Chitin nanofibers were prepared from dry chitin powder by nanofibrillation using a Star Burst instrument employing a high-pressure water jet system. FE-SEM micrographs showed that the nanofibers became thinner as the number of Star Burst passes increased. Fibrillation in an acidic condition made the chitin fibers thinner than those in a neutral condition. The transmittance spectra of chitin nanofiber/acrylic resin composites led us to the same conclusion. In addition, chitin nanofibers prepared by treatment consisting of five Star Burst passes in the neutral condition were thinner than the previously reported nanofibers. X-ray diffraction profiles showed that the Star Burst system did not damage the chitin nanofibers and did not reduce their crystallinity.
Chitin is the main component of the exoskeletons of arthropods such as crab and shrimp, as well as of the cell walls of fungi. It has a complex hierarchical organization consisting of chitin fibers, proteins, and minerals. These fibers were composed of fine nanofiber networks [
FE-SEM micrographs of (a) original chitin powder and (b) chitin nanofibers fibrillated by grinder under acidic condition.
Recently, a new atomizing system developed by Sugino Machine Co., Ltd., has attracted much attention for the production of biopolymer-based nanofibers [
If the Star Burst system efficiently yields chitin nanofibers, it will be a strong candidate for use in the mass production of chitin nanofibers. For this purpose, we expect that treatment under an acidic condition could be a key factor. Accordingly, we have studied the fibrillation of chitin into nanofibers by the Star Burst system under neutral and acidic conditions and characterized them in detail.
Chemical structures of acrylic monomers: A-600, BPE-100, and DCP.
Dry chitin powder was dispersed in water at 1 wt.%. Acetic acid was added to adjust the pH value to 3. The chitin was crushed roughly with a domestic blender. The slurry was stirred for 1 h under vacuum to remove air bubbles. The slurry was passed through the Star Burst system (Star Burst Mini, HJP-25001S, Sugino Machine Co., Ltd.,) equipped with a ball-collision chamber (Figure
Inner structure of ball-collision chamber of Star Burst system.
Fibrillated chitin nanofibers were dispersed in water at a fiber content of 0.1 wt.%. The suspension was vacuum filtered using a membrane filter. The obtained chitin nanofiber sheets were dried by hot pressing at 100°C for 30 min. The dried sheets were cut into 2 cm × 2 cm squares of approximately 44
Infrared spectra of the samples were recorded with an FT-IR spectrophotometer (Spectrum 65, Perkin-Elmer Japan Co., Ltd.,) equipped with an ATR attachment. For field emission scanning electron microscope (FE-SEM) observation, the prepared nanofiber slurry was diluted with the EtOH and dried in an oven to prepare a chitin nanofiber sheet. The sample was coated with an approximately 2 nm layer of Pt by an ion sputter coater and observed by FE-SEM (JSM-6700F; JEOL, Ltd.,) operating at 2.0 kV. The average diameter of the isolated nanofibers was estimated by image analysis. X-ray diffraction profiles of the nanofibers were obtained with Ni-filtered CuK
Star Burst is a water jet atomizing system with super-high pressure. Chitin slurry was compressed by a hydraulic piston, ejected at high pressure (245 MPa) from a nozzle, and collide with ceramic ball in a chamber to atomize the chitin (Figure
Appearances of chitin slurries treated by Star Burst system.
Figure
Fiber thickness of fibrillated nanofibers with and without AcOH after various number of passes.
Number of passes | Fiber thickness (nm) | |
---|---|---|
Without AcOH | With AcOH | |
1 | — | 19.0 (4.9) |
5 | 18.2 (3.4) | 18.0 (3.2) |
10 | 17.3 (3.2) | 16.5 (3.3) |
Standard deviations are given in parentheses.
FE-SEM micrographs of chitin fibers with (a) 1 pass, ((b) and (c)) 5 passes, and ((d) and (e)) 10 passes through Star Burst system without acetic acid. The scale bar lengths are ((a), (b), and (d)) 1000 nm and ((c) and (e)) 300 nm.
Next, the chitin powder in water was passed through the Star Burst system under an acidic condition with AcOH. Even after only one pass, chitin seems considerably fibrillated (Figures
FE-SEM micrographs of chitin fibers after ((a) and (b)) 1 pass, ((c) and (d)) 5 passes, and ((e) and (f)) 10 passes through Star Burst system with acetic acid. The scale bar lengths are ((a), (c), and (e)) 1000 nm and ((b), (e), and (f)) 300 nm.
Table
Figure
FT-IR spectra of chitin fibers after 1, 5, and 10 passes through Star Burst with and without acetic acid.
Figure
Relative degree of chitin nanofiber crystallinity prepared with and without AcOH at various numbers of passes.
Number of passes | Relative crystallinity (%) | |
---|---|---|
Without AcOH | With AcOH | |
Original chitin | 83.7 | |
1 pass | 84.7 | 84.0 |
5 pass | 85.2 | 84.0 |
10 pass | 85.4 | 83.7 |
X-ray diffraction profiles of chitin fibers after 1, 5, and 10 passes through Star Burst with and without acetic acid.
Since the FE-SEM images focused on small parts of the fiber networks, they cannot represent the total morphology of nanofibers. Recently, cellulose nanofiber-reinforced plastic films were reported [
The appearance of transparent chitin nanofiber composite film.
Figure
Regular light transmittances of acrylic resins ((a) A-600, (b) BPE-100, and (c) DCP) reinforced with chitin nanofiber with 1, 5, and 10 passes through Star Burst with (blue dots) and without acetic acid (red dots). Error bars show standard deviations. Black dots show transmittance of acrylic resins reinforced with chitin nanofibers with a previously reported procedure (one pass through a grinder with acetic acid).
In a previous study, we reported that chitin nanofibers were prepared by one pass through a grinder in aqueous AcOH and were complexed with acrylic resins [
We studied the fibrillation of dry chitin powder into nanofibers using the Star Burst system. FE-SEM micrographs of chitin nanofibers and light transmittances of the nanocomposites with acrylic resin showed that the morphology of chitin nanofibers depended strongly on the number of passes through the Star Burst system. The results also showed that treatment under an acidic condition was a key factor in chitin fibrillation into nanofibers effectively. X-ray diffraction profiles of the chitin nanofibers showed that the Star Burst treatment did not reduce their crystallinities, even though the Star Burst system uses a super-high-pressure water jet. The Star Burst system has advantages in quality stability, high-volume production, and low contamination. I expect that this unique system can play a strong role in the commercial use of chitin nanofibers.
This work was financially supported partially by KAKENHI (23750256) of JSPS.