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Polyethylene glycol as an epoxy modifier with extremely high toughening effect: formation of nanoblend morphology.

INTRODUCTION

Epoxy thermosetting polymers have attracted considerable attention due to their outstanding mechanical, chemical, electrical, and thermal properties. This is why they are classified as major engineering materials. They are used extensively in protecting coatings and structural applications including fiber-reinforced composites, electrical laminates, casting, encapsulation, tooling, and adhesives. However, due to their inherent brittleness nature, poor resistance to crack propagation, and low impact strength, their applications have faced obstacles to make them more practical popular [1, 2]. Therefore, many investigations have been conducted on the toughening of epoxy polymers with the hope to broaden their end-use applications.

The main approach for toughening of brittle epoxy is to incorporate a second phase as modifier into epoxy matrix using various experimental methods. The most commonly used modifiers are low-molecular-weight liquid rubbers, engineering thermoplastics, and inorganic nanofillers. Although the use of these modifiers often leads to number of favorable results in terms of impact and fracture properties, specific drawbacks have also been observed.

Reactive-functionalized butadiene-co-acrylonitrile rubbers [3-5] and acrylate-based rubbers [6, 7] are the most commonly used liquid rubbers for epoxy toughening. The rubber toughened epoxy often possesses increased fracture and impact characteristics, but the presence of the rubbery phase often decreases the modulus, glass transition temperature (Tg), and the thermal stability of the material with a further weakened properties at elevated temperatures. Based on numerous studies on the morphological properties of rubber-toughened epoxies, a two-phase microstructure with uniformly dispersed rubber particles of micron size in epoxy matrix is formed.

Engineering thermoplastics such as polyetherimide [8, 9], polyethersulfone [10], poly(ether ether ketone) with pendant alkyl groups [11, 12], and reactively terminated polysulfones [13, 14] are also used extensively for epoxy toughening. Thermoplastics toughened epoxies have received much attention because they generally do not show a significant decrease in their other desired mechanical properties [15]. Toughness improvement for epoxy/ engineering thermoplastic is obtained only at higher thermoplastic contents, where thermoplastic phase separates from resin due to decrease in epoxy/thermoplastic compatibility, and continuous or cocontinuous phase morphology is formed. However, epoxy resins modified with higher thermoplastic contents show a steep increase in viscosity, which causes specific difficulties in the manipulating of the resin [16-18].

Inorganic nanofillers along with thermoplastic polymers are also used for epoxy toughening. Adding two types of modifiers has higher toughening effect than one modifier. In this respect, the ternary nanocomposite with addition of polyethersulfone and organoclay to epoxy has been prepared and studied. It has been reported that fracture toughness of the ternary nanocomposite is higher than that of the binary hybrids prepared with addition of organoclay and polyethersulfone to epoxy [19]. In another work, a combination of polyamide and clay has been used for enhancement of epoxy properties. The ternary nanocomposite with composition of 2% montmorillonite and 20% polyamide has exhibited maximum enhancement [20]. It has been also reported that the incorporation of clay and polyacrylonitrile-butadiene-styrene has a considerable toughening effect on epoxy polymer [21, 22].

The objective of this article is to develop a novel modifier with extremely high toughening effect on epoxy without attenuating other desired characteristics such as tensile properties. Low-molecular-weight liquid polyethylene glycol (PEG) is compatible with epoxy oligomer [23]. Therefore, PEG was considered as a proper candidate for toughening of brittle epoxy. To the best of the authors' knowledge, there is no study on the application of PEG as toughening modifier to a thermosetting polymer in the literature. Tensile, impact, and fracture properties of epoxy/PEG hybrid with different PEG contents were studied. Chemical structure, morphological, and thermal properties of the hybrid were also investigated using Fourier transform infrared (FTIR) spectrometry, scanning electron microscopy (SEM), and differential thermal analysis (DTA), respectively.

EXPERIMENTAL

Materials

Liquid diglycidyl ether of bisphenol A (DGEBA)-type epoxy resin, Epiran 6 with an epoxy equivalent weight of 187 equiv/g, was purchased from Khuzestan Petrochemical Co. (Iran). The curing agent used was triethylene tetramine, provided by Huntsman Chemical Products. PEG with a molecular weight of 600 (PEG 600) was purchased from Sigma-Aldrich Chemical Co.

