RFL adhesive dips

The RFL adhesive dip is prepared by dissolving the resorcinol in water, adding the formaldehyde and then the alkali as condensation catalyst, to produce the aqueous solution of the resin condensate. This generally takes place at room temperature in about 6 hours. 

About 99% of all ethylbenzene produced is converted to styrene, which is used to make SBR for the rubber industry as well as polystyrene and ABS for the plastics industries. Also, styrene is used to make SBVP latex (for use in RFL adhesive dips) as well as to produce styrenic block polymers, such as SIS, SBS, SEES, and SEPS, which are used as thermoplastic elastomers. 

There is practically no adhesion between polyester tire cord and RFL adhesive dip. Poor... 

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. 

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. 

Rayon and nylon are the main fibers with which RFL dips func-tion well directly. For polyester and glass, the textiles require a special treatment, usually involving a polyisocyanate and a reactive silane respectively, to afford appropriate levels of RFL adhesion. In the glass-silane case, the mechanistic contribution is almost surely one of strong adsorption and attachment through the silane s organofunctional end with reactive R-F species. 

Many other one-step adhesive dips for polyester fibers have been patented over the last 15 years. The adhesion activating agents in the RFL dip are usually based on isocyanates or cyanurates such as triallyl cyanurate, triphenyl cyanurate, etc. Other agents are also used based on condensation polymers similar to that used in Pexul. A more detailed discussion on this subject is given in Rubber Chemistry and Technology, Vol. 58, No. 3 (1985). 

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. 

The actual ratios of resorcinol to formaldehyde and of resin to rubber solids may be varied within reasonable limits, the generally accepted optimum ratios being between 1 1.5 and 1 2 for the resorcinol to formaldehyde and between 1 5 and 1 7 for the resin to rubber ratio. Within these limits, it is possible to fine tune the RFL formulation to suit the particular rubber compound with which it is to be used. Generally, however, it is more convenient to use a standard dip formulation for most applications, to avoid excessive complexity, while still maintaining perfectly adequate adhesion levels to a wide range of rubber compounds. For most applications, the preferred latex used is a styrene/ butadiene/vinyl pyridine (VP) terpolymer latex. 

The rayon textile is then dipped through the RFL dip and the impregnated fabric dried and baked, to cure the deposited dip solids. With these fabrics, exposures of 1 - 2 minutes at between 130 °C and 150 °C are sufficient to achieve good adhesion levels, the time and temperatures being to some extent controlled by the weight of the fabric. 

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... 

Additional evidence for sulfur s role was provided by a paper two years ago by K-D Albrecht of Bayer. With both nylon and rayon, RFL dips furnished markedly improved adhesion as the level of sulfur was increased in the carcass compound over the range from one part to three parts. This is shown in Figure 9. 

The final data of interest for the proprietary bonding agent were in regard to peel strength development as a rate process. Here we compared the solvent based adhesive again with a typical RFL dip for laminating nylon strips to SBR (Figure 22). 

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. 

An RFL dip consists of an aqueous resorcinol formaldehyde resin liquid with a ruhher latex. The RF resin is used to achieve good adhesion to the organic fiber substrate whether it be rayon, nylon, or polyester. The rubber latex portion can be a natural rubber latex or a styrene butadiene vinylpyridine (terpolymer) latex or even a neoprene latex. This rubber latex is present to achieve good adhesion with the rubber matrix itself. So the RFL allows good adhesion between the rubber and the textile cord reinforcement whether it is used in the manufacture of passenger tires, truck tires, off-the-road tires, power transmission belts, V-belts, timing belts, or various hose products. 

The textile cord or fabric is first immersed in the RFL dip suspension and then passed through a vertical oven under tension at a predetermined elevated temperature and time. This process is called heat setting, where the adhesive dries and coats the cord or fabric while under tension. 

Certain textile fabrics, such as polyester or aramid, possess low surface activity and many times require a special isocyanate or epoxy pretreatment under tension, in much the same manner that the RFL is applied afterwards. So with polyester or aramid cord, two successive dipping operations may be necessary in order to achieve good rubber-to-fabric adhesion. 

Resorcinol formaldehyde resin is a vital ingredient in preparing the RFL dips. Without the RF resin, it would be most difficult to achieve acceptable adhesion with the textile cord substrate. 

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