Phenolic resins general properties

General Properties. Phenolic resins generally are aqueous solutions of alkaline-catalyzed phenol-formaldehyde polymers. A typical resin would be about 40% solids, containing phenol, formaldehyde, and sodium hydroxide in molar ratios of about 1 2 0.75, and might average 10-50 phenol units linked together. These can be spray dried for application as a dried powder. Phenol-formaldehyde resins are cured with heat under pressure. The resultant bond is highly water resistant and heat resistant. The durability and weatherability of phenolic-bonded wood composites enables them to be rated for exterior use. 

These adhesives are generally based on blends of solid epoxy resins with resole-type phenolic resin. The epoxy resin component is often not the predominant component in the blend, depending on the end properties required. Phenolics are compatible with epoxy resins and will react through the phenolic hydroxyl group. The amount of phenolic resin used is generally much greater than that required to crosslink with the epoxy, so one can debate whether (1) the epoxy toughens the phenolic adhesive or (2) the phenolic increases the heat resistance of the epoxy. 

Epoxy Coreactants. One of the most successful epoxy coreactant systems developed thus far is an epoxy-phenolic alloy. The excellent thermal stability of the phenolic resins is coupled with the valuable adhesion properties of epoxies to provide an adhesive capable of 371°C short-term operation and continuous use at 175°C. The heat resistance and thermal-aging properties of an epoxy phenolic adhesive are compared with those of other high-temperature adhesives in Fig. 15.5. Epoxy-phenolic adhesives are generally preferred over other high-temperature adhesives, such as the polyimides and polybenzimidazoles, because of their lower cost and ease of processing. 

The complex chemistry of phenolic resins is well described by Martin. The general performance qf phenolic resins, however, can be understood by a consideration of the three major reactions which phenol and formaldehyde undergo (reactions 1, 2, and 3). These reactions are varied to yield the desired end properties by controlling catalyst, mole ratio of reactants, degree of reaction, and type of phenol used. Since phenol has three highly reaction positions, these reactions can take place readily at the two ortho and the para positions. Formalddiyde is generally used as 37 per cent formalin. 

MF moldings are superior to UF products in lower water absorption (see Table 4.21), greater resistance to staining by aqueous solutions such as fruit juices and beverages, better retention of electrical properties in damp conditions, better heat resistance, and greater hardness. Compared with the phenolic resins, MF resins have better color range, track resistance, and scratch resistance. MF resins, are however, more expensive than general-purpose UF and PF resins. 

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. 

The most widely used epoxy resins are reaction products of either bisphenol A or a novolac phenolic resin with epichlorhydrin. When used to manufacture corrosion-resistant structures for use in the chemical process industry, epoxy resins are generally hardened with either aromatic or cycloaliphatic amines. The hardeners for epoxy resins are, with few exceptions, added at levels varying from 20phr (parts per hundred resin) to lOOphr. This means that the hardener is actually quite a high proportion of the matrix resin and has quite a profound effect on the mechanical and corrosion properties of the cured resin. Thus the selection of the most suitable hardener is critical to the eventual success of the application. Epoxy resins have viscosities of several thousand mPas at room temperature, which makes it much more difficult to wet out glass fibre efficiently with them than with polyesters. Wet-out therefore involves heating the resin formulation to between 40°C and 60°C to reduce the viscosity to less than 1000 mPas. 

Estrone sulfokiiiase of rabbit liver has been separated from phenol sulfokinase by resin electrophoresis, but its specificity and general properties liave not yet been investigated (Nose and Lipmann, 1958). Further studies on this subject would be valuable, particularly regarding whether or not there are several estrogen sulfokiiiases and whether the phenolic or the alcoholic hydroxyl group is preferentially esterified. 

Chem. Descrip. Polyolefin wax disp. in solvent Uses Surf. modiTrer, lubricant, slip agent, scratch resist, aid for can coatings, coil coatings, general industrial coatings based on polyester, ep-ory ester melamines, epoxy phenolic resins Features High gloss retention Properties Disp. 

For many years, modified woods have been prepared by impregnation with prepolymers such as phenolic resins, followed by curing under heat and pressure. By this means considerable improvement in dimensional stability and mechanical properties may be obtained (Tarkow et a/., 1970), and products of this type are often encountered in articles of commerce, such as knife handles. In general, densely crosslinked thermosetting resins have been used in these applications. The structure of wood itself is briefly considered in Section 9.9. 

Calixarenes, coined by C. D. Gutsche, are the cyclic oligomers produced by condensation of phenols and formaldehyde (1). In other words, they are cyclic phenol resins, but their physical and chemical properties are much different from those of linear phenol resins. Many of the calixarenes are crystalline and generally have poor solubilities in either water or in organic solvents. Their melting points are generally high, while ordinary phenol resins, novolaks, soften below 150°C. 

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