Investigation on the Mechanical and Rheological Properties of Coir Fiber Reinforced Polypropylene

Abstract

In the recent time, due to the environmental consciousness the researchers and the manufacturing industries worldwide have been motivated to look for a new class of materials which could be cost-effective and environmentally friendly providing opportunities to replace the traditional non-renewable and non-biodegradable synthetic reinforcing materials in FRPCs materials. On account of this, in the recent years, natural fibers have been being used as reinforcing agent in thermoplastics for applications in automobile, non-structural elements for low cost housing, consumer products and various civil structures. Consequently, the studies of natural fiber-reinforced plastic composites have been increased.

Coir is an important lingo-cellulosic fiber, which is obtained from coconut fruit. But as reported in literature, the performance of coir fiber as reinforcement in polymer composites is unsatisfactory in terms of tensile properties and not comparable even with the other natural fibers. The improvement in the tensile strength, however, could not be the only criterion for the judgment of the improvement of mechanical strength of a given material. There are other parameters such as flexural strength for the evaluation of the mechanical strength as well. Also, keeping in mind the hard wearing quality and durability and also biodegradable nature of the coir fiber, the composite based on it could be used successfully in non-structural applications. With that end in mind, this work encompasses preparation and characterization of coir-fiber reinforced polypropylene (PP) composites and comparison of the results with those reported in literature.

This work describes firstly, preparation of coir-fiber PP compositions with a wide range of fiber loading and their characterization in terms of some mechanical and physical properties. Secondly, it makes thorough analysis of the applicability of the mechanical models available in the literature for the prediction of tensile modulus and proposes a simple empirical model. Finally, it makes rigorous analysis of the theoretical ground of the rheological study and presents some rheological parameters of the composites under study. K. Begum, Coir Fiber Reinforced Polypropylene Composites

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In the first and initial stage of this work, the mechanical properties of both untreated and treated Coir-fiber PP composites were studied. The coir fibers content was in the range of 0-35 wt%. For both the untreated and alkali treated coir-fiber PP composites the tensile strength showed small improvement as compared to that of pure PP with the increase of fiber loading in the range of 0-25wt%, but beyond that it decreased again. In the case of diazonium salt treated coir fiber PP composites there was a decreasing trend of tensile strength with the increase of fiber loading. Optical microscopic images of the fractured surfaces after tensile test showed that the fibers had been pulled out of the matrix providing evidence for weak interfacial bonding between the fiber and the matrix.

A remarkable improvement in flexural strength has been observed for both untreated and treated coir fiber reinforced PP composites. For all systems, the optimum flexural strength was obtained at low fiber load range and then there was a slight decreasing trend with further increase of fiber loading.

The tensile and flexural modulus increased almost linearly with the increase in fiber content for all the three composite systems under study. Among the three composite systems, the diazonium salt treated coir fiber composites yielded the highest values of the tensile modulus.

The elongation at break was found to decrease with the increase in fiber content showing that the reinforcement made the PP matrix more brittle. Water absorption increased with the increase in fiber content. Relatively poorer water absorption was shown by composites with chemically treated fibers than those with untreated ones. This implied that the hydrophilic nature of the natural fibers had been reduced after chemical treatment.

In the second stage of this work, a discussion has been done on the existing volume fraction based theoretical models with no adjustable parameters and semi-empirical models with one adjustable parameter for the prediction of elastic modulus of FRPCs. The experimental data for the elastic modulus of different FRPCs reported in different specialized research journals have been fitted to these models. The probable variation in the averaged value of K. Begum, Coir Fiber Reinforced Polypropylene Composites

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the modulus of elasticity of the polymer and the fiber has been taken into consideration and the validity of the theoretical models has been tested with an acceptable deviation range of the prediction value. It was found that the models without any adjustable parameter such as the Parallel, Series and Halpin-Tsai models totally failed to predict the elastic modulus within an acceptable deviation factor of 0.1. On the other hand, the models with one adjustable parameter and expressed in terms of volume fraction such Modified Halpin-Tsai and Bowyer-Bader model described the elastic modulus satisfactorily. Finally, a simple mass fraction based model with one adjustable parameter was developed and proposed. This model also successfully predicts the elastic modulus data in the Literature as well as those obtained in our laboratory. The proposed model, being mass fraction-based, is more convenient to work with than any volume-fraction based model, and unlike all other models (theoretical and semi-empirical), it has the potentials to have practical applications in structural material design.

In the final stage of this work, an intensive theoretical and experimental study of rheological behavior of melted coir-fiber PP composites has been conducted. The melt shear viscosity, and elastic properties such as the recoverable shear strain, normal stress and elastic strain have been investigated as a function of shear stress, fiber loading and temperature. Unlike similar studies in the literature, in this work the viscosity has been treated as a stress-independent parameter. Coir fiber-PP composites with fiber content of 0, 10, 20, and 30% were prepared to study the rheological behavior. The study has been carried out at the shear stress range of (1.0-4.4) x10 4 Pa and at a temperature of 220-260C.

The viscosity of melted PP composites followed a power law model. The flow behavior index, n, decreased with the increase of fiber content and slightly increased with temperature. The melt viscosity increased with increasing fiber content and for a particular composition, the viscosity decreased with increasing temperature. The activation energy increased with the increase of fiber content. The die-swell ratio (B) increased with the increase of shear stress and temperature, and decreased with the increasing length to diameter ratio (L/D) of the die. Similar to B the recoverable shear strain S R and the first normal stress difference N 1 induced by the stored energy in the capillary reservoir of the K. Begum, Coir Fiber Reinforced Polypropylene Composites

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composites increased with the increase in the extrusion temperature and shear stress. But for a particular stress and temperature this parameter decreased with the increase in the fiber loading. The elastic strain  1 of the composites increased with the increase in shear stress but decreased with the increase of the extrusion temperature.

The SEM data revealed that the fibers were loosely bound to the polymer matrix and the outer surface of the composite was rough and irregular, making it susceptible to high friction to the wall of the flow channel.