WO2004076151A2

WO2004076151A2 - Process and an extrusion die for eliminating surface melt fracture during extrusion of thermoplastic polymers

Info

Publication number: WO2004076151A2
Application number: PCT/EP2004/001772
Authority: WO
Inventors: Koulikov (Kulikov), Oleg; Klaus Hornung
Original assignee: Koulikov Kulikov Oleg; Klaus Hornung
Priority date: 2003-02-28
Filing date: 2004-02-23
Publication date: 2004-09-10
Also published as: WO2004076151A3; DE10308909A1; EP1606095A2

Abstract

Process and apparatus for substantially eliminating surface melt fracture during extrusion of a thermoplastic polymer such as a molten linear low density polyethylene, by using a die (1, 4) having an elastic coating (2, 3, 5) on its inner surface adjacent the die exit.

Description

IPC⁷: B29C 33/56, B28C 47/00, B29C 47/12, B29C 49/00

Process and an extrusion die for eliminating surface melt fracture during extrusion of thermoplastic polymers Background of the Invention: Field of the Invention:

The present invention generally relates to the processing of thermoplastic polymers by extrusion. More particularly the present invention relates to the manufacture of blown films, tubes, and wire coatings having a good surface appearance. In a more specific aspect, the invention relates to a process of thermoplastic polymers (e.g. polyolefins), which have a narrow molecular weight distribution. The invention further relates to a die design which provides a high rate of defect free extrusion of the molten plastics. Description of the Prior Art:

Modern polymers including polyethylene, polypropylene, polyvinyl chloride, acrylic, etc. are characterized by a narrow molecular weight distribution and advantageous mechanical properties but specially subjected to flow instabilities which occur when the extrusion rate exceeds a certain critical value. For a review of the existing literature see [1 - 7].

The extrudate surface characteristics, in general, show that at low shear stress the emerging extrudate is smooth and glossy. At a critical value of the stress, the extrudate exhibits loss of surface gloss. This loss of gloss is due to fine scale roughness of the extrudate surface which can be observed under a microscope at moderate magnification (20-40X). This condition represents the "onset" of surface irregularities and most investigators believe this to occur at a certain critical linear velocity through the die. At extrusion rates above the critical, two main types of extrudate irregularities can be identified with most polymer melts: surface irregularities and gross irregularities. The surface melt fracture, as the name implies, is confined only to the surface of the extrudate and the core of the extrudate appears to show no irregularity. The available literature on surface melt fracture show the following, (a) The onset of surface melt fracture is relatively independent of the die dimension (diameter, length to diameter ratio L/D, and taper angle at the entry) and the materials of construction of the die. (b) The onset of melt fracture is considerably delayed by increasing the temperature of the melt. (c) Polymers with linear structure (for example, high density polyethylene) show increased tendency to surface melt fracture as compared to those with branched structure.

(d) Polymers with narrow molecular weight distribution show more severe surface melt fracture than those with broad distribution. There is widespread agreement amongst different investigators that surface melt fracture is due to effects at the die exit where the viscoelastic melt is subjected to high local stresses as it leaves the die resulting in cyclic build-up and release of surface tensile forces. As a result, differential recovery occurs between the skin and core of the extrudate.

When the extrusion rate is increased further, the emerging extrudates exhibit gross irregularities (hereafter referred to as gross melt fracture) which are no longer confined to the surface of the extrudates. This is a catastrophic defect in the extrudates and it has received considerable attention in the literature. The term, "Melt Fracture", coined by Tordella [1], was originally intended to describe these gross irregularities. In contrast to the surface melt fracture, gross melt fracture occurs under unsteady conditions with spiraling flow instabilities at the die entry and pressure and flow rate fluctuations prevail. Depending on the molecular characteristics of the polymer, the emerging extrudates show a variety of distortions ranging from those which show some periodicity (alternating smooth and rough, wavy, bamboo, screw thread etc.) to random distortions with no regularity.

The extensive literature on gross melt fracture indicates the following. (a) The onset of gross melt fracture occurs at a critical shear stress and is relatively independent of the die length, die diameter and temperature.

