GB2510930A - Free flowing stannous chloride in thermoplastic carrier - Google Patents

Abstract

Stannous chloride particles are entrained in a thermoplastic polymer wherein the stannous chloride particles are a stannous chloride particle core coated with a layer of stannous oxide. The composition is readily prepared with high levels of stannous chloride in the composition, thought to be due to improved wetting of the particle by the polymer. The composition provides low acidity in contact with water and more even distribution of the stannous chloride in the polymer. The composition may be useful in the preparation of natural and synthetic rubbers, particularly when used in a coextrusion process.

Description

Free flowing stannous chloride in thermoplastic carrier The present invention relates to an improved composition comprising primarily stannous chloride in a thermoplastic polymeric carrier, the composition being more easily prepared and less prone to oxidation.

Stannous chloride, the chlorine salt of tin in the 2+ oxidation state, also known as Tin(ll) chloride is widely used in industry, for example in the production of synthetic rubber. When used for this purpose the stannous chloride is conveniently delivered in the form of a pellet (2-3mm diameter) of thermoplastic polymer in which the stannous chloride is entrained when the polymer is in a fluid state. This enables the stannous chloride, used at a relatively low level in rubber processing, to be more easily handled both because of the increased weight and volume, per unit activity, of the pellet compared to pure stannous chloride but also because the polymer can protect the stannous chloride from atmospheric water and oxygen which can cause the stannous chloride to decompose: SnCI2 + H20 -* Sn(OH)Cl + HCI -(1) 6 SnCI2 + 02 + 2 H20 2 SnCI4 + 4 Sn(OH)Cl -(2) SnCI2+2H20+%02 H2SnO3 +2HCl -(3) The above being examples of representative reactions. As can be seen those reactions give rise to acidic species, for example hydrochloric acid, and as such improved handling of this reactive reductant is facilitated by such entrainment, which may be thought of as being a form of encapsulation, in thermoplastic polymer. Naturally, such compositions are used and transported in the solid-state for subsequent melting and application in a fluid state.

Hence, in subsequent processing of the pellets the thermoplastic polymer melts, due to heating, and the entrained low stannous chloride can be released so as to perform its reactive function.

Whilst the above is known in the art, such as disclosed in GB 2455981, GB 2489123, and EP 10785565 such products are not optimal. For the disclosures of relevance to the present invention can be found in JP 2003 001098, JP 62 025749, JP 62 013457 and WO 2007/073479.

Two significant issues are present related to stannous chloride entrained in thermoplastic polymer. First, stannous chloride, particularly in its dihydrate form, but, also in its anhydrous form, provides a relatively hydrophilic surface (i.e. surface which has a contact angle of less than 90° with water). Many thermoplastic polymers are substantially hydrophobic (i.e. having a surface which has a contact angle of greater than 90° with water). There is therefore an inherent issue with what might be thought of as wetting' of the stannous chloride, so as to efficiently entrain it in the polymer, when introduced into the molten thermoplastic polymer as pad of preparation of the pellets. This gives rise to an uneven distribution of the stannous chloride, preferential location of the stannous chloride at or near the surface of the polymer and difficulties during extrusion processing of molten thermoplastic comprising the stannous chloride in which production of a regular size particle, such as in the production of a masterbatch, is problematic.

When such pellets, being the solid form of the polymer at the cooling down to ambient temperature, as described above are used in the form of a masterbatch in polymer processing then production downtime from clogging and poor particle flow result due to the irregular nature of the pellets produced.

It is one aim of the present invention to provide a method of producing stannous chloride entrained in thermoplastic polymer wherein distribution of the stannous chloride is more even and in which particle size and shape is also more even.

A further issue with stannous chloride entrained in thermoplastic polymer, in the form of palates, is that the pellets can be classified as irritant' due to stannous chloride being present on the pellets surface. This material is readily dissolved out in the presence of water. This leads to a minor amount of decomposition of the stannous chloride so present, but also leads to the production of hydrochloric acid (as illustrated above) which can lead to an irritant' classification of what would otherwise be a material having neutral characteristics associated with the parent thermoplastic polymer.

