Rubber is the name given to elastic, high – molecular compounds capable of changing their shape (deforming) when acted upon by an external force of rapidly resuming their orginal shape when the force is removed. The elastic properties of rubber are due to the fact that the linear macromolecules of the material which normally are of a bent, curled or curved shape easily stretch out on contact under the effect of an applied force; when the force is removed the molecules return to their initial shape and length, maintaining a constant volume.
Rubber is a controlled commodity in India an its production sale & manufacture have to be carried out under licence. Every holding in the country must be registered.
Rubbers are classified into natural rubber and synthetic rubber, the ever increasing demand for rubber and the dependence of many nations on the foreign supplies of natural rubber forced the invention of synthetic rubbers which again can be classed as general purpose and special purpose rubbers. This division is purely nominal. General purpose rubbers are used for making tyres and other rubber articles of mass consumption. Special rubbers can operate under severe conditions for instance at low or at high temperature, resist highly corrosive media (acids, alkalis, oxidants, solvents, etc.). The student will observe that since India is a producing country of natural rubber, synthetic rubber is not much used and we have only few factories.
Nearly 70% rubber used in U.S.A. is synthetic. Synthetic rubber is artificial rubber. Compounded manupulated and vulcansed in the same way as natural rubber. Both the products are alike in elasticity and mechanical properties. While synthetic rubber has ozone resisting properties, it is a poor electrical insulating material.
Rubber has become a material of tremendous economic and strategic importance and in an excellent barometer of industrialisation of the nation. This will be evident from the fact that per capita consumption of rubber is 14 Kg. in U.S.A and 8 Kg. in UK and Germany while it is less then ½ kg. in our country. Transportation, chemical, electrical and electronic industries are all major consumers of rubber. M/S. B.F. Goodrich & Co, of U.S.A claim to make no less than 35000 distinct articles from the material. Its unique importance is due to its physical properties of flexibility combined with great tensile strength.
NATURAL RUBBER. It is got from the plant family Heva Brasiliensis from its latex. Originally Brazil was its home and later introduced to Asia and the tree is now grown in tropical regions of Asia, Africa and America. The rubber tree is sturdy, quick provided and tall and grows on many types of soils provided the soils are deep and well drained. A warm humid equable climate (21*C to 35*C) and a fairly distributed annual rainfall of not less than 200 cm are necessary for the optimum growth of the plant. The trees adopt under slightly varying conditions. The trees have a well developed tap root and laterals. The bark on taoping yields latex. The leaves are trifoliate with long stalks. Pollin- is by insects and the fruits mature in about 5 months time after fertilization. The fruits burst when matured and seeds are scattered upto 15 to 18 metres.
Level land as far as possible should be selected for raising nursery. Rubber plantation in India are situated mostly on slopy and undulating lands. There are well defined agronomic principles for preparing the land, clearing, lining, terracing, drainage and planting distances, weed control which are not of relevance for our study.
Cover crops like leguminous creepers are established and maintained in rubber plantations for the purpose of conserving the soil or maintaining the soil structure and improving or maintaining the soil structure and fertility and to prevent soil erosion.
Latex is obtained from the bark of the rubber tree by tapping. It is a process of “controlled wounding” during which thin shavings of bark are removed. Latex is a hydrosol in which the dispersed particles mainly rubber besides Lutoid and Frey Wyssling are protected by a complex film. Latex vessels are filled with viscous latex under hydrostatic pressure. The latex spilt and or over-flowed on the ground (earth scrap) gets dried and is also collected as a scrap. Normally 10-20% of the total crop constitute the tree lace, shell scrap and earth scrap. It will be generally economic to begin tapping when 70% of the trees in the selected attain the standard girth. Under certain conditions, it takes an average of 7 years to reach this stage.
With a view to impart knowledge on scientific method of tapping a Tappers Training School was opened by the Rubber Research Institute of India in 1965.
