Effects of Heat Treatment Conditions on Microstructure and Mechanical Properties of Halloysite Nanotube (HNT) Filled Epoxy Nanocomposites

ABSTRACT

Halloysite nanotubes (HNTs) are naturally occurring Kaolite group minerals having an aluminosilicate-layer in the form of nanotubes. The composites containing a nanofiller in the form of HNTs used as a reinforcement and an epoxy resin as a matrix is effectively fabricated by using polymer casting method incorporating dispersion strengthening by mechanical stilling action and specimens are prepared as per American Society for Testing and Materials (ASTM) standards. The functional nanofillers are effectively used to enhance the mechanical strength of the composite by restricting matrix disengagement movements. In this regard.

the present work is carried out to evaluate the mechanical properties of the nanocomposites consisting of different Wt.% of HNTs effectively heat-treated at three temperatures viz. Room temperature (RT), 50°C and 70°C and varying in the range of 0 to 10 with an interval of 5. The various properties viz. “density, hardness, tensile, flexural, and impact strength” are investigated through the ASTM procedure. As per the experimental investigation, the mechanical properties of the nanocomposite increases by the incorporation of heat-treated HNT. Further, the study revealed that the 10 Wt.% of HNT with 50°C heat-treated nanocomposite shows superior tensile and flexural strength. However, the critical observation of the results reveal that the impact strength is maximum for 70°C heat- treated nanocomposite synthesized for 5 Wt.% of HNT. It is evident that the properties of the nanocomposite depend on the quantity of functional filler present and temperature of heat treatment.

INTRODUCTION

Nanocomposites are materials which take the benefit of dispersoids, whiskers, and platelet form of nanoparticles used as reinforcement in their fabrication. In general, nanofiller with a size range of several hundred nanometers and any polymer matrix phase, either a thennoset or thermoplastic comprises a nanocomposite. Due to the better interfacial area property, the nanoparticles are the most commonly used reinforcement materials compared to other reinforcements [1,2].

Halloysite nanotube (HNT) is basically a kaolite group naturally occurring mineral which includes aluminosilicate nanotubes in layer forms. HNTs are available in the form of white color nanoparticles with chemical formula Al,Si20. (0H)4*2H,0. HNTs are odorless and more economical compared to other nanofiflers, especially carbon nanotubes [3-6]. Studies revealed that mechanical properties of the polymer matrix can be enhanced by the addition of HNTs, e.g., toughness, and elastic modulus can be enhanced through restrictive matrix dislocation activity [7-10]. The effective dispersion of HNTs in the polymer matrix are known to improve the characteristic features, however, it is very difficult to disperse HNTs effectively into the polymer matrix due to clustemess of the HNTs [11-13]. Therefore, it is a challenge for the researchers to establish the light parameters to achieve a homogenous dispersion of HNTs in the polymer matrix.

Currently, the methods such as mechanical stirring, ultrasonic homogenization, and ball milling are commonly used to disperse nanofillers in the polymer matrix [14]. The mechanical stirring is an easy method to achieve homogenous dispersions with the minimum aggregation of nanofillers in the polymer matrix and also it is more suitable for the production of the nanocomposites commercially [15,16]. In the literature, it is revealed that the HNTs have an appreciable amount of water between Si04 and A106 structures [17-19]. Therefore, the structure, chemical, and mechanical properties of HNT reinforced polymer composites depend majorly upon heat-treated temperatures.

In the present work, a set of experiments is earned out to analyze the dispersion behavior and structural changes in HNTs at different heat-treated temperatures. Further, the mechanical properties of HNT-reinforced epoxy nanocomposites are investigated and from the revelations, it is concluded that the addition of HNT nanofillers with different weight proportions in epoxy improves their mechanical properties than compared to neat composites.

MATERIALS AND METHODS

MATERIALS AND FABRICATION PROCEDURE

HNTs are used as reinforcements in current work and they are acquired from Sigma Aldrich Company, Bengaluru, India. The diameter and length of the HNT’s has a range in between 30 to 70 mn and 1-15 pm, respectively. The morphology of HNT has a tube-like structure with a density of 2.53 g/cc and the surface area is about 65 m2/g. It has a high aspect ratio and low percolation property which makes it convenient to be used as reinforcement for epoxy matrix composites. The vacuum furnace (Figure 1.1) is used to heat treat the HNTs at temperatures, viz. 50°C and 70°C for 10 minutes and are subsequently cooled down over a duration of 1 hour. Table 1.1 shows the compositions and specifications of the constituents of nanocomposite.

