Damage Mechanism Of UV And Freeze-thaw Aging On Mechanical Properties Of Wood Adhesive Interface

Modern wooden architectures possess exceptional characterizations that lies in light,high assembly efficiency and low carbon emission.Applying modern wooden architectures is beneficial for reducing the consume of natural resources and the negative effects on the environments.It also fits in the trend of green construction movement and a low carbon lifestyle,especially considering the background of achieving carbon peaking and carbon neutrality.Wooden engineered composites are now applied as the load-bearing elements in wooden architectures,and the mechanical performance of which affects the safety of wooden architectures significantly.Due to the important role,continuous attention has been poured onto the mechanical performance of wooden engineered composites.Researches have proved that environmental aggressors(i.e.,alternating moisture,temperature and light et al)lead to mechanics degrading of wooden engineered composites,which further results in the sudden fracture of wooden architectures.Wood-adhesive interface exhibits an important role in stress and strain transferring,and the structure and mechanical performance of which affect the macro-mechanical performance of wooden engineered composites.Exploring the effects of ageing-induced effects on the structure and mechanical performance of wood-adhesive interface is beneficial for revealing the ageing-induced mechanics degrading of wooden engineered composites.Also,it can benefit the designing of wooden engineered composites with outstanding mechanical performance and resistant to ageing.On that basis,the safety and durability of wooden architectures can be improved.Hereby,we investigated the morphology,chemical structure and mechanical performance of wood-adhesive interface after UV irradiation and freeze-thawing cycles.The major objective of this work is to revealing the specific mechanism underlying the mechanics degrading of wood-adhesive interface after ageing and the outcomes are as follows:(1)A Nanoindentation technique(NI-mapping)was applied for mapping the mechanical performance of wood-adhesive interface and the results displayed a small variant(3%)as compared with the common-utilized NI methodology,yielding the promising reliability of NI-mapping.A large-scale and high-resolution mapping of mechanical performance of wood-adhesive interface can be obtained from NI-mapping and it revealed that the mechanical performance of wood-adhesive interface reduced as adhesive penetrating.Phenolic adhesive(PF)exhibited a favored penetration as compared with polyurethane adhesive(PUR)due to the small molecular size of PF.NI-mapping also displayed promising in revealing the mechanical performance of cell walls in early wood region and the wood-adhesive interface composed by early wood and adhesive.(2)This chapter focused on the UV-induced degradation and oxidation of wood-adhesive interface.NI-mapping revealed a significant reduction of elastic modulus and hardness of cell walls in wood-adhesive interface(from 20.8 GPa/920.4 MPa to 13.5 GPa/612.7 MPa)with significant reduction ratios of 35% and 33%,respectively.SEM evidenced the formation of cracks and cavities in UV-irradiated wood-adhesive interface.FT-IR and XPS provided undeniable evident that UV irradiation leads to obvious photo-degradation and oxidation of wood and PF adhesive.An interesting finding was that UV irradiation seems to exhibit more effects on wood.The photo-degradation and oxidation were initialed by electron escaping as-induced by UV irradiating phenol rings in lignin and PF adhesive.Following hydrogen transferring reaction resulted to the formation of Phenoxy radicals,singlet oxygen,superoxide anion and hydroxyl radicals.Aforementioned reactive oxygen species(ROSs)accelerated the photo-degradation and oxidation of wood-adhesive interface.(3)The effects of freeze-thawing cycles on the structure and mechanical performance of wood-adhesive interface were investigated in this chapter.The freeze-thawing cycles were proved to be harmful for macro-mechanical performance of wood-adhesive bonds The shear and compression strength of wood-adhesive bonds reduced to 3.3 MPa/51.7 MPa after 5 times of freeze-thawing cycles at-40℃,yielding significant reduction ratios of 38% and 21%,respectively.SEM also revealed cracks in freeze-thawed wood-adhesive interface,which can be attributed to the deformation induced by moisture content changes and water phase transition.SAXS further revealed that the distance between microfibrils elevated from 41.2 (?)(raw)to 42.9 (?)(20 times of freeze-thawing cycles at-20℃)and 43.8(?)(5 times of freeze-thawing cycles at-40℃).It clearly demonstrated the structural disperse of the cluster of cellulose macromolecules.Free water and hydroxy formed hydrated hydrogen bond,which leads to stress in the cellulose macromolecules and further deconstructs the crystalline region.It resulted in the reduced crystalline and mechanics degrading of wood-adhesive interface after freeze-thawing cycles.This paper proposed an investigation into the mechanics degrading of wood-adhesive interface after UV irradiation and freeze-thawing cycles.The outcomes are beneficial for designing the wooden engineered composites with outstanding mechanical performance and resistant to ageing.It will also provide a basis for elevating the safety and durability of wooden architectures.