Crude oils (crude mineral oils) and certain fuels (oils) derived therefrom, e.g. wax-containing fuel oils and diesel fuels, may contain considerable amounts of wax. This wax gradually separates out as such materials are cooled, and at sufficiently low temperatures these materials may solidify completely.

The lowest temperature observed during a standard laboratory test (ASTM D.97) at which the waxy material still flows is called the "pour-point" of the material.

In order to overcome the problems arising from cooling of such materials in storage or in transport, whether in tankers or by pipeline, certain polymers have been developed which when added to such materials depress the pour-point of the material.

Such polymers are typically polymers obtained by polymerisation of at least one olefinically unsaturated compound, which polymers contain aliphatic hydrocarbon side chains of at least 10 carbon atoms, e.g C10-30 aliphatic hydrocarbon side chains. Typically these polymers have molecular weights (Mn) in the range of 10,000 to 1,000,000.

Examples of such polymers are described in GB-A-1154966, GB-A-1161188, GB-A-1285087, GB-A-1410819, EP-A-0120512, EP-A-0236844 and US-A-4547202.

Whilst these polymers have proved to be very effective pour point depressants, and hence useful fluidity improvers, they are typically sold as concentrated solutions in solvents such as toluene or xylene, which solutions may themselves have relatively

high pour points. For dispersion in tanks of crude oil and fuel oil, this normally presents no significant problem. Examples of commercially available fluidity improvers include those available from the Royal Dutch/Shell Group of companies under the trade mark "SHELLSWIM - 5X", a 50%w solution of an alkyl acrylate polymer in xylene (having a pour point of 270C), and under the trade mark "SHELLSWIM - liT", a 50%w solution of an alkyl acrylate/vinyl pyridine copolymer in toluene (having a pour point of 180C).

However, there are circumstances, for example where it may be desired to pass the fluidity improvers themselves along a pipeline in cold environments, e.g. in North Sea or Arctic areas, when it would be advantageous to have concentrated formulations of pour point depressants which would remain readily flowable at low ambient temperatures.

In EP-A-0448166, concentrated formulations have been disclosed containing an alkylacrylate polymer, a surfactant, a polyol and various amounts of water. It appears from said document that the best results (long term storage stability) are obtained when use is made of formulations containing a limited amount of water.

Surprisingly, it has now been found that formulations which may contain high amounts of water constitute attractive pour point depressants, when they contain a thixotropic hydrophilic polymer.

According to the present invention there is provided a polymer composition comprising a dispersion of at least one polymer A obtained by polymerisation of at least one olefinically unsaturated compound, which polymer contains aliphatic hydrocarbon side chains of at least 10 carbon atoms, in a continuous aqueous phase comprising a surfactant and at least one polymer B which comprises a thixotropic hydrophilic polymer.

The polymer compositions according to the present

invention advantageously display a low viscosity under transport conditions and a long term~storage stability.

Suitable polymers A are described in GB-A-1154966, GB-A-1161188, GB-A-1285087, EP-A-0120512, EP-A-0236844 and US-A-4547202. These polymers typically have molecular weights (Mn) in the range 10,000 to 1,000,000 and contain C10-30 aliphatic hydrocarbon side chains.

Preferably polymer A has a molecular weight (Mn) in the range 20,000 to 250,000, more preferably 25,000 to 100,000, e.g. 30,000 to 80,000.

Polymer A is preferably a homo- or copolymer of one or more (C18-30 n-alkyl) acrylates or methacrylates, e.g. 1-docosyl acrylate (C22), 1-octadecyl acrylate (C18) and 1-eicosyl acrylate (C20) . Suitable copolymers are copolymers of one or more (C18-30 n-alkyl) acrylates or methacrylates with a monoolefinically unsaturated compound having a nitrogen containing group, such as acrylonitrile, acrylamide, p-aminostyrene, N-vinylpyrrolidone or 4-vinylpyridine.

Most preferably polymer A is a polymer of one or more (C18-26 n-alkyl) acrylates or a copolymer of one or more (C18-26 n-alkyl) acrylates and 4-vinyl- pyridine).

Various non-ionic and ionic surfactants are suitable for incorporation in compositions according to the invention. Preferably the surfactant is selected from alcohol ethoxy sulphate surfactants and lignosulphonate surfactants. Such surfactants are typically alkali metal or ammonium salts. Alcohol ethoxy sulphate surfactants typically have the formula RO-(CH2CH2O)n-so3-M where m is sodium or ammonium, R is C12-18 alkyl and n is 2 or 3.

Compositions according to the invention suitably contain 0.1 to 20, preferably 1 to 15, more preferably

1 to 10 parts by weight of surfactant per 100 parts by weight of polymer A.

Polymer B comprises a thixotropic hydrophilic polymer. Solutions of such polymers are known to have a high viscosity at a low shear rate and to have a low viscosity at a high shear rate. Suitable polymers B include for instance polysaccharides (e.g. Xanthan Gum) and partly hydrated polyacrylamides. These polymers typically have number average molecular weights (Mn) in the range 1 x 105 to 1 x 107, preferably in the range 5 x 105 to 5 x 106.

Suitably, these polymers have a molecular weight distribution (Mw/Mn, whereby Mw is defined as the weight average molecular weight) in the range 1 to 5, preferably 1 to 2, most preferably 1 to 1.5.

Polymer B is preferably a polysaccharide. Most preferably polymer B is Xanthan Gum.