Vacuum furnace used for heat treatment

FIGURE 1.1 Vacuum furnace used for heat treatment.

TABLE 1.1 Specification of Materials Used in Nanocomposite

Reinforcement

Halloysite Nanotube (HNT)

  • - Sigma-Aldrich
  • - Bengaluru. India
  • - Stock No. NS6130-09-917 -CAS-No.: “1332-58-7”
  • -Formula: “Al,Si,05(0H)4 ■ 2 H.O”
  • - Molecular Weight: “294.19 g/inol”
  • - Purity: 99.9%

Matrix

Epoxy Resin

  • - Atul India Ltd., Gujarat, India
  • - Product: Lapox L-12
  • - Chemical name: Diglycidyl Ether Bisphenol (DGEBA)
  • -Density: 1120 g/cc
  • - Gelation Time at 80°C: 205 minutes
  • - Glass Transition Temperature: 140°C
  • - Cluing time: 4 hrs. at 140°C

Hardener

Tryethylene Tetramiue (TETA)

  • -Atul India Ltd., Gujarat, India
  • - Product: K-6
  • - Density: 954 g/cc

Accelerator

N,N- dimethyl benzylamine (BDMA)

  • - Sigma-Aldrich, Bengaluru. India -CAS-No.: “103-83-3”
  • - Accelerator
  • - Density: “0.9 g/ml at 25°C (lit)”

Heat-treated HNTs considered for present work are thoroughly mixed in the epoxy resin with a hardener in the ratio 100:12 by weight using a mechanical stirrer (as shown in Figure 1.2) maintaining a constant speed of 500 r.p.m., followed by the vacuum cleaning carried out for a duration of 10 min. The mixture is then poured into molds and allowed to cure at RT for 24 hours. Further, post-curing is done at a temperature of 50°C for 2 hours. Three different fractions of HNT (0, 5 and 10 Wt.%) are used to reinforce the epoxy.

There are five samples for each test that are prepared in accordance with the American Society for Testing and Materials (ASTM) standards through the polymer casting method, The specimens fabricated are as shown in Figure 1.3. The details of the constituents included in prepared composites are tabulated in Table 1.2.

TABLE 1.2 Details of the Constituents With Designation for Each of the Sample of the Composites

Weight

Percent

Weight

Percent

Weight

Percent

Untreated

(RT)

HNTs

  • 5
  • 10

50°C Heat- treated HNTs

  • 5
  • 10

70°C Heat-treated HNTs

  • 5
  • 10
Mechanical stirrer for dispersion process

FIGURE 1.2 Mechanical stirrer for dispersion process.

Specimen as per ASTM standards for Mechanical characterization

FIGURE 1.3 Specimen as per ASTM standards for Mechanical characterization.

EXPERIMENTATION

MICROSCOPIC EXAMINATION

The heat-treated HNT particles are inspected in a transmission electron microscope (ТЕМ) equipped with a computer image analysis system; the ТЕМ measurements are carried out with a 1200 EX ТЕМ applying an accelerating voltage of 120 keV.

MECHANICAL PROPERTIES TESTING PROCEDURE

The density of the composites was determined by using a high precision electronic balance (Mettler Toledo, Model AX 205) using Archimede’s principle. Hardness (Shore-D) of the samples was measured as per ASTM D2240, by using a Hiroshima make hardness tester (Durometer). Six readings at different locations were noted and the average value is reported. The tensile measurement was carried out using a universal tensile testing machine (JJ Lloyd, London, United Kingdom, capacity 1-20 kN), according to ASTM D3039. The tensile test was performed at a crosshead speed of 30 mm/min (quasi-static).

The flexural strength of the particulate filled composites is determined on rectangular specimens (90 mm x 12 lmn x 3 mm) in three points bending at RT according to ASTM D790. The span length of the specimens was 70 mm and their loading on a universal testing machine (JJ Lloyd, London, United Kingdom, capacity 1-20 kN) occurred with deformation rate v = 1.3 mm/min.

Izod impact test was earned out using an Aveiy ceast pendulum impact tester (ASTM D256-92). A 7.5 J/m energy haimner with the striking velocity is 3.46 m/s is used. Five samples are tested for each composite type for all the studies and the average value is recorded.

