The structure size of micro-structured plastics as fluidic chips is getting smaller and smaller. The preparation of sandwich structures and covered structures asks for a joining technology which does not affect the structuring. With structures in the range of some micrometers it becomes rather difficult to use adhesives without changing the volume and the shape of channels and cavities. Also the thermal bonding with lasers approaches its limits at these sizes.
Adhesion
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Adhesive interactions |
Adhesive interactions are based on a number of different effects. Mechanical
interlocking is not important on very flat joints. The interface mixing differs in
its importance depending on the materials to be joint. And there are chemical
Interactions (figure): Acid-base interactions (=electron donor-acceptor interactions)
are rather weak but they can be numerous. Chemical bonds are the strongest
interactions but they are also quite demanding. The right partners must meet with a
precise spatial orientation.
Bonding plastics
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Scheme of the adhesive-free bonding process |
Considering the chemical interactions, it seems to be feasible to join two flat
surfaces if they are brought into molecular contact and if there are appropriate
chemical structures on the interface.
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PMMA sheets bonded without adhesive and charged with 100g |
However, there are very few interactions between the surfaces of usual polymers
(upper part of the picture). A surface treatment creates interaction sites (red line
in the picture). If they are brought into molecular contact interactions take place
which can be recorded as an adhesion force (lower picture).
Theoretically there is no limitation for the materials to be joint as long as the
surface chemistry is prepared in an appropriate way.