Generally, polydimethylsiloxane containing vinyl is used as the base rubber and hydrosilicone oil is used as the crosslinking agent. Under the action of platinum catalyst, the elastomer with three-dimensional network structure is formed by hydrosilylation. Because of its excellent high and low temperature resistance, weather resistance and electrical insulation performance, it does not release low molecular by-products in the process of vulcanization, has the characteristics of small shrinkage, deep vulcanization, easy control of cross-linking structure, product can be vulcanized at room temperature and heated vulcanization, etc., which is considered as the preferred material for filling and sealing of electronic and electrical assembly parts. However, the thermal conductivity of silicone rubber is very poor, and the thermal conductivity is only about 0.2w/m · K, which is easy to cause the heat generated by electronic equipment can not be emitted in time, so that the reliability and life of electronic components can be reduced.
Therefore, it is necessary to add a large number of thermal conductive fillers (such as Al2O3, SiC, BN, SiO2, etc.) to use the addition molding silicone rubber as the electronic potting material. In most cases, in order to obtain high thermal conductivity, it is often necessary to modify the surface of the thermal conductive filler to reduce the surface energy and surface polarity of the filler, improve the dispersion of particles, and increase the filling amount, so as to ensure that the silicone rubber has good thermal conductivity and good physical properties at the same time. Lin Xiaodan et al. Found that the thermal conductivity of silicone rubber filled with coupling agent modified magnesium oxide can be improved by nearly 20% when the amount of silane coupling agent is 0.5% of the amount of magnesium oxide; The conductive silicone electronic potting adhesive was prepared by using alumina as the filler. The mechanical properties of the potting adhesive were improved by treating alumina with KH-570, but the thermal conductivity was almost unchanged.
Compared with other fillers, silica fume has advantages in the reinforcement, electrical properties and dispersion of silicone rubber. In this paper, the effects of different coupling agents on the viscosity, mechanical properties, thermal conductivity and electrical properties of the silicone electronic potting adhesive were studied.
01 surface treatment of silica powder
A certain amount of pre drying silicon powder was used in the modified equipment by dry process modification. Under the high speed dispersing machine, the KH 0.3% or KH 570 ethanol solution (mass concentration 50%) was added into the spray media to treat 1H with the mass of 50%. Then, the filler was dried in a vacuum oven at 120 ℃ for 2 hours to prepare active silica powder, which was recorded as KH-560 modified silica powder and KH-570 modified silica powder respectively. (the activation degree should be more than 50%)
02 results and discussion
2.1 effect of silicon powder content on viscosity of silicone electronic potting adhesive
Fig. 1 Effect of silicon powder dosage on viscosity of potting adhesive
It can be seen from Figure 1 that with the increase of the amount of silica fume, the viscosity of the potting glue increases continuously, and the viscosity of the potting glue B and C prepared with the modified silica fume is significantly lower than that of the potting glue a prepared with the ordinary silica fume, because the surface of the silica fume has active hydroxyl group and the particle size is small, it will have chemical bonding and physical adsorption with the silicone rubber in the potting glue, and with the silica fume With the increase of dosage, the interaction force between the silicone rubber and the silicone rubber is also increased, which leads to the viscosity of the potting adhesive rising constantly. However, the surface active hydroxyl groups of the silica micropowder modified by KH-560 and KH-570 are obviously reduced, the interaction force between the filler and the silicone rubber is greatly reduced, and the corresponding viscosity is also reduced, so the viscosity of potting adhesive B and potting adhesive C is significantly lower than that of potting adhesive a. For example, when 180 phr silica powder is added, the viscosity of potting glue C is only 4150mpa · s, while that of potting glue a is 5050mpa · S. the fluidity of the former is obviously better than that of the latter, which is convenient for potting.
