Phosphorylated Epoxy Nanocomposites: Study of Structure and Mechanical Properties

 

Priyanka, J. B. Dahiya*

Department of Chemistry, Guru Jambheshwar University of Science and Technology, Hisar-125001, Haryana

*Corresponding Author E-mail: jbdic@yahoo.com

This paper presents the effect of nanoclay content on structure and mechanical properties of phosphorylated epoxy containing 9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) as flame retardant. The nanoclay up to 4 wt% was blended with phosphorylated epoxy containing 2.5wt% phosphorus content and molded out after curing. The internal structure of prepared samples was analyzed by X-ray diffraction (XRD) and transmission electron microscope (TEM) methods. The surface morphology was analyzed by field emission scanning electron microscope (FESEM). The mechanical properties were measured using Universal Testing Machine (UTM). The results of XRD and TEM reveal mixed type of structure (intercalated and exfoliated) of samples. The mechanical properties (tensile and flexural strength) are found maximum for phosphorylated epoxy sample containing 2.0 wt% nanoclay.

 

 

 

INTRODUCTION:

Epoxy resin is a thermoset polymer. This is used in wide range of applications such as automotive, high barrier packaging for food and electronics, encapsulation of semiconductor devices and aerospace industry^{1, 2}due to its favorable attractive combination of toughness, chemical, thermal and mechanical properties^{3, 4}. However, because of its inherent brittle and flammable nature, the modifiers and additives are generally added to improve their physical, mechanical, thermal and flammability properties.

 

The halogenated flame retardants were used earlier as additives but are not suitable due to their environmental hazards.

 

Literature survey revealed that organophosphorus compounds are more effective flame retardants for epoxy resins and produce less toxic materials than halogenated flame retardants^5-8. In early 1970’s, 9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO)⁹ was developed and then used as reactive flame retardant by incorporating into backbone of epoxy resin.

 

Organically modified nanoclays are used for preparation of polymer nanocomposites to improve affinity between polymer and nanoclay. If the original structure of nanoclay platelets is still retained on blending with epoxy matrix and the interlayer spacing is increased, the intercalated structure of epoxy nanocomposite is formed. But if nanoclay platelets are dispersed individually within epoxy matrix, the exfoliated structure is formed. Exfoliated epoxy nanocomposites exhibit greatly improved mechanical, thermal and physicochemical properties on comparing with conventional composites¹⁰. Many researchers studied the effect of organically modified nanoclay on mechanical properties of epoxy resins¹¹. The combination of nanoclay with phosphorylated epoxy resin may form more effective system for mechanical and all other properties of epoxy resin system.

 

In this work, composites of phosphorylated epoxy containing 2.5wt% phosphorus content were prepared by blending with small amount (up to 4 wt%) of organically modified nanoclay using in-situ polymerization method¹². The dispersion of nanoclay in samples was analyzed by X-ray diffraction (XRD) and TEM methods. The surface morphology of epoxy samples was studied by field emission scanning electron microscope (FESEM). The mechanical properties of epoxy samples were also studied.

 

EXPERIMENTAL:

Materials:

The materials used in this study were diglycidyl ether of bisphenol-A (DGEBA) i.e. epoxy resin; nanoclay namely Nanomer 1.31PS (sodium montmorillonite modified with 15-35 wt% octadecylamine and 0.5-5.0 wt% γ-aminopropyltriethoxysilane with CEC about 145 meq/100gm.) and 4,4ˈ-diaminodiphenyl sulfone (DDS) used as curing agent were purchased from Sigma Aldrich, India. The 9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) used as flame retardant was supplied by TCI Chemicals, India and mold release agent was purchased from Giant Sales Corporation, Gurgaon. All these commercial materials were used as received without further purification.

