Polymer Degradation and Stability (v.136, #C)

In this study, we explore the interest of recycling PA11 and PP unidirectional flax fibre composites. After describing the mechanical performance of the various components of the composite, its tensile properties are investigated before and after recycling. The results indicate a good stability of the modulus and strength of the composites during the multiple recycling steps, which also applies to the fibre properties, as highlighted by nanoindentation tests throughout the recycling process. However, interestingly, the elongation at break of the PA11 composites shows a significant increase with the number of injection cycles, ranging from 5.2 to 14.7%. Further analyses show that the PA11 matrix has highly stable thermal, rheological and mechanical properties, so the matrix cannot be involved in this increased elongation at break. Flax fibers display a steady decrease in length with recycling, especially in the case of PA11 matrix compared to PP matrix. This drastic decline is explained by the high viscosity of the matrix, but above all by the presence of defects (kink bands) in the flax fibers, spaced at intervals of 60 μm, which is consistent with the residual lengths measured at the end of recycling. PA11 and PP flax UD composites prove to be very efficient after recycling, while PA11 biocomposites exhibit an increase of elongation after recycling that makes it a material of first choice in the biocomposites industry.
Keywords: Flax fibre; Recycling; Degradation; Polyamide 11; Polypropylene; Mechanical properties; Rheology;

Degradation of fluoroelastomers in rapeseed biodiesel at different oxygen concentrations by S. Akhlaghi; A.M. Pourrahimi; C. Sjöstedt; M. Bellander; M.S. Hedenqvist; U.W. Gedde (10-19).
The degradation of fluoroelastomers (FKM) based on different monomers, additives and curing systems was studied after exposure to rapeseed biodiesel at 100 °C and different oxygen partial pressures. The sorption of fuel in the carbon black-filled FKM terpolymer was promoted by the fuel-driven cavitation in the rubber. The bisphenol-cured rubbers swelled more in biodiesel than the peroxide-cured FKM, presumably due to the chain cleavage caused by the attack of biodiesel on the double bonds formed during the bisphenol curing. With any of the selected types of monomer, the FKM rubbers absorbed biodiesel faster and to a greater extent with increasing oxygen partial pressure due to the increase in concentration of the oxidation products of biodiesel. Water-assisted complexation of biodiesel on magnesium oxide and calcium hydroxide particles led to dehydrofluorination of FKM, resulting in an extensive fuel uptake and a decrease in the strain-at-break and the Young's modulus of the rubbers. An increase in the CH2-concentration determined by infrared spectroscopy, and the appearance of biodiesel flakes in scanning electron micrographs of the extracted rubbers, were explained as being due to the presence of insoluble biodiesel grafted onto FKM on the unsaturated sites resulting from dehydrofluorination. The extensibility of the GFLT-type FKM was the least affected on exposure to biodiesel because this rubber contained less unsaturation and metal oxide/hydroxide particles.
Keywords: Degradation; Fluoroelastomers; Biodiesel; Chain cleavage; Dehydrofluorination;

Evolution of radial heterogeneity in polyacrylonitrile fibres during thermal stabilization: An overview by Srinivas Nunna; Minoo Naebe; Nishar Hameed; Bronwyn L. Fox; Claudia Creighton (20-30).
Thermal stabilization of polyacrylonitrile (PAN) fibres is an essential step in the carbon fibre manufacturing process. The formation of radial heterogeneity in polyacrylonitrile precursor fibres in the thermal stabilization process is an important issue that is not well understood and needs to be addressed as it affects the quality of the resultant carbon fibres. Hence, in this review we put forward the recent developments on the evolution of radial heterogeneity in the PAN fibres during the thermal stabilization process.
Keywords: Radial heterogeneity; Thermal stabilization; Polyacrylonitrile precursor; Carbon fibres;

