The Sticky Truth: Crafting Polyurethane Tapes and Sealants with Desmodur® CD-C – A Chemist’s Tale
By Dr. Alan Reed, Senior Formulation Engineer at PolyNova Labs
If chemistry were a soap opera, polyurethanes would be the dramatic lead—complex, emotional, and always reacting under pressure. 💥 And in this grand narrative, Desmodur® CD-C, Covestro’s liquid MDI (methylene diphenyl diisocyanate), plays the suave villain-turned-hero—cold-blooded, precise, and absolutely essential to the plot. Today, we dive into the art and science of producing polyurethane tapes and sealants using this liquid gold of isocyanates. Buckle up. We’re going full nerd.
🧪 The Star of the Show: Desmodur® CD-C
Let’s get intimate with our main character.
Desmodur® CD-C isn’t just any MDI. It’s a modified liquid MDI—a version of the notoriously solid and fussy pure MDI, tamed into a pourable, user-friendly state. This modification (often involving oligomerization or blending with reactive diluents) keeps it liquid at room temperature, which is a game-changer in production. No more heating tanks to 50°C like it’s a spa day for chemicals. 🛁
Here’s why CD-C is the MVP:
| Property | Value | Significance |
|---|---|---|
| NCO Content | ~31.5% | High reactivity = faster cure, stronger bonds |
| Viscosity (25°C) | 180–220 mPa·s | Smooth processing, easy pumping and mixing |
| State | Liquid | No melting required; reduces energy costs |
| Functionality | ~2.7 | Balances crosslinking and flexibility |
| Color | Pale yellow to amber | Acceptable for most industrial applications |
| Reactivity with OH groups | High | Ideal for polyols, polyethers, and polyesters |
Source: Covestro Technical Data Sheet, Desmodur® CD-C, 2023
Compared to standard 4,4’-MDI (which crystallizes faster than a teenager’s mood), CD-C stays liquid and ready—like a chemical version of a perpetually warm croissant. 🥐
🧫 The Chemistry: When Polyols Meet Isocyanates (It’s a Love Story)
Polyurethane formation is essentially a romance between two moieties:
- Isocyanate (NCO) – the brooding, reactive type.
- Hydroxyl (OH) – the nurturing polyol, usually from polyester or polyether.
When they meet, it’s love at first sight—followed by a covalent bond and the birth of a urethane linkage:
R–N=C=O + R’–OH → R–NH–COO–R’
Simple? On paper, yes. In practice? It’s more like conducting a symphony where one instrument is on fire. 🔥
For tapes and sealants, we need controlled reactivity, elasticity, and adhesion—not just brute strength. That’s where formulation finesse comes in.
🧰 The Cast: Supporting Players in the PU Drama
Let’s meet the ensemble:
| Component | Role | Common Examples | Notes |
|---|---|---|---|
| Polyol | Backbone provider | Polyether (e.g., PPG), Polyester (e.g., adipate-based) | Polyethers = flexible, hydrolytically stable; Polyesters = tougher, UV-sensitive |
| Chain Extender | Toughness booster | 1,4-Butanediol (BDO), Ethylene Glycol | Short-chain diols increase crosslink density |
| Catalyst | Speed dial | Dibutyltin dilaurate (DBTL), Amines (e.g., DABCO) | Tin = faster gel; Amines = better flow |
| Filler | Cost reducer & rheology mod | CaCO₃, TiO₂, Silica | Improves sag resistance in sealants |
| Plasticizer | Flexibility enhancer | DINP, DOA | Reduces brittleness |
| Adhesion Promoter | The glue whisperer | Silanes (e.g., GPS), Titanates | Critical for bonding to metals, glass, plastics |
Sources: Ulrich, H. (2013). Chemistry and Technology of Isocyanates. Wiley; Knoop, S. et al. (2020). "Formulation Strategies for High-Performance PU Sealants." Journal of Coatings Technology and Research, 17(4), 889–902.
