Polyol and Isocyanate in PU Foam: OHV, %NCO & Index

Polyol and Isocyanate in PU Foam: How the Main Reactive Pair Builds the Network Polyol and isocyanate are the main reactive pair behind every polyurethane foam. Everything else in the formula supports, modifies, controls, or stabilizes what this pair creates. Polyol provides reactive hydroxyl groups. Isocyanate provides reactive NCO groups. When these groups react, they form urethane linkages and build the polymer network that gives polyurethane foam its structure. This network decides whether the foam becomes soft or rigid, flexible or structural, durable or weak, high-resilience or commodity, open-cell or closed-cell, stable or defective. That is why polyol and isocyanate cannot be selected only by name, price, or supplier recommendation. The key numbers must be understood: Polyol OHV Polyol equivalent weight Polyol functionality Isocyanate %NCO Isocyanate equivalent weight Isocyanate index Current CoA values If these numbers are wrong, the formula is wrong. The foam may still rise. The block may still look acceptable. But the chemistry is not running where the engineer thinks it is running. This article explains how polyol and isocyanate build the PU foam network, why OHV and %NCO control the calculation, and why functionality and CoA values decide whether the formula is only correct on paper or actually correct in production. The Main Reaction: Polyol OH + Isocyanate NCO The central polyurethane reaction is simple in concept. A hydroxyl group from the polyol reacts with an isocyanate group from the isocyanate. The result is a urethane linkage. Simplified reaction: R–OH + R′–NCO → R′–NH–COO–R This urethane linkage becomes part of the polymer network. As more OH groups and NCO groups react, the liquid mixture turns into a foam network. That network may be soft and flexible, or hard and rigid, depending on the raw materials and formulation. The reaction is controlled by: How many OH groups the polyol provides How many NCO groups the isocyanate provides How these groups are balanced through index How fast the reaction proceeds through catalyst How the foam structure is stabilized by surfactant How water and crosslinker add additional reactive pathways But the main backbone starts with polyol and isocyanate. If the polyol and isocyanate calculation is wrong, the rest of the formula is built on a weak foundation. Polyol: The Material That Defines the Foam Character Polyol is usually the largest component by weight in a PU foam formula. It provides the main soft or rigid segments of the foam network. The polyol type strongly influences: Flexibility Rigidity Softness Hardness potential Compression set Resilience Recovery Hydrolysis resistance Viscosity Processing behavior Foam durability A flexible foam polyol and a rigid foam polyol are not the same kind of material. Flexible foam polyols usually have lower OHV and longer chains. Rigid foam polyols usually have higher OHV and shorter chains. This difference changes equivalent weight and network density. Typical direction: This is why polyol selection is not only a purchasing decision. It is a network design decision. A polyol change can move the foam from flexible to rigid behavior, from good recovery to poor compression set, or from stable processing to difficult viscosity. Polyol OHV: The Number That Controls Equivalent Weight OHV means hydroxyl value. It tells you how many reactive hydroxyl groups exist per gram of polyol. The OHV value is used to calculate polyol equivalent weight. The formula is: Polyol EW = 56,100 ÷ OHV For example, if the polyol OHV is 51: EW = 56,100 ÷ 51 EW = 1,100 g/eq Equivalent weight means the mass of material that contains one equivalent of reactive OH groups. This is important because polyurethane foam does not react by weight alone. It reacts by equivalents. A 100-part polyol addition is not chemically meaningful until you know the OHV and equivalent weight. If OHV changes, equivalent weight changes. If equivalent weight changes, reactive equivalents change. If reactive equivalents change, isocyanate demand changes. If isocyanate demand changes, the index changes unless the formula is recalculated. That is why every polyol CoA should be checked before formula approval. Do not assume old OHV values are still correct. Polyol Functionality: Why Same OHV Can Produce Different Foam OHV tells you reactive concentration. It does not tell you network architecture. That is where functionality matters. Functionality means the number of reactive OH groups per polyol molecule. For example: A difunctional polyol has about 2 reactive OH groups per molecule. A trifunctional polyol has about 3 reactive OH groups per molecule. Higher-functionality polyols create more network junctions. Two polyols can have the same OHV but different functionality. Example: The index calculation may look similar because the OHV is the same. But the foam network can behave differently because the molecular architecture is different. Functionality affects: Crosslink density Compression set Recovery Resilience Durability Load-bearing behavior Network strength This is why selecting polyol based only on OHV can be dangerous. OHV tells you how much isocyanate is required. Functionality helps tell you what kind of network will form. Both are needed. Isocyanate: The NCO Source That Drives the Reaction Isocyanate provides NCO groups. These NCO groups react with: Polyol OH groups Water Crosslinker OH or amine groups Amines formed during the water reaction This makes isocyanate the reactive partner that drives the foam chemistry. The most common isocyanates in polyurethane foam include: TDI TDI means toluene diisocyanate. It is widely used in flexible slabstock and many flexible molded foam applications. Common TDI 80/20 has around 48.3% NCO , but the actual value should be checked from the current CoA. MDI MDI means methylene diphenyl diisocyanate. It is widely used in rigid foam, molded foam, PIR