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A ACETAL RESINS Introduction Acetal resins are the family of polymers derived primarily from formaldehyde and therefore have a repeat unit, (CH2—O)n. The homopolymers are completely char- acterized by this repeat unit and the name polyoxymethylene is more structurally descriptive. Acetal copolymers have the oxymethylene structure occasionally in- terrupted by the comonomer unit. The designation aldehyde polymers generally refers to polymers of aldehydes larger than formaldehyde. The term polyacetal should be reserved for polymeric products of the reactions of aldehydes and poly- hydric alcohols. Commercialization of formaldehyde polymers did not occur until some 100 years after the first reported observation of them (1). Research during the 1920s contributed much of the background on linear formaldehyde polymers, on which today’s acetal resins are based (2); in 1922 the formation of a white polymer from the sublimation of trioxane was observed (3). However, formaldehyde polymers of sufficient molecular weight to be commercially significant were not prepared until the late 1950s (4). DuPont developed this technology and the end- capping reaction that imparts stability, and launched a new plastic in the family of engineering resins. In 1959 DuPont’s commercial plant went into operation, producing an acetate end-capped homopolymer called Delrin. Shortly thereafter, Celanese researchersCOPYRIGHTED developed an acetal resin basedMATERIAL on the copolymerization of trioxane and cyclic ethers, such as ethylene oxide (5). In 1962 a commercial plant began producing this acetal copolymer, designated Celcon. Today both homopolymers and copolymers are commercially available. DuPont and Ticona are the largest producers (6). The successful development of these engineering resins is better appreci- ated if one considers the modes of degradation that had to be overcome. These have been summarized and include unzippering from the chain ends to yield monomeric formaldehyde, oxidation and chain scission followed by depolymerization, acidic attack on the acetal chain, and thermal scission which occurs above 270◦C (7). Stabilization to prevent or limit these modes of degradation was 1 2 ACETAL RESINS Vol. 1 Table 1. Properties of Formaldehyde and Trioxane Property Formaldehyde Trioxane Refs. molecular weight 30.03 90.08 formula CH2OC3H6O3 density, g/cm3 −20◦C 0.815 8 10◦C1.179 normal boiling point, ◦C −19 114.4 10,11 melting point, ◦C −118 62 9,10 vapor pressure, Antoine constants, Paa A 9.28176 10.040 12,13 B 959.43 1849.920 C 243.392 253.270 heat of vaporization at normal boiling point, kJ/molb 23.3 42.0 9,10 heat of combustion, kJ/molb 561 1488 9,14 critical constantsc temperature, ◦C 137.2 331.4 pressure, MPad 6.784 5.797 flammability in air lower/upper limit, vol% 7.0/73 3.6/29.0 17 ignition temperature, ◦C 430 413.9 17 a ◦ Log10P = A − B/(C + T); where T = C; To convert Pa (kPa) to mm Hg, multiply by 0.0075 (7.5). bTo convert J to cal, divide by 4.184. cEstimated by methods in refs. 15 and 16. dTo convert MPa to psi, multiply by 145. essential to the commercial development of acetal resins. Throughout this discus- sion, the homopolymers and copolymers referred to are generally DuPont’s Delrin and Ticona’s Celcon, respectively. Physical Properties of Monomer Acetal resins are produced from either formaldehyde or its cyclic trimer, trioxane. Formaldehyde is a well-known, widely used commodity chemical. At room temperature, it is a colorless, pungent gas that is highly irritating to the mu- cous membranes of the eyes, nose, and throat. Formaldehyde is not commercially available in the anhydrous monomeric form, and is usually sold in aqueous solution, with methanol added as an inhibitor. Trioxane is the anhydrous trimer of formaldehyde. It is a white, crystalline solid with a pleasant etherlike aroma un- like that of formaldehyde. Trioxane is soluble in alcohols and ethers and will also dissolve in water without depolymerization to formaldehyde. Both formaldehyde and trioxane are flammable. Table 1 lists the physical properties of each. Vol. 1 ACETAL RESINS 3 Chemical Properties of Monomer Formaldehyde is extremely reactive with itself and with other chemicals. It is readily soluble in water, with which it immediately reacts to form methylene glycol, CH2(OH)2, and poly(oxymethylene) glycol, HO(—CH2O—)nH. It can oxidize to formic acid, and in aqueous alkali, the Cannizzaro autooxidation–reduction reaction may produce both formic acid and methanol (18). At ambient temperatures, pure formaldehyde gas polymerizes slowly, yield- ing a white solid; traces of water markedly increase the polymerization rate. Poly- merization of the dry gas can be avoided by maintaining the temperature above 100◦C. Trioxane is chemically stable; pure material shows little decomposition at 224◦C (11). Trioxane readily depolymerizes to monomeric formaldehyde in an acid environment at elevated temperatures. Manufacture of Monomer Formaldehyde is manufactured from methanol. In a typical process, methanol and air are passed across a catalyst bed, consisting