1. Same curing power, different behaviour

Equal active oxygen means equal curing power per kilogram, but not always an identical gel time. M-50A contains a high share of MEKP monomer, which can make it gel faster than M-50 in some resin systems.

2. Storage stability

All three grades share a maximum storage temperature of 25 °C. The SADT differs: 60 °C for M-50 and 55 °C for M-50 VRN. The lower the SADT, the less margin you have in a hot warehouse.

3. Water content and part quality

Butanox M-50 has guaranteed low water content and no polar compounds, which is why Nouryon recommends it for GRP parts that must resist water, such as boats. A cobalt and MEKP cure can also make light-resistant parts possible, which benzoyl peroxide and amine systems cannot.

Applications of Each Grade

1. Butanox M-50: the all-round standard

Gelcoat and laminating resins, lacquers and castings, hand lay-up and spray-up, filament winding, resin transfer moulding and continuous laminating. Nouryon calls it the work horse of GRP curing systems, and that matches what I see in the market.

2. Butanox M-50A: phthalate-free production

Engineered stone, gelcoats, centrifugal casting, coatings, continuous laminating, resin transfer moulding, vinyl esters, spray fillers, and hand lay-up and spray-up. Besides cobalt-accelerated room-temperature cure, it can be used for heat cure of unpromoted UP resin at 100–127 °C.

3. Butanox M-50 VRN: visible dosing

Any process where catalyst addition is checked by eye. The resin turns red when the catalyst is mixed in, so a missed or uneven dosage shows up before the resin gels; the colour then virtually disappears during cure. For more on how colour-indicator MEKP works, see our article on fading MEKP.

Butanox M-50 vs M-50A vs M-50 VRN: How to Choose?

When a customer asks me which grade to buy, I start with three questions:

- Must the finished part or the process be phthalate-free? If yes, choose Butanox M-50A, and run a gel time test in your own resin before switching a production line.

- Do your operators need to see that catalyst is mixed in? If yes, choose Butanox M-50 VRN.

- Neither? Choose Butanox M-50, the standard grade.

If none of the three fits your process, look beyond the M-50 family. Butanox M-60 has about 10 % more active oxygen than M-50, and Butanox LPT-IN gives an extra slow gel for filament winding and marine GRP. For vinyl ester resins, also consider Trigonox V388; our Trigonox V388 vs Butanox M-50 comparison covers that choice in detail.

One practical note: Butanox M-50 VRN has replaced Butanox M-50 VR. If your purchase specification still says "M-50 VR", it is time to update it.

Safety and Handling Considerations

Having worked on organic peroxide safety standards, I would rather you read this section twice than skip it. All three grades are transported as organic peroxide type D, liquid (Division 5.2, UN 3105).

- Keep it cool. Store below 25 °C in closed original containers, away from heat and direct sunlight. Summer warehouses get closer to the SADT than many people expect.

- Never mix MEKP directly with a cobalt accelerator. Add each one to the resin separately and stir in between.

- Avoid contamination. Store MEKP apart from accelerators, reducing agents, acids, alkalis and metal compounds, and never pour unused material back into the original container.

- Watch the date. Under the recommended conditions, Butanox M-50 and M-50 VRN stay within specification for at least 6 months after delivery. Check the M-50A data sheet for its shelf life.

Frequently Asked Questions

Is Butanox M-50A stronger than Butanox M-50?

No. Both have a total active oxygen content of 8.8–9.0 %. M-50A is phthalate-free and has a high MEKP monomer content, which can give faster gel times in some resin systems, so test it before switching.

What is the difference between Butanox M-50 VR and M-50 VRN?

Butanox M-50 VRN has replaced Butanox M-50 VR. Both are MEKP in dimethyl phthalate with a vanishing red indicator that shows mixing and fades during cure.

Does the red colour of Butanox M-50 VRN stay in the part?

No. The red colour is visible while the catalyst is mixed into the resin and virtually disappears during cure.

Can Butanox M-50 be replaced by M-50A without changing the process?

Often, because both have the same active oxygen content. M-50A can gel faster in some resins, so run a gel time test in your own resin and accelerator system first.

Where can I get the TDS and SDS for these grades?

Product data sheets and safety data sheets can be downloaded from the Butanox M-50, M-50A and M-50 VRN product pages, or requested through the inquiry form.

Ava's Perspective: The Right MEKP Is the One That Fits Your Process, Not Your Price List

From my perspective, choosing between M-50, M-50A and M-50 VRN is rarely about chemistry alone, because their curing power is the same. It is about the rules your products must meet, how your operators work, and how well your warehouse can keep the material cool. A phthalate-free requirement, a hand-mixing step that needs a visual check, or a hot storage area will each point to a different grade. When I review a customer's process with these three points in mind, the right choice is usually clear within minutes.

At Evergreen Chemicals, our Organic Peroxide Expert Library works with customers worldwide as an authorized Nouryon distributor, supplying the full Butanox, Trigonox and Perkadox range together with technical selection support and the latest data sheets. See the complete organic peroxide range.

Not sure which Butanox grade fits your line? Send Ava your resin system and process conditions, and our team will recommend a grade and send you the latest TDS and SDS.