RU2253568C2 - Device for molding of thermoplastic materials - Google Patents

Abstract

FIELD: chemical industry; food industry; thermoplastic polymeric materials molding by extrusion.

SUBSTANCE: the invention is pertaining to the field of chemical industry, in particular, to thermoplastic polymeric materials molding by extrusion, mainly to an extrusion of polyethylene and may be used at production of pipes, structural sections, a film. The device contains a nozzle, which base is made out of one material, and on an inner surface of its cavity in the place, which adjoins its outlet, there is a coating or a fixed insert made out of another material, that is elastic as contrasted to a melt of the polymeric material. The length of the coating or the insert is no less than 1/10 of a gap width for the melt squeezing out. The invention ensures an increase of the flawless extrusion rate of melts of the polymers subject to crushing.

EFFECT: the invention ensures an increase of the flawless extrusion rate of melts of the polymers subject to crushing.

2 cl, 15 dwg

Description

FIELD OF THE INVENTION

The inventive device relates to the molding of thermoplastic polymeric materials by extrusion, and mainly to the extrusion of polyethylene, and can be used in the manufacture of pipes, profiles, films.

State of the art

Polymers made using metallocene catalysts are characterized by a narrow molecular weight distribution of the polymer chains, increased strength, stiffness, transparency and lightness, but at higher processing speeds they acquire a roughness known as “shark skin” or “snake skin”, “melt crushing” ( see the review of technical literature in the following works: [1-7]. The appearance of defects on the surface of products limits the speed of processing plastics. The most common in the plastics industry com is polyethylene. Polyethylene obtained on metallocene catalysts is characterized by a narrow molecular weight distribution, it is stronger and more transparent than ordinary polyethylene. According to experts, in 2010 the global use of linear polyethylene obtained on metallocene catalysts will increase to 18 million tons and will reach about half of the total consumption of this type of plastics. Polyethylene is mainly used for the production of film (about 60% of the total volume) by extrusion and blowing. I'm a pipe. During the extrusion of polyethylene with a narrow molecular weight distribution, even at low extrusion speeds (about 10 mm / s), the pipe surface becomes rough, and the thin film obtained by blowing the pipe has a reduced reflection gloss and scatters the transmitted light, which reduces the consumer quality of the product. Similar defects can be observed when processing mixtures of polymeric materials with filler. The appearance of defects on the surface of the melt with increasing extrusion speed limits the performance of machines for the manufacture of film and thin-walled profiles.

Several methods for eliminating surface roughness are known, namely:

- increase in melt temperature;

- the use of additives that reduce the adhesion of thermoplastic material to the walls;

- local heating or cooling of the walls of the nozzle (mouthpiece) of the extruder;

- change the geometry of the nozzle (mouthpiece) of the extruder press;

- the manufacture of a nozzle (mouthpiece) from materials that increase or reduce adhesion to the walls, applying the appropriate coatings or using appropriate inserts.

An increase in the temperature of the molten polyethylene increases the instability of the bubble to inflate with compressed gas and leads to an uneven thickness of the resulting film, which is unacceptable in production. Additionally, an increase in the melt temperature of the polymer material leads to its thermal decomposition, dyeing of the material and loss of mechanical strength.

The use of special additives, for example, fluorinated polymers [8, 9], increases the cost of raw materials and makes it difficult to connect the film by welding during heating, which is necessary for the manufacture of plastic bags.

Heating the outlet cross section of the mouthpiece significantly higher than the melting temperature of the polymer material [10, 11] or local cooling [12] complicates the design of the mouthpiece.

A change in the geometry of the mouthpiece can correspond either to an increase in the thickness of the gap along the entire length of the mouthpiece [13], or to a local increase in the gap near the outlet section of the mouthpiece [14, 15], or to an extension of the mouthpiece. As the gap increases, the degree of inflation of the tube for manufacturing a thin film increases and, accordingly, the degree of orientation of the polyethylene molecules increases, which leads to a decrease in the mechanical uniformity of the film and the resistance of the film to wear. A thicker wall of the tube at the same time requires more time for cooling, which reduces the performance of machines. A longer mouthpiece requires higher pressure to force the melt of the thermoplastic material, which complicates and increases the cost of the extruder press design.

The manufacture of the mouthpiece from materials or coating the surface of the mouthpiece with materials that increase the adhesion of polyethylene to the walls [16, 17], allows to stabilize the extrusion and to obtain a defect-free product at higher extrusion speeds. For example, the manufacture of a mouthpiece made of brass or nickel doped with phosphorus gives better results compared to steel alloyed with chromium or nickel. However, the increase in machine performance when using such materials is negligible. At the same time, it is known [18, 19] that a decrease in the adhesion of polymers to the walls of the mouthpiece, for example due to the use of thin (less than micron thick) coatings with fluorinated polymers or the use of PTFE inserts (fluoropolyethylene, trademark - Teflon) in the mouth of the mouthpiece, allow suppress the formation of defects on the surface and increase the speed of defect-free extrusion several (up to 3.5) times.

The manufacture of a mouthpiece from one material and coating or the use of inserts in the inner cavity of the mouthpiece from another material in order to impart special properties to the inner surface adjacent to the outlet of the mouthpiece is the simplest technological technique to increase the speed of defect-free extrusion. A disadvantage of the known devices and materials used to increase the speed of defect-free extrusion is a small increase in speed.

Despite the fact that the appearance of defects with an increase in the extrusion rate has been known for more than 55 years, and the considerable efforts of researchers to understand the cause of the defects, there is still no consensus on this, and the success in preventing the appearance of defects is insignificant. Two points of view can be distinguished on the cause of defects during extrusion of polyethylene, namely:

1. Defects are formed during the transition of the polyethylene melt from sticking to sliding along the wall. This explanation is supported by experimental data showing that the specific loss of plastic flow due to friction decreases simultaneously with the appearance of defects on the surface of the outlet product. Friction losses can be reduced due to the sliding of the polymer material along a hard and smooth wall near the mouthpiece exit.

