Phenolic Resins: A Century of Progress

The title of this book is selected appropriately to describe a century of progress for phenolic resins. The contributions of Leo Hendrik Baekeland are immense! He was not easily discouraged as he attempted to harness the reaction of phenol and formaldehyde into a manageable, commercial product. In spite of some of the great organic chemists who examined the reaction of phenol and formaldehyde and obtained intractable, useless materials, he prevailed. The legacy of Leo Hendrik Baekeland and his development of phenol formaldehyde resins are recognized as the cornerstone of the Plastics Industry in the early twentieth century whereby phenolic resins and other polymeric systems continue to flourish after a century of robust growth. On July 13, 1907, Baekeland filed his “heat and pressure” patent related to the processing of phenol formaldehyde resins and identified their unique utility in a plethora of applications. Bakelite as coined by Baekeland was considered “the material of a thousand uses!” Phenol formaldehyde resins or Bakelite as they were commonly referred to became the first truly synthetic polymer to be developed. The perseverance and diligence of Baekeland and those who followed him in the ensuing years attest to their significant contributions that are identified with a variety of thermosetting and thermoplastic polymeric materials within the Plastics Industry. Further, it is remarkable that after a century of existence phenolic resins continue their preferred status in a number of applications that were developed during Baekeland’s era to this very day.

Up to the end of the nineteenth century, phenol was recovered primarily from coal tar. With the commercialization of the phenolic resins, the demand for phenol grew significantly. Currently, the cumene-to-phenol process is the predominant synthetic route for the production of phenol. It is accompanied by acetone as a co-product. Cumene is oxidized with oxygen to form cumene hydroperoxide. The peroxide is subsequently decomposed to phenol and acetone, using a strong mineral acid as catalyst. The products are purified in a series of distillation columns. The cumene-to-phenol process is described in more detail in this chapter. An overview is given about synthetic routes via direct oxidation of benzene. None of these alternative routes has been commercialized. The chapter also gives an overview of global supply and use of phenol in 2008. Finally, the main natural sources and synthetic routes for cresols, xylenols, resorcinol, and bisphenol-A are described. These components are used as comonomers for special phenolic resins.

Formaldehyde, as an aqueous solution ranging from 37 to 50 wt%, continues to be the preferred aldehyde for reaction with phenol for the preparation of phenolic resins. Over 30 million metric tons of formaldehyde represent the global worldwide consumption of formaldehyde for an array of products, besides phenolic resins. These include urea formaldehyde resins, melamine formaldehyde resins, polyacetal resins, methylenebis (4-phenyl isocyanate), butanediol, pentaerythritol, and others.

The two basic processes to produce formaldehyde from methanol – the silver catalyst process and the metal oxide process – are described along with the strengths and weaknesses of the respective processes. Furthermore, methanol plant siting location is a factor due to raw material (natural gas) and energy costs.

The controversy regarding the classification of formaldehyde as a human carcinogen remains unsettled. In 2004, the International Agency for Research on Cancer (IARC) of the World Health Organization reclassified formaldehyde from a group 2A substance (probable carcinogen to humans) to a group 1 (carcinogenic to humans) substance. Yet no government regulating agency has classified formaldehyde as a known human carcinogen. The studies that acknowledged formaldehyde to be a human carcinogen are being re-analyzed with additional research by IARC to re-examine its current classification of formaldehyde. By end of October 2009, despite strong disagreement among participants of the voting body, who were evenly split at the vote, IARC concluded that there is sufficient evidence in humans of a causal association of formaldehyde with leukemia. Industry disagrees with this conclusion and believes that the weight of scientific evidence does not support such a determination. A review of all of these data is still in process but impact on possible governmental reclassifications expected to be seen in 2010.

During the ensuing years since the last phenolic resins book was published, many new and remarkable developments have occurred in the realm of phenolic chemistry and are given in this chapter.

A critical examination of the first step or addition step (methylolation) in the preparation of resoles is described and how it can be controlled and compared with the typical resole resin preparations. It provides a vision into the preparation of mineral wool/glass insulation resins and ways to minimize the undesirable dimer/oligomer formation.

