Substrate Reinforced Rubber Structure and Compound for Forming Same

This invention relates to rubber compouding and, more specifically, to the compounding of rubber which is to be bonded to reinforcing substrates. As is well known, the chemical make-up of reinforced rubber compounds is complex, and the demand for performance over widely varying conditions of stress and temperature has caused an increase in the variety of reinforcing substrates being used. Substrates such as polyester, nylon and steel are commonly used to reinforce rubber in the manufacture of pneumatic tires, pressure hoses and the like. It is well known to incorporate certain methylene donors and acceptors in the rubber compound in order to increase the bond between the rubber and the reinforcing substrates. It is also well known that some of the methylene donor-acceptor additions result in degradation of certain reinforcing substrates. For example, in the Endter et al U.S. Pat. No. 3,638,703, dated Feb. 1, 1972, reference is made to the degradation of polyester cord at high temperatures where hexamethylenetetramine is used as a methylene donor with resorcinol being used as a methylene acceptor. To reduce such degradation of polyester cord hexamethylenetetramine was first reacted with certain acids, and then the reaction products were used as methylene donors in the well-known donor-acceptor relationship. The use of resorcinol as an acceptor has not been entirely satisfactory since it must be added to the rubber compound at an elevated temperature, and at this temperature, resorcinol fumes, thereby liberating vapors which are noxious to persons working nearby. To reduce such problems, resorcinol has been prereacted to form the well known resorcinol-formaldehyde (RF) resins which have been used in place of resorcinol as methylene acceptors. However, the use of hexamethylenetetramine as a donor with a RF resin used as an acceptor has not been satisfactory because the adhesion thus obtained between the rubber and the substrate is poor. Accordingly, a search for satisfactory donors for use with RF resins has been conducted for several years. So far as we are aware, the only donor known to be satisfactory for use with RF resins is hexamethoxymethylmelamine which retards the rate of cure of the rubber compound in which the hexamethoxymethylmelamine was incorporated.

In accordance with our invention phenol is reacted with hexamethylenetetramine to produce reaction products which are excellent methylene donors when used with the well known RF resin as an acceptor. We have also found that such reaction products can be used as a methylene donor with resorcinol as the acceptor. Furthermore, excellent adhesion is obtained between the rubber compound and the reinforcing substrate with either of the acceptors. Such use of a reaction product of phenol as a donor was entirely unexpected due to the fact that phenol has always been considered an acceptor, as disclosed in the Endter et al U.S. Pat. No. 3,683,703 mentioned above.

Our methylene donor comprising the reaction products of phenol and hexamethylenetetramine is preferably made by the process described in the Orem et al U.S. Pat. application Ser. No. 500,906, filed Aug. 27, 1974. The present application and said application Ser. No. 500,906 are owned by a common assignee. In accordance with the process disclosed in patent application Ser. No. 500,906 one mole of hexamethylenetetramine may be reacted with one, two or three moles of phenol. Also, our methylene donor may be made by combining aqueous solutions of phenol and hexamethylenetetramine, allowing a precipitate to form, and substantially filtering off and drying the precipitate. However, our present invention relates to the novel use of the reaction product of phenol and hexamethylenetetramine to produce an improved rubber compound and reinforced structure, regardless of the manner in which the products are produced.

The benefits to be gained by the use of the reaction products of phenol and hexamethylenetetramine in rubber compounds of Table 1 reinforced with nylon are shown below.

The composition of Table 1 was maintained constant throughout this particular test, except in one case where resorcinol was eliminated, and various methylene donors were also added on the open mill and in the quantities indicated. From the resulting compounds, test samples were formed around square-woven nylon (greige) cured for sixty minutes at 300.degree. F, and adhesion was determined in a conventional manner well known to those skilled in the art.

No resorcinol was used in Experiment Number 5A to show that the adhesion obtained resulted from the acceptor-donor system and not from the reaction product of phenol and hexamethylenetetramine alone. It is well known that resorcinol alone does not significantly increase the adhesion of rubber compounds to reinforcing substrates.

