the formation of the copolymer to the extrusion of the film. The copolymer can be irradiated as a solid during or after the fluidized bed operation, or as a melt in an extruder before or during extrusion. The copolymer as a granular solid can be irradiated in either granular or pelletized form.

In some cases, LLDPE may be mechanically modified after irradiation to improve its optical properties, i.e., reduce the haze value and increase the gloss and specular transmission. Mechanical shear motion, such as homogenization in a mixer at temperatures below the melting point of the resin, can be used. In this way, homogenized LLDPE is more easily extruded into a tubular shape and has better gloss and clarity. Mechanical modification can be accomplished by methods well established in the art. Mechanical modification should be applied below a level that does not cause deterioration in tensile viscosity or other Theological properties as a result of the modification.

Extrusion

Low level irradiated LLDPE can be extruded into a film using conventional, well-known procedures in the art, such as the blow bubble method, extrusion coating, surface patterning, or slit die.

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11 81109 injection molding. Such techniques are described, for example, in U.S. Patent 4,243,619 (Fraser et al.), Which is incorporated herein by reference.

The present invention provides a method of making a film comprising (a) irradiating LLDPE with low levels of ionizing radiation to the extent that cross-linking to LLDPE occurs, but insufficient to provide significant gelation, and after irradiation, (b) irradiated LLDPE. extrusion film, and (c) film stretching. In one embodiment, the film extruded in step (b) is in the form of a hose and the stretching of the film hose in step (c) occurs by expanding the film bubble. In other embodiments, the extrusion in step (b) is (1) production of a planar film through a slit die by casting, or (2) extrusion coating to form a film coating on a substrate. In a preferred embodiment, the irradiated LLDPE (d) obtained from step (a) is mixed with the non-irradiated LLDPE prior to the extrusion step (b). Alternatively, or in addition to mixing step (d), a certain ratio between irradiated and non-irradiated LLDPE is obtained by irradiating only a portion of the LLDPE in step (a).

LLDPE alloys

In a preferred embodiment, mixtures of irradiated LLDPE and the same or different non-irradiated LLDPE are provided. The term "non-irradiated", of course, includes normal substance that has been the subject of background radiation from the environment. The polyethylene blends of the present invention may contain additives including other resins such as low or high density LCB polyethylenes. Mixtures of irradiated and non-irradiated LLDPE polyethylenes have been found to achieve higher production rates than the values obtained by linear extrapolation to unirradiated and 100% irradiated LLDPE in weight percentages. The preferred LLDPE blend may contain from about 1% to about 95%, preferably from about 5% to about 30%, and most preferably from about 10% to about 25% of irradiated LLDPE. Mixtures of LLDPE with only a portion of the irradiated LLDPE can be obtained as blends, or by adjusting the dosage during irradiation by irradiating only the desired portion of the resin, thereby avoiding or minimizing mixing operations with the non-irradiated LLDPE. When irradiated LLDPE is mixed with a different, non-irradiated LLDPE, it is preferred that the melt index of the irradiated LLDPE before irradiation is lower than that of the non-irradiated LLDPE.

In a typical embodiment, the LLDPE absorbs a radiation dose that rises to about 1 Mrd (10kGy). The irradiated LLDPE is then mixed with some non-irradiated LLDPE and fed to an extruder. The mixture is heated to form a melt and pressed through a nozzle forming a film or web. The film is then stretched or applied using, for example, blow molding in film-tube extrusion, or extrusion coating, surface patterning, or slit die casting by reducing the cross-section by stretching.

Film

The thickness of the film produced by the process of the present invention is usually from about 0.1 mils (0.00254 mm) to about 20 mils (0.508 mm), preferably from about 0.1 mils to about 10 mils (0.254 mm), and most preferably about 0, From 1 mil to about 6 mil (0.1524 mm). The most preferred film is characterized by the following properties: puncture resistance greater than 7.0 in-lb / mil (= 31.14 kJ / m), elongation at break greater than about 400%, tensile strength between about 400 and about 2000 ft-lb / in ^ (33 ... 166 MN / m2) and a tensile strength greater than about 2000 to about 7000 psi (13.8 ... 48.2 MN / m2). In accordance with established practice, the film may contain well-known additives such as lubricants, anti-clogging agents, colorants, antioxidants, stabilizers, fillers, nucleating agents, and the like. The film obtained by the extrusion coating process may further contain well-established additives such as chili roll release agents, and may be extruded directly onto various commonly used substrates such as polyethylene, paper, aluminum foil and the like. .

