Terephthalic chemistry
The three phthalic acids — ortho, iso and tere — are isomers: the same atoms, arranged with their acid groups in different positions on a benzene ring. Orthophthalic resins use phthalic anhydride, isophthalic resins use isophthalic acid and terephthalic resins use terephthalic acid, the para isomer. The position changes the shape of the polyester chain and, with it, how the cured resin behaves.
In Samrat Poly Resins’ 2026 catalogue, the terephthalic resins form the SPR 401 series: SMC and DMC resins, a fire-retardant gelcoat and a pultrusion grade.
How the catalogue describes this grade
The catalogue describes the terephthalic SMC resin as having high heat-resistant capacity, good resistance to corrosive chemicals and excellent mechanical and impact properties. Those are the manufacturer’s qualitative descriptions; no numerical heat, chemical or mechanical values are published for SPR 401 SMC.
Its typical liquid values are published: clear to slightly hazy, pale-yellow to light-amber appearance, viscosity 800–1,500 cPs at 25 °C, specific gravity 1.11–1.15 and styrene 32–38% by weight.
Choosing an SMC family
The catalogue’s selection guide lists SMC resins in the orthophthalic, isophthalic and terephthalic families and chooses among them by the chemistry the compound requires. A compound for a part that sees heat or chemical exposure may justify a different family from one for a dry interior part.
The moulded part’s requirement, not the family name, is the starting point, and the choice is confirmed on the moulded compound.
The ambient gel time is a QC figure
The 12–18 minutes at 25 °C gel time is labelled in the catalogue as an ambient QC value, measured with the reference cure of MEKP 1.5% and cobalt octoate 0.4%. Hot-mould cure is set by the compound’s initiator system and tool temperature, which belong to the compounder.