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Jayamani, E., Hamdan, S., Rahman, M. R., Heng, S. K., and Bin Bakri, M. K. (2014). "Processing and characterization of epoxy/luffa composites: Investigation and chemical treatment of fibers on mechanical and acoustical properties," BioRes. 9(3), 5542-5556.

Abstract

This study focuses on the development of epoxy/luffa composites and the investigation of their mechanical and acoustical properties. The fibers underwent an alkalization treatment, and its effects on the mechanical and sound absorption properties of the composites were measured utilizing a universal testing machine and two-microphone transfer function impedance tube methods. The effects of chemical modifications on the fibers were studied using a scanning electron microscope (SEM). The thermal analyses of composites were conducted using thermo-gravimetric analysis (TGA). The composite’s functional group was identified and evaluated using Fourier transform infrared spectroscopy (FTIR). The sound absorption coefficient of untreated and treated composites across a range of frequencies was very similar. Untreated composites appeared to perform better than those that were treated. Compared with untreated fiber composites, there was an improvement in the tensile strength of the treated fiber composites. The SEM characterization showed that the alkaline treatment changed the morphology of the fibers, resulting in a decrease in the sound absorption coefficients of the composites. The thermal characterization of composites showed that dehydration and degradation of lignin occurred in a temperature range of 40 to 260 °C, and the maximum percentage of cellulose was found to decompose at 380 °C.


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Processing and Characterization of Epoxy/Luffa Composites: Investigation on Chemical Treatment of Fibers on Mechanical and Acoustical Properties

Elammaran Jayamani,* Sinin Hamdan, Md Rezaur Rahman, Soon Kok Heng, and Muhammad Khusairy Bin Bakri

This study focuses on the development of epoxy/luffa composites and the investigation of their mechanical and acoustical properties. The fibers underwent an alkalization treatment, and its effects on the mechanical and sound absorption properties of the composites were measured utilizing a universal testing machine and two-microphone transfer function impedance tube methods. The effects of chemical modifications on the fibers were studied using a scanning electron microscope (SEM). The thermal analyses of composites were conducted using thermo-gravimetric analysis (TGA). The composite’s functional group was identified and evaluated using Fourier transform infrared spectroscopy (FTIR). The sound absorption coefficient of untreated and treated composites across a range of frequencies was very similar. Untreated composites appeared to perform better than those that were treated. Compared with untreated fiber composites, there was an improvement in the tensile strength of the treated fiber composites. The SEM characterization showed that the alkaline treatment changed the morphology of the fibers, resulting in a decrease in the sound absorption coefficients of the composites. The thermal characterization of composites showed that dehydration and degradation of lignin occurred in a temperature range of 40 to 260 °C, and the maximum percentage of cellulose was found to decompose at 380 °C.

Keywords: Composites; Natural fibers; Acoustic absorption; Mechanical Properties; Characterisation

Contact information: Department of Mechanical and Manufacturing Engineering, Universiti of Malaysia Sarawak, Kota Samarahan, 94300, Kuching, Sarawak, Malaysia;

* Corresponding author: elammaranj@gmail.com

INTRODUCTION

Asbestos was one of the first materials used in many industrial applications. According to the 12th Report on Carcinogens (NTP 2011), there are six common types of naturally occurring mineral fibers inside asbestos that have been commercially exploited, namely crocidolite, anthophyllite, chrysotile, amosite, tremolite, and actinolite. Thermal, electrical, and sound insulation made from asbestos materials is widely used in industrial applications. This application has been exploited by increasing the absorption capacity, wear durability, and frictional properties. These indirectly enable the fabrication of paper and felt-type asbestos materials for flooring and roofing product for sound insulation (NTP 2011). However, the use of asbestos material poses a threat to human health and the environment. Because of the health hazard posed by asbestos, the use of asbestos in most applications has been banned in most countries in the world. For example, the members of the European Union voted to ban asbestos use by late 2005 (Kogel et al. 2006). Because of these tight restrictions, many industries have stopped using asbestos materials and pursued alternative materials, such as synthetic fibers.