Different reactivities of the common methylolated phenols and phenol are ranked, and their resulting reactivities differ depending on whether formaldehyde is present or absent in the methylolated phenols.

Use of organic bases such as triethyl amine indicates that a much faster reaction of F with P occurs as compared to the use of NaOH under similar conditions, and primarily ortho directed intermediate 2-methylol phenol is obtained. Further more methylene ether linkages result in the final TEA resole.

Bisphenol F, the simplest oligomer of phenolic novolak, continues to elicit considerable activity related to improved and economical synthetic preparative methods.

A unique, novel novolak process involving heterogeneous/two phase method known as the PAPS process is discussed along with several features and favorable comparisons with existing novolaks. Features such as narrow MWD, high yield, low free phenol, and rapid reaction favor this new process. Current markets that are responding to PAPS novolaks are photoresists and novolak curing agents for epoxy resins.

Nanotechnology is being applied to resoles and novolaks as well as the closely related phenolic materials such as cyanate esters and benzoxazines. With very small amounts of nanoparticles (≤5%), these phenolic materials are significantly “upgraded” with resulting nanomodified phenolics exhibiting higher heat strength, higher modulus, T _g, etc., with many of these resin characteristics carrying over into fiber reinforced composites.

A new phosphorous flame retardant additive known as DOPO is reported and is instrumental in providing UL 94 VO behavior to cured novolak epoxy electrical laminates.

Benzoxazines are emerging as a more desirable phenolic resin system available from resin manufacturers as well as formulators such as Gurit (prepreg) and Henkel (FRP matrix). Benzoxazines undergo ring opening without emitting any volatiles during cure and result in a cured product with excellent dimensional stability, low water absorption, and stable, low dielectric properties (many of which are unavailable from typical phenolic resins). These attractive features have been responsible for its large volume use in electronics and FRP.

An overview of all types of natural products used either as partial phenol replacement, solvent or co-reactant, or as a resin modifier are tabulated along with origin/source, role in resin preparation, and different application areas.

An unusual technical development that attracted the interest of the symposium attendees was delivered by Sumitomo Bakelite researchers at the Commemorate Centennial Baekeland 2007 Symposium held in Ghent, Belgium during September 25, 2007. It was proposed that the 3D polymer network of hexa cured phenolic novolak resin can undergo further improvement based on reported curelastomer, T _g values, and DSC data. It is the view of the researchers based on phenolic resin conformations reached by using the Mark-Houwink-Sakarada equation as well as molecular simulations that higher T _g, exceptional strength, and performance driven characteristics of the resulting cured phenolic systems can be attained if an extended, linear novolak with low amounts of branching is cured with hexa. Conventional novolaks possess a “coiled” structure with branching, and during hexa cure, some novolak sites are sheltered or unavailable for cure and result in a 3D polymer network that is not fully cured in spite of the availability of excess hexa.

A reaction that is being revisited consists of phenylene bisoxazoline (PBO) reacting with novolak, and this promises to lead to the commercialization of a polyaryl ether amide type polymer possessing high T _g, high strength, and toughness exceeding multifunctional epoxy systems.

The capture and release of formaldehyde by aminoalcohols through the formation of an oxazolidine intermediate to facilitate the cure of phenolic resins has been commercialized as is shown in 2 different application areas. A pultrusion system based on novolak and oxazolidine is claimed to operate at line rates comparable to fast polyester pultrusion speeds. The other application involves the use of oxazolidine with PRF resin in the preparation of fiber reinforced FST duct systems.

The appendage of either an allyl or an ethynyl group to a novolak or resole followed by thermal cure leads to unusually high T _g phenolic materials, close to 400°C and is of interest for ultra high performance organic matrix composites.

The characterization of phenolic resins by modern analytical techniques is detailed. Common wet chemical and ISO methods are referenced, yet the chapter’s emphasis resides in the numerous advances in instrumental techniques. For instance, the authors make a special effort to illustrate the use of GC×GC, MALLS detector in GPC, and some LC-MS and TOF-MS applications. Larger sections on NMR and IR indicate the power of these tools for analysis and also contain peak position tables. Thermal analysis techniques, including rheometry, are discussed with experimental procedures and applications. Finally, microscopy, both optical, and SEM/X-ray, with sample preparation insights, are discussed. The chapter cites 115 papers using analytical chemistry techniques in the examination of phenolic polymers.

