A Boron-based Adhesion Aid for Efficient Bonding of Silicone Rubber and Epoxy Resin

WANG Andong, ZHOU Peng,,3, TANG Xiaolin,,3, YI Shengping,,3, ZENG Qihui,ZHANG Zhiqiang, HU Mingjie,,3, LIAO Jun,,3*, HUANG Chi,,3*

(1. College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China; 2. Hubei Key Laboratory of Advanced Aerospace Propulsion Technology, Wuhan 430040, China; 3. Advanced Technology Research Institute (Jinan), Beijing Institute of Technology,Jinan 250300, China)

Abstract: We improved the adhesion between silicon based insulating materials and epoxy resin composites by adding the adhesion promoter cycloborosiloxane (BSi, cyclo-1,3,3,5,7,7-hexaphenyl-1,5-diboro-3,7-disiloxane). The experimental results show that the addition of BSi in the silicone rubber (SR) system significantly increases the tensile shear strength between BSi and epoxy resin (EP), reaching 309% of the original value. On this basis, the mechanism of BSi to enhance the adhesion effect was discussed. The electron deficient B in BSi attracted the electron rich N and O in EP to enhance the chemical interaction, combined with the interfacial migration behavior in the curing process, to improve the adhesion strength. This study provides the design and synthesis ideas of adhesive aids, and a reference for further exploring the interface mechanism of epoxy resin matrix composites.

Key words: boron based adhesion promoter; epoxy resin composites; silicone rubber; adhesion mechanism

1 Introduction

Epoxy resin (EP) composites are multiphase solid materials composed of EP and reinforcement materials such as glass fibers, carbon fibers, microcapsules,nanomaterials and so on[1-3]. High performance EP composites have excellent specific stiffness, mechanical strength, chemical inertness, electrical insulation and corrosion resistance, which plays an extensive and irreplaceable role in cutting-edge fields such as aerospace, electronics and electrical appliances[4-9].However, the poor high temperature performance,especially under extreme high temperature conditions,is gradually becoming one of the main factors restricting the further development and application of EP composites[10-12]. Owing to its resistance to heat,cold, weather, UV and ozone, excellent electrical properties as well as physical inertia, silicone based thermal insulation materials can protect EP composites from the corrosion of internal and external heat sources[13-15]. However, the main chain of RTV,polydimethylsiloxane, embraces many hydrophobic organic groups, which shield the polarity of Si-O bonds. Therefore, SR has low surface free energy. It is very difficult for SR to achieve high adhesion to common polar substrates like EP composites, which limits further applications in this domains[16-18].

In order to improve the adhesion property of SR,many methods have been developed, such as sand blasting, coating[19], plasma treatment[20], polar groups modification[21], introduction of adhesion promoters[22],etc.Among them, the adhesion promoters have groups that can form chemical bonds or physical interactions with SR and EP. During the curing process, it moves to the interface between SR and EP, simply and efficiently improving the adhesion strength between them[23,24]. At present, bisphenol A epoxy resin rich in phenyl structure occupies a major share in EP industry.Yoshizawaet al[25,26]disclosed that the benzene rings play an important role in the adhesion forces working at interfaces between EP and other materials via theoretical chemical calculation. The adhesion interaction of EP with the surfaces of h-BN and graphite are mainly attributed to π - π stacking between benzene ring and large conjugate structure. The interfacial interactions of EP with hydroxylated silica (0 0 1) andγ-alumina (001) surfaces come from hydrogen bonds and OH-π interactions between the hydroxyl groups on the surfaces and the benzene rings of EP.Compared with π - π stacking and OH-π interactions,the interaction between electron deficient boron atom and benzene ring is stronger. In fact, boron is widely used in high temperature resistant adhesives due to its strong electron deficiency and large bond energy with common polar atoms such as nitrogen and oxygen[27-29].Moreover, with high thermal stability, boron based adhesion promoters can ensure them tight bonding at high temperatures. Thus they have broad application prospects.

In previous studies, boron based adhesion promoters have been used to improve the adhesion between SR and polar materials like EP[30-32]. Although some achievements have been made in these works,the research on its mechanism is not in-depth, therefore difficult to give full play to the adhesion promotion of boron atoms. In this study, by using the synthesis methods of high-purity BSi[33-35], boron atoms and silicon atoms were combined as an adhesion promoter,which can not only ensure its dispersion and migration in SR, but also maintain its adhesion on EP surface.At the same time, benzene ring and epoxy group are introduced into the adhesion promoter. On the one hand, it improved its thermal stability and water resistance. On the other hand, it further enhanced the bonding effect via π-π stacking between benzene rings,thereby enhancing its adhesion promotion ability.

In this study, a boron containing adhesion promoter BSi was designed, the chemical composition and structures of BSi were characterized via Fourier transform infrared spectra (FT-IR), nuclear magnetic resonance spectroscopy (1H- and13C-NMR), and X-ray diffraction pattern (XRD). In addition, the adhesion mechanism of BSi was explored under scanning electron microscopy (SEM) and energy dispersion spectrometers (EDS).

