B. Polymers Flashcards
What is the difference between thermoforming and thermosetting polymers?
Thermosetting plastics undergo a chemical change and once formed, cannot be reformed. Thermoforming plastics can be reshaped multiple times
Why are thermoforming plastics able to be reshaped?
TF plastics are loosely entangled with very few cross links, allowing the chains to slide past each other when heated.
Why can’t thermosetting plastics be reshaped?
TS plastics have rigid cross-linked polymer chains that set when heated and cannot be reformed.
Which polymers are thermoforming?
Polypropylene (PP)
High density polyethylene (HDPE)
High impact polystyrene (HIPS)
Polyethylene terephthalate (PET)
Acrylic (PMMA)
Polyvinyl chloride (PVC)
Which polymers are thermosetting?
Polyester resin (PR)
Urea formaldehyde (UF)
Epoxy Resin (ER)
Phenol formaldehyde (PF)
Melamine formaldehyde (MF)
What are the uses of Polypropylene (PP)?
Uses: Kitchen, medical properties, rope
What are the properties of Polypropylene (PP)?
Thermoforming
Properties: Flexible, tough, lightweight, food safe
What are the properties of Acrylic (PMMA)?
Thermoforming
Properties: Tough but brittle when thin. Easily scratched, formed and bonded
What are the uses of Acrylic (PMMA)?
Uses: Car lights, alternative to glass, modern baths, clothing
What are the properties of High density polyethylene (HDPE)?
Thermoforming
Properties: Lightweight, rip and chemical resistant
What are the uses of High density polyethylene (HDPE)?
Uses: Milk bottles, pipes, crates, wheelie bins
What are the properties of High impact polystyrene (HIPS)?
Thermoforming
Properties: Flexible, impact resistant, lightweight, (can be) food safe
What are the uses of High impact polystyrene (HIPS)?
Uses: Yoghurt pots, vacuum forming
What are the uses of polyethylene terephthalate (PET)?
Thermoforming
Properties: Blow moulded, chemically resistant, fully recyclable
What are the uses of polyethylene terephthalate (PET)?
Uses: Drinks bottles, food containers, yarn
What are the properties of Polyvinyl chloride (PVC)?
Thermoforming
Properties: Flexible, easily extruded, tough and resistant to chemicals
What are the uses of Polyvinyl chloride (PVC)?
Uses: Raincoats, pipes, electrical tape
What are the properties of Polyester resin (PR)?
Thermosetting
Properties: Good electrical insulator, hard
What happens when glass is added to Polyester resin (PR)?
PR becomes tough when mixed with glass strands to form Glass reinforced plastic (GRP)
What are the uses of Polyester resin (PR)?
Encapsulation (think microfibre), boat halls (as GRP)
What are the properties of Urea formaldehyde (UF)?
Thermosetting
Uses: High tensile strength, heat resistant, electrical insulator, hard, brittle, easily injection moulded
What are the uses of Urea formaldehyde (UF)?
Uses: Adhesives for binding particle boards, decorative laminates, electrical casings
What are the properties of Epoxy resin (ER)?
Thermosetting
Properties: Good strength to weight ratio, electrical insulator, heat resistant
What are the uses of Epoxy resin?
Uses: Bonding waterproof coatings, electronic circuit boards
What are the properties of Phenol formaldehyde (PF)?
Thermosetting
Properties: Heat resistant, electrical insulator
What are the uses of Phenol formaldehyde (PF)?
Uses: Heat resistant handles, electrical components, snooker balls
What are the properties of Melamine formaldehyde (MF)?
Thermosetting
Properties: Lightweight, hard but brittle
What are the uses of Melamine formaldehyde (MF)?
Uses: Worktops, surfaces, some kitchenware
Why are TS polymers more difficult to recycle than TF polymers?
- The molecular structure of TS plastic is more rigid, making them difficult to reform once ‘set’
- When TS plastic is reheated, it tends to burn rather than melt
What properties do thermoforming plastics have?
- Easy to mould when hot
- Can be formed into complex shapes
- Lightweight
- Strong
- Waterproof
- Corrosion resistant
- Good strength to weight ratio
- Can be injection moulded
What properties of thermosetting plastics have?
- Irreversibly hardened and cannot be reformed
- Good heat, chemical and electrical resistance
- Structural integrity
- Can be reinforced
What do stabilisers do?
Make plastic more resistant to heat and light
What impact does UV light have on plastic?
UV light degrades plastic, turning it brittle and faded. Impacts strength and surface texture
What do plasticisers do?
Give a material improved flexibility
What do plasticisers do when added to PVC?
Improve applications such as sheathing for electrical cables and car interior trim
What are the main stock forms that polymers come in?
Film
Granules/pellets
Foam
Rods/tubes
Powder
Sheets
What is film (SF) used for?
Packaging, bags, labels and sheeting
What are granules and pellets (SF) used for?
Extrusion and injection moulding
What is foam (SF) used for?
Thermal insulation and packaging. Can be flexible or rigid
What are rods and tubes (SF) used for?
Electrical, plumbing, garden, military and medical applications
What is plastic powder (SF) used for?
Resin glues, spray coating and 38 printing. Mixes well with additives and dyes
What are plastic sheets (SF) used for?
Protective surfaces, roofing and signage. Can be cut to size, bent, curved and vacuum formed.
How does vacuum forming work?
- A plastic sheet is placed above the mould and clamped
- The electric heater is turned on to warm the plastic until it becomes flexible
- The air is vacuumed out below the plastic and mould, pulling the plastic to shape around the mould
How does extrusion work?
A continuous flow of molten plastic is pushed through a die to produce a specific shape or profile
What is extrusion used for?
Cables, pipes, mouldings and plastic films for packaging
How does injection moulding work?
- Molten plastic is injected into a mould using an Archimedes screw
- The plastic cools and hardens into the shape of the mould
What sort of product is injection moulding used to make?
Bottle caps, toys, automotive parts, toothbrushes
What are the benefits of injection moulding?
- Can handle complex parts and shapes to produce consistent products in large quantities
- The process can be automated for a high output rate
- Cost efficient
What is resin casting used to make?
Industrial prototypes, precise models, moulds for dentistry, art and jewellery-makers
How does resin casting work?
- Liquid synthetic resin is mixed with a curing agent
- The resin is poured into a mould and allowed to harden
- Relies on gravity to pull the resin into all parts of the mould
What are the advantages and disadvantages of resin casting?
Pros:
- Produces a strong final product
- Quick production and setting time
Cons:
- Use is limited to smaller scale items
What sort of product is blow moulding used to make?
Plastic bottles and containers
How does blow moulding work?
- Heated plastic is extruded into a hollow tube
- The mould closes to grip the parison
- Compressed air inflates the parison
- The parison fills the mould and cools
- The mould opens and the finished product is trimmed
