Resorcinol Formaldehyde Resin Solid

Originally the tire industry used straight resorcinol as the R component in the HRH compound for good steel tire cord adhesion. However, with environmental and safety concerns in handling free resorcinol in the rubber mixing operations, resorcinol is now used in the tire and rubber industry as a solid resorcinol formaldehyde resin (or RF resin), which is somewhat safer to work with. 

These solid resorcinol formaldehyde resins used in HRH compounds are produced from a chemical reaction between resorcinol and formaldehyde as shown in Figure 5.32. 

A-ring of the tannin (194). Resin synthesis conditions varied in the approach to addition of formaldehyde. For example, the tannins could be crosslinked by reaction of a phenol-resorcinol-formaldehyde resin carrying little or no methylol functionality by addition of paraformaldehyde or by reaction of a phenol-formaldehyde prepolymer carrying a comparatively high methylol functionality. Adhesive formulations were similar to those described above. Typically, 100 parts of a 55% solids content water solution of wattle tannin were combined with 0.25 parts of defoamer, 7 parts of paraformaldehyde and 9 to 10 parts of coconut shell powder the pH was adjusted to 6.5 to 7.4 by addition of 40% NaOH. These adhesives also provided exterior quality glue lines. They are exceptional in their tolerance to high moisture content veneer and permit fast curing rates, subjects of particular interest in the plywood industry today. 

A small amount of formaldehyde is used to make resorcinol formaldehyde resin for RFL dips for tire cord adhesion, as well as to make solid RF resins for use in HRH adhesion systems for good steel tire cord adhesion. 

Applications for cold-setting, wood-laminating adhesives initially followed the same approach (47) used for laminating resins from western hemlock (38) (i.e., reaction of tannin with phenol-resorcinol-formaldehyde prepolymers). Improvements resulted through the application of Kreibich s Honeymoon technique (48) wherein one side of the material to be bonded is treated with resin and the other with catalyst. One of the preferred systems (49) was phenol-resorcinol-formaldehyde or tannin-resorcinol-formaldehyde at pH 8 with extra paraformaldehyde on the A-side and tannin at 53% solids or tannin-resorcinol-formaldehyde at pH 12 on the B-side. Such resin systems are currently used to laminate eucalyptus or pine in most South African timber-laminating plants. 

The second is a two-component adhesive " component A is a m-aminophenol modified novolac at a pH of 5 (about 63% solids) and component is a formaldehyde donor used as hardener. The adhesive has gap-filling capability, impact resistance, and is thixotropic. A phenol-resorcinol laminating resin can be used as a primer, with good results. 

Resorcinol differs from other phenols in that it reacts readily with formaldehyde under neutral conditions at ambient temperature. To make stable adhesives, which can be cured at the point of use, they are prepared with less than a stoichiometric amount of formaldehyde. About two thirds of a mole of formaldehyde for each mole of resorcinol will give a stable resinous condensation product. The resin is formed into a liquid of convenient solids content and viscosity. Such solutions have infinite stability when stored in closed containers. Glue mixes formed at the point of use from these solutions, on addition of paraformaldehyde-containing hardeners, will have a useful life of several hours due to two principal factors (1) the paraformaldehyde depolymerizes to supply monomeric formaldehyde at a slow rate, as determined by the pH (2) the availability of the formaldehyde is also controlled by the kind and amount of alcohol in the solvent. Formaldehyde reacts with the alcohol to form a hemiacetal. This reaction is reversible and forms an equilibrium which exerts further control on the availability of the formaldehyde. 

The use of hydroxymethylated resorcinol (HMR) as an adhesion promoter to enhance solid wood bonding is a recent development that holds considerable promise in improving the durability of the bonds between solid wood and adhesives. HMR consists of a mix of methylolated resorcinol monomers and low molecular weight oligomers [1]. Its usefulness as an adhesion promoter has been shown for a number of different wood species and different adhesives formaldehyde-based resins, epoxies and polyurethanes [2-7]. 

The best adhesive formulation for phloroglucinolic tannins, such as pine tannin extracts is, instead, a comparatively new and is also capable of giving excellent results when using resorcinol tannins such as a wattle tannin extract [68-71]. The adhesive gluemix consists only of a mix of an unmodified tannin extract 50 per cent solution to which paraformaldehyde and polymeric nonemulsifiable 4,4 -diphenylmethane diisocyanate (commercial pMDI) are added [68-71]. The proportion of tannin extract solids to pMDI can be as high as 70/30 w/w, but can be much lower in pMDI content. This adhesive is based on the peculiar mechanism by which the pMDI in water, is hardly deactivated to polyureas because it reacts faster with the hydroxymethyl groups of a formaldehyde-based resin, be it a tannin or another resin [69,71]. 

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. 

Resorcinol -161, - ol n (1881) (resorcin, m-dihydrox-ybenzene, 3-hydroxyphenol) C6H6O2. A white, needlelike, water-soluble solid, a benzene derivative (1,3-substituted with two hydroxy group) originally obtained from certain resins, now usually synthesized. When reacted with formaldehyde, produces resins suitable for cold-setting adhesives... 

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