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Abstract

Based on the Plateau-Rayleigh instability mechanism, this study proposes a novel fabrication method for flexible thermoplastic composite prepreg yarns. Using this approach, we successfully prepared carbon fiber/polyether ether ketone (CF/PEEK) prepreg yarns with a unique ‘bead-on-string’ microstructure. The prepreg yarn simultaneously exhibits excellent flexibility and uniform resin distribution, effectively addressing the challenge of thermoplastic resin impregnation. Its superior pliability enables textile processing such as weaving and braiding, offering a novel strategy for developing multidimensional structural thermoplastic composites.

Keywords

Prepreg yarn CF/PEEK thermoplastic composites textile fabric bead-on-string microstructure Plateau-Rayleigh instability

Highlights

• Thermoplastic composite has economic and environmental benefits.

• Wetting fibers with thermoplastic polymers presents a challenge.

• Proposes a novel fabrication method for flexible thermoplastic composite prepreg yarns.

• Enables textile processing (e.g., weaving, braiding) of these prepreg yarns.

• Makes 3D thermoplastic composite structures practical through this prepreg technology.

Introduction

Fiber-reinforced thermoplastic composites (FRTP) offer significant advantages over thermoset composites, including high toughness, impact resistance, short processing cycles, ease of transportation and storage, weldability, remoldability, and environmental friendliness.^1–3 These benefits have led to their increasing applications in aerospace,^4–8 marine engineering,^9–11 and medical fields.^12–15 However, the high viscosity of thermoplastic matrices makes uniform distribution and fiber impregnation challenging. This creates both scientific and practical difficulties in designing and manufacturing continuous fiber-reinforced thermoplastic composites. These limitations have constrained the development of 2D and 3D reinforced thermoplastic composite, consequently restricting their applications primarily to non-load-bearing components.^16–21

Various manufacturing methods have been developed for thermoplastic composite prepregs, including hot-melt impregnation,²² solvent impregnation,^23,24 in situ polymerization,^25–31 commingled fibers,^32–35 film stacking,³⁶ and powder coating.^22,37 For example, SAKAGUCHI et al.³⁴ produced a series of prepreg yarns with varying resin distribution states using braiding technology, demonstrating that shorter matrix flow distances result in improved impregnation quality. Qu et al.²² reduced the resin flow distances by fiber spreading and enhanced impregnation through roller compression. However, the resulting prepregs exhibited high stiffness and brittleness, rendering them unsuitable for subsequent textile processes. STOLYAROV et al.³⁵ employed hybrid fiber technology to further reduce the resin flow distance, but non-uniform mixing occurred and was further exacerbated during subsequent textile processing. Hu et al.³⁶ utilized a film-stacking method to fabricate thermoplastic composites, although this technique is principally applicable only to laminated structures. AGEYEVA et al.¹⁶ provided a comprehensive overview of in situ polymerization devices and manufacturing techniques for thermoplastic composites. While in situ polymerization employs low-viscosity monomers capable of impregnating a wide range of preform architectures, the current range of applicable polymers remains limited, restricting its utility across diverse application fields. In summary, existing methods often yield prepregs that suffer from either poor impregnation due to inhomogeneous mixing or insufficient flexibility due to rigidity and brittleness. This inability to simultaneously achieve adequate impregnation and textile-process compatibility severely limits the fabrication of complex-shaped components.

This study presents a novel design and manufacturing method for flexible thermoplastic composite prepreg yarns based on the Plateau-Rayleigh instability mechanism.³⁸ Driven by melt surface tension, PEEK resin encapsulates carbon fibers and breaks into discontinuous micro-beads, forming a “bead-on-string” prepreg yarn structure. This approach effectively addresses the challenge of insufficient fiber wetting caused by the high melt viscosity of thermoplastic matrices, while preserving the inherent flexibility of the carbon fibers. The developed prepreg yarns are suitable for textile processes such as weaving and braiding, enabling the fabrication of thermoplastic composites with complex geometries. This work establishes a fundamental basis for developing multidimensional structural thermoplastic composites.

