Scholarly article on topic 'Physical and Thermal Characterization of Red Mud Reinforced Epoxy Composites: An Experimental Investigation'

Physical and Thermal Characterization of Red Mud Reinforced Epoxy Composites: An Experimental Investigation Academic research paper on "Materials engineering"

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Procedia Materials Science
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{"Polymer matrix composites" / epoxy / "red mud" / "morphology / effective thermal conductivity."}

Abstract of research paper on Materials engineering, author of scientific article — Johan Banjare, Yagya Kumar Sahu, Alok Agrawal, Alok Satapathy

Abstract The present paper deals with the effect of volume fraction of filler particles on the effective thermal conductivity (keff) of particulate filled polymer composites. A new class of epoxy based composites reinforced with an industrial waste i.e. red mud, with filler content ranging from 0 to 25 vol % has been prepared by conventional hand lay-up technique. keff of these fabricated samples are measured and then compared with the values obtained from themathematical model proposed by the authors in their earlier investigation and also with some existing models. A significantenhancement of about 135% in the value of keff is observedfor maximum filler loading. This comparison tells that,among all the models,the results obtained from the proposed model are in closest approximationwith the measured values. Some physical properties like density and void fraction together with morphological behavior of the entire fabricated specimens are also reported.

Academic research paper on topic "Physical and Thermal Characterization of Red Mud Reinforced Epoxy Composites: An Experimental Investigation"


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Procedía Materials Science 5 (2014) 755 - 763

International Conference on Advances in Manufacturing and Materials Engineering,

AMME 2014

Physical and thermal characterization of red mud reinforced epoxy composites: An experimental investigation

Johan Banjare*,Yagya Kumar Sahu,Alok Agrawal,Alok Satapathy

Dept. of Mechanical Engineering, National Institute of Technology-Rourkela, India


The present paper deals with the effect of volume fraction of filler particles on the effective thermal conductivity (keff) of particulate filled polymer composites. A new class of epoxy based composites reinforced with an industrial waste i.e. red mud, with filler content ranging from 0 to 25 vol % has been prepared by conventional hand lay-up technique. keff of these fabricated samples are measured and then compared with the values obtained from themathematical model proposed by the authors in their earlier investigation and also with some existing models. A significantenhancement of about 135 % in the value of keff is observedfor maximum filler loading. This comparison tells that,among all the models,the results obtained from the proposed model are in closest approximationwith the measured values. Some physical properties like density and void fraction together with morphological behavior of the entire fabricated specimens are also reported.

©2014ElsevierLtd.Thisisanopenaccessarticleunder the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/3.0/).

Selection and peer-review under responsibility of Organizing Committee of AMME 2014

Keywords:Polymer matrix composites, epoxy, red mud, morphology,effective thermal conductivity.

1. Introduction

Polymers are widely utilized by several industries as they show remarkable properties when it comes to corrosion resistance, durability, low density and low fabrication cost. However, because of its low thermal conductivity, their applications in electronic industries are decreasing as the need of faster and denser circuits intensified for fulfilling

* Corresponding author. Tel.: 8269810404; fax: +91-661-2462022. E-mail 0^<

2211-8128 © 2014 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (

Selection and peer-review under responsibility of Organizing Committee of AMME 2014 doi:10.1016/j.mspro.2014.07.325

the future need and unfortunately they cannot effectively dissipate the generated heat which results in thermal failure [Pezzotti et al.,2000].


¿^Effective thermal conductivity kp Thermal conductivity of matrix polymer kf Thermal conductivity of filler material

Volume fraction of filler material pc Density of fabricated composite

/^Density of matrix polymer Pf Density of filler material

So in place of various commonly used polymer materials i.e. polyethylene, polypropylene, epoxy and polyester, polymer with improved thermal conductivity would certainly serve as the better alternate. Improved thermal conductivity in polymers may be achieved either by molecular orientation [Peng and Landel, 1975] or by the addition of conductive fillers. The usefulness of particle filled polymer composites have increased because of their versatile applications in science and engineering for technological development. The use of metal powder as filler in polymers relates chiefly to applications requiring a certain degree of electrical conductivity, magnetic permeability, sound absorption and improved thermal conductivity. Different type of powders are widely reinforced in polymer for increasing thermal conductivity of composite system, few of them are diamond [Cho et al., 2011], silver [Bjorneklett et al., 1992], carbon [Song and Youn, 2006], graphite [Liu et al., 2008], copper [Zhang et al., 2014], aluminium [Agrawal and Satapathy, 2012] etc. From the above literature, it had been seen that for improving thermal conductivity of polymers, metal particles are widely used, but industrial waste like fly ash [Yang et al., 2009] and copper slag [Biswas et al., 2012] are proved to be a good replacement of metal particles in the polymer composites because of their thermal conduction behavior, as fly ash contains carbon in it whereas copper is present in its slag. Also being waste product of industries, they are easily available with no cost involved. In view of this, another industrial waste, red mud which is an waste product generated during alumina production from bauxite by the Bayer's process can be potential filler for improving thermal conductivity of polymers. Though reinforcement of red mud in polymer is already explored by few researchers, but they studied mainly the mechanical and tribological [Gok et al., 2007; Biswas and Satapathy, 2010] aspect of it, whereas no work has been reported till know on thermal characterization of red mud reinforced polymer composite. In this respect, present study deals with usage of micro sized red mud powder as filler material for increasing thermal conductivity of epoxy resin.

