Residual stress effect on degradation of polyimide under simulated hypervelocity space debris and atomic oxygen

Polyimides are used as the outer layer of thermal control insulation blankets covering most of the external spacecraft surfaces that are exposed to space environment. The combined effect of ground simulated hypervelocity space debris impacts and atomic oxygen (AO) on the fracture of polyimide films was studied. A laser-driven flyer system was used to accelerate aluminum flyers to impact velocities of up to 3 km/s. The impacted films were exposed to an RF plasma source, which was used to simulate the effect of AO in the low Earth orbit. Scanning electron microscopy and atomic force microscopy were used to characterize the fracture and surface morphology. When exposed to oxygen RF plasma, the impacted polyimide film revealed a large increase in the erosion rate, the damage being characterized mainly by the formation of new holes. This effect is explained by the formation of residual stresses due to the impact and enhancement of oxygen diffusivity and accumulation. A complementary experiment, in which a stressed polyimide was exposed to RF plasma, supports this model. This study demonstrates a synergistic effect of the space environment components on polymers' degradation, which is essential for understanding the potential hazards of ultrahigh velocity impacts and AO erosion for completing a successful spacecraft mission.     

Atomic oxygen damage behavior of TaC coating-modified C/C composite

Groups of carbon/carbon (C/C) and tantalum carbide (TaC)-coated C/C composite specimens were studied under exposure to atomic oxygen (AO). Low earth orbit ground-based simulator was employed. Degradation mechanisms were investigated by scanning electron microscopy, X-ray photoelectron spectroscopy, and energy dispersive spectrometry. The results indicated that compared to C/C composite, TaC-coated C/C composite exhibited improvements in resistance properties against AO flux, such as 16.7% better strength retention ratio and 57% less mass ablation than non-coated C/C after 40 h of AO exposure. Moreover, AO preferentially consumed tantalum (Ta) atoms than carbon atoms of TaC_x till atomic ratio was in the same proportion. Production of swollen tantalum oxide and crack healing components were obtained under the oxidation effect of high-speed AO. Swelling textures due to exposure were developed following an apical dominance growth behavior. The damage mechanisms of AO on TaC-coated C/C were revealed as chemical reaction first followed by mechanical effect.     

有机硅改性聚酰亚胺复合膜的抗原子氧性能

由端氨丙基聚二甲基硅氧烷(PDMS)、均苯四甲酸二酐(PMDA)和4,4′-二氨基二苯醚(ODA)在N,N′-二甲基乙酰胺(DMAc)中共聚,热亚胺化法制备一系列有机硅改性聚酰亚胺复合材料。通过傅里叶变换红外吸收光谱仪(FTIR)、X射线光电子能谱(XPS)分析材料的化学结构,采用扫描电子显微镜(SEM)观察材料的表观形貌,利用地面原子氧(AO)模拟设备研究了该复合膜的抗原子氧性能。结果表明,有机硅改性聚酰亚胺复合膜在AO累积通量达到2.06×1020atom/cm2时,复合膜的AO剥蚀率可以达到6.69×10-25cm3/atom,约是纯聚酰亚胺薄膜抗原子氧性能的4.5倍。     

TPO based nanocomposites. Part 2. Thermal expansion behavior

The linear thermal expansion behavior of thermoplastic polyolefin, or TPO, nanocomposites based on a polypropylene/elastomer/masterbatch mixture was examined using a thermomechanical analyzer (TMA). For these experiments the masterbatch consisted of a mixture of organoclay and maleated polypropylene. The nanocomposites were prepared in a twin-screw extruder. The effects of both the elastomer domains and the filler particles on the thermal expansion behavior of the nanocomposites were investigated by means of transmission electron microscopy (TEM) and atomic force microscopy (AFM). The addition of elastomer tends to increase the linear coefficient of thermal expansion, CTE. On the other hand, the addition of clay significantly reduces the thermal expansion in both the flow (FD) and transverse directions (TD) of injection molded specimens; however, the extent of reduction of CTE in the FD is much greater than in the TD. The CTE in the normal direction (ND) increases when either the clay or elastomer content is increased. The trends in thermal expansion for the nanocomposites are discussed in terms of the morphology of both dispersed clay and elastomer phases based on TEM and AFM observations and subsequent particle analyses.     