Resoles are phenol-formaldehyde condensation products prepared with a molar ratio of F:P of ≥ 1, basic catalysts, and lead to reactive phenolic resins with methylol functional groups. A wide variety of additives can be added to provide the final resin with the desired properties such as flame retardancy, plasticization, pigmentation, or improve processability by using release agents, wetting agents, and other surfactants. Most resoles are waterborne, but in some certain applications, they can be solvent based with either alcohols or ketones as solvents.

A vast majority of resoles are manufactured by batch conditions, with the reactor size dependent upon the end-use application. Reactors of 50 m³ are used for large volume applications, such as mineral wool or wood binders, while lesser volume reactors are used for other lower volume uses.

The convenience of “in-line” monitoring allows the determination of resin reaction to progress and whether the resinous material is ready for the next stage of the process or ready to be discharged. Typical “in-line” process tests include pH, viscosity, water tolerance, and gel time.

Raw materials of most modern phenolic resin manufacturers are charged automatically via closed systems using mass flow meters so that there is no contact of the potentially very harmful chemicals with the operator. To ensure the prevention of run-away reactions that may occur from the exothermic reaction of phenol with formaldehyde under basic conditions, formalin (37–52%) is added continuously over an extended period of time. This allows an adequate temperature control and gives the possibility to interrupt formalin addition if the temperature is increasing to fast. An alternate method is the staged addition of catalyst which reduces energy in the system but is generally less effective in the prevention of run-away reaction than the continuous formalin addition method.

Novolak resins are produced by reacting formaldehyde (30–55% concentration) with phenol under acidic conditions, with oxalic acid as the preferred catalyst and in special conditions, sulfuric acid. Depending on the batch size, all raw material components can be introduced into the reactor, or when there is an increase in the batch size as well as in the reactor volume, the reaction exotherm is controlled by a gradual addition of formaldehyde. Modern novolak production facilities are automated and programmed for reduced operational cost. A flow diagram of a general production line for the manufacture of novolak is shown. Recovery of the novolak is accomplished by the removal of water and devolatilization of crude novolak to molten, low-free phenol novolak resin which can be isolated as flake or pastille or dissolved in appropriate solvents. Novolak is stored either in a solid flake or pastille form or in solution. Most production is conducted under atmospheric conditions, but there are some recent, novel activities such as pressure in a hermetically-closed reactor reaching 0.1–10 MPa by using the heat of reaction without reflux to shorten reaction time, accelerating dehydration time by flash distillation, and providing economic benefit in the cost of novolak production.

Batch production remains as the predominate method of novolak production; however, there persists some mention of continuous processes for novolak production.

The global environment, in which phenolic resins are being used for wood composite manufacture, has changed significantly during the last decade. This chapter reviews trends that are driving the use and consumption of phenolic resins around the world. The review begins with recent data on volume usage and regional trends, followed by an analysis of factors affecting global markets. In a section on environmental factors, the impact of recent formaldehyde emission regulations is discussed. The section on economics introduces wood composite production as it relates to the available adhesive systems, with special emphasis on the technical requirement to improve phenolic reactivity. Advances in composite process technology are introduced, especially in regard to the increased demands the improvements place upon adhesive system performance. The specific requirements for the various wood composite families are considered in the context of adhesive performance needs. The results of research into current chemistries are discussed, with a review of recent findings regarding the mechanisms of phenolic condensation and acceleration. Also, the work regarding alternate natural materials, such as carbohydrates, lignins, tannins, and proteinaceous materials, is presented. Finally, new developments in alternative adhesive technologies are reported.

