Why are viscosity and molecular weight so important in pet?

In recycled PET, viscosity and molecular weight can determine the workability and quality of the product.

When using a recycled PET resin, both by molding and by blowing and by thermoforming, it is important to understand what relationships exist between the molecular weight and the viscosity of the material.

Speaking of viscosity and molecular weight , it is necessary to return with the mind to the great physicist Isaac Newton who was involved, among other innumerable scientific activities, also in the study of fluid dynamics.

And it is precisely the dynamics of the fluids that somehow also interacts with some rules of behavior in the processing of PET , when we observe the change from the solid state to the semi-fluid state of the heated raw material.

In fact, in the production of a PET object, whether by thermoforming, molding or blowing , the molten mass which is transformed into an extruder, creates flow parameters in which the molecular weight is of great importance.

This value, in a polymer, is to be kept in the utmost consideration as it determines some mechanical properties such as rigidity, strength, toughness, viscosity and viscoelasticity .

If the value of the molecular weight were too low, the mechanical properties of the PET product you want to make would probably be insufficient to achieve an appropriate quality.

Changing the length of the chain leads to a higher molecular weight, with the consequence of an increase in the relationship of the individual polymer molecules and their viscosity, which will affect the processing and the quality of the product.

If we want to take an example in the field of blowing, we can say that the variation in the molecular weight of the polymer will lead to greater or lesser ease in the formation of Parison or the preform.

As we have seen, there is another parameter closely linked to the molecular weight value, which is the viscosity of the molten polymer , or also called flow resistance. An increase in molecular weight generally corresponds to an increase in viscosity in relation to temperature.

The presence of heat , which is used to create the polymer flow, incising the material through an extruder or injector, allows the plastic to soften, increasing in volume and reducing its density.

This involves the separation of the molecules that will move at different speeds , those in the center of the spindle which, not encountering particular obstacles, will have a different speed than the peripheral ones that will come into contact with the walls that contain them, thus creating shear forces ( stress from cutting ) caused by the difference in speed.

We can therefore say that the viscosity of a material is also influenced by its speed , as the plastic materials, at the base temperatures, appear as tangled elements together, and as the flow velocity increases, a greater orientation of the molecules will be created with a reduction in viscosity.

This type of behavior inserts plastic into those fluids called “non-Newtonians” , unlike water which maintains its viscosity even with increasing speed, returning to fluids defined as “Newtonians”.

This makes us understand what happens to a PET fluid that passes through a head, a preform or a Parison, changing its viscosity, reducing the output flow and increasing the cutting forces.

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