Every industrial water system runs two economies at once. The first is water and energy: fewer cycles of concentration means less water used but more pumping and heat loss. The second is deposit control: push the cycles up and the sparingly soluble salts in the water start to precipitate on heat transfer surfaces, and a millimetre of calcium carbonate on a condenser tube costs more in energy than the water saved.
Acrylic acid-2-hydroxypropyl acrylate copolymer (AA-HPA, CAS 55719-33-0) is one of the polymers that makes high-cycle operation possible. It is a low molecular weight, water-soluble polycarboxylate supplied as a 30% solution, and it works by threshold inhibition, crystal distortion and dispersion rather than by chelation. This guide covers what it is, how it is made, how it behaves at the doses actually used, and where it fits relative to polyacrylic acid, sulfonated copolymers and phosphonates.
What Is AA-HPA Copolymer?
AA-HPA is a free-radical copolymer of acrylic acid and 2-hydroxypropyl acrylate. The acrylic acid units provide the carboxylate groups that adsorb onto crystal surfaces and give particles a negative charge; the hydroxypropyl acrylate units introduce hydroxyl functionality and steric bulk that change how the polymer interacts with calcium phosphate and iron oxide deposits.
The result is a polymer that behaves like polyacrylic acid on calcium carbonate scale, but noticeably better than PAA on calcium phosphate and iron oxide – the two foulants that limit cycles of concentration in alkaline cooling programmes. It is supplied as a neutralised aqueous solution at 30% solids, which is the practical form for dosing into a water system.
Chemical Identity and Specification
| Item | Detail |
|---|---|
| Chemical name | Acrylic acid – 2-hydroxypropyl acrylate copolymer |
| CAS Registry Number | 55719-33-0 |
| Chemical class | Water-soluble polycarboxylate; hydroxyl-functional acrylic copolymer |
| Supply form | Aqueous solution, neutralised |
| Appearance | Colourless to yellow liquid |
| Solid content | Min 30.0% |
| Free monomer (as acrylic acid) | Max 0.5% |
| Density (20 deg C) | Min 1.10 g/cm3 |
| pH (1% water solution) | 7.5 +/- 1.0 |
| Limiting viscosity number (30 deg C) | 0.065 – 0.095 dl/g |
Because the product is a polymer, a molecular formula is not a meaningful specification. What matters is the molecular weight range – captured here by the limiting viscosity number – the solids content, and the residual free monomer. Buyers comparing quotations should ask for all three.
What Each Specification Number Means in Service
| Specification | Why it matters |
|---|---|
| Solid content min 30.0% | Sets the active polymer delivered per litre; a low-solids lot silently increases your dosing cost per unit of active |
| Free monomer (as AA) max 0.5% | Residual acrylic acid is the main regulatory and occupational concern; a low free monomer figure is essential for RO and potable-adjacent applications |
| Density min 1.10 g/cm3 | Consistency indicator for a 30% polycarboxylate solution; used for volumetric dosing calibration |
| pH 7.5 +/- 1.0 | Neutralised for safe handling and for compatibility with phosphonate and zinc programmes |
| Limiting viscosity number 0.065-0.095 dl/g | Molecular weight proxy: this is the low-MW range that gives threshold inhibition and dispersion. Higher viscosity means a bigger polymer, which flocculates instead of dispersing |
The viscosity number deserves emphasis because it is the specification most often ignored and most often responsible for a failed trial. A polymer with a limiting viscosity number well above 0.095 dl/g can still reduce scale, but it does so by a different mechanism – it bridges particles and forms fluffy deposits instead of dispersing them. For a cooling water programme the low range is the right range.
How the Polymer Controls Scale
Threshold inhibition
Stoichiometric chelation would require one carboxylate per calcium ion, which is uneconomic at industrial doses. Polycarboxylates instead work at substoichiometric levels: a few milligrams per litre is enough to keep hundreds of milligrams of calcium carbonate in suspension. The polymer adsorbs onto crystal nuclei and blocks the sites where further lattice growth would occur.
Crystal lattice distortion
When the polymer adsorbs onto a growing crystal face, subsequent layers grow imperfectly. The resulting crystals are irregular, softer and far less adherent than the smooth rhombohedral crystals that form without inhibitor. Soft, non-adherent scale is removed by normal flow and blowdown.
