All-cellulose composites based on microfibrillated cellulose and filter paper via a NaOH-urea solvent system

All-cellulose composites were produced by partial dissolution of two cellulosic sources in a mixture of sodium hydroxide and urea at low temperature. Filter paper (FP) and microfibrillated cellulose (MFC) were used to examine the role of the fiber dimensions of the initial reinforcing network on the final mechanical properties of thin self-reinforced all-cellulose composites. The dissolution time was used to control the extent of the transformation. The initial fiber structure was progressively transformed into a thoroughly consolidated composite material. X-ray diffraction and Fourier-transform infrared spectroscopy were able to show that the initial cellulose I allomorph was replaced with a cellulose II allomorph. FP underwent a fast crystallinity loss and transformation to cellulose II. In contrast, the crystallinity of MFC decreased slowly after dissolution. This result was also correlated with a slower allomorphic transformation. The crystallinity of MFC decreased to a level comparable to that of FP after 40 min and it remained comparatively unaffected at extended dissolution times. Comparison between powder and transmission XRD measurements demonstrated that the cellulose II present in all films after dissolution was strongly textured with its (1\(\bar{1}\)0) plane lying parallel to the sample surface despite fibrous microstructural features remaining from the initial substrates. The tensile strength and elastic modulus of FP increased significantly (+690 and +528 %, respectively) after only 20 min, while that of MFC remained relatively unaltered as a function of the dissolution time.