A polyacrylamide viscosity number means nothing on its own. "0.1% solution, 300 mPa·s" and "0.1% solution, 35 mPa·s" can both be correct readings on the same drum of the same grade, taken an hour apart, and neither lab made a mistake. Viscosity of a high molecular weight polymer solution depends on concentration, temperature, shear rate, water salinity, pH, and how long the solution has been sitting — change any one and the number moves, sometimes by a factor of ten.
That is a problem when viscosity is how you check what you bought. Buyers routinely reject a batch because their in-house reading came in below the figure on the certificate, when the real cause was a different spindle speed or tap water instead of deionized. It is also a problem in reverse: a genuinely degraded batch passes because the test was run at a shear rate low enough to hide it. This page covers how to take a reading that means something, what the number should be for each grade, the six variables that move it, and how to raise or lower solution viscosity on purpose when your process needs a target.
If you are choosing a grade rather than verifying one, molecular weight is the parameter you actually want, and our molecular weight guide covers selection. This page is about measurement and control of the solution you already have.
A Viscosity Spec Needs Five Conditions, Not One Number
Any viscosity figure on a certificate of analysis or a data sheet is incomplete unless it states all five of these. When we quote a release figure, all five are on the document, and when a customer sends us a reading that disagrees with ours, the discrepancy is in one of these five about nine times out of ten.
| Condition | Typical test setting | Effect if it drifts |
|---|---|---|
| Concentration | 0.1% or 0.5% w/w active | Strongly non-linear: 0.1% to 0.2% can triple the reading |
| Temperature | 25°C ± 0.5°C, water bath | Roughly 2–3% per °C; a 5°C error is a 10–15% error |
| Shear rate / spindle speed | Brookfield LV, spindle and rpm both stated | Largest single factor: 6 rpm vs 60 rpm can differ 5–10× |
| Make-up water | Deionized, or stated NaCl concentration | Tap water can halve an anionic reading |
| Solution age | Fully hydrated, tested within 2 hours | Under-aged reads low; over-aged reads low again |
The shear rate row is the one that catches people. Polyacrylamide solutions are strongly shear-thinning — the long chains align with the flow and offer less resistance as you spin faster. A 0.1% solution of a 20 million dalton anionic grade in deionized water reads in the low hundreds of mPa·s on a Brookfield LV at 6 rpm, and in the tens of mPa·s at 60 rpm or at the ~7 s⁻¹ shear rate conventionally used for oilfield polymer screening. Both numbers describe the same solution. This is why we publish 200–400 mPa·s at 0.1% in a water treatment context and 30–80 mPa·s at 0.1% for our PHPA oilfield grade — the oilfield figure is at the higher shear rate that field practice uses, not a weaker polymer.
What the Reading Should Be, By Grade
These are the ranges we work to on release, all at 0.1% w/w active in deionized water, 25°C, Brookfield LV at low spindle speed. Use them as an acceptance window, not as a target to hit exactly — a production window of ±15% on the same grade across batches is normal for this chemistry, and any supplier claiming tighter reproducibility on a high molecular weight product is describing a marketing number rather than a measurement.
| Grade family | MW range (M Da) | 0.1% low-shear (mPa·s) | Notes on reading it |
|---|---|---|---|
| APAM high MW | 18–28 | 200–400 | Most salt-sensitive family; DI water is not optional |
| APAM medium MW | 3–8 | 40–120 | Low enough that spindle choice matters for resolution |
| CPAM high charge | 12–20 | 120–300 | Reads lower than APAM at equal MW; charge type differs |
| CPAM medium charge | 8–15 | 80–200 | Paper mill users often test in white water, which reads low |
| NPAM | 8–15 | 60–160 | Least affected by salt and pH; most reproducible test |
| PHPA oilfield | 15–25 | 30–80 at ~7 s⁻¹ | Quoted at field shear rate, not low shear |
Two patterns are worth internalising. Anionic grades give the highest reading per unit of molecular weight in fresh water because the carboxylate groups repel each other and hold the chain extended, so the molecule sweeps a larger hydrodynamic volume. That same property is what makes them collapse in hard or saline water. Nonionic grades have no charge to expand or collapse, so they read lower but far more consistently — if you need a QC test that gives the same answer in your lab and ours without arguing about water, an NPAM grade is the easy case.
