Methods relating to rubber products

The present invention relates to a method for treating rubber materials. In particular the invention relates to a method for devulcanisation of rubber materials.

Vulcanization of rubber is a very old well known industrial process in which an additive such a sulphur is added to rubber to modify the polymer by forming cross links between the polymer chains. Untreated rubber is sticky, deforms on heating and becomes brittle at low temperatures. Vulcanised rubber has superior mechanical properties and is used to manufacture a wide range of products, especially vehicle tyres.

Such products do however have a limited life time and thus very large quantities of waste vulcanised rubber are produced each year, It would be highly advantageous if this waste product could be used in new applications. However, as vulcanisation of rubber is a thermosetting process it is generally difficult to reverse.

A number of devulcanisation processes are known. However rubber obtained by these processes often has inferior properties to vulcanised virgin rubber and thus can be used in limited applications, for example road surfacing or roof tiles. Rubber produced by devulcanisation processes of the prior art produce a rubber material which can be subsequently included in vehicle tyres at only very low levels.

It is an aim of the present invention to provide an improved process for devulcanisation of rubber materials.

According to a first aspect of the present invention there is provided a method for the devulcanisation of a rubber material, the method comprising contacting the rubber material with carbon dioxide and an additive selected from dithiocarbamates, thiazoles, guanidines, thiurams, sulphenamides and mixtures thereof.

The present invention relates to a method of treating a rubber material. Suitably the rubber material comprises waste rubber masterial, that is vulcanised rubber which has been previously used. In some preferred embodiments the rubber material comprises waste rubber sou reed from used rubber tyres.

The rubber material used in the method of the present invention preferably comprises rubber crumb. Rubber crumb is a term well known to those skilled in the art and is used to refer to particulate rubber, typically recovered from vehicle tyres. During the recovery process steel and fluff is removed to leave rubber with a granular consistency. Further processing reduces the size of the particles. The particles are typically sized by passing through a screen with a particular mesh. A 10 mesh screen has 10 holes per inch, a 20 mesh screen has 20 holes per inch, and so on. Thus a rubber crumb having a "10 mesh" size will have a particle having a size of less than 0.1 inch, a "20 mesh" size will have particles of less than 0.05 for example less than 0.033 inch and so on.

Preferably the rubber crumb used in the present invention has a size of 30 mesh or less. The method for devulcanisation of the present invention may use rubber crumb having a size of 40 mesh or less, for example 60 mesh or 80 mesh.

Preferably the rubber material used in the present invention comprises particles having a diameter of less than 2mm, preferably less than 1.5mm, suitably less than 1 mm. Suitably the rubber material comprises particles having a diameter of from 0.2 to 0.6mm. It is a particular advantage of the present invention that the method may be used to devulcanise rubber crumb of a variety of sizes including 30 mesh, 40 mesh, 60 mesh and 80 mesh.

The rubber material used in the present invention may comprise natural rubber, synthetic rubber or a mixture thereof.

In the devulcanisation method of the present invention the rubber material is contacted with carbon dioxide and an additive selected from dithiocarbamates, thiozoles and mixtures thereof. In some preferred embodiments carbon dioxide is used as liquid carbon dioxide. It may be used as supercritical carbon dioxide. Carbon dioxide may suitably be supplied from a cylinder of the compressed gas. It may be supplied as a liquid but once present in the reaction vessel the temperature and pressure will adapt according to the surroundings. The rate of supply of carbon dioxide is suitably selected to provide a desired ratio of carbon dioxide to rubber material. Control of the flow of carbon dioxide can be easily achieved by the person skilled in the art.

In some alternative embodiments carbon dioxide may be used as solid carbon dioxide. In such embodiments said carbon dioxide may be added with other solid ingredients.

