New Acrylic Dispersers in Coatings

Introduction
Uniform dispersion of pigment particles in papermaking coatings and printing composites is a major challenge in the printing and packaging industries. In the paper coating process, it is common to add a large amountof mineral fillers such as calcium carbonate, clay and titanium dioxide to increase whiteness and printability. If the appropriate dispersion factor is not used, the pigment particles quickly become clusters, resulting in a significant increase in viscosity, coating instability, and heterogeneous areas. Acrylic acid-based dispersantsare able to create a thin layer on particles by having carboxyl groups in their structure, reduce surface forces between them and prevent particle accumulation and deposition. These polymers, after being neutralized, make the surface of the particles negatively charged and maximize the stability of the particles by combining the electrostatic repulsion effect and the space barrier created by polymer chains. Recent studies have shown that the use of these dispersants can reduce the consumption of solvent-based resins and allow for replacement with more environmentally friendly materials in water systems. In other words, these factors make it possible to provide coatings with a higher solid percentage and lower viscosity that have significant economic and environmental benefits.

Synthesis Methods

The most common method of synthesis of acrylic dispersants is free radical polymerization. In the process, major monomers such as acrylic acid or methacrylic react with other monomers and aninitiator in the aqueous phase. This reaction is usually performed at temperatures of about ۵۰ to ۸۰ degrees centigrade. Synthesis can be performed in two main forms of solution and emulsion. In solution polymerization, all the components are dissolved in anaqueous phaseand polymerization is carried out. This method can produce polymers with low molecular weight and uniform distribution. However, a neutralization phase may be needed after the reaction is completed to remove or stabilize the remaining active substances. In emulsion polymerization, first the water mixture, monomers and a small amount of emulsifier are prepared and then the initiator is gradually added. In this case, fine polymer grains are formed in the aqueous phase, which shows good stability after neutralization. By adjusting the reaction parameters such as the ratio of monomers, the initiator concentration, temperature and time, the final molecular weight and the polymer structure can be controlled. For example, the use of chain transfer agents such as thiols or chlorinated hydrocarbons is used to reduce molecular weight. Also, techniques such as controlled polymerization, such as the RAFT method or granulation emulsification, have been used to achieve dispersants with more precise structure and improved properties. In general, the use of emulsion polymerization allows for mass production and high scalability, and industrial companies use this method to provide the required dispersants.

Monomer Composition and Polymer Structure

The choice of monomers is vital in the production of dispersants. The general structure of these polymers is a combination of carboxylic units such as acrylic acid or methacrylic acid and aliphatic or aromatic hydrophobic units. neutralization of polymer chain acid groups with an alkaline agent,this leads to the formation of negatively charged ionic groups, which leads to increased surface load and promotion of hydrophilic properties of the chain. This negative charge adheres to the surface of the particles and creates electrostatic repulsion between the particles, which is effective in preventing particle accumulation. Along with acid monomers, the addition of hydrophobic monomers such as ethyl acrylate, butyl acrylate,Styrene or its derivatives is customary for parts of the chain to interact better with the pigment surface and to expand the water-friendly parts of the environment. Also, the use of two-factor monomers such as diacrylates and di-amides allows the production of cross and branch polymer structures. The presence of these transverse bonds and branching structures causes the polymer chains to stick to the surface of the particles, and the bulky side chains fill the space between the particles by hydrating. By adjusting the ratio of acidic to non-acidic units and the percentage of functional monomers, the balance between the solubility of polymer in water and the ability to absorb to the surface of particles is carefully optimized. Finally, the final molecular structure of the dispersants must be in harmony with the type and surface of the pigment to achieve the best stability and performance.

Dispersion Mechanism

The function of acrylic acid dispersants in pigments is based on two main mechanisms of electrostatic stability and space barrier. In the electrostatic mechanism, polymer carboxyl groups create a strong negative charge at the surface of the pigment particles after ionization. When this polymer is absorbed on the surface of the particle, the particle has a similar charge and around it forms a layer of homogeneous ions such as sodium or ammonium ions. The repulsive force between similar particle loads causes particles to distance themselves from each other and prevent contact and clustering. In addition, polymer chains attached to the surface of the particle hold water around the particle and create a hydrated layer that adds extra stability to the system. The second mechanism is related to the steric hindrance. After the particle is coated by polymer-absorbing parts, the remaining long chains expand in the vicinity of the water and find a significant thickness. These hydrated chains act as physical shielding and prevent the two particles from directly approaching each other. In other words, the dispersantsare made up of two parts: the part that sticks to the surface of the particle and the part that expands and swellsin the aqueous phase. The combination of these two mechanisms, on the one hand, creates a significant electrostatic repulsion and, on the other hand, a mechanical barrier that keeps particles stable apart.

Applications

One of the main applications of mineral dispersants is in the paper industry. In this industry, a large percentage of paper coatings are calcium carbonate, clay and titanium dioxide. The use of acrylic acid dispersants allows increasing the concentration of particles in the coating mass without undesirable raising of viscosity. For example, acrylic acid copolymers with the right molecular weight can only create a uniform diffusion of particles in the system by adding less than 0.5 percent to the weight of dry pigment. As a result, it is possible to raise the solid material of the coating 70 percent without the need for organic solvents or traditional fillers. This allows the viscosity of the paper pulp to be maintained and the resin consumption to be reduced, both of which are economically and environmentally important. In addition to the paper industry, these materials have also become widely used in other water-based coatings. In the production of structural and industrial paints, the uniform distribution of inorganic and organic pigments is essential to achieve optimal color quality and prevent particle deposition. For example, in car paint formulations and stainless paints, acrylic acid dispersantssuspend titanium dioxide particles and carbon black during the aqueous phase. In the digital printing and inkjet printing industry, the control of colored particles such as iron oxide and organic pigments is done with the help of these polymers to maintain the uniformity and stability of the printing color. Even in the production of powder paints and electrostatic coatings on metals, the presence of these dispersants leads to better diffusion of metal particles and improved adhesion of the coating on the substrate. In general, due to the high compatibility of these polymers with water systems and their combinability with resins and other additives,their scope of application extends beyond traditional industries and is used in many advanced formulations.

Conclusion

Acrylic acid-based polymers, with their multifunctional properties, represent a good option for pigment dispersion factors, exhibiting excellent abilities to control molecular weight when one changes the concentration of initiator, as well as the addition of a transporter, to exhibit linear or branched structures, and the ability to ionize in anaqueous phasewhich are among the key featuresto ensure their efficacy. Improvements in methods of synthesis in recent years such as controlled emulsion polymerization, branching structures and the addition of functional monomers lead to improvements in their stability and quality of dispersion. Similarly, the convergence of experimental studies and molecular modeling, have facilitated advanced designs of new dispersants by way of quicker modeling and more performance certainty. Nevertheless the understanding of the effects of conditions such as pH, temperature and water hardness on the performance of the materials, requires further investigation. Furthermore, ongoing studies and advanced computational methods, seek to create complementary synthetic strategies, and artificial intelligence to improve higher and less environmentally impactful compounds. In conclusion, the combination of knowledge from polymer chemistry, materials engineering and advances in molecular simulations, will produce coatings with more enhanced performance and new characteristics literally to make printing packages and industrial coatings of tomorrow, more efficient and sustainable.