Thermal properties of automotive polymers III – Thermal characteristics and flammability of fire retardant polymers

Thermal properties and flammability behavior of two grades of fire retardant polypropylene and nylon 66, and their base resins were determined. A nylon 6 base polymer and a nano-composite based on that polymer were also analyzed. Thermal gravimetric analysis showed more complex degradation patterns for the fire retardant grades as compared to the base resin. This was attributed to decomposition of ingredients present in the fire retardant. Degradation of polypropylene in air started at about 100°C lower temperature than degradation in nitrogen. For nylon, the degradation in both atmospheres occurred at approximately the same temperature. Modulated Differential Scanning Calorimetry (MDSC) measurements were used to determine melting and glass transition temperatures, heats of fusion, heat capacity and thermal conductivity. Both phosphorus-based and halogen-based fire retardants modified the ignition, propagation, and melt-dripping behavior of nylon and polypropylene during burning. Incorporation of a nano-filler was found to be ineffective in imparting fire retardancy to nylon 6. Performance of these materials will have to be evaluated in actual vehicle applications and fire exposures before use on a broad scale basis. Received: 25 August 2000 / Reviewed and accepted: 28 August 2000     

Dynamic mechanical,electrical and magnetic properties of ferrite filled styrene-isoprene-styrene

The dynamic mechanical, electrical and magnetic properties of highly filled magnetic polymeric composites containing 75 to 85 wt % barium ferrite in a thermoplastic elastomer matrix styrene-isoprene-styrene (SIS), are reported. The dependence of the properties on the volume fraction of the filler has been investigated. It is shown that the toughness and shore hardness of the composite may be correlated to its dynamic mechanical parameters. The use of coupling agents for surface treatment of ferrites has been shown to improve the magnetic properties of the composite due to better filler dispersion.     

Thermal and electrical properties of some carbon fibers

Two types of carbon materials were studied and their electrical and thermal properties in various conditions of utilisation were determined. Variation of properties was correlated with structure changes.     

Graphite nanoplatelets filled silicone composites with novel electrical and dielectric properties

In the design of medium and low voltage equipment such as cable accessories, generator, motor end windings or bushings, issues with electrical field enhancement occur at interfaces between insulators and conductors, resulting in accelerated material ageing. The purpose of this paper is to present a novel dielectric composite material which has the properties to mitigate this local amplification. It is a functional dielectric which resistivity decreases by several orders with electric field from 10¹⁴ to 10⁹ Ωm up to 1 kV?mm^?1 while the dielectric constant decreases from 15 to 12 in the 10^–2–10⁶ Hz range. This novel material is made with graphite nanoplatelets. It may be used as a resistive or capacitive field grading material in electrical applications.

     

Comparison of rheological and electrical percolation phenomena in carbon black and carbon nanotube filled epoxy polymers

Epoxy nanocomposite suspensions including multi-wall carbon nanotubes (MWCNTs) and carbon black (CB) were produced and investigated by means of combined rheological and electrical analysis. The rheological percolation behaviour was compared to the electrical percolation behaviour. Due to similar dynamic agglomeration mechanisms the difference between the rheological and the electrical percolation threshold in the cured state is identical for MWCNT and CB filled systems. Non-covalent matrix–nanoparticle interactions in uncured epoxy suspensions are negligible since the onset of electrical and rheological percolation in the uncured state coincidence. Furthermore, the electrical percolation threshold in the cured state is always lower than in the uncured state because of the high tendency of CB and MWCNTs to form conductive networks during curing. The difference between rheological and electrical percolation threshold is dependent on the curing conditions. Thus, the rheological percolation threshold can be considered as an upper limit for the electrical percolation threshold in the cured state. Due to the formation of co-supporting networks multi-filler (MWCNTs and CB) suspensions exhibit a similar rheological behaviour as the binary MWCNT suspensions. For both types of suspensions a rheological percolation threshold of around 0.2 and 0.25 wt% was determined. Conversely, the binary CB nanocomposites exhibit a four-times higher percolation threshold of about 0.8 wt%. The difference between the binary MWCNT suspension and the ternary CB/MWCNT suspension in storage shear modulus at high filler concentrations (~0.8 wt%) turns out to be less than expected. Thus, synergistic effects in network formation are already present in the epoxy suspension and get more pronounced during curing.     

