There’s no magic pill (or generic carrier) that can be used in all colorant applications. Do your homework and verify the concentrate composition.
One of the most effective ways to reduce costs in an injection molding operation is to implement the practice of coloring raw materials at the press. Fully compounded colors are usually higher in price than the combined cost of the natural base resin and the color concentrate and lead times for these precolored materials are typically longer. Specialty colors are also increasingly subject to large minimum order quantities.
In addition, use of natural resin plus color concentrate allows for flexibility in adjusting to order quantity changes. Five thousand pounds of fully compounded orange polycarbonate can only be used to make orange product. But 5000 lb of natural material can be combined in any number of ways with various color concentrates to fulfill orders without over-committing to a color that suddenly becomes obsolete or lags in usage due to a change in customer tastes.
Coloring at the press requires the use of a concentrate—a powder or a liquid colorant that is mixed with the natural resin to provide a colored finished part. Color concentrates are still the most common method for imparting color to the molded product and are the subject of this article.
The incompatibility of universals
As their name indicates, concentrates are a highly concentrated form of the colorant mixed with a carrier resin—a binder that allows for easy introduction of the color into the natural resin. In most formulations, approximately half of the concentrate is color and the other half is the carrier. The concentrate is typically introduced at levels of 2-5 lb per 100 lb of resin. Therefore, in the final product the carrier resin can make up approximately 1-2.5% of the final molded part composition.
Many molders consider this to be a minor ingredient, and this leads to a practice that is common in our industry but is often the cause of significant quality problems: the use of “universal” carriers for color concentrates.
Universals, as the name suggests, can be used to color a wide range of raw materials. A typical data sheet for a universal color concentrate will cite utility in all densities of polyethylene and ethylene copolymers such as ethylene-vinyl acetate (EVA), as well as polypropylene, polystyrene, SAN, ABS, PVC, nylons, PET polyesters, and polycarbonate. The composition of the carrier is not mentioned; however, a few properties provided in the literature suggest that it is an ethylene-based material.
If we assume this to be the case, it is credible that the material can be mixed with polyethylenes, ethylene copolymers, and polypropylene. However, anyone who has ever inadvertently mixed a polyethylene or a polypropylene with a styrenic knows that the two systems are incompatible and produce significant delamination. In fact, even some combinations that seem reasonable are incompatible. For example, SAN and ABS can be melt-blended into a uniform mixture, but polystyrene and ABS cannot. Therefore, it is improbable that a concentrate that can be mixed with polyethylene can also be blended with polystyrene or ABS.
The consequences of using carriers that are incompatible with the base resin vary from productivity sacrifices such as extended cycle times to poor performance caused by polymer degradation.
Don’t blame the processing
Many years ago I attempted to use an EVA-based gray concentrate to color a part molded in a PC/PBT polyester alloy. The part contained a long, thin core that formed a recess for a metal blade. This core was difficult to cool even when tooled in beryllium copper and at the tip it typically ran at a temperature greater than 100 deg F above the setpoint temperature of the cooling water. In natural material the cycle time for the part was 32 seconds. When the color concentrate was added, the part distorted badly in the blade area and the cycle time had to be lengthened to 51 seconds, a 37% loss in productivity. When a concentrate was formulated with a PBT polyester carrier, the cycle time returned to 32 seconds.
Combinations that result in polymer degradation are more serious because they reduce the performance of the molded part and may not be obvious from the part appearance or a change in process conditions. A universal black mixed with a polycarbonate produced molded parts with very high melt-flow-rate (MFR) values. The universal was based on low-density polyethylene (LDPE) and when mixed with a polycarbonate with an MFR of 10 g/10 min, it produced molded parts with MFRs of 30-40 g/10 min, an increase of 200-300%.
Initially, this was attributed to the molder’s poor process control. However, an evaluation of the process showed no evidence of problems and when the color was removed and parts were molded with natural resin only, the increase in MFR dropped to 20-25%, a result consistent with good retention of the average molecular weight of the polymer. Changing to a color concentrate based on a PC carrier resin prevented the problem from recurring.
Sometimes the incompatibility produces a combination of processing difficulties and polymer degradation. Recently, a client sent in samples of parts molded in a glass-filled nylon 6. The parts were black but had white blemishes on the surface. When the white blemishes were analyzed by infrared spectroscopy, they proved to be degraded nylon.
