Hybrid Bicomponent Fibres for Thermoplastic Composites: Towards New Intermediate Materials for High Volume Manufacturing using Stamp Forming

Abstract

Hybrid preforms are used in thermoplastic composite manufacturing processes to reduce the potentially long consolidation times caused by the high viscosities of thermoplastic melts. Darcy's law for fluid flow through a porous medium indicates that the negative effects of high viscosities on impregnation time can be offset by reducing the maximum distances the thermoplastic melts must flow for complete consolidation. In thermoplastic composite preforms, the total impregnation length is reduced by increasing the degree of mingling between reinforcement and matrix. Currently, mingling in such preforms is found on the level of the laminate down to the level of the yarn, but may occur on any of the hierarchical tiers found in fibre-reinforced composite materials. The level and quality of mingling in existing arrangements - such as organosheets, commingled yarns, powder-impregnated yarns and fibre impregnated thermoplastics (FITs), co-woven yarns or stacked laminates - greatly influence the flow lengths, cycle times, achievable part complexity, raw material costs, and suitable manufacturing routes. Given the limited selection of commercially available preform architectures, manufacturers must choose between the low cycle times of organosheets and the better drapeability of unconsolidated hybrids, e.g. commingled yarns, in thermoforming. The development of a material architecture which combines the fast processing of fully impregnated products with the flexibility of unconsolidated preforms would render thermoplastic composites significantly more attractive to high volume production markets, e.g. automotive parts. This thesis proposes hybrid bicomponent fibres - which consist of continuous reinforcement fibres individually sheathed in a thermoplastic polymer - as a new class of preform materials for thermoplastic composites. By reducing the scale of mingling between the reinforcement and matrix materials to the level of the fibre, a full wet-out of the fibres is ensured while the unconsolidated nature of the material allows the fibres to shift and deform with respect to each other to ensure drapeability even at room temperature. It is hypothesized that preforms made from hybrid bicomponent fibres can be stamp formed with cycle times similar to those of pre-consolidated blanks. Furthermore, it is expected that the void content of laminates stamp formed from hybrid bicomponent fibre preforms is greatly influenced by sintering mechanisms and the removal and/or collapse of air pockets. The presented research aims to answer these hypotheses by developing suitable methods to manufacture such hybrid bicomponent fibres and by processing them into consolidated laminates. The basic idea of hybrid bicomponent fibres is motivated and introduced in further detail in part I. Part II moves on to discuss materials and their corresponding processing methods for their suitability in realizing bicomponent fibres. A fibre forming approach based on glass-melt spinning combined with an in-line coating process is chosen. Multiple versions of the latter are investigated empirically, namely dip-coating of newly spun glass fibres in either a polymer solution or a sparsely nanofilled polymer melt, as well as the so-called kiss-roll coating method. It is found that drawing glass monofilaments of finite length over a rotating roll which is partially immersed in a dilute polymer solution yields a coating method which can endure high fibre velocities while ensuring the deposition of a sufficiently thick thermoplastic sheath for down-stream conversion into a structural grade composite laminate. The validity of this strategy is proven in the realization of a pilot plant which employs solution kiss-roll coating in-line with melt-spinning of a glass monofilament for the continuous fabrication of bicomponent fibres. Unidirectional layups of specimens of aluminium borosilicate glass fibres clad in polycarbonate produced with the pilot plant were characterized for their consolidation behaviour, the results of which are reported in part III. Supported by theoretical treatments on issues related to void collapse and autohesion, a parameter study on rapid stamp forming of these preforms was performed and complemented with stamp forming trials processing cross-ply layups of different thicknesses. All experiments yielded excellent laminate qualities with void contents <0.7%, supporting the conclusion that issues related to air removal and void collapse are insignificant. Laminates with a consolidated thickness of 1 mm and a fibre volume content of 0.69 were consolidated with holding times inside the press as low as 5 s, illustrating that virgin hybrid bicomponent fibre preforms can be stamp formed with similar process parameters as pre-consolidated blanks. Overall, it is concluded that the concept of hybrid bicomponent fibres as a novel type of preform provides enormous advantages for manufacturing continuous fibre-reinforced thermoplastic polymer composites. The combined value chain of stamp forming solution kiss-roll coated glass fibres offers a first opportunity for the production of continuous fibre-reinforced polymer composites without relying on Darcian impregnation flows anywhere between fibre formation and part production. The research presented in this thesis provides experimental proof for these claims and has established pilot equipment for the continuous spinning of glass/thermoplastic polymer bicomponent fibres, bringing this potentially disruptive technology closer to reality and expediting the adaptation of thermoplastic composites for high volume manufacturing.