Polyethylene and its properties

Plastics are organic polymeric materials (those consisting of giant organic molecules) that are plastic—that is, they can be formed into desired shapes through extrusion, moulding, casting, or spinning. The molecules can be either natural—including cellulose, wax, and natural rubber—or synthetic—including polyethene and nylon. The starting materials are resins in the form of pellets, powders, or solutions; from these are formed the finished plastics.

Polyethylene is probably the polymer you see most in daily life. Polyethylene is the most popular plastic in the world. It is a polymer of ethylene. (a gaseous unsaturated hydrocarbon. It is the simplest alkene) This is the polymer that makes grocery bags, shampoo bottles, children's toys, and even bullet proof vests. For such a versatile material, it has a very simple structure. A molecule of polyethylene is nothing more than a long chain of carbon atoms, with two hydrogen atoms attached to each carbon atom. We can simply draw it like this, only with the chain of carbon atoms being many thousands of atoms long:

Sometimes it's a little more complicated. Sometimes some of the carbons, instead of having hydrogens attached to them, will have long chains of polyethylene attached to them. This is called branched, or low-density polyethylene, or LDPE. When there is no branching, it is called linear polyethylene, or HDPE. Linear polyethylene is much stronger than branched polyethylene, but branched polyethylene is cheaper and easier to make.

Although the raw material for polyethylene comes from coal, a non-renewable resource, polyethylene is environmentally friendly since polyethylene products are durable and recyclable. As well, polyethylene film is used in Tetra Brik cartons which reduce waste. The development of polyethylene has often been credited with initiating the modern plastics industry. Many newer plastics such as polypropylene, polystyrene, and Teflon, are variations of polyethylene. However, the reliability, relative cheapness, and versatility of polyethylene ensure its continued use in the future. While both LDPE and HDPE are white, translucent, waxy textured thermoplastics with excellent resistance to water, chemicals, and electricity, HDPE is much more rigid and resistant to heat and surfactants. These qualities of HDPE make it useful for shampoo, detergent, and bleach bottles.

However, LDPE is cheaper so it is used extensively in products such as packaging film, squeeze bottles, toys, and ink tubes for pens.


Characteristics of polyethylene:

-Mechanical properties: Good resistance to impact over wide temperature range (-40 - 90 C). Limitation is that it has lower creep resistance and inferior fatigue properties compared to polypropylene. -Environmental: Resistant to a wide range of chemicals, most inorganic acids and alkalis at room temperature. Good moisture resistance. Insoluble in organic solvents below 60 C but may be swollen. Limitations are that it is susceptible to weathering, UV light and environmental stress-cracking. Alloying of PE with butyl rubber and
-Thermal properties: Limitation due to very high thermal expansion. Poor temperature capability. -Food and medicine: Non-toxic grades available for use with food stuffs. -Electrical properties: Good electrical properties- not appreciably affected by humidity due to low wear absorption. Power factor virtually unaffected by frequency or temperature. Limitation is that power factor very sensitive to slight degrees of oxidation and to polar additives. -Processing: Ease of processing by all thermoplastic methods. Limitation is the high mould shrinkage (typical for crystalline polymers). Difficult to bond or print on. -Cost: It has a low cost. Low density polyethylene (LDPE) (produced by high pressure and temperature process):
-Tough at much lower temperature (embrittles at -60 C). -Translucent. -Flexible. -Upper service temperature limit of 88 C. -Low warpage. -Improved environmental stress-crack resistance. -Improved heat resistance. High density polyethylene (HDPE) (Produced by low pressure process with special metallic derivatives as catalysts)
-Better chemical resistance. -Better creep resistance. -Five times stiffer than LDPE at room temperature. -Higher temperature service (130 C). -More resistant to permeation. -Can withstand hot water sterilization but not steam. Applications:

LDPE:
-Injection moldings: Kitchen utility ware, toys, process tank liners, closures, packages, sealing rings and battery parts. -Blow moldings: Squeeze bottles for packaging, containers for drugs. -Film: Wrapping materials for food, clothes, etc. -Wire and cable: High frequency insulation, jacketing. -Pipe: Chemical handling, irrigation systems, natural gas transmission. HDPE:
Refrigerator parts, packaging, structural housing panels, pipe, defroster and heater ducts, sterilizable houseware and hospital equipment, hoops, battery parts, blow molded containers including automotive petrol tanks, film wrapping materials, wire cable and insulation and chemical resistant pipe.

Glass filled HDPE: Automotive fender liners, tote boxes, blower and fan casings, chemical pipe fittings and pump components. UHMWPE: Applications requiring abrasion resistant sheet, plate or moldings, e.g. pumps, filtration of coarse media, chemical processing, food cutting boards. LLDPE: Film resins aim at markets for LDPE, EVA. For moldings, applications include rubbish bins and quality kitchen ware.


Sources used in this our project:

www.encarta.co.uk
http://www.psrc.usm.edu/macrog/pe.htm
http://www-classes.usc.edu/engr/ms/125/MDA125/polymers/tsld011.htm
http://www.goodfellow.com/static/A/ET31.HTML
www.google.com.