Mechanisms of antioxidant action. Effect of the polymer medium on the antioxidant efficiency of the dithiolates

Peroxide decomposing antioxidants (e.g. nickel dithiophosphates and thiophosphoryl disulphides) control hydroperoxide formation during processing and on exposure to light. However, these additives are more efficient u.v. stabilizers in polypropylene (PP) than in low density polyethylene (LDPE). It is suggested that this difference results from the more rapid formation of hydroperoxides in the more oxidisable substrate under normal processing conditions. In contrast, nickel xanthates are completely destroyed in PP under the same processing conditions and the transformation products obtained in this case are less effective u.v. stabilizers than the original xanthates. Nickel dialkyl dithiophosphates stabilise both LDPE and PP very effectively, while nickel alkyl xanthates are much less effective u.v. stabilisers in both matrices. However, the difference between the efficiencies of the two dithiolates is much less in the case of LDPE. The nickel dithiophosphates and xanthates effectively synergise with the commercial u.v. absorber Cyasorb u.v. 531 (HOBP) but they show antagonism towards a typical chain breaking antioxidant, Irganox 1076, during u.v. exposure. They are however synergists under thermal oxidative conditions.     

The mechanism of u.v. stabilization of polypropylene films by 2-hydroxy-4-octyloxybenzophenone

The rates of oxygen absorption by unstabilized and u.v.-stabilized polypropylene (PP) films irradiated with a high-pressure mercury lamp have been determined as a function of the light intensity (I). The induction time is nearly proportional to I^?0.5 for unstabilized PP and for PP stabilized with Tinuvin P, and proportional to about I^?0.9 for PP stabilized with 2-hydroxy-4-octyloxybenzophenone (HOBP). The change in concentration of HOBP in PP during the photo-oxidation is determined with radioactive HOBP. During the induction time, the observed decrease obeys zero-order kinetics even when the starting concentration of HOBP is as low as 0.008%. The decrease is caused by reaction of HOBP with the polymer and by fragmentation into gaseous products. These results are interpreted on the basis of the same reaction scheme as that worked out for the uninhibited and inhibited autoxidation of PP. It is concluded that HOBP acts not only as a u.v. absorber or as an excited-state quencher, but also as a scavenger of free radicals.     

Photo-stabilising action of orthohydroxy aromatic compounds in low density polyethylene film: UV absorption versus radical scavenging

The importance of uv absorption versus radical scavenging in the photo-stabilisation of low density polyethylene by various concentrations of 4-methoxy and 4-n-octoxy substituted 2-hydroxybenzophenones and 2(2′-hydroxy-3′-t-butyl-5′-methylphenyl)-5-chlorobenzotriazole is examined using infra-red and second-order derivative uv absorption spectroscopy and hydroperoxide analysis. Unlike our earlier study with polypropylene, the stabilisers were found to be just as effective when used as screens as when directly present in the polymer matrix. This effect was particularly prominent at concentrations >0·1 % w/w under both polychromatic light of λ's > 300 nm and monochromatic light of 365 nm wavelength. Under 254 nm wavelength exposure screening appeared to be dominant at all concentration levels examined > 0·01 % w/w. These data, coupled with rates of photo-decomposition, indicate that in low density polyethylene the additives migrate rapialy to the surface of the polymer whereupon they are capable of operating as both effective screens and radical scavengers; the effects observed are, as expected, wavelength dependent.     

Photo-degradation and photo-stabilisation of the two-phase system poly(vinyl chloride)-polybutadiene

The photo-oxidative degradation of poly(vinyl chloride) (PVC)/polybutadiene (PB) blends has been studied. After uv irradiation, photo-grafting of the PVC and PB phases was observed. Photolysis of PVC accelerates cis-trans isomerisation of PB. Both phases, PVC and PB, are photo-degraded according to free radical oxidation mechanisms. The rates of these processes can be decreased by the addition of metal chelates, such as commercially produced Cyasorb uv light absorber 1084 (2,2′-thiobis(4-t-octylphenolato)-n-butylamine nickel(II)) and Cyasorb uv light absorber 2548 (cobalt dicyclohexyldithiophosphinate).     