Cationic grades sit in between and carry an extra complication: charge density and molecular weight move independently, and viscosity tracks molecular weight far more closely than charge. A high-charge, moderate-MW CPAM can read lower than a low-charge, high-MW one while being the correct product for sludge dewatering. Viscosity is not a proxy for cationic strength, and buyers who use it that way end up rejecting the right grade. Charge has to be measured separately, which is what the charge density guide covers.
The Six Variables That Move Your Reading
1. Concentration, and why it is not proportional
Doubling concentration does not double viscosity. Below a certain concentration the polymer coils are separate and viscosity rises roughly in line with dose; above it the coils overlap and entangle, and viscosity climbs steeply. For a 20M dalton anionic grade that crossover sits near 0.05–0.1%, which is exactly where most plants make up stock solution. Going from 0.1% to 0.2% typically triples the reading rather than doubling it, and 0.5% stock of a high-MW grade is thick enough that dosing pumps start to struggle and the solution will not mix uniformly.
That is the practical reason we recommend 0.1–0.3% make-up for powder grades and never above 0.5%. It is also why a plant that quietly raised stock concentration to save tank turnovers ends up with a viscosity complaint about the product. Check the make-up ratio by weight before anything else.
2. Temperature
Viscosity falls roughly 2–3% per °C over the 15–40°C range, so a reading taken at 30°C on a warm afternoon in a plant lab lands about 10–15% below the same solution at 25°C. Uncontrolled ambient temperature is the most common reason a customer reading disagrees with a certificate. A water bath fixes it, and where there is no bath, recording the temperature alongside the number at least makes the two readings comparable.
Sustained heat is a different problem. Above about 50°C the polymer starts to lose molecular weight through chain scission and oxidation, and that loss is permanent — the solution does not recover viscosity when it cools. Warm make-up water above 40°C, steam tracing on a stock tank, or a solution held hot over a weekend all show up as a viscosity that keeps falling day over day.
3. Salinity and hardness
Dissolved salts screen the charges on an anionic chain, the electrostatic repulsion holding it extended disappears, and the coil contracts. Practical scale: a few hundred ppm of sodium chloride visibly reduces a high-MW anionic reading, and at 20,000 ppm TDS the same solution can read a fraction of its deionized-water value. Divalent calcium and magnesium are far more aggressive than sodium at equal concentration, and above roughly 100 ppm calcium a high-hydrolysis grade can begin to precipitate rather than merely thin.
This is why QC tests must specify the water. A plant testing in hard tap water and comparing against a supplier figure taken in deionized water is comparing two different experiments. It also explains a common field complaint — a polymer that performed on commissioning water underperforms two years later on recycled process water that has accumulated hardness. The polymer did not change. Nonionic and low-hydrolysis grades are the salt-tolerant answer, and for high-salinity oilfield brine the sulfonated chemistries discussed in our EOR guide hold viscosity where standard HPAM will not.
4. pH
Anionic grades need their carboxylate groups ionised to stay extended. Below about pH 4 those groups protonate, the chain coils, and viscosity drops sharply — which is the same reason anionic PAM stops flocculating in acidic circuits and nonionic takes over. Above pH 10, and especially with heat, the amide groups on the backbone hydrolyse to carboxylate over hours to days. Viscosity initially rises as the chain picks up charge and expands, then falls as the backbone itself degrades. A stock solution sitting in a caustic-contaminated tank will show exactly that rise-then-fall curve.
