| Resorcinol-tannin-formaldehyde (RTF) reactions were carried out in aqueous media and adhesives based on RTF were characterized. Rates of reaction were strongly dependent on concentration, temperature, pH, and the mole ratio of resorcinol or tannin to formaldehyde. Viscosities of reaction mixtures were followed up to gelation. Gel time passed through a maximum at about 50% tannin substitution. The reactivity of T-F resins was greater than that of R-F resins. To formulate an adhesive, a styrene-butadiene-vinyl pyridine terpolymer latex (L) was added to the RTF. This promoted the phenolic-aldehyde reaction.;The stiffness and strength of adhesive films decreased slightly with increasing tannin substitution for resorcinol in a standard RFL adhesive. However, adhesive films yielded and had high elongation if more than 60% of tannin was substituted for resorcinol. A marked softening and reduction in toughness occurred at sufficiently high tannin levels. In tannin based adhesives, film strength increased with the introduction of resorcinol, which is thought to bridge reactive sites of tannin molecules, which are far apart. Adhesive films, which were heat-treated to simulate cure, had higher strength than unheated films, especially at low test rate and high temperature.;Finally, the Tire Cord Adhesion Test (TCAT) and T-peel geometries were used to measure the adhesive strength between rubber and either nylon or polyester. Generally, the pecan pith sulfite extract and purified pine bark tannin gave superior results with polyester cord, whereas, the pine bark sulfite extract and the peanut skin tannin were more effective for the nylon. In polyester bonding, an RTFL system and the N-3 system, which is a conventional single dip for polyester, were compared at various rates and temperatures. Over a certain range, the RTFL gave results comparable to the N-3 system. |
