Phenol-Aldehyde Condensations

1. Introduction     49

A. Phenol-Aldehyde Condensates     50

2. Condensation of Phenol with Aldehydes     52

A. Phenol-Formaldehyde Condensations     52

B. Other Phenols     59

C. Reaction Conditions     60

2-1. Preparation of a Phenol-Formaldehyde Resole  63

2-2. Preparation of a Phenol-Formaldehyde Casting Resin  65

2-3. Preparation of a Cresol-Formaldehyde Resole  67

2-4. Preparation of a Phenol-Formaldehyde Novolac  68

2-5. Preparation of Salts of Tris(hydroxymethyl)phenate  70

2-6. Condensation of p-Chlorophenol with Formaldehyde  71

2-7. Dehalogenation of p-Chlorophenol-Formaldehyde Polymers  72

2-8. Preparation of Resole (Used for NMR and GPC Study)  73

D. Other Aldehydes     74

2-9. Preparation of Furfural-Phenol Resin  75

E. Stability     75

F. Curing Conditions for Phenolic Resins     76

G. Compounding with Phenolic Resins     78

H. Dihydroxydiphenylalkanes (Bisphenols)     79

3. Miscellaneous Preparations and Applications     80

References     81

1 INTRODUCTION

This chapter is concerned with the practical aspects of reaction of formaldehyde with phenols to give both methylolated and cross-linked products. The reaction of other aldehydes to give resinous products will be briefly mentioned. The mechanism of these reactions will not be discussed.

A Phenol-Aldehyde Condensates

In 1843, Pira [1] reported that phenol alcohols are converted to resins (called saliretins) on heating. Baeyer [2] in 1872 reported that the reaction of phenols with acetaldehyde in the presence of acid catalysts also gives resinous products. Kleeberg [3] in 1891 reported that formaldehyde undergoes similar reactions. However, Dianin [4, 5] found that acetone reacts with phenol to give a crystalline bisphenol (now known as bisphenol A). In 1874 Lederer [6] and Manasse [7] independently synthesized o-hydroxybenzyl alcohol (saligenin) by the low-temperature alkaline-catalyzed formaldehyde reaction.

Baekeland [8, 9] was granted a patent in 1909 describing his alkaline-catalyzed Bakelite resins ("resoles") and also the acid-cured "novolak" product. He conceived the idea of using counterpressure during hot cure to prevent bubbles and foaming from heat and was able to produce strong cured resinous products.

In connection with nomenclature, Chemical Abstracts uses the terms novolak and resol. Following more popular usage we shall use the terms novolac and resole in this chapter.

Some historical reviews and monographs on phenolic resins are worthwhile sources for additional details [8-13].

Today phenolic resins are a large-volume item achieving sales of approximately 2.6 billion lb in 1990 up from 450 million lb in 1956. The pattern of consumption and sales growth are shown in Tables I and II. The markets for phenolic molding compounds is decreasing in size [14]. As a result of their stability [15] and other useful properties, these resins continue to find widespread use.

Table I

Phenolics: Pattern of Consumptiona

a Reprinted from Mod. Plast. 51, 40 (1974). Copyright 1974 by Modern Plastics. Reprinted by permission of the copyright owner.

b The data for 1990 are from Ref. [14].

Table II

Phenolics: Molding Powder Marketsa

a Reprinted from Mod. Plast. 51, 40 (1974). Copyright 1974 by Modern Plastics. Reprinted by permission of the copyright owner.

b The data for 1990 are taken from Ref. [14].

Much information is being published on the phenolic resins as evidenced by the proliferation of references in a recent Chemical Abstracts issue (8th Collect. Vol. CA 66-75), which lists 22 pages of subject matter under "phenol condensation products."

Some additional references describing the use and versatility of resins based on phenolics are also worthwhile to consult for more details [11, 16].

CAUTION:

(Toxicity): Phenols, formaldehyde, and other aldehydes are toxic and should be handled with adequate ventilation and skin protection [17]. Where possible the use of hydrogen chloride in the presence of formaldehyde or formaldehyde sources (hexamethylenetetramine and the like) should be avoided. Recent reports have indicated that formaldehyde and hydrogen chloride spontaneously react to give the known carcinogen bis(chloromethyl) ether.

In 1983 The American Chemical Society commemorated 75 years of phenolic resin chemistry by hosting a symposium with over 30 papers over a four-day period [19].

A recent (1988) review by Peter W. Kopf should also be consulted [20].

Recently patent activity continued to be strong in all application areas cited earlier for phenol-formaldehyde (P-F) resins. For example P-F sheet molding compositions (SMC) involving B stages or intermediary systems with glass fiber reinforcing have been patented by Reichhold [21]. The sheet is molded to the applications and then pressed at 320°F at 1000 psi. Other activity in this area in Japan [22] is also worth noting.

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