Mechanical properties of an epoxy-based coating reinforced with silica aerogel and ammonium polyphosphate additives

Flame retardant (FR) additives may degrade polymers’ mechanical performance. In this work, FR epoxy composites fabricated based on two popular FR agents of ammonium polyphosphate (APP) and silica aerogel (SAG) are investigated. Several mechanical properties of these composites, including compressive, micro-hardness, and Izod impact, were investigated for different filler loadings. Although the addition of 10 vol.% APP improved compressive modulus, yield strength, and micro-hardness, it degraded the impact strength. The incorporation of SAG made the composites more ductile, improved the impact strength, but deteriorated their compressive properties. Samples containing both SAG and APP demonstrated synergetic effects evident by their enhanced compressive properties and hardness. The findings of this study can guide the design of epoxies with both exceptional FR and mechanical performance.

into the system. These challenges have inspired researchers to investigate how FR additives impact the composites' mechanical properties. [7][8][9]17 Ammonium polyphosphate (APP) is a well-known commercial FR of polymers. However, the effect of APP on the mechanical properties of polymers has not been investigated thoroughly. Khalili et al. 18 reported that the tensile strength of a fiber-reinforced epoxy composite reduced adding APP. Long et al. 8 discovered that epoxy's tensile and impact strength deteriorate using the APP. Atikah et al. 9 showed that APP decreases the flexural strength of kenaf-filled polypropylene but increases its impact strength using low filler content. Lim et al. 7 investigated that although adding APP does not change the glass transition temperature ðT g Þ of a glass fiber/epoxy composite, the storage modulus increases. Riahipour et al. 14 also demonstrated that in comparison with pure epoxy, the transition to a rubbery state of the composite occurs at a higher temperature by adding 20 vol.% of APP. They also showed that the T g of the composite slightly increases at 20 vol.% of APP.
Silica aerogels (SAG) are composed of an interconnected network of channels leading to a mesoporous structure. Extraordinary characteristics of SAGs, such as low density, high specific area, and high thermal insulation, make them potential candidates as a filler in polymer-based composites. 17,19,20 As it is well-known, aerogels' ultra-low density and hydrophobic nature encounter several challenges in the fabrication procedures. To this end, Gupta and Ricci 19 developed a fabrication method based on the encapsulation of the aerogel particles in the epoxy resin. Besides, it has been established that adding SAG into the epoxy resin increases the viscosity of the matrix, thereby leading to more challenging mixing operations. 19,21,22 On the other hand, SAG's unique properties can solve some problems, including poor interfacial interaction and the aggregation of particles. 21, 23 Salimian et al. 24 reported that incorporation of SAG into the epoxy resin increases the fracture toughness, T g , the impact strength, and storage modulus. Riahipour et al. 14 also found that the storage modulus of epoxy reduces filling with 20 vol.% of SAG. Gupta and Ricci 19 discovered that the SAG/epoxy composites with higher density possess a higher compressive modulus. Maghsoudi et al. 17 also demonstrated that adding SAG makes the epoxy more ductile with a higher fracture toughness than the neat resin.
Even though APP is an effective FR for epoxy-based composites, to reinforce the brittle epoxy coatings against the mechanical stresses, we need to study other additives. Herein, this work's impetus is to investigate the synergistic effect of SAG and APP on epoxy-based composites' mechanical properties. In a previous study, we demonstrated that adding 10 vol.% of SAG elevated the T g of the epoxy 12% (from 66.3 to 74.2°C) while at 20 vol.% of SAG the T g decreased to a value close to the T g of neat epoxy (from 66.3 to 66.1°C). 14 Also, we showed that filling the epoxy with 10 and 20 vol.% of APP just increased the T g by 3% (from 66.3 to 68.3°C) and 2.4% (from 66.3 to 67.9°C), respectively. In this paper, in addition to previously discussed thermal and thermomechanical properties, we prepared SAG/APP/epoxy composites with different filler contents and characterized mechanical properties, including impact, compressive, and micro-hardness. The obtained results demonstrate that good interaction between the epoxy resin and SAG particles (at low volume fractions), in addition to the highly porous structure and high internal surface area of SAG, increase all T g , hardness, and impact strength properties of epoxy. Therefore, adding the SAG particles into epoxy/APP coatings can improve the composites' mechanical properties.

Materials
Ammonium polyphosphate (APP) in fine powder with an average size of 5 µm was purchased from Pars Idea Chem Co. Ltd. (Tehran, Iran). The density of the powder particles is 1.98 g/cm 3 based on the supplier's data. Silica Aerogel (SAG) with an average pore size of 20 nm was supplied by Vaspart Co., based in Isfahan, Iran. The material's porosity is higher than 95%, and the density is 0.06 g/cm 3 based on the datasheet provided by the vendor. For the matrix, diglycidyl-ether bisphenol A (Epikote 828) resin (EP) with the equivalent weight of 190 g/eq is mixed with HA-11, a tetra-amine-based hardener with the amine hydrogen equivalent weight of 26.9 g/eq. The mixture's mass ratio was 10:1, and the material is supplied by Shimi Afsoon Co. Ltd. from Tehran, Iran. Tetrahydrofuran (THF) from Aldrich was also employed as the solvent for making the epoxy composites.

Preparation
Three different scenarios were considered to investigate each filler's effect and their synergetic impacts on mechanical properties: APP/EP, SAG/EP, and SAG/APP/EP. Once each case was investigated, one can verify the superposition principle's validity for predicting the composites' mechanical and physical properties. A coding system representative of the filler content was also proposed to identify each sample. Table 1 illustrates the samples' name and their composition.
All the samples were prepared using the solution processing method described in. 19 Before adding the fillers, for a more uniform distribution, the viscosity of the epoxy was lowered by heating up to 50°C. After adding the corresponding amount of additives for each case based on Table 1, using a mechanical mixer, the solution was blended for 1 h at the speed of 2000 rpm. Before adding the hardener, the mixture was heated on a hot plate to remove the solvent. The mixture was then degassed at room temperature using a vacuum oven for 30 min. Then, the HA-11 hardener was added to the mixture and blended for 10 min. The weight ratio of the hardener was 10:1 resin to hardener. Eventually, depending on the mechanical test type, the mixture was poured into the various molds for curing in a vacuum oven for 12 h at room temperature.

Characterization
A rectangular sample shape was used for manual measurement of the density. The sample's mass was obtained using an ultraprecise scale (precision: 0.0001 g) manufactured by Nevada Radwag (USA). A Mitutoyo caliper also measured the sample's dimensions with ±0.1 error tolerance to calculate the volume. A rheometer (TA Instruments AR2000, USA) was employed to measure the epoxy's viscosity at different contents of SAG and APP particles prior to adding the hardener. The viscosity measurements were carried out at 0.1 s À1 shear rate and 25°C. The compressive strength of the block samples was measured with a machine made by ADMET (USA). Testing conditions were according to ASTM D6950 at room temperature. The load was applied to the specimen at the rate of 3 mm per minute. The test continued until the sample started to cripple, which was considered the ultimate strength point. The compression modulus and compression yield strength of each specimen are the other two results of this test. The Vickers hardness test was employed based on ASTM E384 standard to evaluate the samples' surface properties. After cleaning the specimens, the square-based pyramid-shaped indenter was penetrated the surface in 10 different spots of each 20 mm × 20 mm × 3 mm sample with a load of 100 g for 15 s. The instrument used for this test was HMV2 micro-hardness tester made by Shimadzu. The notched Izod impact test was carried out using an impact tester manufactured by ZWICK. The notched specimen dimensions were 63.5 mm × 12.7 mm × 32.5 mm, prepared based on ASTM D256 standard. Samples from the notched Izod impact test were used for fractography. Before taking the images, the fractured surfaces were coated with 4 nm of carbon. Scanning electron microscope (SEM) images were collected using an FEI Q3D scanning electron microscope (Zeiss, Germany).

