Xueying Nai

Functional group effect on flame retardancy, thermal, and mechanical properties of organophosphorus-based magnesium oxysulfate whiskers as a flame retardant in polypropylene

In this paper, magnesium oxysulfate whiskers (MOSw) were reacted with dodecyl dihydrogen phosphate (DDP) to prepare DDP functionalized MOSw (DDPMOSw). The morphology and structure of DDPMOSw were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). Then MOSw and DDPMOSw were respectively incorporated into polypropylene (PP) to obtain hybrid composites via the melt mixing method. SEM results showed that the DDPMOSw were distributed more evenly within the PP matrix than MOSw, resulting in the improved thermal stability, nominal strain at break, and impact toughness of PP/DDPMOSw in comparison with PP/MOSw. The flammability of the composites was investigated using limiting oxygen index (LOI) and cone calorimetry (CC) tests. The results showed that the introduction of DDPMOSw into the PP matrix further increased the LOI, meanwhile, reducing the heat release rate (HRR), smoke production rate (SPR) and CO and CO2 release. SEM and energy dispersive spectroscopy (EDS) analyses of the char residues implied two reasons for the more efficient flame-retardant properties of PP/DDPMOSw than PP/MOSw: (i) DDPMOSw was more thoroughly decomposed to release more nonflammable gases and absorb more heat during the combustion process (gas-phase flame-retardant effect); (ii) DDP induced the formation of the carbonaceous residue, leading to a compact and coherent char based on MgO whiskers backbone (condensed-phase flame-retardant effect). Besides, FTIR results indicated that the phosphorous compound layer in the condensed phase also acted as a protective shield.

Effects of different compatilizers on mechanical, crystallization and thermal properties of polypropylene/magensium oxysulfate whisker composites

Abstract Two kinds of compatilizers, maleic anhydride grafted polyolefin elastomer (POE-g-MAH) and maleic anhydride grafted polypropylene (PP-g-MAH), were incorporated into a polypropylene/magnesium oxysulfate whisker (PP/MOSw) composite. Scanning electron microscopy pictures presented a clear interface between MOSw and the PP matrix in the PP/MOSw composite, while vague interfaces appeared in the PP/iPOE-g-MAH/5MOSw and PP/iPP-g-MAH/5MOSw composites. Dynamic mechanical thermal analysis results indicated that PP-g-MAH was highly compatible with the PP matrix while POE-g-MAH was not. Impact strength results showed that POE-g-MAH had a superior toughening effect on PP/MOSw composites, since the proper interfacial interaction and appearance of β-crystal PP. However, incorporating PP-g-MAH seemed to be conducive to increasing strength and modulus (both for tensile and flexural tests), as evidenced by the greatly raised interfacial adhesion between the PP matrix and MOSw. Quantitative characterization carried out by Turcsányi equation for ternary composites also confirmed that PP-g-MAH efficiently enhanced interfacial interaction, by the proof of higher B values. Therefore, the thermal stability of PP-g-MAH treated composites was far superior to that of PP/iPOE-g-MAH/5MOSw composites. Differential scanning calorimetery and polarized light microscopy results showed that POE-g-MAH promoted PP nucleation, with effects further enhanced with the presence of PP-g-MAH.

Effects of different compatibilizing agents on the interfacial adhesion properties of polypropylene/magnesium oxysulfate whisker composites

The effects of maleic anhydride-grafted polypropylene (PP-g-MAH) and maleic anhydride-grafted polyolefin elastomer (POE-g-MAH) on interfacial adhesion properties of the polypropylene/magnesium oxysulfate whiskers (PP/MOSw) composites were investigated via mechanical, thermal, ATR-FTIR and rheological tests. Although significant increases in yield strength and Young’s modulus were observed in PP-g-MAH treated composites, a sharp decline in these properties was observed in POE-g-MAH treated composites. ATR-FTIR results indicated that esterification occurred between the hydroxyl groups of MOSw and the carbonyls of anhydrides of both compatibilizers but POE-g-MAH was still incompatible with the PP matrix, as verified by the presence of shoulder peaks in DTG curves and numerous voids in SEM micrographs. On the other hand, PP-g-MAH was highly compatible with the PP matrix, as evidenced by the peaks in DTG curves and vague interfaces with wrapped melts on the surface of MOSw. Rheological behaviors also confirmed that introducing PP-g-MAH resulted in a transition from liquid-like to solid-like, which was attributed to the stronger interfacial adhesion between MOSw and the PP matrix. POE-g-MAH treated composites, in contrast to PP-g-MAH, maintained liquid-like rheological behaviors as typical molten polymers. There is likely a MOSw network formed in the PP/15PP-g-MAH/15MOSw composite as suggested by the significant deviation of G′ versus G″ plots and the two crossover frequencies observed in plots of tanδ versus frequency.

