Beta-Scission

Molecular Cleavage

Radical formation on the polymer chain precedes the cleavage step. Once a radical is generated, the carbon-carbon bond at the beta position becomes weak and breaks, producing a shorter polymer chain and a new terminal radical. In polypropylene, beta-scission is the primary mechanism of thermal degradation, leading to a narrower molecular weight distribution.

Viscosity Shift

Melt flow index values rise sharply when polymer chains are cleaved into shorter segments. The occurrence of beta-scission reduces the viscosity of the melt, making it flow faster but lowering its mechanical performance. Processors monitor melt viscosity to ensure that the material has not degraded.

Part Defect

Molded parts suffer from reduced tensile strength and increased brittleness when the polymer chains are too short to form sufficient entanglements. This molecular damage causes part defects such as flashing, warping, and cracking under mechanical load. Compounders utilize peroxide additives to deliberately trigger this process when producing controlled-rheology resins with specific flow properties.

This controlled degradation ensures that the polymer can fill thin-walled molds easily without compromising the overall dimensional stability of the finished product.