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Articles - Rubber Technical Papers


Rajammal, Rubber Technologist

Vulcanisation processes are divided mainly in to two groups. The first consists of moulding methods which involve an integral shaping operation which is completed prior to the onset of crosslinking. The second includes a number of techniques used to cure a previously formed product. During vulcanisation, externally supplied heat flows in to the rubber at a rate controlled by efficiency transfer properties of the rubber. Changes in temperature within a rubber product tend to give a non-uniform state of cure and result in the properties of rubber at the surface of a product being quite different from those at the centre. One of the main objectives of selection and optimisation of vulcanisation processes is for achieving a uniform state of cure with viable production rate.

Vulcanisation like all chemical reactions naturally gets faster as the temperature rises. The velocity of reaction is approximately doubled or the reaction time is approximately doubled or the reaction time is approximately halved for every 8 - 10°C by which the temperature is raised. For the purpose of streamlining operations, it is of course very desirable to use the highest possible vulcanisationo temperatures in order that correspondingly short vulcanisation times may be employed. However, the extent to which this can be done is limited for various reasons. For example, compounds with a very broad vulcanisation plateau permit high vulcanisation temperature, the opposite is true of compounds with a narrow plateau. The greater, the thermal stability of the rubber, the higher the vulcanisation temperature which can be tolerated by the material without its properties suffering. As the mechanical properties of vulcanisates depend on the vulcanisation temperature, in some special case, there will be a temperature limit, which should not be exceeded.

Often when the articles are vulcanised in a press and definite dimenstios have to be adhered to with definite moulds, the vulcanisation temperature cannot be changed because a different temperature would lead to a different degree of shrinkage. Where vulcanisation in hot aire is concerned, the upward temperature limit is fixed by the oxidising action of the oxygen present. Only in certain cases, therefore, can the vulcanisation temperature be raised to such an extent that extremely short vulcanisation times can be used. One of these cases is the continuous vulcanisation method.

I. Vulcanisation in a Press (Moulding methods)

Vulcanisation in a press involves the transfer of heat from the metal of the mould to the article to be vulcanised. This subject is dealt in detail in the chapter, "Different methods of moulding".

II. Vulcanisation by methods other than moulding

It is convenient to group these vulcanisation methods in to two:

1. Batch curing methods in which one batch of material or components is cured at a time and

2. Continuous vulcanisation methods which involves continuous vulcanisation of lengths of material.

II. 1. Batch Curing methods

II. 1.1 Autoclave or steam pan

The autoclave is used for the vulcanisation of extrusions, sheeting and components that are of an unsuitable size for convenient mould curing. Examples of these compoments are rollers and hand formed goods. It is also used for the secondary vulcanisation of large moldings to improve press and mould utilisation. The temperature and pressure required for cure are achieved by the use of steam.

As autoclave is a cylindrical pressure vessel, normally used in the horizontal position. Autoclaves can be of two main types:

1. A jacketed autoclave consists of two large pressure vessels, one inside the other,
Constructed, so that it is possible to fill the inner vessel with an inert atmosphere, such as nitrogen (to prevent polymer degradation through oxidation) and the outer vessel with high-pressure steam to act as the heating medium. The inert gas eliminates oxidation and permits brightly coloured articles with a good surface finish to be produced. It is also referred to as 'gas curing'

2. An unjacketed autoclave consists of a single cylindrical pressure vessel. Steam
Is introduced directly in to the autoclave and may cause condensation marking on the product. Preventive measurers usually result in a dull surface finish.

One of the advantages of an autoclave is that a large number of components can be vulcanised at one time, provided that the curatives used in different materials do not interact. It is usual to support the extrusions and small components by embedding them in talc on trays. Products formed on mandrels and sheeting are cloth wrapped. To prevent distortion. In some instances clamped up mandrels are used.

After loading the autoclave with the components, it is closed. Steam is then very rapidly applied and th e vulcanisation completed for the prescribed cycle. The pan is then cooled while soill under pressure, otherwise blistering of the product may occur. Bulky components usually require a "stepped cure" which involves raising the auto clave temperature to the maximum vulcanisation temperature through a series of steps to give a relatively uniform temperature distribution through the product during heating. This technique is used when bulky mouldings are given a second cure in the autoclave after being vulcanised to the point of dimensional stability in the mould.

