GB2065142A - PTFE for paste extrusion - Google Patents

Abstract

Pipes with walls thicker than 4 mm are made by paste extrusion at a reduction ratio below 20:1 of PTFE powder which is a mixture of 80 to 99% by weight of coagulated dispersion grade PTFE and 20 to 1% by weight of PTFE having a median particle size less than 50 micrometers in the presence of a lubricant. The mixture is preferably a co-coagulation of the two forms of PTFE.

Description

SPECIFICATION PTFE for past extrusion This invention relates to polytetrafluoroethylene compositions suitable for extrusion into tubes.

PTFE tubes are particularly useful for conveying corrosive liquids because PTFE is resistant to most chemicals. It is clearly essential that the tubes are free from leaks and, in addition, it is important that the tubes have good physical properties, e.g. good tensile strength and smooth surfaces. The tubes are commonly made by a process known as "paste extrusion".

Paste extrusion is carried out using a mixture of PTFE powder and a volatile hydrocarbon lubricant.

(The amount of lubricant is usually between 14 and 25% by weight of the total mix.) A preform formed from the mixture is placed in the cylinder of a hydraulic extruder and forced through a cone and die (for a tube an annular die is used). The ratio (cross sectional area of the cylinder: (cross sectional area of the die) is known as the "reduction ratio", and the value of this ratio is of great significance in determining the physical properties of the extrudate.

Thin wall tubes (e.g. 3 mm or less) are extruded at reduction ratios in the range of about 30:1 to 250:1 (or higher) and PTFE powders suitable for use at reduction ratios within this range are readily availab!e. However, for thicker wall tubes limitations in the extruder may make it necessary to use reduction ratios below 20:1 and PTFE powders often give very bad extrudates at such low reduction ratios, particularly when the wall thickness of the tube exceeds about 4 mm. In many cases the tube has cracks or holes so that it is completely lacking utility. Even when there are no holes the pipe has low tensile strength and a rough surface so that the pipe is not of a technically acceptable standard.

According to this invention shaped articles are made by paste extrusion of PTFE powder in the presence of a lubricant, characterised in that the powder is a mixture containing 80 to 99% by weight of coagulated dispersion PTFE and 20 to 1% by weight of PTFE having a median particle size (optical determination) less than 50 micrometers, preferably less than 10 micrometers and the extrusion is carried out at a reduction ratio below 20:1, e.g. below 1 5:1. The method is particularly suitable for making thick-wall pipes, e.g. pipes with walls thicker than 4 mm. The products of the method are part of the invention.

The method described above is based on the surprising discovery that mixtures oi 80 to 99% coagulated dispersion PTFE and 20 to 1% of fine particle PTFE give smooth extrudates even at reduction ratios as low as 20:1. We have made the further surprising discovery that mixtures prepared by co-agulation have certain advantages over mixtures prepared by other methods, e.g. dry mixing.

Therefore, this invention also includes paste extrusion powders made by coagulating 80% to 99% (dry weight) of an unstable PTFE dispersion in the presence of 20% to 1% of a fine particle PTFE powder. The median particle size (optical determination) of the fine particle PTFE is less than 50 micrometers, and preferably less than 10 micrometers.

Coagulated dispersion PTFE is a well known product widely used for paste extrusion of tubes and other products. It is produced by polymerising tetrafluoroethylene in contact with an aqueous phase containing initiator and dispersant. As polymer is produced the dispersant keeps it in suspension so that the product of the reaction is a colloidal dispersion of PTFE in the aqueous phase. This dispersion will coagulate on vigorous stirring or treatment with chemicals but it is sufficiently stable to avoid premature coagulation.

The fine particle component is preferably a low molecular weight PTFE, because this form of PTFE readily comminutes to particle size below 50 micrometers. If necessary the commtiution can be enhanced by milling. Low molecular weight PTFE can be made directly by polymerisation or, preferably, by degrading high molecular weight PTFE. The degradation can be carried out by heating or irradiating.

PTFE polymer which has received a dose of 2 to 50 Mrads of gamma-irradiation is particularly suitable.

Four compositions according to the invention will now be described by way of example.

Three ingredients were used to make the compositions. These were: (1) First grade of fine particle PTFE, hereinafter P 1; (2) Second grade of fine particle PTFE, hereinafter P2; (3) Aqueous dispersion which contained 325 g of PTFE per 1 litre of dispersion.

