Post-baking (also called post-curing) is a common step in the processing of phenolic resins and other thermosetting polymers. It involves heating the molded part after its initial curing to promote further chemical reactions and enhance material properties.
Here’s how post-baking raises the glass transition temperature (Tg) and improves the thermal stability of phenolic parts:
1. Increased Crosslinking Density
- What happens: During post-baking, residual reactive groups (like hydroxymethyl or methylol groups in phenolic resins) undergo further condensation reactions.
- Effect: This increases the crosslinking density of the polymer network.
- Why it matters: A highly crosslinked network restricts molecular mobility, which leads to:
- Higher glass transition temperature (Tg)
- Better thermal and dimensional stability
2. Completion of Cure Reaction
- What happens: In many molding processes (e.g. compression or transfer molding), curing may be incomplete due to limited time or temperature.
- Effect: Post-baking ensures that uncured or partially cured regions fully react.
- Why it matters: A fully cured network resists softening and thermal degradation better, improving thermal endurance and mechanical integrity at elevated temperatures.
3. Reduction of Volatiles and By-products
- What happens: Post-baking helps drive off residual water, alcohols, or formaldehyde (by-products of condensation).
- Effect: Lower internal stress and fewer defects like voids or microcracks.
- Why it matters: Improved thermal stability and reduced risk of outgassing or decomposition at high temps.
4. Network Rigidification
- What happens: The polymer structure becomes more rigid and stable due to tighter molecular packing and crosslinking.
- Effect: Increased stiffness, reduced molecular chain motion.
- Why it matters: This directly raises the Tg and improves creep resistance at elevated temperatures.
Summary Table
| Mechanism | Effect on Tg | Effect on Thermal Stability |
| Increased crosslinking | Raises Tg | Improves heat resistance |
| Completion of cure | Raises Tg | Reduces softening and degradation |
| Removal of volatiles | Minimal direct effect on Tg | Prevents thermal breakdown |
| Network rigidification | Raises Tg | Improves structural stability |