It is a further aim of the present invention to provide a form of stannous chloride entrained in thermoplastic polymer which has reduced decomposition and reduced, or absent, likelihood of being labelled as irritant'.

The present invention provides: A method of providing a composition of stannous chloride entrained in a thermoplastic polymer; the method comprising the steps of: providing a particulate form of stannous chloride comprising a stannous chloride particle core coated with a layer comprising stannous oxide providing a thermoplastic polymer admixing said stannous chloride and thermoplastic polymer at a temperature above the glass transition temperature of the thermoplastic polymer.

The composition is then normally cooled to form a solid pellet for further transportation and subsequent use.

The method is more easily carried out using the stannous chloride in the coated form. In young coated form a maximum level of incorporation of stannous chloride, such as to form a homogeneous composition (as opposed to the composition segregating and leading to mix properly) can be as high as 70% by weight, in contrast to use of uncoated stannous chloride work, a level of around 50% is the most that can be achieved. Efficient mixing is also understood to take place using lower physical energy input.

The present invention also provides stannous chloride entrained in a thermoplastic polymer wherein the stannous chloride a particulate form of stannous chloride comprising a stannous chloride particle core coated with a layer comprising stannous oxide.

In the present invention the particulate form of stannous chloride particle core having the layer comprising stannous oxide my also, in the layer, comprise one or more of comprising stannous hydoxy chloride, stannic oxide, metastannic acid and metastannate of stannous oxide.

A composition according to the present invention, when compared to an equivalent composition in which the stannous chloride is not coated, the thermoplastic being polyethylene, the level of coating being 0.1% by weight and the level of incorporation of the stannous chloride being 40%, as a particle size average of 250pm, provides lower acidity (moles of available acid) when placing 5 g of each composition in 100 ml of water. This indicates improved entrainment of the coated stannous chloride in the polymer and a lower likelihood of the composition being considered suitable for irritant' labelling.

Said particular form of stannous chloride can be produced by the controlled oxidation of the stannous chloride were in limited atmospheric oxygen is made available such that partial oxidation of the stannous chloride arises so as to form stannous oxide in preference to stannic oxide.

Surprisingly, whilst analysis has indicated that the invention involves a coating of stannous oxide the coating has a light yellow colouration, when present at a level of 0.1% by weight or less rather than the black colour disclosed in the literature for this oxide or, of the metastable red form also disclosed in the literature. This provides a relatively neutral colouration such as to avoid discolouration of any product in which the material is subsequently used, such as a light-coloured rubber, and is one reason why the person skilled in the art would not normally contemplate stannous oxide for such use.

The main reaction involved would therefore appear to be 6 SnCl2 + 02 + 2 H20 (I) 2 SnO + 2 HCI -(4) The term particulate form means solid particles of size from lOOnm to 1cm. for routine transportation and use solid particles of size from 1pm to 2mm average particle size as determined using electron microscopy such as using a EEl Quanta 200 FEG scanning electron microscope (SEM) using the Projected area diameter, Pa, particle size measure by all means provide your own method but it must come the full range and give a defined particle size measure down to the lower size. The most preferred particle size range is 10pm to 200pm as in this range the attrition caused by pouring of the particles is sufficiently low as to not significantly impact the integrity of surface treatment provided by representative use of the present invention.

Stannous oxide has the chemical formula SnO.

Stannic oxide has the chemical formula Sn02.

Stannous hydroxy chloride has the formula Sn(OH)Cl.

Metastannic acid has the formula H2Sn5Oii(s).

Metastannate of stannous oxide has the formula SnO.Sn5010.

The layer comprises stannous oxide and optionally one or more of stannous hydroxy chloride, metastannic acid, metastannate of stannous oxide, stannic oxide.

The composition of the layer, the coating layer, may be determined by X-ray photoelectron spectroscopy.

The layer preferably comprises stannous oxide and optionally stannous hydroxy chloride.

