Polymer Degradation and Stability (v.74, #3)

Preface by Dennis Price (395).

Solid state 13C NMR study of the char forming processes in polychloroprene by Caroline M Dick; John J Liggat; Colin E Snape (397-405).
Polychloroprene, also known as neoprene, is an elastomer commonly utilised in the electrical and automobile industries. Its degradation is known to occur predominantly in a two stage process: HCl is lost in the initial step, whilst the second step involves the production of volatile hydrocarbons through chain scission. In this paper we describe the use of solid state 13C NMR as a probe for structural changes in the condensed phase during these degradative steps. Cross polarisation-magic angle spinning (CP-MAS) analysis of virgin polychloroprene and a series of samples degraded at temperatures between 275 and 550 °C reveals that as degradation becomes more advanced there is a steady loss of sp3 carbon with a commensurate growth in sp2 carbon. The bulk of the chlorine loss occurs by 350 °C with the aliphatic carbon lost by 550 °C, by which temperature the residue is essentially aromatic carbon. Dipolar dephasing experiments show that this residue is essentially a network of, on average, tri-substituted phenyl rings.
Keywords: Polychloroprene; Thermal degradation; Char formation; Solid state 13C NMR; DSC; TGA;

In situ NMR investigation into the thermal degradation and stabilisation of PAN by S.C. Martin; J.J. Liggat; C.E. Snape (407-412).
In situ broadline 1H NMR has been used to assess the low temperature degradation of polyacrylonitrile (PAN) under a variety of atmospheres and temperatures. In combination with conventional analytical techniques (solid state 13C NMR and FTIR), the structural and conformational changes produced in the network have been correlated with its thermal behaviour. Degradation has also been shown to be both temperature and time dependent, irrespective of reaction environment. Moreover, it has been demonstrated why air is the preferred medium for stabilisation in the production of fibre precursors. Enhanced stability and resistance to char formation is observed in the air-modified polymer, which also becomes resistant to resoftening during low temperature carbonisation.
Keywords: PAN; Fibres; Degradation; Thermo-oxidation; Stabilisation; NMR;

Combustion behaviour of EVA/fluorohectorite nanocomposites by Marco Zanetti; Giovanni Camino; Rolf Mülhaupt (413-417).
Octadecylammonium (ODA) or aminododecanoic acid (ADA) exchanged fluorohectorite (FH) was dispersed in ethylene–vinyl acetate copolymer (EVA) in an internal mixer. A nanocomposite was obtained in the case of FH/ODA with exfoliation of the FH crystalline layers which were separately distributed in EVA as shown by X-ray diffraction (XRD). Whereas distribution of particles formed by multiple crystalline layers with overall size of the order of 1 micron resulted in the case of FH/ADA. In the horizontal combustion mode of the mass loss calorimeter, the nanocomposite shows acceleration of EVA deacetylation and delayed volatilisation of the resulting polyene. The overall heat release rate is much lower than in the case of EVA as measured by the gas combustion temperature. A partial protection of EVA from the flame is also found in the case of the immiscible composite although in this case the volatilisation of the polymer during combustion occurs at a larger rate than in EVA alone. Cumulation of the silicate on the surface of the burning specimen may create a protective barrier to heat and mass transfer which is however much more effective in the case of ablative reassembling of crystalline layers of the nanocomposite than in that of the particles of the immiscible composite. Dripping of burning particles in vertical combustion is suppressed only in the case of the nanocomposite which reduces the hazard of fire spread to surrounding flammable materials.

A model system consisting of a polyethylene substrate surface treated by vinyltriethoxysilane and by organoboroxo-siloxane (OBSi), and an OBSi-containing intumescent flame-retarded compound (IFR-OBSi) based on polypropylene, ammonium polyphosphate and pentaerythritol were prepared and investigated. After a radio-frequency plasma treatment of the model system its oxygen permeability decreased by about one order of magnitude. According to XPS studies, enrichment of Si took place on the surface of the untreated IFR-OBSi, while after its ignition in a cone calorimeter surface enrichments of N and P were also detected. According to the evolution of the Si 2p peaks, creation of glass-like surface coatings took place on both the model system upon its RF plasma treatment and on IFR-OBSi upon its flame treatment. A small portion of OBSi remained unreacted in the surface layer of the flame-treated IFR-OBSi, which may ensure the plasticity necessary to prevent it from cracking and to ensure improved flame retardancy.
Keywords: Polyolefins; Intumescence; Flame retardants; Organosilicon additive; XPS; Permeation;