Experimental procedure

Materials

The materials used included carbon fibers (Toray T700SC-12000-50C), PEEK powder (Victrex 150P), deionized water, and silicone oil.

Differential scanning calorimetry (DSC) was performed on a PerkinElmer DSC 8000 instrument under a nitrogen atmosphere at a heating and cooling rate of 20°C/min to characterize the glass transition temperature, melting point, and crystallization behavior of the PEEK resin. Thermogravimetric analysis (TGA) was carried out using a PerkinElmer TGA 4000 in an air atmosphere with a heating rate of 20°C/min to evaluate the thermal stability and decomposition temperature of the material. Contact angle measurements were conducted on a JC2000DM instrument at a temperature of (23 ± 2)°C and relative humidity of (50 ± 10)%, using deionized water and silicone oil as probe liquids. Five measurements were taken per sample and averaged to assess the interfacial wettability between the fiber and the resin matrix. The fiber surface morphology was examined using a JEOL JSM-6360 scanning electron microscope to analyze structural and morphological changes before and after desizing.

Design and manufacturing of flexible CF/PEEK prepreg yarns

Inspired by beaded necklaces, a novel prepreg yarn was designed by utilizing the fluidic Plateau–Rayleigh instability. This approach addresses the challenge of thermoplastic resin impregnation while preserving the flexibility of the fibers.

Design of flexible CF/PEEK prepreg yarns

When melted, PEEK resin tends to minimize its surface area under the effect of surface tension. As a result, the cylindrical PEEK melt coating on the carbon fibers spontaneously breaks up into a series of micro-beads, leading to the formation of a “bead-on-string” structured CF/PEEK prepreg yarns. The formation process is shown in Figure 1, with a schematic of the prepreg yarn structure presented in Figure 2.

Figure 1.

Process diagram of microbead formation.

Figure 2.

Conceptual representation of the “bead-on-string” structured prepreg yarn.

The process is primarily governed by the surface tension of the melt, whose magnitude depends on the properties of the melt, the chemical nature of adjacent substances, temperature, pressure, and impurities within the melt. It directly determines the contact angle θ, which can be described by the Young–Laplace equation³⁹:

\cos θ = \frac{γ_{s v} - γ_{S L}}{γ_{L V}}

(1)

where

γ_{s v}

、

γ_{S L}

and

γ_{L V}

represent the surface tension coefficients at the solid-vapor, solid-liquid, and liquid-vapor interfaces, respectively. This demonstrates that bead morphology can be controlled by adjusting the interfacial tension coefficients (

γ_{s v}

、

γ_{S L}

and

γ_{L V}

) at the solid-vapor, solid-liquid, and liquid-vapor interfaces.

Manufacturing of prepreg yarns

The prepreg yarns were prepared using self-developed equipment, following the process illustrated in Figure 3.

The carbon fiber yarn was continuously unwound at 1.5 m/min under controlled tension and first passed through a 650°C thermal treatment zone in air to completely decompose the original epoxy sizing agent (Figure 3). The 12 K tow was then spread to 30 mm width using an ultrasonic spreading unit. A uniform layer of PEEK powder was deposited on both sides of the spread fiber tow via electrostatic spraying (200 kV, 3 μA). The powder-coated yarn was subsequently heated in a 390°C melting zone, where the PEEK melted and self-assembled into a periodic bead-on-a-string structure under the combined effects of surface tension and Plateau–Rayleigh instability. After cooling and solidification, the resulting flexible CF/PEEK prepreg yarn was wound. The overall manufacturing process is schematically illustrated in Figure 4. By precisely regulating the yarn speed and electrostatic spray parameters, the resin content was effectively controlled. The resulting prepreg yarn produced in this study exhibited a PEEK resin content of 50 wt%.

Figure 3.

Prepreg yarn process flow.

Figure 4.

Prepreg yarn preparation process.

Shape control of microbeads on prepreg yarns

According to the Young-Laplace equation, the contact angle of microbeads on fibers can be adjusted by controlling the surface tension coefficients at the solid-gas, solid-liquid, and liquid-gas interfaces. To facilitate practical implementation and enhance the performance of composite materials, we investigated the influence of two parameters—the surface structure of carbon fibers and the molding temperature—on the size of microbeads.

