Air-Plasma treatment of bamboo fiber for polypropylene composite application

196 Journal of Chemistry, Vol. 45 (5A), P. 196 - 200, 2007 Air-plasma treatment of bamboo fiber for polypropylene composite application Received 16 August 2007 Ta thi Phuong Hoa, Do Thi Cuc, Nguyen Hoang An Polymer Centre, Hanoi University of Technology summary In this study, the air-plasma generated by radio-frequency was used to treat bamboo fiber. The treatment parameters were studied to find the suitable condition. The testing on tensile strength, tensile modulus, module Weibull and elongation to break of the fiber, the measurement of contact angles and the SEM analysis showed that at the high frequency of 12 kHz, power 50 W and treatment time of 5 minutes, after air- plasma treatment the tensile strength and tensile modulus of bamboo fiber increased more than double (101.1% and 101.5%), the contact angles increased remarkably and treated fibers have cleaner surface. The adhesion between bamboo fiber and polypropylene (PP) as well as PP containing maleic anhydride grafted PP evaluated by droplet method showed that the interfacial shear strength (IFSS) of the treated fiber increased 18.2% at PP and 39.4% at PP-MA-PP compared with untreated fiber; SEM image showed a better adhesion between fiber and polymer. Compared to treatment by silane coupling agent, air- plasma treatment can improve both the mechanical properties of fiber and adhesion while silane coupling agent only the adhesion. I - Introduction In recent years there has been renewed the interest in using the natural fiber to replace glass fiber to get polymer composite which is eco- friendly. Vietnam is the country where bamboo is abundantly available and of diversified species, polypropylene (PP) has a wide range of application and a high value of property/price therefore there is a good potential to use bamboo fiber for polypropylene composite application to get a motivated combination of environmental friendliness and economical feasibility. However, due to very different natures of natural fiber and polymer there is a poor wet ability and weak interfacial adhesion between fiber and polymer which leads to poor mechanical properties of composite. Surface treatment of reinforcement fiber is one of the effective solutions. Recently cold plasma treatment, which is environmental friendly, is considered as a promising physical treatment method to modify the fiber surface for a better adhesion. In this paper air-plasma has been used to treat bamboo fiber surface for PP-composite application. The mechanical properties, morphology of fiber and adhesion between fiber and PP have been investigated and compared with that of silane treatment [1, 2, 4]. II - Experimental 1. Material - Bamboo fiber was separated from bamboo using mechanical method. - Polypropylene. - Yeniosil Silane GF31. 2. Plasma treatment 197 Usually surface treatment is under low- pressure. However this study is carried out at atmosphere pressure and room temperature in order to simplify as much as possible the technology for application. Plasma is generally created by supplying a sufficient amount of energy to a volume containing a neutral gas. The energy may be supplied in the form of electrical energy, heat, ultra violet radiation or particle beams. In technical plasma device, the input energy is generally supplied as electrical energy [1, 2]. The major structural components of self- made plasma device for this study are a bell (chamber) and an excitation source (Fig. 1) connecting with gas line, vacuum pump and pressure controller. Fig. 1: Plasma treatment device Bamboo fibers reside directly in the chamber housing the plasma. The chamber was filled by air. Plasma excitation sources in this test having high frequency of 12 KHz with the plasma power of 50 W. The investigation was carried out at room temperature, atmospheric pressure. 3. Silane treatment The bamboo fibers were treated with a silane solution (the ratio 10 water to 90 methanol, 0.5% dicumine peroxide, 3% silane by weight). CH3COOH solution was used for adjusting the pH of around 3.5 to 4. After 1 hour treatment fibers were dried at room temperature and then heated for 1 hour at 120 degree Celsius. 4. Tensile test of fiber Bamboo fibers were separated from bamboo and cut approximately 40 mm in length. Both fiber ends were glued on the pieces of paper (paper tabs of size 40x40 mm) for handing purposes. During pulling, the specimens were handled only by paper tabs and the working zone of the fiber was not touched. Before experimenting, fiber diameter was measured on optical microscope with an optical objective of 4-40 times magnification. The test was carried out on a computer connected LLOYD LRX Plus machine. Measurements of load and displacement were used to compute stress strain curves for the fibers. All tests were displacement controlled with the loading rate of 2 mm/min. 5. Determination of Fiber to Resin Interfacial Shear Strength The mechanical adhesion between reinforcing fibers is usually characterized by the interfacial shear strength (IFSS) determined by such test methods as fiber pull-out, microdebond test, fiber fragmentation test etc. In this study droplet pull-out test was conducted to evaluate the interfacial shear strength (IFSS) between the fiber surface and resin. Fibers were dried to allowable moisture content and mounted on the cardboard. Drops of PP resin was heated to melting point and then dripped directly on the fiber. It would solidify in several seconds. The maximal dimension of the drops and embedded length of the fiber were measured using an optical microscope with a gratitude. The separation of the droplet from the fiber surface was made possible by use of two blades adjusted laterally using vernier gauges. Resistance of the droplet against the blades provided the necessary force for generating the shear stress between the resin droplet and the fiber surface. The IFSS,
, is estimated as : Cl d 2 .
