Recent developments in the processing of waste carbon fibre for thermoplastic composites

Abstract

The aim of this paper is to highlight recent developments in the processing of waste carbon fibre for thermoplastic composites. Initially, injection moulding and nonwoven technologies have been used to integrate waste carbon fibres into fibre-reinforced thermoplastic composites. Recently, tape and hybrid yarn spinning technologies have been developed to produce tape and hybrid yarn structures from waste carbon fibre, which are then used to manufacture recycled carbon fibre-reinforced thermoplastics with much higher efficiency. The hybrid yarn spinning technologies enable the development of various fibrous structures with higher fibre orientation, compactness and fibre volume fraction. Therefore, thermoplastic composites manufactured from hybrid yarns possess a good potential for use in load-bearing structural applications. In this paper, a comprehensive review on novel and existing technologies employed for the processing of waste carbon fibre in addition to different quality aspects of waste carbon fibre is presented.

Keywords

Waste carbon fibres polymer matrix composites recycled carbon fibre-reinforced plastics processing technologies hybrid yarn thermoplastic composites

Introduction

Carbon fibre has a very high tensile strength and modulus, good compressive strength, low density, favourable electrical and heat conductivity, low thermal expansion, beneficial chemical and thermal resistance and great electromagnetic wave shielding properties. Therefore, it is widely used in different fields, e.g. aerospace, automotive, wind turbines, sports and medical equipment, in the form of carbon fibre-reinforced plastics (CFRP).^1,2 CFRP applications in these fields have been extended throughout the past few decades. Consequently, the global production of carbon fibre is steadily increasing as well and will reach up to 120.5 ktons in 2022.³

In spite of the steady growth in CFRPs, the problem associated with this fibre involves its waste management, which originates from the composite industry and end-of-life CFRP components. Also, ecological aspects, limited landfill capacity, new legislations and higher production costs for virgin carbon fibre are the driving factors towards recycling and processing of waste carbon fibres.^4–6 Therefore, different recycling methods were developed to reclaim carbon fibre from end-of-life CFRP components, which can be broadly classified as mechanical recycling, thermal recycling (pyrolysis) and chemical recycling (solvolysis).^6–8

Waste carbon fibre from different sources is being used for the manufacturing of fibre-reinforced plastics with thermoplastic and thermoset polymers. Thermoplastic polymers have gained attention as a result of their distinctive advantages over thermoset polymers. These advantages include low density, good storage capability and the availability of the ready-to-use semi-finished product. Additionally, thermoplastic polymers provide less curing complexity, higher production capabilities, ease of recycling and less hazardous chemical emissions during the curing stage. Consequently, the integration of waste carbon fibre into thermoplastic polymers has become an emerging trend in both academia and industry. A number of review papers and studies have been published on the recycling of waste carbon fibre and its integration into thermoset polymer through sheet moulding compound, bulk moulding compound and resin transfer moulding techniques.^6–12 In contrast, no review paper has been reported on the processing of waste carbon fibre for fibre-reinforced thermoplastic composites. Therefore, this paper focuses on technologies for the processing of waste carbon fibre.

Conventionally, injection moulding and nonwoven technologies are used to integrate waste carbon fibre into fibre-reinforced thermoplastic composites. In injection moulding, short recycled carbon fibre is mixed with a thermoplastic matrix and injected into the moulding to develop thermoplastic composites. In contrast, different wet and dry processes are employed for isotropic and anisotropic structures by using thermoplastic fibres in nonwoven technologies, which are then subjected to compression moulding so as to manufacture thermoplastic composites. These composites produced by conventional techniques possess limited performance capacity, low fibre volume content and poor fibre orientation.^12,13

In order to enhance the performance of thermoplastic composites based on waste carbon fibre, tape and hybrid yarn spinning technologies have been introduced in the last decade. The tape or tow structures were reported at the University of Bristol,^14,15 University of Manchester¹⁶ and German Institutes of Textile and Fibre Research (DITF).¹⁷ In addition, the hybrid yarn structure was initially reported at the University of Leeds¹⁸ and the Institute of Textile Machinery and High Performance Material Technology (ITM), TU Dresden.¹⁹ These fibrous structures are subjected to a consolidation process to manufacture recycled carbon fibre-reinforced thermoplastics. The composites produced from hybrid yarn structures exhibit enhanced tensile properties compared to composites based on conventional technologies. The aim of this review is to highlight recent developments in the processing of waste carbon fibre for thermoplastic composites. Moreover, different quality aspects of waste carbon fibre in relation to different recycling techniques are also discussed. Lastly, the mechanical properties of composites developed from these recent technologies are compared with existing technologies.

Market size and volume of carbon fibre and waste

In 2017, the global demand for carbon fibres and CFRP amounted to approximately 70.5 ktons and 114.7 ktons, respectively, generating revenues of approximately US $2.59 and US $14.73 billion, respectively. Major consumption sectors include aerospace, defence, automotive, wind energy, sports equipment and construction industries. The demand for carbon composites has increased tremendously in the automotive and aviation industries compared to other sectors; this is due to the particular lightweight potential of carbon fibres.^20–22 The higher demand of CFRPs causes the composite industry to generate more waste carbon. Thus, it generates approximately 30 to 40% of the total production waste of CFRP. Based on reports, both the USA and UK are generating around 3 ktons waste carbon fibre per year within their composite industries. Within the upcoming two decades, industries such as aerospace, defence, automotive and wind energy are expected to produce massive waste in the category of end-of-life components. In Europe alone, 6000 to 8000 commercial planes are going to retire by 2030. In 2040, 30,000 tons of end-of-life waste will be produced by the energy sector in the form of rotor blades every year in Germany.^23–25 The global demand for carbon fibres and the estimated carbon fibre waste originating from the composite industry and end-of-life components are given in Figure 1.^3,26

Types of waste carbon fibre and their characteristics

Waste carbon fibre can originate from two main sources. The first major source is the composite industry. It generates two types of waste termed as dry waste and wet waste. The dry waste comes from production offcuts, fabric selvedges and bobbin ends. The dry waste has mechanical properties that are identical with those of virgin carbon fibre and can be used directly without any recycling treatment. The wet waste comes from composite industry as well, e.g. in the form of out-of-date prepreg rolls. This waste is contaminated with resin and subjected to recycling treatment prior to reuse. The second major source of carbon fibre waste is retired end-of-life fibre-reinforced plastics components, as for example aircrafts and wind turbine blades. This type of waste is also contaminated with resin and requires recycling treatment prior to processing.^6,27

In order to distinguish between different types of waste carbon fibres, fibres obtained from dry waste, wet waste and end of life-of-life components waste are referred to as Type-I, Type-II and Type-III fibres, respectively, in this paper.

