Polyacetals acetal resins

Although there is a substantial body of information in the pubHc domain concerning the preparation of polyacetals, the details of processes for manufacturiag acetal resins are kept highly confidential by the companies that practice them. Nevertheless, enough information is available that reasonably accurate overviews can be surmised. Manufacture of both homopolymer and copolymer involves critical monomer purification operations, discussion of which is outside the scope of this article (see Formaldehyde). Homopolymer and copolymer are manufactured by substantially different processes for accomplishing substantially different polymerisation chemistries. 

From the time that formaldehyde was first isolated by Butlerov in 1859 polymeric forms have been encountered by those handling the material. Nevertheless it is only since the late 1950s that polymers have been available with the requisite stability and toughness to make them useful plastics. In this period these materials (referred to by the manufacturers as acetal resins or polyacetals) have achieved rapid acceptance as engineering materials competitive not only with the nylons but also with metals and ceramics. 

Since acetal resins are degraded by ultra violet light, additives may be included to improve the resistance of the polymer. Carbon black is effective but as in the case of polyethylene it must be well dispersed in the polymer. The finer the particle size the better the ultra violet stability of the polymer but the poorer the heat stability. About 1.5% is generally recommended. For white compounds and those with pastel colours titanium dioxide is as good in polyacetals as most transparent ultraviolet absorbers, such as the benzophenone derivatives and other materials discussed in Chapter 7. Such ultraviolet absorbers may be used for compounds that are neither black, white nor pastel shade in colour. 

By organic chemistry formalism, polyacetals are reaction products of aldehydes with polyhydric alcohols. Polymers generated from aldehydes, however, either via cationic or anionic polymerization are generally known as polyacetals because of repeating acetal linkages. Formaldehyde polymers, which are commercially known as acetal resins, are produced by the cationic ring opening polymerization of the cyclic trimer of formaldehyde, viz., trioxane [29-30] (Fig. 1.5). 

Polyoxymethylene polymers, POM, commonly known as polyacetals or Acetal resins are linear thermoplastic polymers containing predominantly the -CH -O- repeat unit in their backbone. There are two types of acetal resins available commercially (1) homopolymers made by the polymerization of formaldehyde, followed by endcapping, (2) copolymers derived from the ring opening polymerization of trioxane (a cyclic trimer of formaldehyde), and a small amount of a comonomer such as ethylene oxide. Acetal resins are... 

Synonyms Acetal resin Polyacetal Polyacetal copolymer Polyacetal resin Polyfonnaldehyde Polyot methylene copolymer POM Classification Thermoplastic polymer... 

The enormous commercial success of the polyacetals is owed to their very high resistance to creep and fatigue. The acetal resins show superior creep resistance to the nylons. 

Polyoxymethylene (POM) n. Linear polymers of formaldehyde or oxymethylene glycol with the formula (-OCH2-) , in which n is above 100. Those in the range of 100 < n < 300 are brittle sohds used as intermediates. Those in the range 500 < n < 5000 are acetal resins. Note—Polyoxymethylene is theoretically the simplest member of the generic class of polyacetals. 

Linear, hard, tough synthetic resins produced by the polymerisation of formaldehyde (for acetal homopolymers) or of formaldehyde with trioxane (for acetal copolymers). Acetal resins are also called as polyacetals and are used as substitutes for metals. 

PolyaCGtal. Polyacetals thermally decompose by an acid-catalyzed depolymerization process starting at the chain ends (see Acetal Resins). The polymer structure is stabilized by end capping and introdncing comonomers to interrupt the unzipping. The process is autocatalytic since the liberated formaldehyde is easily oxidized to formic acid, which is a prodegradant. Formaldehyde scavengers and phenolic antioxidants are typically used in polyacetal formulations (42). 

PMMA. See Methacrylic Ester Polymers. POLYACETAL. See Acetal Resins. POLYACRYLAMIDE. See Acrylamide Polymers. 

Polymers of aldehydes are commonly termed polyacetals (or acetal resins) since their structure resembles that of polymers obtained by the acetal-forming reaction between aldehydes and diols ... 

Since the most important commercial polymers of aldehydes are based on formaldehyde, the term polyacetal (or acetal resin) is commonly used as applying solely to polymers of formaldehyde. 

Polyacetal n A polymer in which the repeated structural unit in the chain is of the acetal type. See also Acetal Resin and Polyoxymethylene. 

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