Phenol/formaldehyde products

Some nonconventional bonding methods are based on the use of agricultural by-products, i.e., on nonpetroleum-based materials this use constitutes another advantage. Some nonconventionally bonded materials produce reduced amounts of toxic gaseous materials, such as formaldehyde, that make them preferable to phenol-formaldehyde products and urea-formaldehyde resins. Economically, the nonconventional methods do not offer any particular advantages, although they appear to be competitive with the conventional methods. 

Since the cross-linked polymer of phenol-formaldehyde reaction is insoluble and infusible, it is necessary for commercial applications to produce first a tractable and fusible low-molecular-weight prepolymer which may, when desired, be transformed into the cross-linked polymer [14,44,45]. The initial phenol-formaldehyde products (prepolymers) may be of two types resols and novolacs. 

Levinstein was here referring to the raw materials used to produce macromolecules, the synthetic polymers that followed the introduction of Bakelite and the related Novolak (phenol-formaldehyde products) ... 

In 1933, ICI purchased an interest in urea-formaldehyde manufacturer Croydon Mouldrite Limited, forerunner of ICI (Plastics) Ltd. (1938). ICI took up research into urea-formaldehyde polymers at Billingham (and phenol-formaldehyde products at its dyestuffs division). 

An additive used in varnishes. A thermoplastic phenol-formaldehyde product, produced with excess of phenol in the mixture. 

Some commercially important cross-linked polymers go virtually without names. These are heavily and randomly cross-linked polymers which are insoluble and infusible and therefore widely used in the manufacture of such molded items as automobile and household appliance parts. These materials are called resins and, at best, are named by specifying the monomers which go into their production. Often even this information is sketchy. Examples of this situation are provided by phenol-formaldehyde and urea-formaldehyde resins, for which typical structures are given by structures [IV] and [V], respectively ... 

Phenol—formaldehyde (PF) was the first of the synthetic adhesives developed. By combining phenol with formaldehyde, which has exceptional cross-linking abiHties with many chemicals and materials, and a small amount of sodium hydroxide, a resin was obtained. The first resins soHdified as they cooled, and it was discovered that if it was ground to a powder with a small amount of additional formaldehyde and the appHcation of more heat, the mixture would Hquify and then convert to a permanently hard material. Upon combination of the powdered resin mixture with a filler material such as wood flour, the result then being placed in a mold and pressed under heat and pressure, a hard, durable, black plastic material was found to result. For many years these resulting products were called BakeHte, the trade name of the inventor. BakeHte products are still produced today, but this use accounts for only a small portion of the PF resins used. 

Amino and Phenolic Resins. The largest use of formaldehyde is in the manufacture of urea—formaldehyde, phenol—formaldehyde, and melamine—formaldehyde resins, accounting for over one-half (51%) of the total demand (115). These resins find use as adhesives for binding wood products that comprise particle board, fiber board, and plywood. Plywood is the largest market for phenol—formaldehyde resins particle board is the largest for urea—formaldehyde resins. Under certain conditions, urea—formaldehyde resins may release formaldehyde that has been alleged to create health or environmental problems (see Amino RESINS AND PLASTICS). 

In 1993, worldwide consumption of phenoHc resins exceeded 3 x 10 t slightly less than half of the total volume was produced in the United States (73). The largest-volume appHcation is in plywood adhesives, an area that accounts for ca 49% of U.S. consumption (Table 11). During the early 1980s, the volume of this apphcation more than doubled as mills converted from urea—formaldehyde (UF) to phenol—formaldehyde adhesives because of the release of formaldehyde from UF products. Other wood bonding applications account for another 15% of the volume. The next largest-volume application is insulation material at 12%. 

Particle board and wood chip products have evolved from efforts to make profitable use of the large volumes of sawdust generated aimually. These products are used for floor undedayment and decorative laminates. Most particle board had been produced with urea—formaldehyde adhesive for interior use resin demand per board is high due to the high surface area requiring bonding. Nevertheless, substantial quantities of phenol—formaldehyde-bonded particle board are produced for water-resistant and low formaldehyde appHcations. 

Spheres. HoUow spherical fillers have become extremely useflil for the plastics industry and others. A wide range of hoUow spherical fillers are currently available, including inorganic hoUow spheres made from glass, carbon, fly ash, alumina, and 2h conia and organic hoUow spheres made from epoxy, polystyrene, urea—formaldehyde, and phenol—formaldehyde. Although phenol—formaldehyde hoUow spheres are not the largest-volume product, they serve in some important appHcations and show potential for future use. 

Alkylated phenol derivatives are used as raw materials for the production of resins, novolaks (alcohol-soluble resins of the phenol—formaldehyde type), herbicides, insecticides, antioxidants, and other chemicals. The synthesis of 2,6-xylenol [576-26-1] h.a.s become commercially important since PPO resin, poly(2,6-dimethyl phenylene oxide), an engineering thermoplastic, was developed (114,115). The demand for (9-cresol and 2,6-xylenol (2,6-dimethylphenol) increased further in the 1980s along with the growing use of epoxy cresol novolak (ECN) in the electronics industries and poly(phenylene ether) resin in the automobile industries. The ECN is derived from o-cresol, and poly(phenylene ether) resin is derived from 2,6-xylenol. 

