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Vol. 8. Issue 5.
Pages 3843-3851 (September - October 2019)
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Vol. 8. Issue 5.
Pages 3843-3851 (September - October 2019)
Original Article
DOI: 10.1016/j.jmrt.2019.06.046
Open Access
Fabrication and characterization of two-phase syntactic foam using vacuum assisted mould filling technique
Lukmon Owolabi Afolabi, Nor Azaniza Abdul Mutalib, Zulkifli Mohamad Ariff
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School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, Pulau Pinang, Malaysia
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Table 1. Relative densities and compressive properties of SFs.

Conventional mould casting technique for cell foam production demonstrated trapped air bubbles in the cell foam which are undesirable stress concentration points in the matrix. This study devise a modified process technique of fabricating syntactic foam by eliminating interstitial porosity. Varying epoxy hollow spheres and matrix stoichiometry ratio was involved in the fabrication of syntactic foam. Analysis through characterization of the mechanical properties, macrostructural morphology, cell size distribution, wall thickness and curing conditions was determined. The influence of fabrication techniques on performance of the syntactic foams was also evaluated. From the results the hollow sphere size distribution prevented the interstitial spaces, enhanced density and create foam compactness. The matrix stoichiometry ratios impacts the foam morphology, the optimal formulation was 1:1 because it enhances cross-linking within the matrix and produced a smooth, strong surface microsphere coating with structural rigidity. The relative density, compressive modulus and compressive strength of the syntactic foam are 377.11 kg/m3, 556.39 MPa, 10.40 MPa and 334.61 kg/m3, 37.05 MPa, 4.11 MPa, for vacuum assisted mould filling and conventional mould casting method, respectively. Crack propagation test showed failure started at the center and spread through the edges of the syntactic foam because of good interfacial bonding between sphere and matrixes.

Syntactic foam
Mechanical properties
Vacuum assisted mould technique
Full Text

Nowadays, curtailing size and weight limitations without negative distortion in the requisite characteristics of a materials is a primary focus of design and manufacturing engineers in production of equipment for domestic and industrial applications [1–4]. In engineering applications where weight as a crucial factor in selecting material, structural foams can provide relatively light weight structure, though, with less strength, modulus and water absorption characteristics. Certain structural foams exhibits closed porosity owing to their low density and surface areas which contributes to their excellent mechanical properties [5,6].

The conventional techniques of introducing lightweight materials in matrix is the basic concept in development of syntactic foam (SFs) materials [7,8]. SFs which simply is a polymer composite materials predominantly comprising of hollow microspheres dispersed in a resinous matrix containing trapped air bubbles and voids space created in either closed or open cell structure [6,9,10]. SFs composites with an isotropic arrangement, offers a unique combination of lightweight with good mechanical, thermal, electromagnetic and electrical properties, depending on the specific hollow spheres-matrix types and proportions [11,12]. The matrix used in syntactic foam production are mostly classified into three groups: polymer, metal and ceramic. Polymer matrices, the thermoset polymers in particular, are the most widely utilized matrices in SFs owing to their low temperature processing ability, less solvent sensitiveness and low curing shrinkage [13]. Thermosetting syntactic foams demonstrate many advantageous characteristic compared to the thermoplastics from the processing and application point of view. The important properties to be considered before selection of binder material for syntactic foams are low viscosity, readily controlled gelation time, small exothermal effect during curing, low curing shrinkage, good adhesion, wettability and compatibility with modifiers and fillers [14,15]. Appropriate matrix selection contributes strength, stiffness and bonds untreated hollow spheres towards matrix, improves the matrix rheology and improves curing temperature, time and degree of curing, hydrolytic stability in deep water application, and cost [16–18].