With the exception of substituting a conventional precondensed resorcinol-formaldehyde resin, an RF resin, for the resorcinol, the composition of TABLE 1 was further used in the experiments shown below. Various methylene donors were also added on the open mill and in the quantities indicated. From the resulting compounds, test samples were formed around square-woven nylon (greige) cured for 60 minutes at 300.degree. F, and adhesion was determined in a manner well known to those skilled in the art.

Certain physical properties, indicative of reinforcement of the rubber compound, where also determined on the compounds obtained in Experiments 1B through 4B.

By way of comparison, with the compound obtained in Experiment 1A a stress of 1270 psi was required to produce a 300% elongation, with the elongation being 460%.

The benefits to be gained by the use of the reaction products of phenol and hexamethylenetetramine in rubber compounds of Table 2, reinforced with steel cord are shown below.

The composition was maintained constant throughout this particular test, and various methylene donors were also added on the open mill and in the quantities indicated. From these compounds, test samples were formed around brassed steel tire cord, cured for 45 minutes at 300.degree. F, and adhesion determined in a manner well known to those skilled in the art.

The composition of Table 2 was further used in the experiments shown below, and the amounts of acceptor and/or donor were varied as shown.

The physical properties of certain of the above compounds were also measured to determine whether the reaction product of phenol and hexamethylenetetramine enhanced reinforcement of the compound.

Further tests were made with a composition shown in Table 3.

This composition was maintained constant throughout the experiments shown below with the acceptor and donor being varied, as shown. From the resulting compounds, test samples were formed around brassed steel tire cord, cured for 30 minutes at 300.degree. F, and adhesion determined in a manner well known to those skilled in the art.

The composition of Table 3 was further used in a separate set of experiments shown below. The same control experiments were performed, but an RF resin was used as a methylene acceptor. The amounts of acceptor and/or donor were varied as shown.

As is well known in the art, the use of hexamethylenetetramine as a donor in rubber compounds reinforced with polyester cord tends to degrade the polyester cord at elevated temperatures. The use of the reaction product of phenol and hexamethylenetetramine in rubber compounds of Table 4 reinforced with polyester cord is shown below. The data relative to the use of the reaction product of phenol and hexamethylenetetramine in the compound of Table 4 indicates that no such degradation of polyester cord is experienced when our improved methylene donor is used with either resorcinol or RF resins as acceptors. Experiment 3G discloses the use of a salt of hexamethylenetetramine and toluenesulfonic acid as a methylene donor and is included for comparison with the results disclosed in the Endter et al U.S. Pat. No. 3,638,703 mentioned above.

The composition of Table 4 was maintained constant throughout the experiments shown below wherein the acceptor and/or donor was varied. From the resulting compounds, test samples were cured around nine-inch lengths of polyester cord for 45 minutes at 300.degree. F, and the tensile strength of the cord so cured was determined.

While the major portion of the examples revealed herein deals with the reaction product resulting from the reaction of three moles of phenol with one mole of hexamethylenetetramine, it is also shown that similar results may be expected from reaction products resulting from other molar ratio combinations. In fact, comparison of Experiments 2D, 4D and 5D suggests that the reaction products from the other molar ratio combinations may give adhesion results which are superior to the three-to-one molar reaction product. A comparison of the physical properties of the compounds obtained from these same three experiments similarly suggests that the 1:1 and 2:1 molar reaction products are more reinforcing than the 3:1 product. That is, in the resultant rubber compound, modulus is increased and elongation is decreased. In the interpretation of the results of the experiments revealed herein it should be noted that no attempt was made to optimize all of the components in a particular rubber compound. Furthermore, although only some of the available reinforcing substrates, such as cotton, rayon, nylon, fibreglass, steel, polyester, were included in the experimental work revealed herein, the adhesion obtained with all the substrates studied was excellent.

We wish it to be understood that the limited number of examples herein disclosed should not be interpreted as an attempt to define or delimit the scope of this invention for it will be readily apparent to those skilled in this art that changes and modifications may be made without departing from the spirit of the invention.