Development of Macroporous Silicone Rubber for Acoustic Applications

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Development of Macro-Porous Silicone Rubber for Acoustic Applications Anil Kumar, Asfak Ali Mollah, Anup Kumar Keshri, Manoj Kumar, Kulvir Singh, Krishna Dutt Venkata Shiva Rallabhandi, and Raghunandan Seelaboyina Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02051 • Publication Date (Web): 26 Jul 2016 Downloaded from http://pubs.acs.org on July 29, 2016

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Industrial & Engineering Chemistry Research

Development of Macro-Porous Silicone Rubber for Acoustic Applications Anil Kumar c, Asfak Ali Mollah b, Anup Kumar Keshri c, Manoj Kumar a, Kulvir Singh a, Krishna Dutt Venkata Shiva Rallabhandi b, Raghunandan Seelaboyina a, * a

Centre for Nanotechnology, b Machine Dynamics & Failure Analysis Laboratory, Bharat Heavy

Electricals (BHEL) Corporate Research & Development (R & D), Vikasnagar, Hyderabad500093. India. c

Materials Science and Engineering, Indian Institute of Technology (IIT)-Patna, Bihta-801103.

India.

Abstract This work reports a comprehensive and scalable acoustic material preparation by medium internal phase emulsion templating technique from a single-part room temperature vulcanizable silicone rubber and pore generator (water and mineral oil mixture). By optimizing mixing conditions, pore generator volume and cure schedule macro-porous sound absorbing silicone rubber was developed. By optical and scanning electron microscopy the pore size, their interconnectivity, and heterogeneity was analyzed. The average sound absorption coefficient and transmission loss of ~12.5±1 mm of the optimized porous (~83%) sample with 0.5 volume-ratio water and 3.67 volume% mineral oil measured by impedance tube technique in 50-6400 Hz were ~0.45 and 6.7-42 dB. Insertion loss measurements in 125-6300 Hz with material as cladding inside (100%) and outside (85%) of an acoustical enclosure demonstrated minimum, maximum

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noise reduction of 2.31 dB at 630 Hz, 20.9 dB at 4000 Hz. and 3.55 dB at 400 Hz, 13.71 dB at 2000 Hz. Keywords: Sound absorber/attenuator, Emulsion templating, Liquid silicone rubber (LSR), Single-part room temperature vulcanizable (RTV-1)


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1. Introduction Exposure to excessive or repetitive noise is a potential risk especially for workers employed in heavy engineering industry including thermal and hydro power plants. Working in such conditions over a long period of time can result in hearing loss and other problems

1-4

.

Hence to protect the workers and reduce environmental noise pollution, control measures have been enacted and are enforced by governing bodies specific to various regions or countries 4. So it is mandatory for power plant equipment manufacturer and supplier such as Bharat Heavy Electricals limited (BHEL) to meet specific acoustic guidelines for power plant commissioning. Before an acoustic solution is designed, the dominant source of noise pollution is determined and either passive or active measures are applied to achieve a reduction in noise to acceptable level 15

. Noise reduction techniques involve modification of sound source, path and receiver 1-5. In this

work, the focus is on developing and applying passive noise attenuating macroporous silicone rubber to enclosures (sound path) of electrical equipment such as motors, industrial fans etc. Noise reduction by porous materials prepared from metals, polymers, and ceramics is not new and has been a subject of research by many groups

1-6

. The designer of the absorber/barrier

must know how to choose the proper material. The selection criterion includes the working environment and absorber physical properties like weight and adaptability to environment i.e. whether it can be applied as cladding to the enclosure of electrical equipment. Compared to porous materials prepared from metals and ceramics, polymers are relatively easy to use as claddings because they are flexible (can adapt the shapes of the enclosure), easy to manufacture and less expensive. Further empirical models have also been developed which demonstrate the noise attenuating capability of the porous materials, especially at medium and high frequencies 79

. Acoustic property of porous materials can be tailored by balancing porosity, density, and



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thickness. Several groups have optimized these parameters and reported the developed materials sound absorption coefficient (SAC), transmission loss (TL), and insertion loss 6-23. Porous solid materials are made up of open or closed pores through which sound wave can enter. Open pore materials have individual cells and their shape and sizes are irregular and have a continuous channel of communication with the external surface so they show higher sound absorption coefficient in the wide frequency range 2, 5. Open pore polymer noise absorbing materials are widely prepared from various polymers including polyurethane, silicone rubber, polyester, melamine, and polyimide

