Industrial Plastics Polyethylene and PVC Production Quiz

Polyethylene is formed through the addition polymerization of ethylene molecules Each ethylene molecule contains a carbon to carbon double bond that breaks during the reaction allowing the molecules to link together into a long continuous chain of saturated carbon atoms

Chloroethene commonly known as vinyl chloride is the monomer for PVC It is structurally similar to ethylene but has one hydrogen atom replaced by a chlorine atom C2H3Cl This single substitution significantly alters the physical and chemical properties of the resulting polymer compared to standard polyethylene

In addition polymerization monomers containing double bonds link together without the loss of any small molecules The double bond opens up to form new single bonds with adjacent monomers Because no atoms are eliminated during the process the final mass of the polymer is the sum of the masses of all monomers used in the reaction

Ziegler Natta catalysts allow for the production of linear polyethylene chains with very little branching Because the chains are straight they can pack together tightly resulting in a material that is denser stronger and has a higher melting point than the branched versions of the same polymer

LDPE is synthesized under high pressure which causes the polymer chains to develop numerous side branches These branches prevent the long molecules from packing closely together in a neat crystalline structure The resulting loose arrangement makes the material softer more flexible and transparent ideal for plastic bags and films

The presence of the large polar chlorine atom in the polymer chain gives PVC unique advantages The chlorine makes the plastic resistant to many chemicals and oils and it also makes the material naturally difficult to ignite These properties combined with its insulating ability make it the standard material for electrical wire coating and chemical piping

Pure PVC is a very rigid and brittle material To make it flexible manufacturers add plasticizers small molecules that fit between the polymer chains These molecules act as internal lubricants increasing the space between chains and allowing them to slide past one another which transforms the stiff plastic into a pliable material

Since this is an addition reaction every atom present in the C2H4 monomers is incorporated into the final C2H4n polymer chain There are no byproducts like water or gas released The law of conservation of mass is directly observed as the weight of the starting gas precisely matches the weight of the resulting solid plastic

The repeating unit reflects the structure of the original vinyl chloride monomer after the double bond has been converted into a single bond It consists of two carbon atoms one is bonded to two hydrogens and the other is bonded to one hydrogen and one chlorine atom This repeating pattern extends thousands of times to form the macro molecule

The chlorine atoms in PVC are much larger than the hydrogen atoms in polyethylene These atoms create strong dipole dipole interactions between the chains essentially sticking them together more firmly This increased attraction makes it much harder for the chains to move resulting in a material that is significantly stiffer and tougher than PE

Polyethylene's versatility allows it to be used in various forms HDPE's strength makes it perfect for rigid containers like milk jugs LDPE's flexibility and transparency make it ideal for thin films like plastic wrap and soft squeezable containers Window frames however require the extreme rigidity and weather resistance of PVC

To begin addition polymerization an initiator like a peroxide is often used to produce free radicals These highly reactive species attack the double bond of an ethylene molecule creating a new radical at the end of the chain This triggers a chain reaction where the active center moves to the end of the growing polymer rapidly adding thousands of monomers

Because PVC contains a high percentage of chlorine by weight thermal decomposition or combustion results in the formation of hydrogen chloride gas This gas is highly acidic and corrosive posing risks to human health and the environment during fires or improper waste incineration This factor is a major consideration in industrial waste management

LDPE is traditionally produced using a high pressure process up to 3000 atm which encourages the random branching of chains HDPE is produced at much lower pressures and temperatures because the specialized catalysts like Ziegler Natta do the work of aligning the molecules making the process more energy efficient and the product more organized

The length of the polymer chain is controlled by how often new chains are started versus how often they are stopped Higher initiator concentrations start more chains often leading to shorter average lengths Temperature affects the speed of the reaction and chain transfer agents are chemicals specifically added to stop a growing chain at a desired length to ensure consistent product properties