Phenolic chemistry formaldehyde reaction

Although phenolic resins have been known and widely utilised for over 60 years their detailed chemical structure remains to be established. It is now known that the resins are very complex and that the various structures present will depend on the ratio of phenol to formaldehyde employed, the pH of the reaction mixture and the temperature of the reaction. Phenolic resin chemistry has been discussed in detail elsewhere and will be discussed only briefly here. 

The other root causes were (1) the poor understanding of the chemistry, (2) an inadequate risk analysis, and (3) no safeguard controls to prevent runaway reactions. This EPA case history also summarized seven similar accidents with phenol-formaldehyde reactions during a 10-year period (1988-1997). 

The workhorse of the VLSI industry today is a composite novolac-diazonaphthoquinone photoresist that evolved from similar materials developed for the manufacture of photoplates used in the printing industry in the early 1900 s (23). The novolac matrix resin is a condensation polymer of a substituted phenol and formaldehyde that is rendered insoluble in aqueous base through addition of 10-20 wt% of a diazonaphthoquinone photoactive dissolution inhibitor (PAC). Upon irradiation, the PAC undergoes a Wolff rearrangement followed by hydrolysis to afford a base-soluble indene carboxylic acid. This reaction renders the exposed regions of the composite films soluble in aqueous base, and allows image formation. A schematic representation of the chemistry of this solution inhibition resist is shown in Figure 6. 

BAEKELAND, L. EL (1863-1944). Born in Ghent, Belgium. He did early research in photographic chemistry and invented Velox paper (1893). After working for several years in electrolytic research, he under took fundamental study of the reaction products of phenol and formaldehyde, which culminated in his discovery in 1907 of phenol-formaldehyde polymers originally called Bakelite. The reaction itself had been investigated by Bayer in 1872, but Baekeland was the first to learn how to contiol it to yield dependable results on a commercial scale, The Bakelite Co, (now a division of Union Carbide) was founded in 1910. 

The chemistry of phenolic molding powders begins with the reaction of phenol with formaldehyde. 

A discussion of the various possible facets of the chemistry of the interactions of lignins and carbohydrates with phenol lies beyond the scope of this article. However, in a general way, it can be stated that, because of the excess of phenol in the reaction mixture, the combination with either lignin or carbohydrates is most likely to involve only one of the three reactive positions on the aromatic ring of the phenol. Consequently, the reactivity of the modified lignins and carbohydrates to formaldehyde ought to be enhanced relative to the prephenolyzed material because, for every reactive position lost by combination, two new reactive positions are created by the added phenol moiety. 

This section gives a brief overview of the chemistry of classical phenolics. Classical phenolics refer to the reaction of phenol and formaldehyde in water under basic or acidic conditions. 

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. 

Up to this point, we have presented only a general overview of phenolic chemistry with some illustration of applications. We now turn to a more detailed and quantitative discussion of the methylolation process. Though other aldehydes are occasionally used, formaldehyde is overwhelmingly the aldehyde of choice, and we will limit our discussions to phenol-formaldehyde reactions. 

Our story takes an important turn when W.H. Story applied some of the methods of industrial chemistry to the reaction of phenol and formaldehyde [35]. He constructed a heated pressure reactor with inlet and outlet valves, and an independent source of gas pressure. The system was charged with commercial carbolic acid (phenol) and 40% formaldehyde solution. It was heated at 100 °C and stirred for 8 h. The highly viscous solution was then drawn off and concentrated to drive off water. Further heating led to a solid product that was clear, tough and a good electrical insulator. But, chemically, what was it ... 

Synthetic developments in the area of Supramolecular Chemistry are currently leading to a massive production of new macrocycles. The driving force for this continuous growth is the search for selective hosts to target a particular neutral or ionic species. There is no doubt that the impact produced by the discovery of macrocyclic ligands such as the crown ethers [1] and the cryptands [2] resulted from their cation complexation and this prompted us to consider the thermodynamic characterisation of these systems (mainly cryptands) which has been extensively reported [3-5]. Calixarenes, an important class of macrocyclic compounds, are products of the base-catalysed condensation reaction of p-substituted phenols and formaldehyde [6, 7]. These compounds are characterised by their low solubilities in most solvents, although until recently [8], no quantitative data has been reported. Functionalisation of the lower or upper rim of parent calix[n]arenes has... 