2. Defects are formed immediately after the polyethylene melt exits the mouthpiece due to cavitation of the surface layer of the material. Indeed, the inhibition of plastic flow by friction on the inner surface of the nozzle of the nozzle reduces the speed of movement in the outer layers of the stream. After the material leaves the forming nozzle with an obvious absence of friction, the velocity distribution should become uniform over the product cross section. Thus, after exiting the mouthpiece, acceleration of the outer layers of the plastic flow occurs. Acceleration corresponds to extension of the surface layer, hydrostatic rarefaction, and cavitation [20]. The use of materials providing melt sliding along a solid wall will reduce the difference in velocity between the core of the flow and the surface, respectively, the acceleration of the surface layer of the material at the mouth of the mouthpiece will decrease, and the hydrostatic depression will be reduced to values insufficient for material cavitation.

Closest to the proposed solution is a known method of reducing melt crushing in the processing of polyethylene with a narrow molecular weight distribution by extrusion through a mouthpiece with a surface layer of a special material that increases melt adhesion to a solid wall [16]. In the description of this method there is a description of the device of the mouthpiece designed to produce a cylindrical tube of polyethylene and blow it into a film, which involves the manufacture of the basis of the mouthpiece of one material (mainly stainless steel) and the use of inserts in the inner cavity of the mouthpiece or coating the inner surface of the mouthpiece of another a material (for example, made of brass) that increases the adhesion of the melt to the inner surface.

The technical result achieved by using the known technical solution is to increase the speed of defect-free extrusion.

The disadvantage of this method and device is a small increase in the productivity of machines for the manufacture of film.

Unexpectedly, we established experimentally that coating a metal surface with an elastic material makes it possible to increase the speed of defect-free extrusion, and we propose a new device for molding plastic material.

The inventive device for molding plastic material is aimed at increasing the defect-free extrusion rate of polymer melts.

SUMMARY OF THE INVENTION

This result is achieved by the fact that for extrusion, a mouthpiece is used, the base of which is made of one material, and on the inner surface of the cavity in the part adjacent to the mouth of the mouthpiece, a coating is applied or an insert of another material is fixed, and the device is characterized in that the coating or inner insert is made of resilient material.

A particular case of the implementation of the device is characterized in that the coating or the inner insert is made with a length of at least 1/10 of the width of the gap for extruding the melt.

Distinctive features of the claimed device are:

- the use of a coating or an inner insert of elastic material,

- the implementation of the coating or the inner insert with a length of at least 1/10 of the width of the gap for extruding the melt.

The coating may have a constant or variable thickness. For example, to improve the mechanical engagement of the coating with the base material of the mouthpiece, the surface of the base can be made with a surface relief in the form of, for example, circular or spiral grooves.

In a section parallel to the axis of the device, the insert is preferably made with a rectangular or other profile containing an inner cylindrical surface parallel to the axis of the mouthpiece and with a length of at least 10% of the width of the gap for extruding the melt. The insert can be made in combination with a coating or consist of several parts that are not interconnected.

Brief description of drawings and additional materials

Figure 1 presents the curves of changes in the elastic and viscous module for the molten polyethylene with a change in the frequency of deformation. Figure 2 presents the curves of changes in the elastic and viscous module for silicone rubber with a change in the frequency of deformation. Figure 3 presents schematically embodiments of the device for extrusion: "A" according to known technical solutions using a metal or glass mouthpiece without coating, "B" according to the proposed technical solution using a rubber coating on a metal base of the mouthpiece, "C" - using rubber inserts in the metal case of the mouthpiece. Figure 4 schematically presents embodiments of the mouthpiece: "A" according to the proposed technical solution using short ring inserts, "B" using an annular rubber coating on a glass base of the mouthpiece near its outlet. Figure 5 schematically presents embodiments of the device for extrusion: "A" for the manufacture of pipes, "B" for the manufacture of the rod. Figure 6 presents the curves of pressure changes with increasing speed of extrusion through a mouthpiece made of glass (solid curve). A curve is also presented of the frequency of occurrence of defects on the surface of the product with increasing extrusion speed (dashed line). Figure 7 presents a graph of the dependence of the critical extrusion rate on the melt temperature of polyethylene. The critical extrusion rate here is the maximum speed of defect-free extrusion. On Fig presents a graph of the dependence of the critical extrusion speed on the length of the mouthpiece. Figure 9 shows the pressure variation curve with increasing speed of extrusion of polyethylene through a mouthpiece made of brass with an elastic coating (solid curve) and steel (dashed line). The cross marks the appearance of defects (shark skin) in the case of using a steel mouthpiece. The inclined cross marks the destruction of the elastic coating. In FIG. Figure 10 shows the curves of pressure changes with increasing speed of extrusion of polyethylene through a glass mouthpiece with elastic coatings and without coating. Figure 11 shows the profile of the flow rate during extrusion through a glass mouthpiece with an elastic coating (solid curve) and through a glass mouthpiece without coating (dashed line). The flow rate values are normalized to the extrusion rate. In FIG. Figure 12 shows the curves of changes in the normalized sliding velocity of the polyethylene melt along the elastic surface with increasing extrusion rate for 144 ° С (solid curve), 158 ° С (dashed line), 187 ° С (points). In FIG. Figure 13 shows the pressure variation curves with increasing speed of extrusion of polyethylene through a mouthpiece made of uncoated glass (solid curve), glass coated with silicone rubber (dotted line) and glass coated with fluorinated silicone rubber. In FIG. Figure 14 shows the pressure variation curves with increasing speed of extrusion of polyethylene through a mouthpiece made of uncoated glass (solid curve), glass with a thin (about 0.001 mm) coated silicone rubber (dotted line) and glass with a thick (0.2 mm) coating silicone rubber (dots). Crosses mark the moments of the appearance of surface defects. In FIG. Figure 15 shows the curves of pressure changes with increasing speed of extrusion of polyethylene through a mouthpiece made of uncoated glass (solid curve), of glass with a thin (about 0.001 mm) coating of VITON fluorinated polymer (dashed line) and of glass with a thin coating of TRE- elastic thermoplastic elastomer 120 (dots). Crosses mark the moments of the appearance of surface defects.