(b) The critical stress for gross melt fracture is independent of the molecular weight distribution but the critical average product velocity increases with the width of the distribution.

(c) The die entry can have a significant effect on the critical average product velocity for the onset of gross melt fracture.

(d) The critical average product velocity increases with increasing L/D ratio of the die as well as with increasing melt temperature.

Linear Low Density Polyethylene (LLDPE) resins have essentially a linear molecular structure with a very narrow molecular weight distribution (MWD) in contrast to the conventional high pressure low density polyethylene (HP-LDPE) resins which have long chain branched structure and a much broader MWD. In film applications, products fabricated from LLDPE resins significantly outperform those from HP-LDPE resins because of these differences in molecular architecture. However, extrusion processing of LLDPE with conventional film dies, optimized for HP-LDPE, is limited by the occurrence of severe "melt fracture" at current commercial rates.

Whereas polypropylene (PP) resins generally have better dielectric properties and abrasion resistance than polyethylene (PE) resins, flow instabilities are more pronounced and less easily controlled with polypropylene resins. [8]. Severe melt fracture during extrusion coating of electrical conductors results in a non-uniform thickness of the insulation around the conductor, i.e., eccentricity. Failure to have the conductor consistently positioned at the geometric center of the cable can result in a reduced electrical signal performance. Furthermore, those areas where the thickness of the insulation layer is inadequate are more prone to pinholes and cracking from bending or abrasion. As wire manufacturers continue to push for higher line speeds, eccentricity becomes one of the major limiting factors [9].

There are several methods for eliminating surface melt fracture under commercial film fabrication conditions. These are aimed at reducing the shear stresses in the die and include the following.

(a) Increasing the melt temperature.

(b) Use of slip additives in the resin to reduce friction at the wall.

(c) Local heating or cooling of the die land area and by this of the outer layer of the polymer. (d) Modifying the die geometry.

(e) Modification of the die land material, like use of the materials and coatings to enhance or to reduce surface adhesion of the molten plastics to the die land surface.

Increasing the melt temperature is not commercially useful since it lowers the rate for film formation due to bubble instabilities and heat transfer limitations. Rise in the melt temperature leads to thermal decomposition of the plastics in dead corners and a to a loss of mechanical strength of the product.

Additives for use as processing aids to obtain melt fracture reduction in extrudates [10,11], are expensive and the added cost, depending on the required concentration, may be prohibitive in resins, such as granular LLDPE, intended for commodity applications. Also additives influence the rheological properties of the base resin. In excess amounts they may adversely affect critical film properties including gloss, transparency, blocking and heat sealability characteristics of the product.

Another method to suppress surface fractures could be a local heating of the die lip to temperatures significantly above the melting temperature [12,13] or cooling of an outer layer of the polymer [14] leaving the bulk of the melt at optimum working temperature. However, these methods are difficult to use and control.

Die geometry modifications have been designed to reduce the shear stress in the die land region to be below the critical stress level by enlarging the die gap [15] or just by a local increase of the gap near the die lips [16,17]. Enlarging the die gap results in thick extrudates which must be drawn down and cooled in the film blowing process. While LLDPE resins have excellent draw down characteristics, thick extrudates increase the molecular orientation in the machine direction and result in a directional imbalance and in a reduction of critical film properties such as tear resistance. Also thick extrudates limit the efficiency of conventional bubble cooling systems which result in reduced rates for stable operation.

Coating the die land area with substances which reduce or enhance the adhesion of the polymer material to the die land surfaces [18,19] eliminate surface melt fracturing at higher extrusion rates. For example the use of brass or a composition containing about 80% to 99% nickel and about 1% to 20% phosphorous give better results as compared to common stainless steel. It is known also [20,21] that the use of thin coatings from fluorinated polymers as well as the use of PTFE for the die land area makes it possible to enlarge the rate of defect free extrusion up to 3.5 times.

