Experimental investigation of water absorption in polymer-matrix composite structures
Du Plessis, Sean Daniel
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Polymer-matrix composites (PMCs) are commonly used in modern sailplanes in the primary load-bearing structure. All competition-class sailplanes make use of water ballast tanks in the wings to improve performance in certain weather conditions. Due to the adverse effects that water has on composite materials, they have to be waterproofed. There are a variety of materials that can be used to waterproof built-in ballast tanks. Yet, it is often the case that these waterproof sealants don’t properly protect the underlying layers from water permeating into lower layers and being absorbed by PMCs – causing externally visible deformation. In this study, the water absorption and permeability of PMCs used in sailplane construction were investigated. Samples were made from PMC and core foam which is used in the construction of sailplane wings. These samples were tested for absorption and deformation. A second set of samples made were made with coating and bonding materials (fillers) on-top of a substrate consisting of wood and fibreglass. These samples were used to measure permeability. Seventeen absorption sample variants were made – each of which could be categorised according to material type for result comparison. The categories were gelcoats, fillers, fibre-reinforced composites, peel ply and PVC foam. Absorption in terms of moisture content was 0.28% to 0.29% for gelcoats, 0.50% for Novolac coating, 0.76% to 1.43% for fillers, between 1.70% and 4.88% for fibre-reinforced composite samples, 10.9% for nylon peel ply, and up to 40.7% for PVC foam. The different material samples didn’t all have the same exposed surface area. It was thus necessary to implement a set of criteria to score and rank the different materials by taking the differing surface areas into consideration in conjunction with the moisture content. Similarly the permeability experiment results were categorized in the groups of coating materials (which included gelcoats, Novolac, fluorinated plastic and peelply), PVC foam and fillers. Serving as a point of reference, permeability moisture content of the substrate-only control samples was found to be 3.58%. Two-layer peel ply samples had slightly more at 3.63%. Single-layer fluorinated plastic samples gained 3.56% moisture. Triple-layer Novolac coated samples had 3.21%. PVC foam samples had 2.12%. Triple-layer gelcoat samples ranged between 1.72% and 2.03%, and samples coated with filler materials ranged between 0.61% and 1.77%. These moisture content recordings, however, included moisture gain from moisture in the air that didn’t have to pass through the materials – thus influencing the validity of reporting on these as permeability values. A permeability rank- scoring system, akin to the one used for absorption data, was devised to take atmospheric moisture into account. Evaluation of overall suitability of materials in aqueous environments was then done by jointly considering absorption and permeability characteristics of materials. Materials were compared within two functional, positional categories – fillers and coatings, and PMC’s and foam.
- Engineering