Materials
Cutting chips (approximately 4 × 3 × 0.5 cm) of air-dried Japanese cedar (C. japonica) were used as the raw material for the cedar pulp preparation. A 25 wt% NaOH aqueous solution was purchased from Kobayashi Chemistry Co. Ltd., Fukuoka, Japan. Anthraquinone (2% NaOH solution containing 22% of anthraquinone (1,4-dihydro-9,10-dihydroxyanthracen) as solid content) was donated by Kawasaki Kasei Chemicals Co. Ltd., Kanagawa, Japan. Peracetic acid preparation, Persan MP2-J, was purchased from Kanto Chemical Co. Inc., Tokyo, Japan. Hydrogen peroxide solution used for the hydrogen peroxide bleaching was obtained from Fujifilm Wako Pure Chemical Co. Ltd., Osaka, Japan. Aqueous acryl resin emulsion, having a glass transition temperature of − 8 °C, a viscosity of 800 mPa‧s, and a non-volatile component of 45%, was from GenGen Co. Ltd. Cellulizer CL was purchased from Nagase ChemteX. Co. Ltd., Osaka, Japan.
Preparation of cedar pulp
Based on the previous report [7, 10], the cedar pulp was bleached with oxygen, peracetic acid, and hydrogen peroxide after soda–anthraquinone cooking. A quantity of 5 kg (dry matter, (DM)) of cedar chips was placed in a 55 L-capacity vertical rotary autoclave (No. 2615, Kumagai Riki Co. Ltd., Tokyo, Japan) and 15 kg of a cooking solution containing sodium hydroxide ((NaOH), 24% per DM of chips) and anthraquinone (0.1% per DM of chips) was added. The water content in the chips was subtracted from the preparation of the cooking solution. After cooking at 170 °C for 150 min and being allowed to cool, the chips were squeezed with a rotary squeezing dehydrator (RP-1, Kato Tekkou Co. Ltd., Gunma, Japan) to a solid content of approximately 50%. The obtained pulp was returned to the autoclave, and 10 kg of 1% NaOH aqueous solution was added. After sealing the autoclave, oxygen gas was added at 0.7 MPa, and the autoclave was treated at 90℃ for 60 min while rotating. After the oxygen treatment, the pulp was dehydrated with the rotary squeezing dehydrator and then made into slurry with 3% water. This slurry was defibrated using a slush pulper (No. 2534, Kumagai Riki Co. Ltd.) and was then knot-removed and washed using a flat screen (No. 2625, Kumagai Riki Co. Ltd.) equipped with a 50/1000-inch-wide slit. The washed pulp was collected by a 300-mesh screen and dehydrated with a centrifuge (H-130G, Kokousan Co. Ltd., Saitama, Japan). The washed oxygen-bleached pulp was placed in a slush pulper to make 3% slurry. The pH of the slurry was adjusted to 4.5 with phosphoric acid, and then the peracetic acid was added to 6% per DM of the pulp. After bleaching at 70 °C for 60 min with stirring, NaOH was added to adjust the pH to 7. Then, the slurry was dehydrated with a rotary squeezing dehydrator, defibrated with an integrator (No. 2531, Kumagai Riki Co. Ltd.), washed with a flat screen equipped with an 8/1000-inch-wide slit, and dehydrated with a centrifuge. The dehydrated pulp was put into the slush pulper again, and the slurry concentration was adjusted to 3% with water. Then, the pH was adjusted to 10.5 with NaOH, and 1.5% of hydrogen peroxide was added per DM of the pulp; then, the mixture was bleached at 70 °C for 60 min. After washing with a flat screen with an 8/1000-inch-wide slit, the pulp was dehydrated to a pulp concentration of approximately 30% using a centrifuge. The brightness of the obtained pulp was measured using a spectral reflectance meter (Elrepho 2000A, Datacolor, NJ, USA).
Preparation of CNFs water suspension
The pulp after bleaching was made into 2.4 wt% slurry using ion-exchanged water. The endoglucanase-rich enzyme preparation (Cellulizer CL) was added at a rate of 2 mg (equivalent to protein) per 1 g (DM) of the pulp, and then, the enzymatic reaction was carried out at pH 4–5 and 55 °C for 6 h. The amount of protein contained in the enzyme preparation was quantified by a Quick Start Bradford Protein Assay (Bio-Rad Laboratories Inc., CA, USA) using bovine gamma globulin as a standard. The reaction was terminated by heating at 110 °C to inactivate the enzyme, and the obtained pulp suspension was defibrated to nano-level with a circulating bead mill (LMZ-2, Ashizawa Finetech Co., Ltd., Chiba, Japan). Zirconia beads of φ1 mm were used to defibrate, and the bead mill filling rate in the vessel was set at 85%. A total of 40 kg of the pulp suspension after the enzymatic pretreatment was passed through a vessel that had a peripheral bead speed of 12 m/s at a flow rate of 3 L/min. The entire amount was passed once in a batch system. Then, the one-pass processed product was returned to the apparatus, and the defibration process under a circulate operation was carried out for an amount of time equivalent to 14 passes. The resultant was used as the CNFs water suspension.
