Polyacetal properties

Some typical properties are given in Table 11.13 in comparison with typical properties for nylon 66 (see Chapter 18) and a polyacetal (see Chapter 19) for which it has been suggested that these materials will be competitive. 

The properties of the nylons are considerably affected by the amount of crystallisation. Whereas in some polymers, e.g. the polyacetals and PCTFE, processing conditions have only a minor influence on crystallinity, in the case of the nylons the crystallinity of a given polymer may vary by as much as 40%. Thus a moulding of nylon 6, slowly cooled and subsequently annealed, may be 50-60% crystalline, while rapidly cooled thin-wall mouldings may be only 10% crystalline. 

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. 

Blends or alloys of polyacetals with polyurethane elastomers were first introduced by Hoechst in 1982, who were then followed by other manufacturers. The key features of these materials are their improved toughness with little change in other important properties. There are two aspects with respect to the impact toughness ... 

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. 

Table 3.9 Examples of toughening effects on properties of polyamide and polyacetal...

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. 

Trogamid T can be processed without difficulty by the normal methods used for thermoplastics (5). In this respect it offers fewer problems than polyacetal or polycarbonate, for example. Some Trogamid T properties can be improved by biaxial orientation. The effects, however, are lower than with crystalline polymers. 

To improve the properties of PLA, plasticizers, special additives such as chain-extenders, polymer blends, and composites are commonly investigated. Martin and Averous (10) have studied the effects of various plasticizers on the properties of PLA. Pilla et al. (11-12) have investigated the effects of chain-extenders on the foaming properties of PLA. In addition, a vast number of studies have been conducted to enhance the properties of PLA by blending it with various polymers such as polyethylene oxide (PEO), polypropylene oxide (PPO), polyvinyl acetate, polyolefins, polystyrene, HIPS (high impact polystyrene), polyacetals, polycarbonate, and acrylonitrile butadiene styrene (ABS) (13-26). 

The photostabilisation of polymers still maintains a high level of activity with the mechanistic action and performance of hindered piperidine compounds continuing to attract interest. Some interesting reviews have appeared with one very extensive and highly recommended paper on the mode of action of hindered piperidine light stabilisers while another has dealt with their manufacturing properties. The kinetic reactions and photophysics of light stabilisers have been covered in another review as has the stabilisation of polyacetals. ... 

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. 

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 polymerization parameters influence, sometimes dramatically, the properties of the final product. First of all let us compare the behaviour of the system in the presence and in the absence of polyEO. In the absence of this polyether the reaction mixture containing 45 % wt polyacetal solidifies and cannot be mixed anymore (at least at laboratory scale). In the presence of polyEO even a mixture containing 60 % wt solid polymer is fluid and can be efficiently mixed. 

One may prepare new products differing in properties from the basic polyacetal by functionalization of polyTXN. For example, the following approach may be used ... 

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