T. O'Conor Sloane

“ One type of curing process does away with the air or steam vulcanizer, and substitutes, as the curing agency, a hot bath of liquid. For this purpose a fluid is required that will not act injuriously upon the india rubber, and which will give a curing temperature without boiling away. One favorite liquid is glycerine. This can be heated to the necessary degree and is an excellent substitute for the expensive apparatus often used. For experimental work it is exceedingly convenient. ”

“ If the juice is left to stand in vessels, like milk in a creamery, the globules rise to the surface, and a cream of india rubber can be skimmed off from the surface. If the juice is evaporated over a fire, the water escapes and the india rubber remains. By dipping an article repeatedly in the juice and drying it, a thick or thin coating of india rubber can be developed. Before the modern methods for the manipulation of the gum had been developed, and before the invention of vulcanization, this method was adopted for the manufacture of shoes. ”

“ The blacksmith heats two pieces of iron until they are nearly white hot and are pasty in consistency. On placing them in contact and hammering to force them together they unite so firmly as to be practically one. It is necessary that the surfaces of clean metal should be brought together. If the pressure induced by the hammering is insufficient to bring this about, a flux is added which dissolves the oxide and causes the metal to come in contact with metal and to weld. The analogy with india rubber in its cohesive action is evident. ”

“ It is almost impossible to masticate untreated gum in an ordinary mortar.
The dealers sell a special india rubber for the manufacture of cement and solutions. This is so treated by mastication that it dissolves with great readiness. It is also said that heating under pressure is used to dissolve it in some factories.
Many solvents have been used and none work without some difficulty. Benzole, coal tar naptha, petroleum naptha, carbon disulphide, ether and chloroform, oil of turpentine and caoutchoucin are the best known. The naptha best suited for its solution is termed solvent naptha. ”

“ By pressing the point of a knife against it the period of vulcanization can be told. Before the material is heated it is elastic and resists the pressure of the knife; as heat is applied it becomes soft like putty; as the heat increases it again stiffens and becomes quite elastic. At this point the press can be opened and the sheet and matrix can be taken out or the platen swung aside. On pulling or stripping the sheet from the matrix it will be found to reproduce the model in elastic india rubber to the minutest detail. ”

“ The steam and vapor formed by its liquid contents expand it and press it with great force against the sides of the mould. After curing the mould and bulb are removed from the vulcanizer, cooled by a shower bath of cold water, the mould is opened and the bulb is removed. Often an iron pin is left projecting through the side during the vulcanizing, which pin, when withdrawn, leaves the necessary aperture, or it is perforated. The bulbs are polished by tumbling in a revolving cylinder. Considerable skill and practice are needed to succeed in making hollow bulbs. ”

“ Plaster of paris or dental plaster mixed with dextrine or gum arabic water or the zinc oxychloride cement, already described, is to be recommended for the moulds. They should be made, if deep, in frames or “flasks” of tin, as plaster if unsupported is liable to split open when the rubber is forced home.
For many articles the hot press can be used. Such articles are mats and other thin flat pieces. The rubber stamp sheet is a good material for them. For thicker articles a thicker sheet can be used, and sheet of any gauge can be procured from the maker. ”

“ This rolling of the mixed india rubber into sheets of definite thickness is done by special calendering rolls. The product is termed “mixed sheet.”
In the mixing rolls the incorporation of other material is often brought about. Zinc white, lead sulphide, antimony sulphide, chalk, clay, talc, barium sulphate, plaster of paris, zinc sulphide, lead sulphate, white lead, oxides of lead, magnesia, silica, form a list of ordinary mixing ingredients. These lower the cost of the finished material and are often serious adulterants. ”

“ The press must first be heated to the temperature of boiling water or thereabouts and the moulding is then effected by screwing down the mould screw, upon the sheet and matrix. It is then placed in the vulcanizer and cured.
The manufacturers supply gas regulators which automatically regulate the gas supply. These are worked by the steam pressure. If any one wishes to study the practical manipulation of small steam vulcanizers he can see them in use at any dentist’s office.
There is no need of a steam vulcanizer for ordinary stamp work. ”

“ Pencil Tips.—These are generally little cylinders of india rubber, which fit into a tube that slides over the end of the pencil. They can be thus simply made. A hole is bored in a piece of wood the diameter of and a little more than the depth of the pencil tip. A short cylinder that exactly fits the hole is required for plunger. The gum is put into the hole in little discs, or rolled up into a cylinder, the plunger is placed on top, and the mould put in the press. ”

“ By the press system of curing, a heat far above the vulcanizing temperature may be made to do good work by a very short application. There is71 however danger of burning the work if left in too long. If the air-bath with thermometer or the steam vulcanizer is used, and the heat is kept down to the proper curing temperature, there is no danger of burning the india rubber even if the curing is considerably prolonged. ”

“ It is stated that a solution or pasty mixture of one part of caoutchouc in eleven parts of turpentine with one half part of a hot concentrated solution of sulphur (potassium sulphide) gives on evaporation a film neither tacky nor soft, a species of vulcanization taking place.
It is of much interest to note that an aqueous solution of india rubber has been proposed in which the vehicle is a solution of borax in water. This is well known to be a solvent for shellac and other resins. It has been recommended often as a vehicle for rubbing up india ink. ”

“ While thus intractable by the usual methods, it is the most plastic of materials when properly treated. Its power of reproducing the finest details of a mould, of entering all the intricacies and undercuttings of a design, cause one to feel a peculiar pleasure in working with so responsive a material. ”

