To decide between thermosetting plastics and thermoplastics for high-heat applications, it's crucial to understand their properties and behavior under heat.

Comparison:

• Once cured, thermosetting plastics cannot be remolded or softened by heating. This characteristic is due to the formation of cross-linked, three-dimensional network structures during the curing process.
• This cross-linking provides excellent heat resistance and mechanical strength. Therefore, they are suitable for high-heat applications.

• Thermoplastics can be remelted and reshaped upon heating. They do not form cross-linked structures, which makes them less heat resistant compared to thermosetting plastics.
• These are generally used where remolding is an advantage, but they are less suitable for sustaining high temperatures as they soften when heated.

Conclusion:

For applications requiring high-heat resistance, thermosetting plastics are preferred because they do not soften on heating. This makes them able to maintain structural integrity and performance in high-temperature environments.

Explanation of Options:

• Option 1: "Thermosetting plastics, because they get remoulded upon heating" is incorrect as thermosetting plastics do not remold; they become hard and irreversible after curing.
• Option 2: "Thermoplastics, because they do not soften on heating" is incorrect as thermoplastics do soften and can be reshaped with heat.
• Option 3: "Thermosetting plastics, because they do not soften on heating" is the correct answer given their structural rigidity at high temperatures.
• Option 4: "Thermoplastics, because they are heat resistant" is incorrect. While thermoplastics have some heat resistance, they are not suitable for high-heat applications as they will soften.

This analysis demonstrates why thermosetting plastics are the superior choice for high-heat applications.