Polymer Degradation and Stability (v.75, #2)

During storage and service, exterior tire rubber compounds are subject to aging due to multiple factors. This process cannot be accurately represented by conventional single factor tests. The overall goal of our research is to develop an accelerated, service-simulating, artificial aging test for exterior rubber compounds. During the first part of this work, a cyclic test consisting of a series of single factor sub-tests was proposed and compared to dynamic outdoor aging and two standard single factor test: thermal aging and dynamic ozone aging. Although this cycle test shows promise, it does not account for interactions and is time consuming and inefficient making it unattractive for industrial use. In the present paper, an outdoor accelerated aging simulator is designed, build and tested. It includes the aging factors of heat, ozone, ultraviolet light, dynamic stretching, and aqueous solutions. The chamber is compared against dynamic outdoor aging and the above-mentioned cyclic aging test by characterizing the degradation as a function of aging time. Four formulations of exterior tire rubber compounds are used as experimental materials. The experimental results demonstrate that the chamber runs show excellent correlation with the cyclic and dynamic outdoor aging test. For all the tests completed, the static modulus initially increases, then reaches a maximum, and then decreases. The final chamber test run provided an acceleration factor of roughly 7.5 times the rate at which aging occurs in the dynamic outdoor aging test. It was found that the size of the specimen has a significant affect on the aging rate, which can be utilized to accelerate the aging without elevating the temperature.
Keywords: Rubber aging; Accelerated aging; Dynamic outdoor aging;

Novel oligomeric amines as co-synergists for the photosensitised crosslinking of a triacrylate resin by Norman S Allen; Sergiy V Ryabov; Michele Edge; Sergiy N Kobylinskyy; Alexander A Pokcholenko; Zoia V Voitenko (229-236).
Novel effective oligomeric amine co-synergists and donor–acceptor systems have been developed coupled with an investigation into the effect of their chemical structure and molar ratio with the following proton acceptors—IBBA (isobutyl ester of benzophenone–p-carboxylic acid) and 4-phenylbenzo-phenone (4-PhBP). For this purpose a range of oligomeric amines (OA) with molecular weights from 500 to 4250—polyoxypropylenetetraethyldiamine and polyoxypropylene-N-tetraethylmalediamide,polyoxypropylene-N-tetraethylphthalediamide having end-terminated tertiary amines groups and a different chemical structure on their backbone have been synthesised on the base of Huntsman commercial products—Jeffamine® D Series of diamines (D400 and D2000) and have been characterized by IR-spectra, mass-spectrometry analysis and NMR-methodologies. The kinetics of photoinitiated polymerisation of the triacrylate monomer with the above oligomers were monitored by two methods: real time infra red (RTIR) spectroscopy and microwawe photodielectrometry equipment. The results obtained by these two methods (R p—rate of polymerisation, conversion level, induction time) are in good agreement. Thus, application of the oligomeric amines elaborated were found to be effective for accelerating the photopolymerisation and enabling the polymer properties to be regulated in a number of ways by varying their chemical structure. It has been shown that chemical configuration of amines, molecular weight and molar ratio influenced the rate of UV-light induced polymerisation.
Keywords: Photocuring; Cosynergists; Amines; Photopolymerisation; Photoinitiators;

The chemiluminescence of the polyester, poly(ethylene-co-1,4-cyclohexanedimethylene terephthalate) (PECT) has been determined and related to that from poly(ethyleneterephthalate) (PET). The aliphatic backbone in PECT and PET is found to play a dominant role in controlling the behaviour of the chemiluminescence (CL) emission and can be related to the nature of the hydroperoxidation sites. The CL emission and its profile is also found to be highly dependent upon the thermal and UV oxidative history of the material. Hydroperoxide sites formed during oxidation are found to be responsible for the thermally induced CL. Thermal oxidation of the polymer as measured by hydroperoxide concentration is also shown to be directly related to CL intensity and can predict the behaviour of antioxidants.
Keywords: Chemiluminescence; Thermal oxidation; Photodegradation; Polyester; Antioxidants; Stabilisers;

Polyethylene greenhouse cover aged under simulated sub-Saharan climatic conditions by N Hassini; K Guenachi; A Hamou; J.M Saiter; S Marais; E Beucher (247-254).
The sand wind effect on the durability of low-density polyethylene (LDPE) used as greenhouse cover, kept under sub-Saharan climatic conditions, has been investigated. Sand winds were simulated with an apparatus developed in our laboratory. Infrared, UV visible spectroscopies, contact angle, interferential microscopy, differential scanning calorimetry and permeability measurements were performed on untreated samples and on samples submitted to a sand wind. The data analysis shows that the surface roughness is modified, leading to drastic decreasing of the UV–visible light transmission. This phenomenon is due to the presence of small sand particles inlaid in the sample surface and due to surface erosion. Sand wind does not affect O2 permeation but decreases the rate of water absorption. Finally, in the range of experimental duration investigated in this work, no chemical action due to sand wind is observed.
Keywords: Polyethylene; Ageing; Degradation; DSC; Infrared spectroscopy; Permeation;

