|Michigan Tech's Open Sustainability Technology Lab.|
Purpose[edit | edit source]
This literary review is to investigate the properties and relevant mechanisms of PPET plastic during extrusion. The current reach of the research can be found at the PET Preparation Protocol page.
Basic Knowledge[edit | edit source]
Crystalline Melting Temperature: 260°C
Glass transition: 70°C
Temperatures[edit | edit source]
|Melting||Decomposing||Thermal Degradation||Hydrolytic Degradation||Printer Temperature||Extruder Temperature|
|Leapfrog 3D Printing ||160-220°C|
|Romão ||285–370 °C|
Extrusion Data[edit | edit source]
- One group used a "60 mm single screw venting extruder with a length of 38D [that] has been integrated in a flat film extrusion line for processing PET." It has also been noted that high extrusion rates can result in higher shear stresses and thusly, temperatures.
Searches[edit | edit source]
Google[edit | edit source]
- PET degradation temperature
- Printing with PET filament
- PET Extrusion
- PET Extrusion Conditions
Google Scholar[edit | edit source]
- Hygroscopic PET
- Thermal Oxidation PET
Citation List[edit | edit source]
- Technical paper detailing a comparison between stabilities of various plastics vs. thermo-oxidative degradation.
- Focus on solely PET, the mechanisms of degradation, and PET properties.
- Analyzes and gives data on thermal degradation of PET. 
- Less technical outline and study of thermal degradation. 
- contains 2 slides showing the use of PET as a filament, indicating that the process is possible. 
- Contains temperatures, along with examples and tips for printing with PET. 260°C is said to have resulted in a opaque finish. 
- A study analysing the required temperatures, pressures, and drying amounts for PET to be extruded. An excellent source for how to modify the recyclebot. 
- Also, indicates that drying is not necessary, if the "hopper degassing" method is used, as described on page 296.
- Temperatures, durations, and other conditions to properly dry and process various types of PET. 
- Forum discussion on methods for extruding PET. 
- Document outlining the methods and processes used by Eastapak Polyesters, a company that does work with Crystalline and amorphous PET (APET). It gives information on L/D ratios for extrusion, along with yet another insistence that the PET be dried before processing. 
- Yet another source emphasising how and why PET should be dried, and offering various temperatures and conditions. 
- Puts forth information on temperatures and times for PET drying. 
- Brings to light the point that many additives may be within PET bottles. The plastic in PET Bottles is amorphous, and needs drying to a recommended <100ppm moisture. 
- Highlights what phase transformations can occur inside of the extruder. Indicates that above 140°C, the plastic should become crystalline, and would therefore become highly brittle. Around 260°C the brittle crystal melts. The recommendation then, is that the plastic be kept above the glass transition temperature, but below the temperature at which crystallization begins.
- A video on YouTube showing the PET recycling process at Marglen Industries. The video highlights that proper cleaning of the bottles is extremely important, and that as long as the colors match, different bottles may be recycled together. Marglen also demonstrates that PET filament extrusion is possible, and that once it is extruded, the fiber should be rapidly water-cooled. 
- NAPCOR is the National Association for PET Container Resources. Some general information and statistics about PET recycling can be found on there website, but not a large amount of technical information on PET processing.
- A website published by the University of Tennessee, which shows how crystallinity is affected in polymers. In particular, are the DSC and X-Ray Diffraction sections, which show how both of these instrumental processes are carried out, and how to determine crystallinity from recorded data.
- A study from the Burcham International Corporation website, which lays out tensile strengths, and crystalline properties of various blends of different PET plastics, both recycled and virgin. 
- A YouTube video from the "How It's Made" Show, showing the process to turn PET bottles into polyester fabric. During this process, extrusion of the plastic is said to be undergone at 270°C. The strands that appear after this extrusion are not black and liquid, as has been the case for MOST experiments in Spring 2015. The bottles were separated from the labels and caps, through a buoyancy control. Then the glue for the labels was removed with a caustic soda bath (NaOH). The shredded bottles were dried for 10 hours in a rotating drying oven, and then taken to be extruded. 