Hydrothermal Synthesis of Calcium Borate Whiskers

A type of calcium borate whiskers with highly thermal stability was synthesized by hydrothermal method at 180 °C for 2 h, using borax, calcium chloride and sodium hydroxide as raw reactants. The X-ray powder diffraction (XRD) indicated that as-prepared products were composed of monoclinic Ca2B2O5·H2O phase. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images showed calcium borate whiskers with a length of 50~100 μm and diameter 1.0 ~3.0 μm. Selected-area electron diffraction (SAED) pattern showed that the calcium borate whiskers were single crystalline structure. The adjustment of temperature of hydrothermal reaction, can lead to obvious morphology changes of products, and the possible chemical reaction and growth mechanism were proposed. Furthermore, the analysis of thermogravimetry (TG) showed the calcium borate whiskers released crystal water at 370~425 °C.

Synthesis of Mg2B2O5 whiskers via coprecipitation and sintering process

Mg2B2O5 whiskers with high aspect ratio were synthesized by coprecipitation and sintering process using MgCl2·6H2O, H3BO3, and NaOH as raw materials and KCl as a flux. Their formation process was investigated by thermogravimetry and differential scanning calorimetry (TG-DSC), X-ray diffraction (XRD), and scanning electron microcopy (SEM). It is found that the products synthesized at 832°C are monoclinic Mg2B2O5 whiskers with a diameter of 200–400 nm and a length of 50–80 μm. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) analyses show that the whiskers obtained at 832°C are single crystalline and grow along with the [010] direction. The growth mechanism of Mg2B2O5 whiskers was also presented.

Crystallization, mechanical, thermal and rheological properties of polypropylene composites reinforced by magnesium oxysulfate whisker

Polypropylene (PP) composites containing magnesium oxysulfate whisker (MOSw) or lauric acid (LA) modified MOSw (LAMOSw) were prepared via melt mixing in a torque rheometer. The heterogeneous nucleating effect of LAMOSw was clearly observed in polarized light microscopy (PLM) pictures with the presence of an abundance of small spherulites. MOSw exhibited no nucleation effect and formed a few spherulites with large size. Compared with PP/MOSw composites, PP/LAMOSw exhibited better impact strength, tensile strength and nominal strain at break, ascribing to three possible reasons: (i) more β-crystal PP formed, (ii) better dispersity of LAMOSw in PP matrix and (iii) the plasticizing effect of LA. The results of dynamic mechanical thermal analysis (DMTA) indicated that brittleness of the PP matrix at low temperature was improved by the addition of LAMOSw, while the interfacial interactions between MOSw and PP matrix were actually weakened by LA, as evidenced by the higher tanδ values over the entire range of test temperatures. In terms of the rheological properties of the composites, both the η* and G′ at low frequencies increase with the addition of MOSw or LAMOSw, indicating that the PP matrix was transformed from liquid-like to solid-like. However, a network of whiskers did not form because no plateau was found in the G′ at low frequencies. With low filler content, LAMOSw produced a stronger solid-like behavior than MOSw mainly due to the better dispersion of the LAMOSw in PP matrix. However, for highly-filled composites, the η* of PP/LAMOSw at low frequencies was smaller than that of PP/MOSw composite, since the particle-particle contact effect played a major role.

Reinforcing polypropylene with calcium carbonate of different morphologies and polymorphs

Abstract The influence of calcium carbonate (CaCO3) with different polymorphs (calcite and aragonite) and morphologies (granular and rod-like) on mechanical and crystallization properties of polypropylene (PP) was investigated. Meanwhile, these CaCO3 fillers coated with oleic acid were added in different contents to PP. The results indicate that the tensile strength, flexural strength, modulus, and crystallization property of the filler-treated samples are improved, but the impact strength decreased. The crystallinity of the composites is higher than that of neat PP. Moreover, in the rod shape filler-treated sample, in both whisker species, the mechanical properties of composites are superior to the particles filled. Differential scanning calorimetry, X-ray diffraction, and mechanical tests display that calcite whisker-reinforced composite has higher crystallization enthalpy, melting enthalpy, degree of crystallinity, and mechanical properties than aragonite whiskers and calcite particles filled composites.

