GB1576505A

GB1576505A - Heat recoverable plastics artefacts

Info

Publication number: GB1576505A
Application number: GB733577A
Authority: GB
Original assignee: Coal Industry Patents Ltd
Current assignee: Coal Industry Patents Ltd
Priority date: 1978-01-27
Filing date: 1978-01-27
Publication date: 1980-10-08

Description

(54) HEAT RECOVERABLE PLASTICS ARTEFACTS (71) We, COAL INDUSTRY (PATENTS) LIMITED, a company organised in accordance with the laws of Great Britain of Hobart House, Grosvenor Place, London S.W. 1X 7AE England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to heat recoverable plastics artefacts, and in particular to such artefacts made from polyvinyl chloride (PVC)-based materials.

It has been known for some time that certain plastics materials may exhibit the property of elastic memory, which is exemplified as follows. A plastics material may be formed into one shape, heated, deformed into another shape and cooled while still retained in the deformed shape. The cool deformed shape will remain form-stable for a considerable time if maintained at approximately room temperature. However, when the deformed shape is heated it will recover to its original shape, providing recovery is not restricted.

A limited number of plastics materials can be used in the preparation of heat-recoverable artefacts, and these include polyethylene, polypropylene, PVC, polyamides and some fluorinated polymers.

PVC has been widely used in the past to make heat recoverable artefacts, but these artefacts suffered from the disadvantage that, since PVC is a thermoplastic, they tended to melt during the heat-recovery operation. Also if a naked flame was used to induce heat recovery of the artefacts there was a considerable danger that the melted PVC would degrade. It has now been discovered, however, that the tendency of PVC to melt on heating can be substantially removed by cross-linking the PVC, either by irradiation or by chemical agents, in the presence of a sensitizer.

It has surprisingly been discovered that cross-linked PVC not only has the desired heat resistance properties but also can be made into heat recoverable artefacts having much improved recovery properties.

Therefore according to the present invention there is provided a method of producing an improved heat-recoverable PVC-based artefact, comprising the steps of: a) forming a thermoplastic PVC-based artefact which contains from 1 to 20%, preferably from 1 to 10So, of a sensitizer; b) cross-linking the artefact, preferably by exposure to a high energy electron beam; c) heating the cross-linked artefact to a temperature of from 100 to 1600C; (d) deforming the cross-linked artefact while at the temperature of from 100 to 1 60 C; and (e) cooling the artefact to approximately room temperature while retaining it in its deformed state.

The present invention also provides heat recoverable PVC-based artefacts which have been made by the above method.

PVC-based artefacts made by the method of the present invention have, as expected, good heat resistance properties. The novel and unexpected property of these artefacts lies in their strength of and degree of recovery. For instance, an artefact made according to the invention which has been expanded will shrink back to substantially its original size if not constrained. If the artefact is shrunk onto a substrate the artefact will be very much more difficult to pull off the substrate than would a similar heat shrink PVC-based artefact which has not been cross linked.

Hereinafter wherever "tightness of shrinkage" is mentioned it is to be understood that this is a term used in the art and is not intended to limit the scope of the invention solely to heat-shrinkable artefacts. The invention also comprises a method of making heat-expandable artefacts, and the following paragraph applies in appropriate fashion to such artefacts.

This result of the cross linking of PVC-based heat shrink artefacts is most surprising, and it would not have been predicted that the tightness of shrinkage of a heat shrink artefact onto a substrate should have been improved to such an extent by cross linking. Artefacts made according to the invention have tightnessess of a shrinkage onto a substrate equivalent to, and in some cases better than, those of cross-linked polyolefin artefacts.

It is essential to the process of the present invention to have a sensitiser present in the thermoplastic artefact. Although it is possible to cross-link unsensitized PVC artefacts by using large radiation doses this has a great adverse effect on the artefact, in that the PVC is degraded, with the evolution of noxious fumes of hydrogen chloride, resulting in an unusable artefact. However, in the presence of a sensitizer, it is possible to cross-link the thermoplastic PVC-based artefact using relatively low radiation doses, or by chemical agents.

