Line 119: Line 119:


===Drying Temperatures and Times Table===
===Drying Temperatures and Times Table===
Important temperatures to be mindful of: 260°C is the melting point. 70-80°C is the glass transition temperature. The PET should be crystalline before this point, otherwise, temperatures much above the T<sub>g</sub> will just clump the plastic together and cause it to degrade. The two tables presented here are for, what are believed to be mostly crystalline, and mostly amorphous PET. Newly shredded PET bottles are assumed to be amorphous.
Important temperatures to be mindful of: 260°C is the melting point. 70-80°C is the glass transition temperature. The two tables presented here are for, what are believed to be mostly crystalline, and mostly amorphous PET. Newly shredded PET bottles are assumed to be amorphous.


{| border="1"
{| border="1"

Revision as of 15:28, 4 March 2015

THIS METHOD IS CURRENTLY A WORK IN PROGRESS

Here is outlined the method to shred, clean and dry PET at the MOST lab at Michigan Technological University. For more information on PET mechanisms please see the PET Extrusion literature review.

Shredding and Cleaning

Any PET bottles or containers which are to be to be recycled need to be ground up into smaller pieces first. This is to speed up the drying process, and also, to fit the plastic into the RecycleBot hopper. Approximately 1/4 of the mass of an average water bottle is in the thred and cap region of the main body. This is because this region has to be very resistant to deformation so that the cap can correctly screw on.

The leftover glue from the label on this water bottle.

Shredding and Cleaning: Equipment

  • Shredder
  • Oil-based cleaner to remove glue, see step 2. Peanut butter, nail polish remover, and perfume are all recommended. (NO WD40)
  • Scissors or shears

Shredding and Cleaning: General Procedure

  1. Peel any labels and/or different colored plastics off of the bottle.
  2. Remove any glue from the wrappings off using your cleaner. MOST used => Duct tape. The sticky side, if repeatedly applied and removed, can pull of the glue. This process, however, takes a significant amount of time.
  3. Rinse out your container, and dry, to remove any leftover beverages.
  4. Cut apart bottles and containers to fit into your shredder. The threaded section will most likely need to be removed, until a total-shredder is developed.
  5. Shred the PET chunks. MOST has used a "Chipper Joe", green wood-chipper and a disk shredder, from Staples. The disk shredder has worked the best so far, with consistent results. The wood chipper is not great for water bottles, it regularly becomes jammed, and has inconsistent sizes.
  6. Collect the chunks and store somewhere cool and dry until the re-crystallization step, just as a preventative measure to moisture buildup.

Cutting Techniques

Some containers may be made of a thicker plastic than others, so different methods must be taken to prepare them.

Method 1 (Gatorade Bottles): Tested

  1. Cut apart the bottle as shown in pictures.
    1. Top and bottom off
    2. Cut in half
    3. Trim off curved pieces
  2. Plug in and turn on disk shredder (NW corner of room)
  3. Hold forwards button
  4. Insert slightly curved section into disk shredder.
  5. Losses of 14/24g:
    • The thread
    • Bottom
    • Curved top

Method 2 (Gatorade Bottle): Tested

NOTE: At MOST this doesn't really work, as the bottles decompress inside the wood chipper and jam.

  1. Cut off top and bottom
  2. cut 1 slit in bottle lengthways
  3. Turn on wood chipper
  4. compress and slide into wood chipper
  5. Losses of 10/24g:
    • The thread
    • Bottom

Method 3 (Plastic Water bottle): Tested

  1. Cut top off of bottle
  2. Split into 4 sections
  3. Plug in and turn on disk shredder, in NW corner of room
  4. Hold forwards button
  5. Insert sections lengthways
  6. Losses of:
    • The thread

Method 4 (Water Bottle Threads): Tested

  1. Place a large plastic container under the output of the wood chipper. Alternatively, a cardboard box may be used, but pieces of the plastic may be lost in the seams of the box.
  2. Take the threads from the water bottles that none of the other methods can shred.
  3. Turn on the wood chipper, and WEAR SAFETY GLASSES.
  4. Carefully insert them into the wood chipper.
  5. Turn off the wood chipper when done, and collect the results.
  6. Losses of:
    • None, as we are able to use the losses of a previous method.