In order to reuse waste fibres, mechanical and fibre reclamation technologies (pyrolysis and solvolysis) are employed for recycling. In mechanical recycling, any type of waste carbon (Type-I, Type-II and Type-III) fibres can be subjected to mechanical treatment. Generally, Type-I waste fibres are converted into short chopped fibres (e.g. 3 mm) or milled fibres (e.g. 80 µm). In contrast, Type-II and Type-III wastes that originate from end-of-life components are converted into small pieces by cutting. Subsequently, they are further processed into very short fibres or fibre fragments (e.g. 50 µm to 400 µm) by means of shredding, crushing and milling processes. Hence, the mechanical technology yields recycled carbon fibres in the form of short chopped fibres, milled fibres or fibre fragments. These recycled fibres can be used as fillers and reinforcement in composites.^6,22,27

In fibre reclamation technology, pyrolysis and solvolysis treatments are used for Type-II and Type-III waste fibres. In pyrolysis, waste carbon fibres are exposed to higher temperatures (450℃ to 600℃) with or without medium (oxygen). The temperature selection depends on the type of resin present in the waste. The temperature degrades the matrix, and consequently, fibres are separated from it. In case of solvolysis, carbon waste is treated with a solvent to degrade the matrix under temperature and pressure conditions in the presence of co-solvents, catalysts and additives. The selection of main solvent, co-solvent, catalysts and additives depends on the type of matrix present in carbon waste.^6,27

Currently, mechanical, pyrolysis and solvolysis technologies are exercised commercially. The companies employing these technologies are ELG carbon fibre, CFK valley, SGL group, Hadeg recycling, Adherent technologies, Material innovation technologies, CFR, Vartega and Karborek. These recycling technologies yield different qualities of waste carbon fibres. These quality characteristics play a vital role in the performance of the composite and are measured in terms of fibre length, strength, purity and sizing. Therefore, the characteristics of waste carbon fibre in relation to recycling techniques are discussed in this section.

The length of waste carbon fibres is an important aspect and can be classified into three main categories, i.e. short (<6 mm), medium (6–25 mm) and long (25–300 mm). The waste carbon fibres termed as Type-I can be cut into any desired fibre length. In contrast, the lengths of Type-II and Type-III waste fibres significantly depend on the recycling technique. Generally, mechanical recycling produces short fibres in the form of powder or short fibre fragments, whereas pyrolysis or solvolysis recycling yields random fibre lengths.¹² The determination of carbon fibre lengths is also important because technologies employed for the processing of waste carbon fibres are dependent on fibre length. Particularly in terms of emerging technologies, fibre length becomes an increasingly important aspect as different technical settings (e.g. roller clearance) depend on mean fibre length.²⁸ Unfortunately, existing fibre measuring systems, e.g. high volume instrument and Fibrograph, are unable to measure the length of carbon fibre due to its conductive nature. Therefore, a new approach was introduced to measure the mean and upper half mean lengths of waste carbon fibres.²⁹

The tensile strength of waste carbon fibre is also a crucial characteristic and depends on fibre type and recycling treatment. Generally, Type-I waste fibre has the same strength as virgin carbon fibre. In contrast, recycling treatments, e.g. pyrolysis or solvolysis, reduce the strength of waste fibres depending on the recycling conditions, such as temperature, pressure etc.³⁰ The effect of recycling techniques on the strength of waste carbon fibre is presented in Table 1. Results revealed that solvolysis treatment has less impact on tensile strength as compared to pyrolysis treatment.

Table 1.

Influence of recycling technique on fibre strength.

References	Technique	Fibre type	Strength (%)
Pimenta³⁰	Pyrolysis	Type-II	96
Nahil³¹	Pyrolysis	Type-II	90
Greco et al.³²	Pyrolysis	Type-III	86–88
Stoeffler et al.³³	Pyrolysis	Type-III	90
Holmes³⁴	Pyrolysis	Type-II	90
Pinero-Hernanz et al.³⁵	Solvolysis	Type-II	90–98
Jiang et al.³⁶	Solvolysis	Type-II	97
Bai et al.³⁷	Solvolysis	Type-III	95–96
Maa et al.³⁸	Solvolysis	Type-II	96
Okajima and Sako³⁹	Solvolysis	Type-III	97

The fibre purity and sizing of waste carbon fibres are also considered essential characteristics. Fibre purity refers to the amount of resin residuals remaining on the surface of waste carbon fibre after recycling. Generally, pyrolysis and solvolysis techniques yield waste carbon fibres with different fibre purities. The solvolysis technique provides enhanced fibre purity as compared to the pyrolysis technique.²⁷ The sizing of carbon fibre also plays a critical role for the performance of the composite. Therefore, the surface of virgin carbon fibre is sized with different materials to protect its surface and enhance interfacial bonding between carbon fibre and matrix during consolidation. The sizing material is inherently present on Type-I waste carbon fibre. In contrast, sizing has been vanished from the surface of waste carbon fibres (Type-II and Type-III) after pyrolysis and solvolysis treatments.⁴⁰

Processing of waste carbon fibre for thermoplastic composites

Waste carbon fibres can be processed based on injection moulding, nonwoven, tape development and hybrid yarn spinning technologies as displayed in Figure 2.

Processing by injection moulding

Injection moulding is an established technology used for development of fibre-reinforced thermoplastics. It can also be employed for the production of recycled carbon fibre-reinforced thermoplastics. With this technique, chopped or milled carbon fibres or fibre fragments obtained from any type of waste (Type-I, Type-II or/and Type-III) can be mixed with thermoplastic polymers and injected into a mould to produce composites. Generally, chopped or milled carbon fibres obtained from Type-I waste have very good bonding and reinforcement properties. Therefore, this type of waste does not require any additional processes to improve bonding and reinforcement properties. In contrast, short or milled carbon fibres or fibre fragments obtained directly from Type-II or/and Type-III waste or recovered through fibre reclamation technology (e.g. pyrolysis) have lower bonding and reinforcement properties. Therefore, this type of waste is subjected to different pre-treatment processes, i.e. chemical (e.g. acid, alkali, coupling agent), physical (e.g. plasma, radiation), mechanical (e.g. compounding, extrusion) or combinations thereof. The prime objective of these processes is to enhance interfacial bonding between reinforcement and polymer. In a last step, this mixture or pre-compounded pellets are injected into a mould to produce a composite.^41,42 Many attempts have been reported to improve interfacial bonding between reinforcement and different matrices by applying these treatments.

