Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry (2015) 45, 502–506 C Taylor & Francis Group, LLC Copyright ISSN: 1553-3174 print / 1553-3182 online DOI: 10.1080/15533174.2013.809751
Synthesis and the Surface Resistivity of Carbon Black Pigment on Black Silicone Thermal Control Coating MASOUD BAREKAT1, REZA SHOJA RAZAVI1, and FARIBORZ SHARIFIANJAZI2 1
Department of Materials Engineering, Malek Ashtar University of Technology, Isfahan, I. R. Iran Young Researchers and Elite Club, Najafabad Branch, Islamic Azad University, Najafabad, Isfahan, I. R. Iran
2
Received 19 April 2013; accepted 27 May 2013
Thermal control has become more and more important in the development of the spacecraft technology. The thermal control coatings are exposed to electrostatic charges that develop on the external surface of the spacecraft. In this research, the synthesis, the surface resistivity, and the influence of the type, percent of the pigments, and adhesive properties on the coating were studied. The weight fraction of the pigment in coatings fabricated by carbon black with a sponge-like structure are lower compared to coatings fabricated by carbon black with a nonporous structure also, the surface resistance decreased with increasing pigment contents. Keywords: adhesive properties, polydimethylsiloxane, surface resistivity, synthesis, thermal coating control
Introduction Nowadays, there is an increasing need worldwide for thermal control coating development in order to obtain desired properties in spacecraft. With the development of the spacecraft technology, thermal control has become more and more important in this area. A spacecraft in orbit undergoes extreme temperature cycling due to the direct sun load on one side and deep cold space on the other. This causes a large temperature change on the vehicle, which is in the range of –200◦ C to +200◦ C.[1] Thermal control coatings are important components of the passive thermal control system on spacecraft, which can adjust the surface balance temperature of spacecraft.[2] In most cases, the thermal control coating must dissipate electrostatic charges that develop on the external surface of the spacecraft. The charges would otherwise accumulate to cause arcing and possible damage to or interference with sensitive electronic equipment on or in the spacecraft.[3–5] In order to dissipate electrostatic charge, the thermal control coating must be somewhat electrically conductive, with a surface resistivity on the order of about 109 ohms per square or less.[3] The carbon black/silicone coating is a typical thermal control coating used on spacecraft extensively.[6] The organic silicone is often used as a binder in the carbon black/silicone coating, for it has a unique structure in which the main Si–O chains are Address correspondence to Masoud Barekat, Department of Materials Engineering, Malek Ashtar University of Technology, Isfahan, I. R. Iran. E-mail:
[email protected] Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsrt.
spiral shaped and the good performances such as the electric isolation and the resistance to heat, weather, and aging.[3,4] The carbon pigment provides the black color of the coating, and also is the source of its sufficient electrical conductivity to dissipate electrostatic charges.[3] With the development of the long life and high reliability spacecraft, the application of the black silicone thermal control coating becomes more and more wide. So the surface resistivity of the coating is very important, while the report about it is very little. Carbon black pigment on black silicone has received more attention than any others among the many candidate thermal control coatings investigated. In this study, we report on the synthesis of polydimethylsiloxane and carbon black for black silicone, surface resistivity, and adhesive properties on thermal control coating.
