EP1299221A1

EP1299221A1 - Extrusion head for expanded plastic tubes

Info

Publication number: EP1299221A1
Application number: EP01945285A
Authority: EP
Inventors: Claude Dehennau; Martine Kaszacs
Original assignee: Solvay SA
Current assignee: Solvay SA
Priority date: 2000-06-22
Filing date: 2001-06-19
Publication date: 2003-04-09
Also published as: US7008577B2; AU2001267545A1; US20030164569A1; FR2810577B1; WO2001098057A1; FR2810577A1

Abstract

The invention concerns an extrusion head for making tubes comprising at least an plastic layer expanded by extruding a flow of melted plastic material comprising an expansion agent through an annular passage section provided in the extrusion head and wherein the melted plastic material flow comprising the expansion agent is circumferentially distributed and is welded on itself by recombination of the two fronts of the flow, said extrusion head being provided with a specific heat regulating device enabling to heat or cool the melted plastic material comprising the expansion agent after the flow fronts have been recombined.

Description

Extrusion head for expanded plastic tubes

The present invention relates to an extrusion head for the manufacture of tubes comprising at least one layer of expanded plastic. The present invention also relates to a method of manufacturing such tubes. Tubes (and in general, cylindrical objects) having at least one layer of expanded plastic material have the advantage, compared to tubes of non-expanded plastic material, of being lighter and therefore more economical. In addition, the layer of expanded plastic material can provide a function of thermal and / or sound insulation, protective coating, etc. Many attempts have been made in the past to optimize the expansion rate of the plastic material as well as the mechanical properties of the tubes obtained, in particular by adapting the (co) extrusion extrusion head. Most of these extrusion heads are provided with a single heating device, possibly common to all the layers in the case of coextrusion heads, which makes their optimization nevertheless difficult. US Patents 4,484,883 and US 5,069,612 describe coextrusion heads not specific to expanded plastics, but which can be used for such materials, and which have the advantage of independent temperature regulation for each layer to be extruded. However, within the same layer, the temperature is uniform since there is only one thermal regulation device per layer. Such extrusion heads are suitable for non-linear plastics (LDPE) and / or not or not very crystalline (PS, PNC, PCL), or more generally: for plastics having a processing temperature range expanding sufficiently wide. On the other hand, in the case of linear and semi-crystalline plastics such as HDPE or PP, the molecular tangles that occur during expansion are likely to lead to the tearing of the material flow if the material temperature is too high . In addition, such extrusion heads are difficult to optimize since to avoid the expansion of the plastic material within them, and an uncontrolled expansion at the outlet of the die, they cannot be overheated, whereas if they are not enough, the viscosity of the plastic is too high and the latter may freeze at the die. Consequently, the subject of the present invention is an extrusion head for the manufacture of tubes comprising at least one layer of expanded plastics material which is easy to optimize and regulate and which makes it possible to obtain tubes having good mechanical properties and having a high regularity of cell morphology on the surface and in the heart, whatever the type of plastic material envisaged. The present invention also relates to a method of manufacturing expanded plastic tubes using such an extrusion head, as well as a method of manufacturing sheets from such tubes, by cutting along one of their generatrices . . The present invention therefore relates to an extrusion head for the manufacture of tubes comprising at least one layer of expanded plastic material by extrusion of a flow of molten plastic material comprising an expanding agent through an annular passage section made in, the extrusion head and in which the flow of molten plastic material comprising the blowing agent is distributed circumferentially and is welded on itself by recombination of the two fronts of the flow, said extrusion head being provided with a specific thermal regulation device allowing the heating or cooling of the molten plastic material comprising the blowing agent after the recombination of the fronts of the flow. By tube is meant any cylindrical object such as a pipe; a parison of a bottle; a tabular envelope which by cutting, will give a sheet; a cable sheath ...

