Polyoxymethylene mechanical properties

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 ). 

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]. 

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. 

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. 

The SSE process based on changes in polymer-billet shape is, first of all, characterized by the EDR value. The limited EDR value is determined by polymer type, molecular weight and morphology. For example, for HDPE it may reach more than 40 (1). For polyoxymethylene (POM), polypropylene (PP), PTFE the maximum values of draw ratio equal 10,6, and 4 (20-22), respectively. For polymethylmethacrylate (PMMA), polystyrene (PS), polycarbonate (PC), and other amorphous polymers they are even lower (21,23). The extruded specimens are highly oriented and possess improved mechanical properties. In the case of semicrystalline polymers, the extrusion results in a considerable increase of tensile modulus and strength (1,2). With amorphous polymers, a considerable increase of plasticity is observed alongside with increase of tensile moduli (21,23). 

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. 

The integration of comonomers results in polyoxymethylenes with good thermal stability and only slightly reduced mechanical properties. 

Keywords polyoxymethylene, polyurethane, montmorillonite, nanocomposite, mechanical properties, thermal degradation. 

Title Hybrid effects on mechanical properties of polyoxymethylene Descriptors "Polymers Fracture toughness Strain rate Tensile strength Bending strength Impact resistance Elastic moduli Identifiers Polyoxymethylene Hybrid strength Impact strength ... 

Engineering thermoplastic resins (ETP) are those whose set of properties (mechanical, thermal, chemical) allows them to be used in engineering applications. They are more expensive than commodity thermoplastics and generally include polyamides (PA), polycarbonate (PC), linear polyesters such as polyethylene terephthalate (PET) or polybutylene terephthalate (PBT), polyphenylene ether (PPE) and polyoxymethylene (POM). Specialty resins show more specialized performance, often in terms of a continuous service temperature of 200°C or more and are significantly more expensive than engineering resins. This family include fluoropolymers, liquid crystal polymers (LCP), polyphenylene sulfide (PPS), aromatic polyamides (PARA), polysulfones (P ), polyimides and polyetherimides. 

Carbon fibers cause a deterioration of volume resistivity, and arc resistance in polyamide 6,6. Mineral fillers also decrease the dielectric constant and surface arc resistance in polyamide 6,6. This is confirmed by a comparison of electrical, mechanical, and thermal properties of polyoxymethylene, where the incorporation of 30% carbon fiber reduces volume resistivity and dielectric strength, in addition to a distinct improvement in tensile strength at heat distortion temperature (Table 5.10). 

Effect of Carbon Fiber Addition on Electrical, Mechanical, and Thermal Properties of Polyoxymethylene... 

The Mechanical and Thermal Properties of Polyoxymethylene (POM)/Organically Modified Montmorillonite (OMMT) Engineering Nanocomposites Modified with Thermoplastic Polyurethane (TPU) Compatibilizer... 

Polyoxymethylene or polyacetal is a highly crystalline polymer. POM is most noted for its high stiffness, mechanical strength, abrasion resistance and good resistance to chemicals and solvents. It also displays good low temperature impact resistance, high dimensional stability, favourable frictional properties and low water absorption. 

Acetal homopolymer is a highly crystalline thermoplastic manufactured by polymerization of formaldehyde and capping the two ends of the polymer chain with acetate groups (Table 6.2). It is called polyoxymethylene (POM) and has a backbone consisted of repeating —CH2O—units. Acetal copolymers are produced by copolymerization of trioxane and small amounts of a comonomer. The comonomer randomly distributes carbon—carbon bonds in the polymer chain, which stabilizes the resin against environmental degradation. The low cost of acetals compared to other polymers with similar performance and their mechanical, chemical, and electrical properties, allows them to replace metal and other structural materials in many applications. 

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