Dispersion
Adsorbed polymer gives each particle a net negative charge. The electrostatic repulsion keeps particles apart, and the hydroxyl-bearing side chains add a steric contribution. This is the mechanism that handles iron oxide, clay, silt and corrosion products – deposits that are not “scale” in the chemical sense but foul heat exchangers just as effectively.
Applications in Detail
Open recirculating cooling water
The classic application. AA-HPA is dosed as the dispersant and carbonate-scale component of a programme that also contains a phosphonate (HEDP, ATMP or PBTC) for calcium carbonate threshold inhibition, zinc or phosphate for corrosion control, and an azole for copper alloy protection. Typical AA-HPA dose is 5-20 mg/L product, higher in systems with high iron or phosphate loading.
Boiler water treatment
In low and medium pressure boilers, polycarboxylates disperse hardness salts and iron transport products that enter with the feedwater. They complement, rather than replace, phosphate/polymer internal treatment programmes and coordinated pH-phosphate control.
Reverse osmosis and desalination
RO antiscalants must control calcium carbonate, calcium sulfate and barium sulfate at low dose without fouling the membrane. The low free monomer content and controlled molecular weight matter here; confirm the membrane manufacturer’s approval for the specific product before use.
Oilfield water injection
Injection and produced water systems use AA-HPA to prevent sulfate and carbonate scale in tubing and near-wellbore formation. Compatibility with biocides, corrosion inhibitors and oxygen scavengers has to be checked, since some cationic programmes precipitate anionic polymers.
Detergents and industrial cleaning
In laundry and hard-surface formulations, the same polymer acts as an anti-redeposition agent and calcium carbonate dispersant, improving whiteness retention when builders are limited.
Mineral processing and pigment dispersion
Slurry dispersion and grinding aid applications use low-MW polycarboxylates to reduce viscosity at high solids loading.
Dosing Guidance
| System | Product dose (mg/L) | Active polymer (mg/L) | Co-components |
|---|---|---|---|
| Open cooling, standard | 5 – 15 | 1.5 – 4.5 | Phosphonate, zinc, azole |
| Cooling with high iron or phosphate | 10 – 20 | 3 – 6 | Phosphonate, dispersant booster |
| RO antiscalant | 2 – 5 | 0.6 – 1.5 | None or a second antiscalant for sulfate control |
| Boiler internal | 3 – 10 | 0.9 – 3 | Phosphate, oxygen scavenger |
| Oilfield injection | 5 – 15 | 1.5 – 4.5 | Biocide, corrosion inhibitor, scavenger |
Always confirm whether a quoted dose refers to product or to active solids – a factor-of-three error is easy to make with a 30% solution, and it is the most common cause of a “the polymer did not work” trial result.
Cycles of Concentration and the Saturation Index
The dose that matters is the one that holds the water in a non-scaling state at the cycles of concentration the plant actually runs. Two numbers drive that calculation, and both are worth writing into a programme sheet rather than keeping in a spreadsheet on one engineer’s laptop.
Cycles of concentration compares an indicator ion in the circulating water with the same ion in the make-up. Chloride or conductivity are the usual choices; conductivity is fastest but is affected by chemical addition, so chloride is more reliable for a formal mass balance. At five cycles, everything in the make-up is five times more concentrated in the tower.
The calcium carbonate saturation index combines calcium hardness, total alkalinity, pH, temperature and total dissolved solids into a single number that says how far the water sits above or below calcium carbonate equilibrium. A positive index means the water will deposit unless something interferes; a strongly negative index means it is corrosive. Polymers and phosphonates do not remove the driving force – they allow the system to operate at a positive index without forming adherent scale.
| Parameter | Why it enters the calculation | Practical note |
|---|---|---|
| Calcium hardness | Supplies the cation for calcium carbonate | Measure on the circulating water, not the make-up |
| Total alkalinity | Supplies the carbonate | Alkalinity drives the index more strongly than hardness in most towers |
| pH | Sets the carbonate/bicarbonate balance | A 0.3 pH unit step changes the index materially |
| Temperature | Reduces calcium carbonate solubility as it rises | Use the hot-surface temperature, not the basin |
| Total dissolved solids | Ionic strength correction | Conductivity is a reasonable proxy |
Two operating consequences follow. First, the polymer dose has to be reviewed whenever the plant changes cycles – a dose set at three cycles is under-designed at six. Second, a programme that holds a high positive index without scaling at one site may fail at another with the same index but a higher skin temperature, because the driving force at the hot surface, not in the bulk water, is what deposits.