The Tables show the values of the critical extrusion rate at which defects are formed on the surface of the outlet product at a temperature of 155 ° C: Table 1 - for inelastic mouthpiece material, Table 2 - for elastic coatings, Table 3 - for long rubber inserts, Table 4 - for short inserts, Table 5 - for short and long coatings of the glass mouthpiece, and without coating (temperature 158 ° C), Table 6 - for the ring mouthpiece with and without coating (temperature 158 ° C).

Terminology

The term "plastic material" or, for brevity, "plastic" means a material that can be irreversibly deformed under load without breaking.

The term "thermoplastic polymeric material" or, for brevity, "thermoplastic material", "thermoplastic" means a material based on a polymer that softens and becomes capable of plastic deformation when heated to a temperature lower than its thermal decomposition temperature. In general, a thermoplastic material includes not only the polymer itself, but also various additives: antioxidants, stabilizers, additives to reduce static electricity, organic and inorganic fillers, dyes, plasticizers, additives preventing the adhesion of polymer products, for example, anti-block additives for a polymer film additives improving the processing of polymers, for example oils, etc. Examples of polymeric material include, but are not limited to, polyolefins, fluorinated hydrocarbon polymers, vinyl polymers, acrylic and methyl acrylic polymers and elastomers, polyacetates and their copolymers, polyesters, polyamides, polycarbonates, polysulfonates and polyurethanes, thermoplastic cellulose ethers.

First of all, the proposed technical solution is intended for use in the molding of polyolefins. Examples of polyolefins include, but are not limited to polymers and copolymers of ethylene, propylene, butene, pentene, hexene, as well as copolymers of polyolefin molecules with silanes and with monomers containing polar molecules, which include, but are not limited to vinyl alcohol, acrylic acid, and mixtures these polymers. Practically preferred materials for extrusion are: polyolefins and their copolymers obtained using metallocene catalysts and which are characterized by a narrow molecular weight distribution. The proposed technical solution with some limitations can be used to improve the extrusion of mixtures of crude elastomers with peroxides that are vulcanized by the free radical mechanism, or thermoplastic elastomers based on block copolymers, or by extrusion of mixtures of thermoplastic polymers with elastomers.

In accordance with the proposed technical solution for extrusion, a mouthpiece is used, the base of which is made of hard material, and on the inner surface of the cavity in the part adjacent to the exit, a coating is applied or an insert of elastic material is fixed.

The term "resilient material" means a resilient material compared to a thermoplastic polymeric material.

The molten polymer material exhibits elastic properties for short processes and plastic properties for long ones. To measure the relationship between the plastic and elastic properties of the material, its thin layer is periodically deformed in the direction parallel to the surface [21], i.e. shear strain γ = γ ₀ · SIN (ω t). If F is the force applied to the surface area S in the direction parallel to this surface, then the shear stress σ = F / S. The shear stress in a viscoelastic material is usually described using a G '-elastic modulus and G "- viscous modulus. Then σ = γ ₀ · (G' · sin (ω t) + G" · cos (ω t)). Figure 1 graphically shows for a temperature of 165 ° C the change in the elastic and viscous modulus G ', G "and the change in the ratio between the elastic and viscous modulus G' / G" with the frequency of exposure f = 2π ω for the melt of polyethylene LL1201 XV, which we used in examples of the implementation of the proposed technical solution. Curves were measured with an RT20 Rotovisco instrument [22]. In the limit of low frequencies, the viscous modulus is larger than the elastic modulus, that is, the material behaves like a liquid. In the limit of high exposure frequencies, the viscous modulus is less than the elastic modulus, i.e., the material behaves like an elastic body.

As shown in Example 4 below, extrusion of the melt of this polyethylene at a temperature of 165 ° C causes surface defects with a frequency of occurrence of about 40 Hz. The reason for the appearance of oscillations can be explained as follows. At the mouth of the mouthpiece, the surface layer of the product accelerates and stretches. The elastic tensile energy is released in periodic adherence-slip transitions, i.e. abrupt shifts of the surface layer of the material along the surface of the mouthpiece near its outlet, and in the destruction (crushing) of the surface layer of the polymer by cracks. The ratio between the elastic and viscous modulus for the polyethylene melt at the frequencies of occurrence of defects is close to G ’/ G" = 1.7. The value of 1.7 can be used as a criterion for an elastic material.

If, in accordance with the proposed technical solution, the surface of the mouthpiece near the outlet has a coating of elastic material, then the vibrations are damped by the coating. The coating material must be resilient compared to the polymer melt, i.e. the ratio of the elastic modulus to the viscous modulus for this material should be at least 1.7 times greater than the corresponding value for the polymer melt. Then it can be expected that in a coupled mechanical polymer-coating system, the resonant frequency will decrease, which will provide damping of the vibrations. Additionally, the presence of a coating leads to a redistribution of elastic energy between the polymer and the coating. The larger the G ' _polymer / G'' _coating ratio, the better the vibration damped.

Thus, the coating material can be considered elastic if the ratio of its elastic modulus to the viscous modulus at a characteristic frequency of occurrence of defects exceeds the corresponding value for the polymer melt by 1.7 times. In our case, this means (G ' _coating / G " _coating )> 3. Figure 2 graphically shows the change in the elastic modulus G' and the viscous modulus G" with the frequency of exposure f for silicone rubber, which we used in the examples of implementation of the proposed technical solution . At the characteristic frequencies of occurrence of defects of 40 Hz, the measured ratio G '/ G "> 20. Obviously, such rubber is an elastic material.

The coating of the elastic material during the formation of the polymer melt may vary, for example, wear out. The material does not lose elastic properties up to a thickness comparable to the length of the elastomer molecule, that is, up to 0.02-0.1 microns. Although the deformation of such a thin layer is difficult to measure, the nature of the elastic properties of the material does not qualitatively change at the indicated scales. The presence of a rigid surface changes the elastic properties of the layer only quantitatively, namely, the bond of molecules with a common rigid surface reduces the elastic modulus of the coating as compared to thicker layers.

