Patent Description:
Synthetic rubbers are elastomers which can be made using a water-based process in which monomers are polymerised via free-radical emulsion polymerisation in the presence of surfactants. The resulting emulsions comprise an aqueous dispersion of polymer particles. These particles may be coagulated, separated from the water and then dried to form solid rubber or maintained for use in its emulsion form. Natural and synthetic elastomers based on monomers containing unsaturation are inherently susceptible to oxidative degradation and it is standard practice to add one or more antioxidants to them.

The ageing of organic polymers or vulcanizates thereof can lead to a change in various properties such as increased hardness or brittleness of the polymers. Alternatively, softening, loss of elastomeric properties or of mechanical strength may occur. Cracking, surface changes or other changes, such as the electric properties, may be observed. Undesirable odours and discolouration phenomena also often occur.

In order to prevent or reduce said ageing processes it is known to add antioxidant compounds. These can be grouped into four main categories:.

The effect of anti ageing compounds typically slows down when the polymer or vulcanizate to be protected is exposed to higher temperatures, in particular for longer time periods. It is desirable that the anti ageing compounds do not have a colouring effect, but provide a good colour stability to the rubber or vulcanizate thereof and may be used in combination with peroxide or sulfur based vulcanizing agents. Some of the known anti ageing compounds are toxic. Besides the diphenyl amines this applies to phenolic antioxidants like Vulkanox® BKF which is categorized as H361f, and is suspected of damaging fertility.

As there is an increasing need for high ageing stability of rubbers and vulcanizates thereof with regard to storage and colour stability, in particular on exposure to high temperatures, it is a continuing task to provide new and improved concepts for preventing and reducing the ageing processes in rubbers and vulcanizates. This includes reducing the necessary amount of anti-ageing compounds to the minimum without lowering the stabilizing effect.

Examples of rubbers which can be stabilized according to the present invention are:.

The storage and colour stability of rubbers such as nitrile rubbers, styrene butadiene rubbers ("SBR") or other types are frequently problematic. For the present purposes, storage-stable means that the Mooney viscosity changes as little as possible during prolonged storage times, in particular at high temperatures; and, colour stable means that the rubber shows values ΔE as small as possible as determined according to CIEDE <NUM> after storage at high temperatures.

For elastomers which are manufactured using a water-based process, it is highly desirable to add an aqueous dispersion of the antioxidant to the rubber emulsion. This is to ensure the most effective and efficient incorporation of antioxidant into the final polymer. Currently used antioxidant systems comprise single phenolic antioxidants or a combination of a phenolic antioxidant with a thioester antioxidant. A commonly used phenolic antioxidant is a butylated reaction product of p-cresols and dicyclopentadiene (<NPL>) commonly known as Antioxidant L (available commercially under the trade names Wingstay L, Lowinox CPL and Ionol LC) which has been used as a dispersion either alone or in combination with a thioester such as dilaurylthiodipropionate (<NPL>). Alternatively <NUM>,<NUM>-bis(octylthiomethyl)-o-cresol (<NPL>) commonly known as Antioxidant <NUM> has been used in combination with a phosphite antioxidant, such as trisnonylphenylphosphite (<NPL>), commonly known as TNPP. Another commonly used antioxidant is <NUM>,<NUM>'-methylenebis(<NUM>-tert-butyl-<NUM>-methyl-phenol) (<NPL>); commonly known as Antioxidant <NUM>, which has health and safety concerns relating to health of the unborn child (Risk phrase R61).

There is an ongoing need to improve the protection of such polymers against oxidative degradation because this has a deleterious effect on aesthetic and mechanical properties. There is also a need to respond to health and safety concerns relating to some of the currently available antioxidant technologies.

<CIT> discloses a stabilising mixture of two sterically hindered phenolic esters which remain liquid after having been heated together in order to be of use for stabilisation of solid rubbers. The use of such a preparation is inherently inefficient in the stabilisation of aqueous polymer dispersions as the antioxidant mixture is not fully compatible with an aqueous polymer.

