GB2152427A

GB2152427A - Extruding & cooling a thermoplastic sheathed core of filled polyethylene terephthalate

Info

Publication number: GB2152427A
Application number: GB08500069A
Authority: GB
Inventors: Antoine Baciu
Original assignee: Cibie Projecteurs SA
Current assignee: Cibie Projecteurs SA
Priority date: 1984-01-04
Filing date: 1985-01-03
Publication date: 1985-08-07
Also published as: ES539337A0; GB8500069D0; FR2557499B1; IT8423941A0; ES8700138A1; FR2557499A1; DE3500094A1; JPS60159028A; IT1196359B

Abstract

A core (13) of polyethylene terephthalate containing 0 to 50% mineral fillers and having a semi-crystalline structure, and a coating layer (14, 15) made from at least one other thermoplastics material are co-extruded as a flat sheet or tube. Immediately after extrusion, the coating layer (12) is cooled by treatment by means (20,21,22) having a temperature lower than the crystallisation temperature of the filled polyethylene terephthalate, e.g. of the order of 40 to 80 DEG C, and the coating layer of other thermoplastic performs as a thermal buffer to slow down the equalisation of temperature between the polyethylene terephthalate and the said means (20, 21,22) in such a way as to ensure that the polyethylene terephthalate which is still fluid cools sufficiently slowly to have the required semi-crystalline structure. The treatment means may comprise a cooler (20) which does not effect further shaping, or a moulding means which further shapes the still fluid core, such as a calender (21), or a sheet stamping means (22), or a blow moulding means (Figure 2, not shown) which acts on a tubular extrusion. <IMAGE>

Description

SPECIFICATION Method of producing an article by two-material extrusion and the article thus obtained The present invention relates to a method of manufacturing an article by extrusion, and an article produced by this method. It relates more particularly to the manufacture of articles having a core made from polyethylene terephthalate (or PETP) covered with a coating layer made from another thermoplastic material.

It is known that when PETP is injected at its normal transformation temperature,(which is higher than its plastoelasticity temperature and is for example of the order of 210 to 22000) into a mould heated to a temperature of the order of 130 C, that is to say in conditions in which thermal shock is avoided, at a temperature a little below 140 C, whilst cooling slowly it acquires a semi-crystalline structure. In this condition, it is opaque and very homogeneous and is distinguished by outstanding mechanical and physical properties as well as having very good characteristics as regards chemical resistance and cracking under stress, notably when it contains up to 50% of mineral fillers.

This structure is characterised by a very poor coefficient of thermal expansion ((1.8 x 10-5) and an almost total absence of internal tension which make it largely possible to avoid subsequent deformation in use of the articles obtained, even with an increase in temperature.

In addition, the remarkable rigidity of the parts obtained (bending modulus of the order of 16000 MN/M2) and the high softening point or temperature of bending (of the order of 240 C) make it possible to reduce the usual thickness of articles made from the material and thus produce a saving in weight. Also, PETP is less costly than other crystalline thermoplastic materials.

However, in spite of its qualities when it is treated under these temperature conditions, PETP sometimes shows poor development.

In fact, if it contains high proportions of fillers which largely contribute to its excellent properties these high proportions of fillers can alter the surface of the articles obtained, which have a heterogeneous satin-like appearance rendering them unsuitable in this condition for certain uses which require a smooth surface.

More generally, the necessity to heat the mould and then to carry out slow cooling from the injection temperature to less than 14000 if it is desired to obtain a semi-crystalline product leads on the one hand to an expenditure of energy and on the other hand to low rates which are incompatible with exploitation on an industrial scale. In addition, the temperature of hardening of PETP is much lower than 14000 and the rates are lowered still more because after reaching the crystallisation temperature, it is necessary to wait again before removal from the mould, until the mould, which has been heated to a temperature higher than this temperature, has again reached this temperature.

It is an object of the present invention to remedy these disadvantages of PETP and thus to permit its use on an industrial scale for the production of articles having a smooth surface whilst having the mechanical, physical and chemical characteristics which result from a semi-crystalline PETP structure.