Synthetic fibers are also called man-made fibers and can be created industrially. There are several hundred types of synthetic fiber in the world. Synthetic fibers are often manufactured with cellulose as the starting material (Rouette 2001). Although the manufacturing of synthetic fibers was meant to replace the wide usage of asbestos material in various industries, the results of studies that have been conducted in the laboratory indicate that synthetic fibres can possess the same human health hazard as asbestos material (Su and Cheng 2009). Because of this health risk, other alternatives have been investigated, such as the use of natural renewable fibres rather than synthetic fibres. With the recent increasing attention towards sustainability and environmental awareness, there is a large need to find clean, green, and sustainable materials that can be used as replacements; this is where natural biomass-derived fibres play a vital role. According to Manthey et al. (2010), such natural fibres are inexpensive, easy to process, renewable and they are recyclable. Luffa fiber is a light-weight natural material that has the prospective to be used as an alternative sustainable material for various engineering applications such as acoustic and vibration isolation, impact energy absorption, and packaging (Shen et al. 2013). Cellular materials with hierarchical microstructures have attracted much attention due to their excellent mechanical performance and the potential to achieve multi-functions such as vibration and shock isolation, thermal insulation, catalyst support, and acoustic absorption (Shen et al. 2014). Alkaline treatment, also known as alkaline mercerization, is the most commonly used chemical treatment of natural fiber composites in the preparation of thermoset and thermoplastic reinforced natural fiber composite material. In the alkaline treatment process, the network structure of the hydrogen bonding is altered due to reaction of sodium hydroxide (NaOH). This process is important for increasing the surface roughness of the natural fibres. According to Demir et al. (2006), the alkalization treatment of natural fiber improves adhesion and creates better mechanical properties of reinforced natural composite materials. Moreover, the alkaline treatment process can remove the wax, oils, and lignin at the cell wall surface of the natural fibers. However, there is limited research on the effect of the sound absorption coefficient due to the chemical treatment. Thus, more research on composite materials and natural fibres needs to be performed to better understand the effect of chemical treatment on the sound absorption coefficients.

According to Koizumi et al. (2002), bamboo fiber samples reveal similar sound absorption properties of glass wool fibres. The enclosing surface of bamboo fireboard materials yields high sound absorption properties compared to plywood materials, which have similar densities. The same result can also be seen in composite boards of randomly cut rice straws and wood particles (Mehta and Parsania 2006). It tends to exhibit higher sound absorption properties compared to particleboard, fireboard, and plywood in the frequency range of 500 to 8000 Hz. The use of composite materials made from plant fibres is currently receiving great attention. This is because reinforced natural fiber composites can be superior to reinforce synthetic fiber composites in certain properties, such as being lightweight, biodegradable, combustible, and recyclable. The good physical properties of natural fiber composites have ranked them among high-performance composites, which have environmental and economic advantages (Avella et al. 2000). Sound absorbing materials are chosen in terms of material types and dimensions and also based on the frequency of sound to be controlled (Simon and Pfretzschner 2004). Poly (l-Lactic acid) reinforced ramie fiber shows the sound absorption coefficients of 0.089 to 0.353 in the frequency range of 250 to 1600 Hz (Chen et al. 2010). Polypropylene reinforced with wheat straw had higher sound absorption coefficients (0.03 to 0.2) within the range of 0.3 to 1.8 kHz than that of composites reinforced with jute fiber reinforced polypropylene composites (Zou et al. 2010). Sound absorption coefficients of zein-jute composites showed higher sound absorption (0.06 to 0.8) compared with polypropylene-jute composites between frequency ranges of 1 and 5 kHz (Reddy and Yang 2011). Composite boards made of rice straw; wood particle reinforced commercial urea formaldehyde showed higher sound absorption coefficients than particleboard, fibreboard, and plywood in the frequency range of 500 to 8000 Hz (Yang et al. 2003). Commercial polyurethane reinforced rice straw and waste tire particle composites were found to have higher sound absorption coefficients at frequencies within the range of 2000 to 8000 Hz than particleboard, fibreboard, and rice straw-wood particle composite board (Yang et al. 2004).

