Effects of water soaking-drying cycles on thermally modified spruce wood-plastic composites

Abstract

The overall aim of this work was to gain more insight on the potential of modified wood (TMW) components for use in wood-thermoplastic composites (WPCs). Laboratory-scale TMWPCs were produced, and the effects of severe water soaking-drying cycles on the samples were studied. Water sorption behavior and resulting dimensional and micromorphological changes were also studied, and the results were compared with those of unmodified wood-plastic composites (UWPCs) used as control. The TMW was prepared by cutting a spruce board into half and subjecting one-half to an atmosphere of superheated steam at atmospheric pressure with a peak temperature of 210°C, with the other unmodified wood (UW) half as a control. The TMW and UW components were then prepared by a Wiley mill and thereafter sifted into smaller (mesh 0.20-0.40 mm) and larger (mesh 0.40-0.63 mm) size fractions. A portion of the wood components were also subjected to hydrothermal extraction (HE). Composite samples with these different wood components, polypropylene (PP) matrix, and maleated PP (MAPP) as coupling agent (50/48/2 wood/PP/ MAPP ratio by weight) were then prepared by using a Brabender mixer followed by hot pressing. The matching micromorphology of the composites before and after the soaking-drying cycles was analyzed using a surface preparation technique based on ultraviolet-laser ablation combined with scanning electron microscopy. The results of the water absorption tests showed, as hypothesized, a significantly reduced water absorption and resulting thickness swelling at the end of a soaking cycle for the TMWPCs compared with the controls (UWPCs). The water absorption was reduced with about 50-70% for TMWPC and 60-75% for HE-TMWPC. The thickness swelling for TMWPCs was reduced with about 40-70% compared with the controls. Similarly, the WPCs with HE-UW components absorbed about 20-45% less moisture and showed a reduced thickness swelling of about 25-40% compared with the controls. These observations also were in agreement with the micromorphology analysis of the composites before and after the moisture cycling which showed a more pronounced wood-plastic interfacial cracking (de-bonding) as well as other microstructure changes in the controls compared with those prepared with TMW and HE-UW components. Based on these observations, it is suggested that these potential bio-based building materials show increased potential durability for applications in harsh outdoor environments, in particular TMWPCs with a well-defined and comparably small size fractions of TMW components.

Vacuum formed bio‐based composite materials using polyolefin and thermally modified wood powder

“…It is further difficult to determine whether the polymer has filled the lumena or wood cavities in the wood cell fragments or not, which is usually the case for extruded WPCs. 15,41 However, unfilled lumens did exist (Figure 8). Nevertheless, as mentioned above, the consolidation was high in the sheets, and wood cavities were thus mostly filled or tightly covered with polymer.…”

“…It should be noted that a development of pores has previously been observed for WPC exposed to cycles of water soaking and drying. 15…”

“…At the same time, the compatibility between the filler and the polymer can be increased. 14,15 In several studies, the properties of wood-plastic composites (WPC), prepared from thermally modified wood, have been investigated, such as water uptake and mechanical properties. [15][16][17][18][19][20] The published literature on vacuum forming of biocomposites is very limited and is mainly focused on prediction and modeling properties.…”

“…14,15 In several studies, the properties of wood-plastic composites (WPC), prepared from thermally modified wood, have been investigated, such as water uptake and mechanical properties. [15][16][17][18][19][20] The published literature on vacuum forming of biocomposites is very limited and is mainly focused on prediction and modeling properties. For example, modeling has been used to describe how the properties of polymer matrices, containing unmodified wood components, depend on processing parameters (forming time, temperature, and pressure) and the filler content.…”

Nuclear magnetic resonance analysis of water absorption characteristics and dynamic changes in pore size distribution of wood-plastic composites

“…When the WPC was mixed with 5 wt% nanoparticles, the particles could aggregate on the composite substrate, causing a significant decrease in strength. Kallbom et al (2020) used modified wood to prepare thermoplastic composites and analyzed the water migration behavior of the samples, as well as the resulting changes in macro-size and micro-morphology, through a water soaking-drying cycle measurement. The results showed that the water absorption rate of the modified wood-thermoplastic composite decreased considerably.…”