Surface treatment of the adhered material and bonding
(I) Influence of surface properties
1. Cleanliness: The metal surface is covered with a layer of low cohesion and low surface energy contamination layer and oxide film, which will reduce the bonding strength. Generally, when the metal surface is clean, the contact is very small or even zero.
The contact changes with time after material treatment and is easily contaminated by the environmental atmosphere in the air. The contact angle increases and tends to stabilize after about 5 hours.
2. Roughness: The adhered material surface can be roughened by sanding with a cloth or sandblasting. Appropriate roughening of the surface can increase the bonding strength. The roughening process removes the original surface layer.
A new surface layer (purification) is formed, and the actual bonding area of the sandblasted surface is larger than that of the polished surface. If the surface is too rough, it will reduce the bonding strength because the overly rough surface cannot well infiltrate the adhesive, and the concave areas are prone to residual air, which is unfavorable for bonding.
3. Surface chemical structure: The chemical composition and structure of the surface have significant influences on the bonding performance, durability, and thermal aging performance of the adhered material. The influence of the surface structure on the bonding performance is often achieved by changing the cohesion strength, thickness, porosity, activity, and surface energy of the surface layer.
After acid etching the aluminum surface with chromic acid, the aluminum sample is immersed in 60°C distilled water for cleaning.
It is easy to form an aluminum oxide hydrate mainly composed of AL2O3.3H2O on the surface, with a low cohesion strength and a thickness > 103A, which causes the joint to be damaged, resulting in a significant reduction in strength, and rainbow interference phenomena can be observed on the aluminum sample surface.
When washed in hot tap water, the filling efficiency is very low, and the hydrate is formed less, and the surface does not show rainbow or single crystal patterns, and the bonding strength is higher. Therefore, to obtain high peel strength, the oxide on the aluminum surface should be anhydrous.
(II) Surface Treatment Methods
1. Solvent-based and ultrasonic degreasing methods:
Solvent degreasing method: The common wiping method involves using oil-free cotton (degreasing cotton) to directly wipe the surface of the adhered material with solvents such as acetone, gasoline, and toluene.
Alkali-based degreasing method: The oily surface is treated with hot alkaline solution, such as 30-55% sodium phosphate (Na4P2O7), 10-50% NaOH, 10-6% Na2CO3, or 30-85% alkaline silicate.
Although NaOH is a strong base, its wetting property is not very good and it is not easy to rinse off completely, so its effect is not as good as alkaline silicate; the alkaline silicate consists of SiO2 and Na2O in a 1:2 ratio, providing the best cleaning effect; when the ratio is 1:1, it is suitable for cleaning aluminum materials.
Ultrasonic degreasing method: The components are placed in an ultrasonic device with a power of 20 watts per square centimeter for 20-60 seconds to obtain a sufficiently clean surface.
2. Mechanical processing: Grinding and sandblasting
Sandpaper grinding is simple to operate but has poor uniformity and is difficult to achieve the same repetitive effect.
Sandblasting method can quickly and simply remove surface contaminants and produce surfaces of different roughness.
3. Chemical corrosion method:
After the adhered material has undergone the above treatments, it must be placed in an acid or alkali solution for chemical corrosion treatment. This can further remove residual contaminants on the surface and generate an activated or passivated oxide layer with good cohesive strength on the surface (strictly control the purity of water).
4. Primer coating method:
On the treated surface, a very thin primer coat is applied to improve the adhesive performance.
5. Plasma method
The bonding strength of PE, polypropylene, and polytetrafluoroethylene and other difficult-to-bond materials is greatly improved by treating them with the plasma method. This is because the contact angle is reduced after surface treatment, and the limit surface tension increases, thereby improving the infiltration and adhesion of the adhesive to the material.
(III) Surface treatment and bonding of various materials
1. Metal materials
1.1. Aluminum alloy: Follow the process of degreasing – alkali treatment – chromic acid etching treatment.
- The solvent degreasing method can only remove the dirt on the surface of aluminum and make the natural oxidation layer on the surface become clean;
- The alkali treatment method removes oil stains and dissolves the natural oxide film on the surface in the alkali solution, forming a new oxide film;
- The acid etching treatment can dissolve the natural oxide film on the surface and regenerate a dense and hard oxide film with a thickness of 1-3 um, high internal cohesion strength and strong adhesion, thus having good bonding performance.
