CHAPTER 3 PROPERTIES OF ROSELLE FIBER, SISAL FIBER AND POLYESTER RESIN

Transcription

1 37 CHAPTER 3 PROPERTIES OF ROSELLE FIBER, SISAL FIBER AND POLYESTER RESIN 3.1 INTRODUCTION The plant fiber composites have been used by the human race ever since the onset of civilization as a source of energy, to make shelters, clothes, construct tools and produce weapons. The best example is the use of straw as reinforcement for clay to build walls in ancient Egypt, 3000 years ago. Glue laminated beams were also introduced using a casein adhesive in 1893 in Basel, Switzerland. Some creative designs were made but limited by the shape and weight of the structural elements. As early as 1908, the first composite materials were attempted for the fabrication of large quantities of sheets, tubes and pipes (paper or cotton to reinforce phenol- or melamineformaldehyde resins sheets). In recent years, there has been a renewed interest in the natural fiber as a substitute for synthetic fibers. Natural fibers as reinforcements in polymer matrix composites provide positive environmental benefits with respect to ultimate disposability and raw material utilization. The properties of the composites depend upon the properties of the individual components in the composites. Hence it is essential that the strength of fiber and matrix have to be established. This chapter deals with the fiber separation process and their properties. Moisture absorption of the roselle and sisal fibers in distilled water at room temperature is also studied.

2 38 The matrix material used unsaturated polyester resin and its mechanical properties are studied and presented here. 3.2 FIBER SEPARATION PROCESS The common word for H. sabdariffa (Roselle) is Mesta which produces good fiber of commerce. These are major fiber yielding species in India. Roselle and sisal fibers find traditional, age-old applications in the form of high strength ropes in India. From lost decade, Roselle and Sisal fibers were used traditionally in age-old applications in the form of high strength ropes in India, especially, in Tamilnadu villages. The roselle fibers used as low weight and high strength ropes to lift the heavy weight from Well etc. The sisal fibers used to fix together the coconut leaf and wooden stem while preparing the roof of a house. These fibers have not been really examined from a composite angle at that time. These fibers have been the main source of revenue of the people in this area for more than three decades. In Tamilnadu region, the roselle (Botanical Name: Hibiscus sabdariffa L, Family: Malvaceae), Local names: Pulichchai kerai (Tamil); Lal-ambari (Hindi), fiber is cultivated in many villages to protect the food plants, as a sides for food, medical purposes and specifically for fibers to produce the high strength rope and gift articles etc. (Figure 3.1). This plant is an erect, branched, smooth or nearly smooth annual herb 1 to 2 meters in height. Roselle is used for making tarts, jellies, and wine. The young leaves of the roselle are used as a substitute for spinach, or they may be cooked with fish or meat in making sinigang. Fiber is prepared from the bast of the stem. Sisal (Botanical name: Agave sisalana, Family: Agavaceae) fibers are grown naturally in lakes, stream and river sides in Tamilnadu regions (Figure 3.2a). It is also cultivated like other plants for fiber production (Figure 3.2b).

3 39 Figure 3.1 Roselle plant (Hibiscus sabdariffa L) (a) (b) Figure 3.2 Sisal plants (a) naturally grown, (b) cultivated They give good economic value to the people involved in the roselle and sisal related cultivation and other related work. Sisal fibers are also used for ornamental purposes. Every scrap or part of the roselle and sisal fibers can be utilized for some purpose. Rich quantities of fibers can be produced from the roselle and sisal plants. The fibers from the roselle and sisal plants are separated from roselle stem and sisal leafs by manually and also by mechanically. Manual separation process of roselle and sisal fibers from their plants is shown in Figures 3.3 and 3.4.

4 Extraction of Sisal Fibers Figure 3.3 shows the extraction method of sisal fibers. The sisal leafs are cut from sisal plant and tied into bundles by using bags. Then bags contain the sisal leafs are retted in tanks or River or Well for 3-4 days. The retted leafs are washed in running water and the top portion of the leafs are removed by manually (May by removed mechanicallly) to get the fiber separatly and cleaned and dried in the sun. (a) (b) (c) (d) (e) Figure 3.3 Extraction of sisal fibers (a) sisal plant (b) retting in water for 3-4days (c) removing the top portion of the leafs (d) dried using sunlight, and (e) final form of sisal fibers

5 Extraction of Roselle Fibers The fully grown plant is used to extract fibers. Figure 3.4 shows the extraction procedures of Roselle fibers. For the production of fiber the roselle crops should be harvested at the bud stage. The stalks are tied into bundles and retted in tanks or well for 3-4 days, as in the case of sisal leafs. The retted stem of the roselle plant is washed in running water. Then the fibers are removed from stem and cleaned and dried in the sun. (a) (b) (c) (d) (e) Figure 3.4 Extraction of roselle fibers (a) cultivated roselle plant (b) stalks in the form of bundles (c) retting in water for 3-4days (d) removing the fibers from the stem, and (e) final form of roselle fibers

