This paper presents a new nano-fibrillation method to produce highly expanded microcellular polypropylene (PP) foams with high thermal insulation properties. Polybutylene terephthalate (PBT) ...nano-fibrils were formed in the PP matrix through a melt spinning process to increase the foamability of PP resins which have inherently low melt strength. In this process, the two immiscible polymer phases are extruded and drawn into fibers. In the fiber composite, the PBT nano-fibrillar reinforcement phase typically has an average diameter of less than 200 nm. The results show that the presence of the long aspect-ratio nano-fibrils enhances the foamability of the PP matrix, and the foamed samples achieved high thermal insulation properties (~32 mW∙m−1∙K−1). As the capital investment and development time costs of the existing technologies for improving foamability are high, the new manufacturing method can provide significant economic and technical advantages for producing highly expanded and insulating PP-based foams.
Display omitted
•Nano-fibrillar composites are manufactured by melt spinning.•The microcellular foams are obtained from nano-fibrillar composites through batch foaming.•More than one order of magnitude increase of cell density is observed in nano-fibril reinforced samples.•The expansion ratio of the foams has a great impact on the thermal insulating properties.
The creation and application of PET nanofibrils for PP composite reinforcement were studied. PET nanofibrils were fibrillated within a PP matrix using a spunbond process and then injection molded to ...test for the end-use properties. The nanofibril reinforcement helped to provide higher tensile and flexural performance in solid (unfoamed) injection molded parts. With foam injection molding, the nanofibrils also helped to improve and refine the microcellular morphology, which led to improved performance. Easily and effectively increasing the strength of a polymeric composite is a goal for many research endeavors. By creating nanoscale fibrils within the matrix itself, effective bonding and dispersion have already been achieved, overcoming the common pitfalls of fiber reinforcement. As blends of PP and PET are drawn in a spunbond system, the PET domains are stretched into nanoscale fibrils. By adapting the spunbonded blends for use in injection molding, both solid and foamed nanocomposites are created. The injection molded nanocomposites achieved increased in both tensile and flexural strength. The solid and foamed tensile strength increased by 50 and 100%, respectively. In addition, both the solid and foamed flexural strength increased by 100%. These increases in strength are attributed to effective PET nanofibril reinforcement.
The study aimed to conduct a systematic review and meta-analysis comparing the efficacy of teprenone with control or other drugs for reducing the incidence of gastrointestinal (GI) adverse events in ...patients receiving long-term non-steroidal anti-inflammatory drugs (NSAIDs).
Databases of PubMed, Embase, BioMed Central, CENTRAL, and Google Scholar were searched up to November 10th, 2020 for randomized controlled trials (RCTs) comparing teprenone with control or other drugs. A random-effects model was used for the meta-analysis. Grading of Recommendations Assessment, Development, and Evaluation (GRADE) tool was used for assessing the certainty of evidence.
Seven RCTs were included. Six compared teprenone with control and one with famotidine. Meta-analysis indicated a statistically significant reduced risk of GI ulcers in patients receiving teprenone as compared to control after 12 weeks/3months (RR 0.37 95% CI 0.17, 0.18
= 0%
= 0.01). Pooled data of three open-label studies indicated statistically significant reduction of GI symptoms in patients on teprenone as compared to control at 6 months and 12 months, but not at 3 months. Comparing teprenone with control, our analysis indicated non-significant but a tendency of better reduction in Modified Lanza Score (MLS) with teprenone. The RCT comparing teprenone to famotidine demonstrated better reduction of MLS with famotidine. The certainty of evidence-based on GRADE was deemed to be low.
Low-quality evidence indicates a beneficial role of teprenone in preventing GI injuries in patients receiving long-term NSAIDs. Further high-quality RCTs comparing teprenone with placebo as well as other gastroprotective drugs are needed to strengthen current evidence.
•A theoretical model was developed to predict the electrical conductivity of CPC foams.•Effect of bubble-to-bubble interactions on the electrical conductivity was simulated.•Model prediction agreed ...well with experiments for both solid and foam composites.•At a void fraction of ~20%, the increase of electrical conductivity reached a maximum.•Bubble size and MWCNT length showed a synergistic effect on electrical conductivity.
