Viscoelastic characteristics of short fiber reinforced polybutylene terephthalate

Mechanics of Time-Dependent Materials - The mechanical characteristics of short-fiber-reinforced thermoplastics should be evaluated based on viscoelastic theory because of the time and temperature...     

Characterization of recycled polycarbonate/polybutylene terephthalate blends

Polycarbonate (PC)/polybutylene terephthalate (PBT) blends (CBs) represent a good compromise between the properties of PC and PBT and are among the most popular engineering plastics today. To evaluate the capability of recycled and reprocessed CBs, the physical and mechanical properties of 0 to 20 times reprocessed CBs were characterized, and an attempt was made to modify a 20-time reprocessed CB to reach ≥85% of the functionality of a virgin CB. Generally, the thermal weight loss (30% at 300–450°C and 40% at ~470°C) of the CBs varied little with the reprocessing cycles, reflecting their high thermal stability. The increased melting index (from 18.7 to 92.0 g (10 min)^–1) but decreased stress values (from 6.08 to 4.99 kgf mm^–2), strains (from 79.0 to 29.1%), impact strength (from 144 to 14.7 J m^–1) and torque values (from 82 to 60 N m^–1) with reprocessing cycles suggest that the CBs undergo thermal/mechanical decomposition when reprocessing and become thereafter stiffer. Satisfactory modification of the 20 times reprocessed CB succeeded simply via adding ~30% (w/w) virgin PC and PBT. Adding styrene maleic anhydride and a chain extender failed to improve the stress values of reprocessed CBs, probably due to their weak interaction with the PC and PBT molecules.     

The dielectric properties of a commercial polyvinylchloride

The dielectric response of a commercial polyvinylchloride is examined in terms of the cluster model of dielectric relaxation, and compared with a sample from which the plasticizer had been extracted. An interpretation of the approach to the glass transition in terms of scaling concepts is outlined and related to the hierarchical dynamics of the cluster model. In this picture the dynamics goes over naturally to the dynamics on small size scales. The plasticizer is shown to contribute a quasi-d.c. electrical transport above the glass transition, which at higher temperatures causes the formation of an electrode barrier layer.     

Generation of novel microstructures in rapidly foamed polybutylene terephthalate

A novel technology called Dynamic Decompression and Cooling (DDC) Process for producing foams from semicrystalline polymers has been developed. In the present experiment, a solution of polybutylene terephthalate (PBT) melt and blowing liquid (CHCl₃) is processed under high pressure and temperature. As the system is decompressed above the boiling point of the solvent, phase separation occurs; gas bubbles nucleate out from a metastable regime and grow through evaporation of the volatile phase and diffusion of non-condensable gas. Solidification is achieved by crystallization of the polymer and supercooling of the melt induced by the latent heat of vaporization of the volatile phase. The resulting foams have open, interconnected cell structures with densities of 10–20% of the original materials. Structural characterization with DSC, SEM and X-ray diffraction techniques reveals that the DDC foams are semicrystalline with crystallinity of ca. 35% and possess a variety of micromorphologies as well as crystalline orientations. This structural character is believed to modify considerably the mechanical properties of the fabricated DDC foams.     

Morphology and properties of acrylate styrene acrylonitrile/polybutylene terephthalate blends

The structure and mechanical properties of acrylate styrene acrylonitrile (ASA) and ASA/polybutylene terephthalate (PBT) blends have been studied. The morphology of ASA is found to conform to a previous model. 40/60 and 60/40 blends of ASA/PBT have a two-phase, dispersed morphology while the 50/50 blend is shown to have a co-continuous structure. As processing temperature is increased, the mechanical properties decrease, due to PBT degradation. The 60/40 ASA/PBT blend has very poor impact resistance because of the continuous, degraded PBT matrix. Better mechanical properties are observed for blends with a continuous ASA matrix, particularly in the 50/50 blend. Fracture surface analysis reveals a unique morphology of mushroom-like PBT fibrils for the low processing temperature samples near the crack tip. This is thought to occur due to the competition of cohesion and adhesion of the PBT with the ASA matrix.     

The dielectric response of a polymeric three-component composite

The dielectric behaviour of a composite system of epoxy resin filled with Kevlar fibres and Aluminum powder is investigated in the frequency range 20 Hz to 10 MHz and the temperature interval 10 to 150°C. Dielectric permittivity is increasing with filler content and temperature, being always higher in the low than in the high frequency range. Dielectric permittivity and loss of the composites is mostly affected by interfacial polarization arising from inhomogeneites at interfaces introduced by the fillers. Equations based on the extension of the logarithmic law of mixtures are formulated and their applicability tested with the experimental data obtained.     