Mechanisms of antioxidant action: Synergism between a 2-hydroxybenzophenone uv stabiliser and autosynergistic antioxidants in polypropylene

Synergism between a uv stabiliser (2-hydroxy-4-octoxybenzophenone (I)) and phenolic antioxidants has been studied during the photo-oxidation of polypropylene. It has been found that hindered phenols containing benzylic sulphur (II) are more effective synergists at the same molar concentration than conventional hindered phenols and that optimum synergism occurs at high molar ratios (I/II). The autosynergistic phenols (II) are shown to protect the uv stabiliser against the effects of hydroperoxides under photo-oxidative conditions by catalytically destroying them and scavenging radicals formed from them. The ‘uv absorber’ (I) also appears to deactivate excited species formed in the photo-decomposition of oxidation products of (II).     

Mechanisms of antioxidant action: Metal complexes as u.v. stabilisers in polyethylene

In order to evaluate the importance of u.v. screening, peroxide decomposition and excited state quenching in the mechanism of action of a variety of metal complex stabilisers, screening and additive studies were carried out in high density polyethylene and in model hydrocarbons. All the metal complexes studied were found to be u.v. screening agents when contained in a separate phase between the u.v. source and the polymer and all were rather more effective than a typical commercial u.v. absorber (2-hydroxy-4-octyl-oxybenzophenone, HOBP). However, when used as additives, all the stabilisers had an additional function. In the case of the transition metal dibutyl dithiocarbamates (NiDBC and FeDBC), the additional stabilisation mechanism is consistent with their known peroxide decomposing behaviour. In the case of the 2-hydroxy-acetophenone oxime complexes (NiOx and CuOx), the initial function is a stoichiometric reaction with hydroperoxides to give non-radical products. An amine complex of nickel bis-octylphenol sulphide (NiBOPS) appeared to behave primarily as a screen. Only one of the metal complexes (FeOx) appeared to be capable of functioning as an excited state quencher and this complex is effectively sensitised by triplet carbonyl to give a photo-pro-oxidant effect in cumene. It is concluded that the concept of excited state quenching, although it may occur with some metal complexes, is not a sufficient criterion of u.v. stabilising activity.     

Thiocyanate-selective electrode based on cobalt(II) complexes of pyrazolone heterocyclic Schiff bases

A new solvent polymeric membrane electrode based on pyrazolone heterocyclic Schiff base complexes of Co(II) is described. It shows a preferential response towards thiocyanate over a range of 2.0 × 10^–6 to 1.0 × 10^–1 mol L^–1 with a slope of –60.2 ± 0.6 mV/dec. The selectivity sequence observed is thiocyanate > hydroxide > nitrite > iodide > perchlorate > citrate > bromide > fluoride > chloride > nitrate > acetate > borate > sulfate > phosphate. The selectivity behavior is discussed in view of axial coordination by uv/vis spectroscopy and the transfer process of thiocyanate across the membrane interface is investigated by the ac impedance technique. The electrode was successfully applied to the determination of thiocyanate in human urine as an indicator for distinguishing between smokers and non-smokers.     

Thiocyanate-selective electrode based on cobalt(II) complexes of pyrazolone heterocyclic Schiff bases

A new solvent polymeric membrane electrode based on pyrazolone heterocyclic Schiff base complexes of Co(II) is described. It shows a preferential response towards thiocyanate over a range of 2.0 × 10^–6 to 1.0 × 10^–1 mol L^–1 with a slope of –60.2 ± 0.6 mV/dec. The selectivity sequence observed is thiocyanate > hydroxide > nitrite > iodide > perchlorate > citrate > bromide > fluoride > chloride > nitrate > acetate > borate > sulfate > phosphate. The selectivity behavior is discussed in view of axial coordination by uv/vis spectroscopy and the transfer process of thiocyanate across the membrane interface is investigated by the ac impedance technique. The electrode was successfully applied to the determination of thiocyanate in human urine as an indicator for distinguishing between smokers and non-smokers. Received: 30 September 1997 / Revised: 9 December 1997 / Accepted: 13 December 1997     

