Surface Energy & Wetability
A “wetable” surface is one which is said to have “high surface energy,” liquid can be spread across the surface without the bunching of droplets. Low surface energy materials which do not “wet out” leave liquid droplets standing proud – imagine a freshly waxed car sprayed with water droplets – the drops bunch up at a steep angle to the paintwork.
Surface energy is normally measured in mJ/m² or Dynes. Generally to be able to bond a material it needs to have a surface energy of >36 mN/m (although this can still be pretty hard to bond to even with industrial adhesives).
Table showing surface energy values of common engineering materials. Figures in mJ/m².
As you can see, it is plastics that exhibit the lowest surface energy. This makes them challenging for both industrial adhesives and printing inks to adhere to.
Bond Plastics with Low Surface Energy
To successfully bond plastics with low surface energy a pre-treatment may be required. By modifying the surface of low surface energy plastics through chemical etching, flame, corona or plasma treatment the wettability can be drastically improved. Due to recent developments in adhesive technology giving rise to specialist primers and industrial adhesives which will bond plastics with low surface energy without expensive or hazardous pre-treatment, enabling manufacturers to save having to make large capital expenditure on equipment, space, training and set-up as well as reducing risk in the work place.
Permabond offers POP Polyolefin Primer, which can be brushed on and then the surface can be bonded with cyanoacrylate adhesive, this combination can similarly be used for bonding silicone rubber. Also available from Permabond are two new structural acrylic industrial adhesives, TA4610 and TA4620 which can be used on untreated “problem” plastics and provide high bond strength performance and good resistance to environmental conditions. For further information or technical advice, please contact Permabond’s helpline team.