DE1260766B - Process for the continuous processing of thermoplastics - Google Patents

Description

Process for the continuous processing of thermoplastics The invention relates to a method for the continuous processing of thermoplastics Plastics by pressing or spraying through nozzles.

Thermoplastic resins are generally used for extrusion molding or injection molding the molten resin passed through a tool for shaping, which optionally can be heated from the outside, the mold by rapid subsequent cooling of the molten material is maintained.

Many thermoplastic resins can be produced using this extrusion technique process in the melt, but certain resins, e.g. B. Polyethylenes with high Melt viscosity, are very difficult to measure at economical rates extrusion or injection. Above a limit speed (as »critical Extrusion speed «) one obtains an extreme roughness of the extrudate (referred to as "surface fracture") which makes it unusable for most purposes. The critical extrusion rate can be increased by increasing the temperature increase the melt, but the melting temperature itself is limited by the fact that the pressed body after exiting the outlet opening of the tool should retain the desired shape and that degradation of the polymer is avoided got to.

The outlet opening of the tool is the part of it which last touches the extrudate.

Below the melting temperature is the mean temperature of the melted Understand thermoplastics immediately before it passes through the tool. The extrusion temperature range is defined here as the temperature range in which a uniformly heated thermoplastic resin can be pressed without any mass decomposition is noticeable. The upper limit of the extrusion temperature range any thermoplastic material depends on the heat resistance of the Resin. The lower limit of this range is generally from the softening point of the resin, in which the material forms a viscous mass. the mean melt temperature is the mean temperature of the resin when it passes through Exit opening of the mold. It is an improvement in the quality of the extrusion and the extrusion speed achievable by making the extrusion through a tool undertakes, the outlet opening is heated to such a temperature that on Substantial decomposition of the polymer takes place on the surface of the extrudate. It was previously of the opinion that an extrusion at such temperatures that too decomposition of the resin lead, appearance and other physical properties would adversely affect the extrudate.

According to the invention it is therefore proposed that the nozzle outlet opening to well above the decomposition temperature of the plastic to be processed lying temperature is heated and the extrusion rate is adjusted in this way that the decomposition occurs only on the surface of the nozzle opening Plastic takes place. It is achieved that despite the supply of heat to the Outlet opening of the mold, to a substantial surface degradation of the Resin cause the extrudate not only in bulk and on the surface is able to retain its desired properties, but that the surface of the extrudate can be made smoother, for propelling the resin through the Tool less force is required and a higher extrusion speed can be achieved without a surface break occurring.

The surface degradation strength is determined as follows: One A sample of the resin film 0.152 mm thick and 50 mg in weight is placed on a stainless steel sheet Brought steel 0.79 mm thick. Sheet steel and polymer film, in turn, are in introduced a test device. The test device contains a circulating air stream, which is held at about 150 "C before introducing the sample. The temperature is then at 5 ° C / min. elevated, until the polymer is essentially complete is converted into a volatilized product. The weight loss of the sample will be recorded and in a graph as a function of the temperature applied. Since a thin film (initial thickness 0.152 mm) is used for the determination it can be assumed that the specific weight loss on the material surface originated. The slope of the curve obtained is here as the surface decomposition strength Are defined. Because the temperature of the heated air stream is a function of time is the surface decomposition strength as a percentage of weight loss Calculate volatilization per hour. This size changes with different ones Resins, but the temperature of the nozzle outlet opening of the tool should in general have such a value that the surface decomposition strength is at least 32% Weight loss by volatilization per hour is.

With lower surface decomposition strengths, this size increases the temperature slowly, but above 47 ° / 0 weight loss due to volatilization per hour increases very quickly.

According to one embodiment of the invention, therefore, is close to the outlet opening of the mold a high frequency induction heating element is provided and sufficient heat supplied so that the nozzle outlet opening was operated at such a temperature that the surface decomposition of the emerging thermoplastic material at least 32, in particular 470 / o weight loss due to volatilization per hour amounts to. The temperature of the nozzle outlet opening necessary for decomposition can be determined with the aforementioned test. The heating of the outlet opening of the tool can also be done in a way other than high-frequency inductive. It However, it is necessary that only the nozzle outlet opening of the tool is heated will. Heating the entire mold can degrade the entire mold Polymers lead. It is also useful that the polymeric material with a Propellant processed mixed.

