Enhancement of Electromagnetic Interference Shielding Performance and Wear Resistance of the UHMWPE/PP Blend by Constructing a Segregated Hybrid Conductive Carbon Black-Polymer Network.

The low-percolation-threshold conductive networking structure is indispensable for the high performance and functionalization of conductive polymer composites (CPCs). In this work, conductive carbon black (CCB)-reinforced ultrahigh-molecular-weight polyethylene (UHMWPE)/polypropylene (PP) blend with tunable electrical conductivity and good mechanical properties was prepared using a high-speed mechanical mixing method and a compression-molded process. An interconnecting segregated hybrid CCB-polymer network is formed in electrically conductive UHMWPE/PP/CCB (UPC) composites. The UPC composites possess a dense conductive pathway at a low percolation threshold of 0.48 phr. The composite with 3 phr CCB gives an electrical conductivity value of 1.67 × 10-3 S/cm, 12 orders of magnitude higher than that of the polymeric matrix, suggesting that CCB improves both the electrical conductivity and electromagnetic interference shielding effectiveness (EMI SE) of the composite at the loading fraction over its percolation threshold. The composite with 15 phr CCB presents an absorption-dominated electromagnetic interference shielding effectiveness (EMI SE) as high as 27.29 dB at the X-band. The composite also presents higher tribological properties, mechanical properties, and thermal stability compared to the UP blend. This effort provides a simple and effective way for the mass fabrication of CPC materials with excellent performance.

Introduction

With the rapid development of 5G communication technology, electromagnetic radiation has become a serious problem. As promising substitutes for metal-based electromagnetic interference (EMI) shielding materials, conductive polymer composites (CPCs), because of their light weight, lost cost, corrosion resistance, easy processability, and controllable electrical conductivity, have been extensively used as antistatic, EMI-shielding, and EMI-sensing materials.[1−3] To satisfy the high-quality EMI shielding application requirements, more and more attention has been given to high-electrical-conductivity fillers, such as carbon components including multiwalled carbon nanotubes (MWCNTs),[4−10] graphene nanosheets (GNPs),[11] and carbon fiber (CF).[12] Unfortunately, employing MWCNTs, GNPs, CF, or its hybrid fillers as conductive fillers often faces the intractable problem of requiring high loading, leading to high cost and poor mechanical reliability.[13] Therefore, at low-content conductive filler loading, the main challenge is how to build the high-effective-conductivity pathways. In recent years, tremendous efforts have been devoted to realizing the segregated and double-percolated structure at low filler content via control of the phase morphology of the immiscible polymer matrix and preconstruction of the composite microstructure.[14−17] For instance, Li et al.[18] found that the carbon nanotube (CNT)/ethylene vinyl acetate (EVA)/ultrahigh-molecular-weight polyethylene (UHMWPE) composites with a unique double-percolated conductive structure can form the continuous conductive pathways at only 20 wt % CNT-enriched EVA content. Xie[19] and Gu[20] reported that the continuous honeycomb-like conductive network can effectively form dense conductive channels and improve the electrical conductivity and mechanical properties. Hence, fully utilizing the advantages of the rational microstructure to balance the mismatch between performance and functionality of CPCs is of great importance.[21,22]

Ultrahigh-molecular-weight polyethylene (UHMWPE) is a polymer matrix with unique rheological properties and possesses superior mechanical performances.[23−25] Based on the unique rheological characteristic of UHMWPE, functionalization and material structure design can be achieved by taking advantage of polydispersity of the molecular weight, adjustable soft/hard phase and physical cross-linking density, stress-induced crystallization, crystallization-induced phase separation, and other characteristics. It is reported that the UHMWPE blend with a 10–30% high-fluidity polypropylene (PP) soft phase can greatly improve the processability and mechanical properties of UHMWPE composites.[26,27] Furthermore, PP materials often also serve as second-component polymers to design the segregated and double-percolate structural CPCs, which reduce effectively the percolation threshold of conductive particles via tuning the morphology of multiphase polymer matrices.[28−31] Regrettably, the introduction of the PP component and fillers together into UHMWPE composites often results in deteriorating the high loading sliding wear resistance and high mechanical properties of materials.[32]

