Environmentally Compatible Solvent Cleaning

When it comes to protecting the environment, solvents have developed a questionable reputation. But hydrocarbons, modified alcohols and chlorinated hydrocarbons demonstrate definite advantages for a variety of applications in the field of industrial parts and surface cleaning. Used in combination with the right technique, solvents can not only help meet strict cleaning requirements, but also can be reliable and environmentally sound.

The alternatives for removing processing residues from surfaces are just as varied as the different options for processing materials. Wet chemical cleaning processes with aqueous media and solvents are usually used to meet specific requirements for surface quality. Before deciding to use solvents for cleaning, a series of cleaning tests should be run to determine whether or not the required level of cleanliness can be achieved with a water-based cleaning process. Evaluating the economic efficiency of the process is also a good idea.

Featured Content

A Simpler Way to Calculate Shot Size vs. Barrel Capacity READ MORE

Choosing the Right Robot for Your Plastics Manufacturing Application: 3-, 5-, or 6-axis? READ MORE

Traditional Solvent Applications

Solvents are known to provide effective and efficient cleaning results in four basic areas:
• For heavily oiled parts that would otherwise (in the case of aqueous cleaning systems), require extensive conditioning of the medium and/or high cleaning agent consumption
• For workpieces that are difficult to dry, such as thin-walled and capillary parts, as well as stampings and bent parts. Aqueous cleaning systems with hot-air drying consume large amounts of energy for these kinds of parts.
• In industries such as aviation and aerospace, where process approvals are based on use of solvents in order to assure flawless degreasing
• Whenever cleaning very small parts that require good material compatibility and a non-corrosive cleaning agent (in the electronics industry, for example).

Selecting the Right Solvent

The most common solvents include non-halogenated hydrocarbons (HCs) such as isoparaffin and modified alcohols, and chlorinated hydrocarbons (CHCs) such as perchlorethylene (PERC), trichloroethylene (TRI) and methylene chloride (MC). Hydrocarbons and modified alcohols differ from one another in chemical composition. Some of the substance groups demonstrate non-polar characteristics as well as good oil and grease dissolving capabilities. Others offer polar characteristics and can be mixed with water. Some cleaning agents are formulated with both polar and non-polar characteristics.

The primary application for non-polar HC cleaning agents is degreasing. These agents provide good results in dissolving animal, vegetable and mineral oils and grease, although modified alcohols usually have better degreasing capabilities than pure hydrocarbons. Cleaning agents based on modified alcohols that demonstrate lipophilic and hydrophilic characteristics are suitable for difficult applications such as cleaning lapped parts, sintered metals and top quality, high precision small parts.

Because hydrocarbons and modified alcohols are highly volatile combustible substances that, as a rule, are used at temperatures above the flashpoint in industrial applications, their use requires systems technology including appropriate explosion and fire protection.

Non-combustible chlorinated hydrocarbons demonstrate various physical and chemical characteristics that make them advantageous for certain cleaning tasks. For example, they are very good grease solvents, have minimal surface tension and are chemically stable. Generally, CHCs allow for effective removal of non-polar oils and slightly polar grease from components with complex shapes – without corrosion, oxidation, discoloration, burning, dulling, or other surface impairments. CHCs are typically preferred where critical oils (such as chlorinated and highly sulphurous oils) are carried over into the cleaning medium along with the parts to be cleaned. Over a period of time during the distillation process, these oils result in the formation of acids, which impair the cleaning qualities and reduce the service life of HCs and modified alcohols. In this case, chlorinated hydrocarbons provide the opportunity for restabilization, which is not possible with HCs and modified alcohols.

Because of their low evaporation rates, chlorinated hydrocarbons offer advantages over HCs and modified alcohols for drying. Generally speaking, they dry faster and more completely, making them preferable for parts with critical drying characteristics, such as those with complex geometries and capillaries. Hydrocarbons and modified alcohols, as well as CHCs, are now used in closed circuits, thus assuring safe, sustainable handling.

Vacuum Technology

Today, solvents are used in fully enclosed cleaning systems that fulfill applicable requirements for reduced VOC emissions. Modern systems are equipped with an integrated distillation unit that continuously conditions the solvent, thus assuring consistent cleaning quality.

Cleaning systems that operate under full vacuum have also been growing in popularity. This is at least partly due to the fact that the use of vacuum for combustible solvents (HCs and modified alcohols) eliminates the need for additional explosion protection. Solvent use in reduced pressure atmospheres has also shown to provide additional advantages, such as system safety with regard to leaks and low-temperature distillation.

With these systems, both the solvent and the oil introduced to the process are subject to less stressing. The distillation of CHCs generally takes place at low temperatures–the boiling point of perchlorethylene is approximately 250° F. Fewer cracking products occur as a result, and oils with a boiling point of around 285° F can also be distilled out reliably. In the case of hydrocarbons and modified alcohols, the distillation point lies within a range of 350° F to greater than 390° F, regardless of the solvent used. Therefore, there is a risk of distilling an oil with a low boiling point along with the solvent and impairing the quality of the cleaning agent as a result.

Energy-Saving Distillation Setup

Because solvent regeneration is the most energy-intensive step in this type of cleaning process, modern solvent systems are equipped with heat recovery devices for heat generated during distillation. These devices are being continuously improved. Furthermore, today’s systems are also equipped with automatic distillation power adjustment adapted to actual conditioning requirements, thus further reducing energy consumption and operating costs.

Making Solvent Systems More Flexible

The trend toward full vacuum technology is expanding the possibilities for universal use of solvent-based cleaning systems. For example, systems are available that can be operated with hydrocarbons, modified alcohols or chlorinated hydrocarbons. These systems contribute to investment security for the user, in the event that changing requirements or new components make it necessary to switch to a different solvent.

More than Just a Degreaser

Solvents are traditionally regarded as degreasers. Contaminating particles, such as chips that are trapped in the oil or grease, are generally cleaned away as well because they no longer adhere to the degreased surface. In order to fulfill stricter particle count requirements, the systems are frequently equipped with several media tanks, ultrasonic generators and appropriate filtering technology, as well as special design features. These features might include, for example, an electropolished process chamber, as well as wall rinsing to ensure that any particles that adhere to the process chamber walls are washed away. Systems of this sort are also used in industries such as medical technology that require validated processes.