The Materials Analyst, Part 97: Incompatible color concentrate carriers meet poor mixing practices

The industry myth of a universal carrier for color concentrates simply isn’t true, and the issues with incompatible carriers worsens with a lack of control over the color concentrate mixing process.

In past articles, we have discussed the problems related to using incompatible carriers in color concentrates. There is a myth in the industry that there are universal carriers—polymers that can be used as carriers for color concentrates and blended into any base resin to provide the desired color in the end product. Processors who have spent significant time in the industry have experienced inadvertent contamination. They know that very few materials can be mixed together without negative consequences because very few polymers are naturally miscible. (A chemist’s way of saying that two materials can be blended to produce a uniform product).

There are exceptions. Polyethylenes of different densities can be mixed without creating different phases, and often polyethylene and polypropylene will blend properly. Nylon 6 and nylon 6/6 can be blended uniformly and often are used in the same base compound and as a combination of colorant plus natural material, preferably with nylon 6 as the color concentrate carrier.

But the experience of molding different resins together at the press tells us that these cases are in the minority. Most attempts at blending produce a useless, delaminated mess with very poor properties. Even two materials that are available in commercial blends, such as polycarbonate and polyester, or ABS and nylon, cannot simply be melt blended. The manufacturers of these blends must perform special chemical modifications in order to get these types of combinations to behave in a uniform manner.

So the idea that color concentrates based on a single universal material can be added to any material is just another one of those absurd notions that the industry clings to, because in the short run, it at least appears to save some money. But the problem of incompatible carriers becomes even worse when it is employed by molders who have poor control over the color concentrate mixing process. This article profiles one of those cases.

Ratios letting you down?

When a manufacturer of color concentrates supplies a product, one of the important parameters provided with the product is the letdown ratio, typically given in pounds of concentrate per hundred pounds of resin. The objective of the letdown ratio is to ensure the correct level of colorant in the final product. If there is too little, the color of the product may not match the target or it may lack some functional characteristic. For example, if carbon black is incorporated to provide good outdoor weatherability, a low level of carbon black may result in early failure of the product due to UV degradation. However, too much color may compromise the properties of the base resin. Usually the first property to be affected is impact strength, and almost any color has the ability to reduce the toughness of a polymer if it is incorporated at a level that is too high. So addition of the proper amount of the color concentrate is an important aspect of good process control. When a molded part and the related color concentrate are tested, it is frequently discovered that a concentrate designed to be added at 3 or 4 pounds per hundred has been incorporated at significantly higher levels.

The addition of too much color is enough of a concern. But when the concentrate also contains an incompatible polymer, the problem is compounded. This was the case with a set of good and bad parts that were sent in for evaluation. The intended material was a black 33% glass fiber-reinforced nylon 6/6. One set of samples exhibited satisfactory behavior, while the other set of parts lacked virtually all the typical properties expected of this type of material. Impact was poor, but at the same time, the strength and stiffness of the material were also lacking. The composition of the two sets of parts was examined by both DSC (differential scanning calorimetry) and TGA (thermogravimetric analysis). DSC gives us insight into the state of the polymer and TGA allows us to look at the amount of the filler and, if necessary, to examine the filler either visually or analytically to determine its composition.

The results clearly showed that this was not simply a black nylon 6/6. Figure 1 shows the melting behavior of a good part and Figure 2 gives the same result for the defective product. Both samples show two melting points. The portion of the material that melts at a temperature slightly above 260°C is the nylon 6/6. The -endotherm that peaks near 126°C is attributable to polyethylene. In the good part, the contribution from the polyethylene is relatively small. Based on the -melting behavior of a pure polyethylene, we can estimate that the polyethylene makes up 2-3% of the final part. The amount of polyethylene in the bad part is obviously much higher. The strength of the melting point for the polyethylene in this sample is 15 times higher, which translates to a rather rough estimate of 30-40%. The TGA results will provide us with another opportunity to make that calculation. The corresponding reduction in the strength of the melting point for the nylon 6/6 is quite apparent.

