Careful surface pretreatment is a prerequisite of high-performance adhesive bonding. This is especially true when it comes to achieving good bonds that have to stand elevated temperatures and possible media influences.
As a rule of thumb, bonding surfaces must be free from dust, stain, fat, and moisture. Metals, glass and ceramics should therefore be degreased with good fat dissolving solvents (e.g. acetone, ethyl acetate). If possible, degreasing should be performed in an ultrasonic bath which is more effective that simple wiping or rinsing.
Much higher bond strength can be achieved by additional mechanical roughening of the bond surfaces e.g. with sandpaper (grit size 400) or by sand blasting. After the roughening, degreasing should be performed to remove any newly created dust and particles.
In most cases however, chemical etching processes are applied to get the highest bond strength possible.
For Aluminium the highest bond strengths are obtained (on AlCuMg1 alloy) if pretreated by the Pickling etching process as described in DIN 53281 part 1. Additional improvement of bond strength can be achieved by application of a Polymerics primer onto the pretreated surface to which 5% of water are added 1 hour before application. It is important that the primer type be suitable with the type of adhesive used.
Etching with oxalic acid / sulphuric acid:
| oxalic acid | 10 parts |
| concentrated sulphuric acid | 10 parts |
| dest. water | 80 parts |
Dissolve the oxalic acid in the water and then slowly add the sulphuric acid while stirring the solution (Caution: corrosive!). Heat the solution to 90–95 °C.
Dip the substrates into the solution and observe. When a black film begins to form on the surface leave the substrate for 8 additional minutes in the etch solution.
Note: other types of stainless steel than 1.4301 may not show the black film forming – in this case treat the substrates for max. 30 min.
After etching, remove the substrates from the bath and use a brush to rubb off the black film under fresh water. Then rinse with dest. water and dry the substrates at 100 to 120 °C. Application of the adhesive should take place directly after the pretreatment.
Roughen the bond surfaces with sandpaper (grit size 400), by sandblasting or with metal wool. Degrease with chloroform. Then dip the substrates for 1 minute into the following etch bath (Caution: very corrosive!).
| HF (48–50%) | 15 parts |
| dest. water | 85 parts |
After etching, wash thoroughly with water, rinse with dest. water and dry at 100 to 120 °C.
Careful degreasing of glass surfaces by wiping with ethyl acetate has shown to have very good effects on bond strength. Degreasing with ethyl acetate in an ultrasonic bath is also recommended.
Alternatively, glass substrates can be cleaned at 40 to 50 °C in an ultrasonic bath in conventional water with dishwashing cleaner. Then wash with clear water and rinse with dest. water.
Moisture from the air always forms a thin film on glass substrates which can prevent surable bonding. Therefore removal of this film can lead to better bonds. To remove the water film dry the substrates for 20 min at 100 °C. Application of the adhesive should be performed as quickly as possible in order to prevent the recreation of the water film. Work in a room with low relative humidity is recommended. If this is not possible, a primer can be applied to glass surfaces.
Polyester should be roughened with sandpaper (grit size 400), then freed from created dust (e.g. by rinsing with methanol in an ultrasonic bath), dried and finally impregnated with a primer.
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