Nylon degradation: problems and opportunities: it is probably not an exaggeration to say that the development and commercialisation of linear, thermoplastic polyamides were due in large part to the farsightedness of one company and the brilliance of one man.
When, in 1928, E.I. DuPont de Nemours & Co. recruited the academic chemist Wallace H. Carothers from Harvard University to head a research department, it was an unusual move. That an established chemicals company would carry out fundamental research was a departure for the time, and was seen by some as a potentially expensive gamble. But it was to pay off in spectacular fashion over the next decade.
Carothers was particularly interested in the possibility of creating new thermoplastic polymers through the use of condensation reactions, such as take place between carboxylic acids and alcohols to form esters, and between carboxylic acids and amines to form amides. Research on polyester formation by reacting aliphatic diacids with aliphatic diols produced some interesting materials, but none that appeared promising for commercial applications. It is also interesting to note that while the many experiments were carried out in this area in the late 1920s and early 1930s, Carothers had not investigated the use of aromatic diacids. Thus, he did not discover PET this was found a few years later by other researchers.
Moving on to reactions between aliphatic diacids and aliphatic diamines did, however, lead to success. Among dozen of pairings of diacids and diamines, it was found that the best results, in terms of a usable polymer, were obtained by reacting adipic acid [HO2C(CH2)4CO2H] and hexamethylenediamine [H2N(CH2)6NH2]. Development of this material toward commercialisation was not easy. It was said that visitors to the laboratory were asked not to even look directly at the experimental setup being used to produce test quantities of the polymer, as it was so temperamental that it would cease working at the slightest opportunity.
All the methods required to spin fibre from the polyamide had to be developed from scratch, including stretch-orienting the spun fibres to provide a product with the required properties. There is a (possibly apocryphal) story that this factor was discovered when some researchers played around with the initial sticky mass produced in the laboratory, by throwing samples at the walls. It was noticed that when this "goop" was pulled away from the wall it formed stretched filaments that were considerably less tacky than the original material and showed high toughness and resilience.
Whether this was the case (Carothers must have been absent at the time if it did happen!), further work rapidly established the commercial potential of the material.
This polymer, polyhexamethylene diamine, soon to be named nylon 6,6 (the numbers reflect the number of carbon atoms contributed to the polymer chain by each component), now provided one of the main targets of the research effort, namely a synthetic fibre potentially suitable as an artificial silk for textile manufacture.
Everything about the production of starting materials, polymer and the final fibres was new. DuPont took another gamble by constructing plants for the manufacture of starting materials at Belle, West Virginia, USA, and for polymer production at Seaford, Delaware, USA, despite the fact that America was still locked in the grip of the Great Depression.
Nylon was first used for fishing line and nets, surgical sutures, and toothbrush bristles, but as the fibre-manufacturing techniques were developed and refined, thinner fibres were made possible, and nylon stockings were first exhibited at the 1939 World's Fair in New York. Ladies throughout the USA were, unfortunately, to be quickly disappointed, as the start of World War II meant that the majority of nylon production was earmarked for military use, for example, in webbing and parachutes.
In Germany, the work of Carothers and co-workers had been followed with keen interest. In order to develop similar fibres, and circumvent DuPont intellectual-property claims, I.G. Farben investigated the possibility of using lactams to form polyamides, and rapidly developed their own artificial fibre in the form of polycaprolactam, also known as nylon 6.
From this time, and especially after the war, many other nylons were developed, and these later materials and nylon 6,6 and 6 found wider applications, away from their original fibre use, as engineering plastics.
From the beginning of nylon development, it was observed that under the action of heat or light, these aliphatic polyamides are susceptible to rapid discoloration, followed by loss of physical properties. In particular, it was noted that sample plaques could be considerably discoloured without there being any substantial change in molecular weight or discernible loss in tensile, flexural, or impact strength. It would thus appear that the production of coloured species (chromophores) occurs very early in the degradation, and that an understanding of this phenomenon should lead to information on the initiation and early stages of the degradation process--a factor crucial to the development of effective stabilising strategies.
As with most polymers, the first reaction to take place in the oxidative degradation of a nylon will be abstraction of a hydrogen from the polymer by some initiating species. This will be followed by reaction of the thus-formed alkyl radical with oxygen to form a peroxy radical. This radical can then itself abstract another hydrogen to form a hydroperoxide. The hydroperoxide is easily split into an alkoxy radical and a hydroxyl radical, both of which can abstract further hydrogens from the polymer. The result is thus a cascade of reactions, known as an autoxidation cycle, or chain reaction.
