by Rheology modifiers are common additives used in paint and coating formulations to control the viscosity and flow properties. They work by interacting with the vehicle and pigment components to impart pseudoplastic, thixotropic or anti-settling behavior as needed for the particular application. Understanding the different types of these modifiers and their mechanisms of action is important for formulating high-quality paints and coatments.
Cellulosic Rheology Modifiers
One of the most widely used classes of Rheology Modifiers are cellulosic derivatives such as hydroxyethyl cellulose (HEC), hydroxypropyl cellulose (HPC), and hydroxypropyl methylcellulose (HPMC). These cellulosic polymers have backbone chains that become entangled when at rest, causing an increase in viscosity. However, under shear forces such as stirring or application, the chains disentangle and slide past each other, lowering the viscosity to aid flow and leveling. This provides excellent pseudoplastic and thixotropic behavior for formulations. The hydroxyl groups on the cellulose polymers also interact strongly with water molecules, further adding to the rheological effects.
Acrylic Polymer Rheology Modifiers
Acrylic polymers synthesized from monomers such as acrylic acid, methacrylic acid and their esters are also commonly used as rheology modifiers. Popular examples include polyacrylic acid, polymethacrylic acid and their copolymers. These polymers typically contain both hydrophilic and hydrophobic sections, allowing them to interact strongly with pigment particles as well as the liquid vehicle. The entanglement and disentanglement of polymer chains under shear provide pseudoplastic and thixotropic rheology. Some acrylic polymers are also associative thickening agents that provide additional thickening via hydrophobic interactions.
Organoclay Rheology Modifiers
Organoclays are another important class of these modifiers which work by a different mechanism than polymers. They are composed of clay mineral platelets that have been treated with organic modifiers such as quaternary ammonium cations. This treatment causes the normally hydrophilic clay platelets to organophilic instead. In paints and coatings, the organoclay platelets interact strongly with each other and nearby pigment particles via attractive forces. At rest, a structure forms with the platelets stacked together, increasing viscosity. Under shear, the structure breaks down allowing flow, and then quickly reassembles to solidify the coating. This provides excellent thixotropic behavior to suspend pigments without affecting application and flow.
Associative Thickener Rheology Modifiers
Associative thickeners contain hydrophilic sections that hydrate in water as well as hydrophobic sections that associate together above the Krafft temperature. Well-known examples include alkali-swellable emulsions (ASE) and hydrophobically-modified ethoxylated urethanes (HEUR). In paint formulations, the associative interactions between hydrophobic sections cause chain entanglement that strongly increases viscosity. Under shear, the entanglements break to reduce viscosity for easy application. Upon stopping shear, rapid reformation of associations causes quick setting without sag or settling of the coating. This makes them popular as anti-sag and leveling agents.
Use of Rheology Modifiers in Specific Formulations
The choice of rheology modifier depends on the requirements of the specific formulation. For example, latex paints require pseudoplastic and rapid setting properties to avoid sagging, which can be provided by cellulosics, acrylic polymers or associative thickeners. High-performance industrial coatings need strongly thixotropic behavior to suspend heavy pigment loads without compromising flow or leveling during application – this favors organoclays. Waterborne automotive coatings require shear-thinning as well as good aging resistance, qualities that cellulosic and certain acrylic modifiers deliver. Powder coatings employ cellulosic or fumed silica particles as flow and anticaking agents. Thus formulators must evaluate the end-use conditions and rheological demands of the coating to select the optimal type and concentration of modifier.
Advantages of Using these Modifiers
The use of thses modifiers in paint and coating formulations provides several advantages:
– Improved suspension of pigments without unwanted settling
– Easy brushing, rolling or spraying application along with excellent leveling and flow out of the coating
– Resistance to sagging or slumping of the coating on vertical surfaces
– Fast hardening and drying without compromising application properties
– Stable viscosity over a wide range of shear rates and temperatures
– Long shelf life stability without demixing or separation in the container
– Ability to formulate high-solids, low-VOC coatings using thickening power of modifiers
– Compatibility with various resins and vehicles including water, solvents and 100% solids systems.
In conclusion, careful selection of these modifiers tailored for the demands of specific formulations enables paint and coating manufacturers to develop products with optimal properties for top performance and end-user satisfaction. Understanding modifier chemistry equips formulators to leverage these important additive tools.
