Formation of polymer-bonded nitroxyl radicals in the UV stabilization of polypropylene by a bifunctional hindered amine light stabilizer

The formation of polymer-bonded stabilizer derivatives during photooxidation of isotactic polypropylene, which contains the hindered amine light stabilizer bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate and the corresponding bisnitroxyl radical, was investigated. The photooxidized polypropylene films that contain these additives were studied by ESR, IR, and nitrogen analysis before and after exhaustive solvent extraction of the photooxidized films. ESR showed that under the conditions in use a maximum of 20% of the nitroxyl radicals formed from the hindered amine was bonded to the polymer chain. Regeneration of nitroxyl radicals from the polymer-bonded stabilizer derivatives under photooxidative conditions indicated that the stabilizer was bonded to the polymer chain by O-alkyl-substituted hydroxylamine linkages >N? O? PP.     

Synthesis of a water-soluble macromolecular light stabilizer containing hindered amine structures

A novel water-soluble macromolecular light stabilizer was synthesized by grafting 2-chloro-4,6-bis- [(2,2,6,6-tetramethylpiperidin-4-yl)amino]-l,3,5-triazine onto polyvinylamine.The intermediate 2- chloro-4,6-bis-[(2,2,6,6-tetramethylpiperidin-4-yl)arnino]-1,3,5-triazine and the obtained macromolecular light stabilizer were characterized by ~1H NMR,HRMS,IR and UV spectroscopy.Cotton fabrics dyed with C.l.Reactive Yellow 145,C.I.Reactive Red 195 and C.I.Reactive Blue 19 were finished with the macromolecular light stabilizer,and the lightfastness of the dyes was tested.The results showed that the lightfastness of the reactive dyes was improved by 0.5-1.0 grade after being finished and the macromolecular light stabilizer exhibited good wash fastness and thermal stability.     

Synthesis and characterization of core-shell polyacrylate particles containing hindered amine light stabilizers

A polymerizable hindered amine light stabilizer (HALS) 1,2,2,6,6-pentamethylpiperidin-4-yl acrylate (PMPA) was synthesized through transesterification of 1,2,2,6,6-pentamethylpiperidin-4-ol (PMP) with methyl acrylate (MA). Core-shell latex particles containing HALS moieties in the shell phase were prepared by two-stage seeded emulsion polymerization from n-butyl acrylate (BA), methyl methacrylate (MMA) and PMPA. The Fourier transformed infrared (FTIR) and nuclear magnetic resonance (¹H NMR) analysis showed that PMPA monomer was successfully prepared and was effectively involved in the polyacrylate particles. The surface composition was studied by X-ray photoelectron spectroscopy (XPS), and the results indicated that HALS-containing groups could be distributed on the surfaces of the particles. Transmission electron microscopy (TEM) analysis revealed that the particles obtained presented a core-shell structure with a particle size around 100 nm. Two glass transition temperatures (T_g), assigned to the core phase and the shell phase of the particles, respectively, were observed for both HALS-containing and HALS-free particles, as determined by differential scanning calorimetry (DSC). In addition, the T_g value for the shell phase of HALS-containing particles was 13 °C lower than that of HALS-free particles, indicating the presence of random copolymer between MMA monomer and PMPA comonomer in the shell phase. The thermogravimetry analysis (TGA) and differential thermal gravimetric (DTG) results showed that HALS-containing particles provided an improvement in thermal stability in comparison to HALS-free particles.     

The oxidation of hindered amine light stabilizers to nitroxy radicals in solution and in polymers

Cyclohexane and three polyolefins (PE, PP and EPM) have been oxidized by γ-irradiation, and the concentration of nitroxy radicals derived from a typical HALS added either before or after irradiation has been studied by ESR spectroscopy. The results indicate that HALS is rapidly oxidized by peroxy radicals under irradiation and that afterwards it can be oxidized more slowly by hydroperoxides. This last reaction is, however, very limited.     

