Phenolic cross-linking

To make phenolic resins, you have to first make a phenolic prepolymer, which may be in a liquid or solid form. This can be accomplished by using either a base or an acid catalyst. The prepolymer is a low molecular weight, linear polymer that—and this is the whole key to phenolics—can be further processed at a time of the processors choosing to give the cross-linked phenolic resin.-All you need is a little more heat and pressure. The reactions are shown in abbreviated form in Figure 24-1,... 

One such process involves the thermal decomposition of a diazo compound to give an acid that cross-links phenol formaldehyde resins upon heating, similar to the conventional UV initiated plates used in the industry (Figure 4.3), but other sensitisation methods are also used (see section 4.5). It is also possible to produce plates in a dry resin process by ablation or phase change methods. 

Figure 1.14. Portion of a cross-linked phenol-formaldehyde polymer.

Figure 18. The effect of strong solvents on cross-linked phenolics...

Each of these monomers has more than two reactive sites and can react with up to three others to form a cross-linked polymer that is much stronger and more impact-resistant than the linear polymer. The very first synthetic plastic, Bakelite, was made in 1907 from cross-linked phenol and formaldehyde. Modern phenol-formaldehyde polymers are used as adhesives for plywood more than a billion kilograms are produced per year in the United States. 

C-Stage resin. (resite). The fully cross-linked phenol-formaldehyde resin, which is infusible and insoluble in all solvents. 

Lignins. Lignins are amorphous, cross-linked phenolic polymers that occur uniquely in vascular plants and comprise 20-30% of most wood. Lignins isolated from wood are polydisperse, with molecular weights in the range of thousands to hundreds of thousands (4). Lignins are produced almost exclusively from three cinnamyl alcohols, whose structures are shown in Chart I. These structural units have propylphenyl carbon skeletons and differ... 

As the name implies, these resins are derived from phenol and formaldehyde. The structural formula is shown in Figure 3.7. Phenol-formaldehyde is cross-linked phenolic resin and, in general, has approximately the same basic physical and mechanical properties as other phenolic resins. However, it does not have the impact resistance of the polyesters or epoxies. 

Cross-linked phenol/formaldehyde resins are used as baffle-plates in rocket technol-ogy (cf. Section 3.1.7) due to their low inflammability derived from their aromatic network ... 

Oleoresinous phenolic varnishes are excellent coatings that dry in 2-4 h and show exterior durability, corrosion resistance (especially when aluminum-modified), compatibility, solubility, and good package stability. Recoatability and intercoat adhesion are also excellent. The films are sensitive, however, to strong solvents and concentrated acids and alkalies. Unlike systems containing cross-linking phenolics, films containing these resins remain flexible. 

Typifying the use of PPS is the handle of a carpet-bonding iron (see Figure 9.10a). A similarity between cross-linked phenolics and poly(phenylene sulphide) is that both can withstand operating temperatures up to 250 "C Table 9.1 reproduces some of their properties. 

Kosonen H et al. Self-organized cross-linked phenolic thermosets Thermal and dynamic mechanical properties of novolac/block copolymer blends. Polymer 2001 42(23) 9481-9486. 

Mechanical strength is a property generally associated with secondary walls these walls contain substantial deposits of cellulose microfibrils and lignin, a highly cross-linked phenolic polymer that is synthesized and covalently linked into the wall at the cessation of cell expansion. Secondary walls allow plants to exploit the aerial environment and account for the ability of eucalyptus and redwood trees to reach heights of more than 100 m. 

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