Polymer Degradation and Stability (v.104, #C)

Effects of processing variables on polypropylene degradation and long chain branching with UV irradiation by Yasaman Amintowlieh; Costas Tzoganakis; Savvas G. Hatzikiriakos; Alexander Penlidis (1-10).
A technique has been developed to modify the melt properties of polypropylene (PP). Photoinitiators along with UV irradiation were employed to introduce long chain branching (LCB) and/or crosslinking (CL). Statistically designed experiments were carried out to study the effect of processing conditions, such as photoinitiator concentration, duration of irradiation, UV lamp intensity, cooling air pressure, and photoinitiator type, on rheological properties, molecular weight characteristics and branching level. Samples were evaluated through linear viscoelastic (LVE) measurements, extensional rheometry, gel content, gel permeation chromatography (GPC), and differential scanning calorimetry (DSC). Results clearly indicated that PP can be successfully modified in order to enhance strain hardening behaviour without significant gel formation.
Keywords: Polypropylene; Long chain branching; Crosslinking; UV radiation; Photoinitiator; Rheology;

Corona charging and potential decay on oxidized polyethylene surfaces by Thiago Augusto Lima Burgo; Lia Beraldo Silveira Balestrin; Fernando Galembeck (11-17).
Insulating polymers are highly affected by static electricity, caused by many but only partially understood sources. Charged polyethylene holds charge for many days to weeks that usually goes without monitoring. Here, we describe a simple, non invasive and reliable procedure to detect and survey charge build-up and decay on PE surfaces oxidized by different procedures and characterized by contact angle and staining followed by visible reflectance spectroscopy: corona-charged samples were monitored by scanning with a Kelvin probe and mapping electrostatic potential as a function of time and position. The results show that PE oxidation largely prevents charge build-up and it contributes to faster dissipation of the acquired charge. PE oxidized with KMnO4/H2SO4 solution acquires no detectable corona charge but without harming the PE insulating power along the z-axis, normal to the polymer surface. Since the Kelvin probe technique is a non-contact and non-destructive method, it is potentially useful for monitoring polymer surface oxidation state in online, real time monitoring.
Keywords: Oxidized polyethylene; Kelvin probe; Corona charging; Static electricity; Surface characterization; Anti-static;

Synergistic effect of fumed silica with Ni2O3 on improving flame retardancy of poly(lactic acid) by Jiang Gong; Nana Tian; Xin Wen; Xuecheng Chen; Jie Liu; Zhiwei Jiang; Ewa Mijowska; Tao Tang (18-27).
A novel combined catalyst of fumed silica (SiO2) with Ni2O3 was demonstrated to be more efficient than SiO2 or Ni2O3 alone in enhancing the char yield of poly(lactic acid) (PLA) and improving its char layer structure, which greatly enhanced the flame retardancy of PLA. The results of field-emission scanning electron microscope, transmission electron microscope, X-ray diffraction, Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy revealed that the residual chars mainly consisted of block-like carbon and carbon nanotubes with some surface functional groups. The flame retardancy of PLA and its composites was studied by cone calorimeter test. The heat release rate and total heat release of PLA composites containing both SiO2 and Ni2O3 decreased significantly in comparison with those of neat PLA. The investigation of the synergetic mechanism showed that in the presence of both SiO2 and Ni2O3, a large amount of chars were in situ formed from the catalytic carbonization of degradation products of PLA by Ni2O3 during combustion. This not only reduced the release of flammable degradation products, but also acted as an insulating barrier to prohibit the oxygen and feedback of heat from reaching the underlying substrate. On the other hand, the formation of a percolated network structure of SiO2 and Ni2O3 particles in PLA matrix increased the melt viscosity of PLA and facilitated the formation of a more compacted protective layer, which promoted the flame retardancy of PLA.
Keywords: Poly(lactic acid); Fumed silica; Ni2O3; Flame retardancy; Catalytic carbonization;

