Problem 67 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following can be remelted time and again without producing any change? (a) Bakelite (b) Urea formaldehyde resin (c) PVC (d) Melamineformaldehyderesin

Step-by-Step Solution

Answer

PVC can be remelted and remolded multiple times.

Step 1: Understanding Thermoplastics and Thermosetting Plastics

To solve this question, we need to understand the difference between thermoplastics and thermosetting plastics. Thermoplastics can be melted and remolded multiple times without any chemical change, while thermosetting plastics harden permanently after being heated and cannot be remelted.

Step 2: Identifying the Plastic Types

Let's identify each material in the options: - Bakelite, Urea formaldehyde resin, and Melamine formaldehyde resin are thermosetting plastics. - PVC (Polyvinyl Chloride) is a thermoplastic.

Step 3: Selecting the Correct Answer

Since thermoplastics can be remelted repeatedly without changing its properties, the correct answer among the given options is the thermoplastic, which is PVC.

Key Concepts

Thermosetting Plastics

Thermosetting plastics are a unique class of polymers characterized by their ability to form irreversible bonds when heated. Once set, these plastics cannot be melted again. This is due to the cross-linking nature of their chemical structure, which prevents any reshaping after the initial molding process.

• They are highly resistant to heat and can withstand high temperatures without losing their rigidity.
• They are durable, making them an excellent choice for applications that require long-lasting materials.
• Common types of thermosetting plastics include Bakelite, urea formaldehyde, and melamine formaldehyde resins.

Once cured, thermosetting plastics form a permanently hard material, making them ideal for products like electrical insulators, adhesives, and cookware handles. However, their inflexibility in being remolded limits their recyclability.

Polyvinyl Chloride (PVC)

Polyvinyl Chloride, better known as PVC, is one of the most widely used thermoplastics in the world. Unlike thermosetting plastics, PVC can be repeatedly softened by heating and hardened by cooling. This cyclical process is the cornerstone of what makes thermoplastics like PVC so versatile.

• PVC is renowned for its strength, durability, and resistance to environmental degradation, making it suitable for a vast array of applications.
• Common uses include construction materials, plumbing pipes, electrical cable insulation, and medical devices.
• PVC can be easily colored, cut, shaped, and welded, which enhances its adaptability in manufacturing.

The ability of PVC to be recycled by remelting is a significant advantage, supporting environmental efforts in waste reduction. Its robustness and malleability make it an essential component in modern manufacturing.

Bakelite

Bakelite is a pioneering thermosetting plastic famously known for being one of the first plastics made from synthetic components. Created in the early 20th century, it set the foundation for modern thermosetting polymers.

• This material is formed from a reaction between phenol and formaldehyde.
• Bakelite possesses excellent electrical insulating properties, making it an ideal material for electrical components and household electronics.
• It also exhibits high heat resistance and mechanical strength.

Due to its rigid structure, Bakelite cannot be remelted after setting, which characterizes it as a traditional plastic used for items like radio casings, telephone handsets, and kitchenware. Its unique properties, combined with its historical significance, make Bakelite a classic example of early thermosetting plastic technology.