Autoclaves are pressure vessels and need to comply with the general regulation for pressure vessels which require regular testing and fool proof interlock mechanisms to prevent the door from being opened while there is a pressure in the autoclave. Even very low pressures can exert considerable forces over the door area.

II.1.2. Hot Air Ovens

The majority of rubber compounds, if heated at atmospheric pressure sponge as a result of dissolved air and moisture or degrade owing to oxidation. Ovencuring, is not, therefore, generally applicable as a curing procedure. It is however possible in the case of certain heat resistant polymers eg: silicone and fluorocarbon rubbers for post curing. The hot air post cure serves two purposes.

1. The first is to continue the crosslinking process to improve the physical properties of the rubber product.
2. The second is to drive off volatile materials from components intended for service at high temperature by raising them above their service temperature. Potentially, toxic fumes are disposed of in this way and hence it is important to vent the oven directly to the atmosphere and to ensure that fumes do not re-enter any of the work areas.

The ovens require precise temperature control varying from ±5°C to ±1°C.

II. 1.3. Water Curing

Water curing techniques usually employ an autoclave as a pressure vessel, heating being either from a steam jacked or from steam injection. The autoclave is first loaded with the components, then filled with cold water and steam is then applied and vulcanisation is completed for the prescribed period. This method curing finds application in the vulcanisation of very large hand build rubber lined machinery such as pumps, pipes and chemical tanks. The water supplies the pressure for adequate adhesion of the rubber to metal in addition to acting as the heat transfer medium.

II. 1.4. Lead curing

Moulded length hose is cured by this technique. It involves the formation of a lead sheath around the hose. The sheath of lead is made to surround the cover of the hose by passing through a lead is made to surround the cover of the hose by passing through a lead press or a lead extruder. The lead-sheathed hose is next wound on to a large drum. The lining of the hose is now filled with water, pressure applied and the ends of the hose and leads are clamped. The drum and contents are placed in large vulcanising pans (steam autoclaves) and curing is carried out. The water inside the hose expands and becomes superheated.

The hose is pressed against the lead which not as a mould and hence the name of the process. If the inside of the lead is fluted, then such a finish is imparted on to the hose. If smooth dies are used to form the lead, then of course, a smooth finish is produced. After cure and cooling, the clamps are out from the hose by slitting along its length in a stripping machine. The cured hose is coiled up, tested and inspected and the scrap load is returned to the melting pot for reuse.

II.1.5. Cold Curing

Thin rubber products can be vulcanised by immersion in a carbon disulphide solution of sulphur chloride or by exposure to its vapour.

II.2. Continuous Vulcanisation

II.2.1. High pressure steam tube

In steam tube vulcanisation (often called the cv process), the extradite is unsupported and is subjected to tensions which could cause total failure of a homogenous extradite. For this reason, the technique is only suitable for composite products having a continuous non-rubber reinforcement which remains relatively inextensible at the vulcanisation temperature. This finds two major applications viz., in cables and hoses.

When cable is covered with rubber, the wire core provides a means of drawing the rubber cable. The cable passes from the extruder in to the steel tube which high temperature and high-pressure steam (15kg/cm² or higher). Vulcanisation takes place in this tube. This tube, approximately, 75cm. Long is sealed to the extruder at one end and the other end is sealed with rubber or waterseals to retain the steam pressure. The length of the tube and hence the cure is determined by the line speed of the cable together with the pressure and temperature of the steam in it. The cable passes through a series of mechanical seals or a pressurised water seal to atmosphere. Three types of steam tube are used, inclined, catenary and vertical. The choice between them is determined by the tension needed to hold the product away from the tube wall. The average length of a vertical tube is 50 m. compared with 60 m. for the inclined tube and 100m. for the catenary tube.

II.2.2. Hot Air Tunnel

It is possible to use a single gas heated tunnel for the vulcanisation of cellular extrusions and carpet underlay, which do not have to meet tight dimensional tolerances.

The method is very cheap to run, particularly, if several extrusions are traversing together, but the cost of installation of this type of tunnel is very high. Hot air tunnels usually electrically heated are used for the vulcanisation of silicone rubber products, in which case the exit temperature is 250-300°C.

