Meaning
A localized color change from the original hue to a pale or white appearance occurs when a plastic part is subjected to mechanical strain or impact that exceeds its elastic limit. The phenomenon of stress whitening is caused by the formation of micro-voids, crazes, or micro-cracks that scatter light differently than the surrounding dense polymer matrix. This visual defect indicates that the underlying plastic has undergone localized yielding and has suffered irreversible structural damage.
Deformation Mechanism
When tension or bending is applied to semi-crystalline polymers like polypropylene, the crystalline regions detach from the amorphous zones. These microscopic detachments form small internal voids which act as light-scattering centers. The region appears white because the refractive index differences between air in the voids and the polymer scatter all visible light wavelengths.
Defect Prevention
Product designers must minimize high stress concentrations to prevent this visual and structural defect. Moulders can reduce the susceptibility to stress whitening by optimizing resin selection, such as using block copolymers instead of homopolymers. Adjusting the molding parameters to ensure uniform cooling and lower molded-in stresses also helps maintain the cosmetic integrity of the finished part under load.
Quality Inspection
Visual inspection of molded parts often uses this effect as an indicator of mechanical overload. Areas of white discoloration on a flexed living hinge or a snap-fit connector indicate imminent component failure. This straightforward check allows quality controllers to reject fatigued parts without specialized test equipment.