Mechanisms of antioxidant action: the role of 2-hydroxybenzophenones in photo-oxidizing media

A study of the photo-stability of 2-hydroxy-4-octyloxybenzophenone (HOBP) in the presence of possible photo-sensitizers present in polymers shows that this u.v. stabilizer is readily destroyed by them in the presence of light. HOBP is destroyed by carbonyl compounds and therefore appears to interfere with their photolysis at least partly by a sacrificial quenching process.     

An esca study of the surface chemistry of cellulose nitrates and double based propellants,with particular reference to their degradation in ultra-violet light

Model studies have been carried out on the photo-degradation (λ > 290 nm) of cellulose nitrates of differing degrees of substitution and of double based propellants from the surface point of view. By monitoring the relative intensities of appropriate core levels it is possible to follow the transformation occurring under uv irradiation.     

Ultraviolet absorbers for retarding wool photo-degradation: Sulphonated 2-hydroxybenzophenones and 2,2′-dihydroxybenzophenones

Nineteen sulphonated 2-hydroxybenzophenones and three sulphonated 2,2′-dihydroxybenzophenones have been prepared and compared with a commercially available member of each class of uv absorber as photo-protective agents for wool. Treated fabrics were exposed to Philips ML G/74 lamps and the extent of photo-tendering was assessed by measuring breaking loads and tear strengths. In general, 2-hydroxybenzophenones with 3-alkyl substituents provide better protection against photo-tendering than absorbers lacking 3-alkyl substituents. 2,2′-Dihydroxybenzophenones are more effective than 2-hydroxybenzophenones.

On the basis of effectiveness and ease of synthesis, 2,2′-dihydroxy-4,4′-bis-w-sulphobutyloxybenzophenone (VIIb) shows most promise as a photo-protective agent. At the 5% level of application it trebles the lifetime of wool fabric during exposure to sunlight through window glass. It also retards the photo-tendering and fading of wool fabrics containing either a red or a blue milling acid dye.     

Photo-stabilising action of a p-hydroxybenzoate compound in polyolefins—Part I: Thermal and photo-chemical behaviour in polypropylene film

The photo-stabilising action of a new aliphatic p-hydroxybenzoate light stabiliser, Cyasorb® UV 2908 (American Cyanamid Company), has been examined in polypropylene film, with the aid of a number of related compounds, by both normal and derivative uv absorption, infra-red techniques and hydroperoxide analysis. During processing and oven ageing the stabiliser operates as an effective chain breaking donor, terminating macroalkyl radicals and inhibiting the formation of hydroperoxides. Under both monochromatic (365 nm) and polychromatic (λ′s > 300 nm) irradiation conditions the decomposition of the stabiliser shows a direct dependence on initial hydroperoxide concentration in the film, indicating that it operates as an effective light stable alkoxy and hydroxy radical scavenger. Under ‘direct photolysis’ conditions (254 nm light) the stabiliser does not undergo unfavourable dimerisation reactions like other related p-alkyl substituted phenols. Evidence is also presented to show that the presence of the long aliphatic hydrocarbon chain has a powerful protective effect on the molecule and this is associated with a radical recombination process due to the polymer cage.     

