Polymers- Recycling and Additives

Question 1

Primary recycling

Answer 1

Regrind and reuse. Most efficient method. Causes shear and heat cycle damage. Hard to do with multiple component systems and can contain contaminants

Question 2

Secondary recycling

Answer 2

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

Question 3

Tertiary recycling

Answer 3

Controlled degradation

Depolymerise into useful products such as fuels, oils, gases

Question 4

Quaternary recycling

Answer 4

Controlled degradation

Burn polymer and used heat produced to generate electricity

Question 5

How are thermoset polymers recycled?

Answer 5

Ground or chipped and used as a filler (e.g tyres)

Question 6

Polymer, polymer blend, mixture, plastic

Answer 6

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

Question 7

Why are many plastics hard to recycle?

Answer 7

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

Question 8

Additives- mechanical: fillers

Answer 8

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

Question 9

Additives- mechanical: reinforcements

Answer 9

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

Question 10

Additives- appearance: colourants

Answer 10

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

Question 11

Additives- appearance: black and white

Answer 11

Black: carbon black (high light absorption)
White: TiO2 (high light reflectance)

Question 12

Additives- appearance: special effects

Answer 12

Al powder gives metallic lustre
Mica coated with TiO2 gives pearlescent effect
Fluorescein dye gives fluorescent effect

Question 13

Additives- heat

Answer 13

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

Question 14

Additives- light/UV stabilisers

Answer 14

Can block the light or UV (carbon black, CaCO3, TiO2)
Can retransmit the light/UV.
Benzophenone absorbs UV then transmits the energy as heat

Question 15

Additives- flame retardants

Answer 15

Reduce ignition, control burning, reduce smoke evolution.
Halogenated compounds
Aluminium trihydroxide

Question 16

Additives- anti-static

Answer 16

Reduce tendency of polymer to retain electrical charge. Provide a conductive path away from the surface.
Glycerol monostearate (GMS)
Carbon black

Question 17

Additives- processing: curing and cross-linking

Answer 17

Reacting monomers to form polymers or linking polymer chains together. Example is dicumyl peroxide which initiates and cures unsaturated polyester and cross-links polyethylene

Question 18

Additives- processing: coupling/compatibilising

Answer 18

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)

Question 19

Additives- processing: plasticising

Answer 19

Enhancing flexibility during processing and/or in use.
Dioctyl phthalate (DOP) for PVC
Benzoate esters safer alternative

Question 20

Additives- processing: blowing agents

Answer 20

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

Question 21

Additives- toughening

Answer 21

Increase polymer’s ability to absorb energy.
Rubber or thermoplastic particles
Copolymerise with a tougher polymer (ABS)

Question 22

Additives- anti-bacterial and biocides

Answer 22

Resistant to attack by microorganisms.

Organic copper compounds

Question 23

What are miscible and immiscible polymers?

Answer 23

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

Question 24

Miscibility and Gibbs free energy

Answer 24

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

Question 25

What does compounding of additives into polymers require?

Answer 25

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

Question 26

How is compounding of additives done (2 ways)

Answer 26

Batch mixers for discrete mixing operations

Continuous mixers

Question 27

What is compounding?

Answer 27

Generic term for obtaining a homogeneous blend of polymers or a polymer-additive mixture

Question 28

Steps in compounding

Answer 28

Feeding, mixing, filtering and pelletisation

Question 29

Compounding: feeding

Answer 29

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

Question 30

Compounding: batch mixing

Answer 30

Small scale for particulate solids, doughs or liquid polymers. Heat and shear mix with intermeshing blades

Question 31

Compounding- continuous mixing: single screw extruders

Answer 31

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.

Question 32

Compounding: continuous mixing: twin screw extruders

Answer 32

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

Question 33

Compounding- filtering and pelletisation

Answer 33

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)

Question 34

Thermoplastics processing strategy

Answer 34

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.

Question 35

Thermosets processing strategy

Answer 35

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