Polyimide Composites Research Articles

Influence of cryogenic cooling on milling hole exit of QFRP based on thermal–mechanical coupling

Quartz fiber–reinforced polyimide composites (QFRP) had heterogeneous and anisotropic properties. Because of the gradient change of cutting force at hole exit, some fibers could not be effectively chipping breaking and the defect of delamination burr was often inevitable. In this paper, a cutting force model of fiber fracture stripping considering thermal–mechanical coupling was constructed based on the mechanism of cutting force and thermal action. A series of milling hole tests at different temperatures (from 290 to 80 K) was carried out using liquid nitrogen inner-cooling equipment. Similarly, the hole exit morphology, milling force, and cutting temperature were investigated and the delamination factors were calculated in detail. The results show that the model can explain the reason of hole exit defect, which is attributed to the lack of tangential cutting force and the large gradient change of axial force considering the high cutting temperature. Meanwhile, cryogenic cooling can reduce the effect of cutting temperature for hole exit defect. Although the cutting force is increased, the gradient degree of axial force can be reduced, and the delamination factor can be decreased from 1.08 at low-speed dry cutting to 1.01 of high-speed cryogenic one. At the same time, the increase of tangential force and the decrease of fiber ductility improve the chip breaking ability and the processing efficiency. Conclusion: the spiral hole milling process with cryogenic medium intervention can inhibit the milling hole delamination defects.

Greatly enhanced thermal conductivity of polyimide composites by polydopamine modification and the 2D-aligned structure

Highly thermally conductive polyimide (PI) composites have great potential application in the heat-resistance electronic packaging field, but the poor interface between the PI matrix and fillers limits their application. Herein, we prepared the polydopamine (PDA) modified hexagonal boron nitride (h-BN@PDA)/polyimide (PI) composites by a simple “vacuum filtration - hot pressing” method. In this composite, the interfacial interaction between the PI matrix and h-BN micro-sheets was greatly enhanced after PDA modification, and a 2D-aligned structure was successfully constructed. In this work, the effect of PDA modification on the thermal conductivity, thermal stability and mechanical properties of PI composites was fully investigated. The thermal conductivity of the h-BN@PDA/PI composites rises as the modified filler increases, and reflects a distinct anisotropy on in-plane and through-plane direction. The highest in-plane thermal conductivity, 3.01 W m−1 K−1, is obtained in the 20 vol% h-BN@PDA modified for 6 h (h-BN@PDA(6h))/PI composite, about 1405% higher than pure PI sample, while its T10% reaches 525.2 °C, and the tensile strength and Young's modulus are still high, about 38.80 MPa and 8.33 GPa. Our study provides a feasible fabrication technology for the high-performance thermal management materials.

Non-thermal plasma activation of BPDA-PPD polyimide for improved cell-material interaction

Biocompatible BPA-PPD polyimide is widely used in the packaging of implantable devices. Plasma activation can improve its interaction with the surrounding tissue upon implantation. The influence of He, air, N2 and Ar plasma activation on polyimide's surface hydrophilicity, roughness, topography, composition and cell-surface interaction was evaluated, along with the influence of hydrophobic recovery on such properties. All plasma activations increased the surface hydrophilicity but neither the roughness nor topography changed. The increase was attributed to the incorporated O- and N-functionalities. 24 h after the activations the surface hydrophilicity decreased while maintaining the functionalities, due to the functionalities’ reorientation/migration towards the bulk of polyimide. Air and N2 activations improved the cell-surface interactions with fibroblasts. These were equally influenced by the surface hydrophilicity and the surface functionalities availability. The hydrophobic recovery lowered the initial cell adhesion but not the cell proliferation, as the hydrophobic recovery was progressively reversed in the culture media.

