To distinguish pigment-associated color shifts from resin effects in polypropylene processing, cross the retained acceptable and suspect pigment with the retained acceptable and suspect resin. Include unpigmented controls and compare each formulation’s processing response against its own baseline. First separate an initial shade mismatch from additional change during processing. A repeatable response that follows one pigment directs the next investigation toward that pigment grade; a response confined to one pigment–resin pair requires investigation of the combination. Neither pattern alone proves chemical decomposition.

This guide serves formulation and process engineers investigating color changes after a pigment or resin lot or grade change in unfilled, injection-molded polypropylene (PP). The comparison concerns retained and suspect materials with the same declared pigment identity. Switching between pigment chemistries requires a separate reformulation study. Filled compounds, effect pigments, soluble dyes and weathering failures fall outside this scope.

Identify the Materials and Establish the Starting Difference

Define the organic pigment before interpreting the failure. Quinacridones are one identifiable family; examples include Pigment Red 122 and Pigment Violet 19, discussed in Heubach’s plastics-coloration guide. These names identify possible investigation subjects; they provide no blanket approval of a commercial grade for the intended PP process.

Record the Colour Index designation, exact commercial grade, delivery form and disclosed crystal modification or surface treatment. Keep those identifiers attached to every test result.

Retain acceptable pigment, resin and molded plaques. Record the resin grade and lot, added stabilizers, nucleating or clarifying additives, processing aids and regrind fraction. Here, “resin package” includes the supplied PP and its incorporated additives; the crossover cannot automatically separate polymer structure from an undisclosed additive change.

For masterbatches, obtain pigment content and available carrier and additive information. If these cannot be balanced, identify the trial as a comparison of complete concentrates. Its result cannot isolate pigment identity from concentrate composition.

Prepare baseline plaques under the established acceptable process. Record the initial color difference before introducing a processing challenge. A material that starts at a different shade and then remains stable needs a different investigation from one that initially matches and subsequently drifts.

If retained acceptable materials are unavailable, document the replacement reference and the resulting limitation. A reconstructed reference cannot establish exactly what changed between the original production lots.

Cross the Pigment and Resin Variables

The following is a proposed diagnostic design. It tests which material change the response follows; it is not a standardized pigment heat-resistance method.

Let P0 and P1 denote the retained acceptable and suspect pigment. Let R0 and R1 denote the corresponding resins.

Prepare baseline and challenged specimens for every formulation. Include independent preparations or repeat runs appropriate to the observed process variation. Repeated readings on one plaque assess measurement repeatability; they do not cover preparation or machine variability.

For powder pigments, fix actual pigment mass fraction and the non-pigment formulation. Leave loading unchanged during diagnosis, even if the initial shades differ.

For concentrates, calculate pigment content after letdown and balance carrier and additives where the composition permits. Retain those non-pigment constituents in the blank, replacing omitted pigment with base resin. Removing pigment still changes the system, so the blank cannot reproduce every particle-dependent interaction.

Keep the grade and concentration of titanium dioxide fixed if it is present. A separate white-reduction comparison needs its own matched set.

For blended colors, single-pigment screens can help locate a suspect constituent. Confirm the finding in the complete color formula before applying a correction.

ChemicalCell’s Pigments category supports initial material browsing. Confirm exact commercial-grade identity and relevant polymer data before using a listed material in this comparison.

Reproduce the Relevant Processing Challenge

Select the first challenge from the failure history. For color drift after a production interruption, compare normal residence with a defined longer dwell within the suppliers’ permitted processing conditions.

Record barrel settings, measured melt temperature where available, shot utilization or throughput, screw speed, back pressure, cycle timing and the dwell procedure. Preserve the same preceding compounding history.

SABIC’s polymer-processing guidance describes the time–temperature dependence of colorant and polymer behavior. It also explains that shear contributes to dispersion and can change color, including through particle changes and frictional heating.

On that basis, treat identical machine settings as a starting comparison rather than proof of identical material exposure. Different melt behavior can alter the thermal and mechanical history. If those exposures cannot be matched, report performance under the tested setup and investigate the exposure difference before assigning a chemical cause.

