GB1603638A

GB1603638A - Polymer processing

Info

Publication number: GB1603638A
Application number: GB2964177A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1978-05-31
Filing date: 1978-05-31
Publication date: 1981-11-25

Description

(54) POLYMER PROCESSING (71) I, JEREMY ARCHIBALD BOWMAN, a British subject of Brunel University, Uxbridge, Middlesex, do hereby declare this invention, for which I pray that a patent may be granted to me. and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to the processing of thermoplastic polymers, in particular of amorphous or semi-crystalline thermoplastic polymers.

In the manufacture of finished articles from thermoplastic polymers, it is known to cross-link the polymer after fabrication. The fabricated polymer may be cross-linked by irradiation or by chemical cross-linking. In the case of chemical cross-linking, a cross-linking agent, which is insensitive to the fabrication conditions, is compounded with the polymer prior to fabrication and the fabricated polymer is, for example, heated to a sufficiently high temperature to generate cross-linking.

According to the present invention there is provided a method of processing an amorphous or semi-crystalline thermoplastic polymer which method comprises crosslinking the polymer in particulate form and fabricating the so-crosslinked polymer by injection moulding or fibre spinning such that there is obtained a fabricated polymer of enhanced tensile strength.

During the injection moulding of fibre spinning of the fabrication step there is introduced significant preferred orientation into the polymer before crystallisation or freezing to preserve that orientation. In particular. there may be introduced significant orientation into the melt which is preserved by shock cooling.

It has surprisingly been found that, with cross-linking of the amorphous or semicrystalline thermoplastic polymer in particulate form prior to its fabrication, there may be obtained a fabricated polymer of higher tensile strength than when the same polymer is fabricated in conventional manner without the prior cross-linking.

It is also found that the method of the invention can lead to increase in stiffness or in environmental stress crack resistance of the polymer.

The thermoplastic polymer suitable for use in accordance with the process of the present invention is amorphous or semi-crystalline (which latter can be cross-linked at either low or high degrees or perfection), homo- or copolymeric, or may comprise a filler. In particular the thermoplastic polymer is as-received polymer and may be virgin polymer (i.e.

unprocessed polymer) or regrind polymer (i.e. polymer which has been previously processed and reclaimed for recycling). In the case of regrind polymer there tends to have been some molecular degradation leading to a lowering of tensile strength. Thus the method of the invention offers a means to reverse this degradation.

Generally the polymer is used in the form of granules or powder.

The thermoplastic polymer is suitably a high density polyethylene (HDPE).

The cross-linking of the thermoplastic polymer in accordance with the method of the present invention may be effected by irradiation and/or by chemical cross-linking, for example using an organic peroxide such as one of the Retilox or Di-Cup type: p - C6H4 [C(C113)2O-O-C(C113)3]2 or C6H5(CH3)2C-O-O-C(CH3)2C6H respectively. [Retilox and Di-Cup are Registered Trade Marks].

The chemical cross-linking agent may be compounded with the polymer for example a Banbury blender or a two roll mill. The cross-linking agent may then be exposed to those temperatures, and for those times, which permit the requireddegree of cross-linking. The resulting crepe will then consist of the cross-linked polymer which can next be fabricated.

Cross-linking by irradiation is, however, generally to be preferred, particularly for those applications requiring minimum polymer contamination. Irradiation may be effected by electron beam or y - radiation (for example using a cobalt-60 source), desirably at a temperature which avoids significant fusion of the granules or powder.

The amount of cross-linking introduced must neither be so small as to give an insignificant improvement in tensile strength nor so large as to prevent the fabriction.

Cross-linking may rapidly reduce the melt flow index (MFI) of the polymer leading to incomplete mould filling (in injection moulding) or other fabrication failures.

Accordingly, it is preferred that for successful operation of the method of the present invention the as-received polymer should have a high MFI thereby enabling higher cross-linking without fabrication difficulties arising. (Lower molecular weight polymers generally have a higher MFI but will be more prone to relaxation effects so that their quenching must be more extreme). Where cross-linking is effected by irradiation, it is generally found that doses of up to 50M Rads, typically up to 10M Rads and preferably from 0.5 to 2.0M Rads can be used.

Fabriction conditions are those which introduce and preserve significant preferred orientation (which includes molecular, crystalline and filler orientation). In the case of injection moulding these are low injection speed, low melt temperature, low mould temperature and high packing pressure. In the case of fibre spinning these are low melt temperature, fast cooling and high take off speed.

In particular the present invention provides a method of processing high density polyethylene which method comprises cross-linking high density polyethylene in particulate form by irradiation with 0.2 to 2.5M Rads and fabricating the so-crosslinked polymer by injection moulding or fibre spinning such that there is obtained a fabricated polymer of enhanced tensile strength.

The following Example illustrates the invention.

Example A homopolymeric HPDE (Rigidex 140/60 ex BP; Mw = 65,000; MW/Mn = 4.6; MFI as received 11.8) was irradiated at room temperature as granules with doses of 0.2, 0.5, 1.02, 2.0, 5.0 and 10.0M Rad (from a cobalt 60 source typically at 0.5M Rad/hour). The irradiated granules were then injection moulded using a wide range of barrel temperatures (150 to 250"C) with the mould temperature (40"C), injection pressure (41.1 MNm-2) and cycle times (fill times, 2.5 sec), hold-on pressure time (12.5 sec), cooling time (20 sec) and mould open time (5 sec) remaining constant, to produce a dumb-bell type tensile bar. (For granules irradiated to 5.0M Rad and above, incomplete mould filling occurred at all barrel temperatures while the 2.0M Rad irradiated sample was not mouldable at barrel temperatures of 1500C and 175"C).

