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Abstract

Generally, carbon and glass fibers based composites are used in high-tech end products, but these are not preferred in indoor, outdoor and hygienic furniture applications due to microorganisms attack and prone to fire. In the first section of the research, different percentages (5%, 10%, and 15%) of zirconium phosphate (ZrP) particles were added in epoxy resin and corresponding glass/epoxy composites were fabricated to enhance their flame retardancy (FR) and mechanical properties (tensile, flexural, Charpy impact, and short beam shear). In the second section, different percentages (0.5%, 1%, and 1.5%) of zinc oxide (ZnO) particles were mixed in the epoxy resin and corresponding glass/epoxy composites were fabricated to optimize their antibacterial activity and mechanical performance. 15% concentration of ZrP particles exhibited the maximum flame retardancy and mechanical performance in composites, and 1.5% concentration of ZnO particles exhibited the highest antibacterial activity along with improved mechanical performance. In the third section, two (02) pure glass and carbon, and two (02) glass-carbon/carbon-glass hybrid composites were made with optimized concentrations of both ZrP and ZnO particles. Carbon/epoxy (H2) composite showed the highest mechanical properties in comparison with glass and hybrid composites due to the presence of four layers of carbon reinforcement. These functional hybrid composite-based furniture products can be used in indoor, hygienic (hospitals, schools, and offices), and outdoor furniture applications.

Keywords

Zirconium phosphate zinc oxide flame retardancy antibacterial activity hybrid reinforcement hybrid composite furniture

Introduction

A composite has two major constituents, that is, matrix and reinforcement, which have combined properties over the individual one. Resin and fiber are mixed physically on macroscopical scale to manufacture a composite and different fillers are also added to get certain functionalities.^1–3 Composite materials are replacing heavy metals because of their high strength to weight ratio and low density. These composites are used in low-end to high-tech applications like furniture, doors, wall paneling, automobiles, electronics, construction, aerospace, submarines, etc.^4,5

Matrix shows the surface quality of composites and helps in load transfer. Matrices are available in the form of polymers, ceramics, and metals. Polymer matrices are most commonly used in low-end to high-end applications.⁶ Thermoset and thermoplastic matrices are two types of polymer matrices. Thermoset polymers are those polymers which are not able to remelt and reshape after curing. Epoxy, vinyl ester, phenolic and polyester resins are the types of thermoset polymer matrices.⁷ Thermoplastic polymers are able to remelt and reshape on heating. Polyethylene, polypropylene, and PVB are the examples of the thermoplastic polymer matrices.^8,9 While reinforcement consisting of different synthetic (carbon, glass, aramids, etc.) or natural (jute, hemp, flax, etc.) fibers is the stiff part of the composite which is incorporated into the matrix and provides strength to the composite.¹⁰

Composites having two or more than two different fibers as reinforcements or different types of matrices are known as hybrid composites. Hybridization can also be achieved by adding synthetic and natural fibers or resins in a single composite to get the required characteristics.^11,12 Also, composites can be made flame retardant and antibacterial and other functionalities can be added in the composites by mixing different filler content or fibers or resins having inherent desired properties.^13–15

Gopinath et al., examined the mechanical performance of composite made from jute fibers with polyester and epoxy resins. It was revealed that jute/epoxy exhibited superior mechanical properties than jute/polyester composites. Hence, the jute/epoxy composite was a better selection for automotive applications than jute/polyester composites.¹⁶ Abbas et al., investigated the effect of glass microspheres on the hemp/green epoxy composites and revealed that Charpy impact, flexural and tensile properties were enhanced by adding glass microspheres in the composites.¹⁷ Also, addition of lignin in hemp/epoxy composite lead to the increase in their mechanical performance.¹⁸

Wang et al., investigated the impact of coupling agent (maleic anhydride-grafted polyethylene “MAPE”) with hemp fiber. The yield stress and compressive modulus was increased with the presence of MAPE and by increasing the hemp fiber content.¹⁹ Silver and zinc nanoparticles improved the antimicrobial activity of their corresponding PLA and chitosan-based composites along with other properties.^20,21,22 Furthermore, nanoparticles of silver, magnesium hydroxide, zinc oxide, zinc borate, and layered hydroxides are majorly used as flame retardant materials.²³

Cone calorimeter is used as the most common instrument for research and measurement of the fire retardancy of polymeric materials. It measures the values based on the external heat flux, the peak of heat release rate, thermal feedback, and dimensional distances from the cone heater. UL-94 vertical burn test (v) ratings are used for the cone calorimeter analysis of the thermoplastics.²⁴ Disk diffusion, agar dilution, and broth dilution are well commonly used methods for combating bacteria. Flow cytoflurometric and bioluminescent methods are new methods used in this field of research.²⁵