Effect of fiber surface wettability on microbead size

Under identical processing conditions (temperature: 390°C, hot-melt time: 5 min), flexible prepreg yarns were prepared using three types of carbon fibers with different surface tensions: Epoxy-sized fibers, desized fibers, and desized-activated fibers. The size and shape of microbeads on the three prepreg yarns were examined using microscopy.

Effect of molding temperature on microbead size

Using desized carbon fibers, four prepreg yarns were prepared at molding temperatures of 390°C, 405°C, 420°C, and 435°C, respectively. The microbeads on each prepreg yarn were observed under a microscope, and their differences in size and shape were analyzed.

Mechanical properties of flexible CF/PEEK prepreg yarns

The mechanical property tests of the prepreg yarns were conducted in accordance with GB/T 1446-2005 and GB/T 32788.4-2016, using a CMT4204 universal testing machine (SANS). The testing environment was maintained at (23 ± 2) °C and a relative humidity of (50 ± 10)%.

Tensile strength of flexible CF/PEEK prepreg yarns

The tensile specimens had an effective gauge length of 180 mm, with 40 mm × 40 mm kraft paper tabs bonded at both ends using two-component epoxy adhesive. The loading speed was set at 2 mm/min.

Knot strength of flexible CF/PEEK prepreg yarns

The knot strength specimens had an effective gauge length of 180 mm, with a standard fisherman’s knot tied at the center. 40 mm × 40 mm kraft paper tabs were bonded at both ends using two-component epoxy adhesive, and the loading speed was 15 mm/min (as shown in Figure 4).

Loop strength of flexible CF/PEEK prepreg yarns

The loop strength specimens had a gauge length of 130 mm, with 40 mm × 40 mm kraft paper tabs bonded at both ends using two-component epoxy adhesive. The loading speed was 15 mm/min (as shown in Figure 5).

Figure 5.

Knotted and looped specimens of prepreg yarns.

Results and discussion

Analysis of material properties

Figure 6 shows the DSC results, where a distinct melting peak was observed at 342.73°C, indicating the melting phase transition of the material at this temperature. The corresponding enthalpy change (ΔH) was 43.29 J/g, confirming the absorption of heat during melting. Additionally, a clear crystallization peak appeared at 303.85°C, demonstrating crystallization at this temperature, with an associated enthalpy change (ΔH) of 43.06 J/g, signifying heat release during crystallization.

Figure 6.

DSC curve of PEEK material.

These experimental results demonstrate that the PEEK powder melts at 342.73°C and crystallizes at 303.85°C, providing critical reference data for the preparation of prepreg yarns and the molding of composite materials.

Figure 7 shows the TG results, where below 500°C, the mass of the PEEK powder remained relatively stable, indicating excellent thermal stability within this temperature range. However, when the temperature exceeded 500°C, the mass of the PEEK powder began to decrease, with significant thermal degradation observed between 550°C and 650°C. The maximum mass loss rate occurred near 600°C, identifying this temperature as the primary decomposition temperature of the PEEK powder. Above 600°C, the mass loss rate diminished, likely attributed to the formation of residual products or a slowdown in decomposition reactions. These findings confirm that PEEK exhibits robust thermal stability below 500°C, and the determination of its decomposition temperature provides critical insights for processing and application optimization.

Figure 7.

TG and DTG curves of PEEK material.

Table 1 summarizes the contact angle measurement results, reflect the hydrophilicity and oleophilicity of the materials: As-received epoxy-sized carbon fibers exhibited a hydrophilic contact angle of 11.15 and an oleophilic contact angle of 81.4°, indicating strong hydrophilicity and moderate oil repellency. This suggests excellent wettability for water-based resins. Desized carbon fibers showed a hydrophilic angle of 102.5° and an oleophilic angle of 65.51°, demonstrating significantly reduced hydrophilicity and enhanced oleophilicity, which improves compatibility with oil-based resins. Desized and Mitis acid-activated carbon fibers displayed reduced contact angles for both liquids (hydrophilic: 78.75°; oleophilic: 35.62°), indicating improved hydrophilicity and oleophilicity. However, after exposure to high temperature (390°C, 5 min), these angles increased to 104.17° (hydrophilic) and 52.36° (oleophilic), revealing the thermal instability of the activated functional groups. PEEK material showed a hydrophilic angle of 75.78 and an oleophilic angle of 54.95°, suggesting moderate hydrophilicity and superior oleophilicity.

