UltraViolet light absorbers have been known to plastic formulators, for some time, as a necessary additive to protect plastics from the long term degrading effects of sunlight. Infrared absorbers have been known only to a small group of plastic formulators. However, as the laser finds increased application this relatively unknown group of additives is increasing in use.
As lasers became more powerful, in the late sixties and early seventies, it became evident that laser operators must be protected from the blinding effect of infrared radiation. Depending on the power, and proximity of the laser to the eye, either temporary or permanent blindness could result. At about the same time, with the commercialization of polycarbonate, molders learned to use infrared absorbers in plates for welder’s face shields. This innovation offered high impact strength, protection from infrared radiation and lower cost than the glass plates then in use.
If one wanted to block all infrared radiation, and was not concerned about seeing through the device, one could use carbon black. However, many applications require transmission of visible light as well as blocking infrared wavelengths. Some of these applications include:
Military Eyewear –Powerful lasers are used by the military for range finding and sighting of weapons. It has been reported that during the Iran – Iraq war of the eighties, the Iraqis used the powerful laser range finder on their tanks as a weapon to blind the enemy. It has been rumored that a potential enemy is developing a powerful laser to be used as a weapon, intended to blind enemy troops. The Neodynium/YAG laser emits light at 1064 nanometers (nm), and is used for range finding. Consequently, today soldiers wear goggles with a molded polycarbonate lens incorporating one or more Infrared Absorbers, which absorb intensely at 1064 nm, to protect them from incidental exposure to the Nd/YAG laser.
Medical Eyewear – Certainly, it is important for soldiers to have good visible light transmission in goggles, which block Infrared Radiation. It is even more important that medical personnel using lasers have excellent visible light transmission, while being protected from incidental exposure to the lasers they are using. The infrared absorber selected must be coordinated so that it absorbs light at the wavelength of emission of the laser used. As the use of lasers in medicine increases, the need for protection from the harmful effects of infrared radiation will also increase.
Welder’s Face Plates and Goggles – As mentioned above, this is one of the oldest applications of Infrared Absorbers. In the past, the thickness and impact strength of the face plate was specified by an industry standard. This specification was selected primarily because the infrared absorbers used at the time would burn off if processed at a higher temperature. With the advent of Infrared Absorbers with greater thermal stability, the specification was changed last year to allow eyewear of any thickness.
Electric utility workers face shields – Electric Utility workers can be exposed to intense infrared radiation if there is an arcing of the electric cables. This radiation can be blinding, and in some cases it has been fatal. Face shields incorporating infrared absorbers have been helpful in reducing the tragic effects of some of these accidents. In the past, these face shields had to be made of cellulose acetate propionate, because the infrared absorber would burn off if polycarbonate were used. Recently, due to the advent of more thermally stable infrared absorbers, polycarbonate face shields are being introduced, providing these workers with needed higher impact protection.
High end skiing goggles – Sunlight reflected from snow and ice can have a blinding effect on skiers. In addition to dyes, to tint the goggles, and ultraviolet light absorbers to protect from UVA and UVB radiation, some manufacturers are now adding infrared absorbers to protect from the harmful effects of infrared radiation.
There are many other interesting applications utilizing the special properties of infrared absorbers. These include laser ablated lithographic printing plates, laser welding of plastic film, optical shutters, and security inks.
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 molded polycarbonate. The aminium salts are larger molecules and are more thermally stable than the cyanines. New developments in this chemistry have increased the maximum molding temperature of these absorbers from 480oF to 520oF. Depending on the chemistry of the aminium salts, these 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. 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 polycarbonate molders, are:
Thermal Stability – great care must be taken in formulating and processing polycarbonate containing the aminium salt infrared absorbers. The amount of absorber needed to block the desired amount of radiation must be calculated considering the thickness of the lens. The maximum exposure temperature and time must be determined and carefully observed. If the infrared absorber remains in the molding machine during an “extended coffee break”, the absorber will burn off and the first few pieces molded after the break will be rejected. Some newly developed aminium salt infrared absorbers have allowed the maximum safe molding temperature to be increased from 480oF to 520oF, thereby reducing the number of rejected parts due to burnoff.
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 to the molded part.
Haze – Related to Visible Light Transmission, haze is a critical property in eyewear as it can dramatically reduce visibility. Haze can be caused by impurities in the IR Dye, which do not dissolve in polycarbonate. The newer aminium IR Dyes are produced in such a way that these impurities are completely removed, thereby eliminating haze from this source, and coincidentally improving thermal stability.
Improved Products and Improved Quality – The correct choice of Infrared Absorbers, allows the plastics processor to offer products with improved performance properties and with a consistently high level of quality.
As infrared absorbers are much more expensive than other plastic 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 performance needed. 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.