The method of the present invention involves using an additive selected from dithiocarbamates, thiazoles, guanidines, thiurams, sulphenamindes and mixtures thereof. Suitable dithiocarbamate compounds for use in the method of the present invention include metal salts of alkyl dithiocarbamates especially dialkyl dithiocarbamate salts, diary Ithiocarbamates or di(alkylaryl)thiocarbamates. Preferred are divalent metal salts of dialkyl thiocarbamtes or di(alkylaryl)dithiocarbamates, suitably transition metal salts of dialkyl dithiocarbamates or di(alkylaryl)dithiocarbmates.

Preferred dialkyldithiocarbamates include alkyl groups having 1 to 16 carbon atoms, preferably 1 to 12, more preferably 1 to 8 and most preferably 1 to 4 carbon atoms. Preferred di(alkylaryl)dithiocarbamates include dibenzyl dithiocarbamate.

Preferred dithiocarbamate compounds for use herein are zinc salts. These include zinc dimethyldithiocarbamate, zinc diethyldithiocarbamate, zinc dipropyldithiocarbamate, zinc dibutyldithiocarbamate and zinc dibenzyldithiocarbamate. An especially preferred dithiocarbamate additive is zinc diethyldithiocarbamate.

Any suitable thiazole compound may be used in the method of the present invention. Preferred thiazole compounds include 2-mercaptobenzothiazole or derivatives thereof, for example zinc mercaptobenzathiazole and benzathiazyl disulphide. Other metal salts of mercaptobenzothiazyl could also be used. An especially preferred thiazole is 2- mercaptobenzothiazyl disulfide (MBTS).

Suitable guanidine compounds for use herein include Ν,Ν-diphenyl guanidine and N,N-di- ori 70-tolyl guanidine. Suitable thiuram compounds for use herein include tetramethyl thiurum disulfide, tetraethyl thiuram disulfide, tetrabutyl thiurum disulfide and tetramethyl thiuram monosulfide.

Suitable sulphenamide compounds for use herein include N-cyclohexyl-2-benzothiazole sulphenamide, N-ieri-butyl-2-benzothiozole sulphenamide and 2-(4-morpholinothio)- benzothiozole sulphenamide.

The additive may comprise a mixture of two or more compounds selected from dithiocarbamates, thiazoles, guanidines, thiorams and sulphenamides. Preferably the additive is selected from dithiocarbamates, thiazoles and mixtures thereof.

In some embodiments the additive may comprise a mixture of one or more dithiocarbamate compounds and/or one or more thiazole compounds. In some preferred embodiments the additive comprises at least one dithiocarbamate compound and at least one thiazole compound.

The additive is suitably present in an amount of at least 0.5 parts by weight for every 100 parts by weight of the rubber material, i.e. for every 100g of rubber material at least 0.5g of additive is used. Preferably the additive is used in an amount of at least 1 part by weight per 100 parts of rubber material, suitably at least 1.5 parts, preferably at least 2 parts, more preferably at least 2.5 parts, suitably at least 3 parts, for example at least 3.5 parts. The additive may be present in an amount of up to 15 parts by weight for 100 parts by weight of the rubber material, suitably up to 12 parts, preferably up to 10 parts, more preferably up to 8 parts, preferably up to 7 parts, suitably up to 6 parts for example up to 5 parts.

Where a mixture of additive compounds is present, the above amounts refer to the total of all such compounds.

Where a mixture of dithiocarbamate and thiazole compounds is used the weight ratio of thiazole compounds to dithiocarbamate compounds is preferably at least 0.5:1 , preferably at least 1 :1 , for example at least 1.5:1 , at least 2: 1 or at least 2.5:1.

In some embodiments of the method of the present invention rubber crumb is contacted with from 0.5 to 2 parts by weight of one or more dithiocarbamate compounds and from 1 to 5 parts by weight of one or more thiozaole compounds for every 100 parts of rubber crumb. In the method of the present invention a rubber material is contacted with carbon dioxide and one or more additives. The or each additive and the carbon dioxide may be contacted with the rubber material sequentially or simultaneously.