Cationic grades are the mirror case: they are stable in acid and lose charge and performance as pH climbs past 8–9. If you are seeing viscosity drift on a CPAM stock tank, check whether lime or caustic dosing upstream is carrying over.
5. Shear history
Shear-thinning is reversible — spin the spindle slower and the reading comes back up. Mechanical degradation is not. A centrifugal transfer pump, a partly closed control valve, or a mixer left at high speed permanently breaks chains, and the viscosity you lose is gone. The distinguishing test is simple: measure, let the solution rest 30 minutes undisturbed, and measure again at the same speed. Recovery means shear-thinning. No recovery means the polymer was degraded.
High molecular weight is what makes this acute. A 25M dalton grade loses viscosity through a progressive cavity pump where an 8M grade passes through untouched, which is the real reason plants running centrifuges are advised to step down molecular weight rather than dose harder. Use low-shear progressive cavity or diaphragm pumps on made-up solution, keep mixers at 60–100 rpm after the initial wetting stage, and avoid recirculating stock through a pump loop for hours.
6. Age and oxidation
Fresh solution reads low because hydration is incomplete — a high-MW powder grade needs 45–90 minutes with gentle agitation to reach full viscosity, and a reading at 20 minutes will be well under spec through no fault of the product. That is the most common false rejection we see. Past full hydration, viscosity holds for a day or so and then declines as dissolved oxygen, iron, chlorine, or bacteria attack the backbone. Ferrous iron plus dissolved oxygen is the fastest of these; residual chlorine in make-up water is the most often overlooked.
Practical stock life for a made-up 0.1% high-MW solution is 24 hours, or 8–12 hours where make-up water carries chlorine or iron. Size the tank for one to two days of consumption rather than a week. If your solution is thinning within a shift, test the make-up water for chlorine and iron before suspecting the powder — and check that the dry product is fully dissolving in the first place, since trapped gel reads as low viscosity too. The mechanics of that failure are in our guide to why polyacrylamide lumps, and the standard procedure is in the dissolving method.
A Viscosity Test You Can Defend
This is the procedure we run on release and the one we ask customers to run when a reading is in dispute. It takes about two hours, most of which is unattended hydration time, and it produces a number that two labs can actually compare.
- Weigh, do not scoop. 0.5 g of powder into 499.5 g of deionized water gives 0.1% w/w. Correct for moisture if you are working to tight tolerance — at ≥92% solid content, a nominal 0.5 g of powder is about 0.46 g of active polymer, and ignoring that alone accounts for several percent of disagreement.
- Wet it properly. Start the stirrer at 300–400 rpm to form a vortex, add the powder steadily over 30–60 seconds into the shoulder of the vortex, never as a single dump onto still water.
- Drop to gentle agitation. Reduce to 100–150 rpm once the powder is dispersed and hold for 60 minutes for high-MW grades, 30–45 minutes for medium-MW. High speed throughout is the single easiest way to shear your own sample and then blame the supplier.
- Equilibrate to 25°C in a water bath and confirm with a thermometer in the beaker, not the bath.
- Record the instrument setup. Brookfield LV, spindle number, rpm, and the reading. A number without spindle and rpm is not a result. Take the reading after 60 seconds of rotation once the display stabilises.
- Run a reference alongside. Keep a retained sample of a batch that performed well and test it in the same session. Relative comparison against your own reference removes almost every source of inter-lab error at once.
That last point is the one worth building into procedure. Absolute viscosity numbers travel badly between labs; a ratio against your own retained reference travels perfectly. Per AWWA and WEF practice on polymer acceptance testing, the recommendation is to verify performance with the specific product and the specific water in service rather than relying on a data sheet figure alone, and a side-by-side reference test is the cheapest way to do that. Where a formal method is required, intrinsic viscosity determination under ISO 1628 and the Chinese national standard GB/T 12005.10 for polyacrylamide are the recognised routes, and both report intrinsic viscosity used to derive molecular weight rather than an apparent solution viscosity.