According to the invention, there is provided a method of producing by extrusion an article comprising a core of a first thermoplastic material comprising a polyethylene terephthalate (PETP) containing 0 to 50% by weight mineral fillers, and a layer coating the core, the layer being of at least one second thermoplastic material, the method comprising: bringing the first and second thermoplastic materials to a temperature higher than their respective plastoelasticity temperatures, in sufficient quantities to form respectively the coating layer and the core; extruding the thermoplastic materials together to form a composite material; applying immediately to the composite material thus obtained a treatment involving cooling by means having a temperature lower than the hardening temperature of the second thermoplastic material and also lower than the crystallisation temperature of the polyethylene terephthalate containing the filler, the second thermoplastic material acting as a thermal buffer to slow down the equalisation of temperatures between the polyethylene terephthalate while still fluid, and the cooling means in such a way as to ensure that the polyethylene terephthalate cools sufficiently slowly to have a semi-crystalline structure when solid.

Thus, it is proposed to apply the method known as "multi-material extrusion" to the manufacture of articles having a core of PETP of semi-crystalline structure, having the mechanical, physical and chemical characteristics associated with that structure, and a coating layer made from at least one other thermoplastic material which for its part makes it possible to obtain a smooth surface.

It is known that, if it is desired to obtain an article having a core of a first thermoplastic material and a coating layer of a second thermoplastic material, this method produces a composite material by simultaneously extruding the first and second thermoplactic materials, after they have been brought to a temperature higher than their respective plastoelasticity temperatures, in sufficient quantities to form the coating layer and the core.

The said the composite material thus obtained is then subjected to a treatment involving cooling by means having a temperature lower than the hardening temperature of the second plastic material.

This method is characterised by high production rates which have no comparison with those the moulding processes.

It is also characterised by the application of the cooling treatment, to the composite material obtained by extrusion, immediately after that extrusion. The cooling can consist of simple cooling, or forming by some appropriate means the temperature of which is generally of the order of 6000 +2000.

If PETP injected at its normal transformation temperature into a mould which has been heated above 1300C it is possible to obtain a semi-crystalline structure which is particularly advantageous from the point of view of mechanical, physical and chemical properties. However, if PETP at its normal transformation temperature, that is to say at a temperature higher than its plastoelasticity temperature, is brought into contact with cooling means at 60 C -+20 C this will result in perfectly transparent parts being obtained which have an amorphous structure but which are not suitable for the production of industrial and technical parts because their physical characteristics are too poor, being close to those of polyvinyl chloride.

In order to avoid obtaining such an amorphous structure, it might be possible to carry out the known method of multi-material extrusion taking care to slow down the cooling of the composite material after extrusion.

However, such a method would necessitate considerable slowing down of the extrusion rates and costly means for preventing the composite material from cooling too rapidly, even assuming that it would be possible to find a thermoplastic material to form a coating layer which is capable of giving satisfactory results in these conditions.

Thus, in a quite unexpected manner, the present invention proposes the application of the multi-material extrusion process just as it is known, i.e with cooling applied immediately after extrusion to the obtained composite material. This is a treatment involving cooling of the composite material by the cooling means, that is to say, a temperature of the order of 600C - +200C is used in order to produce articles but in this case, the articles have a core of PETP and this material is caused to have a semicrystalline structure which is of great relevance for industrial and technical parts.

Thus, the present invention proposes to produce an article having a core made from PETP having 0 to 50% of mineral fillers and a layer coating this core of at least one other thermoplastic material by a method which consists of producing a composite material by simultaneously extruding the PETP with its fillers and the other thermoplastic material, each having been brought to a temperature higher than the respective plastoelasticity temperatures, in sufficient quantities to form respectively the coating layer and the core, and immediately applying to the composite material thus obtained a treatment involving cooling by cooling means having a temperature lower than the crystallisation temperature of the polyethylene terephthalate, more precisely, a temperature of the order of 40 to 800C.

In this way, the thermoplastic material acts as a thermal buffer to slow down the equalisation of temperatures between the PETP, which is still fluid, and the cooling means, in such a way as to ensure that the PETP cools sufficiently slowly to have a semi-crystalline structure.

Thus, although the composite material is subjected immediately after extrusion to treatment by the "cold" means if one compares their temperature (of the order of 40 to 800C or thereabouts) with the crystallisation temperature of PETP, (slightly lower than 140 C,), sufficiently slow cooling of the PETP is nevertheless obtained to enable the PETP to acquire the particularly advantageous semi-crystalline structure called for. The amorphous structure usually associated with coming into contact with such "cold" means is avoided as it is desired according to the invention.

