A Detailed Introduction to Hindered-Amine Light Stabilizers (HALS)

Light stabilizers prevent polymer materials from photoaging and greatly extend their service life, with very significant effects. At present, in many long-term outdoor or light-exposed polymer products—such as agricultural plastic films, military equipment, plexiglass, daylighting materials, building materials, light-resistant coatings, medical plastics, bulletproof laminated glass, synthetic fibers, industrial packaging materials, and rubber products—light stabilizers are an indispensable additive component. As the application fields of polymer materials expand, light stabilizers will surely develop further. There are many types of light stabilizers, generally divided into four categories by mechanism: 1. Light screening agents, including carbon black, zinc oxide, and some inorganic pigments; 2. Ultraviolet absorbers, including salicylates, benzophenones, benzotriazoles, triazines, substituted acrylonitriles, and other organic compounds; 3. Quenchers, mainly nickel organic compounds; 4. Free-radical scavengers, mainly hindered-amine derivatives. At present, hindered-amine light stabilizers are the fastest-growing category in the world. Because hindered amine light stabilizers (HALS) have the ability to capture free radicals and decompose peroxides, with a stabilization effect 2-4 times higher than ultraviolet absorbers, since Japan first developed HALS in the 1970s, it has received widespread international attention, been widely developed and applied, and become one of the fastest-growing additives. Entering the late 1990s, with the development of HALS's heat- and oxygen-stabilization functions, HALS found new application fields. 1. Mechanism of HALS HALS is a kind of free-radical scavenger whose light-stabilizing performance exceeds any previous light stabilizer. It can be used as a general high-efficiency light stabilizer for plastics, synthetic fibers, and coatings, and provides good protection for both thick and thin products. Its application in agricultural film is uniquely advantageous: it not only extends the service life of agricultural film but also transmits long-wave UV beneficial to crop growth, and is therefore highly valued. The mechanism of HALS is very complex; it is internationally recognized that HALS achieves photo-protection through the synergy of at least the following mechanisms. 1.1 Capturing free radicals The hindered-amine functional group belongs to an alicyclic amine structure; it itself does not absorb any light with a wavelength above 260 nm, nor can it quench excited-state chromophores. However, after absorbing light in the presence of oxygen, the hindered amine is converted into the corresponding nitroxyl radical NO·, which can not only capture the alkyl active free radicals generated during the photo-oxidative degradation of polymer materials but also has a regenerative function during the light-stabilization process, thereby inhibiting the chain reaction and achieving protection.
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1.2 Decomposing hydroperoxides The presence and accumulation of hydroperoxides is the root cause of initiating polymer photo-oxidative degradation. It is generally believed that after the amine group combines with the hydrogen in the hydroperoxide to form a nitroxyl radical, it then reacts with the active free radical, converting it into a stable alcohol/ketone compound, thereby protecting the polymer material. The specific process is shown in the following formula:
It can be seen that the hydroperoxide-decomposing action of HALS differs from that of phosphorus- and sulfur-based auxiliary anti-oxidants, which are only effective when used together with phenolic anti-oxidants. The former obtains the highly efficient free-radical scavenger NO· while decomposing hydroperoxides, showing the self-synergy of capturing free radicals and decomposing hydroperoxides. 1.3 Capturing heavy metals Polymer materials may have residual metal catalysts, or metal or metal salts mixed in during processing or use, which can promote hydroperoxides to split into free radicals, and the presence of hydroperoxides accelerates the destruction of polymer materials. The nitrogen in HALS has lone-pair electrons that coordinate strongly with metal ions in polymer materials, thereby protecting the polymer material. 1.4 Quenching singlet oxygen Singlet oxygen is an electronically excited-state molecular oxygen with high chemical activity that can initiate degradation reactions of polymers. Most of the singlet oxygen present in polymer materials comes from stable molecular oxygen via photophysical processes, as well as from the decomposition of ozone complexes or peroxides of condensed-ring aromatics.
2. Classification of HALS Light Stabilizers
Divided by the parent molecular structure of HALS, they can be roughly divided into three categories: piperidine derivatives, piperazine derivatives, and imidazolidinone derivatives. At present, piperidine and piperazine derivatives are the most researched and have the largest market demand.
3. Domestic and Foreign Development Trends
3.1 Foreign Development Trends The first climax of HALS R&D appeared in the 1970s; now a second climax has entered, with many second- and third-generation HALS varieties appearing. The development trend shows the following characteristics.
