Mechanical Analysis of Melamine-Formaldehyde Composites
Doctoral thesis, 1999
Planar random fibre composite materials based on melamine-formaldehyde (MF), one of the hardest and stiffest isotropic polymers that is known, have been developed recently on an industrial scale. In this thesis the mechanical behaviour of these materials is systematically investigated for the first time.
Based on microstructural observations, a model predicting stiffness of these ternary materials with various resin/filler/fibre compositions is developed. The results are in agreement with experimental data. In addition, for practical purposes, merit indices for optimising the composition towards minimised weight and material cost are introduced, thus extending the use of the developed model.
The effect of cure temperature and amount of catalyst on the rheokinetical behaviour of the MF resin is investigated using a technique called Torsional Substrate Analysis (TSA), developed for this purpose. TSA measurements revealed several transitions. Rheokinetical data are used to construct Time-Temperature-Transformation (TTT) cure diagrams. High Pressure Differential Scanning Calorimetry (HPDSC) measurements are used to estimate the fractional conversion. Tg of the resin is discussed.
Damage mechanisms in MF composites with inorganic and organic constituents are studied by scanning electron microscopic observations directly under tensile load (in-situ SEM). Debonding of inorganic filler particles and glass fibres is observed on straining. Critical cracks mainly originate at debonded fibres and at fibres located close to the sample surface. On the other hand, debonding of cellulose is not observed. Damage initiation and development are evaluated by conducting cyclic tensile tests with a systematically increasing maximum strain. The unloading modulus is used to define a damage parameter. The resin/filler/fibre composition and the architecture of glass fibre reinforcement are found to have a strong influence on damage behaviour and performance of MF composites.
The mechanical behaviour of several flax fibre reinforced MF composites is studied and compared to that of a glass fibre reinforced grade. Although replacing glass with flax fibres has a somewhat negative effect on tensile performance the difference is small and is compensated for by the lower density and cost, making flax fibre composites a competitive material. Compared to glass, flax fibres constitute a material with a considerably lower damage rate.
The work can serve as a foundation for developing MF composites towards various applications and requirements.