Study of the deformation/cracking of a barrier coating on a flexible substrate

Long considered secondary, food packaging now occupies an important place in the manufacturing and value chain of a product. Throughout the life cycle of food, it plays several roles: protection, storage, transport and preservation.

To meet these needs, food packaging must have different characteristics: no interaction with the contents to avoid any risk of toxicity or change in the physicochemical characteristics of the product, impermeability to oxygen and water vapour...

These requirements vary according to the food, its state (solid, liquid), its temperature, the duration of contact... The packaging shaping stage is an important phase for the professionals of the sector. It is necessary to ensure that the desired characteristics are preserved even after deformation of the materials used. In the case of oxygen or water vapor impermeable packaging, the surface coating applied to the substrate must retain its barrier properties even after stretching and shaping.

Physicochemical analyses will allow us to study these phenomena and to learn more about the deformation and cracking of barrier coatings.

Among these investigation techniques, the PO3D profilometry in traction mode gives access to the measurement of the elongation of the sample at the same time as that of the topography on surfaces ranging from a few tens of µm² to a few mm².
 

Optical profilometry (confocal microscopy/interferometry) is a non-contact imaging method that allows to observe and characterize the topography of surfaces over measurement ranges from a few tens of µm² to a few mm², with a lateral resolution of 200 nm and depth resolutions up to nm. A classical analysis provides 2D and 3D images with the dimensions of the observed features, as well as the roughness parameters of the analyzed surface. Several modes can be used to access different levels of information.

Confocal microscopy allows the imaging of all types of solid samples, interferometry gives access to nanometric vertical resolutions on smooth surfaces. Tensile modulus provides information on the deformation and the cracking threshold of a coating.

In tensile mode, a load cell is used to measure the elongation of the sample while the profilometer acquires the surface topography images. 
The characteristics of the tensile module are as follows: 
- Adjustable jaw spacing speed from 0.8 mm/min to 25 mm/min.
- Force sensor (range 0-500 N), accuracy 0.1 N.
- Displacement sensor (range 50 mm), accuracy 0.01 mm.

TESCAN ANALYTICS has a recognized expertise in the use of PO3D profilometry on all types of materials. Our team of experts works with all industrial sectors to find solutions to their problems.

Figure 1: PO3D image of the topography (127 x 95 µm2) of a barrier coating on a flexible substrate at different elongation levels


Figure 2: Plot of crack density versus elongation rate

 

During the stretching of the film, its deformation (% elongation) is measured and the surface topography of the coating is imaged simultaneously using the PO3D profilometer.

In figure 1, images acquired at 0%, 2% and 8% elongation are shown. The tension is applied vertically. From 2% elongation, cracks start to appear in the coating. At 8 % elongation, very marked striations are clearly observable in the direction perpendicular to the elongation. The cracking threshold of the coating is therefore around 2% elongation.

In figure 2, the plot of the crack density as a function of the percentage of elongation applied to the film is presented. Stretches around the cracking threshold (visible in the inset) were then performed with a step of 0.25% elongation, the cracking then appearing around 2.4% elongation.

In this example, it has been demonstrated that PO3D profilometry associated with a tensile modulus is a non-contact microscopy tool suitable for the study of strain/cracking as a function of elongation of a barrier coating deposited on a flexible polymer substrate.

The PO3D allows to observe and characterize the surface topography of a coating on areas of a few tens of µm² to a few mm², with a lateral resolution of 200 nm and depth resolutions up to nm.

For more applications of PO3D analysis or our other analytical and microscopy techniques, click here.

Other complementary techniques can be used to study the deformation and cracking of a coating such as AFM.