Influence of modified rheology on the efficiency of intumescent flame retardant systems by P Anna; Gy Marosi; I Csontos; S Bourbigot; M Le Bras; R Delobel (423-426).
Optimal concentration of boroxo siloxane elastomer synergist was determined by LOI and cone calorimetric measurements on ammonium polyphosphate and pentaerythritol containing intumescent flame retardant for polypropylene. The increased viscosity of the melt and plasticity of intumescent char due to boroxo siloxane elastomer could be proved by a thermal scanning rheometric investigation. It is presumed, the increased melt viscosity is created by the product of boroxo siloxane-pentaerythritol, formed during the compound preparation, while the improved char plasticity is the result of product formed at high temperature from boroxo siloxane and ammonium polyphosphate.
Keywords: Polyolefins; Intumescence; Flame retardants; Boroxo siloxane; Thermal scanning rheometry;

Flammability parameters of wood tested on a cone calorimeter by Ondrej Grexa; Henrich Lübke (427-432).
The effect of different loads of magnesium hydroxide as flame retardant on the flammability parameters of particleboard was studied. The effect of different flame retardant combinations was also compared. The flammability parameters were measured on a cone calorimeter at the uniform external irradiance of 50 kW m−2 for all modifications. The magnesium hydroxide improved the flammability parameters of particleboard and did not have negative effect on carbon monoxide production. From the different combinations of flame retardants the best retardant effects were measured for the combination of monoammonium phosphate and boric acid.
Keywords: Particleboard; Flame retardant; Heat release rate; Smoke production; Carbon monoxide production;

A simple model of ignition is discussed in the context of char-forming polymers. The model in question assumes that ignition occurs at a critical value of volatile mass flux from the degrading material. The char-forming mechanism chosen for the study is the simplest possible formulation preserving finite-rate kinetics—char and volatile products are formed together in a single step. This simple pyrolysis model has the advantage that the ultimate char yield is known a priori, and is in fact a parameter of the model. Results are presented and discussed for a number of cases, depending on char yield and char thermal properties. Complex ignition behaviour is exhibited by the model and practical consequences of the model predictions are discussed.
Keywords: Char-forming; Critical mass flux; Mathematical model; Polymer pyrolysis;

Flame retarding poly(methyl methacrylate) with phosphorus-containing compounds: comparison of an additive with a reactive approach by Dennis Price; Kelly Pyrah; T.Richard Hull; G.John Milnes; John R. Ebdon; Barry J. Hunt; Paul Joseph; Christopher S. Konkel (441-447).
The flame retardance and thermal stability of a methyl methacrylate (MMA) copolymer reactively modified by copolymerisation of the MMA with diethyl (methacryloyloxymethyl) phosphonate (DEMMP) have been compared with those of poly(methyl methacrylate) (PMMA) containing equivalent amounts of the additive diethyl ethyl phosphonate (DEEP). DEEP can be regarded as having a structure similar to that of a DEMMP comonomer unit and therefore the two compounds might be expected to confer about the same levels of flame retardance to PMMA when used at similar concentrations. The incorporation of 3.5 wt.% phosphorus in both cases raises the limiting oxygen index of PMMA from 17.2 to over 22. However, cone calorimetry shows that the MMA/DEMMP copolymer is inherently more flame retardant than PMMA containing DEEP: the former has a significantly lower peak rate of heat release than the latter (449 and 583 kW m−2, respectively) and gives rise to a greater amount of char. Thermogravimetric analysis (TGA) of the polymers indicates also that the MMA/DEMMP copolymer is more thermally stable than PMMA whilst PMMA containing DEEP is less thermally stable. Dynamic mechanical thermal analysis (DMTA) shows that the MMA/DEMMP copolymer has physical and mechanical properties similar to those of PMMA, whilst the low molecular weight DEEP plasticises PMMA, resulting in a significantly reduced glass transition temperature, T g. A condensed phase mechanism of flame retardance in MMA/DEMMP has been identified.
Keywords: Poly(methyl methacrylate); Phosphorus; Flame retardance; Diethyl ethyl phosphonate; Diethyl (methacryloyloxymethyl) phosphonate;

