Effect of crosslinking on the positive temperature coefficient stability of carbon black‐filled HDPE/ethylene‐ethylacrylate copolymer blend system

Carbon black‐filled high‐density polyethylene (HDPE)/ethylene ethylacrylate copolymer (EEA) blends were prepared and the effect of crosslinking of the blends on the positive temperature coefficient (PTC) stability was investigated. By irradiation and silane‐crosslinking methods, crosslinked composites with various degrees of crosslinking were obtained. Crosslinking of the matrix polymer led to the disappearance of the negative temperature coefficient (NTC) phenomenon. Also, the PTC intensity increased with an increasing degree of crosslinking. The PTC stability of silane‐crosslinked samples was notably improved at heat cycles of 140°C. This was sufficiently improved by both the silane‐ and radiation‐crosslinking methods when they were treated at 85°C. Therefore, the limiting temperature of self‐regulating heat is about 85°C. Both radiation‐ and silane‐crosslinked samples are thought to be of use to the industry.     

Effect of the cross‐linking process on the electrical resistivity and shape‐memory behavior of cross‐linked carbon black filled ethylene‐octene copolymer

The electrical resistance of carbon black (CB) filled ethylene‐octene copolymer (EOC) was monitored during the compression molding step by means of conductivity sensors. It increases strongly during the cross‐linking process due to the de‐agglomeration of CB aggregates. After completion of cross‐linking reaction CB aggregates reagglomerate causing a decay of resistance. The electrically stimulated shape‐memory (SM) behavior was found to be strongly dependent on the extent of electrical resistivity and the rigidity of CB network, which is formed during compounding and subsequent cross‐linking process. The effect of amount of peroxide and cross‐linking conditions like time and temperature on resistivity and the related SM behavior was characterized. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Effects of strain and temperature on the electrical properties of carbon black‐filled alternating copolymer of ethylene‐tetrafluoroethylene composites

Carbon black (CB)‐filled alternating copolymers of ethylene‐tetrafluoroethylene (ETFE) composites were prepared by the melt‐mixing method. The effects of strain and temperature on the electrical resistivity of the composites were elucidated in detail. Our results indicated that the CB content and CB particle size are the two main factors that influence the electrical resistance of the composites when they are subjected to strain. The large particle size CB‐filled ETFE composites with a low CB content show a significant increase in electrical resistivity as a function of strain. A linear relationship between the logarithm of the electrical resistivity of the composites and strain was observed, indicating that tunneling conduction plays an important role when the composites are subjected to strain. The tensile testing results indicated that the CB‐filled ETFE composites have reasonably good tensile properties.     

Dielectric relaxation of conductive carbon black reinforced ethylene‐octene copolymer vulcanizates

The dielectric relaxation behavior of different conducting carbon black‐filled ethylene‐octene copolymer (EOC) vulcanizates prepared by melt‐mixing method has been studied as a function of frequency (100 Hz–5 MHz) over a wide range of temperatures (25–100°C). The effect of filler loading and frequency on AC conductivity, dielectric permittivity, impedance, and dielectric loss tangent (tanδ) has been studied. The nature of variation of the dielectric permittivity with the filler loadings was explained on the basis of interfacial polarization of the filler in the polymer matrix. The effect of filler loading on the real and complex part of the impedance was explained by the relaxation dynamics of the polymer chains in the vicinity of the fillers. The effect of filler and temperature on dielectric loss tangent, dielectric permittivity, AC conductivity, and Nyquist plot was also reported. The bound rubber (BR) value increases with increase in filler loading suggesting the formation of strong interphase, which is correlated with dielectric loss. Thermal activation energy (E_a) was found to be decreasing with the temperature, which follows the Arrhenius relation: τ_b = τ₀ exp(−E_a/K_BT) where τ_b is the relaxation time for the bulk material. From the plot of lnτ_b versus inverse of absolute temperature (1/T), the activation energies (E_a) were found to be 0.37 and 0.44eV, respectively. The percolation threshold was observed with 40 phr carbon black loading. POLYM. COMPOS., 37:342–352, 2016. © 2014 Society of Plastics Engineers     