The coating layer comprises stannous oxide and the layer comprises greater than 30% stannous oxide, preferably more than 50% stannous oxide and more preferably greater than 75% stannous oxide.

Where a mixture of stannous oxide and stannous hydroxy chloride is present the layer comprises greater than 50% of the mixture, or preferably greater than 70% of the mixture, more preferably greater than 90% of the mixture.

Whilst not wishing to be bound by theory it would appear that a coating substantially comprising stannous oxide is sufficient to stop the aggregation of particles, leading to caking, even if some stannous chloride may be present on the particle surface since into particle adhesion and diffusion of water will be sufficiently reduced to hinder the caking action. This would also be appear to be the case regarding stannous hydroxy chloride.

As such it is being found that compositions according to the present invention are less likely to be classified as irritant'. Such classification being carried out by standard methods including the motion of a material in water and determination of pH of that water.

The coating preferably comprises stannous oxide as this is more chemically inert than the stannous hydroxy chloride and is insoluble in water, this reducing water penetration into the core of the particles.

Whilst the coating preferably comprises stannous oxide it more preferably comprises stannous oxide in combination with stannous hydroxy chloride. This is because in the absence of complete conversion of the surface to stannous oxide, or surface abrasion reducing the stannous oxide coating the surface exposed will then comprise some stannous hydroxy chloride, this does not have the low melting point of the stannous chloride dihydrate and as such does not contribute towards caking (caused by the adhesion of adjacent particles).

The term coating refers to the presence of a layer of material covering another material in the form of a relatively thin layer. A conventional definition being that a coating is a thin layer or covering of something, in this case stannous chloride.

The coating need not be homogeneous but is preferably so, the coating need not be complete but is preferably so.

In the present invention the coating is stannous oxide which coats the stannous chloride so that any given particle has a core comprising stannous chloride with an external coating of stannous oxide forming a barrier between the stannous chloride and the surroundings of the particle.

The additional species mentioned previously may also be present. Specifically, the coating may also comprise stannic oxide. The coating may also comprise metastannic acid and/or Metastannate of stannous oxide.

The stannous chloride core of the particles of the present invention preferably comprises stannous chloride at a level of greater than 90% by weight, more preferably greater than 95% by weight, still more preferably greater than 99% by weight most preferably greater than 99.5% by weight of the particles. High levels of stannous chloride are preferred as the ability of stannous oxide to effectively coat other substrates cannot be guaranteed. The composition of the stannous chloride core is preferably homogeneous as this reduces the possibility of occlusions of other materials which may interfere with the coating process.

Further, the present invention is directed to a means of providing a more easily transportable and usable form of pure (within industrial standards which allow defined levels of contamination) stannous chloride and as such low levels of stannous chloride are undesirable as this material represents a non-functional material in further use.

Additional materials which may comprise the stannous chloride core include stannous and stannic salts, indium, lead and silver, transition metals and common anions such as other halogens besides chlorine and oxygen.

The coating layer comprises less than 2% by weight of the particles, more preferably less than 1% by weight, yet more preferably less than 0.5% by weight, most preferably less than 0.05% by weight. It is been surprisingly found that very low coating levels are effective in providing reduced caking and hence improve flow, which indicates that the benefits are a surface effect, i.e. the properties of the surface layer of the particles is modified and modification of the bulk is not required.

The coating layer is preferably of thickness between 1 Onm and 100pm, preferably between lOOnm and 10pm. This level of thickness enables agitation of the particles in water to give rise to normal dissolution of the stannous chloride whilst obtaining the free-flowing characteristics which persist over time when exposed to the atmosphere. The thickness of the coating is preferably determined indirectly by first assuming that the coating is substantially homogeneous, evidenced by the free-flowing characteristics (breaks in the coating would remove this) and a determination of the level of stannous oxide in the particles as a whole in comparison with uncoated particles. Alternative methods include the scanning of broken particles using electron microscopy, suitable equipment being cited as previously.

The presence of the coating layer of stannous oxide gives the appearance of a light yellow colouration to otherwise substantially white stannous chloride particles. The presence of an even colouration is indicative of an even coating.