Rapid determination of oligomeric hindered amine light stabilizers in polymeric materials

Hindered amine light stabilizers are essential for the stabilization of synthetic polymers, particularly for materials used for outdoor applications. Although up to now a considerable number of studies dealing with the analytics of this class of stabilizers had been published, especially the determination of oligomeric hindered amine light stabilizers is still an analytical challenge. In the current work, a fast and simple liquid chromatographic method for the quantitative determination of oligomeric hindered amine light stabilizers is presented. A key aspect of this method is their completely different retention behavior depending on the pH, enabling a single peak elution approach by a pH gradient run. This allows a quantitation with simple UV detection independent of the actual oligomeric composition. Calibration curves within the concentration range relevant for the analysis of real polymer samples (LOQ = 70 mg/L) were constructed with R ² values above 0.99. Spiked extracts from polyolefin samples showed recovery rates between 97.3 and 102.9% for five different commercial hindered amine light stabilizers. Relative standard deviations were between 2.0 and 3.9%. Furthermore, it was demonstrated that the employed approach can be easily adapted for mass spectrometry detection.     

A study of the photolysis of a commercial hindered amine light stabilizer

The photolysis of a commercial hindered amine light stabilizer (HALS) has been studied with a range of experimental methods. The results show that the stabilizer is easily photolyzed and photolysis mechanisms are proposed.     

Determination of a polymeric hindered amine light stabilizer in polypropylene formulated with magnesium hydroxide flame retardant by reactive thermal desorption-gas chromatography.

A polymeric hindered amine light stabilizer (HALS), Tinuvin 622 (MW [symbol: see text] 4000), in PP materials formulated with a magnesium hydroxide flame retardant was determined by reactive thermal desorption (RTD) gas chromatography (GC). Two kinds of the HALS components that were formed through the RTD in the presence of tetramethylammonium hydroxide [(CH3)4NOH, TMAH] were clearly observed in the chromatograms of the PP samples, with negligible interference from the other additives and the PP substrate. Here, the coexisting flame retardant was proved to affect significantly the RTD process of the occluded HALS. As a result, the recovery of the HALS components in the RTD-GC chromatograms of the PP samples increased with increase in the content of the flame retardant. This enhancement of the HALS recovery is attributed mainly to the preferential exposure of the HALS on the surface of the ground PP sample through the interaction between the polymeric HALS and the flame retardant in the molten PP during kneading. In spite of such a considerable action of the flame retardant, the observed intensities of the characteristic peaks of HALS by RTD-GC showed a good linear relationship with the HALS content in the PP samples with constant content of the flame retardant (50 phr); this relationship could be used as the calibration line for the determination of the polymeric HALS in the PP materials containing the flame retardant.     

Highly sensitive determination of a polymeric hindered amine light stabilizer in polypropylene by reactive thermal desorption-gas chromatography using nitrogen-specific detection

Highly sensitive and specific determination of trace amounts of a polymeric hindered amine light stabilizer (HALS) in polypropylene (PP) materials could be established by improving reactive thermal desorption-gas chromatography (RTD-GC) in the presence of an organic alkali, tetramethylammonium hydroxide. By using nitrogen-phosphorus detection, highly selective detection of the HALS-related components was attained. In addition, the use of a polar poly(ethylene glycol) separation column alleviated the adsorption of minor specific pyrolysis products. This modified RTD-GC method allowed the determination of the polymeric HALS (Mr 1900) in PP even for trace concentrations between 100 and 500 ppm, through observing selectively the characteristic products containing a tetramethylpiperidine moiety, which had been impossible to detect under the previous RTD-GC conditions using a non-polar separation column and conventional flame ionization detection.     

