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.     

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.     

Mechanisms of anti-oxidant action: The role of nickel dithiolates in the stabilization of low density polyethylene

Nickel O,O-dialkyldithiophosphates and nickel xanthates stabilize low density polyethylene against both thermal and photo-oxidation. Under thermal oxidative conditions, the initial complexes are rapidly destroyed and the anti-oxidant function is associated with the further oxidation products formed via the corresponding disulphides. Their photostabilizing activity is related primarily to the photostability of the metal complexes themselves which act as light stable reservoirs for the anti-oxidant species.     

Surface modification of polymers. I. Vapour phase photografting with acrylic acid

Surfaces of low density polyethylene, high density polyethylene, and polystyrene have been modified by grafting with acrylic acid. Benzophenone and acrylic acid in the vapor phase were UV-irradiated in the presence of a polymer substrate. Grafting with acrylic acid took place in a thin layer on the surface, thus increasing the wettability of the polymer. After 5 min of irradiation, the contact angle against water had decreased to 20° for polystyrene and 50° for the polyethylene samples. ESCA measurements on samples irradiated for 5 min showed a 90% poly(acrylic acid) coverage of the surface for polystyrene, 63% for low density polyethylene, and 56% for high density polyethylene. Acetone or ethanol were used as carriers of monomer and initiator. Acetone was able to initiate grafting and was found to promote and direct grafting to the surface. The stability of the acrylic acid grafted surfaces was studied by contact angle measurements and ESCA. At room temperature, the grafted layer is confined to the surface, but when the material was heated in air the surface was reshaped into a hydrophobic one. The process was reversible. In aqueous surroundings at elevated temperatures the hydrophilic character of the surface was restored.     

Materiaux composites a base de graves et De liants polymeres—VI. Etude des enrobes a base de liant bitume-polyethylene ozonise et de granulats mineraux

Studies have been made of mixtures consisting of minerals and a binder made of bitumen with polyethylene (pure or recovered; non-ozonized or ozonized). The presence of the polymer leads to materials with better properties, especially compression strength. The bituminous mixtures are less affected by temperature have improved modulus of elasticity and there is better adhesion with the mineral. It is shown that recovered polyethylene, when ozonized, can be useful in bituminous mixtures.     

Chemistry of dicumyl peroxide-induced crosslinking of linear polyethylene

The course of intermolecular reactions induced by peroxide decomposition in linear polyethylene has been determined and compared with similar reaction systems in which the substrate is a lowmolecular-weight alkane. Efficiency of intermolecular coupling is 40% for an alkane and 25–30% for the polymer system. Competing reactions, which reduce coupling efficiency, are mainly disproportionation of the initially formed radicals and to a minor extent further reactions involving the products from the peroxide decomposition. The low efficiencies found are similar to those well studied when initial radicals are produced by radiolysis in polymer and alkane systems.     

Monomer reactivity ratios in graft copolymerization

This paper discusses monomer reactivity ratios in various radiation- and redox-initiated graft copolymerizations. The polymers studied were polyethylene, cellulose acetate, poly(vinyl chloride), polytetrafluoroethylene, poly(vinyl alcohol), and poly(methyl methacrylate); the comonomer mixtures were styrene–acrylonitrile, methyl acrylate–styrene, acrylonitrile–methyl acrylate, and vinyl acetate–acrylonitrile. The polymer–comonomer mixture systems were so chosen as to permit study of both homogeneous and heterogeneous systems. The homogeneous systems included systems of low and high viscosity. The heterogeneous systems included both polymers swollen by the comonomer mixture and polymers not swollen by the comonomer mixture. None of the homogeneous grafting systems studied showed deviations from the normal copolymerization behavior under a variety of experimental conditions. Monomer reactivity ratios in graft copolymerization were the same as the values in nongraft copolymerization. The heterogeneous systems in which the polymer was swollen by the comonomer mixture yielded grafted copolymer compositions which were the same as those in nongraft copolymerization. The heterogeneous grafting system polytetrafluoroethylene/styrene–acrylonitrile showed deviations from normal copolymerization behavior at low degrees of grafting when the reaction was only on the polymer surface. The behavior became normal at higher degrees of grafting when the system approaches that in which the polymer is swollen by the comonomers. In all reaction systems, it was found that the use of radiation to initiate the reaction does not in any way affect the copolymerization behavior of the two monomers in a comonomer pair.     

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.     

Pseudo-halogen behavior of thiophosphoryl azides as a tool for the functionalization of phosphorus macrocycles

Phosphoryl azides are known as azide-transfer agents in organic chemistry; such behavior denotes the pseudo-halogen character of the azide group when linked to a tetracoordinated phosphorus atom, but it was considered up to now only for the functionalization of organic substrates by N₃. We show in this paper that the azide can also be considered as a good leaving group, which facilitates the functionalization of the phosphorus atom which bore it. This reaction is first demonstrated on a small thiophosphoryl azide, then applied to the functionalization of more complex macromolecules, i.e. macrocycles incorporating in their structure thiophosphoryl azido groups.     

