Working vocabulary for the polyurethane industry. Bookmark this page — we update it as the industry evolves.
Reaction & components
A-side. The isocyanate component of a two-component PU system, typically MDI, polymeric MDI (pMDI), or TDI.
B-side. The polyol component, usually a blend of polyols, catalysts, surfactants, blowing agents, and additives like flame retardants.
Isocyanate index. The ratio of NCO equivalents to OH equivalents, multiplied by 100. An index of 105 means 5% excess isocyanate. PIR systems run high indices (200–350) to drive isocyanurate trimer formation.
Equivalent weight. Molecular weight per reactive group. For a polyol it is MW / functionality.
Functionality. Average number of reactive groups per molecule.
Hydroxyl number (OH#). Milligrams of KOH equivalent to the hydroxyl content of one gram of polyol. Inverse of equivalent weight (×56,100).
Foam structure
Closed-cell foam. Cells are discrete bubbles isolated by intact polymer walls — traps gas, blocks moisture, gives high R-value per inch.
Open-cell foam. Cells interconnect — lower R-value per inch, lower density, more sound absorption.
Cell size. Average diameter of the gas bubbles in foam. Smaller and more uniform = better insulation performance.
Density. Mass per unit volume — drives mechanical strength, dimensional stability, and cost.
Cream time, gel time, tack-free time, rise time. The four standard reactivity benchmarks for measuring foam cure.
Performance & testing
K-Factor (λ). Thermal conductivity. Lower is better.
R-Value. Thermal resistance per unit thickness. Higher is better.
LTTR. Long-term thermal resistance per ASTM C1289 — aged R-value used in building codes.
IFD (Indentation Force Deflection). The standard hardness measurement for flexible foam.
Compression set. Permanent deformation after sustained load — critical for cushion durability.
ASTM E84 / UL 723. The Steiner Tunnel flame-spread test.
FSI / SDI. Flame Spread Index and Smoke Developed Index — the two outputs of ASTM E84.
FST. Fire, Smoke, and Toxicity — a combined evaluation increasingly required for healthcare and transit.
Chemistry & additives
PIR. Polyisocyanurate — high-isocyanate-index PU foam with thermally stable trimer structure.
SPF. Spray polyurethane foam, applied in the field as a 1:1 A:B mix through a heated gun.
CASE. Coatings, Adhesives, Sealants, Elastomers — the non-foam side of PU.
TCPP. Tris(1-chloro-2-propyl) phosphate — chlorinated phosphate flame retardant being phased out.
HFO. Hydrofluoroolefin blowing agent — low-GWP replacement for HFCs.
Reactive vs non-reactive additive. Reactive additives carry hydroxyls or other groups that bond into the polymer; non-reactive additives are physically blended.
Surfactant. Silicone-based additive that controls cell size and prevents collapse during rise.
Catalyst. Amine and tin compounds that selectively accelerate the gel (urethane) and blow (urea/CO₂) reactions.
One short message, one business day, real chemistry guidance from a working formulator.
Contact Us