JPH0139598B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of manufacturing a sound absorbing material used in an exhaust silencing device for an internal combustion engine.

Conventionally, sound absorbing materials used in exhaust silencers for internal combustion engines include glass wool and porous metal bodies. However, when these materials are used as sound-absorbing materials, when an internal combustion engine is operated, soot and tar, which are the combustion residues of fuel such as gasoline, adhere to the inside of the sound-absorbing materials, and the sound-absorbing materials become clogged within a relatively short period of time. This has the disadvantage that the sound absorbing performance decreases and the noise emitted from the exhaust silencer increases. In addition, because the surface roughness of the sound absorbing material is large, there is a large flow resistance to the exhaust gas flowing along the surface of the sound absorbing material, which increases the back pressure of the internal combustion engine and reduces the output and efficiency of the internal combustion engine. It was a contributing factor. Furthermore, the sound absorption coefficient of conventional sound absorbing materials rapidly decreases in the low frequency range, which is disadvantageous for use in exhaust muffling devices for internal combustion engines, where sound deadening performance at low frequencies is important.

This invention was made to eliminate the above-mentioned drawbacks, and provides a method for manufacturing a sound absorbing material that prevents the deterioration of sound absorption properties caused by clogging with soot and tar, and further has good sound absorption properties. It is.

The phenomenon of clogging of sound absorbing materials due to soot and tar is
This occurs because part of the exhaust gas flow mixed with soot and tar flows into the sound-absorbing material, and the soot and tar are deposited inside the sound-absorbing material accordingly.
Therefore, soot and tar flow into the sound absorbing material,
The best way to prevent this from accumulating is to have a sound-absorbing material structure that prevents exhaust gas from flowing into the sound-absorbing material, and one way to do this is to form a non-permeable thin film on the surface of the sound-absorbing material to block the inflow of exhaust gas. be able to.
However, forming a thin film makes it difficult for the sound waves themselves to propagate inside the sound-absorbing material, leading to a decrease in sound absorption coefficient. By setting the eigenvalues of the mechanical-acoustic impedance system in the low frequency range where a high sound absorption coefficient is desired, it is possible to improve the sound absorption coefficient in the low frequency range compared to conventional sound absorbing materials.
When used as a sound absorbing material for an exhaust silencer for an internal combustion engine, heat resistance up to a maximum of about 800°C is required. Moreover, the exhaust gas temperature during normal internal combustion engine operation fluctuates over a range of several hundred degrees Celsius.
Thin films are repeatedly subjected to large heat stress. Furthermore, since the fluid force of exhaust gas is also applied, mechanical strength that can withstand this force is required. In particular, the bonding state between the thin film and the porous sound-absorbing base material (hereinafter referred to as porous material) becomes a problem, and if the bonding force is weak,
Heat stress and fluid force cause the thin film to separate from the porous body and scatter.

From the above point of view, the inventor investigated various manufacturing methods and found that metal or inorganic materials (particularly ceramics) are preferable for the thin film in terms of heat resistance and mechanical strength, and that it also improves the bonding strength with the porous body. In order to achieve this, we have found that it is possible to put into practical use a method of forming a thin film by melt-spraying a thin film material onto the surface of a porous body.It should be noted that when sprayed directly onto a porous body, the sprayed material penetrates into the inside of the porous body. However, it was discovered that it is impossible to form a thin film of arbitrary thickness on the surface of a porous body. As a countermeasure, if you close the porous body with a suitable filler before thermal spraying and remove the filler after forming the thermal spray coating, you can create a smooth thin film of any thickness on the surface of the porous body using the thermal spraying method. discovered that it is possible to form

As mentioned above, the present invention involves filling the inside of a porous body with a filler, then coating the surface with a metal or inorganic material by melt spraying, and then removing the filler.
By forming a thin film on the surface of a porous material, it prevents clogging of the sound absorbing material with tar and soot, improves sound absorption characteristics in the low frequency range, and provides a sound absorbing material with good heat resistance and mechanical strength. It is something to do.