“ It is evident that a wooden press could not be used for direct heating. Such a press must be used in a hot chamber or vulcanizer, properly so called. Originally rubber stamps were generally made in chamber vulcanizers.
The next cut shows a combined matrix making, moulding and vulcanizing apparatus of very convenient and compact form and adapted for rapid work. As the press stands in the cut the matrix press is seen in front. A box or chase is carried under its platen by two trunnions, so as to be free to oscillate to a limited extent. The type model is secured in this box. ”

“ The manipulation of pure sheet rubber is simple, yet is liable to lead to disappointment. When two pieces are laid face to face and cut across with a sharp knife, or scissors, the edges will adhere with considerable tenacity. This may be increased by applying some thick solution of india rubber in a volatile solvent, and by manipulating the sheets so as to bring the entire surfaces of the cuts together. ”

“ A proper admixture renders the gum more easily moulded and treated in the shaping processes.
Mixing Rolls.
The next step in the vulcanizing process is the heating of the mass, which step is called “curing.” Up to a temperature in the neighborhood of that of boiling water the mixed rubber can be heated without change except as it is softened. But if the heat is increased it begins to get a little more elastic and less doughy, and eventually becomes “cured” or vulcanized. The temperature for vulcanization is about 284° F. (140° C.) . ”

“ The sulphur bath is of value to the experimenter, enabling him to do his own mixing without expensive apparatus.
Bromine, iodine, chlorine and nitric acid are vulcanizers. A piece of sheet rubber dipped into liquid bromine is instantly vulcanized. Iodine and nitric acid have also been used in commercial work.
Alkaline or alkaline earth sulphides can be employed in solution under pressure for vulcanizing. At a vulcanizing temperature their solutions will answer for thin sheet very well. Polysulphides of calcium have thus been employed. ”

“ In the factory the normal vulcanization is carried out in two steps, mixing and curing. The washed sheet india rubber which has not been masticated and which must be perfectly dry is the starting point, and the mixing rolls shown in the cuts are the mechanism for carrying out the first step. These are a pair of powerful rollers which are geared so as to work like ordinary rolls, except that one revolves43 about three times as fast as the other. They are heated by steam, which is introduced inside of them. ”

“ African gum is of very inferior quality.
The African india rubber vines grow often in dark moist ravines, where no valuable product other than themselves could be cultivated. They are entirely wild. The vines when cut exude an abundance16 of sap, which differs from the South American product in its quickness of coagulation. As it escapes from the wound it at once solidifies and prevents the further escape of juice. The negroes are said to employ the following highly original method of collecting it. They make long gashes in the bark. ”

“ Cement for uniting pieces of the partially cured material may be made by rubbing up some of the untreated scrap with benzole.
At the heat of boiling water, ebonite can be bent to a certain extent, which bend it retains on cooling. When warm an impression of a coin or relief die may be made on it by heavy pressure which it will retain. On heating the image disappears. If before heating the surface is planed off and the piece is heated the image formerly in intaglio will expand into relief.
By the exact process of rubber stamp making111 excellent stereotype plates may be made of ebonite. ”

“ The great lumps of gum are torn to pieces and washed free from gravel and dirt without going to powder, because owing to their elasticity they yield27 and as fast as torn apart the pieces tend to reunite. Again india rubber is mixed with pigments and vulcanizing reagents by a method practically one of grinding or masticating, but the material while it changes its shape, and by the admixture of the various ingredients becomes less strong or easier torn, still remains intact, as it welds together or coheres as fast as disintegrated. ”

“ If the stamp is properly made it is surprising how much force may be used to insert it. If the edges of the brass swinging piece are not rounded there is danger of the composition being cut. The stamp in its brass frame is then swung back over the brass base, where it is secured by a catch. The stamp is now ready for use, as shown in the second figure of the cut.
It is imperative that no aqueous or glycerine ink be employed for continuous work with such stamps. Common printers’ ink is perfectly satisfactory, and the work may be nearly or quite as good as that executed by an india rubber stamp. ”

“ Ebonite is properly the name for black hard rubber, and vulcanite for the colored products such as used by dentists and others.
GUTTA-PERCHA.
Gutta-percha is prepared by coagulation from the juice or sap of several trees, among others the Isonandra gutta, of Borneo and the East Indian Archipelago. The product gutta-percha is identical in composition with india rubber. It is hard at all ordinary temperatures.
Its manufacture includes purification and mastication. It is far more amenable to treatment than is india rubber. ”

“ Vulcanized india rubber is unaffected by changes of temperature within ordinary range. It softens a little on heating. Even hard vulcanite when heated33 can be bent and will retain the bend on cooling. It is exceedingly elastic with elasticity of shape but far less compressible as regards absolute change of volume than the raw gum. It melts at 392° F. (200° C.) It cannot be made to cohere, and no cement has yet been discovered that will satisfactorily unite two surfaces. It is unaffected by light, by ordinary acids and rubber solvents. ”

“ If the gum is masticated or kneaded at the temperature of boiling water a change occurs not well understood, by which its solubility is greatly increased. As solvents many liquids have been named. Oil of turpentine, caoutchoucin, coal-tar, naptha, benzole, petroleum-naptha, coal-tar-naptha, anhydrous ether, many essential oils, chloroform, bisulphide of carbon, pure, or mixed with seven or eight per cent. of alcohol, are among the solvents recommended. A mixture of fifty parts of benzole and seventy parts of rectified turpentine has been given as a solvent for twenty-six parts of the gum. ”

“ Sometimes the juice is said to be collected upon the arms, the dried caoutchouc coming off in the shape of tubes. A clew to the inferiority of African india rubber is afforded by the statement that too deep a cut liberates a gum which deteriorates the regular product if it mixes with it. The drying of the gum is thought to have much to do with its quality and it is highly probable that this affects the African product. ”