• International Journal of Technology (IJTech)
  • Vol 15, No 4 (2024)

Adhesives Type on the Burning Rate and Emission Properties of Honeycomb Sandwich Composite

Adhesives Type on the Burning Rate and Emission Properties of Honeycomb Sandwich Composite

Title: Adhesives Type on the Burning Rate and Emission Properties of Honeycomb Sandwich Composite
I Gusti Agung Ketut Catur Adhi, Ketut Adi Atmika, Ni Made Dwidiani, I Dewa Gede Ary Subagia

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Cite this article as:
Adhi, I.G.A.K.C., Atmika, K.A., Dwidiani, N.M., Subagia, I.D.G.A., 2024. Adhesives Type on the Burning Rate and Emission Properties of Honeycomb Sandwich Composite. International Journal of Technology. Volume 15(4), pp. 999-1011

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I Gusti Agung Ketut Catur Adhi Department of Mechanical Engineering, Mataram University, Nusa Tenggara Barat, 83121, Indonesia
Ketut Adi Atmika 1. Mechanical Engineering Study Program, Udayana University, Bukit Jimbaran 80361, Badung-Bali, Indonesia 2. Technology and Material Research Centre, the Institution of Research and Community Servic
Ni Made Dwidiani Mechanical Engineering Study Program, Udayana University, Bukit Jimbaran 80361, Badung-Bali,
I Dewa Gede Ary Subagia 1. Mechanical Engineering Study Program, Udayana University, Bukit Jimbaran 80361, Badung-Bali, Indonesia 2. Materials Engineering Laboratory, Engineering Faculty, Udayana University, Bukit Jimbaran
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Abstract
Adhesives Type on the Burning Rate and Emission Properties of Honeycomb Sandwich Composite

Sandwich composite is an important part of engineering products capable of replacing metallic composite. It consists of two types of material, namely polypropylene honeycomb core and skin made from jute fiber-reinforced epoxy composite (JFRP), which are joined with glue. This study presented a unique discussion about adhesives that focused on the burning rate and emission performance of SA-A and SD-E. The burning rate performance was assessed with the UL-94 HB test in accordance with the ASTM D 635 standard. Emission value of both adhesives was also examined in line with the ASTM D 2863 standard using a Gasboard-3100P Syngas analyzer. In addition, FTIR and SEM analyses were used to determine the characteristics of the SA-A and SD-E adhesives. The results showed a significant difference in adhesives rates, with SA-A burning 0.5% faster than SD-E in addition to a 0.58% reduced weight loss. Emission test confirmed that both adhesives have similar LOI values of 22.6% and 22.8%, respectively. SA-A adhesives contained LSD, which is dangerous to human health. In conclusion, SD-E adhesives should be used on sandwich composite due to its epoxy-based potential as a flame retardant because SA-A adhesives has more potential to trigger firing due to the fuel content.

Adhesives; Burning rate; Emission; Fire; Sandwich composite

Introduction

Sandwich composite structures are widely used in manufacturing engineering products, and it consist of three components, namely skin, core and adhesives (Jeevi, Nayak, and Abdul-Kader, 2019; Novotný, Doubrava, and Ružicka, 2017). The thick core is covered by a pair of thin skins (Wei et al., 2020). Sandwich composite structures are characterized by the lightweight nature, production ease, strong mechanical properties and are highly used in manufacturing airplanes, automobiles, ships, and packaging, including electrical insulators, energy absorption, and other industrial purposes (Rupani, Acharya, and Jani, 2017).

Failure caused by fire has become increasingly relevant in recent years (Suwondo et al., 2021), despite the numerous benefits of composite materials, one significant disadvantage is the highly inflammable nature (Zhu et al., 2020). Fire performance was determined based on factors such as ignition, self-extinguishing ability, flame spread, burn through, heat release, smoke obscuration, toxicity, and related scenarios. Therefore, the process of assessing the resistance of composite materials is needed to examine the ability to perform the intended load-bearing functions when exposed to fire. Several studies have extensively examined fire performance of composite materials (Ortega et al., 2020; Hörold et al., 2017; Suoware, Ezema, and Edelugo, 2017; Salmeia et al. 2016; Bar, Alagirusamy, and Das, 2015; Szolnoki et al., 2015). However, the most widely accepted study focused on an experiment conducted by incorporating fire retardant into the polymer matrix. The aim was to suppress heat release, increased temperature, and gas emission, perceived as toxic sources, through the solid and gaseous phase mechanism (Ogabi et al., 2021; Kim, Dutta, and Bhattacharyya, 2018).

