Yonglai Lu

Fabrication of highly oriented hexagonal boron nitride nanosheet/elastomer nanocomposites with high thermal conductivity.

A homogeneous dispersion of hexagonal boron nitride nanosheets (BNNSs) in elastomers is obtained by solution compounding methods, and a high orientation of BNNSs is achieved by strong shearing. The composites show high thermal conductivities, especially when BNNS loading exceeds 17.5 vol%, indicating that the material is promising for thermal-management applications which need high thermal conductivity, low dielectric constant, and adequate softness.

Dramatically improved dielectric properties of polymer composites by controlling the alignment of carbon nanotubes in matrix

Aligned multi-walled carbon nanotubes (MWCNTs)/polyvinyl alcohol composite films were prepared by using an easy and controllable electrospinning-in situ film-forming (EF) technique. A high dielectric constant (k), a low dielectric loss, a consistently high breakdown strength, and a high energy density were obtained by using this technique. The dramatically improved dielectric properties are ascribed to the good dispersion and alignment of MWCNTs in the matrix, facilitating the formation of a large number of separated nano-capacitors (high k and low direct current (DC) conductance). For comparison purposes, the same composite films were prepared by solution casting (SC). At the same MWCNT content, the SC method yielded a higher k, but a significantly higher dielectric loss and much lower breakdown strength and energy density because of the random dispersion of MWCNTs in the matrix and the formation of a MWCNT network, which result in a large increase in DC conductance. The formation mechanism of the different microstructures and the relationships between the microstructures and dielectric properties are clarified. Our results indicate that high-performance MWCNTs/polymer dielectric composites can be obtained by controlling the microstructure of the composites by using the EF technique, which widens the applications of dielectric materials.

A Golden Section approach to optimization of automotive friction materials

A Golden Section approach combined with relational grade analysis is proposed as an experimental design tool helpful in the development of new automotive friction materials. Golden Section was used to design the volume fraction of the components systematically. The changes in friction performance (friction coefficient and wear) measured using Friction Assessment and Screening Test (FAST) can be correlated with component variations by use of relational grade analysis. This approach was utilized to optimize a non-metallic friction material containing seven ingredients including two fibers (aramid and slag fiber), four fillers (Al2O3, BaSO4, graphite and nitrile rubber) and one binder (benzoxazine). The volume fraction of seven components was varied simultaneously in order to optimize the parameters of friction coefficient and wear with a minimum number of tests. Three phases were performed to find the optimal proportion of the components. The optimized friction performance was obtained after doing 19 formulation experiments.

Natural rubber/nitrile butadiene rubber/hindered phenol composites with high-damping properties

New natural rubber (NR)/nitrile butadiene rubber (NBR)/hindered phenol (AO-80) composites with high-damping properties were prepared in this study. The morphological, structural, and mechanical properties were characterized by atomic force microscopy (AFM), polarized Fourier transform infrared spectrometer (FTIR), dynamic mechanical thermal analyzer (DMTA), and a tensile tester. Each composite consisted of two phases: the NR phase and the NBR/AO-80 phase. There was partial compatibility between the NR phase and the NBR/AO-80 phase, and the NR/NBR/AO-80 (50/50/20) composite exhibited a co-continuous morphology. Strain-induced crystallization occurred in the NR phase at strains higher than 200%, and strain-induced orientation appeared in the NBR/AO-80 phase with the increase of strain from 100% to 500%. The composites had a special stress–strain behavior and mechanical properties because of the simultaneous strain-induced orientation and strain-induced crystallization. In the working temperature range of a seismic isolation bearing, the composites (especially the NR/NBR/AO-80 (50/50/20) composite) presented a high loss factor, high area of loss peak (TA), and high hysteresis energy. Therefore, the NR/NBR/AO-80 rubber composites are expected to have important application as a high-performance damping material for rubber bearing.