Inquiring of the processor revealed that the purchased material was a natural nylon 6/6 and black concentrate was then being added with the intent of producing parts with a carbon black content of 2%. This practice is somewhat puzzling from an economic standpoint, because natural and black materials are typically sold at the same or very nearly the same price. Going to the expense of sourcing a second component and mixing it when the same result could be obtained from a single compound is not cost effective and introduces a step to the process that the molder was clearly not controlling very well. In addition, the concentrate was obviously being consumed at an alarming rate. And finally, the concentrate was based on polyethylene, so when the concentrate ratio spiked, the result was a non-functional part.

Information through decomposition

The TGA provides more information about composition through controlled decomposition of the product. This allows us to accomplish several things. First, by fully decomposing the polymer, we can measure the total glass fiber content and compare it to the nominal specification of 33%. Second, carbon black can be isolated by first decomposing the polymer in nitrogen and then changing the atmosphere to air. The carbon black will remain stable in the inert atmosphere and then burn off once the oxygen in the air is introduced. Finally, we can sometimes quantify the relative amounts of different polymers in a mixture if they decompose at significantly different temperatures.

Figures 3 and 4 give the TGA results for the two samples. The good part shows the expected amount of glass fiber, 32.28%. Typically the glass content is expected to fall within 2-3% of the nominal value provided in the material specification. The glass content for the bad part is 4.48%. If we do the math, it tells us that the glass-filled nylon makes up approximately 15% of the part. We said that 30-40% of the part appeared to be polyethylene based on the DSC result. Where did the other 40-45% go? It is made up by the carbon black. To the extent that the carrier resin replaced the base polymer in the part, the colorant has a corresponding contribution. The weight loss associated with the carbon in the material is 45.2%.

Now the weight loss in air due to carbon is 5.33% in the good part. But this does not mean that the carbon black loading is this high. We know from experience that nylon, when decomposed in an inert atmosphere (a process known as pyrolysis), will yield about 2% carbon by virtue of the way it breaks down. So we can estimate with a good degree of certainty that the carbon black level is in the range of 3.0-3.5%. This shows that even a good part probably has too much concentrate in it and therefore contains a higher level of polyethylene than desired. However, the part is functional. Polyethylene, however, produces no residual carbon when it is pyrolyzed. So the 45.2% total carbon in the bad part is virtually all carbon black. Between the massive overload of color and the replacement of most of the nylon polymer with polyethylene, the bad part is very far from its intended composition.

There is another interesting detail in the TGA for the bad part. Notice that the polymer decomposition phase breaks up into two distinct steps. The relatively small weight loss that occurs first is attributable to the nylon while the much larger weight is due to the polyethylene, which is a more thermally stable polymer and therefore decomposes at a higher temperature. There is some overlap between the two weight loss steps, so the weight loss values associated with each step are not a perfect representation of the actual recipe, but they are close.

Poor process control

The root cause for the part failure is poor control over the process of mixing color concentrate and base resin, and we should not mix the issues here. The incompatible carrier resin is not the cause of the poor process control. But it adds another element to the problem that complicates matters. Because if there are parts being made with 3% carbon black and parts being made with 45% carbon black, logic and statistics tell us that there are parts in between these two extremes. Hopefully, most of them reside at the lower end of the range. But even if we assume this, it is important to remember that most color concentrates are approximately 50% color and 50% carrier resin. If the color concentrate loading fluctuates during normal production by 2-3%, a concentrate based on a compatible carrier such as nylon 6 will limit that fluctuation to the amount of color being added. Use of an incompatible carrier automatically doubles the degree of variation and increases the likelihood that the product will fail.

Finally, consider the fact that this product can see application temperatures as high as 130°C. Imagine, in addition to the loss in strength, stiffness, and toughness, the effect on performance that occurs when the percentage of polyethylene, a material with a melting point of 126°C, fluctuates between the two extremes represented by these parts. These types of sourcing decisions truly come under the heading of false economy.