Polymers

Question 1

Define and explain the following terms in relation to polymers:

Repeat unit -

Saturated Hydrocarbons -

Double and Triple bonds -

Isomerism -

Free radical polymerization -

Degree of polymerization -

Molecular weight -

Answer 1

Repeat unit - A group of specific elements bonded together that repeatedly appear within a polymer

Saturated Hydrocarbons - Most polymers are made of hydrocarbons( made of carbon and hydrogen), saturated hydrocarbons are when the carbon atoms are singularly bonded to four other atoms.

Double and Triple bonds - These somewhat unstable bonds occur when covalent bonds “share” electrons and will therefore move to more stable structures when possible. double bonding can be found within ethylene and ethene, while triple bonding can be found in Acetylene and ethyne

Isomerism - Two compounds with the same chemical formula can have very different polymer structures, resulting in alternative properties.

Free radical polymerization - When an additional element is polymerized into the polymer. This step is known as initiation and can be used to separate double bonds. Then with this new polymer if additional double bonding polymers are added, it is known as propagation.

Degree of polymerization - DP is the average number of repeat units that occur within a polymer chain

Molecular weight - The mass of a mole of chains. Not all chains in a polymer are of the same length; there is a distribution of molecular weights

This can be used to look for degradation – i.e. high molecular weight chains (long) breaking down to low molecular weight (short) chains

Question 2

What factors of the in-coming raw polymeric material need to be assessed for quality control purposes?

Answer 2

To gain the most comprehensive understanding of the raw material, the following should be assessed:  Flow characterisation under simulated processing conditions  Thermal response  Granule size variation  Residue content, e.g. By ashing or TGA Note that the cost implications of this can be quite significant.

Question 3

Identify the two basic types of polymer and give examples of them

Answer 3

There are two basic types of polymeric materials, (excluding rubbers). These are:

• Thermoplastic polymers are solid at room temperature but become a viscous liquid at elevated temperature. This is a reversible process through cooling and heating without a significant change to the material.

Examples: polyethylene (PE)

polypropylene (PP)

polyamide or nylon (PA)

poly (vinyl chloride) (PVC)

• Thermosetting polymers when initially heated soften and flow for moulding but also undergo a chemical reaction (cross-linking) which hardens the material into a solid. Reheating of the material causes degradation and char. This process is irreversible

Examples: epoxy

polyester (thermoset)

phenolic

Question 4

State some common uses of polymers

Answer 4

• Household furnishings

Carpets, curtains and wallpaper

• Electrical Fittings

Wire Insulation, casings for electrical goods, printed circuit boards

• Household Fittings

Drain pipes, kitchen fittings, window frames, mastics

• Surgical Prostheses

Implants, tooth fillings, contact lenses

• Transport

Bicycles, cars, trains, planes, space-craft

• Sports Materials

Clothing, shoes, athletic tracks

• Domestic

Utensils, containers, non-stick coatings

Question 5

What properties would make up an ideal Engineering polymer

Answer 5

• Low cost and low density-polymers are bought by weight and sold by volume and if the material has a high density then more of it will be required to make a moulded component;
• Easy processing, low mould shrinkage and dimensional stability in use – this is so that components may be made quickly and accurately, and so that components will retain their dimensions in service;
• Transparency – if a material is transparent then the colour range is only limited by the thermal stability of the colourant system;
• High strength, stiffness and impact strength – these are obviously useful in metal replacement applications.
• Fatigue and creep resistance – a major limitation of many polymers is that they will creep, or deform, under continuous loads (which need not be very large);
• Resistance to changes in temperature – what limits the use of many polymers is that components distort, or change shape, on heating. An EP should have good resistance to thermal degradation and to deformation; it should also not become brittle when the temperature is lowered;
• Flame resistance – a major problem with many polymers is their ease of burning and the degradation products produced on burning (e.g. smoke, burning drops etc.);
• Wear resistance – this is important because of the application areas of many EP components e.g. in bearing applications.