= 198 where: d is the fiber diameter, m;  is the average fiber strength at critical, MPa; lC is the critical length related to the average fiber length at saturation fragmentation process, l as: ll C 3 4 = with lC is the average embedded length [3]. Fig. 2: Pull-out test III - Results and discussion 1. Effect of plasma treatment on the mechanical properties of fiber Table 1 presents some mechanical properties of bamboo fibers after plasma treatment with various times. Tab.1: Effect of plasma treatment on the mechanical properties of bamboo fiber Plasma treatment time, min 0 1 5 7 Mean stress (), MPa 181.8 357.8 365.7 235.9 Young’s modulus, GPa 19.72 27.79 39.73 26.32 , % 2.00 1.84 1.48 1.55 Weibull modulus (m) 1.54 1.52 1.44 1.38 Normalizing stress (0), kPa 15.57 16.81 25.38 98.15 The results show a significantly increase of tensile property of fibers after air-plasma treatment. Namely, mean tensile stress of untreated fiber was 181.84 MPa and of treated were 357.87 MPa (after 1 minute) and 365.77 MPa (after 5 minutes), respectively. Thus, stress increased more than double after treated 5 minutes (up to 101.1%) In the same conditions, Young’s modulus increased 101.5% and elongation decreased 25.6%. The tensile fracture surface of bamboo fiber was observed in the scanning electronic micoroscope and shown on the figure 2. We can see that each bamboo fiber was even if small (the average diameter was about 0.1 to 0.4 mm) also set from many single fibers. The fiber bundles arranged quite tightly and isotropic so that sensating as a single fiber. However, the orientation and bond on the whole fiber wasn’t the same and fiber would be demolished at the point that the bond was weakest. Fig. 3: SEM image of the tensile fracture surface of bamboo fiber When producing plasma, there are not only charged and neutral particles bombarding samples, but it also produced light, such as UV and others which may cause the polymerization of lignin, making a significant increase in mechanical properties of fiber due to polymerized lignin- cellulose composite fiber structure. 2. Contact angle It can be seen that plasma treatment can improve the contact angle of fiber with ethylene glycol at nearly the same level as silane treatment, however less in the case of water. 199 Tab. 2: Contact angle of bamboo fiber with water and ethylene glycol Water Ethylene glycol Untreated 39.3 39.1 1 56.4 42.4 2 64.5 43.2 Silane treatment, %silane 3 66.9 44.5 Plasma treatment, 1min 49.4 44.0 3. Interfacial Shear Strength (IFSS) The results in the table 3 show the slightly increasing of IFSS value between bamboo fibers with resin after air plasma treatment. Tab. 3: IFSS between bamboo fibers with PP Plasma treatment time, minAdhesion Un- treated 1 5 7 Silan treatment (3% w) Specific shear strength, MPa 2,06 2,25 2,28 2,18 2.72 IFSS, MPa 3,09 3,38 3,41 3,09 4.3 With un-modified PP, IFSS between fiber and resin increased from 3.085 MPa of untreated fiber to 3.38 MPa after 1 minute treatment, to 3.41 MPa after 5 minutes treatment and then decreased to 3.09 MPa (plasma treated for 7 minutes). With PP grafted MAPP (5% MA), adherence increased from 3.12 MPa to 3.31 MPa of 1 minute treatment, to 3.41 MPa at 5 minutes and got the highest value of 4.35 MPa with plasma treated for 7 minutes, higher than that of silane treatment. It can be seen that for PP plasma treament can increase IFSS only 9.7%, however for PP grafted MAPP 39.52%. 4. Bamboo fiber surface morphology There was clear difference on the plasma untreated fiber surface (a), plasma treated (b) and silane treated (c). Plasma treatment caused a cleaner and smoother fiber surface as showed in Fig. 4. Tab. 4: IFSS between bamboo fibers with PP grafted MAPP Plasma treatment time, min Adhesion Un- treated 1 5 7 Silan treatment (3%w) Specific shear strength, MPa 2,08 2,1 2,27 2,94 2.76 IFSS, MPa 3,12 3,31 3,41 4,35 4.13 In addition, plasma treatment lost the sets of non-orientation on the fiber’s surface. Therefore, the single fibers in the fiber’s bundle would be oriented more closely. That’s fit to increase significantly strain strength of fiber after treated. Simultaneously, the interstice between very close two single fibers also became wider and deeper creating conditions adherence better with resin but unremarkably. Silane treatment also lost the sets of non- orientation on the surface of fiber but the effect got lower at plasma treatment. Silane treatment only made fiber’s surface changed but without changing the inner fiber’s structure, therefore, strain strength of silane treated wasn’t almost changed. IV - Conclusions Air plasma treatment has increased remarkably the tensile properties of bamboo fiber. Both mean stress and Young’s modulus in suitable condition increased more than 101.15 and 101.5%. Plasma treatment also has changed the surface of fibers for a better adhesion. 200 (a) (b) (c) Fig. 4: SEM showing the surface of fiber untreated (a), plasma treatment (b) and silane treatment (c) The adhesion between bamboo fiber and PP has improved. The effect of plasma treatment at PP grafted MAPP is higher than at PP, however less than that of silane treatment at suitable treatment condition of 5 minutes. References 1. Essay (2005). Low-Temperature Plasma Processing of Materials: Past, Present and Future. Plasma Processes and Polymers, Vol. 2: 7 - 15 (2005). 2. D. Sun, G. K. Stylios. Textile Research Journal. Vol. 75(9), P. 639 - 644 (2005). 3. Arnold N. Towo, Martin P. Ansell. Third International Workshop on Green Composites. P. 190 - 194 (2005). 4. X. J. Dai, L. Kviz. Study of Atmospheric and Low Pressure Plasma Modification on the Surface Properties of Synthetic and Natural Fibres, Textile and Fibre Technology (2001).