Enhancement of thermal properties of Kevlar 29 coated by SiC and TiO₂ nanoparticles and their binding energy analysis

2.1 Materials

The following materials are used for the preparation of samples. Richest Group Limited, Chemicals, Shanghai, China provided Kevlar 29 (Aramid Fiber, Color: yellow, Break Strength: 22 g/D), Silicon Carbide nano-powder (Model No.: SiC-001, 99.9% Purity, Cubic form, Gray-green Powder, Bulk density: 0.11 gm/cc, particle size: 40 nm), and Titanium Dioxide nanopowder (Model No.: XF102-1, 99Epoxy (Bisphenol-A, Lapox AH-713 HS, Model No.: B-47, 99Epoxy (Bisphenol-A, Lapox AH-713 HS, Model No.: B-47, 99Epoxy (Bisphenol-A, Lapo Atul Ltd., Maharashtra, India, supplied the Viscosity (650 m Pas, Epoxy Value: 5.50 Eq/kg, Density: 1.17 g/cc, Specific Gravity: 1.10 mix mass) and Hardener (Mixing Ratio: 100:10 (w), Viscosity Density: 1.05 to 1.15 g/cc, Hardener Density: 0.90 to 0.96 g/cc, High-Performance Liquid Chromatography (HPLC) Water Aqua Dihydrogen Oxide (CAS No: 7732-18-5, Chemical Formula: H₂O, HS Code: 2853 90 10) PT. Smart-Lab, Serpong, Tangerang, Indonesia (Molecular Weight: 18.02 g/mol, Batch No: 107703022001, Product Code: H-1077) imported the product. Acetone (Lot: 0059001, Chemical Formula: CH3.CO.CH3, Molecular weight: 58.08, CAS: 67-64-1, Code: A/0676/20) was imported from Fisher Scientific, UK.

2.2 Preparation of coating surface

Surface preparation is an important factor for coating. Improving the adhesion properties between two surfaces always relies on the material surface. The surface of the Kevlar fiber has been done in three steps. These are; a) cutting the fiber mat, b) cleaning the fiber mat, and c) heating the fiber mat. The samples were taken from a roll of Kevlar fiber mat at a dimension of 4 in. × 12 in.. Two solvents were used to complete the cleaning process. To clean the manufacturing coating on the surface of the fiber mat, first it was washed with acetone (CH₃ CO CH₃). The samples were dried in an open atmosphere for 10 min. There is a possibility of ionization after the cleaning with acetone. Finally, the surface of the fiber mat was washed with water and dihydrogen oxide (HPLC) to neutralize the ion. The neutralized samples were dried in an open atmosphere for a specific amount of time. Heating is the final step of the surface preparation. At the last stage of the cleaning process, the samples absorb some moisture content for the addition of water aqua dihydrogen oxide (HPLC). Heating is very essential to remove the moisture content of the samples. The water aqua dihydrogen oxide (HPLC) vaporization temperature is 100 °C. Evaporation has occurred after the addition of heat up to 100 °C, and at 100 °C it produces vapor. To remove this moisture from the fiber mat, it was dried out in an oven for 45 min at 110 °C.

2.3 Preparation of coating solution

Epoxy resin, hardener, silicon carbide nanoparticles, and titanium oxide nanoparticles (Fig. 2) were used to make the coating solution. The mixture was agitated for a given amount of time to get a homogenous solution (see Table 1).

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Fig. 2

Preparation of coating solution.

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2.4 Preparation of coated samples

One of the most popular methods of developing a thin and uniform coating is the dip-coating method. It is also called saturation or impregnation. Although the deep coating is suitable for complex shapes and curved parts, this method is also used for flat and cylindrical surfaces.

In the dip-coating process (Fig. 3) Kevlar fiber mat is dipped into a coating solution bath. Here, step 1 represents the passing of the fiber mat towards the solution bath, and step 2 is the immersion of the mat into the solution so that it can absorb nanoparticles. The coating solution is adhesive. For this, the samples are allowed to stay for 5 min. In step 3, the immersed samples were then allowed to pass through a pad mangle machine to insert the nanoparticles into the fiber portion. The squeezed roller of the machine pressed the sample and straightened it. Curing is an important factor in the polymerization of a polymer matrix. The durability and strength of the composite materials partly depend on the curing method. The ratio of resin to binder affects the curing time. The conditions of the product life cycle, ambient weather, shape, and size of the product, and the specified strength play a crucial part in the curing period. In order to obtain the desired requirements for this work, two-step curing has been done. Firstly, the coated sample is cured in an open atmosphere for 20 min. After the completion of the first step of curing, the samples are placed in an oven. The samples are kept in the oven at 200 °C for two and a half hours, and the samples are also cooled in the oven.

