Field of the Invention

The invention relates to processes for preparing sulphur cross-linked polymer-modified bitumen Background of the Invention Bitumen may be modified by the addition of a polymer to improve the mechanical properties of the bitumen. The most commonly incorporated polymers are thermoplastic elastomers, particularly styrenic block copolymers such as styrene-butadiene-styrene (SBS) and styrene-isoprene- styrene (SIS) . To further improve the mechanical properties of the bitumen, the polymer may be cross- linked by the addition of a cross-linking agent, typically elemental sulphur.

Sulphur cross-linked polymer-modified bitumens emit increased quantities of hydrogen sulphide during storage or when used, when compared to polymer-modified bitumens that have not been cross-linked. A scavenging agent such as zinc oxide may be added to the formulation to reduce the emission of hydrogen sulphide. The present inventors have sought to provide alternative methods of producing cross-linked polymer-modified bitumen, wherein the emissions of hydrogen sulphide are minimised. Summary of the Invention

Accordingly, the present invention provides a process for preparing sulphur cross-linked polymer- modified bitumen, comprising a step of mixing sulphur with bitumen and polymer, characterised in that the sulphur is in the form of pellets of sulphur that comprise at least 0.25wt% carbon black, and in that the amount of sulphur that is mixed with the bitumen and polymer is from 0.5 to 20wt%, based upon the weight of the polymer. _ o ~

The present inventors have found that the pellets of sulphur comprising at least 0.25wt% carbon black (based upon the weight of the sulphur pellet) can be used as a cross-linking agent and hydrogen sulphide emissions are low even in the absence of a separate scavenging agent. Detailed Description of the Invention

The bitumen used in the process of the invention may be a residue from the distillation of crude oil, a cracked residue, naturally occurring bitumen or a blend of various bitumen types. Examples of bitumen that may be conveniently used in the present invention include distillation or "straight run" bitumen, precipitation bitumen, e.g. propane bitumen, oxidised or blown bitumen, naphthenic bitumen, paraffinic bitumen or mixtures thereof. The bitumen in the bitumen composition may be prepared by blending a bitumen with a flux oil, e.g. an aromatic, napthenic or paraffinic flux oil. The penetration at 25°C of the bitumen (as measured according to EN 1426) is preferably between 10 and 400. {If the bitumen is a blend of bitumen and flux oil, the penetration is preferably between 10 and 400 after the flux oil has been blended with the bitumen.)

The weight ratio of bitumen to polymer in the blend of bitumen and polymer is preferably at least 4:1, more preferably at least 8:1. Bitumen is typically the least expensive component in the composition so it is desirable to maximise the quantity of bitumen. However, a weight ratio of greater than 40:1, preferably 20:1 bitumen to polymer, is not desirable as there will be insufficient polymer to improve the mechanical properties of the polymer-modified bitumen.

The polymer is a polymer that can be cross-linked with sulphur. The polymer is preferably a thermoplastic elastomer. The thermoplastic elastomer may be a homopolyiner such as polybutadiene or may be a random copolymer such as styrene butadiene rubber. The thermoplastic elastomer is most preferably a block copolymer comprising at least one polymer block derived from a vinyl aromatic hydrocarbon and at least one polymer block derived from a conjugated diene. The block copolymer may be linear or radial, symmetric or asymmetric, and may have a structure represented by the formula A-B, A-B-A, B-A-B, A-B-A-B etc. wherein A is the polymer block of a vinyl aromatic hydrocarbon and B is the polymer block of a conjugated diene. The block copolymer preferably contains up to 50% by weight of vinyl aromatic hydrocarbon, more preferably from 25-40wt^ of vinyl aromatic hydrocarbon.

The vinyl aromatic hydrocarbon is preferably styrene, o-methylstyrene, p-methylstyrene, p-tert- butylstyrene, 1, 3-dimethylstyrene, α-methylstyrene, vinylnapthalene or vinylanthracene, and most preferably is styrene. Mixtures of vinyl aromatic hydrocarbons may be used.

The conjugated diene preferably has from 4-8 carbon atoms, and more preferably is 1, 3-butadiene, 2-methyl- 1, 3-butadiene (isoprene) , 2, 3-dimethyl-l, 3-butadiene, 1, 3-pentadiene or 1, 3-hexadiene. Most preferably the conjugated diene is 1, 3-butadiene or isoprene. Mixtures of conjugated dienes may be used.

