Thermal properties of high density polyethylene composites with natural fibres: Coupling agent effect

High density polyethylene composites with curaua fibres were prepared using an intermeshing co-rotating extruder and two different coupling agents. The thermal stability of the components was studied by thermogravimetric and differential scanning analysis, as well as by the oxidation induction time. Maleic anhydride grafted polyethylene, used as coupling agent, affected the composite stability more markedly than did poly(ethylene-co-vinyl acetate). However, oxidation induction times were analogous for composites with and without coupling agents. Results also indicated that a higher fibre-matrix interaction precludes the crystallinity enhancement caused by the fibre.     

Modeling of catalytic coke formation in thermal cracking reactors

At the start-up period, the most important mechanism in the coke production with a clean reactor surface is the catalytic mechanism. The study of this mechanism can be very useful for the better comprehension of the coke production process. In this paper, a model was designed for such a production through the utilization of a catalytic mechanism and a kinetic model, capable of interpreting the catalytic coke production on the reactor surface. For the determination of the model reliability, the experimental data related to the naphtha feed, existing in literature, were used. In addition, the constant parameters of the model, the velocity and the activation energy constants, associated to the kinetic model, were calculated. The results of the developed model were in satisfactory agreement with the experimental data. Eventually, the catalytic coke amount in comparison with the total coke production on the reactor surface and its significance were under investigation.     

Stable complex formation with boric acid in pyrolysis of levoglucosan in acidic media

The influence of boric acid or phenylboronic acid on thermal conversion of levoglucosan in acidic sulfolane was studied. Although levoglucosan was converted to levoglucosenone, furfural and 5-hydroxymethyl furfural (total yield: 40 mol%) at 200 °C in sulfolane containing 0.1 wt% H₂SO₄, addition of boric acid enormously lowered the yields of these, and instead caused the formation of a stable complex in more than 70 mol% yield. Thus, boric acid substantially suppressed the acid-catalyzed dehydration and formation of furfurals from levoglucosan through complex formation. From the ¹H NMR spectrum, the chemical structure of this complex was confirmed as -d-glucofuranose cyclic 1,2:3,5-bisborate, which was readily hydrolyzed quantitatively to glucose by the addition of water. Phenylboronic acid also exhibited similar influences.     

Influence of water content on failure of phenolic composites in fire

This paper evaluates the structural performance of flame resistant phenolic matrix composites exposed to fire. Experimental fire tests were performed on a glass-phenolic composite under combined static loading and one-sided radiant heating. The reduction to the tension and compression failure strengths of the phenolic composite was measured in these tests for heat flux conditions ranging from 10 kW/m² (∼225 °C) to 75 kW/m² (∼700 °C). It was discovered that the failure strengths of the phenolic composite decreased rapidly in the event of fire, particularly under compressive loading when failure occurred more rapidly than under tensile loading. The phenolic composite, despite having high flame resistance, loses strength more rapidly and fails sooner than a more flammable vinyl ester composite. The study shows that greater flammability resistance does not necessarily result in better structural performance in fire. The poor structural performance of the phenolic composite was due to explosive delamination damage and cracking caused by vaporisation of water in the matrix phase. It is shown that removing water from phenolic composites by natural or artificial ageing reduces the incidence of delamination cracking and thereby improves the materials' structural performance in fire. It is concluded that phenolic composites do not provide good structural performance in fire, even though they have low flame and smoke properties. However, reducing the water content in the matrix phase below about 10% can greatly improve the structural performance of phenolic composites during fire.     

Degradation patterns and surface wettability of electrospun fibrous mats

Degradation profiles and surface wettability are critical for optimal application of electrospun fibrous mats as drug carriers, tissue growth scaffolds and wound dressing materials. The effect of surface morphologies and chemical groups on surface wettability, and the resulting matrix degradation profiles were firstly assessed for electrospun poly(d,l-lactide) (PDLLA) and poly(d,l-lactide)-poly(ethylene glycol) (PELA) fibers. The air entrapment between the fiber interfaces clarified the effects of various surface morphologies on the surface wettability. Chemical groups with lower binding energy were enriched on the fiber surface due to the high voltage of the electrospinning process, and a surface erosion pattern was detected in the degradation of electrospun PDLLA fibers, which was quite different from the bulk degradation pattern for other forms of PDLLA. Contributed by the hydrophilic poly(ethylene glycol) segments, the degradation of electrospun PELA fibers with hydrophobic surface followed a pattern different from surface erosion and typical bulk degradation.     

Thermal degradation of deoxybenzoin polymers studied by pyrolysis-gas chromatography/mass spectrometry

The thermal degradation behavior of novel ultra-fire-resistant polymers and copolymers containing deoxybenzoin units in the backbone was studied by pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). The polymers were synthesized by the polycondensation of 4,4′-bishydroxydeoxybenzoin (BHDB) with isophthaloyl chloride (to give polyarylates), phenylphosphonic dichloride (to give polyphosphonates), and their mixtures (to give poly(arylate-co-phosphonate) copolymers). The thermal decomposition, under nitrogen conditions, of BHDB-polyarylate was characterized by a simultaneous degradation of both the bisphenolic (deoxybenzoin) and isophthalate sub-units, whereas a three-step decomposition phenomenon was observed for the BHDB-polyphosphonate. BHDB-polymers containing phosphonate groups in the backbone did not show any phosphorus-based volatile decomposition products, whereas the corresponding bisphenol A-based polyphosphonates released volatile decomposition products comprised mainly of phosphorus-containing compounds.     

