Polymer Degradation and Stability (v.103, #C)

Effect of two types of iron MMTs on the flame retardation of LDPE composite by Cong Deng; Jing Zhao; Cheng-Liang Deng; Qiang Lv; Li Chen; Yu-Zhong Wang (1-10).
Both MMT synthesized by hydrothermal method (Fe-OMMT) and MMT prepared by modifying MMT using Fe3+ (Fe-MMT) were used to prepare flame-retardant low-density polyethylene (FR-LDPE) together with an intumescent flame retardant (IFR). The effects of two types of MMTs on the flame retardation of FR-LDPE were investigated by different measurements. Combustion tests showed that the limiting oxygen index (LOI) of LDPE/IFR28/Fe-MMT2 was increased to 34.6 from 29 of LDPE/IFR30, and the UL-94 rating of the former reached V-2 from no rating of the latter. Cone calorimeter (CC) test demonstrated that both Fe-OMMT and Fe-MMT improved the combustion behaviors of LDPE/IFR at about 2 wt%. LDPE/IFR/Fe-MMT had a lower heat release rate (HRR), lower total heat release (THR) and more residues than LDPE/IFR/Fe-OMMT at the same loading in the range from 0.5 to 3 wt%. Scanning electron microscopy (SEM) observation showed that both Fe-MMT and Fe-OMMT could promote the formation of continuous and compact intumescent char layer in LDPE/IFR. Both CC and UL-94 tests vitrified that Fe-MMT had better contribution to IFR in fire resistance test. Thermogravimatric (TG) and Mossbauer spectrum etc. revealed that the different efficiency of Fe-OMMT and Fe-MMT should mainly be resulting from the structure difference between Fe-MMT and Fe-OMMT, in which the essential difference might be the different existing state of Fe in two types of MMTs.
Keywords: Iron; MMT; Intumescent flame retardant; LDPE;

Heterogeneity of accelerated photooxidation in commodity polymers stabilized by HAS: ESRI, IR, and MH study by J. Pilař; D. Michálková; M. Šlouf; T. Vacková; J. Dybal (11-25).
Heterogeneity of the photooxidation processes in plaques made of four commodity polymers, polypropylene (PP), polyethylene (PE), polystyrene (PS) and poly(ethylene-co-norbornene) (Topas®, TP) stabilized with hindered amine stabilizer (HAS) were studied using Electron Spin Resonance Imaging (ESRI) and three independent microscopic-scale methods: scanning electron microscopy (SEM), infrared microscopy (IR) and microhardness (MH). Concentration profiles of nitroxides mapping photooxidation process inside polymer plaques along the direction perpendicular to their surface were determined by electron spin resonance imaging technique (ESRI) in dependence on the duration of accelerated photooxidation. We present data characterizing stabilization activity of Chimassorb® 119, Chimassorb® 944, Tinuvin® 770 in combination with UV stabilizer Tinuvin® 320 or Tinuvin® 327, nitroxide HAS Prostab® 5415, and alkoxyamine derivative of HAS Tinuvin® NOR 371 and compare them with the previously published data. Comparison of profiles of oxidation products, crystallinity and microhardness measured inside the aged plaques made of non-stabilized semicrystalline polymers PE and PP and inside the plaques made of the polymers stabilized with Chimassorb® 944 together with SEM micrographs of the irradiated surface layers were found to confirm efficacy of the stabilizer used.
Keywords: Hindered amine stabilizers; Heterogeneity of photooxidation; Electron spin resonance imaging; Microhardness; Oxidation products; Crystallinity;

