GB2230255A

GB2230255A - Zinc stannate and hydroxystannate

Info

Publication number: GB2230255A
Application number: GB8904031A
Authority: GB
Inventors: Paul Andrew Cusack; Jeremy Arthur Pearce
Original assignee: DEAN RONALD RICHARD
Current assignee: DEAN RONALD RICHARD
Priority date: 1989-02-22
Filing date: 1989-02-22
Publication date: 1990-10-17
Also published as: WO1990009962A1; GB8904031D0

Abstract

Zinc hydroxystannate, suitable for use as a fire-retardant additive in plastics material, is obtained by simultaneously adding to a body of water, (i) an aqueous solution of a water-soluble hydroxystannate and (ii) an aqueous solution of a water-soluble zinc salt. The body of water and/or the starting solution(s) may contain a surfactant. The resultant finely-divided hydroxystannate may be heated to convert it to anhydrous zinc stannate which latter may also be used as fire-retardant additive for polymeric materials.

Description

Stannates and hvdroxvstannates This invention is concerned with improvements in and relating to stannates and, more particularly, is concerned with the preparation of zinc hydroxystannate [ZnSn(OH)63 and zinc stannate (ZnSnO3).

Zinc hydroxystannate may conveniently be prepared in particulate form by a double decomposition reaction between a water-soluble hydroxystannate (generally an alkali metal hydroxystannate such as sodium hydroxystannate) and a water-soluble zinc salt (such as zinc nitrate or zinc chlocide) in an aqueous medium, i.e. by adding an aqueous solution of the soluble hydroxystannate to an aqueous solution of the soluble zinc salt or vice versa.

The resultant precipitated zinc hydroxystannate typically has a particle size of 0.5 to 5 microns, with an average of about 2 microns. The precipiLåted zinc hydroxystannate may be converted to the anhydrous stannate by simple heating, e.g. at temperatures of 200 to 4500C. It has recently been found that the zinc hydroxystarinate and zinc stannate, in particulate form, are useful as fire-retardant additives for polymeric materials, typically in amounts of l to 5 % by weight.

It has now been found, in accordance with the present invention, that a more finely divided zinc-hydroxystannate, having generally improved fire-retardant properties when incorporated in polymeric materials, may be produced by double decomposition by adding, to a body of water, (i) an aqueous solution of a water-soluble hydroxystannate and (ii) an aqueous solution of a water-soluble zinc salt.

According to one embodiment of the invention, therefore, there is provided a process for the production of zinc hydroxystannate which comprises simultaneously and separately adding, to a body of water, (i) an aqueous solution of a water-soluble hydroxystannate and (ii) an aqueous solution of a water-soluble zinc salt.

The water soluble hydroxystannate used as starting material is most conveniently an alkali meal hydroxystannate, especially sodium hydroxystannate. The water-soluble zinc salt may conveniently be, for example, zinc nitrate or zinc chloride although, of course, other water-soluble zinc salts may be used.

The concentration of water-soluble hydroxystannate in the starting solution thereof is suitably from 25 Lo 300 g/Q, preferably about 135 g/Q. The concentration of zinc chloride is suitably from 12.5 to 550 g/Q, preferably about 70 g/Q. The relative volumes of the two solutions and their concentrations should, as will be appreciated, be such as to give substantially stoichiometric amounts of the two reactants to avoid wastage and contamination.

The process of the invention may be carried out as a batch process, i.e. one i which a predetermined volume of each of the two starting solutions is added to a predetermined volume of water. Alternatively, the process may be carried out as a continuous process, for example one in which the two starting solutions are continuously fed, each at a predeermined rate, to a body of water, generally one to which water is also fed at a predetermined rate and from which excess water and precipitated zinc hydroxystannate are removed at a rate to compensate for the rate of introduction of starting solutions and water. In any case, the ratio of the total volume of the starting solutions to the body of water is suitably from 20:1 to 1:1, preferably about 4:1.

The body of water to which the starting solutions are added is preferably stirred or otherwise agitated. In general gentle agitation is preferred since it gives rise to a product of smaller primary particle size.

Similarly, a lower rate of addition of the starting solution generally gives rise to a product of smaller primary particle size.

However, it has also been found, in accordance with a further feature of the invention, that conditions which encourage aggregation of the primary particles, e.g. an increase in agitation rate, can produce a material which has the advantage that when incorporated in transparent or translucent polymeric materials, it does not mackedly affect transparency. This can also be achieved by spray drying a suspension of preformed zinc hydroxys tanna te.

It has been found, in accordance with a further feature of the invention, that a material having improved fire-retardant properties may be obtained by incorporating a suffacLant or dispersant in the body of water and/or in the starting solution(s). Particularly preferred surfactants are polymers of ethylenically unsaturated carboxylic acids and their salts, especially polyacrylic acid and its sodium salt, e.g. the dispersants sold under the trade name "Dispex". The total amount of surfactant used, calculated as a weight percentage based on the dry yield of suitably from 0.1 to 5%, preferably 0.5 to 19. Such dispersants may also be used in the preparation of finely divided zinc hydroxystannate by grinding or mixing preformed zic hydroxystannate in the presence of the surfactant.

