4- -phenol Novolac resin

Void-free phenolic-epoxy networks prepared from an excess of phenolic novolac resins and various diepoxides have been investigated by Tyberg et al. (Fig. 7.37).93 -95 The novolacs and diepoxides were cured at approximately 200°C in the presence of triphenylphosphine and other phosphine derivatives. Network densities were controlled by stoichiometric offsets between phenol and... 

Calculations for a branched advancement synthesis are defined to permit development of well-characterized functionality in the product molecules. In this study, the difunctional epoxy resin monomer used is the diglycidyl ether of bisphenol A, Epon 828, and the multifunctional epoxy phenol novolac resin used is DEN 438. Let ... 

Epoxy Novolac and Other Phenols. Resins of greater functionality than DGEB A can be produced in several ways. Polyols having more than two hydroxyl groups per molecule (e.g., phenol novolac resins) can be reacted with epichlorohydrin to produce epoxy novolac resins with a structure shown in Fig. 2.5. 

Ho et al (1996) examined polyol or polysiloxane thermoplastic polyurethanes (TPUs) as modifiers in cresol-formaldehye novolac epoxy resins cured with phenolic novolac resin for computer-chip encapsulation. A stable sea-island dispersion of TPU particles was achieved by the epoxy ring-opening with isocyanate groups of the urethane prepolymer to form an oxazolidone. The flexural modulus was reduced by addition of TPU and also the Tg was increased due to the rigid oxazolidone structure. Mayadunne et al (1999) extended this work to a series of phenol- and naphthol-based aralkyl epoxy resins. 

The key empirical facts surrounding the dissolution of novolac that have been established are outlined below. It was on the basis of these facts that mechanistic models of novolac dissolution were proposed. The dissolution of novolac, with the exception of a brief initial phase, proceeds with a constant rate and follows case II mass transfer kinetics (also called case II Fickian diffusion). In low-molecular-weight novolacs, the width of the gel layer is on the order of 10 nm. Arcus has demonstrated the existence of this gel layer in a high-molecu-lar-weight phenolic novolac resin, as shown in the laser interferogram trace of Fig. 11.34. This interferogram shows three separate sets of reflections, corresponding to the interface between the polymer matrix and the silicon wafer, the interface between the polymer matrix and the swollen gel, and the interface between the gel layer and the developer solution. ... 

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) ... 

Reaction Path for Synthesis of 4-(l, 1 -Dimethylethyl)phenol Novolac Resin (from ref [121]). 

Isobutyl bis(glycidylpropylether) phosphine oxide has been used as a crosslinking agent for phenolic novolac resin [77] in the production of phosphorus containing novolac-epoxy systems. It was shown that samples containing more than 2% phosphorus content produced a VO material, the industry standard for flame retardancy, bnt phosphorus-free polymers and those containing less than 2% phosphorus were consumed in the first ignition. 

Borowitz, J. Kosfeld, R. About the influence of hexamethylene tetramine content on the dynamic-mechanical properties of the polymeric system Phenolic novolac resin (PF) and nitrile rubber (NBR-29). RheolActa, 1980, 19, 770. 

Epoxy NovolaC Resins. Epoxy novolacs are multifunctional epoxies based on phenolic formaldehyde novolacs. Both epoxy phenol novolac resins (EPN) and epoxy cresol novolac resins (ECN) have attained commercial importance (53). The former is made by epoxidation of the phenol-formaldehyde condensates (novolacs) obtained from acid-catalyzed condensation of phenol and formaldehyde (see Phenolic Resins). This produces random ortho- and para-methylene bridges. 

Epoxy phenol novolac resins. Acute oral studies indicate low toxicity for these resins. Eye studies indicate only minor irritation in animals. The EPN resins have shown weak skin sensitizing potential in humans. 

DOW Chemical Company provided the commercial availability of epoxidized phenol novolac resins in the late 1950 s (6). 

FIGURE 7.6 A multifunctional epoxy phenol novolac resin. 

TABLE VI Series of Furfural-Phenol Novolac Resins"... 

Being a glycidyl ether of a novolac type phenol formaldehyde resin, epoxy phenol novolac resins (Figure 2.39 (a)) are the polymeric multifunctional counterpart of bisphenol F resin. They are supplied as highly viscous through semisolid to solid resins. They are generally prepared by reacting epichlorohydrin with novolac type phenol formaldehyde resins. 

Some examples of multifunctional epoxy resins other than epoxy phenol novolac resins are depicted in Figure 2.39 (b), (c) and (d). Triglycidylisocyanurate is a solid trifunctional epoxy cross-linker used in powder coatings, which provides a higher cross-link density and superior photochemical stability compared to BPA epoxy resins. The use of triglycidyl isocyanurate may present toxicity hazards. 

The average molar mass of phenol-novolac resins is approx. 250 to 900 g/mol, i.e., approx. 4 to 10 phenol groups. Resoles are liquid or solid with a softening temperature as high as approx. 70 °C. The median molar mass of liquid resoles is approx. 300, that of solid resoles approx. 400 to 600 g/mol [5]. 

