Enhanced mechanical and electrical properties of ECR-glass reinforced polyimide composites with incorporation of TiO2 for insulation applications

This paper presents the mechanical behaviour of novel class composites consisting of ECR-glass type reinforced polyimide (PI) composite loaded with TiO2 nanoparticles. ECR glass reinforced PI composites were fabricated by adding TiO2 particles at three different concentrations namely; 2, 4, and 6 wt% using 3D-Turbula dispersion and Spark Plasma Sintering (SPS) method. The morphologies, crystallinity, mechanical, and electrical properties of the produced composites were evaluated using scanning electron microscope (SEM), X-ray diffractometer, nanoindentation test, and LCR meter device. The SEM results revealed that the TiO2 nanoparticles were homogenously dispersed into the PI composites. The mechanical properties, such as hardness, stiffness, and elastic modulus of the pure PI and ECR glass reinforced PI composite was improved by the incorporation of TiO2 nanoparticles. Maximum hardness and elastic modulus values of 2.19 GPa and 13.99 GP, respectively, was observed in ECR reinforced PI composited loaded with 6 wt% TiO2 nanoparticles. In addition, ECR glass reinforced PI composites with 6 wt% TiO2 nanoparticles depicted the lowest dielectric constant (1.18), dielectric loss (1.14) and electrical conductivity (3.16 × 10−6 S/cm). Finally, the findings suggest the easy processability of PI nanocomposites and their potential for mechanical and electrical insulation applications.


Introduction
Polymer based matrix composites are currently attracting the attention of scientists, engineers, researchers, academicians and industries as a promising material in the design and fabrication of structural and insulator components.6][7][8] More so, in the design and fabrication of parts for thermal, mechanical, and tribological applications, PI materials have significantly become beneficial to numerous industries, such as electrical, power, and automotive and transportation as a result of its properties. 9However, as power equipment/structures and electrical systems are developing towards high mechanical loads and high power, the used of polyimide composites become a challenge, for instance insulator core rods.Thus, using suitable reinforcement particulates to improve the mechanical and electrical insulation properties of PI remains a concern to researchers and industries. 3,10Electrical corrosion resistance (ECR) glass is a novel glass fiber, which is similar to E-glass fiber but happened to be boron and fluorine free.2][13][14][15] This mentioned features of ECR-glass positioned its usage in the fabrication of highperformance composites.As such, ECR-glass is at all times used to improve the mechanical and electrical insulation performance of polymer matrix composites.Generally, ECR-glass can increase the hardness, yield strength, elastic modulus of polymer composites and reduced their dielectric constant with improved electrical insulation owing to its intrinsic nature.For instance, study conducted by Wieczorek et al. 16 on the aging resistance of ECR-glass fiber reinforced polymer core rod for use in composite insulators under direct current high voltage revealed no trace of degradation on the composite properties.In the study, after the long-term exposure of the polymer composites reinforced with ECR-glass to direct current voltage field condition in the direction parallel to the ECR-glass for 6000 h.It was captured that the composites depicted no mechanical or electrical insulation deterioration.Thus, it was concluded that ECR-glass remains an appropriate filler material in enhancing the properties of polymer composite insulator core rods for high voltage applications.Velaga et al. 17 study obtained that incorporation of the ECR-glass into polymer composite core rod improves its electrical and thermal insulation, hardness and structural integrity for mechanical load-bearing and high voltage insulation.Indeed, to achieve polymer matrix based composites with superior mechanical and electrical properties for mechanical load-bearing and electrical insulation, two or more types of particulates are usually introduced in the polymer matrix simultaneously.And numerous investigations revealed that glass fiber and micro-and nano-inorganic particles, for example TiO 2 , 18 SiO 2 , 19 Al 2 O 3 3,[20][21][22] and so on can improve the mechanical and electrical properties of polymer composites synergistically.Nallusamy 18 studied the effect of hybrid glass-reinforced fiber with epoxy nanocomposites.The nanocomposites were produced by hand lay-up method with different content of TiO 2 nanoparticles.The results indicate that the addition of the TiO 2 into polymer composite improved its strength and hardness.Sivabharathi et al. 20 evaluated the mechanical properties of glass fiberreinforced polyester composite embedded with Al 2 O 3 particles.The composites were prepared using hand lay-up fabrication process at different concentrations of Al 2 O 3 particles.Characterizing the composites, it was recorded that glass fiber reinforced polyester composite hardness was enhanced by increasing the weight percent Al 2 O 3 particles.Furthermore, study conducted by Yim et al. 19 showed that the introduction of SiO 2 nanoparticles into the glass fiber-reinforced PI based composites primarily enhance its properties.As the SiO 2 containing composites depicted low dielectric constant for insulation with superior mechanical properties.In the aforementioned study, the primary mechanism is that glass fiber with high strength and stiffness could carry load applied on the indenter tip in reducing the load effect of the indenter.Meanwhile, the nanoparticles can share stress within glass fiber in protecting glass fiber from being attacked.As such, it can be deduced that introduction of nanoparticles in the interface of glass fiber and host matrix, the resultant composites could exhibit much better mechanical, dielectric and electrical properties for load-bearing and electrical insulation applications.However, in the present study, to well advance the positive influence of inorganic particulates, a novel technique (SPS) is been adopted for enhancing the properties of ECR/PI by incorporating TiO 2 nanoparticles into the ECR/PI powders using 3D-Turbula dispersion method.This can comprehend the simultaneous incorporation of TiO 2, and ECR glass in the PI based composites.Besides, as ECR-glass addition is in micron size, it is easily dispersed in composites.Thus, the adopted method (3D-Turbula mixer) of introducing TiO 2 nanoparticles into the ECR/PI frame work can solve the issue of agglomeration of TiO 2 nanoparticles accordingly.The fabrication method differs from the past studies where the composites are prepared by hand lay-up method.As a result of this, the morphology and influence of TiO 2 on the mechanical, dielectric, and electrical insulation properties of PI reinforced composites have been examined systematically and discussed in detail.Positively, this research can be a guide in the design and production of high performance polymer composite components for mechanical load-bearing and electrical insulation applications.

