Polymer degradation aliphatic polyamides

Polymers with hetero-atoms in the chain are suitable for chemical recycling of waste materials. In addition to depolymerisation (nylon 6) and solvolysis (nylon 6,6, PETP, PU) the degradation of aliphatic polyamides with dicarboxylic acids, diamines and cyclic anhydrides, especially trimellitic anhydride, becomes more and more important. The utilisation of the obtained fragments is described. 

Nonetheless a few commercially successful noncellulosic membrane materials were developed. Polyamide membranes in particular were developed by several groups. Aliphatic polyamides have low rejections and modest fluxes, but aromatic polyamide membranes were successfully developed by Toray [25], Chemstrad (Monsanto) [26] and Permasep (Du Pont) [27], all in hollow fiber form. These membranes have good seawater salt rejections of up to 99.5 %, but the fluxes are low, in the 1 to 3 gal/ft2 day range. The Permasep membrane, in hollow fine fiber form to overcome the low water permeability problems, was produced under the names B-10 and B-15 for seawater desalination plants until the year 2000. The structure of the Permasep B-15 polymer is shown in Figure 5.7. Polyamide membranes, like interfacial composite membranes, are susceptible to degradation by chlorine because of their amide bonds. 

The examples of PA 6 and 6.6 illustrate the challenges that these polymers create. The classical research into the thermal degradation occurred during the 1950-1970 period, and extensive reviews of this work include those by Kohan25 and Peters and Still.26 Essentially, for all linear, aliphatic polyamides, thermal degradation is influenced by two major factors ... 

Thermal lability of aliphatic polyamides, in general, is influenced by the potential for ring-formation during chain degradation, and this is particularly the case with PA 6.6, in which the adipate repeat unit enables the formation of a six-membered intermediate along the polymer chains with eventual formation of cyclopentanone and its derivatives.29... 

Some synthetic polymers like, polyurethanes, specifically polyether-polyurethanes, are likely to be degraded by microbes but not completely. However, several polymers such as, polyamides, polyfluorocarbons, polyethylene, polypropylene, and polycarbonate are highly resistant to microbial degradation. Natural polymers are generally more biodegradable than synthetic polymers specifically, polymers with ester groups like aliphatic polyesters [1]. Therefore, several natural polymers such as cellulose, starch, blends of those with synthetic polymers, polylactate, polyester-amide, and polyhydroxyalkanoates (PHAs) have been the focus of attention in the recent years [3]. 

Montaudo, G., Mass Spectral Determination of Cyclic Oligomers Distributions in Polymerization and Degradation Reactions, Macromolecules, 24, 5289, 1991. Ballistreri, A., Garozzo, D., Giuffrida, M., Impallomeni, G., and Montaudo, G., Primary Thermal Decomposition Processes in Aliphatic Polyamides, Polym. Deg. and Stab., 23, 25, 1988. 

The comparison of the results obtained in this study for the LCP degradation imder processing temperatures with the peculiarities of some thermally stable polyheteroaiylenes degradation [14] brings to light some common features carbonization of the structure, H2 evolution, improvement in thermo-oxidative stability with transition metal compoimds. That is why we took into accoimt the stabilization of polysulfones, aiyl-aliphatic polyimides, polyamides etc. The approach to such stabilization is based on the following proposals on the mechanisms of the above said polymer degradation ... 

Pavlov, N. N., Kudrjavtseva, G.A. et al. Structural and chemical changes in aliphatic polyamides during artificial ageing. Polymer Degradation and Stability, 24 (1989), p. 389-397... 

Aliphatic polyamides do not absorb light above 290 nm and photo-oxidative degradation of these polymers in that region can be initiated by the presence of such impurities as hydroperoxides, [47, 139] carbonyl groups [1869], a,j -unsaturated carbonyl groups [47, 65, 89, 99, 103] and traces of metal ions [236]. 

Polyamide or polyimide polymers are resistant to aliphatic, aromatic, and chlorinated or fluorinated hydrocarbons as well as to many acidic and basic systems but are degraded by high-temperature caustic exposures. 

The processing of polymers should occur with dry materials and with control of the atmosphere so that oxidative reactions may be either avoided, to maintain the polymer s molar mass, or exploited to maximize scission events (in order to raise the melt-flow index). The previous sections have considered the oxidative degradation of polymers and its control in some detail. What has not been considered are reactions during processing that do not involve oxidation but may lead to scission of the polymer chain. Examples include the thermal scission of aliphatic esters by an intramolecular abstraction (Scheme 1.51) (Billingham et al., 1987) and acid- or base- catalysed hydrolysis of polymers such as polyesters and polyamides (Scheirs, 2000). If a polymer is not dry, the evolution of steam at the processing temperature can lead to physical defects such as voids. However, there can also be chemical changes such as hydrolysis that can occur under these conditions. 

Hydrolytic degradation is especially important in polymers with hydrolyzable links between the CRUs. Thus, polyesters can be saponified to yield the starting materials from which they were formed. Acetal links in synthetic polymers such as polyoxymethylene, or in natural polymers such as cellulose, can be hydrolyzed with acids. However, the resistance to hydrolysis depends very much on the structure of the polymer for example, polyesters of terephthalic acid are very difficult to hydrolyze while aliphatic polyesters are generally easily hydrolyzed. Polyamides are normally much more resistant to hydrolysis than polyesters they may be cleaved by the methods usually employed for polypeptides and proteins. 

Polymers with hydrolysable linkages in the backbone are very useful in a range of degradable materials. For disposable table-wares as cups or expendable packages many of them are still too expensive and do not exhibit the desirable combination of mechanical and chemical properties. Well-known synthetic hydrolysable polymers are polyesters [1], polycarbonates [2], polyanhydrides [2], polyamides [2] and poly(amino acids) [2]. Hydrolysable biopolymers may be cheaper than synthetically produced polymers (e.g. aliphatic polyesters such as polylactides) and many scientists today are looking for new possibilities using such traditional natural polymers as polysaccharides, proteins and lipids. Special interest is focused on poly(P-hydroxybutyrate) and its copolymers [3,4] (see Chapters 9 and 10). Well-known natural products such as Pullulan (a bacterial polysaccharide produced by Aerobasidium pullulans), cellulose acetate and starch, as well as synthetic polyvinyl alcohol are important degradable materials. 

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