Photo-oxidative degradation of organo-functionalized vermiculite clay-reinforced polyimide composites

Herein, accelerated photo-oxidation under UV light of polyimide (PI) films filled with functionalized vermiculite (VMT) clay (i.e., 1–7 wt %.) were examined from 0 to 600 h. Thick films (⁓ 60 µm) were fabricated and exposed to accelerated UV light (λ > 295 nm). The as-synthesized PI-reinforced clay nanocomposites were examined by Fourier transform infrared (FT-IR), scanning surface micrograph (SEM), yellowing index, and thermogravimetric analysis (TGA), respectively. The surface morphology examined by SEM exhibits well exfoliation of VMT clay in the PI matrix. The change in internal chemistry on a higher temperature and clay composition is found to be the result of the aggregation of clay. The yellowing index determined by UV–Vis spectroscopy followed a similar tendency as the carbonyl index. TGA analysis revealed good heat resistance in clay-based PI due to the lower thermal expansion of clay reinforcement. Overall, the data clearly showed a better photo-stability of the developed clay/PI composites over the pristine polymer. It is concluded that clay-based PI could be a better alternative in applications requiring harsh conditions, as clay has improved the oxidative, thermal, and insulation properties of the PI matrix.

Performance of high-temperature thermosetting polyimide composites modified with thermoplastic polyimide

The preparation of polyimide (PI) resin with high heat resistance and toughness is a significant challenge. In this study, thermoplastic PI (TPI) was used to toughen thermosetting PIs, and toughened PI (TPI/PI) blends were prepared. The modified PI resin system exhibited good thermal stability, excellent heat resistance, and high toughness. The results indicated that the TPI/PI blends maintained the curing behavior and characteristics of the PI oligomer. The Tg of the cured TPI/PI blend exceeded 395 °C, and the T5% values were in the range of 533–563 °C, suggesting excellent thermal stability and heat resistance. The maximum impact strength was increased by 46% compared with that of pure PI, indicating the excellent toughening effect of the TPI. Carbon fiber-reinforced PI composites were prepared using the toughening system as a matrix. The compression-after-impact values of the carbon fiber-reinforced PI composites were up to 190 MPa, indicating the excellent toughness of the materials.

Milling hole force of Quartz fiber-reinforced polyimide composite in cryogenic cooling

The machinability of composites had close relationship with cutting force. A milling hole force model of quartz fiber-reinforced polyimide composites (QFRP) was constructed considering cutting microzone temperature. A series of cryogenic milling hole testes were carried out through a liquid nitrogen inner cooling cutting equipment. The surface morphology and cutting force performance at hole entry and exit were analyzed in detail. The results show that helical milling force is mainly related to the axial feed, tangential feed, and mechanical properties of composites in different temperatures and changed slowly with spindle speed. The influence of axial feed on force is larger than that of tangential one. Meanwhile the large cutting force at hole entry and the little force at hole exit have the main reasons of the crimping and delamination defects. After cryogenic is intervened, the milling force is significantly improved, and the defects are effectively restrained. Especially, the hole entry and exit accuracies can be reached 0.03 mm, and there are no low-speed tearing and high-speed ablation defects. Furthermore, the change of force is caused by the change of properties of matrix, and composite at low temperature is the main reason to restrain defects. The cooling medium can avoid the phenomenon of increasing cutting force caused by tool expansion, as well as restraining the ablative defect.

Surface textured polyimide composites for improving conversion efficiency of ultrasonic motor

Two ways to enhance conversion efficiency of the ultrasonic motor (USM) were developed. According to mechanical and tribological properties, an advanced polyimide (PI) friction material was fabricated. In comparison to conventional polytetraflfluoroethylene (PTFE) friction material, the novel PI friction material enhanced efficiency from 29% to 44%, an improvement of 51.7%. The results indicated high friction coefficient and high elastic modulus were essential for improving efficiency. Inspired by the unique topography, the micro-dimples were introduced into PI friction material which improved the efficiency from 44% to 53%, an increment of 20.5%. The optimum parameter was diameter of 200 μm and area density of 7%. The effect of surface texture on tribological behavior and output characteristic of the USM was comprehensively explored.

