Use of polymers which contain units derived from 4-hydroxystyrene or 4-acetoxystyrene as reinforcing resins and adhesion promoters in rubber mixtures

#It is known to add reinforcing resins to mixtures of natural or synthetic rubber to improve their mechanical properties, in particular the hardness and the stress values. Adhesion promoters are used to improve the adhesion of vulcanized rubbers to steel cord, especially to brass-coated or galvanized steel cord. Crosslinkable phenolic resins, in general novolaks, and resorcinol or resorcinol precondensates are used for these purposes in combination with formaldehyde-releasing substances or with substances which react with the phenolic resins, resorcinol or the resorcinol precondensates via methylene or methylol groups with crosslinking, for example hexamethylenetetramine and methylol group-containing or - forming melamine and urea resins, which may be partially or completely etherified. The rubber mixtures can be based on natural rubber or the known synthetic rubbers. The reinforcing resins and adhesion promoters are mixed into the rubber mixtures at elevated temperature. Since the precondensates always contain free phenol or resorcinol, expensive precautions must be taken with this process to ensure that these phenolic materials do not get into the environment. This is particularly the case when using free resorcinol. Moreover, resorcinol precondensates, because of their hygroscopic nature, usually contain water. This makes their incorporation into the rubber mixture more difficult and impairs the properties of the vulcanized material prepared therefrom.

#The object of the invention was, therefore, to make available reinforcing resins and adhesion promoters for rubber mixtures, which, while having at least equally good effectiveness, do not release phenolic materials into the environment and can be easily incorporated into rubber mixtures.

#It has now been found that polymers which contain units which are derived from 4-acetoxystyrene or 4-hydroxystyrene, which may be ring-substituted, are suitable as reinforcing resins and adhesion promoters for rubber mixtures, which is described in more detail in the following text.

#1080 g of xylene, 10.8 g of di-tert-butyl peroxide and 810 g of p-acetoxystyrene (5 mols) are initially introduced into a 3-liter autoclave. 135 g (2.5 mols) of butadiene are pumped into this mixture at room temperature. The mixture is then heated to 140.degree. C. over a period of 1 hour. After holding the reaction mixture at 140.degree. C. for 4 hours the pressure, which initially rose to 8 bars, has fallen to 3.4 bars. To complete the reaction the reaction mixture is then heated to 180.degree. C. for a further half hour, a pressure of 4.4 bars developing. The mixture is allowed to cool to 140.degree. C., the pressure is released and the solvent is distilled off, the temperature of the reaction mixture being increased to 200.degree. C. In order to remove the residual solvent the autoclave is evacuated to 60 mbar at this temperature for a further half hour.

#After cooling, the resultant copolymer is isolated as a glass-like solid resin in 95% yield (relative to the monomers used). It contains the monomer units p-acetoxystyrene and butadiene in the molar ratio 1:0.5, has a melting point of 95.degree. C. and an average molecular weight (M.sub.w) of 52,000.

#Examples 2-5 are prepared analogously to Example 1: