Table of Content:
1. What is “Thermal Modified Wood”?
2. Principle of Thermal Modification
3. Characteristics

What is “Thermal Modified Wood”?

“Carbonized wood” is a colloquial term in China for ultra-high-temperature heat-treated wood.

The basic concept involves heating wood at 160–230°C (typically 180–215°C) using a heat medium to improve wood quality. This reduces its hygroscopicity and water absorption, enhances dimensional stability, biological decay resistance, and weather resistance, creating a high-performance, visually appealing, and eco-friendly wood product.

Strictly speaking, “carbonized wood” should be called thermally modified wood or heat-treated wood. International terms include Thermal Modified Wood or Heat-Treated Wood.

Principle of Thermal Modification

Wood consists of hemicellulose, cellulose, lignin, and small amounts of extractives.

• Hemicellulose is less heat-resistant and degrades first under high temperatures, producing free acetic acid. This acetic acid acts as a catalyst, accelerating the hydrolysis of hemicellulose into sugars.
  As a result:
  
  • Hemicellulose content decreases.
    
  • Free hydroxyl groups (which attract water) are reduced.
    
  • Moisture absorption decreases.
    
  • Dimensional stability increases.
    
  • Less food is available for decay fungi, inhibiting their growth.
    
• Cellulose has both crystalline and amorphous regions. Its hygroscopicity depends on the size of the amorphous regions and the number of free hydroxyl groups.
  During heating:
  
  • Acetic acid degrades microfibrils in the amorphous zones.
    
  • Crystallinity and crystal size increase.
    
  • Hygroscopicity decreases, improving dimensional stability.
    
• Extractives (resins, terpenes, fats, waxes, tannins, phenolics) are volatile at high temperatures. Their evaporation:
  
  • Reduces food sources for fungi, improving decay resistance.
  • Improves coating and bonding performance.
    

CHARACTEREISTICS

1. Significantly reduced hygroscopicity and water absorption
   → Due to a ~51% reduction in hygroscopic hydroxyl groups
   
2. Lower equilibrium moisture content (EMC) and improved dimensional stability
   → EMC reduced by ~50%
   → Tangential/radial swelling decreases, improving dimensional stability by ~42% [6].
   
3. Improved biological decay resistance
   → Hemicellulose degradation and extractive loss cut off the fungal food chain.
   
4. Improved bonding and surface finishing
   → Due to fewer extractives like resin.
   
5. Attractive and uniform color
   → High durability; can mimic expensive woods via different heat treatments.
   
6. Increased hardness and longitudinal compressive strength
   
7. Slight increase in modulus of elasticity (MOE)
   
8. Slight reduction in density (~3%)
   
9. Slight decrease in bending strength (~5%)
   → Lower density = lower strength.
   → But lower moisture content increases strength, partially compensating.
10. Increased brittleness
    → Impact bending strength and shear strength drop by ~25% and ~20% respectively