Selective Laser Sintering 3D Printing: A Way to Construct 3D Electrically Conductive Segregated Network in Polymer Matrix

Selective laser sintering (SLS), which can directly turn 3D models into real objects, is employed to prepare the flexible thermoplastic polyurethane (TPU) conductor using self‐made carbon nanotubes (CNTs) wrapped TPU powders. The SLS printing, as a shear‐free and free‐flowing processing without compacting, provides a unique approach to construct conductive segregated networks of CNTs in the polymer matrix. The electrical conductivity for the SLS processed TPU/CNTs composite has a lower percolation threshold of 0.2 wt% and reaches ≈10⁻¹ S m⁻¹ at 1 wt% CNTs content, which is seven orders of magnitude higher than that of conventional injection‐molded TPU/CNTs composites at the same CNTs content. The 3D printed TPU/CNTs specimen can maintain good flexibility and durability, even after repeated bending for 1000 cycles, the electrical resistance can keep at a nearly constant value. The flexible conductive TPU/CNTs composite with complicated structures and shapes like porous piezoresistors can be easily obtained by this approach.     

Multilayered ultrahigh molecular weight polyethylene/natural graphite/boron nitride composites with enhanced thermal conductivity and electrical insulation by hot compression

A scalable strategy to fabricate thermally conductive but electrically insulating polymer composites was urgently required in various applications including heat exchangers and electronic packages. In this work, multilayered ultrahigh molecular weight polyethylene (UHMWPE)/natural graphite (NG)/boron nitride (BN) composites were prepared by hot compressing the UHMWPE/NG layers and UHMWPE/BN layers alternately. Taking advantage of the internal properties of NG and BN fillers, the UHMWPE/NG layers played a decisive role in enhancing thermal conductivity (TC), while the UHMWPE/BN layers effectively blocked the electrically conductive pathways without affecting the thermal conductive pathways. The in-plane TC, electrical insulation, and heat spreading ability of multilayered UHMWPE/NG/BN composites increased with the increasing layer numbers. At the total fillers loading of 40 wt%, the in-plane TC of multilayered UHMWPE/NG/BN composites with nine layers was markedly improved to 6.319 Wm⁻¹ K⁻¹, outperforming UHMWPE/BN (4.735 Wm⁻¹ K⁻¹) and pure UHMWPE (0.305 Wm⁻¹ K⁻¹) by 33.45% and 1971.80%, respectively. Meanwhile, the UHMWPE/NG/BN composites still maintained an excellent electrically insulating property (volume resistance~5.40×10¹⁴ Ω cm ; breakdown voltage~1.52 kV/mm). Moreover, the multilayered UHMWPE/NG/BN composites also exhibited surpassing heat dissipation capability and mechanical properties. Our results provided an effective method to fabricate highly thermal conductive and electrical insulating composites.     

Electrical,rheological and electromagnetic interference shielding properties of thermoplastic polyurethane/carbon nanotube composites

Electrically conducting rubbery composites based on thermoplastic polyurethane (TPU) and carbon nanotubes (CNTs) were prepared through melt blending using a torque rheometer equipped with a mixing chamber. The electrical conductivity, morphology, rheological properties and electromagnetic interference shielding effectiveness (EMI SE) of the TPU/CNT composites were evaluated and also compared with those of carbon black (CB)‐filled TPU composites prepared under the same processing conditions. For both polymer systems, the insulator–conductor transition was very sharp and the electrical percolation threshold at room temperature was at CNT and CB contents of about 1.0 and 1.7 wt%, respectively. The EMI SE over the X‐band frequency range (8–12 GHz) for TPU/CNT and TPU/CB composites was investigated as a function of filler content. EMI SE and electrical conductivity increased with increasing amount of conductive filler, due to the formation of conductive pathways in the TPU matrix. TPU/CNT composites displayed higher electrical conductivity and EMI SE than TPU/CB composites with similar conductive filler content. EMI SE values found for TPU/CNT and TPU/CB composites containing 10 and 15 wt% conductive fillers, respectively, were in the range ?22 to ?20 dB, indicating that these composites are promising candidates for shielding applications. © 2013 Society of Chemical Industry     

Synergistic effects of conductive carbon nanofillers based on the ultrahigh-molecular-weight polyethylene with uniform and segregated structures