The term “extrusion head” according to the present invention is intended to denote a set of metal blocks and a core comprising a passageway for at least one stream of molten plastic material leaving an extruder. Such an assembly generally comprises at least one block for distributing the material in the form of an annular flow (or distributor), and in the case of a coextrusion head, it generally comprises at least one distributor per layer of material. The extrusion head according to the present invention is generally followed by an outlet block called a die. The distributor and the die, if necessary, are traversed right through by a cylindrical orifice which determines with the core, an annular passage section for the molten plastic. The flow of molten plastic material which is supplied to the extrusion head according to the present invention through an adequate opening, is generally a cylindrical flow full of plastic material under pressure. In the case of a coextrusion head, there are generally as many feed orifices as there are cylindrical material flows. The plastic according to the present invention is preferably an extrudable or thermoplastic plastic. This resin can be of any kind, crystalline or amorphous. These may be so-called convenience plastics, such as polyolefins (PE, PP) or PNC. It can also be so-called specialty plastics, such as fluorinated resins, and in particular PNDF; polycaprolactone (PCL) ... It is preferably semi-crystalline plastics such as PP, HDPE (modified (for example with a silane) or not), PNDF or PCL. These resins can include fillers (fibrous, particulate, etc.) and various usual additives such as plasticizers, flame retardants, stabilizers, etc.

The blowing agent according to the present invention can be of any known type. It can be a so-called “physical” blowing agent, that is to say a gas dissolved in the plastic under pressure and which causes its expansion during expansion at the outlet of the extruder. Examples of such gases are CO ₂ , nitrogen, water vapor, HFCs (such as SOLKAΝE ^® XG87), hydrocarbons (such as butane and pentane) or a mixture thereof. It can also be a so-called "chemical" blowing agent, that is to say a substance (or a mixture of substances) dissolved or dispersed in the plastic material and which, under the effect temperature, releases the gas or gases which will be used to expand the plastic. Examples of such substances are azodicarbonamide, a mixture of sodium bicarbonate and citric acid, etc. The annular passage section made in the extrusion head according to the present invention is preferably preceded by an equalizing device. which is located after the feed opening of the extrusion head. This device can be of any known type. However, it is generally a toroidal groove which acts as a reservoir to provide a constant and regular material flow to the annular passage section, and which therefore has a larger section than this. This toric groove generally has a non-constant section, which. decreases from the entry of the material towards the point of recombination of the fronts of the flow, of. so as to balance the velocity profile of the material after recombination. In this case, the recombination of the fronts of the flow takes place in the groove and therefore, according to the invention, the flow of material is cooled only after passing through this groove.

The thermal regulation device of the extrusion head according to the present invention is of any known type. It is a device for cooling or heating the plastic. This device combines generally an external heating device such as a heating collar (electrical resistance or double jacket with thermal fluid) enclosing the extrusion head, and an internal heating device. Such a device advantageously uses a thermal fluid, which can be oil for example. Consequently, the extraction head according to the present invention preferably comprises at least one channel for the internal circulation of such a fluid. Advantageously, it will include two channels to allow the cooling of the two faces of the layer to be expanded.

The specific thermal regulation device preferably ensures a temperature close to the melting temperature of the plastic material to be expanded (T _f ) within the molten plastic material comprising the blowing agent. This temperature is advantageously at most T _f + 30 ° C, or even at most T _f + 10 ° C. This temperature is generally not lower than Tf, and preferably not lower than Tf + 5 ° C. The expansion of the plastic material is preferably carried out after the exit from the die which is arranged just after the extraction head according to the present invention. To this end, the annular passage section of the extraction head according to the present invention is advantageously narrowed just before the opening through which the molten plastic comprising the blowing agent leaves the extrusion head. This narrowing makes it possible to create sufficient back pressure and to avoid pre-expansion in the die itself. Alternatively, this narrowing of section can take place in the die which follows the extraction head. This narrowing is preferably done by adjusting the thickness of the core. The extraction head according to the present invention consists of one or more materials of suitable mechanical and thermal resistance. They are generally at least partly metals which can be pure or in the form of alloys. By way of example, mention may be made of heat treated stainless steels to increase their hardness. The extraction head according to the present invention can also include elements with low thermal resistance such as Cu-Be alloys (to improve the heat exchange between the thermal fluid and the plastic to be expanded). The extraction head according to the present invention can also include ceramics or air pockets as insulating elements. According to a preferred variant, the extraction head according to the present invention is such that all the surfaces which come into contact with the material to be expanded promote the sliding of this material and avoid flow breaks. A sliding coating, for example made of Teflon resin, on these surfaces is suitable for this purpose.