The nature of the hollow spheres used in the production of the SFs plays a key influencing factor in achieving good mechanical properties, other influencing factors are the hollow microsphere (HMs) sizes distribution, shape, strength, surface defects, surface treatments, volume and wall thickness of hollow spheres [18,19]. They are available in variety of sizes and are grouped into macrospheres, microspheres and nanospheres. In consideration of the hollow spheres selection, properties such as shape, non-cohesive, strong, chemical and moisture absorption resistance, and hydrolytically stability is a priority [20,21]. Basically, hollow spheres are manufactured from glass, polymer, ceramic, carbon and even metal. And glass microspheres are usually preferred as embedded microsphere to the matrix because of the high strength, regularity of the surface, good wetting characteristic, low viscosity and high absorption rate.

Lightweight porous composite such as SFs have gained prominence in manufacturing sector mostly because of their many applications in engineering. For examples marine industries — marine structures, subsea pipping insulation, float-berg, floating berg; aerospace industries — aerospace structures, spacecraft insulation, laggings for optical and sensitive equipment’s; oil and gas industries — offshore and onshore pipelines, flow line lagging, anticorrosion and fire retardant materials for pipes; automobile industries — insulation, dash-boards, transport casing, packing cases; built industries — thermal comfort materials and insulation materials [22–24]. Of special interest to many researcher is the ability of the SFs properties to be skewed towards desired specification for different applications. The mechanical properties of lightweight SFs have been investigated by many researchers, the studies on compression test, tensile test, fatigue and failure analysis and relative density have been reported by several researchers, [16,25–29]. Also the thermophysical properties — thermal conductivity, apparent density, thermal stability, dielectric and acoustic properties have been investigated and reported many researchers, [30–33].

As characteristic of SFs are lightweight and brittle, compressive strength is utmost importance properties of interest. Anbuchezhiyan, et al. [34], reported on the effect of varying volume fraction from 0 to 53% tested under compression and result suggested that tested foams having higher volume fraction contributes to the linear decreased bulk density from 1.5 g/cc to 0.78 g/cc. A comprehensive study on the process parameters for production of SFs using polymer injection mould technic was carried out by Bharath Kumar et al. [35], the results showed the optimized parameters are 20, 40 and 60 wt.% cenosphere. While the optimal cenosphere and HDPE processing parameters are 160 °C temperature and 30 kg/cm2 pressure. Investigation on the mechanical properties of SFs containing hollow carbon microspheres treated with coupling agent was studied by Zhang and Ma [36], result obtained showed that the compressive and flexural strength decrease with increasing filler content, though interfacial adhesion is induced with improved mechanical properties. The coupling agent increases the fracture toughness and the maximum fracture toughness values occurred at 28.12 vol. %. Similarly, a fiber reinforced phenolic microsphere SFs showed enhanced mechanical property according to the study by Huang et al. [23], the stiffness and toughness increased by 35%. The enhancement in the mechanical properties is a function of the wt. % ratio and composition of the filler.

In view of the literatures gaps which mostly emphasized on material types, matrix medium as the most crucial factors in fabrication of SFs. However, few literature reports on the effect of the methods of fabrication of SFs were reported. The present study evaluates SFs produced through Conventional Mould Casting Technique (CMCT) and the modified Vacuum Assisted Mould Technique (VAMT) to determine their influence on the characteristic properties under different matrix composition. In addition, the behavior of the SFs studied by mechanical testing under varying microsphere cell size and distribution.

2Materials and methods2.1List of materials

The epoxy resin based on diglycidyl ether of bisphenol A (DGEBA) the main curing agent in the matrix and was supplied by Euro Chemo Pharma Sdn. Bhd, Penang, Malaysia. Varying sizes of expandable polystyrene beads supplied by San Yong Enterprise Sdn. Bhd, Penang, Malaysia was used in fabrication of the epoxy microspheres.

2.2Material preparation2.2.1Preparation of matrix

The matrices containing varying formulations of epoxy resin clear and epoxy hardener clear in stoichiometric ratios of (2:1, 1:1 and 1:2), were prepared using a laboratory analytical balance for measurement of the epoxy samples and subsequently mixing the samples together and gently stirred. But the optimal formulation ratio of 1:1 was used in the present work.