In [22,23] it was discovered that some fillers from group of foam cell nucleating agents (like boron nitride, fluorinated polymers) when added to the thermoplastic polymer act as very effective processing aids to enable the maximum extrusion rate to be increased before the extrudate surface changes from smooth to rough. Foam cell nucleating agents are normally used to nucleate the formation of voids in polymer extrudate, but without blowing agent being present at the time of extrusion the extruded polymer is unfoamed. The amount of boron nitride and additives of fluorinated polymers can be less than 0.02 wt % to produce unfoamed extrudate at a rate far beyond that found for the polymer alone. Nevertheless the use of micro-powder fillers makes the extruded polymer nontransparent, which is not a problem for wire-coating, but for blown-film production it forms a serious problem.

The most close to the present invention is a patent [19] which describes a process for eliminating surface melt fracture during extrusion of a thermoplastic polymer by using a die having a die land region defining opposing surfaces at least one of which is coated with a composition containing about 80% to 99% nickel and about 1% to 20% phosphorous.

Generally, all the methods described above are effective in postponing the occurrence of sharkskin defects. However further improvement is still needed in order to further increase extrusion rates.

It is an object of the present invention to overcome one or more problems described above and to get high rate of defect free extrusion of the molten plastics. Other objects and advantages may become apparent from the following detailed description taken in conjunction with the drawings, the examples, and the claims.

Summary of the invention: The present invention provides a process of thermoplastic polymer, comprising the steps of heating said thermoplastic polymer above the temperature of melting and extruding the molten polymer through a die gap, said die having a die land region defining opposing surfaces, said thermoplastic polymer having a surface in contact with said opposing surfaces, the improvement wherein at least one of said opposing surfaces in area adjacent to the die orifice is coated with an elastic material, whereby melt fracture is substantially eliminated on the surface of the polymer adjacent to said coated surface.

In one aspect of the invention the polymer is selected from polyolefins with a narrow molecular weight distribution, preferably said polymer is Linear Low Density Polyethylene.

In the preferred operative mode it is desirable to reduce the processing temperature to a range of about 110% to 150% of T₀, where T₀ is the melting point of the polymer.

The invention also resides in an apparatus for processing of thermoplastic polymers including a bin feeder, heaters, press extruder, and a die, preferably an annular die, said die having a die land region defining opposing surfaces, the improvement wherein at least one of said opposing surfaces in area adjacent to the die orifice is coated with an elastic material.

Preferably, both opposing surfaces in area adjacent to the die orifice are coated with an elastic material comprising elastomers selected from the group consisting of: Hydrogenated Nitrile Rubber, Fluorinated Hydrocarbon Elastomers, Perfluorinated Elastomers, Silicone Elastomers, and Fluorosilicone Elastomers.

In another embodiment said elastic material elastic material contains additives in form of powder having low surface energy.

In yet another embodiment the elastic coating has a length along the die axis not less then 10% of the die gap.

Brief description of the drawings: Fig. 1 is a sectional view of tubular dies in following variants. Fig. 1A is a common tubular die from metal, Fig. IB and Fig. IC are dies with elastic coatings suitable for an extrusion process at high velocity without surface defects.

Fig. 2 is a sectional view of a tubular (A) and an annular (B) dies with elastic rings of rectangular cross section pressed to the base of the metal dies. Fig. 3 is a sectional view of an annular die. (7) is a "spider-like" suspension of the core

(8), The core is fixed to the suspension by a screw (9). (10) is the base of the die, (11) is the die collar fixed to the die base by screws (12), (13) is a pressure sensor fixed inside a brass ring (14), (15) is a closing cap, (16) is a pin.

Fig. 4 is an example of a "flow curve", i.e. pressure drop on the die vs. average product velocity.

Fig. 5 shows the temperature dependence of the critical average product velocity, which is the extrusion rate when the extrudate surface changes from smooth to rough.

Fig. 6 shows the dependence of the critical average product velocity on the length of the tubular die. Fig. 7 is the "flow curve" for the case of a die with an elastic coating, as presented in Fig.

IB.

Detailed description of the invention: The present invention involves the discovery that an elastic coating of the die land area adjacent to the die orifice acts as a very effective processing aid to increase the extrusion rate before the extrudate surface changes from smooth to rough.