       These properties need to be considered when studying the combustibility of honeycomb-based composite materials. According to Kim, Dutta, and Bhattacharyya (2018), several factors such as the chemical composition, physical features, load-bearing capacity of adhesives, surface condition of the joints, and the use of materials that do not adhere to health standards significantly affect the toxicity of emitted smoke (Jeevi, Nayak, and Abdul-Kader, 2019; Ledesma et al., 2018). Preliminary studies stated that synthetic adhesives, such as structural silicone, or stiffer substances namely acrylic or epoxy are commonly used for assembling composite materials (Shang et al., 2020; Valente et al., 2019; Ledesma et al., 2018; Machalická and Eliášová, 2017). However, thermosetting types such as phenol-formaldehyde (PF), urea-formaldehyde (UF), and polyurethane (PU) are currently been used due to the water-resistant properties (Chanda, Kim, and Bhattacharyya, 2022; Shavandi and Ali, 2018). The application of adhesives in non-metallic structural materials has numerous benefits, including uniform stress distribution, eliminating the need for drilling, and enabling the bonding of substances with varying mechanical and thermal properties. Sugiman and Sulardjaka (2016), stated that adhesives play a critical role in the bonding of materials. Arenas, Narbón, and Alía (2010), stated that joint strength is inversely proportional to the thickness of adhesives, meaning the shear strength increases as the thickness of adhesives decreases. Davies et al. (2009), examined the physical, chemical, and mechanical properties of Aluminium substrates bonded with epoxy adhesives of varying thicknesses ranging from 0.2 to 1.3 mm. However, there is no proof that varying thickness can weaken composite materials (Kostin, Nasonov, and Zinin,  2021; Momber, Fröck, and Marquardt, 2021; Shang et al. 2020; Jeevi, Nayak, and Abdul-Kader, 2019; Shavandi and Ali, 2018).

       Adhesives are an essential aspect of sandwich composite structures that bond the core and skin. Generally, the liquid types such as SA-A and SD-E are commonly used due to ease of application. Aica-Aibon (SA-A), a kind of poly-chloroprene-based glue with toluene characteristics (C7H8) (Tualeka et al., 2019; Djurendic-Brenesel, Stojiljkovic, and Pilija, 2016), contains lysergic acid diethylamide (LSD) with the chemical formula C20H25N3O. LSD is a synthetic narcotic drug that causes mental disorders when consumed excessively. Liao et al. (2020) stated that when SA-A is burned, it tends to have minimal impact on the surrounding environment. Meanwhile, Dextone-Epoxy (SD-E) is an epoxy-based adhesive dependent on epoxy resin and one of the most essential polymer classes due to the multiple binding capacities provided by the oxirane ring. Due to the significant differences in the source materials, studying the flame and emission of these adhesives becomes interesting. This study mainly focused on examining the impact of flammability and emission on adhesives used in sandwich composite. Furthermore, both adhesives were tested concerning the application process in honeycomb composite to evaluate burning rate and emission. The aim is to investigate the potential roles as triggers for fuel fire and the resulting emission. The tests were carried out based on ASTM D 635 and 2863 standards to determine both adhesives burning rate and emission.

Experimental Methods

2.1. Materials

        Sandwich composite panel comprised three main parts, namely skins, adhesives, and honeycomb core. A typical example is the natural jute fiber, produced by Casthanal Textile CIA in Brazil, shown in Figure 1 (Gupta, Srivastava, and Bisaria, 2015). The fiber has relatively low conductivity, ranging from 0.29 to 0.32/mK, as well as composite matrix materials, namely Bakelite Korea epoxy resin Bakelite-EPR-174 and Justus Kimia-Raya cycloaliphatic amine curing agent Bakelite-EPH-555, shown in Tables 1 and 2.

        Polypropylene honeycomb (PPH), particularly a Nomex 8 mm mesh, was used as the core of composite sandwich. This material has a low density and good performance function as a shear load-carrying core in sandwich composite construction. PPH is a hexagonal structure with equal sides and six interior angles of the same dimension measuring 1200, as shown in Figure 2.

