Dr. Mark D. Soucek
Professor of Polymer Engineering
Professor of Chemistry
Department of Polymer Engineering
- UV-Curable Coatings
- Ureas, Urethanes, Polyesters
- Waterborne Coatings
- Space Coatings
- Powder Coatings Center
- Dry Oils and Ceramer Coatings
Due to the considerable expense of launching space vehicles, there is push to reduce the weight of the satellites and spacecraft. A means of achieving this goal is to utilize lightweight high strength composites to replace load-bearing metallic exterior and infrastructure. The problem of using composites is a lack of durability in a space environment. As demonstrated by LDEF, 1 atomic oxygen etches organic media, in low earth orbit and in geosynchronous orbit high energy UV-light, protons, and electrons bombard the spacecraft. All of which can degrade any organic material.
A coating which can withstand first atomic oxygen etching, then high energy electromagnetic radiation would be ideal. Ceramic coatings are one approach to this problem. As a coating, ceramics are usually porous, and relatively dense with respect to the thickness needed to protect the substrate. A vapor deposited, metallic coating also can be effective against atomic oxygen and high-energy electromagnetic radiation. The drawback to vapor deposited coatings is a lack of self-healing mechanism. When particulate matter impacts upon the satellite or spacecraft, any damage bares the organic substrate leading to rapid erosion or degradation.
Since polysiloxane coating exhibited bothatomic oxygen and UV-resistance, we propose using a polysiloxane backbone for our protective coating. In addition to methyl substituted siloxane polymers, both cyclopentyl and cyclohexyl groups will be prepared. Since phenyl groups cannot be utilized on account of light absorption, cycloaliphatic groups will be used to investigate the effects of glass transition temperature on coating performance. The siloxane pre-polymers will be functionalized with cyclohexyl epoxide and alkoxyl silane groups. From our previous work, only 5-10% alkoxysilane functionalization is needed for interaction with the metal/silicon-oxo cluster phase. Anticipating polymerization of the cycloaliphatic epoxide groups, a statistical mixture of cycloaliphatic epoxide and alkoxy silane groups will be sufficient to add connectivity between the polysiloxane and metal/silicon oxo-cluster phase.
Once prepared, the polysiloxanes will be formulated with pre-hydrolyzed TEOS oligomers, and the zirconium sol-gel precursor zirconium tetra n-propoxide will be used in conjunction with zinc alkoxides to protect the underlying composite form UV-induce degradation. The Soucek group has previously found that TEOS oligomers are amenable to photo-initiated cationic polymerization of cycloaliphatic epoxides, and we have proposed that in addition to condensation reactions the silanol group can also react with the cycloaliphatic epoxide. The Soucek group has also previously reported that zinc acetate intercalates into zirconium-oxo-clusters formed from zirconium tetra n-propoxide during the ceramer coating curing process. 5 The proposed photopolymerization process has three reactive groups: 1) epoxide, 2) alkoxyl silane, 3) pre-hydroylized TEOS oligomers. All three groups commitantly react with each other to form an interlocking network.
The coatings have been engineered to perform a number of functions in a space environment. The first function is to protect the underlying substrate against atomic oxygen erosion. The protective mechanism is illustrated in Figure 1. One of the functions of the polysiloxane polymer backbone, and the silicon/metal-oxo-clusters is to protect the film and substrate from degradation. As previously reported, the protection mechanism involves formation of a silicon oxide layer on the surface of the coating, which inhibits further erosion of the underlying coating. After exposure, the films retained their transparency. The second function of the coating is to be non-yellowing. The cycloaliphatic substitution on the siloxane backbone was chosen for raising the glass transition of the polysiloxane coating while not contributing UV-absorption, and ultimately yellowing. The coating resin (binder) is designed to have minimal UV-absorption, and zinc and zirconium-oxo-clusters are integrated into the coating to block unwanted high energy light or disperse electrical charge.