Improvement of polyimide/polysulfone composites filled with conductive carbon black as positive temperature coefficient materials

In this study, polyimide (PI)/polysulfone (PSF) blends filled with carbon black (CB) were developed for the use as positive temperature coefficient (PTC) materials in order to achieve the volume resistivity as lower than 10⁴ Ω.cm at room temperature. The weight ratios of PI/PSF were various from 100/0 to 10/90 with CB varied from 0 to 20 wt%. The use of conductive filler was reduced when PSF was blended with PI; the blends clearly possessed a percolation threshold decreased by 90%. The electrical conductivity of the CB-filled blends was higher than those of CB-filled pure PI. The transition temperature for PTC material was reported in the range of 180 to 210 °C. The preferential location of CB filler in PI domains could be observed using the optical microscope. In addition, the composites met the standards for the obtained mechanical and thermal properties, exhibiting the potential use as PTC materials. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48482.     

Conductive polymer blends filled with carbon black: Positive temperature coefficient behavior

Electrical conductivity and positive temperature coefficient (PTC) behavior of carbon black (CB) filled incompatible polyblends of ethylene-vinyl acetate copolymer/low density polyethylene (EVA/LDPE) were investigated. In comparison with single polymer systems, more possibilities for tailoring composite performance were brought about with the employment of polymer blends as matrix resins in conductive composites. Based on the concepts of double percolation and two-step percolation, PTC-type composites with balanced performance, improved processability, and reproducibility can be made. Thermodynamical and kinetic factors including interfacial energy, melt viscosity, blending ratio, melt mixing time, sequence of blending as well as CB concentration were shown to be closely related to the ultimate properties obtained.     

Effect of hot air aging on the properties of ethylene‐vinyl acetate copolymer and ethylene‐acrylic acid copolymer blends

The aim of this investigation is to evaluate the effect of hot air aging on properties of ethylene‐vinyl acetate copolymer (EVA, 14 wt % vinyl acetate units), ethylene‐acrylic acid copolymer (EAA, 8 wt % acrylic acid units), and their blends. Attenuated total reflection‐Fourier transform infrared spectroscopy, differential scanning calorimeter (DSC), wide angle X‐ray diffraction, and mechanical tests are employed to investigate the changes of copolymer blends' structures and properties. Increase of carbonyl index derived from ATR measurements with aging time suggests the incorporation of oxygen into the polymeric chain. By DSC measurements, the enthalpy at low temperature endothermic peak (T_m2) of EAA becomes less and disappears after 8 weeks aging, but enthalpy at T_m2 of EVA is not influenced by the hot air aging and remains stable despite of the aging time. For various proportions of EAA and EVA blends, enthalpy at T_m2 decreases as the EAA proportion increases when aging time is 8 weeks; after several weeks of hot air aging, the various blends appear a same new peak just over the aging temperature 70°C which is due to the completion of crystals which are not of thermodynamic equilibrium state. Mechanical tests show that increase of crystallinity and hot air aging deterioration both have influence on the hardness, tensile strength, and elongation at break. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

PTC effect in carbon black‐filled ethylene–propylene–diene terpolymer systems

The positive temperature coefficient (PTC) effects of carbon black (CB)‐filled semicrystalline and amorphous ethylene–propylene–diene terpolymer (EPDM) composites were studied. The semicrystalline EPDM/CB composite exhibited a low PTC effect followed by a pronounced negative temperature coefficient (NTC) effect, while the amorphous EPDM/CB composite exhibited only an NTC effect. By the effect of γ‐ray irradiation, not only was the NTC effect of the composites eliminated, but also a high PTC effect appeared. The PTC intensity reached as high as six orders of magnitude even for an amorphous EPDM/CB composite and the PTC transition temperature decreased with the irradiation dose. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1571–1574, 2001     

Dielectric response of conducting carbon‐black‐filled ethylene–octene copolymer microcellular foams

