Polyphenol formaldehyde resins

Contamination of water by phenol or substituted phenols is very often due to industrial activity, because these are commercially very important chemicals. Phenol, cresylic acids, and cresols are used for making phenol-formaldehyde resins and tricesyl phosphates. Phenol, alkylphenol, and polyphenols are important raw materials for a wide variety of organic compounds, dyes, pharmaceuticals, plastizers, antioxidants, etc. Phenols are also present in effluents from coke ovens, blast furnaces, and shale oil processing [1]. 

Natural polymers crosslinked by synthetic molecules represent many resins used for industrial applications. These are now being more closely examined by solid-state NMR spectroscopy to try to understand more fully what occurs in these systems and how it is possible to improve them. Of industrial importance are the polyphenolic tannin resins crosslinked by hexamethylenetetramine. These principally contain flavan-3-ols (Fig. 15.2.18) in the tannin [21] and have been examined by CP/MAS solid-state NMR spectroscopy. Hexamethylenetetramine was used in preference to formaldehyde as it has showed a much faster rate of reaction. The intermediates in this reaction are tribenzyl-, dibenzyl-<(>, and monobenzylamines some of which rearrange to give the dihydroxydiphenylmethane crosslinking bridges in the resin. The exact nature of the crosslinking process, however, is still in debate and the study was undertaken to try and clarify the issue. To examine this process fully, a comparison was made between pine tannin (high in flavan-3-ol) (Fig. 15.2.19) pine tannin hardened with paraformaldehyde (Fig. 15.2.20) and pine tannin hardened with hexamethylenetetramine (Fig. 15.2.21). 

In woody materials, the long crystalline fibrils of cellulose are bound into a composite structure by lignin, a macromolecule based on polyphenols. Lignin, which is present to the extent of 25-30% in most woods, is a cross-linked polymer rather similar to man-made phenol-formaldehyde resins and may be looked upon as a glue which gives wood its permanent form (Figure 1.1). 

Adams and Holmes discovered that resins of the phenol or polyphenol formaldehyde type were suitable as exchangers. They also prepared the first anion exchangers on the basis of polyamine formaldehyde resins. 

Much interest has centred on the branch of cyclophanes known as calixarenes. They are polyphenol systems that can act as hosts in the formation of inclusion compounds, where a small guest molecule resides completely in a cavity within a single host they are cone-shaped cavitands . Several accounts have appeared of their history. The discovery by Baeyer of a formaldehyde/phenol resin led to Bakelite and to the work of A. Zincke and E. Ziegler, who gave to the first oligomer a tetrameric structure of a calix[4]arene. Later syntheses by Gutsche (1978) led to calixarenes with 4, 6 or 8 phenol residues.107-109... 

The main parameter for the application of tannins as adhesives for wood-based panels is the content of reactive polyphenols and the reactivity of these components towards formaldehyde. Tannins can be used as adhesives alone (with a formaldehyde component as crosslinker) or in combination with aminoplastic or phenolic resins. These resins can react chemically with the tannin component in a polycondensation reaction, form only two interpenetrating networks, or both. The simplest adhesive mix formulation consists of the tannin solution and powdered paraformaldehyde as crosslinker [283]. The addition of paraformaldehyde can cause in the short term a relatively high level of formaldehyde emission. Glue mixes using paraformaldehyde for the production of particleboards with low formaldehyde emission are described and used industrially [284]. In the literature a large number of papers describe the combinations of tannins with synthetic resins (Table 14). 

Baumann also examined the details of the reaction of silicic acid with catechol (163c) as well as with a catechol-formaldehyde condensate which acted as a polyphenol (163d). The latter resin, when not too highly cross-linked, removes soluble silica from slightly alkaline water. 

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