Experimental. Chapter 2. Abstract:

Transcription

1 Chapter 2 Experimental Abstract: This chapter provides a detailed description about the materials and the formulation used for the preparation of nanocomposites. The method of composite fabrication is presented in detail. This chapter also deals with the different characterization techniques used for the morphological analysis and for evaluating the properties such as mechanical, dynamic mechanical, rheological, thermal degradation, dielectric breakdown strength, water resistance, transport properties, surface characteristics and crystallization behaviour of the nanocomposites.

2 94 Chapter Materials Low density polyethylene (PETROTHENE NA951080), with a density of 0.94 g/cm 3 was obtained from Equistar, USA. The cross-linking agent, dicumyl peroxide (DCP) and antioxidant, Irganox 1010 was used. The different spherical nanofillers used were Al 2 O 3, SiO 2, TiO 2, ZnO and SiO 2 +TiO 2 mixture. Nanofillers of 100 % silane, trimethoxyoctyl-reaction product, having around 25 nm in diameter were obtained from Evonik Industries, United States. The nanoclay: Bis (hydrogenated Tallow Alkyl) dimethyl, salt with Bentonite was also obtained from Evonik Industries, United States. Table 2.1: Materials Polymer (matrix or continuous phase or major phase) Nanofillers (dispersed phase or minor phase) Cross-linking agent Low-density polyethylene Al 2 O 3, SiO 2, TiO 2, ZnO and SiO 2 + TiO 2 mixture and clay Dicumyl peroxide (DCP) Antioxidant Irganox Nanocomposite Fabrication XLPE/Al 2 O 3 Nanocomposites XLPE nanocomposites were prepared by melt mixing using dicumyl peroxide as the curing agent. The nanocomposites with 0, 2, 5 and 10 wt% Al 2 O 3 were prepared. The cross-linking agent, DCP 1.5 wt% and antioxidant 0.5 wt% were used. The mixing was done in a Haake mixer at 160 o C and 60 rpm for 12 minutes. The temperature, rotation speed, time

3 Experimental 95 of mixing and the amount of DCP and Irganox were kept constant for all mixes. These parameters have been selected based on the previous findings on this system. The mixed nanocomposites were compression molded in a SHP-30 model hydraulic press with a maximum pressure of 200 kg/cm 2 at 180 C for 5 minutes. High pressure was applied while molding, otherwise the escaped methane, the cross-linking by-product will form pores in the films. Nanocomposites with 2, 5 and 10 wt% Al 2 O 3 content were designated as A2, A5 and A10. Composite of XLPE without nanofiller, designated as X was also prepared for comparison. Table 2.2: Sample codes of XLPE/Al 2 O 3 nanocomposites. Sample details Neat XLPE XLPE + 2 wt% Al 2 O 3 XLPE + 5 wt% Al 2 O 3 XLPE + 10 wt% Al 2 O 3 Sample code X A2 A5 A XLPE/Inorganic Filler Nanocomposites XLPE nanocomposites with Al 2 O 3, SiO 2, TiO 2, ZnO and clay nanoparticles were prepared keeping the filler concentration as 5 wt%. Fabrication method is same that of Nanocomposites are represented as XLPE/Al 2 O 3 - A, XLPE/SiO 2 - S, XLPE/TiO 2 -T, XLPE/ ZnO -Z and XLPE/clay C, where A, S, T, Z and C stands for compositions of Al 2 O 3, SiO 2, TiO 2, ZnO and clay respectively.

4 96 Chapter 2 Table 2.3: Sample codes of XLPE/Inorganic filler nanocomposites. Sample details Neat XLPE XLPE + 5 wt% Al 2 O 3 XLPE + 5 wt% SiO 2 XLPE + 5 wt% TiO 2 XLPE + 5 wt% ZnO XLPE + 5 wt% Clay Sample code X A S T Z C XLPE/Al 2 O 3 /clay and XLPE/SiO 2 /TiO 2 Ternary Nanocomposites For all binary and ternary composites, nanofiller concentration kept fixed as 5 wt%. Fabrication method is same as that of XLPE/clay composite is designated as C, XLPE/ Al 2 O 3 composite is designated as A, Al 2 O 3 and clay in 1:1 ratio composite is designated as A1C and Al 2 O 3 and clay in 2:1 ratio composite is designated as A2C. XLPE/SiO 2 composite is designated as S, XLPE/TiO 2 composite is designated as T and XLPE/ SiO 2 and TiO 2 in 1:1 ratio is designated as ST. Table 2.4: Sample codes of XLPE/ Al 2 O 3 /clay and XLPE/SiO 2 /TiO 2 ternary nanocomposites. Sample details Neat XLPE XLPE + 5 wt% Al 2 O 3 XLPE + 5 wt% Clay XLPE + 5 wt% Al 2 O 3 & Clay in 1: 1ratio XLPE + 5 wt% Al 2 O 3 & Clay in 2: 1ratio XLPE + 5 wt% SiO 2 XLPE + 5 wt% TiO 2 XLPE + 5 wt% SiO 2 &TiO 2 in 1: 1 ratio Sample code X A C A1C A2C S T ST