Examples of porous bodies that can be used in this invention include foamed metals, sintered metals, metal fibers, sintered inorganic materials, and inorganic fibers. Fe―Ni―Cr, Fe
-In addition to alloys such as Cr-Al, Ni-Fe-Cr-Al,
Glass wool, raw wool, ceramic wool, sintered glass, sintered ceramic, etc. can be used. As the thin film material to be thermally sprayed, the above-mentioned constituent materials of the porous base material can be used.

As for the characteristics required of the filler, it is desirable that it be easy to fill into a porous body and that gas release from the filler is small when heated during thermal spraying. In other words, if a large amount of gas is released, it becomes difficult to form a sprayed coating on the surface of the porous body. Furthermore, since it is necessary to remove the filler from the porous body after thermal spraying, it is required that the filler be easy to remove.

In view of the above, it is desirable that the filler be in a liquid state at the time of filling, solidify after filling, emit less gas due to thermal spray heating, and be easily removed by chemical treatment or the like. As a result of the inventor's experimental investigation of materials that satisfy the above requirements, the inventors found that materials such as dehydration-curing inorganic compounds such as calcium sulfate and sodium silicate, low melting point metals such as zinc and aluminum solder, and relatively comparative materials such as acrylic resin and polyethylene were found. Effective materials include those that have a low molecular weight and can be dissolved in an appropriate solvent, or polymeric compounds that are low molecular in the initial state and are polymerized and hardened over time. However, to remove these fillers, dilute acid solutions are recommended for dehydration-curable inorganic compounds and low-melting point metals, and dissolution using solvents is recommended for polymerization-curable organic polymer compounds in terms of removal speed. found it to be effective.

The details of this invention will be described below with reference to Examples.

〔Example〕

A metal porous body (trade name: CELMET, made of Ni-Cr, thickness 3 mm) was used as the porous body. After the porous body was degreased by washing with acetone, an aqueous solution of calcium sulfate was filled into the porous body, and the porous body was heated in a furnace at 120°C. Dry for 30 minutes,
After curing, as shown in FIG. 1, the surface of the porous body 1 to be thermally sprayed is polished to remove calcium sulfate 5 adhering to the surface and expose the skeletal surface 2 of the porous body 1 to the surface. Processed. Next, Ni--Cr powder was sprayed onto the surface of the polished porous body 1 using a plasma spraying device so that the film thickness was about 30 μm.
The porous body 1 after thermal spraying was placed in a 5% aqueous hydrochloric acid solution at a temperature of 100°C and immersed for 20 minutes to remove ammonium sulfate 5 from the porous body 1, washed with water, and dried in a drying oven at 60°C.

FIG. 2 shows the result of an enlarged observation of the cross section of the sound absorbing material of the present invention produced as described above using an electron microscope. That is, 2 is a skeleton of the metal porous body 1, 3 is a hole, and 4 is a thin film formed by thermal spraying.
No residual calcium sulfate was found in pore 3,
It was confirmed that the thin film 4 formed by thermal spraying and the skeleton 2 were completely bonded. Furthermore, various performances were investigated by comparing the sound absorbing material of the present invention with a porous metal material without a thermal spray coating.

Figure 3 shows a comparison of normal incidence sound absorption coefficients when the back air layer is 50 mm using the standing wave tube method (JIS A 1405). In the figure, curve A shows the characteristics of the sound absorbing material of the present invention, and curve B shows the characteristics of the sound absorbing material made only of metal porous bodies. As is clear from Fig. 3, the sound absorbing material according to the present invention has a 1K
It can be seen that the sound absorption coefficient in the low frequency region below Hz is considerably improved.