The desired semi-crystalline structure is obtained with the further advantages normally associated with extrusion, namely, high rates of production, in contrast with rates permitted by moulding at 1300C which is usually ncessary in order to obtain such a structure, and a reduced energy consumption is in comparision with that involved in such hot moulding.

The articles obtained also have smooth surfaces due to the presence of the coating layer, and because of this, they are suitable for a wide range of applications, for example for use in automobile bodywork.

Furthermore, since the coating layer in direct contact with the "cold" means is capable of hardening more rapidly than the PETP core, it is possible to carry out shaping of the obtained multi-layer material when only a skin of the coating layer has become rigid, that is to say even when the PETP has not reached its rigidification temperature much lower that 140 C. This makes it possible to obtain formed articles particularly rapidly.

The invention may be carried into practice in various ways and some embodiments will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows in a schematic sectional view of one embodiment a method according to the invention consisting of extruding a flat composite material and then immediately subjecting it to treatment involving cooling thereof, three variants being il- lustrated.

Figure 2 shows in a similar view a method according to the invention consisting of extruding a composite-material in the form of a sheath which is then immediately subjected to treatment involving cooling, two variants being illustrated.

Figure 1 shows "triple material" extrusion head of known type which has three extrusion channels 2, 3, 4 which have seperate inlets 5, 6, 7 to receive a respective material to be extruded as shown schematically by the arrows 8, 9, 10. The channels 2, 3, 4 become progressively thinner and end as slots which are superimposed flat in order to define one single outlet 11 also in the from of a slot.

Thus, this outlet can produce a flat composite material 12 having a core 13 formed from the material extruded by the channel 2 which is located between the channels 3 and 4 and a coating for this core 13 defined on one side by a layer 14 formed from the material extruded by the channel 3 and on the other side by a layer 15 formed from the material extruded by the channel 4.

Immediately at the outlet 11 of the extrusion head 1 this composite material 12 is calibrated by passing it through a flat die 16 which could be integral with the extrusion head 1. The die 16 has an inlet 17 in the immediate proximity of the outlet 11 from the extrusion head 1, in direct extension of this outlet, in order to recieve the composite material 12 and an outlet 18 for the calibrated composite material.

This outlet 18 constitutes the outlet of the actual extruder 19, which also includes means which are not shown for introducing into the inlet 5, 6, 7 of each of the channels 2, 3, 4 a thermoplastic material which has been brought to a temperature higher than its thermoplasticity temperature and to a specific pressure.

These elements of the extruder 19, and their functioning parameters such as temperature and pressure are known to the expert in the art.

However, in a system in accordance with the present invention, the channel 2, which is intended to form the core 13 of the composite material 12, is supplied at its inlet 5 with PETP containing 0 to 50% of mineral fillers such as mica, biotites or some other similar mineral fillers, and/or glass fibres, and brought to a temperature higher than its plastoelasticity temperature, for example of the order of 210 to 220 C, these figures being given by way of non-limiting example.

The channels 3 and 4 for their part are supplied with other thermoplastic materials chosen in dependence upon the characteristics which it is desired to give to the coating layers 14 and 15, which are also brought to a temperature higher than their plastoelasticity temperature. Thus, the channel 3 and the channel 4 can be supplied with polycarbonate resin, ABS, polystyrene, polybutylene terphthalate or methacrylate. Other thermoplastic materials which lend themselves to coextrusion with PETP can of course be chosen without in any way departing from the scope of the present invention.The temperature at which these materials are introduced into the channels 3 and 4 is for example of the order of 240 C and the assembly of the extrustion head 1 and the flat die 16 is advantageously heated to a temperature such that all the thermoplastic materials introduced into the channels 2, 3, 4 can be kept at a temperature higher than their plastoelasticity temperature until they reach the outlet 18 of the extruder 19.

At the outlet 18 of the extruder 19, the composite material 12 is immediately subjected to a treatment involving cooling which is applied to it by means 20, 21, 22 having a temperature which is not only lower than the hardening temperature of the thermoplastic materials forming the coating layers 14, and 15, in order to cause these latter to harden, but is also lower than the crystallisation temperature of the polyethylene terephthalate, which may contain fillers, although it is desired to give this PETP the required semi-crystalline structure.

For this purpose means 20, 21, 22 which have been brought to a temperature of the order of 40 to 80 C, in dependance upon the brilliance which it is desired to give to the coating layers 14 and 15 are used. The thermoplastic materials which form the coating layers 14, 15 act as a thermal buffer to slow down the aquaiisation of temperatures between the PETP (which forms the core 13 and is still fluid) and these means 20, 21, 22 in such a way as to ensure that the PETP cools sufficiently slowly to have the required semi-crystalline structure.