Glass fiber reinforced polypropylene composites have been flame retarded using a brominated acrylate as both monomer (PBB-MA) and polymer (PBB-PA), associated with Sb2O3 (Sb/Br) and Mg(OH)2. We have shown that the brominated monomer can react either by grafting onto the polymer matrix or onto the glass fibers or by polymerizing around the Sb2O3 particles. We have highlighted that the efficiency of this FR system is connected to its high bromine content and to its reactivity with polypropylene. We have observed that grafting of brominated monomer onto glass fibers does not strongly modify the fire properties of the composites. We have demonstrated that the action of PBB-MA/Sb2O3 can be improved in reinforced PP by combination with Mg(OH)2, which delays ignition. Improvement of the fire properties of the composites has been achieved to the detriment of mechanical properties which are strongly affected by the presence of the brominated polyacrylate.
Keywords: PP/glass fiber composites; Brominated monomer; Magnesium hydroxide; Flame retardancy;

The flame retardant effect of inorganic hydroxides, like aluminum hydroxide and magnesium hydroxide in ethylene vinyl acetate copolymer, was evaluated and compared with that of other inorganic fillers such as boehmite and hydrotalcite. The thermal decomposition of the fillers and the mixtures with EVA polymer were analysed by differential scanning calorimetry (DSC), Thermogravimetry (TG) and X-ray diffraction (XRD). The most significant flame retardant effects, observed using a mass loss calorimeter, indicated that the EVA polymer filled with 50 wt.% of hydrotalcite has the slowest heat release rate and the lowest evolved gas temperature. XRD data, combined with thermal analysis results, indicated that the layered structure of hydrotalcite may play a role in the degradation mechanism. The improved fire resistance of EVA filled with hydrotalcite also results from its intumescent behaviour.
Keywords: Flame retardants; Ethylene-vinyl acetate copolymer; Inorganic hydroxides; Hydrotalcite;

Distribution of a brominated acrylate flame retardant in polypropylene by H Dvir; M Goldraich; M Gottlieb; S Daren; J Lopez Cuesta (465-474).
Thermal polymerization of pentabromobenzylacrylate (PBBMA) in a polypropylene (PP) composite that contains glass fibers and magnesium hydroxide has been studied using scanning and transmission electron microscopy techniques coupled with energy-dispersive spectrometry. The addition of PBBMA imparts flame retardant (FR) properties to the PP composite but also affects adversely its mechanical properties. It is of practical importance to determine the spatial distribution and the extent of polymerization of the FR in the PP composite in order to understand better its role in the system. The methods presented here allow the distinction between the monomeric and polymeric forms of the FR and to determine their spatial distributions. PP itself shows poor adhesion to the glass fibers, which may be improved by the addition of the reactive PBBMA. The latter is polymerized during reactive extrusion through an antimony-catalyzed reaction. Antioxidant hinders self-thermal polymerization but the presence of antimony overcomes this interference. PP shows good adhesion to sized Mg(OH)2 as expected from a properly surface-treated filler.
Keywords: Polypropylene; Flame retardant; Electron microscopy; Bromine; Energy dispersive spectrometry (EDS); Pentabromobenzylacrylate;

Influence of different flame retardants on fire behaviour of modified PIR/PUR polymers by M Modesti; A Lorenzetti; F Simioni; M Checchin (475-479).
The influence of some halogen-free flame retardants (ammonium polyphosphate or ammonium polyphosphate+melamine cyanurate) on fire behaviour of modified polyisocyanurate/polyurethane (PIR/PUR) foams, with a constant isocyanate index equal to 200, have been studied by means of a cone calorimeter. Foams were prepared from isocyanates modified with adipic acid, in order to obtain amide-modified isocyanurate foams that exhibit better thermal stability performance than polyurethane and lower friability than unmodified polyisocyanurate foams. The results obtained demonstrate that an increasing amount of filler causes a slight worsening of physical and mechanical properties. On the other hand, the cone calorimeter results show that the presence of melamine, which causes a rapid decrease of rate of heat release (RHR) and rate of weight loss, considerably improves the fire behaviour of the foams.
Keywords: Amide-modified; Polyisocyanurate foams; Flame retardants; Fire behaviour; Cone calorimeter;

