Characteristics of Plastic Materials Chapter 10

Plastics – synthetic materials capable of being formed and molded to produce finished products.  Derived from many types of organic and inorganic materials  Most often from petroleum base stock or natural gas  Polystyrene – benzene and ethylene, both from petroleum

 New plastics are constantly being developed  Many no longer depend on petroleum  Gargill Dow, LLC invented the first viable plastic made from sugar found in corn and sugar beets  Renewable resource  Corn is milled and starch is separated  Unrefined dextrose sugar is produced from starch  Dextrose is converted into lactic acid  Lactic acid is purified  Used in thermoforming, sheet and film extrusion, blown film processing, fiber spinning, and injection molding  Other plastics being developed using wheat, potatoes, green algae

Unique Characteristics of Plastics  Several characteristics that provide advantages over other materials:  About 1/8 th the weight of steel (next slide)  Highly resistant to corrosion (do not deteriorate when exposed to environment)  Good thermals and electrical insulation (handles, electrical joints)  Minimal friction (when created with smooth surfaces, may never need lubrication)  Reduce energy consumption (use for insulation, saves millions of barrels of oil)

Weight Reduction Examples  1977 – 2 liter plastic bottles weighed 68 grams  Today they weigh 51 grams  25% reduction  lbs of plastics in modern cars  If the 300 lbs of plastic were made of steel the car might weigh 2100 lbs more

The Development of Plastics  First synthetic plastic called Parkesine developed in 1862  English chemist Alexander Parkes  Material could be heated and formed, then when cooled it retained it’s shape  Wanted to use it to replace rubber at a lower cost  Lost financial backing  New York, 1866  Billiards company offered a $10,000 reward for a usable substitute for ivory to make billiards balls  John Hyatt discovered the same combination as Parkes  Patented celluloid in 1869  Was used for dental plates, vehicle windshields and motion picture film  Unfortunately it was highly flammable

 First real industrial application called Bakelite, introduced in 1907  American chemist Dr. Leo Baekeland  Thermoset phenolic resin  Would not burn, boil, melt or dissolve in any known acid or solvent  Could not be remelted  Used through WWII Telephone housings Pan handles

 By 1940, plastic had replaced metals, wood, glass, leather, paper and rubber in many products  Many new varieties of plastics introduced  1981, Malden Mills and Patagonia create a synthetic sheep skin called Polar Fleece

 Plastics made from petrochemical resources create disposal and recycling challenges  Biodegradable polymers (from starches) are designed to be more environmentally friendly  Uses for recycled plastics are growing rapidly  Demand for products greater than the amount of recycled plastics

Making Industrial Stock Resin - any of a class of solid or semi-solid organic products of natural or synthetic origin, generally of high molecular weight with no definite melting point Plastic – one of many natural or synthetic, high polymeric products (excluding rubber), that are capable of flowing under heat and pressure, into a desired shape  Often confused  Manufacturer starts with resin in liquid, pellet, granule, or powdered form, and uses it to form or create the plastic material that will be the final product

Commodity Resins  Standard grade resins  Includes most thermoplastic and thermoset resins  Used to make common day items  Containers  Toys  Household items

Engineering Resins  High performance resins in various grades  Advanced  Intermediate  Commodity  Advanced grades are most resistant to chemical attack, extreme heat, and impact  Scientific lab equipment  Helmets (sports)  Reheatable food containers

Structure of Plastic  Hundreds of resins and compounds  Each manufacturer is able to customize their product for specific applications  All plastics are polymers Polymer – long chain molecule made up of thousands of smaller molecules linked together called monomers Monomer – simple molecules Different combinations of monomers = different polymers

Major Classes of Plastics Thermoplastics  Long discrete chains of molecules that melt to a viscous liquid at a specific processing temperature (240°F - 700°F)  Softening and hardening cycles are reversible  Formed into desired shape, pellet, bar, tube, etc.  Then reformed into products

Thermosets  Chemically reactive in their fluid state and harden through further reaction called curing  Once cured, cannot be returned to a fluid state  Kept partially cured before final forming  High dimensional stability  High temperature resistance  Excellent resistance to solvents

Plastic Memory  The capability of a material to return to its original shape after it is bent or formed  Heat is applied after a material has been deformed to activate it’s memory  PVC has a very high plastic memory

Creep  The slow and continuous increase in length at the point of deformation, over a period of time, when a material is placed under a steady load and constant temperature  Cold flow – when creep takes place at room temp

How Creep Takes Place  A plastic bar, 2 inch square by 6 foot long  Ends are fastened to cement blocks  One pound weight is placed on the center of the bar  The plastic will naturally sag (called elastic deformation)  If the weight is removed quickly the bar will return to its original shape  If the weight is left on the bar for a period of time, the bar will continue to sag at a slow rate