The minimum temperature of the mold can be determined according to the above Determine test, but the optimal temperature is best determined by experiment. A practical way of working is to open the outlet of the tool Apply heat until the desired gloss is achieved.

If an increase in the extrusion speed is not desired, then the outlet of the tool is simply kept at this optimal temperature and the squeezing continued. For many Purposes it is desirable at maximum Exit speed to work in order to maximize the output of the machine achieve. If so desired, the extrusion speed and heat input at the same time reinforced towards the outlet opening of the tool until the desired extrusion speed is achieved. Then, as already described, the optimum temperature of the tool outlet opening becomes determined and maintained for the new extrusion speed. The optimal one The temperature of the mold outlet changes with each resin and with it the intended use of the extrudate, but it can generally be said that at a moderate extrusion rate better results can be obtained if one the tool to 20 to 1000 "C above that determined by the surface degradation test determined minimum temperature of the tool outlet opening is heated. At faster The dwell time of the thermoplastic material in the press is reduced, and in many presses the melting temperature decreases again if one does not supplies additional heat to the mass cylinder or a press with a longer cylinder used.

If extremely high temperatures of the outlet opening are required, you should use a material for the tool that can withstand high temperatures withstands a long time. The tool should also be shaped so that it is heated in the Condition and use is the shape and size you want. With slow pressing the temperature of the outlet opening is limited by the temperature at which degradation of the entire polymer takes place. However, this temperature is extraordinary high, and the extrusion speed is already extremely slow before any Degradation becomes noticeable. At the technically applied stall speeds of wire, 152 to 914 m / min., a failure of the material of the injection molding tool occur before degradation of the entire polymer occurs.

Table 1 shows examples of minimum temperatures of the tool outlet opening and preferred work areas for different types of technically used today Polymers listed; it has been shown that those intended for a specific resin Temperatures apply quite generally to other resins of the same type. For example the values determined for a given low density polyethylene apply, regardless of the molecular weight distribution or the melt viscosity in general for other low density polyethylenes.

Table 1 Preferred Resin type Temperature of the outlet opening, ° C maximum mean Melt temperature minimal I preferred range oc "Low density" polyethylene (0.91 to 0.94) .... 335 360 to 900 300 "High density" polyethylene (0.95 to 0.97) ........ 325 340 to 900 300 Polypropylene ................... .. 250 290 to 750 275 Hexafluoropropylene-tetrafluoroethylene interpolymers. . 480 480 to 1000 425 Ethylene / C3-12 N-olefin copolymer. . ... 340 to 900 300 Polyoxymethylene ............................... ~ 300 to 1000 275 Polyvinyl butyral ..... ..... - 325 to 900 215 Polyhexamethylene adipamide ...................... - 325 to 900 310 In general, -olefin-ethylene copolymers are pressed at the same temperatures of the outlet opening of the tool as the ethylene homopolymers, the more crystalline copolymers being comparable to the "high-density" polyethylene and the less crystalline copolymers being comparable to the homopolymers of the "low density" ethylene.

All thermoplastic materials are suitable for carrying out the invention Resins. Examples of particularly valuable resins are the hydrocarbon resins, such as polyethylene, especially with a melt index in the range from 0.001 to 20, Polypropylene, especially with a melt index of 0.001 to 10, ethylene propylene and other C ,,, - α-olefin copolymers, especially those having at least 75 percent by weight Ethylene, polystyrene; Fluorocarbon resins such as tetrafluoroethylene-hexafluoropropylene interpolymers, especially those with a specific IR ratio of 1.5 to 6.0 and one Melt viscosity in the range from 1.5 103 to 1 106 P, and polyhexafluoropropylene; Polyaldehyde resins, such as polyoxymethylene, particularly having a number average molecular weight of more than 10,000; Polyamide resins such as polyhexamethylene adipamide, especially with a relative viscosity in the range from 40 to 300 (determined on a solution in Formic acid with a polymer content of 8.4 percent by weight), polyhexamethylene sebacamide and polycaprolactam; Polyacrylate resins such as polymethyl methacrylate and polymethyl acrylate; Chlorocarbon resins such as polyvinyl chloride; Polyacetal resins such as polyvinyl butyral and polyester resins such as cellulose acetate and cellulose butyrate. If desired, you can the resins, if desired, also include plasticizers, stabilizers, foaming agents or blowing agents, Contains pigments, fillers and the like, but when performing the surface degradation tests should the polymer without low-boiling additives such as plasticizers and blowing agents, being checked.

