Polyurethane hot melt adhesive based on Diels-Alder reaction

doi:10.1016/j.ijadhadh.2020.102597

International Journal of Adhesion and Adhesives

Volume 100, July 2020, 102597

https://doi.org/10.1016/j.ijadhadh.2020.102597 Get rights and content

Abstract

Traditional hot melt adhesives are thermoplastic resins with non-covalent hydrogen bonds. In this letter, a polyurethane hot melt adhesive (CDI-PUR-DA) was synthesized via introducing thermally reversible covalent Diels-Alder adducts into the polyurethane main chain. Results from hydrogen nuclear magnetic resonance (NMR) analysis and an assessment of rheological properties demonstrated that CDI-PUR-DA exhibited a much lower viscosity than a traditional hot melt adhesive. The viscosity of the polyurethane decreased to 4 Pa s at 110 °C due to rupture of Diels-Alder adducts. However, the viscosity returned to its initial value on cooling due to the regeneration of the Diels-Alder adducts. This approach provides a new method for preparing hot melt adhesives.

Introduction

Hot melt adhesives have been widely employed in a variety of industries, due to their environmental friendliness, good mechanical properties as well as ease of processability [[1], [2], [3]]. Traditional hot melt adhesives are based upon thermoplastic resins such as ethylene vinyl acetate (EVA) [4,5], polyamide (PA) [6,7], polyester [8,9], polyurethane [10,11]and so on. These thermoplastic resins can be used as hot melt adhesive due to the rupture and re-generation of inter-molecular hydrogen bonds during the bonding process [3]. However, the structure of the polymer main chain does not change during the bonding process. Therefore, the viscosity of the hot melt adhesive during the bonding process is mainly due to the structure of the resins themselves.

In this letter, a novel polyurethane hot melt adhesive (CDI-PUR-DA) was synthesized via introducing thermally reversible covalent Diels-Alder adducts into a polyurethane main chain. Upon heating, the viscosity of the polyurethane hot melt adhesive decreased significantly due to the rupture of Diels-Alder adducts in the polyurethane main chain. However, the viscosity returned to its initial value on cooling due to the regeneration of the Diels-Alder adducts [12]. This novel hot melt adhesive based on a Diels-Alder reaction exhibited a much lower viscosity during the bonding process and will provide a new way to prepare hot melt adhesives.

Section snippets

Materials

Furfurylamine (99%, AR.), N,N′-Carbonyldiimidazole (CDI) and BMI (1,1-(methylenedi-1,4-phenylene)bismaleimide, 95%, AR.) were purchased from Aldrich and used as received. Polytetramethylene ether glycol (PTMG, Mn = 1000, CP.) was supplied by DuPont and used after 2 h of drying under a vacuum at 90 °C. N,N-dimethylformamide (DMF) was used after drying with CaH₂ and purified by vacuum distillation. Dichloromethane (99%, AR.) was purchased from China National Medicines and was used as received.

Synthesis of polyurethane prepolymer terminated with furan ring (PMCF)

Results and discussion

Five polyurethanes with varying DA adduct contents were synthesized via Diels-Alder reaction between PMCF and BMI, as shown in Scheme 1.

As shown in Fig. 1a, the presence of the peak at δ = 7.58 ppm, attributed to the urethane group, confirmed the successful reaction between PTMG and CDI. Meanwhile, the characterized chemical shifts of furan ring (a,b,c,d in Fig. 1a) also confirmed the successful reaction between CDI-PTMG-CDI and FA. CDI-PUR-DA exhibited the characterized chemical shifts [12] of

Conclusions

In this letter, a novel polyurethane hot melt adhesive was synthesized via introducing DA adducts into the polyurethane main chain, confirmed by hydrogen nuclear magnetic resonance analysis. The results of rheological properties test demonstrated that the viscosity of CDI-PUR-DA can decreased to 4 Pa s and returned back to the initial value during the bonding procedure. CDI-PUR-DA also exhibited good adhesive properties even after 3 times adhesion and can be used as a hot melt adhesive

Acknowledgement

This project was supported by the Natural Science Foundation of Fujian Province (No.2016J05029).This project was also supported by the Shanghai Leading Academic Discipline Project (No. B202)

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Citation Excerpt :
It is generally found that Young's modulus and/or yield stress increase with increasing crosslinker content (in all cases DA bonds act as crosslinkers) while there seems to be a disparity in the elongation at break behaviour [7,10,13,15,16]. The rheological properties of a crosslinked semicrystalline PUR system with shape-memory properties [12] and of linear thermoplastic polyurethanes have been investigated [20,21]. On the one hand, rheological studies demonstrated the reversibility of the DA reaction with application of temperature.
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Polyurethane hot melt adhesive based on Diels-Alder reaction

Abstract

Introduction

Section snippets

Materials

Synthesis of polyurethane prepolymer terminated with furan ring (PMCF)

Results and discussion

Conclusions

Acknowledgement

Int J Adhesion Adhes

Int J Adhesion Adhes

Int J Adhesion Adhes

Blends of poly (ε‐caprolactone‐b‐lactic acid) and poly (lactic acid) for hot‐melt applications

J Appl Polym Sci

Current research and development status and prospect of hot melt adhesives: a review

Ind Eng Chem Res

Viscoelastic properties and lap shear strength of EVA/aromatic hydrocarbon resins as hot-melt adhesives

J Adhes Sci Technol