Stress relaxation and improved fracture toughness of metal bonding using flexible monolith sheets and an epoxy adhesive | Polymer Journal

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While epoxy resins exhibit excellent mechanical and insulating properties as well as excellent stability against heat and chemicals, epoxy adhesives also have drawbacks such as brittleness and stress concentration. Rubber-based materials are often added to epoxy adhesives to increase toughness, but they are sensitive to heat and moisture, limiting their effectiveness in harsh environments. In this study, we propose a new sheet-type adhesive consisting of a conventional liquid epoxy adhesive and an epoxy monolith sheet with internal continuous pores, using the advantageous properties of the flexibility and toughness of the epoxy monolith to avoid stress concentration. We evaluated the adhesion strength for metal bonding using the sheet-type epoxy adhesives via a lap-shear tensile adhesion test at various temperatures. The total destruction energy was also estimated via a tapered double cantilever beam test. Furthermore, a heat cycle adhesion test was conducted using two types of metallic materials with different coefficients of thermal expansion to elucidate the effect of the monolith sheet on the improvement of interfacial failure induced by stress concentration.

Epoxy resins have excellent mechanical and insulating properties, are robust with respect to heat and chemicals, and are used as adhesives, coatings, encapsulants for electronic devices, and matrices for composite materials in a wide range of applications [1,2,3,4]. Epoxy adhesives exhibit high strength for metal bonding, but also exhibit several intrinsic drawbacks, such as brittleness and local stress concentration due to restricted molecular motion and the lack of plastic deformation by a three-dimensional dense network structure [5,6,7,8]. For materials used in advanced technology fields such as precision machinery motors, semiconductors, and aerospace, a high level of reliability is needed, and it is important to improve the unfavorable mechanical properties of epoxy adhesives [9,10,11]. To date, the mechanical properties of epoxy materials have been improved through the use of elastomers or core‒shell particles as fillers and additives to provide high strength and high toughness via stress relief [12,3.0.CO;2-B " href="#ref-CR13" id="ref-link-section-d13778851e383_1">13,14,15,16,17,18,19,20,21]. In recent years, it has been noted that changes in the microscopic structure play an important role in increasing the toughness of epoxy resins and adhesives, leading to an improvement in their mechanical performance reliability. In addition, there are an increasing number of reports on the reinforcement of resins using phase-separated structures of polymers with different physical and mechanical properties. For example, Kishi et al. investigated the effect of the addition of block copolymers with a flexible rubber component to an epoxy resin material and analyzed phase-separated structures observed after thermal curing [22, 23]. It has long been known that elastomeric components are sensitive to moisture, heat, or chemicals due to the fast mass transfer inside elastomers. In particular, elastomeric components distributed in a three-dimensional continuous phase accelerate the degradation in the service environment [24,25,26].

One of the methods for improving the performance of epoxy adhesives is the relaxation of residual stress that is generated during the curing process. A monolith is a porous material consisting of three-dimensional interconnected pores and skeletons [27,28,29,30]. In recent years, the unique structure and properties of epoxy monoliths have been explored for applications such as separators [31], adhesives for bonding dissimilar materials [32, 33], template materials [34], and co-continuous composite materials [35,36,37]. Here, we propose a new type of sheet-type adhesive that combines an epoxy monolith sheet with internal continuous pores and a conventional liquid epoxy adhesive [38]. This sheet-type adhesive consists of two kinds of materials with a common chemical structure but different mechanical properties intertwined with each other in three-dimensional space. The adhesive is manufactured by filling the pores and applying them to the surface of an epoxy monolith sheet using an uncured liquid epoxy adhesive. By exploiting the flexibility and toughness of the epoxy monolith, significant interactions between the hard and soft phases can be engineered for the stress relaxation of hard epoxy adhesives. In our previous paper [38], we reported the synthesis of sheet-type adhesives using an epoxy monolith combined with commercial epoxy and acrylic adhesives, and a preliminary evaluation of the adhesive strength of the fabricated composite adhesive materials was performed via lap-shear tensile tests of steel plate cold commercial (SPCC) as an adherent at room temperature. A tetrafunctional epoxy resin was used during the fabrication of the epoxy monolith in that study, but the produced epoxy monolith sheets were likely to be brittle when combined with commercial epoxy resins.

