HRH adhesion

One of the original uses for precipitated hydrated silica was its use in the original HRH adhesion compounds. Even though silica is also used today as a very important inorganic filler to improve rolling resistance in tire tread compounds, it is still used extensively in the wire coat or breaker compounds to impart steel tire cord adhesion. 

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. 

Sulfur dioxide is used in the production of resorcinol, which is used to make RF resin for HRH adhesion applications for rubber. 

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. 

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. 

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. 

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 a vital ingredient in the HRH rubber adhesion compounds (such as wire coat stock or breaker stock) in order to achieve good rubber-to-brass-coated steel tire cord adhesion. 

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... 

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

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. 

There are other methylene donors that can be used in HRH. Hexamethylene tetramine (HMT) is the second most common choice for use in HRH. However, the resulting adhesion characteristics will be different when HMT is used in place of HMMM. 

If the HRH formulation has been fine-tuned to take advantage of a cobalt salt such as cobalt stearate, there might be a significant loss in adhesion performance if it were not available. 

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). 

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. 

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