Research on the impact of nano carbon fillers on the morphology of epoxy binder

Original article

A s of today, polymeric composite materials are actively used in various areas: aircraft and space engineering [7–9], instrument and mechanical engineering [10, 11, 33, 34], construction [12–15], oil and gas. It is used as repair materials and protective covering for surfaces to prevent corrosion [1–6, 28–32].

The scope of application of composites directly impacts requirements to their physical and mechanical properties. When developing a composite material, it is important to understand how the components that make up its composition affect its properties. Fillers, by interacting with the matrix, can modify its original structure, thereby giving the composite characteristics different from those of the matrix. This allows for the creation of composite materials with desired physico-mechanical properties.

Unlike macro- and micro-sized fillers, nano-filler have a high modifying power determined by their significant specific area [8, 9, 37–43]. Therefore, relatively small concentration of particles allows to cover the entire surface of the boundary line between the matrix and the dispersion phase. In addition, this feature allows to effectively influence physical and mechanical properties of the composites despite of small amount of the filler.

The aim of this study was to investigate the influence of fillers on the morphology of the epoxy binder and the composite’s ability to restrain crack growth.

THE STUDY OF THE PROPERTIES OF NANOMATERIALS

Fig. 1. The structural formula of ED-20

METHODS AND MATERIALS

Epoxy diane resin ED-20 was used as a polymeric matrix. The structural formula of the resin is presented in Fig. 1. Epoxy resin is an oligomer. It has epoxy groups at the ends of the chain.

Polyethylenepolyamine (PEPA) was used as a hardener. The structural formula of the hardener is presented in Fig. 2. When the material is hardening, it causes a chemical reaction with the hardener’s amino-groups. The reaction disrupts the epoxy group and forms a cross-linked polymer.

The reinforcing fillers are nano-carbon materials, such as fullerens, nano-tubes and graphene. Such a list is determined by different morphology of the fillers’ structures.

Carbon nano-tubes (CNT) are one-dimensional structures. They are rolled up sheets of graphene. They form hollow cylindric molecules. Their specific area is approximately 500-1,000m2/g [16]. The typical structure of CNT is presented in Fig. 3a and 3b.

Fullerenes are the three-dimensional fillers since they are polyatomic molecules with an enclosure and the sur-

Fig. 2. The structural formula of polyethylene polyamine face of a high curvature. Their specific area is approximately 1,340 m2/g [35]. The typical structure of fullerenes is presented in Fig. 3e and 3f.

The diluent of the experiment is a kerosene-based liquid. The liquid has magnetic particles Fe2O3 sized from 5 to 30 nm. This diluent can improve the composite’s movement through narrow channels [1, 36]. The liquid does not form huge agglomerates of magnetic particles, because it has a surface active agent (SAA) in it. The agent is stearic acid C17H35COOH. The acid pulls to O–H+ polar structures to the magnetic particle surface and forms a 2 nm molecular layer around Fe2O3 particles. Flexible non-polar tails of the SAA in the non-polar dispersive environments are pointed away from the particles toward the liquid. (Fig. 4).

The component proportions in the composite materials are chosen based on the previous factorial experiments done by the authors of this research [21]. As a result, the study detects the most suitable patterns for getting the required balance of the adhesive and cohesive strength of the composites.

To evaluate the structure of the composites, this study applies fractographic research to test sample cracks for tensile strength. The test allows to define the destruction and how the composite resits the expansion of the cracks.

The microstructure of the composite materials, as well as the morphology of the hardening nano-carbon fillers, are studied with a scanning electron microscope (SEM) Tescan MIRA3. Initially, the sample surface was sprayed with a thin layer of carbon using Polaron E6700. The approximate thickness is 20 nm.

RESULTS AND DISCUSSION

The surface of the epoxy resin without fillers and its typical structure are presented in Fig. 5.

It is distinct that destruction is caused by insufficient strength of the material since the crack is straight. Few pores do not influence the crack’s expansion. There are also limited areas which prove that the material is flowing while getting destroyed. The long microcracks are found across the whole surface of the crack. They expand in the

THE STUDY OF THE PROPERTIES OF NANOMATERIALS

Fig. 3. The typical structure of nanocarbon fillers: a, b – graphene, c, d – carbon nanotubes, e, f – fullerenes

THE STUDY OF THE PROPERTIES OF NANOMATERIALS

Fig. 4. Fe₂O₃ Particle with Adsorbed Layer of SAA on it

same direction as the main crack (Fig. 5a). Microcracks spread in a coral-like manner (Fig. 5b).

Unlike the epoxy resin without the fillers, if we add carbon fillers to the epoxy resin, it will form numerous pores (Fig. 5a). This is associated with the fact that the nano-fillers have the big specific surface area. As a result, they are able to adsorb the epoxy resin or the hardener. At the same time they alter the stoichiometry in the limited areas. The nano-sized particles also tend for adhesion and can form agglomerates. The size of the agglomerates may reach hundreds of micrometers [11].

Adding graphene to the composite material causes a layered structure (Fig. 6b). The crack in the sample is straight. Therefore, the destruction is caused by strength insufficiency. (Fig. 6a). The composite is found to become stronger because the crack energy disperses to overcome the friction forces between the layers of the composite material (Fig. 6c). Also the energy is dispersed because the crack is branching (Fig. 6b).

Adding CNT as the filler forms a fibred composite material (Fig. 7b). The surface of the crack in the composite with CNT, and its typical structure are pre-

Fig. 5. Typical Macrostructure (a) and Microstructure (b) of Epoxy Resin Crack without Fillers

THE STUDY OF THE PROPERTIES OF NANOMATERIALS

Fig. 6. Typical Macrostructure (a) and Microstructure (b, c) of Crack in Composite with Graphene Filler: Arrows Indicate Branching of Microcrack

Fig. 7. Typical Macrostructure (a) and Microstructure (b, c) of Crack in Composite with CNT Filler

THE STUDY OF THE PROPERTIES OF NANOMATERIALS

Fig. 8. Typical Macrostructure (a) and Microstructure (b) of Crack in Composite with Fullerene Filler: Circles Indicate Places Where Crack Front Goes Around Fullerene Agglomerates

sented in Fig. 7. It is clear that the microcracks expand radially away from the pore concentration area. CNT get attached to the walls and prevent the crack edge from expanding (Fig. 7b). For example, we have similar findings in [26]. The CNT-filled composite material hardens because the crack energy disperses due to friction forces caused by CNTs extracted from the epoxy matrix (Fig 7c).

Adding the fullerenes does not significantly alter the structure of the default matrix (Fig. 5b). However, the sample faces major plastic destruction (Fig. 8a). The fullerene agglomerates effectively obstruct the crack expanding. It forces the crack to go around the agglomerates. (Fig. 8b). This causes the new crack areas, extension of the crack front and boosts the energy necessary for the material to get destroyed. For example, we have similar findings in [27].

CONCLUSION

The research showed that adding nano-carbon fillers (graphene, CNT and fullerenes) as the reinforcing components in the epoxy binder causes altering its structure. The study defines possible mechanisms for hardening of the composite materials due to adding the nano-carbon fillers. The crack energy disperses through the graphene-filled composite due to branching and elongation of the crack. The carbon nano tubes get attached to the walls and prevent the crack edge from expanding. The crack energy may also be dispersed to overcome the frictional forces caused by carbon nano tubes extracted from the epoxy matrix. The fullerene agglomerates effectively obstruct crack expanding. They force cracks to go around the agglomerates. This causes new crack areas. Alongside, the crack front extends while boosting the energy necessary for the material to get destroyed.