Polyoxymethylene

Polyoxymethylene is processed at mass temperatures of 180 to 230 °C, at which it may aiready degrade, Fig. 4.58. The degradation of poiyoxymethyiene ieads to formaldehyde cleavage and proceeds in a zipper-like mechanism, s. Fig. 5.197. This effect is most pronounced in the pure homopolymer. To avoid degradation, care must be taken to keep thermal loading as short as possible. 

Residence time untii beginning of decomposition for poiyoxymethyiene as a function of meit temperature (empiricai vaiues) [599] 

The formaldehyde formed during degradation is very oxidation sensitive and is converted to formic acid by oxygen. Formic acid is a relatively strong acid and as such attacks polyoxymethylene by chain scission. Moreover, formaldehyde cleavage is acid catalyzed, so that bases and antioxidants are indispensable in processing of polyacetals. Fig. 4.59 [38]. 

Influence of co-stabilizers (individual ooncentratlon 0.3% each) during thermal loading of polyoxymethylene copolymers during thermogravimetric testing (220 °C, air, isothermic) [38] 

POM is also sensitive to oxygen. Ether peroxides are formed and decompose under chain scission at elevated temperatures. Degradation products accelerate further chain disintegration [599]. 

The polyoxymethylene (4.1) contains only C—C and C—O bonds and is therefore not expected to absorb light of wavelength longer than 190-220 nm. However, this polymer is not resistant to light and photo-oxidative degradation [826, 1026, 1597]. 

The photodegradation occurs with scission of main chain bonds  

A subsequent reaction is the formation of formaldehyde by a depolymerization (unzipping) reaction  

The photolysis of formaldehyde proceeds to radical and nonradical products, depending upon the wavelength of light  

Photodecomposition of hydroperoxide (OOH) groups subsequently gives end-hydroxyl (OH) and end-formyl(aldehyde) (—C ) chains  

Igarashi and co-workers [61] have studied the thermal decomposition of polyoxymethylene (POM) in vacuum, in air, and in a nitrogen atmosphere by TGA and isothermal techniques and by IR absorption spectroscopy. Besides the pure uninhibited material (POM), the acetylated derivative (POMAc) and commercial Delrin (5000X) were used. In the TGA curves for POM, POMAc, and Delrin, two peaks were obtained. POM showed a large initial peak (earlier than other samples) at about 200 °C and a particular sample completely decomposed in the order POM POMAc Delrin. 

Major polymer applications appliances, automotive (doorhandles, window winders, tank filler necks and caps, carburetor, screw caps for cooling system expansion tanks, fuel piunps) phones (diahng units and slider guideways), pneumatic components, parts of textile machines, shower parts, home electronics and hardware, bearings, cams, containers, pump impellers, rollers, springs, clips, and many other applications 

Important processing methods injection molding, blow molding, rotational molding, extrusion, foam molding, compression molding, transfer molding 

Typical fillers glass fiber, glass beads, carbon fiber, aramid fiber, carbon black, metal flakes, zinc whisker, talc, calcium carbonate, PTFE fiber 

Typical concentration range generally - 20-40 wt% PTFE. aramid fiber - 2-10 wt%, glass fiber 20-30 wt%, glass microspheres - 10-30 wt% 

Special considerations carbon black is the best UV stabilizer printability of POM is obtained by addition of talc or calcium carbonate 0.2-0.3% moisture may reduce thermal stability by 20-30°C (y-sterilization degrades POM rapidly) 

In this case, POM and POMAc showed a large weight loss between 150 C and 

From IR absorption spectroscopy studies, Igarashi and co-workers [1] found that the carbonyl content increased as weight loss increased in the decomposition of POM in air, nitrogen, and in a vacuum. The oxygen present in a vacuum should have little effect on carbonyl formation, so it was assumed that this formation is due to the interchange of an ether linkage into a carhonyl group. 

The following mechanism was proposed [1] for the thermal degradation of POM in the absence of oxygen  

From this mechanism. Equations 3.1-3.4, the decomposition of a stable fragment, C, with carbonyl groups at the chain end may occur during the second stage of the 

This mechanism, Equations 3.5-3.7, can readily account for the fact that the quantity of carbonyl groups formed in the decomposition in air is greater than that in nitrogen. 

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Polyoxymethylene is obtained as a finely divided soHd. The bulk density of the product, which is very important for ease of handling in subsequent manufacturing steps, is influenced by many reaction variables, including solvent type, polymerisation temperature, and agitation. 

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). 