Table 1.

Contact angle test results of raw materials.

Material	Epoxy-sized CF	Desized CF	Meldrum’s acid-grafted CF	Thermally-treated Meldrum’s acid-modified CF	PEEK
Average hydrophilic contact angle	11.15	102.50	78.75	104.17	75.78
Left/right hydrophilic contact angle	11.17/11.13	102.90/102.10	79.07/78.42	104.54/103.80	76.25/75.31
Micrograph
Average oleophilic contact angle	81.40	65.51	35.62	52.36	54.95
Left/right oleophilic contact angle	81.68/81.12	65.51/65.51	35.25/35.98	52.66/52.07	54.95/54.95
Micrograph

The experimental results demonstrate that: The epoxy-sized carbon fibers exhibit strong hydrophilicity and oil repellency; After desizing treatment, the carbon fibers show significantly reduced hydrophilicity and markedly enhanced oleophilicity; The activated carbon fibers display improved both hydrophilicity and oleophilicity to some extent, but demonstrate poor high-temperature resistance; Furthermore, PEEK material exhibits better oleophilicity than hydrophilicity. Therefore, desized carbon fibers is more suitable as reinforcement materials for PEEK composites.

As shown in Figure 8, the carbon fibers exhibited uniform diameters with axial grooves along their surfaces. After desizing, these grooves became more pronounced, suggesting enhanced surface roughness and interfacial bonding potential. Meldrum’s acid-modified, these grooves are between the two above.

Figure 8.

Surface morphology of carbon fibers under different treatments: (a) Epoxy-sized virgin carbon fibers (upper), (b) Desized carbon fibers (lower), (c) Meldrum’s acid-modified CF.

Analysis of Influencing Factors on Microbead Regulation

This experimental study investigated the effects of fiber surface wettability, molding temperature, and microbead size on the diameter-to-height ratio (reflecting contact angle) to optimize the preparation process of flexible prepreg yarns.

Effect of fiber surface wettability

Three types of prepreg yarns (epoxy-sized, desized, and desized-activated fibers) were fabricated under identical processing conditions (Table 2). Microbeads with heights ranging from 150 to 210 μm were selected for dimensional analysis (Table 3). The measured average diameter-to-height ratios were 0.84 for epoxy-sized fibers, 0.83 for desized fibers, and 0.80 for desized-activated fibers. These quantitative results demonstrate that PEEK displays the largest contact angle when deposited on epoxy-sized fibers, an intermediate contact angle on desized fibers, and the smallest contact angle on desized-activated fibers.

Table 2.

Effect of carbon fiber surface wettability on microbead dimensions.

Magnification	Different CF
Magnification	PEEK microbeads formed on epoxy-size filament (390°C)	PEEK microbeads formed on desized filament (390°C)	PEEK microbeads formed on desized-activated filament (390°C)
50×
100×
200×

Table 3.

Influence of fiber surface wettability on microbead size distribution.

