Resole Hexamethylenetetramine

Most phenohc foams are produced from resoles and acid catalyst suitable water-soluble acid catalysts are mineral acids (such as hydrochloric acid or sulfuric acid) and aromatic sulfonic acids (63). Phenohc foams can be produced from novolacs but with more difficulty than from resoles (59). Novolacs are thermoplastic and require a source of methylene group to permit cure. This is usually suppHed by hexamethylenetetramine (64). 

Resol resins thermoset on heating and are used for adhesives. Novolacs require a further source of formaldehyde in the form of hexamethylenetetramine to produce molding powders. Phenolic moldings are resistant to heat, chemicals, and moisture with good electrical and heat insulation qualities. Complex phenols from, e.g., cashew-nut shell liquid, are used in making brake... 

This chapter emphasizes the recent mechanistic and kinetic findings on phenolic oligomer syntheses and network formation. The synthesis and characterization of both novolac- and resole-type phenolic resins and dieir resulting networks are described. Three types of networks, novolac-hexamethylenetetramine (HMTA),... 

The reaction mixture is dehydrated at temperatures as high as 160°C (higher temperatures can be tolerated than with resoles). The prepolymer is cooled, crushed, blended with 5-15% hexamethylenetetramine, (CH2)6N4, and sold to the fabricator. Hexamethylenetetramine, referred to as hexa, is the product of the reaction of 6 mol of formaldehyde and 4 mol of ammonia. Curring occurs rapidly on heating with the formation of both methylene and ben-zylamine crosslinking bridges between benzene rings. The crosslinked network is pictured as... 

The novolac foams are made by mixing the solid (powdered) oligomer (70 parts), carbon filler (30 parts), and hardener (hexamethylenetetramine) in a vibromixer and then press-molding the mixture for one hour at 150 °C, at a compression of 30-40% of the initial volume39). Casting compositions curable at room temperature are made from resol cold hardening oligomers 83). 

Often, the additive is a formaldehyde source such as hexamethylenetetramine (HMTA). On the other hand, a resole is capable of forming a network structure by the application of heat. 

The novolac resins are prepared by using acidic catalysts and a deficiency of formaldehyde. Because this type of resin is less reactive, cross-linking is accomplished by the addition of a curing agent or catalyst. The most common is hexamethylenetetramine or "hexa." The curing agent serves as a latent source of formaldehyde. As in the case of the resoles, volatiles are emitted during the cure. The chemistry of the phenolic resin is old but complex and well documented in the literature (10). 

Novolacs are low-molecular-weight, fusible but insoluble prepolymers (18) prepared by reaction of formaldehyde with molar excess of phenol. Novolacs, unlike resoles, do not contain residual methylol groups. A high-molecular-weight network polymer similar to that of resoles is formed by heating novolac with additional formaldehyde, paraformaldehyde, or hexamethylenetetramine. 

Phenolic resins are also extensively used in the binding of foundry molds. Both resol and novolak resins are used for this application. The sand is coated with the phenolic resin at a rate of 3 to 4%. The PF resin can be used both as an organic solvent solution and in powder form. Coating of the substrate can be done both at ambient or at higher temperature. In higher-temperature coatings novolaks are the preferred resins and in this application, waterborne resins (75% resin) can also be used. Hexamethylenetetramine as well as wax are added. Hexamine is often added separately from the resin to avoid precuring. 

Tertiary amines [70], some inorganic salts [71], ammonia [70], and l,3,5,7-tetraazatricyclo[3.3.3.P ]decane (hexamethylenetetramine) [72] can catalyze the formation of resoles. The basic pH in the reaction mixture produees a phenolate anion. This anion possesses a higher electron density at the ortho and para positions. Therefore, the electrophilic attack of the carbon end of the methanal occurs more efficiently in these positions. Furthermore, the dehydration of the hydroxymethylol group oecurs slower because the cation of the basic catalyst stabilizes the hydroxymethylated phenols. A typical example for such a stabilized structure is given in Seheme 3. 

The early technique for PF manufacturing was based on the expansion of novolac-hexamethylenetetramine (FiMTA) mixtures. Now, the manufacturing is based on acid catalysed process of a resole type PF with added foaming agent and surfactant [43]. 

Phenolic resins such as resol-type resins generate water and formaldehyde during curing as by-products of condensation reactions. Hexamethylenetetramine (HMTA) can be used as a source of formaldehyde for curing novolac-type resins, producing formaldehyde and ammonia as by-products. During curing. 

Resole on the other hand, does not require hexamethylenetetramine, for crosslinking because resole type resin contains relatively large amounts of ether-linkages in its structure. It is, therefore, expected that a blend of polyesterimide and resole will result in a crosslinked product on heating at lower teannpera-ture than novolac. 

It is to be noted that the relative amounts of the various linkages shown above are not intended to have any quantitative significance. Thus the network polymer obtained from the novolak-hexamethylenetetramine reaction has a structure which is predominantly similar to that of the network polymer derived from a resol. 

One-stage resins or resoles contain an adequate number of methylol groups to be cured either thermally (160°-200°C) or by acids. Novolac resins (two-stage resins), which are prepared with an acidic catalyst and less than one mole of formaldehyde per mole of phenol, are permanently soluble and fusible and cure only upon the addition of a curing agent (heat and paraformaldehyde or more preferably hexamethylenetetramine) [13, 78]. 

Usually hexamethylenetetramine (hexa) is used at 0% of the novolac resin. Resin and hexa are ground together before curing the resin. These mixes are stable in the dry state. The physical properties of the cured resole or novolac reach a maximum degree of cross-linking at or near approximately 1.5 1.0 ratio of formaldehyde to phenol. 

The novolak can then be treated with crosslinkers such as hexamethylenetetramine in a mold to give further condensations and a thermoset polymer. With both the resole and the novolak the structures can be affected by the reaction conditions and the choice of catalyst. In particular, the ortho para ratios can vary. Resoles are characterized by a large number of methylol groups which can condense with the phenolic rings to evolve water and give additional methylene bridges in the cure step. 

Phenolic resins are obtained by polymerizing phenol with formaldehyde. When polymerized at low pH (i.e., acidic reaction medium), the resultant material is a straight-chain polymer, normally called novolac. However, under basic conditions, a higher-branched polymer called resole is formed. To cure novolac, a cross-linking agent, hexamethylenetetramine, is required, which has the following chemical formula ... 

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