The basic interest of present work is to study the physical and thermal properties of polymeric materials reinforced with particulate filler, and more particularly their thermal conductivity. Determining the thermal conductivity of composite materials is crucial in number of industrial processes.The keff of a composite material is a complex function of their geometry, the thermal conductivity of the different phases, distribution within the medium and contact between the particles [Kumlutas et al., 2003].In view of this, the present work has been undertaken to study the effect of adding micro-sized red mud powder of elliptical shape on the thermal conductivityof epoxy resin. The measured values are compared with various analytical models including the model proposed by the authors in their earlier work [Banjare et al., 2014]. For physical characterization, effect of filler loading on density and void content are calculated experimentally and analytically. Some SEM micrographs are also presented to study the shape and size of filler particles and their affinity towards matrix material.

2. Theoretical models foreffective thermal conductivity

To predict the keff of composite materials there are several theoretical and empirical models have been proposed. There are many analytical modelsavailable for predicting the effective thermal conductivity of filled polymer composites. The two basic equation of thermal conduction used in composites are of Rule of mixture, derived on the basis of series conduction and parallel conduction respectively, Maxwell [Maxwell, 1954] assumes a random

dispersion of small sphere within a continues matrix to calculate the effective thermal conductivity which hold good for low filler concentrations. Bruggeman [Bruggeman, 1935] derived an equation of thermal conductivity versus the solids loading for spherical fillers in a dilute suspension. Behrens [Behrens, 1968] derived a theoretical model for calculating effective thermal conductivity for elliptic filaments in a square lattice. A few comprehensive review articles too have discussed the applicability of many of these models [Progelhof et al., 1976; Mamunya et al., 2002]. For a two-component composite, the simplest alternatives would be with the materials arranged in either series or parallel with respect to heat flow.

For the parallel conduction model:

kff = (l-^/ )kp +4fkf (1)

For series conduction model:

1 _(1 ) ^

The correlations presented by Eqs. (1) and (2) are derived on the basis of the Rules-of-mixture. Maxwell obtained an exact solution for the conductivity of randomly distributed and non-interacting homogeneous spheres in a homogeneous medium.

kf + 2kp + 20f (kf -kv )^

kf + 2kv(kf -kv)

Thermal conductivities for low filler concentrations are predicted very well using this model; but when there is an increase in filler concentrations, conductive chains is started to form and modelstarts to underestimate the measured values. Some other models which can be used for predicting the effective thermal conductivity of polymer composite system are Bruggeman model

1 ~éf =

Kff - kf

kp - kf

Behrens Theoretical Model:

\p + 2)+(p -1)2^.

keff = kP

(p + 2)-(p -1)

From the above studies it can be seen that there is no single model which can predicts the effective thermal conductivity of particulate filled polymer composite for all filler type, size, shape and concentrations. Model proposed by the authors in their earlier investigation [Banjare et al., 2014] is used for calculate theoretical valueof kefffor typical particulate filled polymer composite and is given as:

k„ k

-P V n J

(kf - K

The above model is developed using the law of minimal thermal resistance and equal law of the specific equivalent thermal conductivity.

3. Experimental details

Epoxy (LY 556) resin is used as matrix material. Its common name is Bisphenol-ADiglycidyl-Ether and it is belonging to the "epoxide" family. Inspite of its low thermal conductivity (0.363 W/m-K) and high CTE (31.0 x 10"7K) it possesses low density (1.1 gm/cm and low dielectric constant (4.3 at 1 MHz) which makes this polymer feasible for microelectronics application.Epoxy (LY 556) resin is used with its corresponding hardener Methylene tetramine (TETA, HY 951). The epoxy resin and the hardener used for the present work are supplied by Ciba Geigy India Limited.An industrial waste red mud powder is used as filler material. Red mud is insoluble product generated after the digestion of bauxite with alkali sodium hydroxide at high temperature and pressure to produce alumina. This process of extraction of alumina is known as Bayer's process. It has wide range of application starting from in the field of building (clay material, cements, ceramics, fired and non-fired building materials, concrete industry), pollution control (treatment of waste water and polluted waste gases), metal recovery (iron, titanium, aluminium, alkali), coagulant, adsorbent, catalyst and in soil remediation. Itpossesses intrinsic thermal conductivity and density values of 11.7 W/m-K and 3.1gm/cm3respectively which make this material suitable for present application.

