MILEX phenolic resin: For Better Brakes and a Smoother Stop

Cars have brakes for the protection of passengers and passersby. The brake pad is crucial to the brake system because it's the part that directly stops the axle from rotating. The secret sauce for a successful brake pad is the resin that binds its parts together. All over the world, for over 30 years, Mitsui Chemical's MILEX™ has been the resin of choice for brake pads. Here's why.

In disc-brake performance, brake pads are the make-or-break factor.

Brake systems play a vital role in keeping us safe on the road. They're mostly invisible to us, though, because they are hidden inside the wheel mechanism. Many people do not know how they work.

Car brakes take the form either of drum brakes or of disc brakes. Both stop the rotation of the wheel through friction, but the drum brake works by pressing the "brake shoe" from inside the wheel, while the disc brake sandwiches the disc rotor, which itself rotates along with the wheel.

Disc brake structure and brake pads

With MILEX™ phenolic resin, not only do brake pads exert sufficient force to stop the car, but the car stops quietly without lurching. The result is a more comfortable drive.

Brake pads are extremely heavy, though, like thick chunks of metal, and they have a rough surface. It's almost hard to believe that resin can have such an impact on them.

Brake pads are made of about 20 components with an array of functions. There are metal oxides that serve as abrasives, for example, to increase the friction with the disk rotor. The brake pad's weight is due to them. Added to these are agents for damping vibration and others for reducing wear. MILEX™ is the material that binds them all together. It accounts for only about 10% of the brake pad's weight, but it is a critical ingredient that enables each of the individual components to function, which enhances the brake pad's performance overall.

Brake pad components

Components Materials
Binders Phenolic resins
Abrasives Metal oxides
Damping agents Cashew dust, rubber
Abrasion inhibitors Barium salt
Lubricants Artificial graphite, natural graphite, sulfide
pH regulators Slaked lime
Scaffolding agents Fibers (resin, metal, ceramics)

MILEX™: the world's choice for over 30 years®

Brake pads have to be able to withstand high temperatures. In the manufacturing process, the brake-pad components are placed in a mold and heated to about 150° C. On the road, the brake pads themselves can also reach temperatures as high as 300° C due to friction with the disk rotor. Phenolic resin's superior heat resistance makes it the ideal binder for brake pads.

Phenolic resins have the drawback, however, of being hard. When the binder resins are too hard, the brakes squeal and the car is apt to lurch as it stops. Too soft a resin, though, weakens the friction that is needed. Phenolic resin has to be precisely soft enough to both maximize the brake's effectiveness and ensure that the car stops quietly without lurching.

For over 30 years, Mitsui Chemical's has been developing the technology to give MILEX™ this "precision softness." Mitsui Chemicals started manufacturing novolak phenolic resin in 1959. To soften the novolak, it blended in rubber that it developed in-house. The resulting product was the first MILEX™, released in 1982. Mitsui Chemicals thereafter made steady improvements to the formulation by modifying the type and quantity of rubber used. Braking became more and more stable with less and less squeal and vibration.

The MILEX™ manufacturing process

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The resin block is ground in stages until the powder is fine enough for use in brake pads.

A powder form of MILEX™ is mixed with other ingredients and melted, and then made into a brake pad with a molding machine.

Mitsui Chemicals uses a prototype of this kind to make a simulated brake pad, in order to evaluate the phenolic resins' performance.

Mitsui Chemicals also continues to explore the possibilities of phenolic resins whose structures differ from that of novolak, with the aim of producing still higher-performance resins. A new binder resin based on phenol aralkyl was introduced to the MILEX™ line in 1987. Due to its particular structure, this resin offers both high heat resistance and "precision softness." The improved heat resistance of the resin enables the brake pad to operate safely at even higher temperatures than before.

As of 2016, MILEX™ has grown to a product line of over 20 formulations. This is because we optimize our binder resins to meet the requirements of the particular climate and driving habits of the countries where they are used. MILEX™ is trusted by automobile manufacturers throughout the world, and widely featured in luxury cars.

Environmental considerations also influence the development of brake-pad components. In recent years, for example, concerns have mounted over the environmental impact of copper powder, which is used as an abrasive, so the amount used in brake pads has been steadily reduced. But copper also has the effect of dissipating heat, so the heat resistance of the binder resin itself must increase as the copper content is decreased.

The performance and conditions required of MILEX™ are always changing with the times. Mitsui Chemicals believes that these changes represent opportunities for further development. We continue to advance, building both on our long experience with resin technology and on our pursuit of new ideas. There inside the wheel mechanism, where its contribution remains unseen, MILEX™ responds to our changing needs and helps keep us safe.

What explains MILEX™'s superior performance?

Phenolic resin has a structure in which phenols are connected by carbon atoms. Novolak, a kind of phenolic resin, has one carbon atom between the phenols. Long strands of resin (black) are intertwined. When rubber (yellow) is added, the rubber molecules enter the gaps, resulting in a softer resin. This softness suppresses the squeal of the brake and also absorbs vibration.

The graph shows the softness level of novolak resins to which three different acrylic rubbers have been added (A, B, C), across changes in temperature. All three lines are lower than the novolak line, showing that they are softer than novolak across the entire temperature range. All the resins gradually lose their softness as the temperature drops, but there are differences in the minimum temperature at which they can continue to maintain the softness required for brake pads. We utilize this difference in properties to propose fine adjustments in the binder resin to correspond to the climate of the area where the car will be used.