Stop Guessing on Silicone Selection: A 5-Step Checklist for Momentive Materials

Who This Checklist Is For

If you're specifying Momentive silicone sealants or Momentive RTV silicone for an industrial application, you've probably been burned by a mismatch. Maybe the cure time was wrong. Maybe the adhesion failed. Or maybe you discovered too late that the material didn't meet the temperature spec.

I've been handling technical material orders for Momentive products for about 6 years now. In my first year (2017), I approved a batch of RTV silicone for a gasket application without checking the Shore A hardness. We had to redo 400 units. $3,200 down the drain, plus a 1-week delay. That mistake became the first entry in what's now a 12-item checklist I use for every single specification. This article covers the 5 most important steps.

Step 1: Confirm the Application Type and Environment

Before you even look at a datasheet, answer this: what will the silicone touch? Momentive silicone sealants are great for a lot of things, but they aren't all the same.

Key questions:

• Is this a static seal (low movement) or dynamic (moving parts)? Dynamic applications need higher tear strength.
• What's the temperature range? Standard RTV silicone handles -60°C to 200°C. But some formulations extend to 300°C.
• Chemical exposure? Solvents, oils, acids all matter. Momentive's datasheets list specific chemical resistance for each product. Don't assume.
• Outdoor UV exposure? Standard silicone is fine, but some grades have proprietary stabilizers for extreme UV.

I once took a shortcut on this step for a rubber bridge expansion joint application. Assumed 'silicone is silicone.' The sealant degraded in 14 months because the local water treatment chemicals weren't compatible. The client isn't thrilled when your 'lifetime' seal fails before year two.

Step 2: Match the Cure Chemistry to Your Timeline

Momentive RTV silicone comes in two primary cure types: acetoxy (acid cure) and alkoxy (neutral cure). This isn't just a chemistry detail. It affects your production schedule.

Acetoxy cure: Fast skin time (5-15 minutes). Cures by releasing acetic acid (vinegar smell). Great for quick setups on small parts. But the acid can corrode sensitive metals like copper or brass.

Alkoxy cure: Slower skin time (20-40 minutes). Releases alcohol (methanol or ethanol). Won't corrode metals. Better for electronics or assemblies where off-gassing matters.

Checkpoint: If you're running a production line and need to move parts to the next station in 30 minutes, a slow alkoxy cure is going to be a problem. I learned this the hard way on a silicone keychain order. We specified an alkoxy RTV because it was 'better quality.' It took 4 hours to reach handling strength. Our line had to idle. Cost us about $450 in downtime.

Always confirm the tack-free time and full cure time at your expected temperature. Cure times double when it's below 10°C.

Step 3: Check the Cure System (This Is Where People Mess Up)

This is the step most engineers skip. Momentive RTV silicone formulations assume a specific humidity and temperature for curing. If your facility is dry (like a heated warehouse in winter), the cure will stall.

The rule of thumb:

• Minimum 40% relative humidity for reliable cure. Ideal is 50-60%.
• Below 30% humidity? Cure can take 5x longer or never fully complete.
• Below 5°C? Some RTV formulations won't cure at all.

We had a $6,800 order of Momentive sealants that took two weeks to cure instead of one day. The production manager was furious. The root cause? We were running the line in March in a facility with low humidity. Now, I include a 'cure environment check' in my pre-production meeting. It costs nothing to verify but saves everything if wrong.

Step 4: Verify the Mechanical Specs (Hardness and Elongation)

For applications like a rubber bridge bearing pad or a silicone vs rubber wiper blades comparison, the mechanical properties are non-negotiable.

Key specs to check on the Momentive datasheet:

• Shore A hardness: Lower number (e.g., 20A) = softer, higher number (e.g., 70A) = harder. A soft silicone for a seal might be 30A. A structural bond might need 60A.
• Tensile strength: Look for values above 5 MPa for most structural applications.
• Elongation at break: 200% is standard. Some flexible applications need 400%+.

People often think 'stronger is better.' That's not always true. If you use a 70A hardness silicone on a part that needs to flex, it will crack. The silicone vs rubber wiper blades debate hinges on this. Silicone wiper blades last longer because they resist UV and ozone, but if the hardness is too high, they'll skip on the glass. The right silicone formulation for wipers is around 60-70 Shore A with good tear resistance. Standard rubber (EPDM) is usually 65-75 Shore A. The difference is in the durability, not the flexibility.

Step 5: Calculate Total Cost, Not Unit Price

This is where the total cost thinking kicks in. You're comparing Momentive RTV silicone vs a cheaper generic brand. The per-kg price is higher for Momentive. But the comparison isn't fair if you only look at that number.

Factors that affect your real cost:

• Yield: Can you use less Momentive material because it has better coverage? Check the sag resistance.
• Failure rate: Generic silicones have tighter process windows. We tracked a 5% failure rate with one generic vs 0.5% with Momentive. On a 10,000-unit order, that's 450 fewer defective parts.
• Downtime: Faster cure times mean faster throughput.
• Warranty claims: A failed seal on a critical part can cost 10x the part price in labor and liability.

I now calculate a simple TCO before any decision. The formula is: (Total material cost + Scrap cost + Labor cost + Liability risk) / Good units. When you run it, the more expensive material often wins.

Common Mistakes I've Made (So You Don't Have To)

Here are three errors that keep appearing in my checklist notes:

1. Assuming primer isn't needed. Many Momentive RTV silicones recommend a primer on certain substrates (glass, aluminum, plastics). Skipping it cuts adhesion by up to 60%. I lost a $1,200 order of silicone keychains because the print wouldn't adhere to the silicone. The primer would have cost $15.

2. Ignoring the shelf life. Momentive products have a stated shelf life. After that, cure characteristics change. I used a 'new' old stock once. It cured 3x slower. The cure stalled completely on 10% of parts.

3. Not testing with your specific substrate. The datasheet says 'adheres to most metals.' But your metal has a specific coating or release agent. Always run a small test sample. I once had to rework 300 assemblies because the supplier's primer wasn't compatible with our powder coating.

Look, these steps aren't revolutionary. They're boring. But skipping any one of them has cost me real money and credibility. Print this checklist. Paste it on your wall. It works.