Stabilization polyoxymethylene

Figure 4.60 Influence of repeated processing on melt Index (MVR) and strain at break, example heat-stabilized polyoxymethylene [622]...

Acetals. Acetal resins (qv) are polymers of formaldehyde and are usually called polyoxymethylene [9002-81-7]. Acetal homopolymer was developed at Du Pont (8). The commercial development of acetal resins required a pure monomer. The monomer is rigorously purified to remove water, formic acid, metals, and methanol, which act as chain-transfer or reaction-terminating agents. The purified formaldehyde is polymerized to form the acetal homopolymer the polymer end groups are stabilized by reaction with acetic anhydride to form acetate end groups (9). 

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). 

Staudinger also found that diacetates of polyoxymethylenes with a degree of polymerisation of about 50 were less stable. Truly high molecular weight polyoxymethylenes (degree of polymerisation -1000) were not esterified by Staudinger this was effected by the Du Pont research team and was found to improve the thermal stability of the polymer substantially. 

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. 

Other carbonyl compounds, such as acetaldehyde or propionaldehyde can also be polymerized to high-molecular-weight products however, their stability is lower than that of polyoxymethylenes with protected end groups. 

The precipitated polyoxymethylene is filtered off, washed with ether and dried in vacuum at room temperature it melts between 176 and 178 °C.The limiting viscosity number is determined on a 1% solution in DMF at 140 °C (Tisp/c 0.08 l/g,corresponding to an average molecular weight of 80,000).The thermal stability can be tested before and after blocking the hydroxy end groups (see Examples 5-7 and 5-13). 

Polyoxymethylene, obtained by the polymerization either of formaldehyde or of 1,3,5-trioxane,is a highly crystalline product which is insoluble in all solvents at room temperature with the exception of hexafluoroacetone hydrate at higher temperatures it dissolves in some polar solvents (e.g., at 130 °C in DMF or DMSO). If the unstable semi-acetal end groups are blocked (see Example 5-7) polyoxymethylene can be processed without decomposition as a thermoplastic at elevated temperatures in the presence of stabilizers. 

Polyacetals. Simionescu and coworkers (28) have extended to polyoxy-methylene the process of grafting vinyl polymers (acrylonitrile and methylmethacrylate). They performed the synthesis using a virbomill at room temperature under vacuum (10.1 Torr). The initial monomer-polymer ratio was 1 -5/1.0 and the degree of vibromill packing 0.44. Before milling the polyoxymethylene granules were dissolved in dimethyl formamide and repredpitated with the aim of stabilizer removal and for reduction of polymer particle size (from 2-2.5 mm to 0.05-0.10 mm). Full details of the reaction have been described (40). 

Fig. 3. Polymerization of acrylonitrile by vibromilling polyoxymethylene, Thermogravimetric analysis of the polyoxymethylene (without stabilizers) and of the interpolymers at different...

Long-chain methanal polymers have become very important as plastics in recent years. The low cost of paraformaldehyde is highly favorable in this connection, but the instability of the material to elevated temperatures and dilute acids precludes its use in plastics. However, the end-capping of polyoxymethylene chains through formation of esters or acetals produces a remarkable increase in stability, and such modified polymers have excellent properties as plastics. Delrin (DuPont) is a stabilized methanal polymer with exceptional strength and ease of molding. 

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. 

There is an enormous difference between a nonstabilized and a stabilized commodity polymer (for instance polyoxymethylene) undergoing a chain-degradation process. Stabilization can occur by various routes, e.g., suppression of weak points interruption of the propagation by a competitive process increase of the termination rate using a radical scavenger such as carbon black, etc. 

Formaldehyde homopolymer is composed exclusively of repeating oxymethylene units and is described by the term polyoxymethylene (POM) [9002-81-7]. Commercially significant copolymers, for example [95327-43-8], have a minor fraction (typically less than 5 mol %) of alkylidene or other units, derived from cyclic ethers or cyclic foimals, distributed along the polymer chain. The occasional break in the oxymethylene sequences has significant ramifications for polymer stabilization. 

In this review article, an account will be presented of the diffia t methods by which high modulus materials have been produced from flexible polymers. Much of the discussion will be concerned with polyethylene, although comparable results have been obtained fot polypropylene and polyoxymethylene, and these will also be considered. The initial stimulus to this research came from the quest for high stiffness, but other properties have also been enhanced, including strength, thermal and chemical stability, and barrier properties. The present article updates and extends previous reviews of progress in this exciting new area of polymer science. 

In Zone A, at monomer concentrations below the equilibrium concentration solid polymer dissolves. This is equivalent to the dissolution of a crystalline low molecular wt. compound in a good solvent. In Zone C the solution is super-saturated, it is above the stability limit, spontaneous nucleation occurs and polymer precipitates from the clear solution. There is a Zone B, about 4—8% above the equilibrium concentration of formaldehyde, where super-saturation is insufficient to cause spontaneous precipitation of polyoxymethylene but where seeds or nuclei of polyoxy-m ethylene can grow when added to the clear solution and can increase in weight and molecular weight. This is the desirable range for the preparation of high molecular weight polyoxymethylene in hydroxylic media. 

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]. 

Butlerov in 1859 and investigated by Nobel laureate Staudinger in the 1920 s. Polyoxymethylene stabilized by acetylation of the hydroxyl end groups (capping) was introduced commercially by DuPont under the trade name of Delrin in the late 195O sA— ... 

A study of the thermal stability of polyoxymethylene was made by Schweitzer et al. [225]. The rate of weight loss was measured at 222°C. Formaldehyde is evolved by a first-order reaction. The decomposition was assumed to take place mainly by unzipping from the chain end, since the rate of weight loss increases with molecular weight. This was confirmed later by Kern and Cherdron [226] who also showed that acetylation of end groups leads to improved thermal properties. 

Figure 28 shows that substituted cinnamic acid derivatives have a relatively low absorption in the 310—320 nm region, so that they are relatively ineffective ultraviolet absorbers. Their main advantage is that they have no phenolic hydroxyl group which could be sensitive to alkali or heavy metal ions. The alkali sensitivity is a severe shortcoming for textile applications. On the other hand, with polyoxymethylene for instance, the thermal degradation can be catalysed by phenols, and with polyvinylchloride, side reactions can occur with metal stabilizers. 

The stable OCH3 end-groups, that are formed in this process, may be responsible for the observation that the polyoxymethylene contains up to 25 % of a thermally stable fraction 48). One has to remember, however, that thermal stability may at least partially be due to the inevitable presence of macrocyclics 101>. 

Structures involving the ether linkage are often employed as blend components. The thermal stability of polyoxymethylene, POM, blends with polyurethane have been studied and these were found to have higher activation energies for decomposition than either of the components [Kumar et al., 1993]. The opposite effect occurred in miscible PEG blends with Phenoxy (the polyhydroxy ether of bisphenol-A) although the degradation process was affected by composition, the addition of the Phenoxy had a negative effect on blend stability [Iriarte et al, 1989]. 

Polyoxymethylene is susceptible to depolymerization, or unzipping, under molding conditions. To improve thermal stability, end capping is essential. The capping of the hydroxyl end groups is achieved by etherification or, preferably, by esterification using acetic anhydride ... 

MAJOR APPLICATION Thermal stabilizer for polyoxymethylene, and stabilizer for polyacetal resin. Because of high amide concentration, nylon 3 shows properties of an excellent formaldehyde scavenger. 

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