1 Introduction

Sandwich composite materials consist of layered structure configurations with three or more layered materials. This structure consists mainly of the core material and the fiber- reinforced polymer (FRP) face-sheets that have good potential for structural purposes such as roofing panels, house partitioning, decking, and cladding. These materials have useful applications in other industries such as aircraft, marine, automotive, and aerospace industries where weight is a major issue or requirement [1,2,3,4].

Different fibers (both natural and synthetic) have been employed as the reinforced face-sheet in sandwich composite. Natural fibers are product of plant remains such as kenaf, jute, banana, flax, hemp, etc., and are regarded as renewable and biodegradable materials which have several economic, environmental, and ecological advantages over synthetic fibers [1, 5,6,7]. Synthetic fibers such as glass and carbon fibers are non-renewable and emit greenhouse gases effect which causes global warming. Different core materials such as nomax honeycomb, kraft honeycomb, polyurethane (PU), geopolymer foam, polyvinyl chloride (PVC), [8, 9], and hourglass lattice core [10], have been hybridized with glass face-sheet to produce sandwich composite material. The study of Jie Shi et al. [10], shows that the structural parameters of hourglass lattice core had different effect on the equivalent stiffness and natural frequencies of the sandwich composite. This core materials have few limitations such as low strength, poor stiffness, and poor load bearing capacity.

In the recent time, syntactic foam composites (SFC) are employed as core material in sandwich composites because of their wide range of acceptability, and good properties. SFC are made from the combination of hollow glass microsphere (HGM) and polymer matrix (epoxy resin) [11]. SFC are solid multiphase material which has a wide range of acceptability in engineering materials due to lightweight, buoyant behaviour, low moisture absorption, valuable mechanical properties, and improved structural characteristics [12]. SFC possess better tensile, flexural, compressive strength [13,14,15,16], thermal stability [17, 18], and electrical properties [18, 19] than the PVC and PU foams cores [20, 21].

Several researchers have studied the mechanical behaviour of sandwich panels under different loading conditions. Chen et al. [22], studied the mechanical behaviour of glass fiber reinforced polymer (GFRP) skin sandwich panel under crushing loading. It was stated that the crushing behaviour, failure modes, and energy absorption of GFRP correlates to the face-sheets layers, fiber volume fractions in both longitudinal and transverse directions. The results further revealed that the energy absorption of sandwich panels can be effectively enhanced by increasing the number of fiber reinforced polymer layers of the lattice web. Furthermore, Parente et al. [5], improved the mechanical advantages of sandwich structure by reinforcing them with nanoclays which has high ion exchange capacity, high aspect ratio and economic advantages. Also, Liu et al. [9] developed a high impact sandwich panel to improve the vulnerability of to low-velocity impact which usually lead to significant degradation of structural integrity.

Orbulov et al. [23, 24], reported the hardness properties of metal matrix syntactic foams (MMSFs) using three major hardness testing methods: global (or Brinell-like) hardness, depth sensing hardness, and dynamic hardness. It was investigated that the hardness properties of the matrices change with the various methods employed, there was no significant difference in the global hardness of the foams. The results further showed that the dynamic hardness properties of the MMSFs were higher than that of the matrices. Furthermore, Manakari et al. [25], stated that the progressive addition of glass microballoons resulted in a steady rise in the hardness properties of pure magnesium matrix.

Hobaica and Cook [26] studied the nature of syntactic foam composites used for buoyancy. They stated that syntactic foams have demonstrated their ability as a supplementary material for buoyancy in present day submarines and can also be applicable for practical means of designing deeper diving submersibles.

According to Cosse et al. [27], the acoustic absorption behaviour coefficient of the foam composites carried out in single screw extruder performed better than the neat matrix within the range of 800 Hz to 1600 Hz. Most of these studies mainly focused on the mechanical properties of the sandwich composites [28,29,30].

The use of syntactic foam core sandwich composite has been previously employed by researchers in marine, structural construction and building applications due to their promising mechanical properties such as compressive, tensile, and flexural behaviour. Also, among many lightweight materials used in marine applications, sandwich structures with syntactic foam core are promising because of lower water uptake in foam. In addition, using syntactic foam as core material enhances weight-saving in marine applications and improves the sandwich structure’s performance [30,31,32,33,34].

Meanwhile, other properties such as water absorption capacity, buoyancy, acoustic etc., which contributed to the sandwich composites applications in structural and marine purposes have not been widely studied. Therefore, in this study, the water absorption capacity, hardness, and acoustic properties of hybrid sandwich syntactic foam composites will be discussed in correlation to the mechanical properties of previously published articles for the purpose of marine and structural applications.

