HRH adhesion systems

Hydrated precipitated silica used with general-purpose elastomers in HRH adhesive systems comes from water glass (sodium silicate), which is made from natural silica (sand) as shown in Figure 5.31, where it is reacted either with sodium hydroxide or sodium carbonate at very high temperatures. 

Historically, HMT was the first methylene donor used in the HRH adhesion system. However, over the years, HMMM has replaced much of the HMT used in HRH. HMT is still used to some extent in HRH adhesion. 

Ammonia is reacted with formaldehyde to produce hexamethylene tetramine, which is used as a methylene donor in the HRH adhesion system for rubber. It is also used to cure novolac phenolic resins in rubber compounds to increase hardness. 

Ammonia reacts with carbon dioxide to form urea, which is a feedstock for the production of melamine, from which the HMMM adhesion promoter for the HRH adhesion system for rubber to steel tire cord is derived. 

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. 

About 3% of formaldehyde production is used to manufacture hexamethylenetetramine, of which a small amount is used by the rubber industry in HRH adhesion systems, as a hardening agent, and as a curative. However, most of the HMT is used as a methylene donor curing agent by the plastics industry. 

Another 3% of formaldehyde production is used in making melamine-formaldehyde resins for the plastics industry. However, a small amount of formaldehyde goes into the production of HMMM, used as a methylene donor in HRH adhesion systems for rubber. 

HMT is used as a methylene donor for HRH adhesion systems and is used with reinforcing novolac phenolic resins to harden rubber compounds. 

Melamine is used to produce HMMM, a methylene donor used in HRH adhesion systems for good rubber-to-metal bonding. 

Hydrated precipitated silica has been used in the rubber industry over 40 years. Not only is hydrated precipitated silica a major inorganic filler used in rubber compounding to impart reinforcement, it is also commonly used as part of the HRH in situ rubber-to-metal adhesion systems just discussed. 

Hexamethoxymethylmelamine is the most commonly selected rubber compound additive to function as the methylene donor in HRH rubber-to-metal adhesion systems. 

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). 

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... 

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. 

While it is possible to get some adhesion using HRH without the hydrated silica, many times insufficient adhesion is imparted. Hydrated precipitated silica is a very important component of HRH, if this system is being used to achieve adequate rubber-to-... 

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. 

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. 

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-... 

Results obtained by Van Ooij in his ESCA studies on the composition of interfaces between rubber and brass" confirm that cobalt salts and HRH form essentially the same inter-facial products with a standard brass surface. Therefore, the mechanism of brass-rubber bonding must be the same for both bonding systems and differences in adhesion values must be explained by a modification of rubber properties (crosslink density, cure rate, modulus, etc.) or the rate of brass attack. Clearly, adhesion of brass to HRH-NR compounds cannot be explained on the basis of hydrogen bonds with the substrate, as in rubber-to-textile bonding. 

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