Natural rubber was an all purpose rubber and due to its acute shortage many synthetic rubbers have been developed and marketed. They are classified under two groups, viz, general purpose rubbers and specially rubbers. The following are the different types of synthetic rubbers.
1. Stryrene butadiene 8. Cis-polysoprene
2. Butyl 9. Polyurethanes
3. Neoprene 10. Acrylates
4. Nitrile 11. Hypalon
5. Silicone 12. Fluro hydrocarbons
6. Thiokol 13. Ethylene propylere
7. Cis-polybutediene 14. Ethylene propylene terpolymers
Styrene buratine rubber (SBR) called ‘Synaprene’, the raw materials used are benzene and ethyl alchol which are obtained indigenously from steel plants and sugarcane industry respectively.
The rated capacity of the plant at Bareilly is 30,000 tonnes of SBR per year. The plant is at present producing five grades of SBR for the manufacture of synthetic rubber, SBR occupies an important place due to its general applicability. They have also started producing nitrile rubber called ‘Chemaprene’.
PROCESSING OF LATEX INTO RUBBER
The crop from the rubber tree is latex, a milky liquid which is obtained by tapping. The latex that flows out from the rubber trees on tapping is channelled into a container (generally coconut cups in kerala) attached to them. Latex is then transferred to clean buckets 2 to 3 hours after tapping.
The latex which gets dried up on the tapping panel (tree lace) and the collection cups (shell scrap) and the latex spilt and or overflowed (earth scrap) are all collected and in fact these scraps constitute 10 – 20% of the total crop.
The latex are highly susceptible to degradation due to contamination on keeping and hence will be processed immediately.
The important forms in which the crop from rubber plantations can be stored and marketed are as follows.
1. Preserved latex and latex concentrates.
2. Dry ribbed sheet rubber.
3. Dry crepe rubbers.
4. Dry technically specified block rubbers
Latox can be processed into any of the above forms but the scrap referred earlier can be processed only into crepe or block rubbers.
PRESERVING INTO PRESERVED OR LATEX CONCENTRATES
Latex is a white or slightly yellowish opaque liquid with a specific gravity varying between .974 and .986. It contains also a variety of non rubber constituents both organic and inorganic in addition to rubber. The proportion vary with clones, nutrition, climate etc. in general the composition is as follows :
Rubber 30 – 40 % Resin 1 – 1.6 %
Protein 2 – 25 % Sugar 1 – 1.5 %
Ash .7 – .9 % Water 55 – 60%
Fresh latex is slightly alkaline or neutral but becomes acidic rapidly and gets coagulated on keeping. Hence fresh latex cannot be kept for long without pre-coagulation.
An anticoagulant is a material added to latex to prevent pre-coagulation before it is processed. Anticoagulents used are Ammona, Sodium Sulphite and Formalin.
A preservative is a chemical which when added to latex can prevent bacterial action in it and stabilises it. While Ammonia is the most popular preservative, it is also used with other chemicals. Field latex preserved with a suitable preservative is termed Preserved field latex. There is no much scope in processing field latex into preserved field latex unless there is a local demand because the cost of packing and transportation will be very high per unit quantity of dry rubber compared to latex concentrates. There are 3 major methods of processing latex into preserved latex concentrates.
1. Latex concentrates by evaporation.
2. By creaming (chiefly used by small scale manufactures.)
3. By centrifugation.
According to rule 48 of Rubber Rules 1955 (as amended) preserved latex and latex concentrates shall be graded and marketed in conformity with such standards as are specified by the Indian Standard Institution from time to time.
DRY RIBBED SHEET RUBBER
Latex is coagulated in suitable containers into thin slabs of coagulam and sheeted through a set of rollers consisting of a grooved set and dried to obtain ribbed sheet rubbers. Depending upon the drying method, sheet rubbers are classified into two
(1) Ribbed smoked sheets,
(2) The air dried sheets (pale amber unsmoked sheets)
The major qunatity of natural rubber produced in this country is marketed in the sheet form and is the oldest and simplest method.
