Mechanical Properties of Short Sisal Fibre Reinforced ...

Mechanical Properties of Short Sisal Fibre Reinforced Phenol Formaldehyde Eco-Friendly Composites

Mechanical Properties of Short Sisal Fibre Reinforced Phenol Formaldehyde Eco-Friendly Composites Maya M.G.1,3, Soney C. George1, 3*, Thomasukutty Jose1, Sreekala M.S.2 and Sabu Thomas3 1Centre for Nanoscience & Technology, Amal Jyothi Engineering College, Kanjirappally, Kerala, India 2Sree Sankara College, Kalady, Mattoor, Kalady-683 574, Kerala, India 3International and Interuniversity Centre for Nanoscience & Nanotechnology, Mahatma Gandhi University, Kottayam, P.D Hills P.O India

Received: 19 June 2015, Accepted: 29 November 2016

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Summary

Natural fibres are a good reinforcing material. Among the various natural fibres, sisal fibre is of particular interest, since its composites have high impact strength besides having moderate tensile and flexural properties compared to other lignocellulosic fibres. Sisal fibre is a promising reinforcement for use in composites on account of its low cost, low density, high specific strength and modulus, no health hazards, easy availability in some countries and renewability. The present work focused on the recent developments in the field of sisal fibre reinforced phenol formaldehyde composites with reference to the properties of sisal fibre, processing techniques, and the physical and mechanical properties of the composites. The mechanical properties of the composites show much improvement by the addition of sisal fibre of optimum fibre length 40 mm and fibre loading 54 wt%. The better fibre matrix interaction is shown by sisal-PF containing 54 wt%. The comparison with other natural fibres shows that sisal fibre is a good reinforcing agent in PF matrix. Flexural strength and impact strength of the composites were examined and are increases with increasing the fibre content. Aging studies of the composites showed a similar trend to that of un-aged samples. Interestingly, water absorption test results show that composite with 54 wt% fibre loading have good interaction with the matrix and contains less voids.

Keywords: Sisal fibre; Composites; Phenol formaldehyde resins; Flexural strength; Volume resistivity *Corresponding Author: E-mail: [email protected] ©Smithers

Information Ltd, 2017

Polymers from Renewable Resources, Vol. 8, No. 1, 2017

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Maya M.G., Soney C. George, Thomasukutty Jose, Sreekala M.S. and Sabu Thomas

INTRODUCTION The interest in using natural fibres as reinforcement in resins and elastomers has increased dramatically during last few years. This is due to the environmental aspects as well as cost reduction of the composites compared to the other reinforcement materials such as carbon fibres and aramids. Natural fibres like sisal fibres, banana fibres etc. were found to be a good reinforcement in different resin systems. The banana fibre reinforced polyester resins show much improvement in their mechanical properties [1]. The banana fibre reinforced phenol formaldehyde resins exhibited superior mechanical properties compared to the synthetic fibre reinforced composites [2]. Phenol formaldehyde (PF) resin was the first resin which is industrialized in the world and it exhibits high stiffness, electrically insulating properties and excellent chemical corrosion resistance.

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Sisal fibre is a good reinforcement material because of its easy availability, low cost, renewability and high specific strength. Sisal fibres are used in various industrial applications for making ropes, baler and binders twine. Ropes and twines are widely employed for marine, agricultural, and general industrial uses. The structure and properties of sisal fibre have been investigated by several researchers [3-6]. Such understanding of structure –property relationship will not only help open up new avenues for these fibres, but also emphasize the importance of this agricultural material, which form one of the abundantly available renewable resources in the world. The characteristics of the sisal fibres depend on the properties of the individual constituents, the fibrillar structure and the lamellae matrix. The fibre is composed of numerous elongated fibre cells that taper towards each end. The fibre cells are linked together by means of middle lamellae, which consist of hemicellulose, lignin and pectin. According to Gram [7], a sisal fibre in cross-section is built up of about 100 fibre cells. Kulkarni et al. [4] state that the number of cells in cross-section of a coconut fibre ranges from 260 to 584 depending on the diameter of the fibre. Figure 1 shows the schematic sketch of sisal fibre cell. Sisal fibres are being used as reinforcing filler in many resins, elastomers and other polymeric systems. The tannin–phenolic resins composite reinforced with 50 wt% of sisal fiber showed high stiffness and low loss modulus and this system gave very good fiber/matrix interface adhesion [8]. The mechanical properties of sisal fiber reinforced polyethylene composite show gradual increases as the fibre content increases [9]. The addition of sisal fiber in the banana reinforced epoxy composites results in 16% increase in tensile strength, 4% increase in flexural strength and 35% increase in impact strength [10]. Thus sisal fibers show better reinforcement in these hybrid composites. The sisal fibers have been used extensively for the reinforcement of elastomers. Addition of sisal

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Figure 1. A schematic sketch of a sisal fibre cell

fibres in rubber matrix shows good reinforcement and improved mechanical, thermal, rheological and ageing properties [11-15].

