PPG 2000
Polypropylene glycol (polyether diol)
- CAS
- Formula
- H[OCH(CH₃)CH₂]ₙOH
- OH no.
- ~56
Used forFlexible polyurethane coatings, prepolymers and sealants.
Hydrolysis-resistant and economical: the default soft segment for many urethanes.
Full profilePolyols are the hydroxyl-bearing partners that react with isocyanates to form polyurethane. Their backbone, chain length and functionality decide whether a film ends up soft and stretchy or hard and chemical-resistant.
A polyol is any molecule with two or more hydroxyl (–OH) groups. In a two-component polyurethane the polyol forms the B-side, or is pre-reacted into an A-side prepolymer, and each –OH reacts with one isocyanate group to form a urethane link.
The backbone sets the character. Polyether polyols such as PPG and PTMEG are flexible and hydrolysis-resistant. Polyester polyols are tougher and more solvent-resistant but can break down in hot, wet service. Acrylic polyols bring outstanding weathering for topcoats, and natural oils like castor oil add moisture tolerance.
Molecular weight and functionality set the network. Long diols create soft, elastic segments; short diols like 1,4-butanediol and triols build hard segments and crosslinks. Most real formulations blend several polyols to hit a target hardness, elongation and pot life.
Soft segments, crosslinkers and hard-segment builders used in polyurethane coatings.
Used forFlexible polyurethane coatings, prepolymers and sealants.
Hydrolysis-resistant and economical: the default soft segment for many urethanes.
Full profileUsed forAdds crosslink density to polyurethane coatings, elastomers and foams.
Triols build network density; diols build flexibility. Formulators blend the two.
Full profileUsed forHigh-performance elastomers and abrasion-resistant prepolymers.
Tougher and more hydrolysis-resistant than PPG, at a higher price.
Full profileUsed forIndustrial polyurethane coatings, adhesives and oil-resistant elastomers.
Tough and chemical-resistant, but more prone to hydrolysis than polyethers.
Full profileUsed forAutomotive refinish clears and 2K aliphatic polyurethane topcoats.
Paired with HDI trimer, it forms the classic high-gloss, UV-stable 2K urethane.
Full profileUsed forMoisture-tolerant polyurethane coatings, potting compounds and flexibilizers.
Naturally hydroxyl-functional: one of the few polyols that comes straight from a plant.
Full profileUsed forHard-segment chain extender in polyurethane elastomers and prepolymers.
Short diols build the hard, ordered segments that give urethanes their strength.
Full profileThe hydroxyl oxygen attacks the carbon of an isocyanate group and its hydrogen shifts to nitrogen, creating a urethane (carbamate) linkage. Alcohols are much slower than amines, so polyurethane formulas lean on catalysts such as bismuth or tin carboxylates and tertiary amines to set gel time.
Two numbers make the math work. The OH number (mg KOH per gram) tells you how many hydroxyls a polyol carries; dividing 56,100 by it gives the equivalent weight. Matching polyol equivalents to isocyanate equivalents, usually at an index of about 100–110, sets the mix ratio.
Because water also reacts with isocyanate, polyols must be kept dry. Many B-sides include molecular sieve or other moisture scavengers to prevent CO₂ bubbles.
R–N=C=OIsocyanate or prepolymerR′–OHPolyol hydroxyl groupR–NH–CO–O–R′Urethane linkageEquivalent weight = 56,100 ÷ OH number
Generalized ratings (5 = best). Each backbone trades off flexibility, durability and cost.
| Backbone | Flexibility | Hydrolysis resistance | Solvent resistance | Weathering | Best for |
|---|---|---|---|---|---|
| Polyether (PPG) | Flexible coatings, sealants, prepolymers | ||||
| Polyether (PTMEG) | Abrasion-resistant elastomers | ||||
| Polyester | Industrial finishes, adhesives | ||||
| Acrylic polyol | UV-stable 2K topcoats | ||||
| Castor oil | Moisture-tolerant coatings, potting |
Most polyols are low-hazard liquids, but they are rarely used alone. The real risks come from the isocyanates and catalysts they are mixed with, and from moisture.
Always follow the product’s Safety Data Sheet, your employer’s safety program and local regulations. This page is general reference information, not a substitute for them.
OH number is the milligrams of KOH equivalent to the hydroxyl content of one gram of polyol. It tells you how much isocyanate the polyol will consume; equivalent weight = 56,100 ÷ OH number, which you need to calculate the mix ratio.
Neither in general. Polyethers resist hydrolysis and stay flexible in the cold; polyesters offer better solvent, oil and abrasion resistance but can degrade in hot, humid service. Many coatings blend both.
Water in the polyol, pigments or substrate reacts with isocyanate to release CO₂. Keeping raw materials dry and adding moisture scavengers such as molecular sieve prevents it.
A short diol such as 1,4-butanediol, or a short diamine, that reacts with isocyanate to build hard, ordered segments, raising hardness, strength and heat resistance.