For processing into sheet rubber, latex is brought to the processing factory before precoagulation sets in. Where the latex is prone to precoagulation a anticoagulant is used. A few drops of solution of anticoagulants are added to the collection cups at the tire of tapping if found necessary. The rest of the required quantity of the solution is added into the collection buckets when they are half full. They are never poured into empty collection buckets.
Latex brought to the factory is strained through 40 and 60 mesh sieves. The volume of latex is measured with a standard vessel and calibrated rod. The dry rubber content (d r c) is found by a special type of hydrometer. For accurate determination of d r c laboratory methods are employed.
Latex is diluted in bulking tanks to a standard consisting of ½ kg. of rubber for every 4 litres of diluted latex. The diluted latex is allowed to stand in the bulking tank for a fixed time (15 to 20 minutes) to sediment the heavy dirt particles. To prevent the surface blackening of the coagulem a small quantity of sodium bisulphide is added in the bulking tank. The diluted latex is drawn out from the bulking tank without disturbing the sedimented layer of impurities into coagulating pans or tanks.
Acetic acid or Formic acid is generally used for coagulation. The acid requirement slightly change under varying conditions like the seasonal change.
After coagulation the coagulam is removed from the pans or tank and thoroughly washed in running water. They are sheeted either in a sheeting battery or smooth roller to a thickness of 1/3 cm and finally passed through the marketing rollers. While sheeting, the coagula are continually washed. The sheets are again washed rubber can be prevented by treating the freshly machined sheet in a dilute solution of para-nitrophenol or santobrite. The wet sheets are allowed to drip on reapers arranged in a well ventilated dripping shed. The sheet should not be exposed to direct sunlight to avoid development of tackiness.
The sheets after 2 to 3 hours of dripping are put in the smoke house, where the temperature is maintained between 100 – 140*F. Sheets are dried gradually whereby blisters are avoided. In addition the croeosotic substances present in the smoke prevent the mould and yeast growth on smoked sheets. On the first day the sheets have to be smoked at a humidity and subsequently (2nd to 4th day) they should be dried at a higher temperature (not exceeding 140*F) with low relative humidity. Thus smoke houses will have 2 chambers. If there are no separate chambers, first day drying is done by placing the sheets or reapers at the lowest region and taking them to upper region on subsequent days. Thus the essential feature of a smoke house and a drying chamber, a furnace outside it and a flue connecting the furnace to the chamber. While 4 days of smoking are generally sufficient, during rainy seasons 5 to 6 days are required. The dried sheets are carefully inspected and graded according to standards published by the Rubber Manufacturers Association in Washington. (RMA 1, 2 etc)
The sheets after grading are packed in bales of known weight and grade.
When sheets are dried in a shed or tunnel in hot air instead of smoke, they are called air dried sheets. Sodium bisulphite is used which prevents enzymic discolouration. These sheets may fetch a better price as they can be used as substitutes for pale latex crepe.
DRY CREPE RUBBERS
When coagulated latex or any form of scrap is passed several times through a minimum of 3 mills with heavy rolls, crunky lace-like rubber which when air dried is called crepe rubber. There are different types of crepe depending upon the starting materials (latex or scrap). The grading of crepe is also done visually as per the standards published by the Rubber Manufacturers Association, Washington. This method or manufacture has the obvious advantage of handling all the different forms in which the crop is collected from a plantation. However a relatively high investment is required and hence it is uneconomical to have such a factory for small holdings.
Dry ribbed sheet rubbers and various forms of crepe rubber packed in conventional methods were found to be ineffective in world markets to compete with technically specified synthetic rubbers which started coming after the fifties. Generally the operations are (1) size reduction (2) de-watering (3) dirt removal (4) drying (5) baling and specified and the rubber is called Technically specified block rubber. This method of processing is a revolutionary departure from the conventional methods in preparation and presentation in small high density bales of rubber having uniform quality for the preparation of special rubber product with constant viscosity, low viscosity etc.
