Impurities removed from the workpiece surface are called pollutants. In different environments, the types of pollutants adhering to workpieces made of different materials are very different, and the situation is complicated. In some cases, the types of pollutants are still relatively fixed, and most of them only adhere to the surface of the workpiece, so targeted and economical means can be adopted to clean them. For example, in the production of metal workpieces, it is necessary to remove metal chips, cutting fluids, or certain grease, abrasive particles, dust, handwriting, etc. attached to the workpiece surface during processing.

In most cases, the types and shapes of workpieces are varied, and the adhered pollutants are solid and liquid, so it is naturally impossible for the cleaning process to be uniform. Therefore, according to the material of the workpiece, we should know the adhesion properties and types of pollutants, and then choose the cleaning method.

1. The nature of adhesion

Due to the different surface energy of workpiece materials and the different properties of pollutants, the degree of adhesion is different. Some adhesion is relatively firm, and some adhesion is less, which can be basically divided into the following three situations.

(1) Mechanical adhesion

Mechanical adhesion is a kind of adhesion caused by roughness of workpiece surface. No matter how smooth and bright the surface of the workpiece is, the surface is still very rough and uneven when viewed under the microscope, so pollutants will adhere to the depressions on the surface. This adhesion caused by surface roughness is called mechanical adhesion. Mechanical adhesion is relatively small and easy to remove. The mechanical adhesion of metal surface is shown in the following figure.

(2) Physical adhesion

Physical adhesion is a kind of adhesion caused by intermolecular interaction.

A positive charge center and a negative charge center can be found in any molecule. If the positive charge center does not coincide with the negative charge center, this molecule is called a polar molecule. Generally, the molecular polarity is measured by the dipole moment μ, μ=qd, and the schematic diagram of dipole moment is shown in the following figure.

For a simple diatomic molecule, if two atoms are the same, the molecule is nonpolar and the dipole moment of the molecule is zero, such as H2 and Cl2. If two atoms are not the same, the molecule is polar, such as HCI, CO molecules, etc. For polyatomic molecules with three atoms or more, whether the molecule has polarity depends on the spatial configuration of the molecule. "intermolecular forces" include both van der waals forces and hydrogen bonds.

a. Vander Waals forces

Vander Waals force is a ubiquitous intermolecular force, including dispersion force, induction force and orientation force (also called dipole force). 

Dispersion force: When two nonpolar molecules are close to each other, the movement of electrons and the vibration of nuclei will cause instantaneous relative displacement of the charges in the molecules, which will lead to the deviation between the center of positive charge and the center of negative charge, resulting in instantaneous dipole and deformation of the molecules. This instantaneous dipole will also induce adjacent molecules to generate instantaneous dipoles, and these two molecules are opposite to each other by this instantaneous dipole. This interaction force is called dispersion force, and the generation of dispersion force is shown in the following figure.

Induction force: When the non-polar molecules and the polar molecules are close to each other, the non-polar molecules are subjected to the action of the dipoles of the polar molecules, and the centers of the positive and negative charges that are originally coincident will also deviate, thus generating the induced dipoles. Mutual attraction occurs between the induced dipoles of non-polar molecules and the intrinsic dipoles of polar molecules. This interaction force between molecules is called induction force, and the generation of induction force is shown in the following figure.

Orientation force: When two polar molecules with intrinsic dipoles approach each other, the dipoles with electrostatic properties repel each other but the opposite ones attract each other, causing the molecules to rotate in an adjusted manner and producing a state of opposite poles. This intermolecular force generated by the orientation of the intrinsic dipoles is called the orientation force, also known as the dipole force. The generation of the orientation force is shown in the following figure.

