RETRACTED ARTICLE: Performance analysis of ethylene-propylene diene monomer sound-absorbing materials based on image processing recognition

Abstract

In order to study the performance of rubber sound-absorbing materials, this study processed the surface of ethylene-propylene diene monomer (EPDM) rubber sound-absorbing materials based on image recognition. Simultaneously, in this study, microscopic images were obtained from macroscopic rubber materials, and the images were processed to become standard images with certain characteristics. In addition, this study combines image processing to obtain pictures related to sound absorption performance. In the identification of rubber sound-absorbing materials, this study used EPDM rubber as the material, and studied the influence of various factors on the sound absorption performance of rubber sound-absorbing materials from the technical point of view and obtained the corresponding processed images. Through research, it is found that the sound-absorbing materials of this study have good sound-absorbing effects based on the control of relevant process conditions, and the image recognition and processing functions can be applied to the research of rubber sound-absorbing materials.

1 Introduction

Noise can have a negative impact on human health. In lighter cases, noise affects people’s quality of life, such as causing fatigue and discomfort, causing irritability, reducing labor productivity, and affecting communication quality. In addition, it damages the human auditory organs, causes deafness, and has adverse effects on the nervous and cardiovascular systems. Particularly strong noise even causes mental illness and death. Based on this, it is necessary to reduce the noise or the use of sound insulation materials to reduce the impact of noise on people. Among them, rubber sound-absorbing materials are a very common [1].

The rubber-based polymer is composed of a polymer with a chain structure. Generally, the polymer has a diameter of several angstroms, and also has long-chain molecules having a length of several thousand, tens of thousands, or even hundreds of thousands of angstroms, which are easily curled into random coils to exhibit flexibility of the polymer. Due to the long-chain structure of the polymer, the molecular weight is not only high but also polydisperse. In addition, it can have different side groups, plus branching, cross-linking, crystallization, orientation, copolymerization, etc., making the polymer’s motion unit multiplicative [2]. Therefore, compared with small molecules, the molecular motion of the polymer can be roughly divided into two sizes of motion units, that is, large-sized units and small-sized units. The multi-layer structure of the rubber polymer and the kinematic characteristics of the flexible molecular chain give the material a special high viscoelasticity, high internal damping characteristics, and molecular designability [3]. As a sound-absorbing material, it has the characteristics of slack time spectrum width, absorption of sound and audio bandwidth, sound absorption performance, and material use performance. What is more remarkable is that when the longitudinal acoustic wave in the air is introduced into the rubber elastic body, the vibration direction of the mass point deviates from the direction of propagation of the sound wave due to the shear deformation caused by the vibration of the polymer elastomer material. This shearing elastic force can change the direction in which the sound propagates, generate transverse waves, and increase the propagation path, which is beneficial to the improvement of the sound absorption coefficient [4]. In addition, compared with other sound-absorbing materials, polymer materials are easier to process by foaming, pressing, and extrusion, and the sound-absorbing structure is designed, which facilitates the simultaneous introduction of damping and other sound-absorbing mechanisms into sound-absorbing materials for sound-absorbing performance design [5].

At present, the design of the sound absorption properties of the polymer mainly focuses on the structural design of the polymer chain: such as the soft segment and hard segment ratio of the molecular chain, the molecular chain configuration, and the winding mode. The main purpose is to enhance the damping properties and intrinsic sound absorption properties of polymer materials. Then, by modifying in a polymer material, such as adding a bubble filler or other foaming process, the polymer material has a certain amount of microcavities, and the sound absorption performance of the material is improved by friction [6]. The rubber used to make the sound-absorbing material is styrene butadiene rubber, neoprene rubber, natural rubber, polyurethane, and the like. Because of the poor damping properties (energy conversion properties) of these rubbers, they rely mainly on the cavity to attenuate the acoustic signal, while the material itself contributes less to the attenuation of the underwater acoustic signal. Therefore, this material is particularly suitable for underwater acoustic absorption materials, and it has the advantages of light weight and easy molding compared with the material added with metal powder. The shortcoming is that the performance of the universal sound-absorbing material in the air is not high, and the elastomer rubber has disadvantages such as poor flame retardancy and aging due to its own problems [7].

