Surface treatment method (below)

Second, mechanical processing method

Mechanical treatment is one of the commonly used treatment methods in industry. There are two main methods of mechanical grinding and mechanical blasting.
(I) Mechanical grinding method

Usually, the loose oxide layer on the bonding surface is first brushed with a wire and then ground with a grinding wheel, an emery cloth, a sandpaper, or a rough burr to obtain a certain roughness on the bonding surface.

The mechanical grinding method is suitable for most materials. Tables 3-5 show the comparison of the models and particle size numbers of domestic water sandpaper, wood sandpaper, and iron sand cloth, respectively, for reference when used. The mechanical grinding method is simple and easy, and the cost is very low. However, it is difficult to obtain the same repetitive results in actual operation, and the reproducibility and uniformity of the operation are poor. Therefore, it is usually used only when the demand is not high.


Table 3 domestic water sandpaper model and size number comparison model 180 220 240 280 320 400 500 600 Abrasive grain number Shanghai products 100 120 150 180 220 240 280 320 Tianjin products 120 150 160 180 220 260 - -

Table 4 Domestic wood sandpaper model number and size number comparison model 00 0 1 3/2 2 5/2 3 4 Abrasive size number Shanghai product 150 120 80 60 46 36 30 20 Tianjin product 160 140 100 80 60 46 36 30

Table 5 Domestic iron abrasive cloth model and size number comparison model 0000 000 00 0 1 2 3 4 Abrasive size number Shanghai product 220 180 150 120 100 60 36 24 Tianjin product 200 180 160 140 100 60 36 30

(b) Mechanical sandblasting Generally, mechanical sandblasting can be adopted in plastic joints that do not affect the assembly size and finish due to mechanical sandblasting, and the joints that do not require high abrasion resistance during use.

Before sandblasting, it is best to remove the oil with a solvent, so that the bonded surface is particularly clean after sandblasting. The mechanical sandblasting method is fast and good, the bonding strength is even and stable, and the operation is also very simple. Among them, the bonding effect of the copper alloy, aluminum alloy and steel is particularly best. However, mechanical sand blasting also has certain limitations. For example, thin film materials, soft porous materials, plated parts, precision parts, and large parts should not be used. In general, mechanical sandblasting is used for medium or slightly smaller cemented joints and rougher cemented joints. In addition, when dust is sprayed on dry sand, it will cause harm to the health of operators. Therefore, labor protection must be paid attention to.

Mechanical sand blasting can be divided into dry and wet methods.

The dry method is simply sandblasting with an abrasive. It is generally used for alloys that are difficult to treat with chemical treatments and where there are more scales and oxides on the surface. The finely powdered abrasive remaining on the surface after dry blasting is blown with clean compressed air or nitrogen.

Wet method is to use abrasive, liquid and air together. Its main advantage is that it can control and reduce the dust flying, and can use spray leaching to remove the residue on the sandblasted surface.
The mechanical blasting method can produce a round or uneven shape on the bonding surface, and its depth is related to the properties of the bonding material, the characteristics and size of the abrasive, and the blast pressure used. Among them, the characteristics and size of the abrasive are the most important factors. For example, sandblasting works best with sharp cutting abrasives. Such abrasives include corundum sand, corundum, quartz sand, and the like. The spherical abrasives, such as glass beads, metal bombs, etc., are less effective. Because the spherical abrasive will cause the surface to produce overlapping layers of metal particles, these loose metal particles adhere to the metal substrate, resulting in poor adhesion. Table 6 shows a comparison of these two abrasive bonding properties. Table 7 is a range of abrasive sizes for several metal grit blasting treatments.

For the sake of simplicity, all metal surfaces except aluminum can be sandblasted with a clean, 0.25-0.40mm steel grit or gravel grit blasted for approximately 0.5 minutes. For aluminum, sand or corundum abrasives can only be used for blasting. When sandblasting is used to treat non-metallic materials, softer non-metallic materials (such as certain thermoplastics) are not suitable to use coarse abrasives to increase their bonding strength because of unevenness and easy cohesion on the surface of soft materials. Destruction, in order to greatly reduce the adhesive strength.

Table 6 Comparison of Adhesion Effect of Glass Beads and Quartz Abrasives Adhesive Material Adhesive Abrasive Peel Strength (N/cm) Aluminum Alloy Aldehyde Phenolic Rubber Glass Beads 18-54 Silica Sand 108-162 Modified Epoxy Glass Beads 0- 27 Quartz sand 180-270

Table 7 Sandblasting Abrasive Size Range Adhesive Material Method Abrasive Size (No.) Steel Dry Method 80-100 Aluminum Wet Method 140-325 Brass Dry Method 80-100 Brass Wet Method 140-325 Stainless Steel Wet Method 140-325 III Chemical treatment

The chemical treatment method is to immerse the adhesive at room temperature or higher temperature in alkaline solution, acid solution or certain inorganic salt solution to remove loose oxides and other contaminants on its surface. For some of the more active metals, proper "passivation" treatment is also required after processing to obtain a firm and stable bonded surface.

The chemical treatment method has the characteristics of high efficiency, economy, stable quality, and large area usage. In addition to roughening and cleaning the surface, it can also produce a surface layer that is chemically resistant and improves the durability of the adhesive joint. For some easily oxidized metals and non-polar materials such as polytetrafluoroethylene, the chemical treatment method is especially the most convenient method from the standpoint of rapidity, reliability, and improved bonding strength.

Table 8 shows the comparison of the bonding strengths of aluminum alloys treated with different surface treatments. As can be seen from the table, the highest bonding strength can be obtained by chemical treatment.

Table 8 Relationship between surface treatment of aluminum alloy and bond strength Treatment method Shear strength (MPa ) Untreated 3.1 Degreasing treatment 5.9 Degreasing, blasting 9.3 Anodizing 13.6 Degreasing, sodium dichromate, sulfuric acid treatment 19.3 Degreasing, sandblasting Sodium dichromate, sulfuric acid treatment 21.7 (Note) Adhesives are two-component epoxy resins containing fillers that must be chemically treated according to different bonding materials. Different processing times and processing temperatures result in different bonding strengths. The disadvantage of the chemical treatment method is that most of the treatment liquids have a certain corrosive effect and may generate harmful gases. Therefore, attention must be paid to ventilation and labor protection during operation.

Fourth, primer method

The primer applied on the glued surface has the following effects:

1. The protective function keeps the adhesive surface clean, prevents corrosion or contamination, and prolongs the storage time of the treated surface.

2, to improve the adhesive properties such as elastic primer and epoxy resin, can improve the brittleness. The elastic primers used in epoxy adhesives include neoprene, nitrile rubber, and polysulfide rubber.

3, fixed film or workpiece positioning

4, to improve the surface adhesion Here mainly refers to the coating of a coupling agent. The coupling agent can form a chemical bond between the glue and the glued material, thereby greatly improving the adhesive strength and the resistance to the environment. However, the use should not be used too much, because not only the coupling agent is generally more expensive, but too much will cause the heat resistance of the entire adhesive layer to decrease.

Table 9 shows the effect of the coupling agent on the bonding strength. From the conditions listed in the table, the coupling agent can increase the bonding strength by a factor of two. At present, commonly used coupling agents are mainly various types of siloxanes, in addition to chromium complexes, phosphate esters, organic acids, and organic amines.

Table 9 Effect of coupling agent on bonding strength

Adhesive type Coupling agent type Strength increase compared to unused coupling Polyurethane KH-550 2.8-7.5 Epoxy resin KH-570 1.6 Phenolic-Butadiene KH-580 2.1-3.3

Source: 21st Century Fine Chemicals Network