2.2.2Preparation of microsphere

The expandable polystyrene beads were divided into three sizes (1–5, 3–7 and 4–8) mm. And the matrix is poured on the tray. Then, the microsphere gradually added at apportioned quantities, thoroughly mixed to achieve constant wetting of resin mixture throughout the microsphere surface. The uncured coated microsphere was then transferred to a tray of dried calcium carbonate powder to coat the microsphere surface and prevent agglomeration of the HMs. The coated HMs were then left overnight at room temperature for pre-cure purpose before proceeding to a post cure process in an oven at 60 °C for 30 min the next day.

2.2.3Preparation of hollow microsphere

The coated microspheres were further heat treated in oven at 120 °C for 90 min to shrink microsphere and create the intended inner hollow structure. The resultant cured HMs were then sieved to remove excess calcium carbonate on their surface.

2.2.4Preparation of syntactic foam

The Teflon mould was completely filled with HMs and then gradually poured into it the matrix to allow complete wettability. In addition, the mould was vacuumed treated simultaneously while introducing the matrix to remove any entrapped air inside the mould. Thereafter the mould was air sealed with silicone grease to avoid leakage and air passage. The final curing of the matrix was through five days, thereafter the mould was opened and the SFs is exposed.

2.3Experimental process

The photo-image of the VAMT and the experimental flow chart describing the procedures in fabricating the syntactic foam is shown in Fig. 1 and 2 respectively.

Fig. 1.

Vacuum Assisted Mould Technique (VAMT) apparatus set-up.

Fig. 2.

Methodology flow chart.

2.4Testing procedures2.4.1Relative density

The ASTM D35-74 standard was adopted to measure the density of all fabricated specimens. The densities of five specimens were measured and the average values and standard deviations were reported.

2.4.2Compression test

Compression test was conducted following the ASTM D35-74 standard. The specimen dimension of 50 × 50 × 25 mm3 was used. The test was performed at room temperature by universal testing machine model INSTRON 5982 at a constant crosshead speed of 5.0 mm/min and specimens compressed up to 60% strains between two parallel flat plates respectively. The test were conducted in triplicate for each batch production of SFs.

2.4.3Thermal decomposition test

Thermal decomposition test was carried out to confirm fluctuation of excess calcium carbonate embedded in the spheres shell. The ASTM D17-26 standard was followed in conducting the test. The test was conducted using universal furnace under controlled heating temperature rising from 30 °C until 70 °C and maintained for 1 h. Measurements is calculated based on the weight loss after subtracting the sample weight before with that of after the burning process. The test was conducted in triplicate and the average values were recorded.

2.4.4Thermogravimetric analyzer

Thermogravimetric analyzer TGA 8000 by PerkinElmer was used to confirm percentage reduction of excess calcium carbonate before and after cleaning. The specimens were heated from 30 to 650 °C, at a heating rate 10 °C/min. Each specimen was placed in an open crucible where nitrogen was used as the purge gas at a flow rate of 20 mL/min.

2.4.5Differential scanning calorimetry

Differential scanning calorimetry DSC 8000 by Perkin-Elmer was used to measure the shift in thermal decomposition and the instrument was calibrated using zinc. The purge gas was nitrogen with a flow rate of 100 mL/min and a pressure of 20 psi.


Kern OZL 464 stereo zoom microscope, x0.7 → 4 x (Hitachi, Japan) is used for the physical and microstructural analysis of the epoxy sphere (ES), epoxy hollow spheres (EHS) and the syntactic foams (SF) samples before and after mechanical testing. Nikon D7000 camera with Nikkor 35 mm F1.8G lens is used for optical imaging.

3Result and discussion3.1Microsphere analysis

Characterization of the HMs under varying stoichiometry of epoxy to hardener weight ratio is presented and discussed based on its appearance and cure characteristic. The stereo zoom microscope was used to capture the physical appearance — outer surface image of the HMs. Several random HMs were selected placed to acquire clear image during the examination to ascertain the optimal epoxy: hardener ratio for the coating system.