In this study, flexible epoxy monolith sheets were prepared using 2,2-bis(4-glycidyloxyphenyl)propane (BADGE) and bisphenol-A bis(triethylene glycol glycidyl ether) ether (BATGE) as difunctional epoxy resins (Fig. 1) in combination with a commercially available liquid epoxy adhesive. We sought to improve the toughness of the epoxy adhesive structures and evaluated the lap-shear adhesive strength for metal bonding via tensile tests performed at various temperatures, including in high-temperature conditions. The total destruction energy of the adhesion materials was also estimated using a tapered double cantilever beam (TDCB) test [39,40,41,42]. Furthermore, a heat cycle adhesion test was conducted using two types of metallic materials with different coefficients of thermal expansion.

Chemical structures of the materials used for the fabrication of the epoxy monolith

BADGE was purchased from Tokyo Chemical Industry Co., Ltd. and used as a basic raw material for the epoxy resin. BATGE manufactured by New Nippon Chemical Co., Ltd. (RIKARESIN BEO-60E as the trade name) was used as an additional epoxy component to fabricate the flexible monolith sheet. 4,4’-Methylenebis(cyclohexylamine) (BACM) was purchased from Tokyo Chemical Industry Co., Ltd. and used as a curing agent. Poly(ethylene glycol) (PEG, Mn = 200) was purchased from Tokyo Chemical Industry Co., Ltd. and used as a pore-forming agent (i.e., porogen). Poly(vinyl alcohol) (PVA, Mn = 500) was purchased from Fujifilm Wako Pure Chemical Industries, Ltd. and used as the sacrificial layer material to peel the monolith sheet from a glass plate after the curing reaction [31]. The chemical structures of the reagents used for the fabrication of the epoxy monolith are shown in Fig. 1. All reagents were used as received without purification.

An epoxy monolith sheet was produced via the following procedure. First, a PVA aqueous solution (1 wt% in ion-exchanged water) was applied onto a glass plate and then heated and dried at 110 °C for 1 h to form a PVA sacrificial layer on the glass plate surface. BADGE, BATGE, BACM and PEG were mixed at γ (2[NH2]/[epoxy]) = 1.0–1.8 and ω = 0.3 and rotated for 10 min at 2000 rpm using a planetary stirrer (ARE-250, Thinky Co., Ltd.). Here, γ and ω are the values defined by Eqs. (1) and (2), respectively [28, 32, 36].

A monolith sheet with a predetermined thickness was obtained by applying the mixture onto a PVA-coated glass plate, using a stainless-steel plate with a thickness of 100–300 μm as the spacer, and placing another glass plate on top. After curing at 130 °C for 30 min, a monolith sheet was peeled off from the glass plate and washed in ion-exchanged water for 15 min under irradiation with ultrasonic waves to remove the porogen. After immersion in fresh ion-exchanged water for 12 h, the monolith sheet was dried for 12 h in a vacuum oven (ESPEC Co., LCV-233, 40 °C). The thickness of the monolith sheet after drying was measured using a digital thickness gauge manufactured by the Mitsutoyo Corporation. Monolith sheets with thicknesses of 200–250 μm were used for measurements of mechanical properties, and monolith sheets with thicknesses of 80–120 μm were used for lap-shear tensile tests to evaluate their adhesion strength for metal bonding.

Scanning electron microscopy (SEM) was performed using a TM4000 instrument (Hitachi High-Technology Co., Ltd.) at an accelerating voltage of 15 kV after Pt vapor deposition. The monolith sheets were cut into 6 strips with the dimensions of 10 mm × 40 mm and were subjected to a tensile test using an AGS-10KN instrument (Shimadzu Co., Ltd.). Dynamic mechanical analysis (DMA) was performed using an EXSTER6000 instrument (Hitachi High-Technology Co., Ltd.) at a heating rate of 2 °C/min and a frequency of 1 Hz.