In some embodiments the rubber material is further contacted with one or more coagents. One preferred class of coagents comprises metal oxides, preferably transition metal oxides. An especially preferred coagent is zinc oxide.

Another preferred class of coagents comprises fatty acids, especially saturated fatty acids having from 12 to 26 carbon atoms. An especially preferred coagent is stearic acid.

Another suitable coagent for use in the method of the present invention is zinc stearate.

In some preferred embodiments the rubber material is contacted with a metal oxide, for example zinc oxide. This is suitably used in an amount of from 0.05 to 10 parts per 100 parts of rubber material, preferably from 0.1 to 5 parts, more preferably from 0.1 to 3 parts, suitably from 0.1 to 1 parts, preferably from 0.1 to 0.5 parts, for example 0.2 to 0.4 parts.

In some preferred embodiments the rubber material is contacted with a fatty acid, for example stearic acid. This is suitably used in an amount of from 0.05 to 10 parts by weight per 100 parts of rubber material, preferably from 0.1 to 5 parts, more preferably from 0.1 to 3 parts, suitably from 0.1 to 1 parts, preferably from 0.1 to 0.5 parts, for example 0.2 to 0.4 parts.

In some especially preferred embodiments the rubber material is contacted with zinc oxide and stearic acid.

In some embodiments the rubber material is further contacted with a peptiser. Peptisers are common additives used in the rubber industry and the skilled person will know the type of compounds which can be used.

Peptisers can be used to scavenge free radicals. Suitable peptisers include aromatic thiols. One preferred peptiser for use herein is bis(2-benzamidophenyl) disulfide which is commercially available under the trade mark Pepton 22. Suitably the peptiser is used in an amount of at least 0.5 parts by weight per 100 parts of rubber material, preferably at least one part, for example at least 1 .5 parts. It may be present in an amount of up to 10 parts per weight per 100 parts of rubber material, suitably up to 7 parts, preferably up to 5 parts for example up to 3 parts. The method of the present invention may be carried out in any suitable reaction vessel.

In some preferred embodiments the rubber material first is contacted with the additives and when used, the one or more coagents and/or one or more peptisers, and this mixture is subsequently contacted with carbon dioxide.

The rubber material, additives, coagents and peptisers may be contacted prior to charging the reaction vessel or they may be contacted in the reaction vessel. In some embodiments these components are thoroughly mixed for example by stirring or other agitation and the mixture then contacted with carbon dioxide.

In such embodiments the reaction vessel is suitably charged with a mixture of rubber material, additive, and optionally coagents and/or peptisers. Each of these materials is suitably provided in powdered and/or crumb form. Suitably carbon dioxide is then supplied to the vessel in liquid form, for example from a cylinder with a control valve. In embodiments in which solid carbon dioxide is used, the reaction vessel may be charged with a mixture of rubber material, additive, carbon dioxide and optionally coagents and/or peptisers all in solid form. Suitably carbon dioxide is supplied in an amount of from 0.1 to 20% by weight compared to the total weight of the rubber material, preferably in an amount of from 0.25 to 15%, preferably from 0.5 to 12%, more preferably from 0.6 to 10%, suitably from 0.7 to 8%.

The devulcanisation method of the present invention is preferably assisted by the application of heat.

The devulcanisation method of the present invention is preferably assisted by application of a shear force to the mixture. Without wishing to be bound by theory it is believed that the addition of carbon dioxide helps improve contact of the additives and if present cogents with the rubber material by expanding the volume of the reaction mixture. In addition it may increase the shear force.

After charging the reaction vessel the contents (i.e. the reaction mixture) are preferably heated to a temperature of at least 40°C, suitably at least 50°C, preferably at least 60°C, for example at least 70°C, at least 75°C or at least 80°C.