Phenolic foam is a unique cellular material that can be utilized in either a fully open cell structure or a completely closed cell structure in a diversity of applications such as open cellular material for floral foam, soil propagation media and/or orthopedic use, and closed cell phenolic foam primarily for thermal insulation. Thus, phenolic foam is much more versatile than other competitive organic foams such as polystyrene and polyurethane with the latter materials being more heavily involved in thermal insulation. Foam processing can consider batch, semi-continuous, or continuous conditions, and the features and weaknesses of the appropriate processes are discussed along with continuous mix heads involving high and low pressure conditions.

The use of phenolic foam for thermal insulation is quite active in Europe, particularly in the UK, and is being revisited in North America because of the efforts of a program sponsored by the US Department of Energy leading to proposed neutral pH foam and improved mechanical strength/performance. The latter is accomplished by the introduction of small amounts of chopped cellulose fibers into the resole resin prior to foaming.

New foam areas are described and include the use of foam for air filtration for dust control, carbon foam prepared by carbonization of phenolic foam and then used for high temperature structural composite sandwich panels or for composite tooling, and pultruded phenolic foam. The latter offers an interesting opportunity as a means of preparing sandwich structures continuously by foaming/pultruding between phenolic glass and carbon fiber prepreg face sheets. In a related matter, fiber reinforced pultruded phenolic foam is being developed to possibly replace balsa core currently used by the US Navy in naval ship sandwich panel structures.

Different test methods are described such as fire testing, insulation efficiency, strength testing, and open/closed cell content. Saturation properties determination for floral foam is detailed.

Mineral wool is considered the best known insulation type among the wide variety of insulation materials. There are three types of mineral wool, and these consist of glass, stone (rock), and slag wool. The overall manufacturing processes, along with features such as specifications and characteristics for each of these types, as well as the role of the binder within the process are described.

Of the variety of mineral wool binders such as sodium silicates, polyesters, melamine urea formaldehyde, polyamides, furane-based resins, and others, phenolic resin-based binders continue to enjoy prominence as the preferred binder for mineral wool.

Optimum conditions are presented for preparing a low viscosity (<50 mPas) resole, infinite water dilutability, solids content (SC) between 45 and 55%, low tetradimer content (≤18%), lowfree phenol (<0.4%), and adequate storage stability by using a molar ratio of F/P of ∼4:1. Various inorganic and organic base catalysts are described along with the strengths and weaknesses of these catalysts. The resulting resole binder contains a high amount of free unreacted formaldehyde and is reduced to zero within a temperature range of 20–40°C by the addition of urea prior to use as a binder. The resulting PFU (phenol-formaldehyde-urea) resin is called “premix” or “prereact.”

The formation of the undesirable tetradimer [bis(4-hydroxy-3,5-dimethylolphenol) methane] and several methods to minimize it from crystallizing within the resole are listed. Even some selective base catalysts that are used to prepare the resole binder provide some enhanced stability against tetradimer precipitation.

Gel times or B-Stage of the resole binder is within 5–20 min and is adjusted to coincide with the overall process (from binder spraying to oven cure). A correct set B-stage enables the binder to flow to the junction points of the mineral wool fibers as the material enters the curing oven and cures within the residence time of the oven to provide the necessary product properties such as recovery, tensile strength, and resistance against ageing.

The emission of various volatile organic components (VOCs) such as monomethylol phenols, trimethyl amine, ammonia, phenol, and formaldehyde occurs at the site of mineral wool production. The generation of the latter three VOCs is shown to occur by resin cure (formaldehyde), the urea responsible for the generation of ammonia, and free phenol due to an unreacted amount within the resin.

The roles of some of the other components (ammonium hydroxide, ammonium sulfate, silane, emulsifier, de-dusting oil, extenders, and water) that are introduced into the final binder mixture are discussed. Ammonium hydroxide brings the pH of the premix binder to a 9–10 pH value at the site of mineral wool production and “temporarily” stabilizes the higher oligomeric species such as dimers and tetramers from precipitating by maintaining them in solution. Ammonium sulfate (AS) is involved in a multiplicity of roles such as release of acidity only at elevated temperature to facilitate resole cure within the curing oven, regulation of the gel time of the resole by pH change (guided by a plot of gel time vs pH on the thermal hardening of the resole from the initial spraying, to binder-coated fibers into the collecting chamber, and finally the curing oven), and provides the mineral wool with its characteristic color from white to yellow, with an intensity of yellow, due to the amount of AS that is present.. The amount of AS can be 1.03–1.3 mol/mol of the basic catalyst used in resin preparation, and this amount contributes to maximum burst strengths.

