Polymer Degradation and Stability (v.115, #C)

Isocyanate-based polyimide foams (PIFs) with different dosages of liquid tri (1-chloro-2-propyl) phosphate (TCPP) and micro-sized hydrotalcites (LDHs) particles alone, as well as different mixing ratios of TCPP and LDHs, were prepared via a one-step process in this work. Limiting oxygen index (LOI) and cone calorimeter test (CCT) results indicated that TCPP exhibited more pronounced flame retardant efficiency than LDHs for isocyanate-based PIFs. However, scanning electron microscopy images and digital macrostructural images results showed that, in contrast to LDHs, when the dosage of TCPP exceeded 10% it caused a clear cracking effects on the macro-cellular structure and opening cell effects on the micro-cellular structure for isocyanate-based PIFs. Because the dramatically volatilization of TCPP during the postcuring process caused obvious cellular contraction in the isocyanate-based PIFs. Meanwhile, the use of TCPP also obviously decreased the thermal stability of isocyanate-based PIFs unlike LDHs. However, when these two flame retardants were used in combination, they could effectively enhance the fire resistance and ensure macro- and micro-cellular structures of the isocyanate-based PIFs, unlike standalone use of TCPP and LDHs. When 10% TCPP was simultaneously used with 10% LDHs, the macro- and micro-cellular structures of the resultant foams were clearly improved compared with foams prepared using TCPP only. These results were believed to be attributable to LDHs dispersion in the foams, which enhanced the strength of cellular windows and skeletons, then restrained the cell contraction. Compared with foams without flame retardants, the fire resistance of the isocyanate-based PIFs prepared with 10% TCPP and 10% LDHs was obviously enhanced; specially, the LOI was enhanced by 29.4%, and the peak of heat release rate (PHRR) decreased by 36.1%. Thus, the use of liquid and solid flame retardants in combination may effectively yield isocyanate-based PIFs with high quality cellular structures and excellent fire resistance.
Keywords: Isocyanate-based polyimide foams; Tri (1-chloro-2-propyl) phosphate; Hydrotalcites; Fire resistance; Cellular structure;

Subcritical water and soluble alkali were applied to hydrolyze thermosetting polyester resins of fiber reinforced plastic (FRP) to recover a styrene-fumaric acid copolymer (SFC) in high yield. SFC is a functional polymer and has the same molecular structure as that of styrene-maleic acid copolymer, which is applied as high value-added additive. Potassium hydroxide (KOH) contributed to accelerate hydrolysis and to provide water solubility to the SFC. The optimized reaction conditions based on the SFC yield were at either a temperature of 230 °C, reaction time of 2 h, and with a KOH concentration of 0.38 mol/L, or 230 °C, 1 h, and 0.72 mol/L for sodium hydroxide (NaOH). It was verified that this method had the potential to recycle thermosetting polyester resin of FRPs to produce SFC.
Keywords: Subcritical water; Thermosetting polyester resin; Recycling; Hydrolysis; Styrene-fumaric acid copolymer; Glycol;

Biodegradation of Gum tragacanth acrylic acid based hydrogel and its impact on soil fertility by Saruchi; Balbir Singh Kaith; Rajeev Jindal; Vaneet Kumar (24-31).
In the present work, biodegradation study of Gum tragacanth-acrylic acid based hydrogel was conducted using soil burial and composting methods. Within 77 days, complete degradation of synthesized hydrogel was observed in the composting method, while 92.29% of degradation was observed in soil burial method. Scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) techniques gave the evidence and progress of biodegradation at different stages. Impact of biodegradation of hydrogels on soil fertility was studied through macroanalysis of soil before and after biodegradation studies. Plant growth of Phaseolus vulgaris showed that test sample soil was better for plant growth in comparison to the garden soil. Synthesized hydrogel was also evaluated for controlled release of fertilizer. Fertilizer release kinetic through the synthesized matrix showed Non-fickian diffusion behaviour.
Keywords: Hydrogel; Gum tragacanth; Soil burial; Composting; Lipase and fertilizer release;

