Mixing

This invention relates to mixing, and in this regard is concerned with mixers and mixing methods using such mixers, of the kind in which two members are mounted for rotation relative one to the other about a central axis, and opposed grooved-surfaces of the two members are spaced apart to define a gap between them and are such that during the relative rotation one or more grooves and lands of each grooved surface are traversed within the gap by one or more grooves and lands of the other grooved surface for subjecting material entered within the gap to shearing and splitting, and the space within the grooves reduces towards the axis and each groove has walls that are mutually inclined to open outwardly from one another.

Mixers of this above-specified kind are known from GB-A-2 142 554 in which material entered through one of the two grooved-members into the mixing zone between them, moves outwardly in the grooves as relative rotation proceeds. The material is discharged from the outer periphery of the two members.

It is an object of the present invention to provide a mixer of said above-specified kind which is of an improved form and which is especially applicable for heavy-duty mixing where, for example, the resultant mix has a viscosity of some 3,000 poise (300 N/m²) or more. Heavy-duty mixers and mixing methods are applicable, for example, in the mastication and blending of rubbers, in the mixing of rubbers with carbon black and other ingredients for vulcanisation, in incorporating plasticisers and other chemicals into polyvinyl chloride, and in mixing fillers into polyethylene and polystyrene.

According to one aspect of the present invention a mixer of the kind specified is characterised in that the two members are mounted within a closely-fitting housing, that the grooving of each member comprises one or more spiral grooves or parts of such grooves for interacting with the traversing grooves of the other member to urge entered material progressively inwardly towards the central axis, and that the grooves are of reducing width and depth inwardly towards the axis for creating increasing pressure on the material as it is urged progressively inwardly along the grooves towards the central axis so that, aided by the inclined walls, it wells up from the grooves into the gap for extensional-shear and distributive mixing in the gap, and in welling up forces return movement of material in the gap outwardly away from the central axis against the movement inwardly of material in the grooves.

There is also provided, according to another aspect of the invention, a method of mixing wherein material to be mixed is entered into a gap defined between opposed grooved-surfaces of two members, and there is relative rotation between the two members such that one or more grooves and lands of each grooved surface are traversed within the gap by one or more grooves and lands of the other grooved surface so as to subject the material entered within the gap to shearing and splitting, and the space between the grooves reduces towards the axis and each groove has walls that are mutually inclined to open outwardly from one another, characterised in that the two members are mounted within a closely-fitting housing, that the grooving of each member comprises one or more spiral grooves or parts of such grooves for interacting with the traversing grooves of the other member to urge entered material progressively inwardly towards the central axis, and that the grooves are of reducing width and depth inwardly towards the axis for creating increasing pressure on the material as it is urged progressively inwardly along the grooves towards the central axis so that, aided by the inclined walls, it wells up from the grooves into the gap for extensional-shear and distributive mixing in the gap, and in welling up forces return movement of material in the gap outwardly away from the central axis against the movement inwardly of material in the grooves.

A mixer and method of mixing is known from DE-U-9410196.5 in which material moves in curved channels between upper and lower discs by the Transfermix process. The channels do not have walls that are mutually inclined to open outwardly from one another. Moreover, there is no urging of material to move along the channels inwardly towards the rotational axis and to well up for extensional, shear and distributive mixing in the gap and forced return movement outwardly away from the rotational axis, in the manner achieved with the mixer and method of the present invention.

The same applies in the case of the mixer and method of mastication known from US-A-1 869 833, in which material entered through a central hopper is fed outwardly and downwardly between cooperating conical, spirally-grooved surfaces of a rotor and stator. Moreover, the space within the grooves in this case reduces outwardly away from the rotational axis of the rotor, rather than inwardly as in the present invention.

The said surfaces of the mixer and the method of mixing of the present invention may be substantially planar faces, and a first of the two members may be mounted for rotation relative to the second member with its grooved face opposed across the gap to the grooved face of the second member. Both members, or only one, may rotate, and the grooving of one or both of them may comprise a multiplicity of spiral grooves each of less than one turn.

