Why Higher Carbon Black Content Does Not Always Mean Higher Blackness
Short Answer
The carbon black percentage on a masterbatch datasheet describes the concentrate, not the finished part. How black the part looks depends on how much carbon black reaches it after dilution, which carbon black grade is used, how well that carbon black is dispersed, and the part's surface, thickness and base resin. A 50% masterbatch dosed at 1% delivers less carbon black to the part than a 40% masterbatch dosed at 2%, and two masterbatches that deliver the same amount of carbon black can still look different.
The Masterbatch Percentage Is Not the Part Percentage
For appearance, the figure that matters is the carbon black content of the final compound. If the masterbatch is the only source of carbon black, that content is the masterbatch's carbon black content multiplied by its addition rate:
| Masterbatch carbon black | Addition rate | Carbon black in the final part |
|---|---|---|
| 40% | 2.0% | 0.80% |
| 50% | 1.0% | 0.50% |
| 50% | 2.0% | 1.00% |
These figures are arithmetic examples, not dosing recommendations. They show why a "50% versus 40%" comparison is meaningless unless the addition rates are stated: at the same addition rate the two grades put different amounts of carbon black into the part, and at different addition rates the higher-loaded grade may deliver less. The full calculation, including let-down ratios, is explained in our guide to the addition ratio of PE black masterbatch.
If the actual carbon black content of a polyolefin part needs to be checked, ASTM D1603-20 describes a gravimetric method for polyethylene, polypropylene and polybutylene plastics. It has composition limits, for example when other non-volatile pigments or fillers are present, so it does not suit every formulation.
Beyond a Certain Level, Extra Carbon Black Adds Little
Blackness does not rise in proportion to carbon black content. Once a part is fully opaque, adding carbon black hides nothing more, and once the surface absorbs most visible light, further additions change the measured lightness only slightly. Where that point lies depends on the carbon black grade, the dispersion, the part thickness and the base resin, so it has to be found by trial rather than assumed.
Higher loadings also have costs beyond the raw material. Depending on the formulation, more carbon black can raise melt viscosity and make the masterbatch harder to disperse, which can show up as specks, streaks or lower jetness in the part. A higher figure on the datasheet is therefore not automatically the better choice.
The Carbon Black Grade Matters as Much as the Amount
Carbon blacks differ in primary particle size and structure. Finer particles generally absorb more light and can give a deeper, often bluer black, but they are also harder to disperse. As a result, a masterbatch with a lower loading of a fine, well-dispersed black can look blacker than one with a higher loading of a coarser general-purpose black. This article focuses on concentration; the influence of particle size and structure is explained in how carbon black particle size affects blackness and opacity.
Dispersion Turns Carbon Black into Color
Carbon black only contributes fully to blackness when its agglomerates are broken down and wetted by the polymer. Undispersed agglomerates add little color and may appear as specks or streaks. Poor dispersion can therefore reduce jetness even when the carbon black content is high, and highly loaded concentrates generally demand more from the dispersion process. Methods for checking this are covered in our guide to evaluating black masterbatch dispersion.
Jetness and Opacity Are Different Targets
"Blacker" can mean two different things, and the right masterbatch depends on which one the product needs:
| Property | What it describes | Where it usually matters |
|---|---|---|
| Jetness (masstone) | How deep and dark the surface looks | Visible housings, appliance parts, automotive interior trim |
| Opacity | How much light passes through the part | Thin film, bags, light-sensitive packaging, covering the color of recycled resin |
A thin film may need a relatively high carbon black concentration simply to block light, while a thick molded part is already opaque at a lower concentration. In that thick part, how black it looks then depends more on the carbon black grade, the dispersion and the surface than on adding more carbon black.
Surface, Thickness and Base Resin Change What You See
- Surface finish: gloss, texture and mold polish change how much light is reflected from the surface, so the same material can look darker or lighter on different finishes and at different viewing angles.
- Thickness: thin film and thin walls can still transmit light, which makes them look less black against a light background.
- Base resin: the natural color and clarity of the resin, fillers such as calcium carbonate or talc, and gray or colored recycled resin all affect the final shade.
- Measurement geometry: whether an instrument includes or excludes specular reflection changes the reading for glossy samples.
How to Compare Blackness Fairly
Comparisons are only meaningful when everything except the masterbatch is kept the same:
- Decide whether you are comparing at the same final carbon black content or at the same cost per part, and record which.
- Use the same base resin lot, machine, processing conditions and part or film thickness.
- Prepare specimens with the same surface finish, ideally the same plaque or mold.
- Measure with the same instrument, geometry, illuminant and observer, and record CIE L* (lower L* means darker) together with a* and b* if undertone matters. The CIE L*a*b* color space is defined in ISO/CIE 11664-4:2019.
- Confirm visually under the same light against an approved reference.
L* values are only comparable within the same measurement setup and specimen preparation. There is no universal L* that defines "good" blackness, and readings from different laboratories or specimens should not be ranked against each other.
Frequently Asked Questions
Is a 50% carbon black masterbatch always blacker than a 40% one?
No. The result depends on the addition rate, which sets the carbon black content of the part, and on the carbon black grade, the dispersion and the part itself. At different addition rates the 40% grade can deliver more carbon black than the 50% grade.
How much carbon black does a part need to look fully black?
There is no universal value. It depends on the thickness, whether the target is jetness or opacity, the base resin and the carbon black grade. Product standards for some applications, such as outdoor or pipe products, can also set requirements. The level should be confirmed by trials on the actual product.
Does a lower L* value always mean better quality?
A lower L* means the sample looked darker in that measurement setup. Quality also covers dispersion, undertone, processing behavior and the mechanical properties of the part, which L* does not measure.
Can adding more carbon black cause problems?
It can. Depending on the formulation and process, higher carbon black levels can raise melt viscosity, increase the dispersion demand and add cost without a visible gain in blackness.
Related Guides
- How carbon black particle size affects blackness and opacity
- How to determine the addition ratio of PE black masterbatch
- How to evaluate black masterbatch dispersion
Matching a Black Masterbatch to Your Blackness Target
Tell us whether the product needs jetness, opacity or both, together with the base resin, thickness, processing method and current addition rate. ZFH offers high color black masterbatch for appearance-critical parts and standard PE/EVA black masterbatch for general polyolefin applications. Contact ZFH to plan a comparison trial under your own conditions.
Technical References
- Cabot — Specialty carbons for plastics
- ISO/CIE 11664-4:2019 — Colorimetry, Part 4: CIE 1976 L*a*b* colour space
- ASTM D1603-20 — Carbon black content in olefin plastics
These references describe general principles and test methods. They are not test results for any ZFH grade, and ZFH does not publish blackness rankings or L* values for its grades in this article.