The crack resistance in polybutylene terephthalate (PBT) at crack initiation and during steady crack growth

The resistance to crack initiation and quasi-static crack propagation is investigated for a polybutylene terephthalate (PBT) using annealed and unannealed three-point bend specimens. The resistance to crack initiation (R _i) is determined based on the generalized locus method which determines the resistance to crack growth including crack initiation utilizing the locus of characteristic points on the load against load-point displacement curves of specimens which differ only in initial crack length. This generalized locus method also enables us to investigate the invariance of the crack resistance value along the locus line. The steady state crack resistance (R _p) during quasi-static crack propagation is determined utilizing the functional relation between total essential energy (U _f) for complete fracture and the initial ligament length. The total essential energy is the sum of the blunting energy and the integration of the resistance to crack propagation with respect to the cracking area. The invariance of the crack initiation resistanceR _i, and the steady state resistanceR _p is discussed based on the experimental results of the annealed and unannealed specimens which show different sizes of crack tip plastic deformation.     

Evaluation of copolymers of polyethylene oxide and polybutylene terephthalate (polyactive): mechanical behaviour

Polyether–polyester segmented block copolymers (Polyactive®) on the basis of polybutylene terephthalate (PBT) and polyethylene oxide (PEO) were mechanically tested. Tensile strength and modulus of elasticity in compressive and tensile deformation were recorded according to ASTM standards. These tests were done in vitro under dry and wet conditions, and after 3, 9 and 25 wk subcutaneous implantation of these materials in goats. Strength and modulus of elasticity were higher with increased contents of PBT in the copolymers. After water uptake, the polymer displayed a lower strength and stiffness. Disintegration of the materials with 70% PEO content and dumb-bell shape was noted at 3 wk. Disintegration of the cylinders of the same material was seen after 25 wk implantation. Of the materials with 60% PEO content, only four of the five dumb-bells had disintegrated after 25 wk implantation. The in vivo test results of all other implants did not show a clinically relevant decrease of strength and stiffness with time after implantation of the copolymers in the goats. Mechanical behavior of the various copolymers seemed mainly determined by the amount and integrity of the PBT phase.     

热塑性聚氨酯弹性体对聚对苯二甲酸丁二醇酯基阻燃复合材料性能的影响

以溴化聚苯乙烯(BPS)为阻燃剂,Sb2O3纳米颗粒(nano-Sb2O3)为协效阻燃剂,聚对苯二甲酸丁二醇酯(PBT)为基体,热塑性聚氨酯弹性体(TPU)为增韧组分,采用球磨分散和熔融共混的方法制备出TPU/nano-Sb2O3-BPS-PBT阻燃复合材料.通过DSC、拉伸、冲击和极限氧指数(LOI)等性能测试,研究...     

聚(丁二酸丁二醇-co-二苯醚二甲酸丁二醇)酯/聚对苯二甲酸丁二醇酯复合材料的制备及性能

聚对苯二甲酸丁二醇酯(PBT)是半结晶热塑性聚合物,具有出色的尺寸稳定性、高的刚度和硬度、良好的耐化学性、力学性能和可加工性等优良特点。但其抗冲击性较低、韧性较差。采用将PBT与具有优异韧性的聚(丁二酸丁二醇-co-二苯醚二甲酸丁二醇)酯(PBSO)进行熔融共混以改善PBT的韧性。首先以二苯醚二甲酸(OBBA)、丁二酸(SA)和丁二醇(BDO)为单体,以钛酸四丁酯为催化剂,采用先酯化后缩聚的两步法合成了PBSO。然后将不同质量比的PBSO与PBT进行熔融挤出共混,研究复合材料的力学韧性、PBSO与PBT的相容性、结晶行为、热稳定性以及流变性能等。FT-IR、1H NMR、GPC测试分析了所合成PBSO的结构。力学性能测试表明,20wt%PBSO/PBT复合材料的拉伸强度为40.3 MPa,断裂伸长率达到82.1%,断裂能增加到24.7 MPa,冲击强度达到较大值23.2 kJ/m²,具有较高的拉伸强度和良好的韧性。动态热机械分析(DMA)和SEM结果发现,PBSO对PBT具有增容效果,PBSO/PBT是部分相容共混体系。差示扫描量热(DSC)和广角X射线衍射(WA...     

The dielectric response of doped semiconducting glasses

Measurements are reported of the dielectric properties of two semiconducting glasses of composition SiO₂ · Na₂O with admixtures of Sb_{2O 5} and SnO₂, covering a wide range of temperatures and a range of six decades of frequency 10^–2 to 10⁴ Hz. Both glasses show a gradual evolution of growing low frequency dispersion (LFD) with increasing temperature, with both the real and imaginary components of the complex susceptibility following a power law of the type ^n–1, with n 1 at the lowest frequencies. There are also clear indications of reversible phase transitions taking place which manifest themselves by changes in the spectral shape of the dielectric response.     

Effect of polybutylene terephthalate (PBT) on impact property improvement of hybrid kenaf/glass epoxy composite

Environmental regulations, costs and lightweight encourage car manufacturers to develop new reliable products. Epoxy provides a reliable fibre impregnation and creates substantial three-dimensional (3D) cross-linking for proper load transmission and impact strength improvement, but their low toughness decreases their energy absorption. Thermoplastic toughening improves the epoxy impact property with a low thermo-mechanical defect. This study, focused on improving the impact property of hybrid kenaf/glass fibre epoxy composite by use of a modified sheet moulding compound (GMT). The results indicated that most of the mechanical properties of developed material were almost the same as those of the GMT, except impact. This result highlights the potential for utilisation of the toughened hybrid bio-composite in some automotive structural components. Moreover, geometric parameters, e.g., cross-section, thickness, and reinforcement ribs suggest an improvement of structural impact resistance to comply with the bumper beam product design specification (PDS).     

