Polymer_2 Flashcards
Additives and Blends Compounding Extrusion (37 cards)
What is an Additive?
A substance added to polymers to add certain properties.
What are Reinforcements additive type? Name some.
Additives which improve mechanical properties.
Short fibres such as glass. Long fibres such as glass, carbon, Kevlar etc.
What are Toughening Agents additive type? Name some.
Additives which improve the toughness of the polymer if it is brittle.
Rubber particles. Copolymerise (e.g. ABS).
What are Fillers additive type? Name some.
Used to ‘bulk’ out or increase the volume while maintaining similar properties.
Extenders such as wood flour or calcium carbonate, talc etc. Enhancers such as mica (heat resistance), carbon black (conductivity).
What are Coupling Agents additive type? Name some.
Used to improve appearance and adhesion qualities of polymer.
Silanes (e.g. RSi(OR)3)
Stearic acic, C17H35-CO2H.
What are Stabilisers additive type? Name some.
Helps prevent polymers from breaking down.
HCl scavengers (e.g. organo-tin compounds). UC protectors (e.g. phenones).
What are Pigments & Dyes additive type? Name some.
Used to change the colour of the polymer.
Colorants such as titanium dioxide (brilliant white), metal oxides.
Also calcium carbonate (white) and carbon black black).
What are Plasticisers, lubricants, processing aids additive type? Name some.
Low molecular weight compounds that help with the plasticising, lubrication or processing of a polymer.
Low molecular weight compounds (e.g. PE in PVC).
What are Curing Agents additive type? Name some.
Used to cure a polymer or improve the cross-linking between linear chains.
Pe can be crosslinked with dicumyl peroxide.
What are Blowing Agents additive type? Name some.
Used to make foams. Low boiling points.
Physical (e.g. pentane in PS).
Chemical (e.g. hydrazine compounds).
What are Flame Retardants additive type? Name some.
Used to retard flames in polymers. Reduced the flammability of polymers.
Halogenated compounds (Cl or Br containing). They are being phased out for environmental and health reasons.
What are Other additive types? Name some.
Biocides reduced biological activity in the polymer. (e.g. organic copper compounds)
Anti-static agents (e.g. fatty acid amines).
What are Polymer Blends and Polymer Mixtures?
Polymer = Pure system
Polymer Blend = Mixture of two or more pure polymers.
Polymer Mixture = Pure Polymer + Anything else.
Plastic = Mixture of one or more pure polymers and one or more additives.
What are Miscible blends?
Complete mixing on a molecular scale. Basic rule: ‘like dissolves like’ (e.g. PET & PBT).
What are Immiscible blends?
Phase separated. Not completely mixed. Most common.
Miscibility is controlled by the Gibbs free energy of mixing (Change in Gmix) and rheological properties. Show this equation.
∆𝐺𝑚𝑖𝑥 = ∆𝐻𝑚𝑖𝑥 −𝑇∆𝑆𝑚𝑖𝑥. Memorise!
Polymers are already highly disorders so mixing them yields little entropic (∆𝑆𝑚𝑖𝑥) benefit.
Therefore enthalpy dominates but there are relatively few combinations of polymers that can physically bond to provide enthalpic (∆𝐻𝑚𝑖𝑥) benefit.
Also need to match viscosity.
Polymer-additive mixtures are required to be homogenous to obtain desired properties. Describe distribution and dispersion.
Good distribution means the sample volume is filled. Good dispersion means particles are separated.
Describe Compounding in terms of polymer mixtures. Also name the processes behind compounding.
A generic term for obtaining a homogenous blend or mixture.
Compounded plastic. Concentrate or ‘masterbatch’.
Process: Feeding Mixing Filtering Pelletisation
Describe Feeding in Compounding. Name the feeders type systems.
A hopper containing polymer granules/powder/ chip/ beads/ pellets.
Additive feeders Solid, gravimetric control -Dust control -Shakers Liquid, volumetric control.
Sometimes ‘bridging’ may occur where the polymer is stuck in the hopper feeders.
Describe Mixing in Compounding. Name the Mixing type systems.
Batch mixers.
- Small scale
- Heat and shear mix with intermeshing blades
Continuous mixers
- Large scale
- Heat and high shear mix with non-intermeshing parallel screws.
Screw extruders
- Heat and shear mixing
- Single screw for large scale mixing of easy to mix compounds.
- Twin screws (co-rotating or counter-rotating) for more efficient large scale mixing.
Describe Filtering in Compounding.
Screens remove contaminants and control flow.
Describe Pelletisation in Compounding.
Creates a sensibly sized solid polymer for use in major processing techniques.
Strand polymer extruded (forced through die, cool, dry, chop).
Describe the Extrusion process after compounding.
Homogenised molten plastic is forced through an open-ended die or aperture.
Continuous process.
Creates parts with a constant cross-section (e.g. tubes, fibres, sheets etc.)
Potentially ‘infinite’ length.
Often a pre-cursor to other processes.
- Sheets for thermoforming
- Pellets for injection moulding
Most thermoplastics can be extruded.
Scrap/Waste from out of specification and edge trim etc.
Describe an Extruder.
Archimedes screw pumps molten polymer along screw.
Flight depth and material channel decreases along the screw.
Polymer molecules shear against each other causing melting.
Also heat/cool the barrel as necessary.
A Breaker plate removes the spiral motion, makes the flow linear.
Screen pack filters for contaminants and controls the flow.
