Polymer Processing and Recycling - Appunti

POLYMER

PROCESSING and

RECYCLING

SUMMARY

POLYMERS..............................................................................................................................6

CLASSIFICATION............................................................................................................ 11

CHAIN GROWTH POLYMERIZATION.............................................................................11

STEP GROWTH POLYMERIZATION.............................................................................. 12

COPOLYMERIZATION.....................................................................................................13

SEMICRYSTALLINE MATERIALS................................................................................... 14

ADDITIVES............................................................................................................................ 17

FILLERS and REINFORCEMENTS................................................................................. 18

POLYMER DEGRADATION.................................................................................................. 24

THERMAL DEGRADATION EXAMPLE - polyvinylchloride............................................. 26

THERMOGRAVIMETRIC ANALYSIS (TGA).................................................................... 26

THERMO-OXIDATIVE DEGRADATION...........................................................................27

PHOTODEGRADATION...................................................................................................29

STABILIZING ADDITIVES.....................................................................................................30

PRIMARY STABILIZERS................................................................................................. 30

STERICALLY HINDERED PHENOLS........................................................................30

SECONDARY STABILIZERS........................................................................................... 31

CHELATING AGENTS..................................................................................................... 32

DEGRADATION of CHLORINATED POLYMERS............................................................ 32

STABILIZERS for CHLORINATED COMPOUNDS.................................................... 33

PHOTO-OXIDATION STABILIZERS................................................................................ 34

UV ABSORBERS....................................................................................................... 34

SCREENERS............................................................................................................. 35

STERICALLY HINDERED AMINES (HALS).............................................................. 35

FLAME RETARDANTS......................................................................................................... 37

HALOGEN BASED FLAME RETARDANTS.................................................................... 38

PHOSPHOROUS BASED RETARDANTS.......................................................................38

AMMONIUM POLYPHOSPHATE...............................................................................39

EXPANDABLE GRAPHITE.............................................................................................. 39

METAL HYDROXIDE....................................................................................................... 39

OXIDATION INDUCTION TIME (OIT)..............................................................................41

PLASTICIZERS..................................................................................................................... 43

RHEOLOGY...........................................................................................................................47

PSEUDOPLASTIC FLUIDS............................................................................................. 48

DEPENDENCE on MOLECULAR WEIGHT...............................................................49

DEPENDENCE on TEMPERATURE..........................................................................50

DEPENDENCE on PRESSURE.................................................................................50

SUMMARY - DEPENDENCE of APPARENT VISCOSITY.........................................51

VISCOELASTICITY..........................................................................................................53

RHEOMETRY........................................................................................................................ 57

EXTRUSION.......................................................................................................................... 69

SINGLE SCREW EXTRUDER......................................................................................... 69

1

TWIN SCREW EXTRUDER............................................................................................. 90

INJECTION MOLDING........................................................................................................ 100

STAGES......................................................................................................................... 100

PVT DIAGRAMS - VOLUME SHRINKAGE....................................................................108

COOLING TIME............................................................................................................. 112

PROCESSING WINDOW - DEFECTS...........................................................................121

PROCESS VARIANTS................................................................................................... 122

EFFECT of MOISTURE................................................................................................. 123

BLOW MOLDING................................................................................................................ 125

INJECTION STRETCH BLOW MOLDING..................................................................... 127

THERMOSETS.................................................................................................................... 132

POLYMER RECYCLING......................................................................................................135

POLYMER WASTE SORTING.............................................................................................138

MACROSORTING..........................................................................................................139

MICROSORTING........................................................................................................... 144

DRY FLOATATION................................................................................................... 144

WET FLOATATION...................................................................................................144

ELECTROSTATIC SEPARATION.............................................................................147

SEPARATION of FERROUS METALS..................................................................... 148

SEPARATION of NON-FERROUS MATERIALS...................................................... 149

PET RECYCLING................................................................................................................ 151

INCREASE the INTRINSIC VISCOSITY........................................................................155

MELT REPROCESSING: DEGASSING...................................................................156

MELT REPROCESSING: CHAIN EXTENDERS...................................................... 156

SOLID STATE POLYMERIZATION.......................................................................... 156

POLYOLEFINS RECYCLING.............................................................................................. 161

HDPE RECYCLING....................................................................................................... 161

