| In electronics industry, in order to improve the stability of the electronic devices, it isoften necessary to encapsulate the electronic assembly components. Addition-cure siliconeencapsulant has broad development prospects due to its excellent electrical insulationproperty, resistance to high temperature and aging resistant performance, good chemicalstability, easy processing, etc. However, common silicone encapsulant had somedisadvantages such as low thermal conductivity, poor flame retardancy and adhesionproperties, which seriously limited its application. To improve these properties of encapsulant,a large amount of thermally conductive fillers, flame retardants and adhesion promoters wereadded, which has a negative effect on the mechanical properties and processability, etc. Inthis dissertation, a novel adhesion-enhancing cross-linker and several types of adhesionpromoters were synthesized, and their structures were characterized. Addition-cure siliconeencapsulant with good thermal conductivity, flame retardancy and adhesion properties wasprepared through selecting appropriate base polymer, thermally conductive filler and flameretardant, etc. The main research contents and results are listed as following:Firstly, electrical insulating and thermal conductive addition-cure silicone encapsulantwas prepared using complex of vinyl end silicone oils with different viscosities, hydrogensilicone oil as cross-linker, alumina (Al2O3), silica power (SP) and silicon carbide (SiC) asthermally conductive fillers. The effect of mass ratio of vinyl end silicone oils, hydridecontent of hydrogen silicone oil, molar ratio of hydrosilyl (SiH) and vinylsilyl (SiVi), typeand content of thermally conductive fillers on the properties of silicone encapsulant wereinvestigated. When mass ratio of vinyl end silicone oil with viscosity of300mPa·s and1000mPa·s was40:60, hydride content of hydrogen silicone oil was0.50wt%and molar ratioof SiH and SiVi was1.2, silicone encapsulant had good mechanical properties. Theencapsulant filled with SP and SiC had better mechanical properties, but the encapsulant withAl2O3had lower viscosity so that it was more suitable for encapsulating process. When thecontent of Al2O3was150phr, silicone encapsulant had good comprehensive performance.Secondly, a novel adhesion-enhancing polyhydrosiloxane containing acrylate groups (MPMS-PHMS) as cross-linker was synthesized by the hydrolysis ofmethacryloyloxypropylmethyldimethoxysilane (MPMS) and the ring opening polymerizationof octamethylcyclotetrasiloxane (D4) and tetramethylcyclotetrasiloxane (DH4). Fouriertransform infrared (FT-IR) spectroscopy,1H nuclear magnetic resonance (1H-NMR)spectroscopy,29Si nuclear magnetic resonance (29Si-NMR) spectroscopy and gel permeationchromatography (GPC) were used to characterize its chemical structure. The effect ofMPMS-PHMS content and cure conditions on the properties of silicone encapsulant wereinvestigated. It was found that silicone encapsulant had higher cross-linking density whenusing MPMS-PHMS as cross-linker. Compared with commercial hydrogen silicone oil(PHMS) cross-linker, MPMS-PHMS could markedly improve both the adhesion strength andmechanical properties of the encapsulant. Appropriately increasing cure temperature andprolonging cure time were favorable to enhance the shear strength of the encapsulant. Theoptimal content of MPMS-PHMS was10phr, and the suitable cure temperature and time were130~150°C and2h, respectively.Thirdly, three types of silanes containing vinyl and epoxy group,(3-glycidoxypropyl)allyloxydimethoxysilane (GAMS),[2,2-bis (allyloxymethyl) butoxy](3-glycidoxypropyl)dimethoxysilane (AGMS) and [3-allyloxy-2,2-bis (allyloxymethyl) propoxy](3-glycidoxypropyl) dimethoxysilane (AAGMS), were synthesized by the transetherificationand used as adhesion promoters. The chemical structures were characterized by FT-IR and1H-NMR. The influence of adhesion promoters on the properties of silicone encapsulant wasinvestigated in detail, heat aging resistance and water resistance of the encapsulant werestudied, and the adhesion mechanism of the encapsulant was explored. It was found thatGAMS, AGMS and AAGMS could not only greatly improve the adhesion strength of theencapsulant but also significantly enhance the mechanical properties of the encapsulant.Among them, AAGMS with more C=C bonds and ether bonds gave the best adhesionstrength and mechanical properties of the encapsulant. The adhesion promoter improved thedispersibility of filler particles in silicone rubber matrix and enhance the bond between fillerand matrix. X-ray photoelectron spectroscopy (XPS) result showed that AAGMS couldmigrate to the bonding interface and produce the adhesion-enhancing effect. When thecontent of AAGMS was1.5phr, silicone encapsulant had good comprehensive performance. The shear strength and tensile strength of the encapsulant were1.14MPa and3.27MPa, whichwas about235%and47%higher than that of the encapsulant without adhesion promoter,respectively. The encapsulant with AAGMS had good heat aging resistance and waterresistance.Fourthly, two types of silane oligomers containing vinyl and epoxy group (PTGM andPTGE) as adhesion promoters were synthesized by the reaction of trimethylolpropanemonoallyl ether (TMPME) with3-glycidoxypropylmethyldimethoxysilane (GPMMS) and3-glycidoxypropylmethyldiethoxysilane (GPMES), respectively. FT-IR,1H-NMR and GPCwere used to characterize their chemical structures. The effect of PTGM and PTGE on theadhesion properties, mechanical properties and viscosity of the encapsulant were investigated,and heat aging resistance and water resistance of the encapsulant were studied. The resultsshowed that PTGM and PTGE could enhance both the adhesion strength and mechanicalproperties of the encapsulant. Compared with PTGE, the encapsulant with PTGM had bettercomprehensive performance.Finally, self-adhesive, halogen-free flame retardant and thermal conductiveaddition-cure silicone encapsulant was prepared by adding AAGMS and aluminiumhydroxide (Al(OH)3) to silicone encapsulant with Al2O3as thermal conductive filler. Theeffect of particle size and content of Al(OH)3and AAGMS content on the flame retardancy,adhesion properties, thermal conductivity, mechanical properties and viscosity of theencapsulant were investigated. It was found that AAGMS could enhance adhesion properties,thermal conductivity and mechanical properties of the encapsulant, but its viscosity increased.Scanning electron microscopy (SEM) showed that AAGMS could improve the dispersibilityof Al2O3and Al(OH)3particles in silicone rubber matrix and enhance the bond between fillerand matrix. Compared with small particle size of Al(OH)3, the encapsulant filled with largeparticle size of Al(OH)3had good flowability, which was beneficial for encapsulating process.When the content of AAGMS was1.5phr, and the particle size and content of Al(OH)3were20μm and60phr, the encapsulant possessed good comprehensive performance. The thermalconductivity was0.583W·m-1·K-1, the shear strength was1.31MPa, the tensile strength was3.43MPa, the elongation at break was52%, the viscosity was7200mPa·s, and the verticalburning test achieved a UL-94V-0rating. |
PDF Full Text Request