Low-molecular-weight 1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU) was used to modify bamboo strips catalyzed by maleic anhydride. In this study, response surface methodology (RSM) was applied to optimize the treatment conditions for DMDHEU modification of bamboo. Three selected parameters, DMDHEU concentration, curing temperature, and curing time, were further optimized using response surface methodology via the box behnken design (BBD). The equilibrium moisture content (EMC), water absorption (WA), anti-swelling efficiency (ASE), mechanical properties, and mold resistances to (Aspergillus niger V. Tiegh, Penicillium citrinum Thom, Trichoderma viride Pers. ex Fr., and Botryodiplodia theobromae Pat.) were evaluated as the dependent variables. The results show that the dimensional stability and mold resistance of bamboo were enhanced remarkably, and the modulus of rupture (MOR) of bamboo was decreased after DMDHEU modification, while the modulus of elasticity (MOE) changed slightly, and the parallel-to-grain compressive strength (CS) was increased. Furthermore, scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and Fourier transform infrared spectroscopy (FTIR) were used to investigate the effect of modification on the bamboo microstructure and functional groups. In the context of SEM, TGA, and FTIR analyses, the thermal stability of bamboo modified with a medium or high concentration of DMDHEU was improved, the starch granules in the parenchyma cells of modified bamboo dissolved, and few resins were distributed in the lumens of treated bamboo.

In this study, 1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU) and furfuryl alcohol (FA) modification catalyzed by acrylic acid were comparatively studied. The effects on physical-mechanical properties and durability against mould of Masson pine and Camphor pine wood were investigated, including weight gain rate (WPG), moisture uptake, equilibrium moisture content (EMC), anti-swelling efficiency (ASE), parallel-to-grain compressive strength (CS), modulus of rupture (MOR), and mould resistance. The wood samples modified with DMDHEU closely retained their original color and texture, whereas the color of furfurylated wood became dark brown and the texture became clearer. The WPG of DMDHEU-treated wood ranged from 17.9% to 29.3%, which was lower than that of furfurylated wood that ranged from 36.7% to 39.3%. The equilibrium moisture content of DMDHEU-modified wood was slightly higher than the untreated wood, while furfurylation decreased the EMC of wood by approximately 50% compared with the untreated wood. The dimensional stability, parallel-to-grain compressive strength, and mildew resistance of Masson pine and Camphor pine improved after both modifications noticeably. It was concluded that acrylic acid can be utilized as a catalyst to perform wood furfurylation and DMDHEU modification.

In this study, bamboo was treated with an optimized surface furfurylation process. With this process, dimensionally stable and highly biologically durable bamboo material could be prepared without mechanical reduction. The anti-swelling efficiency (ASE) could reach 50% with a low weight percent gain (WPG about 13%). By using SEM, nanoindentation, and Imaging FTIR, we demonstrated that this high performance improvement is attributed to the unique furfuryl alcohol (FA) resin distribution pattern in the modified bamboo, namely a higher concentration of FA resin located in the region near to the surface of bamboo, and what is more, the preferred distribution of FA resin within the cell walls of parenchyma cells, which is known to be the weak point of bamboo both for biological durability and mechanical performances. Such graded modified bamboo could be utilized as a reliable engineering material for outdoor applications.

Furfurylation is an effective and green method for wood or bamboo modification that can significantly improve its physical and mechanical properties and the resistance against biological deterioration and the attack of subterranean termites. To elucidate the effect of furfurylation on the physical and multiscale mechanical properties of bamboo, the conditions of the furfurylation process were modified to cause an independent variation of the physical and multiscale mechanical properties in differently-treated bamboo samples. This was achieved by impregnating bamboo samples with solutions containing 15%, 30%, 50%, or 70% furfuryl alcohol (FA) by either of the two impregnation processes, vacuum pressure (V-P) and soaking (S) impregnation, while applying different curing conditions (wet- or dry-curing). The physical properties we measured included the absorption rate, weight percent gain (WPG), swelling efficiency (SE), and anti-swelling efficiency (ASE); the macro-mechanical properties involved the modulus of rupture (MOR), the modulus of elasticity (MOE), parallel-to-grain compressive strength (CS), and tensile strength (TS); the micro-mechanical properties included the tensile strength of bamboo’s vascular bundle and hardness and the indentation modulus of bamboo’s fiber cell walls. Finally, the correlation between the different physical and mechanical properties of the modified bamboo samples was analyzed. The results indicate that V-P impregnation made bamboo more permissible for the penetration of FA, while wet-curing was more conducive to ensuring a high curing rate. The dimensional stability of the bamboo samples treated with a high FA concentration through V-P impregnation and of those furfurylated by the S-Wet process using either medium or high FA concentrations was significantly increased. However, the dimensional stability of the bamboo samples modified with either low or medium FA concentrations decreased in both dry and wet curing. In terms of mechanical strength, furfurylation had little effect on the macro- and micro-mechanical properties of bamboo and was slightly improved in comparison to untreated samples. The results also showed a positive correlation between the macro- and micro-mechanical strength of the modified bamboo samples and a significant negative correlation between the mechanical strength and ASE. In soaking impregnation, the WPG and ASE were positively correlated, while the WPG and CS were negatively correlated. Interestingly, the correlation between the mechanical properties and ASE was not significant. Finally, both V-P-Wet and S-Wet approaches can be recommended for bamboo furfurylation, the former being time-saving and having a high curing rate in FA resin while significantly improving the moisture absorption and mechanical strength of bamboo. The advantage of the latter process is simplicity, a high utilization rate of FA, and a significant improvement in the dimensional stability of bamboo.