Conductive filler/polymer composite foams (CPC) with micro-/nano-scale bubbles have attracted much attention due to their various advantages. Understanding the evolution of electrical behavior in CPC foams is important for their functional application. There have been many experimental investigations of the electrical behavior of CPC foams, but how the electrical conductivity varies with bubble growth is not fully understood. Here, a novel resistive network model based on Monte Carlo simulation to directly predict the electrical conductivity of CPC foams is reported. The model uses multiwalled carbon nanotubes (MWCNTs) as a conductive filler and considers both the intrinsic electrical conductivity of the nanotubes and the electron tunneling effect. The model predictions agreed very well with the experimental data for both the solid and foamed composites. The results showed that the percolation threshold and the conductivity are strongly affected by the relative sizes of MWCNT length and bubble diameter. At relatively low void fractions (up to ~20%), the growth of bubbles with size comparable to the MWCNT length increased the conductivity. Further growth of cells (>30%) resulted in a gradual decrease in the conductivity. This model can be used in the design and optimization of conductive foams.
In this study, we fabricated conductive poly(vinylidene fluoride) (PVDF)/carbon composites simply by dispersing multiwalled carbon nanotubes (MWCNTs) and graphene nanoplatelets into a PVDF solution. ...The electrical conductivity and the electromagnetic interference (EMI) shielding of the PVDF/carbon composites were increased by increasing the conductive carbon filler amounts. Moreover, we also found that the EMI shielding properties of the PVDF/CNT/graphene composites were higher than those of PVDF/CNT and PVDF/graphene composites. The mean EMI shielding values of PVDF/5 wt %-CNT, PVDF/10 wt %-graphene, and PVDF/CNT/graphene composite films with a thickness of 0.1 mm were 22.41, 18.70, and 27.58 dB, respectively. An analysis of the shielding mechanism showed that the main contribution to the EMI shielding came from the absorption mechanism, and that the EMI shielding could be tuned by controlling the films’ thickness. The total shielding of the PVDF/CNT/graphene films increased from 21.90 to 36.46 dB as the thickness was increased from 0.06 mm to 0.25 mm. In particular, the PVDF/carbon composite films, with a thickness of 0.1 mm, achieved the highest specific shielding values of 1 310 dB cm2/g for the PVDF/5 wt %-CNT composite and 1 557 dB cm2/g for the PVDF/CNT/graphene composite, respectively. This was due to the ultrathin thickness. Our study provides the groundwork for an effective way to design flexible, ultrathin conductive polymer composite film for application in miniaturized electronic devices.
Display omitted
•Crosslinked EPDM nanofibres were generated by novel reactive in-situ fibrillation.•Fibrillar nanostructure was stabilized and well preserved beyond re-processing.•BDT of PP was ...achieved at low EPDM contents with marginal loss in rigidity.•Presence of flexible fibrillar network imparts highly efficient energy dissipation.
Incorporation of homogenous rubber microparticles is a widely adopted toughening method for extending the range of applications of polypropylene (PP). Herein, a new toughening strategy with nano-structured rubber inclusion is devised and its efficiency is evaluated against the classical counterpart. The presented methodology integrates two well-established processes, namely vulcanization and in situ fibrillation, to allow scalable fabrication of PP nanocomposites toughened with crosslinked ethylene-propylene-diene terpolymer (EPDM) nanofibrils, which is otherwise technologically challenging. Morphological observation of the PP nanocomposites, supported by the drastic change in rheological behaviour under shear flow, substantiates the self-assembly of physical EPDM network nanostructure throughout the entire composite system above their rheological percolation threshold. The distinct morphology imparts unique characteristics that are vastly different from classical PP blends produced with the equivalent material composition. Massive improvement in macroscopic fracture behaviour is observed as the tensile fracture toughness is markedly enhanced at a comparatively low EPDM loading while the yield strength and elastic modulus are only marginally decreased. In our attempt to explain the superior toughening efficiency of crosslinked EPDM nanofibrillar network, we propose a collective toughening mechanism which is possibly triggered by several micro-scale mechanisms in play. This industrially relevant process potentially opens up a new avenue in material design for high-volume/lost-cost applications in the transportation sectors where excellent strength/toughness balance is indispensable.
In this study, we used a batch-foaming method to prepare closed-cell poly(vinylidene fluoride) (PVDF) foams with tailored microcellular structures. This is a simple, cost-effective, and ...environmentally-friendly method, and it offers a wide range of tunable microcellular structures. The cell size and porosity volume of PVDF foams can be effectively monitored at their saturation temperatures. The PVDF foam prepared at 169.5 °C possesses an ultra-low dielectric constant of k ∼ 1.1 and a thermal conductivity of 0.027 W m −1 K −1 . We used numerous theoretical models to predict the effective dielectric permittivity of the microcellular PVDF foams, and found that the Bruggeman model was very close to the measured results. The ultra-low thermal conductivities of the PVDF foams were the result of a large number of air pockets in the microcellular structure. Based on these results, we concluded that this PVDF foam would be a promising low- k dielectric and heat insulating material.
