FI76955C - FOERFARANDE FOER ATT MINSKA YTANS SMAELTFRAKTUR VID STRAENGSPRUTNING AV ETYLENPOLYMERER. - Google Patents

Description

76955

Method for reducing surface melt fracture in extrusion of ethylene polymers For the preparation of ethylene polymers by extraction with ethylene polymers

Object of the invention

The invention relates to a process for reducing melt fracture, in particular surface melt fracture, in the extrusion of a molten linear ethylene copolymer having a narrow molecular weight distribution under flow rate and melting temperature conditions that would otherwise occur.

Background of the invention

Most commercial, low density, i.e., LD polyethylenes are polymerized in thick-walled autoclaves or tubular reactors at pressures up to 50,000 psi (about 345 MPa) and temperatures up to 300 ° C »The molecular structure of high pressure LD polyethylene is very complex. The changes in the order of its simple building blocks are essentially infinite. High pressure resins are characterized by a confusing molecular structure of long branch chains. These long branch chains have a dramatic effect on the melting rheology of resins. High pressure LD polyethylene resins also have a spectrum of short branch chains, usually 1 to 6 carbon atoms in length, which controls the crystallinity (density) of the resin. The frequency distribution of these short branch chains is such that, on average, most chains have an average number of branches. The branching distribution of short chains characteristic of high pressure LD polyethylene can be considered narrow.

LD polyethylene can exhibit numerous properties. It is flexible and has a good balance of mechanical properties such as tensile strength, impact strength, puncture resistance and tear strength 2 76955. In addition, it maintains its resistance to relatively low temperatures. Certain resins are not yet brittle at -70 ° C. LD polyethylene has good chemical resistance and is relatively inert to acids, bases and inorganic solutions. However, it is sensitive to hydrocarbons, halogenated hydrocarbons, and oils and fats. LD polyethylene has excellent dielectric strength.

More than 50% of all LD polyethylene is made into a film. This film is primarily used in packaging applications such as meat, processed products, frozen foods, ice bags, cooking bags, textile and paper products, shelving, industrial seals, ship bags, pallet surfaces and shrink wraps. Large quantities of wide, thick film are used in construction and agriculture.

Most of the LD polyethylene film is produced by the tube blow molding extrusion method. Film products made by this method range in size from tubes of about 2 inches (about 5.08 cm) or less in diameter and used as hoses or bags to huge bubbles that provide a flat space of up to about 20 feet (about. 6.1 m) wide, and which, when cut along the edge and opened, is up to about 40 feet (about 12.2 m) wide.

Polyethylene can also be prepared from low to medium pressures by homopolymerizing ethylene or copolymerizing ethylene with various o (-olefins) using heterogeneous catalysts based on the transition from different valences of short valances. The branch length is controlled by the concentration of the mixed monomer (s) The frequency distribution of the branch is affected by the nature of the transition metal catalyst 3 Linear LD polyethylene can also be produced by high pressure techniques well known in the art.

U.S. Patent No. 4,302,566 to FJ Karol et al., Entitled "Preparation of Ethylene Copolymers in a Layer Layer Reactor," states that ethylene copolymers having a density of 0.91 to 0.96 have a higher flow index to melt index ratio, or equal to 22 and less than or equal to 32 and a relatively low residual catalyst content can be prepared in granular form with relatively high productivity if the monomer (s) are copolymerized in a gas phase process with a specific high activity Mg-Ti complex catalyst mixed with an inert support .

U.S. Patent No. 4,302,565 to GL Goeke et al., Entitled "Impregnated Polymerization Catalyst, Process for Its Preparation, and Use in Co-Polymerization of Ethylene," states that ethylene copolymers having a density of 0.91 to 0.96 have a flow index. and a melt index ratio greater than or equal to 22 and less than or equal to 32 and a relatively low residual catalyst content can be prepared in granular form with relatively high productivity if the monomer (s) are copolymerized in a gas phase process with a particular high activity Mg-Ti complex catalyst which is impregnated in a porous inert carrier.

Polymers produced, for example, by the processes of said applications using a complex catalyst containing Mg-Ti have a narrow molecular weight distribution, Mw / Mn, greater than or equal to 2.7 or less than or equal to 4.1.

76955 LD polyethylene; rheology

The rheology of polymeric substances depends to a large extent on the molecular weight and molecular weight distribution.

