PT ZONE: PET Drying

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PET Drying


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PET Drying

Question:
We are a large extrusion and thermoforming company; recently we introduced PET products but we are experiencing breakability in our products. We use 100% recycled bottle flakes in our extruder from and we were told a dryer is not required since the extruder has a double vacuum station enough to dry the flakes in the process. Do you think the breakability is related to a insufficient drying? Would a drier improve the impact resistance in our PET products?

Response:
PET is highly hygroscopic and absorbs moisture from the atmosphere. Small amounts of moisture will hydrolyze PET in the melt phase, reducing molecular weight.  PET must be dry just prior to processing, and amorphous PET requires crystallizing prior to drying so that the particles don’t stick together as they go though glass transition. 

Hydrolysis can occur due to moisture and this often can be seen as a reduction in the IV (Intrinsic Viscosity) of the product. PET is "semi-crystalline".  When the IV is reduced, the bottles are more brittle and tend to fail at the “gate” (injection point) during blowing and filling.  It is very possible that due to the initial moisture level in the resin, and the amount removed during vacuum, that a significant amount of moisture still remains as it is reaching its melt phase in the extruder.

In its “crystalline” state it has both crystalline and amorphous portions in its molecular structure. The crystalline portion develops where the molecules can align themselves in a very compact linear structure. In the non-crystalline regions the molecules are in a more random arrangement.  By insuring that your crystallinity is high, prior to processing, the result will be a more uniform and higher quality product.

Another thing to consider is the number if times the PET has been processed. Each time the PET is processed there is a reduction in IV.  Therefore, PET that has been used to make a bottle, recycled and used again, does not have the IV of the original bottle.  Each time the bottle is recycled, the IV is further reduced.  This is why a percentage of virgin resin is often added to increase the products properties.

I recommend that you look at adding both a crystallizer and dryer to your manufacturing.  This will help you gain control over the inconsistencies that you are experiencing.

Mark Haynie: Drying Systems Product Manager, Novatec, Inc.

Question:
What's the typical flake size of regrind in a PET system using a grinder?

Response:
Flake size is determined by the grinder. I have seen customers who think that ¼ inch is perfect and those who think ¾ is best. The smaller the flake the more fines will be produced and the better the material will flow in the drying hopper and vacuum chambers. Extremely fine grinds tend to lead to higher pressure drop in the hopper and can cause reduced air flow and uneven distribution.  Large grinds can sometimes cause uneven hopper material flow.

Don Rainville: Energy Consultant, Novatec, Inc.

Question:
What's the disadvantage of running a 60% regrind mixture in PET applications?

Response:
60% regrind or more is common in extrusion. Most bottle applications are less than 10% for food and beverage and less than 30% otherwise. It depends on the source of the regrind and the end product. Blow molding is more difficult when you use material with different heat history and IV. Fines can also cause processing problems.

The disadvantage of using regrind versus virgin resin is that regrinds have a heat history and have a significantly lower IV (Intrinsic Viscosity).  Lower IV in the finished part causes it to be more brittle/less flexible.  The second disadvantage is that there tends to be more “yellowing” in regrind materials that can cause a color or haziness issue.

Don Rainville: Energy Consultant, Novatec, Inc.

Question:
Is it not more important to take moisture from PET rather than just heat it?  Shouldn’t we be more concerned with moisture control that heat loss in the process?

Response:
Yes, it is more important to remove the moisture than to heat it. However, it is very difficult to measure moisture on-line in a process so time and temperature is generally used to set the moisture level achieved.  For instance, with PET processing, it is generally assumed that if there is 4-6 hours in the drying hopper at 325-350° F, the moisture will be reduced to less than 50 ppm.

Husky machines rely on the heat from drying to process the material to achieve the cycle times they rely on and the quality of the product produced.  Energy that is not in the material at the throat of the machine must be made up by the processing machine in the form of additional shear and through the machine heaters.

Mark Haynie: Drying Systems Product Manager, Novatec, Inc.

Question:
I'm a start-up PET-masterbatch producer .Need your advice in using PET material for producing color-masterbatch. Which type of PET is commonly used ? Amorphous type or crystalline type ? Process is extrusion compounding.

Response:
We think you should talk to your supplier. In general, crystallizing master batch can have a lot of drying/crystallizing issues.  Unless the quantity is so large that you can afford to purchase a crystallizer, buying crystallized master batch is probably preferred.
Mark Haynie: Drying Systems Product Manager, Novatec, Inc.

Question:

From reading your forum it is clear that for APET materials moisture control and material drying is important to control prior to the extrusion process. My question is regarding the finished product. After APET is extruded into a film, does it still have the same sensitivity to moisture or is it more stable? Is there a need to precondition roll stock APET prior to secondary operations such as thermoforming?

Response:

After the PET has been formed into a sheet, there isn’t typically any additional pre-conditioning required before thermoforming.  The sheet will undergo heat in the thermoforming and as long as it doesn’t approach the melt temperature the process just changes the shape of the already formed sheet and the stresses imparted to the sheet tend to give it strength.

Mark Haynie: Drying Systems Product Manager, Novatec, Inc.


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