IE47479B1 - Manufacture of hollow bodies of polyethylene terephthalate - Google Patents

Description

The present invention relates to a method of making hollow bodies of polyethylene terephthalate. The invention also relates to hollow bodies obtained by this method.

The hollow bodies may be especially but not exclusively flasks, bottles and other receptacles intended to contain liquids, especially liquids under pressure.

It has been known for a long time to make hollow bodies of thermoplastics materials by blowing methods. These methods consist of first making a parison, for example by extrusion or injection in a mould, then, after having introduced the parison at a suitable temperature into a blowing mould having two or more parts, to cause expansion by blowing a fluid such as compressed air inside the blowing mould. Frequently the expansion by blowing is preceded by a drawing intended -to Tength'eh-the parison.

It has further been known for a long time to carry out the a · expansion of the hollow bodies in such a manner as to reach a state of the material described as the bioriented state, a state which shows itself by a substantial improvement in the mechanical performance of the material and in general also by a greater stability.

To make bodies having high performances, especially mechanical performances for a minimum v/eight, for example hollow bodies intended to contain liquids under pressure such as beer or soda water, it has been known for a certain time to use polyesters especially polyethylene terephthalate. These materials are however quite difficult to work and it has been necessary to use particular methods under optimum conditions.

As concerns polyethylene terephthalate, which is denoted below by the abbreviation PTE, the methods which have been found satisfactory consist of making a parison or blank which is amorphous, that is to say cooled sufficiently quickly to avoid appearance of a substantial amount of crystallisation, then after having reheated this blank to a temperature close to and preferably greater than the temperature of vitreous transition, carrying out expansion and blowing to a sufficient extent to cause, in at least certain parts of the body, a bioriented state. Preferably the hollow body is then subjected to an operation of thermostabilisation by increase of the temperature to release a certain amount of residual stress.

The applicant has improved this method in making for the parison a preform which has already been subjected to a certain radial and possibly axial expansion, this preform also being maintained in the amorphous state to be later expanded to the required degree. It is thus possible to better control the exchanges of temperature, especially to maintain the material in the amorphous state and to adapt easily the whole method to different requirements, especially geometrical, imposed by the particular shapes of hollow bodies which are required.

It has also already been suggested to make hollow bodies of PTE from blanks which are initially substantially amorphous and to blow the blanks at a temperature greater than the crystallisation temperature so that they are drawn biaxially and crystallised. However there are obtained by this method opaque hollow bodies which are strongly crystallised, which do not give satisfaction for the envisaged use.

The requirement to use a blank or parison which is amorphous, that is to say to carry out a cooling which is sufficiently rapid at the moment of formation of the parison to avoid its crystallisation, requires a certain number of operations and lengthening of the duration of manufacture.

The invention is intended to remedy this disadvantage by providing a method which allows easy manufacture at a high speed of hollow bodies of PTE which have finally a bioriented structure are transparent and are provided with excellent properties of mechanical strength, especially to internal pressure.

According to one aspect of the invention there is provided a method of making hollow transparent bodies of polyethylene terephthalate, comprising the steps of:(a) injecting a hot tubular parison or blank around a mandrel cooled in a first mould, (b) rapidly cooling the blank to a temperature lower than or equal to the temperature of crystallisation of the material and greater than its temperature of vitreous transition, (c) submitting the blank, which is held solely by the mandrel, to a radial and/or axial expansion to a limited extent, (d) separating completely the blank from the mandrel, and (e) introducing the blank into a second mould and submitting it to a rapid axial and/or radial expansion at a rate sufficient to obtain biorientation before substantial crystallisation of the materi al.

The temperature of the material at the moment of the biorientation is preferably greater than 60 or 70°C below the crystallisation temperature but it may possibly be even lower. For a crystallisation temperature of the order of 180°C the temperature is preferably greater than 120° or 130°C. In fact, it is preferable that the temperature should be as high as possible and it may be very close to the crystallisation temperature but in this case the method has to be carried out with particular rapidity.

The method of the invention presents numerous advantages. The improvement in production rate allows omission of the stations for thermal treatment and reheating which are usual in known methods.

Further, because in the method of the invention there exists a crystallisation starting at the moment of the bi orientation, crystals are formed and provide fixing nuclei from which the drawing may be carried out. Further, and surprisingly, there exists less free elastic stress after the biorientation and when it is desired to carry out a thermofixing of the hollow body after blowing this thermofixing is carried out more easily.

Finally the hollow bodies obtained according to the invention have interesting advantages from the point of view of mechanical and physicochemical properties, probably related to their proportion of real crystallinity which is small, which allows addition, to the known properties of hitherto known amorphous hollow bodies such as transparency and resistance to dropping, of other properties inherent in this crystalline state, for example chemical inertness.

The degree of elongation of the material in each direction should be at least equal to 1.5 and preferably greater than 2.5. Of course it is possible, according to the case, to carry out a quasi complete biorientation of the whole of the hollow body or only to biorientate a part of the latter for example in a body in which the neck is left in the non-biorientated state.

In all cases, the method according to the invention should be carried out rapidly to avoid any substantial crystallisation of the material, the speed of crystallisation being higher when it is closer to the temperature of crystallisation.

