Differences in Polymer Structures
Thermoplastics have loose polymer chains that break apart when heat is applied.
Thermosets have rigid, cross linked polymer structures that once set with heat cannot be reshaped.
Thermosets have rigid, cross linked polymer structures that once set with heat cannot be reshaped.
Thermoplastics
Thermoplastics can be heated and reshaped many times.
ABS
P: abrasion resistant, durable and strong. Ap: toys, components that have a lot of movement. |
PET
P: food safe plastic, easy to recycle. Ap: plastic bottles and food packaging. |
ACRYLIC (PMMA)
P: weatherproof, available in all different colours and transparencies. Ap: signage, car headlights. |
POLYPROPYLENE (PP)
P: strong, durable, good impact resistance – good general purpose plastic. Ap: can be injection moulded for complex, solid shapes. |
HIGH IMPACT POLYSTYRENE (HIPS)
P: available in a variety of colours, light weight. Ap: packing trays, toys. |
EXPANDED POLYSTYRENE
P: good heat insulation, impact resistant. Ap: packaging foam, take away cups. |
LOW DENSITY POLYETHYLENE (LDPE)
P: easily recyclable, can be calendared, light weight. Ap: plastic bags. |
HIGH DENSITY POLYETHYLENE (HDPE)
P: chemically resistant, high density. Ap: chemical bottles. |
NYLON
P: very tough and durable, fair abrasion resistance. Ap: tie wraps, components under constant wear. |
UPVC
P: resistant to weather and resists degradation in the sunlight. Ap: window and door casings. |
POLYCARBONATE
P: very strong and tough, high impact resistance. Ap: goggles, safety equipment, riot shields. |
Thermoset
Once set with heat the shape of a Thermoset cannot be changed.
MELAMINE FORMALDEHYDE
P: good insulation and heat resistance. Ap: laminates for counter tops. |
UREA FORMALDEHYDE
P: high electrical insulation, good heat insulation. Ap: electrical fittings e.g. plugs. |
EPOXY RESIN
P: most commonly round as the two part adhesive - one part catalyst which gives the heat to set the thermoset. |
POLYESTER RESIN
P: most commonly used as the bonding agent in glass reinforced plastic. |
Elastomers
Elastomers have the ability to deform and then return to their original shape almost instantly. Their molecules are like springs, which can be compressed.
LSR
Stands for Liquid Silicon Rubber. Ap: kitchen products, medical equipment, phone cases. |
TPE
Stands for Thermoplastic Elastomer. This is the rubbery feeling grip on products. Ap: over mouldings to aid grip. |
Additives
Additives are added to plastics to alter and improve their properties.
FILLERS
• Reduce the bulk of the plastic (makes them cheaper, or can increase strength or hardness) examples are; sawdust and limestone.
FLAME RETARDANT
• Reduce risk of combustion, they create a chemical reaction which can stop combustion.
ANTI-STATIC
• Reduces the effects of static charge that can build up through use.
PLASTICISER
• Reduces the softening temperature and makes them flow easier.
STABILISER
• Reduces the effect of UV light, stops the plastic degrading in sunlight.
• Reduce the bulk of the plastic (makes them cheaper, or can increase strength or hardness) examples are; sawdust and limestone.
FLAME RETARDANT
• Reduce risk of combustion, they create a chemical reaction which can stop combustion.
ANTI-STATIC
• Reduces the effects of static charge that can build up through use.
PLASTICISER
• Reduces the softening temperature and makes them flow easier.
STABILISER
• Reduces the effect of UV light, stops the plastic degrading in sunlight.
Degradable Polymers
Made from renewable materials, they are designed to degrade (breakdown) in certain conditions.
The problem with using degradable polymers and additives is that you then cannot recycle the plastic as it can contaminate other plastics.
The problem with using degradable polymers and additives is that you then cannot recycle the plastic as it can contaminate other plastics.
