Novolac is the {linear} precursor used in paints and as a binder. Bakelite is the {cross-linked} (3D) thermosetting plastic used for electrical switches and handles.
Step 1: Understanding the Concept:
Phenol-formaldehyde resins are formed by the condensation reaction of phenol with formaldehyde in the presence of either an acid or a base catalyst. The reaction starts with the formation of ortho and para hydroxymethylphenol derivatives. Step 3: Detailed Explanation:
1. Novolac: When the molar ratio of formaldehyde to phenol is less than 1 and an acid catalyst is used, a linear polymer is formed. This linear polymer consists of phenol rings linked by methylene (\(-CH_2-\)) bridges, typically at the ortho positions. This is the structure shown in the image.
2. Bakelite: Further heating of Novolac with formaldehyde leads to cross-linking, forming an infusible solid mass called Bakelite.
3. Other Options:
- Neoprene: Polymer of chloroprene.
- Buna-N: Copolymer of 1,3-butadiene and acrylonitrile.
- Acrilan (PAN): Polymer of acrylonitrile. Step 4: Final Answer:
The linear phenol-formaldehyde resin shown is Novolac.
