Green Chemistry
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PAPER
Towards natural- fi bre-based thermoplastic fi lms produced by conventional papermaking †
Cite this: Green Chem. , 2016, 18 , 3324
P. A. Larsson* a,b and L. Wågberg a,b
Materials based on cellulose are predicted to be of great importance in a sustainable society. However, for materials such as paper to replace materials with a higher ecological footprint, they need to be strong, ductile, provide a gas barrier, and, sometimes, also be transparent. Improved properties, or even novel properties, are also important for use outside the conventional markets. This paper describes how cellu- lose fi bres partly derivatised to dialcohol cellulose can be used to fabricate high-density materials by con- ventional papermaking techniques that simultaneously display all the above-mentioned features. The materials produced were characterised with respect to X-ray di ff raction, dynamic mechanical thermal behaviour, visual appearance, oxygen permeability and tensile properties. The highest degree of modi fi - cation studied, resulted in a material with thermoplastic features, a tensile strength of 57 MPa, a strain-at- break of 44% and an oxygen permeability at 80% RH of 23 ml μm (m 2 kPa 24 h) − 1 . At a thickness of 125 μm, these fi lms have a total light transmittance of 78% (87% haze). However, by hot pressing the fi lm for 2 min at 150 °C under a pressure of 16 MPa, and thereby increasing the density, the total transmittance increases to 89% (23% haze). The hot pressing can also be used to fuse individual pieces together, which is useful in many modern packaging applications. Altogether, this work shows how chemical modi fi cation of cellulose fi bres can be used to induce novel properties and improve the range of application, and con- sequently provide an interesting bio-based material with a good potential to replace less sustainable materials.
Received 23rd December 2015, Accepted 25th February 2016 DOI: 10.1039/c5gc03068d
www.rsc.org/greenchem
Introduction Our society is gaining an increased environmental awareness at the same time as producers are continuously focusing on production e ffi ciency and customers are demanding better product and material performance. Cellulose, which is the most abundant polymer on earth, is a fascinating and versatile biopolymer which in di ff erent forms can be expected to be extremely important in a sustainable bioeconomy. 1 However, for cellulose really to be a competitive option in advanced applications, it cannot be used directly in its natural state but needs to be refined or chemically modified to attain suitable properties. An example of such a refinement is to individualise † Electronic supplementary information (ESI) available: FTIR spectra of the final materials. Light transmittance and density of modified papers pressed under various pressures, temperatures and times. Full spectrophotometer data in support of Fig. 5b. Additional information regarding the t -peel test. See DOI: 10.1039/c5gc03068d a Fibre and Polymer Technology, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden. E-mail: per.larsson@polymer.kth.se b BiMaC Innovation, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden
the strong and highly ordered cellulose nanofibrils (CNFs) and cellulose nanocrystals (CNCs) constituting the native cellulose fibre, an approach that has resulted in thousands of scientific papers over the last 15 years. 2 – 5 In several of these papers, CNFs and CNCs have been applied in advanced electronic devices, film and barrier applications, and nanocomposites. Since all forms of native cellulose have limited ductility and are non-thermoplastic, the material processing is typically limited to particle-in-water systems. However, since cellulose is also hydrophilic and has a high propensity to form high- viscosity slurries and hydrogels, especially in the molecular or nanoparticle form, wet processing into the above-mentioned nano- and mesostructured materials can be very complicated on a large scale. It is therefore desirable to find a cellulose- based system which benefits from the high strength of native cellulose, but can be processed by large-scale water-based papermaking-type processes followed by dry-state processing such as hot pressing or hydroforming. Recently, we have shown that a heterogeneous conversion of cellulose to dialcohol cellulose, presumably by forming a shell of dialcohol cellulose that surrounds the crystalline core of each CNF, can be an interesting way to form an in situ com- posite with high strength and ductility. 6,7 If the modified CNFs
3324 | Green Chem. , 2016, 18 , 3324 – 3333
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