A novel alkoxysilane‐modified high solids hydroxyl acrylic polyurethane: Preparation and surface properties

Alkoxysilane‐modified high solids hydroxyl acrylic polyurethane was prepared by solution polymerization. Its structure, surface, and thermal properties were investigated by ¹H NMR, device of contact angle, thermo gravimetric analysis (TGA), atomic force microscopy (AFM), and X‐ray photoelectron spectroscopy (XPS). Research showed that alkoxysilane modified high solids hydroxyl acrylic polyurethane has superior properties that can be used for automotive paints. The contents of silicone in the alkoxysilane‐modified high solids hydroxyl acrylic polyurethane were 1.25, 1.5, 2, and 2.5 wt %. In this study, γ‐methacryloxypropyltrimethoxysilane (MPTS) was chosen as the modifier. Results showed that the contact angles of water and surface roughness on the film of MPTS modified high solids hydroxyl acrylic polyurethane increased, and thermal stability of the film at high temperatures improved with the increasing of the silicone content in the resins. MPTS modified high solids hydroxyl acrylic polyurethane with 2.5 wt % silicone content had better water resistance, better acid resistance, higher hardness, and excellent weatherability. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1866–1871, 2006     

A near edge X-ray absorption fine structure study of oxidized single crystal and polycrystalline diamond surfaces

The influence of oxidizing environments on single crystal diamond and polycrystalline chemical vapor deposited CVD diamond films was studied using the near edge X-ray absorption fine structure (NEXAFS) pre-edge region in both bulk and surface sensitive modes. The NEXAFS of (100) oriented single crystal diamond was measured following (i) exposure to a microwave (MW) hydrogen plasma, (ii) annealing to 1000 °C, (iii) exposure of the as annealed surface to H₂O, and (iv) exposure of the as annealed surface to O₂. From these measurements particular surface bonding configurations have been assigned to features in the pre-edge structure. The NEXAFS of microcrystalline CVD diamond films was studied following different oxidative treatments using (i) a thermal atomic oxygen (AO) environment, (ii) a hyperthermal (5 eV) AO source, and (iii) an RF oxygen plasma exposure. The nature of the surface layer was found to be different for differently oxidized surfaces. These treatments were carried out as part of a study of CVD diamond durability in the low Earth orbit space environment.     

Investigation on polyimide/silica hybrid foams and their erosion resistance to atomic oxygen

A series of polyimide/silica (PI/SiO₂) hybrid foams were prepared by the sol–gel process. Aminopropyltriethoxysilane was used as the coupling agent to enhance the compatibility between PI matrix and SiO₂. Fourier transform infrared spectroscopy and scanning electron microscopy were used to analyze the chemical structure and cellular structure of PI/SiO₂ hybrid foams. The results indicated that the three‐dimensional network of Si O Si was formed in the hybrid foams, and the hybrid foam presented the uniform cellular structure when the SiO₂ content was less than 6 wt%. The thermal stability, dynamic mechanical property, and dielectric property of PI/SiO₂ hybrid foams were investigated by dynamic mechanical analysis, thermogravimetric analysis, and vector network analyzer, respectively. The introduction of SiO₂ improved the thermal stability and increased the storage modulus and glass‐transition temperature. The hybrid foams showed higher dielectric constants compared with the neat PI foam. The erosion resistance to atomic oxygen (AO) of PI/SiO₂ hybrid foams was also evaluated in a ground‐based AO simulator. The surface morphology and chemical structure of PI/SiO₂ hybrid foams before and after AO exposure were investigated by scanning electron microscopy, atomic force microscopy, and X‐ray photoelectron spectroscopy. The results revealed that the inorganic SiO₂ protective layers were formed on the surface of PI/SiO₂ hybrid foams after AO exposure, which could effectively improve the AO erosion resistance of PI/SiO₂ hybrid foams. POLYM. COMPOS., 36:713–721, 2015. © 2014 Society of Plastics Engineers     

