Crosslinking of Resole

The crosslinking of resoles is slightly more straightforward than that of novolaks, if only because there are fewer possible chemical structures involved in the setting reaction or finished structure. Since resoles are prepared under alkaline conditions, crosslinking is generally preceded by neutralisation. This enhances the ease with which network structures can form when the resin is simply heated. 

Finally, various Mannich bases used as catalysts in the crosslinking of oligomers (see also Chap. V, A.2.), are worth mentioning. Their basic properties are applied in the curing of epoxy oligomers- as well as in the production of polyurethanes and, less frequently, in the crosslinking of resols. - Compounds of types 426 and 427 are employed mostly for the above purposes. 

The crosslinking of resoles is slighdy more straightforward than that of novolaks, if only because there are fewer possible chemical... 

Reactions described in equations 27 - 29 occur at higher temperatures during the crosslinking process of resoles. All three processes together contribute to an increase in the content of methylene bridges in the final product They are responsible for crosslinking of resoles at elevated temperatures. 

Quinone raethides are able to react with every hydroxy containing compound (equations 33 - 34) forming different linkages during crosslinking reactions. These reactions are very efiSeient and therefore, they are a source for crosslinking of resoles at elevated temperatures. 

Novolac network degradation mechanisms vary from tiiose of resole networks due to differences in crosslinking metiiods. Nitrogen-containing linkages must also be considered when HMTA (or other crosslinking agent) was used to cure novolac networks. For example, tribenzylamines, formed in HMTA-cured novolac networks, decompose to cresols and azomethines (Fig. 7.50). 

Resols (phenol-formaldehyde resins) are commercially used for effective crosslinking of EPDM in the production of thermoplastic vulcanisates [8]. General studies on rubber crosslinking for different diene rubbers are presented here. 

For instance in case of an interpenetratirrg polymer network consisting of a resole resin and ethylene-vinyl acetate copolymer a phase separation durirrg crosslinking of the resole occurs. The network formed consists of a more or less pure phenolic phase and a mixed phase consisting of resole and an ethylene-vinyl acetate copolymer [183,185]. 

Explain the main differences in crosslinking of novolacs and resoles ... 

When pure polyesterimide is heated v > to 400 C or above in air, no cxosslinking takes place. Hiis is evident from the complete solubility of the heated polymer in the same solvent as before. Novolac and resole, on the other hand, vdien heated to 200 C or above and 150 C or above, respectively in air for about 2 hrs., are found to be insoluble. Hiat the insolubility is due probably to the crosslinking of f ienolformaldehyde resins is further confirmed the swelling of heated novolac or resole in 10% aqueous NaOH solution. 

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. 

But contrary to the findings of PEI-novolac blend we have noticed that the TG profile of resole is not significantly altered in case of PEI-resole blend (Fig. 8). The TG profile of the blend containing resole is not altered because above 150 C resole also behaves as a thentostable resin. The thenrostability of resole is greater than that of polyesterimide due to its enhanced crosslinking tendency. The thermostability of the PEI-resole blend depends on the amount of resole present in the blend. 

In case of resole the crosslinking may also take place through the reaction of C-metiylol groip>s (Structure ) with the carboxyl groipjs of polyesterimide (Structure, Schane II. 

They are self-curing and thus not storable. The heat-curing sequence of resoles is a very complex reaction in which mainly methylene and dimethylene bridges develop. At temperatures above 160 °C, the dimethylene bridges are also transformed into methylene bridges under cleavage of formaldehyde. The formaldehyde leads to additional crosslinking [5], [6]. 

We distinguish the following reaction states during the crosslinking of PF resins A-state Linear initial state resole or novolac liquid or solid meltable and soluble... 

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... 

Used for crosslinking novolacs or catalyzing resole syntheses, HMTA is prepared by reacting formaldehyde with ammonia (Fig. 7.5). The reaction is reversible at high temperatures, especially above 250°C. HMTA can also be hydrolyzed in the presence of water. 

Quinone methides are the key intermediates in both resole resin syntheses and crosslinking reactions. They form by the dehydration of hydroxymethylphenols or dimethylether linkages (Fig. 7.24). Resonance forms for quinone methides include both quinoid and benzoid structures (Fig. 7.25). The oligomerization or crosslinking reaction proceeds by nucleophilic attack on the quinone methide carbon. 

Resole resins are generally crosslinked under neutral conditions between 130 and 200° C or in the presence of an acid catalyst such as hydrochloric acid, phosphoric acid, p-toluenesulfonic acid, and phenolsulfonic acid under ambient conditions.3 The mechanisms for crosslinking under acidic conditions are similar to acid-catalyzed novolac formation. Quinone methides are the key reaction intermediates. Further condensation reactions in resole resin syntheses under basic conditions at elevated temperatures also lead to crosslinking. 

Crosslinking resoles in the presence of sodium carbonate or potassium carbonate lead to preferential formation of ortho-ortho methylene linkages.63 Resole networks crosslinked under basic conditions showed that crosslink density depends on the degree of hydroxymethyl substitution, which is affected by the formaldehyde-to-phenol ratio, the reaction time, and the type and concentration of catalyst (uncatalyzed, with 2% NaOH, with 5% NaOH).64 As expected, NaOH accelerated the rates of both hydroxymethyl substitution and methylene ether formation. Significant rate increases were observed for ortho substitutions as die amount of NaOH increased. The para substitution, which does not occur in the absence of the catalyst, formed only in small amounts in the presence of NaOH. 

A number of analytical techniques such as FTIR spectroscopy,65-66 13C NMR,67,68 solid-state 13 C NMR,69 GPC or size exclusion chromatography (SEC),67-72 HPLC,73 mass spectrometric analysis,74 differential scanning calorimetry (DSC),67 75 76 and dynamic mechanical analysis (DMA)77 78 have been utilized to characterize resole syntheses and crosslinking reactions. Packed-column supercritical fluid chromatography with a negative-ion atmospheric pressure chemical ionization mass spectrometric detector has also been used to separate and characterize resoles resins.79 This section provides some examples of how these techniques are used in practical applications. 

Crosslinking in phenol-formaldehyde resins is carried out on essentially linear prepolymers which have been formed by having one of the components in sufficient excess to minimise crosslinking during the initial step. These prepolymers may be one of two kinds the so-called resoles or the so-called novolaks. 

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... 

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