Once again, the focus fell on the molding process. However, a thorough examination of the molding conditions and the drying practices employed by the molder showed no reason for polymer degradation. But in discussing the process with the molder, they reported difficulty with screw recovery, particularly during startup. A further examination of the raw material showed that it was a salt-and-pepper blend of natural material and a black concentrate.
Check your carrier
Often when a material supplier provides the concentrate in such a dry blend form, the processor assumes that the selection of the color concentrate is done correctly. However, an examination of the color concentrate composition by DSC (differential scanning calorimetry) provided the result in the graph. A melting point of 72.5°C (162.5°F) is not a fingerprint for a nylon. Further evaluation by infrared spectroscopy showed the carrier resin in the concentrate to be EVA. The fact that this is done frequently in the industry does not alter the fact that it presents some significant difficulties for processors.
Consider that glass-filled nylon 6 typically is processed at temperatures of 500-550°F. For nylon 6/6 the temperature range is a little higher and for nylon 4/6 the melt temperature approaches 600°F to accommodate the higher melting point of the base resin. EVA is based on polyethylene, one of the most thermally stable thermoplastics available. However, the vinyl acetate component is not as stable. Most processing guides for EVA warn against using processing temperatures hotter than 518°F to prevent resin degradation. In fact, the vinyl acetate component degrades at a temperature so much lower than that of the polyethylene that the actual vinyl acetate content of an EVA resin can be accurately measured by slowly heating the material in a TGA (thermogravimetric analysis) and observing the early weight loss.
It gets better. When vinyl acetate decomposes, the primary byproduct is acetic acid. Strong acids tend to degrade nylon. At room temperature, nylon is listed as resistant to acetic acid until the concentration becomes very high. However, inside the injection molding machine the temperature is much higher than 73°F and as a rule higher temperatures permit certain types of chemical interactions to occur much more rapidly and aggressively. At 540°F the EVA releases acetic acid and this acid in turn attacks the nylon.
MFR tests showed that the attack did not result in a severe loss in average molecular weight in the bulk of the part. But the reaction was capable of producing the defects found on the part surface. In addition, when the EVA becomes overheated, it turns to a waxy lubricant. This interferes with smooth feeding of the resin into the screw and causes screw slippage and inconsistent recovery.
The cost of poor quality
These are just a few examples that support the avoidance of universal color concentrates. Everyone acknowledges that colorants are contaminants to a base resin. We tolerate them because the ability to mold in color is a major competitive advantage over painting, and we have learned how to adapt the chemistry of the color to that of the appropriate base resin. However, given the fact that in most cases the carrier resin is approximately 50% base resin, it makes sense to minimize the contaminating effect of the color concentrate to the color and not double the effect by also adding an inappropriate carrier resin.
A color concentrate should be based on the resin to which it is being added. There is some wiggle room in this rule. For example, SAN and ABS are compatible; therefore, an ABS concentrate can be based on SAN with minimal impact on properties. A lower-melting nylon works as a carrier for a higher-melting nylon base resin. In blends, such as PC/PBT, a concentrate based on one of the two resins works well. And LDPE or LLDPE carriers are acceptable in HDPE and usually work in polypropylenes as well. However, even in combining polyethylene and polypropylene, problems have been noted in sensitive part geometries such as living hinges.
Anything outside of these general guidelines comes under the heading of buyer beware. If the processing conditions are optimal and the part geometry is simple, colorants based on incompatible carriers may work or may appear to work. As in the case of the PC components mentioned at the beginning of this article, the parts may look fine and then fail functionally for reasons that are not readily apparent.
Of course, the rationale for universals is the same as it is for every ill-advised shortcut: cost. But real costs are measured in total productivity and prevention of quality issues. When these are considered in relation to the minor impact of a more expensive color concentrate, the justification for poor practices does not hold up.
About
Michael Sepe Michael Sepe has worked in the plastics industry since 1975 in a variety of roles involving both manufacturing and research and development. He is an independent consultant based in Arizona with clients throughout North America. He assists clients with material selection, designing for manufacturability, process optimization, troubleshooting, and failure analysis. Learn
more about Michael Sepe.
Check your carrier