In membrane extrusion, two aspects of theological behavior are important: shear stress and elongation. Inside the membrane extruder and die, the polymer melt undergoes strong shear deformations. As the extruder feed screw pumps the melt into and through the diaphragm nozzle, the melt experiences a wide range of shear rates. Most film extrusion processes are thought to subject the melt to shear at speeds of 100 to 5000 s-1. Polymer melts are known to indicate what is commonly referred to as shear thinning behavior, i.e., non-Newtonian flow behavior. As the shear rate increases, the viscosity (ratio of shear stress, X, to shear rate, .λ) decreases. The amount of viscosity decrease depends on the molecular weight, its distribution, and the molecular configuration, i.e., the branching of the long chains of the polymeric material. Branching of short chains has little effect on shear viscosity. In general, high pressure LD polyethylenes have a broad molecular weight distribution and exhibit increased shear thinning behavior at shear rates common in film extrusion. The narrow molecular weight resins used in this invention show reduced shear thinning behavior in extrusion at increasing shear rates. The consequences of these differences are that the narrow distribution resins used in this invention require more power and develop higher pressures during extrusion than high pressure LD polyethylene resins with a broad molecular distribution and corresponding average molecular weight.

The rheology of polymeric materials is usually studied in shear deformation. In a simple cut, the velocity gradient of the deformable resin is perpendicular to the direction of flow. The shape of the deformation is experimentally suitable, 5 76955 but does not provide essential information for understanding the sensitivity of the material in film making processes. The shear viscosity can be determined by the shear stress and the shear rate, i.e.: shear = T-12 / λ where shear = shear viscosity (off) * T 12 = shear stress (dyn / cm ^) X = shear rate (s- ^) determines normal tension and stretching rate, i.e.: \\ stretch = 'u / £ stretch = tensile viscosity (off)' n '= normal tension (dyn / cm ^) £. = stretching speed (s ~ l)

In the extrusion of a narrow molecular weight and high molecular weight ethylene polymer through the extruders, a "melt fracture" occurs, as with other similar polymeric materials, when the extrusion rate exceeds a certain critical value. "Melt fracture" is a common term used in the polymer industry to describe the variations in extrudate irregularities observed in the extrusion of molten polymers. The occurrence of melt fracture severely limits the rate at which acceptable products can be made under commercial conditions. The occurrence of melt fracture was first described by Nason in 1945 and since then several researchers have studied this in an effort to understand the mechanism (s) underlying its occurrence. C. J. S. Petrie and M. M. Denn (American Institute of Chemical Engineers Journal, Vol. 22, p.

209 ... 236, 1976) have presented a critical literature review showing that the current understanding of the mechanism (s) leading to melt fracture is far from complete.

6 76955

The melt fracture characteristic of a molten polymer is usually examined using capillary rheometry. The polymer is forced at a given temperature through a capillary nozzle of known dimensions at a given flow rate. The required pressure is recorded and the surface properties of the penetrating extrudate are examined.

The surface properties of the extrudates of linear LD polyethylene (LLDPE) resins, as determined by a capillary rheometer, are typical of many linear polymers with a narrow distribution. They show that at low shear stresses (less than about 20 psi, about 138 kPa) the extrudates passing through the capillary die are smooth and shiny. At critical shear stresses (approx.

20 .. .22 psi, approx. 138 ... 152 kPa) extrudates indicate loss of surface gloss. The loss of gloss is due to the fine roughness of the surface of the extrudate, which can be observed under a microscope at medium magnification (20 ... 40X). These conditions represent the "onset" of surface irregularities that occur in the die at a critical shear stress. Above the critical stress, two main types of extrusion melt fracture can be detected with LLDPE resins: surface melt fracture and total melt fracture. The melt fracture of the surface occurs in the shear stress range n.

10 .. .65 psi (approx. 68.9 ... 448.2 kPa), and results in an increase in the degree of surface roughness. At its most severe, it occurs as a "hainnah chicken." Surface irregularities occur under apparently stable flow conditions. Thus, no variation in pressure or flow rate was observed. At a shear stress of about 65 psi (about 448 kPa), the flow becomes unstable when both the pressure and the flow rate vary between the two extremes and the outgoing extrudate has a flat and rough surface, respectively. This is the beginning of a total melting fracture and has been the subject of intense research by several scientists due to its severity. With increasing shear stress, the extrudates become completely twisted and show no regularity.

7 76955

Several mechanisms for the occurrence of surface and total melt fracture have been reported in the literature. The melt fracture of a shark skin type surface has been shown to be due to the effects at the nozzle outlet where the viscoelastic melt is exposed to high local stresses as it detaches from the nozzle surface. This results in a cyclic structure and the release of surface forces at the nozzle outlet, resulting in a noticeable surface melt fracture. Another mechanism for surface melt fracture suggests differential recovery due to elasticity as the primary cause between the surface of the extrudate and the core. On the other hand, the total melt fracture has been suggested to be due to the effects of the nozzle flow orifice and / or the nozzle inlet. Proposed mechanisms include: "slip-stick" in the flow orifice region of the nozzle; rupture of the melt in the nozzle inlet region due to exceeding the melting strength; and an increase in helical flow instabilities in the nozzle inlet region.