The terms "rubber", "elastic material", "elastomer" and "elastic material" in the technical literature are synonyms. However, in practice, rubber is called not only a material that already has elastic properties, but also one that can acquire elastic properties as a result of chemical and / or heat treatment. Therefore, it is necessary to distinguish between crude rubber (latex, crude elastomer), which has no bonds between the molecules, which is a plastic material, and the rubber itself, which has bonds between the molecules. These bonds can be covalent, hydrogen, ionic and physical. Covalent bonds connect atoms that have common electrons. Vulcanized rubber (vulcanized elastomer), in which there are covalent bonds between the molecules, does not melt, although it changes its elastic properties when heated. It can be considered as one molecule with a very large molecular weight and complex three-dimensional structure. Rubber, in which polymer chains are connected by bonds that are weaker than covalent bonds, softens when heated and is called a thermoplastic elastomer. Such rubber additionally has increased adhesion to metals, because polar segments of molecules that provide a bond between molecules, at the same time provide a bond of molecules with oxygen atoms in metals and alloys.

The terminology is complicated by the fact that in practice rubber is called not only the elastomer itself, but also a mixture of elastomer with fillers, antioxidants, oils, etc. Therefore, the term "elastic material" corresponds to the term "rubber", implying that it generally has a complex composition.

Many varieties of vulcanized rubber lose their elastic properties when heated, become sticky and decompose at a temperature of 180 ° C-200 ° C, which is typical for the processing of polymeric materials by extrusion. For example, natural rubber cannot be used for elastic coating in this temperature range. Examples of vulcanized rubber grades that retain elastic properties at such temperatures include, but are not limited to, the group consisting of silicone rubber, fluorinated rubber, and fluorinated silicone rubber.

The term "silicone rubber" means a group of artificial elastic materials based on silicon (silanes, siloxanes). Silicone rubbers have high resistance to heat, they can be used for a long time at temperatures up to 235 ° C and can withstand short-term heating up to 315 ° C. The term "fluorinated rubber" means a group of artificial elastic materials based on fluorinated polymers, for example: fluorinated propylene (CF ₃ - CF = CF ₂ ) and fluorinated vinyl (CF ₂ = CF ₂ ). Fluorinated rubbers are highly resistant to heat (up to 230 ° C for a long time, up to 270 ° C for a short time) and are characterized by a slippery surface. The term "fluorinated silicone rubber" means a group of artificial elastic materials based on fluorinated silicone polymers.

Detailed Disclosure of Invention

The essence of the claimed device is illustrated by graphic materials and examples of implementation.

Example 1

The design of embodiments of the mouthpiece is schematically shown in Figure 3 with the following notation:

- A - the design of the mouthpiece made of hard material, made in accordance with known technical solutions,

- B and C - design options for the mouthpiece, made in accordance with the proposed technical solution,

- 1 - rigid base of the mouthpiece,

- 2 - a rubber coating, which is obtained by applying and curing an organosilicon elastomer on the corrugated surface of the metal base of the mouthpiece,

- 3 - an insert of vulcanized rubber, which is fixed in the metal base of the mouthpiece,

- 4 - metal diaphragm for attaching the rubber insert.

Option "A" of the known device was implemented by manufacturing a hole with a diameter of 6 mm in a metal base of the mouthpiece. The hole was polished to reduce the height of the roughness of the relief to 0.1 microns. The length of the mouthpiece was 6, 12, 24, 32, 48, and 96 mm.

Option "B" of the proposed device was implemented by applying a crude organosilicon elastomer to the surface of a threaded hole in the brass base of the mouthpiece and then vulcanizing it. The threaded holes M8 * 1 (cavity diameter varies from 7.2 to 8 mm), M7 * 0.75 (cavity diameter from 6.4 to 7 mm) and two variants of crude organosilicon elastomer were used, namely: one-component compound "Ceresit "manufactured by Henkel KGaA with a maximum operating temperature of up to 315 ° C and a two-component compound" RTV-ME 622 A "manufactured by Wacker-Chemie GmbH. Vulcanization of a one-component compound occurred under the influence of water vapor in the air. After applying a two-component compound, its vulcanization was carried out by heating to 150 ° C.

Option "B" of the proposed device was implemented by using an insert made of vulcanized silicone or fluorinated rubber. The insert was closed on the material inlet side by a diaphragm-sleeve with a hole diameter of 5 mm for material passage.

Example 2

The design of embodiments of the mouthpiece using annular inserts of rectangular cross-section and annular coatings is schematically shown in Figure 4 with the following notation:

- 1 - inelastic base of the mouthpiece,

- 2 - an annular rubber coating, which is obtained by applying and vulcanizing a silicone elastomer on a glass base of the mouthpiece,

- 3 - rubber insert,

- 4 - nut for fixing the annular insert.

Option "A" was implemented using an insert made of silicone or fluorinated rubber. The insert was pressed against the metal base of the mouthpiece with a nut.

Option "B" of the proposed device was implemented by applying a crude organosilicon compound to the inner surface of the glass base of the mouthpiece and subsequent vulcanization. The inner surface of the mouthpiece was coated with silicone rubber by dipping a liquid mixture of xylene and a one-component silicone compound "Elastosil N10" manufactured by Wacker-Chemie GmbH. The coating was dried in air, vulcanized at a temperature of 90 ° C in a humid atmosphere and fixed at a temperature of 200 ° C to remove volatiles from rubber. Vulcanized silicone rubber obtained from the indicated silicone compound is transparent, has a density of about 1.1 g / cm ³ , tensile strength of about 1.6 Pa, shear strength of about 2 Pa, maximum elongation to break of 400%. The coating thickness was varied by the degree of dilution of the compound with xylene. By immersing the tube at different depths in a liquid mixture of a one-component compound with xylene, ring coatings of various lengths were made.

Example 3

The design of the options for the mouthpiece:

- "A" - for extruding a pipe from a melt of a thermoplastic polymer material,

- "B" - for extruding the rod,

5 is schematically represented in FIG. 5 with the following notation:

1 - the supporting body of the mouthpiece with devices for attaching it to the extruder,

2 - distribution insert with holes for the passage of the melt of polyethylene and with annular grooves for uniform redistribution of the melt along the cross section,

3 - screw securing the central core of the mouthpiece to the distribution insert,

4 - a central core with a diameter of 18 mm,

5 - clip, mounted on the supporting body of the mouthpiece,

6 - ring insert of the mouthpiece (nozzle or "collar") with a diameter of 20 mm and a thickness of 5 mm,

7 - screws for attaching the annular insert and holder to the supporting body of the mouthpiece,

8 - pressure sensor,

9 - insert with a mouthpiece in the form of a tube for extruding the rod.

