How to Maximize the Mechanical and Conductive Properties of Recycled Carbon Fiber?

 

 

As the demand for carbon fiber reinforced polymers (CFRP) increases, the environmental pressure of CFRP waste is also increasing. Therefore, recycling and reusing CFRP waste has important environmental and economic benefits.

 

At present, rCF is usually used directly as a filling material in the fields of construction, transportation, sports, etc. rCF is a typical anisotropic reinforcement material with better axial performance than radial performance. To make full use of the axial properties of rCF, aligning rCF is a key step in manufacturing high-performance composites.

 

01 Research Background

 

At present, common fiber alignment methods include dry alignment and wet alignment technology. Dry alignment is usually achieved by using magnetic fields, sound waves and spinning processes. This method has high fiber alignment efficiency, but low alignment quality.

 

A series of studies have shown that the wet alignment process is more suitable for the preparation of rCF aligned mats. Therefore, on the basis of the wet laying process, by optimizing the alignment process parameters, rCF fiber mats with good alignment can be prepared, and then rCF aligned fiber mat reinforced composites with excellent performance can be obtained.

 

rCF not only has the good mechanical properties of vCF, but also exhibits excellent electrical conductivity, which means that rCF has the potential to become a candidate material for the remanufacturing of high-performance composites with electromagnetic interference (EMI) protection. However, there is little research on the effect of the ply structure of rCF fiber mats on EMI shielding performance.

 

In this study, rCF fiber mats with different alignment degrees were selected to obtain rCF fiber mat composites with different electrical conductivity, fiber contact degree and interlayer deflection angle. These differences may lead to different interaction processes of electromagnetic waves within rCF mat composites. Therefore, it is of great significance to study the effect of different ply structures on the EMI shielding performance of rCF mat-reinforced composites.

 

rCF produced from recycling processes is usually in a fluffy and disordered state, which makes it challenging to directly utilize the excellent axial properties of these fibers. In addition, there is a lack of research on the correlation between rCF alignment and process parameters.

 

In this study, a wet alignment device for rCF was established, which enabled the preparation of highly aligned rCF fiber mats. This study clarifies the correlation between rCF alignment and process parameters, thereby achieving optimized and controllable rCF alignment.

 

Taking the composite manufacturing process as a carrier, the anisotropy of the aligned rCF fiber mat was used for the ply structure design, and the synergistic improvement of the mechanical properties and functional properties of rCF remanufactured composites was achieved. These findings not only reflect the effective use of rCF, but also demonstrate the potential for improving performance through structural and process optimization. This provides a new perspective and scientific basis for the high-value recycling and application of rCF.

 

02 Research Preparation

 

2.1 Preparation of rCF fibers

 

As shown in Figure 1A, the rCF alignment device consists of an air pressure control unit and an alignment unit, and the alignment principle is shown in Figure 1B. The wet rCF aligned mat is vacuum filtered and then placed in a box oven for drying to obtain rCF fibers.

 

 

2.2 Preparation of rCF RP

 

Figure 2A describes the preparation process of rCF aligned mats. Under the optimal process parameters, four rCF aligned mats with alignment angles of 0°, 45°, 90° and 135° were prepared. The rCF aligned mats with different alignment angles were combined in different layup orders to prepare composite materials. The five layup orders were 0°, 0° + 45°, 0° + 90°, 0° + 45° + 90° + 135° and 0° + 90° + 45° + 135°.

 

 

The preparation process of rCF alignment felt reinforced composites is shown in Figure 2B. Three methods were used to produce rCF alignment felt reinforced composites, namely hand lay-up combined with hot pressing, hand lay-up combined with vacuum infusion, and vacuum infusion. By comparing the advantages and disadvantages of the three processes, a process suitable for rCF alignment felt was selected.

