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MEC207_Materials_Processing > Polymer_1 > Flashcards

Polymer_1 Flashcards

Introduction to polymers Classification and selection Common thermoplastics Flow properties

1
Q

What does the Ashby Diagram plot?

A

Young’s Modulus against Density.

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2
Q

What does Polymer stand for?

A

Poly = Many; Meros = Parts

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3
Q

How are Polymers & Plastics made?

A

By synthesising polymerising monomers.

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4
Q

What are Polymers made from?

A

Long chain organic molecules. Mainly Hydrocarbons with additional atoms such as Oxygen, Nitrogen and Sulphur.

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5
Q

What is the molecular weight range for Polymers and Plastics? State if it is high or low.

A

10^4 - 10^6. High.

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6
Q

Do Polymers have high or low density?

A

Low.

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7
Q

What is the definition of a Plastic?

A

Any synthetic organic solid that is mouldable. This includes polymers, terms used interchangeably.

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8
Q

What is the difference between Polymers and Plastics?

A

Polymers are more specific and Plastics are more general.

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9
Q

What is the less rigorous definition of a Plastic?

A

A Plastic is one or more Polymer containing one or more additives. Plastic = Polymer(s) + Additive(s)

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10
Q

What is a Homopolymer?

A

Homopolymer: AAAAAAAAAAA. A Polymer consisting of identical monomer units.

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11
Q

What is a Copolymer?

A

Copolymer: ABABABABA, etc. A Polymer consisting of two different monomer units.

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12
Q

What is a Isotactic polymer?

A

A polymer in which all repeating units have the same stereochemical configuration.

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13
Q

What is a Atactic polymer?

A

A polymer in which the repeating units have no regular stereochemical configuration.

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14
Q

What are the difference s between Thermoplastic Polymers and Thermoset Polymers?

A

Thermoplastics polymers can be melts and recycled, whereas Thermoset polymers can not melt and will only degrade at high temperatures.

The structure of Thermoplastics polymers consists of Linear, Branched and some crosslinks. Thermoset polymers consists of mostly crosslinked, 3D networks which will not melt.

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15
Q

What are Elastomers?

A

A special case between Thermoplastics and Thermoset polymers which have a reversible network. E.g. Natural rubber (polyisoprene) or nitrile rubber (copolymer of butadiene and acrylonitrile)

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16
Q

How can Polymers be physically classed as?

A

Polymers can be amorphous (non-crystalline) or semi-crystalline. Both types are quite common. They are dependent on processing & structure.

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17
Q

What do Polymers consist of in terms of phases?

A

Polymers have both primary and secondary phase transitions.
Primary: Melt transition at Tm.
Secondary: Glass transition at Tg.
Critically important to know Tm and Tg for effective processing.

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18
Q

Name the strong chemical bonds between atoms in Polymers. Also give the approximate bonding strength value for these bonds.

A

Covalent bonding along the chain and covalent bonding between chains - crosslinking.
The strong covalent bonds have ~350 kj/mol.

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19
Q

Name the weak physical interaction between molecules. Give the approximate bonding strength value for these bonds.

A

vad der Waal’s interaction between chains. Hydrogen Bonding. Ion-ion interactions.
The weak covalent bonds have ~3-10 kj/mol.

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20
Q

Describe the General selection criteria.

A

Tolerance & Dimensional stability.
Weight & Volume.
Service Life.
Gas and liquid permeability.

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21
Q

Describe the Mechanical selection criteria.

A

Stiffness.
Cyclic loading.
Deflection.
Impact resistance.

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22
Q

Describe the Thermal selection criteria.

A

Operating temperatures

Maximum and minimum service temperatures.

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23
Q

Describe the Environmental selection criteria.

A

Chemical attack.
UV exposure & oxidation.
Erosion.
Flora & Fauna.

24
Q

Describe the Electrical selection criteria.

A

Resistivity.

Antistatic.

25
Q

Describe the Hazards selection criteria.

A

Flammability.

Toxicity.

26
Q

Describe the Appearance selection criteria.

