Novolac phenolics

Table 15. Properties of carbonized syntactic foams (Novolac phenolic oligomer and carbon microspheres) S9)...

Let us apply Eq. (3.86) to phenol - formaldehyde polymers synthesized in an acid medium with a phenol excess (novolacs). Phenol is a trifunctional reactant (A3), the functional groups being the aromatic hydrogens located in positions 2, 4, and 6 of the phenolic ring. Formaldehyde acts as a bifunctional monomer (B2), forming methylene bridges between the reactive positions of phenol. Novolacs are synthesized with a phenol excess, such that gelation does not occur at full formaldehyde conversion. From Eqs (3.83) and (3.86), we obtain... 

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)]. 

All samples were prepared from a commercially available epoxy cresol novolac-phenol formaldehyde novolac-tertia-ry amine based molding compound. Pelletized preforms were heated to 85°C in a RF preheater prior to being transfer molded at 180°C/68 atm. for 90 sec. Molded samples were cooled in air to room temperature and stored in a desiccated environment until testing or subsequent thermal treatment. Post mold curing, PMC, was accomplished in a gravity oven at 175°C for a period of 4 hours. Samples without post mold curing are designated by NPMC. 

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. 

Another, similar group of epoxy resins, called epoxy novolacs, forms from reactions of epichlorohydrin with low molecular weight phenolic novolacs (phenolic novolac resins are discussed in the next section) ... 

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. 

Both resole and novolac phenolics are used in structural adhesives, although novolac phenolics predominate. The resole phenolic structure is obtained by heating phenol in an excess of formaldehyde in the presence of base to obtain a structure similar to that shown below. 

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... 

Novolac Phenol-formaldehyde resins, PE Perstorp Bakelite S.A. 

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.

Boron fibers have also been used in the form of epoxy prepreg tapes in which the filaments are carefully aligned and tensioned to provide a one-fllament-thick tape. Generally, these tapes are composed of 65-70 percent filaments in the epoxy resin matrix. The longitudinal flexural modulus of boron-epoxy laminates made in this way is about 27 X 10 psi, and the flexural strength is 280,000 psi. Boron fibers have also been applied to reinforce epoxy novolacs, phenolics, and polyimides in laminate construction. 

In this chapter, we will discus the synthesis and investigation of some properties of copolymers produced in result of copolymerization of epoxy-silicon oligomers and novolac phenol-formaldehyde oligomers have been carried out. 

SCHEME 11. MODIFICATION REACTION OF NOVOLAC PHENOL-FORMALDEHYDE OLIGOMER BY EPOXY GROUP CONTAINING POLYPHENYLSILSESQUIOXANE OLIGOMER. N=1-2, WHERE, N=1 (XVIII) 2 (XIX). 

Epoxy phenolic adhesives are polyepoxy compounds which possess epoxy reactivity and phenolic resin heat resistance. These are prepared by condensing epichlorohydrin and novolac phenolics (phenol/formaldehyde ratio > 1). A recent Japanese patent issued to Mitsubishi Petrochemical Co. describes the preparation of epoxy resins from epichlorohydrin condensation with phenolic resins prepared from phenol and substituted aldehydes, e.g., vanillin. These resins (23) have high epoxy contents (epoxy equivalent weights >200) and relatively high softening points (85-95°C). 

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... 

Novolacs " (Phenol- Formaldehyde) Fig. 16. Novolac epoxy chemical structure... 

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