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Environ Eng Res > Volume 29(3); 2024 > Article
Sellami and Elleuch: Green composite friction materials: A review of a new generation of eco-friendly brake materials for sustainability

Abstract

Particulate matter (PM) still poses a significant threat leading to air pollution which is responsible for continued damage to human health and the environment. Particulate matter resulting from non-exhaust emissions are considered as viable a source comes mostly from brake pad wear concept. The scientific challenge of this work is to stimulate a new generation of green friction materials to reduce particle emissions having low-environmental impact. The specificity of braking materials, their effects on the environment and human health are studied. To minimize the levels of air pollution, the use of green friction composites collected from natural elements reducing human diseases is discussed. For this purpose, the novel and high-performance friction composite materials synthesized from vegetable and animal waste used as a raw material are reported. The natural powder and fiber treatments, the optimum formulation and the binder materials related to the green friction composites are reviewed. An overview of environmentally ecofriendly green friction materials embedded by reinforcing phases of natural elements exhibiting excellent mechanical and tribological properties are mentioned. Therefore, scientific and industrial efforts should concentrate on the development of green friction materials to minimize the effect of transport related air pollution.

1. Introduction

Particle matters are generated from exhaust and non-exhaust emissions. Brake wear is thus considered as non-exhaust traffic related particles. Component wear - road transport- is defined as a direct source of airborne particles from exhaust emission. Particulate matters from brakes are defined as the airborne created by wear, abrasion, corrosion, and turbulence. It has been proven that large amounts of brake and tire wear particles are emitted as PM10 [1]. Although the regulations are focused on the quantity emitted of these particles into our air, their composition is also a preoccupation. As regards the chemical composition of PM10 from the brakes, it was reported the presence of high concentrations of heavy metals (Fe, Cu, Zn, Sn, Sb) although for the finer fractions PM2.5 is characterized by concentrations of organic elements. Other metals such as Ni, Sn, Cd, Cr, Ti and Sb were also detected although in concentrations lower than 0.1 wt.% [2]. Other studies have shown that brake wear emissions show a potentially significant source of Sb in the environment from (Sb2S3) which is used as a lubricant to reduce noise emissions and provide friction stability [3]. Brake wear emissions have been proven that they can integrate physiological fluids and more serious that they contain carcinogenic substance [4].
Nowadays, environmental respect has drawn research focus to use alternative fibers in brake pad applications. In this trend, several researchers have focused their work on replacing asbestos and other carcinogenic materials in the production of green friction materials. The attractive performance / respect for the environment involved in the production of composite materials in the automotive sector from waste inspired the idea of investing in natural waste as reinforcement for the brake lining. The current direction in friction material’s research is the use of animal or agricultural waste as the raw material for high performance friction composites. Therefore, this article presents a comprehensive review of these studies.

2. Specificity of the Brake Materials

Friction materials are mainly composed of several elements about 10 and up to 30 ingredients with a great diversity in terms of chemical composition, morphology, size distribution, and manufacturing process including a succession of steps: mixing, preforming, hot molding and post curing. All these experimental parameters influence microstructure heterogeneity that must ensure several stringent tribological, squeal and thermal performances for friction materials. Constituents can be divided into four groups, namely fillers, friction modifiers, binder, and reinforcement additives [5, 6]. One of the peculiarities of constituents concerns their synergistic / antagonistic effects [7]. Indeed, the synergies between constituents and the complexity of various phenomena of multi-physical couplings operating at different scales have a marked effect on the micrometric scale regarding the wear mechanisms activated on the surface. Under certain conditions, several films of micrometric thickness were highlighted, and their studies prove that the thickness of the tribofilm is generally less than one micron [8]. These latter films were formed from nanoparticles originating from the fragmentation and mixture of disc and brake lining materials, and probably by chemical reactions between ingredients and oxygen coming from atmosphere with the formation of different kinds of oxides [9]. In fact, wear incites the detachment of particles of variable sizes that can reach under certain conditions, depending on the contact pressure and the sliding speed, a few micrometers to tens of nanometers and even down to a few nanometers [10]. The significant presence of fine-grained particles on the surface of the lining, most often around the plates described above can be trapped in the porosities of the material and remain at least temporarily in contact. They can recirculate and agglutinate on the leading edge of the plates. These particles can also be ejected from contact, thus becoming wearing particles thrown off from brakes. When the brake pads and disc wear, a huge amount of wear debris or particles are released into the atmosphere [11] which depend on the characteristics of friction materials (ingredient’s composition, size and shape) and friction layer (wear process) [12]. The particles emitted or even particulate pollution or PM, is a complex mixture of micro/nano particles [13]. Particulate matter is generally size classified according to their aerodynamic diameter i.e. PM10 (ultrafine particles), PM2.5 (fine particles) and PM0.1 (coarse particles) for particulates with aerodynamic diameter smaller than, respectively, 10 μm, 2.5 μm and 0.1 μm [14]. Even though brake wear particles are emitted due to a mainly mechanical and tribological process and should be found mainly in the coarse fraction, many studies have shown high concentrations of fine and ultrafine particles as shown in Fig. 1 [15, 16].

