The influence of cellulose nano fibers extracted from the fruit of luffa cylindrica (LC) on the tensile, flexural and impact properties of composite materials using poly lactic acid (PLA) processed by micro compounding and injection molding was studied. Preliminary results suggested promising mechanical properties. The impact strength, tensile strength and flexural strength of the composites increased with incorporation of very low content of LC fiber up to 2 wt%. But when the wt of LC fiber in the composite increased (5 wt% and 10 wt%), mechanical strength of the composites reduced probably due to agglomeration of cellulose fibers. However, modulus of composites was enhanced with increase in wt of fiber content in the composites. Before reinforcement, the LC fibers were modified with calcium phosphate in order to explore the possibilities of using these composites in biomedical industries. The novelty of this work is that there is no use of compatiblizer and coupling agent during the processing so that the cost of processing is reduced.
The development of bio composites started in 1980s. Now in the 21st century, increased environmental consciousness demands more renewable based and biodegradable polymers in every sphere. Investigations are still going on to see that these bio composite materials become fit to the life cycle of nature as well as fit into the system of sustainable development. Composite materials consist of one or more discontinuous phases (reinforcements) embedded in a continuous phase (matrix). Bio composites use natural fiber as reinforcement and renewable polymers as matrix. Fully bio degradable polymers like startch, poly hydroxy alkanoates, poly lactic acid and soy based plastics are currently investigated by many research workers throughout the globe [
Nearly 830 million tons of celluloses are produced each year through photosynthesis. Considering that an average plant contains 40% cellulose, the annual bio-based resource will be approximately 2000 million dry tons. Cellulose-based polymer composites are characterized by low cost, desirable fiber aspect ratio, low density, high specific stiffness and strength, biodegradability, flexibility during processing with no harm to the equipment, and good mechanical properties. Unlike synthetic fiber, they reduce the wear of machinery. The tensile strength of natural fibers is substantially lower than that of synthetic fibers but the modulus is of the same order of magnitude. However, when the specific modulus (modulus per unit volume) of natural fibers is considered, the natural fibers show values that are comparable to synthetic fibers [
Use of cellulose fibers in thermoplastics in general has not been extensive due to their low thermal stability during processing, poor dispersion in the polymer matrix, and limited compatibility with the coupling agents. Good interfacial adhesion between the matrix and fiber transfers the stress from the matrix to the fibers improving the mechanical properties of the composites. Much attention has been focused on modification of the fiber by physical and chemical methods. Therefore, the main objective of our study is to evaluate the mechanical properties of PLA-cellulose based “green” composites. The current research uses the fruit of LC, a common tropical fruit of India, as reinforcement in composite materials producing green composites. The novel aspect is in terms of modification of surface of LC fibers by Ca salts, before using these fibers as reinforcement. The modification of the surface of the fiber by Ca salts opens the possibility in using these composites in bio medical applications. Calcium phosphate based bio-materials are extensively used for bone replacement, dental filling, bone tissue engineering, drug delivery etc. [
Kakar et al. in 2015 [
Poly lactic acid (PLA) of grade 4042D (molecular weight Mw ~ 600,000), was purchased from Nature Works, USA. The LC fiber was collected from local forest area. The chemicals such as calcium chloride (CaCl2∙2H2O, 97%), di sodium hydrogen phosphate (Na2HPO4∙2H2O, 99.5%), sodium hydroxide (NaOH), sodium hypochlorite (NaClO) all of AR grade were procured from E. Merck, India.
The fibers of LC were cut into small pieces of length around 2 cm, washed thoroughly to remove impurities like oil, dust etc. These were left for drying at 70˚C in vacuum oven for 20 minutes. The dried LC fibers were subjected to chemical treatments like treatment with alkali followed by bleaching and acid hydrolysis. For alkali treatment, the LC fibers were soaked in a 5% NaOH solution at 80˚C for 1 h. The hydrophilic nature of the natural fibers leads to poor adhesion between fiber and matrix and this is the main drawback in fabrication of composites. In wet conditions, therefore, such composites show very poor mechanical properties. To improve interfacial bonding and to reduce moisture absorption the LC fibers were subjected to treatment with NaOH so that the hemicelluloses and lignin present in the fibers were extracted. In this way the number of −OH groups present in the fiber was reduced leading to increase of hydrophobicity of fibers which strengthen the bonding between fiber and matrix. The alkali treated LC fibers were then bleached with 2% sodium hypochlorite solution. The mixture was continuously stirred for 2 h at 80˚C. Bleaching is mainly used to increase whiteness of the fibers. For acid hydrolysis, the bleached LC fiber/water suspension was prepared and kept on an ice bath. H2SO4 was added slowly under continuous stirring to the suspension placed in an ice water bath, until the final concentration of 60% H2SO4 was reached. The obtained suspension was then heated at 45˚C under continuous stirring for 2 h. In order to remove excess acid the mixture was centrifuged using an ultracentrifuge at 30˚C for 20 minutes with 7000 rpm. Acid hydrolysis leads to the isolation of micro and nano-fibers with a high degree of crystallinity by removing the amorphous regions of the raw cellulose material. Acid hydrolysis decreased the degree of polymerization (DP) and molecular weight of the bleached fibers [
The dried LC fiber was immersed in CaCl2 solution for 12 h at room temperature of 30˚C to deposit Ca on its surface. The LC fiber modified with CaCl2, were re-immersed in Na2HPO4 solution for 12 h at room temperature to deposit compounds of calcium phosphate over it [
Prior to use, the PLA pellets and LC fiber were dried under vacuum at 80˚C for 24 h. The polymer and fiber were mixed mechanically at 100 rpm with a micro-compounding molding equipment (DSM Micro 15 cc compounding system, DSM research, The Netherlands) at 170˚C for 10 minutes. This extruder is equipped with a screw of length 150 mm, L/D of 18, and net capacity of 15 cc. The molten composite samples were transferred after extrusion through a preheated cylinder to the mini injection molder in order to obtain the desired specimen samples for various measurements and analysis. The PLA pellets and LC fibers are mixed in different wt proportion which results in B0, B1, B2 and B3 samples. B0 is the neat PLA. In B1, B2 and B3 samples the LC fibers are in the wt ratio 2%, 5% and 10% respectively. There is no use of compatibilzer and coupling agent during the processing.
