1. Introduction
Rubber, a highly viscoelastic polymer, is an irreplaceable key material for modern aerospace [
1], aviation [
2], automotive [
3], shock absorbers, and soundproofing systems [
4,
5]. However, it has a narrow application in some special occasions, such as high-speed train floating floors, ship or aircraft cabin panels and other cutting-edge industrial fields. Unlike general mechanical fields, these fields require materials with large damping factor, excellent mechanical properties, and also to meet the requirements of sound insulation properties. Preparation of rubber composite materials that meet the requirements of multiple performance indexes and can be applied to more sophisticated industrial fields is an urgent problem to be solved.
In recent years, the design of elastomeric nanocomposites with excellent properties has aroused great interest, and numerous researches have demonstrated that the rubber–filler and rubber–rubber interactions are one of the important approaches to compensate for the single property of rubber materials [
6,
7], including mechanical properties, rheological properties, thermal conductivity, etc. [
8,
9,
10]. Qu et al. [
11] reported that nanoclay (NC) and carbon black (CB) filling natural rubber (NR) could effectively improve the mechanical properties, and this improvement was largely dependent on the NC–CB mesh structures, rather than the strain-induced crystallization during the elongation of natural rubber. Tang et al. [
8] incorporated the organically modified montmorillonite (OMMT) into chlorobutyl rubber (CIIR) and found that the addition of OMMT improved the tensile strength and hardness, as well as the stress relaxation rate, of CIIR nanocomposites. Several studies have also revealed that the combination of mica powder (MP) with other fillers improved the mechanical properties of composites under tensile and tearing modes, such as MP/wollastonite-filled styrene-butadiene rubber (SBR)/natural rubber (NR) [
12] and MP/graphite flakes-filled ethylene propylene diene monomer (EPDM) [
13]. In addition, due to the unique structure of hard glass on the outside and inert gas on the inside, hollow glass beads (HGB) are also widely used to fabricate polymer composites for mechanical reinforcement in different applications, such as nitrile and silicone rubbers [
14,
15].
Generally, for obtaining good sound insulation in occasions, application of sound absorption and isolation materials to attenuate or eliminate sound waves during transmission is an effective way, in addition to controlling the noise sources [
16]. Among various materials, rubber exhibits specific viscoelastic and damping properties that have been proven to play an important role in sound insulation and noise reduction, which can convert the kinetic energy generated by the sound waves incident on the material surface into thermal energy [
17,
18]. Sukontasukkul et al. [
19] took advantage of this viscoelastic characteristic of rubber materials, and attached the viscoelastic polymer sheets (VPS) onto the surface of ceramic tiles to improve the damping and sound insulation effect of ceramic tiles. Najib et al. [
20] studied the correlation between acoustic and dynamic mechanical properties of natural rubber foam, and proved that the viscoelastic and damping properties of matrix had a significant influence on the acoustic efficiency. In addition, it has also been confirmed that the strong interaction between polymers and fillers with unique structures such as high specific surface area and hollow structure can increase the transmission path of sound waves and improve the sound transmission loss of composites [
21,
22]. Based on this, Xia et al. [
23] fabricated the low-density polyethylene (LDPE) filled by mica with high specific surface area, and their results indicated that the LDPE/mica composites exhibited better sound insulation and mechanical properties compared to the pure LDPE. Wang et al. [
24] also pointed out that the introduction of mica contributed to the acoustic friction loss and increased the sound insulation capacity of PVC/mica composites. Zhang et al. [
25] reported that the synergistic effect of polyurethane elastomer and hollow glass microspheres improved both damping and sound insulation properties of composites. In our previous work, the sound insulation properties of six kinds of rubber were investigated in detail, and results showed that a combination of nitrile butadiene rubber (NBR) and chloroprene rubber (CR) had excellent damping and sound insulation properties. In addition, fillers such as montmorillonite (MMT), hollow glass beads (HGB), barium sulfate (BaSO
4), and mica powder (MP) were beneficial to the soundproof properties of rubber composites [
26].
In this paper, we tried to investigate the rubber combination and hybrid fillers, making use of the benefits provided by the individual components, aiming to find out their synergistic effect for the acoustic composites by orthogonal test design. The blending ratio of NBR/CR matrix and fillers content of HGB, MMT, and MP were selected as the test factors, while tensile strength, average transmission loss, and damping peak were employed as the test indexes. The influence of four factors on the test indexes were evaluated quantitatively, and the best formula was given by matrix weight analysis.
2. Raw Materials and Methodologies
2.1. Raw Materials
NBR, brand N41, was supplied by Duokang Industrial Co., Ltd., Shanghai, China (29% acrylonitrile, ρ = 0.96 g/cm3, average molecular weight is 700,000). The CR, brand 230, was gained from Shanxiang Industrial Co., Ltd., Shanghai, China (ρ = 1.24 g/cm3, average molecular weight is 170,000). MP (1250 mesh) was gained from Lingshou County Huayuan Co., Ltd., Shijiazhuang, China. HGB (2500 mesh) was obtained from Yinuo New Material Co., Ltd., Dongying, China. MMT (2500 mesh) was provided by Lingshou County Yuntao Mineral Products Trading Co., Ltd., Shijiazhuang, China.
2.2. Orthogonal Test Design
The orthogonal test is a design method widely used to investigate the multifactor complex system for system design and optimization [
27,
28]. In the present work, considering comprehensively coupling interaction of the three functional material fillers on the rubber compound system, an orthogonal experiment was designed to obtain the optimal formula of sound insulation materials that met the high damping and excellent mechanical properties requirements. The orthogonal experiment was carried out by adopting four factors and five levels, totaling 25 groups of test, as explained in
Table 1. For the detailed process and method of composite materials, the reader can refer to our previous research and work [
26].
2.3. Characterization
2.3.1. Mechanical Properties
Tensile strength was determined with a TS 2005b tensile testing machine (U-CAN Dynatex Inc., Taiwan, China). The test samples were fabricated into dumbbell style and were stretched at a rate of 50 mm·min−1 following the GB/T528-1998 standard. For each composite, the reported values were calculated by taking the average data of 5 samples.
2.3.2. Damping Properties
The damping peak (tan
δmax) was measured under the dynamic mechanical performance test, which was carried out following the GB/T33061.1-2016 on a DMA-150 dynamic mechanical analyzer (NETZSCH GABO Instruments GmbH, Ahlden, Germany). The samples (30 × 6 × 2 mm) were tested in a uniaxial tensile mode with test frequency of 10 Hz, temperature range from −40 to 40 °C, and heating rate of 3 °C min
−1. Accordingly, the damping property is characterized by Equation (1).
where tan
δ is the damping factor,
E″ is the loss modulus, and
E′ is the storage modulus, respectively.
2.3.3. Sound Insulation Properties
Sound insulation property was characterized by the average transmission loss (TL
avg), measured by an SW422/SW477 dual-channel impedance tube (BSWA tech., Beijing, China) within fabricated circular samples (2 mm thickness). The diameter of the SW422 tube was 100 mm with the testing frequency range of ≈63–1600 Hz, and the diameter of the SW477 tube was 30 mm with the testing frequency range of ≈1000–6300 Hz. Each sample was tested 5 times, being rotated a certain angle each time. Accordingly, the transmission loss can be calculated using Equation (2).
where
R is the sound transmission loss and
τ is the transmission coefficient, representing the ratio of transmitted sound energy to incident sound energy.