The lignocellulosic materials are sustainable, environmentally friendly and renewable. The shortage of petroleum and environmental concern has resulted in a considerable increase in the usage of renewable natural resources in recent years. Lignocellulosic natural fibers can be obtained from pulping processes, such as thermal mechanical pulping or chemical pulping. These fibers, processed from wood, kenaf, hemp, jute, sisal, etc.
, may be used as reinforcements in the polymer composites for both non-structural and structural applications, including but not limited to decking, doors, window frames, flooring, fencing, walls, furniture, automobiles and electronic products [1
Kenaf is a warm season, annual fiber crop. It grows in large amounts every year in the United States. For example, the growth of kenaf was about 2300 acres in 1998, 5600 acres in 1999 and there were as much as 7000 acres of kenaf in 2000 in Texas [3
]. It is an underutilized biomass in Texas. Kenaf bast fiber is attractive also due to its high cellulose content and good mechanical properties. The cellulose content of kenaf bast fiber is about 46% to 57% [4
]. The tensile strength and modulus of a single kenaf fiber can be as high as 11.9 and 60.0 GPa, respectively [5
]. Thus, kenaf bast fiber is an excellent resource for newsprint, bond paper, etc.
It is also a good reinforcement candidate for the natural fiber reinforced composites for automotive applications [6
In fabricating thermoplastic polymer composites with the natural fiber reinforcements, the compatibility between the natural fibers and the thermoplastic matrices is a major issue for the natural fiber composites [13
]. Lignocellulosic natural fibers are hydrophilic, containing strongly polarized hydroxyl groups, which are incompatible with hydrophobic thermoplastics. Fiber pulled out is often observed at the flexural surface of natural fiber reinforced polymer composites [16
]. Fiber surface modification, such as plasma treatment can introduce the chemical functional groups, which make natural fiber more compatible with polymer [17
]. Simultaneously, fiber and polymer matrix breakdown can be observed at the flexural surface of treated natural fiber composites. Second, there are many micropores in the cell wall structure of natural fibers. Additional micropores are created during the chemical treatments or pulping due to the removal of some lignin and hemicellulose of the natural fibers [19
]. The presence of these micropores in the cell wall could cause manufacturing defects, such as interfacial failure and air pockets, in the composites.
If the nanoparticles can be introduced onto the fiber surfaces serving as attraction force manipulators to polymer matrixes, the nanoparticles will have a potential to improve the affinity between the natural fiber and the polymer matrix, and thus the physical and mechanical properties of the composites can be enhanced. Therefore, in order to improve the compatibility between natural fibers and thermoplastics and reduce the air pocket defect, micro or nano sized particles can be introduced into the micropores of the fiber cell wall structure through an impregnation process to fill those pores. The nanoparticle impregnation could not only fill the micropores of the fiber cell wall structure minimizing the air bubble defects of the composites, but also introduce the nanoparticles onto the fiber surfaces serving as the affinity sites to improve the compatibility at the fiber and polymer interfaces [20
We have developed an in situ
inorganic nanoparticle impregnation (INI) process to obtain high quality kenaf bast fibers [20
]. The objective of this study is to investigate the effect of impregnated inorganic nanoparticles in the kenaf bast fibers on the fiber properties, such as morphology, chemical components, surface roughness and modulus.
2. Experimental Section
Materials: In the fiber retting experiments, Kenaf stems (Hibiscus cannabinus, L.) were obtained from Kengro Incorporation, Charleston, MS, USA. After the bast and core were separated using a crushing process, the bast was cut into 5.1 cm in lengths. The bast was then dried until an 8% moisture content was achieved. The dried bast was stored in an environment of 22 °C and 50% relative humidity. The retting chemicals, such as sodium hydroxide, were in lab grade, and the acetic acid was in regent grade. The inorganic nanoparticle impregnation chemicals, such as sodium carbonate and calcium chloride, were purchased from Thermo Fisher Scientific Inc. (Waltham, MA, USA).
Chemical Retting of Kenaf Fiber: Kenaf bast fibers with about 8% moisture content were retted with 5% NaOH solution (Fiber:NaOH = 1:30, g/mL) in a hermetical reactor (Parr Instrument Co., Moline, IL, USA, 251M) for an hour at 160 °C and 0.6 MPa. The retting process was taken under the mechanical stirring. The kenaf fibers after the chemical retting were neutralized with 5% acetic acid, washed with water to remove excessive chemicals, and then oven dried.
Inorganic Nanoparticle Impregnation of Kenaf Fiber: Two steps of the ionic salt treatment in the heated and pressurized chamber were used for the impregnation process. The fibers were first impregnated with 0.1 mol/L Na2CO3 (primary salt) (Fiber:Na2CO3 solution = 5:400, g/mL) in a hermetical reactor with mechanical stirring at 70 °C and 0.1 MPa for 30 min. Then secondary ionic salt solution, CaCl2, was added into the reactor. The CaCl2 impregnation process was taken place at 160 °C and 0.7 MPa for 15 min. The molar ratio of Na2CO3:CaCl2 was 1:2. The primary ionic salt (Na2CO3) reacted with the secondary ionic salt (CaCl2) in the fibers to generate CaCO3 nanoparticles in the micropore structure of the fiber cell wall. The nanoparticle crystals may grow out onto the fiber surface from the inner cell wall. The impregnated fibers were washed with distilled water to remove the excess CaCO3 particles and other ions on fiber surface, and then oven dried.
X-ray Photoelectron Spectroscopy (XPS): XPS analysis was performed using a PHI 1600 XPS Electron Scanning Chemical Analysis instrument (Physical Electronics Inc., Chanhassen, MN, USA) with a PHI 10-360 spherical detector. An achromatic Mg K_alpha X-ray source was operated at 300 W and 15 kV. XPS data was collected with PHI surface analysis software version 3.0 and analyzed with CasaXPS analysis software version 2.2.88. High resolution scans were energy referenced to C 1s CHx environment at 285 eV. The specimens were left inside the vacuum chamber for overnight in order to degas them.
Scanning Electron Microscopy (SEM): A JEM2100 field emission scanning electron microscope (FESEM) (JEOL USA Inc., Peabody, MA, USA) was used to study the effects of retting and INI on the morphology of the kenaf fibers. An attached X-ray energy dispersive spectrometer (X-EDS) was used to obtain elemental compositions of CaCO3 nanoparticles in the composites. The SEM samples were coated with gold before SEM measurements. The electron beam spot size used in X-EDS was about 5 nm in diameter.
Atomic Force Microscopy (AFM):
A Bruker Dimension Icon AFM (Bruker Corporation, Camarillo, CA, USA) with ScanAsyst™ (Bruker, Banner La, UK) was used to image the surface topography as well as to determine modulus of the fibers. Images were recorded in the recently released PeakForce QNM (Quantitative NanoMechanics) imaging mode using typical silicon tips (Tap525A, spring constant 200 N/m, Veeco, Santa Barbara, CA, USA). Imaging was performed under ambient conditions. The PeakForce QNM mode relies on the information available in AFM force-distance curves, and the maximum force applied to the sample by the tip is constant. The deflection of the cantilever at this maximum force leads to topography mapping, the stiffness/modulus is extracted from the slope of the retraction curve near zero separation, and the adhesion pull-off force comes from the minimum in the retraction curve [23
]. The image was analyzed using the AFM software (Veeco Instruments, version 6.13).