Phenolics resin

Phenolic resins (phenol-formaldehyde polymers), copolymers of phenol and formaldehyde, were the first fully synthetic polymers made. They were discovered in 1910 by Leo Baekeland and given the trade name Bakelite . 

Two processes, both involving step growth polymerization, are used for the manufacture of phenolic resins. 

The second process (Fig. 1) uses an acid catalyst and excess phenol to give a linear polymer (novolac) that has no free methylol groups for cross-linking. In a separate second part of this two-stage process, a cross-linking agent is added and further reaction occurs. In many instances, hexamethylenetetramine is used, which decomposes to formaldehyde and ammonia. 

Other modifications in making phenolic polymers are the incorporation of cresols or resorcinol as the phenol (Fig. 1) and acetaldehyde or furfural as the phenol. 

Phenolic resins (phenol-formaldehyde resins) are a condensation product of phenols with formaldehyde or, occasionally, with other aldehydes. These resins, which have a broad range of applications, are manufactured in amounts of about 200,000 tons/yr. Phenolic resins are classified as novolaks, which are produced with less than equimolar amounts of formaldehyde and acid catalysts, and Bakelites (A, B, and C), which are obtained with basic catalysts and an excess of formaldehyde. 

Novolaks are soluble resins that melt without chemical change (no heat curing). They are polynuclear phenols with methylene cross-link bridges and molecular weights of 600-1500. Although they can be converted into insoluble resins by means of curing agents such as formaldehyde, heat alone is not effective. 

Bakelite A is a readily soluble hydroxymethyl phenol it is produced at lOO C and has one or more growing ends. With suitable phenols, Bakelite A can be used by heat alone a process carried out at 150-160°C produces the intermediate stage of partially condensed Bakelite B, finally at 160-200 C the completely condensed Bakelite C is produced. 

Phenol is obtained by several methods. The middle oil (180-250 C) of pit coal tar is fractionally distilled, naphthalene is removed by cooling, and the alkali-soluble phenol is obtained by dissolving in caustic soda and then acidifying. Creosote is obtained from the heavy oil, which boils at 250-300°C, or from the wash water of coke production. o-Cresol is separated by multiple rectification, whereas p- and m-cresol can only be separated by chemical means. m-Cresol, unlike p-cresol, can be sulfonated at 130°C with dilute sulfuric acid. 

Phenol can also be produced by fusing the sodium salt of benzene sulfonic acid NaOH at 320°C, which leads to the formation of sodium phenate, sodium sulfite, and water. Saturation with CO2 yields phenol from the sodium phenate. 

Phenolic resins are produced by the reaction of phenols with HCHO. Two types of resin 

Some phenolic resins are brittle and a number of prepreg formulations are available which have been blended with thermoplastics (e.g., methylated nylon) to confer improved toughness. Tack can be introduced by incorporating pol5rvinyl butyral (Pilioform BL24), which will also increase the toughness. The structmre of an idealized resol is shown  

The second type of phenolics are the novalac resins formed by using a molar excess of phenol and an acid catalyst. The simplest form of a novalac resin is Bisphenol F  

Novalac resins cannot condense without the addition of a hardening agent, such as hexamine (hexamethylenetetramine), which will form HCHO  

Phenolic resins cure by a condensation reaction, evolving H2O and care must be taken to release this water (actually as steam) during the cure cycle, or use high pressures to compress the voids, as otherwise, blistering will occur. 

Phenolic resins are mainly used as wood adhesives, laminates, molded parts, insulating varnishes, abrasives, and rigid foams. 

Mathematical models describing formation of novolacs both in batch and continuous reactors have been developed by Frontini et al. [230] and Kumar et al. [231]. They consider the existence of at most one methylol group per molecule, and lump together all isomers with the same unit counts. A Monte Carlo method [232] can also be used in order to obtain a more detailed description at molecular level. 

Reaction is exothermal (AH = —80 kj mol ). Heat of reaction is removed using water reflux, sometimes with a small amount of inert solvent (aromatics are inert only if they carry deactivating groups), and relatively small batch reactors ( 2 to 10 m ) are usually preferred. 

Resol reactors have been the subject of studies, such as Ref. 240, concerning operation in accident situations. In novolac production, formaldehyde can be fed continuously in order to increase safety. 

Phenolic resins were among the first synthetic resins explored by the coating industry, initially used to modify properties of oil-based varnishes as a replacement for some natural hard resins. They are essentially solvent-soluble phenol-formaldehyde condensates with reactive methyllol groups. They are widely used as cross-linkers for thermosetting baking finishes, yielding films with excellent solvent and corrosion resistance properties coupled with favorable mechanical properties. 

Phenolic resins exist in both novalac and resole form. The latter are able to be cured like polyesters with the addition of catalysts and/or heat and so are of greatest interest for general purpose engineering. The available information covers resins cured with acid catalysts and this is assumed unless otherwise stated. 