Preparation of Specimens

To prepare epoxy thermosetting polymer, DGEBA-type epoxy resin was mixed with 19 phr (parts per hundred) of curing agent for 10 min and degassed by a vacuum pump to remove any possible air bubbles. Optimum amount of hardener was selected according to the maximum tensile and impact strengths of the prepared epoxy. The mixture was poured into a silicon mold and cured for 72 h at room temperature followed by postcuring for 3 h at 75[degrees]C and 2 h at 100[degrees]C to ensure complete curing. To prepare epoxy/PEG hybrid, the epoxy resin was mechanically mixed with the desired amount of the PEG in a reaction kettle at room temperature for 15 min. Then, 19 phr of the curing agent (based on epoxy resin) was added and mixed for 10 min. The mixture was degassed again, poured into a silicon mold, and cured in a similar method that mentioned for the epoxy thermosetting polymer before being subjected to mechanical tests.

Characterization

The micrographs of epoxy/PEG hybrids were taken on the impact-fractured surfaces using a Vega Tescan SEM (Czech Republic). The fractured surfaces of samples were etched with tetrachloromethane for 24 h to observe phase morphology in more detail.

The Izod impact strength was measured at room temperature for unnotched specimens with the dimensions of 63.5 mm x 12.7 mm x 7.2 mm as indicated in ASTM D256, using a Ceast-Resil impact tester (Italy). At least five replicates for each sample were tested. For preparation of the homogeneous specimens, a measured weight of the material was poured into a leveled silicon mold and completely degassed using a powerful vacuum pump.

Tensile tests were conducted according to ASTM D638 at room temperature with a Shimadzu 20 kN testing machine (Japan) for the specimens with dimensions chosen according to the Type I of this standard method. The applied crosshead speed was 5 mm/min. Average values were obtained from at least five successful determinations.

The Mode I plane-strain fracture toughness ([K.sub.IC]) of samples were measured by compact tension test according to ASTM D5045. A sharp precrack was created in the compact tension samples by gently tapping a razor blade over a molded starter notch. Tests were carried out using a universal Shimadzu 20 kN testing machine (Japan) at room temperature and at a crosshead speed of 10 mm/min. At least, five replicates were tested for each hybrid composition. The fracture toughness [K.sub.IC] was calculated using the following equations:

[K.sub.IC] = [P.sub.c]/[BW.sup.1/2] f(x); x = a/W (1)

f(x) = (2 + x)(0.886 + 4.64.V - 13.32.[x.sup.2] + 14.74[x.sup.3] - 5.6[x.sup.4])/(1 - x).sup.3/2] (2)

where [P.sub.c], W, B, and a are the load at failure or maximum load, the width of each sample, specimen thickness, and the crack length, respectively.

FTIR spectrometry was used to analyze the potential interactions between epoxy thermosetting polymer and PEG in epoxy/PEG hybrid material. FTIR spectra were recorded on a Bruker Tensor 27 spectrometer (Germany) using KBr pellets coated with thin films of the samples.

DTA was used to explore the thermal behavior of pure epoxy and epoxy/PEG blend. DTA measurements were performed on a Linseis PT10 instrument (Germany) by heating the samples from ~50 to 350[degrees]C in air at a rate of 10[degrees]C/min.

RESULTS AND DISCUSSION

Mechanical Properties of Epoxy /PEG Hybrid

To investigate the effect of PEG modifier on mechanical characteristics of epoxy thermosetting polymer, epoxy/PEG hybrids with different PEG contents were prepared and tensile, impact, and fracture properties of them were determined.

Trends of tensile strength, elongation at break, and tensile modulus of epoxy/PEG hybrid as a function of PEG content are shown in Table 1. It can be seen that the tensile strength and modulus of the hybrid are slightly decreased with increasing PEG content, whereas elongation at break is marginally increased with increasing of PEG content. In this study, the maximum amount of PEG used for the hybrid preparation was 10 phr. The results of the current study reveal that adding PEG to the epoxy thermosetting polymer has no significant effect on the tensile properties.

Figure 1 displays the dependence of Izod impact strength of epoxy/PEG hybrid on PEG content. The impact strength was increased gradually with increasing PEG loading. For example, the hybrid containing 10% PEG exhibited impact strength of 56.3 kJ/[m.sup.2] which is 5.7 times greater than that of pristine epoxy.

Figure 2 illustrates the change in the fracture toughness of epoxy/PEG hybrid versus PEG content. Similar to impact properties, fracture toughness is also increased significantly with increasing PEG content. For instance, the fracture strength of 7.7 MPa [m.sup.1/2] with 540% increase compared to pure epoxy can be observed for the hybrid with only 10% PEG content.

