In recent years, background noise in vehicle interiors has tended to decrease more and more in hybrid and luxury cars. As a result, there is an increased demand for noise reduction capabilities of automotive grease, in addition to the original lubrication function. In this study, we report the mechanism of noise reduction in car air conditioners, and describe a new grease that meets both excellent noise reduction and low temperature performance.
In the collision model, sound pressure is generated by collision velocity and material properties (ex. Poisson ratio, Young's modulus). When the grease is applied over the area around the collision point, collision velocity is reduced because of the viscous resistance of the grease. Therefore, it is effective to use highly viscous base oil for noise reduction. However, the base oil also harmfully influences low temperature performance. That is to say, new grease for air conditioner needs two conflicting properties of low viscosity under low temperature and high viscosity under collision pressure. To accomplish this subject, we improved high-pressure viscosity. High-pressure viscosity is shown in the Barus equation, and affected by atmospheric viscosity and viscosity-pressure coefficient, alpha. Therefore, to meet both excellent noise reduction and low temperature performance, we tried to refine alpha of the base oil.
To refine alpha, we focused base oil refining by high-alpha-polymer addition. We selected PAO(Poly-Alpha-Olefin) and polybutene as base oil that has high alpha and few weaknesses. A new noise reducing grease comprising a mixture PAO and polybutene as the base oil and silica as the thickener has been developed. This grease simultaneously demonstrates excellent noise reducing and low temperature performance.