Enzyme-catalyzed degradation behavior of l-lactide/trimethylene carbonate/glycolide terpolymers and their composites with poly(l-lactide-co-glycolide) fibers by Jianting Dong; Lan Liao; Yan Ma; Li Shi; Gongxi Wang; Zhongyong Fan; Suming Li; Zhiqian Lu (26-34).
A series of high molar mass terpolymers is synthesized by ring-opening polymerization of l-lactide (LLA), 1, 3-trimethylene carbonate (TMC) and glycolide (GA). A totally bioresorbable composite is obtained by reinforcing a terpolymer matrix with plasma treated poly (l-lactide-co-glycolide) (PLGA) short fibers. Solution cast films of the various materials are allowed to degrade at 37 °C at pH 8.5 Tris buffer using proteinase K, in comparison with a PLLA-TMC copolymer and a PLLA homopolymer. Degradation is monitored by using various analytical techniques such as gravimetry, DSC, GPC, 1H NMR and SEM. The results show that the enzymatic degradation rate of PLLA-TMC-GA terpolymers with predominant LLA component is affected by both the average LLA block length (l LLA) and crystallinity. A shorter average LLA block length results in lower crystallinity, which will lead to faster degradation. However, too short average LLA block length (i.e. l LLA ≤4.0) can retard the degradation process. The composition of the copolymers remains unchanged during degradation. In contrast, the molar mass decreases due to hydrolytic chain cleavage in the bulk. Similarly, thermal property changes are observed with increase of the melting temperature and melting enthalpy in most cases. SEM observation strongly supports a surface erosion mechanism. The composite PLTG95/5/5C exhibits lower mass loss rate as compared to neat PLTG95/5/5 due to the presence of PLGA fibers which are non-degradable by proteinase K. On the other hand, the composite shows much faster molar mass loss rate than PLTG95/5/5 because the rapid hydrolytic degradation of PLGA fibers speeds up the degradation of the matrix by internal autocatalysis.
Keywords: Biodegradable; Enzymatic degradation; (l-lactide/trimethylene carbonate/glycolide) terpolymers; Cardiovascular stent;

Impact of photooxidative degradation on the oxygen permeability of poly(ethyleneterephthalate) by Jean-Luc Gardette; Alexis Colin; Serge Trivis; Serge German; Sandrine Therias (35-41).
This article reports a study of the influence of PET photooxidation on the oxygen barrier properties of this polymeric material. It is shown that the photochemically induced oxidation of PET provokes a decrease of the oxygen permeability coefficient, which affects the properties of the material. This effect is shown to result from crosslinking by recombination of macroradicals formed by the photooxidation of PET. The global effect that is observed is thickness-dependent. As a consequence of the attenuation of the UV light that penetrates the sample, photochemical oxidation of PET only occurs in the first 35 microns from the exposed surface, meaning that for samples with a thickness larger than that of the oxidised layer, the permeability decreases at the surface, whereas that of the core of the sample is not modified. This is corroborated by measurements of the variations of permeability coefficients for films with different thicknesses.
Keywords: Photodegradation; Oxygen; Permeability; Crosslinking;

Dynamic mechanical analysis (DMA) measurements have been used for determining the viscoelastic behaviour of polystyrene (PS) nanocomposites containing a 1 wt.% of single- or multi-walled carbon nanotubes, both pristine (SWCNT and MWCNT) and modified by previous treatment with a room-temperature ionic liquid (SWCNTm and MWCNTm). Two sets of multiple frequency tests have been carried out. The Arrhenius relationship has been used to estimate the apparent activation energies for the glass transition. Master curves have been generated using the time–temperature superposition principle. The nanocomposites show an increase of the activation energy up to an 8% with respect to PS, in the order PS + MWCNT > PS + MWCNTm > PS + SWCNTm > PS + SWCNT > PS. The nanophases also induce a displacement of the onset of the storage modulus to longer times following the order PS + SWCNTm > PS + SWCNT > PS + MWCNT > PS + MWCNTm > PS, at 40 °C. At higher temperatures, the storage modulus onset is very similar for all materials. In the case of PS + SWCNTm, the period of time for the onset of the storage modulus is multiplied by a factor of 28 with respect to PS. The surface modification of the SWCNT by the ionic liquid could improve the interfacial compatibility. The results described here show that a very low proportion of carbon nanotubes can effectively improve the long-term stability of PS under service temperature conditions.
Keywords: Polystyrene; Carbon nanotubes; Ionic liquid; Viscoelasticity; Dynamic mechanical analysis; Time–temperature superposition;

