Polyacetal depolymerization

The effect of propagation-depropagation equilibrium on the copolymer composition is important in some cases. In extreme cases, depolymerization and equilibration of the heterochain copolymers become so important that the copolymer composition is no longer determined by the propagation reactions. Transacetalization, for example, cannot be neglected in the later stages of trioxane and DOL copolymerization111, 173. This reaction is used in the commercial production of polyacetal in which redistribution of acetal sequences increases the thermal stability of the copolymers. 

The major disadvantage of chemical depolymerization is that it is almost completely restricted to the recycling of condensation polymers, and is of no use for the decomposition of most addition polymers, which are the main components of the plastic waste stream. Condensation polymers are obtained by the random reaction of two molecules, which may be monomers, oligomers or higher molecular weight intermediates, which proceeds with the liberation of a small molecule as the chain bonds are formed. Chemical depolymerization takes place by promoting the reverse reaction of the polymer formation, usually through the reaction of those small molecules with the polymeric chains. Several resins widely used on a commercial scale are based on condensation polymers, such as polyesters, polyamides, polyacetals, polycarbonates, etc. However, these polymers account for less than 15% of the total plastic wastes (see Chapter 1). 

Celanese joined Du Pont in the market with their proprietary Celcon polyacetal polymer within a year (Figure 3). Celanese managed to obtain basic patent coverage, despite Du Font s prior filing, on the basis of a copolymer variation that led to an enhanced stabilization against thermal depolymerization... 

Though useful polymers can be made by these reactions, their low ceiling temperatures (see p. 599) and consequent tendency to undergo facile depolymerization by an unzipping mechanism pose serious limitations. To overcome this problem the technique of end-capping or end-blocking may be used. Thus poly-oxymethylene (polyacetal), an engineering plastic, prepared from the cyclic acetal... 

Polyacetal can also be stabilized against degradative conditions by copolymerizing trioxane with small amounts of ethylene oxide. This introduces a random distribution of -C-C- bonds in the polymer chain. Hydrolysis of the copolymer with aqueous alkali gives a product with stable hydroxyethyl end groups. The presence of these stable end groups coupled with the randomly distributed C-C bonds prevents polymer depolymerization at high temperature. 

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. 

Degradation of polyacetals may also occur by oxidative attack at random along the chain leading to chain scission and subsequent depolymerization (unzipping). Oxidative chain scission is reduced by the use of antioxidants (see Chapter 1), hindered phenols being preferred. For example, 2,2 -methylene-bis(4-methyl-6-r-butylphenol) is used in Celcon (Celanese) and 4,4 -butylidene bis(3-methyl-6-t-butylphenol) in Delrin (Du Pont). 

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. 

The rate of depolymerizing polyacetal to formaldehyde is the subject of much research, because the rate of depolymerizing is a critical factor in MIM costs, and there is not that much proven information on it. Removal of binder is determined by various factors, namely, binder removal temperature, about 120°C (248°F) catalyst linear or random degradation of polyacetal resin fineness of powder mold part shape and size and catalytic debinding furnace. The catalytic furnace developed for debinding... 

PolyaCGtal. Polyacetals thermally decompose by an acid-catalyzed depolymerization process starting at the chain ends (see Acetal Resins). The polymer structure is stabilized by end capping and introdncing comonomers to interrupt the unzipping. The process is autocatalytic since the liberated formaldehyde is easily oxidized to formic acid, which is a prodegradant. Formaldehyde scavengers and phenolic antioxidants are typically used in polyacetal formulations (42). 

If a polymer is heated to the point where chemical bonds begin to break, radicals will normally be formed, rather than ions. Thus if chain scission takes place to produce free radicals at temperatures above T, rapid radical depolymerization, with production of monomer, is expected. This rapid depropagation is often termed unzipping. Table 1 shows some typical values for Tc for common polymers. They range widely from quite close to room temperature for polyacetal to close to 600°C for polytetrafluoroethylene. In reality, side reactions, which are discussed... 

In the polyacetal copolymer, the presence of an occasional -(CH.-CH.)- will prevent complete catastrophic depolymerization of a polymer chain which has been attacked either by oxygen or acid. A homopolymer chain without this stabilizing influence will completely degrade. Thus, acetal copolymers are inherently more stable than homopolymers. 

Polyacetals have a low ceiling temperature, and are readily depolymerized by unzipping at low temperature (0.4-0.8% min at 222°C for POM). Owing to this low thermal stability, polyacetals can be used only if end-capped with stable groups (acetate or ether). This inherent thermal instability is exploited in an industrial method known as metal injection molding , which allows fine metal powder mixed with a polymer binder to be processed by injection molding, in much the same way as thermoplastic materials [31]. In a procedure based on POM, the binder is removed by thermal devolatilization according to Eq. (62)... 

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