Several new surface treatments and modifications of older, conventional surface treatments have been introduced over the last few years to provide alternatives to the common processes noted above. These processes have been developed for both metals and polymers. These developments have primarily been the result of the introduction of new materials as substrates and ongoing efforts to develop bonding systems that have both higher initial strength and environmental resistance. However, environmental regulations and the emphasis on productivity have also been strong driving factors.
In addition to providing safer and environmentally friendly processes, these newer surface treatments have also been shown to provide for easier and faster processing. They promise a potentially tremendous positive impact on both manufacturing cost and performance properties. The reduced cost impact can be in the form of equipment costs, implementation costs, operational costs, rework costs and storage / waste removal costs.
In the metals surface treatment area, the need to provide a fast, safe, and environmentally friendly process is dominant. Chromium compounds have often been used in many chemical treatments and these materials are detrimental to the environment. Several development efforts are underway for non-chromatic, non-carcinogenic surface preparation processes for metals, and examples are given here to illustrate the trend in these developments.
Much of the development work regarding metal surface preparation has been focused on aluminum. This is understandable since aluminum is an almost ideal substrate for adhesive bonding. This is evident by the large number of adhesive manufacturers that take advantage of aluminum substrates to demonstrate the properties of their products and by the scientists that use aluminum substrates for research into the fundamentals of adhesives and adhesion. However, the use of aluminum exceeds that of any other metal except iron. It is important in virtually all segments of the world economy.
The recognition that chrome is carcinogenic has forced alterations in surface treatment processes. Chromic acid anodizing and FPL etching are being phased out in many locations. Several chromate free etching processes have been developed for bonding metals, particularly aluminum. These provide significant improvement from the environmental and occupational health and safety perspectives. There have been several processes that are suggested as more environmentally friendly alternatives. One such process is a chromate-free etching process (designated PT2) for improved environmental and occupational health and safety perspectives.
Work on sulfuric acid anodizing and sulfuric boric acid anodizing of aluminum is now in progress. In the automotive industry, a pretreatment has been developed for aluminum coil that is nontoxic and compatible with weld bonding. The proprietary treatment is claimed to be as effective as chromium based pretreatment processes on exposure to salt spray.