 

Preparation of phosphorylated epoxy nanocomposites:

Phosphorylated epoxy resin (DGEBA-DOPO)⁶ was prepared blending DGEBA and DOPO in desired amounts at 160°C for 5h. Modified epoxy resin was mixed mechanically in a resin kettle with the appropriate amount (0, 1, 2, 3 and 4 wt %) of nanoclay at 90⁰C for 1h. Then, a stoichiometric amount of the curing agent (DGEBA: DDS- 5:1.78 by wt.) was added and mixed at 120°C for 1h. The blended mixture was poured into pre-heated mold coated with mold releasing agent and then cured at 130°C for 3h in vaccum oven to remove air bubbles. The nanocomposites were post-cured for 3h at 170°C in a hot air oven. All epoxy samples of size 15.5×11.0× 0.26cm³were prepared and subsequently machined to the required sample size for measuring the mechanical properties. The prepared phosphorylated epoxy nanocomposites along with their composition are listed in Table 1.

 

Table 1. Composition of phosphorylated epoxy samples

Sample	Composition
p-EP	Phosphorylated epoxy (2.5% P) + nanoclay (0.0 %)
p-EP1NC	Phosphorylated epoxy (2.5% P) + nanoclay (1.0%)
p-EP2NC	Phosphorylated epoxy (2.5% P) + nanoclay (2.0%)
p-EP3NC	Phosphorylated epoxy (2.5% P) + nanoclay (3.0%)
p-EP4NC	Phosphorylated epoxy (2.5% P) + nanoclay (4.0%)

 

Samples Characterizations:

X-ray diffraction patterns of powdered samples were observed with XRD analytical instrument X’pert-PRO model, Netherlands using nickel filtered Cu-Kα radiation having 0.154 nm wavelength with diffraction angle, 2θ ranging from 3^o to 30^o.

 

TEM instrument (Hitachi H-7500 TEM) was used to observe the internal structure of epoxy nanocomposites at an accelerating voltage of 120 kV. The particles of sample were deposited on mica sheet from sample suspension made in alcohol. The solvent was evaporated by using a lamp. After carbon coating, the film was floated-off the mica and retrieved onto the copper grids and film was placed into specimen chamber of instrument.

 

Field Emission Scanning Electron Microscope (FESEM) SU8010, Hitachi was used to examine the surface morphology of platinum coated samples using an accelerating voltage of 10 kV.

 

The tensile and flexural strengths of the epoxy nanocomposites were measured on a standard computerized Universal Testing Machine (UTM) according to the standard ASTM D638 and ASTM D790, respectively with three specimens for each sample at rate of cross head motion of 50 mm/min at ambient temperature.

 

RESULTS AND DISCUSSION:

Structure of epoxy nanocomposites:

X-ray Diffraction:

The interlayer spacing of nanoclay platelets can be calculated in accordance to the Bragg’s equation (2dsinθ = nλ) from the XRD results. Figure 1 shows the XRD spectra of nanoclay, phosphorylated epoxy (p-EP) and epoxynanocomposites(p-EP_1NC and p-EP_3NC). The objective of XRD analysis was to determine the structure of epoxy nanocomposites. The XRD spectra shows that the nanoclay exhibited a characteristic peak at 2θ = 4.33^o with basal reflection plane 001 having an interlayer distance of 2.03 nm. In the XRD spectra of samples p-EP_1NC and p-EP_3NC, the characteristic peak of nanoclay disappeared completely in the analyzed range of angle of diffraction(3^o-30^o),which suggests the formation of exfoliated or intercalated structure.

 

Figure 1. XRD spectra of nanoclay and phosphorylated epoxy samples p-EP, p-EP_1NC, p-EP_3NC

 

Transmission Electron Microscope (TEM):

TEM images (Figure 2) of pure nanoclay and phosphorylated epoxy nanocomposite (p-EP_4NC) samples were taken to investigate the dispersion state of nanoclay in samples. For each sample, around 9-10 TEM images were taken across the sample grid. Figure 2a indicates the nanoclay platelets in the stacked form. The dark lines representing individual and delaminated nanoclay platelets in TEM image of p-EP_4NC (Figure 2b) suggest that the interlayer distance of platelets increased. In some region of sample grid the exfoliation of silicate layers is seen. TEM images taken on some other region of sample grid of p-EP_4NC show laminated individual nanoclay platelets as shown by circle in Figure 2c, which results the intercalated structure of sample. TEM analysis concludes the formation of mixed type of intercalated and exfoliated structure of sample. The mixed dispersion of nanoclay in a sample may be due to variation in speed of curing reactions of epoxy matrix taking place inside and outside the nanoclay platelets.

