The Details of Molded-in Color in Plastics

By Michael Sepe, Materials Analyst

Ask most industry advocates about the benefits of plastic materials and one of the advantages that will certainly come up early in the conversation is that of molded-in color. The ability to avoid painting is an obvious benefit not only in reduced cost and shortened delivery cycles, but also from the standpoint of regulatory concerns.

But coloring plastic materials is a science all its own. The chemistry of the colorant system must be compatible with that of the polymer, particle size must be effectively controlled, and where color concentrates are being employed care must be taken to avoid problems that can arise when carrier resins are incompatible with the base resin. The requirements have become much more demanding as certain routes to achieving some colors have been effectively outlawed by global pressures to remove heavy metals from the recipe.

Assuming that all of these considerations have been addressed, there are still the mechanics of employing equipment that does an adequate job of achieving a homogeneous melt so that the color is evenly distributed. This becomes particularly important in thin-walled products where local concentrations of color can cause nonuniform properties and weak spots. And then there is the mundane task of ensuring good ongoing control of the metering process so that the correct amount of color is being added.

All color concentrates come with a recommended letdown ratio, usually stated in pounds of concentrate per 100 lb of resin. Ask most processors how much concentrate they are using in a given application and they will confidently point to the label on the color concentrate package that specifies the letdown ratio and recite the specified value. But most processors have also had the experience of running out of color concentrate before the corresponding natural material has been consumed, a sure sign that the color is being used at a higher loading than anticipated.

The reasons for this are varied, but the consequences are not. Higher cost is the most obvious problem. Color concentrates, even in commodity materials, are much more expensive than the base resins to which they are added. Alterations in material properties are another concern. High color loadings can increase part weight, reduce impact strength, and alter the crystallinity of those materials that are capable of crystallizing.

This case study is an example of the consequences of using too much color. It is also a lesson in the dangers of jumping to conclusions based on insufficient data and it shows that specifications and certifications are not always to be trusted.

Degradation: Environment or Colorant?
The failure involves a relatively thin-walled container made of a high-molecular-weight, high-density polyethylene. The part is designed to hold a variety of fluids and had a long history of application success. While the application conditions for the container are not harsh, the storage conditions for the product prior to delivery to the consumer can involve elevated temperatures in warehouses and significant top loads from stacking. The problem manifested as cracking that occurred while the product was in this warehouse environment filled with various products.

Slow crack growth while under constant load is the most common form of field failure for plastic products. It is known as environmental stress cracking, and is essentially a creep-related phenomenon accelerated by the presence of a surface active agent. The rate of deformation is driven by stress and temperature. As these factors increase, the rate of deformation increases and the time required to reach a critical strain is shortened.

A number of mundane factors can cause these inputs to change. Higher stacking columns, reduction in the strength of the cardboard used to make the box that holds the containers, or a change in the box height have all been known to result in a stress state that produces failures in a previously successful product. But during this type of a failure analysis, the material must also be examined.

In this case, the first step was a verification of two very important properties in polyethylene: the density and the melt-flow rate (MFR) or melt index. The density is related to the crystallinity of the material and governs the balance of strength, stiffness, and toughness of the material. The melt index is a relative measure of the average molecular weight, which is a key driver of impact resistance as well as long-term properties such as creep, fatigue, and environmental stress crack resistance.

The raw material was listed as having a nominal melt index of .50 g/10 min and a nominal density of .950 g/cc. The test on the part came back with an MFR of 1.29 g/10 min and a density of .974 g/cc. This led to an initial conclusion that the wrong material had been used to produce the part.

But the part is white! With few exceptions, white color is imparted to plastic materials using a pigment based on titanium dioxide (TiO2). The density of titanium dioxide is a little more than 2.1 g/cc and using a rule of mixtures (which is not always strictly accurate), we can calculate that it would take a pigment loading of 2.13% to raise the density of the material by this amount.

While this is a reasonably high pigment loading, it is certainly not unheard of. But the only way to tell whether or not this represents an excessive use of colorant is to characterize the pigment loading of the concentrate and compare it to the letdown ratio supposedly being used during processing.