The shearing and splitting that arises from the traversing of the grooves and lands of the two surfaces over one another, separates small elements of the material from one another to enhance the mixing that arises from the distribution of the material in larger bulk within the grooves. It is possible to determine the proportions of shear, split and distributive mixing that occur to achieve the desired mix, by suitable choice of the grooving. For example, by narrowing the grooves the proportion of shear mixing can be increased, whereas the proportion of split mixing can be increased by increasing the number of grooves. The proportion of distributive mixing can be increased by increasing the space within the grooves for turbulence of the material to take place.

Shear mixing is of especial advantage in relation to the achievement of mechanochemical processing by which breakdown of the material at molecular level takes place. Thus, according to another feature of the present invention there is provided a method of mixing material wherein the material is subjected to shear between relatively moving surfaces to cause mechanochemical reactions within the material breaking down its molecular structure. Mechanochemical processing of this nature has wide application, but is of especial advantage in the reclaim of rubber from used products; notably vehicle tyres and the vulcanised rubber scrap that arises during tyre manufacture. More particularly, the crosslinked chains of the molecular structure of the used rubber can be broken down sufficiently in this way to solubilise it so that it will go into solution with raw rubber or solvents used in rubber production.

Mixers and methods of mixing in accordance with the present invention will now be described, by way of example, with reference to the accompanying drawings, in which;

• Figure 1 is a sectional side-elevation of a first batch mixer according to the invention;
• Figure 2 is a sectional view taken on the line II-II of Figure 1;
• Figures 3 and 4 are illustrative to an enlarged scale of the relative positions of two scrolled rotor-discs of the batch mixer of Figures 1 and 2 during successive stages of the mixing operation performed thereby;
• Figures 5 and 6 illustrate respectively in perspective and section taken on the line VI-VI of Figure 5, a form of single-spiral scrolling that may be used for the disc-rotors of the batch mixer of Figures 1 and 2;
• Figures 7 and 8 illustrate respectively in perspective and section taken on the line VIII-VIII of Figure 7, a further of scrolling that may be used for the disc-rotors of the batch mixer of Figures 1 and 2;
• Figure 9 is a part-sectional side elevation of a second form of batch mixer according to the present invention.

Referring to Figures 1 and 2, the batch mixer in this case involves two fluted or grooved rotor-discs 1 and 2 that are mounted face to face with a small gap 3 between them, within a closed chamber 4. The discs 1 and 2 are carried within the chamber 4 by individual flanged-shafts 5 that are horizontally aligned axially. The shafts 5, which are journalled in the wall 6 of the chamber 4, are driven in rotation (through gearing or otherwise) by one or more motors (not shown) to turn in opposite senses to one another.

The discs 1 and 2 have identical scroll faces 7 each of which is made up of a series of spiral grooves 8 (eight grooves in the specific example shown in Figure 2) that have outwardly-inclined walls. The grooves 8 are of reducing depth as well as width, towards the disc-centre; the groove-depth preferably reduces to zero at the centre. A removable plug 9 is provided in the top of the chamber-wall 6 to enable the material for mixing to be introduced into the chamber 4 and enter the gap 3 between the counter-rotating scroll faces 7. The direction of rotation of each disc 1 and 2 in relation to the sense of the spiral grooving of its scroll face 7 is such that the material entering the gap 3 is urged inwardly along the grooves 8. The space available to this material reduces with decrease in groove-width and -depth towards the centre, so pressure on it increases as it progresses inwardly. The result is that the inwardly-drawn material ultimately wells up away from the disc-centre and from there moves outwardly of the discs 1 and 2 between the material that is being drawn inwardly along the grooves 8 of the two faces 7. Distributive mixing accordingly occurs within the material as it moves and wells-up upon itself under pressure within the gap 3.