 

Figure 2. TEM images of (a) nanoclay (NC), (b and c) p-EP_4NC sample

 

Surface morphology:

Figure 3(a-c) show the representative FESEM images for phosphorylated epoxy nanocomposites containing 1, 3 and 4 wt% of nanoclay (p-EP_1NC, p-EP_3NC and p-EP_4NC), respectively. As can be seen in Figure 3 (a-c), the surface becomes rougher and textured with increase in content of nanoclay. The presence of bright spots corresponding to the agglomerates of nanoclay in the epoxy matrix confirms inevitable agglomeration in epoxy matrix (Figure 3). The agglomeration of nanoclay in the samples may have taken place during the curing process of the epoxy matrix. As the nanoclay content increases, the free volume available for nanoclay to move around decreases, and this leads to increase of agglomeration and spallation of nanoclay on surface. At the same time, the cross-link density of the epoxy nanocomposites increased due to curing processes and resulted in increasing the tendency for the nanoclay particles to form agglomerates. Therefore, the surface roughness of samples increases with increase in nanoclay content.

 

Figure 3. FESEM images of (a) p-EP_1NC, (b) p-EP_3NC and (c) p-EP_4NC samples

 

 

Mechanical properties:

The tensile and flexural strength values of phosphorylated epoxy and its nanocomposites are listed in Table 2. The variation of these properties is represented graphically in Figures 4 and 5. The tensile strength of phosphorylated epoxy (p-EP) sample is 11.54 MPa, which is increased on addition of nanoclay and reached 19.03 MPa for p-EP_2NC sample. A good interfacial bonding between nanoclay and epoxy matrix is the reason for improving the tensile strength. Further beyond 2wt% addition of nanoclay to phosphorylated epoxy decreases the tensile strength. It may be due to increase in agglomeration of nanoclay in samples with increase in nanoclay content as discussed in surface morphology (Figure 3).

 

Figure 4. Variation of tensile strength of phosphorylated epoxy samples with nanoclay content

 

Figure 5. Variation of flexural strength of phosphorylated epoxy samples with nanoclay content

 

Table 2. Tensile and Flexural strength data of phosphorylated epoxy samples

 

The flexural strength of p-EP sample is 21.8 MPa, which is increased to 29.9 MPa on addition of 2 wt% nanoclay to phosphorylated epoxy. On further addition of nanoclay (beyond 2 wt%) to epoxy matrix, leads to decrease in flexural strength of samples. This is also due to agglomeration of nanoclay in samples on higher nanoclay additions by making epoxy brittle in nature. The tensile and flexural strengths of epoxy samples show ascending trend by increase in the filler content up to 2 wt% nanoclay. Beyond 2 wt% nanoclay addition, a significant decrease in mechanical properties is seen. The improvement in mechanical properties of phosphorylated epoxy samples up to 2 wt% nanoclay reinforcement is attributed to the increase in surface bonding between epoxy and nanoclay.

 

CONCLUSION:

Phosphorylated epoxy nanocomposites were prepared with varying content of nanoclay. The effect of nanoclay content on structure and mechanical properties of samples were examined using XRD, TEM, FESEM and UTM. The XRD and TEM results reveal mixed exfoliated and intercalated structures of phosphorylated epoxy nanocomposites. FESEM images show the rougher surface with increase in amount of nanoclay. The tensile and flexural strengths of epoxy samples show ascending trend by increase in the filler content up to 2 wt% nanoclay. The incorporation of nanoclay to phosphorylated epoxy matrix beyond 2 wt% addition, results a reduction in mechanical properties. Among all the samples, phosphorylated epoxy sample (p-EP_2NC) containing 2.0 wt% nanoclay showed improved mechanical properties.

 

ACKNOWLEDGEMENT:

The research fellowship (JRF) of Council of Scientific and Industrial Research (CSIR), New Delhi, India, to Mrs. Priyanka is gratefully acknowledged.

 

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Received on 22.04.2017         Modified on 10.05.2017

Accepted on 29.05.2017         © AJRC All right reserved