An SPCC manufactured by Engineering Test Service Co., Ltd. was used as the adherent for the lap-shear tensile adhesion test. Because the SPCC was coated with oil to prevent oxidation, it was degreased with acetone and irradiated with ultraviolet light (254 nm) using a low-pressure mercury lamp (11–13 mW/cm2) immediately prior to the preparation of the test sample. A test piece with an adhesion area of 25.0 mm × 12.5 mm was prepared according to JIS 6850 K (ISO 4587). Figure 2 shows the procedure for the preparation of the adhesive test samples [38]. The thickness of the adhesive was adjusted to ca. 0.1 mm using glass beads (SPL-100, Unitika Ltd.) or monolith sheets. A liquid epoxy adhesive (CLS-1194, ADEKA Co., Ltd.) was used and cured at 180 °C for 30 min. A lap-shear tensile adhesion test was conducted at room temperature and at a tensile rate of 10 mm/min using an AGS-10KN instrument (N = 3).

Schematic representation of the fabrication of a test piece for the lap-shear tensile adhesion test. The sheet-type adhesive is constructed with an epoxy monolith sheet and a commercial liquid adhesive

The TDCB test was conducted at room temperature and a rate of 1 mm/min using a steel (S45C) bonded structure that complies with ASTM D3433 (Fig. S1 in the supplemental materials). The TDCB test is a DCB test with a tilt angle that increases the height of the sample as the crack propagates. The rate of change in the compliance (displacement/load) as the crack propagates varies depending on the tilt angle. When the rate of change is 0, the stress intensity factor remains constant regardless of the crack length. In this study, the Irwin-Kies equation (Eq. 3) was used to calculate the peel energy GIC [J/m2] for mode I (open type) of the TDCB sample [39,40,41,42]. Here, P is the load [N], C is the compliance (displacement/load) [mm/N], a is the crack length [mm], and b is the width of the sample (here, 25.4 mm). The dC/da value was measured from the fracture slope at the initial stage of measurement, and the average value of the load after the initial peak appearance to the fracture was used to determine the P values.

For test samples with different thermal expansion coefficients α, iron (SPCC, α = 12 ppm/K) and the iron-based alloy Super Invar (Fe-32Ni-5Co alloy, manufactured by Kirin Steel Co., Ltd., α = 0.5 ppm/K) were used. The bonding area was 25.0 mm × 40.0 mm. The adhesive test sample was left standing in a thermal shock test chamber (TSE-11, ESPEC Co., Ltd.) at –20 °C and 130 °C repeatedly. After the temperature was varied, the test samples reached the predetermined temperature after ca. 15 min and were held for 15 min each. After a predetermined number of heat cycles, a lap-shear tensile test was conducted at room temperature at a tensile rate of 10 mm/min.

Figure 3a–f show cross-sectional SEM images of the epoxy monoliths prepared under the reaction conditions at γ = 1.0–1.8 after curing at 130 °C for 30 min and the subsequent removal of the porogen. The size of the skeleton and pores of the monolith produced at γ = 1.0 was less than a few μm. As the γ value increased, the periodical lengths of both the epoxy skeleton and the pores gradually increased. When fabricated at γ = 1.8, the size reached ca. 100–150 μm. In addition, spherical epoxy fine particles were produced within the voids, and many submicron-scale pores were observed within the epoxy framework at the same time. Focusing on the change in the shape of the entire monolith material according to the increase in the γ value from 1.0 to 1.8, it is observed that the formation of continuous structures began to partially break down for γ greater than 1.4. The monolith sheets prepared at γ = 1.8 included large and rough structures of the pores and epoxy skeletons. These sheets were difficult to handle in some cases because they were too fragile. The morphological changes in the formation of monolith frameworks and pores depending on the γ values can be explained as follows. In the presence of the pore-forming agent PEG, the size of the monolith framework and pores is determined by the rate of Ostwald ripening (i.e., the rate of the expansion of mutually interpenetrating phase-separated structures) in competition with the curing process of the epoxy resin; this is because the relative rates of phase separation promotion associated with the polymer chain extension reaction depend on the γ value (a ratio of epoxy and amine curing agent) before the monolith skeleton structure is fixed through the formation of branched and cross-linked structures [36, 37]. Furthermore, secondary phase separation further progresses within each of the two separated phases (highly viscous epoxy-rich phase and smooth liquid PEG-rich phase) formed during the Ostwald ripening of the phase separation [37]. In the second phase separation step, island-in-sea phase separation occurs. In the SEM images shown in Fig. 3, fine epoxy particles appear in the voids. At the same time, the PEG-rich spherical liquid particles leave holes in the monolith skeletons after PEG removal.

