Solid Tyres

The advantage of employing an organic accelerator such as " Suparac " in solid tyre manufacture is not, at first sight, so apparent as in the case of pneumatics and other goods, but experience has shown that even in the curing of thick masses of rubber (such as solid tyres) the benefits which accrue from their use are considerable. Firstly, if we cannot, owing to problems of heat conduction, cut down the time of cure appreciably, we can reduce the heat: we can work at, say 25 to 32 lbs. instead of at 40 to 50 lbs. This means improved quality, besides saving of steam, etc. Secondly, we can more readily employ " softeners " (such as stearic acid, Stockholm tar, pitch, M.R., and so on), because the toughening effect of the accelerator counteracts (during and after vulcanisation) the softness or " sloppiness,' produced by the softening agent. In other words, the softener is used to make handling, forcing, etc., easier, and the accelerator to "" bring the stock back." It is difficult to give detailed mixings for solid tyres, as in view of variation of conditions and types, it will be better for the individual manufacturer to apply " Suparac " in his own way. But. broadly speaking, it can be used in a manner similar to that described for treads of pneumatic tyres, allowance being made, of course, for the peculiar conditions attaching to the curing of " solids." A few trials in this direction will well repay the time and trouble taken.

Inner Tube Mixes. Red Mixes.- A mix consisting of rubber, crim son and/or golden antimonies, according to colour, etc.. may be used containing not more than 2 to 21/2 parts of sulphur (including the "free " sulphur in the antimonies) (on rubber) and sufficient "Z " " Suparac " (say, 1 to 3 per cent, on rubber) to effect curing in time required.

No inorganic accelerator (i.e., no lime or magnesia) should be added to such a mix.

Greg Mix

The following grey mix is recommended lor a trial :-

Rubber ........ .... ;100

Sulphur .... .... .... ;2

Z. Suparac ........ ;3

Zinc. Oxide .... .... ;2

Stearic Acid .... .... ;11/2

Cure

5 to 1 5 minutes at 40 lbs., according to conditions, or (pre ferably) a correspondingly longer cure at, say, 32 lbs.

" Pure Mixings " (Thread, Strip, etc.) . Extra Rapid Mixings.-

Mix

Pale Crepe ........200

Sulphur .... ... .... 4

Zinc Oxide .... .... 1

Z. Suparac .... ... 8

Cures to a maximum in about 10 minutes at 50 lbs. or 20 minutes at 40 lbs in the press. In moulds heated by direct steam, about half the time (i.e., 5 minutes and 10 minutes) is necessary. With a 10 minutes" rise (in steam), these heats can be cut down to about one minute and two minutes. A point of great importance is the "flat "curing oi* such mixes ; that is to say, there is a wide margin within which extra heat exercises no marked prejudicial effect. For instance, in the press cures at 50 lbs., the extra 5 minutes between 10 minutes and 15 minutes effects very little change in "ten-silrs " in comparison with the differences obtained as a result of heating for 5 and 10 minutes respectively. T he above cures correspond to the maximum effect attainable. For ordinary commercial work it will not be necessary or desirable to vulcanise so fully. That is to say, the time or heat (or both) may be cut down to suit conditions. Alternatively, especially where economy is desired, the percentage of "Suparac "may be considerably reduced.

Heel And Sole Mixings : High-Grade Black

A high-grade, extremely " snappy " and durable, non-blooming mix is made as follows :-

Smoked Sheet....... ;188

Catalpo No. 1 (Colloidal Clay)............ ;164

Zinc Oxide .... .... ;10

Carbon Black .... .... ;10

Sulphur .... .... ;4

Z. Suparac .... .... ;8

Cure

10-15 minutes fpress) at 15 lbs.

If there is any tendency to bloom (i.e., if rubber happens to be abnormally slow curing), this can be overcome by increasing the accelerator somewhat. If 16 of " Suparac " are used instead of 8, as much as 51/2 of sulphur can be employed, without a trace of blooming. If blooming is no objection, the "Z " "Suparac " can be reduced below 8 (down to 4) with a slight increase of sulphur (say to 5 1/2) without decreasing cure materially. The higher the proportion of accelerator, the greater the "scrap " and toughness. Mixings made on the above lines are the strongest, most durable and toughest known, and are recommended as being the most suitable type for replacing " crepe." While the specific gravity is considerably higher than that of " crepe," a sole of the above, say 1/8 in. thick, will be equal in cushioning effect to a "crepe " of double the thickness.