Flexible polyurethane (PU) foams comprising various additive components were synthesized to improve their acoustic performances. The purpose of this study was to investigate the effects of various additive components of the PU foams on the resultant sound absorption, which was characterized by the impedance tube technique to obtain the incident sound absorption coefficient and transmission loss. The maximum enhancement in the acoustic properties of the foams was obtained by adding fluorine-dichloroethane (141b) and triethanolamine. The results showed that the acoustic absorption properties of the PU foams were improved by adding 141b and triethanolamine and depended on the amount of the water, 141b, and triethanolamine.
The gradual increase in the undesirable and hazardous noise level has perplexed our living and working environment; therefore, noise reduction is a very important issue [
PU is a significant sound absorbing material owing to its relatively low density and high porosity. It can be converted into ordinary soft foam, high elastic foam, or other porous materials as required in diverse applications. Viscoelastic polymer foams have excellent performance in the acoustic absorption because they can attenuate certain vibrations and absorb sound energy. Moreover, they can also convert sound and mechanical vibrational energy into heat. When acoustic waves propagate in the foams, energy is decreased as a result of the air friction loss in the cores and then transformed into heat. This energy absorption is limited by the PU morphology, which is an inherent property of the foam and is described by the gradual decrease in energy in the dissemination of the acoustic wave propagation [
Both the sound absorption coefficient and transmission loss are the parameters for measuring the sound absorption performance of materials. The absorption coefficient can be measured using the standard test procedures in the standard of “Standard Test Method for Impedance and Absorption of Acoustical Materials using a tube, two microphones and a digital Frequency Analysis System.” Recent studies have shown the focus on the effect of these sound absorbing properties of PU foams to enhance the acoustic performances (e.g., vibration and acoustic attenuations). The sound absorption properties of the foams have been extensively studied. Zhang et al. investigate the effect of the pore cell size and open porosity on the sound absorption performance of PU porous materials, and experiments were performed to investigate the relationships between the geometrical characteristics of the polyurethane foams and their acoustic performances [
In this study, flexible PU foams consisting of various additive components have been synthesized to enhance sound absorption. The purpose of this paper was to investigate the effect of the various additive components of the PU foams on their sound absorption.
PU foams were synthesized using isocyanate and polyether polyols or isocyanate and polyester polyols. Soft ether PU foams were synthesized to be used as the acoustic and insulation materials in automobiles. The names and major characteristics of the materials used are listed in Table
Major characteristic of the materials.
Materials | Supplier | |
---|---|---|
Isocyanate | MDI | Guangzhou Yuju Chemical Co. China |
Polyether polyol | 3630 | Guangzhou Yuju Chemical Co. China |
Polyether polyol | 330N | Guangzhou Yuju Chemical Co. China |
Catalyst | A1 and A33 | Guangzhou Yuju Chemical Co. China |
Catalyst | TEA | Jiahua Fushun Chemical Co. China |
Surfactant | Silicone 8681 | Guangzhou Yuju Chemical Co. China |
Blowing agent | 141b | Guangzhou Yuju Chemical Co. China |
Methylenediphenyl diisocyanate (MDI) was obtained from Guangzhou Yiju Chemical Company, China. A1 and A33 were used as the amine catalysts. A1, composed of 70% bisdimethylaminoethyl ether solution in dipropylene glycol, is a “foaming” type catalyst. Moreover, A33 is composed of 33% triethylene diamine solution. Water and 141b are both used for blowing agents. Triethanolamine, as the catalyst, played a dominant role in controlling the cell size of the foam. Silicone, widely used in the PU foam industry, was selected as the foam stabilizer and surfactant in this study.
The polyether polyols 330N and 3630, surfactant, catalyst, and deionized water were weighed according to the foam formulation listed in Table
Foam formulation and design.
Main component | Group 1 | Group 2 | Group 3 | Group 4 |
---|---|---|---|---|
(parts by weight) | (parts by weight) | (parts by weight) | (parts by weight) | |
3630 | 28 | 28 | 28 | 28 |
330N | 52 | 52 | 52 | 52 |
MDI | 28.8 | 28.8 | 28.8 | 28.8 |
Catalyst A1 | 0.04 | 0.04 | 0.04 | 0.04 |
Catalyst A33 | 0.9 | 0.9 | 0.9 | 0.9 |
Silicone | 0.9 | 0.9 | 0.9 | 0.9 |
Process for preparing the polyurethane foams.
Prepared foams for testing.
The porosity
The layout for the test equipment for airflow resistance.
According to the study of Benkreira et al. on the porosity [
The layout for the test equipment for porosity.
