Meaning
Thermodynamic and mechanical interactions govern the formation of a weld between two thermoplastic surfaces during the application of heat and pressure. Heat sealing kinetics describes the rate and mechanism of molecular entanglement across an interface when thermal energy is applied. This process involves the melting of surface layers, the diffusion of polymer chains, and the subsequent recrystallization upon cooling.
Measurement typically involves plotting seal strength against dwell time or temperature.
Diffusion Mechanism
The movement of long-chain molecules across the contact plane happens once the material exceeds its melting point or glass transition temperature. Chain mobility increases as heat penetrates the film layers, allowing the boundaries between the two surfaces to disappear. Pressure forces these chains into closer proximity to accelerate the entanglement.
A shorter dwell time requires a higher temperature to achieve the same degree of molecular bonding.
Rapid Cooling
This phase of the cycle locks the diffused chains into a new crystalline or amorphous structure that resists mechanical separation. If the heat sealing kinetics are too slow for the line speed, the seal fails during the filling process because it lacks sufficient hot tack. Overheating causes the polymer to degrade or thin out at the seal edge.
This narrowing creates a weak point known as the squeeze-out effect.
Material Response
Molecular weight distribution and the presence of additives influence how quickly a bond matures under pressure.