Bakelite, a copolymer of phenol and formaldehyde, was the first commercial polymer. It was prepared in 1907 by Leo Bakeland. The chemistry of this reaction was described in Section 24.6. The first step is an addition reaction of a phenolate to formaldehyde to give either of two isomeric hy-droxymethylphenols. 

It is also possible to form polymers which possess very high molecular weights via a process known as crosslinking. The chemistry of one such process which involves the formation of a phenolic polymer is described below. The reaction outlined takes place between phenol and formaldehyde to form an alcohol derivative which then reacts further with additional formaldehyde to build an extremely high molecular weight, insoluble polymer. The crossUnking routes and major mechanisms are shown in the second diagram ... 

Positive-Tone Photoresists based on Dissolution Inhibition by Diazonaphthoquinones. The intrinsic limitations of bis-azide—cyclized rubber resist systems led the semiconductor industry to shift to a class of imaging materials based on diazonaphthoquinone (DNQ) photosensitizers. Both the chemistry and the imaging mechanism of these resists (Fig. 10) differ in fundamental ways from those described thus far (23). The DNQ acts as a dissolution inhibitor for the matrix resin, a low molecular weight condensation product of formaldehyde and cresol isomers known as novolac (24). The phenolic structure renders the novolac polymer weakly acidic, and readily soluble in aqueous alkaline solutions. In admixture with an appropriate DNQ the polymer s dissolution rate is sharply decreased. Photolysis causes the DNQ to undergo a multistep reaction sequence, ultimately forming a base-soluble carboxylic acid which does not inhibit film dissolution. Immersion of a pattemwise-exposed film of the resist in an aqueous solution of hydroxide ion leads to rapid dissolution of the exposed areas and only very slow dissolution of unexposed regions. In contrast with crosslinking resists, the film solubility is controlled by chemical and polarity differences rather than molecular size. 

Another common gas that appears on the list of potential teratogens is formaldehyde. Since it is normally used as a 40% aqueous solution ("formalin"), it is listed in Table 2 with the organic liquids. Only four of the twenty lab manuals use formaldehyde one in a clock reaction, two to test for the presence of the aldehyde group, and the other to make a polymer of the phenol-formaldehyde type. In none of these is the use of formaldehyde essential. There are other simple clock reactions, there are other less hazardous aldehydes, and there are other polymerization reactions that would be more suitable for an introductory chemistry course. 

Several chapters also demonstrate the use of smaller molecular-weight carbohydrates (i.e., monomers) in adhesives. Tony Conner and his colleagues (Chapter 25) explore the partial replacement of phenol-formaldehyde adhesives used to bond wood with various wood-derived carbohydrates. A1 Christiansen (Chapter 26) and Joe Karchesy and his coworkers (Chapter 27) investigate the very complicated chemistry and the practical application of adhesives based on the reaction of a carbohydrate with urea and phenol. Tito Viswanathan (Chapter 28) describes his attempts to utilize a very large carbohydrate waste stream, whey permeates from the processing of cheese, for the production of wood adhesives. 

Neither a historical review nor a complete story of this field of polymer making is presented here. Rather, this is a series of views of the chemistry and procedures, from which one may obtain some idea of the potential as well as of the limitations of the methods. The discussion includes only reactions which yield essentially linear polymers, bypassing polyurethane foams and phenol-formaldehyde condensates. 

Resol-Type Foam Chemistry. Resol is obtained as a result of the reaction between phenols (P) and formaldehyde (F) in the presence of basic catalyst. Generally, the reaction is made at a temperature below... 

Calixarenes are macrocyclic molecules synthesized with high yield by condensation of appropriate arenes and aldehyde derivatives. Calix means bowF in Latin and Greek, and this phrase reflects the shape of the tetramer, which usually adopts a bowl or beaker-like conformation. Gutsche first introduced the name calixarene for this class of compounds [38]. Several authors have exhaustively reviewed the chemistry and synthetic procedures, which lead to different structural modifications of calixarenes [39-42]. In general, three types of calixarenes derivatives are known first, metacyclophanes (type 1) second, those obtained by condensation of formaldehyde with phenol (type 11), and third, those obtained by reaction with resorcinol (type III) (Scheme 6). The latter modifications are also called resorcarenes to distinguish calixarenes of type III from those of type II. 

Although the overall reaction mechanism is generally understood, the vast commercial importance of phenol-formaldehyde resins has seen numerous studies aimed at a more detailed understanding of the chemistry involved and the structures formed. In these studies extensive use has been made of model compounds, that is, compounds in which the reaction pathways are restricted, and these studies will be considered in this section. 

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