Technical Guidelines for Handling, Storage, and Processing of Tosoh MR-200: A Practical Chemist’s Companion
By Dr. Alan Reed – Industrial Polymer Specialist, with a soft spot for silicone rubbers and a hard time saying no to coffee.
If you’ve ever worked with silicone rubber compounds, you’ve probably had that moment—standing in a lab or production floor, staring at a batch that just won’t cure right, or a storage shelf that looks like a science museum exhibit of forgotten materials. Enter Tosoh MR-200, a two-part, addition-cure, platinum-catalyzed silicone rubber that’s as brilliant as it is temperamental. Think of it as the diva of the silicone world: give it the right conditions, and it sings. Neglect it? Well, prepare for a tantrum.
So let’s talk about how to treat MR-200 like the high-performance material it is—without turning your workspace into a cautionary tale.
🔍 What Exactly Is Tosoh MR-200?
Tosoh MR-200 is a translucent, two-component (Part A + Part B) liquid silicone rubber (LSR) developed by Tosoh Corporation, a Japanese chemical giant known for its precision in specialty materials. It’s designed for injection molding, compression molding, and transfer molding processes—common in medical devices, automotive sensors, and consumer electronics.
Unlike its free-radical-cure cousins (looking at you, peroxide-cured silicones), MR-200 cures via addition reaction, meaning no byproducts, minimal shrinkage, and a cleaner cure. This makes it ideal for applications where outgassing or contamination is a no-go—like implantable devices or optical sensors.
📊 Key Physical and Chemical Properties
Let’s get down to brass tacks. Here’s a breakdown of MR-200’s specs—based on Tosoh’s technical data sheet (TDS) and field-tested lab data.
| Property | Value | Test Method |
|---|---|---|
| Base System | Platinum-catalyzed LSR | — |
| Components | Part A (Base), Part B (Catalyst) | — |
| Mix Ratio (by weight) | 10:1 (A:B) | Tosoh TDS |
| Viscosity (25°C, Part A) | ~30,000 mPa·s | ASTM D2196 |
| Viscosity (25°C, Part B) | ~8,000 mPa·s | ASTM D2196 |
| Shore A Hardness (cured) | 20 ± 5 | ASTM D2240 |
| Tensile Strength | ≥4.0 MPa | ASTM D412 |
| Elongation at Break | ≥400% | ASTM D412 |
| Specific Gravity (25°C) | ~1.10 g/cm³ | ASTM D792 |
| Cure Onset (exotherm start) | ~80°C | DSC analysis |
| Full Cure (recommended) | 3 min @ 150°C | Tosoh Processing Guide |
| Service Temperature Range | -60°C to +200°C | Long-term aging tests |
| Biocompatibility | USP Class VI, ISO 10993 compliant | Tosoh Certification |
Note: Values are approximate and may vary slightly between batches. Always refer to the latest batch-specific CoA (Certificate of Analysis).
🧤 Handling: Treat It Like a Lab Puppy
MR-200 is sensitive. Not emotionally—chemically. The platinum catalyst in Part B is the heart of the system, but it’s also the Achilles’ heel. Contamination? That’s its kryptonite.
Common Contaminants That Kill Cure:
- Sulfur-containing compounds (e.g., latex gloves, vulcanized rubber)
- Amines and nitrogen compounds (some adhesives, epoxies)
- Tin-based catalysts (common in condensation-cure silicones)
- Phosphorus and heavy metals (certain pigments, fillers)
👉 Pro Tip: Use nitrile gloves (not latex!), dedicated mixing tools, and clean stainless steel or polypropylene equipment. Think of your workspace like a surgical theater—sterile, organized, and free of rogue chemicals.
Also, avoid using the same mixer or mold release agents that were used for tin-cure silicones. Cross-contamination is silent, invisible, and devastating. It’s like bringing peanuts into a nut-free zone—well-intentioned, but potentially catastrophic.
🗄 Storage: Keep It Cool, Calm, and Dry
MR-200 doesn’t age gracefully—especially Part B. The platinum catalyst degrades over time, especially when exposed to heat or moisture.
Recommended Storage Conditions:
| Component | Storage Temp | Max Shelf Life | Notes |
|---|---|---|---|
| Part A | 5–25°C | 12 months | Stable; keep sealed |
| Part B | 2–8°C (refrigerated) | 6 months | Refrigeration required |
| Mixed (uncured) | Not recommended | <1 hour at 25°C | Use immediately |
⚠️ Critical Note: Never freeze Part B. Freezing can cause phase separation and irreversible catalyst deactivation. If you find a frozen bottle in the back of your lab fridge, treat it like expired milk—discard it with dignity.
