Atomic Force Microscopy investigations of fiber-fiber bonds in paper

Franz Schmied

Research output: ThesisDoctoral Thesis

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Abstract

Paper is one of the most versatile materials accompanying mankind during history, either used for delivering and storing of information as well as to protect food and other goods from the environment. In this thesis, the main focus was put on the investigation of cellulose fibers for kraft paper. Cement bags, for instance, need a high tear strength and a high porosity simultaneously to allow a quick filling in the production line. A comprehensive surface morphology analysis based on atomic force microscopy (AFM) was performed to characterize the individual surface features of single fibers. High-resolution phase images were recorded which revealed the microfibrilliar structure of the fibers. Depending on the cell wall layer, different orientation and ordering of the microfibrils - having diameters of 25 to 35 nm - was visualized. Additionally, a detailed analysis of lignin precipitates based on a watershed algorithm as a function of the so called kappa-number was performed. To gain further insight into the contact area of two bonded single fibers, cross-sectional samples were investigated. Here, single fibers, fiber-fiber bonds, and embedded paper sheets are explored in detail. The individual cell wall layers (P, S1-S3) are visualized and their thickness was determined. Further, the change in orientation of the microfibrils with respect to the main fiber axis from perpendicular to parallel is demonstrated. The main part of the thesis is focused on the development of a method to measure the joint strength of two bonded single fibers based on AFM. Here, a calibrated cantilever was used to apply defined loads into the bonded area. To determine the energy contributions, experiments were performed in a load and a displacement controlled fashion. That allows the determination of elastic (90%) and visco-elastic (1%) energy parts of the total energy input (10^{-11}-10^{-12} kJ). The resulting bonding energy is about 10^{-12}-10^{-13} kJ. Additionally to the bonding energy, the breaking behavior prior to the failure based on different bonding mechanisms was analyzed in detail. Here, force discontinuities are strong hints for mechanical interlocking or fibril bridges. Analysis of the force drops revealed different force regimes for rupturing of single cellulose fibrils, fiber wall delamination, and breaking of microfibril bundles. Besides the measurement of the joint strength, stitched AFM topography reconstructions of the formerly bonded area (FBA) were analyzed. A clear difference between the formerly bonded and unbonded region is recognizable. The FBA is smoother (Wenzel ratio: 1.07) in comparison to the unbonded area (Wenzel ratio: 1.20). In addition to the roughness difference, dangling fibrils are detected in the FBA, especially localized close to the transition between formerly bonded and unbonded regions. This unique combination of joint strength measurement and AFM based inspection of the FBA further supports the assumption that mechanical interlocks and fibrillar bridges are important contributions to the bonding strength.
Translated title of the contributionRasterkraftmikroskopieuntersuchungen an Faser-Faserbindungen in Papier
Original languageEnglish
QualificationDr.mont.
Supervisors/Advisors
  • Teichert, Karl Christian, Assessor A (internal)
  • Schennach, Robert, Assessor A (external), External person
Publication statusPublished - 2011

Bibliographical note

embargoed until null

Keywords

  • AFM
  • kraft pulp
  • fiber-fiber bonds
  • cellulose
  • bond strength

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