Metal-resin bonding mediated by epoxy monolith layer
© The Author(s) 2016
Received: 9 September 2016
Accepted: 23 November 2016
Published: 28 November 2016
An epoxy monolith layer with porous structure is fabricated on the surface of a stainless steel (SUS) plate by polymerization induced phase separation process as the mediator for the bonding of SUS and various thermoplastic resin plates. Bonding strength is evaluated in the presence and absence of the epoxy monolith layer by a tensile lap shear test. The morphology of fracture surfaces is observed by scanning electron microscopy (SEM) in order to clarify the anchor effect of molten resins into the pores of the epoxy monoliths. The bonding strength values are calculated to be 1.2‒2.7 MPa based on an apparent adhesion area for the bonding of SUS with polyethylene, polypropylene, polyoxymethylene and acrylonitrile–butadiene–styrene copolymer in the presence of the epoxy monolith mediator. These values are 2‒30 times higher than those for direct metal-resin bonding. By the SEM observation, stretched needle-like structures were detected on the both fracture surfaces of the resins and the epoxy monoliths. The direct observation of the stretched debris out of the holes located at the monolith surfaces indicates the significant anchor effect for the present metal-resin bonding system. The bonding system mediated by the epoxy monolith layer is conveniently used for the bonding of dissimilar materials such as metals and resins without any special process and apparatus.
Tough and reliable bonding between dissimilar materials is one of the most important and challenging topics in the fields of adhesion and adhesives because metal components used for automobiles, aircrafts, and mechanical parts have increasingly been replaced by lightweight plastics and polymer composites during recent years [1, 2]. Mechanical interaction between rough surfaces accompanying an anchor effect as well as chemical bond formation such as reacting-type adhesives are important to obtain reliable bonding systems. For the dissimilar materials bonding such as metals and resins, closer contact and good affinity are required at an interface between both adherends with completely different surface properties [3–10]. Especially, the surface modification of metals has been considered to be a key process. In fact, many techniques by chemical conversion, chemical etching, plasma and laser treatments, as well as primer coating have been developed in order to realize high-strength adhesion for metal-resin bonding during injection, insertion, and transfer moldings and heat pressing processes [1, 2]. On the other hand, the surface structures of polymers are readily controlled using several phase separation systems. Monolithic porous materials consisting of polymers or inorganic compounds have been prepared by thermally induced phase separation , non-solvent induced phase separation , and polymerization induced phase separation . They are mainly used as separation filters, column packing materials for ion exchange and chromatography, catalyst supports, and electrical or thermal insulators due to their high strength and porosity [14–18]. Among them, it should be noted that Tsujioka and coworkers reported the fabrication of epoxy monoliths with a bicontinuous and highly porous structure and its application to the column packing materials for high-performance chromatographic and reacting systems . The porous epoxy monoliths are expected to be applied to the modification of metal surfaces and adhesion to dissimilar materials including resins, but no report has ever been seen for the research on the dissimilar materials bonding using epoxy monoliths in the literature. In this study, we propose a new type of metal-resin bonding technique using monolithic epoxy resins as the adhesive layer manufactured on a metal surface by polymerization induced phase separation process. The bonding strengths were determined for the bonding system using a stainless steel (SUS) plate and polyethylene (PE), polypropylene (PP), polyoxymethylene (POM), and acrylonitrile–butadiene–styrene copolymer (ABS) as the thermoplastic resins in the presence of an epoxy monolith layer as the mediator for the robust bonding. The morphology of fracture surfaces was also investigated in order to discuss an anchor effect on this bonding system.
Thickness of the resin plates and heat welding conditions used in this study
Thickness of plate (mm)
Heat welding conditions
2,2-Bis(4-glycidyloxyphenyl)propane (BADGE), 4,4′-methylenebis(cyclohexylamine) (BACM), and poly(ethylene glycol) (PEG200, M n = 200) were purchased from Tokyo Chemical Industry, Co., Ltd., Japan and used without further purification. The resin plates (thickness = 2‒5 mm) and SUS430 plate (thickness = 0.5 mm) were purchased from ASONE Co., Japan and cut to a size of 10 mm × 50 mm.
Preparation of epoxy monolith
To a mixture of BADGE and BACM at a ratio of 2[NH2]/[epoxy] (γ value) = 0.83‒1.55, added was PEG200 (70 wt%) as the porogen for the preparation of the epoxy monolith, according to the method described in the literature . The flat epoxy layer was prepared at the condition of γ = 1.0 without PEG200 in the presence of a few drops of ethanol for decreasing the viscosity of a reactant mixture. The mixture spread on a SUS plate was cured at 120 °C for 20 min. After PEG200 was removed with water, the epoxy monolith fabricated on a SUS plate was dried in vacuo at room temperature for 2 h.
Results and discussion
Bond strength of metal-resin specimens with or without epoxy monolith mediator
Bond strength based on apparent adhesion area (100 mm2) (MPa)
Imaginary bond strength based on pore cross-section area (MPa)
Breaking strain for SUS-resin specimen (%)
Modulus for SUS-resin specimen (GPa)
Modulus for resin plates (GPa)
Via epoxy monolith
Via flat epoxy
2.00 ± 0.22
0.63 ± 0.21
28.2 ± 5.5
2.3 ± 0.6
0.4 ± 0.1
2.20 ± 0.31
0.63 ± 0.21
1.6 ± 0.5
0.4 ± 0.02
1.36 ± 0.20
12.6 ± 3.4
1.2 ± 0.4
0.4 ± 0.1
1.22 ± 0.57
17.0 ± 6.4
0.9 ± 0.4
0.8 ± 0.3
2.66 ± 0.74
35.3 ± 20.6
1.4 ± 0.5
0.8 ± 0.1
In this paper, we demonstrated a new metal-resin bonding method using epoxy monolith as the adhesive mediator between metal and resin adherends. The anchor effect for the robust bonding of dissimilar materials was directly confirmed by the SEM observation of the epoxy monolith and resin fracture surfaces. The epoxy monoliths have recently been reported to be useful as the separation materials due to their bicontinuous and highly porous structure. In this study, we proposed the dissimilar materials bonding using the epoxy monolith layers on a metal surface in order to effectively induce an anchor effect during the bonding with various resins. This method requires only conventional chemicals and simple procedures for the bonding process and is expected for application to various types of materials and bonding processes in future. Thus, it has been demonstrated that the bonding system with the epoxy monolith mediator is conveniently used for the bonding of dissimilar materials such as metals and resins without any special process and apparatus.
AM designed the study and prepared the manuscript. FU carried out all the experiments for materials preparation and measurements. Both authors read and approved the final manuscript.
The authors thank to Prof. Masaya Matsuoka and Dr. Yu Horiuchi, Osaka Prefecture University for the surface area determination by the BET method.
The authors declare that they have no competing interests.
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