Hindered Amine Light Stabilisers - Assignment Example

Literature Review for Hindered Amine Light Stabilisers (HALS) Introduction In the modern era plastics are used extensively for many applications. However, the material suffers form a deficiency in that on exposure to sunlight and certain specific artificial light plastic degrades. Exposure to sunlight and in particular ultraviolet (UV) rays reduces the useful life period of plastics (Scott, 1999 (1)). The search to overcome this drawback in plastics led to UV stabilisers as the means to overcome the degradation problem with plastics on exposure to sunlight. The UV stabilisers have been classified into two types, namely the ultraviolet and hindered amine light stabilisers (HALS). According to Zweiful, Maeir, and Schiller 2009, pp.221, “Hindered amine type light stabilisers (HALS) represent the most important development in light stabilization for many polymers (Zweiful, Maeir, & Schiller 2009 (2)). The rationale for use of HALS is derived from the approach of copolymerisation of olefins with functional monomers to overcome drawbacks with the polymers (Kawahara et al, 2008 (3)). History The credit for finding a stable free radical with chemical properties that offered potential protection for plastics from UV light rays goes to Neiman and his fellow researchers, who in 1962 came up with the stable 2,2,6,6-tetramethyl-4-oxopiperidine-N-oxyl. The chemical properties and stability that this free radical demonstrated led to this group of compounds becoming the focus of interest, with assessments being conducted on their ability to assist in the stabilization of polymers. These evaluations showed that the group of N-oxyls were unsatisfactory in polymer stabilization, due to the imparting of the high coloring present in them to the polymers. In spite of this setback with N-oxyls, they mark an important step in the search for polymer stabilisers (Klemchuk, 1990 (4)). This failure increased the vigor in the search for a polymer stabiliser without the disadvantage of coloring of the N-oxyl group. In 1970, the Japanese company Sankyo Company discovered 2,2,6,6-tetramethylpiperidines with light stabilising activity. This discovery and further evaluation showed that the N-H compounds group demonstrated as much efficiency as the N-oxyl group without the disadvantage of coloring of the N-oxyl group. Thus by 1974 the hindered amine light stabiliser HALS – 1, which is a derivative of tetramethylpiperidine became commercially available, and began to be used as a light stabiliser for polymers. Subsequently several derivatives of tetramethylpiperidine were developed and became available for commercial use. Many of the commercially available HALS in use for polymer stabilising are derived from triacetone amine (Klemchuk, 1990 (4)). Figure – 1 Chemical Structure of low MW Hindered Light Amine Stabiliser (SpecialChem, 2010 (5)) Synthesis of HALS Triacetoamine is the founding compound from which all HALS products are synthesised. Way back in 1874 there existed the knowledge that triacetoamine could be derived from acetone and ammonia it took a century for it to be commercially viable (Pospisil, 1990 (6)) the The synthesis of HALS is a three step process. The important active group in HALS is tetramethylpiperidine, which makes the synthesis of tetramethylpiperidine the first step in the synthesis of HALS. Acetone and ammonia in the presence of the catalyst system CaCl2-H2O-NH4Cl combine to produce triacetoamine, which is the intermediary that goes on to yield the tetramethylpiperidine 2,2,6,6-tetramethyl-4-piperidone. This reaction is conducted in the absence of any heating and in an aqueous medium in the presence of Lewis acids, when about 60% to 70% yield of the required product can be expected (Lau & Quing, 1996 (7)). In the second stage 2,2,6,6-tetramethyl-4-piperidone is reduced to 2,2,6,6-tetramethyl-4- piperidinol, employing sodium borane as the reducing agent in a water alkaline medium. The final stage involves the reactions involving 2,2,6,6-tetramethyl-4- piperidinol with different carboxylic acids and their ethers to produce HALS (Irina & Nikolay, 2002 (8)). Thus monomeric