Such films are widely used in typical applications of LLDPEs such as packaging materials including bags, shrink film and the like, as well as laminates.

Rheology of irradiated LLDPE copolymer

The effect of low level irradiation on film fabricability is seen when comparing the Theologies of LLDPE with and without crosslinks. In film extrusion, at least two rheological behavior considerations are important: stretching and shear.

The stretching can be biaxial, such as in a film blowing process, or approximately planar, as in other free surface stretching continuous processes, such as an extrusion coating or die casting process. An approximation to the elongation rheology can be obtained by determining the tensile viscosity. The tensile viscosity is defined as the ratio of the normal stress to the rate of elongation (defiformation rate) as follows: (I) where 'Ί is the tensile viscosity in degrees (1P = 10 Pa.s), 1Γ is the normal stress, in dyn / cm2 (ldyn / cm2 = 10 Pa) ; and £ is the stretch rate, in s-1-.

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Tensile viscosity can be measured by a number of experimental procedures. In the present application, the tensile viscosity is measured in the deformation field of uniaxial stretching using the constant stretching rate method. The method uses a stress-strain tester in which the opposite ends of the molten polymer ring are held in the jaws immersed in a silicone oil bath and drawn apart at an accelerating speed according to the following relationship: i L (t) = L0e £ t (II) where L (t) is the distance between the jaws after t (in seconds); Lq is the distance between the jaws at the beginning; and l is the stretching rate (s “l), which is kept constant. The force sensor measures the load during deformation. This load force is divided by the prevailing approximate cross-sectional area to obtain the normal stress. The tensile viscosity can then be calculated using formula (I) by dividing the normal stress by the stretching rate, and is determined as a function of the transition or time at a temperature of about 150 ° C.

Using this procedure, the tensile viscosity of unirradiated LLDPE can be compared to the tensile viscosity of low level irradiated LLDPE as shown in Figure 1. We have found that irradiation of low level LLDPE results in a qualitatively significant increase in tensile viscosity. In Fig. 1, short times from 0.1 to 1 second correspond to a stretching rate of approximately 1 s -1, and long times as considered to be longer than 10 s correspond to stretching rates of the order of 0.01 s -1. Thus, the increase in tensile viscosity observed is greatest at the relatively low stretching rates typical of blown film hose fabrication. For comparison, Figure 1 is provided with the tensile viscosity of a high density polyethylene (HDPE). The tensile viscosity of is 81109 LLDPE irradiated at low levels with increasing radiation doses is similar to the tensile viscosity of HDPE, indicating that conventional film manufacturing equipment and procedures used to produce HDPE film can be used to improve the radiation of the LPDP according to the present invention. balance in membrane properties.

Shear rheology is also important in film extrusion, especially as the polymer melt passes through the extrusion die, subjecting it to severe shear deformation. Shear rheology can be measured using shear viscosity, which is defined as the ratio of shear stress to shear rate (shear deformation rate) as follows:

^ S = Y (HD

where * 2g is the shear viscosity in degrees (1P = 10 Pa s); Γ is the shear stress, in dyn / cm2 (ldyn / cm2 = 10 Pa); and S 'is the shear rate, in s “l.

Figures 2 and 3 show the shear viscosities of granulated and pelletized LLDPE, respectively, at different radiation dosages. As can be seen, irradiation of LLDPE strongly influences low shear rate rheology, while high shear rate rheology remains relatively unchanged, especially at lower radiation doses. At radiation dosages up to 0.5 Mrd for pelletized LLDPE and up to 1 Mrd for granulated LLDPE, the high shear rate remains essentially unchanged (in terms of shear stress). Since an increase in shear stress at high shear rates causes an undesirable increase in power and melt temperature when operating at the same production rate, this can be read in favor of low level irradiated LLDPE as it provides an increase in elongation rheology without high shear viscosity. in an effort to bring about improvements in film production.