Effect of low earth orbit atomic oxygen exposure on epoxy functionalized siloxanes

Epoxy functionalized siloxanes (EFS) are a novel class of UV curable monomers that can be rapidly photopolymerized to give transparent coatings and composites. Thin films of these materials have been subjected to low Earth orbit exposure to atomic oxygen (AO) aboard the space shuttle. It was found that UV cured samples of all four different EFS monomers exhibited excellent AO resistance both to ambient in-flight conditions as well as exposure at 120°C. Based on SEM, STM, XPS, and weight loss data, it was proposed that AO exposure of these materials efficiently produces a thin layer of SiO_x at the surface of the sample. This layer provides a barrier toward further attack by AO. © 1995 John Wiley & Sons, Inc.     

硅改性水性氟碳金铂漆的研制

研制了一种硅改性水性氟碳金铂漆,给出了涂料配方及性能指标,讨论了树脂及各种助剂对涂料性能的影响.     

Formulation effects on the distribution of pigment particles in paints

Modern water-borne paints are widely used in different areas of applications ranging from high-gloss lacquers to flat, scrub-resistant interior paints. From this point of view, the pigment volume concentration (PVC) is one key-parameter adjusting the desired application properties. In high-gloss paints, for example, a low PVC is required to accommodate the proper surface roughness to achieve a high gloss. Consequently, a high concentration of TiO₂ is needed to obtain a good hiding power at the same time. Flat paints nonetheless are highly filled due to cost reasons preferentially by CaCO₃ and the pigment binding capacity of the binder is crucial. In this work, paint formulations differing in PVC, and the type of binder or dispersing agent were investigated by various techniques concerning the distribution and aggregation of pigment particles, e.g. TiO₂. To get a detailed insight into the structure of the liquid paints and the corresponding dried paint films, suitable analytical tools were applied for characterization. The structure of the liquid paints was analyzed by remission light spectroscopy (RLS), disc centrifugation, cryogenic-replication transmission electron microscopy (Cryo-replica TEM) and cryogenic-scanning electron microscopy (Cryo-SEM). The pigment distribution in the corresponding dried paint films was examined by means of atomic force microscopy (AFM), TEM and RLS. The tendency of the TiO₂-pigments to form aggregates was found to depend on both: first on the type of binder used in the formulation and second on the employed dispersing agent. It is shown that only by adjusting the properties of the binder in combination with common dispersants, it is possible to get well-distributed TiO₂ particles within the paint. Correlation of application properties, e.g. gloss and blocking to the microscopic structure is presented.     

Influence of nm-Thick Atmospheric Plasma Deposited Coatings on the Adhesion of Silicone Elastomer to Stainless Steel

In this study the use of a nm thick plasma polymerised primer layer as a means of enhancing the adhesion of silicone elastomer to stainless steel was investigated. This primer layer was formed from tetraethoxysilane (TEOS) and polyhydrogenmethyl siloxane (PHMS). This primer mixture was nebulized into helium atmospheric pressure plasma formed using the PlasmaStream? system. The resulting primer coatings were characterised using X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy, optical profilometry, scanning electron microscopy, spectroscopic ellipsometry and water contact angle measurements. The adhesion of the silicone elastomer was accessed using 45° peel strength measurements. This study investigated the effect of plasma jet to substrate distance on primer performance. Increased coating oxidation is observed as this gap between the jet orifice and substrate increases. The plasma power, measured using current and voltage probes was observed to increase with gap height. The corresponding coating thickness decreased from 450 to 100 nm as the gap height increased from 2 to 12 mm. Associated with this change there was a 38% reduction in silicone elastomer adhesion to the stainless steel surface, demonstrating that silicone elastomer bonded to the thinner and less oxidised primer layer exhibited superior adhesion.     