Example 1 A 1.63 mm thick copper wire is used an extruder with a cross head, an injection nozzle and a screw press a 1.14 mm thick coating of fluorocarbon polymer overmolded.

A tetrafluoroethylene-hexafluoropropylene interpolymer which can be sprayed in the melt is used as the polymer with a specific IR ratio of about 3.5 and a melt viscosity of 8.12 104P (determined at 380 "C and a shear stress of 0.46 kg / cm2; the determination of the IR ratio mentioned in the present description such as also the melt viscosity of tetrafluoroethylene-hexafluoropropylene interpolymers is described in Belgian patent 560 454) and surface decomposition of 470 / o weight loss by volatilization per hour and 4100 / o weight loss by volatilization per hour at 480 or 550 ° C.

The upper extrusion temperature of this polymer is approximately 400 "C. The wire is attached to the polymer without using the method according to the invention 10.4 m / min. encapsulated; the surface of the coating is extremely rough. this shows that the extrusion rate is above the critical value and a Surface fracture occurs. The pressure in the cross head is 19.0 atmospheres and the temperature the melt 350 "C; the die temperature is 372" C.

In a further attempt, the extrusion coating is carried out on the same Speed, however, the nozzle outlet opening by means of a high-frequency induction heater Adds heat that is designed for 2 kVA at 20 MHz. The energy supply to the nozzle outlet opening takes place from the water-cooled coils of the induction heating element, which in one 3.2 mm from the nozzle surface. When the temperature of the When the nozzle outlet opening reaches 510 ° C, the pressed material is very smooth and shiny; the head pressure has dropped to 14.1 atmospheres and the melting temperature remains at 350 "C.

The heating of the nozzle is now interrupted; in a few minutes the nozzle outlet has cooled down enough to restore the surface break to let occur. During the spraying work with the induction heated nozzle outlet opening a solid white powder collects on the coils of the induction heating element, which has been found to be a fluorocarbon polymer by ultrasound analysis.

From the wire that is overmolded to 510 "C at a nozzle outlet opening a portion of the polymer is removed. The melt viscosity of this material is 8.1 104 P; this shows that no significant degradation of the polymer has occurred is.

Corresponding results are obtained if you work at a speed from 7.6 m / min. the wire is coated with a commercial polyhexamethylene adipamide and keep the melt temperature at 272 "C as well as the exit port using of the high-frequency heating element heated to 400 "C or if you use a plasticized polyvinyl butyral at a melting temperature of 172 "C, using the outlet opening heated to 350 "C.

Example 2 A low density polyethylene (melt index 0.3, density 0.921 and surface decomposition strength of 470 / o weight loss by volatilization per hour and 200% weight loss due to volatilization per hour at 335 and 410 "C) is mixed with a temperature sensitive yellow pigment. A 0.64 mm thick wire is overmolded with the mixture on an extrusion press 0.25 mm thick; the yellow pigment undergoes a noticeable change in color when it is at melting temperatures above 260 "C is sprayed.

The melt is applied to the wire at 240 "C, at 762 m / min. without using the nozzle overheating to help. Collects within 15 seconds a significant amount of polymer on the nozzle edges. Man leads the edges using the high-frequency heating element described in Example 1 to heat, wherein the exit opening takes on a dull red color (i.e. temperature about 700 ° C). The previously dull surface of the sprayed product takes on a very shiny one Texture on, and within 5 seconds the polymer accumulation is at the nozzle edges eliminated.

The head pressure (measured in the cylinder immediately in front of the spray nozzle) drops when heat is supplied to the outlet opening from over 281 to 225 kg / cm2. A No degradation of the yellow pigment was found at this injection speed. The spray speed must be at 15.2 m / min. be reduced, before a degradation of the pigment can be detected with the naked eye.

Example 3 A 1.63 mm thick wire is placed on the same device and following essentially the same procedure as in the example with a 1.14 mm thick coating of an interpolymer of ethylene and octene-1 (containing approx 20 / o bound octene-1, melt index 0.5, density 0.94 and surface decomposition strength 47 and 2900 / o weight loss due to volatilization per hour at 325 and 465 ° C, respectively) encapsulated. This polymer has a low critical extrusion speed, what thus making it impossible to squeeze it out at economic speeds. The results obtained under various conditions are summarized in Table II.