Any elastic material is a complex blend of elastomer or rubber, fillers, and other additives. The terms elastomer and rubber are scientifically identical and interchangeable. Modern elastomers are synthetic rubbers which are generally oil by-products. Most synthetic elastomers are not as elastic as natural rubber, but all can be stretched (or otherwise deformed) in a reversible manner to an extent which easily distinguishes them from all other solid materials.

Examples of synthetic rubber are the following categories: Butyl Rubber, Ethylene- Propylene Rubber, Fluorinated Hydrocarbon Elastomers, Perfluorinated Elastomer, Fluorosilicone Elastomers, Latex Rubber, Neoprene (Polychloroprene), Nitrile Rubber (Acrylonitrile), Hydrogenated Nitrile Rubber, Polybutadiene, Silicone Rubber, Styrene-Butadiene Rubber, Urethane Rubber, etc. . Among them Hydrogenated Nitril Rubber, Fluorinated Hydrocarbon Elastomers, Perfluorinated Elastomer, Silicone Elastomers, and Fluorosilicone Elastomers have outstanding heat stability and chemical resistance.

Hydrogenated Nitrile Rubber is known under following trade names: Therban, Tornac, Zetpol. The properties of Hydrogenated Nitrile Rubber depend on the acrylonitrile content, and on the degree of hydrogenation. They have the general advantage over standard Nitrile Rubber of having higher temperature resistance and higher strength. They have good high temperature oil and chemical resistance and are resistant to amines. They are suitable for use in methanol and methanol/hydrocarbon mixtures if the correct Acrylonitrile level is selected. They have good resistance to hot water and steam. They can have excellent mechanical properties including strength, elongation, and tear. Also, abrasion resistance, compression set, and extrusion resistance. They are reported to be satisfactory up to temperatures around 180°C in oil. Fully saturated grades have excellent ozone resistance. They have poor resistance to some oxygenated solvents and aromatic hydrocarbons.

Fluorinated Hydrocarbon Elastomers, or fluoroelastomers are known under following trade names: Dai-El, Fluorel, Technoflon, Viton. This is a family of elastomers designed for very high temperature operation. They can operate continuously somewhat in excess of 200°C depending on the grade, and intermittently to temperatures as high as 300°C. They have outstanding resistance to chemical attack by oxidation, by acids and by fuels. They have good oil resistance. However, at the high operating temperatures they are weak, so that any design must provide adequate support against applied forces. They have limited resistance to steam, hot water, methanol, and other highly polar fluids. They are attacked by amines, strong alkalis and many Freons.

Perfluorinated Elastomer are known under following trade names: ChemrazR, Kalrez, Perfluor, Simriz, Zalak. These are materials having even greater heat and chemical resistance than the fluoroelastomers. They can be used in extreme conditions up to temperatures around 300°C or even higher with special compounding. Their disadvantages are difficult processing, very high cost and poor physical properties at high temperature. Silicone Elastomers and Fluorosilicone Elastomers subdivide into the following classes: with methyl groups on chain, with methyl and vinyl groups, with methyl and phenyl groups, with methyl and fluorine groups. The outstanding property of these materials is their very wide temperature range. Typically the range is -60°C to 250°C and above. They do not have very good physical properties, but the properties they do have are retained to high temperatures. Fluorosilicone Elastomers have better oil- and water resistance than the others.

The use of Silicon Rubber coatings in heat fixing rollers of copying machines, laser beam printers, fax machines and so forth with working temperature up to 250°C, is well known [24]. Silicone Rubber has low surface energy (from about 21 to about 25 dynes/cm). The elastic coating could have multi-layer structure, e.g. the first layer of Silicon Rubber has a surface coating by Fluorinated Silicone Rubber [25], or by Fluorinated Resin and/or Fluorinated Rrubber [26,27]. The advantage of using coatings from Fluorinated Silicon Rubber and/or from Fluorinated Polymers is in higher surface hardness as compared to virgin Silicone Rubber. In addition, perfluorinated polymers and elastomers do not tend to swell in the presence of oils. Oil and especially Silicon oil is often used in the composition of polymer blends [29].

