Comportamiento mecánico de compuestos termoplásticos PEEK/PEI reforzados con fibra de carbono

Abstract

Polymer matrix composites have been increasingly employed in a wide range of structural elements thanks to their excellent strength-to-weight ratio. In this regard, thermoset resins reinforced with continuous fiber have been the most widely used composites since they have reached a high degree of technological maturity and materials with excellent properties are achieved at a relatively low cost. However, thermoset matrix composites have the disadvantage that they cannot be remolded or recycled, which limits the repair and recovery of these materials. This has led to significant interest from the industry in introducing thermoplastic polymers to manufacture new composite materials. The main advantage of these polymers is that, thanks to their chemical nature, thermoplastics can be re-melted, allowing for more efficient and sustainable production lines. In this thesis work, to broaden the perspective on the use of thermoplastic composites, the polymers polyether-ether-ketone (PEEK) and polyether-imide (PEI) are reinforced with 5H satin carbon fiber fabrics and consolidated by filmbased thermoforming. The PEI/CF and PEEK/CF laminates are characterized and compared from the micromechanical scale, covering fiber, matrix, and interlayer properties, to the macroscopic scale, where their response is studied by tensile, mode I fracture toughness and low-velocity impact tests. In carbon fiber/PEEK composites, the presence of a sizing agent (epoxy coating of the fabrics) considerably decreases the strength of the matrix-fiber interface in PEEK/CF composites, significantly affecting the properties of the material. By contrast, PEI matrices exhibited good adhesion with coated fibers. Subsequently, multilayer PEEK/PEI polymer laminates were fabricated to obtain a hybrid matrix that combines the excellent properties of each thermoplastic. In this thesis, a multilayer polymer with a 50/50 by volume ratio was investigated by tailoring the fabrication parameters to achieve good adhesion at the PEEK/PEI interface and high retention of PEEK crystallinity. Interface adhesion was characterized through three-point bending tests, which showed that a processing temperature below the melting point of PEEK results in a weak interface, susceptible to delamination. By contrast, a consolidation temperature slightly above the melting point of PEEK allowed good adhesion between the two polymers. DSC, DMA, and XRD results show that at this temperature, there is a low interpenetration between the PEEK and PEI chains. In addition to this, the nanoindentation technique was used to study the mechanical properties along the cross-section of the multilayer. Finally, similar PEEK/PEI multilayer polymer architectures were reinforced with carbon fiber fabrics to obtain a novel hybrid matrix composite material. The fabrication of this hybrid composite, named in this thesis as PEEKPEI/ CF, is carried out by thermoforming, using as a reference the parameters established in the fabrication of PEI/CF and PEEK/CF laminates. Subsequent inspections show good impregnation qualities, retaining the heterogeneity between the PEI and PEEK phases (with a volume ratio of 70/30 in this case), which is necessary to maximize the mechanical properties of the matrix. The hybrid composites were subjected to the same mechanical tests performed on the PEI/CF and PEEK/CF laminates. The results show no significant differences for the single-matrix composites, except in low-speed impact situations, where the hybrid composites exhibit higher damage tolerances. The computed tomography inspections have made it possible to study the damage patterns in each case. The thesis results show that PEEK, PEI, and hybrids are viable polymers for manufacturing carbon fiber composites on a laboratory scale. However, this line of research needs to be continued to enable their application on a larger scale.