Most flexible foam problems that get blamed on the isocyanate start with the polyol. It sets the backbone of the polymer, and it decides most of what the finished foam will feel like — how it recovers, how it carries load, how it ages.
Two grades cover the majority of flexible foam production: a conventional slabstock polyol and a high-resilience (HR) grade. This guide covers what actually differs between them, what changes in the foam, and what to check on the specification before you commit to an order.
The short answer
Specify a conventional polyol for standard flexible slabstock — furniture cushioning, mattress and bedding foam, protective packaging, foam laminating. This is the workhorse, and for most slabstock production it is the correct and more economical choice.
Specify a high-resilience polyol when the foam has to feel better and last longer under repeated loading — automotive seating, premium furniture cushioning, higher-specification bedding.
If you are producing standard cushioning and considering HR because it sounds superior, you will probably pay more for properties your application doesn't use.
What's actually different about the molecule
Both grades in a typical flexible foam range are nominally trifunctional polyether polyols — three reactive hydroxyl sites per molecule, built on a propylene oxide backbone. Both Polyether Polyol F3500 and Polyether Polyol F3645 are nominal functionality 3.
The difference is in the chain ends.
Conventional slabstock polyols terminate mostly in secondary hydroxyl groups. High-resilience grades are typically capped with ethylene oxide, which converts most of those chain ends to primary hydroxyls. Primary hydroxyls are appreciably more reactive toward isocyanate than secondary ones.
That single change drives nearly everything else. Higher end-group reactivity means the polymer builds strength earlier in the rise, which supports the more open, more elastic cell structure that gives HR foam its character. HR grades are also generally higher in molecular weight, with a correspondingly lower hydroxyl value.
Neither grade is "better". They are tuned for different jobs.
What changes in the finished foam
Resilience. The defining property, and where the name comes from. HR foam returns more of the energy put into it — the property measured by ball rebound testing under ASTM D3574. Conventional slabstock foam is comparatively more damped.
Comfort factor. Also called sag factor: the ratio of load at 65% compression to load at 25%. HR foams typically show a higher ratio, which is why they feel supportive without feeling hard — the foam yields easily at first and firms up as it compresses. This is the property seat designers are usually chasing.
Durability under repeated load. The more elastic network generally holds its height and support better through fatigue cycling. It's the main reason HR grades dominate automotive seating, where the foam is loaded and unloaded for years.
Cell structure. HR systems tend toward a more open, less uniform cell structure. Useful for breathability; it also means the processing window behaves differently.
Reading the specification
This is where a purchasing decision is actually made. Taking the published F3500 data as a worked example:
| Property | F3500 typical | What it tells you |
|---|---|---|
| Hydroxyl value | 48–52 mg KOH/g | Reactive sites per unit mass. Drives isocyanate demand and, with functionality, crosslink density. Lower value = higher molecular weight = generally softer, more elastic foam. |
| Nominal functionality | 3 | Reactive sites per molecule. Sets network connectivity. |
| Nominal molecular weight | ≈ 3,500 g/mol | Chain length between crosslinks. Longer chains, more elastic network. |
| Viscosity | 500–650 mPa·s @ 25 °C | Pumping and mixing behaviour. Matters for your metering equipment, not the foam. |
| Water content | ≤ 0.05 % | Watch this one. Water reacts with isocyanate, generating CO₂ and consuming NCO you had budgeted for the polymer. |
| Acid value | ≤ 0.05 mg KOH/g | High acidity retards amine catalysis and shifts your cream and gel times. |
| Unsaturation | ≤ 0.06 meq/g | Monofunctional impurity from PO rearrangement. Chain terminators — they cap a growing chain without extending the network, softening the foam and hurting load-bearing. |
| Colour | ≤ 50 APHA | Cosmetic for most work; matters if you're producing white or light foam. |
The single most useful number is the hydroxyl value, because it determines your isocyanate demand. If it drifts between batches and your index doesn't follow, the foam changes — and this is a common cause of "the supplier changed something" complaints that are really an unadjusted formulation.
Unsaturation is the quality tell. It's the figure that separates a well-controlled polyol from a marginal one, and it's the one buyers most often skip.
Why F3645's key numbers aren't published
Comparing the two published specifications is instructive. Water content, acid value, colour and unsaturation limits are identical. Nominal functionality is 3 for both.
The three fields that differ — hydroxyl value, molecular weight and viscosity — are exactly the three marked "confirmed on TDS" for F3645.
That isn't evasion. Those figures are what define a specific HR grade commercially, and they vary by producer. Publishing a single set of numbers would imply a precision that doesn't survive contact with multiple sources. The technical data sheet issued against your enquiry carries the actual figures, and the batch certificate of analysis carries what you actually received.
Which is the right way round. Any published figure — here or anywhere — is indicative. The batch CoA governs. If you're designing a formulation, work from the CoA.
What changes in processing
Switching between grades is not a drop-in substitution.
Reactivity. More primary hydroxyls means a faster reaction with the isocyanate. Cream time, gel time and rise profile all shift, and the catalyst package normally has to be rebalanced. Expect to re-trial.
Isocyanate demand. A different hydroxyl value means a different stoichiometric requirement. Recalculate the index against the actual figure on your CoA — see TDI 80/20, where the 48.0–48.5% NCO content is the other half of that calculation.
Density control. Water is the primary blowing agent through the CO₂-generating reaction; auxiliary blowing agents such as methylene chloride are used where lower density is needed without over-reliance on the water reaction. Changing polyol changes the exotherm, so density targets usually need revisiting too.
Moisture discipline, either way. Polyols are hygroscopic, and absorbed moisture reacts with isocyanate to cause foam defects. Both grades should be stored sealed at 15–30 °C, out of direct sunlight, with dry nitrogen blanketing on bulk storage. This is the most common avoidable cause of defects in plants that otherwise run well.
Choosing, by application
| Application | Grade | Why |
|---|---|---|
| Furniture cushioning, standard | Conventional | Adequate comfort at lower cost |
| Mattress and bedding, standard | Conventional | Damped feel is often preferred |
| Protective packaging foam | Conventional | Energy absorption wanted, not rebound |
| Foam laminating | Conventional | Established slabstock process |
| Automotive seating | High-resilience | Fatigue durability and comfort factor |
| Premium furniture cushioning | High-resilience | Support without hardness |
| Premium mattress and bedding | High-resilience | Recovery and longevity |
If you sit between these — say, mid-market furniture where comfort matters but cost is tight — the honest answer is that it depends on your foam formulation and your market's expectations, and it's worth trialling both rather than deciding from a datasheet.
What to send when you enquire
The quickest route to a useful offer is to state the problem rather than only the product code:
- Application and target properties — density, hardness, whether resilience is specified
- Quantity and packing — drum, IBC, ISO tank or flexitank
- Destination port and Incoterm
- Whether you need the TDS before committing — it's issued on request, always before order confirmation
If you're unsure which grade fits, send the application and target properties and let the specification follow from that. See quality and sourcing for how batch documentation is handled, or send an enquiry with what you have.