Stabilization of Polyformaldehyde

Kern and Cherdron [1, 2] were the first to establish that to obtain stable polyformaldehyde, one must block (esterify) the terminal groups of the polymer and thereby prevent depolymerization from the ends of the chains. As Dudina and Enikolopyan [7] have shown, replacement of the terminal hydroxyl groups by acetyl groups Increases the activation energy of polymer decomposition from 26 to 32 kcal/mole. 

In addition to blocking of the terminal groups, Kern and Cherdron proposed that the monomeric formaldehyde evolved durii decomposition be bonded by suitable additives (polyamides, urea, etc.), in order to prevent its oxidation to formic acid, which promotes acidolytic cleavage of the polymer macromolecules. 

Enikolopyan and Vardanyan [4] proposed that the oxidative destimc-tion of polyformaldehyde be prevented not only by bondii the monomeric formaldehyde liberated, but also by tying up the formic acid formed. 

Kern and Cherdron [2] propose the introduction into the polymer not only of additions of formaldehyde acceptors, but also of antioxidants, which, as has been shown on other polymers, satisfactorily solve the problem of stabilization against oxidation. However, the use of antioxidants was not substantiated in the indicated studies, if we consider that the decomposition of polyformaldehyde proceeds according to an ionic mechanism according to the data of these authors. However, it is known that stabilizing additives of the type of phenols, amines, etc., decelerate processes that proceed only through the formation of free radicals. 

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All the brands of polyamides studied, in conjunction with an antioxidant, manifest considerable effectiveness in the stabilization of polyformaldehyde. ... 

The use of derivatives of phenols, aramines, urea, thiourea, and hydrazines is proposed in the available patent literature [29-35] for the stabilization of polyformaldehyde. 

In evaluating the effectiveness of the action of additives that increase the stability of polyformaldehyde, the optimum ratios of polyamide and... 

As was also shown for polyamides, a break is detected on the curve (Fig. 83). This indicates that the dependence of the induction period on the initial inhibitor concentration is not directly proportional, but is more complex in character the inhibitor is apparently consumed not only for chain termination, but also for side processes (volatilization, oxidation, initiation). An analogous picture is also observed in the stabilization of polyformaldehyde by phenols (Fig. 84). 

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]. 

The introduction of oxygen into the hydrocarbon chain of polymers rednces their thermal stability. This trend is illustrated by the examples of polyformaldehyde (T g = 170 "C), polyethylene oxide (PEO Tjg= 345 °C), isotactic polypropylene oxide (iPPO Tjg = 195 °C) and atactic polypropylene oxide (T = 295 "C) here thermal resistances are lower than those in the corresponding hydrocarbon polymers, namely polyethylene (T g = 406 "C) and polypropylene (T g = 390 °C). 

The differential thermogravimetric analysis (DTGA) curves in Figure 3.2 show that for polyformaldehyde decomposition in an inert atmosphere at 170 C, its maximal rate is achieved at 200 C, while its second maximum is at 209 C. Acetylation of polyformaldehyde enhances its thermal stability the first maximum on the DTGA curves is now at 250 G, while the second is at 285 G. This effect is associated with the fact that polyformaldehyde readily decomposes to monomer because of the presence of polar C-O bonds and OH groups ... 

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 is known from [86, 87], polyformaldehyde undergoes rapid thermooxidative destruction at 200°C. The investigations of N. S. Enikolopyan [88] have shown that the destruction of polyformaldehyde begins with the ends of the molecule, formaldehyde being liberated and then reacting with oxygen, forming formic acid. To stabilize polyformaldehyde, polyamide must be added to it to tie up the formaldehyde. 

The introduction of 1.2% polyamide resin and 0.8% of radical n into acetylated polyformaldehyde exerts a good stabilizing action of polyformaldehyde (Fig. 32). 