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Fig. 3

(a) Working flow chart of pad mangle machine (b) Pad mangle machine (c) Interfacial structure of the coated sample.

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2.5 Characterization of coated samples

3.1 Thermal analysis

Thermogravimetric (TGA) analysis and Differential Scanning Calorimetry (DSC) analysis have been done for analyzing the thermal properties of the uncoated and coated Kevlar fiber samples.

3.1.1

3.1.1 Thermogravimetric (TGA) analysis

Fig. 4 shows the TGA, DSC, and derivative curve of uncoated Kevlar (S1) and coated Kevlar (S2 & S3). To compare the relation of heat flow of the sample, a derivative calculation has been analyzed. Moreover, the derivative TGA (DTg) curve clearly appeared the two step transitions compared with the uncoated Kevlar with epoxy and hardener. At glass transition temperature (Tg), the molecule movement is released (heating) or freezes (cooling). The derivative heat flow of the uncoated sample shows a glass transition temperature of 558.59 °C. The oxidation temperature (T_od) is the temperature at the maximum peak of that material on the derivative weight curve and can be used to determine the material's thermal stability. The oxidation temperature of the uncoated Kevlar is 52.38 °C, whereas the coated samples T_od is 52.43 °C and 53.03 °C respectively. The temperature at which a substance started to decompose chemically is known as its initial decomposition temperature (IDT). The coated sample starts to decompose initially at 348.95 °C and 345.94 °C respectively but the uncoated sample starts to decompose at 342.81 °C. For the uncoated sample DSC thermo-gram shows a peak temperature of 480.01 °C when the sample normalized enthalpy is 770.57 J/g. The normalized enthalpy of samples S2 & S3 are 2404.4 J/g and 3490 J/g with a peak temperatures of 493.69 °C and 605.15 °C i.e., the coated samples can receive more heat compared to the uncoated sample also higher value of enthalpy represents difficulty to breaking hydrogen bonds (Yuan et al., 2015). Due to the higher melting point of coating materials, coated samples receive more heat compared to uncoated sample. The derivative glass transition analysis of the samples exhibits a temperature of 586.59 °C and 612.08 °C which is increased 27.7 °C & 53.49 °C compared to the uncoated samples.

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Fig. 4

Simultaneous TGA, DSC and Derivative curve of (a) uncoated Kevlar with epoxy and hardener (S1), (b) coated Kevlar, Epoxy, and Hardener with 5% (1:1) nanoparticles (S2), and (c) coated Kevlar, Epoxy, and Hardener with 10% (1:1) nanoparticles (S3).

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Sample S3 exhibits the maximum enthalpy and the peak temperature is based on the enthalpy shows for sample S1. Sample S3 shows a maximum end set temperature, derivative glass transition temperature, and covers the maximum % of the area under the derivative curve. For cured coatings, the T_g also is an indicator of flexibility or lack of it. Coatings with high T_g values may be brittle and prone to cracking or chipping. Those with low T_g values may be soft and easily marred and scratched and/or prone to dirt pick-up.

Fig. 5 represents the statistical analysis of the TGA and DSC graphs of samples S1, S2 & S3 with weight (%) and weight (mg). The 4.691 mg mass is considered as a residue equal to 98.52% weight of the S1. The uncoated sample (S1) losses 3.344 mg weight and a weight percentage of 70.23 i.e., the sample shows a residue of 29.77% mass of the uncoated sample at 1000 °C. Whereas the coated samples S2 and S3 at 1000 °C losses 74.24% and 95.94% weight loss respectively, that represents the coated samples are light weight. The results of mean, variance and standard deviation corresponding to different axis represents 2nd step transition rely very closely.

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Fig. 5

Statistical analysis of TGA and DSC curve of (a) sample (S1), (b) sample (S2), and (c) sample (S3).

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The crucial matter of the determination of mean, variance, standard deviation, and relative standard deviation strongly depends on the weight of the sample, so that comparison cannot be drawn corresponding to mean because of different sample mass but comparison can be drawn by standard deviation, variance, and relative standard deviation. The higher value of standard deviation and variance represents the better dispersion of data (i.e., higher magnitude covers the maximum data under the bell shape curve). Relative standard deviation is the ratio of standard deviation and mean of the samples data. Coated samples represent the better statistical data compared to the uncoated samples based on the magnitude of relative standard deviation.

