Studying crystallinity gradients in high performance thermoplastic composites manufactured by automated tape placement

Abstract

Poly(aryletherketone)s (PAEKs) are a family of thermoplastics, used as a matrix in carbon fibre (CF) reinforced composites. These high-performance composites are widely implemented as structural materials in aeronautics, in parts like wing flaps, access panels and floor panels, amongst others. Poly(etherketoneketone) (PEKK), a member of the PAEK family, has aroused interest in recent years due to its excellent properties, as well as the range of available grades of PEKK with different applications. In order for a successful implementation of the material, however, a thorough understanding of the polymer’s crystallisation kinetics and morphology is required. Crystallinity refers to the degree of molecular order in the polymer, which plays a crucial part in mechanical, thermal and chemical properties. While some literature exists covering crystallisation morphology of some grades of unreinforced PEKK, such studies on CF/PEKK composites and their mechanical performance are minimal.

Developing such an understanding is of interest within the context of manufacturing, particularly in fastpaced processes such as automated tape placement (ATP). This is a manufacturing process regularly used in the aerospace industry, designed for fast production, automation and repeatability of part manufacturing. However, such fast-manufacturing techniques limit the time the processed material undergoes high temperatures and loads, not only affecting crystallinity development but also part consolidation. This ultimately can have a negative impact on part quality and performance. ATP processing conditions add a further layer of complexity in the case of semicrystalline thermoplastics such as PEKK, as the generation of uniform crystallinity throughout the bulk of a manufactured part is a key requirement for reliable performance during the service life. It is therefore essential to understand how processing conditions affect the crystallinity development across the final part’s thickness, which can provide insight into the effectiveness of the manufacturing method and potential areas of improvement during the manufacturing process.

This PhD project aims to explore the variation of crystallinity across the thickness of CF/PEKK laminates manufactured via ATP. In order to achieve this, three main objectives were established, which are addressed throughout this body of work. These, along with the work performed and the results, are summarised below.

Firstly, it was necessary to establish an understanding of the crystallisation nature of PEKK. In order to achieve this, an exhaustive study on crystallisation morphology, behaviour and kinetics of unreinforced PEKK and CF/PEKK composites was performed. Different isothermal holding temperatures were tested, ranging from 220-300°C. These affected the crystallisation morphology, with spherulites increasing in size as the isothermal temperature was increased (from ~2 μm at 220°C to ~5 μm at 300°C). All samples were observed to crystallise fully (~25%). The fastest crystallisation kinetics were achieved at 240-260°C, reaching peak crystallisation kinetics after 0.8 minutes. Non-isothermal cooling cycles were proven to influence the extent of crystallisation, with samples crystallising fully (26%) after cooling from the melt at 5°C/min, while remaining amorphous (<2% crystallinity) when cooled at 150°C/min. Modelling of the crystallisation kinetics was also performed, where time-temperature-transformation contour plots were generated for both the isothermal and non-isothermal cooling conditions.

Once an understanding of the crystallisation nature was established, it was of interest to evaluate the impact of different ATP parameters on the matrix-dominated mechanical properties of CF/PEKK laminates. To do this, CF/PEKK composites were manufactured via compression moulding and ATP, where 5 different compression moulding cycles were chosen based on the studied thermal cycles, and fully crystallised laminates were obtained. The effect of different spherulite sizes in the compressionmoulded laminates (as observed in the crystallinity studies) was not evident on the properties investigated, as all compression-moulded laminates (crystallised to the highest extent) performed similarly. A series of ATP laminates were manufactured where the effect of different parameters was tested: tool material (ceramic and mild steel), nip temperature (360, 370, 380 and 390°C), lay-down speed (2 and 4 m/min) and roller pressure (2 and 4 bar). All ATP laminates displayed a significantly lower performance across the investigated mechanical properties when compared to the compression-moulded laminates (ranging from 24 to 73% lower performance across different matrix-dominated properties). When evaluating the impact of individual ATP parameters however, tool material and nip temperatures had the most significant impact on properties, with the ceramic tool and the higher temperatures improving performance.

While the macromechanical testing described above provides an indication of the overall quality of the laminate, this does not solely reflect the crystallinity extent, as mechanical performance can also be influenced by other factors such as void content. Furthermore, this testing does not provide an indication of any variation in crystallinity across the thickness of laminates. Therefore, the final consideration to this work was to study variation in micromechanical properties across different CF/PEKK laminates and their thickness to indirectly evaluate crystallinity content variations throughout a sample’s cross-section. Nanoindentation was selected to do this as it allows to test regions small enough to provide different values for fibre, matrix bulk and fibre/matrix properties. The matrix bulk of compression-moulded laminates, which were found to be fully crystallised, performed significantly better than the ATP laminates (22-53% higher), in line with macromechanical testing results. The central region of the ATP laminates performed better than regions close to the top and bottom surfaces (up to 14%), likely due to a higher crystallinity development in the central area. Additionally, a transition region at the fibre/matrix region was observed in all samples.

The present work provides a diligent study on the crystallisation kinetics and morphology of CF/PEKK composites. This offers a solid understanding of crystalline structure development that can be of great value for optimising manufacturing parameters for semicrystalline thermoplastic composites. It also provides significant experimental evidence on crystalline structure development of CF/PEKK composites under compression moulding and ATP, and their impacts on macro and micromechanical performance. The crystallinity gradients developed across the thickness of the CF/PEKK laminates as an effect of ATP processing are studied via nanoindentation. Such scientific understanding can play a big role in optimising ATP manufacturing parameters and increasing the reliability of CF/PEKK products in high-performance applications.