GB2033418A

GB2033418A - Stabilised Cadmium Pigments

Info

Publication number: GB2033418A
Application number: GB7932563A
Authority: GB
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 1978-09-21
Filing date: 1979-09-20
Publication date: 1980-05-21
Also published as: GB2033418B

Abstract

This invention relates to a method of reducing the amount of cadmium extracted from a plastics material containing cadmium pigment when said material is in contact with a foodstuff, beverage or like substance and it also relates to materials containing cadmium pigments, especially plastics containing cadmium pigments and articles made therefrom. In more detail a stabilised cadmium pigment according to the invention comprises a sulphide, selenide or telluride of cadmium and a second compound having a cation selected from the group comprising Fe, Mn, Cr, Co, Ni, Sn, V, Ti, Sr, Ca and Zn an anion selected from the group consisting of oxides, hydroxides, complex oxides, sulphides, selenides and tellurides.

Description

SPECIFICATION Cadmium Pigments with Transition Metal Stabilisers This invention relates to a method of reducing the amount of cadmium extracted from a plastics material containing cadmium pigment when said material is in contact with a foodstuff, beverage or like substance and it also relates to materials containing cadmium pigments, especially plastics containing cadmium pigments and articles made therefrom.

In this specification the term "cadmium pigment" is used to include cadmium sulphide or mixtures of cadmium sulphide, selenide and telluride which may conveniently be referred to as cadmium sulphoselenide. Cadmium sulphides possess pure yellow colours; cadmium sulphoselenides in various proportions possess colours ranging from orange through mid-red to deep maroon.

Cadmium pigments possess excellent hue and brightness, are not affected by the temperatures at which plastics materials are processed, show no tendency to bleed and offer considerable resistance to fading even when used in very low concentrations. They are therefore of particular value for the pigmentation of thermosetting and thermoplastic resinous and polymeric materials.

One object of the present invention is to reduce to very low levels the solubility of cadmium suiphide when used as a pigment in polymeric materials irrespective of the way in which the cadmium sulphide is combined with any other constituents in the pigment.

A further object of the present invention is to reduce the amount of cadmium extracted from polymeric materials containing cadmium pigments when said pigments are included in articles of manufacture such as toys or when said materials are in contact with foodstuffs, beverages or human body fluids.

According to one aspect of the present invention a stabilized cadmium pigment comprises a first compound comprising a sulphide, selenide or telluride of cadmium and a second compound having a cation selected from the groups comprising Fe, Mn, Cr, Co, Ni, Sn, V, Ti, Zr, Ca, and Zn and an anion selected from the group consisting of oxides, hydroxides, complex oxides, sulphides, selenides and tellurides. Preferably the second compound and the first compound form an intimate homogeneous mixture but in some cases the second compound may penetrate the lattice of the first compound.

We believe that V may be present as an oxy-complex.

The amount of the metal compound used may be up to 20% by weight of the cadmium pigment.

Preferably 0.01 to 5% by weight of cadmium pigment should be used.

Preferably the cations are chosen from Sn, Zn, Mn and Ca and the anions we prefer are oxides and sulphides.

The invention is concerned with thermoplastic plastics materials which are used very widely in the packaging of foodstuffs and beverages and in particular those used to make toys and similar articles of manufacture. Plastics material of particular interest are synthetic polymers e.g. polyethylene, polypropylene, polyvinyl chloride, polystyrene and styrene copolymers and interpolymers, for instance that known as ABS.

According to a second aspect of the present invention a composition of matter comprises a synthetic polymer selected from the group consisting of polyethylene, polypropylene, polyvinyl chloride, polystyrene and styrene copolymers and interpolymers containing from 0.01-20 weight % of a stabilized cadmium pigment according to the first aspect of the invention. More preferably the polymer contains from 0.1 to 5 weight % of stabilized cadmium pigment.

The stability of the cadmium pigments can be assessed by the two tests outlined below. The first test is applicable to food packaging. Polymer plaques with a surface area of 100 cm2, composed of ABS polymer with a 1% loading of pigment (wt/wt), were produced by injection moulding. The conditions of testing the plaques are extremely severe and exceed all conditions that would be likely to be encountered in practice. The plaques are suspended for ten days in a 3% solution of acetic acid in water at a temperature of 600C. This testing medium was selected because of its high strength compared to all commonly occurring foodstuffs and beverages and because of its simplicity.

