Plasticisers: A challenge for bonding
What are plasticizers?
Plasticizers are chemical substances that are added to some plastics to make them more flexible and less brittle. These substances are incorporated into the plastic to increase the elasticity and flexibility of the material. Plasticizers can also be referred to as softening agents.
Where are plasticizers found?
In the production of hot-melt adhesives, raw materials such as base polymers, resins, plasticizers and additives are mixed together at high temperatures. The plasticizers, which typically include medical or technical white oil and polyisobutylene (PIB), are incorporated into the base polymers to achieve the desired flexibility of the final product.
Plasticizers are of course contained in substrates to be bonded. Although plasticizers have generally useful properties, they can be problematic when it comes to bonding. There are two main reasons for this:
Migration: Plasticizers can “migrate” from plastics/substrates and penetrate into adjacent materials/adhesives. When an adhesive is applied to a surface, the plasticizer from the plastic can diffuse into the adhesive. This causes the adhesive to change its properties and become softer. It loses its cohesion (internal strength).
Surface contamination: The plasticizer infiltrates the adhesive joint. This is where the adhesive loses its adhesion to the substrate. Sweating then occurs due to the resulting boundary layer of the plasticizer. If plasticizers from plastics get onto the surfaces to be bonded, they can affect the adhesion of the adhesive to these surfaces by forming a greasy or oily layer that prevents adhesion.
In order to protect bonds from the negative effects of plasticizers, the selection of suitable adhesives is crucial. Reactive hot-melt adhesives or selected polyamides, for example, can be used as they are more compatible with plasticizers. This is particularly important in branches such as the automotive industry and medical technology, where plasticizers are widely used.
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