The following are examples of the means placed immediately at the outlet 18 of the extruder 19 and brought to a temperature of the order of 40 to 800C in order to harden the coating layers 14 and without causing cooling of the PETP to cool in such a way as to result in an amorphous structure of the PETP: a cooler 20 of known type, for example one with circulating water, through which the composite material 12 passes without undergoing any modificaiton of its shape when it is desired to produce an article 23 in the form of a strip or, after cutting, a plate; it will be noted that in this case it is possible according to the present invention to produce not only flat articles such as that described but also various shaped articles; a calender 21 which is formed by two rollers 24 and 25 between which the composite material 12 passes immediately at the outlet 18 of the extruder 19 and which produces a reduction in the thickness of the material to give a flat in a strip of the material 26 which can then be cut; stamping means 22 comprising a punch 27 and a die 28 to transform the flat composite material 12 into a shaped material.

Depending upon the particular case, the cooling device 20, the rollers 24 and 25, the punch 27 and the die 28 are brought to a temperature of the order of 40 to 80 C and the PETP forming the core 13 passes through them without sudden cooling thanks to the thermal buffer ensured by the coating layers 14 and 15.

Naturally, these three examples of cooling means 20, 21 22 should not be considered as limiting, and other means which treat the composite material 12 in the same way, particularly as regards temperature, immediately at the outlet 18 of the extruder 19 could be used without in any way departing from the scope of the present invention.

Referring now to Figure 2, a head is shown schematically at 29 for extruding sheaths.

In a manner which is known per se the extrusion head 29 has three channels 30, 31 32 each of which has an inlet 33, 34, 35 in order to receive by appropriate means (not shown) a respective thermoplastic material which has been brought to a temperature higher than it plastoelasticity temperature and subjected to a specific pressure, as is shown schematically in each case by arrows 36, 37, 38. These three channels join together into one single channel 39 located between a central core 40 and an outer body 41 having the same axis 42.

The channel 39 has an annular cross-section transversely with respect to this axis 42 and a common outlet 43 for all three channels 30, 31, 32 which constitutes the outlet of the extruder. The annular channel 39 narrows for calibration at 44 between the zone where the channels 30, 31 34 join together and the outlet 43.

These arrangements are known in themselves and make it possible to extrude the composite material 45 to 43 in the form of a sheath. The sheath has a lining 46 with a core 47 formed from the thermoplastic material introduced into the channel 34 at the inlet 31, and two coating layers. One coating layer 48 faces towards the interior of the sheath and is formed by the thermoplastic material introduced into the channel 30 at the inlet 33, and one coating layer 49 faces towards the exterior of the sheath and is formed by the thermoplastic material introduced into the channel 32 at the inlet 35.

The thermoplastic material introduced at 34 into the channel 31 and forming the core 47 of the material 45 is a PETP which is introduced into the extrusion head 29 and kept in the extrusion head 29 at a temperature higher than its plastoelasticity temperature, for example at a temperature of the order of 210 to 220 C.

The PETP contains 0 to 50% of mineral fillers in the same manner as in the example described with reference to Figure 1 andior glass fibres, depending upon the required characteristics of the material 45. The thermoplastic material forming each of the coating layers 48 and 49, identical or different, can be chosen in a similar way to the thermoplastic material forming the coating layer 14 or the coating layer 15 in the example shown in figure 1.

The extrusion head 29 is suitably heated to maintain a temperature for example of order of 24000 to keep all the thermoplastic materials introduced through the channels 30, 31 32 at a temperature higher than their plastoelasticity temperature.

Immediately at the outlet 43 of the extruder the material 45 is subjected to a treatment which involves cooling it by means having a temperature lower than the hardening temperature at least of the coating layer 49 turned towards the exterior of the sheath in order to cause the hardening of the sheath. The temperature of these means is also lower than the crystallisation temperature of the polyethylene-terephthalate which may contain fillers and the thermoplastic material forming the coating layer 49 acts as a thermal buffer to slow down the equalisation of temperatures between the polyethylene terephthalate forming the core 47 and these means in such a way as to ensure that the polyethylene terephthalate, which is still fluid, cools sufficiently slowly to have a semi-crystalline structure.