 

03 Research Results

 

3.1 The optimal alignment parameters of rCF alignment felt were determined, including convex nozzle shape, 3 mm nozzle diameter, 50 mm nozzle channel length, 7:1 nozzle upper and lower diameter ratio, and 20 mm nozzle standoff height. These settings achieved a preferred alignment degree (PAD) of 93.07%.

 

3.1.1 Effect of Nozzle Geometry on PAD

 

Figures 3A-C show three different geometric shapes of alignment nozzles made by 3D printing technology. Except for the nozzle geometry, other dimensional parameters are the same. Optical microscope (OM) images of rCF alignment pads prepared using three nozzle shapes are shown in Figures 3E and F. The second-order alignment tensor and alignment ellipsoid plots were selected to further characterize the alignment effects of the three rCF alignment mats, see Figure 3G-I.

 

 

3.1.2 Effect of Nozzle Diameter on PAD

 

Figure 4A shows the effect of nozzle diameter on the PAD of rCF alignment mat.

 

As the nozzle diameter increases, the PAD increases first and then decreases. When the nozzle diameter is 3 mm, the fibers in the rCF alignment mat are well aligned. When the nozzle diameter is greater than 4 mm, the fibers in the rCF alignment mat are disordered. Therefore, during the rCF alignment process, the nozzle diameter should not exceed 3 mm and should not be less than 2 mm.

 

 

3.1.3 Effect of Upper and Lower Nozzle Diameter Ratio and Nozzle Channel Length on PAD

 

Figure 5A shows the effect of the change of the upper and lower nozzle diameter ratio on the diameter and length of the nozzle channel on PAD. As the nozzle diameter ratio increases, the PAD increases first and then decreases. The OM diagram of the rCF alignment felt is shown in Figure 5C. As the nozzle channel length increases, the PAD also shows a trend of increasing first and then decreasing.\

 

 

3.1.4 Effect Of Nozzle Height From Platform On PAD

 

Figure 6A shows the effect of nozzle height from platform on PAD of rCF alignment mat. As nozzle height increases, PAD increases first and then decreases. The OM of rCF alignment mat is shown in Figure 6B, and its orientation ellipse is elongated, indicating that the fibers are well aligned.

 

 

3.2 The rCF-0/EP composite material exhibited the highest tensile strength (195.45 MPa), while the rCF-04/EP composite material not only exhibited the highest flexural strength (247.95 MPa), but also exhibited excellent EMI performance (47.97 dB).

 

3.2.1 Mechanical Properties of rCF oriented felt reinforced composites

 

Table 2 shows the mechanical properties of the composites prepared by three different processes.

(Table 2. Performance comparison of three processes)

The comparison of the tensile and flexural properties of pure epoxy resin (EP), randomly arranged rCF pad reinforced composites (rCF-R/EP) and 0° arranged rCF pad reinforced composites (rCF-0/EP) is shown in Table 3 . Compared with EP, rCF can significantly improve the mechanical properties of EP composites and expand the application scope of EP materials.

 

Compared with rCF-R/EP composites, the tensile strength and flexural strength of rCF-0/EP composites were increased by 104.3% and 53.87%, respectively. In rCF aligned mat-reinforced composites, most of the fibers are aligned along the axis, whereas in rCF random mat-reinforced composites, the fibers are randomly distributed. Therefore, the mechanical properties of rCF-0/EP composites are better than those of rCF-R/EP composites.

 

It was shown that rCF aligned mats with a reasonable layup structure can provide excellent mechanical properties.

 

Figure 8A-E shows schematic diagrams of rCF aligned mat reinforced composites with five different ply structures, namely 0°, 0° + 45°, 0° + 90°, 0° + 45° + 90° + 135° and 0° + 90° + 45° + 135°. Figure 8F,G shows the effect of different ply structures on the mechanical properties along the x-direction. It can be seen from the figure that rCF aligned mats with different ply structures are beneficial to improving the flexural strength of the composite material, and the ply order of the rCF arrangement mat is the main factor affecting the flexural strength.

