JPS6112874B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a porous sound-absorbing material, particularly a method for manufacturing a porous sound-absorbing material with permeable air bubbles that exhibits a high sound absorption coefficient in a low frequency sound range. In recent years, noise pollution represented by, for example, traffic noise has become one of the major social problems, and various preventive measures are being planned. Looking at this noise prevention measure from the viewpoint of material development, there is a strong desire today for the emergence of sound absorbing materials that have excellent weather resistance and a high sound absorption coefficient in the low frequency sound range. In view of such social demands, the present invention aims to provide a method for manufacturing a sound absorbing material having the above-described properties. In accordance with this objective, the method for producing a sound absorbing material according to the present invention includes bubbling an aqueous solution containing a surfactant, and then immediately mixing the foaming liquid with a hydraulic substance to prepare a foam slurry. In the method of manufacturing a porous sound absorbing material by curing this foam slurry, the foaming liquid has a syneresis rate of 20% or more and 75% or less when left standing for 5 minutes after foaming. The feature is that the air bubbles of the porous sound absorbing material are made into through holes by using a material having certain properties. In the foam slurry obtained by mixing a foaming liquid containing a surfactant and a hydraulic substance, the hydraulic substance is uniformly dispersed on the surface of the foam film of closed cells in the foaming liquid immediately after mixing. There is. However, if this foam slurry is left to stand still, some of the hydraulic substances will settle due to the action of gravity over time, while the air bubbles will aggregate due to the surface tension of the foam film.
When the membrane ruptures and becomes coarser and grows into bubbles of a certain size, gas escapes into the atmosphere and the bubbles disappear. However, these phenomena are hindered by tissue immobilization phenomena based on the coagulation of hydraulic substances. Depending on the time of immobilization of this tissue, porous bodies with various structures in which air bubbles penetrate are formed. It is known that the sound absorption performance of a porous sound absorbing material is generally better as the porosity of the structure is higher and the ratio of continuous pores to independent pores is higher.
Based on many years of research on porous structures, the present inventor has found that in order to exhibit high sound absorption performance, especially in the low frequency sound range, in addition to the above conditions, it is necessary to The present invention was completed based on the discovery that it is desirable to have a high content of continuous pores, and that the formation of such penetrating continuous pores is closely related to the aggregation, membrane rupture, and coarsening of bubbles in the foam slurry. It is. The most important point in producing a porous body having the above-described structure is that the timing of the gathering of bubbles, the rupture of the membrane, and the coagulation of the hydraulic material is appropriate. If the solidification of the hydraulic material occurs at a stage where the bubbles are insufficiently assembled, a fine closed-cell structure will be formed; If the timing is significantly delayed, large pores will be formed within the structure, or the foaming phenomenon will further result in a tissue with significantly less pore content, and in either case, the sound absorption performance will be extremely poor. The material becomes porous. In order to form a porous structure with continuous pores penetrated by air bubbles, it is necessary for the air bubbles to actively aggregate, rupture, and coarsen while the air bubble slurry is left standing, which leads to the solidification of the hydraulic material. Before fixation of the tissue, hundreds to thousands of bubbles gather and the membrane ruptures, thereby achieving penetration of the pores, that is, communication. Therefore, the foaming liquid constituting the foam slurry must have appropriate defoaming properties, in other words, the ability to aggregate bubbles, rupture membranes, and coarsen the foam, and must have hydraulic properties suited to the defoaming rate. It is necessary for the substance to solidify. It has been found that characteristics such as bubble aggregation and membrane rupture in a foam slurry are expressed by the syneresis rate of the foaming liquid used. Here, the syneresis rate refers to the amount of liquid that separates when an aqueous solution containing a surfactant is foamed under the same conditions as when preparing the foamed slurry and is allowed to stand still. Refers to volume ratio. Therefore, the syneresis rate is also an index representing the defoaming property of the foaming liquid. The syneresis rate is measured, for example, as follows: 100 ml of foaming liquid is placed in a measuring cylinder with a capacity of 100, left to stand, and the volume of the liquid that does not contain bubbles separated at the bottom is read over time. From numerous experiments conducted by the present inventors, we have found that characteristics such as bubble aggregation and membrane rupture in a foam slurry are hardly affected by the type of hydraulic substance or surfactant; It was found that it depends on the liquid properties. At the initial stage of syneresis, fine air bubbles actively gather, the membrane ruptures, and the membrane becomes coarse, and this process is extremely important for the formation of a porous body with continuous pores penetrated by air bubbles. In order to form a continuous pore porous body with a pore size of 400 to 2000 μm that is effective as a sound absorbing material contemplated by the present invention, active aggregation of bubbles, membrane rupture, and coarsening occur in the initial stage after foaming. Specifically, the syneresis rate after standing for 5 minutes is 20 or more.
