At all stages from the raw material to the finished article rubber will burn. it hardens and cracks at low temperature, becomes tackly at moderate heat and degenerates into a gum substance around 90*C, and in the process gives off fumes and smoke which considerably hamper fire fighting operations. Rubber is easily damaged by heat and hence has a poor salvage value. The degree of hazard considerably varies according to the size of the risk and the nature of articles manufactured.
Rubber is ignited easily. At about 120*C it begins to soften and at about 150*C it begins to melt and at about 170*C it change into a thick brown oil with a characteristic odour. Above 300*C cracking occurs following structural breakdown with formation of volatile liquid and inflammable gases which produce explosive mixture with air.
Ignition usually occurs at about 300*C but with all these quotations the actual temperature depends upon the class and quality of rubber concerned, the processing treatment it has received, the nature and manufacture and finally upon whether the rubber is reinforced with textile materials, for this has a marked influence on the inflammability of the finished product. The ease with which rubber can be ignited was stressed by Dr. Firth in the first of the Fire Protection Association Journals. Tests were made on eighteen specimens of prepared rubber containing varying proportions of fillers and softners. Each specimen could be readily ignited by a match, but the behaviour and tendency to spread was influenced by the composition of the material. Three interesting experiments in connection with the ingnition of motor tyres were also detailed by Dr. Firth’s concluding summary is worthy of special note. He said: “It would appear that the ease of ignition of rubber and goods containing a fair percentage of rubber is not realized, except perhaps by those directly connected with the rubber industry”.
Apart from the ease of ignition, reference should be made to the great heat which is evolved when rubber burns. It has a calorific value of 17,000 British thermal units per 1b., as compared with corresponding values of 6000 for straw, 7000 for paper, 7200 for cotton, 8000 for leather and wood, and 9200 for silk. Thus copious quantities of water are necessary to cool the rubber below the ignition point – a requirement which is all the more difficult because of water resisting properties of the material.
Further, when rubber burns, it evolves intense heavy black noxious fumes, which obscure the seat of the outbreak and seriously impede any attempts to deal with it. in enclosed spaces these fumes may produce an explosion.
RAW MATERIALS AND FINISHED GOODS STORE
The raw materials of the rubber factory might be said to comprise crude rubber, chemicals, cotton cloth, naptha and carbon bisulphide etc. the accumulation of empty boxes increases the risk of fire. When sulphur is stored, it will result in fumes in case of fire, preenting easy fire fighting operations. The chemical store prevents hazard according to the nature of storage and there is always the possibility of serious water damage following fire brigade operations.
Cotton cloth stored for use in the spreading machines has to be perfectly dry and hence cloth warehouse usually contains a drying room. Such room should be in fire proof compartment.
Naptha is very volatile and heavier than air and hence any bulk storage should be under-ground tanks with pumps attached. Carbon bisulphide is not used in such great quantities as naptha. Consequently the under-ground system is not adaptable. Any bulk storage of solvents should be confined to tanks located underground or in detached buildings. Where necessary tanks above ground should be provided with catch pits or retaining the walls to prevent escaping fluid reaching the factory. All tanks, pipe lines and pumping equipment should be earthed because static charges are produced by the flow of solvents. All electrical equipments should be flame proof and ventilation should be adequate. Cleanliness is necessary to prevent spontaneous combustion of materials liable to be affected like oily rags, paper and other foreign matter. Wide separation to the extent possible is desirable, since apart from minimizing the spread of fire, the work of the fire brigade in dealing with the fire becomes easier.
MANUFACTURE
The twin dangers during manufacture are the use of inflammable solvents with the resultant hazard and secondly the production of static electricity. About 12 gallons of solvent is required for each ton of rubber and the hazards of solvent and precautions necessary for storage has been dealth with in previous paragraph.
HAZARDS OF STATIC ELECTRICITY
In a large number of cases, the charges generated involve no hazard but the presence of dust or inflammable vapour creates a hazard. The usual precautions are earthing, humidification and ionisation of the air.
On mills and refiners this does not matter, because there is no inflammable vapour. On the dipping, spreading, calendering and tyre-building machines, however, where solvent is used and an explosive mixture lies over the equipment a small spark may cause an explosion or start a fire.
The speed at which the machines operate, combined with the large surfaces under treatment, cause enormously high potentials – measured in tens of thousands of volts – to build up, unless adequate provision is made to discharge them safely to earth and in practice this is not easy to accomplish.
When dissimilar materials have been rubbed together so that electrification occurs, equal but opposite charges are in a state of tension, and are prevented from rushing together by the dielectric which separates them. In a machine, the dielectric may be air, a layer of rubber or fabric, or merely a film of lubricating oil separating a charged spindle from its bearing.
If the potential increases, however, or the thickness of the dielectric is reduced sufficiently, the dielectric is disrupted by a discharge in the form of an instantaneous vivid flash or spark. Even a minute spark formed in this manner is capable of igniting inflammable vapour, and such vapours cover the surface of the solution-tanks and troughs through which the material passes.
