Phenol Formaldehyde (Bakelite)

Definition

Phenol formaldehyde is the oldest synthetic polymer, belonging to the class of thermosetting resins. It is formed by the condensation polymerization reaction between phenol and formaldehyde in the presence of either an acid or a base catalyst. It is widely known by its trade name, "Bakelite."

Main Content

1. Types of Phenol Formaldehyde

• Novolac: These are linear polymers formed when the molar ratio of phenol to formaldehyde is greater than one, usually in the presence of an acid catalyst.
• Resoles: These are branched or cross-linked polymers formed when the molar ratio of phenol to formaldehyde is less than one, usually in the presence of a base catalyst.

2. Chemical Structure and Bonding

• The monomer units are linked by methylene (-CH₂-) bridges.
• The formation involves electrophilic substitution at the ortho and para positions of the phenol ring.

3. Characteristics

• It is a hard, infusible, and insoluble solid.
• It exhibits high thermal stability and excellent electrical insulation properties.

Working / Process

1. Condensation Polymerization

• The process begins with the reaction of phenol with formaldehyde.
• The reaction produces ortho-hydroxy benzyl alcohol or para-hydroxy benzyl alcohol as the initial monomers.

2. Formation of Linear Chains (Novolac)

• If an acid catalyst is used, the hydroxy benzyl alcohol monomers undergo continuous condensation to form long, linear chains.
• These chains are then converted into a hard, infusible mass by adding more formaldehyde during the heating stage.

       OH                OH              OH
        |                 |               |
  CH2--Ph--CH2--Ph--CH2--Ph--CH2--Ph--CH2--Ph
                                          |
                                          OH
    [Linear structure of Novolac]

3. Cross-linking (Thermosetting)

• The linear novolac chains are treated with hexamethylenetetramine (HMTA) or excess formaldehyde and heated.
• This creates extensive three-dimensional cross-links between the polymer chains, turning it into a rigid, non-melting plastic known as Bakelite.

Advantages / Applications

• Due to its high heat resistance and electrical insulating properties, it is extensively used in the manufacturing of electrical switches, plugs, and switchboards.
• It is used in making handles for pressure cookers, frying pans, and other kitchen utensils because of its low thermal conductivity.
• It is used in industrial applications for making gears, protective coatings, and as a bonding agent for laminated wood and plywood.

Summary

Phenol formaldehyde (Bakelite) is a thermosetting polymer created through the condensation reaction of phenol and formaldehyde. It is characterized by its rigid, three-dimensional cross-linked network that prevents it from melting once cured. It serves as a vital material in electrical and heat-resistant components. Important terms: Condensation polymerization, Novolac, Resoles, Cross-linking, and Thermosetting resin.