Sisal fibre reinforced phenol formaldehyde resins showed much improvement in their mechanical properties. The modified and unmodified sisal fibres and their composites showed better mechanical properties [16, 17]. The alkali and silane surface treatment improved the surface adhesion between sisal fibre and PLA matrix, and resulting in composite with improved mechanical properties [18]. The main highlights of this work compared to the previous work are the improved mechanical properties of PF/sisal composites without any surface treatment. The surface adhesion between fibre and PF matrix was improved by the prepreg route of composite synthesis. The novelty in this work is the justification of the effect of fiber length (up to 50 mm) and very high fiber loading (up to 70 wt%) on the mechanical properties of the PF composites. It is important to mention that in most of the studies, the fiber content is limited to 40 wt% and fiber length up 30 mm. In the present study the effects of fibre length and fibre loading on the mechanical and impact properties of phenol-formaldehyde resin have been investigated in detail. The role of sisal fibre on the electrical and water absorption properties of the composites has been studied. The effect of thermal ageing on the tensile properties of the composites has also has been examined. Finally, the properties of sisal/PF composites have been compared with those of coir and oil palm/PF composites


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MATERIALS AND METHODS

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Sisal Fibres

Sisal fibres are collected from a plant Agave Veracruz and are available in southern parts of India especially in Andhra Pradesh, Karnataka, Tamilnadu and Kerala. The physical and mechanical characteristics of sisal fibres were given in Table 1 and the chemical composition of sisal fibres typically includes 66-72% cellulose, 12% hemi cellulose, 10-14% lignin, 11% moisture content. The characteristics of the fibres depend on the properties of the individual constituents, the fibrillar structure and the lamellae matrix. The fibre is composed of numerous elongated fusiform fibre cells that taper towards the end. The fibre cells are linked together by means of middle lamellae, which consists of hemicelluloses, lignin and pectin. Table 1. The physical and mechanical characteristics of sisal fibres Fibre

Diameter (micro-m)

Density (Kg/m3)

Tensile Strength (MPa)

Tenacity (MPa)

Elastic Modulus (GPa)

Elongation at Break (%)

Sisal

50-200

1450

400-700

568-640

09-12

05-14

Phenol Formaldehyde Resin

Phenol formaldehyde (PF) resole type resin is used and is procured from INDITAL Pvt. Ltd., Chennai. The physical, chemical and mechanical characteristics of the PF resin are given in Table 2. Table 2. Typical properties of phenol formaldehyde resin Liquid Resin Appearances

Cured Resin Reddish brown clear liquid

Density (g/cc)

1.3

300 cps

Tensile strength

10

380%

Young’s Modulus (MPa)

375

33 mts 30 sec


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Viscosity @ 250C Water Tolerance Gel Time @ 1210C pH Specific Gravity

8.4

Flexural Strength (MPa)

10

1.12-1.16

Flexural Modulus (MPa)

1875

Composite Preparation The sisal fibre – PF resin composites were prepared by hand layup method followed by compression moulding. Prepreg route was followed for the 4



preparation of composites. Fibres are chopped into desired lengths (10, 20, 30, 40 and 50 mm) and dried at 60-700C for about two hours. These chopped fibres were uniformly spread in a mould cavity. Mould was then closed and pressure was applied to form a single mat. The mat was then impregnated in the resin and the prepreg was kept at room temperature up to semi-cured stage. It was then pressed at 1200C to get a three dimensionally crosslinked network. Composites with different fibre loading (38, 46, 54 and 70 wt%) were prepared and properties were evaluated.