The gap between the central core 4 and the annular insert 6 in option "A" of the proposed device is 1 mm. A rubber coating was applied to the surface of the central core 4 and the annular insert 6 by dipping the parts into xylene diluted one-component silicone compound "Elastosii N10", then vulcanizing in a humid atmosphere and fixed in the same way as described in the description of Example 2 "B".

Example 4

We used ExxonMobil LL1030 XV for extrusion. The polyethylene used is characterized by the following parameters: density - 0.918 g / cm ³ , melting point - 121 ° C, melt index - 0.5 g in 10 minutes, the extrusion temperature recommended by the manufacturer is 180-200 ° C. A piston extruder with the diameter of the chamber for the material is 60 mm, with a length of 200 mm, with a maximum pressure in the chamber of 400 atm. The extruder piston was driven by a hydraulic cylinder. The material chamber is equipped with heaters providing heating up to 210 ° С, and the temperature of the chamber walls was measured with a thermometer - thermocouple. Additionally, the melt temperature in the chamber was measured with a contact thermometer, and the temperature of the outlet product and the temperature of the mouthpiece were measured with a non-contact thermometer - a pyrometer. The mouthpiece was heated by an additional heater. The pressure in the chamber was measured by a sensor with an accuracy of 0.5% in the range from 0 to 100 atm. The movement of the piston was controlled by a computer and measured with a position sensor with an accuracy of 5 μm. Electrical signals from a position sensor and a pressure sensor were converted into a digital signal by an analog-to-digital converter with an accuracy of 24 bits and were transmitted to the serial ports of the computer every 0.6 seconds. Parameters and measurements were controlled using the LabView 6.0 software package and a Pentium 4 computer.

The linear velocity of the output product was gradually increased from 1 mm / s to 350 mm / s. The output product was shot with a SONY Digital 8 DCR-TRV110E digital camera with a frequency of 25 frames per second. The product was illuminated with a stroboscope synchronized with the camera in order to exclude a blurry image due to product movement. Shooting began simultaneously with the beginning of extrusion, so the appearance of defects on the surface of the product could be correlated with high accuracy with the linear velocity of the output product.

For extrusion used mouthpieces made in accordance with the known technical solution of brass, presented in Figure 3 as option "A", with a length of 12 mm and 24 mm and with a diameter of a cylindrical cavity for the movement of the melt 6 mm The mouthpiece was secured in accordance with the embodiment shown in FIG. 5 “B” in order to measure the pressure immediately before its inlet. It has been reported that defects appear on the surface of the product even at an average linear melt flow rate in the mouthpiece of 3.8 mm / s. These defects have a periodicity and with increasing speed of extrusion grow both in amplitude and period. With a further increase in the speed of extrusion, the output product is an alternation of smooth and defective areas. Above a certain threshold of extrusion speed, the product again acquires a smooth, defect-free surface, and the pressure in the chamber drops. The extrusion of the material at such speeds is unstable to the occurrence of oscillations of speed and pressure, therefore, the industrial processing of polymer melts is not performed under such conditions. With a further increase in the extrusion rate, the product acquired large-scale distortions and irregular defects located on its surface. Thus, the defect-free extrusion speed of LL1030 XV polyethylene when using a mouthpiece of known design with a diameter of 6 mm and a length of 24 mm at a temperature of 155 ° C is limited by the critical linear extrusion speed of 3.8 mm / sec.

Additionally, we extruded ExxonMobil LL1201 XV linear polyethylene at 165 ° C through a transparent glass tube 32 mm long and 6 mm inner diameter. The polyethylene used is characterized by the following parameters: density - 0.926 g / cm ³ , melting point - 123 ° C, melt index - 0.7 g in 10 minutes, extrusion temperature recommended by the manufacturer -180 -200 ° C. When observed in crossed polarizers at the moment the appearance of surface defects near the outlet tip of the tube, a bright spot 0.5 mm long along the flow axis was recorded, which showed the appearance of a predominant orientation of the polymer molecules. The critical linear rate of appearance of surface defects was 5.4 mm / s. The appearance of the preferential orientation of the polymer indicates the stretching of the material near the exit of the mouthpiece. Similarly to the case of a brass mouthpiece, the product emerging from the glass mouthpiece at very low speeds had a smooth glossy surface, then a surface roughness appeared. With an increase in the extrusion rate, the roughness scale increased both in length and in elevation. With a further increase in speed, areas of the defective surface followed areas of a smooth surface and the product resembled a bamboo trunk. It has already been established by other researchers [22] and this is confirmed by our observations that such a relief corresponds to periodic transitions of the flow from adhesion to sliding along the entire wall of the mouthpiece. Figure 6 presents a graph of pressure changes with increasing extrusion speed for a glass mouthpiece (solid curve). The values of the frequency of the surface relief on the product, i.e. the ratio of the extrusion rate [mm / s] to the period of the surface relief [mm] were measured and shown in FIG. 6 by a dashed line. As can be seen from this graph, the frequency of occurrence of defects varies little with increasing linear extrusion rate. Pressure oscillations and large-scale surface distortions are observed at velocities of about 120 mm / sec and correspond to “stick-slip” transitions along the entire surface of the mouthpiece. However, the values of the frequency of occurrence of defects at these speeds and at low speeds are close. This observation supports the explanation that surface defects appear due to periodic “stick-slip” transitions near the mouthpiece exit. The frequency of occurrence of defects observed for a temperature of 165 ° C and for a glass mouthpiece 32 mm long lies in the range from 25 to 60 Hz with an average frequency of 40.2 Hz. The ratio of the elastic modulus to the viscous modulus for the molten polyethylene at this frequency is 1.7.

