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Surfactants for Waterborne Adhesives and Sealants

Posted on 11/29/2017 9:36:50 AM By Deb Bhattacharjee
  

Surfactants are surface active agents lowering surface tension or interfacial tension between two liquids or between a liquid and a solid. Chemically, these are amphiphilic organic compounds containing hydrophobic groups which are water insoluble, or oil soluble, representing tails and hydrophilic groups which are water soluble or oil-insoluble, representing heads.

Surfactants impart very critical performance properties in preparation of the waterborne adhesive and sealants and in its end use application properties.  One of the most important role of surfactants is to improve surface wetting by reduction of static or equilibrium surface tension which is relevant when the system is at rest. However, the ability to reduce surface tension under dynamic conditions, like spraying, rolling, brushing or high speed gravure is of great importance in applications where high surface creation rates are utilized. Surfactants also enhance surface coverage while reducing surface defects arising from contamination, difficult surfaces and poor rheology property of certain adhesives.It also protects polymer particles from agglomeration due to external stress like extreme high and low temperature during shipment and storage, high shear during application, etc.Other roles include flow enhancement, dispersion of additives by increased wetting and defoaming capability.

Based on chemical structures, surfactants can be classified into two broad groups. Polar hydrophilic head groups and non-polar hydrophobic tail groups which would make it soluble in polar and non-polar solvents and oil respectively.The relative size, shape and length of the hydrophobic and hydrophilic moieties determine its properties.  For example, hydrophilic groups could be anionic, cationic, amphoteric and non-ionic, whereas the hydrophobic groups are typically comprised of hydrocarbon and fluorocarbon chains and a combination of these or silicone chains.

Anionic surfactants contain negatively charged functional groups at its head, typical examples are sulfate, sulfonate, carboxylate, sulfo-succinate, phosphate esters, etc.  These are commonly used and are relatively inexpensive.  However, these are sensitive to electrolytes and pH changes.  Cationic surfactants have inferior emulsion stabilization efficiency and are less commonly used in adhesives formulation.  Like anionic surfactants, these are also sensitive to electrolytes and pH changes. Amphoteric surfactants contain both cationic and anionic centers attached to the same molecule. Nonionic surfactants are mostly polyethylene glycol ethers.   It could also be block copolymers of polyethylene and polypropylene or polybutylene glycol ethers.They are commonly used in adhesive formulation, though rarely used in emulsion polymerization due to inferior efficiency in creating stable emulsion.  These are less sensitive to electrolyte and pH changes and are usually utilized in combination with anionic surfactants to provide a complimentary method of colloidal stabilization. HLB (hydrophile-lipophile balance) is an important selection criteria in forming stable emulsions.

So far, we have talked about commonly used small molecules which act as surfactants.  Now, we would switch gear to discuss how a few specifically designed small molecules could be incorporated in larger polymer like polyurethane backbone to allow them to form a stable colloidal dispersion.

Dimethylolpropionic acid (DMPA) and tertiary amine diols are incorporated in polyurethane backbone by reacting with isocyanates.  DMPA has very poor solubility in polyols, thus requiring solvents in PU dispersion manufacturing process.  It is then neutralized with tertiary amines leading to stable dispersion at pH >8.  At lower pH, it leads to coagulation. The added advantage of this approach is that the carboxylic acid group can be available for further crosslinking with epoxy, carbodiimide, etc.  Dihydroxy and diamino sulfonates, when incorporated in polyurethane prepolymer, offer alternate method of stabilization of polyurethane dispersion at lower pH and formulation flexibility.  There is a very different, relatively new, continuous mechanical dispersion process using high shear to stabilize finely formed prepolymer droplets.  Advantages of this process include the utilization of highly reactive aromatic isocyanate, it is solvent free, and it leads to high solids, 45 to 55 wt.% solids, polyurethane dispersions.  This knowledge can be utilized to form other high solids polymer dispersions like polyolefin dispersion or hybrid composite dispersions including PU/acrylates.

All the advantages surfactants bring in adhesive formulation do not come without some drawbacks or disadvantages.  One of the major issues come with the water sensitivity of the resulting adhesive through ionic or ethylene oxide polar groups.Some external surfactants could migrate to the interface leading to the loss in adhesive strength.Many common surfactants work fine in static environment but perform poorly in offering stabilization of latex particle in more dynamic situation, like high speed application.  These lead to new opportunities in surfactant development which are environmentally friendly, biodegradable and reactive or polymerizable surfactants, especially in emulsion polymerization.