Polymer Processing

Question 1

What are monomers

Answer 1

Monomers are the repeating units that link together to form a polymer chain

Question 2

Recall the diagrammatic representation of Elastomers and Polymers

Answer 2

Polymers: Collection of single chains with side groups
Elastomers: Collection of cross-linked polymer chains, the cross links reduce the effective length of the polymer chains and increase stiffness.

Question 3

Compare and contrast the mould-ability of elastomers and polymers

Answer 3

Elastomers cannot be remoulded as the molecular structure is fixed at the point of polymerisation. Whereas Thermoplastics (polymers) have no cross links and can be remoulded.

Question 4

What are thermoplastics

Answer 4

Thermoplastics are a type of polymer that becomes soft at higher temperature. At 1.5Tg the weak Van Der Waals bonds between polymer chains break down and viscous flow begins.

Question 5

How does crystallinity determine the physical properties of a thermoplastic

Answer 5

• Density: The closer molecular packing of crystalline regions makes them denser than their amorphous equivalent.
• Young’s Modulus: Increases with crystallinity. For highly crystalline polymers the Young’s modulus remains high throughout Tg until the temperature is sufficient to break the Van Der Waals bonds in the crystalline regions.

Question 6

On what factors does polymer crystallisation depend on.

Answer 6

The two main factors that determine the degree of crystallisation are temperature and time. Above the melting temperature, Tm, the polymer is a stable viscous liquid with randomly orientated molecules. Below the melting temperature, Tm, the polymer is unstable and the difference in free energy between the amorphous and crystalline states provides a driving force for crystallisation. Whilst this driving force increases with the degree of undercooling however crystallisation is a diffusional transformation and is dependent on temperature. This gives rise to the C-curves (trade-off).
Below Tg, the VDW forces become stronger and mobility of the chains is restricted this inhibits further crystallisation.

Question 7

What factors give rise to the C curves

Answer 7

The trade off between the undercooling needed for the thermodynamic driving force for crystallisation (Gibbs change more negative) and the temperature fluctuations for diffusion

Question 8

How can we control the degree of crystallisation

Answer 8

1. Molecular structure of monomer - complex molecules with side groups are slow to re-arrange which means the C-curves are pushed to longer time-frames.
   Example: PMMA (Perspex) - difficult to crystallise
2. Controlling the cooling rates - this can be achieved either by reducing the section thickness of the part to avoid slow cooling at the centre which could result in crystallisation. Or, chill the mould directly by using a water-cooled metal mould.

Question 9

Why are polymers a commonly used material is materials manufacturing

Answer 9

Low cost
Low density 
Low melting points
Environmental and corrosion resistance 
Reasonable material properties

Question 10

How does crystallisation occur in polymers

Answer 10

Occurs by the nucleation and growth of lamellar crystallites within the amorphous regions. Radially arranged lamellar crystallites form structures called spherulites.

Question 11

Outline injection moulding

Answer 11

Injection moulding is applied to thermoplastics.

• Heat polymer granules until they form a viscous liquid
• Squeeze polymer through a two-part mould
• When solidification is complete release the mould

Question 12

Why and how do we control cooling rates in Injection Moulding

Answer 12

Cooling rates impact the degree of crystallinity which affects the material properties of our component. High cooling rates can be achieved by using a small section thickness and (avoiding slow cooling in the centre) and chilling the mould (use a metal mould).

Question 13

Outline the features of the TTT diagram for polymers

Answer 13

The c-curves represent the percentage fraction of crystallisation
Above the melting point the polymer is amorphous (stable)
Below the melting point the polymer is unstable with a lower free energy which acts as the driving force for crystallisation
The critical cooling rate is the minimum rate to avoid the onset of crystallisation

Question 14

What is orientation strengthening and draw a stress-strain diagram to illustrate its effect (around Tg)

Answer 14

This is the process of stretching the polymer to align the molecular chains which significantly increases the strength and stiffness.
The stress-strain diagram starts with a linear increase with constant E, until yielding. After yielding orientation strengthening occurs as the molecular chains untangle and begin to align with the loading direction. Once the chains are aligned the stiffness of the polymer rapidly increases (higher than the initial E in the elastic regime).

Question 15

Outline the two manufacturing processes that exploit orientation strengthening

Answer 15

• Fibre drawing

- Blow moulding (Stretch and Film)

Question 16

How does fibre drawing work?

Answer 16

Fibre drawing uses a screw extruder to produce a polymer filament which is drawn out aligning the molecular chains by applying tension. The resulting polymer fibres have high strength and stiffness.

Question 17

Where are drawn polymer fibres used?

Answer 17

• fabrics
• clothing
• ropes
• sails
• parachutes

Question 18

Outline stretch-blow moulding

Answer 18

• Parison (injection moulded preform) is preheated and transferred into a mould
• A stretch rod elongates the preform
• Compressed air inflates the preform
• Mould is separated and component ejected

Question 19

Where is stretch blow moulding used and how is the temperature chosen?

Answer 19

Stretch blow moulding is primarily used for plastic bottle manufacture. In bottle manufacturing the temperature is kept just above T_g to restrict crystallinity so as to not comprise on optical transparency of the bottle. However, some crystallinity is important to improve the mechanical properties of the bottle.

Question 20

Outline the film blowing process

Answer 20

Film blowing is a combination of injection moulding and stretch blow moulding. First the polymer granules are heated to form a viscous liquid which is extruded through a narrow tube. The extruded polymer is then inflated by internal pressure and using a series of guide rolls, wound on a drum.

Question 21

Where is film blowing used and what makes it appropriate?

Answer 21

Film blowing is used in the manufacture of plastic bags and films. The orientation strengthening allows for improved mechanical properties whilst keeping weight low - ideal for packaging and carrier bags.

Question 22

At what temperature of polymer deformation processing techniques occur

Answer 22

Just above the glass transition temperature

Question 23

What are thermosets

Answer 23

Thermosets are similar to elastomers however they have a higher degree of cross-linking with a higher stiffness. The molecular structure is fixed at the point of polymerisation so they can be moulded once only.

Question 24

How do the different stages in stretch-blow moulding affect the stiffness in different directions.

Answer 24

The elongation and inflation states provide different molecular alignment in the hoop-wise and longitudinal directions. We can think of a bottle as a pressure vessel so for an internal pressure p, the hoop stress is twice as large as the longitudinal stress.