Low level irradiation of LLDPE produces a sufficient number of crosslinks to increase the tensile viscosity of LLDPE while maintaining substantially the same high shear rate shear viscosity compared to the corresponding non-crosslinked LLDPE. At a stretching rate of 0.1 s and 1. ) at 1 ° C, the tensile viscosity of the low level irradiated LLDPE of the present invention increases by more than about 20%, preferably by more than about 50%, and more preferably by more than about 100%. At a shear rate of 200 s -1, at a temperature of 210 ° C, such low level irradiated LLDPE retains a high shear shear viscosity of substantially the same order of magnitude as less than about 20%, preferably less than about 10%, and more preferably than the non-crosslinked form of LLDPE. less than about 5% above the reference value.

We have found that the shape of LLDPE during low-level irradiation affects conversion. LLDPE is produced, for example using gas phase fluidized bed polymerization, as a small, granular solid which is relatively porous and is referred to in this application as a granular form. However, the granular LLDPE can then be treated by melt pelletization, whereby the resin is subjected to melting and shear stress in a mixing extruder or pump to produce a modified form which is referred to as pelletized in this application. The treated pelleted form differs from the untreated granular form in that the former resin (I) is less porous, i.e., has a lower surface area to volume ratio, and consequently a higher density, (2) is different at the molecular level with more chain-

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17 81109 chain entanglements) than in the untreated granular form. Figures 2 and 3 illustrate that LLDPE in granular form has a greater ability to absorb higher levels of radiation without increasing the high shear rate shear viscosity when contrasted with the pelleted form of the same polymer. In contrast, Figure 4 shows that although both granular and pelletized LLDPE show a decrease in melt and flow indices with increasing radiation dosage, this decrease is significantly greater for the pelleted form than for the granular form. This characteristic difference provides a greater increase in bubble stability in film fabrication by blowing the film tube while comparing pelletized LLDPE irradiated at the same low level of dosage to the corresponding granulated one. It follows that the form of the LLDPE to be irradiated can be selected either on the basis of the best possible absorption capacity or on the basis of the greatest rheological change, depending on which property is more desirable for the LLDPE to be used in each case.

The molecular weight study, as shown in Examples 36-46, reveals further characteristic differences between low-irradiated pelletized and granular LLDPE, as there is a definite, clear increase in the molecular weight values of low-irradiated, pelletized LLDPE, while the molecular weight values of the granular LLDPE either do not change or there is only an apparent, negligible decrease.

We have also found that mixtures containing a relatively small proportion, preferably less than 30% by weight of the mixture, of irradiated LLDPE with non-irradiated LLDPE achieve dramatic increases in film production rate. These increases are significantly greater than those that would be expected from combinations of these two resins. Figure 5 graphically shows these very useful increases in film production rate. The maximum production rates of 18 81109 film using unirradiated LLDPE under four different film manufacturing conditions (identifiable as points A, B and D and area B) are compared to the maximum production rates using 100% irradiated LLDPE (identifiable as points A ', B' and D 'and area The maximum production rates between these values (shown in the straight areas A-A ', D-D' and the hatched areas B-B 'and C-C') indicate the area in which the values of the irradiated and non-irradiated mixtures could be expected to settle, without the synergistic combined effect of the mixtures. The maximum production rates for some mixtures of irradiated and non-irradiated LLDPE are shown in the figure (values marked with an X, the subscripts of which identify the corresponding production conditions). As can be seen, with these LLDPE blends, the production rates greatly exceed the corresponding expected values, allowing the use of substantially improved production rates despite the presence of only relatively small percentages of irradiated LLDPE. The commercial benefits of such significant increases in production rates in combination with the potential cost savings from irradiating only a portion of LLDPE are quite significant.

Without wishing to be bound by any particular theory, we believe that low-level irradiation of LLDPE causes crosslinks in the form of long side branches, producing star-shaped molecules such that free radicals are attached to adjacent polymer chains. Such molecules are believed to allow for greater chain intertwining than T-shaped molecules in low-irradiated, conventional types of low-density LCB polyethylenes, much of which have side branches that are too short for chain intertwining to occur. In addition, the absence of high molecular weight chain ends and the higher proportion of 81109 tertiary carbon atoms at short side branch locations in LLDPE are believed to provide both (1) lower susceptibility to intramolecular crosslinks (than larger LCB molecules with large chain ends). tendency to gel, that (2) a higher degree of crosslinking (than conventional LCB polyethylenes with fewer short side branches in the molecules).