Conductive carbon black (CCB) is a nanoscale, low-cost conductive particle. They are prone to aggregating into larger clusters and chainlike aggregated structures due to the interaction of adjacent particles under the effect of van der Waals forces and electrostatic forces. Hence, based on its assembly ability, it easily forms conductive networks in rubber or other polymers and efficiently improves the comprehensive properties of the composites.[33−35] To our knowledge, methods for incorporation of CCB nanoparticles with a high specific surface area into immiscible UHMWPE/PP blends to prepare CPCs with good properties are rare in the literature to date.[28,36−39] In this context, the binary UHMWPE/PP (95/5) blend serves as a matrix. CCB is selected as a multifunctional filler, playing the role of a conductive and wear-resisting additive in composites. We prepared UHMWPE/PP/CCB (UPC) composites with different CCB contents via adopting the high-speed mechanical mixing and hot-compression methods. We found that the CCB content has a significant effect on electrical conductivity, EMI performance, and high-loading long-term sliding tribological properties of CPC materials. Hence, the effects of CCB content on the microstructure, rheological, thermal, and mechanical properties, electrical percolation threshold, and electromagnetic interference (EMI) shielding performance of UPC composites are systematically investigated.

Results and Discussion

Morphologies of CCB Particles, UP (95/5) Blend, and UPC Composites

The morphologies of CCB nanoparticles, the UP (95/5) blend as well as UPC composites are revealed in Figure . Figure a shows that the CCB particles are spherical with a diameter ranging from 10 to 30 nm; these nanoparticles are apt to aggregate into larger clusters due to the interaction between particles under the effect of van der Waals force and electrostatic force.[40]Figure b shows that PP phases located at the adjacent UHMWPE boundary regions are pulled out and there exist significant gaps on the cryo-fractured surface of the UP (95/5) blend. These results indicate insufficient interfacial bonding between UHMWPE and PP, leading to poor mechanical properties. In Figure c,e, a small number of CCB aggregated particles preferentially forms unevenly conductive networks in the binary phase interface. Then, on increasing the CCB concentration, the CCB–polymer hybrid network supported by a high-fluidity polymer phase is uniformly dispersed in the melt defects between UHMWPE granules, as presented in Figure g. Moreover, owing to the strong affinity of CCB particles to polymer chains, a dense and perfect hybrid CCB–polymer network through interconnecting many segregated pathways of the CCB–polymer hybrid phase (marked with red ellipses) at the interface layer between UHMWPE particles is formed, as observed in the high-magnification images. Likewise, the results in Figure g,i further testify that the interconnected segregated structures of the conductive composites are gradually formed on increasing the amount of CCB containing UP (95/5) matrices from 5 to 10 phr. Going further, on increasing the CCB content to 10 phr, the UPC10 composite exhibits great CCB interface bridging polymer hybrid networks, as observed at higher magnifications in Figure k,l. This can be due to the diffusion of the high fluidity of the PP phase enhancing the strong interfacial interaction between CCB and the UHMWPE matrix, further interconnecting the adjacent UHMWPE granules. Numerous interface structure defects of UPC composites are eliminated compared to the low CCB loading, which improves the mechanical properties of the UPC composite. This transformation is attributed to the large specific surface area and dense graphite-like crystalline structure of CCB nanoparticles; the high specific surface area of CCB nanoparticles was confirmed by the Brunauer–Emmett–Teller (BET) method (Figure S1 and Table S1). The graphitization degree of CCB nanoparticles was further analyzed by Raman spectroscopy analysis (Figure S2); a high graphitization degree is beneficial to the improvement of electrical properties of UPC composites. To further provide intuitive insights into the conductive networks of UPC composites, the OM images in Figure S3 show the morphology of the segregated UPC composites. Therefore, this method adopted in this study not only guarantees the good mechanical properties of the composites but also facilitates construction of a segregated and double-percolated structure at low conductive percolation threshold values of 0.5–1 phr, and the composites are expected to have great potential for application in electromagnetic shielding.

Figure 1

Scanning electron microscopy (SEM) images of CCB nanoparticles (a), the cryo-fractured surface of the UP (95/5) blend (b), the cryo-fractured surface of the UPC0.5 composite with different magnifications (c and d), the cryo-fractured surface of the UPC1 composite with different magnifications (e and f), the cryo-fractured surface of the UPC5 composite with different magnifications (g and h), and the cryo-fractured surface of the UPC10 composites with different magnifications (i–l). The region within the red circle and arrow demonstrates the images with different magnifications of the CCB–polymer hybrid conductive network.

