Polypropylene/basalt thick film composites: structural, mechanical and dielectric properties

2.1 Materials

PP was supplied from Petkim Industry (Turkey). The melting temperature of PP is 165°C and the density is 0.905 g/cm³.

VBR was collected from Van City in Turkey. The chemical analyses of the VBR powder was already been identified by X-ray fluorescence (XRF) measurements performed by the equipment model X-123SDD from AMPTEK (Bedford, MA, USA) in our previous work (8). The chemical content of the VBR powder is given in Figure 1. According to Figure 1, the sample contains 41.668% SiO₂ and 6.998% (Na₂O+K₂O).

Figure 1:

Chemical composition of the samples.

As is known, the classification of rocks is largely based on two diagrams: quartz, alkali-feldspar, plagioclase, and foids (QAPF) diagram for plutonic rocks and the total alcali silica (TAS) diagram for volcanic rocks. According to the TAS classification of the volcanic rocks, if the rock contains SiO₂ between 41% and 45% and has (Na₂O+K₂O) content less than 7% (w/w), the sample is considered as “tephrite basanite”. From this point of view, the VBR powder used in this work was determined as “tephrite basanite”.

The particle size distribution of VBR, 10% of the particles’ size is smaller than 1.905 μm, 50% of the particles’ size is smaller than 17.378 μm, and 90% of the particles’ size is smaller than 60.256 μm (measured by a Mastersizer, Malvern Hydro 2000MU, Malvern Instruments, UK).

2.2 Preparation of PP/VBR thick film composites

VBR was mechanically doped to PP with different weight percentages (varying from 0.5 wt.% to 20 wt.%) to obtain the PP/VBR mixtures. Then the mixtures were ground with IKA A11 in a basic analytical mill with the rotational speed of 10,000 rpm. The process of grinding was performed in a chamber with a volume of 250 ml by using 30 mm steel balls. The mixing time per sample was 15 min. After the grinding process, each mixture becomes a homogeneous powder form with the maximum grain size of 10 mm. The powder forms of the composites were compressed by an electrical heated press. The hot press procedure included two steps. The first step was the pre-heating of the samples at 438 K for 20 min. The second step was the compression of the samples at 438 K for 10 min under 15 MPa pressure. The thickness of the samples prepared were between 71 μm and 100 μm. Finally, the thick films of pure PP, PP/0.5 wt.% VBR, PP/1.0 wt.% VBR, PP/5.0 wt.% VBR, PP/10.0 wt.% VBR, PP/20.0 wt.% VBR were cooled in a flow of cold water. The preparation process of the thick film composites are given schematically in Figure 2.

Figure 2:

Schematic representation of the preparation process of PP/VBR thick film composites.

2.3 Fourier-transformation infrared (FT-IR) spectroscopy measurements

Fourier-transform infrared (FTIR) spectroscopy analyses of the PP/VBR thick film composites were performed using a Perkin Elmer Spectrum 400 FTIR spectrometer. All spectra of the samples were taken between 4000 and 400 cm⁻¹ wave number, with a resolution of 2 cm⁻¹ after four scans. The FTIR measurements were performed in the transmittance mode.

2.4 Thermogravimetric measurements

The thermogravimetric analyses of PP/VBR thick film composites were performed using a Perkin-Elmer Pyris Diamond thermogravimetric-dynamic temperature analyzer (TG-DTA) with programmed heating at 10°C/min from 30°C to 700°C. TG thermograms of the samples were also recorded under nitrogen atmosphere.

2.5 Scanning electron microscope (SEM) measurements

The surface morphologies of the PP/VBR composite thick film samples were determined using a Zeiss-EVO^® LS 10 model scanning electron microscope (SEM).

2.6 Mechanical measurements

The mechanical properties of PP/VBR composites were evaluated using a Lloyd Instruments LF Plus Single Column Universal Materials Testing Machine according to ASTM D638-10 standard. To determine Young’s modulus, tensile strength, percentage strain, energy at the break and stiffness the crosshead speed was 50 mm/min at 23°C±3°C temperature. The thickness of PP/VBR composite films was measured using a Mitutoyo micrometer.