In-vitro and in-vivo degradation studies of freeze gelated porous chitosan composite scaffolds for tissue engineering applications by Saad B. Qasim; Shehriar Husain; Ying Huang; Maksym Pogorielov; Volodymyr Deineka; Mykola Lyndin; Andrew Rawlinson; Ihtesham Ur Rehman (31-38).
Tissue engineering approaches have been adapted to reconstruct and restore functionality of impaired tissue for decades. Porous biomimetic composite scaffolds of Chitosan (CH) with hydroxyapatite (HA) for bone regeneration have also been extensively studied in the past. These porous scaffolds play a critical role in providing successful regeneration by acting as a three-dimensional template for delivering nutrients and metabolites and the removal of waste by products. The aim of the current study was to investigate in-vitro and in-vivo degradation rates of porous freeze gelated chitosan (CH) and CH hydroxyapatite scaffolds by scanning electron microscopy (SEM) to observe for morphological changes, Fourier Transform Infrared Spectroscopy (FTIR) in conjunction with photo-acoustic sampling (PAS) accessory for the analysis of chemical changes, pH analysis and UV–Vis spectroscopy of degraded supernatant. SEM results showed significant alterations in the surface morphology. FTIR-PAS spectra showed changes in the finger print region and glycosidic bonds showed signs of breakage. pH values and UV–Vis spectroscopy of the degraded supernatant were indicative of CH bonds scission in neat samples. HA incorporated specimens showed more stability. Histological sections performed after in-vivo implantation also showed greater cellular infiltration and delayed degradation profiles by HA loaded samples. Within 30 days of implantation, neat CH scaffolds showed complete in-vivo biodegradation. The current findings show the advantage of adding hydroxyapatite to porous templates which enhances hard tissue regeneration. In addition, it allows easy and cost effective fabrication of bioactive composite scaffolds.
Keywords: Chitosan; Porous; Scaffolds; Degradation; Hydroxyapatite; In-vivo;

Thermal degradation of two series of chemically well-defined polyurethane (PU) elastomers have been studied using thermogravimetry coupled with a mass spectrometer (TG-MS), differential thermal analysis (DTA) and differential scanning calorimetry (DSC). The samples were synthesised such that they contained two different soft segments (SSs) that are of great interest for aerospace applications. At a given soft fragment, four different hard segments (HSs) based on single diisocyanate molecules (i.e., isophorone diisocyanate (IPDI), toluene-2,4-diisocyanate (TDI), 4,4′-diphenyl methane diisocyanate (MDI) and hexamethylene diisocyanate (HMDI)) were used with no chain extenders. Each segmented PU family was prepared by stoichiometric reactions of macrodiol hydroxyl-terminated polybutadiene (HTPB) and a metallocene-derived one (i.e., (ferrocenylbutyl)dimethylsilane grafted on HTPB, which is also known as Butacene). The structure of the macroglycols and the prepared PUs were studied using Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopies. Both the catalytic effect of the ferrocenyl group and the influence of the HS nature on the thermal behaviour of these PUs polymers have been elucidated. In addition, the gaseous volatile products released during the thermal decomposition process have been investigated, and these products are of interest in propulsion chemistry. Understanding the structure-property relationships of these advanced organometallic PUs is relevant for application as a solid composite propellant binder.
Keywords: Thermal decomposition; Metallo-polyurethane; Thermogravimetry; Mass spectrometry; Volatile products;

The presence of cross-linked networks in rubber creates a tremendous problem for recycling and reusing of waste rubber. Fast transformation of cross-linked natural rubber (NR) into liquid natural rubber was carried out by light pyrolysis in temperature range from 240 to 300 °C in variable time (in range: 1–30 min). The transformation efficiency was evaluated by measuring the sol fraction and the cross-link density of the gel fraction. Over 80 wt% sol fraction was obtained at 280 °C in 15 min and 100 wt% sol fraction was obtained at 300 °C in 3 min. The chemical structure of sol fraction was characterized by gel permeation chromatography, nuclear magnetic resonance and infrared spectroscopy. The molecular weight of sol fraction in cross-linked NR processed at 280 °C and 300 °C decreased when gel fraction is over 50%. The nuclear magnetic resonance results showed that mono-sulfidic linkages occurred mainly in the sol fraction of liquid NR obtained at 300 °C. Isomerization from cis to trans form of isoprene unit in NR increased with extending pyrolysis time.
Keywords: Cross-linked natural rubber; Recycling; Light pyrolysis;

Low melt strength and toughness of poly (l-lactide) (PLLA) limited its large-scale application. In this work, a facile method was proposed and demonstrated to a feasible route to solve these problems. A series of PLLA-based block copolymers PLLA-block-poly (butylene succinate)-block-PLLA (PLLA-b-PBS-b-PLLA), and chain extender (PLLA-block-poly (glycidyl methacrylates))3 (PLLA-b-PGMA)3 were synthesized and used for the improvement of the melt strength and toughness of PLLA by in situ reactive blending. The structure and composition of the copolymers were confirmed by nuclear magnetic resonance spectra, infrared spectra and gel permeation chromatography. The elongation at break increased from 4.2% for PLLA to 234% for the blend containing 40% block copolymer and 5% chain extender, and remarkably maintained their strength. Rheological analysis showed that the blends exhibited the strong strain-hardening behavior. Measurements of the linear viscoelastic properties of the melt blends suggested that the chain extender promoted the development of chain branching. DSC data showed that the crystallization was not perfect after addition of PLLA-b-PBS-b-PLLA and (PLLA-b-PGMA)3 compared with neat PLLA. SEM measurements revealed the improved interface adhesion of the blends. The introduction of PLLA-b-PBS-b-PLLA and (PLLA-b-PGMA)3 imparted both high toughness and high melt strength to the PLLA.
Keywords: Polylactide; Melt strength; Toughness; Reactive blending; Compatibility;