Electrical Conductivity

UPC composites with tunable electrical conductivity prepared using compression molding methods are shown in Figure . Figure a shows the changes in the electrical conductivity of the prepared composites with different loadings of CCB. With the content of CCB increasing from 0.1 to 1 phr, the conductive polymer composites change from being an insulator to a semiconductor and the electrical conductivity increases from 10–15 to 10–3 S/cm, which is an increase of 12 orders of magnitude. As the CCB content continues to increase to 3 phr, the UPC composite converts from being an insulator to a conductor, which indicates typical percolation behavior forming a perfect CCB conductive network. Additionally, to demonstrate the outstanding electrical conductivity of the UPC3 composites, a white-light-emitting diode could be lighted by the circuit assembled using the obtained UPC3 composite as shown in Figure b. The conductive mechanism of composites is further verified by the modified classical percolation theory. According to the percolation theory, σ = σ0 (φ – φc), where φ is the volume fraction of the fillers, φc is the volume percolation concentration, σ and σ0 represent the electrical conductivity of the composites and the conductive fillers, respectively, and t represents the critical exponent reflecting the dimensionality of the system.[41] According to log–log plots (the inset illustrates fitting of the data with the percolation equation), it is clear that φc is 0.48 phr, and t is estimated to be 2.11 ± 0.13 for the segregated UPC composites, which indicates the presence of a three-dimensional conductive network. The corresponding conductivity percolation phenomenon appears near 0.48 phr CCB so the composite has a relatively low percolation value.

Figure 2

(a) Electrical conductivity of UP (95/5) filled with various CCB loadings; the inset illustrates fitting of the data with the percolation equation. (b) Digital images of the circuit assembled using the compression molding UPC3 composite.

Electromagnetic Interference Shielding Effectiveness (EMI SE)

The EMI performances of the total SE (SE, Figure a), absorption SE (SE, Figure b), as well as reflection SE (SE, Figure c) values of the UPC composites were also compared and analyzed. On increasing the content of CCB, both SE and SE values increase, while the SE values increase slowly. The corresponding SE value of the UPC15 composite was enhanced to 27.29 dB, far above that of UPC1, but also higher than that of the target value needed for commercial applications (20 dB). It can be clearly seen that the SE values are much higher than the SE values, which indicates that SE plays a leading role in the loss of electromagnetic waves. The differences between SE and SE values with increased addition of CCB are ascribed to the degree of interfacial polarization and multiple conductive pathways of the segregated UPC composites. The primary mechanism of EMI shielding is usually a reflection of the electromagnetic radiation incident due to the interaction of EMI radiation with free electrons on the surface of the shield. Absorption is usually the secondary mechanism of EMI shielding. The electric dipoles in the shield interacted with the electromagnetic fields in the radiation. With the rise of the CCB content, there was an improvement in electrical conductivity, the EMI SE values of the UPC composites showed a significant increase, and the number of conductive pathways gradually developed, making the conductive network increasingly denser.[33] At the same time, the number of carriers increased, and these charge carriers directly interacted with the incident electromagnetic waves, which significantly increased the reflection of UPC composites to the electromagnetic waves. Resultantly, the improved electrical conductivity and EMI SE values show a charge conduction mechanism: the tunneling conduction mechanism takes effect below the percolation threshold value, and the direct contact mechanism of high-surface-area CCB conductive particles plays a dominant role in transferring charge when it is above the percolation threshold value. To further clarify the EMI shielding mechanism in the segregated UPC composites, SE, SE, and SE values for the composite are investigated. As shown in Figure d, among these obtained composites, the SE values of UPC composites increase with the enhancement of CCB content. These enhancements are mainly due to the rise of SE, and the major contribution to SE of UPC composites is SE rather than SE.[42] For instance, the SE, SE, and SE of the composite with 15 phr CCB loading are 27.29, 19.26, and 8.03 dB, respectively, which indicates that the contribution of absorption loss to the total EMI SE (70.6%) is much higher than that from reflection loss (29.4%). These results suggest that the primary EMI shielding mechanisms for UPC composites with high dielectric loss are an absorption-dominated shielding mechanism.

