Advances in rubber granulation.

Rubber granulation has important applications in processes such as dissolving, extruding and mixing. In these processes, unvulcanized rubber which is either a compound or raw material can be used. There are also techniques for reclaiming material and generating secondary use rubber using a wide range of size reduction equipment.

The use of granular or particulate rubber has been limited up to now by two factors. The first has been the conventional method of preparing material. The second has been the development of processing technologies that exploit the benefits of granular rubber.

Conventional granulating equipment

Traditionally, rubber granulating has been a dirty and noisy operation. For these health and safety reasons alone, it has been avoided at all costs in most factories. The operation is also labor and capital intensive, usually requiring a guillotine feeding one or more than one granulator, depending on the required granule size. All these items of equipment need to be operated and maintained. Technically, the conventional design of granulators imposes restrictions in terms of the granule size that can be produced and the effects of heat degradation on the rubber due to friction in the cutting action.

The uses of granular rubber

The main applications where granular rubber is currently used include dissolving for the manufacture of adhesives and sealants, paints, oil lubricants (viscosity index improvers), modified bitumen and modified polymers (high impact polystyrene). Extruders for profile, injection molding or compounding can be fed with a granular feedstock. With mixing, a continuous mixer (such as MVX) requires a granular feedstock, which can also be used for open or internal mills.

The materials which are granulated are all types of natural and synthetic rubber in bale form, compound (typically in sheet form) and semi-cured rubber, such as tire cord (friction) and flash.

In-house or custom granulating?

Where granulating is required, one route is to use the services of a custom or trade processor. Although convenient in many cases, this can be an expensive option, substantially adding to material costs. A minimum batch size may represent many weeks of production. With the constant drive to reduce stock levels and the pressure of just-in-time (JIT) production methods, this can be a disadvantage. There can be problems with shelf life, since material will want to agglomerate despite the use of a partitioning agent. Also, there are risks of cross-contamination as different materials pass through the same equipment.

These points are not intended as criticisms of custom granulating, which is often performed to the highest quality standards. They are merely the concerns of any production manager faced with such a choice.

When using the latest granulating techniques, the advantages of an in-house operation are:

* A lower processing cost;

* material is granulated only when required, which minimizes storage and agglomeration problems;

* improved material utilization due to less wastage; and

* integration of the granulation process into a comprehensive materials handling system, improving overall production control and product quality.

Granulation techniques

The latest techniques are embodied in the design of rubber granulators, such as the Blackfriars Model 525. A key feature is the `lantern' rotor. This has a hollow construction with three knife beams supporting tangentially mounted rotating (rotor) knives. The design minimizes the frictional effect of working material and producing heat, rather than cutting it. Because of this, the heat generation is low, being typically only a 10 [degrees] C rise in temperature. Another important feature is the expansion box at the rear of the cutting chamber. This accommodates the large volume of material rapidly produced when a 35 kg bale is digested into the granulator. At this moment, the bale is cut like a loaf of bread by the hollow rotor acting as a high speed guillotine. The slices then fill the expansion box before falling back and being granulated between the rotor knives and the stationary knives.

With this principal, a suitably designed machine can process a 35 kg bale, to give granules down to a size of 3 mm to 4 mm, if necessary, in one operation using only a 55 kw or 75 kw (75 hp/100 hp) drive. The output from the machine varies with granule size. A small granule has a high surface area of cut for a given mass, and so is produced at a lower rate than a large granule. The granule size is varied simply by using different screenplates which are positioned below the lower half of the rotor. These can be quickly accessed and changed.

Energy efficiency for this type of equipment is good. The specific granulating energy to produce 10 mm granules is typically only 0.04 kwh/kg.

A partitioning agent can be added during granulation. Depending on the type selected, this both assists the cutting action and provides a coating on the rubber to prevent agglomeration after granulation. There are several types that are used, and the choice will depend on the application, the type of rubber and the required granule size. Sometimes no partitioning agent is required. The types of agent are:

* Dry powder, such as chalk, starch, zinc oxide, PVC, etc.;

* wet powder such as a slurry mix, or more preferably an aqueous dispersion;

* water soluble, such as a liquid anti-tack; and

* water only.

Maintenance for a machine such as the Model 525 is minimal. Knife changing is the only major task, and this is required typically every 300 to 1,000 tons processed. Once changed, knives can be resharpened many times. Current development in knife construction and materials is further extending the interval between knife changes.

The granulator as part of a system

A rubber granulator can lend itself well to automation. Bales of rubber can be depalletized manually, with a power assisted lifting device or by a robot onto an infeed conveyor.

The length of the conveyor and the granulating rate will dictate the duration of unmanned operation. If necessary, a metal detector is positioned on the conveyor to ensure no stray metal enters the granulator. A suitable design of sound attenuating enclosure will reduce the noise within the working area to the required legal levels.

The granulator operates on a cycle. The cycle time is determined by the required output and particle size, together with the type of rubber and partitioning agent. The cycle is initiated by a bale entering the feed hopper. This then controls the infeed conveyor and partitioning agent. For dry powders, a volumetric screw feeder is used, and for liquids there is a pressurized dispensing system.

Granule take-off is preferably by a lean phase pneumatic extraction system. In addition to conveying the material, it has the advantage of drawing air through the granulator. This cools the material, increases the output and produces a more consistent granule size. Where a liquid partitioning agent is used, the material is conveyed through a fan into an air separator. If powder is added, the system is indirect, with granules discharged from the air separator through a rotary valve. The exhaust air and excess powder are then drawn through a filter unit, and the powder which is collected can then be reused. With a suitable conveying system, one granulator at floor level can feed several dissolving tanks or storage vessels.

With many items of equipment, which are all interrelated, a programmable controller is used to sequence, time and monitor the system.

Experience has shown that a bale granulating system of this type can produce with one operator in four hours what a conventional guillotine and granulator might give in eight hours with two operators.

Future developments

Extending the applications for granular rubber will depend on process developments. In some applications, there are obvious advantages, such as dissolving rubber where there is an optimum granule size to give the fastest dissolve time with lowest solvent temperature.

The application with the greatest potential is for internal mixing. Granular rubber could offer reduced mixing times and energy consumption. It also facilitates the use of fully automated materials handling and batch weighing equipment.

Most importantly, it gives consistent mixing. When rubber in a bale form is dropped into an internal mixer, the initial mix pattern may vary from one batch to another. If the rubber is in granular form, this pattern will always be consistent from one batch to another. Also, natural rubber can vary from one bale to another. Pre-blending the granules improves consistency.

Other size reduction equipment

There is a wide variety of size reduction equipment for rubber, much of which is used for material reclaim and generating rubber for secondary uses. The types of equipment available include:

* Heavy duty granulators, used for scrap rubber such as tire tread strippings;

* multi-knife granulators which are particularly suitable for fine granulating scrap down to 1 mm;

* sound deadened granulators, ideal for low output applications where rubber can be pre-cut by guillotine;

* dicers and band granulators for producing a regular shaped granule from a compounded strip;

* disc grinding mills, intermeshing two disc mills for grinding vulcanized scrap to 30 to 40 mesh crumb; and

* attrition mills, suitable for unvulcanized rubber to be ground down to 500 microns.

Conclusion

Granulating technology has advanced considerably in terms of equipment design and ease of use in the production environment. The challenge now exists for the rubber industry to utilize the benefits of granular rubber within its processes and to improve automation.