Formaldehyde, from polyacetals

A whole family of polymers is derived from formaldehyde polyacetal (polymethylene oxide), phenol-formaldehyde, urea-formaldehyde and melamine-formaldehyde. It is interesting to note that the other components (phenol, urea and melamine) are also products of coal pyrolysis. 

Commercial polyacetal copolymers contain 0.1 to 15 mole percent of a cyclic ether, commonly ethylene oxide or 1,3-dioxolane. Typical catalysts for this reaction are BF, or its ether complexes. In 1964, Weissermel and coworkers[5] showed that in the copolymerization of trioxane with ethylene oxide, the latter was almost completely consumed before any visible polymer was observed. During this stage of the polymerization, soluble prepolymers of ethylene oxide could be isolated [6], These prepolymers consisted primarily of oligomers with mono-, di-, and tri-ethylene oxide units. Celanese workers in 1980[7] verified also the presence of cyclic ethers, predominately 1,3-dioxolane and 1,3,5-tri-oxepane, as part of the reaction mixture. These are likely formed as reaction products of ethylene oxide and monomeric formaldehyde generated from the opening of the trioxane ring. 

The growth in use of polyacetals for automotive applieations has been much more influenced by its replacement of metal rather than other polymers. There is a potential threat to polyacetals from polyamide. Some OEMs may be encouraged to switeh from POM to polyamide for interior applications such as instrument panels, grilles and switehes, because of the higher formaldehyde emissions from POM. 

From the time that formaldehyde was first isolated by Butlerov in 1859 polymeric forms have been encountered by those handling the material. Nevertheless it is only since the late 1950s that polymers have been available with the requisite stability and toughness to make them useful plastics. In this period these materials (referred to by the manufacturers as acetal resins or polyacetals) have achieved rapid acceptance as engineering materials competitive not only with the nylons but also with metals and ceramics. 

Polyacetals are among the aliphatic polyether family and are produced by the polymerization of formaldehyde. They are termed polyacetals to distinguish them from polyethers produced by polymerizing ethylene oxide, which has two -CH2- groups between the ether group. The polymerization reaction occurs in the presence of a Lewis acid and a small amount of water at room temperature. It could also be catalyzed with amines ... 

By organic chemistry formalism, polyacetals are reaction products of aldehydes with polyhydric alcohols. Polymers generated from aldehydes, however, either via cationic or anionic polymerization are generally known as polyacetals because of repeating acetal linkages. Formaldehyde polymers, which are commercially known as acetal resins, are produced by the cationic ring opening polymerization of the cyclic trimer of formaldehyde, viz., trioxane [29-30] (Fig. 1.5). 

Polyformaldehyde. Polyformaldehyde or polyacetal is made by two different processes. Delrin is made from formaldehyde by anionic polymerization catalyzed by a tertiary amine. The homopolymer is end-capped with acetic anhydride. Celcon is made from trioxane cationic copolymerization using boron trifluoride catalyst and ethylene oxide (2-3%) as the comonomer. Boron trifluoride is a Lewis acid that associates with trioxane and opens up the six-membered ring. Ethylene oxide provides the end capping. Without an end cap, polyformaldehyde is thermally unstable and loses formaldehyde units. 

Polyacetals form a different subclass of compounds with oxygen in the backbone chain. In this group are included polymers that contain the group -0-C(R2)-0- and can be formed from the polymerization of aldehydes or ketones. A typical example of a polymer from this class is paraformaldehyde or polyformaldehyde or polyoxymethylene (CH20)n. Polyoxymethylene can be prepared by anionic catalysis from formaldehyde in an inert solvent. Acetylation of the -OH end groups of the polymeric chain is common since it improves the thermal stability of the polymer. Some results reported in literature regarding thermal decomposition of these polymers are indicated in Table 9.2.1 [1]. 

Figure 1.42. Weight loss from polyacetal during processing due to liberation of formaldehyde and inhibition by an acid scavenger combined with a hindered phenol antioxidant. Adapted from Zweifel (1998).

Polyoxymethylene polymers, POM, commonly known as polyacetals or Acetal resins are linear thermoplastic polymers containing predominantly the -CH -O- repeat unit in their backbone. There are two types of acetal resins available commercially (1) homopolymers made by the polymerization of formaldehyde, followed by endcapping, (2) copolymers derived from the ring opening polymerization of trioxane (a cyclic trimer of formaldehyde), and a small amount of a comonomer such as ethylene oxide. Acetal resins are... 

The main application fields of aldehyde scavengers are PET (more specifically wafer bottles made from PET), and polyacetals such as POM or polymers synthesized from formaldehyde such as urea-formaldehyde resins. 

Poly(oxymethylene), with the monomeric unit -(rCUiO, can be produced from formaldehyde, HCHO, or its cyclic trimer, trioxane (1,3,5-trioxacyclohexane). The commerical polymer obtained from formaldehyde is also known as polyacetal homopolymer, and that from trioxane is known as polyacetal copolymer. 

There is a series of polymers having a chemical structure — [(CHR) —O— which are derived as polyacetal resins, and are known as polyalkyene oxides or polyalkylene glycols. In the above structure, the polymer with R=H and M = 1 is polyoxymethylene, which is known as Delrin. This material is a high polymer of formaldehyde, which is terminated by an ether or ester function added to stabilize the final product. Other manufactured products include copolymers with ethylene oxide or propylene oxide. The IR and Raman spectra of polyoxymethylene are shown in Reference Spectrum 55. A strong peak at 1098 cm and a doublet at 936 and 900 cm in the IR spectrum are assigned to the C—O—C stretching vibration. It is not possible to determine if the sample is a homopolymer or copolymer from this spectrum. 

Carothers was one of the first to exploit the polyacetal-forming reaction in his attempt to prepare polyformals by the reaction of formaldehyde [3] with glycols. He found that diols below 1,5-pentanediol led to cyclic formals whereas the principal product from 1,6-hexanediol and from higher molecular weight diols was a polyformal [3]. Similar products were formed by the acetal interchange reaction when appropriate diols were used [Eq. (4)] [4]. 

Polyacetals derived from diols or polyols using formaldehyde or its derivatives are known as polyformals. Cyclic formals are formed when a five-, six-, or seven-membered ring can be formed [40]. 

Bruno J S and Vigo T L, Thermal properties of insolubilized polyacetals derived from non-formaldehyde crosslinking agents , Thertnochim. Acta, 1994,243(2), 155-9. 

After nylon, the next engineering polymers to be commercially introduced were polyacetals [7,8]. Polyacetals are polymerized from formaldehyde and... 

Polyacetals were among the first synthetic polymers studied by Staudinger in the 1920s. Polyformaldehydes prepared by both anionic polymerization of formaldehyde and cationic polymerization of TOX were, however, thermally unstable. Thus, they were not commercialized until the late 1950s when the reasons for the thermal instability were better imder-stood and the stabilization methods were developed. The DuPont process, based on anionic formaldehyde polymerization, involves esterification of unstable hemiacetal end groups from which degradation starts, while in the Celanese-Hoechst process TOX is copolymerized with a few percent of... 

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