5 Experimental XLPE/ZnO Nanocomposites Fabrication method is same that of Nanocomposites with 2, 5 and 10 wt% ZnO content were designated as Z2, Z5 and Z10. Table 2.5: Sample codes of XLPE/ ZnO nanocomposites. Sample details Neat XLPE XLPE + 2 wt% ZnO XLPE + 5 wt% ZnO XLPE + 10 wt% ZnO Sample code X Z2 Z5 Z Characterization Techniques Morphology and Structure Transmission Electron Microscopy (TEM) The dispersion of nano fillers in polymer nanocomposites was investigated using TEM. The micrographs of the nanocomposites were taken in JEOL JEM transmission electron microscope with an accelerating voltage of 200 kev. Ultrathin sections of bulk specimens (about 100 nm thickness) were obtained at -120 o C using an ultra microtome fitted with a diamond knife X-Ray Diffraction (XRD) The effect of nanoparticles on the crystallization of XLPE were examined using a wide angle X-ray diffractometer (WAXD) with Ni filtered CuK source having wavelength, λ=0.154 nm operated at 40 KV and 30 ma, and 2 theta from 1 to 10 o (D8 Advance, Bruker AXS, Germany).

6 98 Chapter Mechanical Properties Tensile Tests Tensile tests were performed at room temperature using EOLEXOR 500 N/Auto sampler (ASSS) (GABO Qualimeter Testanlagen Gmbh) machine. The span length and cross head speed used for the testing were 40 mm and 50 mm/min, respectively. Yield strength and Young s modulus were calculated by plotting nominal stress vs. nominal strain curve Tensile Tests at 120 o C The stress-strain behaviour of all the nanocomposites at 120 o C was also analysed to estimate the network density of the system Thermal Analysis Thermogravimetric Analysis (TGA) The thermal stability and decomposition characteristics of nanocomposites were tested using Netisch TG209 F3 Tarsus at N 2 atmosphere. The heating rate was 10 o C/minutes from 25 o C to 800 o C. Decomposition temperature and the amount of residue were estimated from TGA Differential Scanning Calorimetry (DSC) Thermal properties of the samples were studied using Pekin Elmer, DSC- 7 Instrument. Samples of about mg were analyzed by heating and cooling at constant rate of 10 C/min under nitrogen atmosphere to avoid thermal degradation. The heating interval was C. Melting temperature, crystallization temperature, H, percentage crystallinity and lamellar thickness can be measured from the DSC data.

7 Experimental 99 Percentage Crystallinity & Lamellar Thickness Percentage crystallinity Xm is defined by the ratio between Hm/(1-χ) (where Hm is the melting enthalpy and χ is the content of nanoparticle) and the heat of fusion of purely crystalline form of PE Hm 0 = J/g. Xm = [ Hm/ (1-χ)]/ Hm Lamellar thickness is estimated using Gibbs-Thomson equation as Tm(D)/Tm(α) = 1-(2γVm/HmD)..2.2 Where Tm(D) is the melting temperature of a lamellar crystal with a thickness of D, Tm(α) denotes the corresponding bulk value, γ is the interface energy of the lamellar crystal and the surroundings, Vm is the molar volume of the crystal, and Hm is molar melting enthalpy [1] Nonisothermal Crystallization Kinetic Studies using Differential Scanning Calorimetry (DSC) Nonisothermal crystallization experiments were done using Pekin Elmer, DSC-7 Instrument. The temperature range used was 30 o C to 180 o C with a heating rate of 25 o C/minute and at cooling rates of 2 o C, 5 o C, 10 o C and 20 o C. The thermal history was removed by heating the samples from room temperature to 180 o C followed by a hold up at 180 o C for 2 minutes [2]. The protocol of the experiment is summarized below [Figure 2.1]. All samples were cooled to room temperature for complete evaluation of crystallization. All tests were performed in the standard DSC mode.

