Topic 3 Polymers

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Topic 3

  • Polymers

  • Materials Technology

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Topic 3 Polymers

3.1 Overview of Polymers

3.2 Molecular Structures and Properties of Polymers

3.3 Additives to Polymers

3.4 Common Polymers and Applications

3.5 Processing of polymers

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Topic 3 Polymers

3.1 Overview of Polymers

3.2 Molecular Structures and Properties of Polymers

3.3 Additives to Polymers

3.4 Common Polymers and Applications

3.5 Processing of polymers

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What is a Polymer?

  • The term "Polymer" originates from Greek.

    • "Poly" means "many"

    • "Mer" means "a unit"

  • Therefore, polymer refers to a collection of many units (macromolecule).

  • In material sciences, polymers are long-chained molecules or macromolecules.

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Overview of Polymers

  • Polymers are macromolecules formed by the chemical bonding of large numbers of smaller molecules (monomers).

  • Example: Polyethene (PE).

    • Polyethene molecule structure: -CH2-CH2-CH2-CH2-…

    • Mer (repeating unit): [–CH2-CH2-]

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Naturally Occurring Polymers

  • Examples of naturally occurring polymers include:

    • Hair

    • DNA

    • Spider silk

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Synthetic Polymers

  • Non-naturally occurring or man-made polymers are called synthetic polymers.

  • Common examples include various plastics found in daily life, such as:

    • Tyres

    • Cups

    • Synthetic rubber

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Polymerization

  • Synthetic polymers are produced through a chemical reaction called polymerization.

  • This process occurs in a controlled reaction chamber under specific conditions of temperature and pressure, ensuring proper linking of monomers to form polymers.

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Polymerization of Polyethylene

  • Polyethylene is produced from the monomer ethene, existing as gas at room temperature.

  • Ethene molecules' random movement and weak Van der Waals forces govern their interactions.

  • The ethene molecule has two carbon atoms with a double covalent bond, which is unstable and can be broken by heating.

  • Polyethylene is a widely used polymer in the production of various plastic products.

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Mechanism of Polymerization: Polyethene (PE)

  • During polymerization, the C=C bonds break.

  • The free electrons combine with other ethene molecules, forming a chain of polymers.

  • This process continues as more ethene molecules are added, leading to a macromolecule.

  • Monomer: Ethene | Polymer: Polyethene

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Classification of Polymeric Materials

  • Types of polymeric materials include:

    • Plastics

    • Elastomers

    • Thermoplastic

    • Thermosets

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Overview of Polymeric Materials

  • Polymeric materials combine carbon with oxygen, hydrogen, nitrogen, and other elements.

  • Characteristics:

    • Easily shaped under heat and pressure.

  • Plastics:

    • A varied group of synthetic materials, molded into shapes.

  • Thermoplastics:

    • Can be melted and re-formed.

  • Thermosets:

    • Cross-linked polymers that do not melt once formed (e.g., silicone, phenolics).

  • Elastomers:

    • Lightly cross-linked macromolecules with rubbery, flexible properties (e.g., butyl rubber, natural rubber).

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Thermoplastics

  • Properties:

    • Soften when heated and harden upon cooling.

    • The process is reversible without significant chemical changes.

  • Capable methods include:

    • Injection molding

    • Fused deposition modeling (FDM)

  • Limitations:

    • Poor resistance to high temperatures.

    • Low strength but ductile.

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Thermosets

  • Characteristics:

    • Harden during the first heating and do not soften upon reheating.

    • Covalent linkages form during initial heat treatment, creating cross-links.

  • Generally, thermosets exhibit high thermal stability but can degrade at high temperatures.

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Topic 3 Polymers Summary

3.1 Overview of Polymers

3.2 Molecular Structures and Properties of Polymers

3.3 Additives to Polymers

3.4 Common Polymers and Applications

3.5 Processing of polymers

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Molecular Structures and Properties of Polymers

  • Polymer: High molecular weight made up of small units (monomers).

  • Monomer: Low molecular weight compound connecting to form polymers.

  • Homopolymer: Formed from one monomer (all repeating units are the same).

  • Copolymers: Made up of 2 or more different monomers.

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Physical Properties of Polymers

  • Depend on molecular weight/shape and chain structure.

  • Main structures:

    • Linear (HDP, PVC, Nylon)

    • Branched (LDPE)

    • Crosslinked (Rubber)

    • Network (Kevlar, Epoxy)

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Polymer Chain Layout

  • Amorphous Polymers:

    • No ordered arrangement, no definite melting point.

  • Crystalline Polymers:

    • Ordered arrangement, distinct melting point, higher melting strength.

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Polymer Crystallinity

  • Polymers are often partially crystalline.

  • Reasons include:

    • Size of molecules can lead to crystalline regions within amorphous areas.

  • Semi-crystalline polymers exhibit increased tensile strength as crystallinity increases.

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Molecular Weight (M)

  • Molecular weight is defined as the mass of one mole of molecular chains.

  • Common polymers have a molecular weight ranging from 10,000 to 10,000,000.

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Effects of Molecular Weight on Properties

  • Higher molecular weight generally increases entanglement forces.

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Benefits of High Molecular Weight

  • Higher molecular weight results in:

    1. Increased ductility

    2. Enhanced tensile and impact strength

    3. Increased viscosity

    4. Increased chemical resistance

  • Examples for comparison: Polyethylene vs. Candles.

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Challenges with High Molecular Weight

  • Processing becomes harder as viscosity increases, impacting flow.

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Trade-offs in Manufacturing

  • Achieving desired properties versus ensuring material is easy to process. Examples include safety glasses versus CDs and DVDs.