The following lines explain the factors affecting sound absorption coefficients of materials. According to Koizumi et al. (2002), as sound absorption coefficient of composites increased, the fiber diameter decreased. This is because thin fibers can move more easily than thick fibers in response to sound waves. One of the most significant characters that determine the sound absorbing features of a fibrous material is the specific flow resistance per unit thickness of the material. A study by Ibrahim and Melik (1978) showed the increase of sound absorption only at low frequencies, as the material gets thicker. A study conducted by Koizumi et al. (2002) showed the increase of sound absorption coefficients in the middle and higher frequency as the density of the sample increased. Castagnede et al.(2000) demonstrated that compression of fibrous mats decreases the sound absorption properties. Tortuosity is a measure of the elongation of the passageway through the pores, compared to the thickness of the specimen. According to Knapen et al. (2003), tortuosity explains the influence of the inner construction of a material on its acoustical properties. The number, size, and type of pores are the important factors that one should consider while studying sound absorption mechanism in porous materials. This study focused on the evaluation of reinforced untreated and treated Luffa fiber epoxy composites, which includes the sound absorption coefficients and mechanical properties. The features of the composite were evaluated using scanning electron microscopy (SEM) to look into the morphology, thermo-gravimetric analysis (TGA) to examine the thermal stability, Fourier transform infrared (FTIR) to see the functional groups involved, and tensile strength testing to ascertain the mechanical properties.

EXPERIMENTAL

Materials

Premixed epoxy resin BBT-7892, which is the product of Bisphenol-A and epichlorohydrin, was supplied by Borneo Indah (Malaysia) Sdn. Bhd. This type of epoxy resin has a low reactivity, yellowish color, and slow curing.

The luffa fibres were obtained from local sources in Kuching, Sarawak, Malaysia. For chemical treatment of fibres, pellets of sodium hydroxide (NaOH) were used, which have a low reactivity and are soluble in distilled water.

Methods

Fiber preparation

The luffa fiber can be extracted from the Luffa cylindrica plant in two ways, by either naturally drying on the plant itself or by cutting it when it has matured and drying under the sun. When luffa is dried, the hard top part of the luffa needs to be cut off to remove the seed inside the luffa pod. Striking the luffa pod against a hard wall will remove the skin and the seed. Later, the luffa is sprayed or soaked with water to remove the sap color. Because the luffa fiber is in the form of a sponged pod, it was dried before being chopped into smaller sizes (1 mm to 10 mm) for use in specimen preparation. The good specific energy absorption of luffa sponge is attributed partially to its light base material as well as a higher densification strain. Due to the high strength-to-weight ratio of cellular materials, luffa sponge can be used as a good packaging material and an excellent energy dissipation material (Shen et al. 2012).