In the previous works, different particles were added in the composites to enhance the mechanical performance, antibacterial activity, and flame retardancy of polymeric composites. To the best of author’s knowledge, no study was found on the simultaneous use of zirconium phosphate (ZrP) and zinc oxide (ZnO) particles to optimize the flame retardancy, antibacterial activity, and mechanical properties of glass-carbon hybrid composites. In the first section, different concentrations of ZrP particles were used in the epoxy resin to get optimized mechanical properties and flame retardancy of composites. While in the second section, different concentrations of ZnO particles were added in the epoxy resin to get optimized antibacterial activity and mechanical performance of their corresponding glass/epoxy composites. In the last third part, glass, carbon, and two (02) hybrid (glass-carbon) epoxy composites were prepared with enhanced antibacterial activity, flame retardancy, and mechanical properties.

Experimentation

Materials

Two types of woven reinforcements, that is, glass (1/1 plain weave having 514 g/m² areal density) and carbon (2/1 twill weave having 520 g/m² areal density) were used in this study. Glass was got from the Qingdao Feelongda Ltd., China, while carbon reinforcement was imported from Formosa Plastics, Taiwan. EPIKOTE™ 816 (HEXION, USA) epoxy resin and EPOTEC TH 7301 (Aditya, Thailand) hardener were utilized for composite manufacturing. Zirconium phosphate (ZrP) and zinc oxide (ZnO) particles imported from Sunshine, China, were utilized as fillers to enhance the flame retardancy (FR) and antibacterial activity, respectively. The particle sizes of the ZrO and ZrP were 343.2 nm and 812.3 nm, respectively.¹⁴

Composite fabrication

Design of experiment 1 (DOE 1)

In the 1st section of this study, flame retardancy (FR) and different mechanical properties were tested of the fabricated composites to optimize the ZrP percentage. Different percentages (5%, 10%, and 15%) of ZrP particles were mixed in the epoxy on the weight of resin and were applied on woven glass plies. Four layered symmetrical glass/epoxy composites were fabricated by hot compression molding technique. 13-ton pressure and 120°C temperature were set on a compression machine for 40 min to get void-free composites. Reference composite sample without the addition of ZrP was also formed for comparison purpose. The fiber volume fraction of composites was calculated using equation (1) and maintained at 46% ± 0.7¹⁴

V_{f} = \frac{\frac{m_{f}}{d_{f}}}{\frac{m_{f}}{d_{f}} + \frac{m_{r}}{d_{r}} + \frac{m_{p}}{d_{p}}}

(1)

V_f indicates the fiber volume fraction, and mass of particles, resin, and fibers are indicated by m_p, m_r, m_f, respectively, while d_p, d_r, d_f, represents the densities of particle, resin, and fiber, respectively. Experimental design for optimization of ZrP particles is given in Table 1.

Table 1.

Experimental design for the optimization of ZrP particles.

	Coding	Zirconium phosphate (%age) in epoxy resin
1	ZrP0/R	0
2	ZrP1	5
3	ZrP2	10
4	ZrP3	15

Design of experiment 2 (DOE 2)

In the 2nd section, different mechanical testing and antibacterial activity were performed of the fabricated composites to optimize the ZnO percentage. Different percentages (0.5%, 1%, 1.5%) of ZnO particles were mixed in the epoxy on the weight of resin and were applied on woven glass plies. Composite manufacturing method, parameters (temperature, time, and pressure), stacking sequence, number of plies, and fiber volume (V_f%) of composites were maintained the same as in DOE1. Reference sample was also fabricated without ZnO particles for comparison purpose. Experimental design for optimization of ZnO particles is provided in Table 2.

Table 2.

Experimental design for the optimization of ZnO particles.

	Coding	Zinc oxide (%age) in epoxy resin
1	ZnO0/R	0
2	ZnO1	0.50
3	ZnO2	1
4	ZnO3	1.5

Design of experiment (DOE 3)

In the 3rd section of this study, optimized percentage of ZrP and ZnO particles achieved from DOE 1 and DOE 2, respectively, were mixed in the epoxy resin and four-layered glass, carbon, and two (02) carbon-glass hybrid epoxy-based composites were fabricated. Composite manufacturing method, parameters (temperature, time, and pressure), number of plies, and V_f% of composites were maintained in the same way as in DOE1. Stacking sequence of carbon and glass plies in hybrid composites is given in Table 3. In this part, hybrid composites were fabricated with improved flame retardancy and mechanical properties antibacterial activity.

Table 3.

Experimental design for the hybrid composite.

