Cross-linked vs. linear polymers: which of the following is a classic three-dimensional, cross-linked thermoset formed from phenol and formaldehyde?

Introduction:
Polymer networks created by cross-linking form rigid, insoluble thermosets. Recognizing examples of such thermosets versus linear thermoplastics is fundamental in materials selection and processing.

Given Data / Assumptions:

• Bakelite is produced by condensation of phenol with formaldehyde.
• Cross-linked structure prevents melting and dissolving.
• Thermoplastics deform on heating; thermosets do not.

Concept / Approach:
Phenol–formaldehyde systems cure into a three-dimensional network. PET, HDPE, Nylon-6, and PP are linear or lightly branched thermoplastics that soften with heat and can be reprocessed under controlled conditions.

Step-by-Step Solution:
Identify the thermoset: phenol–formaldehyde (Bakelite) → cross-linked network.Eliminate thermoplastics: PET, HDPE, Nylon-6, PP are not cross-linked in standard grades.Select Bakelite as the correct cross-linked example.

Verification / Alternative check:
Handbooks classify Bakelite among oldest commercial thermosets used in electricals and molded articles.

Why Other Options Are Wrong:

• PET, HDPE, Nylon-6, PP: predominantly linear polymers; properties derive from chain packing, not network cross-links.

Common Pitfalls:
Assuming crystallinity equals cross-linking; crystallinity is ordered packing of linear chains, not covalent network formation.

Final Answer:
Bakelite (phenol–formaldehyde)