Open access peer-reviewed chapter
Comparison of asbestos-based and resin-based brake materials.
Abstract
The need for ecologically friendly and high-performing materials in car brake pads has prompted a lot of research into resin-based composites. Since they play a crucial part in defining the mechanical, thermal, and tribological characteristics of brake friction materials, resins—especially phenolic resins—have been extensively researched. The resin-based materials used in brake pads are thoroughly examined in this chapter, with an emphasis on how they affect important characteristics including noise reduction, wear resistance, and thermal stability. The backdrop emphasizes how health and environmental concerns have led to a shift from asbestos-based to resin-based composites. The importance of resin matrices in improving durability and performance under severe braking circumstances is emphasized in the introduction. The effects of resin composition, curing behavior, and filler and reinforcement alterations on wear and friction properties are examined in depth. The developments in benzoxazine resins, which provide better mechanical qualities, greater thermal stability, and less environmental effect than traditional phenolic resins, are given particular emphasis. These resins have the potential to solve issues including the demand for sustainable substitutes and resin deterioration at high temperatures. In order to create next-generation brake friction materials, future research should concentrate on refining benzoxazine resin compositions, investigating their compatibility with fillers and additives, and using their special qualities. Insights from this evaluation are intended to help producers and researchers develop and enhance the longevity and performance of vehicle brake pads.
Keywords
- resin-based materials
- brake friction pads
- tribological properties
- thermal stability
- sustainable composites
1. Introduction
Brake friction materials are the essential components of vehicle braking systems, directly impacting safety by enabling controlled deceleration and stopping. These materials, used in brake pads and linings, rely on their ability to sustain reliable friction across diverse conditions, including extreme temperatures, heavy loads, and environmental exposure. Their composition typically blends binders, fillers, friction modifiers, and reinforcing agents, each selected to balance durability, thermal stability, and performance. Over time, advancements in these materials have focused on meeting stricter safety regulations, extending lifespan, and optimizing efficiency under real-world demands [1].
The shift away from asbestos marks a critical turning point in brake material history. Once favored for its heat resistance and durability, asbestos was phased out globally due to its carcinogenic risks. This led to the development of safer alternatives, such as resin-based composites. Phenolic resins, in particular, emerged as a key binder, offering robust mechanical strength and thermal resilience. Modern innovations now include non-asbestos organic (NAO) compounds, semi-metallic blends, and ceramic-based materials, each designed for specific applications—from everyday passenger vehicles to high-performance automotive systems [2, 3].
Beyond performance metrics, brake friction materials significantly influence noise levels, vibration, and environmental sustainability. Reliable braking minimizes accident risks, while material choices affect particulate emissions and recyclability. As automotive technology evolves—including the rise of electric vehicles and autonomous systems—the demand grows for materials that combine longevity, heat dissipation, and eco-friendly profiles. Continuous research into advanced composites and manufacturing techniques remains vital to address emerging challenges, ensuring braking systems align with both safety imperatives and sustainable innovation [4].
1.1 Historical background
For much of the twentieth century, asbestos was the preferred material for brake friction applications due to its exceptional heat resistance, durability, and cost-effectiveness. Its fibrous structure made it highly effective in maintaining brake pad performance under extreme conditions, ensuring longevity and reliability. However, the discovery of its severe health risks—such as lung cancer, asbestosis, and mesothelioma—sparked widespread concern and led to a reevaluation of its use in brake systems (Table 1).
| Property | Asbestos-based materials | Resin-based composites | Reference |
|---|---|---|---|
| Heat resistance | Excellent | Very good (improves with additives) | Carlevaris et al. [5] |
| Health risks | High (linked to diseases) | Low | Saracci [6] |
| Environmental impact | High | Moderate (improving with bio-resins) | Carlevaris et al. [5] |
| Durability | High | High | Carlevaris et al. [5] |
| Cost | Low | Moderate to high | Carlevaris et al. [5] |
| Customization | Limited | High | Carlevaris et al. [5] |
Table 1.