Coding	Optimized percentages of ZnO and ZrP	Stacking sequence
H1	Optimized	{GGGG}
H2	Optimized	{CCCC}
H3	Optimized	{GCCG}
H4	Optimized	{CGGC}

G: Glass, C: Carbon

Testing

Different mechanical tests, that is, tensile, flexural (3-point), pendulum (Charpy) impact, and short beam shear (SBS) were performed to investigate the mechanical performance of composites in three (03) different experimental designs. Horizontal and vertical flame retardancy tests were performed in DOE 1 and DOE 3. Qualitative antibacterial test was utilized to study the antibacterial activity in DOE 2 and DOE 3.

Tensile test (ASTM D3039) having dimensions of 25 mm wide and 250 mm long, flexural test (3-point, ASTM D7264) with sample dimensions of 13 mm wide and 120 mm long, and short beam shear test (ASTM D2344) with 5 mm wide and 20 mm long sample size were performed on the UTM (Z100 All-round, Zwick) having capacity of 100 KN.^26–28 Pendulum impact test (ISO 179) was performed using Charpy impact tester (Zwick/Roell, HIT 50P) having 10 mm wide and 100 mm long sample dimensions.²⁹ Each test was performed three times and average of results and curves were reported.

EN ISO 15025 standard test method was used for the horizontal³⁰ and ASTM D6413 was used for vertical³¹ flame retardancy tests. AATCC 147 test method was utilized to determine antibacterial activity against Staphylococcus aureus (S. aureus) bacterial strain.³²

Results and Discussion

Design of experiment 1

In the 1st section, flame retardancy and different mechanical tests (tensile, flexural, Charpy impact, and SBS) of developed glass/epoxy composites loaded with ZrP particles were performed and compared.

Horizontal and vertical flame retardancy (FR)

The horizontal and vertical FR testing outcomes are presented in Table 4. From the results, it is evident that the thermal stability of composites was increased with the increase of ZrP percentage.³³ Because FR is due to the presence of ZrP in the composites, and its increasing percentage enhanced this impact. Flame and afterglow durations were reduced as the concentration of ZrP was raised from zero to 15% for both horizontal and vertical flame retardancy. Char development is one of the characteristics of ZrP particles as on thermal decomposition. Phosphate acts as a flame retardant material by producing water, and carbon dioxide which are known to reduce fire propagation. Therefore, it forms inflammable gases acting as an insulating layer which restricts the free access of oxygen and thus, makes materials fire retardant.³⁴ The lowest values of flame and afterglow durations were showed by the ZrP3 sample with 15% loading of ZrP particles while the highest values were showed by the ZrP0 without ZrP particles.¹⁴

Table 4.

Flame retardancy testing results for ZrP-loaded composites.

Horizontal FR
#	Description	ZrP0/R	ZrP1	ZrP2	ZrP3
1	Flame duration(s)	3	3	2	1
2	Afterglow duration(s)	3	3	2	1
Vertical FR
1	Flame duration(s)	170	155	135	85
2	Afterglow duration(s)	40	36	28	22

FM: flame retardancy

Tensile testing

The effect of ZrP particles on tensile stress in the glass/epoxy composites is shown in Figure 1, which showed the tensile stress versus extension curves. The tensile stress of composite samples was increased with the increase of ZrP percentage in this samples. It revealed that the composite containing the highest percentage of ZrP, that is, 15% gave the highest tensile stress followed by 10% and 5% ZrP and least was observed in ZrP0 composite sample. With the addition of ZrP, chain bonding disrupts and their clusters are formed which enhances the tensile properties of the ZrP-loaded composites.³⁵ A slight increase in the stiffness of composites is also observed as the ZrP percentage is increased.

Figure 1.

Tensile stress versus extension curves of ZrP-loaded composites.

Tensile modulus, maximum force, and tensile strength of ZrP-loaded glass/epoxy composites were increased as the %age of ZrP was raised from zero to 15% in the composites as given in Table 5. A composite containing 15% ZrP demonstrated the highest value of tensile strength and modulus along with maximum force. The tensile strength of the composite was increased to 44%, from 135.22 to 244.79 MPa as the %age of ZrP was raised to 15%. ZrP particles could absorb a significant proportion of force and also improved the fiber-matrix adhesion. Covalent bonding is also enhanced by the reaction of ZrP with epoxide group.^36–38 That is why tensile properties were improved with the increase in ZrP percentage.

Table 5.

Tensile testing results of ZrP-loaded composites.