Mixing by shear and splitting also take place within the gap 3 between the counter-rotating discs 1 and 2. These further modes of mixing arise more particularly from the cyclic changes of relative position of the opposed scroll faces 7 occurring at each location. The relative positions of the faces 7 at two stages of the cycle are illustrated by Figures 3 and 4.

In the condition illustrated in Figure 3, the grooves 8 of the two faces 7 are in register with one another, so that the material at this location is at this stage squeezed from between the intervening lands 10 into the spaces afforded by the pairs of opposed grooves 8. As the relative rotation progresses towards the condition illustrated in Figure 4, for which the grooves 8 of each face 7 are brought into register with the lands 10 of the other, there is a progressive transfer of the material in both directions into the smaller spaces provided by the grooves 8 under the lands 10. The transfer, which is enhanced by the outwardly-inclined walls of the grooves 8, not only results in splitting up of the material to enhance mixing but subjects the material at this location to shear forces. The material leaking up the outwardly-inclined walls of the grooves 8 onto the moving lands 10 is extended or stretched in the shearing process and this extension of the material in shear causes mechanochemical reactions which further the mixing process by rupturing the molecular structure of the constituents of the material.

The shear and split mixing continues to take place at each location as the relative rotation of the discs 1 and 2 progresses through the condition illustrated in Figure 4 to return to that illustrated in Figure 3. Moreover, as the material at each location is being subjected to this mixing so the same sequence of cyclical change as represented in Figures 3 and 4 is occurring, but with different phasing, at adjoining locations spaced radially inwards and outwards from that location. The consequence is that not only is the material subjected to mixing movement across the grooves 8 and lands 10, but also along them. The directions of rotation of the discs 1 and 2 are chosen in relation to the hand of the spiral form of their scroll faces 7 to bring about, as referred to above, a drawing down of the material towards the centre of the discs 1 and 2. Thus, the mixing by shearing and splitting takes place in conjunction with and accompanied by the distributive mixing that occurs by movement of the material down the grooves 8.

The mixing process creates substantial heat and it is in general necessary to provide for the wall 6 and the discs 1 and 2 to be cooled; no such provision is shown in Figures 1 and 2 for simplicity. Furthermore, mixing may desirably take place in a controlled atmosphere, and a gas inlet 11 and a gas outlet 12 to the chamber 4 are accordingly provided to facilitate this. As shown in Figure 1, the gas outlet 12 is conveniently provided in the plug 9, and the inlet 11 is similarly provided in a plug 13 that is removed from the wall 6 when the product of the mixing process is to be drawn off from the bottom of the chamber 4.

The operation of the mixer of Figures 1 and 2 has been reproduced in the laboratory using rotor-discs 1 and 2 of some 60 mm in diameter and having eight grooves 8 that decrease in depth linearly from 10 mm at the outer edge to zero at the centre. It has been found that when the gap 3 between the discs 1 and 2 (for example, of between 0.2 mm and 5 mm) is filled to the optimum of about 50%, a motor drive of about 1 horsepower (745.7 W) at fifty revolutions per minute is adequate for successful execution of the mixing process on a rubber material of about 3,000 poise (300 N/m²).

The scroll faces 7 of the discs 1 and 2 of the mixer of Figures 1 and 2 are each made up of a multiplicity of spiral grooves 8 of short angular extent. However, a single spiral groove of one or more turns may be used instead; the form of a rotor-disc that is grooved with a single, plural-turn spiral groove 14 is illustrated in Figures 5 and 6. Where a plural-turn spiral groove such as the groove 14 is used, the force urging material to the centre is less than where a short-sector pattern of multi-start grooving, like that illustrated in Figure 2, is used. The groove-walls, which are generally outwardly-inclined, are preferably straight in section so as to allow easy flow of material in and out, and the bottom of each groove is preferably rounded for the same purpose. Split mixing may be enhanced by increasing the inclination angle of the groove-walls so that the grooving tends to a U-shape in section, whereas distributive mixing may be enhanced by using less rounded bottoms to the grooves.