If you look at the outlet opening under the conditions of the experiment a operated nozzle heat supplies, the head pressure drops considerably, is the plating work possible at much higher speeds and the coating becomes smooth and shiny (Experiment b). Surface degradation of the polymer is due to smoke formation and condensation an oily substance on the induction heating coil.

The nozzle outlet opening is allowed to cool and the speed is increased of the coating process to 36.6 m / min. (Experiment c). The coating is very rough, what shows the occurrence of surface fracture.

The conditions are then changed (experiment d); the coating becomes smooth and shiny, and the head pressure is considerably reduced. An increase in the nozzle outlet temperature leads to an even further drop in the head pressure and to a further improvement in the extrudate (experiment e). The properties of the wire coatings obtained using the invention are found to be at least as good as those of coatings made from the same polymer when applied by conventional methods. Table II Spray, screw, coating, screw nozzle application speed head pressure temperature exit to a quality of the coating test duration temperature of a coating quality UlMin. m / min. kg / cm '0C 0C heating element a 42 14.3 10.5 240 255 no very rough (Surface fracture) b 39 21.6 3.5 220 360 yes smooth and shiny c 55 36.6 14.1 200 255 no very rough (Surface fracture) d 55 36.6 10.5 198 400 yes smooth and shiny e 55 36.6 7.0 198 545 yes very smooth and very shiny If a 0.43 mm thick coating is applied with the same polymer to a 0.64 mm thick wire using a wire extrusion nozzle with a diameter of 1.65 mm in the outlet opening, surface fracture even occurs when using an injection molding tool without external heating a speed as low as 4.6 m / min. on. When using the overheated outlet opening, on the other hand, the spraying work can be carried out at more than 152 m / min. take place, and the pressed material remains smooth and shiny.

Example 4 A foamed polyethylene film is made from a polyethylene low density (melt index 2.1, density 0.9225 g / cm², containing 1.5 percent by weight, based on the total mass of azodicarbonamide as blowing agent) produced by the resin mass through a 15.2 cm wide film injection nozzle with a slot height of 0.508 mm, which is fed by an extruder, which has a screw press having a mixing zone. The melt is at 273 ° C and the outlet opening heated to 450 ° C.

The film obtained has a smooth and glossy surface, a Thickness of 0.51 mm and a density of 0.36 g / cm³.

The method according to the invention is suitable for the production of Articles such as pipes and hoses (whereby, although often not necessary, also heating of the mandrel is possible), monofilaments, blown films, flat films and web material, if desired, made of foamed material, for covering articles such as wire, Cloth, paper, nets and cardboard or cardboard, if desired, with a foamed one Coating, for filling molds such as those used in injection molding (where the article is of course not yet his when it passes through the outlet opening final shape) and for shaping bottles by passing through an overheated Tool presses out a tube and then forms the same into the bottle. Generally The process is suitable in all cases where a resin is required at high speed or to produce a product with a smooth surface through an outlet opening should be driven.

In injection molding, the invention has the advantage of preventing melt breakage of the resin before deforming. A melt fracture of the resin when passing through through the outlet opening in the mold affects the physical properties the molded body disadvantageous. The invention leads to a significantly better product quality and increases the production capacity of the manufacturing apparatus. today in the Machines that are in use can be left in most cases without much Adjust changes.

It ultimately leads to a very high level of adaptability Plastic deformation work to the desired purpose.

Claims (3)

Claims: 1. Method for the continuous processing of thermoplastics by pressing or spraying through nozzles, d a d u R c h ek ek e n n z e i c h n e t that the nozzle outlet opening to a far over the temperature of the decomposition temperature of the plastic to be processed is heated and the extrusion speed is adjusted so that the decomposition only takes place on the surface of the plastic passing through the nozzle opening. 2. The method according to claim 1, characterized in that with a such a temperature of the nozzle outlet opening is worked that the surface decomposition of the emerging thermoplastic material at least 32, in particular 470in Weight loss by volatilization per hour is. 3. The method according to any one of claims 1 and 2, characterized in that that the material is processed with a propellant added. Considered publications: »Kunststoffe«, Vol. 45, 1955, H. 10, p. 426.