The first problem to be solved was, "Is there a particularly vulnerable position in the nylon structure from which a hydrogen will be preferentially abstracted?" The general structure of nylon 6,6 can be written as: -(C=O)CH2CH2CH2CH2-(C=O)-NH-CH2CH2CH2CH2CH2CH2-NH-.
While some authors believe that initial hydrogen abstraction can occur randomly, i.e., from any -CH2-group, it is more generally believed that abstraction occurs preferentially at the N-vicinal methylene group, i.e., that which is immediately adjacent to the -NH- group. Taking such an initial reaction as a starting point, various schemes have been posited to account for the early appearance of chromophoric species in degrading samples of nylon.
The most likely sequence appears to involve the formation of double-bonded species including nitrogen atoms (azomethines), followed by formation of conjugated unsaturated species due to the ease of polymerisation of the azomethines, resulting in the formation of highly coloured oligoenimine sequences. Formation of the coloured oligomers can occur both within and between chains, which also provides a possible explanation as to why high degrees of colour can appear before any appreciable loss in physical properties is noted.
As the autoxidation cycle proceeds, however, the number of chain-scission reactions increases, and begins to predominate. The nylon molecular-weight loss then accelerates, resulting in catastrophic loss of sample integrity once the molecular weight has reduced by 30%-40%.
In the absence of oxygen, and to some extent in conjunction with the above reactions, direct chain breaking via non-radical routes also takes place. In this case the reaction proceeds by hydrogen transfer via cyclic (usually six-membered) intermediates. These "back-biting" reactions may involve any of a number of possible hydrogens on the polymer chain, and the result is splitting off from the polymer of cyclic species such as monomers, dimers, trimers, etc.
Detailed studies on the thermal degradation of nylon 6 show that such reactions result in the formation of caprolactam, the reaction taking place in two ways: either from the chain ends (rapid) or within the polymer chain (slow). Under normal circumstances, non-oxidative thermal breakdown of nylons occurs predominantly via these non-radical routes, and thus is not susceptible to suppression by use of standard stabilising additives.
In the case of photooxidation of nylons, the reaction will depend on the wavelength of light being absorbed, which in turn will depend on the presence of chromophoric species. At low wavelengths, this can be the amide group itself, while impurities, such as coloured species created during manufacture or processing, can account for energy absorption at longer wavelengths. Chain-scission reactions here occur again by a mixture of radical and non-radical routes, although the specific pathways involved may differ somewhat from those occurring in thermal processes.
The first stabilisation system specific to nylons appears to have been discovered serendipitously. At the time of the initial development of nylons, the means to melt-process such relatively high-melting-point polymers were also only just under investigation, and other means of producing usable materials were being sought. Attempts were made to solution-cast films or solution-spin fibres, but it was found that the high levels of hydrogen bonding in the polyamides led to great difficulty in dissolving the polymer in any but the most noxious, expensive, high-boiling solvents. This led to problems with costs, safety, and getting rid of the solvent from the polymer once it had performed its task.
It was discovered that nylons could, however, be dissolved in simple, inexpensive, solvents such as lower alcohols if these also contained high concentrations of copper salts. While the resultant films were highly coloured by the salts, it was noted that they had increased resistance to thermooxidation and photo-oxidation. Researchers at I.G. Farben found that only low concentrations, below 1 wt%, were needed to successfully stabilise the nylons. Further developments in the 1950s and 1960s, including the use of co-stabilisers along with the copper salts (e.g., alkali metal halides, phosphates, and acetates), led to inexpensive, effective, stabiliser packages for nylon. These continue to be used, although such stabilisers do impart a degree of colouration to the nylon substrate, and they are also highly soluble in water, which can result in their being lost from the host polymer rather easily when in contact with aqueous media.
That copper salt should help prevent oxidation of nylon was in itself a puzzle. Incorporation of low levels of metal ions, especially copper, into most polymers, for example the polyolefins, actually causes a considerable acceleration of oxidative degradation of the substrate. A clue to how copper salts act as stabilisers was available from the initial use of such species to aid in the dissolution of nylon in simple solvents. It would appear that the copper salt interfered with the hydrogen bonding between the nylon chains, possibly through complexation with the amide groups, thus rendering the polymer soluble. It was, however, a number of years before experimental evidence was found that this interaction is at least part of the reason behind the effectiveness of copper salts as nylon stabilisers. Most recent work suggests that copper ions can complex with the amide group in nylon, and may also complex with hydroperoxides formed on the N-vicinal methylene group, assisting with their breakdown to non-radical, harmless, species.
