Polymer Degradation and Stability (v.93, #9)

Calendar (I).

Profluorescent nitroxides: Sensors and stabilizers of radical-mediated oxidative damage by James P. Blinco; Daniel J. Keddie; Tim Wade; Philip J. Barker; Graeme A. George; Steven E. Bottle (1613-1618).
In this study, three profluorescent nitroxides 1,1,3,3-tetramethyldibenzo[e,g]isoindolin-2-yloxyl (TMDBIO), 1,1,3,3-tetramethyl-2,3-dihydro-2-azaphenalene-2-yloxyl (TMAO) and 5-[2-(4-methoxycarbonyl-phenyl)-ethenyl]1,1,3,3-tetramethylisoindoline-2-yloxyl (MeCSTMIO) were tested as probes for radical-mediated damage in polypropylene arising from both UV and thermally initiated sources. These nitroxides possess a very low fluorescence quantum yield due to quenching by the nitroxide group; however, when the free-radical moiety is removed by reaction with alkyl radicals (to give an alkoxyamine), strong fluorescence is observed. The results obtained from this profluorescent nitroxide trapping technique compare favourably with other methods of monitoring degradation, provided the appropriate probe is chosen for the conditions of oxidation, signalling an indication of damage well before other techniques show any response. The technique was also applied to the monitoring of crosslinked polyester coating resins. Differentiation in the oxidative stability of the resins was evident after as little as 200 min where other monitoring techniques require up to 300 h of accelerated degradation. This highlights the sensitivity of this method as well as demonstrating the scope of this technique to assess polymer stability.
Keywords: Profluorescent nitroxide; Fluorescence; Polypropylene; Crosslinked polyester; Thermal oxidation; Photo-oxidation;

The melt stabilising efficiency of antioxidants with different structures based on hindered phenols, phosphite esters, phosphonite and a lactone was examined during multi-pass extrusions at 265 °C in three metallocene ethylene-1-octene copolymers (m-LLDPE) having different extent of short chain branching (SCB) and one Zeigler copolymer (z-LLDPE) containing the same level of SCB corresponding to one of the m-LLDPE polymers. The effect of the different antioxidants, when used separately and in combination, was investigated by characterising the changes in the polymer's rheological behaviour, colour formation and structural changes based on unsaturated groups and carbonyl content during five multi-pass extrusions. The results showed that all stabilisation systems examined offered higher efficiency in the metallocene polymers compared to the Zeigler. The effect of the extent of SCB in the metallocene polymers on the stabilising efficacy of the antioxidant systems was also examined, and it was shown that it had a significant effect, with both single and combinations of antioxidants giving higher efficiency in the m-LLDPE polymer containing higher extent of SCB. The presence of the lactone HP136 in mixtures containing hindered phenol–phosphite antioxidant systems gave a higher melt stabilisation efficiency than in its absence and this has been attributed to a co-operative antioxidant reaction steps that take place between the antioxidants resulting in the possible regeneration of the lactone antioxidant through a redox reaction. In all the metallocene PE polymers examined, the biologically hindered phenol, Irganox E, was shown to be more effective than the conventionally hindered phenol Irganox 1076, when examined alone or in combination with phosphite esters.
Keywords: Stabilisation; m-LLDPE; Melt extrusion; Antioxidants;

Photocatalytic titania based surfaces: Environmental benefits by Norman S. Allen; Michele Edge; Joanne Verran; J. Stratton; Julie Maltby; Claire Bygott (1632-1646).
The use of photocatalytic titania nanoparticles in the development of self-cleaning and de-polluting paints and microbiological surfaces is demonstrated. In the former case surface erosion and sensitised photooxidation is shown to be controlled by the use of catalytic grades of anatase nanoparticles. For environmental applications in the development of coatings and cementitious materials for destroying atmospheric pollutants such as nitrogen oxides (NOX) stable substrates are also illustrated with photocatalytic nanoparticles. Here porosity of the coatings through calcium carbonate doping is shown to be crucial in the control of the effective destruction of atmospheric NOX gases. Good environmental stability of the coatings is also crucial for long-term durability and this aspect is examined for a variety of material substrates. For the development of microbiological substrates for the destruction of harmful bacteria/fungi effective nanoparticle anatase titania is shown to be important with hydrated high surface area particles giving the greatest activity. Data from commercial pilot studies is used to signify the important practicalities of this type of new technology.
Keywords: Nanoparticles; Pigments; Titanium dioxide; Anatase; Rutile; De-pollution;