        The study used adhesives SA-A and SD-E supplied by PT AICA- Indonesia and PT Dextone Lemindo, Japan, respectively. SA-A, composed of 10 to 99 wt.% ethylene-vinyl acetate copolymer adhesives, is produced by dissolving rubber in liquid solution, thereby producing a yellow colour. Although SA-A showed outstanding substrate adhesion characteristics of the toluene (C7H8) class, it contained an LSD compound hazardous to human health. The specific properties of these adhesives are shown in Table 3.
 


Figure 1 a) Seedbed process of jute fiber, b) chemical bonding of epoxy bisphenol A

Figure 2 Geometry and cross-section of honeycomb

After SD-E is an epoxy-based adhesive containing epichlorohydrin bisphenol-A (DGEBA) in the formulation. Epoxy resin is characterized by low-molecular-weight comprised of oxirane or epoxide rings as functional groups, imparting thermosetting properties. This makes epoxy resin a commonly and widely used material in various applications, including adhesives, coatings, semiconductor packaging, and composite matrices. The specific SD-E adhesive characteristics are shown in both Tables 2 and 3.

Table 1 Mechanical and physical properties of jute fibre as reinforcement in the shell of sandwich composite

Table 2 Properties of epoxy and hardener

Table 3 Properties of Synthetic adhesives

2.2.   Sandwich composite structures

Sandwich composite panel used in the experiment was 350 mm x 350 mm in size. A panel with three layers of 2 mm thick jute fiber reinforcement was used as the skin of composite. In addition, the panel was manufactured using the vacuum injection process (VaRTM), with the core bonded to both skins using two different types of synthetic adhesives. Figure 3 and Table 4 show sandwich composite and specimen structures using both SA-A and SD-E adhesives, respectively.

Figure 4 shows the geometry of the sample and burning test schema following the ASTM D 635 standards. The specimen used was composed of three segments measuring 125 mm in length. The first segment, positioned 25 mm from the free end, was the initial burning area. The subsequent and final ones, which served as burning test and clamping areas, were 75 mm and 25 mm long, respectively.

Table 4 Sandwich composite materials

Figure 3 Bottom-part sandwich composite panels with SA-A adhesives and top-part sandwich composite panels with SD-E adhesive
Figure 4 a) Size and design of sandwich composite with SAA and SDE adhesives, b) The burn test process of the samples carried out with UL-94HB in the combustion chamber

2.3. Analytical Method

The burning resistance of the sample was tested under the UL-94HB (Chukwunwike et al. 2019) and LOI value (Lee,  Salit, and Hassan, 2014) following the ASTMD 2863 standards. UL-94HB is a standard for evaluating the flammability safety of plastic materials used as parts of devices. The test was conducted through slow burning in a horizontal position of the specimen, shown by the HB code. In addition, emission gas was tested using a Syngas analyzer gas bord-3100P. The LOI value was determined following equation 1 (Szolnoki et al., 2015):
Sandwich composite burning propagation was determined using equation 2 as follows:

        Burning rate (Misnon et al., 2018) was determined using equation 3:

       where LOI is the limiting oxygen index (%) (Parmar et al. 2014), O2 and N2 are denoted as oxygen and nitrogen. L is the distance between two marking lines (75 mm), t is fire spreading time (minutes), FP is the flame propagation (mm/second), Dp is the propagation distance (mm), Pt is fire propagation time (second), and Lt is burning time (second).

The weight loss of the specimen was determined using equations 4 and 5:

where W is the total weight loss after burning (gram/sec), w is the weight loss of the specimen during the burning process, w0 is the initial weight (gram), w1 is the final weight (gram), and t is the burning time (second).

2.4.  FT-IR spectroscopic analysis

    The characteristics of adhesives in sandwich composite were measured using Fourier transform infrared spectroscopy (FTIR) spectra obtained with a Shimadzu spectrometer. The signal resolution of the FTIR was 1 cm-1, and a minimum of 16 scans were obtained and averaged within the range of 500 to 4000 cm-1.

