Special Topics Review

Progress and application of ionic conductive elastomer adhesive

Zhang Qing, Huang Jijun

(College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences, Beijing  100049, China)

【Abstract】With the rapid development of flexible electronics and wearable smart devices, one of their important components, flexible mechanical sensors have become the main ideal interface for connecting biological systems with traditional electronic devices. Due to their ability to simulate the perception characteristics of human skin, they can achieve stable monitoring of various multi-dimensional physiological signals in the human body. As the core material of such sensors, stretchable flexible conductors have become the key limiting factors for the rapid development of flexible mechanical sensors. It is urgent to overcome inherent weaknesses and break through development bottlenecks to meet the growing demand. In practical applications, stretchable flexible conductors often need to be attached to the skin or various substrates, and thus stable adhesion and self-healing are key characteristics. As a new type of flexible conductors, all-solid ionic conductive elastomer adhesive has become one of the ideal candidate core materials for intelligent flexible electronic devices because it solves the problems of hydro/organogel water evaporation and solidification, possible liquid leakage in ionic gel through squeezing, as well as the difficulties in balancing the conductivity and mechanical properties. In this paper, the adhesion mechanism and self-healing mechanism of ion conductive elastomer adhesive, as well as the types and synthesis methods of main elastomer adhesive in recent years were reviewed, and their representative applications in multiple fields such as flexible electronics were introduced. At the same time, the challenges and problems in the current development and practical application of elastomer adhesive were discussed, providing feasible basis and beneficial insights for the molecular design of high-performance elastomer adhesive and their application in flexible electronic products.

【Keywords】ionic conductive elastomer; adhesive; flexible electronics; stretchable flexible conductor; self-healing; solid-state polymer electrolyte

Research progress on chemical modification methods of epoxy adhesive

Nie Zhenyu, Han Lu, Jiang Zhengyang, Kan Chengyou

(Department of Chemical Engineering, Tsinghua University, Beijing  100084, China)

【Abstract】Epoxy resin is a widely-used adhesive material, but the traditional epoxy adhesive often fails to meet various performance requirements of today, and needs to be modified before application. In this paper, the chemical modification methods of epoxy adhesive in the recent ten years, such as the synthesis of new epoxy resins and curing agents, composite modification, etc. were categorized and summarized. Based on the research hotspots in recent years, the future development direction of epoxy adhesive was analyzed and prospected.

【Keywords】adhesive; epoxy resin; chemical modification method

Research Report 

Strengthening-toughening epoxy resin system with paraffin: an experimental and molecular dynamics simulation study

Sun Yi¹, Chen Hongfeng¹, Li Yan³, Luo Yanlong², Lian Qingsong¹

(1. Shanxi Province Key Laboratory of Functional Nanocomposites, North University of China, Taiyuan  030051, Shanxi, China; 2.College of Science, Nanjing Forestry University, Nanjing  210037, Jiangsu, China; 3. Department of Materials Application Research, AVIC Manufacturing Technology Institute, Beijing  100024, China)

【Abstract】A special epoxy resin (D12) with flexible alkyl side chains was synthesized through thiol-ene click chemistry, and then using it as a compatibilizer, n-eicosane (C20, paraffin model compound) was uniformly mixed into the epoxy resin matrix (DGEBA). A series of D12 and C20 synergistic toughening epoxy resin systems (EP_DA-X systems) were obtained by curing. The strengthening-toughening mechanism of DGEBA/D12/C20 ternary system was systematically studied by combining experimental and molecular dynamics simulation methods. The research results showed that, ⑴When the mass ratio of D12 and C20 was 1∶2 and their mass fraction in the curing system was 2.5%, the system had the best comprehensive performance, with the tensile strength of 99.3 MPa, the tensile modulus of 2.8 GPa, and the elongation at break of 4.5%. All three were higher than the pure EP system, achieving a synergistic improvement in the strength, modulus, and toughness of epoxy resin. ⑵There was a microphase separation structure in the EP_DA-X system, and it had been preliminarily proven that the microphase separation structure in the system was caused by the self-assembly of C20 around the D12 side chain. The physical entanglement structure formed by the self-assembly effect could provide additional physical crosslinking points for the system, thereby achieving a synergistic improvement in the strength, modulus, and toughness of epoxy resin. ⑶Molecular dynamics simulation was used to calculate the Young's modulus of all systems, and it was found that the modulus of EP_DA-X system was higher than that of the pure EP system. The calculation results of the radial distribution function of EP_DA-X system indicated that the main chain interaction between C20 and D12 was weak, but the side chain interaction with D12 was strong. This further confirmed the self-assembly effect of C20 around the side chains of D12, thereby proving the mechanism of synergistically strengthening-toughening of epoxy resin by C20 and D12 self-assembly at the molecular and atomic scales.

