Phenol-formaldehyde reaction monomers

Phenolics. These plastics allow the preparation of both random prepolymers, such as Baekelands A stage and true structopendant prepolymers, commonly known under the term novolaks (Figure 6). Novolaks permit one to take advantage of the newer prepolymer technology. Monomers are phenol, cresols, and formaldehyde. Molecular weights of the novolaks are between 300 and 700. Novolaks are obtained through careful selection of reaction conditions and catalysis of the phenol-formaldehyde reaction. Molecular weight, as well as the ratio of 2,2 - and 2,4 -links, can be controlled. These structural factors, studied extensively by Wood (28), have an eflFect on the physical properties of the cured polymer network. 

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. 

Alkyl-substituted phenols have different reactivities than phenol toward reaction with formaldehyde. Relative reactivities determined by monitoring the disappearance of formaldehyde in phenol-paraformaldehyde reactions (Table 7.3) show that, under basic conditions, meta-cresol reacts with formaldehyde approximately three times faster titan phenol while ortho- and para-cresols react at approximately one-third the rate of phenol.18 Similar trends were observed for the reactivities of acid-catalyzed phenolic monomers with formaldehyde. 

The polymerization and crosslinking of phenol-formaldehyde is a highly useful industrial process. However, the reactions that take place are quite difficult to handle in a quantitative manner for a number of reasons. The assumption of equal reactivity of all functional groups in a monomer, independent of the other functional groups in the molecule and of whether the others are reacted, is dubious in this polymerization. Consider, for example, the routes by which trimethylolphenol (XXIb) can be produced in this system ... 

For example, novolacs are phenolic resins obtained by the condensation of phenol (trifunctional monomer) and formaldehyde (bifunctional monomer), using a stoichiometric excess of phenol so that formaldehyde is completely consumed without leading to gelation. In a second step, instead of directly adding the necessary formaldehyde, the polymer network is formed by reaction with hexamethylenetetramine (a condensation product of formaldehyde and ammonia usually called hexa), through a complex set of chemical reactions (Chapter 2). 

A number of early homogeneous membranes were made by simple condensation reactions of suitable monomers, such as phenol-formaldehyde condensation reactions of the type ... 

Several chapters also demonstrate the use of smaller molecular-weight carbohydrates (i.e., monomers) in adhesives. Tony Conner and his colleagues (Chapter 25) explore the partial replacement of phenol-formaldehyde adhesives used to bond wood with various wood-derived carbohydrates. A1 Christiansen (Chapter 26) and Joe Karchesy and his coworkers (Chapter 27) investigate the very complicated chemistry and the practical application of adhesives based on the reaction of a carbohydrate with urea and phenol. Tito Viswanathan (Chapter 28) describes his attempts to utilize a very large carbohydrate waste stream, whey permeates from the processing of cheese, for the production of wood adhesives. 

The functionality may vary with reaction conditions. For example, in base-catalyzed copolymerization of phenol and formaldehyde, both monomers are bifunctional at ambient temperature, but phenol becomes trifunctional if the temperature is raised sufficiently. Copolymerization at ambient temperature can produce essentially linear, liquid, resole-type "prepolymers" of low molecular weight. Upon acidification and heat-curing, methylene and ether crosslinks formed by the now trifunctional phenol units transform the polymer into an insoluble resin [7] (see next page). The original Bakelite was such a "thermosetting" product. 

A condensation polymer is one in which the repeating unit lacks certain atoms which were present in the monomer(s) from which the polymer was formed or to which it can be degraded by chemical means. Condensation polymers are formed from bi- or polyfunctional monomers by reactions which involve elimination of some smaller molecule. Polyesters (e.g., 1-5) and polyamides like 1-6 are examples of such thermoplastic polymers. Phenol-formaldehyde resins (Fig. 5-1) are thermosetting condensation polymers. All these polymers are directly synthesized by condensation reactions. Other condensation polymers like cellulose (1-11) or starches can be hydrolyzed to glucose units. Their chemical structure indicates that their repealing units consist of linked glucose entities which lack the elements of water. They are also considered to be condensation polymers although they have not been synthesized yet in the laboratory. 