The glass frit with low softening temperature, acting as one of the fluxing agents, is a key material to determine the microstructure of polysiloxane elastomer-based ceramizable composites. The effect of the glass frit with the softening temperature of 480 °C on the properties of the ceramic polysiloxane elastomer was investigated. The appearance of the ceramic residues was observed by a digital camera. The morphology of the impact fracture surface was observed by scanning electron microscopy (SEM). The ceramic mechanism of the ceramic residue was revealed by X-ray diffraction (XRD) analysis. The results showed that the linear shrinkage, as well as flexural strength and impact strength of the ceramic residue increased as the content of the glass frit increased from 0 to 30phr. SEM analysis showed that the structure of the ceramic residue was converted from sea-island phase to a uniform matrix with the increased content of the glass frit. XRD analysis showed that quartz and MgSiO3 crystals were generated in the ceramic residues via the eutectic reaction between the mica and the glass frit.
Keywords: Ceramics; Glass frit; Polysiloxane elastomers; Sintering; Scanning electron microscopy; X-ray diffraction;

Quantitative determination of volatile organic compounds formed during Polylactide processing by MHS-SPME by Rómulo Salazar; Sandra Domenek; Cédric Plessis; Violette Ducruet (80-88).
Polylactide (PLA), a bio-based polyester, has been used in wide applications including food packaging. Nevertheless, it is well known that mass transfer occurs between packaging polymer and foodstuff leading to safety and quality issues. In this sense, volatile organic compounds (VOCs) present in packaging materials can migrate to the food in contact, changing its sensorial properties. Up to date, no study has focused on quantification of VOCs in PLA during its processing, which needs an optimized methodology to measure compounds at very low concentrations. In this study, different PLA samples in form of pellets, extruded films and thermoformed samples were studied in order to determine the VOCs present in each step of processing and to quantify them using headspace extraction methodology (MHS-SPME). Several volatile organic compounds were determined such as aldehydes, ethanol, acetone, acetic acid and lactides. Among the VOCs identified, three compounds were quantified: acetaldehyde; 2-methyl-2-propanol; 2,3-pentanedione. Acetaldehyde and 2,3-pentanedione increased after the extrusion and then decreased or disappeared after thermoforming. The results showed that residual acetaldehyde in PLA could be an important marker for the industry in the selection of PLA grades.
Keywords: PLA; Polylactide; VOCs; Extrusion; Thermoforming; SPME;

Fire retardant behaviour of halogen-free calcium-based hydrated minerals by F. Laoutid; M. Lorgouilloux; L. Bonnaud; D. Lesueur; P. Dubois (89-97).
The flame retardant effect of hydrated lime, partially and completely hydrated dolomitic limes in polyethylene and ethylene vinyl acetate copolymers was evaluated and compared to that of magnesium hydroxide and aluminium hydroxide. An important decrease of the peak of heat release rate as measured by cone calorimeter test was observed for Ca-based composites. Thermogravimetric and X-Ray Diffraction analysis indicated that the calcium hydroxide fraction plays an important role in the generation of an intumescent mineral residue during the combustion.
Keywords: Calcium-based hydrated minerals; Flame retardant; Polyolefins;

Radiation-induced crosslinking of polyamide11 in the presence of triallylisocyanurate by Naotsugu Nagasawa; Takanori Tago; Hisaaki Kudo; Mitsumasa Taguchi (98-102).
Crosslinking structures were formed in electron beam irradiated polyamide11 (PA11) without and with triallyl isocyanurate (TAIC). The gel fractions of PA11 without TAIC steeply increased above the dose of 190 kGy, and saturated to 76.2% at higher than 750 kGy. PA11 was ascertained as a radiation crosslinking type polymer. TAIC was found to promote the induction of crosslinking of PA11 main chains. The network structure of PA11 was effectively formed in 3 phr (per hundred resin) TAIC at the dose of 10 kGy. The crosslinked PA11 increased the thermal stability, as demonstrated by retention of its original shape at temperatures even higher than 200 °C.
Keywords: Polyamide11; Radiation crosslinking; Triallyl isocyanurate; Heat resistance;