3. Environmental Health Effect of Braking Systems

Sanders estimate PM emission and airborne particles fraction as 50% of the total wear brake [17]. The study of the city of London concerning PM emissions show that non exhausted particulate matter size ranges from coarse, fine to extend ultrafine particles [18]. Specifically, brake wear particles are generated in a wide range of sizes, including PM10, PM2.5 and PM0.1. Certainly, particle size determines are an important parameter for the human health consequences. In fact, Particles represent a great danger to human health. Indeed, for coarse particles, we are exposed to diseases in the respiratory system. In the less coarse case, we are faced with bronchial and lung diseases. On the other hand, fine particles can easily enter the lungs and can cause diseases related to vascular inflammation and possibly lung cancer. Furthermore, concerning PM2.5, the sedimentation of these ultra-fine particles is very slow and when released to air, particles propagate through the atmosphere along kilometers [19]. These nanoparticles are even more dangerous for human health since they have a high capability to absorb organic molecules and can penetrate more deeply into lung [20] and reach the intimate structure of tissues and organs and thus induce cancer and serious pathologies. They can reach the deepest part of the lungs, such as alveoli, and enter the bloodstream [21, 22]. In fact, High concentration of airborne particles has adverse health effects including, respiratory, skin and cardiovascular diseases [23, 24] which can cause lung cancer in the worst case [25]. Other authors focused their studies on PM2.5 effects on human health. Kim presents two types of effects: moderate health effects such as lacrimation, eye pain, impaired vision and hazardous effects such as pneumonia and diseases of the respiratory system [26, 27]. In addition, nanoparticles can also spread through the blood and move to other vital organs such as the liver and the kidneys [28]. In fact, several research studies the effect of the size of PM on human health as shown in Table 1.

4. Legislations

Nowadays, for environmental concerns, new legislations have forced brake pad industries to produce environmentally friendly materials for their products. Toxicologists classify particulate pollutants as ultrafine, fine and coarse, although regulators, namely World Health Organization (WHO) [35], the United States Environmental Protection Agency (USEPA) and the European Union (EU) describe polluting particulates as particulate matter (PM) classified based on the aerodynamic diameter of the particles. Each class has its own composition, properties, and very specific effects on the environment and human health [36]. Several researcher include brake wear in their Particulate Emission Models and both utilize a 13 mg/mile emission factor for PM10 brake wear debris [37]. The World Health Organization (WHO) studies the effects of particulate matter on human health. It is estimated that PM harmful air quality that effects on human health. In fact, it consists of the most frequent causes of death for around 2 million people every year [38]. The World Health Organization has described the unhealthy effects of PM regardless of short-term and long-term exposure and includes breathing difficulties which in some cases cause asthma, heart disease, high blood pressure and lung cancer and can some cases have caused death [39]. In fact, the chemical composition of the particles is responsible for a number of potential harmful effects on human health. Although there is insufficient information regarding the effects of PM on human health. The World Health Organization agrees that the chemical composition of PM presents several carcinogenic elements classified according to the aerodynamic diameter of the particles. Indeed, PM2.5 formed by carbon black and certain metals such as Fe, Cu, Ni are currently considered to be responsible for several diseases [40]. American Conference of Governmental Industrial Hygienists [41] assumed that refractory ceramic fibers may causes cancers to humans by the IARC [42] and can induce lung diseases. The WHO assessed the harmful effects of various particulate generated by brake pad wear, including carbon elements [38]. The National Institute for Occupational Safety and Health has recently presented research on mineral elements and asbestos fiber impacts on both human health and air quality [43]. Legislations are usually interested in the particulate brake wear emission since their harmful impact on human health. Therefore, the improvement of human consciousness about environmental concern has led to consider natural fibers as eco-friendly attractive reinforcement for green friction material.

