Resole phenolic resins synthesis

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),... 

A Statistical study of phenol-formaldehyde resol resin synthesis reaction... 

For phenolic resins of the resols type, numerous voids (a few micrometers in size) are formed during the synthesis. The average diameter and distribution of voids depend strongly on the cure cycle their presence, together with the high values of crosslink density, explain the low fracture resistance of these networks (Wolfram et al., 1999). 

Only a few studies have tackled the problem of deriving a detailed kinetic model of the phenol-formaldehyde reactive system, mainly because of its complexity. In recent years, a generalized procedure has been reported in [11,14] that allows one to build a detailed model for the synthesis of resol-type phenolic resins. This procedure is based on a group contribution method and virtually allows one to estimate the kinetic parameters of every possible reaction taking place in the system. 

Phenolic resins for plywood are typically caustic-catalyzed phenol-formaldehyde resoles. Typical resins are water solutions containing 40-44 percent resin solids composed of 23 - 25 percent phenol, 5-7 percent sodium hydroxide and 10 - 12 percent formaldehyde. The ratio of formaldehyde to phenol in a resin has a decided effect on performance characteristics as does the synthesis procedure used in manufacturing the resin. 

The cross-linked polymers obtained from the addition-cure approach are often a complex arrangement of atoms bonded in heterocyclic and carbocyclic rings. However, the objective in the preparation of these systems is not simplicity. It is to obtain systems with the desirable properties of phenolic resins retained and the undesirable properties improved or removed. Voids are an undesirable result in the synthesis of both resols and novolacs. Hence, addition-cure phenolic resins are designed to avoid this result. Ease and flexibility of processing are also sought in the addition-cure systems. 

Byun, H.Y. Choi, M.H. Chung, I.J. Synthesis and characterization of resol type phenolic resin/layered silicate nanocomposites. Chem. Mater. 2001, 13, 4221-4226. 

When unsubstituted phenols are used in the synthesis of resols, the resins formed are a mixture of monomeric and polymeric hydroxymethylphenols, as shown in reactions (25) and (26). Since formaldehyde will react at the ortho and para positions to the phenolic hydroxyl, the composition of resols formed will depend on the P/F ratio and various reaction conditions, such as time and temperature. Resols are generally neutralized or made slightly acidic before the crosslinking reaction is carried out. Heat curing is the most important curing process and is usually performed at... 

The final structure of resins produced depends on the reaction condition. Formaldehyde to phenol (F/P) and hydroxyl to phenol (OH/P) molar ratios as well as ruction temperahne were the most important parameters in synthesis of resols. In this study, the effect of F/P and OH/P wt%, and reaction temperature on the chemical structure (mono-, di- and trisubstitution of methyrol group, methylene bridge, phenolic hemiformals, etc.) was studied utilizing a two-level full factorial experimental design. The result obtained may be applied to control the physical and chemical properties of pre-polymer. 

Phenol-formaldehyde resins using prepolymers such as novolaks and resols are widely used in industrial fields. These resins show excellent toughness and thermal-resistant properties, but the general concern over the toxicity of formaldehyde has resulted in limitations on their preparation and use. Therefore, an alternative process for the synthesis of phenolic polymers avoiding the use of formaldehyde is strongly desired. 

J. Monni, R Niemela, L. Alvila and T.T. Rakkanen, Online monitoring of synthesis and curing of phenol-formaldehyde resol resins by Raman spectroscopy. Polymer, 49, 3865-3874 (2008). 

While novolac links may form at elevated temperature during resole synthesis (Scheme 1.12), the preferred synthesis of novolac resins takes place under acid conditions (pH from 1 to 4) and with an excess of phenol. In this case the methylol intermediates cannot be isolated since they react rapidly to give methyene-bridged structures with relative molar mass <2000, as shown in Scheme 1.15. 

Unlike resoles, which show a definite preference for methylolation and condensation at the para position, ring positions in novolacs are less differentiated. The normal ratio of o,p-, and /j,p-linkages in a novolac will be 1 2 1. This may be affected by the choice of catalyst, and much work has been done to control this aspect of novolac synthesis, with the emphasis on producing highly or/Zio-Iinked resins. In some cases, the judicious choice of protic acid may lead to the desired result. More commonly, a Lewis acid salt is chosen as the catalyst. These are usually divalent metal salts of acetates or similar small carboxylates. Zinc acetate is probably the most common example. Often resins made using these salts cannot be cleanly characterized as resole or novolac. They may have a resole molar ratio and a novolac pH or they may be made near neutral conditions. As mentioned before, commercial phenolic polymers showing 85% ortho linkage are available. Solvent choices may also be important to determination of substitution patterns. 

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