The extraction head according to the present invention can be used for the extraction of monolayer tubes of expanded plastic. Advantageously, this extrusion head will be used for coextrusion and will allow the application of at least one other layer of expanded or unexpanded plastic material inside and / or outside the layer of expanded plastic material. The purpose of these layers may be to increase the mechanical and / or chemical resistance, the surface appearance, the roughness, the thermal or sound insulation, etc. The plastic material constituting these layers may be identical to that of the plastic material. expanded, but is generally different. It can also be recycled plastic. These layers may also contain fillers and / or various usual additives as described above for the layer of expanded plastic. The extraction head according to the present invention is therefore particularly suitable for manufacturing multilayer tubes comprising at least one layer of expanded plastics material and at least one layer of unexpanded plastics material. Preferably, the layer of unexpanded plastic material is also heated or cooled by a specific thermal regulation device. A coextrusion head made up of blocks independently regulated in temperature by specific devices is particularly advantageous. These blocks are preferably isolated from each other by suitable means. To this end, they can be separated by vacuum zones. Preferably, these blocks are designed such that they allow the cooling or layers adjacent to the layer of foamed plastic ^• (in order to avoid losing the benefit of the cooling of the latter). The device for cooling the layer of expanded plastics material is advantageously arranged in such a way that it also cools the adjacent layers. This way of proceeding makes it possible to prevent the expanded layer from heating up in contact with the other layers and therefore, at least partially to lose the benefit of its cooling.

In the case where the tube comprises an external layer of unexpanded plastic, this layer is advantageously not cooled in such a way. excessive in the extrusion head. In fact, excessive cooling could have the effect of freezing this layer too much and in doing so, inhibiting the expansion of the inner layer. The external layer of unexpanded plastic material is preferably applied to the multilayer structure just before the exit of the extraction head. Its viscosity will be adapted to allow optimal expansion of the layer of expanded plastic. The present invention is illustrated in a nonlimiting manner by FIG. 1.

This figure represents a longitudinal section (parallel to the axis) in a coextrusion extraction head provided for extruding a two-layer tube with an internal layer of unexpanded plastic and an outer layer of expanded plastic. The flow of cylindrical plastic material which contains the blowing agent (1) and which leaves the extradeuse (not shown) opens into a toric groove (2) where it is distributed circumferentially so as to combine its two fronts at the point (2 '). The section of the groove is smaller at point (2 ') than at point (2) so as to balance the velocity front of the material which leaves the throat all along the circumference (following the pressure drop observed all along the path of the material in the throat). The extraction head comprises cavities (3) intended to contain the oil for cooling the material to be expanded after recombination of the fronts of the flow, as well as for cooling the layer of unexpanded plastic material (7) on the side where it is welded to the layer to be expanded. This procedure makes it possible to prevent the layer of unexpanded plastic material, which is too hot, from interfering with the cooling of the layer to be expanded. Vacuum zones (4) are provided for isolating the blocks heated by heating collars (5) from the oil-cooled blocks. The diameter of the annular orifice which extends from the groove decreases as the plastic material flows from the groove towards the core (6) and joins the flow of unexpanded plastic material (7) which forms the inner layer of the tube. The section of the annular orifice narrows and is minimal at point (8) and up to the exit of the die (10), so as to ensure a back pressure preventing the expansion of the external layer in the die. The internal temperature of the coextrusion head at the place of cooling is controlled by a thermocouple (9). The tubes obtained by the process according to the present invention find their application in various fields such as the transport of waste water. In this case, these are for example tubes having an inner layer of unexpanded PE and an outer layer of expanded PE which ensures the resistance of the tube to stones present in the soil where these tubes are buried. It can also be PE tubes intended to convey hot water for heating in particular and which are thermally insulated by an outer layer of expanded PE.

As explained above, by tube is meant any hollow cylindrical object. Thus, it may be the sheath of an electric cable, for example based on expanded PNDF. It can also be sheets obtained by cutting these cylinders according to one of their generatrices and which find their application in decoration, one automobile, building.