In response to these health concerns, regulatory bodies like the Environmental Protection Agency (EPA) and the Occupational Safety and Health Administration (OSHA) began restricting asbestos use in brake pads during the 1970s and 1980s. This regulatory shift prompted the development of safer alternatives, with resin-based composites emerging as a leading solution. Phenolic resins, in particular, became a cornerstone of modern brake materials due to their thermal stability, mechanical strength, and ability to endure the high temperatures generated during braking. Over time, resin-based materials gained dominance, as they could be enhanced with fillers and additives to improve wear resistance, reduce noise, and optimize frictional performance. This era also saw the rise of non-asbestos organic (NAO) materials, semi-metallic compounds, and ceramic composites, each offering customizable properties to meet the specific needs of different vehicles and driving conditions [7]. In recent years, the focus has expanded to enhancing the sustainability and performance of resin-based composites. Innovations such as bio-based resins and advanced resin technologies, including benzoxazine resins, have gained attention. Benzoxazine resins, for instance, provide superior thermal stability, low moisture absorption, and enhanced mechanical properties compared to the traditional phenolic resins, making them ideal for high-performance brake pads. Researchers are also exploring the integration of these resins with fillers such as graphite, aramid fibers, and metal particles to further refine their tribological properties and environmental compatibility [5]. These advancements underscore the ongoing evolution of brake friction materials, balancing performance, safety, and sustainability in modern automotive systems.
1.2 Scope and objectives of the review
This review examines the use of resin-based materials in brake pads, focusing on their contribution to improving mechanical strength, thermal stability, and tribological performance. It highlights recent advancements in bio-based and modified resins, assesses their environmental impact, and addresses key challenges such as resin degradation under extreme conditions. Additionally, the review proposes future strategies to enhance performance, sustainability, and the development of eco-friendly materials, aiming to meet the evolving demands of the automotive industry.
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2. Types of resins in brake friction materials
Resins play a vital role in brake friction materials, serving as binders that integrate various components while significantly impacting thermal, mechanical, and tribological performance. Among the most widely used resins in brake friction materials are follows.
2.1 Phenolic resins
Phenolic resins, synthesized from phenol and formaldehyde, are widely used in brake pads due to their excellent thermal stability, mechanical strength, and cost efficiency. They typically constitute 10–20% of the brake pad composition and exhibit key properties such as thermal stability, wear resistance, friction stability, noise reduction, and mechanical strength [8]. However, they can degrade at high temperatures, releasing volatile organic compounds (VOCs), prompting the development of modified versions [9]. Rubber-modified phenolic resins, for instance, enhance toughness, reduce noise, improve wear resistance, and are more eco-friendly, addressing environmental concerns while maintaining performance [10].
2.2 Modified phenolic resins
Modified phenolic resins are an advanced version of traditional phenolic resins, developed to address their limitations such as brittleness, limited wear resistance, and VOC emissions. These resins are enhanced with various components, including rubber, cashew-based materials, graphite, reinforcements, and nanomaterials [11]. They typically make up 15–25% of brake pad composition, with the exact percentage varying based on the type of modification and specific performance requirements.
Modified phenolic resins significantly improve properties such as toughness, flexibility, wear resistance, thermal stability, noise and vibration reduction, friction stability, and eco-friendliness. Rubber-modified phenolic resins, for instance, reduce brittleness and enhance durability, while cashew-based phenolic resins lower environmental impact by reducing VOC emissions. Overall, modified phenolic resins offer a more efficient, durable, and environmentally friendly alternative to conventional phenolic resins, making them a superior choice for modern brake pad applications [9, 12, 13].