Fig. 3 shows the comparison of HMs surfaces. Fig. 3b predominantly exhibit smooth and consistent appearance as compared to Fig. 3a and c. This is because cross-linking occurred in equal stoichiometry proportion of the epoxy resin and amine hardener in the matrix deviation in term of network density. Thermal analysis evaluated through the DSC curves demonstrated that stable peak exothermic event occurs at stoichiometry equivalent ratio of 1:1. This means the degree of cure and thermal stability within the cross-linked matrix was obtained at equal proportion of epoxy rich and amine hardener and smoother microsphere surface.

Fig. 3.

Appearance image of hollow microsphere surface at (a) epoxy/hardener ratio 2:1 (b) epoxy/hardener ratio 1:1 and (c) epoxy/hardener ratio 1:2.


Meanwhile for the amine rich formulation (ratio 1:2), incomplete curing reaction was formed and this resulted in partial collapse of the spheres which implies that the shell was not strong enough to hold the structure. The sphere distribution is impacted by the shapes of the spheres because of the volume fraction and average distance among epoxy HMs within the matrix. Fig. 4 shows the cure characteristic curves of the matrices. The endothermic heat flow for the matrix formulation of ratio 1:1 showed the lowest heat peak at 22 mW and at higher heating temperature range of 85–92 °C. Though the highest endo-heat flow recorded was 30.8, 27.6 and 23.7 mW for the matrix formulations 2:1. 1:1 and 1:2 respectively.

Fig. 4.

Curing characteristic curves of epoxy/hardener at different weight.

3.2Contributions of microsphere sizes

The relative density and compressive stress of the SFs with three types of HMs sizes is presented and discussed. The specimen were tagged A, B and C representing the 0.1–2 mm, 2.1–4 mm and 4.1–6 mm size range respectively. The results showed 377.10, 443.29 and 627.37 kg/m3 for specimens A B and C respectively. A swift uptrend density is observed for the three specimens when the spheres were embedded within the matrix. This implies smaller sizes of the spheres enhances density as the case in the specimen A.

In practice, reduction of microsphere size increase the ratio of surface to volume properties because smaller sphere size contributes to interfacial adhesion properties and agglomeration behavior. In other words, the size of HMs contribute to the density and compressive strength of the SFs [13,35,37,38].

3.2.1Syntactic foam analysis

The main concern in production process of SFs is the presence of interstitial void spaces and distribution of the microspheres (close packing) in a regular shape within the resin. Fig. 5 shows the SFs formed using CMCT and VAMT. From Fig. 5, the SFs fabricated from the VAMT showed more evenly distributed HMs within the matrix because absence of vacuum allowed compactness of the HMs. whereas, the CMCT exhibits layered separation with uneven HMs distribution. Fig. 5 (a) Surface morphology of cured SFs CMCT and (b) surface morphology of cured SFs VAMT.

Fig. 5.

Surface of cured epoxy SFs from CMCT and VAMT methods.


Physical inspection of the SFs showed presence of resin-rich region at the bottom phase because of low density of the microspheres compared to the resin, which tends to float to the top surface. Contrastingly in SFs from the VAMT, the HMs were uniformly distributed and fully encapsulated within the matrix with less porosity and air gaps. The CMCT is characterized by poor wetting of the HMs, thus leaving significant empty spaces between the HMs and forms void spaces in the foam structure.

3.3Physical properties analysis3.3.1Cell distribution

Fig. 6 represents the cell morphology of the varying size distribution of the HMs in the SFs produced from CMCT and the VAMT. The variation in size range of the HMs influenced the SFs distribution packing in a manner which allows uneven stress/strain concentration influencing the foam performance. The internal structure of HMs is very important in determination of cell characteristic and their distribution in SFs. Thus, the quantification based on cells measurement analysis using image J showed the different size range classification.

Fig. 6.

(a) CMCT cell morphology — (i) BASF 403 (ii) BASF 303 (iii) B-Normal. (b) VAMT cell morphology — (i) BASF 403 (ii) BASF 303 (iii) B-Normal.