Dielectric response of conducting carbon‐black‐filled ethylene–octene copolymer microcellular foams has been investigated with variation of blowing agent and filler loading in the frequency range of 100 Hz to 5 MHz and temperature range from 30 to 100°C. With increase in blowing agent loading, the dielectric permittivity increases for both unfilled as well as carbon‐black‐filled microcellular foams. The experimental complex impedance plots were compared with model‐fitted plots obtained by taking an equivalent circuit of (CQR) (CR).The values of R_b (bulk resistance), R_gb (grain boundary resistance), bulk capacitance (C_b), and grain boundary capacitance (C_gb) at different temperatures were calculated and compared with experimental values. The relaxation time due to bulk effect (τ_b) has been calculated from relaxation frequency (f_r). The dc conductivity (σ_dc) decreases with rise in temperature indicating the existence of positive temperature coefficient of resistance in the material. The activation energy (E_a) calculated from the relaxation time due to bulk effect (τ_b) was found to be 0.446 eV, whereas it was 0.363 eV from the dc conductivity plot in the temperature range of 30–100°C. POLYM. COMPOS., 37:3398–3410, 2016. © 2015 Society of Plastics Engineers     

Effects of thermal ageing treatment and antioxidants on the positive temperature coefficient characteristics of carbon black/polyethylene conductive composites

Polyethylene (PE)‐filled with carbon black (CB) is a prototypical composite that displays resistance switching. These materials can exhibit either a positive temperature coefficient (PTC) or negative temperature coefficient (NTC). The CB‐filled semicrystalline polymer composites ideally need antioxidants, which stabilize the composites against thermooxidative degradation, because they should be resistant to the severe conditions of high temperature. The characterization of PTC materials is affected by the crystallinity of the polymer, and the crystallinity of the polymer is changed with thermal ageing treatment. Thermal ageing of PTC samples was conducted in an oven in the range 50–140°C, in air. The composites, containing 0.5–3% (by weight) Irganox 1076 (Ciba‐Geigy), were irradiated under nitrogen at room temperature with different doses of gamma rays from a ⁶⁰Co source. The resulting composites were analyzed by differential scanning calorimetry, gel fractionation, X‐ray diffraction, and dynamic mechanical analysis. The conductivity of the composites depended on the amounts of antioxidants and the duration of the thermal ageing treatment. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2316–2322, 2003     

Scratch deformation characteristics of micrometric wollastonite‐reinforced ethylene‐propylene copolymer composites

The scratch deformation behavior of neat and wollastonite‐containing ethylene‐propylene copolymer composites has been studied by electron microscopy and atomic force microscopy techniques. The study indicates that the severity of plastic deformation during scratch testing in reinforced ethylene‐propylene copolymers is a function of debonding/detachment of wollastonite mineral particles from the ethylene‐propylene matrix. The resistance to scratch deformation was evaluated in terms of scratch hardness, scratch depth, average scratch roughness, and change in gray level before and subsequent to scratching. The data suggests that resistance to scratch deformation follows the sequence: coated + coupled wollastonite‐containing EP copolymer > fine wollastonite‐containing EP copolymer > coated wollastonite‐containing EP copolymer > coarse wollastonite‐containing EP copolymer > neat EP copolymer. EP copolymer containing coated wollastonite and coupling agent is characterized by highest scratch hardness and minimum scratch depth and scratch roughness. The visibility of scratch, quantified in terms of gray level, suggests that coated + coupled wollastonite‐containing copolymer exhibits significantly reduced susceptibility to stress whitening, and is characterized by a lower gray level in the scratch‐deformed regions. In the present case of wollastonite‐containing copolymer composites, the resistance to scratch deformation follows a trend similar to that of gray level or scratch visibility. Polym. Eng. Sci. 44:1738–1748, 2004. © 2004 Society of Plastics Engineers.     

