Polymers- Recycling and Additives Flashcards
Primary recycling
Regrind and reuse. Most efficient method. Causes shear and heat cycle damage. Hard to do with multiple component systems and can contain contaminants
Secondary recycling
Sort waste by hand or automatic systems. Shred and separate. Can sort by density differences (PE, PP float on water) or induced charges. Quite inefficient
Tertiary recycling
Controlled degradation
Depolymerise into useful products such as fuels, oils, gases
Quaternary recycling
Controlled degradation
Burn polymer and used heat produced to generate electricity
How are thermoset polymers recycled?
Ground or chipped and used as a filler (e.g tyres)
Polymer, polymer blend, mixture, plastic
Polymer is a pure system
Polymer blend is mixture of two or more pure polymers
Mixture is pure polymer and anything else
Plastic is mixture of one or more pure polymers and one or more additives
Why are many plastics hard to recycle?
Often blends of polymers, mixtures with multiple additives, parts with multiple polymers in their construction, parts with polymers and other materials in their construction
Means no economically viable to recycle
Additives- mechanical: fillers
Increase volume at low cost. May also improve other properties. Often increase density so add weight.
Chalk improves stiffness and gives whiteness
Kaolin for coupling
Talc improved stiffness and impact strength
Additives- mechanical: reinforcements
Improve mechanical properties by adding fibre reinforcements. Can adjust aspect ratio of fibres and affects processability
Can be short fibres like E-glass
Can be long fibres like E-glass, Carbon, Kevlar
Additives- appearance: colourants
Absorb/reflect certain visible wavelengths (dyes and pigments) or transmit certain wavelengths (transparent dyes). Dyes tend to be soluble (compatible with polymer) while pigments tend to be insoluble.
Metal oxide pigment powders
Yellow dyes based on anthraquinone
Additives- appearance: black and white
Black: carbon black (high light absorption)
White: TiO2 (high light reflectance)
Additives- appearance: special effects
Al powder gives metallic lustre
Mica coated with TiO2 gives pearlescent effect
Fluorescein dye gives fluorescent effect
Additives- heat
Heat stabilisers (anti-oxidants). Prevent oxidation of polymers by heat during processing or in use.
Dibutyl tin oxide in PVC (radical scavenger)
Hindered by poly-phenols (found in most fruit and veg
Additives- light/UV stabilisers
Can block the light or UV (carbon black, CaCO3, TiO2)
Can retransmit the light/UV.
Benzophenone absorbs UV then transmits the energy as heat
Additives- flame retardants
Reduce ignition, control burning, reduce smoke evolution.
Halogenated compounds
Aluminium trihydroxide
Additives- anti-static
Reduce tendency of polymer to retain electrical charge. Provide a conductive path away from the surface. Glycerol monostearate (GMS) Carbon black
Additives- processing: curing and cross-linking
Reacting monomers to form polymers or linking polymer chains together. Example is dicumyl peroxide which initiates and cures unsaturated polyester and cross-links polyethylene
Additives- processing: coupling/compatibilising
Surface modification of fibres or filler to enable better adhesion to the polymer. Fillers often inorganic so hard to combine with organic polymer.
Organi-silanes for treatment of glass fibres
Stearic acid is amphiphilic (has hydrophobic and hydrophilic end)
Additives- processing: plasticising
Enhancing flexibility during processing and/or in use.
Dioctyl phthalate (DOP) for PVC
Benzoate esters safer alternative
Additives- processing: blowing agents
Create a cellular structure (foam) within the polymer during processing.
Physical: low boiling hydrocarbons like pentane
Chemical: azodicarbonamide released N2 at high temp
EPS used CFCs then HCFCs now CO2
Additives- toughening
Increase polymer’s ability to absorb energy.
Rubber or thermoplastic particles
Copolymerise with a tougher polymer (ABS)
Additives- anti-bacterial and biocides
Resistant to attack by microorganisms.
Organic copper compounds
What are miscible and immiscible polymers?
Miscible polymer blends are completely mixed on a molecular scale. These are rare. Follows like dissolves like rule. They have one Tg
Immiscible blends are phase separated on some scale and are far more common. They may have two Tg. Mixing often temperature dependent leading to phase diagrams
Miscibility and Gibbs free energy
ΔGm=ΔHm-TΔSm
Requires negative ΔGm for mixing. Polymers are highly disordered so ΔSm is small and so enthalpy dominates. Intermolecular bonding reduces ΔHm via van der waals interactions or hydrogen bonding. Also have to match viscosity
What does compounding of additives into polymers require?
Good dispersive mixing (particles separated): separation of additive agglomerates or aggregates into smaller, more effective particles achieved by high shear force to mixture.
Good distributive mixing (sample volume filled): additive spread throughout entirety of polymer by stretching, dividing and/or reorienting the mixture.
Good dispersion often results in good distribution.
Homogeneous mixture for desired properties
How is compounding of additives done (2 ways)
Batch mixers for discrete mixing operations
Continuous mixers
What is compounding?
Generic term for obtaining a homogeneous blend of polymers or a polymer-additive mixture
Steps in compounding
Feeding, mixing, filtering and pelletisation
Compounding: feeding
Hopper contains polymer granules/powder/chip/beads/pellets. There is an additive feeder. Solids done by gravimetric control (dust control, shakers). Liquids done by volumetric control
Compounding: batch mixing
Small scale for particulate solids, doughs or liquid polymers. Heat and shear mix with intermeshing blades
Compounding- continuous mixing: single screw extruders
Single tapered screw inside a tight-fitting barrel. Heat and high shear melt and mix. Large scale but only for easy to mix compounds. Vents in barrel allow volatiles to escape leading to better final product.
Compounding: continuous mixing: twin screw extruders
Two tapered screws inside a tight-fitting barrel. Heat and high shear mix with two intermeshed screws. Large scale, more efficient version of single screw as there is better pumping efficiency. Intermeshing or non-intermeshing of screws. Co-rotating or counter-rotating
Compounding- filtering and pelletisation
Filtering: screens before the die remove contaminants and control flow.
Pelletisation: after the die, one or more strands are solidified in a water bath and then pelletised. Creates sensibly sized solid polymer (with additives in) for use in major processing techniques (like injection moulding)
Thermoplastics processing strategy
Polymer already synthesised. Heat until soft or molten so it has usable viscosity. Homogenise (polymer blends+additives). Force into mould controlling the temperature, shear and pressure. Cool.
Cost of buying initial polymer. Process is just moulding so is simple. High viscosity.
Thermosets processing strategy
Heat resin (monomers) until pourable (usable viscosity). Mix with curing agent (+additives). Cast into mould. Cure controlling heat and time (cure schedule). Process involves polymerisation adding to complexity. Lower viscosity so ideal for composites
