What is the carbonyl index and how is it measured?
The carbonyl index is an infrared spectroscopy measurement of oxidation in a polymer: the intensity of the carbonyl absorption produced by oxidation is expressed as a ratio against a stable reference peak in the same spectrum, a peak belonging to a part of the polymer unaffected by degradation. Expressing the carbonyl absorption as that ratio normalizes for sample thickness and instrument variation.
The carbonyl index is a relative number rather than an absolute concentration of carbonyl groups, which is why a carbonyl index is only meaningful against a control.
Why is ultraviolet weathering damage concentrated in a thin surface layer of a polymer?
Ultraviolet weathering damage is concentrated in a surface layer because ultraviolet radiation is absorbed within a shallow depth of the polymer surface, so the photochemistry is concentrated there. The degraded surface layer also becomes progressively more absorbing as it forms, partially shielding the polymer beneath it.
The result is a steep oxidation gradient in the weathered polymer: heavily oxidized material at the exposed surface, essentially unaffected material a fraction of a millimeter in, and a transition between the two whose depth grows slowly with ultraviolet exposure.
Why does bulk testing get ultraviolet degradation of a plastic part wrong?
Bulk testing gets ultraviolet degradation wrong because it averages a thin, severely degraded surface skin into a large volume of intact polymer. Dissolving or grinding a whole weathered specimen and measuring an average dilutes the degraded skin and produces a number that suggests the part is fine.
The same error affects mechanical testing of weathered polymers: a bulk tensile specimen may retain most of its strength while the surface layer is brittle enough to initiate cracks that then propagate into sound material. Characterizing ultraviolet weathering damage requires depth resolution, not an average.
How is an oxidation depth profile obtained from a weathered polymer part?
An oxidation depth profile is obtained from a weathered polymer part in one of two common ways: by repeating surface-sensitive infrared measurement in attenuated total reflectance mode after removing successive layers of the part, or by cutting a thin cross-section perpendicular to the exposed face and examining it along its depth by infrared imaging. Infrared measurement in attenuated total reflectance mode samples only the outermost few micrometers of the polymer, so repeating that measurement after removing successive layers builds a profile.
Infrared imaging of a thin cross-section cut perpendicular to the exposed face gives the oxidation gradient in a single map. Cross-section infrared imaging is more informative than layer-by-layer attenuated total reflectance measurement, and it requires more sample preparation.
Why is stabilizer depletion the leading indicator of ultraviolet degradation?
Stabilizer depletion is the leading indicator of ultraviolet degradation because the polymer itself degrades slowly while its ultraviolet absorbers and radical scavengers remain, and oxidation accelerates markedly once the additive package near the surface is exhausted. Ultraviolet absorbers and radical scavengers are consumed as they do their job.
Measuring residual stabilizer, by extraction and chromatography or by oxidative induction measurement, indicates where a polymer sits on that degradation curve. A residual stabilizer measurement can show that a part is close to the acceleration point while it still looks acceptable.
How do you tell what ultraviolet degradation cost a polymer part mechanically?
Determining what ultraviolet degradation cost a polymer part mechanically takes mechanical testing alongside the chemical measurement: a carbonyl index profile establishes that degradation occurred, and mechanical testing establishes what that degradation did. Impact and flexural testing are more sensitive to an embrittled surface skin than tensile testing, because the surface is where bending stress peaks and where impact damage initiates.
Testing specimens with the weathered surface in tension, and comparing them against the same specimens with the weathered surface removed, isolates the contribution of the degraded surface layer.
What can serve as a control for a weathered polymer part when no unexposed material survives?
When no unexposed material of the same grade, color and lot survives, the interior of the weathered polymer part is a usable control for its surface, and an unexposed face of the same part provides a comparison that shares the entire manufacturing and service history except for exposure. Unexposed material of the same grade, color and lot remains the ideal reference.
Unexposed faces that can serve as a control include a sheltered underside, a region that was covered by adjacent hardware, and a surface that faced away from the sun. Documenting which surface was used as the control matters as much as the carbonyl index measurement itself.
Can a carbonyl index tell you how long a part was exposed to ultraviolet light?
Not cleanly: a carbonyl index does not convert cleanly into elapsed time. The relationship between oxidation and exposure depends on the specific polymer, its stabilizer package, color, temperature history and local ultraviolet intensity, and none of those factors are constant across a service life.
A carbonyl index profile supports statements about the extent of degradation and comparisons between specimens. Statements about precise duration require assumptions, and those assumptions should be stated rather than embedded.
What should be preserved from a UV-weathered polymer part?
A UV-weathered polymer part should be preserved with its exposed surface untouched: not cleaned, sanded, repainted or wiped, since the outermost layer carries the entire finding. Sheltered or unexposed portions of the same part should be kept intact for use as controls, along with unused material from the same lot where any exists.
The orientation and installed position of the weathered part should be recorded before removal, because which face was exposed, and in what direction, is not recoverable afterward.
This article is general technical orientation, not a failure analysis, an engineering opinion, or advice on any specific matter. Determining the cause of a particular incident requires hands-on examination by a credentialed expert.