The single-stage organic phosphating system is reducing the cost and impact on the environment by pretreating metal substrates.
by Mary Carpenter
The preparation of metal surfaces is from every point of view a dirty operation. Improvements have been made in the painting phase, including notable changes from conventional paints to high solid paints and powder coatings. However, the pretreatment of metal surfaces has not changed significantly for many years.
With the beginning of the year 2000, a different kind of pretreatment was introduced in North America. The fur technology developed in Italy in the 70s by inventor Dr. Gianluigi Guidetti, owner of Paikor SRL (now Chemtec srl) in Milan, Italy, who designed the phosphating system expressly to be "green" (environmentally friendly). Understanding the problems affecting conventional metal finishing systems, Eng. Guidetti decided to analyze the pre-treatment problem thoroughly, from a new, more environmentally friendly point of view, and also from a cost-saving perspective. The result: a one-step organic phosphating system* that is changing the way many companies that prepare and paint metal products by shifting their focus on the pretreatment process.
Basic technology
The system is in a single stage and without rinsing, and can be done by immersion or by sprinkling. It operates at room temperature (from 16 to over 38°C [60 to >100°F] at the Plant). The liquid is applied to the part for 60 to 90 seconds, then drained and/or blown, dried and ready to be painted. You can also store the part for weeks before giving it the finish because it will not rust.
A common misunderstanding, based on conventional technology, is that a one-stage process cannot produce good results. In the conventional system, each stage performs a different function and cannot be combined with another stage. Therefore a water-based single-stage system could carry out only one step, such as cleaning, but not others such as rinsing, phosphating, or applying a seal. However, the one-stage phosphating system is not water-based, and all the steps can be combined due to their different chemistry.
In just one minute of treatment time, several different functions are developed: Oils and dirt (powders and metal particles) are washed from the parts by solvents, which simultaneously solubilize the oils. Then the phosphoric acid in the bath reacts with the metal surface, creating a layer of inorganic phosphate on the metal substrate. In steel, that layer of phosphate is iron phosphate, while in galvanized metal it is zinc phosphate. A layer of phosphate is not created in aluminum; however, a surface is developed that promotes adhesion and corrosion resistance that comes from cleaning, etching and sealing.
After washing and creating the phosphate layer, the piece is ready to be removed from the bathroom. The excess solution is drained from the part (the excess can also be blown) and is recycled into the tank to be reused. The particles that are removed from the pieces at the beginning of the one-minute treatment are separated from the bath by a bag-like filtering system.
The next stage of the process occurs during drying. As previously stated, the oils are removed from the parts and solubilized into the solvents during treatment. A special resin in the bath is able to absorb up to four times its own weight in oils. This property translates into an absorption capacity of up to an average of four times the amount of oils found in cold rolled steel. The oils removed from the parts by the solvents in the bath are transferred to the resin and that resin, in turn, comes out of the bath in the treated parts.
When the parts are dry, the resin remains on the surface and performs two important functions: First, it forms a continuous seal on the parts to prevent oxidation for weeks and even months before painting. Second, it chemically captures the oils (which cannot migrate to the surface and interfere with adhesion) and uses them as a plasticizer. Captured oils give flexibility to the resin and increase resistance to cracking and chipping and thus improving adhesion to the paint.
Environmental profile
The one-step organic phosphating process is basically an autonomous system that uses its own materials in proportion. Due to its characteristics, the bathroom never needs to be emptied, nor recharged, but simply the level is replenished when needed. The particles that are in the pieces when they arrive, are filtered; the oils are continuously removed as they are trapped by the resin and taken out of the bath in the pieces; and resin, fluid and phosphoric acid are used in proportion. The process is so stable that laboratory analysis of the bathroom only needs to be carried out every two months instead of every hour or daily, as is required with conventional pretreatment systems.
No water is used in the process, and no subsequent rinsing is required. This means that no contaminants enter the water flow and no treatment facilities or permits are required. The process completely avoids the issue of phosphates as effluents as well as other garbage from the Plant that could be found in the waste water.
In contrast to a conventional system, the single-stage organic phosphating process does not create sludge or other polluting solids. The only solids that are discarded are the particles that come into the parts in the first place.
In a conventional water-based system, process oils removed from parts are a significant contaminant. In the organic phosphating pretreatment system, the oils are not skimmed or removed, but are, on the contrary, captured and used as a plasticizer in the sealing resin.
Because the process operates at room temperature, the pretreatment bath does not need natural gas or any other thermal energy. This factor increasingly represents significant cost savings, and also a benefit to the environment.
The process also requires a minimum of electrical energy because it is a one-stage, low-pressure, low-exhaust technology that uses few small flow pumps and few fans, compared to conventional pretreatment methods. In a sprinkler system, the spray pressure is only 3 to 5 psi (it is actually a spray bath). Exhaust is minimal and only enough to create a small negative air pressure from the Plant into the tunnel.
While the single-stage organic phosphating system uses solvents instead of water to perform the cleaning function, solvents serve several goals both in the process and with the environment. Their chemical characteristics make them highly efficient at absorbing the normal range of oils used by industry. In addition, solvents are low emission because they have high flammability points (ranging from 65 to >100°C [149 to >212°F]) and high boiling points (from 170 to 229°C [338 to 444°F]), as well as low vapor pressures (from 0.85 to 0.02 mm of mercury). To put the vapor pressure problem in context, regulations in San Diego, CA allow only the use of solvents with a pressure of less than 20 mm mercury (assuming the solvents meet other requirements, which single-stage organic phosphating chemists meet). Regulations in other jurisdictions allow the use of the process based on the high boiling points of solvents.
Also, solvents are environmentally friendly because they do not contain hazardous air pollutants, carcinogens or mutagens, halogenated compounds or chlorofluorocarbons, or ozone-depleting substances.
The only substances in solvents that go into the atmosphere are volatile organic compounds VOVs (VOCs), and even those are emitted at a low level due to their low vapor pressure and high flammability points. Taking a mid-range production volume as an example, a user who treats 50,000 square feet of metal in a plant appropriately designed for spraying will emit only 170 pounds of VOCs. In an immersion plant that treats the same amount of metal, emissions will be just a little above 100 pounds. Emissions don't go up much in a plant that treats many more square meters, because the air in the spray zone quickly becomes saturated and almost all the vapors return to the tank.
* Mary Carpenter is president of Carpenter Chemicals and Químicas Carpenter de México, both distributors of the single-stage organic phosphating system. For more information about this technology in Mexico, call +52 33 3699 6537, send an email to [email protected], or visit the www.cc-lc.com page. For Colombia, call ESIN Andina S.A.S. on +57 331 202 4347 or send mail to [email protected]
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