To discuss the synergistic effects of mixed conductive filler on nanocomposites, different structural carbon nanofiller/ultrahigh-molecular-weight polyethylene (UHMWPE) hybrid nanocomposites with uniform and segregated structure were prepared by using ethanol-assisted dispersion, hydrazine reduction, and hot-pressing methods. Scanning electron microscopy and polarized optical microscopy images of the nanocomposites fracture showed that the complete conductive channels could be formed in segregated nanocomposites prepared by powder mixing method. By contrast, the discontinuous electric path could be observed in the homogeneous nanocomposites prepared by the solution method. The test of conductivity performance demonstrated that the percolation threshold of carbon black (CB)/UHMWPE and multiwalled carbon nanotubes (MWCNTs)-CB/UHMWPE nanocomposites with segregated structure were 0.42 and 0.18 vol %, which were lower than those of the nanocomposites with uniform structure (4.91 and 2.62%). The electrical conductivity of MWCNTs-CB/UHMWPE nanocomposites with segregated structure reached to 3.0 × 10⁻² S m⁻¹ with the filler content of 1.5 vol %. In addition, the results of differential scanning calorimetry indicated that the crystallinity of UHMWPE decreased slightly with the addition of mixed filler. All of the study showed that the conductivity of MWCNTs-CB/UHMWPE nanocomposites with segregated structure has better electrical conductivity than the uniform. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47317.     

Preparation and electrical conductivity of graphene/ultrahigh molecular weight polyethylene composites with a segregated structure

Graphene-coated ultrahigh molecular weight polyethylene (UHMWPE) powders were prepared by a two-step process. The first step is to coat UHMWPE polymers with graphene oxide (GO) sheets. The second step is to reduce GO on the powders to graphene. The two-step process can effectively prevent the aggregation of graphene during reduction. The resultant graphene/UHMWPE mixtures were hot pressed at 200 °C to obtain the composites with a segregated structure. The composites exhibit high electrical conductivity at a very low percolation threshold (0.028 vol.%). Our method provides a new route for preparing electrical conductive graphene/polymer composites with low percolation threshold.     

Composites of Ultrahigh‐Molecular‐Weight Polyethylene with Graphene Sheets and/or MWCNTs with Segregated Network Structure: Preparation and Properties

UHMWPE/MWCNT and UHMWPE/GNS composites with a segregated network are prepared. TEM and SEM images indicate that the conducting fillers are distributed on the UHMWPE surface and form a segregated conducting network. The percolation threshold of UHMWPE/GNS composites is ≈0.25 wt% and that of UHMWPE/MWCNT composites is 0.20 wt%. The electrical conductivity of UHMWPE/GNS composites is almost four orders of magnitude lower than that of the UHMWPE/MWCNT composites. For equivalent concentrations of GNS and MWCNT, the composites with hybrid fillers exhibit a lower percolation threshold and a higher conductivity than that with GNS or MWCNT alone. Due to the high strength of the fillers and the segregated network structure, the mechanical properties of the composites first increase and then decrease with increasing filler content.

     

Preparation and properties of carbon nanotube/binary‐polymer composites with a double‐segregated structure

A carbon nanotube (CNT)/poly(methyl methacrylate) (PMMA)/ultrahigh molecular weight polyethylene (UHMWPE) composite containing a double‐segregated structure was formalized by means of a facile mechanical mixing technology. In the composite, the CNTs were decorated on the surfaces of PMMA granules, and the CNTs decorated granules formed the continuous segregated conducting layers at the interfaces between UHMWPE particles. Morphology observations confirmed the formation of a specific double‐segregated CNT conductive network, resulting in an ultralow percolation threshold of ～0.2 wt %. The double‐segregated composite containing only 0.8 wt % CNT loading exhibited a high electrical conductivity of ～0.2 S m^?1 and efficient electromagnetic shielding effectiveness of ～19.6 dB, respectively. The thermal conductivity, temperature‐resistivity behaviors, and mechanical properties of the double‐segregated composites were also studied. This work provided a novel conductive network structure to attain a high‐performance conducting polymer composite at low filler loadings. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39789.     