We include a viscosity figure with its full test conditions on every batch certificate of analysis, alongside solid content ≥92% and residual monomer ≤0.05%, from three-stage testing at our Zhengzhou plant under ISO 9001/14001/45001. If a reading disagrees with ours, send the batch number and your test conditions and we will re-run the retained sample against the same setup.
Low Viscosity: Product Problem or Plant Problem?
Work down this table before raising a claim. In our experience the great majority of low-viscosity complaints resolve to make-up conditions, and the pattern of the fault tells you which one.
| What you observe | Most likely cause | Check / fix |
|---|---|---|
| Low at 20 min, normal at 90 min | Incomplete hydration, not a defect | Extend ageing to 60 min minimum for high MW |
| Low, and gel particles on a 1 mm screen | Fisheyes — polymer locked undissolved | Slow the feed, feed into a vortex, lower concentration |
| Low in plant water, normal in DI water | Salinity or hardness in make-up water | Test water TDS and Ca/Mg; consider NPAM or low-hydrolysis grade |
| Falls over hours in the stock tank | Chlorine, iron, or bacterial attack | Test residual chlorine and iron; cut stock tank hold time |
| Drops across a transfer pump | Mechanical shear degradation | Change to progressive cavity or diaphragm; no recovery after rest confirms it |
| Recovers after 30 min rest | Shear-thinning only — no damage | Standardise spindle speed and retest |
| Low with DI water, correct ageing, 25°C | Genuine batch or grade issue | Send batch number, COA and test conditions to the supplier |
Two more that look like viscosity faults and are not. A yellowed or ammonia-smelling powder has degraded in storage, usually from humidity or heat, and no make-up procedure recovers it — that is a shelf life and storage question. And a solution with correct viscosity that flocculates poorly is a grade or dose problem, not a viscosity problem; take that to a jar test and to the dosage calculation guide instead.
Controlling Viscosity On Purpose
Sometimes the reading is not a QC check — you need the solution to sit at a particular viscosity because the process demands it. Two opposite problems come up.
Stock too thick to handle. Symptoms are a dosing pump that cavitates or trips, a stock tank that mixes as a rotating plug with dead zones, and polymer that will not disperse into the process stream at the injection point. Fixes in order of preference: lower the make-up concentration to 0.1%, add an in-line dilution loop so the stock is cut to 0.02–0.05% just before injection, and only then consider dropping to a lower molecular weight grade. In-line dilution is the one that solves the most problems at once — it improves dispersion at the injection point as well as pumpability, and dispersion is usually where the real dosing loss is.
Solution not viscous enough for the duty. This is mainly an oilfield concern, where viscosity is the product rather than a side effect. Options, again in order: confirm hydration is complete before assuming a product shortfall, raise polymer concentration, then move up in molecular weight, then change chemistry if brine is the limiting factor. In drilling and polymer flooding, hydrolysis degree is the additional lever — higher hydrolysis buys viscosity in fresh water and loses it in hard brine, which is the tradeoff our drilling mud additive guide works through, and the same tradeoff governs friction reducer selection in slickwater fracturing.
In water treatment, resist the instinct to chase viscosity as a performance target. Viscosity is a symptom of molecular weight, and molecular weight is only one of the things that decides whether a flocculant works — charge type and charge density often matter more, particularly on organic sludge. High viscosity on a belt press feed can actively hurt you by making the polymer hard to disperse into the sludge, which is covered in belt press polymer selection. Per SPE practice in polymer flooding, solution viscosity should be specified at reservoir shear and salinity conditions rather than at laboratory reference conditions, and the water treatment analogue is to judge the polymer on floc and filtrate in your own water rather than on a viscosity figure.