6-23

. Compared to the above materials silicone rubber is

preferred by power plant equipment manufacturers because of its application as insulation to various components (good dielectric material). Further, its low flammability, good elasticity, resistant to UV radiation and weathering and hydrophobic nature are advantageous for cladding of equipment enclosures. Sound absorbing porous structures from silicone rubber is not new and has been reported by various groups. However, most of the reported work is based on two-part polymer materials and relatively hazardous foaming agents (chlorofluorocarbon compounds) or pore generator (substance yielding porosity)

18-22

. Whereas, this work reports the macroporous

silicone rubber development with single part LSR and relatively non-hazardous pore generator by emulsion templating method that consists of generating pores by utilizing inverted water-inoil emulsion technique

22-30

. By this method highly interconnected heterogeneous macroporous

polymers can be produced by optimizing the polymer and pore generator content, mixing conditions and cure schedule (combination of temperature and time). This work reports the optimization of the above parameters to achieve a macroporous silicone rubber acoustic material with relatively less thickness (~12.5 mm, traditional materials: 30-100 mm

6-23

). A good sound

absorber is defined as material with average SAC greater than 0.2 19, and the material developed


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in this work demonstrated average SAC of ~0.45. The macroporous silicone rubber preparation method reported here is scalable i.e. smaller diameter (Ø) samples (100 mm) required for SAC and TL and larger samples (400 x 500 mm) required for insertion loss measurements were prepared by up-scaling the mixing vessel, dissolver blade, volume of silicone rubber, pore generator and mold size. Similar acoustic properties were observed in both large and small samples thereby demonstrating the stability of the process.

2. Experimental 2.1. Materials A poly(dimethylsiloxane) (PDMS) variant i.e. Powersil-567 of Wacker silicones, which is a one-component room temperature vulcanizable (RTV-1) LSR was used as the matrix for preparing the sound absorbing material. The reason for choosing LSR, a dielectric material as the polymer matrix is its compatibility and applicability in electrical industry. Deionized (DI) water (W) was used as the primary pore generator due to its non-toxic, eco-friendly, and relatively inexpensive nature. In addition to DI water, mineral oil (O) (Apar Industries, India-Power oil TO335, CAS number: 64742-55-8) was also mixed with LSR to achieve interconnected heterogeneous porosity. A mechanical stirrer IKA (Eurostar power control-visc, speed range: 502000 rpm) was used for uniformly dispersing the internal phase in LSR. The mixing was performed with a dissolver blade of Ø 42 mm (IKA R1303) for smaller and 96 mm for larger samples. Metro ark silicone 17 was used as the mold-releasing agent.



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2.2. Preparation of heterogeneous macro-porous silicone rubber The macroporous silicone rubber (MPSR) was prepared by mixing Powersil-567 LSR and pore generator mixture (figure 1). The porous nature of the polymer is dependent on the concentration difference between LSR to pore generator, their mixing and cure schedule. Hence, optimization to determine the LSR and pore generator content to achieve optimum porosity (>80%) required for good sound absorption (average SAC>0.2) was performed by preparing a series of samples by mixing various volume ratio of water = ⁄ ∗ 100 and volume% of mineral oil % = ⁄ + +

in LSR. Depending on the sample volume in the beaker, the overhead stirrer mixing conditions i.e. blade type (dissolver), the distance from the bottom of the beaker to blade (20-30 mm), speed (1000-2000 rpm) and mixing time (5-10 minutes) were optimized. The smaller samples (Ø 100 mm, figure 2a-2b) were prepared in a standard 250 ml glass beaker with a dissolver blade of Ø 42 mm and 105 ml of mil base (total volume of LSR and DI water). The 105 ml mil base was optimum for mixing in a 250 ml beaker with Ø 42 mm dissolver blade and was also sufficient to prepare four Ø 100 mm samples per batch. The larger (500 x 400 mm, figure 2c) samples were prepared in ~5000 ml beaker with dissolver blade of Ø 96 mm and ~2400 ml mil base. The Ø 100 mm samples with mineral oil in addition to DI water were prepared by adding the predetermined volume (1-5 ml) to the 105 ml mil base. In case of larger samples the mineral oil content was proportionately increased (90-94 ml) and added to 2400 ml mil base. After mixing the LSR and pore generator mixture, the as prepared emulsion with entrapped air is transferred into a cylindrical or a rectangular mold (depth: 15 mm, figure 1c-1d) and left for curing in an atmospheric oven with a circulating fan at 35-40 ̊C for 20-24 hours. In the mold the top surface is exposed to the atmosphere and bottom surface is in contact with the