Materials
An amorphous raw PI powder supplied by Xi'an Lyphar Biotech Co Ltd, China of purity 99.95%, with a particle size of 18-25 μm was used as the based polymer matrix.The ECR-glass powder (40-70 μm) as one of the reinforcing phase material used for the study was supplied by Hebei Yuniu Fiberglass Manufacturing Co. Ltd, China.The second reinforcing material, which is TiO 2 nanoparticles (30-50 nm) of purity 99% plus was supplied by Hongwu International Group Ltd, China.

Fabrication of PI composites
The ECR/PI loaded with 2, 4, and 6 wt% TiO 2 nanoparticles samples were produced using a KCE-FCT-HHPD 25 SPS sintering machine made in Germany.Prior to sintering, the nanoparticles were dispersed into the PI composites via 3D-Turbula blending method, and this was performed for 2 h in ensuring homogeneous dispersion of the powder particles.In accomplishing the blending process, the blended samples were weighed by an electronic weighing scale of sensitivity of 0.01 g in order to produce PI composites component of thickness 10 mm.Sintering was performed under a vacuum using graphite die mould of 30 mm in diameter at 320°C sintering temperature, 30 MPa pressure, 9 min dwell time, and 5°C/min heating rate to ensure temperature homogeneity inside the sample.In all the experiments, the sintering temperature was monitored using K-thermocouple, which was placed 2 mm from the internal die surface.For the fabrication of the pure PI and 5 wt% ECR/PI samples, the same sintering process was performed.

