Monomer phenols

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. 

There is a compelling reason to integrate PMMA and phenol-formaldehyde because the monomers phenol and acetone are both made from cumene oxidation (previous chapter). Therefore, one makes one mole of phenol for every mole of acetone, and a producer would have to sell one of these monomers if he did not have an integrated process to produce both polymers or some other products. 

As indicated previously, a significant feature of these resins is the very low content (<0.1%) of the monomers, phenol and formaldehyde. The molecular weights of resins used in plywood manufacture are higher, and the viscosity is in the range of 700 (wet process) or 450 mPa s (dry process) (9). In the manufacture of particle boards, which consist of —95% by weight of wood chips bonded together by the adhesive, the viscosity of the resin used is much lower, about 130 mPa s (9). In both cases, the resin has a dry solids content of 40-50%. 

Monomer phenols in grape and wine are usually analyzed by HPLC using a reverse phase C18 column (usually 250 x 4 mm, 5 p,m) operating close to or at room temperature. 

Phenolic resins Chlorinated rubber Synthetic latices Chlorinated paraffin Polyisocyanates Reactive monomers Phenolic varnishes Natural resins... 

Fig. (7). UV spectra and absorbance features of flavan-3-ol monomer, phenolic acids and tlavonoids.

The electropolymerization technique, applied to oxidizable monomers (phenols and their derivatives) and developed in the seventies to obtain homogeneous thin dielectric fdms [2], was extended to heterocyclic conducting polymers as early as 1979 with polypyrrole (PPy) [3], and some time later to poly aniline (PANI, 1981) [4,5] and poly thiophene (PT, 1982) [6]. It was shown that adherent homogeneous conductive sohd... 

Formaldehyde reacts readily with several types of active-hydrogen monomers (phenol, urea, and melamine) to form highly cross-hiiked thermoset plastics. They form a family in their fundamental chemistry, and they form complementary families in terms of materials properties, markets, and practical applications. 

The kinetics and mechanism of retardation and inhibition has been reviewed by Bamford, Tiidos and Foldes-Berezsnich, Eastmond, Goldfinger et and Bovey and Kolthoff Gommon inhibitors include stable radicals, oxygen, certain monomers, phenols, qirinones, phenothiazine, nitro and nitroso compoimds, and certain hansition metal salts. Some inhibition constants (kjkp) are provided in Table 6. Absolute rate constants (i ) for the reactions of these species with simple carbon-centered radicals are sirmmarized in Table 7. 

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 ... 

This particular representation makes it easy to visualize formaldehyde as a step-growth monomer of functionality 2. Our principal interest is in the reactions of formaldehyde with the active hydrogens in phenol, urea, and melamine, compounds [II] [IV], respectively ... 

The hydrogen atoms shown in these monomers (only those underlined in phenol) are the active hydrogens, so these compounds have nominal functionalities of 3, 4, and 6, respectively. Note that a monosubstituted phenol would have a functionality of 2 and would be incapable of crosslinking. 

At first glance it appears that these systems do conform fully to the discussion above this is an oversimplification, however. The ortho and para hydrogens in phenol are not equal in reactivity, for example. In addition, the technology associated with these polymers involves changing the reaction conditions as the polymerization progresses to shift the proportions of several possible reactions. Accordingly, the product formed depends on the nature of the catalyst used, the proportions of the monomers, and the temperature. Sometimes other additives or fillers are added as well. 

I ovolac Synthesis and Properties. Novolac resins used in DNQ-based photoresists are the most complex, the best-studied, the most highly engineered, and the most widely used polymers in microlithography. Novolacs are condensation products of phenoHc monomers (typically cresols or other alkylated phenols) and formaldehyde, formed under acid catalysis. Figure 13 shows the polymerization chemistry and polymer stmcture formed in the step growth polymerization (31) of novolac resins. 

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.

Fig. 28. Traditional duv-resist design using derivatives of polyhydroxystyrene. Monomer (a) contributes hydrophilic character to the polymer, and its acidic phenol group enhances aqueous base solubiUty monomer (b) provides acid-labile pendent groups.

The yield of acetone from the cumene/phenol process is beUeved to average 94%. By-products include significant amounts of a-methylstyrene [98-83-9] and acetophenone [98-86-2] as well as small amounts of hydroxyacetone [116-09-6] and mesityl oxide [141-79-7]. By-product yields vary with the producer. The a-methylstyrene may be hydrogenated to cumene for recycle or recovered for monomer use. Yields of phenol and acetone decline by 3.5—5.5% when the a-methylstyrene is not recycled (21). 

The reaction with sodium sulfite or bisulfite (5,11) to yield sodium-P-sulfopropionamide [19298-89-6] (C3H7N04S-Na) is very useful since it can be used as a scavenger for acrylamide monomer. The reaction proceeds very rapidly even at room temperature, and the product has low toxicity. Reactions with phosphines and phosphine oxides have been studied (12), and the products are potentially useful because of thek fire retardant properties. Reactions with sulfide and dithiocarbamates proceed readily but have no appHcations (5). However, the reaction with mercaptide ions has been used for analytical purposes (13)). Water reacts with the amide group (5) to form hydrolysis products, and other hydroxy compounds, such as alcohols and phenols, react readily to form ether compounds. Primary aUphatic alcohols are the most reactive and the reactions are compHcated by partial hydrolysis of the amide groups by any water present. 

