Introduction
The polymerisation is defined as the chemical process in which the monomers are joined together to form polymers. Normally, it takes several thousands of monomers to make a single polymer. Polymerisation is the chemical process of monomers joining together to form polymers, often it takes many thousands of monomers to make a single polymer.
Types of polymerization & polymerization reactions–
2 types of polymerization reactions are listed below:
A. Addition Polymerization
where monomers add on to each other with the addition of a catalyst, these are usually alkenes such as ethene and propene. Alkenes can act as monomers because they have a double bond.
B. Condensation Polymerisation
This is when monomers join or polymerise with a byproduct such as water, carbon dioxide or ammonia. This usually requires two different types of monomers that join alternately.
- Linear chain– The chains when heated can flow easily this is a good example of the chain of a thermoplastic. Properties often associated with this type of chain are a weak material, ductile with a low density and melting point.
- Branched chain– Branched chains will not flow as easily when heated they will have a higher melting point than linear chains and typically have the properties of a slightly less ductile material (stronger and stiffer).
- Crossed linked– The Cross linked bonding retains its shape when heated and is a good example of the bonding found in thermo set plastics. These also have a higher melting point than linear and branched bonds, they are usually a harder material but also more brittle.
The two main types of polymers are thermo sets and thermo plastics. Thermo plastics can be re-shaped after heating where as thermo sets cannot. Thermo plastics have the benefit of being recyclable and generally cheaper and easier to process than thermo set plastics.Applications where polymers may be used.
- Acrylic: Acrylic is a polymer called poly(methyl methacrylate) and is often used as an alternative to glass, examples of these include small fish tanks/aquariums visors in goggles and crash helmets covers for skylights and as it has good weatherproofing qualities it is commonly used for outdoor signage.
2. High Density Polythene (HDPE): Babies baths, kitchen equipment, children’s toys, fabric filaments. Polythene is cheap and acid resisting. A strong polymer which softens at aroiund 120 degrees celcius.
3. Low Density Polythene (LDPE): Used in plastic bags, film and packaging. LDPE is a more flexible polymer than HDPE. This softens at around 85 degrees celcius.
4. PVC (Poly Vinyl Chloride): Industrial and domestic piping, artificial leather (e.g. shoes), protective clothing. PVC is reasonably hard and hard wearing it is also fairly cheap.
5. The Ziegler: Natta polymerization of ethyleneEthylene gas is pumped under pressure into a reaction vessel, where it polymerizes under the influence of a Ziegler-Natta catalyst in the presence of a solvent. A slurry of polyethylene, unreacted ethylene monomer, catalyst, and solvent exit the reactor. Unreacted ethylene is separated and returned to the reactor, while the catalyst is neutralized by an alcohol wash and filtered out. Excess solvent is recovered from a hot water bath and recycled, and a dryer dehydrates the wet polyethylene to its final powder form.