From waste to tape: Inverse vulcanization of sulfur and solvent-based depolymerization for preparation of pressure-sensitive adhesives

Abstract

Upcycling is an eco-friendly method that turns waste into valuable items. Elemental sulfur, a waste substance produced in large amounts (70 million tons per year), can be converted into valuable polymers using inverse vulcanization. However, the traditional method had significant limitations, such as limited consumption and weak strength. To address this problem, we newly fabricated sulfur-based pressure-sensitive adhesives (PSAs). PSAs are soft and tacky materials that should have low modulus, and they are used extensively (3.1 million tons per year). We prepared sulfur-based PSAs in four steps: polymerization, dissolution, coating/drying, and curing. In the polymerization step, castor oil and oleic acid were employed to facilitate inverse vulcanization. The resulting polymer then underwent solvent-based depolymerization to be coated in a film form. After drying and ultra-violet (UV)-induced curing, the tapes were successfully produced. Significantly, we adjusted the PSA's crosslinking density by changing the oleic acid content and UV dose. As the amount of mono-functional oleic acid decreased or the UV dose increased, the resulting PSA's crosslinking density increased. The best conditions led to PSAs with excellent adhesion performances and creep resistance, similar to commercial tapes. Moreover, the prepared PSAs exhibited self-healing properties, suggesting the feasibility of high-performance PSAs like remoldable PSAs and conductive sensors.