5. Concept of Green Friction Materials

By dint of the wear mechanisms occurring in the Brake system, abrasive particles are released into the environment. Recently, studies proved that some of the commonly used essential ingredients for brake pad materials have started raising environmental concerns that endanger the wellbeing of humans and air quality [44]. In the context of reducing the PM coming from brake wear, many issues have been satisfactorily addressed for the friction material in order to find optimal formulation for ecological brake systems. Significant improvements must be considered such as the respect of different criterion in the selection of constituent of the friction material. These criteria must be aligned with the regulations on environmental and ecological risk. To meet these criteria different environment and healthy friendly alternative materials for brake pad must be considered [45]. Furthermore, numerous efforts were deployed by scientific researchers to address environmental issues and the need to protect human health using natural fibers in brake pad formulation.

5.1. Sustainable Design of Friction Composite Materials from Vegetable Waste

Most green materials are based on fibers derived from agricultural waste since their economic importance and significant environmental challenge. These agricultural fibers have shown their efficiencies for the manufacture of composite materials in the brake pads industries, and this through several properties, namely their low cost, their availability, their high resistance, their nature respectful of the environment and their durability [46]. This has led to a lot of research concerning their treatments for extracting fiber from natural resources profitably employed in environmental and health monitoring. Several plants have shown great potential in the exploitation of fibers as alternative raw materials for industrial uses (banana, corn, pineapple, hazelnut, coir, corn stalk, bamboo, etc.) [47]. Recently, plenty of research focused on the potential of vegetable waste reinforced brake material with restrictions coming from environment. Their scientific studies were focused on physical, mechanical and tribological properties of brake pad alternative materials, which could be very important because of their allied nature to the structure of fibers as shown in Table 2.

5.2. Sustainable Design of Friction Materials from Animal Resource

Development of innovative friction materials via utilization of marine resources could be part of the solutions to reducing environmental pollution. In this context, many scientific researchers are still on the quest to find suitable green material from animal resource for brake materials to guarantee environmental respects. Bala uses in their contribution Cow hooves as reinforcing components in brake pad material. In this study, the cow hooves were washed and properly sun-dried. The cow hooves can also be dried in an electric vacuum oven for three hours at 250°C to remove contaminating oil, crushed using pestle and mortar then grounded into powder and finally sieved using mesh size of 710 μm. After several test, results show that composite with 15% pulverized cow hooves, 35% epoxy resin present good properties such as wear resistance, water and oil absorption and friction material with 10% cow hoof and 7% epoxy resin gave the better properties for hardness and friction coefficient [64]. Other studies were based on the use of cow dung as a natural ingredient for industrial application [65] as shown in Fig. 2. To be used as reinforcing material, cow dung was dried for 30 min in 1% NaOH solution and washed with distilled water then dried for 7 days in the air. Finally, it was screened with a sieve of 40–60 mesh size and dried again at 70°C. Tribological performance showed that 6 wt.% cow dung fibers are the optimum mass ratio providing best performing [66].
Eggshells are also used as reinforcement material to produce ecological friction material since they are composed of 95% calcium carbonate in the form of calcite [67]. Edokpia developed biodegradable material with eggshell and Arabic gum as binder. To remove its membrane, egg was washed, and sun dried then milled and finally sieved and retained on a 125 μm sieve. The investigated combination of eggshell and Arabic gum shows that 18 wt.% of Arabic gum exhibits good physical, thermal and wear properties [68]. Periwinkle shell is a marine waste product produced from the consumption of a greenish-blue marine periwinkle [69]. The ecofriendly utilization of periwinkle shell is considered as a challenge for scientific [70]. In the study, periwinkles shell was grounded and sieved into fine particles (Fig. 3). The composite composed of 35% phenolic resin and shell [71] with different sieve was characterized. The best results of compressive strength, hardness and density is obtained for periwinkle shell particles of 100 μm size compared with 125 μm size propounded by Yawas [72]. Inversely getter oil soak, water soak and wear rate are reached with 350 μm particle’s size by Elakhame compared with 710 μm confirmed by Yawas.
Aquatic waste from the sea is a major threat to the coastal areas that lead to accumulation causing difficulties in living there. The crab shells, the major solid waste in coastal areas, can be treated properly and used as filler in brake friction composites since they are mainly constituted by calcium carbonate and chitin biopolymer (Fig. 4). The Crab shells were washed with distilled water, grounded then sieved in the range of 210–250 BSS (British Sieve Size). The sieved powder was soaked in a solution of CHCl3 and CH3OH for 1h to remove the fat. Further, the crab shell powder was immersed in 5 wt.% of NaOH solution for twenty-four hours for deproteination. Then, the powder was treated in 4 wt.% of HCl solution for 1h for decarbonation, filtrated and finally dried at room temperature. Thus, the crab shell powder was successfully processed without any organic compounds, with the help of chemical reagents [74]. Singaravelu research proves that crab shell powder content (12 wt.%) composite showed good thermal stability, friction characteristics and wear resistance. In fact, the higher bonding characteristics of the crab shell with resin enhance the transferring of the heat to surrounding thereby reducing the frictional heat generated in the interface [75].

