HPMA (Hydrolyzed Polymaleic Anhydride) is a low-to-medium molecular weight water-soluble polymer widely used as a scale and corrosion inhibitor in water treatment, especially in high-temperature and high-hardness conditions.
1. Chemical Structure
HPMA is produced by the hydrolysis of polymaleic anhydride. Its structure consists of a carbon-carbon backbone with carboxylic acid side groups.
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Basic Repeating Unit:
The polymer chain is primarily composed of maleic acid units in a hydrolyzed form, resulting in a structure with two carboxyl (–COOH) groups per repeating unit. The exact structure can vary due to isomerization during synthesis.However, due to the polymerization process, the structure is not strictly alternating and may include branching or minor rearrangements.
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Key Features:
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High carboxyl density: Two carboxyl groups per repeating unit, providing strong chelation and dispersancy.
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Allies (C–C bonds) provide stability against thermal and oxidative degradation.
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Typically supplied as an aqueous solution (partial sodium or ammonium salt form) with a light yellow to brown appearance.
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2. Molecular Weight Range
HPMA is intentionally synthesized as a low molecular weight polymer to optimize its performance as a threshold inhibitor and dispersant.
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Typical Range: ~400 – 5,000 Da (g/mol)
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Most commercial products have a weight-average molecular weight (Mw) between 500 and 3,000 Da.
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The number-average molecular weight (Mn) is often lower (~300–1,500 Da), indicating a relatively narrow distribution.
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Why Low Molecular Weight?
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Higher carboxyl functionality per unit weight → better scale inhibition via crystal distortion.
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Improved solubility and stability in high-ion-strength water.
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Enhanced ability to adsorb onto scale nuclei and disperse particles.
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3. Comparison with Similar Polymers
| Polymer | Typical Mw Range | Carboxyl Density | Key Features |
|---|---|---|---|
| HPMA | 500 – 3,000 Da | Very high (2 COOH per unit) | Excellent CaCO₃ inhibition, high-temperature stability (>175°C) |
| PAA (Polyacrylic Acid) | 1,000 – 15,000 Da | High (1 COOH per unit) | General-purpose dispersant, cost-effective |
| PASP (Polyaspartic Acid) | 2,000 – 10,000 Da | Moderate | Biodegradable, moderate scale inhibition |
4. Functional Implications of Structure
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Scale Inhibition: Carboxyl groups adsorb onto CaCO₃/CaSO₄ crystals, distorting growth and preventing hard scale.
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Dispersancy: Anionic charges suspend particles (clay, corrosion products) via electrostatic repulsion.
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Corrosion Mitigation: Indirectly reduces under-deposit corrosion by keeping surfaces clean; may enhance performance of Zn²⁺ or phosphonate-based inhibitors.
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Thermal Stability: Stable in boiler water and cooling systems up to ~175–200°C due to C–C backbone and absence of labile bonds.
5. Typical Applications
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High-temperature boiler water treatment (low-pressure to medium-pressure systems).
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Cooling water systems with high scaling tendency.
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Reverse osmosis (RO) antiscalant formulations.
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Detergent builders (as a phosphate-free alternative).
Summary
HPMA is a low-MW polycarboxylate with high carboxyl density, derived from hydrolyzed polymaleic anhydride. Its molecular weight typically ranges from ~500 to 3,000 Da, optimized for efficient scale inhibition and thermal stability. This structure makes it particularly effective in challenging water conditions where conventional polymers (e.g., PAA) may underperform.