Quick answer. To accept a polyol shipment with confidence, match the supplier's Certificate of Analysis (CoA) against your agreed specification lot by lot, independently re-test hydroxyl (OH) value, water content, and viscosity on a retained sample, and reject any lot whose OH number drifts beyond ±3 mg KOH/g from your baseline. A disciplined CoA + OH + lot-consistency + retained-sample SOP is what separates a stable foam or elastomer line from unexplained density and cure failures.
Polyol is the backbone of every polyurethane system, and it is also the raw material most sensitive to lot variation. A shift of a few points in hydroxyl value or a fraction of a percent in water content changes your isocyanate index, your cream and gel times, and ultimately the density and hardness of the finished part. As a manufacturer that ships polyol combinations, catalysts, surfactants, and flame retardants directly to formulators worldwide, we have built the incoming-inspection SOP below from thousands of real acceptance decisions. Use it to protect your own line.
Why polyol acceptance is a procurement problem, not just a lab problem
Most buyers treat the CoA as a formality — a PDF filed with the packing list. That is where the money leaks. The CoA is a supplier's declaration, and its value depends entirely on whether you (a) know what the specification should say, and (b) verify the critical parameters yourself. A polyol that is 2% out on OH value will still pour, still foam, and still look fine — until your rejects climb three weeks later and the batch that caused it is already consumed. Acceptance discipline turns a silent quality risk into a documented, reversible decision.
The four pillars of the SOP are: the Certificate of Analysis, the hydroxyl (OH) value, lot-to-lot consistency, and retained samples. Each pillar has a pass/fail gate. If any gate fails, the lot is quarantined, not consumed.
Pillar 1 — Reading the Certificate of Analysis correctly
A usable polyol CoA is traceable to a specific production lot, dated, and signed, and it reports every parameter that appears in your purchase specification — not a generic marketing datasheet. Before you accept a first shipment, agree with your supplier on the exact parameters, methods, and tolerances that will appear on every CoA. Vague CoAs are a leading indicator of vague quality control.
At minimum, insist that each polyol lot's CoA carries the parameters below with the test method named:
| Parameter | Typical unit | Reference test method | Why it matters to your line |
|---|---|---|---|
| Hydroxyl (OH) value | mg KOH/g | ASTM D4274 / ISO 14900 | Sets stoichiometry and isocyanate index |
| Water content | % (w/w) | ASTM E203 (Karl Fischer) | Acts as a blowing agent; shifts density and cell structure |
| Viscosity (25 °C) | mPa·s | ASTM D445 / ISO 3219 | Affects mixing, metering, and flow |
| Acid value | mg KOH/g | ASTM D4662 | Interacts with catalyst; signals degradation |
| Color (APHA / Pt-Co) | — | ASTM D1209 | Contamination and oxidation indicator |
| Unsaturation | meq/g | ASTM D4671 | Impacts network structure in flexible foam |
Cross-check that the lot number on the CoA matches the lot number stenciled on the drum or IBC, and that the production or fill date is consistent with your lead time. A CoA whose lot number does not physically match the container is an automatic quarantine — no exceptions. The authoritative methods for hydroxyl number are published by ASTM International (D4274) and, internationally, by ISO 14900; naming the method on the CoA is what makes two labs comparable.
Pillar 2 — Verifying hydroxyl (OH) value independently
The OH value is the single most important number on a polyol CoA because it directly determines how much isocyanate you need. Trusting the supplier's figure without any independent check is a risk you carry into every batch. For a first shipment, or any new lot from a new production campaign, re-test OH value on your own retained sample and compare it to the CoA figure.
Set a documented tolerance and hold to it. A practical acceptance band for most rigid and flexible polyols is:
- Green (accept): your measured OH within ±2 mg KOH/g of the CoA value and within your spec range.
- Amber (investigate): deviation of 2–3 mg KOH/g — re-test, review the calculation, and check the index impact before releasing.
- Red (quarantine): deviation greater than ±3 mg KOH/g, or any result outside the purchase specification.
Translate the deviation into index terms so purchasing and production speak the same language: a higher-than-expected OH value means the polyol carries more reactive groups, so a formulation run at a fixed isocyanate quantity will be effectively under-indexed, softening the part and slowing cure. Quantifying the shift in index — not just noting a number is "a bit off" — is what makes the acceptance decision defensible.
