Handout 6: Polymer Processing Flashcards
Classify the different polymers additives and what they do.
- Processing Additives - Stabilisers (to prevent degradation at high T) decomposition or other reaction),Lubricants, Viscosity depressants
- Flexibilisers - Plasticisers (e.g. PVC plasticized by small molecules such as tricresyl phosphate TCP)
- Anti-ageing additives - Antioxidants Ultraviolet stabilisers (e.g. carbon in rubber)
- Surface property modifiers - Antistatic agents
- Coatings - Metallising, ceramic surface (powder bonded on to polymer), barrier layers (e.g. gas impermeable).
- Optical property modifiers - Pigments and dyes
- Fire retardants - Ignition inhibitors Self-extinguishing additives Smoke suppressants
- Foaming agents - Blowing agents. For making polymer foams
- Fillers - Increase rigidity (elastic modulus) Increase creep resistance (e.g. glass fiber in PTFE) ‘Bulk out’ expensive polymer with cheap inert substance.
Give examples of polymer blends that improves the original polymers properties.
- Vinyl chloride + vinyl acetate co-polymerised produce a polymer which is less brittle than PVC (chain irregularities lead to less dense packing; extra free volume allows more chain mobility).
- Styrene-acrylonitrile co-polymer reinforced with polystyrene-butadiene co-polymer, to make ABS.
- Styrene + butadiene to make High Impact Polystyrene (HIPS): polystyrene is brittle, polystyrene-butadiene ‘precipitates’ are rubbery giving higher toughness.
What are the average costs of plastic?
Polymer costs depend on specification, grade and quantity. Very roughly, most high-volume virgin polymers cost £1100-£1800 / tonne – and fluctuate with the oil price. (Steel prices are very approximately £200-400 per tonne).
Name the four general polymer shaping processes, explain them briefly and name analogous metal forming processes. to them.
- Extrusion - Continuous process. Very common. Polymer melted and compressed by the screw in a barrel screw extruder, and forced out of dies in wide range of shapes, geometries and sizes. Extrusion with hydraulic ram.
- Injection Moulding - Discontinuous process. Molten polymer forced into metal mould (die). - Pressure die casting.
- Thermoforming - Thermoplastic sheet heated to plasticise; then formed to shape by pressure - Creep forming.
- Rotational moulding - Rotate heated mould containing polymer granules to form thin shell. - Centrifugal casting (limited to pipes).
Name and explain the range of extrusion based processes.
- Granules - After the polymer is synthesised (usually from oil), it is passed on to processors in the form of granules. The polymer is extruded through multi-hole dies to form laces 3-6mm in diameter. The laces are cooled and chopped into regular granules – used as feedstock for other processes perhaps after blending with additives/fillers.
- Filament, thread and fibre - Extrusion through a circular die, but extrudate is stretched to align polymer chains.
- Thick film/sheet - Multilayer films in which different properties of polymers can be combined. Extrusion of one or more polymers through a long slit can be followed by quenching: -With air jets or on to cooled metal rolls Product may then be stacked (panels) or rolled (sheet). Air-cooled surfaces are rough; smooth surfaces made by cooling in contact with a metal surface.
- Coating - With a different coating geometry, sheets of fabric or paper can be coated with polymer (often LDPE). Examples include waterproof fabric, flooring, polymer-coated cardboard for food packaging.
- Pipes and Hollow Sections - The melt flow is divided and then exits the die. The torpedo helps the polymer to divide and flow round the mandrel. The extrudate may remain as a pipe, or it may be immediately turned into some other shape.
- Extrusion blow moulding - A tube of polymer is extruded vertically from an annular die. A mould is clamped around the tube while it is still hot, and the tube is inflated with compressed air until it fills the mould. After cooling it is removed from the mould and cut from the remaining tube. Widely used for containers e.g. milk bottles, barrels, drums. Material usage not optimized: wall thickness is variable; not much molecular alignment.
- Film Blowing - Most polymer film is made by blowing, which allows biaxial alignment of the polymer molecules in the film. Molten polymer is extruded through an annular die and inflated, providing circumferential hoop stresses and resulting in molecular alignment in this direction. Tensile stress is provided by pulling rolls, resulting in molecular alignment is this direction as well.