A

Transparency.

Surface finish.

27
Q

Describe the Manufacture selection criteria.

A

Processing route.

28
Q

Describe the Economics selection criteria.

A

Cost of materials.
Cost of processing.
Speed.
Energy.

29
Q

Name the seven Thermoplasic polymers that can be recycled and their numbers.

A
  1. Polyethylene Terephthalate (PET)
  2. High Density Polyethylene (HDPE)
  3. Polyvinyl Chloride (PVC)
  4. Low Density Polyethylene (LDPE)
  5. Polypropylene (PP)
  6. Polystyrene (PS)
  7. Others (Polycarbonates, ABS, Nylons, Polyurethanes, PMMA etc.)
30
Q

Name the ‘Commodity Polymers’.

A

Polyethylene (PE) [HDPE and LDPE].
Polypropylene (PP).
Polyvinyl Chloride (PVC).
Polystyrene (PS).

Note: Use there if you can.

31
Q

Name the ‘Engineering Polymers’.

A

Polyesters [Polyethylene Terephthalate (PET) etc.]
Polycarbonate (PC).
Acrylonitrile Butadiene Styrene (ABS).
Polyamides (PA) [Nylon 6, Nylon 6,6 etc.]

32
Q

Name the ‘Speciality Polymers’.

A

Polyurethanes (PUR).
Polytetrafluoroethylene (PTFE).
Polymethylmethacrylate (PMMA).

33
Q

What are High Density Polyethylene Polymers (HDPE)?

A

Same repeating unit as LDPE.

Semi-rigid, tough, cheap, moisture/chemical resistant, gas impermeable.

Packaging, bottle caps, containers, pipes etc.

34
Q

What are Low Density Polyethylene Polymers (LDPE)?

A

Same repeating unit as HDPE.

Flexible, tough, cheap, moisture/chemical resistant, gas impermeable.

Food bags, squeezy bottles, films etc.

35
Q

What are Polypropylene Polymers (PP)?

A

Rigid, opaque, cheap, moisture/chemical resistant, good melting point.

Can be brittle so may need toughening.

Packaging, containers, labelling, pipes, textiles and fibres, laboratory equipment, automotive parts, reusable items etc.

36
Q

What are Polyvinyl Chloride Polymers (PVC)?

A

Unplasticised (UPVC) is rigid.

Plasticised is more flexible.

Window and door frames, pipes, gutters, credit cards etc. (For Unplasticised Polyvinyl Chloride)

Wire coatings, shoes, inflatables, bouncy balls etc. (For Plasticised Polyvinyl Chloride)

37
Q

What are Polystyrene Polymers (PS)?

A

Rigid, transparent, cheap, chemically resistant.

Brittle so often needs toughening.

Rigid packaging, disposable cups & yoghurt pots etc.

Most famous as an expanded foam: Expanded Polystyrene (EPS)

38
Q

What are Polyethylene Terephthalate Polymers (PET)?

A

Rigid, thermally stable, gas & liquid impermeable.

Bottles (drinks, oils, chemicals) packaging films etc.

39
Q

What are Polycarbonate Polymers (PC)?

A

Rigid, transparent.

Health concerns?

Safety glass, compact discs, windows, medical devices, large water bottles etc.

40
Q

What are Acrylonitrile Butadiene Styrene Polymers (ABS)?

A

Co-polymer which adds toughness to brittle polystyrene.

Lego, car dashboards, keyboards etc.

41
Q

Give the Tensile stiffness (Young’s Modulus) for all Thermoplastic Polymers in GPa.

A 
LDPE - 0.3
HDPE - 1
PP - 2
PS - 2.5
PVC - 2.5
PET - 4
42
Q

Give the Tensile strength (UTS) for all Thermoplastic polymers in MPa.

A 
LDPE - 5
HDPE - 15
PP - 25
PS - 30
PVC - 20
PET - 80
43
Q

During the processing of Thermoplastic polymers, what factors affect the outcome?

A

Temperature, pressure, strain rate etc.

44
Q

What is the processing of polymers dominated by?