Sample number	Epoxy-size CF			Desized CF			Desized-activated CF
Sample number	Microbeads height (μm)	Microbeads diameter (μm)	Aspect ratio (D/H)	Microbeads height (μm)	Microbeads diameter (μm)	Aspect ratio (D/H)	Microbeads height (μm)	Microbeads diameter (μm)	Aspect ratio (D/H)
1	158.71	134.09	0.84	158.69	134.07	0.84	154.58	119.02	0.77
2	162.81	136.83	0.84	158.71	129.96	0.82	155.97	123.12	0.79
3	164.16	138.17	0.84	159.40	132.16	0.83	158.71	121.78	0.77
4	168.26	139.54	0.83	161.45	136.83	0.85	161.45	127.25	0.79
5	173.76	147.77	0.85	164.87	136.32	0.83	169.65	136.80	0.81
6	176.49	145.01	0.82	173.93	147.80	0.85	179.23	145.01	0.81
7	180.57	152.00	0.84	175.15	149.12	0.85	180.66	146.40	0.81
8	183.31	153.27	0.84	177.11	145.85	0.82	183.32	151.87	0.83
9	183.31	155.96	0.85	180.60	149.14	0.83	186.05	152.00	0.82
10	183.33	153.21	0.84	180.60	153.21	0.85	190.15	145.11	0.76
11	190.17	161.45	0.85	181.96	146.40	0.80	192.89	153.31	0.79
12	191.52	161.52	0.84	183.31	150.48	0.82	192.89	158.71	0.82
13	192.91	164.16	0.85	184.70	151.87	0.82	198.38	161.45	0.81
14	192.96	161.42	0.84	188.83	151.87	0.80	198.38	161.47	0.81
15	193.01	161.52	0.84	189.06	156.92	0.83	199.05	159.24	0.80
16	195.64	167.17	0.85	190.00	157.70	0.83	200.70	160.56	0.80
17	195.66	164.35	0.84	190.15	156.05	0.82	201.10	158.71	0.79
18	196.00	164.64	0.84	204.36	170.32	0.83	202.46	161.45	0.80
19	201.11	164.16	0.82	157.34	127.25	0.81	185.29	221.44	0.84
20	205.20	174.00	0.85	156.05	132.72	0.85	175.07	209.15	0.84
Mean	/	/	0.84	/	/	0.83	/	/	0.80
Coefficient of variation (CV)	/	/	1.03%	/	/	1.86%	/	/	2.64%

The findings demonstrate that fiber surface wettability significantly affects microbead morphology. Enhanced wettability corresponds to reduced contact angles and flatter microbead geometries.

Effect of molding temperature

Four flexible prepreg yarns were fabricated using desized carbon fibers at distinct molding temperatures (390°C, 405°C, 420°C, and 435°C), as detailed in Table 4. Microstructural analysis was performed on microbeads within the height range of 110-160 μm, where both height and diameter measurements were conducted to calculate the diameter-to-height ratios (Table 5). The experimental results demonstrated consistent diameter-to-height ratios across all processing temperatures: 0.82 (390°C), 0.83 (405°C), 0.83 (420°C), and 0.83 (435°C). Statistical evaluation confirmed no significant differences (p > .05) in the microbead morphology parameters among the four prepreg yarn specimens.

Table 4.

Impact of molding temperature on microbead dimensional characteristics.

Temperature	Magnification
Temperature	50×	100×	200×
390°C
405°C
420°C
435°C

Table 5.

Statistical analysis of temperature effects on microbead morphology.

