Thermal stabilizers polyoxymethylenes

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. 

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). 

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. 

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]. 

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. 

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. 

Acetal polymers are formed from the polymerization of formaldehyde. They are also given the name polyoxymethylenes (POMs). Polymers prepared from formaldehyde were studied by Staudinger in the 1920s, but thermally stable materials were not introduced until the 1950s, when DuPont developed Dehin. Hompolymers are prepared from very pure formaldehyde by anionic polymerization as shown in Fig. 2.1. Amines and the soluble salts of alkali metals catalyze the reaction. The polymer formed is insoluble and is removed as the reaction proceeds. Thermal degradation of the acetal resin occurs by unzipping with the release of formaldehyde. The thermal stability of the polymer is increased by esterification of the hydroxyl ends with acetic anhydride. An alternative method to improve the thermal stabihty is copolymerization with a second monomer, such as ethylene oxide. The copolymer is prepared by cationic methods developed by Celanese and mar-... 

Copolymeiization is often used to alter the properties of homopolymers and to acUeve specific performance. For example, the flow behavior of PVC is considerably improved by incorporating vinyl acetate as comonomer. Similarly, the thermal stability of polyoxymethylene is improved considerably by incor-... 

The integration of comonomers results in polyoxymethylenes with good thermal stability and only slightly reduced mechanical properties. 

Therefore, it is important to convert these unstable hemiacetal end groups into thermally stable end groups. This is done by acetylizing with, e.g., acetic acid anhydride. Polyoxymethylene exhibits sufficient thermal stability for processing (extrusion or injection molding) only after the unstable acetal groups have been... 

An advantage of using nylon-3 as a stabilizer for polyoxymethylene is that nylon-3 exhibits excellent thermal stability because of its high amide concentration. Furthermore, a polymer composition which contains nylon-3 shows negligible decoloration when the polymer melt remains in an injection machine for a long time. Also nylon-3 does not deposit onto the surface of the injection mold because of its high melting point. These three points are the characteristic merits of nylon-3 for this application. 

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. 

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. 

ReactiOTis with monofunctional reagents are for example carried out in order to increase the thermal and/or chemical stability of the end groups (Polyoxymethylenes, Example 5.7). Reactions with bifunctional reagents can be used to enlarge the degree of polymerization or to synthesize block copolymers (see Sect. 4.2.1). 

The trimer of formaldehyde, 1,3,5-trioxane, has been converted to a polyoxymethylene under anhydrous conditions in ethylene dichloride solution using boron trifluoride etherate or stannic chloride as the initiator. The product needs to be stabilized to prevent thermal degradation back to formaldehyde. The capping of this polymer was carried out in dimethyl-formamide solution with an excess of acetic anhydride and a small amount of a high-boiling tertiary amine such as A, A-dimethylcyclohexylamine or... 

Influence of co-stabilizers (individual ooncentratlon 0.3% each) during thermal loading of polyoxymethylene copolymers during thermogravimetric testing (220 °C, air, isothermic) [38]... 

Acetal polymers, also known as polyoxymethylene (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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