Investigation on electrical properties of polyvinyl acetate/graphite adhesive by joule heating and hall effect tests

https://doi.org/10.1016/j.mtcomm.2020.101680Get rights and content

Abstract

decreasing and increasing electrical resistivity of materials by increasing temperature named Negative thermal coefficient (NTC) and positive thermal coefficient (PTC), respectively, are two critical properties for thermistor materials. In this research, the PTC/NTC behavior of conductive adhesive are investigated by performing two tests: Hall Effect and Joule heating tests. In Hall Effect, the temperature increases by the heating element, while in Joule Heating, the ohmic resistance of adhesive leads to increasing the temperature. The adhesives are made of polyvinyl acetate as binder and 40, 50, 60, and 70 wt.% Graphite (G) as filler. Characterizations include the lap-shear strength, electrical conductivity, and Archimedes porosimetry tests, as well as Fourier-transform infrared spectroscopy, Raman spectroscopy, and Scanning Electron Microscopy analysis to investigate the change of chemical composition and crystallinity of carbon adhesives before and after the Joule Heating test. There are little information about the physical mechanisms of NTC/PTC behavior in literature. NTC behavior of carbon adhesives is interpreted by two physical mechanisms. The first is based on increasing temperature on the regions named hot spots, thereby decomposing polymer to amorphous Carbon (validate by experimental analyses) and the second, decreasing polymer viscosity and, consequently, s rotating and aligning the G particles, validated by mathematical modeling.

Introduction

There are three different behaviors of electrical resistivity on the materials via heating; The Positive Temperature Coefficient (PTC), Negative Temperature Coefficient (NTC), and Zero Thermal Coefficients (ZTC). In PTC-type, the resistivity increases with temperature, and in NTC-type, the resistivity decreases with temperature.

PTC/NTC behavior of conductive polymeric composites can be utilized in many applications, such as thermistors, heating electrodes, shielding materials, actuators, self-healing plastics, antistatic products, overcurrent protectors and self-regulation heaters, and electromagnetic radiation absorbing materials [1,2].

The mechanisms of PTC/NTC behaviors are called pyro resistivity have not yet been recognized [[1], [2], [3], [4], [5]]. In the literature, the PTC/NTC behavior of polymer-based carbon composites have been related to the phase transition [6], polymer expansion of polymer [6], crystallinity in polymer chains [3,4,7], shape, distribution, size, and aspect ratio of fillers [8].

For application in a bipolar plate and other materials that need to higher conductivity, NTC behavior is appropriate. Because, over time, the conductivity of the composite increases, thereby decreasing interface contact resistance and improves the performance curve. In thermistors also the resistivity changes by increasing temperature. Thermistors have high applications in electronic devices and are used as a temperature sensor.

There are many research, Multiwall Carbon Nanotube (MCNT) [4,9,10], carbon Black (CB) [1,11,12] carbon fiber [2,13], metallic powder [8,13,14], and graphene [3,15,16] have been filled into different polymers such as High Density Poly Ethylene (HDPE) [1,7,17], Polyvinylidene fluoride or polyvinylidene difluoride (PVDF) [2,15,16,18], polypropylene (PP), epoxy [8], and nylon [14]. However, in all of them, direct heating has been used for study on PTC/NTC. Few articles studied indirect heating via the Joule 1Heating Effect or Hall Effect tests for the NTC/PTC behavior conductive composites [5].

Tang et al. [1] reported that carbon black (CB) filled ethylene/ethyl acrylate copolymer (EEA) composites shows a PTC behavior. Li et al. [11] observed that the composite of HDPE, LDPE, and ethylene/vinyl acetate (EVA) as three binders and two kinds of carbon blacks (CB), carbon fiber as fillers show a PTC behavior before 120 °C and NTC behavior after that. Xin-lei et al. [19]reported that Carbon fiber (CF) filled low-density polyethylene (LDPE) composites resulted in PTC behavior up to 110 °C and NTC behavior after that. NTC region can be effectively eliminated by crosslinking under γ-ray radiation so that the crosslinked composite exhibits a higher PTC intensity and PTC transition temperature than the uncrosslinked composite.

