The effect of silica microparticles and maleic anhydride on the physic-mechanical properties of epoxy matrix phase

1 Introduction

Recently, much work has been reported in the open literature focusing on the reinforcement of thermoplastic and thermosetting polymers by the addition of organic or inorganic particles [1, 2]. The addition of nanoparticles into polymeric composites has been widely investigated due to the advances of nanotechnology and new processing techniques [3].

Thermoset polymers, especially epoxy resin, have been applied in several industrial applications in which high stiffness and adhesive strength are demanded. On the other hand, epoxy resin is rather brittle and has poor fracture toughness. For this reason, the addition of fibres/particles into thermoset polymer can be used to enhance strength and toughness for several structural applications [4]. Silica, alumina, and glass particles have been used as filler to reinforce polymeric composites. Epoxy-silica composite has been used as structural material, especially in the electronics, automotive, and aerospace industries, to dissipate heat and prevent static charge [5].

The mechanical performance of fibrous composites made of polymeric matrices reinforced with ceramic particles has been the focus of much research, especially for those composites manufactured with epoxy resin and glass fibres [6–12]. The particle addition increases the stiffness of the matrix, consequently affecting the effective properties of the hybrid composites, such as Young’s modulus [1, 6, 7], toughness [1], tensile strength [6], flexural strength [8], compressive strength [9, 10], impact strength [6, 11], and glass transition temperature [10], besides improving the interface conditions [11]. It is well known that the interface condition plays an important role in the mechanical behaviour of the material, especially at elevated temperatures [1]. In general, nanoparticle additions on fibrous composites made of epoxy matrices are set at low percentages (<10 wt%) [1, 6–9]; however, major percentage additions from 15 wt% [12, 13] up to 30 wt% [10] of silica nanoparticles have also been investigated on compressive performance of hybrid composites.

A variety of polymers has been used as the matrix phase of composites reinforced with natural fibres [14–18]. Characteristics such as weak interfacial adhesion, low melting point, and poor resistance towards moisture make natural fibres less attractive than synthetic fibres. In order to improve the fibre-matrix adhesion of biocomposites, several methods have been applied to modify the natural fibre surface, such as graft copolymerization of monomers onto the fibre surface and the use of maleic anhydride copolymers, alkyl succinic anhydride, stearic acid, etc. Coupling agents, such as silanes, titanates, zirconates, and triazine compounds, have improved fibre-matrix adhesion [19]. Although certain bonding techniques can provide adequate static strength, biocomposites are not usually durable when exposed to hot, moist environments [20]. Kalia et al. [21] investigated pretreatments of fibre-reinforced composites, revealing an increase in fibre-matrix adhesion and, consequently, improvements in their tensile properties. Naik and Mishra [22, 23] and Mishra et al. [24] studied the effect of maleic anhydride on sisal fibres and banana fibres. Water absorption was significantly decreased by the chemical treatment. Mechanical properties, such as modulus of elasticity, hardness, and impact resistance, increased with the addition of maleic anhydride, demonstrating its capacity as a compatibilizing agent.

Although polymeric composites made of synthetic fibres and nanoceramic/microceramic particles have been studied, hybrid composites made with natural fibres have not been reported in the open literature. For this reason, the addition of silica microparticles into the polymeric phase was firstly investigated in this work. Maleic anhydride was used to enhance the physical adhesion between phases. A full factorial design was performed to evaluate the main and the interaction effects of maleic anhydride and silica particles on the compressive properties and density of the modified epoxy matrix.

2 Materials and methods

The polymeric composites were composed of epoxy matrix, supplied by Resiqualy Company (São Paulo, Brazil), and a particulate phase of silica microparticles, supplied by Moinhos Gerais Company (Minas Gerais, Brazil). The silica particles were classified by sieving process in the monomodal range of size 0.037 mm. The physic-mechanical properties of the silica particles were provided by the supplier (see Table 1).

Experimental factors (levels) such as weight fraction of the silica microparticles (0, 20, and 33.3 wt%) and weight fraction of maleic anhydride (0 and 2 wt%) were investigated, providing a full factorial design of 2¹3¹, resulting in six experimental conditions (ECs; see Table 2). Preliminary tests were conducted in a laminated composite of natural fibres and epoxy resin with different contents of silica particles in order to set the upper level of silica particles (33 wt%), which provides an acceptable rheology for the lamination process. The level of maleic anhydride was chosen based on preliminary mechanical tests by adding 2 and 5 wt% into the epoxy matrix. The maleic anhydride level of 2 wt% exhibited superior mechanical strength.

