Moisture-cured solvent free silylated poly(ether-urea) pressure-sensitive adhesives (PSAs) for use as skin adhesives for application in transdermal drug delivery (TDD)

Abstract

Improving the adhesion and tack of pressure-sensitive adhesives (PSAs) remains an ongoing challenge. Polyureas often have stronger hydrogen bonding relative to, the more commonly used, polyurethanes which are quite common in adhesive applications. An increased adhesion can reduce the requirements for additives such as tackifier resins and fillers to achieve high levels of adhesion required in certain applications. This approach not only necessitates tedious optimisations but also introduces compatibility issues especially in drug-in-adhesive formulation development. In this work, novel silylated polyurea moisture-curable PSAs are introduced with moisture curing of silyl terminated polyether–urea prepolymers. Prepolymer variants were synthesised using a solvent and catalyst-free step-growth polymerisation by reacting the commercially available polyetheramine Jeffamine® D-4000 with isophorone diisocyanate (IPDI) to yield diprimaryamino-terminated polymers. The polymers’ average molecular weight was controlled by adjusting the –NCO/–NH₂ molar ratio. Subsequently, the amino terminal ends were post-functionalised with 3-isocyanatopropyltrimethoxysilane (IPTMS) to attain the silylated moisture-curable prepolymer variant crosslinkers (PUXL). The variants were able to cure to form (–Si–O–Si–) crosslinked PSA gels in the presence of a titanium catalyst and moisture. These materials were characterised by a range of techniques including rheology and thermal characterisation. Although their thermal properties remained unaltered, their adhesion and tack increased by increasing the –NCO/–NH₂ molar ratio exceeding the borders of the Chang's classification windows for removable PSAs. This was achieved without the requirement for any additional tackifyer resins. Peel test and rolling ball tack tests agreed with variants demonstrating superior adhesion compared to commercially available transdermal adhesive products. Finally, no cold flow effects were noticed a result of the crosslinked network/gel.