Poplar wood (Populus euramericana cv. “I-214”) has been impregnated by pulse dipping at 0.7–0.8 MPa for 30 min with a mixture of methylolurea and sodium silicate, and the sol modifier has been cured within the wood micropores by in situ gel polymerization by kiln drying, so that a Si-O-Si framework was formed. The treated wood acquired higher mechanical strength and its hygroscopicity was lowered. It was demonstrated by X-ray diffraction that sodium silicate crystallized within the interfibrillar region of the cell wall. Fourier transform infrared spectra showed that reactions occurred between the wood-OH, Si-OH, and N-CH2-OH from methylolurea to form C-O-Si and C-O-C bonds. As visible by scanning electron microscopy (SEM), the Si-O-Si framework was embedded in the pretreated wood. Moreover, SEM-energy-dispersive X-ray spectroscopy analysis revealed that the modifier formed layers from various thicknesses ranging from a thin layer on the cell walls up to big amounts filling the lumen.

Impregnation of the cell wall with various chemicals continues to attract interest. For most studies, the method of impregnation is the vacuum-pressure process which is limited by the specimens' size in thickness and the requirement for low moisture content. In the present study, we impregnated fast-growing poplar (Populus euramericana) green wood by pulse− pressure at 0.7−0.8 MPa for 50 min with phenolic methylolurea, and then the modifier in situ polymerized within the wood cell wall by kiln drying. The microscopy analysis showed that the pulse−pressure impregnation method could be feasible because of the presence of a wide-bore capillary column system in the poplar xylem. As a result, the chemical treatment reduced the hygroscopicity and increased the dimensional stability of wood. Reactions mainly happened within the interfibrillar amorphous region of the cell wall. Among the reactions were etherification and esterification since the covalent bonds formed between modifier methylol groups, wood hydroxyl, and acetic acid derived from hemicelluloses.

Poplar is a fast-growing tree with a short cultivation time; however, there are some disadvantages to poplar such as its low density and low dimensional stability. The aim of this work was to study the physical and chemical performance of poplar wood treated by its impregnation with functional composite modifiers followed by compression drying. The results showed that this approach not only significantly increased the dimensional stability of the wood but also enhanced its bending strength and compressive strength parallel to the wood grain. Fourier transform infrared analysis showed that the intensity of the hydroxyl (-OH) and carbonyl (C=O) absorption peaks decreased, which provided evidence that the -NHCH2OH of the modifier reacted with the wood's carboxyl and hydroxyl groups. The position of the XRD peaks did not change, which indicated that the crystallinity structure of cellulose was not noticeably affected by chemical modification. The TGA showed that the thermal stability of the modified wood was improved, while FESEM analysis showed that the wood cell lumens and vessels were filled with chemicals.

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