Solvent-based cleaning agents refer to organic solvents that are insoluble in water. Non-aqueous cleaning agents used in precision industrial cleaning mainly consist of organic solvents such as hydrocarbons (petroleum-based), chlorinated hydrocarbons, fluorinated hydrocarbons, bromine hydrocarbons, alcohols, silicone oils, and terpenes.
The main indicators for evaluating solvents include KB value (butanol value of pine resin), AP (aniline point), SP (solution parameter), surface tension, density, viscosity, boiling point, flash point, and exposure concentration. A high KB value, meaning a solvent with strong dissolving power, does not necessarily indicate a good cleaning agent.
A solvent-based cleaning agent suitable for industrial cleaning must meet the following conditions:
1. Chemically stable, not easily reacting with the object being cleaned;
2. Low surface tension and viscosity, strong penetrating power;
3. Low boiling point, capable of self-drying;
4. High flash point, non-flammable;
5. KB value should not be too high to avoid miscibility with the cleaning agent;
6. Low toxicity, safe to use;
7. Non-ODS and low GWP (Global Warming Potential), environmentally friendly.
If we disregard point 7 above, the best cleaning agents are CFCs (Freon) and TCAs (Thioglycolic Acid). They have good chemical stability, can be stored for a long time without deterioration; they are ineffective against most metals, plastics, and paints, and will not dissolve them; they have high flash points, low toxicity, and are safe to use; they have low surface tension, strong penetration, and strong cleaning ability; they have low boiling points, fast evaporation rates, and cleaned workpieces can dry on their own, generally without the need for drying, thus they are widely used in various industrial cleaning applications. However, their only drawback is ozone depletion, and therefore they are being phased out.
Although various alternatives have been developed worldwide, no single cleaning agent can yet rival CFCs. They all have their own problems. The main alternatives currently in use include:
(1) Hydrocarbons
Hydrocarbon solvents, also known as hydrocarbon solvents in Japan, contain only hydrogen. Based on their molecular structures, various performance-specific products have been developed, and they are widely used in Japan. The advantages of this type of cleaning agent are: non-ODS, strong cleaning power for oil stains, good penetration, odorless or with a slight special odor, low toxicity, easy wastewater treatment, recyclable, and inexpensive. The disadvantages are: some flammability and explosiveness, slow drying, high requirements for cleaning equipment, and large initial investment. Ideally, vacuum cleaning should be used to reduce surface tension and improve cleaning ability, followed by vacuum drying. However, the equipment cost is high, operation is complex, and efficiency is relatively low.
(2) Chlorinated hydrocarbon solvents
The main chlorinated hydrocarbon cleaning agents used are trichloroethylene, dichloromethane, and tetrachloroethylene. This type of solvent is widely used in the cleaning field, mainly due to the following advantages and characteristics:
Very low ODP value (Ozone Depletion Potential), meaning it hardly damages the ozone layer; non-flammable under normal conditions, posing no risk of fire or explosion (unlike dichloromethane, which may explode under prolonged intense sunlight); strong dissolving power for metalworking oils, greases, and other oily stains; also causes swelling or dissolution in plastics and rubber; low viscosity and surface tension, strong penetrating power, allowing it to penetrate narrow crevices and thoroughly dissolve and remove attached contaminants; low boiling point and low heat of vaporization, suitable for steam cleaning, and self-drying after cleaning; waste liquid can be separated by distillation and recycled; can use cleaning processes and equipment similar to or comparable to CFCs and TCAs, with simple operation, high efficiency, and low operating costs; the disadvantage is its high toxicity, generally limited to below 50 PPM in the air, and many occupational safety regulations explicitly restrict its use.