The effect of thermo-oxidation on the continuous stress relaxation behavior of nitrile rubber by Jiaohong Zhao; Rui Yang; Rossana Iervolino; Bas van der Vorst; Stellario Barbera (32-37).
Continuous stress relaxation of nitrile rubber (NBR) was characterized in air and N2 at high temperatures (125 °C and 165 °C). Stress increase (“rebound”) after decay was observed in air, but not in N2. Chemical structure, crosslinking density and hardness investigations were carried out to gain insights into the underlying mechanism that governs such behavior. Results indicate that at later stage thermo-oxidative crosslinking prevailed over chain scission causing stress rebound. The effect of atmosphere, temperature and original strain on the stress relaxation is also discussed.
Keywords: Nitrile rubber (NBR); Thermo-oxidative aging; Stress relaxation; Rebound;

The influence of carbon nanotubes on the combustion toxicity of PP/intumescent flame retardant composites by Qiangjun Zhang; Jing Zhan; Keqing Zhou; Hongdian Lu; Wenru Zeng; Anna A. Stec; T. Richard Hull; Yuan Hu; Zhou Gui (38-44).
In recent years, carbon nanotubes (CNTs) have emerged as a promising candidate for improving the flame retardancy of polymer materials, as well as other physical properties. However, few researches have been focused on the influence of this nanoscale material on the combustion toxicity of polymer composites during combustion. In this work, the fire toxicity of polypropylene (PP) composites with intumescent flame retardants (IFRs) and CNTs has been investigated by a Purser Furnace apparatus, which is called steady state tube furnace (SSTF) and enables different fire stages to be created. The Thermo gravimetric analyzer (TGA) and derivative thermo gravimetric analysis (DTG) data indicate that the thermal stability of PP composites was increased by the addition of IFRs or CNTs. However, the SSTF results show that PP samples with IFR or CNTs or both, produced much more carbon monoxide (CO) compared to neat PP during all fire stages, resulting in a much lower CO2/CO ratio. Furthermore, an interesting finding is that the effect of CNTs on the smoke production and CxHy yield of the PP samples during the combustion changes with the combustion equivalence ratio. It indicates that the presence of CNTs promote the formation of smoke particulates from hydrocarbon, but this effect only exist when oxygen supply is not adequate. It is also concluded that the air ventilation and combustion temperature play significant roles in the fire effluent production of PP samples and the morphology of soot particulates.
Keywords: Intumescent fire retardant; Steady state tube furnace; Smoke; CO; Hydrocarbon; CNTs;

The kinetics of poly(ethylene terephthalate) depolymerization by glycolysis has been commonly modeled as a second-order reaction. However, the occurrence of an activation stage and anomalous kinetic behavior does not fit into this reaction model. Utilizing the concept of reference theoretical master plots, we show in terms of generalized kinetics that the chain depolymerization can be controlled by a nucleation-based kinetic behavior. We associate the reported anomalous “activation” stage and chain scission-induced recrystallization in PET glycolysis with the induction period and chain reordering processes during polymer nucleation and growth. The kinetic profile and microscopic images of the polymer surface during degradation support the mechanism of sporadic nucleation and lamellar thickening. Using the matching nucleation-based conversion function we obtained an activation energy of 29 kJ/mol, the closest value reported so far to the experimental and simulated activation energy of thermal and glycolytic breakage of a PET ester bond.Display Omitted
Keywords: PET glycolysis; Depolymerization; Generalized kinetics; Master plot;

High impact polystyrene (HIPS) based composite filled by magnesium hydroxide (MH) and microencapsulated red phosphorus (MRP) with an alternating layered structure was prepared through layer-on-layer laminating. One of the two alternating layers consists of HIPS matrix filled by MH and MRP and the other layer is composed of pure HIPS. Thermo-oxidative degradation behavior and fire performance of the alternating composite were investigated in detail. It is shown that the flame retardants are dispersed alternately along the thickness direction of the composite. Thermo-oxidative degradation of the alternating composite is hindered appreciably. The alternating composite can not only produce more residue upon thermal degradation and burning, the residue is also more continuous and compact than that produced by its homogeneous counterpart with identical loading of flame retardant. Due to shielding effect of the compact char residue layer, thermal transmission along the thickness direction of the alternating composite is impeded noticeably under forced flaming conditions. On the whole, the alternating composite exhibits better thermo-oxidative stability, flame retardancy, smoke suppression and decreased toxic gas release in contrast with its homogeneous counterpart containing the same amount of flame retardant. This work provides another method to further improve the flame retardancy and smoke suppression of polymer composites.
Keywords: High impact polystyrene; Thermal degradation; Flame retardancy; Smoke suppression; Alternating structure;