Compositions according to the invention suitably contain 0.01 to 20, preferably 0.1 to 10, more preferably 0.2 to 5, parts by weight of polymer B per 100 parts by weight of polymer A. These amounts include any minor amounts of solvents such as ethanol which may be present in the surfactant chosen for incorporation in the composition.

Other additives may be incorporated in minor amounts in the compositions of the invention such as anti-oxidants, anti-corrosion agents or metal deactivators.

Compositions according to the invention suitably contain more than 20, preferably more than 50, more preferably more than 80, parts by weight of water per 100 parts by weight of polymer A.

The presence of even small amounts of thixotropic polymers have a beneficial effect in terms of viscosity reduction under transport conditions.

Suitably, the continuous aqueous phase may comprise

two or more different types of thixotropic hydrophilic polymers.

The invention further provides a process for the preparation of polymer compositions of the invention, which process comprises emulsifying a mixture of polymer A, the surfactant and polymer B at a temperature above the melting point of the polymer A, and cooling the resulting emulsion to a temperature below the melting temperature of polymer A.

The upper temperature limit for the process will be determined by the lowest decomposition temperature of the various components of the mixture. T g afflrylar:o polymers as polymer A, alcohol ethoxy sulphate surfactants and/or lignosulphonate surfactants, and temperatures for emulsifying mixtures in the range 500C to 1000C has been found to be very effective, temperatures in the range 60 to 1000C being preferred.

Further in accordance with the invention there is provided the use of a composition of the invention in depressing the pour point of a crude mineral oil or a wax-containing fuel oil or diesel oil.

The polymer compositions of the invention have particularly useful application according to the invention in inhibition and/or reduction of wax deposition in crude oil pipelines. The fact that the present aqueous compositions are fluid at lower temperatures than simple solutions of the relevant polymer at corresponding concentrations in solvents such as toluene or xylene confers significant improvements in handleability in lower temperature environments. Moreover, since the present aqueous formulations contain little or none of such solvents they constitute very attractive materials from the safety standpoint (flashpoint), when compared with conventional formulations.

The invention will be further understood from the

following illustrative Examples, in which:- Polymer I is an alkyl acrylate polymer, mp 510C, of molecular weight (Mn) in the range 49,000 to 76,000 measured by gel permeation chromatography (GPC) (polyacrylate scale) prepared from a blend of linear alkyl acrylates comprising 1-docosyl acrylate, 1-octadecyl acrylate and 1-eicosyl acrylate.

"DOBANOL 25-3A/60" (trade mark) ethoxy sulphate is a 60%w solution in aqueous ethanol (2:1 w/w) of ammonium (C12-15 alcohol) ethoxy sulphates containing 3 mol ethylene oxide/mol (density at 200C 1.04 kg/l).

Examples 1 to 6 Water (44.6 g), "DOBANOL 25 - 3A/60" ethoxy sulphate (5 g) and partly hydrated polyacrylamide (PHAM) (0.4 g) were stirred to give a homogenous solution at ambient temperature (200C) in a 150 ml beaker. Polymer I (50 g) was added and the bottle was transferred to a thermostatic oil bath at 800C. The two- phase mixture was emulsified using an "ULTRA TURRAX T25" (trade mark) laboratory emulsifier (1.8 cm dispersing tool, at 24,000 r.p.m.) for 1 minute. The resulting emulsion of Polymer I in the aqueous phase was cooled by removing the bottle from the oil bath and allowing it to cool in air to ambient temperature (20°C), to yield the desired composition as a readily flowable suspension.

Additional suspensions (Examples 2 to 6) were prepared in a similar manner to the above suspension, except that in Example 4 Xanthan gum was used instead of PHAM and that hexadecyltrimethylammonium bromide (HDTMAB) was used instead of ethoxysulphate; in comparative Example 5 glycerol was used instead of water and PHAM, the temperature of the thermostatic oil bath was 960C and the mixture was emulsified for 2 minutes; and in comparative Example 6 glycerol was used instead of PHAM, the temperature of the thermostatic

oil bath was 1010C and the mixture was gently emulsified for 1 minute, after which water was added. Details of all six suspensions and their preparation are given in Table 1.

In each case the suspensions were stable at ambient temperature (200C) for at least one month. The suspensions were all liquid at least down to 50C. The suspensions were tested by addition to a Far East crude oil and assessment of the pour point according to ASTM D.97. The pour point of the undoped crude oil was 240C, for examples 1-3, and 30"C for examples 4-6.

The test results of the suspensions of Examples I to 6 are summarised in Table II.

Viscosity was measured at 20 OC with a cone and plate system in a Carri-Med "CSL 500" Rheometer.

It will be clear from Table II that the suspensions in accordance with the invention (Examples 1-4) display an attractive pour point activity together with an improved low viscosity when compared with suspensions falling outside the scope of the present invention (Examples 5 and 6).

Table I Example pbw water pbw pbw pbw pbw pbw ethoxysulphate HDTMAB PHAM Xanthan gum glycerol 1 89.6 10 - 0.4 - - 2 89.8 10 - 0.2 - - 3 89.2 10 - 0.8 - - 4 93.4 - 4 - 1.6 - 5 - 4 - - - 96 6 15 4 - - - 46 pbw = parts by weight/100 parts polymer I Table II Example Viscosity Rate of addition of suspension pour point of treated at 20°C (Pa.s) to Far East crude oil oil (kg polymer I/kg oil) (°C) 1 0.17 1000 12 2 0.08 1000 18 3 0.60 1000 12 4 0.16 925 9 5 13.7 2160 15 6 1.9 2470 12