The effective utilization of original natural fibers as indispensable components in natural resins for developing novel, low-cost, eco-friendly biocomposites is one of the most rapidly emerging fields of research in fiber-reinforced composite. The objective of this study is to investigate the interfacial adhesion properties, water absorption, biodegradation properties, and mechanical properties of the kenaf/soy protein isolate- (SPI-) PVA composite. Experimental results showed that 20 wt% poly (vinyl alcohol) (PVA) and 8 wt% glutaraldehyde (GA) created optimum conditions for the consolidation of the composite. The increase of interfacial shear strength enhanced the composites flexural and tensile strength of the kenaf/SPI-PVA composite. The kenaf/SPI-PVA mechanical properties of the composite also increased with the content of cross-linking agent. Results of the biodegradation test indicated that the degradation time of the composite could be controlled by the cross-linking agent. The degradation rate of the kenaf/SPI-PVA composite with the cross-linking agent was lower than that of the composite without the cross-linking agent.
Biodegradable polymers that are derived from natural resources are potential substitutes for existing petroleum-based synthetic polymers, owing to their low cost, easy availability, and complete biodegradability [
Soy protein is commercially available as SPI, soy protein concentrate (SPC), and soy flour (SF). Chemically, SPI contains 90% protein, while SPC contains 70% and 18% carbohydrates and 6% ash, with fiber and moisture making up the remaining components. SF contains up to 55% protein and 32% carbohydrates. Soybean protein contains several amino acids, such as glutamic acid, arginine, lysine, cysteine, and aspartic acids, which have polar groups. These groups can act as useful cross-linking and/or hydrogen bonding sites to improve the mechanical properties of soy protein polymers. In the present research, SPI was modified with GA to increase its mechanical and physical properties and to improve thermal stability and processability as a matrix for composite fabrication. Several researchers have studied the cross-linking of GA with proteins and confirmed the reaction mechanisms [
Schematic structure of the glutaraldehyde (GA) cross-linked soy flour.
Another approach to modify the moisture sensitivity of SPI films is to blend other natural or synthetic polymers with SPI materials [
Biocomposites composed of natural fibers and synthetic or natural polymer matrices have gained recent attention due to their cost effectiveness, low density, biodegradability, ready availability, energy recovery, and CO2 sequestration [
Among others, kenaf offers the particular advantages of a fiber crop, including rapid growth in various climatic conditions and, subsequently, the prompt accumulation of carbon dioxide [
Until recently, studies on SPI/PVA blends reported the use of plasticizers and cross-linking agents to increase mechanical and physical properties, improve thermal stability, reduce moisture absorption, and improve processability as a resin for composite fabrication. Biocomposites reinforced with kenaf fiber that use a SPI/PVA blend have not been reported, despite their many advantages of biodegradability, biocompatibility, chemical resistance, and excellent physical properties. In this study kenaf nonwovens were used to fabricate environment friendly biocomposites using modified SPI-based resins. SPI was modified using PVA in order to improve the interfacial bonding of kenaf nonwoven/SPI and then cross-linked with GA to improve its mechanical properties and water resistance. The biodegradation of the biocomposites was analyzed to determine the effect of PVA and the cross-linking agent on the composting of the kenaf composites.
SPI with an approximate protein content of 90% was supplied by Solea Company, USA. GA (grade II, 25% solution), glycerol, and PVA (98% hydrolyzed, average molecular weight (Mw) of 89,000–98,000) were obtained from Sigma-Aldrich Chemical Company. Kenaf nonwovens with an areal density of 400 g/m2 were obtained from KumHa Co. Ltd. (Korea).
The SPI/PVA films were prepared using a casting method. First, the SPI (10 wt%) and PVA (5 wt%~20 wt%) were dissolved in water (pH 8) and cured at 70°C for 25 minutes. Second, 20 wt% glycerol was added according to the optimum condition [
Kenaf nonwovens were dewaxed by soaking them in a mixture of ethanol and benzene (1 : 2) at 50°C for 5 hours and then washed with distilled water and air dried. The dewaxed nonwovens were immersed in a 10 wt% aqueous sodium hydroxide solution at 30°C for 1 hour and then washed with distilled water and dried. To remove any remaining moisture, the pretreated kenaf nonwoven was cut into 20 cm by 20 cm pieces and dried at 100°C for 2 hours in a dryer and at 80°C for 3 hours in a vacuum oven. Kenaf nonwoven reinforced composites were fabricated using SPI (10 wt%)/PVA (15 wt%) resins added glycerol (20 wt%)/GA (0 wt%~25 wt%). The manufacturing process of the SPI/PVA film and kenaf/SPI-PVA composite are shown in Figures
Manufacturing process of the SPI/PVA film.
Manufacturing process of the kenaf/SPI-PVA composite.
The initial weight of the kenaf/SPI-PVA composite with the cross-linking agent was measured and the specimen submerged in distilled water at 25°C for 12 hours. Water absorption was calculated using (
To investigate the effects of PVA and the cross-linking agent on the mechanical properties of SPI, tensile and flexural (three-point bending) tests were performed at a cross-head speed of 5 mm/min using an Instron (model 4467), according to ASTM D638 and ASTM D790, respectively. The value of each mechanical property was determined by an average of ten specimens.
