2002 Progress Report: Plasma Polymerization: A Novel, Environmentally-Compatible Process for Surface Engineering of MetalsEPA Grant Number: R829579
Title: Plasma Polymerization: A Novel, Environmentally-Compatible Process for Surface Engineering of Metals
Investigators: Boerio, F. James , Bengu, B. , Gupta, M.
Institution: University of Cincinnati
EPA Project Officer: Richards, April
Project Period: March 11, 2002 through March 10, 2006
Project Period Covered by this Report: March 11, 2002 through March 10, 2003
Project Amount: $300,000
RFA: Technology for a Sustainable Environment (2001) RFA Text | Recipients Lists
Research Category: Nanotechnology , Sustainability , Pollution Prevention/Sustainable Development
The objective of this research project is to develop plasma polymerization as a novel, environmentally compatible process for surface engineering of aluminum prior to structural adhesive bonding or finishing operations. New surface engineering processes that result will enable chromates and liquid wastes from surface engineering of aluminum by conventional processes to be eliminated from the nation's waste stream at the source.
Plasma-polymerized, silicalike films can be formed on aluminum and other metal substrates using hexamethyldisiloxane as a monomer and oxygen as a coreactant. Films containing relatively few hydroxyl groups can be formed from plasmas that have a low monomer-to-oxygen ratio, while films with relatively numerous hydroxyl groups can be formed from plasmas with high monomer-to-oxygen ratios. Low-hydroxyl, plasma-polymerized silicalike films form strong bonds to aluminum substrates and inhibit their corrosion during exposure to water at 60°C. Interestingly, the adhesion of adhesives to the silicalike films depends on the chemical composition of the adhesives. Thus, a one-part epoxide adhesive with a dicyandiamide curing agent formed strong, durable bonds to the low-hydroxyl, plasma-polymerized silicalike films, whereas a two-part epoxide adhesive with a polyamide/amine curing agent formed bonds with poor durability to similar films. However, application of a thin film of a silane coupling agent such as -glycidoxypropyltrimethoxysilane (-GPS) to a low-hydroxyl, plasma-polymerized silicalike film enabled the two-part adhesive to form strong, durable bonds to the silicalike film. Gaseous effluents from an hexamethyldisiloxane/oxygen plasma consist mostly of oxygen and trace amounts of hexamethyldisiloxane as well as nitrogen, water vapor, and carbon dioxide.
During the next year, we plan to initiate long-term stressed durability tests on adhesive joints prepared from aluminum substrates coated with low-hydroxyl, plasma-polymerized silicalike films. These tests, which are commonly used in the automobile industry, are very aggressive because a stress is applied to the joints while they are exposed to warm water. Such tests will provide an excellent evaluation of low-hydroxyl, plasma-polymerized films as primers for structural adhesive bonding of aluminum.
We also will begin an investigation of high-hydroxyl, plasma-polymerized films as primers for structural adhesive bonding of aluminum. An investigation of these films will provide important insight regarding the significance of hydroxyl groups at the interface between an adhesive and plasma-polymerized, silicalike films. We will investigate the use of duplex films wherein the film adjacent to the aluminum substrate has low hydroxyl content, while the film adjacent to the adhesive has high hydroxyl content.
We plan to investigate the molecular structure of interfaces between selected adhesives and plasma-polymerized silicalike films. These investigations will help us to determine the molecular-level basis for the outstanding adhesion of epoxides cured with dicyandiamide to low-hydroxyl, plasma-polymerized silicalike films and the relatively poor adhesion of epoxides cured with polyamide amines to similar films. This knowledge will help us to devise improved plasma-polymerized films for environmentally compatible pretreatment of metals such as aluminum.
It is very desirable to conduct all pretreatment processes in the plasma reactor. Thus, we will expand our investigation of plasma processes for removing carbonaceous contamination from aluminum, and if necessary, removing and replacing the native oxide on aluminum with a more stable oxide.
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