A RESISTIVE MASTIC FOR SPARKING PLUGS WITH INCORPORATED RESISTORS

Abstract: The resistive mastic for sparking plugs with incorporated resistors comprises a glass constituent based on boron silicate and alkali metals made up of a low-melting glass having a dilatometric softening point between 500 and 600°C, and a high-melting glass having a softening point which is at least 100C degrees higher than that of the low melting glass, titanium dioxide, dextrin and a further organic binder.

Description: The present invention relates to a resistive mastic for sparking plugs with incorporated resistors. In order to screen sparking plugs to prevent the radiation of energy at a frequency which affects and prevents reception and transmission by radioelectrical equipment, it is known to use a mastic as a suppressor, the mastic being inserted in the insulating body between the central electrode and the current supply pin and sealing them as a result of its hot fusion. The use of resistive mastics including components of glass, titanium dioxide and an organic binder is known from the literature. The resistance of such mastics varies greatly, however, when they are subjected to discharge at high tension and high temperature. It is possible to avoid these variations by means of stabilisation processes but these involve further operations, in addition to the normal production cycle, with a consequent increase in costs. The particular object of the present invention is to produce a resistive mastic whose physical characteristics remain stable with time. In view of this object, the subject of the invention is a resistive mastic characterised in that it comprises: - a glass constituent based on boron and alkali metals silicate, constituted by: a) a quantity of from 7 to 38% by weight of the total composition of the mastic of a low-melting glass having a dilatometric softening point between 500 and 600°C and a transformation temperature above 500°C, b) a quantity of from 43 to 90% by weight of the composition of the mastic of a high-melting glass having a dilatometric softening point at least 100C degrees higher than that of the low-melting glass, - a quantity of from 1 to 19% by weight of titanium dioxide and - a first organic binder selected from the group consisting of dextrin, cellulose derivatives and vinyl alcohols in a quantity such as to give the mastic a resistivity value of no more than 20 Kohm and a second organic binder which has substantially no affect on the resistivity value. In particular, a high-melting glass composition is selected which has a viscosity/temperature curve such that, during hot pressing, which is normally carried out at between 800 and 900°C, the glass remains substantially inert so as to create a structure which delimits the conductive mastic part which is normally interposed between the resistive mastic and the terminals of the central electrode and the core. The low-melting glass correspondingly has a viscosity/temperature curve such that it fuses completely at the hot pressing temperature, thus ensuring the compactness of the resistive mixture and good anchorage thereof to the ceramic body of the plug. Compositions which can be used for the two types of glass are shown in the table below by way of non-limiting example, expressed as percentages by weight. It is preferable that the high-melting glass should have a lower total alkali metal oxide content than that of the low-melting glass. The first organic binder is preferably constituted by dextrin, whose inclusion in the mastic composition causes an appreciable variation in its resistance, which is inversely proportional to the dextrin content. To give a stable resistance value, it is particularly preferred that the dextrin to titanium dioxide ratio should be within the range 0.06:1 to 0.23:1, and more preferably 0.09:1 to 0.15:1. The second binder, which does not affect the resistivity, is generally constituted by a wax emulsion. According to another characterising aspect, the resistive mastic can be obtained by the mixing of: i) a quantity of between 40 and 70% by weight of the total composition of the high-melting glass, with a particle size of between 0.2 and 0.4 mm, ii) a quantity of between 20 and 55% by weight of the total composition of a mixture, previously ground to a particle size of less than 75 µm and including the low-melting glass, titanium dioxide and some of the high-melting glass, iii) the quantities specified above of the first and second organic binders. The mixture ii) preferably includes from 30 to 70% of low-melting glass; from 15 to 41% of high-melting glass and a quantity of from 5 to 35% by weight of titanium dioxide with a particle size of less than 10 µm, the percentages being percentages of the total quantity of the mixture ii). The following example describes the preparation of a mastic composition according to the invention. Example A glass B, having the composition given below, is milled to a particle size of between 5 and 15 µm Glass B SiO₂ 58.8% by weight B₂O₃ 22.5% by weight Al₂O₃ 8.5% CaO 0.3% Na₂O 5.6% K₂O 2.8% Transformation temperature 500°C. Softening temperature 560°C. The high-melting glass, having the composition given below, is then crushed and a fraction having a particle size of between 0.2 and 0.4 mm is selected. Glass A SiO₂ 64.0% B₂O₃ 25.0% Al₂O₃ 1.5% CaO 6.0% MgO 0.6% Li₂O 2.6% Na₂O 0.1% K₂O 0.2% Transformation temperature 545°C. Softening temperature 700°C. The following materials are then introduced into a vessel: glass B 56% glass A less than 0.2 mm 30% Titanium dioxide 14% The mixture thus obtained is then milled to a particle size of less than 75 µm. 45 parts by weight of high-melting glass and 5 parts by weight of wax emulsion are added to 49 parts by weight of the milled mixture. The mixture is pelletised by dampening with a solution of dextrin at a concentration such as to add 1% of dextrin, screened so as to obtain a fraction having a particle size of less than 0.75 mm, and dried in an oven at 120°C. The mastic thus obtained is used as a resistive mass in sparking plugs with incorporated resistors, the mass being introduced into the insulator of the plug by conventional pressing techniques. Tests carried out have shown, in particular, that the resistance is very stable when the plug is subjected to the following set of acceptance tests: a) test discharging for 1 minute at 15 KV at ambient temperature, 3000 sparks/minute b) heating at 350°C for 15 hours c) test discharging at 15 KV, 300°C for 50,000,000 discharges at 50Hz. In particular it was possible to confirm that the resistance and the stability thereof depend on the ratio of the dextrin to the titanium dioxide and on the quantities thereof relative to the total mass. With reference to the resistive mastic-described in the example given above, the following variations in resistance, expressed as percentages of the initial value R O , were found in the acceptance tests: initial resistance R O = 5.0 Kohm test a): ΔR A : ≦ 10% R O test b): ΔR B : ≦ 10% R A test c): ΔR C : ≦ 15% R A