2.3 Benzoxazine resins
Benzoxazine resins are advanced thermosetting polymers offering superior thermal stability and mechanical properties compared to phenolic resins, with a reduced environmental footprint. They are synthesized
2.4 Bio-based resins
Bio-based resins are sustainable, eco-friendly alternatives to synthetic resins, offering a reduced carbon footprint while maintaining brake pad performance. Derived from renewable sources such as plant oils, lignin, cashew nut shell liquid (CNSL), and cellulose, they are engineered to replace the petroleum-based resins. Key types include CNSL, lignin-based, soybean-based, furan-based, and cellulose-based resins [15]. Typically making up 10–20% of brake pad composition, their proportion varies based on application needs and blending with other resins. Bio-based resins enhance properties such as thermal stability, wear resistance, friction stability, noise reduction, and toughness while reducing the environmental impact [16].
Their advantages include renewable sourcing, reduced greenhouse gas emissions, and compatibility with fillers and fibers, enabling high-performance, eco-friendly brake pads. By integrating bio-based resins, the automotive industry can achieve greener, sustainable braking solutions without sacrificing quality or safety [17].
2.5 Epoxy resins
Epoxy resins are widely used in brake friction materials due to their excellent adhesive, mechanical, and thermal properties. Composed of epoxide compounds like diglycidyl ethers of bisphenol A (DGEBA), they are cured with amines or anhydrides and often reinforced with materials such as glass fibers or silicon carbide to enhance strength and reduce brittleness [18]. In brake pads, epoxy resins typically constitute 5–15% of the composition, depending on application needs and compatibility with other resins in hybrid systems. Key properties include strong adhesion (20–25%), thermal stability (15–20%), wear resistance (15–20%), friction stability (10–15%), mechanical toughness (15–20%), and noise reduction (10–15%) [9].
Advantages of epoxy resins include versatility, high adhesive strength, customization, cost-effectiveness, and resistance to thermal degradation. However, challenges such as brittleness and environmental concerns persist. Research into bio-based epoxy resins aims to address these issues, offering sustainable alternatives without compromising performance [19]. Overall, epoxy resins remain a critical component in advancing brake friction materials.
2.6 Thermoplastic resins
Thermoplastic resins, such as polyimides, polyetherimides, polyamide (nylon), polyetheretherketone (PEEK), and others, are increasingly used in brake pads due to their unique properties. These materials can soften upon heating and harden upon cooling, making them recyclable and more environmentally friendly compared to thermosetting resins. They contribute to improved toughness, wear resistance, and noise reduction, and are particularly effective in high-temperature environments [9].
Typically comprising 5–20% of the brake pad composition, thermoplastics enhance mechanical properties and environmental benefits. However, challenges such as thermal softening, high costs for high-performance variants, and lower bonding strength compared to thermosetting resins exist [19]. Emerging bio-based thermoplastics aim to address these issues by offering sustainable options.
The balance between performance and cost is crucial, as high-performance thermoplastics like PEEK can be expensive. Despite this, the potential for more sustainable manufacturing and recyclability is significant. Ongoing research in bio-based thermoplastics may lead to wider adoption in the automotive industry, potentially overcoming current limitations and paving the way for more eco-friendly and high-performance brake pads [20].
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3. Effect of resins on key properties of brake pads
3.1 Tribological properties
Vehicle safety and operational performance are directly influenced by the tribological properties of brake pads, which are the essential components of automobile braking systems. These properties include recovery behavior, wear resistance, fade resistance, and friction efficiency. In brake pad compositions, resins are crucial binders that have a significant impact on the material’s mechanical strength and thermal responsiveness. Various resin types have different effects on these tribological qualities. These include advanced polybenzoxazine resins, modified phenolic versions, and traditional phenolic resins. This study examines the effects of resin selection and modification on important performance metrics, such as wear durability, fade resistance at high temperatures, post-fade recovery capabilities, and frictional consistency, drawing on recent research. The results offer valuable insights into how to optimize brake pad design for increased longevity and safety. Brake pads are critical for vehicle safety, with their performance dependent on the resins used as binders.