Density and rheology
The composites' density as a function of the fillers' volume fraction is presented in Figure 1(a). As expected, the density decreases by filling the composite with ultra-light and high-porous SAG particles. Adding 5, 10, and 20 vol.% of SAG reduces  the density 1.7, 4.3, and 9.6%, respectively. However, some phenomena, like the resin diffusion into the aerogel's pores and the collapse of some SAG particles attributed to the high mixing shear force, restrict further density reduction. 17,19,20 Accordingly, the density of the 5SAG-15APP sample is only 0.63% lower than 20APP. Figure 1(b) exhibits shear viscosity values obtained from the epoxy resin incorporating SAG and APP particles before adding the hardener. The resin's viscosity increases with the volume fraction of both SAG and APP; however, the increase is more drastic once the aerogel particles are added to the resin. One potential reason is that the nanochannel structure of SAG confines the epoxy polymer chains' movement more seriously, resulting in higher resistance in the epoxy's flow. 21 Besides, we realized that the resin's viscosities values are close at 5 vol.% of both APP and SAG while at 20 vol.% of fillers, the SAG particles cause a viscosity more than two times higher than APP.
Morphology of impact fracture Figure 2 represents SEM images of the samples' cross-sections fractured by the impact test. The fracture surface of pure epoxy (Figure 2(a)) shows that the parallel cracks have been propagated in a smooth surface with slight plastic deformation, demonstrating that the fracture was brittle. 17,25 The small particles shown in the image are likely resin shards or dust particles. 26 Figure 2(b) represents the morphology of the fracture surface of the 20APP sample. Some large APP particle sizes shown in the image indicate that the shear force is not strong enough to separate the APP particles due to the mixture's high viscosity. The existence of the APP particles without any epoxy-covered layer demonstrates that the compatibility of APP and epoxy is weak. 27 The dark spots observed at the broken surface reveal that the APP particles are removed easily from the matrix due to the weak adhesive bonding between APP and epoxy. 6,11 The SEM image taken from the 20SAG specimen ( Figure 2(c)) demonstrates that, unlike APP, the specific molecular structure of silica aerogel particles (porous nature with an interconnected network of channels) facilitates the particles' engulfing and encapsulating in the epoxy matrix. 14,19 In other words, interfacial adhesion between SAG and epoxy is noticeable. Also, the wave-like features shown on the surface indicate that adding SAG particles makes the epoxy more ductile. 17 However, we observe some large pores and agglomerates (shown with red dashed ellipses) on the surface, possibly resulting from the high resin viscosity seen at 20 vol.% of SAG. Therefore, the trapped air pockets in the matrix are unable to leave the matrix and emerge as the micro-voids observed in the SEM image. This high viscosity level also makes incorporating the fillers into the matrix hard and eventually leads to aggregation of the SAG particles. 28 We can also observe micro-cracks around the voids, demonstrating that they act as stress concentration spots at the impact fracture.
The morphology of the specimen containing 5 vol.% SAG and 15 vol.% APP is presented in Figure 2(d). In comparison with 20APP, the APP particles are more engulfed in the matrix at the 5SAG-15APP sample. Besides, the SEM image shows more wave-like plastic deformation at the fracture surface. Overall, it can be concluded that adding SAG particles into the APP specimens makes the composites more ductile.