8 100 Chapter C 2 minutes C 25 0 C/mt (a) 2 0 C/minute (b) 5 0 C/minute (c) 10 0 C/minute (d) 20 0 C/minute 30 0 C 30 0 C Figure 2.1: Protocol for DSC crystallization kinetics experiment. From the heating and cooling curves the melting and crystallization parameters were estimated. These include (a) melting peak temperature (Tm), (b) onset of crystallization (Tc onset), crystallization peak temperature (Tp), enthalpy of fusion ( H f ), and percentage crystallinity (Xc) Swelling Studies Circular samples (diameter 2 cm) of ASTM standard D5890 were weighed and immersed in toluene contained in test bottles with airtight stoppers kept at 80 o C temperature. Initial weight, swollen weight and deswollen weight were taken on a highly sensitive electronic balance [3]. Cross-link density was calculated from swellings experiments using Flory Rehner equation. Cross-link Density: According to the theory of swelling of cross-linked polymers, strong bonds, such as the chemical cross-links between the XLPE chains, prevent the molecules becoming completely surrounded by the fluids, but cause swelling. Cross-link density is frequently calculated from equilibrium swelling data by means of the Flory-Rehner equation

9 Experimental 101 and this equation relating swelling behaviour to the kinetic theory through the polymer solvent interaction parameter or Huggins factor [4]. Cross-link density υ = 1/2M c Where M c is the molecular weight of polymer between cross-links M c = [-ρ r V s V 1/3 rf ] / [ln (1-V rf ) + V rf + χv 2 rf ] Where ρ r is the density of polymer, V s is the molar volume of solvent and V rf is the volume fraction of polymer in the solvent-swollen sample and is given by: V rf = [(d-fw)/ ρ r ] / [(d-fw) / ρ r ) + (A s / ρ s )]..2.5 Where d is the deswollen weight, f is the volume fraction of the filler, w is the initial weight of the sample, ρ r is the density of polymer, As is the amount of solvent absorbed. In equation (2.4), χ is the interaction parameter and is given by Hildebrand equation χ = β + [ V s (ρ s ρ p ) 2 / RT ] Where β is the lattice constant, R is the universal gas constant, T is the absolute temperature, ρ s is the solubility parameter of solvent and ρ p is the solubility parameter of polymer Viscoelastic Characteristics Dynamic Mechanical Analysis Dynamic mechanical properties were analysed in shear mode using Home-made machine in MATEIS, INSA de Lyon, France. Rectangular samples at frequency 1 Hz, angle 10-2, couple range 10-5 to 10-8 were tested. Temperature range starting from 100 K to 400 K at the heating rate of 1K/minute was used for the measurement [5]. Liquid N 2 atmosphere is

10 102 Chapter 2 used to reduce the temperature to 100 K. Storage modulus, loss modulus and tan δ were calculated using DMA. (i) Estimation of Coefficient c The effectiveness of fillers on storage moduli of the composites can be represented by co-efficient such as c = (E G /E R ) composite / (E G /E R ) resin..2.7 Where E G and E R are the storage modulus values below glass transition and above glass transition respectively [6]. The lower the value of the constant c, the higher the effectiveness of the filler. The measured E values at 100 K and 300 K were employed as E G and E R respectively. (ii) Estimation of Constrained Region The constrained length in each sample can be estimated from the height of the tan delta peak [7]. For linear viscoelastic behaviour, the relationship among the energy loss fraction of the polymer nanocomposite (W) and tan δ is given by the following equation. W = π tan δ / (π tan δ +1)..2.8 The energy loss fraction W at the tan δ peak is expressed by the dynamic viscoelastic data in the form. W = [(1- C) W 0 ] / (1-C 0 ) Where C is the volume fraction of the constrained region, W 0 and C 0 denote the energy fraction loss and volume fraction of the constrained region of neat XLPE. This equation can be rearranged as follows.

