It is a recurring well known problem that undesirable yellowing exists within polyester fibers and plastics. Furthermore, with the advent of adding UV absorbers within certain polyester containers and/or bottles as protectants for liquids and foodstuffs kept therein, this yellowing problem is compounded. Since UV absorbers absorb visible light most heavily in the low wavelength violet and indigo range of wavelengths, they produce a resultant yellow appearance within polyester. If such a plastic is clear and/or uncolored, the yellow appearance reduces the aesthetics thereof, particularly within thicker plastic portions (such as the bottom or bottlecap support within the neck of a polyester bottle), thus reducing the desirability of such an article from widespread commercial use.
The yellowing of the polyester by itself has been effectively reduced in the past, at least from a straightforward neutralization standpoint, through the utilization of certain types of toners that are incorporated into the target polyester in order to mask, hide, or neutralize the yellow color within the visible spectrum. Such toners for polyester must not exhibit extraction, must not be susceptible to degradation due to exposure to light, humidity, temperature, and other such drastic conditions. Such bluing agents should also exhibit a minimal degree of thermal degradation (or, conversely, excellent thermal stability) during polyester manufacture, desirably at any stage during plastic production, but acceptably at any stage of the article manufacturing process. In addition, the toner must have minimal adverse effects on the physical properties of the polyester polymer, such as in terms of reducing the intrinsic viscosity or transparency thereof.
One of the most prevalent compounds for this purpose is cobalt acetate. However, such a compound exhibits disadvantageous characteristics that limit its desirability for such an application. For example, cobalt acetate toned materials tend to be unstable during storage and are particularly susceptible to temperature and humidity, and tends to undergo an undesirable color shift toward yellow. Further, when high cobalt concentrations are needed to mask the yellow color of some polymers there is a tendency to impart a gray hue to the polymer. This grayness is believed, without intending on being bound to any specific scientific theory, to result from the broad range of wavelength over which this compound absorbs at a relatively high level. This effect appears to be attributable to the extremely broad half-height bandwidth thereof. As a result, and with such a relatively high absorption level over such a wide range of wavelengths, the brightness of the target polyester is compromised and the appearance thereof is dulled. Such a compound is also limited in its additive levels within polyesters by governmental mandate due to suspect effects of cobalt in relatively high amounts within such end-use articles, among other problems.
Additional toners include costly and rather suspect types within U.S. Pat. No. 4,745,174. Disclosed therein are certain 1-cyano-3H-dibenz-isoquinoline-2,7-diones that are effective as bluing agents generally; however, they are also expensive to manufacture and exhibit potential environmental and toxicological issues relative to their manufacture and use.
U.S. Pat. Nos. 5,384,377 and 5,372,864 both disclose mixed compound toner systems requiring red anthraquinones and blue anthraquinones. Such mixtures are polymerized into the target polyester (thereby exhibiting no migration within or therefrom) and provide a certain degree of effective neutralization of yellowing. However, as noted in greater detail below, such mixtures of compounds also generate a dullness or grayness within the target polyester that is undesirable to a certain aesthetic level. As with the cobalt acetate above, the combination of red and blue color synergistically produce a broad absorption spectrum with a rather wide half-height bandwidth. The resultant absorption peaks exhibited by such a combination are favorable to yellow neutralization, however, the high absorption levels exhibited for wavelengths not complementary to the generated yellow within the target polyester also dulls the resultant plastic. Furthermore, this combination is primarily utilized through polymerization within the target resin at the polyester polymerization stage. Although such compounds may be introduced at later stages of polyester production, these compounds are not disclosed as liquids, only as solid colorants. Thus, in addition to the prior art discussed above, nothing has been disclosed providing liquid solutions or dispersions of such combinations with UV absorbers to provide an easy-to-incorporate and/or -handle formulation of such type for the polyester manufacturer. To date, nothing has been taught nor fairly suggested providing a single compound for bluing purposes within polyesters that provides effective yellowness neutralization as a heavy metal-free liquid additive, and exhibits a narrow half-height bandwidth in order to provide a finished clear polyester with very high brightness and hue angle levels.