Multifunctional epoxy resins such as aromatic glycidyl ether resins and aromatic glycidyl amine resins are commercially available. Commercially important epoxy phenol Novolac resins and epoxy cresol Novolac resins are prepared from excess epichlohydrin and phenol-formaldehyde or o-cresol-formaldehyde resins. The high functionality of these Novolac resins increases crosslink density and improves thermal stability and chemical resistance ... 

The resins, solvents and curing agents used in modem conductive epoxy adhesives are chosen to minimize residual vapors which can be released after cure. Modem epoxy adhesives generally consist of high purity Bis-A epoxies cured with a phenolic novolac resin. These mixtures do not contain boron or fluorides, and generate less ammonia or other corrosive vapors than epoxies available before 1982. 

Novolac resins are broadly used in electronics because their functionality higher than two increases the crossimkmg density and yields cured resins exhibiting enhanced chemical and physical properties. Mixtures of epoxy resins and phenol novolacs 61 are excellent structural adhesives in the aerospace industry. However, the phenolic hydroxyl groups are not very reachve at moderate temperatures and most systems include catalysts or accelerators. Classical adhesive compositions are prepared by mixing a solid epoxy resin, t5rpicahy an epoxidized phenol novolac resin (60 parts), a phenol novolac resin (40 parts), a solvent such as 2-butoxyethanol or butylcehosolve acetate, an imidazole catalyst, and silver flakes. 

Epoxy novolac resins are produced by glycidation of the low-molecular-weight reaction products of phenol (or cresol) with formaldehyde. Highly cross-linked systems are formed that have superior performance at elevated temperatures. 

Two-Stage Resins. The ratio of formaldehyde to phenol is low enough to prevent the thermosetting reaction from occurring during manufacture of the resin. At this point the resin is termed novolac resin. Subsequently, hexamethylenetetramine is incorporated into the material to act as a source of chemical cross-links during the molding operation (and conversion to the thermoset or cured state). 

I ovolac Synthesis and Properties. Novolac resins used in DNQ-based photoresists are the most complex, the best-studied, the most highly engineered, and the most widely used polymers in microlithography. Novolacs are condensation products of phenoHc monomers (typically cresols or other alkylated phenols) and formaldehyde, formed under acid catalysis. Figure 13 shows the polymerization chemistry and polymer stmcture formed in the step growth polymerization (31) of novolac resins. 

Fig. 1. Structures of commercial epoxy resins (a) phenolic novolac epoxy resin, (b) glycidated polybasic acid, (c) glycidated polyamine (A,A,A, A -tetraglycidyl-4,4 -diaminodiphenylmethane [28768-32-3] (TGMDA)), and (d) glycidated bisphenol A.

A hard carbon with high capacity can be made from epoxy novolac resin [12]. The epoxy resins used cost about US 2.50 per pound and give pyrolysis yields between 20 and 30%. However, it is well known that phenolic (or phenol-formaldehyde) resins can be pyrolyzed to give hard carbons with a yield of over 50% [42]. In addition, these resins cost about USSl.OO per pound. Phenolic resins therefore offer significant cost advantages over epoxy resins, so we... 

The substitution of water-borne versions of these primers is increasing as environmental restrictions on the use of organic solvents become stricter. These are generally aqueous emulsions of epoxy novolac or phenolic based resins stabilized by surfactants [34]. Non-ionic surfactants are preferred, as they are non-hygroscopic in the dried primer films. Hygroscopic ionic surfactants could result in excessive water absorption by the primer film in service. 

The term novolac refers to the early use of phenolic to replace expensive shellac-based coatings. Novolacs are now those resins made at formaldehyde-to-phenol molar ratios of less than one-to-one. They are generally, though not always, manufactured under acidic conditions. Sulfuric or oxalic acids are most often chosen as catalyst though aromatic sulfonic acids and phosphoric acid are also quite common. Many other acids are used for special purposes. The finished novolac resin is incapable of further polymerization or crosslinking and therefore... 

Free phenol is a major concern in the manufacture of novolac resins. This is true for several reasons. The strongest drivers are probably EPA classification of phenol as a Hazardous Air Pollutant and worker safety concerns. However, free phenol also has significant technical effects on such parameters as melt flow characteristics. In this role, free phenol may undermine the desired effects of a molecular weight design by increasing flow beyond the desired point. Since free phenol is often variable, the effects on flow may also cause variation in product performance from batch to batch. Fig. 18 shows the effects of free phenol on the flow across a series of molecular weights. Free phenol contents between 1 and 10% are commonly seen. In recent years, much work has been aimed at reducing the free phenol. 

Phenolics or phenol-aldehydes include the important commercial phenolic resin bakelite based on phenol and formaldehyde. A one-step process produces resol resin from more than one molecule of formaldehyde per phenol molecule. A two-step process uses an excess of phenol to produce novolacs - resins that have no reactive methylol groups and must be mixed with an aldehyde o undergo further reaction. 

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