Polymers
Table of Contents
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Introduction
- A Polymer is a material that is made from many repeating molecular chains.
- There are two main ways of forming polymers:
- Addition polymerisation: The same monomer is linked repeatedly without changing its chemical deposition. Produces no by-products.
- Condensation polymerisation: Different monomers react to form a polymer. Usually water is created as a by-product.
- It is separated into three basic types:
Thermosoftening Polymers
- These remelt after being heated.
- This is because their molecular chains are held by weak Van der Waals forces rather than a molecular bond.
- This also means it has directional properties
- Their melting points are quite low: around 100 degrees
- Common properties include: resilient, flexible and optically transparent
- Common examples are: Polyethylene, acrylonitrile butadiene styrene (ABS), Polyvinyl Chloride (PVC) and acrylic
Elastomers
- This is when the molecular structure does have crosslinking, but not complete chains
- These tend to deform under tensile stress or heatand have high elasticity
- The most common types of elastomers are rubbers
- For use in high-wear conditions, it can be stiffened through a process known as vulcanisation
- This is when an agent (usually about 4% Sulfur) is introduced to the elastomer with pressure and heat.
- The result is a rubber that is more rigid and hard.
- This occurs because the sulfur completes the chains and thus increases the crosslinking.
- Vulcanised rubber is commonly used in car tyres, and can be made into a composite with steel wire and polyester for greater rigidity.
Thermosetting Polymers
- These undergo a non-reversable change when heated and cannot be resoftened by heat.
- Their molecular bonds have cross-linking by strong covalent bonds
- Common properties are: tough, heat-resistant and ductile
- Common examples are: epoxy resins, polypropylene and bakelite.
- While thermosoftening polymers are made by additional polymerisation, thermosetting polymers are made through condensation polymerisation.
Engineering Textiles
- These are polymers drawn into a fabric-like state
Polyester
- Strong, resilient and is resistant to water absorption (hydrophobic).
- It is used in helium airships, some tyres, and car parts such as fan belts and radiator hoses.
Aramid Fibres
- Aramid fibres have excellent strength qualities but tend to have poor temperature resistance.
- They are used in aircraft manufacture and bulletproof vests.
- Some famous examples include Nomex and Kevlar.
Teflon
- PTFE (Teflon) is fire esistant
- It's used in dry lubrication and in filters for engines.
Methods of manufacturing Polymers
Injection moulding
- Whenever you think polymers, you think injection moulding.
- Molten polymer is forced into a cavity of the desired shape through pneumatics.
- It's cooled to solidify
- Then the cavity can be taken apart and the object can be removed.
- If needed, the product can be machined for greater accuracy.
- It's often used with thermosoftening polymers as they're easier to form.
- This method is incredibly cost efficient, fast and simple for mass production
- Most single-piece polymer equipment is made in this method.
Extrusion
- Very similar to the metal forming process
- Let this picture explain everything:
- Also used with thermosoftening polymers.
Blow Moulding
- Used for making plastic bottles.
Calendaring
- Similar to rolling
- This was used for making films, sheets, tiles and other flat plastics.
- Recently, it has been used in making composite materials, replacing the coating liquid with a viscous polymer (e.g. PVC)
Rotational Moulding
- Used to make hollow, stress free items.
- Used with thermosoftening polymers.
Compressional moulding
- An unpolymerised polymer preform (powder and fillers) is compressed into a mould with applied heat
- The heat and pressure polymerise the it
- An ejector pin or separatable mould ejects the product.
- This is used with thermosoftening polymers and elastomers
- Can be polymerised in a different chamber to the moulding chamber in what is known as transfer moulding.
- This allows further heating and curing to be done.
Examples and applications
page revision: 7, last edited: 07 Oct 2011 09:36