Kinetics of the electrochemical degradation of polypyrrole by R Mazeikiene; A Malinauskas (255-258).
The kinetics of the electrochemical degradation of polypyrrole has been investigated in aqueous solutions, containing HNO3 or NaNO3. For HNO3 solution, it has been found that the first-order degradation rate constant (k) is ca. 10−5 s−1 within the electrode potential limits of 0.3 to 0.6 V vs. Ag/AgCl. Over 0.6 V, a steep increase of k with increasing electrode potential was found, reaching ca. 9.8 V−1 for logk. For NaNO3 solutions, the k value ranges from 2.8·10−5 to 3.4·10−3 s−1 within the electrode potential limits of 0.3 to 0.9 V, and a linear slope of logk vs. E of 5.1 V−1 was found within the E limits of 0.4 to 0.9 V.
Keywords: Polypyrrole; Degradation; Stability; Kinetics;

We have used Doppler broadening spectroscopy to investigate the effects of filling various rubber polymers with different amounts and types of carbon blacks and of silica. Carbon Black has important reinforcing properties for rubbers used by the tyre industry specifically, therefore, understanding these properties is essential as well as critical. Silica is also being used as a rubber filler to replace or complement Carbon Black, and was also investigated. The S parameter was measured for various types of Carbon Black and silica as a function of the relative weight used in the samples. The behaviour of the S parameter was analysed in terms of positron annihilation and positronium formation for the various samples. Results show direct correlations between the S parameter and the various physical characteristics of Carbon Black and of silica.
Keywords: Positron annihilation; Carbon Black; Silica; Rubber polymer; Doppler broadening;

The behaviour of some polyalkenamers under thermal oxidation was examined. Gel content and spectral data were used to characterise thermal stability over the temperature range from 110 to 160 °C. The main kinetic parameters (oxidation induction time, period of propagation stage, oxidation rate, activation energy of oxidation and life-time of precursors) were calculated. Detailed considerations on the degradation process are presented and the differences between cis and trans configurations are emphasised.

Kinetics of thermal degradation of 6FDA based copolyimides—I by P Santhana Gopala Krishnan; Rohit H Vora; S Veeramani; Suat Hong Goh; Tai-Shung Chung (273-285).
Two series of linear aromatic copolyimides containing hexafluoro isopropylidine moieties (i.e. 6F) were synthesized by reacting 2,2-bis (3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) with various mole percents of 2,3,5,6-tetramethyl-1,4-phenylene diamine (Durene diamine) and 2,6-diamino toluene (DAT) or m-phenylene diamine (mPDA) in N-methyl-2-pyrrolidone (NMP). These copolyimides were characterised by GPC, XRD, DSC, TMA, DMA and TGA. Copolymer composition of these copolyimides was determined using 1H-NMR. T g calculated using Fox equation was compared with experimentally found values. All copolyimides were found to be amorphous. Activation energy was determined for the thermal degradation of these copolyimides using Coats–Redfern and Chang equations and were found to follow first order kinetics in air and in nitrogen. Inherent viscosity and T g, decreased with decrease in mole ratio of durene diamine whereas activation energy and frequency factor [ln (A)] were found to increase with decrease in mole ratio of durene diamine in [6FDA+Durene diamine+mPDA] and [6FDA+Durene diamine+DAT] copolyimide series. CO2 was the main decomposition product in air and CHF3 in nitrogen atmosphere. Amount of CHF3 evolved increased with increase in durene content in copolyimide series.
Keywords: 6FDA; Polyimide; Thermal degradation; Kinetics; Activation energy;

Catalytic degradation of polypropylene: effect of dealumination of clinoptilolite catalyst by J.R Kim; Y.A Kim; J.H Yoon; D.W Park; H.C Woo (287-294).
The performance of several differently treated clinoptilolite zeolites in the degradation of polypropylene was investigated in this study. The composition of degradation products, especially liquid fractions resulted over studied catalysts, was compared to understand the role of catalysts. The degradation was carried out in a semi-batch reactor with a mixture of polypropylene and catalysts at 400 °C. The liquid products from the catalytic degradation of polypropylene are distributed in a narrower range of carbon numbers compared with those obtained by thermal degradation. With dealuminated clinoptilolite catalysts, the distribution of liquid products shifted to lower numbers of carbon atoms due to the increase of the portion of medium acid sites and micropore diameter. The increase of temperature accelerated the cracking of intermediately degraded fragments to lower hydrocarbons.
Keywords: Polypropylene; Catalytic degradation; Clinoptilolite; Dealumination;

