Restoring force
Force that drives an elastic system back to its rest position after a deflection.
The restoring force when bonding with hot-melt adhesive is influenced by several factors. Restoring force refers to the adhesive’s ability to restore to its original shape or elasticity after application and hardening.
The chemical composition of the hot-melt adhesive is crucial for its restoring force. Some adhesives have a natural elasticity and can return to their original shape after loading, while others are more brittle and have less restoring force.
Example: Cardboard box closure
The restoring force describes the urge of the lid flap to return to its original position. .
If the paper material has additional strong restoring forces on the lid flap, these must be taken into account when selecting the adhesive.
Substrate material: The material to which the hot-melt adhesive is applied plays an important role. Different materials have different elasticities and adhesion properties, which can affect the restoring force of the adhesive.
The restoring force is material-specific and depends on the type of creasing. The material is intentionally weakened at the crease, in order to reduce bending resistance at the crease. If the fibre composite at the crease is destroyed by pre-breaking the paper material, the forces acting are also lower. If the box is overfilled during packaging, the restoring forces on the lid flaps are increased. The resulting tension places high demands on the bonded seam. The selection of the hot-melt adhesive must therefore be adapted to the entire processing process.
Temperature: Hot-melt adhesives are applied at elevated temperatures and then harden when they cool. The temperature between application and curing can influence the restoring force. Higher temperatures can cause the adhesive to become more flexible and elastic.
Adhesives differ in their behaviour during the phase transition from liquid to solid. If the period of time for this phase transition is too long, the cardboard will spring open again: the cohesion of the adhesion is not sufficient during the bonding process to absorb the tension of the restoring forces.
Loading: The way in which the bonded component is stressed after bonding can reduce the restoring force. Repeated loading or deformation of the component can reduce the restoring force over time.
The storage of the packaging material and the transport routes of the packages also affect the adhesive properties. What if the box is, for example, stacked and the adhesive seam is loaded with weight?
The cardboard absorbs the moisture from the air and usually quickly reaches the ambient temperature (important, for example, for frozen packaging or warm filling). The diffused moisture affects the stability and adhesion of the box from both the inside and the outside.
Changes in the quality of the paper due to environmental influences, temperature fluctuations and transport movement also influence the selection of the adhesive.
The precise restoring force of a hot-melt adhesive depends on a variety of factors, including its composition, its processing, the environment and the properties of the substrate. It is important to consider these factors to ensure that the adhesive bond can withstand the required demands.