Fig. 6 represents the mass loss at various temperatures of uncoated and coated samples. At a temperature of 300 °C for uncoated samples 8.38 wt% that depicts the entire degradation, which was primarily owing to the composites' low boiling additive and moisture content loss. These types of proof were observed by Jawaid et al. (2018). At 500 °C the sample S2 shows higher mass loss and sample S1 shows a lower mass loss, at 700 °C & 900 °C the sample S3 shows higher mass loss and sample S1 shows a lower mass loss, and at 1000 °C the sample S3 shows a higher mass loss, i.e., lower residual mass. Sathishkumar et al. (2021) mentioned less material degradation for lower activation energy.

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Fig. 6

Mass loss at various temperature.

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3.1.2

3.1.2 DSC analysis

DSC is well renowned for its ability to expose the thermal characteristics of solid materials, specifically polymers and plastics (Lakatos et al., 2022). Fig. 7 shows the DSC comparison of all samples. In DSC thermo-gram the 1st step transition shows a lower amount of peak compared to the 2nd step transition. Here also shown a magnification from 400 °C to 700 °C for 2nd step transition. In the DSC curve, sample S3 represents a higher heat flow of 4.169 W/g compared to all samples, whereas the uncoated samples heat flow −0.06 W/g and the maximum heat flow withstood up to 605 °C for samples S3. Sample S1 and S2 exhibits exothermic peaks, but sample S3 show an endothermic peak. The endothermic peak indicates the higher activation energy. All the sample provides heat to the system whereas the sample S3 takes heat from the system i.e., the sample S3 indicates it can be used at high temperature compared to the other samples.

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Fig. 7

(a) DSC Comparison of S2 with respect to sample (S1), (b) DSC Comparison of S3 with respect to sample (S1).

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3.2 FTIR analysis

Fig. 8 (a), (b), and (c) shows the FTIR analysis of all samples. In FTIR analysis, the uncoated sample represents the wavelength of 889, 1244, 1510, and 2928 cm⁻¹, a lower change of wavelength due to the presence of nanoparticles in coated samples but appeared same functional group which represents there is no unreacted bond of the polymer (Guo et al., 2018). The analysis of Fig. 8, exhibits a range of functional group of strong alkanes (C—H, stretching vibration) range 2850–2970 cm⁻¹ (Noobut and Koenig, 1999; Ngono et al., 1999) due to the presence of epoxies, medium aromatic (C—H, benzene) range 675–995 cm⁻¹ (Ngono et al., 1999), strong alcohols, ethers and carboxylic acid (Symmetric stretching, C—O) range 1050–1300 cm⁻¹ (Chike et al., 1993), and aromatic ring (C⚌C, aromatic nucleus) range 1500–1600 cm⁻¹ (Yang et al., 2015) by ignoring the lower peak. Fig. 8 (d) represent the comparison of FTIR analysis to show the variation of the peak, strong alkanes and aromatic ring absorbance peaks appear broadly in cases of coating samples because the coating materials absorbs the infrared ray. In FTIR figures it has been clearly observed that when the sample is uncoated the peak length are small whereas coating is considered broader peak length is observed. The frequency absorbent level is high in case of coated materials as compared to uncoated material. The higher the frequency absorbent level, the higher the quality of the materials is confirmed; however, the quality of the material depends on the type of coated material and percentage of nanoparticle inclusion.

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Fig. 8

(a) FTIR analysis of S1, (b) FTIR analysis of S2, (c) FTIR analysis of S3, (d) FTIR comparison of all sample.

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3.3 FESEM analysis

Morphology of the sample S1-S3 with different scales (100 $\mu m$ , 10 $\mu m$ , 1 $\mu m$ , and100nm) and magnifications (25X to 3000X) as shown in Fig. 9. 10% nanoparticles exhibit a better nanoparticles distribution compared to 5% nanoparticles. In morphology analysis here the shape, texture, and distribution of materials have been analyzed. It is clear that the nanoparticles show the inhomogeneity’s for the particle distribution. Among these, sample S5 shows a better homogeneity of the particles. Homogeneity of the particle distribution represents better surface morphology (Alyami et al., 2017; Zhao et al., 2021). The titanium oxide nanoparticles appeared less compared to the silicon carbide nanoparticles.

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Fig. 9

Morphology of samples.