Through the above research, new ideas for developing new sound-absorbing materials can be provided. Therefore, the development of this research has important and necessary significance for solving the problem of noise pollution. Combined with the above review, it can be found that the conductive phase/piezoelectric phase/elastomer piezoelectric damping sound absorbing composite material is a new technology, which is very promising because of its high internal friction, high damping and sound absorption coefficient, and excellent comprehensive performance. But there are still some problems. First, the overall performance of the damping sound absorption coefficient needs to be further improved. Second, due to the large number of phases and long preparation process, the influence of the relative composite properties is not clear. This study analyzes rubber sound-absorbing materials based on image analysis to solve the shortcomings of traditional rubber sound-absorbing materials.

2 Test methods

In this study, a moderately sized deketoximine type room temperature vulcanized silicone rubber matrix 107 was used as a prepolymerized monomer to prepare a matrix for piezoelectric damping sound-absorbing composites. Studies have shown that the greater the elasticity of the matrix polymer, the better the sound absorption performance of the matrix and the composite, while the excessive elasticity causes other properties to decrease. Therefore, when designing a piezoelectrically damped sound-absorbing composite material, it is desirable that the matrix has a higher elasticity to facilitate the sound-absorbing damping performance, and it is not desirable to reduce other mechanical properties due to excessive elasticity [8]. In this section, the prepared RTV substrate is modified to achieve a comprehensive improvement of flame retardancy, heat resistance, electrical resistivity, and mechanical properties, and is an alternative substrate for the preparation of piezoelectric damping sound-absorbing composite materials.

The powder material was dried in a vacuum oven at 105 °C/5 h until the constant weight was weighed on the analytical balance. The 107-base glue prepolymer in the drum is opened, and the calculated amount of glue is weighed according to Table 1 by the reduction method. It was poured into a three-necked flask and treated under agitation at 220 °C/6 h. After the water was sufficiently removed, the heating was stopped, and 120# gasoline was added at a time 2–3 times the volume of the glue. The heating was stopped, and the stirring was continued until uniform. The desired auxiliary was added, stirring was continued for 1–2 h, the mixture was poured out, and the bottle was sealed. If necessary, the bottle mouth needs to be opened, the mold is injected, and the solidified RTV blank sample can be obtained by standing at room temperature for 1–2 days. The above experimental procedure was repeated to obtain a series of RTV samples [9].

Table 1 Add amount of additives

Firstly, all pre-added powders, nano-SiO₂, D4-nano-SiO₂, DBDPE/Sb₂O₃, DBDPO, Al (OH)₃, are dried in a vacuum oven at 105 °C/5 h until the constant weight is weighed on the analytical balance. After that, it is put into the grinder according to the calculation amount. Then, the RTV base prepolymer is shaken, the cap is opened, and the grinder is injected into the grinder. The grinder is started, the temperature is set at 80–120 °C, and the powder and base rubber are dispersed and kneaded for 30–60 min. After that, it was transferred into a three-necked flask, and under the action of stirring, 120# gasoline was added in a mixing colloid volume of 2–3 times. After the dispersion is uniform, the mixture is decanted, some of the solvent is dried, and placed in the grinder again. A calculated amount of a crosslinking agent, a coupling agent, a catalyst, and the like are added to the ground colloid, and the mixture is ground for 60–90 min to fully crosslink the base rubber and seal the bottle.

A piezoelectric ceramic composite having a PZT volume fraction of 20% to 60% is prepared by using an elastomeric RTV as a matrix and a conventional PZT as a piezoelectric phase. The content of conductive phase NG was fixed to study the influence of PZT and RTV ratio on the comprehensive performance of piezoelectric damping composites. All powder materials are dried at 95 °C for 2–4 h. We do the ingredients according to the ratio of NG:piezoelectric phase:RTV prepolymer = 3: x: (100-x) (x is 0, 20, 30, 40, 50, 60 wt.%, respectively). The raw materials are ball milled on a planetary ball mill at a speed of 350 r/min for 6–8 h, and the solvent is dried after discharge. The elastic mixture prepolymerization system is further extruded and mixed on a three-roll mill, and then a wafer-shaped sample having a thickness of 10 cm and a thickness of 10 cm is pressed by a self-made tableting mold on a powder tableting machine. After the multimeter was tested for non-conduction, the upper and lower surfaces were coated with electrodes and polarized in a silicone oil at a high pressure of 8–10 kV for 15 min to obtain a sample [10].