After the ten days' exposure to the 3% acetic acid, the plaques are removed and the acetic acid analysed for cadmium content. The technique of atomic absorption spectrometry is used but, although this is itself a sensitive technique, a modification has been incorporated into it in order to detect the cadmium at the extremely low levels at which it is present.

According to the method of the modification, a measured amount of cadmium-contaminated acid solution is taken and the cadmium is pulse-electro-deposited from the solution onto an inert, precious metal probe, typically of iridium, which is then introduced into the hydrogen/oxygen flame of the atomic absorption spectrometer.

The second test is based on the United Kingdom Toys (Safety) Regulations 1 973. It simulates what shouid occur if a child chews and swallows part of a toy made of a polymer containing a cadmium pigment.

A quantity of pigmented polymer is comminuted and the resulting particles are collected in a clean container. The particles are filtered successively through two sieves, the first having an aperture of 0.5 mm and the second an aperture of 0.355 mm. Those particles retained by the sieve, having an aperture of 0.355 mm are collected for use as the sample.

An aqueous suspension is prepared which comprises the said sample with 50 times the mass of the said sample of 0.07 m hydrochloric acid. The said suspension is shaken for 1 minute after which the acidity of the suspension is tested and if the pH is more than 1.5, the suspension is acidified with an aqueous solution of 2 m hydrochloric acid added dropwise until the pH is 1.5 or less. The suspension is shaken continuously in the dark for one hour and then allowed to stand for 1 hour after which the suspension is filtered. The filtrate is analysed for cadmium.

The preparation of the cadmium pigments will now be described.

To prepare an aqueous suspension of freshly precipitated cadmium sulphide, a solution of cadmium sulphate, 17.76 g of 3 CdSO48 H2O in 100 cm3 water is made up and acidified with 0.5 cm3 conc. H2SO4. A sodium sulphide solution (Ca 1 6.7 g Na2S 9 H2O in Ca 100 cm3 H2O) is added until all the cadmium is precipitated.

Cadmium sulphide is used as an example of a pigment. In the methods of preparation of stabilised cadmium pigments outlined below the indicated use of cadmium sulphide does not preclude the use of cadmium selenide or cadmium telluride as alternatives where required for colour purposes.

Example 1 A freshly prepared aqueous suspension of cadmium sulphide is prepared as above. Zinc sulphate (0.44 g ZnS04 7 H2O) is added to the suspension and dissolved. To precipitate the zinc as the sulphide from the resulting solution, a second solution, an aqueous solution of sodium sulphide, is added to slight excess whilst stirring the solution containing the zinc ions. This is then filtered and the resulting powder retained by the filter is washed, dried and calcined.

The cation in the example above was zinc but it may be chosen from the following group of Mn, Fe, Co, Ni, Ca, Sn, V, Ti and Zr. The anion in the example above was sulphide but it may be chosen from the following group of oxides, hydroxides, selenides, tellurides and complex oxides.

Example 2 A freshly prepared aqueous suspension of cadmium sulphide is prepared as above. A second solution, comprising an aqueous solution of sodium thiostannate prepared by stirring 0.19 g SnCI2 2 H2O and 0.7 g Na2S 9 H2O in 100 cm3 of water and allowing the solution to stand until it clears, is added to the first solution slowly with stirring. The resulting solution is filtered and the powder retained by the filter is washed dried and calcined.

The cation in the example above was tin but it may be chosen from the following group Fe, Ni, V and Zn. The anion in the example above was sulphide but it may be chosen from the following group of oxides, hydroxides, selenides, tellurides and complex oxides.

Example 3 An aqueous suspension of cadmium sulphide, 10 g in 200 cm3 of water, that has been calcined is prepared, and ferrous sulphide which has been ground and sieved through a 200 mesh sieve, 0.157 g, is added to the aqueous suspension. Using a high shear mixer the suspension is mixed thoroughly. The suspension is filtered and the powder retained by the filter is washed and dried.

The cation in the example above was iron but it may be chosen from the group: Ca, Co, Mn, Ni, Sn, V, Ti, Zr and Zn. The anion in the example above was sulphide but it may be chosen from the following group of selenide, telluride, oxide, hydroxide and complex oxide.