Fig. 7 describes the cell size measurement in the SFs produced using the CMCT and VAMT by analyzing and quantifying the image captured according to their classification ranges. Analysis through micromorphology of the VAMT fabricated SFs shows wider and even distribution of the HMs with the smaller HMs filling the void space between bigger spheres and thus forming a closely packed cells locked within polymer matrix. Whilst, CMCT, showed uneven cell distribution with trapped air bubbles and void spaces. In the study of Skibinski et al. [39], 74% efficiency was garnered at maximum relative volume when spheres with the same size were packed together either in cubic or hexagonal close packing. Similarly, conclusion from the study of Dando et al. [40] highlighted the significance of closely packed microspheres within the matrix as it influences the internal properties. The wide cell size distribution range in the SFs made by VAMT is preferred over the maximize spheres packing in the CMCT-made SFs. The volume fraction is inversely proportional to the density of the SFs as stated by Ullas et al. [41]. This implies tuning the size range of the HMs promotes different volume fraction in the SFs specimen and subsequently able to alter the end properties of the foam.

Fig. 7.

(a) Cell size distribution — SFs via CMCT. (b) Cell size distribution — SFs via VAMT.


The VAMT yielded foam with higher density compared to CMCT as shown in Table 1. For varying cell size distribution (different HMs sizes), the density of the VAMT SFs was higher than the CMCT. This infers that cell distribution contribute less compared to the processing technique used. The density values observed are 42.53 kg/m³ and 31.25 kg/m³ for the sample A and B for different technique. The density increased by 15% and 40% for the specimen B and C respectively, as the cell-size ranges reduced. Similarly, the compressive modulus increased with decrease in cell-size range with 61% and 79% for specimen B and C while for the compressive strength, a 26% and 33%. The influence of the fabrication technique on the foam properties varies with different properties and are inconsistent with the enhancement [42,43], but however, VAMT fabrication process improved the density of the SFs because of the close-packed formation and sphere size range distribution. Table 1 shows the physical of the SFs with different process techniques. SFs produced by VAMT offers higher physical and compressive properties than the CMCT.

Table 1.

Relative densities and compressive properties of SFs.

Specimen identification  Density (Kg/m3Compressive modulus (MPa)  Compressive strength (MPa) 
A-normal unsupported (CMCT)  334.6  37.04  4.10 
B-normal unsupported (VAMT)  377.1  556.39  10.40 
BASF 303 unsupported (CMCT)  411.3  26.12  9.46 
BASF 303 unsupported (VAMT)  443.3  779.37  22.14 

Sample A and B represents the treated and untreated epoxy syntactic foam (SFs) microspheres.

The presence of less interstitial voids and absence of air bubbles contributed to the enhanced properties recorded for the VAMT. The voids-free characteristics influenced the final properties of specimen in both physical and compressive properties. A significant deviation in density impacts the compressive strength value of the specimen. For example, the BASF 303 samples displayed compressive modulus of 779.3 MPa and 26.12 MPa for the VAMT and CMCT respectively. The contributive factor to the disparity of the modulus between the SFs is the fabrication technique, the VAMT had no interstitial void and allows deep matrix penetration in between the HMs thereby enhancing properties. Waddar et al. [44], affirmed that wettability and deep penetration of matrix improves the quality of the SFs properties. There is disparity in the values of the compressive modulus in B-Normal unsupported samples for VAMT (779.37 MPa) and CMCT (37.04 MPa). The compressive strength values for both samples are 22.14 and 10.40 MPa for the VAMT and 9.46 and 4.10 MPa for the CMCT technique.

The disparity in the compressive modulus can be traced to the morphology, even arrangement of the SFs cells distribution. In the case of VAMT, the HMs are layered and closely packed. The lattice bond — wettability, through the spheres matrix spread through the SFs better than in the CMCT. The compressive modulus is attributed to effective wettability and cells arrangement within the SFs [45]. However, the process technique have less impact on the relative density, the values attained for the BASF and B-Normal samples are 377.1 and 443.3 kg/m3 for the VAMT and 334.6 and 411.3 kg/m3 CMCT. Many other studies [8,17,46,47] affirms the linear relationship between filler content (matrix) and density of the foam, implying the density of the foam is directly proportional to the compressive strength.