Experimental Materials The hydroxyl-terminated poly dimethyl siloxane (PDMS, with 18000 M.W., Gelest USA), methyl tris (methyl ethyl ketoximino) silane (Gelest, USA), and dibutyl tin dilaurate (Gelest, USA) were used as a basic polymer, a curing agent, and a catalyst, respectively. Carbon black (Printex xe2 and Printex V) as pigment was supplied by Degussa, Germany. Dow Corning 1200 (Dow Corning, USA) and analytical grade xylene (Merck, Germany) were used as primer and solvent, respectively.[7,8]
Surface Resistivity of Black Silicone
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Table 1. Compositions of samples studied. Sample
PDMS (Part by weight)
Pigment type
Pigment (part by weight)
Cross-linker (part by weight)
Catalyst (Part by weight)
100 100 100 100 100 100 100
— Printex xe2 Printex xe2 Printex xe2 Printex V Printex V Printex V
0 2 3 4 8 10 12
8 8 8 8 8 8 8
0.6 0.6 0.6 0.6 0.6 0.6 0.6
RTV-MB0 RTV-MB1 RTV-MB2 RTV-MB3 RTV-MB4 RTV-MB5 RTV-MB6
Result and Discussion
Materials Sample Preparation To prepare coating, PDMS was mixed with carbon black pigment. Carbon black pigments were dispersed in PDMS with a pearl mill apparatus at 800 rpm. The particle size was controlled with a grindometer according to the ASTM D1210 standard. The milling continued until the grindometer value reached to 0 μm. Xylene was added to the mixture during preparation. Oxime-substituted silanes were added to the mixture. The mixture was refluxed for 72 h in 80◦ C. The product was a fluid polydimethylsiloxane end blocked with ((CH3)2C = NO)2 Si(CH3)O0.5 units.[9] Subsequently, dibutyltin dilaurate catalyst was added to the mixture. The compositions of samples are presented in Table 1. The aluminum (Al 6061) plates (10 × 10 cm2) were first pretreated with anodic treatment (sulfuric acid anodic) according to ASTM D1730. After pretreatment, spraying Dow Corning 1200 primer, spraying adhesive AminoPropylTriEthoxySilane agent, and adding AminoPropylTriEthoxySilane agent to 1 wt% were used for adhesion improvement. The silicone rubber coatings were accurately sprayed onto the pretreated Al plates. The uniformity of thickness was controlled using coating thickness gauge. The coated plates were dried at air for 2 h for solvent evaporation and the completion of curing which was placed in a chamber at 50◦ C and RH = 70% for 14 days.
Fourier Transform Infrared Spectroscopy Figure 1 shows the FTIR spectra of pristine PDMS and PDMS cured with oximesilane. As can be observed in Figure 1, the peaks at 3435 and 3700 cm−1 in pristine PDMS are associated with the HOH and SiOH, respectively. The intensity of the bands at 3435 and 3700 cm−1 is very weak due to concentration of HOH and SiOH in PDMS is very low. The silica network is characterized by the strong absorptions at 1023, 1097, and 2903 cm−1, corresponding to the Si-O-Si asymmetric mode. The peaks at 1261, 1410, and 2963 cm−1 in pristine PDMS are associated with the CH3 asymmetric stretching and CH3 symmetric stretching mode.[10–13] Figure 1 shows the FTIR spectra of cured PDMS. Comparison of the FTIR spectra of pristine and cured PDMS indicated that the intensity of the bands at 1023 cm−1 (Si-O-Si asymmetric), 1261, and 1410 cm−1 (CH3 symmetric and asymmetric stretching) increased distinctively and HOH and SiOH peaks were not observed. The absence of characteristic narrow peak of SiOH groups at 3700 cm−1 in cured PDMS proved the complete reaction of SiOH groups in resin with oxime silane. The absence of characteristic broad peak of HOH groups at 3435 cm−1 in cured PDMS proved the reaction of HOH groups in hydrolysis.
Sample Characterization A JASCO FTIR-6300 spectrometer (JASCO Inc., Japan) with a resolution of 0.07 cm−1 was used to characterize pristine PDMS polymer and the PDMS was cured with oxime silane sample, in the range of 400–4000 cm−1. Surface resistivity of the charge dissipative or conductive coatings was investigated with a HMN118 LCR Meter coupled with a bi electrode cell. Copper foil electrodes 5.0 ± 0.1 mm wide were set apart a distance L0 = 20.0 ± 0.1 mm. The physical contact of the electrodes with the sample was accomplished by affixing the electrodes to the surfaces of the samples with a strip of tape. Surface resistivity, ρ s , was calculated using Eq. 1: ρ S = R(τ ) × B/L0 .