McNally et al. deployed melt compounding to load recycled carbon fibres in polyethylene (PE) matrix and produce composites. Furthermore, the influence of different fibre contents on the mechanical properties of thermoplastic composites was reported. The study concluded that mechanical properties increase with higher fibre content.⁴³ Han et al. developed biodegradable composites based on recycled carbon fibre and poly(L-lactic acid) (PLLA) matrix. The melt extrusion process was used to mix recycled carbon fibres in thermoplastic matrix for composite manufacturing. The crystallization kinetics and mechanical properties of thermoplastic composites were intensively addressed in this study.⁴⁴ Wong et al. developed recycled carbon fibre-reinforced polypropylene (PP) composites via injection moulding and studied the effect of coupling agent (maleic anhydride grafted PP) on mechanical properties.⁴⁵ Chen et al. applied a chemical treatment to recycled carbon fibres and subsequently mixed them with polybutylene terephthalate (PBT) polymer for the production of thermoplastic composites. The effect of the amount of waste carbon fibre on the mechanical properties of composites was also reported. The results revealed that a higher fibre content in injection moulding leads to improved mechanical properties.⁴⁶

Feng et al. modified the surface of recycled carbon fibre with two chemical treatments, thus generating polyamide (PA) thermoplastic composites. Firstly, it was treated with acid and secondly with epoxy macromolecular coupling agent. The combination of these treatments had a significant effect on mechanical properties. Furthermore, crystallization kinetics was also studied, and the influence of the fibre volume fraction on the mechanical properties of recycled carbon fibre PA composites was also reported. This study also suggests that a higher fibre content in composites yields better mechanical properties.⁴⁷ Yan et al. also applied a coupling agent to recycled carbon fibre to enhance the interfacial bonding of polycarbonate (PC)/acrylonitrile butadiene styrene copolymer (ABS) alloy composites.⁴⁸ Furthermore, Han et al. modified the surface of recycled carbon fibre by applying a 3-Glycidoxypropyltrimethoxysilane coupling agent. In a next step, the mechanical properties of thermoplastic composites based on polybutylene succinate (PBS) matrix were investigated. Furthermore, the crystallization behaviour and kinetics of composites were also investigated.⁴⁹

Stoeffler et al. integrated recycled carbon fibre into polyphenylene sulfide (PPS) thermoplastic matrix through melt compounding, which was followed by injection moulding.³³ Lee et al. employed plasma treatment aiming at the surface modification of recycled carbon fibre. Subsequently, it was incorporated into PP matrix for the manufacturing of thermoplastic composites. The study concludes that plasma treatment successfully modifies the surface of recycled carbon fibres and improves the mechanical properties of composites.⁵⁰ Hirayama et al. produced PP-based thermoplastic composites made of recycled carbon fibre by extrusion and injection processes.⁵¹ Luo et al. introduced a new concept to manufacture core/shell pellets structures based on long carbon fibres and PA polymer. These core/shell pellets were further processed by injection moulding to produce thermoplastic composites.⁵²

Summary

A summary of the above mentioned studies along with mechanical properties and achieved fibre volume fractions is given in the Table 2. These studies revealed that injection moulding has the ability to produce recycled CFRPs with a wide range of thermoplastic polymers, e.g. PE, PP, PA, PC, PLLA, PPS, PBT and PBS. In spite of this uniqueness, the injection moulding technology is only able to process short fibres and produce composites with a low fibre volume fraction as shown in Table 2. Therefore, the tensile strength of injection moulding composites is very limited. The range of tensile properties of thermoplastic composites is between 20 MPa and 200 MPa. This may also correlate with short length fibres and random fibre orientation in the composite structure. It is nearly impossible to control fibre orientation during injection moulding. Therefore, new technologies are exercised or developed to increase the resource efficiency of waste carbon fibres.

Table 2.

Mechanical properties of thermoplastic composites based on injection moulding.

References	Fibre type	Matrix	V_f	Tensile Properties
			V_f	Modulus	Strength
			%	GPa	MPa
McNally et al.⁴³	Type-III	PE	30	0.73	22
Han et al.⁴⁴	Type-III	PLLA	15	11.8	110
Wong et al.⁴⁵	Type-II	PP	30	14.7	50.6
Chen et al.⁴⁶	Type-III	PBT	25	12.7	150
Feng et al.⁴⁷	Type-III	PA-6	25	20	170
Yan et al.⁴⁸	Type-III	PC, ABS	20	16.6	131
Han et al.⁴⁹	Type-III	PBS	20	6.2	58
Stoeffler et al.³³	Type-II	PPS	40	30.3	191
Lee et al.⁵⁰	Type-III	PP	15	–	–
Hirayama et al.⁵¹	Type-III	PP	07	2.1	30.9
Luo et al.⁵²	Type-I	PA-6	24	13.2	146

PE: polyethylene; PLLA: poly(L-lactic acid); PP: polypropylene; PBT: polypropylene; PA-6: polyamide-6; PC: polycarbonate; ABS: polybutylene succinate; PBS: polybutylene succinate; PPS: polybutylene succinate.

Processing by nonwovens technology

Different nonwoven technologies, broadly classified into wet and dry nonwoven technologies, were used to produce oriented fibrous structures for recycled carbon fibre-reinforced thermoplastic composites.⁵³

Wet technology

The most common wet process used for the processing of waste carbon fibre is the paper making technology. Short carbon fibres are dispersed in a solvent with the help of a stirrer or propeller. Then, these fibres are filtered, dried and solidified to produce an isotropic structure. Finally, it is combined with thermoplastic films, nonwovens, or matrix to form a thermoplastic composite.⁵⁴ Szpieg et al. introduced a paper making technique for the development of a waste carbon fibrous structure. In this study, a recycled polypropylene film was used as matrix. The resulting carbon fibre mats and matrix film were stacked to produce a composite by press forming.⁵⁵

Turner et al. used the paper making technique to produce waste carbon fibre mats based on bi-component PA fibres. The sheath and core of bi-component fibres were composed of PA-6 and PA-66 polymers, respectively. The waste carbon fibres and thermoplastic fibres were mixed in water to produce a fibrous mat. Finally, these mats were dried and subjected to compression moulding to produce recycled carbon fibre-reinforced thermoplastic composites.⁵⁶ Moreover, Wei et al. used waste carbon fibres and PA fibres to produce an isotropic hybrid fibrous structure through the paper making technique. The effect of different moulding pressures on the mechanical properties of composites was also studied in Wei et al.⁵⁷ Wölling et al. processed waste carbon fibres and thermoplastic fibres by a commercial wet laying process to produce thermoplastic composites. The influence of fibre orientation in the machine and cross direction on the mechanical properties of thermoplastic composites was studied as well. The results revealed that tensile strength and the modulus of thermoplastic composites show an isotropic behaviour in the machine and cross direction. The ratio of mechanical properties in the machine and cross direction is almost equal to 1. This behaviour is associated with random fibre orientation in nonwoven structures.⁵⁸

Dry technology

Carding is a common drylaid nonwoven technology and widely used for the production of highly oriented nonwovens structures from hybrid materials based on waste carbon fibre and thermoplastic fibres. During this process, blended fibres are fed to the carding machine to produce a fibrous sheet termed as web. Subsequently, multiple sheets of web are combined to prepare an anisotropic nonwoven structure with the help of cross lappers, which are suitable for the manufacturing of thermoplastic composites. A card machine consists of several metallic wire cylinders and orients the fibres in the machine direction through carding action. It also has the ability to mix hybrid fibres at the fibre-to-fibre level. This unique hybrid anisotropic structure enhances interfacial bonding between reinforcement and thermoplastic fibres in composites.⁵³ Therefore, numerous studies involved the processing of waste carbon fibre by carding.