Tyre manufacturing industry consumes over 65% of natural rubber produced. The 2 basic requirements are (1) large volume of supply (2) technologically attractive product at an economic price. It may be of interest for the student to note that powers exercisable by the Indian Standards Institution (certification marks) Act 1952 shall in relation to rubber and rubber products manufactured in India be exercisable by the Rubber Board having its Head Office in Kottayam. There is also a Rubber Research Institute.
PROCESSES IN RUBBER WORKS
The plantation rubber after processing arrives in smoked sheets. The work carried out in Rubber works may roughly be divided into 3 basic stages
(1) Preparation of the rubber Composition
(2) Forming the article and (3) Vulcanising them.
A main condition for producing high- grade rubber articles is uniform distribution of all the ingredients in the composition. To make the mixture more plastic and thus facilitate blending the rubber with aring the rubber composition, conditions should be observed (temperature, mechanical effects. etc) under which the rubber macromolecules do not suffer any changes and the other components of the mixture do not capacity, enclosed mixers is usually done in large capacity, enclosed mixers in the form of a chamber in which two rotors (shafts of drums) of oval or trihedral shape and provided with a multitude of fingers rotate in opposite directions at different speeds. The chambers and the rotors have device for removing the heat released during plastification of the rubber. Sometimes roll-mills are used for mixing the rubber composition. To avoid premature vulcanization the sulphur is introduced towards the end of the mixing operation.
The rubber composition is formed into an endless ribbon of uniform thickness by passing it between calendar rollers; It is then cut into sheets. The parts of rubber articles or blanks from which articles are made are cut or stamped out of the rubber sheets or from rubberized cloth. The methods used in processing the parts and blanks into the finished articles vary widely, depending on the shape, use and conditions of service of the articles.
Rubber compositions are also formed by extruder machines, in which they are pressed through holes of various shapes and sizes. This is how rubber tubes and sleeves are made.
New developments in the field of rubber-making are determined primarily by the requirements of automobile transport and the aviation industry. However, utilization of rubber in other branches of industry, in transportation, agriculture, in the home etc. is also continuously growing. Not only are the amounts but also the diversity of the rubbers used increasing, they are designed for work under all sorts of conditions.
Motor-car and aircraft tyres and other commercial rubber articles demand increased wear-resistance and improvements in the other useful properties of rubber in particular its resistance to low and, especially, high temperatures and to various chemical reagents. In this connection much attention is being devoted to synthetic rubber with a strictly regular structure similar to that of natural rubber. Compositions made from rubber with such an orderly structure of the macromolecules possess high strength and elasticity over a wide range of temperatures.
The utilization of new raw materials for producing the monomers, and polymerization methods are undergoing constant improvement.
The work carried out at a rubber works may be divided as follows
1) Removing raw rubber from craftes.
2) Tearing : if bales of plantation rubber.
5) Compounding (i.e. adding, colours and other chemicals)
6) Masticating or mixing the rubber and other ingredients.
7) Calendering to form sheets of rubber.
8) Forming into shapes of the manufactured articles such as shoes, tyres, hose etc or forming into a dough and spreading to a cotton cloth for water proofing.
9) Curing or Vulcanising.
The student will thus note that production of rubber goods is a complicated process. Chirstopher Columbus on landing in America Centuries ago found the natives producing shoes in a peculiar way. Instead of shaping them by means of moulds as at present, they used to dip their own feet in the latex and then allow the latex to coagulate by keeping the feet over the smoke produced by fire wood. Well, we will now rever to the modern process.
(1) & (2) The removal of the rubber from the crates is a hand operation.
