Author: [Bruce Zhou]
Affiliation: [Jota Machinery Composites Material Prepreg Solution]
Corresponding Author: [jotamachinery@gmail.com]
Published : December 09 , 2025

Abstract
Poly(etherketoneketone) (PEKK) has emerged as a key matrix for high-performance thermoplastic composites, particularly in aerospace structures produced by automated tape placement, press moulding and welding. In contrast to poly(etheretherketone) (PEEK), PEKK offers tunable crystallisation behaviour through variation of the terephthalic/isophthalic (T/I) ratio, but this tunability complicates process design and property prediction. This review consolidates current knowledge on crystallinity in neat PEKK and carbon-fibre-reinforced PEKK (CF/PEKK). The molecular structure, polymorphism and semicrystalline morphology of PEKK are first summarised. Isothermal and non-isothermal crystallisation kinetics are then examined, with emphasis on dual (primary and secondary) crystallisation and the influence of T/I ratio. The review subsequently focuses on CF/PEKK composites, discussing fibre-induced nucleation, transcrystalline layers and their still-debated role in interfacial and matrix-dominated properties. Crystallisation models based on Avrami, parallel Avrami, Hillier and derivative Hillier formulations, along with non-isothermal extensions, are critically assessed for their suitability to PEKK and CF/PEKK. Finally, the paper outlines processing–structure–property relationships relevant to industrial manufacturing and identifies priority research needs, including quantitative non-isothermal kinetics for CF/PEKK, a clearer mechanical assessment of transcrystallinity and integrated models that couple realistic thermal cycles with microstructure evolution.

Keywords
PEKK; PAEK; crystallinity; CF/PEKK composites; T/I ratio; transcrystallinity; crystallisation kinetics; Avrami model; Hillier model; automated tape placement.

1. Introduction

High-performance thermoplastic composites based on the poly(aryletherketone) (PAEK) family offer an attractive combination of mechanical performance, chemical resistance, weldability and recyclability for aerospace and other demanding applications. Among these, poly(etherketoneketone) (PEKK) has gained increasing attention as a matrix for carbon fibre composites, competing directly with the more established poly(etheretherketone) (PEEK). PEKK’s appeal stems from its high glass transition temperature, excellent thermal stability and its ability to be processed as a thermoplastic, in contrast to thermoset matrices.

However, structural performance and durability in service depend strongly on the crystallinity of the thermoplastic matrix. For PEKK, crystallinity is not a fixed property but is governed by three coupled factors: molecular architecture (especially the terephthalic/isophthalic, T/I, ratio), processing conditions (heating, isothermal holds and cooling rate) and the presence of reinforcement. Unlike PEEK, PEKK exhibits pronounced dual crystallisation and polymorphism, leading to complex melting behaviour. When combined with carbon fibres, additional phenomena such as transcrystallinity at the fibre–matrix interface further modify local microstructure and interfacial properties.

These features introduce both opportunities and challenges. Tunable crystallisation allows PEKK grades to be tailored for specific processing windows, but it also makes it more difficult to predict properties from process parameters alone. For engineers designing automated tape placement (ATP) cycles or press moulding routes, and for researchers developing predictive models, a consolidated view of structure, crystallisation kinetics and modelling approaches is essential.

This paper reviews current understanding of crystallinity in neat PEKK and CF/PEKK composites, with frequent comparison to PEEK where the PEKK literature is incomplete. Emphasis is placed on (i) molecular structure, polymorphism and morphology, (ii) isothermal and non-isothermal crystallisation kinetics, (iii) the particular role of carbon fibres and transcrystallinity, and (iv) kinetic and microstructural models relevant to process design. The overall goal is to map the present state of the art and identify gaps that must be addressed to enable reliable processing–structure–property control in industrial PEKK composites.

2. Literature Review

2.1 Molecular structure and T/I ratio in PEKK

PEKK belongs to the PAEK family, whose members consist of aromatic rings linked by ether and ketone groups. The relative proportion of ketone to ether functions influences chain stiffness and melt viscosity. PEKK incorporates a higher ketone content (≈67%) than PEEK (≈33%), leading to stiffer chains and generally slower crystallisation. A distinctive feature of PEKK is its tunable T/I ratio, reflecting the relative use of para-substituted terephthalic (T) and meta-substituted isophthalic (I) acid in synthesis. Higher T content (e.g. 80/20 T/I) promotes straight chains, higher melting temperatures and faster crystallisation. Increasing I content (e.g. 60/40) lowers melting temperature and facilitates processing but reduces attainable crystallinity and increases cooling-rate sensitivity.

Commercial PEKK grades, such as Arkema’s KEPSTAN® series, are typically classified by their T/I ratio and melt viscosity level. The 6000 series (60/40) is often targeted for applications requiring lower processing temperatures, while the 7000 (70/30) and 8000 (80/20) series address higher-temperature, high-crystallinity needs.