A thermal degradation model was developed for blends of Poly(ethylene terephthalate) (PET)/Poly(methyl methacrylate) (PMMA) subjected to non-isothermal (dynamic) thermogravimetry at four heating rates (5, 10, 15 and 20 °C/min). The model developed enabled the assessment of pre-exponential factor (A) and apparent activation energy (E a ) for each polymer individually in the blend after developing a mathematical expression based on an integral solution, which was related to both polymer fractions and their blending characteristics in the form of kinetic parameters. The unique approach presented in this study showed that the apparent activation energy of PET (E a1) in the blend (240–270 kJ/mol) is always higher than PMMA's apparent activation energy (E a2, 140–170 kJ/mol), which is attributed to the degradation mechanism of the blend and the latter being of a lower melting point hence degrading faster. It was also observed that the activation energy of both polymers increased with PET composition and decreased with higher heating rates, which can be attributed to PET acting as an inhibitor to PMMA in the blends.
Keywords: PET; PMMA; Kinetics; Activation energy;

Effect of titanium dioxide on chemical and molecular changes in PVC sidings during QUV accelerated weathering by Teng-Chun Yang; Takafumi Noguchi; Minoru Isshiki; Jyh-Horng Wu (33-39).
Titanium dioxide (TiO2) is the most essential additive in polyvinyl chloride (PVC) matrixes used for outdoor applications. In this study, we primarily investigate the effect of TiO2 on the chemical and molecular changes in PVC sidings during QUV accelerated weathering. The results of this study show that carbonyl and polyene groups were generated on the surface of all of the specimens after 480 h of accelerated weathering but that loading TiO2 into PVC successfully inhibited the increase in the number of these oxidative groups over the entire exposure period compared to PVC without TiO2. In addition, a significant decrease in the number average molecular weight (M n) and the formation of an insoluble gel were observed for PVC without TiO2 after accelerated weathering. However, the time required for the M n to decline increased with the amount of TiO2 that was loaded into the PVC matrix, and no insoluble gel was observed during weathering. Furthermore, the crystallinity of PVC without TiO2 increased noticeably after 1920 h of accelerated weathering, whereas no significant change was observed in the crystallinity of PVC with TiO2. These results demonstrate that simultaneous chain scission and crosslinking reactions occurred in PVC without TiO2, whereas only chain scission occurred in PVC containing TiO2. In addition, the chain scission of PVC without TiO2 was initiated earlier than for PVC with TiO2, producing shorter and more mobile chains that underwent secondary crystallization. Accordingly, TiO2 acted as a UV absorber and a radiation screener such that the probability of chain scission was reduced for TiO2-loaded PVC sidings.
Keywords: PVC siding; Titanium dioxide; Chain scission; Crystallinity; QUV accelerated weathering;

Improvement of the thermal stability of branched poly(lactic acid) obtained by reactive extrusion by F. Carrasco; J. Cailloux; P.E. Sánchez-Jiménez; M.Ll. Maspoch (40-49).
One-step reactive extrusion-calendering process (REX-calendering) has been used in order to obtain sheets of 1 mm from poly(lactic acid) modified with a styrene-acrylic multifunctional oligomeric agent. In a preliminary internal mixer study, torque versus time has been monitored in order to ascertain chain extender ratios and reaction time. Once all the parameters were optimized, reactive extrusion experiments have been performed. An enhanced general analytical equation has been developed in order to evaluate the kinetic parameters of the thermal degradation of PLA sheets manufactured by reactive extrusion. This improvement has consisted of replacing the n-order conversion function by a modified form of the Sestak–Berggren equation f(α) = c (1 − α) n α m that led to a better adjustment of experimental data and also adequately represented the conventional mechanisms for solid-state processes. The kinetic parameters so obtained have been compared to those determined by conventional differential methods and n-order reaction kinetics. Given that the thermal degradation of PLA has been argued to be caused by random chain scission reactions of ester groups, the conversion function f(α) = 2 (α 1/2 − α), corresponding to a random scission mechanism for L = 2 (as well as other functions for L values up to 8), has been tested. Once optimized the kinetic model, the thermal degradation kinetics of sheets obtained by REX-calendering process was compared to that of conventional sheets and polymer matrix.Display Omitted
Keywords: Branched poly(lactic acid); Kinetics; Thermal degradation; Enhanced general analytical equation; Random scission; Reactive extrusion;

