Polyoxymethylene , modified

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. 

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),... 

Polyoxymethylenes have a marked tendency to undergo thermal depolymerization with loss of formaldehyde. To prevent thermal depolymerization, polyoxymethylenes are structurally modified, the two possibilities being acetylation to block the reactivity of the end groups of co-polymerization with cyclic ethers, e.g., ethylene oxide. Polyacetals are also sensitive towards autoxidation, which invariably leads to depolymerization as a result of chain scission. The formaldehyde released by depolymerization is very likely to be oxidized to formic acid, which can catalyze further depolymerization. 

Table 19.31 Properties of some ctnnmercial polyoxymethylene/impact modifier blends ...

MAC Glass fiber-reinforced polyoxymethylene, POM Modified Plastics... 

A thermal treatment has been used by Sasakawa et al. to modify the surfaces of polyoxymethylene (POM) and acrylonitrile-butadiene-styrene (ABS) resins [162]. The results showed that the heat treatment eliminates oxygen from POM to form hydrocarbons at the surface while it oxidizes the surface of PBD and PS. 

COMPARISON WITH COMPETITIVE MATERIALS Notched impact resistance better then impact modified polyamide, PTFE, and PTFE modified polyoxymethylene, abrasion resistance better than all above mentioned materials. 

Most polymers used today are thermoplastics. Poiypropylene (PP), polyethylene (PE), polyethylene terephthalate (PET), and polystyrene (PS) often find application as low-end consumer items, packaging or others. Technical parts are produced mostly from acrylonitrile-butadiene-styrene-copolymer (ABS), polyamide (PA), polybutylene terephthalate (PBT), polyoxymethylene (POM), polyether sulfone (PES), polycarbonate (PC), polyphenylene sulfide (PPS), polytetrafluoroethylene (PTFE), polyether ether ketone (PEEK), or polyimide (PI). Polyvinyl chloride (PVC) is a material often used in building construction, especially for roofing membranes, window frames, and pipes, and its properties (rigid or flexible) are generally modified by additives. 

Impact-modified polyoxymethylene exhibits less hydrolysis resistance than basic grades. 

These have often been modified by addition of PE. Blends with polyoxymethylene showed complex anomalous behavior due to interfacial phenomena [34]. PE appeared useful as a melt flow promoter in polyphenylene ether [19]. Dispersion of PE in polycarbonate improves melt flow and energy absorption for automotive applications [19], so PC producers offer such grades commercially [35] fine stable polyethylene domains may be produced by adding PE-PS or SEBS block copolymers [36,37]. 

Figure 16.28 Thermal energy required to heat thermoformable amorphous and crystalline polymers to their respective forming temperatures. HOPE, High density polyethylene at 960 kg/m MDPE, Medium-density polyethylene at 945 kg/m LDPE, Low-density polyethylene at 920 kg/m POM,- Polyoxymethylene PA-6, Polycaprolactam or polyamide PP, Homopolymer polypropylene, PS, General purpose polystyrene MIPS, Medium-impact or rubber-modified polystyrene ABS, Polyacrylonitrile-polybutadiene-polystyrene terpolymer PMMA, Polymethylmethacrylate FPVC, Flexible polyvinyl chloride RPVC, Rigid polyvinyl chloride.

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

POM elastomer Impact modified grades of polyoxymethylene offered by duPont and Celanese elastomer not disclosed 108... 

Polyoxymethylene (Acetal) Impact Modified Homt lymer POMI... 

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