Table 2 shows the piezoelectric properties of a series of 3 wt.% NG/PZT/RTV composites with different PZT contents before and after secondary high-voltage polarization. The added piezoelectric powder had d33 = 434 pC/N [11].

Table 2 Piezoelectric properties of NG/PZT/RTV composites before and after polarization

It can be seen from the table that before the secondary high-voltage polarization, the series of 3 wt.% NG/PZT/RTV composites with different PZT content also have partial piezoelectric properties, but it is lower than the same system after polarization. Meanwhile, the piezoelectric properties increase with the increase of PZT content, reaching a maximum value of 68 pC/N at a PZT content of 40 wt.% [12]. Thereafter, the PZT content continues to increase and the piezoelectric properties are extremely reduced. After the secondary high-voltage polarization, the piezoelectric properties of the composites are higher than those before the polarization of the same system, and the variation of the size with the PZT content is consistent with that before polarization. The maximum piezoelectric performance also occurs at a PZT content of 40 wt.%, which is 74 pC/N. Before the secondary high-voltage polarization, the reason why the 3 wt.% NG/PZT/RTV system has piezoelectric properties is that the added PZT itself has a high piezoelectric coefficient, and the piezoelectric properties of the piezoelectric material have characteristics that do not change with the shape of the material [13]. It is speculated that the piezoelectric performance after the secondary high-voltage polarization is improved because the voltage of the secondary polarization is much higher than the initial polarization voltage of the PZT, so that the remaining domains that are not easily deflected under high voltage are sufficiently polarized, and the insulated RTV is just to prevent the occurrence of breakdown [14].

The surface analysis of rubber sound-absorbing materials by image processing is a microscopic topography of a series of 3 wt.% NG/PZT/RTV composites with different raw materials and different PZT contents. Figure 1 shows an example of image processing of a rubber sound-absorbing material surface. Among them, Fig. 1a is the raw material of the rubber sound-absorbing material, and Fig. 1b is the part of the rubber sound-absorbing material selected as the research object. After material property analysis by image processing, the microscopic state of the rubber material is shown in Fig. 1c that is, the sound absorption performance analysis can be performed in combination with image analysis.

Fig. 1

Example of image processing of rubber sound absorbing material surface

3 Results

First of all, different doses of foaming agent will affect the performance of rubber sound-absorbing materials in the preparation. Then, 2-mm-thick rubber SEM photographs of different foaming agents are shown in Fig. 2. The analysis of Fig. 2 shows the influence of the amount of foaming agent on the microstructure of rubber, which is also an important part of rubber sound absorption research.

Fig. 2

Effect of the amount of blowing agent on the microstructure of rubber

The vulcanization characteristic curve of the foamed EPDM rubber is shown in Fig. 3 when the vulcanization temperature is 150 °C to 180 °C for 25 min.

Fig. 3

Vulcanization curve of foamed EPDM rubber at different temperatures

Figure 4 is a microscopic topography of a series of 3 wt.% NG/PZT/RTV composites with added raw materials and different PZT contents.

Fig. 4

Microscopic topography of a series of 3wt.%NG/PZT/RTV composites with different raw

The effect of different vulcanization times on the damping properties of the foamed EPDM rubber is shown in Fig. 5, where the vulcanization temperature was 165 °C.

Fig. 5

Effect of different vulcanization time on damping properties of foamed EPDM rubber

The effect of different vulcanization temperatures on the damping properties of the foamed EPDM rubber is shown in Fig. 6, where the vulcanization time is 30 min.

Fig. 6

Effect of different vulcanization time on damping properties of foamed EPDM rubber

The effect of carbon black on the SEM image of the foamed EPDM rubber is shown in Fig. 7.

Fig. 7

The effect of carbon black on the SEM image of the foamed EPDM rubber

The effect of filler type on the damping properties of carbon black reinforcing foamed EPDM rubber is shown in Fig. 8. Among them, the amount of carbon black is 10 phr, and the amount of filler is 20 phr.

Fig. 8

Effect of filler types on damping properties of carbon black reinforcing foamed EPDM rubber

The effect of the type of filler on the sound absorption properties of carbon black reinforced foamed EPDM rubber is shown in Fig. 3. At this time, the amount of carbon black is 10 phr.

