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 SSP - Solid State Polycondensation


Increasing the IV value by Polycondensation
First, what in the hell does this means ?
Here what books are saying:
A polymerization in which the growth of polymer chains proceeds by condensation reactions between molecules of all degrees of polymerization.
Now, there are quite many publication on the subject but here we like only to make you understand what this process does in relation to recycling process.
As written before, Polycondensation put together molecules to re-form the polymer chain.
Doing this, it does transform also some contaminants into part of the polymer chain with undesirable side effects and here are the most common:
Diethylene glycol (DEG)
DEG is one of the most important side products of the polycondensation process.
PET which is directly produced by esterification of Terephthalic acid usually contains more DEG than PET produced by transesterification of DMT (Chen, 1998 a). Most of the DEG is formed during the preheating of the melt- polycondensation by intermolecular dehydration of Ethylene glycol. The dehydration of Ethylene glycol is catalysed by acids and therefore promoted by a high concentration of carboxyl end-groups.
In the course of polycondensation DEG is also co-polymerised in the polyester chain causing a decrease in melting temperature of PET.
In general, the melting point of PET decreases by about 5 C for each percent increase in DEG concentration.
The incorporation of DEG in the polymer PET also deteriorates mechanical properties, the hydrolytic and light stability as well as the thermal and oxidative degradation behaviour of the material ).
There are other possibilities for the formation of DEG during the polycondensation and manufacturing process of PET.
Carboxyl and Vinyl end groups
Another important side reaction during the production of the polyester PET is the degradation of polymer chains by random thermal cleavage of ester links forming carboxyl end groups and vinyl end groups respectively.
The amount of carboxyl end groups is an important factor due to the susceptibility to hydrolysis of the polymer since these end-groups have a catalytic effect in the process as already mentioned above.
Vinyl end-groups may also polymerise to polyvinyl esters which are responsible for the (dis)coloration of PET.
The vinylester end group of PET can be transesterified under the formation of Acetaldehyde.
The Acetaldehyde concentration of PET granules is of great importance when manufacturing PET bottles for soft drinks especially when used for carbonated mineral water.
Migrating to the beverage, retained Acetaldehyde can cause an intensive off-flavour.
Formation of Acetaldehyde as a by-product of the polycondensation process of PET.
Acetaldehyde is not only formed during the polycondensation process but also occurs during blow molding of PET bottles from PET resin.
All this means it is not only a solid contamination matter but a chemical one.
That's why not a single washing line manufacturer has the FDA approval; he just cannot have it
SSP Solid State Polycondensation is a step further Melt State Polycondensation and can achieve an higher molecular weight and increase the IV up to over 1.
The longer the chain, the higher the molecular weight and thus the IV with which is measured.
The different SSP systems available.
SSP can be done in two different ways; first is the continuous one, normally used with high production rates, and the batch process.
Both systems are using only heat and time so no chemical additives are necessary for the process.
We like continuous system much better than batch processing but, like any other case, there is a good and a bad part on both.
Continuous process is absolutely perfect if starting point (the IV value) is constant; in fact according to heat and time IV will increase of the same value but if the value is not constant, the final one will not be as well.
On the batch process instead, you can change the time for each batch depending on what you put inside the machine; the cost per Kg is a little bit higher but the result is absolutely constant, doesn't matter which IV you did start with.