For this reason, we believe that LLDPE copolymers have a unique ability to achieve low levels by irradiation of desired molecular changes that are clearly different from a simple change in molecular weight compared to conventional, low density LCB polyethylenes.

Application examples

The following examples illustrate the manufacturing methods and products of the present invention and are not intended to limit the scope of the invention in any way.

Irradiation procedure LLDPE was irradiated in, up to the dose shown in the examples, according to the following procedure using a 2,000,000 V, 500 W van de Graaf electron accelerator. This accelerator was equipped with a beam wiper (scanner) that deflected the electron beam at a frequency of 200 Hz (200 s “1), the deflection direction being at right angles to the direction of movement of the LLDPE. The LLDPE was exposed to the electron beam by a stainless steel chain belt running 3 in. (76.2 mm) below the irradiation window. The LLDPE was fed to the conveyor by means of a vibrating feeder attached to the hopper. The width of the LLDPE stream fed was adjusted to range from 8 to 12 in (20.3-30.5 cm) depending on the desired production rate. The wiping width of the electron beam was set so that the entire width of the material became irradiated. The thickness of the layer of material was set so that the incoming and outgoing radiation dose with respect to the resin surface through which the irradiation takes place was made equal. The electron accelerator was operated at 2,000,000 V and jet currents from 100 to 250 mA, depending on the absorbed dose. The speed of the conveyor belt ranged from 10 in / min (0.423 cm / s) to about 500 in / min (21.15 cm / s) or higher. Different absorbed doses were obtained by combining different values of jet flow, wiping width and product flow rate, as well as substance-flow geometry. The width of the material stream with respect to the wiping width of the jet was set so that an absorption efficiency of 90% at the edges was achieved. The layer thickness in the material stream was set to achieve a depth efficiency of 90%. The actual layer thickness of the material stream was determined by the bulk density of the granular polymer and the requirement for equivalence of entry and exit doses. The maximum dose between entry and exit levels was approximately 40% higher than equivalent entry-exit doses. The average dose received for the entire product stream was 30% higher. The absorbed dosage shown in the embodiments is an entry-exit dosage. The conveyor belt and hopper were continuously purged with nitrogen gas.

Extrusion Procedure LLDPE film was prepared using the following procedures. The LLDPE was loaded into the extruder feed hopper and dropped by gravity into the plasticizing extruder feed zone. The LLDPE was transported, melted, pressurized, and pumped through a ring die as a substantially homogeneous melt.

In the manufacture of a blown membrane hose, the internal, air-generated pressure expands the bubble generated in the melt as expressed by the Blow up ratio (BUR) assigned to each run. The membrane is also cooled by means of air-cooled chillers and is drawn through receiving rollers which control the receiving speed and which close the flow of bubbles in the direction of travel. The film is then wound on a spool while sampling the product to be wound.

Application Examples 52-54 of the extrusion coating were carried out using a 2.5 in (63f5 mm) extruder with a cylinder length to diameter ratio of 27: 1 and a 24: 1 length to diameter polyethylene screw. The extruder had a 50 hp (36.75 kW) dynamic drive and the cylinder had 5 heating zones. The nozzle used was an end-fed 12 in (304.8 mm) semi-coathanger type nozzle with a final parallel flow orifice length of 3/4 in (19.0 mm) and a nozzle gap setting of approximately 20 mils (0.508 mm). ). The coating unit had a 28 in (71.12 cm) lamination station and included a payoff roll, a preheating drum, a cooling roll, and a winding roll. The extrusion speed was controlled by the extruder rotation speed settings. The coating thickness was adjusted by the line speed of the coating unit. Extruder cylinder temperature settings ranged from 500 to 600 ° F (260-316 ° C), typically about 600 ° F. LLDPE was extruded using the following extrusion coating conditions: substrate 40 Lb (18.16 kg) kraft paper; nozzle temperature 600 ° F (315.6 ° C); press roll pressure 100 lb / Lineal-in (175N / run-cm); rise in 0 in (0 mm); draw span 3 in (76.2 mm); coating speed as reported in ft / min (0.508 cm / s); coating thickness 1.5 mil (0.0381 mm); freeze roll water temperature 55 ° F (12.8 ° C); freezer roll water flow 90 gal / min (341 1 / min); and the cooled portion of the screw is neutral.