4 Discussion and analysis

It can be seen from Fig. 2 that when the amount of the foaming agent is 3 phr, the number of cells is small, and most of the cells are closed cells. As blowing agent continues to increase, the number of cells continues to increase and gradually changes from closed to open. When the amount of blowing agent is 9 phr, most of them are mixed pore structures foaming agent is 12 phr, serious parallel pore phenomenon has appeared, and the cells are easily collapsed, and larger open pore structures are formed. When the amount of the foaming agent is 15 phr, the cells are dense at this time, the cell morphology is not uniform, and a pore phenomenon occurs [15]. These different cell structures are all affected by the difference in the amount of blowing agent.

It can be seen from Fig. 3 that the vulcanization temperature and the vulcanization time have a great influence on the torque of the foamed EPDM rubber. When the vulcanization temperature is 150 °C, the vulcanization temperature is low, the scorch time is extremely short, the foaming has not started, and the rubber has begun to vulcanize. When the foaming agent starts to decompose, the degree of crosslinking is already high, and it is difficult to form cells, so it cannot be vulcanized at this temperature. When the vulcanization temperature is 180 °C, the vulcanization process will be over vulcanized in less than 10 min. At this time, the vulcanization rate drops rapidly, and the foaming agent decomposes in a large amount, which causes the gas generated by the decomposition to substantially destroy the EPDM rubber matrix, causing the cells to collapse, and the resulting foamed EPDM rubber has major defects. Therefore, it cannot be vulcanized at this temperature. Observing a few curves, it can be found that when the vulcanization time is 15 min, the vulcanization period is basically reached, and the vulcanization rate is faster [16]. Therefore, the effect of vulcanization at a vulcanization temperature of 155 °C to 175 °C for 15 min to 35 min on the damping sound absorption properties of the foamed EPDM rubber will be discussed.

It can be seen from Fig. 4 that PZT has a higher degree of crystallization, complete lattice, clear grain boundaries, and full grain. The pure RTV elastomer has a smooth, flat section with no defects, holes, and cracks, and presents the cross-section characteristics of the elastic material. For the series of 3 wt.% NG/PZT/RTV composites, with the increase of PZT content, the interface compatibility between 3wt.%NG/30wt.%PZT/RTV and 3wt.%NG/40wt.%PZT/RTV is better, followed by 3wt.%NG/20wt.%PZT/RTV and 3wt.%NG/50wt.%PZT/RTV.

The effect of different vulcanization time on the damping properties of foamed EPDM rubber is shown in Fig. 5, where the vulcanization temperature is 165 °C. It can be seen from Fig. 5a that the foamed EPDM rubber has the largest storage modulus when the vulcanization time is 15 min. At other curing times, the storage modulus is not much different. This is because when the vulcanization time reaches 15 min, the vulcanization rate is already fast, the degree of crosslinking is already high, but the foaming is still in the early stage, and the gas pressure generated by the decomposition of the blowing agent is mostly insufficient to open the cross-linked EPDM rubber matrix, so that the cells are difficult to form. At this time, the foamed EPDM rubber has a small internal cell and a small number. When the vulcanization time is 20 min, at this time, the vulcanization rate of the foamed EPDM rubber is close to that of vulcanization at 15 min, but the foaming agent is decomposed in a large amount, and the internal pressure generated is too large, which breaks the cross-linking, and finally forms many cells. Thereafter, when the vulcanization time is 25 min, 30 min, and 35 min, the vulcanization rate and the foaming speed are both fast, the foaming and vulcanization reach a dynamic balance, and a large number of cells having a uniform pore diameter are produced [17].

It can be seen from Fig. 5b that the vulcanization time has a certain influence on the damping properties of the foamed EPDM rubber. At a temperature of − 25 °C, the maximum loss factor, the effective damping temperature range is − 65 °C~ 20 °C, and there is a peak at − 45 °C, a unique liquid-liquid transition peak of EPDM rubber. Under different curing time, their effective damping temperature range is not much different, but the loss factor is different. The vulcanization times corresponding to the loss factor from large to small are 30 min, 35 min, 25 min, 20 min, and 15 min, respectively. When the vulcanization time is 15 min, the loss factor of the foamed EPDM rubber is the lowest. When the vulcanization time is 20 min, the loss factor is slightly higher, and the vulcanization time has a higher loss factor at 25 min and 35 min. At 30 min, there is a maximum loss factor with a value of 1.44. The reason for this phenomenon is similar to 3.2(A). When the energy passes through the small and dense cells, there is greater energy loss. When the vulcanization time is 15 min, the generated cells are too small, the energy loss is small, and the damping performance is poor. When the vulcanization time is 20 min, the cells are too large and the energy loss is small. However, when the vulcanization time is 30 min, the generated cells are small and dense, and when the energy is passed, a large internal friction is generated by friction with the wall of the hole, waveform conversion, relaxation, etc., and the damping performance is improved.

