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

Short answer

(c) PVC is the answer, as it is a thermoplastic.

Step-by-step solution

Understanding Thermoplastics and Thermosetting Plastics

Bakelite, urea formaldehyde resin, and melamine formaldehyde resin are thermosetting plastics. Thermosetting plastics cannot be remelted once they have been set, meaning they form a permanent solid structure after the initial formation. On the other hand, thermoplastics like PVC can be melted and remolded multiple times without undergoing significant chemical changes.

Identifying the Thermoplastic

PVC, or polyvinyl chloride, is known for being a thermoplastic. This type of plastic can be repeatedly heated until melted and then cooled to form a solid, enabling it to be remelted and remolded as needed without any inherent change in its chemical structure.

Conclusion of the Analysis

Since the exercise asks which material can be remelted time and again without change, the material that fits this description is the thermoplastic. Among the given options, PVC is the only thermoplastic and therefore can be remelted and reshaped multiple times.

Key concepts

thermosetting plastics

Thermosetting plastics are a unique class of polymers that form irreversible bonds once they have been set. These polymers undergo a chemical change when heated for the first time, causing them to harden permanently. This process is due to the formation of cross-links between polymer chains. So, once these plastics are heated and shaped, they cannot be remelted or reshaped.
Here are some properties and uses of thermosetting plastics:

• Durability: They are very durable and resistant to heat and wear, making them ideal for use in industries requiring high heat resistance.
• Rigidity: Once set, they remain hard and cannot be softened by heating again. This rigidity is beneficial for constructing permanent objects like electrical insulators and kitchenware.
• Examples: Common examples include Bakelite, urea formaldehyde resin, and melamine formaldehyde resin.

Thermosetting plastics offer advantages for specific applications, but if reshaping or remelting is required, thermoplastics would be the better choice.

polyvinyl chloride (PVC)

Polyvinyl Chloride, or PVC, is one of the most versatile and widely used thermoplastics available today. Unlike thermosetting plastics, PVC can be heated until melted and then cooled to form a solid multiple times without chemical change. This property is what makes thermoplastics like PVC particularly useful in a myriad of applications.
Here’s why PVC stands out:

• Reusability: It can be remelted and reshaped multiple times, making it environmentally favorable and cost-effective for recycling.
• Applications: It is used in plumbing pipes, electrical cable insulation, clothing, and even in packaging.
• Adaptability: The chemical structure allows blending with other materials to enhance properties such as flexibility and tensile strength.

PVC's unique ability to be remelted without changing its inherent properties makes it among the most flexible materials for repeated use.

chemical structure

The chemical structure of polymers plays a crucial role in determining whether a plastic falls into the thermoplastic or thermosetting category. At the heart of these functionalities is the arrangement of polymer chains and the types of bonds that hold them together.
Understanding a few key principles can help clarify:

• Thermosetting Plastics: These contain polymer chains bonded through cross-links, forming a rigid 3D network that resists reshaping. The cross-linking occurs during the initial heat-setting process and is typically irreversible.
• Thermoplastics: Such as PVC, feature linear or branched polymer chains that are not interconnected by strong covalent bonds. This structure allows them to soften upon heating, which makes them moldable and remeltable.
• Significance: Adjustments to chemical structures allow engineers to design materials with specific properties for targeted applications.

By comprehending the chemical structure, we better understand how different types of plastics perform and can choose the appropriate one for our needs.