Synergistic combinations of HALS have gained increasing importance for stabilization of polyolefins. Some of the effects observed have been presented to illustrate the possibilities of such combinations. The study has been extended beyond the solutions used commercially. This was aimed at gaining better understanding of the physico-chemical phenomena involved in synergism between stabilizers of the same basic structure. It has been found that synergism or antagonism between two HALS can be detected by examining one single combination of the two HALS. It is the 1:1 combination of the two components that gives the most reliable results. In addition, one single test criterion seems sufficient for valid conclusions. Furthermore, it looks as if the results obtained in one PP type were valid with all the PP types so far available and polyolefins in general. There is no synergism usually if two low molecular mass HALS are combined. Many of these combinations show just additive effects but there can be strong antagonism with specific combinations. Combinations of low and high molecular mass HALS give much more often synergism than combinations of two low molecular mass HALS. This has been attributed to the complementary effects resulting from rapid diffusion of the low molecular mass HALS to the surface layers where the need for stabilizers is highest with the resistance to migration and extraction of high molecular mass HALS. Nonetheless, synergism is not general with such combinations, they may give just additive effects or even antagonism. Of course, this explanation of synergism is not valid for combinations of two high molecular mass or polymeric HALS. If synergism is observed with such combinations, it must result from very specific interaction or specific protection mechanisms of the components. Although there are many examples for such synergistic combinations in the patent literature, the effect is not at all general. As a consequence of a considerable amount of work, synergism can be understood to some extent for combinations of the polymeric HALS-2 with HALS-3 and high molecular mass HALS of similar structure. However, most experimental observations of synergism and antagonism cannot yet be explained.
Keywords: HALS; Low molecular mass HALS; High molecular mass HALS; Polyolefins; HALS combinations; Synergism; Antagonism; Additive effects; Mechanisms;

Specific combinations of UV absorbers of different classes show pronounced synergism in polyolefins. The effects depend on the particular combination and sometimes the particular polyolefin. Combinations of an oxanilide with a benzophenone or a benzotriazole or a hydroxyphenyltriazine are systematically synergistic, in polyethylene and in polypropylene. The same holds for combinations of a benzophenone with a hydroxyphenyltriazine. This is not so for combinations of benzotriazole with hydroxyphenyltriazine UV absorbers. The data show them to yield mainly just additive effects. Combinations of benzophenones with benzotriazoles show complex behavior depending on the particular polyolefin. The ternary combinations of the UV absorbers examined are mainly synergistic. The reasons for the results are discussed.
Keywords: UV absorbers; Benzophenones; Benzotriazoles; Oxanilides; Hydroxyphenyltriazines; Polyolefins; Synergism; Antagonism; Additive effect; Mechanisms;

Rotational moulding is used to manufacture hollow plastic products. The process offers many advantages to the designer, but it is hampered by a strong dependence on trial and error methods to achieve good part quality at economic production rates. During rotational moulding, the polymer is subjected to relatively high temperatures for long periods of time in the presence of air. This can lead to degradation of the polymer at the inner free surface of the moulded article, with consequent deterioration of the mechanical properties of the part. The processing conditions that lead to degradation vary with factors that affect the heating rate, such as the type of mould used. In this work a method is proposed to predict the onset of degradation, on the basis that this occurs when the concentration of anti-antioxidant in the polymer reaches zero. Good agreement between the experimental and predicted optimum processing temperature was obtained for two grades of polyethylene stabilised with two different antioxidant systems. Using the method described, it is now possible to identify the best rotational moulding conditions for a particular polymer so that more efficient cost-effective parts can be produced.
Keywords: Rotational moulding; Melt degradation; Polyethylene; Oxidation induction time;

