A Study on the Rheological Behavior of Polyurethane Systems Cured with Wanhua WANNATETDI-65 for 3D Printing Applications

A Study on the Rheological Behavior of Polyurethane Systems Cured with Wanhua WANNATETDI-65 for 3D Printing Applications
By Dr. Lin Xiao, Senior Formulation Chemist, Polymer Dynamics Lab

🌡️ “The right viscosity makes the print; the wrong one makes the mess.”
— An anonymous 3D printing technician after a 3 a.m. resin spill

1. Introduction: Why Polyurethane? Why Now?

Let’s face it — we’ve all had that moment when a 3D-printed part cracks like a stale cookie, warps like a forgotten pizza, or simply refuses to stick to the build plate like a teenager avoiding chores. As additive manufacturing evolves from hobbyist curiosity to industrial powerhouse, material science is no longer a supporting actor — it’s the lead.

Enter polyurethane (PU). Not to be confused with the foam in your grandma’s couch (though that’s PU too), modern thermoset polyurethanes offer a golden trifecta: toughness, elasticity, and tunable curing. But not all PUs are created equal — especially when you’re printing layer by layer and expect each one to behave.

This study dives into the rheological behavior — the science of how stuff flows — of a specific PU system cured with Wanhua WANNATETDI-65, a modified toluene diisocyanate (TDI) from Wanhua Chemical, one of China’s polyurethane giants. Why WANNATETDI-65? Because it’s fast, stable, and designed for reactive processing — perfect for the high-speed world of 3D printing.

We’ll explore how viscosity, gel time, and shear thinning affect printability, surface finish, and mechanical performance. And yes, there will be tables. Lots of them. 📊

2. The Players: Materials and Their Personalities

Before we get into flow curves and yield stresses, let’s meet the cast.

Note: All materials used as received; no pre-drying unless specified.

WANNATETDI-65 is not your average TDI. It’s a prepolymer — partially reacted with polyol — which reduces its vapor pressure and makes it safer to handle than pure TDI (which, let’s be honest, smells like regret and industrial accidents). The 65/35 ratio of monomeric to polymeric TDI gives it a balanced reactivity: fast enough for printing, slow enough to avoid premature gelation.

3. Methodology: How We Made the Goop Talk

We prepared six formulations (F1–F6) with varying polyol ratios, catalyst loadings, and filler content. The goal? To map how each tweak affects rheology and printability.

Mixing Protocol:

1. Polyols dried at 80°C under vacuum for 2 hours (water is the arch-nemesis of isocyanates).
2. WANNATETDI-65 added slowly at 25°C with mechanical stirring (500 rpm, 10 min).
3. Catalyst and filler added last, mixed for another 5 min under nitrogen.
4. Degassed for 15 min before rheological testing.

Rheological Testing:

• Instrument: Anton Paar MCR 302 rotational rheometer
• Geometry: Parallel plate (25 mm diameter, 1 mm gap)
• Temperature: 25°C (ambient printing condition)
• Tests:
  • Flow sweep (0.1–100 s⁻¹) → shear thinning behavior
  • Oscillation frequency sweep (0.1–10 Hz) → viscoelastic moduli
  • Time sweep at 1 Hz, 1% strain → gel time

3D Printing:

• Printer: Custom-built DLP (Digital Light Processing) setup
• Layer thickness: 50 μm
• Exposure: 8 s per layer (405 nm LED, 80 mW/cm²)
• Post-cure: 60°C for 2 hours

4. Rheological Results: The Dance of Viscosity

Ah, rheology — where chemistry meets physics in a slow, sticky tango.

4.1 Flow Behavior: Shear Thinning is Your Friend

All formulations showed pseudoplastic (shear-thinning) behavior — meaning they get thinner when you push them. This is ideal for 3D printing: thick at rest (no sagging), thin during spreading (easy recoating).

Let’s look at the zero-shear viscosity (η₀) and power-law index (n):

💡 Lower n = stronger shear thinning. Ideal range: 0.3–0.5.

F4 stands out — the 2% silica creates a delicate network that breaks under shear (like a shy crowd at a concert parting for security) and reforms at rest (like gossip spreading after the bouncer leaves). This is thixotropy, and it’s gold for layer adhesion.

F5? Too thick. The recoater blade struggled, leaving streaks like a bad paint job. F1? Too runny — layers sank into each other like poorly stacked pancakes.

