November 19

chapter 9

Polymers (Plastics)

Polymer (poly=many mer-units)

• joining many small molecules
  • we call these monomers
• into an extremely large molecule
  • typically 5000-100,000 monomers

• Virtually all important polymers have a C-backbone; Silicones are the important exception (Si)

• Nature beat us to the idea
  • cellulose-- cotton fibers, much of wood pulp
  • rubber-- (latex of the rubber tree)
  • proteins-- meat, enzymes
    • fibers like silk, wool, spider webs
• Man has been manufacturing polymers
  • only for about 120 years

The term Plastic needs to be reviewed

• Originally meaning-- bendable, stretchable, able to be shaped
  • chewing gum is plastic, wood is not
• Term "plastics" generally now refers
  • to all polymers
    • some are truly plastic
    • others are quite rigid

• Generally, polymers fall into two broad classes
  • thermoplastic materials--
    • soften on heating
      • can be stretched, shaped, rolled, molded
      • most harden on cooling
  • thermosetting materials --
    • once formed, they stay
    • heating will char, but not soften

From a manufacturing point of view

• thermoplastics are easier and more versatile
  • make polymer, form pellets
  • deliver to users
  • then heat and mold the material
• thermosetting materials
  • must be shaped while being prepared
  • pour reaction mixture into a mold
  • reshaping = cutting, drilling...

• however... thermosetting materials hold up well
  • generally harder, heat resistant
  • (make better pot handles, for example)

History--

Modifying natural polymers

• Cellulose-- (cotton, fine paper)
  • little change on heating
  • not soluble
  • can't form objects from it
• Nitrocellulose
  • treat cotton with Nitric Acid
  • looks about the same afterwards
  • becomes explosive, if highly nitrated
  • now dissolves in solvents like ether
• you can still buy nitrocellulose lacquer
  • dissolved, sprayed or brushed
  • forms coating as solvent evaporates
  • shiny, waterproof
• can peel off thin films (painted on glass)
  • but relatively brittle

discovery of plasticizers (1870)

• addition of another chemical
• mixture is moldable, flexible
• will then harden
• material remains solid
• often more flexible than pure material

• nitrocellulose + camphor = first synthetic ivory
  • Celluloid
  • used initially for billiard balls
  • became useful as clear plastic films
    • about the time photography was maturing
      • George Eastman (founded Eastman Kodak)
      • replaced glass plates with flexible film
      • developed the camera we know today

• unfortunately, nitrocellulose film (movie film)
  • extremely flammable
  • deteriorates in 50 years
    • breaks, cracks, separates from emulsion,

• since 1930's film has been cellulose acetate
  • another modification of cellulose

Bakelite -- Mix phenol and formaldehyde

• gets hot, hardens as it cools
• discovered 1907
  • often mixed with wood fibers or minerals
  • less brittle, provides color
    • material is hard, waterproof,
    • electrical insulator, ok to 300^oC
• fresh material makes a good glue (for 15 min)
  • (several similar materials: phenol, resorcinol, urea)
  • still dominant material in plywood and particle board (used as glue, filler)
    • one problem-- tendency to release some unreacted formaldehyde

Rayon is the first of the semi-synthetic fibers

• cotton fibers are relatively coarse
• silk is stronger, smoother and finer fiber

• In early 1900's three separate processes discovered
  • treat cotton chemically
  • make it soluble
  • force solution through tiny holes
  • regenerate cellulose, but much finer
• this is Rayon
  • still manufactured in much same way
  • more closely resembles silk
    • sheen and luster, draping

• This is roughly the status through early 1930's
  • bakelight
  • modified cellulose
  • plexiglas (clear plastic)
  • plastic= minor player (metals, wood, cardboard)
  • reliance on naturally occurring polymers
    • silk, wool, cotton
    • rubber