An interesting application of the present invention consists in sound insulation in the automotive field. The present invention is also illustrated in a nonlimiting manner by Examples 1 to 3 detailed below. In each of these examples, two bilayer tubes with internal diameters 32 mm and external diameters 40 mm were extradited under identical conditions. These tubes include:

1. an internal layer with a thickness of 1 mm based on unexpanded resin for tubes: ELTEX ^® TU B 125 resin, HDPE from SOLNAY,

MI 2.16 (Melt Index under 2.16 kg according to ISO 1133) less than 0.15 g / 10 min, MI 5 equal to 0.48 g / 10 min, average molecular weight distribution width and a density equal to 952 kg / m ³

2. an outer layer with a thickness of 3 mm based on expanded resin which is:

- for a tube, HE 1102 resin from BOREALIS, an HDPE resin developed for the extrusion-foaming application and already containing a nucleating agent, which has an MI 2.16 of 5 g / 10 min and a density of 950 kg / m ³ - for the other tube, the resin of ELTEX ^® A4040 SOLNAY, a HDPE resin not intended for expansion and having an MI of 2.16 to 4 g / 10min and a density of 944 kg / m ^3. The extruded tubes are cooled in the open air and the foam can therefore expand freely in each case. Comparative example 1

The tubes were extradited using a conventional die, fitted with a unique thermal regulation device set at 180 ° C. The density difference between the expansion resin and the standard resin is relatively large: 30%. The type of temperature control is not suitable for standard resin which has the appearance of an over-expanded resin with irregular cells. Comparative example 2

The implementation technique used was identical to that described in Comparative Example 1, but the overall regulation temperature this time was 140 ° C.

The appearance of the foam cells is more regular but the surface of the tube is more irregular and has flow breaks because the material is too cooled on the surface. In addition, the weld line of the fluxes formed after the material to be expanded has entered the coextrusion head is more visible than in the first case.

Example 3 (in accordance with the invention) A coextrusion head such as that shown in the figure was used, with the temperature of the heating collars fixed at 180 ° C. to avoid flow breaks and poor welding of the flow lines ; and the temperature of the oil circuits fixed at 140 ° C. in order to cool the material to be expanded to the optimum temperature for expansion. The distribution of cells is regular. There is no apparent surface flow break or apparent weld line. Thanks to the optimization of the expansion temperature, the foam obtained from standard HDPE is of a quality comparable to that obtained from the resin developed for foaming. In addition, the density difference between the two resins is reduced by 30 to 16%, which is advantageous from an economic point of view because, although the density of the standard resin foam is still greater than that of resin for foaming, the difference between the two is sufficiently reduced so that the material cost of the foam is favorable to the standard resin.

Claims

R E V EN D I C A T I O N S

1 - Extraction head for the manufacture of tubes comprising at least one layer of expanded plastic material by extrusion of a flow of molten plastic material comprising an expanding agent through an annular passage section formed in the extraction head and in which the flow of molten plastic comprising the blowing agent is distributed circumferentially and is welded on itself by recombination of the two fronts of the flow, characterized in that the extrusion head is provided with a device specific thermal regulation allowing the heating or cooling of the molten plastic comprising the blowing agent after the recombination of the fronts of the flow.

2 - Extraction head according to any one of the preceding claims, comprising at least one channel for the internal circulation of a thermal fluid.

3 - Extraction head according to any one of the preceding claims, in which the specific regulation device provides a temperature between T _f + 30 ° C and T _f within the molten plastic comprising the blowing agent , T _f being the melting temperature of the plastic.

4 - Extraction head according to any one of the preceding claims, characterized in that the annular passage section has a narrowing just before the opening through which the molten plastic comprising the blowing agent leaves the head d 'extrusion.

5 - Head extrasion according to any one of the preceding claims, wherein all surfaces of the head of extrasion that come into contact with the material to be foamed comprise a Teflon ^® resin coating.

6 - Extraction head according to any one of the preceding claims, further comprising a passageway for at least one layer of unexpanded plastic material inside and / or outside of the layer of expanded plastic material . 7 - Extraction head according to claim 6, in which the layer of unexpanded plastic material is also heated or cooled by a specific thermal regulation device ".

8 - A method of manufacturing tubes comprising at least one layer of expanded plastic material using an extraction head according to any one of the preceding claims.

9 - The method of claim 8, wherein the plastic is PP, HDPE, PNC, PCL or PNDF.

10 - The method of claim 9, wherein the specific thermal control device is used to cool the molten plastic comprising the blowing agent.