27 April 2001Jamila Richardson1 Polyethylene

27 April 2001Jamila Richardson2 Starting Up  Polyethylene was first produced by British company Imperial Chemical Industries. Preparation required temperatures up to 200 deg C and pressures up to 2000atm.  Karl Ziegler prepared a high molecular weight polyethylene at room temperature in 1952 while working at Dacron Industries.  ICI polyethylene had shorter, branched chains, and was waxy and easily deformed. Ziegler’s puppy was had and rigid, and could easily be easily drawn into fibers.  Giulio Natta extended Ziegler’s research, eventually showing how the geometry of polyethylene could be controlled by certain catalysts (produced by Ziegler).

27 April 2001Jamila Richardson3 Ethylene, the interesting molecule  Ethylene, because of its double bond, is more reactive than ethane. Double bonded carbons make molecule more susceptible to bonding with other molecules.  When approached by another molecule, the double bond splits; the carbons are joined by a single bond, leaving the the molecule free to bond with both carbons.  Ethylene is used in the ripening of fruit; it stimulates the metabolic processes of the fruit by dissolving into the cell membrane, therefore increasing its permeability

27 April 2001Jamila Richardson4 Formation and Structure  Ethylene-Ethylene bond yields polyethylene (CH 2 CH 2 ) n  Crystalline regions lie in alternating fashion with amorphous regions, which creates scattering of light polyethylene  Pure polyethylene, low- and high-density polyethylene (LDPE, HDPE)

27 April 2001Jamila Richardson5 High-Density Polyethylene  Absence of branching results in a more closely packed structure, more crystalline, higher density, and chemical resistance slightly higher than that of LDPE.  Resists alcohols, acids, bases, esters, and aldehydes  Specific Gravity: 0.94 to 0.97  Melting point 130 to 135 deg C  Carbon chains can are 10,000 to 100,000 carbon atoms long  Translucent

27 April 2001Jamila Richardson6 Low-density Polyethylene (LDPE)  Small amount of branching on the chain gives a more open structure  Melting point of 109 to 125 deg C  Does not react at room temperature  Resists alcohols, esters, acids, and bases. Limited resistance to aldehydes.  Translucent to opaque  Used for plastic food or garment bags, spray bottles, plastic lids.

27 April 2001Jamila Richardson7 Pure Polyethylene  Low density arises from minimal amount of branching in chain, gives a more open structure.  Translucent to opaque  Resists acids, bases, esters, alcohols, hydrocarbons. Limited resistance to aldehydes.  Does not react at room temperature  Translucent to opaque.

27 April 2001Jamila Richardson8 Properties  Poylethylene is a solvent for fats, oils, and grease. Dissolving occurs slowly.  Electrons are tightly trapped in their C-C and C-H bonds, which results in an inability of electrical current to flow.  Also results in inability for water and ions to penetrate interior of solid. Thin film of polyethylene photographed using polarized light. The pattern arises form the presence of spherulites, or regions where the polymer molecules have aggregated into spheres. Picture and caption from Molecules

27 April 2001Jamila Richardson9 Other (Fun) Facts  Polyethylene is a thermoplastic. That is, it becomes soft and malleable when heated, and hard and solid when cooled. (So, basically, it’s a plastic. Wow.)  Polyethylene is an addition polymer. The double bonds allow for splitting and bonding with other molecules.

27 April 2001Jamila Richardson10 R-E-C-Y-C-L-E, Recycle...  Polyethylene does not biodegrate unless first oxidized.  Polyeofins are highly susceptible to photooxidation, or oxidation caused by the exposure to light. Hindered-amine light stabilizers are then used to prevent photooxidation. Polyethylene film, which is widely used for packaging, is formed by extruding the molten plastic through a ring-like gap and inflating it like a balloon. Photo and caption from Molecules

27 April 2001Jamila Richardson11 Well, that’s it.  I hope you enjoyed my presentation. BYE!