RFL adhesive system

As with most rubber composites the adhesion system used in power transmission belts is based on an resorcinol/formaldehyde/latex (RFL) type system. General overviews of RFL adhesion systems from a tyre cord perspective have been previously published by Takeyama and Matsui [31], and more recently by Solomon [32]. Bonding with RFL systems is achieved through applying an RFL coating to the fibre structure prior to the... 

Natural and SBR latexes were first used in RFL adhesive systems. However it was found that latexes containing vinyl pyridine terpolymer (B/ S/VP) gave high levels of adhesion of fibers to rubber. Fig. 2 illustrates the chemical composition of this latex. 

The most common adhesive system used for bonding continuous fibers and fabrics to rubber is resorcinol-formaldehyde latex (RFL) system. In general, RFL system is a water-based material. Different lattices including nitrile and SBR are used as the latex for the adhesive system. 2-Vinylpyridine-butadiene-styrene is the common latex used in the adhesive recipe. RFL system is widely being used in tires, diaphragms, power transmission belts, hoses, and conveyor belts because of its dynamic properties, adhesion, heat resistance, and the capacity to bond a wide range of fabrics and mbbers. 

The standard aqueous treatment for textile adhesion is based on resorcinol/formaldehyde resin/latex (RFL) dip systems. A typical formulation for such an adhesive dip is given in Table 9.1. 

Before synthetic textile cord was developed, the rubber industry was using casein or animal blood to promote adhesion between cotton cords and rubber. However, when rayon started to be used in the manufacture of tires in 1931, a new adhesive system was needed to promote a good rubber-to-rayon bond. In 1938, W.H. Charch and D.B. Maney of DuPont developed a new adhesive system that involved dipping rayon cord into what is today referred to as an RFL dip. This RFL dip has changed and been modified for the newer synthetic cords discussed earlier however, basically it is still very similar to what was originally developed seven decades earlier. 

Benzene is used as a feedstock to produce resorcinol, a very important chemical that is used to manufacture resorcinol formaldehyde (RE) resin for RFL dips for tire cord adhesion, as well as RF resin for HRH adhesion systems for rubber-to-metal adhesion, and for the production of TIER for curing polyurethane rubber. 

Methanol is oxidized to formaldehyde to make RF resins (for RFL dips for cord adhesion to rubber), methylol-terminated PF resins (to cure butyl rubber), RF resins (for HRH rubber-to-metal adhesion system), HMMM (as a methylene donor in HRH rubber-to-metal adhesion system), IMP (to cure polyurethane elastomers), and 1,4-butanediol (to cure polyurethane elastomers). 

Most of the aramid adhesive systems initially used were based on a first-step dip of either an epoxide or a blocked polyisocyanate and then a second or top coat of RFL. Wenghoefer in his patent disclosed a somewhat different first step dip which consisted of bis(j3-azido-formyl oxyethyl)isophthalate. After drying, the cord thus treated, was then dipped in a phenolic adhesive and cured at 230 C for 60 seconds. A patent issued to Van Gils uses a one-step dip based on vinyl pyridine latex, trimethylol-phenol, resorcinol-formaldehyde resin, and... 

Two papers, by separate groups working simultaneously, described the structure of the resorcinol-formaldehyde-latex (RFL) adhesives used for bonding rubber to tire cords. Rahrig [565] suggested that the RFL adhesives are two phase systems with an interpenetrating network... 

Resorcinol formaldehyde latex (RFL) cord dips have only a limited application within the general rubber goods industry and for adhesion to be achieved with synthetic fibres it is necessary to use the systems developed by Bayer and Degussa. These systems are based upon a combination of resorcinol, a formaldehyde donor and a hydrated silica filler (commonly called the RFK system). This system is incorporated as dry ingredients into the rubber compound and is activated by the application of heat. 

Cords used for reinforcement of such products as drive belts, V-belts and hose require treatment with RFL to achieve the ultimate adhesion to the rubber compound. Although many simple systems can be built in house it can often be preferable to purchase a custom built unit. The dipping and drying units are usually built in tandem, with let-off and wind-up units attached. 

Again the standard RFL system is ideal for use with polyamide yarns and fabrics. With these fibres, however, it is necessary to use higher levels of VP latex than with rayon, a minimum of 80% VP latex being required to obtain the optimum levels of adhesion. This is illustrated in Table 9.3. 

One of the earlier but very successful systems was the DuPont D417 or Shoaf system [6], using an epoxy product with a blocked isocyanate as a first stage, to react with the polyester surface, giving improved adhesion when followed with the RFL second stage dip. The basic formulation for this is given in Table 9.4. 

The use of a solvent solution of an isocyanate as the predip for polyester textiles, followed by an aqueous RFL dip has already been described. The main solvent systems employed today involve the use of rubber cements or solutions, generally including adhesion promoters such as isocyanates, to give the required levels of adhesion between the applied... 

More indirect support comes from the observation that at least one non-sulfur vulcanization system for NR does not result in adequate adhesion to RFL treated textiles. When rubber was crosslinked with a diisocyanate-nitrosophenol adduct (or diurethane" system),... 

Sulfuric acid is used in the sulfonation of benzene to form resorcinoi, which is used to produce RF liquid resins for RFL dips (needed to achieve good rubber-to-fabric adhesion) and RF resins to be used in rubber compounding in HRH systems to achieve good rubber-to-metal adhesion. 

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