Polypropylene glycol (polyether diol)
Polypropylene glycol is the most common soft-segment polyol in polyurethanes: flexible, hydrolysis-resistant and economical.
Polypropylene glycol (PPG) is a polyether made by polymerizing propylene oxide, ending in hydroxyl groups. Grades are named by approximate molecular weight; PPG 2000 has a molecular weight near 2,000 g/mol and an OH number around 56 mg KOH/g. It is a clear, low-viscosity liquid.
PPG diols form flexible soft segments in:
Amine-terminated versions of the same backbone are the polyetheramines used in spray polyurea B-sides.
From an OH number of 56, the equivalent weight is about 1,000 g/eq (56,100 ÷ 56). The ether backbone resists hydrolysis and stays flexible at low temperatures, but polyethers are more susceptible to oxidative and UV degradation than some other backbones.
PPG is generally low in hazard, but it is hygroscopic: absorbed water will react with isocyanate and cause bubbling. Keep containers closed and consider nitrogen blanketing for long storage.
Polyetheramines such as D-2000 are essentially PPG chains with the terminal hydroxyl groups converted to amines, which makes them react far faster with isocyanates.
Water in the polyol reacts with isocyanate to form CO₂ gas, causing bubbles and lowering the effective mix ratio.