Catalytic fire retardant nanocomposites by Alberto Fina; Sergio Bocchini; Giovanni Camino (1647-1655).
In this paper the chemical activity of carbon nanotubes and polyhedral oligomeric silsesquioxane during thermal degradation and combustion of polymer nanocomposites is addressed. Indeed, polymer–nanofiller systems may exhibit chemical effects capable of thermal stabilisation of polymers as well as reduction of combustion rate and heat released, owing to catalytic effects induced by the nanofillers at high temperature.Carbon nanotubes in the presence of oxygen are shown to promote oxidative dehydrogenation in polyethylene with production of a stable surface layer of carbon char that provides an effective oxygen barrier effect. A similar action is performed by metal-containing polysilsesquioxanes dispersed in polypropylene.With either carbon nanotubes or metal POSS, partial carbonisation of the polymer matrix occurs during combustion, subtracting part of the organic polymer from combustion, targeting one of the major fire retardancy aim.
Keywords: Fire retardancy; Carbon nanotubes; Polyhedral Oligomeric Silsesquioxanes (POSS); Carbonisation; Nanocomposites;

Potential flame retardants, MgAl-LDH and ammonium polyphosphate (APP), were added to neat polystyrene (PS) individually or in combinations at weight fractions no greater than 10%. Structural morphologies of MgAl-LDH and the corresponding PS nanocomposites were established via X-ray diffraction (XRD) and transmission electron microscopy (TEM). Thermogravimetric analysis (TGA) and cone calorimetry were used to study the thermal stability and fire performance of the composites. Time to ignition is greatly reduced for PS composites when compared to the virgin polymer. Synergistic effects were observed in both TGA and cone calorimetry for formulations containing both MgAl-LDH and APP. Physical and chemical interactions between MgAl-LDH and APP are responsible for the observed synergy in thermal stability and fire performance.
Keywords: Polystyrene; Ammonium polyphosphate; Layered double hydroxide; Synergy;

Studies of degradation enhancement of polystyrene by flame retardant additives by Mark W. Beach; Nelson G. Rondan; Robert D. Froese; Bruce B. Gerhart; John G. Green; Bill G. Stobby; Andrey G. Shmakov; Vladimir M. Shvartsberg; Oleg P. Korobeinichev (1664-1673).
Experimental methods as well as thermodynamic modeling techniques were utilized to explore potential gas and condensed-phase contributions of various flame retardant (FR) additives with polystyrene polymer. FR additives investigated include hexabromocyclododecane (HBCD), triphenyl phosphine oxide (TPPO), triphenyl phosphate (TPP), triphenyl phosphine sulfide (TPPS), and sulfur. Flame studies of fundamental FR activity were also employed using molecular beam mass spectrometry analysis of FR active species directly in a flame system. The flame studies show direct evidence for active bromine (HBr, Br) species for HBCD and active phosphorous species (HPO2, PO, PO2 HPO3) species for TPPO and TPP which provide high potential for gas-phase activity for these FR additives. Various experimental measurements were also done to assess the degradation species and the degree of degradation of polystyrene by the FR additives. These studies support enhanced degradation of the base polystyrene polymer by the FR additive as a major pathway for condensed FR activity for HBCD and sulfur FR additives. Phosphorous based structures appear to show little enhancement of polystyrene degradation.
Keywords: Condensed phase; polystyrene degradation; Flame retardants; Gas phase; Radical; Phosphorus; Sulfur;

