RU2028361C1 - Hermetic composition - Google Patents

Abstract

FIELD: electronic engineering. SUBSTANCE: hermetic composition has, wt. -% : compound on the basis of low-molecular siloxane rubber 75-96; oligodimethylsiloxane with kinematic viscosity 5-150 mm²/c at 20 C 4-25. Hermetic substance shows enhanced stability of resistance under condition of increased moisture and high-tension voltage. EFFECT: enhanced quality of composition. 2 dwg, 4 tbl

Description

 The invention relates to materials for sealing products, in particular products of electronic equipment, in order to protect them from mechanical damage, electrical breakdowns, as well as other destructive effects in the conditions of operation of the equipment at high supply voltages and high humidity.

The well-known Vixint K-68 compound (TU 38.103508-81) containing low molecular weight siloxane rubber (SKTN-TU38.103508-81) with methyl methyl units and finely divided silica (GOST 14922-77) as a reinforcing polymer and thermostabilizing filler. The main disadvantage of vulcanizate "Viksint K-68" is a small elasticity (elongation break of about 80%) and the impossibility to use it for sealing products operated at a temperature below -60 ° ^C.

These disadvantages were partially eliminated in the compound UV-7-21 (OST 38.03238-81), taken as a prototype of the inventive sealing composition and containing low molecular weight methylphenylsiloxane rubber (SKTNF-TU38. 103470-80) as a polymer base, and finely dispersed dioxides as fillers silicon (GOST 14922-77) and titanium (TU6-09-2166-77). UV Compound 7-21 has a temperature range from -110 to 250 ° ^C at an elongation of about 100% gap. Thus, the UV-7-21 compound is superior to the Vixint K-68 compound in terms of elasticity and frost resistance. A significant drawback of the UV-7-21 compound is the significant loss of electrical resistance by its vulcanizate under the influence of heat and high humidity. At the same time, leakage currents of sealed products increase and their operating parameters deteriorate until the products fail during electrical breakdown.

The aim of the invention is to increase the stability of the electrical resistance of the vulcanizate compound in conditions of high humidity and high supply voltage. This goal is achieved in that the sealing composition, including a compound based on low molecular weight siloxane rubber, additionally contains oligodimethylsiloxane with a kinematic viscosity at 20 ^about 5-150 mm ² / s in the following ratio, wt.%:
Compound Based
low molecular weight siloxane rubber 75-96
Oligodimethylsiloxane
with kinematic viscosity at 20 ^о С 5-150 mm ² / s 4-25
Experimentally it has been found that the addition oligodimetilsiloksana a kinematic viscosity at ²⁰ ° C of 5-150 mm ² / s at indicated ratios can achieve this goal.

 When the content of oligodimethylsiloxane in an amount of less than 4 wt.% There is no significant increase in the stability of electrical resistance in comparison with the prototype. When the oligodimethylsiloxane content is more than 25 wt.%, The specific volume resistance is stable over the entire considered range, however, the mechanical strength of the vulcanizate deteriorates at the same time (Table 1), which makes it structurally unacceptable.

For a sealing composition containing oligodimethylsiloxane with a kinematic viscosity of more than 150 mm ² / s, the necessary degassing (removal of air inclusions from the volume of the compound before use) is not possible, because due to the high viscosity of the composition, its vulcanization occurs faster than removal of air inclusions from the volume of the compound. Due to the duration of the necessary degassing, the increasing viscosity of the composition containing oligodimethylsiloxane with a kinematic viscosity of more than 150 mm ² / s leads to the technological unsuitability of the compound for sealing the product.

