Online Exclusives
Organic-Based Advanced Pretreatments
2/11/2010
Phosphorous and heavy metals are coming under increasingly strict discharge restrictions. Organic-based advanced pretreatments are viable alternatives which also provide significant operational advantages over conventional pretreatments.
By Gary Nelson, Product Manager, Chemetall US Inc., www.chemetallamericas.com
Phosphorous and heavy metals are coming under increasingly strict discharge restrictions. Organic-based advanced pretreatments are viable alternatives which also provide significant
operational advantages over conventional pretreatments.
They are environmentally friendly because they have no zinc; have no or low phosphorous; provide energy savings because they operate at ambient temperature; require less capital investment since fewer stages are needed; require a smaller bath initial charge since a smaller tank is used; have simple operation (have wide robust operating windows, produce almost no sludge, produce no scale, have simple waste treatment); are versatile since they work with most substrates and paints; match the paint performance of the process replaced; and finally, the bottom line is cost savings.
Silanes are made from silicon, the fourteenth element in the periodic table. The basic formula is SinH2n+2, but this structure has no use in a pretreatment line since it does not bond to paint or metals. What’s needed instead are organofunctional silanes. These materials are hydrolyzed, that is, reacted with water, by the chemical supplier. A partial condensation reaction follows, and the result is a molecule with one end that bonds to metals and the other to paint resins. During coating dry off and/or paint cure, the silane coating crosslinks to provide a durable coating.
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Further development of silane coatings involves the use of different types of organofunctional silanes, mixtures of organofunctional silanes, controlling the hydrolysis and condensation during manufacture, and the addition of group IV-B metals such as zirconium and titanium.
Phosphonate coatings are made of select phosphonate molecules along with titanium, zirconium and proprietary activating ingredients. The phosphonate has to have the desired characteristics of corrosion inhibition (film forming), stability in hard water, must bond to specific metals and paints, and have limited complexation with hard water salts. The chemical components must be carefully balanced in order to get the optimization of paint performance.
Many factors should be considered when choosing an advanced pretreatment. They include washer materials of construction, water quality, phosphorous discharge limits, and paint performance, which vary with substrate and paint. Each possible application should be tested for performance. The performance test used to select the advanced pretreatment should be applicable to the end use. For example, neutral salt spray may be a good test to use for items that will be used in a marine environment. For automotive, certain cyclic corrosion tests may be more applicable. For less critical items that will be used in a dry indoor application, cross hatch adhesion may be all that is necessary.
For corrosion testing, it is important to follow these guidelines in testing: Test both current and advanced pretreatment processes side by side, test either the production substrate or if the production substrate is not consistent, use lab test panels and include lab control panels. Coat all panels with the same paint, test all panels in the same test cabinet and test all panels for one time period. Finally, use replicate panels for better confidence in the results.
Water quality is important. Calcium, chloride, sulfate, and other hard water salts can react with or reduce performance of pretreatments. Contaminants from the cleaner such as phosphorous, silicate and complexants can also cause problems. Good rinsing is critical as is good cleaning.
When preparing to switch to an advanced pretreatment, remember to plan carefully, make any washer modifications such as re-routing exit halos and/or counterflows, order all special titration supplies and automation equipment early, chemically descale rinse thoroughly at all stages, and align nozzles. When making the switch, check water quality before adding chemical, read and understand MSDSs, and use proper personal protective equipment. It is a good idea to retain bath samples and run test panels to baseline the new process. Once the switch is made, operator training is needed. It is also a good idea to post SPC charts of pertinent operating parameters near the washer.
The replenishment of the bath can be done through use of pH controllers, or in some cases, conductivity controllers. Both require careful calibration of the controllers on a regular basis.
Finally, throughout the process of converting, it is important to communicate clearly with your pretreatment supplier. Performance requirements and limitations such cost, substrates, water quality, paint, equipment materials of construction, and number of stages should be communicated clearly to all.