Lead Coating for Curing.—Garden hose made in long lengths formerly cured in a manner similar to that used for belts, is now commonly cured by an entirely different method. The as sembled hose is run through a lead press and covered with a lead sheath. A Soo ft. length thus covered is wound on a drum and water connections leading outside the heater are made at both ends. A number of the drums are placed side by side in a hori zontal, cylindrical vulcanizer and hot water under pressure run through the hose, while steam is admitted to the vulcanizer. In this manner heat is applied simultaneously from both sides of the rubber structure, and the internal water compacts the hose against the lead sheath. The lead covering is stripped, melted and used again for covering. The lead covering method may be also ap plied to other articles, such as buggy tires.
Open Steam Curing.—Articles which require no further form ing than is produced in building operations may be vulcanized without enclosure in molds. Steam is permitted to come into contact either with the bare goods or with wet fabric wrappings around them. Tubes of stationers' band stock are hung on metal rods placed in a horizontal heater and vulcanized by steam coming into direct contact with the rubber. Air brake hose is built on mandrels, wrapped tightly with wet cloth and vulcanized by steam directly surrounding the wrapped hose. Pressure may be applied during vulcanization in open steam to articles built on hollow forms. Boots and shoes, for example, built on hollow, perforated lasts, each of which is connected in the vulcanizer with a vent to the atmosphere, receive from the steam around them not only the heat necessary for vulcanization, but mechanical pressure which forces out the air between the shoes and the forms and between the separate pieces composing the goods.
Water Curing.—Some rubber goods are cured immersed in hot water under pressure. The pressure may be equal to or greater than that of saturated steam at the same temperature. Rubber sheet to be cut into thread is wrapped with wet cloth on drums, covered with a vulcanized rubber sheet and cured under water. To secure a smooth surface on sheets of hard rubber the unvulcanized stock is rolled on sheeted tin, a number of these slabs piled together and vulcanized under water.
Air Curing.—Varnished footwear and shoes containing wool fabrics have long been cured by heating in air. Originally the goods were heated simply by air at atmospheric pressure rising from steam pipes in the bottom of a large chamber holding sev eral cars of goods. Several hours were required for these cures. More recently, horizontal vulcanizers have been used of the same construction as open steam vulcanizers. The air is circulated under pressure by pumps and is heated before it enters the vul canizer. This method not only shortens the time of vulcaniza tion but improves the quality of the vulcanized rubber. By use of perforated lasts vented to the atmosphere outside the heater, or by connecting the vents with vacuum pumps, differen tial pressure outside and inside the lasts is secured, with a bene ficial compacting of the goods against them. Litharge used in rather large proportions was once considered necessary to pro duce vulcanization of "dry heat" compositions. Organic accelera tors have been found more effective and much cheaper. Auto topping, sheetings, and other goods are also cured in air.
Combination Air and Steam Curing.—Rubber goods which stain when vulcanized in open steam by the usual procedure may be cured by first warming in air and then introducing steam. This reduces or eliminates condensation of water on the surface of the goods and prevents staining.
Sulphur Chloride Curing.—Vulcanization by means of sul phur mono-chloride may be carried out as "cold vulcan ization" in solutions of this agent. Cold vulcanization is used for thin articles, such as surgeons' gloves and finger cots. The articles are made by repeated dipping of forms in a rubber ce ment, allowing the solvent of the cement to evaporate after each dip. When the desired thickness is attained, the vulcanization is secured by dipping the articles on the forms into a 2% to 3% solution of sulphur chloride. This process of vulcanization was discovered by Alexander Parkes, of Birmingham, England, in 1846. Sulphur chloride is used also in vapour form for curing gloves, dental dam, or dress-shield stock. The goods are placed in a closed vulcanizing room and the sulphur chloride vapour, produced by warming the liquid, is allowed to act on them. To remove excess curing agent, ammonia gas is sent into the room.