2 Experimental

2.1 Materials and chemical reagents

Phenylboronic acid was purchased from Shanghai McLean Biochemical Technology Co., Ltd.Diphenylsilanediol was purchased from Shanghai Bide Pharmaceutical Technology Co., Ltd. Room temperature vulcanized liquid silicone rubber of condensed type (RTV) was prepared in laboratory.E51 epoxy resin (E51) was purchased from Baling Petrochemical Co., Ltd. m-Phenylenediamine (MPD)was purchased from China Pharmaceutical Group Co.,Ltd.

2.2 Preparation of materials

2.2.1 Preparation of BSi

A mixture of equal molar amount of phenylboronic acid and diphenylsilanediol was refluxed under 110 ℃,with continuous removal of water via a Dean and Stark apparatus. After reflux reaction for 12 h, the solvent toluene was removed by vacuum distillation. The white powder product was recrystallized with the mixture of chloroform and toluene (volume ratio 1:3) to remove impurities and obtained white powder crystals (Scheme 1).

Scheme 1 Synthetic route of BSi

2.2.2 Curing test of SR and preparation of tensile and shear samples

The mixture of boron containing adhesion promoter and RTV-A was kneaded in the kneader for 4 h to make them fully mixed. Then add RTV-B of corresponding quality according to the mass ratio of 10:1 and then mix it evenly by hand. The raw materials used in each group of experiments are shown in Table 1.

Table 1 Specific scheme of curing test

Table 2 The content of each element inside SR of BSi-1.5

Table 3 Contents of various elements in a small amount of B and Si aggregates at SR and EP interfaces in BSiO-1.5

Table 4 Content of elements at B and Si non aggregation points at SR and EP interfaces of BSiO-1.5

Table 5 Contents of various elements in a large number of B and Si aggregates at the SR and EP interfaces of BSiO-1.5

The above mixed rubbers were applied to one end of the cleaned epoxy resin sheet and spliced in pairs to obtain a shear sample. The remaining mixed rubbers were spread on the PTFE plate to prepare rubber samples. All the simples were vulcanized under controlled condition of 25 ℃, RH 60% for 48 h. Then,surplus RTV to the shear samples was removed by a scalpel. The rubber samples were cut into tensile spline by cutter.

2.3 Characterization and catalytic performance test

A Nicolet iS50 FT-IR (Thermo, America) was used to record the FT-IR spectra of two boron-based adhesion promoters BSi and its raw materials in the wave number range of 4 000 to 400 cm-1. All the liquid samples above were coated on the surface of KBr pellets. The chemical structures of BSi were characterized via AVANCE III HD 400MHz nuclear magnetic resonance spectroscopy (1H-,13C-Bruker,Germany). DMSO was used as solvent in the detection.The crystal parameters of the BSi were characterized by means of Rigaku-Miniflex600 X-ray powder diffraction(XRD) instrument. According to GB/T 528-1992, the tensile strengths and elongations at break of SR were via UTM6503 electro-mechanical universal testing machines (Shenzhen Sansi Zongheng Technology Co., Ltd., China). According to GB/T 13936-2014, the tensile shear strengths of specimens were measured by this machine.

3 Results and discussion

3.1 Fourier transform infrared spectra

The infrared spectrum proved the structure of BSi and the change of bonding environment during the raw material reaction. The test results are shown in Fig.1.For phenylboronic acid, the peak at around 3 242 cm-1is attributed to the O-H bonds stretching vibrations;peaks at around 1 603, 1 498, and 1 439 cm-1result from ring breathing vibrations of benzenes. Besides,the peak at around 1 346 cm-1corresponds to the B-O bonds stretching vibrations. Peaks at around 693 and 635 cm-1result from B-O bonds bending vibrations.For diphenylsilanediol, the peak at around 3 183 cm-1is attributed to the O-H bonds stretching vibrations.Peaks at around 1 590, 1 485 and 1 428 cm-1result from ring breathing vibrations of benzenes. The peak at around 1 118 cm-1corresponds to the Si-O bonds stretching vibrations. As the reaction proceeds, the peak value caused by the stretching vibration of O-H bond disappears, and the single peak of the stretching vibration of Si-O bond of silanol transforms into double peaks at 1 086 and 1 024 cm-1, which indicates that the condensation reaction of phenylboric acid and diphenylsilanediol takes place, which corresponding to the disappearance of silicon hydroxyl and boron hydroxyl and the formation of Si-O-B bond. In addition, the product retains the respiratory vibration(1 602, 1 493 and 1 440 cm-1), tensile vibration (1 332 cm-1) and bending vibration (694 cm-1) of the B-O bond of benzene, which proves the successful synthesis of the final product BSi.