Figure 3

(a) EMI SE versus CCB loading as a function of frequency for the UPC composites, (b) microwave reflection, and (c) microwave absorption of UPC composites as a function of CCB loading in the frequency range of 8.2–12.4 GHz. (d) Comparison of SE, SE, and SE at the frequency of 8.2 GHz for UPC composites with various CCB loadings.

Moreover, we all know that EMI shielding mechanisms mainly include absorption (A), reflection (R), and transmission (T). To better understand the shielding mechanism, the R, A, and T coefficient values of all of the samples are shown in Figure S4. In Figure S4a, the R values of UPC10 and UPC15 composites are 0.80 and 0.84, respectively. Thus, these two composites are all reflection-dominated due to the R values exceeding 0.8.[43] Apparently, the UPC15 composite has the highest R coefficient due to the integral structures of the CCB–polymer hybrid network. Then, the T, R, and A of the UPC composites as a function of CCB content are presented in Figure S4b–d. As the CCB content increases, T always keeps at quite a low level. R exhibits a considerable increase with CCB content, which is attributed to the impedance mismatch at interfaces caused by the increasing conductivity. A gradually decreasing trend is observed for A values caused by the huge increase of R. Thus, it is concluded that UPC composites show an absorption-dominated mechanism.[8]

Tribological Properties

The tribological property of composites is characterized by the friction coefficient (FC) and wear mass loss (WML).[44]Figure shows the influence of CCB content on Shore D hardness (SDH), wear mass loss (WML), friction coefficient (FC), as well as worn surfaces of UP (95/5) blend and UPC composites under dry wear conditions. In Figure a–c, it is noticed that the SDH of UPC composites increases with the rise of the CCB content. A significant reduction in WML with a decrease in 163.8%, from 0.4971 ± 0.056 g for the UP (95/5) blend to 0.0286 ± 0.0048 g for UPC1 composites, is observed (Figure b). All of the UPC composites exhibit a lower FC than the UP (95/5) blend (Figure c). This result is mainly attributed to the high strength and high modulus of CCB playing the role of the load action and solid lubrication on the wear surfaces of the UPC composites, which helps to reduce the friction and wear efficiently.[45] As a consequence, the friction behavior of the composites is mainly determined by CCB. Under dry wear testing conditions, the FC variation tendency of the composites with different CCB contents and UP (95/5) blends is similar. For instance, the FC is relatively low at first, increases slightly with sliding time, and quickly goes to the steady state. Moreover, the friction behavior of the UP (95/5) blend and UPC composites is very unstable. This may be ascribed to the contact area increasing with sliding time and the friction heat making the surface of the UP (95/5) blend soft; therefore, the adhesion wear component becomes strong.[46] From Figure d–i, we found that the UP (95/5) blend is dominant for adherence, plowing, plastic deformation, and fatigue wear, while the worn surfaces of UPC composites with the different CCB content are relatively smooth, exhibit fine abrasive wear mechanism and a few drawing out of CCB particles from the wear surface. This result suggests that the friction mechanism of UPC composites changes from the viscous wear caused by fatigue wear to abrasive wear with the increase of CCB content. In summary, the optimal amount of CCB for an excellent wear-resistant composite is 1 phr; hardness has no contribution to the improvement in the wear performance of the UPC composites. Despite the FC under the dry sliding condition reducing slightly with the addition of the CCB content, this trend is very distinct, suggesting that the enhancement of the wear performance is influenced by other factors rather than hardness and the friction coefficient. The small number of scratches, plowing, plastic deformation, and drawing out of CCB from the worn surface of the composites are probably mainly ascribed to the wear surface softening from frictional heating during the high-load long-term sliding wear process.[47,48]

Figure 4

Effect of CCB content on Shore D hardness (a), wear mass loss (b), and friction coefficients (c) of the UP (95/5) blend (200 rpm, 30 kg, 60 min). SEM images of the worn surfaces: (d) UP (95/5) blend; (e) UPC0.5 composite; (f) UPC1 composite; (g) UPC3 composite; (h) UPC5 composite; and (i) UPC10 composite.