In the process of the invention, the precipitated hydroxystannate may be recovered from the body of water by any convenient liquid/solid separation technique, such as, for example, filUration, centrifugation or decantation. The resulting product may be dried and, if desired, further heated (e.g. to a temperature of 2000CLo 450 OC) in order to convert it to the anhydrous zinc stannate.

The zinc hydroxystannate or zinc stannate produced in accordance with the invention is finely divided and thus typically has a primary particle size of less than 0.2 microms.

The hydroxystannate or stannate obtained in accordance with the invention finds particular use as a fire-retardant additive for polymeric materials. Thus, zinc hydroxystannate and zinc stannate have been found to give good flame- and smoke-retardant performances and do not themselves give rise to undesirable by-products under the action of heat. Thus, the zinc hydroxystannate or zinc stagnate may act synergistically when combined with halogenated materials in retarding flame propagation and suppressing formation of smoke and toxic gases. They are particularly effective in halogenated polyester resins and in rigid and plasticized PVC.Thus, for example, in tests using brominated polyesters, amounts of the order of about 2% of the hydroxystannate or stannate, produce slightly better flame retardancy than a conventional antimony trioxide additive under the same conditions and approximately twice the reduction in smoke and carbon monoxide evolution. The tin compounds not only suppress the total amount of smoke and carbon monoxide but also retard their rate of evolution. Further, zinc hydroxystannate and zinc stannate also exhibit flame and smoke-retardant properties in halogen-free plastics or elastomeric formulations, particularly where the polymer contains a relatively high level of inorganic filler such as alumina trihydrate.

The materials of the invention are suitably incorporated in polymeric materials in amounts of 1 to 5%, preferably about 2 % by weight; those obtained in the presence of a surfactant being used aL the lower end of this range.

In order that the invention may be well understood tsle following examples are given by way of illustration only.

Example 1 1 LiLre of 0.5 M Na2Sn(OH)6 solution (67.5 g/Q) and 1 litre of 0.5 M ZnC12 solution (62.5 g/Q) were added dropwise, simultaneously, to 500 ml of stirred water. A white precipitate was formed.

The resultant mixture was separated by centrifuging at 1500 rpm for 5 minutes and the supernatent liquor was decanted off. The precipitate was washed in about 4 litres of distilled water three Limes, separating each Lime by centrifuging at 3000 rpm for 30 minutes and decanting. It was then dried at llO"C in an oven overnight. The dried cake was crushed in a morcar and pestle to give 130.99 g (92% yield) of a fine white powder (UF ZHS).

ExamPle 2 1 Litre of 1M Na2Sn(OH)6 solution (135 g/Q) and 1 litre of 1M ZnCl2 solution (125 g/Q) were added dropwise, simultaneously, to 500 mls of stirred water, to which has been added 14.3 mls of a 109 by weight Dispex N40 solution. This amount of surfactant was that calcuulated to be 0.5S by weight on expected dry yield of ZnSn(OH)6. A white precipitate was formed.

The resultant mixture was separated by centrifuging at 1500 rpm for 5 minutes ad the supernatant liquor decanted off. The precipitate was washed in about 4 litres of distilled water three times, separating each Lime by centrifuging at 3000 rpm for 5 minutes and decanting. It was then dried at 1100C in an oven overnight. The cake was crushed in a mortar and pestle to give 270.3 g (95% yield) of a fine white powder (UF ZHS*).

ExamPle 3 A quantity of a powder prepared as in Example 1 was heated aL 4000C for 4 hrs. A 19% weight loss was observed and anhydrous ZnSnO3 was produced, which was also a fine white powder. (UF ZS*).

Test ResulLs The powder samples were incorporated into a commercial 28% brominated polyester resin (Stypol R126/6 supplied by Freeman Chemicals, Ellesmere Port), using a Silverson shear mixer. Flammability measurements (LOI) are shown below.

Limiting Oxygen Index (LOI) 1% loading 2% loading ZHS (commercial grade) 45.5 53.5 UF ZHS 50.5 53.1 UF ZHS* 54.0 59.5 ZS (commercial grade) - 54.6 UF ZS* - 64.7

Claims

CLAIMS: 1. A process for the production of zinc hydroxystannate which comprises simultaneously and separately adding, to a body of water, (i) an aqueous solution of a water-soluble hydroxystannate and (ii) an aqueous solution of a water-soluble zinc salt.
2. A process as claimed in claim 1 in which the body of water and/or the solution of water-soluble hydroxystannate and/or water-soluble zinc sale also contain a surfactant.
3. A process as claimed in claim 2 in which the surfactant is polyacrylic acid or a salL thereof.
4. A process as claimed in any one of the preceding claims in which the zinc hydroxystannate obtained is subsequently heated to convert it to zinc stannate.
5. A process as claimed in claim 1 substantially as hereinbefore with reference to the Examples.
6. A polymeric material containing, as a fire-reLardant additive, zinc hydroxystannate or zinc stannate when obtained by a process as claimed in any one of the preceding claims.