| Aqueous adhesives with aziridine crosslinkers Photoinitiated adhesives with aziridine crosslinkers Todays pressure sensitive adhesives are so strong, they make predecessor products look inadequate. From shear holding capabilities originally designed for modest loads at room temperature, high performance pressure sensitive tapes can now support loads at elevated temperatures for a week. This improved performance is generally delivered by crosslinking the pressure sensitive adhesive (PSA) although careful formulation is needed to retain high levels of tack and adhesion. However, that strength comes at a price. For acrylic PSAs crosslinking is generally achieved by free-radical copolymerization of multifunctional unsaturated groups with other monomers, and covalent or ionic crosslinking through the functional monomers, such as acrylic acid. A variety of different alternatives have also been used as crosslinking agents, like polyfunctional acrylates and acetophenones, as well as photo-initiated crosslinkers like benzophenones and triazines. However, these all have various drawbacks, like high volatility, incompatibility with certain polymer systems and generation of corrosive or toxic by-products. They can also discolour, need a separate photoactive compound to initiate the crosslinking reaction, or are highly sensitive to oxygen. Aziridine crosslinkers are an interesting alternative that avoids these problems, allowing water-based adhesives that eliminate the volatile organic compound (VOC) emissions seen with solvent based alternatives. They are also readily produced by Michael addition of an aziridine compound with an acryloyl compound.
When used with acid-functional acrylate and methacrylate (collectively known as (meth)acrylate) copolymers, the aziridine group is thought to react with the pendant acid functional groups to form a carboxyethyleneamino linkage. The amine produced by the ring opening can crosslink the copolymer by forming an ionic or covalent bond with an adjacent acid group. Reactions between the secondary amine from the aziridine and ester groups from the ester monomer units can also produce covalent amide linkages. To produce aqueous adhesives, the aziridine crosslinkers include water solubilizing chemical groups like tertiary amino groups and polyethylene glycol chains, making a self-emulsifying product. Dont hesitate to react about this Article
Any conventional radical polymerization can prepare (meth)acrylates to be blended with aziridine crosslinkers, with emulsion polymerization, as ever, representing a powerful tool for making aqueous formulations.2 This generally proceeds by first making a monomer premix with more than 85 parts of a (meth)acrylic acid ester monomer, where the ester group has 4 to 12 carbon atoms, such as isooctyl acrylate. Up to 15 parts of an acid functional monomer can also be included, helping form a salt with basic solubilizing groups in the aziridine crosslinker. Acrylic and methacrylic acids wide availability and low cost make them obvious choices, but if even stronger acids are desired, ethylenically unsaturated sulfonic acids and phosphonic acids can be used.
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