The fabric is drawn over a roller under a spreader knife, which can be set at varying distances from the roll, carry ing from a cement feed a thin layer of the cement, and then over steam heated pipes to evaporate the solvent. This process is re peated until the required thickness of rubber coating is built up. By this method the rubber surfacing is applied to shoe cover and backing cloth, balloon fabrics, printers' blankets for offset work, and similar products. Thin coatings similarly applied are used to improve the adhesion to fabric of rubber layers later applied on a calender. Cements applied by brushing are extensively used in the manufacture of rubber articles, notably footwear.
Some grades of rubber—wild varieties and low grade plantation rubber—must be washed to remove sand, bark, or other impurities, before they can be utilized in the manufac ture of rubber goods. For this purpose mechanical washers are used. The ordinary two-roll washer is the commonest type. The rubber is first formed into a relatively thick sheet by pass ing through a "cracker," a small mill with coarsely corrugated rolls. No water is used on the rubber in this operation. It is then passed repeatedly between more finely corrugated rolls on a washer while a stream of cold water plays over it, loosening the dirt and washing it into the sewer. Finally the rubber is sheeted out thin on a similar washer with the rolls set close together,— the "finisher." The rubber is ready for the drier after leaving the "finisher." In place of the corrugations once used, projections of pyramid form have been adopted for the roll surfaces.
washing, rubber must be dried before it is used in the mixing operation. Three methods of drying are in common use, forced drying, vacuum drying, and humidified air drying. Forced drying consists in forcing air at 90° to roo° F. through chambers in which sheets from the washers are suspended from wooden rods. The time of drying varies from 2 to 12 days, dependent upon the kind of rubber and the purpose for which it is to be used. Vacuum drying consists of heating the sheeted rub ber laid in pans on steam plates in a vacuum chamber. Time and temperature of drying vary. High-grade plantation rubber will dry in 21 hr. under a vacuum of 27 in. with steam in plates at 281° F. Ordinary vacuum drying units will accommodate 2,500 lb. per charge. Humidified air drying is secured by circulating air at a relative humidity of 30% to 35% at 17o° F. through a chamber containing rubber in trays placed in racks. In a unit of 50,00o lb. capacity the rubber will dry in 12 to 14 hours under these conditions. Reclaimed rubber comes to the rubber goods
manufacturer in dry condition. In rare cases it is dried in a vacuum drier at temperatures of 240° to 26o° F. Damp pigments are easily dried by storage in a hot room at 160° to 170° F. for 10 to 15 hours. Oils, waxes, tars and other fusible materials may be readily dried by holding them in the melted state at 25oe to 27o° F. for a few hours. Fabric is dried by passing over a series of heated rolls or plates. These are known as "drum" ("can") driers or "shelf" driers. The fabric is passed through the driers at the rate of 35 yd. to 55 yd. per minute. Two passes through the drier are necessary for ordinary belt fabrics and in excep tional cases three or more passes are required.
Fairly large quantities of vul canized rubber waste are used in cheap articles without treatment by any chemical reclaiming process. Preparation for incorpora tion in rubber mixtures is carried out on grinders consisting of two horizontal rolls of different diameters, the larger rotating with peripheral speed about three times that of the smaller. The rolls are set tight together and coarsely pulverized material fed into the machine. The small clearance between the rolls and the great difference in surface speed produce a powerful grinding action. The material is passed between the rolls repeatedly until suffi ciently fine for use. To facilitate handling of the material, fluxes,—pine tar, pitches, or rubber,—may be run through with the vulcanized scrap during the last few passes. This causes the material to form a sheet. A cracker, similar to that used in washing, but having only one roll corrugated, is used for break ing up large masses of vulcanized rubber and similar waste rub ber material in preparation for the grinders. Hard rubber may be ground to a fine dust either on roll mills or in grinders similar to those used in grinding coal. This dust is employed very exten sively in the preparation of various ebonite mixtures.
Preliminary to mixing operations, rubber is plasticized on two-roll mills, in internal mixers or in large machines of the extrusion type. Softening is caused by oxidation. This is promoted on cooled mills through activation of the oxygen of air by electric discharges resulting from mechan ical action. In the extrusion type (Gordon plasticators) oxida tion is promoted because of the high temperature attained by the rubber, up to 35o° F. Plasticizing may be hastened by addi tion to the rubber of small amounts of certain chemicals, notably thio-phenolic compounds.