3.1.1 Friction performance
Phenolic resins: Friction coefficient (μ) ranges from 0.35 to 0.45. Modified phenolic resins: Enhanced stability with μ of 0.42–0.48. Polybenzoxazine resins: Superior thermal stability, maintaining μ of 0.40–0.46 under high temperatures [21].
3.1.2 Temperature influence on friction
Phenolic resins experience a drop in μ at higher temperatures (0.30–0.35 at 300–400°C), indicating thermal degradation [22]. Polybenzoxazine-based pads retain μ above 0.38 even at 350°C, showcasing better thermal retention [23].
3.1.3 Wear performance
Phenolic resins shows wear rates of 2.1–3.5 × 10−7 mm3/Nm [7, 24]. Modified phenolic resins reduces wear by 15–25%, with rates of 1.8–2.7 × 10−7 mm3/Nm [25]. Polybenzoxazine resins: Best wear resistance, with rates of 1.5–2.2 × 10−7 mm3/Nm [26].
3.1.4 Fade behavior
Phenolic resins: 30–40% fade at 300–350°C [27]. Modified phenolic resins: Improved fade resistance, maintaining μ above 0.35 at high temps [28]. Polybenzoxazine resins: Minimal fade (10–15%) at 350°C, demonstrating superior stability [29, 30].
3.1.5 Recovery behavior
Phenolic resins: Recover 75–80% of original friction after cooling [31]. Modified Phenolic resins: Improved recovery to 85–90% [31]. Polybenzoxazine resins: Near-full recovery at 95%, with faster recovery times (15–20 seconds) [32]. Polybenzoxazine resins outperform phenolic and modified phenolic resins in thermal stability, wear resistance, fade resistance, and recovery behavior, making them ideal for high-performance braking applications.
3.2 Thermal stability
Brake pads are essential for vehicle safety, and their performance under high temperatures is crucial. Resins, acting as binders, hold the friction materials, fibers, and fillers together. However, at extreme temperatures (often exceeding 400°C during intense braking), resins can degrade, leading to reduced braking efficiency and increased wear. Understanding the types of resins and their thermal behavior is vital for optimizing brake pad performance.
3.2.1 Types of resins and their properties
Phenolic resins
Decomposition temperature : 300–350°C Degradation Products: CO, CO2, and phenolic char [33, 34].Characteristics : Widely used but less heat-resistant compared to modified versions.
Modified phenolic resins
Decomposition temperature : 350–400°C.Degradation products : CO2 and aromatic hydrocarbons. Characteristics: Offer improved thermal stability and fade resistance [35].
Polybenzoxazine (PBZ) resins
Decomposition temperature : 400–450°C. Degradation products: Char and low volatile gases.Characteristics : Superior thermal stability and minimal fade, making them ideal for high-temperature applications [11].
Bio-based resins
3.2.2 Stages of resin degradation
In Stage 1 (150–250°C), minor mass loss due to moisture evaporation and weak bond breaking. In stage 2 (300–400°C), major thermal degradation with resin chains breaking, forming char and releasing gases. In stage 3 (>450°C), complete decomposition, leading to mechanical failure and increased wear.
3.2.3 Methods to improve thermal performance
Resin modification
Novolac resin addition that enhances heat resistance, increasing decomposition onset temperature. Polybenzoxazine blends improves thermal stability and maintains stable friction coefficients at high temperatures.
Thermally stable fillers
Carbon fibers and graphene nanoparticles were enhance heat dissipation and char formation, improving stability up to 450°C. Ceramic fillers forms heat-resistant layers, increasing decomposition temperatures.
Resin cross-linking
Increases cross-link density, delaying thermal degradation and reducing mass loss.