Formation of rodlike structures of water between oppositely charged ions in decane and polyethylene

The Gibbs ensemble Monte Carlo method has been combined with the connectivity altering osmotic Gibbs ensemble to study water solubility and clustering in decane and polyethylene. We show that the presence of oppositely charged ion pairs that have fixed positions in the hydrocarbon matrices leads to an order of magnitude increase in the water solubility. This is important to a wide range of technical applications, since the uptake of the water leads to an increase in volume--or expansion--of the hydrocarbon phase which, in the case of polyethylene, may change the polymer properties and lead to water treeing. The increase in solubility is largest when the ions are sufficiently close so that rod-shaped clusters of water molecules form between the ions.     

Melt stabilisation of Phillips type polyethylene, Part III: Correlation of film strength with the rheological characteristics of the polymer

Different aspects of the melt stabilising effect of various antioxidant packages were studied in a Phillips type polyethylene in the work described in this series of papers. The polymer was stabilised with various combinations of a phenolic antioxidant with phosphite, phosphonite, and phosphine type secondary antioxidants and processed by multiple extrusions followed by film blowing. After determining the role of the antioxidants in the melt stabilisation process and the effect of antioxidant consumption on polymer properties the correlation of the rheological characteristics of polymer with the tear and impact strengths of films is discussed in this paper. The Elmendorf tear strength of films measured in transverse direction, which is sensitive to long chain branching, correlates closely with viscous compliance determined by creep recovery experiments and the ratio of melt flow indices measured at high and low loads. The relationships are independent of the type and amount of antioxidant added. This makes possible the prediction of the effect of various antioxidant packages on film properties on the basis of rheological measurements. The correlation between the dart drop impact strength of the films and the rheological characteristics of the polymer is less reliable.     

UV stabilising synergies between carbon black and hindered light stabilisers in linear low density polyethylene films

The combined effects of selected carbon black pigments and hindered light stabilisers (HALS) on the UV stabilities of linear low density polyethylene film have been studied under UVA and UVB fluorescent radiation sources. While the presence of HALS do not change the chemistry of film photodegradation, whether they are low or high molecular variants, their presence significantly extends film lifetime relative to the sum of the effects of carbon black and HALS individually. These lifetime extensions may be defined in terms of a synergy factor defined with respect to film time to lose a specific percentage of a tensile property, namely t₂₀, the time to lose 20% of initial elongation-at-break, or the carbonyl index associated with this condition. It is proposed that possible causes of this synergy are a result of the UV screening effect of the carbon black particles which provide lower concentrations of polymer radicals for the HALS component to interact with and/or an accompanying thermal stabilising effect by the latter as a consequence of the higher polymer local temperature during irradiation of pigmented films.     

Simultaneous analysis of thiols, sulphides and disulphides in wine aroma by headspace solid-phase microextraction-gas chromatography.

This paper deals with the improvement of a headspace solid-phase microextraction (HS-SPME) method, developed in a previous work, in order to analyse, simultaneously, thiols, sulphides and disulphides in wines. This can be achieved by applying Carboxen-polydimethylsiloxane fibres and a cryogenic trap to focus the analytes. Under optimum conditions, the HS-SPME procedure developed shows low limits of detection for the sulphides and disulphides studied (0.05-3 micrograms/l) and the thiols can also be analysed and detected at very low levels (0.5-1 microgram/l) with acceptable recoveries and repeatability.     

Influence of the repeated extrusion on the degradation of polyethylene. Structural changes in low density polyethylene

The influence of the repeated extrusion on the molecular parameters of low density polyethylene (LDPE) Bralen NA 7-25 was studied. Virgin polyethylene was submitted up to 20 extrusion cycles and the processed samples were fractioned using precipitation fractionation. Non-fractionated samples and the individual polymer fractions were characterized by their weight average molar masses M_w (static light scattering), number average molar masses M_n (osmometry) and limiting viscosity numbers [η] (viscometry). Rheological properties in terms of shear viscosity curve, zero shear viscosity and flow activation energy were also determined by using high pressure capillary rheometer. The course of the changes in molecular parameters of LDPE is influenced both by the initial polymer structure and by the changes induced by the mechano-chemical degradation. The suggested degradation mechanisms during multiple extrusion of Bralen are chain scission predominating in the early stage of processing followed by recombination of macromolecules resulting in crosslinking and formation of microgel, which is clearly notable for the samples extruded 3-20 times.     

Modification of the electrokinetic properties of reversible electrophoresis gels for the separation and preparation of DNA

The effect of adding linear polymers to a novel reversible electrophoretic was measured. Reversible gels are formed using the polyanionic carbohydrate polymer, gellan gum. Gellan gum forms strong stable gels in the presence of divalent cations or diamines. The gels are reversible (return to solution) by changing the ionic environment or pH. Gellan gum is an anionic polymer, and the electrophoresis gels have considerable electroosmotic flow (EOF) toward the negative electrode. We measured the EOF in gellan gum electrophoresis gels as a function of gel concentration, buffer composition, and linear polymer additive. The linear polymers used in this study were polyethylene oxide and hydroxyethyl cellulose. Both polymers reduced EOF in the gels, in a manner dependent on molecular weight. Polymers with high molecular weight were more effective at reducing EOF. The addition of polymers increased the resolution of low molecular weight DNA. Native gellan gum resolved DNA from approx 50,000 to 1000 bp. Addition of the polymers resolved DNA down to approx 50 bp, in some instances. The influence of the polymers on circular plasmid DNA was also investigated. Addition of high molecular weight polyethylene oxide reduced the electrophoretic mobility of the nicked circular form compared to the supercoiled form.     