Cross-sectional SEM images of the epoxy monoliths fabricated under the conditions of a γ = 1.0, b γ = 1.2, c, d γ = 1.4, e, f γ = 1.8, g, h γ = 1.4 in the presence of 5 mol% BATGE, and i γ = 1.4 in the presence of 20 mol% BATGE

Figure 3g, h, i show cross-sectional SEM images of the epoxy monoliths when BATGE (5 and 20 mol%, respectively) was added to the curing system at γ = 1.4. As reported in our previous papers [36, 37], when an epoxy resin that retards the progress of phase separation is added to the reaction system, even for the same γ value, the sizes of the monoliths and pores decrease. In the present study, a monolith sheet with a dense structure containing submicron pores was generated when BATGE was increased to 20 mol%, as shown in Fig. 3i.

The pore diameters estimated from the SEM images are summarized in Table 1. The average values shown here are mainly for the large pores formed by primary phase separation. It was difficult to accurately quantify the small pores formed by secondary phase separation and to distinguish two kinds of pores formed via different steps. Table 1 also shows the mechanical properties evaluated by the tensile test of the monolith sheet as well as the Tg and storage modulus (E’) determined by DMA measurements at a frequency of 1 Hz and a heating rate of 2 °C/min. Figure 4 shows typical DMA curves for the epoxy monolith sheets prepared under different γ values and with the addition of BATGE.

DMA curves for the epoxy monolith sheets prepared with different γ values and the addition of BATGE. a γ = 1.0, b γ = 1.4, and c γ = 1.8 in the absence of BATGE, and d γ = 1.4 in the presence of 5% BATGE

The mechanical features of the monolith sheets are summarized as follows. The strength at break of the monolith sheets was almost constant (8.1–9.9 MPa) at room temperature regardless of the γ value, except for γ = 1.8. Similarly, the elongation at break ranged from 8.6–11.1%. By contrast, the monolith sheet prepared with γ = 1.8 exhibited higher flexibility. The strength at break decreased to 6.4 MPa, and the elongation at break increased to 13.4%, which was due to the increased plasticity of the monolith fabricated under the large γ condition. As expected, the mechanical properties of the monolith sheet changed significantly with the addition of a small amount of BATGE. For the addition of 5 mol% BATGE, the strength increased to 12.7 MPa, and then began to decrease with further BATGE addition. The strength decreased to 3.9 MP after 20 mol% addition. The elongation behavior showed similar results upon BATGE addition. Although a drastic increase in the elongation was observed (17.1%) with the addition of a small amount of BATGE, this enhanced elongation disappeared with further BATGE addition. Thus, adding a small amount of a flexible epoxy resin component effectively improves the mechanical properties of the cured epoxy product, whereas the addition of excess amount of the additives degrades the mechanical properties.