Pentadecane functionalized graphite oxide sheets as a tool for the preparation of electrical conductive polyethylene/graphite oxide composites

Covalent functionalization of pentadecane-decorated thermally reduced graphite oxide (GO) sheets has been studied as a tool for the preparation of polyethylene/GO composites exhibiting rheological and electrical percolation thresholds. It was accomplished through pentadecane based radical addition onto unsaturated bonds located on the GO sheets' surface using dicumyl peroxide as hydrogen abstractor. This chemical functionalization influences the affinity of the formed pentadecane grafted GO sheets for various solvents. Then, the compounding of the composites pentadecane grafted GO/PE was performed at a processing temperature of 140 °C with 25, 20, 15, 10, 8 and 5 wt% loadings. Rheological and electrical percolation thresholds were found between 10 and 15 wt% for polyethylene/pentadecane functionalized graphene oxide composites while the composite graphite/PE at the same loading percentage did not reach any percolation threshold.     

Electrical conductivity and rheological behavior of multiphase polymer composites containing conducting carbon black

Multiphase polymer composites of carbon black (CB), polypropylene (PP) and low density polyethylene (LDPE) were prepared by melt‐mixing method to reduce the amount of CB in the conductive composites. SEM images showed that CB preferably located in LDPE phase and formed electrically conductive path. The measurement of conductive properties showed that the ternary materials possessed lower percolation than binary composites of CB/PP or CB/LDPE, the former was ～6 wt% and the latter was 9–10 wt%. Positive temperature coefficient (PTC) effects of the binary and ternary composites were investigated, indicating that the latter exhibited a relatively high PTC intensity. A rheological percolation estimated by a power law function is 2.66 wt% of CB loading, suggesting an onset of solid‐like behavior at low frequencies. This difference between the electrical and rheological percolation thresholds may be understood in terms of the smaller CB–CB distance required for electrical conductivity as compared with that required to impede polymer mobility. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Fabrication of electrical and thermal conductive thermoplastic polyurethanes-based nanocomposite with azide polyurethane as interfacial compatibilizer

Electrical and thermal conductive polymers have aroused extensive interest in research recently due to their hi-tech applications in the fields of novel electronics. A novel electrical and thermal conductive nanocomposite (MWCNTs@PU/TPU) made with multiwall carbon nanotubes (MWNTs) and thermoplastic polyurethanes (TPU) by using azide polyurethane (PU) as interfacial compatibilizer. The MWNTs could form well-developed electrical and thermal conductive networks in the TPU matrix. The developed nanocomposite inherited advantageous properties from its constituents, namely the high conductivity and diathermancy from MWNTs, and the high mechanical properties from the TPU. Conductivity tests showed that, compared with neat MWCNTs/TPU, the electrical conductivity of MWCNTs@PU/TPU was significantly enhanced (up to 3.4 × 10⁻⁶ S/cm), with incorporating only 3.0 wt% MWCNTs@PU. And most importantly, the thermal conductivity was greatly improved by about 46.4% when the MWCNTs@PU loading was 6.0 wt%.     

Segregated highly conductive linear low-density polyethylene/graphene nanoplatelet composite through aqueous dispersing and self-leveling method

Construction of segregated structure is an effective way of preparing highly conductive composite. Here, we report an environmentally friendly method to prepare highly conductive linear low-density polyethylene/graphene nanoplatelets composite with segregated structure (s-LLDPE/GN) and low GN content. Firstly, GN coated LLDPE granules are prepared through aqueous dispersing and gradually drying. Then, the s-LLDPE/GN composites are obtained by melting and self-leveling without extra pressure. This method not only favors the forming of GN segregated network, but also allows certain fusing of the polymer at the interfaces that contribute to good mechanical strength of the composite. The electrical conductivity of the composite increases to 4.5 S/m when the GN content is 1.0 wt%. The composite with 0.5 wt% GN shows 10⁻¹ S/m electrical conductivity, and retains 82% of the tensile strength of pure LLDPE. The facile and green process in this method can be applied in many other polymer composites and show high potential for industrial application.     

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.     

