Silicone is a thermoset, not a thermoplastic. It does not melt, it cannot be reshaped, and if someone tells you to "just heat it back up," you're talking to someone who hasn't worked around cured silicone. That one fact drives everything — why Dow Corning 737 neutral-cure sealant behaves the way it does, why O-ring boss fittings either seal or leak, and why matching Dow Corning silicone colours is harder than buying a Pantone fan deck.
I'm a quality and brand compliance manager at a rubber and plastics manufacturer. In our Q1 2024 quality audit, I reviewed more than 200 unique material specifications and rejected 12% of first deliveries — mostly for shore hardness drift, poor color consistency, and paperwork gaps. Almost every rejection points back to a fundamental misunderstanding of what silicone is at the polymer level.
Is Silicone Thermoplastic or Thermoset? The Straight Answer
Thermoplastics soften when heated and harden when cooled. You can melt, mold, and recycle them. Thermosets form a crosslinked network when they cure; once that network is set, heat doesn't make them flow again. They degrade before they melt.
Silicone rubber is a thermoset. The polymer backbone is polysiloxane — silicon-oxygen bonds with methyl groups attached — and curing creates crosslinks that turn a liquid gum into a solid elastomer. Heat accelerates curing, but heat does not un-cure it. Push cured silicone far enough and it chars, not melts.
Why does this matter? Because the thermoset answer changes how you spec, install, and repair. A thermoplastic part can be welded or remolded; a thermoset part can't. A thermoplastic scrap bin is an option; silicone scrap is a disposal decision.
Now, before someone emails me: yes, there is a material called TPSiV (thermoplastic silicone vulcanizate). It blends silicone with a thermoplastic carrier and is technically processable as a thermoplastic. But it's a hybrid, not pure silicone. Standard silicone elastomers — LSR, RTV, HTV, and one-part sealants like Dow Corning 737 — are thermosets. If a spec sheet just says "silicone elastomer," assume thermoset.
It's tempting to think thermoset vs. thermoplastic is textbook vocabulary. In practice, I've watched engineers design around a silicone part as if it could be remelted. It can't. That's the oversimplification that causes real failures.
What "Thermoset" Means for Dow Corning 737 Silicone Neutro
Take Dow Corning 737. If you're in Brazil, you've seen it listed as "dow corning 737 silicone neutro" on Mercado Livre. It's a one-part, neutral-cure silicone sealant. The neutral cure is the point: unlike acid-cure sealants that release acetic acid (the vinegar smell), 737 cures without corrosive byproducts, so it's a no-brainer for metal flanges, plastic housings, and electronics.
Because it's a thermoset, 737 has a finite working time and a cure window. Clean the surface, apply, and leave it alone. It skins over quickly, then cures through, depending on temperature and humidity. What you can't do is reheat it to soften it later. Removal means cutting or scraping. That's a feature if you want a permanent seal, and a limitation if you want a joint you can open next month.
I have mixed feelings about buying critical sealants on a marketplace. On one hand, the price is often right and delivery is fast. On the other hand, I've seen relabeled product with the wrong viscosity sold as "737." The word "neutro" is doing heavy lifting — if a listing doesn't clearly say neutral cure, that's a red flag. For production work, buy from a licensed distributor and record the batch number. For a Saturday repair, take your chances.
After about four years of chasing quality issues, I've come to believe traceability is protection, not paperwork. The batch number on that tube can save you a leak investigation later.
The O-Ring Challenger: Why Boss Fittings Leak
Now let's talk about O-ring boss fittings, because this is where I see the most well-intentioned mistakes. An O-ring boss (ORB) fitting uses straight threads with an O-ring compressed between the fitting face and the port seat. The threads only provide clamping force. The O-ring does the sealing.
Here's the O-ring challenger I deal with regularly: it's not getting the right O-ring — it's installation damage. In our 2022 audit of hydraulic leak complaints, 7 out of 10 traced back to nicked, twisted, or dry-installed O-rings. The O-ring material was fine. The assembly process was the problem.
And here's the part that surprises most people: sealant on the O-ring is usually wrong. A properly seated O-ring doesn't need chemical help. The conventional wisdom says "a little extra sealant is harmless." My experience says it's how you get leaks that show up after a month, plus fittings that won't come apart later. If you want to protect the threads, use a compatible silicone grease — not a thick sealant like 737.
SAE J514 defines ORB port dimensions and dash sizes. If you're replacing a fitting, don't trust "it looks the same." Mismatched seat angles and thread pitch are deal-breakers, and no sealant will bridge that gap.
So where does 737 fit in an O-ring world? On straight-thread fittings, it can serve as a thread sealant and anti-corrosion layer when the application allows. But it is not a substitute for a correctly sized, correctly durometer O-ring. Bottom line: choose the elastomer for the fluid and temperature, install the O-ring without damage, and let the seal do the sealing.
Dow Corning Silicone Colours: Color Matching Is a Process Problem
One more topic that comes up constantly: "dow corning silicone colours." Color matching in cured silicone is more complicated than most buyers expect. Silicone is tinted with pigment masterbatches before curing, and the final color shifts with processing conditions — mold temperature, cure time, even the surface finish of the mold.
Industry standard color tolerance for brand-critical colors is Delta E less than 2. Delta E of 2–4 is noticeable to trained observers; above 4 is visible to most people. — Pantone Color Matching System guidelines
But here's the nuance: Delta E describes color, not surface texture. A glossy silicone part and a matte silicone part with the same pigment formulation won't look the same to a colorimeter or to your customer. (Note to self: always compare samples with the same texture and thickness.)
It took me four years and too many rejected batches to understand that silicone color consistency is a process variable, not a paint formula. Specifying a Pantone reference is only step one. Define the acceptable Delta E on the actual part surface, under controlled lighting, using a calibrated device — and remember that the thermoset curing step is the wildcard. Same pigment, different cure temperature, visibly different result.
When Silicone Is the Wrong Answer
Because I review specs all day, I'll also tell you when silicone is not the right call. Silicone O-rings have limited abrasion resistance and don't perform well in petroleum-based fluids. For a hydraulic system with hot mineral oil, FKM or HNBR is often the better choice. And an RTV sealant like 737 shouldn't sit permanently submerged in fuel or concentrated acids. The right material in the wrong application fails predictably.
Also read the listing carefully. Not every product that says "dow corning 737 silicone neutro" is the genuine article, and not every silicone sealant is neutral cure. Get an acetoxy-cure product and you could be corroding the components you meant to protect.
So, is silicone thermoplastic or thermoset? Thermoset — always in its final cured form. Is that the answer to every problem? No, and it's not supposed to be. Use it with its limits in mind: cure it as specified, respect the O-ring boss fitting assembly, verify your materials, and treat color as a quality parameter — not a color swatch you can eyeball at 3 p.m. on a Friday.