2.2 effect of silicon powder content on electrical properties of silicone electronic potting adhesive
The effect of silica powder content on the tensile strength of potting adhesive is shown in Figure 2. It can be seen from Figure 2 that with the increase of the amount of silica fume, the tensile strength of the potting adhesive increases first and then decreases, and the maximum value appears when the silica fume is 180 phr. This is because the micro silicon powder is a semi reinforcing material of silicone rubber. With the increase of the amount of micro silicon powder, the interaction between the micro silicon powder and silicone rubber increases, so the tensile strength of the potting adhesive increases. But when the amount of micro silicon powder is more than 180 phr, the micro silicon powder will be scattered unevenly due to the excessive viscosity of the potting adhesive, resulting in local agglomeration, which leads to the decrease of the tensile strength of the potting adhesive.
In addition, it can be seen from Figure 2 that the tensile strength of potting adhesive a, potting adhesive B and potting adhesive C increases in turn with the same amount of silica powder. This is because after the surface treatment, the compatibility between the modified silica powder and the silicone rubber is obviously improved, and the interface interaction force is also increased, so the tensile strength of the potting adhesive B and C filled with the modified silica powder is better than that of the potting adhesive a filled with the ordinary silica powder. However, the tensile strength of potting adhesive C is higher than that of potting adhesive B. This is due to the vinyl contained in KH-570, a silica fume modifier filled with potting adhesive C, which can react with silicone oil containing hydrogen, and further improve the interface interaction between silica fume and silicone rubber.
Fig. 2 Effect of silicon powder content on the tensile strength of potting adhesive
Fig. 3 Effect of silicon powder content on breaking elongation of potting adhesive
The effect of the amount of silica powder on the elongation at break of potting adhesive is shown in Figure 3. It can be seen from Fig. 3 that the elongation at break of potting adhesive decreases with the increase of the amount of silica powder. This is due to the increase of the amount of silica powder and the corresponding increase of the interaction between silica powder and silicone rubber, resulting in the enhancement of the free sliding restriction between polysiloxane polymer chains. In addition, it can be seen from Fig. 3 that the elongation at break of potting adhesive a, potting adhesive B and potting adhesive C decreases in turn when the amount of silica fume is the same, which is also caused by the increase of the interaction force between silica fume and silicone rubber in potting adhesive a, potting adhesive B and potting adhesive C, and the increase of the limited degree of free sliding between polysiloxane polymer chains.
2.3 effect of silicon powder content on thermal conductivity of silicone electronic potting adhesive
It can be seen from Figure 4 that the thermal conductivity of the potting adhesive increases with the increase of the amount of silica powder. This is because with the increase of the amount of silica fume, the volume fraction of silica fume in the potting adhesive increases correspondingly, the distance between particles decreases, and the heat transfer resistance decreases, so the thermal conductivity increases rapidly at the beginning; but when the amount of silica fume reaches a certain degree, an effective heat conduction network has been formed in the system, then the amount of silica fume increases, and the thermal conductivity of potting adhesive increases Slow down. In addition, it can be seen from Fig. 4 that the thermal conductivity of potting adhesive a, potting adhesive B and potting adhesive C increases in turn with the same amount of silica fume. This is due to the "coupling" effect of KH-560 and KH-570, which improves the interface compatibility of silicone rubber and filler, reduces the interface defects and possible voids, and reduces the thermal resistance of the system.
Fig. 4 Effect of silicon powder content on thermal conductivity of potting adhesive
03 summary
With the increase of the amount of silica powder, the viscosity, thermal conductivity and relative permittivity of silicone electronic potting adhesive increased, the elongation at break and volume resistivity decreased, while the tensile strength increased first and then decreased. The properties of silicone electronic potting adhesive are improved by coupling agent, and the potting adhesive prepared by KH-570 is better than that prepared by KH-560.
180 parts of KH-570 modified silica powder were added into the potting adhesive, and the potting adhesive has better comprehensive properties. At this time, the viscosity of the potting adhesive is 4150mpa · s, the tensile strength is 3.73mpa, the elongation at break is 61%, the thermal conductivity is 0.63w/m · K, the relative dielectric constant is 3.96, and the volume resistivity is 2.86 × 1014 Ω· cm.
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