6. Challenge of Green Friction Materials

Natural fibers are made from a variety of renewable plant and animal resources. Considering their chemical composition rich in cellulose, which is hydrophilic in nature, inducing an affinity for water that limits its use as a reinforcement for friction materials [76]. Green composite swells when it absorbs the water, and after subsequent drying, the shrinking of the fiber occurs causing the separation of the fibers from the matrix decreasing the strength of friction materials [77]. Therefore, the natural fibers do not meet the reinforcement function decreases. Kabir also proves that these hydrophilic fibers are incompatible with some resins of hydrophobic nature [78]. Thus, it is fundamental to improve the interfacial adhesion between the resin and the fibers during water absorption. Several scientific researchers have exploited a powerful method for the application of green friction materials to improve the interfacial adhesion between the resin and the natural fibers. Silane treatment is widely used for friction composite. It consists of a silane surface treatment based on coupling agent (mostly trialkoxysilanes) as a bonding aid to form a bridge of chemical bonds between the natural fiber and the binder [79]. M. Asim proves that applying silane treatment for pineapple leaf fiber (PLF) and Kenaf Fiber (KF)/phenolic formaldehyde (PF) composites showed very good fiber/matrix bonding and lesser fiber peeling off. By dint of good interfacial bonding, it was speculated that the silane treatment of 50 wt.% fiber loading KF and PLF/phenolic composites enhance their mechanical properties [80]. The other treatment was applied to natural fiber, which is an alkaline treatment. This method consists of treatment with 5 wt.% NaOH in order to improve the fiber to matrix adhesion and thus enhance the compatibility of the natural fibers to the polymer matrix accompanied with removal of large amounts of lignin and hemicelluloses [81]. Rajan demonstrated that alkaline and silane treatment induces the removal of hemicellulose, lignin and impurities from the Prosopis Juliflora fiber (PJF). The Silane-treatment increases shear strength and fiber degradation temperature by 10°C to reach 346°C, which are an essential requirement for friction composite. Obviously, results reveal that composite based on silane treated prosopis juliflora fiber exhibit better properties than that based on alkaline treated fiber and raw composite [82]. Recently, alkaline and silane treated Prosopis Juliflora (PJ) bark fiber in the epoxy modified Phenolic composite was subjected to tribological characterization by Rajan [83]. Results have demonstrated that Silane treatment for composite based on Prosopis Juliflora fibers are most suitable for the brake pad materials where frictional heat can exceed 250°C. Rajan has also applied the silane fiber treatment to shell more specifically scallop shell and periwinkle shell powder. It was concluded that treated shell powder could be an alternative for brake pad materials especially for heavy vehicles [84]. Javeed studied the tribological effect of alkaline treatment of friction composite reinforcement with Alkaline Treated Areva Javanica Fiber. It was found that after an alkaline treatment, the impurities were removed so that the NaOH penetrated the surface, which made the surface rougher [85]. The alkaline-treated Areva Javanica fiber has an increase in cellulose content, the fiber is more fibrillated, the thermal stability is increased from 298°C to 310°C and the tribological performances of the green friction composite are higher and more stable [86].
Atiqah investigated the impact of alkaline treatment, silane treatment and alkaline-silane treatment on Sugar palm fiber. The results revealed that silane treated fiber exhibits the best interfacial stress strength and thus better mechanical properties. It has been shown also that fiber treatments, especially silane treatment help to develop high performance sugar palm fibers reinforced thermoplastic polyurethane matrix [87]. Several studies are concerning the choice of the adequate matrix used with natural fiber. In this trend, investigation was carried out with an epoxy-modified phenolic novolac binder that has been shown to be thermally more stable than straight phenolic novolac. S. Rajan confirms that 12 wt.% of phenolic resin modified by epoxy in the friction composite formulation offer better tribological properties [88]. Y. Liu confirms these results by proving that friction material based on modified phenolic resin acquire good thermal properties and good friction and wear performance than those using straight phenolic resin as binder [89]. Similarly, alkylbenzene-modified Phenolic presents good thermal properties than the straight Phenolic resin [90]. Another study shows that boron-modified Phenolic resin exhibits better mechanical properties than conventional one [91].