Cristina et al. [20] observed that polyvinylidene fluoride (PVDF) matrix composites filled with graphite fiber or carbon fiber show PTC behavior before 150 °C and NTC behavior after 150 °C. Here, it was revealed that the addition of 2% CF into the 7% PVDF composite could effectively eliminate the NTC phenomenon.

In another work [21] the effect of electro-beam irradiation on PTC/NTC behaviors of carbon blacks/HDPE was investigated. The electron beam leads to crosslinking the polymer. It was specified that the 25 °C up to 150 °C shows PTC and, after that shows NTC intensity. The researchers have converted the NTC behavior to ZTC behavior of the conducting polymer composites by a dosage of 60 kGy that creates an effective crosslinking.

In another research [22], the reverse behavior in comparison, most studies are observed. The composite of carbon fiber as filler and cement as matrix shows NTC behavior up to 70 C and after that PTC behavior. Increasing CF percentage from 20 to 80 wt.% leads to increasing PTC intensity or PTC slope.

Park et al. [7] in other research were used that the nylon, polyvinylidene fluoride, polyester, and polyacetal, as matrix and Carbon black and nickel powder as filler. All polymers showed PTC up to 190, 160, 200, and 170 °C, respectively. But after that, NTC behavior was observed. The crosslinking structure enhanced electrical stability and decreased the NTC effect of the composites.

Many types of research established that the use of external forces such as magnetic field and electric field could lead to the alignment of fillers along the field direction and increasing the electrical conductivity of composite along the field direction and decrease the conductivity vertical to the field direction. The difference of the present work with these references [[23], [24], [25], [26]] is that in the current work, the composite is solid, while in these references, the polymers are liquid during field emission. Fig. 10a, b, shows the alignment of G particles before and after directed in the electrical field.

It can be seen in the above literature, in about NTC or PTC behavior of polymer-based composites, there is little information about the physical mechanisms of NTC/PTC behavior of polymer composites. It needs more investigation on this subject. This research will follow the study on the Hall Effect and Joule Heating Effect of Polyvinyl acetate (PVAc) polymer filled with Graphite. PVAc/G adhesive with different G values will be manufactured, and electrical conductivity, adhesion strength, and porosity values will be determined. PTC/NTC behavior of PVAc/G composites with different filler values have been investigated by Hall Effect and Joule Heating tests. Raman and FT-IR analyses will examine changes in the chemical composition. A detailed description of the physical mechanism upon the observed results have been introduced, and the difference between Hall Effect and Joule Heating Effect tests have been described. In order to validate the presented claims, the mathematical model has been introduced for DC. This model has been reported for the first time on the system of the polymer/G system. The difference between the present model with the work of Wu et al. [27] is that the model of Wu was written for AC. Besides, the filler was graphene, while in present work is G with different dimensions and optimized equations.

Section snippets

Materials

Graphite has been purchased from Merck Co. with a particle size of 50 μm, powder density 2.35 g/cm3, bulk density 20−30 g/100 mL. Polyvinyl acetate has been purchased from Resinsazan Co. is a white color with a density of 0.934 g/cm3.

Sample production method

The size of the stainless steel samples was 1 cm × 1 cm×3 mm. The samples were then sanded with 200–1000 sandpaper, then were washed with acetone. The carbon conductive adhesive was prepared by mixing 40, 50, 60, and 70 wt.% of G with PVAc. To decrease the adhesive

FESEM

The surface area of PVAc/50 G can be seen in Fig. 2a. It can be seen there is some porosity in the bulk of adhesive. G particles can be seen darker than polymer.

Lap-shear test

Fig. 3 shows that the maximum strength of adhesion for the adhesives have been decreased by increasing G loading values. That is due to that G is a soft material and reduces adherence of PVAc to a metallic plate. Besides, the surface area below each curve, indicating the toughness value of adhesive, shows that by increasing G value in

Conclusions

In this research, the conductive adhesive has been manufactured by using G as filler and PVAc as polymer. The mechanical property, porosity, and electrical conductivity values were determined. It was observed that the lack of hot compression in the production process, as well as G agglomeration, leads to creating some porosities within the bulk composite that increase the percolation threshold.

To find NTC or PTC behavior of this adhesive, Hall Effect and Joule Heating tests were performed.

Declaration of Competing Interest

Reza Taherian is supervisor of Zahra Samiei in Master thesis. This research is as a result of research of this project.

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