The effect of hindered amine light stabilizers on the photooxidative stability of high-density polyethylene

The photoprotective effectiveness of various polymeric and nonpolymeric hindered amine light stabilizers (HALS) was determined by exposing samples of high-density polyethylene (HDPE) containing these additives to ultraviolet (UV) light and measuring the resultant oxygen uptake characteristics. Values of the initial quantum yield for oxygen uptake calculated for these formulations indicate that the higher molecular weight HALS compounds are less effective photostabilizers than the nonpolymeric HALS, and this is partly attributable to their decreased mobility in the polymer matrix. It was further found that the addition of an ultraviolet absorber (UVA) to a formulation containing a polymeric HALS compound enhances its photostability, although this phenomenon may be partly due to synergism between the UVA and the antioxidant, the latter having been added as part of the base stabilization. The antagonism which exists between certain sulfur-containing antioxidants and HALS compounds was also investigated and it was found that the lower molecular weight sulfur-containing antioxidants exhibit the greatest degree of antagonism. The results confirm that the mobility in the polymer matrix of the stabilizer system can serve as an explanation of its effectiveness. The article provides evidence that the technique of oxygen uptake monitoring is a sensitive and rapid method of assessment of polymer photostability in the presence of stabilizer systems.     

Quantitative analysis of an oligomeric hindered amine light stabilizer in polypropylene by matrix-assisted laser desorption/ionization mass spectrometry using a solid sampling technique

A small amount of an oligomeric hindered amine light stabilizer (HALS) (Adekastab LA-68LD) in polypropylene (PP) materials was directly determined by solid sampling matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) using an internal standard method. First the matrix reagent (dithranol), 20 mg, and the empirically selected internal standard, angiotensin I (MW = 1296.5), 5 microg, were premixed in the solid state. The matrix mixture was then co-ground with the PP sample containing the HALS in liquid nitrogen using a freezer mill. The powdered sample mixture was spotted on the sample plate, suspended in ion-exchanged water, dried to adhere on the plate, and subjected to MALDI-MS. Three series of the HALS components accompanied by the oxidized species were clearly observed as their molecular ions (M*+)) along with that of the internal standard in the mass spectra. A fairly good linear relationship (R2 = 0.9991) with a relative standard deviation of ca. 11% was observed between the relative peak intensities of the HALS components and the HALS contents ranging from 0.1-2.5 wt%, which could be used as the calibration line to determine the HALS content in PP composites directly by MALDI-MS. The UV-exposed PP composite samples were evaluated by this method to interpret the photostabilizing action of HALS in the PP materials based on the observed change in the relative abundances of the original and oxidized HALS components as a function of UV-exposure time.     

Reactions of stabilizer compounds. III. Oxidative reactions of some 2′-hydroxy-2-phenylbenzotriazole light stabilizers in the AIBN-initiated autoxidation of cumene

Oxidative decay of number of commercially important 2′-hydroxy-2-phenylbenzotriazole light stabilizers during the AIBN-initiated autoxidation of cumene at 65°C is described. The reactivity of the hydroxyphenylbenzotriazoles studied increased in the order 2-(2′-hydroxy-5′-methyl)phenylbenzotriazole (Ia) < 2-(2′-hydroxy-3′,5′ -di-tert-pentyl)phenylbenzotriazole (Ie) < 5-chloro-2-(3′,5′-di-tert-butyl-2′-hydroxy)phenylbenzotriazole (Ic) < 5-chloro-2-(3′-tert-butyl-2′-hydroxy-5′-methyl)phenylbenzotriazole (Ib). The major product from the reaction of Ib was identified as the cumylperoxycyclohexa-2,5-dienone (III). The possible occurrence of these reactions during the degradation of stabilized polymers is discussed.     

The determination of small amounts of organic hydroperoxides in the presence of hindered amine light stabilizers and their nitroxyls

A method for the quantitative determination of hydroperoxide (ROOH) in the presence of hindered amine light stabilizers (HALS) and/or their nitroxyl free radical derivatives has been elaborated. The method is based on the quantitative reduction of hydroperoxides by triphenyl phosphine. The resulting compounds (alcohols) are then determined by GLC using the internal standard technique. The method has been tested on the hydroperoxides derived from 2,4-dimethylpentane. Its sensitivity and reproducibility appear to be comparable with other methods for ROOH determination but, unlike the latter, it has the advantage that its results are not influenced by the presence of HALS and/or their nitroxyl radical derivatives in the analyzed medium.     