Although the anionic polymerization mechanism is the predominant one for the cyanoacryhc esters, the monomer will polymerize free-radically under prolonged exposure to heat or light. To extend the usable shelf life, free-radical stabilizers such as quinones or hindered phenols are a necessary part of the adhesive formulation. 

Eor most polymer applications the removal of the inhibitors from the monomer is unnecessary. Should it be requited, the phenolic inhibitors can be removed by an alkaline wash or by treatment with a suitable ion-exchange resia. Uninhibited MMA is sufftcientiy stable to be shipped under carehiUy controlled temperature and time restrictions. Uninhibited monomers should be monitored carehiUy and used promptiy. 

Phenol. This is the monomer or raw material used in the largest quantity to make phenoHc resins (Table 1). As a soHd having a low melting point, phenol, C H OH, is usually stored, handled in Hquid form at 50—60°C, and stored under nitrogen blanket to prevent the formation of pink quinones. Iron contamination results in a black color. 

Gas chromatography (gc) has been used extensively to analyze phenoHc resins for unreacted phenol monomer as weU as certain two- and three-ring constituents in both novolak and resole resins (61). It is also used in monitoring the production processes of the monomers, eg, when phenol is alkylated with isobutylene to produce butylphenol. Usually, the phenoHc hydroxyl must be derivatized before analysis to provide a more volatile compound. The gc analysis of complex systems, such as resoles, provides distinct resolution of over 20 one- and two-ring compounds having various degrees of methylolation. In some cases, hemiformals may be detected if they have been properly capped (53). 

Phenol. Phenol monomer is highly toxic and absorption by the skin can cause severe blistering. Large quantities can cause paralysis of the central nervous system and death. Ingestion of minor amounts may damage kidneys, Hver, and pancreas. Inhalation can cause headaches, dizziness, vomiting, and heart failure. The threshold limit value (TLV) for phenol is 5 ppm. The health and environmental risks of phenol and alkylated phenols, such as cresols and butylphenols, have been reviewed (66). 

Alkylphenols have been substituted for phenol as chain teaninatois in polycarbonates. In this role, PTBP (14) competes with the diol monomer for reactive chlorocarbonate sites. The ratio of butylphenol to diol controls the molecular weight of the polymer. 

Ammonia is used in the fibers and plastic industry as the source of nitrogen for the production of caprolactam, the monomer for nylon 6. Oxidation of propylene with ammonia gives acrylonitrile (qv), used for the manufacture of acryHc fibers, resins, and elastomers. Hexamethylenetetramine (HMTA), produced from ammonia and formaldehyde, is used in the manufacture of phenoHc thermosetting resins (see Phenolic resins). Toluene 2,4-cHisocyanate (TDI), employed in the production of polyurethane foam, indirectly consumes ammonia because nitric acid is a raw material in the TDI manufacturing process (see Amines Isocyanates). Urea, which is produced from ammonia, is used in the manufacture of urea—formaldehyde synthetic resins (see Amino resins). Melamine is produced by polymerization of dicyanodiamine and high pressure, high temperature pyrolysis of urea, both in the presence of ammonia (see Cyanamides). 

Hyperbranched polyurethanes are constmcted using phenol-blocked trifunctional monomers in combination with 4-methylbenzyl alcohol for end capping (11). Polyurethane interpenetrating polymer networks (IPNs) are mixtures of two cross-linked polymer networks, prepared by latex blending, sequential polymerization, or simultaneous polymerization. IPNs have improved mechanical properties, as weU as thermal stabiHties, compared to the single cross-linked polymers. In pseudo-IPNs, only one of the involved polymers is cross-linked. Numerous polymers are involved in the formation of polyurethane-derived IPNs (12). 

Caprolactam [105-60-2] (2-oxohexamethyleiiiiriiQe, liexaliydro-2J -a2epin-2-one) is one of the most widely used chemical intermediates. However, almost all of the aimual production of 3.0 x 10 t is consumed as the monomer for nylon-6 fibers and plastics (see Fibers survey Polyamides, plastics). Cyclohexanone, which is the most common organic precursor of caprolactam, is made from benzene by either phenol hydrogenation or cyclohexane oxidation (see Cyclohexanoland cyclohexanone). Reaction with ammonia-derived hydroxjlamine forms cyclohexanone oxime, which undergoes molecular rearrangement to the seven-membered ring S-caprolactam. 

Donation of a proton to the reactant often forms a carbenium ion or an oxonium ion, which then reacts ia the catalytic cycle. For example, a catalytic cycle suggested for the conversion of phenol and acetone iato bisphenol A, which is an important monomer used to manufacture epoxy resias and polycarbonates, ia an aqueous mineral acid solution is shown ia Figure 1 (10). 

Another elastomer to find use is the substitution product of phenol and -ethylphenol along with an aHylic monomer to provide cross-link sites (5). This is trademarked EYPEL-A elastomer [66805-77-4] by Ethyl Corp., and designated PZ by ASTM. The substitution ratio is roughly 52 mol % phenol, 43% j )-ethylphenol, and 5% aHylic substituent. 

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