Damage behavior of atomic oxygen on zirconium carbide coating modified carbon/carbon composite

Zirconium carbide (ZrC) components were induced as coating modification on carbon/carbon (C/C). These ZrC–C/C specimens were investigated after atomic oxygen (AO) exposure for different assessment times, low earth orbit (LEO) ground-based environmental simulator was employed. The results indicate that ZrO₂ is the major production generated by the AO chemical reaction with the ZrC coating. Upon further exposure to AO, the production of ZrO₂ would drop off, then exfoliate easily, due to the mechanical impacting effect. Then the exposed graphite matrix and carbon fiber get corroded. Amorphous diamond-like carbon (DLC) is detected by X-ray photoelectron spectroscopy during AO exposure. Bending strength performance increased by 25% under AO exposure at first 10 h, then dropped by 52.1% from 10 h to 30 h of AO exposure. The AO damage mechanisms of ZrC–C/C composites are revealed.     

Preparation and characterization of silicone resin nanocomposite containing CdSe/ZnS quantum dots

We report the preparation of the core/shell cadmium selenide/Zinc sulfide quantum dots (CdSe/ZnS QDs)‐silicone resin nanocomposite through the solution‐mixing method, followed by thermal hydrosilylation. After dispersing QDs into Dow Corning two‐component silicone resins (OE6630A and OE6630B at 1:4 mixing ratio by weight), the resins were cured at 150°C for 1.5 h to produce QD‐silicone resin nanocomposites. The curing behavior of the silicone resins resulting from the thermal hydrosilylation was studied using differential scanning calorimetry (DSC). The properties of the QD‐silicone resin nanocomposites were investigated by ultraviolet–visible (UV–vis), fluorescence, confocal laser scanning microscopy (CLSM), atomic force microscopy (AFM), and thermogravimetric analysis (TGA) measurements. The QDs that contain trioctylamine (TOA) as the original ligand can poison the Pt catalyst in the resins and inhibit the curing process by increasing the exothermic peak temperature, at which a lower heat of hydrosilylation is observed. Incorporating a small amount of CdSe/ZnS QDs (0.1 wt%) can greatly improve the thermal stability of the silicone resins. Moreover, CdSe/ZnS QDs tend to form clusters that are relatively homogeneously distributed in a cured silicone resin, offering good optical properties of 11.2 lm W⁻¹ luminous efficiency and 14.6% photoluminescence conversion efficiency (PCE) in light emitting device (LED) test. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

调湿抗菌内墙涂料的研制

以苯丙乳液与硅溶胶组成有机-无机复合基料，以硅藻土、高吸水性树脂、海泡石粉组成调湿材料，在非吸湿颜填料、负离子添加剂及助剂的配合下，制备成调湿抗茵内墙涂料。该涂料不但具有普通内墙涂料的一般物理性能，而且还具有调湿防结露功能，同时还能释放负离子、抑茵抗茵、祛除有害气体、清新空气，是一种新型环保、节能的内墙涂料。     

High-quality and efficient transfer of large-area graphene films onto different substrates

We used a two-layer structure consisting of polyethylene terephthalate (PET) and silicone to transfer graphene grown by chemical vapor deposition onto various rigid and flexible substrates through dispersive adhesion. It only takes a few seconds to transfer graphene from PET/silicone to the target substrates at ambient conditions. And the recycling of the PET/silicone decreases the production cost greatly. The transferred graphene films were characterized by optical and atomic force microscopy, Raman spectroscopy, electrical analyses, and optical transmittance measurements, and the results show that the graphenes transferred by PET/silicone have a cleaner and more continuous surface, lower doping level, and higher optical transmittance and conductivity than those transferred by thermal release tape. Considering its high efficiency, low cost, large area and high quality, the PET/silicone transfer method would be particularly useful for graphene’s electronic applications such as field-effect transistors and transparent conducting electrodes.     

防污涂料的研究及其发展

叙述了海洋防污涂料的类型与研究现状,介绍了新型环保的低表面能防污涂层技术在海洋防污方面的应用,重点介绍了有机硅涂料、有机氟涂层以及绿色防污涂料国内外研究进展和应用现状,展望了海洋防污涂料技术发展前景.     