Compressive properties
Since polymeric coatings are exposed to frequent local or global compressive stresses, 29,30 studying a coating's compression properties are crucial. The standard stress-strain results of specimens loaded in uniaxial compression are shown in Figures 3  and 4, and Table 2. Even though the epoxy materials behave in a brittle-like fashion under tension, 6 the strain-stress curves show a ductile behavior, with yield and ultimate points, under compression, consistent with Kucharek et al. 31 observations. An interesting trend is observed in the APP content samples. Although all compressive properties of the composite initially improve in the 10APP specimen, adding 20 vol.% of APP deteriorates the matrix's compressive properties, Figure 3(a). On the other hand, adding the SAG particle into the resin matrix has worsened the composite's compressive properties, except its failure strain (Figure 3(b).
It is expected that adding the rigid APP particles makes the matrix more brittle but enhances its compressive strength, 32,33 which is the case for the 10APP specimen. Adding 10 vol.% of APP improves compressive modulus and yield strength (16% and 6%, respectively) while its failure strain reduces by 2.6%. Based on the percolation theory, the matrix zone around each filler particle can join together once the particles' distances become small enough. These connected zones create a percolation network against stress concentrations, which improves the compressive modulus. 6,34 In the same filler content, uniform dispersion of the additives leads to shorter distances between particles, and consequently, higher compressive properties. 28,35 On the other hand, at higher APP concentration, 20 vol.%, the particles' inevitable aggregation leads to the lower yield strength, modulus, and failure strain. 32 We can also explain the deterioration of the compressive properties based on the percolation theory. Once particles aggregation occurs, the distances between particles start to increase. As a result, the applied load will not transfer well from the matrix to the particles' zones, and consequently, the composite will fail at lower stresses and plastic deformations. The trapped air pockets left in the matrix due to high viscosity at 20 vol.% of APP also could be the other reason for the compressive properties' deterioration.
It can also be noticed that the pure system's compressive properties deteriorate considerably with the increase in the SAG content, Figure 3(b). The composites show a less distinct yield point compared to the pure resin by adding SAG particles. Besides, at the lower volume fraction, 10 vol.%, the compressive modulus, yield strength, and ultimate strength decrease 13.2, 16.3, and 7.7%, respectively. At 20% volume fraction, these properties have been reduced remarkably, 45.9, and 45.6, and 38%, respectively. Silica aerogels are materials with interconnected networks of channels. 36 Their mesoporous structure of SAG makes them an ideal option to increase the porosity of the composite. Consistent with our results, previous studies also demonstrate that both compressive yield strength and modulus decrease with enhancing the porosity of the composite. 31,37,38 Besides, as Gupta et al. 19 illustrated, the compressive modulus is higher in composites with higher density. Therefore, as expected, the compressive modulus is reduced in the specimens with lower densities. Besides, the high viscosity level seen at 20 vol.% of SAG deteriorates the additives' dispersion and, subsequently, the compressive properties. Increasing viscosity is slighter in the 10SAG specimen in comparison with 20SAG. Therefore, the compressive properties have been worsened more dramatically in the 20-volume faction of the filler content. It is also observed that, unlike APP, SAG particles interestingly do not impact the matrix's failure strain, which is consistent with Jumahat et al.'s 16 results. The higher rigidity of APP particles compared to SAG can explain this observation. Figure 4 illustrates the synergistic effect of the SAG and APP on the compressive properties. The slope of the stress-strain curve in the elastic region (shown in Figure 4(a)) indicates that the epoxy resin synthesis containing both SAG and APP particles has almost similar compressive modulus with the neat resin. Figure 4(b) shows that the 5SAG-15APP specimen has higher yield strength over the 20APP and 20SAG samples, demonstrating the SAG and APP's synergistic effect on the yield strength at 20 vol.% of the fillers' content. It also can be found that adding more SAG into the matrix (the 10SAG-10APP specimen) has deteriorated the yield strength by 7.7% compared to the 5SAG-15APP sample. Besides, compared with the pure matrix, mixed samples' failure strain results confirm that the elongation at break increases slightly (7.8% and 4.6% increment, respectively, in the specimens 10SAG-10APP and 5SAG-15APP).

