S2: 1/2 Polymers, Ceramics, and Processing

Flashcards covering material classification, polymer processing methods, molecular weight calculations, ceramic powder processing, and fibre spinning technologies.

What is the definition of thermoplastics?

A class of polymers that become pliable or mouldable above a specific temperature and return to a solid state upon cooling.

List five common types of thermoplastics and provide one common use for each.

1. Polyethylene (PE): Packaging, bottles, toys. 2. Polypropylene (PP): Automotive parts, textiles, packaging. 3. Polyvinyl Chloride (PVC): Pipes, cable insulation, flooring. 4. Polystyrene (PS): Disposable cutlery, CD cases. 5. Polyethylene Terephthalate (PET): Beverage bottles, clothing fibres.

How do thermosets differ from thermoplastics regarding heat treatment?

Thermosets undergo a chemical change when heated, creating a 3D network that makes the process irreversible; once cured, they cannot be melted or reshaped.

What are elastomers?

Polymers with high elasticity that can be stretched several times their original length and return to their original shape when released, such as natural rubber or silicone elastomers.

What is the primary difference between raw materials and starting materials in the manufacturing life cycle?

Raw materials are taken directly from natural sources (e.g., ores, oil) and must undergo mechanical and chemical processes to become starting materials (e.g., metal slabs, ceramic powders, polymer pellets) which are ready to be formed into shapes.

What does a Life Cycle Assessment (LCA) track?

LCA is used to track products through various stages including material production, manufacturing, use/maintenance, and decommissioning/disposal.

What are the five main steps in producing final polymer products from crude oil?

1. Extraction 2. Refinement 3. Cracking 4. Polymerisation 5. Further processing (e.g., Extrusion, injection moulding).

In polymer processing, what is 'cracking'?

The process of breaking long-chain hydrocarbons into short-chain ones (like converting decane to propylene) using heat or a catalyst.

What specific carbon chain range is identified for Naphtha for plastics?

$$C_5-C_{17}$$

What is the definition of biopolymers?

Polymers made from renewable biomass sources, such as starch or fermentation of corn and sugar cane.

Why is Polylactic Acid (PLA) considered carbon neutral?

Because plants absorb the $CO_2$ emitted by burning or degradation during their growth process, resulting in no net increase in emissions.

Compare Addition and Condensation Polymerisation.

Addition polymerisation involves successive addition of building blocks with no by-products. Condensation polymerisation occurs in a step-wise manner and releases by-products like water.

Define 'Isotactic', 'Syndiotactic', and 'Atactic' in the context of polymer tacticity.

Isotactic: side groups are on the same side. Syndiotactic: side groups are on alternating sides. Atactic: side groups are arranged randomly on both sides.

What does the degree of polymerisation ($DP$) represent?

The average number of repeat units in a polymer chain.

Provide the formula for Number Average Molecular Weight ($M_n$).

$M_n = \frac{\sum_{i=1}^{\infty} N_i M_i}{\sum_{i=1}^{\infty} N_i}$ where $N_i$ is the number of molecules with molecular weight $M_i$.

What is Molecular Weight Distribution (MWD)?

The ratio of weight average molecular weight ($M_w$) to number average molecular weight ($M_n$), expressed as $MWD = \frac{M_w}{M_n}$.

In polymer processing, what are plasticizers used for?

They are primarily used in PVC and other rigid polymers to increase flexibility and workability.

Distinguish between the transitions $T_g$ and $T_m$.

$T_g$ is the glass transition temperature for amorphous regions. $T_m$ is the melting point for crystalline regions.

Name the three main types of intermolecular 'weak bonds' in order of decreasing strength.

1. Hydrogen bonding 2. Dipole-dipole forces 3. London (dispersion) forces.

What is thermal diffusivity and why is it important for polymer processing?

It measures how quickly heat spreads through a material. Since polymers have low thermal diffusivity ($10^{-7}\,m^2/s$), processing requires careful design for heating/cooling and thin layer structures.

Define 'viscous dissipation' in polymer processing.

The irreversible process where work done by a fluid on adjacent layers due to shear forces is transformed into heat (friction-thermal energy).

Name the three functional zones of a single-screw extruder.

1. Solid conveying zone (transport/compaction) 2. Transition zone (melting) 3. Metering zone (pumping/homogenizing).

What is a key difference in the parison (preform) between extrusion blow moulding and injection blow moulding?

In extrusion blow moulding, the parison is a freely suspended molten hollow tube. In injection blow moulding, it is a hollow shape resembling a test tube.

What temperature range is required for the thermoforming of amorphous polymers?

Slightly above the glass transition temperature ($T_g$).

What are two advantages of transfer moulding over compression moulding for thermosets?

It improves temperature distribution during shaping and accelerates polymerisation, making it better for thick or complicated shapes.

List four general properties of ceramics.

1. Brittle 2. High compressive strength 3. Poor electrical/thermal conduction 4. Chemical insensitivity.

Why is powder processing used for ceramics instead of melting?

Ceramics have very high melting points making melt processes inaccessible; powders also allow for minimal waste, controlled porosity, and avoidance of segregation effects.

What is 'comminution' in ceramic processing?

The reduction of particle size by crushing, milling, or grinding (e.g., ball milling).

How is $D_{50}$ defined in powder characterization?

The diameter where half of the particle population lies below that value.

What are the four steps in processing traditional ceramics?

1. Preparation of powders 2. Shaping of wet clay 3. Drying 4. Firing.

Identify four ceramic shaping processes and their associated water content levels.

1. Slip casting ($25\%-40\%$ water) 2. Plastic forming ($15\%-25\%$ water) 3. Semi-dry pressing ($10\%-15\%$ water) 4. Dry pressing ($<5\%$ water).

Name the three fundamental types of synthetic fibre spinning processes.

Melt spinning, Wet spinning, and Dry spinning.

In electrospinning, what is the 'Taylor cone'?

The critical point of eruption where a stream of liquid erupts from the surface of a charged droplet due to electrostatic repulsion counteracting surface tension.

Why is solution (gel) spinning used for Ultra-High Molecular Weight Polyethylene (UHMW-PE)?

Because the high molecular weight makes it too viscous for melt spinning or standard wet spinning processes.