Degradative and morphological characterization of POSS modified nanohybrid polyurethane elastomers by James P. Lewicki; Krzysztof Pielichowski; Małgorzata Jancia; Edyta Hebda; Rebecca L.F. Albo; Robert S. Maxwell (50-56).
Reported here is the synthesis and thermal characterization of a series of polyhedral oligomeric silsesquioxane (POSS) modified polyurethane elastomers. A novel polyurethane architecture has been synthesized which incorporates a partially opened POSS-diol cage (disilanolisobutyl POSS) directly into the methylene di-isocyanate/poly(tetramethylene) glycol urethane network as a substitute chain extender moiety without the need for an alkyl tether. The effects of the inclusion of a sterically hindered and rigid silsesquioxane cage structure on both the non-oxidative thermal stability and micro-phase segregated morphology of the resultant polyurethane elastomer have been studied extensively over a range of POSS inclusion levels by means of pyrolysis-gas chromatography/mass spectroscopy (Py-GC/MS) and differential scanning calorimetry (DSC). The results of analytical pyrolysis assays of the polyurethane systems clearly demonstrate that low levels of POSS substitution (<10 wt. %) lead to a significant increase in both the onset temperature of thermal de-polymerization and a reduction in the yield of volatile degradation products. A characterization of the products of degradation demonstrate that the POSS modified elastomers show some subtle differences in thermal degradation mechanism, yielding increased levels of propenal and decreased levels of ethane when compared with an unmodified control. POSS inclusion enhances the hard-block crystallinity at low levels and DSC analysis demonstrates that the peak thermal stability of the systems corresponds with a maximum in hard-block crystallinity (at a level of 4–6 wt. % POSS). At higher mass fractions we observe a breakdown in the phase separation of the systems and a decline in hard-block crystallinity, which correlates with an observed decrease in the primary onset degradation temperature. The major mechanistic pathways of degradation (urethane bond depolymerisation followed by secondary radical degradation of the monomers) have been shown to be insensitive to the presence of POSS in the matrix. Rather, it is physical changes in the morphology of the elastomer systems, as a consequence of POSS inclusion that are responsible for the observed improvements in the thermal stability of these materials.
Keywords: Polyurethane; POSS; Thermal degradation; Block copolymers;

The thermo-oxidation of poly(isobutylene) (PIB) has been investigated by using accelerated heat aging in the 40–100 °C range and functional group analysis by transmission Fourier Transform Infrared (FTIR) spectroscopy. Time–temperature superposition (TTS) and the Arrhenius approach were used to calculate activation energies. Oxidation product evolution was continuous within the temperature range. PIB displayed excellent TTS and linear Arrhenius behavior leading to an energy of activation (E a) range of 116–125 kJ/mol for the growths of the carbonyl (ketone), hydroxyl and β-lactone groups. The similarity between the E a values strongly suggests that the reactions that they comprise are interrelated, agreeing with the currently accepted oxidation mechanisms of PIB.
Keywords: Poly(isobutylene); Butyl rubber; Thermo-oxidation; Activation energy; Arrhenius; FTIR;