Question 6

State the advantages and disadvantages of thermosetting and thermoplastic polymers

Answer 6

Thermosets

FOR

• Cold cure resins simplify processing
• Low-pressure moulding means cheaper tooling
• Contact moulding suitable for large mouldings and low-volume production
• Good temperature and fire resistance

AGAINST

• User must control chemical reactions and cure process
• Liquid resins have limited shelf-life
• Health hazards from resin handling
• Recycling not easy
• Resins can be brittle, giving composites low toughness

Thermoplastics

FOR

• Can be processed quickly by hot pressing or injection moulding
• Minimal knowledge of resin chemistry needed
• Available as solid pellets which are safe to handle with a long shelf-life
• Ductility gives tougher composite materials
• Waste can be recycled
• Good environmental resistance

AGAINST

• High-temperature, high pressure moulding requires expensive tooling and sensitive controls
• Expensive tooling is only cost effective for high-volume production
• Resins soften and may burn at high temperature
• Temperature and chemical resistance varies widely

Question 7

Identify the chemical composition, graphical structure and boiling point of the following compounds:

Methane

Ethane

Propane

Answer 7

Question 8

Draw the repeat unit for:

Polyethylene (PE)

Answer 8

Question 9

Draw the repeat unit for:

Poly(vinyl chloride) (PVC)

Answer 9

Question 10

Draw the repeat unit for:

Polytetrefluoroethylene (PTFE)

Answer 10

Question 11

Draw the repeat unit for:

Polyproylene (PP)

Answer 11

Question 12

Draw the repeat unit for:

Polystyrene (PS)

Answer 12

Question 13

Draw the repeat unit for:

Poly(methyl methacrylate) (PMMA)

Answer 13

Question 14

Draw the repeat unit for:

Phenol-formaldehyde (Bakelite)

Answer 14

Question 15

Draw the repeat unit for:

Poly(hexamethylene adipamide) (nylon6,6)

Answer 15

Question 16

Draw the repeat unit for:

Poly(ethylene terephthalate) (PET, a polyester)

Answer 16

Question 17

Draw the repeat unit for:

Polycarbonate (PC)

Answer 17

Question 18

Polymers can have many different molecular structures and bonding that affect their properties. Discuss the four common forms of polymer chains.

Answer 18

Linear polymers - Where the polymer chains are arranged in a linear fashion. These can be tangled (like a bowl of spaghetti) or they can be ordered (like building blocks) e.g. HDPE

Linear branched polymers - Where side reaction during polymerization cause branches to form reducing the density of the polymer e.g. LDPE

Network arrangement occurs where a trifunctional mer unit is polymerized. These form thermosetting plastics.

Cross-linking occurs in rubbers (known as vulcanisation)

Question 19

Define copolymers and identify their forms

Answer 19

Question 20

What are the effects of crystallinity on polymers

Answer 20

• The more crystalline the polymer, the stiffer, harder, less ductile, denser and less rubbery the polymer is. They are also more resistant to solvents and heat.
• Increase in density is due to the more ordered packing in a highly crystalline structure.
• Optical properties are also affected by the degree of crystallinity. Polystyrene is amorphous and clear; Polypropylene is crystalline and cloudy.
• Secondary bonding (hydrogen or polar bonds) between polymer chains, or large side groups, stop the chains slipping past each other and this increase strength.

Question 21

Identify the four commodity plastics commonly used for the creation of everyday products

Answer 21

1. Polyethylene (PE)
2. Polypropylene (PP)
3. Polyvinyl Chloride (PVC or Vinyl)
4. Polystyrene (PS) – General Purpose Polystyrene – also includes Styrofoam or expanded polystyrene [EPS])

Question 22

Name the polymer associated with the following symbols

Answer 22

1. PET = Polyethylene Terephthalate
2. High-density Polyethylene
3. Polyvinyl chloride
4. Low-density Polyethylene
5. Polypropylene
6. Polystyrene
7. Other plastics

Question 23

Discuss the use and properties of:

Polethylene

Answer 23

Price – HDPE is one of the cheapest polymers in terms of cost.