We prepared poly(vinylidene fluoride) (PVDF)/carbon/Ni-chain composites by dispersing Ni chains, and either carbon nanotubes (CNTs) or graphene nanoplatelets (GNPs) into a PVDF solution. The ...electrical conductivity and the electromagnetic interference (EMI) shielding properties of the PVDF/CNT/Ni-chain and the PVDF/GNP/Ni-chain composites were increased by increasing the Ni-chain filler content. The electrical conductivity of the PVDF/CNT/10 wt%Ni-chain composite was lower than the PVDF/CNT/6 wt%Ni-chain composite. We attributed this abnormality to the Ni chains having blocked the CNT connections, when there was a high Ni-chain content. Furthermore, the PVDF-based composites' EMI shielding properties were effectively tuned by controlling the films' thicknesses. The total shielding of the PVDF/CNT/6 wt%Ni-chain and the PVDF/GNP/8 wt%Ni-chain composite films increased from 23.6 to 57.3 dB and from 22.7 to 55.8 dB, as their thicknesses were increased from 0.3 mm to 0.6 mm, respectively. The synergetic relationship between the Ni chains and the carbon materials (CNT or GNP), meant that the main EMI shielding mechanisms of the PVDF/carbon/Ni-chain composites had resulted from the absorption process. Moreover, these composites possessed high thermal conductivity, which can convert microwave energy into Joule heating systems. Thus, these PVDF-based composite films can be used to make high-efficiency EMI shielding devices that can rapidly dissipate heat.
Display omitted
There is an urgent need for dielectric-based capacitors to manage the increase in storage systems related to renewable energy production. Such capacitors must have superior qualities that include ...light weight, a high dielectric constant, and ultra-low dielectric loss. Poly(vinylidene fluoride) (PVDF)/carbon (carbon nanotube (CNT) or graphene nanoplatelet (GnP)) nanocomposite foams are considered promising alternatives to solid PVDF/carbon nanocomposites. This is because they have excellent dielectric properties, which are due to the preferred orientation of their carbon materials occurring in the foaming process. In the PVDF/carbon foams, their microcellular structure significantly influenced their electrical conductivity and dielectric properties. In the PVDF/CNT composite foams, the electrical conductivity was increased by an increased degree of foaming that was below a critical foaming degree. The CNTs even formed conductive networks and this caused current leakage. Thus, in the PVDF/CNT foam sample with an expansion ratio of 4.0 where a high dielectric constant of 80.6 was obtained, a relatively high dielectric loss of 3.51 was observed at the same time. In the PVDF/GnP composite foams, the presence of a microcellular structure forcefully increased the distance between GnPs. This induced and produced the insulating quality of the PVDF/GnP foams. In addition, the parallel graphene nanoplatelets that accompanied this process were close together, and they isolated the polymer layer, or air, as a medium between themselves. An unprecedentedly high dielectric constant of 112.1 and an ultra-low dielectric loss of 0.032 at 100 Hz were obtained from the PVDF/GnP composite foam with a high expansion ratio of 4.4 due to charge accumulation at the aligned conductive filler/insulating polymer (or air bubble) interface.
In this study, novel poly(vinylidene fluoride) (PVDF)/multiwalled carbon nanotube (MWCNT) nanocomposite foams with various foaming degrees (void fractions) were prepared using a home-made batch ...foaming process. The effects of the foaming degree in the range of 45.7% to 84.3% on the electrical conductivity, the dielectric permittivity, and the microwave absorption (MA) properties of the nanocomposite foams were investigated in great detail. The electrical conductivity declined linearly with an elevated void fraction, and the PVDF/MWCNT foams with a void fraction below 50% showed a higher electrical conductivity than the unfoamed PVDF/MWCNT nanocomposite counterpart. The dielectric permittivity was also effectively tuned by the foaming degree. Consequently, it could adjust the impedance match to promote MA. The PVDF/MWCNT foam with a void fraction of 69.5% (FC3) exhibited outstanding MA properties. An effective bandwidth (reflection loss below −10 dB, and 90% microwave attenuation) of 8.5 GHz in the measured frequency of 18-26.5 GHz was obtained. A minimal reflection loss (RL) of −34.1 dB could be seen with a thickness of 1.7 mm. The excellent MA properties of these lightweight foam absorbers with ultra-low MWCNTs in the PVDF polymer were attributed to the good impedance match, the greater interfacial polarization, the high conduction loss, and the multiple reflection and scattering mechanisms.
PVDF nanocomposite foams with ultra-low MWCNT content exhibit high-efficiency microwave absorption properties with a light weight, strong and wide-band absorption, and small-thickness properties.
PDF