The reaction mixture may be heated to a temperature of up to 150°C, for example up to 130°C, suitably up to 120°C, preferably up to 1 10°C, for example up to 100°C, suitably up to 95°C, for example up to 90°C.

The reaction mixture is preferably heated in the reaction vessel for a period of at least 0.1 minutes, preferably at least 0.5 minutes, suitably at least 1 minute. It may be heated in the reaction vessel for a period of up to 30 minutes, suitably up to 20 minutes, preferably up to 10 minutes.

Preferably the reaction mixture is subjected to a shear force.

In some embodiments the method of the present invention further involves the use of a processing aid. This is suitably delivered to the vessel after it has been charged with the rubber material, additive, carbon dioxide and other optional components.

The time for which the reaction mixture is heated in the reaction vessel may be regarded as the reaction time. The processing aid is suitably added after at least 10% of the reaction time has elapsed, suitably after at least 20%, for example at least 30%. Preferably it is added during the period when 50 to 70% of the reaction time has elapsed.

A preferred processing aid is virgin rubber, that is rubber which has not previously been used and is not vulcanised. This may be obtained from natural and/or synthetic sources. When virgin rubber is used as a processing aid the method of the present invention this is preferably used in finely divided form.

Not all methods of the present invention include the use of virgin rubber as a processing aid. Whether such a processing aid is needed and the amount used depends on the process conditions and the intended subsequent use of the devulcanised product.

One standard material prepared by the present invention no processing aid is needed.

In some embodiments the processing aid, suitably virgin natural rubber is added in an amount of from 2 to 15, preferably 5 to 12 parts by weight per 100 parts of rubber material. Rubber produced by this method may be used to form a material which can be readily formed into sheets.

In some embodiments from 10 to 30 parts, for example 10 to 15, suitably about 20 parts by weight of processing aid are added per 100 parts of rubber material. The product produced by this method can be later extruded and a good surface finish on the product may be observed.

The method of the present invention may be carried out in any suitable vessel. In preferred embodiments the method is carried out in an extruder, for example of the type typically used in the manufacture of plastics materials. Suitable extruders for use in devulcanisation processes are known to those skilled in the art and include single screw extruders, twin screw extruders, counter rotating screw extruders, co-rotating screw extruders and combinations thereof. The method of the present invention may be carried out in a standard extruder or a specialist extruder can be used.

The reaction mixture may be passed once through a single extruder. It may be passed two or more times through the same extruder. Alternatively it may be passed through a series of different extruders.

The extruder is suitably designed to deliver an appropriate shear force to the reaction mixture.

The method of the present invention suitably involves charging the extruder with 100 parts by weight of vulcanised waste rubber crumb, from 1 to 5 parts, preferably 2.5 to 3.5 parts of 2- mercaptobenzothiozyl disulfide; from 0.5 to 4 parts, preferably 1 to 1.5 parts of zinc diethyldithiocarbamate; from 0.1 to 1 , preferably 0.2 to 0.5 parts of zinc oxide; from 0.1 to 1 , preferably 0.2 to 0.5 parts of stearic acid; and from 0.5 to 4, preferably 1.5 to 2.5 parts of peptiser. These components may be premixed and are suitably delivered to the extruder via a hopper. In preferred embodiments a substantially homogeneous mixture is fed into the hopper.

In embodiments in which carbon dioxide is used in solid form this may also be premixed with the other solid ingredients and delivered to the extruder via a hopper. Alternatively the solid carbon dioxide may be added to the hopper separately.

The present invention further provides a modified single screw extruder as described in example 2. The design of this screw helps retain carbon dioxide within the extruder and provides good shear.

According to a second aspect of the present invention there is provided a devulcanised rubber product obtained by the method of the first aspect.

The devulcanised rubber product of the present invention offers significant advantages over devulcanised rubber products of the prior art. In particular the product of the present invention can be used directly in a standard vulcanisation process. No further modification of the material is usually necessary and changes to standard vulcanisation processes are not normally needed. The devulcanised rubber product of the second aspect is suitably a non-uniform free-having solid with no regular shape. It may have a sticky consistency and have an appearance resembling cotton wool.