The sensitizer is a polyfunctional unsaturated monomer which can form cross-links between the polymeric chains of the PVC. Preferred sensitizers are derived from acrylic or methacrylic acid, for instance, trimethylol-propane-trimethacrylate, tetraethylene glycol dimethacrylate, ethyleneglycol dimethacrylate and pentaerythritol tetraacrylate. Most preferably, trimethylolpropane triacrylate is used as the sensitizer.

Preferably the thermoplastic artefact is cross-linked by exposure to a dose of from 1 to 1 OM rads, more preferably from 3 to 5 M rads, of radiation. Alternatively, the thermoplastic artefact may be cross-linked by an organic peroxide such as for example dicumyl peroxide.

The thermoplastic artefact may be formed by dip moulding, extrusion or injection moulding, using any of the commonly used techniques available to those skilled in the art.

Preferably the thermoplastic artefact contains up to 30 % of a plasticiser, such as for instance, dibutyl phthalate, tricresyl phosphate, propylene sebacate, epoxidised soya bean oil, trixylyl phosphate, dinonyl phthalate, di-iso-octyl adipate, or di-ethyl hexyl phthalate.

Radiation may be supplied to cross-link an artefact from a radiosotope emitting p radiation, but is preferably supplied from a linear accelerator or other electron beam radiation generator. Preferably it is ensured that the artefact is uniformly irradiated by rotating it during the irradiation.

The steps (c), (d), and (e) of the statement of invention are well known within the art of heat-recoverable plastics. The heating in step (c) is preferably effected by placing the artefact in an electric oven. Preferably the artefact is heated to a temperature of from 100 to 1400C.

The artefact may be deformed mechanically, for instance by using mandrels. Alternatively, pneumatic or hydraulic pressure may be applied inside an artefact located in a cavity of appropriate design to cause it to expand and adopt the shape of the cavity. Preferably, especially if the artefact is deformed mechanically, the mandrels or deforming means are heated to approximately the same temperature as the artefact.

The artefact is conveniently cooled by immersion in a bath of water at ambient temperature. However any other convenient cooling method, for instance forced air cooling, may be used.

It is envisaged that artefacts made according to the invention may be used for any of the normal purposes for which PVC-based heat recoverable artefacts are suitable, for instance in covering terminations, joints and unions in electrical cables and gas pipes or for rope splicing etc.

A more specific example of the invention will now be described, but this example is. not intended to limit in any way the scope of the present invention.

EXAMPLE Tubular mouldings having various sensitizer contents were made by the following procedure. Cylindrical Dural * formers were heated in an air oven to 235"C and were then immersed in a bath of a PVC plastisol containing 2, 3 or 5 % of trimethylolpropane triacrylate.

The PVC plastisol contained 27% plasticiser which was a blend of sebacate - and pentaerythritol-derived plasticisers. The formers were immersed in the plastisol at a rate of about 2 cm/ sex in order to prevent air being entrained in the plastisol. The formers remained in the plastisol for about 30 secs and then were withdrawn at a rate of about 4 cm/sec. This treatment left the formers coated with a layer of PVC plastisol about 2.5 mm thick. After excess plastisol had drained off, the coated formers were transferred to an air oven at 2350C and were retained therein for about 4 min, by which time the PVC plastisol had gelled. The coated formers were then immersed for about 10 secs. in cold tap water. This hardened the surfaces of the artefacts and thereby facilitated their removal from the former.

Each artefact was then exposed to radiation by suspending the artefact in front of the window of an 800 watt 4 MeV linear accelerator. The linear accelerator was supplied with a beam current of 9jaA and gave a dose rate of 1 M rad per minute. Each artefact was exposed to the radiation for 3 minutes, and was rotated through 1800 after 1.5 minutes to ensure that all parts of the artefact were equally irradiated. The total radiation dose given was 3 M rad.