Thermal and electrical properties of FeNi/Cu composites

Abstract

Powder metallurgy FeNi/Cu composites with low thermal expansivity and high electrical (thermal) conductivity were fabricated. The effects of Cu content, FeNi particle size, sintering temperature, and rolling reduction on the coefficient of thermal expansion (CTE) and electrical (thermal) resistivity were investigated. The results show that the CTE and electrical resistivity were affected by the volume fraction of the components, the particular properties of the FeNi alloy, diffusion between the FeNi particles and the Cu particles, and the distribution of the Cu particles and the FeNi particles. The experiments indicated that the FeNi/Cu composites could be used as heat sinks in welding type bolt silicon rectifier tubes.     

Computer model for the elastic properties of short fibre and particulate filled polymers

A finite difference type of approach is proposed for the study of the elastic properties of fibre and particulate reinforced composites. The effects of the fibre modulusE ₁ and aspect ratio (I/d) on the composite modulus are studied. In that respect, the ratio (E₁/E_m/(I/d), whereE _m is the matrix modulus, is shown to be an important index of short fibre composites. Our results for the dependence of the composite modulus onE ₁,I/d and volume fraction of the fibres are compared to available experimental data on foams and particulate filled polymers. A good agreement with experiment is found, which is much better than that obtained with the help of the widely used semi-empirical Halpin-Tsai equation.     

Model representation of filled polymers

We have investigated the mechanical properties and dielectric relaxation in polyamide-6 composites with fiber glass and mica. We propose a new model for filled polymers, assuming that the material consists of two interpenetrating continuous phases. The first phase is the polymer sorbed on the surface of the filler particles, mechanically stronger and having a higher modulus of elasticity. The second phase is the unsorbed polymer. The calculated mechanical characteristics of the composites (tensile strength and modulus of elasticity) agree well with experimental data. Translated from Izmeritel'naya Tekhnika, No. 8, pp. 53–56, August, 1998.     

Preparation and electrical transport properties of In filled and Te-doped CoSb3 skutterudite

Nanopowders with nominal compositions of Co₄Sb_11.5Te_0.5 and In_0.5Co₄Sb_11.5Te_0.5 were prepared via hydrothermal synthesis at 180 °C for 48 h, then heat treated and finally hot pressed at 625 °C and 80 MPa for 1 h in vacuum to form bulk samples. The phase compositions of the samples were determined by X-ray diffraction. Hall Effect measurement of the samples was carried out at room temperature. The fracture surface of the samples was observed by field emission scanning electron microscopy. The electrical conductivity and the Seebeck coefficient of the samples were measured from room temperature to around 748 K. The In-filled and Te-doped CoSb₃ sample with longer time annealing before hot pressing had much better electrical transport properties with the highest power factor of 38.4 μWcm^?1 K^?2 around 573 K.     

The effect of particle shape on the mechanical properties of filled polymers

The existing models for predicting the elastic moduli of polymers dispersed with particles of shape other than spheres and continuous fibres are reviewed. The applicability and limitation of these equations are discussed. The emphasis of the review is to seek a unified understanding and approach to the effect of particle shape at finite concentration on the elastic moduli, thermal expansion coefficient, stress concentration factor, viscoelastic relaxation modulus and creep compliance of filled polymers. The effects of anisotropic particle shape on mechanical properties of polymeric composites are clearly illustrated. Attention is also drawn to the relationship between elastic moduli, thermal expansion, creep elongation and stress relaxation moduli.     

The electrical properties of epoxy resin composites filled with Cnts and carbon black

This work introduces an experimental activity related to the realization of an epossidic nanostructured material that develops the function of covering for electronic circuits in aeronautical field. This covering meets the demand of protection of these circuits from possible troubles of electromagnetic nature. In order to realize this covering we used an epoxy resin as matrix (Epon 828) loaded with conductive nanofillers or carbon nanotubes (Cnts). To check the efficiency of the coating we have considered the carbon black, filler widely used as a conductive covering for screenings. We have considered different percentages of the different fillers, precisely 0.1%, 0.25% and 0.5% wt (% valued in comparison to the weight of the resin). From every mixture 12 samples have been obtained (the size of every sample is 10 mm x 10 mm x 10 mm). Every sample has been subjected to electrical measurements, that have concerned the measurement of current intensity and resistance (so as to allow the evaluation of the enhancement of the conductivity), through the application of different values of voltage. The results have demonstrated that the epoxy matrix loaded with Cnts yields higher values of electrical conductivity than the same matrix loaded with carbon black.     