Among polar molecules, orientation force, induction force and dispersion force play a role; Between polar molecules and nonpolar molecules, only the induction force and dispersion force; Between nonpolar and nonpolar molecules, only dispersion force is at work. For example, in water molecules, the orientation force, induction force and dispersion force accounted for 76.9%, 4.1% and 19.0%, respectively. In the hydrogen chloride molecule, the orientation force accounts for 14.4%, the induction force accounts for 4.2%, and the dispersion force accounts for 81.4%. In the ammonia molecule, the orientation force accounts for 45.0%, the induction force accounts for 5.3%, and the dispersion force accounts for 49.7%. In the hydrogen iodide molecule, the orientation force accounts for 0.1%, the induction force accounts for 0.4%, and the dispersion force accounts for 99.5%.

b. Hydrogen bond

When the H atom and the X atom are combined into X-H by covalent bond, the common electron pair is strongly biased to one side of the X atom, so that the H atom becomes a "naked" proton without an electron cloud, showing quite strong positive electricity. Moreover, the H atom has extremely small radius and extremely high potential energy, and is extremely easy to attract and infiltrate the electron cloud of the Y atom with lone pair in another molecule and very large electronegativity, thus forming a bond with the general formula X-H ... Y. This acting force is called hydrogen bond.

Hydrogen bond is a polar bond. Two conditions must be met to form hydrogen bond: One is that there must be H atom in the molecule and form covalent bond with element X (such as F, O and N) with great electronegativity; Second, there are atoms with large electronegativity and small radius with lone pair in the molecule.

A wide range of substances can form hydrogen bonds, for example, HF, H2O, NH3, inorganic oxo acids and organic carboxylic acids, alcohols, amines, protein and some synthetic high molecular compounds and other substances (or molecular chain) between the existence of hydrogen bonds. In the case of H2O, on the other hand, O atom has a stronger ability to attract electrons than H atom, and the common electron pair is biased towards O atom, making one end of O atom relatively negatively charged and one end of H atom relatively positively charged. Its two O-H bonds form an angle of about 104.5. Since O atom is at one end of the molecule, the charge distribution of the whole molecule is asymmetric. Therefore, H2O molecule is a polar molecule. The following figure shows the hydrogen bond diagram of water molecules.

In the molecules of organic substances such as aldehydes, ketones, acetaldehyde, and acetone, hydrogen and oxygen atoms are present, but carbon atoms having a small electronegativity are directly connected to the hydrogen atoms, so that hydrogen bonds cannot normally be formed between the molecules of these same compounds.

The bond energy of a hydrogen bond is much weaker than that of a chemical bond and is of the same order of magnitude as the intermolecular force. However, the existence of hydrogen bonds between molecules greatly enhances the molecular interaction. Liquid with hydrogen bond between molecules, general viscosity is bigger. For example, polyhydroxy compounds such as glycerol, phosphoric acid, and concentrated sulfuric acid can form numerous hydrogen bonds between molecules, and these substances are usually viscous liquids, thus increasing the adhesion strength.

If hydrogen bonds are formed between liquid molecules, association phenomena may occur. For example, liquid HF contains, under normal conditions, besides simple HF molecules, complex molecules (HF)n linked together by hydrogen bonds. Such as nHF(HF)n, where n can be 2,3, 4, .... This phenomenon is called molecular association, which is formed by combining several simple molecules into a complex molecule without changing the chemical properties of the original substance. The result of molecular association also increases the intensity of adhesion.

c. Other forces

In addition to the van der waals forces and hydrogen bonds described above, there are also electronic attractive forces, electrostatic attractive forces due to contact potential differences between the contaminant and the workpiece surface, and capillary forces due to the presence of liquid films. The energy of physical adhesion is much stronger than that of mechanical adhesion. The energy for physical adhesion is generally 20.92 kJ/mol for the even force, 2.092 kJ/mol for the induced force, 41.84 kJ/mol for the dispersion force, and 50.208 kJ/mol for the hydrogen bond. The calculation shows that when the plane distance between two ideal molecules is 1 mm, the attractive force between molecules can reach 10 ~ 100 MPa; If the intermolecular distance is 0.3~0.4 nm, the attractive force can reach 100~1000 MPa.