 

 

 

Figure 9 shows the cross-sectional microstructure of the rCF-0/EP, rCF-09/EP, and rCF-0491/EP flexural specimens. SEM analysis of the cross-sections of the composite specimens with different layup structures showed the presence of small holes caused by fiber pullout.

 

This indicates that the adhesion between the fiber and the epoxy resin needs to be strengthened. Insufficient interfacial adhesion directly affects the overall mechanical properties of the composite.

 

 

 

3.2.2 Conductive Properties Of Composites

 

The effect of different ply structures on the conductivity (σ) of rCF aligned mat reinforced composites is shown in Figure 10, where the fiber alignment of rCF-9/EP composites is perpendicular to the test current conduction direction. The bonding state of rCF in the composite is the main factor affecting the conductivity.

 

In rCF-9/EP composites, the arrangement direction of rCF is mostly perpendicular to the conduction current, so it is difficult to form a complete conductive network. However, in rCF-0/EP composites, the arrangement direction of rCF is mostly parallel to the conduction current, with good conductivity, σ is 2.28S/m.

 

Compared with rCF-0/EP composites, the conductivity of rCF-04/EP composites, rCF-09/EP composites, rCF-0491/EP composites, and rCF-0914/EP composites increased by 93.45%, 35.53%, 78.51%, and 112.72%, respectively. This is mainly because most of the rCF in the composite material is in a cross-lap state, which can effectively form a conductive network and further exert the excellent conductive properties of rCF.

 

 

 

3.2.3 EMI Shielding Performance Of rCF Oriented Mat Reinforced Composites

Total shielding effectiveness (SET) (SEET) is an effective parameter to measure the ability of CFRP to attenuate electromagnetic waves. The EMI shielding mechanism is represented by the reflection coefficient (R) and the absorption coefficient (A). The reflection mechanism mainly depends on the conductivity of the composite material. The absorption mechanism depends on the interaction between the EMI and rCF of the composite material and the incident electromagnetic wave.

 

Figure 11A shows the total shielding effectiveness (SET) graph of rCF aligned mats with five different layup structures. The results show that the SEET can improve the performance of the composite material by orderly variation of the interlayer deflection angle. Figure 11B shows the R and A graphs of five composite materials with different layup structures. This shows that the ordered layup design of rCF aligned mats in composite materials helps to achieve higher R and lower A, and reduce the secondary electromagnetic pollution caused by reflected electromagnetic waves, while improving the SET of the composite material. Therefore, at the same thickness and the same rCF content, the layup structure of rCF oriented mat reinforced composite materials is the main factor affecting their performance.

 

 

Figure 12A shows the layup structure of the rCF-0491/EP composite. As shown in Figure 12B, due to the impedance mismatch of electromagnetic waves passing through the composite, the reflection mainly occurs on the surface of the composite.

 

 

 

04 Conclusion

The mechanical properties and EMI shielding efficiency of composites can be effectively improved by strategically optimizing the arrangement and laminate structure design of rCF mats. Future improvements will involve enhancing the bonding performance between the fiber and the resin matrix, studying the mechanical properties and EMI shielding efficiency of composites with different rCF volume fractions, and combining rCF with other electromagnetically responsive materials to improve the wave absorption efficiency of composites.

 

05 Literature Information

The researchers are from Nanjing Institute of Technology and the School of Mechanical Engineering of Hefei University of Technology. The research results were recently published in Polymer Composites under the title “Investigation of alignment and layup optimization of recycled carbon fiber mats on mechanical and electromagnetic shielding properties of composites”.

 

The research work was financially supported by the Jiangsu Key R&D Program (BE2021709), the Humanities and Social Sciences Foundation of the Ministry of Education of China (23YJCZH024), the National Natural Science Foundation of China (51705237), the Qinglan Project, and the Jiangsu Postgraduate Research and Practice Innovation Program (SJCX23_1179).