Mechanical Tests Test specimens were cut from the composite sheets. Tensile testing was carried using FIE electronic tensile testing machine according to ASTM D 638-76 (12 × 1.2 cm) at a strain rate of 50 mm/min. Three point flexure properties were also tested using the same machine according to ASTM D 790. Impact tester of Ceast Torino, Italy was used to test the notched Izod impact strength of the composites as per ASTM D 256 (10 cm × 1.2 cm). Hardness of the composites was checked on a Barcol Hardness according to ASTM D 2240. Density of the samples was determined using displacement method according to ASTM D 792

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Water Absorption Studies

For water absorption experiments (as per ASTM 570 method) rectangular samples (10×10 mm) were cut from the composite sheets. Corners of the samples were curved to avoid non-uniform water diffusion. The thickness of the samples was measured. Samples were immersed in normal water with pH 7. Increase in weight of the samples was noted at specific time intervals. This process was continued till equilibrium was reached. The values were found to be perfectly reproducible. The mole percent uptake Qt for water by 100 g of the polymer was plotted against the square root of time. The Qt can be expressed as:

(1)

M∞ Mass of composite sample at equilibrium, Mw Relative molecular mass of water and Mo Initial mass of the sample. The water absorption of polymers was calculated as number of moles of water absorbed by 100 g of the polymer.


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Ageing Studies Composite samples were cut into specified dimension according to ASTM D 638-76 for mechanical testing. It was then placed in an air oven at 1000C for about three days. Samples were then allowed to cool at room temperature. Tensile properties of the aged samples were tested.

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Electrical Property Measurements

The volume and surface resistivity of the composite materials were determined as per the procedure in ASTM D257 using a LCR meter. The specimen to electrode dimensions and the width of the sample were measured with an accuracy of ±1% for determining volume resistivity. The volume and surface resistivity are calculated as follows: (2)

r-Volume resistivity in ohm/cm, Rv-Volume resistance in ohm, A-Area of guard electrode in cm2, L-Thickness of the specimen in cm.

(3)

s-Surface resistivity in ohm-cm, Rs-Volume of Resistance in ohm, r-Effective perimeter in cm, f the guarded electrode, g - Distance in cm in between the electrode.

RESULTS AND DISCUSSIONS

Tensile Behavior of Sisal Fibre/PF Composites Effect of Fibre Length Tensile strength of the composites gives a measure of the ability of a material to withstand forces that tend to pull it apart and this determines to what extent the material stretches before breaking. Tensile modulus, an indication of the relative stiffness of a material, can be determined from the stress-strain diagram. Stress-strain behavior of the composites for different fibre lengths on application of tensile stress is shown in Figure 2. The stress-strain curve of the neat 6



sample shows a brittle nature. In the case of composites, at first there is a linear deformation and thereafter a nonlinear behavior was observed [19]. This deviation from linearity accounts for the decreased brittleness of the composite. The effective tensile modulus and yield stress of the composites were found to increase with increase in fibre length up to 50 mm. We have optimized the fibre length as 40 mm because the incorporation and processing is very difficult at much longer fiber length. The tensile strength and the tensile modulus show maxima at a fibre length of 50 mm. As compared to gum sample an increase of 730% in tensile strength and 252% in tensile modulus is observed for 50 mm fibre length. This is due to the increased fibre –polymer interaction than fibre-fibre interaction. At lengths higher than the critical fibre length, the effective stress transfer is not possible due to the fiber to fiber entanglements and contacts. At lengths below the critical length, the fiber easily debonds form the polymer matrix. The concept of critical fibre length and effect of fibre length on the properties of short fibre reinforced polymer composites were reported [20, 21].

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Figure 2. Stress- strain characteristics of sisal fibre/PF composites on the application of tensile stress under various fibre lengths [fibre loading 54 wt%]

There is dramatic improvement in the percentage elongation of the phenolformaldehyde resin by sisal fibre reinforcement. This may be due to the higher intrinsic elongation properties of the fibre. Maximum value of elongation at break is observed for composite prepared from 30 mm fibre length, (results


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are shown in Table 3). The deformation of fibre in a composite is dependent on fibre length because of the difference in stress distribution within the composite. The fibre elongation may become fully effective only at 30 mm length. Above this length composite failure may occur without full elongation of the fibre due to other factors such as entanglement and fiber to fiber contacts.