Bamboo is a material with excellent development prospects. It is increasingly used in furniture, decoration, building, and bridge construction. In this study, Furfurylated bamboo bundles and phenol-formaldehyde resin were used to make bamboo-scrimber composites (BSCs) via molding-recombination and hot-pressing processes. The effects of the impregnation mode, furfuryl-alcohol concentration, and curing temperature on the various physical–mechanical properties and durability of the composites were evaluated. Scanning-electron microscopy (SEM) was used to observe the microstructural differences. Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) were employed to investigate changes in the chemical constituents. The heat resistance was also investigated using thermogravimetric analysis. The results showed that the density of the furfurylated BSC increased by up to 22% compared with that of the BSC-C with the same paving mode. The furfurylated BSCs had lower moisture contents: the average moisture content of the furfurylated BSCs was 25~50% lower than that of the BSC-C. In addition, the furfurylated BSCs showed better dimensional stability and durability, since the decay-resistance grade of the BSCs was raised from decay resistance (class II) to strong decay resistance (class I). In terms of the mechanical properties, the furfurylation had a slight negative effect on the mechanical strength of the BSCs, and the modulus of rupture (MOR) and horizontal shear strength (HSS) of the BSCs were increased to a certain extent under most of the treatment conditions. In particular, the highest HSS for indoor use and MOR of the furfurylated BSCs increased by 21% and 9% compared with those of the untreated BSCs, respectively. The SEM results indicated that the FA resin effectively filled in the bamboo-cell cavities and vessels, and the modified bamboo-parenchyma cells were compressed more tightly and evenly. The FTIR and XPS spectroscopy showed that the hydroxyl group of carboxylic acid of the bamboo-cell-wall component reacted with that of the furan ring, and the cellulose and hemicellulose underwent acid hydrolysis to a certain extent after the furfurylation. Overall, the present study highlights the potential of furfurylation as a modification method to enhance BSC products. Further research should focus on improving the ability of furfurylated BSCs to prevent the growth of Botryodiplodia theobromae. Additionally, the influence of furfuryl-alcohol resin on the bonding strengths of PF adhesives should be further clarified.

Using the ERA5 (the fifth major global reanalysis produced by the European Centre for Medium-Range Weather Forecasts) data and the T-PCA (Principal Component Analysis in T-mode) objective classification method to classify the 850-hPa geopotential height, we summarize four conceptual models of large-scale synoptic weather types over East Asia. By combining this with the daily precipitation observation data of 36 meteorological stations in Guangdong, South China, during summer (June to August) of 2014–2018, we found that summer precipitation in Guangdong Province is closely related to the position of the northwestern Pacific subtropical high and the strong upward motion of the warm airflow over the Pearl River Delta. It is further revealed the regulation effect of different weather patterns on summer precipitation in Guangdong Province and their urban–rural differences. More specifically, both urban and rural areas have a decreasing proportion of light rainfall and an increasing proportion of heavy and torrential rainfall, which are mainly regulated by the trend of frequency changes of four different weather types: Type 1 (47.39%) and Type 2 (32.39%) days are decreasing year by year, modulating the trend of light rainfall, while Type 3 (13.26%) and Type 4 (6.96%) days are steadily increasing, dominating the trend of heavy rainfall. In addition, it was further found that the frequency of light rainfall is decreasing more significantly in cities compared to that in rural areas, while the proportion of heavy and stormy rainfall is increasing more significantly, which is closely related to the effects of rapid urbanization.