Industrial chemistry - Polymers Flashcards
Polymers
Polymers are formed by the chemical bonding of a very large number of relatively small molecules referred to as monomers.
Main 2 types of polymers and their examples
Natural - Natural rubber, blood and enzymes
Synthetic - P.V.C, Bakelite, Nylon, Teflon
Classification of polymers according mechanical properties and their definitions
Elastomers and Plastromers
If a certain polymer regains their original shape when the mechanical force applied is removed, such a polymer is called an elastomer.
Plastromers are made to maintain their shape permanently.
Classification of polymers according to the action of heat
Thermoplastic and thermosetting
Thermoplastic polymers are thermally stable and their shape can be removed using heat.
The forces of attraction between chains are weak. e.g: PVC, PP, PS
Thermosetting polymers can never be removed to change shape through heated once their original shape and texture is determined. e.g: Bakelite, urea formaldehyde
Polyethylene (PE) monomer
Ethylene n(CH2=CH2)
Polyvinylchloride (PVC) monomer
Vinyl chloride n(CH2=CHCl)
Polytetrafluoro ethylene (PTFE) monomer
Tetrafluoro ethylene (TFE) n(CF2=CF2)
Polystyrene (PS) monomer
Styrene n(C6H5CH=CH2)
Polymerization
Production of polymers from monomers
According the reaction in polymerization, 2 types of polymers
Addition polymers
Condensation polymers
Addition polymer
If a polymer is formed by the bonding of monomers by an addition reaction, such a polymer is known as an addition polymer
The molar mass of the monomer is equal to the molar mass of the repeating unit
Condensation polymers
During the formation of condensation polymers, a condensation reaction occurs.
In addition to the polymer, small molecules of low mass are produced.
A number of water molecules equal to the number of ester bonds formed are released. If the volume of the water produced can be measured correctly, the number of ester bonds formed could be calculated.
Types of condensation polymers and their examples
- Long chain- Poly-amide, Polyester, Terylene
2. 3-dimensional - Bakelite-phenol formaldehyde, Urea formaldehyde
According to the nature of the structure , polymers can be classified as (characteristics)
Linear - less stable, single chained and can be packed easily. High in density and has high melting points.
Branched - Cannot be packed very closely. Very low density and melting point. Less elastic.
Network - Presence of cross bonds. Less elastic and fragile. Can be very closely packed. Very high density and melting point.
Semicrystalline polymers
Polymers having certain amount of crystalline regions
e.g: Polythene
Light does not penetrate well through membranes of semi crystalline polymers. Crystalline regions in them scatter light and reduce transparency. Light does not scatter through semicrystalline layers. Therefore they are transparent.
HDPE and LDPE
High density polythene - Linear polythene molecules pack closely with each other and have more high density areas
Low density polythene - Their density is low and have more amorphous regions.
E,g: for artificial rubber
Nitryl rubber
Plastics
Polymers with limited elastic properties
e.g: PVC, PET, PP and PE
When stretched beyond the elastic limit its shape changes irreversibly.
Types of plastics
Thermosetting - Have a molecular structure arranged as a network e.g: Phenol-formaldehyde (bakelite)
Thermoplastics - Composed of linear polymers or branched polymers. Can be softened by heating e.g: PVC, PE and PS
Natural rubber
Obtained by coagulating latex of rubber tree (Hevea braziliensis)
Contains 60-65% water and 30-35% rubber.
In rubber latex rubber occurs as a colloidal solution in which very small rubber particles are dispersed. Simple sugars and salts are also dissolved in this colloidal solution.
There is a lipid and protein layer around a rubber particle. Inner to it lie rubber molecules. Since –COO- groups associate the outer layers of a rubber partical its outer surface is negatively charged.
Coagulation of latex
In addition of acids H+ ions neutralize the –COO- groups and the surface of the particles become electrically neutral. Then the particles combine together and settle down as a mass. This is known as ‘coagulation of latex’.
Microbial activity and preventing coagulation of rubber
Since the rubber latex contains salts, sugar, amino acids etc. It is a suitable medium for microbial action.
coagulation can be prevented by basifying the latex by adding an ammonia solution. Ammonia prevents the medium becomes acidic and stabilizes the negative charges around the rubber particles. Under these basic conditions the microbial action is suppressed.
Reason for the elasticity of natural rubber
Existence of cis-polyisoprene chains
Vulcanisation of rubber
Changing the elastic property of rubber as required industrially and strengthening it, it is heated with 1-3% sulphur.
Ebonite
Ebonite is obtained when rubber is heated with 25-35% sulphur by weight. Ebonite doesn’t show elastic properties. This is due to the fact that activity associated with the double bonds in rubber molecules form large number of crosslinks with sulphur atoms.
Additives
The substances used to improve the qualities of the end product and reduce its production cost in the production of polymers are referred to as additives.
e.g: carbon black, ZnO and CaCO3
Plastics definition
Polymers with limited elastic properties
Terylene is formed by
Ethylene glycole and terephthalic acid
Urea-formaldehyde production
nontransparent thermosetting resin or polymer. It is produced from urea and formaldehyde
Terylene characteristics and uses
Thermoplastic polymer
Used as a substitute for natural fibres such as cotton and wool
Used in the production of textiles, photographic films and audio tapes
Bakelite uses
Used to produce certain parts of electrical equipments such as switch and plug.
Insulating wires