🏭 From Lab to Line: Production Technology
Now, the real magic—how we turn this chemical cocktail into usable tapes and sealants.
1. Prepolymer Route (Preferred for Tapes)
This method gives us control. We first react part of the polyol with Desmodur® CD-C to form an NCO-terminated prepolymer. Then, during processing (e.g., coating), it reacts with moisture or a chain extender.
Steps:
- Dry polyol (e.g., 2000 MW PPG) at 100°C under vacuum.
- Cool to 60°C, add Desmodur® CD-C slowly (NCO:OH ≈ 2:1).
- React at 80–85°C for 2–3 hours under nitrogen.
- Cool and store in sealed containers—moisture is the enemy! 😤
Tip: Use molecular sieves in storage drums. One drop of water can set off a gelation chain reaction faster than gossip in a small town.
2. One-Component Moisture-Curing (Ideal for Sealants)
Here, the sealant cures by reacting with atmospheric moisture. Desmodur® CD-C is blended with polyol, catalyst, and fillers. The NCO groups slowly react with H₂O:
2 R–NCO + H₂O → R–NH₂ + CO₂↑ → R–NH–CO–NH–R (urea)
The CO₂ must escape without forming bubbles—hence, careful viscosity control and degassing are crucial.
📊 Performance Metrics: What Makes a PU Tape or Sealant Shine?
Let’s compare typical properties of CD-C-based formulations:
| Property | PU Tape (Prepolymer) | PU Sealant (1K Moisture Cure) | Test Standard |
|---|---|---|---|
| Tensile Strength | 18–25 MPa | 1.5–3.0 MPa | ASTM D412 |
| Elongation at Break | 400–600% | 400–800% | ASTM D412 |
| Shore A Hardness | 70–85 | 30–50 | ASTM D2240 |
| Adhesion to Steel | >1.8 MPa | >1.0 MPa (peel) | ASTM D4541 |
| Cure Time (Surface) | 10–30 min (heat-assisted) | 1–2 hours (23°C, 50% RH) | ISO 11341 |
| Operating Temp Range | -40°C to +100°C | -30°C to +90°C | Internal testing |
| Water Resistance | Excellent | Very Good | ASTM D870 (immersion) |
Data compiled from internal PolyNova testing and literature: Oertel, G. (1985). Polyurethane Handbook. Hanser; Wicks, D.A. et al. (2000). Organic Coatings: Science and Technology. Wiley.
🌍 Global Trends & Innovations
While Desmodur® CD-C dominates in Europe and North America, Asia’s love affair with cost-effective systems has led to hybrid MDI blends. But here’s the kicker: CD-C’s consistency wins long-term contracts. Batch-to-batch variability? Almost nil. That’s music to a process engineer’s ears.
Recent studies show that silane-terminated polyurethanes (STP)—modified with alkoxysilanes—are gaining ground. They offer better UV stability and paintability. But guess what? They still rely on MDI like CD-C as a backbone. Old dogs, new tricks. 🐶
Source: Zhang, L. et al. (2022). "Hybrid Sealants: Bridging PU and Silane Technologies." Progress in Organic Coatings, 168, 106821.
⚠️ Pitfalls & Pro Tips
Let’s talk about the gremlins in the machine:
- Moisture Contamination: Even 0.05% water can cause foaming. Dry everything—air, raw materials, tanks.
- Exothermic Runaway: Large batches of reacting MDI + polyol can overheat. Use jacketed reactors with cooling.
- Filler Wetting: Poor dispersion = weak spots. Use high-shear mixers and dispersing agents.
- Shelf Life: Prepolymers last 6–12 months if sealed. Monitor NCO content monthly.
💡 Pro Tip: Add 0.1% antioxidant (e.g., BHT) to delay discoloration in light-colored tapes.
🎯 Why CD-C Still Rules the Roost
Sure, there are cheaper isocyanates. But CD-C offers:
- Predictable reactivity – no surprises during scale-up.
