Several trends are emerging in the metal cleaning market. Formulators and users are looking for extremely low-foam cleaners for high-pressure spray cleaning, reported Hedoire, who is technical sales manager for Solvays Novecare line of metal process additives. At the same time, they want to maintain or even increase cleaning performance.

End users are seeking ways to reduce operational costs, as well. Cleaning at lower temperatures can reduce energy consumption as well as carbon dioxide emissions, which is especially relevant to auto body cleaning. Extending the lifetime of the cleaning bath-without losing performance-can reduce maintenance time and expense.

The ability to recycle cleaning solution also reduces costs. Better control of a solutions demulsification or emulsification behavior improves this property. In an abstract submitted to the colloquium, Hedoire explained that soil builds up in the cleaning bath over time. An emulsifying cleaning system holds the soil within the cleaning solution, while a demulsifying system releases the soil so it can be removed, for example, by skimming. These properties are tied to the nature of the soil as well as the cleaning formulation.

Metal Cleaning Basics

The last step in production of metal parts is pre-treatment application, which then allows for application of paint or other coatings. In order to perform this operation, it is necessary for the surface to be perfectly clean. Any defect, impurity or contamination on the metal surface will affect the final surface quality, said Hedoire.

Many metalworking fluid additives adsorb onto the metal surface, making cleaning an even bigger challenge. Fluid formulators often request cleaners that are compatible with their specific products, Hedoire told the January gathering.

Metal cleaning formulations have to be fine-tuned for every type of cleaning application and condition, he continued. Cleaning operations can vary by method, such as immersion or spray, and involve different cleaning machines as well as part sizes ranging from very large to very small.

Four interdependent parameters affect cleaning efficiency: chemistry, mechanics, temperature and time. Cleaning bath chemistry refers to the fluids composition and concentration, in addition to contamination levels. Cleaning mechanics may involve agitation or spray pressure, and more complex mechanics make the cleaning process more challenging. The temperature of the cleaning bath affects its reaction rate and viscosity. And time, of course, is money, Hedoire reminded his audience.

Every time one parameter is modified, we have to modify another parameter in order to keep the same cleaning efficiency, he went on. The drive to decrease temperature means another parameter must be modified to maintain cleaning performance. Increasing cleaning time is not an option, as users want to clean faster. Mechanics are generally fixed by the type of process being used. The only remaining option is to modify the chemistry.

Cleaner Composition

Typical metal cleaners are either solvent based or water based.

Hydrocarbon solvents, which include white or mineral spirits and normal and iso-paraffins, have excellent solvency and low surface tension that enables the cleaner to penetrate crevices or small drill holes. However, the accompanying volatile organic compounds necessitate use in loss-free vacuum equipment, Hedoire explained.

Halogenated solvents, such as perchloroethylene, dichloromethane and hydrofluoroethers, have good capacity to dissolve oils and organic solids, but present health and safety hazards, so open applications are banned in Europe. Despite this, they are still in use for demanding cleaning operations that require optimum surface quality and short cleaning time. Polar solvents include esters, glycol ethers, alcohols and ketones.

There is a trend to replace solvent based cleaners with water based cleaners, said Hedoire. These cleaners are acid, neutral or alkaline. Acid cleaners (pH 2-6) are good for removing scale and oxides, and are used for pickling. Neutral cleaners (pH 8-10) are suitable for non-ferrous metals, as they do not stain, and can also be used for iron and steel alloys if steps are taken to prevent corrosion. Alkaline cleaners (pH 10-13) saponify natural oils and grease, or turn them into soaps, which boosts cleaning. These are suitable for iron and steel alloys.

According to Hedoire, major cleaning formulations are neutral and alkaline cleaners.

In typical aqueous cleaning formulations, inorganic compounds and salts called builders make up a major portion of the solution. Builders include sodium hydroxide, potassium hydroxide, carbonates, phosphates, silicates and borates, and are added at a rate of 15 to 60 grams per liter. Builders maintain pH levels and reduce water hardness by complexing calcium and magnesium cations. They also provide basic cleaning performance, such as dispersion of particles and-in alkaline cleaners-saponification of greases and fatty oils.

But basic cleaning performance is not enough, so surfactants are added at a concentration of 0.5 to 6.0 g/L. Surfactants improve the cleaners wetting ability and detergency by reducing interfacial tension between oil and water. They also improve oil emulsification.

Cleaners may also include additives like corrosion inhibitors and anti-foam agents.

Balancing Act

Two types of surfactants are typically used in cleaning formulations. Primary surfactants are non-ionic surfactants with good wetting and detergency performance, Hedoire explained. Co-surfactants, which are used at a lower treat rate, have limited cleaning performance but can be used to control foam generated by primary surfactants.

Formulators must strike a balance for each application. There is no ideal surfactant, said Hedoire. There is a trade-off between cleaning performance and low foam. These two kinds of surfactants have to be formulated together in order to develop cleaning formulations with good cleaning performance and good foam control, he elaborated.

Foam control has become more important with the trend toward lower cleaning temperatures. The main issue with the decrease in temperature is the increase of foam, lamented Hedoire. At temperatures over 60 degrees Celsius (140 F), no foam is generated during the cleaning process. But end users are pushing to lower cleaning temperatures to 30 or 40 C without giving up foam control or cleaning performance.

Cloud point is the temperature above which an aqueous surfactant solution becomes opaque or cloudy. Surfactants with a higher cloud point generate more foam. The solution, Hedoire stated, is to blend primary surfactants, which are mostly high cloud point, with low cloud point co-surfactants to control the foam.

To find the right blend, Brussels-based Solvay used a cleaning performance test and a foaming evaluation test, both based on methods used by Germanys automobile manufacturers association, to screen how nine surfactants performed with different alloys, soils and temperatures.

In the cleaning performance test, a cloth was used to apply oil to metal plates, which were either cleaned immediately or after aging at 170 C for 15 minutes. The cleaning solution was prepared with 15 g/L of builder, 0.75 g/L of surfactant, and deionized water, adjusted to pH 12 (alkaline). Oil was added at 1 g/L as a contaminant. The cleaning solution was heated to 60 C or 45 C. The plates were dipped in the bath for 2 to 6 minutes, then rinsed with deionized water for 15 seconds. Cleaning performance was evaluated with the water break test. If the water beads or appears blotchy, the plate is not clean. A single, well-defined line or no line at all indicates a clean plate.

To evaluate foaming tendency, cleaning solution was prepared as in the cleaning performance test, but with 2 g/L of oil as a contaminant and pH 10 (neutral/alkaline). The solution was aged for 24 hours and poured into a 250-milliliter graduated cylinder, then hand-shaken 10 times. The foam volume was measured after 15 seconds, 30 seconds, 1 minute, 1.5 minutes, 2 minutes, 4 minutes and 5 minutes.

The test results showed a clear dependence on the type of oil used and also on the type of additives, Hedoire noted.

In the end, he said, the company was able to develop a formulation with less than 1 mL of foam remaining after 5 minutes that maintained cleaning performance.

In addition, the chemical manufacturer says it has developed a proprietary primary surfactant that produces low foam above 60 C and below 40 C, and only medium foam at temperatures in between, along with good cleaning performance. This surfactant is very helpful when the previous method of mixing high cloud point and low cloud point surfactants wont work, Hedoire offered.

The company continues its research to keep up with performance demands. The dream is to be able to clean at room temperature, Hedoire told his audience. But in this case, we foresee some issues, one of them being bacterial growth.