CNFs/acryl resin composite preparation
A prototype of the CNF-containing undercoat paint was prepared by combining the water-based acryl resin emulsion with the CNFs water suspensions. Two suspensions were mixed so that the content of the resin-derived non-volatile component in the mixture was 9 wt% and the concentration of the CNFs was 1.5 wt%. A 50-g mixed suspension was prepared, and the mixture was agitated at 10,000 rpm for 10 s using a homogenizer (AM-11, Nihonseiki Kaisha Ltd., Tokyo, Japan). Additional CNF-containing composites were also similarly prepared, with concentrations of 0.5 wt% and 1 wt%.
Film preparation
Especially ordered polyethylene apparatuses with separable bottoms and sides (Kyoto Jushi-Seiko Co., Ltd., Kyoto, Japan) were used to obtain the self-supporting films. The composites prepared by mixing were left to stand overnight at room temperature before use to defoam the bubbles that had been generated by stirring. Each 10 g of composite was gently cast into the apparatus that was 7 cm in diameter (0.26 g of composite/cm2). To prepare the films for the light transmittance analyses, the apparatuses that were 4.5 cm in diameter were also used with the same amount per unit area. The films were prepared by drying in a room at 24 °C for at least seven days, and then they were carefully peeled from the apparatuses. The thicknesses of the films were measured using a thickness gauge (NO. 7331, Mitutoyo Co. Ltd., Kanagawa, Japan).
Atomic force microscopy (AFM)
The surface properties of the films, including CNFs, were observed using an SPM-9700HT scanning prove microscope (Shimadzu Co. Ltd., Kyoto, Japan). A portion of the diluted specimen was dried at room temperature on a clean mica substrate. The images were scanned in a tapping mode using silicon micro-cantilevers (OMCL-AC200TS, Olympus Co. Ltd., Tokyo, Japan). To observe the surface condition of the film on the wood panel, a CNFs/acryl resin composite was applied to the cedar-wood board with a spray-type coating apparatus to a thickness of the dried film should be approximately 10 μm. After the painted film had dried, it was cut out with a cutter so that the thickness of the base wood was 1 mm or less, and it was fixed to the sample holder with a double-sided tape for AFM observation.
The sizes of the images were 1 × 1 μm or 2 × 2 μm, and flattening of the image processing was only carried out before measuring the thickness of the fibers. The average fiber thickness was calculated from the measured values at 157 locations.
Light transmittance
The light transmittance of each film was determined from 220 to 850 nm using a Shimadzu UV-2400 spectrometer equipped with an integrating sphere attachment ISR-2200 (Shimadzu Co. Ltd.). The measurement was carried out in duplicate, and the average value of the light transmittance at 365 nm was obtained.
Tensile testing
The tensile strength and elongation-at-break for the films were assessed using a testing machine, EZ-Graph (Shimadzu Co. Ltd.), with a 500-N load cell. The films were cut into strips of 10 × 50 mm. The strips were aged at 22 °C and 50% RH for at least two days prior to measuring. The initial gauge length was 10 mm, and the crosshead speed was 100 mm/min. The results are expressed as the means of the four to five specimens.
Oxygen gas permeability
A film for the measurement of the oxygen gas permeability was mounted on a differential pressure gas permeameter (BT-3, Toyoseiki Co. Ltd., Tokyo, Japan). Using oxygen gas of 99.9% or more, the measurement was carried out at a test temperature of 23 °C. The gas pressure on the high side was 100 kPa, and the permeation area had a diameter of 30 mm. The average value was calculated for each of the three films. The analyses were carried out at the Chemicals Evaluation and Research Institute, Tokyo, Japan.
Electron spin resonance (ESR) spectrometric measurement
An air-dried wood sample (DM of 94.4%) of approximately 1 × 1 × 30 mm was cut out from cypress and adjusted to 15 mg. A film was formed by adding 4.13 g of acrylic resin or CNFs/acryl resin composite to a homemade apparatus that had a diameter of 4.5 cm; this was dried at 45 °C. Four pieces of wood samples of the adjusted weight were placed on the dried membrane; then, 1.5 g of each suspension was added so that the pieces of wood were immersed; they were then dried at 45 °C. Next, a further 1.5 g of the suspension was added and dried to obtain the film-coated wood specimens. The film was extracted from the apparatus, and the coated specimen was carefully cut out, taking care not to peel off the coating around the wood sample. Since the weight of the coated wood specimen with the CNFs/acryl resin composite was 95 mg, the weight of an acryl resin-coated wood specimen was also adjusted to 95 mg by excising the resin film remaining around the piece of wood. Since the weight of the wood specimen was 15 mg, 80 mg of the resin film samples was also prepared using some sheets that were obtained by cutting each of the CNFs/acryl resin composite film and the acrylic resin film to a size of approximately 2 × 2 × 30 mm; the weight was adjusted by excising each of the resin pieces. Another set of specimens was prepared with the same weight as the coated wood specimens, and ESR analyses were performed in duplicate.
The ESR (JES FA200, JEOL Co. Ltd., Tokyo, Japan) measurement was performed using a quartz test tube with a diameter of 5 mm. After setting the specimen in the test tube, it was treated with ultraviolet (UV) irradiation for 2 min using an ultra-high pressure mercury lamp (USH-500SH, Ushio Inc., Tokyo, Japan). Immediately after the UV irradiation, the radicals were measured by ESR, and the changes in radical formation before and after irradiation with each specimen were compared. Using manganese (Mn) as a standard, the spectra were measured with 336 ± 7.5 mT, modulation amplitude of 0.35 mT, time constant of 0.1 s, microwave output of 1 mW, and the measurement time was 1 min.