Characterization and measurements
The densities of the sintered samples were obtained using a digital Densimeter device (JA5003 J).X-ray diffractometer (XRD) (X'pert PRO PANalytical) was employed for the determination of the crystallinity index, amorphous region/pattern of PI, ECR/PI, and TiO 2 -containing PI composites.The XRD was carried out using Cu Kα radiation and a scanning speed of 5°C/min.The dispersion of the TiO 2 into the PI matrix based fiber reinforced composites was evaluated by scanning electron microscope (SEM) (VEGA3 TESCAN).The SEM was performed on thin carbon-coated composite samples and conducted at an accelerated voltage of about 20 kV.The mechanical properties of the composites were determined using a nanoindentation tests in accordance with ASTM D785 standard.Applied load of 200 mN, penetration, holding, and retracting time of 20 s for each was adopted.The average mean of five tests per sample was incorporated in this research.The nanoindentation operation is based on Oliver and Pharr method; meanwhile, the hardness H (GPa), stiffness S (mN/nm), and elastic modulus E (GPa) of samples were extracted using the equations described in Ref. 23.The dielectric and electrical resistance of the samples were ascertained at room temperature using an LCR meter (B and K 891) at a frequency range of 100 to 50 kHz.However, the dielectric constant (ε I ), dielectric loss (ε II ), and electrical conductivity (σ) were determined using equations ( 1)-(3). 24 containing PI based reinforced fiber composites did not depict much diffraction peaks, hence demonstrating the amorphous behaviour and low degree of crystallinity of the pure PI and nanocomposites.Glass fiber as an amorphous substance on the other hand evident the amorphous nature of the composites. 25The degree of crystallinity or crystallinity index of the samples was determined using equation (4). 26c ¼ ð0:24=β002Þ 3 (4)

Structure and morphology
Where X c is the crystallinity index, β002 is the Full Width at Half Maximum (FWHM) of (002) reflection, which was obtained using Highscore plus software.Table 1 presents crystallinity index of the pure PI, ECR/PI, and TiO 2 -ECR/PI nanocomposites.It can be seen that the crystallinity of the ECR/PI reduced by the addition of 5 wt% ECR glass particulates.Meanwhile, ECR/PI loaded with 6 wt% TiO 2 drastically reduced in crystallinity when compared with the pure PI, ECR/PI, and other ECR/PI loaded with 2 and 4 wt% TiO 2 nanoparticles.Herein, the reduction in the crystallinity of the 6 wt% TiO 2 -ECR/PI nanocomposites could be attributed to the uniform dispersion of the TiO 2 in the composite framework structure couple with its affinity to materials. 27Therein, the presence of TiO 2 nanoparticle at these percentages function as a hindrance of polymer crystallization, and this is in agreement with the study reported elsewhere. 28igure 2(a)-(e) presents the SEM image of the pure PI and the PI composites.The SEM results revealed that the nanoparticles were uniformly dispersed into the PI matrix material without form of agglomeration, and all of the particles are separate in size i.e., the ECR micro particles and TiO 2 nanoparticles.In all the microstructure images, it can be seen that the nanocomposites depicted negligible grain growth.0][31] Furthermore, the SEM microstructure of 6 wt% shows that the TiO 2 nanoparticles are distributed uniformly into the PI material in comparison with other weight percentage of reinforcement materials, such as 0, 2, and 4 wt% nanoparticles as can be seen in Figure 2.However, with aid of the SEM results, there is evidence that the  32 where TiO 2 hybridized with SiO 2 .One can agree with this owing to the fact that ECR glass constitutes of metallic oxides with SiO 2 tetrahedron as the backbone. 33Figure 2(f) shows the SEM-EDS of the selected TiO 2 -containing glass fiber-reinforced PI matrix nanocomposites.The SEM-EDS results revealed the elemental composition of the nanocomposites to be carbon (C), oxygen (O), titanium (Ti), silicon (Si), calcium (Ca), and aluminium (Al), hence validate that the glass fiber used in the study is boron-free glass fiber.

