How to Keep Moisture Away From Your APET Sheet Line

The PET that’s used in sheet extrusion (amorphous PET, or APET) is pretty much the same polymer that’s used for injection stretch-blow molding. While it may have been modified somewhat to address some issues not encountered in blow molding and its intrinsic viscosity (IV) may not be as high, it is essentially the same material. 

APET offers sheet processors high strength, low cost, and purity, along with optical clarity and sparkle, which optimize the display of the packaged products, thus providing an attractive appearance on the store shelf. Clear APET sheet is typically thermoformed to produce a wide range of transparent packaging for food, medical, consumer, hardware and more.

It’s ubiquity notwithstanding, PET poses a few challenges that processors must overcome in order for the end product to achieve the desired properties. Chief among these challenges is drying. PET is hygroscopic; it will quickly absorb moisture from the surrounding atmosphere until a state of equilibrium is reached, after which it must be dried prior to melt-phase processing in the extruder. If PET is extruded without proper drying, the residual moisture will result in hydrolytic degradation, which will compromise the structural integrity of the APET sheet and thermoformed products. 

The drying requirements for APET provided by the material suppliers are the same as those given for any other PET application. Depending upon the manufacturer’s recommendations, drying temperatures should be in the range of 300 F to 350 F for 4 to 8 hr with the dewpoint of the drying air in the range of -20 F to -40 F. The final residual moisture content most often specified by the material suppliers is in the range of 50 ppm to 30 ppm (0.005% to 0.003%) or less. 

Most if not all of the suppliers of packaging-grade PET offer a material grade that has been specifically formulated to provide improved properties for APET sheet applications, such as a slow crystallization rate along with good optical clarity and sparkle. The virgin pellets used may range from 0.70 to 0.84 IV. Virgin PET pellets used to produce APET sheet are crystalline (milky white in color) and can be dried “as is” at temperatures of up to 350 F. The issues involved with handling the PET virgin pellets are the same as handling any other virgin PET: The resin will absorb moisture when exposed to the atmosphere during shipping and storage and therefore must be dried prior to processing. Figure 1 shows a typical drying-analysis graph depicting the moisture-loss curve for crystalline PET pellets when dried at 300 F. 

CHALLENGES WITH SCRAP 
APET sheet extrusion and thermoforming will typically generate large quantities of scrap flake. Most of the in-house flake is generated from the extruded edge trim and web skeleton remaining after thermoforming. If the products being manufactured are square or rectangular, they can be made to fit or nest nicely in the sheet, thus minimizing the scrap area between the thermoformed products. However if the products being thermoformed are round, oval, or have irregular shapes that cannot be made to nest with each other, the area between the thermoformed products will increase and therefore the scrap rate could easily exceed 50% of the extruder throughput rate. Some additional reclaim flake may come from scrap rolls of APET sheet that the quality-control people have decided do not meet their minimum standards.

Many APET sheet extruders purchase reclaimed PET flake from outside sources, which may require special handling precautions depending upon the history of the material. 
There are several challenges associated with reusing the in-house generated flake that must be addressed, such as:

 1. Conveying and flow characteristics of APET flake.
 2. Fines generated by granulating and handling the APET flake.
 3. Mass-flow characteristics of the APET flake.
 4. The complications associated with drying the APET flake to the recommended final moisture level of 50 ppm or less.

The irregular size, shape, and light bulk density of APET sheet flake make material handling and conveying a challenge. APET flake is flat and tends to layer when stored in silos and day bins. When discharging the flake from the bin into the conveying-system pick-up tube, it may tend to bridge and interrupt material flow. Opening the pick-up tube adjustment to promote better material flow into the conveying line will often cause the conveying line to overfill and flood, which results in surging and plugging of the conveying system. 

One method that can be used to improve the flow characteristics of the flake and reduce bridging is to granulate the flake to a smaller particle size. Rather than using a granulator screen with 3/8-in. or larger holes, use a screen with smaller holes (5/16 in.). This will improve the conveying characteristics of the flake, but the downside is that it will also increase the amount of fines generated. The additional fines can be minimized with meticulous granulator maintenance.

Achieving mass flow of the material through the drying hopper is a prerequisite for controlling the drying time of the material as it travels through the drying hopper. The large flat surface area and light bulk density of the APET flake, coupled with the counterflow drying air being forced up through the bed of material within the drying hopper, may tend to interfere with the orderly downward mass flow of the material. 