Cone calorimeter study of high performance fibres—application to polybenzazole and p-aramid fibres by Serge Bourbigot; Xavier Flambard; Franck Poutch; Sophie Duquesne (481-486).
This work investigates the fire behaviour of high performance fibres. Poly-p-phenylenediamine-terephtalamide fibres (PPT) and poly(p-phenylene-2,6-benzobisoxazole) (PBO) fibres as knitted fabrics are evaluated using the cone calorimeter as fire model. The investigation of the fire performance of the PPT and PBO fibres shows the excellent behaviour of PBO in comparison with PPT. The contribution to fire growth of PBO is very low whatever the external heat flux is. In comparison with PPT, smoke obscuration of PBO is dramatically reduced as well as the production of carbon oxides during the combustion of the materials.

Flame retardant formulations for cotton by E. Lecoeur; I. Vroman; S. Bourbigot; T.M. Lam; R. Delobel (487-492).
This paper evaluates and compares the flame retardant behaviour of the following flame retardants cellulose fabric: Ignilys FDR [Feutrie S.A.], diguanidine hydrogen phosphate and 3-aminopropyltriethoxysilane. Furthermore the durability to laundering processes is tested. The thermal behaviour of FR-treated cellulosic fabric is studied through TGA analysis.
Keywords: Flame retardant; Cotton; Durability; Phosphorus; Nitrogen, Thermogravimetric analysis; LOI; Burner test;

Thermal degradation of polyurethane and polyurethane/expandable graphite coatings by Sophie Duquesne; Michel Le Bras; Serge Bourbigot; René Delobel; Giovanni Camino; Berend Eling; Chris Lindsay; Toon Roels (493-499).
This study deals with the thermal degradation of polyurethane (PU) and polyurethane/expandable graphite (PU/EG) coatings. EG represents a new generation of intumescent additives which provides good fire retardancy to various materials and in particular to PU. The thermal analyses have shown that the thermo-oxidative degradation as well as the pyrolysis of PU are not affected by the presence of EG. On the other hand, they have enabled the determination of the temperatures which correspond to different steps of the development of the intumescence : formation, stabilisation and degradation of the intumescent shield. Spectroscopic analysis of the residues resulting from each of these steps has demonstrated that the structure of the char is similar for PU and PU/EG and consists in a carbonaceous polyaromatic species. However, the results have suggested that the char formed from PU/EG may contained trapped free radicals.
Keywords: Thermal degradation; Fire retardancy; Expandable graphite; Polyurethane coating;

Unsaturated triglyceride oil (sunflower) was obtained from commercial sources. It was epoxidized and characterized by chemical and spectroscopic methods. The thermo-oxidative degradation of poly(vinyl chloride) at 180 °C was investigated. The effects of different metal carboxylate combinations (Ba/Cd and Ca/Zn stearates) in the presence of epoxidized sunflower oil on static heat treatment of PVC have been studied. In order to compare the efficiency of the epoxidized sunflower oil with these metal soap stabilizers, thermal stabilities were evaluated on the basis of evolved hydrogen chloride determined by potentiometric titration and the degree of discoloration are discussed. The development of color and the dehydrochlorination rates were compared for PVC sheets containing epoxidized sunflower oil (ESO) or epoxidized soya bean oil (ESBO). It was found that ESO exerted a stabilizing effect on the degradation of PVC. Furthermore, the thermal degradation of PVC with metal soaps and ESO was investigated by thermogravimetric analysis (TG) and differential thermogravimetric analysis (DTG).
Keywords: Poly(vinyl chloride); Metal carboxylates; Epoxidized sunflower oil; Thermal stabilization; Thermo-oxidative degradation;

Characterisation of smoke in expanded polystyrene combustion by Mario Rossi; Giovanni Camino; MariaPaola Luda (507-512).
The smoke formed in the cone calorimeter during the combustion of expanded polystyrene with and without fire retardant has been analysed. Gaseous products were condensed from the smoke and identified by GC–MS. The liquid components adsorbed on solid particles of the smoke were examined by microscopy-FTIR. The solid charred particles were characterised by SEM and X-ray microanalysis. It has been shown that pyrolysis dominates over thermal oxidation during combustion. The charred particles are essentially composed of carbon rich structures with negligible oxidation having occurred. The fire retardant somewhat modifies the characteristics of the combustion of expanded polystyrene and the composition of the smoke.
Keywords: Expanded polystyrene, Combustion, Smoke, Flame retardant;