The method of the present invention may be a batch process or a continuous process. Preferably it is a continuous process. In some preferred embodiments the devulcanised rubber product of the second aspect may be fed directly into a vulcanisation apparatus and immediately revulcanised. Suitable revulcanisation methods will be known to the person skilled in the art and include compression moulding, transfer moulding and injection moulding. A third aspect of the present invention provides a revulcanised rubber product obtained by the vulcanisation by a standard technique of the devulcanised rubber product of the second aspect. In some embodiments to obtain the revulcanised rubber of the third aspect, the devulcanised product of the second aspect may be mixed with virgin rubber. The weight ratio of virgin rubber to the product of the second aspect may be from 95:5 to 5:95. The products of the second and third aspects have been found to have superior properties compared with similar products of the prior art and in many cases the properties of the product of the second and third aspects compare favourably with those of virgin rubber.

The elongation of the product of the second aspect is at least 50% of that of virgin rubber of the same compound, preferably at least 60%, more preferably at least 70%, for example at least 75%.

The elongation of the product of the third aspect is at least 50% of that of virgin rubber of the same compound, preferably at least 60%, more preferably at least 70%, for example at least 75%.

Elongation may be measured by standard test method ISO 37 (BS9003, Part A2).

The tensile strength of the product of the second aspect is at least 50% of that of virgin rubber of the same compound, preferably at least 60%, more preferably at least 70%, for example at least 75%.

The tensile strength of the product of the third aspect is at least 50% of that of virgin rubber of the same compound, preferably at least 60%, more preferably at least 70%, for example at least 75%.

Tensile strength may be measured by standard test method IS037 (BS9003, Part A2).

The abrasion resistance of the product of the second aspect is at least 50% of that of virgin rubber of the same compound, preferably at least 60%, more preferably at least 70%, for example at least 75%.

The abrasion resistance of the product of the third aspect is at least 50% of that of virgin rubber of the same compound, preferably at least 60%, more preferably at least 70%, for example at least 75%.

Abrasion resistance may be measured by standard test method IS04649 using the DIN abrader. The compression set of the product of the second aspect is at least 50% of that of virgin rubber of the same compound, preferably at least 60%, more preferably at least 70%, for example at least 75%. The compression set of the product of the third aspect is at least 50% of that of virgin rubber of the same compound, preferably at least 60%, more preferably at least 70%, for example at least 75%.

Compression set may be measured by standard test method IS0815 (BS903, Part A6).

The tear strength/tear resistance of the product of the second aspect is at least 50% of that of virgin rubber of the same compound, preferably at least 60%, more preferably at least 70%, for example at least 75%. The tear strength/tear resistance of the product of the third aspect is at least 50% of that of virgin rubber of the same compound, preferably at least 60%, more preferably at least 70%, for example at least 75%.

Tear strength/tear resistance may be measured by the standard test method ISO 34 (BS903, Part A3).

The hardness of the product of the second aspect is at least 50% of that of virgin rubber of the same compound, preferably at least 60%, more preferably at least 70%, for example at least 75%.

The hardness of the product of the third aspect is at least 50% of that of virgin rubber of the same compound, preferably at least 60%, more preferably at least 70%, for example at least 75%. Hardness may be measured by the standard IRHD dead-load instrument test described in IS048 (BS903, Part A26).

The resilience of the product of the second aspect is at least 50% of that of virgin rubber of the same compound, preferably at least 60%, more preferably at least 70%, for example at least 75%.

The resilience of the product of the third aspect is at least 50% of that of virgin rubber of the same compound, preferably at least 60%, more preferably at least 70%, for example at least 75%. Resilience may be measured by the standard test method IS04662 for Pendulum tests.

