Phenol formaldehyde resins crosslinking

Crosslinking may occur during the polymerization reaction when multifunctional groups are present (as in phenol-formaldehyde resins) or through outside linking agents (as in the vulcanization of rubber with sulfur). 

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 route to crosslinked phenol-formaldehyde resins via resoles corresponds to that used by Baekeland in his original commercial technique. They now tend to be used for adhesives, binders, and laminates. The resole... 

From this brief discussion it is clear that crosslinking in phenol-formaldehyde resins is complicated and no individual specimen of these materials can be characterised well at the molecular level. Crosslinking is irregular and variable, though it gives rise to a material having sufficiently acceptable properties that it became the first commercially important plastic material indeed, as mentioned in Chapter 1, these resins continue to retain some commercial importance in certain specialised applications. 

Phenol, along with formaldehyde, is used to produce a very important and versatile group of polymers known as phenolics or phenol-formaldehyde resins. These resins can be either thermoplastic or thermosetting, depending on the amount of formaldehyde used. A larger ratio of formaldehyde to phenol promotes crosslinking to produce more rigid materials. 

Thermosets differ molecularly from thermoplastics in that their individual chains are anchored to one another through crosslinks. The resulting network creates cohesive materials that demonstrate better thermal stability, rigidity, and dimensional stability than thermoplastics. Some examples of traditional thermosets are melamine-formaldehyde resins, which are used to treat fabrics to make them wrinkle-free, and Bakelite (a phenol-formaldehyde resin), a historically important polymer used in many applications, such as costume jewelry, electrical switches, and radio casings. 

With one exception [447], only sulphonated resins were used as catalysts in kinetic studies of esterification and transesterification, the resins being almost exclusively styrene—divinylbenzene copolymers in one case, a sulphonated phenol—formaldehyde resin was also used [433]. The main factors determining the catalytic activity are (i) the concentration of functional groups in protonated form (— S03H groups) and (ii) the degree of crosslinking of the copolymer (characterised by the divinylbenzene content). 

The importance of crosslinked polymers, since the discovery of cured phenolic formaldehyde resins and vulcanized rubber, has significantly grown. Simultaneously, the understanding of the mechanism of network formation, the chemical structure of crosslinked systems and the motional properties at the molecular level, which are responsible for the macroscopic physical and mechanical properties, did not accompany the rapid growth of their commercial production. The insolubility of polymer networks made impossible the structural analysis by NMR techniques, although some studies had been made on the swollen crosslinked polymers. 

Dase-catalyzed phenol-formaldehyde resins polymerized with a mole ratio of formaldehyde to phenol greater than one pose an interesting molecular weight characterization problem. This system is a dynamic one with active methylol end groups. Branched and crosslinked structures are formed, and in general, the separation of the resin from the reaction mixture is difficult. Figure 1 illustrates the chemical nature of a resole resin. 

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. 

Based on these results, it can be concluded that phenol-formaldehyde resins modified with 0.6 to about 1.0-mole of carbohydrate per mole of phenol and cured at neutral conditions can bond wood with acceptable dry- and wet-shear strengths, and wood failures. Also, reducing as well as non-reducing carbohydrates can be used as modifiers for neutral phenol-formaldehyde resins. It was found that the resins formulated under neutral conditions are very light in color and would thus be useful in the preparation of decorative products. Carbohydrate modifiers are incorporated into the resin via ether linkages between the hydroxyls of the carbohydrate and the methylol groups in the resin. Apparently carbohydrates, at least in theory, can participate in a crosslinked network. 

A pyrogram for a crosslinked phenol-formaldehyde resin (made with a basic catalyst) was done similarly to that for other polymers exemplified in this book, at 600° C in He and with the separation on a Carbowax column and mass spectral detection (see Table 4.2.2). The peak identification for the chromatogram shown in Figure 8.3.1 is given in Table 8.3.1. 

Figure 8.3.1. Pyrogram of a crosslinked phenol-formaldehyde resin sample. Pyrolysis done on 0.4 mg material at 60(f C in He, with the separation on a Carbowax type column.

Phenol reacts readily with formaldehyde to give trimethylolphenol (2,4,6-tris(hydroxy-methyl)phenol), which undergoes further alkylative polymerization in the presence of acid catalysts (equation 38). Thus-formed phenol-formaldehyde resins (prepolymers) can be used to crosslink a variety of polymers. This is a broad area of industrial significance. 

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