The effect of adding stabilizers to a family of thermoplastic elastomeric blends of natural rubber and polyethylene has been investigated using UV exposures of samples both unstressed and under tensile elongation. Samples were prepared both with dicumyl peroxide crosslinking agent and with no crosslinking agent. The stabilizers used were (i) isopropyl paraphenylene diamine (IPPD); (ii) a high molecular weight phenolic anti-oxidant; and (iii) a commercially available blend of two high molecular weight hindered amine stabilizers. The crosslinked samples showed better photo-resistance than the compounds with no dicumyl peroxide. All of the stabilizers produced considerable improvement in photo-resistance. The most effective one was IPPD (a stabilizer for rubber) even though the continuous phase in these blends is polyethylene.
Keywords: Natural rubber–polyethylene blends; Photo-degradation; Thermoplastic elastomers; UV-stabilizers;

In the stochastic model for infectious spreading of oxidation in a polymer, the conventional view of a homogeneously oxidising polymer with associated rate parameters is replaced by a system consisting of localized zones of oxidation in a continuous unoxidised substrate (a 3-dimensional matrix) with the rate of degradation controlled by the probability of passing the infection to an adjacent site, and the infectious lifetime of the site. The populations in the epidemiological model for spreading of oxidation, p i (the infectious, oxidising fraction), p r (the remaining, unoxidised fraction) and p d (the dead or oxidised fraction) are calculated for a particular site. Oxidation-time profiles are determined by calculating the sum of the component fractions over the entire matrix volume for different activities (i.e. temperatures). Profiles of the infectious fraction, p i, at different activities were found to show features of an accumulating hydroperoxide fraction whereas the true infectious fraction should be more correctly represented by the reactive fraction of the hydroperoxides, p i , at a particular temperature. This model has been tested by comparison with the reactive hydroperoxide profile as determined by chemiluminescence of polypropylene doped with 9,10-diphenyl anthracene to yield the infectious spreading parameters, their temperature dependence, as well as the profiles for the diffusion of the infectious species.
Keywords: Heterogeneous oxidation; Polypropylene; Chemiluminescence; Infectious spreading;

Luminescence is known to be a very sensitive technique for determining the stability of materials. In order to judge the applicability of this method to determining by how much a polymer has already been (pre-)oxidized, the relationship between oxygen uptake and luminescence results is determined. To determine the robustness of this possible relationship, the influence of polymer type, stabilizers, sample geometry and degradation temperature is measured. Several polymers (unstabilized polypropylene (PP) powder, stabilized PP films, unstabilized low-density polyethylene (LDPE) powder, unstabilized LDPE granules, unstabilized polybutylene terephthalate (PBT) powder, unstabilized polyamide 46 (PA46) powder, and unstabilized polyamide 6 (PA6) granules) were degraded at different temperatures, and the oxygen uptake and chemiluminescence in oxygen and nitrogen were measured. The best relationships between oxygen uptake and luminescence results were found for the total light intensity (TLI) measured in a ramp experiment in nitrogen. Especially at the beginning of the oxidation, a straight-line relationship could be obtained between the consumption of oxygen and the TLI. However, this relationship is polymer-, temperature-, and geometry-dependent. Thus ramp CL experiments cannot be used to quantify the degradation of a sample without a previous correlation curve under the same ageing conditions being available. Ramp experiments can be used to determine differences in the oxidation state of samples of comparable shape and with a comparable degradation history.

The synthesis of new stabilizer compounds (a combination of 2,2,6,6-tetramethylpiperidine and 2-hydroxyphenylbenzotriazole in one molecule) is reported. Four polymerizable combined stabilizers as well as two unsaturated triazinyl-2,2,6,6-tetramethylpiperidines and two unsaturated triazinyl-2-hydroxyphenylbenzotriazoles as individual stabilizers were synthesized. Their copolymers and the ter-copolymers of the individual stabilizers with methyl methacrylate were obtained. Chemical bonding of the stabilizers in the polymer was confirmed spectrophotometrically. The influence of these additives on the photostability of the copolymers was studied. The participation of the combined stabilizers in the polymerization did not significantly affect the molecular weight or polydispersity of the copolymers. A significant stabilizing effect against photodegradation was found.