Figure 1 presents the change in the electrical resistivity of the vulcanizates of the compound based on the UV-7-21 compound with a different content of PMS-10 polymethylsiloxane liquid (GOST 23175-78) from the exposure time in the heat and moisture chamber (day). The content of oligodimethylsiloxanes (PMS-10) is:
1 - 25 wt.%
2 to 15 wt.%
3 to 6 wt.%
4 - 0 wt.% Tests were carried out at a supply voltage of U = 25 kV. 2 shows the change in the specific volume resistance of vulcanizates sealing compositions based on siloxane compounds with various different content oligodimetilsiloksanov a kinematic viscosity at ²⁰ ° C of 5-150 mm ² / s (GOST 23175-78), the time of exposure to the heat and moisture chamber, day:
5 - "Styrosil" + 20 wt.% PMS-150 + 3 wt.% Cat. N 21
6 - "Vixint K" + 20 wt.% PMS-50 + 5 wt.% Cat. N 68
7 - UV-7 + 20 wt.% PMS-5 + 5 wt.% Cat. N 19
8 - UV-7 + 20 wt.% PMS-100 + 3 wt.% Cat. N 21
9 - "Styrosil" + 3 wt.% Cat N 21
10 - UV-7 + 3 wt.% Cat N 19
11 - "Vixint K" + 5 wt.% Cat. N 68.

 The tests were carried out at a supply voltage of U = 25 kV.

PRI me R 1. 85 g of paste UV-7 is thoroughly mixed with 15 g of polymethylsiloxane liquid PMS-10 (GOST 23175-78). To the mixture add 3 ml of catalyst K-21, OST 38.03239-81, carry out degassing at a residual pressure of 10 ^-2 ... 10 ^-3 atm. within 10-15 minutes and make the appropriate samples. Three days after vulcanization, the samples were tested. The data are given in table 1.

 All other examples illustrating the properties of the inventive sealing composition are given in table.2.3, and are also presented in Fig.1,2, curves 1-4, 5-8.

 Table 2 shows that for sealing compositions containing oligodimethylsiloxane in an amount of more than 25 wt.% Leads to a noticeable decrease in the tensile strength of the vulcanizate, which makes these sealing compositions structurally unacceptable.

Analysis of the curves shows that administration oligodimetilsiloksana a kinematic viscosity at ²⁰ ° C of 5-150 mm ² / s in the claimed range significantly affects the stability of volume resistivity vulcanizate sealing composition under exposure at elevated humidity and temperature (relative humidity 98% temperature 40 ° ^C). Moreover, within the claimed limits of the content of oligodimethylsiloxane, the stability of the specific volume resistance of the vulcanizates under conditions of high humidity and temperature is quite high.

 For curing sealing compositions based on low molecular weight siloxane rubbers, any cold curing catalysts containing organic tin salts, a crosslinking agent (tetraethoxysilane or ethyl silicate) and an organic solvent (toluene, hexane, etc.), i.e. any organotin catalyst (OST 38.03239-81). All of the above is confirmed by the above examples. In this case, the sealing compositions containing oligodimethylsiloxane in an amount of from 4 to 25 wt.%, The stability of the specific volume resistance is maintained under conditions of high humidity and temperature at high supply voltages (Fig. 1,2).

PRI me R 2. 100 g of paste UV-7 is thoroughly mixed with 4 g of catalyst N 19, OST 38.03239-81 carry out degassing at a residual pressure of 10 ^-2 . ..10 ^-3 atm for 10 ... 15 min. Full cure of the vulcanizate occurs within no more than 10 hours.

 All other examples illustrating the properties of the inventive sealing composition are summarized in table 4, and are also presented in figure 2, curves 9-11.

Claims (1)

A SEALING COMPOSITION, comprising a compound based on low molecular weight siloxane rubber, characterized in that, in order to increase the stability of electrical resistance under conditions of high humidity and high supply voltage, it additionally contains oligodimethylsiloxane with a kinematic viscosity at 20 ^o C 5 - 150 mm ² / s at the following ratio of components, wt.%:
Compound based on low molecular weight siloxane rubber - 75 - 96
Oligodimethylsiloxane with kinematic viscosity at 20 ^o C 5 - 150 mm ² / s - 4 - 25

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