wed, Nov 28

Although many plastics exist few see much use; the Big Six dominate

• Presumably a combination of
  • strength
  • cost
  • ease of fabrication
  • special features
    • transparent?
    • oxygen permeable?
    • rigid vs flexible?
    • maximum temperature?
• Five of the six are structurally similar
  • C=C ethylene
  • core, addition polymer
  • Polyethylene-- simply start with ethylene
  • PVC or Polyvinyl chloride-- replace one H with Cl
  • Polystyrene-- replace one H with bulky C₆H₅ ring
  • Polypropylene-- replace one H with -CH3
    • and then there is a Polyester-- specifically PET (PETE)

Polyethylene-- most widely used

• #4. low density (soft, inexpensive)
  • plastic bags, wrap-- flexible, stretch
• #2. high density (more rigid)
  • bottles: milk, laundry detergent, etc.
  • typically by adding crosslinking
• (actually can make a range of densities)
• (often add minerals to make opaque, add color)
  • relatively easy to recycle
  • collect, clean, sort
  • cheap at point of manufacture
  • expensive at post consumer locations
    • shred into chips
    • reheat and mold
    • seldom as reliable as starting material
    • never fully free of other polymers
    • hard to remove mineral content
    • typically mixed colors (= ugly shade)
    • never sure of purity (not for food)
      • recycled into less demanding products
      • picnic tables, deck materials

PVC Polyvinyl chloride

• replacing H with Cl seems minor but increases polarity and forces between molecules
  • (Vinyl chloride, inhaled, is carcinogen-- at factory)
• material is much more rigid than PE
  • plastic water pipe
  • bottles (thinner than PE for same strength)

Polystyrene

• two extreme applications
• as clear plastic (CD Jewel boxes are typical)
  • transparent, no coloration
  • rigid; easily molded ; softens if hot (90^oC)
    • but... brittle (low impact resistance)
    • dissolves in many solvents, cleaners
• as plastic foam
  • foamed coffee cups, foam blocks
    • rigid, light weight, easily molded
    • holds liquids w/o leaking
    • inexpensive material, once foamed
  • stock (syrene) is more expensive than ethylene
• Formed Polystrene
  • recycling (post consumer) is very difficult
    • foam is bulky, hard to collect
    • not all foam is polystyrene
      • polyurethanes are common
  • so many different applications for nonfoamed polystyrene
    • seldom recognized, sorted

• demonstration: Polystyrene beads into acetone
  • solvent softens, partly dissolves
  • gas bubbles escape
  • volume collapses
  • can probably dissolve 50:1( volume of foam to volume of solvent)

Polyethylene terephthalate (ester) = PETE

• a polyester (acid + alcohol)
  • di-ol and di-carboxylic acid
  • ethylene glycol (common in antifreeze)
  • terephthalic acid
• Properties
  • transparent material
  • relatively rigid
  • high impact strength,
  • resists stretching
  • impervious to gases (especially oxygen)
• best known in plastic pop bottles
  • want a clear bottle
  • light weight
  • easily molded, special shapes
    • much of molding adds strength
    • allows use of less plastic, lighter
  • seal out oxygen
  • notice: need to store product under pressure
    • PE would gradually expand
    • need non-stretch material
  • rely on filled bottles for strength
    • stacking cases of product
    • CO₂ gas pressure provides much of the support
• used also for audio and video tape
  • strong film, resist stretching
• recycling: large volume of material available
  • often close to point of sale/use
  • relative ease to recognize and sort
    • can shred, separate chips from other plastics
    • (PE caps need to be separated)
    • generally by flotation (density)
  • rarely reused for beverages
    • often spun into fibers, insulating fleece, carpets

How to sort post consumer recyclables?

• manufacturing wastes are easier to handle
  • defective products
  • excess production
  • unused stock materials
    • (when shut down/start a machine)
  • material is 100% identified
  • material is clean, unused
  • material is concentrated
• post consumer--
  • material is dirty, unknown contents, paper labels
  • material is mixed: many plastics, metal caps
    • requires manual sorting
      • or expensive machine sorting
      • read the # label on container?
    • sorting, cleaning, chipping cost may be close to cost of petro feed stock

Look at book-- economics of paper vs. plastic cup

• Try to look at full picture... all costs
  • paper is renewable resource / petroleum isn't
  • paper is biodegradable / plastics are not
    • but paper really doesn't degrade in modern landfill

• paper manufacture is a dirty process
  • logging and erosion
  • logging and transport = fuel required
  • pulping -- logs into wood
    • make wood chips
    • cook in sulfite or other chemicals
    • wash and collect cellulose fibers
    • form paper
  • waste water treatment
  • paper mills are ugly locations
    • air quality, water quality

• but refineries are also ugly locations
  • (air quality issues)

• ask about petroleum use only
  • about equal amounts for paper or foam plastic cup
  • used differently (raw material vs source of heat and transportation)

Environmental Degradation

• most plastics have poor UV resistance
• left outdoors they develop surface crazing
  • crazing = fine cracks
• outdoor products add UV absorbers
  • would eventually crumble
  • still remain mostly polymer
  • might end up as powder
  • probably won't biodegrade (some do)
    • (Soil with 25-50% polymers = good agriculture)
• degradation could be desirable
  • plastic webs for six packs frequently trap small animals
  • would be best if they broke up spontaneously
• can't really rely on UV (usually covered)
  • some efforts to introduce degradable sections in polymers
  • deliberately make a "flawed" material