A further possible form of rotor disc is illustrated in Figures 7 and 8. The form in this case is based on a plural-turn spiral groove 17 with outwardly-inclined walls comparable with that of the rotor illustrated in Figures 5 and 6, but has an inclined flat 18 milled into the spiral pattern to leave only two half-turns of the grooving. Distributive mixing is enhanced using rotors of this form.

Although both grooved faces 7 rotate in vertical planes in the mixer of Figures 1 and 2, it is not necessary for there to be rotation of both, or for any special orientation of them. A form of batch mixer (suitable, for example, for recycling waste in a rubber factory) in which just one of grooved faces rotates, and the rotation is in a horizontal plane, is illustrated in Figure 9 and will now be described.

Referring to Figure 9, the mixing chamber 20 in this case is within a cup-shaped member 21 that has spiral-grooving 22 corresponding to that of the discs 1 and 2 internally of its base 23 (as with the discs 1 and 2, the modified forms of grooving described with reference to Figures 5 to 8 may be used instead). The member 21 is mounted on the vertical shaft 24 of a motor 25 for rotation in a horizontal plane relative to the piston-rod 26 of a fixed hydraulic-ram 27. The rod 26 is axially aligned with the shaft 24 and carries a grooved disc 28 that is entered within the cylindrical side-wall 29 of the member 21. The disc 28 is held against rotation and is located on the rod 26 with its grooving 30 (with outwardly-inclined groove-walls) facing the grooving 22 at a small distance from the base 23 of the member 21 within the chamber 20.

Rotation of the member 21 relative to disc 28 under controlled drive from the motor 25 creates conditions for good distributive, split and shear mixing within the chamber 20 between the opposed groovings 22 and 30. The edge of the disc 28 has grooves or flutes 31 that are inclined in relation to the direction of rotation of the member 21 such that any material which during mixing tends to escape from the chamber 20 up the side-walls 29, is urged back into the space between the groovings 22 and 30. The inside of the side-wall 29 may also be fluted to the same end.

The disc 28 can be withdrawn from the member 21 so as to allow access for loading and emptying the mixing chamber 20, and can be returned to establish the desired small spacing between the groovings 22 and 30, by simple operation of the ram 27. A water-jacket 32 is provided for cooling the member 21, and a water-chamber 33 for cooling the disc 28. Water is circulated, through the jacket 31 and chamber 32 under control of a pump 34.

A choice of the proportions of the different modes of mixing effective in the mixing action, needs to be made. For example, it is important to have extensional-shear mixing for masticating rubbers and incorporating aggregated carbon black into them. This is achieved in the context of the outwardly-inclined groove-walls by having narrow gaps between the relatively-rotating surfaces and 25% or more of their surfaces in the form of lands. On the other hand, a machine with high distributive and split-action mixing is useful for uniform mixing of easily-separated non-reinforcing filler particles such as chalk or clay into polyethylene or plasticiser into polyvinyl chloride, or of the chemicals causing vulcanisation in rubbers.

When it is wished to comminute a fibre while mixing it into a polymer, a high proportion of shear and split-action mixing is desirable. On the other hand, when it is desired to mix easily-separated materials with minimum energy, a high proportion of distributive and split-action mixing is advantageous.

An important application of heavy-duty mixers is in the mixing of raw rubber with compounding ingredients. Some fifteen million tons of rubber are so treated annually. However, the current range of mixers have obvious deficiencies in that they do not have consistent mixing action and tend to overheat in spite of the provision of water cooling. The rise in temperature, for example by up to 150 degrees Celsius, can affect the material being mixed to the extent that the mixed product may vary significantly in its properties from one batch to another when a batch mixer is involved, and from one part to another of the output of a continuous mixer. Mixers according to the invention have been found to have a more consistent mixing action and related temperature characteristic so that the quality of the mixed product is less subject to variation and is more readily reproducible.

