Polyformaldehyde properties

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),... 

Since the painting aspect is not the prime reason for the selection it follows that the type of paint and the method of application must be suited to the plastic further, the properties that make a material useful in engineering often also make it harder to paint satisfactorily. As an indication, generally it is easier to paint ABS and polycarbonate than nylon, and easier to paint nylon than polyethylene or polypropylene. Other plastics that can give problems in painting include acetal (polyformaldehyde) and poly(phenylene oxide). 

Delrln (du Pont polyformaldehyde) has excellent chemical, mechanical, and electrical properties. It is easily machineable, tough, and resilient even at low temperatures. Although there are no good adhesives for joining Delrin pieces, it is weldable and easily molded. Chemically stable enough for continuous use at temperatures up to 85°C, it is also inert to most organic solvents. 

The most stable polyacetal polymer is polyformaldehyde (or polyoxymeth-ylene, POM) this is the only polyacetal that has reached commercial production. This resin has unique properties (e.g., selflubrication) and is very widely used in automotive applications such as engineering plastics. Acetals are widely used engineering thermoplastics with high load-bearing characteristics and low coefficients of friction. Currently, over 200 million lb of acetals are molded and extruded in the United States. 

The composition and thermal data of typical polymers and tensile properties of their respective nonsterile monofilaments are summarized in Tables 3.3 and 3.4, respectively. The monofilaments were then radiochemically sterilized using 5 and 7.5 kGy gamma radiation and a polyformaldehyde package insert as described earlier by Correa et al. ° The tensile properties of typical sterile monofilament sutures and their BSR profiles were determined after incubation in a phosphate buffer at 37°C and pH 7.4 to simulate their in vivo profile. The tensile properties and BSR data of a typical radiochemically sterilized monofilament suture are summarized in Table 3.5. 

The thermal destruction of polyformaldehyde with acetylated terminal groups was conducted on a circulation setup at a temperature of 300°C. Since the basic product of the thermal destruction of polyformaldehyde is monomeric formaldehyde, a successful performance of the analysis necessitated its separation from the other decomposition products, the amounts of which were two to three orders of magnitude smaller than the amount of the formaldehyde formed. For this a method was developed for separating formaldehyde, utilizing its property of spontaneous polymerization to Eu-polyoxymethylene at the temperature of... 

Haloaldehyde polymerization provides a new and quite different approach for the study of the polymerization behavior of aldehydes. The properties of per-haloaldehyde polymers are also substantially different from those of polyformaldehyde or of higher aliphatic polyaldehydes. We are discussing here the results of our work on the preparation and polymerization of nine (fluoro, chloro, and bromosubstituted) perhaloacetal-dehydes with special emphasis on the stereoregularity of the polmers obtained. ( 7 )... 

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. 

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