The particulate composites were fabricated manually by hand mixing in three stages. The resin and hardener were firstly mixed, and then in the second and third stages, maleic anhydride and the silica microparticles were added, respectively. A mixing time of 5 min was set for each stage. The randomization procedure was adopted during the sample fabrication and the experimental tests. This randomization let an arbitrary ordering of the ECs, avoiding the event that noncontrolled factors affect the responses [25, 26].

Four specimens were fabricated for each EC. Two replicates and six ECs were performed running the total of 48 specimens. The replicate consists of the repetition of the EC, which offers an estimate of the experimental error of the individual response. The extension of this error is important in deciding whether there are significant effects attributed to factor action [25, 26]. The software Minitab 14 was used to manipulate the data using the design of experiment and analysis of variance (ANOVA) tools.

Variables such as mixing time (5 min), curing time (7 days), room temperature (∼22°C), and type of matrix (epoxy resin) were kept constant in the experiment. The following responses were investigated: compressive strength (σ_C), modulus of elasticity (E), and apparent density (ρ_A).

The compression tests were carried out according to ASTM D695 [27] in order to determine the mechanical strength and modulus of elasticity of the composites. The mechanical tests were carried out using a DL30000 EMIC machine using a test speed of 5 mm/min. Figure 1 exhibits the compressive test specimens for each EC (C1 up to C6). The apparent density of the composites was determined based on the Archimede’s principle [28].

Figure 1

Compressive specimens for the setup conditions.

3 Results

The experimental results for replicates 1 and 2 are shown in Table 3. The results of the ANOVA for the investigated responses are given in Table 4. Table 4 exhibits the p values and F values. If the p value is less than or equal to 0.05, then the effect is considered significant. An α level of 0.05 is the level of significance with a 95% reliability of the effect being significant. The F values reveal the intensity of each significant factor on the response.

The main effect of a factor might be interpreted individually only if there is no evidence that it does not interact with other factors [25, 26]. When one or more interaction effects of superior order are significant, the interacting factors might be considered jointly and not separately. All p values less than or equal to 0.05 are captured in italics in Table 2. The significant effects are shown on the main effect plot or interaction plot provided by the Minitab software. These graphics are not a typical scatterplot of data but illustrate the statistical analysis and provide the variation on the significant effects. The value of R² shown in the ANOVA (Table 4) indicates that the adjustment of the models is satisfactory. Larger values of adjusted R² suggest models of greater predictive ability [25, 26, 29].

3.1 Apparent density

The apparent density data varied from 1.15 to 1.35 g/cm³ (see Table 3). The main factor ‘silica addition’ and the interaction ‘chemical additive×silica addition’ were significant, showing p values of 0.000 and 0.008, respectively (see Table 4). The F value reveals that the factor silica addition more evidently affected the density of the composites. Figure 2 shows the interaction effect plot for apparent density of the composites.

Figure 2

Interaction effect plot of silica addition and chemical additive for the apparent density response.

Percentage increases of 10.6% and 15.8% were verified for mean apparent density when 20 and 33.3 wt% of silica microparticles, respectively, were added in the epoxy matrix. This behaviour can be attributed to the density of the silica particles (∼2.2 g/cm³, see Table 1) being superior to that of epoxy resin (∼1.16 g/cm³). The interaction between silica particles and maleic anhydride provided a small increase in apparent density when the composite was fabricated with 33.3 wt% of silica and 2 wt% of maleic anhydride. However, the difference was <1% (∼0.6%), which indicates that this interaction has no practical significance.

3.2 Compressive strength and modulus

Figure 3 exhibits the mechanical behaviour of the C1 to C6 composites. A ductile behaviour was verified for all setup conditions; however, maleic anhydride provided an increase in strength and the silica particles provided an increase in toughness, which were observed through the curve attenuation over the maximum strength peak.

Figure 3

Stress/strain curves of the C1 to C6 composites under compressive loadings.