The surface after acid etching treatment is often washed clean in tap water and appears a uniform water film. Wait for cleaning and dry at below 70°C. 2Al(OH)3+3H2SO4 → Al2(SO) 4+6H2O
Na2CrO7+ H2SO4 → Na2SO4+2CrO3+ H2O
2Al+2CrO3 → Al2O3+ Cr2O3
American standard treatment formula for H2SO4 – sodium dichromate solution:
Na2CrO7 1 (2.4%) (66-71°C) / 10-12 parts / concentrated H2SO4 10 (22.4%) (66-71°C) / 10-12 parts / distilled water 30 (73.2%) (66-71°C) / 10-12 parts
British standard treatment formula: Cr2O3 5% or 0 (60-65°C) / 30 parts Na2CrO7 0 or 7.5% (60-65°C) / 30 parts concentrated H2SO4 127.3% or 27.3% (60-65°C) / 30 parts / distilled water 67.7% or 65.5% (60-65°C) / 30 parts
1.2. Carbon steel: It is prone to oxidation in humid air and forms Fe2O3.n H2O.
(1) Solvent degreasing -Soaking in a solution of 1 part concentrated HCl and 1 part H2O at room temperature for 5-10 minutes – Polishing and drying – Drying at 93°C for 10 minutes.
(2) Solution degreasing – Soaking in a solution of 4 parts Na2CrO7, 10 parts concentrated H2SO4 and 30 parts distilled water at 71-77°C for 10 minutes – The rest is the same as (1).
(3) Solution degreasing – Soaking in a solution of 30 parts sodium sulfate， 3 parts laundry detergent and 96.7 parts water at 60-65°C for 5-10 minutes – Polishing and drying at 100-105°C.
1.3, Stainless Steel
(1) After degreasing, immerse in a 100 parts concentrated H2SO4 and 3 parts Na2CrO7 solution at 60-65°C for 15 minutes, then wash and dry.
(2) After degreasing, immerse in a 100 parts concentrated HCl, 4 parts H2O2 (30%), 20 parts formaldehyde (40%), and 90 parts H2O at 65°C for 10 minutes. Wash and then place in a concentrated H2SO4 (100 parts), Na2CrO7 (10 parts), and H2O (30 parts) at 65°C for 10 minutes, then wash and dry.
(3) After degreasing, immerse in a concentrated HCl (2 parts), hexamethylenetetramine (5 parts), H2O2 (1 part), and H2O (20 parts) at 65-70°C for 10 minutes, then wash and dry.
1.4. Titanium Alloy:
(1) After degreasing, it is treated at room temperature for 1-2 hours in 2.7g of 50% H2O2, 9.6g of NaOH and 1000ml of H2O. Then it is washed, dried and air-dried.
(2) After degreasing, it is immersed at room temperature for 2 minutes in 841ml of concentrated HCL, 63ml of concentrated HF acid and 89ml of H3PO4. Then it is washed, dried (for 10-15 minutes at 82-93°C). Adhesives such as epoxy, phenolic, polyimide, and benzimidazole-based polybenzene can be selected for high-temperature bonding.
1.5. Magnesium Alloy: Magnesium is a very lightweight metal with high surface activity, which easily forms an oxide film.
Magnesium is resistant to alkalis, but it reacts easily with methyl esters. Therefore, degreasing solvents can only be acetone, trichloroethylene, etc.
(1) After degreasing, immerse in NaOH (80%) at 60-80°C for 5-10 minutes, wash, and place in 1.8 parts Ca(NO3)2, 24 parts chromic acid, and 123 parts water at room temperature for 5-10 minutes, wash and dry.
(2) After degreasing, immerse in Cr2O3 (17 parts), NaNO3 (20 parts), 105 parts acetic acid, and 100 parts water at room temperature for 3 minutes, wash and dry. Use EPOXY – polyamide or EPOXY – rubber type adhesives.
1.6. Copper: Copper is prone to corrosion in humid environments, especially when epoxy glue is cured with ethylenediamine or DTA, the corrosion effect on copper is even more significant.
(1) After degreasing, immerse in the mixed solution at room temperature for 3 minutes, wash and dry, and use immediately (the mixed solution is concentrated HCL: FeCL3: H2O = 50:20:30).
(2) After degreasing, immerse in solution HNO3 for about 15 minutes, wipe until all corrosion areas disappear completely, dry and use immediately. Choose EPOXY, polyamide, organic silicon, phenolic, chloroprene rubber and nitrile rubber, etc.
1.7. Zinc
(1) After degreasing, immerse in HCL (6%) at room temperature for 4 minutes, wash and dry immediately.
(2) After degreasing, immerse in the mixed solution at 38°C for 5 minutes, wash and dry. (The mixed solution is Na2CrO7: concentrated H2SO4 : H2O = 1:2:8), use epoxy, polyurethane, organic silicon, phenolic, etc. adhesives.
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