Acetals-polyoxymethylene resins

Cyclic ether and acetal polymerizations are also important commercially. Polymerization of tetrahydrofuran is used to produce polyether diol, and polyoxymethylene, an excellent engineering plastic, is obtained by the ring-opening polymerization of trioxane with a small amount of cycHc ether or acetal comonomer to prevent depolymerization (see Acetal resins Polyethers, tetrahydrofuran). 

Acetals. Acetal resins (qv) are polymers of formaldehyde and are usually called polyoxymethylene [9002-81-7]. Acetal homopolymer was developed at Du Pont (8). The commercial development of acetal resins required a pure monomer. The monomer is rigorously purified to remove water, formic acid, metals, and methanol, which act as chain-transfer or reaction-terminating agents. The purified formaldehyde is polymerized to form the acetal homopolymer the polymer end groups are stabilized by reaction with acetic anhydride to form acetate end groups (9). 

Acetal Resins. Acetal resins (qv) are poly (methylene oxide) or polyformaldehyde homopolymers and formaldehyde [50-00-0] copolymeri2ed with ahphatic oxides such as ethylene oxide (42). The homopolymer resin polyoxymethylene [9002-81-7] (POM) is produced by the anionic catalytic polymeri2ation of formaldehyde. For thermal stabiUty, the resin is endcapped with an acyl or alkyl function. 

Polyoxymethylene, also referred to as acetal resin or POM, is obtained either by anionic polymerization of formaldehyde or cationic ring-opening copolymerization of trioxane with a small amount of a cyclic ether or acetal (e.g., ethylene oxide or 1,3-dioxolane) [Cherdron et al., 1988 Dolce and Grates, 1985 Yamasaki et al., 2001]. The properties and uses of POM have been discussed in Sec. 5-6d. 

Polyoxymethylene Ionomers. Ionic copolymers have been prepared from trioxane and epichlorohydrin, followed by reaction with disodium thioglycolate (76). The ionic forces in these materials disrupt crystalline order and increase melt viscosity (see Acetal resins). 

The decorative laminates described in the previous chapter are made with selected thermosetting resins while resins of this type can be moulded and extruded by methods similar to those outlined in the present and the next chapter the materials employed for these processes predominantly are thermoplastic. Many such plastics can be moulded and extruded under suitable conditions, the most important in terms of quantities used being those that combine properties satisfactory for the purpose with convenience in pro-cessing-especially the polyolefins (polyethylene and polypropylene), poly(vinyl chloride), and styrene polymers and blends. Other plastics with special qualities, such as better resistance to chemical attack, heat, impact, and wear, also are used—including acetals (polyformaldehyde or polyoxymethylene), polyamides, polycarbonates, thermoplastic polyesters like poly(ethylene terephtha-late) and poly(butylene terephthalate), and modified poly(phenylene oxide),... 

Acetal resin is a general term used for high molecular weight polymers and copolymers derived from formaldehyde. The name of polyoxymethylene conforms better with the structure of the repeat unit (—OCH2—) . In... 

The polyoxymethylenes are presently widely used in different areas. Approximatively one-third of the market is represented by homopolymers and two-thirds by copolymers. Homopolymers are produced by anionic polymerization of formaldehyde using amines, alkoxides, and other types of anionic initiators. The details of these polymerizations will not be discussed in this paper, although some of their properties will be compared to those of copolymers which are obtained by cationic copolymerization of trioxane with cyclic ethers or cyclic esters. Comprehensive reviews on general aspects of synthesis and properties of acetal resins are available [158-162],... 

The many commercially attractive properties of acetal resins are due in large part to the inherent high crystallinity of the base polymers. Values reported for percentage crystallinity (x ray, density) range from 60 to 77%. The lower values are typical of copolymer. Polyoxymethylene most commonly crystallizes in a hexagonal unit cell (9) with the polymer chains in a 9/5 helix (10,11). An orthorhombic unit cell has also been reported (9). The oxyethylene units in copolymers of tiioxane and ethylene oxide can be incorporated in the crystal lattice (12). The nominal value of the melting point of homopolymer is 17S°C, that of the copolymer is 165°C. Other thermal properties, which depend substantially on the crystallization or melting of the polymer, are listed in Table 1. See also reference 13. 