A novel reactive flame-retarded epoxy resin system was prepared by copolymerizing diglycidyl ether of bisphenol-A (DGEBA) with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), N,N'-bismaleimide-4,4'-diphenylmethane (BDM) and 4,4'-diamino-diphenyl sulfone (DDS). Curing behavior, thermal and flame-retardant properties of the cured epoxy resins were investigated by differential scanning calorimeter (DSC), thermogravimeric analysis (TGA), limited oxygen index (LOI) measurement, UL94 test and cone calorimeter. The results indicated that phosphaphenanthrene group was introduced into the multicomponent system by addition reaction of DOPO with BDM. Compared with traditional DOPO-DGEBA systems, the EP/DDS/BDM/DOPO thermosets showed greatly improved glass transition temperatures (210–223 °C). The results of combustion tests indicated that the addition of BDM or DOPO into DGEBA could improve the flame resistance of the thermosets. Most importantly, the flame-retardant property was further improved when BDM and DOPO coexisted in the epoxy resin systems. For example, compared to the control samples, the EP/DDS/BDM/DOPO-15 thermoset displayed better flame retardancy with higher LOI value and UL94 rating, lower peak of heat release rate (pk-HRR) and average of effective heat of combustion (av-EHC) under the same content of BDM and phosphorus, strongly confirming the synergistic effect of BDM and DOPO. In addition, in a particular proportion, BDM and DOPO synergistically functioned in the condensed-phase and gaseous-phase at the same time. The flame retardant mechanism was studied by TGA and cone calorimeter coupled with the analysis of the char residues.
Keywords: Epoxy resin; DOPO; Maleimide; Synergistic effect; Flame retardant;

In vitro degradation of an aromatic polyanhydride with enhanced thermal properties by Sabrina S. Snyder; Theodore J. Anastasiou; Kathryn E. Uhrich (70-76).
Polyanhydrides have been studied as a drug delivery vehicles due to their surface-eroding behavior which results in zero-order release. However, many polyanhyrides have thermal and solubility properties that make them difficult to formulate for these applications. Poly[α,α′-bis(ortho-carboxyphenoxy)-para-xylene] (oCPX) is an aromatic polyanhydride that has thermal and solubility properties enabling facile processing. The polymer's in vitro degradation profile exhibited an induction period up to 10 days in which degradation product concentration in the media was minimal, followed by a period of stable release of the biocompatible degradation product. Scanning electron microscope images and molecular weight changes of the polymer matrices confirm that this polymer is primarily surface-eroding. The combination of thermal properties, solubility, polymer degradation time, and erosion mechanism indicate that poly(oCPX) is be a suitable matrix candidate for extended, controlled drug delivery.
Keywords: Biocompatible; Biodegradable; Polyanhydride; Aromatic polyanhydride; Surface erosion;

The combination of aluminum trihydroxide (ATH) and melamine borate (MB) as fire retardant additives for elastomeric ethylene vinyl acetate (EVA) by Carmen Hoffendahl; Gaëlle Fontaine; Sophie Duquesne; Frank Taschner; Martin Mezger; Serge Bourbigot (77-88).
Fire retardancy and smoke release of ethylene vinyl acetate (vinyl acetate content of 60%, EVA) with aluminum trihydroxide (ATH) and melamine borate (MB) was evaluated by mass loss cone calorimetry, limiting oxygen index (LOI), UL-94 and a home-made smoke test and compared to materials already reported in literature. It was found that the combination of ATH and MB results in improved fire retardant properties as well as in decrease smoke release in comparison to the virgin polymer. To evaluate the contribution of MB in the material, the decomposition mechanism of EVA-ATH-MB was examined. First, using scanning electron microscopy (SEM), it was shown that ATH and MB are evenly dispersed in the polymeric matrix. The thermal decomposition of EVA-ATH-MB was investigated by thermogravimetric analysis (TGA). Gases released in a fire scenario were analyzed using mass loss calorimeter coupled with a Fourier transform infrared spectrometer (MLC-FTIR). The condensed phase of EVA-ATH-MB was examined using solid state nuclear magnetic resonance (NMR) of 13C, 27Al and 11B. Moreover, thermal conductivity of EVA-ATH-MB in function of the temperature was measured and compared to that of EVA-ATH materials containing melamine (MEL) or melamine phosphate (MP) instead of MB. It was found that EVA-ATH-MB is protected through a gas and a condensed phase mechanism, whereas MB contributes to the formation of a protective layer due to formation of boron oxide, boron nitride and BNO structures at the sample surface. The formed boron species further lead to decrease of the thermal conductivity in the range of the ignition temperature of the polymer leading to ignition at longer times of EVA-ATH-MB in comparison to the other tested materials.
Keywords: Flame retardancy; Ethylene vinyl acetate; Aluminum trihydroxide; Melamine borate; Solid state NMR; MLC-FTIR;

