I. Acidity
Cleaning agents currently used in the market, such as those containing fluorocarbons, chlorinated hydrocarbons, bromine hydrocarbons, hydrocarbons, and alcohols, must have their acidity strictly controlled during production. Furthermore, to prevent changes during storage and use, various stabilizers and antioxidants must be added at the factory to stabilize them and prevent decomposition during use and heating. This is because the acidic substances released from the decomposition of these substances, such as HF, HCl, and HBr, become acidic when they come into contact with trace amounts of water. Increased acidity not only affects the cleaned workpiece, causing the metal surface to lose its luster and damaging components, but also, under the long-term action of acidic media, stainless steel equipment will corrode. Therefore, stabilizers must be added to the above-mentioned cleaning agents. If the stabilizer is not chosen well or the amount added is insufficient, the acidity of the cleaning agent will increase or oxidative decomposition will occur during repeated heating and contact with the atmosphere, deteriorating the quality of the cleaning agent and leading to corrosion of the workpiece.
The stabilizers or antioxidants typically added at the factory include the following types: phenols, alcohols, and epoxy compounds, with the addition amount generally within 0.05% to 5%. The acidity at the time of manufacture is controlled below 10 ppm (0.001%).
Acidity testing is conducted according to GB4120.3, equivalent to the international standard ISO-1393, using the acid-base titration method. Since many cleaning agents are insoluble in water, they cannot be titrated with alkaline solutions; instead, a water washing method is used to titrate the water solution washed off the solvent-based cleaning agent. This method is relatively cumbersome. Alternatively, anhydrous ethanol alkaline solution can be used for direct titration, as many cleaning agents are soluble or partially soluble in ethanol. This method is simple, reliable, and convenient for workshop testing.
II. Moisture
The water content of solvent-based cleaning agents is also a very important indicator. In principle, the less water, the better. However, large-scale production and use cannot achieve complete anhydrousness, and each solvent has a certain water content. Some solvents are completely miscible with water, such as lower alcohols, ketones, and some ethers; others have a certain saturation solubility at a specific temperature, such as hydrocarbons, fluorinated hydrocarbons, chlorinated hydrocarbons, and bromine hydrocarbons. When water exceeds this saturation solubility, stratification occurs, drawing stabilizers from the solvent into the aqueous layer, thus deteriorating the stability of the solvent (cleaning agent).
In the presence of water, free Fˉ, Clˉ, and Brˉ ions in the cleaning agent react with water to form acids, creating a vicious cycle during heating, leading to increasingly higher acidity. Therefore, acidity and moisture content in cleaning agents are two interdependent indicators that directly affect the quality of cleaned workpieces. For example, if a workpiece loses its luster or develops watermarks after cleaning, this is related to acidity and moisture content. Currently, solvent-based cleaning agents produced domestically and internationally have moisture content controlled below 100 ppm (0.01%).
Trace moisture determination is performed according to GB6283-86, i.e., the Karl Fischer method. Alternatively, the simpler cloud point method, i.e., according to GB4120, can be used. Gas chromatography can also be used for analysis.
III. Toxicity
Toxicity is a crucial indicator of whether a cleaning agent can be used. While it doesn't directly affect the workpiece being cleaned, it has profound implications for humans and the environment, making it a primary factor in our decision-making. The toxicity of a cleaning agent is determined based on animal and human physiological experiments. Strictly speaking, all chemical solvents are toxic, differing only in degree. The degree of toxicity, the safe range for humans, and necessary precautions can only be determined through rigorous animal testing—specifically, the tolerable dosage per kilogram of animal body weight. Although many chemicals have undergone toxicity testing and are documented, modern cleaning agents are not pure products, and data in existing materials can only serve as a reference. Mixtures require actual animal testing to determine safe concentration ranges, establishing safety precautions and protocols for both manufacturers and users.
Toxicological testing primarily involves oral and inhalation experiments. In acute oral toxicity tests on rats, the higher the LD50 (median lethal dose), the lower the toxicity. Similarly, in acute inhalation LD50 (median lethal dose) on rats, the higher the permissible concentration in the workpiece environment. Based on these measured data, safety standards and protective measures for the workpiece environment using this cleaning agent can be formulated. Toxicological experiments should be conducted by the health and epidemic prevention department. The testing and pricing should be carried out according to GB1560-1995 or GB5044-85.
IV. Non-volatile substances (residues)
The amount of non-volatile substances in the cleaning agent directly affects the cleanliness of the cleaned workpiece. Since the workpiece is immersed in the cleaning solution and then allowed to air dry, the non-volatile substances in the cleaning solution may directly adhere to the workpiece. For electronic components, this will affect their electrical performance, especially for microcircuit devices such as PCB boards. The number of ions adhering to the board is measured in μg/cm² (NaCl μg/cm²), and the lower the better; generally, it is controlled below 10 ppm at the factory.
This is based on the enterprise standards for single-solvent-based cleaning agents produced domestically and internationally. The testing method is carried out according to GB6324.2-86, similar to the method in ISO759-1981. A simple testing method is to drop a few drops of cleaning agent onto a glass lens, allow it to evaporate naturally, and observe the residue on the lens. This will give a rough estimate of the amount of non-volatile substances in the cleaning agent. Of course, this is only a qualitative test.
The above is a discussion of several common and important indicators for solvent-based cleaning agents. Of course, cleaning agents also have many other technical indicators, such as insulation, flammability, volatility, KB value, and swelling effect on materials, which vary depending on the object being cleaned.