Thermoplastic resoles

Leo Baekeland obtained thermoplastic resoles (one-stage resins) by adding stoichiometric quantities of formaldehyde to phenol and heating gently under alkaline conditions. He also produced thermoplastic novolacs (two-stage resins) by using less than stoichiometric quantities of formaldehyde and heating under acid conditions. 

These thermoplastic resoles and novolacs are mixed with lubricants, pigments and additives, such as wood flour. The molding compound is converted to an infusible resin by heating it under pressure in a mold. A typical sequence of chemical reactions associated with the formation of this complex, three-dimensional polymer is shown in Figure 15.4. Typical properties of phenolic plastics are shown in Table 15.4. 

Most phenohc foams are produced from resoles and acid catalyst suitable water-soluble acid catalysts are mineral acids (such as hydrochloric acid or sulfuric acid) and aromatic sulfonic acids (63). Phenohc foams can be produced from novolacs but with more difficulty than from resoles (59). Novolacs are thermoplastic and require a source of methylene group to permit cure. This is usually suppHed by hexamethylenetetramine (64). 

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. 

Reactions with Aldehydes and Ketones. An important use for alkylphenols is ia phenol—formaldehyde resias. These resias are classified as resoles or aovolaks (see Phenolic resins). Resoles are produced whea oae or more moles of formaldehyde react with oae mole of pheaol uader basic catalysis. These resias are thermosets. Novolaks are thermoplastic resias formed whea an excess of phenol reacts with formaldehyde under acidic conditions. The acid protonates formaldehyde to generate the alkylating electrophile (17). 

Other uses of 4-methylphenol include its conversion to a benzotriazole uv stabilizer, 2-(2 -hydroxy-5 -methylphenyl)benzotriazole [2440-22-4] (45). The benzotriazole-based uv stabilizer makes possible the extended use of thermoplastics in outdoor appHcations. Other minor appHcations for 4-methylphenol include its use in the production of novolak or resole phenoHc resins. It is also used in the production of certain dyes and fragrances (Table 3). 

Resinous adducts, 10 394 Resinous odor, 3 229t Resins. See also Epoxy resins Lacquer resins Novolac resins Phenolic resins Resole resins Thermoplastic resins acidic cation-exchange, 12 191 advanced materials, 1 693 antilipemic agents, 5 141 aromatic glycidyl amine, 10 372—373 chromatographic, 14 383-384 for coatings, 7 95-107 derived from furfuryl alcohol, 12 271— 272... 

Resols (phenol-formaldehyde resins) are commercially used for effective crosslinking of EPDM in the production of thermoplastic vulcanisates [8]. General studies on rubber crosslinking for different diene rubbers are presented here. 

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 ... 

Phenolic resins are prepared by a step-growth polymerisation of formaldehyde and phenol or phenol derivative using an acid or a base catalyst. The product type and the quality largely depend on the ratio of the reactants used and the nature of the catalyst. Phenolic resins are available in two varieties 1) novolac, which is a thermoplastic type and can be used as it is or can be cured with hexamethylene tetramine (HMTA) to get a crosslinked structure. This can also be viewed as a reactive intermediate, and can be transformed into other groups so different types of structures can be generated and 2) resole, which is a multifunctional reactive compound and can be cured thermally without a catalyst or an acid catalyst. 

Novolac can be used as a thermoplastic and can be readily modified by reacting with the terminal hydroxyl groups. Modification of resole is comparatively difficult due to a gelling problem. 

Addition of p-cresol formaldehyde (PCF) into phenolic/NBR blends resulted in rednction in the domain size of the dispersed phase and improvement in mechanical properties [244]. PCF resin has an intermediate polarity compared with NBR and resole and can react faster with NBR. Therefore, PCF molecules are likely to be concentrated at the phenolic/NBR interface and act as an external compatibilising agents [245]. Thus compatibility and chemical bonding between NBR and phenolic resin is improved, leading to the enhancement in properties. The other materials used as toughening agents of phenolic resin include elastomers such as natural rubber and nitrile rubber [246, 247], reactive liquid polymers [248] and thermoplastics such as polysulfone, polyamide, polyethylene oxide [249, 250]. 

The final state of a phenolic resin varies dramatically from thermoplastic to thermoset and from solid to liquid, and includes solutions and dispersions. With a hulk process, resole resins, in neat or concentrated form, must be produced in small hatches ( 2-10 m ) in order to maintain control of the reaction and obtain a uniform product. On the other hand, if the product contains a large amount of water, such as liquid plywood adhesives, large reactors (20 m ) can be used. Melt-stable products such as novolaks can be prepared in large batches (20-40 m ) if the exotherms can be controlled. Some reactors are reportedly as large as 60 m (Ref. 9, p. 83). 

Dry Blending. Phenol-formaldehyde resin still in its resole thermoplastic state is cold-pulverized and then perfectly mixed with fillers and other additives in a two-roll mill heated 80-115°C. Once homogenized, the blend stays on the warmer roll. It is then possible to cut it, since during homogenization, resin condensation continues going partially from resole to resite, and this forms a cohesive layer. Once cold, the blend can be pulverized. 

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