Efficiency and mechanism of phosphorous antioxidants in Phillips type polyethylene

The stabilising efficiency of three phosphorous secondary antioxidants of different chemical structures (phosphonite, phosphite and phosphine) was compared in a Phillips type polyethylene. The polymer was processed by six consecutive extrusions in the presence of 700 ppm primary antioxidant and 700 ppm phosphorous compound. The consumption of the secondary antioxidant was followed quantitatively by Fourier transform infrared (FTIR) spectroscopy. The properties of the polymer were characterised by FTIR spectroscopy, colour and rheological measurements, as well as by the determination of its residual thermo-oxidative stability. The results of the experiments proved that the chemical reactions occurring in the first extrusion of the polymer are different from those taking place in the further processing operations. The rate of antioxidant consumption and the chemical reactions of the polymer are strongly affected by the type of the phosphorous secondary antioxidant. The analysis of the results indicated that the three stabilisers must act according to different mechanisms. The investigated phosphine showed the best melt stabilising efficiency, while phosphonite was found to protect the polymer most effectively from discoloration.     

Effect of the presence of silica nanoparticles in the coefficient of thermal expansion of LDPE

The effect of the presence of alumina microparticles and silica nanoparticles on the coefficient of thermal expansion (CTE) of films of low density polyethylene (LDPE) based composites was investigated. A new method based on the use of an atomic force microscope (AFM) is proposed for measuring nano-thermal expansion of films to finally obtain the CTE in polymer based materials. Nanocomposites based on silica nanoparticles and LDPE were prepared by mixing those constituents by high energy ball milling (HEBM). Pure alumina microparticles come from the milling tools used to mix the components of the composites. When silica nanoparticles are used as nanofiller of LDPE the effectiveness on reducing the CTE (about a 40% of CTE reduction) is higher than that obtained when high amount of alumina microparticles are present in the LDPE. Only when high amount of silica nanoparticles and low amount of alumina microparticles are present, the reduction of CTE expected from the Levin model is in accordance with the experimental results. This effect was associated to the high surface to volume ratio of nanoparticles considering uniform dispersions of them within the polymer. The region of polymer between particles must be so thin (few nanometers) that constraint effects must play an important role on reducing the chain mobility and therefore the thermal expansion.     

Smoke suppression for polystyrene

Studies have been made of the smoke-forming tendency of burning polystyrene and of the influence thereon of some established flame retardants, of a non-aromatic polymer and of a number of relatively unreactive oxides and other similar materials. The flame retardants generally increase smoke formation but decrease the rate of consumption of the polymer and, in order to separate the two effects, measurements have been made of the maximum amount of smoke generated and of the quantity of smoke formed per unit weight of polystyrene burnt. The incorporation of high-density polyethylene does not significantly reduce smoke production until a substantial proportion of the aromatic polymer has been replaced. At the relatively low levels used, most of the inert solids decrease only slowly both flammability and smoke formation but pyrogenic silica, even when present in very small concentrations, has a striking smoke-suppressant influence, the magnitude of which is directly related to its total available surface area.     

Étude ultrasonore de mélanges diphasiques de polyéthylène basse densité et de polystyrène atactique

Ultrasonic experiments have been performed on two-phase polymer blends (low density polyethylene/polystyrene). The damping coefficient goes through a maximum and the velocity through a minimum for a 60% PS blend. The excess of damping may be attributed to a scattering of the waves by particles or by the presence of voids in the blends.     

Determination of captopril and its mixed disulphides in plasma and urine by high-performance liquid chromatography

A high-performance liquid chromatographic method has been developed which enables sensitive determination of captopril and its mixed disulphides in plasma and urine after oral administration of a new antihypertensive agent, 1-(D-3-acetylthio-2-methylpropanoyl)-L-prolyl-L-phenylalanine (DU-1219, I). Captopril is derivatized with a new reagent, N-(4-benzoylphenyl)maleimide and the derivative is extracted with chloroform and assayed using a liquid chromatograph equipped with an ultraviolet detector at 254 nm. Mixed disulphides of captopril with thiol compounds such as cysteine, glutathione and plasma proteins are reduced with tributylphosphine to form captopril, followed by derivatization with N-(4-benzoylphenyl)maleimide. Accurate determinations are possible over a concentration range of 10-500 ng/ml captopril in plasma, and 100-2500 ng/ml captopril in urine. The coefficients of variation of captopril in plasma (200 ng/ml) and urine (500 ng/ml) are 3.7% and 2.6%, respectively, and those of mixed disulphides of captopril are similar to those of captopril. Plasma levels and urinary excretion of captopril and its mixed disulphides in healthy volunteers following single oral administration of I (50 mg) have also been determined.