Polyacetal performance

The polyacetals have a good record of performance in uses involving hot air and hot water. Plumbing components have been used for hot water service in the range 60-80°C and applications in hot air well in excess of 90°C. Recommended maximum continuous use temperatures are given in Table 19.4... 

Polyacetals. Simionescu and coworkers (28) have extended to polyoxy-methylene the process of grafting vinyl polymers (acrylonitrile and methylmethacrylate). They performed the synthesis using a virbomill at room temperature under vacuum (10.1 Torr). The initial monomer-polymer ratio was 1 -5/1.0 and the degree of vibromill packing 0.44. Before milling the polyoxymethylene granules were dissolved in dimethyl formamide and repredpitated with the aim of stabilizer removal and for reduction of polymer particle size (from 2-2.5 mm to 0.05-0.10 mm). Full details of the reaction have been described (40). 

Recently, this salt (being a model of one of the possibie isomeric forms of the active centers in polyacetals) was used to initiate the polymerization of 1,3-dioxolane and 1,3-dioxepane. According to the H- and C-NMR spectra, the following species coexist in equilibrium at 70 C when 1,3-dioxolane is used (measurements were performed below monomer equilibrium concentration when polymerization was not possible ) ... 

Fortunately, the deficiencies of both the classic thermosets and general purpose thermoplastics have been overcome by the commercialization of a series of engineering plastics including polyacetals, polyamides, polycarbonate, polyphenylene oxide, polyaryl esters, polyaryl sulfones, polyphenylene sulfide, polyether ether ketones and polylmides. Many improvements in performance and processing of these new polymers may be anticipated through copolymerization, blending and the use of reinforcements. 

The photostabilisation of polymers still maintains a high level of activity with the mechanistic action and performance of hindered piperidine compounds continuing to attract interest. Some interesting reviews have appeared with one very extensive and highly recommended paper on the mode of action of hindered piperidine light stabilisers while another has dealt with their manufacturing properties. The kinetic reactions and photophysics of light stabilisers have been covered in another review as has the stabilisation of polyacetals. ... 

Steam sterilization (autoclaving) is performed under high steam pressure at relatively low temperature (125-130°C). However, if the polymer is subjected to attack by water vapor, this method cannot be employed. PVC, polyacetals, PE (low-density variety), and polyamides belong to this category. 

Acetal, (Polyacetal) Poly-oxymethylene (POM) Acetal is a polymer obtained through an addition reaction of formaldehyde — (CH2—0) . It excels in mechanical performance and is regarded as a prominent engineering polymer. It appeared in 1959 with the commercial name Delrin . A short time later a useful copolymer was also developed with a cyclic ether like ethylene oxide. The monomer formaldehyde is a gas produced mostly by oxidizing methanol, and it is very useful in thermoset polymers like phenol, urea and melamine-formaldehydes. For high purity it is initially converted to trioxane or paraformaldehyde. The polymerization is carried out by ionic mechanism, wherein the monomer is dispersed in an inert liquid (heptane). The molecular weights reach 20,000 to 110,000. 

Use of polyacetal gears is increasing because of their high performance, dimensional stability, high fatigue, chemical resistance at temperatures up to 90 °C, and excellent lubricity against metals and plastic materials in automotives and precision gears [7]. 

Polyacetals, also referred to as polyoxymethylenes (POMs) or polyfonnalde-hydes, are a semicrystalline engineering thermoplastic polymerized as a homopolymer and copolymer. The homopolymer and copolymer have somewhat different molecular structures and performance values. The difference between performance values is narrowing with new formulations (compounds). Polyacetal engineering thermoplastics were introduced to the world in 1956 with the potential of replacing metals, aluminum, brass, and cast zinc, which polyacetals continue to do. 

All crystalline thermoplastics nylon, polyester, polyacetal, as well as polypropylene, display a similarly large loss in modulus as temperature is increased. As a consequence they are not able to perform under stress at even slightly elevated temperatures. The answer to this problem is found in IPN blends of the crystalline resin with amorphous... 

In principle, engineering plastics and high performance polymers are affected by these trends in the same way, as the margin development for polyesters and polyacetal shows (see Figure 1.10.)... 

The degree of wear/friction improvement depends on the particular plastic to which the PTFE is being added [41], As examples, the above referenced study showed that the addition of 15% low MW PTFE to a 6/6 polyamide, a polyacetal resin, and a polycarbonate reduced the specific wear rates by 97%, 46%, and 97%, respectively. The dynamic friction coefficients were reduced by 76%, 48% and 97%, respectively. Although these are reasonable comparisons, wear and friction performance can depend on the exact contact conditions such as velocity, pressure, and countersurface composition. Commercial compounds of thermoplastics with PIFE will vary depending on the application requirements but an addition level of 15-20% is usually considered optimum for the best wear performance. 

Such distannoxanes have been found to be highly active and selective catalysts for (trans)esterification reactions of methyl esters or acids with a twofold excess of alcohol and 0.5 mol% of distannoxane, either solventless or in toluene.A similar 100% yield in esters could be observed with a stoichiometric amount of alcohol in fluorous solvent that permitted recycling of the catalyst without loss of its performances. " The proposed mechanism consists in the activation of the alcohol via Sn-X (Y) ligands exchange to Sn-OR ones (Scheme 21.13b), while the ester is coordinated through the ketonic o>ygen atom to an adjacent tin centre (Scheme 21.13c). A similar dinuclear activation was reported for the melt transesterification of a variety of polyesters and polyacetates in the presence of such distannoxane catalysts. 

Table 1.17 Effect of Molding Conditions on Structure and Mechanical Performance of Polyacetal...

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