Particles described as stannous chloride particles are particles which comprise at least 90% stannous chloride, or preferably at least 95% stannous chloride, yet more preferably at least 99% stannous chloride and most preferably greater than 99.5% stannous chloride Stannous chloride particles for use in the present invention consist of tin salts in combination with minor impurities of up to 1%, more preferably only up to 0.5%, most preferably only up to 0.1% by weight. The minor impurities are preferably inorganic salts and more preferably soluble inorganic salts. The impurities may be elemental carbon, indium and silicon. Since the coating comprises oxygen than oxygen is not, as such, an impurity of the particles. However, the level of oxygen (as 02) is preferably 2% or less, more preferably 1% or less, most preferably 0.5% or less by weight. The composition of the particles of the present invention may also comprise a small amount of tin in the tin IV oxidation state. This term is understood to be associated with oxygen.

The provision of coated stannous chloride particles for use in the present invention can take place using a method of preparing a particulate form of stannous chloride in which particles comprising a stannous chloride comprising core coated with a layer comprising stannous oxide as the coating species the method comprising the steps: providing stannous chloride in particulate form heating the stannous chloride to between 100°C and 200°C exposing the stannous chloride to a limited amount of atmospheric air over a period of 1 to 6 hours to an extent such that the stannous chloride is reacted to form the aforementioned coating species to the extent of forming less than 2% by weight of the particles.

Controlling the extent to which the stannous chloride is reacted to provide the desired reaction extent is a simple matter of trial and error analysis for any given piece of equipment. Unhindered access of atmospheric air during the reaction does not give rise to the coated particles of the present invention but merely decomposes the stannous chloride to a mixture of species, primarily stannic oxide.

The method is preferably cariied out wherein the atmospheric air is of between 1 and 60% relative humidity.

The stannous chloride of particulate form has the composition as defined above for the core of the particles.

The atmosphere to which the stannous chloride is exposed is ambient air with humidity in the range of 0 to 50% at the stated temperature. The ambient air preferably has humidity in the range of Ito 10% as the presence of some water is understood to facilitate the reaction to form the coated particles.

The stannous chloride is preferably heated to between 100°C and 12000.

The stannous chloride is preferably agitated during the heating process using a stirrer. The particular form of stirrer is not considered to be significant provided that it does not serve to significantly reduce particle size. A significant reduction is a reduction were average particle size measurements fall outside the 90% confidence interval for the equivalent unagitated sample.

A first example method of the preparation of the particular form of coated stannous chloride according to the present invention is as follows.

Glass lined vessel with steam jacket and zirconium agitator, as supplied by Pfaudler and having a capacity of 250 litres, termed an Oyster is provided. The vessel comprises a jacket capable attached to a source of steam at 3 bar (temperature 133°C)so as to provide heating of the contents of the vessel.

Into this vessel particulate anhydrous stannous chloride having particle sizes in the range bum to lUOum as measured using based upon a representative sample. The material is however also passed through a 0.5 mm screen to remove any agglomerates that may be instantly present. 45kg of the stannous chloride was provided in the vessel. The stannous chloride is derived from a process having the same parameters as disclosed role for producing the reference sample of uncoated stannous chloride.

The vessel is then heated to 100-120°C are maintained at that temperature for 6 hours with exposure to the atmosphere. Heating is continued until the colour of the stannous chloride has a b* value > 4 as measured by a DR Lange Colour Meter.

Cooling water was then applied to the jacket to reduce the temperature to <70°C and the product was packed into 25 kg pails with plastic liners after screening through a 0.5 mm screen. Two representative batches were produced by this route yielding 425 kg per batch (113764/20 and 113764/21).

A second example method of the preparation of the particulate stannous chloride of the present invention, this time starting from an aqueous solution is as follows: Stannous chloride solution (600kg) was charged to the oyster and dried under vacuum by application of steam at 4 bar to the jacket with agitation for 6 hours until stannous chloride anhydrous was produced. The vacuum was then removed and the product dried for a further 5-6 hours without vacuum at 100-120°C.