2.7 Dielectric measurements

Thick film composites were placed between two gold electrodes whose surfaces wholly overlapped the faces of the samples. The diameter of the cylindrical electrodes was 20 mm. Dielectric measurements were performed using an HP 4194A Impedance Analyzer between 100 Hz and 15 MHz at room temperature with a high accuracy (0.17% typ.) The results were transferred to computer with GPIB data cable and simultaneously recorded using a computer.

3.1 Structural analysis

The typical FTIR spectra of pure PP and VBR powder are shown in Figure 3A. The assignment for transmittance bands for pure PP are listed in Table 1. As shown in Figure 3A, the characteristic VBR peaks were also observed between 600 and 1200 cm⁻¹. The wide peak observed at around 1000 cm⁻¹ for VBR is attributed to an asymmetric stretching of Si-O which generally occurs between 900 and 1200 cm⁻¹ (17). The second main peak observed at 700cm⁻¹ also corresponds to the Fe-O bonds (18). The other peaks determined at 595, 656cm⁻¹ and 715cm⁻¹ can be assigned to the Al-O stretching mode and symmetric bending of Al-O-H, respectively (19).

Figure 3:

FT-IR graphics of (A) pure PP and VBR (B) PP/VBR thick film composites.

The FTIR spectra of PP/VBR thick film composites are shown in Figure 3B. It was observed that the characteristic peaks which identify PP between 900 and 1200 cm⁻¹ become wider with increasing basalt doping. This effect is mainly due to increasing the SiO₂ content of the composites with basalt doping. Moreover, a new peak, which was not observed for pure PP, was determined for PP/VBR composites between 650 and 750 cm⁻¹ due to basalt additive. This new peak can be attributed to Al₂O₃ (715 cm⁻¹) and Fe₂O₃ (700 cm⁻¹) which are the other two components with high concentration in VBR.

3.2 Thermal analysis

The TG results of the samples are given in Figure 4. As shown in Figure 4, a one-step pattern of thermal degradation occurred for each sample approximately between 300°C and 500°C.

Figure 4:

TG graphics of PP/VBR thick film composites.

According to TG results, the pure PP sample loses almost all its weight at 480°C. On the other hand, the PP/VBR composites have lower mass loss depending on the basalt content of the samples which results in the higher residue weight percentages (see Table 2). The increasing mass residue with increasing basalt content at the temperatures higher than 480°C can be explained by the fact that except for K₂O and P₂O₅, all oxides in the basalt have very high melting temperatures (>1100°C) (see Table 3).

3.3 Morphological analysis

The SEM images of the composites are given in Figure 5. The VBR is shown in Figure 5A and PP is shown in Figure 5B while Figure 5C–G shows the PP/VBR composites. As shown in Figure 5B, pure PP with a thick film form has an almost smooth surface. It has been clearly observed that VBR powder grains are not homogenously distributed on the pure PP. This results in a surface roughness for VBR additive concentration which is higher than 1%. However, it has been determined that 0.5% VBR doping does not noticeably affect the surface morphology of pure PP. The apparent increase in surface roughness observed for the PP/VBR composites, which have a VBR content higher than 1%, causes a degradation in the mechanical properties. This prediction was also confirmed by our mechanical results.

Figure 5:

SEM images of VBR, PP and PP/VBR thick film samples.

3.4 Mechanical analysis

Stress-percentage strain curves of pure PP and PP/VBR composites are given in Figure 6. According to the initial slope of the tensile curves, Young’s modulus are calculated and summarized in Table 4.

Figure 6:

Stress-percentage strain curve of PP and PP/VBR thick films.

The other mechanical properties including tensile strength, percentage strain and energy at break and stiffness were also determined from Figure 6 and are given in Table 4.

According to Table 4, it was determined that the mechanical properties of PP polymer is improved by 0.5 wt.% VBR doping. In other words, the highest tensile strength, percentage strain and energy at break were achieved for PP/0.5 wt.% VBR composite. Moreover, the higher than 0.5 wt.% VBR content makes the mechanical properties of PP polymer worse. In this respect, it was determined that PP/0.5 wt.% VBR composite has the optimum mechanical properties for applications. The optimum concentration percentage of VBR for achieving maximum mechanical performance for PP is also in good agreement with the general opinion for inorganic additives proposed by Boydağ et al. (20), (21), (22).