A train of two or more mixers with different mixing characteristics and operating under different mechanical and temperature conditions enables optimum mixing to be achieved during different phases of the overall process. For example, in the compounding of rubbers, a first mixer may provide a high proportion of extensional-shear mixing to masticate the rubber and disperse the filler. The second machine may then have a high proportion of distributive mixing and operate at lower temperatures to avoid the danger of "scorch" for vulcanisation chemicals, so as to enable finer control of viscosity and elasticity of the final compounded stock to be achieved.

Blending of rubbers is also a frequent operation in making compounded stock. However, polymers are inherently insoluble in each other and form domains of each rubber with different amounts of filler and vulcanising chemicals in each type of domain. Mixers according to the invention have been found to give more intimate mixes of smaller domains. Also, by mixing in the absence of oxygen and other radical acceptors, the rubber radicals can combine with each other to form block polymers. The block polymers act as solubilising agents for the individual rubbers and so as to result in a more intimate mix.

Incomplete mixing manifested as "fish eyes" often occurs on mixing polyvinyl chloride powder with plasticisers. This is obviated by using a mixer according to the invention with a high proportion of shear mixing; the resultant compound is particularly valuable for subsequent extrusions of clear tubing. The provision of a high proportion of shear mixing is also of advantage in the mixing of polyethylene with carbon-black fillers in that it avoids the otherwise often-experienced incomplete mixing which gives rise to non-uniform distribution (on a microscopic scale) of the filler and diminution of physical properties of the final product.

The mixers according to the invention can also be used to treat combinations of materials not treatable satisfactorily by conventional mixers. For example, they may be applied to mixing a high proportion of straw with polyethylene; mixes containing 75% straw, suitable for use as a moulding compound and having a density approaching unity, have been achieved. Also, they may be used for mixing rubber chips with polystyrene in a blend that is sufficiently intimate to enable a toughened polystyrene comparable with that achieved by adding latex, to be realised.

However, a significant application of the mixing technique of the present invention is in the context of mechanochemical reclaim of rubber by which a soluble material having good rubber properties is reclaimed from used rubber products by breaking down the network of crosslinked chains using mechanical forces imposed on the rubber at molecular level. When the crosslinks and chains in the network are reduced below the Flory gel point, the product becomes soluble in solvents for the rubber, including, for example, asphalt and bitumen.

A major application for this method is in the reclaim for re-use of rubber from used vehicle tyres. It may be used also for recycling the vulcanised rubber scrap produced in manufacture of tyres and other rubber products.

A soluble reclaim cannot be made by conventional mills or mixers. These can break up a tyre and the metal and textile can be removed by magnets and sieves. However, the rubber is left in a crumb or chip form with substantially all its crosslinks preserved. The crumb or chip, which is insoluble in the solvents for rubber, is accordingly insoluble in the rubber of any fresh formulation and its particulate form is preserved through to the second vulcanisate.

In accordance with the method of the present invention, the rubber for reclaim is supplied to the mixer in chip form mixed with a minor proportion of raw rubber or other material over 3,000 poise (300 N/m²). The repetitive shearing of the added, matrix material is accompanied by shear of the rubber chips. The shearing eventually results in a mechanochemical break-down of the chains and/or crosslinks of the rubber material of the chips to below the Flory gel point, so that it becomes soluble and goes into solution with the raw rubber or other added, matrix material.

The tread of bus, truck, off-the-road and aircraft tyres usually involves unblended natural rubber, and there is advantage in utilising just this part of the tyre for mechanochemical treatment. The tread rubber can be cut for processing as strips from the cap and/or undertread of the tyre circumference, or may be in the form of buffings, chips or crumbs from these two parts of the tyre. The rubber involved has good physical properties which are to a large extent preserved in the mechanochemical reclaim. This reclaimed material when incorporated to some 20% in a new rubber formulation, has been found to result in a material having a tensile strength in excess of some 75% of what would otherwise have been realised.