Design and preparation of high-aspect-ratio zinc borate whiskers and their effects on mechanical properties of PP nanocomposite

Well-dispersed, high-aspect-ratio 4ZnO·B2O3·H2O whiskers (4ZnO·B2O3·H2OHARw) were synthesized by a facile hydrothermal method with the aid of ZnSO4·7H2O. The formation mechanism was investigated, and the results showed that Zn2+ and SO42− ions played a crucial role in the control of 4ZnO·B2O3·H2OHARw extended along [010] direction. X-ray diffraction (XRD) analysis, high-resolution transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS), and thermogravimetry (TG)-differential scanning calorimetry (DSC) results confirmed that the powder was 4ZnO·B2O3·H2O. Scanning electron microscopy (SEM) and TEM showed that the average aspect ratio of the product was about 100. Nanocomposites of surface-modified 4ZnO·B2O3·H2OSM-HARw/PP were prepared and their mechanical properties measured. The results suggested that the mechanical properties of 4ZnO·B2O3·H2OSM-HARw/PP composites were superior to composites made with low-aspect-ratio whiskers (4ZnO·B2O3·H2OSM-LARw), which displayed poorer mechanical properties even at low addition.

Effect of magnesium oxysulfate (MOS) morphology on the crystallization, mechanical, and rheological properties of polypropylene/MOS composites

Polypropylene (PP) composites containing magnesium oxysulfate particle (MOSp), magnesium oxysulfate whisker (MOSw), or magnesium oxysulfate sector (MOSs) were prepared via melt blending method. Scanning electron microscopy results showed that three magnesium oxysulfate (MOS) fillers all dispersed homogeneously in PP matrix and displayed vague and fuzzy interfaces. Wide-angle X-ray diffraction (WXRD) patterns showed that MOSp induced the most amount of β-PP, which was supported by polarized light microscopy (PLM) photographs. Moreover, PLM photographs also showed that the presence of MOSp, MOSw, or MOSw decreased the PP spherulites, especially for MOSp. As such, mechanical tests showed that incorporation of MOSp into PP matrix greatly improved the impact strength and least lowered the nominal strain at break. The yield strength and Young’s modulus of composites were greatly enhanced with MOSw. Two possible reasons for this phenomenon are rigidity of MOSw and microstructure of composites. Rheological properties were measured via small amplitude oscillatory shear. The results showed that PP melts containing MOSw exhibited significant yield stress and “shear-thinning” behaviors, which indicated the formation of MOSw network and the transition from “liquid-like” PP matrix to “solid-like” composites. The rheological results greatly proved the enhancement in tensile properties of MOSw-incorporated composites.

Mechanical and flame retardant properties of isotactic polypropylene/magnesium oxysulfate whisker composite:

Isotactic polypropylene (iPP) composites containing magnesium oxysulfate whisker (MOSw) or lauric acid-modified MOSw (LA-MOSw) were prepared via melt mixing in a torque rheometer. Scanning electron microscopy pictures showed that the interface between MOSw and iPP matrix was defined, whereas a vague interface was seen in the iPP–LA-MOSw composites. Mechanical properties of these two groups of composites were investigated in terms of tensile, notched impact, and flexural behavior aspects for the purposes of studying toughening effect of MOSw and LA-MOSw. Tensile results showed that yield strength of composites further reduced with the presence of LA, indicating the decrease in interfacial interaction bewteen iPP matrix and MOSw. As such, LA-MOSw performed better than MOSw in toughening of iPP matrix. Flexural strength and modulus of iPP–MOSw composites increased sharply with the increase in MOSw content, while less dependence on the LA-MOSw content indicated that MOSw was deemed beneficial to increase the stiffness. In addition, flammability properties were investigated by cone calorimetry experiment. The results showed that the peak heat release rate apparently reduced with addition of MOSw or LA-MOSw. Besides, iPP–LA-MOSw composites showed higher specific extinction area values than iPP–MOSw composites, which meant the weaker smoke suppression effect of LA-MOSw. It was chiefly because of the incomplete combustion caused by the continuous and complete charred (MgO) shield. The presence of LA was another possible reason.