Enhancements in crystallinity, thermal stability, tensile modulus and strength of sisal fibres and their PP composites induced by the synergistic effects of alkali and high intensity ultrasound (HIU) treatments

doi:10.1016/j.ultsonch.2016.07.008

Ultrasonics Sonochemistry

Volume 34, January 2017, Pages 729-742

https://doi.org/10.1016/j.ultsonch.2016.07.008 Get rights and content

Highlights

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Alkali and ultrasound treated sisal fibres reinforced PP composites were prepared.
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Ultrasound removed the amorphous materials from the surface of sisal fibres.
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Crystallinity of the sisal fibres increased by more than 10%.
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Thermal stability increased by 38.5 °C after the combined treatments.
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Tensile modulus and strength increased by 50% and 10% respectively.

Abstract

In this investigation, sisal fibres were treated with the combination of alkali and high intensity ultrasound (HIU) and their effects on the morphology, thermal properties of fibres and mechanical properties of their reinforced PP composites were studied. FTIR and FE-SEM results confirmed the removal of amorphous materials such as hemicellulose, lignin and other waxy materials after the combined treatments of alkali and ultrasound. X-ray diffraction analysis revealed an increase in the crystallinity of sisal fibres with an increase in the concentration of alkali. Thermogravimetric results revealed that the thermal stability of sisal fibres obtained with the combination of both alkali and ultrasound treatment was increased by 38.5 °C as compared to the untreated fibres. Morphology of sisal fibre reinforced composites showed good interfacial interaction between fibres and matrix after the combined treatment. Tensile properties were increased for the combined treated sisal fibres reinforced PP composites as compared to the untreated and pure PP. Tensile modulus and strength increased by more than 50% and 10% respectively as compared to the untreated sisal fibre reinforced composite. It has been found that the combined treatment of alkali and ultrasound is effective and useful to remove the amorphous materials and hence to improve the mechanical and thermal properties.

Graphical abstract

Introduction

Over the last few decades, more attention is given to cellulose-based fibres as reinforcement fillers in the polymer composites owing to the environmental pollution caused by the extensive usage of synthetic fibres such as glass fibres to produce composite materials [1]. Unlike synthetic fibres, natural fibres possess light weight, high strength, and are readily available, environmentally friendly and biodegradable. Utilisation of natural fibres for the generation of composites has many advantages, for example, recyclability, eco-friendly products etc.

Many different types of natural fibres are being exploited for the production of biodegradable polymer composites [2], [3], [4], [5], [6]. More than thousand types of natural fibres are already available [7]. Among these, sisal fibres are one of the best reinforcing materials for polymer composites owing to their higher cellulose content (78%), good tensile strength (>600 MPa), easy availability and low cost [8]. However, similar to other natural fibres, sisal fibres are poorly compatible with the matrices of thermoplastics, due to their hydrophilic surface which in turn reduces the thermal and mechanical properties of sisal fibres reinforced thermoplastics. Especially in the fibre-matrix interfaces, hydrophilicity of the fibres causes poor mechanical strength and hence limits their applications [9], [10]. Parparita et al. [11] studied the mechanical and rheological properties of different lignocellulosic fibre materials reinforced polypropylene composites. It has been observed that using different types of fibrous materials tensile modulus improved by more than 10% for fibre reinforced polypropylene composites.

Surface modification of natural fibres is often the followed method for the removal of amorphous materials from the fibre surface which increases the compatibility between fibre-matrix interfaces in the fibre-reinforced polymer composites. Alkali treatment reduces the fibre diameter by removing the amorphous materials from the fibre surface and increases the rough morphology. Thus, the removal of amorphous materials exposes cellulose and its hydroxyl groups and leads to more hydroxyl bonding between fibre surface and polymer matrix thereby enhancing the mechanical and thermal properties of the fibre reinforced polymer composites. Sisal fibres were treated with an alkali solution (NaOH) and the thermal and mechanical properties of sisal fibres reinforced cardanol-based biocomposites were studied. Treatment with 5% NaOH improved the thermal stability up to 12 °C, whereas with 10% NaOH the thermal stability increased by 18 °C [12]. Cyras et al. [13] studied the alkaline and acetylene treatments on sisal fibres and observed that the alkali treatments on sisal fibres increased the thermal stability due to the removal of lignin from the fibre surface. Asumani et al. [14] studied the alkali and silane treated kenaf fibre reinforced polypropylene composites. It has been noted that the tensile strength and modulus increased significantly by 25% and 11% respectively after treatment with 5% alkali.