Chemiluminescence accompanying the oxidation of hemicelluloses by Lyda Matisová-Rychlá; Jozef Rychlý; Anna Ebringerová; Katarína Csomorová; Anna Malovíková (1674-1680).
On the series of hemicelluloses such as xyloglucans and xylans the effect of O–CH3 substitution and initial molar mass on chemiluminescence intensity measured under non-isothermal conditions in oxygen and nitrogen from 40 to 220 °C was examined. It was ascertained that the rate constants of oxidation determined from non-isothermal chemiluminescence experiments describe quite well the relative stability of the respective hemicellulose. Similarly as in the case of cellulose and pullulan increasing molar mass has an adverse effect on the intensity of the light emission.
Keywords: Polysaccharides; Xyloglucan; Xylan; Chemiluminescence; Thermogravimetry; Oxidation;

Impact of photosensitized oxidation and singlet oxygen on degradation of stabilized polymers by Jan Pospíšil; Stanislav Nešpůrek; Jan Pilař (1681-1688).
Photooxidation, an important component of polymer weathering, involves excited polymer-inherent chromophores and photoactive additives and impurities. Quenching of excited species by ground state molecular oxygen, the common component of air, results in the formation of singlet state molecular oxygen 1O2. This active form of oxygen is a strong oxygenation agent attacking stabilized polymers. Resulting transformations of the polymer matrix and additives have a negative effect on the service life of the material. The formation of 1O2, its properties, and reactions and products arising from stabilizers are outlined.
Keywords: Photosensitized oxidation; Singlet molecular oxygen; Oxygenation; Transformation of stabilizers;

A kinetic evaluation of the thermal oxidation of a phenol stabilised polybutadiene by M. Coquillat; J. Verdu; X. Colin; L. Audouin; M. Celina (1689-1694).
The thermal oxidation of hydroxy telechelic polybutadiene stabilised with 2,2′-methylene-bis(4-methyl-6-tert-butylphenol) was studied at 100 °C using weight changes and stabiliser quantification by liquid chromatography. The extended induction time relative to the unstabilised sample and the initial rate of stabiliser depletion were found to be proportional to the initial stabiliser concentration.Previously published kinetic models, based on the hypothesis that the stabiliser is only consumed by reaction with peroxy radicals and that the stationary state assumption is appropriate, were examined and found insufficient to explain the observations. An improved model was suggested assuming the contribution of a phenol oxygen reaction that results in a competing oxidation of the stabiliser itself. Experimental and theoretical arguments in favour of this model refinement are proposed.
Keywords: Polybutadiene; Phenolic antioxidant; Thermal oxidation; Kinetics;

Post electron-beam irradiation oxidation of orthopaedic UHMWPE by L. Costa; I. Carpentieri; P. Bracco (1695-1703).
The mechanisms and products of the oxidation process induced by electron-beam irradiation of UHMWPE were thoroughly investigated in the present study. The e-beam technique has the peculiarity of dispensing the whole radiation dose in a very short time and it allows following the post-irradiation oxidation process at room temperature, which represents a major advantage for the present purpose.Thin sections of orthopaedic grade UHMWPE GUR 1050 were irradiated at doses ranging from 5 to 100 kGy in vacuum, air or pure oxygen, depending on the aim of the following analyses. After irradiation, the samples were stored in liquid nitrogen until the measurements began. Chemical and physical changes caused to the polymer by irradiation were investigated by FTIR, derivatisation techniques and spectral subtractions.As reported in the literature, secondary hydroperoxides and ketones were found to be the main oxidation products, along with carboxylic acids and alcohols. Nevertheless, in our case, the kinetics of formation of oxidation products were inconsistent with previously reported mechanisms. An alternative oxidation mechanism, including the simultaneous formation of hydroperoxides and ketones, is proposed. The thermal stability of hydroperoxides is also discussed in detail and the similarities between the post-irradiation oxidation process and the thermal oxidation process are highlighted.
Keywords: UHMWPE; Electron beam; Irradiation; Oxidation; Polyethylene;

