Chemiluminescence of polysiloxane cushions: Correlation with physical properties

Abstract

Cellular cushions are routinely used to maintain the shapes and relative positions of parts with widely different thermal expansion properties. At Lawrence Livermore National Laboratory (LLNL) and Bendix Corporation, Kansas City (BKC) cushions that are a silica-filled polysiloxane random terpolymer made from diphenyl, dimethyl, and methylvinyl siloxane subunits are primarily used. The cellular structure is formed by a leachable urea pore former. Cellular silicone cushions were selected for our purposes over other elastomers and foams because of their superior thermal properties, long term stability, and lower compression set (the ability to recover their original thickness when a load is released). At LLNL, temporarily filled rather than hydrogen-blown cushions are used because the porosity, and thus crucial mechanical properties, is more easily adjusted.Until 1978, the molding compound and raw materials from which the cushions are made were purchased from a n outside vendor and molded into parts at BKC. When these materials became unavailable, both BKC and LLNL were forced to find other suppliers and to develop the technology necessary to make the cushions. Since that time, a large effort has been made to replace and improve the original material and to optimize processing conditions.14 LLNL is primarily responsible for the materials development including improving and predicting the long-term chemical and physical properties of different cushion materials.Traditional life-testing consists, in part, of accelerated aging in the form of compression set testing? In the past, cushion samples have been compressed to a known thickness, stored at 150°C for 24 h, allowed to recover, and their initial and final thicknesses compared. Historically, most of the data at 150°C from the cushions in which urea is used as a pore former correlated poorly with lower temperature data. Conversely, the high temperature data for hydrogen blown cushions did correlate well. Chemiluminescence was used to study this apparently anomalous behavior.Chemiluminescence is a process in which a photon is released as the result of the decay of a n excited state (e.g., a ketone) formed from an exothermic oxidation reaction. With the introduction of very sensitive photomultiplier tubes and modern photon counting equipment, it became apparent that very faint chemiluminescence is a virtually universal property of oxidizable organic substances. This very faint chemiluminescence is probably due to ongoing degradation reactions in the material, and because chemiluminescence can be detected so sensitively these reactions can be monitored tens to hundreds of years before their effect would become apparent on a macroscopic scale. Aging reactions in a myriad of substances have been studied using chemiluminescence.68 We are reporting the correlation of chemiluminescence, a microscopic process, and compression set, a macroscopic physical property of cellular silicone cushions. EXPERIMENTAL MaterialsCellular silicone cushions are made from a raw gum (SE-54, Ge...

Possible cause of chemiluminescence in silicone rubber

“…In fact, it has been reported in several papers that CL is induced in polysiloxane cushions and silane by the oxidation of Si. [31][32][33][34] To summarize the present research, it is likely that the CL in SiR is induced by the oxidative degradation that forms SiO 2 and the cross-links via Si-O-Si bonds. Although the well-known CL origin of organic polymers is oxidation of carbon that induces carbonyls, 5) the oxidation of silicon, which induces no carbonyls, can be a possible cause of CL in SiR.…”