Cornacchia et al. developed a thermal process for the recovery of carbon fibre from end-of-life sources. The waste carbon fibres were integrated into hybrid fibrous mats by a carding mechanism and converted into recycled carbon fibre-reinforced polypropylene composites by consolidation.⁵⁹ Wei et al. manufactured and analysed the mechanical properties of PA composites that were reinforced with waste carbon fibre card webs. A comparison between the mechanical properties of recycled carbon fibre-reinforced PA composites based on paper making and the carding processes was provided by Wei et al. as well.^60,61

Yin et al. fabricated a hybrid web from waste carbon and PA-66 fibres on a carding machine. In a next step, unidirectional as well as cross directional composites were produced by using compression moulding, and mechanical properties were also analysed in machine direction and cross direction. The results of this study concluded that composites possess an anisotropic behaviour in the machine and cross direction. It correlates with the fibre orientation which is higher in machine direction than in cross direction.⁶² Holmes reported the manufacturing of lightweight automotive components from waste carbon fibre mats. These carbon fibre mats were developed on a carding machine by mixing of waste carbon fibres and thermoplastic fibres. Subsequently, recycled carbon fibre-reinforced PA composites were produced by compression moulding. The influence of the fibre volume fraction on the mechanical properties of thermoplastic composites was also described in Holmes.³⁴ The results indicate that thermoplastic composites prepared with a high fibre volume fraction exhibit enhanced mechanical properties.

Lützkendorf et al. also developed hybrid composites based on waste carbon fibres. Thus, waste carbon fibres were mixed with natural and thermoplastic fibres to produce carded mats. Subsequently, hybrid mats made of natural, waste carbon fibre, and thermoplastic fibres were converted into hybrid composites. This study concluded that thermoplastic composites may be suited to the manufacturing of automotive interiors and reduce the weight of automotive components by up to 30% compared to natural fibre composites, while simultaneously maintaining favourable mechanical properties.⁶³ Wölling et al. also reported on recycled carbon fibre-reinforced thermoplastic composites created by carding.⁵⁸

To explore the potential of waste carbon fibre, many research institutes and companies across the world have addressed nonwoven structures made from waste carbon fibres.⁶⁴ The list of research institutes and companies includes the Saxon Textile Research Institute, ITV Denkendorf, RWTH Aachen, ITA Augsburg, University of Applied Sciences, Niederrhein, Reutlingen University, University of Nottingham, Fibre Institute Bremen, ELG England, Carbon Conversion USA, Sigmatex, England, SGL Automotive Carbon Fibres GmbH & Co. KG and Tenowo GmbH. Recently, special card machines for the processing of waste carbon fibres were developed by Dilo Group GmbH, Autefa Solutions GmbH, and Tatham Ltd UK.

Summary

It can be concluded from Table 3 that thermoplastic composites based on wet nonwoven technologies have tensile properties that are identical with those of injection moulding composites (15 MPa to 150 MPa). These can also be accompanied by poor fibre orientation, short fibre length and low fibre volume fraction. Therefore, the dry nonwoven technology was favoured for the processing of waste carbon fibre to fibre-reinforced plastics.

Table 3.

Mechanical properties of thermoplastic composites based on wetlaid nonwovens.

References	Method	Fibre type	Matrix	Vf	Tensile properties
				Vf	Modulus	Strength
				%	GPa	MPa
Szpieg et al.⁵⁵	Paper	Type-III	rPP	25	4.2^a	15^a
Turner et al.⁵⁶	Paper	Type-II	PA-6	25	17²	150²
Wei et al.⁵⁷	Paper	Type-II	PA-6	20	–	–
Wölling et al.⁵⁸	Wetlaid	Type-III	PP	20	11^b	75^b
Wölling et al.⁵⁸	Wetlaid	Type-III	PP	20	12^c	78^c

PA-6: polyamide-6; rPP: recycled polypropylene.

Isotropic.

Machine direction.

Cross direction.

Table 4 represents the mechanical properties of thermoplastic composites prepared by means of the dry nonwoven technology. More detailed information regarding the fibre type, matrix type and achieved fibre volume fraction can be seen in Table 4. The summary shows that fibre-reinforced thermoplastics based on drylaid nonwovens have better mechanical properties than injection moulding and wetlaid nonwoven-based composites. These properties are varied with respect to fibre type, matrix type and fibre volume fraction. For example, Yin et al. used Type-I waste carbon fibre in their studies and reported superior mechanical properties. The composite strength and modulus also depend on the fibre volume fraction. Yin et al. and Holmes et al. reported that both the strength and modulus of thermoplastic composites increased with a higher fibre volume fraction. Moreover, the matrix type also crucially affects mechanical properties, i.e. composites impregnated with PA matrix show more favourable mechanical properties than those impregnated with polypropylene matrix. The mechanical properties of composites are also affected by the degree of fibre orientation. A carding machine has the ability to orient 85% of fibres in the longitudinal direction (machine direction) and mix the blended fibres at the fibre-to-fibre level. Therefore, nonwoven structures produced from carding technology enable improved interfacial bonding in composite structures compared to wetlaid nonwovens, thus leading to enhanced mechanical properties. The processing of waste carbon fibres to drylaid nonwovens and their integration into fibre-reinforced plastics is still in preliminary stages and enormous work is required to understand the mechanical properties of thermoplastic composites.

Table 4.

Mechanical properties of thermoplastic composites based on drylaid nonwovens.

References	Method	Fibre type	Matrix	V_f	Tensile properties
				V_f	Modulus	Strength
				%	GPa	MPa
Cornacchia et al.⁵⁹	Carding	Type-III	PP	20–50	–	–
Wei et al.⁶⁰	Carding	Type-III	PA-6	40	22^a	120^a
Wei et al.⁶⁰	Carding	Type-III	PA-6	40	17^b	80^b
Yin et al.⁶²	Carding	Type-I	PA-66	23	30^a	350¹
	Carding	Type-I	PA-66	23	10^b	220^b
	Carding	Type-I	PA-66	32	35^a	400^a
	Carding	Type-I	PA-66	32	15^b	200^b
Holmes³⁴	Carding	Type-III	PA-66	30	23.7^a	178^a
Holmes³⁴	Carding	Type-III	PA-66	40	26.5^a	217^a
Lützkendorf et al.⁶³	Carding	–	PP	20	–	–
	Carding	–	PP	30	–	–
	Carding	–	PP	40	–	–
Wölling et al.⁵⁸	Carding	Type-III	PP	20	17^a	150^a
Wölling et al.⁵⁸	Carding	Type-III	PP	20	151^b	125^b

PP: polypropylene; PA-6: polyamide-6.

Machine direction.

Cross direction.

Processing by tape technology

The use of tape or tow structures was also evaluated for the manufacturing of recycled CFRP composites in the last decade. The primary objective of this technology is to provide a pre-consolidated prepreg for the rapid production of thermoplastic composites. These structures can be developed through new or existing technologies adding necessary modifications.

