CS201523B2 - Method of production of contractibile polyethylene pipes - Google Patents

Abstract

1380325 Moulding polythene pipe MAGYAR KABEL MUVEK & MUANAGIPARI KUTATO INTEZET 24 Nov 1972 [26 Nov 1971] 54374/72 Heading B5A Heat shrinkable polythene pipe capable of shrinking to internal diameters below 0À5 mm. is made by drawing down pipe of 1 to 3 mm. internal diameter, exposing the drawn pipe to 12 to 20 M rad of radiation, to partly cross-link the polythene, dilating the pipe at 108‹ to 190‹ C. in a gauge of 0À4 to 0À8 mm. diameter in the presence of a non-swelling lubricant, and cooling the pipe before removal from the gauge to freeze it in the dilated condition. An extruded pipe 2 which has been drawn down and irradiated by a Cobalt 60 source or by electrons accelerated by a Van de Graaff generator is fed by rollers 6 into a gauge pipe 1 immersed in a tank 4 of heated glycerol or diethylene glycol. The heating fluid enters the gauge pipe and acts as a lubricant while the pipe 2 is dilated by gas from cylinder 8. The end of the gauge pipe is cooled naturally or artificially to set the dilated tube 3 being drawn off by rollers 7. The pipe is shrunk by heating to 110‹ C. on to cables to repair insulation or on to capillary tubes or syringe parts to couple them together.

Description

SUMMARY OF THE INVENTION The present invention relates to a process for the production of heat-shrinkable polyethylene pipes of less than 0.5 m in diameter.

According to the state of the art, the manufacturing technology of plastic pipes, for example polyethylene pipes, with a small diameter, consists in that the pipe material which has been extruded to a larger diameter is retrofitted to the desired diameter by axial extension. As is known, the tubes produced in this manner return approximately to the extruded diameter under the effect of heat. The reason for this is that the deformation caused by extrusion accumulates the tubes as internal stresses. Tubes made in this manner are therefore only suitable for ^one · use at room temperature.

For the production of heat-shrinkable polyethylene-based tubes, methods are known in which the polyethylene tube is irradiated with energy-rich radiation or is amplified by chemical means during extrusion or after extrusion, and the thus produced crosslinked tube is expanded, for example by causing endless the pipe extends through the heat chamber, the still warm pipe is continuously expanded in the next chamber by the combined action of vacuum and pressure, and then cooled in the third chamber. Under these conditions, for example, in British Patent Specification No. 1,075,704, it is possible to obtain heat-shrinkable tubes having a non-widened inner diameter of about 2.5-12 mm and a widened diameter of about twice. When heated above the melting point of the crystals, these tubes shrink to their own diameter, about 2.5-12 mm.

In this way, it is not possible to produce shrinkable polyethylene pipes with a uniform wall thickness whose diameter before or after shrinking is 0.5 mm or less and whose internal diameter is less than 0.5 mm ^. enlargement is 3 times larger than before enlargement.

The manufacture of plastic pipes with an inside diameter of 0.5 mm · or · · smaller also requires a special process in industry.

This object is achieved by extending the thicker tube with an inner diameter of, for example, 3 mm to the desired diameter, for example less than 0.5 mm, by rapid pulling. However, the result of this method is the stresses that occur during the drawing-off, accumulate in the tube as> stresses, due to which the tube, when heated in use, returns to a diameter approximately equal to the extruded diameter. It is clear that the polyethylene tube is crosslinked when extruded by means of a radiation-rich or chemical route if it is to be used as a shrinkable tube when heated. • shrink to the extruded dimension and not to the stretched dimension.

Furthermore, the pipe expansion method of the cited patent is unsuitable for expanding cross-linked polyethylene pipes of very small diameter and low wall thickness with a large expansion ratio (3 to G times), since the thin walls formed during the process [5-100 microns], tear due to the friction on the caliber wall.

