In open-mold unsaturated polyester resin (UPR) lamination, cyclohexanone peroxide (CHP) is metered into a resin pre-accelerated with cobalt octoate. The working dosage is 1.0–2.0 phr CHP with 0.2–0.5 phr cobalt octoate 6% metal content on resin mass. The redox pair decomposes to alkoxy radicals, initiating styrene–unsaturated polyester copolymerization at 25–30°C. Gel time under ASTM D2471 is held between 15 and 35 min for hand lay-up and spray-up. Peak exotherm is limited to 160°C in laminates thicker than 4 mm by reducing CHP to the lower end of the range and by allowing interpass cooling to 40–45°C. Direct mixing of undiluted CHP with cobalt accelerator is not permitted because neat contact can cause rapid decomposition. A wall-mounted catalyst doser or gun-mounted metering system injects CHP at 0.5–2.0% of resin flow, while a chopper gun deposits glass and catalyzed resin simultaneously in spray-up lines.

The glass reinforcement schedule uses 300–600 g/m² E-glass chopped strand mat or woven roving. Laminating rollers consolidate the laminate and remove trapped air before gelation. Fiber content after cure is checked by ASTM D2584 and typically falls in the 30–40 wt% range. Styrene emissions from open-mold operations in the United States are subject to 40 CFR Part 63 Subpart WWWW; the lower exotherm of CHP relative to methyl ethyl ketone peroxide (MEKP) reduces monomer boil-off at the laminate surface. Cured laminates are qualified to ISO 527-2 for tensile strength, ISO 178 for flexural strength, and ASTM D256 for Izod impact. Typical tensile strength is 60–80 MPa, flexural strength is 100–150 MPa, and Barcol hardness after 24 h is 35–45. Terminal parts include boat hulls, chemical storage tanks, cooling tower shells, truck body panels, and sanitary ware.

Indicative effect of CHP dosage on pot life and peak exotherm for an orthophthalic UPR with 0.4 phr cobalt octoate (6% Co) at 25°C, ASTM D2471 method.

Why Does Cobalt-to-CHP Ratio Dictate Through-Cure in Castings?

Unfilled and lightly filled UPR castings are sensitive to the cobalt-to-CHP ratio because both under-acceleration and over-acceleration produce subsurface defects. A standard casting formulation uses 1.0–2.0 phr CHP and 0.2–0.4 phr cobalt octoate 6% metal. At 25°C, gel time is adjusted to 20–60 min for clear casting and 10–30 min for pigmented casting. If cobalt is below 0.15 phr, through-cure remains incomplete in sections above 10 mm; if CHP exceeds 2.5 phr, peak exotherm can exceed 170°C and internal cracking occurs. Clear embedment molds are therefore fitted with thermocouples and the exotherm is capped at 120–140°C. Residual styrene after cure is determined by gas chromatography under ISO 4901; post-cure at 60–70°C for 2–4 h lowers residual monomer and residual CHP decomposition byproducts.

Certain pigments adsorb cobalt and change gel time. Carbon black and iron oxide are the most aggressive; compensation of 0.05–0.1 phr cobalt octoate is made only after gel-time verification under ASTM D2471. Molds with intricate undercuts are demolded at 50–70% of final hardness to reduce breakage; full hardness is reached after 24 h at ambient temperature. Castings are tested by ISO 527-2 for tensile strength, ISO 178 for flexural strength, and ISO 868 for Shore D hardness. Terminal products include buttons, decorative knobs, paperweights, transparent embedments, and display items. Compliance for decorative castings is handled through REACH (EC) No 1907/2006 and CLP Regulation (EC) No 1272/2008; food-contact castings require separate migration testing and are outside the standard CHP-cured resin specification unless explicitly demonstrated.

Mineral-Filled Unsaturated Polyester Solid Surface Casting

Solid surface matrices are compounded with 55–65 wt% alumina trihydrate (ATH) and, in some grades, 0–20 wt% silica or quartz filler. CHP is added at 0.8–1.5 phr and cobalt octoate at 0.2–0.4 phr. ATH releases water at temperatures above 180°C; peak exotherm is therefore limited to 140°C to preserve opacity, reduce porosity, and prevent crack formation in 10–25 mm slabs. Vacuum mixing at –0.09 to –0.095 MPa removes entrained air before catalyst addition. The catalyst is mixed last under low shear because high shear raises batch temperature and reduces pot life to 10–20 min. Vibration tables with 0.1–0.3 mm amplitude and 3,000–6,000 rpm consolidate the filled mix in closed molds. Demolding occurs at 60–90 min, followed by 24 h at 25°C and post-cure at 70–80°C for 2–3 h.

Finished solid surface sheets are qualified under ANSI/ICPA SS-1-2018 or ISO 19712:2017. Tensile strength is tested by ISO 527-2, flexural strength by ISO 178, water absorption by ASTM D570, and surface hardness by ISO 868. A 12 mm cast sheet typically shows flexural strength of 45–60 MPa and tensile strength of 20–30 MPa. Flame-spread classification under ASTM E84 may be required for commercial interior installations. Terminal products are sinks, vanity tops, worktops, laboratory casework, and wall panels. Translucency is controlled by refractive index matching or mismatch between the ATH filler and the cured resin, so CHP-related exotherm and residual monomer variation directly affect color consistency between casts.