Definitions and test methods

The abbreviations used in the following application examples are based on the definitions given in the following table.

Label Description of meaning LLDPE A Linear, low density ethylene / l-butene copolymer with a melt index of 2 dg / min and a density of 0.918 g / cm 2 in granular form.

HDPE A High density polyethylene with a melt index of 0.2 dg / min. LLDPE B LLDPE A in pelleted form.

LLDPE C Linear, low density ethylene / l-butene copolymer with a melt index of 20 dg / min in granular form.

LLDPE D Linear, low density ethylene / l-butene copolymer with a melt index of 1 dg / min, in granular form.

MD Machine direction (in manufacture) TD Transverse direction (in manufacture)

The test procedures for theological and membrane properties were based on the following methods:

Impact strength with drop arrow: ASTM D-1709 (g / mil) (0.03939 g / μm)

Elmendorf tear strength: ASTM D-1922 (g / mil) (0.03939 g / μm) Film haze: ASTM D-1003-6 (% haze)

Flow Index (FI): ASTM-D1238 Condition Change Condition F (dg / min)

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23 81 1 09

Maximum production rate: measured as the maximum constant speed of the blown film hose that can be achieved with a given extruder without film supports other than guides and flattening frame and with a given nozzle (expressed as mass production per unit length of nozzle circumference).

Melt Flow Ratio (MFR): Flow index / melt index.

Melt Index (MI): ASTM D-1238 Condition Option E (dg / min).

Puncture resistance: ASTM D-1922 (in-lb / mil = 4450 kJ / m). Tensile Strength: ASTM D-1822 (ft-lb ^ / in)

Tensile Strength and Elongation: ASTM D-882 (%)

Gloss at 45 ° reflection angle: ASTM D-2457-70. Shear stress: measurements were made using capillary rheometry and measured as pressure drop between capillary ends multiplied by capillary diameter divided by four times its length (unit psi = 6895 NnT ^), Shear rate: based on the third power is multiplied by silicon, (unit s-1).

Measurements of both shear stress and shear rate were performed by procedures known in the art, for example, from Cogswell, Polymer Melt Rheology, Halstead Press, 1981, page 31.

Examination of Rheology (Examples 1-46) Examples 1-24: Tensile Viscosity Using the irradiation and Theological Test procedures described above, the tensile viscosities of unirradiated and irradiated (at different dosages) LLDPE A polyethylenes were studied. The tensile viscosities of HDPE A polyethylene were studied for comparison. The test results are shown in Tables 1-4. Figure 1 was tabulated as a result of linear extrapolation of the means of the values shown in Examples 1-24.

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Examples 25-32: Shear Viscosity Using the above irradiation procedures and Theological tests, shear stress and shear rate values were determined for granular and pelletized LLDPEs irradiated at different levels. The results are shown in Tables 5 and 6 for granular and pelleted forms, respectively. Figures 2 and 3 show linear combinations of these shear values using logarithmic coordinate scales.

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Performance Examples 33-35: Melt flow index measurements. Using the irradiation procedures and Theological experiments described above, three types of LLDPE were irradiated at varying dosages and the melt index and flow index values were measured with the results given in Table 7. A comparison between the granulated (Example 33) and the pelletized (Example 34) 4 , which plots the values of the melt and flow index (on a logarithmic scale) as a function of the radiation dose. The melt and flow indices of pelletized LLDPE fall significantly more than the values of the corresponding granulated LLDPE.

Table 7 Effect of irradiation dosage on melt and flow index values Irradiation-Melt-Flow-Melt-flow ^ ,. dosage index index ratio (Mrad) _ (dg / min) (dg / million) _

Example 33: LLDPE A (Granular, melt index 2) 0 1.97 50 25.4 0.25 1.27 45.5 35.8 0.5 1.1 42.2 38.4 1-0 0.7 40.5 57.9 1 * 5 0.53 32.8 61.9 2.0 0.31 28.7 92.6 2.5 0.28 24.2 86.4 3.0 0.18 20.3 112.8 3.5 0.16 18.5 115.6

Example 34: LLDPE B (Pelletized) 0 1.6 44.9 28.1 0.1 1.45 44.4 30.6 0.25 1.20 39.8 33.2 0.5 0.61 31. 50.8 1.0 0.16 15.8 99.