Long-term performance of poly(vinyl chloride) cables, Part 2: Migration of plasticizer by M. Ekelund; B. Azhdar; M.S. Hedenqvist; U.W. Gedde (1704-1710).
Cable samples with plasticized poly(vinyl chloride) insulations were aged in air at temperatures between 80 and 155 °C. The concentrations of the plasticizer (di-(2-ethylhexyl) phthalate, DEHP) in the insulations of the aged cables were determined by extraction of samples in tetrahydrofuran followed by analysis of the extract by liquid chromatography. The plasticizer concentration data for different ageing times were analysed by numerical methods, fitting Fick's second law with a concentration-dependent diffusivity. The analysis showed that the transport of the plasticizer to the surrounding air phase was controlled by diffusion at 120 and 155 °C with an activation energy of 89 kJ mol−1. The evaporation of the plasticizer from the outer boundary was rate controlling at lower temperatures (≤100 °C). The rate of evaporation was initially constant and independent of the plasticizer concentration at both 80 and 100 °C. The activation energy for the initial DEHP loss rate from PVC at these temperatures was the same as that obtained for evaporation of pure DEHP on a glass plate at 60–100 °C measured by thermogravimetry, 98 ± 2 kJ mol−1. Furthermore, the evaporation rate of pure DEHP on a glass plate was also of the same order of magnitude as the rate of plasticizer loss from the cable insulation. Extrapolation of the plasticizer loss rate data (from the cable at 80 °C and from pure liquid DEHP at temperatures between 60 and 100 °C) to 25 °C predicted a maximum loss of plasticizer of 1% over 25 years. This is in accordance with earlier presented data and with the data presented in this report.
Keywords: Long-term prediction; Evaporation; Plasticizer migration; Poly(vinyl chloride);

Colour changes in pigmented Nylon-6 systems containing copper iodide by Stephen W. Bigger; Mark Gribben; Allan Rogers (1711-1714).
The fading and colour changes in glass-filled polyamide formulations during ageing in water at 80 °C are reported. Formulations were stabilized with CuI and a UVA system and contained cobalt blue and phthalocyanine blue pigments. The results suggest that CuI causes an “aubergine” colour development in the absence of pigments whose origin is attributed to oxides and complexes of copper that are produced during thermal processing and degradation processes. Ageing in water causes dramatic fading along with a colour shift towards the green. This effect is attributed to leaching and/or hydrolysis of copper-containing species. It was found that an increase in CuI stabilizer produces an increased level of fading in un-pigmented samples. However, as the pigment loading is increased there is a concomitant reduction in the extent of fading and the extent of the green shift suggesting that pigments may mask ageing-induced colour changes in these systems. The susceptibility of phthalocyanine-pigmented polyamines to colour change is not adversely affected by CuI.
Keywords: Polyamide; Copper halide; CuI; Ageing; Fading;

Dominating reactions in the degradation of HDPE during long term ageing in water by Ildikó Kriston; Enikő Földes; Peter Staniek; Béla Pukánszky (1715-1722).
A Phillips type high density polyethylene was extruded six times without additives and the compression molded plates prepared from the granules were stored in distilled water for 12 months. Specimens withdrawn from the containers at regular intervals were thoroughly characterized with various methods including the determination of weight changes, color, MFI, functional group content (FTIR), molecular weight (GPC), thermal (DSC) and mechanical (tensile) properties. The results proved that all reactions taking place during the storage of HDPE in distilled water are related to each other; the correlation of all functional groups formed or consumed in them is surprisingly close. The amount of oxygen present determines the direction of reactions, larger oxygen content leads to chain scission, to an increase of methyl content and to the formation of carbonyl groups. Most of these reactions go through double bonds, their number decreases during storage. In spite of the large number of reactions proposed in the literature, only one or two dominating reactions determine the changes in the chain structure of the polymer and thus the properties of the final product under the conditions of this study. Any variation in the conditions of storage is reflected in the properties of the polymer. Stabilizers used under extractive conditions must be stable against hydrolysis and should trap oxygen centered radicals.
Keywords: Phillips polyethylene; Storage in water; Degradation reactions; Oxidation; Properties;