Smoke composition using MLC/FTIR/ELPI: Application to flame retarded ethylene vinyl acetate by Franck-Estime Ngohang; Gaëlle Fontaine; Laurent Gay; Serge Bourbigot (89-109).
This paper is devoted to the interpretation of smoke data in terms of the fire behaviour of ethylene vinyl acetate unfilled and filled with aluminium trihydroxide (EVA and EVA/ATH), under different fire scenarios: 25, 35, 50 and 75 kW/m2. Flammability parameters, evolved gases and soot particles were evaluated for both materials (EVA and EVA/ATH) as a function of applied heat flux, using a combined Mass Loss Cone, Fourier Transform Infrared spectroscopy and Electrical Low Pressure Impactor (MLC/FTIR/ELPI) in a simultaneous analysis bench test. It was found that the EVA formulation leads to the formation of a carbonaceous layer (char) at low heat flux (25 kW/m2), which blocks the release of decomposition gases, including flammable gases, thereby delaying the onset of piloted ignition of EVA relative to EVA/ATH. Total Heat Release (THR) measurements during tests performed on EVA show that this value remains constant at all heat fluxes, which is consistent with the fact that no residue of the EVA samples remains at the end of the fire tests. In the case of EVA/ATH however, the measured THR is proportional to the external heat flux because the quantity of combusted material increases as a function of the thermal stress applied. Calculation of the average effective heat of combustion (AEHC) of the EVA and EVA/ATH material yielded similar values, in both cases, under all four fire scenarios. With respect to gas phase analysis, the concentration of acetic acid (CH3COOH) release in the smoke of EVA (from the de-acetylation of vinyl acetate) was found to increase as the heat flux decreases, with a significant difference at 25 kW/m2 relative to 35, 50 and 75 kW/m2. Concerning EVA/ATH, it appears that there is a transformation of CH3COOH to acetone (CH3COCH3) attributed to the catalytic effect of Al2O3, which increases as the heat flux increases. Calculation of gas species yields revealed that the CO yields of EVA/ATH are higher than those of neat EVA, which can be ascribed to the incomplete combustion reaction of the flame retarded material (EAV/ATH) as compared to neat EVA; an exception was nevertheless noted at 25 kW/m2, where carbonization of EVA was also reported. In relation to ex situ analysis of particle size distribution and concentration in the EVA and EVA/ATH smoke, it was shown that ultrafine and fine particles are present in much higher concentration than particles above 1 μm (≥1 μm) for both formulations at all applied heat fluxes. Nonetheless, the more ultrafine particles (those of 6 nm) were solely detected in the EVA/ATH smoke, suggesting that the ATH flame retardant additive may promote the release of these ultrafine particles.
Keywords: Smoke; Flame retardant; Aluminum trihydroxide; EVA; FTIR; ELPI;

Study on a novel multifunctional nanocomposite as flame retardant of leather by Yuanping Jiang; Jiaxun Li; Bo Li; Hongyan Liu; Zhengjun Li; Lixin Li (110-116).
The key of leather flame retardant technology is the development of flame retardant materials. A novel intumescent flame retardant (IFR), as the flame-retardant intermediate, was firstly synthesized, then a novel nanocomposite was successfully prepared from the IFR and montmorillonite modified by cetyl trimethyl ammonium bromide (CTAB) and collagen. Its structure and properties were characterized by XRD and FT-IR. The flame retardant was added to leather and its effects on flame-retardant properties of leather were studies by vertical burning test, limiting oxygen index (LOI) test and cone calorimeter test, respectively. The thermal stability and morphology of the flame-retardant leather were characterized by TG and SEM. The results showed that the novel nanocomposite has good flame-retardant properties and can enhance the fire retardancy of leather effectively.
Keywords: MMT; IFR; Leather; Flame retardant; Nanocomposite;