At the room temperature curing epoxy resin (LY 556) and the corresponding hardener (HY951) are mixed in a ratio of 10:1 by weight as per recommendation. An industrial waste red mud powder with average size 70- 80 ^m is reinforced in epoxy resin to prepare polymer composite. Conventional hand-lay-up technique is used for preparation of polymer composite specimens. The dough (epoxy filled with red mud powder) is then slowly decanted into the glass molds, coated beforehand with uniform thin film of silicone-releasing agent. The composites are cast in these molds so as to get disc-type cylindrical specimens (diameter 50 mm, thickness 3 mm). Composites of five different compositions (5, 10, 15, 20, and 25 vol % of red mud powder) are made. The castings are left to cure at room temperature for about 24 hours after which the glass molds are broken and samples are released. Table 1 show the various sample prepared on the basis of volume fraction of particulate filler.

Table 1. List of Particulate-filled polymer composites fabricated by hand-lay-up technique. Sample Composition by hand lay-up technique

1. Epoxy+5 vol% Red mud particle

2. Epoxy+10 vol% Red mud particle

3. Epoxy+15 vol% Red mud particle

4. Epoxy+20 vol% Red mud particle

5. Epoxy+25 vol% Red mud particle

4. Characterization

The distribution characteristics of filler particles into the matrix for redmud/epoxy composites have been studied directly using scanning electron microscope (SEM) JEOL JSM-6480LV. The composite samples are mounted on stubs with silver paste. To enhance the thermal conductivity of the samples, a thin film of platinum is vacuum-evaporated onto them before the photomicrographs are taken.The density of composites is measured by Pycnometer in present investigation. Pycnometer works on Archimedes principle. It uses a working liquid with well-known density, such as water. Pycnometer can used to determine the density of homogeneous solid object that does not dissolve in working liquid. First pycnometer is filled with distilled water. The volume of water that is in the pycnometer and the stopper is given by

V = ^L (7)

where mH2o is experimentally determined weight of water (empty pycnometer weight is subtracted). After that weight of pycnometer is measuredtogether with inserted composite specimen i.e. mc+ms. After that water is added and weight m'H20 [(measured weight) -(mc+ms)]is determined. The volume of added water V'H2o can be obtained by:

-yf ^^ tiiii

The volume of measured solid composite Vcis the difference between the volume of water that fills the empty pycnometer Vand volume F^owhich is given as:

V _ V _ V- _ mH20 ~ H20 Vc ¥ ¥ H 20

PH 20 (9) Density of measured object pc can be then calculated as mc

= T c (10)

where mc = Mass of fabricated composite

Vc = Volume of fabricated composite pc = Density of fabricated composite

Unitherm™ Model 2022 is used to measure thermal conductivity of a variety of materials. These include polymers, ceramics, composites, glasses, rubbers, some metals and other materials of low-to-medium thermal conductivity. Non-solids, such as pastes or liquids, can be tested using special containers. Thin films can also be tested accurately using a multi-layer technique. The tests are in accordance with ASTM E-1530 standard. Circular sample of 50mm diameter and 3mm thickness with flat surface on both sides were used for the measurement. In Unitherm™ Model 2022 the material is held under uniform compressive load between two polished surfaces, controlling each sample at a different temperature. The lower surface is part of a calibrated heat flow transducer. Heat flow directs from upper surface through sample to lower surface, establishing axial temperature gradient in stack. On reaching thermal equilibrium, temperature difference across the sample surfaces is assessed along with heat flow transducer output. The sample thickness value is then measured and used to estimate thermal conductivity. The temperature drop through sample is measured using sensors on metal surface layers on either side.In Unitherm™ 2022, transducers measure value of heat flux Q and temperature difference between upper and lower plate. Thus thermal resistance between surfaces can be evaluated. Providing different thickness and known cross-sectional area as input parameters, the sample thermal conductivity can be calculated.

