Pet

 

Here below just  few lines on further processing clean flakes.

A clean dry PET flake, according to the final application, can be directly fed into an extruder and undergo an SSP process to modify, better saying, increase the IV 

 

PELLETIZING


If flakes have not been washed well, separated, dried and all the other steps a recycling line is supposed to do, the extruder will deliver a dark or brown pellet anyway.

This to tell that if the extruder doesn't perform well, in most cases is not the extruder fault but the problem should be found in some earlier stage of the process.

A pelletizing line should have these characteristics:

A good venting device with a powerful vacuum pump to increase the degassing effect; if venting are two even better.

An oversized screenchanger because sometimes, if material has not been washed well, and this will happen, you need to get production anyway and a big screenchanger helps a lot.

The ratio between Length of the screw and Diameter and screw design can vary and there are many theories about this subject.

Single or twin type extruder can be used depending from the final application of the material. The main difference is that the material at the end of the machine is a compound and not only a melt.

 

 

SSP

—solid state polycondensation—

 

 

There are many publications on the subject but here we like only to make you understand

 what this process does in relation to the recycling process.

In PET manufacturing, in polycondensation reactions and in the polymer applications such as blow molding, some by products  or degradation products are formed. The ones found are Ethylene glycols, Acetaldehyde, Carboxyl and Vinyl end groups.

Particularly considering the recycling process, all this means it is not only a solid contamination matter but also a chemical one.

Polycondensation assemble molecules to re-form the polymer chain and this will also include the by products or contaminants.

Doing this, it does transform also contaminants into part of the polymer chain with undesirable side effects like deterioration of mechanical properties, the hydrolytic and light stability as well as the thermal and oxidative behaviour of the material.

SSP can be a batch or a continuous process performed in either case only with temperature and time. With the batch process, one can change the time for each batch in relation to what is fed inside the machine; the cost per Kg is a little bit higher but the result is absolutely constant, independently from the starting  IV.

 


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