It is usually accepted that a potential difference of about 30,000 volts will produce a spark one inch in length in air, the actual voltage depending upon the shape of the surfaces or electrodes used and the condition of the air. As voltages of 30,000, 50,000 are often produced the possibility of a spark-discharge taking place is not surprising.
The prevention of electrification is not possible. It is natural phenomenon which occurs automatically when dissimilar materials are rubbed together, but the creation of a spark can be prevented if the charges can be dispersed safety to earth as they are formed.
To accomplish this, all moving parts of the machine should be bonded to the frame, and the whole unit bonded to earth. Similarly, a earthed collector should wipe the surface of the material as close as possible to each point at which it has parted contact with a roller or doctor. By this means the dissimilar materials are both kept at earth potential. Operators should not wear rubber soles and heels, because the electrification which they acquire from handling charged surfaces may cause a spark when contact is made with parts of the machine.
In practice, however, it is not always easy to make an efficient, bonding of all parts to an effective earth, and it is still more difficult to collect from every part of the surface of the material. Thus high-resistance contacts to earth may be caused by dirty or rusty contacts, films of oil, or when bending wires snap or work loose.
Frequently it is assumed that a machine is earthed merely because it is bedded in a concrete floor. Following a series of small fire son a large spreading machine, it was maintained that the machine was not effectively earthed, but the engineer disagreed because an electric light worked when one turn. However, finally, he was persuaded to borrow an electrostatic voltmeter, and he found a potential difference of about, 38,000 volts between the machine and a good independent earth. Heavy copper tapes channels in the floor to substantial copper plates buried outside in damp soil and ashes, and the voltage immediately fell to just under 3000.
Collecting the charge from the surface of materials which have been coated with rubber is still more difficult, because the collectors become similarly coated. Also, direct contact with the material would mark the newly coated surface. Earthed collectors provided with a series of points are, therefore, placed as close as possible to the surface without making contact, as it has been fond that a sparkles brush discharge induces the charge into the collector, and so to earth.
Protective devices for all machines should be designed specially for the work to be done, properly fitted and carefully maintained, as otherwise, the alleged protection may increase the fire-risk by creating the spark which it is intended to eliminate.
In America, Static neutralizers designed to apply equal but opposite charges to neutralize those formed on the machines are in use, but if a careful balance is not maintained the creation of excessive charges may defeat the protection.
The addition of a very small percentage of ethyl alcohol to rubber solutions makes them sufficiently conductive to aliminate static charges, but this does not prevent the formation of charges on the material to be coated until coating has taken place, and the solvent is still inflammable.
Finally, a further alternative, and a very affective method of protection, is the use of humidifying plant which introduces sufficient moisture to produce a slight film on every surface, and thus enables the charges to leak safely to earth. This appears to be the only fool-proof system, of eliminating the risk, but its use in United Kingdom is not as popular as in America, because U.K. dot not suffer to the same extent from the long dry spells which are conducive to the production of electrification.
A similar fire-risk occurs on tyre-building machines, where successive layers of rubberized fabic, and the outer tread, are firmly moulded into the modern tyre. In order to improve the adhesion between the layers the operators wipe the surface of the material with a pad dipped in naptha, texine, or similar spirit. When the machine rotates slowly, the voltage of the static charge is about 500, but recent tests show that when the machine is speeded up and brake suddenly the charge rises to between ten and eleven thousand volts.
A further feature of risk is the use of a small pot of solvent on each machine. In large factories the containers are usually filled from portable spirit-pumps and at night all unused solvent is collected : though in practice operators usually guage their requirements so accurately that little remains for collection.
Charges of static electricity are formed on calenders used for drying fabric and on machines which wind textile material for inspection purposes. They are not important, however, because of the absence of inflammable vapour.