P 1 is a PTFE powder sold by Imperial Chemical Industries Limited as a solid lubricant under the code 'Fluon' L171. ('Fluon' is a registered trade mark.) P2 is also sold by Imperial Chemical Industries Limited as a solid lubricant powder, but under the code 'Fluon' L169. Both powders have median particle size (optical determination) of about 3 to 5 micrometers.

The aqueous dispersion was obtained directly by polymerising tetrafluoroethylene in the presence of an aqueous phase containing a dispersant. The dispersion is that coagulated by ICI to obtain a grade of PTFE, sold as 'Fluon' CD123, which is used to make good quality tubes at reduction ratios above 30:1. CD123 does not give satisfactory thick-wall tubes at reduction ratios below 20:1 (from conventional extruders).

The four compositions to be exemplified were all prepared by substantially the same method using in all cases the method comprising (a) the aqueous dispersion, diluted with water to 110 g PTFE/litre, was placed in the coagulation vessel; (b) in a separate vessel, the fine particle PTFE powder (P 1 or P2) was added to a portion of ethyl alcohol and the mixture was stirred to produce a suspension. The volume of alcohol was about 5 to 10% of the volume of the aqueous dispersion plus diluent water used in (a). The amount of fine particle PTFE powder was calculated from the amount of PTFE in the aqueous dispersion to give the concentration required in the final product, (c) the alcoholic suspension was transferred to the coagulation vessel and added to the aqueous dispersion.The contents were stirred gently to ensure mixing, (d) the mixture was stirred vigorously (400 rpm) to cause co-coagulation. Coagulation occurred after about 7 minutes and stirring was continued for 10 minutes afterwards.

(e) the coagulate was separated from the liquid phase, washed and dried for 8 hours at 2000C.

The dry powders are products of the invention. They were mixed with a lubricant and used for extrusion, in accordance with the method of the invention, as exemplified below.

The amounts used in the preparations were as shown in Table 1.

TABLE 1

Mix Code N1 M2 M3 M4 Volume Water (iitre) 10B 10.8 10.7 107 Volume Dispersion (litre) 36 3.5 3B 3.6 Total Aqueous Phase (litre) 14.3 14.3 14.3 14.3 Fine particle powder Type P1 P1 P2 P2 Mass (g) 150 75 150 75 Volume Alcohol (litre) 1 1 1 1 % Wt dispersion PTFE 90 95 90 95 % Wt fine particle PTFE 10 5 10 5 The four mixes M1, M2, M3 and M4 were mixed with 18% (by weight of the total mix) of a naphtha lubricant used for paste extrusion. The mixes were extruded into tube at reduction ratio 12:1 from an extrusion cylinder of diameter 114 mm fitted with a mandrel of diameter 38 mm, a die of diameter 57 mm and a core pin of diameter 48 mm (giving tubes with wall thicknesses of 4.5 mm). The tubes, which had a good appearance, were smooth and free from cracks 'Fluon' CD123 (coagulated from the same batch of aqueous dispersion used to prepare M1, M2, M3 and M4 was dry mixed with P1 and P2 in the same proportions as used in mixes M1 to M4. These dry mixes were extruded into tubes using the method described above for M1 to M4. The extrusions provide a further exemplification of the method of the invention.

Tubes from the dry mixes were similar in appearance to those from the co-coagulated mixes but, when the tubes were viewed in a glass-sided oven at 3800C, their melts were not transparent indicating voiding. By contrast the melts of tubes prepared from M1 to M4 were substantially transparent. This observation indicates that the co-coagulated mixes are better than the dry mixes and this was confirmed by tensile strength and elongation measurement on the sintered tubes.

Comparative tests were carried out using 'Fluon' Cud 123 coagulated from the same batch of aqueous dispersion used to prepare the mixtures specified in Table 1.

The tubes obtained from CDl 23 were very rough, cracked and showed signs of delamination.

They were so clearly of unacceptable quality that no tests were carried out.

Claims (1)

1. CLAIM

   1. A method of making a shaped article by paste extrusion of PTFE powder in the presence of a lubricant, characterised in that the powder is a mixture containing 80 to 99% by weight of coagulated dispersion PTFE and 20 to 1% by weight of PTFE having a median particle size less than 50 micrometers and the extrusion is carried out at a reduction ratio below 20:1.