What Is Black Masterbatch, and What Carbon Black Concentration Does It Need?

For several seasons, the plant had been extruding black polyethylene irrigation hose with the same black masterbatch. A new supplier offered a black that was cheaper per kilogram; the sample looked just as intense, and the supervisor programmed it at the usual percentage. The hose came out a deep black, passed the pressure test and shipped on time. A few months later, in fields exposed to full sun, growers reported cracks at the bends and next to the fittings. The laboratory found the cause: the new concentrate carried considerably less carbon black, with a coarser particle, because it was formulated for coloring. The hose had enough black to look black and very little to withstand the sun. The plant replaced several kilometers of installed hose, and the distributor began requesting quotes from another manufacturer.

Black masterbatch concentrates carbon black in a carrier resin. For coloring, less than 1% in the part is usually enough; for outdoor exposure, the industry works with 2 to 3% and a fine particle.

Two blacks that look identical in pellet form and a hose that cracks in the sun

Black masterbatch carries carbon black dispersed in a carrier resin and is dosed over natural or recycled resin. It performs four jobs: coloring the part black, giving it full opacity, covering the variable shade of a recycled resin and, with the appropriate loading, protecting it from ultraviolet radiation.

In our article on white masterbatch, we explained how titanium dioxide determines its opacity. In black, carbon black plays that role, and the amount that colors a part is much smaller than the amount that protects it from the sun. In the opening case, that gap was hidden behind an intense black.

Carbon black content in the pellet and in the part

Every discussion about concentration involves two figures: the loading of the concentrate, meaning what share of each kilogram of masterbatch is carbon black, and the content in the finished part. The let-down ratio connects them. A masterbatch with 40% carbon black, dosed at 2%, leaves 0.8% carbon black in the part. At the same let-down percentage, a 25% concentrate leaves 0.5%. To the eye, both parts can come out equally black.

That is why a change of supplier requires comparing the carbon black content that reaches the part, in addition to the percentage programmed in the feeder.

Carbon black: the pigment that colors, opacifies and protects in a single material

Elemental carbon in very fine particles

Carbon black is elemental carbon in the form of very fine particles. It is produced by the incomplete combustion or controlled thermal decomposition of heavy hydrocarbons, and the furnace process is the most widespread in the industry. Depending on the grade, primary particles measure from about ten to about one hundred nanometers, and they join into cluster-shaped aggregates.

Carbon black absorbs visible and ultraviolet light. That is why it colors with a very small amount and provides full opacity. The same absorption turns it into a barrier: ultraviolet radiation stays in the outer layers of the material, and much less of it reaches the polymer inside. In addition, its surface captures part of the free radicals that initiate degradation.

Fine or coarse particle: intensity, protection and dispersion

Fine particles give a deeper black, with a bluish undertone, and protect better against ultraviolet light because they spread more absorbing surface over the same weight. In exchange, they require more work to disperse. Coarse particles disperse more easily and give a black with a gray or brown tendency. Choosing the grade is a technical decision: a black for pails and one for outdoor pipe can look identical and carry different carbon blacks.

Coloring or withstanding the sun: the function sets the concentration

Black for color: less than 1% in the part

To color a part of normal thickness black, less than 1% carbon black in the part is usually enough. Beyond a certain content, the eye stops perceiving a difference, because the part already absorbs all the light it receives. In thin film the required content rises, since there is less material between the light and the background.

General-purpose black masterbatches usually carry between 25% and 50% carbon black and are dosed at low percentages. For pails, caps, household items and indoor parts, that level meets color and opacity requirements, and the purchase is decided by the intensity and undertone of the black at the lowest cost per part.

Black for outdoor use: 2 to 3% in the part

Sun protection calls for considerably more. Specifications for polyethylene pipe, geomembranes and cable jacketing usually set a carbon black content of 2 to 3% in the part, with a fine particle grade and good dispersion. Below that range, the part can look just as black and degrade prematurely, like the hose in the opening case. Above it, protection increases very little and the part begins to lose toughness and elongation.

That range changes the let-down calculation. With a 40% concentrate, reaching 2.5% carbon black in the part requires dosing slightly more than 6%. With a 25% concentrate, the let-down ratio rises to 10%, and with it more carrier resin goes in and the cost per part increases. The price per kilogram of the concentrate says little on its own; the useful comparison is the cost of the carbon black that reaches the part.