Claims: 1. A resistive mastic for sparking plugs with incorporated resistors, characterised in that it comprises: - a glass constituent based on boron and alkali metals silicate, constituted by: a) a quantity of from 7 to 38% by weight of the total composition of the mastic of a low-melting glass having a dilatometric softening point between 500 and 600°C and a transformation temperature above 500°C, b) a quantity of from 43 to 90% by weight of the composition of the mastic of a high-melting glass having a dilatometric softening point at least 100C degrees higher than that of the low-melting glass, - a quantity of from 1 to 19% by weight of titanium dioxide and - a first organic binder selected from dextrin, cellulose derivatives and vinyl alcohols in a quantity such as to give the mastic a resistivity value of no more than 20 Kohm and a second organic binder which has substantially no affect on the resistivity value. 2. A resistive mastic according to Claim 1, characterised in that the first organic binder is dextrin and in that the ratio by weight of the dextrin to the titanium dioxide is between 0.07:1 and 0.23:1. 3. A resistive mastic according to Claim 2, characterised in that the second organic binder is constituted by a wax emulsion. 4. A resistive mastic according to Claim 1 or Claim 2, characterised in that the high-melting glass has a lower alkali metal oxide content than that of the low-melting glass. 5. A resistive mastic according to any one of Claims 1 to 3, characterised in that it is obtainable by the mixing of: i) a quantity of between 40 and 70% by weight of the total composition of the high-melting glass, with a particle size of between 0.2 and 0.4 mm, ii) a quantity of between 20 and 55% by weight of the total composition of a mixture previously ground to a particle size of less than 75 µm and including the low-melting glass, titanium dioxide and some of the high-melting glass, iii) the first and second organic binders. 6. A resistive mastic according to Claim 5, characterised in that the mixture ii) comprises from 30 to 70% of low-melting glass, from 15 to 40% by weight of high-melting glass and a quantity of from 5 to 35% by weight of titanium dioxide, the percentages being percentages of the total quantity of the mixture ii). 7. A resistive mastic according to any one of Claims 1 to 5, characterised in that the titanium dioxide has a particle size below 10 µm. 8. A sparking plug with an incorporated resistor, in which the resistor is constituted by a mastic according to any one of Claims 1 to 7.

Inventors: LATTE, GIOVANNI MICHELE, ROSSI, ROBERTO
Applicants: INDUSTRIE MAGNETI MARELLI S.P.A.
Application year: 1988
Main classification: H01T13
IPC codes: H01T13