Zhongguo Zhao

Preparation of alumina-coated graphite for thermally conductive and electrically insulating epoxy composites

Herein, highly thermally conductive and insulating epoxy composites were reported. Firstly uniform alumina-coated graphite flakes were successfully prepared by a two-step coating method of chemical precipitation with the aid of a sodium dodecyl sulfonate (SDS) surfactant using an inorganic precursor (aluminum nitrate) as the starting material. Then the alumina-coated graphite particles were incorporated into the epoxy resin. The thermal conductivity value of epoxy/alumina-coated graphite composite shows a significant increase from 0.22 W mK−1 (neat epoxy) to 0.64 W mK−1 by a factor of approximately 3 at the filler loading of 18.4%. Moreover, due to the presence of the alumina nanolayers coating on the graphite surface, epoxy/alumina-coated graphite composites could retain high electrical volume resistivity of >1010 Ω cm up to high filler contents, which was much higher than that of epoxy/graphite composites (<105 Ω cm) at the same filler loadings. And they still could be regarded as insulators.

Unusual hierarchical structures of micro-injection molded isotactic polypropylene in presence of an in situ microfibrillar network and a β-nucleating agent

The microstructural and mechanical properties of isotactic polypropylene (iPP), in situ PET microfibrils, and β-nucleating agent blends obtained from micro-injection molding were investigated via polarized light microscopy, differential scanning calorimetry, scanning electron microscopy, and two-dimensional wide-angle X-ray diffraction. The results indicate that addition of PET microfibrils markedly increases crystallization temperatures, and increases the thickness of the final oriented layer. Introduction of PET microfibrils to β-nucleation agent-nucleated iPP samples leads to formation of oriented β-crystals epiphytic on the surface of PET fibers in the inner region; this feature improves adhesion between the fiber and the matrix and simultaneously improves the strength and toughness of the final PP/0.5/15 microparts (e.g., the tensile strength increased by 12 MPa and the elongation at break increased by 1.2%) compared with those of iPP microparts. Taken together, the results of this study introduce an alternative approach to optimize the properties of MIM parts.

New understanding of the hierarchical distribution of isotactic polypropylene blends formed by microinjection-molded poly(ethylene terephthalate) and β-nucleating agent

Blends of isotactic polypropylene (iPP) and poly(ethylene terephthalate) (PET) were prepared by using a special injection molding process named microinjection molding (MIM). Interestingly, a strong continuous shear flow imposed on the melt of iPP/PET directly promotes the formation of the in situ PET microfibrils under microinjection molding. The hierarchical structures, including the shish-kebab-like structure, β-cylindrite, β-spherulite and α-spherulite are simultaneously formed in the iPP/PET microparts, which are closely related to formation of row-nuclei induced by the strong shear flow that is further amplified by incorporating in situ PET microfibrils. A surprising synergetic effect is observed between PET microfibrils and β-NA, resulting in the coexistence of shish-kebab, shish-kebab-like β-cylindrite, β-cylindrite and oriented β-crystal epiphytic on the surface of PET fibers in PP/0.1/15 microparts for the first time. Mechanical properties (e.g., tensile strength increased by 10.4 MPa) of the specimen are significantly improved compared with that of the iPP microparts because of the abundant hierarchical structures. A schematic model of the formation of hierarchical distribution of β-crystals via PET and β-NA addition is thus proposed.

Effective in situ polyamide 6 microfibrils in isotactic polypropylene under microinjection molding: significant improvement of mechanical performance

Microparts of isotactic polypropylene (iPP)/polyamide 6 (PA6) blends were prepared with a particular injection molding method known as microinjection molding (MIM). Continuous and strong shear action exerted on the melts of iPP/PA6 directly promoted the formation of in situ PA6 microfibril in MIM. Moreover, hierarchical structures, namely, spherulite, cylindrites, and transcrystallization, were observed in the microparts. The synergetic effect of PA6 in situ microfibrils, β-nucleating agent (β-NA), and strong shear action even induced more oriented β-crystals around the surface of PA6 microfibrils in the core layer and markedly increased the β-crystal content. Results showed that adding PA6 and β-NA markedly raised the crystallization temperature of iPP, and the effect of PA6 microfibrils was evidently more pronounced than that of PA6 spherical particles in conventional blends which implies more nucleation sites on the microfibrils. Moreover, strong orientation of iPP molecular chain was also confirmed by 2D-WAXD. It is well worth mentioning that the mechanical property was remarkably improved by these special morphology and crystalline structures.

