Polymers and Their Classification

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Questions and Answers

What type of polymer is formed from a single type of monomer?

Copolymer

Block polymer

Homopolymer (correct)

Graft polymer

Which of the following processes is used for the fabrication of plastic?

Lamination forming

Filtration molding

Extrusion molding (correct)

Vacuum casting

What is a drawback of natural rubber?

High recyclability

Low thermal resistance

Poor weather resistance (correct)

Very low cost

Which type of copolymer has a sequence of blocks of the same monomer followed by blocks of another?

Block copolymer Signup and view all the answers

Which property of polymers indicates their effectiveness in resisting chemical reactions?

Chemical resistance Signup and view all the answers

What is the correct sequence of steps in the injection moulding process?

Material mixed, resin heated, plunger presses resin, parts removed Signup and view all the answers

What is not a characteristic of the injection moulding process?

Requires no heating in the process Signup and view all the answers

Which material is not typically used in the injection moulding process?

Metal alloys Signup and view all the answers

What is the pressure range applied during the injection moulding process?

70 Mpa – 200 Mpa Signup and view all the answers

Which of the following is an application of injection moulding?

Plastic toys Signup and view all the answers

What characterizes isotactic polymers?

All R groups are on the same side of the chain. Signup and view all the answers

Which type of plastic is known for being flexible and can be molded multiple times?

Thermosoftening plastics Signup and view all the answers

What is a primary characteristic of thermosetting plastics?

They are rigid and cannot be remolded. Signup and view all the answers

How do temperature and pressure affect polymer chains?

They affect the length and branching of the polymer chain. Signup and view all the answers

What happens to a polymer below its glass transition temperature (Tg)?

The polymer remains brittle with limited movement in its segments. Signup and view all the answers

Which of the following factors can NOT significantly affect the properties of plastics?

Refrigeration during storage. Signup and view all the answers

What primarily distinguishes elastomers from other types of plastics?

Their low glass transition temperature (Tg). Signup and view all the answers

What effect do additives have on polymers?

They may modify the elasticity of the polymer chains. Signup and view all the answers

What is the primary characteristic of the materials used in compression moulding?

They form strong bonds and are heat resistant. Signup and view all the answers

What is a major disadvantage of compression moulding compared to injection moulding?

It has a longer cycle time. Signup and view all the answers

What is the purpose of the preheating step in the compression moulding process?

To liquefy the plastic before moulding. Signup and view all the answers

Which of the following describes the conditions for temperature during compression moulding?

It varies from 300ºF to 375ºF. Signup and view all the answers

What distinguishes extrusion moulding from other moulding processes?

It uses a die to create continuous lengths of product. Signup and view all the answers

What type of materials can be used in extrusion moulding?

Only thermoplastic pellets, granules, or powder. Signup and view all the answers

What characteristic does not describe compression moulding?

Short cycle time. Signup and view all the answers

Which of the following is an advantage of compression moulding?

It minimizes internal stress during production. Signup and view all the answers

Study Notes

Polymers

Large molecules composed of repeating units called monomers.
Examples: Polyethylene, PVC, Buna-S
Properties: Lightweight, high strength-to-weight ratio, low thermal conductivity, low electrical conductivity, resistance to chemicals, variety of colours and transparencies, low cost, recyclability, and flammability.

Polymer Classification

Based on Monomer Unit:
- Homopolymer: Prepared from a single monomer.
  - Examples: Polyethylene, PVC
  - Types: Linear, Branched, Cross-linked, Network (3D)
- Copolymer: Prepared from two or more monomers.
  - Examples: Buna-N, Nylon-66
  - Types: Random, Alternating, Block, Graft
Based on Polymerization Process:
- Addition Polymer (Chain Growth): Formed by addition reaction.
  - Examples: Polyethylene, PVC
- Condensation Polymer (Step Growth): Formed by linking condensation reaction.
  - Examples: Nylon-66, Terylene
Based on Stereoisomerism:
- Isotactic: All R groups are on the same side of the chain.
- Syndiotactic: R group occupies alternate sides of the chain.
- Atactic: R group occupies random sides of the chain.

Plastics

Properties: Doesn't conduct electricity, poor heat conductors, unreactive to water, air and chemicals, fall between elastomers and fibers, harder to stretch than elastomers, easily molded and shaped.
Factors Affecting Plastic Properties:
- Reaction Conditions: Affect the chain length and branching of the polymer chain.
- Monomer: Affects the types of forces between polymer chains.
- Additives: Can lubricate polymer chains, join them with cross-links, or prevent decay.

Types of Plastics

Thermosoftening Plastics (Thermoplastics):
- Do not contain cross-links.
- Flexible and stretchy.
- Low melting point.
- Can be molded and shaped multiple times.
- Examples: Polyethylene, Natural Rubber.
Thermosetting Plastics:
- Contain cross-links.
- Rigid.
- High melting point.
- Can only be molded once.
- Examples: Aircraft components, Racing car components, Sports equipment.

Glass Transition Temperature (Tg)

Different polymers exhibit different segment movements on their backbones.
The ease of segment movement is dependent on their structure and physical environment.
Each polymer requires different energy levels for segment movement.
Below Tg, segments lack sufficient energy to move, making the polymer brittle.
Above Tg, polymers become flexible.
Tg relates to the amorphous regions of the polymer, not crystalline regions.

Moulding

Compression Moulding:
- Calculated plastic amount is placed in a mold and closed under pressure.
- The mold is heated, and plastic flows around the mold, solidifying due to strong bonds.
- Used for thermosetting plastics.
- Process: Preheating and pressurizing.
- Advantages: Low cost, uniform density, improved impact strength, dimensional accuracy.
- Disadvantages: Long curing time, uneven parting lines, scrap cannot be reprocessed.
Injection Moulding:
- Material is fed into a heated barrel, mixed, and forced into a mold cavity where it cools and hardens.
- Process: Resin is heated to liquid state, pressurized into mold cavities, cooled, and then removed.
- Advantages: High production rates, dimensional control, short cycle times.
- Applications: Plastic products (toys, containers, combs), rubber materials (seals, hoses).
Extrusion Moulding:
- Plastic is pushed through a die, producing a continuous length of a product with the same cross-section.
- Process: Thermoplastic pellets are heated and liquefied in a barrel with a screw mechanism, and then extruded through a die.
- Applications: Solid rods, channels, tubing, pipe, window frames, coated electrical wire.

Rubber

Natural Rubber (Latex):
- Drawbacks include low tensile strength, poor resistance to heat and solvents.
Compounding of Rubber (Vulcanization):
- Process of heating rubber with sulfur to improve its elasticity, strength, and durability.
Types of Rubber:
- Buna-S: Synthesized from butadiene and styrene, used in tires, hoses, belts.
- Silicone Rubber: Highly flexible, resistant to heat and chemicals, used in sealants, gaskets, implants.
- Polyurethane Rubber: Durable, abrasion-resistant, used in foams, wheels, coatings.

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Description

This quiz explores the fascinating world of polymers, their properties, and the various methods of classification based on monomer unit and polymerization process. Test your knowledge on examples and types of homopolymers and copolymers, as well as addition and condensation polymers. Perfect for students studying this vital topic in chemistry.