Micro-hardness properties
In 1951, Tabor proposed a correlation between hardness and yield strength in metals. 39 However, plenty of studies have demonstrated that Tabor's relation cannot be applied to all materials. 27,40 Polymeric composites particularly have complex behavior under stress. In particulate-filled polymers, many factors, including additives' characteristics and contents, matrix type, and interfacial bonding, determine the material properties. 27 Our results show that the hardness and compressive yield strength for APP contains samples follow a similar trend, consistent with Suwanprateeb et al. 27 We also observe an optimum point for SAG specimens accrued at 5 vol.%. After that, the hardness values reduce below the pure matrix's hardness, comparable to what was observed in the compressive yield strength.
The results reveal that the micro-hardness increases with APP loading up to 10 vol.%, Figure 5(a). Then, the hardness value drops 9.1% in the 20APP specimen. The mechanical dispersion techniques cannot distribute the particles uniformly into the matrix at the high level of filler contents. Consequently, the aggregated particles disable the effects of the reinforcements. On the other hand, particles can act as a retardant against the chemical reactions at high filler contents. 41 Therefore, incomplete curing of the matrix can also explain the hardness reduction at 20 vol.% of APP. Figure 5(a) also presents the hardness results of SAG composites. Previous studies have demonstrated that adding silica particles improve the hardness. 17,36 However, Veena et al. 42 observed an optimum point in the hardness of the epoxy matrix when the silica content increases. Our results show that the hardness of the composite improves 6.3 percent, loading SAG  particles up to 5 vol.%. Conversely, comparable with the compressive strength results, the hardness values reduce 3.5 and 28.8 percent, respectively, at 10 and 20 vol.% of SAG compared with neat epoxy. Increasing cross-linking density due to matrix-SAG interactions can explain increment at 5 vol.%. 43 However, at 10 and 20% volume fractions, high-porous SAG particles raise the viscosity to high levels, making particles' uniform dispersion difficult. Therefore, poor distribution of the particles compensates or even reverses the effect of reinforcements. This problem is more severe in the 20 vol.% because, at higher viscosity levels, the removal of the air pockets trapped in the matrix becomes more difficult.
The hardness results of the specimens containing both APP and SAG particles are presented in Figure 5(b). The results illustrate that, at 20 vol.% of APP content, replacing 5 vol.% of APP with SAG particles, which is termed the 5SAG-15APP sample, improves the hardness by 2% over the 20APP specimen. However, a further 5 vol.% increment of SAG content, which is termed the 10SAG-10APP sample, deteriorates the hardness by 7.4% compared to the 20APP sample. Since SAG elevates the viscosity level more seriously, the particle dispersion will be poor at the specimens with higher SAG contents leading to lower hardness values. Correspondingly, the hardness of the 5SAG-10APP sample is 8.3 percent higher than 10SAG-5APP at 15 vol.% of fillers content. Altogether, the hardness results of mixed samples indicate that APP's effect on the composites' hardness is more dominant than SAG.

