Polyacetal

An amorphous polymer, PMMA has a solubility parameter of about 18.8 MPaX and is soluble in a number of solvents with similar solubility parameters. Solvents include ethyl acetate (8 18.6 MPa)0, ethylene dichloride (8 20.0 MPa, trichloroethylene (8 19 MPaX), chloroform (8 19 MPa 0, and toluene (8 20 MPaX). The polymer is attacked by mineral acids but is resistant to alkalis, water and most aqueous inorganic salt solutions. A number of organic materials although not solvents may cause crazing and cracking (e.g. aliphatic alcohols). 

Polyacetal can be divided into two basic types, acetal homoploymer and acetal copolymer. Both homopolymer and copolymer are available in a range of molecular weights (M = 20 000-100 000). The homopolymer is a polymer of formaldehyde with a molecular structure of repeated oxymethylene units (Staudinger, 1932). Large-scale production of polyformaldehyde, i.e. polyacetal, commenced in 1958 in the USA (US Patent 2 768 994,1956) (British patent 770 717,1957). Delrin (1959) was the first trade mark for this polymer by Du Pont Company. The copolymers were introduced by the Celanese Corporation of America, and the first commercial product named Celcon (1960). One of the major advantages of copolymerization is to stabilize polyacetal because the homopolymer tends to depolymerize and eliminate formaldehyde. The most important stabilization method is structural modification of the polymer by, for example, copolymerization with cyclic ether. 

As can been seen polyacetal has a very simple structure of a polyether. Unlike polyethylene, polyacetal has no branching, and its molecules can pack more closely together than those of polyethylene. The resultant 

The enormous commercial success of the polyacetals is owed to their very high resistance to creep and fatigue. The acetal resins show superior creep resistance to the nylons. 

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Although there is a substantial body of information in the pubHc domain concerning the preparation of polyacetals, the details of processes for manufacturiag acetal resins are kept highly confidential by the companies that practice them. Nevertheless, enough information is available that reasonably accurate overviews can be surmised. Manufacture of both homopolymer and copolymer involves critical monomer purification operations, discussion of which is outside the scope of this article (see Formaldehyde). Homopolymer and copolymer are manufactured by substantially different processes for accomplishing substantially different polymerisation chemistries. 

Table 5. World Supply/Demand for Polyacetal Resins for 1988 ...

With aldehydes or their derivatives, butanediol forms acetals, either 7-membered rings (1,3-dioxepanes) or linear polyacetals the rings and chains are easily intraconverted (126,127). 

Other Rea.ctions, The photolysis of ketenes results in carbenes. The polymeriza tion of ketenes has been reviewed (49). It can lead to polyesters and polyketones (50). The polymerization of higher ketenes results in polyacetals depending on catalysts and conditions. Catalysts such as sodium alkoxides (polyesters), aluminum tribromide (polyketones), and tertiary amines (polyacetals) are used. Polymers from R2C—C—O may be represented as foUows. 

Fig. 5. Flexural modulus—temperature curves of C, polysulfone and B, polyethersulfone compared to the moduli curves of A, polyacetal D, heat-resistant...

Heteroatom Chain Backbone Polymers. This class of polymers includes polyesters, which have been widely studied from the initial period of research on biodegradable polymers, polyamides, polyethers, polyacetals, and other condensation polymers. Their linkages are quite frequendy found in nature and these polymers are more likely to biodegrade than hydrocarbon-based polymers. 

Similar polyacetals were prepared by BASF scientists from CO-aldehydic aUphatic carboxyUc acids (189,190) and by the addition of poly(hydroxycarboxyhc acid)s such as tartaric acid to divinyl ethers (191) as biodegradable detergent polymers. 

The distinctions between these homopolymers arise from the different ways in which the monomer units are hooked together in polyacetal chains. Starch (qv), plant nutrient material, is composed of two polysaccharides a-amylose and amylopectin. cx-Amylose is linear because of exclusive a (1 — 4) linkages, whereas amylopectin is branched because of the presence of a (1 — 6) as well as a (1 — 4) links. The terms linear and branched refer only to primary stmcture. 

In the above examples the polymerisation takes place by the opening of a carbon-carbon double bond. It is also possible to open carbonyl carbon-oxygen double bonds and nitrile carbon-nitrogen triple bonds. An example of the former is the polymerisation of formaldehyde to give polyformaldehyde (also known as polyoxymethylene and polyacetal) (Figure 2.3). 

There are thus no solvents at room temperature for polyethylene, polypropylene, poly-4 methylpent-l-ene, polyacetals and polytetrafluoroethylene. However, as the temperature is raised and approaches F , the FAS term becomes greater than AH and appropriate solvents become effective. Swelling will, however, occur in the amorphous zones of the polymer in the presence of solvents of similar solubility parameter, even at temperatures well below T. ... 

Weak links, particularly terminal weak links, can be the site of initiation of a chain unzipping reaction. A monomer or other simple molecule may be abstracted from the end of the chain in such a way that the new chain end is also unstable. The reaction repeats itself and the polymer depolymerises or otherwise degrades. This phenomenon occurs to a serious extent with polyacetals, polyfmethyl methacrylate) and, it is believed, with PVC. 

By deactivating the active weak link. For example, commercial polyacetal (polyformaldehyde) resins have their chain ends capped by a stable grouping. (This will, however, be of little use where the initiation of chain degradation is not at the terminal group.)... 

Some polymers such as the polyacetals (polyformaldehyde) and poly(methyl methacrylate) depolymerise to monomer on heating. At processing temperatures such monomers are in the gaseous phase and even where there is only a small amount of depolymerisation a large number of bubbles can be formed in the products. 

Gaseous monomers may also be trapped within the processing equipment and accidents have occurred as a consequence of the resulting pressure buildup. In the case of the polyacetals and poly(vinyl chloride) it is reported that at elevated temperatures these materials form a more or less explosive combination so that it is important to separate these materials rigorously at the processing stage. 

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. 

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