Density, nanoindentation and mechanical properties of the composites
Prior to material characterization, the sintered samples density was determined using densimeter device (JA5003J), which function with the Archimedes principle.As can be seen in Table 1, addition of the ECR in the PI matrix does not have much significant effect on the composite density.However, introducing the TiO 2 nanoparticles into the ECRreinforced PI yielded higher density of about 9% when compared to those of pure PI and ECR/PI samples.It is worthy to note that increased in the wt% of the nanoparticles showed no difference in the density (Table 1), 4 to 6 wt% TiO 2 in particular.All these observation indicates that the SPS short sintering time in achieving full densification promotes lightweight and near zero grain growth with a controlled microstructure.And hence the improved properties recorded in the TiO 2 -containing ECR reinforced PI nanocomposites. 34However, the nanomechanical analysis was conducted on the sintered samples in order to ascertain the hardness, elastic modulus, and stiffness by adopting nanoindentation test.Figure 3 displays the load against displacement or penetration depth curves of the composites.The loading plots revealed that loading was accomplished quite smoothly without any discontinuities, demonstrating no crack on the material surface during the loading process.The TiO 2 reinforced ECR/PI nanocomposites shifted to lower penetration depth values in comparison with the pure PI and ECR/PI without TiO 2 nanoparticles.Meanwhile, ECR/PI loaded with 6 wt% TiO 2 nanoparticles depicted the lowest penetration depth as showed in Figure 3, thus illustrating an enhanced mechanical load-bearing capacity of the ECR/PI composites.The load against displacement curves were evaluated utilizing the inbuilt software provided by 'Micro Material' in determining hardness, modulus, and stiffness.These values were calculated using the below equations, as reported in Refs.23 and 35.
Where H represent the hardness, F is the maximum load applied, A equals the contact area at maximum load, E r is the reduced modulus usually obtained from the indenter machine, E is the sample elastic modulus, E i is the modulus of the diamond indenter (1141 GPa), υ is the Poisson ratio of the PI (0.34), υ i is the Poisson ratio of the indenter (0.07), S is the stiffness, ΔL is the change in load, and Δh is change in the penetration depth.To compare the obtained response properties after nanoindentation testing, the hardness, elastic modulus, and stiffness of the sintered samples tested are presented in Figure 4. From Figure 4, it can be seen that incorporation of the TiO 2 nanoparticles in the PI materials improved its hardness, modulus and stiffness.The maximum hardness and elastic modulus in the sintered samples were recorded in ECR/PI composites loaded with 6 wt% TiO 2 nanoparticles, which were 2.19 ± 0.06 and 13.99 ± 0.23 GPa, respectively.The enhanced hardness and elastic modulus of the ECR/PI containing TiO 2 reinforcements could be attributed to the nanoparticle incorporation due to the free volume reduction in the PI matrix after sintering.Moreover, study conducted by Refs.36, 37 and 38 evident that addition of nanoparticles do take up the free volume in host matrix, which in turn improves the resultant material properties.Again, uniform dispersion of the nanoparticles in the PI matrix material with improved load transfer in turn result to high plastic deformation resistance of the nanocomposites.Owing to the fact that the ECR having SiO 2 as backbone, 33 and TiO 2 nanoparticles will synergistically result to the plastic deformation resistance of the composites. 39,40Furthermore, the improved elastic modulus and stiffness of ECR/PI loaded with 4 and 6 wt% TiO 2 on the other hand could be ascribed to active and strong interfacial interaction between the reinforcement and PI matrix 41 using 3D Turbula dispersion and SPS.In addition, fabrication process parameter remains a contribution to the improved properties of the thermoplastic polymer composites. 42More so, TiO 2 is known to be an inorganic material with good stress transfer properties and as such could play a vital role of reinforcing effect in carrying part of the stress in protecting ECR from being damage from the external load applied. 43However, contributes in the improved mechanical properties of the PI composites when compared to those of the pure PI. 44