However, a vital shortcoming of this poisoner for reprocessing into objects is its extreme instability at temperatures below the melting point ( 100°C) the polymer decomposes readily, liberating monomeric formaldehyde. Hence the reprocessing of polyformaldehyde can be accomplished only after preliminary stabilization of the product. 

A study of the effectiveness of the inhibitors showed [19] that phosphites and sulfides (Fig. 77) exert no appreciable inhibiting action on the thermooxidative destruction of polyformaldehyde. Aramine (Fig. 78) and phenol (Fig. 79) derivatives are extremely effective in the stabilization of this polymer (data are cited in comparison with the stable polymer). 

A study of the dependence of the duration of the induction period in the oxidation of polyformaldehyde on the weight concentration of the stabilizing additive showed that at first the induction period increases with increasing concentration of the stabilizing additive, but then it remains practically constant (Fig. 83). 

The inhibition of the oxidation of polyformaldehyde by radical-type stabilizers is also indirect evidence in favor of the occurrence of the process of destruction of polyformaldehyde according to a radical mechanism. 

Polyacetals, as distinct from polyformaldehyde, have not been exploited to their fullest potential as other polymers have and much research still remains to be done [17]. Some typical uses are as prepolymers for polyurethane elastomers [18, 19], in dimensional stabilization of cellulosic fabrics [20], paper [21], and segmented cellulose films [21] as synethetic lubricants [22] as coatings for wire [23] as wash-and-wear finishes [24], in solvent-resistant coatings [25] and in castings, laminates, coatings [13, 26], and varnishes [27]. 

Purser stabilization of acetal polymers also includes the addition of antioxidants and acid scavengers. Polyacetals are subject to oxidative and acidic degradation, which leads to molecular weight decline. Once the chain of the homopolymer is ruptured by such an attack, the exposed polyformaldehyde ends may decompose to formaldehyde and acetic acid. 

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. 

Polyacetaldehyde, a mbbery polymer with an acetal stmcture, was first discovered in 1936 (49,50). More recentiy, it has been shown that a white, nontacky, and highly elastic polymer can be formed by cationic polymerization using BF in Hquid ethylene (51). At temperatures below —75° C using anionic initiators, such as metal alkyls in a hydrocarbon solvent, a crystalline, isotactic polymer is obtained (52). This polymer also has an acetal [poly(oxymethylene)] stmcture. Molecular weights in the range of 800,000—3,000,000 have been reported. Polyacetaldehyde is unstable and depolymerizes in a few days to acetaldehyde. The methods used for stabilizing polyformaldehyde have not been successful with poly acetaldehyde and the polymer has no practical significance (see Acetalresins). 

Polymers produced by methods as described above have thermal stabilities many times greater than those obtained by the earlier bulk and solution methods of Staudinger. Staudinger had, however, shown that the diacetates of low molecular weight polyoxymethylenes (I) (polyformaldehydes) were more stable than the simple polyoxymethylene glycols (II) (Figure 19.2). 

Polyformaldehydes (polyoxymethylenes, polyacetals) These are physically similar to general purpose nylons but with greater stiffness and lower water absorption. There are no solvents, but swelling occurs in liquids of similar solubility parameter. Poor resistance to u.v. light and limited thermal stability are two disadvantages of these materials. 

Acetal Resins. Acetal resins (qv) are poly (methylene oxide) or polyformaldehyde homopolymers and formaldehyde [50-00-0] copolymerized with aliphatic oxides such as ethylene oxide (42). The homopolymer resin polyoxymethylene [9002-81-7] (POM) is produced by the anionic catalytic polymerization of formaldehyde. For thermal stability, the resin is endcapped with an acyl or alkyl function. 

Acetal polymers, also known as POM or polyacetal, are formaldehyde-based thermoplastics that have been commercially available since the 1960s. Polyformaldehyde is thermally unstable. It decomposes on heating to yield formaldehyde gas. Two methods of stabilizing polyformaldehyde for use as an engineering polymer were developed and introduced by DuPont, in 1959, and Celanese in 1962. 

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