![]() Two surfaces joined by a short connector may require high force to separate them by breaking the bonds of the connector, but the total energy required to separate them is relatively low due to the short extension (b). 6 As shown below (a), two of these structural motifs built up in growing biomaterials are sacrificial bonds and hidden lengths. In contrast, studies of biological materials reveal diverse hierarchical structures that often impart superior mechanical performance. 4,5 Moreover, polymer particles of waterborne PSAs derived by random emulsion polymerization generally lack defined internal structures. 2 These hydrophilic domains create weak boundary layers between polymer particles that are thought to decrease adhesion, cohesion, and water resistance. Nonetheless, aqueous phase components such as surfactants, water-soluble polymer, and salts remain in the interstices between particles, 3 while hydrophilic functional groups on the polymer surface can form membranes between polymer particles in the ultimate film. 2 Eventually, the particles deform to create a void-free film with interdiffusion of polymer chains across the particle interfaces. The aqueous phase surrounds the polymer particles, and the charged and hydrophilic components on the particles provide electrostatic and steric barriers to particle coalescence.Īs this dispersion is applied and dried to create a PSA tape, label, or other article, the evaporation of water diminishes the aqueous phase and brings the particles closer together. Salts, such as buffers or polymerization byproducts, and rheology modifiers, which are often hydrophilic polymers, are further components of the aqueous phase of a typical PSA dispersion. Surfactants that are added to PSA dispersions not only to aid polymerization but also to improve dispersion stability and coatability are present both at particle surfaces and in the aqueous phase. ![]() ![]() 1 Water-soluble monomers such as acrylic acid (AA) or 2-hydroxyethyl acrylate may copolymerize with more hydrophobic monomers so that the resulting polymer is amphiphilic, displaying hydrophilic groups on the polymer particle surface, or they may form hydrophilic polymers that remain dissolved in the aqueous phase.Ĭommon initiators such as persulfate salts form sulfate polymer end-groups that also occupy the polymer–water interface. ![]() The emulsion polymerization process utilizes hydrophilic components such as water-soluble monomers, water-soluble initiators, surfactants, rheology modifiers, and salts that remain in the final PSA dispersion either associated with the polymer particles or free in the aqueous phase. Typically, the major component of a PSA dispersion is a hydrophobic copolymer of acrylic, styrenic, or other vinyl monomers made by emulsion polymerization. Waterborne pressure-sensitive adhesives (PSAs) are the largest class of PSAs by production volume, and most waterborne PSAs comprise polymer dispersions in water. ![]()
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