Scorching Or Premature Curing

Nevertheless, the following methods of dealing with " scorching" or curing up in stock may be of utility to those not familiar with the subject.

(a) Where not impracticable (on account of colour or other reason) the addition of a small quantity of Litharge (1-2 per cent., calculated to the rubber, is generally sufficient) is to be recommended. Trials have shown that " Z " 41 Suparac " mixes so treated can be kept for months without curing up.

(h) If litharge cannot be used, and it is necessary to carry stocks for a length of time, the " master-batch " method is recommended. This is now very widely employed not only with " Suparac " but with other accelerators. The two batches may be made up to suit requirements but perhaps as good a way as any is to reserve a small part of the rubber and to work the "Z " " Suparac " well into this (after thorough mastication of the rubber). The other batch contains the balance of the rubber, the sulphur, minerals, etc. Appropriate quantities of the two batches are cut off and put through the warming-up mills as and when required.

(c) The use of softeners has already been described.

(7) Batching Out. As a rule " scorching," when it occurs, can i be traced to the great (and quite superfluous) heat stored up in the " bank " when mixing is carried out in the old-fashioned way. The modern practice in many of the world's largest mills is to pass the mix continuously between the rolls, so avoiding all bank formation. Alternatively, the mix may be cut off and batched out thin, this being repeated as often as is requisite for good mixing. The main point is the cooling effect that this batching-out process exercises on the mix.

(e) Where the "master-batch" method is not used, it is generally preferable to add the sulphur last- that is. after all other mixing ingredients. Sulphur is readily "soluble " in rubber, and does not require as much working to mix it as minerals, etc.

Proportions Of Sulphur And Accelerator

As a rule it is desirable to reduce the sulphur to a minimum, particularly when " blooming " is to be avoided or where service conditions are such that an excess of sulphur is likely to lead to deterioration. It should be borne in mind that for any quantity of sulphur there is a minimum of accelerator that will prove effective, and conversely for any given quantity of accelerator a certain minimum of sulphur is requisite in order to attain the full curing effect.

In a general way it may be said that in "Suparac " mixings the sulphur can be kept down as low as 11/2 to 2 parts per 100 of rubber, and that for each unit of sulphur, to 2 units of " Z " " Suparac " should be employed. In many cases it may not be desirable to reduce the sulphur content so far, in others it is essential : in others again it is preferable to use a higher percentage of " Suparac."

" Incompatibles." - " Suparac " should not he used with white " substitute," but may be usefully employed in mixings containing brown " substitute "

II. Tyre Mixings : Tread Mix.--

Smoked Sheet ........ 55

Carbon Black ....... 12

Zinc Oxide ........ 30

Sulphur........ ;11/2 to 2

Z. Suparac .... .... 1

The above gives a satisfactory cure in 10-15 minutes at 40 lbs., or correspondingly longer at lower heats (25 to 35 lbs. are suggested).

Side Wall

Smoked Sheet .... .... ;98

Carbon Black ........ ;12

Zinc Oxide ........ ;30

Sulphur .... .... .... ;3

Z. Suparac .... 11/2 ;to 2

Mineral Rubber .... ;1

Cure.-As for Tread.

Friction And Skin Coat

Shin.-

Smoked Sheet .... .... 50

Sulphur ... .... .... 11/2

Zinc Oxide .... .... 1

Z. Suparac .... .... 1

Cure.-As for Tread.

Friction,-

Smoked Sheet .... ;.... ;50

Sulphur .... .... ;.... ;11/2

Zinc Oxide .... ;.... ;1

Z. Suparac .... ;.. ;1

Stockholm Tar Pitch ;... ;21/2 Cure.-As lor Tread.

Notes. Curing

The cures of 10-15 minutes given above are too short for ordinary works practice. A minimum " rise " of 20 minutes should be given (according to size and type of tyre), and curing should be effected at 20 to 32 lbs. according to local condition of mould stocks, type of tyre, and the like. The actual cure at these heats can readily be ascertained by a few preliminary trials. In a general way it may be stated that curing speed is doubled for every 18 deg. rise in temperature, that is to say, for example, curing at 286° F. will be twice as fast as at 268° P. The life and strength of the " cord " is much improved by curing at the lower heats suggested. In larger sizes and balloons it is often preferable, instead of giving a prolonged " rise" to adjust the curing agents so that the various parts of the tyre may, having regard to the unequal distribution of heat, cure to the same degree in the same time. Such adjustments, however, must be tried out by the manufacturer.

Storage Of Stock And Re-Milling Of Scrap

Experiments under factory conditions have shown that, for example, the scrap from the tread rubber detailed above could be readilv re-milled and re-used arter 5 months' storage, and that such scrap could be readily re used in both calenders and forcing machines without showing any signs of curling up (see also remarks with regard to usage of litharge above). A further point noted with the mixes described was that no " blooming" occurred. This is of special importance from the point of view of ensuring that no lessening of adhesion between " cord" (or canvas) and rubber occurs.