Joining of Plastics by Adhesive Bonding - What Needs to be Considered

We live in a world where plastics are omnipresent and can be found in almost every product of everyday life. There are many reasons for this - for example, their simple processing and low density. They are also inexpensive and often require less energy than alternative materials in their manufacturing and processing. In a large number of cases, however, a product does not consist of just one (plastic) part but is composed of components made from different materials.

In order to form the ready part, components need to be joined together. Modern bonding technology is the predestined process for joining different materials. But, for every joining technique, the necessary technical characteristics must be fulfilled and at the same time high productivity, quality and reliability must be ensured.

Therefore, it is important to have an in-depth understanding of the interaction between the plastic parts to be joined and the adhesive required to form a reliable bond.

First, let's start by understanding the parameters to be taken into account while designing a bonding process.

What Influences the Bondability in Plastics?

A large number of plastics available on the market which are in use for various applications in different industries do not allow any general statement regarding the bondability of plastics. Undoubtedly, bondability depends on the type of polymer, but there are other factors that must be taken into account as well.

The formation of adhesive forces is a question of surface properties of the part which needs to be bonded. The adhesive forces, which ultimately determine whether an adhesive provides sufficient adhesion to a plastic surface, show only a very short range of less than 1 nm. These can be polar or polarizable molecule groups that are capable of forming hydrogen bonds or allowing for the so-called Van der Waals forces.



Having the short range of such forces in mind; it is obvious that adhesive and part surface has to come in close contact to allow interaction.

• A prerequisite is a good wetting of the surface by the adhesive, which requires that the surface tension of the part surface needs to be higher than the one of the adhesives in a liquid state.
• In addition, it is required that the functional groups on the part surface can interact with those present in the adhesive - surface and adhesive have to match each other.

Polymer Additives

As already mentioned, the type of base polymer plays an important role. But, plastics as synthetically produced materials are typically formulated for different reasons with a wide variety of additives which can be:

• Pigments
• Fillers or fibers
• Internal release agents
• Plasticizers
• Antistatic agents
• Antioxidants
• UV stabilizers, and
• Flame retardants or processing aids like:
  
  • Defoamers
  • Rheology control agents, and
  • So forth

These additives play an important role in regard to surface tension (wettability) and formation of adhesion - especially when they are present on the surface.

Continue reading to further explore factors that affect bondability and learn about the key best practices used to achieve a reliable adhesion in plastics.