5. Results and discussion

5.1 Morphology

The micro structure of filler i.e. red mud powder is shown in figure.l (a). From the figure.l(a)it can be confirmed that the shape of filler particles isalmost elliptical in nature. The surface morphology of fabricated composites i.e. red mud/epoxy is shown in figurel. (b).In thismicrograph, filler content is of 25 vol%.From thefigures it is very much clear that the distributions of micro-sized particles in epoxy resin for the fabricated sampleare more or less can also observed from SEM images that further increase of filler content into matrix material beyond 25% volume fraction is quite difficult task, as increase in filler content reduces the inter-particle distance up to the limit that particle start to interfere with each other, which may degrade the properties of filler as well as composite because of improper wetting.

Figure. l.SEM images of (a) red mud powder and (b) red mud/epoxy composite

5.2 Density

Density is a material property which is of prime importance in several weight sensitive applications. Thus, in many such applications polymer composites are found to replace conventional metals and materials primarily for their low densities. Density of a composite depends on the relative proportion of matrix and the reinforcing materials. As in the present case, the density of the filler is higher than the pure epoxy, it is important to note the increase in density of the composite. The variation in density for composite is measured experimentally byPycnometer. It is also calculated theoretically using following equation [Agrawal and Sathapathy, 2014]:

Pc = (W/)Pp +tfPf (11)

The above equation is of rule of mixtures, where fa is the volume fraction of filler, pc, pj, and pp are the density of composite, filler material and matrix respectively.

Figure.2 shows the effect of volume fraction of fillers in the density of epoxy matrix composite. It is clear from the graph that as the volume fraction of the filler is increasing the density of the composite is increasing. There is always a difference between the measured and the theoretical density values of a composite due to the presence of voids and pores. These voids significantly affect some of the mechanical properties and even the performance of composites. Higher void contents usually mean lower fatigue resistance, greater susceptibility to water penetration and weathering [Agarwal and Broutman, 1990].

1.7 1.6 IT 1-5

15 1-3

Q 1.2 1.1 1

0 5 10 15 20 25

Red mud content (Vol %)

Figure.2. Effect in density with volume fraction of filler particle

Table. 2 shows the theoretical and experimental density with void fraction of red mud filled epoxy composite. From the table it can be seen that with the increases of filler particle void fraction also increases.

Table 2. Theoretical and Experimental densities of fabricated sample in g/cm3 with void contain

Sample Filler content (vol %) Theoretical density pa, Experimental density p6xp Void %

1 5 1.2 1.16 3.33

2 10 1.3 1.23 5.38

3 15 1.4 1.32 5.71

4 20 1.5 1.39 7.33

5 25 1.6 1.48 7.50

5.3 Thermal conductivity

Figure. 3 shows the variation in the value of keff when micro-sized red mud powder is added in epoxy matrix. It shows the comparison between the values obtained from various established theoretical model, experimental values

andobtained from proposed model. It is clear from the figure that there is significant increase in the value of keff as the concentration of red mud particles is increasing. Also it is clear that while none of the established model are predicting the keff values correctly, only the model proposed by the authors are precisely predicting the measured values. Though these approximation is only up to 20 vol% of filler loading. As the volume fraction of filler increases beyond this, the particles begin to touch each other, resulting in the formation of conductive path, due to which a sudden jump in the value of thermal conductivity is observed. The limiting filler content (volume fraction) at which such sudden rise in keff of the composite is noticed is called the percolation threshold of that particular filler in the resin. For epoxy/red mud composite, the percolation threshold reaches when filler content increases beyond 20 vol %. Beyond this no theoretical model is estimating the conductivity value correctly including the model proposed by the authors previously. This can be attributed to the fact that, while deriving the correlation, the authors have not taken care of the inter-connectivity between the filler particles which are built up at higher filler concentrations in the real composite.

"¡5 £ <u

1.4 1.2 1

0.8 0.6 0.4 0.2 0

- * Rule of Mixture model — Bruggeman's model —■— Experimental values —•— Maxwell's model —x— Proposed model

- ___

ytf^ --1--*- i i i i

Red mud content (Vol %)

Figure 3. Comparison inthermal conductivity of red mud/epoxy: Rule of mixture, Maxwell's model, Bruggman model

proposed model and Experimental values.

6. Conclusions

Based on the experimental and analytical work reported above, it can be concluded that:

• The set of epoxy/red mud composites can be successfully fabricated by simply hand lay-up technique.

• The model proposed by authors in their previous work is predicting the keff values of fabricated composites precisely up to percolation threshold.

• An improvement of about 135% in the value of thermal conductivity is recorded with addition of 25 vol % of red mud filler in epoxy resin.

• From the SEM images it is clear that filler particle mud is more or less elliptical in shape and their distribution is uniform over the epoxy resin.

• These above fabricated composites can find their potential applications in electronics encapsulation, printed circuit board, heat sink etc.


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