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Effects of Temperature on Properties

  • Focus on:

    • Melting point

    • Boiling point

  • For thermoplastics:

    • Glass Transition Temperature (Tg)

    • Melting Temperature (Tm)

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Glass Transition Temperature (Tg)

  • The temperature where an amorphous polymer changes from glassy to rubbery form.

  • Can lead to brittleness below certain temperatures (e.g., polypropylene).

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Failure Modes due to Temperature

  • Evidence of material yielding versus breaking under stress due to temperature variations.

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Melting Temperature (Tm)

  • Specific to crystalline/semi-crystalline materials where distinct melting temperature occurs.

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Amorphous vs Crystalline Polymers

  • Differences in how these states behave under heat regarding melting characteristics.

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Temperature and Deformation

  • Overview of how temperature affects the transition between solid states in polymers.

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Topic 3 Polymers Summary

3.1 Overview of Polymers

3.2 Molecular Structures and Properties of Polymers

3.3 Additives to Polymers

3.4 Common Polymers and Applications

3.5 Processing of polymers

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Additives to Polymers

  • Foreign materials added to alter properties:

    • Filler: Increases bulk, reduces cost, improves resistance.

    • Plasticiser: Lowers Tg for improved ductility.

    • Stabiliser: Prevents degradation.

    • Colorant: Provides color.

    • Lubricant: Reduces friction.

    • Flame Retardant: Lowers flammability.

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Topic 3 Polymers Summary Again

3.1 Overview of Polymers

3.2 Molecular Structures and Properties of Polymers

3.3 Additives to Polymers

3.4 Common Polymers and Applications

3.5 Processing of polymers

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Common Polymers and Applications

  • Identifying various common polymers and their repeating units:

    • Polyethylene (PE)

    • Polyvinyl chloride (PVC)

    • Polytetrafluoroethylene (PTFE)

    • Polypropylene (PP)

    • Polystyrene (PS)

    • Others including PMMA, Bakelite, Nylon, PET, PC.

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Polyethylene (PE)

  • Characteristics:

    • Cheap, tough, flexible

    • Good chemical resistance

  • Applications vary by types:

    • LDPE: Bags for bread, groceries.

    • HDPE: Containers, toys, detergent bottles.

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Polypropylene (PP)

  • Similar strength to HDPE but easier to mold.

  • Excellent fatigue properties and chemical resistance.

  • Applications include:

    • Gasoline tanks, luggage, containers.

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Polyvinyl Chloride (PVC)

  • Plasticized version is low strength, used in coatings.

  • Rigid version is stronger, used in chemical storage and piping.

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Polystyrene (PS) & Polyamide (Nylon)

  • PS: Brittle, used in toys and electronics.

  • Nylon: Strong, used in various high-friction applications.

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ABS and Acrylics

  • ABS: Tough, used in panels and helmets.

  • Acrylics: Transparent, used in lenses and signage.

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Thermoset Materials

  • Common thermoset polymers:

    • Alkyds: Hard and stiff, used in electronic encapsulation.

    • Phenolics: High stiffness, used in electrical components.

    • Silicones: Chemically inert and water-repellent.

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Topic 3.5 Polymer Processing

  • Materials Technology

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Topic 3.5 Polymers Processing

3.5.1 Plastic Manufacturing Processes

3.5.2 Injection Moulding

3.5.3 Extrusion

3.5.4 Compression Moulding

3.5.5 Transfer Moulding

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Plastic Manufacturing Processes

  • Used to convert plastic raw materials into finished products.

    • Processing of Thermoplastic:

      • Raw materials in forms easy to remelt for various methods (e.g., injection moulding).

    • Processing of Thermoset:

      • Permanently set during polymerization; not re-meltable.

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Injection Moulding

  • Process includes melting and injecting thermoplastic into a mould cavity.

  • The plastic solidifies upon cooling.

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Injection Moulding Process

  • Polymer granules are fed, melted, and injected into the mould.

  • The product solidifies before being released.

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PLASTIC INJECTION MOLDING

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Advantages and Disadvantages of Injection Moulding

Disadvantages:

  1. High initial machinery costs.

  2. Design restrictions must be followed.

  3. Less cost-effective for low volumes.

Advantages:

  1. Fast process with short cycle time.

  2. Suitable for mass production.

  3. Complex shapes achievable.

  4. Good repeatability.

  5. Flexible design by changing moulds.

  6. Variety of thermoplastics available.

  7. Minimal finishing needed.

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Extrusion Process

  • Transforms solid polymer into a molten state for shaping.

  • Cross-sectional shapes configured through a die.

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Extrusion Overview

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Advantages and Disadvantages of Extrusion

Disadvantages:

  1. Uniform cross-sectional shape only.

  2. Limited shape complexity.

Advantages:

  1. Suitable for mass production.

  2. Low cost compared to molding processes.

  3. Short production lead time.

  4. Long parts produced continuously.

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Compression Moulding

  • Widely used for making thermosetting products.

  • Compound is placed in the mold, heated under pressure.

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Compression Moulding Overview

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Advantages and Disadvantages of Compression Moulding

Disadvantages:

  1. Lower production rate than injection moulding.

  2. Challenges in complex geometries.

Advantages:

  1. Lower capital costs.

  2. Low mold maintenance.

  3. Good surface finish.

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Transfer Moulding Process

  • Heats and forces thermosetting material into a pre-heated mold.

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Transfer Moulding Overview

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Advantages and Disadvantages of Transfer Moulding

Disadvantages:

  1. More expensive than compression moulding.

  2. Lower production rates compared to injection moulding.

Advantages:

  1. Lower equipment costs.

  2. Shorter cycle times.

  3. Tighter dimensional tolerances.

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