Spreading

A process largely used for the production of waterproof garments. The unvulcanised rubber is converted into a paste or dough by treatment with coal tar naphtha, and successive coats are applied to the fabric, the solvent being evaporated from the film after each application.

Spewing

See Tubing.

Slock

A term commonly used in the works to denote compounding unvulcanised rubber.

Substitute,-Also known as " fac-tice." Rubber substitutes are pre-pared from vegetable oils by the action of sulphur (giving brown substitute) or sulphur chloride (white substitute). Used largely in the production of erasers.

Tubing

A process also known as " extruding," "forcing," or " spewing" which consists in forcing the rubber, after mastication and compounding, through a die Of the desired shape.

Vulcanisation

Also known technically as " curing." A remarkable physical or chemical change effected in the properties of rubber by the action of sulphur and certain sulphur compounds, the tensile strength and elastic modulus becoming greatly increased, and its resistance to changes of temperature and to the action of organic liquids and other chemical re-agents greatly improved.

The following are the important technical methods of vulcanisation :

(1) ;Hot Cure, or Sulphur Vulcanisation. The raw rubber is mixed with sulphur and usually other ingredients (see Mastication, Compounding, Compounding Ingredients), and then heated. The various methods of heating are described under Dry Heat Cure, Press Cure, and Open Steam Cure. During this process the rubber com bines with about two to three and a half per cent, of sulphur (see Vulcanisation Coefficient).

(2) ;Cold Cure. See page 326. (31 Gas Cure. See page 326. Vulcanisation Coefficient.- This represents the amount of sulphur which has entered into chemical combination with 100 parts of rubber during vulcanisation, and affords an index of the degree of vulcanisation and of the probable stability of the product.

Vulcanised Rubber

See Rubber.

Vulcanite

Also known as " ebonite," or " hard rubber." A hard tough product obtained by the vulcanisation of rubber with a large proportion of sulphur, so that the vulcanisation coefficient may reach nearly fifty. Usually black and susceptible to a high polish, but occasionally coloured by the addition of pigments. (The Rubber Growers' Association.)

Notes On Storage Of Rubber

Goods

To get the longest life and service from rubber goods, keep them away from light, heat, acids, oils and grease. Do not store the goods high up near the ceiling, as this may become a hot position. Pack in wooden or card cases, never in metal cases, wood and card are non-conductors of heat. Do not allow the stock to be packed anyhow : lay it flat and have no weight on the top of it. Do not show in sunlight, arrange to keep that off the goods. Should acid get on the goods, wash with an alkali at once, then with soda and water, then with plenty of clean water. Wash off any oil, or grease, with strong soapy water. Use petrol, or benzine, to clean the goods, if and when required, and wash afterwards with strong soap and water, and rinse well. To give a good appearance rub over with a mixture of glycerine and methylated spirit. Be careful never to lay the goods out, or hang them up in a distorted, twisted, or bent condition, otherwise damage is sure to result.

Rubber Mixings

The examples of mixings below are intended to be typical of the requirements in various branches of the industry. In some instances manufacturers may find that they can use these mixings as given ; in other cases there should be no difficulty in adapting them to special purposes as regards cost, gravity, service requirements, and the like. Needless to say, in every case, small trials should be made by the user in order that he may allow for his special conditions as regards mixing, stock, vulcanising plant, and the requirements under service conditions.

The particulars as to mixings and methods of using " Surparac " set out below are the result of detailed research and experience covering a period of three years.

" Surparac " consists of an active accelerating substance dispersed in accordance with British Patent No. 170,682 by scientific method on a carrier base. The active substance is not merely mixed with the carrier base, but is synthesized on the same, thus ensuring absolute homogeneity, and avoiding the excessive concentral ion which makes active accelerators so difficult to handle.

The active substance is a condensation product of a strong organic base (piperidine) with CS2, and is, therefore, of the dithio-carbamate or rather carbothionolate type.

The carrier base is, in the case of " Standard," a colloidal clay of special quality, and in the " Z " grade a high-class zinc oxide.

A Few Genera! Hints

Rubber technologists are agreed that the main advantage of the organic accelerator lies in the culling down of the healing effect, which is detrimental to quality and life. Time of cure and heat, therefore, are the two variables to be studied. An accelerator may be used (a) to cut down time, {b) to reduce the heat. Thus for comparatively small or thin articles we can cut down the time of a cure, by using " Superac," from, say 2 hours to 20 minutes, or from 1 hour to 10 minutes. In the case of massive articles, e.g., tyres, it may not be practicable to cut down time to more than a moderate extent, but we can effect a saving in steam and reduce the prejudical influence of heat by curing, say, at 20 or 30 lbs. instead of at 40 or 50 lbs.

Improved Mixing Conditions

Use Of Softeners

While a good deal has been heard about " scorching " or premature curing where accelerators have been funintelli-gently) used, there are many who do not yet appreciate the fact that the toughening and strengthening effects of an accelerator such as " Suparac " can fand are very largely, particularly in America) be made use of indirectly to facilitate the mixing, calendering, and forcing operations. These can be made easier by using a small quantity, say 2 to 5 per cent., of a softening agent such as stearic acid or Stockholm tar pitch. Without the accelerator such softened mixes remain soft and " sloppy " after curing, but " Suparac " possesses the property of " bringing them back," i.e., of toughening them sufficiently during the cure to make up for the softening effect which is such a valuable factor at the earlier stages of manufacture. If, therefore, manufacturers wish, with a view, say, to speeding up or facilitating mixing and forcing, or to avoiding any possibility of scorching or premature curing, to soften their mixes, an addition of a softener such as stearic acid (2-3 per cent, is usually sufficient) or neutral Stockholm tar pitch (3-5 per cent.) or good mineral rubber is recommended. Such softeners, subject to there being no specific objection to them, can be applied to the types of mixings described hereafter.