Figure 1: Depiction of Formation and Function of Protective Silicon Oxide Layer and Metal-Oxo-Clusters
Inorganic/organic hybrid coatings known as ceramers are nanophase separated metal-oxo clusters connected to a continuous organic polymer via a phase coupling agent. For protective space coatings, it is proposed to use a photo-curable polysiloxane continuous phase with silicon-oxo clusters with zinc and zirconium-oxo clusters derived from sol-gel precursors. The nanophase silicon-oxo clusters will provide mechanical stiffness, and the nanophase zinc and zirconium-oxo clusters will provide additional protection and filtering from high-energy radiation and charge distribution. A ceramer approach for the composite resin will result in new nanophase reinforced composite matrices. The ceramer approach imbues into the coatings a self-healing mechanism.
1. Dworak, D.P.; Soucek, M.D. (2003) “Protective Space Coatings: A Ceramer Approach for Nanoscale Materials” Prog. Org. Coat. Vol. 47, 448.
2. Dworak, D.P.; Soucek, M.D. (2004) “Synthesis of Cycloaliphatic Substituted Silane Monomers and Polysiloxanes for Photo-Curing” Macromolecules37(25), 9402.
Protective Space Coatings: A Ceramer Approach for Nanoscale Materials
David P. Dworak and Dr. Mark D. Soucek
Department of Polymer Engineering, The University of Akron, Akron, OH 44325
There is a concerted and focused push to develop protective space coatings for vehicles in low earth and geosyncronous orbit. The space environment is not suitable for organic materials due to atomic oxygen, high energy particles, and deep UV-light being able to degrade polymeric organic resins. An inorganic/organic hybrid coating, known as a ceramer, will be fabricated using a polysiloxane binder and nanophase silicon/metal-oxo-clusters derived from sol-gel precursors. The significant features of this coating are its ability to self-heal, deflect high-energy particles, protect against deep UV-light, and be optically transparent. Photo differential scanning calorimetry will also be used to investigate the effects of temperature, UV light intensity, sol-gel precursor concentration, and exposure time have on the rate of polymerization. Fourier transform infrared spectroscopy and H 1 NMR was used to characterize the synthesis of the polysiloxanes. The rate of polymerization was found to increase as temperature, intensity, sol-gel precursor concentration, and exposure time were increased.
Synthesis of Cycloaliphatic Substituted Silane Monomers and Polysiloxanes for Photo-Curing
D. P. Dworak and M. D. Soucek*
The University of Akron
Department of Polymer Engineering
250 S. Forge Street
Akron, OH 44325-0301
A synthetic scheme was developed to prepare cationically polymerizable methyl, cyclopentyl, and cyclohexyl substituted polysiloxanes. Initially, the desired cycloalkene and dichlorosilane were reacted at high pressure (approx. 250 psi) and high temperature (120° C) to yield the desired cycloaliphatic dichlorosilane. The chlorosilane monomers underwent an oligomerization to produce cyclic oligomers of low molecular weight (~2,000 g/mol). Polysiloxanes were produced through the acid catalyzed ring opening polymerization of the cyclic oligomers to yield high molecular weight polysiloxanes (~45,000 g/mol). The polysiloxanes were then functionalized with a cycloaliphatic epoxy and alkoxy silane groups via hydrosilation. Monomers, oligomers, and polymers were characterized by 1H and 29Si NMR, FT-IR, and electrospray ionization mass spectroscopy. The photo-induced curing kinetics and activation energies were investigated using photo-differential scanning calorimetry. Differential scanning calorimetry was used in order to observe any physical changes in the films that are brought about due to the variation of the pendant groups. The cycloaliphatic substituents raised the glass transition temperature and affected the curing kinetics when compared to a methyl substituted polysiloxane. The activation energies were found to be 144.8 ± 8.1 kJ/mol for the methyl substituted and 111.0 ± 9.2 and 125.7 ± 8.5 kJ/mol for the cyclopentyl and cyclohexyl substituted polysiloxanes.
Keywords: cyclopentyldichlorosilane; cyclohexyldichlorosilane; dicyclopentyldichlorosilane; dicyclohexyldichlorosilane; polysiloxanes; hydrosilation; kinetics