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Thursday, September 28, 2006

C - Cleaner

Question/problem:
What factors should be considered when choosing a detergent?

Solution:
This addresses the fifth variable "CLEANER" in Alconox's acronym BATHCARD, factors that contribute to successful cleaning. Click here to learn more.


CLEANER
The cleaner or detergent used should match to the desired cleaning method, the surface and types of soils being cleaned. For instance, a low-foaming detergent should be used for spray or machine cleaning, a good anti-redeposition detergent for soak and ultrasonic cleaning and a high emulsifying and wetting detergent for manual cleaning. The detergent, temperature, and degree of agitation should be strong enough to remove the soil to the desired level of cleanliness without harming the substrate being cleaned.

It is very important to choose a low-foaming or non-foaming detergent when cleaning in or with a machine that relies on spraying for mechanical agitation. Foam is caused by agitation at an air/solution interface when a foaming agent is present. It may build up and spill over from the machine creating a mess. It will also build up on the substrate and interfere with the mechanical cleaning energy of the spray. Finally, foam may get sucked into recirculation pipes causing problems with pumps in the machine.

Surfactants are often foaming agents. Most aqueous cleaners have surfactants in them. There are three basic types of aqueous cleaners that are suitable for machine washing: cleaners with no surfactant, cleaners with non-foaming surfactants, and cleaners with low or controlled-foam surfactants. There are important differences among these types of cleaners. Remember that foam forms in the presence of an agitated foaming agent where air is present. Many soils are foaming agents. In particular, soap formed by saponifiers, in electronic solder flux cleaning, is a foaming agent. A surfactant-free cleaner will not protect against foam formed by soils. Clean only non-foaming soils with surfactant-free cleaners. A nonfoaming cleaner usually has a nonionic polymer surfactant. These surfactants come out of solution at elevated temperatures and form an oil slick on top of the solution. The oil slick is a barrier to air contact preventing foam from forming or being stable. These cleaners will suppress foam from soils. They only work properly if the temperature is hot enough. Find out the minimum temperature at which to use these cleaners.

Finally, there are controlled foam cleaners which usually have limited foam suppressing capabilities. The surfactants themselves do not foam excessively, but they are not be able to control much foam resulting from soils.

It is critical that the detergent be scientifically formulated to clean effectively and to rinse away without leaving interfering residues. A scientifically formulated detergent will typically have appropriate surfactant ingredients and non-depositing rinse-aids. The surfactant should have sufficient surface tension lowering properties to assist in proper rinsing. A surface tension below 35 dynes per centimeter for the cleaning solution, as used, is often sufficient for good rinsing. Non-depositing rinse-aids can help complete a formulation to meet the rinsing requirements of critical cleaning.

In addition, detergents should be manufactured according to appropriate quality-control procedures. In many critical cleaning applications it is desirable to choose a detergent with a lot number tracking system and with certificates of analysis available from the manufacturer. These certificates document each lot of detergent to assure consistency and quality control and to prevent cleaning failure from inconsistent manufacturing or unannounced formulation changes. It is also desirable to choose a detergent from a manufacturer who maintains quality control of raw materials and who retains samples of each detergent lot to be able to respond to concerns about a particular batch.

The detergent should be widely available and economical to use (for optimum economy, a concentrated detergent is typically used at 1:100 to 2:100 dilutions). The detergent concentrate should be diluted according to the manufacturer's instructions. Typically, warm (about 50 degrees C) or hot (about 60 degrees C) water is used. Ambient temperature water may be acceptable, especially for presoaking. For difficult soils, very hot water should be used (over 65 degrees C), and the recommended detergent concentration doubled.

Chemistry Bath-life extension and control

To avoid potential for cross contamination, only freshly made up cleaning solutions should be used for the highest levels of critical cleaning. For industrial critical cleaning applications, high levels of cleaning can also be achieved with extended bath life. In general, a pH change of 1 unit towards neutral indicates an exhausted cleaning solution. Bath life can be extended by physical filtration of particulates, cooling and settling of sludge and skimming of oils. Bath life can also be extended by adding one half as much detergent, of the initial load, after partially depleting the cleaning life of the bath. With frequent daily use, detergent solutions can rarely be used longer than a week even with these bath life extension techniques. Conductivity, pH and % solids, by refractometer, can be used to control bath detergent concentration.