RESULTS AND DISCUSSION

MICROSTRUCTURE OF HEAT-TREATMENT HNTS

ТЕМ images of HNTs reinforced epoxy nanocomposites are presented in Figure 1.4(a), (b), and (c), respectively. In both heat-treated and untreated conditions, the most of HNTs particles are uniformly dispersed at nanometer scale in an epoxy matrix. HNTs are multi-wall nanotubes consisting of aluminosilicate layers, which are curved and closely packed. The outer diameters, inner diameters, and lengths of the nanotubes are about 30-180 nm, 10-30 rim, and 2-10 pm, respectively. The distinctive boundaries between each layer can be clearly visible is as shown in Figure 1.4(a).

The HNTs are heat-treated at temperatures viz. untreated HNTs (at RT), 50°C and 70°C, Figure 1.4(b) shows that HNTs particles are irregular in shape and also formed agglomeration by itself. Further, as the heat treatment temperature increases the structure of the HNTs formed like amoiphous, which are even more undefined in shape is as shown in Figure 1.4(c). Therefore, from this study, it was concluded that the hydroxyl gr oups of HNTs are eliminated due to dehydration as the heat temperature increases. And also, temperature more than 50°C influences extensively on the structural change of the HNTs and predicted that these structural changes should be strongly monitoring the behavior of the HNTs which was used as a reinforcement material for the epoxy matrix.

ТЕМ images ofHNT reinforced epoxy nanocomposites. (a) Untreated (RT) with 10 Wt.% HNT. (b) heat-treated at 50°C with 10 Wt.% HNT, (c) heat-treated at 70°C with 10 Wt.% HNT

FIGURE 1.4 ТЕМ images ofHNT reinforced epoxy nanocomposites. (a) Untreated (RT) with 10 Wt.% HNT. (b) heat-treated at 50°C with 10 Wt.% HNT, (c) heat-treated at 70°C with 10 Wt.% HNT.

DENSITY OF EPOXY/HNT NANOCOMPOSITES

In order to obtain optimized dispersion conditions for improving mechanical properties, the specimens for various tests are synthesized using heat-treated HNTs at various temperatures. The density of a composite depends on the relative proportion of matrix and reinforcing materials. Figure 1.5 shows an increasing trend in density observed with the increasing content ofHNT particles in the epoxy matrix. By contrast, a marginal improvement in density is observed in nanocomposites of both untreated and heat-treated HNTs and also the structural changes of HNTs at various temperatures which affect significantly on the density of Epoxy/HNT nanocomposites are inferred. The highest density is observed in the case of 10 Wt.% HNTs-Epoxy system at a heat-treated temperature of 70°C due to better dispersion of the reinforcements as well as enhanced agglomeration.

Density of HNT filled epoxy nanocomposites

FIGURE 1.5 Density of HNT filled epoxy nanocomposites.

HARDNESS OF EPOXY/HNT NANOCOMPOSITES

The surface hardness is considered to be one of the important factors to be determined and it has a major effect on the wear rate of the composites. The test result shows that there is an increase in the hardness value of the nanocomposites by the incorporation of HNT content. The hard nature of epoxy with exfoliation of polymer chains in between two plates of nano-clay (HNT) makes the surface of composite veiy hard due to which the indentation of the indenter is quite difficult. The critical observation of the graph as seen in Figure 1.6 reveals that there is no specific trend with variation in heat-treated temperatures. Improvement in hardness is observed only in nanocomposite that contained 50°C heat-treated HNT as the reinforcement and for 70°C heat-treated HNTs, nanocomposites exhibited a decrease in value even with the addition of 10 Wt.% HNT. This signified that the structural changes of HNTs at various temperatures are involved to obtain a formidable chemical combination between epoxy and HNTs.

Hardness of epoxy HNT nanocomposites

FIGURE 1.6 Hardness of epoxy HNT nanocomposites.

TENSILE BEHAVIOR OF EPOXY/HNTS NANOCOMPOSITES

From Figure 1.7, it is evident that the tensile strength of composites is found to increase with HNT reinforcement compared to virgin epoxy due to the restriction of the mobility and defonnability of the epoxy and also the formation of ordered exfoliation of polymer chains in between the interstitial spacing of nanoclay (HNT). The addition of heat-treated HNT in epoxy increased the interfacial stiffness and static adhesion strength of the composites compared to neat, which constitutes to transfer the elastic deformation to a great extent. The results showed a decrease in tensile strength in 70°C heat-treated HNT due to poor bonding at the interface between epoxy and HNT nanoparticles. This low degree of interfacial interaction will result in a decrease in tensile strength even with the higher concentration of HNT reinforcements.

