Thermoplastic rubber

In friction materials such as brake pads and clutch facings, powdered nitrile rubber (PNBR) has become a key additive to improve friction performance with its dual role of "elastic buffer + performance blending". This specially powdered nitrile rubber, with its excellent oil resistance, heat resistance and dispersibility, can accurately balance the "hardness and toughness" and "wear resistance and braking stability" of friction materials, and is the core driver of friction materials from "simple compound" to "performance synergy".
1. Characteristics of powdered nitrile rubber for friction materials: tailor-made for friction scenarios
Powdered nitrile rubber is a powdered elastomer made of nitrile rubber (NBR) by low-temperature crushing or spray drying, and "special for friction materials" means that its performance indicators have been targeted and optimized, and are highly compatible with the working environment of friction materials.
1. Core performance: Dual advantages from molecular structure to powder form
Molecular structure characteristics: Nitrile rubber is copolymerized by butadiene and acrylonitrile, and acrylonitrile content (ACN) is its core indicator - the acrylonitrile content of special friction materials is mostly 33%-45% (medium and high ACN type), which gives it three major advantages:
Oil resistance: The polarity of the acrylonitrile group enables it to resist oil stains that may be contacted during braking (such as brake oil leakage), and the volume change rate is ≤5% (immersed in 150℃ brake oil for 24 hours);
Heat resistance: Glass transition temperature (Tg) ≥-20℃, it can still maintain elasticity in the 150-300℃ environment where the friction material works (no glass transition embrittlement);
Compatibility: The polar structure has a strong affinity with binders such as phenolic resin and epoxy resin in the friction material, and it is not easy to have interface separation.
Advantages of powder form: Compared with block nitrile rubber, powder form (particle size is mostly 50-200μm) brings significant processing convenience:
Excellent dispersibility: Powder particles can be evenly embedded in the matrix (resin + filler) of the friction material, avoiding the "local excessive elasticity" or "insufficient rigidity" caused by uneven dispersion of block rubber;
High mixing efficiency: It can be quickly mixed with other components (such as steel fiber, graphite, calcium carbonate) in the dry mixing stage without pre-plasticization, and the mixing time is shortened by more than 30%;
Controllable dosage: Powder form is easy to accurately measure (the addition amount can be controlled within the range of 2%-8%), and the elastic modulus of the friction material can be accurately adjusted.
2. The core role in friction materials: balancing "friction performance" and "user experience"
The core requirements of friction materials are "stable friction coefficient (μ), excellent wear resistance, low noise, and no scratches", and powder nitrile rubber optimizes these indicators in all aspects through "elastic buffering" and "interface regulation".
1. Regulating friction coefficient: making braking smoother
The friction coefficient of friction materials needs to remain stable at different temperatures and pressures (such as the requirement of μ=0.35-0.45 for automobile brake pads at 200-400℃). The role of powdered nitrile rubber is reflected in:

High temperature elastic compensation: When the friction temperature rises (such as continuous braking), the resin matrix may soften. At this time, the elastic network of powdered nitrile rubber (special grade with Tg higher than 150℃) can maintain the structural integrity of the material and avoid a sudden drop in the friction coefficient ("thermal decay" phenomenon);

Microscopic interface filling: Powder particles fill the gap between rigid fillers (such as alumina) and resin to reduce the fluctuation of friction coefficient caused by "hard point contact" during braking, so that the μ value deviation is controlled within ±0.05.

For example: In the formula of heavy truck brake pads, adding 5% acrylonitrile content 40% powdered nitrile rubber can increase the friction coefficient retention rate at 300℃ by 20%, reducing the risk of brake failure during continuous downhill braking.
2. Improve wear resistance and impact resistance
The wear of friction materials mainly comes from "abrasive wear" (hard particle scratches) and "fatigue wear" (stress concentration from repeated braking). Powdered nitrile rubber alleviates this through dual mechanisms:

Elastic buffering: When the friction interface is impacted by hard particles, the elastic deformation of powdered nitrile rubber can absorb part of the energy, reduce the generation and expansion of matrix cracks, and reduce the wear by 15%-25%;
Enhance interface bonding: The powder particles have a strong affinity with the resin, which can enhance the wrapping force of the resin on the fiber and filler, avoid "filler shedding" during braking to form abrasive particles, and further reduce wear.

