Meaning
Analytical sampling techniques based on internal reflection measure the infrared spectra of solid or liquid materials without extensive sample preparation. In the characterization of molded polymers and films, attenuated total reflectance provides a non-destructive method to identify resin composition and surface contamination. The infrared beam passes through a high-refractive-index crystal in contact with the sample, creating an evanescent wave that penetrates the material surface before returning to the detector.
The measurement is effective only when the sample makes tight physical contact with the crystal surface.
Spectroscopic Method
High pressure is applied to the sample to ensure uniform contact with the internal reflection element. Common crystal materials include zinc selenide, germanium, and diamond. The choice of crystal determines the spectral range and the depth of penetration.
Polymeric parts with irregular surfaces require higher clamping force to achieve clean spectra.
Depth Analysis
Penetration of the evanescent wave is limited to a few microns beneath the polymer surface. This shallow depth allows surface-specific degradation or oxidation to be measured independently of bulk properties. Weathered components often display higher carbonyl indices near the surface.
Molding Assessment
Additive migration can be identified by analyzing the surface chemistry of injection molded parts. Molders use the technique to detect slip agents or flame retardants that have bloomed to the exterior. This prevents issues during subsequent printing or bonding operations.
