The nanotech revolution rolls on and in the finishing world nanotechnology has made its mark mainly in the pretreatment area. In general, nanotechnology pretreatments—and there are several available from different suppliers—result in an ultra-thin conversion coating layer on the order of nanometers thick versus microns thick for a conventional pretreatment. The thinner layers seem to improve corrosion resistance by providing improved paint adhesion
Aluminum is everywhere, and for most of the 20th century hexavalent chromium was used to protect it from corrosion and enhance paint adhesion. Nalco Chemical Co. (Naperville, IL) says most of the chrome-free aluminum finishing products on the market don’t perform as well as those containing hex chrome, especially when it comes to protecting bare aluminum.
Enter Nalco’s Tech Bond sol-gel nanotechnology pretreatment, a patented process for all types of aluminum alloys. The product is chrome-free, and is said to provide very good corrosion resistance while maintaining the natural aesthetics of the base metal. According to the company, the nanotechnology pretreatment can exceed industry standards for corrosion protection and paint performance of aluminum surfaces while reducing total operating costs.
According to Nalco, conventional conversion coating processes etch and deoxidize the natural oxide layer of aluminum, replacing it with the conversion coating. The problem is, the conversion coating actually offers less corrosion protection than the natural aluminum oxide layer.
In contrast, Tech Bond’s nanotech formulation penetrates nanopores in the aluminum oxide layer, Nalco says. The pretreatment molecules self-assemble up to 100 layers of highly cross-linked polymer, clogging pores and providing enhanced corrosion protection.
Coating thickness developed with the product depends only on the working solution concentration. Users who desire corrosion control on bare aluminum surfaces can use more pretreatment, while users who want paint or powder pretreatment can use lower concentrations. Organic functional groups in the polymer bond directly with any applied coating or paint.
The key is sol-gel coating technology, which has been used mainly in high-tech industries and applications such as aerospace, optics and semiconductors. In sol-gel, a small molecule dispersion (the sol) forms a cross-linked polymer (the gel) upon drying. Tech Bond sol-gel products produce a very thin coating 50 to 500 nanometers thick.
According to Nalco, the product’s formulation is unique in the market. It contains no metals or phosphates, and it doe not form a crystalline layer like conventional phosphate pretreatments. The company says it does not require dwell time to build a film, does not sludge, does not require a battery of tests to operate, and is ELV, RoHS and WEEE-compliant.
The process involves four wet stages: clean, rinse, deionized water rinse, and coat. It can be applied in conventional spray and immersion systems, using wet spray gun techniques, or simply by wiping. Nalco says the technology can cut pretreatment operating costs by up to 50%, because eliminating etching and deoxidation stages reduces the cost of chemicals, water, energy and waste treatment.
Nalco says tests on a variety of aluminum alloys, paint systems and surface conditions show the product excels in corrosion tests against chromate and non-chrome competing products. The company has tested the product on hundreds of aluminum alloys in a variety of situations.