What Is Color Migration in Plastics and How Is It Prevented from the Formulation Stage?

The cap came off the machine with the exact red that quality approved on the plaque. Two weeks later, in the customer's warehouse, the stacked caps left a pink shadow on the white boxes and the first return arrived with a photograph attached. The entire lot went into quarantine, the line is waiting for an explanation and the pigment that was bought on price is now the one costing the most.

Migration is the movement of pigment within the part or toward its surface, where it transfers to the materials in contact. It is prevented with a pigment whose resistance is validated for your resin and your process temperature.

The three routes by which color leaves the part

A correctly chosen pigment is trapped in the polymer matrix and stays there for the entire service life of the part. When that fails, the color finds one of three exit routes, and each one leaves a different trace that your quality team can recognize with the naked eye.

Bleeding: the pigment that dissolves in the matrix

Some dyes and low-resistance organic pigments are partially soluble in the polymer or in the plasticizers the formula carries. At process temperature, and later with the heat of the warehouse, that dissolved fraction travels through the part and passes to any material that touches it: another part, a package, a label. The pink shadow on the white box is bleeding. In a part stacked under pressure, prolonged contact accelerates the transfer and the defect appears weeks after inspection.

Blooming: the pigment that surfaces

When the pigment concentration exceeds what the matrix can retain, the excess crystallizes on the surface of the part over time. It appears as a haze or a fine powder that comes off to the touch and stains the operator's gloves. The part loses gloss and the transferred color ends up on the hands of the end user. This mechanism is the one most often mistaken for a process defect, when in fact it originates in pigment selection and dosing.

Extraction by contact with oils, solvents and food

The third route appears when the part coexists with a medium that dissolves the pigment: oils, cleaning solvents, greases or food. The medium extracts the color from the surface and carries it away. In containers and parts with food contact, this route also has a regulatory dimension: only pigments that comply with the applicable reference may be used, normally FDA for the intended use, and that decision is made at the quotation stage, not after the return.

Why the process pushes the pigment out of place

Even with a suitable pigment, three plant conditions open the door to migration. All three can be controlled, and all three appear frequently in the lots that reach our laboratory for diagnosis.

Overdosing: the excess the matrix does not integrate

Every polymer retains a finite amount of pigment. When dosing is raised to "gain coverage" or to correct a shade by eye, the excess remains free inside the part and is the first to migrate. Our standard masterbatch is typically dosed between 1% and 5% depending on pigment loading and target shade; in round numbers, one ton of resin is colored with about 25 kilograms of masterbatch at 2.5%. The exact percentage is defined during development, and raising it on the floor without validation is the most common way of provoking blooming.

Temperature out of range

Every pigment has a thermal limit. Above it, organic pigments degrade and shift shade, and the more mobile fractions gain freedom to move within the matrix. Our standard concentrates work between 150 and 250 degrees Celsius; a temperature profile that exceeds that range in the dosing zone or at the nozzle turns a stable pigment into one that migrates. The machine's temperature record is the first piece of evidence we request when a lot arrives with a problem.

Additives that compete for the surface

Slip agents, antistatics, antiblock and plasticizers do their job precisely by migrating to the surface of the part. When combined with a pigment of limited resistance, those additives drag color with them or open the way for the pigment to follow. That is why it is advisable to declare all the additives in the formula when requesting a color development, and why, when color and additive are integrated in the same masterbatch, the laboratory formulates them together and tests them together.

Flexible compounds: the highest-risk case

Compounds with plasticizer, such as flexible PVC and some thermoplastic elastomers, deserve separate mention. The plasticizer acts as a permanent solvent inside the part and readily dissolves medium-resistance organic pigments, which then migrate along with it toward the surface or toward the material in contact. In a hose, a seal or a flexible toy, migration is the rule if the pigment was not selected for that system. For these parts the laboratory starts from pigments with high migration resistance, almost always inorganic or combinations, and confirms the result with the contact test on the actual compound.

Selecting the right pigment from the formulation stage

Migration is prevented at the formulation bench, with a pigment whose migration resistance is validated for that resin and that process temperature. That validation is different for polyolefins, for engineering plastics and for plasticized compounds, and that is why the same pigment can be excellent in one resin and a problem in another.

Organic pigments offer greater brightness and tinting strength at lower dosing; inorganic pigments provide greater opacity and thermal and light resistance, and in general higher migration resistance. Many formulations combine both to achieve the shade with the stability the application demands. The choice depends on the resin, the process temperature and the end use, and it is made with data, not with the price per kilogram.

What the pigment's technical data sheet must include

Before approving a pigment for a part that is stacked, packaged or in contact with food, request and review four items on its data sheet: thermal resistance, with the recommended process range; lightfastness, rated on the blue wool scale from 1 to 8, where 8 is the highest performance; migration resistance, with the method used to evaluate it; and food contact status. In our basic line catalog each masterbatch indicates whether or not it is recommended for food contact, so that decision does not depend on anyone's memory.

How we validate it before you commit volume

Color development at Delta Pigmentos starts from your shade reference, your exact polymer, your conversion process and your end use, including food contact and outdoor exposure. With that information the laboratory formulates and measures against the standard. For parts with migration risk we add a contact test: the pigmented plaque is placed in contact with a white substrate under temperature and pressure, and the amount of color transferred is evaluated. A pigment that stains in that test does not leave the laboratory.

Afterwards, you validate the shade and the behavior on your own machine, with your real temperature profile, before committing volume. Our procedures are audited and certified under ISO 9001-2015, and color matching is carried out as an authorized Pantone partner. It is the difference between discovering migration in the customer's warehouse and ruling it out in the laboratory.

Frequently asked questions

Does migration appear immediately or over time?

It can take days or weeks to become visible. Warehouse heat, stacking pressure and prolonged contact with other materials accelerate it, which is why a part that comes off the machine perfect can stain a month later. The laboratory contact test reproduces those conditions before the part reaches the customer.

Does lowering the dosing solve migration?

It helps when the cause was excess pigment, and it is usually the first thing reviewed. If the pigment has low migration resistance for that resin, it migrates even at low dosing; in that case the solution is to change the pigment, with the corresponding shade matching.

Does the problem apply to masterbatch or only to powder pigment?

It applies to pigment in any of its presentations. Masterbatch, arriving predispersed and dosed in a controlled way, reduces the risk of local overdosing and uneven dispersion, but the pigment it carries must be the right one for your resin and your process. That is why development is done with your polymer and your end use on the bench.

At Delta Pigmentos we have manufactured masterbatch for 24 years, with our own laboratory, Pantone reference and ISO 9001-2015 certified procedures. If a colored part is staining what it touches, or you want to rule out that risk before launching a new part, write to us on WhatsApp with your polymer, your process and your shade reference; we will respond with a technical proposal and a development timeline.