Colored polymer components provide both functionality and aesthetics in many applications. For example, colors are often used to differentiate between product type, size, and model.
Color can also provide compounds with a cosmetic quality appearance or an eye-catching special effect look. However, there are certain intricacies and potential problems that can arise when incorporating color into a polymer.
Fundamentals must be observed if things are to work properly. This is because the discipline of coloring plastics is more complex than what is commonly thought and there is a science to the process.
Let's take a look at the fundamentals of the coloring process and ways to overcome problems that can arise when attempting to color polymers.
Developing Full Proof Colorant Package for a Polymer
When working with all types of polymers, color and appearance must be adequately engineered, just like any other desired thermal or mechanical property. The ability to achieve the desired color can be affected by the polymer itself or the combination with:
- Other polymers
- Modifiers
- Additives, or
- Stabilizers
Even if the color can be achieved in the particular system of interest, other performance attributes such as UV stability, flammability or mechanical properties may be adversely affected as well.
There are several key items that are essential in developing a colorant package for any polymer.
#1. Avoid any Chemical Incompatibility
The first item to be considered is the avoidance of any chemical incompatibility between the chemistry of the polymer and the chemistry of the colorant system. Melt processing of a polymer involves the use of significantly elevated temperatures and large inputs of mechanical energy. Chemical reactions proceed more rapidly at elevated temperatures and, therefore, any chemical interactions that can occur between a polymer and a colorant system will happen very rapidly during a process like
injection molding.
An example of this type of negative interaction occurs between polycarbonate and TiO
2, a primary ingredient in white colorants. However, in this case, it is not really the TiO
2 that is the issue but compounds such as hydroxides of aluminum and potassium that are constituents of the mineral that is removed from the ground.
These hydroxides are chemically basic in nature and polycarbonate does not withstand well in environments where bases are present. When polycarbonate is exposed to typical processing temperatures of 300°C and greater, degradation occurs rapidly. To overcome this issue, suppliers of TiO
2 pigments treat the material to neutralize the basic components that are present in the TiO
2.
TiO2 Colorant
In some instances, the adverse interaction or reaction is not between the base polymers and the colorant but with a
minor constituent in the material. For example, cadmium-free red colorants can be used with a general purpose
glass-filled nylon.
On the other hand, when these same pigments are used with
impact-modified grades of nylon, problems with the loss of ductility are observed. The conclusion is that the
same colorant that works well for general purpose nylon is not suitable for use with the same formulation with impact modification.
#2. Check the Thermal Stability of the Colorant System
The next consideration in selecting a colorant system for a particular plastic is the thermal stability of the colorant system that is being utilized. The chemistry of the colorant system needs to be capable of surviving the temperature at which the polymer formulation is going to be processed. Thus, a colorant that can be used for polyolefins like
polyethylene or polypropylene, where processing temperatures will be relatively low, may not be suitable for a polymer such as a polycarbonate or
polysulfone, for which typical processing temperatures are significantly higher.
It is important to select a colorant system that can tolerate the processing temperatures that are being utilized.
#3. Control the Amount of Colorant Used
The retention of good mechanical properties is a function of the amount of the colorant that is incorporated into the base polymer. Colorants can act as contaminants to the polymer in that regard, but often they are tolerated as contaminants because they assist in the achievement of the desired effect.
But, for each specific combination of polymer and colorant, there is a certain concentration limit as to how much color can be added before the properties of the base polymer are negatively affected. In most instances, the first polymer property that is usually affected is the ductility. An addition of 1 - 2 % of a colorant is usually not detrimental to the
polymer properties as long as there is acceptable chemical compatibility between the polymer and the colorant. However, amounts of the colorant above that level should be generally avoided to guarantee that there is no loss in the properties of the base polymer.
#4. Consider the Effect of Colorant on Crystallinity of Polymer
An effect of a colorant on a polymer’s behavior that needs to be considered is the influence of the colorant on the manner in which the crystallinity develops in a semi-crystalline polymer. Many colorants are natural nucleating agents for polymers such as
polypropylene. Nucleation typically changes the crystal structure of the material and also changes the mechanical performance, shrinkage and cycle time in injection molding processes.
In general, nucleated materials can be cycled faster and they shrink less than the corresponding non-nucleated polymers. At the same time, they also tend to have a lower impact resistance. This is a direct result of the manner in which the colorants change the crystal structure of the base polymer.
#5. Incorporate the Colorant in the Optimum Manner
The manner in which the color is incorporated into the base resin is also important. Colored polymer components can be produced from masterbatch concentrates in which the color is blended into the natural polymer prior to
molding or extruding.
In addition, they can be made by the use of precolored polymers via melt blending and
compounding. For example, engineering resins that require high masterbatch let down ratios benefit from precoloring because pigments are completely polymerized into a resin package and are used as supplied.
Polymer Color Masterbatches
Select the Right Coloring Method
Selection of one coloring method over the other depends on factors, such as:
- Economics
- Secondary operations, and
- End application of the colored product
Fully compounded color is often the preferred method. This is because, it bypasses the problems that often arise when color is added as a minor second ingredient in the form of a pellet, concentrate, a dry color or a liquid. Many of those issues are associated with the attainment of a uniform dispersion of the color throughout the polymer and, hence, the final product.
But, the economics of the coloring of the polymer where the part is being manufactured is more favorable for several reasons. These include a lower cost per pound as well as shorter lead times that are associated with the final product or products.
It does need to be remembered that the color represents a separate ingredient in the polymer formulation. To obtain the best results, the carrier resin for the color needs to be compatible with the base resin with which it is being mixed. For example, this means that if coloring of
polycarbonate is desired, the color concentrate pellets should be made of polycarbonate, not polyethylene or polypropylene. This will help guarantee that the dispersion of the colorant is uniform throughout the polymer.
However, there is no such thing as a universal carrier resin. Very few polymers generally mix well with other polymers. If the control over the proper ratio of the materials in the final mixture is not optimal, the amount of contaminant that is represented by the color will fluctuate and problems can develop. These issues can be reduced through the use of
metering weight feeders that will minimize the variation in the loading level of the color in the final product.
Ensure Good Mixing of Color with Optimized Molecular Weight of Carrier Resin
The molecular weight of the carrier resin also needs to be optimized to ensure good mixing and dispersion of the color in the polymer formulation. Even when the carrier resin is compatible with the base resin, it is often the case that the melt flow rate or MFR of the carrier resin is 8 to 10 times higher than that of the base material. This has the overall effect of introducing a low molecular weight constituent into the final part. This reduces the properties of the part as well as the processing window that is available for the production of the part.
Further, the use of dramatically different viscosity materials leads to segregation rather than mixing.
This issue is generally resolved by closely matching the melt viscosities of the carrier resin and the base resin to guarantee good mixing of the color in the final polymer formulation.
Conclusion
As has been discussed in this Article, there are several key challenges that are faced with the use of colored plastics for different applications. Some of those challenges are based on the
specific chemistry of the colorant system that is being utilized. Other challenges are focused on the
polymer type that is being colored in the formulation and issues that are associated with the use of particular polymers. Those challenges can be placed into the general category of materials challenges.
Several approaches to limit the effects of these concerns on the performance of the colored plastic have been presented and explained.
The other type of challenge that has been discussed is related to the
equipment that is typically used to add color to a plastic. Specifically, in the use of masterbatches, it is recommended that
metering weight feeders be employed to minimize the variation in the loading level of the color. This will improve the consistency and overall quality of the final colored plastic. As such, the performance of the produced part of the colored polymer will exhibit the desired characteristics and features that are wanted by the manufacturer.