Dispersion, moisture and carrier: where a black masterbatch succeeds or fails

Carbon black agglomerates

Carbon black particles tend to cluster, and the masterbatch extrusion must break up those agglomerates and wet them with the carrier resin. An agglomerate that survives appears in the part as a speck, a streak or a dull area. In thin film it becomes a weak point that breaks when stretched, and in pipe it is a site where a crack can start.

Poor dispersion also reduces protection, because clustered carbon black leaves areas with less barrier against ultraviolet light. Screen pack pressure and microscope inspection of a thin film reveal the dispersion before the problem reaches the customer.

Moisture absorbed by carbon black

The large surface area of carbon black attracts moisture from the environment. A black masterbatch that spent weeks in open bags can carry enough water to cause bubbles or silver streaks in injection molding and holes in film. It is advisable to store it in closed bags and, if the data sheet indicates it, to dry it before the production run.

Carrier resin at high let-down ratios

At a 2% let-down ratio, the carrier goes unnoticed in most polyolefins. At 6% or 10%, its melt flow index and its compatibility with the base resin become noticeable in pipe welding, in film strength and in process stability. That is why a black for outdoor use is best formulated with a high loading and a carrier from the same family as the resin of the part.

How to specify a black masterbatch before buying it

Before approving a new black or changing suppliers, we recommend defining these six data points and comparing the options at the same cost per finished part:

• Function: coloring, outdoor protection, covering a recycled resin or dissipating static charge; each one calls for a different carbon black content in the part.

• Concentrate loading: a thermogravimetric analysis confirms it, and together with the let-down ratio it gives the content in the part.

• Particle grade: fine for outdoor use and for a deep black, according to the supplier's technical data sheet.

• Dispersion: screen pack pressure, specks in thin film and microscope inspection.

• Moisture: behavior in the machine at the processing temperature and storage conditions at your plant.

• End use: food contact, sun exposure, wall thickness and base resin, virgin or recycled.

With these data, the black is chosen for what it does in the part throughout its service life.

The black masterbatch we manufacture at Delta Pigmentos

At Delta Pigmentos we manufacture black masterbatch in our own plant. In the Basic Color Line, MB-Negro LGA-1023A is recommended for food contact under FDA 21 CFR 178.3297 and works in HDPE, LDPE and PP, in injection molding, film extrusion, extrusion blow molding and compression molding, with a processing range of 150 °C to 250 °C.

For outdoor parts, recycled material or a specific process, we define the function with you before the shade: the carbon black content the part needs, the loading of the concentrate and the let-down ratio for your wall thickness. When the application requires it, we add ultraviolet protection from our additives line. Our Quality Control, Design and Development laboratory prepares the sample, you validate it on your machine, and we produce under ISO 9001-2015 certified procedures, shipping anywhere in the country from Santa Anita, Jalisco.

Frequently asked questions

How much black masterbatch is dosed in a part?

It depends on the loading of the concentrate and on the function. For color, it is dosed at low percentages; for outdoor use, it is calculated from the carbon black content the part requires, in the typical range of 2 to 3%. The exact percentage is defined with trials at the actual thickness.

Is carbon black enough to protect a part from the sun?

In polyolefins, a fine-particle carbon black, well dispersed and at the appropriate content, is one of the most effective protections against ultraviolet light. For severe exposure or thin-walled parts, it is advisable to complement it with antioxidants or stabilizers. Protection is sized according to the thickness, the exposure and the service life expected from the part.

Is black masterbatch suitable for food-contact packaging?

It depends on the carbon black grade and on the complete formulation of the concentrate. Our MB-Negro LGA-1023A is recommended for food contact under FDA 21 CFR 178.3297. It is advisable to state the end use from the quotation stage, along with the polymer and the process.

At Delta Pigmentos we have been manufacturing black, white, color and functional masterbatch in our own plant for 24 years, with our own laboratory and ISO 9001-2015 certified procedures. If your black part will be used outdoors, uses recycled resin or needs a deeper black, write to us on WhatsApp from https://deltapigmentos.com with your polymer, your process, the wall thickness, the sun exposure and the end use; we will respond with a technical proposal. Every request is reviewed within the following 24 business hours.