Bio-Degradable Polymers
BIOPOL
• Breaks down over time through exposure to various micro-organisms, used for slow release medication, bone fixings, packaging.
POLYLACTIDE (PLA)
• Thin and transparent – perfect for carrier bags, bottles and film, even sutures.
STARCH-BASED POLYMERS
• Made from renewable materials e.g corn, potatoes. Used to make capsules for medicine.
• Breaks down over time through exposure to various micro-organisms, used for slow release medication, bone fixings, packaging.
POLYLACTIDE (PLA)
• Thin and transparent – perfect for carrier bags, bottles and film, even sutures.
STARCH-BASED POLYMERS
• Made from renewable materials e.g corn, potatoes. Used to make capsules for medicine.
Oxo-Degradable Polymers
BIO-BATCH
• These have short degradation times – usually around 5 years - much better than the 100 years predicted for oil based polymers.
• The additives break down the polymers into fine powders to be absorbed by micro-organisms.
• The additive is included in plastics such as PET, PP and PS.
• The biggest issue is that if added to a plastic then that plastic is no longer recyclable.
• These have short degradation times – usually around 5 years - much better than the 100 years predicted for oil based polymers.
• The additives break down the polymers into fine powders to be absorbed by micro-organisms.
• The additive is included in plastics such as PET, PP and PS.
• The biggest issue is that if added to a plastic then that plastic is no longer recyclable.
Water Soluble Polymers
• Products such as liquid detergent tablets will degrade when in contact with warm water.
• They also make large laundry bags to put in clothes that don’t break down until in wash – useful for hospital staff who would never come into contact with contaminated clothes.
• They also make large laundry bags to put in clothes that don’t break down until in wash – useful for hospital staff who would never come into contact with contaminated clothes.
Fabrication Methods
How plastics are joined together.
Plastic Welding
Watch a Video
Bonding with Adhesives
EPOXY RESIN:
• Mixture of two parts, one of which is a catalyst which causes a heat reaction (sets with heat as it is a thermoset!) the two parts react and create a solid bond. • Will join any materials together. CONTACT ADHESIVE: • The materials to be glued are both coated with contact adhesive, left to cure slightly, then the materials are clamped together until the adhesive fully cures. • Best for joining sheet materials, woods and plastics. |
Forming Methods
How plastics are formed into shapes.
Line Bending
Vacuum Forming
Thermoforming
Calendaring
Redistribution Methods
How molecules in the plastic are redistributed to create new shapes.
The only process that can be used to redistribute Thermoset plastics is Compression Moulding.
The only process that can be used to redistribute Thermoset plastics is Compression Moulding.
Blow Moulding
BLOW MOULDING
1. A tube of heated plastic drops into the mould (parison). 2. The moulds close and seal. 3. Hot air is then blown in – forcing the material onto the mould walls. 4. The moulds cools. 5. The moulds separate and the product is released. Watch a Video. |
Injection Moulding
INJECTION MOULDING
1. Plastic granules are placed in the hopper. 2. The motor turns the archemedean screw and moves the granules to the heaters. This melts the plastic. 3. Once a charge is prepared the melted plastic is forced into the die. Watch a Video. |
Rotational Moulding
Compression Moulding
COMPRESSION MOULDING
1. A “Slug” (this is compressed powder) is placed into the mould. 2. The mould heats to the correct temperature and then closes. 3. This holds the material under pressure until the time it takes to “cure” the plastic. 4. The mould opens and the product is ejected – it can then cool after. Watch a Video. |
Finishing Methods
PAINTING:
• The use of acrylic paints (e.g. spray paints)
• Chrome effects
PIGMENTS AND STABILISERS
• Pigments to add colours
• Stabilisers make plastics stronger or less likely to degrade
• The use of acrylic paints (e.g. spray paints)
• Chrome effects
PIGMENTS AND STABILISERS
• Pigments to add colours
• Stabilisers make plastics stronger or less likely to degrade
Theory Lesson PowerPoint
Click below to download the Independent Learning guide for Plastics.
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