As part of the proposed trial, use a documented cleaning and purge procedure and repeat the reference combination during the sequence. Investigate a drifting reference before interpreting differences between materials.

If dispersion remains a competing explanation, examine sections or suitable thin specimens for agglomerates, streaks and spatial variation. Run a separate dispersion comparison while recording thermal exposure. A shade change after additional mixing can reflect color development; it does not independently establish improved heat resistance.

Read Heat Ratings in Their Actual Test Context

Heubach’s 2022 organic-pigments shade card reports injection-molding heat-resistance data in HDPE at one-third standard depth of shade, with 1% titanium dioxide. Its stated endpoint is the temperature at which a five-minute dwell produces a color difference of ΔE*ab = 3.

One-third standard depth describes the color-depth basis; it is not a fixed pigment mass fraction shared by every grade.

These are the supplier’s test conditions and endpoint. They do not establish a universal PP acceptance tolerance or a permitted production residence time. Check the candidate grade’s data in the intended polymer and compare the actual melt history before using a rating to dismiss the failure.

Measure the Processing Response Against Each Formulation’s Baseline

Use the same mold, plaque thickness, texture, cooling procedure and conditioning interval. Keep measurement location, backing, aperture, geometry, illuminant and observer settings consistent.

Datacolor’s appearance guidance identifies gloss, thickness, opacity and surface characteristics as factors affecting evaluation. Backing also contributes when the specimen is translucent. Control these conditions before interpreting the difference chemically.

Retain spectral data and the individual L*, a* and b* coordinates. For every formulation, compare the challenged plaque with its own baseline and record the direction of change.

Datacolor’s explanation of CIELAB and CIEDE2000 distinguishes the color coordinates from the equations used to calculate total difference. Two similar total differences can have different coordinate changes. Use one agreed equation throughout; ΔE*ab and ΔE00 values cannot be exchanged directly.

This also limits the use of the resin blank. Subtracting its total ΔE from the colored compound’s total ΔE does not yield a pigment-degradation quantity: the values describe differences between separate pairs of optically different specimens. Use blank behavior as comparative evidence.

Where gloss differs, compare specular-included and specular-excluded sphere measurements if available. Datacolor’s gloss guidance explains how those modes help investigate surface contributions. Neither mode identifies pigment chemistry, and an instrumental color pass does not waive the required finished appearance.

Use the Pattern to Choose the Next Investigation

The following interpretations are proposed investigation paths. Apply them only to reproducible changes relative to each formulation’s baseline, with composition, processing and measurement differences accounted for.

A stable blank cannot exclude a resin-dependent interaction that requires pigment. A discolored blank also cannot quantify its contribution to the colored specimen.

Heubach’s technical guide discusses organic-colorant interactions with polymer additives and impurities, together with pigment-induced effects on polyolefin crystallization. These provide competing explanations to investigate before diagnosing pigment decomposition.

If cooling-dependent opacity or dimensional changes accompany the color shift, compare controlled cooling trials with thermal or morphological evidence. Differential scanning calorimetry (DSC) can support the crystallization investigation. As METTLER TOLEDO explains, DSC records heat flow during a controlled temperature program. In this multicomponent compound, treat a changed thermal event as evidence requiring interpretation; DSC alone cannot establish that the pigment has decomposed.

If molecular decomposition remains the leading hypothesis, obtain chemical evidence suited to that pigment in the actual formulation. A color measurement cannot identify which of several competing mechanisms produced the observed shift.

Verify the Correction in the Same PP System

Select the correction from the demonstrated failure path. Establish reproducible dispersion where dispersion is implicated. Investigate the resin package where its contribution follows R1. Qualify the complete combination where the failure is confined to P1/R1.

Repeat the corrected formulation against the acceptable reference under normal production and the relevant challenge. Evaluate initial shade acceptance separately from processing-induced change, and confirm the finished part’s other required properties.

The next step is the controlled crossover using identified acceptable and suspect materials. Carry its result forward with the exact pigment, resin, carrier, additive package and processing conditions. Approval applies to the verified combination and manufacturing route; it does not establish interchangeability across all grades sharing the pigment name.