2 Materials

For the hybrid face-sheets, natural fiber (kenaf) purchased from CSIR, Port Elizabeth, and synthetic fiber (glass) procured from AMT Composite, Durban. T60 HGM with density of 0.6 g/cm3, melting point of 625 ℃ and floating point of 94 ℃ was used for the core SFC fabrication [35]. The detailed address of supplier and properties of the materials used has been stated in the previous published works by the authors as found in [11, 30]. The matrix used is epoxy resin LR 20 and hardener LR 281 in the ratio 10 to 3 as required by the supplier.

2.1 Method and fabrication

The method of fabrication has been previously reported by the authors [30] as suspension of epoxy/HGM for the fabrication of the syntactic foam core used with thickness of 20 mm. The ratio of the HGM in the resin was maintained as 5vol–25vol% and designated as EPT60-X. EP and T60 are the resin and HGM names respectively, while X is the ratio of filler in the composition. Hybrid face-sheets were also suspended before adding the core for the sandwich composites fabrication. The core and the face-sheets were bonded by hand lay-up before vacuum bagging for proper bonding and to remove excess matrix. The dimension of the slab and the details of plies used as been previously discussed by the author [30].

2.2 Scanning electron microscopy (SEM) analysis

The microstructural analysis of the specimen for surface morphology was conducted using scanning electron microscopy (SEM) with the aid of a Zeiss EVO 1 HD 15 Oxford instrument X-max SEM instrument.

2.3 Hardness test

The hardness test of the syntactic foam composites (SFC) and sandwich syntactic foam composites (SSFCs) was conducted using Barber Colman GY2J 934.1 Barcol impresser hardness tester according to ASTM D 2583 test specification. Twenty (20) readings of indentation were taken per sample and the average reading taken for consideration.

2.4 Water absorption properties

The water absorption procedure was performed to determine the extent to which the composite materials can withstand moisture conditions. Five samples per specimen were prepared according to ASTM D570-98. Six separate periods were used as time frame to determine the wettability of the composite materials and the wet values were compared to the dried sample values using the formula in Eq. (1).

$$ \% water \,absorbed \,\left( P \right) = \left( {{\raise0.7ex\hbox{${\left( {wet \,weigth\,\left( {W_{f} } \right) - dry \,weigth \,\left( {W_{i} } \right)} \right)}$} \!\mathord{\left/ {\vphantom {{\left( {wet \,weigth\,\left( {W_{f} } \right) - dry \,weigth \,\left( {W_{i} } \right)} \right)} {\left( {dry \,weight \left( {W_{i} } \right)} \right)}}}\right.\kern-0pt} \!\lower0.7ex\hbox{${\left( {dry \,weight \,\left( {W_{i} } \right)} \right)}$}}} \right) \times 100 $$
(1)

The saturation of the absorbed water weight was gained after 720 h, and the maximum weight gained taken as M(s). The initial readings after 24 h were taken as M(d), and the diffusion coefficient M(d) at maximum moisture absorption of the composite materials was expressed with Eq. (2).

$${M}_{\left(d\right) }= {M}_{(s)}-{M}_{\left(t\right)}$$
(2)

2.5 Buoyancy properties

Buoyancy is a measure of an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. The neat epoxy, SFCs, and the sandwich syntactic foam composites (SSFCs) samples were submerged in a test-tube containing water of 400 ml. The submerged SFCs and SSFCs were of the same weight for proper comparison. Three specimen each was submerged in the test-tube filled with water shown in Fig. 1. The force of buoyancy arises because of variations of density in the fluid subject to gravity. The force of buoyancy is given by the Eq. 3.

$${F}_{b} = {V}_{o} \times D \times {F}_{g}$$
(3)

where Fb (N) is the buoyancy force acting on the object, Vo (m3) is the volume of the submerged object, D (Kg/m3) is the density of the fluid the object is submerged in, and Fg is the force of gravity (N/Kg).