HALS can be got for different applications. For example, to create a monomeric HALS with vinyl functionality a vinyl group is inserted with the help of an ester exchange involving 2,2,6,6-tetramethyl-4- piperidinol and methyl methacrylate (Lau & Quing, 1996 (7)). Properties of HALS The vast range of applications of polyolefin’s in the modern world would not have been possible, but for the contribution of HALS in the advances to polymer light stabilisation, with particular emphasis on the outdoor applications of the polyolefin’s (Coaker, 2000 (9)). In addition to the versatility of the HALS products, they also offer cost benefits in comparison to the other light stabilising products consisting of nickel quenchers and UV absorbers these light stabilisation benefits of HALS are derived from the properties of the various HALS developed. More than thirty HALS structures are currently in use. All these HALS structures have one thing in common. The common thread in all these HALS structures is that they are founded on the tetramethyl piperidinyl moiety. Hence they all possess the nitroxyl active site, which is responsible for the stabilization activity of these HALS structures (Vulic et al, 1999 (10). Normally HALS products are solids that are colorless, unless they are made from epoxides, when they are liquids with a yellowish tinge (Salamone, 1999 (11)). From the very outset it was understood that HALS light stabilising activity was not based on absorption of UV light and so were different from the UV light absorbers. The initial proposed mechanism for the light stabilising activity of HALS known as the Denisov cycle made the HALS transformation products of nitroxyl radicals and hydroxyl amino ethers the key contributors to the stabilising properties of HALS. This proposed mechanism was in keeping with the commonly accepted alkyl and alkylperoxy-radicals pathway for hydrocarbon degradation. Put in simple terms the Denisov cycle consisted of the nitroxyl radicals of HALS combining with the free radicals present in the polyolefin’s and regenerating itself. The nitroxyl radicals of HALS go through several of such turnovers till they finally loses their activity. However, evidence over time from the use of HALS pointed to the stabilising activity to be more complex than the anti-degradation mechanism of the Denisov cycle. This has led to several other mechanisms also being attributed to the light stabilising activity of HALS. These mechanisms consist of transition metals complexation; hydroperoxides and hydroperoxide complexation; and the quenching of polymer-oxygen charge transfer complexes. Putting the properties by which gives light stabilising activity in a nutshell from the body of knowledge that has surfaced through continued use are scavenging of free radicals, deactivation of hydroperoxide, and the creation of charge transfer complexes with oxygen. These light stabilising properties are negatively impacted by the presence of organic thioether costabilisers, halogen flame retardants, and an acidic environment (Pritchard, 1998 (12)). All HALS products contain the nitroxyl active site and so have similar properties in light stabilising. The performance variances demonstrated by the different HALS products stems from their differences in physiochemical properties. The variables of HALS structure that have an impact on the performance are molecular weight, molecular weight distribution, solubility in the proposed matrix, compatibility with the proposed matrix, inherent light stability of the supporting structure, and active sites molar concentration. The additional factors that impact on the performance of HALS are related to piperidinyl nitrogen sterile hindrance of activation kinetics and basicity of the matrix (Vulic et al, 1999 (10)). Since HALS are not conditioned by the Beer-Lambert relationship of absorption and concentration, they prove to be equally efficient in light stabilising at the surface of the coating and at the interior of the coating. Quite often it is observed that their mode of functioning is complementary to ultraviolet absorbers providing synergy when in combination (Ravichandran, Cliff & Renz, 2004 (13)) Applications HALS was developed for the light stabilisation of polymers, and the primary application has remained in light stabilisation of polymers with particular emphasis on polymer applications exposed to sunlight and other specific degrading light sources. As the applications of polymers widened so has the development of later HALS that have extended the application of HALS into more areas of polymer applications such as coatings for protection in the automotive industry and protection of wood furniture products. Essentially the application of HALS is based on the particular polymer employed, the surface material and the required protection and built in at the design stage. The wide range of later developed HALS with more specific durable light stabilising ability in polymers and surface materials has led to specific HALS products being used for different applications. The first generation HALS still remain the most suitable answer for light stabilisation of thick section polyethylene. With properties of low volatility and resistance to extraction the second generation HALS find applications in light stabilising of thin polypropylene and polyethylene items, such as films and fibers. The third generation HALS, which are a combination of monomeric and oligomeric products with enhanced UV protection property is used in applications to provide greater light stabilising in place of single components. The fourth generation of HALS with properties of good thermooxidative and UV light stabilising ability are used in applications where NOx interactions are a problem. The fifth generation HALS with more sites of activity and miscibility are used in thin and thick polymer sections like fibers, coatings, and panels. Recent evaluation of HALS show promise in the light stabilisation off rubbers or rubber applications (Lee, Sanders & Neri, 1998 (14)). In addition to very good light stabilising activity, HALS products also demonstrates very good long term thermal stabilizing effects and are being used in polymer applications exposed to sunlight to derive both these benefits (Lewin, 2007 (15)). Applications of HALS products are not restricted to polymers and rubbers. They have been found useful as additives to varnish as a protection from thermally induced auto-oxidation in the conservation of wood and wood products (Umney & Rivers, 2003 (16). The molecular mass of the various HALS products has an influence on their stabilising effect in polymers. When thick polymers are taken into consideration, there is decrease in the performance of stabilisers with a molecular mass 500 to 800. Hence, there is the need to select the right HALS for optimum stabilisation in different applications (Gugumus, 1999 (17)). Furthermore, the concentration of HALS in the polymer is indicative of the duration of the light stabilising effect. Measurement of the concentration of HALS in the polymer is possible to provide this indication (Dorfman, 2005 (18)). HALS products are however not suitable for application in polymers, where the polymers are exposed to high temperatures like steam and water in artesian bore and pipeline applications. Hydrolysis is responsible for the degradation of the HALS stabiliser (Scheirs, 2000 (19)) Reactions of HALS Suppression of photo-oxidation is the key to the light stabilising effect of HALS. The photo-oxidation effect from HALS is a multi-step process. The first step consists of the parent amine being oxidised to nitroxide. In the second step nitroxides effectively scavenge carbon free radicals developing aminoethers. Typical action in this reaction is the competition that the nitroxides give oxygen to prevent the degradation of the polymer. These amino ethers interact with peroxy radicals to regenerate the nitroxides. Initially it was believed that the product of this interaction were peroxides, but subsequently it was found by Klemchuk and Gande that the product was the more thermally stable and less photoactive ketones. In addition, the aminoether peroxy radical interaction offers competition to the hydrogen abstraction reaction further preventing the progress of degradation of the polymer. Every