High performance discontinuous fibre method

A new process was developed for the production of tapes or tows at the University of Bristol named High Performance Discontinuous Fibre Method (HiPerDiF). Initially, this process was employed for tow structures from short virgin carbon fibres. Later on, it was also used for the processing of waste carbon fibres. This process introduces a new concept to orient carbon fibres under wet conditions. Hence, it comprises a fibre suspension tank, feeding control system, fibre orientation head as well as drying and prepreg sections. In this method, waste carbon fibres are dispersed in water with the help of a propeller and pumped to the fibre orientation head through a feed control system. The fibre feed control system uses shooting jets to align the fibres. Finally, aligned fibre tows or tapes are dried and applied for the development of composites by impregnating with resin films.^14,15

Tapper et al. developed a recycling process for discontinuous carbon fibre polypropylene composites. In the first phase, carbon fibres and a polypropylene matrix were recycled. Then, these recycled materials were used to produce thermoplastic composites based on the HiPerDiF method.⁶⁵ The mechanical properties of recycled thermoplastic composites are presented in Table 5.

Table 5.

Mechanical properties of thermoplastic composites based on tape structures.

References	Method	Type	Matrix	V_f	Tensile properties
				V_f	Modulus	Strength
				%	GPa	MPa
Tapper et al.⁶⁵	HiPerDiF	Type-III	PP	26	39¹	270¹
Akonda et al.¹⁶	Carding	Type-I	PET	53	26.5^a	295^a
Akonda et al.¹⁶	Carding	Type-I	PET	53	25.5^b	300^b
Hehl¹⁷	Carding	Type-I	PA-6	50	–	–
Khurshid et al.⁶⁶	Carding	Type-I	PA-6	43	72¹	605¹

PP: polypropylene; PA-6: polyamide-6; PET: polyethylene terephthalate/Polyester.

Machine direction.

Woven ply.

Carding technology

A tape structure can be directly produced on a card machine by replacing the coiling unit with a consolidation unit. Alternatively, it can be produced by installing a consolidation unit in the drafting stage (during drawing). These pre-consolidated tapes can be further developed into woven structures and then used for the fabrication of thermoplastic composites. Akonda et al. introduced the development of a tape structure by means of a modified carding machine for recycled carbon fibre-reinforced polyester composites. In this study, waste carbon fibres and polyester fibres were mixed on a carding machine, whereby tape was directly produced by replacing the coiler unit with a thermal consolidation unit. Subsequently, tapes were converted into a woven structure followed by compression moulding resulting in a composite.¹⁶ Hehl described a tape structure produced by integrating a consolidation unit between drafting rollers after completion of the carding process. In a next step, a woven structure was developed from these tapes, and thermoplastic composites were produced through compression moulding.¹⁷ Khurshid et al. also introduced a new concept for the production of unidirectional tape structures based on carding and drawing processes. Initially, card slivers based on waste carbon and PA fibres were developed on a carding machine and subsequently subjected to a drawing process to produce a homogeneous, uniform sliver. Subsequently, multiple slivers were doubled and consolidated through a thermo fixation unit to develop a unidirectional tape structure. Finally, a thermoplastic composite was fabricated by combining multiple layers of tape structures.⁶⁶

Summary

The summary, tabulated in Table 5, reveals that thermoplastic composites prepared from tape structures have better mechanical properties than composites prepared from nonwoven structures. This is due to tape fibrous structures providing better fibre orientation with good fibre distribution. Nevertheless, the tape development technology is at its initial development stage, and extensive research is still required to fully grasp the potential of tape structures for fibre-reinforced plastics. The data presented in Table 5 are further analysed in terms of other technologies in the state of the art section.

Processing by hybrid yarn spinning technologies

Recently, hybrid yarn composed of waste carbon and thermoplastic fibres has been explored to maximize the resource efficiency of waste materials. These hybrid yarns can be processed on conventional and advanced spinning machinery to develop fibrous structures with higher fibre orientation, compactness and fibre volume fraction. In the case of this technique, blended fibres (waste carbon and thermoplastic fibres) are subjected to carding and drawing processes to produce hybrid slivers. Subsequently, these slivers were subjected to different spinning technologies (wrap, flyer and friction) to spin hybrid yarn. Finally, hybrid yarns are wound onto a winding frame and subjected to compression moulding to produce unidirectional thermoplastic composites.^67–69

Hybrid yarn produced by wrap spinning

Wrap spinning allows producing hybrid yarn structures with higher fibre orientation and uniformity. This makes wrap spun yarn a suitable candidate for unidirectional recycled carbon fibre-reinforced thermoplastic composites. The wrap spinning process consists of a drafting, wrapping and package winding section. The hybrid drawn sliver is drafted by a series of rollers to achieve the desired number of fibres in the yarn structure. Then, attenuated fibres are wrapped by a filament called binder through a hollow spindle.⁶⁷ The principal diagram, the bobbin of hybrid yarn, and a longitudinal view of spun yarn are displayed in Figure 3.

In wrap spinning, fibres are bound in a yarn structure without a twisting mechanism. Therefore, all fibres are aligned in parallel within the yarn body. Furthermore, the linear density of the selected binder yarn is also very low. It produces a yarn structure that is uniform, particularly compared to other spinning techniques. For this purpose, wrap spun hybrid yarn based on waste carbon and a polypropylene fibre was investigated by Akonda et al. The waste carbon and polypropylene fibres were processed on a modified cotton carding machine to produce a hybrid sliver. In addition, hybrid yarn was produced based on wrap spinning by winding a binder yarn around drafted fibres. Finally, wrap spun hybrid yarn was converted into a recycled carbon fibre-reinforced thermoplastic composite through compression moulding. The effect of the fibre volume fraction on the tensile properties of composites was described in this study as well.¹⁸ Goergen et al. also presented hybrid yarn produced from waste carbon and PA fibres and developed non crimp fabrics for the manufacturing of recycled carbon fibre-reinforced thermoplastic composites.^24,70

Hybrid yarn produced by flyer spinning

A flyer machine produces a twisted and uniform fibrous structure termed as roving with a linear density of 200 tex to 3500 tex. This machine consists of a drafting, twisting and winding process. The drawn slivers are drafted by 3/3 or 4/4 rollers to achieve the desired number of fibres in the cross section. Subsequently, the drafted fibrous assembly is twisted by a flyer and wound onto a bobbin.²⁸ The principal diagram, the bobbin of hybrid yarn, and a longitudinal view of spun yarn are displayed in Figure 4.