Impact properties
The impact strength of composites using different fillers' contents is discussed in this section. Since high test speed is employed in the impact test, the fracture mechanism is totally different from the compression and hardness tests. 17 Therefore, the composite's strength could be completely different from what was observed in the compression and hardness tests. Overall, three processes determine the strength of composites made by polymeric materials when they are exposed to an impact type of loading. The impact energy is absorbed by plastic deformation of matrix, debonding at the additive-matrix interface, and fracture at the additive particles. 33 The impact strength is controlled by the process that needs the least level of energy to occur.
The impact strength of the epoxy matrix deteriorates by adding APP particles, shown in Figure 6. Compared with pure EP, the strength decreases 5.7, 21.1, and 46.1% by adding 5, 10, and 20 vol.% of APP, respectively. It has been reported that the APP particles act as stress concentration sites when the composite is exposed to the impact loads. 8,9,11,12 Therefore, the composite becomes brittle by adding APP and fractures at lower plastic deformations. Besides, as the SEM image demonstrates (Figure 2(b)), weak interaction between the matrix and APP particles could also explain impact strength deterioration. 9,44 In this case, low absorbed energy at the reinforcements-matrix interface leads to the fracture.
As illustrated in Figure 6(a), pure epoxy's impact strength enhances up to 10 vol.% SAG content. Next, we observe a strength reduction at the 20SAG specimen. Impact strength improvement by adding SAG particles into epoxy resin also has been reported by other researchers. Salimian et al. 21 claimed that the interfacial interaction between the resin and particles and subsequently the impact strength of the composite improve due to two incredible characteristics of silica aerogel particles, large-scale porosity, and high internal surface area. Maghsoudi et al. 17 and Albooyeh et al. 36 stated that the matrix's strength enhances against the shock waves in the epoxy-silica aerogel composites because the porous structure of the particles propagates the energy away from the impact zone. Reversely, the impact strength decreases 28% by adding 20 vol.% SAG compared to the 10APP specimen. As discussed earlier, adding mesoporous silica aerogel particles into the polymer mixtures significantly increases viscosity. 31,45 Consequently, the probability of forming aggregated particles enhances because the particles cannot distribute uniformly across the substrate. These aggregated spots act as stress concentration sites in the impact type of energy. Besides, at high SAG contents, the trapped air bubbles inside the substrate are favorite places for the micro- crack formation (see Figure 2(c)). Moreover, Salimian et al. 24 claimed that the impact strength deterioration at high SAG contents could be attributed to the reduction in the cross-linking degree of the epoxy resin. Zhang et al. 12 also indicated that the impact strength deteriorates at the high filler contents due to disturbing the matrix's continuity. Figure 6(b) illustrates the synergetic effect of the impact strength in epoxy-SAG-APP composites. The results show that the impact strength values of all specimens containing both SAG and APP are located between the strengths of the composites with one filler. In specimen 20APP, the impact strength improves 96.4% by replacing 5 vol.% of APP with SAG and making the 5SAG-15APP specimen. Further 5 vol.% replacement of APP with SAG, which is termed the 10SAG-10APP sample, increases the improvement to 135.7 percent. Also, the impact strength of the 5SAG-15APP and 10SAG-10APP are, respectively, 5.7 and 26.9% higher than pure epoxy. At 15 vol.% volume fraction of additives, the impact strength of 10SAG-5APP is 41.4% more than the specimen 5SAG-10APP, while this difference is lower at 20 vol.% volume fraction of fillers (the impact strength of 10SAG-10APP is 16.6% higher than 5SAG-15APP). These observations demonstrate that adding SAG particles into APP composites enhances the impact strength, and this increment is more effective at lower filler contents.

Conclusions
This work investigates the mechanical and physical properties of particulate composites composed of SAG, APP, and EP resin. Samples were prepared in the neat form, SAG/EP and APP/EP, or mixed ones (SAG/APP/EP) with different filler loadings. In a previous study, 14 it has been demonstrated that APP is more effective in improving the flame retardant performance of epoxy. Here, we show that replacing a fraction of APP with SAG is beneficial. The incorporation of SAG into the epoxy decreases the composite's density. Our results indicate that the 5SAG-15APP sample achieves higher compressive properties than the 20APP and 20SAG, demonstrating the synergistic effect of combining SAG and APP. Besides, the elongation at break of APP/EP composites enhances by adding the SAG particles. Adding a low volume fraction of SAG into the APP/EP composite slightly enhances the composite's hardness. The Izod impact results indicate that by replacing 5 vt% of the APP with SAG in the 20APP specimen, the impact strength improves by 96.4%. Consistent with impact results, the SEM images of fracture surfaces demonstrate that adding a low volume fraction of SAG makes the brittle APP/EP composite more ductile.

Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding
The author(s) received no financial support for the research, authorship, and/or publication of this article.