Where viscosity genuinely is the specification, our APAM range at 18–28 million daltons and the PHPA oilfield grade at 25–35% controlled hydrolysis are the two lines we hold to a viscosity release window rather than only a molecular weight window. We run 100,000 tons per year across three lines from our 15,000 m² plant in Zhengzhou with a 500 kg MOQ and 7–10 day lead time on common grades, so a viscosity-critical trial does not require a container order.
Frequently Asked Questions
What viscosity should a 0.1% polyacrylamide solution have?
For a high molecular weight anionic grade at 18–28 million daltons, 200–400 mPa·s measured in deionized water at 25°C on a Brookfield LV at low spindle speed. Medium MW anionic reads 40–120, high-charge cationic 120–300, nonionic 60–160. All of those figures collapse if you test in hard tap water, test warm, or spin the spindle fast, so the conditions matter as much as the number.
Why is my polyacrylamide viscosity lower than the certificate says?
In order of likelihood: the solution has not finished hydrating (high-MW powder needs 45–90 minutes), you tested in tap water instead of deionized, the sample was warmer than 25°C, or your spindle speed is higher than the one on the certificate. Rule those four out before suspecting the product. If the reading is still low with deionized water, 60 minutes of gentle ageing, and 25°C, it is worth raising with your supplier against a batch number.
Does higher viscosity mean better polyacrylamide?
No. Viscosity mostly reports molecular weight, and higher molecular weight is only better for duties where bridging or mobility control is the mechanism — mineral settling, drilling, polymer flooding. For sludge dewatering, charge density decides performance and a high-viscosity low-charge product will underperform a lower-viscosity high-charge one. High viscosity also shears more easily in pumps and centrifuges, so on some equipment it is a liability.
How does temperature affect polyacrylamide solution viscosity?
Roughly 2–3% lower per °C of rise across the normal 15–40°C working range, and that part is reversible. Above about 50°C the polymer begins to degrade by chain scission and the viscosity loss becomes permanent. Keep make-up water at 25–35°C, and do not steam-trace or heat a stock tank to speed dissolution.
Why does salt water reduce polyacrylamide viscosity?
Dissolved ions screen the charges along the polymer chain, so the electrostatic repulsion that holds it extended disappears and the coil contracts into a smaller hydrodynamic volume. Anionic grades are the most affected, calcium and magnesium several times more aggressive than sodium at the same concentration. Nonionic grades have no charge to screen and hold viscosity far better in saline or hard water.
Can I recover viscosity in a solution that has thinned?
Only if the cause was reversible. Shear-thinning recovers within about 30 minutes of rest, and a cooled solution recovers what it lost to temperature. Mechanical degradation from a pump, oxidative attack from chlorine or iron, and thermal degradation above 50°C all break chains permanently and nothing restores them. The rest test tells you which you have: measure, rest 30 minutes, measure again at the same spindle speed.
How long does made-up polyacrylamide solution hold its viscosity?
A 0.1% high molecular weight solution in clean water holds for about 24 hours. Make-up water carrying residual chlorine, ferrous iron, or bacteria cuts that to 8–12 hours. Size stock tanks for one to two days of use rather than a week, and if viscosity is falling within a single shift, test the make-up water for chlorine and iron first.
What instrument do I need to measure polyacrylamide viscosity?
A rotational viscometer of the Brookfield LV type with a spindle set covers everything a plant lab needs and is what most certificates are referenced to. Record spindle number and rpm with every reading. Efflux-cup and flow-time methods are too crude for shear-thinning polymer solutions. Intrinsic viscosity, which is what molecular weight is actually derived from, needs a capillary viscometer and the ISO 1628 or GB/T 12005.10 method rather than a rotational instrument.
Viscosity reading not matching the COA?
WhatsApp: +86 187-3759-0940 · Email: info@chinapolyacrylamide.com
Send us the batch number plus your test conditions — concentration, water type, temperature, spindle and rpm — and we will re-run the retained sample from that batch under your setup and send both results. Free samples available for viscosity-critical qualification. MOQ 500 kg, 7–10 day lead time on common grades.
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