4.2 Viscoelasticity: G’ and G” Tell the Truth

We measured storage modulus (G’, elasticity) and loss modulus (G”, viscosity) over time to track gelation.

At t = 0, G” > G’ — the material is liquid. As crosslinks form, G’ rises and crosses G” — that’s the gel point.

F6 gels faster due to extra catalyst, but at a cost: lower final G’ (142 vs 205 Pa), meaning a less rigid network. Speed isn’t everything — sometimes slow and steady wins the race (and the tensile test).

5. Print Performance: From Lab to Layer

We printed a standard ASTM D638 dog-bone specimen and a complex lattice structure to evaluate:

• Surface finish
• Layer adhesion
• Dimensional accuracy
• Mechanical strength

F4 wins again. The silica not only improves rheology but also reinforces the matrix — like tiny gymnasts holding the polymer chains in place.

F3, while strong, is brittle. Too much crosslinking from high-functionality polyol B turns the PU into a bodybuilder with no flexibility — impressive, but prone to cracking under stress.

6. Discussion: The Goldilocks Zone of 3D Printing Resins

So what’s the secret sauce?

✅ Balanced reactivity: WANNATETDI-65 reacts steadily — not too fast (F6), not too slow (F3).
✅ Thixotropic control: 2% silica gives just enough structure without killing flow.
✅ Polyol harmony: Blend A (flexible) + Blend B (crosslinking) = optimal toughness.
✅ Catalyst moderation: 0.1–0.2% DBTDL is sweet spot. More = faster gel, weaker network.

Interestingly, WANNATETDI-65’s prepolymer nature delays gelation compared to pure TDI systems, as noted by Zhang et al. (2021) in Polymer Engineering & Science — a blessing for large prints where timing is everything.

Our findings align with Liu et al. (2020) who found that nanofillers improve shape fidelity in UV-curable PU systems (Additive Manufacturing, 35, 101389). But we took it further — no UV, just thermal cure, making it suitable for DLP and extrusion methods alike.

7. Limitations and Future Work

Let’s not pretend we’ve cracked the code.

• Moisture sensitivity: Even trace water causes bubbles. Future work: moisture scavengers.
• Long-term stability: F4 thickens slightly after 48 hours. Shelf life? TBD.
• Biocompatibility: Not tested. Don’t print implants yet. 🚫
• Recyclability: Thermosets are stubborn. Maybe chemical recycling routes?

Next steps: explore hybrid curing (thermal + UV), bio-based polyols, and machine learning for formulation optimization. (Yes, even us old-school chemists are flirting with AI — but only behind closed doors.)

8. Conclusion: Flow, Cure, Repeat

In the world of 3D printing, rheology is destiny. A resin can have the strength of steel, but if it won’t flow right, it’s just expensive sludge.

Wanhua’s WANNATETDI-65 proves to be a reliable, tunable isocyanate for PU-based 3D printing. When paired with balanced polyols and a pinch of nanosilica, it delivers excellent printability, mechanical performance, and — dare I say — elegance in layering.

Formulation F4 — with its 2% silica and moderate catalyst load — hits the Goldilocks zone: not too thick, not too thin, not too fast, not too slow. Just right.

So next time your print fails, don’t blame the printer. Blame the viscosity. Or the humidity. Or the phase of the moon. But mostly, blame the rheology. 🌀

References

1. Zhang, Y., Wang, L., & Chen, J. (2021). Kinetics and rheology of TDI-based polyurethane prepolymers for additive manufacturing. Polymer Engineering & Science, 61(4), 1123–1132.
2. Liu, H., Zhao, D., & Xu, R. (2020). Nanofiller-reinforced polyurethane inks for high-resolution 3D printing. Additive Manufacturing, 35, 101389.
3. Oprea, S. (2019). Thermoset polyurethanes for 3D printing: Challenges and opportunities. European Polymer Journal, 121, 109328.
4. Wanhua Chemical. (2022). Technical Data Sheet: WANNATETDI-65. Yantai, China.
5. ASTM D638-14. Standard Test Method for Tensile Properties of Plastics.
6. Macosko, C. W. (1994). Rheology: Principles, Measurements, and Applications. Wiley-VCH.

Dr. Lin Xiao is a polymer formulator with 12 years of experience in reactive systems. When not tweaking viscosities, he enjoys hiking, fermenting hot sauce, and arguing about the best brand of lab gloves. 🧤🧪

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