7. Conclusion

From these overviews, it is concluded that scientific efforts are geared towards using natural fiber as reinforcement for environmental and health related concerns. Currently, novel composite material formulations ensuring required performance of standard brake pads are studied for environmental concerns in order to reduce the particulate matter in the air. A comprehensive review of application of natural elements from vegetable and animal waste as possible reinforcement for green composite friction materials has been highlighted in this study. The physical, mechanical and tribological properties of these ecofriendly brake pads were compared favorably with standard brake pads. Physical and chemical treatments were applied to overcome the poor adhesion fiber/matrix, low thermal stability, and incompatibility of natural fibers with organic matrix. The agro and animal waste can be strongly used as a replacement for toxic ingredients in the formulation of brake pad materials when added with other elements to guarantee good combination of performances. It is expected that this purpose-built innovation in friction material will complement future formulations to reduce particulate emissions needed to meet environmental regulations.

Acknowledgments

The authors gratefully acknowledge the Tunisian Ministry of Higher Education and Scientific Research for their continuous support of research at the Laboratory of Electro-Mechanical Systems of Sfax.

Notes

Author Contributions

S.A. (Associate professor) wrote the article. E.R. (Professor) revised the article.

Conflict of Interest Statement

The authors declare that they have no conflict of interest.

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Fig. 1
PM mass concentration in function of particles size distribution [16]
/upload/thumbnails/eer-2023-226f1.gif
Fig. 2
Cow dung fiber a) Morphology b) SEM) image of a vertical cross section [65]
/upload/thumbnails/eer-2023-226f2.gif
Fig. 3
Periwinkle shell [73]
/upload/thumbnails/eer-2023-226f3.gif
Fig. 4
Blue crab a) shell b) powder [75]
/upload/thumbnails/eer-2023-226f4.gif
Table 1
Effect of the size of PM on human health
Brake wear Particle matter Effects References
PM10 lung cancer, cardiovascular and respiratory problems, [16]
PM2.5 Heart rate variability [16]
< PM2.5 Translocation of Blood in liver, kidneys and more dangerously brain [19]
cardiovascular diseases and pulmonary inflammation [29]
PM10; PM2.5 high blood pressure and provokes inflammatory [30]
Brake wear rich in copperand iron oxides Inflammatory reactions in bronchial branch [31]
Iron Iron induce conjunctivitis and inflammation of the retina in the eye [32}
Copper copper causes irritation of the respiratory tract and eyes and in the worst cases kidney failure [33]
Nickel Nickel is a carcinogenic substance, causing cancers of different organ such as prostate, lung and nose. [34]
Table 2
Physical, mechanical and tribological properties of brake pad using alternative materials
Natural fibers Treatment Results References
Banana soaking 24 h in NaOH solution-Washing with distilled waterdrying in an oven. 60°C for 5h-cutting at 1–5 mm length Quantity less than 10 wt.% improves friction performance and stability [48]
Corn stalk fiber Soaking 24 h in 3% NaOH solution-Washing with distilled water-drying in an oven. 60°C for 5 h-cutting at 4–5 mm length 7 wt.% in the composite exhibited excellent friction and wear performance. [49]
borassus and tamarind fruit fibers Washing with distilled water-baking in an oven at 50°C – Soaking 2 h in NaOH solution and neutralization with hydrochloric acid Increase the hardness of the composites. [50]