A twisted roving structure based on waste carbon fibres for fibre-reinforced thermoplastic composites was examined at the ITM. Researchers of this institute developed a complete process chain for the spinning of hybrid yarn from waste carbon fibres ranging from 40–100 mm and modified the entire spinning setup, including carding, the spinning machine and spun hybrid yarns. For this purpose, a laboratory-scale carding machine was installed, and a hybrid web structure was produced by mixing waste carbon fibre and PA-6 fibres. The hybrid card web was further processed on a drawing and roving machine to produce hybrid yarn. The technological and kinematical parameters of carding, drawing and flyer were also optimized. In addition, the effects of fibre length, level of twist and fibre volume fraction on the tensile properties of hybrid yarn were investigated.⁶⁹

The influence of twist level and fibre volume fraction on the mechanical properties of recycled carbon fibre-reinforced thermoplastic composites was presented in Hengstermann et al.⁷¹ The study revealed that the mechanical properties of thermoplastic composites were significantly affected by the fibre volume fraction. The maximum composite strength was achieved at a 50% fibre volume fraction. Furthermore, the effect of the fibre length distribution on the mechanical properties of recycled carbon fibre-reinforced thermoplastic composites was evaluated. It is interpreted from the results that fibre length has only a slight effect on the mechanical properties of composites. This is attributed to the fact that a laboratory scale carding machine was employed in this study, which was not designed for the processing of longer fibres. Therefore, long fibres exhibit more pronounced damages caused by the carding process, and a composite composed of higher fibre length shows slightly better mechanical properties.⁷² In another study, the influence of fibre sizing on the mechanical properties of thermoplastic composites was addressed.⁷³ In this research, hybrid yarn was developed with waste carbon fibre sized with thermoplastic sizing (T), thermoset sizing (E) and without sizing (W) material, and unidirectional composites were developed and evaluated subsequently. The results revealed that thermoplastic composites fabricated from waste carbon fibre sized with thermoplastic sizing possess more favourable mechanical properties. Hasan et al. introduced a new approach to prepare composites composed of waste carbon fibres. With this approach, waste carbon fibre was mixed with water-soluble PVA fibre, and hybrid yarn was produced on a flyer machine. In the following step, the hybrid yarn was subjected to water treatment to eliminate water-soluble fibres.⁷⁴

Miyake et al. presented an alternative method to produce hybrid slivers based on a combination of drafting and combing processes called Gill box for staple carbon and thermoplastic fibres. A hybrid sliver was used to spin hybrid yarn for recycled carbon fibre-reinforced thermoplastic composites. In addition, the distribution of fibre orientation in composite structures was investigated. The study highlights that a composite structure based on hybrid yarn has unidirectional properties.⁷⁵ These results prove that thermoplastic composites based on flyer spun yarn offer tensile properties of up to 1000 MPa.

Hybrid yarn produced by friction spinning

Friction spinning is a type of open-end-spinning technique used for the production of hybrid yarn with higher fibre orientation. This technique additionally allows for the generation of core-sheath yarn. During twisting, a filament is inserted into the core of the yarn to stabilize the spinning process and increase yarn strength. The core component (staple fibre and endless filament) is permanently confined to the central axis and covered by staple fibres forming the sheath structure. This spinning technique involves two perforated spinning drums moving in opposite directions to contact the yarn surface. Suction air pressure is provided to press the fibres against the surface of the spinning drums. The amount of suction air pressure affects the amount of torque acting on the yarn. A higher air suction pressure through the friction drums leads to increased frictional forces between the fibre assembly (sleeve) and the spinning drum surfaces, thereby increasing the torque acting on the fibre sleeve, which results in a more compact yarn structure. Other advantages of using DREF-3000 friction spun hybrid yarns for the manufacturing of CFRP include multi-material yarn constructions enabling customized yarn and enhanced composite properties. This machine can produce hybrid yarn at a rate of up to 250 meters per minute.^67,76 The principal diagram, the bobbin of hybrid yarn and a longitudinal view of spun yarn are displayed in Figure 5. To understand the effect of the yarn structure on composite properties, friction spun yarn was also investigated at the ITM. Hasan et al. developed friction spun core-sheath hybrid yarn to produce recycled carbon fibre-reinforced PA composites. The core of the hybrid yarn comprised PA filaments, whereas the sheath consisted of carbon and PA fibres.^68,77

Furthermore, investigations provide results on hybrid yarn produced from waste carbon and PA fibres through ring, flyer, rotor, roving air jet and friction spinning technologies. A comparative analysis of these spinning technologies in terms of drafting range, twisting mechanism, yarn quality, yarn fineness and production capacity was also presented. This study shows that flyer, friction and wrap spinning are suitable for the processing of hybrid yarn. These spinning technologies provide both a favourable yarn quality and good productivity.⁷⁸

Summary

These studies show that flyer, wrap and friction spinning technologies have the ability to develop hybrid yarn for thermoplastic composites. The composites prepared from hybrid yarns offer promising tensile properties. This can be attributed to the fact that hybrid yarn technology provides excellent fibre orientation and higher structural compactness. Furthermore, the configuration of different settings involved in pre-spinning and spinning processes, e.g. the level of twist, production speed, draft distribution and roller surfaces, is essential to achieve hybrid yarn with optimum tensile properties. These parameters must be chosen carefully for the gentle processing of waste carbon fibre. A summary of different studies discussed in this section is displayed in Table 6. It provides an overview of hybrid yarn spun on different spinning systems for the purpose of thermoplastic composites. Furthermore, different parameters used for the processing of hybrid yarn, e.g. fibre type, matrix type, level of twist, fineness and achieved fibre volume content, are also given. The summary shows that the mechanical properties of composites based on hybrid yarn are affected by fibre type, matrix type, fibre length, fibre volume fraction and spinning type. Generally, thermoplastic composites developed from Type-I waste carbon fibre and PA matrixes with 50% fibre volume fraction provide excellent mechanical properties (tensile strength and modulus) in composites. However, the influence of fibre length on the mechanical properties of composites based on hybrid yarn has not yet been fully established. This overview also shows that through hybrid yarn technology, significant improvements can be achieved in the mechanical properties of thermoplastic composites based on waste carbon fibre.

Table 6.

Mechanical properties of thermoplastic composites based on hybrid yarn.