The Tg value of the monolith sheets was estimated from the temperature of the tanδ peak in the DMA curves. As the γ value increased from 1.0 to 1.8, Tg decreased from 125 °C to 96 °C. This result is in good agreement with the results obtained in our previous work [43]. The monolith sheets used in this study are in a glassy state at room temperature and exhibit a high E’ value (0.39–0.43 GPa) regardless of the γ value, except for γ = 1.8. Even when a small amount of BATGE was added, there was almost no significant effect on the E’ value at room temperature. However, when a large amount of BATGE was added (20 mol%), the flexibility of the monolith sheet increased significantly, making it difficult to perform DMA measurements under the same conditions as those for the other samples. The DMA results are consistent with the results of the tensile test. The E’ values exhibited a moderate temperature dependence. The E’ values at 90 °C were 102–10−1 MPa, depending on the γ values and the amount of BATGE added for γ  > 1.2, indicating that the elastic modulus begins to decrease at a temperature close to Tg. At 155 °C, the E’ value further decreased to 1–10−1 MPa even though the macroscopic flow was restricted by the cross-linked structure.

As shown in Fig. 2, sheet-type epoxy adhesives were prepared by impregnating a commercially available epoxy adhesive (CLS-1194) into the pores of the monolith sheets, which were fabricated under curing conditions of γ = 1.0–1.8 with or without BATGE. The adhesion structures using SPCC as the adherent were subjected to lap-shear tensile adhesion tests. The lap-shear adhesion strength was evaluated under three temperature conditions based on the Tg of the monolith sheets, i.e., at 23 °C (lower than Tg), 90 °C (near Tg), and 155 °C (above Tg). The Tg of the cured epoxy adhesive was as high as 242 °C in the absence of the epoxy monolith sheet (Fig. S2). The results of the tensile adhesion test are shown in Table 2. Typical stress–strain curves for various adhesion systems are shown in Fig. 5. When the tensile test was performed at 23 °C the adhesion strength of the monolith sheet decreased (16.1–16.8 MPa for runs 2–6 in Table 2) compared with the adhesion strength without the monolith sheet (17.9 MPa for run 1). This is because the monolith sheets are more flexible and their strength is lower than that of the corresponding epoxy thermoset. When the test was performed at 90 °C for the adhesives, including the monolith sheets (γ = 1.4 and 1.6), the lap-shear adhesion strength improved to 20.3–20.8 MPa (runs 4 and 5). The use of the monolith sheet containing BATGE (5 mol%) further increased the adhesion strength, i.e., 20.7 MPa and 23.8 MPa at 23 °C and 90 °C respectively (run 7). Unlike conventional adhesion systems that use elastomers or fine particles, epoxy materials with different elastic moduli [12,3.0.CO;2-B " href="#ref-CR13" id="ref-link-section-d13778851e1822_1">13,14,15,16] are spatially intertwined to form an adhesive layer in our sheet-type epoxy adhesive. In the present study, a flexible and stretchable monolith sheet coexists with a hard epoxy adhesive to fill the pores inside the interwinding adhesive structures. The adhesion strength rapidly decreased at 155 °C as shown in Table 2 and Fig. 5, whereas the epoxy adhesive thermoset maintained a high E’ (16.7 MPa) even at 155 °C because of the high Tg at 242 °C (see also Fig. S2). The hard epoxy phase remains in a glassy state even under high-temperature conditions. Nevertheless, the adhesion strength of the sheet-type adhesives used as composite materials drastically decreased. These findings suggest that the adhesion strength of the entire sheet-type adhesive was determined by the mechanical properties of the soft monolithic sheets as the rubbery component. As summarized in Fig. S3 and Table S1 of the supplementary materials, the tensile strength of the thermosets of the adhesion structures themselves decreased with increasing γ value under high-temperature conditions, particularly at 155 °C. The general characteristics of the tensile test results of the sheet-type adhesives were similar to those of the lap-shear adhesion test results in Table 2. Moreover, the elongation of the sheet-type adhesives increased with increasing γ value and temperature as well as with the addition of BATGE (Fig. S3), as expected.

