SM

Lecture_9___10_-_Polymers___plastics_handouts

Polymers

Definition

  • A polymer is a large molecule made up of hundreds or thousands of atoms.

  • Formed by joining two or more kinds of small repeating units called monomers into chain or network structures.

Structure

  • Long molecules consisting of small molecules (monomers).

Examples of Polymers

  • Polyethylene (PE):

    • Repeat Unit:

      • C-C-

      • H

      • H

  • Polypropylene (PP):

    • Repeat Unit:

      • H

      • C=C=C=C=C=C·

      • H

  • Poly(Vinyl Chloride) (PVC):

    • Structure includes:

      • H

      • CI

      • C-C—C—C—C

      • and others.

Polymer Characteristics

Size

  • Polymer molecules are very large (macromolecules).

  • Typical structure includes long and flexible chains of C atoms as a backbone.

Types of Polymers

  1. Homopolymers:

    • Consists of the same type of monomers.

  2. Copolymers:

    • Consists of more than one type of monomer.

Types of Polymer Chain Structures

  1. Linear Polymers:

    • Repeat units linked end to end forming a single chain.

    • Flexible; comparable to a mass of spaghetti.

    • Examples: polyethylene, nylon.

  2. Branched Polymers:

    • Contains branches of similar or different monomers along the main chain.

  3. Cross-Linked Polymers:

    • Chains are connected by covalent bonds.

    • Common in rubber and elastomers.

  4. Network Polymers:

    • Form three or more active covalent bonds, creating a 3D network.

    • Examples: epoxies used in paints, glues, and coatings.

Types of Copolymers

  • Regular Copolymers: A-B-A-B-A-B-

  • Random Copolymers: A-B-A-B-A

  • Block Copolymers: A-A-A-A-B-B-B

Modifications and Additives

  • Polymers are rarely used alone; they are modified or mixed with additives, forming materials termed plastics.

  • Plastics are typically high molecular mass polymers, derived from natural or synthetic organic compounds, competing with metals and glass.

Natural Plastics

  • Sources:

    • Animals' horns (Casein, glue)

    • Animals' milk (Formaldehyde, glue)

    • Insects (Shellac, French polishing)

    • Plants (Cellulose, table tennis balls)

    • Trees (Latex, rubber)

Synthetic Plastics

  • Primary source: Crude oil. Gas products from refining can be broken down into monomers via polymerization.

Classification of Polymers (Plastics)

  • 2 Main Classes:

    • Thermoplastic: Chemical structure remains unchanged during heating; commercially significant (70% of plastics).

    • Thermosetting Plastics: Undergo a curing process causing permanent change (cross-linking); cannot be remelted.

Thermoplastics

  • Characteristics: Long chain molecules with few entanglements.

  • Behavior upon heating: Molecules move apart, allowing reshaping. When cooled, they harden back to their original shape.

  • Plastic Memory: Reheating generally allows thermoplastics to return to original shape unless damaged.

Thermosetting Plastics

  • Characteristics: Heavily cross-linked, forming rigid structures.

  • Cannot be reshaped after the initial heating.

  • Examples: Polyester resin, melamine.

Properties of Plastics

Thermoplastics

  • Reversible softening and hardening.

  • Characteristics inversely proportional to temperature (e.g., stiffness).

Thermosets

  • Irreversible hardening due to strong cross-linking.

  • Generally stronger, more rigid, heat resistant, but brittle.

Elastomers

  • Exhibit exceptional elastic deformation with near-complete recovery after deformation.

  • Chemical forms include natural rubber (latex) and synthetic silicone rubber.

Physical State of Polymers

  1. Melt or Rubber-Like State: At high temperatures, becomes viscous liquid allowing flow.

  2. Amorphous Glassy State: Reduced flexibility at lower temperatures, becomes brittle.

  3. Partially Crystalline State: Cooling allows favorable molecular arrangements.

Polymer Crystallinity

  • Polymers can be partially crystalline, affecting their strength and temperature resistance.

Plasticizers

  • Substances that help polymers become more flexible by separating polymer chains.

Strengthening of Plastics

  1. Crystallization: Applies to thermoplastic materials without cross-links.

  2. Cross-Linking: Provides enhanced resilience in rubber and thermosetting resins.

  3. Increased Rigidity: By attaching bulky groups to carbon chains to reduce bendings, or by mixing monomers to make co-polymers.

Physical Properties of Plastics

  1. Specific gravity: 0.90 – 2.2.

  2. Tensile strength comparable to metals, but with low Young's modulus of elasticity.

  3. Creep and degradation increase with temperature.

  4. All plastics are combustible, with some having high "spread of flame" properties.

  5. Plastic as insulators in electrical applications but prone to dust attraction.

  6. Significant thermal expansion compared to steel.

  7. Very low moisture absorption.

  8. Durability affected by polymer type, grade, and environmental exposure.

Processing of Plastics

Methods:

  • Continuous Processes:

    • Extrusions, Film blowing, Sheet casting, Spraying, Calendering.

  • Discontinuous/Batch Processes:

    • Compression molding, Injection molding, Blow molding, Extrusion molding, Rotational molding.

Compression Molding

  • Molding material is placed in an open heated mold and pressed to achieve shape.

  • Lower cost and good for larger parts.

Injection Molding

  • Common for manufacturing electronic product casings.

  • High production rates but expensive setup.

Blow Molding

  • Produces hollow products; suitable for thin-walled materials.

Rotational Molding

  • For hollow plastic products; provides stress-free components.

Shrinkage in Molding Processes

  • High thermal expansion leads to significant shrinkage during cooling. Factors affecting shrinkage:

    • Use of fillers, injection pressure, compaction time, and molding temperature.