3.1.1 High Molecular Weight High-molecular-weight HALS can improve migration and extraction resistance, increase compatibility with polymers, and also reduce the toxicity and basicity of HALS. At present, many polymeric high-molecular-weight HALS have been commercialized, such as Tinuvin 622, 2,4-dihydroxybenzophenone 944, and GW3346. These three HALS have been approved by the U.S. FDA for use in plastic products that contact food, expanding the application scope of HALS.
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But because such HALS are difficult to control in degree of polymerization, their quality varies greatly; some HALS have many undispersed crystalline points when applied to films, which has a certain impact on processing.
3.1.2 Low Basicity Traditional HALS light stabilizers have an N-H group on the piperidine ring, giving them a certain basicity, which limits their application in acidic resins, acidic compounding agents, and acidic environments. To broaden the application scope of HALS, low-basicity research must be carried out. The main approach at present is to change the substituent groups on the piperidine ring into substituted alkyl and substituted alkoxy groups: the N-alkylation (N-R) HALS, when entering the light-stabilization chain cycle, due to the presence of the alkyl group, causes the acidic group to be sterically hindered from contacting the active nitrogen, i.e., reduces the basicity. And the N-alkoxy (NOR) HALS not only reduce the electron cloud density of the active nitrogen due to the introduction of the alkoxy group, thereby reducing the reactivity of the nitrogen, but also, because their structure can directly enter the chain cycle in which the hindered amine exerts its stabilizing effect, avoid the problem that the process of traditional hindered amines generating nitroxyl radicals is delayed or blocked by chemicals, damaging the chain cycle that exerts the light-stabilization activity. Many varieties of N-R and NOR HALS have appeared, such as 2,4-dihydroxybenzophenone 119, Tinuvin 123, Tinuvin 317, and 2,4-dihydroxybenzophenone 2020.
3.1.3 Multi-Functionality In order to improve the efficacy of HALS products and further expand their application scope and use efficiency, people have also conducted functionalization research on HALS, and some products have appeared on the market, for example Tinuvin 492 is drug-resistant; Hostavin N30 is beneficial to the heat-sealability of film products; Tinuvin 6922 is beneficial to the transparency of film products; 2,4-dihydroxybenzophenone 944 has excellent thermal-aging resistance, etc. In recent years, reactive HALS have also emerged: using reactive groups to bond HALS onto the polymer main chain, showing a "permanent" light-stabilization effect. Such products are very expensive and are still in the research stage. 3.2 Domestic Production and Development Trends China's development of HALS began in 1974. In the late 1970s, the Shanxi Institute of Chemical Industry took the lead in launching the first domestic HALS variety, GW540, which organically combines the hindered-amine functional group with a phosphite structure and is a multifunctional HALS that still dominates the domestic consumer market. Other series of products include GW508, GW608, GW650, and GW310. The products of the Beijing Institute of Chemical Industry include Jing 5 and Jing 6; the Institute of Chemistry of the Chinese Academy of Sciences has PDS, etc. PDS is a polymeric hindered-amine light stabilizer with good light-stabilization effect, heat resistance, extraction resistance, and compatibility with resins, but its anti-oxidation is poor; when used with certain anti-oxidants, it may still produce an antagonistic effect like other traditional HALS. In the past 10 years, China has built and commissioned more than 10 sets of HALS raw-material and product units; in addition to supplying the domestic market, some raw materials and products have also entered the international market. The newly developed domestic next-generation polymeric light stabilizer GW622 has a 300 t/a unit built at the Huashan Auxiliary Factory in Chaoyang District, Beijing; the Beijing Chemical Industry Research Institute combined GW622 with the composite anti-oxidant B215 to form a non-toxic anti-aging system, replacing the flawed GW540-2002 system for use in polyethylene agricultural film, with an anti-aging effect of more than 18 months; the GFW100 HALS developed and produced by the Institute of Chemistry of the Chinese Academy of Sciences has good compatibility with resins and safe processing, and can be used in low-density polyethylene agricultural film and cable manufacturing; the Shanxi Institute of Chemical Industry successfully developed the process route for synthesizing triacetone amine directly from acetone and ammonia, and on this basis undertook the development of the polymeric HALS GF901 and GF902 and the reactive HALS GW-628 and UV-419 using triacetone amine as raw material, and has now achieved breakthrough progress; the non-basic HALS variety GW757 is also under exploration. In addition, other domestic research institutes have also achieved certain results in HALS research. With the rapid development of China's petrochemical industry, polyolefin output will increase sharply, and the minimum demand for various stabilizers will also increase accordingly. It is estimated that in 2005 and 2010, China's demand for light stabilizers will exceed 3,500 t and 5,000 t respectively. It can be seen that in the next few years, China's demand for light stabilizers will grow rapidly, while some existing products are facing elimination, which will inevitably cause a large shortage of light stabilizers, especially HALS products.