In contrast to the linear thermoplastic polymers, which are soluble and fusible, the cross-linked network polymers are insoluble and infusible. They are formed from polymerizing systems containing monomers or prepolymers with a functionality of three or more. A good example is the phenol-formaldehyde resin systems. The cross-linking reaction takes place in the bond under applied pressure and heat, and the whole adhesive bond might consist of only one super giant molecule. Such resins are, therefore, called thermosetting resins. 

Despite the favoring of formaldehyde hydrate by this equilibrium, there is sufficient free formaldehyde available for an ortho or para position of phenol to add to the highly electrophilic carbon of formaldehyde. As formaldehyde is consumed by this process, the equilibrium is displaced to the left providing further formaldehyde for reaction until all the phenol potential functionalities are taken up or all the formaldehyde is consumed. The structures of the phenol-formaldehyde polymers produced are difficult to study because the final product is infusible and insoluble. However, current thinking is that all possible monomer links can occur in a typical Bakelite sample (Eq. 21.30). 

Epoxies can be formed from several types of monomers, but often start with epichlorohydrin and bisphenol-A, as shown in Fig. 4.11, because the costs of the ingredients are low. Again the initial reaction is carried out to a low degree of polymerization. The system is then stabilized and diluted with monomer to achieve a workable consistency. Further reaction is carried out with the addition of an amine, amide, urea-formaldehyde, or phenol-formaldehyde. 

Step growth condensation reactions involving multifunctional reactants. More or less densely crosslinked products such as epoxy and phenol-formaldehyde resins may be prepared by the reaction of multifunctional monomers or prepolymers with each other or with other appropriate molecules. 

Synthetic polymers Synthetic polymers are obtained from their respective monomer(s) or reactants by chemical reactions in the laboratory. Most polymers fall into this category. Some examples are polyethylene, polypropylene, phenol-formaldehyde resin and styrene-butadiene rubber. 

Phenol-formaldehyde resins can cause several types of damage to the skin. The most frequently reported effects are different types of contact dermatitis such as irritant contact dermatitis and allergic contact dermatitis. Depigmentation and contact urticaria have also been described. Reaction products, such as monomers and dimers, or remaining raw materials, such as phenols and aldehydes, are causative agents. 

Simultaneous patch-test reactions to p-tert-hutyl-phenol-formaldehyde resin and p-ferf-butylcatechol have been reported (Estlander et al. 1998), and p-tert-butylcatechol has been shown to be a component in at least some resins based on p-ferf-butylphenol-formal-dehyde (Zimerson and Bruze 1998). This can indicate a connection to other sources of p-ferf-butylcatechol, which is used as an antioxidant (Gellin et al. 1970) and as a stabiliser in different plastic monomers (Macfar-lane et al. 1990). 

The most commonly known phenolic composite group is phenol formaldehyde polymers (phenoplasts). They are produced by polycondensation of a phenol and a mixture of phenols (phenol and phenol derivatives like cresol-resorcinol or para tertiary butyl phenol) with an aldehyde, usually formaldehyde and hexamethylene tetramine. Reaction of formaldehyde with phenol (up to 3 moles of formaldehyde can react with one mole of phenol - phenol acts as a three functional monomer) yields methylol groups in the ortho and para positions of the phenol molecule. In a further reaction, the methylol groups condenses with another molecule of phenol to form a methylene bridge. In practice, a prepolymer (usually a powder) is prepared first which is then cured later to the shape of the article in the mould. 

So, lignan molecules may be considered a bifunctional monomer for purposes of a reaction of the phenol-formaldehyde condensation type. 

Scaleup of polycondensation reactions involving multifunctional monomers (for example, phenol/formaldehyde) in tubular reactors has proven especially difficult. Even though the overall stoichiometry (for example, formaldehyde at 75 mol% of the entering phenol concentration) is set to avoid crosslinking, locally crosslinked regions near the tube walls can result when the diffusion group, is too... 

For the previons condensation reaction, both ethylene glycol and dimethyl terephthalate are bifunctional. However, condensation reactions can include trifunctional or higher functional monomers capable of forming crossUnked and network polymers. The thermosetting polyesters and phenol-formaldehyde, the nylons, and the polycarbonates are produced by condensation polymerization. Some polymers, snch as nylon, may be polymerized by either techniqne. 

---