Silicon Rubber which is designed to work at elevated temperatures includes a filler, a heat resistance improver, etc. . The heat resistance improver may include, e.g., carbon black, graphite, fluorinated carbon, and iron oxide. The filler is mostly a silica-based inorganic filler, e.g. silica fume, but in accordance with our invention the rubber composition could include micro-powder of a low surface energy filler selected from the group of fluorinated polymers, Boron Nitrate having hexagonal crystal form, graphite, molybdenum disulfide, tungsten disulfide, talc in total amount of about 0,1 to about 80 wt. %. The use of low surface energy fillers improves the wear resistance of the coatings [29]. In addition it works as a processing aid to suppress deterioration of the product surface at high rate of extrusion. The elastic coating or the elastic insert could consist of several parts not connected to each other.

The thickness of the coating could vary along the die land.

"Polyolefins" is the generic term used to describe a family of polymers derived by the polymerization of propylene and ethylene gases, and the family includes following materials: polypropylene, polyethylene, blends of polypropylene and polyethylene, ethylene propylene copolymers, and ethylene propylene rubber copolymers. Every Polyolefin resin consists of a mixture of large and small chains, i.e., chains of high and low molecular weights. The molecular weight of the polymer chain generally is in the thousands. The average of these is called, quite appropriately, the average molecular weight. The relative distribution of large, medium and small molecular chains in a polyolefin resin is important to its properties. When the distribution consists of chains close to the average length, the resin is said to have a "narrow molecular weight distribution".

To the composition of the thermoplastic polymer can be also added various materials which include such as pigments, lubricants, antioxidants, antiblock agents, and the like in amounts well known in the art. Preferably it comprises additives in form of powder having low surface energy. Examples of low surface energy additives include but not restricted by fluorinated polymers, Boron Nitrate having hexagonal crystal form, graphite, molybdenum disulfide, tungsten disulfide, talc, and mica. A total amount of low surface energy additives could be about 0,1 to about 80 wt. %.

One tentative explanation for the delay of surface roughness for the case of a die with elastic walls could be as follows. Under stick boundary conditions the surface layer of the product undergoes a large stretching due to the velocity increase from zero at the wall inside the die channel to an average velocity outside the channel. At the same time the core of the product decelerates from a higher value inside the channel to the lower one outside. Thus one could distinguish between core and skin layers of the product. In the core the material is under compression and it decelerates while in the skin layer the material is under strain and it accelerates. Tensile deformation of the surface layer reduces its thickness and this may lead to an "adhesion failure" of the product from the die wall just near the die exit. A fast propagating shear fracture separates the product from the wall and triggers its fast slip along the wall. The slipping material already inside the channel undergoes large strain and then a stretching after its discharge from the die. A local tensile deformation leads to a local depression in the product surface.

Outside the die this depression may grow across the skin layer toward the product core to form a crack. Thus the "adhesion failure" may lead to a "cohesion failure" of the skin layer, e.g. to a rupture of the skin layer outside the die. Such a shear fracture or the "adhesion failure" will not happen in the case of an elastic wall of the die because the elastic material has very low value of Young's Modulus. In this way the rubber coating of the die wall would delay the onset of surface defects. Examples: The proposed apparatus for forming thermoplastic polymers in a most general variant contains the following known parts and blocks necessary for its operation: a bin feeder, heaters, press extruder, that are means for delivering molten thermoplastic polymers to a die, and the die itself. Within the framework of the present invention the means for delivering molten thermoplastic polymers are considered as known from the state of art. Therefore, they are not presented in the drawings. The essence of the proposal is expressed only in modifications of the die design and only variants of the die design are presented in the Figures. In the examples below the elastic coating is realized as a rubber coating or as a rubber ring. Example 1

The Example demonstrates the conventional design of the tubular die (Fig. 1A) and variants of the die design made in accordance with the present invention (Fig. IB and Fig. IC), wherein (1) is the metal base of the die, (2) is a rubber coating, (3) is a rubber ring of a rectangular cross section. The metal base of the die in Fig. IB was made from brass with helical thread throats

