Phenolic resin chemistry development

As a family of resins originally developed in the early twentieth century, the nature and potential of phenolic resins have been explored thoroughly to produce an extensive body of technical literature (1-9). A symposium sponsored by the American Chemical Society commemorated 75 years of phenolic resin chemistry in 1983 (10), and in 1987 the Phenolic Molding Division of the Society of the Plastics Industry (SPI) sponsored a conference on phenolics in the twenty-first century (1). Exciting new developments continue as new systems are developed for carbon-carbon composites, aramid honeycombs, and new derivative chemistries such as cyanate esters and benzoxazines. New U.S. patents with phenolic resins in the claims are growing at about 150 patents per year. 

Phenolic resins were the first totally synthetic plastics invented. They were commercialized by 1910 [I]. Their history begins before the development of the structural theory of chemistry and even before Kekule had his famous dreams of snakes biting their tails. It commences with Gerhardt s 1853 observations of insoluble resin formation while dehydrating sodium salicylate [2]. These were followed by similar reports on the behavior of salicylic acid derivatives under a variety of reaction conditions by Schroder et al. (1869), Baeyer (1872), Velden (1877), Doebner (1896 and 1898), Speyer (1897) and Baekeland (1909-1912) [3-17]. Many of these early reports appear to involve the formation of phenolic polyesters rather than the phenol-aldehyde resins that we think of today. For... 

From this result on MRS, we expected that a combination of phenolic-resin-based resist and aqueous alkaline developer would lead to etching-type dissolution and non-swelling resist patterns. In this paper, we report on a new non-swelling negative electron beam resist consisting of an epoxy novolac, azide compound and phenolic resin matrix (EAP) and discuss the radiation chemistry of this resist. 

The early aerospace adhesives were primarily based on epoxy resin chemistry. However, unique applications requiring high temperatures and fatigue resistance have forced the development of epoxy-phenolic, epoxy-nitrile, epoxy-nylon, and epoxy-vinyl adhesives specifically for this industry. The aerospace industry has led in the development and utilization of these epoxy-hybrid adhesives. 

Alkyd chemistry lends itself to further modification beyond choice of polyol, dibasic acid, and drying oil. Vinyl-modified alkyds, for example, are produced for more durable and quicker drying films, although with some sacrifice in crosslinking rate and consequent development of solvent resistance. Styrene, vinyl toluene, and methyl methacrylate are the most commonly used modifiers. In the presence of a free radical initiator, vinyl polymer will graft onto the alkyd. Tack-free time (i.e. a surface-dry film) may be reduced from 4 to 6 hoius for an unmodified alkyd to 1 hour in styrenated form. Acrylics, silicones, phenolic resins, and natural resins are likewise used to tailor film gloss, flexibility, durability, and drying time for certain applications. 

The workhorse of the VLSI industry today is a composite novolac-diazonaphthoquinone photoresist that evolved from similar materials developed for the manufacture of photoplates used in the printing industry in the early 1900 s (23). The novolac matrix resin is a condensation polymer of a substituted phenol and formaldehyde that is rendered insoluble in aqueous base through addition of 10-20 wt% of a diazonaphthoquinone photoactive dissolution inhibitor (PAC). Upon irradiation, the PAC undergoes a Wolff rearrangement followed by hydrolysis to afford a base-soluble indene carboxylic acid. This reaction renders the exposed regions of the composite films soluble in aqueous base, and allows image formation. A schematic representation of the chemistry of this solution inhibition resist is shown in Figure 6. 

The initial development of polymeric MDI as a particleboard binder predates the above work but it is, we believe, based upon the same chemistry. Exterior or structural particleboard has been manufactured in North America and in Europe for years with predominantly phenol-formaldehyde binders and, in a few cases, with melamine-modified urea-formaldehyde binders. The familiar urea-formaldehyde resins are subject to hydrolysis and are thus suitable for interior board only. 

The 4-acetyl-3,5-dioxo-l-methylcyclohexane carboxylic acid (ADCC) linker 2n [128] was developed to provide a support for anchoring primary amines via a preformed handle method. The anchor is stable toward acids and bases including piperidine and l,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and is thus compatible with Fmoc chemistry. The amine can be released by rapid hydrazinolysis on treatment with 2% hydrazine in DMF. A formamidine linker 2o has been obtained from Merrifield resin by reaction with the appropriate phenol. Secondary amines can be anchored to the support by an amidine exchange reaction and after further transformations cleaved by treatment with hydrazine [129]. 

Tricarbonyliron diene complexes have found many uses in synthetic chemistry but their synthesis is often not easy. Knolker has developed a range of tricarbonyl(7] -l-aza-l,3-butadiene) iron complexes that are excellent transfer agents for the Fe(CO)3 complexation of 1,3-dienes, and showed their versatility. As an extension to this work, Knolker and Gonser have prepared a polymer-supported l-aza-l,3-butadiene 321 by reaction of Merrifield s resin with phenolic l-aza-l,3-butadiene 320, formed from cinnamaldehyde and /> ra-hydroxyaniline (Scheme 105). The corresponding tricarbonyl iron complex 322 was formed by treatment of 321 with an excess of Fe2(CO)9 in THF using ultrasound. The iron complex was subsequently used efficiently as a transfer agent for the tricarbonyliron complexation of 1,3-dienes. 

The purpose of this book is to provide, in one volume, an overview of structural adhesives. One chapter will be devoted to each of the major classes of structural adhesives, emphasizing the chemistry of the base resin and the main end uses for the adhesives of that class. The choice of systems is restricted to synthetic resins that are of current industrial interest for structural bonding. Some, such as the phenolics and epoxies, have been used successfully for many years and are of considerable industrial importance. Others, notably the structural acrylics and cyanoacrylates, are generating much interest and will probably become more widely used for industrial applications in the future. The newer polymers, for high-temperature-resistant adhesives, are currently of limited use most activity in these systems is at present still in the research and development stage. The desire for higher-temperature-resistant materials is creating much interest in these polymers and adhesives based on them will undoubtedly become important in the future. 

Around the turn of the twentieth century, modern atomic theory wreis developed, and chemistry became a mainstream science through which new materials could be produced. Each new material engendered new apphcations, and each new application played to a demand for stiU newer materials, mostly derived from coal tar, of which a ready supply existed. The final key requirementwreis the discovery and development of polymerization. The first completely synthetic polymer, compounded from phenol and formaldehyde, was developed in 1907 by Belgian chemist Leo Hendrik Baekeland. It proved to be the elusive material needed to expedite the mass production of consumer goods. Soon, many other new materials were created from polymerization, which led to the development of the modem plastics industry. These versatile resin materials were used in a variety of applications, from the synthetic fibers used to make cloth to essential structural components of modern space and aircraft. 

During the first decade of the twentieth century, Leo Baekeland developed the first practical process for making molded objects from phenol-formaldehyde resins. The company he formed was named General Bakelite Co. and he called his product bakelite. The development of these early materials was largely an art because very little was known about the chemistry and physical changes that led to the final products. In the mid-1920s Hermann Staudinger hypothesized that... 

Figure 9 Overview of the structure and chemistry of two-component DNQ-novolac resists. The polymer resins in these resists are novolacs (which are soluble in both organic solvents used for film casting and aqueous alkaline solutions used for development) that are made by co-condensation of phenols (i.e., typically m- and p-cresol) and formaldehyde. The sensitizer in these resists are substituted DNQs which inhibit the dissolution of novolac and which upon exposure to UV light transform into carboxylic acids that generally increase the dissolution of novolacs in aqueous alkaline solutions.

---