Enhanced dielectric properties in polyimide nanocomposites containing barium titanate@ polydopamine core-shell nanoparticles

In this work, we prepared the polyimide (PI) nanocomposite films containing barium titanate @polydopamine (BT@PDA) nanoparticles within a core-shell structure by solution casting method. The dielectric constant of these nanocomposite films is significantly improved compared to pure PI film, and increases with the increase of BT nanoparticles. And, the breakdown strength of these nanocomposite films is better than that of untreated BT/PI films at the fixed BT content. The PI composite film containing 1 vol% of BT@PDA nanoparticles exhibits the highest energy storage density of 1.94 J/cm3 under an electric field of 315 kV/mm, which is 42% over the pristine PI (1.37 J/cm3 at 295 kV/mm) and about 162% greater than biaxially-oriented polypropylene (BOPP) (≈1.2 J/cm3 at 640 kV/mm). The presence of PDA shell effectively improves the dispersion uniformity and interfacial compatibility of BT nanoparticles in the PI matrix, giving rise to lower local electric field distortion in the composites. Besides, the BT@PDA/PI nanocomposite films have excellent dielectric thermal stability in the temperature range from 25 to 200 °C and thermal stability up to 500 °C. The BT@PDA/PI composites fabricated in this work have a great potential to be used in the high-temperature film capacitors.

Laser-Based Fabrication of Carbon Nanotube–Metal Composites on a Polymer Substrate: Experimental Study and Characterizations

Abstract Flexible electronic devices have several advantages and multiple current or potential applications. However, the reliability and durability of their metal components (which often exist) may suffer from large and repeated strains during many applications. Carbon nanotube (CNT)-metal composite films that were laser-sintered on flexible substrates were shown to be very promising in addressing the reliability and durability issues. However, to the authors’ best knowledge, CNT–metal interfaces in a laser-sintered CNT–metal composite film on a polymer substrate and the composite–polymer interface have not been sufficiently characterized and understood. In this paper, CNT–silver composite films were produced on polyimide substrates by laser sintering, and the fabricated samples were characterized through scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. Under the conditions studied, it has been found that: (1) for the CNT–silver interfaces in the composite, a significant amount of Ag–C chemical bonds is unlikely to exist, and (2) for the composite–polyimide interface, mechanical interlocking is expected to play an important role in the interfacial adhesion, while a significant diffusion of silver into the polyimide substrate is not observed. Besides, chemical reactions have likely occurred around the interface, causing the formation of Ag2O due to the reaction between silver from the composite and oxygen (in a certain form) from the polyimide substrate.

Tribological properties of polyimide composites reinforced with fibers rubbing against Al2O3

Reinforcing fillers are of great importance in tribological performance and tribofilm formation of polymeric composites. In this study, the tribological properties of aramid particle (AP) and short carbon fiber (SCF) reinforced polyimide (PI) composites were added to hexagonal boron nitride (h-BN), and silica (SiO2) nanoparticles sliding against alumina were comprehensively investigated. When sliding occurred with AP-reinforced PI composites, the tribological properties were not closely depended on the pressure × velocity (p × ν) factors and the nanoparticles. The interactions between AP and its counterpart could not induce tribo-sintering of the transferred wear debris. As such, the tribofilm seemed to be in a viscous state, leading to higher friction and wear. However, the inclusion of hard SCF into the PI matrix changed the interfacial interactions with alumina. A robust tribofilm consisting of a high fraction of silica was generated when the SCF-reinforced PI was added to the SiO2 nanoparticles. It exhibited a high load-carrying capability and was easily sheared. This caused a significant decrease in the friction and wear of the PI composite at 8 MPa·1m/s. Moreover, due to their high melting point, few h-BN nanoparticles were observed in the tribofilm of the SCF-reinforced PI when hexagonal boron nitride was added.

Nonlinear stress–strain behavior of unidirectional carbon fiber/ phenyl–ethynyl terminated KAPTON-type polyimide, TriA-X, composites at elevated temperatures

A phenyl–ethynyl terminated polyimide resin based on KAPTON-type backbone structures, ‘TriA-X’, polyimide is promising as a matrix resin of heat-resistant carbon fiber reinforced plastic composites, but its mechanical properties remain unclear. This study examined off-axis stress–strain behavior of unidirectional carbon fiber (CF)/TriA-X polyimide composites under tensile load at room temperature (RT), 100°C, 150°C, and 200°C to elucidate the matrix resin plastic deformation behavior. Comparing the rupture stress and rupture strain of off-axis specimens at RT and 100°C, they decreased concomitantly with increasing test temperature. Comparison of the rupture stress at 100°C, 150°C and 200°C revealed no significant difference in the rupture stress. A one-parameter plasticity model proposed by Chen and Sun with temperature-dependence of the plasticity model parameters elucidated the nonlinear stress–strain behavior. Model results agreed well with the nonlinear stress–strain behaviors of the CF/TriA-X composites at RT, 100°C, 150°C, and 200°C.