In the clutch face (often subjected to sliding impact), adding 3%-4% of high Mooney viscosity (ML100℃ 1+4≥80) powdered nitrile rubber can extend the service life of the face by more than 30%.
3. Reduce braking noise and scratches
Noise and scratches are caused by "high-frequency vibration" and "hard contact" at the friction interface, and powdered nitrile rubber resolves them through "damping effect":

Damping and vibration reduction: The internal friction characteristics of nitrile rubber (loss factor tanδ is 0.3-0.5) can absorb the vibration energy during braking, reduce the generation of high-frequency noise (2000-5000Hz), and reduce the braking noise by 5-10 decibels;
Flexible contact: Powdered nitrile rubber forms a "micro-elastic layer" on the friction surface, reducing the scratch marks caused by direct contact with rigid materials and protecting the surface finish of the brake disc/drum.

3. Specialized modification and formula adaptation: precise adjustment for scenarios
The diversity of friction materials (such as automotive brake pads, industrial brakes, and elevator brake shoes) requires "customized modification" of powdered nitrile rubber to match the core needs of different scenarios.
1. Key modification directions
Crosslinking degree control: Through pre-crosslinking treatment (adding a small amount of sulfur or peroxide), the powdered nitrile rubber forms a partial crosslinking network, which improves the compression permanent deformation performance at high temperature (150℃×22h compression permanent deformation ≤25%), which is suitable for continuous high temperature braking scenarios;
Particle size classification: According to the density requirements of different friction materials, different particle sizes such as 50μm (fine powder, suitable for high-density formula), 150μm (medium powder, general type), and 200μm (coarse powder, suitable for low-density, high-porosity formula) are provided to ensure uniform dispersion;
Functional group grafting: Introducing polar groups such as carboxyl (-COOH) or epoxy groups to enhance the chemical reactivity with resins (such as phenolic resins) and improve the interface bonding strength, which is especially suitable for high-load friction materials.
2. Typical formula adaptation cases
Passenger car brake pads: focus on "low noise and smooth braking", add 2%-4% of acrylonitrile content (33%-36%) and 100μm powdered nitrile rubber in the formula, and cooperate with graphite (10%) and aramid fiber (5%) to balance the friction coefficient and comfort;
Industrial brake friction pads: emphasize "high wear resistance and impact resistance", use high cross-linking degree and 150μm powdered nitrile rubber (addition amount 5%-8%), with steel fiber (20%) and alumina (15%) to cope with frequent braking under heavy load;
Elevator brake shoes: require "low wear and no sparks", use fine powder (50μm) powdered nitrile rubber (addition amount 3%), compound with vermiculite (20%) and rubber powder (10%) to reduce scratches on elevator brake wheels.
4. Processing technology adaptation: synergy from mixing to molding
The powder form of powdered nitrile rubber enables it to be seamlessly integrated into the traditional processing flow of friction materials, and even optimizes process efficiency.

Mixing stage: No plasticization is required, and it is directly dry-mixed with resin, filler, fiber, etc. The powder particles can quickly absorb resin due to their large surface area (5-10 times larger than block rubber), reduce the "agglomeration" phenomenon, and improve the uniformity of mixing by 40%;
Hot pressing molding: Under hot pressing conditions of 150-180℃ and 10-20MPa, powdered nitrile rubber and resin melt synchronously to form a continuous elastic network, avoiding local bubbles caused by uneven melting of block rubber;
Post-processing stage: During the curing process at 200-220℃, the stability of powdered nitrile rubber (thermal weight loss rate ≤5%) ensures that it does not decompose, maintains the integrity of the elastic network, and ensures the dimensional stability of the friction material.
5. Development trend of powdered nitrile rubber for friction materials: Advancing to "high performance + environmental protection"
With the improvement of the performance requirements of friction materials for new energy vehicles (low braking frequency but high single braking energy) and high-end equipment, special powdered nitrile rubber is upgrading in the following directions:

High temperature resistance: By increasing the acrylonitrile content (≥45%) or blending with fluororubber, the long-term use temperature exceeds 300℃, which is suitable for the "high energy recovery" scenario of the braking system of new energy vehicles;
Environmentally friendly production: Using aqueous suspension polymerization instead of traditional solvent method to reduce VOC emissions, and developing degradable powdered nitrile rubber to meet EU REACH and other environmental regulations;
Functional compounding: Compounding friction performance regulators (such as molybdenum disulfide and boron nitride) with powdered nitrile rubber to make "integrated functional powder", simplify the formula system, and improve performance stability.

From the smooth braking of passenger cars to the safe parking of heavy machinery, the powdered nitrile rubber used as a friction material uses "elasticity" as a pen to outline a performance curve of "stable, wear-resistant and low noise" on the rigid friction interface, becoming the core strength of the "hardness and flexibility" in friction materials.