In view of the above circumstances, procedures known from the literature do not produce tubes for fitting to a core of less than 0.5 mm diameter, possibly with an extension ratio of over 3.

The purpose of the invention is to produce polyethylene pipes which can be mounted on a core having a diameter of less than 0.5 mm and whose expansion or contraction ratio is 3-8.

The invention is based on the finding that this object can be achieved by extending the polyethylene tube to the desired size after extruding but before crosslinking by irradiation, irradiating it with a radiation dose of 12-20 Mrad and then subjecting it to a 3-8 fold extension.

The invention is further based on the finding that the desired expansion ratio can be achieved by expanding in a heated caliber using a suitable lubricant and drawing the expanded tubular material in the caliber until cooling below the melting point of the crystals. In one part of the procedure, a heated calibrated caliper is used in the other.

The shrinkable tubes according to the invention are produced in the following manner.

The extrusion of the tubes is carried out in the conventional manner. The extruded tube having a diameter of 1-3 mm is still stretched while hot. By stretching, the diameter of the pipe is reduced to less than 0.5 mm.

Stretching is about itself, our way. However, no shrinkable material can be made from the elongated product, partly because it melts, secondly, because due to the shortened geometric form, at the time of extrusion by the specified chemical bonds, the material does not contract to the elongated state but to the extruded state.

For the production of shrinkable plastic tubes with a diameter of less than 0.5 mm, heat-stable crosslinks must be formed in tubes extruded to a larger diameter and then stretched. This can be achieved by irradiation with energy-rich radiation, for example by gamma-radiation of the Co> ⁶⁰ isotope or by electron radiation using an accelerator. (Even when crosslinked with chemical initiators they form a state that is determined by the size of the extruder.)

When using a small dose of radiation, a small number of crosslinks is not sufficient to achieve sufficient shrinkage. If the radiation dose is too high, no crosslinked crystalline part is retained in the more crosslinked grid structure. Upon cooling following expansion, the defioirtmation of the force bonds does not accumulate in the form of tensile stress and the tube does not retain its expanded state at room temperature.

The required dose of irradiation is, depending on the stabilizer content of the polyethylene used, 12-20 Mrad.

Crosslinks formed by radiation: (a) form a heat-stable and non-fusible material; (b) are not subject to aging due to time; (c) affect other mechanical and electrical properties of the material.

The irradiation of the material is heated above the melting point of its crystalline - not its crosslinked - part, ie above 108 ° C, and in caliber, the continuously running tube is expanded by the internal pressure of the gas or liquid to the desired degree. The crystalline part solidifies in the cooled part of the caliber and holds the deformation of the crosslinks in the form of stresses. A surprising finding is that the material, after a later deformation, n-shrinks to the extruded dimension but to the dimension fixed during irradiation.

The difficulty in expanding is that the tube expanded to 300 - 600% of 1 to 5 mm diameter, whose wall thickness is usually 10 - 500 μ, and whose heat resistance is low, is compressed by the gas pressure used for expanding to the gauge wall . The friction that results is tearing a tube which is not sufficiently strong in the warm state. Instead of conventional lubricants to reduce friction, special lubricants are used which do not swell the polyethylene tube. Such lubricants are, for example, polyhydric aliphatic alcohols. Lubrication is performed by dipping the caliber in a bath of the desired lubricant or applying the lubricant to the caliber by another method.

Special features of the extension method are as follows:

a) Due to the small size of the pipe, heating the caliber is necessary.

(b) Due to the reduced tensile strength caused by heating, the adhesion of the material to the gauge shall be reduced by the use of lubricants.

c) With respect to the lubricant, this must not cause the material to swell and be washable with water.