Mechanisms of antioxidant action: The effect of thermal processing on the antioxidant activity of sulphur compounds

The nickel dithiophosphates and their corresponding disulphides are shown to be more effective uv stabilisers for LDPE than the closely related nickel xanthates and disulphides (dixanthogens). In general, the nickel complexes in both series are more effective uv stabilisers than the disulphides.Evidence is presented to show that the better performance of the dithiophosphates than the xanthates is associated with their higher solubility in the polymer. For the same reason, nickel dithiolates containing long alkyl substituents are more effective uv stabilisers than those containing shorter alkyl substituents.Thermal processing adversely affects the photo-stabilising performance of nickel dithiolates whereas the performance of the less efficient disulphides is enhanced under the same conditions. In the former case, the uv stability of the polymer is shown to be directly proportional to the amount of the nickel complex left after processing. In the case of the disulphides, on the other hand, the evidence suggests that increasing the severity of their processing increases the extent of oxidation to products which are more effective uv stabilisers than the parent disulphides.     

Some mechanistic considerations of hindered amine uv stabilisers

Hindered amines are good uv stabilisers. Tetra-substituted hindered amines are usually better uv stabilisers than tri-substituted ones. Among bicyclo tri-substituted amines the uv stabilisation activity appears to depend on the degree of hindrance around the nitrogen atom and the way in which the two rings are fused together.This conclusion is based on the finding that 2,2,4-trimethyl-trans-decahydroquinoline (trans-TDQ) is a better uv stabiliser than cis-TDQ in polypropylene. The same trend was found between 1-(2-hydroxyethyl)-2,2,4-trimethyl-trans-decahydroquinoline (trans-HETDQ-1) and cis-HETDQ-1. The difference in the uv stabilisation activity of these isomers is explained in terms of the relative stability of the nitroxyl radical of trans and cis TDQ.     

The molecular complex of dibenz[a,c]anthracene and trinitrobenzene

The crystal structure of the complex between the polycyclic aromatic hydrocarbon di-benz[a,c]anthracene and 1,3,5-trinitrobenzene is reported. The crystals are triclinic, space group P¯1 with unit cell dimensionsa=7.277(2) å,b=11.237(6) å, andc=13.902(5) å,= 104.13(4),=96.04(3), and =95.15(2). Diffraction data were measured at 121(2) K. The structure was determined and refined toR ₁=0.046. The structure consists of layers containing both dibenz[a,c]anthracene and 1,3,5-trinitrobenzene molecules, interconnected within a layer by C-H O interactions. Layers stack on one another so that dibenz[a,c]anthracene molecules are sandwiched between 1,3,5-trinitrobenzene molecules and vice versa. The average distance between molecules in these sandwiches is 3.23 å.     

Photo- and thermochromic 2-amino-2H-chromenes

A number of N-aryl- and N-alkylimines of 2-hydroxy-5,6-benzocinnamaldehyde were synthesized. The cyclic 2-amino-2H-chromene structure in nonpolar solvents was established for them by IR, UV, and PMR spectroscopy. It is shown that the transition to polar solvents leads to the establishment of a tautomeric ring-chain equilibrium. The equilibrium thermal transformations and phototransformations of some N-aryl- and N-alkylimines were studied.Communication 12 from the series Photo- and Thermochromic Spirans. See [1] for Communication 11.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1035–1044, August, 1980.     

Enhancing the performance of a linear Fresnel reflector using nanofluids and internal finned absorber

The objective of this paper is to investigate thermal efficiency enhancement methods in a linear Fresnel reflector (LFR) with evacuated tube receiver. The primary reflectors of the collector are flat mirrors of 27 m² total net aperture, while the secondary reflector has a parabolic shape. The working fluid is Syltherm 800, and the analysis is performed for temperatures up to 650 K. The use of nanofluids and internal fins is the investigated thermal enhancement methods in the receiver of the LFR. The examined nanofluid is Syltherm/CuO for concentrations 2, 4 and 6%, while the examined internal fins are 8 longitudinal fins which are symmetrically located in the absorber. The LFR is examined using nanofluids and pure thermal oil in smooth or finned absorber. According to the final results, the maximum thermal efficiency enhancement is up to 1% and it is greater for higher operating temperature levels. The use of internal fins is better enhancement method compared to the use of nanofluids, while the combination of these two techniques leads to the highest possible performance. For the inlet temperature of 600 K with 200 L min^?1 flow rate, the thermal efficiency enhancement with 4% nanofluid and finned absorber is found 0.82%, while it is found 0.61 and 0.28% with finned absorber with pure oil and 4% nanofluid with smooth absorber, respectively.