Under commercial film manufacturing conditions (shear stress between about 25 ... 65 psi, about 172 ... 448 kPa), conventional blow film nozzles exhibit a predominantly shark skin type surface melt fracture with LLDPE resins, resulting in commercially unacceptable products.

There are several methods for eliminating surface melt fracture under commercial film making conditions. They are aimed at reducing the shear stress in the die and include: raising the melting temperature; modifying the geometry of the nozzle; and the use of lubricating additives in the resins to reduce friction on the wall. Raising the melting temperature is not commercially viable because it reduces the rate of film formation due to bubble instabilities and heat transfer limitations. Another method for eliminating surface melt fracture is described in U.S. Patent No. 3,920,782. However, this method is difficult to use and control.

The invention of U.S. Patent No. 3,920,782 is apparently based on the inventor's conclusion that the onset of surface melt fracture under his working conditions on his resins is a function of essentially exceeding a critical linear velocity on his resins through his nozzles at his working temperatures. However, in the method of the present invention, the onset of surface melt fracture on the applicant's resins under his working conditions is primarily a function of exceeding the critical shear stress.

U.S. Patent No. 3,382,535 discloses a means of designing dies for use in high speed extrusion coating of wires and cables with plastics such as polypropylene, HD and LD polyethylene together with copolymers which are susceptible or sensitive to the die extruder. The nozzles of this patent are designed to avoid the total melt fracture of the plastic coating of the extruded conductor that occurs at significantly higher stresses than the surface melt fracture during film formation.

The invention of U.S. Patent No. 3,382,535 is for designing cone angles of a nozzle inlet so as to provide a curved nozzle configuration (Figures 6 and 7 of the patent) that tapers in the flow direction of the resin. However, this method, in fact, by reducing the cone angle of the die, causes an increase in the critical shear rate through the die in the processed resin. This reduces overall distortions only as a function of the inlet angle to and / or into the nozzle. The surface melt fracture is insensitive to the cone angles of the nozzle inlet, and the present invention relates to reducing the surface melt fracture as a function of the building materials in the nozzle flow orifice region, including the nozzle outlet.

U.S. Patent No. 3,879,507 discloses a means of reducing melt fracture when extruding a foamable mixture into a film or sheet. This method involves increasing the length of the orifice flow orifices and / or slightly tapering the orifice while maintaining or decreasing the orifice, which must obviously be relatively narrow compared to the prior art (see Section 4, lines 2 to 6) and on the order of 0.025 inches, or 25 mils. (approx. 0.64 mm) (Chapter 5, line 10). A melt fracture like this is caused by premature bubble formation on the surface. However, this melt fracture is completely different from the melt fracture that occurs when processing LLDPE resins for film formation. In other words, the melt fracture is not caused by the Theological properties discussed here. Nozzle modifications are designed to reduce the shear stress in the nozzle flow orifice region below the critical stress level (about 20 psi, about 138 kPa) either by widening the nozzle orifice (U.S. Patent Nos. 4,243,619 and 4,282,177) or by significantly heating the nozzle edge to temperatures. The widening of the die orifice results in thick extrudates, the cross-section of which must be reduced by stretching and which must be cooled in the film-blowing process. While LLDPE resins have excellent cross-sectional reduction properties, in thick extrudates the molecular orientation increases in the machining direction and results in a direction-dependent imbalance and a decrease in critical film properties such as tear strength. Thick extrudates also limit the efficiency of conventional bubble cooling systems, resulting in lower speeds in stable operation.

Wide-aperture technology also has other disadvantages. The required orifice is a function of the extrusion rate, the melt index of the resin, and the melting temperature. The wide aperture configuration is not suitable for processing conventional low density polyethylene (HP-LDPE) resins. Thus, changes in the orifice of the nozzle are required to accommodate the flexibility expected by a manufacturer with such a line.

The heated edge concept is intended to reduce stresses at the nozzle outlet and includes extensive modifications that require effective insulation of the hot edges from the rest of the nozzle and pneumatic ring.

