Friction Material basics : Part 2

 In the previous post "Friction material basics: Part 1", we discussed on what is a friction material, its classification based on the composition and finally the current challenges and future scope. Please refer below for the link to "friction material basics: part 1"

https://polymerscientia.blogspot.com/2022/06/friction-material-basics-part-1.html

In this post "friction material basics: Part 2", we will discuss on the types of binders used in friction material formulation, what are all the advantages of each binder and finally about the current challenges and future scope. 

Straight phenolic Novolak resin:

Novolak powder resin is widely used in most of the friction material applications like Brake pads, Brake lining, railway brake blocks,clutch facings and other friction products. In the application where the wet mixing is the process, novolak powder resin is used in partially quantity along with the liquid resins. The main advantage of Novolak powder resin is it's higher shelf life which is usually 6 months (can be extended based on storage conditions). The novolak powder resin has higher shelf life because of the stable methylene bridges in the structure. Hexamethylene tetramine (Hexamine) is a commonly used curing agent which is generally premixed with the resin by the manufacturer. 

The critical parameters of the Novolak powder resin are Pellet / Inclined flow, melting temperature, hexamine content and gel time. One should select a right product based on the application requirements.

Liquid Resole Resin

The liquid resole resins are used in the applications where the wet mixing is a process. The applications where wet mixing generally used are woven based clutch facings, railway brake blocks, composite friction materials for industrial applications. One challenge with the liquid resole resins are lower shelf life which starts from few days to few months highly depending on the storage conditions. The critical parameters for a resole resins are viscosity, solids,water tolerance, water content, type of solvent system and pH. Selection of right solvent system and viscosity levels is very critical in manufacturability of product. So, one must select a product based on application requirements. 

Modified resins:

Friction materials requires few peculiar properties compared to other composite products. Brake noise is one of the critical issue in passenger car disc pads for which we need the resins to have a better noise absorption behavior. NBR modified novolak resins are predominantly used for such requirements. Similarly modified resins are used for various specific reasons like to improve the water repellency, to improve the flexibility, to improve the reactivity of the resin to reduce the cure time. For clutch applications, modified resins are used to improve the bonding bonding between the glass/copper/viscous yarns with other filler materials. For brake pad adhesive application, to improve the flexibility modified phenol based resins are used. In summary, modified resins are prominently used in friction material to meet the specific demands of the customers and inturn we have to keep in mind that they are also expensive.

Current Challenges and future scope

Most of the resole resins used in the friction material industry are solvent based which is not eco-friendly.Though Eco-friendly resins are available in the market, friction material industry is still expected to move towards the positive direction by collaborating with the resin manufacturers.

The perennial issue with the phenolic resin is the excess of free-phenol & free formalin content in the resins. Phenol is a corrosive chemical which can cause rashes to the operators handling the chemical.Free Formalin can be observed only in resole resins which is an irritant and can cause eye burning sensation.Some how it can be controlled by the addition of scavengers but upto to a certain level only.

The main drawback of the phenolic resins are its curing process which releases gases like ammonia because of condensation reaction. New type of resins are available which is expected to replace most of the phenolic resin market which doesn't require a curing agent (Hexamine). Currently, the penetration of the product is slower because of challenges in the manufacturing and the expensive raw materials. As the demand buildsup, we can also expect the cost to drop. This type of resins has a potential to replace the century old material. Let the time answer this question. 

For more queries on the Friction material basics: Part 2 please write to hemapolymercs@gmail.com or comment in this blog.

For further information on this topic on development of friction material, please read friction material basics: Part 3

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