1.5 - 2.65

Example 35: LLDPE C (Granular, melt index 1) 0 0.91 23.9 26.3 0.2 0.61 23.4 38.4 0.4 0.49 21.6 44.1 0.5 0.46 20 43.5 33 81 1 09

Embodiment 35 shows the melt index and flow index values for an irradiated, higher molecular weight LLDPE, denoted LLDPE C, having a melt index prior to irradiation of approximately 1.0 dg / min. Compared with Example 33, LLDPE A having a melt index of 2.0, the percentage decrease in the melt index of higher molecular weight LLDPE C at the same dosage level is greater. Thus, higher molecular weight resins can provide significant improvements in rheological properties at lower radiation doses than can be achieved with lower molecular weight LLDPE.

Examples 36-46: Molecular weight changes due to irradiation of LLDPE.

Following the above irradiation procedures and standard molecular weight test methods, different LLDPE copolymers were subjected to different levels of irradiation and the molecular weight was measured, the results being listed in Table 8. The molecular weight of irradiated, granular LLDPE remained unchanged or unchanged. Conversely, the molecular weight of irradiated, pelletized LLDPE increases by a certain, clearly determinable amount. As with the melt and flow index values, the molecular weight values of the irradiated LLDPE show variation due to the granulation form of the resin to be irradiated.

34 81 1 09

Table 8 Effect of irradiation on molecular weight

Number average Weight average number molecular molecular weight.

Radiation- F -3 p 1Q3 dosage n Mwxi0

Example LLDPE Form (Mrad) (g / mol) (g / mol) 36 A Granulated 0 24 83 37 A 0.35 24.0 82.2 38 A 0.5 22.6 74.3 39 A 0.5 * 24.0 79.7 40 A 0.5b 23.0 78.0 41 B Pellet 0 21.9 78.9 42 B 0.5 24.7 93.6 43 B 1 25.7 104.8 44 C Granulated 0 15.2 47.9 45 C 0.5 15.7 47.8 46 C 2 15.1 53.5 * - using a layer thickness of 3/8 in (9.525 mm) during irradiation b - using a layer thickness 1/8 in (3.175 mm) during irradiation

II

35 81 1 09

Expansion of Blown Film (Examples 47-50) Examples 47-50: Film Extrusion These examples show studies to find the maximum production rate of a stable film, the results of which are shown in Table 9. In Examples 47-48, a 2.5 in (63.5 mm) screw extruder was used. either with or without additional melt pumping. The nozzle was 3 inches (76.2 mm) in diameter and used a single lip pneumatic ring. Air at room temperature was used for cooling in these examples. The maximum stable velocity was determined after the membrane had run continuously for at least 20 min without ripple, membrane bubble diameter variations, and other large-scale irregularities. The study was performed using unalloyed LLDPE.

In Example 47, it was found that the use of a mixture containing 10% of 0.5 billion irradiated LLDPE D improved the production rate by 50% over the rate achievable with non-irradiated LLDPE D. In all examples, the rate was exceptionally improved with the use of mixtures containing relatively little, i.e. 10-25%, irradiated components, and much more than could have been predicted from the linear propagation ratio.

Examples 48 and 49 illustrate that exceptional production rates of the type described can be achieved using mixtures of different LLDPE species. Both LLDPE copolymers with a melt index of 1 dg / min and 2 mg / min showed a significant improvement with the addition of irradiated LLDPE with a melt index of 1 dg / min.

The results of Embodiment 49 illustrate that higher production rates can be achieved using irradiated LLDPE grades in mixtures that have been subjected to higher radiation doses 36 81 1 09 than using 100% LLDPE irradiated at a lower dose.