Effect of electron‐beam irradiation on ethylene–methyl acrylate copolymer

Ethylene–methyl acrylate copolymer (Elvaloy 1330) was irradiated by an electron beam at different levels of radiation both in the presence and absence of a trimethylolpropane trimethacrylate sensitizer at various dosages of incorporation. The mechanical, thermal, and electrical properties of these samples were compared. The mechanical properties were observed to reach an optimum maximum around 6 Mrad of irradiation and 1 phr of sensitizer incorporation. Furthermore, an increase in either the radiation dose or the sensitizer level helped very little to further modify the properties. The thermal properties as determined by the thermogravimetric analysis and differential scanning calorimetry studies were quite supportive of the observation made during the study of the mechanical properties. The thermal stability of the irradiated samples underwent an increase with increasing electron‐beam dosage. In a manner similar to those of the mechanical properties, the increase in thermal stability was found to reach a maximum at a particular level of treatment and sensitizer incorporation, beyond which there was marginal or no effect at all. The α transition temperature underwent a substantial increase with increasing crosslink density, as evidenced by the increase in gel content with increasing proportion of electron‐beam radiation dose. The other glass‐transition temperature, however, appeared to remain unaffected. The electrical properties, as described by the dielectric constant, volume resistivity, and breakdown voltage, appeared to be affected very little by the electron‐beam radiation. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Effect of coupling agents on rice‐husk‐filled HDPE extruded profiles

Lignocellulosic composites are diversifying their applications into various fields as they can meet the requirements of the respective applications by changing the matrix, fiber resource and processing ingredients. In this research work we explored the potential of extruded rice‐husk‐filled high density polyethylene (HDPE) composite profiles for structural applications. The structure and the properties of the interface in fiber‐reinforced composites play a crucial role in determining the performance properties of the composites. An optimum degree of adhesion between the fiber and the matrix is required for efficient stress transfer from the matrix to the fiber. Generally, coupling agents are used to improve the adhesion between lignocellulosic filler and the polymer matrix in structural composite materials. In this study, four different coupling agents based on ethylene‐(acrylic ester)‐(maleic anhydride) terpolymers and ethylene‐(acrylic ester)‐(glycidyl methacrylate) terpolymers were used to enhance the performance properties of the composites. The results indicated that these coupling agents enhanced the tensile and flexural strength of the composites significantly, and the extent of the coupling effect depends on the nature of the interface formed. Incorporation of coupling agents enhanced the resistance to thermal deformation and the water absorption properties of the composite, whereas it reduced the extrusion rate significantly. Among the four coupling agents used, EGMA1—the one with a glycidyl methacrylate functional group and without any methyl acrylate pendant group on the polymer backbone—was found to be the best coupling agent for the rice‐husk‐filled HDPE composites. Copyright © 2004 Society of Chemical Industry     

Deformation and energy absorption characteristics of microcellular ethylene‐octene copolymer vulcanizates

Compressive stress‐strain properties of unfilled, CaCO₃, silica and aluminum silicate filled closed‐cell microcellular ethylene‐octene co‐polymer vulcanizates were studied with variation of blowing agent loading (density). With decrease in density, the compressive stress‐strain curves for microcellular vulcanizates behave differently from those of the solid vulcanizates. The stress‐strain properties are found to be strain rate dependent. The log‐log plots of relative compressive moduli versus relative density of the microcellular vulcanizates show a fairly linear correlation. The energy absorption behavior was also studied from the stress‐strain properties. The efficiency, E, and Ideality parameter, I, were evaluated. These parameters were plotted against stress to find the maximum efficiency and maximum ideality region, which will make these materials suitable for cushioning and packaging applications. The cushioning factor, C, for microcellular vulcanizates has also been evaluated for various systems.     

Study on composition and characteristics of maleated ethylene‐octene copolymer prepared by reactive extrusion on the morphology and properties of polyamide 6/ethylene‐octene copolymer blends

Blends of polyamide 6 (PA6) and elastomeric ethylene‐octene copolymer (EOR), with and without maleated EOR (EOR‐MA) were studied. EOR‐MA with various amounts of grafted MA and gel content were prepared by reactive extrusion. The effects of EOR‐MA characteristics and composition on the morphology, thermal and mechanical properties of the blends were investigated. EOR‐MA was found to promote the toughness efficiency of PA6 remarkably. High impact resistance was achieved by the use of EOR‐MA containing less than 2% gel. The content of MA grafted on EOR‐MA in the range of 0.5%–1.0% gave a similar effect on the blend properties. The blend containing 20% of EOR grafted with 1% MA exhibited twenty times higher impact strength (1000 J/m) than pure PA6 (55 J/m). The presence of EOR‐MA in the blends led not only to a drastic reduction in the dispersed particle size, but also to some changes in fracture mechanisms, thus enhancing the impact resistance of the blends.     

