Polyether polyols & block polyethers: defoaming, PU

Polyether polyols and block polyethers sit behind many foam-related processes — both the foams you want (polyurethane) and the foam you do not (process entrainment). Understanding the polyol backbone and the block structure tells you whether a grade will stabilise, level or knock down foam.

PPG, PEG and random polyethers

Poly(propylene glycol) (PPG) is more hydrophobic and lowers surface activity, which is why medium- and low-molecular-weight PPG shows defoaming character. Poly(ethylene glycol) (PEG) is hydrophilic and water-soluble, used to tune balance and handle. Random polyethers blend the two backbones so one molecule can be tuned for wetting, levelling or low foam depending on the EO/PO ratio.

See PPG-400 for a propylene-oxide backbone and Random Polyether BPE-1600 for a tuned EO/PO random structure.

Block polyethers as defoamers

Pluronic-style block polyethers (EO-PO-EO) behave as defoamers or foam suppressors when the PO block dominates and the molecular weight is tuned so the molecule spreads at the air-liquid interface and breaks the lamellae. Low-foam grades are used in textiles, metal cleaning and CIP where entrained foam slows throughput. The same molecule can foam or defoam depending on concentration and temperature, so the working window matters.

Butanol-started polyethers such as Butanol Polyether are another route to low-foam, solvency-tuned structures.

Foam levelling in PU systems

In polyurethane, the polyether polyol is the backbone of soft foam and elastomers, and a silicone or polyether surfactant controls cell opening and uniformity. The right surfactant levels the rise and prevents collapse or coarse cells. Here the polyether is not the defoamer — it is the matrix, and the additive manages the foam structure.

Selection tips

• For process defoaming, choose a PO-rich or block polyether tuned to your temperature and concentration window.
• For PU, select the polyol molecular weight for the hardness/elasticity target and add a cell-control surfactant.
• Match hydrophilicity to the medium: more EO for water-based, more PO for solvent or hydrophobic systems.

Polyethylene glycols and the rest of the polyether family

Polyethylene glycols, polypropylene glycols and the random and block copolymers between them are one family, not a product list. They share a polyether backbone and differ in which oxide was polymerised onto it. That single choice - ethylene oxide, propylene oxide, a random mixture, or a segmented block - decides water solubility, surface activity and therefore whether the grade carries, wets, levels or defoams.

What the backbone actually changes

Ethylene oxide gives a water-soluble, uncharged, chemically quiet chain: the polyethylene glycols. Propylene oxide gives a more hydrophobic chain with a methyl side group, so a polypropylene glycol is water-soluble only while it is short, and the longer it grows the more it lowers surface activity instead of raising it. That is why medium and low molecular weight PPG shows up as a defoaming base rather than as a solubiliser.

Mix the two oxides in one chain and you get an adjustable molecule. A random copolymer carries the two units in no particular order and behaves somewhere between its parents, tunable by the EO/PO ratio. A block copolymer keeps the two as segments, which makes the molecule interface-active in a way neither parent is - the usual starting point when the job is knocking foam down rather than making it.

PEG, polyethylene glycols and PEG glycol: one family, more than one name

The plural forms are catalogue conventions rather than different substances: a supplier's "polyethylene glycols" list is the same family as its PEG grades, and PEG glycol is an older trade shorthand for the same polymer. The naming itself, and the grade-number convention that goes with it, is set out in the guide to polyethylene glycol rather than repeated here.

A family map you can start a selection from

Read the published values in that map as grade-level figures. Where a grade is documented on this site with series-level reference values rather than a dedicated specification row, this page does not upgrade them into a grade-specific number - the random polyether entry is the case in point, and its own product record says so explicitly.

One figure that travels across the whole family

Whatever the backbone, the hydroxyl value is the figure that keeps its meaning. It counts chain ends per unit of mass, so it falls as the chain lengthens: PEG-4000 is published at 26-32 mgKOH/g and PPG-400, a tenth of its chain length, at 255-312 mgKOH/g. That inverse relationship is what makes the figure comparable across two different backbones - PEG-4000 and PPG-400 - and it is the number to quote when you specify a polyol by reactivity rather than by name.

Starting points in this range

The polyether grades documented here are Polyethylene Glycol 4000 (PEG-4000) and Allyl Polyethylene Glycol APEG-1000 on the ethylene-oxide side, Polypropylene Glycol 400 (PPG-400) on the propylene-oxide side, and Random Polyether BPE-1600 with Butanol Polyether across the mixed and block structures. For the same family once it has been ethoxylated further into a surfactant, continue with the nonionic and anionic comparison; where these grades are used in wet processing rather than in a formulation, the roles they play are set out in polyether grades in textile processing.

How to brief a polyether selection

• Water-based or solvent-based, because that is what turns the EO/PO ratio from a preference into a constraint.
• Foam wanted or foam unwanted: polyurethane systems want the foam structure controlled, defoaming applications want it gone, and the same backbone serves both.
• The target the grade has to hit - viscosity, hardness, elasticity or pour point - since that sets the molecular weight band.
• The service temperature window, which decides how much EO is needed to keep the grade soluble in use.
• Whether a reactive end group is needed, because a functionalised grade is a different selection from a polyol of the same chain length.

Takeaway

Polyether performance is a tuning problem, not a single product. Start from whether you want foam (PU) or none (defoaming), then set the EO/PO balance and molecular weight. Compare PPG-400, Random Polyether BPE-1600 and Butanol Polyether to set a starting point.