Ni filtered Cu Kα radiation having wavelength 0.1542 nm was generated at 40 KV and 35 mA using WXRD/ SHIMADZU/JAPAN. The X-ray diffractograms were recorded from Bragg angle 10˚ to 80˚ at room temperature of 28˚C by goniometer equipped with scintillation counter at a scanning speed of 10˚/minute.
The tensile and flexural properties of the composite specimen were measured with Universal testing machine, (3382 Instron, UK) according to ASTM D638 and ASTM D790 respectively. System control and data analysis were performed using Datum software. Notched izod impact strength of the specimens was evaluated using an impactometer (Tinius olsen, USA) as per ASTM-D 256 with a notch depth of 2.54 mm and notch angle of 45˚. All results given are the average values of five measurements.
XRD pattern of injection molded PLA, reinforced with chemically treated LC fiber at 5 wt% (sample B2) is shown in
The natural fibers are rigid in nature and their rigidity is more compared to neat PLA which leads to increase in modulus of the composites with the incorporation of fibers. Hence reinforcement of LC fibers in to the PLA matrix will enhance the modulus of all the composite samples. The ductile behavior of all the composite samples, B1, B2, B3 were found to be reduced due to incorporation of rigid fibers. Data in
Impact strength of composite materials are indicators of toughness of the materials. PLA is a brittle polymer and the low impact strength of PLA polymers is the main drawback for technical applications. Therefore most current
Sample | Tensile stress at maximum load (MPa) | Tensile strain at yield (%) | Tensile modulus (MPa) | Energy at maximum load (Joules) |
---|---|---|---|---|
B0 | 30.425 | 1.32 | 2242.61 | 1.1234 |
B1 (2%) | 36.447 | 1.58490 | 2827.61997 | 1.29249 |
B2 (5%) | 35.603 | 1.44661 | 2930.452 | 1.14323 |
B3 (10%) | 33.298 | 1.38646 | 2997.452 | 1.03722 |
Sample | Flexural stress at maximum flexure load (MPa) | Flexural strain at yield (%) | Flexural modulus (MPa) |
---|---|---|---|
B0 | 36.12 | 1.12 | 3226.12 |
B1 (2%) | 48.64381 | 1.43 | 3624.819 |
B2 (5%) | 42.182 | 1.151 | 3939.352 |
B3 (10%) | 37.206 | 1.16 | 3765.363 |
research on PLA bio composite seeks to improve the impact property to a level that satisfies some specific applications. Data in
Sample | Break value (in J) | Strength (J/m) |
---|---|---|
B0 | 0.0564 | 22.6543 |
B1 | 0.0902 | 28.1937 |
B2 | 0.0481 | 15.0614 |
B3 | 0.0364 | 15.0181 |
A maximum of 19% enhancement in tensile strength and a maximum of 34.66% enhancement in flexural strength were reported for sample B1 (2 wt%) compared to that of the neat PLA (sample B0). This enhancement was achieved only with reinforcement of very low content of treated LC fiber in the composites and without using any compatibilizer or coupling agents, thereby reducing the cost of processing leading to very light wt materials having high strength to mass ratio. Use of DSM micro compounding molding techniques reduced the experimental processing time. The present work transformed the low priced, readily available and agricultural product LC fiber into a high value product with low cost and low processing time. The maximum tensile stress was reported to be 36.447 Mpa and this value is comparable to tensile stress of a soft tissue, articular cartilage [
The authors thank Laboratory of Advanced Research in Polymeric Materials (LARPM), CIPET, Govt. of India for providing facilities and equipments for making this study.
ChhatrapatiParida,Sarat KumarDash,PinakiChaterjee, (2015) Mechanical Properties of Injection Molded Poly(lactic) Acid—Luffa Fiber Composites. Soft Nanoscience Letters,05,65-72. doi: 10.4236/snl.2015.54008