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Table 3. Variation of mechanical properties of sisal fibre/PF composites with fibre length [fibre loading 54 wt%]

Fibre length Tensile strength Elongation at Tensile Modulus Impact Strength (mm) (MPa) break (%) (MPa) (KJ/m2) 10 20 30 40 50

30

2.8

450

9.5

47

3.7

525

10

53

6.1

650

11.1

62

4.8

920

12

84

5.9

1300

12.7

Effect of the Fibre Loading

Table 4 show the effect of varying percentage of fibre weight on the mechanical properties of sisal fibre/PF composites. The tensile strength and modulus of the neat polymer is 10 and 360 MPa respectively because the gum sample shows a brittle behavior. As the fibre content increases, the mechanical strength of the composite increased and showed optimum value at 54 wt% fibre loading. Chow [22] studied the stress –strain behavior of polymer composites as a function of filler concentration, strain rate and temperature. Both tensile strength and modulus increase linearly up to 54 wt% of fibre followed by a decrease at higher fibre loading. At higher fibre loading there is a chance of phase separation and agglomeration of fibres, thereby reducing the effective aspect ratio. Crack initiation and its propagation will be easier at higher loadings, which decrease the fibre-matrix adhesion. As compared to gum sample, there is an increase of 521% in tensile strength and 193% in tensile modulus [19]. Elongation at break increases with fibre loading unto 38% and later decreases. It reaches a minimum value at 54 wt%. An increase of 120% was observed at 38 wt% and an increase of 95% at 54 wt% compared to gum sample. Stress-Strain characteristics of sisal fibre/PF composites on application of tensile stress under various fibre loading [fibre length 40 mm] is shown in Figure 3.

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Figure 3. Stress-strain characteristics of sisal fibre/PF composites on application of tensile stress under various fibre loading [fibre length 40 mm]

Table 4. Variation of mechanical properties of sisal fibre/PF composites with percentage of fibre by weight [fibre length 40 mm] Fibre Loading (Wt %) 0 38 46 54 70

Tensile Strength (MPa)


Tensile Modulus (MPa)

Impact Strength (KJ/m2)

Hardness (BIU)

10

2

380

6.2

23

45

4.4

560

7.5

26

53

4

915

9

38

60

3.8

1100

9.6

43

51

-

700

14

52

Flexural Properties

Effect of the Fibre Loading

Flexural strength is the ability of the material to withstand bending forces applied perpendicular to its longitudinal axis. The flexural modulus is a measure of the stiffness during first or initial part of the bending process. Flexural stress is a combination of compression and tension. The effect of the fibre loading on the stress-strain characteristics of the composites on application of flexural stress is shown in Figure 4. The difference in stress-strain characteristics


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of the composites, on application of tensile and flexural stress, implies that the type of stress applied to a system plays an important role in non-linear deformations [23]. The flexural stress-strain curve for the neat sample indicates low stiffness and brittle nature. The yield stress for gum sample is very low compared to composites. Composites prepared from fibre loading of 54 wt% exhibit maximum flexural strength and flexural modulus. There is an increase of 1088% in flexural strength of the composite compared to neat PF sample up on the addition of 54 wt percent of fibre.

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Figure 4. Stress-strain characteristics of sisal fibre/PF composites on application of flexural stress under various fibre lengths [fibre loading 54 wt%]

Impact Behavior The fracture toughness of composite is perhaps its most characteristic property. It is manifested in impact tests. The toughness of a fibre composite is mainly dependent on the fibre stress-strain behavior. Fibres with excellent mechanical properties show high work of fracture. Impact behavior is a measure of the energy required to cause damage and the progress of failure within the composite.

Effect of Fibre Length Table 3 shows a linear increase in impact strength with fibre length keeping fibre loading constant. This can be attributed to the weaker interfacial bond

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formed as the fibre length is increased to higher values. Since the interfacial bond is weak debonding occurs and extra energy is needed to do the work of debonding. Greater force is required to propagate a crack through the interface during impact. Upon impact loading, fibre fracture may occur and the broken ends of the fibres have to be pulled out as the fracture proceeds, which require additional energy.

Effect of Fibre Loading With increase in fibre loading the impact strength is increases, which is shown in Table 4. The high impact strength at high fiber loading is associated with large extent of microscale debonding taking place at the fiber/ matrix interface at high fiber loading.

Hardness and Density

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The Barcol hardness of the sisal fibre reinforced phenol-formaldehyde resin composites are given in Table 4. Barcol hardness values are increasing with increases of fibre loading. Due to the increase of fibre content, the composite becomes stiffer and harder. As compared to the matrix polymer, the fiber has superior mechanical properties such has hardness, modulus, strength etc. Therefore the additional high modulus fiber increases the hardness of the composite. The density values of the composites are considerably less than that of cured resin. As in the case of tensile and flexural properties the maximum density is exhibited by composite sample having 54 wt% of fiber content and the value is 1.2 g/cc.