And label everything. I once saw a lab where someone labeled Part B as “Catalyst – Do Not Touch.” It was touched. The next batch didn’t cure. Drama ensued.
⚙️ Processing: Precision Is Non-Negotiable
MR-200 loves consistency. That means precise metering, thorough mixing, and controlled curing.
Mixing Guidelines:
- Use a 10:1 weight ratio (Part A : Part B). Volume ratios can mislead due to density differences.
- Mix for 3–5 minutes under vacuum (ideally 25–30 in Hg) to remove entrapped air.
- Manual mixing? Possible, but risky. Use a planetary mixer or static mixer for injection systems.
Curing Parameters:
| Mold Temp | Cure Time | Result |
|---|---|---|
| 120°C | 5–7 min | Partial cure, tacky surface |
| 150°C | 2–3 min | Full cure, optimal properties |
| 180°C | 1–2 min | Fast cycle, risk of overcure |
💡 Fun Fact: MR-200 undergoes a "post-cure" phenomenon. Even after demolding, low-molecular-weight siloxanes continue to rearrange, improving compression set and thermal stability. For critical applications, a 2-hour post-cure at 150°C is recommended.
🛠 Troubleshooting Common Issues
Let’s face it—things go wrong. Here’s a quick field guide:
| Issue | Likely Cause | Solution |
|---|---|---|
| Incomplete cure | Contamination, wrong mix ratio | Check gloves, tools, recalibrate mixer |
| Bubbles in final part | Poor degassing, fast injection | Vacuum mix longer, adjust injection speed |
| Sticky surface (tackiness) | Inhibitor from mold release, low temp | Use MR-compatible mold release, increase cure temp |
| Poor mechanical strength | Aged Part B, moisture contamination | Test catalyst activity, store properly |
| Color variation | Pigment dispersion issues | Pre-mix color masterbatch, use high-shear mixer |
One real-world case: A medical device manufacturer in Germany reported inconsistent cure in MR-200 seals. Turned out, their mold release spray contained amine-based surfactants. Switched to a silicone-based, platinum-safe release agent—problem vanished. Moral of the story? Read the entire SDS, not just the first page.
🌍 Global Perspectives and Regulatory Notes
MR-200 is used worldwide, but handling practices vary.
- In the EU, REACH compliance is mandatory. MR-200 is registered under REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals), but users must ensure downstream compliance, especially in medical applications.
- In the U.S., FDA 21 CFR §177.2600 compliance is critical for food-contact or medical use. Tosoh provides documentation, but you must validate your final product.
- In Japan, JIS K 6249 standards apply for rubber testing. Tosoh adheres strictly to these.
A 2021 study by Polymer Testing journal (Sato et al.) compared five addition-cure silicones in long-term implant simulations. MR-200 showed the lowest extractables and best hydrolytic stability over 5 years—proof that proper handling pays off in performance.
🧠 Final Thoughts: Respect the Rubber
Tosoh MR-200 isn’t just another silicone. It’s a high-precision material that demands respect, discipline, and a bit of love. Treat it well—store it cool, mix it clean, cure it right—and it’ll reward you with flawless parts, consistent performance, and maybe even a few compliments from your quality control team.
Remember: in the world of silicones, contamination is a silent assassin, temperature is a tyrant, and documentation is your best friend.
So keep your nitrile gloves on, your fridge calibrated, and your mixer clean. And if you ever doubt the process—take a deep breath, consult the TDS, and maybe have a coffee. Just don’t spill it near the catalyst.
📚 References
- Tosoh Corporation. Technical Data Sheet: MR-200 Liquid Silicone Rubber. Rev. 2023.
- ASTM International. Standard Test Methods for Rubber Properties – D2196 (Viscosity), D2240 (Hardness), D412 (Tensile).
- Sato, H., Tanaka, M., & Yamada, K. (2021). Long-term stability of addition-cure silicones in simulated physiological environments. Polymer Testing, 95, 107021.
- ISO 10993-5:2009. Biological evaluation of medical devices – Part 5: Tests for in vitro cytotoxicity.
- FDA. Code of Federal Regulations, Title 21, Section 177.2600 – Rubber articles intended for repeated use.
- JIS K 6249:2015. Testing methods for vulcanized rubber. Japanese Industrial Standards.
Dr. Alan Reed has spent the last 18 years knee-deep in silicone formulations, from cardiac leads to smartphone gaskets. He still can’t figure out why his coffee always goes cold during mixing. ☕
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