The great initial success with the copper and copper co-stabiliser additives seems to have engendered an "if it ain't broke, don't fix it" attitude towards research into nylon stabilisation. While patents from the 1950s to the 1980s covered a wide range of species claimed to be efficacious for the protection of nylons, it was not until the 1990s that a systematic study examined the effectiveness of "standard" antioxidants such as hindered phenols, aromatic amines, and piperidine derivatives.
Hindered phenols and aromatic amines are so-called chain-breaking donor (CB-D) primary antioxidants, operating by capturing hydroperoxy radicals and converting these to non-radical species; daughter species of both types can also further react with active species in the polymer. Hindered phenols are, in fact, largely ineffective in nylons, possibly owing to their being insufficiently stable at the processing temperature of these polymers. Aromatic amines are very effective antioxidants in nylons but unfortunately are themselves highly coloured and more water-extractable than other organic stabilisers.
Piperidine derivatives (more commonly referred to as hindered amine stabilisers, HAS) provide good stabilisation, although they are not effective in the very early stages of the oxidation. This is because the initial piperidine (> NH) structure is not the most effective stabilising derivative; more effective are the daughter species, formed during the oxidation, nitroxyl (> NO) and hydroxylamine (> NOH), which form some time after addition and act as radical scavengers. HAS are particularly effective when reacted into the nylon, either by inclusion of a diacid or diamine derivative thereof into the preparation of the polymer (see picture on previous page), or by reacting an acid or amine derivative with an amine or acid chain end on the polymer molecule. HAS are especially useful as photostabilisers.
Other photostabilisers, such as the common UV absorbers the 2-hydroxybenzophenones and 2(2'-hydroxyphenyl)benzotriazoles, are less useful in nylons than would be indicated by their efficiency in other polymers. It is suggested that this is due to incompatibility problems and/or interference with the mechanism they use to carry out their UV-absorbing function by the hydrogen-bonding potential of the nylon host. Other photostabilisers that have more recently been commercialised have been noted as possible additives for nylons, but further study is required to evaluate their efficiency. These include hydroxyphenyltriazines (similar mechanism to the older absorbers), cyclic iminoesters, oxalic acid diarylamides, and cinnamamides.
Another factor that can allow the use of certain additives to stabilise the nylons is the profound effect on stability of the type and ratio of end groups present in the nylon chains. It is not a simple case of favouring amine ends over acid, or vice versa, however; both have problems. A preferred additive route is to react both with species that either provide an inert chain end or add a stabilising moiety as chain end.
As many of us were taught in early organic chemistry classes, the formation of an ester or amide from a carboxylic acid and an alcohol or amine is a reversible reaction: CH3(C = O) - OH + CH3NH2 [left and right arrow]) CH3(C = O)NHCH3 + H2O.
Logically it should thus be possible to "hydrolyse" nylon back to its starting materials and recover valuable chemical feedstock from a polymer product that is not to specification for some reason (post-industrial) or which has reached the end of its useful life (post-consumer). There is, of course, a world of difference between a simple amide species as shown above and a very-high-molecular-weight polyamide like a nylon.
DuPont itself, early in the development of the nylons, patented means of recovering starting materials from the polymer by either acid-catalysed or base-catalysed hydrolysis. The methods available at this time were energy-intensive, gave impure products, and created large amounts of waste materials. In any case, at this time the tonnage of scrap material was too low and there was little environmental incentive to pro vide the impetus for further research.
By the 1990s, the product and applications for nylon had increased dramatically, and there was increasing awareness of the finite amount of natural resources available. This led to a reappraisal of the chemical recycling of nylon, especially in the case of nylon 6. Many of the larger manufacturers, and organisations dealing with specific products such as carpets, began setting up pilot schemes and even large plants for this purpose.
While a number of these ventures proved to be technically successful, problems with supply of scrap materials and fluctuating prices in the world chemicals market have meant that economic success has been less forthcoming. The jury is still out on the eventual overall success of chemical recycling of nylons.
Research continues: Various means of reducing the energy required to break down the polymer to its constituent parts by chemical means, and the use of biochemical approaches, such as enzymes, are under intense scrutiny.
From an academic background in polymer sciences at St. Andrews University, Scotland, Stuart Fairgrieve entered into industrial research with Cookson Group, becoming senior researcher in plastics. In 1996 he set up SPF Polymer Consultants. He is the author of many academic papers and monographs, including Degradation and Stabilisation of Polyamides, published by RAPRA Technology, April 2008. He is also the principal inventor of many current U.S. patents. He can be contacted at email@example.com
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|Date:||Jul 1, 2008|
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