Quick answer. Aromatic polyester polyols for spray polyurethane foam (SPF) typically run 200-320 mgKOH/g hydroxyl value, 3,000-8,000 mPa·s viscosity at 25°C, and 2.0-2.3 average functionality, producing closed-cell rigid foam with LOI above 20% without added flame retardant. Against polyether polyols in the same OH range, polyester grades give higher char yield and lower flame spread under ASTM E84, at the cost of higher raw viscosity and a shorter usable pot life once mixed. When benchmarking a new supplier against an established aromatic polyester line, request hydroxyl value, acid value, and water content on every drum's certificate of analysis, not density alone.
A polyester polyol that drifts 15 mgKOH/g from its data sheet forces a formulator to re-tune the A-side/B-side ratio, and that re-tuning eats a production week on a running line. This guide lays out the parameters that predict foam behavior, shows where aromatic DEG/PET-based polyester polyols diverge from polyether alternatives, and covers the compliance and sourcing terms that matter when qualifying a second or third supplier.
What Hydroxyl Number and Functionality Actually Control
Hydroxyl value sets the isocyanate index calculation directly: NCO index = (NCO equivalents / OH equivalents) × 100. A polyol quoted at 280 mgKOH/g but delivering 265 mgKOH/g shifts the index by roughly 5 points at a fixed MDI dose, which shows up as under-cured, tacky foam at the substrate interface within the first 48 hours.
Hydroxyl value is measured by phthalic anhydride or pyridine-based esterification per ASTM D4274. Reputable suppliers run this on every production batch, not on a retained master sample from qualification. Ask for the batch-specific COA, not a generic spec sheet PDF.
Average functionality for phthalic anhydride/diethylene glycol (DEG) aromatic polyester polyols sits at 2.0-2.3. Below 2.0, crosslink density drops and closed-cell content falls below 90%, which raises long-term k-factor drift in roofing and wall SPF applications. Functionality above 2.4 raises brittleness and increases friability under freeze-thaw cycling, a common field complaint on cold-climate spray jobs.
Polyester vs Polyether Polyol for Rigid SPF: Spec-by-Spec Comparison
Formulators evaluating a switch need the dimensions laid out side by side, not narrative claims. The table below compares aromatic polyester polyol (phthalic anhydride/DEG type, the chemistry behind commercial aromatic polyester lines used in closed-cell SPF) against sucrose/glycerine-initiated polyether polyol at comparable OH ranges.
| Parameter | Test Method | Aromatic Polyester Polyol | Polyether Polyol (rigid grade) |
|---|---|---|---|
| Hydroxyl value | ASTM D4274 | 200-320 mgKOH/g | 350-470 mgKOH/g |
| Viscosity @25°C | ASTM D2196 / D4878 | 3,000-8,000 mPa·s | 300-800 mPa·s |
| Acid value | ASTM D4662 | ≤1.0 mgKOH/g | ≤0.1 mgKOH/g |
| Average functionality | Calculated | 2.0-2.3 | 3.0-4.5 |
| Water content | ASTM D4672 (Karl Fischer) | ≤0.1% | ≤0.1% |
| Aromatic ring content | Structural (NMR) | High (phthalate backbone) | None |
| Flame spread index | ASTM E84 | 20-25 (unmodified) | 75-150 (unmodified) |
| Closed-cell content | ASTM D2856 | 90-95% | 90-95% |
| Hydrolysis resistance | 85°C/85% RH aging | Moderate — acid value must stay ≤1.0 | High — ether linkage is hydrolytically stable |
The acid value line is the one buyers skip and shouldn't. An aromatic polyester with acid value above 1.5 mgKOH/g autocatalyzes ester hydrolysis in the drum during ocean freight through tropical humidity, and viscosity can climb 10-15% by the time a 20-ton IBC shipment clears customs. Specify acid value ≤1.0 mgKOH/g on the purchase order, not just on the reference data sheet.