Embodiment 50 using a 4.5 in (114.3 mm) extruder shows maximum stable production rates, but the extrusion conditions differ from the previous examples. The maximum velocities were limited by the uniformity of the die flow and the distribution of the freezing air, where the irregularities led to the asymmetry of the membrane bubble. Such maximum production rate limitations are similar to those that occur when low density LCB polyethylenes are used under similar conditions. This suggests that production rates comparable to low density LCB copolymers can be achieved using low level irradiated LLDPE. The production rates achieved in this embodiment represent up to a 57% rate increase for a blend containing 19% at a dosage of 0.4 billion irradiated copolymers compared to unirradiated LLDPE.

Examination of the properties of the film: Example 51: The properties of the film prepared in Example 50 were examined by the test procedures shown in Table 10 and its results.

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39 81 1 09

Extrusion Coating (Examples 52-54): These examples illustrate elevated production rates using low level irradiated LLDPE for extrusion coating and applying the testing procedures described previously. Extrusion speeds of 36 lb / h (16/14 kg / h) or approximately 3 lb / h (1.36 kg / h-2.54 cm) using a 12 in (30.48 cm) slot die produced the highest travel speeds and reduction values ("neck-in values"), as shown in the table.

Table 11

Extrusion Coating Results Radiation- Maximum Dosing Driving Speed, (ft / min) Neck-in (in)

Eg LLDPE (Billion) (x0.508 cm / s) (x25.4 mm) 52 A 0 85 3 1/2 0 120 3 3 / 4a 53 B 0.5 85 2 3/4 0.5 120 2 3 / 4b 0.5 160 _a 54 B 0.25 85 2 5/8 0.25 120 2 3/4 0.25 180 _a a = unstable b = slightly edge ripple

The results show up to a 30% improvement in neck-in speed with irradiated samples, and up to a 50% increase in maximum running speed before draw resonance occurred, compared to the non-irradiated control.

Claims (36)

