How much colorant to add to cast polyurethane

Ask this question anywhere and you get a percentage. One number, no conditions attached, as if every pigment behaved the same way in every system. The number is usually in the right neighborhood and it is still the wrong answer, because the loading that gives you the shade you want depends on which pigment is doing the work.

Here are the sourced starting figures, what the percentage is actually a percentage of, and what happens when you go past the band.

First, percent of what

This is the question that causes more bad trials than the number itself, and almost nobody states their basis.

The commonly published cast polyurethane figures are on the total system mixture, meaning polyol plus isocyanate plus everything else. Other figures in circulation, including patent language, are on the weight of polyol alone. On a system running near a one-to-one ratio those two bases differ by roughly a factor of two. That is not a rounding difference. That is the difference between a shade that lands and one that comes out washed.

So ask, every time, and state your own basis when you hand a number to someone else. A rate quoted without its basis is not a rate.

The sourced numbers

For cast polyurethane: one to two percent of the total system mixture is typical, with four percent as a working maximum. That is the tightest figure in open publication and it agrees well with the band across our own product documentation, which runs one to five percent for standard liquid colors. Our band is wider because it covers pigments of very different strength.

Within that band, our lab's working rule of thumb is roughly one percent for blacks and roughly three percent for chromatic colors. Grades carrying a UV and HALS package run higher, two to six percent, because a meaningful fraction of what you are metering in is stabilizer rather than pigment. Carbon black grades sit around one to four percent. One effect grade in the line runs as low as 0.1 percent.

For flexible PVC plastisol: two percent, a 50 to 1 let-down. For PVC masterbatch: generally one to four percent. Worth noting that every source expresses these as a percentage or a ratio and none of them expresses colorant in phr, even though phr is the standard basis in PVC for plasticizers. If someone hands you a phr figure for colorant, ask where it came from.

Why one percentage cannot cover every color

Pigments differ enormously in tinting strength and hiding power, and the spread is much larger than most published guidance admits.

Carbon black is the extreme case at one end. It absorbs across the whole visible spectrum, so a small fraction of a percent already reads dark and full black arrives at a loading that would leave most other pigments looking like a tint. Titanium dioxide is the extreme at the other end: white is achieved by scattering rather than absorbing, scattering is inefficient compared with absorption, and a solid opaque white therefore needs a lot of pigment. That is why whites are the loading problem in almost every line, and why whites are the grades most likely to need the polyol carrier, since more pigment means more carrier riding along with it.

Between those two sit the chromatic pigments, and they split again. Inorganic pigments such as the iron oxides are opaque and forgiving. Many organic pigments, particularly the clean bright reds and yellows, are transparent by nature. A transparent organic red will not reach full opacity at any sensible loading on its own. It reaches it with white behind it, which changes the shade, which is why the answer to "how do I get this red more opaque" is usually a reformulation rather than a bigger scoop.

A single percentage quoted for all colors is an average across things that are not alike. It is useful for planning a purchase. It is not useful for hitting a target.

What goes wrong above the band

The upper limit is not arbitrary. Three separate things start to bite.

Physical performance. Higher pigment loads can degrade the physical performance of the cured part. Pigment is solid filler that is not part of the network, and past a point it interferes with the properties you cast a urethane to get.

Cure behavior. Gel time may slow in pre-colored Shore A durometer systems. The usual remedies are adding catalyst to the polyol or coloring immediately before each batch rather than holding pre-colored material. If a soft system started running slow after a color change, that is where to look first.

The index, if your carrier is a polyol. A polyol-carried dispersion brings hydroxyl into the formulation, and hydroxyl consumes isocyanate. Below roughly one part per hundred the contribution is small enough that most formulators reasonably ignore it. Above that, it has to be folded into the isocyanate index or the part comes out soft and under-cured, and the shop spends a week blaming the wrong ingredient. A plasticizer carrier has no active hydrogen and does not enter that arithmetic at all. Which carrier you are running, and what each one costs you, is covered in choosing the carrier for a polyurethane pigment dispersion.

Where we will not give you a number

For PU foam, TPU, and polyurea there is no defensible universal let-down rate, and we are not going to invent one. The rate moves with the system, the density, and what the finished surface has to look like. The suppliers who handle this question honestly say the same thing: the addition rate is calculated for the job. Send the system and we will establish it on the bench.

The same goes for a phr figure in PVC. No source expresses colorant that way, so neither will we.

How to land on your number in one afternoon

Run a ladder rather than a single shot. Take the published starting figure for your system, mix three small batches at that rate, at half again, and at double, and cast a plaque from each. Cut the plaques, because cast color goes all the way through and a cut face tells you what the part will look like after machining. Read them side by side against your standard under one light source, and take the lowest loading that holds the shade rather than the one that looks best, because the lowest loading is the one that costs you the least in properties and in money.

Write down the basis you used. That single line saves the next person a week.

Frequently asked

How much colorant do you add to cast polyurethane?

One to two percent of the total system mixture is the typical figure, with four percent as a working maximum. Our own product documentation across the line runs a 1 to 5 percent band, wider because it covers pigments of very different strength. Blacks land near the bottom of that band and chromatic colors near the middle. The number is a starting point for a trial, not a specification.

Is the percentage of the polyol or of the total system?

Ask, every time. The commonly quoted cast polyurethane figures of 1 to 2 percent typical and 4 percent maximum are of the total system mixture. Other published figures, including patent language, are expressed on the weight of polyol alone. On a system running near a one-to-one ratio those two bases differ by roughly a factor of two, which is the difference between a shade that lands and one that does not.

Why can't one percentage cover every color?

Because pigments differ enormously in tinting strength and hiding power. Carbon black hides at a fraction of the loading a titanium dioxide white needs, and a transparent organic red may not reach full opacity at any sensible loading without a white behind it. A single percentage quoted for all colors is an average of things that are not alike.

What happens if I add more colorant than the recommended range?

Higher pigment loads can degrade the physical performance of the cured part. Gel time may slow in pre-colored Shore A durometer systems, which is usually handled by adding catalyst to the polyol or by coloring immediately before each batch. And if the carrier is a polyol, above roughly 1 part per hundred its hydroxyl contribution has to be folded into the isocyanate index rather than ignored.

What is the colorant addition rate for plastisol and flexible PVC?

Two percent, a 50 to 1 let-down, is the published figure for flexible PVC plastisol. PVC masterbatch is generally quoted at 1 to 4 percent. Every source expresses this as a percentage or a let-down ratio, not as phr, even though phr is standard in PVC for plasticizers.

What is the let-down rate for PU foam, TPU, or polyurea?

There is no universal figure, and we will not invent one. The rate depends on the system, the density, and what the part has to look like. Send the system and the target and we will establish the rate on the bench instead of guessing.

Get the rate with the color

Every color we match ships with the addition rate it was matched at, on the basis it was measured on. Custom matching is free, samples ship at 0.5 kg, and there is no minimum order. Send the resin system and the target and we will tell you the number for your system rather than the average for everyone's.