Water Absorption

The water absorption characteristics of sisal fibre-PF resin composites are displayed in Figure 5. The mole percent uptake of water is plotted against time. All the sorption curves are similar in nature and which indicate that the sorption characteristics of all the composites are uniform. Water sorption linearly increases with time, and after about 360 minutes all the sorption curves attains the saturation state. The maximum water uptake was exhibited by composites having 46 wt% fibre loading and the minimum sorption was shown by composite having 54 wt% of fibre loading. All others have intermediate values. This indicates that composites with 54 wt% fibre loading having the best fibre-matrix interaction and least voids. Good fibre-matrix interactions restrict the swelling process in composites. Hence the swelling process can be used as a measure of estimating fibre-matrix adhesion.


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Figure 5. Variation of Qt mol percentage of sisal fibre/PF composites with time with percentage of fibre by weight [fibre length 40 mm]

Effect of Thermal Ageing

The influence of thermal ageing on tensile properties of sisal/PF composite is carried out as a function of fibre loading. The stress-strain curves after ageing is displayed in Figure 6. The stress-strain curves of aged sisal/PF composite are similar in nature to that of unaged sample. In Figure 7, tensile strength values of both aged and unaged samples are compared. In both cases tensile strength values increase with increase of fibre loading and reaches a maximum at 54 wt% and then decreases. But it is very clear from the figure that tensile strength decreases upon thermal ageing. About 24% of reduction in tensile strength is observed for sisal/PF composites having 54 wt% of fibre content. The decrease in properties is associated with weakening of the interfacial interactions up on ageing process.

Electrical Testing The resistivity of lignocellulosic fibres depends on the moisture content, crystalline and amorphous component present, presence of impurities, chemical composition, cellular structure and microfibrillar angle etc. Phenol formaldehyde is an amorphous polymer and it has high volume resistivity. The surface and volume resistivity values of sisal fibre reinforced PF composites 12



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Figure 6. Stress-strain characteristics of sisal fibre/PF composites on application of tensile stress under various fibre loading after ageing [fibre length 40 mm]

Figure 7. Comparison of tensile strength values of aged and un-aged sisal fibre/PF composites as a function of fibre loading

as a function of fibre loading are shown in Figure 8. It is very clear from the figure that both surface and volume resistivities are increasing with increase of fibre loading. These results indicate that PF/sisal composites could be used in electrical applications as low coast excellent insulators


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Figure 8. Variation of surface resistivity and volume resistivity of sisal fibre/PF composites with percentage of fibre by weight [fibre length 40 mm]

Comparison of the Strength Properties of Sisal Fibre/PF Composites With Other Short Natural Fibre Reinforced PhenolFormaldehyde Composites

The tensile strength and flexural strength of coir fibre reinforced phenol formaldehyde resins [21, 22] and oil palm fibre reinforced PF composites are compared with those of sisal fibre reinforced PF resins. The comparison indicated that the sisal fibre /PF resin composites has higher tensile and flexural strength values as compared to other composites. Maximum flexural strength in the reported work is 47.61 MPa for oil palm fibre reinforcement and 53 MPa for coir fibre reinforcement while sisal fibre reinforcement gives the values of 116.82 MPa. These data clearly indicate that sisal/PF composites could be used for high performance applications.

CONCLUSIONS Utilization of sisal fibre will lead to the formation of a new composite product. Tensile, flexural and impact properties of the sisal fibre/PF composites were investigated as a function of fibre length and fibre loading. All these mechanical properties showed improvement upon reinforcing with the sisal fibre. The elongation, brittle nature and buckling characteristics of PF resin were considerably improved by incorporating sisal fibre. 14



The present study reveals that sisal fibre represents a potential reinforcement in resol type phenol-formaldehyde resin. The mechanical and electrical properties of the composites show very good improvement up on the addition of the fibre. The maximum tensile and flexural properties were observed at 50 mm fibre length. The impact strength showed linear enhancement with increase in fibre length up to 50 mm. However, for the best balance of mechanical properties, the optimum fibre length and fibre loading are considered to be 40 mm and 54 wt% respectively. The density, hardness and electrical properties are found to increase with fibre loading. Thermally aged samples show decrease in tensile properties compared to the unaged samples. Composites having 54 wt% fibre content exhibit less water uptake compared with other samples. This indicates that there is better fibre-matrix adhesion in sisal-PF composites containing 54 wt% of fibre. Compared to other natural fiber reinforced PF composites, sisal fiber/PF composites show excellent properties. Finally, it is important to add that these composites can be successfully used for high performance door applications in mechanical and electrical fields.

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