II.2.3 Fluidised Bed

The principle upon which the Fluidised bed operates if the floation of particles of an inert material such as sand or very small glass balls (ballotini) in an air stream, giving fluidity to the system. Two types of fluidised beds are available, one of which operates at atmospheric pressure and one which operates at pressures upto approximately 0.7 Mpa. The former type is used for homogenous or reinforced extradites, whereas the latter has been developed specifically for cable and hose.

Fluidi sed beds are on line vulcanisation processes designed to accept a product direct from the extruder. The vulcanisation chamber or troug contions a bed of heated fludised particles, through which continuous extruded rubber sections, may be easily passed without resistance. The bed of particles of ballotini, a sand like materials is heated initially by steam and once the operating temperature has been obtained, air is substituted for steam and the heating is continued electrically. This causes the particles to become suspended on thesteam/air stream and separated from each other. In this state, the bed takes on all the attributes of a liquid, objects can float on the surface of the bed or sink depending on their density and convection currents can be set up in the bed. Thus externally applied heating will be transferred to objects in the bed by convection heat transfer. The bed provides support also to the extrudate, thus ensuring minimal distortion. The product to be vulcanised floats near the surface of the bed and cure takes place in a short time. The cure time of a fluidised bed extrudate is a function bed length and have-through speed. Operating temperature upto 240°C may obtained. The length of the bed and temperature can be varied to suit specific manufacturing conditions. To prevent porosity of the compounds, when vulcanised at high temperatures, desiccants (caloxol) may be added in the compound during mixing.

As for any vulcanisation, venting of fumes is essential and most fluidised beds are enclosed ensuring that all the fumes generated are drawn off and vented to the atms. The fluidised bed cheap, used for many types, and efficient for long runs.

II.2.4. Molten Salt Bath (Liquid Curing Method)

In principle this method is applicable to solid and cellular extrusion. Molten salt batch an cutectic mixture of inorganic salts, usually potassium nitrate and sodium, nitrate, at a temperature of 150-250°C. Salt baths are very similar in external forms and arrangements to the fluidised bed. However, the salts are much denser than rubber compounds requiring the use of rollers or a submerged conveyor belt to hold the extrudate under the surface of the molten salt. Furthermore, the density of the salts obviates the necessity for the use of the desiceant necessary with fluidised bed. Any salt adhering to the extrusion is removed by means of a water wash tank.

II.2.5. Microwave curing

Microwave curing has the advantage of providing uniform and rapid heating
Throughout a product. The majority of microwave units are designed to accept extrudates, but they are also used to preheat conveyor belting prior to press vulcanisation and drum. Vulcanisation. In the hose in the hose industry, its use is limited for curing of only non-metallic hoses because of the metallic interference.

Microwave vulcanisation units consist of a microwave section followed by a hot air section, the function of the latter section is to maintain the temperature established by microwave heating. The whole system is about 7 m. in length and a conveyor belt mode from materials not susceptible to microwave heating is used to transport the extrudate through it.

In this connection, microwave curing is neither better nor worse than LCM and fluidised bed techniques. The system is more clean, heating is more uniform and do not require the latter cleansing operations of either the LCM and fluidised bed techniques.

Non-polar rubbers such as NR, SBR and EPDM are poor absorbers of microwaves and becomes only viable propositions for microwave vulcanization when mixed with carbon black or with a polar polymer such as NBR or PVC. Two further problems are dimensional stability and oxidation. During vulcanisation, the extrudate is unsupported and can destroy after heat softening, simply by the effect of itsown weight and it will then be vulcanised in the distorted shape. Surface oxidation may occur with NR and SBR due to the high temperature, although modern antioxidants provide sufficient fume extraction system is required and microwave emission levels are to be monitored at regular intervals.

II.2.6. Continuous Drum Cure

Continuous Drum Cure is used for the vulcanisation of sheet material, flooring and belting. It is an offline process due to the through put rate being much lower then that of calendering or spreading, Two machines in common use are the Berst off and Roto Cure.

The sheated material is fed between a large heated curing drum and a tension steel belt which holds the sheet around the circumference of the drum. And imparts a good surface finish to the product. On emergence, it is fully cured. The speed of rotation of the curing drum is variable and is adjusted to achieve the full vulcanisation of the material in the time of its passage through the machine.





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