The amount of heat required for melting varies considerably between resins, with the semicrystalline resins requiring more heat to melt their crystalline phase. For example, acetal or polyoxymethylene, POM, has a fairly high heat of fusion (AH = 160-200 kJ/kg). Examples of total heat energy required for forming are POM = 435-475, high density polyethylene, HOPE, = 720, PA-66 = 756-786 kJ/kg. 

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 Polyformaldehyde Polyoxymethylene copolymer POM... 

Acetal polymers are formed from the polymerization of formaldehyde. They are also given the name polyoxymethylenes (POMs). Polymers prepared from formaldehyde were studied by Staudinger in the 1920s, but thermally stable materials were not introduced until the 1950s, when DuPont developed Dehin. Hompolymers are prepared from very pure formaldehyde by anionic polymerization as shown in Fig. 2.1. Amines and the soluble salts of alkali metals catalyze the reaction. The polymer formed is insoluble and is removed as the reaction proceeds. Thermal degradation of the acetal resin occurs by unzipping with the release of formaldehyde. The thermal stability of the polymer is increased by esterification of the hydroxyl ends with acetic anhydride. An alternative method to improve the thermal stabihty is copolymerization with a second monomer, such as ethylene oxide. The copolymer is prepared by cationic methods developed by Celanese and mar-... 

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. 

Figure 1 Cost-related (specific) flexural strength of major thermoplastics, versus cost-related (specific) thermal tolerance. The unit cost is the market price in US cents (1992) of 1 cm plastics. The thermal tolerance is the temperature difference (AT) over room temperature (AT — T - room T), by which temperature (7 ) the flexural modulus is equal to 1 GPa. Designations, abbreviations WFRP-S, wood fiber reinforced PP (S type) of AECL, Canada (See Table 1) PMMA, polymethylmethacrylate PVC, pol)winyl chloride PS, polystyrene PP, polypropylene UP, unsaturated polyesters PA-GF, glass fiber (35%) reinforced polyamide PHR, phenolic resin EP, epoxy resin ABS, acrylonitrile/butadiene/styrene copolymer UF, urea/formaldehyde LDPE, low density polyethylene PC, polycarbonate POM, polyoxymethylene CAB, cellulose acetate butyrate LCP, liquid crystal polymers PEEK, polyether-etherketone PTFE, polytetrafluorethylene.

An example of the importance of end-group control is in acetal resin or polyoxymethylene (POM). As normally prepared from cationic polymerization of sym-trioxane, this polymer has —OH end groups, which readily initiate depropagation to formaldehyde (2) by a molecular mechanism (9) ... 

The blends described in the EDCPB provide a cross section of commercial alloys available in Asia, Europe, and North America. The focus is on blends with the five principal engineering resins polyamides, thermoplastic polyesters, polycarbonates, polyoxymethylenes (acetals), and polyphenylene ethers. There are but few examples of the commodity (and these mainly with polypropylene) as well as with high performance specialty resin blends. This may leave a wrong impression of the global blend industry. 

These materials form yet another class of polymer which may be prepared by more than one polymerization method. For example polyoxymethylene (polyformaldehyde, polymethanal, acetal resin) may be prepared by double-bond polymerization from formaldehyde or by ring-opening of trioxane ... 

Acetal resin is either a homopolymer or copolymer of formaldehyde. The homopolymer is sometimes called polyoxymethylene or POM. It is made by polymerizing either anhydrous formaldehyde or its cyclic trimer triox-ane. The polymerization can he done using either acidic or basic catalysts. At least some of the end groups are hydroxyl groups and with heat, the resin... 

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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