Investigation of the Effect of Mechanical Activation of Powder Components on the Properties of Polyimide Composites

This study presents the results of an analysis of the influence of joint mechanical activation of a polyimide matrix and tungsten (IV) oxide (WO2) on the mechanical properties of their composites. Mechanical activation of the powder components of the matrix and the filler is performed in a vortex jet mill. The ultimate tensile strength, tensile modulus and relative elongation at break of the composites are all investigated. When using mechanical activation, an increase in tensile strength of 9% is observed with a content of 30 wt.% WO2 filler and of 12% with a content of 70 wt.% WO2 filler compared to composites in which the joint mechanical activation of components is not used. Using scanning electron microscopy, it is shown that the use of mechanical activation by dispersing in a jet-vortex mill made it possible to achieve a uniform distribution of highly dispersed filler based on tungsten dioxide in a polyimide matrix.

Controlling thermal conductivities and electrical insulation properties of carbon nanofiber/polyimide composites using surface coating techniques

AbstractIn the present work, sandwich‐structured composite sheets consisting of a core layer of carbon nanofiber (CNF)/polyimide (PI) and an outer layer of either pristine PI or hexagonal boron nitride (hBN)/PI were proposed and characterized for the possibility that they could enhance the effective thermal conductivity (TC) of PI composite sheets while also retaining their electrical insulation properties. The sandwiched structures were prepared using a dip coating method. The electrical insulation properties were evaluated via the breakdown voltage (BDV) of direct current. By using hBN with an average size of 0.5 μm as filler for the outer layer, a composite sheet with a highly effective level of TC was prepared. The BDV achieved more than 2 kV, when the outer layer had a total thickness of approximately more than 60 μm even at 30 vol% of hBN_0.5 content. The tensile strengths of the sandwich‐structured sheets indicated the values between each single layer of the 10 vol%‐CNF/PI and hBN_0.5/PI composites.

사다리형 폴리실세스퀴옥세인과 POSS를 포함하는 하이브리드 폴리이미드 복합체의 합성 및 특성 분석

1,3-bis(4-aminophenoxy)benzene(APB)과 3,3",4,4"-benzophenonetetracarboxylic dianhydride(BTDA)의 조성에 여덟개의 페닐기를 함유한 케이지 구조의 실세스퀴옥세인(octaphenyl-POSS, OPS)과 페닐과 에폭시 그룹이 각각 8대2 비율로 함유된 사다리형 폴리실세스퀴옥세인(LPS)을 혼합하여 내열성 및 절연성이 우수한 하이브리드 폴리이미드 복합체를 제조하였다. 제조된 하이브리드 폴리이미드 필름은 OPS의 중량을 2.5 wt%로 고정시키고, LPS의 중량비를 다양(1.5, 2.5, 3.5 wt%)하게 변화시켜 합성하였다. OPS는 물리적인 결합을 이용하여 poly(amic acid)(PAA) 용액에 균일하게 분산시켰으며, LPS의 경우에는 화학적인 결합을 이용하기 위하여 LPS의 에폭시 그룹과 PAA의 아민 그룹과의 에폭시-아민 반응을 이용하였다. 얻어진 하이브리드 폴리이미드 복합체의 내열 특성, 광학 성질, 유전상수와 모폴로지를 측정하기 위하여 퓨리에 변환 적외선 분광기(FTIR), 열 중량 분석기(TGA), 분광광도계(microspectrometer), 임피던스기(RF impedance) 그리고 전계방사형 주사전자현미경(FE-SEM)과 주사탐침현미경(AFM)을 사용하였다. 또한 제조된 하이브리드 폴리이미드 매트릭스 내에 Si 원소가 균일하게 분산돼 있는 것은 EDS mapping으로 확인하였다. In this study, we used the POSS cage with eight phenyl groups (octaphenyl-POSS, OPS) and ladder-like polysilsesquioxane (LPS) having phenyl and epoxy groups in 8:2 ratio in the composition of 1,3-bis(4-aminophenoxy)benzene (APB) and 3,3",4,4"-benzophenonetetracarboxylic dianhydride (BTDA) to synthesize hybrid polyimide composites with high thermal stability and excellent electrical insulating property. The hybrid polyimide composites had various LPS weight ratios (1.5, 2.5, 3.5 wt%) with the fixed 2.5 wt% of OPS. OPS was homogeneously dispersed in the solution of poly(amic acid) (PAA) via physical bonding interaction and the epoxy groups of LPS were chemically reacted with the residual terminal amines of PAA. The thermal and optical properties, dielectric constant, morphology of the hybrid polyimide composites were determined using infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), microspectrometer, RF Impedance, field emission scanning electron microscope (FE-SEM), and atomic force microscope (AFM). Also, we confirmed that Si atom was uniformly dispersed in polyimide matrix by using EDS mapping.