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mold surface. The circulating fan helps in uniform distribution of heat resulting in the formation of a relatively smooth top layer i.e. surface exposed to atmosphere. In absence of the circulating fan a wrinkled top layer with open pores was observed. Curing/polymerization of Powersil-567 RTV-1 LSR occurs by condensation cure (Oxime) from the top surface towards the bottom. The cure mechanism involves an oxime functional cross linker (Methyl-O,O',O''-butan-2-ontrioximo-silane for Powersil-567) in the presence of catalyst (tin) and humidity. During the curing schedule 2-butanone oxime (methyl ethyl ketoxime, MEKO) is released, hence the oven was placed in a fume hood to remove MEKO from the lab environment. In the mold, the top surface develops ~500 µm completely cured i.e. non-sticky membrane. The completely cured top skin layer prevents the remaining material underneath the membrane from completing the curing process. This results in partially cured and sticky structure with pore generator mixture and entrapped air distributed throughout its volume. In the next step, the samples are removed and placed back in the mold with the bottom surface exposed to the atmosphere and top (skin layer) in contact with the mold. Cure schedule as shown in figure 1f is followed, this was observed to aid the escape of entrapped air and pore generator in the form of gas/vapor (water vapor, mineral oil) resulting in the formation of the porous structure of ~12.5 mm thickness. If the optimized cure schedule was not maintained and if the mold containing the polymer and pore generator mixture is left in the open atmospheric conditions it took 72 hours for complete curing with most of the water collected at the bottom of the mold. This led to a nonuniform, rigid, less porous (30-40%) and thin (4-6 mm) samples. The SAC and TL (data not reported here) of such samples were relatively less than the samples prepared with optimum conditions.



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2.3. Macro-porous silicone rubber characterization 2.3.1. Porosity The porosityΦ 3 of the MPSR was calculated using the following formula: Φ=

= 1−

(1)

where, Vpore is pore volume (m3), Vtotal is total volume (m3), ρa is the density of the porous material (kg/m3) and ρm is the density of the solid portion of material (kg/m3) i.e. solid silicone rubber. The density of the sample was calculated from its mass and volume.

2.3.2. Pore morphology The microstructure and morphological characterization of the samples were carried out with optical (figure 2e) and scanning electron (SEM-ZEISS, EVO18) microscopy (figure 2f-2g). Samples for SEM analysis were gold coated to minimize the charging effect. By utilizing imageJ software the SEM and optical microscope images were analyzed (~160 pores) to determine the average size of the pores (figure 2h) present in the volume of the macroporous silicone rubber.

2.3.3. Flow resistivity Flow resistivity (σ), of the macroporous silicone rubber sample, was carried out as per ASTM C 522-03 using flow resistivity test rig on Ø 100 mm sample. The measurements (temperature 25±1 °C and humidity 45±1%) were carried out at different flow rates and the differential pressure was measured across the surfaces of samples and flow resistivity value was computed as per formula is given below 3. ∆

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(2)

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where ∆" (pa) is differential pressure measured across the sample, V (sL/min) is volume velocity, volumetric airflow rate and d (m) is thickness of the sample.

2.3.4. Acoustic properties For the macroporous silicone rubber samples, two types of acoustic properties i.e. SAC (figure 3-5) and TL (figure 6) were studied. Measurement of SAC was performed for low frequencies i.e. 50-1600 Hz (large tube, Ø 100 mm) and high frequencies i.e. 500-6400 Hz (small tube, Ø 29 mm) separately using Bruel & Kjaer (B&K) two microphone impedance tube equipment (4206). The measurement was executed without an air gap between sample back side and support piston, sample front side and the front end of the tube. The test was performed according to the standard procedure detailed in ASTM E1050 across 50-6400 Hz in two geometries i.e. once with the incident sound normal/perpendicular to the top surface and in another to the bottom surface of the macroporous silicone rubber (figure 6d). Additionally, according to ASTM E2611-09, a four-microphone impedance tube setup was used to obtain the transmission loss measurements across 50-6400 Hz with BSWA make SW477. Each of the tests was repeated with at least five samples to ensure the consistency of the sample preparation process.