Fig. 1
figure 1

Buoyancy measurement procedure set-up for the composite materials with (a) measurement of water before submerging the specimen, b specimen submerged in the water, and (c) measurement of submerged specimen + water

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2.6 Acoustic properties

Acoustic measurement was done to measure the sound absorption capacity of sandwich composite, the extent to which a composite material can emit or retain external noise. The measurement was conducted in accordance to ASTM E1050-12 [27, 36] standard, using an impedance tube test of 100 mm diameter. The measuring range was taken as 30dBA to 130dBA using frequency range of 50 – 1600 Hertz. Five readings of sound level “P” (dB) were taken per sample and are used to determine the sound pressure “Lp” (dB) using Eq. (4).

$${L}_{p }= {20 log}_{10} P +94 (dB)$$
(4)

3 Results and discussion

3.1 Fracture morphology of sandwich composites

Figure 2 shows the fractured images of the sandwich syntactic foam composites for the skin and core destructive test with (a) showing the fractured kenaf fibers and matrix porosity in the arrowed region, (b) shows the clustered glass fibers due to compression loading and debris, (c) shows both the fractured kenaf and glass fibers in the composite, and (d) shows the clustered kenaf fibers and fractured glass fibers. This was responsible for the trends observed in the mechanical properties of the sandwich composites. Similar observation was reported by Sharba et at. [37] where they used hybrid kenaf and glass fibers to improve the mechanical properties of the sandwich composite.

Fig. 2
figure 2

SEM images of the fractured surfaces for (a) kenaf-syntactic foam core-kenaf (KK), b glass- syntactic foam core -glass (GG), c glass/kenaf- syntactic foam core -kenaf/glass (GK), and (d) kenaf/glass- syntactic foam core -glass/kenaf (KG) orientations, respectively

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3.2 Hardness properties of syntactic foam composite and sandwich syntactic foam composite

3.2.1 Hardness properties of syntactic foam composites

Figure 3 presents the average hardness result of the SFCs for the five-volume compositions of HGM (EPT60-5, EPT60-10, EPT60-15, EPT60-20, and EPT60-25) compared to the neat epoxy (EPT60-0). The SFCs increased in hardness value compared to the neat epoxy by 15.1%, 10%, 18.2%, 11.1%, and 7.0% for EPT60-5, EPT60-10, EPT60-15, EPT60-20, and EPT60-25 respectively. This is due to the fact that the addition of HGM into the resin increases the stiffness of the composites. Similar observation was earlier reported by Orbulov and Németh [23], where the hardness properties of metal matrix syntactic foam (MMSF) for the Al99.5 matrix increased compared to the neat matrix material. The highest average hardness value is 27.5Hv at EPT60-15, this could imply that the SFCs at EPT60-15 were more sensitive to deformation than other composites. The increased hardness values imply that the SFCs can better withstand hardness conditions than neat epoxy when used as reinforcement in composite materials. The increased hardness properties of the SFCs can be related to their improved mechanical properties than the neat epoxy as previously stated by the author [11]. Also, Anbuchezhiyan, Mohan [38] reported that syntactic foam synthesized by the reinforcement of hollow glass microspheres and magnesium alloy matrix increased in hardness properties with a mass fraction of the HGM in the matrix alloy due to the occurrence of the reinforcement. Moreover, Manakari, Parande [25], reported a steady increase in the hardness value of SFC with the addition of GMB particles up to 107 Hv at 25vol% which is 127.7% more than that of the pure Mg (47 Hv). The hardness readings of EPT60-15 are worth noticing as they have better surface toughness compared to other compositions of SFC. This contributed to their load bearing along with making the step of elastic distortion long and exceeding a critical load yielding in the composites. The crucial limit of the clustered size of HGM at higher volume fraction (20vol% and 25vol%) resulted in the decrease of hardness value after attaining highest value at 15vol% of the SFC leading to decrease function of the reinforcement [39]. Similar case was studied by Kumar [40], where he reported that the hardness value of SFC increased with an increase in volume fraction of glass microballoons up to an optimum limit of 10vol% and then decreased.

Fig. 3
figure 3

Hardness properties of syntactic foam composites and neat epoxy

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3.2.2 Hardness properties of sandwich syntactic foam composites

Figure 4 presents the Barcol hardness readings of the sandwich syntactic foam composite. Fifteen indentations were done on each sample and the readings taken. The GK sandwich structure shows the highest hardness value of 49.800 Hv, which is 41% increase, while KK has the lowest hardness value of 29.267 Hv. This is because the addition of glass and kenaf fibers can better improve the hardness properties of the sandwich material. The order of hardness strength is GK > GG > KG > KK. The highest hardness strength by GK shows that hybrid glass/kenaf fibers can possibly enhance good hardness resistance than plain kenaf or glass fiber when used as reinforcement in sandwich composites. An increase in fiber layers and weight, contributed to the increasing hardness property which is possibly attributed to the alignment and the length of the fiber [41]. The reduced hardness strength in KK is possibly attributed to the lower mechanical properties and high porosity associated with interface and the stress concentration in the matrix.