Quinone dioximes, alkylphenol disulfides, and phenol—formaldehyde reaction products are used to cross-link halobutyl mbbers. In some cases, nonhalogenated butyl mbber can be cross-linked by these materials if there is some other source of halogen in the formulation. Alkylphenol disulfides are used in halobutyl innerliners for tires. Methylol phenol—formaldehyde resins are used for heat resistance in tire curing bladders. Bisphenols, accelerated by phosphonium salts, are used to cross-link fluorocarbon mbbers. 

The initial phenol-formaldehyde reaction products may be of two types, novolaks and resols. 

Today the phenol-formaldehyde moulding compositions do not have the eminent position they held until about 1950. In some, important applications they have been replaced by other materials, thermosetting and thermoplastic, whilst they have in the past two decades found use in few new outlets. However, the general increase in standards of living for much of this period has increased the sales of many products which use phenolics and consequently the overall use of phenol-formaldehyde moulding powders has been well maintained. 

The method for producing formaldehyde was described in Chapter 19. In aminoplastics manufacture it is used in the form of formalin (36-37% w/w CH2O). As in the case of phenolic resin production, formalin with both high and low methanol content is used according to the needs of the manufacturer. The low methanol content formalin is more reactive but is also less stable and must be used soon after its preparation. For this reason some resin manufacturers prefer to use formalin with a high 7-10%) methanol content. 

At one time urea-formaldehyde was used extensively in the manufacture of plywood but the product is today less important than heretofore. For this purpose a resin (typically U-F molar ratio 1 1.8)-hardener mixture is coated on to wood veneers which are plied together and pressed at 95-110°C under pressure at 200-800 Ibf/in (1.38-5.52 MPa). U-F resin-bonded plywood is suitable for indoor application but is generally unsuitable for outdoor work where phenol-formaldehyde, resorcinol-fonnaldehyde or melamine modified resins are more suitable. 

Prior to 1890, formaldehyde was not commercially available [2]. Thus the first phenol-formaldehyde resins were made using formaldehyde equivalents such as methylene diacetate or methylal [2,20]. The first true phenol-formaldehyde resin was made by Kleeberg at the direction of Emil Fisher in 1891 [2,21]. Saliginen (o-hydroxymethyl phenol) was recognized as a condensation product of phenol and formaldehyde in 1894 and was the subject of United States patents in 1894 and 1896 [22,23]. 

By far the preponderance of the 3400 kt of current worldwide phenolic resin production is in the form of phenol-formaldehyde (PF) reaction products. Phenol and formaldehyde are currently two of the most available monomers on earth. About 6000 kt of phenol and 10,000 kt of formaldehyde (100% basis) were produced in 1998 [55,56]. The organic raw materials for synthesis of phenol and formaldehyde are cumene (derived from benzene and propylene) and methanol, respectively. These materials are, in turn, obtained from petroleum and natural gas at relatively low cost ([57], pp. 10-26 [58], pp. 1-30). Cost is one of the most important advantages of phenolics in most applications. It is critical to the acceptance of phenolics for wood panel manufacture. With the exception of urea-formaldehyde resins, PF resins are the lowest cost thermosetting resins available. In addition to its synthesis from low cost monomers, phenolic resin costs are often further reduced by extension with fillers such as clays, chalk, rags, wood flours, nutshell flours, grain flours, starches, lignins, tannins, and various other low eost materials. Often these fillers and extenders improve the performance of the phenolic for a particular use while reducing cost. 

Reasonable procedures for manufacturing resoles and novolacs are presented in subsequent sections. These procedures utilize the a concept known in the industry as programmed formaldehyde addition to avoid the problems mentioned above as well as aiding in control of the exothermic reactions resulting from the manufacture of the desired phenol-aldehyde products. These reactions are also extremely exothermic. 

An example of what can happen in a production situation is provided in Fig. 1. This photo shows the devastation resulting from a phenol-formaldehyde reactor explosion that occurred at the Borden Chemical plant in Demopolis, Alabama on June 28, 1974. In this explosion, the stainless steel reactor was blown to bits. The reactor operators control room was obliterated. Two people were killed and several others were injured. All nearby property was demolished and windows were broken in homes for a distance of five miles from the plant. 

While discussing ethers we should mention that the presence of unreacted anisoles or methyl anisoles is highly undesirable in the manufacture of phenol-formaldehyde resoles. These materials tend to be unreactive relative to phenol under normal resole conditions. They are also volatile and have odors detectable at very low concentrations. They have been the source of worker complaints and costly claims in the wood products industry. Benzophenones and methyl phenyl ketones are also common phenol contaminants that are problematic in this regard. 

Alkaline co-condensation to yield commercial resins and the products of reaction obtained thereof [93,94] as well as the kinetics of the co-condensation of mono methylol phenols and urea [104,105] have also been reported [17]. Model reactions in order to prove an urea-phenol-formaldehyde co-condensation (reaction of urea with methylolphenols) are described by Tomita and Hse [98,102, 106] and by Pizzi et al. [93,104] (Fig. 1). 

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