Sample number	390°C			405°C			420°C			435°C
Sample number	Microsphere height (μm)	Microsphere diameter (μm)	Aspect ratio (D/H)	Microsphere height (μm)	Microsphere diameter (μm)	Aspect ratio (D/H)	Microsphere height (μm)	Microsphere diameter (μm)	Aspect ratio (D/H)	Microsphere height (μm)	Microsphere diameter (μm)	Aspect ratio (D/H)
1	112.21	88.92	0.79	112.18	90.29	0.80	113.58	93.02	0.82	112.31	93.06	0.83
2	114.91	93.06	0.81	121.78	99.90	0.82	113.58	93.11	0.82	114.94	95.76	0.83
3	119.02	95.77	0.80	127.22	101.23	0.80	114.91	93.06	0.81	117.68	95.76	0.81
4	121.75	98.50	0.81	127.23	105.65	0.83	117.65	98.50	0.84	119.05	99.87	0.84
5	123.13	102.60	0.83	134.06	109.58	0.82	117.65	98.50	0.84	124.49	102.64	0.82
6	124.49	102.61	0.82	134.07	112.21	0.84	119.05	97.14	0.82	124.49	105.37	0.85
7	125.85	102.61	0.82	134.09	110.94	0.83	123.15	98.53	0.80	127.25	102.64	0.81
8	127.25	105.37	0.83	134.09	109.47	0.82	124.49	102.64	0.82	127.25	102.60	0.81
9	127.25	102.60	0.81	134.13	112.18	0.84	128.66	108.11	0.84	132.69	109.47	0.83
10	129.99	109.65	0.84	136.80	114.94	0.84	129.96	105.34	0.81	134.09	112.18	0.84
11	132.69	109.47	0.83	136.83	112.21	0.82	134.07	112.21	0.84	135.43	112.18	0.83
12	132.72	110.88	0.84	139.56	117.68	0.84	136.81	116.28	0.85	136.80	114.91	0.84
13	134.17	112.21	0.84	139.56	117.68	0.84	136.83	112.18	0.82	136.83	115.04	0.84
14	136.97	112.31	0.82	149.11	124.61	0.84	142.27	120.41	0.85	138.19	119.02	0.86
15	140.93	117.65	0.83	149.11	124.67	0.84	145.03	120.38	0.83	140.93	114.92	0.82
16	142.30	114.94	0.81	149.11	124.49	0.83	146.40	124.49	0.85	142.30	120.51	0.85
17	146.37	123.19	0.84	150.58	124.49	0.83	149.21	121.78	0.82	145.03	117.65	0.81
18	149.11	123.12	0.83	151.87	128.71	0.85	150.48	124.61	0.83	154.73	129.99	0.84
19	154.58	119.02	0.77	155.96	132.81	0.85	153.21	131.36	0.86	147.16	176.51	0.83
20	155.97	123.12	0.79	160.05	136.81	0.85	145.10	176.46	0.82	158.04	188.72	0.84
Mean	/	/	0.82	/	/	0.83	/	/	0.83	/	/	0.83
CV	/	/	2.31%	/	/	1.71%	/	/	1.89%	/	/	1.71%

The data suggests that molding temperature has negligible influence on microbead morphology. Although higher temperatures increase melt surface tension, only the contact angle at solidification (glass transition temperature) is preserved. Since PEEK’s Tg remains constant across these processing temperatures, the final microbead characteristics show minimal variation.

Effect of microbead size

Experimental observations revealed an inverse correlation between microbead size and diameter-to-height ratio (contact angle). The theoretical foundation for this phenomenon can be explained through Young-Laplace equation analysis. While the Young-Laplace equation inherently assumes infinitesimal droplet dimensions (neglecting gravitational effects), thereby theoretically predicting size-independent contact angles, our experimental results demonstrate measurable contact angle variations with droplet size. This apparent contradiction originates from the additional pressure (termed Young-Laplace pressure), mathematically expressed as:

Δ P = γ (\frac{1}{R_{1}} + \frac{1}{R_{2}})

(2)

where ΔP represents the additional pressure arising from surface tension-induced curvature; γ denotes the liquid’s surface tension coefficient (units: N/m); R₁ and R₂ correspond to the principal radii of curvature of the liquid surface.

The Young-Laplace pressure equation demonstrates that decreasing the principal radii of curvature (R₁ and R₂) leads to increased ΔP. This enhanced pressure promotes molecular spreading at the fiber-liquid interface, with the following mechanistic consequences: Microbeads with smaller dimensions exhibit proportionally reduced curvature radii, which leads to significantly increased Laplace pressure (ΔP) that effectively drives enhanced surface wetting behavior, ultimately resulting in smaller microbeads displaying characteristically flatter morphologies with measurably reduced contact angles. Therefore, in experimental practice, it is essential to select appropriate droplet sizes and maintain controlled environmental conditions to ensure result reliability.

Performance analysis of prepreg yarns

Cross-sectional Area Calculation Formula for Prepreg Yarns:

S = S_{1} + S_{2} = \frac{t_{1}}{ρ_{1}} + \frac{t_{2}}{ρ_{2}}

(3)

where: A - Cross-sectional area of prepreg yarn (unit: mm²); t₁, t₂ - Linear density of fiber and matrix material respectively (unit: g/km); ρ₁, ρ₂ - Density of fiber and matrix material respectively (unit: kg/m³).