<CIT> describes stabilizer compositions comprising <NUM>) sulfide having one or more sulfide groups -CH<NUM>-S-CH<NUM>-R wherein R is C<NUM>-C<NUM> alkyl or alkyl alkanoate or <NUM>,<NUM>-bis(n-octylthiol)-<NUM>-<NUM>'-hydroxy-<NUM>',<NUM>'-di-tert. butylanilino)<NUM>,<NUM>,<NUM>-triazin and at least two hindered phenols (<NUM>) and (<NUM>) one of which (<NUM>) is less sterically hindered than the other (<NUM>). Such compositions can be incorporated into polymers to make polymer additives. These polymer additives can provide polymeric products having improved physical and mechanical properties. They are used in high concentrations of <NUM> to <NUM> % by weight of the polymer and the focus lies on stabilizing acrylate-based rubbers. The use thereof shows some synergistic effect, however, there is neither any showing of an improved stability of the rubber's molecular weight nor any disclosure or teaching of how to reduce the amount of the stabilizing system.

<CIT> discloses a combination of (i) a phenolic antioxidant, (ii) a thiodipropionic acid ester and (iii) a phosphite. Alkyl substituted phenyl phosphites such as TNPP (tri-nonylphenylphosphite) are emphasized. Nowadays these phosphites, are considered harmful in view of their toxic by-products, in particular nonylphenol. <CIT> does not disclose whether the stabilizers are suited to increase the colour stability of the polymers.

<CIT> discloses the use of a combination of.

<CIT>describes aqueous emulsions which comprise at least (A) one phenolic antioxidant, and/or (B) one thio dipropopionic acid ester and/or (C) an organic phosphite, a surfactant being a salt of an organic acid, and an alcohol.

<CIT> discloses the stabilization of a) methylmethacrylate-butadiene polymers styrene graft polymers using b) a sterically hindered phenolic antioxidant of formula (I), (II) or (III) or a mixture thereof
<CHM>
with n being from <NUM> to <NUM>
<CHM>
and c) a thioether differing from that of formula (II).

<CIT> discloses salts of sterically hindered phenols which are used for stabilizing rubbers. These stabilizers are characterized by comprising at least two phenolic hydroxy groups. Preferred sterically hindered phenolic compounds are those of the following formula
<CHM>
wherein R<NUM>, R<NUM>, and R<NUM> may identical or different, R<NUM>, R<NUM> are C<NUM>-C<NUM> alkyl or C<NUM>-C<NUM> cycloalkyl and R<NUM> is hydrogen, C<NUM>-C<NUM> alkyl or C<NUM>-C<NUM> cycloalkyl. However, there is no disclosure regarding improvement of the stabilizing efficiency by using specific stabilizers in combination.

<CIT> discloses liquid stabilizing mixtures for organic polymers comprising:.

The stabilising mixture is said to be liquid and is prepared by heating the components together. An inherent problem associated with this mixture is an insufficient compatibility with aqueous polymer dispersions. It is disclosed in <CIT> that such a mixture may be used in combination with further stabilizers, and many classes of compounds are listed. There is no disclosure of any specific combination of stabilizers which might improve the stabilizing effect on rubbers synergistically.

<CIT> discloses low-dust granules of plastics additives, comprising a) a phenolic antioxidant, an organic phosphite or phosphonite, a phosphonate, a sterically hindered amine or a UV absorber, individually, or a mixture of these compounds, and b) at least one epoxy compound which is solid at room temperature. The granules are particularly suitable for stabilizing polymers, especially polyolefins such as polypropylene or polyethylene.

In summary, it can be said that no non-toxic stabilizing system has been described which provides an improved stability for rubbers with regard to Mooney viscosity stability.

It is therefore an object of the present invention to provide a stabiliser which gives unsaturated rubbers good storage stability with regard to Mooney viscosity and colour stability, which does not include toxic or environmentally hazardous compounds and at the same time affords good vulcanization profile and advantageous mechanical properties.

According to a first aspect of the present invention, there is provided a stabiliser according to claim <NUM>. In preferred formulations the phosphite antioxidant, where present, is selected from the group consisting of:.

The present invention provides aqueous dispersions of antioxidants and combinations of antioxidants which may improve aesthetic and mechanical properties of polymers compared to those available using currently available technologies.

When compound (d) is present the amount is <NUM>-<NUM>% of the total mass of antioxidant, more preferably <NUM>-<NUM>% of the total mass of antioxidant.

In a preferred composition the ratio by weight of (b):(c) is in the range of about <NUM>:<NUM> to about <NUM>:<NUM>, more preferably about <NUM>:<NUM> to about <NUM>:<NUM>, especially about <NUM>:<NUM>.