【Keywords】molecular dynamics simulation; toughening mechanism; epoxy resin; paraffin, self-assembly

Diffusion behavior and effect of plasticizer in acrylate pressure sensitive adhesive

Wu Jihong¹, Zhou Qiyan¹, Zhang Xiaohui¹, Shi Yifeng², Qian Jun¹

(1.School of Materials Science and Engineering, East China University of Science and Technology, Shanghai  200237, China; 2.Shanghai Hongdingfang Technology Co., Ltd., Shanghai  200237, China)

【Abstract】In order to investigate the diffusion behavior of plasticizer dioctyl phthalate (DOP) in acrylate pressure sensitive adhesive, the pressure sensitive adhesive system consisting of 2-ethylhexyl acrylate (2-EHA), methyl acrylate (MA), and acrylic acid (AA) was prepared. Using methods such as molecular weight distribution (GPC), dynamic mechanical analysis (DMA), thermogravimetric analysis (TG), and mechanical testing, the effects of initiator content, crosslinking agent content at different plasticizer concentrations, and different plasticizer and tackifying resin contents under the same crosslinking agent content on the performance of pressure sensitive adhesive were investigated. The research results showed that when the molecular weight increased, the diffusion behavior of plasticizer DOP slowed down and tended towards equilibrium. As the crosslinking degree of adhesive sample increased, the diffusion behavior of plasticizer DOP would be significantly slowed down. When the contents of crosslinking agent and DOP were changed simultaneously, both crosslinking agent and DOP would reduce the initial tack and peel strength. After DOP entered the acrylate adhesive layer containing the tackifying resin system, the adhesion behavior would also change with the content of the tackifying resin.

【Keywords】plasticizer; pressure sensitive adhesive; diffusion

Process and Application

Preparation and properties of inorganic filler modified wheat protein/tannin composite adhesive

Cheng Zenghui, Wang Jifu, Zhang Daihui, Wang Chunpeng, Chu Fuxiang

(Institute of Chemical Industry of Forest Products, CAF; Key Lab of Biomass Energy and Material, Jiangsu Province; Key Lab. of Chemical Engineering of Forest Products, National Forestry and Grassland Administration; National Engineering Research Center of Low-Carbon Processing and Utilization of Forest Biomass; Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, Nanjing  210042, Jiangsu, China)

【Abstract】Formaldehyde-free bio-based wood adhesive was prepared by using wheat protein (WG) and condensed tannin (CTs) as raw materials. Using montmorillonite, kaolin, heavy calcium carbonate, light calcium carbonate and hydroxyapatite as fillers for modification, the filler-modified composite adhesive was prepared by the blending method. The influence of inorganic filler type on the properties of WG/CTs composite adhesive was investigated, and water-resistance bonding strength, scanning electron microscopy (SEM), thermogravimetric analysis, insolubility in boiling water, viscosity and other tests were analyzed. The research results showed that after adding fillers, the viscosity of WG/CTs composite adhesive increased to varying degrees, and the thermal stability of adhesive was also improved compared to the control group adhesive. There were significant differences in the water resistance of adhesive modified with different inorganic fillers, but the wet strength of plywood all exceeded 0.7 MPa, meeting the requirements of the national standard  of Class II plywood. The test results of insolubility in boiling water showed that light calcium carbonate improved the water resistance of adhesive. SEM images and crack assessment of adhesive layer showed that heavy calcium carbonate and light calcium carbonate had good compatibility with the adhesive, indicating that calcium carbonate filler had no effect on the toughness of adhesive layer and was a good filler choice.