Strength Calculation Formula for Prepreg Yarns:

σ = \frac{P}{S}

(4)

where: σ - Strength of prepreg yarn (unit: MPa); P - Maximum load (unit: N); S - Cross-sectional area of prepreg yarn (unit: mm²). The calculation of arithmetic mean, standard error and coefficient of variation (CV) shall comply with GB/T 1446.

Tensile testing of prepreg yarns

Figure 9 illustrates the force-displacement curves obtained from the tensile testing of prepreg yarns, with the five distinct curves representing the results from five individual test specimens. The experimental data reveal two significant observations: (1) a well-defined positive correlation exists between the tensile load and corresponding displacement, and (2) all five specimens demonstrate nearly identical mechanical response trends. These results quantitatively confirm that the material exhibits linear elastic deformation behavior under external tensile loading, with the deformation magnitude being directly proportional to the applied force within the elastic regime.

Figure 9.

Tensile strength performance of prepreg yarns.

The tensile property test data of prepreg yarns are summarized in Table 6. Comprehensive analysis of the experimental results reveals that the prepreg yarns exhibit an average maximum tensile force of 1219.14 N (CV = 3.79%), demonstrating excellent load-bearing consistency. The materials show outstanding tensile performance with an average tensile strength of 1354.60 MPa (CV = 3.79%) and an average tensile modulus of 13507.14 MPa (CV = 23.88%). Furthermore, the average fracture strain reaches 6.69% (CV = 28.25%), indicating both high deformability and relatively stable failure characteristics. These quantitative results systematically characterize the mechanical behavior of prepreg yarns under tensile loading conditions.

Table 6.

Tensile test results of prepreg yarns.

Parameter	Maximum force (N)	Tensile strength σM (MPa)	Tensile modulus Et (MPa)	Fracture tensile strain εB (%)
1#	1285.06	1427.84	18657.00	2.94
2#	1192.31	1324.79	11847.05	7.82
3#	1185.38	1317.09	15864.02	7.18
4#	1263.59	1403.99	10374.74	7.81
5#	1169.35	1299.28	10792.91	7.70
Mean	1219.14	1354.60	13507.14	6.69
Standard deviation (SD)	46.17	51.30	3225.24	1.89
Coefficient of variation (CV)	3.79%	3.79%	23.88%	28.25%

Knot strength testing of prepreg yarns

The force-displacement curves from knot strength testing are presented in Figure 10, showing four distinct loading profiles corresponding to individual knotted specimens. While the average maximum tensile force values demonstrate good consistency (199.82 ± 31.97 N, CV = 0.16), the deformation processes exhibit notable variations, potentially attributable to differences in initial knot tightness. This observation warrants further experimental verification.

Figure 10.

Knot strength behavior of prepreg yarns.

Table 7 presents the knot strength test results of the prepreg yarns. Statistical analysis of the experimental data indicates the knotted specimens exhibit an average maximum tensile force of 199.82 N (CV = 16.21%), and an average tensile strength of 222.02 MPa (CV = 16.21%).

Table 7.

Knot strength test results of prepreg yarns.

Parameter	Maximum (N)	Tensile strength σM (MPa)
1#	233.01	258.90
2#	175.31	194.79
3#	230.51	256.12
4#	160.44	178.27
Mean	199.82	222.02
Standard deviation (SD)	32.38	35.98
Coefficient of variation (CV)	16.21%	16.21%

Loop strength testing of prepreg yarns

Figure 11 presents the force-displacement curves obtained from loop strength testing of prepreg yarns, where five distinct curves correspond to five individual looped specimens. The experimental results demonstrate highly consistent loading profiles with minimal variation, as evidenced by the closely clustered curves. This observation indicates excellent reproducibility in the loop strength performance of the tested materials.

Figure 11.

Loop strength characteristics of prepreg yarns.

Quantitative analysis of the loop strength test data (Table 8) reveals an average maximum tensile force of 350.05 N (CV = 11.32%) and an average tensile strength of 388.94 MPa (CV = 11.32%). The low variation coefficients (≤11.32%) confirm superior mechanical consistency under looped configurations, highlighting the material’s structural reliability.

Table 8.

Loop strength test data of prepreg yarns.