Thermal degradation investigation of poly(ethylene terephthalate)/fibrous silicate nanocomposites

Based on the fibrous silicates (palygorskite, PT) organically modified by water-soluble polyvinylpyrrolidone (PVP), poly(ethylene terephthalate) (PET) nanocomposite with good dispersion of the PT nano-particles was prepared via in situ polycondensation. The thermal degradation behavior of PET and PET/PT nanocomposite was investigated by thermogravimetric analysis (TGA) under non-isothermal conditions at various heating rates in air and nitrogen, respectively. The apparent activation energies of the samples were evaluated by Kissinger and Flynn-Wall-Ozawa method. It is suggested that, during thermal decomposition in nitrogen, the clay as a mass-transport protective barrier can slow down degradation of polymer, but the catalytic effect of metal derivatives in clays may accelerate the decomposition behavior of PET. The combination of these two effects determines the final thermal stability of nanocomposite. However, in air atmosphere, the oxidative thermal stability of PET/PT nanocomposite was obviously superior to that of pure PET.     

Pyrolysis of poly(phenylene vinylene)s with polycaprolactone side chains

The thermal degradation characteristics of a new macromonomer poly(-caprolactone) with central 4,4′-dicarbaldehyde terphenyl moieties and poly(phenylene vinylene)s with well defined (-caprolactone), (PPV/PCL) as lateral substituents were investigated via direct pyrolysis mass spectrometry. The unexpectedly high thermal stability of the macromonomer was attributed to intermolecular acetylation of benzaldehyde yielding a hemiacetal and causing a crosslinked structure during the pyrolysis. Increased thermal stability of the PCL chains was detected for all samples. The increase in stability of PCL chains was much more pronounced than was detected for poly(p-phenylene)-graft-poly(-caprolactone) copolymer (PPP/PCL); the upward temperature shift was about 100 °C for PPV/PCL and only 20 °C for PPP/CL. This pronounced effect may be due to higher thermal stability of PPV compared to PPP and the decrease in steric hindrance for PPV with PCL side chains.     

Synthesis and thermal properties of the thermosetting resin based on cyano functionalized benzoxazine

A novel thermosetting resin based on cyano functionalized benzoxazine (BZCN) has been synthesized from 2,6-bis(4-diaminobenzoxy)benzonitrile phenol and formaldehyde by solution reaction. The structure of the monomer is supported by FTIR, ¹H NMR and ¹³C NMR spectra, which have exhibited that the reactive benzoxazine rings and cyano group exist in the molecular structure of BZCN. The curing reactions of BZCN are monitored by the disappearance of the nitrile peak and the tri-substituted benzene ring that is attached with oxazine ring peak at 2231 and 930 cm⁻¹, respectively. The complete cured materials could achieve char yields up to 70% at 800 °C in nitrogen atmosphere, above 64% at 600 °C in air (20% oxygen) environments and the glass transition temperature up to 250 °C. The thermally activated curing polymerization reaction of BZCN follows multiple polymerization mechanisms via the ring-opening polymerization of oxazine rings and the triazine ring-formation of cyano groups, which contribute to the stability of the polymer.     

Effect of metal compounds on thermal degradation behavior of aliphatic poly(hydroxyalkanoic acid)s

Effect of metal compounds on the thermal degradation behaviors of poly(3-hydroxybutyric acid) (P(3HB)), poly(4-hydroxybutyric acid) (P(4HB)), and poly(?-caprolactone) (PCL) was investigated by means of thermogravimetric and pyrolysis-gas chromatograph mass spectrometric analyses. Na and Ca compounds accelerated a random chain scission of P(3HB) molecules resulting in a decrease of thermal degradation temperature, whereas the contribution of Zn, Sn, Al compounds to the thermal degradation of P(3HB) was very small. In contrast to P(3HB), Zn, Sn and Al compounds induced the thermal degradation of PCL at lower temperature range by catalyzing the selective unzipping depolymerization from ω-hydroxyl chain end. Transesterification reaction of PCL molecules could be facilitated by the presence of Ca compound, while the gravimetric change was detected at almost identical temperature region regardless of the content of Ca compound. According to the lactonizing characteristic of monomer unit, the thermal degradation of P(4HB) progressed by the cyclic rupture via unzipping reaction from the ω-hydroxyl chain end or/and random intramolecular transesterification at the main chain with a release of γ-butyrolactone as volatile product. Each of metal compounds used in this study was effective to catalyze the cyclic rupture of P(4HB) molecules, and the degradation rate was accelerated by the presence of metal compounds.