The use of phenolic resin for the impregnation of a carrier material such as paper or fabric based on either organic or inorganic fibers was and still is one of the most important application areas for liquid phenolic resins. Substrates like paper, cotton, or glass fabric impregnated with phenolic resins are used as core layers for decorative and technical laminates and for many other different industrial applications. Nowadays, phenolic resins for decorative laminates used for furniture, flooring, or in the construction and transportation industry have gained significant market share. The Laminates chapter mainly describes the manufacture of decorative laminates especially the impregnation and pressing process with special emphasis to new technological developments and recent trends. Moreover, the different types of laminates are introduced, combined with some brief comments as they relate to the market for decorative surfaces.

The Composites market is arguably the most challenging and profitable market for phenolic resins aside from electronics. The variety of products and processes encountered creates the challenges, and the demand for high performance in critical operations brings value. Phenolic composite materials are rendered into a wide range of components to supply a diverse and fragmented commercial base that includes customers in aerospace (Space Shuttle), aircraft (interiors and brakes), mass transit (interiors), defense (blast protection), marine, mine ducting, off-shore (ducts and grating) and infrastructure (architectural) to name a few. For example, phenolic resin is a critical adhesive in the manufacture of honeycomb sandwich panels. Various solvent and water based resins are described along with resin characteristics and the role of metal ions for enhanced thermal stability of the resin used to coat the honeycomb. Featured new developments include pultrusion of phenolic grating, success in RTM/VARTM fabricated parts, new ballistic developments for military vehicles and high char yield carbon–carbon composites along with many others. Additionally, global regional market resin volumes and sales are presented and compared with other thermosetting resin systems.

The quality that all phenolic composites offer is the capability to provide a fire safe, light weight composition at a relatively low material cost. Phenolic composites are commonly used in high temperature environments and wherever they can be applied to protect human life from fire. These crucial applications generally require extensive development and testing, allowing suppliers to demand a premium for many of their products. There are a variety of innovative methods to make a myriad of products for a number of industries, and each of the processes and applications are discussed in detail. Specifically, new processes such as the Quickstep™ Rapid Curing Process and the Double Vacuum Bag process have been developed and are expected to expand the use of phenolic resins in composites by reducing cost through improved efficiency and increased structural component size.

The Composites chapter examines both the advantages and disadvantages associated with the use of phenolics, along with the technologies that have been developed to enhance their attributes and mitigate their drawbacks.

The historical development of the abrasives industry is noteworthy considering its evolution from a rudimentary beginning to a relatively mature industry. Presently, hardly a material exists that has not been exposed to a grinding operation. It can be stated without exaggeration that the abrasives industry represents a basic building block for all other branches of industry.

There are two basic types of abrasives: bonded abrasives or grinding wheels and coated abrasives. As one currently examines the abrasives market, the global market for abrasives is showing a downward trend for bonded abrasives while coated abrasives are increasing globally. In developing countries like China, India, and Thailand that are fostered by a robust economy, abrasives are increasing in use and are produced for industries such as construction, machinery, and automotive. Parallel to this activity, more abrasive production is moving from industrial nations such as US, Japan, and some European nations to these developing countries.

The performance of bonded abrasives and coated abrasives as they are used industrially is recognized by their physical behavior, stress, and heat generated during use. By examining the microstructure of both types of abrasives, abrasive performance characteristics are improved by a judicious evaluation of abrasive components. Bonded abrasive structure consists of two basic components such as grain and bond for most wheels, some voids or pores. The grain must be self-renewing during operation; bond created by phenolic resin provides strength and heat resistance as well as uniformity in bond strength to facilitate regeneration of grain during grinding, and finally the pores of the bonded abrasive draw out the grinding dust or debris. Too weak or too strong bond strength either facilitates rapid wheel wear or the pores are filled with debris and the grinding efficiency is reduced. “Trouble-shooting” suggestions are provided in the manufacture of bonded abrasives as they relate to plant environment (humidity/temperature) and adjustments of specific characteristics of liquid and powdered resins.