U.S. Patent No. 3,125,547 describes a polyolefin blend comprising the addition of a fluorocarbon polymer, which provides improved extrusion properties and non-melt-breaking extrudates at high extrusion rates. This is based on the inventor's conclusion that the slip-adhesion phenomenon at high extrusion speeds and the resulting herringbone pattern on the surface of the extrudate are due to poor lubrication in the die mouth. The use of a fluorocarbon polymer is intended to promote lubrication and reduce the stresses associated with it in order to obtain melt-unbreakable extrudates. However, the present invention is based on the exact opposite argument, in which the lack of adhesion rather than the lack of lubrication at the polymer / metal-li interface in the die orifice region is to blame for both surface and total melt fracture with LLDPE resins. The present invention is thus directed to improving adhesion at an interface with the right choice of nozzle building material in the flow orifice region, including an orifice outlet, and the use of an adhesion promoter in the resins to achieve a reduction in melt fracture in the extrudates. The practice of U.S. Patent No. 3,125,547 greatly reduces stresses through nozzles constructed of conventional materials, which apparently suggests that the change in the theological properties of the polyolefin resin is due to the presence of a fluorocarbon polymer. In the method of the present invention, which comprises a building material of a different orifice flow orifice region, a reduction in melt fracture is achieved without significantly affecting the properties of the resin.

U.S. Patent No. 4,762,848 to 11,765,955 describes the use of polyvinyl octadecyl ether as a processing transducer to provide a smoother extrudate surface and improved film properties with HD polyethylene resins. However, this additive has been found to be unsuitable for reducing melt fracture with LLDPE resins.

Additives used as processing aids to reduce melt fracture in extrudates are expensive and the additional cost, depending on the concentration required, may be a barrier to resins such as granular LLDPE for consumer goods applications. Additives affect the rheological properties of the base resin and in excess can adversely affect critical film properties such as gloss, transparency, adhesion and heat seal properties of the product.

More specific objects of the present invention appear from the appended claims, wherein in the process of the invention melt fracture, especially surface melt fracture, can be substantially reduced by adding an adhesion promoter to the polymer, extruding a narrow molecular weight distribution surfaces terminating in a nozzle orifice, wherein at least one and preferably both opposing surfaces of the nozzle flow orifices, preferably including the nozzle outlet, are made of stainless steel by providing at least one and preferably two stainless steel surfaces in contact with the melt adhesive, the ratio of the length to the width of the orifice of the nozzle is about 35: 1 ... 60: 1, preferably about 45: 1 ... 55: 1. The benefit of the present invention arises from the finding, 76955, that the primary mechanism for initiating melt fracture in LLDPE resins is the onset of polymer melt sliding on the die wall. The slip is caused by the collapse of the adhesion at the polymer / metal interface under flow conditions and occurs at critical shear stress. Adhesion is a surface phenomenon that is strongly dependent on the nature of the surfaces and the proximity of the surfaces in contact. Thus, procedures that provide good adhesion at the wall interface of the flowing polymer / mouthpiece result in a reduction in surface melt fracture with LLDPE resins. Improvements in adhesion can be achieved either by choosing the right resin nozzle building material on the right or by using adhesion promoters in the resin formulation of the given substance, or by a right combination of both. According to the invention, it has been found that the use of a stainless steel surface in the nozzle flow orifice region, together with the use of a large nozzle orifice length and adhesion promoter in the resin, reduces melt fracture when making films from LLDPE resins at narrow nozzle orifices at commercial speeds.

In the case where only one of the opposite surfaces of the nozzle flow opening is constructed of stainless steel, the melt fracture of the surface is reduced or eliminated at the boundary between the polymer surface and the stainless steel surface. If both opposite surfaces of the nozzle flow port are constructed of stainless steel, then both surfaces of the polymer would have a reduced melt fracture.

Films suitable for packaging applications must demonstrate a balance of key properties for broad end-use benefit and broad commercial acceptance. These properties include the optical quality of the film, such as turbidity, gloss and transparency properties. Mechanical resistance properties such as puncture resistance, tensile strength, impact strength, stiffness and tear strength are important. Vapor permeability and gas permeability properties are important considerations in the packaging of perishable goods. The behavior of the film in conversion and packaging equipment is influenced by the properties of the film, such as coefficient of friction, adhesion, heat sealability and bending resistance. LD polyethylene has a wide range of applications, such as food and non-food packaging applications. Typically, sacks made of LD polyethylene include ship sacks, woven sacks, laundry and dry cleaning sacks, and waste sacks. LD polyethylene film can be used in drum seals for numerous liquid and solid chemicals and as a protective lining inside wooden louvre boxes. LD polyethylene film can be used in a variety of agricultural and horticultural applications, such as protecting seedlings and shrubs such as straw cover, storing fruits and vegetables. In addition, the LD polyethylene film can be used in construction applications, such as as a moisture or vapor barrier. Furthermore, the LD polyethylene film can be coated and printed on for use in newspapers, books, etc.

Demonstrating a unique combination of the above properties, high pressure LD polyethylene is the most important of the thermoplastic packaging films. Its use is about 50% of the total use of such films in packaging. Films made of the polymers of this invention, primarily ethylene-hydrocarbon blended polymers, provide an improved combination of end-use properties and are particularly suitable for many applications where high pressure LD polyethylene is already used.

