Methyl Ethyl Ketone Peroxide (MEKP, CAS 1338-23-4) is the most widely used initiator for room-temperature curing of unsaturated polyester and vinyl ester resins — ubiquitous in FRP hand lay-up, spray-up, casting and resin crafts. This guide covers curing mechanism, half-life, formulation and safety.
Identity
- Chemical name: methyl ethyl ketone peroxide / 2-butanone peroxide
- CAS: 1338-23-4 (formulations often contain dimethyl phthalate 131-11-3 as phlegmatiser)
- Common forms: ~30–50% solutions in phthalate plasticiser (e.g. MEKP-925, MEKP-30)
Mechanism & half-life
With a cobalt (e.g. cobalt octoate) or amine promoter, MEKP decomposes at room temperature to generate radicals that trigger styrene crosslinking in UP/vinyl ester resins. Its 10-hour half-life temperature is ~60 °C (typical literature value; formulation-dependent), enabling ambient-to-medium-temperature cure with gel time tuned by promoter level and temperature.
Applications
- Room-temperature curing of FRP hand lay-up, spray-up, winding and moulding
- Artificial stone, castings, resin crafts and repair putties
- Ambient curing of vinyl ester anti-corrosion resins
Formulation & handling
- MEKP and cobalt promoter must be added separately and metered independently — never mix the two directly (fire/explosion hazard).
- Dosage depends on resin and process; in cold conditions increase promoter gradually and avoid a single large addition that causes runaway exotherm.
- Control batch size and heat removal; cast thick parts in layers.
Safety & storage
- Strong oxidiser, sensitive to heat, shock and reducing agents; avoid contamination with metals, promoters and strong bases.
- Store cool and dark (typically ≤ 25 °C), follow the SDS SADT and stack limits; handle in well-ventilated areas.
- Keep separate from cobalt/amine promoters.
Selection
For fast ambient cure choose MEKP + cobalt; for 60–90 °C medium-temperature cure consider BPO; for higher temperatures use TBPB/TBPEH/DCP. Common equivalents include Norox MEKP-30/925. See the half-life chart and the BPO guide.
Note: half-life and formulation values are typical references; confirm against the supplier TDS/SDS and plant trials.