HALS stabilizers

From the standpoint of stabilization, complex formation is certainly an advantage. It means namely that the HALS stabilizer is already located preferentially at the site where degradation is initiated. 

Most plastics e.g. polyolefins and polystyrenes and their derivatives such as ABS (acrylonitrile-butadiene-styrene) and SAN (styrene-acrylonitrile) are supplied by the manufacturers in ready-to-use form with most of the above-mentioned stabilizers or simply need to be additionally stabilized with other additives, e.g. antistatic agents and HALS stabilizers, as required. On the other hand, in the case of other materials (e.g. PVC) it is the end user who adds the additives, pigments or preparations. This is normally done on fluid or high-speed mixers, although in the past gravity mixers or tumble mixers were also used. The mixture is then homogenized on mixing rolls, kneaders, planetary extruders or twin-screw kneaders and further processed. 

Apart from paints, P.R.224 is also used in polyacrylonitrile spin dyeing. Application in the spin dyeing of polypropylene is compromised by the fact that medium to high pigment concentrations accelerate the degradative action of light on HALS stabilizers (Sec. 3.4.1.4). 

Special considerations chemical composition of filler surface affects nucleation of filler traces of heavy metals decrease thermal stability and cause discoloration siuface free energy of fillers determines interaction large difference in thermal properties of fillers and polymer may cause stress hydrotalcite is used as acid neutralizer with stabilizing packages anatase titanium dioxide decreases UV stability presence of transition metals (Ni, Zn, Fe, Co) affects thermal and UV stability calcium carbonate and talc were found to immobilize HALS stabilizers in PP with organic masterbatches such as ethylene diamine phosphate V-0 classification can be obtained with 20-25 wt%, at the same time tensile strength and impact strength are substantially reduced... 

Stabilization. For example, calcium stearate may play the role of an associate thermal stabilizer when used in a system with calcium salts of fatty acids. These stabilizers use combinations of two or more metals - one of which (e.g. zinc) produces metal chlorides which accelerate PVC degradation. The presence of large amount of calcium salts helps to convert this chloride to calcium chloride which does not increase the degradation rate of PVC. Also, calcium carbonate can react with hydrogen chloride which is produced as PVC degrades. On the other hand, inclusion of fillers which contain admixtures of metals such as iron, nickel, copper, etc. reduces PVC thermal stability. Fillers also affect UV stabilization by adsorption of HALS stabilizers which immobilizes them and prevents them from performing as radical scavengers. 

This process is very important in the HALS stabilization mechanism. It should, however, be considered only as a minority pathway in diarylamines just because of the lower stability of the corresponding diarylnitroxides. This results in the participation of the latter in side reactions leading to antioxidant ineffective species, e.g. benzoquinone (BQ) and nitrobenzene ( 7 ). Transformation of diarylnitroxide into a mixture of diarylamine and N-aryl-1,4-benzoquinone monoimine-N-oxide (4. 8) seems therefore to be a more probable pathway regenerating partly amine antioxidant than the hydroxylamine/nitroxide cyclical process (Scheme 2). 

HALS was based on the discovery that the 2,2,6,6-tetramethyl-l-piperidinyloxy, free radical (TEMPO) (1)), which already was known as an effective radical scavenger [46,47], was a very effective UV stabilizer too [48,49]. However, due to its physical and chemical properties TEMPO itself did not led to practical use. TEMPO is colored and will impart color to the to be stabilized polymer, it is thermally unstable and volatile [49]. Furthermore, it reacts with phenolic antioxidants present in many polymers leading to a reduction of processing and/or long-term heat stability. The discovery that compounds in which the /V-oxyl functionality was replaced by a N—H functionality also showed good UV stabilization activity was the key finding that led to the development of HALS stabilizers [49]. 

Although HALS stabilizers were developed as UV stabilizer it is more and more recognized that these molecules could also impair long-term heat stability. Especially were phenolic antioxidants cannot be used due to their discoloration HALS is used to protect the polymer against long-term heat degradation [50]. 

Nowadays, there are many HALS stabilizers commercial of which the majority is based on 2,2,6,6-tetra-methyl-4-piperidinyl moieties. The first commercial type (LMW-HALS-1) is relatively low in molecular weight, which caused that due to its high volatility it is not suitable for thin applications. To overcome this problem oligomeric HALS types were developed. One of the drawbacks of the piperidinyl moiety of HALS stabilizers is that it is basic causing that it can react with acids and forms a not stabilizing salt. Consequently, the effectiveness of HALS in systems where acids are present or can be formed is limited. To beat this problem less basic HALS types as, for example, N-O-R types were developed. 

Almost all HALS stabilizers are derivatives of 2,2,6,6-tetramethylpiperidin-4-one, commonly known as triacetoneamine (TAA) (2) ... 

As shown in Scheme 17.9, TAA derivatives can be made by hydrogenation or hydroamination with ammonia, butylamine or hexamethylene diamine leading to intermediates for the most commercial HALS stabilizers as, for example, HMW-HALS-2, HMW-HALS-4, or HMW-HALS-6 [53],... 