Ultra‐high compression and wear resistant hybrid filled polyimide composite: Synergistic effect of Fe2O3 decorated RGO

AbstractHerein, the tribological performance, thermal and compression resistance behavior of polyimide (PI) reinforced by Fe2O3 decorated reduced graphene is systematically investigated. The remarkable synergistic effect of Fe2O3 decorated reduced graphene oxide (RGO) is demonstrated in its PI wear resistance, and PI/RGO/Fe2O3 composites show good thermal stability and much higher compression resistant ability than PI, PI/RGO, and PI/Fe2O3 composites when the filling contents are same. Additionally, the PI/RGO/Fe2O3 composites also exhibited ultra‐wear‐resistant properties under high load condition, and the lowest wear rate is 3.18 × 10−8 mm3N−1 m−1, which is an order of magnitude lower than that of pure PI. The investigation of its tribological mechanism also showed strong synergistic effect and interface force of Fe2O3 decorated RGO, which contribute to its high‐performance friction‐reducing behaviors. These findings give an inside view to Fe2O3 decorated RGO and its polyimide composites, and open an avenue for the graphene oxide (GO) based composite to act as compression wear‐resisting solid fillers and lubricants when polymer composite with excellent compressive, thermal and tribological properties is required.

Highly Thermally Conductive Dielectric Nanocomposites with Synergistic Alignments of Graphene and Boron Nitride Nanosheets

AbstractElectrically insulating polymer dielectrics with high energy densities and excellent thermal conductivities are showing tremendous potential for dielectric energy storage. However, the practical application of polymer dielectrics often requires mutually exclusive multifunctional properties such as high dielectric constants, high breakdown strengths, and high thermal conductivities. The rational assembly of 2D nanofillers of boron nitride nanosheets (BNNS) and reduced graphene oxide (rGO) into a well‐aligned micro‐sandwich structure in polyimide (PI) composites is reported. The alternating stacking of rGO and BNNS synergistically exploits the large difference in their electrical conductivities to yield a high dielectric constant with a moderate breakdown strength. Moreover, the distinctively separated rGO and BNNS layers give rise to higher thermal conductivities of composites than those containing mixed fillers because of reduced phonon scattering at the interfaces between two identical fillers, as verified by molecular dynamics simulations. Consequently, the micro‐sandwich nanocomposite prevails over the PI film with a simultaneously high dielectric constant of ≈579, a high energy density (43‐fold higher than PI) and an excellent thermal conductivity (11‐fold higher than PI) at a low hybrid filler content of only 2.5 vol%. The multifunctional nanocomposites developed in this work are promising for flexible dielectrics with excellent heat dissipation.

