2. Phenolic and phenolic compounds are substances which have one or more aromatic rings with one or more hydroxyl substituents on the ring, including functional derivatives such as esters, methyl ethers, glycosides and other derivatives. Phenol (hydroxybenzene) is the simplest example of a phenolic compound, but most phenolic compounds have two or more hydroxyl groups and are bioactive substances occurring widely in food plants that are eaten regularly by substantial numbers of animals and people and have been found to be safe compounds. Included in the definition of phenolics are polyphenols having complex substitution patterns, compounds having condensed rings, and phenolics containing one or more amine groups and/or carboxylic acid groups.
Polymers can be formed from the functionalized phenolics that are difunctional, that is those species having more than one hydroxyl group. Polymers of the functionalized phenolic compounds also have specific ranges over which they release the active phenolic groups.
[Phenolics, US Patent 8,372,882 (2/12/2013)]
1. “Phenol formaldehyde resins (PF) are synthetic polymers obtained by the reaction of phenol or substituted phenol with formaldehyde. Phenolic resins are mainly used in the production of circuit boards. They are better known however for the production of molded products including pool balls, laboratory countertops, and as coatings and adhesives. In the form of Bakelite, they are the earliest commercial synthetic resin
Phenol-formaldehyde resins, as a group, are formed by a step-growth polymerization reaction that can be either acid- or base-catalysed. Since formaldehyde exists predominantly in solution as a dynamic equilibrium of methylene glycol oligomers, the concentration of the reactive form of formaldehyde depends on temperature and pH.
Phenol is reactive towards formaldehyde at the ortho and para sites (sites 2, 4 and 6) allowing up to 3 units of formaldehyde to attach to the ring. The initial reaction in all cases involves the formation of a hydroxymethyl phenol:
HOC6H5 + CH2O → HOC6H4CH2OH
The hydroxymethyl group is capable of reacting with either another free ortho or para site, or with another hydroxymethyl group. The first reaction gives a methylene bridge, and the second forms an ether bridge:
HOC6H4CH2OH + HOC6H5 → (HOC6H4)2CH2 + H2O
The diphenol (HOC6H4)2CH2 (sometimes called a "dimer") is called bisphenol F, which is itself an important monomer in the production of epoxy resins. Bisphenol-F can further link generating tri- and tetra-and higher phenol oligomers.
Novolacs are phenol-formaldehyde resins with a formaldehyde to phenol molar ratio of less than one. The polymerization is brought to completion using acid-catalysis such as oxalic acid, hydrochloric acid or sulfonate acids. The phenol units are mainly linked by methylene and/or ether groups. Novolacs are commonly used as photoresists. See also photolithography. The molecular weights are in the low thousands, corresponding to about 10-20 phenol units.
Hexamethylenetetramine or "hexamine" is a hardener added to crosslink novolac. At a temperature >90 °C, it forms methylene and dimethylene amino bridges.”
Resoles are base-catalysed phenol-formaldehyde resins with a formaldehyde to phenol ratio of greater than one (usually around 1.5). Phenol, formaldehyde, water and catalyst are mixed in the desired amount, depending on the resin to be formed, and are then heated. The first part of the reaction, at around 70 °C, forms a thick reddish-brown tacky material, which is rich in hydroxymethyl and benzylic ether groups.
(Phenolic Resins, Wikipedia, 4/5/2013)
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Roger D. Corneliussen
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Copyright 2013 by Roger D. Corneliussen.
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* Date of latest addition; date of first entry is 2/18/2013.