Abstract
Poly(hydroxy urethane)s (PHUs) based on 5-membered cyclic carbonates have emerged as sustainable alternatives to conventional isocyanate-based polyurethanes. However, while from the point of view of sustainability they represent an improvement, their properties are still not competitive with conventional polyurethanes. In this work, the potential of PHUs as reversible hot-melt adhesives is discussed. We found that with a judicious choice of reagents (i.e., the dicyclic carbonate and diamine), the detrimental hydrogen bonding between the soft segment of the chains and the pendant hydroxyl groups was partially avoided, thus imparting PHUs with hot-melt adhesion properties (i.e., adhesion at elevated temperatures and cohesiveness at a temperature lower than Tg/Tm). The importance of a balanced hard to soft segment ratio, along with the relevance of the chain extender in the final properties, is highlighted. Addition of aliphatic diamines (HMDA, 1,12-DAD) resulted in rubbery materials, while the employment of cycloaliphatic (CBMA) or aromatic ones (MXDA, PXDA) led to materials with hot-melt adhesive properties. The thermoreversibility of all compositions was assessed by rebonding specimens after lap-shear tests. Lap-shear strength values that were comparable to the virgin adhesives were observed. The breaking and reformation of hydrogen bonding interactions was demonstrated by FTIR measurements at different temperatures, as well as by rheological frequency sweep experiments. In order to mitigate the negative impact of the low molar mass PHUs and to enhance the service temperature of the adhesives, a hybrid PHU was prepared by adding a small amount of an epoxy resin, which acts as a cross-linker. These hybrid PHUs maintain the thermoreversibility displayed by thermoplastic PHUs while providing better adhesion at elevated temperatures. We believe that this work provides some important insights into the design of PHU-based hot-melt adhesives.
Highlights
Hot-melt adhesives (HMAs) are typically solvent-free thermoplastic materials or lightly cross-linked thermosets, which are characterized by their solid state at low temperatures, while presenting low viscosity and good flowing above this temperature.[1]
In our quest to hot-melt adhesives, a Poly(hydroxy urethane)s (PHUs) made of a rigid diamine such as m-xylylenediamine (MXDA) combined with resorcinol dicyclic carbonate (RdiCC) was initially synthesized at 80 °C for 24 h (Scheme 1)
Hot-melt adhesives based on PHUs have been prepared
Summary
Hot-melt adhesives (HMAs) are typically solvent-free thermoplastic materials or lightly cross-linked thermosets, which are characterized by their solid state at low temperatures, while presenting low viscosity and good flowing above this temperature.[1] They provide great bond strengths in short periods upon cooling, which makes them very attractive materials when fast processing is required. They are relatively easy to handle, economical, and clean running. They present excellent adhesion on surfaces that are difficult to adhere, such as low surface roughness materials
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