Parameter	Maximum (N)	Tensile strength σM (MPa)
1#	285.57	317.31
2#	349.81	388.67
3#	358.01	397.79
4#	346.60	385.11
5#	410.25	455.83
Mean	350.05	388.94
Standard deviation (SD)	39.64	44.04
Coefficient of variation (CV)	11.32%	11.32%

The tensile, knot, and hook strength measurements of the prepreg yarn confirm its flexibility satisfies the requirements of textile processing, including weaving, braiding and so on. Based on the prepreg yarn (with 50% resin weight content), we trial-woven a three-layer 2.5D fabric. The woven fabric is shown in Figure 12, further demonstrating the weavability of this prepreg yarn.

Figure 12.

Weavable experiment with this prepreg yarn.

The developed flexible thermoplastic prepreg yarns exhibit three significant advancements: (1) stable mechanical performance in straight, knotted, and looped states, (2) successful resolution of the brittleness limitation inherent in conventional thermoplastic prepregs, and (3) full compatibility with textile manufacturing processes (weaving, braiding, and knitting). This technological breakthrough addresses two critical challenges: thermoplastic resin impregnation difficulties and textile processing compatibility. The achieved combination of mechanical robustness and processing flexibility establishes a fundamental platform for developing multidimensional thermoplastic composite structures, representing a substantial advancement in flexible composite materials technology.

Conclusions

The flexible thermoplastic composite prepreg yarns developed in this study demonstrate excellent compatibility with textile manufacturing processes, including weaving, braiding, and knitting, enabling the fabrication of 2D and 3D thermoplastic composite structures with enhanced adaptability to various service conditions. Furthermore, this technology exhibits broad applicability to different reinforcing fibers and thermoplastic matrices, allowing for material and structural modifications to meet diverse application requirements. When combined with subsequent textile processes, this approach facilitates the application of thermoplastic composites in load-bearing components, with significant potential in aerospace, rail transportation, sports equipment, and construction industries. Thermoplastic composite materials can be recycled and melt reshaped, providing great potential for reducing the lifecycle carbon footprint compared to thermosetting composite materials.

Key technological advancements include:

(1) The “bead-on-string” prepreg yarn design effectively addresses the challenges associated with thermoplastic resin impregnation and prepreg yarn weaving.

(2) The strength and toughness of the bead-on-string prepreg yarns satisfy the requirements for multidimensional textile manufacturing, providing a viable strategy for developing multidimensional thermoplastic composites.

(3) The surface wettability of carbon fibers significantly influences microbead morphology, offering a potential approach to control microbead geometry and load transfer within composites.

(4) Molding temperature shows negligible effects on microbead morphology.

These findings establish a fundamental framework for expanding the structural complexity and application scope of thermoplastic composites, while addressing critical processing challenges in conventional thermoplastic composite manufacturing.

Supplemental Material

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Jing Li

Supplemental Material

Supplemental material for this article is available online.

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Supplementary Material

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Design and manufacturing of weavable CF/PEEK prepreg yarns based on the Plateau–Rayleigh instability mechanism

Abstract

Keywords

Highlights

Introduction

Experimental procedure

Materials

Design and manufacturing of flexible CF/PEEK prepreg yarns

Design of flexible CF/PEEK prepreg yarns

Manufacturing of prepreg yarns

Shape control of microbeads on prepreg yarns

Effect of fiber surface wettability on microbead size

Effect of molding temperature on microbead size

Mechanical properties of flexible CF/PEEK prepreg yarns

Tensile strength of flexible CF/PEEK prepreg yarns

Knot strength of flexible CF/PEEK prepreg yarns

Loop strength of flexible CF/PEEK prepreg yarns

Results and discussion

Analysis of material properties

Analysis of Influencing Factors on Microbead Regulation

Effect of fiber surface wettability

Effect of molding temperature

Effect of microbead size

Performance analysis of prepreg yarns

Tensile testing of prepreg yarns

Knot strength testing of prepreg yarns

Loop strength testing of prepreg yarns

Conclusions

Supplemental Material

Footnotes

Declaration of conflicting interests

Funding

ORCID iD

Supplemental Material

References

Supplementary Material