2.2 Polymorphism and morphology

PEKK exhibits polymorphism, with at least two orthorhombic crystal forms reported. Form 1, usually associated with melt crystallisation, has a two-chain unit cell with edge-to-face phenyl ring interactions. Form 2, often linked to cold or solvent crystallisation and high-T/I grades, exhibits face-to-face π–π interactions, with both one-chain and two-chain unit cell variants proposed in the literature. These polymorphs manifest as multiple melting peaks in differential scanning calorimetry (DSC) and may co-exist within the same specimen depending on thermal history.

At the microstructural level, PEKK forms the familiar semicrystalline morphology seen in many thermoplastics: lamellae that assemble into spherulites. Optical microscopy studies of 60/40 PEKK show nearly instantaneous nucleation followed by radial spherulitic growth. The resulting spherulite size and number density are sensitive to the melt-holding time and temperature before crystallisation and to the subsequent cooling rate. Short melt holds or anneals produce high nucleation densities and finer spherulitic textures, whereas long high-temperature holds lead to large, sparse spherulites.

2.3 Isothermal crystallisation and dual melting behaviour

Isothermal DSC studies on neat PEKK, particularly 60/40 and 70/30 grades, exhibit dual melting peaks on subsequent heating. The lower-temperature endotherm, located around 10–20 °C above the isothermal hold temperature, is attributed to less perfect lamellae and secondary interlamellar crystallites, whereas the higher-temperature peak near 300 °C corresponds to more perfect primary crystals. The overall crystallinity shows a maximum at an intermediate isothermal temperature (e.g. around 240–250 °C for 60/40) and decreases at higher temperatures where nucleation is sparse and lamellar thickening dominates.

PEKK typically achieves lower crystallinity and crystallises more slowly than PEEK under comparable conditions, consistent with its higher chain stiffness and the presence of meta linkages.

2.4 Non-isothermal crystallisation and grade dependence

Non-isothermal DSC data reveal strong differences among PEKK grades. For 60/40 PEKK, crystallinity drops sharply with increasing cooling rate: values near 28% at ≤1 °C·min⁻¹ decrease to a few percent at 10 °C·min⁻¹ and become negligible under quenching. High-T/I grades such as 80/20 maintain high crystallinity even at cooling rates up to 40–60 °C·min⁻¹ and are therefore less sensitive to cooling conditions.

Such behaviour has important consequences for processing. 60/40 PEKK demands careful cooling control or isothermal holds to achieve sufficient crystallinity, whereas 80/20 PEKK can develop a crystalline network under faster cooling but at the cost of higher melt temperatures.

2.5 CF/PEKK composites and transcrystallinity

When carbon fibres are introduced, crystallisation behaviour becomes more complex. Fibres can serve as heterogeneous nucleation sites, particularly at high isothermal temperatures where bulk nucleation is limited. Under appropriate thermal conditions (extended melt hold and moderate cooling), lamellae may grow preferentially normal to the fibre surface, producing a transcrystalline layer. The propensity for transcrystallinity depends strongly on fibre type and surface energy. High-modulus, graphitic, low-energy fibres often give rise to pronounced transcrystalline bands, whereas more polar, PAN-based fibres may support only conventional spherulitic growth near the interface.

Studies using single-filament debond tests have reported increased interfacial debond forces in the presence of transcrystalline layers, suggesting improved adhesion. However, laminate-level results on interlaminar shear strength (ILSS) and transverse properties are less consistent. At high fibre volume fractions, transcrystalline fronts from neighbouring fibres can impinge to form planes of weakness, and residual stresses may accrue at the interface, complicating the simple picture of “stronger is always better”.

2.6 Crystallisation modelling

Crystallisation kinetics in PEKK and CF/PEKK have been modelled using various extensions of the Avrami framework. Isothermal data are often fit using parallel Avrami formulations that represent primary and secondary crystallisation as independent processes, or using Hillier-type models that treat secondary crystallisation as sequential within existing crystals. More recent derivative-based approaches reduce sensitivity to experimental artefacts early in the isotherm.

Non-isothermal behaviour has been addressed using Ozawa-type or Nakamura-type transformations, parallel Avrami models with temperature-dependent rate constants and, in some cases, Tobin formulations that separate heterogeneous and homogeneous nucleation contributions. However, for PEKK the presence of dual crystallisation and strong cooling-rate sensitivity limits the reliability of simple non-isothermal models.

Microstructural simulations, such as pixel-based growth models and unit-cell models for ATP, provide additional insight into spherulite and transcrystalline development but remain largely qualitative.

