Investigation of sizing - from glass fibre surface to composite interface

Helga Nørgaard Petersen

    Research output: Book/ReportPh.D. thesis

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    Abstract

    Composites are far from a new invention, and have through time taken many shapes. From a simple hay clay house to advanced nano particle containing composites for advanced material applications. Since the industrialisation in the late 1800’s the use of fibre reinforced composites have increased significantly. The usage span wide, from furniture and car components to construction materials. Even though, the concept of composites is well known and widely applied, the fundamental principles of the interaction of the constituents, in the composites are still not fully understood. This thesis is a part of Danish Center for Composite Structures and Materials for wind turbine blades who work towards improving composites. Since wind turbine blades are the basis of the DCCSM it is the materials used here that are the focus, explicitly glass fibres and epoxy matrix. Glass fibre composites greatly dominate the fibre reinforced composite industry due to the combination of their relatively high stiffness and low production cost. During manufacturing the glass fibres are applied a coating, called sizing, for protection of the fibres and for compatibility with the polymer matrix. The sizing is located at the interface between glass fibre and polymer matrix. Despite the importance of this interface, in regards to the stress transfer, which is responsible for the reinforcing effect of fibres, very little research address how the interface is affected and how it can be controlled. This thesis covers an analysis of the sizing from the glass fibre surface to the interface in composites.
    Through soxhlet extraction with acetone it was possible to remove a part of the sizing from the glass fibres for analysis. By burning off the sizing at 565 ºC a higher mass loss was obtained than from the extraction, indicating that a part of the sizing might be covalently bonded to the glass fibre surface. The investigation of the sizing extract by ATR-FTIR and TGA-MS revealed the presence of a DGEBA film former as one of the components of the sizing. Glass plates were successfully coated with the organosilanes APTMS and GPTMS in order to mimic the surface of the glass fibres. The non-planar surface of glass fibres yields difficulties in some analysis e.g. determination of contact angle. The plates displayed a clear difference in contact angle after being coated towards a more polar surface.
    An investigation of the adhesion between fibre and matrix analysed by microbond testing and the determination of the IFSS was conducted varying the amine:epoxide group ratio in the matrix and the testing temperature. IFSS was found to be affected by both parameters. A maximum IFSS was observed around the stoichiometric ratio of amine:epoxide group (1:1). The presence of amine or epoxide groups in the sizing will affect the ratio at the interface and in all probability also the IFSS with a decrease in IFSS as the result. Furthermore, the testing temperature influenced the IFSS. The highest values were obtained at room temperature. Above the glass transition temperature the dependency of the amine:epoxide group ratio changed to become linear. Two different microbond setups were used for the determination of the IFSS and a difference was detected. It was explained by the difference in loading procedure; one had constant strain rate and the other constant load rate. Additionally the duration of the microbond test might also influence the determination of the IFSS.
    The influence on the mechanical properties stiffness, strength and J-integral by changes in the chemistry of the interface was investigated. The stiffness of single glass fibres increased after the removal of sizing by extraction but also when the sizing was removed by burning. This could partly be explained by the sizing being less dense than the glass fibres. For the burned glass fibres compactment of the glass structure also yields an increase in stiffness. The fibre strength was less affected by the extraction of sizing but burning drastically decreased the strength. The enlargement of surface flaws after the removal of the protective sizing is given as the cause of the decrease in strength. Coating of fibres after extraction of the original sizing by the organosilane GPTMS resulted only in insignificant changes of stiffness and strength of single glass fibres. However the effect on the adhesion measured by the J-integral was remarkable. Small scale specimens were successfully used for the DCB testing and the determination of the J-integral. The GPTMS modified fibres displayed significant higher interface adhesion in comparison to the fibres with the original sizing. From this it had been proved that the original sizing is far from the optimal when it comes to facilitating a strong adhesion between glass fibre and matrix.
    Original languageEnglish
    PublisherDTU Nanotech
    Number of pages84
    Publication statusPublished - 2017

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