GB2215336A

GB2215336A - Particulate metal filled plastics composition

Info

Publication number: GB2215336A
Application number: GB8804626A
Authority: GB
Inventors: Peter Challenger
Original assignee: Individual
Current assignee: Individual
Priority date: 1988-02-27
Filing date: 1988-02-27
Publication date: 1989-09-20
Also published as: GB8804626D0

Abstract

A plastics composition comprises, 40 to 95 per cent by weight of iron powder or iron alloy powder combined with a minor proportion of a dissimilar metal powder being chosen from those metals which will form ions preferentially to iron when exposed to an aqueous electrolyte whilst in contact with iron, and 60 to 5 per cent by weight of an organic plastics material or polymer. The preferred dissimilar metal is zinc. The composition shows improved resistance to corrosion in contact with water compared to a similar composition not containing the dissimilar metal, and is suitable for producing shaped articles such as fishing weights.

Description

PARTICULATE METAL FILLED PLASTICS COMPOSITION This invention relates to a particulate metal filled plastics composition.

Plastics materials have gained wide acceptance because of their ability to be shaped accurately and reproducibly, for instance by injection moulding or extrusion. Typical plastics materials such as polypropylene and nylon are characterised by low density, flexibility, low thermal and electrical conductivity and by being non-magnetic. There are however occasions on which the desirable features of plastics materials, i.e. ability to be shaped, are required combined with properties such as high density which are not normal. The physical properties of a plastic material or polymer can be altered without losing its ability to be shaped, by combining it with a filler. A well-known example is the use of glass fibre filler to increase stiffness.

Plastics materials or polymers may be combined with particulate metal fillers to impart properties such as high density, thermal and electrical conductivity, or magnetic susceptibility which are not normally associated with plastics materials. One such example is in materials to produce moulded weights for fishing tackle, where high density is required. The lowest cost particulate metal available for this purpose is iron powder. However iron is susceptible to corrosion when in contact with water and air by development of hydrated ferric oxide or rust. Moulded plastics items made from iron powder filled plastics materials do not rust readily as the iron is protected by encapsulation in the plastics material. However, if iron particles are exposed, rust can develop.This can occur at the injection point or gate of a moulding, on areas where excess material or flash is trimmed or as a result of abrasion in use. Iron powder filled plastics are therefore unsuitable for applications such as fishing weights where frequent contact with water is to be expected.

This problem of corrosion or rust development can be overcome by using a particulate metal such as copper powder which is not susceptible to corrosion in contact with water. However such materials are significantly more expensive than iron powder.

The present invention accordingly provides a plastics composition of improved resistance to corrosion in contact with water comprising: i) 40 to 95 per cent by weight of a particulate metal filler containing iron powder or iron alloy powder combined with a minor proportion of a dissimilar metal powder being chosen from those metals which will form ions preferentially to iron when exposed to an aqueous electrolyte whilst in contact with iron.

ii) 60 to 5 per cent by weight of an organic plastics material or polymer.

Dissimilar metal powders which may be combined with iron powder in this way include magnesium, aluminium, and zinc but the preferred material is zinc powder.

Organic plastics materials or polymers which may be used in this composition include all those plastics materials normally used in combination with particulate fillers but preferred materials are thermoplastic polymers such as polyolefines and polyamides.

It will be obvious to those knowledgeable of plastics materials that the plastics material component of this composition may incorporate additives such as plasticisers, lubricants, stabilisers, pigments, fibrous reinforcements and non-metallic fillers as well as the polymer or polymerisable component.

The particulate metal filler may be combined with the plastics material by physically mixing the metal filler with the particulate plastics material but it is preferred than an homogenous composition is produced by combining the particulate metal filler with the plastics material in the molten state by using a machine such as an internal mixer, twin screw, or single screw extruder. The techniques for producing such an homogenous composition are well known.

The metal filled plastics composition thus produced may be shaped by any of the well known techniques such as injection moulding, compression moulding, or extrusion, to form the required shaped article.

It is well known that iron articles may be protected from corrosion by coating with zinc, a process known as galvanising. It may be assumed therefore that iron powder may be similarly protected by coating each iron particle with zinc. A surprising feature of this invention is that an adequate degree of protection occurs merely by blending a minor proportion of zinc or similar metal powder with the iron powder in the composition. Sufficient electrical contacts are developed to ensure galvanic protection of the iron particles.

An example of the effectiveness of this invention is shown by the results of the following experiment in which a series of plastics compositions was produced by compounding the following formulations in an internal mixer.

Formulation No: 1 2 3 4 5 6 Parts by weight: Polypropylene polymer 98 98 98 98 98 98 Zinc Stearate Lubricant 2 2 2 2 2 2 Iron Powder 790 782 773 748 705 622 Zinc Powder 0 7.8 15.5 39 78 155 Approx. Ratio Iron:Zinc 100:0 99:1 98:2 95:5 90:10 80:20 All these formulations contain approximately 48 per cent by volume of metal powder.

Moulded test pieces 10mm by 5mm by 3mm were produced by compression moulding. All surfaces were thoroughly abraded with coarse emery paper to expose metal powder at the surface. The test pieces were then immersed separately in tap water and 10 per cent aqueous sodium chloride solution in test tubes open to the air, and observed for the development of visible rust. The results were as follows: Formulation No: 1 2 3 4 5 6 Days to develop visible More than surface rust, in water 4 5 7 20 20 20 Days to develop visible More than surface rust, in 10% NaCl 1 1 2 4 5 20 The observations were discontinued after 20 days.

Claims

1 A plastics composition comprising: i) 40 to 95 per cent by weight of a particulate metal filler containing iron powder or iron alloy powder combined with a minor proportion of a dissimilar metal powder being chosen from those metals which will form ions preferentially to iron when exposed to an aqueous electrolyte whilst in contact with iron.

ii) 60 to 5 per cent by weight of an organic plastics material or polymer.

2 A plastics composition as claimed in Claim 1 wherein the dissimilar metal is zinc.

3 A plastics composition as claimed in Claim 1 or Claim 2 wherein the ratio of dissimilar metal to iron or iron alloy lies between 1:99 and 20:80.

4 A plastics composition as claimed in any preceding claim wherein the organic plastics material or polymer is a thermoplastic polymer.

5 A plastics composition as claimed in Claim 4 wherein the thermoplastic polymer is a polyolefine.

6 A plastics composition as claimed in Claim 5 wherein the thermoplastic polymer is polypropylene.

7 A plastics composition as claimed in Claim 4 wherein the thermoplastic polymer is a polyamide.

8 A shaped article produced from a plastics composition according to any of the preceeding claims.