of a silver or a metal oxide catalyst (19). The reactor product, consisting of formaldehyde, unconverted methanol, and the by-products, carbon monoxide and dioxide, methyl formate, and formic acid, is cooled and then scrubbed with water in an absorption tower. The aqueous product is then purified by distillation. Anhydrous formaldehyde used in the manufacture of acetal homopolymer must be extremely pure. Even very low concentrations of residual formic acid or hydroxy compounds, such as water and methanol, in the monomer can result in unacceptably low molecular weight in the finished product; great care is taken to eliminate these species in monomer production. In order to obtain the necessary purity, the formaldehyde process is modified to scrub the product gases with cyclohexanol in a series of columns. The formaldehyde is absorbed and forms cy- clohexyl hemiformal with the cyclohexanol (20). Residual formaldehyde and cyclohexanol carried over from the absorption columns are scrubbed out by water before the remaining gases are discharged. The cyclohexanol from the absorption columns containing the hemiformal, along with some water and methanol, is sent to a distillation column in which the water-cyclohexanol azeotrope and methanol are removed overhead at 95◦C. Bottoms from this column are then sent to a py- rolysis column, where they are heated to 160◦C in order to decompose the hemiformal; formaldehyde gas goes overhead. Removal of trace cyclohexanol and any remaining water is accomplished by condensation after chilling the formaldehyde gas to 1◦C. The purified monomer must then be introduced to the polymerization reactor immediately. On a laboratory-scale basis, an effective way of generating anhydrous formaldehyde is to depolymerize trioxane in the vapor phase over a supported acidic catalyst, such as phosphoric acid. Using nitrogen as a carrier gas, trioxane vapors are passed over the catalyst at 200◦C. The formaldehyde product can then be further purified by passing through a series of U-tubes held at −15◦C. 4 ACETAL RESINS Vol. 1 Acetal copolymer is commonly produced from trioxane, which is made from aqueous formaldehyde in the presence of a strong acid catalyst. Dilute formaldehyde solution is concentrated to 60% and introduced into the reactor, which contains 6 wt% sulfuric acid. The product is then distilled; a trioxane- formaldehyde-water azeotrope is taken overhead and the residue returned to the reactor. Extraction of the distillate with solvents, such as benzene or methylene chloride, separates the trioxane from the water and most of the formaldehyde. The dilute formaldehyde solution is sent to recovery; separation of the trioxane from the solvent is accomplished by distillation. If necessary, the trioxane can be further purified by another distillation and treatment with molten sodium (21). Polymerization Mechanism. Formaldehyde is very reactive and readily polymerizes with ionic initiation. Either anionic or cationic materials such as amines, phosphines, onium salts, sulfur compounds, mineral acids, halogens, and Lewis acids, etc (22, 23), will initiate the polymerization reaction, as illustrated below: Ionizing radiation also initiates polymerization (24). However, this method has not been used commercially. In the case of an anionic initiator, the reaction then proceeds as follows: The reaction propagates by addition of the monomer to the double bond. The polymer molecule continues to grow until it encounters an impurity capable of undergoing chain transfer. Reaction of the growing chain with the impurity terminates the chain and releases an ionic species, which then initiates another polymer chain. Because of chain transfer, the amounts of impurities present in the monomer must be rigorously controlled in order to obtain products of the de- sired molecular weight. A detailed review of reaction mechanisms and kinetics has been published (25). In commercial practice, it is believed that anionic ini- tiators are used to produce acetal homopolymers; cationic polymerization is more complex because of the nature of the cation. Trioxane is also readily polymerized, but a cationic initiator is employed. The polymerization of trioxane also differs from that of formaldehyde in that an induction period is observed during which, although a reaction is occurring, no visible polymer is formed. One explanation is that the cationic catalyst associates with an oxygen atom of the trioxane ring and the complex then undergoes ring opening. The cationic oligomer thus formed reversibly depolymerizes releasing Vol. 1 ACETAL RESINS 5 formaldehyde. An equilibrium concentration is built up, which then remains con- stant during the polymerization (26,27): O O + + + R + O R—O ROCH2OCH2 + CH2O O O Propagation occurs by the ring opening of the activated species and the addition of trioxane monomer. Chain transfer with impurities can occur. Recent investigations have shown that in the boron trifluoride dibutyl etherate-initiated trioxane polymerization, a cocatalyst such as water is necessary for reaction (28).

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