Example 5

To study the temperature dependence of the critical extrusion rate, extrusion of the LL1030 XV polyethylene melt was carried out through a brass mouthpiece with a diameter of 6 mm and a length of 24 mm, shown in Figure 3 as option "A", and the melt temperature was varied. Figure 7 presents a graph of the critical linear extrusion rate with changing melt temperature. It can be seen that in the range of 180-200 ° C, which is recommended by the manufacturer of polyethylene for extrusion, the critical speed increases. The polyethylene used for the production of blown film can have a melting point in the range from 100 ° C to 125 ° C, and the extrusion temperature recommended by the manufacturer is 60-100 ° C higher than the melting temperature. The temperature range recommended by the manufacturer is limited from below by the appearance of surface defects at a low extrusion rate, and from above by the instability of inflation of the melt film and thermal decomposition of the polymer.

Example 6

To study the dependence of the critical linear extrusion speed on the length of the mouthpiece, the extrusion was carried out through a stainless steel mouthpiece, shown in Figure 3 as option "A", with a diameter of 6 mm and a length of 0 (aperture) to 96 mm. The melt temperature was fixed at 160 ° C. FIG. 8 is a graph of the critical linear extrusion rate with a change in the length of the mouthpiece. It can be seen that with a change in the length of the mouthpiece in the range from 0 to 96 mm, the critical velocity slowly increases.

Example 7

To study the effect of the material of the inner surface of the mouthpiece on the appearance of defects during extrusion, inserts 12 mm long and 6 mm in diameter were made of the following materials: Stainless steel, Brass, Teflon, Boron Nitride (BN). Boron Nitride, which is known for its high temperature resistance and slippery surface due to a crystalline structure like graphite, was kindly provided for measurement by Henze BNP GmbH. Table 1 shows the values of the critical extrusion rate for the studied materials at a temperature of 155 ° C. It is seen that the highest value of the critical extrusion rate is achieved using boron nitride. However, even when using this material, the increase in the speed of defect-free extrusion is negligible compared to using a stainless steel mouthpiece. The critical speed values are presented in Table 1 for comparison.

Table 1 Insert Material (Length) The value of the critical extrusion rate at a temperature of 155 ° C, mm / s Stainless steel (12 mm) 3.8 Brass (12 mm) 5,0 Teflon (12 mm) 5.2 Boron Nitride (BN, 12 mm) 5.9

Example 8

To demonstrate the proposed technical solution, extrusion was carried out through a mouthpiece coated with elastic material - option "B" of the implementation of the proposed device shown in Fig. 3. For extrusion, mouthpieces with a length of 12 mm and 24 mm, with an inner diameter of 6 mm, were used. Fig. 9 is a solid line showing a graph of pressure changes with increasing extrusion speed for the case of using the proposed design of a 12 mm mouthpiece with an elastic coating on a threaded hole M8 * 1 at a temperature of 155 ° C. It has been reported that defect-free extrusion of polyethylene occurs up to a speed of 114 mm / sec, at which mechanical destruction of the coating occurs. For comparison, there is also a graph of pressure changes with increasing speed for a steel mouthpiece 12 mm long and 6 mm in diameter, used in Example 7. For a steel mouthpiece, surface defects (shark skin) appeared at a speed of 3.8 mm / sec. When using a coating of two-component organosilicon compound “RTV-ME” and a threaded hole M7 * 0.75, mechanical destruction did not occur up to the maximum achieved value of the average linear extrusion rate of 300 mm / s, but surface defects appeared at a speed of 117 mm / s. Compared to using a steel mouthpiece, defect-free extrusion was observed at a speed 30 times greater. The critical speed values are presented in Table 2 for comparison.

table 2 Insert Material (Length) The value of the critical extrusion rate at a temperature of 155 ° C, mm / s Ceresit Coating (12 mm) 114 Coating "RTV-ME" (24 mm) 117

Example 9

To demonstrate the proposed technical solution, extrusion was carried out through the mouthpiece with an insert of elastic material - option "B" of the implementation of the proposed device, shown in Figure 3. For a 12 mm long silicone rubber insert, defect-free extrusion at a temperature of 155 ° C was observed with an increase in the average linear extrusion speed to 112 mm / s. For a 6 mm long silicone rubber insert, defect-free extrusion at a temperature of 152 ° C was observed with an increase in the average linear extrusion speed to 45 mm / s. The critical speed values are presented in Table 3 for comparison. The destruction of the elastic insert was not observed up to the maximum achieved value of the linear extrusion speed of 340 mm / sec.

Table 3 Insert Material (Length) The value of the critical extrusion rate at a temperature of 155 ° C, mm / s Silicone rubber (12 mm) 112 Silicone rubber (6 mm) 45

Example 10

To demonstrate the proposed technical solution, extrusion was carried out through a mouthpiece with an elastic insert 1.5 mm long combined with a mouthpiece 24 mm long — an embodiment of the proposed device shown in FIG. 4 "A". Surface defects were formed at a speed of 27 mm / s, i.e. less than for a long insert. Using an insert of slippery fluorinated rubber allows defect-free extrusion at speeds up to 29 mm / sec. The critical speed values are presented in Table 4 for comparison.

Table 4 Insert Material (Length) The value of the critical extrusion rate at a temperature of 155 ° C, mm / s Silicone rubber (1.5 mm) 27 Fluorinated rubber (1.5 mm) 29th

Example 11

A glass tube of 6 mm inner diameter and 32 mm long was coated on the inside with silicone rubber according to example 2, option "B". Coatings were made with the following lengths from the outlet of the tube: 0.5; 1.5; 6; 18 and 32 mm. The coating thickness was about 0.1 mm. Uncoated tubes and a tube coated only with a flat exit surface, i.e. with a coating length along the tube axis of not more than 1% of the tube diameter. Extrusion of the polyethylene melt was carried out at a temperature of 158 ° C. The characteristic curves in the logarithmic coordinates of speed / pressure are shown in Fig. 10 for the following series of coating lengths: only the tube outlet had a coating of 6; 18; 32 mm. The appearance of roughness on the surface of the product is shown by straight crosses in the case when there are characteristic curves, and inclined at 45 ° crosses for other cases. As the length of the coating increases, the pressure drop across the tube for a given speed decreases. The critical speed values are presented in Table 5 for comparison. It can be seen that already with a coating length of 0.5 mm, i.e. about 10% of the diameter, defects form at a speed of about 7 times higher compared to an uncoated mouthpiece.