The compressive strength and modulus of elasticity of the composites varied from 61.99 to 77.05 MPa and from 1.68 to 2.25 GPa, respectively (see Table 3). The ANOVA results (Table 4) reveal that the main factors ‘chemical additive’ and ‘silica addition’ and the interaction ‘chemical additive×silica addition’ exhibited significant effects (p<0.05) on the compressive strength of the composites. The F value of 446.17 (Table 4) indicates that the chemical addictive exhibited a superior effect on the response. Figure 4 exhibits the interaction effect plot for compressive strength. The addition of silica microparticles reduced the compressive strength of the material. Percentage reductions of 5.6% and 6.6% were observed for mean compressive strength by adding 33.3 wt% of silica for the material containing 0 and 2 wt% of maleic anhydride, respectively.

Figure 4

Interaction effect plot of ‘chemical additive’ and ‘silica addition’ for the compressive strength response.

Maleic anhydride was able to increase the compressive strength of the materials. Percentage increases of 14.9% and 13.6% were observed for mean compressive strength when maleic anhydride was added to those composites fabricated with 0 and 33.3 wt% of microsilica particles, respectively. An opposite behaviour was observed between the chemical additive levels (0 and 2 wt%) when 20 wt% of silica particles was added. The main reduction in compressive strength for those composites fabricated with no chemical additive occurred between the silica levels of 20 and 33.3 wt%. On the other hand, by adding a chemical additive, the main reduction in the compressive strength occurred when the silica addition of 20 wt% was set; subsequently, no relevant variation was observed when 33.3 wt% of silica was added.

The main factors ‘chemical additive’ and ‘silica addition’ significantly affected the modulus of elasticity, showing p values of 0.000 (see Table 4). However, based on the F value (162.48), the silica addition factor revealed a superior effect on the response.

Figure 5 shows the main effect plot of ‘chemical additive’ for the modulus of elasticity. Maleic anhydride was able to increase the mean value of the stiffness nearly to 6.6%, as shown in Figure 5.

Figure 5

Main effect plot of ‘chemical additive’ for modulus of elasticity response.

Figure 6 shows the main effect plot of ‘silica addition’ for modulus of elasticity. Based on the works reported in the open literature [1, 6, 7, 10, 12, 13], the addition of ceramic particles, especially in nanoscale, into epoxy matrix improves the stiffness of the material, which can be attributed to the higher stiffness of the silica compared with the epoxy matrix. As expected, in this work, the silica microparticles increased the elastic modulus of the material. Percentage increases of 10.1% and 23.4% were observed for mean elastic modulus when adding silica particles at 20 and 33.3 wt%, respectively.

Figure 6

Main effect plot of ‘silica addition’ for modulus of elasticity response.

These results indicate that the hybridization of silica particles and fibrous composites fabricated with epoxy matrix is promising for the improvement of the stiffness of the material because the silica particles increase not only the elastic modulus of the epoxy matrix [10] but also the physical adhesion between fibres and the matrix [11]. Despite that maleic anhydride has not been able to improve the adhesion between silica particles and epoxy matrix, this chemical additive could be more effective when dealing with biocomposites, affecting directly the surface of natural fibres, as previously reported [22–24]. However, the effect of maleic anhydride on hybrid biocomposite properties will be the scope of future investigations.

4 Conclusions

A full factorial design was performed to evaluate the effect of microsilica particles and maleic anhydride on the physic-mechanical properties of epoxy resin used as matrix phase in composite materials. The main conclusions of this work are as follows:

• While the maleic anhydride did not affect the apparent density of the composites, the silica microparticles exhibited a significant effect, increasing the density of the composites with the increase in the level of silica addition. Although the statistical analysis shows that the interaction between silica and maleic anhydride was significant, this interaction provided changes of <1% on the apparent density of the composites, revealing no practical significance.

• An interaction effect was significant on the compressive strength of the composites. The chemical additive was able to increase the compressive strength of the composites. On the other hand, the silica microparticle addition decreased the compressive strength of the composites. However, the stress-strain curves revealed that the presence of silica particles was able to increase the toughness of the polymeric phase.

• The main factors chemical additive and silica addition significantly increased the compressive modulus of the composites.

• Finally, the addition of silica microparticles and maleic anhydride into the epoxy matrix phase for biocomposite applications is promising, being able to enhance the stiffness and the physical fibre-matrix adhesion, respectively.