Plantation rubber being free from foreign substances does not undergo washing process but is often placed in a drying room for a few hours to soften the rubber slightly and facilitate the next operation which is the opening out of or ‘cracking’ the bale of sheets the machine used consists of a part of horizontal iron rollers, one clothed with teeth or grooved and the other smooth. The bale is thrown on the top of the rollers which revolve inwards and so grip a portion of the bale, tearing it asunder sheet by sheet. The sheets pass between the rollers and are delivered into a tray under the machine. The rubber is then ready to be mixed with various Chemicals, Colour and filling materials required.
It is now necessary to mix with the rubber various substances according to its ultimate from as a manufactured article. The most important is the sulphur used in the vulcanization. There are also the catalysts or accelerators, filling materials such as carbon black, materials for increasing the tensile strength, deodorising agents, and pigments for coloring the finished article. The list varies according to each rubber manufacturer’s own methods. Literally thousands of Compounding formulas are available and many manufacturers use one hundred or more different combinations to meet their requirements. Often the precise mix is a closely guarded secret. Secrecy is maintained by numbering the ingredients so that the employees who know the proportions do no know the actual number of ingredients corresponding to the code numbers.
The compounding materials must now be thoroughly kneaded into the rubber. The machine used for this process is very similar to the washing machinery, except that the rollers are hollow and can be filled with either hot or cold water and that there is no spray or water used. During this process the mixture must be kept warm and consequently the machine is started up with the rolls filled with hot water. As the process proceeds the friction of the mixture between the rolls causes the mixture to heat and the cold water is substituted for the hot water in the rolls. The operator feeds the rubber and compounding material between the top of the rollers and continually scoops the drugs on to the rubber until the mixture is thoroughly mixed. The machines used are called masticators, mixing mills and refiners. A more modern type of mixer or masticator is of an enclosed type consisting of a hopper superstructure into which the ingredients are fed. A plunger forces the materials down into an enclosed mixing blades revolves and masticates the mixture by forcing it against the walls of the chamber. The chamber has a water jacket into which hot or cold water can be injected.
The preparation for vulcanization various according to the nature of the finished article. If sheet rubber is required, the rubber is pressed into blocks by passing through steam heated sheeting calenders and further reduced into sheets by another operation of calendering. From sheet rubber many articles can be shaped; hot water bottles, shoe soles, floor rubber and mats are a few of the hundreds of shapes which may be cut.
If the rubber has to be mounted on canvas as for example, for the manufacture of hose pipes, the operation is performed by a steam heated friction calendar. That is a calender in which the bowls move at different speeds. The rubber is spread over one of the bowls and a roll of canvas passed between that bowl and another bowl. The difference in speed offers resistance between the cloth and the rubber, in consequence of which the canvas is covered with a sheet of rubber. By pressing two such rubber covered sheets together a doubling mahine which consists of two rollers waterproof cloth with cloth on both sides can be produced.
Other forms of calenders consist of machines in which one of the rollers contains a pattern from which impressions are made on the sheet of rubber. An embossing calender of this description is used for preparing the soles of rubber shoes.
In the manufacturing of shaped lengths of rubber, such as solid tyres, the form is obtained by passing the masticated rubber through on extruding machine. The rubber is placed in a hopper and forced through a heated die mouth by means of a revolving screw. The shaped article is received on a table covered with french chalk. It is possible to feed into this machine, lengths of wire to be discharged through the die mouth in the centre of the length of moulded rubber. This is the manner in which wire finds its way into the centre of a solid pram tyre.
In some cases the sheet rubber is placed in moulds so that all manner of fancy shapes can be obtained. Moulds are utilised in the making up of motor tyres. The canvas is placed on the mould and smeared with a solution of rubber and naptha, commonly known as rubber solution, or cement. On this base is built by hand, layer by layer, the shape of the tyre and the outer portion of the mould placed in position.
Rubber solution is utilized extensively for jointing. In this way the various parts of a hot water bottle, a rubber shoe, and many other articles which have been cut or shaped from sheet rubber are cemented together by smearing the portions to be joined, by hand, called thumbling and pressing the two portions together. Articles made in this way are ready for vulcanization, unless a glossy surface is required as, for example, in a wellington boot. Gloss is obtained by dipping the article in a bath of boiled linseed oil and naphtha or camphine.