The effects of zirconium silicate as a fire retardant reinforcement in the mixture of expandable graphite (EG), ammonium poly phosphate (APP), melamine, boric acid, bisphenol A epoxy resin BE-188(BPA) and ACR Hardener H-2310 polyamide amine are presented. Different formulations were developed to study the effects of zirconium silicate on char expansion, heat shielding, char morphology and composition after fire test. The coatings were tested at 950 °C using Bunsen burner for 1 h. The results show state that the zirconium silicate enhanced fire protection performance of intumescent coating. The morphology of the char was studied by Field emission scanning electron microscope (FESEM) after furnace fire test. X-ray Diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) results showed the presence of graphite, borophosphate; boron oxide and boric acid in the char. Thermogravimetric analysis (TGA) showed that zirconium silicate enhanced residual weight of char. X-ray photoelectron spectroscopy (XPS) analysis showed that 5% zirconium silicate enhanced the carbon content up to 60.87% and lowered oxygen content to 28.09% in the residual char which proved helpful in improving the fire resistance performance of coating. Pyrolysis analysis confirmed that IF5-ZS releases less gaseous products concentration compared to IF-control coating.
Keywords: Expandable graphite; Intumescent coating; Zirconium silicate; Pyrolysis analysis;

Evidence of surface accumulation of fillers during the photo-oxidation of flame retardant ATH filled EVA used for cable applications by J.-F. Larché; G. Gallot; L. Boudiaf-Lomri; C. Poulard; I. Duemmler; M. Meyer (63-68).
This paper is devoted to the photo-oxidation of a fire retardant polymer used for cable jacketing and an emphasis was put on understanding how highly filled composites (Ethylene-Vinyl Acetate filled with Aluminium Tri-Hydroxide) behave under UV irradiation. Samples have been irradiated during several thousand hours and the degradation has been studied at the surface but also within the polymer bulk. The degradation of the polymer part, because of a massive chain scissions process, leads to a progressive enrichment of the inorganic part at the surface which then acts as a screen for the bulk. Thus, the degradation is very heterogeneous along the polymer thickness and the degradation profiles well correlate with the evolutions of the mechanical properties (i.e. mechanical properties are not influenced if the degradation profile remains small compared with the total sample thickness).
Keywords: Photo-oxidation; UV ageing; Halogen free; Flame retardant; EVA; Aluminium hydroxide;

Aminolytic depolymerization of polyarylsulfones by Linghui Wang; Yue Cui; Nachuan Wang; Hong Zhang; Baoku Zhu; Liping Zhu; Youyi Xu (69-74).
Aminolysis reaction between polyarylsulfones and organic amines was investigated. A bimolecular nucleophilic substitution mechanism was proposed. The nucleophilic reagent attacks the aryl ether bond carbon on the aryl ring carrying substituents with strong electron-attracting character. 1H NMR and 1H–1H COSY NMR spectroscopies were adopted to characterize the chemical structure of the products, the terminal groups especially. The effect of polyarylsulfone structure on the reactivity was studied by comparing the reaction rates. The aminolysis kinetics of polyethersulfone with ethylenediamine at different temperature and reagent concentration was studied in detail. The molar mass of the aminolyzed product was determined by GPC and theoretical deduction, and they showed great consistency.
Keywords: Polyarylsulfone; Aminolysis; Chemical degradation; Aromatic nucleophilic substitution;

