HRH systems

Several years ago it was discovered that a tricomponent system (1,3) consisting of hexamethylenetetramine, resorcinol, and high surface area hydrated silica (HRH system) when incorporated into a rubber compound, brought about extremely good adhesion to many types of fabrics. Subsequently, the system was also evaluated in elastomer composites for... 

Figure 6. Stress-strain curves for 50-volume rayon composite with and without HRH system...

Figure 7. Thin section of 50-volume rayon composite without HRH system stretched to 25% elongation...

The effect of the HRH system on adhesion is further illustrated by the micrographs (Figures 7-11) of the same rayon-natural rubber composite with and without HRH. Figures 7-9 show a thin section of the composite without HRH stretched to various elongations with the force applied parallel to the direction of orientation. Many voids form as the strain is increased owing to fiber-matrix bond failures. Both the number and size of voids increase with increasing strain. 

Figures 10 and 11 show the identical composite with HRH stretched to 25 and 100% elongation. In this case only two relatively small voids were formed and not until a strain of 100% elongation was reached. This is a clear demonstration of the effectiveness of the HRH system, especially when it is noted that at any given strain level, the HRH composite was under considerably more stress than the non-HRH composite owing to its considerably higher modulus. Unfortunately, it was impossible to determine the actual stress levels incurred for each micrograph since the rate of deformation could not be controlled and the samples were considerably thinner than those used in Figure 6.

The HRH systems that are used in rubber compounding today are very proprietary however, they generally consist of several rubber compounding additives, which will now be discussed. 

The following ASTM standards can be used to classify different grades of hydrated precipitated silica used in rubber compounding and HRH systems. 

Resorcinol formaldehyde resin is an extremely important component of the HRH system for achieving good rubber-to-brass-plated steel tire cord adhesion. This RF resin functions as a resorcinol donor in the rubber compound that reacts with the... 

HMMM is commonly used in HRH systems for good adhesion. It is probably the most commonly used methylene donor, which reacts with the resorcinol component of the HRH to achieve the desired adhesion. 

While it is possible to get some adhesion using HRH without the hydrated silica, many times insufficient adhesion is imparted. Thus hydrated precipitated silica is a very important component of the HRH system if it is used to achieve adequate rubber-to-metal adhesion. This is particularly true with rubber-to-brass-coated steel tire cord adhesion where there are very few practical alternatives. 

Resorcinol reacts with formaldehyde to produce RF resin for HRH systems in rubber compounds to achieve good rubber-to-metal steel tire cord adhesion, as well as good textile (polyester, nylon, rayon) cord adhesion. 

Sodium hydroxide is used to make resorcinoi to produce RF resin (which is used in HRH systems to achieve better rubber-to-metai adhesion). 

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. 

Most of the tire companies used cobalt salts and/or a variation of the HRH system in their steel cord skim compounds. 

HRH Systems. A review of currently available bonding agents such as HRH and modifications thereof, is given by Weaver." HRH designates the use of hexamethylenetetramine (hexa), resorcinol, and a hydrated silica as additives to rubber in order to improve adhesion to brass coated wire. Normally about 1.5 phr hexa, 2.5 phr resorcinol, and 15 parts of silica are added to a wire skim stock. Pre-... 

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