Table 5 Coating Length, mm The value of the critical extrusion rate at a temperature of 158 ° C, mm / s No coverage 5.8 Only exit 8.1 0.5 41 1,5 56 6 112 eighteen 156 32 162

Example 12

The melt velocity profile of LL1201 XV polyethylene was measured inside an uncoated glass tube and a coated glass tube at a distance of 0.5 mm from the tube exit. Particles of boron nitride (BN) having white color, the structure of thin flakes, and a size of about 30 μm were selected as tracing particles. The displacement of the tracer particles was recorded using a video camera equipped with a micro lens. The camera resolution was about 1.5 microns per pixel. The camera was focused on the inner wall of the tube, and then, using a micrometer table, it was moved to the center of the stream. The error in determining the radial coordinate was about 0.2 mm and was caused by the extended focus area of the microscope. The relative error in the speed measurements was about 20% and is due to the mechanical vibrations of the camera relative to the tube and the instability of the flash backlight. The resulting velocity profile is shown in FIG. 11. The velocity profile is normalized to the average product speed V ₀ , and the distance from the center of the tube is normalized to the radius of the tube. The difference in the velocity profile for a tube with an elastic coating (solid curve) and a clean tube (dashed line) is clearly visible. In the case of a clean tube, the rate of polymer melt near the tube wall tends to zero, while in the case of a rubber-coated tube, the melt glides along the wall. Silicone rubber has the chemical composition of organically modified quartz oxide, i.e. close to the composition of the glass, but the difference in the behavior of the molten polyethylene material near the rigid glass wall and near the wall covered with elastic silicate material is striking.

In FIG. Figure 12 shows the curves of changes in the normalized sliding velocity of the polyethylene melt along the elastic surface with increasing extrusion rate for 144 ° С (solid curve), 158 ° С (dashed line), 187 ° С (points). It can be seen that with increasing temperature of the melt of polyethylene, the sliding speed decreases.

Example 13

For extrusion, Exceed 1018 EB linear low-density polyethylene obtained with metallocene catalysts was used. A glass tube with an inner diameter of 6 mm and a length of 32 mm was used, coated with silicone rubber by dipping into an Elastosil N10 compound diluted with xylene and a glass tube coated with fluorinated silicone rubber by mechanical application of a one-component compound FRV 1106 from GE Silicones. The use of the mouthpiece with an elastic coating corresponds to option "B" of the implementation of the proposed device, shown in Fig.3. Fluorinated silicone rubber contains a filler (iron oxide) and when diluted with xylene, the filler particles coagulate, which leads to roughness of the coating. When applying the compound mechanically, it was also not possible to avoid scratches and surface irregularities. Vulcanization and fixation of the coating was carried out similarly to the silicone rubber coating in Example 2. Using fluorinated silicone rubber gives a more rigid coating compared to silicone rubber.

Extrusion was carried out at a temperature of 186 ° C. FIG. 13 shows the characteristic curves obtained for an elastic coated glass tube and for a clean glass tube. Crosses mark the occurrence of defects on the output product: 207 mm / s for fluorinated silicone rubber, 284 mm / s for silicone rubber and 7.9 mm / s obtained by extrusion through an uncoated glass tube for comparison.

Example 14

In an additional experiment, we tried to see during extrusion of a melt of polyethylene LL1201 XV at a temperature of 186 ° C the difference in the appearance of defects on the surface of the output product for thick (0.2 mm) and thin (of the order of 0.001 mm) elastic coatings. Thin coatings were applied using a highly diluted xylene rubber compound. In FIG. Figure 14 shows the characteristic curves obtained for an uncoated glass tube (solid curve), with a thin elastic coating (dashed line), and for a thick elastic coating (point). Crosses mark the occurrence of defects on the output product: 6.2 mm / sec for a clean glass tube, 172.3 mm / sec for a thin coating and 240 mm / sec for a thick elastic coating. Lower values of the maximum defect-free extrusion rate for the case of thin coatings indicate that the elastic properties of a thin layer of material are worse than a thick layer. It is also seen from the characteristic curves that when thin elastic coatings are used, the pressure drop across the tube is greater than when using thick coatings.

Example 15

A glass tube of 6 mm inner diameter and 32 mm long was internally coated with a TRE-120 thermoplastic elastomer manufactured by Wacker-Chemie GmbH or a Viton® FreeFlow fluorinated polymer manufactured by DuPont. Coatings were mechanically applied to a clean glass tube at a temperature of about 240 ° C and leveled by extrusion of a polyethylene melt at a temperature of 220 ° C to obtain thin coatings of comparable thickness. The fluorinated polymer "Viton® FreeFlow" is characterized by low adhesion to the polyethylene melt, but at a defect rate of 40 Hz and for a temperature of 165 ° C the ratio of elastic modulus to viscous modulus G '/ G' '= 1.8, which is close to the corresponding value for molten polyethylene G '/ G' '= 1.7. The thermoplastic elastomer "TRE-120" is characterized by the ratio of the elastic modulus to the viscous modulus G ’/ G’ ’= 3.3, i.e. at least 1.7 times the corresponding value for LL1201 XV polyethylene melt. In FIG. Figure 15 shows the characteristic curves obtained by extrusion of a LL1201 XV polyethylene melt at a temperature of 165 ° C using an uncoated glass tube (solid curve), with a thin coating of fluorinated polymer (dashed line), and with a thin elastic coating with a thermoplastic elastomer (dots). Crosses mark the occurrence of defects on the output product: 6.7 mm / s for a clean glass tube, 17.8 mm / s for thin coating with fluorinated polymer and 65.3 mm / s for thin coating with an elastic thermoplastic elastomer. The lower values of the maximum defect-free extrusion rate for the case of using thin coatings with fluorinated polymers as compared to thin coatings with an elastic thermoplastic elastomer indicate that the choice of material for coating with a ratio of elastic modulus to viscous modulus is at least 1.7 times greater than the corresponding value for polymer melt can significantly increase the speed of defect-free extrusion. It is also seen from the characteristic curves that when thin elastic coatings are used, the pressure drop across the tube is less than when using coatings of fluorinated polymers of comparable thickness.

