Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C48/00—Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor
  • B29C48/25—Component parts, details or accessories; Auxiliary operations
  • B29C48/30—Extrusion nozzles or dies
  • B29C48/3001—Extrusion nozzles or dies characterised by the material or their manufacturing process
  • B29C48/3003—Materials, coating or lining therefor
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C48/00—Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor
  • B29C48/03—Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor characterised by the shape of the extruded material at extrusion
  • B29C48/09—Articles with cross-sections having partially or fully enclosed cavities, e.g. pipes or channels
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C48/00—Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor
  • B29C48/03—Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor characterised by the shape of the extruded material at extrusion
  • B29C48/09—Articles with cross-sections having partially or fully enclosed cavities, e.g. pipes or channels
  • B29C48/10—Articles with cross-sections having partially or fully enclosed cavities, e.g. pipes or channels flexible, e.g. blown foils
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C48/00—Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor
  • B29C48/03—Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor characterised by the shape of the extruded material at extrusion
  • B29C48/06—Rod-shaped

Definitions

• IPC 7 B29C 33/56, B28C 47/00, B29C 47/12, B29C 49/00
• 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.
• 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.
• 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 die entry can have a significant effect on the critical average product velocity for the onset of gross melt fracture.
• 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.
• LLDPE resins significantly outperform those from HP-LDPE resins because of these differences in molecular architecture.
• 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.
• 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.
• these methods are difficult to use and control.
• 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.
• substances which reduce or enhance the adhesion of the polymer material to the die land surfaces 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.
• 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.
• 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.
• the polymer is selected from polyolefins with a narrow molecular weight distribution, preferably said polymer is Linear Low Density Polyethylene.
• the processing temperature In the preferred operative mode it is desirable to reduce the processing temperature to a range of about 110% to 150% of T 0 , where T 0 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.
• 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.
• elastomers selected from the group consisting of: Hydrogenated Nitrile Rubber, Fluorinated Hydrocarbon Elastomers, Perfluorinated Elastomers, Silicone Elastomers, and Fluorosilicone Elastomers.
• said elastic material elastic material contains additives in form of powder having low surface energy.
• the elastic coating has a length along the die axis not less then 10% of the die gap.
• 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
• 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.
• 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.
• 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. .
• 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.
• 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.
• 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".
• 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.
• additives in form of powder having low surface energy 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. %.
• 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.
• 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.
• 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
• 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.
• 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 Example demonstrates the extrusion of thermoplastic polymers with use of a conventional die design.
• 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.
• 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.
• Fig. 1A length - 24 mm, diameter - 6 mm, temperature of extrusion - 145°C
• the Example demonstrates the temperature dependence of the critical average product velocity for the onset of sharkskin.
• 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.
• Example 5 The Example demonstrates length dependencies of the critical average product velocity for the onset of sharkskin.
• the Example demonstrates material dependencies of the critical average product velocity for the onset of sharkskin.
• the material of the die is a contributing factor and defects could be delayed by the use of materials with low surface energy.
• BN Boron Nitrate
• Critical average product velocities for "micro surface roughness” are presented in Table 2 .
• 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] .
• the Example demonstrates the extrusion process when using the proposed die design presented in Fig IB.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Manufacturing & Machinery (AREA)
• Extrusion Moulding Of Plastics Or The Like (AREA)

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• 2003
  • 2003-02-28 DE DE10308909A patent/DE10308909A1/de not_active Withdrawn
• 2004
  • 2004-02-23 EP EP04713535A patent/EP1606095A2/de not_active Withdrawn
  • 2004-02-23 WO PCT/EP2004/001772 patent/WO2004076151A2/en not_active Ceased

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