It can be seen from Fig. 6a that the foamed EPDM rubber has the highest storage modulus when the vulcanization temperature is 160 °C. When the vulcanization temperature is 170 °C, 155 °C, the storage modulus is lower. When the vulcanization temperature is 165 °C, 175 °C, the storage modulus is the lowest. According to the analysis of the EPDM rubber vulcanization characteristic curve, when the vulcanization time is 30 min, the vulcanization temperature is in the range of 155 °C to 175 °C, and the vulcanization rate is not much different. However, at this time, the foaming speed has a large increase with the increase of the vulcanization temperature, which will have a great influence on the damping properties of the foamed EPDM rubber. As shown in Fig. 6b, the highest loss factor is 1.491 when the vulcanization temperature is 160 °C. However, in the whole effective damping temperature range, the loss factor is smaller than the loss factor of 165 °C, and the maximum loss factor of the foamed EPDM rubber is 1.44 at a vulcanization temperature of 165 °C, which is slightly smaller than the vulcanization temperature of 160 °C. Therefore, when the vulcanization temperature is 165 °C, the foamed EPDM rubber has the best damping performance. This phenomenon occurs because the foaming agent is largely decomposed at a vulcanization temperature of 170 °C and 175 °C, and the generated gas pressure is too large, destroying most of the cross-linking bonds. At the same time, a large number of cells collapse, and a parallel hole phenomenon occurs, and many defective cells are formed, which further reduces the damping performance. When the vulcanization temperature is 155 °C, the decomposition rate of the foaming agent is too low, the gas pressure is insufficient to break the cross-linking bonds, the generated cells are less, and the energy loss is also small. When the vulcanization temperature is 160 °C and 165 °C, the decomposition rate of the foaming agent reaches a certain level, and the vulcanization rate of the EPDM rubber reaches a dynamic balance. At this time, more cells with more uniform pore diameter will be generated. When the energy passes, there will be a large loss, and the damping performance will be further improved.

The effect of the amount of carbon black on the SEM photograph of the foamed EPDM rubber is shown in Fig. 7. It can be seen from the figure that when carbon black is used to reinforce foamed EPDM rubber, the cells are larger, and when the amount of carbon black is 10 phr, more cells are formed, and the walls of the holes are thicker, but uneven. As the amount of carbon black increases, the structure of the cells begins to deteriorate and gradually forms a pore phenomenon. When the amount of carbon black is more than 30 phr, the cells collapse, most of the cells produce a parallel pore phenomenon, and there are major defects, and the pore walls are also thinner and thinner, which eventually leads to only a small amount of closed-cell foam inside the foamed EPDM rubber. At the same time, a large number of parallel pore foams cause gas to escape and the damping sound absorption performance is degraded.

The effect of filler type on the damping properties of carbon black reinforcing foamed EPDM rubber is shown in Fig. 8. Among them, the amount of carbon black is 10 phr, and the amount of filler is 20 phr. It can be seen from Fig. 8a that after the addition of the rare earth oxide filler, the storage modulus of the foamed EPDM rubber decreases rapidly, and the fastest decline in the storage modulus is the Gd₂O₃-filled foamed EPDM rubber. When Y₂O₃, CeO₂, and La₂O₃ are filled with foamed EPDM rubber, the difference of storage modulus is small, and the addition of inorganic filler CaCO₃ has little effect on the storage modulus of foamed EPDM rubber. It can be seen from Fig. 8b that the Gd₂O₃-filled foamed EPDM rubber has the highest loss peak, the maximum loss factor value is 1.5 MPa at − 20 °C, and the effective damping temperature range is − 65 °C to 20 °C. Followed by Y₂O₃-, CeO₂-, CaCO₃-, and La₂O₃-filled foamed EPDM rubber, the damping effect is the worst. This phenomenon may be caused by the fact that the rare earth filler is a rigid particle, and the energy generated by the vibration generates more heat in the foamed EPDM rubber than the rigid particle, which causes a further increase in internal friction. The poor damping performance of La₂O₃ may be due to its poor compatibility with the EPDM rubber matrix, resulting in a large amount of agglomeration, resulting in reduced damping performance.

