MASTICATION & MIXING

Processing is a general term which includes all the operations which are carried out on the rubber and which alter its physical shape or chemical composition.

The raw polymer, natural or synthetic can be softened either by mechanical work, termed action, by heat or by chemicals known as peptisers. The increase of plasticity or decrease of viscosity brought about by mastication and peptisers is permanent; by heat it may be permanent or temporary, depending on the nature of the polymer. When the rubber contains all the ingredients needed it is known as a compound or preferably a mix. If some ingredients have been withheld deliberately, the partially completed compound becomes a master batch. The master batch is converted to the compound by the addition of the withheld ingredients, which are usually the vulcanising or curing ingredients.

MACHINERY

Open Mixing Mill; The back roll is driven from the gear box or back shaft and the front roll from the back roll through roll end gears. The gears will come out of mesh on a singly geared mill if the nip ie. The distance between the rolls is set too wide furthermore, there is always a danger of stripping the teeth if the nip is opened so that the gears are working on the tips of the teeth.

Friction Ratio

The speeds of the two rolls are often different, the friction ratio depending upon the mills use. For natural rubber mixing a ration of 1:1.25 for the front to back roll is common. High friction ratios are used for refining compounds and even speed rolls on feed mills to calendars ie. When mills are used other than for mixing. For mixing some if the synthetic rubbers, a near even speed is best or even an immense friction ratio, ie. Less than 1.0. Other synthetic rubbers are very difficult to mix on mills, so internal mixers are often used for these.

COOLING

Two alternative types of cooling are used. The principal, one employees cored rolls ie. Water is sprayed on to the outside of an auxiliary drilled central core, with peripherally drilled rolls the water is circulated through a labyrinth of passages about 50 mm under the roll surface.

SAFETY

Mills are usually fitted with various safety devices, some to protect the operator, the other to protect the mill.

ANCILLARY ATTACHMENTS

Nills are fitted with a metal tray under the rolls to collect droppings from the mill and with guides or checks which are plates fitted to the ends of rolls to prevent rubber being contaminated with grease etc. Other attachments which may be added, depending upon the use of the mill are:

1. A mill apron, which replaces the mill tray and consists a belt running under the nip rolls up to the top of the back roll the belt returns to the mill any powders powders which fall from the nip.

2. A stock blender, which is a device for blending the rubber mechanically instead of depending upon the operator.

3. Cutting knives mounted against the roll to produce strips far feeding other equipment or for removing compound from the mill.

4. Scraper blades to remove soft stocks.

INTERNAL MIXERS

Because of its versatility, rapid mixing and large throughout, is tending to replace the mill as batch mixer. The details of internal mixer are not iscussed here, as it has no importance for small scale rubber industries.

MASTICATION

The mastication stage in processing is, in general only applied to Natural Rubber and the internal mixer is an efficient machine for this. The energy can be put into the rubber quickly the shorter the masticating cycle; the limitations to shortening the mastication cycle are the time to load the machine and the handling of the rubber after discharge from the mixer.

The shape of the rubber being loaded and the bulk density8 of the rubber affect the loading of the machine. The modern 331/3 kg bale of natural rubber is ideal for loading whole into the type of mixer or alternatively rubber cut from larger bales to the same shape. The low bulk density of certain crepes means that to obtain similar batch weights, larger volumes must be loaded into the machine than for other higher density rubbers. These low density rubbers can take longer to load than to masticate and hence are at a economic disadvantage in processing.

The handling and storage of masticated rubber in sheet form is not very satisfactory unless the viscosity and dismension stability of the rubbers can be assured. Mastication costs money it reduces the viscosity, but at the same time sults in a reduction in many physical properties of the finished vulcanisate. If the viscosity reduction is obtained by using softeners then a saving in compound volume cost results. The reduction in physical properties of the vulcanisate is probably no greater from using softeners than from extra or separate mastication of the polymer.

With the availability of constant viscosity and low viscosity natural rubber and of synthetic rubbers of suitable viscosity for direct mixing, the need for mastication may well be eliminated in due course.

Mill Mixing

In operation, the ingoing side of the nip is at the top of the rolls and rubber is added to the nip. A band of rubber comes through the nip and is formed around the front roll.

Varying degrees of difficulty arise in forming these bends, depending upon the polymer. Natural rubber if not formasticated is difficult to band initially, but after a time, depending upon mill nip and temperatures, a band form. This starts with many holes and after continued working, the band becomes smooth. Powders process oils etc. are now added into the nip and any which fall into the tray under heath the rolls are returned to the nip until they are absorbed by the rubber.

The compound is blended to homogenise the additives and give adequate distribution of the ingredients. Mills generally give good ingredient dispersion.

Mills have a fairly high power in put. A 1.5-m mill ie. With rolls 105 m long will handle about 50 kg of rubber at sp.gr. 1.0-1.2 and will require 45-75 kw A 2.1 m mill handle 100-150 kg of compound and needs a motor of 115-225 kw. The majority of electric power is converted to heat so, to prevent compounds vulcanising on the mill with the heat produced to rolls are water cooled.

INTERNAL MACHINE MIXING

With internal mixers having low rotor speeds and low ramthrust, a batch of compound can take from say 5 min. to 10 min. to mix. An error of ½ min. does not result in a marked temperature increase in the compound when dumped. However as mixing cycles are reduced it becomes increasingly important to control the time because this same time error on a short cycle of say 2 min. Is far more important than on a longer cycle of 10 min.

MASTERBATCHING

It is generally accepted that masterbatching improves the physical properties of those compounds where a high degree of carbon black dispersion must be achieved. Where a master batch can be carried from that used in the compound eg: the masterbatch recipe may (a) be the same as the compound recipe but without curatives; (b) contain only rubber, filler and some softeners, or (c) be richer in filler than the final compound.

Each of these types of masterbatch is capable of improving black and filler dispersion in a tyre tread or other reinforced compound, but type © has been formed less satisfactory than the other twc. The problem is the dispersion in the hard filler masterbatch of the relatively soft polymers added in the next stage.

CONVERSION OF MASTERBATCHES

Masterbatches can be converted to compounds by the addition of curatives using either a mill, an internal mixer or a continuous mixer. A 200 LT internal mixer can use 20 rev/min. 30 rev/min or 40 rev/min for this operation, depending upon the cooling and associated handling facilities. The curing ingredients themselves may be masterbatches, there is considerable divergence of opinion in the industry as to which system gives the best distribution of ingredients.

Reference: rubber Technology & Manufacture by
- C.M. Blow