(1)
where R (τ ) = measured resistance at time τ = 1 min, B = electrode width, and L0 = distance between electrodes.[5]
Adhesive Properties The produced adhesive experiments of black silicon coating were shown that coating adhesive has not been depend on the variety of pigment percent, crosslinker percent, and catalyst. Adhesion increased with decreasing molecular weight of the polymer samples. The surface treatment and using of adhesive improvement agent have been more impressive in adhesive resistance of these coatings. As can be seen in Figure 2, the pull off experiments were carried out for spraying Dow Corning 1200 primer, spraying adhesive AminoPropylTriEthoxySilane agent, and adding Amino PropylTriEthoxySilane agent to 1 wt%, respectively. The black layers have shown the fixed layer of adhesion. It is illustrated the adhesion layer just fixed in Figure 2. So the optimum adhesion obtained from spraying Dow Corning 1200 primer. These results were presented in the last research.[14]
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Fig. 1. FTIR Spectra of (a) illustrated pristine PDMS and (b) PDMS cured with oxime silane.
Surface Resistivity The surface resistances of all samples were evaluated. In each sample, three specimens were tested, and the average of the three results is presented in Table 2. It can be seen that the electrically conductive carbon pigment particles are embedded in a
dielectric silicone-based polymer matrix to form an electrically conductive composite coating. The interparticle connectivity, contacts, and pathways between the pigment particles form electrically conductive paths through the otherwise-dielectric polymer.[3,4] Also, the results showed that the weight fraction of the pigment in coatings fabricated by Printex xe2 were lower
Surface Resistivity of Black Silicone
505
Fig. 2. Image of pull off experiment for investigated adhesive strength: (A) spraying Dow Corning 1200 primer, (B) spraying adhesive AminoPropylTriEthoxy Silane agent, (C) adding AminoPropylTriEthoxySilane agent to 1 wt%.
compared to coatings fabricated by Printex v. The Printex xe2 carbon pigment is preferably formed by the agglomeration of carbon particles in to a sponge-like structure having a high degree of porosity and a high specific surface area. The specific surface area is about 800–1000 m2/g. The Printex v carbon pigments have a dense, less open, and substantially nonporous structure with a specific surface area that is typically about 100 m2/g. When the pigment particles are relatively small and dense, a large weight fraction of the particles is necessary to achieve a specified level of electrical conductivity. On the other hand, when the particles have a porous, sponge-like structure so that their sizes are large for their weight, the specified level of electrical conductivity can be achieved with a much smaller weight fraction of the carbon particles than would required for conventional, substantially nonporous, particles. The thickness of the sponge-structure walls is small current flows, this small wall thickness and consequent small current flow area are fully operable and acceptable and in fact desirable due to the excellent connectivity between the particles.[3,4] Additionally, as the polymer matrix dries and cures, internal stresses tend to separate the pigment particles. The porous, sponge-like particles used with the present approach maintain their electrically conductive path better than do conventional carbon particles in these circumstances.[3,4] According to Table 2, in both coatings fabricated by Printex xe2 and Printex v, the surface resistances decreased with increasing pigment contents. Increasing pigment contents increased the interparticle connectivity, contacts, and pathways between the pigment particles and decreased surface resisTable 2. Surface resistances of all samples. Sample RTV-MB0 RTV-MB1 RTV-MB2 RTV-MB3 RTV-MB4 RTV-MB5 RTV-MB6
Surface resistance 6.0 × 1013 2.4 × 108 4.0 × 107 6.4 × 106 7.4 × 108 2.3 × 107 8 × 106
tances. If too much carbon pigment is present the mechanical properties of the coatings are unacceptably low, adhesion of the coating to the substrate is reduced and the flexibility of the coating is reduced below acceptable levels. In comparison to previous works[5] illustrated in Table 2, surface resistance of coatings fabricated in this study are comparable to common conductive black thermal control coatings.a
Conclusions In conclusion, the weight fraction of the pigment in coatings fabricated by carbon black with a sponge-like structure are lower compared to coatings fabricated by carbon black with a non porous structure. The electrically conductive carbon pigment particles are embedded in a dielectric silicone-based polymer matrix to form an electrically conductive composite coating. The optimum adhesion obtained from spraying Dow Corning 1200 primer. The surface resistances decreased with increasing pigment contents.
References 1. Babel, H.W.; Jones, C.; David, K. Design properties for state-of-theart thermal control materials for manned space vehicles in leo. Acta Astronaut. 1996, 39, 369–379. 2. Xiao, H.; Li, C.; Yang, D.; He, S. Optical degradation of silicone in ZnO/silicone white paint irradiated by