Investigations on the thermal stability of a diazonium ion on solid support by Stefan Bräse; Stefan Dahmen; Crisan Popescu; Maarten Schroen; Franz-Josef Wortmann (329-335).
We have synthesised a diazonium ion starting from 2-chloro-5-aminobenzyl alcohol, and investigated in detail its thermal stability. The DSC runs at seven different heating rates give evidence of a single decomposition reaction occurring around 90–120 °C, whose kinetics were analysed by both the iso-conversional differential and integral method, the invariant kinetic parameters method, Kissinger's method, and a differential isothermal method. All of these methods led to similar values for the kinetic parameters and showed that the first-order mechanism fits the data best. The values of the kinetic parameters were further used for calculating the half-life time of the ion at various temperatures. The result of the stability prediction at 60 °C was compared with that of an analytical isothermal test yielding a good agreement between prediction and experiment.
Keywords: Diazonium ion; Non-isothermal kinetics; Thermal analysis; DSC; Half-life time prediction;

Thermal degradation of polyacrylic acid in dilute aqueous solution by Louis Lépine; Roland Gilbert (337-345).
The thermal degradation of polyacrylic acid of molecular weight (M w) 5660 (PAA 5K) has been studied between 180 and 260 °C, representing the heat conditions prevailing at the high pressure heaters (185 °C) and steam generators (260 °C) of the steam-condensate cycle of CANDU nuclear power plants. The degradation does not follow a simple first-order kinetics but probably a rather complex mechanism that includes decarboxylation and another process that attacks the integrity of the polymer chain. The degradation seems to be rapid at first with up to 10% lost in the first hour required to reach the set temperature in the pressure vessel and a 50% loss after approximately 5 h at 260 °C. The major thermal decomposition products of PAA are identified as carbon dioxide in the gas phase, while in the liquid phase the degradation primarily yields carbonate/bicarbonate, with the presence of low-molecular-weight organic acids, including acetate and to a lesser extent formate. Also, the size exclusion chromatography (SEC) analysis tends to indicate the production of smaller polymer chains, which are likely to have fewer charged carboxyl groups and possibly unsaturated double bonds. These results suggest that, along with the decarboxylation mentioned in the literature, another degradation mechanism that would attack the integrity of the polymer chain is likely taking place, especially at a high pH level. The acetate and formate seem to play an intermediate role since their concentration levels have been shown to increase at first but later decrease to a lower level during the course of our tests.
Keywords: Polyacrylic acid; Degradation; Water; Steam generator; Decomposition products;

Films of biodegradable aliphatic polyesters, poly(ε-caprolactone) (PCL), poly[(R)-3-hydroxybutyrate] (R-PHB), and poly(L-lactide) (PLLA) were prepared by solution-casting and annealing from the melt. Their biodegradation in static seawater controlled at 25 °C was investigated using polarizing optical microscopy, gravimetry, gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and tensile testing. The change in weight loss, tensile strength, and Young's modulus revealed that the biodegradabilities of the aliphatic polyesters in the controlled seawater decreased in the order: PCL>R-PHB>PLLA. The results of gravimetry, GPC, and DSC showed that the biodegradation of PCL and R-PHB films proceeds via surface erosion mechanisms. The polarizing optical microscopy indicated that the PCL and R-PHB films were biodegraded inhomogeneously on the film surface where the marine microbes attached, resulting in pore formation. The crystalline residues of PCL and R-PHB films could not be traced by GPC even when large weight losses occurred. Polarizing optical microscopy and GPC indicated that the decreased tensile strength and Young's modulus of the PCL and R-PHB films are attributed to the formation of pores and cracks during biodegradation. The biodegradation of the PLLA films was insignificant even after immersion in the controlled seawater for 10 weeks and the initial crystallinity had no significant effects on the biodegradability of the PLLA films, excluding the tensile properties change. The biodegradation of these aliphatic polyester films in the controlled static seawater could not be traced by GPC and DSC measurements.
Keywords: Biodegradation; Seawater; Polycaprolactone; Polyhydroxybutyrate; Polylactide;

The (bio)degradation of aliphatic polyesters, poly(ε-caprolactone) (PCL), poly[(R)-3-hydroxybutyrate] (R-PHB), and poly(L-lactide) (PLLA) films in natural dynamic seawater was investigated using polarizing optical microscopy, gravimetry, gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and tensile testing. The results were compared with those reported for the polyester films biodegraded in the controlled static seawater to reveal the physical effects in the natural dynamic seawater on the degradation of the aliphatic polyesters. The gravimetry and tensile testing showed that the mechanical stresses and strains in the natural dynamic seawater caused mechanical destruction or degradation of the films, resulting in seemingly accelerated (bio)degradation of all the films compared with that in the controlled static seawater. Such mechanical acceleration effects in the natural dynamic seawater were higher for the R-PHB and PLLA films with relatively high T g than for the PCL films with relatively low T g, when their (bio)degradation was monitored by gravimetry. The polarizing microscopy showed that the R-PHB films were (bio)degraded predominantly at the centers of and between the spherulites. The tensile strength and Young's modulus were lower for the initially crystallized PLLA films than for the initially amorphous PLLA films when compared at the same degradation time.
Keywords: Biodegradation; Seawater; Poly(caprolactone); Poly(hydroxybutyrate); Polylactide;

