IE41886B1 - A method of manufacture by extrusion of sections of structural foamed thermoplastics - Google Patents

Abstract

1470267 Extruding longitudinally oriented foamed sections SOC CHIMIQUE DES CHARBONNAGES 3 June 1975 [5 June 1974] 23958/75 Heading B5A [Also in Division C3] A foamed thermoplastics section, having a ratio of density to that of the unfoamed material of 0À4 to 0À9, and which is oriented and has cells which are elongate, in the direction of the longitudinal axis of the section, is produced by extruding the thermoplastics containing the blowing agent at a temperature corresponding to the "rubbery flow" region, D, Fig. 1, cooling the extrudate around its periphery immediately at the outlet of the die and to a temperature of the "rubbery plateau", C, and drawing it through a metallic sizing die and a following cooling zone, with the extrusion and sizing die passages corresponding in shape and size to that of the section, and the ratio of extrusion speed to that at which the section is drawn from the sizing die corresponding to the density ratio of the section. The process requires an elongation of the order of 100% of the surface regions of the extrudate, obtained by the cooling to a temperature of the "rubbery plateau" of the material. For density ratios of 0À4 to 0À6, the temperature of the remainder of the extrudate may be that of the rubbery flow region, D. The peripheral cooling is preferably effected by jets of cold fluid directed at pressures which may be constant around the periphery in the case of a circular section rod, or may vary around the periphery, as in Specification 1,450,990, for sections such as that of the example, Fig. 2, where a specified foamable polystyrene homopolymer composition is extruded from the die P, and cooled by jets 1-7. The thermoplastics may also comprise polyvinyl chloride or polystyrene, or copolymers comprising one thereof.

Description

The present invention relates to a method of manufacture by extrusion of sections of structural foamed thermoplastics material, that is to say which have a density ratio of the density of the foamed material to the density of the solid material between 0.4 and 0.9. The method is suitable for use with polystyrene, polyvinyl chloride and copolymers containing one of these two substances, and generally of polymers having a sufficiently extended rubbery plateau (as defined hereinafter).

The invention also relates to sections produced which have an anisotropic structure, characterised especially by the fact that the cells present in the section of structural foamed thermoplastics have shapes extended in the longitudinal direction.

Various methods of producing by extrusion sections of structural foamed thermoplastics material are already known, generally using costly and complicated apparatus, especially as regards the design and construction of the dies. In all these methods, the expansion takes place either from the exterior towards the interior or from the interior towards the exterior, and in these cases the expansion may be restricted close to the edges by the presence or the formation of solid polymer.

The method according to the invention differs from the prior methods in that the expansion at the outlet of the extrusion die takes place only in the longitudinal direction. This permits a particularly simple design of the die, the geometry of the section and the dimensions of which correspond to those of the section to be obtained. The same thing is true of a metallic shaping device which is used in combination with the die and which has the shape and dimentions of the section to be obtained. - 2 41886 According to the invention there is provided a method of manufacturing a section of structural foamed thermoplastics material having a density ratio of the densityof the foamed material to the density of the solid material of between 0.4 and 0.9, in which the foamable ® thermoplastics material with its blowing agent and the necessary additives is extruded through a die passage having the shape and the dimensions of the desired section while it is at a temperature corresponding to the rubbery flow region, the extruded product is cooled rapidly over its surface immediately at the outlet of said die passage to a temperature corresponding to the rubbery plateau of the material and the extruded product is drawn through a metallic shaping die passage having the shape and the dimensions of the desired section and with a drawing speed higher than the extrusion speed, the ratio of the extrusion speed to the drawing speed corresponding to said density ratio, whereby the section of structural foamed thermoplastics material is orientated and has cells elongated in the direction of drawing.

For example, in order to obtain a section having a density ratio of 0.50, the linear speed of the extruded product at the outlet of the metallic shaping die should be twice the speed of extrusion of the extruded product at the outlet of the extrusion die.

In order to prevent any radial swelling of the extruded product, which would result in deformation of the section, and in order to prevent bubbles of gas from bursting at the surface, thus giving a bad surface appearance to the section, the external cooling of the polymer may be effected by jets of cold fluid distributed around the periphery of the extruded product. The jets are located as close as possible to the outlet of the die. When extruding complex sections, it is advantageous that the jets of cold fluid be at different pressures at different - 3 locations around the periphery of the extruded product, in order to stop the expansion at the cooled zone, as described in United Kingdom Patent Specification No. 1,450,990.

During the extrusion, the thermoplastics material must have a visco-elastic behaviour such that it can withstand without breaking, an elongation at least equal to that which corresponds to the longitudinal expansion which it is desired to obtain. In his work “Properties and Structures of Polymers, Wiley & Sons, New York, London (1960) Chapters 2 and 4, pages 71 to 73 and especially on the curve shown on page 73, A.V. Tobolsky distinguishes five regions of visco-elastic behaviour of polymers as a function of temperature.

This curve is reproduced in Fig. 1. of the accompanying drawings.