Based on the results of the degradation of unstabilized PP and the effectivity of HALS stabilizers in different model systems it is postulated that HALSs are mainly effective in systems in which peracids can be formed [50]. According to Step et al. [63,64] this is because in the nitroxide regeneration mechanism the peracyl radical plays a key role (Scheme 17.13). 

In many stabilizer formulations, different types of stabilizers are used. As processing stabilizer phenolic antioxidants and phosphites are applied, for long-term heat stability phenolic antioxidants and thioethers are used and for UV stability, combinations of HALS with other types of UV stabilizers can be applied. HALS stabilizers show interactions with these types of stabilizers that can lead to synergisms as well as antagonisms. 

Since the levels of flame-retardants used in practice (up to 30%) exceeds many times those of HALS (up to 1%), even a small conversion of the flame-retardant to HBr can cause complete transformation of HALS to its ammonium salt [ 109]. This leads to total deactivation of HALS and mins the HALS stabilizing performance [110],... 

Although in many cases organic stabilizers are used to protect polyamides against photo-oxidation, in a recent study it was evidenced that a mixture of CuCl2 and KI has a far better long-term stabilizing efficiency than a HALS Stabilizer (HMW-HALS-2) [132],... 

In contrast to polyamides in polystyrene phenolic antioxidants are not able to reduce the decrease in tensile strength during UV irradiation [141]. UVAs as well as HALS stabilizers are effective in polystyrene (PS). The best protection can be reached by using combinations of a UVA and a HALS. According to Gugumus [133,134] the synergism between UVA and HALS is distinct when the time until a AYI = 5 is taken as failure criterion (see Table 17.12). However, when as criterion the yellowness index after 1600 h in a Weather-Ometer is taken, the synergism between a UVA and a HALS is much smaller (see Fig. 17.14). 

P. Gijsman and A. Dozeman, Comparison of the UV-degradation chemistry of unstabilized and HALS-stabilized polyethylene and polypropylene, Polym. Degrad. Stab. 1996, 53, 45-50. 

J.H. Khan and S.H. Hamid, Durability of HALS-stabilized polyethylene film in a greenhouse environment, Polym. Degrad. Stab. 1995, 48, 137-142. 

P. Delprat, X. Duteurtre, and J.-L. Gardette, Photo-oxidation of unstabilized and HALS-stabilized polyphasic ethylene-propylene polymers, Polym. Degrad Stab. 1995, 50, 1-12. 

Steinlin and Saar reported the effect of various concentrations of pigments on the light stability of fine-denier polypropylene yarn (72 denier, 24 filament) containing varied levels of Tinuvin 770 [145]. Although Tinuvin 770 is not a preferred HALS stabilizer for fine-denier fibers, the data presented are of considerable interest. (The authors pointed out that Tinuvin 622 and Chimassorb 944 are preferred for polypropylene fibers.) The amount of energy required to reduce yarn strength by 50% was determined. Tinuvin 770 was added at 0.25,... 

Figure 4.16 Cyclic mechanism for HALS stabilization through radical trapping. AIcchcl and carbonyl speoies are also produced, which can have degrading effects,...

Sample Preparation for Nitroxide Measurements in HALS Stabilized Coatings 264... 

The first step, R6, converts the HALS initially added to the clearcoat, parent HALS, into inhibition cycle, R7 and R8, products. These reactions compete with R2 and R3 lowering the stationary radical concentration, which in turn lowers the hydroperoxide concentration and the photooxidation rate. The rate constants and radical concentrations are such that only a small fraction (—5%) of the HALS stabilizer is in the form of nitroxide. Although nitroxides are thermally stable, they are not pho-tolytically stable. Nitroxides absorb light, and excited-state nitroxides can abstract hydrogen atoms to initiate free-radical formation. These reactions have been discussed in detail. "Reactions R9 and RIO are important both for the nitroxide decay measurement of free radical formation and in limiting the ultimate effectiveness ofHALS.i° i5... 

Both the nitroxide decay measuranents of free-radical photoinitiation rates and nitroxide kinetics during HALS stabilization depend on accurate, quantitative measurements of nitroxide concentrations in cross-linked polymers. Quantification of radical concentrations by ESR requires a suitable primary standard, careful sample preparation, a reference standard with which to monitor spectrometer performance, and most important, reproducible positioning of the samples in the resonance cavity of the spectrometer. Most of the experiments described here were carried out with a Bruker-IBM ER 200 D spectrometer equipped with an Aspect 2000 Data System. Because these coatings are cured at temperatures as high as 130°C, the primary nitroxide standard, which was introduced into the coating prior to cure, had to be... 

Different HALS stabilizers were added to coatings and cured on quartz plates or disks described above. Nitroxide concentration was quantified as a function of exposure time via the primary standard, I. The total amount of parent HALS and HALS-based inhibition cycle products was determined by oxidizing CH2CI2 extracts with p-nitroperbenzoic acid. " ... 

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