2.5.  Scanning electron microscopy (SEM)

     The residue from the horizontal burning test on sandwich composite was subjected to SEM analysis using a JSM-6510 from Japan. In addition, the residue surface was coated with gold before being tested.

Results and Discussion

3.1. FTIR analysis

        Figure 5 shows the FTIR spectra of the SA-A and SD-E adhesives used to bond the skin to the core of the sandwich composite. The test results show different performances for each adhesive, with SA-A and SD-E adhesives being rubber-based and monomers from the diglycidylether bisphenol A (DGEBA) family, respectively.

Figure 5 FT-IR spectra pattern adhesives of SA-A and SD-E 

        Analysis of the FTIR spectra comparing the SA-A (red line) and SD-E adhesives (blue line) shows significant differences. The O-H stretching region in the absorbance range of 2500 cm-1 to 3000 cm-1 showed there was a peak shift of 2936 cm-1 and 2924 cm-1 for SD –E and SA-A. However, within an absorbance range of 2000 cm-1 to 2500 cm-1, SD-E had increased energy compared to SA-A, depicting a greater tendency for bond breaking, thereby contributing to enhanced stability and reduced flammability. The characteristic peaks for C-O-C ether and aromatic C-C stretching were observed at 1036 cm-1 and 1509 cm-1, respectively. The stretching C=C in the aromatic ring was observed at 1608 cm-1, while C-H bending bands occurred within the absorbance range of 500 cm-1 to 1000 cm-1.

3.1. Burning Rate

        The recent focus on building fire has proven the importance of understanding the material types used (Suwondo et al., 2021). Building materials and equipment types have been identified as the main factors contributing to the frequency of these fire (Nugroho, Latief, and Wibowo, 2022). The combustibility of a material depends on various factors related to the constituents, which impact characteristics such as heat release, flame spread, and ignitability (Pausas, Keeley, and Schwilk, 2017). In addition, the presence of adhesives used to bond the skin and core of sandwich composite was considered fire reaction properties, as shown in Table 5.

Table 5 Characteristics of combustion products and weight loss of sandwich composite with SDA and SDE adhesives on sandwich composite
 

Figure 6 Comparison of adhesives on burn test and weight loss of honeycomb composite

       Figure 6 shows a bar graph of burning rate and weight-loss combination of sandwich composite using SA-A and SD-E adhesives. The use of dark colour shows there is a significant difference in burning rate between sandwich composite using SA-A and SD-E. This was caused by the different chemical compound properties of both adhesives, as shown in Figure 5. SA-A is an elastomer-based adhesive that can function as thermoplastic and thermosetting types, depending on the required cross-linked structure. It is also characterized by rapid adhesion and belongs to a group of elastomers with rubber-based adhesives, including butyl, butadiene, styrene-butadiene, and nitrile rubber, as well as silicone, and neoprene (Wang et al., 2022). However, adhesiveness and cohesion levels are limited at temperatures greater than 70°C, and a stabilizer is needed to withstand environmental effects such as UV and ozone. Adhesives can be dried at normal temperatures and has good heat, water, and chemical resistance, depending on the contents of the hardening compounds. SA-A adhesives characterized based on FTIR analysis, showed features of a rubber-based composition, for example, C=C aromatic bonds observed at a wavenumber of 1656.92 cm-1. Meanwhile, the aliphatic C-H or C-C hydrocarbon groups that trigger the flame were observed in the wavenumber range of 2000 cm-1 to 2500 cm-1 with a prominent peak at 2358.08 cm-1 and an intensity of 79.64 mm. SD-E adhesives are epoxy-based, mainly synthesized from active hydrogen reactions in phenols, alcohols, amines, and acids with epichlorohydrin under well-controlled conditions. Generally, SD-E adhesives were prepared by packing epoxy and curing agent composition separately before use, with curing occurring briefly after mixing in accordance with the mixing ratio. Epoxy adhesives such as SD-E often have a higher glass transition temperature, making it suitable for applications requiring high-temperature resistance. In FTIR analysis, the epoxy tends to react with amines and amides (NH) at frequencies ranging from 3140 to 3320 cm-1. The deformation band of C-O occurred at 828.46 cm-1 within the wavenumber range of 750 cm-1 to 1000 cm-1. In the spectrum range, characteristics of the aromatic ring were observed, double chain, benzene and C-C aromatics, C-O-C chain from the ether, C-O chain of oxirane group, and CH2 chain were formed at wavenumbers of 1606 cm-1, 1506 cm-1, 1031 cm-1, 913 cm-1, 862 cm-1 and 769 cm,-1 respectively. The stretching of hydroxyl groups of O-H was depicted by broadband at 3500 cm-1, suggesting the presence of species and dimers of high molecular weight. In addition, a band corresponding to the ether linkage was observed within the 1000 to 1100 cm-1 spectrum range.