Cooling water was applied to the jacket to reduce the temperature to <70°C and the product was packed into 25 kg pails with polyethylene plastic liners which are sealed by tying in a knot (termed an overhand or figure of eight knot) after screening through a 0.5 mm screen. Two batches were produced by this route yielding 425 kg per batch (113764/20 and 113764/21).

The stannous chloride anhydrous produced was sent to Wells plastic for processing into a masterbatch.

A 25kg pail (15 Litre Curtech Click Pack Pail) of each batch was retained at William Blythe and the level of caking assessed every month.

A reference sample of uncoated stannous chloride was produced for comparative purposes following a very similar procedure: Stannous chloride (% by weight solution) solution (600kg) was charged to the oyster and dried under vacuum (Vacuum applied of 90 mmHg)by application of steam at 4 bar to the jacket with agitation for 6 hours at which time stannous chloride anhydrous was produced. The vacuum was then released and extraction applied to the oyster and cooling water was applied to the jacket to reduce the temperature of the product to <70°C. The product was packed into 25 kg pails with plastic liners after screening through a 0.5 mm screen. One batch was produced by this route yielding 425 kg (113764/19).

Results Analysis data of the samples is as follows

Table 1A

% Colour Colour Batch %SnCI2 %Sn LOD (L) (b*) 113764/19 99.6 62.33 0.01 96.26 1.68 113764/20 99.7 62.42 0.01 95.21 7.18 113764/21 99.5 62.29 0.02 95.43 6.02

Table 2B

ppm ppm ppm ppm ppm ppm ppm ppm Batch Fe Pb Sb Cu Na K Mg Ca 113764/19 16 8 1 0 11 0 1 6 113764/20 14 8 1 0 9 0 0 5 113764/21 14 8 1 0 9 0 0 5 The assay of the free flowing product is identical to the standard product at >99.5%. This shows that only a very small amount of the Sn2 has been oxidised.

The analytical data indicates that the free flowing stannous chloride has been oxidised but only very slightly. The XPS data shows that the oxidation has mainly occurred on the surface of the stannous chloride. It is postulated that this change in the surface chemistry of the stannous chloride has made it less hydroscopic and as a result prevented caking and made the product free flowing.

The present invention relates to a composition comprising tin (II) chloride, entrained in or (at least partially) encapsulating by a thermoplastic polymer.

The tin (II) chloride has the formula SnCI2.2H20, or simply SnCI2 for the anhydrous form. The stannous chloride (tin (II) chloride) used in the invention is preferably anhydrous tinOl) chloride.

For the purposes of the present invention the tin chloride is entrained in thermoplastic polymer such that the thermoplastic polymer predominantly and preferably completely surrounds the tin chloride particles, to the extent to which the particles are completely surrounded then there may be considered as encapsulated. The term entrained is used since the key process is the then entraining, mixing in of, stannous chloride particles in fluid thermoplastic.

However, for the purposes of describing the resulting pellets or other solid physical form a stannous chloride particle physically retained in thermoplastic polymer is further the purposes of the present application considered to be entrained.

In the present invention the term thermoplastic polymer means a polymer that becomes reversibly plastic upon heating. The term thermoplastic polymer, whilst preferably being a single polymer, either copolymer or homopolymer, may comprise one or more polymers. A single polymer, more preferably homopolymer is preferred. This is because homogeneity of the thermoplastic polymer enhances lO homogeneity and hence even distribution of the stannous chloride in the composition of the present invention.

The polymer is a water insoluble polymer. Preferred water insoluble polymers are ethyl vinyl acetate (EVA) polyethylene and polypropylene as they are particularly hydrophobic. The composition of the present invention may consist of polypropylene and tin (II) chloride having the coating mentioned hearing. The polypropylene may comprise crystalline polypropylene.

In the present invention encapsulation means that material, the encapsulated material, here tin(ll) chloride is surrounded by an encapsulate, here a thermoplastic polymer to form a capsule. An encapsulated material may typically comprise small amounts of material (<10%, preferably <1%) which are not encapsulated (entrained) and incidental to the encapsulated material, this material may be considered as entrained, if not encapsulated.

