Paper Microstructure

Introduction
Paper is the main ingredient of papercrete and thus its properties depend on papers microstructure. Wood fragments are thermometrically or mechanically treated  to dissolve the lignin binder and free the cellulose fibers. Paper is then made by pressing the pulp to remove excess water. Paper is an anisotropic material and the quality and strength of its fibers differs depending on several factors:
– the type of wood
– the percentage of recycled paper the amount of water in the pulp. Without water the fibers fall apart (no binding between them)
– the pulp that the paper was made
– the way of pulping (Chemical or mechanical)
– the speed of drying
Today half of the paper fiber utilized in current production come from recovered fibers. Yet recovered fibers are inherently less strong.
Moving the pulp means orienting the fibres
Microstructure
Paper is a natural polymer which consists of wood cellulose the most abundant organic compound on the planet. Cellulose is made of units of the monomer glucose (polysaccharide). The links in the cellulose chain are a type of sugar: ß-D-glucose. Despite containing several hydroxyl groups , cellulose is water insoluble. The reason is the stiffness of the chains and hydrogen bonding between two -OH groups on adjacent chains. The chains pack regularly in places to form hard, stable crystalline regions that give the bundled chains even more stability and strength. This hydrogen bonding forms the basis of papercrete’s strength. By applying a force on the paper the hydrogen bond between the water and the cellulose molecule is broken. Coating cellulose fibers with Portland cement creates a cement matrix, which encases the fibers for extra strength to the mix.
Linear polyglucose joined by β-1-4 hydrogen bonds
Paper is a disordered – time dependent material. The irregular structure makes it difficult to assign fixed values for strength and stiffness and to predict the minimum. Strength depends on the water content and thus changes with temperature and humidity.
A lot of research has been carried out about how size, time, water content and orientation of the fibers affect the behavior of the final product, but they won΄t be further discussed. It is enough to know that the properties of paper depend on many factors and that΄s why, in combination with the inconsistency of manufacture, property values can widely differ. Representative I include a strain [%] – stress [Mpa] graph which shows the effect of fiber orientation on the total strength.

 

Effect of fiber orientation
Nanotechnology
Nanotechnology presents potential opportunities to create better materials and products. Lars Berglund from the Royal Institute of Technology in Stockholm has developed  porous cellulose nanopaper of remarkable high toughness.
As we have already mentioned paper is made of pulping wood. The mechanical process damage the individual cellulose fibres and greatly reduce their strength. Normal paper processing, generates relatively large cellulose microfibers with defects which break easily apart. That’s why the tensile strength of normal paper has a maximum of 30 MPa –normal paper has less than 1 MPa-.
To produce stonger paper, we have to get the cellulose fibres smaller without damaging them. Nanopaper results in a pulp, with about 1,000 times smaller than the typical fibres, by breaking them down using enzymes and then beating the pulp mechanically. The cellulose nanofibers are treated with carboxymethanol. These groups form hydrogen bonds, adding strength to the material. They are still able to slip and slide over each other to dissipate strains and stresses.
The defect-free matrix, make cellulose nanopaper a superstrong material. Its tensile strength 214 MPa, can compare with that of structural steel (250 MPa).
The potential is pretty clear. As long as mass-production fabrication costs are as low as the inventors claim, nanopaper will open the way for a wide use of paper as a construction material. It will replace expensive carbon fibres and provide a basis for reinforcement in the construction industry. Until then we have to deal with the fact that papercrete is an imperfect material.
Figure 1. FE-SEM micrographs of (a) a cellulose nanofibril film surface showing a fibrous network (scale bar is 1.5 μm), (b) the cross section of a fracture surface of a film showing a layered structure (scale bar is 2 μm), and (c) a fracture surface viewed perpendicular to the film surface (scale bar is 1 μm). These films were dried from water suspension. The film in (a) is prepared from DP-1100 and the other two are prepared from DP-800.
(a) a cellulose nanofibril film surface showing a fibrous network (scale bar is 1.5 μm),  (b) the cross section of a fracture surface of a film showing a layered structure (scale bar is 2 μm),  (c) a fracture surface viewed perpendicular to the film surface (scale bar is 1 μm). These films were dried from water suspension. The film in (a) is prepared from DP-1100 and the other two are prepared from DP-800.

 

 

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About Menelaos Kokkinos

Menelaos Kokkinos TU Delft mgkokkinos@gmail.com
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