Bio-based and hybrid resins
Lignin-based resins lowers emissions and maintain integrity up to 370°C. Epoxy-phenolic hybrids enhance heat resistance and reduce weight loss at high temperatures (Table 2).
| Resin type | Thermal decomposition (°C) | Mass loss at 400°C (%) | Fade resistance at 350°C (%) | Reference |
|---|---|---|---|---|
| Phenolic resin | 300–350 | 35 | 40 | Bijwe et al. [26] |
| Modified phenolic resin | 350–400 | 20 | 25 | Kumar et al. [35] |
| Polybenzoxazine (PBZ) | 400–450 | 10 | 10 | Yusubov [11] |
| Bio-based Resin | 250–350 | 30 | 35 | Di Confiengo and Faga [37] |
Table 2.
Performance comparison of different resins.
Polybenzoxazine resins excel in thermal resistance, followed by modified phenolic resins. Bio-based resins offer environmental benefits but may lack the durability of synthetic options. The choice of resin significantly impacts brake pad performance under high temperatures. Enhancing resin properties through modification, fillers, and cross-linking can improve thermal stability and braking efficiency, ensuring safer and more reliable braking systems.
3.3 Mechanical properties
Brake pads endure considerable mechanical stresses, such as tension and compression, which are critical to their durability and performance. The specific resin used as a binder significantly influences the pad’s tensile and compressive strengths, as well as its overall structural integrity. This chapter explores the effects of various resins on the mechanical characteristics of brake pads, analyzing their composition, impact on strength, and methods for achieving optimal performance.
Mechanical properties of brake pads are as below.
3.3.1 Tensile strength
Tensile strength is a critical parameter in assessing a brake pad’s ability to withstand pulling forces without breaking. Standard phenolic resins exhibit a tensile strength in the range of 6–12 MPa [38]. Modified phenolic resins show improvements, reaching 12–18 MPa, due to enhanced cross-linking and reinforcement [39]. Polybenzoxazine resins (PBZ) provide 20–25 MPa, offering superior strength compared to phenolic-based composites [40].
3.3.2 Compressive strength
Compressive strength defines a brake pad’s ability to resist crushing forces during braking pressure application. Phenolic resin-based brake pads exhibit a compressive strength range of 50–80 MPa [1]. Modified phenolic resins, reinforced with fillers, improve this to 90–120 MPa [26]. Polybenzoxazine resins offer 130–150MPa, showing superior resistance under high-pressure conditions [41].
3.3.3 Effect of resin content and composition
Brake pad performance is affected by an increase in resin concentration. Materials with compressive strength below 60 MPa, increased porosity, brittleness, and resin content under 15% are recommended for lightweight applications [42, 43]. The ideal resin content ranges from 15 to 30%, which balances strength, increased toughness, and decreased wear. Tensile strength ranges from 10 to 18 MPa, while compressive strength lies between 90 and 120 MPa [37]. More than 30% resin content improves flexibility at the expense of hardness and heat resistance. When polymeric nature is excessive, compressive strength decreases (<80 MPa) [44].
Reinforcements and fillers enhance resins’ mechanical qualities. A 20% improvement in compressive strength is achieved with graphene nanoparticles and increases resistance to cracking and increased hardness [43]. Compressive strength and wear resistance are increased by 15–25% using ceramic fillers (alumina, silicon carbide) [37]. A 30% increase in tensile strength without sacrificing flexibility is possible with carbon fiber reinforcement [45].
In terms of compressive and tensile strength, polybenzoxazine resins perform better than conventional phenolic resins. For the greatest mechanical performance, the resin content should be tuned between 15 and 30 percent. Brake pad strength and durability are greatly increased by reinforcements such as carbon fibers, graphene, and ceramics [46].
3.4 Noise and vibration
For the comfort and safety of cars, brake noise and vibration are serious problems. The contact between the brake pads and rotors is the source of these issues. In order to reduce noise and dampen vibrations, the resin binder that keeps the brake pad components together is crucial. Numerous tests of various resin formulas have been conducted to see how they impact squeal reduction, vibration damping, and noise suppression. The usefulness of phenolic, modified phenolic, and polybenzoxazine resins in lowering braking noise and improving comfort has been demonstrated in real-world trials.
