(A) The effect of surface characteristics
1. Cleanliness: The metal surface is adsorbed with a layer of low surface energy, low pollution and oxide film, which will reduce the bonding strength. In general, when the metal surface is clean, the contact is small or even zero. After the material is treated, the contact changes with time, and it is easily polluted by the ambient atmosphere in the air. The contact angle increases, and the contact angle tends to be stable after about 5H.
Contact angle and bonding strength before and after surface treatment
Metal treatment method Contact angle (degrees) Shear strength Mpa Aluminum untreated degreased H2SO4/Na2Cr2O7 H2SO4/Na2Cr2O7+ high temperature baking 67 67 0 78 17.2 19.3 36.3 25.5 Stainless steel untreated degreased H2SO4/Na2Cr2O750-75 67 10 36.6 44.3 49.7 Ti untreated degreased H2SO4/Na2Cr2O750-75 61-71 10 9.5 22.4 43.22. Roughness The surface of the adhered material is polished or grit-blasted with gauze. Appropriately roughening the surface can increase the bond strength. The roughening process removes the pre-existing surface layer. Form a new surface layer (purification), and the sandblasted surface has a larger actual bonding area than the polished surface, the surface is too rough but will reduce the bonding strength, because too rough surface can not be good infiltration of the adhesive, the recess is easy Residual surface air is not good for bonding.
Adhesive Weight Ratio Curing Conditions (oC/H) Upa Sandblasting Polishing Epoxy-Polyamide 70/30 74/16 58.3 27.8 60/40 74/16 57.7 34.6 50/50/ 74/16 59.0 38.2 Epoxy-DTA 100/ 10 60/96 85.0 61.9 3. Surface chemical structure The chemical composition and structure of the surface have an important influence on the bonding properties, durability, and heat aging properties of the adhered material. The influence of the surface structure on the bonding performance is often achieved by changing the cohesion of the surface layer. Intensity, thickness, porosity, activity and surface energy are achieved.
After the aluminum surface is etched with chromic acid, the aluminum sample is soaked in distilled water at 60oC. It is easy to generate Al2O3.3H2O-based alumina hydrate on the surface. The cohesive strength is low and the thickness is >103A. The joints were damaged, the strength was greatly reduced, and single-crystal or powder-like patterns and rainbow-color interferences were observed on the surfaces of the aluminum test pieces. When cleaning in hot tap water, the filling efficiency is very low, and the hydrate formation is less, the iris color or single crystal spectrum is not visible on the surface, and the bonding strength is high. Therefore, to obtain high peel strength, the aluminum surface oxide should be anhydrous.
(b) Surface treatment methods
1, solvent alkali and ultrasonic degreasing method:
Solvent degreasing method: Commonly used scrubbing method is to directly scrub the surface of the adhered material with oil-free cotton (absorbent cotton) and solvent such as acetone, gasoline, toluene and the like.
Alkaline degreasing: The oily surface is treated with a hot caustic 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 wettability is not very good, and it is not easy to rinse clean. Its effect is not as good as that of alkaline silicate; alkaline silicate is mixed with SiO2 and Na2O. When the ratio is 1:2, the cleaning effect is the best; when the ratio is 1:1, it is suitable for the cleaning of aluminum materials.
Ultrasonic degreasing method: General parts are treated with ultrasonic waves with a power of 20 W/cm2 for 20-60 seconds to obtain a sufficiently clean surface.
2, machining: grinding and sandblasting
The sanding operation is simple, but the uniformity is poor and it is difficult to obtain the same repetitive effect.
Sandblasting removes surface dirt quickly and easily and produces surfaces with different roughness.
3, chemical corrosion method:
After the above-mentioned treatment, the adhered material must be placed in an acid or alkaline solution for chemical etching. This can further remove the residual dirt on the surface and make the surface produce an activated or passivating oxide layer with good cohesive strength (strictly controlling the purity of the water).
4, coating primer method:
On the treated surface, a thin primer is applied first to improve the bonding performance.
5, plasma method
The bonding strength of hard-bonded materials such as PE, polypropylene, and polytetrafluoroethylene is greatly improved by the plasma method. This is because the contact angle decreases after surface treatment, and the ultimate surface tension increases, thereby improving the adhesive's penetration and adhesion. force.
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