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 is the material that can be remelted multiple times without change.

Step 1: Understanding the Question

The question asks us to identify a material which can be reheated and remelted multiple times without changing its properties. This requires knowledge about types of polymers, specifically thermoplastics and thermosetting plastics.

Step 1: Identifying Key Characteristics of Polymers

Thermoplastics can be remelted and reshaped multiple times. In contrast, thermosetting plastics harden irreversibly upon heating and cannot be remelted. This property helps us distinguish materials in this context.

Step 2: Analyzing the Options

Let's look at the options: (a) Bakelite - a thermosetting plastic. (b) Urea formaldehyde resin - a thermosetting plastic. (c) PVC (Polyvinyl Chloride) - a thermoplastic. (d) Melamine formaldehyde resin - a thermosetting plastic.

Step 3: Selecting the Thermoplastic

Among the given options, only PVC is a thermoplastic, which means it can be reheated and remelted multiple times without changing its properties.

Key Concepts

thermosetting plastics

Thermosetting plastics are a category of polymers known for their permanent chemical bond formation upon being heated. Unlike thermoplastics that can be melted and reshaped multiple times, thermosetting plastics harden irreversibly. This happens because of cross-linking between polymer chains, which provides them with high structural strength and stability.
Once a thermosetting plastic is set into a certain shape, it cannot be melted again. Instead, when subjected to heat after solidifying, these plastics tend not to melt or reshape but might burn or decompose.

• Examples include Bakelite, Melamine, and Urea formaldehyde resin.
• These materials are extensively used in applications requiring heat resistance.

Thermosetting plastics are often used in the manufacturing of electrical insulators, adhesives, and kitchen products due to their durability.
Understanding the characteristics of thermosetting plastics is essential for choosing the right materials for heat-resistant applications.

polymer properties

Polymer properties generally dictate how materials behave under different conditions. These properties are influenced by the structure, composition, and molecular arrangement of the polymers. One of the key distinctions between polymers is whether they are thermoplastics or thermosetting plastics, each possessing unique characteristics.
Thermoplastics have linear or slightly branched chain structures which allow them to be flexible and re-moldable upon heating. Key properties include:

• Recyclability – they can be reheated and reshaped.
• Resistance to solvents and oils.
• Ability to withstand physical impact without breaking easily.

In contrast, thermosetting plastics are highly stable and do not soften once set. Their properties make them more suited to high-temperature applications.
Understanding polymer properties is crucial when selecting materials for specific industrial and consumer applications.

polyvinyl chloride (PVC)

Polyvinyl chloride (PVC) is one of the widely used types of thermoplastics. This polymer is known for its versatility, affordability, and wide range of applications, from construction to healthcare.
PVC is distinct from other plastics because it combines chlorine with ethylene, making it more flame resistant than other alternatives such as polyethylene. It is highly valued for:

• Being lightweight yet durable.
• Its resistance to moisture and chemicals.
• The ability to be molded into various shapes and forms through heating and cooling processes.

PVC can be made flexible or rigid, catering to different needs, such as plumbing pipes, credit cards, or flooring materials. Despite its benefits, care must be taken in its use and disposal due to environmental concerns associated with chlorine.
The recyclability and economic value of PVC make it a popular choice for manufacturing various products.