WO1993019916A1

WO1993019916A1 - Processing polymer compositions

Info

Publication number: WO1993019916A1
Application number: PCT/GB1993/000667
Authority: WO
Inventors: Stefan Verne
Original assignee: Bicc Public Limited Company
Priority date: 1992-04-03
Filing date: 1993-03-31
Publication date: 1993-10-14
Also published as: GB9207326D0; AU3896393A

Abstract

Filled polymer compositions, especially low-smoke-and-fume cable insulation compounds, are subjected to reduction in cross-section by shear flow in the solid state (plastic flow) in order to improve dispersion of the filler. The reduction ratio is at least 10:1 and the shear flow is maintained for at least 0.3 sec. Preferably the softening point is not exceeded, but if it is the exposure time is limited to one minute; no substantial excursion above the decomposition temperature of any ingredient is allowed. ''Conform'' extrusion is the preferred way of achieving the required flow.

Description

Processinq Polymer Compositions This invention relates to the processing of polymer ■t compositions containing fillers, by which is meant particulate ingeredients that are solid at processing i temperatures. It relates to compositions which are

5 thermoplastic, but which may be intended for subsequent crosslinking.

The properties of such filled polymer compositions may be critically dependent on the state of dispersion of the solid particles, and many desirable fillers are difficult 10 to disperse uniformly because their affinity for the polymer base is relatively low and a great deal of shearing may be needed to break down agglomerations of particles in direct particle-to-particle contact; indeed mixing at low shear rates may tend to form aggregates of some fillers. 15 The point is well illustrated by cable-making compositions based on non-polar polymers filled with "conductive" grades of carbon black, in which the frequency of particle-to-particle contacts can be so high that a composition containing not much more than 10% of the filler 20 is electrically conducting. In some cases (as when a conductive screen composition is required) this may be beneficial; more often the effect is deleterious, as for example when fillers with a high affinity for water (such as alumina trihydrate and magnesium hydroxide used to make 25 flame-retardant compositions that produce relatively small amounts of smoke and of hazardous fumes under fire conditions) tend to agglomerate in chains facilitating penetration of moisture into the composition and degrading its performance when exposed to water. Conventional ways >_** 3 of improving dispersion, such as the use of "high speed" mixers, are difficult to apply to this type of composition, because the time/temperature requirements of the mixing process risk breakdown of the filler with evolution of water that may result in unwanted porosity and/or interfere 35 with a crosslinking process.

In accordance with the present invention, a method of processing a polymer composition that contains filler particles in an incompletely dispersed state to increase the degree of dispersion thereof comprises subjecting the composition to shear flow in the solid state for a period of at least 0.3sec by passing through a restricted passage in which its cross-sectional area is reduced by a ratio of at least 10ϊl while limiting exposure of the material to temperatures in or above the softening range to a maximum of one minute and avoiding any substantial exposure to temperatures above the temperature at which decomposition of any ingredient occurs at a material rate.

The restricted passage may in some cases be the die orifice in which a useful extruded product is formed; in other cases it may be an auxiliary orifice upstream of the die orifice, or the process of the invention may be a preliminary (or "off-line") process forming a raw material for subsequent fabrication by an independent conventional extrusion process or by some other fabrication technique, such as injection-moulding or blow-moulding.

Conventional screw extrusion is unsuited operation in the solid state; ram extrusion could be used in some instances, but we much prefer to use the "Conform" process for continuous friction-actuated extrusion, which is well- established as a process for extrusion of aluminium and copper and has been proposed for certain very specialised plastics-extrusion applications (for example the extrusion of polyethylenes of extremely high molecular weight that are formed as powders and cannot be processed by ordinary screw extruders) . In the Conform process, a working chamber is formed between a grooved rotatable wheel and a stationary member (the "shoe") that projects into the groove for part of the circumference (usually less than a quarter) . The surface area of the wheel-groove exposed in the working chamber exceeds that of the shoe, and this results in a nett force tending to drive material in the working chamber in the direction of the rotation of the wheel. Such motion is restricted by a member (the "abutment") which at least partially closes the downstream end of the working chamber, and this results in the generation of very high pressures immediately upstream of the abutment. An orifice is normally formed either in the shoe a short distance from the abutment ("radial" extrusion) or in the abutment itself ("tangential extrusion"). For processing of metals, radial extrusion has usually been preferred as this makes it easier to provide the necessary high restraining force on the abutment, but for processes in accordance with the invention tangential Conform extrusion is preferred as giving more appropriate flow conditions. If radial extrusion is used the flow length (from wheel groove to die) should preferably be minimised. The grip length, i.e. the length of wheel groove completely filled with the polymer composition, may need to be greater than for the extrusion of metals, and this may require a Conform machine with a circumferentially longer shoe than has been usual hitherto. By use of the Conform process, extrusion ratios in excess of 100 and exposure times above the softening range less than 10 seconds can be easily achieved. The composition prior to processing in accordance with the invention will usually be a solid pre-dispersion of the filler in the polymer in rod, granule or other form, but in some cases a mixture of polymer powder and filler powder might be fed directly to Conform extrusion. Example

A mixture of 5 parts of an extrusion-grade ethylene/ vinyl acetate copolymer (70%VA) containing conventional stabilisers and 6 parts of a high-purity paper-making grade

2 of alumina trihydrate of BET surface area about 11m /g was premixed in a conventional extruder-mixer to give a composition whose electrical breakdown strength rapidly declined on exposure to hot water. The mixture was formed into granules of nominal particle size 5mm and extruded with a Conform extrusion machine with a working chamber nominally 9mm square and a tangentially arranged circular extrusion orifice 0.5mm in diameter (reduction ratio 400), without preheating, at a rate of about 5 litres per hour to give a product with significantly enhanced electrical performance after exposure to hot water. The residence time of the material in the transition zone between the wheel groove and the die, where the bulk of the work is applied, was estimated as 0.4 second, and the whole process took place below the softening temperature of the polymer.

Claims

1 A method of processing a polymer composition that contains filler particles in an incompletely dispersed state to increase the degree of dispersion thereof comprising subjecting the composition to shear flow in the solid state for a period of at least 0.3sec by passing through a restricted passage in which its cross-sectional area is reduced by a ratio of at least 10:1 while limiting exposure of the material to temperatures in or above the softening range to a maximum of one minute and avoiding any substantial exposure to temperatures above the temperature at which decomposition of any ingredient occurs at a material rate.

2 A process as claimed in claim 1 in which the orifice is a die orifice in which a useful extruded product is formed.

3 A process as claimed in claim 1 being a preliminary process forming a raw material for subsequent fabrication.

4 A process as claimed in any one of claims 1-3 in which the composition is extruded by Conform extrusion. 5 A process as claimed in any one of claims 1-3 in which the composition is extruded by tangential Conform extrusion.

6 A method of processing a polymer composition that contains filler particles in an incompletely dispersed state to increase the degree of dispersion thereof substantially as described by way of example.