The interfacial shear strength (IFSS) between the kenaf fiber and SPI was measured by a microdroplet debonding test using an Instron test system (model 4467) equipped with a 500 N load cell. The microdroplet test specimen of kenaf fiber with SPI was made to composite at 80°C for 24 hours. Figure
Photograph and schematic diagram of the microdroplet debonding test sample and test setup.
The surface and fracture morphologies of the kenaf/SPI-PVA composite were observed with a scanning electron microscope (SEM, S4700, HITACHI).
A Fourier transform infrared spectrophotometer (FT-IR, Bruker Optic GmbH, ALPHA-P) equipped with an attenuated total reflectance (ATR) accessory was used to examine the surface composition of the uncross-linked and cross-linked kenaf/SPI-PVA composite. The spectra were recorded in the transmission mode in the range of 4000–500 cm−1. FT-IR spectra were measured at a spectral resolution of 4 cm−1, and the spectra were obtained with an accumulation of 128 scans for a high signal-to-noise ratio.
Test specimen of the kenaf/SPI-PVA composites containing the cross-linking agent was prepared in equal amounts of the size of 20 mm × 20 mm. They were dried in a vacuum oven for 2 hours. Specimens were mixed with the compost in a test bath (as shown in Figure
Schematic diagram of biodegradation test chamber.
Figures
Tensile properties of the SPI film without PVA content.
Tensile properties of the SPI/glycerol film with PVA content.
The FTIR spectra of the SPI/glycerol film with and without PVA are shown in Figure
FTIR spectra of SPI/glycerol without and with PVA.
Figure
Figures
SEM photographs of surface of (a) SPI/glycerol without PVA, (b) SPI/glycerol film with PVA.
SEM photographs of the fractured surface of the SPI/PVA film after the tensile test are shown in Figure
SEM photographs of tensile fracture surface of (a) neat SPI/glycerol film, (b) SPI/glycerol film with PVA (5 wt%), (c) SPI/glycerol film with PVA (15 wt%), and (d) SPI/glycerol film with PVA (20 wt%).
The FTIR spectra of the uncross-linked and cross-linked kenaf/SPI-PVA composite are shown in Figure
FTIR spectra of the uncross-linked and cross-linked kenaf/SPI-PVA composite.
The interfacial adhesion strength was examined whether the addition of GA could improve adhesion between the kenaf fiber and the SPI. The average of thirty specimens was used to evaluate the IFSS as a manifestation of adhesion strength. The IFSS of the kenaf/SPI-PVA composite according to the added amount of GA is shown in Figure
Interfacial adhesion properties of the kenaf/SPI-PVA composites with GA content.
The effect of the IFSS on tensile properties of the kenaf/SPI-PVA composites is shown in Figure
Relationship between tensile strength and IFSS of the kenaf/SPI-PVA composites.
Flexural and tensile properties of the kenaf/SPI-PVA composite with GA are shown in Figures
Flexural properties of the kenaf/SPI-PVA composites with GA content.
Tensile properties of the kenaf/SPI-PVA composites with GA content.
In Figure
Water absorption properties of the cross-linked kenaf/SPI-PVA composite with the cross-linking agent after immersion in a water tank at 25°C for 12 hours are shown in Figure
Water absorption of cross-linked kenaf/SPI-PVA composites with GA content.
SEM photographs of the surface and fracture surface of the kenaf/SPI-PVA composites are shown in Figure
Surfaces (left, ×500) and fracture surfaces (right, ×100) of the kenaf/SPI-PVA composites with GA content: (a) 0 wt%, (b) 8 wt%, (c) 16 wt%, and (d) 25 wt%.
The weight changes of the kenaf/SPI-PVA composite specimen, according to biodegradability, are shown in Figure
Weight loss (%) of (a) kenaf/SPI-PVA composite without GA and (b) kenaf/SPI-PVA composite having 8 wt% of GA with compost time.
Photographs of the specimen under biodegradation conditions are shown in Figure
Photographs of the kenaf/SPI-PVA composite with compost time: (a) kenaf/SPI-PVA composites without GA, (b) kenaf/SPI-PVA composites with GA.
In this study, the kenaf/SPI-PVA compositeswere prepared with plasticizers and a cross-linking agent. Their interfacial adhesion properties, water absorption, biodegradation, and mechanical properties were analyzed. Results were as follows: Increase of tensile strength in the kenaf/SPI-PVA composites indicated a good adhesion between kenaf fiber and SPI/PVA when GA of 8 wt% was added to the kenaf/SPI-PVA composites. Through the use of the cross-linking agent, the water absorption of the kenaf/SPI-PVA composite decreased. When GA 16 wt% was added, water absorption of the kenaf/SPI-PVA composite decreased significantly. The optimum preparation condition for the kenaf/SPI-PVA composite was established at PVA 15 wt% as a plasticizer and GA 8 wt% as a cross-linking agent. In the biodegradation test, degradation was controlled by the cross-linking agent GA. The degradation rate of the kenaf/SPI-PVA composite with GA was lower than the composite without GA, because the cross-linking between the kenaf fiber and SPI/PVA in the composite restricted its biodegradation.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This research was financially supported by the Fundamental R&D Program for Technology of the Graduate Student Education Program for Research of Hybrid and Super Fiber Materials through the Ministry of Trade, Industry & Energy (MOTIE), and Korea Institute for Advancement of Technology (KIAT) (N0000993).