Example 4 An aqueous suspension of cadmium sulphide, 10 g in 200 cm3 of water, that has been calcined is prepared. The suspension is acidified with approximately 0.5 cm3 concentrated sulphuric acid. Zinc sulphate, 0.44 g ZnSO, 7 H2O, is added to the aqueous suspension and allowed to dissolve. An aqueous solution of sodium sulphide, containing approximately 0.40 g of Na2S . 9 H2O, is slowly added to slight excess to the aqueous suspension while stirring the suspension. This is then filtered and the powder retained by the filter is washed and dried.

The cation in the example above was zinc but it may be chosen from the group La, Co, Mn, Fe, Ni, Sn, V, Ti and Zr. The anion in the example above was sulphide but it may be chosen from the group selenide, oxide and hydroxide.

Example 5 An aqueous suspension of cadmium sulphide, 10 g in 200 cm3 of water, that has been calcined is prepared. The suspension is acidified with approximately 0.5 cm3 concentrated sulphuric acid. A solution of sodium thiostannate is prepared by stirring 0.19 g SnCI2 2 H2O and approximately 0.7 g Na2S 9 H2O in 100 cm3 of water and allowing it to stand till it clears. This solution of sodium thiostannate is slowly added, with stirring to the suspension of cadmium sulphide. The suspension is filtered and the powder retained by the filter is washed and dried.

The cation in the example above was tin but it may be chosen from the group V, Zn, Ni, and Fe.

The anion in the example above was sulphide but it may be chosen from the following group of oxides, hydroxides, selenides, tellurides and complex oxides.

The examples above describe a number of methods of preparing cadmium pigments but are not an exhaustive list. Other methods by which the cadmium pigment is prepared may be used.

In the above description two tests for measuring the amount of cadmium that may be extracted from a plastic containing a cadmium pigment are outlined. Results from these tests show how the amount of cadmium that can be extracted from a plastic containing a cadmium pigment is reduced when a stabilised cadmium pigment as disclosed in this specification is used.

Results from the first test.

Polymer plaques, comprising ABS polymer with a 1% loading of pigment in polymer (wt/wt), were produced by injection moulding.

Amount of cadmium extracted Sample in ng. cm-3 calcined cadmium sulphide -- untreated 1 9.4 calcined cadmium sulphide+SnS2 added after calcination 4.1 calcined cadmium sulphide+SnS2 added before calcination 8.4 calcined cadmium sulphide -- untreated 1 7 calcined cadmium sulphide+SnO2 added before calcination 12.8 Results from second test.

Polymer plaques, comprising ABS polymer with a 1% loading of pigment in polymer (wt/wt) were produced by injection moulding.

Amount of cadmium extracted Sample ppm calcined cadmium sulphide - untreated 85 calcined cadmium sulphide with 1% (wt/wt) SnS2 added after calcining 45 calcined cadmium sulphide with 1% (wt/wt)SnO2addedafter calcining 32 calcined cadmium sulphide with 1% (wt/wt) SnO2 added before calcining 35

Claims

1. A stabilised cadmium pigment comprising a first compound comprising a sulphide, selenide or telluride of cadmium and a second compound having a cation selected from the group comprising Fe, Mn, Cr, Co, Ni, Sn, V, Ti, Zr, Ca, and Zn and an anion selected from the group consisting of oxides, hydroxides, complex oxides, sulphides, selenides and tellurides.

2. A stabilised cadmium pigment according to Claim 1, wherein the first and second compounds form an intimate homogeneous mixture.

3. A stabilised cadmium pigment according to Claim 1, wherein the second composition penetrates the lattice structure of the first compound.

4. A stabilised cadmium pigment according to any preceding claim, wherein the second compound is present in an amount up to 20 weight percent of the pigment.

5. A stabilised cadmium pigment according to Claim 4, wherein the second compound is present in an amount between 0.01 and 5 weight percent of the pigment.

6. A composition of matter comprising a synthetic polymer selected from the group consisting of polyethylene, polypropylene, polyvinyl chloride, polystyrene and styrene copolymers and interpolymers containing from 0.01-20 weight percent of a stabilised cadmium pigment according to any one of Claims 1 to 5.