Glass fibres have many applications such as blades for wind turbines, glass fibre boats etc. Researchers from DTU Nanotech have shown that by coating the glass fibres the composite properties can be improved. In future, this might lead to the development of e.g. lighter/longer blades resulting in more energy efficient wind turbines.
During the course of manufacturing, surface treatment (sizing) is applied to the glass fibres e.g. to protect the fibres and to ensure compatibility between the fibres and the composites in which they are included. During the past three years, PhD Student Helga Nørgaard Petersen has examined the effect of the sizing on the properties of both glass fibres and composites. Helga explains: ”In fibre reinforced composites, glass fibres induce strength to a polymer matrix. The adhesion between the fibre and the matrix needs to be strong in order to transfer the stress applied to the material and the adhesion is closely linked to the interface between the coated fibre and the matrix. Despite the importance, the nature and behaviour of the fibre/matrix interface is poorly understood and documented in the literature. With my project, we have come closer to a deeper understanding of the correlation between the sizing and the adhesion and thereby the properties of the fibre/matrix interface”.
Two different methods were used for quantifying the adhesion between the fibre and the matrix: The microbond test and the Double Cantilever Beam (DCB) test.
Sizing affects adhesion and composite properties
The microbond test is widely used in various forms for the determination of the so-called Interfacial Shear Stress (IFSS), which is a measure of the amount of energy needed to separate matrix and fibre. The test is performed by pulling a single glass fibre out of a micro droplet of epoxy resin. The test was performed at the University of Strathclyde in Glasgow, where Helga was on a 6 months’ research stay establishing a collaboration with James Thomason’s Advanced Composites Group researching in the field of composite performance, the interface between fibre and matrix and on sizing. At Strathclyde, Helga investigated how the IFSS was affected by changing the ratio between amine and epoxide groups of the epoxy resin.
The results confirmed that a change in the amine:epoxide group ratio would affect the IFSS greatly and that the maximum IFSS appeared at around the stoichiometric ratio of 1:1. From the study it was concluded that the adhesion between the fibres and the matrix could be controlled by varying the ratio of amine and/or epoxide groups in the glass fibre sizing.
Changing the surface chemistry leads to stronger adhesion
In collaboration with the Composites and Material Mechanics section at DTU Wind Energy, mechanical tests were carried out, including the DCB test where the so-called J-integral was determined. DCB tests were conducted on composite laminates specimens. “The J-integral is a measure of how much energy is needed for a crack to propagate along a composite beam causing fibres to bridge across the fracture” says Helga, and continues “a stronger adhesion will yield a higher J-integral, as more energy is needed for the debonding of the bridging fibres, and for the crack to propagate through the matrix”. The DCB test was found to be sensitive enough to detect changes in the adhesion as a result of changing the chemistry of the glass fibre surface.
Coating the glass fibres used in the composite laminates for the DCB specimens with an epoxy functionalised organosilane resulted in a remarkable increase in the adhesion between fibre and matrix. This proved the possibility of enhancing the composite properties by chemically changing the glass fibre surface.
Head of the Danish Centre for Composite Structures and Materials for Wind Turbines (DCCSM) Professor Bent F. Sørensen from DTU Wind Energy says that “Helga’s results show that it is possible to obtain an enhanced fracture resistance of the composite by controlling the fibre/matrix interface. This is a result of great importance for us on the path towards enabling the development of yet larger wind turbine rotor blades in the future”.
Helga Nørgaard Petersen’s research is part of the DCCSM centre financed by the Danish Research Council for Strategic Research. The research was done in collaboration with DTU Wind Energy (COM Section) and the University of Strathclyde. Helga Nørgaard Petersen handed in her PhD thesis in February 2017.