Mechanical Properties

Figure shows the mechanical properties of UP (95/5) and UPC composites. In Figure , it is clear that the yield strength, tensile strength, and elongation at break values of the UP blend are 21.66 MPa, 23.45 MPa, and 105%, respectively. When 3 phr CCB content is added in the UPC composite, the yield strength, ultimate tensile strength, and elongation at break values possess optimal performance, as shown in Figure a–c. This is attributed to the formation of network structures between highly polar CCB nanoparticles and polymer chains, and low-dimensional carbonaceous nanofillers as nucleation agents induce polymer crystallization, which improves the tensile strength and elongation at break of the UPC materials.[49] When the CCB content is beyond 3 phr, the mechanical properties of the UPC composite slightly decline. This is ascribed to the agglomerated CCB nanoparticles being located at the interface of the incompatible UP (95/5) blend and the weak interface compatible between the highly polar CCB particles and nonpolar polymer matrices in the UP blend filled with a high content of CCB. Therefore, incorporation of a certain amount of highly polar CCB particles into the UP blend effectively enhanced the mechanical properties of the UPC composite. The mechanical reinforcement can be further understood by the stronger interfacial adhesion of highly polar CCB particles and polymer matrices. Moreover, SEM images in Figure also testified that the CCB–polymer layer dominated the hybrid conductive networks, confirming the strong interfacial adhesion between the adjacent UHMWPE domains.

Figure 5

Yield strength (a), ultimate tensile strength (b), elongation at break (c), and stress–strain (d) curves of UP (95/5) and UPC composites.

Rheological Properties

Generally speaking, nanofiller-filled composites show a storage modulus that increases at low frequency and storage modulus as a function of the frequency curves deviates from linearity, which generally is referred to as the so-called second plateau of the storage modulus.[50]Figure S5 shows the variation of storage modulus (a), loss modulus (b), complex viscosity (c), and corresponding changes in tan δ as a function of frequency (d) for UPC composites and the UP (95/5) blend. In Figure S5a, on increasing the CCB content from 0.5 to 1 phr, the storage modulus tends to decrease. This phenomenon is attributed to the UPC composites behaving less elastic than the UP (95/5) blend. On the other hand, due to the broader molar mass distribution of commercial UHMWPE, the higher adhesion probability of the long chains to the CCB surface decreases the storage modulus of the polymer melt, in accordance with an earlier study.[51] On increasing the CCB content beyond 1 phr and the CCB concentration from 3 to 10 phr, the storage modulus gradually increases at low frequency and becomes independent of frequency. It is known from the literature that the appearance of the second plateau of the storage modulus is a correlation to the formation of the CCB assembly network within composites.[50] Moreover, it can also be speculated that the strong interaction between the polymer chain segments and the filler possibly inhibits the disentangled UHMWPE from achieving the thermodynamic equilibrium melt state, which leads to an increase in the storage modulus.[52] From Figure S5b, it can be noticed that an analogical variation also occurs in the relationship between the loss modulus and frequency. Thus, when the mass fraction of CCB is 1, a rheological network forms between the polymer–filler–polymer due to the viscous flow of the polymer and the slip of the conductive fillers.[53] Combined with the above percolation behavior and the microstructure analysis of UPC composites, the results further illustrated that the network structure formed by the polymer-conductive filler hybrid layer and the rheological network formed by the polymer-conductive filler are synchronous. Figure S5c shows that the complex viscosity of the composites decreases with increasing frequency, which indicates that UPC composites are typical shear-thinning systems. From Figure S5d, it is clear that the content of CCB in UPC composites has a special contribution to the phase angle (tan δ); the response of tan δ is presented as an arc curve in the low-frequency region. When the content of CCB is in the range of 0.5–1 phr, the gradually formed rheological percolation network in the composite changed from a metastable state to a stable state. When the content of the CCB is 5 phr, the rheological network between the polymer and filler in conductive composites is continuously infiltrated to reach a stable state. The difference in tan δ can be ascribed to the strong interaction between the polymer layer and fillers.[52]

Differential Scanning Calorimetry (DSC)