Thermal and dielectric properties of thermal and microwave cured thermoset polymers

 We compare microwave and thermal curing of DGEBA thermosets hardened with BDMA catalyzed HHPA. The glass transition temperature and (complex) dielectric constant are monitored throughout the process, for variable hardener contents and curing times. Received: 5 December 1997 / Accepted: 2 January 1998     

Enhanced electrical conductivity and mechanical properties of ABS/EPDM composites filled with graphene

Acrylonitrile–butadiene–styrene (ABS)/ethylene–propylene–diene monomer (EPDM) composites reinforced with graphene nanoplatelets (GN) were fabricated by the direct melt blending, dried premixing and wet premixing process, respectively. The electrical resistivity, tensile strength, impact strength, microstructure, thermal stability, glass transition temperature and morphology of fracture surface of composites were investigated. In case of direct melt blending process, the maximum tensile strength with minimum impact strength is obtained. But this result is reversed while the fabrication of composites by wet premixing process. SEM results show that GN is prior to distributing in the continuous ABS phase. The percolation threshold could be significantly decreased from 11.8 wt% to 6.6 wt% when prepare composites by wet/dried premixing process instead of melt blending.     

Synthesis of expanded graphite filled polyaniline composites and evaluation of their electrical and electrochemical properties

A series of conducting polyaniline/expanded graphite (PA/EG) composite was synthesized by in situ polymerization of aniline in acid medium followed by the addition of expanded graphite in various proportions (1, 2 and 3 wt%). The synthesized samples were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, thermogravimetric analysis, ultraviolet–visible absorption, X-ray diffraction and by electrical conductivity measurements. The dc electrical conductivities of the composites were dramatically increased compared with pure polyaniline and found to be 0.50 × 10² S/cm to 6.11 × 10² S/cm. The PA/EG composites showed a reversible electrochemical response up to 150th repeated cycles as revealed by the cyclic voltametry study.     

Effect of ion exchange on the electrical properties of talc filled polypropylene composite

Variation of electrical properties during aging of talc filled polypropylene composite ion-selective electrodes was studied over a time interval of about one month. The measurements were made in the low-frequency range (0.5 Hz to 10 kHz). The results show that the impedance decreases with frequency and exhibits a Cole-Cole-plot behavior. The aging has improved the electrical conduction mechanism and reduced markedly the impedance. Also, the complex dielectric constant showed significant change with frequency and aging. This behavior was attributed to surface and bulk effects operating simultaneously.     

MWNTs/PU复合超细纤维的热性能及导电性能

采用静电纺丝技术制备了多壁碳纳米管/聚氨酯(MWNTs/ PU)复合超细纤维,并收集成无纺布薄膜,采用热失重分析仪(TGA)和动态力学分析仪(DMA)分析了纤维的热稳定性。利用数字高阻计(PC68)和LorestaGP电阻计测量了纤维薄膜的直流电导率随MWNTs含量的变化关系。为了研究该多孔薄膜的动态电学性能,同时采用Agilent 4294A阻抗分析仪测试了纤维薄膜的电导率在40 Hz～110 MHz频率范围内的变化关系,并与浇注试样的结果进行比较。结果表明,随MWNTs 在 PU 纤维中含量的增加,复合纤维的热稳定性提高。当10MWNTs质量分数达40 %时,PU的电导率提高近10 倍。     

Modelling damage accumulation in filled polymers

A variant of the non-linear endochronic viscoelasticity theory is developed wherein the reduced-time function depends on a damage parameter, equivalent strains or stresses, as well as on some functions enabling one to distinguish between loading and unloading processes. Some combinations of the first and second invariants of strain and stress tensors are used as the equivalent strains or stresses.
The existence of flaws is taken into account by the introduction of special equivalent stresses which are dependent on principal tensile stresses and the characteristics of the material.
As a criterion of damage equivalence in non-linear viscoelastic materials, the condition of a specific dissipation equality is introduced. Experiments for the determination of material functions entering the constitutive equations are outlined.
The proposed variant of the theory has made it possible to realistically explain the results of creep and constant loading rate tensile tests and creep tests to rupture under conditions of two-step loading sequences acting on filled polymeric materials.