Preserving Cellulose Structure: Delignified Wood Fibers for Paper Structures of High Strength and Transparency. 
To expand the use of renewable materials, paper products with superior mechanical and optical properties are needed.  Although beating, bleaching, and additives are known to improve industrially produced Kraft pulp papers, properties are limited by the quality of the fibers.  While the use of nanocellulose has been shown to significantly increase paper properties, the current cost associated with their production has limited their industrial relevance.  Here, using a simple mild peracetic acid (PAA) delignification process on spruce, we produce hemicellulose-rich holocellulose fibers (28.8 wt %) with high intrinsic strength (1200 MPa for fibers with microfibrillar angle smaller than 10°).  We show that PAA treatment causes less cellulose/hemicellulose degradation and better preserves cellulose nanostructure in comparison to conventional Kraft pulping.  High-density holocellulose papers with superior mechanical properties (Young's modulus of 18 GPa and ultimate strength of 195 MPa) are manufactured using a water-based hot-pressing process, without the use of beating or additives.  We propose that the preserved hemicelluloses act as "glue" in the interfiber region, improving both mechanical and optical properties of papers.  Holocellulose fibers may be affordable and applicable candidates for making special paper/composites where high mechanical performance and/or optical transmittance are of interest.