40 81 1 09 A well-suited polyethylene blend comprising a substantially linear, narrow molecular weight distribution and low density crosslinked copolymer of ethylene and one or more alpha-olefins, characterized in that the copolymer is substantially non-gelling and has a high degree of crosslinking to provide sufficient crosslinking. shear viscosity at high shear rates compared to the corresponding non-crosslinked polyethylene. Polyethylene blend according to claim 1, characterized in that the copolymer has an increased tensile viscosity stretching at a rate of 0.1, -1 s-1 and a stretch of 1 at a temperature of 150 ° C which is more than about 20% higher than the corresponding tensile viscosity of the non-crosslinked form of the copolymer , and a high shear shear viscosity at a shear rate of 200 s-1 at a temperature of 210 ° C that is less than about 20% greater than the corresponding high shear shear viscosity of the non-crosslinked form of the copolymer. Polyethylene blend according to claim 2, characterized in that it has a tensile viscosity of more than about 50% and a high shear shear viscosity of less than about 10% higher than the corresponding values of the non-crosslinked form. Polyethylene blend according to claim 2, characterized in that it has a tensile viscosity of more than about 100% and a high shear rate of less than about 5% higher than the corresponding values of the non-crosslinked form. Polyethylene blend according to claim 1, characterized in that the copolymer has a narrow molecular weight distribution of between one and about 10. «81109 The polyethylene blend of claim 1, characterized in that the copolymer has a gel content of less than about 1% by weight of the polymer. Polyethylene blend according to claim 6, characterized in that the gel content of the copolymer is less than about 0.1% by weight of the polymer. Polyethylene blend according to Claim 1, characterized in that the alpha-olefin comonomers have from four to eight carbon atoms. Polyethylene blend according to Claim 1, characterized in that the alpha-olefin comonomer is one or more compounds selected from the group consisting of 1-butene, 4-methyl-1-pentene, 1-hexene, 1-heptene and 1-octene. Polyethylene blend according to one of the preceding claims 1 to 9, characterized in that it further comprises a linear, narrow-molecular-weight and low-density copolymer which does not crosslink ethylene and one or more alpha-olefins. Polyethylene blend according to claim 10, characterized in that the amount of said crosslinked copolymer is between about one and about 95% by weight of the blend. Polyethylene blend according to claim 10, characterized in that the amount of said crosslinked copolymer is from about 5 to 30% by weight of the blend. Polyethylene blend according to claim 10, characterized in that the amount of said crosslinked copolymer is between about 10 and about 25% by weight of the blend. 42 81 1 09 Plastic film, characterized in that it consists of a polyethylene mixture according to one of the preceding claims 1 to 13. A method of improving the well-forming properties of a polyethylene blend comprising a linear, narrow molecular weight distribution and low density copolymer of ethylene and one or more alpha-olefins by irradiation, characterized in that the mixture is irradiated until the copolymer is exposed to a low level and absorbed radiation in an amount insufficient to provide significant gelation but sufficient to crosslink the copolymer to provide higher tensile viscosity and substantially equal shear viscosity at high shear rates compared to the corresponding non-crosslinked polyethylene. The method of claim 15, characterized in that the amount of absorbed radiation is between about 0.05 and about 2 Mrd (0.5-20 kGy). The method of claim 15, characterized in that the amount of absorbed radiation ranges from about 0.1 to about 1 Mrd (1-10 kGy). The method of claim 15, characterized in that the amount of absorbed radiation is between about 0.25 and about 0.5 Mrd (2.5-5 kGy). Process according to Claim 15, characterized in that the copolymer is irradiated as a granular solid or in molten form. Process according to Claim 19, characterized in that the copolymer is irradiated in granular form. Process according to Claim 19, characterized in that the copolymer is irradiated in the form of pellets. 43 81 1 09 22. A method of making a plastic film from a polyethylene blend, comprising extruding and stretching ethylene and one or more alpha-olefins of a substantially linear, low molecular weight distribution and low density crosslinked copolymer, characterized in that the method comprises (a) ethylene and irradiating the linear, narrow molecular weight distribution and low density copolymer of one or more alpha-olefins with a low level of ionizing radiation in an amount insufficient to provide significant gelation but sufficient to provide crosslinking rates with greater shear viscosity and substantially equal tensile viscosity and substantially equal to the corresponding non-crosslinked polyethylene followed by irradiation (b) extrusion of the irradiated copolymer melt into a film, and (c) stretching the obtained film. A method according to claim 22, characterized in that the amount of absorbed radiation is sufficient to increase the tensile viscosity of the copolymer but below a level which would cause a significant increase in the high shear rate shear viscosity therein. A method according to claim 22, characterized in that the amount of absorbed radiation is below a level which provides a gel content of 1% or more by weight of the copolymer in the copolymer. The method of claim 22, characterized in that the amount of absorbed radiation ranges from about 0.05 to about 2 Mrd (0.5-20 kGy). «81109 The method of claim 22, characterized in that the amount of absorbed radiation ranges from about 0.1 to about 1 Mrd (1-10 kGy). The method of claim 22, characterized in that the amount of absorbed radiation ranges from about 0.2 to about 0.5 Mrd (2-5 kGy). Process according to Claim 22, characterized in that the irradiation of the copolymer takes place at any stage between polymerization and extrusion. Process according to Claim 22, characterized in that the copolymer is irradiated either as a granular solid or as a melt. Process according to Claim 29, characterized in that the copolymer is irradiated in the form of granules. Process according to Claim 29, characterized in that the copolymer is irradiated in the form of pellets. A method according to claim 22, characterized in that the film is extruded in step (b) as a film hose, and the tubular film is stretched in step (c) by expanding the blown film bubble. A method according to claim 22, characterized in that the film extrusion in step (b) is (1) slit die casting to form a planar film, or (2) an extrusion coating to form a film coating on a substrate. The method of claim 22, further comprising mixing the non-crosslinked, linear, low density, narrow molecular weight ethylene / alpha-olefin copolymer with the irradiated copolymer in step (a) prior to extrusion in step (b). A method according to claim 22, characterized in that in step (a) only a part of the copolymer is irradiated, whereby a certain ratio is formed between the irradiated and the non-crosslinked copolymer. A plastic film prepared by the method of claim 22, characterized in that it is prepared by steps comprising (a) irradiating a linear, narrow molecular weight distribution and low density copolymer of ethylene and one or more alpha-olefins with low level ionizing radiation in an amount equal to: insufficient to provide significant gelation but sufficient to provide cross-links to provide higher tensile viscosity and substantially equal shear viscosity at high shear rates compared to the corresponding non-cross-linked polyethylene followed by irradiation (b) extrusion of the irradiated copolymer stretch. 46 81 1 09