Pillar 3 — Controlling lot-to-lot consistency
A single in-spec CoA tells you nothing about drift. Polyol lot consistency is a trend, not a snapshot. The discipline that top formulators use is simple: maintain a running control chart of OH value, water, and viscosity across every lot you receive from a given supplier and grade. A lot can be inside the specification and still signal trouble if it jumps from one edge of the band to the other.
Build a lot-consistency log with, at minimum, these columns:
| Field | Purpose |
|---|---|
| Lot number & fill date | Traceability to a production campaign |
| CoA OH value | Supplier declaration |
| Your measured OH value | Independent verification |
| Delta vs. rolling average | Drift detection (flag > ±3) |
| Water & viscosity | Secondary drift indicators |
| Disposition | Accept / investigate / quarantine |
Two or three consecutive lots trending in the same direction is your early warning that the supplier's process has shifted — often a feedstock or catalyst change upstream that never reaches your inbox. Raising it before it becomes a reject spike is the entire point of tracking consistency. This is also where sourcing from a manufacturer that controls its own polyol blending pays off: a direct producer can trace a drift back to a specific raw-material batch and correct it, where a trading intermediary can only forward your complaint. Impurity profiles and hazard data for many polyol feedstocks are searchable through the ECHA information on chemicals database, which is useful when a color or acid-value shift suggests a contamination event rather than normal variation.
Pillar 4 — Retained samples and shelf-life discipline
Every accepted lot should leave behind a sealed, labeled retained sample stored under controlled conditions for the life of the material plus a margin — typically 12 months for standard polyols. The retained sample is your evidence locker. When a finished-part failure surfaces weeks later, a retained sample lets you re-test the exact lot and determine whether the raw material or the process was at fault, instead of arguing from memory.
Good retained-sample practice includes: an airtight container purged of moisture (polyols are hygroscopic), a label bearing lot number, receipt date, and measured OH value, storage away from heat and light, and a first-in-first-out consumption rule so you never pull an aged lot ahead of a fresh one. Pair this with real inbound sampling technique — draw from the container, not from a supplier-provided pre-fill — so the sample truly represents what you received.
Putting the SOP on one page
The acceptance flow, start to finish: receive shipment → match lot number to CoA and to container → confirm every spec parameter is present and in range → draw a representative sample → independently re-test OH value (and water on first lots) → log the delta against your rolling average → dispose (accept / investigate / quarantine) → seal a retained sample. Any single failed gate stops consumption. Documented, reversible, and boring — exactly what a raw-material acceptance process should be.
If you are building or tightening this SOP for your own line, our technical team supplies grade-specific specifications, method references, and lot histories with every order. Explore our polyol product range to see the parameters and tolerances we certify on each lot, or contact us for a custom combination matched to your index and cure targets.
FAQ
Q: What OH value tolerance should I accept on a polyol CoA?
For most rigid and flexible polyols, treat ±2 mg KOH/g from your baseline as clean, 2–3 mg KOH/g as an investigate zone, and anything beyond ±3 mg KOH/g — or outside the purchase specification — as a quarantine. Always convert the deviation into its effect on isocyanate index before releasing the lot.
Q: Which test method should the CoA reference for hydroxyl value?
ASTM D4274 or ISO 14900. Naming the method matters because titration conditions affect the result; two labs only produce comparable numbers when they run the same method.
Q: Do I really need to re-test OH value if the CoA already reports it?
Re-test at least the first lot of every new grade or production campaign, and any lot flagged by your consistency chart. The CoA is a supplier declaration; independent verification on a retained sample is what makes acceptance defensible when a failure surfaces later.
Q: How long should I keep retained polyol samples?
Store a sealed, moisture-purged retained sample for the material's shelf life plus a margin — typically about 12 months for standard polyols — labeled with lot number, receipt date, and measured OH value, so you can re-test the exact lot if a downstream failure appears.
Q: Why does buying polyol from a direct manufacturer improve lot consistency?
A direct producer controls the blending and can trace a drift in OH value or color back to a specific feedstock batch and correct it. A trading intermediary can only forward your complaint, which lengthens the feedback loop and leaves the root cause unaddressed.