Influence of organophosphorous silica precursor on the thermal and fire behaviour of a PA66/PA6 copolymer by J. Sahyoun; V. Bounor-Legaré; L. Ferry; R. Sonnier; A. Bonhommé; P. Cassagnau (117-128).
An organophosphorus silica filler was generated in situ the molten PA66 copolymer (Cop-PA) during extrusion process without adding solvents. Nanocomposites synthesis is based on hydrolysis–condensation reactions of diethylphosphatoethyltriethoxysilane precursor (SiP). The thermal stability of Cop-PA/SiP nanocomposites and the major degradation products were studied by thermogravimetric analysis (TGA) and TGA coupled with Infrared spectrometry (TGA/FTIR). The Pyrolysis Combustion Flow Calorimeter (PCFC) and cone calorimeter were used to investigate the fire behaviour of the nanocomposites. Results showed a decrease by more than 50% of the peak of heat release rate (PHRR) and the formation of an expanded char layer with the addition of only 0.91wt% of silicon and 0.96wt% of phosphorus by this original route of fillers dispersion.
Keywords: PA66 copolymers; Diethylphosphatoethyltriethoxysilane; Reactive extrusion; Sol–gel; In situ nanocomposites; Fire-retardancy;

Thermo-oxidative resistant nanocomposites containing novel hybrid-nanoparticles based on natural polyphenol and carbon nanotubes by R. Arrigo; N. Tz. Dintcheva; M. Guenzi; C. Gambarotti; G. Filippone; S. Coiai; S. Carroccio (129-137).
Quercetin (Q), a natural antioxidant molecule, is physically immobilized onto multi-walled carbon nanotubes (CNTs) bearing covalently-linked long-chain alkyl functional groups, and the so obtained hybrid-nanoparticles are used to prepare Ultra High Molecular Weight PolyEthylene-based nanocomposite films with enhanced thermo-oxidation resistance. The effective immobilization of the Q molecules is confirmed by spectroscopic (micro-Raman, ATR-FTIR, and FTIR) and thermo-gravimetric analyses, and the influence of the nanoparticles on the rheological behaviour and thermo-oxidative stability of the nanocomposites are investigated. Rheological analyses (linear viscoelasticity and stress relaxation tests) and morphological observations reveal that the Q-functionalized CNTs disperse better than bare CNTs in the host matrix. Quercetin confirms to be an excellent anti-oxidant for polyethylene, but the study of the thermo-oxidation behaviour shows that a remarkable stabilizing action only emerges when Q is physically immobilized on the CNTs. In particular, a ten-fold increase of the onset of degradation phenomena in thermo-oxidative environment was found. Such an excellent result is due to a synergic effect stemming from the physical interaction between Q and CNTs, which cannot provide a similar stabilizing action if used separately. In particular, we argue that the process of physical immobilization of the Q molecules causes the formation of structural defects onto outer CNTs surfaces, thus remarkably improving the CNTs radical scavenging activity and probably promoting Q regeneration. In addition, CNTs seem acting as efficient nano-carriers for the quercetin molecules, improving the dispersion of the latter in the host matrix in spite of their poor solubility.
Keywords: Quercetin; Multi-walled carbon nanotubes; Thermo-oxidation resistance; Ultra-high molecular weight polyethylene (UHMWPE); Nanocomposites;

Development of pyrolysis models for charring polymers by Jing Li; Junhui Gong; Stanislav I. Stoliarov (138-152).
Controlled atmosphere, radiation-driven gasification experiments were conducted on a series of synthetic polymers including poly(acrylonitrile butadiene styrene), poly(ethylene terephthalate), poly(methyl methacrylate)-poly(vinyl chloride) alloy (Kydex) and polyetherimide. Mass loss rate and non-radiated surface temperature of coupon-sized material samples were measured simultaneously and recorded as a function of time. These temperature data were combined with the results of broadband radiation absorption measurements and previously conducted thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) to characterize the transport of thermal energy inside the gasifying materials through inverse modeling. Subsequently, complete pyrolysis models, based on the kinetics and thermodynamics of the thermal decomposition derived from the TGA and DSC experiments, were formulated and employed to predict the mass loss rate histories obtained at 30–90 kW m−2 of external radiant heat flux simulating fire exposure. Satisfactory predictions were obtained for all materials with the exception of polyetherimide, which highly intumescent behavior introduced large uncertainties in the gasification conditions.
Keywords: Polymer flammability; Burning rate; Thermophysical properties; Intumescence; ThermaKin;