- A YouTube video showing the process and quality control at Amut Recycling. They use a wet grinding technique. Re-crystallized PET can be seen towards the end of the video, in large metal tins. 
Hindrances[edit | edit source]
Current work results in a low viscosity, brittle and discolored polymer.
Hygroscopy[edit | edit source]
Degradation[edit | edit source]
PET plastics degrade in a number of ways: 
General Thermal Degradation[edit | edit source]
Thermo-Mechanical[edit | edit source]
The stresses from the extrusion process may cause further degradation of the polymer.
Thermo-Oxidative[edit | edit source]
Hydrolyzing[edit | edit source]
This form of degradation necessitates the drying of the PET prior to processing, also known as "pre-drying."
Current Planned Approach and Conclusions[edit | edit source]
PET needs to be shredded and dried. Drying prevents hydrolyzation. Also, the melt temperature will need to be controlled,
to just slightly above 260°C, to prevent thermal degradation. Degradation is undesirable, as the PET's viscosity likely lowers as it is degraded further, due to lower molecular mass, and a lower viscosity means that extrusion of the PET into filament is much more difficult.
Drying is critical for PET. Every source emphasizes it.
Temperature ranges as of 9/9/2014 are: 70°C<T<290°C
Temperature range as of 9/12/2014 is: 70°C<T<250°C. The PET does not need to be a liquid, as opposed to just being softened to allow extrusion. Too high temperatures will both break any semi-crystalline and amorphous bonds, and potentially degrade and break the bonds within the PET molecules itself.
Copolymers may assist in the process, as they can potentially reduce the crysallinity of the PET, allowing it to melt at a lower temperature, and preventing degradation. However, this may impact how the PET can be printed then, as it is at a lower temperature.
- "PET is a nice material to print: it has a wide temperature range and prints from 160°C to 210°C without any problems.I had no problems at all getting the material to stick to the glass plate, I use ABS slurry a lot and just rubbing the glass with a little it of acetone (leaving a slight ABS coat) give a perfect bond. I have to lift a corner of a printed piece to remove it from the bed. I accidentally increased the temperature all the way up to 260°C resulting in a lot of oozing and an opaque/white finish. At 200°C the bonding with the raft is too good making it impossible to remove the raft from the object. Printing without a raft is no problem at all even for small parts."
Summary of Findings 1/12/2015[edit | edit source]
PET requires drying before it can be processed.The material can degrade thermally, oxidatively, and hydrolytically. If one of these degradation processes is allowed to carry out to an excessive extent, the PET molecules will scission, or break, reducing their length. A reduction in the average length of a polymer, its molecular weight, will cause the viscosity of the plastic to drop, as it is more difficult for the polymer chains to latch on to one another.
- The plastic can be dried via heating, to evaporate the water out of the material. If the apparatus used to heat the plastic does not convect and vent strongly however, the plastic will instead be heated and still wet, causing hydrolytic degradation.
- Desiccant packs can be used, along with heating, to pull the water out, provide that the desiccant used absorbs water more readily than PET. 300°F-350°F at a -20°F- -40°F dewpoint, for 4-6 hours. 
- A vacuum could be applied, to evaporate the water out, without increasing the temperature of the material. The length of time for this would likely be slightly longer the 4-6 hours mentioned above, to achieve similar results, as there is no heat introduced.
2. Dry (Either with a vacuum or a convecting desiccant oven)
Crystallinity[edit | edit source]
Original work on PET recycling indicated that the crystallinity of the plastic which was to be extruded, was significant. It has progressed to the realization that industrial PET recycling operations do not sort the plastic based upon crystallinity, and upon testing, it was actually been found to be of no significant effect to extrusion. In the future, perhaps it will be evident that the different crystalline levels expressed in PET waste plastic can be utilized to enhance the process, but as of March 2016, PET waste plastic crystallinity will no longer be considered.
Acknowledgements[edit | edit source]
Funding for this research has been provided by a Charles and Carol McArthur Research Internship, through the Materials Science and Engineering department and Michigan Technological University.
References[edit | edit source]
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