The crystallization and melting behavior of the UP (95/5) blend with various CCB contents were investigated by DSC. Figure S6 presents the melting and crystallization curves of the prepared UP (95/5) and UPC composites. The calorimetric results including the crystallization temperature (Tc), melting temperature (Tm), melting enthalpy (ΔHm), and crystallinity (Xc) are presented in Table S2. Figure S6a shows an obvious change in the crystallization temperature (Tc) with increasing CCB content. In Table S2, the calculated results of crystallinity show that the crystallinity of composites is higher than that of the UP (95/5) blend (53.5%). According to the literature reports, the increase in crystallinity can be attributed to nanofiller works in two mechanisms in the crystallization process of the filled polymer composites:[26] (1) the nanofillers serve as nucleation agents and play an active role in the heterogeneous nucleation of polymer blends, which promotes crystallization; and (2) the nanofillers affect the motion of polymer chains and thus change the ordered rearrangement of the composites. Thereby, from the increase in crystallinity, we speculate that CCB particles can possibly be used as nucleating agents in the crystallization process of the UP (95/5) blend; thus, the heterogeneous nucleation effect is achieved.[54] In Figure S6b, two obvious melting peaks appeared in the melting curves. The first melting peak at 137 °C represents the actual melting temperature of UHMWPE, and the second one at 165 °C can be ascribed to the melting temperature of PP; the melting peak of PP disappears gradually on increasing the CCB content. This result suggests that a small amount of PP is possibly located in the amorphous region of UHMWPE, reducing the viscosity of UHMWPE and promoting the untangling of the entanglement.[27]

Thermal Stability

Figure S7 shows the thermal gravimetric curves of the UP (95/5) blend and UPC composites under a nitrogen atmosphere. The detailed data are also listed in Table S3. The UP blend degrades significantly at a temperature of 482.60 °C, whereas the weight loss and degradation rate of its composites decrease with the content of CCB, indicating that the presence of CCB nanoparticles in the polymer matrix delays the thermal degradation of the UP (95/5) blend. The initial degradation temperature (Tonset) and the maximum degradation temperature (Tmax) both increase with increasing CCB content. Compared with the UP (95/5) blend, the Tonset and Tmax of the composite with 10 phr CCB increase from 448.23 and 482.60 °C to 458.91 and 488.24 °C, respectively. The residue at 600 °C for the composites also increases with increasing CCB content. These findings indicate that the incorporation of CCB improves the thermal stability and strong interface interaction of the UPC composites. At the same, the large amount of CCB serves as a thermal barrier in the polymer matrix and restricts heat transfer between polymer chains, which delays the thermal decomposition progress of the UP (95/5) blend.[55] This result is consistent with other literature reports on the effect of carbon-filler-modified polymer composites. Besides, this strong interfacial interaction between the filler and polymer is further demonstrated by the result of the rheological and thermal properties of the UPC composite.

Conclusions

In conclusion, the UPC composites with a hybrid CCB–polymer network were successfully prepared by the high-speed mechanical mixing method and the compression-molded process. The morphological observation revealed that the formation of a dense hybrid conductive filler network is beneficial to the electrical conductivity and electromagnetic interference shielding effectiveness (EMI SE) of UPC composites, leading to a low percolation threshold. The critical exponent t value obtained from the electrically conductive percolation threshold mechanism further suggested that the UPC composite possesses a three-dimensional conductive network. The CCB loading fraction is higher than that of the percolation threshold (0.48 phr), exhibiting intense improvements in conductivity and electromagnetic interference shielding effectiveness (EMI SE). The UPC composite with 10 phr CCB is adequate for EMI shielding commercial applications (20 dB), and the UPC composite with 15 phr CCB presents a more satisfying EMI SE of 27.29 dB at the X-band. The EMI shielding mechanism for UPC composites is an absorption-dominated shielding mechanism. More importantly, when the addition of CCB into the UP (95/5) blend is 1 phr, the abrasion resistance of the UPC1 composite under a high load of 30 kg for 60 min is improved by nearly 164% in its highest value. Furthermore, the thermal stability and rheological and mechanical properties of the composites are also improved on increasing the CCB content. These results are attributed to the strong interaction existing between the polymer and CCB nanoparticles and forming the polymer–filler hybrid segregated networks. Overall, this low-cost CCB as a conductive filler of thermal-plastic materials has great potential for the development of antielectrostatic and electromagnetic interference shielding materials, actual industrial production, and applications.

Experimental Section

Materials and Methods

CCB (F900A) nanoparticles were purchased from Tianjin Ebory Chemical Co., Ltd. UHMWPE with the trade name SLL-2 with a molecular weight range from 2.00 to 3.00 × 106 g/mol was supplied from Shanghai Lian Le Chemical Industry Science and Technology Co., Ltd. Polypropylene (PP), PPH-T03, with the melt flow rate of 3 g/10 min (2.16 kg, 230 °C), was provided by the Maoming branch of China petroleum and chemical corporation.

