Adhesives in space environment

Abstract

Adhesives are widely used on a spacecraft for several reasons. They allow a significant mass reduction and, using an adhesive allows. The combination of several properties (as the mechanical fixation and some electrical function). One of their main drawbacks is that they are organics and thus they react with space environment. Among all the components of the space environment which could lead to material degradations, the three most important are the energetic charged particles (protons and electrons particles with different fluxes and energies), the electromagnetic radiation from the direct solar flux (and most specifically in the Ultra Violet wave length) and the Atomic Oxygen (ATOX) coming from the photodissociation of the oxygen molecules of the atmosphere by the solar electromagnetic radiation. These factors lead to physicochemical degradation of the adhesives, which can affect their functional properties. All the adhesives used in spacecraft have to be validated on ground before launch and their behavior in space environment must be evaluated. Test facilities are developed to simulate charged particles, UV, and ATOX constraints. Even if it is not possible to be 100% representative of the in-orbit conditions, tests, methodologies and specifications exist to correctly evaluate the degradation and to take into account the adhesives end of life properties, in the spacecraft design. Atoms and molecules along the charged particles trajectory can be either excited or ionized. The primary effects are generally deformation, embrittlement, and discoloration, which impact the mechanical integrity of the adhesive. The modification of mechanical properties is generally the result of the competition between chain scissions and crosslinking in the macromolecular chains. The main UV effect on adhesives is generally a degradation of their optical properties (e.g., solar cell cover glass). Indeed, during the interaction of a photon with an atom or a molecule of the material, the photon can either be absorbed or diffused with possible loss of energy. These photochemical effects may result in a change of color of the material, leading to an increase of solar absorptance. Atomic Oxygen might also have detrimental effects on adhesives, mainly erosion of surfaces and oxidation.