Acrylic thickeners are synthetic, high molecular weight copolymers of acrylic acid that are used to adjust the rheology of aqueous systems. The simple handling of the mostly liquid thickeners and their easy activation via the adjustment of the pH value (increased from the acidic to the slightly basic range) are, in addition to a good price-performance ratio, the decisive advantages for their universal use and their wide distribution in the market.
When weak bases, such as ammonia or amine alcohols, are added, the acidic groups of the polymers are completely or partially neutralized and become water-soluble in their salinized form. In this process, the polymer chains dissolve and distribute themselves homogenously in the aqueous phase (increase in volume). There, depending on the molecular weight, they get entangled to varying degrees. Depending on the dosage, a thickening effect up to a colorless, transparent gel is achieved.
The rheological behavior is distinctly shear-thinning (pseudoplastic). As emulsion polymers, acrylic thickeners are generally sensitive to frost. Partially neutralized products can overcome this drawback, show easier incorporation and better compatibility. In addition, products in 100 % powder form are available as an alternative to aqueous products.
The market distinguishes between two different types of acrylic thickeners, the so-called ASE and HASE types.
ASE grades (Alkali Swellable Emulsions) are the more frequently used grades, which are characterized by their strong thickening effect in water and good color stability. However, they show disadvantages in water sensitivity (e.g. whitening/wet rub resistance). Specially modified ASE grades can show advantages in the orientation of effect pigments due to their viscoelastic behavior.
HASE grades (Hydrophobically modified Alkali Swellable Emulsions) contain additional hydrophobic components in the polymer chain which, in addition to the thickening mechanism via solution and chain entanglement, also exhibit associative behavior – in analogy to polyurethane thickeners (HEUR). This results in advantages in terms of lower water sensitivity, higher brush resistance (ICI) and lower spattering tendency, for example in roller application. Disadvantages may be, depending on the stabilization of the coating system, inferior color stability and loss of gloss due to interaction with pigments and fillers (unwanted flocculation).