        The average combustion length of the SA-A and SD-E adhesives was 75 mm and 72 mm, respectively. The results showed the carbon composition of the SA-A adhesives possessed higher fuel propagation capacity compared to the SD-E, which had greater fire resistance. This result is in line with the study conducted by Chanda, Kim, and Bhattacharyya (2022), focusing on the significant influence of adhesive formulation materials on fuel value. The burning rate of each sandwich composite sample using the SA-A adhesives was 0.5% faster than the SD-E. The difference was caused by certain thermos-physical properties, such as the reaction rate depending on the temperature threshold or higher amount of oxygen (O2) during the combustion process, leading to increased heat energy (Ogabi et al., 2021). Table 5 shows the burning time of the two adhesives used.

        In Figure 6, the result of the weight loss measures in the bar graph is shown in grey. The result is similar to the burning rate, and the weight loss of SA-A adhesive samples has a higher value than SD-E, with a significant difference of 0.58%. This difference directly correlates with the degradation of the samples caused by increased heat and smoke production. The higher weight loss and low percentages of mass residues observed in the SA-A samples were attributed to the pyrolysis and burning phases, depicting a prolonged exposure to heat flux. These results are in line with previous studies (Vinod et al., 2022).

Table 6 The average values of chemical elements of SA-A and SD-E adhesives

3.2. Burning Emission

Table 6 shows emission from burning sandwich composite, which were tested to determine the environmental effects of burning adhesives. When burned, adhesives in composite emits carbon dioxide (CO2) and carbon monoxide (CO) as combustion products, which is proven by the results of the FTIR test shown in Figure 5. However, both SA-A and SD-E adhesives generated the same quantity of oxygen, measured at 20.9 mg/m3. The NO2 produced by the SA-A adhesives is slightly greater than SD-E by 0.01%, the two materials have similar LOI values of approximately 22.6% and 22.8%, respectively. The difference in burning behavior was caused by the higher carbon content in SD-E, which formed a denser carbon layer, preventing heat transfer during the burning process. There is a definite relationship between burning rate and oxygen content, with a threshold of 21%. Materials with an LOI less than this threshold are typically regarded as flammable or combustible and would burn readily in an open-air setting, while a higher index showed lower flammability (Raajeshkrishna and Chandramohan, 2020). When the oxygen content drops below the critical LOI values, the material ceases to burn because the available oxygen is insufficient for combustion. However, a high LOI value, minimizes burning potential and oxygen concentration, in order to sustain combustion (Misnon et al., 2018). LOI is a distinguishing characteristic of materials, often used to rank the relative flammability of polymer composite materials (Raajeshkrishna and Chandramohan, 2020; Chukwunwike et al., 2019; Bhattacharyya and Kim, 2017).

3.3. Burning residue

Figures 7a and b show the charcoal from sandwich composite with SA-A and SD-E adhesives, respectively. The characteristics of the charcoals depend on the quantity of lignin and cellulose content in the jute fiber and properties of the polypropylene in the core. Furthermore, Charcoal produced from poly-propylene is more reflective and contains the following compounds CO, CO2, HC2, NO2, and CxHx. The elements produced from combustion are also described in the spectral range of the FTIR test for each adhesive applied in the bonding of sandwich composite. This implied that the chemical breakdown of each adhesive sample contributed to the formation of oxides and other degradation products, forming the crosslinking network that produced the coal. Meanwhile, the LSD element contained in the SA-A adhesives was in the form of CxHx, which is harmful when inhaled.

        Figure 8 shows the result of the SEM test conducted on the charcoal samples obtained after burning, showing distinctive structural characteristics. The charcoal samples from SA-A and SD-E adhesives had rubber and polymer-based structures that are not agglomerated, respectively, as shown in Figures 8a and b.