Brake noise is a complex problem that has been thoroughly researched to find practical ways to reduce it while preserving or improving the lifetime and performance of brake pads. Three main types of brake noise can be distinguished: low-frequency groan noise (100–500 Hz) associated with stick-slip interactions, high-frequency squeal noise (1–10 kHz) brought on by frictional instability, and judder and vibration from structural resonances brought on by inconsistent material.
Recent studies have explored various resin modifications and materials to mitigate brake noise. Phenolic resin brake pads exhibited high noise levels (80–85 dB). Modified resins with cashew dust or rubberized resins reduced noise by 5–10 dB due to increased damping. Optimal resin content (15–30%) minimized noise while providing structural stability [47]. Chauhan et al. worked with standard phenolic resins showing high noise propensity (~82 dB). Cashew-modified resins reduced noise to 75 dB, a 7 dB improvement. Siloxane treatment further reduced noise by 2–4 dB, making eco-friendly brake pads comparable to commercial options [48]. Phenolic resin brake pads had squeal noise of 90 dB at high frequencies (5–8 kHz) identified by Masoomi. TPE-modified resin composites reduced squeal noise to 70 dB, a 20 dB reduction. Best damping was achieved at 25% TPE content, balancing noise suppression and pad durability [49].
Kalel and colleagues discovered Noise levels were lowered by 5–8 dB using phenolic resin pads with aramid fibers. Because they absorbed more energy, the reinforcements made of nylon fibers produced the lowest noise levels (around 65 dB). The finest compromise between noise reduction and durability was offered by hybrid resin systems (phenolic + PBZ) [50]. The noise levels of standard phenolic resins were greater (over 80 dB). Brake pads made of polybenzoxazine showed reduced noise (~68 dB) and better friction characteristics. Noise frequency shifts were controlled by Young’s modulus and pad stiffness [51]. The best noise reduction (~65 dB) is shown by fiber-reinforced resins and PBZ. Composites made of TPE may reduce noise levels by up to 20 dB, which makes them useful for high-performance braking applications. Only phenolic resins generate the most noise (80–85 dB), necessitating noise control modifications.
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4. Fillers and additives
Resin composites in brake pads are critical for achieving optimal mechanical strength, thermal stability, and tribological performance. The addition of fillers and additives, such as carbon fibers, aramid fibers, and metal oxides, enhances the resin matrix, improving braking efficiency, wear resistance, and thermal performance. This review analyzes the role of fillers in resin composites, their influence on brake pad performance, and methods to optimize filler-resin interactions based on working research papers.
4.1 Fillers
The mechanical strength, thermal conductivity, and friction characteristics of brake pads are enhanced by fillers. Different fillers have different interactions with resins, which can result in differences in noise reduction, wear resistance, and performance (Table 3).
| Filler type | Function in brake pads | Function in brake pads | Friction coefficient (μ) | Wear rate (×107 mm3/Nm) | Noise reduction (dB) | Reference |
|---|---|---|---|---|---|---|
| Carbon fibers | Enhances strength, reduces wear | +30% wear resistance, +20% mechanical strength | 0.38–0.42 | 2.0–2.5 | 5–8 dB | Ding et al. [52] |
| Aramid fibers | Absorbs impact, improves noise damping | −8 dB noise, +15% crack resistance | 0.36–0.40 | 1.8–2.3 | 8–10 dB | Pritchard and Cousins [53] |
| Metal oxides (Fe2O3, Al2O3, TiO2) | Increases thermal stability, reduces fade | +50°C heat resistance, −25% fade rate | 0.42–0.45 | 1.5–2.0 | 3–5 dB | Brown [54] |
| Graphene nanoparticle | Improves friction stability, reduces wear | −30% wear rate, +10% friction stability | 0.40–0.44 | 1.2–1.8 | 6–9 dB | Mullaikodi et al. [44], Kazemi-Fakhr [55] |
| Ceramic fillers | Enhances hardness, maintains friction at high temperatures | +20% durability, +15% compressive strength | 0.43–0.47 | 1.0–1.5 | 2–4 dB | Bakhori et al. [56] |
Table 3.