An improvement in any property of a particular film, such as eliminating or reducing surface melt fracture or improving the extrusion properties of the resin, or an improvement in the film extrusion process itself, is of paramount importance in accepting this film to replace high pressure LD polyethylene in many end applications.

Drawings

Figure 1 shows a cross-section of a circular ring of a helical / lattice-like nozzle.

14,769 5 5

Figure 2 shows a cross-section of a helical nozzle.

Figure 3 shows the structure of the flow opening area of the nozzle, in which the opposite stainless steel surfaces are arranged with recesses made of stainless steel.

Figure 4 shows the structure of the nozzle flow orifice area, in which the opposite stainless steel surfaces are arranged with a stainless steel collar and a stainless steel mandrel which are constructed of stainless steel throughout.

According to the invention, there is provided a method of reducing surface melt fracture in the extrusion of a molten linear ethylene polymer having a narrow molecular weight distribution at flow rate and melting conditions under conditions in which to the surfaces terminating in the orifice, the at least one of said opposite surfaces being made of stainless steel so as to obtain at least one stainless steel surface in the vicinity of the polymer, and wherein the ratio of the length of the orifice to the width of the orifice is about 35: 1 to 60: 1 and preferably n 45: 1 to 55: 1, which reduces the melt fracture on the surface of the polymer adjacent to said stainless steel surface.

Preferably, both opposite surfaces are made of stainless steel.

Description of a Preferred Embodiment of Nozzles Nozzles

Preferably, the molten ethylene polymer can be extruded through a die such as a helical annular die, a slit 76955 die, etc., more preferably an annular die having a narrow die orifice greater than about 5 mils (about 0.13 mm) and more preferably 5 ... 40 mils (ca. 0.13 ... 1.0 mm). Preferably, in processing LLDPE resins, it is no longer required to extrude the molten ethylene polymer through a die having an orifice greater than about 50 mils (about 1.27 mm) and less than about 120 mils (about 3.05 mm). as disclosed in U.S. Patent 4,243,619. Conventionally, the structure of the orifice flow orifices has been largely based on materials made of nickel or hard chrome plated steel.

Figure 1 is a cross-sectional view of a helical / lattice annular die 10 through which a molten thermoplastic ethylene polymer is extruded into a single layer film, tube or tube. The die body 12 includes channels 14 that lead to the outlet of the polymer die. When the molten thermoplastic ethylene polymer is extruded, it spreads as it passes through the die channels 14.

Referring to Fig. 2, a cross-section of a threaded nozzle is shown a threaded portion J, a flow inlet portion H, and a nozzle flow port G. Referring to Figs. 1 and 2, the nozzle outlet has a nozzle outlet generally indicated by reference numeral 16. Off is limited to a nozzle outlet 18 formed by opposite surfaces of the nozzle edges 20 and 20 'extending to the nozzle flow orifice surfaces 22 and 22'.

As shown in Figures 3 and 4, the flow orifice region of the nozzle is of such a construction that the opposing surfaces are made of stainless steel as opposed to conventional nickel or chrome plated steel. The surfaces can be arranged with interposable insert pieces 24 which are firmly attached to the spindle and collar. The inserts may be removably attached to the 16 76955 mandrel and collar in any suitable manner, such as by arranging the threaded portions internally for inserts that threadably connect the threaded portions in associated engagement with the surface of the corresponding mandrel or collar, the length of the inserts being preferably n.

45 ... 55 times the width of the opening between opposite surfaces. Other procedures for arranging the stainless steel surface can be utilized, such as by making the entire mandrel and collar from stainless steel, as shown in Figure 4.

The adhesion promoter for the stainless steel surface can be incorporated into the film-forming resin mixture as one additive, and conventional adhesion promoters such as compounds containing amines, esters and methacrylates can be used. A preferred adhesion promoter for use in this invention is a diethylated tertiary amine of a fatty acid (commercially available as Kemamine AS 990, Witco Chemical Corporation, Memphis, Tennessee). The amount of adhesion promoter used generally ranges from about 50 to 3000 ppm, preferably from about 300 to 800 ppm.

When a diethylated tertiary amine of a fatty acid is used, the recommended amounts are between about 50 and 1500 ppm and preferably about 800 ppm.

Preferably, the adhesion promoter may be part of a base charge containing antistatic and other process aids added to the film-forming resin.