Most preferably the polymer is styrene-butadiene- styrene or styrene-isoprene-styrene. Alternatively, the polymer may be a polymer comprising glycidyl functional groups; preferably either a copolymer wherein at least some of the monomers are glycidyl-containing monomers, or a grafted polymer wherein glycidyl-contaiπing groups have been grafted onto a polymer backbone. Preferred copolymers are copolymers of ethylene and glycidyl acrylate or methacrylate, or terpolymers of ethylene, alkyl acrylate or methacrylate and glycidyl acrylate or methacrylate. The alkyl acrylate is preferably Ci-e alkyl. Preferred grafted polymers comprise glycidyl acrylate or methacrylate grafted onto a polyethylene backbone.

The molecular weight of the polymer is preferably from 50,000 to 500,000.

Sulphur, in the form of pellets of sulphur that comprise at least 0.25wt% carbon black, is added to the blend of bitumen and polymer. The amount of sulphur that is added is from 0.5 to 20wt%, preferably from 1 to 5wt%, based upon the weight of the polymer.

Reference herein to pellets is to any type of sulphur material that has been cast from the molten state into some kind of regularly sized particle, for example flakes, slates or sphere-shaped sulphur such as prills, granules, nuggets and pastilles or half pea sized sulphur. The sulphur pellets typically comprise from 50 to 99wt% of sulphur, based upon the weight of the sulphur pellets, preferably from 60wt% and most preferably from 70wt%; and typically to 95wt%, and preferably to 90wt%. A preferred range is from 60 to 90wt%.

The sulphur pellets comprise at least 0.25wt% carbon black. As described in WO 03/14231, the carbon black plasticises the sulphur, i.e. the sulphur in the pellets is typically amorphous rather than crystalline. The sulphur pellets preferably comprise from 0.25 to 2wt% carbon black, and more preferably comprise from 0.4 to 0.8wt% carbon black. According to WO 03/14321 such sulphur pellets are added as the majority component to bitumen to manufacture paving binders. It does not teach that minor amounts of sulphur pellets that contain at least 0.25wt% carbon provide crosslinking in polymer- modified bitumen or inhibit hydrogen sulphide formation. The sulphur pellets preferably comprise sulphur that has been degassed to remove hydrogen sulphide.

The sulphur pellets may comprise other components, for example, they may comprise amyl acetate in a concentration of at least about 0.08wt% based upon the weight of the pellet. They may also comprise free radical inhibitors and/or redox catalysts, e.g. from 0.05 to 6wt% of a compound selected from iodine, amine compounds, copper salts, copper oxides, iron salts, iron oxides, cobalt salts and cobalt oxides. They may also comprise waxes such as paraffin waxes of ethylene bis stearamide. In one preferred embodiment, the sulphur pellets consist of sulphur and carbon black only.

Sulphur pellets are mixed with bitumen and polymer. Preferably the polymer is added to molten bitumen, and mixed under lower shear to form a blend of bitumen and polymer, and the sulphur pellets are subsequently added and mixed under low shear. Alternatively, the sulphur pellets may be added to molten bitumen and the polymer may be added subsequently, preferably with high shear mixing.

The sulphur pellets are preferably mixed with the bitumen and polymer at a temperature of about 180 ⁰ C, and preferably not more than 200 ⁰ C.

The invention will now be described by reference to examples which are not intended to be limiting of the invention. Example 1

A cross-linked polymer-modified bitumen formulation was prepared comprising 76.8wt% bitumen (50/70 grade), 18.8wt% SBA (a very hard grade of bitumen), 3.2wt% styrene-butadiene-styrene (DlIOl from Kraton®) , l.lwt% paraffin wax and 0.1wt% sulphur pellets comprising carbon black.

5Og of the bitumen formulation was placed in a metal tube, which was then sealed. The formulation was heated to 180 ⁰ C for 24 hours and then cooled. The hydrogen sulphide formed in the formulation was measured by using a Drager pump and reaction tubes that change colour when exposed to hydrogen sulphide. This indicated the presence of 475ppm hydrogen sulphide. Comparative Example 1

A cross-linked polymer-modified bitumen formulation was prepared comprising 76.8wt% bitumen (50/70 grade) , 18.8wt% SBA, 3.2% styrene-butadiene-styrene (DlIOl from Kraton®), l.lwt% paraffin wax and 0.17wt% of elemental sulphur and zinc oxide.

5Og of the bitumen formulation was placed in a metal tube, which was then sealed. The formulation was heated to 18O ⁰ C for 24 hours and then cooled. The hydrogen sulphide formed in the formulation was measured by using a Drager pump and reaction tubes that change colour when exposed to hydrogen sulphide. This indicated the presence of lOSppm hydrogen sulphide. The hydrogen sulphide reading in this instance was lower, but a scavenging agent (zinc oxide) was incorporated in this formulation.