Types of fillers used in resin composites.
The maximum friction stability is offered by metal oxides and ceramics; however, they may produce more noise. Carbon and aramid fibers provide high wear resistance while reducing noise. Although they are costly, graphene fillers increase wear resistance and friction.
4.1.1 Optimization of filler-resin interactions
The next section discusses ways to improve the interactions between filler and resin in brake pads. Surface treatment of fillers: Silane-treated carbon fibers increase strength by 20% by improving adherence to phenolic resins. Metal oxide fillers with nanocoatings improve dispersion and 15% reduce heat deterioration. Filler combinations that are hybrid carbon fibers and aramid combine to balance wear resistance and noise reduction. While preserving little wear, graphene + metal oxides maximize friction stability. Epoxy-modified phenolic resins enhance filler dispersion and reduce fading by 30%, two examples of resin modification for improved bonding. Rubberized resins improve resistance to impacts and lower noise levels by 5–8 dB. Together, these techniques improve brake pad performance, durability, and noise reduction by improving filler-resin interactions.
4.2 Additives and modifiers
Lubricants, stabilizers, and friction modifiers are among the additives and modifiers used in resin composite brake pads to maximize braking performance. By enhancing wear reduction, temperature resistance, and friction stability, these additives guarantee steady braking effectiveness in a range of circumstances. Based on current research, this analysis looks at how various additives affect resin curing and overall brake pad performance.
4.2.1 Lubricants
Lubricants decrease brake fade by lowering wear and frictional heat. Kumar et al. investigated several resin composites based on lubricants in brake pads. Graphite-reinforced pads kept μ between 0.38 and 0.42 while reducing wear by 30%. Under high-load braking, MoS2-enhanced pads increased fade resistance by 25%. Composites injected with boron nitride had a 50°C improvement in heat resistance, reducing overheating. MoS2 and graphite reduce wear while maintaining steady friction. By improving thermal stability, boron nitride lessens brake fade [57].
4.2.2 Stabilizers
Stabilizers increase resistance to thermal degradation, halting the breakdown of resin at elevated temperatures. ZnO and SiC stabilizers in resin-based brake pads were assessed by Bilvatej et al. By increasing oxidation resistance by 40°C, ZnO-stabilized pads decreased material deterioration. SiC reinforcements improved mechanical stability by 20% by increasing compressive strength. Cashew-based resin compositions improved NVH characteristics by reducing braking noise by 10 dB. Long-term stability is ensured by ZnO and SiC’s protection against heat degradation. In environmentally friendly brake pads, cashew dust reduces noise and increases durability [58].
4.2.3 Friction modifiers
The coefficient of friction (μ) is stabilized by friction modifiers under different braking circumstances. Blau investigated friction modifiers based on copper and aluminum oxide in resin composites. AlO3-modified brake pads improved grip by increasing μ from 0.35 to 0.42 [59]. Although there are regulatory concerns, copper-based compounds increased fade resistance by 30%. By reducing friction variations, graphene additions increased pad life by 20%. By increasing friction uniformity, alumina and graphene lessen brake squeal. Because of rules, copper compounds are being phased out even though they boost fade resistance [12].
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5. Environmental and economic considerations
The choice of resins in brake pads significantly impacts both environmental sustainability and economic feasibility. Traditional synthetic resins, such as phenolic resins, are derived from petrochemical sources and contribute to carbon emissions. In contrast, bio-resins offer an eco-friendly alternative with reduced environmental impact but face challenges in cost and performance. This review evaluates the environmental challenges of synthetic resins, the role of bio-resins in reducing carbon footprints, and the economic feasibility of advanced resin technologies.