time a nitroxide aminoether interaction happens two radicals are extracted from the oxidation cycle. The efficiency at which HALS works in hindering photoxidation is dependent on the comparative rates at which oxygen versus nitroxide occurs with the carbon radical and the comparative rates at which hydrogen abstraction occurs versus reaction with aminoether. It must be remembered that it is not necessary for the stabilizing reaction to win over the destabilizing reaction on every occasion. It is enough that the carbon radical reacts with nitroxide more often in comparison to reaction with oxygen than the hydroperoxide in the polymer decomposing to free radicals for HALS to provide its beneficial effect of light stabilisation to the polymer (Bauer, 2001(20)). Literary References 1. Scott, G. 1999, Polymers and the Environment, The Royal Society of Chemistry, Cambridge. 2. Zweiful, H., Maeir, R. D. & Schiller, M. 2009, Plastics Additives Handbook, Sixth Edition, Hanser Verlag, Munich, Germany. 3. Kawahara, N., Saito, J., Matsuo, S., Kaneko, H., Matsugi, T. & Kashiwa, N. 2008, ‘Polymer Hybrids Based on Polyolefin: Synthesis, Structures and Properties’, pp. in New Frontiers in Polymer Synthesis, ed. S. Kobayashi, Springer-Verlag, Berlin, 79-120. 4. Klemchuk, P. P. 1990, ‘History of Stabilizing Additives for Polymers’, in Oxidation Inhibition in Organic Materials – Volume I, eds. Jan Pospisil & Peter P. Klemchuk, CRC Press, Boca Raton, pp.11-32. 5. SpecialChem. 2010, ‘Hindered Amine Stabilisers’, [Online] Available at: http://www.specialchem4adhesives.com/tc/uv-light-stabilizers/index.aspx?id=hals (Accessed June 2, 2010). 6. Pospisil, J. 1990, ‘Antioxidants and Related Stabilisers’, in Oxidation Inhibition in Organic Materials – Volume I, eds. Jan Pospisil & Peter P. Klemchuk, CRC Press, Boca Raton, pp. 33-60. 7. Lau, W. W. Y. & Quing, P. J. 1996, ‘Hindered Amine Light Stabilisers, Monomeric’, in Polymeric Materials Encyclopedia, Volume 5, ed. Joseph C. Salamone, CRC Press, Boca Raton, pp.3054-3067. 8. Irina, S. & Nikolay, P. 2002, ‘Using Hindered-Amine Light Stabilisers for Protecting of Rubbers under Various Types of Aging’, in Polymer Aging at the Cutting Edge, eds. G. E. Zaikhov, A. L. Bouchachenko & V. B. Ivanov, Nova Science Publishers, New York, pp.85-100. 9. Coaker, A. W. 2000, ‘Poly(Vinyl) Chloride’, in Applied Polymer Science 21st Century eds. Clara D. Craver & Charles E. Carraher Jr., Elsevier Science Ltd., Oxford pp.107-184 10. Vulic, I., Samuels, S. B., Wagner, A. H. & Eng, J. M. 1999, New Breakthroughs in Hindered Amine Light Stabiliser Performance, Paper submitted at the International Plastic Additives and Modifiers Conference, October 1999, Prague, Rapra Technology Limited, Shropshire, UK, pp.1-9. 11. Salamone, J. C. 1999, Concise Polymeric Materials Encyclopedia, CRC Press, Boca Raton. 12. Pritchard, G. 1998, Plastic Additives: An A-Z Reference, Chapman & Hall, London. 13. Ravichandran, R., Cliff, N. & Renz, W. L. 2004, ‘Light and Heat Stabilisers for Coatings’, in Handbook of Coating Additives, eds. John, J. Florio & Daniel, J. Miller, Second Edition, Marcel Dekker Inc., New York, pp.159-234. 14. Lee, R. E., Sanders, B. M. & Neri, C. 1998, ‘Stabilizing Effects of Hindered Amines’, in Conference Proceedings at ANTEC 1998, April 1998, Atlanta, Society of Plastic Engineers, Connecticut, pp.2881-2885. 15. Lewin, M. 2007, Handbook of Fiber Chemistry, Third Edition, CRC Press, Boca Raton. 16. Umney, N. & Rivers, S. 2003, Conservation of Furniture, Butterworth-Heinemann, Oxford. 17. Gugumus F. 1999, ‘Aspects of the Impact of Stabilizer Mass on Performance in Polymers, Polymer Degradation and Stability, vol.66, no.1, pp.133-147. 18. Dorfman, R. M. 2005, ‘Thermal Spray Coatings’, in Handbook of Environmental Degradation of Materials, Volume 0-8155, ed. Myer Kutz, William Andrew Inc., New York, pp.405-422. Schiers, J. 2000, Failure Analysis of Polymers: A Practical Approach, John Wiley & Sons, Ltd., Chichester, UK. 19 Bauer, D. 2001, ‘Stabilization of Coatings’, in Plastics and Coatings: Durability-Stabilization Testing, ed. Rose A. Ryntz, Hanser Gardner Publications, Cincinnati, Ohio, pp.99-115. Read More