Determining Letdown Ratio
This also helps to unravel the mystery of the MFR increase. A material that begins with an MFR of .50 g/10 min and ends up in a part at 1.29 g/10 min would typically be considered degraded. But color concentrates are often intentionally made with a higher MFR to promote good mixing during processing. So while we are establishing the pigment loading of the color concentrate, we can measure the MFR. This also provides us with two methods for establishing the letdown ratio that was actually used during processing.

The intended letdown ratio of the color concentrate was 2 lb per 100 lb of resin. We are already in trouble here because if our calculation of 2.13% pigment in the container were correct, the concentrate would have to be more than 100% pigment (2/102 = 1.96%). The essential questions then are how much pigment is in the color concentrate and what is the carrier resin for the concentrate.

The specification for the color concentrate lists the total pigment loading as 50%, with 45% being titanium dioxide and the other 5% calcium carbonate. This throws a bit of a wrinkle into the calculations because the density of calcium carbonate is somewhat higher than that of TiO2 at 2.83 g/cc. It also helps a little bit because it puts the composite density of the pigment system at 2.24, so now it is at least physically possible, although unlikely, for the concentrate to provide this much pigment.

Thermogravimetric analysis (TGA) can be used to provide an accurate breakdown of the composition for the color concentrate. Some may wonder why a simple ash test would not suffice. The reason has to do with the properties of calcium carbonate. While titanium dioxide is completely stable at the temperatures needed to decompose the carrier resin, calcium carbonate undergoes partial decomposition at elevated temperatures, breaking down into carbon dioxide (CO2) and calcium oxide (CaO). Only the CaO remains as ash; the CO2 is lost along with the polymer. Unless we know the ratio of TiO2 and calcium carbonate ahead of time, in which case there is little point in performing the test, then we cannot with certainty use the amount of residue to deduce the quantities of the two components.

Fallible Certifications
Figure 1 shows the result of the TGA test on the color concentrate. As it turns out, this particular lot of color concentrate does not reflect the specified recipe. The total ash content is 51.61%, which is certainly within the industry tolerance for a nominal pigment loading of 50%. However, the small weight loss of .685% near 700°C shows that there cannot be 5% calcium carbonate in the concentrate. If there were, 2.2% of the sample would have decomposed at this point in the test.

The weight loss here is associated with a calcium carbonate content of 1.56%. The remaining 50.74% of the ash is TiO2. This is relatively trivial in the context of determining the root cause of the failure; however, it does point out the problems that can arise when relying too much on certifications. In addition, TiO2 is more expensive than calcium carbonate. Therefore, the color concentrate manufacturer’s costs for making this material are higher than they should be.

The results of a similar test on the molded container are shown in Figure 2. This answers the question of the real letdown ratio. The actual ash content in the part is 2.31%, quite close to the value of 2.13% that we calculated based on our density measurement. More importantly, taking the ratio of 2.31% in the part to 51.61% for the concentrate shows that almost 4.5% of the molded part is color.

Now that we have the real letdown ratio used when the part was produced, we can measure the melt index of the color concentrate and calculate its effect on the final MFR of the molded part. The melt index of the color concentrate was measured at 15.22 g/10 min. (The certification said 10.45 g/10 min). Again, applying a simple rule of mixtures where 95.5% of the part is the original material with a melt index of .50 and 4.5% is made of the concentrate, we get an MFR value of 1.16 g/10 min. This is before any processing has been done. Now the value for the molded part of 1.29 g/10 min appears quite benign and we have a complete picture of the true state of our materials and our part.

It should be emphasized that this treatment does not conclusively link the cracking problem to the pigment overload. However, because the addition of a high level of pigment is a known contributor to a loss in toughness, it must be considered as a possible root cause. An examination of the fracture surface under high magnification may help to account for the role of the pigment in the failure. At the very least, the high pigment loading represents a lack of control over the process and an incurring of additional cost.

September, 2006 - Reprinted with permission from Injection Molding Magazine. Copyright © Canon Communications LLC.