【Keywords】wheat protein; condensed tannin; inorganic filler; adhesive; bonding strength

Preparation and characterization of EPI wood adhesive with long opening period

Yu Guixun, Huang He

(College of Chemistry, Chemical Engineering and Materials Science of Soochow University, State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials, Suzhou    215123, Jiangsu, China)

【Abstract】A styrene-acrylateemulsion (SAEP2)was prepared by a specific emulsion polymerization process, and it was applied in wood adhesive. The influence of the polymerization degree and concentration of polyvinyl alcohol, the amount of isocyanate curing agent, and the amount of calcium carbonate powder on the opening period and bonding strength of adhesive was systematically investigated. The research results showed that the opening period of adhesive was shortened with the increase of the polymerization degree and concentration of polyvinyl alcohol, the amount of curing agent and calcium carbonate powder. When m(SAEP2)∶m(10% BP17 aqueous solution)∶m(calcium powder)=1∶1∶1, and the amount of isocyanate curing agent was 13%, the wet strength of the prepared adhesive SAEP2 met the standard (up to 5.4 MPa), and the opening period was as long as 150 min.

【Keywords】wood adhesive; opening period; styrene-acrylate emulsion; polyvinyl alcohol

Material Science

 Preparation of surface grafting block polymers onto nano silica microspheres and their performance study in acrylic structural adhesive

Huang Bing, Sun Yitong, Li Haiping, Ge Yujing

[Kangda New Material (Group) Co., Ltd., Shanghai  201419, China]

【Abstract】Using silica microspheres as templates, the surface was modified with active functional groups. By initiating controllable/"active" radical polymerization on the surface, and continuous polymerization of styrene and 2-hydroxyethyl methacrylate, nano silica microspheres grafted with block polymers on the surface were obtained, which were then characterized. The surface modified silica microspheres were used in acrylic structural adhesive system and they were compared with the unmodified silica microsphere system. The research results showed that the target product was synthesized successfully through structural characterization. The surface modified silica microspheres were used in the acrylic structural adhesive system, and the performance of polymer modified nano silica in tensile strength, elongation at break, and shear strength was superior to that of unmodified nano silica. Nano silica had the strengthening and toughening effect in the curing system, and its performance was better after surface grafting for polymer modification.

【Keywords】nano silica microsphere; modification; grafting; adhesive

Study on the electrical resistivity of nano calcium carbonate for silicone sealant

Tian Wei¹, Wu Jiawei² ,Li Hua³ ,Wang Lihong¹

(1. Zhejiang Tianshi Nanotechnology Co., Ltd., Hangzhou  311600, Zhejiang, China; 2. Shanghai Huitian New Material Co., Ltd., Shanghai  201600, China; 3. Hangzhou Jiande Ecol-environmental Monitoring Station, Jiande  311600, Hangzhou, China)

【Abstract】Nano calcium carbonate (NCC) was synthesized by the carbonification method, and the effects of raw material origin, crystal shape control agent, type and content of surface treatment agent, process water quality on the electrical resistivity of NCC were investigated. The research results showed that the type and content of surface treatment agent had the most significant impact on the electrical resistivity of NCC, followed by raw material origin, and the influence of crystal shape control agent and process water quality was relatively small. This was because the type and concentration of ions adsorbed or entrapped in nano calcium particles were the main factors affecting the resistivity of nano calcium. The mechanism of influence was that, under the action of the electric field, Na⁺ and OH^- introduced by the process migrated and transported through the capillary tube formed between the NCC particles, causing ion tunneling through the material matrix. The high concentration and molar conductivity intensified the resistivity attenuation of nano calcium. The concentrations of Mg²⁺, Fe²⁺, Ca²⁺, and CO₃^2- introduced by the raw material were low, they were difficult to undergo ion tunneling migration, could only be dissolved in traces of free water to form a sparse distribution of the "salt pool", which relied on interconnection to migrate and conduct electricity, and thus had less impact on electrical resistivity.

【Keywords】silicone sealant; nano calcium carbonate; electrical resistivity