Study of high density polyethylene (HDPE) pyrolysis with reactive molecular dynamics by Xiaolong Liu; Xiaoxia Li; Jian Liu; Ze Wang; Bin Kong; Xiaomin Gong; Xiaozhen Yang; Weigang Lin; Li Guo (62-70).
PE pyrolysis is considered as an important process for converting plastic solid waste into fuel or other valuable chemicals to alleviate the environmental burdens. A computational approach can be of help for fully understanding the mechanisms and reaction pathways during PE pyrolysis in various industrial applications. In this paper, ReaxFF molecular dynamics was for the first time applied in simulating the pyrolysis of a High Density Polyethylene (HDPE) model system with 7216 atoms. Detailed reaction mechanisms and generation pathways of primary gas molecules were obtained by further analysis of the trajectories within a total of 250 ps simulation in NVT ensemble at 2000–3000 K using a newly created program, VARMD. The product profile and primary gas products evolution tendencies with temperature agree reasonably with the observations in Py-GC/MS experiments and literature data. The overall kinetics of ∼ C31 were analyzed and used to predict the reaction time for 90% weight loss of HDPE thermolysis that falls in around the reported time observed experimentally in the literatures. The reaction mechanisms based on the detailed reactions from the simulation trajectories are in broad agreement with what reported in literatures. The work presented here validates that ReaxFF MD is a promising method to help building a systematic understanding of the detailed chemical reactions in pyrolytic process for polymers.
Keywords: ReaxFF MD; HDPE pyrolysis; Chemical reaction mechanism; Reactive molecular dynamics; VARMD;

The performance and mechanism of brominated and halogen free flame retardants in formulations of glass fibre reinforced poly(butylene terephthalate) composites were investigated. This study entailed four glass fibre reinforced PBT formulations, namely, (i) glass fibre reinforced PBT without flame retardant, (ii) glass fibre reinforced PBT combined with one brominated flame retardant (10 wt% of brominated polystyrene), (iii) glass fibre reinforced PBT combined with one halogen free flame retardant (15 wt% of aluminium diethyl phosphinate), and (iv) glass fibre reinforced PBT combined with two halogen free flame retardants (15.5 wt% of aluminium diethyl phosphinate and 2.5 wt% of natural montmorillonite clay modified with methyl, octadecyl, bis-2-hydroxyethyl quaternary ammonium salt). Their fire performance is compared using LOI, UL-94, thermogravimetric analysis (TGA), differential thermogravimetry (DTG), evolved Fourier transform infrared gas analysis (TGA-FTIR), Fourier transform infrared solid residue analysis (FTIR-ATR), and cone calorimetric studies. The results of this study show that aluminium diethyl phosphinate combined with natural montmorillonite clay, chemically modified with a quaternary ammonium salt, provides an alternative to brominated flame retardants in glass fibre reinforced PBT composites.
Keywords: Poly(butylene terephthalate); Aluminium diethyl phosphinate; Methyl, octadecyl, bis-2-hydroxyethyl quaternary ammonium salt; Brominated polystyrene; Halogen free flame retardant; Flammability;

A series of flame retardant ethylene-vinyl acetate (EVA) composites, with different content of magnesium hydroxide (MH) and hollow glass microspheres (HGM), have been prepared. The synergistic flame retardant effects of HGM with MH in EVA/MH blends have been studied by limiting oxygen index (LOI), UL-94 tests, cone calorimeter test (CCT), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM), respectively. The LOI and UL-94 results show that when 2.00 wt% magnesium hydroxide (MH) is substituted with HGM, the LOI value can be raised to the maximum value of 42.1 among all samples. The data obtained from the CCT test indicate the heat release rate (HRR) of EVA/MH/HGM sample with 1.00 wt% HGM reduced by 37% when compared with EVA/MH sample without HGM. The digital photo graphs and scanning electron microscope (SEM) of char residues after CCT give positive evidence that the synergistic flame retardant effects between HGM and MH can be described that HGM mixed with MH can decrease the melt viscosity of EVA/MH/HGM, and promote the homogeneous dispersion of MH. HGM also can promote the formation of compact charred layers and prevent the charred layers from cracking, which effectively protects the underlying materials from burning. The TGA results reveal that EVA/MH/HGM samples show higher thermal stability at high temperature than EVA/MH sample.
Keywords: Synergistic; Hollow glass microspheres; Magnesium hydroxide; Ethylene-vinyl acetate;

