Phenolic resins novolac

In other systems, such as in some Friedel-Crafts alkylations, ortho-substitution is desirable. For example, extensive alkylation at both the ortho-and para-positions of phenol with formaldehyde in the presence of an acid catalyst yields highly branched novolac phenolic resin prepolymers [Eq. (26)]. 

Phenolic Resins. Phenolic resins (qv) are formed by the reaction of phenol [108-95-2], C H O, and formaldehyde [50-00-0], CH20. If basic conditions and an excess of formaldehyde are used, the result is a resole phenolic resin, which will cure by itself liberating water. If an acid catalyst and an excess of phenol are used, the result is a novolac phenolic resin, which is not self-curing. Novolac phenolic resins are typically formulated to contain a curing agent which is most often a material known as hexamethylenetetraamine [100-97-0], C6H12N4. Phenolic resin adhesives are found in film or solution... 

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. 

The phenolic resins are condensation products of phenol and formaldehyde [144-146, 148]. These materials were among the earliest commercial synthetic plastics. Two different methods [144-146] are used to prepare them. In the first one, the condensations are base catalyzed, while in the second one, they are acid-catalyzed. The products formed with basic catalysts are called resols and with acidic ones novolacs. Phenolic resins are used widely in coatings and laminates. The pure resins are too friable for use as structural materials by themselves. They become useful plastics, however, when filled with various fillers. 

Further condensation can be obtained by heating the resole again to continue the reaction of the methylol groups. This further condensation, however, results in the evolution of gas which could result in a porous bondline. This evolution of small molecules limits the utility of resole phenolics in structural adhesives. Novolac phenolic resins have a structure similar to that shown in Pig. 1 and they are obtained by heating formaldehyde in the presence of excess phenol. As one can see, the novolac structure contains no residual methylol groups. Novolacs will not cure with themselves as do resoles, but they can be cross-linked by means of hexamethylene tetramine ( HEXA"). This is also shown in Pig. 1. 

Fig. 1. Chemistry of Formation and Curing of Novolac Phenolic Resins...

The novolac phenolic resins described above can also be used to generate materials known as novolac epoxies by the reaction of the novolac with epichlorohydr in. Th ese materials use latent catalyst systems based on aromatic amines. The reactions which take place will be discussed below. Table 1 also contains information about the physical properties of adhesive bonds made with adhesives based upon novolac epoxies, AF-143 and AP-147. These adhesives have similar chemistries, but AF-147 is a more flexible version of AF-143. This is, of course, to be ascertained by the higher peel strength of AF-147. The high strength values at... 

Zhang, Z., Ye, G., Toghiani, H., Pittman Jr, C.U. Morphology and thermal stability of novolac phenolic resin/clay nanocomposites prepared via solution high-shear mixing. Macromol. Mater. Eng. 295, 923-933 (2010)... 

Figure 2.6 Structural formula for single-stage novolac phenolic resins.

The two-step (Novolac) phenolic resins are formed with phenol, but with less formaldehyde in an aqueous catalyst, so that the solid product has capacity for further cross-linking. With the addition of a curing agent containing formaldehyde, the product becomes fully cross-linked high... 

Oil-soluble non-heat-reactive novolac phenolic resins are produced from a substituted phenol such as/>-phenylphenol,/>-terf-butyl-phenol or p-nonylphenol with a lower ftp ratio. They are designed to be used with drying oil-based varnishes as hard components. Due to the availability of a larger variety of synthetic varnishes, the usage of such varnishes has been declining fast. 

Phenolic resin (PF) is the technically most important representative of the class of condensafion resins, ft is created in several process steps from the monomer base materials carbolic acid and formaldehyde. Here, we distinguish between resols and novolacs. Phenolic resins are often processed in the form of free-flowing molding compounds under pressure at elevated temperatures individual cases of liquid processing are also known. Melamine resins, urea resins, and furan resins are also condensation resins created from formaldehyde. 

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. 

HMT is used as a methylene donor for HRH adhesion systems and is used with reinforcing novolac phenolic resins to harden rubber compounds. 

Many resins are used in making oleoresinous varnishes. These include polyterpene resins, petroleum resins, coumarone-indene resins, gmn rosins, wood rosins, tall oil rosins, rosin soaps, rosin esters, novolac phenolic resins and rosin modified phenolic resins. 

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