6 42 This method has been traditionally followed for fiber separation. These fibers are also separated by mechanical crushing between the rollers and followed by cleaning with motorized combing device. The separated fibers are then dried under sunlight. During the separation of fibers from the plants, a large quantity of fibrous waste is produced, in which the roselle fibrous wastes are used as a fertilizer for other plants during cultivation, but the sisal fibrous wastes have no value. For the present study the fibers are separated from the roselle and sisal plants by traditional method. The fibers thus obtained have lot of impurities and these impurities are cleaned by the motorized combing device. The fibers and the waste materials are collected separately. The fibers are subjected to different mechanical test and environmental condition to study their mechanical properties and the environmental effect on mechanical properties. The bundle of roselle and sisal fibers used to fabricate roselle and sisal fiber hybrid polyester composites is shown in Figures 3.3 and PROPERTIES OF ROSELLE AND SISAL FIBER The roselle fibers are and light gold in color. The shape varies from fiber to fiber, and also non uniform. The length of the fiber varied from 1 m to 2 m. Fiber thickness is measured using an electron (Digital microscope) microscope for 25 samples and it was found to be varying between 0.13mm to 0.24mm which depends on the age and area of cultivation of the plant. The density was calculated using Archimedes principle and it was found to be around 1.45 g/cm 3. The sisal fibers obtained from the leaf of sisal plants are white/golden white in color. They can be twisted in to yarns and ropes in wet conditions. Fiber thickness, length and strength depend upon the age and location of the plant. The length varied from 0.5m to 1m and diameter is

7 43 between 0.21mm to 0.29mm. The density of the fiber was found to be around 1.51 g/cm Tensile Properties of Fibers The cleaned and dried single fiber was mounted along the centerline of a slotted paper window as shown in Figure 3.5. The ends of the paper window were clamped in the grips of the Single Yarn Testing machine with gauge length of 20 mm and its mid section was cut off during loading. The load is applied with the crosshead speed of 1mm/min till the fiber break and the breaking point load values were recorded. 20 samples were tested and the results are tabulated in Tables 3.1 and 3.2. From the load deflection curve the strength and modulus values were calculated. It is observed that the strength of the roselle fibers was not uniform and it varied from to MPa. The modulus was also in the range of to GPa. The percentage of elongation varies between 0.5 and 0.8. It was observed that the strength of the sisal fibers was also not uniform and it varied from MPa to MPa. The modulus was also in the range of 7.460GPa to GPa. The percentage of elongation varies between 1.0 to 1.5. The Strength of the fibers depends mainly on the fibrillar structure, micro fibrillar angle and the cellulose content. The relation between the elongation and the fibrillar angle is є = x 10-2 θ+7.7x10-2 θ 2 (3.1) σ = θ+12.22w (3.2) where θ is microfibrillar angle and W is cellulose content.

8 44 Figure 3.5 Methodology of tensile test of roselle and sisal fibers Table 3.1 Tensile properties of roselle fibers as received Fiber Diameter specimen (mm) Load (N) Deflection (mm) Strength (MPa) Strain % Elongation Modulus (GPa)

9 45 Table 3.2 Tensile properties of sisal fibers as received Fiber specimen Diameter (mm) Load (N) Deflection (mm) Strength (MPa) Strain % Elongation Modulus (GPa) WATER ABSORPTION CHARACTERISTICS OF FIBERS Water Absorption Kinetics of Roselle and Sisal Fibers Water or moisture uptake in natural fiber reinforced polymer matrix composites has a deleterious effect on their mechanical properties. The growth of natural fiber composites is not without challenges. The hydrophilic nature of natural fibers is a potential cause of poor interfacial adhesion

10 46 between fibers and matrix. Understanding the moisture diffusion mechanisms in natural fiber composite materials is essential for the improvement of their durability. As a raw material for polymer composites, the water-absorption behavior of roselle and sisal fibers has to be comprehensively investigated. Water-diffusion characteristics of roselle and sisal fibers in water have been investigated and the results are discussed. About 10g of dried fibers having an approximate length of 50mm is taken for the water absorption examination. The samples were immersed in raw water at room temperature. Increase in weight of the samples was noted at specific time intervals. This process was continued until equilibrium is reached. The molar percentage uptake Q t for water for 100g of polymer was plotted against the square root of time. Q t W W 18W 2 1 (3.3) 1 where 18 is the relative molecular mass of water. When equilibrium is reached Q t is taken as the molar percentage uptake at infinite time, Q (Bhagavan et al 2003). The water absorption of fibers was calculated as the number of moles of water absorbed by 100g of the fiber. The major factors that control the interaction between fibers and water are diffusion, permeability and sorption. Roselle and sisal fibers are lingo cellulose, they contains hemi-cellulose, lignin, pectin, waxy material and water-soluble substances. The swelling behavior of natural fibers is greatly affected by its morphology as well as physical and chemical structures. The water penetration through the microspores of the fiber surface by capillary action was explained (Sreekala 2001). The fibers have a porous internal structure. The penetrating water enters into the fiber structure and stay in the pores medium. Roselle fiber contains less waxy (0.5) materials when compared to sisal fibers. The waxes present on the fiber surface reduce the moisture