Mechanisms for the surface colour protection of bamboo treated with chromated phosphate by Shang-Tzen Chang; Ting-Feng Yeh; Jyh-Horng Wu (551-557).
The attractive green appearance of bamboo culm discolours easily without any protective treatment. Chromated phosphate (CP) was used successfully to protect the green surface of bamboo. To further understand the mechanisms of this protective treatment, three model compounds including cellobiose, arabinose and guaiacol, in place of the chemical components of bamboo, were used to react with CP. FTIR with liquid attenuated total reflectance accessory (liquid ATR) was conducted to investigate the reaction between the model compounds and CP. The results revealed that guaiacol had the greatest reactivity toward CP, followed by arabinose and cellobiose. Green precipitates were formed in the solution, as reaction time increased. This green complex was similar to a green pigment, chromic phosphate (CrPO4), in chemical structure. This green complex contributed to the green-colour protection of bamboo treated with CP by precipitation and chemical bonding to its epidermis.
Keywords: Discolouration; Green-colour protection; Bamboo; Chromated phosphate (CP); FTIR; Model compound;

Photo-degradation depth profiles in 3 mm thick polypropylene injection mouldings containing (i) hindered amine light stabilizer (HALS); (ii) titanium dioxide (TiO2) pigment; and (iii) both HALS and TiO2, have been compared with those obtained with similar mouldings containing no ultraviolet (UV) stabilizer or pigment. Laboratory UV exposures produced steep degradation profiles in unstabilized samples with very little degradation occurring near the centre even after prolonged exposure. In samples stabilized by HALS the degradation was almost uniform through the thickness. At the surface, the level of degradation in the stabilized bars was much less than in unstabilized bars whereas in the centre the degradation was much more than in unstabilized bars. This is attributed to diffusion-controlled oxidation. The presence of TiO2 confined degradation to a thin region close to the exposed surface.
Keywords: Polypropylene; Photo-degradation; Depth profiling; HALS; TiO2;

Molecular mechanism of the inhibition reaction of lactam peroxy radicals with phenoxy and diphenylaminyl radicals yielding cyclic imide was studied. Ab initio quantum chemical methods—MP2 and DFT were used for models where benzene rings were replaced with methyl groups to find the transition state between the complex of starting radicals and the complex of products. Both methods give qualitatively similar results. For calculations of real systems with phenyl substituents, only DFT method was used. A transition state was found for the reaction of both types of radicals. The reaction can occur through a four-membered ring in the reaction center, breaking the O–O bond of the lactam peroxy radical HCOO with simultaneous transfer of hydrogen from the carbon atom to the departing oxygen. The OH formed combines with the phenoxy or diphenylaminyl radical. The activation barriers for the suggested inhibition are very low (less than 30 kcal mol−1). Final gains of energy for the reaction were found to be about 70 kcal mol−1 for diphenylaminyl radical and about 50 kcal mol−1 for phenoxy radical. Thus, both reactions should be exothermic.
Keywords: Ab initio calculations; Lactams; Reaction mechanism; Transition state;

An alternating styrene-maleic anhydride copolymer was hydrolysed to obtain styrene-maleic acid copolymer. The degradation of the product was studied and the thermal stability of hydrolysed copolymer compared with that of parent copolymer. It was found that alternating styrene-maleic acid copolymer degraded in three stages. FTIR spectra of heated film of copolymer as well as mass spectrometry of volatile products of the decomposition indicate that dehydration is the main reaction, which takes place at the first stage of degradation. However, the decomposition is not only the simple regeneration of the maleic anhydride units. Heating of the styrene-maleic acid copolymer to 250 °C causes an increase of its glass temperature in comparison to T g of the parent copolymer. During heating at 140 °C the hydrolysed styrene-maleic acid copolymer loses its solubility in polar solvents, presumably due to crosslinking. The volatile products of the last two stages of degradation were studied by mass spectrometry.
Keywords: Poly(styrene-maleic anhydride); Poly(styrene-maleic acid); Thermal degradation; Thermogravimetry; Mass spectroscopy; FTIR spectroscopy;

Polymer Yearbook 17 by N.C Billingham (585-586).

Handbook of Polymer Degradation by N.C Billingham (585).

Polymer Modifiers and Additives by N.C Billingham (586-587).

Calendar of events (589-590).