References	Spinning	Fibre types	Fibre lengths	Matrixes	V_f	Hybrid yarn	Hybrid yarn tenacity	Tensile properties
					V_f	Hybrid yarn	Hybrid yarn tenacity	Modulus	Strength
					%	Tex/Twist	cN/Tex	GPa	MPa
Akonda et al.¹⁸	Wrap	Type-III	50	PP	15	1000/–	–	21	105
Akonda et al.¹⁸	Wrap	Type-III	50	PP	28	1000/–	–	32	160
Hengstermann et al.⁶⁹	Flyer	Type-I	60	PA-6	30	800/118	18.1	–	–
	Flyer	Type-I	60	PA-6	50	800/118	25.9	–	–
	Flyer	Type-I	60	PA-6	70	800/118	35.6	–	–
Hengstermann et al.⁷²	Flyer	Type-I	40	PA-6	50	800/102	19	72	740
	Flyer	Type-I	60	PA-6	50	800/102	20	72	760
	Flyer	Type-I	80	PA-6	50	800/102	23	73.6	800
Hengstermann et al.⁷¹	Flyer	Type-I	60	PA-6	30	800/87	–	35	444
	Flyer	Type-I	60	PA-6	50	800/87	–	68	838
	Flyer	Type-I	60	PA-6	70	800/87	–	74	274
	Flyer	Type-III	65	PA-6	50	800/87	–	38.5	801
Miyake et al.⁷⁵	Spinning	Type-I	200	PP	12.8	–/–	–	12.2	78
Goergen et al.⁷⁰	Spinning	Type-III	25	PA-6	50	–/–	–	84	1100
Hasan et al.⁷⁴	Flyer	Type-I	60	–	–	800/80	38
Hasan et al.⁷⁴	Flyer	Type-I	60			800/118	22
Hasan et al.⁷⁷	Friction	Type-I	40	PA-6	28	800/–	22	–	–
	Friction	Type-I	40	PA-6	24	800/–	15	–	–
	Friction	Type-I	40	PA-6	42	800/–	40	–	839
Hasan et al.⁶⁸	Friction	Type-I	60	PA-6	36	800/–	–	74	960
Hasan et al.⁶⁸	Friction	Type-I	60	PA-6	41	800/–	–	85	1030
Hengstermann et al.⁷³	Flyer	Type-I-T	80	PA-6	48	800/–	–	82	1110
Hengstermann et al.⁷³	Flyer	Type-I-E	80	PA-6	49	800/–	–	84	990

PP: polypropylene; PA-6: polyamide-6.

Discussion

State of the art

Waste carbon fibres can be processed by means of injection moulding, nonwoven, tape development and hybrid yarn spinning technologies. These processing technologies produce different fibrous structures that yield thermoplastic composites with a wide range of mechanical properties. The mechanical properties of thermoplastic composites are affected by fibre source, fibre type, fibre characteristics, properties of the composite structure, processing technologies and their technological parameters.

The main sources of waste carbon fibres are the composite industry and end-of-life components. The composite industry processes different qualities of virgin carbon fibres, for example low modulus (LM), standard modulus (SM), intermediate modulus (IM), high (HM) and ultra-high modulus (UHM) fibres. Similarly, waste fibres recovered from end-of-life components also possess different qualities. Waste recovered from aeronautics end-of-life components has different mechanical properties compared to waste recovered from other types of end-of-life components. Therefore, the fibre source has a great influence on the properties of recycled fibre-reinforced plastics.

Moreover, the fibre type affects the properties of recycled fibre-reinforced plastics. Generally, Type-I waste fibre has mechanical properties that are identical with those of virgin carbon fibre. In contrast, Type-III waste carbon fibre is recycled and therefore has lower mechanical properties than virgin carbon fibre. The effect of the fibre type on the mechanical properties of a thermoplastic composite based on hybrid yarn was reported by Hengstermann et al. Results suggest that a thermoplastic composite prepared from Type-I fibres provides slightly enhanced mechanical properties compared to a composite containing Type-III waste fibre.⁷¹

The characteristics of waste carbon fibre play a decisive role for the properties of the resulting composite. The most significant characteristics are fibre strength, fibre length and fibre sizing. Generally, waste fibres with higher fibre length yield enhanced mechanical properties. For instance, the fibre length used in injection moulding is very low compared to the length used in the nonwoven technique. Consequently, a thermoplastic composite based on nonwoven technology exhibits more favourable mechanical properties than injection moulded composites. The effect of fibre length and fibre sizing on mechanical composite properties was investigated by Hengstermann et al. Based on their results, it is concluded that thermoplastic composites manufactured with higher fibre length exhibit slightly better tensile properties.⁷² Moreover, it is also suggested that thermoplastic composites fabricated from waste carbon fibres sized with thermoplastic polymer have improved tensile properties as compared to composites developed from waste carbon fibres sized with thermoset sizing.⁷³ The effect of fibre sizing on the tensile properties of composites is presented in Figure 6.

The properties of the composite structure, for example the degree of fibre orientation, have a significant effect on the performance of thermoplastic composites. For instance, a fibrous structure produced using the drylaid nonwoven technology has good fibre orientation in comparison with wetlaid nonwovens. This property is responsible for higher mechanical properties in thermoplastic composites. A comparative study of the mechanical properties of thermoplastic composites based on wet and dry nonwovens was presented by Wölling et al. Their results lead to the conclusion that thermoplastic composites based on drylaid nonwovens possess better mechanical properties when compared to composites produced from wetlaid nonwovens.⁵⁸ This phenomenon can be attributed to the fact that fibre orientation is higher in composites based on drylaid nonwovens. Based on this study, the relationship between fibre orientation and mechanical properties is illustrated in Figure 7.

The behaviour of fibre orientation in injection moulding and hybrid yarn structures is completely different when compared with nonwovens. In injection moulding, the distribution of fibre orientation is completely random. Therefore, composites based on injection moulding technology exhibit an isotropic behaviour. In contrast, the distribution of fibre orientation in hybrid yarns is unidirectional. In fact, all fibres bound in a yarn structure are oriented in the longitudinal direction. Therefore, composites based on hybrid yarn techniques show an anisotropic behavior.⁷⁵

The fibre volume content is another important property that affects the mechanical properties of composite structures. In injection moulding, the influence of fibre content on the mechanical properties of composites was investigated by various researchers.^{43,44,46,47,52} These studies concluded that composite structures loaded with a higher amount of waste carbon fibres yields higher mechanical properties (tensile strength and modulus). Thermoplastic composites composed of nonwoven structures show a similar trend in Holmes³⁴, Yin et al.⁶² These studies also concluded that thermoplastic composites containing a higher amount of waste carbon fibre possess enhanced mechanical properties. The impact of the fibre volume fraction on the mechanical properties of composites based on hybrid yarn was reported in Hengstermann et al.⁷¹ In this study, composites were developed with 30%, 50% and 70% fibre volume fraction, and the mechanical properties of these composite were evaluated. The results highlighted that thermoplastic composites with a 50% fibre volume fraction offer excellent tensile strength. In contrast, a composite produced with 70% fibre volume fraction was not completely impregnated, thus exhibiting poor tensile properties. In contrast, the elastic modulus was improved by increasing the fibre volume content according to this study. An overview on the mechanical properties of composites based on injection moulding, drylaid nonwovens and hybrid yarn versus fibre volume fraction is presented in Figure 8.

The processing technology is a key factor that determines the mechanical properties of thermoplastic composites. Generally, injection moulding is a well-established technology that produces composite structures with a low fibre volume content and random fibre orientation. Therefore, composites based on this technology have very limited performance capacities.^42–51 In contrast, the nonwoven technology produces isotropic and anisotropic fibrous structures with a higher fibre volume fraction (20%–40%). These structures produce thermoplastic composites with tensile properties of up to 400 MPa.^54–61 The tape development technology has just emerged in the last few years. It can process short, medium and long waste carbon fibres. Moreover, it has the potential to produce fibrous structures with better fibre orientation and higher fibre composition. The research in this field is still very limited and further technical developments are required to explore its hidden potential.