Preferably, the duration separating the making of the hot blank and the operation of biorientation is preferably less than 30 seconds, for example 10 to 12 seconds.

In consequence, it is advantageous to carry out the final blowing in 15 a mould having cold walls to obtain at the end of expansion, or even during expansion, a temperature sufficiently low to avoid any later crystallisation.

Other advantages and characteristics of the invention will appear from a reading of the description which follows, given by way of nonlimiting example, and referring to the accompanying drawing which shows schematically apparatus for carrying out the method according to the invention.

Example 1 There is used a polyethylene terephthalate having a melting point of 257°C, a crystallisation temperature of 176°C and a vitreous transition temperature of 57°C.

By means of a known injection device there is made in an injection mould, about a core or mandrel which is cooled, a tubular blank having a closed bottom. The temperature of the plastics material on injection is 280 to 300°C.

The duration of moulding of the blank in the mould is such that on opening of the mould the material is at a temperature from 140 to 150°C.

The mandrel is then displaced to an unsticking station where there is blown a fluid such as compressed air through the mandrel to achieve a small radial expansion of the blank which remains however attached to the mandrel by a neck portion.

When the blank arrives in the blowing mould, it is at a temperature from 120 to 130°C. There is then carried out an axial drawing to 2.5 times the original length by means of a stem sliding through the mandrel and carrying with it the bottom of the blank in a manner known as such. Immediately after or simultaneously there takes place the blowing itself, the blank being deformed until it takes the required shape of the article desired. At the end of blowing the temperature of the hollow body is less than the temperature of vitreous transition. Finally, at a fourth station, the mandrel which no longer carries the hollow body has its temperature reduced to a low value, for example 30°C.

There is thus obtained a hollow transparent body with a small degree of crystallinity, and experience shows that the residual elastic stresses are relatively small so that the operation of thermofixing, when required, is simplified.

By way of example the duration of the cycle may be the following for a blank having a 3.6 mm thickness.

I) Duration of injection in the injection mould Variation of temperature Cooling and holding at injection station 4 seconds - 300 to 280°C 8 seconds 10 Variation of temperature Transfer Variation of temperature - 280 to 150°C 2 seconds - 150 to 145°C Π) Unsticking and thermal conditioning Temperature variation 10 seconds - 130 to 125°C 15 Transfer 2 seconds HI) Blowing drawing Variation of temperature Total duration of cycle 10 seconds - 125 to 30°C 36 seconds The numerical values indicated in the Example may be different 20 according to the characteristics of the material used or the thicknesses of the material, especially in that the blanks or parisons having a thinner wall may be cooled more rapidly.

In general it is desirable to work at a high temperature very close to the crystallisation temperature. The higher the temperature, the faster the cycle should be carried out.

The method of the invention may be carried out by means of the apparatus shown in the accompanying drawing, which shows a device i rotatable about the vertical axis 2 in steps of 90°. Each face of the device has a battery of four parallel horizontal mandrels 3. Each 5 battery passes in succession by steps to a moulding station 4 comprising an injection mould having four units and means to feed the plastics material into the units about the mandrels. Preferably means for cooling the mandrels and the units are provided.

The mandrels then pass through 90° to a station indicated generally by 5. When they are immobilised at this station there is carried out through the inside of the mandrel, a limited blowing by fluid under pressure which contributes to unsticking of the parison with respect to the mandrel. This unsticking is preferably such that it produces a play between the parison and the mandrel of the order of 0.6 mm.

By a fresh rotation of 90°C, the four mandrels are brought to a transfer station 6 having a plurality of channels 7 and suitable means for expelling the parisons towards the channels which bring them directly to a moulding station 8 comprising a mould having four units 9.

The mould, in the example described, comprises two parts each having four half moulds and means for blowing through the neck. The blowing is carried out according to a known method following which the finally shaped hollow bodies are demoulded and transferred by removal means 10.

Claims (7)

1. A method of making hollow transparent bodies of polyethylene terephthalate, comprising the steps of:(a) injecting a hot tubular parison or blank around a mandrel 5 cooled in a first mould, (b) rapidly cooling the blank to a temperature lower than or equal to the temperature of crystallisation of the material and greater than its temperature of vitreous transision, (c) submitting the blank whi:ch is held solely by the mandrel, 10 to a radial and/or axial expansion to a limited extent, (d) separating completely the blank from the mandrel, and (e) introducing the blank into a second mould and submitting it to a rapid axial and/or radial expansion at a rate sufficient to obtain biorientation before substantial crystallisation of the 15 material.

2. Method according to Claim 1, wherein the temperature of the material during expansion to achieve biorientation is greater than 60° or 70°C below the crystallisation temperature.

3. Method according to Claim 2, wherein said temperature of the 20 material during expansion to achieve biorientation is greater than 120°C.

4. Method according to Claim 1 or Claim 2, wherein the temperature of the material during expansion to achieve biorientation is close to the crystallisation temperature.

5. Method according to any one of Claims 1 to 4, wherein the 25 duration between making the blank and the expansion to achieve biorientation is less than 30 seconds.

6. Method of making hollow transparent bodies, substantially as hereinbefore described with reference to the accompanying drawing.

7. Hollow bodies obtained by the method according to any one of Claims 1 to 6.