Coding	Tensile modulus (MPa)	Maximum force (N)	Tensile strength (MPa)
ZrP0 (R)	22220.50 ± 30.38	7794.33 ± 12.54	135.22 ± 2.34
ZrP1	23892.80 ± 35.45	5445.45 ± 15.56	112.08 ± 2.45
ZrP2	24011.06 ± 40.56	7996.16 ± 18.87	219.07 ± 3.45
ZrP3	26014.73 ± 52.67	8628.99 ± 20.70	244.79 ± 4.47

Flexural testing (3-point)

Figure 2 showed the flexural stress versus deformation curves of ZrP-loaded glass/epoxy composites. This can be observed that flexural stress was increased as the %age of ZrP was raised in the composites. The maximum value of flexural stress was observed in 15% ZrP-loaded composite followed by 10% and 5%. With the addition of ZrP, chain bonding disrupts and their clusters are formed which enhances the flexural properties of composites.³⁵

Figure 2.

Flexural stress versus deformation curves of ZrP-loaded composites.

Flexural strength and modulus of ZrP-loaded glass/epoxy composites are tabulated in Table 6. Flexural strength and flexural modulus were increased with the increase of ZrP percentage in the composites. 45% flexural strength and 40% flexural modulus were increased as ZrP percentage was increased from zero to 15%.

Table 6.

Flexural testing results of ZrP-loaded composites.

Coding	Flexural strength (MPa)	Flexural modulus (MPa)
ZrP0 (R)	138.16 ± 2.23	12982.24 ± 14.43
ZrP1	186.29 ± 2.54	15088.78 ± 22.76
ZrP2	221.73 ± 3.78	18726.18 ± 28.89
ZrP3	254.18 ± 4.87	21972.28 ± 35.90

Impact testing (Charpy)

Force versus displacement, force versus test time, and work versus displacement curves of ZrP-loaded glass/epoxy composites are shown in Figure 3(a)–3(c) respectively. Maximum force value was observed in 15% ZrP-loaded glass/epoxy composite followed by the 10%, 5% ZrP-loaded composites and least value of force was observed in composite without ZrP.

Similar trend of increasing maximum force was observed in force versus test time as shown in Figure 3(b) and work done during Charpy impact test is shown in Figure 3(c). Table 7 showed the impact energy values of ZrP-loaded composites of glass/epoxy composites. Impact energy values were increased with the rise of ZrP percentage in the composites. The impact energy was improved to 40% as the ZrP percentage was increased from zero to 15% in the composites.¹⁴

Table 7.

Charpy impact energy of ZrP-loaded composites.

Coding	Impact energy (KJ/m²)
ZrP0 (R)	56.24 ± 1.34
ZrP1	65.64 ± 1.75
ZrP2	73.71 ± 2.46
ZrP3	94.57 ± 3.77

Short beam shear (SBS) testing

The effect of addition of ZrP particles in the glass/epoxy composites on short beam shear stress is shown in Figure 4. SBS stress was increased with the rise of ZrP percentage. The highest SBS stress was observed with the 15% ZrP loadings followed by 10%, 5% ZrP loadings and least value of SBS stress was observed in reference sample.

Figure 3.

(a) Force versus displacement, (b) Force versus test time, and (c) work versus displacement curves of ZrP-loaded composites.

Figure 4.

Short beam shear stress versus deformation curves of ZrP-loaded composites.

Table 8 showed the maximum force and short beam shear strength values in ZrP-loaded composites. Addition of ZrP particles improved the maximum force and short beam shear strength values. The highest value of maximum force and SBS strength was observed in 15% ZrP-loaded composites followed by 10%, 5% ZrP loadings and least values were observed in reference sample. 43% maximum force and 39% SBS strength values were increased with the increase of ZrP percentage from zero to 15%.

Table 8.

Short beam shear testing results of ZrP-loaded composites.

Coding	Maximum force (N)	Short beam shear strength (MPa)
ZrP0 (R)	48.15 ± 0.32	3.82 ± 0.03
ZrP1	52.78 ± 0.63	3.96 ± 0.04
ZrP2	78.09 ± 0.57	5.76 ± 0.04
ZrP3	85.84 ± 1.11	6.32 ± 0.03

Design of experiment 2

In the 2nd section, different mechanical tests (tensile, flexural, Charpy impact, and short beam shear) and antibacterial activity of glass/epoxy composites loaded with ZnO particles were performed and compared.

Tensile testing

The effect of ZnO particles on the tensile stress of glass/epoxy composites is shown in Figure 5 which showed the tensile stress versus extension curves of composite samples. The tensile stress of composites was increased by increasing the ZnO percentage in the epoxy resin.³⁹ Figure 5 showed that the highest percentage (1.5%) of ZnO gave the highest tensile stress followed by 1% and 0.5% ZnO loadings and the least value was showed by reference sample. Because the addition of ZnO particles improves the load distribution and fiber-matrix interface in the composites, that enhanced the tensile properties. Also, curves showed that stiffness of composites was also improved with the rise of ZnO percentage.