Ventilation
Adequate Friction due to stones in the bales, or Para balls may cause sparks in the cutting or cracking machines but fires seldom result because there are no inflammable gases in this department and the rubber does not ignite easily. The washing department is free from hazards. However, care should be taken in connection with the electric motors which usually drive the machines. Screen protected drip proof motors are preferred in this situation. The heat in the drying rooms seldom exceeds 100*F. Steam heat with good ventilation is usually employed. This, at the low temperature, presents little hazard but it must be remembered that the rooms are often lined with wood and are filled with wooden racks – plenty of well dried fuel for a fire should one commence. Masticating does not in itself present any special features of hazard, but it must be noted that the mixture becomes heavily charged with electricity during the process. In no case must the rubber in this state be mixed immediately with a solvent. It is usual to allow the masticated rubber to cool. The table on which it is placed should be covered with zinc and earthed. Calendering is another innocuous process. The heat of the bowls does not exceed 130*F. The real hazards of the rubber factory commence after calendering; that is, with the processes involving the use of a solvent. In the making up of articles from sheet rubber, the joints are made by cementing with rubber solution. It is usual to see three-quarters of the factory’s employees in this room, each employee using an open tin of solution. The naptha is evaporating all day long and as the vapour is heavier than air, it accumulates in pockets under tables and machines and creeps along the ground. A fire starting here would involve the room with astounding rapidity. Rooms in which rubber solution is used should not communicate with buildings containing naked flames of any description. Ventilation should be at ground level. Also flame proof electrical equipments should be used in this area. When work ceases for the day, the tins of solution should be removed to a fire-proof compartment. Solution tins of a self-closing type are an improvement on the open tin, especially if the tins are so constructed that it is not necessary to lift the lid in order to reach the solution. In order to be effective it is necessary that self-closing tins should be of a type easy for the operative to handle, otherwise the operative may damage the tins by removing the lids in order to make his work easier. Solution or dough mixing is another process where the atmosphere becomes charged with naptha fumes. It is advisable to choose a well ventilated building with a light roof for this process. The building should be isolated from the main factory. Easily the most hazardous process is carried on in the spreading room. It is essential to the process that the naptha be evaporated from the dough by the heat of the spreading table. Under certain conditions, electric sparking may result due to static electricity sparking may result due to static electricity, and that is one of the risks inherent in the spreading department. The risks inherent in the spreading department. The cotton cloth, passing over the spreading machine, if subject to friction may become positively charged with electricity, whilst the negative charge collects on the naptha. On reaching a certain point of electric tension, a spark may be discharged and ignition ensue. Atmospheric conditions favourable to such electrical discharge are found on a dry, cold, clear day. Means should, therefore, be adopted to prevent accumulation of electricity. This is usually done by earthing by means of copper wire. Also static electricity eliminator may be provided to remove the static charges. This is another department which needs isolation. Light roof construction is prepared as a source of explosion venting. Also flame proof electrical fittings must be used. The amount of naptha in the atmosphere can be considerably reduced by fitting a canopy over each table with metal ducts connected to an exhaust system in which fans draw the vapour from the top of each table. Sometimes, this is connected with naptha recovery plant in which the paptha is absorbed by activated carbon and afterwards recovered. This system is a great improvement and certainly reduces the amount of naptha in the air, thus lessening the fire hazard although even with the most up-to-date plant fitted there is the possibility of naptha collecting in pockets under the tables. The naptha recovery plant is usually in a detached building and rightly so. Modern methods are fairly safe and consists of driving the naptha from the carbon by a jet of steam and distilling the naptha by reducing the temperature. Older methods are far more hazardous; as for example, the method of using a table similar to a spreading machine, fitted with a canopy on which cold water is allowed to run. The cooling effect of the water liquefies the spirit which is collected from the end of the table in a petrol can. Rooms in which articles are moulded or coated by dipping in dilute solutions of rubber in naptha or carbon bisulphide present the same hazards of explosion. Exhaust trunks should be fitted over such machines; similarly, if any roller mills are used for working dough, such machines should be connected to the exhaust system. The heat used in vulcanization by the hot system seldom exceeds 250 deg.F. In cold cure rooms the main risk is the use of carbon bi-sulphide combined with the fact that the cabinets in which the material is treated are often constructed of wood, so the process in itself cannot be termed specially hazardous. The danger of this Department lies rather in the heat radiated from the plant. Proximity of pipes and heaters to woodwork should be avoided. Further, the interior of the heater should be kept clean as fires have occurred by ignition of refuse, such as tickets and labels in the heater. If varnishing is done, we again have the hazard of vapour the naphtha or camphine. The preparation of the varnish, which may include the boiling of the linseed oil, should be done in a detached building. Cold curing is another hazardous process if carbon bisulphide is used. This process should be isolated and methods of exhausting the vapour used. Reclaiming Factories present very grave hazards. This process is usually carried on by unskilled workmen in premises not at all suitable for the purpose. In grinding or powdering the waste hard substances are often encountered, with the resultant spark. Further a considerable amount of dust and carbon is thrown into the atmosphere, thereby adding to the risk of explosion. Considerable hazard is also met here in connection with the drying arrangements. Particles of ground rubber fall through the meshed trays on to the steam pipes so that if the pipes are not cleaned regularly fire will eventually ensue. This is another branch of the industry subject to explosion risk. The rubber in the reforming works is ground finer than in a Reclaiming Factory with the result that such works are very liable to explosions. A further hazard results from the use of carbon bisulphide in the coating of the finished articles. In reforming risks a plant for exhausting vapours is seldom installed on account of the small size of works involved. Many goods must undergo a finishing process by being pared, smoothed or trimmed. For this purpose the most hazardous machines are emery wheels on account of the dust thrown out. Such dust, especially form an explosive mixture in the air. Emery wheels should be fitted with an exhaust system for drawing the dust out of the room.DRYING
MIXING
CALENDERING
SOLUTIONING OR CEMENTING
SPREADING
VULCANIZATION
VARNISHING
COLD CURING
RUBBER RECLAIMING
RUBBER REFORMING
FINISHING