The melt fracture is reduced at the boundary between the polymer surface and the stainless steel surface. As a result, the method can be used with the invention disclosed in U.S. Patent 4,348,349, issued September 7, 1982. Preferably, therefore, the melt fracture can be reduced on one side of the film by passing the molten polymer through a die orifice region where only the film surface the melt fracture to be reduced or eliminated is in contact with the stainless steel surface. This allows the production of multilayer films in which one layer is formed of LLDPE and the other layer is formed of a resin which is not susceptible to melt fracture under the conditions of use. Thus, by the method of the present invention, the LLDPE resin can be passed through a die in contact with a stainless steel surface, while a resin not susceptible to melt fracture is extruded in contact with another surface of the die flow orifice to produce a multilayer film with reduced melt fracture on both outer surfaces.

As previously mentioned, the surface of the die orifice region adjacent to the polymer is made of stainless steel. Several other types of materials were tested in an attempt to reduce melt fracture. These materials included: conventional chrome-plated steel; titanium nitride coated steel; pure copper; zinc coated steel; Beryllium copper; carbon steel (4140); and nickel steel (4340). In the presence of an adhesion promoter, none of the above surfaces proved to be as effective in reducing melt fracture even on a long stainless steel surface of the flow opening length.

Diaphragm Extrusion I. Blow Diaphragm Extrusion

Films formed as described herein can be extruded by a tube blow molding extrusion method. In this method, a polymer with a narrow molecular weight distribution is extruded through an extruder. This extruder may have an extruder feed screw having a length to diameter ratio of 15: 1 to 21: 1, described in U.S. Patent 4,343,755 to John C. Miller et al., Entitled "Method for Extruding Ethylene Polymers." This application describes that this extruder feed screw includes feed, modification and measuring parts. Alternatively, the nozzle 76955 press feed screw may include a mixing section, such as that described in U.S. Patent Nos. 3,486,192; 3,730,492 and 3,756,574, which are incorporated herein by reference. Preferably, the mixing section is placed at the end of the feed screw.

An extruder that can be used in this context can have a length to diameter ratio of 18: 1 to 32: 1. The extruder feed screw used in the present invention may have a length to diameter ratio of 15: 1 to 32: 1. For example, when an extruder feed screw having a length to diameter ratio of 18: 1 is used in a 24: 1 extruder, the remaining space in the extruder cylinder can be partially filled with various plugs, torpedoes, or static mixers to reduce the residence time of the polymer melt.

The extruder feed screw may also be of the type described in U.S. Patent No. 4,329,313. The molten polymer is then extruded through the die, as shown below.

The polymer is extruded at a temperature of about 165-260 ° C. The polymer is extruded upwards in a perpendicular direction in tubular form, although it can be extruded downwards or even laterally. After the polymer is extruded through an annular die, the tubular film is applied to the desired extent, cooled or allowed to cool and flattened. The tubular film is flattened by passing the film through a collapsible frame and a series of stretch rollers. These stretch rollers are used to form means for withdrawing the tubular film from the annular die.

Inside the tubular bubble, an overpressure of a gas, such as air or nitrogen, is maintained. As is known from conventional film making methods, the gas pressure is adjusted to impart the desired degree of expansion to the tubular film. The degree of expansion, measured as the ratio of the circumference of the fully expanded tube to the circumference of the nozzle ring, is between 1: 1 ... 6: 1 and preferably 1: 1 ... 4: 1. The tubular extrudate is cooled by conventional methods such as air cooling, water cooling or in a molding cylinder.

The stretch thinning properties of the polymers used herein are excellent. Draw-down, defined as the ratio of the orifice to the product of the film thickness and the blow ratio, is considered to be less than about 250. Very thin films can be made from these polymers with high stretch thinning even when said polymer is highly contaminated with foreign particles and / or gel. Thin films of about 0.5 to 3.0 mils (about 0.0127 to 0.076 mm) can be made to produce critical MD elongations greater than about 400 to 700% and TD greater than n 500 ... 700%. In addition, these films are not considered "cracking". "Splittiness" is a qualitative term that describes the notched tear resistance of a film at high strain rates. "Crackability" reflects the rate of crack propagation. It is the final property of use of certain types of films and is not well understood in its basic aspects.

As the polymer exits the circular die, the die-cap is cooled and its temperature drops below its melting point and solidifies. The optical properties of the extrudate change as crystallization occurs and a frost line is formed. The location of this crystallization limit above the circular nozzle is a measure of the cooling rate of the film. This cooling rate has a very obvious effect on the optical properties of the film prepared here.

The ethylene polymer can also be extruded into a rod-like or other solid cross-section using the same die geometry only on the outer surface. In addition, the ethylene polymer can also be extruded into a tube through circular dies.