In this curve A designates the glassy region, B the transition region, C the rubbery plateau, D the rubbery flow region and “E the liquid flow region. A large elongation without breaking is only possible in the third region (C) rubbery plateau and in the fourth region (D) rubbery flow region. In general, for a large expansion, that is to say for density ratios in the range 0.4 to 0.6, it will be necessary that the mass of the polymer is at a temperature corresponding to the rubbery flow region, whereas the surface of the extruded product which is subjected to large stresses and which should have a relatively high modulus while being capable of withstanding without rupture an elongation of the order of 1005!, will be at a temperature corresponding to the rubbery plateau.

In order that the density ratio of the expanded foamed polymer to the density of the solid polymer shall be the same as the ratio of the speed of extrusion to the speed of traction drawing, it is preferred that the expansion is practically terminated when the section passes into the cold metallic shaping die, that is to say the distance between the - 4 41886 extrusion die and the metallic shaping die is such that the expansion 1s practically completed at the inlet to the metallic shaping die.

The installations for carrying out the method according to the invention comprise an extrusion machine, a die, a cold-fluid jets unit, a metallic shaping die, a cooling zone and a drawing or traction device for the section produced, these various units or apparatus being arranged relative to each other in such a manner as to obtain the conditions necessary for the process.

The following example is given by way of illustration with reference to Fig. 2 of the accompanying drawings, showing a complex section of structural foamed thermoplastics material which is manufactured by the method of the invention.

In this drawing, the reference P has been used to indicate the geometry of the section of the die, while the references 1 to 7 indicate blowing devices located around the periphery of the extruded product.

EXAMPLE In this example, there are utilized a polystyrene homopolymer having a number average molecular weight of 240,000 and a weight molecular weight of 500,000. This polymer had a rubbery plateau between ITO and 2o 170°C., its modulus then varying between 10? and 106 dynes per sq.cm.

The operation was carried out on a Samafor extrusion machine with a single screw of 65 mm. in diameter, with four heating zones; the screw had one stage of compression.

Temperature profile of the extrusion machine: First region: inlet body 149°C Second region: first central body 169°C Third region: second central body 180°C Fourth region: outlet body 175°C - 5 4 & 8 8 S This range ensures the pi astifi cation of the polymer and the decomposition of the blowing agent.

Temperature profile of the die: First zone: die inlet 170°C Second zone: die outlet 190°C The choice of these temperatures makes it possible to obtain an expandable extruded product, the mass of which has such a temperature that it can be subjected to an elongation of 100%.

The geometry P of the outlet section of the die is shown in the figure. This section is identical with the section of the product to be obtained. This is also true for the geometry of the section of the metallic shaping die.

The blowing pressures at the blowing devices 1 to 7, which are of the kind described in United Kingdom Patent Specification No. 1,450,990 previously referred to are as follows: Cooling device top left (1) blowin g pressure; 300 millibars centre (2) II “ 400 II ri ght (3) II 500 II right W II 50 II11 bottom right (5) II 200 II11 left (6) II 400 II11 left (7) II 50 II Formulation of the extruded polystyrene Parts by weight Polystryene homopolymer 100 Lubricant: stearic acid 0.140 Nucleation agent: Micronized talc 0.50 Blowing agent: azo-dicarbonamide 0.25 - 6 41886 The drawing speed was 1.20 meters per minute, which, for a density of 0.5 grams per cc. corresponds, for the extruded product, to a speed of 0.60 meter / minute at the outlet of the die. The distance from the die to the metallic shaping die is 150 mm.

By the method of the invention, it is also possible to obtain cylindrical sections, especially rods, by creating a substantially uniform pressure around the periphery of the extruded product. Similar results are obtained with polymers of the polyvinyl chloride type and copolymers containing polystryene units or polyvinyl chloride units.

According to the invention, sections of structural foamed thermoplastics material products are obtained which have an orientated cellular structure, elongated in the longitudinal direction.

Claims (7)

1. A method of manufacturing a section of structural foamed thermoplastics material having a density ratio of the density of the foamed material to the density of the solid material of between 0.4 and 0.9, in which the foamable thermoplastics solid material with its blowing agent and the necessary additives is extruded through a die passage having the shape and the dimensions of the desired section while it is at a temperature corresponding to the rubbery flow region as hereinbefore defined the extruded product is cooled rapidly over its surface immediately at the outlet of said die passage to a temperature corresponding to the rubbery plateau as hereinbefore defined of the material and the extruded product is drawn through a metallic shaping die passage having the shape and dimensions of the desired section and with a drawing speed higher than the extrusion speed, the ratio of the extrusion speed to the drawing speed corresponding to said density ratio, whereby the section of structural foamed thermoplastics material is orientated and has cells elongated in the direction of drawing.

2. A method as claimed in Claim 1, in which the cooling is effected by means of jets of cold fluid distributed around the periphery of the extruded product.

3. A method as claimed in Claim 2, in which jets of cold fluid are at different pressures at different locations around the periphery of the extruded product.

4. A method as claimed in any one of Claims 1 to 3, in which the polymer is selected from the group comprising polystyrene, polyvinyl chloride, and copolymers containing one of these two substances.

5. A section of structural foamed thermoplastics material prepared by the method of Claim 1 and having an anisotropic structure, and wherein the cells present in said sections have shapes extended in the longitudinal direction.

6. A method of manufacturing a section of structural foamed thermoplastics material, substantially as herein described with reference to the - 8 41886 accompanying drawings.

7. A method of manufacturing a section of structural foamed thermo plastics material, substantially as herein described with reference to the Example.