Phenol - Formaldehyde

Phenol-formaldehyde laminates are generally used with mineral acids, salts, and chlorinated aromatic hydrocarbons. When graphite is used as a filler, the laminate is suitable for use with hydrofluoric acid and certain fluoride salts. Refer to Table 3.17 for the compatibility of phenol-formaldehyde with selected corrodents. Reference [1] provides a more comprehensive listing. 

Phenol formaldehyde resins were among the first completely synthetic polymers made [73]. They have found wide commercial application due to their ablative properties, with potential use as high-temperature polymers [74, 75]. Phenolics were among the first synthetic polymers to be filled with fillers such as cotton fibres. 

As manufactured, these polymers usually contain various types of fillers and this has obvious food contact implications. The fillers are used to reduce cost, improve shock resistance and to reduce mould shrinkage. Phenol-formaldehyde resins are resistant to common solvents, acids, aqueous salt solutions and hot oils, although water and alcohols cause slight swelling. As far as food contact applications are concerned the main use of these resins is in the manufacture of closures for containers such as jars and bottles. Direct contact is less likely in these applications because of the interposition of a liner between the closure and the container. The resins are also used for moulding the handles of kitchen implements. 

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In practice, synthetic polymers are sometimes divided into two classes, thermosetting and thermo-plMtic. Those polymers which in their original condition will fiow and can be moulded by heat and pressime, but which in their finished or cured state cannot be re softened or moulded are known as thermo setting (examples phenol formaldehyde or urea formaldehyde polymer). Thermoplastic polymers can be resoftened and remoulded by heat (examples ethylene polymers and polymers of acrylic esters). 

Ion-exchange resins. The constituent phenolic hydroxyl groups in the insoluble phenol-formaldehyde resins react with cations of salts ... 

The formation of various types of phenol - formaldehyde resins (the so-called Bakelite) may be demonstrated by the following experiments. 

Phenol-Formaldehyde Resin. Phenol-formaldehyde resin is prepared as follows ... 

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 ... 

Industrial phenol-formaldehyde polymerization is a complex process, but the following reactions suggest the successive stages and the possible linkages involved ... 

Table 5.7 Calculated Values of a and f for Phenol-formaldehyde Resins Formed from Different Proportions of Reactants and Based on Experimental Values of n and n .

Technological advances continue to be made, several recent patents describe advanced phenol—formaldehyde—furfuryl alcohohol biader systems (68—70). These systems are free of nitrogen compounds that can be detrimental to metal iategrity. Systems with extended bench life have also been developed (71). 

Fig. 13. Polymerization chemistry of phenol—formaldehyde condensation synthesis of novolac resia. The phenol monomer(s) are used ia stoichiometric excess to avoid geUation, although branching iavariably occurs due to the multiple reactive sites on the aromatic ring.

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. 

For exterior appHcations, where water exposure is expected, phenol—formaldehyde (PF) or phenol—resorcinol—formaldehyde (PRF) adhesives are used. Only small quantities of this type of hardwood plywood are made, primarily for marine use. 

The adhesive used in virtually all softwood plywood has a phenol—formaldehyde (PF) base to provide an exterior-grade, durable, waterproof bond. Thus, most grades of plywood can be used in stmctural appHcations. A very small percentage of softwood plywood is made using interior-grade adhesive systems, and this material is used in interior cabinetry, furniture, and shelving. 

The synthesis in 1912 of phenol—formaldehyde resins by Baekeland, which forms the basis of many modern day adhesives. 

Although the use of simple diluents and adulterants almost certainly predates recorded history, the use of fillers to modify the properties of a composition can be traced as far back as eady Roman times, when artisans used ground marble in lime plaster, frescoes, and po22olanic mortar. The use of fillers in paper and paper coatings made its appearance in the mid-nineteenth century. Functional fillers, which introduce new properties into a composition rather than modify pre-existing properties, were commercially developed eady in the twentieth century when Goodrich added carbon black to mbber and Baekeland formulated phenol— formaldehyde plastics with wood dour. 

The first cellular synthetic plastic was an unwanted cellular phenol—formaldehyde resin produced by early workers in this field. The elimination of cell formation in these resins, as given by Baekeland in his 1909 heat and pressure patent (2), is generally considered the birth of the plastics industry. The first commercial cellular polymer was sponge mbber, introduced between 1910 and 1920 (3). 

Fig. 3. Effect of density on compressive modulus of rigid cellular polymers. A, extmded polystyrene (131) B, expanded polystyrene (150) C-1, C-2, polyether polyurethane (151) D, phenol—formaldehyde (150) E, ebonite (150) E, urea—formaldehyde (150) G, poly(vinylchloride) (152). To convert...

Foams prepared from phenol—formaldehyde and urea—formaldehyde resins are the only commercial foams that are significantly affected by water (22). Polyurethane foams exhibit a deterioration of properties when subjected to a combination of light, moisture, and heat aging polyester-based foam shows much less hydrolytic stabUity than polyether-based foam (50,199). 

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). 

Phenol—formaldehyde resins are used as mol ding compounds (see Phenolic resins). Their thermal and electrical properties allow use in electrical, automotive, and kitchen parts. Other uses for phenol—formaldehyde resins include phenoHc foam insulation, foundry mold binders, decorative and industrial laminates, and binders for insulating materials. 

It should be possible to form linear noncross-linked polymers of melamine—formaldehyde or phenol—formaldehyde by reaction of one mole of the patent with one mole of formaldehyde, but this is generally not the case. The melamine crystal itself is very insoluble in water and only becomes soluble as the formaldehyde molecules add on. If much less than 1.5 moles of formaldehyde pet mole of melamine ate used, the aqueous resin solution is very unstable. 

Organic cross-linkers, which include glyoxal (48) and formaldehyde (qv), have also been used. Use of hypohaUte salts (49) and epichlorohydrin (50) promotes gel stabiUty. Phenol—formaldehyde cross-linking systems have been used to produce stable acrylamide copolymer gels at temperatures above 75°C and brine hardness levels above 2000 ppm (51). 

Strong-Acid Catalysts, Novolak Resins. PhenoHc novolaks are thermoplastic resins having a molecular weight of 500—5000 and a glass-transition temperature, T, of 45—70°C. The phenol—formaldehyde reactions are carried to their energetic completion, allowing isolation of the resin ... 

Special resoles are obtained with amine catalysts, which affect chemical and physical properties because amine is incorporated into the resin. For example, the reaction of phenol, formaldehyde, and dimethylamine is essentially quantitative (28). 

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%. 

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