Abstract
Polypropylene (PP) is a multifunctional and widely applied polymer. Nevertheless, its low energy surface and poor adhesion are well-known and might impair some prospective applications. Aiming to overcome these limitations, PP composites can be applied as a tool to enhance PP surface energy and then increase its practical adhesion. In this work, Kraft lignin (KL) was chemically modified and blended with PP. In short, KL was hydroxypropylated and further reacted with acetic anhydride (A-oxi-KL) or maleic anhydride (M-oxi-KL). Lignin modifications were confirmed by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). PP-composites with different lignin contents, as well as pristine PP, were characterized in terms of their thermal behavior, morphology, surface energy, and practical adhesion by DSC, scanning electron microscopy (SEM), contact angle measurement, and peeling tests, respectively. Lignin incorporation did not affect the PP degree of crystallization. The lignin modifications led to a better compatibility with the PP matrix and surface energies up to 86% higher than neat PP. Increases of up to 66% in the peel strength were verified. Composites with M-oxi-KL showed the best adhesion performance, confirming the lignin functionalization is an efficient approach to improve the practical adhesion of PP films.
Highlights
Some remarkable differences confirmed the successful Kraft lignin (KL) hydroxypropylation: (i) the increase in the peak intensities and the emergence of new peaks in the wavenumber region between 3000 and 2800 cm−1, which are related to asymmetric and symmetrical stretching of CH2 and CH3 groups [11]; (ii) the increase in the intensity of the peak around 1370 cm−1, which is associated with the bending of CH2 and CH3 groups [21]; and (iii) the increase in the peak intensity at 1120 cm−1, which is assigned to stretching of C-O bonds of the ether groups [22]
KL esterification via reaction with maleic anhydride was confirmed by the following evidence observed when KL and M-oxy-KL spectra are compared: (i) the emergence of a weak peak centered at 1765 cm−1 (C=O stretching of aromatic ester); (ii) the increase in the intensity of the peak around 1260 cm−1 and the increase in intensity and width of the peak centered at 1030 cm−1, both associated with the stretching of C-O bonds of ester groups [8]
Hydroxypropylation followed by esterification with maleic anhydride and acetylation with acetic anhydride reduced the Kraft lignin Tg from 165 to 141 ◦C and 105 ◦C, respectively, and improved the compatibility between Kraft lignin and the PP matrix
Summary
Polypropylene (PP) is a multifunctional and low-cost polymer widely used in industrial applications. It presents low surface energy because of its non-polar chemically stable structure, which leads to adhesion hindrance, such as coating failures or flexible laminated layers delamination. Polymer blending can be used as a strategy to alter the wettability of PP and increase the strength of PP adhesive joints. The combination of PP with renewable resource materials has been intensively investigated as a promising approach to reduce petroleum-based dependency, collaborating to a reduced environmental impact. The wettability and surface roughness of polypropylene can be improved when filled with wood flour [1]. Potato starch, and chitosan were reacted with PP for membranes application, and the new materials changed their hydrophobicity, besides mechanical properties improvements [2]
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