In addition, the hybrid yarn technology is an innovative technology that has been introduced within the last decade. This technology offers promising advantages over injection moulding, nonwovens and tape development technologies. Furthermore, it allows the processing of medium to long waste carbon fibres and produces a twisted and compact fibrous structure with excellent fibre orientation.^66–75 In hybrid yarn technology, carding is the first process to orient fibres in longitudinal direction through carding action. This carding action takes place between different metallic clothing that damages the carbon fibres. Therefore, the speed of metallic rollers, technical gauges, feeding rate, draft and sliver linear density must be optimized prior to processing. In the second stage, these card slivers are doubled and drafted on the drawing machine. The amount of doubling, break draft, main draft, production speeds and rubber cots hardness are key factors for the damage free processing of carbon fibre. Finally, this sliver is subjected to different spinning machines to generate hybrid yarn. Critical factors involve the level of twist, twisting angle, roller gauges, roller pressure, roller hardness, draft and production speeds. Therefore, these processes require modifications for the gentle processing of waste carbon fibre.⁶⁹

Initially, many attempts have been reported to produce hybrid yarn from waste carbon fibre for thermoplastic composites. However, these studies produced thermoplastic composites with poor mechanical properties. Later on, extensive research has reported on the development of hybrid yarn for thermoplastic composites. For this purpose, necessary modifications were implemented for the gentle processing of waste carbon fibres. Subsequently, the effects of fibre length,⁷² fibre type,⁷¹ modifications,⁶⁹ spinning type,^18,68,71 level of twist and fibre volume fraction⁷¹ on the tensile properties were investigated. Finally, a combination of optimized parameters led researchers to develop a high-performance hybrid yarn for thermoplastic composites by using flyer and friction spinning systems.^68,71 Results revealed that composites produced from hybrid yarn structures achieve tensile properties of up to 1000 MPa. An overview on the mechanical properties of recycled carbon fibres-reinforced thermoplastic based on drylaid nonwovens, tape and hybrid yarn technologies is presented in Figure 9. This figure leads to the conclusion that hybrid twisted structures have a great potential as compared to non-twisted hybrid structures, e.g. nonwovens, since they have a significant influence on the mechanical properties of composites. However, their hidden potential has not yet been fully explored, so that further efforts are required to enhance the hybrid effect in composites structures.

Application and future challenges

Recycled carbon fibre-reinforced thermoplastics are of particular interest to the automotive industry compared to other sectors, e.g. construction and sports. This is due to the hidden potential of these thermoplastics to reduce weight and fuel consumption. Moreover, they enable much needed CO₂ reductions. This potential encourages large automotive companies to build partnerships with carbon fibre manufacturers for the supply of low-cost carbon fibres for automotive applications. Waste carbon fibres are cost-efficient and provide mechanical properties that are similar to those of virgin carbon fibres. For these reasons, waste carbon fibres have been gaining considerable attention within the automotive industry.^79,80

Currently, several companies across the globe, e.g. BMW, Volkswagen, Nissan, Ford, Toyota and General Motors, are trying to reduce weight by use of fibre-reinforced plastics in different sections of the automotive, as for example interiors, exteriors, power system, suspensions, engine and body. This trend will increase the consumption of fibre-reinforced plastics from 200 to 350 kg per automobile. Recently, BMW has formed a joint venture with the SGL group in terms of carbon fibres, whereby simultaneously consuming a large amount of virgin carbon fibres for the fabrication of composite parts. BMW also utilizes waste carbon fibres for fibre-reinforced plastics. The ELG, being a recycled carbon fibre supplier, also cooperates with different automotive companies and research institutions for the application of recycled carbon fibre in the automotive industry.⁸¹ A list of prototypes and demonstrators produced by different companies and research institutions for automotive applications is provided in Table 7. It shows that so far many attempts have been carried out to reduce weight in automobiles by use of recycled carbon fibre-reinforced thermoplastics.

Table 7.

Applications of recycled carbon fibre-reinforced thermoplastics in the automotive industry.

Processing technologies	Prototype/components	Fibre types	References
Injection moulding	Engine oil pan	Type-I	⁷⁹
Injection moulding	Dashboard parts	Type-II	⁸²
Injection moulding	Engine covers	Type-I,III	⁸³
Injection moulding	Headlight support	Type-II	⁸⁴
Carding nonwoven	Car door panel	Unknown	⁶³
Wetlaid nonwoven	Car interior	Unknown	⁸¹
Carding nonwoven	Car interior	Unknown	⁸³
Carding nonwoven	Car interiors	Type-II	⁸⁴
Nonwoven	Car seat back	Type-III	⁸⁵
Tape technology	Bracket	Type-I	¹⁷
Hybrid yarn	Car door panel	Type-I	⁸⁶
Hybrid yarn	Suspension spring turret	Type-I	⁸⁷

The integration of recycled carbon fibre-reinforced thermoplastics in the automotive industry is a challenging task. It involves many complexities and requires collective efforts from technologists, materialists and manufacturers. Firstly, recycling and processing technologies should be organized to control the complete process chain of the product. Secondly, parts or products developed from these processing technologies require standardization and testing. These products must meet industry standards and win the confidence of engineers and technologists. Thirdly, these processing technologies must be suitable for mass production and fulfil the needs of the market. Finally, manufacturing costs involved in these processing technologies should be as low as possible due to financial viability being a great concern for developers and consumers.⁸⁸

Conclusion

This paper presented a comprehensive review on the state of the art in technologies employed for the processing of waste carbon fibres for fibre-reinforced thermoplastic composites. Furthermore, the mechanical properties of thermoplastic composites processed by different technologies were analysed and discussed. This analysis revealed that hybrid yarn spun by means of flyer and friction spinning technologies provides a unique fibrous structure when compared to nonwovens and tape structures. Therefore, this novel type of hybrid yarn yields unidirectional thermoplastic composites with promising tensile properties. Furthermore, it provides an opportunity to develop recycled carbon fibre-reinforced thermoplastics for non-critical structural applications.

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.

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Recent developments in the processing of waste carbon fibre for thermoplastic composites – A review

Abstract

Keywords

Introduction

Market size and volume of carbon fibre and waste

Types of waste carbon fibre and their characteristics

Processing of waste carbon fibre for thermoplastic composites

Processing by injection moulding

Summary

Processing by nonwovens technology

Wet technology

Dry technology

Summary

Processing by tape technology

High performance discontinuous fibre method

Carding technology

Summary

Processing by hybrid yarn spinning technologies

Hybrid yarn produced by wrap spinning

Hybrid yarn produced by flyer spinning

Hybrid yarn produced by friction spinning

Summary

Discussion

State of the art

Application and future challenges

Conclusion

Footnotes

Declaration of Conflicting Interests

Funding

References