LDPE RECYCLING........................................................................................................ 163

POLYPROPYLENE (PP) RECYCLING..........................................................................164

MULTIPLE CHOICE QUESTIONS...................................................................................... 165

QUESTIONS........................................................................................................................ 179

N.B.: italic titles represent topics which won’t be in the final exam (anyway, you must

verify this statement by yourself at the end of the course when the professor sends the list

of the subjects). 2

CIRCULAR ECONOMY

Circular economy is a general term to define an economic system that is restorative and

regenerative by design. Basically, it is a zero-waste economy where every product is

consumed and disposed without leaving a trace.

CIRCULARITY of PLASTICS

The circular economy of plastics is a model for a closed system that promotes the reuse of

plastic products, generates value from waste and avoids sending recoverable plastic to

landfills. Plastics waste can be used for:

●​ production of new raw material of polymer synthesis

●​ production of plastic parts and products

●​ generate energy whenever recycle is not feasible

ECODESIGN

Ecodesign is a design approach based on the efficient employment of materials and

resources, which allows to reduce both the environmental impact due to the production

stage and the amount of waste produced, operating on products durability and recyclability.

According to this principle it is necessary to consider the product disassembling and recycle

at the end of its useful life, already during the design stage.

BIOPLASTICS

Bioplastics are a large family of different materials which are biobased, biodegradable or

both.

Bio-based means that the material is derived from biomass (e.g. corn, cellulose ecc) and

therefore the opposite origin with respect to fossil based materials.

Biodegradable instead means that the material is largely susceptible to hydrolysis

(remember that also some fossil based materials are biodegradable and there are bio-based

materials which are not biodegradable).

Usually the production of bioplastics have higher production costs than the fossil

counterparts since:

●​ they are produced with less consolidated production processes

●​ they have a reduced production scale (higher weight of the fixed costs)

BIOBASED FEEDSTOCKS

Biomass can be divided into 4 generations:

●​ First generation

composed by starch and oils

➢​ used to produce biodiesel or bioethanol

➢​ advantages:

➢​ ■​ cost efficient since they are easily processed

disadvantages:

➢​ ■​ in conflict with food

■​ large land use

■​ we can no more use this type of biomass because of regulations 3

●​ Second generation

composed by lignocellulosic feedstocks or non-edible oils (waste oils)

➢​ not be suitable for human consumption

➢​ grow on small land

➢​ not require large amount of fertilizers

➢​

➢​ advantages:

➢​ st

■​ more sustainable than 1 generation

disadvantages:

➢​ ■​ difficult to process (higher costs)

■​ lignin is a waste

used to produce bioethanol, syngas or bio oils

➢​

●​ Third generation

obtained by microalgae [microalghe]

➢​ not used on industrial scale

➢​ used to produce biodiesel

➢​ advantages:

➢​ ■​ small land use

disadvantages:

➢​ ■​ higher costs

●​ Fourth generation

really similar to third generation

➢​ obtained by modified microorganisms

➢​ 4

POLYMERS

A polymer is a macromolecular material, with high molecular weight, constituted by many

repetitive units (monomers).

Polymers do not have a gaseous state, in fact by increasing the temperature we would see

the breaking of the polymers in much lighter parts that could or not evaporate.

More commonly, by increasing the temperature a polymer will find a molten state

(viscoelastic liquid because it has a hybrid behaviour between a liquid and an elastic

material), which is also the state where the polymer processing is operated.

At low temperatures, polymers can appear as:

●​ glassy solid

○​ disordered structure

○​ amorphous polymers

●​ semi-crystalline solid

○​ ordered structures together with amorphous portions

PROPERTIES of the LONG CHAIN

If a material is composed by long chain structures it will probably have:

●​ chain entanglements that reduces chain mobility

●​ secondary physical interactions like dispersion forces or dipole interactions that

could be relevant in such long chains (the higher the MW the higher the physical

interactions) if the macromolecules are very close to each other (low steric

hindrance). These interactions are important for:

○​ melting point (the point at which these forces are overcome by heat)

○​ solubility of thermoplastics

We have a list of polymers from the one with the lowest to the highest interaction

strength:

