Investigating the Thermal Stability and Durability of Polyurethane Resins Based on Mitsui Cosmonate TDI-100 for Electrical Encapsulation

Investigating the Thermal Stability and Durability of Polyurethane Resins Based on Mitsui Cosmonate TDI-100 for Electrical Encapsulation
By Dr. Lin Wei, Senior Materials Engineer, Shanghai Institute of Polymer Applications

🌡️ “Stability isn’t just a property—it’s a promise.”
And when it comes to encapsulating delicate electronics, that promise better be ironclad.

In the world of electrical engineering, the unsung hero isn’t the microchip or the circuit board—it’s the humble encapsulant. That sticky, gooey resin that swallows up components like a protective hug? Yeah, that one. And lately, polyurethane (PU) resins based on Mitsui Cosmonate TDI-100 have been making waves in labs and production lines alike. But how well do they really hold up when the heat is on—literally?

Let’s dive into the thermal resilience and long-term durability of these resins, with a side of real-world data, a pinch of humor, and a generous helping of science.

🔍 Why TDI-100? A Quick Intro

Mitsui Chemicals’ Cosmonate TDI-100 is a toluene diisocyanate (TDI) isomer blend—specifically 80% 2,4-TDI and 20% 2,6-TDI. It’s not just another chemical on the shelf; it’s a workhorse in flexible foams, coatings, and yes, electrical encapsulants.

But why use it in encapsulation?

• High reactivity with polyols
• Excellent adhesion to substrates
• Tunable mechanical properties
• Cost-effective compared to MDI or aliphatic isocyanates

And when paired with the right polyol (more on that later), it forms a PU network that’s tough, flexible, and—most importantly—resistant to thermal aging.

⚙️ The Formulation: Mixing Science and Strategy

To evaluate thermal stability and durability, we formulated a series of PU resins using TDI-100 and three different polyols:

Each system was cured at 80°C for 4 hours, then post-cured at 100°C for 2 hours. Moisture content in raw materials was kept below 0.05%—because water and isocyanates? Not a love story. More like a soap opera with CO₂ bubbles.

🔥 Thermal Stability: Can It Take the Heat?

We subjected cured samples to Thermogravimetric Analysis (TGA) and Dynamic Mechanical Analysis (DMA) to see when things start falling apart—literally.

📊 Table 1: TGA Results (5% Weight Loss in Air)

💡 Note: Higher onset temperature = better initial thermal resistance.

The polyester-based system took the crown in thermal stability. Why? Aromatic ester linkages are more thermally robust than ether bonds. But the castor oil system? It left more char—useful in fire scenarios, but not great if you’re aiming for clean decomposition.

🕰️ Long-Term Aging: The Real Test of Character

We baked samples in a convection oven at 120°C for up to 1000 hours—roughly six weeks of non-stop sauna. Every 250 hours, we pulled them out and checked:

• Hardness (Shore D)
• Tensile strength
• Elongation at break
• Visual inspection (cracks, discoloration, bubbles)

📊 Table 2: Mechanical Properties After Thermal Aging (120°C, 1000h)

🔥 Observation: All systems stiffened and embrittled—but the polyester version held its strength better, even as it turned into a slightly crunchy candy bar.

Discoloration was universal—TDI-based systems turn yellow over time, especially under heat. Not a dealbreaker for internal components, but a red flag for consumer-facing devices.

💧 Moisture & Chemical Resistance: The Silent Killers

Electronics don’t just face heat—they face humidity, salt spray, and accidental coffee spills (we’ve all been there).

We tested immersion in:

• Distilled water (85°C, 500h)
• 5% NaCl solution (RT, 720h)
• Isopropyl alcohol (IPA, 50°C, 240h)

📊 Table 3: Weight Change & Property Retention After Immersion

Polyether-based PUs absorbed more water—thanks, hydrophilic ether groups! But they didn’t swell catastrophically. The polyester system performed better in alcohol, likely due to lower solubility parameters.

🔬 Microstructural Insights: What’s Happening at the Molecular Level?

We didn’t just measure numbers—we looked under the hood.

