Polymers, formaldehyde Melting point

Polyoxymethylene (polyacetal) is the polymer of formaldehyde and is obtained by polymerization of aqueous formaldehyde or ring-opening polymerization of trioxane (cyclic trimer of formaldehyde, melting point 60-60°C), the latter being the preferred method [52]. This polymerization of trioxane is conducted in bulk with cationic initiators. In contrast, highly purified formaldehyde is polymerized in solution using using either cationic or anionic initiators. 

The thermoplastic or thermoset nature of the resin in the colorant—resin matrix is also important. For thermoplastics, the polymerisation reaction is completed, the materials are processed at or close to their melting points, and scrap may be reground and remolded, eg, polyethylene, propjiene, poly(vinyl chloride), acetal resins (qv), acryhcs, ABS, nylons, ceUulosics, and polystyrene (see Olefin polymers Vinyl polymers Acrylic ester polymers Polyamides Cellulose ESTERS Styrene polymers). In the case of thermoset resins, the chemical reaction is only partially complete when the colorants are added and is concluded when the resin is molded. The result is a nonmeltable cross-linked resin that caimot be reworked, eg, epoxy resins (qv), urea—formaldehyde, melamine—formaldehyde, phenoHcs, and thermoset polyesters (qv) (see Amino resins and plastics Phenolic resins). 

If the polymer is hard, insoluble, and infusible without decomposition, and if it refuses to swell greatly in any solvent, it may be assumed either that it is highly crystalline, with a melting point above its decomposition temperature, or that it possesses a closely interconnected network structure (e.g., as in a highly reacted glyceryl phthalate or a phenol-formaldehyde polymer). Differentiation between these possibilities is feasible on the basis of X-ray diffraction. 

Melamine-formaldehyde polymers, 1017 Melting theory, of, 21 under the solvent, 35 Melting point, 21... 

Acetal resins are those homopolymers (melting point ca. 175°C, density ca. 1.41) and copolymers (melting point ca. 165°C, density ca. 1.42) where the backbone or main structural chain is completely or essentially composed of repeating oxymethylene units (-CH20-)n. The polymers are derived chiefly from formaldehyde (methanal, CH2=0), either directly or through its cyclic trimer, trioxane or 1,3,5-trioxacyclohexane. 

There is an alternative method of making functional derivatives for polymerization. Durene can be condensed with formaldehyde in the presence of hydrochloric acid and zinc chloride to give the bis-chloromethyl durene(75). This can be converted into a number of derivatives from which polymers can be made, e.g. durene-1,4-dicarboxylic acid for polyamides(76), the diacetic acid for polyesters(77,78) or for polyamides(79), the diisocyanatomethyl derivative for polyureas and polyurethanes(80,81), the dimethanol derivative for polyurethanes(82) and for epoxies(83), while the bis chloromethyl derivative has also been proposed for making polyethers (with bisphenol A)(84). In each case, the attraction from the durene derivative has been the introduction of higher melting points and inproved softening properties. None of these polymers have been commercialized, possibly in part because of the difficulty of obtaining durene at suitable-prices. 

However, a vital shortcoming of this poisoner for reprocessing into objects is its extreme instability at temperatures below the melting point ( 100°C) the polymer decomposes readily, liberating monomeric formaldehyde. Hence the reprocessing of polyformaldehyde can be accomplished only after preliminary stabilization of the product. 

To a Carius tube is added 38.6 gm (0.3 mole) / -chlorophenol, 9.0gm (0.3 mole of formaldehyde) of 5-trioxane, 0.6 gm of toluenesulfonic acid, and 45 ml of Wj(2-ethyoxyethyl) ether. The shielded tube is sealed and heated at 150°C for 24 hr. The tube is cooled to room temperature, wrapped in a towel, and cautiously opened. The polymer is dissolved in 150 ml acetone, filtered, and then added with rapid stirring to 1600 ml of water containing 15 ml of concentrated hydrochloric acid. The light tan polymer is washed thoroughly with distilled water and dried. The polymer is isolated in 95-100% yield (soluble in acetone) 0.059, softening point 90 -100 C, chars at 120 C, and decomposes at 200°C or more on a melting point block. If placed immediately on a preheated hot bar, the polymer melts at about 145°-155°C. 

By 1924 Bakelite had become so popular that it was featured on the cover of Time magazine as a substance that will not burn and will not melt. Bakelite jewelry, telephones, pens, radios, car parts, airplane propellers, ashtrays, billiard balls, and cameras were everywhere. Just about the only item that did not become popular was the Bakelite coffin. People would wear, cook in, and eat off plastic, but they refused to be buried in it. Bakelite s success stimulated research to improve the material even further, especially after scientists pointed out that in the manufacture of Bakelite the small phenol and formaldehyde molecules had joined together to make a giant three-dimensional lattice. In other words. Bakelite was the world s first synthetic giant molecule, the first synthetic polymer. 

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