Coated abrasives are grain bonded on a backing material. The latter can be paper, vulcanized paper, cloth, or a non-woven. Coated abrasives are single layers of grain on backing material and quite different from bonded abrasives which are bonded aggregates of grains in a circular configuration. Grains of coated abrasives are not replaced; they just become eroded or possibly expelled. Coated abrasives lifetime ceases when the grain is nearly or completely eroded. The key phenolic resin bond requirement is to provide high bond strength between the grain and the backing material. Different resins both phenolic and non-phenolic are used in either the make coat or the size coat. Manufacturing conditions, various phenolic resins, and different backing materials are mentioned for the production of coated abrasives.

Attractive schematics within the chapter provide relationships of both liquid (viscosity, MW, water/solvents) and powdered resins (MW, hexa content, particle size) characteristics in guiding the manufacture of various grinding wheels with attractive wheel features such as good wear resistance, good self sharpening, good water dilutability, and good handling. Similar manufacturing features are mentioned for coated abrasives.

Friction materials such as disk pads, brake linings, and clutch facings are widely used for automotive applications. Friction materials function during braking due to frictional resistance that transforms kinetic energy into thermal energy. There has been a rudimentary evolution, from materials like leather or wood to asbestos fabric or asbestos fabric saturated with various resins such as asphalt or resin combined with pitch. These efforts were further developed by the use of woven asbestos material saturated by either rubber solution or liquid resin binder and functioned as an internal expanding brake, similar to brake lining system. The role of asbestos continued through the use of chopped asbestos saturated by rubber, but none was entirely successful due to the poor rubber heat resistance required for increased speeds and heavy gearing demands of the automobile industry. The use of phenolic resins as binder for asbestos friction materials provided the necessary thermal resistance and performance characteristics. Thus, the utility of asbestos as the main friction component, for over 100 years, has been significantly reduced in friction materials due to asbestos identity as a carcinogen. Steel and other fibrous components have displaced asbestos in disk pads. Currently, non-asbestos organics are the predominate friction material. Phenolic resins continue to be the preferred binder, and increased amounts are necessary to meet the requirements of highly functional asbestos-free disk pads for the automotive industry. With annual automobile production exceeding 70 million vehicles and additional automobile production occurring in developing countries worldwide and increasing yearly, the amount of phenolic resin for friction material is also increasing (Fig. 14.1).

In recent years, increased fuel efficiency of passenger car is required due to the CO₂ emission issue. One of the solutions to improve fuel efficiency is to lower the car body weight. It means that the weight of car components must be decreased. In the case of reduced weight for friction parts, the load applied to the friction parts would be higher (more heat also) and trend would lead to phenolic resins with improved heat resistance.

Lithographic technology by photoresist is an important technique for today’s electronic industries. In the manufacture of a semiconductor and a liquid crystal display (LCD), a photoresist is used as the key photo sensitive material. Phenolic resin is the base polymer that controls photoresist characteristics and plays an active role in these vital electronic industries.

In this chapter, we discuss phenolic resins for photoresists, meta–para cresol novolaks, important properties for photoresist such as meta–para ratio, molecular weight, and alkali dissolution rate. Moreover, various types of phenolic resins for photoresists and production technology are also discussed.

Phenolic Molding Compounds continue to exhibit well balanced properties such as heat resistance, chemical resistance, dimensional stability, and creep resistance. They are widely applied in electrical, appliance, small engine, commutator, and automotive applications. As the focus of the automotive industry is weight reduction for greater fuel efficiency, phenolic molding compounds become appealing alternatives to metals. Current market volumes and trends, formulation components and its impact on properties, and a review of common manufacturing methods are presented. Molding processes as well as unique advanced techniques such as high temperature molding, live sprue, and injection/compression technique provide additional benefits in improving the performance characterisitics of phenolic molding compounds. Of special interest are descriptions of some of the latest innovations in automotive components, such as the phenolic intake manifold and valve block for dual clutch transmissions. The chapter also characterizes the most recent developments in new materials, including long glass phenolic molding compounds and carbon fiber reinforced phenolic molding compounds exhibiting a 10–20-fold increase in Charpy impact strength when compared to short fiber filled materials. The role of fatigue testing and fatigue fracture behavior presents some insight into long-term reliability and durability of glass-filled phenolic molding compounds. A section on new technology outlines the important factors to consider in modeling phenolic parts by finite element analysis and flow simulation.

