By James Ford, Senior Chemist, Color Resolutions International
As has been stated many times over the centuries, all that glitters may not be gold. In the case of metallic printing inks, it is usually aluminum, copper, and/or zinc. Not so long ago, when printing metallics on corrugated stocks, many printers only saw trouble, and avoided metallic inks whenever they could. However, printers today have many more options than existed just a few years ago. The pigments are more stable, the ink chemistries have improved, and the regulatory issues are more understood. If you have not looked at metallic inks in the past few years, it may be time to give them another chance. Just like with any other ink system, the best choice of metallic formulations will depend upon the particular job which is being considered.
In general, metallic pigments are not like other pigments. The particle shape and size are much more important. Most metallic pigments can be thought of as flakes, and it is this flake structure that allows for more uniform reflectance, therefore brightness, than conventional (organic or inorganic) pigments that scatter light. Also, they tend to be larger in size than conventional pigments, with 4 to 12 micron sizes being common in the flexographic printing realm. However, this shape and size has made them historically more difficult to run on press. Previously, the inks were typically made using resins providing better protection, but little resolubility, such as polyurethane resins. Thus, the particles might plug aniloxes more easily, or may not transfer as well to the substrate. There were also more problems with piling and foaming on press, as well as handling and storage issues. In the past, the printer had to walk a fine line between having enough base (ammonia, amine, etc.) to keep the resin in solution, and keeping the pH low enough to minimize corrosion. Due to the problems running metallic golds using copper/ zinc alloys, many printers chose to use tinted silvers to imitate the golds, or switched to pearlescent pigments. Pearlescent pigments are relatively inert, and tend to cause little problems in printing. Like other metallics, they do have a tendency to settle, but do not generally cause stability or gassing issues. However, the visual effect from a pearlescent silver or gold is often inferior to what could be achieved with a true metallic version. With recent improvements in metallic pigment and resin chemistries, one is able to develop a more stable pigment particle with much better on-press stability. Newer ink formulations can run on press for much longer periods of time without adjustments, and they tend to hold up to shear better than previous versions.
CRI's James Ford comparing printed samples to the Pantone Metallic Formula Guide
Metallics can be formulated in many degrees of brilliance. An obvious consideration is that the brighter/shinier effects are generally more expensive. For example, workhorse silvers can cost on the order of a few dollars a pound, while some of the mirror-image metallics may run as much as $150 per pound. Much of the variation in expense is related to the methods required to manufacture the higher reflective pigments. All metallics should be treated to as little shear as possible to maintain the brilliance. However, the high end metallics may require special blending equipment, such as ribbon mixers to maintain the sheen. Another factor affecting the brilliance with aluminum pigments stems from the difference between leafing and non-leafing pigments. Leafing pigments appear brighter because they rise, or leaf, to the surface of the coating in order to show the brightest effects. The down side to this is that since leafing pigments will be exposed on the surface at a much greater proportion, they will tend to rub off more when any abrasive pressure is applied. The best rub performance for a metallic on mottled or natural kraft is often achieved by using a non-leafing pigment in conjunction with a good rub-resistant wax. Overprints can be used to protect the metallic inks from abrasion, but these have a tendency to dull the brilliancy to some degree. It may be necessary to use a brighter pigment when an overprint is to be used.
Metallic pigments and dispersions used in flexographic printing inks.
Aluminum pigments, which are used to make metallic silvers, may react in high pH environments to generate aluminum oxides and hydrogen gas. This could be a significant problem with storage, as well as during printing, since some inks contain resins which must be kept at a somewhat higher pH level in order to keep the resin in solution. Copper and zinc alloy pigments, used to make metallic golds, have historically generated numerous viscosity stability issues, as well as the bad trait of tarnishing quickly under high pH conditions. Metallic pigments are manufactured in a way to try and protect the surfaces from shear and oxidation effects using various surface treatments. Significant advancements have been made in the past few years to develop better means of protecting the metallic pigments, as well as developing more stable ink vehicles for use with metallic inks. Overall, the new pigment and resin chemistries enable the formulation of metallic inks which are less sensitive, more stable, as well as more resoluble on press. However, one still needs to be cautious of the basic metal chemistry, and should heed the instructions given by their ink supplier. For example, it is always advisable to eliminate or minimize the use of pH adjusters with metallic inks, and if anything is added to the metallic inks, allow the inks to degas for a period of time before placing back into storage. Also, if the metallic inks need to be toned with conventional pigments, it is always best to ask the ink supplier for a list of suitable toners. Dispersions with high pH levels and dispersions of pigments containing metal salts, such as calcium lithol or methyl violet, should be avoided since the metal salts may react with the metallic aluminum and bronze pigments used in the metallic inks.
The bronze pigments used to make the “gold” metallics will contain somewhere between 10 and 25% copper, along with a certain amount of zinc. Both of these may be regulated from a wastewater discharge standpoint. We have even come across a few issues relating to aluminum discharge limits. There are printers that just will not be able to use bronze or aluminum pigments without some special type of effluent treatment system. Beyond this, many metallic formulations contain a small amount of alcohol or other solvents as carryover from the refining/ polishing of the metal. Alcohol has been used to develop pigment pastes that are more easily incorporated into inks, and have a tendency to reduce foaming. These VOC levels have often been a problem for printers due to air permit restrictions. However, new technologies have enabled the development of low VOC alternatives without drastically impacting the brilliance of the metallic particles. Formulations once resulting in VOC levels in the 6 to 7% by weight range can often be replaced with versions as low as 0.3% by weight. There are even options available that utilize sustainable resin chemistries as part of the ink composition.
The brighter metallic pigments tend to have less opacity, so these would be unsuitable for kraft and some mottled stocks. Since the brighter pigments are more expensive, one would also not want to use the brighter pigments to print on such substrates, anyway. A good balance will need to be made between brilliance and coverage depending on the chosen substrate. Aniloxes with line counts between 180 and 440 have been run successfully on corrugated substrates, including coated and non-coated stocks, but best results are obtained when the max particle length of the metallic pigment is limited to one-fifth of the anilox cell width. The composition of the doctor blade needs to be considered. Many blade suppliers will tell you that metallic inks should not be run on composite or graphite blades. These tend to show excessive banding when used with metallic inks. In the end, it is important to remember that with metallic inks, the reflectance is vital. High shear environments, excessive use of opaque waxes and other additives, will have negative effects on the brilliance and should be avoided.
Pigment and resin chemistry advancements have led to the improvements seen in recent metallic ink formulations. The new formulations will run better, look better, and will be friendlier to the environment. There are even flexographic post-print options that can approach the level of quality and brightness seen with hot stamping. We hope that these reflections enlighten your mind to consider a brighter future with metallic inks.
James Ford is the Senior Chemist and Special Projects Technical Manager for Color Resolutions International in Fairfield, Ohio. Color Resolutions International (CRI) is an international packaging ink company focused on water-based and UV curing flexo inks for corrugated, flexible packaging, envelopes, folding cartons and tags and labels. For more information about CRI, visit www.colorresolutions.com or call toll free at (800) 346-8570.