In preferred embodiments a minimum amount of component (c) is employed.

Compositions in accordance with the second and third aspects of this invention have the advantage that stabilised rubbers are whiter in colour and better preserved.

According to a fourth aspect of the present invention a method of preparation of a stabiliser according to claim <NUM> is provided.

A particularly preferred compound from group (b) is octadecyl-<NUM>-(<NUM>,<NUM>-di-tert-butyl-<NUM>-hydroxyphenyl) propionate, commonly referred to as Antioxidant <NUM>. This compound has <NPL>.

A particularly preferred compound is <NUM>-methyl-<NUM>,<NUM>-bis [octylthiomethyl]-phenol, commonly referred to as Antioxidant <NUM>. This compound has<NPL>.

In advantageous embodiments of the invention the antioxidants are a mixture of Antioxidants L, <NUM> and <NUM>.

In a further embodiment, the antioxidant mixture can also be composed of any two of these antioxidants and optionally a phosphite of Formula IV.

The phosphite antioxidant (d) preferably comprises one or more phosphites selected from the group consisting of:.

The composition may further comprise one or more additional antioxidants or synergists. Preferred additional antioxidants or synergists are selected from the group consisting of: tocopherols, phenolics, thioesters, phosphonates, triazines, amines, benzyl compounds, and ascorbic acid. Other additives, for example light stabilisers, catalysts and optical brighteners, may also be admixed with these antioxidants and synergists.

In normal industrial practice it is desirable to use aqueous dispersions of the above compositions in order to maximise compatibility and efficiency of incorporation. A dispersion may be defined as a heterogeneous mixture of organic material suspended in water. In order to provide these, further ingredients may be required and comprise one or more of: water, surfactants, thickening agents, preservatives and adjuvants such as antifoams.

Surfactants may be anionic, non-ionic or cationic in nature. A preferred surfactant may comprise a mixture of a fatty acid, for example oleic acid and alkali, for example aqueous potassium hydroxide.

A preferred thickening agent may be a hydrocolloid, preferably xanthan gum. This is preferred in order to ensure long term stability during storage and transportation.

Alternatively a solution of such mixtures in water or organic solvent may be employed.

A stabiliser dispersion of the present invention may be manufactured by heating and mixing the melted ingredients. A surfactant may be added. A suitable surfactant may comprise a mixture of long chain aliphatic acid, for example, oleic acid and alkali, for example, aqueous potassium hydroxide. The resultant mixture may be stirred with hot water and a hydrocolloid, for example xanthan gum to form a suspension which may be cooled. Any further ingredients, for example a biocide may be added before packaging and storage or distribution.

The stabiliser of the present invention may be used to stabilise a wide variety of rubbers and other polymeric materials, including: nitrile rubbers; partially or fully hydrogenated nitrile rubbers; styrene-butadiene copolymers, including carboxylated derivatives; polychloroprene; polybutadiene; isobutylene-isoprene copolymers; brominated isobutylene-isoprene copolymers; chlorinated isobutylene-isoprene copolymers; butadiene-C<NUM>-<NUM>-alkyl acrylate copolymers; acrylonitrile isoprene rubbers; styrene isoprene rubbers; styrene isoprene rubbers; polyisoprene; natural rubbers; epoxidized natural rubber; and mixtures thereof.

The preparation of the nitrile rubbers by polymerization of the above mentioned monomers is adequately known to those skilled in the art and is comprehensively described in the literature. Nitrile rubbers which can be used for the purposes of the invention are also commercially available, e.g. as products from the product range of the Perbunan® and Krynac® grades of Lanxess Deutschland GmbH.

Rubbers with the previously disclosed stabilisers and in particular stabilized nitrile rubbers surprisingly show very good storage and colour stability, a positive processing behaviour and unchanged positive mechanical and dynamical properties.

Good storage stability of a rubber is given if the Mooney viscosity is as stable as possible over a prolonged period of time, especially at relatively high temperatures.

The storage stability is usually determined by storing the unvulcanized rubber for a defined period of time at elevated temperature (also referred to as hot air storage) and determining the difference between the Mooney viscosities before and after this storage at elevated temperature. Since the Mooney viscosity of rubber usually increases during hot air storage, the storage stability is characterized by the difference of Mooney viscosity after storage minus Mooney viscosity before storage. This will be shown on the example of stabilized nitrile rubbers in the following:
The stabilized rubbers may be prepared according to the invention by bringing the stabiliser components into contact with the rubber.

