UltraViolet light absorbers have been known to formulators of inks and coatings, for some time, as a necessary additive to protect the ink or coating from the long term degrading effects of sunlight. Infrared absorbers have been known only to a small group of formulators of inks and coatings. However, as the laser finds increased application this relatively unknown group of additives is increasing in use.
Infrared Security Inks – In the ‘90’s, the proliferation of counterfeits of expensive consumer products such as watches, perfumes and others spurred the development of inks, which could verify the authenticity of the product. Early in this decade, especially since 9/11, the need to authenticate financial transaction cards, driver’s licenses, passports and currency has accelerated. There are many possible techniques and chemical compounds, which can be used for product authentication. One could use chemicals, which glow on exposure to UltraViolet or InfraRed light. In the case of our new US currency dyes, which change color on exposure to the light of a copying machine is used, thereby making it difficult to counterfeit the currency. Infrared inks used in hidden bar codes readable only by and infrared readers, authenticates the item as well as storing information about the product. Of course, the authenticator can use several of these techniques to further confuse the counterfeiter. US Patent 6,581,839 describes the use of an “invisible optically recognizable compound is an infrared ink …, which can be detected by a sensor in an ATM or card assembly line”.
Laser Ablation of Lithographic Printing Plates – Recent patent literature is replete with references to the use of infrared absorbing dyes used in the laser ablation of lithographic printing inks. In this case, the infrared absorber is dispersed in a pre-polymeric material. A computer directs very fine lasers at the pre-polymeric material. The infrared absorbers absorb energy intensely at the wavelength of emission of the laser, and ablate with a sudden increase in temperatures, perhaps several hundred degrees centigrade in a nanosecond, and initiate the polymerization reaction. In this way, an image can be transferred directly from the computer to printing plate very quickly. US Patent 6,730,457 describes: “Photo thermal conversion materials absorb radiation and convert it to heat. … Useful infrared absorbing compounds typically have a maximum absorption wavelength greater than 750 nm and less than 1300nm. … The photo thermal conversion material may be either a dye or a pigment. … Dyes, especial dyes with high extinction coefficients in the range of 750 nm to 1200 nm, are preferred.”
Window Film Coating – The infrared region of the spectrum is the region, which generates heat from sunlight. Infrared absorbers are being coated onto plastic film, sold as window film in countries such as Vietnam, Cambodia and Thailand, where air conditioning is just now arriving en masse. The infrared radiation of the sun is blocked by the infrared absorber, and convected away from the building, thereby reducing the heat load on the air conditioning system.
The three major groups of chemicals used as infrared absorbers are the cyanines, aminium salts and the metal dithiolenes. The cyanines are rather small molecules and therefore do not have the thermal stability to be used in high temperature applications. The aminium salts are larger molecules and are more thermally stable than the cyanines. Depending on the chemistry of the aminium salts, these dyes can have infrared absorption spectra, which range from very broad to fairly narrow. The metal dithiolenes are the most thermally stable, but have the disadvantage of being very expensive, and limited solubility in solvents. Some have absorption spectra, which are very narrow. If not synthesized properly, the metal dithiolenes can give off a foul sulfur smell during processing.
The properties of infrared absorbers, which are of greatest importance to printing applications, are:
Solvent Solubility – As metal complexes, the dithiolenes have limited solubility in most ink and coating vehicles. The aminium and cyanine dyes are soluble is a fairly broad range of solvents, particularly those with oxygen or nitrogen in their molecule. These dyes tend to be more soluble in aromatic solvents and very limited solubility in aliphatic solvents. The cyanines, which are smaller molecules, tend to have better solubility than the aminium dyes.
Color – Most applications would prefer infrared absorbers with no color. Unfortunately, nature has, thus far, not been so kind. Most dyes, which absorb broadly in the near infrared region tend to be green. There are a few browns and pinks, However, with a little creativity, it is possible to mask the color of the infrared absorber. Some do so by printing the infrared absorber on a lower layer, and then printing the picture or text on top of it. Others print the infrared absorber on an area of the credit cark on picture which is green.
Absorptivity – is the measure of infrared blocking power of the absorber per unit of weight, at a specific wavelength. The higher the absorptivity, the more blocking power. It is important that the supplier of infrared absorber have good batch-to-batch consistency of absorptivity. If not, you will be reformulating with each batch of absorber.
Visible Light Transmission (VLT) – In most applications you want to minimize transmission of infrared light, from 800 nm to 2000nm, and maximize visible light transmission from 450nm to 800nm. The human eye is most sensitive to light in the region of 490nm to 560nm. Unfortunately all available infrared absorbers absorb some visible light as well infrared light, and add some color, usually green.
Thermal Stability – If the ink, or coating, will be exposed to temperatures above 150oF, some care must be taken in selecting the infrared absorber. These dyes are sensitive to temperature, and will “burn off” if exposed to high temperatures for too long a period. The dithiolene dyes tend to have the best thermal stability, and the cyanines the least.
Ph – These dyes are also sensitive to acids and bases. They like a Ph close to neutral. Therefore, some consideration of possible interactions with other additives in the formulation, should be taken. For example, the use of a HALS compound, to protect the ink or coating from deterioration by sunlight, can cause problems. The HALS compounds are strong bases, and will react with these dyes, deactivating them. This problem can be avoided by using the benzophenone or benzotriazole class of UV Absorbers, which have a more neutral Ph. This interation can also be avoided by printing the infrared absorber in a different layer than the HALS compound.
As infrared absorbers are much more expensive than other additives ($/gram instead of $/lb), it is very important that the formulator take great care to formulate precisely to avoid waste, and to attain the maximum performance. It is equally important that the processor carefully develop the necessary processing conditions to avoid producing off-spec products. It can be a challenging task, but can result in high value added quality products.