DMA analysis and wood bonding of PVAc latex reinforced with cellulose nanofibrils

Suspensions of commercial refined beech pulp (RBP) were further processed through mechanical disintegration (MD-RBP), chemical modification (CM-RBP) and through chemical modification followed by mechanical disintegration (CM-MD-RBP). Nanocomposites were prepared by compounding a poly(vinyl acetate) (PVAc) latex adhesive with increasing contents of the different types of nanofibrils, and the resulting nanocomposites were analyzed by dynamic mechanical analysis (DMA). Also, the suitability of using the CM-RBP fibrils to formulate PVAc adhesives for wood bonded assemblies with improved heat resistance was studied. The presence of cellulose nanofibrils had a strong influence on the viscoelastic properties of PVAc latex films. For all nanocomposites, increasing amounts of cellulose nanofibrils (treated or untreated) led to increasing reinforcing effects in the glassy state, but especially in the PVAc and PVOH glass transitions. This reinforcement primarily resulted from interactions between the cellulose fibrils network and the hydrophilic PVOH matrix that led to the complete disappearance of the PVOH glass transition (tan δ peak) for some fibril types and contents. At any given concentration in the PVOH transition, the CM-MD-RBP nanofibrils provided the highest reinforcement, followed by the MD-RBP, CM-RBP and the untreated RBP. Finally, the use of the CM-RBP fibrils to prepare PVAc reinforced adhesives for wood bonding was promising since, even though they generally performed worse in dry and wet conditions, the boards showed superior heat resistance (EN 14257) and passed the test for durability class D

Arrangement of Cell-Wall Constituents in Chemically Treated Norway Spruce Tracheids

The cell-wall of tracheids in conifer wood has evolved to provide both water conduction and mechanical strength to the standing tree. However, its structure at the nanometer level is not yet accepted beyond doubt, and little is known about the interactions between the cell-wall components. In the present study, the fracture pattern of the S2 layer of Norway spruce tracheids was observed by field emission scanning electron microscopy (FE-SEM) after pretreatment of the cell wall with various alkali solutions, acetic and nitric acid, and ASAM delignification. The resulting cell-wall arrangements were also studied in ultra-thin sections of unfractured samples with transmission electron microscopy (TEM). In the case of untreated samples (reference), radial fracture patterns—perpendicular to the compound middle lamella—were regularly observed. A treatment with 10% and 18% NaOH or 24% KOH at room temperature—associated with a slight decrease of glucomannan—resulted in the disappearance of these radial fracture formations. As the severity of the alkali treatment increased and acid and ASAM delignification was applied, concentric alignments in the cell wall became more and more discernable. The increasing loss of hemicelluloses and lignin therefore led to distinct changes in the fragmentation patterns of the cell walls. In addition, reduction in strength and stiffness were determined for all chemically treated cell walls. It is concluded that even slight changes in cell-wall constitution influence the interactions of the cell-wall components and thus fracture mechanics and ultrastructural appearance of wood cell walls

DMA analysis and wood bonding of PVAc latex reinforced with cellulose nanofibrils

ISSN:1572-882XISSN:0969-023