M7*0.75 and M8*l. The rubber coatings were produced by potting a one component silicone rubber "Ceresit" from Henkel KGaA (maximum working temperature is 315°C) [30] or by potting a two component silicone rubber "RTN-ME 622A" from Wacker-Chemie GmbH [31]. The coatings were vulcanized by heating at 150°C. The rubber rings for the die design in Fig. IC were cut from silicon rubber or fluorinated rubber (Viton) tubes with 6 mm inner diameter. The rings were glued inside the metal frame and closed from the entrance side by diaphragms of 5 mm diameter. Example 2

The Example demonstrates the following variants of the die design with rubber rings of rectangular cross section: A tubular die (Fig. 2A) and an annular die (Fig. 2B), wherein (4) is the metal base of the die, (5) is a rubber ring of the tubular die, (6) are male and female screws to press the rubber rings (5) to the die base (4).

The use of short rubber rings pressed to the metal base of the die allows easy exchange of the rings and cleaning of the die face. Example 3

The Example demonstrates the extrusion of thermoplastic polymers with use of a conventional die design. We used LL1030XV - a commercial LLDPE from ExxonMobil Chemical [32], which is specially recommended for blown film production. Some of its properties are listed in Table 1.

For extrusion at controlled flow rate we used a hydraulically driven press extruder from LOOMIS PRODUCTS [33] with a barrel of 60 mm in diameter and 200 mm in length. Maximum pressure in the barrel could be up to 400 bars. The temperature of the barrel was controlled by heaters and measured by a thermocouple thermometer. The procedure to load the barrel was as follows: Portions of LLDPE granules were loaded to the heated barrel (200 - 210°C) and were molten after evacuation of the barrel. This procedure was repeated until the barrel was completely filled with LLDPE melt. The melt temperature was measured also inside the barrel by a contact .. thermocouple thermometer. The die temperature and the temperature of the extruded product was measured by a non-contact infrared pyrometer. The experiments were done at temperatures between 135 and 210°C. The pressure was measured in the bottom part of the barrel with a pressure transducer from WINTEC [34] which has a linearity of 0.5% in the range from 0 to 100 bar. The position of the piston was measured with a transducer from BALLUFF [35] which has a precision of about 5 micrometers. Analog outputs of the pressure and position transducers were digitized every 0.6 sec with 24 bit precision by an Analog-to-Digital converter LTC2400 from LINEAR TECHNOLOGY [36] and delivered to a Pentium IV Computer through Serial Ports. An analog output from the board PCI6023E from NATIONAL INSTRUMENTS [37] was used to control the flow rate through the die in the range from 0.5 to 250 mm/sec. The measurements were automated using Lab View software.

The extrusion rate was gradually increased from 0.1 to 250 mm per sec. The flow curve for the case of a tubular brass die of the conventional design (Fig. 1A, length - 24 mm, diameter - 6 mm, temperature of extrusion - 145°C) is presented in Fig. 4 by a continuous line. At very low average product velocity the extruded product has a glossy surface and then at an extrusion rate of about 2.4 mm per sec small scale defects ("micro surface roughness") appeared. With rise in the extrusion rate some periodicity becomes visible ("sharkskin defects"). With further rise in average product velocity the product consists of periodic alterations of defect and smooth surface ("stick- slip" defects). Because of the time averaging of the pressure values (present in the data acquisition) we could not see any oscillations of the pressure but only a change in the slope of the flow curve. At a certain velocity the extrusion gives a defect-free product with glossy surface and the pressure drops. This could be a consequence of a transition to continuous slip inside the die. At higher average product velocity the product gets some irregular grooves on its surface and crumbs of polymer accumulate on the exit face of the die. The number of these grooves grows until the whole surface gets disturbed by many crater like cavities ("gross melt fracture"). Example 4