Figure 5.

Tensile stress versus extension curves of ZnO-loaded glass/epoxy composites.

Tensile modulus, maximum force, and tensile strength of ZnO-loaded glass/epoxy composites were also increased as the ZnO percentage was increased as given in Table 9. Composites having 1.5% ZnO content showed the highest tensile modulus, maximum force, and tensile strength. The tensile modulus, maximum force, and tensile strength were increased to 43%, 17%, and 50%, respectively, when ZnO particles were increased from zero to 1.5%.

Table 9.

Tensile testing results of ZnO-loaded composites.

Coding	Tensile modulus	Maximum force	Tensile strength
	(MPa)	(N)	(MPa)
ZnO0 (R)	22220.50 ± 32.23	7794.33 ± 09.54	135.22 ± 2.11
ZnO1	32838.97 ± 45.43	5231.69 ± 11.45	147.23 ± 2.67
ZnO2	35524.86 ± 40.45	12114.15 ± 16.76	387.28 ± 3.12
ZnO3	39596.58 ± 72.65	9379.15 ± 21.12	274.24 ± 4.43

Flexural testing (3-point)

The impact of ZnO particles on the flexural stress of glass/epoxy composite is shown in Figure 6, which showed the flexural stress versus deformation curves of composites. It is evident that flexural stress was increased with the increase in ZnO percentage from zero to 1.5%. The highest value of flexural stress was observed in 1.5% ZnO concentration followed by 1% and 0.5%, and least value was showed by reference sample. Also, curves showed that stiffness of composites was also increased with the increase of ZnO percentage in composites. A similar increasing trend of flexural strength and modulus was increased with the increase of ZnO percentage⁴⁰ as given in Table 10. Flexural strength and modulus were increased to 37% and 34%, respectively, with the increase in ZnO percentage from zero to 1.5%.

Figure 6.

Flexural stress versus deformation of ZnO-loaded composites.

Table 10.

Flexural testing results of ZnO-loaded composites.

Coding	Flexural strength (MPa)	Flexural modulus (MPa)
ZnO0 (R)	138.16 ± 2.78	12982.24 ± 21.89
ZnO1	167.40 ± 2.43	15225.58 ± 27.78
ZnO2	179.78 ± 3.43	16099.46 ± 34.45
ZnO3	222.71 ± 4.23	19768.93 ± 42.23

Impact testing (Charpy)

Force versus displacement, force versus test time, and work versus displacement curves of ZnO-loaded glass/epoxy composites are shown in Figures 7(a)–(c), respectively. It is evident that maximum force values were increased with the increase of ZnO percentage. The highest value of force was observed in 1.5% ZrP-loaded composites, followed by 1% and 0.5% ZrP loadings and the least value of force was observed in reference composite sample. A similar increasing trend of force with test time was observed in Figure 7(b). Figure 7(c) curves displayed the efficacy of adding ZnO particles in the composites. Both stiffness and energy absorption were increased with the increase in ZnO percentage. The work done curves showed that resistance to crack was increased with the rise of ZnO percentage showing more steeper curves, while for reference sample curve was less steep.

Figure 7.

(a) Force versus displacement, (b) Force versus test time, and (c) Work versus displacement curves of ZnO-loaded composites.

Table 11 showed impact energy values of ZnO-loaded glass/epoxy composites. Impact energy values were increased with increase of ZnO percentage in the composites. The highest amount of impact energy was absorbed by 1.5% ZrP-loaded composites.⁴⁰ The impact energy value was increased to 37% with the increase in ZnO percentage from zero to 1.5%.

Table 11.

Charpy impact energy of ZnO-loaded composites.

Coding	Impact energy (KJ/m²)
ZnO0 (R)	56.24 ± 1.79
ZnO1	61.27 ± 1.98
ZnO2	71.04 ± 2.11
ZnO3	90.79 ± 2.34

Short beam shear (SBS) testing

The impact of ZnO particles on the SBS stress of glass/epoxy composite is shown in Figure 8, which showed the SBS stress versus deformation curves of composites. The highest SSB value of stress was observed with the 1.5% ZnO loadings followed by 1%, 0.5% ZrP loadings and least value of SBS stress was observed in reference sample. Table 12 highlighted the maximum force and SBS strength values of ZnO-loaded composites, both values were increased with the increase in ZnO percentage. The highest values were showed by the 1.5% ZnO-loaded composites and least values were showed by reference composite sample. Maximum force and SBS strength values were increased to 61% and 48%, respectively, with the rise of ZnO percentage from zero to 1.5%.