3. Methodology

This paper is a critical literature review rather than an experimental investigation. The methodology therefore consists of three main stages: literature collection, thematic organisation and cross-comparison with modelling frameworks.

1. Literature collection
   Peer-reviewed journal articles, conference proceedings, doctoral theses and open-access preprints relating to PEKK, PAEK crystallisation and CF/PEKK composites were collected, with Composites Part B, Polymer, Polymer Engineering & Science and similar journals as core sources. The review by Pérez-Martín et al. on PEKK crystallinity and CF/PEKK composites served as a central reference, complemented by earlier and subsequent works on PEEK and PEKK crystallisation, as well as papers on transcrystallinity and kinetic modelling.
2. Thematic organisation
   The literature was organised into five thematic blocks:
   (i) molecular structure and grade classification (T/I ratio, ketone content),
   (ii) polymorphism and spherulitic morphology in neat PEKK,
   (iii) isothermal and non-isothermal crystallisation kinetics,
   (iv) CF/PEKK composites and interfacial morphology,
   (v) kinetic and microstructural models. Within each block, PEKK data were extracted and, where necessary, compared with corresponding PEEK studies to fill conceptual gaps.
3. Cross-comparison and synthesis
   Crystallinity levels, melting peak structures and kinetic parameters reported in different studies were compared to identify trends and inconsistencies, with particular attention to the role of T/I ratio and thermal history. Modelling approaches were evaluated based on their ability to capture dual crystallisation behaviour and to handle both neat resin and composite data. The implications of these findings for industrial processing were then synthesised into a qualitative processing–structure–property framework.

No new experimental data were generated; the emphasis is on extracting consistent physical interpretations and practical design guidance from existing work.

4. Results

4.1 Summary of structural and morphological findings

The reviewed studies consistently show that PEKK’s higher ketone content and variable T/I ratio introduce richer crystallisation behaviour than PEEK. PEKK can form at least two polymorphic crystal forms, with melt crystallisation generally favouring form 1 and cold or solvent crystallisation favouring form 2. Optical and DSC data for 60/40 and 70/30 grades confirm a dual crystallisation process, evident in dual melting peaks and deviations from single-stage Avrami behaviour.

In CF/PEKK composites, the presence of carbon fibres introduces transcrystalline layers under certain thermal histories, but the extent and uniformity of these layers depend markedly on fibre type, surface treatment, fibre volume fraction and cooling conditions.

4.2 Isothermal and non-isothermal kinetics in neat PEKK

Isothermal DSC data for 60/40 and 70/30 PEKK grades show:

• A distinct optimum crystallisation temperature, typically in the mid-200 °C range, at which crystallinity reaches a maximum of about 25–30%.
• Dual melting behaviour, with a low-temperature endotherm associated with secondary structures and a higher-temperature endotherm near 300 °C.
• Avrami exponents around 3 for primary crystallisation, consistent with instantaneous nucleation and three-dimensional spherulitic growth, and exponents near 1–2 for secondary processes.

Non-isothermal studies reveal that:

• 60/40 PEKK is highly sensitive to cooling rate; crystallinity collapses at moderate to high rates, leading to substantial cold-crystallisation exotherms on reheating.
• 80/20 PEKK maintains relatively high crystallinity over a wider range of cooling rates, confirming its suitability for faster processes at the expense of higher processing temperatures.

4.3 CF/PEKK composites: kinetics and interfacial morphology

For CF/PEKK laminates, the literature reports:

• Crystallisation rates similar to or slightly faster than the neat resin at high isothermal temperatures, reflecting fibre-induced nucleation.
• Comparable or modestly reduced crystallinity relative to neat PEKK at a given cooling rate, particularly at high fibre volume fractions.
• Formation of transcrystalline bands along fibre surfaces under conditions involving high melt temperatures, long melt holds and relatively slow cooling.

Matrix-dominated properties such as shear modulus, Young’s modulus at off-axis orientations and elevated-temperature stiffness increase with matrix crystallinity. Comparisons between amorphous and crystallised CF/PEKK laminates highlight dramatic losses in modulus at temperatures near or above the glass transition when crystallinity is low, underscoring the need for adequate crystallisation in high-temperature service applications.

Single-filament tests show that transcrystallinity can raise debond forces by roughly 40–50% compared with amorphous interfaces. Laminate-level ILSS and transverse strength data, however, are mixed, indicating a more complicated relationship between transcrystallinity and global mechanical performance.

4.4 Performance of kinetic models

For neat PEKK:

• Single-stage Avrami fits adequately capture early isothermal crystallisation but fail to reproduce the full dual-stage behaviour.
• Parallel Avrami and Hillier-type models provide better fits, especially when combined with derivative-based analysis that reduces error from experimental transients.
• Reported exponents from derivative Hillier analyses align with primary three-dimensional spherulitic growth and lower-dimensional secondary processes.

