Investigation of damping and toughness properties of epoxy-based nanocomposite using different reinforcement mechanisms: polymeric alloying, nanofiber, nanolayered, and nanoparticulate materials

1 Introduction

In the last decade, using nanoparticles in composite community was one of the most attracting subjects to achieve better physical properties [1–6]. What makes these materials unique and different from other reinforcements is their nanometer size that leads to high specific surface in such cases up to more than 1000 m²/g. This characteristic showed that new generation of nanocomposites shows extraordinary mechanical, electrical, and thermal properties. Due to appearance and shape of nanomaterials, three principal categories can be introduced: particulate, fibers, and platelet nanomaterials. Al₂O₃, TiO₂, and SiO₂ are commonly used nanoparticles. Carbon nanotube (CNT) is the most famous example for nanofiber, and clay is a popular instance for layered nanomaterials. Each of the categories has some advantages; for example, the important indicator of nanoparticles such as silica is their specific area, or the addition of small amount of CNTs can enhance the matrix properties such as tensile stiffness [7]. Epoxy polymer because of good properties such as low cost, high strength, low viscosity, low shrinkage during cure, low creep, and good adhesion to many substrates, is one of most applicable materials in various applications such as adhesives, construction materials, composites, laminates, coatings, aircraft, and spacecraft industries. A common method for toughening epoxy resins is based on blending with engineering thermoplastic polymers such as polysulfone [8], polyetherimide [9], and polyethersulfone [10]. It has been well established for many years that the incorporation of a second microphase of a dispersed rubber (e.g., [11–15]) or a thermoplastic polymer (e.g., [16–19]) into the epoxy polymer can increase their toughness. Using reinforcements such as adding nanomaterials and polymeric alloying in epoxy resin is one of the new and famous ways to improve the mechanical properties of these types of materials. An alternative approach for increasing mechanical properties of epoxy resins is adding rigid inorganic nanoparticles without affecting the glass transition temperature of the epoxy. Various nanofillers such as silica (SiO₂) [20, 21], alumina (Al₂O₃) [22], and titania (TiO₂) [23] are also employed for this purpose. Incorporation of inorganic nanofillers improves the fracture toughness to some extent. Increased toughness can be achieved up to certain filler contents and further addition of them may not enhance toughness. CNTs [24, 25] have a high potential to improve the mechanical, physical, and electrical properties of polymers. They exhibit an exceptionally high aspect ratio in combination with low density [25], as well as high strength and stiffness [26–29], which make them a potential candidate for the reinforcement of polymeric materials. These issues are the interfacial bonding and especially the proper dispersion of the individual CNTs in the polymeric matrix. Investigations focusing on the interfacial bonding have been performed by Wagner and coworkers [30, 31]. They performed pullout experiments of individual CNTs embedded in a polymer matrix with a specially developed test to evaluate the interfacial shear strength of the nanotube polymer system. The interfacial adhesion to the polymer can be enhanced by chemical functionalization of the nanotube surface. Molecular dynamics simulations by Frankland et al. [32] predicted an influence of chemical bonding between the nanotubes and the matrix on the interfacial adhesion. The dispersion of the CNTs in the matrix system is another challenge to overcome for nanotube-reinforced polymers. Nanoscaled particles exhibit an enormous surface area being several orders of magnitude larger than the surface of conventional fillers. This surface area acts as interface for stress transfer but is also responsible for the strong tendency of the CNTs to form agglomerates. An efficient exploitation of their properties in polymers is therefore related to their homogeneous dispersion in the matrix, a breakup of the agglomerates, and a good wetting with the polymer. The properties of conventional composites are controlled by the constitutive mechanical, electrical, and optical properties of fiber, matrix, and interface. Achieving high-performance nanocomposites, a variety of further challenges have to be overcome. A meshwork of interactions has to be clarified, outlined by the properties of the particle, matrix, and the manufacturing process in particular. Epoxy/clay nanocomposites are a large class of new materials, where the inclusion of clay as nanoplatelet in epoxy matrices has enhanced the tensile and compressive properties of the matrix. The fillers also cause an increase in chemical resistance, barrier properties, and dimensional stability [32]. Most of the research on epoxy-based nanocomposites has focused on their elastic properties. Relatively little attention has been given to their damping mechanisms and toughness ability. The main goal of this article was to explore the damping and toughness properties of epoxy-based nanocomposites reinforced with different type of fillers, such as polymeric alloying, nanofiber, nanolayered, and nanoparticulate materials, and to also compare these systems with each other based on mechanical view.