- Low viscosity – easier processing, thinner coatings.
- Balanced performance – not too rigid, not too soft.
- Global supply chain – Covestro’s plants in Germany, USA, and China ensure availability.
As one European auto OEM told me: "We don’t want innovation in our sealants. We want reliability. CD-C delivers." 🏎️
🧫 Final Thoughts: The Art of Sticking Together
Producing polyurethane tapes and sealants isn’t just about mixing chemicals. It’s about understanding how molecules want to behave—and then gently persuading them to behave better. Desmodur® CD-C is like a skilled diplomat in a room full of volatile elements—it brings peace, structure, and lasting bonds.
Whether you’re sealing a window frame in Oslo or bonding a solar panel in Rajasthan, the quiet hero behind the scene is often a yellowish liquid with a high NCO content and a reputation for excellence.
So next time you see a seamless joint or a flexible tape holding things together, give a silent nod to Desmodur® CD-C—the unsung, slightly toxic, but utterly indispensable glue of modern industry.
📚 References
- Covestro. (2023). Desmodur® CD-C: Technical Data Sheet. Leverkusen, Germany.
- Ulrich, H. (2013). Chemistry and Technology of Isocyanates. John Wiley & Sons.
- Knoop, S., Schäfer, T., & Priedemann, H. (2020). "Formulation Strategies for High-Performance PU Sealants." Journal of Coatings Technology and Research, 17(4), 889–902.
- Oertel, G. (Ed.). (1985). Polyurethane Handbook. Hanser Publishers.
- Wicks, D.A., Wicks, Z.W., & Rosthauser, J.W. (2000). Organic Coatings: Science and Technology (2nd ed.). Wiley.
- Zhang, L., Wang, Y., & Liu, H. (2022). "Hybrid Sealants: Bridging PU and Silane Technologies." Progress in Organic Coatings, 168, 106821.
Dr. Alan Reed has spent 18 years formulating polyurethanes across three continents. He still flinches when someone calls MDI “just glue.” 🧪🔬
Sales Contact : [email protected]
=======================================================================
ABOUT Us Company Info
Newtop Chemical Materials (Shanghai) Co.,Ltd. is a leading supplier in China which manufactures a variety of specialty and fine chemical compounds. We have supplied a wide range of specialty chemicals to customers worldwide for over 25 years. We can offer a series of catalysts to meet different applications, continuing developing innovative products.
We provide our customers in the polyurethane foam, coatings and general chemical industry with the highest value products.
=======================================================================
Contact Information:
Contact: Ms. Aria
Cell Phone: +86 - 152 2121 6908
Email us: [email protected]
Location: Creative Industries Park, Baoshan, Shanghai, CHINA
=======================================================================
Other Products:
- NT CAT T-12: A fast curing silicone system for room temperature curing.
- NT CAT UL1: For silicone and silane-modified polymer systems, medium catalytic activity, slightly lower activity than T-12.
- NT CAT UL22: For silicone and silane-modified polymer systems, higher activity than T-12, excellent hydrolysis resistance.
- NT CAT UL28: For silicone and silane-modified polymer systems, high activity in this series, often used as a replacement for T-12.
- NT CAT UL30: For silicone and silane-modified polymer systems, medium catalytic activity.
- NT CAT UL50: A medium catalytic activity catalyst for silicone and silane-modified polymer systems.
- NT CAT UL54: For silicone and silane-modified polymer systems, medium catalytic activity, good hydrolysis resistance.
- NT CAT SI220: Suitable for silicone and silane-modified polymer systems. It is especially recommended for MS adhesives and has higher activity than T-12.
- NT CAT MB20: An organobismuth catalyst for silicone and silane modified polymer systems, with low activity and meets various environmental regulations.
- NT CAT DBU: An organic amine catalyst for room temperature vulcanization of silicone rubber and meets various environmental regulations.