Burnham has discovered that, by combining the sap of the mangrove (the cativo of the United States of Colombia) with rubber in variable proportions, he is enabled to produce permanently elastic waterproof compounds. These compounds are capable of being made into waterproof varnishes by the use of the solvents and additional ingredients ordinarily employed, and are also capable of conversion into products of greater or less hardness by the ordinary process of vulcanisation, either with or without the addition of the whiting, white-lead, or other ingredients. The so-called "cativo" is semi-solid at the ordinary temperature of the atmosphere, but becomes fluid at about 130° F. (54 1/2° C). In preparing this material to serve as the basis of the compound, it is raised to a temperature slightly above that of boiling water for the purpose of diminishing the strength of its natural odour, and is then strained through a bag-filter to separate it from insoluble impurities. When strained and purified, it exhibits a clear reddish-brown colour. In this condition it is mixed with rubber, either by using appropriate solvents, such as carbon bisulphide or mineral naphtha, or by the employment of hot kneading rolls, by means of which the mixing is effected mechanically.
In the latter case the temperature is elevated until the mixture acquires the desired degree of plasticity. The compound may be used as the base for a waterproof varnish, in which case it is preferable to effect the mixing by dissolving the cativo and rubber (say, 3 parts cativo to 1 of rubber) in carbon bisulphide, naphtha, or other solvent, to which may be added linseed oil, tar, or asphalt. When it is desired to produce a vulcanised product, it is better to effect the mixture of the cativo and rubber mechanically by the use of the ordinary kneading rolls. By vulcanising the compound with, say, 5 per cent. of sulphur, a strong elastic product results. For special purposes the proportion of sulphur may be largely increased, so that the product will acquire additional hardness. Metallic oxides, carbonates, or other solid substances may be mixed with - the compound for increasing its weight or bulk, or for other purposes. For example, a product suitable for application to canvas may be made by using the following ingredients: 23 parts cativo, 23 of rubber, 36 whiting. 7 white-lead, 5 1/2 litharge, 5 1/2 sulphur.
The vulcanisation of the mixture is effected by subjecting it to a temperature of about 275° F. (135° C).