Adolph Von Baeyer was the first chemist (in the latter part of the nineteenth century) to discover the reaction between phenol and formaldehyde in the presence of aqueous sodium hydroxide. Later on Leo Bacheland has studied on this reaction and was able to show that it had a very high molecular weight.
There are the most versatile materials in the entire family of polymers. Their versatility is due partly to the low cost of raw materials and partly because of the many forms in which these can be made to fit specific requirements. Its raw materials are phenol and formaldehyde. Phenol formaldehyde resins are the oldest of the commercially important polymers were patented in 1909 by L.H. Backland and are thus known as Bakellite. Phenol is obtained as a byproduct from the reduction of bituminous coal.
Cresols and xylenols are sometime used in place of or with phenol to develop special resin formulations. Formaldehyde. normally a gas, is produced from methyl alcohol by several methods. But it is used in the form of fortnalin which is nothing but a 37 to 40% water solution of formaldehyde. Phenol reacts with aldehyde to give condensation products if there are free positions on the benzene ring at ortho and para to the hydroxyl group. Phenol-formaldehyde plastic or Backelite is a high molecular weight substance wherein several phenol rings are linked together by methylene (-CI 12-) groups. This is obtained by polymerization of a monomer obtained by condensation of one molecule of phenol and one molecule of formaldehyde using basic or acidic catalysts. The following sequence of reaction may be considered for this polymerization.
1) Reaction of phenol with formaldehyde to yield monomer, or hydroxymethyl phenol. This reaction will take place at the ortho or para position and is known as methylol derivatives.
These products, which may be considered the monomers for subsequent polymerization, arc formed in neutral or alkaline condition. These two conditions are described as follows:
1 -NOVOLAC FORMATION
The condensed and bridge formation of polymer is called novolac formation. In case of phenol formaldehyde. methylol derivatives condense with phenol to form dihydroxydiphenyl methane.
The reaction will proceed at a minimum at a pH of 4-5. Hydroxymethylphenol with a second molecule of phenol with the loss of water to yield a compound where two rings are joined via a —CH2- group. As three positions in each phenol ring (i.e. two ortho and one pan) are further susceptible to attack the sequence of reaction continues further to afford phenol-formaldehyde resins known as bakelites which are cross-linked three dimensional polymers. These are called novolacs with the structure, where ortho and pan links occur at random.
By controlling the degree of polymerization phenol-formaldehyde plastic can be obtained in different forms. However, a pure phenolic resin is rarely moulded without modification to overcome limitations imposed by the moulding process and end-use requirements. Phenolic resins are brittle, and have high shrinkage caused by polymerization and elimination of water, dimensions are difficult to control. To overcome these difficulties resin manufacturers resort to use of additives such as fillers, binders, hardening agents, plasticizers etc. to produce a desired compound. Each filler results in a different compound. Molecular weights may range as high as 1000, corresponding to about ten phenyl residues.
2 RESOLE FORMATION
The polymers of phenol and formaldehyde containing alcohol groups are called resoles. The methyl phenols in the presence of alkaline catalysts and with more fonnaldehyde condense either through methylene linkages or through ether linkages. At the end of the reaction subsequent loss of formaldehyde may occur with methylene bridge formation. The four major reactions in phenolic resin are as follows:
(a) Addition to give methylol phenols;
(b) Condensation of a methylol phenol and a phenol to give a methylene bridge:.
(c) Condensation of two methylol groups to give an ether bridge; and
(d) Decomposition of ether bridges to methylene bridges and formaldehyde, the latter reacting again by the first mod.
Tetraethyl ammonium hydroxide
Usually, the amount of basic catalyst used is 0.05 moles per kg of reactants.
Production of this type of resin is soluble and fusible but containing alcohol groups are called resoles. The formation of resoles and novolacs respectively, leads to the production of phenolic resins by one stage and two-stage processes.