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Clean workpieces boost productivity.

Clean workpieces boost productivity

Vapor degreasing as we know it is on the way out. To help you find alternatives, you'll need to know some cleaning basics.

Cleaning is simply the removal of dirt, but the concepts of clean and dirty are relative to the operations to be performed. For example, when a machined part goes into storage, a rust preventive is applied. In storage, the part is considered clean. However, if the rust-preventive coating interferes with the next operation, the part is considered dirty, and a cleaning step is required.

The four basic types of chemical solutions used in industrial cleaning are alkali, acid, neutral, and organic solvents (including oil-water emulsions).

Alkali cleaners

Alkali cleaners are water-based and considered the workhorses of all industrial cleaners. They're popular because they can perform a wide variety of jobs effectively. Personal soaps, household detergents, and "miracle" cleaning sprays are all alkali cleaners. They have an in-use pH between 8 and 14. To be effective, they should soften or condition the water in which they are used to prevent scale problems, and they should be able to buffer the solution so the pH does not vary over a wide dilution range. Good products operate at temperatures from ambient to 200 F, to reduce energy consumption.

Cleaning ability is based on one or more of three functions: saponification, wetting, and deflocculation. Saponification is the reaction between an alkali and a fatty acid to form a soap. Some cleaners perform this reaction with the fats in the soils they are removing, as in the case of alkali paint strippers. The caustic-based material in the stripper attacks the fatty acid binder in the paint to form a soap. This in turn provides a wetting action that undercuts the paint coating and releases it from the surface. The caustic cleaner reverses the role of the fatty acid paint binder through saponification.

Wetting, which is what soap does, is very similar to emulsification. The emulsifier pulls the soil into a water system so it can be flushed away. An emulsifier is a long-chain molecule that attracts oil at one end and water at the other end. Each end of the molecule repels the other.

The repulsion principle is important when the cleaner contains a surface-active agent, or surfactant. The surfactant latches onto greasy soils that otherwise would not mix in water, pulling them away from the surface being cleaned. Because the water-repelling end of the molecule does not want to remain in the water, the grease-soaked emulsifier floats to the solution surface, where it can be flushed away.

Deflocculation is a process whereby soils are split out of solution. This is important where the cleaning solution is contaminated with soils that might be redeposited on the cleaned surface. In spray washing systems, the deflocculated soil floating on the reservoir surface can be periodically removed, while clean solution is drawn from below the soil layer for spraying. But in a soak tank, the clean parts must be lifted out of the tank through the soil layer, redepositing soil on them. To prevent this, we recommend that the removed soil be suspended in the solution and removed by filtration.

Water hardness is another key factor. Many of the dissolved salts that cause hardness stay in solution and don't interfere with cleaning, but temporary hardness, caused by salts that come out of solution, can form a sediment. The sediment, in turn, may contaminate the cleaning solution and redeposit soil onto the parts to be cleaned. It can also clog spray nozzles and other cleaning devices.

A good alkali cleaner solves this problem by using sequestrants and chelators. Sequestrants block the ions that cause temporary hardness, preventing them from taking part in any reaction and thus eliminating precipitation of salts. In the opposite mode, chelators cause a chemical reaction that changes temporary hardness into permanent hardness; the salts that react with the chelator join the solution.

Non-alkali cleaners

Non-alkali, or acid, cleaners have an in-use solution pH below 6. They are used primarily to remove mineral deposits, rust, scale, and other metal oxides. Also, acid cleaners remove hard-water deposits from washers and soak tanks.

Typical products are inhibited-acid cleaners for descaling or rust stripping of ferrous metals. They are formulated with fast-acting ingredients. Good ones are designed with operator safety in mind.

Acid cleaners attack most metals, and this complicates the selection of spray and tank equipment. Also, the cleaners react not only with rust, scale, and other metal oxides, but with the base metal of the work-piece itself. Parts may go out of size tolerance in some cases. In other instances, the acidic reaction may lead to hydrogen embrittlement--important if parts are under residual stress or will be used under stress.