It is envisaged that in commercial practice an irradiation chamber will be provided through which the artefacts may be continuously passed. The radiation chamber may contain a large number of separate sources of radiation which will obviate the need to rotate the artefacts.

The rate at which the artefacts pass through such a radiation chamber will be arranged to ensure that the appropriate radiation dose is given to each artefact.

The artefacts were then heated to a temperature of 100"C and expanded by use of cylindrical PTFE mandrels, heated to 100"C and having diameters greater than the diameters of the artefacts. The expanded artefacts were cooled while still on the mandrels, and then removed therefrom to leave a PVC-based heat-recoverable artefact made according to the invention.

The following test was then carried out on a variety of heat recoverable cylindrical artefacts all made using the same formers and mandrels. The formers had an external diameter of 9 mm, and the mandrels an external diameter of 20 mm. One batch of artefacts was made according to the invention and another was made according to the prior art from plasticised non-cross-linked PVC.

The test rig comprised a pair of cylindrical Dural * rods having a length of 70 mm and an external diameter of 12.5 mm. Each Dural * rod had screwed into the centre of one face thereof a steel rod 60 mm long and of 5 mm external diameter. One of the Dural * rods was provided with an axial hole about 4 mm in diameter. The two steel rods are capable of being clamped into an extensometer.

The test procedure was as follows. The Dural * rods were located coaxially and with their free end faces touching. A test artefact 80 mm long, which had been formed with an internal diameter of 9 mm and expanded to an internal diameter of 20 mm, was located around the Dural *rods. This arrangement was placed in an oven at 1350for 15 mins, which caused the test piece to shrink onto and grip the Dural * rods. The test piece was arranged so that 60 mm of its length was located on one Dural * rod and the remaining 20 mm of its length was located on the other Dural * rod. A jubilee clip was fastened onto the Dural * rod having the 60 mm length of the test piece on it. This was to ensure that the test piece would not slip off the Dural * rod. The steel rods were than clamped into the extensometer, which was activated to draw the Dural * rods apart axially. The presence of the 4 mm diameter hole ensured that no spurious results were obtained due to the creation of a vacuum between the two faces of the Dural * rods. The diameters of duplicate artefacts after they had been heatshrunk with no constraint were also recorded. The results of these tests are shown in the Table below.

TABLE Amount of Wall Force required to Average diameter sensitizer** Radiation thickness pull moulding of fully used dose of moulding off rod recovered moulding (%) (M rad) (mm) (Kg) (mm) 1 2.0 2 1.8 5 3 2-2.5 2.1 9.0 3 2.1 5 1.7 1 1.6 3 2 2-2.5 1.2 9.0 3 1.8 5 1.6 1 1.1 2 2 2-2.5 1.1 9.0 3 1.2 5 1.3 Blank 1 - 0.5 Blank 2 - 2-2.5 0.4 90 Blank 3 - ' 0.8 t Trimethylolpropane triacrylate Blanks 1 to 3 were artefacts made from PVC but which were not cross-linked. The remaining samples were made according to the invention, and coontained the amount of sensitizer shown in the first column of the Table.

It can be seen from these results that cross-linked PVC-based heat recoverable items have a greatly increased tightness of shrinkage onto a substrate when compared to similar artefacts made from PVC which has not been corss-linked, and recover to their original dimentions.

* Dural is a registered Trade Mark.

Herein, all percentages and proportions are by weight, unless otherwise stated.

WHAT WE CLAIM IS: 1. A method of producing an improved heat-recoverable PVC-based artefact, comprising the steps of: (a) forming a thermoplastic PVC-based artefact which contains from 1 to 20% of a sensitizer, (b) cross-linking the artefact, (c) heating the cross-linked artefact to a temperature of from 100 to 1600C, (d) deforming the cross-linked artefact while at the temperature of from 100 to 1600, and (e) cooling the artefact to approximately room temperature while retaining it in its deformed state.

2. A method according to claim 1, wherein the sensitizer is present in an amount of from 1 to 10%.

3. A method according to either one of claims 1 and 2, wherein the sensitizer is an acrylic or methacrylic acid derivative.