Tensile strength of epoxy'HNT uanocomposites

FIGURE 1.7 Tensile strength of epoxy'HNT uanocomposites.

FLEXURAL STRENGTH OF EPOXY/HNT NANOCOMPOSITES

Epoxy/lieat-treated HNT reinforced nanocomposites exhibit an increasing trend in flexural strength as shown in Figure 1.8. This is due to a positive effect of HNT on the performance of epoxy resin correlated with the unique characteristics of the HNT to impart better flexural rigidity. The uniform dispersion and a strong interfacial bonding between modified or heat-treated HNT and epoxy register better flexural value compared to neat and untreated composites. There is a drop in flexural strength value of nanocomposites at higher heat- treated temperature (70°C) due to interfacial slippage between nanotubes and epoxy matrix during the bending of the composites due to the inert surface of nanotubes that leads to improvement in the flexural rigidity of the composite specimens at the neutral fiber of the HNTs thereby enhancing its flexural strength.

Flexural strength of epoxy HNT nanocomposites

FIGURE 1.8 Flexural strength of epoxy HNT nanocomposites.

IMPACT ENERGY OF EPOXY/HNT NANOCOMPOSITES

Figure 1.9 shows the variation of the impact energy of epoxy composites with different conditions of HNTs in the composites. Under impact

Impact energy of epoxy/HNT nauocomposites

FIGURE 1.9 Impact energy of epoxy/HNT nauocomposites.

loading situation, a gradual improvement in impact strength with the addition of HNT in different Wt.% is noticed in the composites due to better compatibility between epoxy and HNT that will be the important factor to obtain the nanocomposites with a high level of dispersion in order to provide better impact strength and toughness. The impact strength of composite having a 70°C heat treated HNT with 5 Wt.% composition reinforcement in epoxy is higher as compared to other grades of composites. The further higher weight percentage of HNT leads to an increase in brittleness of epoxy resin which decreases the impact strength of the epoxy composites.

CONCLUSIONS

hi this study, the microstructure and mechanical properties are evaluated for heat-treated HNTs synthesized with the epoxy matrix. The primary objective is to generate a strong interfacial bond between the epoxy and the HNTs through an optimal synthetic process. There are three types of heat treatments incorporated for HNTs; viz. untreated HNTs (at RT), 50°C heat-treated HNTs and 70°C heat-treated HNTs. The mechanical stirring process is effectively employed to disperse the HNT in epoxy resin thoroughly. Experimentally, the inferences drawn from the analysis of the results are as follows: [1]

Graphical illustration

FIGURE 1.10 Graphical illustration.

HNT added to the epoxy nanocomposite registered highest impact resistance value of 5.61 J/m.

• Further, the density and hardness of the composite specimens improved with the addition of reinforcements and incorporation of heat treatment conditions irrespective of loading and other attributes (Figure 1.10).

ACKNOWLEDGMENTS

The authors are grateful to the management of M/s. Brakes India Limited, Nanjangud, and Karnataka, India for providing the required facilities for conducting the present study. The authors are also thankfully acknowledged VTU-RRC Belagavi, Karnataka, India for their co-operation and encouragement to cany out this research work.

KEYWORDS

  • • epoxy
  • • halloysite nanotube (HNT)
  • • heat treatment
  • • mechanical properties
  • • polymer stir casting
  • • vacuum furnace

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  • [1] The epoxy nanocomposites are successfully fabricated reinforcingwith HNT particles by polymer stir casting method, the additionof HNT in epoxy nanocomposites exhibits an improvementin the mechanical properties in both untreated and heat-treatedconditions. • The mechanical properties viz. hardness, tensile, and flexuralstrength are increasing for 50°C heat-treated condition with 10wt.% of HNT incorporated in the epoxy matrix, Thus it couldbe proposed as the optimum mixture ratio for epoxy nanocomposites for a number of applications requiring above mentionedattributes. • An improvement in the impact performance of epoxy nanocomposites is observed with respect to varying filler loading conditionsand heat treatment temperatures. The 70°C heat-treated 5 wt.%
 
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