Neutral types

Neutral cleaners normally have a pH between 6.0 and 8.0. They often have a strong tendency to foam. Because of their mild reaction, safety, ease of disposal, and overall effectiveness with easily emulsified soils, they are good for hard scrubbing.

Water-based neutral cleaners generally remove light grease and oil. They are formulated for a wide range of industrial and vehicle cleaning applications ranging from hand cleaners to floor scrubbers.

Organic solvents

Organic-solvent cleaners have been very popular, but their future use is limited because of difficulty in handling, application, and disposal--especially in the face of the Montreal Protocol and OSHA and EPA regulations. However, when used properly, they can be extremely effective and nonpolluting.

Solvent cleaners are usually highly volatile, so parts dry quickly with little or no residual film. However, unless properly controlled, the rapid volatility can cause an unventilated area to fill with potentially toxic fumes that may also be a fire hazard.

Solvent cleaners dissolve difficult soils, such as heavy grease, oil, and tar--in difficult-to-reach recessed areas. Although organic-solvent cleaners will remove only soils that are soluble in them, they cut through and dissolve greasy soil better than any other cleaner, because they pull the grease into solution.

Four basic processes using solvent cleaners are: vapor degreasing, water-seal soak systems, solvent-water emulsions, and oil-water emulsions. Vapor degreasing can flush out long, thin recessed holes in parts such as carburetor bodies that might trap alkali or other types of cleaners and their residues. The solvent can move through the part, remove the soil, and leave the part clean and dry.

Water-seal soak systems use a layer of water on the surface of the solvent to prevent it from evaporating. They are used for heavy-duty cleaning, where alkali cleaners lack the penetrating strength. Applications include degreasing engine blocks and paint stripping.

Solvent-water emulsions offer a unique combination of performance properties. With the help of an emulsifying agent, a solvent or oil normally incompatible with water can be uniformly distributed throughout a volume of water. With an emulsifier to hold it in suspension, the cleaner has the characteristics of both a solvent and water-based cleaners.

Emulsion cleaners are a compromise between solvent and water-based cleaners. They will remove soils that are cleaned only by water as well as those oils and greases that are cleaned only by solvents. But they're not as strong as individual solvent or water-based cleaners.

Mild oil-water emulsions normally handle in-process parts cleaning when extended rust protection is required. They're safe for both ferrous and nonferrous metals, and they usually provide in-process rust protection for 30 to 60 days.

Both oil- and solvent-based emulsions remove light soils and leave behind a good rust-protective film. Solvent emulsions provide better cleaning than oil emulsions, but have shorter bath life and rust protection. Emulsion cleaners require agitation to keep the emulsions stable, and they are susceptible to rancidity and unpleasant odors.

Efficient cleaning

Whether using an alkali, acid, neutral, or solvent product, cleaning happens because of chemical activity. As with most chemical reactions, cleaning is affected by time, temperature, concentration, agitation, and rinsing. These factors are interrelated, although temperature is the most important factor affecting efficiency. And heating is the most expensive factor in terms of costs for energy, equipment, maintenance, and worker protection.

In some cleaning operations, high temperatures produce convection currents in the solution to provide agitation without mechanical means. The higher the temperature, the higher the level of agitation. Spray washers also can scrub.

Temporary hardness in the rinse water can contaminate the surface being cleaned. In fact, hardness could be more of a problem here than in the cleaner bath, because there are no hardness controls in fresh water to prevent precipitation. Heating the rinse accentuates the problem, and, in extreme cases, it may be necessary to use softened or deionized water.

Finally, consider physical form. Both granular and liquid product will clean equally well, but powdered cleaners typically dissolve slowly and require physical agitation and heat to get completely into solution. And, they can be dusty, resulting in handling problems. Liquid cleaners, on the other hand, are easy to measure, and they mix into solution immediately.

PHOTO : Bearings require cleaning before final inspection.

Harold Diez Research Chemist Castrol Industrial Inc Chicago, IL
COPYRIGHT 1991 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991 Gale, Cengage Learning. All rights reserved.

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Author:Diez, Harold
Publication:Tooling & Production
Date:Jul 1, 1991
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