Another method of obtaining the finished articles is by making use of a rubber solution. For the manufacture of gloves and similar articles moulds are dipped in a dilute rubber solution and withdrawn with a coating of solution on them. The solvent, usually bisulphide of carbon quickly evaporates and leaves the article shaped in the mould ready for vulcanization.
Waterproof cloth is made by an operation called spreading. A dough is made by mixing the mastricated rubber with naphtha. When the naphtha is added the rubber swells. The mixture is left until the swelling reaches its maximum and is then formed into a solution called dough by agitating the mixture. The spreading machine consists of a metal table about the size of a billiard table, the top of which is a hollow chest heated by steam to about 100 deg.F. A roll of cloth which has previously been dried and smoothed by passing through a steam heated chamber is placed in slots at one end of the table. The clothe is passed between a roller and a blade called a doctor over the top of the table, round another roller at the other end of the table, and thence under the table to be rolled on a roller located below the original roll of cloth. Dough is placed on the cloth in contact with the doctor and the cloth slowly would from the original roll to the delivery roll. The doctor spreads the dough evenly over the surface of the cloth. Whilst passing over the surface of the table the heat from the steam chest evaporates the naphtha, leaving the cloth covered with a surface of dried rubber.
(9) VULCANIZATION OR CURING
All the goods mentioned in the preceding paragraphs must now be vulcanized in order to make the rubber impervious to the action of small changes in temperature and to fix the goods permanently in the shape in which they have been moulded. The percentage of sulphur mixed with the rubber in the masticator determines whether the vulcanized product will be soft or hard. In fact by adding 40 per cent of sulphur, the rubber would come out of the vulcanizing process in the form of vulcanite.
Curing is carried on by tow methods. One involves the application of heat and is usually called vulcanizing, steaming or heating. The other is a cold process and is termed cold curing. Vulcanization by heat is carried out in closed vulcanizing pans or cylinders constructed of steel. The pans have been left in the pans a few hours, the combination of the sulphur is complete and the vulcanized rubber articles are taken out.
For tyres and belting it is usual to use steam presses, in which the pressure is obtained by the closing up of the steam heated platens of the press by hydraulic power.
Cold curing is performed by placing in a bath of sulphur chloride and carbon bisulphide. After treatment, deposites of sulphur are washed from the articles with alkali. Sometimes, for example, in the cold curing of water proofed cloth, the cloth is dried and the carbon bisulphide evaporated by passing the cloth over steam heated cylinders.
After vulcanization, the goods are finished by trimming of joints, polishing, paring and the like.
MANUFACTURE OF RUBBER ARTICLES
DIRECT FROM LATEX
Rubber particles are negatively charged with electricity and will collect round an anode if an electric current is passed through latex. This property is occasionally made use of in the manufacture of articles such as cycle inner tubes. A current is passed through the bath with the result that the latex coagulates on the rod. When removed the rod is covered with a film of rubber, which if dried in a drying chamber, is a length of rubber tubing. Vulcanization is not necessary when this process is used.