• one other process, used for millennia
  • oils like teak, linseed
  • turn into hard coats after painting
    • varnish, if used alone
    • lacquer ware, when used to build objects
      • Chinese and Japanese lacquer ware made from sap of poison ivy relative
    • if mixed with pigments for color and resistance called paint

• reactive oil consists of moderate sized molecules (10-20 C atoms)
  • contain some double bonds
  • react with oxygen in air
    • double bond turns into a link between monomers
    • liquid (small molecules) turns solid (large molecules)

• still basis of solvent based varnishes and paints
  • (water based varnish and paints are quite different)

(monomers) linked together

Proteins-- basically small linked amino acids

• amino acid is small molecule with two ends
  • one end is an amine similar to ammonia
  • other end is a carboxylic acid
    • these will react to form an amine
    • loss of water
  • with an amino acid, each end is active
    • and can start another amide bond

• proteins are amazing complex sequences of these amino acids

• In 1930's People at DuPont wanted to try to make synthetic polymer like a protein

• but keep it simple, limit it to one consistent pattern
• they chose a diamine and a dicarboxylic acid
  • first product was called 6-6 Nylon
  • hexadiamine and adipic acid
• this was thermoplastic
• could be drawn into fine fibers
• nylon stockings and WW-II parachutes
• could also be molded (combs, gears, cups)
• could be produced from petroleum and from corn cobs

In the 1940-50 catalysts were developed

• double bonds are always a good candidate to form links
  • ethylene .....polymerizes to form polyethylene
  • polythene is British name

• easy to make
• very easily heated and molded
  • limited uses-- bit too soft for many roles
• Discovery of cross linking and chain branching

Now there are hundreds of polymers known and manufactured

• can blend or mix , make wider assortment six or eight represent the bulk of materials
• some have no market (cheaper or better replacements exist)
• some have specialty markets
  • (dacron in heart valves, for example)
  • Fluorinated-- like Teflon (inert, high temperature)

• as a rule, natural polymers are labor intensive
  • growing and harvesting cotton
  • raising and shearing sheep
  • raising silk worms (moths) and
    • unspinning the cocoon
  • also involve risk and long term investments
  • the product is somewhat variable
    • somewhat take what you get
    • (breeding programs help to refine the strain)
  • collected material must be spun into fibers
    • too short to weave otherwise

• natural rubber is similar story
  • raising trees is large investment if farmed
  • rubber tappers work hard to collect latex "ball"
  • sources of natural rubber limited, easily disrupted
  • (during WW-II Burma, Thailand and South America)
• wood-- can cut and drill, not form
  • can soak, separate and mold
  • (paper, papier mache)

Petrochemicals--

• ready supply of starting materials
• can typically prepare the polymer one wants
• properties are quite uniform
• fibers are typically miles in length
• thermoplastics can be molded, extruded, rolled into shape
  • complex shapes often stronger than solid material
  • I- beam is as strong as solid bar of same size
  • at lower cost, lower weight (cars like this)
  • extrusion can form complex shapes inexpensively

• In many areas, synthetics have taken over the market
  • plastic bottles vs. glass or cans
  • carpets (wool = traditional, but small fraction of market)
  • cotton still important fabric, but not dominant
  • synthetic rubber dominates car tires, etc.

Plastic recycling

• much as we'd like to believe in recycling
• it's a marginal business
  • collecting , shipping used plastic is costly
  • uses relatively large amount of fuels
  • requires careful sorting
  • serious restrictions on used plastic with foods
    • milk bottle to oil cans ok, not vice versa
• thermoplastics can be collected, washed, shredded
  • then used to mold or make fibers
• (thermosetting polymers have little or no recycle value)
• must sort: (mixed polymers are an engineering disaster)
  • depending on oil prices, cheaper to start with raw oil
  • typically cheaper to burn for heat
  • economy is driven by reducing disposal costs or for public relations purposes
• notice that most recycling is not post consumer material
  • it's factory waste
  • concentrated
  • known material (no sorting needed)