The breathing or degassing operation is a fundamental necessity in the hot pressing of friction materials to ensure structural integrity. It is required because the curing process of phenolic resin generates volatile gases and moisture that, if trapped, inevitably lead to material failure. By momentarily releasing pressure, you allow these byproducts to escape rather than becoming embedded defects within the composite.
The "breathing" step acts as a relief valve for chemical byproducts. By allowing volatiles to escape during curing, you prevent the formation of internal voids and stress, directly translating to superior crack resistance and reliability in the final product.
The Chemistry of Curing and Gas Generation
Managing Phenolic Resin Byproducts
The core issue lies in the chemical nature of the phenolic resin used in friction materials. As the resin undergoes the curing process under heat, it naturally generates volatile gases and moisture as byproducts.
The Function of Intermittent Pressure Release
The laboratory press must perform intermittent pressure releases, known as breathing operations. This mechanical action creates a momentary pathway for these trapped gases and moisture to exit the mold cavity effectively.
Structural Integrity and Performance
Eliminating Internal Defects
If the gases are not expelled, they remain trapped within the cooling material. This leads to the formation of internal pores (voids) and can cause delamination, where the layers of the composite separate.
Alleviating Residual Stress
Trapped pockets of gas create internal pressure points. By degassing the material, you significantly alleviate this residual stress, ensuring the material's internal structure is uniform and relaxed.
Improving High-Temperature Reliability
The removal of defects and stress directly impacts performance. A properly degassed material exhibits improved crack resistance, which is critical for maintaining reliability under the harsh conditions of high-temperature braking.
The Consequence of Omission
The Risk of Invisible Weaknesses
While breathing adds a step to the cycle, omitting it is a critical error. Skipping this operation leaves the material vulnerable to catastrophic failure under load, as internal voids and stresses act as initiation points for cracks that may not be immediately visible upon ejection.
Ensuring Quality in Friction Materials
If your primary focus is Structural Durability:
- Ensure breathing cycles are sufficient to fully expel moisture, as this prevents the internal pores that lead to cracking.
If your primary focus is High-Performance Reliability:
- Prioritize degassing to eliminate residual stress, ensuring the material holds together under the thermal shock of heavy braking.
Mastering the breathing operation is the difference between a cohesive, reliable composite and a defective part prone to separation.
Summary Table:
| Feature | Impact of Degassing/Breathing |
|---|---|
| Structural Integrity | Prevents internal pores, voids, and delamination |
| Chemical Management | Releases volatile gases and moisture from phenolic resins |
| Stress Relief | Alleviates residual internal stress for a uniform structure |
| Product Reliability | Enhances crack resistance under high-temperature conditions |
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References
- Tej Singh. An integrated multicriteria decision making framework for the selection of waste cement dust filled automotive brake friction composites. DOI: 10.1038/s41598-023-46385-5
This article is also based on technical information from Kintek Press Knowledge Base .
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