2.3.5. Insertion Loss Sound attenuation capability of the macroporous silicone rubber over 125-6300 Hz frequency range was carried out by insertion loss measurement setup (figure 7). The setup consisted of a mild steel box enclosure 500 x 500 x 500 (L x W x T mm, sheet metal thickness: 3 mm) having a B&K Omni-power sound source (type 4292-L) inside the enclosure. The enclosure



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box was prepared by welding five plates together and the bottom plate had an opening of Ø 52 mm to accommodate the sound source connection. The box was supported by four steel legs of both Ø and length ~60 mm. Rubber pads ~10 mm were placed underneath the legs to minimize the vibrational effects. The lid plate was bolted to the top, and a thin rubber layer was placed between the lid and the flange to prevent sound leakage. The measurement was taken in a relatively large room (5 x 5 x 3 m). Sound intensity measurements were made by placing five B&K microphones (4187 1/4-inch microphone with 2670 preamplifier) perpendicular to the center enclosure/box walls at 1 m distance. Data was acquired with B&K PULSE type 3560-C (portable data acquisition unit) and sound signal (swept sine 125-6300 Hz) was amplified by B&K power amplifier 2716C with the gain set at 18 dB. The insertion loss measurement was made with a sound source inside the bare enclosure/box followed by cladding the macroporous silicone rubber slabs (400 x 500 x 12.5 mm) inside as-well-as outside.

3. Results and Discussion 3.1 Medium Internal Phase Emulsion (MIPE) For acoustic applications the morphology of macroporous silicone rubber i.e. pore type (open or closed), their size distribution, and the amount of porosity (%Φ) are important. Hence in this work, they were optimized by mixing of various percentage pore generator to the solvent of Powersil-567 (C7-C10 isoalkanes). This technique referred as emulsion templating approach, was first developed by Unilever laboratories in 1980’s

27

. Depending on the volume percentage

(% = 100 × $ ⁄$ + ) of pore generator in the polymer matrix three types of emulsions i.e. high (>74%), medium (30-70%) and low (0.55, phase separation between internal phase and LSR was observed.

3.2 Acoustic properties measurement The acoustic properties of the samples are measured with B&K impedance tube setup in the low (50-1600 Hz) and high (500-6400 Hz) frequency was studied by preparing circular specimens of Ø 100 and 29 mm having a thickness of ~12.5±1 mm. Smaller Ø 29 mm samples were punched with a metal die from the Ø 100 mm samples. The as-prepared samples have different thickness layers on both faces i.e. 80%) by water-in-oil emulsions techniques22-30. The addition of diluents such as oil to LSR and DI water MIPE has been reported to serve two purposes

24

. First, it can significantly decrease the viscosity (probably by mixing with C7-C10

isoalkanes) of the prepolymer system which is required for achieving relatively uniform mixing of polymer and pore generator. Second, it can act as a pore formation agent, and the porous beads could be formed due to the evaporation of the diluent in the polymerization system, thus enhancing the density, interconnectivity, and heterogeneity of the pores which can enhance SAC and TL

22-30

. To determine the optimum content of mineral oil, various Vo% (0.94, 1.87 and

3.67) was added to DI water (VRw of 0.40, 0.45, 0.50 and 0.55) and mixed with LSR and the resulting samples SAC was measured (figure 4-5). From figure 4c, it can be observed that sample prepared with 0.5VRw DI water and 3.67Vo% of mineral oil demonstrated relatively higher average SAC of ~0.45 which is the best among all the measured samples. Further, the sample also has maximum SAC of 0.982 at 2000 Hz (bottom side, figure 4c) and 0.63 at 400 Hz (top side, figure 5c). To correlate the relatively higher average SAC of SR-0.5VRw-3.67Vo% sample further investigations as explained below were carried out. Figure 2e illustrates the structure of SR-0.5VRw-3.67Vo% sample, where three distinct regions i.e. skin, transition, and core are clearly observed. Analysis of these three regions was 12 ACS Paragon Plus Environment

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carried out by an optical microscope and SEM, which are shown in figure 2d, 2f-2c. The relatively non-porous skin layer of

Development of Macroporous Silicone Rubber for Acoustic Applications

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