The interaction of a phenolic anti-oxidant and an aromatic amine in a thermo-oxidative ageing process by Julien Barret; Pieter Gijsman; Josefien Swagten; Ronald F.M Lange (367-374).
An interaction of the sterically hindered phenolic anti-oxidant 1,3,5-trimethyl-2,4,6-Tris(3,5-di-t-butyl-4-hydroxybenzyl)-benzene, 1a with the aromatic amine 4,4′-bis(α,α-dimethyl-benzyl)diphenylamine, 2a in co-poly(ether esters) has been observed. This interaction is successfully analyzed by heating 1a and/or 2a in the polyether-mimicking solvent bis(2-methoxyethyl)ether using various analytical techniques, e.g. IR, HPLC–PDA, GC–FID, GC–MS and LC–MS. As well as a detailed insight of the action of the individual anti-oxidants, it has been shown that the quinone structure of the oxidized sterically hindered phenolic anti-oxidants are only stable in the absence of radical species, which results for the 1a2a system in the stabilisation of the oxidized 1a.
Keywords: Interaction; Phenolic anti-oxidant; Aromatic amine;

A comparative study of the thermal decomposition of apple pulp in the absence and presence of phosphoric acid by F Suárez-Garcı́a; A Martı́nez-Alonso; J.M.D Tascón (375-383).
Apple pulp pyrolysis in the presence of phosphoric acid (chemical activation) represents a promising way for utilisation of this residue from apple juice and cider production. Thermogravimetry and infrared spectroscopy have been used to compare the pyrolysis behaviour of apple pulp, alone and impregnated with 60 wt.% H3PO4. Phosphoric acid promotes the degradation of the biopolymers constituting apple pulp, lowering the temperatures necessary for their degradation. At low temperatures, the main chemical transformation is dehydration leading to anhydrocellulose. The chemical structure collapses during cellulose degradation (300–350 °C in absence of phosphoric acid and 155–205 °C in presence of phosphoric acid). At higher temperatures progressive aromatisation and decarboxylation of the char take place. At 500 °C, carbonyl bands are only found in the spectra of chars from apple pulp impregnated with phosphoric acid, indicating that this reagent introduces oxygenated functions (alcohol and/or ether) into the solid pyrolysis residues.
Keywords: Apple pulp; FTIR; Phosphoric acid; Pyrolysis; TG;

Chemiluminescence from thermal oxidation of poly (2,6-dimethyl-1,4-phenylene oxide) by L Matisová-Rychlá; J Rychlý; G.A George (385-396).
Chemiluminescence (CL) accompanying the oxidation of poly(2,6dimethyl-1,4-phenylene oxide) in oxygen and decomposition of the products of its pre-oxidation in nitrogen was examined from the viewpoint of the relation to the rate of oxidation. Light emission typically decays in both oxidizing and inert atmosphere from an initial maximum value to some steady value. The initial decay is rather fast. It has been shown that the intensity of the initial CL depends not only on the concentration of oxygen in the surrounding atmosphere but also on the degree of pre-oxidation and on the conditions of exposure of the sample to visible light. The mechanism of the polymer oxidation taking into account the terminal phenoxyl groups, formation of hydroperoxides and the decomposition of charge transfer complexes between oxygen and oxygen in hydroxyl or ether groups is discussed.
Keywords: Chemiluminescence; Poly (phenylene oxide); Thermal oxidation; Degradation;

Flammability of polyamide-6/clay hybrid nanocomposite textiles by Serge Bourbigot; Eric Devaux; Xavier Flambard (397-402).
The flammability of polyamide-6 (PA-6)/clay nanocomposites used as textile fabrics has been investigated. PA-6/clay nanocomposite (PA-6nano) was prepared by melt blending and had an exfoliated structure. TG curves suggest that PA-6 is slightly stabilised between 450 and 600 °C. PA-6nano was processed via melt spinning to make multifilament yarns. Textiles have been evaluated as knitted fabrics and it is shown that the heat release rate of PA-6nano evaluated with the cone calorimeter at 35 kW/m2 is reduced by 40% in comparison with pure PA-6. This result offers a new promising route for flame retarding textiles with a permanent effect (laundry resistance) at relatively low cost and keeping the basic properties of the textiles.
Keywords: Polyamide-6; Nanocomposite; Montmorillonite; Flammability; Flame retardancy; Textile;