Resols cured crosslinked

There are some reports proposing that structures not present in the aqueous resole are formed during cure, to include 2,2-diphenyl ethanol and 1,1,3,3-tetraphenyl propane crosslink structures [132,158,159]. However, these have not... 

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). 

Due to the presence of reactive CH2OH groups, resol oligomers may be converted into highly crosslinked products without the addition of hardeners. Heat curing is conducted at T 130 200°C. The polycondensation mechanisms are complex and different bridges are possible CH2-0-CH2-and CH2. The latter is thermodynamically the most stable. Therefore the methylene bridges are the prevalent crosslinks in cured resols. 

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). 

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 functionality may vary with reaction conditions. For example, in base-catalyzed copolymerization of phenol and formaldehyde, both monomers are bifunctional at ambient temperature, but phenol becomes trifunctional if the temperature is raised sufficiently. Copolymerization at ambient temperature can produce essentially linear, liquid, resole-type "prepolymers" of low molecular weight. Upon acidification and heat-curing, methylene and ether crosslinks formed by the now trifunctional phenol units transform the polymer into an insoluble resin [7] (see next page). The original Bakelite was such a "thermosetting" product. 

Resoles are synthesized from a phenol to formaldehyde mole ratio less than one. They will harden (cure) on heating and in this respect contrast with Novolacs, which require an additional crosslinking agent for curing to occur. 

Research has been aimed at understanding the mechanism of these linking reactions. This includes the reactivity of the ortho and para sites, possible intermediates involved in these linking reactions and behaviour of these higher units to further crosslinking. Attempts have been made to link the properties of the cured resin or carbon derived from these resins to the initial resin formulation and structure. As the crosslinking in a resole is very complicated, various model compounds have been used to investigate the chemistry. 

Many different structures have been identified within cured resole resins. The most common crosslink is the methylene bridge, though ethers can also be present in significant amounts . Phenoxy bridge , and carbonyl and methyl groups " , have also been identified within the cured structure. 

Bifunctional monomers, such as A-A, B-B and A-B, yield linear polymers. Branched and crosslinked polymers are obtained from polyfunctional monomers. For example, polymerization of formaldehyde with phenol may lead to complex architectures. Formaldehyde is commercialized as an aqueous solution in which it is present as methylene glycol, which may react with the trifunctional phenol (reactive at its two ortho and one para positions). The type of polymer architecture depends on the reaction conditions. Polymerization imder basic conditions (pH = 9-11) and with an excess of formaldehyde yields a highly branched polymer (resols. Figure 1.8). In this case, the polymerization is stopped when the polymer is still liquid or soluble. The formation of the final network (curing) is achieved during application (e.g., in foundry as binders to make cores or molds for castings of steel, iron and non-ferrous metals). Under acidic conditions (pH = 2-3) and with an excess of phenol, linear polymers with httle branching are produced (novolacs). 

Cellobond phenolic resins are available from Borden Chemicals in the UK. These resole resins are made with an excess of formaldehyde to produce a water-based polymer capable of crosslinking or curing purely by the application of heat. A range of grades is available for processing via the full range of composite techniques into inherently fire resistant products without the addition of flame retardants or fillers. 

Phenolic resins are prepared by a step-growth polymerisation of formaldehyde and phenol or phenol derivative using an acid or a base catalyst. The product type and the quality largely depend on the ratio of the reactants used and the nature of the catalyst. Phenolic resins are available in two varieties 1) novolac, which is a thermoplastic type and can be used as it is or can be cured with hexamethylene tetramine (HMTA) to get a crosslinked structure. This can also be viewed as a reactive intermediate, and can be transformed into other groups so different types of structures can be generated and 2) resole, which is a multifunctional reactive compound and can be cured thermally without a catalyst or an acid catalyst. 

Phenol-formaldehyde resins resemble a group of chemicals that contain the formaldehyde structure but are not necessarily associated with formaldehyde allergy. Resols and novolacs are distinguished within that group. While the chemical curing of novolacs requires the presence of formaldehyde to react with the phenol terminate group, resols do not. Resols are intermediates, which, in turn, need heat to cure (Malten 1984). Para-substituted phenol resins do not crosslink but adhere readily under pressure. Among... 

Phenolic resins are the reaction products of formaldehyde (usually in the form of aqueous formaldehyde), with phenol. Sometimes alkyl phenols, such as cresol and xylenol, are incorporated to reduce crosslink density and brittleness, although reactivity is also reduced. Phenolic resins are offered in two forms, novolacs and resols, characterised by their different methods of manufacture and curing. 

Resols possess a plurality of methylol groups through which crosslinking during curing primarily occurs. In contrast, novolacs are cured through reaction of the vacant ortho and para positions on the phenolic nuclei and require a co-reactant. 

Curing reactions applied to epoxy prepoiymers, unsaturated polyesters, resoles, and novolacs make use of three general classes of prepoiymers which are distinguished by the number and location of sites of functional groups available for subsequent crosslinking reactions. These three general classes have been defined as discussed in the following sections. 

The five AC powders used in the present study were produced after carbonisation of ground, crosslinked, fully cured, polymer (resol) precursor at 1073 K under Nitro i atmosphere for 60 minutes [9], One of the samples is used without further treatment (Sample PI), while the other four samples, prior to their characterisation, were activated at 1073 K, under CO2 flow, to different degrees (wei t loss), as follows Sample P2 - 6.5%, Sample P3 — 10%, Sample P4 -16% and Sample P5 -30% weight loss, respectively. 

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. 

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