The Example demonstrates the temperature dependence of the critical average product velocity for the onset of sharkskin. We carried out measurements to verify the temperature influence for the polyethylene we used. The die was made of brass with L = 24 mm and D = 6 mm. The barrel was filled at a temperature of 210°C and cooled down to the extrusion temperature (between 130 and 206°C). The time from one measurement to the next was as long as 4 hours to get a homogeneous heat distribution within the barrel. Pressure was then applied by the hydraulic piston and the onset of surface sharkskin defects was detected at a certain value of the average product velocity. A plot of the critical average product velocity for the onset of sharkskin versus melt temperature is shown in Fig. 5. Maximum sharkskin formation is observed at temperatures between 145 and 150°C. In a narrow temperature interval between the melting temperature and the temperature of maximum sharkskin formation the flow resistance drops significantly and defect-free extrusion is possible for higher average product velocity. Polymer materials are usually extruded at as low temperature as possible of the melt, but there is a risk of solidification of the polymer inside the extruder and in practice the lower limit of the polymer melt temperature is above the temperature of maximum sharkskin formation. Exxon Mobil [32] recommends temperatures of extrusion between 180 and 200°C because the critical average product velocity of extrusion is rising in this range. Example 5 The Example demonstrates length dependencies of the critical average product velocity for the onset of sharkskin. We have determined the apparent critical average product velocity necessary for the onset of the sharkskin defect for a tubular stainless steel die with inner diameter D= 6 mm for various lengths L as well as for a sharp diaphragm. The melt temperature was 160°C. The long dies had a spiral heater around them to keep the surface of the tube evenly heated. Results are shown in Fig. 6, together with values for the barrel pressure P. The figure shows that there is only a small change in the critical average product velocity when changing the die length from 0 to 96 mm. Correspondent pressure values show a much larger relative variation from 2 bars for the case of the diaphragm to above 90 bars for the case of the longest die (96 mm). Therefore in the following discussions we will use the apparent average product velocity (8*V/D) rather than the apparent shear stress (P/4)/(L/D) to characterize critical situations for the onset of instabilities. Example 6

The Example demonstrates material dependencies of the critical average product velocity for the onset of sharkskin. As mentioned above the material of the die is a contributing factor and defects could be delayed by the use of materials with low surface energy. To show this effects for our polymer we made extrusion with tubular dies (L = 12 mm, D = 6mm) from stainless steel, brass, Teflon, and Boron Nitrate (BN). Critical average product velocities for "micro surface roughness" are presented in Table 2 . We could see that the appearance of the micro surface roughness for the brass die is delayed to higher velocities as compared to the case of stainless steel die, as found before in [4]. A die from Teflon results in a further delay of the micro roughness. The die from BN provides the highest defect-free rate of extrusion. This material (CDBN - with 97% content of BN) was provided by Henze BNP GmbH [38] . Example 7

