Functionalised Fibres as a Coupling Reinforcement Agent in Recycled Polymer Composites

Abstract

This study addresses the structure-property relationship within the green concept of wood fibres with cellulose nanofibre functionalised composites (nW-PPr) containing recycled plastic polyolefins, in particular, polypropylene (PP-r). It focuses especially on the challenges posed by nanoscience in relation to wood fibres (WF) and explores possible changes in the thermal properties, crystallinity, morphology, and mechanical properties. In a two-step methodology, wood fibres (50% wt%) were first functionalised with nanocellulose (nC; 1-9 wt%) and then, secondly, processed into composites using an extrusion process. The surface modification of nC improves its compatibility with the polymer matrix, resulting in improved adhesion, mechanical properties, and inherent biodegradability. The effects of the functionalised WF on the recycled polymer composites were investigated systematically and included analyses of the structure, crystallisation, morphology, and surface properties, as well as thermal and mechanical properties. Using a comprehensive range of techniques, including X-ray diffraction (XRD), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), zeta potential measurements, and dynamic mechanical analysis (DMA), this study aims to unravel the intricate interplay of factors affecting the performance and properties of the developed nanocellulose-functionalised wood fibre-polymer composites. The interfacial adhesion of the nW-PPr polymer composites, crystallisation process, and surface properties was improved due to the formation of an H-bond between the nW coupling agent and neat PP-r. In addition, the role of nW (1.0 wt%) as a nucleating agent resulted in increased crystallinity, or, on the other hand, promoted the interfacial interaction with the highest amount (3.0% wt%, 9.0% wt%) of nW in the PP-r preferentially between the nW and neat PP-r, and also postponed the crystallisation temperature. The changes in the isoelectric point of the nW-PPr polymer composites compared to the neat PP-r polymer indicate the acid content of the polymer composite and, consequently, the final surface morphology. Finally, the higher storage modulus of the composites compared to neat r-PP shows a dependence on improved crystallinity, morphology, and adhesion. It was clear that the results of this study contribute to a better understanding of sustainable materials and can drive the development of environmentally friendly composites applied in packaging.

Keywords: crystallisation; functionalised wood fibres; mechanical properties; nanocellulose; recycled polypropylene structure properties.

Figure 1

X-ray diffraction (XRD) diffraction pattern of the PP-r polymer and nW-PPr composites.

Figure 2

(a) The comparative ATR-FTIR spectra of nC, WF, PP-r, and nW-based PP-r composites over the large wavenumber range. (b) Dependence on the intensity of the peak at 1027 cm⁻¹ on the nW addition content representing the stretching vibration of the C-O and C-O-C groups of the nW-PPr composites.

Figure 3

The proposed structure of the nW-PPr composites.

Figure 4

(a) TGA profile/thermograms, and (b) dTG curves of neat PP-r, nC, and PPr filled composites with nW.

Figure 5

(a) DSC curves by scan 2—heating and cooling at 10 K/min upon erasing the thermal history. The heat flow has been adjusted to the samples’ masses. The vertical dash-dotted lines represent the shifted temperature positions of the nW-PPr composites’ thermal point regarding neat PP-r. (b) Filler loading effect on T_c estimated from melting.

Figure 6

DSC thermograms of non-isothermal crystallisation of PP-r polymer and composites nW-PPr1, nW-PPr3, and nW-PPr9 composites at various cooling rates: (a) The temperature of crystallisation dependence on heating rate (2.5, 5.0, 10.0, and 20.0) of neat PP-r and nW-PPr1, nW-PPr3, and nW-PPr9. (b) The comparison of DSC curves by crystallisation and at the different cooling rates for the samples PP-r and composites nW-PPr9. The heat flow values have been normalised to the mass of each sample.

Figure 7

Plots of X(t) versus T for samples: (a) PP-r comparing with nW-PPr9; (b) nW-PPr1.

Figure 8

SEM images of (a) nW-PPr1, (b) nW-PPr3, and (c) nW-PPr9.

Figure 9

(a) Surface zeta potential as a function of pH for the nW, PP-r, and composite materials. The zeta potential for neat nC is shown (b) for comparison purposes.

Figure 10

DMA curves of PP-r and nW-PPr1, nW-PPr3, and nW-PPr9 composites as a function of temperature.