Synthesis and thermal decomposition kinetics of moisture curable polyurethane films as a reinforcing material for cultural relics

Xing Zhao,Liping Zheng,Ziming Wang,Liqin Wang,Lang Guo,Qing Niu,Yanni Ma

doi:10.1038/s41598-020-78705-4

Abstract

In situ consolidation is the most common treatment to conserve cultural relics, but materials for preserving fragile organic cultural relics in humid archaeological excavation sites are scarce. To solve the problem, a moisture-curable polyurethane (MCPU) prepolymer was synthesized by reacting isophorone diisocyanate with polyethylene glycol 600. The standard acetone–dibutylamine method, Fourier transform infrared spectroscopy, gel chromatography and thermogravimetric analysis were utilized to determine the change in isocyanate groups before and after the reaction, the prepolymer molecular weight, the thermal decomposition kinetic parameters and the MCPU film lifetime. The results showed that the number-average molecular weight of the prepolymer was 749, and the weight average molecular weight was 1684. Isophorone groups in the prepolymer react with moisture in the air to form colorless, transparent, flexible films. The thermal decomposition of the MCPU films was a first-order reaction, and the decomposition process consisted of two stages. The Dakin equation was used to obtain the thermal aging equation lg t = 4600.82/T − 8.07, meaning that at 15 °C, the sample has an approximately 150-year lifetime. A new conservation material was developed, and its thermal decomposition kinetics were studied, which are significant for the conservation of fragile organic cultural relics in humid environments.

Highlights

In situ consolidation is the most common treatment to conserve cultural relics, but materials for preserving fragile organic cultural relics in humid archaeological excavation sites are scarce
In a high-humidity environment, B72 and polyvinyl butyral (PVB) whiten easily[10,11], which can change the appearance of cultural relics, whereas epoxy r esins[12] and organosilicones are generally used for masonry preservation and are rarely implemented in the reinforcement of fragile organic cultural relics
A moisture-curable polyurethane (MCPU) prepolymer was synthesized for use in high-humidity environments, and the thermal decomposition kinetics of the MCPU films were studied, providing a scientific basis for the selection of cultural relic reinforcement materials and research on the aging process of these materials

Summary

Introduction

In situ consolidation is the most common treatment to conserve cultural relics, but materials for preserving fragile organic cultural relics in humid archaeological excavation sites are scarce. A wide variety of reinforcing materials for cultural relics are available, and the most commonly used organic materials, include Paraloid B72 (B72)[2,3,4,5], polyvinyl butyral (PVB)[6], epoxy resin7, organosilicone[8,9], etc These materials are generally employed in a normal-humidity environment amenable to physical or chemical reactions that provide long-term protection for cultural relics. Moisture-curable polyurethane (MCPU) is a polyurethane that contains isocyanate end groups and is widely used in waterproof coatings for buildings, anti-corrosion materials, bio-pharmaceutical materials, electronic materials, wood adhesives, etc.[15] This material has rarely been used for the conservation of cultural relics. A MCPU prepolymer was synthesized for use in high-humidity environments, and the thermal decomposition kinetics of the MCPU films were studied, providing a scientific basis for the selection of cultural relic reinforcement materials and research on the aging process of these materials

Methods

Results

Conclusion