| The influence of drying on planted Poplar lumber (Populus tomentosa) strengthened with low molecular weight phenol-formaldehyde (PF) resin was studied in this dissertation. The physical and mechanical properties of control and PF impregnated lumber were investigated, pre-drying methods were air drying (Air-dry), conventional drying (CV-dry) and high temperature drying (HT-dry); and re-drying methods after impregnation with PF resin solution were CV-dry and HT-dry. Moisture transfer characterisitics was studied in drying process of impregnated Poplar lumber. The mechanism of influence of drying on impregnated Poplar lumber was also investigated through microscopic structure observation. The objective of this research is to improve physical and mechanical properties of planted lumber, to increase the added value, and to extend application field of low density planted lumber. The main research results of this dissertation are as follows:(1) Influence of drying methods to Poplar lumber properties and microscopic structureThe drying quality up to grade II in accordance with national standard (GB/T 6491-1999) is 96.5% for CV-dry, 86.7% for HT-dry (highest temperature: 120℃), and 84.0% for Air-dry. Influence of the three drying methods to MOR and MOE is not significantly different; HT-dry lumber's toughness was 28.6% lower than that of Air-dry lumber. HT-dry method evidently reduced wood hardness of Poplar lumber in comparison of the other two drying methods. The shrinking rate and EMC of HT-dry lumber was 20% lower than the other drying methods. But swelling rate of HT-dry Poplar lumber in water absorption was higher. The moisture content of Air-dry lumber after water absorption was 20% higher than that of CV-dry and HT-dry lumber, i.e. Air-dry Poplar lumber is easier for water impregnation. The dimensional stability sequence is as follows: HT-dry > CV-dry > Air-dry. The surface wettability sequence is: Air-dry > HT-dry > CV-dry.The influence of drying methods to microscopic structure was investigated. Under the condition of CV-dry, microcracks occurred in middle lamella between cell walls of fibers, and the microcracks were connected together to form minicracks, and V-type microcracks were observed occasionally in cell wall of fiber; all these cracks were invisible under unaided eyes. As for HT-dry lumber, besides the minicracks, the V-type microcracks and internal checks inside cell wall are widespread, some even enlarged to checks in radial cell wall.(2) Low molecular weight PF impregnation and influence of combined drying technics (predrying– redrying) on physical and mechanical properties of impregnated poplar lumber When impregnating Poplar lumber, the concentration of low molecular weight PF was 20%, vacuum-pressure technics and apparatus were used. The weight percentage gain (WPG) was around 23% that was similar for lumbers with different pre-drying methods. The drying time of impregnated lumber was almost doubled than control lumber. When Air-dry is as predrying, the influence of redrying methods on drying quality of impregnated Poplar lumber is not significantly different; but for CV-dry and HT-dry as predrying, the qualified proportion of HT-dry-redry-samples was lowered 10% and 8% than that of CV-dry-redry-samples respectively. After PF impregnation, the density of lumber is higher, and density uniformity is more even across the earlywood and latewood than that in control samples. The MOE and MOR of Poplar lumber were significantly improved by PF impregnation, but the toughness sharply decreases, it was 77% lower than that of control sample. The hardness of impregnated lumber was 70% higher than control samples. Anti-shrinking efficiency of impregnated lumber was 24.8%, Moisture excluding efficiency was 21.2%; the Anti-swelling efficiency was 37.7% in water absorption, and the ratio of tangential-radial swelling rate was beyond 30%. The dimensional stability and anisotropic property can be greatly improved, but the surface wettability was worse than control lumber.Air-dry - CV-dry was an optimum combined technics for improving MOE, MOR and toughness. As for the hardness, HT-dry - CV-dry combined technics was better. For dimensional stability, EMC and wettability, the optimum combined technics is to adopt Air-dry as predrying and HT-dry as redrying.(3) The moisture transfer properties in drying process of PF impregnated Poplar lumberThe diffusion coefficient and drying rate of PF impregnated Poplar lumber were significantly lower than that of control samples. There was no significant difference in diffusion coefficient and drying rate between quatersawn and flatsawn impregnated lumber. The thinner the impregnated lumber is, the smaller the diffusion coefficient; but the drying rate was decreasing with the increasing of thickness of impregnated lumber. The diffusion coefficient of PF impregnated lumber decreases with the decreasing of MC, it is not in accordance with that of control samples, in which the diffusion coefficient increased firstly with decreasing of MC, and it reached the highest when MC was around 17%, then it decreased with the decreasing of MC. Evaporating ability of humid air condition have stronger influence on diffusion coefficient of impregnated lumber than that of control samples.(4) The mechanism of drying of impregnated lumberContrast to the control lumber, in which shrinking should be occur in drying process even in high MC stage, the dimension changed differently with PF impregnated lumber drying. The impregnated Poplar lumber swelled when drying was in high MC stage, this indicated that it was a bulking process of PF in wood drying process in high MC stage. The shrinking rate of PF impregnated lumber was 6.15% lower than that of control samples in low MC stage, and this resulted in an improved wood recovery by PF impregnation. The proportion of PF resin presented in cell wall was influenced by predrying and redrying combined technics, Air-dry-HT-dry resulted in 52.12% PF resin in cell wall, it is the best combined drying technics.(5) The mechanism of influence of impregnation to wood propertiesThe microscopic observations were conducted for impregnated Poplar lumber. The cured PF resin presented in different forms in lumina of vessels, fibers and ray parenchyma cells. There were pillar-shape, ring-shape and granule shape in lumina of vessels. A few scattered vessel cells were full-filled with PF resin in their lumina– form the pillar type. Most of PF scattered in vessels was in granules shape, and cured PF packed the apertures of bordered pits among vessels and some of pits between vessels and fibers. The cured PF resin in fibers was in flat hummock shape, granules and irregular flake shape coated to the surface of fiber lumina. As for the cured PF in parenchyma ray cells, it is unevenly distributed, some are full-filled, some are part-filled, and some are empty. There is not full-filled-PF-type vessel in lumber with Air-dry-HT-dry combined technics, and this is in accordance with the high proportion of PF presented in cell wall as calculated. The PF packing of pits between vessels, coating of inner surface of fiber lumina and bulking to the cell wall will stuff part of the macro-capillary system and micro-capillary system, which is responsible for moisture transferring in drying process, and decreased the permeability of lumber. These resulted in the lower diffusion coefficient and decreasing drying rate in drying process of PF impregnated Poplar lumber. Meanwhile these are the reasons for the improvement of dimensional stability and other properties of impregnated lumber.(6) Analysis of mechanism for low molecolur weight thermosetting resin impregnation of low density hardwoodThe structure of hardwood results in that impregating process is a multi-stage filtering process of resin. The high molecular weight resin was kept in vessels by pits, and lower molecular weight resin coated in inner surface of lumina of fiber and parenchyma cells, only small molecular weight resin finally bulk cell walls of fiber and parenchyma cells. |
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