11 Experimental 103 The volume fraction of the constrained region, C is given by C = 1 [(1-C 0 ) W / W 0 ] C 0 is taken to be 0. The height of the tan δ peak is used to calculate W according to Eq. (2.8) Rheology The rheological measurements were performed on a stress controlled rheometer (REOLOGICA Instruments AB). Frequency sweep measurements were carried out over a frequency range of rad/s with a strain amplitude 0.01%. The storage modulus was measured in the frequency sweep experiments. The strain sweep measurements were carried out for % strain, with an angular frequency 10 rad/s Contact Angle Studies Contact angle measurements were carried out in a SEO Phoenix instrument. Measurements were carried out with water (triply distilled) and DMSO on samples of size cm 3 at room temperature. The volume of the sessile drop was maintained as 5 µl in all cases using a microsyringe. The contact angle was measured within s of the addition of the liquid drop with an accuracy of ±1 [8]. Also contact angles were measured with definite time intervals for a single drop and the measurements were recorded as snap shots Diffusion Studies The experiments were done at 3 different temperatures of 70, 80 and 90 o C using toluene as the solvent. Circular samples (diameter 2 cm) were weighed and immersed in toluene contained in test bottles with airtight stoppers [9]. At the expiration of the specified time, the samples were removed from the sample bottles, wiped free of adhering solvent and

12 104 Chapter 2 weighed using an electronic balance. The weighing was continued till equilibrium solvent uptake was attained Dielectric Studies Dielectric Strength Breakdown electric field testing was performed on an ac dielectric strength tester (France). The specimens were immersed in the pure silicon oil, between 25 mm and 20mm diameter copper ball electrodes. The lower electrode was connected to earth and an increasing ac voltage (60 Hz) with a rate of 1 kv/s was applied to the upper electrode until the sample failed [10-11]. Under a quasi-homogeneous field (sphere-sphere electrode configuration), the field to breakdown E depends on the maximum voltage V and specimen thickness d. E = V/d. The data were treated by using a two parameter Weibull statistical distribution method. The weibull statistical distribution in the case of ramp voltage test can be written as P = 1 exp [-(E/E 0 ) β ] Where E is the experimental breakdown strength, P is the cumulative probability of electrical failure, β is the shape parameter, which is related to the scatter of the data (inverse function of the variation of the data), E 0 is the characteristic breakdown electric field that represents the breakdown strength at the cumulative failure probability of 63 %, the scale parameter which is often used to compare the dielectric breakdown of various samples with one another.

13 Experimental Dielectric Constant and Dielectric Loss Dielectric spectroscopy measurements were performed using a Solatron dielectric instrument. Sample thickness is around 0.25 mm in all cases. Permittivity and loss tangent were recorded in the discrete frequencies ranged from 1 Hz to 1MHz at room temperature. Silver paste was applied on the front and rear surfaces of the samples. 2.4 References 1. Olmos, D.; Dominguez, C.; Castrillo, P. D.; Gonzalez-Benito, J. Polymer, 2009, 50, Abbasi, S. H.; Hussein, I. A.; Parvez, M. A. Journal of Applied Polymer Science, 2011, 119, Schick, C.; Wurm, A.; Mohammed, A.; Thermochimica Acta, 2003, 396, Meera, A. P.; Said, S.; Grohens, Y.; Thomas, S. Journal of Physical Chemistry: C, 2009, 113, Josef, K.; Jancar, K. J. Polymer Engneering and Science, Varghese, H.; Bhagawan, S. S.; Thomas, S. Journal of Applied Polymer Science, 1999, 71, Vijayan, P.; Puglia, D.; Kenny, J. M.; Thomas, S. Soft Matter, 2013, 9, Abraham, R.; Varughese, K. T.; Isac, J.; Thomas, S. Macromolecular Symposia, 2012, 315, Wilson, R.; George, S. M.; Maria, H. J.; Plivelic, S. T.; Kumar, A.; Thomas, S. Journal of Physical Chemistry: C, 2012, 116,

14 106 Chapter Smith, R. C.; Liang, C.; Landry, M.; Nelson, J. K.; Schadler, L. S. IEEE Transactions on Dielectrics and Electrical Insulation, 2008, 15, Murakami, Y.; Nemoto, M.; Okuzumi, S.; Masuda, S.; Nagao, M. IEEE Transactions on Dielectrics and Electrical Insulation, 2008, 15, 33-39