Coated vs. naked red phosphorus: A comparative study on their fire retardancy and smoke suppression for rigid polyurethane foams by Zhi-Jie Cao; Xue Dong; Teng Fu; Shi-Bi Deng; Wang Liao; Yu-Zhong Wang (103-111).
Red phosphorus (RP) and the coated RP with melamine formaldehyde resin (MFcP) were foamed with isocyanates to prepare polyurethane foam (PUF). The cell size of PUF/RP became uneven and more spherical, and the cell struts became thicker. The cells of PUF/MFcP were much more uniform in size and more spherical in shape. Addition of flame retardants increased the densities, compressive strength and thermal conductivity of the foams, but decreased the blind hole percents. Thermal gravimetric analysis (TGA) tests exhibited that decomposition of both PUF/RP and PUF/MFcP took place at lower temperatures than that of neat PUF. However, the maximum decomposition temperature (Tmax) for PUF was increased from 300 °C to ca. 340 °C when RP or MFcP was added. Load of RP or MFcP with 80 pphp or higher increased the limiting oxygen index (LOI) of PUF to higher than 25% and pass UL-94 V-0 grade. In the cone calorimetry tests (CC), It indicated that PUF/MFcP100 released much less heat and smoke than PUF/RP100 and PUF. The smoke release of PUF/MFcP100 was reduced by almost 75% than that of PUF/RP100. The toxic gas release of PUF/MFcP100 was reduced more than 50% than that of PUF/RP100. The smoke emission of PUF increased significantly after addition of RP comparing with that addition of MFcP greatly relief the situation. The corresponding mechanism was proposed.
Keywords: Red phosphorus; Coating; Polyurethane foam; Smoke suppression; Flame retardancy;

Hydrolysis of waste monomer casting nylon catalyzed by solid acids by Wei Wang; Linghui Meng; Kunyue Leng; Yudong Huang (112-120).
Decomposing the waste polyamide material onto economically-valuable monomer under the relatively mild hydrothermal conditions is a key technology for the development of wastes recycling. This reaction is traditionally catalyzed by homogeneous acids which would result in the difficult separation and equipment corrosion. We first indicated the solid catalysts for the hydrolysis of waste monomer casting nylon in subcritical water, including a series of γ-Al2O3 supported solid acid catalysts and some commercial zeolites. Zeolite Hβ-25 exhibited the highest activity among the H-form zeolites, and a better recyclability than the γ-Al2O3 supported solid acids. According to the kinetic analyses and reaction pathway exploration, the generation and consumption of linear oligomers, which are the intermediate products, were accelerated when using the zeolite Hβ-25 because of the microporous structure.
Keywords: Catalyzed hydrolysis; Solid acids; Waste nylon; Reaction kinetics;

Characterization of thermally annealed PEEK and CFR-PEEK composites: Structure-properties relationships by Marco Regis; Anuj Bellare; Tommaso Pascolini; Pierangiola Bracco (121-130).
The structure-properties correlation of neat and carbon fiber reinforced (CFR) PEEK, subjected to thermal annealing, was investigated. The polymer and its composites (30% w/w carbon fibers, PAN or pitch based) underwent annealing treatments at 200, 225, 250, 275 and 300 °C. The resulting morphology was investigated by Differential Scanning Calorimetry (DSC), Fourier Transform InfraRed (FTIR) spectroscopy, Wide Angle X-ray Diffraction (WAXD) and Small Angle X-ray Scattering (SAXS), while the flexural peak load from a bending test and the reduced elastic modulus from nanoindentation measurements were chosen as markers of the mechanical properties. The morphological investigation showed that annealing induces an increase in crystallinity, by a combination of thickening of the existing lamellae and nucleation and growth of new, thinner lamellae. Although the mechanical properties showed highly significant differences depending upon the reinforcement type, we were able to induce a significant increase (form 9–12%) in the flexural peak load with the annealing treatments, regardless of the material formulation. The reduced elastic modulus of all PEEK and CFR PEEK formulations also increased with increasing the annealing temperature.In summary, this investigation provides an evidence of how appropriate thermal treatments can be used to tune the mechanical properties of both PEEK and CFR PEEK.
Keywords: PEEK; Carbon fiber; Crystallization; Annealing;