The morphologies of the CCB particles, UP (95/5) blend, and the cryo-fracture surfaces of the UPC composite were observed using a cold-field emission scanning electron microscope (FE-SEM) (Regulus 8100, Hitachi, Japan). After samples were fractured in liquid nitrogen, the cryo-fracture surfaces of samples were cut and sprayed with gold for 160 s before observation. The voltage was 10 kV, and the working distance was 8–10 mm.

The electrical conductivity of specimens higher than 10–6 S/m was measured using a four-point probe resistivity measurement (RTS-9, Guangzhou Four Probe Technology Co., Ltd., China). Below 10–6 S/m, the electrical conductivity was measured using a high-resistance meter (ZC-90G, Shanghai Taiou Electronics Co., Ltd., China) according to GB/T1410–2006. The electromagnetic interference shielding performance of the UPC composites samples depended on scattering parameters, which corresponded to the reflection and transmission of transverse electromagnetic waves, and was tested by a Vector network analysis (N5244A, Agilent) with the waveguide method at the X-band frequency range (8.2–12.4 GHz) according to ASTM D5568-08. The obtained scattering parameters were used to calculate EMI SE values.[56−59]The sliding wear tests were examined on an M-200 wear tester (Beijing, China) at room temperature according to GB3960-83. The sliding pair consisted of a tested specimen block and a 45# steel ring (Ra = 0.8 μm). The diameter of the steel ring was 40 mm. Under the force of 294 N, the tests were carried out with the axis rotation speed of 200 rpm. The tests were done more than three times for each sample with a sliding time of 60 min.

The Shore D hardness of samples was measured according to the GB/T 2411-2008 standard. Samples for tensile tests were cut into dumbbell shapes according to the GB/T 1040.2-2006 standard. These samples were tested by a mechanical testing machine (W2Y-240, SANS) at 25 °C with a cross-head speed of 50 mm/min. The measurements were repeated five times to get the average values.

Rheological experiments were carried out using a stress-controlled rheometer (DHR-2, TA Instruments) equipped with parallel-plate geometry (diameter of 25 mm). The frequency sweep of the UP (95/5) blend and UPC composites was measured at 200 °C from 100 to 0.01 rad/s. The strain was set to 0.5% in the linear viscoelastic region.

Crystalline and melting properties of the composites were performed on a DSC (Q20, TA Instruments). The degree of crystallinity (Xc) was calculated using eq (26,60)

where ΔHm is the fusion enthalpy calculated from the area of the endothermic melting peak, ΔHm0(289.3 J/g) is the theoretical heat of fusion for a 100% crystalline UHMWPE, and ω represents the UHMWPE weight fraction in the blends.

The thermal stability analysis of the samples was detected by thermal gravimetric analysis (TGA Q50, TA Instruments) from room temperature to 600 °C at a heating rate of 10 °C/min and nitrogen gas flow of 90 mL/min.

Preparation of UPC Segregated Composites

The fabrication process of segregated UHMWPE/PP/CCB (UPC) composites is shown schematically in Figure . CCB particles, UHMWPE powder, and PP pellets were dried in a vacuum oven at 80 °C for 6 h before processing. Then, the UHMWPE/PP (95/5) blend and UHMWPE/PP/CCB (95/5/x) composites were mechanically mixed in a high-speed mixer (800Y, Yongkang Platinum Ou Hardware Products Co. Ltd., China) with a rotation speed of 34 000 rpm for 50 s at room temperature, in which x (from 0.5 to 15 phr) represents the CCB content expressed in parts per hundred of the resin (phr). Afterward, UHMWPE/PP (95/5) and UHMWPE/PP/CCB (95/5/x) compounds were compression-molded at 200 °C for 25 min on a flat vulcanizing machine (ZG-80 T, Dongguan Zheng gong Electromechanical Equipment Technology Co. Ltd., China), followed by cold-molding at room temperature and molding pressure of 17 MPa. The obtained specimens were denoted UPC0.5, UPC1, UPC3, UPC5, UPC7, UPC10, and UPC15, respectively. Moreover, the contrast of the UHMWPE/PP (95/5) sample was marked as UP (95/5).

Figure 6

Preparation schematic of UPC segregated composites.