The dielectric response function of copper formate tetrahydrates

The feasibility of the method of observation of response function is shown. By a direct observation of response function of Cu(HCOO)₂·4H₂O single crystals, the frequency dependence of dielectric constants and the relaxation times are obtained. It is shown that a critical slowing down occurs as the antiferroelectric transition temperature is approached, and that the crystal seems to be monodispersive and polydispersive below and above the transition temperature, respectively.     

Random response of a viscoelastic dielectric elastomer spherical balloon

The viscoelastic behavior of the dielectric elastomers (DEs) plays an important role in the time-dependent mechanical deformation. The increasingly demands to precise utilities and controls of the DEs are motivating the researchers to insight into the influence of the viscoelasticity on the dynamical response. In this paper, the random response of a DE spherical balloon subjected to electrical and/or mechanical disturbances is analytically studied. Based on a rheological model of two parallel units, the governing equations of the stretch ratio of the deformed and undeformed radius of the spherical balloon are derived from the Lagrange equation. Firstly, the influences of the pressure in balloon and the voltage on the membrane on the equilibrium positions are discussed. The big pressure and high voltage will both induce static instability. Then, by expanding the motion equations around the static equilibrium position, the relation of the resonant frequency with the viscoelasticity are revealed. Finally, an approximation procedure is adopted, which pave the way to adopt the stochastic averaging. The probabilistic density function (PDF) of the stretch ratio around the equilibrium position is obtained. The influences of system parameters, i.e., viscoelasticity, voltage, pressure, on the probability distribution are investigated in detail. The accuracy of the proposed procedure are also been evaluated by the Monte-Carlo simulation (MCS).     

The description of superconductivity in terms of dielectric response function

A critical temperatureT _c of a superconducting transition is calculated for a rather general form of the electron-electron interaction. It is shown that even if both the energy and momentum dependence of the interaction is included, the equation determiningT _c coincides formally with the corresponding equation of the BCS theory. The kernel of this equation is a smooth real function of its variables; it is expressed through ρ(k, E), the spectral density of the inverse dielectric function of the system. The expression forT _c is written in terms of ρ(k, E); this enables us to analyze the dependence of the critical temperature on the properties of the metal in a normal state. Some simple models illustrating the results are considered, and a discussion of the limits onT _c is given.     

High-temperature dielectric response of ferroelectric relaxors

It has long been considered that polar nanoregions in relaxors form at Burns temperature T(d) ≈ 600K. High-temperature dielectric investigations of Pb(Mg(1/3)Nb(2/3)) O(3) (PMN) single crystal, PMN-PbTiO(3) ceramics, and (Pb,La) (Zr,Ti)O(3) ceramics reveal, however, that dielectric dispersion, detected around 600K, is due to the Maxwell-Wagner-type contributions of surface layers. The intrinsic response was analyzed in terms of the universal scaling, taking into account the asymptotic and the correction-to-scaling behavior, and the results imply much higher T(d) or formation of polar nanoregions in a broad temperature range. High values of the dielectric constant indicate, however, that polar order already exists at the highest measured temperatures of 800K. The obtained critical exponents indicate critical behavior associated with universality classes typically found in spin glasses.     

Pulse response functions of dielectric susceptibility

As dielectric response in the time domain is becoming increasingly of experimental relevance and as such responses for quite a number of well-established susceptibility formulae are still unknown, we examine in this paper how pulse response functions may be calculated from susceptibility by use of various integral transform methods. We need to specialize some parameters in many cases to keep the mathematics sufficiently tractable. The asymptotic behaviour of pulse responses are then classified. Finally we comment on the adequacy of the Shin-Yeung response in the time domain.     

High-temperature creep response of a commercial grade siliconized silicon carbide

Creep studies conducted in four-point flexure of a commercial siliconized silicon carbide (Si-SiC, designated as Norton NT230) have been carried out at temperatures of 1300, 1370, and 1410°C in air under selected stress levels. The Si-SiC material investigated contained 90% -SiC, 8% discontinuous free Si, and 2% porosity. In general, the Si-SiC material exhibited very low creep rates (2 to 10×10^–10 s^–1) at temperatures 1370°C under applied stress levels of up to 300 MPa. At 1410°C, the melting point of Si, the Si-SiC material still showed relative low creep rates (0.8 to 3 × 10^–9 s^–1) at stresses below a threshold value of 190 MPa. At stresses >190 MPa the Si-SiC material exhibited high creep rates plus a high stress exponent (n=17) as a result of slow crack growth assisted process that initiated within Si-rich regions. The Si-SiC material, tested at temperature 1370°C and below the threshold of 190 MPa at 1410°C, exhibited a stress exponent of one, suggestive of diffusional creep processes. Scanning electron microscopy observations showed very limited creep cavitation at free Si pockets, suggesting the discontinuous Si phase played no or little role in controlling the creep response of the Si-SiC material when it was tested in the creep-controlled regime.