Use of ultrasound to determine variation of carbon dispersion in carbon black filled HDPE melts

Abstract

An ultrasonic technique was used to determine whether differences in the degree of dispersion of carbon filler in high density polyethylene (HDPE) melts could be identified. Ultrasonic transit time measurements were used to identify differences between three grades of HDPE with different degrees of filler dispersion (designated high, medium and low dispersion). Tests were carried out on static melts (off line) over a range of temperatures and pressures, and during extrusion (inline). Tests carried out on samples of static melt showed a significant increase in ultrasonic transit time for the poorly mixed (low dispersion) sample, compared with the medium and high dispersion samples. Differences measured during off line tests were not observed during extrusion tests.     

Conductivity of polyolefins filled with high‐structure carbon black

The electrical conductivities of various polyolefins filled with a high‐structure carbon black (CB) were studied. Typical percolation behaviors were observed in all of the materials studied. At a critical CB content, which defined the percolation threshold, CB formed conductivity pathways, and resistivity fell sharply from a value characteristic of an insulator into the range of 10–100 Ω cm. The dependence of the percolation threshold on the matrix viscosity was understood in terms of competing effects on CB dispersion during blending and CB flocculation during compression molding. For the conditions used in this study, polypropylene with a melt flow index of about 50 was optimum. Flocculation in the quiescent melt was studied directly by atomic force microscopy. Conductivity pathways formed over time by CB agglomeration. The temperature dependence of the percolation time was described by an Arrhenius relationship. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1799–1805, 2005     

Effect of organoclay content on the mechanical properties of ethylene‐vinyl acetate copolymer/ multi‐walled carbon nanotube/organoclay foams

The main objective of this study is to obtain ethylene‐vinyl acetate copolymer (EVA)/multi‐walled carbon nanotube (MWCNT)/organoclay foams with improved mechanical properties without increase of their density, compared with EVA/MWCNT foams. MWCNT content was fixed at 5 phr in this study. To achieve the objective, EVA was melt‐mixed with MWCNTs and organoclays in a bench kneader. And the obtained EVA/MWCNT/organoclay mixtures were mixed with chemical blowing agent and cross‐linking agent in a two roll‐mill. After being mixed in a two roll‐mill, the mixtures were put in a mold and the foams were obtained by compression‐molding. The effect of organoclay content on the mechanical properties and surface resistivity of EVA/MWCNT (5 phr)/organoclay foams was investigated. The addition of 1 phr organoclays to the EVA/MWCNT (5 phr) foams resulted in the improvement of tensile strength, 100% tensile modulus, tear strength, and compression set without increase of the density. However, further increase in content of organoclay (3 phr) leaded to a deterioration of mechanical properties. Therefore, determining the optimal content of organoclay was very important in order to achieve the main objective of this study. POLYM. COMPOS., 2013. © 2013 Society of Plastics Engineers     

Electrical properties and morphology of carbon black‐filled HDPE/EVA composites

The sensitive effect of weight ratio of the high‐density polyethylene (HDPE)/ethylene‐vinylacetate copolymer (EVA) on the electrical properties of HDPE/EVA/carbon black (CB) composites was investigated. With the EVA content increasing from 0 wt % to 100 wt %, an obvious change of positive temperature coefficient (PTC) curve was observed, and a U‐shaped insulator‐conductor‐insulator transition in HDPE/EVA/CB composites with a CB concentration nearby the percolation threshold was found. The selective location of CB particles in HDPE/EVA blend was analyzed by means of theoretical method and scanning electron micrograph (SEM) in order to explain the U‐shaped insulator‐conductor‐insulator transition, a phenomenon different from double percolation in this composite. The first significant change of the resistivity, an insulator‐conductor transition, occurred when the conductive networks diffused into the whole matrix due to the forming of the conductive networks and the continuous EVA phase. The second time significant change of the resistivity, a conductor‐insulator transition, appeared when the amorphous phase is too large for CB particles to form the conductive networks throughout the whole matrix. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010