Figure 8.

Short beam shear stress versus deformation curves of ZnO-loaded composites.

Table 12.

Short beam shear testing results of ZnO-loaded composites.

Coding	Maximum force (N)	SBS strength (MPa)
ZnO0 (R)	48.15 ± 2.28	3.82 ± 0.05
ZnO1	103.97 ± 1.10	6.64 ± 0.06
ZnO2	109.37 ± 2.10	7.07±0.02
ZnO3	124.71 ± 2.23	7.45 ± 0.03

Antibacterial activity

The antibacterial activity of reference (R) and ZnO particles loaded composites was tested against the S. aureus bacterial strain as demonstrated in Table 13. We have added three different concentrations of ZnO particles (0.5%, 1%, and 1.5%) during composite manufacturing. However, irrespective of the concentration, the antibacterial activity was same (as per sample size). This confirms that the prepared composite sample had only contact killing mode for antibacterial action while no leaching of ZnO particles was observed. This could be attributed to the fact that the ZnO particles were entrapped within the resin and were unable to diffuse in the agar bed. As far as the application of the composite is concerned, the purpose of addition of ZnO particles was that it could inhibit the growth of any microbe available on the surface ensuring neat and microbe-free composites. The mode of action of ZnO in killing the microbes has already been well described in the literature.⁴¹ However, the impact of ZnO loading was evident on the other properties of the composites, for example, with the increase of ZnO percentage from 0.5% to 1.5%, tensile, flexural, Charpy impact, and SBS properties were also increased which have been discussed under the different mechanical test headings.

Table 13.

Effect of ZnO loading on antibacterial efficacy of composites.

Coding	Zone of inhibition (mm)
ZnO0 (R)	0
ZnO1	10
ZnO2	11
ZnO3	11

Optimization with hybridization (DOE 3)

In the 3rd section, optimized percentages of ZrP and ZnO achieved from DOE 1 and DOE 2, respectively, were mixed in the epoxy resin, and glass, carbon, and two (02) hybrid (glass and carbon) epoxy-based composites were prepared. Flame retardancy, mechanical tests (tensile, flexural, harpy impact, and short beam shear), and antibacterial activity were performed of these composites.

Horizontal and vertical flame retardancy (FR)

Horizontal and vertical FR results are provided in Table 14. Optimized %age of both ZrP and ZnO particles, that is, 15% and 1.5%, respectively, were utilized in these composites. H2(CCCC) sample displayed lower values of flame and afterglow durations due to the presence of carbon on the outer sides as compared to the H1(GGGG) sample in both vertical and horizontal flame retardancy tests. Because carbon fibers thermal stability is higher than the glass fibers.^33,34 Similarly, H4(CGGC) sample showed lower values of flame and afterglow durations in comparison with H3(GCCG) sample due to the presence of carbon layers on the outer sides. So, thermal stability of H4 composite sample is higher than H3 sample.^14,42

Table 14.

Flame retardancy testing results for ZrP and ZnO-loaded composites.

Horizontal FR
#	Description	H1	H2	H3	H4
1	Flame duration(s)	3	1	2	1
2	Afterglow duration(s)	3	1	2	1
Vertical FR
1	Flame duration(s)	90	74	86	76
2	Afterglow duration(s)	40	28	38	26

FR: flame retardancy

Tensile testing

Figure 9 showed the tensile stress versus extension %age curves with optimized percentages of ZrP (15%) and ZnO (1.5%) particles of glass(H1), carbon(H2), and glass-carbon(H3) and carbon-glass(H4) hybrid composites. The trend of increase in the tensile stress is mainly due to the carbon and glass reinforcements. Furthermore, ZnO particles improve the interfacial bonding between matrix and reinforcements, and ZrP particles enhance the matrix strength by forming a microcluster in structure. The combined result of both particles helped in improving the tensile properties of the hybrid composites. The hybrid composite H2 showed the highest tensile stress because it has four layers of carbon reinforcements. Carbon fiber is a strong manmade fiber due to its higher mechanical strength. Carbon fiber helped in bearing the initial load and enhanced tensile properties of H2 hybrid composite.⁴³ H1 composite showed the 2nd highest peak of tensile stress due to presence of four layers of glass reinforcement. While H3 and H4 containing two, two plies of carbon and glass reinforcements showed the least values of tensile stress. Within H3 and H4 composite, H3 showed better tensile stress values as compared to the H4 composite sample. With the increase of carbon layers, the stiffness of the composite was also increased as shown in Figure 9. H2 curve indicated the highest steep behavior as compared with the other composite curves.