Typical properties of a cast resin are listed in Table 4.7. 

The viscosity range selected depends upon manufacturing process and application. These resins are cold cured by adding a liquid (acid-based) catalyst. Though cure is possible at 20 C, it is preferable to post cure in the range 60 —80 C to obtain dimensionally stable components. 

For large mouldings, low shrinkage, which is characteristic of this resin, is an important design consideration. The high heat distortion temperature combined with excellent fire resistance (section 8.6) has allowed these materials to become more widely used. 

Manufacture was achieved by contact moulding through hand lay-up additional time allowed for good wet-out. 

As a family of resins originally developed in the early twentieth century, the nature and potential of phenoHc resins have been explored thoroughly to produce an extensive body of technical Hterature (1 8). A symposium sponsored by the American Chemical Society commemorated 75 years of phenoHc resin chemistry in 1983 (9), and in 1987 the PhenoHc Mol ding Division of the Society of the Plastics Industry (SPI) sponsored a conference on phenoHcs in the twenty-first century (1). 

PhenoHc resins are prepared by the reaction of phenol or substituted phenol with an aldehyde, especially formaldehyde, in the presence of an acidic or basic catalyst. Their thermosetting character and the exotherm associated with the reaction presented technical barriers to commercialization. In 1900, the first U.S. patent was granted for a phenoHc resin, using the resin in cast form as a substitute for hard mbber (10). 

Early phenoHc resins consisted of self-curing, resole-type products made with excess formaldehyde, and novolaks, which are thermoplastic in nature and require a hardener. The early products produced by General BakeHte were used in molded parts, insulating varnishes, laminated sheets, and industrial coatings. These areas stiH remain important appHcations, but have been joined by numerous others such as wood bonding, fiber bonding, and plywood adhesives. The number of producers in the 1990s is approximately 20 in the United States and over 60 worldwide. 

Phenol. This is the monomer or raw material used in the largest quantity to make phenoHc resins (Table 1). As a soHd having a low melting point, phenol, C H OH, is usually stored, handled in Hquid form at 50—60°C, and stored under nitrogen blanket to prevent the formation of pink quinones. Iron contamination results in a black color. 

The most widely used process for the production of phenol is the cumene process developed and Hcensed in the United States by AHiedSignal (formerly AHied Chemical Corp.). Benzene is alkylated with propylene to produce cumene (isopropylbenzene), which is oxidized by air over a catalyst to produce cumene hydroperoxide (CHP). With acid catalysis, CHP undergoes controUed decomposition to produce phenol and acetone a-methylstyrene and acetophenone are the by-products (12) (see Cumene Phenol). Other commercial processes for making phenol include the Raschig process, using chlorobenzene as the starting material, and the toluene process, via a benzoic acid intermediate. In the United States, 35-40% of the phenol produced is used for phenoHc resins. 

The phenolic resins may be considered to be the first polymeric products produced commercially from simple compounds of low molecular weight, i.e they were the first truly synthetic resins to be exploited. Their early development has been dealt with briefly in Chapter 1 and more fully elsewhere.  

Phenolic moulding powders, which before World War II dominated the plastics moulding materials market, only consumed about 10% of the total phenolic resin production by the early 1990s. 

In recent years there have been comparatively few developments in phenolic resin technology apart from the so-called Friedel-Crafts polymers introduced in the 1960s and the polybenzoxazines announced in 1998 which are discussed briefly at the end of the chapter. 

Phenolic resins are also widely known as phenol-formaldehyde resins, PF resins and phenoplasts. The trade name Bakelite has in the past been widely and erroneously used as a common noun and indeed is noted as such m many English dictionaries. 

Various workers have discussed flame retardancy in phenolic resins [49-52]. Alkyl ammonium treated montmorillonite [52], silica [50], and polysiloxane [51], have all been studied as flame-retardants for phenolic resins. 

Chiang and co-workers [50] studied the flame retardance of phenolic resin-silica nanocomposites. The char yields of the polymer were observed to increase when the tetraethoxysilane content of the polymer was increased. LOI and UL-94 [13] tests revealed that the hybrid possessed excellent flame resistance. 

Lin and co-workers [51] observed that cnring phenolic resins with epoxies instead of with hexamethylene tetramine yields polymers which have almost the same flame retardance as polymers produced with hexamethylene tetramine curing. They also have toughness, stiffness, good thermal stability, excellent flame retardance and low glass transition temperature (Tg). 

Analytical and kinetic studies of photo-oxidation in the solid state of phenolic resins (phenol-formaldehyde polycondensates) (4.36) show that photoreaction involves two processes [1845]  

Photolysis of the phenolic groups leads to the formation of quinone methide structures  

Photo-oxidation of dimethylene ether linkages causes the formation of hydroperoxy groups, which are further converted into ester and formate 

Lyim and co-workers [31] studied the effect of PETP on the thermo-oxidative stability of Novolac-type phenolic resins using TGA, DSC and FTIR. It was found that the incorporation of PETP in hlends improves stability. Degradation of polymer is avoided up to 370 °C. 

