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Figure 1. The preparation process and properties of PVA/CNF coated paper.
ARL, Switzerland). The samples were scanned at 40 kV and 30 mA in a 2 h range between 10° and 40° using Cu Kα radiation (λ = 15.4 × 10 –2 nm) at 1°/min 41 . The crystallinity index (CrI) was calculated according to the empirical method developed by Segal (1959) using the Eq. (1) 42 .
I 200 − I am I 200
(1)
CrI ( % ) =
where I 200 is the maximum intensity of the (200) plane (I 200 , 2θ = 22°) that represents the crystalline and amor- phous material, and I am is the minimum intensity between plane (110) and (200) (I am , 2θ = 18°) that represents the amorphous material. The viscosity and pH of the coatings were measured with a viscometer (ROTAVISC lo-vi Complete, IKA, Germany) and a pH meter (PHS-3E, Shanghai Yidian Scientific Instruments Co., Ltd., China). The oil/grease resistance of paper was measured using the TAPPI T559 standard. The contact angle of paper was measured according to ASTM D 724–1999 standard, using a surface contact angle tester (OCA40, Dataphysics, Germany). Water absorption of paper (Cobb test) was determined using Paper Cobb Water Absorption Tester (PWA-01, Sichuan Changjiang Paper Instrument Co., Ltd., China) according to TAPPI T441 standard. The tensile strength of paper was determined using a computerized tensile tester (TTM, Hangzhou Qingtong Boke Automation Technology Co., Ltd., China) according to the TAPPI T494 standard. The surface morphology of the paper was observed using a scanning electron microscope (Phenom pro, Phenom Scientific Instruments (Shanghai) Co., Ltd., China). ƤǤ Morphology analysis. Figure 2 shows the optical microscope images of BFs and EBFs. Compared with BFs, the diameter of EBFs did no change significantly, about 20 μm. The average length of the EBFs was 0.4 mm, which was 1.1 mm smaller than the diameter of the BFs (Table 1). The reason for this phenomenon was that the amorphous region of cellulose was hydrolyzed by cellulase 43 . The reduction of fib- er length was beneficial to the exfoliation of microfibrils from the fibers during the preparation of nanocellulose. Figure 3a shows the image of CNF tested with the atomic force microscope (AFM). As shown in Fig. 3a, the CNF was a filamentous structure. And the length and diameter of CNFs were mostly distributed between 1–3.5 μm and 18–42 nm, with an average length of about 2.4 μm and an average diameter of about 28.7 nm (Fig. 3b,c). FTIR spectroscopy analysis. Figure 4 shows the Fourier transform infrared (FTIR) spectra of BFs, EBFs, and CNFs. It could be seen from the figure that EBFs, CNFs, and BFs had similar characteristic peaks. They all con-
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