Mechanical Properties – HDPE is generally considered to have poor mechanical properties with the exception of impact strength.

Chemical Resistance – HDPE has excellent chemical resistance. It is one of the most chemically resistant thermoplastic resins. This is the reason it is commonly used in bottles for detergents and other household chemicals. Note: it is flammable.

Heat Resistance– Tg -135°C, max operating temp 90°C, melt 140°C, processing ~170°C, degradation ~350°C (Tg covered later in the lecture)

Optical - Translucent and poor weathering resistance

Question 24

Discuss the use and properties of:

Polypropylene

Answer 24

Better mechanical properties than PE, Tg -18°C, max operating temperature 125°C, flammable, translucent, semi-crystalline, good chemical resistance, BUT poor weather resistance.

Used for plastic chairs, plastic bottles and caps, reuseable containers and rope.

Question 25

Discuss the use and properties of: Polystyrene

Answer 25

Structure- The large benzene ring structure adds rigidity to the polymer chain. Clear, hard brittle, amorphous. Chemical resistance- Poor compared to PE and PP. PS is flammable. Temperature resistance- Tg 95°C, max operating 90°C, melt ~240°C Used for packaging reinforcement, plastic cutlery and yogurt pots

Question 26

Discuss the use and properties of: Polyvinyl chloride (PVC)

Answer 26

Forms – PVC comes in a wide variety of forms: •Plasticized PVC (PPVC) •Rigid PVC (uPVC) •Semi-Rigid PVC The mechanical properties and hardness of PVC are frequently modified with the use of plasticizers. Flammability- uPVC has a very low natural combustibility and is usually self-extinguishing. This is a very advantageous property to have in buildings and homes. Electrical Properties- PPVC has very poor electrical properties making it very useful for wire insulation among many other uses. Good mechanical Properties- twice the stiffness of PE. Weatherability- PVC has a good weather resistance which makes it great for many outdoor uses like pipes and windows. Temperature resistance - Tg 80°C, max operating 125°C, processing 180°C, at high temp is susceptible to degradation (HCI released).

Question 27

Discuss the use and properties of Nylons (Polyamides) (PA)

Answer 27

* Higher crystallinity * Sharp melt-point * Strength (\>95 MPa) * Problem: Absorbs water Used in tents, parachutes and plastic gears

Question 28

Discuss the use and properties of polycarbonate

Answer 28

* Solvent sensitivity: Poor (but this is nice for joining) * Clear: Except for UV yellowing, slight crystallinity * Hard but still quite .. * Ductile: Nailed, sawed, drawn, punched, sheared, drilled * Tough: Helmets, light covers, windows, roadside signs, bulletproof shields * Dimensional stability: Low creep * Electrical resistance: Good Used for safety goggles, helmets, sunglasses and DVD's

Question 29

Discuss the use and properties flouropolymers (PTFE)

Answer 29

•PTFE strengthened by the tight bond between the fluorine and the carbon atoms (F is very electronegative) –Slippery (anti-stick surfaces) –Chemical inertness –High temperature melting (330oC) –Non-flammable –High electrical resistance –Very dense and has a high melt viscosity Used for frying pans and wire coatings

Question 30

Identify the following thermo-analytical methods and what values are acquired from them: Differential scanning calorimetry (DCS)

Answer 30

• Differential scanning calorimetry (DSC) - thermal transitions (e.g. glass transition temperature (Tg), crystallization temp (Tc) and melt temp (Tm)). DSC measures the difference in the amount of heat required (heat capacity) to increase the temperature of a sample vs a reference as a function of temperature. When the sample undergoes a physical transformation (such as a phase transition), more or less heat capacity than in the reference to maintain both at the same temperature. For example, as a solid sample melts to a liquid it requires heat. Therefore the heat capacity of the sample vs the reference will increase. If the sample undergoes an exothermic process (such as crystallization) less heat is required to raise the sample temperature to that of the reference. •The liquid-glass transition (or glass transition) is a reversible phase transition in amorphous materials from a hard and relatively brittle state into a molten or rubber-like state. (Can also happen in materials with crystalline and amorphous content, but only the amorphous content is changing.)

