| Conductive silver paste(CSP) is mainly composed of silver powder and resin. Because of its good processability, adhesion and biocompatibility, CSP is a kind of key materials in the electronics industry. However, the high content of silver powder, the poor stability of mechanical properties, and some other shortcomings restrict its application. The properties of CSP are effected by both silver powder and resin. The effect of silver powder has been studied by many scholars, but the study about resin was less. In this paper, some urethane adducts were synthesized with different diols and different diisocyanates, and were blocked by blocking agents to obtain blocked polyurethane prepolymers(BPUPs). The CSPs were prepared with BPUPs as the resin phase and flake silver powder as the conductive phase. The effects of silver content, agents, type of diols and diisocyanates on the properties of CSPs were investigated. And the dispersion state of silver powder was analyzed with SEM. The main contents and the results are as follows.(1) Some urethane adducts were synthesized with different diols and different diisocyanates, and were blocked by blocking agents to obtain blocked polyurethane prepolymers(BPUPs). The BPUPs were characterized by fourier transform infrared spectroscopy and gel permeation chromatography(GPC). Effects of reaction conditions and reactants were investigated. The results showed that the reaction rate was increased with the rising of temperature and catalyst content. The catalytic efficiency of dibutyltin dilaurate(DBTDL) was higher than that of triethylamine. The type of diols has obvious effect on the addition reaction rate. Polyether diols reacted at lower rate than polyester diols, except for the polytetramethylene glycol. The optimal reaction temperature and time of addition reaction were 40 ℃ and 2~4 h. The suitable catalyst was dibutyltin dilaurates. When methyl ethyl ketoxime was used as the blocking agent, the blocking reaction temperature and time were 60 ℃ and 2 h. BPUPs synthesized with polyether diols has smaller molecular weight and lower viscosity. When the reaction activity of two-NCO groups at diisocyanates showed more difference, the BPUPs have smaller molecular weights.(2) CSPs were prepared with BPUPs blocked by methyl ethyl ketoxime as the resin phase and silver flake as the conductive phase. Effects of curing condition, silver content and agents on the properties of CSPs were investigated. The results showed that the conductivity and hardness of CSPs increased, the folding resistance was improved at first and then declined, with the rising of curing temperature. The conductivity increased, and the folding resistance was improved at first and then declined, with the rising of silver content. The addition of flatting agent and antifoaming agent improved the appearance and the processability. The optimal curing temperature and time were 150 ℃ and 60 min, the suitable silver content was 60 wt%.(3) A series of CPSs were prepared with different BPUPs. The effects of BPUPs structure on the properties of CSPs were investigated. The results showed that the CSPs prepared with BPUPs showed good adhesion to polyethylene terephthalate. Type of diols has obvious effect on the properties of CSPs. The conductivity and folding resistance of CSPs based on polyether BPUPs were better than those of CSPs based on polyester BPUPs. But the hardness of the later was higher. The comprehensive property of CSP with BPUP derived from 2, 4-toluene diisocyanate was better than that from other diisocyanates. The SEM results showed that the silver powders were dispersed more uniformly in polyether BPUPs, which gave the CSPs higher conductivity. |
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