The experimental methods according to ASTM E-1050 were used to determine the normal sound absorption performance [
Apparatus of the two-microphone impedance tube for the ASTM-E-1050: absorption testing.
The test sample is mounted at one end of a straight, rigid, smooth, and airtight impedance tube; the other end of the tube is connected to a sound source (loudspeaker). Plane waves are generated in the tube by a sound source (random, pseudorandom sequence or chirp), and the sound pressures are measured at two locations near to the sample. Sound waves propagate as the plane waves in the tube, hit the sample, get partially absorbed, and then get subsequently reflected. The test measures the sound pressure close to the sample at two different positions to obtain the acoustic transfer function of the two-microphone signals. The complex acoustic transfer function of the two-microphone signals is determined and used to compute the normal incident complex reflection factor, normal-incidence sound absorption coefficient, and transmission loss of the test material. The acoustic absorption coefficient is defined as the ratio of the acoustic energy absorbed by the foam to the acoustic energy incident on its surface, and it depends on the frequency. The absorption coefficient was calculated as the average value of the values obtained from four cylindrical foam pieces (100 mm in diameter and 20 mm in thickness) in the frequency range 100–6,000 Hz, according to the procedure. The sound waves were perpendicular to the surface of the foams. Each of the tests was repeated at least four times to obtain consistent and precise results [
Example of the layout for the test equipment. (1) Microphone A, (2) microphone B, (3) test specimen, (4) impedance tube, (5) sound source, (6) amplifier, (7) signal generator, and (8) frequency analysis system.
In this study, water was used as the primary blowing agent as well as the chemical blowing agent in the PU, and it plays a dominant role in controlling the absorption coefficient. In the first experiment, water was added to investigate the influential trends on the absorption coefficient of the PU foams according to the recipes listed in Table
Main components and properties of the foam.
Main component | Group 1 | Group 2 | Group 3 | Group 4 |
---|---|---|---|---|
(parts by weight) | (parts by weight) | (parts by weight) | (parts by weight) | |
3630 | 28 | 28 | 28 | 28 |
330N | 52 | 52 | 52 | 52 |
MDI | 28.8 | 28.8 | 28.8 | 28.8 |
Catalyst A33 | 0.9 | 0.9 | 0.9 | 0.9 |
Catalyst A1 | 0.04 | 0.04 | 0.04 | 0.04 |
Silicone | 0.9 | 0.9 | 0.9 | 0.9 |
TEA | 1.0 | 1.0 | 1.0 | 1.0 |
Water | 3.8 | 4.1 | 4.4 | 4.7 |
Density (kg/m3) | 80.2 | 71.8 | 69.9 | 64.0 |
Airflow resistivity | 18,600 | 15,400 | 14,600 | 11,300 |
Porosity | 74 | 77 | 83 | 89 |
Figure
Curves showing the sound absorption character of the PU foams with varying water contents. (a) Group 1 with water 3.8 parts by weight, (b) Group 2 with water 4.1 parts by weight, (c) Group 3 with water 4.4 parts by weight, and (d) Group 4 with water 4.7 parts by weight.
Curves of transmission loss of the PU foams with varying water contents. (a) Group 1 with water 3.8 parts by weight, (b) Group 2 with water 4.1 parts by weight (c) Group 3 with water 4.4 parts by weight, and (d) Group 4 with water 4.7 parts by weight.
According to the results of the above experiments, when the water content is controlled within a particular range, the sound absorption performance is improved by increasing water content. However, when the water content continued to increase, the sound absorption performance decreased. The primary reason for the change in the sound dampening is the content of water in the foams. The average porosity of the foams increases and the density becomes smaller with increasing water content. Therefore, the reasonable control of the water content is necessary to improve the sound absorption properties of the foams.
In the second experiment, the type and amount of foaming agent was changed to investigate their effects on the sound absorption properties of the foams by adding 141b agent. The components and properties of the foams are listed in Table
Main components and properties of the foams.
Main component | Group 1 | Group 2 | Group 3 | Group 4 |
---|---|---|---|---|
(parts by weight) | (parts by weight) | (parts by weight) | (parts by weight) | |
3630 | 28 | 28 | 28 | 28 |
330N | 52 | 52 | 52 | 52 |
MDI | 28.8 | 28.8 | 28.8 | 28.8 |
Catalyst A33 | 0.9 | 0.9 | 0.9 | 0.9 |
Catalyst A1 | 0.04 | 0.04 | 0.04 | 0.04 |
Silicone | 0.9 | 0.9 | 0.9 | 0.9 |
TEA | 1.0 | 1.0 | 1.0 | 1.0 |
141b | 5.0 | 5.0 | 5.0 | 5.0 |
Water | 3.8 | 4.1 | 4.4 | 4.7 |
Density (kg/m3) | 81.2 | 75.0 | 64.8 | 61.9 |
Airflow resistivity | 17,800 | 16,500 | 13,800 | 9,800 |
Porosity | 72 | 79 | 85 | 91 |
Figure
Curves of the sound absorption character of the PU foams with varying water with the 141b. (a) Group 1 with water 3.8 parts by weight, (b) Group 2 with water 4.1 parts by weight, (c) Group 3 with water 4.4 parts by weight, and (d) Group 4 with water 4.7 parts by weight.