Water-soluble polymers contaminating industrial effluents create potential pollution concerns in water resources. In particular, polyvinyl alcohol (PVA), which is quite abundant in wastewater effluents, is toxic and cannot be effectively treated by biological processes. The present study investigates the photo-oxidative degradation of aqueous PVA solutions in a UV/H2O2 photochemical reactor and the effect of hydrogen peroxide feeding strategies on the photoreactor performance. The total organic carbon (TOC) removal and the reduction of polymer molecular weights are determined for different initial polymer and hydrogen peroxide concentrations. By treating an aqueous solution containing 500 mg/L of a 130 kg/mol PVA and H2O2/PVA mass ratio of 10, the results show that nearly 87% of TOC removal was achieved when the photoreactor was operated for 2 h in stepwise operation (semi-batch). Accordingly, the number average molecular weight of the polymer is reduced to 10.9 kg/mol (91.6%). The oxidation reaction without UV light, however, is much less effective and results in merely 43% TOC removal and 21% reduction of polymer molecular weights. As for the case where the UV reactor was operated with no hydrogen peroxide, almost no PVA polymer degradation is achieved. This study also shows that the feeding strategy of hydrogen peroxide can have a significant impact on the degradation of PVA molecules in the UV/H2O2 process.
Keywords: Advanced oxidation process; UV/H2O2 process; Polyvinyl alcohol degradation; Water-soluble polymer; Feeding strategy;

The thermal degradation behavior and fire performance of a series of halogen-free flame-retardant high impact polystyrene (HIPS) composites containing magnesium hydroxide (MH) and microencapsulated red phosphorus (MRP) were studied by various means. It is shown that the oxidation of MRP has much influence on the thermal degradation behavior of the composite. This oxidative reaction releases heat and causes mass gain when the composite is degraded in air. No exothermic peak and mass gain peak appear in nitrogen under identical conditions. The HIPS composite containing both MH and MRP generate a smooth, compact, continuous and stable charred residue layer upon degradation in air and in flame. The residue is mainly composed of amorphous magnesium phosphate and black carbon. At the same loading level of 50 wt%, the composite filled by either MH or MRP individually burns out, drips vigorously and fails to pass the UL-94 test. However, the HIPS/MH/MRP composite self-extinguishes quickly and reaches the V-0 rating with ease. There is appreciable synergism between MH and MRP on the flame retardancy of HIPS. The combination of both flame retardants enhances the charring capacity and fire retardancy of the composite and decreases the loading of flame retardants remarkably. The flame retardant effect occurs mainly in the condensed phase.
Keywords: High impact polystyrene; Magnesium hydroxide; Microencapsulated red phosphorus; Thermal degradation; Flame retardancy;

A novel cyclotriphosphazene-based epoxy compound (PN-EPC) as a halogen-free reactive-type flame retardant was synthesized via a two-step synthetic route. The chemical structures and compositions of the cyclotriphosphazene precursor and the final product were characterized by 1H, 13C, and 31P NMR spectroscopy, Fourier transform infrared spectroscopy, mass spectroscopy, and elemental analysis. A series of thermosetting systems based on a conventional epoxy resin and PN-EPC were prepared, and their thermal curing behaviors were investigated. These epoxy thermosets achieved a significant improvement in glass transition temperature and also gained the good thermal stability with a high char yield. The incorporation of PN-EPC could impart an excellent nonflammability to the epoxy thermosets due to a synergistic flame retarding effect as a result of the unique combination of phosphorus and nitrogen from the phosphazene rings, and these epoxy thermosets achieved the high limiting oxygen indexes and the UL-94 V-0 rating when 20 wt.% of PN-EPC was added. The study on flame-retardant mechanism indicates that the pyrolysis products of phosphazene rings acted in both the condensed and gaseous phases to promote the formation of intumescent phosphorus-rich char on the surface of the epoxy thermosets. Such a char layer can supply a much better barrier for underlying thermosets to inhibit gaseous products from diffusing to the flame, to shield the surface of the thermosets from heat and air, and to prevent or slow down oxygen diffusion. As a result, the resulting epoxy thermosets obtained an excellent nonflammability.
Keywords: Cyclotriphosphazene-based epoxy compound; Synthesis; Thermosetting system; Curing kinetics; Flame retardancy;