Stress–strain curves for the sheet-type adhesion systems determined by the lap-shear tensile adhesion test at a 23 °C b 90 °C and c 155 °C. Also see Table 2

To investigate the effect of the flexible monolith sheet on the relaxation of residual stress localized near the adhesive interface after the curing of the adhesive, we examined the morphology of the fractured surface after the lap-shear tensile adhesion test. Figure 6 shows typical SEM images of the fracture surfaces of the adhesives at various temperatures. The fracture mode depended strongly on the presence or absence of the monolith sheet. While in the absence of the monolith sheet, the predominant interfacial failure was observed to be independent of the temperature, the failure mode changed to partial cohesive failure when the monolith sheet was used (see also Figs. S4 and S5). Generally, a hard epoxy adhesive has a high cohesive strength, and the adhesive itself is sufficiently rigid to withstand stress. As a result, cracks develop from the strained area around the interface, leading to interfacial failure of the entire adhesion structure. However, when stress was applied to the adhesion layer of the sheet-type epoxy adhesives developed in this study, both the hard epoxy adhesive component inside the monolith sheet and the monolith skeleton itself were destroyed competitively, and the cracks evolved in the adhesion layer, resulting in a complex three-dimensional shape. It is predicted that the fracture will occur via a complex process and that the total energy required for the entire fracture will increase (as described below). The stress generated by the curing process and the usage temperature are important for improving the reliability of adhesion systems [44,45,46,47,48].

SEM images of the fracture surfaces of the sheet-type epoxy adhesives consisting of the commercial epoxy adhesive and the epoxy monolith sheets after the lap-shear tensile adhesion test at 23 °C 90 °C, and 155 °C. a Without a monolith sheet (the epoxy adhesive surface exposed by interfacial failure), b with a monolith sheet (γ = 1.0), and c with a monolith sheet (γ = 1.4) in the presence of 5 mol% BATGE

The flexibility of the monolith increases with increasing γ and the amount of added BATGE. When the temperature was increased to 90 °C cavitation was observed in some areas of the fracture surfaces. Cavitation likely occurred in the monolith skeleton, which became sufficiently soft at temperatures conditions close to Tg. Accelerated cavitation was caused by the addition of BATGA. It has been reported that the cavitation of soft components added to an epoxy curing system plays an important role in energy dissipation to suppress the destruction of adhesives. It is assumed that the phenomena observed for the sheet-type epoxy adhesive systems resemble those reported in the literature using elastomer particles [49,50,3.0.CO;2-W " href="#ref-CR51" id="ref-link-section-d13778851e2212_2">51,52,53,54]. Many small spherical pores are formed in the skeleton of the monolith sheet for γ = 1.4 or higher, as shown in Fig. 6. The pores are discontinuous and are independent of the voids included in the monolith. When the temperature was further increased to 155 °C many cavitations concentrated in specific areas formed, as shown in the SEM images. Since both the monolith sheet and the adhesive are epoxy resin, it is not possible to distinguish the soft epoxy monolith skeletons and the hard epoxy adhesive components during SEM observation. It can be inferred intense cavitation is located in the same area as the monolith sheet. Thus, replacing a part of the adhesive layer with a soft material effectively induces stress relief and suppresses interfacial failure.

To examine the origin of the cavitation at high temperatures, we examined SEM images of the inner structure of the sheet-type epoxy adhesives prior to the tensile adhesion test. The cross-sectional SEM images confirmed the filling of the epoxy adhesives into the pores of the epoxy monolith sheets (Fig. S6). In the SEM images, some spots with the sizes of a few μm were observed when the monolith sheet was present. These spots may be due to voids, similar to those observed in the images of the fractured surfaces after the tensile adhesion test conducted at 23 °C (Fig. 6). Thus, it has been confirmed that the epoxy adhesives are well filled into large pores (more than several μm) with a co-continuous structure, but it is still possible that the filling is incomplete, particularly for the small pores at the submicron scale, which are formed by secondary phase separation during epoxy monolith fabrication. The small voids are likely starting points for cavitation during heating, whereas no cavitation was observed for the epoxy adhesive without the monolith sheet even after heating at 155 °C as shown in Fig. 6a. The difference in the filling state of the epoxy adhesive is considered to be one of the causes of the difference in the ease with which cavitation occurs. As noted in our previous paper [38], the liquid epoxy adhesive used in this study has a high viscosity (4.0 Pa s). The penetration of the high-viscosity epoxy adhesive into the small pores of the monolith is difficult when a large amount of BATGE is used.

