Polyacetal, mechanical properties

It must, however, be stressed that for design purposes such data have little value. Like the nylons, which are also widely used for load-bearing light engineering applications, the polyacetals exhibit a small but finite creep under load. It is thus necessary to consider mechanical properties under those main headings. 

Some typical properties of polyester-glass laminates are given in Table 25.1. From these figures it will be seen that laminates can have very high tensile strengths. On the other hand some laminates made by hand lay-up processes may have mechanical properties not very different from those of thermoplastics such as the polyacetals and unplasticised PVC. 

Polyacetals are sensitive to high-energy radiation and sterilization by this process can only be used if high mechanical properties are not required impact resistance in particular is altered. Exposure to doses exceeding 30kJ/kg causes yellowing and a decrease of the impact resistance. 

Rigid sub-assembly components, e.g. filter canister bodies, and mountings for valves, eyepieces and speech modules, were originally made of metal but tend more now to be made of polymers. In the SI0, polyacetal is used as it is not only chemically hard but also has mechanical properties that allow snap-fitting, which reduces production costs and allows easy replacement of parts. 

Polyacetals are produced by reacting formaldehyde. These are also sometimes called polyoxymethylene (POM) and known widely as Delrin (DuPont). These polymers have a reasonably high molecular weight (>2 xlO g/mol) and have excellent mechanical properties. More importantly, they display an excellent resistance to most chemicals and to water over wide temperature ranges. 

M. Thompson, M. Northmore-Ball, K. Tanner, Tensile mechanical properties of polyacetal after one and six months immersion in Ringer s solution, 1. Mater. Sci. Mater. Med. 12 (2001) 883-887. 

Compatibilized blends of hydroxylated polyoxymethylene (polyacetal) with carboxylic acid-functionalized PP have been prepared (Chen et al. 1991). The formadtHi of ester linkages between the polymers was proposed. For example, blends comprising hydroxylated polyoxymethylene and muconic acid-grafted PP were made into film by calendering at 200 °C to provide compositions with markedly improved mechanical properties compared to similar blends containing unfunctionahzed PP or unfunctionalized polyoxymethylene. 

Bowman [192] conducted a systematic study of the structure-property relations of injection molded polyacetals (polyoxymethylene or POM) and observed correlations between process conditions, structures and mechanical properties. Barrel temperature effects were studied as they are known to influence both microstructure and mechanical properties [193]. Increased barrel temperature was shown to reduce the outer skin layer while increasing the extent of the equiaxed, unoriented core, resulting in a decreased tensile yield strength parallel to the injection direction. 

Bowman [157] conducted a systematic study of the structure-property relations of injection molded polyacetals (polyoxymethylene or POM) and observed correlations between process conditions, structures and mechanical properties. 

Thompson MS, Northmore-BaU MD, Tanner KE (2001) Tensile mechanical properties of polyacetal after one and six month s immersion in ringer s solution. J Mater Sci Mater Med 12 883-887... 

Other exceptions include polymers with restricted symmetry which can, however, erystallize in a helical macroconformation because of electrostatic interactions between molecular groups of the main chain ( intrachain interactions). However, for sueh helicity to occur, the chain should exhibit a great mobility, which is the ease, for example, in the family of polyethers in the latter case, the dipole attraetion due to —groups is responsible for the stability of the crystalline state poly(ethylene oxide) (-CH2-CH2-0-) crystallizes in a I2 helix (c = 1.94 nm), whereas poly(oxymethylene) (-CH2-0-) (also called polyformaldehyde or polyacetal ) does in a 9s helix (c = 1.72 nm). In the latter case, multiple dipole interactions contribute to stiffen the chains and enhance mechanical properties of the corresponding materials. 

One of the attractions of the fracture mechanics approach is that the results are geometry independent. This, of course, is subject to conducting tests with valid specimens that meet the dimensions criteria set out in the standards. Considerable work has been conducted with various materials to verify how appropriate these specimen size criteria are for specific polymers and to a.scertain how sensitive the test results are to changes in specimen dimensions and to deviations from the criteria. The specimen thickness is the geometric variable that is considered most, since it governs whether the fracture is plane-strain or not. The effect of specimen thickne.ss B on fracture parameters, e g., K, G, J. and CTOD. has been examined for uPVC [23.61], polycarbonate [23,63], HIPS [64], HOPE [37,59,60,], ABS [65], and ABS PC blends [74], The effect of a/IF on fracture toughness properties is also considered for some of these materials, e g, uPVC [25], HOPE [59.60], polyacetal [68], and polyether sulphone (PES) [70]. 

These polymers have excellent mechanical, thermal, and chemical properties due to their stiffened main backbone chains. Polyacetals and polysulfones are being tested as implant materials, while polycarbonates have found their applications in the heart/lung assist devices, food packaging, etc. 

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