Phenol-Formaldehyde Resol by the One-Stage, Aniline Method

Submitted by: J. Nelson 1 Checked by: L. M. Kaderabeck and A. F. Shepard 2

1. Procedure A 3 l three-neck, round-bottomed flask (Note 1) is equipped with a Teflon® or stainless steel paddle-type stirrer about 3-4 in long rotating at 300-500 rpm, thermometer, efficient bulb-type reflux condenser, and heating mantle. To this is charged phenol (940 g, 10 mol, 99% purity), aniline (70 g, 0.75 mol, Note 2), formaldehyde (1130 g of 37.2% solution, 14 mol, Note 3) and barium hydroxide octahydrate (110 g of a 28.5% hot water solution. The pressure is reduced to 300-350 torr and the reactants are heated slowly to a reflux temperature of 80o and maintained there for 50 min. The reflux condenser is then replaced with a condenser set for distillation, and the resin is dehydrated at 10-20 torr to a final temperature of 80-90o. As the dehydration proceeds, the molecular weight and viscosity of the condensate increase progressively and the resin becomes increasingly sensitive to further heating. A practical test to determine the extent of condensation is the so-called stroke cure test. A small amount of the resin (ca 0.5 g) is removed, spread on a metal hot plate maintained at 150o surface temperature, and stroked with a spatula until the sol-gel transition occurs. The sol-gel transition is indicated by a rapid increase in the rubberiness or gel character of the film. When the "gel time," determined in this manner, falls to 65-85 s,the apparatus is quickly disassembled and the resin is poured in a thin layer into a large shallow vessel and covered with heavy aluminum foil to provide rapid cooling (Note 4). The cooled, solidified product can be broken into lumps or can be ground with suitable fillers, pigments, lubricants, and pigments for use as a thermosetting molding compound (Note 5). The resin should be broken or ground just before use, because one-stage resins tend to fuse together on standing.

2. Notes 1. A commercially available standard resin pot or reaction kettle of suitable size fitted with a heating mantle is desirable. 2. The function of aniline is to slow condensation, improve grindability, and improve rheological properties during subsequent molding. 3. This formulation is designed specifically for uninhibited, essentially methanol-free, commercial formalin solution. Other grades of formalin may be used but are less reactive,and the reaction

167 168 Macromolecular Syntheses, Collective Volume 1 exotherm and final resin properties will not be the same. Some increase in weight of formalin charged might be required. 4. The resin should be transferred in a good hood; ca 900 g of aqueous distillate will have collected by the time of transfer. 5. Molded products from one-stage phenol-formaldehyde resin have better chemical resistance, better solvent resistance, and less odor than two-stage resin products. One-stage resins are often soft, and mixing with additives and fillers generally requires a roll mill.

3. Methods of Preparation A variety of alkaline catalysts may be used for preparation of one-stage resins. They include alkali metal hydroxides, alkaline earth hydroxides, weaker alkalies such as magnesium and zinc oxides and hydroxides, ammonium hydroxide, quaternary ammonium hydroxides, and various organic amines (alkyl and alkanol amines). Each produces a different rate of reaction and different product properties. Phenolic resins may also be prepared in essentially non-aqueous media using polyformaldehyde (paraform or trioxane). Such procedures are limited in terms of catalysts that may be used, but are more efficient in that the dehydration stage is eliminated or greatly shortened. Water-soluble phenolic resins are prepared with formaldehyde in moderate excess, strong aqueous alkali as catalyst, and short reflux times. The dehydration step is either shortened or dispensed with entirely. Phenolic resins suitable for casting are prepared using a large excess (2-3:1) of formaldehyde and an alkali metal hydroxide as catalyst. After reaction the mixture is neutralized to about pH 6, usually with a weak organic acid, and a polyol such as glycerol or glycol is added to improve the clarity of the product. Curing normally involves heating in an open mold at about 75o for extended periods of time. Many alkylated or arylated phenols, or mixtures of phenols (e.g., cresols, xylenols, phenyl phenols, alkoxyphenols) can be used to prepare phenol-formaldehyde resins. Optimum conditions for making the resins vary as do their properties. If the substituents are large alkyl groups or aryl groups, resins soluble in organic solvents can be produced.

4. References 1. General Electric Company, Plastics Department, Pitsfield, MA 01201. 2. Hooker Research Center, Niagara Falls, NY 14302.