hazelnut shells Drying - pulverization with a high-speed grinder-sieving (400 μm) Good compressibility and high level of friction coefficient and good stability [51]
Sawdust Drying for about 1 month after collection-milling into powder using a ball milling machine-sieving The lower the sieve grades of sawdust, the better are mechanical and friction properties. [52]
Coir fiber (coconut fiber) Soaking 1h in 5% Na OH solution - washed with distilled water-drying process-cutting to 7 mm of length and storing with silica gel. Composite with 20 vol % coconut fiber and 46% of epoxy resin show similar frictional properties than commercial brake pad [53]
Rattan-Fibre dipping into 2 vol.% of NH3H2O and 4 vol.% of CON2H4 for 60 min at 100°C - soaking for 40 min at 65°C in 5 vol.% NaOH solution-washing in distilled water-neutralization for 20 min with 2 vol.% H2SO4 - drying in an oven at 90°C for 3 h 5% of rattan fiber exhibits better wear resistance [54]
abaca fibre Immersion in 3 wt.% NaOH solution for 90 min - soaking 40 min in the H2SO4 solution (1 wt.%) - washing in distilled water - drying in an oven at 75°C for 40 min – cutting of different lengths Short fiber (5mm and 10 mm of lengths) improve the wear resistance of the friction composites [55]
Aramid pulps Extraction with acetone for 24 h in a Soxhlet extractor -drying at 70°C for 5 h in an air oven Optimal aramid: carbon fiber mass ratio is 75:25 which enhance mechanical properties and friction stability of the composites associated with good thermal stability. [56]
Hemp fiber Immersion with 2% NaOH solution for 24 h-washing with distilled water fibers - drying for 5 h in an oven at 60°Ccutting into short fiber of 1–5 mm length 5 wt.% of Hemp fiber content enhance the friction and wear performance of the composite [57]
Flax and basalt fibres drying at 80°C for 30 min in a hot air oven-soaking 1 h in 5% NaOH solution-washing with distilled water-drying in hot air oven for 5 h at 80°C-soaking - washing with acetone solution-drying in atmospheric air - drying at 80°C for 5 h. Due to the thermal characteristics of basalt fiber and its bonding nature composite show a good wear resistance. [58]
Areva javanica fiber Extraction from its seed-soaking with 3 % alkaline solution for 1 h -Good physical, chemical, and mechanical properties
-Good dispersion od fiber in the matrix
-Good wear resistance
[59]
pineapple leaf fiber Soaking for 24h in 5 wt.% of NaOH - washing with distilled water-drying in oven for 5h at 60°C-cutting to a length of 2–6mm 5 wt.% pineapple fiber content enhance highest friction performance and wear resistance [60]
Bagasse/ banana peel Bagasse: Soft milled for 30 min in a ball mill
Banana peel: drying in oven at 75°C for 8h-ball milling for 5h
5 wt.% of bagasse content improves hardness value. Composite with 5wt.% of bagasse and 5wt.% of nanoalumina particle show good friction and wear behavior. [61]
Palm Kernel Shell Collection-Extraction-suspension in a solution of caustic soda (sodium hydroxide) for 24 h Composite with 40% epoxy-resin, 10% palm kernel shell presented good properties (physical, mechanical and tribological) [62]
Durian Fruit Skin and Teak Leaves Durian fruit skin fiber: cutting into pieces 5×5 cm - drying in the sun for 36 h-stoving 1h at temperature 100°C-cutting Teak leaves: heating with the oven at a lower temperature of 50°C.-cutting composition 40% of durian fruit skin fiber, 10% magnesium oxide, 40% teak leaf, and 10% polyester resin have identical properties with standard brake pad [63]
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