The principal objects of adding rubber to bitumen are to give the bitumen elasticity, to increase its ductility and to reduce its susceptibility to temperature changes. The effectiveness of rubber in bringing about these desirable changes depends on the extent to which the rubber dissolves in the bitumen. To dissolve read ily, the rubber must be unvulcanised and be in a finely divided state. Rubber exists in this condition in the form of latex, and it is known to add rubber in this form both to bitumen emulsions and to hot bitumen and to add rubber in the forms of vulcanised and unvulcanised rubber powder to hot bitumen and to asphalt paving mixtures during the process of mixing.
In the preparation of asphalt paving mixtures using bitumen in which rubber in the form of latex has already been incorporated, I find that If, for example, from 2 to 5% by Weight of rubber in the form of latex is added directly to hot bitumen, for instance at a temperature of 150 C., there is a rapid evoluticn of steam and consequent foaming but this may be controlled by stirring and the water in the latex may be driven off by maintaining the composition at a temperature exceeding 100 C. If the temperature of the composition is not allowed to exceed 165 C., the resulting water-free rubberised bitumen will be very elastic, will have a high ductility and have a much reduced tendency to flow under the influence or heat. Even at a temperature of 165 C., the material will be highly viscous. If the temperature during manufacture is raised, for instance to 190 C., the viscosity will be reduced and the rubberised bitumen will iicw readily even when the temperature is reduced once more to 165 C. The resulting material will not, however, have the high elasticit ductility and the reduced susceptibility to temperature changes of a composition which had not been subjected to temperatures over 165 C.