It has been found necessary to use a foaming liquid in the range of 75% by volume. In other words, if a foaming liquid with this value of less than 20% by volume is used, the aggregation, membrane rupture, and coarsening of bubbles in the initial stage will be too inactive, and when such a foam slurry is cured, small bubbles with a high pore content will be formed. On the other hand, most of the pores contained therein are closed cells, and the content of continuous pores penetrated by cells is very small.
Therefore, a porous body with high sound absorption performance cannot be obtained. On the contrary, the syneresis rate after standing for 5 minutes was 75% by volume.
If a foaming liquid exceeding The sound absorption performance is also poor. Typical examples of hydraulic substances used in the present invention include portland cement, alumina cement, and slag. In order to adjust the timing between the active aggregation, membrane rupture, and coarsening of the bubbles in the foam slurry and the solidification of the matrix, it is preferable to select a slurry whose initial solidification time is within 2 hours. Further, the type of surfactant used is not limited as long as it satisfies the conditions for the foaming liquid described above. Of course, it must not have an adverse effect on the curing reaction of the hydraulic material. Specific examples of surfactants used in the present invention include sodium dodecylbenzenesulfonate and sodium nonylphenoxydiethoxyethyl sulfate. These surfactants may be used in combination with a foam stabilizer such as polyvinyl alcohol or methyl cellulose, if necessary. Examples of the present invention will be described below. [Example 1] An aqueous solution having the composition listed in Table 1 was prepared with an internal volume of 500
The mixture was placed in a batch mixer and stirred for 3 minutes using a stirring blade with a diameter of 300 mm (rotation speed: 750 r.P.m) to foam. The syneresis characteristics of this foamed liquid were as shown by curve A in FIG. 1, and the syneresis rate was 44% when it was allowed to stand for 5 minutes after foaming. Next, while stirring this foaming liquid under the same conditions as above, a hydraulic substance mixture having the composition shown in Table 1 is added.
A foam slurry was prepared by quantitatively feeding 50 kg using a cement metering feeder over a period of 2 minutes, and stirring was continued for 1 minute. Immediately after stirring, add bubble slurry to a volume of 350mm ₃
It was cast into a mold. After adding the hydraulic substance mixture to the foaming solution, solidification started 16 minutes later, and it reached a strength sufficient to be demolded in about 40 minutes. After demolding, steam curing was carried out at a temperature of 55°C, and a porous body with permeable air bubbles was obtained. As shown in Table 1, the physical properties of this porous body are as follows: the average pore diameter is 1200μ, 500Hz
The normal incidence sound absorption coefficient is 93%, and the pressure gradient method (here, the pressure gradient method refers to the flow rate of air with a viscosity of μpoise with a pressure difference of Δpg/cm ² in the axial direction of a cylinder with a base area of Scm ² and a height of Hcm). It is a method to calculate air permeability Kp=H・Q・μ/ΔP・S by flowing at Qcm ³ /sec. However, 20
The air permeability measured by μ=181×10 ⁻⁶ poise at °C was 4.3×10 ⁻³ cm ³ /sec ⁻² . [Comparative Example 1] A porous body was prepared in the same manner as in Example 1 except for the composition of the foaming liquid. In this example, the foaming liquid has high syneresis (the 5-minute value is
77%), the continuous coarsening of pores occurred actively, but on the other hand, the cement holding power of the foam decreased, and large pores were formed in the center of the molded product during the solidification process of the cement structure, and large pores were formed in various places. Cracks were observed and it was difficult to put it into practical use. [Comparative Example 2] A porous body was prepared in the same manner as in Example 1 except for the composition of the foaming liquid. Since the foaming liquid in this example has low syneresis (3% in 5 minutes), tissue immobilization occurs at a stage where continuous coarsening due to membrane rupture of the pores in the foam slurry is insufficient.
Therefore, although the pores in the porous body are uniformly dispersed, most of them are closed cells, and the sound absorption coefficient is low, making it impractical as a sound absorbing material. [Examples 2 to 5, Comparative Examples 3 to 6] According to the method described in Example 1, different types of hydraulic substances and surfactants were used to create various bubble-pierced samples as shown in Table 1. A porous body was prepared. The physical properties of the obtained porous body are shown in the table.

【table】

[Table] From the experimental results described above, all porous materials prepared from surfactant aqueous solutions with syneresis rates in the range of 20% or more and 75% or less when left to stand for 5 minutes after foaming have a negative impact in the low frequency sound range. It can be seen that it exhibits excellent sound absorption coefficient.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the syneresis characteristics of various foaming liquids.

Claims (1)

[Claims] 1. After foaming an aqueous solution containing a surfactant, immediately mix this foaming liquid with a hydraulic substance to prepare a foam slurry, and manufacture a porous sound absorbing material by hardening this foam slurry. In this method, the foaming liquid has a property of having a syneresis rate of 20% or more and 75% or less when left standing for 5 minutes after foaming to penetrate the bubbles of the porous sound absorbing material. A method for producing a porous sound absorbing material characterized by having pores.