Poly(p-phenylene-2,6-benzobisoxazole) (PBO), a fiber of extraordinary tensile modulus and strength, has been found to degrade quickly under moisture. Many studies have suggested that the likely mechanism of the degradation is that the residual phosphoric acid that is trapped inside the fiber during the production process reacts with the oxazole bonds and breaks the chains. However, there has been no direct study of the structure of the residual phosphorous (P). The present report investigates the chemical structure and physical state of the residual P in PBO fiber using solid-state 31P nuclear magnetic resonance (ssNMR) spectroscopy. The residual P is found to exist mainly in the form of phosphoric acid (PA) and its various forms of hydrated or dehydrated derivatives. Moisture drives the reaction along the direction of phosphorous anhydride → polyphosphoric acid → PA → hydrated PA, while increasing temperature in the absence of moisture reverses this direction. Hydration increases the mobility of PA molecules, thus the ability to migrate, and promotes hydrolytic degradation, while higher temperature in the absence of moisture removes water and immobilizes PA. The residual P species likely reside on the surface of the nanovoids in the PBO fiber, easily accessible by gaseous water molecules but inaccessible by aqueous media. These studies provide direct evidence to confirm the proposed contribution of the residual P to the hydrolytic degradation mechanisms reported in the literature. Further, ssNMR may play a role in assessing future strategies mitigating this crippling problem for PBO fiber which would otherwise be an excellent choice for soft body armor applications.
Keywords: Poly(p-phenylene-2,6-benzobisoxazole); PBO fiber; Hydrolytic degradation; Phosphorous-31 NMR; Solid-state NMR;

Mechanism of enhancement of intumescent fire retardancy by metal acetates in polypropylene by Yan Zhang; Xiaonan Li; Zhengping Fang; T. Richard Hull; Antonios Kelarakis; Anna A. Stec (139-145).
The effects of cobalt acetate (CoAc), manganese acetate (MnAc), nickel acetate (NiAc) and zinc acetate (ZnAc) as fire retardant additive in intumescent polypropylene (PP) formulations containing PP/ammonium polyphosphate (APP)/pentaerythritol (PER) are reported. The limiting oxygen index (LOI) and vertical burning (UL94) tests and cone calorimetry were used to quantify the enhancement. Environmental chamber rheometry, thermal gravimetric analysis and the morphology of the residual char were used to investigate the mechanism of enhancement. The incorporation of small quantities of metal acetates had a significant influence on the fire behaviour. As an example, 0.7 wt% MnAc improved the UL 94 rating of PP/APP + PER (mass ratio 100/25, with APP/PER = 3/1) sample from V-2 to V-0, while 1 wt% MnAc reduced the peak heat release rate and the total heat release by 18% and 12% in the cone calorimeter. Rheological data, cone calorimetry, and photographs of the residual char showed how the fire retardancy of the systems were affected by the melt viscosity, which depended on the loading of metal acetate. During thermal decomposition, the metal acetates promote the crosslinking of the polymer and the fire retardant, reinforcing the protective intumescent layer. While, the effect is most potent at the optimal metal loadings. At higher MnAc loadings, the benefit of a stronger char is overwhelmed by the adverse effect of crosslinking on the transition char layer. Thus, this paper offers a new insight into the mechanism of the intumescent fire retarded PP system.
Keywords: Fire; Intumescent flame retardant; Polypropylene; Metal acetate; Rheology; Mechanism;

Synergistic effects of nitric acid exposure, elevated temperature, applied flexural stress and time were examined in the Stress Corrosion Cracking (SCC) of widely used unidirectional Glass Reinforced Polymer (GRP) composites. The composites were composed of two different E-CR glass fibers with a vinylester-epoxy resin as the matrix with E-glass fibers in the same matrix used for a baseline comparison. SCC damage, as measured by the number of fiber cracks, amount of metallic ion leaching from the fibers, and the reduction of residual flexural strength and stiffness properties of the composites, affected the composites differently depending on their composition. As expected, the E-CR composites greatly outperformed the E-glass composites under all conditions. Long term exposure of the E-CR composites did not greatly increase the amount of stress corrosion fiber damage; instead, interface issues and matrix decomposition led to reduced mechanical properties over extended periods of time. The synergistic effect of the combined testing conditions was found to be much stronger in the E-glass than the E-CR glass composites.
Keywords: Synergistic degradation; Stress corrosion cracking; E-CR glass fiber; Glass reinforced polymer composite;