2 Materials and methods

2.3 Characterization

The impact resistance was carried out according to un-notched specimens with the dimensions of 63.5, 12.7, and 7.2 mm as indicated in ASTM D256 using a SIT-50 Izod impact machine from Santam Co. (Iran). At least five replicates for each sample were tested. For the specimens of damping test, the authors used Izod impact samples according to ASTM D256 before using for impact test. The Laser Doppler Vibrometer OMETRON VH300+ shown in Figure 1 was employed for measuring vibration. In the current test, calculating damping coefficients and natural frequencies are based on Stochastic Subspace Identification-Data Driven (SSI-Data) as first introduced by Overschee and De Moor and modified by Peeter and Brinker [33]. Based on this method, the specimen was considered as cantilevered beam and excited environmentally. All time-dependent responses were collected in Block Henkel Matrix and converted into individual past and future matrix. In this step, to make connection between responses, the future matrix portrait on past matrix created projection matrix. By severance singular value decomposition (SVD) of projection matrix, observability matrix and Kalman states will be calculated and the collection of polar of system matrix will be achieved. In this step as shown in Figure 2, calculation of damping coefficients and natural frequencies, the stabilization diagram will be used. The stabilization diagram is a tool to show polars of systems in different order [30]. All these steps were calculated by software. A scanning electron microscope (SEM) was performed using SEM 1530 from LEO to examine the fracture surface morphology of all prepared composite at their optimum concentration. The samples were sputtered with gold prior. These images were taken to evaluate the nanoparticles dispersion in the resin and also probable structural defect.

Figure 1

Vibration measuring (A) the Laser Doppler Vibrometer OMETRON VH300+ (B) specimen.

Figure 2

Stabilization diagram in SSI-Data method for (A) neat epoxy, (B) epoxy/4 wt% HIPS, (C) epoxy/5 wt% SiO₂, (D) epoxy/1.5% MWCNT, and (E) epoxy/2% clay.

3 Results and discussion

In this work, the effects of four different mechanisms of reinforcing on damping coefficient and impact resistance of DGEBA have been studied. Table 1 contains the results of damping and impact strength. The results show that epoxy with 5 wt% silica in both first and second modes have maximum value of damping coefficient in comparison with other silica weight percentage loading. However, the impact strength occurred at a maximum value of 3.5 wt%. In these optional concentrations, it was observed that damping coefficient in first and second modes and also impact strength were increased up to 32%, 76%, and 80% of neat value, respectively. For epoxy/MWCNT at the best concentration (1.5 wt%), increase of damping coefficient in first and second modes is about 8% and 48% of neat value, and for impact strength, it is 49% of neat value. The results achieved by damping and Izod impact tests presented in Table 1 for epoxy/clay showed that, at the optimum amount of clay loading (i.e., 2 wt%), damping coefficient in first and second modes and also impact strength were increased up to 18%, 37%, and 55% of neat value, respectively. Finally, Table 1 demonstrates these parameters for adding HIPS as reinforcement to epoxy resin. The damping coefficient enhancement in optimum situation (i.e., 4 wt%) was 45% for first mode and 83% for second mode of neat value. Also, it can be observed that the impact strength is improved considerably up to 145%. It is completely obvious that the most improvement in damping coefficient and impact resistance occurred for polymeric alloying via HIPS between four mentioned mechanisms. Generally, with comparing the results, it seems that using HIPS as a material for polymeric alloying is the most appropriate way to enhance damping coefficient and impact strength of epoxy resins. A similar behavior was found in [11], where thermoplastic filler increased the toughness; however, they may have detrimental effect on other mechanical characteristics. Also, nanoparticles may cause improvement in impact and damping resistance but not as effective as elastomeric particles. These improvements may be attributed to enhancement of the interfacial interaction. As nanoparticles get smaller, cracks interact with more particles and also better interfacial properties.