The composition of the present invention, may have tin(ll) chloride present in an amount from 1 to 60 by weight %, and the thermoplastic polymer is present in an amount from 99.9 to 40% by weight. Preferably, the tin(ll) chloride is present in an amount from 10 to 50 by weight %, and the thermoplastic polymer present in an amount from 90 to 50% by weight. Most preferably, the tin(ll) chloride is present in an amount from 15 to 35 byweight%, and the thermoplastic polymer is present in an amount from 85 to 65 % by weight. Most preferably, the tin(ll) chloride is present in an amount of 20 % by weight, and the thermoplastic polymer, preferably polypropylene, is present in an amount of 80 % by weight. Not only can the above compositions provide a free flowing, non-hygroscopic and non-toxic form of tin (II) chloride but the bulking up of the material by the encapsulate enables more accurate dosing of material as a greater weight is added to provide a given activity of tin salt.

In another aspect of the present invention there is provided a process for producing the composition of stannous chloride entrained in a thermoplastic polymer.

In the present invention, particulate tin (II) chloride is preferably in the form of crystals having no occluded or entrained air or water. The particulate tin(ll) chloride is anhydrous tin(ll)chloride. The particle size of the particulate tin (II) chloride is preferably between 10 and 2,000pm, more preferably between 50 and 500 pm. If a flake is used then a size range of 2mm to 10mm is preferred as it is less prone to agglomeration in charging the extruder and on storage and has lower hygroscopicity.

Particle size was measured using a Mastersizer 2000 with a Scirocco 200DM dry dispersion unit. The analysis was carried out at 0.5 bar air pressure and a feed rate of 50-60%.

II

The properties defined for the tin (II) chloride and the thermoplastic polymer and of the composition as provided herein apply to the composition of the invention, to the method of production and to uses of the present invention.

The composition of the invention, such as produced by the process of the present invention, may be used, for example, in the production of an elastomeric material for the purposes of crosslinking a polymer mixture comprising natural rubber by means of extrusion the polymer mixture with the composition at a temperature above the melting point of the encapsulating thermoplastic polymer. The thermoplastic polymer is preferably polypropylene. The polymer mixture preferably comprises linked natural rubber and polypropylene. The composition of the invention may be mixed with kaolin and chopped rubber in a premix hoper before co-extrusion with polypropylene.

A masterbatch is a concentrated mixture of pigments and/or additives, encapsulated during a heat process into a carrier resin which is then cooled and cut into a granular shape. Masterbatch allows the processor to colour raw polymer or add components economically during the plastics manufacturing process. The present invention encompasses masterbatch comprising polymer and one or more of the inorganic salts disclosed herein as hereinbefore disclosed.

Claims (10)