Figure 9.

Tensile stress versus extension (%) curves of hybrid composites.

Tensile modulus, maximum force, and tensile strength of hybrid composites are given in Table 15. A similar increasing and decreasing trend of tensile modulus, maximum force, and tensile strength was observed. Results showed that H2 composite exhibited the highest value of tensile modulus followed by H4, H3, and H1 composites, respectively. Also, H2 sample presented the highest values of maximum force and tensile strength followed by H1, H3, and H4 samples, respectively. Because H2 sample consists of only carbon reinforcements while H1 consists of only glass reinforcements.¹⁴

Table 15.

Tensile testing results of hybrid composites.

Coding	Tensile Modulus (MPa)	Maximum force (N)	Tensile strength (MPa)
H1	37086.23 ± 97.22	9870.17 ± 33.43	293.22±5.56
H2	99582.32 ± 160.51	10258.94 ± 43.09	326.77 ± 4.56
H3	48814.51 ± 120.76	9370.02 ± 51.34	237.60 ± 3.43
H4	67477.59 ± 150.34	8886.17 ± 45.54	202.99 ± 4.34

Flexural testing (3-point)

Figure 10 showed the flexural stress versus deformation %age curves having optimized percentages of ZrP (15%) and ZnO (1.5%) particles of glass(H1), carbon(H2), and glass-carbon(H3) and carbon-glass(H4) hybrid composites. The trend of increase in the flexural stress mainly depends on the carbon and glass reinforcements. The ZnO and ZrP particles improved the interfacial bonding between matrix and fibers, and matrix strength is enhanced by forming a microcluster. The hybrid composite H2 showed the highest value of flexural stress followed by H4, H3, and H1, respectively. H2 hybrid composite gave higher flexural stress because of carbon layers. The carbon is a strong manmade fiber due to its higher mechanical strength it helped in bearing the initial load and maximum force on deformation and enhanced flexural properties.

Figure 10.

Flexural stress versus deformation curves of hybrid composites.

The flexural modulus and flexural strength values of H1, H2, H3, and H4 hybrid composites are given in Table 16. H2 hybrid composite gave the highest flexural strength and flexural modulus followed by H4, H3, and H1, respectively.

Table 16.

Flexural testing results of hybrid composites.

Coding	Flexural strength (MPa)	Flexural modulus (MPa)
H1	105.34 ± 3.45	14119.14 ± 90.23
H2	483.89 ± 7.76	56485.50 ± 165.89
H3	195.56 ± 5.67	17215.28 ± 120.54
H4	251.34 ± 4.43	40784.79 ± 150.65

Impact testing (Charpy)

Force versus displacement, force versus test time, and work versus displacement curves of H1, H2, H3, and H4 composites are presented in Figures 11(a)–(c), respectively. Since percentages of ZnO and ZrP particles were same in all hybrid composites, so the maximum force and work done values depend upon the type of reinforcement in the composites. Figure 11(a) showed that hybrid composite H3 exhibited the uppermost value of maximum force followed by H2, H4, and H1, respectively. Carbon fiber has higher stiffness as compared to the glass fiber. A similar increasing or decreasing of maximum force versus time was observed in Figure 11(b). Figure 11(c) indicated the work done versus displacement behavior of hybrid composites during Charpy impact. The curves clearly showed the efficacy of glass and carbon reinforcements. Work done behavior was altered with the change of glass, carbon, and hybrid stacking of both reinforcements. H3 presented the greatest value of work done followed by H1, H4, and H2 samples, respectively.

Figure 11.

(a) Force versus displacement, (b) force versus test time, and (c) work versus displacement curves of hybrid composites.

The impact energy values of H1, H2, H3, and H4 hybrid composites are given in Table 17. The H3 hybrid composite exhibited the highest amount of impact energy followed by H1, H4, and H2 hybrid composites, respectively. H3 composite had hybrid stacking of glass and carbon reinforcements while H2 had four layers of carbon reinforcements. Figure 12 showed the fracture analysis of composite samples after the Charpy impact test. H3 samples showed less damage of carbon layers placed inside the glass layers, while outer glass layers were fractured showing higher impact energy. Both H2 and H4 samples containing carbon on outer side showed higher damage and less impact energy.

Table 17.

Charpy impact energy of hybrid composites.

Coding	Impact energy (KJ/m²)
H1	55.56 ± 0.75
H2	40.98 ± 0.97
H3	60.24 ± 0.85
H4	44.41 ± 0,78

Figure 12.

Microscopic analysis of composites after Charpy impact test.