The use of phenolic resins with an elastomer or a rubber often involves phenolic resins based on alkylated phenolic monomers. This chapter discusses the details of the products used and describes the various applications where this practice is commonly found. It also attempts to explain why each choice is made and to compare the choices made to alternate technologies used to accomplish similar goals.

This chapter reprises the authors' original work and covers the ubiquitous uses of phenolic resins as sand mold and core binders in the metalcasting (foundry) industry. An overview of the economic and technical significance of metalcasting is provided along with a simplified description of the process of casting. A description of all synthetic organic resins used as foundry binders is provided with an overview of the chemistry and coremaking process. Where appropriate, significant advancements made in each process are described. A new section on emissions from phenolic resins used as foundry binders is provided. Finally, comments on the future of phenolic resins in this field are described.

Refractories are used in furnaces and boilers that process steel, cement, or glass as well as incinerators that operate at high temperatures. A variety of binders is used when refractories are manufactured. In this chapter, the use of phenolic resin as a binder for refractories is described. There are several factors that support the use of phenolic resins in comparison to other refractory binders. These include the following:

Recycling systems are classified into those employing typically three methods, and the progress of each method is described. In mechanical recycling, powders of phenolic materials are recovered via a mechanical process and reused as fillers or additives in virgin materials. The effects to flowability, curability, and mechanical properties of the materials are explained. In feedstock recycling, monomers, oligomers, or oils are recovered via chemical processes and reused as feedstock. Pyrolysis, solvolysis or hydrolysis, and supercritical or subcritical fluid technology will also be introduced. When using a subcritical fluid of phenol, the recycled material maintains excellent properties similar to the virgin material, and a demonstration plant has been constructed to carry out mass production development. In energy recovery, wastes of phenolic materials are used as an alternative solid fuel to coal because they are combustible and have good calorific value. Industrial wastes of these have been in practical use in a cement plant. Finally, it is suggested that the best recycling method should be selected according to the purpose or situation, because every recycling method has both strengths and weaknesses. Therefore, quantitative and objective evaluation methods in recycling are desirable and should be established.

There are some disturbing signs that appear on the horizon as phenolic resins enter their second century of existence. The large area of wood adhesives application (~60% of the total volume of phenolic resins in North America) is under intense pressure due to many factors that are contributing to continuing reduction in the sales volume of wood adhesives. These factors include the known slow cure speed of phenolic resins compared to Urea Formaldehyde (UF), Melamine Formaldehyde (MF), or Methylene Diphenyl Isocyanate (MDI); installation of new machinery/ equipment with fast continuous lines; continued decrease in plywood consumption at the expense of Oriented Strand Board (OSB) where phenolic resin is the preferred adhesive for plywood; further reduction in formaldehyde emissions through California Air Resources Board (CARB) Phase I and Phase II; uncertainty of whether formaldehyde will be identified as a human carcinogen pending the anticipated 2009 study; and the environmental movement to reduce or eliminate formaldehyde-containing resins in wood and thermal insulation consumer products (U.S. Green Building Council and other Environmental groups like the Sierra Club). Consumers are being urged by environmental organizations to purchase composite wood products with lower formaldehyde emission levels or none at all. This is illustrated by examining the news media reports after the Hurricane Katrina in 2005. The home trailers provided by the Federal Emergency Management Agency (FEMA) that were used for Louisiana and Mississippi residents after Katrina hurricane as temporary housing further accelerated concerns over formaldehyde emissions since higher than typical indoor exposure levels of formaldehyde in travel trailers and mobile homes were determined for the FEMA trailers.