○​ hydrocarbon based polymers (weak dispersion forces)

○​ polar polymers (dipole interactions usually with heteroatoms)

○​ polymers with hydrogen bonds

○​ ionomers (Coulomb energy)

●​ glass transition temperature T → the mobility of the chain is dependent from

G

temperature in fact we would have:

○​ glass like behaviour if the temperature is lower than T (T < T )

G G

○​ rubber like behaviour for temperature higher than T (T > T )

G G

●​ Viscoelastic properties → some polymers combine property of liquids like viscosity

with properties of solid materials like elasticity

●​ Semi-crystalline properties → some polymers combine features of amorphous and

crystalline materials

IONOMERIC POLYETHYLENE

An ionomer is a thermoplastic copolymer that contains few ions.

The apolar chains group together while ionic polar groups attract each other, forming clusters

of ionic heads among non-polar structures → ionomers are physically crosslinked. 5

The difference between chemical crosslinked polymers is their reversibility (applying heat

the ionic groups are broken down).

MOLECULAR WEIGHT

Polymers are macromolecules that can have different lengths so they are polydispersed

(have a molecular weight distribution) that will determine the properties of the material

during application and processing.

Usually we need to find a compromise between the mechanical properties of the material

and its viscosity, since the higher the viscosity and the more difficult the processing will be.

The number average molecular weight is the arithmetic mean of the MW weighted on the

Σ Σ

ℎ

number of molecules:

= = = =

° Σ Σ

where N is the number of molecules that have a molecular weight M .

i i

The weight average molecular weight consider also the fact that longer molecules weigh

2

Σ

more on the overall MW and it is

= Σ

The polydispersity index is .

= > 1

The polydispersity index is important for the designing of a polymer with particular

characteristics. Generally: 6

AMORPHOUS and SEMI

CRYSTALLINE POLYMERS

Semicrystalline materials are composed by:

●​ a crystalline fraction, built in lamellae (chains that folds together with a prevalent

direction)

●​ an amorphous fraction, characterized by disordered chains that are present in

between lamellae

Usually lamellaes and amorphous parts create a more complex structure called spherulite.

Thermoplastics at the solid state could be:

●​ (b) semi-crystalline polymers that have a glass transition temperature and a melting

point

○​ usually ∼ 0, 6 ·

○​ lower specific volumes or higher density

○​ the crystalline part will melt at T (constant temperature process) while the

m

amorphous part starts to transition at T and the process is not isothermal

g

●​ (a) amorphous polymers which usually have a lower density. They act as:

○​ glass at T<T G

○​ rubber at T>T G 7

Weight crystallinity degree: 1 1

−

ρ ρ =

1 1

−

ρ ρ

Volumetric crystallinity degree: ρ−ρ

= Φ

ρ −ρ

GLASS TRANSITION TEMPERATURE

The glass transition temperature is the starting point of the chain mobility, when there is

sufficient thermal energy to facilitate the breaking of intermolecular interactions and

sufficient free volume to promote the movement of segments.

This basically means that the material loses rigidity at the T but the macromolecules center

g

of gravity prevents flow phenomena.

Usually a polymer has low T if:

G

●​ it has weak chain interactions

●​ its chains can easily rotate

●​ it has abundant free volumes

The T is affected by:

g

●​ molecular weight:

○​ for low MW, T increases rapidly as we increase the chain length

g

○​ at higher MW it approaches an asymptotic level

●​ chain stiffness: higher stiffness (usually given by aromatic groups) means higher T

g

because the movement of chains is more difficult

●​ presence of crosslinks increase T because they lead to have small free volume

g

●​ pendant highly steric groups reduce chain mobility, so their presence will increase

T g

●​ crystallinity: the more crystalline is a polymer the higher will be T because there is

g

less free volume

●​ plasticizers decrease T since they “block” the interactions between chains

g

●​ random copolymers and blend: their T is an average of the different T of the

g g

1

components → 1 2

= +

,1 ,2

8

CLASSIFICATION

THERMOPLASTICS

Thermoplastics are those polymers that have linear or branched chains which are bounded

with chain entanglements or secondary interaction but not chemical bonds.

They can melt and are usually soluble so they can be processed with standard technologies.

They differentiate in:

●​ se