The morphological evolution and β-crystal distribution of isotactic polypropylene with the assistance of a long chain branched structure at micro-injection molding condition

Long chain branching polypropylene (LCBPP) has good application prospect but the synergistic effects of the long chain branching (LCB) structure and complex flow field on the crystallization behavior of LCB polymers are still elusive. In the present work, microparts of isotactic polypropylene (iPP)/LCBPP blends were prepared by microinjection molding (MIM). The effect of the LCB structure on the morphology evolution and the β-crystal distribution in microparts were investigated. Interestingly, adding LCBPP facilitates the formation of oriented β-crystals along the shear direction in the core layer of the microparts but unexpectedly, decreases the relative content of β-crystals due to the heterogeneous nucleation effect of LCBPP. Moreover, the introduction of LCBPP can also improve the orientation degree of the molecular chains and shrink the thickness of the core layer thereby inevitably suppressing the “skin–core” structure which might optimize the properties of MIM parts.

Thermal oxidative and ozone oxidative stabilization effect of hybridized functional graphene oxide in a silica-filled solution styrene butadiene elastomer.

Hybridization of modified functional graphene oxide (fGO) in silica-filled solution styrene butadiene rubber (SSBR) endows preferable tensile and dynamic properties before and after thermal oxidative aging, and similar mechanical hysteresis performance compared with the composites without fGO. The preventing mechanism of fGO is attributed to its intrinsic peroxy radical scavenging and gas barrier abilities, which significantly reduces the peroxy radical concentration and oxygen permeability of nanocomposites and then prolongs oxidative induction time (OIT), characterized by differential scanning calorimetry (DSC). The ozone resisting effect of different loadings of fGO on nanocomposites have also been investigated by Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) after ozonization under 50 ppm ozone concentration. As a result, incorporation of fGO apparently suppresses both the formation of oxygenic groups of the olefinic elastomer and crack morphology extension upon ozonization. We propose that fGO protects the SSBR elastomer from ozone attack through the conjugated delocalized π-bonds of the fGO instead of the C[double bond, length as m-dash]C bonds of the elastomer matrix being attacked, and the compared experiments, characterized by X-ray photoelectron spectroscopy (XPS), confirm that this presumption is perhaps reasonable. Moreover, more than 3 phr incorporation of fGO in nanocomposites deteriorates the chemical and mechanical properties of the elastomer during the thermal oxidation and ozonization because of the cleavage influence of oxygenic groups on peroxy radicals.

New insight into the flocculation behavior of hydrophilic silica in styrene butadiene rubber composites

The flocculation behavior of hydrophilic silica in styrene butadiene rubber composites has been carefully analyzed by rheology methodology. An evident increment of the elastic modulus (G′) can be observed over a critical temperature for unmodified composites due to a significant filler network composed of loose silica clusters, while the increment of G′ for modified composites is slight. Still, the flocculation behavior is confirmed by nonlinear dynamical strain sweeps and Atomic Force Microscopy (AFM). Thereafter, modified and unmodified silica filled composites, with varied processing temperature, are vulcanized respectively, and corresponding fatigue crack growth tests are implemented. A modified composite with a processing temperature of 130 °C possesses the smallest exponent law, b, and dc/dn at a given tearing energy (T). We deduce that fatigue crack growth changes from a local stress concentration mechanism originating from severe silica flocculation within unmodified composites to a crack deflection growth mechanism originating from moderate silica flocculation within modified composites. Based on the crack tip morphology investigations at T = 5, 10, 15, 20 kJ m−2, it can be proposed that the crack tip morphology has a tear energy dependence, closely related to the flocculation behavior of silica.