Question 31

Identify the following thermo-analytical methods and what values are acquired from them: Thermogravimetric analysis (TGA)

Answer 31

TGA is performed to determine changes in weight with respect to change in temp. • Degradation temperatures, • Absorbed moisture content of materials, • Level of inorganic and organic components, • Decomposition points of solvent residues.

Question 32

Identify the following thermo-analytical methods and what values are acquired from them: Dynamic mechanical analysis (DMA) - stiffness

Answer 32

Dynamic Mechanical Analysis determines elastic or storage modulus (E'), viscous modulus (E'') and damping coefficient (Tan δ) as a function of temperature. DMA identifies transition regions in plastics, such as the glass transition, and may be used for quality control or product development.

Question 33

What is the purpose of the melt flow index (MFI)

Answer 33

Defined as the mass of polymer, in grams, flowing in ten minutes through a capillary of a specific diameter under a fixed pressure and at a specific temperature. We measure g/10 min. Thermoplastic exposed to different conditions to melt (e.g. 5 min at 190oC for PE; 6 min at 230°C for PP). Then a piston extrudes the molten polymer at a fixed pressure through a die capillary. Weight (g) of extruded polymer measured after 10 min.

Question 34

Describe giving examples, what factors affect the inherent viscosity of a polymeric material.

Answer 34

* Molecular Architecture * Molecular chain length * Molecular weight distribution * Additive system employed

Question 35

Describe TWO methods for the determination of flow behaviour in a polymer. What advantages and disadvantages do these methods have in a polymer manufacturing environment?

Answer 35

1) Capillary Rheometer The basic principle is that a thermoplastic sample (originally in the shape of granules, powder or flakes) is made fluid by heating and forced to flow out of a cylinder through a capillary die. The measured quantity is normally the generated pressure under steady state conditions. A flow curve is the typical output, obtained by interpolation of several experimental data. 2) Melt Flow Indexer It is defined as the mass of polymer, in grams, flowing in ten minutes through a capillary of a specific diameter and length by a pressure applied via prescribed alternative gravimetric weights for alternative prescribed temperatures

Question 36

Identify the different methods polymers are recycled

Answer 36

* Process Scrap – almost 100%, one material, no contamination, no transport, possible degradation * Post consumer, separation, different grades, contamination, transport, food contact, colour * Chemical recycling e.g. PET back to purified terephthalic acid (PTA) not currently commercially viable

Question 37

Discuss the issues with polymer recycling

Answer 37

Contamination: Most recovered plastic bales only allow a 2% contamination. In particular paper labels are a big source of contamination on plastic film since there is no automated process to remove them. Quantity / Weight / Volume: Containers to hold a ton of plastic are huge! This creates a logistical challenge to collect and process the material and increases recycling cost. Sorting: Plastics that have been sorted into one specific grade are more valuable than a mixture of different grades of plastic. Most polymers simply cannot be mixed in the recycling/manufacturing process and are typically used in low-grade products.

Question 38

Why should we recycle polymers and what solutions would improve polymer recycling

Answer 38

Environmental benefits of using recycled polyethylene include: * Reduction of energy consumption by two-thirds * Production of only a third of sulphur dioxide and half of nitrous oxide * Reduction of water used by nearly 90% * Reduction of carbon dioxide generation by two and a half times Only around 10% of plastic waste is currently recycled! Solutions: * Standardise production: reduce no. of materials, additives, packaging, labels * New technology for cleaning, sorting, recycling * New product designs and methodologies for use of mixed materials * Viable recycling scenarios for composites

Question 39

Identify three forms of polymer processing

Answer 39

Typical processing routes are : –Injection Moulding –Extrusion –Blow Moulding