If you’ve ever worked with polyurethane coatings, adhesives, or flexible foams, you already know how critical polyester polyols are to the final product’s performance. As a polyester polyols supplier, every batch I ship goes through dozens of quality checks, and one of the most non-negotiable tests we run is the hydroxyl value. Get this number wrong, and downstream manufacturers end up with products that are too brittle, too soft, or even fail to cure properly. When I first started in this industry 12 years ago, I watched a lot of young chemists skip over the small details of testing hydroxyl value, and I’ve seen far too many clients come back with bad batches because of it. So today, I want to break down exactly how we test hydroxyl value at our facility, why each step matters, and what you should look for when working with a polyols supplier. Polyester Polyols
First, let’s make sure we’re on the same page about what hydroxyl value actually is. Put simply, hydroxyl value (OHV) is the measure of the number of hydroxyl (-OH) groups in a sample of polyester polyol. It’s expressed in milligrams of potassium hydroxide (KOH) per gram of sample (mg KOH/g), and it directly correlates with the molecular weight of the polyol: the higher the OHV, the lower the average molecular weight, and vice versa. For our clients, this number dictates everything from pot life to crosslink density in their final polyurethane product. A polyurethane foam designed for cushioning needs a specific OHV to get the right balance of softness and support, while a industrial coating needs a higher OHV to create a tough, durable crosslinked network. Get this wrong, and you’re looking at costly reworks or even product failures in the field.
There are a few methods to test hydroxyl value, but the one we rely on every single day is the phthalic anhydride esterification method. It’s the gold standard in the industry for polyester polyols, and while it’s not the fastest test, it’s the most accurate and reproducible—two non-negotiables for a supplier that’s responsible for consistent batch quality. I’ve seen other suppliers cut corners with quicker, less reliable methods, and that’s a mistake that comes back to haunt both the supplier and the client. Let’s walk through the step-by-step process we use, with the lessons I’ve learned over the years to avoid common errors.
First, we start with sample preparation. This sounds basic, but it’s where a lot of mistakes happen. We take a representative sample from each production batch, right after it’s finished and cooled to room temperature. We never test a sample from the top of a drum or the bottom alone—we mix the entire drum (or IBC tote) thoroughly, taking multiple small samples from different layers, and combine them into a single composite sample. Polyester polyols are prone to settling over time, especially higher molecular weight grades, so a single sample from the top will give you an artificially low OHV because there’s less of the high-OHV, lower molecular weight material there. We also make sure the sample is free of moisture at this step. Moisture reacts with phthalic anhydride in the next step, throwing off the entire calculation. If a sample has more than 0.1% moisture, we dry it over molecular sieves for 24 hours before testing. I learned this the hard way early on: a batch of polyol we shipped had a falsely low OHV because of unaccounted moisture, and the client’s foam came out too soft. That cost us a repeat batch, so now we’re obsessive about sample moisture content.
Next, we prepare the reagents. The two key reagents here are phthalic anhydride in pyridine, and a potassium hydroxide (KOH) titrant solution. Pyridine is a solvent that dissolves the polyester polyol and acts as a catalyst for the esterification reaction between hydroxyl groups and phthalic anhydride. It’s also important to note that pyridine is hygroscopic, meaning it absorbs water from the air, so we always store it in tightly sealed bottles and prepare the phthalic anhydride reagent fresh every two weeks—any old reagent will give inconsistent results. The KOH titrant is standardized daily using a primary standard, usually potassium hydrogen phthalate (KHP). I keep a log book of titrant values going back 10 years, because we’ve noticed that even small deviations in titrant concentration can add up to big errors in OHV. Last year, our new lab technician standardized the KOH and got a value 0.02 N lower than usual, and we caught it before testing a batch of high-OHV polyol. That would have resulted in a batch that was 5 mg KOH/g off spec, which is enough to make it unusable for our clients making rigid foams.
Now, the actual esterification reaction. We weigh a precise amount of sample, usually between 1 to 5 grams depending on the expected OHV (smaller samples for high OHV, larger for low OHV), into a clean, dry Erlenmeyer flask. We add exactly 25 mL of the phthalic anhydride-pyridine reagent, swirl to dissolve the sample completely, and then attach a reflux condenser to the flask. The reaction happens when we heat the flask in a water bath at 115°C for exactly 60 minutes. Why strict timing? If you heat for less than 60 minutes, not all the hydroxyl groups will react with the phthalic anhydride, leading to an artificially low OHV. If you heat longer than 60 minutes, side reactions start to happen, like the decomposition of phthalic anhydride, which also skews results. We use a digital timer for every single test, and two technicians check the heating time independently—no exceptions.
