The eco-friendly BehrOxal? process simplifies process management, improves ecobalance of the coating process, contributes to industrial safety, improves the corrosion protection of the evaporator and inhibits the buildup of odours.
Since July 01, 2007, it is no longer permissible to use chrome (VI) in vehicles. Two years ago, Behr developed a new, completely chrome-free process called BehrOxal® for coating the evaporators of HVAC (heating, ventilation and air conditioning) modules. Not only does this eco-friendly BehrOxal® process satisfy the EU End-of-Life Vehicle Directive that came into force on July 01, 2007, but also
• simplifies process management
• improves the ecobalance of the coating process
• contributes to industrial safety
• improves the corrosion protection of the evaporator
• inhibits the buildup of odours
In contrast to the yellow chromate coating process, the BehrOxal® process consists of only three process steps. In the first step, the evaporator is intensely heated and then sprayed with dilute aqueous BehrOxal® solution (Figure 1). The solution evaporates and spreads over the entire surface of the evaporator. A closed, homogeneous passivation layer is thus created on the surface. During the yellow chromate coating process, microfissures form that create contact surfaces for water, and, for example, de-icing salt. Those microfissures are a starting point for corrosion. In contrast, there are no such fissures in the BehrOxal® layer. In addition, the residual flux from the brazing process is not laboriously removed prior to this step, but instead is directly integrated into the BehrOxal® layer (Figure 2). This eliminates time-consuming pre-treatment and post-treatment of the components. The BehrOxal® process cycle times can thus be matched precisely to the rhythm of the production line, allowing the operation to be fully incorporated into the production process.
Unlike other alternatives to the yellow chromate coating process, BehrOxal® requires no poisonous or carcinogenic chemicals such as hydrofluoric or chromic acids – a boon for the environment and the health of employees. The BehrOxal® process does not require any heavy metals or corrosives. This eliminates the need for the disposal of fluoride-containing wastes and sludge contaminated with heavy metals, thus considerably enhancing worker safety.
Studies show that the BehrOxal® process sustainably protects the environment in all its process steps when compared with the yellow chromate coating process and other alternative coating processes. There is no disposal of sludge containing chrome (VI) or fluoride from the production process. The addition of heavy metals can be completely avoided by avoiding chromic acid as a chromate coating agent. The ecological sustainability of the BehrOxal® process is evident even when compared with other available alternative chrome (VI)-free coating processes. This is primarily due to the smaller amounts of water and chemicals used which have no aggressive and environmentally damaging properties.
In addition to the benefits offered by the process, including improved worker safety and ecological sustainability, BehrOxal® also provides enhanced corrosion protection. This was demonstrated using the SWAAT corrosion testing method (Figure 3).
Unlike a yellow-chromate-coated evaporator, which has a perceptible or almost perceptible cement-like odour, an evaporator with a BehrOxal® coating has no or an almost imperceptible individual odour. In odour tests based on the VDA 270, recommendation of the VDA (German Association of the Automotive Industry), a plate-and-fin evaporator with BehrOxal® coating was given a rating of between 1 and 2, while a yellow-chromate-coated evaporator only managed a score of between 2 and 3.
During operation of an air conditioning unit, water condenses on the surface of the evaporator. Depending on the evaporator, up to 200g of water can accumulate. When the air conditioning unit is turned off and the vehicle is stationary, it can be a while before the evaporator dries out completely, depending on the weather conditions and location. Contaminants in the air such as dirt, pollen etc. can be deposited on the wet evaporator surface during operation, and can thus form, together with the condensation, a growth medium for microorganisms.
It is therefore important to improve the water run-off in the evaporator. This is realised on the one hand using an appropriate evaporator geometry, and on the other through the use of a hydrophilic evaporator surface. Water forms large droplets on a hydrophobic surface that block the narrow gaps between the evaporator fins. This restricts airflow, thus imparing cooling performance. In addition, droplets can be swept along by the airflow and can reach the passenger cabin. When the air conditioning unit is in reheat operating mode, the windows can suddenly become misted with condensation.
On the hydrophilic BehrOxal® layer, flat droplets are first formed that then join up to create a thin film that readily runs off from the surface and does not hinder the airflow (Figure 4). The efficiency of the evaporator is not affected, thanks to the self-cleaning effect, and contaminants on the evaporator surface are washed away. Additionally, the evaporator can also be configured as a more compact and thus lighter unit. The trade journal Automotive News honoured BehrOxal® with the coveted 2007 PACE (Premier Automotive Suppliers’ Contribution to Excellence) Award, an independent award for an innovative and eco-friendly coating technology.