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3.4 X-ray Photoelectron Spectroscopy (XPS)

XPS is a powerful measurement approach because it not only indicates what elements are present but also what other factors they may be bound to i.e., chemical state, density, electronic shape, and state (Rahmayeni et al. 2019). Fig. 10, and Table 2 shows the X-ray Photoelectron Spectroscopy survey of binding energy before and after etching of samples S1, S2, and S3. Without etching, the specific structural properties cannot be identifying in the polish state. To assess the quantitative and qualitative elemental composition of coating etching has done. After etching XPS analysis of the uncoated sample shows Si 2p (102.73ev), S 2p (169.15ev), C 1s (285.31ev), N 1s (399.76ev), O 1s (532.78ev), and Na 1s (1072.09ev). In cases of sample S2; C 1s (285.12ev), N 1s (399.87ev), O 1s (533.01ev), Na 1s (1072.52ev) appeared after etching but 0.56% Si 2p appeared before etching. Sample S3 shows Si 2p (102.78ev), Cl 2p (199.4ev), C 1s (285.16ev), N 1s (399.93ev), O 1s (532.92ev), and Na 1s (1072.28ev) after etching. The carbon leads the maximum percent of element because the Kevlar fiber contains carbon a lot. Here Sulphur represents the impurity of the coated sample. From the C 1s spectrum shows the 298.0 to 280.45 eV after etching which assigned the presence of alkanes (C—H), medium aromatic (C—H), and aromatic ring (C⚌C), also O 1s spectrum ranges are 542.08–526.29 eV ensured the presence of strong alcohols, ethers and carboxylic acid (C—O) group respectively (Li et al., 2017). These results are consistent with the FTIR analysis shown in earlier (Fig. 8). The elemental analysis may vary in different methods. In XPS analysis only the upper portion of the coating are investigated, whereas the EDS analysis can represent a better elemental analysis due to higher penetration of electron (Mashtalyar et al., 2022).

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Fig. 10

XPS survey of before and after etching (a) S1 (b) S2, and (c) S3.

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Fig. 11 shows the peak analysis of all samples before and after etching. Before etching uncoated sample gives five visible peaks but after etching gives six peaks, for coated sample (S2) seven and four visible peaks. Here carbon and oxygen elements show the organic O-C⚌O, C—O, C—C, C—H, and C—O-H compounds. The sodium element shows a higher binding energy because it can emit the last electron which is very close from the nucleus (Chiara et al., 2018). Less oxidation after the etching affects the C constituent a lot. The elemental composition of the carbon constituent increases whereas the oxygen constituent decreases (Table 3). It seems clear, the peak fitting attributes visible peak after the etching condition for sample S3 (see Table 4).

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Fig. 11

XPS peak of (a) S1 before etching, (b) S1 after etching, (c) S2 before etching, (d) S2 after etching (e) S3 before etching, and (f) S3 after etching.

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3.5 Activation energy

From equation (2), Activation energy of S0, $$\mathrm{E}=-2.303\mathrm{R}\times \mathrm{s}\mathrm{l}\mathrm{o}\mathrm{p}\mathrm{e}=-2.303\times 8.3144\times -0.57772=11.06\mathrm{K}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}$$

Fig. 12 shows the relationship of $\log \left(-log\frac{\left(1-\alpha \right)}{T^2}\right)$ and $\frac{1000}{T}$ to determine the slope and Table 2 represents the activation energy of all samples. The slope of the linear fitting multiplied with the universal gas constant provides the activation energy of composites. The coefficient of correlation (R²) for all fitting curves ranges from 0.96356 to 0.96386, indicating high dependability. The lower activation energy of the samples indicates a shorter diffusion barrier, better water absorption and less material degradation for the composite (Sathishkumar et al., 2021; Carra and Carvelli, 2015). The results of the activation energy completely satisfy the statistical analysis of the samples. In this works, sample S3 represents a higher mass loss i.e., higher degradation of materials with higher activation energy.

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Fig. 12

Slope calculation of (a) S1, (b) S2, and (c) S3.

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3.6 Comparison of present and previous studies

The comparison showed that the glass transition temperature of the coated samples shifted maximum from 559.13 °C to 580.51 °C, whereas the previous study showed a glass transition temperature of 560.27 °C. In the case of the end set temperature, the coated samples increased by 30.67 °C compared to the uncoated sample, whereas the previous study showed an end set temperature of 590 °C. The derivative Tg shifted from 558.89 °C to 612.08 °C, while the previous study showed the derivative Tg at 599.5 °C. The range of binding energy increased from 450 eV to 970 eV compared with the previous study as shown in Table 5.