Why HDT Alone Can’t Predict Heat Resistance in Humid Environments

In plastic material selection, “heat resistance” is often treated as a stable property: HDT, RTI, long-term service temperature, glass transition temperature (Tg)… Datasheets list these values, giving the impression they are fixed.

However, real-world applications show that in humid environments, a material’s heat resistance is not constant—it can drop significantly. Many failures are due to the combination of humidity + heat, which reduces the material’s true heat resistance. Ignoring this factor can lead to serious consequences.

Understanding How Humidity Reduces Heat Resistance

The mechanisms differ for various plastics, but generally fall into three main categories:

1. Water Absorption Lowers Tg (“Premature Softening”)

For engineering plastics where Tg dominates performance—such as PC, PA, PBT, ASA—water acts as a plasticizer. It penetrates polymer chains, increases chain mobility, lowers Tg, and consequently decreases HDT, RTI, and deformation temperature.

Examples:

• PC (dry): Tg ≈ 147°C → severe moisture absorption can reduce Tg by 13°C
• PET: Tg drops from 75°C to 69°C

Tip: Datasheet values measured in dry conditions are often overly optimistic.

2. Vicat Softening Point Changes After Water Absorption

Below is a table showing the change in Vicat softening temperature for different materials after water absorption under standard laboratory conditions:

3. Decreased Crystallinity or Disrupted Crystal Structure

Semi-crystalline plastics may experience structural disturbances after water uptake:

• Water affects crystal defects
• Increases the proportion of the amorphous phase
• May interfere with recrystallization behavior

Result: The material’s heat deformation temperature or melt mechanical properties are worse than expected.

4. Disruption of Hydrogen Bonds in PA Materials

For PA (polyamide) materials, water absorption disrupts hydrogen bonds between chains, significantly reducing heat resistance and mechanical performance.

Example: Flexural modulus decreases after 60 days at 23°C, 50% R.H.—indicating a reduction in heat resistance.

5. Hydrolysis: Dual Loss of Strength and Heat Resistance

Some plastics are prone to hydrolysis under high-temperature, high-humidity conditions:

• Typical examples: PBT, PLA, PESU, PSU, PPSU, PC
• PA can experience chain scission in high-temperature steam

Once hydrolysis occurs:

1. Molecular weight decreases
2. Tg drops
3. Mechanical properties and heat resistance decline simultaneously

Real-world observations:

• PA66: 125°C dry heat is fine, but after one week at 85°C/85% RH, tensile strength drops 20–40%
• PBT: Hydrolysis occurs at 85/85, while 120°C dry heat shows stable performance
• PC: Loss of transparency and microcracks at 85/85
• TPU: Softening under moist-heat is 2–3 times greater than under dry heat

Five Key Questions When Selecting Plastics for Humid Environments

To ensure reliable performance, check the following:

1. Operating conditions: Dry or humid? Datasheet values are usually measured in dry conditions.
2. Water absorption: How much water is absorbed? How much does Tg drop?
   • Water content (0.1%–3% can make a huge difference)
   • Tg vs. humidity curve
   • HDT vs. water content curve
3. Temperature-humidity coupling: Will the environment combine heat + moisture?
   • Electronics: heating + condensation
   • Automotive: engine bay humidity + temperature cycling
   • Outdoor: daytime sun / nighttime dew
   • Medical: repeated steam sterilization
4. Hydrolysis susceptibility: Materials with ester or carbonate bonds require:
   • Stabilizers
   • Hydrolysis-resistant grades
   • Avoid long-term exposure above 70–85°C under humid conditions
5. Manufacturer-provided long-term moist-heat data: Focus on:
   • Strength retention at 85/85 RH
   • Wet-state mechanical properties
   • Heat deformation temperature after saturated water absorption
   • Steam sterilization cycle durability (PA, PEEK, PPSU)

Conclusion: The Hidden Variables in Material Selection

The biggest danger in material selection is invisible variables. Datasheet values may look safe, but in humid environments, they may no longer exist. Humidity transforms “design values” into actual performance, and true engineering quality depends on actual performance.

Key takeaway: For water-absorbing plastics, heat resistance comes in two sets—dry-state and wet-state. Material selection must consider both.