An important branch of the rubber industry when the price of rubber is high, is the department dealing with waste or scrap rubber. Before waste rubber can be remoulded, it must be freed of all impurities and made plastic. The process by which these changes are effected < called Reclaiming, Rejuvenating or The old rubber is cut up by means of revolving knives or by shears. The scraps of rubber are then reduced to a powder by means of various grinding machines in which emery wheels form a prominent part. In some of these machines magnets are fitted to attract pieces of metal. The next process is to remove the fabric and one of the following methods is used. The ground up rubber and fabric is boiled for two or three hours in a tank of sulphuric acid. The acid chars the fabric which can be removed if the rubber is washed in a machine called a riffler, i.e. a series of boxes arranged in steps down which water runs. Another method is by boiling in an alkali. Caustic soda is usually used and has the effect of making the fabric soluble in water. Moisture is driven off by spreading the ground rubber on wire trays above a current of hot air, obtained usually by forcing air over steam heated coils. The sulphur is removed by devulcanization – a process resembling vulcanization except that resin oils are mixed with the rubber. The devulcanized rubber is now compounded, masticated, and formed into articles by processes similar to those used for raw rubber. Some modern developments in Rubber processing are given below : A modern development which is likely to extend is the use of high-frequency and infra-red heating for pre-heating moulding powders and welding thermoplastic sheets and laminated plastics, but this is usually effected by self-contained units which do not introduce any special feature of risk other than that pertaining to all electrical equipment. There is another modern development which is of special interest to the textile, paper, and boot and shoe trades. The preservatives usually employed to prevent attack by bacteria in latex are negatively charged, but in this new method the charge is reversed. The reversal causes the rubber particles to carry a positive charge, even when in an alkaline solution, and this has an important commercial bearing because textile fibres are negative to such solution. In Positex, therefore, particles of rubber are deposited on a textile fibre under conditions which permit an easily regulated rubber coating. However, this process does not appear to introduce any new feature of risk. Still another modern process is the manufacture of pliofilm, material similar in appearance to cellophane and used for similar purposes. Briefly it is produced by milling rubber in benzol, with which it is mixed to form a cement. As required the latter is mixed with hydrochloric acid in water-jacketed mixers, after which the excess acid is neutralized by the addition of soda ash, the material is filtered, and is then spread in the form of a film. Obviously very great risk is inseparable from this process, in consequence of the naphtha fumes. The use of flameproof electrical equipment combined with an adequate exhaust system is essential.
The old rubber is cut up by means of revolving knives or by shears.
The scraps of rubber are then reduced to a powder by means of various grinding machines in which emery wheels form a prominent part. In some of these machines magnets are fitted to attract pieces of metal.
The next process is to remove the fabric and one of the following methods is used.
The ground up rubber and fabric is boiled for two or three hours in a tank of sulphuric acid. The acid chars the fabric which can be removed if the rubber is washed in a machine called a riffler, i.e. a series of boxes arranged in steps down which water runs.
Another method is by boiling in an alkali. Caustic soda is usually used and has the effect of making the fabric soluble in water.
Moisture is driven off by spreading the ground rubber on wire trays above a current of hot air, obtained usually by forcing air over steam heated coils.
The sulphur is removed by devulcanization – a process resembling vulcanization except that resin oils are mixed with the rubber. The devulcanized rubber is now compounded, masticated, and formed into articles by processes similar to those used for raw rubber.
Some modern developments in Rubber processing are given below :
A modern development which is likely to extend is the use of high-frequency and infra-red heating for pre-heating moulding powders and welding thermoplastic sheets and laminated plastics, but this is usually effected by self-contained units which do not introduce any special feature of risk other than that pertaining to all electrical equipment.
There is another modern development which is of special interest to the textile, paper, and boot and shoe trades. The preservatives usually employed to prevent attack by bacteria in latex are negatively charged, but in this new method the charge is reversed. The reversal causes the rubber particles to carry a positive charge, even when in an alkaline solution, and this has an important commercial bearing because textile fibres are negative to such solution. In Positex, therefore, particles of rubber are deposited on a textile fibre under conditions which permit an easily regulated rubber coating. However, this process does not appear to introduce any new feature of risk.
Still another modern process is the manufacture of pliofilm, material similar in appearance to cellophane and used for similar purposes. Briefly it is produced by milling rubber in benzol, with which it is mixed to form a cement. As required the latter is mixed with hydrochloric acid in water-jacketed mixers, after which the excess acid is neutralized by the addition of soda ash, the material is filtered, and is then spread in the form of a film.
Obviously very great risk is inseparable from this process, in consequence of the naphtha fumes. The use of flameproof electrical equipment combined with an adequate exhaust system is essential.