High intensity ultrasound (HIU) treatment is the application of sound waves to the liquids in the frequency range from 20 to 1000 kHz [15]. It is well known that ultrasonication is one of the important and eco-friendly methods for various technological applications [16]. Ultrasonication induces cavitation which is the formation, growth and violent collapse of bubbles. Millions of formed bubbles collapse in the solution simultaneously which generates very high temperatures and pressures locally. The resultant effects of stable cavitation cause the etching effects on the fibre surface and results in the removal of amorphous materials from the fibre surface. The effect of ultrasound in degrading the linkages of cellulose and amorphous materials on the surface of fibres has been reported [17]. It has been proved that the removal of hemicellulose and lignin from lignocellulosic materials could be improved by applying HIU [18], [19], [20], [21].

Moshiul Alam et al. [22] and Akindoyo et al. [23] studied the simultaneous effects of both alkali and ultrasound treatments on oil palm empty fruit bunch fibres and reinforced with poly (lactic acid), where they observed a significant improvement in the mechanical, thermal and interfacial properties of the resultant composites.

Looking at the fruitful effects of ultrasound, in this investigation, sisal fibres were treated with different concentrations of alkali, HIU as well as simultaneous application of both the alkali and HIU treatments (Fig. 1) and their resultant effects on the morphology and thermal properties were studied and also the tensile and impact properties of untreated and treated short sisal fibres reinforced polypropylene composites were investigated.

Section snippets

Materials

General purpose injection moulding grade polypropylene (Titanpro 6331) was used with the melt flow index of 14 g/cm³ and with the density of 0.9 g/cm³. Sisal fibres were obtained from vibrant nature, Chennai, India. Commercial grade NaOH and acetic acid were used for the alkali treatment.

Alkali treatment

The alkali solution was prepared with different concentrations of NaOH (w/v) (0%, 3%, 5%, 7%, 9% and 15%) with distilled water. At first, the clean and dried sisal fibres were soaked in an alkali solution at room

Fourier-transform infrared spectroscopy (FTIR)

FTIR was conducted in between 400 and 4000 cm⁻¹ with 0.4 cm⁻¹ resolution by using PerkinElmer Spectrum RX1 to confirm the surface modification of alkali, ultrasound and the combination of alkali and ultrasound treated sisal fibres. For this, fibre samples were grounded well with KBr (∼2% by weight) and pressed into a pellet with the thickness of about 1 mm. All FTIR spectra were recorded in Transmittance units.

Morphological studies

The surface morphology of untreated, ultrasound treated and the combination of alkali

Effects of alkali and HIU treatments on sisal fibres

Alkali treatment is one of the most common and efficient chemical methods used for cleaning and/or mercerising the surface of natural fibres [26]. On the surface of the sisal fibres, free space exists between the cellulose and amorphous materials, which is responsible for the absorption of moisture. Absorption of moisture leads to the reduction in the mechanical and thermal properties of the fibres. Scheme 1 shows the alkali reaction with sisal fibres in which Na⁺ ions from the alkali solution

Conclusions

Sisal fibres possess light weight, good tensile strength and are cheaper and biodegradable. However, due to the presence of hemicellulose, pectin and lignin on the surface of fibres, the fibres become more hydrophilic which leads to poor compatibility with the polymer matrix. Thus, sisal fibres were successfully surface modified with alkali and ultrasound treatments as these treatments removed the hemicellulose, pectin and lignin and led to an improvement in the mechanical and thermal

Acknowledgements

Authors would like to thank Fundamental Research Grant Scheme (FRGS) for the funding support (FRGS/1/2013/SG05/UNIM/01/1).

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Enhancements in crystallinity, thermal stability, tensile modulus and strength of sisal fibres and their PP composites induced by the synergistic effects of alkali and high intensity ultrasound (HIU) treatments

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials

Alkali treatment

Fourier-transform infrared spectroscopy (FTIR)

Morphological studies

Effects of alkali and HIU treatments on sisal fibres

Conclusions

Acknowledgements

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