Fig. 1. Example of (a) luffa sponge and (b) chopped luffa fiber

Specimen preparation

There were a total of 40 specimens prepared for the sound absorption test and 32 specimens prepared for the tensile test. Both sets of specimens were divided into two classes, untreated and treated. For untreated specimens, the luffa fiber was rinsed with distilled water and dried in an oven at 60 °C for 48 h. For treated specimens, the luffa fiber was immersed in a 5 % NaOH solution at 25oC for 48 h. The purpose of immersing the luffa fiber inside the alkaline solution was to remove impurities and to increase the surface roughness of the fiber. The surfaces of an untreated fiber are covered with a layer of substances, which may include pectin, lignin, and other impurities. After sodium hydroxide treatment most of the lignin and pectin had been removed, resulting in a rough surface with some fibrils (Sgriccia et al. 2008). The immersed luffa fiber was cleaned with distilled water and dried in an oven at 60 °C for 48 h. Specimens for the sound absorption test were prepared by the following hardener to epoxy ratios 1:2, 1:4, 1:6, 1:8, and 1:10 to test the influence of binder on sound absorption. In this research a hardener to epoxy ratio of 1:4 was used as the control specimen for the tensile test. A circular mould with a diameter of 25 mm and thickness of 4.5 mm was used to fabricate the sound absorption specimens. For curing purposes, the mould was cold pressed under a pressure of 7 MPa using a hydraulic press for 24 h. For the tensile test, a mould with a thickness of 5 mm and cross-sectional area of 72.5 mm2 was used. The compositions for both tensile and sound absorption specimens were set at 5/95, 10/90, 15/85, and 20/80 wt. % of luffa/epoxy.

Composite testing

The sound absorption properties of the composites were assessed using a locally fabricated and calibrated two-microphone transfer-function method according to ASTM E1050-10 (2012), as shown in Fig. 2. This setup was employed to measure different acoustical parameters in the range of 500 to 6000 Hz. A loudspeaker was placed as a sound source at one end of the tube, and the test material was placed at the opposite end to measure the sound absorption properties. An impedance tube is a rigid, straight and smooth cylindrical pipe composed of two sections or tubes, a transmitting, and a receiving tube to test a material’s acoustic absorption coefficient (α) by producing a sound wave incident on the material being tested; the difference between the incident and reflected wave is then measured. Based on Muehleisen (2005), the two-microphone method measures the magnitude and phase difference of the pressure reflection coefficients that are used to measure the sound absorption coefficients of composites.

The TGA was performed on a TA-60WS workstation analyser (Shimadzu Corp.; Kyoto, Japan) at a heating rate of 10 °C/min. Specimens were examined under flowing nitrogen (80 mL/min) over a temperature range of 30 to 900 °C. According to Monteiro et al.(2012), thermal analysis studies of composites are important to understand the relationships between the structural properties and the production of composite materials, especially in the wide field of applications based on reinforced fibre composites. The morphological studies of the chemically treated luffa fibres were observed using a JEOL JSM-6390LA SEM (Tokyo Japan) with a field emission gun and an accelerating voltage of 5 kV to collect images of the surface of composites. The test specimens were sliced and mounted on aluminum stubs with double sided adhesive tape and sputter coated with gold for 5 min to a thickness of approximately 10 nm under 0.1 torr and 18 mA to make the sample conductive. Micrographs were recorded at different magnifications to ensure clear images. The FTIR spectroscopy was performed on a Shimadzu FTIR-8101 spectrometer in the range from 4000 cm-1 to 400 cm-1. The FTIR was used to collect and understand the functional groups of the composite materials. Tensile testing was performed with a LS-28011-50 Universal Testing Machine (T-machine Technology Co., LTD, Taiwan) using ASTM D638 – 10 (2012) as the control specimen.

Fabrication of two-microphone impedance tube

To fabricate the two-microphone transfer function impedance tube, all of the criteria mentioned in ASTM E1050-10 (2012) were used as a standard reference. Calculations were performed to ensure the equipment had a working frequency from 500 to 6000 Hz. According to ASTM E1050-10 (2012), to maintain the plane wave propagation, the frequency upper limit is defined in Eq. 1,

 (1)

where fu is the upper frequency limit (Hz), c is the speed of sound in the tube (m/s), d is the diameter of the tube (m), and K is a constant with a value of 0.586. The spacing between the two microphones can be improved by creating a large gap; however, the microphone spacing must be smaller than the shortest half-wavelength needed. This can be determined with Eq. 2,

 (2)

where is the microphone spacing (m), c is the speed of sound (m/s), and fu is the upper frequency limit (Hz).