Incoming Inspection and Jar Testing
A water treatment polymer is bought on numbers, and a programme is defended with numbers, so the two should connect. Four checks on arrival cover most failure modes:
- Solid content. Confirm 30.0% min. A lot at 26% delivers 13% less polymer per litre and quietly underdoses the whole programme.
- Density at 20 deg C. Min 1.10 g/cm3 – used to calibrate volumetric dosing pumps.
- pH of 1% solution. 7.5 +/- 1.0. Drift outside the window suggests incomplete neutralisation or contamination.
- Limiting viscosity number. 0.065-0.095 dl/g. This is the measurement that confirms you received a dispersant-grade polymer rather than a flocculant-grade one.
Jar testing then connects the specification to performance. A simple static test – synthetic or filtered system water at the target cycles, dosed across a range of polymer concentrations, held at the system temperature for a defined period – will show at which dose the water stays clear and which dose produces haze. It is not a substitute for a coupon or heat-transfer monitor trial, but it catches gross errors before a drum ever reaches the dosing tank.
| Test | What it tells you | Limitation |
|---|---|---|
| Static jar test at system temperature | Whether the dose suppresses visible precipitation | No flow, no heat flux – optimistic |
| Coupon deposit rate | Actual deposition in the system | Slow; weeks to a usable signal |
| Polymer residual | Whether the product reaches the system | Does not show whether it is working |
| Heat-transfer monitor | Deposit effect on the thing you care about | Requires installed equipment |
AA-HPA Compared With Other Water Treatment Polymers
| Polymer | CaCO3 scale | Ca-phosphate | Iron oxide dispersion | Chlorine tolerance | Typical use |
|---|---|---|---|---|---|
| AA-HPA copolymer | Good | Good | Good | Moderate | General cooling and boiler programmes |
| Polyacrylic acid (PAA) | Good | Moderate | Moderate | Moderate | Cost-driven carbonate scale control |
| AA-AMPS copolymer | Good | Good | Good | Good | High temperature, high pH, chlorinated systems |
| Phosphonates (HEDP/ATMP) | Very good (threshold) | Moderate | Poor | Consumed by chlorine | Threshold inhibition backbone |
| Maleic anhydride copolymers | Good | Good | Good | Good | Higher temperature and iron-rich systems |
Handling, Storage and Safety
The product is a neutralised aqueous polymer solution, so the handling profile is mild compared with acid monomers: no significant vapour hazard, no exotherm on dilution. The practical controls are splash protection for eyes, standard gloves, and avoidance of prolonged skin contact.
| Item | Practice |
|---|---|
| Storage temperature | 5 – 35 deg C; do not freeze – freezing can separate the polymer irreversibly |
| Containers | Keep closed; the solution supports microbial growth if diluted and left standing |
| Dilution | Dilute with clean water and use within a short period; do not store dilute working solutions for weeks |
| Incompatibility | Cationic polymers and quaternary biocides can precipitate anionic polycarboxylates – dose at separate points |
| Shelf life | Confirm per batch; viscosity or colour change indicates ageing |
Packaging, Supply and Documentation
| Item | Detail |
|---|---|
| Standard pack | 200 kg polyethylene drum |
| Bulk pack | 1000 kg IBC on request |
| Documents | COA per batch (appearance, solids, free monomer, density, pH, limiting viscosity number), SDS in English, shipping documents |
| Samples | Available for jar tests and pilot dosing trials |
Frequently Asked Questions
Is AA-HPA a chelating agent? No. It works by threshold inhibition, crystal distortion and dispersion at doses far below stoichiometric; chelants such as EDTA bind calcium one-to-one and are used for different purposes.
Can it replace phosphonate in my programme? Partly. Phosphonates are stronger threshold inhibitors for calcium carbonate in some conditions, while the polymer is far better on iron oxide and particulate dispersion. Most programmes use both.
Does chlorine destroy the polymer? Oxidising biocides attack polycarboxylates, though AA-HPA tolerates typical halogen residuals better than many alternatives. Avoid slug dosing at high concentration into the polymer feed point.
What is the difference between AA-HPA and AA-AMPS? AMPS introduces sulfonate groups, which improve tolerance of high calcium, high temperature and chlorine. AA-HPA is normally the lower-cost general-purpose choice.
How do I convert 30% solution to active dose? Multiply the product dose by 0.30. A 10 mg/L product dose delivers about 3 mg/L active polymer.
Contact PolyBlueChem for specifications, samples, COA and quotations.