Transparency, mechanical resistance, barrier effect can vary by changing the degree of crystallinity

We have addressed, in previous articles, some important aspects in the use of PET for the production of manufactured articles, such as the viscosity and molecular weight or the main degradation phenomena of PET.

In this article we see another important aspect, which concerns the management of the degree of crystallinity of PET and how, its variation, it can influence many structural factors, such as the transparency of the manufactured articles, the structural and mechanical aspects and the barrier effect towards the components that the product will contain.

To get straight into the technical aspects, we can say that PET is a semi-crystalline polymer, this means that its solid structure is made up from an amorphous phase, in which the macromolecules that constitute it are arranged in static balls, and from a crystalline phase, in which the chains are arranged in a precise geometric shape.

That said, we can see that PET is a polymer that can be subjected to crystallization, but, like all polymers, it it will never reach it completely due to the very nature of the macromolecules that compose it and their irregularity.

The chains, in fact, tend to arrange themselves towards minimal intermolecular distances, as the general principle that regulates the aggregation of macromolecules for the formation of a crystalline structure is the formation of inter and intra-chain interactions, through regularity of the torsional angles of the macromolecule.

The relationship between the two phases depends on many factors, such as the intrinsic characteristics of the material and the thermal processes it has undergone.

During the crystallization phase of PET, the macromolecules form a lamellar structure, in which the chains fold back on themselves in an orderly manner , but, at the same time, the creation of disorderly external areas occurs.

The PET, being formed by these two phases, is arranged and organized in domains, in which the two phases coexist, creating a maximum thermal crystallization limit of the 50-60% and, in some cases, it is necessary to use nucleating agents to reach the limit value.

Recalling that non-optimal crystallization of polymers can lead to a certain opacity of the manufactured articles, we can say that PET has a low crystallization speed and, for this reason, combined with other properties, it has had a rapid diffusion in the world of packaging.

During PET processing, the crystallization peak can be reached at a temperature of about 160 - 170 °C, but there is also another system to reach this phase, which is the mechanical one.

In fact, with mechanical stretching operations at a certain temperature, an induced crystallization is created, which consists of a forced orientation of the macromolecules in the direction of stretching.

In the uniaxial orientation, in which the effort is applied in a single direction, structures called fibrils are formed, in the biaxial one, in which the effort has two components perpendicular to each other, large and flat crystals (plates) are formed.

This phenomenon is influenced by four main factors:

- The amount of stretch

- The speed of the ironing

- The temperature

- The molecular weight

The combination of these four entities determine the characteristics of the PET and, consequently, the quality of the same, thus, to define a parameter that can characterize the product as a result of these combinations, an indicator defined as "degree of crystallinity" is used, with which we want to indicate the percentage of material that is in the crystalline phase with respect to the total quantity taken into account.

In particular, an increase in the degree of crystallinity leads to greater packing and, thanks to the presence of crystalline domains which act as physical nodes of the lattice, there is an improvement in the mechanical properties.

At the same time, as we have already said, an increase in the crystallinity of the product can lead to a certain opacity of the same, due to the different refractive indices, in fact, this must be taken into serious consideration if you want to produce transparent bottles.

But we must also pay attention to the size of the crystals, in fact, two containers with the same degree of crystallization can have different transparencies or opacities , thus, the larger the crystals, the greater the chances of producing opaque bottles.

Crystallization by stretching is linked to the phenomenon of strain hardening, which involves an increase in the mechanical, thermal properties and barrier resistance of polyethylene terephthalate, determining the success in the production of containers.

the point that identifies the beginning of this phenomenon is defined as Natural Stretch Ratio (NSR).

Consequently, when blowing a preform, a degree of deformation (stretch ratio) equal to or slightly higher than the NSR must be achieved, to be able to have the increase in properties necessary to obtain a light and conforming product.

Another important factor to take into account when blowing preforms, which affects the crystallization of the material, is the presence of water.

In fact, if the water content in PET can theoretically reach 1% of its weight, it must be considered that its presence can vary the physical, mechanical and barrier.

This occurs because water is a plasticizer which affects the orientation of the material, thermal stability and, therefore, also on the crystallization induced by stretching, creating a flow situation between the macromolecules, reproducing a similarity with a polymer of lower viscosity.

The percentage of water also affects the natural stretch ratio and, therefore, the properties of the finished product, with the same axial stretch and radial, a preform containing water will have lower properties, as if blown at a higher temperature.

Machine translation. We apologize for any inaccuracies. Original article in Italian.