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

Paraformaldehyde [30525-89-4] is a mixture of polyoxymethylene glycols, H0(CH20) H, with n from 8 to as much as 100. It is commercially available as a powder (95%) and as flake (91%). The remainder is a mixture of water and methanol. Paraformaldehyde is an unstable polymer that easily regenerates formaldehyde in solution. Under alkaline conditions, the chains depolymerize from the ends, whereas in acid solution the chains are randomly cleaved (17). Paraformaldehyde is often used when the presence of a large amount of water should be avoided as in the preparation of alkylated amino resins for coatings. Formaldehyde may also exist in the form of the cycHc trimer trioxane [110-88-3]. This is a fairly stable compound that does not easily release formaldehyde, hence it is not used as a source of formaldehyde for making amino resins. 

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. 

In the above examples the polymerisation takes place by the opening of a carbon-carbon double bond. It is also possible to open carbonyl carbon-oxygen double bonds and nitrile carbon-nitrogen triple bonds. An example of the former is the polymerisation of formaldehyde to give polyformaldehyde (also known as polyoxymethylene and polyacetal) (Figure 2.3). 

Besides being commercially referred to as polyacetal materials polyformaldehydes are also often known as polyoxymethylenes and are the simplest type of a family of aliphatic polyethers. 

Polymers produced by methods as described above have thermal stabilities many times greater than those obtained by the earlier bulk and solution methods of Staudinger. Staudinger had, however, shown that the diacetates of low molecular weight polyoxymethylenes (I) (polyformaldehydes) were more stable than the simple polyoxymethylene glycols (II) (Figure 19.2). 

Staudinger also found that diacetates of polyoxymethylenes with a degree of polymerisation of about 50 were less stable. Truly high molecular weight polyoxymethylenes (degree of polymerisation -1000) were not esterified by Staudinger this was effected by the Du Pont research team and was found to improve the thermal stability of the polymer substantially. 

Chemical Designations - Synonyms Fonnaldehyde polymer Polyformaldehyde Polyfooxymethylene Polyoxymethylene glycol Chemical Formula HO(CHjO) H. 

Poly(ethylene terephtlhalate) Phenol-formaldehyde Polyimide Polyisobutylene Poly(methyl methacrylate), acrylic Poly-4-methylpentene-1 Polyoxymethylene polyformaldehyde, acetal Polypropylene Polyphenylene ether Polyphenylene oxide Poly(phenylene sulphide) Poly(phenylene sulphone) Polystyrene Polysulfone Polytetrafluoroethylene Polyurethane Poly(vinyl acetate) Poly(vinyl alcohol) Poly(vinyl butyral) Poly(vinyl chloride) Poly(vinylidene chloride) Poly(vinylidene fluoride) Poly(vinyl formal) Polyvinylcarbazole Styrene Acrylonitrile Styrene butadiene rubber Styrene-butadiene-styrene Urea-formaldehyde Unsaturated polyester... 

Polyformaldehydes (polyoxymethylenes, polyacetals) These are physically similar to general purpose nylons but with greater stiffness and lower water absorption. There are no solvents, but swelling occurs in liquids of similar solubility parameter. Poor resistance to u.v. light and limited thermal stability are two disadvantages of these materials. 

Processability Styrene-acrylonitrile, methacrylate-butadiene-styrene, chlorinated polyethylene, PVC-ethyl acrylate, ethylene-vinyl acetate, chlorinated polyoxymethylenes (acetals)... 

Thin polymer films may also be investigated by TEM and high resolution images are obtained for e.g. thin films of liquid crystalline polymers [64]. Usually thin microtome cuts from bulk samples are investigated, but also epitaxial growth of polyoxymethylene on NaCl [152], chain folding of polyethylene crystals [153], epitaxial crystallization of polypropylene on polystyrene [154] or monomolecular polystyrene particles [155] are observed. The resolution is, however, in most cases not comparable to STM. 

Two-shot techniques for acyclic diene metathesis, 435-445 for polyamides, 149-164 for polyimides, 287-300 for polyurethanes, 241-246 for transition metal coupling, 483-490 Anionic deactivation, 360 Anionic polymerization, 149, 174 of lactam, 177-178 Apolar solvents, 90 Aprotic polar solvents, 185, 338 Aprotic solvents, low-temperature condensation in, 302 Aqueous coating formulations, 235 Aqueous polyoxymethylene glycol, depolymerization of, 377 Aqueous systems, 206 Ardel, 20, 22... 

Poly f p-oxybenzoyl-co-p-phenylene isophthalate]), 113-114 Poly(2,2 -oxydiethylene adipate), 29 Polyoxymethylene glycol, aqueous, 377 Poly(oxytetramethylene) (PTMO), 53 Poly (p-pheny lene). See also Poly(para-phenylene)s dendronized, 520-521 synthesis of, 491-494 synthesis of water-soluble, 493 Poly(phenylene ether sulfone) chains,... 

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