     

Depletion mechanism of antioxidants in MDPE-clay nanocomposites under thermal aging

The depletion behavior of two types of hindered phenolic antioxidants (AO), Irganox^® 1010 (I-1010) and Irganox^®1076 (I-1076), in medium density polyethylene (MDPE)/nanoclay composite was evaluated by incubating samples in a forced air oven at 85 °C. The presence of 4 wt% nanoclay accelerated the depletion of both types of AO, particularly at the surface region of the sample. However, the depletion mechanism in the interior of sample was governed by the AO molecular structure. For samples containing the bulky Irganox®1010, OIT decreased exponentially with aging time consistent with a first order reaction. In contrast, an increase of OIT was detected in first 60 days of heat aging for sample containing I-1076 and afterward the OIT decreased slowly with aging time. The hypothesis for the initial increase of OIT is that the relatively small and linear structure of I-1076 may enable it to be trapped inside the nanoclay galleries and then subsequently released into the polymer matrix during heat aging.     

Efficient protocol for isolation and purification of different soyasaponins from soy hypocotyls

Soyasaponins are naturally occurring triterpenoid glycosides associated with many biological activities. The aim of the present study was to develop an effective method for isolation and purification of differently glycosylated, acetylated, and 2,3‐dihydro‐2,5‐dihydroxy‐6‐methyl‐4H‐pyran‐4‐one (DDMP)‐conjugated soyasaponins from soy hypocotyls. Both gel filtration using Sephadex LH‐20 chromatography (Amersham Pharmacia Biotech AB; elution phase: methanol, flow rate: 3.0 mL/min, sample loading: 60 mg) and high‐speed countercurrent chromatography (stationary phase: n‐butanol–acetic acid (5.0%, v/v), mobile phase: water flow rate: 3.0 mL/min, sample loading: 100 mg) could effectively fractionate isoflavones and soyasaponins from the crude extract with yield of soyasaponin complexes 20.5 mg and 22.3 mg, respectively. After fractionation, the soyasaponin complexes could be purified further using preparative HPLC to separate individuals. A total of nine soyasaponins, triacetyl soyasaponin Ab (yield 1.55%, HPLC purity >98%), Aa (2.68%, >99%), Ab (18.53%, >98%), Ae (0.85%, >98%), Ba (0.63%, >91%), Af (1.12%, >85%), Bb (3.45%, >98%) and Be (0.59%, >76.8%) were obtained. DDMP‐conjugated groups, αg (2.06%, >85%), βg (7.59%, >85%), and γg (0.29%, >85%) that were very labile even in mild conditions, were also collected. The method described here can be used as an effective protocol to separate different soyasaponins occurring in the original sample.     

β‐Functionalized meso Tetrahalothien‐2‐ylporphyrins: Synthesis,Spectral, and Electrochemical Properties

Two types of β‐ functionalized (mono nitrated and perbrominated) meso tetrakis(5‐halothien‐;2‐yl)porphyrins, which can be used as precursors for the synthesis of other asymmetric and highly substituted porphyrins, have been synthesised and characterized. Introduction of a nitro group at the β‐ position shifted soret band 11–16 nm to the red region and redox potentials to > 170 mV for oxidation and > 250 mV for reduction anodically. Perbromination of halothienylporphyrins lead to enhanced bathochromically shifted uv‐visible spectral bands, but had only marginal influence on oxidation potentials. Effect of mono nitro group and eight bromo groups on the electronic properties of the porphyrins is attributed, respectively to, the electron deficiency created in the porphyrin π‐ system and the nonplanar conformation induced by the bulky bromo groups.