Fig.1 Infrared spectra of phenylboronic acid, diphenylsilanediol and BSi

3.2 Nuclear magnetic resonance spectroscopy

In order to further prove that the synthesized material has a theoretical structure, NMR testing was carried out to verify (Fig.2). In the1H-NMR spectrum of BSi, peaks at 8 to 7 ppm result from benzene rings.In the13C-NMR spectrum of BSi, peaks at 138 to 125 ppm are also attributed to benzene rings. The above results indicate that BSi has benzene rings. In addition, there are no other organic groups containing hydrocarbons, which indicates that the reaction between phenylboronic acid and diphenylsilanediol is not related to benzene ring, and it can also prove that condensation reaction occurs between them.

Fig.2 BSi NMR hydrogen pattern (a) and BSi NMR carbon pattern (b)

3.3 X-ray diffraction pattern

X-ray diffraction (XRD) test helps to increase the understanding of the crystal parameters of BSi.The results of BSi are shown in Fig.3. As can be seen,the crystal diffraction peaks of BSi mainly appear at 10.42°, 15.40°, 16.42°, 17.40°, 18.22°, 19.32°,19.98°, 20.24°, 21.06° and 23.12°. All the positions of the above diffraction peaks are consistent with those reported in Ref.[35]. Meanwhile, most peaks are sharp and have strong strength, which illustrates its high crystallinity.

Fig.3 BSi XRD spectrum and its standard spectrum

3.4 Mechanical test

On the basis of understanding the crystal structure and bonding environment of BSi, BSi and commercially available SR were mixed in a certain proportion to further explore their influence on SR properties and their adhesion interaction with EP, and the mechanical properties and tensile shear strength of SR before and after addition were compared, as shown in Fig.4. As can be seen in Figs.4(a) and 4(b),after adding the promoters, the tensile strength and elongation at break of SR decreased slightly. However,because BSi has good compatibility with silicone rubber, both of them held steady against the content of promoters in the range of no more than 2%. It can be seen that the introduction of appropriate BSi into the silicone rubber system has little impact on its bulk properties. As shown in Fig.4(c), with the increase of accelerator content, the tensile shear strength between SR and EP first increased and then decreased, and reached the maximum when the accelerator content was 1.5%. This situation is due to the strong adhesion of BSi to EP, and its introduction into silicone rubber system can significantly improve the adhesion between SR and EP; At the same time, the introduction of too much BSi will affect the bulk performance of SR,especially the interface between SR and EP. Because of the large concentration of BSi at the interface, the interface performance is more inclined to the har and brittle characteristics of BSi crystals, which is different from the SR bulk, but is not conducive to the adhesion between SR and EP. Therefore, the BSi-1.5 added with appropriate amount of BSi can achieve a significant improvement effect of 309%.

Fig.4 Effect of BSi introduction on SR tensile strength (a)elongation at break (b) and tensile shear strength (c)

3.5 Mechanism analysis of adhesion improvement by promoter

In order to prove the rationality of the molecular design of the material, BSi-1.5 with the strongest promotion effect was selected, and the boron content in SR and at the interface between SR and EP was measured by EDS to verify that the promoter can migrate to the interface between SR and EP, thus playing a role in the interface interaction. The test results are shown in Fig.5. As can be seen in Figs.5(a)and 5(b), inside SR, the boron content is about 0.24%,and its distribution is relatively uniform. As shown in Figs.5(c)-5(f) and Tables 3-5, at the EP interface, the elements are distributed unevenly. There are structures rich in B and Si elements, in which the boron content is up to 2.79%. The uniform distribution of B in SR proves that BSi is well dispersed in SR and has good compatibility with SR system. In addition, there are sites rich in B and Si at the interface between SR and EP, which indicates that BSi can migrate to the interface between SR and EP and agglomerate. It is suggested that during the curing process of SR, the promoter BSi migrated to the interface between SR and EP, and aggregated together to form a small rivet point that closely adheres SR and EP. The specific adhesion mechanism can be attributed to the fact that the adhesive BSi can attract N and O in EP to enhance chemical interactions due to the electron deficient property of B. During the curing process, it moves to the interface between SR and EP to simply and effectively improve the adhesion strength between them.

Fig.5 Distribution (a) and element content (b) of boron element in SR in BSi-1.5, SEM photos (c) of the interface between SR and EP in BSi-1.5, comparison of element content of B and Si aggregates at SR and EP interfaces in BSi-1.5 in point scan mode (d and e), and comparison of element content of B and Si non aggregation points at SR and EP interfaces in BSi-1.5 in face scan mode

4 Conclusions

In a word, the adhesion accelerator BSi of silicone rubber and epoxy resin was successfully synthesized.The chemical composition and crystal structure of BSi were confirmed by FT-IR,1H-,13C-NMR and XRD.In addition, the influence of BSi on improving the adhesion interaction between SR and EP was discussed through relevant mechanical tests. The results show that the proper addition of BSi can enhance the tensile shear strength between SR and EP, and significantly enhance the bonding effect. Based on the EDS results,it is proved that the adhesion between SR and EP is caused by the formation of coordination bonds and the effect of interfacial migration. This study provides experience for exploring SR/EP composites with high bonding ability.

Conflict of interest

All authors declare that there are no competing interests.