4. A method according to claim 3, wherein the sensitizer is selected from the group consisting of trimethylolpropane trimethacrylate, tetraethyleneglycol dimethacrylate, ethyleneglycol dimethacrylate and pentaerythritol tetraacrylate.

5. A method according to claim 3, wherein the sensitizer is trimethylolpropane triacrylate.

6. A method according to any one of the preceding claims, wherein the artefact is cross-linked by exposure to a high energy electron beam.

7. A method according to claim 6, wherein the artefact is exposed to a dose of from 1 to 10 M rads of radiation.

8. A method according to claim 7, wherein the artefact is exposed to a dose of from 3 to 5 M rads of radiation.

9. A method according to anyone of claims 6 to 8, wherein the radiation to cross-link the artefact is supplied from a radiosotope emitting p radiation.

10. A method according to any one of claims 6 to 8, wherein the radiation to cross-link the artefact is supplied from a linear accelerator or other electron beam radiation generator.

11. A method according to any one of claims 6 to 10, wherein the artefact is uniformly irradiated by rotating it during the irradiation.

12. A method according to any one of claims 1 to 5, wherein the thermoplastic artefact is cross-linked by an organic peroxide.

13. A method according to claim 12, wherein the peroxide is dicumyl peroxide.

14. A method according to any one of the preceding claims, wherein the thermoplastic artefact is formed by dip moulding, extrusion or injection moulding.

15. A method according to any one of the preceding claims, wherein the thermoplastic artefact contains up to 30% of a plasticiser.

16. A method according to claim 15, wherein the plasticiser is selected from the group consisting of dibutyl phthalate, tricresyl phosphate, propylene sebacate, epoxidised soya bean oil, trixylyl phosphate, dinonyl phthalate, di-iso-octyl adipate, and di-ethyl hexyl phthalate.

17. A method according to any one of the preceding claims, wherein the heating in step (c) is effected by placing the artefact in an electric oven.

18. A method according to any one of the preceding claims, wherein, in step (c), the artefact is heated to a temperature of from 100 to 1400C.

19. A method according to any one of the preceding claims, wherein the artefact is deformed mechanically.

20. A method according to claim 19, wherein the artefact is deformed by using mandrels.

21. A method according to any one of claims 1 to 18, wherein the artefact is deformed by applying hydraulic or pneumatic pressure inside the artefact located in a cavity of appropriate design, to cause it to expand and adopt the shape of the cavity.

22. A method according to any one of the preceding claims, wherein the deforming means are heated to approximately the same temperature as the artefact.

23. A method according to any one of the preceding claims, wherein the artefact is cooled by immersion in a bath of water at ambient temperature.

24. A method according to claim 1, substantially as hereinbefore described.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. Blanks 1 to 3 were artefacts made from PVC but which were not cross-linked. The remaining samples were made according to the invention, and coontained the amount of sensitizer shown in the first column of the Table. It can be seen from these results that cross-linked PVC-based heat recoverable items have a greatly increased tightness of shrinkage onto a substrate when compared to similar artefacts made from PVC which has not been corss-linked, and recover to their original dimentions. * Dural is a registered Trade Mark. Herein, all percentages and proportions are by weight, unless otherwise stated. WHAT WE CLAIM IS:

1. A method of producing an improved heat-recoverable PVC-based artefact, comprising the steps of: (a) forming a thermoplastic PVC-based artefact which contains from 1 to 20% of a sensitizer, (b) cross-linking the artefact, (c) heating the cross-linked artefact to a temperature of from 100 to 1600C, (d) deforming the cross-linked artefact while at the temperature of from 100 to 1600, and (e) cooling the artefact to approximately room temperature while retaining it in its deformed state.

13. A method according to claim 12, wherein the peroxide is dicumyl peroxide.

24. A method according to claim 1, substantially as hereinbefore described.

25. A heat recoverable artefact when made according to the method of any one of claims

1 to 24.