Explain ram injection moulding and how it differs from conventional injection moulding.
The molten polymer accumulates in a chamber in front of the screw, and only once the chamber is full is the whole charge ejected into the mould cavity. This is achieved by the whole screw moving forwards, acting as a hydraulic ram. Once the polymer in the mould is solid, the screw is drawn back so that the molten polymer can be collected again.
Explain some important variables in injection blow moulding.
- Injection pressure: The polymer shrinks as it cools and crystallizes. High pressures allow some compensation.
- Component thickness: Increased thickness reduces cooling rates, so increases shrinkage. This is especially important in semicrystalline polymers, as the longer cooling time allows more crystal nucleation and growth. Crystalline regions have higher density than amorphous regions, so the shrinkage is increased.
- Hold-on times: Time that the die is under pressure. Longer time allows for counterbalancing of shrinkage (as long as the polymer is still molten at gate).
- Mould temperature: Moulds are generally water-cooled to minimize cycle time. Increasing mould temperature reduces cooling rate, increasing crystallisation. Implications for shrinkage.
Explain stretch blow moulding.
Used for hollow containers. An injection-moulded pre-form is heated and stretched longitudinally (giving axial alignment of polymer chains) before also being inflated (chain alignment in the hoop direction). This biaxial alignment gives higher strength and stiffness so the polymer walls of the bottle can be made thinner. More uniform wall thickness than extrusion blow moulding.
Explain the process thermoforming.
Used for a wide range of thermoplastic parts in simple shapes: heated sheet deforms onto a mould. Various methods can be used to achieve this (vacuum, air pressure, moving die) Examples: housing for appliances, automotive panels; also used for packaging: cups, trays etc.
Explain the process rotational moulding.
- Hollow mould is charged with thermoplastic granules, then heated and rotated. The granules adhere together to form the component (but do not fully melt). Typically used for PE or PP parts.
- Cheap: does not need expensive equipment; moulds made from aluminium sheet so easily made and customised; can be large. External heat provided by hot gas.
- There is no molecular alignment.
- Product hollow, but with poor control of wall thickness.
- Typical products: Traffic cones, tanks, barrels, canoes, pallets, toys.
What is the effect of molecular chain length on polymer properties?
What is the effect of molecular shape on polymer properties?
What is the effect of side groups on chain backbone?
- Longer chains give higher strength, higher softening temperatures, often higher crystallinity
- Linear chains show less entanglement and are more likely to crystallize
- Influences how easily chains are able to move past each other under stress. Large side groups inhibit crystallization giving an amorphous polymer
Explain the different types of bonds found in polymers and the how the affect polymers.
- Van der Waals bonds - Weak ‘proximity bonds’ which break and re-form easily around TG. - Number and nature determine glass transition temperature TG. Provide polymer rigidity below TG.
- Entanglements - Friction between polymer chains. - Important for strength above TG
- Secondary bonds
- Thermosets have many secondary bonds and therefore cannot melt, retaining stiffness when heated
- Elastomers and rubbers have a few.
- Thermoplastics have none (and thus melt when heated).
Why will two of the same polymers from different companies have different properties?
Because of variation in molecular weight, branching etc. the ‘same polymer’ from different manufacturers may well have different properties. Any particular polymer may have a range of physical properties (Tg, elastic modulus, strength, failure strain, toughness, transparency) which will also depend on processing history,
What are the propertis of polymer crystalline regions?
What forms doe crystalline regions take?
Crystalline regions have denser chain packing than amorphous material, with higher elastic modulus and tensile strength.
the maximum amount of crystallinity does not normally exceed about 80%. Crystals may be dispersed through the material, or may form regions with spherical symmetry (spherulites):combined crystalline/amorphous region.
Draw a specific volume vs temperature graph for different plastic cooling rates and explain each one.
A: Fast cooling: Fully amorphous (no crystallization)
B: Intermediate cooling rate: Partially crystalline
C: Slow cooling: “Fully” crystalline (as much crystallinity as possible in the system)
Compare amourphous and crystalline polymers by drawing a modulus/temperature graph.
