Polyoxymethylene properties

Staudinger relentlessly championed the molecular, or primary valence, viewpoint in the years which followed. He supported his original contentions with the observation that hydrogenation of rubber, as well as its conversion to other derivatives, does not destroy its colloidal properties. In contrast to association colloids, high polymers (or macromolecules as he chose to call them ) exhibit colloidal properties in all solvents in which they dissolve. Polyoxymethylenes were ex-... 

ISO 9988-1 2004 Plastics - Polyoxymethylene (POM) moulding and extrusion materials -Part 1 Designation system and basis for specifications ISO 9988-2 1999 Plastics - Polyoxymethylene (POM) moulding and extrusion materials -Part 2 Preparation of test specimens and determination of properties... 

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. 

These compounds are polyoxymethylene blycols. Walker (Ref 1, p 65 Ref 3, p 142) gives a table describing the properties of various formaldehyde polymers... 

Long-chain methanal polymers have become very important as plastics in recent years. The low cost of paraformaldehyde is highly favorable in this connection, but the instability of the material to elevated temperatures and dilute acids precludes its use in plastics. However, the end-capping of polyoxymethylene chains through formation of esters or acetals produces a remarkable increase in stability, and such modified polymers have excellent properties as plastics. Delrin (DuPont) is a stabilized methanal polymer with exceptional strength and ease of molding. 

Polyoxymethylene (POM) is, again, a crystalline polymer, with a melting point of about 180 °C. Its mechanical properties enable it to gradually replace metals in a number of applications. Many technical parts are being made from POM, such as gear wheels, bars, automotive accessories, parts of several apparatuses and machines. The polymer is used as such (e.g. Delrin ), but also as a copolymer with a small amount of ethylene oxide (e.g. Celcon and Hostaform ). 

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),... 

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],... 

Polyoxymethylenes have a low water absorption (0.6% at 20° C). Copolymers are very stable to long exposure in hot water and their properties remain almost unchanged after 1 year of immersion at 82° C. In order to be protected against UV radiation, suitable additives such as benzophe-none or benzotriazole may be included. 

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. 

In this review article, an account will be presented of the diffia t methods by which high modulus materials have been produced from flexible polymers. Much of the discussion will be concerned with polyethylene, although comparable results have been obtained fot polypropylene and polyoxymethylene, and these will also be considered. The initial stimulus to this research came from the quest for high stiffness, but other properties have also been enhanced, including strength, thermal and chemical stability, and barrier properties. The present article updates and extends previous reviews of progress in this exciting new area of polymer science. 

Weld lines (also known as knit lines) are a potential source of weakness in molded and extruded plastic products. These occur when separate polymer melt flows meet and weld more or less into each other. Knit lines arise from flows around barriers, as in double or multigating and use of inserts in injection molding. The primary source of weld lines in extrusion is flow around spiders (multiarmed devices that hold the extrusion die). The melt temperature and melt elasticity (which is mentioned in the next section of this chapter) have major influences on the mechanical properties of weld lines. The tensile and impact strength of plastics that fail without appreciable yielding may be reduced considerably by in doublegated moldings, compared to that of samples without weld lines. Polystryrene and SAN copolymers are typical of such materials. The effects of weld lines is relatively minor with ductile amorphous plastics like ABS and polycarbonate and with semicrystalline polymers such as polyoxymethylene. Tliis is because these materials can reduce stress concentrations by yielding [22]. 

These crystal modifications differ in their molecular and crystal structures as well as in their physical properties. Many types of crystalline modifications are reported, including a stable orthorhombic phase and metastable monoclinic phase for PE a, and y forms for isotactic polypropylene (/-PP) trigonal and orthorhombic phases for polyoxymethylene a and y forms for Nylon 6 and others. Poly(vinylidene fluoride) (PVF), for example, appears in at least four types of crystalline modification (Lovinger, 1985 Dunn Carr, 1989). 

The predicted properties of polyoxytrimethylene are identical to the results calculated by adding up the predicted extensive properties of polyethylene and polyoxymethylene, and using the resulting extensive properties to calculate the intensive properties. 

A study of the thermal stability of polyoxymethylene was made by Schweitzer et al. [225]. The rate of weight loss was measured at 222°C. Formaldehyde is evolved by a first-order reaction. The decomposition was assumed to take place mainly by unzipping from the chain end, since the rate of weight loss increases with molecular weight. This was confirmed later by Kern and Cherdron [226] who also showed that acetylation of end groups leads to improved thermal properties. 

For HPA, removed from the sorbents hematite, silica and polyoxymethylene, the molecular structure has been compared with that of the native molecule, on the basis of their circular dichroism spectra (11). It was found that after desorption the helix content of HPA is some twenty to thirty percent lower. This reduction is virtually independent of the type of sorbent and the desorption method. It suggests that the change in the helix content is related to properties of the protein molecule itself. It is still not clear to which extent the adsorption and the desorption step affect the protein structure. It Is furthermore interesting that the helix reduction is larger for the samples with lower F -values. This supports the earlier conclusion that a reduced Fp value reflects further structural rearrangements in the protein molecule. It is noted that the decrease In the helix content of desorbed HPA found by us is considerably less than that reported by others (19). 

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. 

Comparison of five properties of un-reinforced grades of polyamide 66, polyamide 612 and polyoxymethylene, on a polar plot (redrawn from Du Pont data from CAMPUS). 

All TP or TS matrix property can be improved or changed to meet varying requirements by using reinforcements. Typical thermoplastics used include TP polyesters, polyethylenes (PEs), nylons (polyamides/ PAs), polycarbonates (PCs), TP polyurethanes (PURs), acrylics (PMMAs), acetals (polyoxymethylenes/POMs), polypropylenes (PPs), acrylonitrile butadienes (ABSs), and fluorinated ethylene propylenes (FEPs). The thermoset plastics include TS polyesters (unsaturated polyesters), epoxies (EPs), TS polyurethanes (PURs), diallyl phthalates (DAPs), phenolics (phenol formaldehydes/PFs), silicones (Sis), and melamine formaldehydes (MFs). RTSs predominate for the high performance applications with RTFs fabricating more products. The RTPs continue to expand in the electronic, automotive, aircraft, underground pipe, appliance, camera, and many other products. 

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