A

Flow properties of the melt. Polymer melts are shear-thinning fluids.

45
Q

Name the three types of flow that are examined.

A

Bulk, Elongational and Shear Flow.

46
Q

What value of the Bulk Modulus (K) for polymer melts is often around?

A

~1 GPa

47
Q

Define the Bulk Flow.

A

It relates the applied hydostatic pressure (P) to the volumetric strain (∆V/V) as below.

K = P/(∆V/V)

48
Q

An example for Bulk Flow calculation.

In injection moulding the pressure applied to the melt is often on the order of 10^3 atm. What is the volumetric strain experienced by the melt?

A

∆V/V = P/K = (10^3*1.01x10^5)/10^9 = 0.101

So, the melt contracts by ~10% under this pressure and therefore as mould filled at 1000 atm will contain ~10% more polymer than one filled at atmospheric pressure.

49
Q

What does the Elongational Flow describe?

A

Combined with Shear Flow, the Elongational Flow decribes the flow of polymers during processing.

50
Q

Consider a polymer melt being pulled through a tube at a constant temperature.

A force (F) is required to pull the melt with varying cross-sectional area (A) creating a tensile stress (𝜎𝑡) in the melt. Which equation does this form?

A

One of the Elongational Flow equation.

𝜎𝑡 = F/A

51
Q

The velocity (v) at which a polymer melt moves varies creating a constant what?

A

Volumetric flow rate.

Q=vA

52
Q

The Elongational viscosity (𝜆) can be defined in terms of what? Also state the equation.

A

The Elongational viscosity (𝜆) can be define in terms of the applied force and either the velocity or cross-sectional area gradient in the z-direction.

𝜆 = (F/A)/(dv/dz)
or
𝜆 = (F/v)/(dA/dz)

In this way, the viscosity can be determined by measuring the force on an area and velocity of the polymer melt.

53
Q

An example of Elongational Flow calculation.

Determine the viscosity of a polymer melt being extruded through a die with a force of 12 N creating an extrudate moving at a velocity gradient of 6 s-1 with a cross-sectional area of 4x10^-6 m^2.

A

𝜆 = (F/A)/(dv/dz) = (12/(4x10^-6)/6 = 5x10^5 Pa. s

For the same polymer melt, the force required to maintain a velocity of 1 ms-1 when the cross-sectional area gradient is 6x10-6 m is

𝜆 = (F/v)/(dA/dz)

F = (𝜆/v)(dA/dz) = (5x10^5/1)6x10^-6 = 3N

54
Q

What will the response to an applied tensile stress in a polymer will result in?

A

It will result in either a Newtonian or non-Newtonian behaviour.

Typically, stresses lower than 10^3 Pa result in Newtonian behaviour; stresses higher than 10^3 result in non-Newtonian behaviour.

55
Q

State the Polymer melt shear stress (𝜏) equation for a Newtonian fluid.

A

𝜏 = 𝜇 ሶ 𝛾 where

𝜏 - Shear Stress
𝜇 - Shear viscosity
ሶ 𝛾 - Shear Strain rate

56
Q

For a non-Newtonian fluid, the polymer melts can be treated as ‘power law fluids’ instead. State the alternative Shear stress equation and when it applies.

A

𝜏 = C (ሶ 𝛾)^𝑛 where

n and C are constants
ሶ 𝛾 - Shear strain rate

In non-Newtonian melts n < 1 and in Newtonian melts n = 1 where C=𝜇.
This non-Newtonian characteristic of polymer melts is know as pseudoplasticity or shear-thinning and is very advantageous in their processing.

57
Q

What is the shear stress (𝜏w)equation for a polymer melt flowing down a capillary of radius 𝑅 and length 𝐿 with a pressure gradient along the capillary (dP/dZ)?

A

𝜏𝑤 = R/2 * dP/dz

Scaling up, for Newtonian and non-Newtonian liquids flowing in a circular pipe, the following expression holds

dP/dz = -dP/dL

MEC207_Materials_Processing (22 decks)

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