Seven grades of the same product, all “polyamine, light yellow liquid”, differing only in solid content and viscosity. To a purchasing department they look like a reason to buy the cheapest. To a plant chemist they are a selection variable that decides whether the coagulation programme works at all.
This guide is about choosing between grades A to G of polyamine (CAS 42751-79-1): what viscosity actually measures, why charge density moves the opposite way, and how to pick a grade from the water you are treating rather than from the price list.
Viscosity is a proxy for molecular weight
In a polymer solution at fixed solid content, viscosity rises with molecular weight. That is the only reason a viscosity ladder exists in this product family:
| Viscosity band | Relative molecular weight | Charge density per unit mass | Dominant mechanism |
|---|---|---|---|
| 50-200 mPa.s (grade A) | Lowest | Highest | Charge neutralisation |
| 200-500 (B) | Low | Very high | Charge neutralisation |
| 600-1000 (C) | Medium | High | Mixed |
| 1000-3000 (D) | Medium-high | Medium | Mixed to bridging |
| 3000-6000 (E) | High | Medium-low | Bridging |
The counter-intuitive part: raising the molecular weight lowers the charge density, because the extra mass is backbone, not cationic sites. A plant that switches from grade B to grade E to “get a stronger product” often gets worse clarification – the polymer now bridges instead of neutralising, and the colloids it was supposed to destabilise are still charged.
Start from the problem, not the grade
| If the problem is… | The mechanism you need | Grade |
|---|---|---|
| Dissolved colour, fine colloids, low turbidity | Charge neutralisation | A or B |
| Anionic trash in a paper mill | Charge neutralisation (maximum charge per kilogram) | A |
| Municipal wastewater, mixed suspended solids | Charge plus some bridging | C or D |
| Coarse industrial suspension, slow settling | Bridging, large flocs | D or E |
| Oily water with stable emulsion | Charge neutralisation at the droplet surface | B or C |
What grades F and G are for
Grades F and G carry 40% solids instead of 50%, at viscosity bands that overlap C and D. They exist for two practical situations:
- Plants that dilute on site. If you are making up a 1% working solution anyway, paying to ship ten percent more water is a choice, not a requirement – F and G reduce freight and storage volume per unit of active.
- Dosing pump range. A high-viscosity 50% product can fall outside the suction capability of a small metering pump; the 40% version at the same viscosity band is easier to move.
What F and G are not is a cheaper way to buy the same chemistry. Per kilogram of active polymer, they cost what they cost – the saving is in logistics, not in the molecule.
The dosing calculation that trips people up
Because the product is a solution, the active dose is a fraction of the product dose, and the fraction is not the same across the series:
| Grade | Solids | Product dose for 5 mg/L active |
|---|---|---|
| A-E | 50% | 10 mg/L of product |
| F-G | 40% | 12.5 mg/L of product |
A programme transferred from a 50% grade to a 40% grade without recalculating runs 20% under-dosed – and the operator concludes the new grade is weaker. It is not; the arithmetic changed.
Jar test protocol for grade selection
- Characterise the water. Turbidity, colour, pH, alkalinity, and above all the charge demand (colloid titration or streaming current).
- Pick three grades from opposite ends and the middle – A, C, E is a good first screen. Testing all seven wastes time and reagents.
- Run a dose series for each – typically 1, 2, 5, 10, 20, 50 mg/L product – at the plant’s actual rapid-mix intensity.
- Record settling rate and settled turbidity, not just clarity at one minute. Floc size and settling velocity are what the clarifier sees.
- Find the overdosing point deliberately. Cationic polymers restabilise colloids above an optimum dose; knowing where that is prevents a plant upset later.
Common selection errors
| Error | What happens | Fix |
|---|---|---|
| Buying on solid content alone | A 50% low-charge product underperforms a 40% high-charge one | Compare on charge demand, not on solids |
| Choosing the highest viscosity “for strength” | Bridging chemistry applied to a charge problem – poor clarification | Match mechanism to problem |
| Dosing neat into a pipe | Local overdosing, gummy deposits, restabilisation | Dilute to 0.1-1% and dose into turbulent flow |
| Adding polyamine after PAM | Both chemicals wasted; the anionic flocculant coats particles first | Polyamine first at rapid mix |
| Storing drums in the sun | Viscosity drift, dosing error | 5-35 deg C, shaded, sealed |
Frequently asked questions
Does a higher viscosity mean a better product? No – it means a higher molecular weight and lower charge density. Which is “better” depends on whether your problem is charge or floc size.
Can I blend grades to make an intermediate viscosity? Physically yes, but you lose the clean relationship between specification and performance, and your COA no longer describes what is in the tank. Choose the grade you need.
Why is viscosity measured at 3# spindle, 60 rpm? Because polymer solutions are non-Newtonian – the reading depends on shear rate. Without the spindle and speed, a viscosity figure is not a specification.
How do I convert from a 50% to a 40% grade? Multiply the product dose by 1.25 to keep the same active dose (50/40). Then re-check settling, because the dosing window also narrows.
Contact PolyBlueChem for a grade-screening sample set.