3.5 Dielectric analysis

The complex dielectric function is defined by the real and imaginary components dielectric constant:

The relation between the real and imaginary component of complex dielectric function is given by the loss factor:

The frequency dependences of the real part of complex dielectric function of thick film composites are given in Figure 7.

According to Figure 7, regardless of the VBR doping percentage, all samples exhibit almost frequency independent behavior between 100 Hz and 1 MHz. This behavior is also agreement with the scientific literature for dielectric properties of pure PP (9). It was observed that VBR doping lowers the magnitude of the real part of dielectric constant up to a critical doping percentage of 20.0 wt.% VBR. Especially, 0.5 wt.% VBR doping results in the lowest ε′ values. From this point of view, PP/0.5 wt.% VBR composite may be suggested as a low dielectric constant material for next generation of microelectronic devices. As is known, a choice of suitable low dielectric material has a crucial role on both device’s performance and lifetime (23).

Figure 7:

The variation of the real component of complex dielectric function of the PP based samples.

In Figure 7, the variation of dielectric strength with respect to VBR doping rate is shown in the table. The dielectric strength, Δε was calculated by Eq. [3]

where ε_s and ε_∞ are the limiting low and high frequency dielectric constants, respectively. As shown in Figure 6, dielectric strength first decreases with increasing VBR doping and reaches its minimum value for 0.5 wt.% VBR doping concentration and then starts to increase with increasing VBR doping rate. The decrease in the value of dielectric strength for 0.5 wt.% VBR doping can be interpreted as the fact that the molecular alignment is getting easier relative to other PP thick film composites.

Figure 8 also shows the frequency dependence of the imaginary component of complex dielectric function of pure PP and PP/VBR thick film composites. According to Figure 8, the samples have considerably low dielectric loss and a relaxation peak observed at high frequency region. The relaxation frequency, f_c, of pure PP at which a maximum dielectric loss was observed, shifts to higher frequencies up to 1.0 wt.% VBR doping and then f_c decreases to lower frequencies with increasing VBR doping percentages (see Table in Figure 8). In addition, a sharp relaxation peak observed at 10.626 MHz for PP/1.0 wt.% VBR composite can be attributed to the occurrence of vibrational relaxation process in the material.

Figure 8:

The variation of the imaginary component of complex dielectric function of the samples.

3.6 Analysis of alternative current conductivity

The frequency dependence of alternative current (ac) conductivity was investigated in terms of Jonscher’s power law defined by Eq. [4]:

where σ_dc corresponds to conductivity at ω=0, A is pre-exponential constant, ω is the angular frequency and s is frequency exponent which varies between 0 and 1 (24), (25), (26). “Joncher’s Power Law” was used for analysis to determine ac conduction mechanisms of wide variety of materials including solid electrolytes, amorphous solids, molten ion conductors, molecular materials, etc. (27).

The angular frequency dependences of ac conductivity of the thick film samples in natural logarithmic scale are given in Figure 9. As shown in Figure 9, σ_ac(ω) curves of all samples show a linear increase with increasing angular frequency in ln-ln scale but no significant change on the value of ac conductivity was observed due to VBR powder doping.

Figure 9:

The lnσ_ac versus frequency curves of thick film composites.

The frequency exponent value, s, was calculated by the slope of the curves given in Figure 9. The σ_dc values for each sample were also calculated using a linear fit performed with Origin Lab 2015. The related conductivity parameters are listed in Table 5.

By referring s parameter values at the vicinity of 0.99, the charge transport mechanism of the samples is suggested as the nearly constant loss (NCL) model (28). As is known, the NCL model deals with ac conductivity which depends nearly linearly on frequency. Essentially, the NCL model is the consequence of the nearly frequency independent dielectric loss just like in our case (see Figure 8).

This type of ac conductivity is generally observed for glasses and ceramics, some polymers exhibit the NCL type of ac conductivity mechanism (29), (30). Several explanations have been suggested for the origin of NLC in the scientific literature. From the microscopic point of view, this behavior may be attributed to the cooperative “jellyfish-type” movements of atom groups (31), (32), (33). According to the NCL model, angular frequency dependence of ac conductivity for PP thick film samples has two terms: σ_ac(ω)=σ_dc+Aω. From this point of view, it has been proved that ac conductivity part of each PP/VBR thick films is directly proportional to angular frequency.