Recrystallization

The opaque section has a higher degree of crystallinity, while the clear is less so. The whole sample was heated to 140°C. Differencs within the same part pose questions as to why the same material appears to undergo different phase transformations.

Amorphous PET retains more water than crystalline PET, and also tends to clump together when dried. Temperatures for drying are intended to be for crystalline PET, rather than the amorphous form. Because of these factors, the amorphous PET which is collected from beverage bottles needs to be recrystallized first.

The opaque pieces are recrystallized, while the clear are still amorphous. The whole sample was subject to 160°C.

Currently, the recommended method of doing this is to agitate and heat the amorphous shredded plastic at 180°F for around 1 hour [1]. This will increase the crystallinity of the plastic, and prepare it for drying.

Some success so far has been found in the re-crystallization of the threaded sections of the bottles. After removing these sections from the bottle, grinding them with a wood chipper, and subjecting to a temperature of 160°C for around 10 minutes, the plastic turned opaque, indicating an increase in crystallinity.

Upon further testing, it appears that this process works at 140°C, and only on the threads, rather than the walls of the bottle, as shown in the picture to the right. It is unknown what the cause of this is, as of 2/11/2015, but it is unlikely due to different materials in the bottle, as this would heavily complicate the commercial-industrial recycling process.

Drying

PET needs to be dried prior to extrusion in a Recyclebot as otherwise it will undergo rapid hyrdolyzation at high temperatures, and reduce the molecular mass.

Drying: Equipment

  • Convection Drying Oven (Any oven will generally work, ovens at home run off the same convection principles of heating and moving air. If using a home oven, be careful of temperatures, and be aware that there could potentially be burning plastic inside of it, if proper precautions are not taken)
  • Shredded PET
  • Drying tray (Any metal tray to hold the PET shreds)

Drying: Procedure

  1. Turn on the oven
  2. Evenly distribute the shredded PET in the drying tray. This exposes more surface area to enhance evaporation speed.
  3. Open the oven door.
  4. Place the tray inside the oven.
  5. Close the oven door.
  6. Set the temperature on your oven to a value as shown by the table below. This table is a work in progress, please feel free to contribute and check!
  7. Wait for an amount of time also shown in the table.
  8. Switch off the oven then carefully open the door after letting it cool for a few minutes.
  9. Quickly place the PET in an airtight container, such as inside two (2) layers of plastic bags that are twisted and tied shut. As long as the PET is not allowed to reabsorb much water from the air, any arrangement will work.
  10. Store PET in a cool dry place until it is to be used. One excellent method is to place the dry PET inside a bag, which also has dessicant. This will help to remove any moisture that may have accrued whilst handling it.

Drying Temperatures and Times Table

Important temperatures to be mindful of: 260°C is the melting point. 70-80°C is the glass transition temperature. The two tables presented here are for, what are believed to be mostly crystalline, and mostly amorphous PET. Newly shredded PET bottles are assumed to be amorphous.

Amorphous-

Bottle Walls

1 Hour 2 Hours 3 Hours 4 Hours 5 Hours 6 Hours
82°C

Slightly above Tg

No Change No Change No Change
100°C
125°C
140°C Slight melting No change
160°C Edges opaque Same as 2 hours
170°C
190°C No change
200°C No change
220°C 50% volume reduction,

discoloration

250°C


Amorphous-

Bottle Threads

1 Hour 2 Hours 3 Hours 4 Hours 5 Hours 6 Hours
82°C

Slightly above Tg

100°C
125°C
140°C Completely Opaque
160°C Completely

opaque

170°C


Crystalline 1 Hour 2 Hours 3 Hours 4 Hours 5 Hours 6 Hours
82°C

Slightly above Tg

100°C
125°C
150°C
175°C
200°C
225°C
250°C
275°C
300°C
Cookies help us deliver our services. By using our services, you agree to our use of cookies.