Next, the two types of sound absorbing materials mentioned above are formed into a cylindrical shape,
It was installed in the exhaust silencing system of a commercially available domestically produced passenger car (displacement 1800c.c.), and after driving approximately 10,000 km, the silencing performance was measured according to JIS D 1616, and the initial value before actual driving was determined. Figure 4 shows a characteristic diagram for comparison. In the figure, curves C and D indicate the degree of sound attenuation reduced from the initial value when using a sound absorbing material consisting only of metal porous bodies and when using the sound absorbing material of the present invention. As can be seen from the figure, the performance of the product using the sound absorbing material of this invention has hardly decreased from the initial performance even after 10,000 km of actual driving, but the performance of the product using only the sound absorbing material of porous metal body has not decreased after 10,000 km of actual driving. Later, the silencing performance deteriorated significantly in most frequency bands. Furthermore, after measuring the silencing performance, both sound-absorbing materials were taken out and visually observed, and it was found that the sound-absorbing material of the present invention had only a thin layer of soot attached to its surface, but the material with only a metal porous material had soot. had penetrated into the interior, causing severe clogging. Further, the thin film was also completely bonded to the porous body, and no peeling or damage of the thin film was observed.

[Example 2] A metal porous body (product name: CELMET, made of Ni-Cr, thickness 3 mm) was used as the porous body. After cleaning the porous body with acetone and degreasing, an acrylic resin (for example, Acrytet manufactured by Mitsubishi Rayon) was used. A porous body is filled with an acrylic resin solution dissolved in a solvent (for example, chloroform), and after the solvent evaporates and hardens, the surface of the porous body to be sprayed is polished until the skeleton of the porous body is exposed, and Ni- Cr powder was sprayed to a film thickness of approximately 30 μm using a plasma spraying device.
The porous body after thermal spraying was immersed in a 700 form solvent for about 5 hours to remove the acrylic resin in the porous body, and then dried. As a result of measuring various properties of the sound absorbing material, it showed almost the same properties as in Example 1.

By the way, in the above example, the thickness of the thin film was 30 μm.
We have shown the case of , but it is better to increase the thickness to improve the sound absorption coefficient in the lower frequency region, and it is better to make it thinner to improve the sound absorption coefficient in the higher frequency region. However, it was experimentally confirmed that the sound absorption properties deteriorate when the thickness exceeds 250 μm. Therefore, an improvement in the sound absorption coefficient can be achieved by adjusting the thickness of the thin film according to the required sound absorption properties.

The sound absorbing material of the present invention is intended to be used in an exhaust silencing device for an internal combustion engine, but depending on how it is used, it may also be used as a sound absorbing material in other equipment such as a combustor. Additionally, it can be used in environments with a lot of powder such as dust and cement.

As explained above, according to the method of manufacturing a sound absorbing material according to the present invention, the sound absorbing material formed into a thin film by melt spray coating has no deterioration in sound deadening performance, and furthermore, by selecting the thermal spraying time, the film thickness can be adjusted to any desired thickness. The sound absorbing properties can be easily improved because of the ability to select the desired temperature.Furthermore, before thermal spraying, a filler that releases less gas due to heating during thermal spraying and is easy to remove after thermal spraying is installed inside the surface of the porous material to be thermally sprayed. By filling it in advance, the formation of a thin film becomes more reliable.

[Brief explanation of drawings]

1 and 2 are enlarged sectional views of a sound absorbing material manufactured according to an embodiment of the present invention, and FIGS. 3 and 4 are characteristic diagrams of the sound absorbing material according to an embodiment of the present invention. 1... Porous body, 4... Thin film, 5... Filler. Note that the same reference numerals in the figures indicate the same or corresponding parts.

Claims (1)

[Claims] 1. A step of applying and hardening a filler made of a liquid dehydration-curable inorganic compound, a low-melting point metal, or a polymer compound to the surface of a porous sound-absorbing base material made of a metal or inorganic material, and closing the porous sound-absorbing base material with the porous sound-absorbing base material. A step of polishing the surface of the material until this base material appears, and a process of melting and spraying a metal or inorganic material onto the polished surface of the porous sound-absorbing base material to form a thin film with a predetermined thickness within the range of several tens of μm to 250 μm. A method for producing a sound absorbing material, comprising: a step of removing a filler in the porous sound absorbing base material.