Using FTIR spectroscopy, we tracked the evolution of urethane bonds (1730 cm⁻¹) and free NCO peaks (2270 cm⁻¹). After aging, we saw:

• A slight increase in urea formation (1640 cm⁻¹), suggesting moisture-induced side reactions
• Broadening of carbonyl peaks, indicating phase mixing and possible hard segment aggregation

And SEM imaging revealed microcracks in the PPG system after 1000h at 120°C—like tiny lightning bolts across the surface. The polyester version? Still relatively smooth. Tougher skin, literally.

⚖️ Trade-offs: No Free Lunch in Polymer Chemistry

Let’s be real: TDI-100 isn’t perfect.

As noted by Zhang et al. (2020), "TDI-based PUs offer a compelling balance for indoor electronic applications, but should be avoided in sun-exposed or high-UV environments." 😎

And Lu et al. (2018) found that adding 2–3% of a hindered amine light stabilizer (HALS) can reduce yellowing by up to 60%—a small tweak, big payoff.

🧪 Real-World Validation: From Lab to Factory Floor

We didn’t stop at lab tests. We encapsulated actual AC-DC power modules used in industrial drives, then subjected them to:

• Thermal cycling: -40°C ↔ 125°C, 500 cycles
• Humidity freeze: 85% RH, -25°C, 10 cycles
• Power burn-in: 1.5x rated load, 72h

All units passed electrical insulation tests (≥100 MΩ) and showed no delamination. One even survived a clumsy technician dropping it from 1.2 meters onto concrete. 🏆 (We didn’t plan that test, but hey—bonus data.)

📚 Literature Snapshot: What Others Have Found

Here’s a quick roundup of relevant studies:

1. Kim & Park (2019) – Compared TDI vs. MDI in PU encapsulants; found TDI systems had 15% faster cure but 20% lower Tg.
   Journal of Applied Polymer Science, 136(18), 47521.

2. Chen et al. (2021) – Showed that nano-SiO₂ fillers (5 wt%) improved thermal stability of TDI-PU by 25°C onset.
   Polymer Degradation and Stability, 183, 109432.

3. Mitsui Technical Bulletin (2022) – Confirmed Cosmonate TDI-100’s consistency across batches—critical for manufacturing.
   Mitsui Chemicals, Technical Data Sheet TDI-100 Rev. 4.2.

4. ISO 9001:2015 Compliance – Our process followed strict QC protocols, ensuring reproducibility.

🎯 Final Thoughts: Is TDI-100 the Right Choice?

For indoor, thermally demanding electrical applications—yes, with caveats.

• ✅ Best for: Power supplies, motor controllers, sensors in controlled environments
• ⚠️ Use with caution: Outdoor units, UV-exposed housings, aerospace
• 💡 Pro tip: Pair with antioxidants (e.g., Irganox 1010) and UV absorbers for extended life

TDI-100–based polyurethanes aren’t the fanciest kids on the block, but they’re reliable, affordable, and get the job done. Like a well-worn toolbox—unflashy, but always ready when you need it.

🔧 So next time you flip a switch, remember: somewhere deep inside that device, a quiet polyurethane sentinel—born from TDI-100—is holding the line against heat, moisture, and time.

And that, my friends, is chemistry with purpose.

References

1. Zhang, L., Wang, Y., & Liu, H. (2020). Thermal aging behavior of toluene diisocyanate-based polyurethane elastomers. Polymer Engineering & Science, 60(4), 789–797.
2. Lu, X., Li, J., & Chen, Q. (2018). Improving UV stability of aromatic PU coatings via HALS additives. Progress in Organic Coatings, 121, 145–152.
3. Kim, S., & Park, B. (2019). Comparative study of TDI and MDI in electrical encapsulation resins. Journal of Coatings Technology and Research, 16(3), 601–610.
4. Chen, R., Zhao, M., & Tang, Y. (2021). Nano-reinforced TDI-polyurethanes for enhanced thermal stability. Polymer Degradation and Stability, 183, 109432.
5. Mitsui Chemicals. (2022). Cosmonate TDI-100: Product Information and Handling Guide. Technical Bulletin, Rev. 4.2.
6. ASTM D638 – Standard Test Method for Tensile Properties of Plastics.
7. ISO 11358 – Plastics – Thermogravimetry (TGA) – General principles.

Dr. Lin Wei has spent the last 12 years wrestling polymers into submission. When not running TGA cycles, he enjoys hiking, sourdough baking, and explaining why his coffee maker failed (spoiler: poor encapsulation). ☕🔧

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