Fire Performance: Why the Aromatic Backbone Passes Char and LOI Targets
Phthalic anhydride contributes benzene rings directly into the polymer backbone. On combustion, that aromatic content chars instead of volatilizing into fuel, which is why unmodified aromatic polyester SPF formulations reach a Limiting Oxygen Index above 20% (ASTM D2863) where an equivalent polyether formulation needs 8-12 phr of added flame retardant to hit the same number.
This matters directly on cost: cutting flame retardant loading by 6-10 phr on a 1,000-ton annual SPF polyol volume removes a meaningful line item without touching the isocyanate index or foam density. It also matters on building-code compliance, where roofing and wall assemblies are tested to ASTM E84 flame spread and smoke developed indices as a system, not a raw material.
Density and compressive strength move together with formulation, not chemistry class alone. Typical rigid closed-cell SPF core density runs 28-48 kg/m³ with compressive strength 150-350 kPa at 10% deformation (ASTM D1621), regardless of polyester or polyether base — the polyol class changes fire behavior and hydrolysis resistance, not core density.
Handling, REACH Registration, and Isocyanate Exposure Controls
Polyester polyols for export into the EU need REACH registration under the relevant tonnage band; buyers should request the registration number and check status directly against ECHA's REACH registry rather than accepting a supplier's self-declared compliance statement.
The isocyanate side of the SPF system, not the polyol, carries the acute occupational exposure risk during application. OSHA's isocyanates guidance sets the framework for respiratory protection and air monitoring that spray crews follow on job sites; polyol suppliers should reference this in their SDS revision history, since an SDS that hasn't been updated to the current GHS revision is a red flag on documentation discipline generally.
Drums should be stored below 30°C and away from moisture ingress — aromatic polyester polyol absorbs atmospheric moisture through an opened bung faster than polyether grades because of the ester linkage's polarity, and moisture content above 0.15% causes visible CO2 microcell defects in the foam.
Custom OH Tuning, MOQ, and Lead Time for Manufacturer-Direct Supply
A trading-house polyol arrives at whatever OH value the last production run happened to produce. A manufacturer that blends in-house can hold a target OH value within ±5 mgKOH/g across batches and adjust the target itself — a formulator moving from a 250 OH system to a 300 OH system to raise crosslink density for a colder-climate spec doesn't need to requalify a whole new supplier.
Typical terms for manufacturer-direct aromatic polyester polyol supply: MOQ 1-3 tons for a trial batch versus 20 tons minimum from most trading intermediaries, lead time 12-18 days ex-works for standard OH grades, COA issued per batch with hydroxyl value, acid value, viscosity, and water content. rPET-based recycled-content polyester polyol variants are available at the same OH range for buyers targeting a recycled-content declaration on a green-building submission, with acid value control being the main process variable to hold tight in that stream.
See BlendPolyol's aromatic polyester polyol grades for spray foam for current OH ranges and batch documentation available for trial-order qualification.
FAQ
Q: Can I substitute an aromatic polyester polyol directly for a polyether polyol at the same OH value without reformulating?
No. Functionality, viscosity, and reactivity profile differ enough that the isocyanate index and mix ratio both need re-verification through a bench pour before a line trial, even at matched hydroxyl value.
Q: What acid value should I reject a polyester polyol shipment at?
Above 2.0 mgKOH/g per ASTM D4662 indicates active ester hydrolysis in transit; foam from that batch typically shows reduced dimensional stability within 30 days and should be quarantined, not blended into production stock.
Q: Does higher viscosity in polyester polyol require different spray equipment?
Above roughly 6,000 mPa·s at 25°C, proportioner preheat and hose trace temperature need adjustment to hold output ratio accuracy; most high-pressure SPF rigs handle this with a 5-8°C preheat increase over polyether-grade settings.
Q: How does recycled-content (rPET) polyester polyol compare on hydroxyl value consistency?
Batch-to-batch OH variance runs slightly wider on rPET feedstock — typically ±8-10 mgKOH/g versus ±5 mgKOH/g on virgin phthalic anhydride/DEG — because feedstock purity varies by collection stream; request COA data across at least three batches before committing volume.
Q: What's the realistic shelf life for an unopened polyester polyol drum?
12 months from production date stored below 30°C with the bung sealed; acid value and water content should be re-checked before use on any drum held longer than 9 months.
