2.4 Characterization
2.4.1 Characterization of SCG
Mobile phone camera (Vivo S7), Scanning electron microscope (Vltra 55), and optical microscope (Nikon eclipse ts100) were used to observe the morphologies of SCG. Fourier transform infrared spectrometer was used to analyze the functional groups of SCG before and after heat treatment. Three kinds of SCG powder samples were ground with KBr at a mass ratio of 1:100 and pressed into thin discs. KBr was used as a carrier, and the spectral range is between 4000 and 500 cm-1 with 32 scans. The Water contact angle of SCG was measured by a video contact angle instrument (JY-82A). The residual oil content of SCG, NSCG, and ASCG was determined by using a Soxhlet extraction device and petroleum ether was selected as the solvent. If the refractive index of the petroleum ether after extraction is the same as that of the original petroleum ether, it indicates that the oil extraction is complete. The calculation formula of residual oil content is as follows:
\(\omega=\frac{m_{\text{SCG}}-m_{\text{Oil}}}{m_{\text{SCG}}}\times 100\%\)(1)
where mSCG is the mass of SCG and mOilis the mass of residual oil.
The calculation formula of relative weight loss of NSCG and ASCG is as follows:
\(M=\frac{m_{1-m_{2}}}{m_{1}}\ \times 100\%\) (2)
where m1 is the mass of SCG before heat treatment and m2 is the mass of SCG after heat treatment.
2.4.2 Characterization of SCG/PP composites
The morphologies of SCG/PP composites was observed by optical microscope (Nikon eclipse ts100). The water contact angle was tested with a video contact angle meter (JY-82A). The samples were dried at 50 ℃ for 24 h according to GB-T 1034-2008 to test the water absorption rate. The weight of the dried sample (m0) was measured with an analytical balance with a precision of 0.01 g. The samples were immersed in tap water at room temperature for 24 h and then removed to measure their weight. The calculation formula of water absorption rate is: water absorption rate = (mt - m0)/ m0×100 %, where m0 represents the initial weight and mt represents the weight of the sample after soaking for 24 h. All tests were performed at room temperature. A thermogravimetric analyzer (NETZSCH TG 209 F1) was used to analyze the thermal decomposition process of samples. The temperature was increased from 30 °C to 600 °C at a rate of 20 ℃· min-1 under the nitrogen atmosphere. Thermal analysis of PP, SCG/PP, NSCG/PP, and ASCG/PP composites was performed using differential scanning calorimetry (DSC 25, TA Instruments instrument) equipped with a liquid nitrogen cooling system (REFRIGERATED COOLING SYSTEM 90). 3-8 mg samples were taken respectively. First, the samples were heated from room temperature to 200 °C at a rate of 10 °C·min-1 for 5 minutes, then cooled to room temperature at a cooling rate of 10 °C·min-1, and then heated to 200 °C at the same rate of 10 °C·min-1 for 5 minutes. The minimum nitrogen flow is 50 mL·min−1. The melting enthalpy was obtained during the second heating process and crystallization parameters were obtained through the cooling stage. Crystallinity Xc = (ΔHf/ΔH0) × 100%, where ΔHf is the enthalpy of melting and ΔH0is the enthalpy of melting of crystalline PP with crystals of 100% (ΔH0=209 J·g-1). Undercooling degree ΔT= Tm-Tc, where Tm is the equilibrium melting point and Tc is the crystallization temperature. The mechanical properties of the dumbbell-shaped samples were tested by a universal testing machine (INSTRON 5934) at a rate of 50 mm· min-1according to ASTM D882. Each sample was tested in parallel five times.