There are a few things that can be done to prevent the drying air from interfering with the mass flow through the drying hopper. First, reduce the particle size of the flake as described above. Another method that has a successful track record is to adjust the volume of drying air flowing through the drying hopper to match the material throughput of the drying system. Excess airflow through the drying hopper will not only affect the material mass flow through the drying hopper, it will also consume more energy than is needed to dry the material. 

Large amounts of fines are generated when processing APET flake materials—it’s the nature of the beast. Granulator knife-blade sharpness and gap clearance are critical issues in the generation of fines: Knife sharpness must be maintained, and the gap between the rotor knives and stationary knives must be held to a minimal clearance to allow the granulator to cut the flake rather than pulverize it.

Sheet processors commonly evacuate a granulator with a conveying blower attached directly to the discharge of the granulator. This blower pulls the flake from the granulator discharge, through the conveying blower’s impeller, and then blows the flake to its destination. This type of conveying is not a good practice with PET, as the blower’s impeller will tend to pulverize the flake, creating a large amount of fines. A vacuum-conveying system provides the means to control the conveying velocity from the granulator to its final destination, keeping it at a reasonably low velocity (just above the flake’s pickup velocity), which will reduce the generation of fines and snake skins. 

Drying PET at high temperatures (300 F to 350 F) is prerequisite to achieving the low final moisture level of 50 ppm or lower before processing. This temperature range is not a problem when drying crystalline PET (pellets or flake), which will not begin to soften until it is heated to near its melting temperature, typically above 455 F.

However, when the dried, crystalline PET enters the extruder and goes through melt-phase processing, the crystal structures within the polymer will break down and the polymer will return to the clear, transparent, amorphous state. The sheet exiting the extruder is quickly cooled and frozen in the amorphous state, which is why it is commonly referred to as APET sheet. The biggest challenge in reusing the APET sheet flake from the edge trim and web skeleton is in drying it. 

When the amorphous APET flake is heated it reacts differently than crystalline PET. Amorphous PET has a glass-transition temperature starting at about 180 F, depending upon its composition. When it is heated to its glass-transition temperature it will begin to soften, fuse, and agglomerate, which would result in flow stoppage in a drying hopper. Therefore amorphous flake must be crystallized prior to high-temperature drying in order to prevent agglomeration in the drying hopper. 

CRYSTALLIZING APET FLAKE
Crystallizing is accomplished by heating the PET to a level above its glass-transition temperature. The higher the crystallizing temperature is, the faster the amorphous flake will crystallize. When crystallizing the flake it must be agitated to prevent it from agglomerating. As the flake changes from the amorphous state to the crystalline state it will change from a clear, transparent flake to milky white due to crystal formation within the polymer. During crystallization, the surface of the flake hardens and will not soften again until it is heated to its melting temperature. 

Once crystallized, the APET flake can then be dried at the high temperatures required to achieve the low final moisture levels of 50 ppm or lower. Figure 2 is a typical drying analysis graph showing the moisture-loss curve for crystalline PET flake when dried at 300 F. Notice the low initial moisture content of the crystalline flake; this is due to crystallizing the flake with hot (350 F) ambient air, which provides some limited drying but not enough to achieve the low moisture levels required for processing.

The equipment needed to crystallize the APET flake is costly and takes up valuable floor space within the plant, which has forced some processors to experiment with drying the flake at lower temperatures (150 F) prior to processing. Unfortunately, drying PET to the low final moisture level of 50 ppm or lower at a drying temperature below the PET’s glass-transition temperature is not possible under any circumstances.

Extruding PET that has been dried in such a manner will result in some level of hydrolytic degradation, lower IV, and less-than-optimal physical properties of the extruded sheet. If you want to achieve a final moisture level in the range of 50 ppm or lower and the benefits it provides, you must first crystallize and then dry at the recommended temperatures. Figure 3 is a drying-analysis graph showing the poor performance of drying amorphous PET flake at 150 F.  

APET sheet extruders that have achieved some level of success in processing APET flake without recrystallizing it can attribute that success to the fact that the physical requirements of the product they are making are not quite as demanding as for many other APET products. And in addition they are still most likely experiencing some quality issues that come and go along with the ambient weather conditions. The cold winter months when the ambient humidity is low are their most problem-free months.

Most of their processing and quality problems occur during the hot, humid summer months of June, July, and August. The time between the cold dry winter and hot humid summer can be either good or bad, depending mostly on the weather conditions.