Free alkalinity titration can be used to control bath life of alkaline cleaners where the soil being cleaned depletes free alkalinity-as is often the case with oily soils.

The process:
  • Titrate a new solution to determine free alkalinity
  • Titrate the used solution to determine the percent drop in free alkalinity
  • Add more detergent to the bath to bring the free alkalinity back to the level of the new solution. (For example if the initial solution is made up with 100 ml of cleaner concentrate and a 25% drop in free alkalinity is observed, try adding 25 ml of cleaner concentrate to recharge your solution.)

Perform a new free-alkalinity titration to confirm the recharge the first few times this recharging method is used. This is to ensure that the detergent being used is linear with respect to free alkalinity depletion. This form of bath life extension cannot run indefinitely, sludge will eventually form. Fresh solutions must be made up periodically. Bath lives can also be extended using conductivity. Most cleaners contain conductive salts which can be detected using conductivity. Once the conductivity response of the detergent is determined, the depletion of those conductive salts can be measured. Many cleaner manufacturers can supply the curves of detergent concentration versus conductivity. By adapting these curves to your conditions and measuring the conductivity, detergent depletion and dilution can be determined. This determination can be used to figure out how much detergent to add to the cleaning solution to restore cleaning performance. Typically, this kind of measure the bath and recharge with detergent process can be done 2-3 times before a new bath is needed. Keep in mind that, the bath will ultimately reach a point where it forms sludge (or where some other failure occurs). At that point, the bath must be dumped and a complete batch of new cleaner made up. The time to dump the bath, and start over, is generally determined using some sort of cleanliness measurement and defined in terms of number of parts cleaned or time period of bath use. Conductivity does not typically detect the point of cleaning failure, but only detects concentration of cleaner present, whether depleted or not. The following table gives specific examples of concentration vs. conductivity for several Alconox-brand cleaners. Use this data to derive the concentration of detergent from measured conductivity. Note that, conductivity is temperature dependent. Detergent solutions do not have the same slope as many default settings on temperature-correcting conductivity meters. For best results, allow hot detergent solutions to cool to a consistent temperature for comparison.

Chemistry, cleaning and corrosion inhibition

Corrosion, during cleaning, is accelerated by the same variables that accelerate cleaning: heat, aggressive chemicals, time, and agitation. To reduce metal corrosion (in approximate order of importance) use less heat, lower pH detergent, shorter cleaning time and less agitation. In general, use the mildest pH detergent to limit metal corrosion.

Higher pH detergents may have metasilicate corrosion inhibitors making them suitable for cleaning soft metals such as aluminum. In general, to reduce plastic corrosion, use less aggressive cleaners, with less solvent or surfactant character; lower concentrations of cleaners; lower cleaning temperatures; less contact time; and finally, less agitation. After aqueous cleaning, metal corrosion can occur during rinsing and drying. Corrosion inhibitors can be added to rinse water provided that inhibitor residue does not interfere with clean surfaces. Using hot rinse water (to keep clean surfaces hot) and rapid heat or vacuum drying, speeds drying and minimizes metal corrosion. Forced air drying, drying with a hot oxygen-free gas such as nitrogen and using air knives, that physically remove rinse water, can also minimize corrosion. When rinsing mild steel with hot water and drying with hot air, "flash rusting" can occur. The corrosion actually occurs during rinsing as a result of dissolved oxygen in the rinse water. In some instances, lowering the water temperature or drying temperature can help avoid corrosion. For instance, in a case where flash rusting on mild steel had been occurring, the rusting was avoided by lowering the temperature of the rinse water from 150 degrees F to 120 degrees F maintaining an ambient air drying system. Flash rusting can also be avoided by using a solvent, such as isopropyl alcohol, to rinse with rather than water. Adding corrosion inhibitors, to rinse water, can also prevent corrosion but the corrosion inhibitor may leave residue during rinsing.

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