The working principle of Hydrolyzed Polymaleic Anhydride (HPMA)

Hydrolyzed Polymaleic Anhydride (HPMA) works primarily through two interconnected mechanisms: chelation/complexation of scale-forming ions and crystal distortion (lattice distortion) of developing mineral scales.

Chelation and Complexation

HPMA is a low-molecular-weight polymer rich in carboxyl groups. These active groups can chelate with hardness ions in water, such as calcium (Ca²⁺) and magnesium (Mg²⁺), forming soluble complexes. This keeps these ions in solution and reduces their tendency to precipitate as insoluble salts. Research indicates that HPMA can interact with Ca²⁺ via complexation, thereby improving the degree of supersaturation that the solution can tolerate before scaling occurs.

Crystal Distortion and Dispersion

Beyond simple chelation, HPMA interferes directly with the growth of scale crystals:

Adsorption onto crystal nuclei: HPMA anions adsorb onto the surface of calcium carbonate crystal nuclei.

Lattice distortion: This adsorption disrupts the normal, orderly growth of the crystal lattice, causing the resulting crystal structure to become distorted or irregular. These distorted crystals are less adherent and more easily disrupted.

Dispersion: After adsorption, the crystal nuclei acquire a negative charge. The resulting electrostatic repulsion between these charged particles prevents them from aggregating into large, hard deposits. Instead, they remain dispersed as loose, flowable sludge that can be removed from the system.

Additional Practical Effects

Scale morphology modification: HPMA has been shown to change both the morphology and crystal structure of calcium carbonate precipitates.

Old scale removal: Its dispersing action also helps to loosen and strip existing scale deposits, facilitating their removal.

In summary, HPMA functions as a scale inhibitor by "tying up" scale-forming ions and by "poisoning" the growth of scale crystals, rendering them harmless and dispersible rather than allowing them to form hard, insulating deposits on equipment surfaces.