Temperature dependence of electrical resistivity for carbon black filled ultra-high molecular weight polyethylene composites prepared by hot compaction

In this article, the temperature dependence of electrical resistivity is studied for carbon black (CB)/ultra-high molecular weight polyethylene (UHMWPE) composites. A new positive temperature coefficient (PTC) material with a very low percolation threshold is produced by the hot compaction method. The very low percolation threshold can be attributed to the segregation of CB in the interfacial regions of UHMWPE particles. The percolation threshold decreases with the increase of the molecular weight of UHMWPE, and with the decrease of the particle size of CB. For CB filled lower molecular weight UHMWPE (145M) composites, the PTC temperature, at which a sharp increase in the resistivity of the composite occurs, decreases with the increase of CB size. However, for a higher molecular weight UHMWPE (630M) filled with CB, the second PTC effect is observed and the negative temperature coefficient (NTC) effect is eliminated. A mechanism is proposed to explain these phenomena based on the optical microscopy and TEM observations. It can be concluded that the degree of the intermixing between CB and UHMWPE particles plays an important role in determining the electrical properties of the composites.     

Enhanced positive temperature coefficient in amorphous PS/CSPE-MWCNT composites with low percolation threshold

In this work, amorphous polystyrene/chlorosulfonated polyethylene composites doped with multiwalled carbon nanotubes (PS/CSPE-MWCNT) were constructed by in situ polymerization to form semi-interpenetrating networks. The MWCNTs showed excellent dispersion and selective location in the PS regions. High electrical conductivity and low percolation threshold (0.89 wt %) for the composites were achieved. An enhanced positive temperature coefficient (PTC) behavior for amorphous PS/CSPE-MWCNT composites was first reported, nearly without a negative temperature coefficient (NTC) effect when the conductive fillers were beyond the percolation threshold, similar to those of crystalline polymer composites. Moreover, a PTC intensity of more than five orders of magnitude and excellent repeatability of the PTC effect were achieved. This study offers new insight into the development of novel PTC materials with low percolation threshold. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47053.     

A bio-based compatibilizer for improved interfacial compatibility in thermally conductive and electrically insulating graphite/high density poly(ethylene) composites

Graphite is a thermally conductive filler. However, when dispersed into high density poly(ethylene) (HDPE) resin, graphite particles tend to agglomerate and requires a compatibilizer to achieve desired thermal/physical properties. In this study, oleic acid (OA), a bio-based additive and polyethylene-polyamines (PEPA) were used to synthesize a new compatibilizer, PEPA-g-OA, containing numerous  NR₂ groups. The experimental results showed that PEPA-g-OA can significantly improve the compatibility between graphite particles and the HDPE matrix due to uniform dispersion of graphite in the HDPE matrix. When the graphite content was 25 wt%, the thermal conductivity of the composite recorded 1.2 W m⁻¹ K⁻¹ (three times that of neat HDPE) and the volume resistivity was 1.8 × 10⁹ Ω cm, indicating excellent electrical insulation. Compared to the composites with no graphite content, the properties of the composites with 25 wt% graphite content exhibited narrower melting and crystallization peaks, more stable mechanical properties, and higher ultraviolet aging resistance. Synthesized new bio-based compatibilizer and thermally conductive and electrically insulating composites developed in this study can be useful in different industrial fields for the preparation of the next generation composites.     

Comparison of electrical properties between multi-walled carbon nanotube and graphene nanosheet/high density polyethylene composites with a segregated network structure

Multi-walled carbon nanotube (MWCNT)/high density polyethylene (HDPE) and graphene nanosheets (GNS)/HDPE composites with a segregated network structure were prepared by alcohol-assisted dispersion and hot-pressing. Instead of uniform dispersion in polymer matrix, MWCNTs and GNSs distributed along specific paths and formed a segregated conductive network, which results in a low electrical percolation threshold of the composites. The electrical properties of the GNS/HDPE and MWCNT/HDPE composites were comparatively studied, it was found that the percolation threshold of the GNS/HDPE composites (1 vol.%) was much higher than that of the MWCNT/HDPE composites (0.15 vol.%), and the MWCNT/HDPE composite shows higher electrical conductivity than GNS/HDPE composite at the same filler content. According to the values of critical exponent, t, the two composites may have different electrical conduction mechanisms: MWCNT/HDPE composite represents a three-dimensional conductive system, while the GNS/HDPE composite represents a two-dimensional conductive system. The improving effect of GNSs as conducting fillers on the electrical conductivity of their composites is far lower than theoretically expected.     