As shown in Figure
Curves of transmission loss of the PU foams of varying water contents and 141b. (a) Group 1 with water 3.8 parts by weight, (b) Group 2 with water 4.1 parts by weight, (c) Group 3 with water 4.4 parts by weight, and (d) Group 4 with water 4.7 parts by weight.
Curves of sound absorption character of the PU foams with two different compositions, one containing water without 141b and the other containing water with 141b.
TEA acts as a catalyst in the reaction system. According to the effects of TEA on the foaming reaction with increasing TEA content, the foaming time decreases, and the catalytic ability to enhance the foaming reaction to produce CO2 strengthens. Meanwhile, the cell size and gas proportion both increase. When the TEA content is <7 parts by weight, the foaming reaction is stronger than the gel reaction. Thus, the porosity of the foams increases with increasing TEA content. This is very likely because of the domination of the crosslinking reaction. Therefore, the system can be optimized to enhance the sound absorption properties of the foam by varying the amount of TEA. As a result, in the third experiment, the amount of TEA was varied to investigate the effects on the foam performance; the major components of the materials are listed in Table
Main components and properties of the foams.
Main component | Group 1 | Group 2 | Group 3 | Group 4 |
---|---|---|---|---|
(parts by weight) | (parts by weight) | (parts by weight) | (parts by weight) | |
3630 | 28 | 28 | 28 | 28 |
330N | 52 | 52 | 52 | 52 |
MDI | 28.8 | 28.8 | 28.8 | 28.8 |
Catalyst A33 | 0.9 | 0.9 | 0.9 | 0.9 |
Catalyst A1 | 0.04 | 0.04 | 0.04 | 0.04 |
Silicone | 0.9 | 0.9 | 0.9 | 0.9 |
141b | 5.0 | 5.0 | 5.0 | 5.0 |
Water | 3.8 | 3.8 | 3.8 | 3.8 |
TEA | 1.8 | 2.1 | 2.4 | 2.7 |
Density (kg/m3) | 81.6 | 78.3 | 74.36 | 70.5 |
Airflow resistivity | 17,100 | 16,700 | 15,200 | 14,900 |
Porosity | 74 | 82 | 87 | 89 |
Figure
Curves of absorption coefficients of the PU foams with added triethanolamine. (a) Group 1 with 3.8 parts by weight, (b) Group 2 with 4.1 parts by weight, (c) Group 3 with 4.4 parts by weight, and (d) Group 4 with 4.7 parts by weight.
In this study, flexible PU foams with varying amounts of additive components were synthesized to improve sound damping and absorption. To investigate the effect of the various additive components on the sound absorption of the foams, the correlations between the contents of additive components, absorption coefficients, and transmission losses were studied. The two-microphone impedance tube was applied to measure the sound absorption and transmission loss. This study has the following conclusions: When water was used as the only blowing agent in the foams, the sound absorption properties improved with increasing water content in a certain range. When the water content was 4.4 parts by weight, the sound absorption coefficient reached a maximum of 0.935. Excessive water will increase porosity of the foams, thus lowering its density and airflow resistivity, thereby decreasing sound absorption and dramatically increasing transmission loss. The addition of the foaming agent 141b dramatically enhanced the absorption performance of the foams. When the amount of 141b was 1 part by weight, an increase in water content gradually increased the sound absorption performance and reached the highest value of 0.985 without the declining trend of the curves with excessive water content. The addition of 141b decreased the airflow resistivity and increased the porosity in some sense. Adding TEA has certain effects on the density of the foams and the absorption performance was enhanced. When more TEA was added, good quality foams were obtained. 141b and TEA show the enhancing effects on the acoustic properties of the foams.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This study was supported by the National Natural Science Foundation project (no. 51205152), Jilin Provincial Natural Science Foundation project (20140101075JC), Specialized Research Fund for the Doctoral Program of Higher Education (20120061120036), China Postdoctoral Science Foundation Funded project (2012M520675), Jilin Province “Chunmiao” Talents Scheme, China Postdoctoral Science Foundation Funded project (2013T60322), and China Automobile (Beijing) Vehicle Lightweight Technology Institute.