As mentioned above, the variations of impact and fracture properties of epoxy/PEG were found to have similar trends versus PEG content. Fracture and impact properties were significantly improved with the incorporation of maximum 10% low-molecular-weight PEG, in a way that it can be concluded that a strong bonding between two phases at interfacial area is present. This can be an explanation for such a significant improvement in the considered properties. In other words, these properties are mainly affected by the interfacial interactions. In contrast, tensile properties are reduced slightly with the addition of low-molecular-weight PEG into epoxy matrix. It is important to note that the changes of tensile properties for the hybrid versus PEG content are much less than those of fracture and impact properties. It proves that tensile properties have low dependency on the interfacial interactions. Thus, tensile properties for the hybrid depend on the constituents' properties and percentages. In the present study, the modifier with the maximum amount of 10% was used for the hybrid preparation and so relatively little changes in tensile properties were observed. Such behaviors have been observed in other published studies for epoxies toughened by certain engineering thermoplastics [24-26],

An important observation emerging from this study is that the magnitude of improvements for impact and fracture properties using PEG modifier are significantly higher than the best reported values with other modifiers such as hydroxyl-terminated polyfether ether ketone) with pendant methyl groups [12], carboxy-terminated butadiene-acrylonitrile rubber [27], and hyperbranched polyester [28], Furthermore, lower amounts of PEG modifier (maximum 10%) have been used in this work.

Morphological Properties of Hybrid

The Morphology of epoxy/PEG hybrid was studied using SEM. The impact fractured surface of the hybrid was etched with tetrachloromethane to get a more detailed picture of the morphology for PEG phase in epoxy matrix. Upon etching, PEG is dissolved in the solvent while epoxy thermosetting phase remains unchanged.

Figure 3 shows the micrograph of the hybrid with 4% PEG content. It can be seen that phase separation occurs, forming a morphology with the dispersion of nanosized PEG particles in epoxy matrix. PEG nanoparticles are nearly spherical and have an average diameter of 85 nm using SEM measurements. Due to formation of a special morphology, epoxy/PEG hybrid can be named as nanoblend. Morphologies reported for epoxy hybrids toughened by common modifiers such as low-molecular-weight rubbers and engineering thermoplastics include the dispersion of micron or submicron sized modifier phase in epoxy matrix [3-5]. Continues or cocontinuous modifier phase in epoxy matrix have been also observed for the hybrids toughened by higher modifier content [16-18],

Significant increase in impact and fracture properties of epoxy/PEG hybrid compared to unmodified epoxy can be attributed to dispersion of PEG nanoparticles which act through various mechanisms such as crack pinning, particle bridging, crack path deflection, and microcracking that have been proposed and discussed in more detail elsewhere [29, 30], A part from toughening mechanism, the interaction between epoxy and PEG phases in interfacial area has a significant role in epoxy toughening. Two factors mainly affect the interfacial interaction: the extent of interfacial area and the type of interaction between two phases at interfacial area including chemical and physical bonding. It is clear that the interfacial interaction increases with increasing interfacial area. The total surface area of nanoparticles is much larger than that of microparticles. The presence of PEG nanoparticles with large surface area in epoxy matrix enables them to have further interactions with the thermosetting polymer. Hence, adding low amounts of PEG into epoxy matrix leads to superior enhancement in impact and fracture properties. For the second factor, according to the scanning electron micrograph for the etched sample (Fig. 3), a good adhesion between PEG nanoparticles and epoxy matrix can be observed. It indicates that there is a strong interaction between epoxy and PEG phases. Determination of the type of interaction needs further experimental evidences, such as FTIR spectra, which is discussed in the following section.