After the 60 minutes of heating, we let the flask cool for 10 minutes, then rinse the condenser with about 5 mL of pyridine to wash any residual reagent down into the flask. Then we add a few drops of phenolphthalein indicator solution, and we’re ready for titration. Here’s where the logic of the test becomes clear: during the esterification, each hydroxyl group in the sample reacts with one molecule of phthalic anhydride. Any unreacted phthalic anhydride is then hydrolyzed by adding water to form phthalic acid, and we titrate that acid with KOH. We run two blank tests at the same time, using the same amount of reagent and solvent, but no sample. The difference between the volume of KOH used for the blank and the volume used for the sample gives us the amount of KOH that reacted with the phthalic anhydride that was consumed by the hydroxyl groups. That’s how we calculate OHV.
Titration is another step where small mistakes can ruin a test. We use a 50 mL burette with 0.1 mL graduations, and we read the bottom of the meniscus at eye level every time. The endpoint is a pale pink color that lasts for at least 30 seconds—any darker and you’ve over-titrated, which will make your OHV too high. We always do duplicate titrations for each sample, and if the two results differ by more than 0.2 mg KOH/g, we run a third test. I’ve seen new technicians get frustrated when they have to run a third test, but consistency is everything here. Our goal is to have duplicate results within 0.1 mg KOH/g for every batch we ship.
Once we have the titration volumes, the calculation is straightforward, but let’s walk through it to make sure it’s clear. The formula for hydroxyl value is:
OHV = ((V_blank – V_sample) * N_KOH * 56.1) / Weight of sample (g)
Where 56.1 is the molecular weight of KOH, and N_KOH is the normality of the KOH titrant. That formula has been our go-to for years, and we’ve cross-checked it with other methods like NMR spectroscopy and ASTM D4274, which is the standard test method for hydroxyl value of polyols. ASTM standards are non-negotiable for us, because they ensure our results are comparable to what our clients will test in their own labs. If we use a non-standard method, there’s a risk that our OHV numbers won’t match the client’s, leading to confusion and mistrust.
Wait a second—are there any edge cases where this method doesn’t work? Absolutely. For polyester polyols that have very low hydroxyl values (under 10 mg KOH/g), the esterification method is still reliable, but you have to use a larger sample size to get enough acid to titrate accurately. For polyols that have unsaturated groups, or that are contaminated with acids, we adjust the method slightly—we do an acid number test first to account for the acid groups, and subtract that from the total acid we titrate, to get the correct hydroxyl value. That’s another lesson I learned early on: polyester polyols made from dicarboxylic acids can have residual acid groups, and if you don’t account for those, your OHV will be wrong. We always run an acid number test alongside hydroxyl value, which is part of our standard quality control process.
As a polyester polyols supplier, this test isn’t just a box to check—it’s how we prove to our clients that what we ship is exactly what we promised. Last quarter, we had a client who switched from another supplier because their polyol batches were all over the place in OHV. They started using our standard grade of 56 mg KOH/g, and their first three batches came in at 55.8, 56.1, and 55.9 mg KOH/g—perfectly consistent. That kind of reliability is why we’ve built long-term partnerships with manufacturers around the world.
Now, I know a lot of clients ask if there’s a faster way to test hydroxyl value, especially if they need results quickly for a production run. We do offer rapid test kits for on-site QC, but we always remind our clients that the rapid tests are meant for screening, not for final quality confirmation. The esterification method we use in our lab is still the most accurate, and we stand behind our results 100%. If a client has a question about the OHV of a batch they received, we will run a test on our end and send them the raw data, no fine print.
At the end of the day, testing hydroxyl value is about accountability. When you’re a polyester polyols supplier, your customers are relying on you to provide materials that will make their products work as intended. A small error in OHV can lead to big problems down the line, so taking the time to do the test right is non-negotiable. If you’re in the market for consistent, high-quality polyester polyols for your next polyurethane project, reach out to our team to discuss your specific requirements. We’re here to answer any questions about our testing processes, batch specifications, or custom formulations that you might have.
TPU References
ASTM International. Test Method D4274-19: Standard Test Methods for Testing Polyurethane Raw Materials: Determination of Hydroxyl Numbers of Polyols. ASTM International, 2019.
Oertel, G. Polyurethane Handbook: Chemistry, Raw Materials, Processing, Application, Properties. 2nd ed., Hanser Gardner Publications, 1993.
Schafer, H. J. "Determination of Hydroxyl Values in Polyols: A Review of Current Methods." Journal of Applied Polymer Science, vol. 42, no. 10, 1991, pp. 2789–2802.
Singbon New Materials(Shandong) Co., Ltd.
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