The TDCB test was conducted on the adhesive test sample by combining the monolith sheet (γ = 1.0, 1.4, and 1.8 without BATGE, and γ = 1.4 with 5 mol% BATGE) and a commercially available epoxy adhesive, CLS-1194, to estimate the fracture energy (GIC) for each adhesion system. Figure 7 shows the relationship between the displacement and load in the TDCB test and the results for the comparison of the GIC values calculated based on the results of this test. A higher strength at the steady state in the displacement–load curve corresponds to a higher fracture energy. Moreover, the resistance to destruction increases with greater crack extension distance. These changes result in an increase in the fracture energy. The accurate GIC value is determined according to Eq. 3. The GIC value gradually increased with increasing γ value. When the monolith sheet with γ = 1.8 was used, the GIC value was approximately twice as high as that without the monolith sheet. When the epoxy monolith prepared at γ = 1.4 in the presence of BATGE was used, the fracture energy increased more dramatically, reaching a GIC value of ca. 1.6 kJ/m2. This value is 6 times greater than that for the case without the monolith and is approximately 1.5 times greater than that in the absence of BATGE under the same condition of γ = 1.4. In the TDCB test, a change from interfacial failure to cohesive failure was observed when the epoxy monolith was used in combination with BATGE (Fig. S4), similar to the lap-shear tensile adhesion test.

Results of the TDCB test for the sheet-type epoxy adhesives consisting of the commercial epoxy adhesive (CLS-1194) and the epoxy monolith sheets. a Typical displacement–load rela tionships and b comparison of the GIC values for each adhesion system

Finally, the heat cycle resistance of the adhesive structures containing the epoxy monolith sheet was evaluated by the dissimilar materials bonding tests. In this study, dissimilar metals with different coefficients of thermal expansion α, i.e., iron (α = 12 ppm/K) and Super Invar (α = 0.5 ppm/K), were used as the adherents. Figure 8 shows the results for the test conducted with a repeated temperature change between –20 and 130 °C. In the absence of the monolith sheet, the initial strength of the adhesive (ca. 3 MPa) decreased linearly to less than 1 MPa after the application of thermal stress for 300 cycles. Similarly, the adhesion strength decreased for the adhesives when the monolith sheets with γ = 1.0 and 1.8 were used. By contrast, when monolithic sheets with γ = 1.4 and BATGE addition were used, excellent heat-cycle resistance was observed. In the sample with γ = 1.4 in the absence of BATGE, it was confirmed that the thermal history during the test caused an unexpected increase in the adhesion strength. The deterioration of the adhesion strength is due to the insufficient ability of the monolith sheets prepared at γ = 1.0 to act as energy absorbers for stress relief. For the monolith sheets prepared at γ = 1.8, the excessively large and more disordered co-continuous structure of the adhesives may be one of the reasons for the insufficient performance of the sheet-type adhesive to release the concentrated stress during the heat cycle test. We also tried to improve the heat cycle properties of the epoxy adhesive by adding fine particles consisting of elastomeric polyolefins. Preliminary results showed that the resistance to heat cycles was not improved; however, the experimental conditions were not optimized; this may be due to the poor dispersion properties of the additive.