FTIR Studies

FTIR spectrometry was applied to reveal the presence or absence of any chemical interaction between epoxy matrix and PEG. Figure 4 shows the absorption spectra of epoxy, PEG and epoxy/PEG materials. The most prominent features for pure epoxy are the absorption bands located at 1116 and 830 [cm.sup.-1] correspond to stretching of hydrogen bonded C-OH group and out-of-plane wagging of OH (alcohol) group, respectively. The important characteristic bands of pure PEG are located around 3359, 2872, and 1107 [cm.sup.-1], which can be assigned to stretching of hydrogen bonded O-H group (in PEG or absorbed water), C-H of alkyl group and C-O-C of aliphatic ether, respectively. FTIR spectrum of epoxy/PEG is very similar to that of pure epoxy with small shifts in certain bands. Considering all possible reactions among the epoxy, the hardener and PEG, they do not generate new functional groups, therefore shifts in position and changes in intensities of OH and C-O-C bands in epoxy/PEG hybrid compared to pure epoxy and PEG are required to be investigated. The intensity of OH out of plane vibration located around 830 [cm.sup.-1] is considerably higher than that of pure epoxy. This may explain OH group concentration increase due to the reaction between epoxy and PEG. Furthermore, it is expected that the absorption band of aliphatic ether located around 1107 [cm.sup.-1] should be observed in the hybrid spectrum. Interestingly, the broad band of aliphatic ether (1107 [cm.sup.-1]) observed in PEG spectrum is not present in the hybrid spectrum. It may be possible that the characteristic peak of aliphatic ether shifts to higher wave numbers because of interaction between epoxy and PEG and overlaps with absorption peak of C-OH group located around 1115 [cm.sup.-1] and so have higher intensity compared to pure epoxy. These results show strong interaction between epoxy and PEG in the prepared hybrid material.

DTA Studies

DTA thermograms for pure epoxy and epoxy/PEG hybrid are presented in Fig. 5. Pure epoxy shows a broad endothermic transition between 68 and 100[degrees]C due to the loss of moisture content in the epoxy structure. An exothermic broad peak between 270 and 290[degrees]C may be related to the postcuring reaction in the epoxy. As expected, the glass transision temperature ([T.sub.g]) was not obseved because of the rigid and cross-linked structure of the epoxy. For Epoxy/PEG hybrid, two exothermic peaks around 80 and 278[degrees]C were observed. DTA scans for pure epoxy and epoxy/PEG hybrid reveals a new exothermic peak around 80[degrees]C in the blend thermogram. Considering the fact that the melting and boiling points of pure PEG are 4-8 and 200[degrees]C, respectively, the observed peak around 80[degrees]C cannot be related to the phase transition. It can be attributed to the strong interaction between PEG and epoxy matrix in the blend structure. Again, DTA studies confirm a strong interaction between epoxy matrix and PEG as supported by SEM images and FTIR spectrometry.

CONCLUSIONS

Fracture and impact properties of the epoxy are improved significantly through adding low amounts of PEG (maximum 10%). At the same time, no deteriorative changes were observed in tensile properties of the epoxy. Morphological properties of epoxy/PEG hybrid were obtained through SEM studies and a two-phase morphology was revealed in which nanosized PEG domains were uniformly distributed in the epoxy matrix. This unique morphology for the epoxy-based hybrid material was named as nanoblend. The results obtained from SEM, FTIR, and DTA studies confirm the existence of a strong interaction between epoxy and PEG phases at interfacial area which could be considered as a major factor affecting epoxy toughening.

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Siamak Zavareh, (1) Golnaz Samandari (2)

(1) Department of Applied Chemistry, Faculty of Basic Sciences, University of Maragheh, Maragheh, Iran

(2) R&D Group, Atrin Industries Co., Tabriz, Iran

Correspondence to: S. Zavareh; e-mail: zavareh@maragheh.ac.ir Contract grant sponsor: University of Maragheh.

DOI10.1002/pen.23733

Published online in Wiley Online Library (wileyonlinelibrary.com).

TABLE 1. Tensile properties of epoxy/PEG hybrid as a
function of PEG content.

PEG         Tensile              Tensile           Elongation
(%)      strength (MPa)       modulus (MPa)       at break (%)

0      46.0 [+ or -] 4.9    1157 [+ or -] 58    5.3 [+ or -] 0.8
2      45.2 [+ or -] 3.9    1144 [+ or -] 84    5.4 [+ or -] 0.5
4      46.2 [+ or -] 4.2    1139 [+ or -] 58    5.6 [+ or -] 0.7
6      44.3 [+ or -] 4.3    1130 [+ or -] 56    5.6 [+ or -] 0.8
8      43.2 [+ or -] 4.7    1122 [+ or -] 67    5.8 [+ or -] 0.7
10     42.1 [+ or -] 4.0    1114 [+ or -] 55    6.0 [+ or -] 0.6
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Author:Zavareh, Siamak; Samandari, Golnaz
Publication:Polymer Engineering and Science
Article Type:Report
Date:Aug 1, 2014
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