Igarashi, I. Mita and H. Kambe, Bulletin of the Chemical Society Japan, 1964,37, 8,1160. 

Kavarskaya, A.B. Blyumenfeld and S.I. Levantovskaya, Thermal Stability of Heterochain Polymers, Khimya, Moscow, Russia, 1977. [in Russian] 

Major polymer applications aircraft interiors, automotive (pump housings, transmission reactors, timing pulleys), marine, construction, coatings, adliesives. carbonless copy paper, abrasives, friction materials, laminates, foimdry resins, battery separators, wood bonding, composites, foam, hollow spheres 

Important processing methods lamination, molding, coating, compounding 

Typical fillers wood flour, glass fiber, carbon fiber, mica, wollastonite, mineral wool, talc, magnesium hydroxide, graphite, molybdenum sulfide, carbon black, cashew shell particles, alumina, chromium oxide, brass and copper powder, iron particles, steel fiber, ceramic powder, rubber particles, aramid, wollastonite, cellulosic fiber, lignin 

Auxiliary agents stearates, fluoropolymers, carboxylic groups-containing copolymers which reduce viscosity of filled polyester (BYK-W 995)  

Methods of filler pretreatment lignin treated by methylolation decreases the rate of cure of phenolic adhesives carbon fiber was anodicaUy oxidized and subjected to various treatments with coupling agents to improve interfacial interaction with phenolic resins and oxidative stability of carbon fibers titanate coupling of oxidized fibers resulted in improved adhesion to matrix and enhanced thermal stability of fibers  

Composites containing a polymer matrix are a valuable class of materials often used in high-temperature applications - phenolic resins and epoxies can be considered useful polymer matrix materials in that respect, as described in the subsequent sections. 

Phenolic resins are thermosetting polymers with high chemical resistance and thermal stability but low toughness and mechanical resistance. Phenolic resols have intrinsic 

During degradation, the reactions of chain scission and further crosslinking occurred simultaneously, although it was not possible to uncouple the two contributions and to determine the activation energy of each partial reaction. The global activation energy of 

One of the most important areas of application of the solid-state NMR technique is the investigation of the structures of cross-linked amorphous materials in cases where X-ray diffraction technqiues are not applicable. Polymeric resins are one such important class of materials. A lot of work has been done in this area by several investigators 36,37 38 since the beginning of the 80. Some solid-state NMR data of phenolic resins are presented in Fig. 10. Comparison with liquid state data for 

In Fig. 11 spectra of resorcinol resins 36) are given. In comparison with prepolymers, it shows that the completely hardned resin is more susceptible than that of uncured resin to the conditions of prepolymer synthesis. Because of only moderate resolution in the aromatic region the spectral pattern is fairly similar for the two resins. 

Phenoplasts manufactured on tlie basis of thermoreactive phenolformalde-hyde resin, harden on heating up to 120-170°C and then become insoluble. They are usually reinforced witli asbestos fiber. 

The phenolic/asbestos laminates (used up to 200°C) have excellent resistance to most mineral and organic acids but are attacked by strong oxidizing agents such as nitric and concentrated sulfuric acids and strong alkalis such as sodium and potassium hydroxide. Tanks, scrubbers, columns, pumps, pipes, etc., are fabricated from phenolic/asbestos laminates. 

While working with another associate, Nathanial Thurlow, Baekeland developed techniques for controlling the condensation of phenol and formaldehyde. He wrote little about the science of polymers, yet, it is evident, from his first publication in Industrial and Engineering Chemistry in 1909, that he had a better understanding of functionality than his illustrious predecessors. 

His heat and pressure patent demonstrated that he recognized the need to maintain pressure on the resin while converting it from an A and B stage to a C stage infusible plastic. The A stage resin was produced by the condensation of phenol and formaldehyde in the presence of an alkali. 

Baekeland used the term resole to describe resins made with alkaline catalysts. Those made with acid catalysts were called novolacs. However, the A stage 

Leo Baekeland demonstrated that he was a professional chemist by discussing his inventions before the New York Section of the American Chemical Society in 1909. He was the recipient of the first Chandler award in 1914 and served as national president of the ACS in 1924. 

It is of interest to note that George Eastman used Bakelite for the end panels of his Kodak camera in 1914 and that the Hyatt Burrou s Billiard Ball Co. replaced Celluloid with Bakelite for its billiard balls in 1912. 

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Phenol was first isolated m the early nineteenth century from coal tar and a small por tion of the more than 4 billion lb of phenol produced m the United States each year comes from this source Although significant quantities of phenol are used to prepare aspirin and dyes most of it is converted to phenolic resins used m adhesives and plastics... 

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