Fig. 4
figure 4

Hardness properties of sandwich syntactic foam composites for the KK, GG, GK, and KG layering sequence

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3.3 Water absorption of SFC and sandwich syntactic foam composites

3.3.1 Percentage water absorption of SFC.

Table 1 shows the percentage of the SFC water intake as the vol% of the HGM increases compared to the water intake of the matrix. The absorption capacity of the SFC is defined as the role played by the absorbency, superficial, and subsurface states on the material. It was noted that when the specimen was immersed into the water basin, the SFC surface was not entrapped by the moisture before the proper diffusion process. The superficial setups was not applicable in explaining the SFC water dispersion means since it cannot explain the main computations required for the diffusivity of the composite material [42].

Table 1 Mean values for the percentage water absorption of syntactic foam composites (%)
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From Table 1, equal time setup was used for all the SFC arrangements together with the 0% composite. They were submerged together into the water basin and left throughout the stipulated time. The final time \({T}_{(s)}\) was captured after 720 h and the first wet recorded time \({M}_{(t)}\) were measured after 24 h. The absorbed water intake proportion increased as the time of immersion changes for all the SFC specimens due to their longer hours of staying in the water basin. This shows the variation in the SFC densities and void fraction [11]. The EPT60-15 has the highest proportion of water intake after 24 h which was 0.27%. Table 1 shows the raw value for the mean water absorption results for the SFC filled with HGM at 5-25vol% fraction for the period of 24 to 720 h. Table 2 shows the diffusion coefficient properties as calculated from the mean water absorption values for the neat epoxy and the SFC. The value of \({M}_{(d)}\) for the 0% composite and the ratio of \({M}_{(d)}/\surd {T}_{(s)}\) was 0.572% and 0.021%/√h respectively which was more than the SFC samples. By observation, EPT60-20 shows the highest values of \({M}_{(d)}\) and \({M}_{(d)}/\surd {T}_{(s)}\) which are 0.471% and 0.018%/√h respectively. The values of \({M}_{(d)}\) varies as the volume fraction of HGM increases in the SFC which correlates the relationship between the density and the specific properties of syntactic foam composites. The highest value of \({M}_{\left(d\right)} and {M}_{(d)}/\surd {T}_{(s)}\) observed from the neat epoxy sample was an indication that the neat epoxy resin was more porous and absorbed water than the respective SFC samples. Reduced water absorption rate of SFC shows that addition of HGM restricted water from entering the SFC due to insolubility of HGM in water [42,43,44,45].

Table 2 Diffusion coefficient properties of syntactic foam composites
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3.3.2 Percentage water absorption of sandwich syntactic foam composites

Tables 3 and 4 show the water absorption values and diffusion coefficient values for the SSFC respectively. From Table 3, the percentage water intake of the SSFC (KK, GG, GK, and KG) was compared and the mean values recorded. The same procedure observed for SFC was repeated for SSFC samples.

Table 3 Mean values for the percentage water absorption of sandwich syntactic foam composites
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Table 4 Diffusion coefficient properties of sandwich syntactic foam composites
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The percentage of water absorption at GG, GK, and KG increased with time while that of KK shows no trend of time increase, this can be related to the higher porosity at KK and the sandwich varying densities. The percentage of water absorbed by KK was the highest for all the time frames. From Table 4, KK showed the highest value of diffusion coefficient \({M}_{(d)}\) and the ratio \({M}_{(d)}/\surd {T}_{(s)}\) of 3.784% and 0.1410%/√h respectively compared to GG with the lowest water absorption and diffusion coefficient of 0.593% and 0.0221%/√h respectively. The higher values of \({M}_{\left(d\right)} and {M}_{(d)}/\surd {T}_{(s)}\) observed at KK due to higher porosity leading to higher water absorption than other sandwich composites. Furthermore, higher water absorption rate is a result of using natural fibers as face-sheets reinforcement which justified the buoyancy results. The reduced water absorption rate of GG, GK, and KG shows that SSFC water absorption rate can be controlled or minimized by the combination of kenaf/glass fibers as reinforcement which implies that they can be employed for structural and marine purposes [44,45,46].