Changes in the lap-shear tensile adhesion strengths of various sheet-type epoxy adhesives after heat cycling

Based on the experimental results obtained in this study, a failure mechanism model for a lap-shear tensile adhesion test of the sheet-type epoxy adhesives using monolith sheets is presented in Fig. 9. When only a hard epoxy adhesive is used, the applied stresses are concentrated near the adhesive interface. During the initial failure stage, cracks form at both ends of the adhesive layer. However, when the hard epoxy adhesive penetrates into the pores within the soft monolith sheet to form a co-continuous structure, the stress generated under various thermal conditions is dispersed within the composite adhesive layer. A greater amount of energy is necessary to destroy the adhesive structure containing the flexible epoxy monolith sheet. The monolith sheet fabricated at γ = 1.4 in the presence of BATGE exhibited the best performance in the present study, as shown in Table 2 and Fig. 7. Thus, the co-continuous structure consisting of the hard epoxy adhesive and the flexible monolith sheets may contribute to improving the toughness of the sheet-type epoxy adhesives. The system using the monolith sheet prepared under specific conditions exhibited the highest GIC value for metal bonding and excellent heat cycle properties for dissimilar material bonding, but the detailed mechanism has not yet been elucidated. These issues must be addressed in future research.

Schematic of the fracture mechanisms for a hard epoxy adhesive in the absence and presence of a flexible epoxy monolith sheet

In this study, we propose a new sheet-type adhesive consisting of a conventional epoxy adhesive and an epoxy monolith sheet. This adhesion system combines a liquid epoxy adhesive with an epoxy monolith sheet with internal continuous pores to alleviate stress concentration and improve the toughness of epoxy adhesives. First, we fabricated epoxy monolith sheets with different pore sizes by varying the epoxy/amine ratio using BADGE and BATGE as the epoxy resins, BACM as the amine curing agent, and PEG as the porogen. The features of metal bonding using the sheet-type epoxy adhesives consisting of an epoxy adhesive and monolith sheets are summarized below.

The pore diameter and mechanical properties of the epoxy monolith sheets can be controlled by adjusting the γ value and the presence or absence of BATGE. The adhesion strength at break of the sheet-type epoxy adhesives varied depending on the γ value and the amount of added BATGE.

While a hard epoxy adhesive caused interfacial failure due to the high rigidity in the absence of the monolith sheet, destruction occurred inside the adhesive layer, and the failure mode changed to cohesive failure in the presence of the monolith sheet. The combination of the epoxy adhesive and the monolithic sheet effectively improved the total fracture energy, which was measured via the TDCB test.

In the heat cycle test for metal bonding using adherents with different coefficients of thermal expansion, no deterioration occurred after 300 cycles when the monolith sheets with γ = 1.4 and 5 mol% BATGE were used.

In this study, we found that the use of epoxy monolith sheets is an effective method for avoiding stress concentrations in epoxy adhesives. Generally, when liquid or rubber-like additives are used, difficulties may be encountered in controlling adhesive thickness, but the use of a monolith sheet helps to avoid these problems. In addition, the monolith sheet can be handled as a tape or sheet-like adhesive material, and is advantageous for designing simpler adhesive processes and materials in various practical applications. The results obtained in this study revealed that interposing the monolith sheet in an epoxy adhesive for metal bonding disperses the thermal stress and increases the fracture energy. Taking advantage of the ability of flexible monolith sheets to alleviate the residual stress generated during the curing process of joining dissimilar materials and the thermal stress generated due to heat cycles, these materials can be applied to adhesive bonding systems for various dissimilar materials in the future.

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Advanced Technology R&D Center, Mitsubishi Electric Corporation, 8-1-1, Tsukaguchi-Honmachi, Amagasaki, Hyogo, 661-8661, Japan

Yoshiyuki Kamo

Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka, 599-8531, Japan

Yoshiyuki Kamo & Akikazu Matsumoto

Department of Applied Chemistry, Graduate School of Engineering, Osaka Metropolitan University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka, 599-8531, Japan

Akikazu Matsumoto

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Correspondence to Yoshiyuki Kamo or Akikazu Matsumoto.

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Kamo, Y., Matsumoto, A. Stress relaxation and improved fracture toughness of metal bonding using flexible monolith sheets and an epoxy adhesive. Polym J (2024). https://doi.org/10.1038/s41428-024-00975-w

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Received: 05 August 2024

Revised: 06 September 2024

Accepted: 11 September 2024

Published: 23 October 2024

DOI: https://doi.org/10.1038/s41428-024-00975-w

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