White and black are the most commonly used colors, and percentages from 1 to 5% are common (even more is used to make conductive blacks). There are many varieties of these, with different particle size, opacity and tendency to clump (agglomerate). With most other colorants, you’re in Monet’s world—very little is needed to get the desired effect, and colorants are generally more expensive than resins. This is the economic justification for concentrates, as they allow mixing ratios on the order of 1 in 20 (which is hard enough) rather than the 1 in 200 that might be needed with “dry coloring.” The dry colors may be cheaper, but the concentrate gets them in uniformly and saves money that way.
Carriers affect melt viscosity
Liquid colorants are often best of all, but may cost even more, and require special equipment and attention to the carrier, which affects both cost and mixing, with solid as well as liquid. It may also affect physical properties. I once heard of lard (yes, pig fat) being used as a colorant carrier because it reacted during extrusion and was less harmful to strength than another less-compatible carrier. And carriers affect melt viscosity, which relates to the rule that viscosities of minor/major components (at appropriate shear rates and temperatures) should be in proportion to the proportions. In other words, the minor component must thin out more quickly to flow more easily through the spaces between the major particles. It sounds hard to implement precisely, but if you use melt index, especially HLMI, you are better off following this rule than just buying what someone says is OK.
There are other ways to get better mixing, too: Static mixers, higher pressure in melt zones (tighter screens, lower temperatures) and special screws. Internal water cooling of single screws can do this, and it is reversible and controllable. Twin-screw lines are often promoted as PVC mixers; this is true for many designs, with the higher cost per unit output justified by lower formulation costs, as less stabilizer is needed as melt temperature is lower. Feeders matter, as continuous proportional feed may do away with the need for premixing.
Finally, layering (coating or co-extrusion or laminating or combinations of these) is useful where color is needed only on the surface.
Allan Griff is a veteran extrusion engineer, starting out in tech service for a major resin supplier, and working on his own now for many years as a consultant, expert witness in law cases and especially as an educator via webinars and seminars, both public and in-house. He wrote the first practical extrusion book back in the 1960s as well as the Plastics Extrusion Operating Manual, updated almost every year, and available in Spanish and French as well as English. Find out more on his website, www.griffex.com, or e-mail him at [email protected].
Griff conducts live seminars across the country. The next ones are scheduled for Houston on June 25 and Chicago on June 27. Seminars in your plant are also available. If you can’t attend his live events, he offers a Virtual Seminar, which can be seen at anytime, anywhere. E-mail Griff at the address listed above for more information.