The products of the second and third aspects may be used in a wide range of applications for general rubber goods manufacture. It can for example be used in the manufacture of vehicle tyres, gaskets, hoses and seals.

Useful products that may be manufactured using the rubber products of the second and third aspects include: various types of tyre, for example cycle tyres, air craft tyre treads, agricultural semi-pneumatic tyres and solid tyres for fork-lift trucks; tyre compounds, for example carcass compounds, side wall compounds and cover strip compounds; conveyor belt covers; wiper blades; blow-out preventors; hoses; soft-linings for example for equipment used to handle slurries and sand blasting; footwear soles and toe cap compounds; trawler bobbins; chemically resistant linings for storage tanks; handle grips; pipe gaskets and seals; drive coupling rubbers; puncture proof tyre inserts; ferrules and chair feet; solid balls for valves; instrument boots; off-shore damp pads; and non metallic springs, for example anti-vibration mountings, engine mounts, bridge bearings, earthquake protectors for buildings, dock fenders, rail track pads and suspension bushes. In the manufacture of some of the above products the devulcanised product of the second aspect may be blended with one or more further components prior to revulcanisation and/or the revulcanised product of the third aspect may be blended with one or more further components. Methods of manufacturing products of this type are well known to those skilled in the art.

The invention will now be further described with reference to the following non-limiting examples.

Example 1

The following composition was fed via a hopper into a modified single screw extruder:

Vulcanised waste rubber crumb (40 mesh) 100 parts by weight

2-mercaptobenzothiazyl disulfide 3.0 parts by weight

Zinc diethyldithiocarbamate 1.2 parts by weight

Zinc Oxide 0.3 parts by weight

Stearic acid 0.3 parts by weight

Peptiser Pepton 22 2.0 parts by weight The composition was very well mixed by stirring prior to delivery into the extruder. The extruder used can be seen in figure 1 and is described in detail in example 2.

Liquid carbon dioxide was delivered at a rate of 200g/hour via inlet valves 5 to provide a concentration inside the extruder equivalent to 4% by mass of the mass of rubber.

The extruder was heated to 85°C and the screw rotated at a speed of 15 rpm.

The resultant product was revulcanised by a standard technique and found to have the following properties, each measured by the standard tests referred to in the description.

Test Value

Tensile strength 14.6 MPa

Elongation 295 %

Hardness 68

Tear strength 24.4 N/mm

Compression set 13.9 %

Abrasion resistance 166 mm ³

Example 2

The present inventors have developed a modified single screw extruder having the structure shown in cross-section in figure 1 . The extruder comprises a barrel 1 having a screw 2 running throught the centre thereof. Rubber crumb and additives are fed into barrel of the extruder via hopper 3 into a feed zone 4. Liquid carbon dioxide may be delivered to the extruder at injection ports 5. When solid carbon dioxide is used this may be fed into the barrel of extruder via hopper 3. The screw is of cylindrical cross section and has a diameter that varies along its length. At the area of the barrel 4 into which the rubber crumb and additives are delivered (the feed zone) the screw diameter 10 is at its narrowest. The diameter then increases slowly along tapered region 6 (a compression zone) to a maximum at 7. Rotation of the screw causes material to be pushed along the barrel of the extruder in the direction indicated by arrow A. The taper at 6 reduces the volume in the barrel of the extruder able to accommodate material thus causing compression of the material. This compression is continued along region 8 (a high shear zone). A sharp decrease in the screw diameter at 9 provides a decompression zone. The screw then returns to its narrowest dimeter at 10. As can be seen from figure 1 the modified single screw extruder of the present invention provides a number of small compression zones and high shear zones.

Figure 2 shows a single screw extruder of the prior art which comprises a single feed zone, a single compression zone and a single metering zone. The modified single screw extruder of the present invention has been found to provide significant advantages over the screw of the prior art. For example carbon dioxide is retained within the extruder more effectively and the product obtained has significantly improved properties.