(3) Chemical Adhesion

Due to the interaction of residual electronic valence between pollutants and workpiece surface, similar to the formation of chemical bonds, this adhesion is called chemical adhesion. The energy of chemical adhesion is relatively large, and the adhesion is relatively firm. The energy of chemical bonds formed between the contaminants and the workpiece surface was 585.76–1046 K/mol for ionic bonds, 62.76–711.28 Ki/mol for covalent bonds and 112.97–347.27 KJ/mol for metallic bonds.

2. Types of pollutants

The contaminants are various and the adhesion to the surface of the workpiece is also diverse and can be classified into the following four categories according to their characteristics.

(1) Particulate Polllution

The particles have a certain shape and volume, and the particle size distribution is very wide. Each solid substance may become dust and float in the sky with the wind and adhere to the surface of the workpiece. Common metal filings, plastic powder, glass powder, dust, fiber, hair, skin filings, grinding powder, polishing powder, etc., these contaminants come from the processing procedures and the surrounding environment. Some particles are free-fall deposition on the surface of the workpiece, adhesion is not very strong; Some are deposited on the surface of the workpiece due to liquid evaporation, adhesion is very strong; Some of them are ground or polished with great mechanical force on the surface of the workpiece, adhesion is very strong. The dust floating in the air often has static electricity on the surface of the particles due to friction, and the static adhesion is very strong and difficult to remove. The adhesion mode and adhesion strength of particle contamination on the surface of workpiece are shown in the following figure.

The smaller particles, the larger the specific surface area, the larger the surface energy, the more solid adhesion, the more difficult to remove. In general, the size of dust in the atmosphere is 0.01–20 μ m, and the thickness of human hair is about 70 μ m. Workpiece surface easy adhesion of solid particles, this is because the solid particles have strong adhesion ability, such as side length is 1 cm cube, when dispersed into colloidal particles, its total surface area of 600m2, namely increased by 1 million times. An increase in surface area means an increase in surface energy.

(2) Organic pollution

Organic contaminants are compounds of carbon whose atoms can be linked by chain, ring, branching, and cross-linking by covalent bonds. The covalent nature of organic matter, make this material has a melting point, low boiling point, soluble in organic solvents and insoluble in water, flammable, solution is not conductive, usually exist isomers, etc. Common organic pollutants are antirust oil, grease, wax, oil, asphalt, paint film, paint, adhesive, rosin flux, etc. The adhesion of organic pollutants to the surface of the workpiece is sometimes semi-solid, and the adhesion to the surface of the workpiece is very strong.

(3) Inorganic pollution

Inorganic pollution mainly refers to the corrosion of metal pollution, generally form ion reaction, reactant high melting point and boiling point, can be dissolved in water and insoluble in organic solvents, the solution conductive and not easy to burn. For example, the surface of the active metal and acidic solution or alkaline solution oxidation reduction reaction, the presence of salts in the reactants, belong to the ionic substance. Ionic substances encounter some tiny water trace will immediately form an electrolytic cell, produce micro current, the micro current will make the device short circuit, cause corrosion. Even in the absence of moisture, as an electrolyte, the contaminated charge on the ion surface may cause a redistribution of charge within the device. Inorganic pollution is the main cause of metal workpiece corrosion, such as rust, copper rust, etc.

(4) Microbial pollution

Microorganisms are living organisms that can be viruses, rickettsia, fungi, protozoa, algae. They can proliferate and form organic deposits and particulate contamination that can have a significant impact on product quality. The average size of bacteria is 0.3–0.5 μ m, that of virus is 0.003–0.05 μ m, that of rickettsia is 0.2–0.5 μ m and that of protozoa is 10–100 μ m. The adhesion of pollutants on the surface of actual workpiece is relatively complex, with both particle adhesion and liquid adhesion. Both organic adhesion and inorganic adhesion; Both polar pollution and nonpolar pollution; Both physical and chemical adhesion. It must therefore be carefully analysed.