3.4 Buoyancy properties of syntactic foam composites and sandwich syntactic foam composite

3.4.1 Buoyancy properties of syntactic foam composites

Figure 5 shows the buoyancy force (Fb) (N) of the SFC compared with the neat resin. Table 5 presents the detailed values of the results used in calculating the Fb. The Fb of the SFC increased with increasing volume fraction of HGM from EPT60-5 to EPT60-15 before it decreased further at EPT60-20 and EPT60-25. The Fb of the neat epoxy was the highest at 0.0939N with ~ 65% more than the EPT60-5. The result shows that neat epoxy will require more energy to float in water compared to the SFC composites. This may be related to water absorption results discussed in Sect. 3.3.1 where the coefficient of diffusion of neat resin is high compared with SFC due to higher water absorption after submerging in water for 720 h. This means that as the neat epoxy sank in the water, the volume of the water that spills over will was equal to the volume of the resin sample. Furthermore, the presence of HGM in the SFC enhanced the floating capacity because Fb is greater than the force of gravity (G) [47, 48].

Fig. 5
figure 5

Buoyancy force (N) of Syntactic Foam Composites and the Neat Epoxy

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Table 5 Syntactic Foam Composites Buoyancy Properties
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3.4.2 Buoyancy properties of sandwich syntactic foam composites

Figure 6 presents the buoyancy force (Fb) of the SSFC for KK, GG, GK, and KG orientations. Table 6 shows the detailed values of the results used in calculating the Fb. The Fb of the KK was the highest followed by the GG, then the KG and the GK, which are 0.3296N, 0.2747N, 0.2619N, and 0.2502N. The highest Fb at KK could mean that when it is submerged in the fluid, it can easily absorb water, therefore, it requires more Fb for an up-thrust to float. Following this observation, a relationship may therefore be established between the buoyancy and water absorption results discussed in Sect. 3.3.2 where the coefficient of diffusion of KK was more than other sandwich composites due to more water absorption after 720 h. Similarly, KK will sink faster when submerged because of the nature of the kenaf/kenaf fibers as reinforcement while the combination of KG fibers reduced the water absorption rate of the SSFC [48, 49].

Fig. 6
figure 6

Buoyancy force (N) of sandwich syntactic foam composites for the KK, GG, GK, and KG layering sequence

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Table 6 Sandwich syntactic foam composites buoyancy properties
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3.5 Acoustic properties of sandwich syntactic foam composites

The acoustic properties of the sandwich syntactic foam composites are presented in Table 7 and the mean values in Table 8. The sound pressure (Lp) was calculated from the sound level (P) measurement values. The sound level increased up to 61.58 dB at GG, about a 15% increase to the sound level at KG, 52.06 dB. The sound pressure of the sandwich composites does not show much appreciable difference in value for all the sequence. This is an indication that the four sequences, KK, GG, GK, and KG can be used to investigate sound radiation vibration of a sandwich composite for accurate precision of sound pressure value [27, 50].

Table 7 Acoustic properties of sandwich syntactic foam composites
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Table 8 Mean values for acoustic properties of sandwich syntactic foam composites
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4 Conclusion

In this study, the hardness, water absorption, buoyancy, and acoustic properties of SFC and SSFC were discussed. The hardness strength of SFC increased with respect to the decreasing neat epoxy for all the volume fraction of HGM with maximum of 18.2% at EPT60-15. It was observed that hybrid kenaf/glass fibers in GK and KG improved the strength of the sandwich composite by 41% and ~ 10% respectively compared to KK and GG sandwich composites.

The addition of HGM in the SFCs reduced the water absorption diffusion coefficient and ratio of Md/√Ts by 15.2% and 57.1% compared to the neat epoxy respectively, which is an indication that HGM can be used to improve the porosity of the neat epoxy. KK sandwich composite was more porous than other sequence with diffusion coefficient of 3.784% and Md/√Ts of 0.1410% due to hydrophilic nature of kenaf fiber was when submerged in water. However, the combination of hybrid kenaf/glass fiber as reinforcement improved the water absorption capacity of sandwich syntactic foam composite.

Also, HGM reduced the SFC buoyancy force by 64.6% compared to the neat epoxy with 0.0939N buoyancy force due to their porous nature as expressed in water absorption test and SEM. Thus, it required more force to float when submerged in water. Also, KK sandwich composite had the highest buoyancy force of 0.3296 N compared to other compositions reflecting from the porous nature observed in water absorption.

Finally, acoustic properties of the SSFC shows that the GG with sound level (P) of 61.58 dB is more than other sequence due to the nature of the glass fiber while there is no appreciable difference in the sound pressure for all the sandwich sequences due to the closed gap range of their values. This is an indication that KK, GG, GK, and KG as sandwich composite is applicable for acoustic vibration measurement for structural purposes.