5.1 Environmental impact
Although phenolic resins are prized for their durability and heat stability, they pose serious sustainability issues. In addition to releasing volatile organic compounds (VOCs), their manufacture has a significant carbon impact, generating 2.5 to 3.0 kg of CO2 per kilogram of resin. Upon thermal breakdown, these non-biodegradable resins release toxic gasses and remain in landfills for more than 50 years. They also release recognized carcinogens such as formaldehyde and benzene, and they contribute to air pollution by releasing harmful brake wear particles such as PM2.5 and PM10. Creating biodegradable substitutes, improving recycling techniques, and streamlining industrial procedures to cut emissions are some possible ways to solve these problems. Engineering solutions might also focus on reducing brake wear-related particulate pollution.
To balance the performance advantages of the resins with their effects on the environment and human health, cooperation between material scientists, environmental specialists, legislators, and business executives is crucial [34, 60]. A sustainable substitute for synthetic resins are bio-based resins that are made from natural sources such as lignin, cashew nutshell liquid (CNSL), and soy-based polymers. Compared to phenolic resins, lignin- and cashew-based resins cut CO2 emissions by 50–70%. About 90% biodegradability is attained by soy-based resins, although their heat resistance is reduced [34, 61, 62].
5.2 Economic considerations
Market demand, processing techniques, and the availability of raw materials all affect the resin manufacturing costs. Although they are the least expensive choice, phenolic resins are hazardous to the environment. On the other hand, cashew-based resins have high manufacturing costs because of a lack of raw ingredients, whereas bio-resins are 1.5 to 2 times more expensive yet have sustainability advantages.
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6. Conclusion
The review emphasizes the critical role of resin formulations in optimizing brake pad performance, focusing on thermal stability, mechanical strength, noise damping, tribological behavior, environmental sustainability, and economic viability. Heat-resistant resins such as polybenzoxazine (PBZ) and silicon-based composites demonstrate the exceptional durability at high temperatures (350–400°C), significantly reducing thermal degradation and brake fade, which is crucial for safety and performance.
The incorporation of nanofillers such as graphene and carbon nanotubes (CNTs) enhances wear resistance by up to 50% and improves the thermal stability, leveraging their excellent strength and thermal properties. Bio-based resins, derived from materials such as lignin, cashew, and soy, offer environmental benefits by reducing CO2 emissions by 50–70% compared to conventional phenolic resins, while also improving biodegradability. However, their performance under extreme conditions compared to synthetic resins is a consideration.
Hybrid resin systems, blending synthetic and bio-based materials, provide a balanced approach to performance, sustainability, and cost-effectiveness. Advancements in testing methodologies, such as acoustic emission and infrared thermography, are vital for accurately assessing material performance in real-world conditions.
Challenges include scalability, cost implications, manufacturing complexity, and long-term durability of advanced materials. The availability and dependency on bio-based materials, as well as their performance in varied environmental conditions, are additional considerations. Economic viability is crucial, as the adoption of new materials may be hindered by high production costs.
In conclusion, the review highlights the importance of resin formulations, advanced fillers, hybrid systems, and improved testing in enhancing brake pad technology. Addressing challenges related to scalability, cost, manufacturing, and long-term performance is essential for realizing the full potential of these advancements.
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7. Research directions and future scope in brake pad resins
Resins are used as binders to keep the components of brake pads, which are crucial for vehicle safety. Numerous resins, including polybenzoxazine (PBZ), bio-based resins, modified phenolic, and phenolic, each have special qualities. Common but less heat-resistant are phenolic resins; PBZ and modified variants provide the better thermal stability. Although bio-based resins are environmentally beneficial, they cannot be as durable as synthetic ones. Techniques including resin modification, the use of thermally stable fillers, and cross-linking to postpone deterioration are all parts of improving thermal performance. Modified phenolic resins come in second to PBZ resins in terms of heat resistance. In order to guarantee sustainable, high-performance brake solutions, future research focuses on creating affordable, long-lasting bio-resins, recycling techniques, and standardized testing.
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