Methylene donor

Reinforcing Resins. Reinforcement and stiffness of a compound can also be achieved with the use of reactive resins. Resins consisting of two-component systems of resorcinol or resorcinol condensation products and a methylene donor such as hexamethoxymethylmel amine (HMMM) or hexamethyltetramine (HMT) are the most popular in tires. These materials can be prereacted and added to the formula, or for more effective results they can react in situ ie, they can be added separately into the formula and react when the tire is vulcanized. 

Resoles are usually those phenolics made under alkaline conditions with an excess of aldehyde. The name denotes a phenol alcohol, which is the dominant species in most resoles. The most common catalyst is sodium hydroxide, though lithium, potassium, magnesium, calcium, strontium, and barium hydroxides or oxides are also frequently used. Amine catalysis is also common. Occasionally, a Lewis acid salt, such as zinc acetate or tin chloride will be used to achieve some special property. Due to inclusion of excess aldehyde, resoles are capable of curing without addition of methylene donors. Although cure accelerators are available, it is common to cure resoles by application of heat alone. 

Resorcinol will react rapidly with hydroxymethyl as well as free formaldehyde thus only a resin with low-to-nonexistent methylol will be stable in its presence. Such a resin is not likely to show good cure speed unless some sort of latent, protected methylene donor is present. This approach may be viable in a user s mill where the storage time after resorcinol addition is relatively short. If the resorcinol is dissolved, in-line mixing with the PF would be a reasonable approach. 

The RFK mechanism can be used with loomstate nylon and rayon fabrics, but polyesters will require a pre-treatment to achieve good adhesion. Hexamethylene tetramine should be avoided as the methylene donor when polyester fabrics are being used, as aminolysis of the ester linkages in the polyester can occur, which would cause significant degradation of fabric and adhesion to take place under severe service conditions. Hexamethylolmelamine ethyl ether should be used as the methylene donor in this case. 

In the biosynthesis of serine from glycine, (25) serves as the methylene donor. The reverse of this reaction is important in the catabolism of serine and provides a major source of the one-carbon units needed in biosynthesis (80MI11003). In addition to tetrahydrofolate, pyridoxal phosphate is required as a coenzyme in this transformation. The topic will be taken up again in the next section. 

Furfural-resorcinol oligomers have been known for many years (15,16). The reaction velocity of furfural with resorcinol is the fastest compared to other hydroxy substituted aromatics. Furfural-resorcinol polymers, when heat cured, give excellent bonding strengths, especially under conditions of high humidity. Their oligomers, even in the presence of acids, show good shelf life until reacted with a suitable methylene donor. 

Use A methylene donor for improving rubber-to-fabric adhesion. 

Cohedur. [Bayer] Methylene donor and resorcinol bonding agent for fabrics and cord iHjnded to rubber. 

Phenolic resins are of two types, reactive and nonreactive. Nonreactive resins tend to be oligomers of alkyl-phenyl formaldehyde, where the para-alkyl group ranges from to C4 to C9. Such resins tend to be used as tackifying resins. Reactive resins contain free methylol groups. In the presence of methylene donors such as hexamethylenetetramine, crosslink networks will be created, enabling the reactive resin to serve as areinforcing resin and adhesion promoter. 

Phenolic novolak resins and high styrene SBR resins are used for reinforcing and increasing the hardness and modulus of rubber compounds. Resorcinol novolak resins are used as a part of the adhesion system between rubber and brass plated steel cord or organic fibers. Both phenolic novolak and resorcinol novolak resins require the addition of a methylene donor such as hexamethoxymethylmelamine (HMMM) or hexamethylenetetramine (HMTA) to fully crosslink and become a thermoset. Phenol, alkyl phenols, and resorcinol can be reacted in bulk or in a polymeric formulation with methylene donors. Typical donors are 2-nitro-2methylpropanol (NMP), HMTA, and HMMM, used to produce a thermoset resin network in the... 

The results of labeling experiments in tissues and callus cultures of various Malva species indicate the sequence of reactions in Fig. 3 for the biosynthesis of fatty acids containing a three-membered carbocyclic ring (Yano et al., 1972a). Oleic acid is converted to dihydrosterculic acid, methionine, presumably as S-adenosylmethionine, being the methylene donor. The desat-... 

Michael Addition. The Michael addition is a conjugate nucleophilic addition of a carbanion to electron-deficient olefins (Fig. 13) (54). A base is used to form the carbanion by abstraction of protons from activated methylene donors which attack the olefin acceptor forming an enolate ion. The mechanism is 1,4-addition of a nucleophile to the conjugated system. 

Many rubber composites require that the rubber adheres permanently to a substrate such as steel, brass, textiles or other media. There are a number of established proprietary additives, which may be added to the rubber compound, which function in this capacity. The resorcinol-formaldehyde-silica (RFS) system is commonly used to achieve adhesion to metals. This system relies on the interaction of resorcinol (or a resorcinol donor), a methylene donor snch as hexamethylene tetramine and precipitated silica. Other systems that may rely on cobalt complexes or blocked isocyanates may also be used. 

Precipitated silica has wide applications in integral bonding systems based on resorcinol and methylene donor (the RES system) or for systems based on cobalt complexes [55]. High levels of adhesion to steel, zinc plate, brass and a variety of textile substrates can be achieved with these systems. 

Hoff reported on the use of certain melamine resins that were able to function as standalone or one-component systems when used in steel wire belt applications [39]. One-component melamine resins were described that were capable of forming a network in the rubber without the need for a co-reactant such as resorcinol. The performance of these new resins was compared to that of some classic two-component methylene donor-methylene acceptor systems. The new, one-component systems gave good initial and aged adhesion, equivalent tensile and dynamic mechanical properties and superior crack growth resistance than the currently used two component systems. 

Resin systems have been employed extensively for many years in applications such as the bonding of textiles to rubber, in hoses and belts where they have been used as tackifiers, reinforcers, curing agents and adhesion promoters. The addition of resin and/or cobalt depends upon the application and performance requirements of the rubber article that is to be bonded. Traditionally, resorcinol in combination with a methylene donor such as hexamethylene tetramine (HMT Figure 7.6) was utilised either alone or in compounds containing cobalt salts. On heating, HMT decomposes to produce ammonia and formaldehyde that reacts with the resorcinol to produce a stable, highly crosslinked polymeric network. 

As stated previously, cobalt salts can be used with or without a resin/silica system and for the latter a combination of all three is required to obtain the desired adhesion and rubber physical properties under all ageing conditions. The concentration of resin is variable but it generally falls in the range of one to four parts per hundred rubber and the methylene donor level is approximately in the same range. For optimum compound properties, it is usual for the resin/donor ratio to be greater than one. Cobalt metal levels generally fall in the range 0.1 to 0.3 parts, whilst silica may used at levels from 0 to 55 parts. 

These systems rely on the inclusion into the rubber matrix of resorcinol (or a resorcinol/ formaldehyde precondensate) together with a methylene donor. The addition of these two components will give a moderate level of adhesion, this can be greatly enhanced by... 

The original methylene donor used for this system was hexamethylene tetramine. While this was perfectly adequate as a methylene donor from the adhesion standpoint, it also significantly affected the cure system, and thus alternative materials have been used, especially hexamethoxy methyl melamine. 

In addition, a third class of chemical additives is used in the HRH rubber formulations. This third additive is known as a methylene donor because it actually reacts with the resorcinol formaldehyde resin during the mixing in the rubber compound. The most common methylene donor used by the rubber industry today is hexamethoxymethylmelamine (HMMM). However, sometimes another chemical, hexamethylene tetramine (HMT), may be used. 

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. 

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. 

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