The thermoshrinkable films shrink under the effect of heat because during their implementation stresses imposed on the film are abruptly frozen. These are oriented polymer films that, when reheated, release the accumulated stresses.

The phenomenon takes place in three stages :

• the film enters the oven: thermal expansion during heating ;
• the film reaches T_softening: the internal tensions are released, the film shrinks;
• the film comes out of the oven and cools.

Today, ecological and economic considerations push the formulators to develop thermoshrinkable films of lower thickness or made of recycled/biobased/biodegradable materials.

INTRODUCTION

Heat-shrinkable films have been used for decades in many industries : the paper industry to wrap some books to protect or prevent their reading, the food industry to cover certain foods (cheese, meat, vegetables), the setting under film of palletized freight, the boats protection...

PE-based materials are mainly used and are highly bi-oriented during the process. Other plastics generally used are PVC, PS, PP, PA6 and polyesters (eg PET). In some cases, several polymers are associated with each other. The polymers used in packaging must fulfill three functions: presentation, preservation and processability.

The presentation seduces the potential buyer with graphics and colors, and quickly synthesizes the segment in which the customer usually buys.

The conservation ranges from a few days to 24 months, with or without cold chain, which implies different levels of barrier (O₂, H₂O, UV) but also welds with a certain tightness.

La processabilité est l’aptitude du matériau à être utilisé sur les technologies disponibles : sachets préfabriqués, machine verticale ou horizontale, machine de thermoformage ou de pliage, operculage de barquettes, compression...

IMPLEMENTATION PROCESSES

The implementation of heat-shrinkable films can be carried out by two processes: blown film extrusion and flat dye extrusion.

In the first case, the bubble is obtained by blowing hot air into a polymer tube at the outlet of an annular die. In the second case, a system of cooled rollers makes it possible to obtain the targeted orientation.

FILMS MICROSTRUCTURE 

In the manufacture of thermoshrinkable films, the macromolecules are stretched, oriented and frozen in a state of low entropy. If heat is applied to the unstressed film, the macromolecules will quickly compress through a coordinated set of molecular motions and return to a high entropy state, restoring the stored elastic stress. 

Thus, the retraction is mainly due to the relaxation of the extension of the amorphous chains during the passage of the T_G and/or the orientation of the crystals during the passage of the melting temperature T_F. The shrinking force will be greater as the heat treatment will be performed in the vicinity of the characteristic temperatures.

Processing conditions influence

Thermoshrinkable films generally have a bi-oriented structure.The intensity of the orientation is given by the stretching force in the machine direction TUR and in the transverse direction BUR, the TUR being predominant on the BUR. The dosage of these two parameters will make it possible to switch from a mono-oriented film machine direction to a biaxially oriented film. When the BUR increases, the retraction increases in the transverse direction TD and decreases in the machine direction MD.

Moreover, the greater the frost line height FLH, the more the lamella are oriented according to TD.

Material influence (structure, composition)

The microstructure of the film depends on the composition of the material. The structure of the polymer (branching, molar mass, Ip) is of paramount importance. In addition, the additives (plasticizers, fillers, etc.) modify the shrinkage by interfering in particular with the entanglements.

THE RETRATECH : CHARACTERIZATION OF THERMOSHRINKABLE FILMS 

The RETRATECH is designed to control the retraction settings plastic films.It is also useful for the film manufacturer to develop and test new materials, for the user who can adapt its packaging parameters according to the different films received, and perform a check reception and effective selection.

It is in accordance with ISO 14616, DIN 53369 and 53125.

The RETRACTECH allows to :

• Check on receipt of films;
• Control the quality ;
• Optimize new formulations;
• Reduce the wastes;
• Improve the packaging process.

The method consists of subjecting two to four test pieces to a thermal cycle simulating the passage of a package in a shrink tunnel. One of them is connected to a force sensor, the others to an incremental position sensor. A specific software makes it possible to follow the evolution of the force and the displacement.

If you want more information about RETRATECH, go HERE.