20 76955 II. Slit extrusion of the film

Films formed as disclosed herein can also be extruded by slit extrusion of the film. This method of film extrusion is well known and comprises extruding a molten polymer sheet through a slit die and then cooling the extrudate using, for example, a cooling roller or a water bath. The nozzle is shown below. In the cooling roll method, the film can be extruded horizontally and placed on top of the cooling roll, or it can be extruded down and pulled under the cooling roll. The cooling rates of the extrudate in the cooling roll process are very high. The cooling roll or water bath cooling is so rapid that the extrudate cools below its melting point, the crystals form very rapidly, the supramolecular structures have little time to grow, and the spherolites are considered very small in size. The optical properties of the slit extruded film have been vastly improved compared to films using a slower cooling rate in the tube blown film extrusion process.

The mixture temperatures in the film slit extrusion process generally rise much higher than those in the tube blow molding process. Melt strength is not a method limitation in this film extrusion method. Both shear viscosity and tensile viscosity are lower. The film can generally be extruded at higher outlet rates than that used in the blow film process. Higher temperatures reduce shear stresses in the die and raise the threshold for melt fracture to occur.

Film The thickness of the film prepared by the method of the present invention is greater than about 0.10 to 20 mils (about 0.0025 to 0.51 mm), preferably greater than about 0.10 to 10 mils (about about 0.002 to 0.25 mm), preferably greater than about 0.10 to 4.0 mils (about 0.002 to 0.1 mm). The following features are characteristic of a 0.10 to 4.0 mil (about 0.002 to 0.1 mm) film: puncture resistance greater than about 7.0 in-lbs / mil (approx. 317.5 kg cm / mm); elongation at break greater than about 400%; tensile strength greater than about 500 ... 2000 ft-lbs / in3 (about 4.22 ... 16.9 cm * and tensile strength greater than about 2000 ... 7000 psi (approx.

13.8 ... 48.3 MPa).

Various conventional additives such as glidants, anti-caking agents and antioxidants can be mixed into the film according to conventional practice.

ethylene polymer

The polymers that can be used in the process of this invention are homopolymers of ethylene, or copolymers containing mostly ethylene (at least 80 mol%) and less (up to 20 mol%) one or more C3-C8 olefins. C3 ... C8 olefins should have no branching at any of their carbon atoms closer than the fourth carbon atom. The most suitable C3-C8 olefins are propylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene and 1-octene.

The ratio of the flow index to the melt index of the ethylene polymers is greater than or equal to about 22 and less than or equal to about 50, and preferably greater than or equal to about 25 and less than or equal to about 30. The ratio of the flow index to the melt index is another way to indicate molecular weight distribution. A range of flow index to melt index (MFR) greater than or equal to 22 and less than or equal to 50 thus corresponds to a Mw / Mn value of about 2.7 to 6.0.

The homopolymers have a density greater than or equal to about 0.958 and less than or equal to about 0.972, and preferably greater than or equal to about 0.961 and less than or equal to about 0.968. The density of the copolymers is greater than or equal to about 0.89 and less than or equal to about 0.96, and preferably greater than or equal to 0.917 and less than or equal to about 0.955, and preferably greater than about 76955 22 or greater than about 0.917 and less. or equal to 0.935. The density of the copolymer at a given melt index level of the copolymer is primarily controlled by the amount of C3 to C8 mixed monomer copolymerized with ethylene. In the absence of the mixed monomer, ethylene would be homopolymerized with the catalyst of this invention to produce homopolymers having a density greater than or equal to 0.96. Thus, the gradual addition of larger amounts of mixed monomer to the copolymers causes a gradual decrease in the density of the copolymer. The amount required for each different C3 to C8 mixed monomer to obtain the same result varies from one mixed monomer to another under the same reaction conditions.

The fluidized bed polymers of this invention are granular materials having a settled bulk density of about 15 to 32 lb / ft (about 240 to 513 g / l) and an average particle size of the order of about 0.005 to 0 .0 inches (approx. 0.13 to 1.52 mm).

The polymers best suited for film making purposes by the process of this invention are copolymers, and in particular those copolymers having a density greater than or equal to about 0.917 and less than or equal to about 0.924; and a standard melt index greater than or equal to about 0.1 and less than or equal to n.

5.0.

Films made by the method of this invention have a thickness greater than 0.1 (about 0.0025 mm) and less than or equal to 10 mils (about 0.25 mm), and preferably greater than 0.1 (about 0.0025 mm). ) and less than or equal to 5 mils (approximately 0.13 mm).

After describing the general nature of the invention, the following examples illustrate a few embodiments specific to the invention. It is to be understood, however, that the present invention is not limited to the examples, since the invention may be practiced 23 7 6 9 5 5 using various modifications.

Example 1 This example illustrates typical methods for extruding ethylene polymers into tubes.