Short beam shear (SBS) testing

SBS stress of hybrid composite is shown in Figure 13. H2 composite showed the uppermost value of SBS stress followed by H4, H3, and H1, respectively. The rise in the SBS stress mainly depends upon the types of reinforcement and their stacking sequence. H2 composite had four layers of carbon reinforcements. Carbon is a strong manmade fiber due to its higher stiffness. It bears the initial load and maximum force on deformation and enhanced SBS properties of H2 hybrid composite. H4(CGGC) and H3(GCCG) hybrid samples displayed intermediate values of SBS stress due to the presence of two layers of carbon and two layers of glass reinforcements in different stacking pattern. H2(CCCC) composite demonstrated the greatest value of SBS stress and the least value was exhibited by H1(GGGG). Table 18 showed the maximum force and SBS strength values of composites. Similar increasing trend of maximum force and SBS strength was also observed. H2 sample revealed the greatest values of maximum force and SBS strength followed by H4, H3, and H1 samples, respectively.

Figure 13.

Short beam shear stress versus deformation curves of hybrid composites.

Table 18.

SBS testing results of hybrid composites.

Coding	Maximum force (N)	Short beam shear strength (MPa)
H1	80.67 ± 2.27	5.47 ± 0.32
H2	317.14 ± 7.76	17.34 ± 1.12
H3	153.46 ± 4.43	9.85 ± 0.78
H4	226.71 ± 6.16	13.41 ± 1.10

Antibacterial activity

As described earlier, the effect on ZnO percentages was not significant on the antibacterial activity due to the non-leaching nature of the particles (entrapped within polymeric matrix). However, the addition of 1.5% ZnO loading showed promising increase in the mechanical properties of the prepared composites. Therefore, the sample codes H1, H2, H3, and H4 were prepared by adding 1.5% ZnO particles for having better mechanical performance. It is already evident from Table 19, all samples showed the exact zone of inhibition as per its sample size, that is, H4 sample showed 12 mm zone as per its sample size while sample H2 showed 10 mm zone as per its cutting size.¹⁴

Table 19.

Zone of inhibition of different hybrid composites.

Coding	Optimized percentages of ZnO (1.5%) and ZrP (15%)	Stacking sequence	Zone of inhibition (mm)
H1	04 plies of glass	{GGGG}	10
H2	04 plies of carbon	{CCCC}	10
H3	02 plies of glass (outer) and 02 plies of carbon (inner)	{GCCG}	12
H4	02 plies of carbon (outer) and 02 plies of glass (inner)	{CGGC}	12

Conclusions

In the 1st section, different percentages (5%, 10%, and 15%) of ZrP particles were mixed in epoxy resin and their corresponding woven glass/epoxy composites were fabricated. The composite containing 15% ZrP particles showed the highest horizontal and vertical flame retardancy, that is, lowest values of flame and afterglow durations, and mechanical (tensile, Charpy impact, flexural, and short beam shear) properties followed by 10% and 5% ZrP particle composites, respectively. The least values of flame retardancy and mechanical properties were observed in reference composite without ZrP particles.

In the 2nd section, different percentages (0.5%, 1%, and 1.5%) of ZnO particles were mixed in epoxy resin and woven glass/epoxy composites were made. The composite containing 1.5% ZnO particles showed the highest mechanical (tensile, Charpy impact, flexural, and short beam shear) properties followed by 1%, 0.5% ZnO particle-loaded composites, respectively. Antibacterial activity, that is, clear zone of inhibition, was same with three different percentages (0.5%, 1%, and 1.5%) of ZnO particles. The least values of mechanical properties were observed in the reference composite without ZnO particles with poor antibacterial activity. 1.5% ZnO particles concentration was selected from this section having the highest mechanical properties and better antibacterial activity.

In the 3rd section, glass(H1), carbon(H2), glass-carbon(H3), and carbon-glass(H4) hybrid composites were fabricated with epoxy resin containing optimized percentages of ZrP (15%) and ZnO (1.5%) particles. H2 composite showed the better mechanical properties as compared to the H4, H3, and H1 hybrid composite samples, due to presence of four layers of high modulus carbon reinforcement. All composite samples showed better antibacterial activity and samples containing four layers of carbon or having carbon on outer sides of composite showed better flame retardant properties.

Footnotes

Acknowledgements

The authores are thankful to the lab staff of National Center for Composite Materials at National Textile University, Faisalabad-Pakistan

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) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors are thankful to the Higher Education Commission of Pakistan for funding this research work under Technology Development Fund (TDF03-112).

ORCID iDs

Yasir Nawab

Khubab Shaker

Muhammad Umair

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