Question

Which of the following are characteristics of thermosetting polymers? (A) Heavily branched cross linked polymers (B) Linear slightly branched long chain molecules (C) Become infusible on moulding so cannot be reused (D) Soften on heating and harden on cooling, can be reused

Short answer

The characteristics of thermosetting polymers are described by options A: Heavily branched cross-linked polymers and C: Become infusible on molding so cannot be reused.

Step-by-step solution

Option A: Heavily branched cross linked polymers

Thermosetting polymers are known for their high degree of cross-linking which provides them with increased stability and rigidity. Cross-linking in thermosetting polymers is established during the polymerization process. Therefore, option A correctly describes a characteristic of thermosetting polymers.

Option B: Linear slightly branched long chain molecules

This characteristic corresponds to thermoplastic polymers, not thermosetting polymers. Thermoplastic polymers have linear and slightly branched long chain molecules, while thermosetting polymers have highly cross-linked structures. So, option B does not describe a characteristic of thermosetting polymers.

Option C: Become infusible on moulding so cannot be reused

Thermosetting polymers become infusible and insoluble once they have been molded. This means that they cannot be melted and reshaped, making it impossible to reuse them like thermoplastic polymers. This is an important characteristic of thermosetting polymers, so option C is correct.

Option D: Soften on heating and harden on cooling, can be reused

This characteristic corresponds to thermoplastic polymers, not thermosetting polymers. Thermoplastic polymers can be softened on heating, allowing them to be reshaped, and harden on cooling, making them reusable. Thermosetting polymers, however, do not soften upon heating and cannot be reused. Therefore, option D does not describe a characteristic of thermosetting polymers. In conclusion, the characteristics of thermosetting polymers are well-described by options A and C.

Key concepts

Cross-Linking in Polymers

Understanding the nature of polymers is essential for many areas of science and technology. Cross-linking is a critical process that occurs in certain types of polymers, which significantly alters their physical properties.

In the world of polymers, cross-linking refers to the creation of chemical bonds between the long chains of molecules that make up the polymer. These bonds can be visualized as bridges connecting different chains, transforming the polymer from a linear, entangled mass into a more structured, three-dimensional network. The high degree of cross-linking in thermosetting polymers is responsible for their unique characteristics, such as enhanced stability, strength, and resistance to heat and chemicals.

During the production of thermosetting polymers, once the cross-linking process is initiated, often through heat or a chemical reaction, the material undergoes a permanent transformation. It becomes 'set' in its final form and cannot be reshaped or remolded - this is a one-way journey. This is why products made from thermosetting polymers are so durable and why they're chosen for applications that demand materials that can withstand harsh conditions without deforming.

Thermoplastic vs Thermosetting

When comparing thermoplastic and thermosetting polymers, it's like comparing ice to a baked clay pot.

Thermoplastic polymers are like ice; they melt when you heat them and solidify when cooled. This makes them very versatile in their applications, allowing for processes such as injection molding, extrusion, and thermoforming. They consist primarily of linear, slightly branched long chains that can slide past each other when heated, making them soft and pliable. Polymers like polyethylene and polypropylene are classic examples of thermoplastics, widely used in packaging, toys, and many household items.

On the other hand, once thermosetting polymers are formed, they're more akin to a clay pot baked in a kiln - they can't be remelted and reshaped. Their rigid and cross-linked structure gives them a permanent shape, which makes them suitable for high-stress and high-temperature applications such as electrical insulation and car parts. Epoxy and phenolic resins are good examples of thermosetting polymers.

Properties of Thermosetting Polymers

Delving into the properties of thermosetting polymers unveils their role in creating robust and enduring materials. These polymers exhibit a myriad of characteristics that render them indispensable for specific uses.

Since thermosetting polymers are heavily cross-linked, they possess a high degree of rigidity and structural integrity. They are resistant to high temperatures, often maintaining their shape and form even when exposed to extreme heat where other materials might fail. This resistance also encompasses chemicals - thermosetting polymers can withstand corrosive substances, which makes them ideal for coatings and protective layers.

Moreover, they do not soften upon heating, a property known as being infusible. This infusibility, paired with their insolubility, means that once set, these polymers are virtually unchangeable without destruction. Thus, they cannot be recycled in the same manner as thermoplastics. Though this may seem like a drawback, it underscores their utility in applications where permanency is crucial - from circuit boards to kitchen countertops.