Example 16

For extrusion, the mouthpiece shown in FIG. 5 option "A" for pipe extrusion. Extrusion of polyethylene was carried out at a temperature of 158 ° C. First, an uncoated mouthpiece was used. In the second embodiment, the annular insert (collar) was coated with silicone rubber as described in Example 2. In the third embodiment, both the central core and the mouthpiece collar were covered. In the case where there was no coating, surface defects occurred at an extrusion rate of about 4 mm / s. In the case when only the annular insert was coated with rubber, defects appeared on the inner side of the pipe, and on the outer side they were not up to the extrusion speed of 89 mm / sec. In the case when both surfaces were coated with rubber, no defects were observed up to a speed of 89 mm / s. The critical speed values are presented in Table 6 for comparison.

Table 6 Coating Material (Length) The value of the critical extrusion rate at a temperature of 158 ° C, mm / s Without cover 4 Silicone rubber 89

So, the use of the proposed device can increase the defect-free extrusion rate. The length of the insert or coating of elastic material can be as little as 10% of the gap width for the polymer melt, since the mechanical stresses leading to pressure and velocity fluctuations of the material are concentrated in the zone of contact of the flow with the walls of the mouthpiece near its outlet. An increase in the length of the elastic coating or insert along the polymer stream improves the quality of the product and further increases the defect-free extrusion rate. The use of the proposed device allows extrusion at a low temperature. Reducing the temperature of extrusion will increase the stability of inflation of the film, to avoid thermal decomposition of the polymer in the stagnant zones of the extruder. Instead of practicing the previously exceeded extrusion temperature above the melting temperature of the polymer material by 60-100 ° C, extrusion can be performed with the melting temperature exceeding just 20-60 ° C.

The proposed device has obvious significant differences from the prototype. Namely, a coating or insert of resilient material is used.

The effect of suppressing tears on the outer surface during extrusion through a mouthpiece with an elastic coating or insert was not previously known from the patent and special literature.

As a result of the patent search, no known technical proposals were found, characterized by a combination of distinctive features.

In the implementation example, the achievement of a technical result is demonstrated - an increase in the speed of defect-free extrusion of polyethylene.

Implementation examples indicate industrial applicability.

So, the proposed device is new, has significant differences from the prototype, is industrially applicable, and therefore, it is characterized by all the necessary features of the invention.

Literature

1. J.P. Tordella, SPE J., (Feb. 1956), 36

2. J.P. Tordella, J. Appl. Phys, 27, (1956) 454

3. E.R. Howells, J. J. Benbow, Trans. Plast. Inst. 30 (1962) 240

4. A.V. Ramamurthy, J. Reol. 30 (1986) 337

5. T. J. Person, M. M. Denn, J. Rheol. 41 (1997) 249

6. C. Venet, B. Vergnes, J. Rheol. 41 (1997) 873

7. J.W. H. Kolnaar, A. Keller, J. Non-Newt. Fluid Mech. 67 (1996) 213

8. P.S. BIatz, et.al. US Patent No. 3,125,547 "Extrudable composition consisting of a polyolefin and a fluorocarbon polymer", 17/03/1964, US Class - 525/199

9. Chisholm, et al. US Patent No. 5,854,352 "Melt Fracture Reduction", 12/29/1998, MKI - C 08 L 027/12

10. Bentivoglio, et al. US Patent No. 4,830,595 "Die-lip heater", 16/05/1989, MKI - B 29 C 047/86

11.Bentivoglio, et al. US Patent No. 4,842,504 "Lip heater for plastic extrusion die", 06/27/1989, MKI - B 29 C 047/86

12. Cogswell, US Patent, No. 3,920,782 "Sharkskin", 18/11/1975, MKI - B 29 C 17/07

13. Fraser, et al., US Patent, No. 4,243,619 "Process for making film from low density ethylene hydrocarbon copolymer", 06/01/1981, MKI - B 29 D 007/04

14. Kurtz, et al., US Patent, No. 4,282,177 "Method for reducing sharkskin melt fracture during extrusion of ethylene polymers", 04/08/1981, MKI - B 29 D 023/04

15. Kurtz, et al., US Patent, No. 4,360,494 "Process for reducing melt fracture during extrusion of a molten narrow molecular weight distribution, linear, ethylene copolymer", 11/23/1982, MKI - B 29 D 023/04

16. Ramamurthy, US Patent No. 4,522,776 "Process for substantially eliminating surface melt fracture when extruding ethylene polymers", 11/06/1985, MKI - B 29 F 03/4

17. Pawlowski, et al., US Patent No. 4,948,543 "Process for eliminating surface melt fracture during extrusion of thermoplastic polymers", 08/14/1990, MKI - B 29 C 033/60

18. Kissi, et al., "Effect of surface properties on polymer melt slip and extrusion defects", Journal of Non-Newtonian Fluid Mechanics, 52 (1994), 249-261

19. Kissi, et al., "Sharkskin and cracking of polymer melt extrudates", 68 (1997), 271-290

20. B. Tremblay, "Sharkskin defects of polimer melts, role of cohesion and adhesion", J. Reol. 35 (1991) 985-998

21. A.Yu. Grosberg, A.R. Khokhlov. Physics in the world of polymers. M., "Science", 1989

22. http://www.thermo.com/haake

23.K.P. Adewale, A.I. Leonov, Modeling spurt and stress oscillations in flow of molten polymers, Rheol. Acta 36 (110-127) 1997.

Claims (2)

1. Device for extrusion of a thermoplastic polymeric material subject to crushing of the melt, including a mouthpiece, the base of which is made of one material, and on the inner surface of the cavity in the part adjacent to the exit, a coating is applied or an insert made of another material, elastic compared with a melt of polymer material, a length of at least 1/10 of the gap width for extruding the melt. 2. The device according to claim 1, characterized in that silicone rubber is selected as the material of the elastic coating.

Images