Such incorporation may be achieved in various manners.

The addition of the components in any of the aforementioned steps may be realized in different ways:
Each component (b), (c) and (d) can be added separately. Different mixtures of the three components can be combined. Typically the three components, whether added singly or in any type of combination, are used as a dispersion or emulsion.

Percentages and other amounts referred to in this specification are by weight unless indicated otherwise. Percentages are selected from any ranges quoted to total <NUM>%.

The invention is further described by means of example but not in any limitative sense.

Antioxidant <NUM> (<NUM>% w/w) and Antioxidant <NUM> (<NUM>%w/w) were added to a vessel and heated with high shear stirring. When the mixture was molten poly-dicyclopentadiene-co-p-cresol (<NUM>%w/w) was added. The mixture was heated to <NUM> for <NUM> minutes and cooled to <NUM>. A mixture of oleic acid (<NUM>%w/w) and <NUM>% strength aqueous potassium hydroxide (<NUM>%w/w) was added and the mixture was stirred for <NUM> minutes. Hot water at <NUM>(<NUM>%) was added at a rate of <NUM>/minute. When <NUM>% of the total water charge remained to be added, xanthan gum (<NUM>%) was sprinkled onto the solution and the remaining water was added. The mixture was allowed to cool to <NUM>, biocide (Acticide (MBL) <NUM>%) was added with stirring and the mixture was packaged for use or distribution as required.

The resultant antioxidant was a solid in water suspension having a viscosity of <NUM> - 450cP and pH <NUM>-<NUM>, particle size d95%<<NUM> and a total solids content of <NUM>-<NUM>%.

Antioxidant <NUM> (<NUM>% w/w) was added to a vessel and heated with high shear stirring. At <NUM>, poly-dicyclopentadiene-co-p-cresol (<NUM>%w/w) was added. The mixture was heated to <NUM> for <NUM> minutes and cooled to <NUM>. A mixture of oleic acid (<NUM>%w/w) and <NUM>% strength aqueous potassium hydroxide (<NUM>%w/w) was added and the mixture was stirred for <NUM> minutes. Hot water at <NUM>(<NUM>%) was added at a rate of <NUM>/minute. When <NUM>% of the total water charge remained to be added, xanthan gum (<NUM>%) was sprinkled onto the solution and the remaining water was added. The mixture was allowed to cool to <NUM>, biocide (Acticide (MBL) <NUM>%) was added with stirring and the mixture was packaged for use or distribution as required.

The resultant antioxidant was a solid in water suspension having a viscosity of <NUM> - 600cP and pH <NUM>-<NUM>, particle size d <NUM>% <<NUM> and a total solids content of <NUM>-<NUM>%.

Antioxidant <NUM> (<NUM>% w/w) was added to a vessel and heated with high shear stirring. At <NUM> Antioxidant <NUM> (<NUM>% w/w) was added. The mixture was heated to <NUM> for <NUM> minutes and cooled to <NUM>. A mixture of oleic acid (<NUM>%w/w) and <NUM>% strength aqueous potassium hydroxide (<NUM>%w/w) was added and the mixture was stirred for <NUM> minutes. Hot water at <NUM>(<NUM>%) was added at a rate of <NUM>/minute. When <NUM>% of the total water charge remained to be added, xanthan gum (<NUM>%) was sprinkled onto the solution and the remaining water was added. The mixture was allowed to cool to <NUM>, biocide (Acticide (MBL) <NUM>%) was added with stirring and the mixture was packaged for use or distribution as required.

Claim 1:
A stabiliser for an organic polymer mixture, comprising one or more compounds from group (b) and one or more compounds from group (c) wherein (b) and (c) are as follows;
(b) Antioxidant <NUM> (octadecyl-<NUM>-(<NUM>,<NUM>-di-tert-butyl-<NUM>-hydroxyphenyl)propionate; CAS <NUM>-<NUM>-<NUM>);
(c) Antioxidant <NUM> (<NUM>-methyl-<NUM>,<NUM>-bis [octylthiomethyl]-phenol; CAS <NUM>-<NUM>-<NUM>),
wherein the ratio by weight of (b):(c) is in the range <NUM>:<NUM> to <NUM>:<NUM>.