Dispersion and Filler Network Structure of Fibrillar Silicates in Elastomers

The dispersion and filler network of fibrillar silicate (FS) in elastomers were studied. The results showed that a good dispersion of FS in matrix during mechanical blending in unvulcanized composites contributed to a strong FS filler network, different from that of traditional reinforcing fillers. Meanwhile, the filler re-aggregation during vulcanization caused by the overlapping and intertwining of FS further strengthened the filler network. The factors including Mooney viscosity and molecular polarity of elastomer, type and amount of silane coupling agents used for filler modification, that may influence the filler network, were studied. Our study helps us to understand the mechanism for the formation of filler network of FS in elastomers and provides guidance for the preparation of high performance FS/elastomer composites.

A combined molecular dynamics simulation and experimental method to study the compatibility between elastomers and resins

C5 and C9 petroleum resins are widely used in the rubber industry and their softening, tackifying and reinforcing effects highly depend on their compatibility and interaction strength with the rubber matrix. Herein, we chose five commercially used petroleum resins and two industrial solution polymerized styrene-butadiene rubbers (SSBR). By employing atomistic molecular dynamics (MD) simulation, the influence of resin composition on the compatibility was studied. Results show that different compatibility orders obtained from the solubility parameter (δ), binding energy (Ebinding), mean square displacement (MSD), and the related self-diffusion coefficient (Ds) match well with each other, and are consistent with our experimental solubility parameter data. More importantly, by calculating the non-bond energy (Enon-bond) between single resin chain and rubber units (styrene unit, trans-1,4 unit, cis-1,4 unit, and vinyl unit), it was found that the styrene unit has the strongest interaction with resins, while the cis-1,4 unit has the weakest, which fits well with the solubility parameter result that resins have better compatibility with SSBR than cis-polybutadiene rubber (cis-BR). This chain/unit level MD method saves much time compared to the traditional chain/chain level method. In general, by combining MD simulation and experiments, our work provides some guidance to a compatibility investigation between rubbers and resins, and may promote design and development of high-performance resins and other new materials.

Synchronously Tailoring Strain Sensitivity and Electrical Stability of Silicone Elastomer Composites by the Synergistic Effect of a Dual Conductive Network

The use of conductive polymer composites (CPCs) as strain sensors has been widely investigated. A wide range of strain sensitivities and high repeatability are vital for different applications of CPCs. In this study, the relations of the conductive filler network and the strain-sensing behavior and electrical stability under fatigue cycles were studied systematically for the first time based on the conductive polymethylvinylsiloxane (PMVS) composites filled with both carbon nanotubes arrays (CNTAs) and carbon black (CB). It was proved that the composites could be fabricated with large strain-sensing capability and a wide range of strain sensitivities by controlling the volume ratio of CNTA/CB and their amounts. Additionally, the CNTA/CB/PMVS composite with 3 vol % content of fillers showed high sensitivity (GF is 10 at 60% strain), high repeatability (the relative standard deviation (RSD) of the max R/R0 value is 3.58%), and electrical stability under fatigue cycles (value range of R/R0 is 1.62 to 1.82) at the same time due to the synergistic effects of the dual conductive network of CNTAs and CB. This could not be achieved by relying on a single CNTA or CB conductive network. This study may provide guidance for the preparation of high performance CPCs for applications in strain sensors.

16 – Processing of macro, micro and nanocomposites

: This chapter surveys some recent researches on two main classes of processing methods for the preparation of rubber-based nanocomposites: (a) directly compounding nanoparticles into the matrix; and (b) in-situ generation of the nanodispersion phase during vulcanization through in-situ radical polymerization of metallic salts of unsaturated carboxylic acid. The characteristics of microstructure and properties of the corresponding rubber nanocomposites related to processing conditions are reviewed. The microstructural evolutions of rubber-based nanocomposites, including rubber filled with carbon black or silica and rubber/layered silica nanocomposites during storage and the curing process, and their influence on properties of the nanocomposites, are also introduced.