Resols heat cure

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. 

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. 

To a flask equipped with a reflux condenser and mechanical stirrer are added 47 gm (0.5 mole) of phenol, 80 ml of 37% aqueous formaldehyde (1.0 mole), and 100 ml of 4 TV sodium hydroxide. The reaction mixture is stirred at room temperature for 16 hr and then heated on a steam bath for 1 hr. The reaction mixture is cooled and the pH adjusted to 7.0. The aqueous layer is decanted from the viscous brown liquid product and the wet organic phase is taken up in 500 ml of acetone and dried over anhydrous MgS04 followed by molecular sieving. The dried acetone product solution is filtered and evaporated to yield a water-free light brown syrup. The IR spectrum of the uncured resole resin dried at room temperature for 3 days is shown in Fig. 5. The same resole resin cured for 3 hr at 120 "C in air is shown in Fig. 6. 

They are self-curing and thus not storable. The heat-curing sequence of resoles is a very complex reaction in which mainly methylene and dimethylene bridges develop. At temperatures above 160 °C, the dimethylene bridges are also transformed into methylene bridges under cleavage of formaldehyde. The formaldehyde leads to additional crosslinking [5], [6]. 

When unsubstituted phenols are used in the synthesis of resols, the resins formed are a mixture of monomeric and polymeric hydroxymethylphenols, as shown in reactions (25) and (26). Since formaldehyde will react at the ortho and para positions to the phenolic hydroxyl, the composition of resols formed will depend on the P/F ratio and various reaction conditions, such as time and temperature. Resols are generally neutralized or made slightly acidic before the crosslinking reaction is carried out. Heat curing is the most important curing process and is usually performed at... 

Heat resistance is an important characteristic of the bond. The strength of typical abrasive stmctures is tested at RT and at 300°C. Flexural strengths are between 24.1 and 34.4 MPa (3500—5000 psi). An unmodified phenoHc resin bond loses about one-third of its room temperature strength at 298°C. Novolak phenoHc resins are used almost exclusively because these offer heat resistance and because the moisture given off during the cure of resole resins results in undesirable porosity. Some novolaks modified with epoxy or poly(vinyl butyral) resin are used for softer grinding action. 

PhenoHc and furfuryl alcohol resins have a high char strength and penetrate into the fibrous core of the fiber stmcture. The phenoHc resins are low viscosity resoles some have been neutralized and have the salt removed. An autoclave is used to apply the vacuum and pressure required for good impregnation and sufficient heat for a resin cure, eg, at 180°C. The slow pyrolysis of the part foUows temperatures of 730—1000°C are recommended for the best properties. On occasion, temperatures up to 1260°C are used and constant weight is possible even up to 2760°C (93). 

Most processors of fiber-reinforced composites choose a phenol formaldehyde (phenoHc) resin because these resins are inherently fire retardant, are highly heat resistant, and are very low in cost. When exposed to flames they give off very Htde smoke and that smoke is of low immediate toxicity. PhenoHc resins (qv) are often not chosen, however, because the resole types have limited shelf stabiHty, both resole and novolac types release volatiles during their condensation cure, formaldehyde [50-00-0] emissions are possible during both handling and cure, and the polymers formed are brittle compared with other thermosetting resins. 

Resoles can be cured by the addition of base or by heat alone. Their shelf life is thus limited, which is a significant deterrent to their use in fiber-reinforced composites. Resoles are often used in unreinforced appHcations in electronics and high moisture areas. 

Resoles are usually those phenolics made under alkaline conditions with an excess of aldehyde. The name denotes a phenol alcohol, which is the dominant species in most resoles. The most common catalyst is sodium hydroxide, though lithium, potassium, magnesium, calcium, strontium, and barium hydroxides or oxides are also frequently used. Amine catalysis is also common. Occasionally, a Lewis acid salt, such as zinc acetate or tin chloride will be used to achieve some special property. Due to inclusion of excess aldehyde, resoles are capable of curing without addition of methylene donors. Although cure accelerators are available, it is common to cure resoles by application of heat alone. 

While phenol is the most common monomer for novolac manufacture, it is far more common to see incorporation of other phenolic materials with novolacs than with resoles. Cresols, xylenols, resorcinol, catechols, bisphenols, and a variety of phenols with longer alkyl side chains are often used. While most resoles are made with a single phenolic monomer, two or more phenolic materials are often seen in novolac formulae. These additional monomers may be needed to impart special flow characteristics under heat, change a glass transition temperature, modify cure speed, or to adjust solubility in the application process among others. 

Curing Catalysts. If resol is heated to a high temperature, it can be cured without catalyst but in order to make curing it at an ambient temperature (room temperature), an acidic catalyst must be used. Both organic acid and inorganic acid catalysts can be used. Inorganic acids used include hydrochloric acid, sulfuric acid, and phosphoric acid, whUe benzene sulfonic acid, toluene sulfonic acid, and phenol sulfonic acid, are organic acids used. 

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. 

The composites based on the PFO resols cured ester type (UP-632, XXIV) and cychc acetal type (UP-612, XXVI) CECs exhibit higher mechanical strength and heat resistance in comparison with similar materials cured with dicarboxylic acid anhydrides [86]. 

Phenolic Resins. Phenolic resins react with high molecular mass epoxy resins on heating. Such systems are used as gold lacquers to line food containers. The similarly synthesized bisphenol A resols produce colorless coatings with a higher chemical resistance, less odor during cure, and less alteration of the taste of food in contact with the coating. Flexibility is, however, lower than with standard phenolic resins. 

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