Photostabilization study of ethylene-butyl acrylate copolymers functionalized in the molten state with hindered amine light stabilizers (HALS)

Hindered Amine Light Stabilizers (HALS), N-methylated and O-alkylhydroxylamine, were successfully anchored onto an ethylene-co-butyl acrylate (BA) by a transesterification reaction in the molten state. The content of HALS bonded to the EBA was calculated from the atomic ratio (N/C) determined by X-ray Photoelectron Spectroscopy (XPS). After reaction times of 5–30 min in the molten state the content of bonded HALS attained values from 0.17 to 0.63%. The modified EBA with HALS were studied by Chemiluminescence emission and the photostabilization effect was evaluated under accelerated ageing in the presence of pesticides. The antioxidant effect of HALS bonded to EBA was evaluated by determination of Carbonyl Index (CI) and retention of elongation at break as a function of exposure time and pesticide treatment. The stability of the EBA with anchored HALS exhibited higher performance as shown by low CI-values and correlated with the content of bound HALS and the thermal history of the materials. Also, mechanical properties correlated well with the CI results and the photostabilization of bound HALS was effective until 3150 h of exposure time using pesticides.     

A study of free-radical formation in reactions of aliphatic nitro compounds using radical-trap methods 3. Generation of α-halodinitromethyl radicals and their identification as nitroxyl radicals by ESR methods

Conclusions Radical-trapping methods have been used to show that the photodecomposition of the halotrinitronethanes XC(N0₂)₃ (X=F, Cl, Br) in solution (acetonitrile, diglyme) proceeds through C-N bond cleavage. Electron spin resonance methods have been used to detect and identify the short-lieved XC(NO₂)₂ radicals (X=F, Cl, Br) in the form of the corresponding nitroxyl radicals.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1793–1798, August, 1978.     

ESR imaging and FTIR study of thermally treated poly(acrylonitrile-butadiene-styrene) (ABS) containing a hindered amine stabilizer: Effect of polymer morphology, and butadiene and stabilizer content

Electron spin resonance imaging (ESRI) was applied to the study of thermal degradation at 393 K of poly(acrylonitrile-butadiene-styrene) (ABS) prepared by emulsion polymerization and containing 25% wt butadiene (ABS-25B). The polymer was doped with 1 or 2% wt Tinuvin 770 as the hindered amine stabilizer (HAS). The spatial distribution of the HAS-derived nitroxide radicals, obtained by 1D ESRI, was initially homogeneous, but became heterogeneous through sample depth with increasing treatment time, t. The spatial variation of ESR line shaping with sample depth was visualized by 2D spectral-spatial ESRI. ESR spectra along the sample depth, obtained by nondestructive (“virtual”) slicing of the 2D images, were used to deduce the relative intensity of nitroxide radicals present in two dynamically distinct sites; the sites were assigned to butadiene-rich (fast component) and SAN-rich domains (slow component), respectively. 1D and 2D ESRI allowed the determination of the extent of degradation within morphologically-distinct domains as a function of sample depth and treatment time. The results from the ESRI experiments were substantiated by attenuated total reflectance (ATR)-FTIR spectroscopy of the outer layer (500 μm thick) of the polymer. Both techniques indicated faster degradation of polymer samples that contained the higher HAS content, 2% wt. Comparison with the results obtained for a parallel study of ABS prepared by mass polymerization and containing 10% wt butadiene (ABS-10B) indicated clearly that the rate of degradation of the polymer prepared by emulsion polymerization (ABS-25B) is significantly reduced. This result can be explained by the formation of cross-linked “composite” networks during emulsion polymerization, which leads to greater thermal stability.     

ESR studies of the photodegradation of polyethylene containing trapped allylic free radicals

Changes produced in γ-irradiated polyethylene by subsequent ultraviolet irradiation have been investigated by ESR measurements, ultraviolet spectroscopy, and viscometric determination of average molecular weight. The photoinduced changes depend on the wavelength of irradiation. Upon irradiation at wave length greater than 3900 Å, main-chain scission occurs by reaction of trapped allylic radicals: A reduction in molecular weight sndicated by this reaction was verified by fractionation experiments and molecular weight determinations.