11 47 absorption. Due to porous and less waxy material, the roselle fibers have the large initial uptake due to capillary action. The diffusion of water in water cellulose system is reported to be non-fickian or anomalous and two-stage absorption behavior is reported in natural fibers (Sreekala 2001). During the present investigation it was observed that both roselle and sisal fibers exhibited single stage behavior is observed in water, which is shown in Figure 3.6. The initial stage of water penetration is by capillary action, which is shown by the linear portion of the curve, and the second linear portion of the moisture absorption is up to 360 minute. It is due to the late filling of micro pores. The equilibrium sorption is higher for roselle fibers, which due to pores and less waxy materials. Figure 3.6 Comparison of moisture absorption curves of roselle and sisal fibers Properties of the Matrix Thermoset resins are usually liquids or low melting point solids in their initial form. By its three dimensional cross linked structure, they have high thermal stability, chemical resistance, good dimensional stability and

12 48 also high creep properties. The most common thermosetting resins used for composite manufacturing are unsaturated polyesters, epoxies, vinyl esters and phenolics. Unsaturated polyester is economical as is used due to its excellent process ability and good cross linking tendency as well as mechanical properties when cured (Regnier and Mortaigne 1995; Mortaigne et al 1999) and due to these reasons unsaturated polyester has been chosen. The typical properties of the unsaturated polyester are listed in Table 3.3. Unsaturated polyester as matrix for the current investigation was tested at Saint-Gobain Vetrotex India Ltd. Table 3.3 Typical properties of unsaturated polyester resin matrix Appearance Yellow viscous liquid Specific 25 C 1.1 Viscosity (a) FC-4 30 C 110 (b) Brookfield 25 C RVT model 480 Volatile content C 42.5 Acid value (Mg.KOH/G) 6.97 To find the mechanical properties a plate was cast with neat resin mixed with accelerator and catalyst in mould of 150 mm 20 mm 3 mm. Figure 3.7 shows the fabricated neat resin sample. Tensile test was carried out using computerized FIE universal testing machine. Six samples were tested and the average strength was estimated as 24 MPa and the tensile modulus to be MPa. For the flexural strength, the samples were tested by three point loading and flexural strength of 28 MPa was observed. The flexural modulus of the samples was observed as 1.07 GPa. Impact test was carried out on Izod impact testing machine. The impact strength was observed as 0.49 KJ/m 2.

13 49 Figure 3.7 Fabricated neat resin sample 3.5 ALKALI TREATMENT OF ROSELLE AND SISAL FIBERS The quality of a fiber reinforced composite depends considerably on the fiber-matrix interface because the interface acts as a binder and transfers stress between the matrix and the reinforcing fibers. Strong interfacial bonding can be developed as result of good wetting of the fibers by the matrix and the formation of a chemical bond between the fiber surface and the matrix. In order to develop composites with good mechanical properties and good environmental performance, it is necessary to impart hydrophobicity to the fibers by mechanical treatments, surface treatments and chemical treatments. This results in increase of the strength of the composite specimen. Many studies were carried out to improve the properties of the composites. Mwaikambo et al (2002) studied the alkalisation or acetylation of plant fibers resulting in the changes of the surface topography of the fibers and their crystallographic structure. Dewaxing method has been used to remove waxy substances from sisal fiber surfaces. Soxhlet extraction is one technique adopted. It was found that the properties of the fiber are not enhanced but the fiber matrix bond is improved. The fiber was washed with sodium hydroxide prior to any treatment. The sodium hydroxide opens up the cellulose structure allowing the hydroxyl groups to get ready for the reactions. During washing with

14 50 sodium hydroxide, the wax, cuticle layer and part of lignin and hemicellulose were removed. The major reaction takes place between the hydroxyl groups of cellulose and the chemical used for the surface treatment. The fiber treatment resulted in the decrease of the properties of the fiber, but increase in the strength of the specimen as a whole. As per the literature reviews about the alkali treatment of natural fibers reinforced composites, it was confirmed that the treatment of the fibers with NaOH solution is the most suited one. The alkali treatment process has some critical parameters like: 1. Alkali used 2. Concentration of the solution 3. Treatment duration Here any two of the parameters need to be fixed such that the variation in the properties of the composite can be studied carefully. The concentration of solution and treatment duration plays major role. There is a positive effect cited when the concentration increases up to certain limit, beyond that the value drops suddenly. In this case, the roselle and sisal fibers are treated in 10% of alkali solution (NaOH) for 2 h, 4 h, 6 h and 8 h. Figure 3.8 shows the treated roselle and sisal fibers. (a) Roselle (b) Sisal Figure 3.8 Alkali treated roselle and sisal fibers

15 SUMMARY The fiber separation process and its physical and mechanical properties were studied. From the test it was observed that the fiber size and the strength are not uniform. The moisture absorption characteristic of the fibers was also studied. From the study it was observed that the equilibrium sorption is higher for roselle fibers when compared with sisal fibers. The mechanical properties of the matrix material were studied. The roselle and sisal fibers are treated with alkali solution for different duration and then used to fabrication of the composites.