Poly(vinylidene fluoride)/thermoplastic polyurethane flexible and 3D printable conductive composites

This work focus on the development of polymeric blends to produce multifunctional materials for 3D printing with enhanced electrical and mechanical properties. In this context, flexible and highly conductive materials comprising poly(vinylidene fluoride)/thermoplastic polyurethane (PVDF/TPU) filled with carbon black-polypyrrole (CB-PPy) were prepared by compression molding, filament extrusion and fused filament fabrication. In order to achieve an optimal compromise between electrical conductivity, mechanical properties and printability, blends composition was optimized and different CB-PPy content were added. Overall, the electrical conductivities of PVDF/TPU 50/50 vol% co-continuous blend were higher than those found for PVDF/TPU 50/50 wt% (i.e., 38/62 vol%) composites at same filler content. PVDF/TPU/CB-PPy 3D printed samples with 6.77 vol% filler fraction presented electrical conductivity of 4.14 S m⁻¹ and elastic modulus, elongation at break and maximum tensile stress of 0.43 GPa, 10.3% and 10.0 MPa, respectively. These results highlight that PVDF/TPU/CB-PPy composites are promising materials for technological applications.     

Bridged double percolation in conductive polymer composites: an electrical conductivity, morphology and mechanical property study

Conductive polymer composites are ubiquitous in technological applications and constitute an ongoing topic of tremendous commercial interest. Strategies developed to improve the level of electrical conductivity achieved at a given filler concentration have relied on double-percolated networks induced by immiscible polymer blends, as well as mixtures of fillers in a single polymer matrix, to enhance interparticle connectivity. In this work, we combine these two strategies by examining quaternary composites consisting of high-density polyethylene (HDPE), ultrahigh molecular weight polyethylene (UHMWPE), graphite (G) and carbon fiber (CF). On the basis of our previous findings, we examine the electrical conductivity, morphology, thermal signature and mechanical properties of HDPE/UHMWPE/G systems that show evidence of double percolation. Upon addition of CF, tremendous increases in conductivity are realized. The mechanism by which this increase occurs is termed bridged double percolation to reflect the role of CF in spanning non-conductive regions and enhancing the continuity of conductive pathways. At CF concentrations above the percolation threshold concentration, addition of G promotes increases in conductivity and dynamic storage modulus in which the conductivity increases exponentially with increasing modulus.     

Conducting aluminum‐filled nylon 6 composites

This work is concerned with the preparation and characterization of composite materials prepared by compression molding of a mixture of aluminum flakes and nylon 6 powder. The electrical conductivity, density, hardness and morphology of composites were investigated. The electrical conductivity of the composites is < 10⁻¹¹ S/cm unless the metal content reached the percolation threshold, beyond which the conductivity increased markedly by as much as 10¹¹. The volume fraction of conductive filler at the percolation threshold was calculated from experimental data, by fits to functions predicted by the percolation theory. Decreasing the average particle diameter of filler leads to increased percolation threshold (it varies from 23 to 34 vol% for the three different fillers studied) and decreased maximal conductivity of composites. The density of the composites was measured and compared with values calculated assuming different void levels within the samples. Furthermore, it is shown that for certain sizes of particle filler, the hardness decreases initially with the increase of metal concentration, possibly because of poor surface contact with the nylon matrix, but, starting from a certain value, there is a hardness increase. For the smallest particle filler, the hardness of samples is not influenced by the presence of the filler.     

Elastomeric composites based on ethylene–propylene–diene monomer rubber and conducting polymer-modified carbon black

Thermally stable elastomeric composites based on ethylene–propylene–diene monomer (EPDM) and conducting polymer-modified carbon black (CPMCB) additives were produced by casting and crosslinked by compression molding. CPMCB represent a novel thermally stable conductive compound made via “in situ” deposition of intrinsically conducting polymers (ICP) such as polyaniline or polypyrrole on carbon black particles. Thermogravimetric analysis showed that the composites are thermally stable with no appreciable degradation at ca. 300°C. Incorporating CPMCB has been found to be advantageous to the processing of composites, as the presence of ICP lead to a better distribution of the filler within the rubber matrix, as confirmed by morphological analysis. These materials have a percolation threshold range of 5–10 phr depending on the formulation and electrical dc conductivity values in the range of 1 × 10⁻³ to 1 × 10⁻² S cm⁻¹ above the percolation threshold. A less pronounced reinforcing effect was observed in composites produced with ICP-modified additives in relation to those produced only with carbon black. The results obtained in this study show the feasibility of this method for producing stable, electrically conducting composites with elastomeric characteristics. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers