Phenolic resin systems, cure

When cured with room temperature curing system these resins have similar thermal stability to ordinary bis-phenol A type epoxides. However, when they are cured with high-temperature hardeners such as methyl nadic anhydride both thermal degradation stability and heat deflection temperatures are considerably improved. Chemical resistance is also markedly improved. Perhaps the most serious limitation of these materials is their high viscosity. 

Ammonia is reacted with formaldehyde to produce hexamethylene tetramine, which is used as a methylene donor in the HRH adhesion system for rubber. It is also used to cure novolac phenolic resins in rubber compounds to increase hardness. 

Foam. PhenoHc resin foam is a cured system composed of open and closed ceUs with an overall density of 16—800 g/cm. Principal appHcations are in the areas of insulation and sponge-like floral foam. The resins are aqueous resoles cataly2ed by NaOH at a formaldehyde phenol ratio of ca 2 1. Free phenol and formaldehyde content should be low, although urea may be used as a formaldehyde scavenger. 

The methylated maleic acid adduct of phthalic anhydride, known as methyl nadic anhydride VI, is somewhat more useful. Heat distortion temperatures as high as 202°C have been quoted whilst cured systems, with bis-phenol epoxides, have very good heat stability as measured by weight loss over a period of time at elevated temperatures. The other advantage of this hardener is that it is a liquid easily incorporated into the resin. About 80 phr are used but curing cycles are rather long. A typical schedule is 16 hours at 120°C and 1 hour at 180°C. 

Recently, a two-part cross-catalyzed system has been developed that takes advantage of both the acceleration abilities of resorcinol resin and ester [179], The term cross-catalyzed is applied because the phenolic resin contains an accelerator-crosslinker for the resorcinol resin while the resorcinol resin carries an accelerator for the PF, in addition to itself being capable of improving PF cure speed. In each part, the resin carrier for the accelerator is not susceptible to acceleration by the material contained. It is only when the systems are mixed that the accelerators are activated. This system is faster and lower in cost than most of the resorcinol accelerators and gives better bonds (in wood products) than the ester cure alone [179], Another variant of the resorcinol approach utilizes resorcinol-glutaraldehyde resins [180-182],... 

The PVF is made by acidic reaction between poly(vinyl alcohol) (PVA) and formaldehyde. The poly(vinyl alcohol) is, in turn, made by hydrolysis of poly(vinyl acetate) or transesterification of poly(vinyl acetate). Thus, residual alcohol and ester functionality is usually present. Cure reportedly occurs through reaction of phenolic polymer hydroxyls with the residual hydroxyls of the PVA [199]. The ester residues are observed to reduce bond strength in PVF-based systems [199]. This does not necessarily extend to PVF-P adhesives. PVF is stable in strong alkali, so participation of the acetals in curing is probably unimportant in most situations involving resoles. PVF is physically compatible with many phenolic resins. 

Sulfur cross-links have limited stability at elevated temperatures and can rearrange to form new cross-links. These results in poor permanent set and creep for vulcanizates when exposed for long periods of time at high temperatures. Resin cure systems provide C-C cross-links and heat stability. Alkyl phenol-formaldehyde derivatives are usually employed for tire bladder application. Typical vulcanization system is shown in Table 14.24. The properties are summarized in Tables 14.25 and 14.26. 

Ester-cured alkaline phenolic system. The resin is an alkaline phenolic resin (essentially the same as the self-hardening resins of this type). Sand is mixed with the resin and blown or manually packed into a core box. A vaporized ester, methyl formate, is passed through the sand, hardening the binder. The total resin and peroxide addition is 1.5%. Compression strengths of 5000 kPa (700 psi) are possible. 

Typically tape or film epoxy adhesives are modified with synthetic thermoplastic polymers to improve flexibility in the uncured film and toughness in the cured adhesive. Epoxy resins can also be blended with phenolic resins for higher heat resistance. The most common hybrid systems include epoxy-phenolics, epoxy-nylon, epoxy-nitrile, and epoxy-vinyl hybrids. These hybrid film adhesives are summarized in Table 13.2, and structural properties are shown in Table 13.3. 

It is noted that the gel time decreased as the formaldehyde/phenol ratio of the phenolic resin in the system increased. Furthermore, the gel time curve of the lignin-phenolic system was, in general, similar to that of phenolic resins measured in the previous section. The similarity of the gel time curves (Figure 3) may indicate that the phenolic resin plays the major role in affecting the cure speed of the lignin-phenolic resin system even though the phenolic resin consisted of only 25% by weight in the system. 

Reactions with Model Compounds. To test whether carbohydrates were actually reacting with the phenolic resin, the reaction of methyl xyloside (III) and saligenin (V) under neutral conditions was studied. This reaction system was used as a model for the curing reaction. 

Modification of the phenol ring can affect both the kinetics and the properties of the phenol-formaldehyde system. Substitution at the ortho and/or para position of the phenol will limit the extent of cross-linking and hence the mechanical properties of the cured phenolic resin. A summary of current work on these modified-classical phenolic resins is given. 

Including 4-bromophenol in the phenol-formaldehyde resol system impacts the cross-link density of the cured product. In a systematic study of this copolymer, a comparison was made among the polymers obtained using phenol only, a 9 1 mole ratio of phenol to 4-bromophenol and a 1 1 mole ratio of phenol to 4-bromophenol. Comparisons included measurement of interlaminar shear strength and cone calorimetry tests of composites prepared using these phenolic resins and S2-glass fiber plain weave. 

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. 

Phenolic resins continue to be an important material at both the commercial and the research levels. These complex systems are fascinating not only because of their current usefulness, but also because of their potential to become even more useful materials of the future. The classical phenol-formaldehyde system is deceptively simple and lends itself to much variation depending on factors such as pH, molar ratio of reactants, preparation/cure temperature, and curing agents. The desire to enhance the properties of phenolic resins and expand the processing options has led to considerable work on modified-classical and nonclassi-cal phenolic resins. 

The modified-classical phenolic resins are particularly noteworthy for their effect on the mechanical and thermal properties of the cured resin. The processing of these systems is very similar to the classical systems. The nonclassical phenolic resins utilize phenol, but in many cases give a cured product with a chemical structure having little resemblance to the classical system. In future articles, it would be best to develop independent grouping for addition-cure phenolics resins. This redefinition of polymer types is not within the scope of this entry and instead, this interesting class of polymers has been viewed based on current designations. 

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