Coumarone and indene resins are used, to some extent, in linoleum, for impregnating roofing felt, in electrical and friction tapes, paper and cloth sizing, printing inks, brake linings, adhesives, artificial leather, oil cloth, and shoe polishes. As a substitute for chicle as much as 10 percent may be incorporated in the chewing gum mixture. Their application in molded articles is very limited because of their brittleness and low tensile strength. Production in the United States. There are three domestic makers of these resins. Statistics of production and sales cannot be published without disclosing the operations of individual companies. The output, however, has increased appreciably in recent years and this type of synthetic resin is now among the most important produced. Imports into and exports from the United States. There have been no recorded imports of coumarone and indene resins in recent years. This is understandable because the duty alone would usually be more than the domestic price.[4] Official export statistics do not separately record these resins, although quantities are exported to nearby countries, including Canada. 8. PETROLEUM RESINS Considerable research work has been done on the synthesis of resins from petroleum. It has long been known that cracked petroleum distillates, when stored for a time, have a tendency to form gums. This tendency is so pronounced that inhibitors are added to arrest such formation. These gums are of little value as resins, but it is possible to obtain good varnish resins by oxidation or controlled polymerization of certain distillates of petroleum cracking. By carefully controlling operations, resins of varied properties are obtained and several of them have become commercially important. The unsaturated compounds, largely olefins and diolefins, present in highly cracked petroleum distillates can be polymerized, with certain catalysts. The resin produced depends upon the types of unsaturated hydrocarbons present and upon the conditions of polymerization. Properties and uses. Several types of petroleum resins are on the market, one made from the “polymer slop” obtained in the high temperature, vapor-phase cracking operation, and the other prepared primarily for the production of resin. The former is marketed under the trade name Petropol and the latter as Santoresin. The “Petropol” resins are marketed in two grades, No. 1158 and No. 2138. The specifications for these are as follows: