Abstract
Knowledge of the mechanical behaviors of polymer film in humid environments is of great significance for predicting the long-term performance of emulsion polymer isocyanate (EPI) as a high-performance wood adhesive. A tri-copolymer latex was cross-linked by the general polymeric methylene diisocyanate (p-MDI) and aqueous emulsified isocyanate (EMDI) at different loadings for preparing EPI. Furthermore, a series of uniaxial tension tests under different relative humidity (RH) were carried out on cured EPI samples before and after post-curing treatment, and the corresponding chemical structure, as well as the microstructure of polymers, was investigated in detail. In addition, a constitutive equation was formulated to calculate the viscoelastic characteristics of the adhesive layer. The results indicate that the EPI films reveal various kinds of intrinsic deformation as RH increases, and the tensile rupture stress and stiffness would obviously decrease, even at cross-linker weight ratios of up to 20%. Furthermore, the moisture resistance could be markedly improved by increasing the isocyanate content and post-cure. Importantly, EMDI-cross-linked film not only exhibits much better mechanical properties than that containing p-MDI at 0–80% RH, but is also more sensitive to post-cure. Finally, the derived viscoelastic model could efficiently track moisture-dependent stress-strain curves of EPI films, and the obtained relaxation time further reveals the influence mechanism of isocyanate and post-cure on the mechanical response of the cured polymer under moist conditions.
Highlights
Emulsion polymer isocyanates (EPI) are composed of a water-based polymer emulsion and an isocyanate cross-linker
The samples were first equilibrated at 0% RH for 60 min, and the relative humidity in the testing chamber was adjusted to fixed values of 0%, 20%, 40%, 60%, and 80% RH
The mechanical behaviors of the bonding layer are crucial to the comprehensive understanding of the complicated deformation response of EPI adhesive in a moist environment
Summary
Emulsion polymer isocyanates (EPI) are composed of a water-based polymer emulsion and an isocyanate cross-linker. One is a pioneering investigation made by Taki et al [11,12] They reported the relationship between the mechanical properties of cured adhesives and bond strength of EPI with different cross-linking densities over a wide temperature range. In another experiment, Umemura et al performed dynamic mechanical analysis (DMA) measurements on cured EPI films, and observed a very small loss in storage modulus from 20 to 70 ◦ C [13]. A series of uniaxial tension tests under different RH were performed to determine the mechanical properties of EPI cured samples before and after post-cure treatment, and the glass transition relative humidity (RHg ), chemical structure, and microstructure of polymers were checked using DMA RH scans, Fourier transform infrared spectroscopy (FTIR) and optical microscopy respectively. A constitutive equation was formulated to quantitatively describe the moisture-effected stress-strain curves of EPI films, and the obtained relaxation time was used to unveil the effect mechanism of cross-linker and post-cure on the mechanical properties of the polymer in humid environments
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