The ethylene-butene copolymer was prepared according to the method of U.S. Patent 4,302,566 and is available from Union Carbide Corporation under the trademark Bakelite GRSN 7047. The copolymer also contained 800 ppm by weight of Kemamine AS 990 available from Witco Chemical Corporation, Memphis, Tennessee. The copolymer had a nominal density of 0.918 g / cm 2, a nominal melt index of 1.0 dg / min and a nominal flow index to melt index ratio of 26. The copolymer was formed into a tube by passing the resin through a conventional 2.5 inch (about 6.35 cm) diameter screw extruder. , which had a polyethylene feed screw in the Maddock mixing compartment described in U.S. Patent No. 4,329,313, and thence to a conventional hard chrome-plated steel nozzle with a 1,375-inch (about 3.493 cm) flow orifice, a 3-inch (about 7.62 cm) collar diameter the spindle diameter was 2.92 inches (about 7.417 cm), with a nozzle orifice of 40 mils (about 1.02 mm). The sides of the die orifice were parallel to the flow axis of the polymer melt. The resin was extruded through the die at a rate of 52 lbs / h (about 23.6 kg / h) at 219 ° C.

A 1.5 mil film was prepared at a blow ratio of 2 and the height of the film at the crystallization limit was 12 inches (about 30.5 cm) using a conventional double lip pneumatic ring. Several melt fractures were observed on both sides of the tube with an average surface roughness of 12 micrometers (about 0.31 microns) in the machining direction and 20 micrometers (about 0.51 microns) in the transverse direction as measured by Bendix Proficorder.

With this conventional 40 mil (about 1 mm) nozzle orifice, a surface melt fracture was observed at all speeds greater than 24 lbs / h (about 10.9 kg / h).

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Example 2 This example demonstrates the improved results of Example 1 using the surface of a long stainless steel (304) in the nozzle flow orifice region.

The ethylene-butene copolymer was similar to Example 1 and contained 800 ppm of Kemamine AS 990. The copolymer was formed into a tube by passing the resin through a conventional 2.5-inch (about 6.35 cm) diameter screw extruder and into a stainless steel die. 304) surfaces on opposite surfaces of the nozzle flow orifice area. The length of the orifice flow orifice was 2.2 inches (about 5.59 cm) and the orifice orifice was 40 mils (about 1 mm). The sides of the nozzle flow port were parallel to the flow axis of the polymer melt. The resin was extruded at a rate of 52 lbs / h (about 23.6 kg / h) at 220 ° C. A 1.5 mil (ca. 0.038 mm) film was prepared at a blow ratio of 2 and the film height at the crystallization limit was 13 inches (ca. 33.0 cm) using a conventional twin lip air ring. Very little surface melt fracture was observed on both sides of the film, with an average surface roughness of 4 micrometers (about 0.1 microns) in the machining direction and 5 micrometers (about 1.3 microns) in the transverse direction.

Profreorder Ilia measured.

On long surfaces of stainless steel (304) in the nozzle flow orifice region, no surface melt fracture was observed even at speeds as high as 47 lbs / h (about 21.3 kg / h).

Claims (9)

A method of substantially reducing surface melt fracture during extrusion of a narrow linear ethylene copolymer with narrow molecular weight distribution under flow rate and melt temperature conditions which would otherwise provide such surface melt fracture, adding an adhesive polymer to the ethylene and adding an adhesive polymer to the process. ), whose flow channel region (G) defines resistive surfaces (22, 22 ') which terminate in a nozzle opening (18), characterized in that at least one of said resistive surfaces (22, 22') is made of stainless steel sh, obtaining at least one 27 7 6 9 5 5 surface of stainless steel at the molten polymer, and the ratio between the length of the flow channel (G) in the nozzle (10) and the width of the nozzle opening (18) is about 35: 1. * 60: 1, whereby the melt fracture decreases the pS surface of the polymer located at the stainless steel surface. 2. A process according to claim 1, characterized in that the adhesion promoter is a diethoxylated tertiary amine of a fatty acid. 3. Process according to claim 2, characterized in that the diethoxylated tertiary amine of fatty acid is added to the ethylene polymer in an amount of about 50 ... 1500 ppm. 4. A method according to claim 1, characterized in that the surface of stainless steel in the flow duct area (G) is provided by insertion pieces (24) which are attached to the nozzle and collar of the nozzle. 5. Method according to claim 4, characterized in that the length of the insert pieces (24) is about 45 to 55 times the width of the nozzle opening (18). 6. A method according to claim 1, characterized in that the surface of stainless steel is arranged by producing the pin and collar of the stainless steel. 7. Method according to claim 1, characterized in that the distance between the lips of the nozzle (20, 20 ') is in the range 0.005 ... 0.040 turn (about 0.127 ... 1.016 mm). 8. A process according to claim 1, characterized in that the copolymer is a copolymer between more than or equal to 80 mole percent ethylene and less than or equal to 20 mole percent of at least one C 3 ... C 8 alpha olefin.