Electrical properties characterization
To produce PI nanocomposites with electrical insulation properties suitable for electronic device or insulator core rod application, PI nanocomposites with low dielectric constant and dielectric loss are urgently needed.However, the influence of varying TiO 2 nanoparticles on the dielectric and electrical properties for the TiO 2 -ECR/PI composites was carried out.Figure 5(a) and (c) showed the dependency of dielectric characteristics of the nanocomposites with different TiO 2 content on the frequency.For the PI composites reinforced with or without TiO 2 , a decrease in dielectric constant is noticed as the frequency increases from 100 Hz to about 10 kHz, and then followed by a stable response.Thus, the space charge polarization, which generates from the nanocomposites interface could be attributed to the dramatically reduction of the nanocomposites dielectric constant as it could result to more or less charge barrier. 45Basically, suppressing of space charge in polymer material remains a great significance in enhancing the electrical properties of polymer dielectric for insulation applications. 46Interface formation between the particle and dipoles provides traps for trapping of charge carriers, basically in polymer nanocomposites.Thus in the sintered PI nanocomposites, the trapping of a large number of charge carriers is expected, and ECR having SiO 2 as a backbone on the other hand could effectively act as a charge carrier trap.The dielectric constant for the pure PI, ECR/PI, 2 wt% TiO 2-ECR/PI, 4 wt% TiO 2 -ECR/PI, and 6 wt% TiO 2 -ECR/PI samples at 50 kHz were 3.21, 2.71, 1.76, 1.54, and 1.18, respectively.Also, it is worthy to note that the introduction of the TiO 2 nanoparticles in the PI matrix material contributed to the low dielectric loss and conductivity properties of the nanocomposites over the pure PI and ECR/PI samples (Figure 5(c)).Comparing the dielectric constant, dielectric loss, and electrical conductivity of the nanocomposites measured at 50 kHz with the ECR/PI as can be seen in Figure 5(a)-(c).ECR/PI reinforced with 6 wt% TiO 2 nanoparticles depicted the lowest dielectric constant, dielectric loss, and electrical conductivity.This relates to the XRD results obtained as the nanocomposites possess low degree of crystallinity, which evidence their amorphous/insulation behaviour.In detail, the values of the dielectric constant, loss, and electrical conductivity decreased from 2.71 (ECR/PI) to 1.18, from 9.28 to 1.14, and from 2.59 × 10 À5 to 3.16 × 10 À6 (S/cm), respectively.This demonstrates that the high affinity of the reinforcements to the PI matrix, effectively restrict the mobility of PI chains, electron movement within the composites with reduced orientation polarization and electronic distortion. 47The result also indicates that the reduced electrical conductivity of the nanocomposites is mainly caused by a degradation of the charge transport within the host matrix charge transfer sites, and similar has been reported elsewhere. 48

Conclusions
The novel ECR glass reinforced PI composites with varying concentration of TiO 2 nanoparticles with enhanced mechanical and electrical insulation properties were successfully developed using SPS sintering technique.The PI composites with ECR and TiO 2 nanoparticles were more effective than raw ECR glass in improving the mechanical properties.The hardness, elastic modulus, and stiffness remarkably increased with ECR and TiO 2 loading and reached their maximum values of 2.19 GPa, 13.99 GPa, and 0.17 mN/nm with 6 wt% TiO 2 -5 wt% ECR/PI nanocomposites, corresponding to 80.9%, 76.2%, and 30.8% improvements, respectively compared to pure PI.Incorporation of the ECR and TiO 2 nanoparticles reinforcements in the PI matrix based composites effectively restrain the mobility of PI chain, however, the electronic distortion of the composites.And this leads to the low dielectric constant, dielectric loss, and electrical conductivity of the TiO 2 -containing ECR reinforced PI nanocomposites when compared to the pure PI and ECR reinforced PI composites without TiO 2 .The analysis of the mechanical reinforcement efficiency indicated that the simultaneous addition of ECR and TiO 2 in the PI composites display a better ability to improve its hardness and elastic modulus than most of the reported functionalized glass fiber in the past years.The improved mechanical properties were predominantly attributed to the intrinsic nature of TiO 2 and enhanced interfacial interaction.Meanwhile, the nanocomposites depicted more electrical insulation behaviour based on the dielectric and electrical conductivity data obtained in the study.

Figure 1 Figure 1 .
Figure1shows the XRD patterns of the produced samples.The XRD was conducted to examine the amorphous and degree of crystallinity of the nanocomposites.In this study, the composites nature of bonding and structure of the resultant composites were characterized at different content of TiO 2 nanoparticles.X'Pert Highscore plus software was used to analyze the XRD results.The XRD pattern of pure PI, ECR/PI, and TiO 2

Table 1 .
Physical and mechanical properties of the produced polyimide composites.
TiO 2 nanoparticles hybridized with ECR glass/PI particles.Such observation is in agreement with the study reported previously,