By Doris Schulz
Where environmental protection is concerned, solvents do not enjoy a very good reputation. However, 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, cleaning with solvents is not only requirements-oriented, it’s reliable and environmentally sound as well.
The alternatives for removing processing residues from surfaces are just as varied as the different options for processing materials. Specified requirements for surface quality must frequently be fulfilled, to which end wet chemical cleaning processes with aqueous media and solvents are usually used. Before deciding to use solvents for cleaning, it is advisable to run a series of cleaning tests to determine whether or not the required level of cleanliness can be achieved with a water-based cleaning process. It is also a good idea to conduct an evaluation of economic efficiency.
There are basically four areas where advantageous cleaning results are obtained with solvents:
The most common solvents include nonhalogenated hydrocarbons (HCs) such as isoparaffin and modified alcohols, as well as chlorinated hydrocarbons (CHCs), for example perchlorethylene (per), trichloroethylene (TRI) and methylene chloride (MC).
Hydrocarbons and modified alcohols differ from one another with regard to chemical composition. Some of the utilised 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. This allows for the formulation of polar, as well as non-polar cleaning agents.
The primary application for non-polar HC cleaning agents is degreasing. They’re distinguished by good dissolving performance for animal, vegetable and mineral oils and grease, although modified alcohols usually have better degreasing capabilities than pure hydrocarbons.
Cleaning agents based on modified alcohols which demonstrate lipophilic and hydrophilic characteristics are suitable for difficult applications such as cleaning lapped parts, sintered metals and top quality, high-precision small parts.
Due to the fact that hydrocarbons and modified alcohols are highly volatile, combustible substances which, as a rule, are used at temperatures above the flashpoint in industrial applications, systems technology including appropriate explosion and fire protection is required.
Non-combustible chlorinated hydrocarbons demonstrate various physical and chemical characteristics which make them advantageous for certain cleaning tasks. For example, they’re very good grease solvents, have minimal surface tension and are chemically very stable. As a rule, CHCs allow for effective removal of non-polar oils and slightly polar grease from components with complex shapes – without corrosion, oxidation, discolouration, burning, dulling or other surface impairments.
CHCs are used with preference where critical oils (e.g. chlorinated and highly sulphurous oils) are carried over into the cleaning medium along with the parts to be cleaned. During the distillation process, these oils result in the formation of acids over a period of time, which impair the cleaning qualities and reduce the service life of HCs and modified alcohols.
In this case, chlorinated hydrocarbons provide the opportunity of restabilisation, which is not possible with HCs and modified alcohols. Where drying is concerned, chlorinated hydrocarbons offer advantages in comparison with HCs and modified alcohols thanks to their low evaporation rates.
Generally speaking, they dry faster and more completely, making them preferable for parts with critical drying characteristics -- for example complex geometries and capillaries. Hydrocarbons and modified alcohols, as well as CHCs, are now used in closed circuits, thus assuring safe, sustainable handling.
Today, solvents are used in fully closed cleaning systems which fulfil applicable requirements for reduced VOC emissions (volatile organic compounds). At the same time, modern systems are equipped with an integrated distillation unit which continuously conditions the solvent, thus assuring consistent cleaning quality.
A trend towards cleaning systems which are operated under full vacuum has been apparent for quite some time now as well. On the one hand, this results from the fact that the use of vacuums for combustible solvents (HCs and modified alcohols) eliminates the need for additional explosion protection.
On the other hand, advantages in the area of process engineering speak in favour of using solvents in reduced pressure atmospheres. Among others, these include system safety with regard to leaks, and low temperature distillation.
Consequently both the solvent and the oil introduced to the process are subjected to less stressing.
The distillation of CHCs generally takes place at low temperatures – the boiling point of perchlorethylene is approximately 120°C (280°F). Less cracking occurs as a result, and oils with a boiling point of, for example, 140°C can also be distilled out reliably. In the case of hydrocarbons and modified alcohols the distillation point lies within a range of 175°C (347°F) to greater than 200°C (392°F) regardless of the utilised solvent, and thus one runs the 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.
Due to the fact that solvent regeneration is the most energy-intensive step in this type of cleaning process, modern solvent systems are equipped with heat recovery devices for the heat generated during distillation. These devices are being continuously improved. Furthermore, today’s systems are also equipped with automatic distillation power adjustment, which is adapted to actual conditioning requirements, thus further reducing energy consumption and operating costs.
The trend towards full vacuum technology is expanding the possibilities for universal use of solvent-based cleaning systems. For example, systems are available which 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.
Solvents are traditionally regarded as degreasers. Contamination in the form of particles such as chips which 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 fulfil 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 include, for example, an electropolished process chamber, as well as wall rinsing which assures that any particles which adhere to the process chamber walls are washed away. Systems of this sort are also used for validated processes, for example in the field of medical technology.