The Example demonstrates the extrusion process when using the proposed die design presented in Fig IB. We used a "Ceresit" compound and a thread M8*l with L = 12 mm. Polyethylene was extruded through the dies and a resulting flow curve for the first variant is presented in Fig. 6. The extrusion was defect-free up to the moment of the mechanical break-up of the elastic coating inside the die. The moment of this break-up is marked on the plot by a cross. A comparison of the flow curves of Fig. 4 and Fig. 7 shows that the last one is much closer to a straight line. The difference could be attributed to an absence of "stick-slip" transitions inside the die in the presence of an elastic coating. When using the two component rubber on a thread M7*0.75 the die was not destroyed up to 250 mm/sec (apparent average product velocity - 330 s- 1), which is the maximum limit for the extruder used. Resulting values for the critical average product velocity are summarized in Table 3. Shark-skin was completely absent. Therefore the critical average product velocity values, contained in the table are related to such instabilities, which first occur after defect-free extrusion. These instabilities, occurring at very high extrusion rates, have a markedly different outlook as compared to the case of rigid dies. In the "Ceresit"- case we could not observe any defect, therefore we take the last value before the rupture of the insert occurred. The data clearly show that elastic walls could delay the onset of surface defects to an extent which is far beyond that has been achieved up to now with rigid dies. Example 8 The Example demonstrates the extrusion process when using the proposed die design presented in Fig IC and Fig. 2A. Rubber inserts were cut from Silicon and Viton tubes at lengths of 1.5, 6, and 12 mm. The inserts are mechanically stable in the investigated range of flow rates. Resulting values for the critical average product velocity are summarized in Table 4. Example 9 The Example demonstrates the extrusion process when using the rigid annular die and the proposed die design presented in Fig. 3. Resulting values for the critical average product velocity are summarized in Table 5. The annular die has a "spider-like" suspension of the core. The die collar is produced with the possibility to be adjusted to a certain position in cross-axis direction in order to get an even thickness of the extruded tube. The gap of the rigid die was 1.0 mm, a die land length is of 5 mm, and a product diameter is of 20 mm. In the second variant the die collar was coated with Silicon rubber by immersing it in fluid Silicon compound diluted by Xylene, heating, and polymerising at temperature about 200°C. For better adhesion it was treated by an open flame to oxidise the metal surface before potting. The thickness of the coating was about 0.2 mm. On the product "shark-skin" defects are suppressed only at the outer surface. In the third variant the die core was also coated. No surface deterioration is visible up to about 50 mm/sec for the die gap 0,6 mm and 1.5 mm. The silicon rubber coatings are mechanically stable in the investigated range of flow rates.

Summary for the Examples 1 to 9:

In the present experiments we first investigate flow curves and extrusion instabilities for traditional dies from steel, brass, glass, Teflon and BN to have a safe reference for the present experimental conditions. Then a novel die design is proposed to suppress those irregularities. Using this novel die design defect-free extrusion is demonstrated up to velocities 25 to 35 times higher as compared to the case of rigid dies of the same diameter and length. The use of elastic elements has the additional advantage of allowing a reduction of the processing temperature. This would improve the stability of the blown bubbles and reduce thermal decomposition of the polymer in dead corners of the press extruder. The experiments were performed with LLDPE, and the results therefore may contribute to an improvement of extrusion of thermoplastic polymers, which have a narrow molecular weight distribution. Especially the results may have an impact on the improvement of polymer processing in tubular film blowing , injection molding, fiber spinning, as well as on measurements of viscosity values in rheometers.

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Table 2: Critical average product velocity values for the onset of surface defects.

Table 3: Critical average product velocity values for the onset of surface defects.

Table 4: Critical average product velocity values for the onset of surface defects.

Table 5: Critical average product velocity values for the onset of surface defects.

Claims

Claims:

1. A process of thermoplastic polymer, comprising the steps of heating said thermoplastic polymer above the temperature of melting and extruding the molten polymer through a die gap, said die having a die land region defining opposing surfaces, said thermoplastic polymer having a surface in contact with said opposing surfaces, the improvement wherein at least one of said opposing surfaces in area adjacent to the die orifice is coated with an elastic material, whereby melt fracture is substantially eliminated on the surface of the polymer adjacent to said coated surface.

2. A process according to claim 1, wherein said polymer is selected from polyolefins with a narrow molecular weight distribution, preferably said polymer is Linear Low Density Polyethylene.

3. A process according to claim 1, wherein heating of said thermoplastic polymer is made to a temperature in the range of about 110% to 150% of T₀, where T₀ is the melting point of said polymer.

4. An apparatus for processing of thermoplastic polymers including a bin feeder, heaters, press extruder, and a die, preferably an annular die, said die having a die land region defining opposing surfaces, the improvement wherein at least one of said opposing surfaces in area adjacent to the die orifice is coated with an elastic material.

5. An apparatus according to claim 4, wherein said elastic material comprises elastomers selected from the group consisting of: Hydrogenated Nitrile Rubber, Fluorinated Hydrocarbon Elastomers, Perfluorinated Elastomers, Silicone Elastomers, and

Fluorosilicone Elastomers.

6. An apparatus according to claim 4, wherein said elastic material contains additives in form of powder having low surface energy.

7. An apparatus according to claim 4, wherein said elastic coating has a length along the die axis not less then 10% of the die gap.