For CF/PEKK:

• Limited kinetic datasets restrict firm conclusions, but existing work suggests that Avrami/Hillier frameworks can be adapted without major structural changes to the equations.
• Non-isothermal models such as Ozawa or simple extensions of Avrami show curvature and parameter instability for PEKK, reflecting their inability to handle dual crystallisation and strong cooling-rate effects.

Microstructural simulations, though simplified, confirm that fibres accelerate local crystallisation and can generate transcrystalline structures consistent with experimental observations.

5. Discussion

The literature reviewed here paints a coherent picture of PEKK as a tunable but intrinsically complex semicrystalline matrix. The adjustable T/I ratio enables material designers to trade processability against crystallisation robustness, but this flexibility requires careful alignment between grade selection, processing route and performance targets.

From a structural standpoint, PEKK’s polymorphism and dual crystallisation arise from the interplay between chain stiffness and packing constraints imposed by the backbone and T/I ratio. From a kinetic standpoint, the coexistence of primary and secondary crystallisation stages, each with distinct thermal dependencies, complicates the application of classical models originally developed for simpler polymers. Derivative-based Hillier analyses appear to offer a practical compromise between physical realism and mathematical tractability for isothermal conditions, but further validation is still needed, especially for composite systems.

In CF/PEKK laminates, the presence of carbon fibres modifies nucleation, growth and local morphology. Transcrystallinity, while often discussed qualitatively as a sign of “good adhesion”, has a more nuanced effect. At low to moderate fibre volume fractions and with appropriate fibre surface treatments, transcrystalline layers can improve interfacial shear strength and delay debonding. At higher fibre volume fractions, however, transcrystalline fronts can coalesce into planar weaknesses, potentially reducing transverse toughness and promoting delamination under multi-axial loading. The net effect on laminate-scale mechanical performance is thus highly context-dependent.

For manufacturing, differences between neat and composite crystallisation behaviour and between high- and low-T/I grades must be explicitly considered. In autoclave and press moulding, where thermal cycles can include controlled isothermal holds, targeting the temperature region of maximum crystallisation rate is feasible, particularly for 60/40 and 70/30 grades. In ATP or induction welding, where each tape layer experiences rapid reheating and cooling, achieving uniform and adequate crystallinity becomes more challenging, particularly for low-T/I grades. Post-crystallisation annealing may be required to reach the desired crystallinity and stabilise properties.

Current kinetic models provide useful qualitative guidance but remain limited in three respects. First, most are calibrated only for neat PEKK or for a narrow range of processing conditions. Second, they often neglect secondary effects such as latent heat release, thermal degradation and stress development during crystallisation. Third, their extension to non-isothermal composite conditions typical of large structures is still in its infancy. Microstructural simulations address some of these issues at a local scale but are not yet integrated with full-scale process models.

Addressing these limitations will require coordinated efforts: systematic kinetic studies of CF/PEKK under realistic thermal cycles; micromechanical testing to isolate the effect of transcrystallinity from that of bulk crystallinity; and multi-scale modelling frameworks that couple heat transfer, crystallisation and mechanical response.

6. Conclusion

Crystallinity in PEKK and CF/PEKK composites is controlled by a combination of molecular architecture, thermal history and reinforcement. PEKK’s tunable T/I ratio allows crystallisation behaviour to be adjusted for specific processing windows, but it also introduces dual-stage crystallisation and polymorphism not present in PEEK. Isothermal studies show that derivative Hillier-type models, supported by morphological observations, can capture primary and secondary crystallisation in neat PEKK. Non-isothermal behaviour remains more difficult to predict, particularly at industrially relevant cooling rates.

In CF/PEKK composites, fibres act as both nucleation sites and mobility constraints, yielding crystallisation kinetics and final morphologies that differ from those of the neat matrix. Transcrystalline layers form under certain combinations of fibre type and thermal history and can enhance local interfacial strength, but their impact on laminate-scale performance is not yet fully resolved.

From a practical perspective, successful application of PEKK in high-performance composites requires: (i) appropriate grade selection based on T/I ratio and expected thermal cycle, (ii) process windows that ensure adequate matrix crystallinity, especially for high-temperature service, and (iii) careful control of fibre surface treatments and volume fractions to balance interfacial strength and toughness.

Future work should focus on building quantitative processing–structure–property maps for CF/PEKK under realistic ATP, press moulding and welding conditions; refining kinetic models to address dual crystallisation under non-isothermal composite conditions; and developing integrated simulation tools that can guide process design for complex structures. Achieving these goals will allow PEKK to fully realise its potential as a matrix for robust, weldable and recyclable aerospace-class composites.

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