At 0.6–0.8 mm wet film thickness, air-sprayed gelcoat formulated with CHP sets in 8–20 min at 25°C when the resin contains 0.3–0.5 phr cobalt octoate and 1.5–2.5 phr CHP. The spray mix is adjusted to 0.8–1.2 Pa·s under ISO 2884-1, with thixotropic index of 2.5–4.0 to prevent sag on vertical mold surfaces. Plural-component spray equipment or batch mixing is used; pot life after catalyst addition is 12–25 min. The gelcoat is deposited at 400–600 µm dry film thickness as a barrier layer before structural laminate is applied. Surface tack is eliminated by incorporating a small amount of paraffin or by using an air-curable resin, because atmospheric oxygen inhibits CHP/cobalt cure in the outer 10–20 µm. Cured film hardness is checked by ISO 868, adhesion by ISO 2409, and gloss retention after accelerated aging. Styrene emissions from gelcoat spraying are covered under 40 CFR Part 63 Subpart WWWW in the United States and Directive 2010/75/EU in the European Union. Terminal products include marine hull gelcoats, wind energy nacelle covers, shower trays, and exterior cladding panels.

When Vinyl Ester Linings Are Post-Cured After Room-Temperature CHP Initiation

Vinyl ester resins used for chemical-resistant linings are accelerated with cobalt and dimethylaniline (DMA) before CHP is added at 1.0–2.0 phr. The cobalt level is 0.3–0.5 phr and DMA is 0.05–0.1 phr. DMA secures surface cure but compresses pot life to 15–30 min at 25°C. Catalyzed resin is therefore applied by roller or airless spray within the first third of pot life. Lining thickness of 0.5–2.0 mm is built in multiple passes; each pass is allowed to reach 35–45°C internally before the next is applied. After 24 h ambient cure, the lining is post-cured at 70–80°C for 4 h to complete methacrylic conversion and improve solvent resistance. Peak exotherm is limited to 150°C to avoid microcracking at the steel–laminate interface.

Chemical resistance is qualified by ASTM C581 with 90-day exposure to representative acid, caustic, and solvent fluids. Hardness retention, flexural strength retention, and visual rating are recorded. Pull-off adhesion to blasted carbon steel is tested by ISO 4624; values of 5–7 MPa are expected where surface preparation meets Sa 2.5. Terminal products include tank linings, sumps, scrubbers, and secondary containment areas. In such service, CHP-initiated room-temperature cure is selected when post-cure heat is available but on-site pot life and moderate exotherm are required. Storage of the peroxide must remain below 30°C and away from cobalt and DMA concentrates to avoid pre-reaction.

Exotherm Management Determines Repairability of Polymer Concrete Overlays

Polymer concrete repair mortars use an unsaturated polyester or vinyl ester binder with 10–13 wt% polymer content and graded silica or basalt aggregate. CHP is added at 1.0–2.0 phr based on resin weight, with cobalt octoate at 0.3–0.5 phr and DMA at 0.05–0.1 phr for low-temperature repairs at 5–15°C. Aggregate moisture is held below 0.1% because water consumes cobalt and retards cure. A forced-action mixer blends the resin component with aggregate for 2–3 min before catalyst injection; placement is completed within 10–15 min after catalyst addition. Overlays are screeded and troweled at 5–25 mm thickness. The lower exotherm of CHP relative to MEKP becomes critical in sections above 15 mm; peak exotherm is held under 90°C to prevent thermal shrinkage cracks and debonding from concrete. Compressive strength after 24 h is tested by ASTM C579, with typical values of 80–110 MPa; flexural strength is tested by ASTM C580 and often exceeds 20 MPa. Terminal products include chemical-plant drains, trench drains, bridge overlay repairs, and secondary containment floors.

For closed-mold perfusion and resin transfer molding (RTM) operations, CHP is selected when low exotherm is required for thick cores or dimensionally stable tools. The resin is formulated at 0.15–0.30 Pa·s at 25°C for infusion; CHP dosage is 0.8–1.5 phr with cobalt octoate 0.2–0.4 phr. Injection time is kept below 50% of pot life to avoid viscosity rise in feed lines; pot life at 25°C is 25–45 min. Tooling is held at 30–45°C to stabilize gel time and through-cure. The low exotherm allows infusion of 20–40 mm core structures without exceeding 140°C in the laminate. After fill, the part remains under –0.07 to –0.09 MPa vacuum until gelation to prevent air ingress. Cured fiber volume fraction is measured by ASTM D3171 or ISO 1172 and is typically 50–60 vol% for carbon fiber RTM. Mechanical properties are qualified by ISO 527-2, ISO 178, and ISO 14130. Terminal products include wind turbine root sections, marine structural members, composite doors, and pump housings.