1. Claims, 1. A method of providing a composition of stannous chloride entrained in a hydrophobic thermoplastic polymer; the method comprising the steps of: providing a particulate form of stannous chloride comprising a stannous chloride particle core coated with a layer comprising stannous oxide providing a hydrophobic thermoplastic polymer admixing said stannous chloride and thermoplastic polymer at a temperature above the glass transition temperature of the thermoplastic polymer.
2. 2. The method of claim 1 wherein the water insoluble thermoplastic polymei is polypropylene or polyethylene.
3. 3. The method of claim 1 or claim 2 wherein tin (II) chloride is present in an amount from 1 to 60 by weight %, and the polymer present in an amount from 99 to 40% by weight.
4. 4. The composition of any of claims 1, 2 or 3 wherein the particle size of the particulate tin (II) chloride is between 10 and 2,000pm.
5. 5. The method of any preceding claim, wherein the process of admixing comprises: (i) providing an extruder, the temperature within said extruder (a) increasing along the length of said extruder, and (b) being sufficient, in combination with the pressure exerted within, to melt the thermoplastic polymer; (ii) feeding a mixture of the particulate tin(ll) chloride and the thermoplastic polymer into said extruder; (Di) feeding the mixture of particulate tin(ll) chloride and at least one thermoplastic polymer along the length of said extruder such that said thermoplastic polymer melts, and said tin(ll) chloride becomes dispersed within said at least one thermoplastic polymer; (iv) extruding the molten at least one thermoplastic polymer / tin(ll) chloride mixture through a die, wherein cooling of said extruded mixture results in setting of said thermoplastic polymer, encapsulating said tin(ll) chloride; and (v) chopping the extruded thermoplastic polymer encapsulated tin(ll) chloride to form pellets.
6. 6. The method of any preceding claim wherein the admixture comprising the entrained stannous chloride is cooled to ambient temperature using water so as to provide a solid pellet.
7. 7. A method of any preceding claim wherein the stannous chloride is anhydrous stannous chloride
8. 8. The composition of claim 1 wherein the particle size of the particulate tin (II) chloride is between 10pm and 500pm.
9. 9. Stannous chloride entrained in a thermoplastic polymer wherein the stannous chloride is a particulate form of stannous chloride comprising a stannous chloride particle core coated with a layer comprising stannous oxide.
10. 10. Use of the composition of any of claims 1 to 8, or the composition of claim 8, in the production of an elastomeric material for the purposes of crosslinking a polymer mixture comprising natural or synthetic rubber by means of extrusion of said composition with the polymer mixture at a temperature above the melting point of the composition.AMENDMENTS TO THE CLAIMS HAVE BEEN FILED AS FOLLOWS: Claims, 1. A method of providing a composition of stannous chloride entrained in a hydrophobic thermoplastic polymer; the method comprising the steps of: providing a particulate form of stannous chloride comprising a stannous chloride particle core coated with a layer comprising stannous oxide providing a hydrophobic thermoplastic polymer admixing said stannous chloride and thermoplastic polymer at a temperature above the glass transition temperature of the thermoplastic polymer.2. The method of claim 1 wherein the water insoluble thermoplastic polymer is polypropylene or polyethylene.3. The method of claim 1 or claim 2 wherein tin (II) chloride is present in an amount from 1 to 60 by weight %, and the polymer present in an amount from 99 to 40% by weight.4. The method of any of claims 1, 2 or 3 wherein the particle size of the particulate tin (II) chloride is between 10 and 2,000pm.5. The method of any preceding claim, wherein the process of admixing comprises: (i) providing an extruder, the temperature within said extruder (a) increasing along the length of said extruder, and (b) being sufficient, in combination with the pressure exerted within, to melt the thermoplastic polymer; O (H) feeding a mixture of the particulate tin(ll) chloride and the thermoplastic polymer into said extruder; (iii) feeding the mixture of particulate tin(ll) chloride and at least one thermoplastic polymer along the length of said extruder such that said thermoplastic polymer melts, and said tin(ll) chloride becomes dispersed within said at least one thermoplastic polymer; (iv) extruding the molten at least one thermoplastic polymer! tin(ll) chloride mixture through a die, wherein cooling of said extruded mixture results in setting of said thermoplastic polymer, encapsulating said tin(ll) chloride; and (v) chopping the extruded thermoplastic polymer encapsulated tinOl) chloride to form pellets.6. The method of any preceding claim wherein the admixture comprising the entrained stannous chloride is cooled to ambient temperature using water so as to provide a solid pellet.7. A method of any preceding claim wherein the stannous chloride is anhydrous stannous chloride 8. The method of claim 1 wherein the particle size of the particulate tin (II) chloride is between 5Opm and 500pm.9. Stannous chloride entrained in a thermoplastic polymer wherein the stannous chloride is a particulate form of stannous chloride comprising a stannous chloride particle core coated with a layer comprising stannous oxide.10. Use of the composition of any of claims 1 to 8, or the composition of claim 8, in the production of an elastomeric material for the purposes of crosslinking a polymer mixture comprising natural or synthetic rubber by means of extrusion of said composition with the polymer mixture at a temperature above the melting point of the composition. (4 r