Tuning Nanofillers in In Situ Prepared Polyimide Nanocomposites for High‐Temperature Capacitive Energy Storage

AbstractModern electronics and electrical systems demand efficient operation of dielectric polymer‐based capacitors at high electric fields and elevated temperatures. Here, polyimide (PI) dielectric composites prepared from in situ polymerization in the presence of inorganic nanofillers are reported. The systematic manipulation of the dielectric constant and bandgap of the inorganic fillers, including Al2O3, HfO2, TiO2, and boron nitride nanosheets, reveals the dominant role of the bandgap of the fillers in determining and improving the high‐temperature capacitive performance of the polymer composites, which is very different from the design principle of the dielectric polymer composites operating at ambient temperature. The Al2O3‐ and HfO2‐based PI composites with concomitantly large bandgap and moderate dielectric constants exhibit substantial improvement in the breakdown strength, discharged energy density, and charge–discharge efficiency when compared to the state‐of‐the‐art dielectric polymers. The work provides a design paradigm for high‐performance dielectric polymer nanocomposites for electrical energy storage at elevated temperatures.

A comparative investigation on different silane coupling agents modified sericite mica/polyimide composites prepared by in situ polymerization

To make clear the effect and mechanism of sericite mica (SM) modified by silane coupling agents (SCA) on the morphology and properties of polyimide (PI) composites, the SM modified by different SCA including 3-aminopropyltriethoxysilane (KH550), 3-glycidoxypropyltrimethoxysilane (KH560) and 3-methacryloxypropyltrimethoxysilane (KH570), respectively, with equal grafting rate was prepared and used to prepare a series of PI/SM composites with the same SM content (5.0 wt%) by in situ polymerization. The effects of different SCA on the structure and properties of PI composites were studied by means of FT-IR, XRD, SEM, X-ray photoelectron spectroscopy, TGA, tensile and dielectric tests, and their influence mechanism was analyzed. The results showed two-phase interfacial adhesion between KH550-SM and PI matrix was the strongest. PI/KH550-SM composite exhibited the best excellent thermal and mechanical properties, the lowest dielectric constant and loss. The investigation would be of great value for improving the interfacial effects between SM and PI matrix and developing PI composites in relevant application areas.

Tribological properties of polyimide coating filled with carbon nanotube at elevated temperatures

AbstractThis work mainly focuses on the development and characterization of polyimide (PI) based composite coatings filled with multi‐walled carbon nanotubes (MWCNT) on a steel substrate. The tribological properties of PI/MWCNT composite coatings at different temperatures ranging from room temperature (RT) to 200°C were investigated. Also, the influence of MWCNT and thermal & mechanical properties of PI composites coatings were measured. The addition of MWCNT could reduce the friction coefficient as well as wear volume at elevated temperatures. The PI/3MWCNT composites coatings show better wear‐resistant properties as compared to other composites. Also, the PI/MWCNT composites showed improved mechanical properties such as micro‐hardness and nanoindentation. Formation of an organic‐inorganic structure due to the inclusion of MWCNT, enhance the mechanical properties and reduce the thermo‐degradation of composites coatings. The glass transition temperature (Tg) of the PI/MWCNT composites was improved as the MWCNT content increases from 0.1 to 5 wt%.

Thermal conductivity and mechanical properties of polyimide composites with mixed fillers of BN flakes and SiC@SiO2 whiskers

AbstractPolyimide (PI) composites with mixed fillers of BN flakes and SiC whiskers exhibit enhanced thermal conductivity and mechanical properties. In order to improve dispersion and interaction of these mixed fillers within the PI matrix, BN flakes were modified by a titanate coupling agent while SiC whiskers were oxidized at 750°C for 60 minutes to produce SiC@SiO2 followed by silane coupling agent modification. PI composites reached a maximum thermal conductivity of 0.95 W/m K at volume fraction of mixed fillers of 27.6 vol% when the weight ratio of BN flakes to SiC@SiO2 whiskers was 1:4. The enhanced thermal conductivity is likely attributed to the formation of heat conductive networks constructed by BN flakes and SiC@SiO2 whiskers and the improved interfacial affinity between fillers and matrix. The optimized Nielsen‐mold confirms the distribution and morphology of fillers affect the thermal conductivity of PI composites. In addition, SiC whiskers enhanced the mechanical property of PI composites and the influence of fillers on the mechanical property was further elaborated.