It may be advantageous also to reclaim just the rubber in the lower part of the tyre called the apex. This is enclosed in a textile ply that is covered by rubber on the outside, and whereas it is feasible to remove this ply by cutting prior to-treatment, the whole, including the ply, may be treated. In the latter case, the ply is broken up during the mechanochemical reclaim so that the resultant mix is a fibre-reinforced rubber product that has application, for example in the damp course of a building, where load-bearing and resistance to creep are desired.

The mixing method may also be carried out on the radial and cross-ply parts of the tyre with or without adjoining tread. This is especially so if the plies are of textile. Nevertheless, it can also be carried out with metal plies if the grooved rotor discs of the batch mixer are of a suitable alloy so as not to be damaged by the metal fibres. Moreover, the whole tyre after being stripped of the bead wire can be recycled, the flattened tyre being, for example, chopped into generally rectangular strips having a width of about 50 mm for feeding into the mixer for mechanochemical processing. The result is a solubilised rubber containing all the polymeric and non-polymeric constituents of the tyre with randomly oriented fibres from the broken tyre plies. If required, the fibres can be oriented by extrusion or calendering.

If the mixing is carried out in air, oxygen or ozonized air, the treated material is relatively soft and tacky, i.e. self-adhering, which may be advantageous in certain applications where adhesion of parts is required during product manufacture; it is the oxygen content that determines the degree of tackiness. If air, or more particularly, oxygen, is excluded from the mixing chamber by use of a nitrogen, argon, carbon dioxide or other inert-gas atmosphere in the mixing chamber, or is removed by evacuation, the treated material is relatively stiff and has a generally non-tacky surface; enhanced physical properties such as strength and modulus of elasticity, of virgin-rubber formulations can be achieved using the reclaimed material as an ingredient.

The mixing process can be readily controlled to give the optimum output product. In particular, the properties of the output product may be affected by the speed of relative rotation of the grooved parts within the mixing chamber. It has been found, for example, that when operating with an air atmosphere in the mixing chamber, the problem of the rubber material becoming so oxidised that it sticks to the grooved parts, can be avoided while still producing a satisfactorily solubilised product, simply by reducing the speed of relative rotation. The degree of shearing involved to produce solubility may also affect the physical properties of the resultant solubilised rubber.

The mixing carried out according to the present invention is mechanically and not chemically induced. Hence the different chemical reactivities of rubbers, for example non-polarity of butyl rubber compared with the polarity of nitrile rubber, is not of major consequence. What is of consequence is the strength of the crosslink or main-chain bond and the amount of shear which can be imposed on it to cause it to rupture. As most vulcanisates have sulphur-sulphur bonds in the crosslinks and these are weaker than main-chain bonds, it is explicable that all sulphur vulcanisates breakdown to become soluble at rates of the same order. The rates of breakdown correlate more with the bulk viscosities of the rubbers than with any chemical property and within a rubber type, for example a vulcanisate made from crepe rubber breaks down faster than one from SMR20 natural rubber grade.

However, the method of the invention is not only applicable to sulphur-sulphur crosslinked vulcanisates. Carbon-bond crosslinks of peroxide-cured rubbers (for example butyl rubber of inner tubes of tyres) and polyisocyanate crosslinks of polyurethanes also undergo breakdown by the treatment according to the invention to produce ultimately soluble materials.

Bitumens and asphalts can be used with the vulcanised rubber as matrices for shearing instead of, or together with, raw rubber. Non-polar rubbers such as styrenebutadiene and natural rubber are soluble in bitumens and asphalts and confer increased viscosity at ambient temperature and during abnormally hot weather. They also increase the adhesion of the bitumen or asphalt to aggregate and other solid constituents of road-marking composites, and increase the flexibility against cracking or composites in applications such as on bridge roadworks.