The damping property of the rare earth oxide Gd₂O₃-filled foamed EPDM rubber is best due to the magnetic properties of Gd₂O₃. After being filled into the rubber, when the external vibration can pass through the rubber, the rubber will undergo a certain deformation, which causes the movement of the Gd₂O₃ magnetic powder to be restrained and increases the relative displacement. At the same time, the total magnetization of the rubber changes rapidly, eventually causing macro eddy current loss in the rubber. In addition, the reversible movement of the domain wall and the microscopic eddy current generated by the rotation of the magnetization vector and the magnetic hysteresis damping caused by the irreversible movement of the domain wall can transform the vibration energy into heat dissipation, and finally achieve a good damping effect.

The effect of the type of filler on the sound absorption properties of carbon black reinforced foamed EPDM rubber is shown in Fig. 9, at which time the amount of carbon black is 10 phr. It can be seen from Fig. 9 that the sound absorption performance of the foamed EPDM rubber is obviously improved after the filler is added, and the foamed EPDM rubber filled with CaCO₃ has the worst effect. Gd₂O₃-filled foamed EPDM rubber has the best sound absorption performance, the maximum sound absorption coefficient is 0.39 at 400 Hz, and the sound absorption coefficient is increased by 0.2 when no filler is added, followed by Y₂O₃-, CeO₂-, and La₂O₃-filled foamed EPDM rubber. Filler is one of the reasons that affect the sound absorption performance of rubber materials. Different rare earth fillers have different effects on the sound absorption properties of foamed EPDM rubber, which are related to the structure and properties of rare earth itself. Yttrium and Gadolinium are densely arranged hexagonal structures, Cerium is a face-centered cubic structure, and Lanthanum is a double hexagonal structure. Since Yttrium and Gadolinium are magnetic, their magnetic damping increases the dissipation of acoustic energy in the foamed EPDM rubber, thereby improving the sound absorbing performance. Secondly, the compatibility of different rare earths with EPDM rubber matrix is also different. The compatibility of bismuth with EPDM rubber matrix is the worst, and the compatibility of other rare earths with EPDM rubber is not good. Therefore, there must be a better modification method to improve the compatibility of the rare earth with the EPDM rubber matrix.

Fig. 9

Effect of filler type on sound absorption performance of carbon black reinforcing foamed EPDM rubber

5 Conclusion

In order to study the properties of rubber sound-absorbing materials, this study analyzed the production process of rubber materials and obtained the following results: This study investigated the effect of carbon black on the damping properties of foamed EPDM rubber. The results show that only a small amount of carbon black will increase the damping performance, and the addition of carbon black will result in a decrease in sound absorption performance. The more carbon black added, the more obvious the sound absorption performance decreases. At the same time, this study discussed the effect of filler types on the damping properties of carbon black reinforced foamed EPDM rubber. It was found that Gd₂O₃-filled carbon black reinforced foamed EPDM rubber has the best damping performance and sound absorption performance; Y₂O₃-, CeO₂-, and La₂O₃-filled foamed EPDM rubber has poor sound absorption performance; and inorganic filler CaCO₃-filled foamed EPDM rubber has the worst sound absorption performance. This study investigated the effect of filler loading on the sound absorption properties of carbon black reinforced foamed EPDM rubber. The results show that the effect of the filler on the sound absorption properties of the foamed EPDM rubber is generally the more the filler is used. The worse the sound absorption effect, the best sound absorption effect when the filler dosage is 10 phr. In addition, this study investigated the effect of material thickness on the sound absorption properties of carbon black reinforced foamed EPDM rubber. The results show that the thicker the foamed EPDM rubber, the better its sound absorption performance. When the thickness of the foamed EPDM rubber is 60 mm, the maximum sound absorption coefficient is 0.66 at 400 Hz, and the effective sound absorption band is 250 to 2000 Hz.