3.3.3Comprehensive stress–strain curves

Inference from previous studies has shown that the HMs volume fraction, concentration and sizes contribute to the compressive strength of SFs. Also, the production techniques impacts the compressive strength of SFs [48–51]. The representative compressive stress–strain curves for all types of epoxy HMs SFs produced by VAMT and CMCT methods is presented in Fig. 8a and b. The SFs exhibits identical stress–strain profiles consisting of a linear elastic region followed by a strain softening region that is characterized by a slight drop in stress. It is evidence from the Figures the stress–strain curves for both the VAMT and CMCT methods followed identical upward stress pattern for the different volume fraction of HMs in the SFs. The modulus and peak strength of the SFs are closely identical but with variation in the peak and modulus values. The higher modulus and peak strength are obtained for lower-HMs volume fraction and for thicker walled HMs for both production methods. From literatures, compressive strength of composites have been expressed as the first peak in the stress–strain curves [52–54].

Fig. 8.

(a) Syntactic foam stress–strain curve. (b) Syntactic foam stress–strain curve.


The compressive modulus values are measured as the slope of the initial linear region of the stress–strain graphs [16,55,56]. From the results, the compressive modulus of SFs increases with decreasing η as demonstrated by both the VAMT and CMCT samples. It is also seen that the modulus decreases as the volume fraction of the HMs types increases. The compressive modulus for the VAMT samples are BASF 103 (556.4 MPa), BASF 203 (505.15 MPa), BASF 303 (533.28 MPa) and BASF 403 (518.22 MPa). On the other hand, the compressive modulus of the CMCT samples are BASF 103 (337.04 MPa), BASF 203 (313.45 MPa), BASF 303 (325.14 MPa) and BASF 403 (309.62 MPa). The stress pattern in the CMCT SFs is higher when compared to the VAMT SFs and these can be traced to the impact of the fabrication method. The stress plateaus exhibited in the VAMT and CMCT SFs are synonymous with most SFs. And the high energy absorption was due to the large compressive strain contain in the epoxy microsphere SFs. The VAMT exhibited higher energy absorption and does displayed delayed failure compared to the CMCT produced SFs. Inherently, the impact of the varying volume fraction and load concentration on SFs is relatively lesser compared to the fabrication techniques. In this case, the VAMT fabrication improved compactness between the microspheres spaces, reduced minimally the stress concentration points that usually is caused by interstitial void spaces within the SFs matrices.


The outcome of this study describes the relevance of processing techniques on the performance of the epoxy syntactic foam characteristics. The physical and mechanical properties of the ESF are deeply influenced by the method of fabrication. The VAMT method, unlike the CMCT method, can effectively remove the entrapped bubbles during production process. Thus eliminating formation of interstitial voids and air gaps which create stress concentration points within the SFs. This was achieved as a result of efficient wettability (i.e. epoxy resin able to fully penetrate the gaps in between the HMs obtained using the innovative VAMT technique.

In addition, the empty space between the spheres is easily filled by matrix because of effective cell-size range distribution. Whereas, the CMCT exhibits much lower density due to presence of the interstitial voids which mitigate good wettability of the matrix. Also, during mixing of the matrix, air pockets was created which is difficult to be degassed thereby reducing the quality of the produced SFs. The MHs preparation and material characteristics has demonstrated the relative impact on the SFs performance. Likewise, the formulation ratio of the epoxy hardener and epoxy resins significantly contributed to the HMs characteristics. The optimized formulation ratio reached in this study is ratio 1:1 where effective cross-link level was achieved together the post heat treatment of the SFs. The curing time is significantly reduced at this equal ratio for either the heat treated or atmospheric curing o the SFs. Overall, the VAMT produced SFs demonstrated improved mechanical and physical properties compared to the CMCT SFs. The VAMT technique was capable of producing cell foams with enhanced properties and holds advantage of simplicity and effectiveness.

Conflicts of interest

The authors declare no conflicts of interest.


The authors would like to thank Universiti Sains Malaysia for the financial support provided through the Research University (RUI) grant (ref. no.: 1001/PBAHAN/80114041).

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