The enlargement process is explained in the following:

The irradiated polyethylene tube 2 of diameter d is passed through a pair of rubber rollers 6 serving for transport to caliber 1. The raw tube is exposed to the internal gas pressure from the inlet side (or the downstream side). The tubular material 2 reaches bath 4 at point A of gauge 1. The feed rate is controlled so that the tube reaches the desired temperature of 108 ° C at point В. At this point, the tube is expanded to the gauge size by a tug ¹ obtained from the gas cylinder 8, then cooled on the wall or the room temperature of the cooled gauge as it passes through the section V-C. The solidifying " crystalline, non-crosslinked " portion of the material accumulates tensile stresses that occur as the material expands and the material retains. The caliber is heated by a conduit 5, for example by means of a medium flowing through the thermostat.

For example, diethylene glycol, used as a heating medium, may also be used as a lubricant to reduce adhesion to the gauge wall that occurs when a slowly setting tube passes through the B-C section. The finished shrinkable tube B with a pair of tightening rollers 7 is transferred to the winding device? The shrink miniature polyethylene tube produced in this way retains its expanded shape, unlike PVC-based tubes, even at storage temperatures between -200 ° C and +100 ° C. The high-grade axial expansion allows its use also for wrapping or insulating turned objects whose shape deviates substantially from the cylindrical shape.

In use, the shrinkable tube is threaded onto the article to be wrapped and then heated to a temperature above 110 ° C. In this case, the pipe of the tube is to shrink to its original, irradiated, fixed dimension and adheres firmly to the object.

Heating may be carried out by: .a) hot air; (b). gas flame · (city gas .or gas cylinder); c) radiant heat.

Shrinkable tubes. produced in the manner explained herein can also be shrinked back such that the resulting outer shell faces considerable internal gas and liquid pressures. When the back-shrink product has a dimension of 300 to 600% relative to the irradiation dimension, the resulting shrinkage stress is subject to considerable internal gas pressures. This pressure can be determined with a very good approximation where p is the internal overpressure that the shrinkable tube still resists,

V pipe wall thickness in cm,

D pipe diameter in cm, σ. Shrinkage stress in kp / cm ² , which can be determined from vztahu - .0,2 + 0,5 dg where σ is the tensile strength of the material.

The shrinkage pressure thus calculated is independent of the core material on which it shrinks.

Example 1

High-pressure polyethylene tube extruded on an extruder with a 2 mm diameter tool. is still drawn out on the pipe with an outer diameter of 0.5 mm and an inner diameter of 0.2 mm and this is. on a van de Graaff electron emitter illuminated with a dose of 1: 6 Mrad. The irradiated tube is expanded to a tube with a diameter of 2.5 mm in a bath of heated diethylene glycol and then cooled.

Example 2

The tube extruded on the extruder with a 1.5 mm diameter tool is still drawn through the tube to a 0.8 / 0.4 tube. mm, this. is irradiated with Coi60 at a dose of 17 Mrad and, after irradiation, expanded to a 4 mm tube by stretching. to a caliber found in a heated glycerin bath and then wound up.

The shrinkable tubes produced according to the examples can be used according to their dimensions:

1. when insulating cables to sheath the interface of the outer protective layer or damaged areas resulting from repairs;

2. in medical and laboratory practice, for the manufacture of sterile pipette and syringe junctions;

3. for connecting capillary tubes.

Claims (3)

the subject of the invention 1. A process for the production of shrinkable polyethylene pipes having an inside diameter of less than 0.5 mm, characterized in that a polyethylene pipe having an inside diameter of 1 to 3 is produced by the present extrusion process. mm, this narrows to a desired inner diameter by elongation, and is preferably irradiated with electron radiation at a dose of 12 to 20 Mrad, per. the use of a polyethylene non-swelling lubricant, preferably using diethyleneglycol, is used. at a temperature of 108 DEG to 190 ^DEG C., it extends to a caliber of 0.4 to 8 mm corresponding to a given extent of expansion, and, after cooling, is withdrawn from the caliber. 2. The method of claim 1, wherein the expansion gauge is heated in a heated lubricant bath. 3. The method according to. 1, marked by '. that the expanded tube is drawn on the caliber until it cools below the crystalline melting point. after lyethylene.