systems, spray foam, and many CASE applications. MDI grades often have lower %NCO than TDI, commonly around the low 30% range depending on grade. The important point is that TDI and MDI are not interchangeable by name or weight. Their %NCO values differ. Their equivalent weights differ. Their reactivity profiles differ. Their application windows differ. A change from TDI to MDI, or from one MDI grade to another, is a formulation event. It must be recalculated. Isocyanate %NCO and Equivalent Weight The key isocyanate specification is %NCO . %NCO tells you how much reactive NCO is present in the isocyanate material. The formula for isocyanate equivalent weight is: Isocyanate EW = 4,200 ÷ %NCO For TDI at 48.3% NCO: EW = 4,200 ÷ 48.3 EW = 86.96 g/eq For an MDI grade at 31.5% NCO: EW = 4,200 ÷ 31.5 EW = 133.33 g/eq This shows why isocyanate identity matters. The same number of parts does not give the same NCO equivalents if %NCO changes. If the formula uses an old %NCO value, the actual index may shift. If the CoA %NCO differs from the TDS midpoint, the formula should use the current CoA value for accurate calculation. This is especially important in quality-sensitive production. A small %NCO difference can move the actual index enough to change hardness, cure, compression set, and exotherm. Isocyanate Index: The Ratio That Holds the Formula Together The isocyanate index is the ratio between NCO equivalents and reactive hydrogen equivalents. A simple formula is: Index = NCO equivalents ÷ Reactive H equivalents × 100 Index 100 means stoichiometric balance. Index 105 means 5% excess NCO. Index 95 means NCO deficit. In polyurethane foam, the index affects many properties at once. It can influence: Hardness Cure Foam strength Compression set Exotherm Cell stability Brittleness Surface quality Final network balance Reactive hydrogen equivalents usually come from: Polyol Water Crosslinker Other reactive additives, if present NCO equivalents come from: TDI MDI Other isocyanate components, if present Catalyst and surfactant normally do not enter the index calculation. They are critical to foam quality, but they do not normally contribute stoichiometric equivalents. This distinction is important. If the index is wrong, catalyst cannot truly fix it. Catalyst can change speed. It cannot correct the chemical ratio. Why CoA Values Matter More Than Old TDS Values A technical data sheet is useful. But a certificate of analysis is closer to production reality. The TDS usually gives typical values. The CoA gives the measured values for a specific batch or drum. For polyol, the important CoA value is often OHV. For isocyanate, the important CoA value is %NCO. If the formula uses old TDS values while production uses current batch material, the formula may not run at the intended index. Example: These changes may look small. But they can move equivalent calculations. And when equivalents move, index moves. The foam responds to actual chemistry, not paperwork. A formula calculated from old values can produce: Hardness drift Cure variation Compression set change Exotherm change Density or rise behavior changes Batch-to-batch inconsistency This is why CoA review should be part of production control, not only supplier documentation. Isocyanate Moisture Exposure: Silent Index Drift Isocyanates are moisture sensitive. NCO groups react with water. If an isocyanate drum is exposed to atmospheric moisture, some NCO groups can be consumed before production. That means active %NCO can drop. The formula may still use the old %NCO value. But the drum may no longer behave like that value. This creates silent index drift. Possible causes include: Drum left open Poor resealing Humid storage area Long exposure time Damaged packaging Transfer system moisture Poor nitrogen blanketing where required Condensation during handling The exact %NCO loss depends on exposure conditions, humidity, time, temperature, and storage practice. The important point is not to guess a fixed loss value. The important point is to control exposure and verify quality when risk exists. If active %NCO drops and the formula is not adjusted, the actual index may be lower than intended. That can affect: Cure Hardness Compression set Network formation Foam stability Final properties Isocyanate handling is part of formulation control. Polyol and Isocyanate Change-Control Checklist Before approving a polyol or isocyanate change, check these points. Do not approve a raw material change only because the name looks similar. The foam does not read the name. It reacts to OHV, functionality, %NCO, equivalent weight, index, reactivity, and processing condition. Practical Calculation Workflow Use this workflow when reviewing polyol and isocyanate in a PU foam formula: Read current polyol OHV from CoA. Calculate polyol EW using 56,100 ÷ OHV . Confirm polyol functionality. Read current isocyanate %NCO from CoA. Calculate isocyanate EW using 4,200 ÷ %NCO . Calculate reactive H equivalents from polyol. Add reactive H equivalents from water and crosslinker. Calculate NCO equivalents from isocyanate. Calculate actual index. Compare actual index with target index. Review reaction timing and catalyst package. Run production trial if raw material grade changed. This workflow prevents the most common calculation problem: The formula says one index, but production is actually running another. If your foam hardness, cure, compression set, or batch consistency changed after a polyol or isocyanate change, the issue may be hidden in OHV, functionality, %NCO, equivalent weight, or actual index. PolymersIQ can help review your polyol and isocyanate data, recalculate the formula from current CoA values, and identify whether the foam is running at the index and network design you intended. Contact PolymerIQ for a polyol and isocyanate formula review

Polyol and Isocyanate in PU Foam: How the Main Reactive Pair Builds the Network

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