The micrograph of fracture surface of pure and filled epoxy resins at relative optimum weight percentage of four different types of reinforcements after tensile testing is shown in Figure 3. Figure 3A depicts a fracture surface for the neat epoxy matrix, which characterized that a large smooth area that contain cracks in different planes can be seen. The SEM photographs obtained for fracture study of 3.5 wt% nanosilica loading after tensile test are presented in Figure 3B. The image shows homogeneous dispersion of nanoparticles in epoxy resin. Through agglomeration of nanoparticles in a region yields to stress concentration point and rupture start point; thus, the good particle dispersion can improve mechanical properties. Generally, the presence of more van der Waals forces between more particles would limit its dispersion capability and would result in production of agglomerates of particles in the matrix. This may explain that, with exceedance from optimal fraction content, the main interaction becomes between particles and not between particles and matrix and it causes agglomeration of nanoparticles and also decreases the mechanical properties. Figure 3C shows the quality of dispersion of HIPS in epoxy resin. From the figure, it is obvious that the existence of thermoplastic phase can play important role in front of crack propagation under impact load. These improvements may be attributed to the fact that existence of thermoplastic or rubber phase in epoxy polymer may cause delay between crack propagation and break moment. The reason of this delay may be related to big size of thermoplastic or rubber particles and substitution of these particles in direction of crack propagation. The state of dispersion of MWCNT in the epoxy nanocomposite is corroborated by SEM micrograph in Figure 3D. Good dispersion with minimum degree of agglomeration can be seen in this figure. For explanation of multiwall nanotube behavior, the static adhesion strength as well as interfacial stiffness has to be considered, and this characteristic plays a vital role in the capability of composite to transfer stress and elastic deformation from the matrix to the filler [31]. The tensile fracture surface of epoxy filled with organoclay is shown in Figure 3E. In comparison with neat epoxy, addition of clay nanoplatelet makes crack surface rough and this roughness causes the resistance in front of propagation of crack to be high and the crack has not propagated so easily. Also, better exfoliation is a very effective parameter to improve quality of mechanical behavior of epoxy/clay nanocomposite.

Figure 3

Scanning electron micrographs of fracture surface for (A) neat and samples having (B) 3.5 wt% SiO₂, (C) 4 wt% HIPS, (D) 1.5 wt% MWCNT, and (E) 1 wt% clay.

4 Conclusion

In the current study, epoxy resin reinforced with four different reinforcements were prepared. These four mechanisms use MWCNT as nanofiber, clay as nanolayer, SiO₂ as particulate nanofiller, and HIPS for polymeric alloying. Such composites are experimentally characterized by means of damping, Izod impact tests, and also microscopy testing. Generally, comparison between results led authors to conclude that, between mentioned mechanisms, using HIPS as a polymeric alloy is the most appropriate way to enhance these properties of epoxy resin. The results show that inclusion of HIPS thermoplastic polymer up to 4 wt% considerably improves damping and toughness properties of epoxy polymer. In this optional concentration, damping coefficient in first and second modes is increased up to 45% and 83% of neat value and impact strength increased up to 145% of neat value. These improvements may be resulted from ability of HIPS to absorb and propagate the energy in all area specimens. On the contrary, existence of thermoplastic or rubber phase in epoxy polymer may cause delay between crack propagation and break moment. The reason of this delay may be related to big size of thermoplastic or rubber particles and substitution of these particles in the direction of crack propagation. Also, some improvements are observed for other reinforced mechanisms. The main reason for this phenomenon is related to interfacial forces between nanoparticle and epoxy matrix. As nanoparticles get smaller, there are interactions with more particles and also better interfacial properties. Also, SEM analysis on tensile fractured samples indicated the absence of particle aggregation at low nanofiller content and reinforcing effect in terms of increased mechanical properties.