Examples of suitable cooling means are shown in the drawings and may comprise: a cooling device 50 of known type for example one with circulating water, through which the material 45 passes immediately at the outlet 43 of the extuder when it is desired to obtain an article 51 in the form of a tube with a multi-layer lining; -blowing means 52 also arranged immediately at the outlet 43 and having two mould halves 53, 54 initially separated from one another and located on either side of the material 45 as is shown schematically at A. The two halves 53, 54 are closed on a section 55 of this material before air 56 is injected into this section 55 in order to flatten it against the respective imprints 57, 58 of the two mould halves 53, 54, as shown schematically at B.When the two mould halves 53 and 54 are separated again a parison is released which can either be used as it is when the article to be produced is to be in the form of a receptacle having a multi-layer lining with a core of PETP, or can be divided into two halves 59, 60, as shown schematically at C, each of these halves then constituting an article comparable to that obtained by stamping at 22 in the case of the embodiment illustrated in Figure 1.

These means 50 and 52 only constitute one nonlimiting example of means capable of being used immediately at the outlet 43 of the extruder in order to treat a material 45 by cooling it without in any way resulting in an amorphous structure of the PETP.

These means 50 and 52, like any which could be substituted for them, are advantageously maintained at a temperature of the order of 40 to 8000 depending upon the brilliance which it is desired to give the coating layer 49 facing the exterior. However, the use of this coating layer 49 as a thermal buffer guarantees that the core 47 cools sufficiently slowly for the PETP which forms it to have the required semi-crystalline structure.

It will be easily understood that by using a method according to the present invention, both in the variants which have been described and in other variants which would not depart from the scope of the present invention, it is possible to produce articles of various shapes having a core of PETP with a semi-crystalline structure showing the advantages which result from this, and a coating of another thermoplastic material chosen to provide the desired surface characteristics of the article.

This covers numerous applications, for example, the production of automobile body parts, protective casings etc.

Although the method according to the invention has been described with reference to triple material extrusion, it will be understood that the number of materials coextruded is not critical provided that the material coming out of the extruder has, in addition to a core of PETP, at least one coating layer made from another thermoplastic material capable of acting as a thermal buffer as has been defined.

Claims

1. A method of producing by extrusion an article comprising a core of a first thermoplastic material comprising a polyethylene terephthalate (PETP) containing 0 to 50% by weight mineral fillers, and a layer coating the core, the layer being of at least one second thermoplastic material the method comprising: bringing the first and second thermoplastic materials to a temperature higher than their respective plastoelasticity temperatures, in sufficient quantities to form respectively the coating layer and the core; extruding the thermoplastic materials together to form a composite material; applying immediately to the composite material thus obtained a treatment involving cooling by means having a temperature lower than the hardening temperature of the second thermoplastic material and also lower than the crystallisation temperature of the polyethelyene terephthalate containing the filler, the second thermoplastic material acting as a thermal buffer to slow down the equalisation of temperatures between the polyethylene terephthalate while still fluid, and the cooling means in such a way as to ensure that the polyethylene terephthalate cools sufficiently slowly to have a semi-crystalline structure when solid.

2. A method as claimed in Claim 1, in which the temperature of the cooling means is of the order of 4000 to 8000.

3. A method as claimed in Claim 1 or Claim 2 in which the composite material which is extruded is in the form of a flat sheet.

4. A method as claimed in Claim 3 in which the cooling treatment includes shaping the flat composite material immediately after extrusion and before rigidification of the PETP by the cooling means.

5. A method as claimed in Claim 4 in which the flate composite material is shaped by stamping.

6. A method as claimed in Claim 3 in which the cooling treatment includes subjecting the flat composite material immediately after extrusion and before rigidification of the PETP to calendering by the cooling means.

7. A method as claimed in Claim 1 or Claim 2 in which the composite material is extruded in a tubular form.

8. A method as claimed in Claim 7 in which the cooling treatment includes shaping the tubular composite material immediately after extrusion and before rigidification of the PETP by the cooling means.

9. A method as claimed in Claim 8 in which the cooling treatment includes blowing in a mould.

10. A method of producing an article by extrusion substantially as herein specifically described with reference and as shown in Figure 1 or Figure 2 of the accompanying drawings.

11. An article obtained by carrying out a method according to any preceeding claim, the article having a core of polyethylene terephthalate containing 0 to 50% mineral fillers and a coating layer of another thermoplastic material, the core having a semi-crystalline structure.