The effect of changing the ratio of aluminium diethyl phosphinate (as Exolit OP1230) and zinc stannate (as Flamtard S) present within a glass-reinforced high temperature polyamide (HTPA/GF) at a constant total flame retardant level of 15 wt% is shown to have fire performance properties that depend on the ratio of both these agents. Aluminium diethyl phosphinate (AlPi) alone at 15 wt% gives LOI > 40 vol% and a V-0 rating. The introduction of zinc stannate (ZS) at levels up to 3.75 wt% maintains the LOI at about 40 vol% and the V-0 rating. Similarly, glow wire ignition temperatures and cone calorimetry results, in terms of minimal peak heat release rate, show that optimum fire performance also occurs at the [ZS] ≤ 3.75% and [AlPi] ≥ 11.25% condition.Smoke generation reduces only slightly when highest levels of AlPi are present compared with the HPTA/GF sample and then reduces further as the ratio [ZS]/[AlPi] increases. Tensile and impact properties are also optimal for the [ZS] ≤ 3.75 wt% and [AlPi] ≥ 11.25 wt% condition.At the mechanistic level, TGA studies show that for HPTA/GF, the presence of oxygen increases char residue in the 450–550 °C region compared to when heated under nitrogen. Air oxidation further increased residues above 450 °C when AlPi is present and confirms other published work that the reported volatilization occurring during pyrolysis under nitrogen is now in competition with its oxidation to aluminium phosphate. Thus residues above 450 °C in air comprise both increased char from HTPA plus residual aluminium phosphate with the latter remaining above 600 °C after the former has oxidized.
Keywords: Zinc stannate; Aluminium diethyl phosphinate; Polyamide; Flammability; Cone calorimetry; Tensile properties;

Hydrolytic degradation of highly crosslinked polyaromatic cyanate ester resins by Vivek V. Marella; James A. Throckmorton; Giuseppe R. Palmese (104-111).
This work presents a robust method for the study of the hydrolytic degradation of a polyaromatic cyanate ester thermoset. A detailed understanding of the kinetics of formation of phenolic degradation intermediates was determined both by fractional increase in weight and near infrared spectroscopy following exposure to controlled moisture and temperature environments. Evidence of decreased cross-linking density by chain scission degradation was also obtained based on the decrease in glass transition temperature (T g). The effects of catalyst type and concentration on both reaction kinetics and T g were also determined, linking hydrolysis events directly to T g degradation. It was found that low conversion of hydrolyzable bonds can lead to significant decreases in T g values and that the type of catalyst used significantly influences the rate of hydrolysis. The experimental techniques developed in this work were found to be a reliable and reproducible method to characterize the hydrolysis reaction, and can be extended to other cyanate ester systems.
Keywords: High temperature; Hydrolysis; Degradation; Cyanate ester; Infrared spectroscopy;

Structure and thermal behavior of PMMA–polysilsesquioxane organic–inorganic hybrids by Hudson Wallace Pereira Carvalho; Ana Flávia Suzana; Celso Valentim Santilli; Sandra Helena Pulcinelli (112-119).
PMMA–polysilsesquioxane (PMMA–Sil) class II organic–inorganic hybrids were prepared by the sol–gel method from a PMMA-based polymer precursor containing trimethoxysilane groups. An analysis was made of the effect of siloxane content, adjusted by addition of tetraethyl orthosilicate (TEOS), on the structure and thermal stability of the dried gels. 13C nuclear magnetic resonance measurements confirmed PMMA as the organic phase, while 29Si measurements revealed the presence of both T and Q silicon species, the most abundant being T2 and Q3. X-ray diffraction results showed that the inorganic SiO2 phase was amorphous, while small angle X-ray scattering analyses indicated that the average gyration radius size of the silicate particles and the correlation distance between the particles increased with greater addition of TEOS. Thermal stability was improved by increasing the amount of the inorganic phase. This effect was more evident under an air atmosphere than under N2, indicating that the silicate phase acted to limit oxygen diffusion.
Keywords: Hybrid nanocomposites; NMR; SAXS; Thermal stability; PMMA; TEOS;