Polyhedral oligomeric silsesquioxane polyimide nanocomposites for color filters and flexible conductive films

Color filters and conductive films are widely used in spacecraft, while the lack of lightweight, flexibility, and atomic oxygen (AO) durability confine their potential applications in low earth orbit. In this study, a clear poly(amic acid) with an empirical 20 wt% polyhedral oligomeric silsesquioxane (POSS) solid content is designed for transparency and AO durability. Red, green, blue, and yellow dyes are reinforced with small amounts of 1–5 wt% in POSS polyimides for color filters. A silver nanowire network film is infiltrated onto the POSS polyimide for conductive film. Erosion depth upon hyperthermal AO exposure, surface morphology, surface chemistry, optical transparency, and conductivity have been systematically investigated. The erosion yields of all 20 wt% POSS polyimides decrease by an order of magnitude when subjected to 2.32 ± 0.05 and 2.39 ± 0.06 × 10²⁰ atoms cm⁻² AO fluences, as passivating SiO_x networks are formed on film surface. The small-amount dye additives into polyimides do not introduce obvious changes in AO durability and surface chemistry. The silver nanowire infiltrated POSS polyimide film shows a 65.7% transmittance at 550 nm and a sheet resistance of 8.50 ± 0.36 Ohm square⁻¹. This study reveals promising attempts of POSS-polyimide-based color filters and flexible conductive films for potential space applications.     

The degradation behavior of C/SiC composites fabricated by polymer impregnation and pyrolysis process under simulated space atomic oxygen environment

Carbon fiber reinforced silicon carbide (C/SiC) composites fabricated by polymer impregnation and pyrolysis (PIP) have been exposed in simulated space atomic oxygen (AO) environment for up to 15 h. The mechanical properties and chemical composition of PIP C/SiC composites have been studied. The results show that the mass loss of the composites increases at the beginning and then decreases as the exposure time lasts. The flexural properties of C/SiC composites have no obvious changes after up to 15 h exposure in AO. C/SiC composites have been oxidized slightly by AO. The amorphous carbon in the matrix has been oxidized to CO or CO₂ gas and SiC has been oxidized to SiO gas and SiO₂.     

Synthesis of silicone–acrylic resins and their applications to superweatherable coatings

Silicone–acrylic resins were synthesized to prepare superweatherable paints for building materials. The raw materials used were n‐butyl acrylate, methyl methacrylate, and n‐butyl methacrylate as acrylic monomers and 3‐methacryloxypropyltrimethoxysilane (MPTS) as a silicone monomer reactive with the acrylic monomers. Acrylic copolymers were synthesized such that their glass‐transition temperatures were adjusted to 30°C and their MPTS contents were varied to 10, 20, and 30 wt %. As the content of silicone and MPTS increased, average molecular weight and viscosity increased, and thermal stability at high temperatures improved. When we tested the properties of coatings by blending the synthesized silicone–acrylic resins with a white pigment, adhesion was superior with various substrates, and their properties were suitable on the whole. Weatherability was tested by an outdoor exposure test with a weather‐ometer and an accelerated weathering tester, and their results showed that silicone–acrylic resin composed of 30 wt % MPTS was a superweatherable coating. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 1614–1623, 2001     

Radiation damage behavior of carbon/carbon composite in Low Earth Orbit environment

To understand the degradation behavior of Carbon/Carbon(C/C) composite in LEO radiation environment, groups of 2D-C/C composites samples were exposed to ground-based Low Earth Orbit(LEO) simulating facilities. The LEO radiating environment covers part of Van Allen inner belt, which contains atomic oxygen(AO) and high-energy proton radiating regions. These two kinds of radiating sources were arranged for simulating the real LEO radiating conditions regarding to the first orbit movement speed of serving space shuttles. Changes of micro-structure, mass loss, surface roughness, chemical construction and thermal physical properties after environmental assessment were analyzed and compared to understand the damage behavior of LEO radiation on C/C composite. It was found that AO is the main factor of mass loss in LEO radiating particles. The charged high-energy protons have an aggravating damage ability with energetic AO, damage situations of degree in orbit down process is more serious than in orbit rising process. The LEO radiation damage mechanisms of C/C composite are revealed and expounded either.