The as-moulded tensile bars were next tested in uniaxial tension at room temperature at a cross-head speed of 0.5 mm/min.

Figure 1 of the accompanying drawings shows the marked reduction of MFI with increasing radiation dose, 2.5M Rad being identified as the maximum dose that this particular polymer could usefully sustain in the method of the present invention.

Figure 2 of the accompanying drawings shows the increase in strength of the tested tensile bars at different barrel temperatures. It will be seen that the tensile strength of the bars increases markedly with increasing dose, for a given barrel temperature; and that, for a given dose, raising the barrel temperature tends to reduce the effect.

WHAT I CLAIM IS: 1. A method of processing an amorphous or semi-crystalline thermoplastic polymer which method comprises cross-linking the polymer in particulate form and fabricating the so-crosslinked polymer by injection moulding or fibre spinning such that there is obtained a fabricated polymer of enhanced tensile strength.

2. A method according to claim 1 wherein the cross-linking is effected by irradiation of the polymer with up to 10M Rads.

3. A method according to claim 2 wherein the irradiation is effected with electron beam or y-radiation.

4. A method according to claim 1 wherein the polymer is chemically cross-linked.

5. A method according to claim 4 wherein an organic peroxide is used as chemical cross-linking agent.

6. A method according to any one of claims 1 to 5 wherein the polymer is a high density polyethylene.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. The chemical cross-linking agent may be compounded with the polymer for example a Banbury blender or a two roll mill. The cross-linking agent may then be exposed to those temperatures, and for those times, which permit the requireddegree of cross-linking. The resulting crepe will then consist of the cross-linked polymer which can next be fabricated. Cross-linking by irradiation is, however, generally to be preferred, particularly for those applications requiring minimum polymer contamination. Irradiation may be effected by electron beam or y - radiation (for example using a cobalt-60 source), desirably at a temperature which avoids significant fusion of the granules or powder. The amount of cross-linking introduced must neither be so small as to give an insignificant improvement in tensile strength nor so large as to prevent the fabriction. Cross-linking may rapidly reduce the melt flow index (MFI) of the polymer leading to incomplete mould filling (in injection moulding) or other fabrication failures. Accordingly, it is preferred that for successful operation of the method of the present invention the as-received polymer should have a high MFI thereby enabling higher cross-linking without fabrication difficulties arising. (Lower molecular weight polymers generally have a higher MFI but will be more prone to relaxation effects so that their quenching must be more extreme). Where cross-linking is effected by irradiation, it is generally found that doses of up to 50M Rads, typically up to 10M Rads and preferably from 0.5 to 2.0M Rads can be used. Fabriction conditions are those which introduce and preserve significant preferred orientation (which includes molecular, crystalline and filler orientation). In the case of injection moulding these are low injection speed, low melt temperature, low mould temperature and high packing pressure. In the case of fibre spinning these are low melt temperature, fast cooling and high take off speed. In particular the present invention provides a method of processing high density polyethylene which method comprises cross-linking high density polyethylene in particulate form by irradiation with 0.2 to 2.5M Rads and fabricating the so-crosslinked polymer by injection moulding or fibre spinning such that there is obtained a fabricated polymer of enhanced tensile strength. The following Example illustrates the invention. Example A homopolymeric HPDE (Rigidex 140/60 ex BP; Mw = 65,000; MW/Mn = 4.6; MFI as received 11.8) was irradiated at room temperature as granules with doses of 0.2, 0.5, 1.02, 2.0, 5.0 and 10.0M Rad (from a cobalt 60 source typically at 0.5M Rad/hour). The irradiated granules were then injection moulded using a wide range of barrel temperatures (150 to 250"C) with the mould temperature (40"C), injection pressure (41.1 MNm-2) and cycle times (fill times, 2.5 sec), hold-on pressure time (12.5 sec), cooling time (20 sec) and mould open time (5 sec) remaining constant, to produce a dumb-bell type tensile bar. (For granules irradiated to 5.0M Rad and above, incomplete mould filling occurred at all barrel temperatures while the 2.0M Rad irradiated sample was not mouldable at barrel temperatures of 1500C and 175"C). The as-moulded tensile bars were next tested in uniaxial tension at room temperature at a cross-head speed of 0.5 mm/min. Figure 1 of the accompanying drawings shows the marked reduction of MFI with increasing radiation dose, 2.5M Rad being identified as the maximum dose that this particular polymer could usefully sustain in the method of the present invention. Figure 2 of the accompanying drawings shows the increase in strength of the tested tensile bars at different barrel temperatures. It will be seen that the tensile strength of the bars increases markedly with increasing dose, for a given barrel temperature; and that, for a given dose, raising the barrel temperature tends to reduce the effect. WHAT I CLAIM IS:

1. A method of processing an amorphous or semi-crystalline thermoplastic polymer which method comprises cross-linking the polymer in particulate form and fabricating the so-crosslinked polymer by injection moulding or fibre spinning such that there is obtained a fabricated polymer of enhanced tensile strength.

7. A method of processing high density polyethylene which method comprises

cross-linking high density polyethylene in particulate form by irradiation with 0.2 to 2.5M Rads and fabricating the so-crosslinked polymer by injection moulding or fibre spinning such that there is obtained a fabricated polymer of enhanced tensile strength.

8. A method according to claim 7 substantially as described in the Example.

9. Polymer processed by the method claimed in any one of the preceding claims.